WorldWideScience
1

Chiral N-heterocyclic carbene (NHC) ligands in Pd-catalyzed cross-coupling reactions  

OpenAIRE

New chiral N-heterocyclic carbene (NHC) ligands have been developed and applied in the asymmetric Pd-catalyzed intramolecular a-arylation of amides producing 3,3-disubstituted-, spiro-, and aza-spirooxindoles in high yields (up to 99%) and excellent enantioselectivities (up to 97% ee). For the first time a palladacycle intermediates were isolated and fully characterized including X-ray crystal structure determinations. Analysis of these solid state structures demonstrates the importance of bo...

Katayev, Dmitry

2012-01-01

2

An N-heterocyclic carbene phenanthroline ligand: synthesis, multi-metal coordination and spectroscopic studies.  

Science.gov (United States)

Dimetal complexes of a new N-heterocyclic carbene/phenanthroline ligand have been synthesized. Coordination of both ruthenium and rhenium to the phenanthroline moiety in combination with platinum at the carbene moiety are reported. Steady-state and time-resolved optical absorption and photoluminescence spectra were obtained for the complexes. These results illustrate significant changes occur with the incorporation of the second metal, and that the specific metal bound to the phenanthroline moiety is important to the photophysical characteristics of this system. PMID:25604044

Webster, Alyssa A; Prasad, Shyamal K K; Hodgkiss, Justin M; Hoberg, John O

2015-02-10

3

Functionalised N-heterocyclic carbene complexes  

CERN Document Server

N-heterocyclic carbenes (NHCs) have found increasing use as reagents for a range of organic transformations and in asymmetric organocatalysis. The performance of these molecules can be improved and tuned by functionalisation. Functionalised carbenes can anchor free carbenes to the metal site, introduce hemilability, provide a means to immobilise transition metal carbene catalysts, introduce chirality, provide a chelate ligand or bridge two metal centres. NHC can be attached to carbohydrates and campher, derived from amino acids and purines, they can be used as organocatalysts mimicking vitamin

Kühl, Olaf

2010-01-01

4

N-Heterocyclic Carbenes: Versatile Second Cyclometalated Ligands for Neutral Iridium(III) Heteroleptic Complexes.  

Science.gov (United States)

With 2-(2,4-difluorophenyl)pyridine (dfppy) as the first cyclometalated ligand and different monoanionic N-heterocyclic carbenes (NHCs) as the second cyclometalated ligands, 16 blue or greenish-blue neutral iridium(III) phosphorescent complexes, (dfppy)2Ir(NHC), were synthesized efficiently. The obtained Ir(III) complexes display typical phosphorescence of 455-485 nm with quantum yields up to 0.73. By modifying the phenyl moiety in the NHCs with electron-withdrawing substituents (e.g., -F or -CF3) or replacing it with N-heteroaromatic rings (pyridine or pyrimidine), the HOMO-LUMO gaps are broadened, and the emissions shift to the more blue region accordingly. Furthermore, to extend the application scope of NHCs as the second cyclometalated ligands, five other Ir(III) complexes from blue to red were synthesized with different first cyclometalated ligands. Finally, the organic light-emitting diodes using one blue emitter exhibit a maximum current efficiency of 37.83 cd A(-1), an external quantum efficiency of 10.3%, and a maximum luminance of 8709 cd m(-2). Our results demonstrate that NHCs as the second cyclometalated ligands are good candidates for the achievement of efficient phosphorescent Ir(III) complexes and corresponding devices. PMID:25495711

Li, Tian-Yi; Liang, Xiao; Zhou, Liang; Wu, Chen; Zhang, Song; Liu, Xuan; Lu, Guang-Zhao; Xue, Li-Sha; Zheng, You-Xuan; Zuo, Jing-Lin

2015-01-01

5

New N-Heterocyclic Carbene Ligands in Grubbs and Hoveyda-Grubbs Catalysts  

Science.gov (United States)

A series of N-heterocyclic carbene (NHC) ligands bearing aliphatic amino side groups were synthesized and reacted with the Grubbs first generation catalyst. Reactions involving symmetrical, aliphatic NHCs did not allow the isolation of any pure NHC substituted complexes due to their instability. Unsymmetrical NHCs having a planar mesityl group on one amino side reacted with Grubbs catalyst in a favorable manner, and the resulting complexes were stable enough to be isolated. X-ray crystallographic analysis demonstrated that the mesityl group is co-planar with the phenyl ring of the benzylidene, which indicates that a ?-? interaction between the mesityl arm and the benzylidene moiety might constitute an important structural element. Catalysts substituted with an NHC derived from a primary or secondary amino-group were found to surpass the parent-complex for the ROMP of cycloocta-1,5-diene. The catalyst substituted with an NHC derived from tBu-NH2 was considerably less metathesis active. Also new N-alkyl-N'-(2,6-diisopropylphenyl) heterocyclic carbenes were synthesized. These NHC ligands revealed a different reactivity towards Grubbs complexes than the hitherto reported imidazolinylidenes: (i) facile bis(NHC) coordination was found, and (ii) both NHCs on the bis(NHC) complexes can be exchanged with a phosphine, thereupon regenerating the Grubbs first generation complex. Furthermore, a comparison between the classical Hoveyda-Grubbs complexes and complexes substituted with N-alkyl-N'-(aryl) heterocyclic carbenes demonstrates that the introduction of one aliphatic group into the NHC framework does not improve the catalytic activity in any of the tested metathesis reactions. The introduction of two aliphatic amino side groups enhances the reactivity in the ROMP reaction while the increase of steric interactions lowers the CM activity. The lower activity of the N-alkyl-N'-(2,6-diisopropylphenyl) heterocyclic carbene complexes compared with the N-alkyl-N'-mesityl heterocyclic carbene complexes, may analogously be attributed to a more demanding steric environment. While small differences in donor capacities might cause a significantly different catalytic behavior, it is thus plausible that subtle steric differences exert a more determining influence on the activity of the catalysts. In addition, the obtained results confirm that the NHC's amino side groups play a pivotal role in determining the reactivity, selectivity as well as the stability of the corresponding catalysts.

Monsaert, Stijn; Ledoux, Nele; Drozdzak, Renata; van der Voort, Pascal; Verpoort, Francis

6

Cytotoxic gold(I) N-heterocyclic carbene complexes with phosphane ligands as potent enzyme inhibitors.  

Science.gov (United States)

Organometallic gold complexes with N-heterocyclic carbene (NHC) ligands have been demonstrating promising properties as novel anticancer agents. Gold(I) NHC complexes containing different phosphanes as secondary ligands were shown to trigger strong cytotoxic effects in cancer cells, and their effective uptake into the cells was quantified by atomic absorption spectroscopy. Moreover, the new compounds strongly inhibited the activity of the seleno-enzyme thioredoxin reductase (TrxR) and of the zinc-finger enzyme poly(ADP-ribose) polymerase?1 (PARP-1). In the case of TrxR inhibition, their activity depended clearly on the size of the alkyl/aryl residues of phosphorus atoms. Density functional theory (DFT) calculations showed that the Au-P bond of the triphenylphosphane complex [Au(I) (NHC)(PPh3 )]I had a lower bond dissociation energy compared to trialkylphosphane complexes [Au(I) (NHC)(PR3 )]I, indicating a higher kinetic reactivity of this particular compound. In fact, [Au(I) (NHC)(PPh3 )]I triggered an enhanced inhibitory activity against PARP-1. PMID:24677779

Rubbiani, Riccardo; Salassa, Luca; de Almeida, Andreia; Casini, Angela; Ott, Ingo

2014-06-01

7

Gold(I) Complexes of N-Heterocyclic Carbene Ligands Containing Benzimidazole: Synthesis and Antimicrobial Activity  

OpenAIRE

Gold(I) N-heterocyclic carbene (NHC) complexes were obtained in good yields from the corresponding silver complexes by treatment with [AuCl(PPh3)] following the commonly used silver carbene transfer route. The silver complexes were synthesized from the benzimidazolium halide salts by the in situ reactions with Ag2O in dichloromethane as a solvent at room temperature. All gold complexes have been characterized by 1H-NMR, 13C-NMR and IR spectroscopy and elemental analysis. Au-NHC complexes were...

?lknur Özdemir; Nazan Temelli; Selami Günal; Serpil Demir

2010-01-01

8

Rhenium and technetium tricarbonyl complexes of N-heterocyclic carbene ligands.  

Science.gov (United States)

A strategy for the conjugation of N-heterocyclic carbene (NHC) ligands to biomolecules via amide bond formation is described. Both 1-(2-pyridyl)imidazolium or 1-(2-pyridyl)benzimidazolium salts functionalized with a pendant carboxylic acid group were prepared and coupled to glycine benzyl ester using 1-ethyl-3-(3-(dimethylamino)propyl)carbodiimide. A series of 10 rhenium(I) tricarbonyl complexes of the form [ReX(CO)3(?N)] (?N is a bidentate NHC ligand, and X is a monodentate anionic ligand: Cl(-), RCO2(-)) were synthesized via a Ag2O transmetalation protocol from the Re(I) precursor compound Re(CO)5Cl. The synthesized azolium salts and Re(I) complexes were characterized by elemental analysis and by (1)H and (13)C NMR spectroscopy, and the molecular structures for one imidazolium salt and seven Re(I) complexes were determined by single-crystal X-ray diffraction. (1)H NMR and mass spectrometry studies for an acetonitrile-d3 solution of [ReCl(CO)3(1-(2-pyridyl)-3-methylimidazolylidene)] show that the monodentate chloride ligand is labile and exchanges with this solvent yielding a cationic acetonitrile adduct. For the first time the labeling of an NHC ligand with technetium-99m is reported. Rapid Tc-99m labeling was achieved by heating the imidazolium salt 1-(2-pyridyl)-3-methylimidazolium iodide and Ag2O in methanol, followed by the addition of fac-[(99m)Tc(OH2)3(CO)3](+). To confirm the structure of the (99m)Tc-labeled complex, the equivalent (99)Tc complex was prepared, and mass spectrometric studies showed that the formed Tc complexes are of the form [(99m/99)Tc(CH3CN)(CO)3(1-(2-pyridyl)-3-methylimidazolylidene)](+) with an acetonitrile molecule coordinated to the metal center. PMID:25280253

Chan, Chung Ying; Pellegrini, Paul A; Greguric, Ivan; Barnard, Peter J

2014-10-20

9

Photophysical properties of trans-platinum acetylide complexes featuring N-heterocyclic carbene ligands.  

Science.gov (United States)

A series of trans-N-heterocyclic carbene (NHC) platinum(ii) acetylide complexes of the form (ICy)2Pt(R)2 (where ICy = 1,3-bis-(cyclohexyl)imidazol-2-ylidene and R = 1-Ethynyl-4-(phenylethynyl)benzene (PE2), 2-(9,9-Diethyl-9H-fluoren-7-yl)benzo[d]thiazole (BTF), and 9,9-Diethyl-7-ethynyl-N,N-diphenyl-9H-fluoren-2-amine (DPAF), respectively), were synthesized via Hagihara reaction of the unprotected aryl-acetylide ligands with trans-(ICy)2PtCl2 () in 47-73% yield. Precursor was generated in a one-pot synthesis via formation of a silver carbene precursor followed by transmetallation, and was obtained in high yield (95%). The single-crystal X-ray structures of , were determined and analyzed. The photophysical properties of were compared to their respective tributyl phosphine (PBu3) analogues. The optical properties of the series were studied by UV-Vis spectroscopy, photoluminescence spectroscopy, nanosecond transient absorption spectroscopy, and open aperture nanosecond z-scan. Coupling of the organic chromophores to the platinum center affords efficient intersystem crossing as concluded by the complexes' low fluorescence quantum yields, efficient phosphorescence and intense T1 - Tn absorption. Open aperture z-scan with 606 nm, 10 ns laser pulses showed comparable optical attenuation relative to a standard sample of (PBu3)2Pt(DPAF)2 (). Pulse limiting was achieved via a dual-mechanism of two-photon absorption (2PA) coupled with excited-state absorption (ESA). TD-DFT Computations were also employed for to give greater insight into the nature of the singlet-singlet electronic transitions. PMID:25019204

Winkel, Russell W; Dubinina, Galyna G; Abboud, Khalil A; Schanze, Kirk S

2014-12-21

10

Nickel N-heterocyclic carbene complexes in homogeneous catalysis  

OpenAIRE

Described in this thesis are the investigations into the chemistry of N-heterocyclic carbene (NHC) ligands and transition-metal complexes thereof. Specifically, a variety of N-heterocyclic carbene complexes of nickel were prepared, characterized and used as catalysts in three types of homogeneous catalytic processes. First, nickel(II) complexes of monodentate NHC ligands were successfully used as catalysts in the hydrosilylation of internal alkynes. Second, nickel(II) complexes bearing bident...

Berding, Joris

2009-01-01

11

Electronic Structure Determination of Pyridine N-Heterocyclic Carbene Iron Dinitrogen Complexes and Neutral Ligand Derivatives.  

Science.gov (United States)

The electronic structures of pyridine N-heterocyclic dicarbene ((iPr)CNC) iron complexes have been studied by a combination of spectroscopic and computational methods. The goal of these studies was to determine if this chelate engages in radical chemistry in reduced base metal compounds. The iron dinitrogen example ((iPr)CNC)Fe(N2)2 and the related pyridine derivative ((iPr)CNC)Fe(DMAP)(N2) were studied by NMR, Mössbauer, and X-ray absorption spectroscopy and are best described as redox non-innocent compounds with the (iPr)CNC chelate functioning as a classical ? acceptor and the iron being viewed as a hybrid between low-spin Fe(0) and Fe(II) oxidation states. This electronic description has been supported by spectroscopic data and DFT calculations. Addition of N,N-diallyl-tert-butylamine to ((iPr)CNC)Fe(N2)2 yielded the corresponding iron diene complex. Elucidation of the electronic structure again revealed the CNC chelate acting as a ? acceptor with no evidence for ligand-centered radicals. This ground state is in contrast with the case for the analogous bis(imino)pyridine iron complexes and may account for the lack of catalytic [2? + 2?] cycloaddition reactivity. PMID:25328270

Darmon, Jonathan M; Yu, Renyuan Pony; Semproni, Scott P; Turner, Zoë R; Stieber, S Chantal E; DeBeer, Serena; Chirik, Paul J

2014-10-13

12

N-Heterocyclic Carbene-Catalyzed Oxidations  

OpenAIRE

N-Heterocyclic carbenes catalyze the oxidation of allylic and benzylic alcohols as well as saturated aldehydes to esters with manganese(IV) oxide in excellent yields. A variety of esters can be synthesized, including protected carboxylates. The oxidation proceeds under mild conditions, with low loadings of a simple triazolium salt pre-catalyst in the presence of base. Substrates containing potentially epimerizable centers are oxidized while preserving stereochemical integrity. The acyl triazo...

Maki, Brooks E.; Chan, Audrey; Phillips, Eric M.; Scheidt, Karl A.

2009-01-01

13

Influence of pyrazolate vs N-heterocyclic carbene ligands on the slow magnetic relaxation of homoleptic trischelate lanthanide(III) and uranium(III) complexes.  

Science.gov (United States)

Two isostructural series of trigonal prismatic complexes, M(Bp(Me))3 and M(Bc(Me))3 (M = Y, Tb, Dy, Ho, Er, U; [Bp(Me)](-) = dihydrobis(methypyrazolyl)borate; [Bc(Me)](-) = dihydrobis(methylimidazolyl)borate) are synthesized and fully characterized to examine the influence of ligand donor strength on slow magnetic relaxation. Investigation of the dynamic magnetic properties reveals that the oblate electron density distributions of the Tb(3+), Dy(3+), and U(3+) metal ions within the axial ligand field lead to slow relaxation upon application of a small dc magnetic field. Significantly, the magnetization relaxation is orders of magnitude slower for the N-heterocyclic carbene complexes, M(Bc(Me))3, than for the isomeric pyrazolate complexes, M(Bp(Me))3. Further, investigation of magnetically dilute samples containing 11-14 mol % of Tb(3+), Dy(3+), or U(3+) within the corresponding Y(3+) complex matrix reveals thermally activated relaxation is favored for the M(Bc(Me))3 complexes, even when dipolar interactions are largely absent. Notably, the dilute species U(Bc(Me))3 exhibits Ueff ? 33 cm(-1), representing the highest barrier yet observed for a U(3+) molecule demonstrating slow relaxation. Additional analysis through lanthanide XANES, X-band EPR, and (1)H NMR spectroscopies provides evidence that the origin of the slower relaxation derives from the greater magnetic anisotropy enforced within the strongly donating N-heterocyclic carbene coordination sphere. These results show that, like molecular symmetry, ligand-donating ability is a variable that can be controlled to the advantage of the synthetic chemist in the design of single-molecule magnets with enhanced relaxation barriers. PMID:24650296

Meihaus, Katie R; Minasian, Stefan G; Lukens, Wayne W; Kozimor, Stosh A; Shuh, David K; Tyliszczak, Tolek; Long, Jeffrey R

2014-04-23

14

N-Heterocyclic carbene metal complexes: photoluminescence and applications.  

Science.gov (United States)

This review covers the advances made in the synthesis of luminescent transition metal complexes containing N-heterocyclic carbene (NHC) ligands. The presence of a high field strength ligand such as an NHC in the complexes gives rise to high energy emissions, and consequently, to the desired blue colour needed for OLED applications. Furthermore, the great versatility of NHC ligands for structural modifications, together with the use of other ancillary ligands in the complex, provides numerous possibilities for the synthesis of phosphorescent materials, with emission colours over the entire visible spectra and potential future applications in fields such as photochemical water-splitting, chemosensors, dye-sensitised solar cells, oxygen sensors, and medicine. PMID:24604135

Visbal, Renso; Gimeno, M Concepción

2014-05-21

15

Water-soluble luminescent cyclometalated gold(III) complexes with cis-chelating bis(N-heterocyclic carbene) ligands: synthesis and photophysical properties.  

Science.gov (United States)

A new class of cyclometalated Au(III) complexes containing various bidentate C-deprotonated C^N and cis-chelating bis(N-heterocyclic carbene) (bis-NHC) ligands has been synthesized and characterized. These are the first examples of Au(III) complexes supported by cis-chelating bis-NHC ligands. [Au(C^N)(bis-NHC)] complexes display emission in solutions under degassed condition at room temperature with emission maxima (?max ) at 498-633?nm and emission quantum yields of up to 10.1?%. The emissions are assigned to triplet intraligand (IL) ???* transitions of C^N ligands. The Au(III) complex containing a C^N (C-deprotonated naphthalene-substituted quinoline) ligand with extended ?-conjugation exhibits prompt fluorescence and phosphorescence of comparable intensity with ?max at 454 and 611?nm respectively. With sulfonate-functionalized bis-NHC ligand, four water-soluble luminescent Au(III) complexes, including those displaying both fluorescence and phosphorescence, were prepared. They show similar photophysical properties in water when compared with their counterparts in acetonitrile. The long phosphorescence lifetime of the water-soluble AuIII complex with C-deprotonated naphthalene-substituted quinoline ligand renders it to function as ratiometric sensor for oxygen. Inhibitory activity of one of these water-soluble Au(III) complexes towards deubiquitinase (DUB) UCHL3 has been investigated; this complex also displayed a significant inhibitory activity with IC50 value of 0.15??M. PMID:24957269

Hung, Faan-Fung; To, Wai-Pong; Zhang, Jing-Jing; Ma, Chensheng; Wong, Wai-Yeung; Che, Chi-Ming

2014-07-01

16

Synthesis and characterization of divalent manganese, iron, and cobalt complexes in tripodal phenolate/N-heterocyclic carbene ligand environments.  

Science.gov (United States)

Two novel tripodal ligands, (BIMPN(Mes,Ad,Me))(-) and (MIMPN(Mes,Ad,Me))(2-), combining two types of donor atoms, namely, NHC and phenolate donors, were synthesized to complete the series of N-anchored ligands, ranging from chelating species with tris(carbene) to tris(phenolate) chelating arms. The complete ligand series offers a convenient way of tuning the electronic and steric environment around the metal center, thus, allowing for control of the complex's reactivity. This series of divalent complexes of Mn, Fe, and Co was synthesized and characterized by (1)H NMR, IR, and UV/vis spectroscopy as well as by single-crystal X-ray diffraction studies. Variable-temperature SQUID magnetization measurements in the range from 2 to 300 K confirmed high-spin ground states for all divalent complexes and revealed a trend of increasing zero-field splitting |D| from Mn(II), to Fe(II), to Co(II) complexes. Zero-field (57)Fe Mössbauer spectroscopy of the Fe(II) complexes 3, 4, 8, and 11 shows isomer shifts ? that increase gradually as carbenes are substituted for phenolates in the series of ligands. From the single-crystal structure determinations of the complexes, the different steric demand of the ligands is evident. Particularly, the molecular structure of 1-in which a pyridine molecule is situated next to the Mn-Cl bond-and those of azide complexes 2, 4, and 6 demonstrate the flexibility of these mixed-ligand derivatives, which, in contrast to the corresponding symmetrical TIMEN(R) ligands, allow for side access of, e.g., organic substrates, to the reactive metal center. PMID:24299260

Käß, Martina; Hohenberger, Johannes; Adelhardt, Mario; Zolnhofer, Eva M; Mossin, Susanne; Heinemann, Frank W; Sutter, Jörg; Meyer, Karsten

2014-03-01

17

Synthesis and Characterization of Divalent Manganese, Iron, and Cobalt Complexes in Tripodal Phenolate/N-Heterocyclic Carbene Ligand Environments  

DEFF Research Database (Denmark)

Two novel tripodal ligands, (BIMPNMes,Ad,Me)? and (MIMPNMes,Ad,Me)2–, combining two types of donor atoms, namely, NHC and phenolate donors, were synthesized to complete the series of N-anchored ligands, ranging from chelating species with tris(carbene) to tris(phenolate) chelating arms. The complete ligand series offers a convenient way of tuning the electronic and steric environment around the metal center, thus, allowing for control of the complex’s reactivity. This series of divalent complexes of Mn, Fe, and Co was synthesized and characterized by 1H NMR, IR, and UV/vis spectroscopy as well as by single-crystal X-ray diffraction studies. Variable-temperature SQUID magnetization measurements in the range from 2 to 300 K confirmed high-spin ground states for all divalent complexes and revealed a trend of increasing zero-field splitting |D| from Mn(II), to Fe(II), to Co(II) complexes. Zero-field 57Fe Mo?ssbauer spectroscopy of the Fe(II) complexes 3, 4, 8, and 11 shows isomer shifts ? that increase gradually as carbenes are substituted for phenolates in the series of ligands. From the single-crystal structure determinations of the complexes, the different steric demand of the ligands is evident. Particularly, the molecular structure of 1?in which a pyridine molecule is situated next to the Mn–Cl bond?and those of azide complexes 2, 4, and 6 demonstrate the flexibility of these mixed-ligand derivatives, which, in contrast to the corresponding symmetrical TIMENR ligands, allow for side access of, e.g., organic substrates, to the reactive metal center.

Ka?ß, Martina; Hohenberger, Johannes

2014-01-01

18

Stepwise Strategy to Cyclometallated Pt(II) Complexes with N-Heterocyclic Carbene Ligands: A Luminescence Study on New ?-Diketonate Complexes.  

Science.gov (United States)

The imidazolium salt 3-methyl-1-(naphthalen-2-yl)-1H-imidazolium iodide (2) has been treated with silver(I) oxide and [{Pt(?-Cl)(?(3) -2-Me-C3 H4 )}2 ] (?(3) -2-Me-C3 H4 =?(3) -2-methylallyl) to give the intermediate N-heterocyclic carbene complex [PtCl(?(3) -2-Me-C3 H4 )(H$\\widehat{CC}$*-?C*)] (3) (H$\\widehat{CC}$*-?C*=3-methyl-1-(naphthalen-2-yl)-1H-imidazol-2-ylidene). Compound 3 undergoes regiospecific cyclometallation at the naphthyl ring of the NHC ligand to give the five-membered platinacycle compound [{Pt(?-Cl)($\\widehat{CC}$*)}2 ] (4). Chlorine abstraction from 4 with ?-diketonate Tl derivatives rendered the corresponding neutral compounds [Pt($\\widehat{CC}$*)(L-O,O')] {L=acac (HL=acetylacetone) 5, phacac (HL=1,3-diphenyl-1,3-propanedione) 6, hfacac (HL=hexafluoroacetylacetone) 7}. All of the compounds (3-7) were fully characterized by standard spectroscopic and analytical methods. X-ray diffraction studies were performed on 5-7, revealing short Pt?Pt and ?-? interactions in the solid-state structure. The influence of the R-substituents of the ?-diketonate ligand on the photophysical properties and the use of the most efficient emitter, 5, as phosphor converter has also been studied. PMID:25412763

Fuertes, Sara; García, Hector; Perálvarez, Mariano; Hertog, Wim; Carreras, Josep; Sicilia, Violeta

2014-11-20

19

N-heterocyclic carbenes in porous networks  

OpenAIRE

Ziel dieser Arbeit war die Darstellung sowie Charakterisierung N-heterocyclischer Carbene in porösen Netzwerken. Es konnte gezeigt werden, dass ein direkter Nachweis der Carbene durch gängige spektroskopische Nachweismethoden nicht eindeutig gegeben ist. Durch die Verwendung verschiedener Aminoverbindungen (1,4-Diaminobenzol, Melamin, Tetra(4-aminophenyl)methan und Meso- tetra-p-aminophenylporphyrin) mit unterschiedlichen Geometrien, wurde der Versuch unternommen N-heterocyclische Carbene i...

Thiel, Kerstin

2012-01-01

20

Synthesis and characterization of N-heterocyclic carbene complexes of uranium(III).  

Science.gov (United States)

Reaction of [(((Ad)ArO)(3)tacn)U(III)] (1) or [((Me(3)Si)(2)N)(3)U(III)] (3) with tetramethylimidazol-2-ylidene (Me(4)IMC:) yields novel N-heterocyclic carbene complexes [(((Ad)ArO)(3)tacn)U(III)(Me(4)IMC:)] (2) and [((Me(3)Si)(2)N)(3)U(III)(Me(4)IMC:)] (4). Uranium complexes 2 and 4 represent the first examples of compounds with an N-heterocyclic carbene ligand coordinated to a low-valent uranium center. The paramagnetic complexes 1, 2, and 4 were characterized by (1)H NMR, UV-vis-NIR, and EPR spectroscopy as well as SQUID magnetization measurements and X-ray diffraction analyses. DFT studies indicate a significant degree of pi-bonding in the U(III)-carbene entity. PMID:14753804

Nakai, Hidetaka; Hu, Xile; Zakharov, Lev N; Rheingold, Arnold L; Meyer, Karsten

2004-02-01

21

N-Heterocyclic Carbene Complexes in Additions to Multiple Bonds  

Science.gov (United States)

The use of N-heterocyclic carbene (NHC) complexes as homogeneous catalysts in addition reactions across carbon-carbon double and triple bonds and carbon-heteroatom double bonds is described. The discussion is focused on the description of the catalytic systems, their current mechanistic understanding and occasionally the relevant organometallic chemistry. The reaction types covered include hydrogenation, transfer hydrogenation, hydrosilylation, hydroboration and diboration, hydroamination, hydrothiolation, hydration, hydroarylation, allylic substitution, addition, chloroesterification and chloroacylation.

Danopoulos, Andreas A.

22

A benzimidazole-based N-heterocyclic carbene derived from 1,10-phenanthroline.  

Science.gov (United States)

[reaction: see text] A catalytically active palladium-complexed tetracyclic N-heterocyclic carbene (NHC) was prepared in three steps from commercially available 1,10-phenanthroline by using a reduction-cyclization-deprotonation sequence. The new carbene framework is a prototype for the development of a series of chiral N-heterocyclic carbenes. PMID:15387568

Metallinos, Costa; Barrett, Fred B; Chaytor, Jennifer L; Heska, Mary E A

2004-09-30

23

Silver(I) complexes of mono- and bidentate N-heterocyclic carbene ligands: Synthesis, crystal structures, and in vitro antibacterial and anticancer studies.  

Science.gov (United States)

A series of benzimidazole-based N-heterocyclic carbene (NHC) proligands {1-benzyl-3-(2-methylbenzyl)-benzimidazolium bromide/hexafluorophosphate (1/4), 1,3-bis(2-methylbenzyl)-benzimidazolium bromide/hexafluorophosphate (2/5) and 1,3-bis(3-(2-methylbenzyl)-benzimidazolium-1-ylmethylbenzene dibromide/dihexafluorophosphate (3/6)} has been synthesized by the successive N-alkylation method. Ag complexes {1-benzyl-3-(2-methylbenzyl)-benzimidazol-2-ylidenesilver(I) hexafluorophosphate (7), 1,3-bis(2-methylbenzyl)-benzimidazol-2-ylidenesilver(I) hexafluorophosphate (8) and 1,3-bis(3-(2-methylbenzyl)-benzimidazol-2-ylidene)-1-ylmethylbenzene disilver(I) dihexafluorophosphate (9)} of NHC ligands have been synthesized by the treatment of benzimidazolium salts with Ag2O at mild reaction conditions. Both, NHC proligands and Ag-NHC complexes have been characterized by (1)H and (13)C{(1)H} NMR and FTIR spectroscopy and elemental analysis technique. Additionally, the structure of the NHC proligand 5 and the mononuclear Ag complexes 7 and 8 has been elucidated by the single crystal X-ray diffraction analysis. Both the complexes exhibit the same general structural motif with linear coordination geometry around the Ag centre having two NHC ligands. Preliminary in vitro antibacterial potentials of reported compounds against a Gram negative (Escherichia coli) and a Gram positive (Bacillus subtilis) bacteria evidenced the higher activity of mononuclear silver(I) complexes. The anticancer studies against the human derived colorectal cancer (HCT 116) and colorectal adenocarcinoma (HT29) cell lines using the MTT assay method, revealed the higher activity of Ag-NHC complexes. The benzimidazolium salts 4-6 and Ag-NHC complexes 7-9 displayed the following IC50 values against the HCT 116 and HT29 cell lines, respectively, 31.8 ± 1.9, 15.2 ± 1.5, 4.8 ± 0.6, 10.5 ± 1.0, 18.7 ± 1.6, 1.20 ± 0.3 and 245.0 ± 4.6, 8.7 ± 0.8, 146.1 ± 3.1, 7.6 ± 0.7, 5.5 ± 0.8, 103.0 ± 2.3 ?M. PMID:25461313

Haque, Rosenani A; Choo, Sze Yii; Budagumpi, Srinivasa; Iqbal, Muhammad Adnan; Al-Ashraf Abdullah, Amirul

2015-01-27

24

The Depolymerization of Poly(Ethylene Terephthalate) (PET) Using N-Heterocyclic Carbenes from Ionic Liquids  

Science.gov (United States)

The depolymerization of the plastic polyethylene terephthalate (PET or PETE) is described in this laboratory procedure. The transesterification reaction used to depolymerize PET employs a highly efficient N-heterocyclic carbene catalyst derived from a commercially available imidazolium ionic liquid. N-heterocyclic carbenes are potent nucleophilic…

Kamber, Nahrain E.; Tsujii, Yasuhito; Keets, Kate; Waymouth, Robert M.; Pratt, Russell C.; Nyce, Gregory W.; Hedrick, James L.

2010-01-01

25

Electronic structure and ionization energies of palladium and platinum N-heterocyclic carbene complexes.  

OpenAIRE

Density functional methods have been used to calculate the geometries, electronic structure and ionization energies (IE) of N-heterocyclic carbene complexes of palladium and platinum, [M(CN2R2C2H2)2](M = Pd, Pt; R = H, Me, Bu t). Agreement with X-ray structures (R = Bu t) was good. Calculated IE agreed well with the photoelectron (PE) spectra (R = Bu t); metal bands were calculated to be within 0.25 eV of the experimental values, whereas the higher lying ligand bands deviated by up to 0.9 eV....

Green, Jc; Herbert, Bj

2005-01-01

26

'Classical' and 'Abnormal' Bonding in Tin (II) N-Heterocyclic Carbene Complexes  

OpenAIRE

Reaction of Sn(OTf)2 (OTf-?OSO2CF3-) with one and two equivalents of the N-heterocyclic carbene (NHC) 1,3-bis(2,6-diisopropylphenyl)-imidazol-2-ylidene (IPr) yielded the complexes [Sn(IPr)(OTf)2] (1) and [Sn(IPr)(aIPr)(OTf)][OTf] (2), respectively. Both species were characterised by single crystal X-ray diffraction, multi-element NMR spectroscopy, and elemental analysis. Both compounds display an NHC ligand bonded to the tin(ii) metal centre via the C2 carbon in a 'classical' mode, while 2 ...

Turbervill, Rsp; Goicoechea, Jm

2013-01-01

27

Highly enantioselective [4 + 2] cyclization of chloroaldehydes and 1-azadienes catalyzed by N-heterocyclic carbenes.  

Science.gov (United States)

Highly functionalized dihydropyridinones were synthesized via the N-heterocyclic carbene-catalyzed enantioselective [4 + 2] annulation of ?-chloroaldehydes and azadienes. Hydrogenation of the resulted dihydropyridinones afforded the corresponding piperidinones with high enantiopurity. PMID:23023792

Jian, Teng-Yue; Sun, Li-Hui; Ye, Song

2012-11-14

28

C(sp3)-H activation without a directing group: regioselective synthesis of N-ylide or N-heterocyclic carbene complexes controlled by the choice of metal and ligand.  

Science.gov (United States)

N-Ylide complexes of Ir have been generated by C(sp(3))-H activation of ?-pyridinium or ?-imidazolium esters in reactions with [Cp*IrCl2]2 and NaOAc. These reactions are rare examples of C(sp(3))-H activation without a covalent directing group, which-even more unusually-occur ? to a carbonyl group. For the reaction of the ?-imidazolium ester [3H]Cl, the site selectivity of C-H activation could be controlled by the choice of metal and ligand: with [Cp*IrCl2]2 and NaOAc, C(sp(3))-H activation gave the N-ylide complex 4; in contrast, with Ag2O followed by [Cp*IrCl2]2, C(sp(2))-H activation gave the N-heterocyclic carbene complex 5. DFT calculations revealed that the N-ylide complex 4 was the kinetic product of an ambiphilic C-H activation. Examination of the computed transition state for the reaction to give 4 indicated that unlike in related reactions, the acetate ligand appears to play the dominant role in C-H bond cleavage. PMID:25164979

Cross, Warren B; Razak, Sunnah; Singh, Kuldip; Warner, Andrew J

2014-10-01

29

Asymmetric catalysis with N-heterocyclic carbenes as non-covalent chiral templates.  

Science.gov (United States)

N-heterocyclic carbenes are a class of persistent carbenes stabilized by adjacent heteroatoms that are part of a heterocycle. They play a central role in multiple enzymatic biosynthetic reactions that involve thiamine diphosphate. Inspired by this biocatalysis machinery, N-heterocyclic carbenes have emerged as one of the most versatile classes of organocatalysts for organic reactions. However, the asymmetric synthesis of carbon-carbon bonds through a non-covalent interaction mechanism has not been previously established for chiral carbenes. Here, we report an N-heterocylic carbene-catalysed, highly enantioselective process that uses weak hydrogen bonds to relay asymmetric bias. We find that catalytic amounts of hexafluoroisopropanol are the critical proton shuttle that facilitates hydrogen transfer to provide high-reaction rates and high enantioselectivity. We demonstrate that a successful asymmetric reaction of this type can be accomplished through a rational design that balances the pKa values of the substrate, the carbene precursor and the product. PMID:24599089

Chen, Jiean; Huang, Yong

2014-01-01

30

Copper(i), silver(i) and gold(i) complexes of N-heterocyclic carbene-phosphinidene.  

Science.gov (United States)

N-heterocyclic carbene stabilized phosphinidene IMes·PPh has been used as a bridging ligand to isolate a group of closely related molecules involving all three coinage metal ions. The bis-copper(i) chloride, bis-silver(i) chloride and bis-copper(i) bromide adducts of IMes·PPh crystallize as halide ion bridged octanuclear molecules while the [IMes·PPh](AuCl)2 adduct remains monomeric. These molecules feature long P-C(carbene) bonds in the range 1.822(3)-1.843(4) Å, near the typical P-C single bond length region. PMID:25410496

Adiraju, Venkata A K; Yousufuddin, Muhammed; Rasika Dias, H V

2014-11-20

31

The Potential of N-Heterocyclic Carbene Complexes as Components for Electronically Active Materials  

OpenAIRE

The application of N-heterocyclic carbene complexes as active sites in materials other than catalysis has been remarkably scarce. Inspired by the — often misleading — ‘carbene’ label, which implies a substantial degree of M=C ? bonding, we have been interested in evaluating the potential of N-heterocylclic carbene complexes as building blocks for constructing electronically active materials. Electron mobility via the metal-carbon bond has been investigated in monometallic imidazol-2-...

Schuster, Oliver; Mercs, Laszlo; Albrecht, Martin

2010-01-01

32

Synthesis and antimicrobial studies of silver N-heterocyclic carbene complexes bearing a methyl benzoate substituent  

OpenAIRE

Due to the properties of silver as an antimicrobial, our research group has synthesized many different silver carbene complexes. Two new silver N-heterocyclic carbene complexes derived from 4,5-dichloroimidazole and theobromine bearing methyl benzoate substituents were synthesized by in situ carbene formation using silver acetate as the base in the reaction. The new compounds were fully characterized by several methods including NMR spectroscopy and X-ray crystallography. Preliminary antimicr...

Knapp, Amanda R.; Panzner, Matthew J.; Medvetz, Doug A.; Wright, Brian D.; Tessier, Claire A.; Youngs, Wiley J.

2010-01-01

33

End-on and side-on ?-acid ligand adducts of gold(I): carbonyl, cyanide, isocyanide, and cyclooctyne gold(I) complexes supported by N-heterocyclic carbenes and phosphines.  

Science.gov (United States)

N-heterocyclic carbene ligand SIDipp (SIDipp = 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene) and trimesitylphosphine ligand have been used in the synthesis of gold(I) cyanide, t-butylisocyanide, and cyclooctyne complexes (SIDipp)Au(CN) (3), (Mes(3)P)Au(CN) (4), [(Mes(3)P)(2)Au][Au(CN)(2)] (5), [(SIDipp)Au(CN(t)Bu)][SbF(6)] ([6][SbF(6)]), [(SIDipp)Au(cyclooctyne)][SbF(6)] ([8][SbF(6)]), and [(Mes(3)P)Au(cyclooctyne)][SbF(6)] ([9][SbF(6)]). A detailed computational study has been carried out on these and the related gold(I) carbonyl adducts [(SIDipp)Au(CO)][SbF(6)] ([1][SbF(6)]), [(Mes(3)P)Au(CO)][SbF(6)] ([2][SbF(6)]), and [(Mes(3)P)Au(CN(t)Bu)](+) ([7](+)). X-ray crystal structures of 3, 5, [6][SbF(6)], [8][SbF(6)], and [9][SbF(6)] revealed that they feature linear gold sites. Experimental and computational data show that the changes in ?-acid ligand on (SIDipp)Au(+) from CO, CN(-), CN(t)Bu, cyclooctyne as in [1](+), 3, [6](+), and [8](+) did not lead to large changes in the Au-C(carbene) bond distances. A similar phenomenon was also observed in Au-P distance in complexes [2](+), 4, [7](+), and [9](+) bearing trimesitylphosphine. Computational data show that the Au-L bonds of "naked" [Au-L](+) or SIDipp and Mes(3)P supported [Au-L](+) (L = CO, CN(-), CN(t)Bu to cyclooctyne) have higher electrostatic character than covalent character. The Au?L ?-donation and Au?L ?-back-donation contribute to the orbital term with the former being the dominant component, but the latter is not negligible. In the Au-CO adducts [1](+)and [2](+), the cationic gold center causes the polarization of the C-O ? and ? orbitals toward the carbon end making the coefficients at the two atoms more equal which is mainly responsible for the large blue shift in the CO stretching frequency. The SIDipp and Mes(3)P supported gold(I) complexes of cyanide and isocyanide also exhibit a significant blue shift in ?(CN) compared to that of the free ligands. Calculated results for Au(CO)Cl and Au(CF(3))CO suggest that the experimentally observed blue shift in ?(CO) of these compounds may at least partly be caused by intermolecular forces. PMID:23273108

Celik, Mehmet Ali; Dash, Chandrakanta; Adiraju, Venkata A K; Das, Animesh; Yousufuddin, Muhammed; Frenking, Gernot; Dias, H V Rasika

2013-01-18

34

N-heterocyclic carbenes bearing two, one and no nitrogen atoms at the ylidene carbon: insight from theoretical calculations.  

Science.gov (United States)

Electronic structure, thermodynamic stability and ligand properties in LRh(CO)2Cl complexes of a series of N-heterocyclic carbenes (NHCs) were studied at the DFT level. The systems under study are: imidazolin-2-ylidene (1), imidazolidin-2-ylidene (2), cyclic(alkyl)(amino)carbene (CAAC, 3), pyrazolin-3-ylidene (4), pyridin-2-ylidene (5), and pyridin-4-ylidene (6). The main structural feature influencing the properties of these species is the number of nitrogen atoms at the ylidene carbon. A decrease of the number of nitrogen atoms on the one hand leads to an increase in donor ability and ligand-to-metal bond strength, but lowers the stability of the NHC on the other hand. The number of nitrogen atoms can be taken as a key parameter for the classification of carbenes into 2N-NHC, 1N-NHC and r-NHC (r = remote). PMID:20449144

Tukov, Aleksandr A; Normand, Adrien T; Nechaev, Mikhail S

2009-09-21

35

Proton-induced generation of remote N-heterocyclic carbene-ru complexes.  

Science.gov (United States)

The proton-induced Ru?C bond variation, which was previously found to be relevant in the water oxidation, has been investigated by using cyclometalated ruthenium complexes with three phenanthroline (phen) isomers. The designed complexes, [Ru(bpy)2 (1,5-phen)](+) ([2](+) ), [Ru(bpy)2 (1,6-phen)](+) ([3](+) ), and [Ru(bpy)2 (1,7-phen)](+) ([4](+) ) were newly synthesized and their structural and electronic properties were analyzed by various spectroscopy and theoretical protocols. Protonation of [4](+) triggered profound electronic structural change to form remote N-heterocyclic carbene (rNHC), whereas protonation of [2](+) and [3](+) did not affect their structures. It was found that changes in the electronic structure of phen beyond classical resonance forms control the rNHC behavior. The present study provides new insights into the ligand design of related ruthenium catalysts. PMID:25382152

Fukushima, Takashi; Fukuda, Ryoichi; Kobayashi, Katsuaki; Caramori, Giovanni F; Frenking, Gernot; Ehara, Masahiro; Tanaka, Koji

2015-01-01

36

Isolation of an imino-N-heterocyclic carbene/germanium(0) adduct: a mesoionic germylene equivalent.  

Science.gov (United States)

An autoionization of germanium dichloride/dioxane complex with an imino-N-heterocyclic carbene ligand (L) afforded a novel germyliumylidene ion, [(L)GeCl](+)[GeCl3](-), which was fully characterized. Reduction of the germyliumylidene ion with potassium graphite produced a cyclic species [(L)Ge], which can be viewed as both a Ge(0) species and a mesoionic germylene. X-ray diffraction analysis and computational studies revealed one of the lone pairs on the Ge atom is involved in the ??system on the GeC2N2 five-membered ring. It was also confirmed that the nucleophilic behavior of [(L)Ge] as a two lone-pair donor. PMID:25283638

Su, Bochao; Ganguly, Rakesh; Li, Yongxin; Kinjo, Rei

2014-11-24

37

Electrocatalytic Reduction of CO2 with Palladium Bis-N-heterocyclic Carbene Pincer Complexes.  

Science.gov (United States)

A series of pyridine- and lutidine-linked bis-N-heterocyclic carbene (NHC) palladium pincer complexes were electrochemically characterized and screened for CO2 reduction capability with 2,2,2-trifluoroethanol, acetic acid, or 2,2,2-trifluoroacetic acid (TFA) as proton sources. The lutidine-linked pincer complexes electrocatalytically reduce CO2 to CO at potentials as low as -1.6 V versus Ag/AgNO3 in the presence of TFA. The one-electron reduction of these complexes is shown to be chemically reversible, yielding a monometallic species, with density functional theory studies indicating charge storage on the redox-active ligand, thus addressing a major source of deactivation in earlier triphosphine electrocatalysts. PMID:25337973

Therrien, Jeffrey A; Wolf, Michael O; Patrick, Brian O

2014-12-15

38

N-Heterocyclic carbene/Brønsted acid cooperative catalysis as a powerful tool in organic synthesis  

Directory of Open Access Journals (Sweden)

Full Text Available The interplay between metals and N-heterocyclic carbenes (NHCs has provided a window of opportunities for the development of novel catalytic strategies within the past few years. The recent successful combination of Brønsted acids with NHCs has added a new dimension to the field of cooperative catalysis, enabling the stereoselective synthesis of functionalized pyrrolidin-2-ones as valuable scaffolds in heterocyclic chemistry. This Commentary will briefly highlight the concept of N-heterocyclic carbene/Brønsted acid cooperative catalysis as a new and powerful methodology in organic chemistry.

Rob De Vreese

2012-03-01

39

Electrochemical flow-reactor for expedient synthesis of copper-N-heterocyclic carbene complexes.  

Science.gov (United States)

An electrochemical flow-cell for highly efficient and selective generation of Cu(I)-N-heterocyclic carbene complexes under neutral and ambient conditions is reported. The feasibility of the flow-cell is demonstrated through the electrochemical synthesis of [Cu(IMes)Cl] and subsequent in situ flow directly into hydrosilylation reactions, with equal efficiency to the purified catalyst. PMID:25476754

Chapman, Michael R; Shafi, Yarseen M; Kapur, Nikil; Nguyen, Bao N; Willans, Charlotte E

2014-12-23

40

Dual reactivity of N-heterocyclic carbenes towards copper(II) salts.  

Science.gov (United States)

Complexes of N-heterocyclic carbenes (NHCs) with copper(II) halogenides are unstable. Upon formation, these complexes decompose to give haloamidinium salts. Contrastingly, O-substituted copper(II) NHC complexes are fairly stable. A series of new five-, six- and seven-membered ring NHC complexes of Cu(OAc)(2) have been synthesised and characterised in the solid state. PMID:21327287

Kolychev, Eugene L; Shuntikov, Viacheslav V; Khrustalev, Victor N; Bush, Alexander A; Nechaev, Mikhail S

2011-03-28

41

Synthesis and Isolation of a Stable, Axially-Chiral Seven-Membered N-Heterocyclic Carbene  

OpenAIRE

A chiral seven-membered N-heterocyclic carbene (NHC) has been synthesized from its phenol adduct (NHC-HOPh) by a novel base-induced ?-elimination method, and its donor strength has been determined from the IR stretching frequencies of the NHC-Rh(CO)2Cl complex.

Scarborough, Christopher C.; Guzei, Ilia A.; Stahl, Shannon S.

2009-01-01

42

N-Heterocyclic Carbene-Promoted Rauhut]Currier Reactions between Vinyl Sulfones and ?,?-Unsaturated Aldehydes  

OpenAIRE

N-heterocyclic carbenes (NHCs) promote the addition of 1,1-bis(phenylsulfonyl)ethylene to the ?-position of ?,?-unsaturated aldehydes. The proposed reaction pathway for this Rauhut-Currier-type reaction includes an unusual conjugate addition of an NHC to a bis-vinyl sulfone.

Atienza, Roxanne L.; Scheidt, Karl A.

2011-01-01

43

N-Heterocyclic Carbene-Promoted Rauhut]Currier Reactions between Vinyl Sulfones and ?,?-Unsaturated Aldehydes.  

Science.gov (United States)

N-heterocyclic carbenes (NHCs) promote the addition of 1,1-bis(phenylsulfonyl)ethylene to the ?-position of ?,?-unsaturated aldehydes. The proposed reaction pathway for this Rauhut-Currier-type reaction includes an unusual conjugate addition of an NHC to a bis-vinyl sulfone. PMID:22399824

Atienza, Roxanne L; Scheidt, Karl A

2011-08-19

44

Mild and rational synthesis of palladium complexes comprising C(4)-bound N-heterocyclic carbenes.  

Science.gov (United States)

Oxidative addition of pyridyl-functionalised 4-iodoimidazolium salts to palladium(0) gives catalytically active complexes in which the N-heterocyclic carbene is bound to the palladium(II) centre in a non-classical bonding mode via C(4). PMID:17283796

Kluser, Evelyne; Neels, Antonia; Albrecht, Martin

2006-11-21

45

Mild and rational synthesis of palladium complexes comprising C(4)-bound N-heterocyclic carbenes  

OpenAIRE

Oxidative addition of pyridyl-functionalised 4-iodoimidazolium salts to palladium(0) gives catalytically active complexes in which the N-heterocyclic carbene is bound to the palladium(II) centre in a non-classical bonding mode via C(4).

Kluser, Evelyne; Neels, Antonia; Albrecht, Martin

2007-01-01

46

Sterically driven synthesis of ruthenium and ruthenium-silver N-heterocyclic carbene complexes.  

Science.gov (United States)

A straightforward and efficient synthetic route to novel Ru N-heterocyclic carbene (NHC) complexes by transmetallation of non-bulky silver NHC to ruthenium dicarbonyl tetraarylcyclopentadienone is described. The same procedure with sterically demanding NHC leads to unprecedented heterobimetallic Ru-Ag(NHC) complexes. PMID:25338159

Cesari, C; Conti, S; Zacchini, S; Zanotti, V; Cassani, M C; Mazzoni, R

2014-12-14

47

Bifunctional N-heterocyclic carbene-catalyzed highly enantioselective synthesis of spirocyclic oxindolo-?-lactams.  

Science.gov (United States)

The N-heterocyclic carbene-catalyzed Staudinger reaction of ketenes with isatin-derived ketimines was investigated. The bifunctional NHCs with a free hydroxyl group were demonstrated as efficient catalysts for the reaction, giving the corresponding spirocyclic oxindolo-?-lactams in high yields with excellent diastereo- and enantioselectivities. PMID:24856000

Zhang, Han-Ming; Gao, Zhong-Hua; Ye, Song

2014-06-01

48

Synthesis, characterization, and reactivity of furan- and thiophene-functionalized bis(N-heterocyclic carbene) complexes of iron(II).  

Science.gov (United States)

The synthesis of iron(II) complexes bearing new heteroatom-functionalized methylene-bridged bis(N-heterocyclic carbene) ligands is reported. All complexes are characterized by single-crystal X-ray diffraction (SC-XRD), nuclear magnetic resonance (NMR) spectroscopy, and elemental analysis. Tetrakis(acetonitrile)-cis-[bis(o-imidazol-2-ylidenefuran)methane]iron(II) hexafluorophosphate (2a) and tetrakis(acetonitrile)-cis-[bis(o-imidazol-2-ylidenethiophene)methane]iron(II) hexafluorophosphate (2b) were obtained by aminolysis of [Fe{N(SiMe3)2}2(THF)] with furan- and thiophene-functionalized bis(imidazolium) salts 1a and 1b in acetonitrile. The SC-XRD structures of 2a and 2b show coordination of the bis(carbene) ligand in a bidentate fashion instead of a possible tetradentate coordination. The four other coordination sites of these distorted octahedral complexes are occupied by acetonitrile ligands. Crystallization of 2a in an acetone solution by the slow diffusion of Et2O led to the formation of cis-diacetonitriledi[bis(o-imidazol-2-ylidenefuran)methane]iron(II) hexafluorophosphate (3a) with two bis(carbene) ligands coordinated in a bidentate manner and two cis-positioned acetonitrile molecules. Compounds 2a and 2b are the first reported iron(II) carbene complexes with four coordination sites occupied by solvent molecules, and it was demonstrated that those solvent ligands can undergo ligand-exchange reactions. PMID:25188636

Rieb, Julia; Raba, Andreas; Haslinger, Stefan; Kaspar, Manuel; Pöthig, Alexander; Cokoja, Mirza; Basset, Jean-Marie; Kühn, Fritz E

2014-09-15

49

Coordination chemistry of highly hemilabile bidentate sulfoxide N-heterocyclic carbenes with palladium(II).  

Science.gov (United States)

Imidazolium salts, [RS(O)-CH2 (C3 H3 N2 )Mes]Cl (R=Me (L1a), Ph (L1b)); Mes=mesityl), make convenient carbene precursors. Palladation of L1a affords the monodentate dinuclear complex, [(PdCl2 {MeS(O)CH2 (C3 H2 N2 )Mes})2 ] (2a), which is converted into trans-[PdCl2 (NHC)2] (trans-4a; N-heterocyclic carbene) with two rotamers in anti and syn configurations. Complex trans-4a can isomerize into cis-4a(anti) at reflux in acetonitrile. Abstraction of chlorides from 4a or 4b leads to the formation of a new dication: trans-[Pd{RS(O)CH2(C3H2N2)Mes}2](PF6)2 (R=Me (5a), Ph (5b)). The X-ray structure of 5a provides evidence that the two bidentate SO-NHC ligands at palladium(II) are in square-planar geometry. Two sulfoxides are sulfur- and oxygen-bound, and constitute five- and six-membered chelate rings with the metal center, respectively. In acetonitrile, complexes 5a or 5b spontaneously transform into cis-[Pd(NHC)2(NCMe)2](PF6)2. Similar studies of thioether-NHCs have also been examined for comparison. The results indicate that sulfoxides are more labile than thioethers. PMID:25263615

Yu, Kuo-Hsuan; Wang, Chia-Ching; Chang, I-Hsin; Liu, Yi-Hung; Wang, Yu; Elsevier, Cornelis J; Liu, Shiuh-Tzung; Chen, Jwu-Ting

2014-12-01

50

N-heterocyclic carbene (NHC) ligands annulated to free-base porphyrins: modulation of the electronic properties of the NHC upon protonation or metallation of the porphyrin.  

Science.gov (United States)

NHC ligands annulated to free-base porphyrins can be reversibly switched between electron-poor and electron-rich states upon protonation and deprotonation of the inner nitrogen atoms of the porphyrin. Metallation of the macrocycle with nickel(II) locks the peripheral NHC ligand in its electron-rich state. PMID:21249236

Lefebvre, Jean-François; Lo, Mamadou; Leclercq, Dominique; Richeter, Sébastien

2011-03-14

51

Accesses to electronic structures and the excited states of blue luminescent copper(I) complexes containing N-heterocyclic carbene ligands: a DFT/TDDFT exploitation.  

Science.gov (United States)

The ground electronic states and photophysical properties of three designed Cu(I) complexes [Cu(ImNHC)(POP)](+) (1), [Cu(methyl-ImNHC)(POP)](+) (2), and [Cu(BenzImNHC)(POP)](+) (3); where [ImNHC?=?3-methyl-1-(pyridin-2-yl)-1H-imidazol-2-ylidene; methyl-ImNHC?=?3-methyl-1-(pyridin-2-ylmethyl)-1H-imidazol-2-ylidene; BenzImNHC?=?3-methyl-1-(pyridin-2-yl)-1H-benzimidazol-2-ylidene], have been investigated using density functional theory (DFT) and time-dependent density functional theory (TDDFT). The results reveal that the presence of the methylene spacer in the NHC ligands has a more direct effect on the distribution of frontier molecular orbitals while the elongation of ? conjugation provided by the fused imidazole in the NHC ligands has a negligible effect. The UV-vis absorption spectra of all the complexes are well produced by TD-DFT calculations based on the charge transfer amount calculations and the corresponding band assignments are discussed. Importantly, the triplet energy calculations demonstrated that complex 2 would be a highly efficient blue emitter with the deep-blue of 440 nm. PMID:25123713

Li, Qiang; Zhao, Feng; Xu, Shengxian; Xia, Hongying; Wang, Jinglan; Wang, Yibo

2014-09-01

52

New stable aryl-substituted acyclic imino-N-heterocyclic carbene: synthesis, characterisation and coordination to early transition metals.  

Science.gov (United States)

The synthesis of the bulky 1-(1-arylimino-2,2-dimethylpropyl)-3-(aryl)imidazolium salt from the corresponding imidazole and the activated imidoyl chloride is presented. The absence of acidic protons adjacent to the iminic carbon allowed for the first isolation of an imino-N-heterocyclic carbene of this ligand class. The free carbene was isolated, structurally characterised, and coordinated to titanium, zirconium, hafnium and chromium. The resulting metal halide complexes were fully characterised and were tested at room temperature and atmospheric pressure for their activity as ethylene polymerisation catalysts. The Zr(IV) complex was found to be the most active with a productivity of 140 kg PE mol M(-1) h(-1). PMID:22006062

Larocque, Timothy G; Badaj, Anna C; Dastgir, Sarim; Lavoie, Gino G

2011-12-21

53

N-heterocyclic carbene-catalyzed hydroacylation of unactivated double bonds.  

Science.gov (United States)

An intramolecular N-heterocyclic carbene (NHC)-catalyzed hydroacylation of unactivated double bonds is reported. Systematic variation of the catalyst structure revealed an N-mesitylthiazolylidene annulated with a seven-membered ring to be especially reactive. This NHC enables a unique C-C bond-forming reaction to afford substituted chroman-4-ones in moderate to excellent yields, even ones containing all-carbon quaternary centers. PMID:19807174

Hirano, Keiichi; Biju, Akkattu T; Piel, Isabel; Glorius, Frank

2009-10-14

54

Alkyne-Azide Cycloaddition Catalyzed by Silver Chloride and “Abnormal” Silver N-Heterocyclic Carbene Complex  

OpenAIRE

A library of 1,2,3-triazoles was synthesized from diverse alkynes and azides using catalytic amounts of silver chloride instead of copper compounds. In addition, a novel “abnormal” silver N-heterocyclic carbene complex was tested as catalyst in this process. The results suggest that the reaction requires only 0.5% of silver complex, affording 1,2,3-triazoles in good yields.

Ortega-arizmendi, Aldo I.; Eugenia Aldeco-Pérez; Erick Cuevas-Yañez

2013-01-01

55

Bimacrocyclic concave N-heterocyclic carbenes (NHCs): synthesis, structure, and application in catalyses  

OpenAIRE

Abstract Imidazolinium moieties have been incorporated into bimacrocycles to generate precursors for concave N-heterocyclic carbenes (NHCs). Starting from 2-nitroresorcinol and alkenols, symmetric concave imidazolinium salts 1 were obtained. Bimacrocyclization was achieved via ring closing metathesis (RCM). In an analogous fashion, axially chiral concave imidazolinium salts 2 were obtained by using a naphthalene bridgehead devoid of local C2-symmetry. The concave NHCs derived from ...

Lu?ning, Ulrich; Winkelmann, Ole

2009-01-01

56

N-Heterocyclic Carbenes : catalys in the chemistry of silicones Cyclic Carbodiphosphoranes : synthesis, coordination and reactivity  

OpenAIRE

In a first chapter, we are interested in the use of N-Heterocyclic Carbenes like nucleophilic organic catalysts for ring opening polymerization of octamethylcyclosiloxane and for reactions of polycondensation.In a second chapter, we carry out a bibliographical review on coumpounds presenting two cumulated ylides functions : carbodiphosphoranes.In a third chapter, we present the synthesis and the coordination of new cyclic carbodiphosphoranes. They are recognized to be strong -dono...

Marrot, Se?bastien

2007-01-01

57

Oxidative Enantioselective ?-Fluorination of Aliphatic Aldehydes Enabled by N-Heterocyclic Carbene Catalysis.  

Science.gov (United States)

Described is the first study on oxidative enantioselective ?-fluorination of simple aliphatic aldehydes enabled by N-heterocyclic carbene catalysis. N-fluorobis(phenyl)sulfonimide serves as a an oxidant and as an "F" source. The C?F bond formation occurs directly at the ??position of simple aliphatic aldehydes, thus overcoming nontrivial challenges, such as competitive difluorination and nonfluorination, and proceeds with high to excellent enantioselectivities. PMID:25348753

Li, Fangyi; Wu, Zijun; Wang, Jian

2015-01-01

58

Synthesis of New Chiral N-Heterocyclic Carbenes and Abnormal Carbenes  

OpenAIRE

Chapter 1 Carbenes have fascinated organic chemists ever since the first evidence of their existence. The isolation and first crystallographic analysis of a stable NHC by Arduengo, in 1991, has had a major impact on the application of NHCs in chemistry. NHCs are widely used as ligands for transition metals, but also as catalysts in their own right. The aim of this project is to synthesise novel monodentate chiral NHCs 1 in order to induce a high level of stereocontrol in a s...

Levy, Jean-noel

2011-01-01

59

Chelated Bis-N-heterocyclic Carbene Platinum and Palladium Units as Tunable Components of Multinuclear Complexes.  

Science.gov (United States)

Heterometallic trinuclear M2M' complexes (M = Rh, Ir; M' = Pt, Pd) containing a platinum or palladium moiety with chelated bis-N-heterocyclic carbene ligands, [(MCp*)2{M'(bisNHC-Cn-R)}(?3-S)2](BPh4)2 (M = Rh, Ir; M' = Pt, Pd; bisNHC-Cn-R = methylene-, ethylene-, or propylene-bridged bis(N-alkyl-imidazolylidene)), were synthesized by reacting bis(hydrosulfido)platinum(II) or palladium(II) complexs with bisNHC-Cn-R and hydroxo-bridged dinuclear complexes [(MCp*)2(?-OH)3](BPh4), whose dinuclear structures remained intact during the formation of the trinuclear complexes, which was confirmed by using electrospray mass spectrometry and NMR spectroscopy. The heterometallic trinuclear M2M' complexes with a variety of alkylene bridges in bisNHC-Cn-R showed two reversible reduction waves in the cyclic voltammogram, and the second reduction potentials were affected by the alkylene chain lengths, which caused different dihedral angles between the imidazolylidene rings and the coordination plane of the platinum or palladium center. The M2M' complexes, except for those containing the platinum unit with the ethylene-bridged bisNHC ligands, showed dynamic behavior in solution due to the flapping wing motion of the NHC ligand moieties. Although activation parameters obtained from line-shape analyses on variable-temperature NMR spectra of the complexes suggested that the flapping wing motion occurred without bond cleavage, large negative ?S(‡) values were obtained for the complexes with the palladium unit with the ethylene-bridged ligand, suggesting that the Pd-Ccarbene bond cleavage, accompanied by coordination of solvent molecules, occurred. PMID:25563064

Maeda, Yuri; Hashimoto, Hideki; Kinoshita, Isamu; Nishioka, Takanori

2015-01-20

60

Mixed phosphine/N-heterocyclic carbene palladium complexes: synthesis, characterization and catalytic use in aqueous Suzuki-Miyaura reactions.  

Science.gov (United States)

A series of N-heterocyclic carbene (NHC)/PR3 palladium(II) and palladium(0) complexes has been synthesized and fully characterized. X-ray crystallographic data have allowed comparison of ligand steric properties. The NHC ligand was found to vary its steric properties as a function of the phosphine co-ligand. These complexes display interesting catalytic properties in the Suzuki-Miyaura reaction performed in aqueous media. The pre-catalyst [PdCl2(IPr)(XPhos)] (IPr = N,N'-bis-(2,6-diisopropylphenyl)imidazol-2-ylidene; XPhos = 2-dicyclohexylphosphino-2',4',6'-triisopropylbiphenyl) was found to be the most efficient system, promoting the coupling of a wide range of aryl chlorides with boronic acids in aqueous media with a typical catalyst loading of 0.03 mol%. PMID:23348096

Schmid, Thibault E; Jones, Dale C; Songis, Olivier; Diebolt, Olivier; Furst, Marc R L; Slawin, Alexandra M Z; Cazin, Catherine S J

2013-05-28

61

Carbene rearrangements in three-coordinate N-heterocyclic carbene complexes of cobalt(II) bis(trimethylsilyl)amide.  

Science.gov (United States)

The synthesis and molecular structures of the cobalt(II) N-heterocyclic carbene (NHC) complexes [(NHC)Co{N(SiMe3)2}2], where NHC = 1,3-bis(diisopropylphenyl)imidazolylidene (IPr) (6), 1,3-bis(mesityl)imidazolylidene (IMes) (7), and 1,3-bis(tert-butyl)imidazol-2-ylidene (I(t)Bu) (8), are reported. Complexes 6-8 are rare examples of three-coordinate cobalt NHC complexes. The steric congestion within the coordination environments of the cobalt(II) centers in 6 and 7 results in the longest Co-C(NHC) distances currently known. Investigating the thermal stability of 6-8, we have found that the steric congestion in 6 is such that heating the complex to reflux in toluene prompts a rearrangement from the normal, C2-bonding mode of the IPr ligand to the corresponding "abnormal" or mesoionic bonding mode. The rearrangement results in formation of [(aIPr)Co{N(SiMe3)2}2] (9), which is the first cobalt complex of an abnormal NHC ligand. The Co-C bond in 9 is 0.06 Å shorter than the analogous bond in 6, suggesting that, although the rearrangement occurs due to the spatial demands of the IPr ligand, there is also a thermodynamic driving force in terms of the Co-C bond. In contrast to the case for 6, complex 7 is stable with respect to the normal-to-abnormal rearrangement. Refluxing complex 8 in toluene results in activation of a tert-butyl substituent, which is eliminated as isobutene, followed by formation of the 1-tert-butylimidazole complex [((t)BuIm)Co{N(SiMe3)2}2] (10). PMID:25203858

Day, Benjamin M; Pal, Kuntal; Pugh, Thomas; Tuck, Jessica; Layfield, Richard A

2014-10-01

62

Synthesis and antimicrobial studies of silver N-heterocyclic carbene complexes bearing a methyl benzoate substituent  

Science.gov (United States)

Due to the properties of silver as an antimicrobial, our research group has synthesized many different silver carbene complexes. Two new silver N-heterocyclic carbene complexes derived from 4,5-dichloroimidazole and theobromine bearing methyl benzoate substituents were synthesized by in situ carbene formation using silver acetate as the base in the reaction. The new compounds were fully characterized by several methods including NMR spectroscopy and X-ray crystallography. Preliminary antimicrobial efficacy studies against Pseudomonas aeruginosa, Staphylococcus aureus, and Escherichia coli were conducted. The results of this study demonstrated antimicrobial efficacy of the two complexes comparable to silver nitrate, showing their potential for use in the treatment of bacterial infections. PMID:21218156

Knapp, Amanda R.; Panzner, Matthew J.; Medvetz, Doug A.; Wright, Brian D.; Tessier, Claire A.; Youngs, Wiley J.

2010-01-01

63

Systematic strategy for designing immidazolium containing precursors to produce N-heterocyclic carbenes: a DFT study.  

Science.gov (United States)

A series of cationic N-heterocyclic carbene (NHC) precursors that can be utilized as fluorescent chemosensors for carbon dioxide capture were investigated by density functional theory (DFT) calculations. Activation energy barriers for the reactions of the cationic NHC precursors and hydrogen carbonate (HCO3(-)) based on intrinsic reaction coordinate (IRC) profiles as well as proton affinity of the precursors were compared. The calculated proton affinity of 1-ethyl-3-methylimidazol-2-yliene was in good agreement with experimental one within the margin of error. We clarified main factors to lower the activation energy barrier based on the correlation among the number of N-heterocyclic functional group, aromatic ring size, and structural characteristics for the candidate compounds. On the basis of the results, it was verified that some of our model systems spontaneously generate NHCs without any specific catalyst. PMID:25594878

Baek, Kyung Yup; Jo, Ji Hye; Moon, Jong Hun; Yoon, Juyoung; Lee, Jin Yong

2015-02-01

64

N-heterocyclic carbene complexes of au, pd, and pt as effective catalysts in organic synthesis.  

Science.gov (United States)

N-Heterocyclic carbenes (NHC) have been developed in the last 20 years as effective alternatives to classical tertiary phosphines in transition metal-catalyzed transformations. The rapid development of this area is a result of synergistic interactions between experimental and computational chemists. Indeed, computer modeling has proven extremely useful in rationalizing large amount of experimental data, and thus has permitted to accelerate the pace at which this chemistry has been developed. In this review, we will discuss catalytic transformations involving NHC-containing gold, platinum, and palladium complexes. Particular attention is drawn to the fundamental insights that computational chemistry provided to rationalize mechanistic aspects of these processes. PMID:21290215

Correa, Andrea; Nolan, Steven P; Cavallo, Luigi

2011-01-01

65

Catalytic homogeneous hydrogenation of olefins using N-heterocyclic carbene complexes of rhodium and iridium  

OpenAIRE

A synthesis of a small library of rhodium(I) and iridium(I) complexes that combine different types of N-heterocyclic carbenes with pyridines and phosphines was developed. The complexes were tested for the hydrogenation of the simple olefins, 1-octene, cyclohexene, 1-methyl-1-cyclohexene and 2,3-dimethyl-2-butene. Three major trends were observed: first, complexes having iridium and a combination of a phosphine and a NHC prove to be more effective than those with a combination of a pyridine an...

Vazquez-serrano, Leslie D.

2004-01-01

66

Complexes of borane and N-heterocyclic carbenes: a new class of radical hydrogen atom donor.  

Science.gov (United States)

Calculations suggest that complexes of borane with N-heterocyclic carbenes (NHC) have B-H bond dissocation energies more then 20 kcal/mol less than free borane, diborane, borane-THF, and related complexes. Values are in the range of popular radical hydrogen atom donors like tin hydrides (70-80 kcal/mol). The resulting prediction that NHC borane complexes could be used as radical hydrogen atom donors was verified by radical deoxygenations of xanthates by using either AIBN or triethylborane as initiator. PMID:18611014

Ueng, Shau-Hua; Brahmi, Malika Makhlouf; Derat, Etienne; Fensterbank, Louis; Lacôte, Emmanuel; Malacria, Max; Curran, Dennis P

2008-08-01

67

N-Heterocyclic Carbene Complexes of Rh: Reaction With Dioxygen Without Oxidation  

Energy Technology Data Exchange (ETDEWEB)

The reaction of oxygen with rhodium complexes containing N-heterocyclic carbenes was found to give dioxygen complexes with rare square planar geometries and unusually short O-O bond lengths. Analysis of the bonding in these complexes by Rh L-edge X-ray absorption spectroscopy (XAS), Raman spectroscopy, and DFT calculations provides evidence for a bonding model in which singlet oxygen is bound to a Rh(I) d{sup 8} metal complex, rather than the more common Rh(III) d{sup 6} peroxo species with octahedral geometry and O-O bond lengths in the 1.4-1.5 {angstrom} range.

Praetorius, J.M.; Allen, D.P.; Wang, R.; Webb, J.D.; Grein, F.; Kennepohl, P.; Crudden, C.M.

2009-05-21

68

N-heterocyclic carbene complexes of Rh: reaction with dioxygen without oxidation.  

Science.gov (United States)

The reaction of oxygen with rhodium complexes containing N-heterocyclic carbenes was found to give dioxygen complexes with rare square planar geometries and unusually short O-O bond lengths. Analysis of the bonding in these complexes by Rh L-edge X-ray absorption spectroscopy (XAS), Raman spectroscopy, and DFT calculations provides evidence for a bonding model in which singlet oxygen is bound to a Rh(I) d8 metal complex, rather than the more common Rh(III) d6 peroxo species with octahedral geometry and O-O bond lengths in the 1.4-1.5 A range. PMID:18321109

Praetorius, Jeremy M; Allen, Daryl P; Wang, Ruiyao; Webb, Jonathan D; Grein, Friedrich; Kennepohl, Pierre; Crudden, Cathleen M

2008-03-26

69

Sulfur-Functionalized N-Heterocyclic Carbene Complexes of Pd(II: Syntheses, Structures and Catalytic Activities  

Directory of Open Access Journals (Sweden)

Full Text Available N-heterocyclic carbenes (NHCs can be easily modified by introducing functional groups at the nitrogen atoms, which leads to versatile coordination chemistry as well as diverse catalytic applications of the resulting complexes. This article summarizes our contributions to the field of NHCs bearing different types of sulfur functions, i.e., thioether, sulfoxide, thiophene, and thiolato. The experimental evidence for the truly hemilabile coordination behavior of a Pd(II thioether-NHC complex has been reported as well. In addition, complexes bearing rigid CSC-pincer ligands have been synthesized and the reasons for pincer versus pseudo-pincer formation investigated. Incorporation of the electron-rich thiolato function resulted in the isolation of structurally diverse complexes. The catalytic activities of selected complexes have been tested in Suzuki-Miyaura, Mizoroki-Heck and hydroamination reactions.

Dan Yuan

2012-03-01

70

Phosphorescent iridium(III)-bis-N-heterocyclic carbene complexes as mitochondria-targeted theranostic and photodynamic anticancer agents.  

Science.gov (United States)

Mitochondria-targeted compounds represent a promising approach to target tumors selectively and overcome resistance to current anticancer therapies. In this work, three cyclometalated iridium(III) complexes (1-3) containing bis-N-heterocyclic carbene (NHC) ligands have been explored as theranostic and photodynamic agents targeting mitochondria. These complexes display rich photophysical properties, which greatly facilitates the study of their intracellular fate. All three complexes are more cytotoxic than cisplatin against the cancer cells screened. 1-3 can penetrate into human cervical carcinoma (HeLa) cells quickly and efficiently, and they can carry out theranostic functions by simultaneously inducing and monitoring the morphological changes in mitochondria. Mechanism studies show that these complexes exert their anticancer efficacy by initiating a cascade of events related to mitochondrial dysfunction. Additionally, they display up to 3 orders of magnitude higher cytotoxicity upon irradiation at 365 nm, which is so far the highest photocytotoxic responses reported for iridium complexes. PMID:25477176

Li, Yi; Tan, Cai-Ping; Zhang, Wei; He, Liang; Ji, Liang-Nian; Mao, Zong-Wan

2015-01-01

71

Oxyanion steering and CH-? interactions as key elements in an N-heterocyclic carbene-catalyzed [4 + 2] cycloaddition.  

Science.gov (United States)

The N-heterocyclic carbene catalyzed [4 + 2] cycloaddition has been shown to give ?,?-unsaturated ?-lactones in excellent enantio- and diastereoselectivity. However, preliminary computational studies of the geometry of the intermediate enolate rendered ambiguous both the origins of selectivity and the reaction pathway. Here, we show that a concerted, but highly asynchronous, Diels-Alder reaction occurs rather than the stepwise Michael-type or Claisen-type pathways. In addition, two crucial interactions are identified that enable high selectivity: an oxyanion-steering mechanism and a CH-? interaction. The calculations accurately predict the enantioselectivity of a number of N-heterocyclic carbene catalysts in the hetero-Diels-Alder reaction. PMID:22765294

Allen, Scott E; Mahatthananchai, Jessada; Bode, Jeffrey W; Kozlowski, Marisa C

2012-07-25

72

Synthesis of Well-Defined Copper "N"-Heterocyclic Carbene Complexes and Their Use as Catalysts for a "Click Reaction": A Multistep Experiment that Emphasizes the Role of Catalysis in Green Chemistry  

Science.gov (United States)

A multistep experiment for an advanced synthesis lab course that incorporates topics in organic-inorganic synthesis and catalysis and highlights green chemistry principles was developed. Students synthesized two "N"-heterocyclic carbene ligands, used them to prepare two well-defined copper(I) complexes and subsequently utilized the complexes as…

Ison, Elon A.; Ison, Ana

2012-01-01

73

Stereoelectronic effects in C-H bond oxidation reactions of Ni(I) N-heterocyclic carbene complexes.  

Science.gov (United States)

Activation of O2 by the three-coordinate Ni(I) ring-expanded N-heterocyclic carbene complexes Ni(RE-NHC)(PPh3)Br (RE-NHC = 6-Mes, 1; 7-Mes, 2) produced the structurally characterized dimeric Ni(II) complexes Ni(6-Mes)(Br)(?-OH)(?-O-6-Mes')NiBr (3) and Ni(7-Mes)(Br)(?-OH)(?-O-7-Mes')NiBr (4) containing oxidized ortho-mesityl groups from one of the carbene ligands. NMR and mass spectrometry provided evidence for further oxidation in solution to afford bis-?-aryloxy compounds; the 6-Mes derivative was isolated, and its structure was verified. Low-temperature UV-visible spectroscopy showed that the reaction between 1 and O2 was too fast even at ca. -80 °C to yield any observable intermediates and also supported the formation of more than one oxidation product. Addition of O2 to Ni(I) precursors containing a less electron-donating diamidocarbene (6-MesDAC, 7) or less bulky 6- or 7-membered ring diaminocarbene ligands (6- or 7-o-Tol; 8 and 9) proceeded quite differently, affording phosphine and carbene oxidation products (Ni(O?PPh3)2Br2 and (6-MesDAC)?O) and the mononuclear Ni(II) dibromide complexes (Ni(6-o-Tol)(PPh3)Br2 (10) and (Ni(7-o-Tol)(PPh3)Br2 (11)) respectively. Electrochemical measurements on the five Ni(I) precursors show significantly higher redox potentials for 1 and 2, the complexes that undergo oxygen atom transfer from O2. PMID:24972307

Poulten, Rebecca C; López, Isidoro; Llobet, Antoni; Mahon, Mary F; Whittlesey, Michael K

2014-07-21

74

Metal Complexes of a Boron-Dipyrromethene (BODIPY)-Tagged N-Heterocyclic Carbene (NHC) as Luminescent Carbon Monoxide Chemodosimeters.  

Science.gov (United States)

Several metal complexes with a boron dipyrromethene (BODIPY)-functionalized N-heterocyclic carbene (NHC) ligand 4 were synthesized. The fluorescence in [(4)(SIMes)RuCl2 (ind)] complex is quenched (?=0.003), it is weak in [(4)PdI2 (Clpy)] (?=0.033), and strong in [(4)AuI] (?=0.70). The BODIPY-tagged complexes can experience pronounced changes in the brightness of the fluorophore upon ligand-exchange and ligand-dissociation reactions. Complexes [(4)MX(1,5-cyclooctadiene)] (M=Rh, Ir; X=Cl, I; ?=0.008-0.016) are converted into strongly fluorescent complexes [(4)MX(CO)2 ] (?=0.53-0.70) upon reaction with carbon monoxide. The unquenching of the Rh and Ir complexes appears to be a consequence of the decreased electron density at Rh or Ir in the carbonyl complexes. In contrast, the substitution of an iodo ligand in [(4)AuI] by an electron-rich thiolate decreases the brightness of the BODIPY fluorophore, rendering the BODIPY as a highly sensitive probe for changes in the coordination sphere of the transition metal. PMID:25487747

Kos, Pavlo; Plenio, Herbert

2015-01-12

75

Monomeric bis(anilido)iron(II) Complexes with N-heterocyclic carbene ligation: synthesis, characterization, and redox reactivity toward aryl halides.  

Science.gov (United States)

Using monodentate N-heterocyclic carbenes as the ancillary ligands, seven monomeric bis(anilido)iron(II) complexes [(IPr(2)Me(2))(2)Fe(NHAr)(2)] (IPr(2)Me(2) = 2,5-diisopropyl-3,4-dimethylimidazol-1-ylidene; Ar = Ph, C(6)H(4)-2-Pr(i), Mes, C(6)H(3)-2,6-Cl(2), Dipp) and [(IPr)Fe(NHAr)(2)] (IPr = 2,5-di(2,6-diisopropylphenyl)imidazol-1-ylidene; Ar = C(6)H(3)-2,6-Cl(2), Dipp) have been prepared by the one-pot reactions of [Fe(Mes)(2)](2) with the corresponding N-heterocyclic carbenes, and anilines. These high-spin diamido complexes have been fully characterized by (1)H NMR, solution magnetic susceptibility, UV-vis, IR, X-ray diffraction, cyclic voltammetry, as well as elemental analysis. The strong affinity of the N-heterocyclic carbene ligands toward ferrous centers, and the steric protection exerted by the NHC ligands are the key factors to stabilize these bis(anilido)iron complexes in a monomeric manner. Reactivity studies revealed the four-coordinate complex [(IPr(2)Me(2))(2)Fe(NHMes)(2)] can react with 1 equiv of 1-iodo-3,5-dimethylbenzene or 1-bromo-3,5-dimethylbenzene in C(6)D(6) and THF-d(8) to furnish 1-C(6)D(5)-3,5-Me(2)C(6)H(3), and 5-D-1,3-Me(2)C(6)H(3), respectively. Under similar conditions, the three-coordinate compound [(IPr)Fe(NHDipp)(2)] is inert toward these halides. PMID:23249144

Wang, Xiaojie; Mo, Zhenbo; Xiao, Jie; Deng, Liang

2013-01-01

76

N-heterocyclic carbene formation induced fluorescent and colorimetric sensing of fluoride using perimidinium derivatives.  

Science.gov (United States)

In this study, two perimidinium derivatives (1 and 2) were designed, synthesized, and developed as efficient fluorescent and colorimetric chemodosisensors for F(-) in DMSO or more competitive media (DMSO containing 10?% water). In the presence of F(-) , the yellow and non-fluorescent solution of 1/2 became colourless and exhibited strong blue fluorescence. This unique spectroscopic behaviour of 1/2 towards F(-) was attributed to the formation of N-heterocyclic carbene deprotonated by F(-) , which immediately reacted with water to give a colourless and fluorescent carbinol. Interestingly, it was found that this carbinol intermediate was unstable and further underwent a redox disproportionation to generate two other optically changed compounds. All the proposed mechanisms for the sensing process have been carefully confirmed by experiments. PMID:25346203

Zhang, Dawei; Yang, Haiqiang; Martinez, Alexandre; Jamieson, Kelsey; Dutasta, Jean-Pierre; Gao, Guohua

2014-12-15

77

Group 12 Metal Complexes of N-Heterocyclic Ditopic Carbanionic Carbenes  

OpenAIRE

Reaction of the N-heterocyclic carbene 1,3-bis(2,6-diisopropylphenyl) imidazol-2-ylidene (IPr) with M(mes)2 (M = Zn, Cd) in diethyl ether afforded the Lewis acid-base adducts [M(IPr)(mes)2] (M = Zn (1), Cd (2)) in quantitative yields. An analogous reaction between Hg(mes)2 and IPr failed to form the desired 1:1 adduct, [Hg(IPr)(mes)2], as evidenced by NMR spectroscopy. Reduction of 1 and 2 with KC8 afforded K+ salts of the anionic complexes [{:C[N(2,6-iPr 2C6H3)]2(CH)C}2M(mes)] - (M = Zn (3),...

Waters, Jb; Turbervill, Rsp; Goicoechea, Jm

2013-01-01

78

Expedient syntheses of the N-heterocyclic carbene precursor imidazolium salts IPr·HCl, IMes·HCl and IXy·HCl  

OpenAIRE

Abstract The 1,3-diaryl-imidazolium chlorides IPr·HCl (aryl = 2,6-diisopropylphenyl), IMes·HCl (aryl = 2,4,6-trimethylphenyl) and IXy·HCl (aryl = 2,6-dimethylphenyl), precursors to widely used N-heterocyclic carbene (NHC) ligands and catalysts, were prepared in high yields (81%, 69% and 89%, respectively) by the reaction of 1,4-diaryl-1, 4-diazabutadienes, paraformaldehyde and chlorotrimethylsilane in dilute ethyl acetate solution. A reaction mechanism involving a 1,5-dipolar ele...

Hintermann Lukas

2007-01-01

79

Expedient syntheses of the N-heterocyclic carbene precursor imidazolium salts IPr·HCl, IMes·HCl and IXy·HCl  

Directory of Open Access Journals (Sweden)

Full Text Available Abstract The 1,3-diaryl-imidazolium chlorides IPr·HCl (aryl = 2,6-diisopropylphenyl, IMes·HCl (aryl = 2,4,6-trimethylphenyl and IXy·HCl (aryl = 2,6-dimethylphenyl, precursors to widely used N-heterocyclic carbene (NHC ligands and catalysts, were prepared in high yields (81%, 69% and 89%, respectively by the reaction of 1,4-diaryl-1, 4-diazabutadienes, paraformaldehyde and chlorotrimethylsilane in dilute ethyl acetate solution. A reaction mechanism involving a 1,5-dipolar electrocyclization is proposed.

Hintermann Lukas

2007-08-01

80

Imidazolium hydrogen carbonates versus imidazolium carboxylates as organic precatalysts for N-heterocyclic carbene catalyzed reactions.  

Science.gov (United States)

Imidazolium-2-carboxylates (NHC-CO(2) adducts, 3) and (benz)imidazolium hydrogen carbonates ([NHC(H)][HCO(3)], 4) were independently employed as organic precatalysts for various molecular N-heterocyclic carbene (NHC) catalyzed reactions. NHC-CO(2) adducts were obtained by carboxylation in THF of related free NHCs (2), while the synthesis of [NHC(H)][HCO(3)] precursors was directly achieved by anion metathesis of imidazolium halides (1) using potassium hydrogen carbonate (KHCO(3)) in methanolic solution, without the need for the prior preparation of free carbenes. Thermogravimetric analysis (TGA) and TGA coupled with mass spectrometry (TGA-MS) of most [NHC(H)][HCO(3)] precursors 4 showed a degradation profile in stages, with either a concomitant or a stepwise release of H(2)O and CO(2), between 108 and 280 °C, depending on the nature of the azolium and substituents. In solution, NHC generation from both [NHC(H)][HCO(3)] salts and NHC-CO(2) adducts could be achieved at room temperature, most likely by a simple solvation effect. Both types of precursors proved efficient for organocatalyzed molecular reactions, including cyanosilylation, benzoin condensation, and transesterification reactions. The catalytic efficiencies of NHC-CO(2) adducts 3 were found to be approximately 3 times higher than those of their [NHC(H)][HCO(3)] counterparts 4. PMID:23092332

Fèvre, Maréva; Coupillaud, Paul; Miqueu, Karinne; Sotiropoulos, Jean-Marc; Vignolle, Joan; Taton, Daniel

2012-11-16

81

Intramolecular cyclotrimerization of triynes catalyzed by N-heterocyclic carbene-CoCl2/Zn or -FeCl3/Zn.  

Science.gov (United States)

[reaction: see text] Triynes 1 could effectively be cyclotrimerized to annulated benzenes 2 by treatment with a catalytic amount of zinc powder, N-heterocyclic carbene, and CoCl(2) or FeCl(3). PMID:15987206

Saino, Naoko; Kogure, Daisuke; Okamoto, Sentaro

2005-07-01

82

N-heterocyclic carbene-catalyzed [4 + 2] cycloaddition of ketenes and 3-aroylcoumarins: highly enantioselective synthesis of dihydrocoumarin-fused dihydropyranones.  

Science.gov (United States)

The N-heterocyclic carbene-catalyzed [4 + 2] cyclization of ketenes and 3-aroylcoumarins has been developed to give the corresponding dihydrocoumarin-fused multisubstituted dihydropyranones in high yield with good diastereoselectivity and high enantioselectivity. PMID:23117299

Jian, Teng-Yue; Chen, Xiang-Yu; Sun, Li-Hui; Ye, Song

2013-01-01

83

N-heterocyclic carbene gold(i) and silver(i) complexes bearing functional groups for bio-conjugation.  

Science.gov (United States)

This work describes several synthetic approaches to append organic functional groups to gold and silver N-heterocyclic carbene (NHC) complexes suitable for applications in biomolecule conjugation. Carboxylate appended NHC ligands (3) lead to unstable Au(I) complexes that convert into bis-NHC species (4). A benzyl protected carboxylate NHC-Au(I) complex was synthesized but deprotection to produce the carboxylic acid functionality could not be achieved. A small library of new alkyne functionalized NHC proligands were synthesized and used for subsequent silver and gold metalation reactions. The alkyne appended NHC gold complex readily reacts with benzyl azide in a copper catalyzed azide-alkyne cycloaddition reaction to form the triazole appended NHC gold complex . Cell cytotoxicity studies were performed on DLD-1 (colorectal adenocarcinoma), Hep-G2 (hepatocellular carcinoma), MCF-7 (breast adenocarcinoma), CCRF-CEM (human T-Cell leukemia), and HEK (human embryonic kidney). Complete spectroscopic characterization of the ligands and complexes was achieved using (1)H and (13)C NMR, gHMBC, ESI-MS, and combustion analysis. PMID:25490699

Garner, Mary E; Niu, Weijia; Chen, Xigao; Ghiviriga, Ion; Abboud, Khalil A; Tan, Weihong; Veige, Adam S

2014-12-23

84

N-Heterocyclic carbene-palladium(II)-1-methylimidazole complex catalyzed Mizoroki-Heck reaction of aryl chlorides with styrenes.  

Science.gov (United States)

A well-defined N-heterocyclic carbene-palladium(II)-1-methylimidazole [NHC-Pd(II)-Im] complex 1 was found to be an effective catalyst for the Mizoroki-Heck reaction of a variety of aryl chlorides with styrenes. Both activated and deactivated aryl chlorides work well to give the corresponding coupling products in good to excellent yields by using tetrabutylammonium bromide (TBAB) as the ionic liquid. PMID:23209531

Gao, Ting-Ting; Jin, Ai-Ping; Shao, Li-Xiong

2012-01-01

85

N-Heterocyclic carbene–palladium(II)-1-methylimidazole complex catalyzed Mizoroki–Heck reaction of aryl chlorides with styrenes  

OpenAIRE

A well-defined N-heterocyclic carbene–palladium(II)-1-methylimidazole [NHC-Pd(II)-Im] complex 1 was found to be an effective catalyst for the Mizoroki–Heck reaction of a variety of aryl chlorides with styrenes. Both activated and deactivated aryl chlorides work well to give the corresponding coupling products in good to excellent yields by using tetrabutylammonium bromide (TBAB) as the ionic liquid.

Gao, Ting-ting; Jin, Ai-ping; Shao, Li-xiong

2012-01-01

86

N-Heterocyclic carbene–palladium(II)-1-methylimidazole complex catalyzed Mizoroki–Heck reaction of aryl chlorides with styrenes  

Science.gov (United States)

Summary A well-defined N-heterocyclic carbene–palladium(II)-1-methylimidazole [NHC-Pd(II)-Im] complex 1 was found to be an effective catalyst for the Mizoroki–Heck reaction of a variety of aryl chlorides with styrenes. Both activated and deactivated aryl chlorides work well to give the corresponding coupling products in good to excellent yields by using tetrabutylammonium bromide (TBAB) as the ionic liquid. PMID:23209531

Gao, Ting-Ting; Jin, Ai-Ping

2012-01-01

87

Enantioselective sulfonation of enones with sulfonyl imines by cooperative N-heterocyclic-carbene/thiourea/tertiary-amine multicatalysis.  

Science.gov (United States)

Many hands make light work: In an organocatalytic asymmetric sulfonation of enones, the activation of a sulfonyl imine by an N-heterocyclic carbene (NHC) catalyst led to the release of a sulfinic anion, which underwent nucleophilic addition to the enone. The enantioselectivity of the process was controlled by a chiral thiourea/amine co-catalyst through anion recognition and hydrogen-bonding interactions. Tol=p-tolyl. PMID:24115649

Jin, Zhichao; Xu, Jianfeng; Yang, Song; Song, Bao-An; Chi, Yonggui Robin

2013-11-18

88

Suzuki-Miyaura Reactions Catalyzed by C2-Symmetric Pd-Multi-Dentate N-Heterocyclic Carbene Complexes  

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Full Text Available Suzuki-Miyaura coupling reactions are promoted by Pd complexes ligated with C2-symmetric multi-dentate N-heterocyclic carbenes derived in situ from Pd(OAc2 and imidazolium salts. Good to excellent yields were obtained for aryl bromides as substrates. Turnover numbers of up to 105 could be achieved with 5 × 10?4 mol% of Pd(OAc2/1 × 10?3 mol% NHC precatalyst in 24 h.

Zhengning Li

2012-10-01

89

N-Heterocyclic carbene–palladium(II-1-methylimidazole complex catalyzed Mizoroki–Heck reaction of aryl chlorides with styrenes  

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Full Text Available A well-defined N-heterocyclic carbene–palladium(II-1-methylimidazole [NHC-Pd(II-Im] complex 1 was found to be an effective catalyst for the Mizoroki–Heck reaction of a variety of aryl chlorides with styrenes. Both activated and deactivated aryl chlorides work well to give the corresponding coupling products in good to excellent yields by using tetrabutylammonium bromide (TBAB as the ionic liquid.

Ting-Ting Gao

2012-11-01

90

Catalytic boracarboxylation of alkynes with diborane and carbon dioxide by an N-heterocyclic carbene copper catalyst.  

Science.gov (United States)

By the use of an N-heterocyclic carbene copper(I) complex as a catalyst, the boracarboxylation of various alkynes (e.g., diaryl alkynes, aryl/alkyl alkynes, and phenylacetylene) with a diborane compound and carbon dioxide has been achieved for the first time, affording the ?,?-unsaturated ?-boralactone derivatives regio- and stereoselectively via a borylcupration/carboxylation cascade. Some important reaction intermediates were isolated and structurally characterized to clarify the reaction mechanism. PMID:22909063

Zhang, Liang; Cheng, Jianhua; Carry, Béatrice; Hou, Zhaomin

2012-09-01

91

[3+2] Fragmentation of an [RP(5)Cl](+) cage cation induced by an N-heterocyclic carbene.  

Science.gov (United States)

The cage compound [DippP5 Cl][GaCl4 ] (Dipp=2,6-diisopropylphenyl) reacts with an NHC (N-heterocyclic carbene) by an unprecedented [3+2] fragmentation of the P5 (+) core. This yields an imidazoliumyl-substituted P3 species featuring a triphosphaallyl anion motif and a neutral P2 compound. The mechanism of the fragmentation reaction was elucidated by means of experimental and quantum chemical methods. PMID:24038818

Holthausen, Michael H; Surmiak, Sabrina K; Jerabek, Paul; Frenking, Gernot; Weigand, Jan J

2013-10-11

92

N-Heterocyclic Carbene Catalyzed Enantioselective ?-Fluorination of Aliphatic Aldehydes and ?-Chloro Aldehydes: Synthesis of ?-Fluoro Esters, Amides, and Thioesters.  

Science.gov (United States)

The asymmetric fluorination of azolium enolates that are generated from readily available simple aliphatic aldehydes or ?-chloro aldehydes and N-heterocyclic carbenes (NHCs) is described. The process significantly expands the synthetic utility of NHC-catalyzed fluorination and provides facile access to a wide range of ?-fluoro esters, amides, and thioesters with excellent enantioselectivity. Pyrazole was identified as an excellent acyl transfer reagent for catalytic amide formation. PMID:25394812

Dong, Xiuqin; Yang, Wen; Hu, Weimin; Sun, Jianwei

2015-01-01

93

Mechanistic Investigation of the Ruthenium–N-Heterocyclic-Carbene-Catalyzed Amidation of Alcohols and Amines  

DEFF Research Database (Denmark)

The mechanism of the ruthenium–N-heterocyclic-carbene-catalyzed formation of amides from alcohols and amines was investigated by experimental techniques (Hammett studies, kinetic isotope effects) and by a computational study by using dispersion-corrected density functional theory (DFT/ M06). The Hammett study indicated that a small positive charge builds-up at the benzylic position in the transition state of the turnover-limiting step. The kinetic isotope effect was determined to be 2.29ACHTUNGTRENUNG(!0.15), which suggests that the breakage of the C"H bond is not the rate-limiting step, but that it is one of several slow steps in the catalytic cycle. Rapid scrambling of hydrogen and deuterium at the a position of the alcohol was observed with deuterium-labeled substrates, which implies that the catalytically active species is a ruthenium dihydride. The experimental results were supported by the characterization of a plausible catalytic cycle by using DFT/M06. Both cisdihydride and trans-dihydride intermediates were considered, but when the theoretical turnover frequencies (TOFs) were derived directly from the calculated DFT/M06 energies, we found that only the trans-dihydride pathway was in agreement with the experimentally determined TOFs.

Makarov, Ilya; Fristrup, Peter

2012-01-01

94

Expanded ring N-heterocyclic carbene adducts of group 15 element trichlorides: synthesis and reduction studies.  

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Reactions of the expanded ring N-heterocyclic carbene, 6-Dip (:C{N(Dip)CH2}2CH2, Dip = 2,6-diisopropylphenyl), with group 15 element trichlorides have yielded the monomeric complexes, [(6-Dip)ECl3] (E = P, As or Sb), two examples of which (E = P and Sb) have been crystallographically characterised. Reduction of [(6-Dip)PCl3] with KC8 yielded the unusual tetraphosphorus dicationic complex, [(6-Dip)2(?-P4)]Cl2, the X-ray crystal structure of which shows it to be an ion-separate salt. The compound can also be prepared from the direct reaction of excess 6-Dip with PCl3. Treatment of the cyclic amidinium salt, [6-MesH]Br (6-MesH = [HC{N(Mes)CH2}2CH2](+), Mes = mesityl) with KC8, leads to reductive coupling of the heterocycle and formation of the hindered bis(hexahydropyrimidine), (6-MesH)2. An X-ray crystallographic analysis of (6-MesH)2 shows the compound to have a long central C-C bond, while an electrochemical analysis reveals it to undergo an irreversible two-electron oxidation in dichloromethane solutions. PMID:25166429

Sidiropoulos, Anastas; Osborne, Brooke; Simonov, Alexandr N; Dange, Deepak; Bond, Alan M; Stasch, Andreas; Jones, Cameron

2014-10-21

95

On the electronic impact of abnormal C4-bonding in N-heterocyclic carbene complexes  

OpenAIRE

Sterically similar palladium dicarbene complexes have been synthesized that comprise permethylated dicarbene ligands which bind the metal center either in a normal coordination mode via C2 or abnormally via C4. Due to the strong structural analogy of the complexes, differences in reactivity patterns may be attributed to the distinct electronic impact of normal versus abnormal carbene bonding, while stereoelectronic effects are negligible. Unique reactivity patterns have been identified for th...

Heckenroth, M.; Neels, A.; Garnier, M. G.; Aebi, P.; Ehlers, A. W.; Albrecht, M.

2009-01-01

96

Gold(III) N-heterocyclic carbene complexes mediated synthesis of ?-enaminones from 1,3-dicarbonyl compounds and aliphatic amines.  

Science.gov (United States)

A series of gold(III) N-heterocyclic carbene complexes [1-(R(1))-3-(R(2))imidazol-2-ylidene]AuBr(3) [R(1) = i-Pr, R(2) = CH(2)Ph (1c); R(1) = mesityl, R(2) = CH(2)Ph (2c); R(1) = i-Pr, R(2) = CH(2)COt-Bu (3c), and R(1) = t-Bu, R(2) = CH(2)COt-Bu (4c)] act as effective precatalysts in the synthesis of ?-enaminones from 1,3-dicarbonyl compounds and primary amines under ambient conditions. Specifically the 1c-4c complexes efficiently catalyzed the condensation of a variety of cyclic as well as acyclic 1,3-dicarbonyl compounds, namely, acetyl acetone, benzoylacetone, 2-acetylcyclopentanone, and ethyl-2-oxocyclopentanecarboxylate with primary aliphatic amines, viz., methylamine, ethylamine, n-propylamine, i-propylamine, and n-butylamine, yielding ?-enamines at room temperature. Interestingly enough, the more electrophilic gold(III) 1c-4c complexes exhibited superior activity in comparison to the gold(I) counterparts 1b-4b. A comparison along a representative 4a-c series further underscored the importance of gold in the reaction as both the gold(I) 4b and gold(III) 4c complexes were more effective than the silver analogue 4a. The density functional theory (DFT) study revealed that the strong ?-donating nature of the N-heterocyclic carbene ligand results in a strong C(carbene)-Au(III) interaction in the 1c-4c complexes. PMID:21254751

Samantaray, Manoja K; Dash, Chandrakanta; Shaikh, Mobin M; Pang, Keliang; Butcher, Ray J; Ghosh, Prasenjit

2011-03-01

97

Investigation of the Role of Electrogenerated N-Heterocyclic Carbene in the Staudinger Synthesis in Ionic Liquid  

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Full Text Available Electrogenerated N-heterocyclic carbene (NHC, obtained by cathodic reduction of Bmim-BF4, behaves as organocatalyst and base in the Staudinger synthesis from an acyl chloride and a deactivated imine in ionic liquid to yield ?-lactams. The effect of many parameters (temperature, amount of electricity, substituents, presence of an external base has been evaluated and a tentative mechanism for the Staudinger synthesis in a very polar medium like the ionic liquid reported. The yields of isolated ?-lactams are good, starting from non-electrophilic imines, and predominantly trans lactams are obtained with a good diastereomeric ratio.

Marta Feroci

2011-12-01

98

C?C coupling of ketones with methanol catalyzed by a N-heterocyclic carbene-phosphine iridium complex.  

Science.gov (United States)

An N-heterocyclic carbene-phosphine iridium complex system was found to be a very efficient catalyst for the methylation of ketone via a hydrogen transfer reaction. Mild conditions together with low catalyst loading (1?mol?%) were used for a tandem process which involves the dehydrogenation of methanol, C?C bond formation with a ketone, and hydrogenation of the new generated double bond by iridium hydride to give the alkylated product. Using this iridium catalyst system, a number of branched ketones were synthesized with good to excellent conversions and yields. PMID:25589169

Quan, Xu; Kerdphon, Sutthichat; Andersson, Pher G

2015-02-23

99

N-heterocyclic carbene catalyzed carba-, sulfa-, and phospha-Michael additions with NHC·CO? adducts as precatalysts.  

Science.gov (United States)

N-heterocyclic carbene catalyzed Michael additions have been revisited with 1,3-dialkyl- or 1,3-diarylimidazol(in)ium-2-carboxylates, that is, NHC·CO2 adducts, as the source of the free NHC catalysts in solution. Using these precatalysts, a number of efficient carba-, sulfa-, and phospha-Michael additions were achieved very conveniently, without the need for an external strong base to generate the NHC by deprotonation of an azolium salt. To further expand the scope of the procedure, some NHC-catalyzed sulfa-Michael/aldol organocascades were also investigated. PMID:24568358

Hans, Morgan; Delaude, Lionel; Rodriguez, Jean; Coquerel, Yoann

2014-03-21

100

A cationic rhodium(I N-heterocyclic carbene complex isolated as an aqua adduct  

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Full Text Available The title complex, aqua[1,3-bis(2,6-diisopropylphenylimidazol-2-ylidene](?4-cycloocta-1,5-dienerhodium(I tetrafluoridoborate, [Rh(C8H12(C27H36N2(H2O]BF4, exihibits a square-planar geometry around the Rh(I atom, formed by a bidentate cycloocta-1,5-diene (cod ligand, an N-heterocylcic carbene and an aqua ligand. The complex is cationic and a BF4? anion balances the charge. The structure exists as a hydrogen-bonded dimer in the solid state, formed via interactions between the aqua ligand H atoms and the BF4? F atoms.

Ashley L. Huttenstine

2011-09-01

101

Palladium-N-heterocyclic carbene (NHC)-catalyzed asymmetric synthesis of indolines through regiodivergent C(sp3)-H activation: scope and DFT study.  

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Two bulky, chiral, monodentate N-heterocyclic carbene ligands were applied to palladium-catalyzed asymmetric C-H arylation to incorporate C(sp(3))-H bond activation. Racemic mixtures of the carbamate starting materials underwent regiodivergent reactions to afford different trans-2,3-substituted indolines. Although this CAr-Calkyl coupling requires high temperatures (140-160?°C), chiral induction is high. This regiodivergent reaction, when carried out with enantiopure starting materials, can lead to single structurally different enantiopure products, depending on the catalyst chirality. The C-H activation at a tertiary center was realized only in the case of a cyclopropyl group. No C-H activation takes place alpha to a tertiary center. A detailed DFT study is included and analyses of methyl versus methylene versus methine C-H activation is used to rationalize experimentally observed regio- and enantioselectivities. PMID:25262613

Katayev, Dmitry; Larionov, Evgeny; Nakanishi, Masafumi; Besnard, Céline; Kündig, E Peter

2014-11-10

102

Contrasting reactivities of silicon and germanium complexes supported by an N-heterocyclic guanidine ligand.  

Science.gov (United States)

We report the synthesis of an acyclic two-coordinate germylene supported by two bulky and electron donating N-heterocyclic guanidine [IPr?N](-) ligands (IPr = [(HCNDipp)2C:]; Dipp = [2,6-(i)Pr2C6H3]), and its reactivity with molecular hydrogen to form IPr?NH, which presumably proceeds via the unstable intermediate [H2Ge(N?IPr)2]. Our attempts to isolate the corresponding silylene [:Si(N?IPr)2] led to an unexpected ligand activation/rearrangement process involving N-C(aryl) bond cleavage within the N-heterocyclic guanidine ligand; this transformation was also studied by computational methods. PMID:25621845

Lui, Melanie W; Merten, Christian; Ferguson, Michael J; McDonald, Robert; Xu, Yunjie; Rivard, Eric

2015-02-16

103

Sterically Encumbered Iridium Bis(N-heterocyclic carbene) Systems: Multiple C-H Activation Processes and Isomeric Normal/Abnormal Carbene Complexes  

OpenAIRE

The reaction of [Ir(coe)2Cl]2 (coe = cyclooctene) with the N-heterocyclic carbene N,N?-bis(2,6diisopropylphenyl)imidazol-2- ylidene (IPr) in tetrahydrofuran under anaerobic conditions leads to the formation of two main products, the planar four-coordinate Ir(I) complex Ir(IPr)(IPr?)Cl (1) formed by dehydrogenation of one of the IPr isopropyl substituents (to give the mixed NHC/alkene donor IPr?) and the trigonal bipyramidal Ir(III) system Ir(IPr)(aIPr)(H)2Cl (6), which features both "no...

Tang, Cy; Smith, W.; Vidovic, D.; Thompson, Al; Chaplin, Ab; Aldridge, S.

2009-01-01

104

Tungsten(VI) N-Heterocyclic Carbene Complexes: Synthetic, Structural, and Computational Study  

Science.gov (United States)

The reaction of WOCl4 with 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (Idipp) leads to an orange solid whose spectroscopic data are consistent with the 1:1 adduct [WOCl4(Idipp)]. Computational studies at the DFT level further support this formulation. Exposure of this compound to the atmosphere results in rapid hydrolysis to various imidazolium salts. If air diffuses very slowly into solutions of [WOCl4(Idipp)], it also undergoes slow hydrolysis to form [WO2Cl2(Idipp)]. This has been crystallographically characterized and is the first five-coordinate, 1:1 adduct of WO2Cl2. This complex has also been subject to DFT calculations, and its metal–ligand bonding has been explored. The carbene–metal interaction is primarily ?-donor in nature. The mechanism of the hydrolysis has also been probed by computational methods, revealing a plausible, low-energy reaction pathway. PMID:24882916

2011-01-01

105

A comparative study on dinuclear and multinuclear Ni(II), Pd(II), and Pt(II) complexes of a thiolato-functionalized, benzannulated N-heterocyclic carbene ligand.  

Science.gov (United States)

Dimeric thiolato-bridged Ni(II) and Pt(II) NHC complexes 2 and 4 have been synthesized from ligand precursor A through a combined and in situ deprotonation/hydrolysis protocol of a thioester-functionalized benzimidazolium salt in the presence of the respective metal salts. Reactivity studies of 2 and 4, and their previously reported Pd(II) analogue 1a toward either Me3OBF4, NaOH, or Na2S·9H2O revealed clear differences. Complex 2 decomposed when treated with Me3OBF4. On the other hand, its reaction with aqueous NaOH solution in the presence of NaBF4 yielded trinuclear [Ni3S3O] complex 6, which possesses an interesting [Ni3S3] triangle with a capping ?3-oxido ligand. Pt(II) analogue 4 was converted to the tetranuclear [Pt4S4] macrocycle 5 when treated with Me3OBF4, in analogy to the result from 1a, while no defined products could be isolated when 4 was treated with either NaOH or Na2S·9H2O. Pd(II) analogue 1a reacted with Na2S·9H2O to give the tripalladium [Pd3S3S] complex 7 bearing a capping ?3-sulfido ligand. PMID:23679658

Yuan, Dan; Huynh, Han Vinh

2013-06-01

106

Synthesis and Characterization of Di- and Tetracarbene Iron(II) Complexes with Chelating N-Heterocyclic Carbene Ligands and Their Application in Aryl Grignard–Alkyl Halide Cross-Coupling  

OpenAIRE

A series of new and known bis(imidazolium) chloride and bromide salts bridged by either a methylene group (1?8, 10a,b) or an ethylene group (9a,b) and bearing different N substituents (Me, Et, Bn, tBu, Mes) have been reacted with [Fe{N(SiMe3)2}2]2 to yield the four-coordinate iron(II) complexes [LFeX2] (11?20; X = Cl, Br; L = chelating bis(imidazolylidene) ligand). Molecular structures of six of these complexes have been characterized by X-ray crystallography, and ...

Meyer, Steffen; Orben, Claudia Manuela; Demeshko, Serhiy; Dechert, Sebastian; Meyer, Franc

2011-01-01

107

A chiral 6-membered N-heterocyclic carbene copper(I) complex that induces high stereoselectivity.  

Science.gov (United States)

A chiral 6-membered annulated N-heterocyclic (6-NHC) copper complex that catalyzes ?-borylations with high yield and enantioselectivity was developed. The chiral 6-NHC copper complex is easy to prepare on the gram scale and is very active, showing 10,000 turnovers at 0.01 mol % of catalyst without significant decrease of enantioselectivity and with useful reaction rates. PMID:20919706

Park, Jin Kyoon; Lackey, Hershel H; Rexford, Matthew D; Kovnir, Kirill; Shatruk, Michael; McQuade, D Tyler

2010-11-01

108

Activation of carbon dioxide and carbon disulfide by a scandium N-heterocyclic carbene complex.  

Science.gov (United States)

A Sc NHC complex readily activates three equivalents of CO2 showing 'Frustrated Lewis Pair' type reactivity with each metal-carbene bond, but whilst CS2 is also activated by the labile carbenes, no metal involvement is observed. PMID:24162511

Arnold, Polly L; Marr, Isobel A; Zlatogorsky, Sergey; Bellabarba, Ronan; Tooze, Robert P

2014-01-01

109

Expanded-ring N-heterocyclic carbenes efficiently stabilize gold(I) cations, leading to high activity in ?-acid-catalyzed cyclizations.  

Science.gov (United States)

A series of six- and seven-membered expanded-ring N-heterocyclic carbene (er-NHC) gold(I) complexes has been synthesized using different synthetic approaches. Complexes with weakly coordinating anions [(er-NHC)AuX] (X(-) = BF4(-), NTf2(-), OTf(-)) were generated in solution. According to their (13)C?NMR spectra, the ionic character of the complexes increases in the order X(-) = Cl(-) < NTf2(-) < OTf(-) < BF4(-). Additional factors for stabilization of the cationic complexes are expansion of the NHC ring and the attachment of bulky substituents at the nitrogen atoms. These er-NHCs are bulkier ligands and stronger electron donors than conventional NHCs as well as phosphines and sulfides and provide more stabilization of [(L)Au(+)] cations. A comparative study has been carried out of the catalytic activities of five-, six-, and seven-membered carbene complexes [(NHC)AuX], [(Ph3 P)AuX], [(Me2S)AuX], and inorganic compounds of gold in model reactions of indole and benzofuran synthesis. It was found that increased ionic character of the complexes was correlated with increased catalytic activity in the cyclization reactions. As a result, we developed an unprecedentedly active monoligand cationic [(THD-Dipp)Au]BF4 (1,3-bis(2,6-diisopropylphenyl)-3,4,5,6-tetrahydrodiazepin-2-ylidene gold(I) tetrafluoroborate) catalyst bearing seven-membered-ring carbene and bulky Dipp substituents. Quantitative yields of cyclized products were attained in several minutes at room temperature at 1?mol?% catalyst loadings. The experimental observations were rationalized and fully supported by DFT calculations. PMID:24682976

Morozov, Oleg S; Lunchev, Andrey V; Bush, Alexander A; Tukov, Aleksandr A; Asachenko, Andrey F; Khrustalev, Victor N; Zalesskiy, Sergey S; Ananikov, Valentine P; Nechaev, Mikhail S

2014-05-12

110

Bis-N-heterocyclic carbene (NHC) stabilized ?6-arene iron(0) complexes: synthesis, structure, reactivity, and catalytic activity.  

Science.gov (United States)

Reaction of FeCl2 with the chelating bis-N-heterocyclic carbene (NHC) bis-(N-Dipp-imidazole-2-ylidene)methylene (abbreviated {((Dipp)C:)2CH2}) (Dipp = 2,6-di-isopropylphenyl) affords the complex [FeCl2{((Dipp)C:)2CH2}] (1) in high yield. Reduction of complex 1 with excess KC8 with a 10-fold molar excess of PMe3 affords the Fe(II) complex [FeH{((Dipp)C:)2CH2}(PMe3)(?(2)-PMe2CH2)] (2) as a mixture of three stereoisomers. Complex 2, the first example of any iron(II) complex bearing mutually an NHC and PMe3 ligand, is likely obtained from the in situ, reductively generated 16 VE Fe(0) complex, [Fe{((Dipp)C:)2CH2}(PMe3)2] (2'), following intramolecular C-H activation of one of the phosphorus-bound CH3 groups. Complex 2 is unstable in aromatic solvents and forms, via a novel synthetic transformation involving intramolecular reductive elimination and concomitant PMe3 elimination, the Fe (0) arene complex [Fe{((Dipp)C:)2CH2}(?(6)-C6D6)] (4-d6) in C6D6. Complex 4-d6 represents the first example of an NHC stabilized iron (0) arene complex. The transformation from 2 to 4-d6 can be accelerated at higher temperature and at 60 °C forms immediately. Alternatively, the reduction of 1 in the presence of toluene or benzene affords the complexes [Fe{((Dipp)C:)2CH2}(?(6)-C7H8)] (3) and [Fe{((Dipp)C:)2CH2}(?(6)-C6H6)] (4), selectively and in good yields. DFT calculations characterizing the bonding situation in 3 and 4 reveal similar energies of the HOMO and LUMO orbitals, with the LUMO orbital of both complexes located on the Dipp rings of the bis-NHC. The HOMO orbital reflects a ?-back-bonding interaction between the Fe(0) center and the chelating NHC ligand, while the HOMO-1 is associated with the arene interaction with the Fe(0) site. The calculations do not suggest any noninnocence of the coordinated arene in either complex. Moreover, the (57)Fe Mössbauer spectrum of 4 at 80K exhibits parameters (? = 0.43 mm·s(-1); ?EQ = 1.37 mm·s(-1)) which are consistent with a five-coordinate Fe(0) system, rendering 3 and 4 the first examples of well-defined authentic Fe(0)-?(6)-arene complexes of the type [Fe(?(6)-arene)L2] (L = ?(1 or 2) neutral ligand, mono or bidentate). Some reactivitiy studies of 3 are also reported: The reaction of 3 with excess CO selectively yields the five-coordinate piano-stool complex [Fe{((Dipp)C:)2CH2}(CO)3] (6) in near quantitative yields, while the reaction of complex 3 with C6D6 under heating affords by toluene elimination 4-d6. The catalytic ability of 4 was also investigated with respect to amide reduction to amines, for a variety of substrates using Ph2SiH2 as a hydride source. In all cases good to excellent yields to the corresponding amines were obtained. The use of 4 as a precatalyst represents the first example of a well-defined Fe(0) complex to effect this catalytic process. PMID:24195449

Blom, Burgert; Tan, Gengwen; Enthaler, Stephan; Inoue, Shigeyoshi; Epping, Jan Dirk; Driess, Matthias

2013-12-01

111

Visible-light photoredox catalysis: selective reduction of carbon dioxide to carbon monoxide by a nickel N-heterocyclic carbene-isoquinoline complex.  

Science.gov (United States)

The solar-driven reduction of carbon dioxide to value-added chemical fuels is a longstanding challenge in the fields of catalysis, energy science, and green chemistry. In order to develop effective CO2 fixation, several key considerations must be balanced, including (1) catalyst selectivity for promoting CO2 reduction over competing hydrogen generation from proton reduction, (2) visible-light harvesting that matches the solar spectrum, and (3) the use of cheap and earth-abundant catalytic components. In this report, we present the synthesis and characterization of a new family of earth-abundant nickel complexes supported by N-heterocyclic carbene-amine ligands that exhibit high selectivity and activity for the electrocatalytic and photocatalytic conversion of CO2 to CO. Systematic changes in the carbene and amine donors of the ligand have been surveyed, and [Ni((Pr)bimiq1)](2+) (1c, where (Pr)bimiq1 = bis(3-(imidazolyl)isoquinolinyl)propane) emerges as a catalyst for electrochemical reduction of CO2 with the lowest cathodic onset potential (E(cat) = -1.2 V vs SCE). Using this earth-abundant catalyst with Ir(ppy)3 (where ppy = 2-phenylpyridine) and an electron donor, we have developed a visible-light photoredox system for the catalytic conversion of CO2 to CO that proceeds with high selectivity and activity and achieves turnover numbers and turnover frequencies reaching 98,000 and 3.9 s(-1), respectively. Further studies reveal that the overall efficiency of this solar-to-fuel cycle may be limited by the formation of the active Ni catalyst and/or the chemical reduction of CO2 to CO at the reduced nickel center and provide a starting point for improved photoredox systems for sustainable carbon-neutral energy conversion. PMID:24033186

Thoi, V Sara; Kornienko, Nikolay; Margarit, Charles G; Yang, Peidong; Chang, Christopher J

2013-09-25

112

Preparation of Ti(IV) fluoride N-heterocyclic carbene complexes.  

Science.gov (United States)

1,3,4,5-Tetramethylimidazol-2-ylidene (L(Me)) and 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene (L(iPr )) readily form complexes of trans-TiF4(L(Me))2 (1) and of trans-TiF4(L(iPr))2 (4) with TiF4 in THF, respectively. Complex 1 has been used as a precursor for preparing the Ti(IV) fluoride carbene complexes [{TiF2(L(Me))(NEt 2)}2(mu-F)2] (2) and (TiF4(L(Me))2)(NacNacLi) (3) (NacNac = HC(CMeN(2,6- iPr2C6H3))2). Complex 2 was prepared from the reaction of 1-3 equiv of 1 and 1 equiv of Ti(NEt2)4 or by reacting TiF4 with Ti(NEt2)4 and L(Me) in toluene. Complex 3 has been prepared from 1 and NacNacLi in toluene. Reaction of 1 and AlMe3 in toluene results in ligand transfer and formation of AlMe3(L(Me)). Complex 4 is unstable in solution at room temperature and degrades with formation of [HL(iPr)][TiF5(L(iPr))] (5). Complexes 1, 2.2CH2Cl2, 4, and 5 were characterized by single crystal X-ray structural analysis, elemental analysis, IR and NMR spectroscopy, and mass spectrometry. The relative basicities of L(Me), L (iPr), and the donor ligands THF, pyridine, DMSO, and H2O as well as [Cl](-) and [F](-) toward the Ti(IV) pentafluoride anion were established by NMR and confirmed by density functional theory (DFT) calculations. L(Me) and L(iPr ) are more basic than the mentioned molecular donors and more basic than chloride, however less basic than fluoride. PMID:18260620

Nikiforov, Grigory B; Roesky, Herbert W; Jones, Peter G; Magull, Jörg; Ringe, Arne; Oswald, Rainer B

2008-03-17

113

Theoretical investigations of the reactivities of four-membered N-heterocyclic carbene analogues of the group 13 elements.  

Science.gov (United States)

The potential energy surfaces for the chemical reactions of four-membered N-heterocyclic group 13 heavy carbeneoid species have been studied using density functional theory (Becke, 3-parameter, Lee-Yang-Parr (B3LYP)/Los Alamos National Laboratory 2-Double-Zeta (LANL2DZ)). Five four-membered group 13 heavy carbeneoid species, iPr2NC(NAr)2E:, where E ¼ B, Al, Ga, In, and Tl, have been chosen as model reactants in this work. Also, three kinds of chemical reactions, CAH bond insertion, alkene cycloaddition, and dimerization, have been used to study the chemical reactivities of these group 13 fourmembered N-heterocyclic carbeneoid species. In principle, our present theoretical work predicts that the larger the ffNEN bond angle of the four-membered group 13 iPr2NC(NAr)2E: species, the smaller the singlet–triplet splitting, the lower the activation barrier, and, in turn, the more rapid its chemical reactions to various chemical species. Moreover, our theoretical investigations suggest that the relative carbenic reactivity decreases in the following order: B > Al > Ga > In > Tl. That is, the heavier the group 13 atom (E), the more stable its fourmembered carbeneoid toward chemical reactions is. As a result, our computations predict that the four-membered heavy group 13 iPr2NC(NAr)2E: species (E ¼ Al, Ga, In, and Tl) should be both kinetically and thermodynamically stable, and can be readily synthesized and isolated at room temperature. Furthermore, the singlet–triplet energy splitting of the four-membered group 13 iPr2NC(NAr)2E: species, as described in the configuration mixing model attributed to the work of Pross and Shaik, can be used as a diagnostic tool to predict their reactivities. The results obtained allow a number of predictions to be made. PMID:22167870

Wu, Chi-Shiun; Su, Ming-Der

2012-01-01

114

N-heterocyclic carbene catalyzed annulation of benzofuran-2,3-diones and enals: a concise synthesis of spiro-bis-lactone.  

Science.gov (United States)

The N-heterocyclic carbene catalyzed annulation of benzofuran-2,3-diones and enals via homoenolate intermediates is described. The reaction provided a direct and efficient method for the synthesis of spiro-bis-lactones. The ketone-carbonyl group annulated products and the ester-carbonyl group annulated products can be obtained as major products with good yields by convenient catalyst regulation. Furthermore, commercially available thiazolium salt can also catalyze this reaction with modest yield. PMID:23868545

Wang, Ze-Dong; Wang, Feng; Li, Xin; Cheng, Jin-Pei

2013-09-14

115

Novel benzyl-substituted N-heterocyclic carbene-silver acetate complexes: synthesis, cytotoxicity and antibacterial studies.  

Science.gov (United States)

From the reaction of 1-methylimidazole (1a), 4,5-dichloro-1H-imidazole (1b(I)) and 1-methylbenzimidazole (1c) with p-cyanobenzyl bromide (2a), non-symmetrically substituted N-heterocyclic carbene (NHC) [(3a-c)] precursors, 5,6-dimethyl-1H-benzimidazole (1d) and 4,5-diphenyl-1H-imidazole (1e) with p-cyanobenzyl bromide (2a) and benzyl bromide (2b), symmetrically substituted N-heterocyclic carbene (NHC) [(3d-f)] precursors were synthesised. These NHC-precursors were then reacted with silver(i) acetate to yield the NHC-silver complexes (1-methyl-3-(4-cyanobenzyl)imidazole-2-ylidene)silver(i)acetate (4a), (4,5-dichloro-1-(4-cyanobenzyl)-3-methyl)imidazole-2-ylidene)silver(i)acetate (4b), (1-methyl-3-(4-cyanobenzyl)benzimidazole-2-ylidene)silver(i)acetate (4c), (1,3-bis(4-cyanobenzyl)5,6-dimethylbenzimidazole-2-ylidene) silver(i) acetate (4d), (1,3-dibenzyl-5,6-dimethylbenzimidazole-2-ylidene) silver(i) acetate (4e) and (1,3-dibenzyl-4,5-diphenylimidazol-2-ylidene) silver(i) acetate (4f) respectively. Three NHC-precursors 3c-e and four NHC-silver complexes 4b and 4d-f were characterised by single crystal X-ray diffraction. Preliminary in vitro antibacterial activity of the NHC-precursors and NHC-silver complexes was investigated against Gram-positive bacteria Staphylococcus aureus, and Gram-negative bacteria Escherichia coli using the qualitative Kirby-Bauer disk-diffusion method. NHC-silver complexes have shown very high antibacterial activity compared to the NHC-precursors. All six NHC-silver complexes were tested for their cytotoxicity through MTT based in vitro tests on the human renal-cancer cell line Caki-1 in order to determine their IC?? values. NHC-silver complexes 4a-f were found to have IC?? values of 6.2 (±1.0), 7.7 (±1.6), 1.2 (±0.6), 10.8 (±1.9), 24.2 (±1.8) and 13.6 (±1.0) ?M, respectively. These values represent improved cytotoxicity against Caki-1, most notably for 4c, which is a three times more cytotoxic than cisplatin (IC?? value = 3.3 ?M) itself. PMID:21135954

Patil, Siddappa; Deally, Anthony; Gleeson, Brendan; Müller-Bunz, Helge; Paradisi, Francesca; Tacke, Matthias

2011-01-01

116

Reactivity of coordinatively unsaturated bis(N-heterocyclic carbene) Pt(II) complexes toward H(2). Crystal structure of a 14-electron Pt(II) hydride complex.  

Science.gov (United States)

The reactivity toward H2 of coordinatively unsaturated Pt(II) complexes, stabilized by N-heterocyclic carbene (NHC) ligands, is herein analyzed. The cationic platinum complexes [Pt(NHC')(NHC)](+) (where NHC' stands for a cyclometalated NHC ligand) react very fast with H2 at room temperature, leading to hydrogenolysis of the Pt-CH2 bond and concomitant formation of hydride derivatives [PtH(NHC)2](+) or hydrido-dihydrogen complexes [PtH(H2)(NHC)2](+). The latter species release H2 when these compounds are subjected to vacuum. The X-ray structure of complex [PtH(IPr)2][SbF6] revealed its unsaturated nature, exhibiting a true T-shaped structure without stabilization by agostic interactions. Density functional theory calculations indicate that the binding and reaction of H2 in complexes [PtH(H2)(NHC)2](+) is more favored for derivatives bearing aryl-substituted NHCs (IPr, 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene and IMes = 1,3-dimesityl-1,3-dihydro-2H-imidazol-2-ylidene) than for those containing tert-butyl groups (I(t)Bu). This outcome is related to the higher close-range steric effects of the I(t)Bu ligands. Accordingly, H/D exchange reactions between hydrides [PtH(NHC)2](+) and D2 take place considerably faster for IPr and IMes* derivatives than for I(t)Bu ones. The reaction mechanisms for both H2 addition and H/D exchange processes depend on the nature of the NHC ligand, operating through oxidative addition transition states in the case of IPr and IMes* or by a ?-complex assisted-metathesis mechanism in the case of I(t)Bu. PMID:24716606

Rivada-Wheelaghan, Orestes; Roselló-Merino, Marta; Ortuño, Manuel A; Vidossich, Pietro; Gutiérrez-Puebla, Enrique; Lledós, Agustí; Conejero, Salvador

2014-04-21

117

Spectroscopic and electrochemical correlations in triangular ruthenium clusters containing N-heterocyclic ligands  

International Nuclear Information System (INIS)

A series of clusters of general formula [Ru sub(3) O (OOCCH sub(3)) sub(6) L sub(3)] sup(+), where L = N-heterocyclic ligands, were synthesized and characterized based on elemental analysis. UV-VIS and IR spectra. Voltametric studies revealed the existence of up to six acessible oxidation states, with a high degree of electronic delocalization. The Ru sub(3) O trigonal center possesses many delocalized electrons and can be visualized as a source of electrons. The ligands coordinated to the clusters tune their redox potentials, determine the differences in their electronic spectra, and are responsible for the special conditions required for their synthesis. (author)

118

Cyclodimerization versus polymerization of methyl methacrylate induced by N-heterocyclic carbenes: a combined experimental and theoretical study.  

Science.gov (United States)

The activation behavior of two N-heterocyclic carbenes (NHCs), namely, 1,3-bis(isopropyl)imidazol-2-ylidene(NHCiPr) and 1,3-bis(tert-butyl) imidazol-2-ylidene (NHCtBu), as organic nucleophiles in the reaction with methyl methacrylate (MMA) is described. NHCtBu allows the polymerization of MMA in DMF at room temperature and in toluene at 50?°C, whereas NHCiPr reacts with two molecules of MMA, forming an unprecedented imidazolium-enolate cyclodimer (NHCiPr/MMA=1:2). It is proposed that the reaction mechanism occurs by initial 1,4-nucleophilic addition of NHCiPr to MMA, generating a zwitterionic enolate 2, followed by addition of 2 to a second MMA molecule, forming a linear imidazolium-enolate 3 (NHCiPr/MMA=1:2). Proton transfer, generating intermediate 5, followed by cyclization and release of methanol yielded the aforementioned zwitterionic cyclodimer 1:2 adduct 7, the molecular structure of which has been established by NMR spectroscopy, X-ray diffraction, and mass spectrometry. This unexpected difference between NHCtBu and NHCiPr in the reaction with MMA (polymerization and cyclodimerization, respectively) can be rationalized by using DFT calculations. In particular, the nature of the NHC strongly influences the cyclodimerization pathway, the cyclization of 5 and the release of methanol are the discriminating step and limiting step, respectively. In the case of NHCtBu, both steps are strongly disfavoured compared with that of NHCiPr (energetic difference of around 14 and 9?kcal?mol(-1), respectively), preventing the cyclization mechanism from a kinetic viewpoint. Moreover, addition of a third molecule of MMA in the polymerization pathway results in a lower activation barrier than that of the limiting step in the cyclodimerization pathway (difference of around 14?kcal?mol(-1)), in agreement with the formation of polymethyl methacrylate (PMMA) by using NHCtBu as nucleophile. PMID:24604823

Nzahou Ottou, Winnie; Bourichon, Damien; Vignolle, Joan; Wirotius, Anne-Laure; Robert, Fredéric; Landais, Yannick; Sotiropoulos, Jean-Marc; Miqueu, Karinne; Taton, Daniel

2014-04-01

119

N-Heterocyclic carbene-palladium(II)-1-methylimidazole complex catalyzed Suzuki-Miyaura coupling of benzylic chlorides with arylboronic acids or potassium phenyltrifluoroborate in neat water.  

Science.gov (United States)

An easily available N-heterocyclic carbene-palladium(II)-1-methylimidazole complex showed efficient catalytic activity in the Suzuki-Miyaura coupling of benzylic chlorides with arylboronic acids or potassium phenyltrifluoroborate in neat water under mild conditions, providing an alternative method for the synthesis of diarylmethane derivatives, which widely exist in molecules with pharmaceutical activities and are also frequently found as subunits in supramolecules. Under the optimal conditions, all reactions performed well to give the desired products in moderate to almost quantitative yields in an environmentally benign medium within 12 h, extending their applications toward potentially industrial processes. PMID:23381388

Zhang, Yun; Feng, Meng-Ting; Lu, Jian-Mei

2013-04-14

120

Quinone-annulated N-heterocyclic carbene-transition-metal complexes: observation of pi-backbonding using FT-IR spectroscopy and cyclic voltammetry.  

Science.gov (United States)

A new N-heterocyclic carbene architecture comprising a 1,4-naphthoquinone annulated to 1,3-dimesitylimidazolylidene (NpQ-NHC) was synthesized in two high yielding steps from commercially available starting materials. The free NpQ-NHC was characterized (solution and solid-state) and was used to synthesize various Rh and Ag complexes that ranged in pi-electron density. Enabled by the quinone moiety, the pi-systems of these complexes were analyzed using infrared spectroscopy and cyclic voltammetry. In contrast to previous reports, pi-backbonding was found to be non-negligible and was directly influenced by the metal's electronic character. PMID:17177396

Sanderson, Matthew D; Kamplain, Justin W; Bielawski, Christopher W

2006-12-27

121

[bmim][Br] as a solvent and activator for the Ga-mediated Barbier allylation: direct formation of an N-heterocyclic carbene from Ga metal.  

Science.gov (United States)

The room-temperature ionic liquid (RTIL) [bmim][Br] has been found to be an excellent green and inexpensive medium for the Ga-mediated allylation of aromatic and aliphatic aldehydes and ketones. The RTIL activated the metal via formation of a Ga-N-heterocyclic carbene complex that assisted in the completion of the reaction at ambient temperature with only 0.5 equiv of Ga and 1.2 equiv of allyl bromide with respect to the carbonyl substrates. The present protocol required a much shorter time than those reported in the literature using other metals and solvents and proceeded with good yields and excellent selectivity. PMID:23198818

Goswami, Dibakar; Chattopadhyay, Angshuman; Sharma, Anubha; Chattopadhyay, Subrata

2012-12-21

122

Dehydrogenation of saturated CC and BN bonds at cationic N-heterocyclic carbene stabilized M(III) centers (M = Rh, Ir).  

OpenAIRE

Chloride abstraction from the group 9 metal bis(N-heterocyclic carbene) complexes M(NHC)(2)(H)(2)Cl [M = Rh, Ir; NHC = IPr = N,N'-bis(2,6-diisopropylphenyl)imidazol-2-ylidene or IMes = N,N'-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene] leads to the formation of highly reactive cationic species capable of the dehydrogenation of saturated CC and BN linkages. Thus, the reaction of Ir(IPr)(2)(H)(2)Cl (1) with Na[BAr(f)(4)] in fluorobenzene generates [Ir(IPr)(2)(H)(2)](+)[BAr(f)(4)](-) (4) in whic...

Tang, Cy; Thompson, Al; Aldridge, S.

2010-01-01

123

Helical metal cage of silver(I) and mercury(II)-N-heterocyclic carbene complexes of 1,4-phenylene-bis{imidazo[1,5-a]pyridine-4-iumhexaflurophosphate}  

Science.gov (United States)

Novel N-heterocyclic carbene (NHC) architecture of 1,4-phenylene-bis{imidazo[1,5-a]pyridine-4 iumhexaflurophosphate}, [1(HPF6)2] has been synthesized in two steps from commercially available starting materials. The p-phylene bridged free annulated imidazolium salt has been synthesized and characterized both in solution and solid-state and has been used to synthesize silver(I) and mercury(II)-NHC complexes using the basic metal precursors. The [Ag2(1)2][PF6]2 (2) was synthesized using [1(HPF6)2] by Ag2O method in acceptable yield. The dinuclear Hg(II) complex [Hg2(1)2][PF6]4 (3) consisting of two biscarbene ligand has been synthesized from Hg(OAc)2 and [1(HPF6)2]. The helical nature of Hg(II)-NHC (3) has been established from solid state X-ray structure.

Rana, Bidyut Kumar; Bertolasi, Valerio; Pal, Satyanarayan; Mitra, Partha; Dinda, Joydev

2013-10-01

124

Antibacterial properties of water-soluble gold(I) N-heterocyclic carbene complexes.  

Science.gov (United States)

The antibacterial properties of water-soluble gold(I) complexes [1-methyl-3-(3-sulfonatopropyl)imidazol-2-ylidene]gold(I) chloride (C1), [1-mesityl-3-(3-sulfonatopropyl)imidazol-2-ylidene]gold(I) chloride (C2), [1-(2,6-diisopropylphenyl)-3-(3-sulfonatopropyl)imidazol-2-ylidene]gold(I) chloride (C3) and [1,3-bis(2,6-diisopropyl-4-sodiumsulfonatophenyl)imidazol-2-ylidene]gold(I) chloride (C4) and the respective ligands were assessed by agar diffusion and broth macrodilution methods against Gram-positives Staphylococcus aureus, Enterococcus faecalis and Micrococcus luteus and the Gram-negative bacteria Yersinia ruckeri, Pseudomonas aeruginosa and Escherichia coli. Viability after treatments was determined by direct plate count. The bactericidal activity displayed by C1 and C3 was comparable to that of AgNO3. PMID:24662463

Fernández, Gabriela A; Vela Gurovic, María S; Olivera, Nelda L; Chopa, Alicia B; Silbestri, Gustavo F

2014-06-01

125

Effect of chelating ring size in catalytic ketone hydrogenation: facile synthesis of ruthenium(II) precatalysts containing an N-heterocyclic carbene with a primary amine donor for ketone hydrogenation and a DFT study of mechanisms.  

Science.gov (United States)

A half-sandwich ruthenium(II) complex, [Ru(?(6)-p-cymene)(C-NH(2))Cl]PF(6) (4·PF(6)), containing an N-heterocyclic carbene (NHC) with a primary amine donor (C-NH(2)) which chelates through the carbene carbon and the amine nitrogen to form a 6-membered ring was synthesized in a one-pot reaction starting from a primary-amine functionalized imidazolium salt 2. Complex 4·PF(6) catalyzed the hydrogenation of ketones using 2-propanol or H(2) as the reductant. A maximum turnover frequency of 1062 h(-1) and a turnover number of 1140 at 5 h were achieved for the transfer hydrogenation of 3'-chloroacetophenone in 2-propanol at 75 °C. A cationic hydride-amine complex 5, [Ru(?(6)-p-cymene)(C-NH(2))H]PF(6), was synthesized, and this reacted very slowly with acetophenone unless first activated by an alkoxide base. Computational studies by DFT methods suggested that the poor reactivity of the hydride-amine complex 5 was attributed to a large barrier for the transfer of its H(+)/H(-) couple to a ketone for bifunctional catalysis. An inner-sphere mechanism, which involves a decoordinated amine group of the C-NH(2) ligand, was computed to be a feasible energetic pathway in comparison to the computed outer-sphere bifunctional mechanism. This explains the catalytic activity and selectivity that is observed for the newly synthesized ruthenium(II) catalysts. PMID:22728720

Ohara, Hisashi; O, Wylie W N; Lough, Alan J; Morris, Robert H

2012-08-01

126

Synthesis, spectroscopic studies and reactivity of triphenylphosphine ruthenium (II) complexes with N-heterocyclic ligands  

International Nuclear Information System (INIS)

Reported is the chemistry of triphenylphosphine ruthenium (II) complexes of general formula RuCl2(PPh3)2L2 and RuCl2(PPh3)2A, obtained from the reaction of RuCl2(PPh3)3 with N-heterocyclic ligands L, or A (of ambidentate nature). The electronic spectra exhibit two strong metal-to-ligand charge-transfer bands, ascribed to the b1(dxz)->b1(pi) and a2(dxy)->a2(pi) transitions, and a third, weak band ascribed to the b2(dyz)->a2(pi) transition. The electronic states and the vibrational modes of the complexes were characterized by means of their resonance Raman and infrared absorption spectra. Thermogravimetric and thermodifferential analysis indicated that the melting process is succeeded by an exothermic reaction, and that the weigh loss starts to occur only after this step. The complexes dissociated in CHCl3 solution, showing preferential labilization of the phosphine ligands, as in the case of the hydrogenation catalyst Ru(PPh3)3Cl2. In the presence of CO, RuCl2(CO)2L2 complexes were gennerated. Several derivatives were isolated and characterized. (author)

127

Reaction of an N-Heterocyclic Carbene-Stabilized Silicon(II) Monohydride with Alkynes: [2+2+1] Cycloaddition versus Hydrogen Abstraction.  

Science.gov (United States)

An in depth study of the reactivity of an N-heterocyclic carbene (NHC)-stabilized silylene monohydride with alkynes is reported. The reaction of silylene monohydride 1, tBu3 Si(H)Si?NHC, with diphenylacetylene afforded silole 2, tBu3 Si(H)Si(C4 Ph4 ). The density functional theory (DFT) calculations for the reaction mechanism of the [2+2+1] cycloaddition revealed that the NHC played a major part stabilizing zwitterionic transition states and intermediates to assist the cyclization pathway. A significantly different outcome was observed, when silylene monohydride 1 was treated with phenylacetylene, which gave rise to supersilyl substituted 1-alkenyl-1-alkynylsilane 3, tBu3 Si(H)Si(CH?CHPh)(C?CPh). Mechanistic investigations using an isotope labelling technique and DFT calculations suggest that this reaction occurs through a similar zwitterionic intermediate and subsequent hydrogen abstraction from a second molecule of phenylacetylene. PMID:25413306

Eisenhut, Carsten; Szilvási, Tibor; Breit, Nora C; Inoue, Shigeyoshi

2014-11-20

128

Mixed N-Heterocyclic Carbene-Bis(oxazolinyl)borato Rhodium and Iridium Complexes in Photochemical and Thermal Oxidative Addition Reactions  

Energy Technology Data Exchange (ETDEWEB)

In order to facilitate oxidative addition chemistry of fac-coordinated rhodium(I) and iridium(I) compounds, carbene–bis(oxazolinyl)phenylborate proligands have been synthesized and reacted with organometallic precursors. Two proligands, PhB(OxMe2)2(ImtBuH) (H[1]; OxMe2 = 4,4-dimethyl-2-oxazoline; ImtBuH = 1-tert-butylimidazole) and PhB(OxMe2)2(ImMesH) (H[2]; ImMesH = 1-mesitylimidazole), are deprotonated with potassium benzyl to generate K[1] and K[2], and these potassium compounds serve as reagents for the synthesis of a series of rhodium and iridium complexes. Cyclooctadiene and dicarbonyl compounds {PhB(OxMe2)2ImtBu}Rh(?4-C8H12) (3), {PhB(OxMe2)2ImMes}Rh(?4-C8H12) (4), {PhB(OxMe2)2ImMes}Rh(CO)2 (5), {PhB(OxMe2)2ImMes}Ir(?4-C8H12) (6), and {PhB(OxMe2)2ImMes}Ir(CO)2 (7) are synthesized along with ToMM(?4-C8H12) (M = Rh (8); M = Ir (9); ToM = tris(4,4-dimethyl-2-oxazolinyl)phenylborate). The spectroscopic and structural properties and reactivity of this series of compounds show electronic and steric effects of substituents on the imidazole (tert-butyl vs mesityl), effects of replacing an oxazoline in ToM with a carbene donor, and the influence of the donor ligand (CO vs C8H12). The reactions of K[2] and [M(?-Cl)(?2-C8H14)2]2 (M = Rh, Ir) provide {?4-PhB(OxMe2)2ImMes?CH2}Rh(?-H)(?-Cl)Rh(?2-C8H14)2 (10) and {PhB(OxMe2)2ImMes}IrH(?3-C8H13) (11). In the former compound, a spontaneous oxidative addition of a mesityl ortho-methyl to give a mixed-valent dirhodium species is observed, while the iridium compound forms a monometallic allyl hydride. Photochemical reactions of dicarbonyl compounds 5 and 7 result in C–H bond oxidative addition providing the compounds {?4-PhB(OxMe2)2ImMes?CH2}RhH(CO) (12) and {PhB(OxMe2)2ImMes}IrH(Ph)CO (13). In 12, oxidative addition results in cyclometalation of the mesityl ortho-methyl similar to 10, whereas the iridium compound reacts with the benzene solvent to give a rare crystallographically characterized cis-[Ir](H)(Ph) complex. Alternatively, the rhodium carbonyl 5 or iridium isocyanide {PhB(OxMe2)2ImMes}Ir(CO)CNtBu (15) reacts with PhSiH3 in the dark to form the silyl compound {PhB(OxMe2)2ImMes}RhH(SiH2Ph)CO (14) or {PhB(OxMe2)2ImMes}IrH(SiH2Ph)CNtBu (17). These examples demonstrate the enhanced thermal reactivity of {PhB(OxMe2)2ImMes}-supported iridium and rhodium carbonyl compounds in comparison to tris(oxazolinyl)borate, tris(pyrazolyl)borate, and cyclopentadienyl-supported compounds.

Xu, Songchen [Ames Laboratory; Manna, Kuntal [Ames Laboratory; Ellern, Arkady [Ames Laboratory; Sadow, Aaron D [Ames Laboratory

2014-12-08

129

DFT investigation on mechanisms and stereoselectivities of [2 + 2 + 2] multimolecular cycloaddition of ketenes and carbon disulfide catalyzed by N-heterocyclic carbenes.  

Science.gov (United States)

The first theoretical investigation using density functional theory (DFT) methods to study the detailed reaction mechanisms of stereoselective [2 + 2 + 2] multimolecular cycloaddition of ketene (two molecules) and carbon disulfide (CS2, one molecule) which is catalyzed by N-heterocyclic carbene (NHC) is presented in this paper. The calculated results indicate that this reaction occurs through four steps: the complexation of NHC with ketene (channel 1a) rather than with CS2 (channel 1b), addition of CS2 (channel 2b) but not dimerization of ketene (channel 2a), formal [4 + 2] cycloaddition with a second molecule of ketene (channel 3a) rather than intramolecular [2 + 2] cycloaddition (channel 3b), and finally regeneration of NHC. The second step is revealed to be the rate-determining step. Moreover, the stereoselectivities associated with the chiral carbon center and the carbon double bond are predicted to be respectively determined in the second and third steps and respectively R and E configurations dominated, which are in good agreement with the experimental results. Furthermore, the possible mechanisms of the identical [2 + 2 + 2] cycloaddition catalyzed by N,N-dimethylpyridin-4-amine (DMAP) have also been investigated to help understand the ring closure mechanism proceeding in the third step. PMID:24188481

Zhang, Wen-Jing; Wei, Dong-Hui; Tang, Ming-Sheng

2013-12-01

130

Injection, transport, absorption and phosphorescence properties of a series of platinum (II) complexes with N-heterocyclic carbenes: a DFT and time-dependent DFT study.  

Science.gov (United States)

The ground and excited states, charge injection/transport, and phosphorescence properties of five N?heterocyclic carbine-functionalized Pt(II) complexes were investigated by using the DFT method. By analyzing the nonradiative (k nr) rate constant and energies at [Formula: see text] and [Formula: see text] states, it is possible to forecast that BC5 with the pyrrole ligand has a higher phosphorescence quantum yield than any of the other four complexes. Thus, we consider that BC5 will be an efficient phosphorescent material that has balanced electron/hole-transport performance as well as high phosphorescence quantum yield. The calculated results indicate that, for the studied complexes, the nature of the ligand strongly affected the energy of the emissive state and was able to tune the emission color. We hope that our study will aid better understanding of the structure-property relationship of phosphorescent Pt (II) complexes and provide constructive information for designing novel and highly efficient OLED materials in the future. PMID:25227448

Li, Leijiao; Liu, Xiaojuan; Feng, Jing; Song, Shuyan; Zhang, Hongjie

2014-10-01

131

Expanded-ring N-heterocyclic carbenes for the stabilization of highly electrophilic gold(i) cations.  

Science.gov (United States)

Strategies for the synthesis of highly electrophilic Au(I) complexes from either hydride- or chloride-containing precursors have been investigated by employing sterically encumbered Dipp-substituted expanded-ring NHCs (Dipp=2,6-iPr2 C6 H3 ). Thus, complexes of the type (NHC)AuH have been synthesised (for NHC=6-Dipp or 7-Dipp) and shown to feature significantly more electron-rich hydrides than those based on ancillary imidazolylidene donors. This finding is consistent with the stronger ?-donor character of these NHCs, and allows for protonation of the hydride ligand. Such chemistry leads to the loss of dihydrogen and to the trapping of the [(NHC)Au](+) fragment within a dinuclear gold cation containing a bridging hydride. Activation of the hydride ligand in (NHC)AuH by B(C6 F5 )3 , by contrast, generates a species (at low temperatures) featuring a [HB(C6 F5 )3 ](-) fragment with spectroscopic signatures similar to the "free" borate anion. Subsequent rearrangement involves B?C bond cleavage and aryl transfer to the carbophilic metal centre. Under halide abstraction conditions utilizing Na[BAr(f) 4 ] (Ar(f) =C6 H3 (CF3 )2 -3,5), systems of the type [(NHC)AuCl] (NHC=6-Dipp or 7-Dipp) generate dinuclear complexes [{(NHC)Au}2 (?-Cl)](+) that are still electrophilic enough at gold to induce aryl abstraction from the [BAr(f) 4 ](-) counterion. PMID:25336057

Phillips, Nicholas; Dodson, Tristan; Tirfoin, Rémi; Bates, Joshua I; Aldridge, Simon

2014-12-01

132

N-heterocycle carbene (NHC)-ligated cyclopalladated N,N-dimethylbenzylamine: a highly active, practical and versatile catalyst for the Heck-Mizoroki reaction.  

Science.gov (United States)

The wide dissemination of catalytic protocols in academic and industrial laboratories is facilitated by the development of catalysts that are not only highly active but also user-friendly, stable to moisture, air and long term storage and easy to prepare on a large scale. Herein we describe a protocol for the Heck-Mizoroki reaction mediated by cyclopalladated N,N-dimethylbenzylamine (dmba) ligated with a N-heterocyclic carbene, 1,3-bis(mesityl)imidazol-2-ylidene (IMes), that fulfils these criteria. The precatalyst can be synthesized on approximately 100 g scale by a tri-component, sequential, one-pot reaction of N,N-dimethylbenzylamine, PdCl2 and IMes.HCl in refluxing acetonitrile in air in the presence of K2CO3. This single component catalyst is stable to air, moisture and long term storage and can be conveniently dispensed as a stock solution in NMP. It mediates the Heck-Mizoroki reaction of a range of aryl- and heteroaryl bromides in reagent grade NMP at the 0.1-2 mol% range without the need for rigorous anhydrous techniques or a glovebox, and is active even in air. The catalyst is capable of achieving very high levels of catalytic activity (TON of up to 5.22 x 10(5)) for the coupling of a deactivated arylbromide, p-bromoanisole, with tBu acrylate as a benchmark substrate pair. A wide range of aryl bromides, iodides and, for the first time with a NHC-Pd catalyst, a triflate was coupled with diverse acrylate derivatives (nitrile, tert-butyl ester and amides) and styrene derivatives. The use of excess (>2 equiv.) of the aryl bromide and tert-butyl acrylate leads to mixture of tert-butyl beta,beta-diarylacrylate and tert-butyl cinnamate derivatives depending on the substitution pattern of the aryl bromide. Electron rich m- and p-substituted arylbromides give the diarylated products exclusively, whereas electron-poor aryl bromides give predominantly mono-arylated products. For o-substituted aryl bromides, no doubly arylated products could be obtained under any conditions. Overall, the active catalyst (IMes-Pd) shows higher activity with electron-rich aryl halides, a marked difference compared with the more commonly used phosphane-Pd or non-ligated Pd catalysts. PMID:19421449

Peh, Guang-Rong; Kantchev, Eric Assen B; Zhang, Chi; Ying, Jackie Y

2009-05-21

133

Speciation of cobalt, nickel and zinc-ethylenediaminediacetate complexes with N-heterocycle ligand - Synthesis, spectroscopic and structural studies  

Science.gov (United States)

Cobalt, nickel and zinc N-heterocycle chelated ethylenediaminediacetates [Co(edda)(phen)]·(NH 4Cl)·1/3H 2O ( 1), [Ni(edda)(phen)]·3H 2O ( 2), [Co(edda)(bpy)]·(NH 4Cl)·2H 2O ( 3), [Ni(edda)(bpy)]·4H 2O ( 4), [Zn(edda)(bpy)]·4H 2O ( 5) (H 2edda = ethylenediaminediacetic acid, phen = 1,10-phenanthroline, bpy = 2,2'-bipyridine) were isolated in pure forms and characterized spectroscopically. The X-ray structural analyses reveal that neutral mononuclear complexes contain both edda and phen/bpy ligands. Divalent metal ions are octahedrally bound by a bidentate phen or bpy ligand, and a tetradentately edda through carboxyl and amine groups. The co-existences of phen/bpy and edda in the complexes show no decomposition between phen/bpy and edda ligands. This is supported by the trace of 13C NMR spectra with less-bound zinc complex. Further comparisons of the bond distances between Co 2+/Ni 2+/Zn 2+ ions with edda indicate that cobalt and nickel interact much stronger than those of zinc complexes.

Ji, Jin-Long; Huang, Li-Qing; Cai, Yan; Yu, La-Jia; Zhou, Zhao-Hui

2011-05-01

134

Nitrosyl and carbene iron complexes bearing a ?(3)-SNS thioamide pincer type ligand.  

Science.gov (United States)

The previously reported monochelate iron complex with ?(3) SNS thioamide pincer ligand, 2,6-bis(N-2,6-bis(diphenylmethyl)-4-isopropylphenyliminothiolate)pyridine (L(DPM)), [Fe(THF)2(?(3)-L(DPM))], gave novel complexes, [Fe(NHC)(?(3)-L(DPM))] and [Fe(NO)2(?(3)-L(DPM))], by substitution reactions with N-heterocyclic carbene (NHC) and NO molecules, respectively. The X-ray crystal structure of the [Fe(NHC)(?(3)-L(DPM))] complex revealed a unique square planar iron(ii) complex, which was determined to be in an intermediate spin state (S = 1) in benzene from the Evans method. The [Fe(NO)2(?(3)-L(DPM))] complex was determined to have a trigonal bipyramidal geometry from X-ray analysis and was indicated to be diamagnetic from the (1)H NMR spectrum. The ?(NO) stretching vibration of this complex showed two peaks at 1840 cm(-1) and 1790 cm(-1), and also the Fe-N-O bond angles were 168.9(2)° and 168.03(19)°. These findings suggest that the two coordinated NO molecules have neutral radical character, and they are antiferromagnetically coupled with the high-spin iron center. PMID:25407757

Suzuki, Tatsuya; Matsumoto, Jun; Kajita, Yuji; Inomata, Tomohiko; Ozawa, Tomohiro; Masuda, Hideki

2015-01-21

135

N-Heterocyclic Carbene-Catalyzed Diastereoselective and Enantioselective Reaction of 2-Aroylvinylcinnamaldehydes with ?,?-Unsaturated Imines: Complete Control and Switch of Diastereoselectivity by N-Substituents of Catalysts.  

Science.gov (United States)

Highly diastereoselective and enantioselective reactions between 2-aroylvinylcinnamaldehydes and ?,?-unsaturated imines were achieved under asymmetric catalysis of chiral triazole carbene catalysts. In the presence of N-anisylindeno[2,1-b]triazolo[4,3-d][1,4]oxazinium salt, the reaction of 2-aroylvinylcinnamaldehydes with ?,?-unsaturated imines afforded indeno[2,1-c]pyran-1-one derivatives 4 with 90%-99% ee, while enantiopure indenocyclopentan-1-ones 5 (>99% ee) were obtained under the catalysis of N-mesitylindeno[2,1-b]triazolo[4,3-d][1,4]oxazinium salt. A slight variation of an N-substituent on triazole carbenes was found to switch completely the diastereoselectivity of the reaction for the formation of indeno[2,1-c]pyran-1-ones. PMID:25584883

Wang, Zi-Tian; Zhao, Yuan; Wang, Zhan-Yong; Cheng, Ying

2015-02-01

136

A user-friendly, all-purpose Pd-NHC (NHC=N-heterocyclic carbene) precatalyst for the negishi reaction: a step towards a universal cross-coupling catalyst.  

Science.gov (United States)

We have developed the first user-friendly Negishi protocol capable of routinely cross-coupling all combinations of alkyl and aryl centers. The use of an easily synthesized, air stable, highly active, well-defined precatalyst PEPPSI-IPr (1; PEPPSI=pyridine-enhanced precatalyst preparation, stabilization and initiation; IPr=diisopropylphenylimidazolium derivative) substantially increases the scope, reliability, and ease-of-use of the Negishi reaction. All organohalides and routinely used pseudohalides were excellent coupling partners, with the use of chlorides, bromides, iodides, triflates, tosylates, and mesylates resulting in high yield of the coupled product. Furthermore, all reactions were performed by using general laboratory techniques, with no glove-box necessary as the precatalyst was weighed and stored in air. Utilization of this methodology allowed for the easy synthesis of an assortment of sterically encumbered biaryls and druglike heteroaromatics, demonstrating the value of the PEPPSI-IPr system. Furthermore, this is also the first time Pd-NHC (NHC=N-heterocyclic carbene) methodology has surpassed the related phosphine-ligated Negishi processes both in activity and use. PMID:16568493

Organ, Michael G; Avola, Stephanie; Dubovyk, Igor; Hadei, Niloufar; Kantchev, Eric Assen B; O'Brien, Christopher J; Valente, Cory

2006-06-14

137

Three-coordinate nickel(I) complexes stabilised by six-, seven- and eight-membered ring n-heterocyclic carbenes: synthesis, EPR/DFT studies and catalytic activity.  

Science.gov (United States)

Comproportionation of [Ni(cod)(2)] (cod = cyclooctadiene) and [Ni(PPh(3))(2)X(2)] (X = Br, Cl) in the presence of six-, seven- and eight-membered ring N-aryl-substituted heterocyclic carbenes (NHCs) provides a route to a series of isostructural three-coordinate Ni(I) complexes [Ni(NHC)(PPh(3))X] (X = Br, Cl; NHC = 6-Mes 1, 6-Anis 2, 6-AnisMes 3, 7-o-Tol 4, 8-Mes 5, 8-o-Tol 6, O-8-o-Tol 7). Continuous wave (CW) and pulsed EPR measurements on 1, 4, 5, 6 and 7 reveal that the spin Hamiltonian parameters are particularly sensitive to changes in NHC ring size, N substituents and halide. In combination with DFT calculations, a mixed SOMO of ?3d?z?2? and ?3d?x?2-y?2? character, which was found to be dependent on the complex geometry, was observed and this was compared to the experimental g values obtained from the EPR spectra. A pronounced (31)P superhyperfine coupling to the PPh(3) group was also identified, consistent with the large spin density on the phosphorus, along with partially resolved bromine couplings. The use of 1, 4, 5 and 6 as pre-catalysts for the Kumada coupling of aryl chlorides and fluorides with ArMgY (Ar = Ph, Mes) showed the highest activity for the smaller ring systems and/or smaller substituents (i.e., 1>4?6?5). PMID:23292787

Page, Michael J; Lu, Wei Y; Poulten, Rebecca C; Carter, Emma; Algarra, Andrés G; Kariuki, Benson M; Macgregor, Stuart A; Mahon, Mary F; Cavell, Kingsley J; Murphy, Damien M; Whittlesey, Michael K

2013-02-01

138

?-Hydroxy coordination of mononuclear vanadyl citrate, malate and S-citramalate with N-heterocycle ligand, implying a new protonation pathway of iron-vanadium cofactor in nitrogenase.  

Science.gov (United States)

Unlike the most of ?-alkoxy coordination in ?-hydroxycarboxylates to vanadium, novel ?-hydroxy coordination to vanadium(IV) has been observed for a series of chiral and achiral monomeric ?-hydroxycarboxylato vanadyl complexes [VO(H2cit)(bpy)]·2H2O (1), [VO(Hmal)(bpy)]·H2O (2), [VO(H2cit)(phen)]·1.5H2O (3), [VO(Hmal)(phen)]·H2O (4), and [(?)VO(S-Hcitmal)(bpy)]·2H2O (5), [VO(H2cit)(phen)]2·6.5H2O (6), which were isolated from the reactions of vanadyl sulfate with ?-hydroxycarboxylates and N-heterocycle ligands in acidic solution. The complexes feature a tridentate citrate, malate or citramalate that chelates to vanadium atom through their ?-hydroxy, ?-carboxy and ?-carboxy groups; while the other ?-carboxylic acidic group of citrate is free to participate strong hydrogen bonds with lattice water molecule. The neutral ?-hydroxy group also forms strong intermolecular hydrogen bonds with water molecule and the negatively-charged ?-carboxy group in the environment. The inclusion of a hydrogen ion in ?-alkoxy group results in the formation of a series of neutral complexes with one less positive charge. There are two different configurations of citrate with respect to the trans-position of axial oxo group, where the complex with trans-hydroxy configuration seems more stable with less hindrance. The average bond distances of VOhydroxy and VO?-carboxy are 2.196 and 2.003Å respectively, which are comparable to the VO distance (2.15Å) of homocitrate in FeV-cofactor of V-nitrogenase. A new structural model is suggested for R-homocitrato iron vanadium cofactor as VFe7S9C(R-Hhomocit) (H4homocit=homocitric acid) with one more proton in homocitrate ligand. PMID:25240212

Chen, Can-Yu; Chen, Mao-Long; Chen, Hong-Bin; Wang, Hongxin; Cramer, Stephen P; Zhou, Zhao-Hui

2014-12-01

139

Cyclic (Amino)[bis(ylide)]carbene as an Anionic Bidentate Ligand for Transition Metal Complexes  

OpenAIRE

The 3-triphenylphosphonio-N-(2,6-diisopropylphenyl)-pyrole reacts with two equivalents of methyl lithium to afford a lithium adduct in which a cyclic (amino)[bis(ylide)]carbene, a novel type of NHC, acts as a 1,4-bidentate ligand via the carbene center and the exocyclic ylidic carbon. This species readily undergoes transmetallation reactions, which allows for the synthesis of a variety of transition metal complexes.

Asay, Matthew; Donnadieu, Bruno; Baceiredo, Antoine; Soleilhavoup, Michele; Bertrand, Guy

2008-01-01

140

Tuning the electronic and ligand properties of remote carbenes: a theoretical study.  

Science.gov (United States)

The effect of annulation and carbonylation on the electronic and ligating properties of remote N-heterocyclic carbenes (rNHCs) has been studied quantum-chemically. The thermodynamic stability of these complexes has been assessed on the basis of their hydrogenation and stabilization energies, while HOMO-LUMO gaps are used to measure the kinetic stabilities. Annulated/carbonylated rNHCs are found to be weaker ? donors but better ? acceptors compared with the parent rNHCs. The reactivity of these rNHCs has been studied by evaluating their nucleophilicity and electrophilicity indices. The nucleophilicity values are in good agreement with the ? basicities of all of the rNHCs. The (31)P NMR chemical shifts of the corresponding rNHC-phosphinidene adducts have been calculated and found to correlate well with the ? acidities of these rNHCs. PMID:25340967

Borthakur, Bitupon; Rahman, Taskia; Phukan, Ashwini K

2014-11-21

141

Synthesis and Antimicrobial Activity of Novel Ag-N-Hetero-cyclic Carbene Complexes  

OpenAIRE

A series of imidazolidinium ligand precursors are metallated with Ag2O to give silver(I) N-heterocyclic carbene complexes. All compounds were fully characterized by elemental analyses, 1H-NMR, 13C-NMR and IR spectroscopy techniques. All compounds studied in this work were screened for their in vitro antimicrobial activities against the standard strains: Enterococcus faecalis (ATCC 29212), Staphylococcus aureus (ATCC 29213), Escherichia coli (ATCC 25922), Pseudomonas aeruginosa (ATCC 27853) an...

?lknur Özdemir; Emine Özge Özcan; Selami Günal; Nevin Gürbüz

2010-01-01

142

Pyrido[1,2-a][1,2,4]triazol-3-ylidenes as a new family of stable annulated N-heterocyclic carbenes: synthesis, reactivity, and their application in coordination chemistry and organocatalysis.  

Science.gov (United States)

General synthetic avenues to the pyrido-annulated triazolium salts with different steric and electronic properties have been developed. This architecture can be readily altered with different N-alkyl or aryl substituents at the N2 position of the triazole ring and modifications to the pyridine backbone. Deprotonation of the triazolium salts 12 with NaH led to formation of stable carbenes 11 at room temperature as clearly demonstrated through ESI mass spectra and by observation of the characteristic (13)C NMR resonance for the carbene carbon at delta = 202-208 ppm. In sharp contrast, treatment of these triazolium salts with K2CO3 led to dimerization of free carbenes 11. The dimeric enetetramine (11b)2 could react with elemental sulfur to deliver the corresponding thiourea 16 in toluene at 80 degrees C in good yield. A silver complex with the pyrido[1,2-a][1,2,4]triazol-3-ylidene is described, and the molecular structure of complex 17 was established by X-ray crystallography. The triazolium salts 12 turned out to be powerful catalysts in catalytic benzoin condensations and transesterifications at 25 degrees C. The catalytic activity was largely dependent on the steric and electronic nature of the R(1) and R(2) substituents of the triazolium salt. We rationalized that this type of triazolium-catalyzed benzoin condensations should undergo the "traditional" Breslow mechanism rather than the pathway of the dimer (11)2 as the real catalytic species. PMID:18844416

Ma, Yajun; Wei, Siping; Lan, Jingbo; Wang, Jingzhi; Xie, Rugang; You, Jingsong

2008-11-01

143

Carbene-stabilized beryllium borohydride.  

Science.gov (United States)

The reaction of N-heterocyclic carbene, L:, with BeCl(2) quantitatively yields L:BeCl(2)1 (L: = :C{N(2,6-Pr(i)(2)C(6)H(3))CH}(2)). The carbene-stabilized beryllium borohydride monomer L:Be(BH(4))(2)2 is prepared by the reaction of 1 with LiBH(4). Compound 3, prepared by the reaction of 2 with Na(2)[Fe(CO)(4)]·dioxane, represents an unusual "dual reduction" of the imidazole ring (i.e., hydroboration of the C?C backbone and hydrogenation of the C2 carbene center). PMID:22670857

Gilliard, Robert J; Abraham, Mariham Y; Wang, Yuzhong; Wei, Pingrong; Xie, Yaoming; Quillian, Brandon; Schaefer, Henry F; Schleyer, Paul V R; Robinson, Gregory H

2012-06-20

144

Synthesis and Antimicrobial Activity of Novel Ag-N-Hetero-cyclic Carbene Complexes  

Directory of Open Access Journals (Sweden)

Full Text Available A series of imidazolidinium ligand precursors are metallated with Ag2O to give silver(I N-heterocyclic carbene complexes. All compounds were fully characterized by elemental analyses, 1H-NMR, 13C-NMR and IR spectroscopy techniques. All compounds studied in this work were screened for their in vitro antimicrobial activities against the standard strains: Enterococcus faecalis (ATCC 29212, Staphylococcus aureus (ATCC 29213, Escherichia coli (ATCC 25922, Pseudomonas aeruginosa (ATCC 27853 and the fungi Candida albicans and Candida tropicalis. The new imidazolidin-2-ylidene silver complexes have been found to display effective antimicrobial activity against a series of bacteria and fungi.

?lknur Özdemir

2010-04-01

145

Three-dimensional supramolecular networks constructed from Keggin anions and metal-organic complex with unpredicted oxidation of an N-heterocycle ligand  

Science.gov (United States)

Two novel Keggin polyoxometalate compounds, [Cu 2(PCA) 4(H 2O) 2][SiW 12O 40]·8H 2O ( 1) and [Cu(PCA) 2(H 2O) 4][Cu(PCA) 3(H 2O)][SiW 12O 40]·H 2O ( 2) (PCA = pyridine-4-carboxylic acid), were synthesized by in situ generation of pyridine-4-carboxylic acid ligand from 1,3-bis(4-pyridyl)propane precursor under hydrothermal conditions. Compounds 1 and 2 were structurally characterized by elemental analysis, IR and UV spectrography, TGA and single-crystal X-ray diffraction analysis. The variable-temperature magnetic susceptibility was measured at 2-300 K for 1. The compound 1 consists of polyoxoanions [?-SiW 12O 40] 4-, dimer cations [Cu 2(PCA) 4(H 2O) 2] 4+ and lattice water molecules. The water molecules link [?-SiW 12O 40] 4- and [Cu 2(PCA) 4(H 2O) 2] 4+ into three-dimensional (3D) architecture via hydrogen bonds. Polyoxoanions of 2 are linked into 1D chains through hydrogen bonds. Moreover, 1D chains are further extended into 3D supramolecular networks by hydrogen bonds and ?···? stacking interactions of pyridine-4-carboxylic acid molecules. The variable-temperature magnetic measurement of 1 shows an antiferromagnetic interaction in the 2-300 K temperature range.

Li, Jia; Chen, Ya-Guang; Zhang, Chun-Jing; Kong, Qing-Jiao

2009-03-01

146

The Orientation and Stereodynamic Paths of Planar Monodentate Ligands in Square Planar Nickel N2S Complexes  

OpenAIRE

The well-established presence of histidine donors in binding sites of Ni-containing biomolecules prompts the study of orientational preference and stereodynamic nature of flat monodentate ligands (L = imidazoles, pyridine and an N-heterocyclic carbene) bound to planar N2SNi moieties. Square planar [N2SNiL]n+ complexes are accessed through bridge-splitting reactions of dimeric, thiolate-S bridged [N2SNi]2 complexes. The solid state molecular structures of three mononuclear products, and three ...

Jenkins, Roxanne M.; Singleton, Michael L.; Leamer, Lauren A.; Reibenspies, Joseph H.; Darensbourg, Marcetta Y.

2010-01-01

147

N-Heterocyclic Carbene Complexes in Dehalogenation Reactions  

Science.gov (United States)

Catalytic dehalogenation represents an underdeveloped transformation in M-NHC chemistry with a small number of reports detailing the reactivity of Co, Ru, Ni and Pd catalysts. In situ generated nickel and palladium NHC complexes catalyse the hydrodechlorination of aryl chlorides. Lower coordinate Ni complexes are proposed to operate in the hydrodefluorination of mono- and poly-fluorinated substrates. The single example of Ru-NHC catalysed hydrodefluorination of fully and partially fluorinated aromatic substrates is characterised by an unusual regioselectivity. The highly regioselective dehydrohalogenation of relatively unreactive alkyl halide substrates is achieved with a cobalt NHC catalyst.

Mas-Marzá, Elena; Page, Michael J.; Whittlesey, Michael K.

148

Fischer and N-heterocyclic carbene complexes of tungsten(0)  

OpenAIRE

Access to this dissertation is restricted until 2014-09-07 Copyright © 2012, University of Pretoria. All rights reserved. The copyright in this work vests in the University of Pretoria. No part of this work may be reproduced or transmitted in any form or by any means, without the prior written permission of the University of Pretoria E12/9/111/gm

Pretorius, Rene

2012-01-01

149

A spiroborate-based anionic bis-N-heterocyclic carbene.  

Science.gov (United States)

A twisted mono-cationic bis-benzimidazolium salt was serendipitously isolated from the dehydrative condensation of 5,6-dihydroxyl-1,3-dimesityl-benzimidazolium and 1,4-benzenediboronic acid. Subsequent deprotonation of the benzimidazolium salt led to the formation of a spiroborate-linked free bis-NHC, which was further transformed into the corresponding diborane adduct and the di-Rh complex. PMID:24395106

Su, Jia-Hong; Lee, Gene-Hsiang; Peng, Shie-Ming; Chiu, Ching-Wen

2014-02-28

150

Carbene-, Vinylidene and Allenylidene-Complexes of Ruthenium with Heteroscorpionate Ligands  

OpenAIRE

Im Rahmen der vorliegenden Dissertation wurden neutrale Carben-, Vinyliden- und Allenyliden-Komplexe mit Bispyrazolylacetato-Liganden synthetisiert und charakterisiert. Die erhaltenen Komplexe wiesen fast durchgängig eine für Organometallverbindungen sehr hohe Stabilität gegenüber Sauerstoff und Wasser auf. Für die Benzyliden-Komplexe wurde Aktivität als Katalysatoren in der Ringschlussmetathese gefunden. Fragen zur strukturellen Isomerie wurden mittels zweidimensionaler NMR-Experimente...

Kopf, Henning

2008-01-01

151

A persistent P,N-heterocyclic carbenew  

OpenAIRE

Conjugate acids of cyclic (amino)(phosphino)carbenes (P-NHCs) have been prepared, and several di erent processes have been observed during their deprotonation, which include the formation of a metastable P-NHC, an azomethine ylide, and a bicyclic phosphirane.

Frey, Guido D.; Song, Maoying; Bourg, Jean-baptiste; Donnadieu, Bruno; Soleilhavoup, Michele; Bertrand, Guy

2008-01-01

152

A molecular iron catalyst for the acceptorless dehydrogenation and hydrogenation of N-heterocycles.  

Science.gov (United States)

A well-defined iron complex (3) supported by a bis(phosphino)amine pincer ligand efficiently catalyzes both acceptorless dehydrogenation and hydrogenation of N-heterocycles. The products from these reactions are isolated in good yields. Complex 3, the active catalytic species in the dehydrogenation reaction, is independently synthesized and characterized, and its structure is confirmed by X-ray crystallography. A trans-dihydride intermediate (4) is proposed to be involved in the hydrogenation reaction, and its existence is verified by NMR and trapping experiments. PMID:24877556

Chakraborty, Sumit; Brennessel, William W; Jones, William D

2014-06-18

153

Ligand-controlled regiodivergent nickel-catalyzed annulation of pyridones.  

Science.gov (United States)

The 1,6-annulated 2-pyridone motif is found in many biologically active compounds and its close relation to the indolizidine and quinolizidine alkaloid core makes it an attractive building block. A nickel-catalyzed C?H functionalization of 2-pyridones and subsequent cyclization affords 1,6-annulated 2-pyridones by selective intramolecular olefin hydroarylation. The switch between the exo- and endo-cyclization modes is controlled by two complementary sets of ligands. Irrespective of the ring size, the regioselectivity during the cyclization is under full catalyst control. Simple cyclooctadiene promotes an exo-selective cyclization, whereas a bulky N-heterocyclic carbene ligand results in an endo-selective mode. The method was further applied in the synthesis of the lupin alkaloid cytisine. PMID:25378295

Donets, Pavel A; Cramer, Nicolai

2015-01-01

154

Synthetic and Thermodynamic Investigations of Ancillary Ligand Influence on Catalytic Organometallic Systems. Final Report  

Energy Technology Data Exchange (ETDEWEB)

During the grant period we have been involved in synthesizing and experimentally determining solution enthalpy values associated with partially fluorinated ligands. This has lead to the publication of manuscripts dealing with synthetic, calorimetric and catalytic behavior of partially fluorinated ligands. The collaboration with Los Alamos researchers has lead to the publication of catalytic results in sc CO{sub 2} which have proven very interesting. Furthermore, we have also examined ligands that behave as phosphine mimics. The N-heterocyclic carbenes have been explored as alternatives for tertiary phosphines and have resulted in the design and construction of efficient palladium and nickel system capable of performing C-C and C-N cross coupling reactions. The initial studies in this areas were made possible by exploratory work conducted under the DOE/EPSCoR grant.

Nolan, Steven

2003-03-20

155

Spectral, structural, and electrochemical properties of ruthenium porphyrin diaryl and aryl(alkoxycarbonyl) carbene complexes: influence of carbene substituents, porphyrin substituents, and trans-axial ligands.  

Science.gov (United States)

A wide variety of ruthenium porphyrin carbene complexes, including [Ru(tpfpp)(CR(1)R(2))] (CR(1)R(2) = C(p-C(6)H(4)Cl)(2) 1 b, C(p-C(6)H(4)Me)(2) 1 c, C(p-C(6)H(4)OMe)(2) 1 d, C(CO(2)Me)(2) 1 e, C(p-C(6)H(4)NO(2))CO(2)Me 1 f, C(p-C(6)H(4)OMe)CO(2)Me 1 g, C(CH==CHPh)CO(2)CH(2)(CH==CH)(2)CH(3) 1 h), [Ru(por)(CPh(2))] (por=tdcpp 2 a, 4-Br-tpp 2 b, 4-Cl-tpp 2 c, 4-F-tpp 2 d, tpp 2 e, ttp 2 f, 4-MeO-tpp 2 g, tmp 2 h, 3,4,5-MeO-tpp 2 i), [Ru(por)[C(Ph)CO(2)Et

Li, Yan; Huang, Jie-Sheng; Xu, Guo-Bao; Zhu, Nianyong; Zhou, Zhong-Yuan; Che, Chi-Ming; Wong, Kwok-Yin

2004-07-19

156

The Synthesis of Some Perhydrobenzimidazolinium Salts and Their Application in Pd-Carbene Catalyzed Heck and Suzuki Reactions  

Directory of Open Access Journals (Sweden)

Full Text Available Novel 1,3-dialkylperhydrobenzimidazolinium chloride salts were prepared as precursors of N-heterocyclic carbenes 3a-e by reacting N,N’-dialkylcyclohexandiamine, triethyl orthoformate and ammonium chloride. The salts were characterized spectroscopically and the complexes formed in situ from Pd(OAc2 and 3 have been tested as catalysts in homogenous Heck and Suzuki reactions.

Murat Yi?it

2009-06-01

157

[Au(dien)(N-heterocycle)](3+): Reactivity with Biomolecules and Zinc Finger Peptides.  

Science.gov (United States)

The reaction of [Au(dien)(N-heterocycle)](3+) (AuN4) coordination compounds with simple amino acids and zinc finger proteins is reported. Compared to [AuCl(dien)](2+), NMR studies show that the presence of a more substitution-inert N-donor as the putative leaving group slows the reaction with the sulfur-containing amino acids N-acetylmethionine (NAcMet) and N-acetylcysteine (NAcCys). Lack of ligand dissociation upon reaction with NAcCys indicates, to our knowledge, the first long-lived N-heterocycle-Au-S species in solution. Reactions with zinc finger proteins show a higher reactivity with the Cys3His zinc finger than with Cys2His2, likely due to the presence of fewer aurophilic cysteines in the latter. Of the Au(III) compounds studied, [Au(dien)(DMAP)](3+) (DMAP = 4-dimethylaminopyridine) appears to be the least reactive, with ESI-MS studies showing the presence of intact zinc fingers at initial reaction times. These results, in combination with previously reported characterization and pH dependency studies, will further aid in optimizing the structure of these AuN4 species to obtain a substitution-reactive yet selective compound for targeting zinc finger proteins. PMID:25531886

Spell, Sarah R; Farrell, Nicholas P

2015-01-01

158

The ketimide ligand is not just an inert spectator: heteroallene insertion reactivity of an actinide-ketimide linkage in a thorium carbene amide ketimide complex.  

Science.gov (United States)

The ketimide anion R2C=N(-) is an important class of chemically robust ligand that binds strongly to metal ions and is considered ideal for supporting reactive metal fragments due to its inert spectator nature; this contrasts with R2N(-) amides that exhibit a wide range of reactivities. Here, we report the synthesis and characterization of a rare example of an actinide ketimide complex [Th(BIPM(TMS)){N(SiMe3)2}(N=CPh2)] [2, BIPM(TMS)=C(PPh2NSiMe3)2]. Complex 2 contains Th=C(carbene), Th-N(amide) and Th-N(ketimide) linkages, thereby presenting the opportunity to probe the preferential reactivity of these linkages. Importantly, reactivity studies of 2 with unsaturated substrates shows that insertion reactions occur preferentially at the Th-N(ketimide) bond rather than at the Th=C(carbene) or Th-N(amide) bonds. This overturns the established view that metal-ketimide linkages are purely inert spectators. PMID:25044515

Lu, Erli; Lewis, William; Blake, Alexander J; Liddle, Stephen T

2014-08-25

159

Phosphines with N-heterocyclic boranyl substituents.  

Science.gov (United States)

A lithio-diazaborole reacted with diamino-chlorophosphines via metathesis to yield previously unavailable phosphinoboranes bearing amino substituents at both the phosphorus and boron atoms, and with Ph2PCl and Mes*PCl2via chloride transfer and reductive PP coupling to give a chloro-diazaborole and the corresponding diphosphine or diphosphene, respectively. Diazaboroles with phenylphosphino- and PH2-substituents were nonetheless accessible via inverse metathesis upon treatment of a bromoborane precursor with phosphides PhnPH2-nM (n = 0-2, M = Li, K). The products were characterised by spectroscopic data and in most cases by single-crystal X-ray diffraction studies which show the molecules to exhibit strongly pyramidal coordination at the phosphorus atom and long BP bonds of 1.93-1.95 Å. The insensitivity of the BP distance towards substituent effects and the tolerance of large sterically induced torsional twists along the BP bond axis suggest the presence of pure single bonds without any contribution from P?B dative ?-interactions. This view was confirmed by DFT studies which indicate further that the molecules lack a significant electrophilic character at boron but may act as potential ?-donor/?-acceptor ligands through the phosphorus atom. PMID:24142130

Kaaz, M; Bender, J; Förster, D; Frey, W; Nieger, M; Gudat, D

2014-01-14

160

Catalytic Hydrodechlorination of Benzyl Chloride Promoted by Rh-N-heterocyclic Carbene Catalysts.  

Science.gov (United States)

The rhodium(I) complexes [Rh(Cl)(COD)(R-NHC-(CH2 )3 Si(OiPr)3 )] [COD=cyclooctadiene; R=2,6-diisopropylphenyl (1?a); n-butyl (1?b)] are effective catalyst precursors for the homogeneous hydrodechlorination of benzyl chloride using HSiEt3 as hydrogen source. This reaction is selective to the formation of toluene. However, in presence of a stoichiometric amount of potassium tert-butoxide (KtBuO) the formation of mixtures containing toluene together with 17-19?mol?% of the C?C homocoupling product, namely PhCH2 CH2 Ph, is observed. A mechanism proposal based on experimental insights and theoretical calculations at the DFT level that allows explanation of the experimental findings is included. Moreover, the heterogeneous catalytic system based on catalyst 1?a supported on MCM-41 has been demonstrated to be effective for the solvent-free hydrodechlorination of benzyl chloride using HSiEt3 and HSiMe(OSiMe3 )2 . PMID:25504980

Lázaro, Guillermo; Polo, Víctor; Fernández-Alvarez, Francisco J; García-Orduña, Pilar; Lahoz, Fernando J; Iglesias, Manuel; Pérez-Torrente, Jesús J; Oro, Luis A

2015-02-01

161

Cationic Rh and Ir complexes containing bidentate imidazolylidene-1,2,3-triazole donor ligands: synthesis and preliminary catalytic studies.  

Science.gov (United States)

A series of new cationic Rh(I), Rh(III) and Ir(III) complexes containing hybrid bidentate N-heterocyclic carbene–1,2,3-triazolyl donor of general formulae [Rh(CaT)(COD)]BPh4 (2a–d), [Rh(CaT)(CO)2]BPh4 (3a–d) and [M(CaT)(Cp*)Cl]BPh4 (M = Rh, 4a–d; M = Ir, 5a–c), where CaT = bidentate N-heterocyclic carbene–triazolyl ligands, COD = 1,5-cyclooctadiene and Cp* = 1,2,3,4,5-pentamethylcyclopentadienyl, were synthesised. The imidazolium–1,2,3-triazolyl pre-ligands (1a–c and 1e–i) were readily prepared using the Cu(I) catalysed ‘click reaction’ between phenyl azide or benzyl azides with propargyl functionalised imidazolium salts. The single crystal solid state structures of complexes 2a–d; 3a–b; 4a–d and 5a–b confirm the bidentate coordination of the NHC–1,2,3-triazolyl ligand with the NHC coordinating via the ‘normal’ C2-carbon and the 1,2,3-triazolyl donor coordinating via the N3? atom to form six membered metallocycles. These complexes are the first examples of Rh and Ir complexes containing the hybrid NHC–1,2,3-triazolyl ligands which exhibit a bidentate coordination mode. A number of these complexes showed limited efficiency as catalysts for the intramolecular hydroamination of 4-pentyn-1-amine to 2-methylpyrroline. PMID:23959044

Vuong, Khuong Q; Timerbulatova, Marina G; Peterson, Matthew B; Bhadbhade, Mohan; Messerle, Barbara A

2013-10-21

162

Iron(II) complexes of ditopic carbanionic carbenes.  

OpenAIRE

Reaction of dimesityliron(II) (Fe2(mes)4) with the N-heterocyclic carbenes 1,3-bis(2,6-diisopropylphenyl)-imidazol-2-ylidene (IPr) and 1,3-bis(2,6-dimethylphenyl)hexahydropyrimidin-2-ylidene (6-Xyl) afforded the novel trigonal planar complexes [Fe(IPr)(mes)2] (1) and [Fe(6-Xyl)(mes)2] (2), respectively. Both species were structurally characterized by single crystal X-ray diffraction and display structures and magnetic responses consistent with a quintet ground state (S = 2). Reaction of 1 wit...

Musgrave, Ra; Turbervill, Rs; Irwin, M.; Herchel, R.; Goicoechea, Jm

2014-01-01

163

Stereoelectronic basis for the kinetic resolution of N-heterocycles with chiral acylating reagents.  

Science.gov (United States)

The kinetic resolution of N-heterocycles with chiral acylating agents reveals a previously unrecognized stereoelectronic effect in amine acylation. Combined with a new achiral hydroxamate, this effect makes possible the resolution of various N-heterocycles by using easily prepared reagents. A transition-state model to rationalize the stereochemical outcome of this kinetic resolution is also proposed. PMID:24839065

Hsieh, Sheng-Ying; Wanner, Benedikt; Wheeler, Philip; Beauchemin, André M; Rovis, Tomislav; Bode, Jeffrey W

2014-06-10

164

Intramolecular C-H/O-H bond cleavage with water and alcohol using a phosphine-free ruthenium carbene NCN pincer complex.  

Science.gov (United States)

Transition metal complexes that exhibit metal-ligand cooperative reactivity could be suitable candidates for applications in water splitting. Ideally, the ligands around the metal should not contain oxidizable donor atoms, such as phosphines. With this goal in mind, we report new phosphine-free ruthenium NCN pincer complexes with a central N-heterocyclic carbene donor and methylpyridyl N-donors. Reaction with base generates a neutral, dearomatized alkoxo-amido complex, which has been structurally and spectroscopically characterized. The tert-butoxide ligand facilitates regioselective, intramolecular proton transfer through a C?H/O?H bond cleavage process occurring at room temperature. Kinetic and thermodynamic data have been obtained by VT NMR experiments; DFT calculations support the observed behavior. Isolation and structural characterization of a doubly dearomatized phosphine complex also strongly supports our mechanistic proposal. The alkoxo-amido complex reacts with water to form a dearomatized ruthenium hydroxide complex, a first step towards phosphine-free metal-ligand cooperative water splitting. PMID:25266279

Prokopchuk, Demyan E; Tsui, Brian T H; Lough, Alan J; Morris, Robert H

2014-12-15

165

Propane activation by palladium complexes with chelating bis(NHC) ligands and aerobic cooxidation.  

Science.gov (United States)

The development of efficient aerobic oxidation methods remains a challenge for the selective functionalization of C-H bonds in alkanes. Herein we report the development of a C-H functionalization procedure for propane by using a palladium catalyst with chelating bis(N-heterocyclic carbene) ligands in trifluoroacetic acid together with a vanadium co-catalyst. Halides play a decisive role in the reaction. The experimental results are presented together with supporting kinetic data and an isotope effect. The reaction can be run with dioxygen as the oxidant if vanadium salts and halides are present in the reaction mixture. Experimental as well as computational results favor a mechanism involving C-H activation by palladium(II), followed by oxidation to palladium(IV) by bromine. PMID:24482080

Munz, Dominik; Strassner, Thomas

2014-02-24

166

A diruthenium ?-carbido complex that shows singlet-carbene-like reactivity.  

Science.gov (United States)

Low-temperature deprotonation of the cationic ?-methylidyne complex [(Cp*Ru)2(?-NPh)(?-CH)][BF4] (Cp* = ?(5)-C5Me5) with KN(SiMe3)2 affords a thermally unstable ?-carbido complex [(Cp*Ru)2(?-NPh)(?-C)] (2), as evidenced by trapping experiments with elemental S or Se and (13)C NMR spectroscopic observation. The reactivity of 2 toward CO2, Ph2S(+)CH2(-), EtOH, and an intramolecular C-H bond indicates that the ?-carbido carbon in 2 has an ambiphilic (nucleophilic and electrophilic) nature consistent with the formulation of 2 as the first example of a transition-metal-substituted singlet carbene. DFT study suggests that the Ru substituents in 2 are stronger ?-donor and weaker ?-donor to the carbene center than amino substituents in N-heterocyclic carbenes. PMID:25365114

Takemoto, Shin; Ohata, Jun; Umetani, Kento; Yamaguchi, Masahiro; Matsuzaka, Hiroyuki

2014-11-12

167

Tantalum catalyzed hydroaminoalkylation for the synthesis of ?- and ?-substituted N-heterocycles.  

Science.gov (United States)

Unprotected secondary amines are directly alkylated by C-H functionalization adjacent to nitrogen, thereby opening new routes toward the synthesis of ?- and ?-alkylated N-heterocycles. ?-Alkylated piperidine, piperazine, and azepane products are prepared from heterocycles and alkenes in an atom-economic reaction with excellent regio- and diastereoselectivity. ?-Alkylated N-heterocycles are synthesized via a scalable one-pot alkylation/cyclization procedure generating 3-methylated azetidines, pyrrolidines, and piperidines. PMID:23600625

Payne, Philippa R; Garcia, Pierre; Eisenberger, Patrick; Yim, Jacky C-H; Schafer, Laurel L

2013-05-01

168

Electronic spectra of N-heterocyclic pentacyanoferrate(II) complexes in different solvents, studied by multiconfigurational perturbation theory.  

Science.gov (United States)

Ligand-field and charge-transfer spectra of N-heterocyclic pentacyanoferrate(II) complexes [Fe(CN)5L]n? were investigated using multiconfigurational perturbation theory. The spectrum of [Fe(CN)5(py)]3– was studied in detail under vacuum and in the following polarizable continuum model (PCM) simulated solvents: acetone, acetonitrile, dimethylsulfoxide (DMSO), ethanol, methanol, and water. The ligand-field states proved to be rather insensitive to the solvent environment, whereas much stronger solvent effects were observed for the charge-transfer (CT) transitions. The nature of the intense band was confirmed as a metal-to-ligand charge transfer originating from a 3d(xz)????(b1)* (L)-orbital transition. The difference between the calculated and experimental transition energy of this CT transition is minimal for aprotic solvents, but increases strongly with the solvent proton donor ability in the protic solvents. In an attempt to improve the description of this CT state, up to 14 solvent molecules were explicitly included in the quantum model. In DMSO, the spectra of complexes with ligands L (where L is pyridine, 4-picoline, 4-acetylpyridine, 4-cyanopyridine, pyrazine, and N-methylpyrazinium) correlate very well with the experiment. PMID:23992205

Formiga, André Luiz Barboza; Vancoillie, Steven; Pierloot, Kristine

2013-09-16

169

N-Heterocyclic carbene–palladium(II)-1-methylimidazole complex catalyzed Mizoroki–Heck reaction of aryl chlorides with styrenes  

OpenAIRE

A well-defined N-heterocyclic carbene–palladium(II)-1-methylimidazole [NHC-Pd(II)-Im] complex 1 was found to be an effective catalyst for the Mizoroki–Heck reaction of a variety of aryl chlorides with styrenes. Both activated and deactivated aryl chlorides work well to give the corresponding coupling products in good to excellent yields by using tetrabutylammonium bromide (TBAB) as the ionic liquid.

Ting-Ting Gao; Ai-Ping Jin; Li-Xiong Shao

2012-01-01

170

Orientation and stereodynamic paths of planar monodentate ligands in square planar nickel N2S complexes.  

Science.gov (United States)

The well-established presence of histidine donors in binding sites of Ni-containing biomolecules prompts the study of orientational preference and stereodynamic nature of flat monodentate ligands (L = imidazoles, pyridine and an N-heterocyclic carbene) bound to planar N(2)SNi moieties. Square planar [N(2)SNiL](n+) complexes are accessed through bridge-splitting reactions of dimeric, thiolate-S bridged [N(2)SNi](2) complexes. The solid state molecular structures of three mononuclear products, and three monothiolate bridged dinickel complexes, reveal that the plane of the added monodentate ligand orients largely orthogonal to the N(2)SNiL square plane. Variable temperature (1)H NMR characterization of dynamic processes and ground state isomer ratios of imidazole complexes in their stopped exchange limiting spectra, readily correlate with density functional theory (DFT)-guided interpretation of Ni-L rotational activation barriers. Full DFT characterization finds Ni-L bond lengthening as well as a tetrahedral twist distortion in the transition state, reaching a maximum in the NHC complex, and relating mainly to the steric hindrance derived both from the ligand and the binding pocket. In the case of the imidazole ligands a minor electronic contribution derives from intramolecular electrostatic interactions (imidazole C-2 C-H(delta+)- - S(delta-) interaction). Computational studies find this donor-acceptor interaction is magnified in O-analogues, predicting coplanar arrangements in the ground state of N(2)ON(imid)Ni complexes. PMID:20507173

Jenkins, Roxanne M; Singleton, Michael L; Leamer, Lauren A; Reibenspies, Joseph H; Darensbourg, Marcetta Y

2010-06-21

171

On the reactivity and stability of electrogenerated N-heterocyclic carbene in parent 1-butyl-3-methyl-1H-imidazolium tetrafluoroborate: Formation and use of N-heterocyclic carbene-CO2 adduct as latent catalyst  

International Nuclear Information System (INIS)

A simple electrolysis (under galvanostatic conditions) of the room temperature ionic liquid 1-butyl-3-methyl-1H-imidazolium tetrafluoroborate, BMIm-BF4, yields, after bubbling CO2 into the catholyte, the adduct NHC-CO2. The considerable stability of this NHC-CO2 adduct, at room temperature, in the parent ionic liquid as solvent, has been compared with the one of free NHC in the same BMIm-BF4. The BMIm-BF4 solution containing NHC-CO2 adduct, suitably triggered (US irradiation or 120 °C), is able to release free NHC. The NHC-CO2 adduct usefulness has been demonstrated using it as efficient latent catalyst, in BMIm-BF4 as solvent, in the benzoin condensation and in the oxidative esterification of cinnamaldehyde with benzyl alcohol

172

N-heterocyclic carbene-palladium(II)-1-methylimidazole complex catalyzed amination between aryl chlorides and amides.  

Science.gov (United States)

We report herein that amides are excellent N-sources in the NHC-Pd(II)-Im complex 1 catalyzed amination of aryl chlorides. In the presence of KO(t)Bu, various aryl chlorides and amides can react smoothly to give the corresponding aminated products in moderate to almost quantitative yields at room temperature within 6 h. PMID:23020672

Chen, Wen-Xin; Shao, Li-Xiong

2012-10-19

173

Concise formal synthesis of (-)-7-deoxyloganin via N-heterocyclic carbene catalysed rearrangement of ?,?-unsaturated enol esters.  

Science.gov (United States)

NHC catalysed rearrangement of ?,?-unsaturated enol esters derived from formyl acetates and cyclopentyl annulated ?,?-unsaturated acids provides the cyclopentapyranone core of (-)-7-deoxyloganin (1) with diastereo- and chemoselectivity in 6 steps starting from (-)-citronellal. The elaboration to the natural product has been investigated using two new approaches. The most successful intercepts our previous work on (-)-7-deoxyloganin (1) allowing completion of a formal total synthesis in 10-steps. PMID:22024770

Candish, Lisa; Lupton, David W

2011-12-01

174

N-Heterocyclic carbene–palladium catalysts for the direct arylation of pyrrole derivatives with aryl chlorides  

Directory of Open Access Journals (Sweden)

Full Text Available New Pd–NHC complexes have been synthesized and employed for palladium-catalyzed direct arylation of pyrrole derivatives by using electron-deficient aryl chlorides as coupling partners. The desired coupling products were obtained in moderate to good yields by using 1 mol % of these air-stable palladium complexes. This is an advantage compared to the procedures employing air-sensitive phosphines, which have been previously shown to promote the coupling of aryl chlorides with heteroarenes.

Ismail Özdemir

2013-02-01

175

N-heterocyclic carbene/Lewis acid strategy for the stereoselective synthesis of spirocyclic oxindole-dihydropyranones.  

Science.gov (United States)

Under the cooperative catalysis of NHC/Lewis acid, the mild, straightforward [4 + 2] annulation of ?-bromo-?,?-unsaturated aldehydes bearing ?-H with isatin derivatives gave spirocyclic oxindole-dihydropyranones stereoselectively. This approach is particularly attractive due to the concise construction, avoidance of external oxidants, and the potential utilization value of final products in molecular biology and pharmacy. PMID:24987936

Xiao, Zhaoxin; Yu, Chenxia; Li, Tuanjie; Wang, Xiang-Shan; Yao, Changsheng

2014-07-18

176

Pyrene-based bisazolium salts: from luminescence properties to janus-type bis-N-heterocyclic carbenes.  

Science.gov (United States)

A series of pyrene-based bisazolium salts have been obtained starting from 4,5,9,10-tetrabromo-2,7-di-tert-butylpyrene. The synthetic procedure to the pyrene-bisazoliums (PBIs) reveals an unexpected behavior, as a consequence of the presence of the alkyl groups (alkyl=Me, Et, n-Pr, and n-Bu) coming from the trisalkoxyformate in the final products, instead of the expected tBu of tAmyl groups from the starting tetra-aminated pyrenes. All bisazoliums show fluorescence properties, with emissions in the range of 370-420?nm, and quantum yields ranging from 0.29 to 0.41. The PBIs were used as bis-NHC precursors in the preparation of a series of dirhodium and diiridium complexes, which have been fully characterized. The electrochemical studies on selected dimetallic complexes reveal that the electronic communication between the metals through the polyaromatic linker is negligible. PMID:24753109

Gonell, Sergio; Poyatos, Macarena; Peris, Eduardo

2014-07-28

177

The Orientation and Stereodynamic Paths of Planar Monodentate Ligands in Square Planar Nickel N2S Complexes  

Science.gov (United States)

The well-established presence of histidine donors in binding sites of Ni-containing biomolecules prompts the study of orientational preference and stereodynamic nature of flat monodentate ligands (L = imidazoles, pyridine and an N-heterocyclic carbene) bound to planar N2SNi moieties. Square planar [N2SNiL]n+ complexes are accessed through bridge-splitting reactions of dimeric, thiolate-S bridged [N2SNi]2 complexes. The solid state molecular structures of three mononuclear products, and three monothiolate bridged dinickel complexes, reveal that the plane of the added monodentate ligand orients largely orthogonal to the N2SNiL square plane. Variable temperature 1H NMR characterization of dynamic processes and ground state isomer ratios of imidazole complexes in their stopped exchange limiting spectra, readily correlate with DFT-guided interpretation of Ni-L rotational activation barriers. Full DFT characterization find Ni-L bond lengthening as well as a tetrahedral twist distortion in the transition state, reaching a maximum in the NHC complex, and relating mainly to the steric hindrance derived both from the ligand and the binding pocket. In the case of the imidazole ligands a minor electronic contribution derives from intramolecular electrostatic interactions (imidazole C-2 C-H?+- - S?? interaction). Computational studies find this donor-acceptor interaction is magnified in O-analogues, predicting coplanar arrangements in the ground state of N2ONimidNi complexes. PMID:20507173

Jenkins, Roxanne M.; Singleton, Michael L.; Leamer, Lauren A.; Reibenspies, Joseph H.; Darensbourg, Marcetta Y.

2010-01-01

178

More sustainable formation of C-N and C-C bonds for the synthesis of N-heterocycles.  

Science.gov (United States)

Heterocycles made green: New methodologies for the synthesis of pyrroles were recently developed based on domino Ir- and Ru-catalyzed amination and alkylations of alcohols. The concept provides a greener approach to interesting N-heterocyclic compounds. PMID:23775609

Schranck, Johannes; Tlili, Anis; Beller, Matthias

2013-07-22

179

Fluorocarbon and hydrocarbon N-heterocyclic (C5-C7) imidazole-based liquid crystals.  

Science.gov (United States)

By using three synthetic protocols, a series of fluorocarbon and hydrocarbon N-heterocyclic imidazole-based liquid crystals (LCs) and related imidazolium-based ionic liquid crystals (ILCs) have been prepared. The ring size of the N-heterocycle and the length of the N-terminal chain (on the imidazolium unit in the ILCs) were modified, and the influence of these structural parameters on liquid-crystal phases was investigated by means of polarizing optical microscopy (POM), differential scanning calorimetry (DSC), and X-ray diffraction (XRD). These new ILCs exhibit a disordered smectic phase (SmA), good thermal stabilities, a broad smectic phase range, a high dipole moment, relatively low melting points, but high clearing points and strong emission fluorescence relative to imidazole-based LCs. These encouraging results have led us to believe these fluorocarbon and hydrocarbon N-heterocyclic imidazole-based LCs and related imidazolium-based ILCs could be used as new liquid-crystalline materials. PMID:25256069

Chen, Hongren; Hong, Fengying; Shao, Guang; Hang, Deyu; Zhao, Lei; Zeng, Zhuo

2014-12-01

180

Iron(II) complexes of ditopic carbanionic carbenes.  

Science.gov (United States)

Reaction of dimesityliron(II) (Fe2(mes)4) with the N-heterocyclic carbenes 1,3-bis(2,6-diisopropylphenyl)-imidazol-2-ylidene (IPr) and 1,3-bis(2,6-dimethylphenyl)hexahydropyrimidin-2-ylidene (6-Xyl) afforded the novel trigonal planar complexes [Fe(IPr)(mes)2] (1) and [Fe(6-Xyl)(mes)2] (2), respectively. Both species were structurally characterized by single crystal X-ray diffraction and display structures and magnetic responses consistent with a quintet ground state (S = 2). Reaction of 1 with KC8 in THF afforded K(+) salts of the anionic complex [{:C[N(2,6-(i)Pr2C6H3)]2(CH)C}2Fe(mes)](-) (3) and the homoleptic organometallic anion [Fe(mes)3](-) (4). By contrast, reduction of 2 resulted in extensive decomposition and intractable product mixtures. Complex 3 is coordinated by two ditopic carbanionic carbenes via the C4/C5 position while the C2 position retains unquenched carbenic character and remains vacant for further coordination. This was corroborated by reacting solutions of 3 with one and two equivalents of triethylaluminium (AlEt3) which resulted in the formation of [{Et3Al:C[N(2,6-(i)Pr2C6H3)]2(CH)C}{:C[N(2,6-(i)Pr2C6H3)]2(CH)C}Fe(mes)](-) (5) and [{Et3Al:C[N(2,6-(i)Pr2C6H3)]2(CH)C}2Fe(mes)](-) (6), respectively. Both of these species were structurally characterized as [K(2,2,2-crypt)](+) salts. PMID:24185197

Musgrave, Rebecca A; Turbervill, Robert S P; Irwin, Mark; Herchel, Radovan; Goicoechea, Jose M

2014-03-21

181

Self-assembly of N-heterocyclic derivatives of divalent germanium, tin, and lead.  

Science.gov (United States)

A new class of exceptionally stable asymmetric N-heterocyclic germylenes, stannylenes, and plumbylenes has been successfully isolated and characterized by single-crystal X-ray diffraction analysis and multinuclear NMR spectroscopy. Their stability results from tetrameric supramolecular aggregation through strong intermolecular Npy ?E(II) (E=Ge, Sn, Pb) interactions involving the nitrogen atom of a neighboring pyridine moiety. The electronic structures and stabilities of the prepared divalent derivatives of Ge, Sn, and Pb in monomeric and aggregated forms are discussed based on theoretical investigations. PMID:25346531

Zabula, Alexander V; Rogachev, Andrey Yu; West, Robert

2014-12-01

182

A Heteroleptic Ferrous Complex with Mesoionic Bis(1,2,3-triazol-5-ylidene) Ligands: Taming the MLCT Excited State of Iron(II).  

Science.gov (United States)

Strongly ?-donating N-heterocyclic carbenes (NHCs) have revived research interest in the catalytic chemistry of iron, and are now also starting to bring the photochemistry and photophysics of this abundant element into a new era. In this work, a heteroleptic Fe(II) complex (1) was synthesized based on sequentially furnishing the Fe(II) center with the benchmark 2,2'-bipyridine (bpy) ligand and the more strongly ?-donating mesoionic ligand, 4,4'-bis(1,2,3-triazol-5-ylidene) (btz). Complex?1 was comprehensively characterized by electrochemistry, static and ultrafast spectroscopy, and quantum chemical calculations and compared to [Fe(bpy)3 ](PF6 )2 and (TBA)2 [Fe(bpy)(CN)4 ]. Heteroleptic complex?1 extends the absorption spectrum towards longer wavelengths compared to a previously synthesized homoleptic Fe(II) NHC complex. The combination of the mesoionic nature of btz and the heteroleptic structure effectively destabilizes the metal-centered (MC) states relative to the triplet metal-to-ligand charge transfer ((3) MLCT) state in 1, rendering it a lifetime of 13?ps, the longest to date of a photochemically stable Fe(II) complex. Deactivation of the (3) MLCT state is proposed to proceed via the (3) MC state that strongly couples with the singlet ground state. PMID:25504660

Liu, Yizhu; Kjaer, Kasper S; Fredin, Lisa A; Chábera, Pavel; Harlang, Tobias; Canton, Sophie E; Lidin, Sven; Zhang, Jianxin; Lomoth, Reiner; Bergquist, Karl-Erik; Persson, Petter; Wärnmark, Kenneth; Sundström, Villy

2015-02-23

183

Cyclic (Alkyl)(Amino)Carbenes (CAACs): Stable Carbenes on the Rise.  

Science.gov (United States)

Conspectus Carbenes are compounds that feature a divalent carbon atom with only six electrons in its valence shell. In the singlet state, they possess a lone pair of electrons and a vacant orbital and therefore exhibit Lewis acidic and Lewis basic properties, which explains their very high reactivity. Following the preparation by our group in 1988 of the first representative, a variety of stable carbenes are now available, the most popular being the cyclic diaminocarbenes. In this Account, we discuss another class of stable cyclic carbenes, namely, cyclic (alkyl)(amino)carbenes (CAACs), in which one of the electronegative and ?-donor amino substituents of diaminocarbenes is replaced by a ?-donating but not ?-donating alkyl group. As a consequence, CAACs are more nucleophilic (?-donating) but also more electrophilic (?-accepting) than diaminocarbenes. Additionally, the presence of a quaternary carbon in the position ? to the carbene center provides steric environments that differentiate CAACs dramatically from all other ligands. We show that the peculiar electronic and steric properties of CAACs allow for the stabilization of unusual diamagnetic and paramagnetic main group element species. As examples, we describe the preparation of room temperature stable phosphorus derivatives in which the heteroatom is in the zero oxidation state, nucleophilic boron compounds, and phosphorus-, antimony-, boron-, silicon-, and even carbon-centered neutral and cationic radicals. CAACs are also excellent ligands for transition metal complexes. The most recent application is their use for the stabilization of paramagnetic complexes, in which the metal is often in a formal zero oxidation state. Indeed, bis(CAAC)M complexes in which the metal is gold, copper, cobalt, iron, nickel, manganese, and zinc have been isolated. Depending on the metal, the majority of spin density can reside either on the metal or on the carbene carbons and the nitrogen atoms of the CAAC ligand. In contrast to diaminocarbenes, the higher basicity of CAACs makes them poor leaving groups, and thus they cannot be used for classical organocatalysis. However, because of their superior electrophilicity and smaller singlet-triplet gap, CAACs can activate small molecules at room temperature, such as CO, H2, and P4, as well as enthalpically strong bonds, such as B-H, Si-H, N-H, and P-H. Lastly, excellent results have been obtained in palladium, ruthenium, and gold catalysis. CAAC-metal complexes are extremely thermally robust, which allows for their utilization in harsh conditions. This property has been used to perform a variety of gold-catalyzed reactions in the presence of basic amines, including ammonia and hydrazine, which usually deactivate catalysts. PMID:25515548

Soleilhavoup, Michèle; Bertrand, Guy

2015-02-17

184

Unprecedented silicon(II)?calcium complexes with N-heterocyclic silylenes.  

Science.gov (United States)

The first N-heterocyclic silylene (NHSi) complexes of any s-block element to date are reported for calcium: [(?(5)-C5Me5)2Ca?:Si(O-C6H4-2-(t)Bu){(N(t)Bu)2CPh}] (6) and [(?(5)-C5Me5)2Ca?:Si(N(t)BuCH)2] (7). Complexes 6 and 7 are isolable in a facile way upon reaction of the corresponding free N-heterocyclic silylenes (NHSis) with [(?(5)-C5Me5)2Ca] (2). Complexes 6 and 7 were fully characterised by spectroscopic means and the single crystal X-ray diffraction analysis of 6 is also reported. Analysis of the bonding situation by DFT methods including a Bader Atoms in molecules (AIM) analysis is also reported. The bonding interaction between the Si and Ca centres in complexes 6 and 7 can best be viewed as ?-donor-acceptor interactions, with a considerable ionic contribution in the bond. The reactivity towards the oxygen containing substrates THF and benzophenone is also discussed. PMID:25382093

Blom, Burgert; Klatt, Günter; Gallego, Daniel; Tan, Gengwen; Driess, Matthias

2015-01-14

185

Quantitative description of structural effects on the stability of gold(I) carbenes.  

Science.gov (United States)

The gas-phase bond-dissociation energies of a SO2 -imidazolylidene leaving group of three gold(I) benzyl imidazolium sulfone complexes are reported (E0 =46.6±1.7, 49.6±1.7, and 48.9±2.1?kcal?mol(-1) ). Although these energies are similar to each other, they are reproducibly distinguishable. The energy-resolved collision-induced dissociation experiments of the three [L]-gold(I) (L=ligand) carbene precursor complexes were performed by using a modified tandem mass spectrometer. The measurements quantitatively describe the structural and electronic effects a p-methoxy substituent on the benzyl fragment, and trans [NHC] and [P] gold ligands, have towards gold carbene formation. Evidence for the formation of the electrophilic gold carbene in solution was obtained through the stoichiometric and catalytic cyclopropanation of olefins under thermal conditions. The observed cyclopropane yields are dependent on the rate of gold carbene formation, which in turn is influenced by the ligand and substituent. The donation of electron density to the carbene carbon by the p-methoxy benzyl substituent and [NHC] ligand stabilizes the gold carbene intermediate and lowers the dissociation barrier. Through the careful comparison of gas-phase and solution chemistry, the results suggest that even gas-phase leaving-group bond-dissociation energy differences of 2-3?kcal?mol(-1) enormously affect the rate of gold carbene formation in solution, especially when there are competing reactions. The thermal decay of the gold carbene precursor complex was observed to follow first-order kinetics, whereas cyclopropanation was found to follow pseudo-first-order kinetics. Density-functional-theory calculations at the M06-L and BP86-D3 levels of theory were used to confirm the observed gas-phase reactivity and model the measured bond-dissociation energies. PMID:25236486

Ringger, David H; Kobylianskii, Ilia J; Serra, Daniel; Chen, Peter

2014-10-27

186

Enzymatic hydrolysis of long-chain N-heterocyclic fatty esters.  

Science.gov (United States)

Incubation of a 1-pyrroline ester [viz. methyl 8-(5-hexyl-1-pyrroline-2-yl)octanoate, 1] with bakers' yeast (Saccharomyces cerevisiae) gave the corresponding free fatty acid (1a, 52%). The C = N bond of the 1-pyrroline was not reduced by the yeast. Complete hydrolysis of compound 1 was successful using lipase of Candida cylindracea (CCL) or Lipolase (Rhizomucor miehei) under stirred or ultrasound condition. Fatty esters containing a pyrrolidine [viz. methyl 8-(cis/trans-5-hexyl-pyrrolidine-2-)octanoate, 2] or N-methyl pyrrolidine [viz. methyl 8-(cis-5-hexyl-N-methyl-pyrrolidine-2-)octanoate, 3] system in the alkyl chain were not hydrolyzed by either CCL or Lipolase, unless conducted in an ultrasonic bath. The hydrolytic activities of the enzymes appeared to be strongly affected by the stereochemistry of the N-heterocyclic ring system. Chemical hydrolysis of compounds 1-3 gave the corresponding fatty acid N-HCl salts. PMID:8569439

Lie Ken Jie, M S; Syed-Rahmatullah, M S

1995-11-01

187

Cyclization of ?3-benzylpalladium intermediates derived from carbene insertion.  

Science.gov (United States)

Migratory insertion of benzylidene carbene ligands into arylpalladium(II) species generates ?(3)-benzylpalladium intermediates that can cyclize to generate five- and six-membered rings with new sp(3) centers. The reaction tolerates a range of arene functional groups and stabilized enolates. The products generated through this reaction are 1-arylindanes and 1-aryltetralins that are common to a range of natural products. PMID:25279856

Gutman, Eugene S; Arredondo, Vanessa; Van Vranken, David L

2014-10-17

188

6?-N-Heterocyclic Substituted Naltrexamine Derivative NAP as a Potential Lead to Develop Peripheral Mu Opioid Receptor Selective Antagonists  

OpenAIRE

A 6?-N-heterocyclic substituted naltrexamine derivative, NAP, was proposed as a peripheral mu opioid receptor (MOR) selective antagonist based on the in vitro and in vivo pharmacological and pharmacokinetic studies. To further validate this notion, several functional assays were carried out to fully characterize this compound. In the charcoal gavage and intestinal motility assay in morphine-pelleted mice, when administered 0.3 mg/kg or higher doses up to 3 mg/kg subcutaneously, NAP significa...

Yuan, Yunyun; Stevens, David L.; Braithwaite, Amanda; Scoggins, Krista L.; Bilsky, Edward J.; Akbarali, Hamid I.; Dewey, William L.; Zhang, Yan

2012-01-01

189

Chelating C4-bound imidazolylidene complexes through oxidative addition of imidazolium salts to palladium(0)  

OpenAIRE

Oxidative addition of donor-functionalised 4-iodoimidazolium salts to palladium(0) provides a selective route for the preparation of abnormal chelating N-heterocyclic carbene complexes and enables the introduction of a variety of donor groups. The activation of the C4 position does not necessitate protection of the imidazolium C2 position, thereby leaving this site available for further modification. While metallation of the unsubstituted C2 position of the N-heterocyclic carbene ligand was u...

Kru?ger, Anneke; Kluser, Evelyne; Mu?ller-bunz, Helge; Neels, Antonia; Albrecht, Martin

2011-01-01

190

Multi-component reactions involving group 6 Fischer carbene complexes: a source of inspiration for future catalytic transformations.  

Science.gov (United States)

The ability of heteroatom stabilized Fischer carbene complexes (FCCs) to participate in multicomponent reactions (MCRs) has become a characteristic of such organometallics, particularly of chromium carbenes. This feature article updates the main results in this field during the last lustrum, highlighting the ability of FCCs for the construction of densely functionalized frameworks, mainly through the successive incorporation of unsaturated moieties (alkynes, CO ligands,…) in a sequential manner. Examples where up to seven components are coupled will be presented. PMID:20835424

Fernández-Rodríguez, Manuel Ángel; García-García, Patricia; Aguilar, Enrique

2010-11-01

191

Metal-ligand cooperation on a diruthenium platform: selective imine formation through acceptorless dehydrogenative coupling of alcohols with amines.  

Science.gov (United States)

Metal-metal singly-bonded diruthenium complexes, bridged by naphthyridine-functionalized N-heterocyclic carbene (NHC) ligands featuring a hydroxy appendage on the naphthyridine unit, are obtained in a single-pot reaction of [Ru2(CH3COO)2(CO)4] with 1-benzyl-3-(5,7-dimethyl-1,8-naphthyrid-2-yl)imidazolium bromide (BIN?HBr) or 1-isopropyl-3-(5,7-dimethyl-1,8-naphthyrid-2-yl)imidazolium bromide (PIN?HBr), TlBF4, and substituted benzaldehyde containing an electron-withdrawing group. The modified NHC-naphthyridine-hydroxy ligand spans the diruthenium unit in which the NHC carbon and hydroxy oxygen occupy the axial sites. All the synthesized compounds catalyze acceptorless dehydrogenation of alcohols to the corresponding aldehydes in the presence of a catalytic amount of weak base 1,4-diazabicyclo[2.2.2]octane (DABCO). Further, acceptorless dehydrogenative coupling (ADHC) of the alcohol with amines affords the corresponding imine as the sole product. The substrate scope is examined with 1 (BIN, p-nitrobenzaldehyde). A similar complex [Ru2(CO)4(CH3COO)(3-PhBIN)][Br], that is devoid of a hydroxy arm, is significantly less effective for the same reaction. Neutral complex 1?a, obtained by deprotonation of the hydroxy arm in 1, is found to be active for the ADHC of alcohols and amines under base-free conditions. A combination of control experiments, deuterium labeling, kinetic Hammett studies, and DFT calculations support metal-hydroxyl/hydroxide and metal-metal cooperation for alcohol activation and dehydrogenation. The bridging acetate plays a crucial role in allowing ?-hydride elimination to occur. The ligand architecture on the diruthenium core causes rapid aldehyde extrusion from the metal coordination sphere, which is responsible for exclusive imine formation. PMID:24715433

Saha, Biswajit; Rahaman, S M Wahidur; Daw, Prosenjit; Sengupta, Gargi; Bera, Jitendra K

2014-05-19

192

N-heterocyclic carbene-palladium(II)-1-methylimidazole complex-catalyzed direct C-H bond arylation of (benz)imidazoles with aryl chlorides.  

Science.gov (United States)

(Benz)imidazoles can be efficiently functionalized by (hetero)aryl chlorides via direct C-H bond arylation in the presence of a well-defined NHC-Pd(II)-Im complex. Under the optimal conditions, various activated, unactivated, and deactivated (hetero)aryl chlorides were successfully applied as the arylating reagents to achieve the 2-(hetero)aryl (benz)imidazoles in acceptable to high yields, giving a facile and alternative methodology for the direct C-H bond arylation of (benz)imidazoles. PMID:24869774

Gu, Zheng-Song; Chen, Wen-Xin; Shao, Li-Xiong

2014-06-20

193

Direct C-H bond arylation of (benzo)oxazoles with aryl chlorides catalyzed by N-heterocyclic carbene-palladium(II)-1-methylimidazole complex.  

Science.gov (United States)

The direct C-H bond arylation of (benzo)oxazoles with aryl chlorides was achieved catalyzed by a well-defined NHC-Pd(II)-Im complex. Under the optimal conditions, various aryl chlorides were successfully applied as the arylating reagents to achieve the 2-aryl (benzo)oxazoles in acceptable to high yields, providing a convenient and alternative method for the direct C-H bond arylation of (benzo)oxazoles and enriching the chemistry of the NHC-Pd(II) complex in organic synthesis. PMID:24670076

Shen, Xiao-Bao; Zhang, Yun; Chen, Wen-Xin; Xiao, Zheng-Kang; Hu, Ting-Ting; Shao, Li-Xiong

2014-04-01

194

N-heterocyclic carbene-palladium(II)-1-methylimidazole complex-catalyzed Suzuki-Miyaura coupling of aryl sulfonates with arylboronic acids.  

Science.gov (United States)

A well-defined NHC-Pd(II)-Im complex 1 was found to be an effective catalyst for the Suzuki-Miyaura coupling of aryl sulfonates including tosylates and phenylsulfonates with arylboronic acids, giving the desired coupling products in good to high yields. Acceptable yields can also be achieved even by using the less reactive mesylates as the substrates. It is worthy of noting here that this is the first example of NHC-Pd(II) complex-catalyzed Suzuki-Miyaura coupling of aryl sulfonates with arylboronic acids, enriching an inexpensive, convenient, and alternative method for the synthesis of biaryl compounds. PMID:22804630

Wang, Zhan-Yong; Chen, Gao-Qi; Shao, Li-Xiong

2012-08-01

195

Suzuki–Miyaura cross-coupling reaction of 1-aryltriazenes with arylboronic acids catalyzed by a recyclable polymer-supported N-heterocyclic carbene–palladium complex catalyst  

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Full Text Available The Suzuki–Miyaura cross-coupling reaction of 1-aryltriazenes with arylboronic acids catalyzed by a recyclable polymer-supported Pd–NHC complex catalyst has been realized for the first time. The polymer-supported catalyst can be re-used several times still retaining high activity for this transformation. Various aryltriazenes were investigated as electrophilic substrates at room temperature to give biaryls in good to excellent yields and showed good chemoselectivity over aryl halides in the reactions.

Guangming Nan

2010-06-01

196

Novel Synthesis of 3-Phenyl-chromen-4-ones Using N-Heterocyclic Carbene as Organocatalyst: An Efficient Domino Catalysis Type Approach  

International Nuclear Information System (INIS)

Herein is reported a simple and efficient synthesis of isoflavones starting from various substituted phenacyl bromides and salicylaldehydes in presence of NHC. The mechanism involved domino catalysis type approach with consumption and regeneration of catalyst in two catalytic cycles. This method proved to be very lucrative and gives very good yield. The method described here represents an environmentally benign alternative to classical approach

197

Novel Synthesis of 3-Phenyl-chromen-4-ones Using N-Heterocyclic Carbene as Organocatalyst: An Efficient Domino Catalysis Type Approach  

Energy Technology Data Exchange (ETDEWEB)

Herein is reported a simple and efficient synthesis of isoflavones starting from various substituted phenacyl bromides and salicylaldehydes in presence of NHC. The mechanism involved domino catalysis type approach with consumption and regeneration of catalyst in two catalytic cycles. This method proved to be very lucrative and gives very good yield. The method described here represents an environmentally benign alternative to classical approach.

Mishra, Priya; Singh, Sarita; Ankit, Preyas; Fatma, Shahin; Singh, Divya; Singh, Jagdamba [Univ. of Allahabad, Allahabad (India)

2013-04-15

198

Umpolung reactions in an ionic liquid catalyzed by electrogenerated N-heterocyclic carbenes. Synthesis of saturated esters from activated ?,?-unsaturated aldehydes.  

Science.gov (United States)

The umpolung reaction of ?,?-unsaturated aldehydes to saturated esters has been carried out in an ionic liquid by organocatalysis of electrogenerated NHC. The roles of solvent, precatalyst and proton donor of the ionic liquid have been verified and good to high yields of esters have been obtained using a "green" and mild methodology. PMID:22517053

Feroci, Marta; Chiarotto, Isabella; Orsini, Monica; Pelagalli, Romina; Inesi, Achille

2012-05-28

199

The Regio- and Stereospecific Intermolecular Dehydrative Alkoxylation of Allylic Alcohols Catalyzed by a Gold(I) N-Heterocyclic Carbene Complex  

OpenAIRE

A 1:1 mixture of (IPr)AuCl [IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidine] and AgClO4 catalyzes the intermolecular dehydrative alkoxylation of primary and secondary allylic alcohols with aliphatic primary and secondary alcohols to form allylic ethers. These transformations are regio- and stereospecific with preferential addition of the alcohol nucleophile at the ?-position of the allylic alcohol syn to the departing hydroxyl group and with predominant formation of the E stereoisomer...

Mukherjee, Paramita; Widenhoefer, Ross A.

2013-01-01

200

The regio- and stereospecific intermolecular dehydrative alkoxylation of allylic alcohols catalyzed by a gold(I) N-heterocyclic carbene complex.  

Science.gov (United States)

A 1:1 mixture of [AuCl(IPr)] (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidine) and AgClO(4) catalyzes the intermolecular dehydrative alkoxylation of primary and secondary allylic alcohols with aliphatic primary and secondary alcohols to form allylic ethers. These transformations are regio- and stereospecific with preferential addition of the alcohol nucleophile at the ?-position of the allylic alcohol syn to the departing hydroxyl group and with predominant formation of the E stereoisomer. The minor ? regioisomer is formed predominantly through a secondary reaction manifold involving regioselective ?-alkoxylation of the initially formed allylic ether rather than by the direct ?-alkoxylation of the allylic alcohol. PMID:23348826

Mukherjee, Paramita; Widenhoefer, Ross A

2013-03-01

201

Synthesis of chiral mono(N-heterocyclic carbene palladium and gold complexes with a 1,1'-biphenyl scaffold and their applications in catalysis  

Directory of Open Access Journals (Sweden)

Full Text Available Axially chiral mono(NHC–Pd(II and mono(NHC–Au(I complexes with one side shaped 1,1'-biphenyl backbone have been prepared from chiral 6,6'-dimethoxybiphenyl-2,2'-diamine. The complexes were characterized by X-ray crystal structure diffraction. The Pd(II complex showed good catalytic activities in the Suzuki–Miyaura and Heck–Mizoroki coupling reactions, and the (S-Au(I complexes also showed good catalytic activities in the asymmetric intramolecular hydroamination reaction to give the corresponding product in moderate ee.

Min Shi

2011-05-01

202

Computational and experimental investigations of CO2 and N2O fixation by sterically demanding N-heterocyclic carbenes (NHC) and NHC/borane FLP systems.  

Science.gov (United States)

The sterically demanding NHCs 1,3-di-tert-butylimidazolin-2-ylidene (1a), 1,3-di-tert-butyl-4,5-dimethylimidazolin-2-yildene (1b), and also the corresponding frustrated Lewis pair combinations 1a,b/B(C6F5)3 react readily with CO2 to form the NHC·CO2 (5a,b) and the NHC·CO2·B(C6F5)3 (9a,b) adducts, respectively. However, N2O activation and isolation of the NHC·N2O adduct (6) was only possible for NHC 1a. On heating, the NHC·N2O adduct 6 degrades to 1a, N2O, N2 and the urea derivative 7. Nevertheless, an NHC·N2O adduct of 1b was obtained with the FLP system 1b/B(C6F5)3. In contrast, for the FLP combination 1a/B(C6F5)3, N2O coordination appears to be slower than the self-deactivation. Hence, only the self-deactivation product 3 was observed under an N2O atmosphere. DFT calculations give insights into the CO2 and N2O activation process with 1a,b and 1a,b/B(C6F5)3. PMID:24217215

Theuergarten, Eileen; Bannenberg, Thomas; Walter, Marc D; Holschumacher, Dirk; Freytag, Matthias; Daniliuc, Constantin G; Jones, Peter G; Tamm, Matthias

2014-01-28

203

Rh-Catalyzed rearrangement of vinylcyclopropane to 1,3-diene units attached to N-heterocycles  

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Full Text Available Dienes embedded in quinolizidine and indolizidine structures can be prepared in four steps from cyclic nitrones and bicyclopropylidene. The key intermediates ?-spirocyclopropanated N-heterocyclic ketones, generated via a domino 1,3-dipolar cycloaddition/thermal rearrangement sequence, were converted by Wittig methylenation to the corresponding vinylcyclopropanes (VCPs, which underwent rearrangement to 1,3-dienes in the presence of the Wilkinson Rh(I complex under microwave heating. The previously unexplored Rh(I-catalyzed opening of the VCP moiety embedded in an azapolycyclic system occurs at high temperature (110–130 °C to afford the corresponding 1,3-dienes in moderate yield (34–53%.

Alberto Brandi

2011-03-01

204

Synthesis, structure and properties of 2D lanthanide coordination polymers based on N-heterocyclic arylpolycarboxylate ligands.  

Science.gov (United States)

The reaction of 3-(2,4-dicarboxyphenyl)-2,6-pyridinedicarboxylic acid (H4dppd) with rare earth nitrates under hydrothermal conditions generated a series of new two-dimensional (2D) coordination polymers, namely {[La(Hdppd)(H2O)2·(H2O)2]n (1), [Ln2(Hdppd)2(H2O)4·(H2O)3]n [Ln = Sm (2), Eu (3)] and [Ln(Hdppd)(H2O)3·H2O]n [Ln = Gd (4), Tb (5), Dy (6), Ho (7), Er (8)] [Hdppd = 3-(2,4-dicarboxyphenyl)-2,6-pyridinedicarboxylic trivalent anion]}. The complexes were characterized by X-ray single-crystal diffraction, infrared spectroscopy, elemental analysis and thermogravimetric analysis. Luminescence spectroscopy of 3 and 5 showed bright red and green luminescences due to the 4f(n)-4f(n) transitions in Eu(3+) and Tb(3+) respectively, although the luminescence lifetime is shortened by non-radiative decay due to the presence of coordinating water molecules. The magnetic properties of 2-8 were measured and discussed. Compound 6 exhibits frequency dependent out-of-phase signals, and ferromagnetic coupling exists in 8. PMID:25331555

You, Li-Xin; Wang, Shu-Ju; Xiong, Gang; Ding, Fu; Meert, Katrien W; Poelman, Dirk; Smet, Philippe F; Ren, Bao-Yi; Tian, Yan-Wen; Sun, Ya-Guang

2014-12-14

205

Steriese en elektroniese effek van metaalbevattende substituente op Fischer-karbeenmetaalbondelverbindings The steric and electronic effects of metal-containing substituents on Fischer carbene metal clusters  

Directory of Open Access Journals (Sweden)

Full Text Available Die insluiting van verskillende oorgangsmetaalfragmente by komplekse word reeds die afgelope aantal jare wyd bestudeer om die effek van die verskillende metaalfragmente op die reaktiwiteit van die ligande en die chemie van die komplekse te ondersoek.Novel Group VI transition metal Fischer carbene complexes containing three different transition metal substituents, all in electronic contact with the carbene carbon atom, were synthesised.

Editorial Office

2009-09-01

206

Synthesis, Crystal Structure and Luminescent Property of A Novel Cd(II) Coordination Polymer with Bis-imidazole Ligand  

International Nuclear Information System (INIS)

The key to the successful design of metal-organic coordination polymers is the judicious selection of organic ligand. Recently, polydentate aromatic nitrogen heterocyclic ligands with five-membered rings have been well-studied in the construction of supramolecular structure for their N-coordinated sites apt to coordinating to transition metals. Similar to six-membered N-heterocyclic ligands, the azole-based five-membered N-heterocyclic ligands, such as imidazoles, triazoles and tetrazoles have been extensively employed in the construction of various coordination polymers with diverse topologies and interesting properties. The bis(azole) ligands in which N-donor azole rings (imidazole, triazole, or tetrazole) are separated by alkyl, (CH2)n, spacers are good choices for flexible bridging ligands. The conformational flexibility of the spacers makes the ligands adaptable to various coordination networks with one-, two-, and three dimensional structures

207

Synthesis, Crystal Structure and Luminescent Property of A Novel Cd(II) Coordination Polymer with Bis-imidazole Ligand  

Energy Technology Data Exchange (ETDEWEB)

The key to the successful design of metal-organic coordination polymers is the judicious selection of organic ligand. Recently, polydentate aromatic nitrogen heterocyclic ligands with five-membered rings have been well-studied in the construction of supramolecular structure for their N-coordinated sites apt to coordinating to transition metals. Similar to six-membered N-heterocyclic ligands, the azole-based five-membered N-heterocyclic ligands, such as imidazoles, triazoles and tetrazoles have been extensively employed in the construction of various coordination polymers with diverse topologies and interesting properties. The bis(azole) ligands in which N-donor azole rings (imidazole, triazole, or tetrazole) are separated by alkyl, (CH{sub 2}){sub n}, spacers are good choices for flexible bridging ligands. The conformational flexibility of the spacers makes the ligands adaptable to various coordination networks with one-, two-, and three dimensional structures.

Zhou, Yong Hong [Huaibei Normal Univ., Huaibei (China)

2013-04-15

208

Syntheses and structural characterizations of 24-membered dimetal (Mn, Ni, Fe) macrocyclic complexes and the C-S bond formation between acetylacetone and a mercapto N-heterocycle.  

Science.gov (United States)

An organic ligand 2,5-di(3-pentanedionylthio)-1,3,4-thiadiazole (H2L) reacts with metal (Mn, Ni, Fe) salts, resulting in 24-membered dimetal macrocyclic complexes [MnL(H2O)(dmso)](2).2dmso, [NiL(H2O)(dmf)](2).2dmf, [MnL(dmf)2]2 and [Fe2L2(solvent)2(SO4)] (solvent=dmso; H2O ; dmf). Di-manganese macrocyclic complexes [MnL(dmf)(dmso)]2 and [MnL(H2O)2](2).6H2O can also be obtained directly by aerobic assembly reaction of MnCl2, dipotassium 1,3,4-thiadiazole-2,5-dithiolate (K2tdadt) and acetylacetone (H2acac) in various solvents, accompanying a C-S bond formation between acetylacetone and the mercapto N-heterocycle. Disulfide has been considered as the intermediate in the assembly reaction. Meanwhile an assembly reaction including MnCl2, 2-mercaptobenzimidazole and H2acac has produced an organic compound 2-(3-pentanedionylthio)benzimidazole with a new C-S bond. These dimetal complexes have similar macrocyclic structures, in which solvent molecules and sulfate coordinate to the octahedral metal in trans-configuration, whereas a pair of water molecules are located in octahedral cis-positions for owing to a small steric effect. A host cavity of sufficiently large size exists in the macrocyclic structure to trap the solvent molecules and the sulfate anion. The IR spectra have been used to assign the solvent molecules trapped and the sulfate anion which is shown as a bridged bidentate ligand. Thermal analyses show the stability of the macrocyclic backbone below 200 degrees C and gradual release processes of the trapped solvent molecules. Decomposition and oxidation of the dimetal macrocycle backbone occur at 300-500 degrees C, resulting in a metal sulfate. Further decomposition led to metal oxide at 500-600 degrees C. PMID:17028714

Zhang, Xiaofeng; Chen, Hui; Ma, Chengbing; Chen, Changneng; Liu, Qiutian

2006-09-01

209

Computational Chemistry Studies on the Carbene Hydroxymethylene  

Science.gov (United States)

A density functional theory computational chemistry exercise on the structure and vibrational spectrum of the carbene hydroxymethylene is presented. The potential energy curve for the decomposition reaction of the carbene to formaldehyde and the geometry of the transition state are explored. The results are in good agreement with recent…

Marzzacco, Charles J.; Baum, J. Clayton

2011-01-01

210

Chemical and thermal stability of N-heterocyclic ionic liquids in catalytic C-H activation reactions.  

Science.gov (United States)

(1)H-NMR spectrum analyses are applied to study the chemical and thermal stability of selected N-heterocyclic ionic liquids within the reaction system that can highly efficiently activate a C-H bond of methane and convert it into the C-O bond in methanol. Our results indicate that under such reaction conditions involving using a powerful Pt-based catalyst and strong acidic solvent, the aromatic ring of an imidazolium cation becomes unstable generating an ammonium ion (NH(4)(+)). Our results also suggest that the instability of the imidazolium ring is more chemically (participation in reactions) than thermally based. Modifications of the aromatic ring structure such as pyrazolium and triazolium cations can increase the chemical/thermal stability of ionic liquids under these reaction conditions. PMID:24942984

Chen, Guanyi; Kang, Shujuan; Ma, Qisheng; Chen, Weiqun; Tang, Yongchun

2014-11-01

211

Syntheses, structural, computational, and thermal analysis of acid-base complexes of picric acid with N-heterocyclic bases.  

Science.gov (United States)

Four new acid-base complexes using picric acid [(OH)(NO2)3C6H2] (PA) and N-heterocyclic bases (1,10-phenanthroline (phen)/2,2';6',2"-terpyridine (terpy)/hexamethylenetetramine (hmta)/2,4,6-tri(2-pyridyl)-1,3,5-triazine (tptz)) were prepared and characterized by elemental analysis, IR, NMR and X-ray crystallography. Crystal structures provide detailed information of the noncovalent interactions present in different complexes. The optimized structures of the complexes were calculated in terms of the density functional theory. The thermolysis of these complexes was investigated by TG-DSC and ignition delay measurements. The model-free isoconversional and model-fitting kinetic approaches have been applied to isothermal TG data for kinetics investigation of thermal decomposition of these complexes. PMID:24011225

Goel, Nidhi; Singh, Udai P

2013-10-10

212

On the synthesis, characterization and reactivity of N-heteroaryl-boryl radicals, a new radical class based on five-membered ring ligands.  

Science.gov (United States)

The synthesis and physical characterization of a new class of N-heterocycle-boryl radicals is presented, based on five membered ring ligands with a N(sp(2) ) complexation site. These pyrazole-boranes and pyrazaboles exhibit a low bond dissociation energy (BDE; B?H) and accordingly excellent hydrogen transfer properties. Most importantly, a high modulation of the BDE(B?H) by the fine tuning of the N-heterocyclic ligand was obtained in this series and could be correlated with the spin density on the boron atom of the corresponding radical. The reactivity of the latter for small molecule chemistry has been studied through the determination of several reaction rate constants corresponding to addition to alkenes and alkynes, addition to O2 , oxidation by iodonium salts and halogen abstraction from alkyl halides. Two selected applications of N-heterocycle-boryl radicals are also proposed herein, for radical polymerization and for radical dehalogenation reactions. PMID:24623534

Tehfe, Mohamad-Ali; Schweizer, Stéphane; Chany, Anne-Caroline; Ysacco, Cédric; Clément, Jean-Louis; Gigmes, Didier; Morlet-Savary, Fabrice; Fouassier, Jean-Pierre; Neuburger, Markus; Tschamber, Théophile; Blanchard, Nicolas; Lalevée, Jacques

2014-04-22

213

Conformationally constrained N-heterocyclic phosphine-diimine with dual functionality.  

Science.gov (United States)

Condensation of octahydro-2,2'-bipyrimidine with P(NMe2)3 gave a 1,3,2-diazaphospholidine-4,5-diimine 4a in which the "open" (exo/exo) conformation of the diimine unit was enforced by incorporation into a tricyclic molecular backbone. The coordination behavior of this potentially ambident ligand was sampled in reactions with ([(nbd)W(CO)4] and [CpCo(CO)2]) and pnictogen halides ECl3 (E = P, As, Sb). While PCl3 reacted under ring metathesis, all other reactions gave isolable complexes of composition (4a)MLn (MLn = W(CO)5, CpCo(CO), AsCl3, SbCl3); attempted recrystallization of the As-adduct yielded a complex (4a)(AsCl3)2 which was also accessible from reaction of 4a with 2 equiv of AsCl3. Single-crystal X-ray diffraction studies revealed that the ligand in [(4a)W(CO)5] and [(4a)CpCo(CO)] binds through its phosphorus lone-pair; [(4a)SbCl3] and [(4a)(AsCl3)2] contain a T-shaped ECl3 unit which binds to the chelating diimine moiety, and associate further via chloride bridges to give centrosymmetric dimers. Reactions of 4a with excess metal substrates gave no evidence that formation of bimetallic complexes with ?-bridging 1?(2)(N,N')-2?P-coordination is feasible; the extra AsCl3 moiety in [(4a)(AsCl3)2] avoids this coordination mode by interacting with the peripheral chlorides of the central core. The observed selectivity suggests that ligand 4a specifically addresses transition metal centers with low positive charge and some back-bonding capacity through the phosphorus lone-pair, and electrophiles that behave essentially as "pure" Lewis acids through the diimine unit. This assumption was confirmed by DFT studies which disclosed further that binding of the first metal center deactivates the opposite binding site and thus strongly inhibits the formation of dinuclear complexes. PMID:23514329

Mourgas, Georgios; Nieger, Martin; Förster, Daniela; Gudat, Dietrich

2013-04-01

214

Microwave-assisted synthesis of N-heterocycles and their evaluation using an acetylcholinesterase immobilized capillary reactor  

Scientific Electronic Library Online (English)

Full Text Available SciELO Brazil | Language: English Abstract in portuguese Os etenoditioacetais polarizados são conhecidos como importantes intermediários em muitos processos de síntese. Neste trabalho, reporta-se a anelação livre de metal de transição de 1,1-bis(tiometil)-2-nitroetileno com hidroxilalquilaminas e alquildiaminas. Esta metodologia permite a síntese direta d [...] e N-heterociclos tais como imidazolidinas, oxazolidinas e benzoxazóis sob irradiação de micro-ondas. Estes compostos foram avaliados como inibidores da acetilcolinesterase usando um reator capilar contendo a enzima imobilizada acoplado a um espectrômetro de massas. Abstract in english Polarized ketene dithioketals have been recognized as useful building blocks in many synthetic operations. In this work, a transition-metal-free annulations of 1,1-bis(thiomethyl)-2-nitroethylene with hydroxylalkylamines or alkyldiamines have been reported. This methodology provides a directed appro [...] ach to N-heterocycles, e.g., imidazolidines, oxazolidines and benzoxazoles under microwave conditions. These compounds were evaluated as acetylcholinesterase inhibitors by using an enzyme immobilized capillary reactor-tandem mass spectrometry.

Diego P., Sangi; Julia L., Monteiro; Kenia L., Vanzolini; Quezia B., Cass; Marcio W., Paixão; Arlene G., Corrêa.

2014-05-01

215

A fullerene-carbene adduct as a crystalline molecular rotor: remarkable behavior of a spherically-shaped rotator.  

Science.gov (United States)

A new fullerene structure was recently obtained from the reaction of a Lewis basic N-heterocyclic carbene (NHC) and the Lewis acidic C60. The molecular features of the zwitterionic adduct can be described as a molecular rotor with the fullerene cage acting as the rotator that spins about one distinct axis given by its C-C single bond linkage with the imidazolium heterocycle stator. A detailed structural analysis of the compound by means of single-crystal X-ray diffraction (XRD) revealed significant differences in the packing motifs of solvent-free and solvent-containing crystals. Variable temperature single-crystal XRD experiments (80 K ? T ? 480 K) carried out to investigate the rotational dynamics of the fullerene group in the higher quality solvent-free structure revealed atomic displacement parameters consistent with fast rotation of the highly symmetric fullerene in the solid state, whereas the imidazolium unit remains in a fixed position and therefore represents the stator. DFT and semiempirical calculations were applied to get insight into the profile of the rotational potential of the fullerene unit, particularly considering interactions with the neighboring molecules in the crystal lattice. The results indicate that the crystal environment leads to the presence of one lowest energy minimum that is connected to seven others that are slightly higher in energy through rotational barriers of approximately 1.5-2.5 kcal mol(-1). PMID:24852314

Lorbach, Andreas; Maverick, Emily; Carreras, Abel; Alemany, Pere; Wu, Guang; Garcia-Garibay, Miguel A; Bazan, Guillermo C

2014-07-01

216

Ionic origins of carbenes in space  

International Nuclear Information System (INIS)

The author reports experiments which suggest that a much higher proportion of the ion chemistry in partially ionized regions in space is directed towards the formation of carbene molecules than has previously been recognized. Plausible reaction paths are identified, which lead to both known and previously unknown carbenes, such as the cyclopropenylidene ring and long carbon-chain molecules. The cyclopropenylidene ring has recently been reported to be ubiquitous in the Galaxy, whereas long carbon-chain molecules have been implicated in the origin of diffuse interstellar lines. (author)

217

Easy access to uranium nucleophilic carbene complexes  

International Nuclear Information System (INIS)

Metathesis reactions of UCl4 with Li2C(Ph2PS)2 in Et2O only afforded the tris-carbene complex [{Li(OEt2)}2-U{=C(Ph2PS)2}3](1), while the bis- and mono-carbene compounds [U{=C(Ph2PS)2}2(THF)2] (2) and [{Li(THF)2}2U{=C(Ph2PS)2}Cl4] (3) were obtained by treatment of UCl4 with Li2C(Ph2PS)2 in a mixture of THF and toluene. The bis- carbene complex 2 was also obtained either from the comproportionation reaction of 1 and UCl4 or protonolysis reaction of U(NEt2)4 with H2C(Ph2PS)2 and was transformed into the mono- carbene complex [U{=C(Ph2PS)2}Cl2(THF)2] (4) by further reaction with UCl4. The utility of these complexes as precursors is illustrated by the synthesis of the bis-cyclopentadienyl derivative [Cp2U{=C(Ph2PS)2}] (5) by treatment of 3 with TlCp. The crystal structures of [U{=C(Ph2PS)2}2(py)2]center dot 1.5py.0.5THF and 5.toluene are reported. (authors)

218

TOF-SIMS analysis of N-heterocyclic compounds-implications for the Rosetta/COSIMA Mission  

Science.gov (United States)

The European Rosetta spacecraft launched on March 2004 will reach the comet 67/P-Churyumov-Gerasimenko in 2014 to perform the most exhaustive study ever achieved on comets (Glassmeier et al. 2007). The COmetary Secondary Ion Mass Analyser (COSIMA) carried on board will focus on chemical analysis of cometary dust grains collected in situ on a variety of metallic targets. COSIMA's goals are the in-situ characterisation of the atomic, molecular and isotopic composition of cometary dust grains. These analyses will provide inorganic and organic data about pristine solar system material (Kissel et al. 2007). COSIMA is a time-of-flight secondary ion mass spectrometer (TOF-SIMS) using an indium liquid metal ion source. Spectra inter-pretation of complex unknown samples with COSIMA instrument will be difficult due to the limited mass resolution (M/M of 2000 at m=100 amu) which excludes unambiguous compounds identification and requires preliminary work with reference samples. To help the interpretation of COSIMA spectra calibration laboratory work is in progress using the twin of the COSIMA flight instrument located at Katlenburg-Lindau (Germany) and the prototype of COSIMA in Orléans (France). Here we report the analysis of 100-400m deposits of organics performed with the COSIMA prototype based in Orléans. The organics are expected to be minor peaks, making their identification not simple. Nucleobase molecules, especially the purine bases, are among the building of the replicating molecules of life (DNA and RNA). Scour the universe in an attempt to find such a natural source of nucleobases is of fundamental significance in considerations of the origin of life (Lazcano Miller 1996) Therefore we focused our work on N-heterocyclic compounds which are believed to be present in comets or which belong to the chemical family of such molecules (Krueger Kissel 2006). Experiments have been performed on nine compounds: adenine, guanine, imidazole, 2-aminopyrimidine, cytosine, isocytosine, thymine, 4-aminopyrimidine and uracil. The first step was to characterize all these pure ni-trogenous organic molecules by their high statistics of positive and negative spectra. Deep examination has then been performed to look for peculiarity: we tried to differentiate i) the fragmentation peaks of a complex molecule from the quasi-molecular peak of pure compounds at the same mass and ii) characteristic features between isomers. For almost all compounds, highly intensive parent-like secondary ions of the general composition [M+H]+ or [M-H]-and characteristically large fragment ions correlated with functional groups were emitted. None possible ambiguities were revealed between the peaks from the actual molecule and the same one in a substructure form: they have two different spectral features. The two molecules were even distinguishable in binary mixture experiments. References: Glassmeier KH et al. (2007) Space Sci. Rev. 128, 1-21. Kissel J et al. (2007) Space Sci. Rev. 128, 823-867 Lazcano A Miller S.L. (1996) Cell 85, 793-798 Krueger F.R. Kissel J. (2006) Eds Springer, p 325-339

Le Roy, Léna; Briois, Christelle; Thirkell, Laurent; Cottin, Hervé; Fray, Nicolas; Poulet, Gilles; Hilchenbach, Martin

219

NHC?SiCl4 : An Ambivalent Carbene-Transfer Reagent.  

Science.gov (United States)

The addition of BCl3 to the carbene-transfer reagent NHC?SiCl4 (NHC=1,3-dimethylimidazolidin-2-ylidene) gave the tetra- and pentacoordinate trichlorosilicon(IV) cations [(NHC)SiCl3 ](+) and [(NHC)2 SiCl3 ](+) with tetrachloroborate as counterion. This is in contrast to previous reactions, in which NHC?SiCl4 served as a transfer reagent for the NHC ligand. The addition of BF3 ?OEt2 , on the other hand, gave NHC?BF3 as the product of NHC transfer. In addition, the highly Lewis acidic bis(pentafluoroethyl)silane (C2 F5 )2 SiCl2 was treated with NHC?SiCl4 . In acetonitrile, the cationic silicon(IV) complexes [(NHC)SiCl3 ](+) and [(NHC)2 SiCl3 ](+) were detected with [(C2 F5 )SiCl3 ](-) as counterion. A similar result was already reported for the reaction of NHC?SiCl4 with (C2 F5 )2 SiH2 , which gave [(NHC)2 SiCl2 H][(C2 F5 )SiCl3 ]. If the reaction medium was changed to dichloromethane, the products of carbene transfer, NHC?Si(C2 F5 )2 Cl2 and NHC?Si(C2 F5 )2 ClH, respectively, were obtained instead. The formation of the latter species is a result of chloride/hydride metathesis. These compounds may serve as valuable precursors for electron-poor silylenes. Furthermore, the reactivity of NHC?SiCl4 towards phosphines is discussed. The carbene complex NHC?PCl3 shows similar reactivity to NHC?SiCl4 , and may even serve as a carbene-transfer reagent as well. PMID:25363673

Böttcher, Tobias; Steinhauer, Simon; Lewis-Alleyne, Lesley C; Neumann, Beate; Stammler, Hans-Georg; Bassil, Bassem S; Röschenthaler, Gerd-Volker; Hoge, Berthold

2015-01-01

220

Oxygenation of ruthenium carbene complexes containing naphthothiophene or naphthofuran: spectroscopic and DFT studies.  

Science.gov (United States)

The aryl propargylic alcohol 1-[2-(thiophen-3-yl)phenyl]prop-2-yn-1-ol (1a) is readily prepared from 2-(thiophen-3-yl)benzaldehyde. In the presence of visible light, treatment of 1a with one-half mole equivalent of [Ru]Cl ([Ru]=Cp(dppe)Ru) (dppe=1,2-bis(diphenylphosphino)ethane) and NH4PF6 in O2 affords the naphtha[2,1-b]thiophene-4-carbaldehyde (4a) in high yields. The cyclization reaction of 1a proceeds through the formation of the carbene complex 2a that contains the naphtha[2,1-b]thiophene ring, which is isolated in a 1:1 stoichiometric reaction. The C-C bond formation between the inner carbon of the terminal triple bond and the heterocyclic ring is confirmed by structure determination of 2a using single-crystal X-ray diffraction analysis. Facile oxygenation of 2a by O2 yields the aldehyde product 4a accompanied by the formation of phosphine oxide of dppe. Oxygen is most likely activated by coordination to the ruthenium center when one PPh2 unit of the dppe ligand dissociates. This dissociated PPh2 unit then reacts with the coordinated oxygen nearby to generate half-oxidized dppe ligand and an unobserved oxo-carbene intermediate. Coupling of the oxo/carbene ligands followed by demetalation then yields 4a. Presumably the resulting complex with the half-oxidized dppe ligand continuously promotes cyclization/oxygenation of 1a to yield the second aldehyde molecule. In alcohol such as MeOH or EtOH, the oxygenation reaction affords a mixture of 4a and the corresponding esters 5a or 5a'. Four other aryl propargylic alcohols 1b-e, which contain thiophen-2-yl, isopropenyl, fur-3-yl, and fur-2-yl, respectively, on the aryl ring are also prepared. Analogous aldehydes 4b-e are similarly prepared from 1b-e, respectively. For oxygenations of 1b, 1d, and 1e in alcohol, mixtures of aldehyde 4, ester 5, and acetal 8 are obtained. The carbene complex 2b obtained from 1b was also characterized by single-crystal X-ray diffraction analysis. The UV/Vis spectra of 2a and 2b consist of absorption bands with a high extinction coefficient. From DFT calculations on 2a and 2b, the visible light is found to populate the LUMO antibonding orbital of mainly Ru=C bonds, thereby weakening the Ru=C bond and promoting the oxygenation/demetalation reactions of 2. PMID:23929767

Tsai, Fu-Yuan; Lo, Ji-Xian; Hsu, Hsin-Tzu; Lin, Ying-Chih; Huang, Shou-Ling; Wang, Ju-Chun; Liu, Yi-Hong

2013-11-01

221

Chelating C4-bound imidazolylidene complexes via oxidative addition of imidazolium salts to palladium(0)  

OpenAIRE

Oxidative addition of donor-functionalised 4-iodoimidazolium salts to palladium(0) provides a selective route for the preparation of chelating abnormal N-heterocylic carbene complexes and enables the introduction of a variety of donor groups. The activation of the C4 position does not necessitate the imidazolium C2 position to be protected, leaving this site available for further modification. While metallation of the unsubstituted C2 position of the N-heterocyclic carbene ligand was unsucces...

Kru?ger, Anneke; Kluser, Evelyne; Mu?ller-bunz, Helge; Neels, Antonia; Albrecht, Martin

2012-01-01

222

Rhodium carbene complexes as versatile catalyst precursors for Si-H bond activation.  

Science.gov (United States)

Rhodium(III) complexes comprising monoanionic C,C,C-tridentate dicarbene ligands activate Si-H bonds and catalyse the hydrolysis of hydrosilanes to form silanols and siloxanes with concomitant release of H(2). In dry MeNO(2), selective formation of siloxanes takes place, while changing conditions to wet THF produces silanols exclusively. Silyl ethers are formed when ROH is used as substrate, thus providing a mild route towards the protection of alcohols with H(2) as the only by-product. With alkynes, comparably fast hydrosilylation takes place, while carbonyl groups are unaffected. Further expansion of the Si-H bond activation to dihydrosilanes afforded silicones and polysilyl ethers. Mechanistic investigations using deuterated silane revealed deuterium incorporation into the abnormal carbene ligand and thus suggests a ligand-assisted mechanism involving heterolytic Si-H bond cleavage. PMID:22162081

Krüger, Anneke; Albrecht, Martin

2012-01-01

223

From bis(N-Alkylimidazole) to bis(NH–NHC) in rhenium carbonyl complexes  

OpenAIRE

Two birds, one stone: A deprotonation–protonation sequence using only one molar equivalent of base and acid transforms two N-alkylimidazole molecules into N-heterocyclic carbene ligands (see scheme; black?C, gray?H, green?F, blue?N, red?O, cyan?Re, yellow?S).

Huertos, Miguel A.; Pe?rez, Julio; Riera, Luci?a

2010-01-01

224

Methylation of secondary amines with dialkyl carbonates and hydrosilanes catalysed by iron complexes.  

Science.gov (United States)

Methylation of secondary amines was achieved using dimethyl carbonate or diethyl carbonate as the C1 source under the catalysis of well-defined half-sandwich iron complexes bearing an N-heterocyclic carbene ligand. The reaction proceeded under mild conditions in the presence of hydrosilanes as the reductants, and the amines were obtained with good to excellent isolated yields. PMID:25285339

Zheng, Jianxia; Darcel, Christophe; Sortais, Jean-Baptiste

2014-11-25

225

Easy access to uranium nucleophilic carbene complexes  

Energy Technology Data Exchange (ETDEWEB)

Metathesis reactions of UCl{sub 4} with Li{sub 2}C(Ph{sub 2}PS){sub 2} in Et{sub 2}O only afforded the tris-carbene complex [{l_brace}Li(OEt{sub 2}){r_brace}{sub 2}-U{l_brace}=C(Ph{sub 2}PS){sub 2{r_brace}3}](1), while the bis- and mono-carbene compounds [U{l_brace}=C(Ph{sub 2}PS){sub 2{r_brace}2}(THF){sub 2}] (2) and [{l_brace}Li(THF){sub 2{r_brace}2}U{l_brace}=C(Ph{sub 2}PS){sub 2}{r_brace}Cl{sub 4}] (3) were obtained by treatment of UCl{sub 4} with Li{sub 2}C(Ph{sub 2}PS){sub 2} in a mixture of THF and toluene. The bis- carbene complex 2 was also obtained either from the comproportionation reaction of 1 and UCl{sub 4} or protonolysis reaction of U(NEt{sub 2}){sub 4} with H{sub 2}C(Ph{sub 2}PS){sub 2} and was transformed into the mono- carbene complex [U{l_brace}=C(Ph{sub 2}PS){sub 2}{r_brace}Cl{sub 2}(THF){sub 2}] (4) by further reaction with UCl{sub 4}. The utility of these complexes as precursors is illustrated by the synthesis of the bis-cyclopentadienyl derivative [Cp{sub 2}U{l_brace}=C(Ph{sub 2}PS){sub 2}{r_brace}] (5) by treatment of 3 with TlCp. The crystal structures of [U{l_brace}=C(Ph{sub 2}PS){sub 2}{r_brace}2(py){sub 2}]center dot 1.5py.0.5THF and 5.toluene are reported. (authors)

Tourneux, Jean-Christophe; Berthet, Jean-Claude; Thuery, Pierre; Ephritikhine, Michel [CEA, IRAMIS, SIS2M, CNRSUMR3299, 91191, Gif-sur-Yvette (France); Mezailles, Nicolas; Le Floch, Pascal [Laboratoire ' Heteroelements et Coordination' , Ecole Polytechnique, CNRS, 91128, Palaiseau Cedex (France)

2010-07-01

226

Mutagenic activity of some platinum and ruthenium complexes with N-heterocyclic ligands in salmonella typhimurium Ta 1530 and Ta 98  

International Nuclear Information System (INIS)

The mutagenic activity of some platinum and ruthenium complexes with 2,2'-biquinoline (b iq) and 2-(2' -pyridyl) quinoline (p q) was examined in strains of salmonella typhimurium Ta 1530 and Ta 98. The complexes cis-[Pt(Nn)X2)] (Nn=b iq, X=CI; Nn=p q, X=CI, Br), Pt3(b iq)2I6 and me r-[Ru(b iq)2CI3].2H2O exhibit significant mutagenic activity while me r-[Ru(p q)CI3(C2H5OH)], [Ru(p q)CI4]. 3a-2a and [Ru(p q) (DMSO)2CI2].0.5H2O show much weaker mutagenic activity. the platinum complexes appear to be more active via induction of frameshift than base substitution mutation while me r-[Ru(b iq)2CI3].2H2O is highly mutagenic via base substitution. (authors). 16 refs., 2 tabs

227

Direct C-H bond arylation of fluorenes with aryl chlorides catalyzed by N-heterocyclic carbene-palladium(II)-1-methylimidazole complex and further transformation of the products in a one-pot procedure.  

Science.gov (United States)

We report here the NHC-Pd(II)-Im complex 1-catalyzed direct C-H bond functionalization of the C9 position of fluorenes with aryl chlorides and further transformation of the resulting products in a one-pot procedure. Under the optimal conditions, arylated fluorenes can be obtained in moderate to almost quantitative yields using various activated and unactivated (hetero)aryl chlorides as the arylating reagents. Furthermore, if the mixture from the arylation reaction is exposed to air, the C9-oxidized products can be obtained in acceptable to good yields in a one-pot procedure. In addition, alkyl groups can also be efficiently introduced to the above mixture from the arylation reaction, producing further C9-alkylated products in good to almost quantitative yields in a one-pot procedure, thus providing an expedient, inexpensive and practical strategy for the mono- and di-functionalization of fluorenes. PMID:25231668

Ji, Ya-Yun; Lu, Li-Li; Shi, Yu-Chun; Shao, Li-Xiong

2014-11-14

228

N-heterocyclic carbene-palladium(II)-1-methylimidazole complex catalyzed ?-arylation of oxindoles with aryl chlorides and aerobic oxidation of the products in a one-pot procedure.  

Science.gov (United States)

NHC-Pd(II)-Im complex 1 was found to be an effective catalyst for the ?-arylation of unprotected oxindoles with aryl chlorides to give products 4 in 44-98% yields under a N2 atmosphere. Furthermore, if the reactions were first performed under conditions identical to those for the ?-arylation reaction for 12 h and then exposed to air for another 3 h, 3-aryl-3-hydroxy-2-oxindoles 5 can be obtained in 49-84% yields in a one-pot procedure. PMID:23451859

Xiao, Zheng-Kang; Yin, Hui-Ying; Shao, Li-Xiong

2013-03-15

229

Two-component, three-molecule reactions between 2,3-dihydroinden-1-ones and aryl chlorides catalyzed by an N-heterocyclic carbene-palladium(II)-1-methylimidazole complex.  

Science.gov (United States)

The two-component, three-molecule reactions between 2,3-dihydroinden-1-ones and aryl chlorides were achieved in the presence of an NHC-Pd(II)-Im complex. Under the optimal conditions, all reactions performed well to give the corresponding products in moderate to high yields. PMID:24228996

Yin, Hui-Ying; Liu, Meng-Yuan; Shao, Li-Xiong

2013-12-01

230

Three-coordinate cobalt(IV) and cobalt(V) imido complexes with N-heterocyclic carbene ligation: synthesis, structure, and their distinct reactivity in C-H bond amination.  

Science.gov (United States)

The reaction of the cobalt(0) alkene complex [(IMes)Co(?(2):?(2)-dvtms)] (1) (IMes = 1,3-bis(1',3',5'-trimethylphenyl)imidazol-2-ylidene, dvtms = divinyltetramethyldisiloxane) with 2 equiv of DippN3 (Dipp = 2,6-diisopropylphenyl) afforded the cobalt(IV) imido complex [(IMes)Co(NDipp)2] (2), which could be oxidized by [Cp2Fe][BAr(F)4] (Ar(F) = 3,5-di(trifluoromethyl)phenyl) to give the cobalt(V) imido species [(IMes)Co(NDipp)2][BAr(F)4] (3). The molecular structures of all these complexes were established by single-crystal X-ray diffraction studies. Characterization data and theoretical calculations suggest ground spin states of S = (1)/2 and S = 0 for the cobalt(IV) and cobalt(V) species, respectively. When heated, the cobalt(IV) imido species was converted to a cobalt(II) diamido complex via an intramolecular C-H bond amination reaction, but the cobalt(V) species was stable under similar conditions. The different outcomes suggest that a high oxidation state does not guarantee C-H bond activation reactivity of late-transition-metal imido species. PMID:25330361

Zhang, Long; Liu, Yuesheng; Deng, Liang

2014-11-01

231

Sacrificial spacer and non-covalent routes toward the molecular imprinting of 'poorly-functionalized' N-heterocycles  

International Nuclear Information System (INIS)

A comparison of three different methods for the imprinting of small aromatic heterocycles containing only a single nitrogen atom, for the preparation of specific analytical phases, was carried out. A conventional non-covalent approach to the imprinting of pyridine using methacrylic acid as the functional monomer was compared with two sacrificial spacer methods, in which heterocycles were imprinted as covalent template analogues. The results of binding experiments showed that discrimination based on ligand size was possible when polymers were prepared using a silyl ester-based template. The most selective polymer was able to bind pyridine in preference to quinoline or acridine which is opposite to the trend predicted by the pKHB values for the three ligands. Curve fitting of the isotherm for pyridine binding to this polymer to the Langmuir model gave an approximate Kd of 1.1±0.1 mM and a binding site concentration of 57±2 mmol g-1. Acridine binding did not show saturation behaviour and was non-specific and cooperative in nature

232

Chemistry of transition metal carbene complexes: nucleophilic substitution reactions of cyanamide anion to Fischer carbene complexes.  

Science.gov (United States)

Rate constants for the reactions of cyanamide anion N?C-NH(-) with Fischer carbene complexes of the type (CO)(5)M=C(XR)C(6)H(4)Z with M = Cr and W, XR = SMe and OMe, Z = NMe(2), OMe, Me, H, F, Cl and CF(3) in 50% MeCN-50% water (v/v) at 25 °C are reported. N?C-NH(-) shows a much higher reactivity towards these carbene complexes than OH(-), primary aliphatic amines (e.g. n-butylamine) or secondary alicyclic amines (e.g. piperidine) but is slightly less reactive than thiolate ions (e.g. HOCH(2)CH(2)S(-)). The alkoxy carbene complexes were found to react faster than the thiomethyl derivatives, consistent with previous findings for alkoxide ion, CH(CN)(2)(-), OH(-), amine and thiolate ion nucleophiles. Hammett ? values are 3.00 ± 0.08 (k(1)) and 2.98 ± 0.08 (k(2)) for Cr-OMe-Z-N?C-NH(-) reactions and 0.94 ± 0.05 (k(1)) for Cr-SMe-Z-N?C-NH(-) reactions. The ? values for the reaction of Cr-OMe-Z and Cr-SMe-Z with CH(CN)(2)(-) and DABCO (1,4-diazabicyclo[2.2.2]octane) (in 50% MeCN-50% H(2)O (v/v) are comparable to the present reactions. The much higher reactivity and hence much higher ? value for methoxy carbene complexes over the corresponding thiomethyl derivatives is best explained by considering the substituent effects not only on the transition state (TS) but also on the reactant, and consistent with the previously observed pattern. Higher k(1)(W)/k(1)(Cr) ratios for (thiomethyl)carbene complexes as well as methoxy and ethoxycarbene complexes are related to the intrinsic rate constant which is higher for tungsten-carbene complexes than the corresponding Cr ones resulting in an enhanced ratio. This can also be explained by considering the electronegativity of Cr and W, which is higher for the latter; as a result the negative charge on the central metal atom is more localized in case of W causing destabilization of the TS and hence higher reactivity. PMID:23076623

Gangopadhyay, Sumana; Mistri, Tarun; Dolai, Malay; Alam, Rabiul; Ali, Mahammad

2013-01-14

233

Cyclic Alkyl Amino Carbene (CAAC) Ruthenium Complexes as Remarkably Active Catalysts for Ethenolysis.  

Science.gov (United States)

An expanded family of ruthenium-based metathesis catalysts bearing cyclic alkyl amino carbene (CAAC) ligands was prepared. These catalysts exhibited exceptional activity in the ethenolysis of the seed-oil derivative methyl oleate. In many cases, catalyst turnover numbers (TONs) of more than 100?000 were achieved, at a catalyst loading of only 3?ppm. Remarkably, the most active catalyst system was able to achieve a TON of 340?000, at a catalyst loading of only 1?ppm. This is the first time a series of metathesis catalysts has exhibited such high performance in cross-metathesis reactions employing ethylene gas, with activities sufficient to render ethenolysis applicable to the industrial-scale production of linear ?-olefins (LAOs) and other terminal-olefin products. PMID:25522160

Marx, Vanessa M; Sullivan, Alexandra H; Melaimi, Mohand; Virgil, Scott C; Keitz, Benjamin K; Weinberger, David S; Bertrand, Guy; Grubbs, Robert H

2015-02-01

234

Fundamental spectroscopic studies of carbenes and hydrocarbon radicals  

Energy Technology Data Exchange (ETDEWEB)

Highly reactive carbenes and carbon-chain radicals are studied at millimeter wavelengths by observing their rotational spectra. The purpose is to provide definitive spectroscopic identification, accurate spectroscopic constants in the lowest vibrational states, and reliable structures of the key intermediates in reactions leading to aromatic hydrocarbons and soot particles in combustion.

Gottlieb, C.A.; Thaddeus, P. [Harvard Univ., Cambridge, MA (United States)

1993-12-01

235

The reactions of anthronylidene carbene with some heterocyclic compounds  

International Nuclear Information System (INIS)

The action of the anthronylidene carbene, generated by photochemical decomposition of 9-diazo 10-anthron, on four heterocyclic compounds (furan, thiophene, 1-methyl-pyrrole and 2,5-dihydrofuran) has been examined. Two classical carbene reactions have been observed: the addition on double bond (furan, thiophene, 1-methylpyrrole) and hydrogen atom abstraction of the heterocyclic compound (2,5-dihydrofuran). In the case of furan and thiophene, the cyclo-propanic compound resulting from the addition is spontaneously transformed into an ethylenic derivative by valence isomerization. The furan derivative undergoes a cis-trans isomerization, while the thiophene one undergoes an extra carbene attack. In the case of 1-methylpyrrole, the corresponding cyclo-propanic compound undergoes a ring cleavage, followed by a hydrogen atom migration leading to the formation of a substituted anthron. Only an allylic hydrogen atom selective abstraction of heterocyclic compound takes place in the reaction of anthronylidene carbene with 2,5-dihydrofuran. The asymmetrical coupling of radicals so obtained yields the corresponding substituted anthron. (author)

236

Phosphaalkene vs. phosphinidene: the nature of the P-C bond in carbonyl-decorated carbene ? PPh adducts.  

Science.gov (United States)

Treatment of dichlorophenylphosphine with two equivalents of carbonyl-decorated carbenes results in a two-electron reduction of the phosphorus centre concomitant with carbene oxidation to afford novel phosphaalkenes as confirmed via crystallographic, spectroscopic, and DFT analyses. PMID:23925541

Rodrigues, Roberta R; Dorsey, Christopher L; Arceneaux, Chelsee A; Hudnall, Todd W

2014-01-01

237

Stereoselective synthesis of 5-substituted pyrrolo[1,2-c]imidazol-3-ones: access to annulated chiral imidazol(in)ium salts.  

Science.gov (United States)

A two-step synthesis of N-heterocyclic carbene (NHC) precatalysts by diastereoselective or enantioselective lithiation of pyrrolo[1,2-c]imidazol-3-ones followed by POCl(3)-induced salt formation is described. The resulting 3-chloro-pyrroloimidazol(in)ium salts may be coordinated to palladium(II) upon NHC generation with t-BuLi at low temperature. The method may facilitate exploitation of these compounds as chiral organocatalysts or ligands in metal catalysis. PMID:19961188

Metallinos, Costa; Xu, Shufen

2010-01-01

238

Fluoride, bifluoride and trifluoromethyl complexes of iridium(i) and rhodium(i).  

Science.gov (United States)

Herein we report robust methods for the preparation and full characterisation of a range of Ir(i) and Rh(i) fluoride and bifluoride complexes using N-heterocyclic carbenes (NHCs) as ancillary ligands. The processes that link the fluoride and the bifluoride species are investigated and reports of the first Ir-bifluoride and Ir(i)-NHC and Rh(i)-NHC trifluoromethyl complexes are revealed. PMID:25358404

Truscott, Byron J; Nahra, Fady; Slawin, Alexandra M Z; Cordes, David B; Nolan, Steven P

2015-01-01

239

Regioselective allene hydroarylation via one-pot allene hydrosilylation/Pd-catalyzed cross-coupling.  

Science.gov (United States)

Advances in hydroarylation have been achieved by the development of a one-pot regioselective allene hydrosilylation/Pd(0)-catalyzed cross-coupling protocol. The regioselectivity is primarily governed by N-heterocyclic carbene (NHC) ligand identity in the hydrosilylation step and is preserved in the subsequent cross-coupling reaction. This methodology affords streamlined access to functionalized 1,1-disubstituted alkenes with excellent regiocontrol. PMID:25275880

Miller, Zachary D; Montgomery, John

2014-10-17

240

Synthesis and characterization of distonic radical, biradical and carbene ions  

OpenAIRE

The synthesis and characterization of distonic radical, biradical and carbene anions were studied with the use of a flowing afterglow-triple quadrupole instrument. A new synthetic strategy for gas-phase synthesis of negative ions with multiple open valence sites was developed. This method, which is based on the reaction of organic anions with molecular fluorine, allowed effective synthesis of a series of negative ions of biradicals and triradicals that are of great theoretical importance. The...

Hu, Jun

1997-01-01

241

Deprotonation of a borohydride: synthesis of a carbene-stabilized boryl anion.  

Science.gov (United States)

An acidic hydride! Thanks to the presence of a ?-acceptor cyclic alkyl amino carbene and of two electron-withdrawing nitrile groups, a borohydride reacts with a base to give a carbene-stabilized boryl anion, which reacts with carbon and metal electrophiles at the boron center. Dipp = 2,6-diisopropylphenyl, KHMDS = potassium bis(trimethylsilyl)amide. PMID:23765789

Ruiz, David A; Ung, Gaël; Melaimi, Mohand; Bertrand, Guy

2013-07-15

242

Assembly and tunable luminescence of lanthanide-organic frameworks constructed from 4-(3,5-dicarboxyphenyl)pyridine-2,6-dicarboxylate ligand  

International Nuclear Information System (INIS)

Highlights: ? A new N-heterocycle muticarboxylate ligand 4-(3,5-dicarboxyphenyl)pyridine-2,6-dicarboxylic acid (H4dpda) was developed. ? Several lanthanide metal–organic frameworks (Ln = Eu, Gd, Tb, Dy) and Eu/Tb mixed MOFs were synthesized. ? The ligand H4dpda can provide efficient sensitization for the lanthanide ion Tb(III) and Eu(III). ? Luminescence color of the mixed MOFs can be easily tuned by varying the molar ratios of Eu(III)/Tb(III). - Abstract: A novel N-heterocycle multicarboxylate ligand 4-(3,5-dicarboxyphenyl)pyridine-2,6-dicarboxylic acid (H4dpda) was synthesized, and then reacted with lanthanide salts to yield a series of lanthanide metal–organic frameworks, [Ln(Hdpda)(H2O)4]·0.5(H2O) (Ln = Eu(1), Gd(2), Tb(3), Dy(4)) and [Tb1?xEux(Hdpda)(H2O)4]·0.5(H2O) (x = 0.1–10 mol%). Single crystal X-ray diffraction and powder XRD patterns confirm these MOFs are isostructural. Luminescent measurements suggest that the ligand can provide efficient sensitization for the lanthanide ion Tb(III) and Eu(III) in the mixed lanthanide MOFs. Additionally, the luminescence color of the mixed MOFs can be easily tuned from green to green–yellow, yellow, orange, red–orange and red by varying the molar ratio of Eu(III)/Tb(III).

243

A Theoretical Study of Uranium(IV) Bis-Methyl Complexes: Towards the Predictive Formation of a Transient Uranium(IV) Carbene Complex  

Energy Technology Data Exchange (ETDEWEB)

In summary, we have shown that the first step of ortho C-H activation of pyridine N-oxide mediated by L{sub 2}U(IV)-(CH{sub 3}){sub 2} can occur with the three ligands L (Cp, CpTMS, and Cp*) to form an {eta}{sup 2}(C, O) ortho-metallated complex. Also, we have shown that for L=Cp and CpTMS it is possible to form a very stable uranium (VI) formaldehyde complex going through an uranium(IV) Schrock-type carbene intermediate. This is in contrast with the ligand Cp*: the formation of uranium(IV) formaldehyde complex is not possible with the Cp* owing to a high barrier at the H{sub 2}C-O repulsive coupling transition state. This high barrier can be explained by the steric hindrance of Cp* compared with Cp and CpTMS. Moreover, the electronic repulsion between the two negative charges (carbene and oxygen) is enhanced in the case of Cp*. (authors)

Yahia, A.; Castro, L.; Maron, L. [Univ Toulouse 3, Lab Phys and Chim Nanoobjets, INSA, F-31077 Toulouse (France); Yahia, A. [Univ Montpellier 2, Inst Chim Separat and Marcoule, UMR 5257, CEA, CNRS, ENSCM, Ctr Marcoule, F-30207 Bagnols Sur Ceze (France)

2010-07-01

244

Conformation analysis of triphenylphosphine in trans and cis triphenylphosphine-substituted Fischer carbene complexes  

Science.gov (United States)

The synthesis and selected crystal structures of novel (M = Cr or Mo) and known (M = W) [(CO)4(PPh3)M = C(OEt)R] complexes, M = Cr, R = 2-thienyl (1), 2-furyl (2); M = Mo, R = 2-thienyl (3), 2-furyl (4); M = W, R = 2-thienyl (5), 2-furyl (6), are presented. Experimental crystal structures and DFT calculations of selected trans- and cis-triphenylphosphine-substituted Fischer carbene complexes, illustrate that the minimum energy conformation of triphenylphosphine (PPh3) in octahedral [(CO)4(PPh3)ML]-complexes generally have distinct features that can be described in terms of the "plane of nadir energy", a plane linking all points of minimum steric compression between the ligands. The generally observed orientation of PPh3 involves a correlated feathering of the phenyl groups with the PCipso bond of one phenyl group orientated near parallel to the nadir plane, and a meta carbon (Cm) of the other two phenyl groups orientated as near as possible to the nadir plane, orthogonal to the first. Although the orientation of PPh3 in 6-trans, [(CO)4(PPh3)W = C(OEt)2-furyl], deviates from this, DFT correctly calculated the unexpected and not generally observed PPh3 orientation.

Landman, Marilé; Levell, Tamzyn; van Rooyen, Petrus H.; Conradie, Jeanet

2014-05-01

245

SO2- yet another two-faced ligand.  

Science.gov (United States)

Experimentally known adducts of SO2 with transition metal complexes have distinct geometries. In the present paper, we demonstrate by a bonding analysis that this is a direct consequence of sulfur dioxide acting as an acceptor in one set, square-planar complexes of d(8) and linear two-coordinated complexes of d(10) transition metals, and as a donor with other compounds, well-known paddle-wheel [Rh2(O2CCF3)4] and square-pyramidal [M(CO)5] (M = Cr, W) complexes. Bonding energy computations were augmented by the natural bond orbital (NBO) analysis and energy decomposition analysis (EDA). When the SO2 molecule acts as an acceptor, bonding in the bent coordination mode to the axial position of the d(8) or the d(10) metal center, the dominant contributor to the bonding is LAO(S) (Lewis Acidic Orbital, mainly composed of the px-orbital of the S atom) as an acceptor, while a dz(2) orbital centered on the metal is the corresponding donor. In contrast, the distinct collinear (or linear) coordination of the SO2 bound at the axial position of [Rh2(O2CCF3)4] and/or [M(CO)5] is associated with a dominant donation from a lone pair localized on the sulfur atom, ?*(Rh-Rh) and/or empty LAO(M) (mainly composed of the dz(2) orbital of the metal), respectively, acting as an acceptor orbital. The donor/acceptor capabilities of the SO2 molecule were also checked in adducts with organic Lewis acids (BH3, B(CF3)3) and Lewis bases (NH3, N(CH3)3, N-heterocyclic carbene). PMID:25475579

Li, Jingbai; Rogachev, Andrey Yu

2015-01-21

246

On the stability of perfluoroalkyl-substituted singlet carbenes: a coupled-cluster quantum chemical study.  

Science.gov (United States)

A series of trifluoromethyl-substituted carbenes R-C(:)-CF3 (R = NMe2, OMe, F, PMe2, P(NMe2)2, P(N(Pr-i)2)2, SMe, Cl); (dimethylamino)(perfluoroalkyl)carbenes Me2N-C(:)-R (R = CF3, C2F5, n-C3F7, i-C3F7, and t-C4F9) and symmetrically substituted carbenes R-C(:)-R (R = NMe2, OMe, F, PMe2, SMe, Cl) have been investigated by means of quantum chemistry methods. Different levels of approximation were used, including the CCSD(T) approach also known in quantum chemistry as the "golden standard", in combination with three different basis sets (TZVP, cc-pVDZ, cc-pVTZ). Relative stabilities of carbenes have been estimated using the differences between the singlet and triplet ground state energies (?EST) and energies of the hydrogenation reaction for the singlet and triplet ground states of the carbenes. The latter seem to correlate better with stability of carbenes than the ?EST values. The (13)C NMR chemical shifts of the methylidene carbon indicate the more high-field chemical shift values in the known, isolable carbenes compared to the unstable ones. This is the first report on the expected chemical shifts in the highly unstable singlet carbenes. Using these criteria, some carbene structures from the studied series (as, for instance, Me2N-C(:)-CF3, Me2N-C(:)-C3F7-i) are proposed as good candidates for the experimental preparation. PMID:24471524

Rozhenko, Alexander B; Schoeller, Wolfgang W; Leszczynski, Jerzy

2014-02-27

247

Reactivity of Tp(Me2) -supported yttrium alkyl complexes toward aromatic N-heterocycles: ring-opening or C-C bond formation directed by C-H activation.  

Science.gov (United States)

Unusual chemical transformations such as three-component combination and ring-opening of N-heterocycles or formation of a carbon-carbon double bond through multiple C-H activation were observed in the reactions of Tp(Me2) -supported yttrium alkyl complexes with aromatic N-heterocycles. The scorpionate-anchored yttrium dialkyl complex [Tp(Me2) Y(CH2 Ph)2 (THF)] reacted with 1-methylimidazole in 1:2 molar ratio to give a rare hexanuclear 24-membered rare-earth metallomacrocyclic compound [Tp(Me2) Y(?-N,C-Im)(?(2) -N,C-Im)]6 (1; Im=1-methylimidazolyl) through two kinds of C-H activations at the C2- and C5-positions of the imidazole ring. However, [Tp(Me2) Y(CH2 Ph)2 (THF)] reacted with two equivalents of 1-methylbenzimidazole to afford a C-C coupling/ring-opening/C-C coupling product [Tp(Me2) Y{?(3) -(N,N,N)-N(CH3 )C6 H4 NHCH?C(Ph)CN(CH3 )C6 H4 NH}] (2). Further investigations indicated that [Tp(Me2) Y(CH2 Ph)2 (THF)] reacted with benzothiazole in 1:1 or 1:2 molar ratio to produce a C-C coupling/ring-opening product {(Tp(Me2) )Y[?-?(2) :?(1) -SC6 H4 N(CH?CHPh)](THF)}2 (3). Moreover, the mixed Tp(Me2) /Cp yttrium monoalkyl complex [(Tp(Me2) )CpYCH2 Ph(THF)] reacted with two equivalents of 1-methylimidazole in THF at room temperature to afford a trinuclear yttrium complex [Tp(Me2) CpY(?-N,C-Im)]3 (5), whereas when the above reaction was carried out at 55?°C for two days, two structurally characterized metal complexes [Tp(Me2) Y(Im-Tp(Me2) )] (7; Im-Tp(Me2) =1-methyl-imidazolyl-Tp(Me2) ) and [Cp3 Y(HIm)] (8; HIm=1-methylimidazole) were obtained in 26 and 17?% isolated yields, respectively, accompanied by some unidentified materials. The formation of 7 reveals an uncommon example of construction of a C?C bond through multiple C-H activations. PMID:24339020

Yi, Weiyin; Zhang, Jie; Huang, Shujian; Weng, Linhong; Zhou, Xigeng

2014-01-13

248

Nitrene-carbene-carbene rearrangement. Photolysis and thermolysis of tetrazolo[5,1- a ]phthalazine with formation of 1-phthalazinylnitrene, o-cyanophenylcarbene, and phenylcyanocarbene  

DEFF Research Database (Denmark)

1-Azidophthalazine 9A is generated in trace amount by mild FVT of tetrazolo[5,1-a]phthalazine 9T and is observable by its absorption at 2121 cm-1 in the Ar matrix IR spectrum. Ar matrix photolysis of 9T/9A at 254 nm causes ring opening to generate two conformers of (o-cyanophenyl) diazomethane 11 (2079 and 2075 cm-1), followed by (o-cyanophenyl)carbene 312, cyanocycloheptatetraene 13, and finally cyano(phenyl)carbene 314 as evaluated by IR spectroscopy. The two carbenes 312 and 314 were observed by ESR spectroscopy (D|hc = 0.5078, E|hc = 0.0236 and D|hc = 0.6488, E|hc = 0.0195 cm-1, respectively). The rearrangement of 12 â., 13 â., 14 constitutes a carbene-carbene rearrangement. 1-Phthalazinylnitrene 310 is observed by means of its UV-vis spectrum in Ar matrix following FVT of 9 above 550 C. Rearrangement to cyanophenylcarbenes also takes place on FVT of 9 as evidenced by observation of the products of ring contraction, viz., fulvenallenes and ethynylcyclopentadienes 16-18. Thus the overall rearrangement 10 ?11 ? 12 â., 13 â., 14 can be formulated. © 2013 American Chemical Society.

HØj, Martin; Kvaskoff, David

2014-01-01

249

Selectivity control in alkylidene carbene-mediated C-H insertion and allene formation.  

Science.gov (United States)

Regioselectivity of alkylidene carbene-mediated C-H insertion was explored utilizing electronic, conformational, steric, and stereoelectronic effects. Relying on these factors, highly regio- and chemoselective carbene insertion reaction of C-H bonds in different environments could be obtained. The observed selectivity clearly indicates that an electronic effect plays a more important role than steric effect. In general, C-H bonds in conformationally rigid cyclic environments are less reactive than those in acyclic systems toward carbene insertion, and in this situation, a competing intermolecular reaction between alkylidene carbene and trimethylsilyldiazomethane led to the formation of allenylsilanes. The formation of allenylsilane becomes more favorable as the concentration of reaction becomes higher, as well as the C-H bonds undergoing insertion becomes electronically and conformationally deactivated. PMID:21244086

Zheng, Jun-Cheng; Yun, Sang Young; Sun, Chunrui; Lee, Nam-Kyu; Lee, Daesung

2011-02-18

250

Oxidative Addition at a Carbene Center: Synthesis of an Iminoboryl-CAAC Adduct.  

Science.gov (United States)

The reaction of a cyclic (alkyl)(amino)carbene (CAAC) with dichloro- and dibromobis(trimethylsilyl)aminoborane results in the formation of haloiminoborane-CAAC adducts. When the iodo analogue is used, an oxidative addition at the carbene center affords a cationic iminoboryl-CAAC adduct, featuring a boron-nitrogen triple bond. Similar salts are also obtained by halide abstraction from the chloro- and bromoiminoborane-CAAC adducts. The reactivity of all of these compounds towards CO2 is discussed. PMID:25414006

Dahcheh, Fatme; Stephan, Douglas W; Bertrand, Guy

2015-01-01

251

Synthesis and Structure-Activity Correlation Studies of Metal Complexes of α-N-heterocyclic Carboxaldehyde Thiosemicarbazones in Shewanella oneidensis  

Directory of Open Access Journals (Sweden)

Full Text Available This investigation involved the synthesis of metal complexes to test the hypothesis that structural changes and metal coordination in pyridine thiosemicarbazones affect cell growth and cell proliferation in vitro. Thiosemicarbazones are well known to possess antitumor, antiviral, antibacterial, antimalarial, and other activities. Extensive research has been carried out on aliphatic, aromatic, heterocyclic and other types of thiosemicarbazones and their metal complexes. Due to the pronounced reactivity exhibited by metal complexes of heterocyclic thiosemicarbazones, synthesis and structural characterization of di-2-pyridylketone 4N-phenyl thiosemicarbazone and diphenyl tin (Sn and platinum (Pt complexes were undertaken. Shewanella oneidensis MR-1, a metal ion-reducing bacterium, was used as a model organism to explore the biological activity under aerobic conditions. A comparision of the cytotoxic potential of selected ligand and metal-complex thiosemicarbazones on cell growth in wild type MR-1 and mutant DSP-010 Shewanella oneidensis strains at various concentrations (0, 5, 10, 15, 20 or 25 ppm was performed. The wild type (MR-1 grown in the presence of increasing concentrations of Sn- thiosemicarbazone complexes was comparatively more sensitive (mean cell number = 4.8 X 108 + 4.3 X 107 SD than the DSP-010, a spontaneous rifampicillin derivative of the parent strain (mean cell number = 5.6 x 108 + 6.4 X 107 SD under comparable aerobic conditions (p=0.0004. No differences were observed in the sensitivity of the wild and mutant types when exposed to various concentrations of diphenyl Pt- thiosemicarbazone complex (p= 0.425 or the thiosemicarbazone ligand (p=0.313. Growth of MR-1 in the presence of diphenyl Sn- thiosemicarbazone was significantly different among treatment groups (p=0.012. MR-1 cell numbers were significantly higher at 5ppm than at 10 to 20ppm (p = 0.05. The mean number of DSP-010 variant strain cells also differed among diphenyl Sn- thiosemicarbazone complex treated groups (p=0.051. In general, there was an increasing trend in the number of cells from about 5.0 X 108 cells (methanol control group to about 6.0 X 108 cells (25ppm. The number of cells in methanol control group was significantly lower than cell numbers at 20ppm and 25ppm (p = 0.05, and numbers at 5ppm treatment were lower than at 20 and 25ppm (p = 0.05. Furthermore, a marginally significant difference in the number of MR-1 cells was observed among diphenyl Pt- thiosemicarbazone complex treatment groups (p = 0.077, and an increasing trend in the number of cells was noted from ~5.0 X 108 cells (methanol control group to ~5.8 X 108 cells (20ppm. In contrast, the DSP-010 variant strain showed no significant differences in cell numbers when treated with various concentrations of diphenyl Pt- thiosemicarbazone complex (p = 0.251. Differences in response to Sn- metal complex between MR-1 and DSP-010 growing cultures indicate that biological activity to thiosemicarbazone metal complexes may be strain specific.

Quintell Tillison

2005-04-01

252

Crystal structure of {3-[3,5-bis(2,6-dimethylphenyl-1,2-phenylene]-1-(2,6,2??,6??-tetramethyl-1,1?:3?,1??-terphenyl-5?-ylimidazol-2-ylidene}chlorido(?6-p-cymeneruthenium(II benzene disolvate  

Directory of Open Access Journals (Sweden)

Full Text Available The title compound, [Ru(C47H43N2Cl(C10H14]·2C6H6, crystallized with two independent molecules of benzene. One of the N-aryl moieties of the N-heterocyclic carbene (NHC ligand underwent cyclometallation to form a five-membered ruthenacycle. The complex has a three-legged piano-stool structure with two C atoms incorporated in the five-membered ruthenacycle and a Cl atom as legs. The ruthenacycle is essentially coplanar with the imidazole ring of the NHC ligand, making a dihedral angle of 0.85?(8°.

Shohei Sase

2014-12-01

253

Crystal structure of {3-[3,5-bis­(2,6-di­methyl­phen­yl)-1,2-phenyl­ene]-1-(2,6,2??,6??-tetra­methyl-1,1?:3?,1??-ter­phen­yl-5?-yl)imidazol-2-yl­idene}chlorido­(?6-p-cymene)ruthenium(II) benzene disolvate  

Science.gov (United States)

The title compound, [Ru(C47H43N2)Cl(C10H14)]·2C6H6, crystallized with two independent mol­ecules of benzene. One of the N-aryl moieties of the N-heterocyclic carbene (NHC) ligand underwent cyclo­metallation to form a five-membered ruthenacycle. The complex has a three-legged piano-stool structure with two C atoms incorporated in the five-membered ruthenacycle and a Cl atom as legs. The ruthenacycle is essentially coplanar with the imidazole ring of the NHC ligand, making a dihedral angle of 0.85?(8)°. PMID:25553006

Sase, Shohei; Ikehara, Yuriko; Goto, Kei

2014-01-01

254

Crystal structure of {3-[3,5-bis-(2,6-di-methyl-phen-yl)-1,2-phenyl-ene]-1-(2,6,2'',6''-tetra-methyl-1,1':3',1''-ter-phen-yl-5'-yl)imidazol-2-yl-idene}chlorido-(?(6)-p-cymene)ruthenium(II) benzene disolvate.  

Science.gov (United States)

The title compound, [Ru(C47H43N2)Cl(C10H14)]·2C6H6, crystallized with two independent mol-ecules of benzene. One of the N-aryl moieties of the N-heterocyclic carbene (NHC) ligand underwent cyclo-metallation to form a five-membered ruthenacycle. The complex has a three-legged piano-stool structure with two C atoms incorporated in the five-membered ruthenacycle and a Cl atom as legs. The ruthenacycle is essentially coplanar with the imidazole ring of the NHC ligand, making a dihedral angle of 0.85?(8)°. PMID:25553006

Sase, Shohei; Ikehara, Yuriko; Goto, Kei

2014-12-01

255

Substitution effects on the formation of T-shaped palladium carbene and thioketone complexes from Li/Cl carbenoids.  

Science.gov (United States)

The preparation of palladium thioketone and T-shaped carbene complexes by treatment of thiophosphoryl substituted Li/Cl carbenoids with a Pd(0) precursor is reported. Depending on the steric demand, the anion-stabilizing ability of the silyl moiety (by negative hyperconjugation effects) and the remaining negative charge at the carbenic carbon atom, isolation of a three-coordinate, T-shaped palladium carbene complex is possible. In contrast, insufficient charge stabilization results in the transfer of the sulfur of the thiophosphoryl moiety and thus in the formation of a thioketone complex. While the thioketones are stable compounds the carbene complexes are revealed to be highly reactive and decompose under elimination of Pd metal. Computational studies revealed that both complexes are formed by a substitution mechanism. While the ketone turned out to be the thermodynamically favored product, the carbene is kinetically favored and thus preferentially formed at low reaction temperatures. PMID:24664573

Molitor, Sebastian; Feichtner, Kai-Stephan; Kupper, Claudia; Gessner, Viktoria H

2014-08-18

256

Hydrolysis of Fischer carbene complexes. Steric effects of the pi-donor group.  

Science.gov (United States)

Rate constants for the hydrolysis of Fischer carbenes (CO)5Cr=C(OR)Ph (R = n-propyl, neopentyl, isopropyl, and menthyl) in 50% MeCN-50% water (v/v) at 25 degrees C are reported. The rate constants for the addition of -OH to the carbene carbon are 5.3, 3.7, 0.84, and 0.01 M(-1) s(-1), respectively. These rate constants give linear correlations with the corresponding rate constants for the hydrolysis of esters such as acetate, benzoate, and formiate. The slopes of the plots of the observed rate constants for the carbenes vs the rate constants for the esters are 1.4 and 1.2 for acetate and benzoate, respectively, indicating that the factors that decrease the reactivity of the two types of compounds are similar, but the carbenes show higher sensitivity. The rate constants are well correlated with several steric parameters giving a value of -3.84 for the Charton's psi parameter. The results show that the steric bulkiness of the R group is the main factor determining the reactivity differences for these carbene complexes. PMID:16526789

Zoloff Michoff, Martin E; de Rossi, Rita H; Granados, Alejandro M

2006-03-17

257

Complexation of trivalent cationic lanthanides by N.O donor ligands: physico-chemical studies of the association and selectivity in solution  

International Nuclear Information System (INIS)

The aim of this work is to study the complexation of f-elements in solution by ligands incorporating N-heterocyclic donors. These ligands display interesting properties for the selective separation of An(III)/Ln(III) have been studied to obtain a better understanding of the coordination properties with f-elements and to develop more selective extractants. The hepta-dentate ligand tpaam shows an affinity for Ln(III) similar to the tetradentate ligand tpa in water even when the three additional amide groups are bonded to the metal. Even though the complexation with tpa is exothermic, that with tpaam is endothermic with a more positive entropy. The dehydration of the cation disfavours the formation of Ln(III) complexes with ligands containing weak donors. The analysis of the solution paramagnetic relaxation times of the tpaam complexes is in agreement with data in the solid-state. There is little difference between the formation constants of the Ln3+ complexes with different ligands (tpaam, tpzen, tpa and tpza) as determined by UV-vis spectrophotometry in anhydrous acetonitrile. The limitations encountered during this study are intrinsic to the ligands studied. The preliminary study of two tetrapodal ligands containing acid and pyridine groups (Lpy)or pyrazine (Lpz) show the formation of 1:1 complexes in water. Analysis of the formation constants of the corresponding Gd(III) complexes shows that replacement of a pyridine group by pyrazine result in a loss of stability of 1.6 logarithmic units. (author)

258

Assembly and tunable luminescence of lanthanide-organic frameworks constructed from 4-(3,5-dicarboxyphenyl)pyridine-2,6-dicarboxylate ligand  

Energy Technology Data Exchange (ETDEWEB)

Highlights: Black-Right-Pointing-Pointer A new N-heterocycle muticarboxylate ligand 4-(3,5-dicarboxyphenyl)pyridine-2,6-dicarboxylic acid (H{sub 4}dpda) was developed. Black-Right-Pointing-Pointer Several lanthanide metal-organic frameworks (Ln = Eu, Gd, Tb, Dy) and Eu/Tb mixed MOFs were synthesized. Black-Right-Pointing-Pointer The ligand H{sub 4}dpda can provide efficient sensitization for the lanthanide ion Tb(III) and Eu(III). Black-Right-Pointing-Pointer Luminescence color of the mixed MOFs can be easily tuned by varying the molar ratios of Eu(III)/Tb(III). - Abstract: A novel N-heterocycle multicarboxylate ligand 4-(3,5-dicarboxyphenyl)pyridine-2,6-dicarboxylic acid (H{sub 4}dpda) was synthesized, and then reacted with lanthanide salts to yield a series of lanthanide metal-organic frameworks, [Ln(Hdpda)(H{sub 2}O){sub 4}]{center_dot}0.5(H{sub 2}O) (Ln = Eu(1), Gd(2), Tb(3), Dy(4)) and [Tb{sub 1-x}Eu{sub x}(Hdpda)(H{sub 2}O){sub 4}]{center_dot}0.5(H{sub 2}O) (x = 0.1-10 mol%). Single crystal X-ray diffraction and powder XRD patterns confirm these MOFs are isostructural. Luminescent measurements suggest that the ligand can provide efficient sensitization for the lanthanide ion Tb(III) and Eu(III) in the mixed lanthanide MOFs. Additionally, the luminescence color of the mixed MOFs can be easily tuned from green to green-yellow, yellow, orange, red-orange and red by varying the molar ratio of Eu(III)/Tb(III).

Zhang, Wenqian; Yu, Jiancan [State Key Lab of Silicon Materials, Cyrus Tang Center for Sensor Materials and Applications, Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027 (China); Cui, Yuanjing, E-mail: cuiyj@zju.edu.cn [State Key Lab of Silicon Materials, Cyrus Tang Center for Sensor Materials and Applications, Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027 (China); Rao, Xingtang; Yang, Yu [State Key Lab of Silicon Materials, Cyrus Tang Center for Sensor Materials and Applications, Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027 (China); Qian, Guodong, E-mail: gdqian@zju.edu.cn [State Key Lab of Silicon Materials, Cyrus Tang Center for Sensor Materials and Applications, Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027 (China)

2013-02-25

259

Rotational spectrum and theoretical structure of the carbene HC4N  

Science.gov (United States)

Following a high-level coupled cluster calculation, the rotational spectrum of the bent HC4N singlet carbene was detected in a supersonic molecular beam by Fourier transform microwave spectroscopy. The three rotational constants, the leading centrifugal distortion constants, and two nitrogen hyperfine coupling constants were determined to high accuracy. The rotational constants agree with those calculated ab initio to better than 0.5%. Like the isoelectronic C5H2 carbene of similar structure, HC4N was found to have fairly large centrifugal distortion and a large inertial defect. The calculated dipole moment of HC4N is 2.95 D.

McCarthy, M. C.; Apponi, A. J.; Gordon, V. D.; Gottlieb, C. A.; Thaddeus, P.; Daniel Crawford, T.; Stanton, John F.

1999-10-01

260

Rhodium (II) carbene C-H insertion in water and catalyst reuse  

International Nuclear Information System (INIS)

A five-session laboratory experiment is described for the synthesis of a beta-lactam via Rh(II) catalysed intramolecular C-H insertion of a alpha-diazo-alpha-ethoxycarbonyl acetamide. The metallo-carbene, responsible for the C-H bond activation, was generated from the diazo substrate and the catalyst Rh2(OAc)4. The high stability and solubility of the catalyst and the exclusive C-H insertion of the Rh-carbene allows the synthesis of this important heterocycle in water and the catalyst reutilization. (author)

261

Synthesis and Characterization of Nitrogen-Rich Macrocyclic Ligands and an Investigation of Their Coordination Chemistry with Lanthanum(III).  

Science.gov (United States)

Derivatives of the ligand 1,4,7,10-tetraazacyclododecane (cyclen) containing pendant N-heterocyclic donors were prepared. The heterocycles pyridine, pyridazine, pyrimidine, and pyrazine were conjugated to cyclen to give 1,4,7,10-tetrakis(pyridin-2-ylmethyl)-1,4,7,10-tetraazacyclododecane (L(py)), 1,4,7,10-tetrakis(3-pyridazylmethyl)-1,4,7,10-tetraazacyclododecane (L(pyd)), 1,4,7,10-tetrakis(4-pyrimidylmethyl)-1,4,7,10-tetraazacyclododecane (L(pyr)), and 1,4,7,10-tetrakis(2-pyrazinylmethyl)-1,4,7,10-tetraazacyclododecane (L(pz)), respectively. The coordination chemistry of these ligands was explored using the La(3+) ion. Accordingly, complexes of the general formula [La(L)(OTf)](OTf)2, where OTf = trifluoromethanesulfonate and L = L(py) (1), L(pyd) (2), L(pyr) (3), and L(pz) (4), were synthesized and characterized by NMR spectroscopy. Crystal structures of 1 and 2 were also determined by X-ray diffraction studies, which revealed 9-coordinate capped, twisted square-antiprismatic coordination geometries for the central La(3+) ion. The conformational dynamics of 1-4 in solution were investigated by variable-temperature NMR spectroscopy. Dynamic line-shape and Eyring analyses enabled the determination of the activation parameters for the interconversion of enantiomeric forms of the complexes. Unexpectedly, the different pendant N-heterocycles of 1-4 give rise to varying values for the enthalpies and entropies of activation for this process. Density functional theory calculations were carried out to investigate the mechanism of this enantiomeric interconversion. Computed activation parameters were consistent with those experimentally determined for 1 but differed somewhat from those of 2-4. PMID:25526533

Wilson, Justin J; Birnbaum, Eva R; Batista, Enrique R; Martin, Richard L; John, Kevin D

2015-01-01

262

Low-lying electronic states of unsaturated carbenes. Comparison with methylene  

International Nuclear Information System (INIS)

The lowest singlet and triplet electronic states of methylene, H2C:, vinylidene, H2CC:, and vinylidenecarbene, H2CCC:, are studied theoretically at the spin-unrestricted Hartree--Fock (HF) and spin-unrestricted many-body perturbation theory (MBPT) levels of approximation. Calculations at both levels predict a triplet ground state for methylene and singlet ground states for vinylidene and vinylidenecarbene. MBPT singlet--triplet energy splittings are found to be -13.1 kcal/mol for methylene, 51.1 kcal/mol for vinylidene, and 48.7 kcal/mol for vinylidenecarbene. A comparison of the effects of correlation on electronic structure is made between the parent carbene, methylene, and each of these unsaturated carbenes. From electronic structure trends observed in H2C:, H2CC:, and H2CCC:, singlet ground states are predicted for the extended unsaturated carbenes H2CCC:, and H2CCCC:, the next members of the series. The importance of d-type carbon polarization functions in basis sets used for the calculation of electronic structure properties in unsaturated carbenes is demonstrated by comparison with results in which d functions are not included in the basis. 4 tables, 54 references

263

How to make the ?0?2 singlet the ground state of carbenes.  

Science.gov (United States)

Successful strategies have previously been developed to stabilize the ?(2)?(0) singlet states of carbenes, relative to ?(1)?(1) triplet states. However, little or no attention has been paid to the stabilization of the ?(0)?(2) singlet states. We present two simple strategies to stabilize the ?(0)?(2) singlet states of carbenes, relative to both the ?(2)?(0) singlet and ?(1)?(1) triplet states. These strategies consist of destabilization of the carbene ? orbital by two, adjacent, sp(2) nitrogen lone pairs of electrons and stabilization of the carbene 2p-? orbital by incorporating it into a five-membered ring, containing two double bonds, or into a six-membered ring, containing two double bonds and a sixth atom that has a low-lying empty ? orbital. B3LYP, CASPT2, and CCSD(T) calculations have been performed in order to assess the success of these strategies in creating derivatives of cyclopenta-2,4-dienylidene and cyclohexa-2,5-dienylidene with ?(0)?(6) singlet ground states. Differences between the calculated geometries and binding energies of the Xe complexes of the ?(0)?(6) singlet ground state of 2,5-diazacyclopentadienylidene (5) and the ?(2)?(0) singlet states of CH2 and CF2 are discussed. PMID:24007553

Chen, Bo; Rogachev, Andrey Yu; Hrovat, David A; Hoffmann, Roald; Borden, Weston Thatcher

2013-09-18

264

Helicene-grafted vinyl- and carbene-osmium complexes: an example of acid-base chiroptical switching.  

Science.gov (United States)

The first helicene-based carbene-osmium complex has been prepared from a vinyl-osmium derivative and this system has been shown to behave as a potential acid-base triggered chiroptical switch. PMID:24322581

Anger, Emmanuel; Srebro, Monika; Vanthuyne, Nicolas; Roussel, Christian; Toupet, Loïc; Autschbach, Jochen; Réau, Régis; Crassous, Jeanne

2014-03-18

265

Synthesis of Transient and Stable C-Amino Phosphorus Ylides and Their Fragmentation into Transient and Stable Carbenes  

OpenAIRE

Only basic phosphines, such as tris(dimethylamino)phosphine, allow for the synthesis of a stable acyclic ?-amino phosphonium salt 1c, which upon deprotonation with butyllithium affords the corresponding stable C-amino phosphorus ylide 2c. In contrast, cyclic ?-amino phosphonium salts 5a and 5b are stable despite the presence of weakly basic triarylphosphine fragments. They are prepared by intramolecular insertion of the carbene center of (amino)(phosphonio)carbenes into the CH bond of a pho...

Canac, Yves; Conejero, Salvador; Soleilhavoup, Miche?le; Donnadieu, Bruno; Bertrand, Guy

2006-01-01

266

Oxidation-promoted activation of a ferrocene C-H bond by a rhodium complex.  

Science.gov (United States)

The oxidation of a rhodium(I) complex containing a ferrocene-based heterodifunctional phosphine N-heterocyclic carbene (NHC) ligand produces a stable, planar chiral rhodium(III) complex with an unexpected C-H activation on ferrocene. The oxidation of rhodium(I) to rhodium(III) may be accomplished by initial oxidation of ferrocene to ferrocenium and subsequent electron transfer from rhodium to ferrocenium. Preliminary catalytic tests showed that the rhodium(III) complex is active for the Grignard-type arylation of 4-nitrobenzaldehyde via C-H activation of 2-phenylpyridine. PMID:23467475

Labande, Agnès; Debono, Nathalie; Sournia-Saquet, Alix; Daran, Jean-Claude; Poli, Rinaldo

2013-05-14

267

Palladium-Catalyzed ?-Arylation of Arylketones at Low Catalyst Loadings.  

Science.gov (United States)

A general catalytic protocol for the ?-arylation of aryl ketones has been developed. It involves the use of a preformed, bench-stable Pd-N-heterocyclic carbene pre-catalyst bearing IHept as an ancillary ligand, and allows the coupling of various functionalized coupling partners at very low catalyst loading. Careful choice of the solvent/base system was crucial to obtain optimum catalyst performance. The pre-catalyst was also successfully tested in the synthesis of an industrially relevant compound. PMID:25414140

Marelli, Enrico; Corpet, Martin; Davies, Sian R; Nolan, Steven P

2014-12-22

268

Phosphine modified cobalt hydroformylation  

Energy Technology Data Exchange (ETDEWEB)

An ongoing challenge in phosphine modified cobalt hydroformylation is the fundamental understanding of the electronic and steric properties of phosphine ligands that influence the selectivity and activity of the catalytic reaction. A series of acyclic and cyclic phosphines have been prepared and tested in phosphine modified cobalt hydroformylation of 1-octene. Molecular modelling on a series of phospholanes showed some interesting theoretical and experimental correlations. We also evaluated the use of N-heterocyclic carbenes as an alternative for phosphines in modified cobalt hydroformylation. (orig.)

Rensburg, H. van; Tooze, R.P.; Foster, D.F. [Sasol Technology UK, St. Andrews (United Kingdom); Janse van Rensburg, W. [Sasol Technology, Sasolburg (South Africa)

2006-07-01

269

Imidazole to NHC rearrangements at molybdenum centers. An experimental and theoretical study  

OpenAIRE

Both manganese and rhenium complexes of the type [M(bipy)(CO)3(N-RIm)]+ (bipy=2,2’-bipyridine) undergo deprotonation of the central CH group of the N-alkylimidazole (N-RIm) ligand when treated with a strong base. However, the outcome of the reaction is very different for either metal. For Mn, the addition of the equimolar amount of an acid to the product of the deprotonation affords an N-heterocyclic carbene (NHC) complex, whereas for Re, once the deprotonation of the central imidazole CH g...

Brill, Marcel; Huertos, Miguel A.; Pe?rez, Julio; Riera, Luci?a

2011-01-01

270

Palladation of diimidazolium salts at the C4 position: access to remarkably electron-rich palladium(II) centers  

OpenAIRE

Palladation of C2-protected diimidazolium salts with Pd(OAc)? afforded complexes comprising C4-bound N-heterocyclic dicarbene ligands. The reactivity of these complexes towards Lewis acids (AgBF?, AgOAc) and Brønsted acids (H?SO?, H?PO?, HOAc) revealed that abnormal C4 bonding of the carbenes markedly increases the nucleophilicity of the coordinated palladium center as compared to C2 bonding. Despite its formal +2 charge, the palladium center in these complexes is best described ...

Heckenroth, Marion; Kluser, Evelyne; Neels, Antonia; Albrecht, Martin

2008-01-01

271

Highvalent and organometallic technetium and rhenium compounds  

International Nuclear Information System (INIS)

Diagnostic methods in nuclear medicine allow a detailed description of morphological organ structures and their function. The beta emitting isotope Tc-99 has optimal physical properties (140 keV gamma rays, half-life 6 h) and is therefore used for radiopharmaceuticals. The thesis is concerned with the search for new technetium complexes and their reproducible production. The (TcO3) core is of main interest. The second part of the thesis deals with organometallic technetium and rhenium complexes with carbonyl ligands and N-heterocyclic carbenes that show stability in aerobic aqueous solutions.

272

Engineering carbene rearrangements in crystals: from molecular information to solid-state reactivity.  

Science.gov (United States)

Although severely restricted, reactions in crystals have delivered some of the most impressive examples of chemical control and provided an ideal medium for detailed mechanistic studies. However, to take full advantage of their potential and avoid their shortcomings, a systematic approach for the reliable design of reaction in crystals is required. To that end, strategies based on structural and energetic information contained at the molecular level have been explored with reactions involving carbene rearrangements. PMID:12859210

Garcia-Garibay, Miguel A

2003-07-01

273

Metathesis of alkenes using ruthenium carbene complexes / K.N.G. Mtshatsheni  

OpenAIRE

In this study different ruthenium carbene complexes, Grubbs 1 and 2 were investigated for the reactions of 1-, 2-, 3-octene and 1-tetradecene. All reactions were conducted in glass mini reactors equipped with Mininert® valves under a dry nitrogen atmosphere. A heating block was used to heat the reactors. Gas chromatography was used to determine the product composition of reaction mixtures. The influence of reaction temperature, solvents and catalyst concentration on the rea...

Mtshatsheni, Kgomotso Ntombizodwa Gina

2005-01-01

274

Formation of carbene and carbine particles during decomposition of molybdenum benzyl derivatives  

International Nuclear Information System (INIS)

A qualitative composition of hydrocarbon products of interaction of benzyl magnesium chlorides and MoCl5 in diethylene and dibutyl ethers has been studied. Hydrocarbon reaction products mainly consist of toluene and dibenzyl and small quantities of totan and stilbene. Total residue after the reaction contains some quantity of ''benzyl'' groups bound with Mo or Mg. Carbene and carbine particles disappear at the expense of secondary reactions with organo-metallic compounds of Mo or Mg

275

Steady state and laser flash photolysis studies of ferrocenyl group VI fischer-carbenes and metalloporphyrins  

OpenAIRE

Chapter one begins with a theoretical description of the electronic structure of the various compounds studied in this thesis, which includes group VI Fischer carbenes, ferrocene and meso-substituted porphyrins. This is followed by an introduction to the principles of photophysics and non-lmear optics. Chapter one finishes with an outline of the experimental techniques of laser flash photolysis and electric field induced second harmonic generation (EFISHG). Chapter two opens with a litera...

Rochford, Jonathan

2005-01-01

276

Catalysis with metalloporphyrins : asymmetric oxidation and carbene transfer reactions  

OpenAIRE

The goal of green chemistry is to promote the research and the development of innovative chemical technologies in order to reduce the contamination of environment. Significant progress in this field has been made by using relatively non toxic transition metal and chiral ligands to catalyze enantioselective transformations. In this work, we have reported the combination of hydrogen peroxide as environmentally benign and atom-economically oxidant with water-soluble metalloporphyrins (Fe, Mn) fo...

Srour, Hassan

2013-01-01

277

A theoretical study of the mechanism of the addition reaction between carbene and azacyclopropane  

Directory of Open Access Journals (Sweden)

Full Text Available The mechanism of the addition reaction between carbene and azacyclopropane was investigated using the second-order Moller–Plesset perturbation theory (MP2. By using the 6-311+G* basis set, geometry optimization, vibrational analysis and the energy properties of the involved stationary points on the potential energy surface were calculated. From the surface energy profile, it can be predicted that there are two reaction mechanisms. The first one (1 is carbene attack at the N atom of azacyclopropane to form an intermediate, 1a (IM1a, which is a barrier-free exothermic reaction. Then, IM1a can isomerize to IM1b via a transition state 1a (TS1a, in which the potential barrier is 30.0 kJ/mol. Subsequently, IM1b isomerizes to a product (Pro1 via TS1b with a potential barrier of 39.3 kJ/mol. The other one (2 is carbene attack at the C atom of azacyclopropane, firstly to form IM2 via TS2a, the potential barrier is 35.4 kJ/mol. Then IM2 isomerizes to a product (Pro2 via TS2b with a potential barrier of 35.1 kJ/mol. Correspondingly, the reaction energy for the reactions (1 and (2 is –478.3 and –509.9 kJ/mol, respectively. Additionally, the orbital interactions are also discussed for the leading intermediate.

XIAOJUN TAN

2010-05-01

278

Phosphorus-stabilized titanium carbene complexes: synthesis, reactivity and DFT studies.  

Science.gov (United States)

The synthesis of two novel titanium carbene complexes from the bis(thiophosphinoyl)methanediide geminal dianion 1 (SCS(2-) ) is described. Dianion 1 reacts cleanly with 0.5 equivalents of [TiCl4 (thf)2 ] to afford the bis-carbene complex [(SCS)2 Ti] (2) in 86?% yield. The mono-carbene complex [(SCS)TiCl2 (thf)] (3) can also be obtained by using an excess of [TiCl4 (thf)2 ]. The structures of 2 and 3 are confirmed by X-ray crystallography. A strong nucleophilic reactivity towards various electrophiles (ketones and aldehydes) is observed. The reaction of 3 with N,N'-dicyclohexylcarbodiimide (DCC) and phenyl isocyanate leads to the formation of two novel diphosphinoketenimines 8?a and 8?b. The bis-titanium guanidinate complex 9 is trapped as the by-product of the reaction with DCC. The X-ray crystal structures of 8?a and 9 are presented. The mechanism of the reaction between complex 3 and DCC is rationalized by DFT studies. PMID:25339253

Lafage, Mathieu; Heuclin, Hadrien; Le Goff, Xavier-Frédéric; Saffon-Merceron, Nathalie; Mézailles, Nicolas

2014-12-15

279

Explicit role of dynamical and nondynamical electron correlation on singlet-triplet splitting in carbenes  

Energy Technology Data Exchange (ETDEWEB)

Density functional theoretical studies have been performed on carbene systems to determine the singlet-triplet splitting and also to explore the role of electron correlation. Using an approximate method of separation of dynamical and nondynamical correlation, it is found that dynamical and nondynamical electron correlation stabilizes the singlet state relative to the triplet for halo carbenes in both BLYP and B3LYP methods. Calculations performed on higher homologues of methylene suggest that beyond CH(CH{sub 3}), both the electron correlations have leveling effect in stabilizing the singlet state relative to the triplet. It has also been observed while dynamical electron correlation fails to provide any substantial degree of stabilization to the singlet states of higher homologues of methylene in B3LYP method, an opposite trend is observed for nondynamical counterpart. Among the larger systems studied (9-triptycyl)({alpha}-naphthyl)-carbene has the highest stability of the triplet state whereas bis-imidazol-2-ylidenes has the most stable singlet state. Interestingly, the values of the dynamical electron correlation for each state of each system studied are different for the two methods used. The reason behind this apparent discrepancy lies in the fact that the coefficients of the LYP part in B3LYP and BLYP functionals are different.

Seal, Prasenjit [Department of Chemistry, University of Calcutta, 92, A.P.C. Ray Road, Kolkata - 700 009 (India); Chakrabarti, Swapan [Department of Chemistry, University of Calcutta, 92, A.P.C. Ray Road, Kolkata - 700 009 (India)], E-mail: swapanchem@yahoo.co.in

2007-02-14

280

Role of quantum mechanical tunneling on the ?-effect of silicon on carbenes in 3-trimethylsilylcyclobutylidene.  

Science.gov (United States)

Quantum mechanical tunneling (QMT) is increasingly being realized as an important phenomenon that can enhance the rate of reactions even at room temperature. Recently, the ability of a trimethylsilane (TMS) group to activate 1,3-H shift to a carbene from a ?-position has been demonstrated. Direct dynamical calculations (using canonical varitational transition state theory) inclusive of small curvature tunneling (CVT-SCT) show that QMT plays a decisive role in such 1,3-hydrogen migration in both the presence and absence of TMS. The presence of a TMS group reduces the activation energy of 1,3-H shift reaction via 1,3-equatorial interaction of the TMS group with the carbene. Tunneling across the smaller barrier enhances the overall forward rate of the reaction. The Arrhenius plot for the reaction shows substantial curvature in comparison to the CVT mechanism at room temperature. Arrhenius plots for the kinetic isotope effects (KIEs) for the ?-deuterated and per deuterated 3-trimethylsilylcyclobutylidene also show strong deviations from the classical over the barrier mechanism. The magnitude of the KIE is suggestive of QMT from the vibrational excited states of the carbenes. PMID:24467461

Karmakar, Sharmistha; Datta, Ayan

2014-03-01

281

DFT STUDY ON THE MECHANISM OF THE ADDITION REACTION BETWEEN CARBENE AND GLYCINE  

Scientific Electronic Library Online (English)

Full Text Available SciELO Chile | Language: English Abstract in english The mechanism of addition reaction between the singlet carbene and glycine has been investigated at the B3LYP/6-311+G* level of theory, including geometry optimization and vibrational analysis for the involved stationary points on the potential energy surface. Computational results suggest that ther [...] e are two reaction pathways (1) and (2) during the addition process. In the pathway (1), carbene attacks the C=O bond from the H¹C¹C² side of glycine to form an intermediate (IM1), which is a barrier-free exothermic reaction. Then, IM1 isomerizes to a product (Pro1) via a transition state (TS1) with a potential barrier of 25.7 kJ/mol. Similarly, in the pathway (2), carbene attacks the C=O bond from the H²C¹C² side of glycine to form an intermediate IM2. Subsequently, IM2 isomerizes to Pro2 via TS2, where the Pro2 and Pro1 are enantiomers actually. The calculated potential barrier of 51.3 kJ/mol is higher than that of the pathway (1). Correspondingly, the reaction energy for the both pathways is -258.5 kJ/mol. Additionally, the atoms in molecules (AIM) theory has also been performed to characterize the bonding interaction and structural features for the addition reaction.

XIAOJUN, TAN; WEIHUA, WANG; PING, LI; JINSONG, GU; GUORONG, WANG; DANGSHENG, WANG; TAO, LI.

282

Ruthenium-catalyzed synthesis of functional conjugated dienes via addition of two carbene units to alkynes.  

Science.gov (United States)

The reaction of a variety of alkynes with N(2)CHSiMe(3), in the presence of Cp*RuCl(cod) as the catalyst precursor, leads to the general formation of functional conjugated dienes. This selective formation results from the ruthenium-catalyzed creation of two carbon-carbon double bonds in a single step under mild conditions. Terminal alkynes produce 1,4-bistrimethylsilylbuta-1,3-dienes with Z stereoselectivity for the less hindered double bond whereas disubstituted alkynes favor E-configuration for the same double bond. Diynes react also as monoalkynes, and only one triple bond is transformed to give disilylated dienynes. The reaction can be applied to the in situ desilylation in methanol and formation of monosilylated dienes. The catalytic formation of 1,4-bisfunctional buta-1,3-dienes can also take place with N(2)CHCO(2)Et and N(2)CHPh. The reaction can be understood by addition of two carbene units to triple bonds. An initial [2 + 2] addition of the Ru=CHSiMe(3) bond with the alkyne triple bond leads to an alkenyl ruthenium-carbene species capable of coordinating a second carbene unit to produce conjugated dienes. PMID:20459106

Le Paih, Jacques; Vovard-Le Bray, Chloé; Dérien, Sylvie; Dixneuf, Pierre H

2010-06-01

283

Enantioselective synthesis of beta-trifluoromethyl-beta-lactones via NHC-catalyzed ketene-ketone cycloaddition reactions.  

Science.gov (United States)

The highly diastereo- and enantioselective synthesis of beta-trifluoromethyl-beta-lactones bearing two contiguous stereocenters was realized by chiral N-heterocyclic carbene-catalyzed formal cycloaddition reaction of alkyl(aryl)ketenes and trifluoromethyl ketones. PMID:19681591

Wang, Xiao-Na; Shao, Pan-Lin; Lv, Hui; Ye, Song

2009-09-17

284

Palladation of diimidazolium salts at the C4 position: access to remarkably electron-rich palladium(II) centers.  

Science.gov (United States)

Palladation of C2-protected diimidazolium salts with Pd(OAc)2 afforded complexes comprising C4-bound N-heterocyclic dicarbene ligands. The reactivity of these complexes towards Lewis acids (AgBF4, AgOAc) and Brønsted acids (H2SO4, H3PO4, HOAc) revealed that abnormal C4 bonding of the carbenes markedly increases the nucleophilicity of the coordinated palladium center as compared to C2 bonding. Despite its formal +2 charge, the palladium center in these complexes is best described as a Lewis base. The abnormal carbene bonding mode induces new reaction patterns such as the formation of a Pd-Ag adduct. Based on metallation studies including the palladation of a dissymmetric diimidazolium salt, a rationale for the selective activation of the C4-H bond in the diimidazolium precursor salts is proposed. PMID:18985257

Heckenroth, Marion; Kluser, Evelyne; Neels, Antonia; Albrecht, Martin

2008-11-28

285

Making oxidation potentials predictable: coordination of additives applied to the electronic fine tuning of an iron(II) complex.  

Science.gov (United States)

This work examines the impact of axially coordinating additives on the electronic structure of a bioinspired octahedral low-spin iron(II) N-heterocyclic carbene (Fe-NHC) complex. Bearing two labile trans-acetonitrile ligands, the Fe-NHC complex, which is also an excellent oxidation catalyst, is prone to axial ligand exchange. Phosphine- and pyridine-based additives are used for substitution of the acetonitrile ligands. On the basis of the resulting defined complexes, predictability of the oxidation potentials is demonstrated, based on a correlation between cyclic voltammetry experiments and density functional theory calculated molecular orbital energies. Fundamental insights into changes of the electronic properties upon axial ligand exchange and the impact on related attributes will finally lead to target-oriented manipulation of the electronic properties and consequently to the effective tuning of the reactivity of bioinspired systems. PMID:25333790

Haslinger, Stefan; Kück, Jens W; Hahn, Eva M; Cokoja, Mirza; Pöthig, Alexander; Basset, Jean-Marie; Kühn, Fritz E

2014-11-01

286

Gold-catalyzed tandem cycloisomerization/functionalization of in situ generated ?-oxo gold carbenes in water.  

Science.gov (United States)

A gold-catalyzed tandem cycloisomerization/functionalization of in situ generated ?-oxo gold carbenes in water has been developed, which provides ready access to highly functionalized indole derivatives from o-alkynyl anilines and ynamides. Importantly, gold serves dual catalytic roles to mediate both the cycloisomerization of o-alkynyl anilines and the intermolecular oxidation of ynamides at the same time, thus providing a new type of concurrent tandem catalysis. The use of readily available starting materials, a simple procedure, and mild reaction conditions are other notable features of this method. PMID:25229883

Shen, Cang-Hai; Li, Long; Zhang, Wei; Liu, Shuang; Shu, Chao; Xie, Yun-Er; Yu, Yong-Fei; Ye, Long-Wu

2014-10-01

287

Electrochemical illumination of thienyl and ferrocenyl chromium(0) Fischer carbene complexes  

OpenAIRE

A series of ferrocenyl and thienyl mono- and biscarbene chromium(0) complexes 1–6 were synthesised. The complexes were characterised both spectroscopically and electrochemically, and the single crystal X-ray structure of 3 was determined. Electrochemical measurements in CH2Cl2 revealed that the carbene double bond of 1–6 is reduced to an anion radical, ?Cr–C? at formal reduction potentials

Westhuizen, Belinda; Swarts, Pieter J.; Strydom, Ian; Liles, David C.; Fernandez, Israel; Swarts, Jannie C.; Bezuidenhout, Daniela Ina

2013-01-01

288

Synthesis and reactivity of metal carbene complexes with heterobiaryl spacer substituents  

OpenAIRE

Mono- and binuclear Fischer carbene complexes, [M(CO)5{C(OR)Ar-ArX}], X = H, {C(OR)M(CO)5}; M, M = W or Cr; R = Me, Et or (CH2)4OMe; Ar = thiophene, N-methylpyrrole or furan units 1–20, were synthesized. For this purpose, mono-, bi- or stepwise lithiated bithiophene, N,N-dimethylbipyrrole, thienylfuran and N-methyl(thienyl)pyrrole were reacted with chromium and tungsten hexacarbonyl precursors. Dilithiation in the 2- and 9-positions of N-methyl(thienyl)pyrrole could not be achieved. Alkylat...

Lotz, Simon; Crause, Chantelle; Olivier, Andrew John; Liles, David C.; Go?rls, Helmar; Landman, Marile; Bezuidenhout, Daniela Ina

2008-01-01

289

Coinage metals binding as main group elements: structure and bonding of the carbene complexes [TM(cAAC)2] and [TM(cAAC)2](+) (TM = Cu, Ag, Au).  

Science.gov (United States)

Quantum chemical calculations using density functional theory have been carried out for the cyclic (alkyl)(amino)carbene (cAAC) complexes of the group 11 atoms [TM(cAAC)2] (TM = Cu, Ag, Au) and their cations [TM(cAAC)2](+). The nature of the metal-ligand bonding was investigated with the charge and energy decomposition analysis EDA-NOCV. The calculations show that the TM-C bonds in the charged adducts [TM(cAAC)2](+) are significantly longer than in the neutral complexes [TM(cAAC)2], but the cations have much higher bond dissociation energies than the neutral molecules. The intrinsic interaction energies ?Eint in [TM(cAAC)2](+) take place between TM(+) in the (1)S electronic ground state and (cAAC)2. In contrast, the metal-ligand interactions in [TM(cAAC)2] involve the TM atoms in the excited (1)P state yielding strong TM p(?) ? (cAAC)2 ? backdonation, which is absent in the cations. The calculations suggest that the cAAC ligands in [TM(cAAC)2] are stronger ? acceptors than ? donors. The trends of the intrinsic interaction energies and the bond dissociation energies of the metal-ligand bonds in [TM(cAAC)2] and [TM(cAAC)2](+) give the order Au > Cu > Ag. Calculations at the nonrelativistic level give weaker TM-C bonds, particularly for the gold complexes. The trend for the bond strength in the neutral and charged adducts without relativistic effects becomes Cu > Ag > Au. The EDA-NOCV calculations suggest that the weaker bonds at the nonrelativistic level are mainly due to stronger Pauli repulsion and weaker orbital interactions. The NBO picture of the C-TM-C bonding situation does not correctly represent the nature of the metal-ligand interactions in [TM(cAAC)2]. PMID:25394669

Jerabek, Paul; Roesky, Herbert W; Bertrand, Guy; Frenking, Gernot

2014-12-10

290

Bis-mixed-carbene ruthenium-thiolate-alkylidene complexes: synthesis and olefin metathesis activity.  

Science.gov (United States)

A series of bis-carbene Ru-hydride species, including (IMes)(Im(OMe)2)(PPh3)RuHCl () and (SIMes)(Me2Im(OMe)2)(PPh3)RuHCl () were prepared and subsequently shown to react with aryl-vinyl-sulfides to give the bis-carbene-alkylidene complexes: Im(OMe)2(SIMes)RuCl(SR)([double bond, length as m-dash]CHCH3) (R = p-FC6H4 (), p-(NO2)C6H4 ()), Im(OMe)2(IMes)RuCl([double bond, length as m-dash]CHCH3)(SPh) (), Me2Im(OMe)2(SIMes)RuCl([double bond, length as m-dash]CHCH3)(SPh) (), Im(OMe)2(SIMes)(F5C6S)RuCl([double bond, length as m-dash]CHR) (R = C4H9 (), C5H11 ()). The activity of these species in the standard Grubbs' tests for ring-opening metathesis polymerization, ring-closing and cross-metathesis are reported. Although these thiolate derivatives are shown to exhibit modest metathesis activities, the reactivity is enhanced in the presence of BCl3. PMID:25462569

Dahcheh, Fatme; Stephan, Douglas W

2014-12-23

291

Palladium-Catalyzed Heck Coupling Reaction of Aryl Bromides in Aqueous Media Using Tetrahydropyrimidinium Salts as Carbene Ligands  

Directory of Open Access Journals (Sweden)

Full Text Available An efficient and stereoselective catalytic system for the Heck cross coupling reaction using novel 1,3-dialkyl-3,4,5,6-tetrahydropyrimidinium salts (1, LHX and Pd(OAc2 loading has been reported. The palladium complexes derived from the salts 1a-f prepared in situ exhibit good catalytic activity in the Heck coupling reaction of aryl bromides under mild conditions.

?smail Özdemir

2010-01-01

292

Carbene Spin Multiplicity in Solution Probed Using Time-Resolved EPR Spectroscopy  

Scientific Electronic Library Online (English)

Full Text Available SciELO Mexico | Language: English Abstract in spanish Espectroscopia de resonancia paramagnética electrónica (EPR por las siglas en inglés) fue utilizada para detectar los radicales libres formados mediante reacciones de abstracción de átomos de hidrógeno de diversos carbenos generados fotoquímicamente en soluciones de n-hexano y a temperatura ambiente [...] . En particular, se describe el análisis de tres carbenos que fueron generados por fotoexcitación de un bifenilo con dos grupos diazo. Se identificaron señales EPR de dos radicales alquilo y uno bencílico. La estructura de dichos radicales se confirmó mediante un proceso de simulación computacional utilizando parámetros espectroscópicos (factores-g, constantes de acoplamiento hiperfino) reportados en la literatura. Experimentos previos de absorción óptica han llevado a la conclusión de que los carbenos estudiados presentan un estado basal de tipo triplete, lo que se confirma al observar las señales EPR de emisión a campo bajo pero de absorción a campo alto. Además, se encontró que los espectros presentan un espín fuertemente polarizado como consecuencia de un mecanismo CIDEP, y que la intensidad de las señales está relacionada con los rendimientos cuánticos asociados a la producción de los carbenos iniciales. Como era de esperarse, un tercer sustrato, que reacciona vía una reacción concertada de tipo Wolff a través de un reordenamiento del intermediario singulete producido en el estado excitado, no dio lugar a ninguna señal EPR. Así mismo, cuando las reacciones de los tres carbenos se llevan a cabo en acetonitrilo, la multiplicidad del espín se invierte de modo que se genera un estado basal.singulete, por lo que no se observan señales EPR. Los resultados descritos en este trabajo son consistentes con aquellos obtenidos mediante fotólisis láser. Abstract in english Time-resolved (CW) EPR spectroscopy at X-band has been used to detect free radicals created by H-atom abstraction reactions of photochemically generated carbenes in n-hexadecane solutions at room temperature. Three different carbenes were studied, formed via loss of nitrogen after photo-excitation o [...] f a substituted biphenyl diazo precursor. The TREPR signals from two different alkyl radicals and the benzylic-type radical from the carbene are detected for two of the three diazo substrates. The radicals were unambiguously identified by spectral simulation using literature parameters (g-factors and hyper-fine coupling constants). Previous transient optical absorption spectroscopy experiments have concluded that these two carbenes have triplet ground states, which is confirmed by the low field emissive, high field absorptive phase of the TREPR signals reported here. The spectra are strongly spin polarized by the Radical Pair Mechanism of CIDEP, and their overall intensities track with the measured quantum yields for carbene production. A third substrate, known to react via a concerted Wolff reaction or excited state rearrangement via a singlet state, showed no TREPR signal, as expected. Reactions run in acetonitrile, where the spin multiplicity is reversed to give a singlet ground state, did not lead to detectable TREPR signals for all three substrates. The observations are consistent with previously published laser flash photolysis results.

Jin, Wang; Natalia V., Lebedeva; Matthew S., Platz; Malcolm D. E., Forbes.

2009-09-01

293

Development of catalysts and ligands for enantioselective gold catalysis.  

Science.gov (United States)

During the past decade, the use of Au(I) complexes for the catalytic activation of C-C ?-bonds has been investigated intensely. Over this time period, the development of homogeneous gold catalysis has been extraordinarily rapid and has yielded a host of mild and selective methods for the formation of carbon-carbon and carbon-heteroatom bonds. The facile formation of new bonds facilitated by gold naturally led to efforts toward rendering these transformations enantioselective. In this Account, we survey the development of catalysts and ligands for enantioselective gold catalysis by our research group as well as related work by others. We also discuss some of our strategies to address the challenges of enantioselective gold(I) catalysis. Early on, our work with enantioselective gold-catalyzed transformations focused on bis(phosphinegold) complexes derived from axially chiral scaffolds. Although these complexes were highly successful in some reactions like cyclopropanation, the careful choice of the weakly coordinating ligand (or counterion) was necessary to obtain high levels of enantioselectivity for the case of allene hydroamination. These counterion effects led us to use the anion itself as a source of chirality, which was successful in the case of allene hydroalkoxylation. In general, these tactics enhance the steric influence around the reactive gold center beyond the two-coordinate ligand environment. The use of binuclear complexes allowed us to use the second gold center and its associated ligand (or counterion) to exert a further steric influence. In a similar vein, we employed a chiral anion (in place of or in addition to a chiral ligand) to move the chiral information closer to the reactive center. In order to expand the scope of reactions amenable to enantioselective gold catalysis to cycloadditions and other carbocyclization processes, we also developed a new class of mononuclear phosphite and phosphoramidite ligands to supplement the previously widely utilized phosphines. However, we needed to judiciously design the steric environment to create "walls" that enclose the gold center. We also successfully applied these same considerations to the development of binuclear carbene ligands for gold. Finally, we describe the design of bifunctional urea-monophosphine ligands used in a gold-catalyzed three-component coupling. PMID:24228794

Wang, Yi-Ming; Lackner, Aaron D; Toste, F Dean

2014-03-18

294

Ruthenium complexes with chiral tetradentate PNNP ligands: asymmetric catalysis from the viewpoint of inorganic chemistry.  

Science.gov (United States)

This is a personal account of the application of ruthenium complexes containing chiral tetradentate ligands with a P(2)N(2) ligand set (PNNP) as catalyst precursors for enantioselective "atom transfer" reactions. Therewith are meant reactions that involve bond formation between a metal-coordinated molecule and a free reagent. The reactive fragment (e.g. carbene) is transferred either from the metal to the non-coordinated substrate (e.g. olefin) or from the free reagent (e.g. F(+)) to the metal-bound substrate (e.g.beta-ketoester), depending on the class of catalyst (monocationic, Class A; or dicationic, Class B). The monocationic five-coordinate species [RuCl(PNNP)](+) and the six-coordinate complexes [RuCl(L)(PNNP)](+) (L = Et(2)O, H(2)O) of Class A catalyse asymmetric epoxidation, cyclopropanation (carbene transfer from the metal to the free olefin), and imine aziridination. Alternatively, the dicationic complexes [Ru(L-L)(PNNP)](2+) (Class B), which contain substrates that act as neutral bidentate ligands L-L (e.g., beta-ketoesters), catalyse Michael addition, electrophilic fluorination, and hydroxylation reactions. Additionally, unsaturated beta-ketoesters form dicationic complexes of Class B that catalyse Diels-Alder reactions with acyclic dienes to produce tetrahydro-1-indanones and estrone derivatives. Excellent enantioselectivity has been achieved in several of the catalytic reactions mentioned above. The study of key reaction intermediates (both in the solid state and in solution) has revealed significant mechanistic aspects of the catalytic reactions. PMID:20676450

Mezzetti, Antonio

2010-09-14

295

N, N?-Olefin Functionalized Bis-Imidazolium Gold(I) Salt Is an Efficient Candidate to Control Keratitis-Associated Eye Infection  

Science.gov (United States)

Keratitis treatment has become more complicated due to the emergence of bacterial or fungal pathogens with enhanced antibiotic resistance. The pharmaceutical applications of N-heterocyclic carbene complexes have received remarkable attention due to their antimicrobial properties. In this paper, the new precursor, 3,3?-(p-phenylenedimethylene) bis{1-(2- methyl-allyl)imidazolium} bromide (1a) and its analogous PF6 salt (1b) were synthesized. Furthermore, silver(I) and gold(I) -N-heterocyclic carbene (NHC) complexes [Ag2LBr2/Au2LBr2; 2a/3a], [(Ag2L2)(PF6)2/(Au2L2)(PF6)2; 2b/3b] were developed from their corresponding ligands. All compounds were screened for their antimicrobial activities against multiple keratitis-associated human eye pathogens, including bacteria and fungi. Complexes 2a and 3a showed highest activity, and the effectiveness of 3a was also studied, focusing eradication of pathogen biofilm. Furthermore, the structures of 1a, 2a and 3b were determined using single crystal X-ray analysis, 2b and 3a were optimized theoretically. The mechanism of action of 3a was evaluated by scanning electron microscopy and docking experiments, suggesting that its target is the cell membrane. In summary, 3a may be helpful in developing antimicrobial therapies in patients suffering from keratitis-associated eye infections caused by multidrug-resistant pathogens. PMID:23554886

Samanta, Tapastaru; Roymahapatra, Gourisankar; Porto, William F.; Seth, Saikat; Ghorai, Sudipta; Saha, Suman; Sengupta, Jayangshu; Franco, Octávio L.; Dinda, Joydev; Mandal, Santi M.

2013-01-01

296

Low-spin hexacoordinate Mn(III): synthesis and spectroscopic investigation of homoleptic tris(pyrazolyl)borate and tris(carbene)borate complexes.  

Science.gov (United States)

Three complexes of Mn(III) with "scorpionate" type ligands have been investigated by a variety of physical techniques. The complexes are [Tp(2)Mn]SbF(6) (1), [Tp(2)*Mn]SbF(6) (2), and [{PhB(MeIm)(3)}(2)Mn](CF(3)SO(3)) (3a), where Tp(-) = hydrotris(pyrazolyl)borate anion, Tp*(-) = hydrotris(3,5-dimethylpyrazolyl)borate anion, and PhB(MeIm)(3)(-) = phenyltris(3-methylimidazol-2-yl)borate anion. The crystal structure of 3a is reported; the structures of 1 and 2 have been previously reported, but were reconfirmed in this work. The synthesis and characterization of [{PhB(MeIm)(3)}(2)Mn]Cl (3b) are also described. These complexes are of interest in that, in contrast to many hexacoordinate (pseudo-octahedral) complexes of Mn(III), they exhibit a low-spin (triplet) ground state, rather than the high-spin (quintet) ground state. Solid-state electronic absorption spectroscopy, SQUID magnetometry, and high-frequency and -field electron paramagnetic resonance (HFEPR) spectroscopy were applied. HFEPR, in particular, was useful in characterizing the S = 1 spin Hamiltonian parameters for complex 1, D = +19.97(1), E = 0.42(2) cm(-1), and for 2, D = +15.89(2), E = 0.04(1) cm(-1). In addition, frequency domain Fourier-transform THz-EPR spectroscopy, using coherent synchrotron radiation, was applied to 1 only and gave results in good agreement with HFEPR. Variable-temperature dc magnetic susceptibility measurements of 1 and 2 were also in good agreement with the HFEPR results. This magnitude of zero-field splitting (zfs) is over 4 times larger than that in comparable hexacoordinate Mn(III) systems with S = 2 ground states. Complexes 3a and 3b (i.e., regardless of counteranion) have a yet much larger magnitude zfs, which may be the result of unquenched orbital angular momentum so that the spin Hamiltonian model is not appropriate. The triplet ground state is rationalized in each complex by ligand-field theory (LFT) and by quantum chemistry theory, both density functional theory and unrestricted Hartree-Fock methods. This analysis also shows that spin-crossover behavior is not thermally accessible for these complexes as solids. The donor properties of the three different scorpionate ligands were further characterized using the LFT model that suggests that the tris(carbene)borate is a strong ?-donor with little or no ?-bonding. PMID:23259486

Forshaw, Adam P; Smith, Jeremy M; Ozarowski, Andrew; Krzystek, J; Smirnov, Dmitry; Zvyagin, S A; Harris, T David; Karunadasa, Hemamala I; Zadrozny, Joseph M; Schnegg, Alexander; Holldack, Karsten; Jackson, Timothy A; Alamiri, Ahmad; Barnes, Diane M; Telser, Joshua

2013-01-01

297

Generation of carbene complexes in decomposition of organometallic compounds of transition metals  

International Nuclear Information System (INIS)

A method of forming carbine complexes initiating polymerization of cycloolephines with use being made of a redox system is considered herein. It has been established that one of such systems may involve decomposition of compounds of the general formula Rsub(3)ZCHsub(2)WXsub(n). There is described the decomposition of an organo-metallic compound, accompanied by formation of radicals (CH3)SicH2WCL5 initiating polymerization of cycloolephines at temperatures down to -78 deg C. The decomposition products are identified by gas-liquid chromatography, paramagnetic resonance and mass-spectrometry techniques. The composition of the decomposition products is given. It is concluded that some organo-metallic compounds of transition metals may serve as sources of carbenes in solutions at low temperatures

298

Evidence for the cyclic CN2 carbene in the gas phase.  

Science.gov (United States)

3-Halodiazirine-3-carboxylic acids (c-CN2XCOOH, X = Cl or Br) were prepared from their esters and converted to the corresponding sodium salts. Collision-induced dissociation (CID) of the carboxylate ions led exclusively to the loss of CO2 and the resulting c-CN2X(-) ions dissociated to c-CN2 carbene at low energies. The bond dissociation energy (BDE) for c-CN2Br(-) was found to be less than 8 kcal/mol using CID of the anion generated by electrospray ionization of the carboxylate. The analogous difluoro system (CF2XCOOH/CF2X(-)/CF2) exhibits similar dissociative behavior. All experimental BDEs are in very good agreement with MP4/aug-cc-pVTZ calculations. PMID:25296289

Hanzlová, Eva; Vá?a, Ji?í; Shaffer, Christopher J; Roithová, Jana; Martin?, Tomáš

2014-10-17

299

C1 polymerisation and related C-C bond forming 'carbene insertion' reactions.  

Science.gov (United States)

In this critical review we summarise the currently available 'C1 polymerisation' techniques as valuable alternatives for 'C2 polymerisation' in the preparation of saturated main-chain carbon-based polymers. C1 polymerisation involves the growth of polymers from monomers delivering only one functionalised carbon unit (C1 monomers; typically 'carbene precursors') in each chain-growth step, which contrasts with common polymerisation of C=C bond containing substrates (C2 monomers). In the general introduction (section 1) we comment on the availability of C1 monomers and the most important differences between C1 and C2 polymerisation techniques, highlighting the opportunities provided by C1 polymerisation to prepare new polymer structures. In section 2 we describe several Lewis acid mediated C1 polymerisation reactions based on diazocompounds and sulfur ylides as C1 monomers. Some of these are 'living polymerisation' reactions which allow the synthesis of functional telechelic block-copolymers and polymethylenic homo-polymers with a wide variety of different functional end-groups at both polymer chain-ends in a controlled fashion. Miscellaneous related reactions of other C1 monomers are described in section 3. Transition metal mediated C1 polymerisation methods in section 4 allow the polymerisation of polar functionalised C1 monomers. Several homogeneous and heterogeneous catalysts are capable of producing polymers from a variety of diazo compounds, some of which even allow the formation of stereoregular polar functionalised polymers (which is a major challenge in traditional olefin polymerisation). An overview of the current state-of-the art, challenges and opportunities, as well as an overview of the available mechanistic information (sections 4 and 5) is provided. Some related C-C bond forming reactions proceeding via (migratory) carbene insertion (section 6) are also discussed (106 references). PMID:20419216

Jellema, Erica; Jongerius, Anna L; Reek, Joost N H; de Bruin, Bas

2010-05-01

300

Photolysis of 4-chlororesorcinol in water: competitive formation of a singlet ketene and a triplet carbene.  

Science.gov (United States)

Research on photoinduced reactions of halogenated phenols is of interest for environmental photochemistry and for synthetic organic chemistry. Previous studies have uncovered interesting mechanistic features, including ring contraction from benzene to cyclopentadiene from ortho derivatives and two-electron processes forming carbocations and carbenes from para derivatives. In the present work, we studied the aqueous photochemistry of 4-chlororesorcinol (1), which combines the conformational properties of both types of derivatives, using nanosecond transient absorption spectroscopy and photoproduct analysis. The absorption spectra obtained upon pulsed laser excitation of 1 showed the occurrence of both o-Cl and p-Cl elimination, the first observed transients being the ketene 3-hydroxy-6-fulvenone (2, ?(max) = 255 nm) and the carbene 2-hydroxy-4-oxo-2,5-cyclohexadienylidene (3, ?(max) = 405 and 395 nm). The reactivities of 2 and 3, the spectra of the secondary transients and the analysis of the final products showed that the two HCl elimination pathways take place concurrently. Most probably, the bifurcation step is the competition between intersystem crossing on the molecular level and o-Cl elimination on the singlet surface; p-Cl elimination proceeds on the triplet surface. Remarkably, the quantum yield of p-Cl elimination from 1 is lower by one order of magnitude compared to that found in para-halogenated phenols, while that of o-Cl elimination from 1 is comparable to ortho-halogenated phenol. To explain this result, we propose that o-Cl elimination is the major deactivation step, forming an intermediate singlet cation which is able to recombine to ground state 1, thereby limiting the observed photochemical quantum yields. PMID:20859579

Richard, Claire; Krajnik, Paul; Grabner, Gottfried

2010-11-14

301

Metal nanoparticles functionalized with metal-ligand covalent bonds  

Science.gov (United States)

Metal-organic contact has been recognized to play important roles in regulation of optical and electronic properties of nanoparticles. In this thesis, significant efforts have been devoted into synthesis of ruthenium nanoparticles with various metal-ligand interfacial linkages and investigation of their electronic and optical properties. Ruthenium nanoparticles were prepared by the self-assembly of functional group onto bare Ru colloid surface. As to Ru-alkyne nanoparticles, the formation of a Ru-vinylidene (Ru=C=CH--R) interfacial bonding linkage was confirmed by the specific reactivity of the nanoparticles with imine derivatives and olefin at the metal-ligand interface, as manifested in NMR, photoluminescence, and electrochemical measurements. Interestingly, it was found the electronic coupling coefficient (beta)for strongly depend upon such metal-ligand interfacial bonding. Next, such metal-ligand interfacial bonding was extended to ruthenium-nitrene pi bonds on ruthenium colloids, which were investigated by XPS. The nanoparticles exhibited a 1:1 atomic ratio of nitrogen to sulfur, consistent with that of sulfonyl nitrene fragments. In addition, the nanoparticle-bound nitrene moieties behaved analogously to azo derivatives, as manifested in UV-vis and fluorescence measurements. Further testimony of the formation of Ru=N interfacial linkages was highlighted in the unique reactivity of the nanoparticles with alkenes by imido transfer. Extensive conjugation between metal-ligand interfacial bond results in remarkable intraparticle charge delocalization on Ru-alkynide nanoparticles, which was manipulated by simple chemical reduction or oxidation. Charging of extra electrons into the nanoparticle cores led to an electron-rich metal core and hence red-shift of the triple bond stretching mode, lower binding energy of sp hybridized C 1s and dimmed fluorescence of nanoparticles. Instead, chemical oxidation resulted in the opposite impacts on these properties. By taking advantage of such extensively conjugated metal-ligand bonding and effective intraparticle charge delocalization of ruthenium nanoparticles, Ru=carbene nanoparticles functionalized with multiple moieties by olefin metathesis reactions was further exploited for metal ion sensing. When the nanoparticles were co-functionalized with 1-vinylpyrene and 4-vinylbenzo-18-crown-6, upon the binding of metal ions into the crown ether cavity, the emission intensity of the nanoparticle fluorescence from the conjugation of vinylpyrene was found to diminish, with the most significant effects observed with K+ ions. In the case of ruthenium nanoparticles co-functionalized with pyrene and histidine derivative moieties through Ru=carbene pi bonds. The selective complexation of the histidine moiety with transition metal ions led to marked diminishment of the emission intensity from conjugation of pyrene. Of all the metal ions tested, the impacts were much more drastic with Pb2+ , Co2+ and Hg2+ than with Li +, K+, Rb+, Mg2+ Ca 2+ and Zn2+ ions. These were ascribed to the selective binding of 18-crown-6 to potassium ions or complexation of histidine derivative to transition metal ions, where the metal ions led to polarization of the nanoparticle core electrons to the metal surface and hence impeded intraparticle charge delocalization. Functionalization of semiconductor with metal nanoparticles could be exploited to remarkably enhance their photo catalytic performance. Before this exploration, in the last chapter, the impacts of the TiO2 nanocrystalline structure on the photocatalytic activity were then examined by using the reduction of methylene blue in water. It was found that in the presence of anatase and brookite crystalline phase, TiO2 nanotube arrays exhibited the highest photo catalytic activity. This is ascribed to synergistic coupling of the anatase and brookite crystalline domains, which led to effective charge separation upon photoirradiation.

Kang, Xiongwu

302

Role of gold(I) ?-oxo carbenes in the oxidation reactions of alkynes catalyzed by gold(I) complexes.  

Science.gov (United States)

The gas phase structures of gold(I) complexes formed by intermolecular oxidation of selected terminal (phenylacetylene) and internal alkynes (2-butyne, 1-phenylpropyne, diphenylacetylene) were investigated using tandem mass spectrometry and ion spectroscopy in conjunction with quantum-chemical calculations. The experiments demonstrated that the primarily formed ?-gold(I) vinyloxypyridinium complexes readily undergo rearrangement, dependent on their substituents, to either gold(I) ?-oxo carbenenoids (a synthetic surrogate of the ?-oxo carbenes) or pyridine adducts of gold(I) enone complexes in the condensed phase and that the existence of naked ?-oxo carbenes is highly improbable. Isotopic labeling experiments performed with the reaction mixtures clearly linked the species that exist in solution to the ions transferred to the gas phase. The ions were then fully characterized by CID experiments and IRMPD spectroscopy. The conclusions based on the experimental observations perfectly correspond with the results from quantum-chemical calculations. PMID:25068382

Schulz, Ji?í; Jašíková, Lucie; Skríba, Anton; Roithová, Jana

2014-08-13

303

A spectroscopic study of the reaction of the carbene, tetrabromocyclopentadienylidene, with O 2 in low temperature matrices  

Science.gov (United States)

The photolysis of tetrabromodiazocyclopentadiene ( 2) in low temperature Ar and N 2 matrices generates the carbene, tetrabromocyclopentadienylidene ( 3). In O 2-doped matrices, ( 3) reacts with oxygen yielding first tetrabromocyclopentadienone O-oxide ( 4), identification of which was aided by experiments with isotopically labelled oxygen, and then tetrabromocyclopentadienone ( 5) and tetrabromo-?-pyrone ( 6). The matrix uv-visible and infrared absorption spectra of tetrabromocyclopentadienone O-oxide ( 4) are reported.

Dunkin, Ian R.; McCluskey, Adam

1994-01-01

304

Synthesis and characterization of reactive intermediates: The study of biradical, carbene, nitrene and carbyne negative ions in the gas phase  

OpenAIRE

The synthesis and characterization of carbene, nitrene, carbyne and biradical negative ions were studied using the flowing afterglow-triple quadrupole apparatus. These include the negative ions of phenylcarbene, 3-ethoxyphenylnitrene, phenylcarbyne and 2,4-dimethylenecyclobutane-1,3-diyl, respectively. Various experimental gas-phase acidities, electron affinities and halide dissociation energies were combined in thermochemical cycles to derive the heats of formation of phenylcarbene, phenylca...

Hill, Brian Trenton

2000-01-01

305

Density functional computations of the cyclopropanation of ethene catalyzed by iron (II) carbene complexes Cp(CO)(L)Fe?CHR, L D CO, PMe3, R D Me, OMe, ph, CO2Me  

Science.gov (United States)

Density functional theory has been used to study the Fe-catalyzed cyclopropanation of Fe-carbene complexes with ethene. All the intermediates and transition states were optimized completely at the B3LYP/6-31+G(d,p) level. Calculation results confirm that the cyclopropanation of Fe-carbene complexes with ethene involves the two reaction paths I and II. In the reaction path I, the double bond of ethene attacks directly on the carbene carbon of Fe-carbene complexes to generate the cyclopropane. In the reaction path II, ethene substitution for PMe3 or CO in the Fe-carbene complexes leads to the complexes M2; and the attack of one carbon of ethene on the carbene carbon results in the complexes M3 with a Fe bond C bond C bond C four-membered ring, and then generates the cyclopropane via the elimination reaction. For Fe-carbene complexes A, C, D, E, and H, the main reaction mode is the reaction path I; for Fe-carbene complexes B, F, and G, the main reaction mode is the reaction path II.0

Wang, Fen; Meng, Qingxi; Li, Ming

306

rac-cis-Dicarbonylchlorido{1-[2-(diphenylphosphanyl-?Pbenzyl]-3-(phenyl-?C1imidazol-2-ylidene-?C2}ruthenium(II dichloromethane monosolvate  

Directory of Open Access Journals (Sweden)

Full Text Available In the title compound, [Ru(C28H22N2PCl(CO2]·CH2Cl2, the RuII atom exhibits a distorted octahedral coordination geometry. The N-phenyl group of the ligand has undergone orthometalation; as a result, the tridentate phosphane-functionalized N-heterocyclic carbene ligand is coordinating in a meridional fashion. This complex is of interest with respect to transfer hydrogenation catalysis and also provides an example of C—H activation behavior in late transition metal complexes. The dichloromethane solvent molecule is disordered over two sets of sites with an occupancy ratio of 0.873?(14:0.127?(14.

Gregory J. Domski

2012-09-01

307

Catalytic asymmetric syntheses of quinolizidines by dirhodium-catalyzed dearomatization of isoquinolinium/pyridinium methylides--the role of catalyst and carbene source.  

Science.gov (United States)

Convenient access to highly enantioenriched substituted quinolizidines has been achieved by chiral dirhodium(II) carboxylate-catalyzed dearomatizing formal [3 + 3]-cycloaddition of isoquinolinium/pyridinium methylides and enol diazoacetates. Coordination of Lewis basic methylides to dirhodium(II) prompts the rearrangement of the enol-carbene that is bound to dirhodium to produce a donor-acceptor cyclopropene. The donor-acceptor cyclopropene is in equilibrium with the dirhodium-bound enol-carbene and undergoes both enantioselective [3 + 3]-cycloaddition from the dirhodium-bound enol-carbene and diastereoselective [3 + 2]-cycloaddition by uncatalyzed reaction of the cyclopropene with isoquinolinium or pyridinium methylides. Increasing the mol % of catalyst loading suppresses the [3 + 2]-cycloaddition pathway. PMID:23889041

Xu, Xichen; Zavalij, Peter Y; Doyle, Michael P

2013-08-21

308

Novel 7-phenylsulfanyl-1,2,3,4,10,10a-hexahydro-pyrazino[1,2-a]indoles as dual serotonin 5-HT2C and 5-HT6 receptor ligands  

DEFF Research Database (Denmark)

Novel 7-phenylsulfanyl-1,2,3,4,10,10a-hexahydro-pyrazino[1,2-a]indoles are synthesized using a six-step protocol. Notably, the synthesis route make use of a new and improved ring-closing methodology for the assembly of the hexahydro-pyrazino[1,2-a]indole scaffold, which is based on intramolecular C-H insertion of a carbene. The compounds act as dual serotonin 5-HT2C- and 5-HT6-ligands.

Krogsgaard-Larsen, Niels; Jensen, Anders A.

2010-01-01

309

Inserção C-H de carbenóides de ródio em água e reutilização do catalisador Rhodium (II carbene C-H insertion in water and catalyst reuse  

Directory of Open Access Journals (Sweden)

Full Text Available A five-session laboratory experiment is described for the synthesis of a beta-lactam via Rh(II catalysed intramolecular C-H insertion of a alpha-diazo-alpha-ethoxycarbonylacetamide. The metallo-carbene, responsible for the C-H bond activation, was generated from the diazo substrate and the catalyst Rh2(OAc4. The high stability and solubility of the catalyst and the exclusive C-H insertion of the Rh-carbene allows the synthesis of this important heterocycle in water and the catalyst reutilization.

Nuno R. Candeias

2007-01-01

310

Inserção C-H de carbenóides de ródio em água e reutilização do catalisador / Rhodium (II) carbene C-H insertion in water and catalyst reuse  

Scientific Electronic Library Online (English)

Full Text Available SciELO Brazil | Language: Portuguese Abstract in portuguese [...] Abstract in english A five-session laboratory experiment is described for the synthesis of a beta-lactam via Rh(II) catalysed intramolecular C-H insertion of a alpha-diazo-alpha-ethoxycarbonylacetamide. The metallo-carbene, responsible for the C-H bond activation, was generated from the diazo substrate and the catalyst [...] Rh2(OAc)4. The high stability and solubility of the catalyst and the exclusive C-H insertion of the Rh-carbene allows the synthesis of this important heterocycle in water and the catalyst reutilization.

Nuno R., Candeias; Pedro M. P., Gois; Carlos A. M., Afonso.

1768-17-01

311

Rhodium (II) carbene C-H insertion in water and catalyst reuse; Insercao C-H de carbenoides de rodio em agua e reutilizacao do catalisador  

Energy Technology Data Exchange (ETDEWEB)

A five-session laboratory experiment is described for the synthesis of a beta-lactam via Rh(II) catalysed intramolecular C-H insertion of a alpha-diazo-alpha-ethoxycarbonyl acetamide. The metallo-carbene, responsible for the C-H bond activation, was generated from the diazo substrate and the catalyst Rh{sub 2}(OAc){sub 4}. The high stability and solubility of the catalyst and the exclusive C-H insertion of the Rh-carbene allows the synthesis of this important heterocycle in water and the catalyst reutilization. (author)

Candeias, Nuno R.; Gois, Pedro M.P.; Afonso, Carlos A.M. [Instituto Superior Tecnico, Lisboa (Portugal)]. E-mail: carlosafonso@ist.utl.pt

2007-07-01

312

Cyclopentadienylidene: aromaticity and antiaromaticity of the ?- and ?-cyclic conjugation systems in cyclic conjugated carbenes  

International Nuclear Information System (INIS)

The feasibility was discussed for the prediction of electronic structural characteristics of cyclic conjugated carbenes on the basis not only of the ?-cyclic conjugation system but also of the ?-cyclic conjugation system. The features found are in accord both with the results of quantum chemical calculations and the available experimental data. As an example, a detailed examination was carried out on cyclopentadienylidene. MINDO/3, MNDO and ab initio calculations with the STO-3G and 4-31G bases showed that the ground state of cyclopentadienylidene is a triplet (3B1, planar C/sub 2v/ structure). The reasons were discussed for the lability of the planar 6? and 4? electron C/sub 2v/ geometric configurations of the singlet 1A1 state which do not correspond to minima on the potential energy surfaces of C5H4. The scheme for the isomerization of acoplanar C/sub s/ structures of the singlet state of cyclopentadienylidene was shown to be the inverted scheme for the interconversion of 6? and 4? electron forms. The bifurcation of the completely symmetrical pathway of this interconversion leads to C/sub s/ structures

313

Electrochemical illumination of thienyl and ferrocenyl chromium(0) Fischer carbene complexes.  

Science.gov (United States)

A series of ferrocenyl and thienyl mono- and biscarbene chromium(0) complexes 1-6 were synthesised. The complexes were characterised both spectroscopically and electrochemically, and the single crystal X-ray structure of 3 was determined. Electrochemical measurements in CH2Cl2 revealed that the carbene double bond of 1-6 is reduced to an anion radical, (-)Cr-C? at formal reduction potentials 0.95 V. The reactivity towards follow-up chemical reactions of the anodically produced Cr(II) species is much higher than the reactivity of the cathodically produced radical anions as the latter was still observably reoxidised to the parent Cr=C species at fast scan rates. The ferrocenyl group is oxidised electrochemically reversibly to ferrocenium at larger potentials than the electrochemically reversible oxidation of the Cr(0) centre to Cr(I). That all redox centres in 1-6 are involved in one-electron transfer steps was confirmed by comparing the ferrocenyl voltammetric wave with those of the other redox centres in linear sweep voltammetric experiments. The ferrocenyl group was electrochemically shown to stabilise the Cr=C centre almost as much as the NHBu, and much more than the ethoxy and thienyl groups. PMID:23417243

van der Westhuizen, Belinda; Swarts, Pieter J; Strydom, Ian; Liles, David C; Fernández, Israel; Swarts, Jannie C; Bezuidenhout, Daniela I

2013-04-21

314

Relativistic effects in the cationic platinum carbene PtCH + 2  

Science.gov (United States)

Fully relativistic four-component Dirac-Fock Coulomb calculations in conjunction with a second-order perturbational estimate for the correlation energy have been performed in benchmark calculations on geometric and electronic structures as well as the binding energy of the cationic platinum carbene complex PtCH+2. The relativistic stabilization of this species amounts to as much as 50 kcal/mol and the combination of relativistic and correlation effects shorten the Pt-C bond length by nearly 1 bohr, changing the bond order from one to two. The relative importance of spin-free and spin-dependent relativistic effects on the geometry, the electronic structure, and the binding energy is evaluated by comparison to the Douglas-Kroll method. Relativistic effective core potentials are shown to describe the spin-free effects reliably. The best theoretical estimate for the bond dissociation energy underestimates the experimental value by 13% due to truncation errors in the one- and n-particle space treatments. The mixed Hartree-Fock/density-functional method Becke3-Lee-Yang-Parr performs surprisingly well with respect to the structure and binding energy of the target molecule.

Heinemann, Christoph; Schwarz, Helmut; Koch, Wolfram; Dyall, Kenneth G.

1996-03-01

315

Ligand modeling and design  

Energy Technology Data Exchange (ETDEWEB)

The purpose of this work is to develop and implement a molecular design basis for selecting organic ligands that would be used tin applications for the cost-effective removal of specific radionuclides from nuclear waste streams.

Hay, B. [Pacific Northwest Lab., Richland, WA (United States)

1996-10-01

316

CAAC Boranes. Synthesis and characterization of cyclic (alkyl) (amino) carbene borane complexes from BF3 and BH3  

OpenAIRE

In situ formation of two cyclic (alkyl) (amino) carbenes (CAACs) followed by addition of BF3•Et2O provided the first two examples of CAAC–BF3 complexes: 1-(2,6-diisopropylphenyl)-3,5,5-trimethyl-3-phenylpyrrolidin-2-ylidene trifluoroborane, and 2-(2,6-diisopropylphenyl)-3,3-dimethyl-2-azaspiro[4.5]decan-1-ylidene trifluoroborane. These CAAC–BF3 complexes are robust compounds that are stable to ambient laboratory conditions and silica gel chromatography. They were characterized by spectr...

Monot, Julien; Fensterbank, Louis; Malacria, Max; Laco?te, Emmanuel; Geib, Steven J.; Curran, Dennis P.

2010-01-01

317

Complexation of lanthanides, actinides and transition metal cations with a 6-(1,2,4-triazin-3-yl)-2,2':6',2''-terpyridine ligand: implications for actinide(III)/lanthanide(III) partitioning.  

Science.gov (United States)

The quadridentate N-heterocyclic ligand 6-(5,5,8,8-tetramethyl-5,6,7,8-tetrahydro-1,2,4-benzotriazin-3-yl)-2,2'?:?6',2''-terpyridine (CyMe(4)-hemi-BTBP) has been synthesized and its interactions with Am(III), U(VI), Ln(III) and some transition metal cations have been evaluated by X-ray crystallographic analysis, Am(III)/Eu(III) solvent extraction experiments, UV absorption spectrophotometry, NMR studies and ESI-MS. Structures of 1:1 complexes with Eu(III), Ce(III) and the linear uranyl (UO(2)(2+)) ion were obtained by X-ray crystallographic analysis, and they showed similar coordination behavior to related BTBP complexes. In methanol, the stability constants of the Ln(III) complexes are slightly lower than those of the analogous quadridentate bis-triazine BTBP ligands, while the stability constant for the Yb(III) complex is higher. (1)H NMR titrations and ESI-MS with lanthanide nitrates showed that the ligand forms only 1:1 complexes with Eu(III), Ce(III) and Yb(III), while both 1:1 and 1:2 complexes were formed with La(III) and Y(III) in acetonitrile. A mixture of isomeric chiral 2:2 helical complexes was formed with Cu(I), with a slight preference (1.4:1) for a single directional isomer. In contrast, a 1:1 complex was observed with the larger Ag(I) ion. The ligand was unable to extract Am(III) or Eu(III) from nitric acid solutions into 1-octanol, except in the presence of a synergist at low acidity. The results show that the presence of two outer 1,2,4-triazine rings is required for the efficient extraction and separation of An(III) from Ln(III) by quadridentate N-donor ligands. PMID:22729349

Lewis, Frank W; Harwood, Laurence M; Hudson, Michael J; Drew, Michael G B; Sypula, Michal; Modolo, Giuseppe; Whittaker, Daniel; Sharrad, Clint A; Videva, Vladimira; Hubscher-Bruder, Véronique; Arnaud-Neu, Françoise

2012-08-14

318

Controlled CO release using photochemical, thermal and electrochemical approaches from the amino carbene complex [(CO)?CrC(NC?H?)CH?].  

Science.gov (United States)

Multimodal photo, thermal and electrochemical approaches toward CO release from the amino carbene complex [(CO)5CrC(NC4H8)CH3] is reported. Picosecond time resolved infrared spectroscopy was used to probe the photo-induced early state dynamics leading to CO release, and DFT calculations confirmed that CO release occurs from a singlet excited state. PMID:25188765

McMahon, Suzanne; Rochford, Jonathan; Halpin, Yvonne; Manton, Jennifer C; Harvey, Emma C; Greetham, Gregory M; Clark, Ian P; Rooney, A Denise; Long, Conor; Pryce, Mary T

2014-10-21

319

Competitive pathways in the reaction of lithium oxy-ortho-quinodimethanes and Fischer alkoxy alkynyl carbene complexes: synthesis of highly functionalised seven-membered benzocarbocycles.  

Science.gov (United States)

Up to four different outcomes have been found for the reaction between 1-oxy-ortho-quinodimethanes (oQDMs) and alkoxy alkynyl Fischer carbene complexes (FCCs). The product formed depends on the structure of both reagents and on the reaction solvent. The pathways can be topologically classified as a [4C+2C], a [3(2C+O)+3C], and two different [4C+3C] processes and, in all these sequences, 1-oxy-oQDMs behave as enolates or as vinylogous enolates. The reaction of Choy and Yang's unsubstituted oQDM 1 with tungsten alkynyl FCCs is solvent controlled; thus, selective formation of benzocycloheptenones can be achieved in THF, whereas exclusive synthesis of benzocycloheptene ketals is reached in diethyl ether. On the other hand, THF is the solvent of choice to form benzocycloheptene ketals when an alkyl or aryl group is placed at position?1 of the oQDM in its reaction with tungsten carbene complexes; however, a pyranylidene carbene complex is formed when a chromium carbene complex is used. Alternatively, the presence of bulky alkoxy groups in the FCC component favours a Diels-Alder aromatisation sequence, which leads to 1-naphthyl FCCs. Furthermore, the isolation and the characterisation of several deuterated compounds by labelling experiments have provided some insight into the reaction pathways, and mechanisms consistent with those findings have been established and several reaction intermediates have been identified. PMID:21207574

García-García, Patricia; Novillo, Carlos; Fernández-Rodríguez, Manuel A; Aguilar, Enrique

2011-01-10

320

Pull-push mechanism for the 1,2-hydrogen rearrangement of carbenes. Substituent and deuterium isotope effects for thermal decomposition of 1-phenyl-2-diazopropanes  

International Nuclear Information System (INIS)

Intramolecular and intermolecular deuterium isotope effects have been measured for the thermal carbene H rearrangements of three substituted 1-phenyl-2-diazopropanes-1-d in hexane at 250C. The isotope effects, k/sub H//k/sub D/, vary from ca. 1.2 to 1.5, increasing with electron-withdrawing substituents, for the formation of both cis- and trans-?-methylstyrenes. Product ratios were determined for these compounds as well as the corresponding undeuterated ones, permitting calculation of intermolecular primary and secondary isotope effects. In addition, the competition among the cis and trans products and the product of H migration from the terminal methyl group permitted determination of Hammett rho values of ca. -1 for the competition between benzylic and terminal migration. The results strongly indicate a ''pull-push'' mechanism, which can be pictured roughly as electrophilic attack on the C--H bond by the phantom p orbital of the carbene along with backside nucleophilic attack by the carbene unshared electron pair to push the H away and form the ? bond. The data are believed to be consistent only with a nonzero barrier for the carbene hydrogen rearrangement

321

Domino Michael addition-carbene rearrangement-cyclization reaction of 1-alkynyl(aryl)-lambda3-bromanes with 2-mercapto-1,3-benzazoles.  

Science.gov (United States)

Exposure of 1-alkynyl[p-(trifluoromethyl)phenyl](tetrafluoroborato)-lambda3-bromanes to 2-mercaptobenzimidazole or benzothiazole in dichloromethane at 0 degrees C under argon resulted in a domino Michael addition-carbene rearrangement-cyclization reaction to produce directly tricyclic heterocycles in high yields, whereas the reaction with 2-mercaptobenzoxazole afforded 1-alkynyl sulfides. PMID:16220178

Ochiai, Masahito; Tada, Norihiro

2005-10-28

322

Synthesis, characterization and luminescence studies of gold(I)–NHC amide complexes  

Science.gov (United States)

Summary A flexible, efficient and straightforward methodology for the synthesis of N-heterocyclic carbene gold(I)–amide complexes is reported. Reaction of the versatile building block [Au(OH)(IPr)] (1) (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) with a series of commercially available (hetero)aromatic amines leads to the synthesis of several [Au(NRR’)(IPr)] complexes in good yields and with water as the sole byproduct. Interestingly, these complexes present luminescence properties. UV–vis and fluorescence measurements have allowed the identification of their excitation and emission wavelengths (?max). These studies revealed that by selecting the appropriate amine ligand the emission can be easily tuned to achieve a variety of colors, from violet to green. PMID:24204434

Gómez-Suárez, Adrián; Nelson, David J; Thompson, David G; Cordes, David B; Graham, Duncan; Slawin, Alexandra M Z

2013-01-01

323

High-density monolayers of metal complexes: preparation and catalysis.  

Science.gov (United States)

Catalysts are one of the key materials for realizing a sustainable society. However, we may encounter problematic cases where conventional catalyst systems cannot provide effective solutions. We thus believe that the establishment of novel methods of catalyst preparation is currently necessary. Utilization of high-density monolayers of molecular metal complexes is our strategy, and we expect that this methodology will enable facile and systematic screening of unique and efficient catalysts. This Personal Account describes our challenges to establish such an immature method in catalyst preparation as well as the related background and perspective. Preparation and catalysis by high-density monolayers of Rh complexes with N-heterocyclic carbene, structurally compact phosphine and diisocyanide ligands on gold surfaces are presented. The catalytic application of a high-density Pd-bisoxazoline complex prepared on a single-crystal silicon surface is also shown. Uniquely high catalyst turnover numbers and high chemoselectivities were observed with these catalyst systems. PMID:25112780

Hara, Kenji; Sawamura, Masaya; Fukuoka, Atsushi

2014-10-01

324

3,3?-Di-n-butyl-1,1?-(p-phenylenedimethylenediimidazolium bis(hexafluorophosphate  

Directory of Open Access Journals (Sweden)

Full Text Available The asymmetric unit of the title N-heterocyclic carbene compound, C22H32N42+·2PF6?, consists of one half of the N-heterocyclic carbene dication and one hexafluorophosphate anion. The dication lies across a crystallographic inversion center. The imidazole ring is twisted away from the central benzene ring, making a dihedral angle of 76.23?(6°. The hexafluorophosphate anions link the cations into a three-dimensional network via intermolecular C—H...F hydrogen bonds. A weak C—H...? interaction further stabilizes the crystal structure.

Rosenani A. Haque

2010-04-01

325

Ligand discovery: Docking points  

Science.gov (United States)

The flexibility and structural dynamics of proteins pose a big challenge for those trying to discover new bioactive compounds. Now, by using guiding crystallographic data, a method that uses the energetic balance between protein conformers to weight docking scores is shown to aid the hunt for new ligands.

Barril, Xavier

2014-07-01

326

Carbene complexes of zirconium. Synthesis, structure, and reactivity with carbon monoxide to affort coordinated ketene  

International Nuclear Information System (INIS)

Treatment of Cp2Zr(L)(CO) (Cp = C5H5; L = PMe3, CO) with Cp*2ZrH2 (Cp* = C5Me5) affords zirconium oxycarbene complexes, Cp2(L)Zr double bond CHO single bond Zr(X)Cp*2 (L = PMe3, X = H, I; L = CO, X = H), that represent some of the first examples of group 4 metal-to-carbon multiple bonding. The first X-ray diffraction structure determination of a zirconium carbene complex, that of Cp(PMe3)Zr double bond CHO single bond Zr(H)Cp*2.C6H6, is reported (C2/c, a = 27.318 (4) A, b = 19.895 (3) A, c = 19.932 (5) A ? = 132.188 (10)0, Z = 8) and shows a very short Zr-C bond length of 2.117 (7) A. Treatment of Cp2(CO)Zr double bond CHO single bond Zr(H)Cp*2 with CH3 I or Cp2(PMe3)Zr double bond CHO single bond Zr(I)Cp*2 with CO affords the zirconium substituted enediolate zirconacycle, Cp*2ZrOCH double bond C(Zr(I)Cp2)O, which has been characterized by an X-ray diffraction study (P21/c, a = 15.866 (4) A, b = 10.673 (3) A, c = 20.561 (4) A, ? = 105.5 (2)0, Z = 4). This complex most likely forms by coupling of the zirconoxycarbene and a carbonyl to give a metal-coordinated ketene intermediate that subsequently rearranges to the isolated product. An isotopic crossover experiment has demonstrated that ssover experiment has demonstrated that the new carbon-carbon bond is formed in an intramolecular coupling step. The ketene intermediate can be trapped by dissolving Cp2(CO)Zr double bond CHO single bond Zr(H)Cp*2 in pyridine, giving Cp2(py)Zr(O double bond C double bond CHOZr(I)Cp*2). Treatment of the isolated ketene complex with CH3I in benzene gives the enediolate zirconocycle; in pyridine Cp2(py)Zr(O double bond C double bond CHOZr(I)Cp*2) can be observed spectroscopically. 38 references, 4 figures, 5 tables

327

CAAC Boranes. Synthesis and characterization of cyclic (alkyl (amino carbene borane complexes from BF3 and BH3  

Directory of Open Access Journals (Sweden)

Full Text Available In situ formation of two cyclic (alkyl (amino carbenes (CAACs followed by addition of BF3•Et2O provided the first two examples of CAAC–BF3 complexes: 1-(2,6-diisopropylphenyl-3,5,5-trimethyl-3-phenylpyrrolidin-2-ylidene trifluoroborane, and 2-(2,6-diisopropylphenyl-3,3-dimethyl-2-azaspiro[4.5]decan-1-ylidene trifluoroborane. These CAAC–BF3 complexes are robust compounds that are stable to ambient laboratory conditions and silica gel chromatography. They were characterized by spectroscopy and X-ray crystallography. In contrast, a CAAC complex with borane (BH3 was readily formed in situ according to 1H and 11B NMR analysis, but did not survive the workup conditions. These results set the stage for further studies of the chemistry of CAAC boranes.

Julien Monot

2010-08-01

328

Electronic structure and slow magnetic relaxation of low-coordinate cyclic alkyl(amino) carbene stabilized iron(I) complexes.  

Science.gov (United States)

Cyclic alkyl(amino) carbene stabilized two- and three-coordinate Fe(I) complexes, (cAAC)2FeCl (2) and [(cAAC)2Fe][B(C6F5)4] (3), respectively, were prepared and thoroughly studied by a bouquet of analytical techniques as well as theoretical calculations. Magnetic susceptibility and Mössbauer spectroscopy reveal the +1 oxidation state and S = 3/2 spin ground state of iron in both compounds. 2 and 3 show slow magnetic relaxation typical for single molecule magnets under an applied direct current magnetic field. The high-frequency EPR measurements confirm the S = 3/2 ground state with a large, positive zero-field splitting (?20.4 cm(-1)) and reveal easy plane anisotropy for compound 2. CASSCF/CASPT2/RASSI-SO ab initio calculations using the MOLCAS program package support the experimental results. PMID:25072104

Samuel, Prinson P; Mondal, Kartik Chandra; Amin Sk, Nurul; Roesky, Herbert W; Carl, Elena; Neufeld, Roman; Stalke, Dietmar; Demeshko, Serhiy; Meyer, Franc; Ungur, Liviu; Chibotaru, Liviu F; Christian, Jonathan; Ramachandran, Vasanth; van Tol, Johan; Dalal, Naresh S

2014-08-27

329

CASSCF and CASPT2 calculations of hole-doped amines with triplet carbene groups. Possibilities of high- Tc organic ferrimagnets  

Science.gov (United States)

Ab initio CASSCF and CASPT2 calculations were carried out for hole-doped amines with triplet carbene groups such as monocations of bis(methylene) and tris(methylene) amines in order to confirm previous spin polarization (SP) and delocalization (SD) rules for ion-radicals. The ground states for the hole-doped bis(methylene) and tris(methylene) amines were calculated to be quartet and sextet, respectively. The low (LS)- and high (HS)-spin crossover occurred upon hole doping, being consistent with the SD rules. Implications of the calculated results are discussed in relation to possibilities of organic ferri- and ferro-magnets composed of CT complexes with radical substituents.

Yamanaka, S.; Okumura, M.; Nagao, H.; Yamaguchi, K.

1995-02-01

330

Galanin receptors and ligands.  

OpenAIRE

The neuropeptide galanin was first discovered 30?years ago. Today, the galanin family consists of galanin, galanin-like peptide (GALP), galanin-message associated peptide (GMAP), and alarin and this family has been shown to be involved in a wide variety of biological and pathological functions. The effect is mediated through three GPCR subtypes, GalR1-3. The limited number of specific ligands to the galanin receptor subtypes has hindered the understanding of the individual effects of each r...

KristinEmmy BirgittaWebling; TamasBartfai; ÜloLangel

2012-01-01

331

Radiobiology with DNA ligands  

International Nuclear Information System (INIS)

The paper deals with the following topics: labelling of DNA ligands and other tumour-affinic compounds with 4.15-d 124I, radiotoxicity of Hoechst 33258 and 33342 and of iodinated Hoechst 33258 in cell cultures, preparation of 76Br-, 123I-, and 221At-labelled 5-halo-2'-deoxyuridine, chemical syntheses of boron derivatives of Hoechst 33258.III., Gadolinium neutron capture therapy

332

Radiobiology with DNA ligands  

Energy Technology Data Exchange (ETDEWEB)

The paper deals with the following topics: labelling of DNA ligands and other tumour-affinic compounds with 4.15-d {sup 124}I, radiotoxicity of Hoechst 33258 and 33342 and of iodinated Hoechst 33258 in cell cultures, preparation of {sup 76}Br-, {sup 123}I-, and {sup 221}At-labelled 5-halo-2`-deoxyuridine, chemical syntheses of boron derivatives of Hoechst 33258.III., Gadolinium neutron capture therapy.

Weinreich, R.; Argentini, M.; Guenther, I.; Koziorowski, J.; Larsson, B.; Nievergelt-Egido, M.C.; Salt, C.; Wyer, L. [Paul Scherrer Inst. (PSI), Villigen (Switzerland); Dos Santos, D.F.; Hansen, H.J. [Zurich Univ., Inst. of Organic Chemistry, Zurich (Switzerland)

1997-08-01

333

Photochemical generation, intramolecular reactions, and spectroscopic detection of oxonium ylide and carbene intermediates in a crystalline ortho-(1,3-dioxolan-2-yl)-diaryldiazomethane.  

Science.gov (United States)

Photochemical excitation of a diaryldiazomethane with an ortho-acetal both in solution and in crystals led to products that originate from the expected diarylcarbene and form an intramolecular oxonium ylide. While crystals strongly favored the formation of a benzocyclobutane by intramolecular hydrogen abstraction in the triplet carbene, reactions in benzene led exclusively to products that derive from the oxonium ylide. Studies in methyl cyclohexane glasses and in mixed crystals at 77 K led to the spectroscopic detection of triplet carbene (3)C, which partially transformed into a new species that we assign as the sought-after oxonium ylide Y. The formation of a formally ionic intermediate suggests that the scope of reactions by reactive intermediates in crystalline solids may be broader than it is generally assumed. PMID:19262929

Garcia-Garibay, Miguel A; Dang, Hung

2009-03-21

334

Comparison of Two Tetrapodal N, O Ligands: Impact of the Softness of the Heterocyclic N-Donors Pyridine and Pyrazine on the Selectivity for Am(III) over Eu(III)  

International Nuclear Information System (INIS)

To quantify the impact of the N-donor softness on the coordination of f elements in aqueous solution, and in particular on the selectivity for Am(III) over Eu(III), we have designed the two tetrapodal hexa-dentate ligands N, N-bis(2-pyridylmethyl)ethylenediamine-N', N'-diacetic acid (L-py) and N, N-bis(2-pyrazyl-methyl)ethylenediamine-N', N'-diacetic acid (L-pz). These ligands bear two hard acetate groups to provide stability to the An(III) and Ln(III) complexes and two N-heterocyclic soft groups to provide Am(III) versus Eu(III) selectivity. They only differ in their N-donor moieties, pyridine or pyrazine. The proton NMR and potentiometric analyses performed on the lanthanide complexes of the two ligands indicate that a unique metallic complex, LnL, is formed and that LnLpy+ and LnLpz+ have the same structure in water. Furthermore, the hydration numbers of the europium and terbium ions in these complexes, measured by luminescence decay, have the same value (q=3), indicating that the two ligands act as hexa-dentate donors in both systems. As expected, the softer pyrazine-based ligand gives less stable complexes than the pyridine-based ligand with the hard Ln(III) cations. The fragment N(CH2pz)2 containing two pyrazine functions has a very low contribution to the stability of the lanthanide complexes, even though the pyrazine groups are coordinated to the cation in water. The stabilities of the americium(III) complexes were dies of the americium(III) complexes were determined by potentiometry and are greater than those found for the isoelectronic europium complexes. The selectivity for Am(III) over Eu(III) increases from 60 to 500 when the pyridine-containing fragment N(CH2py)2 is substituted by the pyrazine-containing fragment N(CH2pz)2, which demonstrates that the selectivity for Am(III) over Eu(III) is significantly enhanced when the softness of the N-hetero-cycle increases from pyridine to pyrazine. These new hydrophilic ligands present attractive selectivities for Am(III) over Eu(III) that could make them good candidates for the selective back extraction of Am(III) from organic solutions containing 4f and 5f elements. (authors)

335

Rhodium carbene complexes as versatile catalyst precursors for Si–H bond activation  

OpenAIRE

Rhodium(III) complexes comprising monoanionic C,C,C-tridentate dicarbene ligands activate Si–H bonds and catalyse the hydrolysis of hydrosilanes to form silanols and siloxanes with concomitant release of H2. In dry MeNO2, selective formation of siloxanes takes place, while changing conditions to wet THF produces silanols exclusively. Silylethers are formed when ROH is used as substrate, thus providing a mild route towards the protection of alcohols with H2 as the only by-product. With alkyn...

Kru?ger, Anneke; Albrecht, Martin

2012-01-01

336

Ligand Cooperation in the Formal Hydrogenation of N2O Using a PCsp2P Iridium Pincer Complex.  

Science.gov (United States)

A new PCcarbeneP pincer ligand with 2,3-benzo[b]thiophene linkers connecting the flanking dialkyl phosphine donors to the central carbene can be attached to Ir(I). The chloro derivative reacts with N2O with loss of N2 to form an iridaepoxide species by addition of an oxygen atom to the Ir?C linkage. This compound reacts with H2 to afford the oxidative addition product, in which the hydride ligands are trans to the Ir-O bond. Heating this dihydride results in slow release of H2O; kinetic and spectroscopic studies show that conversion of the dihydride to its isomer, in which the hydrides are cis to the Ir-O bond, is required for H2O elimination to take place. Together, these reactions constitute the stoichiometric conversion of N2O and H2 to N2 and H2O; further mechanistic studies suggest ways to make the system catalytic. PMID:25642725

Doyle, Lauren E; Piers, Warren E; Borau-Garcia, Javier

2015-02-18

337

Melatonin: functions and ligands.  

Science.gov (United States)

Melatonin is a chronobiotic substance that acts as synchronizer by stabilizing bodily rhythms. Its synthesis occurs in various locations throughout the body, including the pineal gland, skin, lymphocytes and gastrointestinal tract (GIT). Its synthesis and secretion is controlled by light and dark conditions, whereby light decreases and darkness increases its production. Thus, melatonin is also known as the 'hormone of darkness'. Melatonin and analogs that bind to the melatonin receptors are important because of their role in the management of depression, insomnia, epilepsy, Alzheimer's disease (AD), diabetes, obesity, alopecia, migraine, cancer, and immune and cardiac disorders. In this review, we discuss the mechanism of action of melatonin in these disorders, which could aid in the design of novel melatonin receptor ligands. PMID:24792719

Singh, Mahaveer; Jadhav, Hemant R

2014-09-01

338

One-pot asymmetric synthesis of quaternary pyrroloindolones through a multicatalytic N-allylation/hydroacylation sequence.  

Science.gov (United States)

An intramolecular, quaternary carbon center forming hydroacylation of ?-substituted acrylates has been discovered. This interesting transformation can be readily incorporated into a multicatalytic tandem process enabled by a combination of nucleophilic tertiary amine and N-heterocyclic carbene catalysis. With no additional stoichiometric base required, this transformation affords the quaternary pyrroloindolones with high levels of enantioselectivity. PMID:25079294

Lu, Hong; Lin, Jun-Bing; Liu, Jin-Yu; Xu, Peng-Fei

2014-09-01

339

Nickel-catalysed carboxylation of organoboronates.  

Science.gov (United States)

A nickel/N-heterocyclic carbene (NHC) catalysed carboxylation of aryl-, heteroaryl- and alkenylboronates, affording the corresponding carboxylic acids, has been developed. This transformation proceeds under one atmosphere of CO2 with a broad range of substrates and exhibits good functional group compatibility. PMID:24915842

Makida, Yusuke; Marelli, Enrico; Slawin, Alexandra M Z; Nolan, Steven P

2014-07-28

340

Photoswitchable NHC-promoted ring-opening polymerizations.  

Science.gov (United States)

The UV-induced photocyclization of a dithienylethene-annulated N-heterocyclic carbene precatalyst enabled photoswitchable ring-opening polymerizations of ?-caprolactone and ?-valerolactone. The polymerizations proceeded efficiently in ambient light, however UV irradiation attenuated the reaction rate (k(amb)/k(UV) = 59). Subsequent visible light exposure reversed the photocyclization and restored catalytic activity. PMID:23665923

Neilson, Bethany M; Bielawski, Christopher W

2013-06-18

341

NHC-catalysed highly selective aerobic oxidation of nonactivated aldehydes  

Directory of Open Access Journals (Sweden)

Full Text Available This publication describes a highly selective oxidation of aldehydes to the corresponding acids or esters. The reaction proceeds under metal-free conditions by using N-heterocyclic carbenes as organocatalysts in combination with environmentally friendly oxygen as the terminal oxidation agent.

Lennart Möhlmann

2013-03-01

342

NHC-catalyzed oxidative ?-addition of ?,?-unsaturated aldehydes to isatins: a high-efficiency synthesis of spirocyclic oxindole-dihydropyranones.  

Science.gov (United States)

This manuscript discloses an efficient construction of the spirocyclic oxindole-dihydropyranone scaffold via the N-heterocyclic carbene (NHC)-catalyzed oxidative ?-functionalization of ?,?-unsaturated aldehydes bearing ?-H with isatin derivatives. The ready availability of the starting materials, easy work-up, mild reaction conditions and the potential utilization value of the products make this strategy attractive. PMID:24518966

Liu, Rui; Yu, Chenxia; Xiao, Zhaoxin; Li, Tuanjie; Wang, Xiangshan; Xie, Yuanwei; Yao, Changsheng

2014-03-28

343

NHC-catalysed highly selective aerobic oxidation of nonactivated aldehydes  

OpenAIRE

This publication describes a highly selective oxidation of aldehydes to the corresponding acids or esters. The reaction proceeds under metal-free conditions by using N-heterocyclic carbenes as organocatalysts in combination with environmentally friendly oxygen as the terminal oxidation agent.

Hlmann, Lennart M.; Stefan Ludwig; Siegfried Blechert

2013-01-01

344

Traditional and ion-pair halogen-bonded complexes between chlorine and bromine derivatives and a nitrogen-heterocyclic carbene.  

Science.gov (United States)

A theoretical study of the halogen-bonded complexes (A-X···C) formed between halogenated derivatives (A-X; A = F, Cl, Br, CN, CCH, CF3, CH3, H; and X = Cl, Br) and a nitrogen heterocyclic carbene, 1,3-dimethylimidazole-2-ylidene (MeIC) has been performed using MP2/aug'-cc-pVDZ level of theory. Two types of A-X:MeIC complexes, called here type-I and -II, were found and characterized. The first group is described by long C-X distances and small binding energies (8-54 kJ·mol(-1)). In general, these complexes show the traditional behavior of systems containing halogen-bonding interactions. The second type is characterized by short C-X distances and large binding energies (148-200 kJ·mol(-1)), and on the basis of the topological analysis of the electron density, they correspond to ion-pair halogen-bonded complexes. These complexes can be seen as the interaction between two charged fragments: A(-) and (+)[X-CIMe] with a high electrostatic contribution in the binding energy. The charge transfer between lone pair A(LP) to the ?* orbital of C-X bond is also identified as a significant stabilizing interaction in type-II complexes. PMID:25188812

Donoso-Tauda, Oscar; Jaque, Pablo; Elguero, José; Alkorta, Ibon

2014-10-01

345

Two novel mixed-ligand complexes containing organosulfonate ligands.  

Science.gov (United States)

The structures reported herein, viz. bis(4-aminonaphthalene-1-sulfonato-kappaO)bis(4,5-diazafluoren-9-one-kappa(2)N,N')copper(II), [Cu(C(10)H(8)NO(3)S)(2)(C(11)H(6)N(2)O)(2)], (I), and poly[[[diaquacadmium(II)]-bis(mu-4-aminonaphthalene-1-sulfonato)-kappa(2)O:N;kappa(2)N:O] dihydrate], {[Cd(C(10)H(8)NO(3)S)(2)(H(2)O)(2)].2H(2)O}(n), (II), are rare examples of sulfonate-containing complexes where the anion does not fulfill a passive charge-balancing role, but takes an active part in coordination as a monodentate and/or bridging ligand. Monomeric complex (I) possesses a crystallographic inversion center at the Cu(II) atom, and the asymmetric unit contains one-half of a Cu atom, one complete 4-aminonaphthalene-1-sulfonate (ans) ligand and one 4,5-diazafluoren-9-one (DAFO) ligand. The Cu(II) atom has an elongated distorted octahedral coordination geometry formed by two O atoms from two monodentate ans ligands and by four N atoms from two DAFO molecules. Complex (II) is polymeric and its crystal structure is built up by one-dimensional chains and solvent water molecules. Here also the cation (a Cd(II) atom) lies on a crystallographic inversion center and adopts a slightly distorted octahedral geometry. Each ans anion serves as a bridging ligand linking two Cd(II) atoms into one-dimensional infinite chains along the [010] direction, with each Cd(II) center coordinated by four ans ligands via O and N atoms and by two aqua ligands. In both structures, there are significant pi-pi stacking interactions between adjacent ligands and hydrogen bonds contribute to the formation of two- and three-dimensional networks. PMID:18599971

Li, Mingtian; Huang, Jun; Zhou, Xuan; Fang, Hua; Ding, Liyun

2008-07-01

346

Cobalt carbene ion: reactions of Co+ with C2H4, cyclo-C3H6, and cyclo-C2H4O  

International Nuclear Information System (INIS)

An ion beam apparatus is employed to study the formation of the cobalt carbene ion, CoCH2+. This ion is produced in the endothermic reaction of cobalt ions with ethene and cyclopropane and in an exothermic reaction with ethylene oxide. A model is proposed to account for the dependence of experimental cross sections on relative kinetic energy for the endothermic reactions. Using this model to interpret the experimental results, a bond dissociation energy D 0(Co+-CH2) = 3.7 +- 0.3 eV is derived

347

Enhanced targeting with heterobivalent ligands.  

Science.gov (United States)

A novel approach to specifically target tumor cells for detection and treatment is the proposed use of heteromultivalent ligands, which are designed to interact with, and noncovalently crosslink, multiple different cell surface receptors. Although enhanced binding has been shown for synthetic homomultivalent ligands, proof of cross-linking requires the use of ligands with two or more different binding moieties. As proof-of-concept, we have examined the binding of synthetic heterobivalent ligands to cell lines that were engineered to coexpress two different G-protein-coupled human receptors, i.e., the human melanocortin 4 receptor (MC4R) expressed in combination with either the human delta-opioid receptor (deltaOR) or the human cholecystokinin-2 receptor (CCK2R). Expression levels of these receptors were characterized by time-resolved fluorescence saturation binding assays using Europium-labeled ligands; Eu-DPLCE, Eu-NDP-alpha-MSH, and Eu-CCK8 for the deltaOR, MC4R, and CCK2R, respectively. Heterobivalent ligands were synthesized to contain a MC4R agonist connected via chemical linkers to either a deltaOR or a CCK2R agonist. In both cell systems, the heterobivalent constructs bound with much higher affinity to cells expressing both receptors, compared with cells with single receptors or to cells where one of the receptors was competitively blocked. These results indicate that synthetic heterobivalent ligands can noncovalently crosslink two unrelated cell surface receptors, making feasible the targeting of receptor combinations. The in vitro cell models described herein will lead to the development of multivalent ligands for target combinations identified in human cancers. PMID:19671749

Xu, Liping; Vagner, Josef; Josan, Jatinder; Lynch, Ronald M; Morse, David L; Baggett, Brenda; Han, Haiyong; Mash, Eugene A; Hruby, Victor J; Gillies, Robert J

2009-08-01

348

Site-directed ligand discovery  

Science.gov (United States)

We report a strategy (called “tethering”) to discover low molecular weight ligands (?250 Da) that bind weakly to targeted sites on proteins through an intermediary disulfide tether. A native or engineered cysteine in a protein is allowed to react reversibly with a small library of disulfide-containing molecules (?1,200 compounds) at concentrations typically used in drug screening (10 to 200 ?M). The cysteine-captured ligands, which are readily identified by MS, are among the most stable complexes, even though in the absence of the covalent tether the ligands may bind very weakly. This method was applied to generate a potent inhibitor for thymidylate synthase, an essential enzyme in pyrimidine metabolism with therapeutic applications in cancer and infectious diseases. The affinity of the untethered ligand (Ki?1 mM) was improved 3,000-fold by synthesis of a small set of analogs with the aid of crystallographic structures of the tethered complex. Such site-directed ligand discovery allows one to nucleate drug design from a spatially targeted lead fragment. PMID:10944209

Erlanson, Daniel A.; Braisted, Andrew C.; Raphael, Darren R.; Randal, Mike; Stroud, Robert M.; Gordon, Eric M.; Wells, James A.

2000-01-01

349

Cis-interactions between Notch and its ligands block ligand-independent Notch activity.  

Science.gov (United States)

The Notch pathway is integrated into numerous developmental processes and therefore is fine-tuned on many levels, including receptor production, endocytosis, and degradation. Notch is further characterized by a twofold relationship with its Delta-Serrate (DSL) ligands, as ligands from opposing cells (trans-ligands) activate Notch, whereas ligands expressed in the same cell (cis-ligands) inhibit signaling. We show that cells without both cis- and trans-ligands can mediate Notch-dependent developmental events during Drosophila oogenesis, indicating ligand-independent Notch activity occurs when the receptor is free of cis- and trans-ligands. Furthermore, cis-ligands can reduce Notch activity in endogenous and genetically induced situations of elevated trans-ligand-independent Notch signaling. We conclude that cis-expressed ligands exert their repressive effect on Notch signaling in cases of trans-ligand-independent activation, and propose a new function of cis-inhibition which buffers cells against accidental Notch activity. PMID:25486593

Palmer, William Hunt; Jia, Dongyu; Deng, Wu-Min

2015-01-01

350

Novel synthesis of tripodal borate ligands  

OpenAIRE

Poly(azolyl)borate ligands have proven to be extremely popular ligands since their introduction by Trofimenko in the late 60´s. The basic skeleton of these ligands involves usually three heterocycle units linked to a central boron apex via the azole nitrogen atoms. These ligands have been applied in diverse research areas such as homogeneous catalysis, materials science and bio-inorganic chemistry. More than 2000 papers, including books and reviews regarding the properties of ...

Sanchez Perucha, Alejandro; Perucha, Alejandro Sanchez

2007-01-01

351

Reactivity of halide and pseudohalide ligands  

International Nuclear Information System (INIS)

Reactivity of halide and pseudohalide (cyanide, azide, thiocyanate, cyanate) ligands tending to form bridge bonds in transition metal (Re, Mo, W) complexes is considered. Complexes where transition metal salts are ligands of other, complex-forming ion, are described. Transformation of innerspheric pseudohalide ligands is an important way of directed synthesis of these metal coordination compounds

352

DNA and RNA quadruplex ligands.  

Science.gov (United States)

Guanine-rich nucleic acids can adopt unusual structures called guanine quadruplexes (G4) based on stacked guanine quartets. Both RNA and DNA backbones are compatible with G4 formation. As RNA and DNA quadruplexes may be recognized by ligands, it is important to understand the rules that govern the stability and specificity of these complexes. We explore the binding of a pyridine dicarboxamide derivative to various oligoribo- and oligodeoxyribo-nucleotides. PMID:18776226

De Cian, Anne; Gros, Julien; Guédin, Aurore; Haddi, Meriem; Lyonnais, Sébastien; Guittat, Lionel; Riou, Jean-François; Trentesaux, Chantal; Saccà, Barbara; Lacroix, Laurent; Alberti, Patrizia; Mergny, Jean-Louis

2008-01-01

353

Dithiocarbamate ligand stabilised gold nanoparticles  

OpenAIRE

Dithiocarbamates are used as adsorbates for the preparation of gold nanoparticles. A range of mono- and tetrakis-functionalised dithiocarbamate ligands containing alkyl, benzyl and resorcinarene cavitand substituents are synthesised. Nanoparticles are prepared by a two-phase synthesis and are characterised by 1H NMR spectroscopy, UV-Visible spectroscopy, transmission electron microscopy (TEM), energy dispersive X-ray analysis (EDX) and elemental analysis. The colloids are found to be stable o...

Vickers, Ms; Cookson, J.; Beer, Pd; Bishop, Pt; Thiebaut, B.

2006-01-01

354

Tumor Targeting via Integrin Ligands.  

Science.gov (United States)

Selective and targeted delivery of drugs to tumors is a major challenge for an effective cancer therapy and also to overcome the side-effects associated with current treatments. Overexpression of various receptors on tumor cells is a characteristic structural and biochemical aspect of tumors and distinguishes them from physiologically normal cells. This abnormal feature is therefore suitable for selectively directing anticancer molecules to tumors by using ligands that can preferentially recognize such receptors. Several subtypes of integrin receptors that are crucial for cell adhesion, cell signaling, cell viability, and motility have been shown to have an upregulated expression on cancer cells. Thus, ligands that recognize specific integrin subtypes represent excellent candidates to be conjugated to drugs or drug carrier systems and be targeted to tumors. In this regard, integrins recognizing the RGD cell adhesive sequence have been extensively targeted for tumor-specific drug delivery. Here we review key recent examples on the presentation of RGD-based integrin ligands by means of distinct drug-delivery systems, and discuss the prospects of such therapies to specifically target tumor cells. PMID:24010121

Marelli, Udaya Kiran; Rechenmacher, Florian; Sobahi, Tariq Rashad Ali; Mas-Moruno, Carlos; Kessler, Horst

2013-01-01

355

Tumor targeting via integrin ligands  

Directory of Open Access Journals (Sweden)

Full Text Available Selective and targeted delivery of drugs to tumors is a major challenge for an effective cancer therapy and also to overcome the side effects associated with current treatments. Overexpression of various receptors on tumor cells is a characteristic structural and biochemical aspect of tumors and distinguishes them from physiologically normal cells. This abnormal feature is therefore suitable for selectively directing anticancer molecules to tumors by using ligands that can preferentially recognize such receptors. Several subtypes of integrin receptors that are crucial for cell adhesion, cell signaling, cell viability and motility have been shown to have an upregulated expression on cancer cells. Thus, ligands that recognize specific integrin subtypes represent excellent candidates to be conjugated to drugs or drug carrier systems and be targeted to tumors. In this regard, integrins recognizing the RGD cell adhesive sequence have been extensively targeted for tumor specific drug delivery. Here we review key recent examples on the presentation of RGD-based integrin ligands by means of distinct drug delivery systems, and discuss the prospects of such therapies to specifically target tumor cells.

HorstKessler

2013-08-01

356

Radioiodinated ligands for dopamine receptors  

International Nuclear Information System (INIS)

The dopamine receptor system is important for normal brain function; it is also the apparent action site for various neuroleptic drugs for the treatment of schizophrenia and other metal disorders. In the past few years radioiodinated ligands for single photon emission tomography (SPECT) have been successfully developed and tested in humans: [123I]TISCH for D1 dopamine receptors; [123I]IBZM, epidepride, IBF and FIDA2, four iodobenzamide derivatives, for D2/D3 dopamine receptors. In addition, [123I]?-CIT (RTI-55) and IPT, cocaine derivatives, for the dopamine reuptake site are potentially useful for diagnosis of loss of dopamine neurons. The first iodinated ligand, (R)trans-7-OH-PIPAT, for D3 dopamine receptors, was synthesized and characterized with cloned cell lines (Spodoptera frugiperda, Sf9) expressing the D2 and D3 dopamine receptors and with rat basal forebrain membrane preparations. Most of the known iodobenzamides displayed similar potency in binding to both D2 and D3 dopamine receptors expressed in the cell lines. Initial studies appear to suggest that by fine tuning the structures it may be possible to develop agents specific for D2 and D3 dopamine receptors. It is important to investigate D2/D3 selectivity for this series of potent ligands

357

Quantum-chemical investigation of the nature of metal-carbene and metal-olefin bonds in complexes [ClnM(C2H4)]- (M=Mo,Pd)  

International Nuclear Information System (INIS)

On the basis of electron structure calculation using Huckel method comparative investigation of the nature of metal-ethylidene and metal-ethylene bonds in complexes [Cl5Mo(C2H4)]- and [Cl3Pd(C2H4)]-, and also carbene complex stability regarding olefine izomerization is carried out. It is shown that metal-ethylidene bond in the above complexes is stronger than metal-ethylene one, and Mo(4)-ethylidene bond is stronger than Pd(2)-ethylidene one. It is supposed that relative stability of alkylidene structures may be one of the reasons of catalytic activity of Mo(4) compounds in olefine metathesis and oligomerization reactions in which carbene complex is considered as an intermediate

358

Reactivity of a frustrated lewis pair and small-molecule activation by an isolable Arduengo carbene-B{3,5-(CF3)2C6H3}3 complex.  

Science.gov (United States)

Tris[3,5-bis(trifluoromethyl)phenyl]borane reacts with the sterically demanding Arduengo carbenes 1,3-di-tert-butylimidazolin-2-ylidene and 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene to form isolable normal adducts. In the case of 1,3-di-tert-butylimidazolin-2-ylidene, the adduct exhibits dynamic behaviour in solution and frustrated-Lewis-pair (FLP) reactivity. Fast cleavage of dihydrogen and THF, the C-H activation of phenylacetylene, and carbon dioxide fixation were achieved by using solutions of this adduct in benzene. This adduct is stable at room temperature in the absence of suitable substrates; however, thermal rearrangement into an abnormal carbene-borane adduct can be observed. In contrast, the 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene adduct exhibits no evidence of FLP reactivity or of dissociation in solution. DFT calculations confirmed the experimental behaviour and stability of these carbene-borane adducts. PMID:23150467

Kolychev, Eugene L; Bannenberg, Thomas; Freytag, Matthias; Daniliuc, Constantin G; Jones, Peter G; Tamm, Matthias

2012-12-21

359

A universal rule for organic ligand exchange.  

Science.gov (United States)

Most synthetic routes to high-quality nanocrystals with tunable morphologies predominantly employ long hydro-carbon molecules as ligands, which are detrimental for electronic and catalytic applications. Here, a rule is found that the adsorption energy of an organic ligand is related to its carbon-chain length. Using the density functional theory method, the adsorption energies of some commonly used ligand molecules with different carbon-chain lengths are calculated, including carboxylate, hydroxyl, and amine molecules adsorbed on metal or metal oxide crystal surface. The results indicate that the adsorption energy of the ligand molecule with a long carbon chain is weaker than that of a smaller molecule with same functional group. This rule provides a theoretical support for a new kind of ligand exchange method in which large organic ligand molecules can be exchanged by small molecules with same functional group to improve the catalytic properties. PMID:25335915

You, Hongjun; Wang, Wenjin; Yang, Shengchun

2014-11-12

360

Synthesis of 3,3-Disubstituted Oxindoles by Palladium-Catalyzed Asymmetric Intramolecular ?-Arylation of Amides: Reaction Development and Mechanistic Studies.  

Science.gov (United States)

Palladium complexes incorporating chiral N-heterocyclic carbene (NHC) ligands catalyze the asymmetric intramolecular ?-arylation of amides producing 3,3-disubstituted oxindoles. Comprehensive DFT studies have been performed to gain insight into the mechanism of this transformation. Oxidative addition is shown to be rate-determining and reductive elimination to be enantioselectivity-determining. The synthesis of seven new NHC ligands is detailed and their performance is compared. One of them, L8, containing a tBu and a 1-naphthyl group at the stereogenic centre, proved superior and was very efficient in the asymmetric synthesis of fifteen new spiro-oxindoles and three azaspiro-oxindoles often in high yields (up to 99?%) and enantioselectivities (up to 97?%?ee; ee=enantiomeric excess). Three palladacycle intermediates resulting from the oxidative addition of [Pd(NHC)] into the aryl halide bond were isolated and structurally characterized (X-ray). Using these intermediates as catalysts showed alkene additives to play an important role in increasing turnover number and frequency. PMID:23893893

Katayev, Dmitry; Jia, Yi-Xia; Sharma, Akhilesh K; Banerjee, Dipshikha; Besnard, Céline; Sunoj, Raghavan B; Kündig, E Peter

2013-07-25

361

Canonical and non-canonical Notch ligands  

OpenAIRE

Notch signaling induced by canonical Notch ligands is critical for normal embryonic development and tissue homeostasis through the regulation of a variety of cell fate decisions and cellular processes. Activation of Notch signaling is normally tightly controlled by direct interactions with ligand-expressing cells and dysregulated Notch signaling is associated with developmental abnormalities and cancer. While canonical Notch ligands are responsible for the majority of Notch signaling, a diver...

D’souza, Brendan; Meloty-kapella, Laurence; Weinmaster, Gerry

2010-01-01

362

Photoredox catalysts: synthesis of the bipyrazine ligand.  

Science.gov (United States)

The bipyrazine ligand is often employed in photoredox catalysts in order to increase the excited state oxidation potential of the catalyst. However, literature syntheses of the ligand are cumbersome and typically lead to low yields. This hampers use of the desired catalysts. We report here an efficient copper based synthesis of the bipyrazine ligand that affords the product in 65-76% yield on a multigram scale. PMID:25569563

Graaf, Matthew D; Moeller, Kevin D

2015-02-01

363

Development of immobilized ligands for actinide separations  

International Nuclear Information System (INIS)

Primary goals during this grant period were to (1) synthesize new bifunctional chelating ligands, (2) characterize the structural features of the Ln and An coordination complexes formed by these ligands, (3) use structural data to iteratively design new classes of multifunctional ligands, and (4) explore additional routes for attachment of key ligands to solid supports that could be useful for chromatographic separations. Some highlights of recently published work as well as a summary of submitted, unpublished and/or still in progress research are outlined

364

Autocrine signal transmission with extracellular ligand degradation  

International Nuclear Information System (INIS)

Traveling waves of cell signaling in epithelial layers orchestrate a number of important processes in developing and adult tissues. These waves can be mediated by positive feedback autocrine loops, a mode of cell signaling where binding of a diffusible extracellular ligand to a cell surface receptor can lead to further ligand release. We formulate and analyze a biophysical model that accounts for ligand-induced ligand release, extracellular ligand diffusion and ligand–receptor interaction. We focus on the case when the main mode for ligand degradation is extracellular and analyze the problem with the sharp threshold positive feedback nonlinearity. We derive expressions that link the speed of propagation and other characteristics of traveling waves to the parameters of the biophysical processes, such as diffusion rates, receptor expression level, etc. Analyzing the derived expressions we found that traveling waves in such systems can exhibit a number of unusual properties, e.g. non-monotonic dependence of the speed of propagation on ligand diffusivity. Our results for the fully developed traveling fronts can be used to analyze wave initiation from localized perturbations, a scenario that frequently arises in the in vitro models of epithelial wound healing, and guide future modeling studies of cell communication in epithelial layers

365

Ligand binding by PDZ domains  

DEFF Research Database (Denmark)

The postsynaptic density protein-95/disks large/zonula occludens-1 (PDZ) protein domain family is one of the most common protein-protein interaction modules in mammalian cells, with paralogs present in several hundred human proteins. PDZ domains are found in most cell types, but neuronal proteins, for example, are particularly rich in these domains. The general function of PDZ domains is to bring proteins together within the appropriate cellular compartment, thereby facilitating scaffolding, signaling, and trafficking events. The many functions of PDZ domains under normal physiological as well as pathological conditions have been reviewed recently. In this review, we focus on the molecular details of how PDZ domains bind their protein ligands and their potential as drug targets in this context.

Chi, Celestine N.; Bach, Anders

2012-01-01

366

Macrocyclic ligands for uranium complexation  

International Nuclear Information System (INIS)

A highly preorganized 24-macrocycle containing biuret, thiobiuret and pyridine subunits has been prepared by high dilution ring-closure procedures. Intermediate products to this macrocycle have been utilized to extend this synthetic route to include further representatives where solubility and stability will be influenced by substituent variation. A 1:1 complex has been formed from uranyl acetate and a quinquepyridine derivative, this representing a new type of ligand for the uranyl ion. A very convenient synthetic procedure that will allow the incorporation of these macrocycles into polymeric systems has been developed for the introduction of a vinyl substituent into the 4-position of the pyridine ring. Using triflate, vinyltributyltin and Pd0 chemistry, this procedure should make a variety of substituted 4-vinylpyridines available for the first time. 3 refs

367

Selective synthesis of carbonylated N-heterocycles by homogeneous and heterogeneous multi-catalysis  

OpenAIRE

The one-pot selective synthesis of 4-quinolones and indoxyls was achieved through a carbonylative Sonogashira coupling followed by cyclization. A study in homogeneous catalysis revealed that the nature of catalysts in presence controlled the selectivity toward each compounds. Whereas the first coupling step is palladium catalyzed, the cyclizations require organic nucleophilic species. Thus, 4-quinolones were obtained by one-pot multi-catalysis {[Pd]+amine} and indoxyls by one-pot tandem catal...

Genelot, Marie

2011-01-01

368

Coupling Reactions of ?-Bromocarboxylate with Non-Aromatic N-Heterocycles  

Directory of Open Access Journals (Sweden)

Full Text Available The conditions for the C-N bond forming reaction (C-N coupling reaction between ?-bromocarboxylate and nitrogen-containing non-aromatic heterocyclic rings under heterogeneous copper(I oxide catalysis are investigated in this paper. All the generated compounds were fully characterized by IR, NMR and MS spectroscopy. Ab initio/DFT calculations of partial charges on nitrogen atoms in all the discussed heterocycles and on C(2 of carboxylate under applied conditions were predicted. These in silico results correlate relatively with the experimental observations.

Jozef Csollei

2009-08-01

369

Synthesis, characterization, and crystal structure of two ?,?-unsaturated ketones with N-heterocycle  

Science.gov (United States)

Two ?,?-unsaturated ketones: C22H16N2O ( 1), ( E)-3-(4-(1 H-benzo[d]imidazol-1-yl)phenyl)-1-phenylprop-2-en-1-one and C19H16N2O2 ( 2), ( E)-3-(4-(1 H-pyrazol-1-yl)phenyl)-1-(4-methoxyphenyl)prop-2-en-1-one were synthesized and characterized by X-ray diffraction analyses, EA, 1H NMR-, and IR-spectra. In the crystal structure of 1, molecules are linked through intermolecular hydrogen bonds, C-H⋯? and ?⋯? interactions forming a 3-D network, while in the crystal structures of 2, molecules are assembled by the C-H⋯O, C-H⋯N hydrogen bonds and C-H⋯? interactions into an infinite 3-D supramolecular structure. A particularly interesting feature of the packing in the structure of 2, is the extensive contribution of C-H⋯? interactions, involving two hydrogen atoms from each of the methyl groups of the methoxy substituents.

Wang, G.-F.; Sun, S.-W.; Gong, Q.-J.; Sun, H.

2014-12-01

370

Transient negative ions in benzene. Some N-heterocyclic and mono-substituted derivatives  

International Nuclear Information System (INIS)

Electron transmission spectroscopy is used to study transient negative ions or shape resonances in various benzene derivatives. Because of the long lifetime of these ions (? > 10-14 S) the vibrational structure of their first two electronic states is observed superposed on the total electron cross section curves in the energy range 0-6 eV and the corresponding adiabatic electron affinities are determined. The comparison of the first electron affinity with the first ionization potential and the energy on the first excited state of each of the derivatives is used to characterize the 'donor' substituents on the benzene ring. As a complementary study, these derivatives are studied in the liquid phase using polarography (cyclic voltametry). The linear correlation established between polarographic potentials measured in dimethyl formamide and the electron affinities was used to deduce electron affinities for several molecules which are difficult to measure in the gas phase. (author)

371

A functional triazine framework based on N-heterocyclic building blocks  

OpenAIRE

Covalent organic frameworks constitute a subclass of polymeric materials offering enhanced porosity, functionality and stability. In this work a covalent triazine framework based on bipyridine building blocks is presented, along with a comprehensive elucidation of its local structure, porosity, and capacity for metal uptake. A typical synthesis was carried out under ionothermal conditions at 400-700 degrees C using ZnCl2 as a Lewis acidic trimerization catalyst. A high degree of local order a...

Hug, Stephan; Tauchert, Michael E.; Li, Shen; Pachmayr, Ursula E.; Lotsch, Bettina V.

2012-01-01

372

New iodocuprates(I) with N-heterocyclic molecules as the cations  

Energy Technology Data Exchange (ETDEWEB)

Under the hydrothermal conditions, the reactions between CuI, KI and bp/bpp (bp=4,4?-bipiperidine, bpp=1,3-bis(4-piperidyl)propane) in an acidic alcohol solution produced three new organically templated iodocuprates(I) as [H{sub 2}bp]{sub 2}[Cu{sub 2}I{sub 6}] 1, [tmbp][Cu{sub 2}I{sub 4}] 2 and [tmbpp] 2 [Cu{sub 4}I{sub 8}]·2H{sub 2}O 3 (tmbp{sup 2+}=N,N,N?,N?-tetramethyl-4,4?-bipiperidinium; tmbpp{sup 2+}=N,N,N?,N?-tetramethyl-1,3-bis(4-piperidyl)propane dication). X-ray analysis revealed that (i) tmbp{sup 2+} and tmbpp{sup 2+} in compounds 2 and 3 originated from the complete N-alklation of bp/bpp with CH{sub 3}OH; (ii) templated by H{sub 2}bp{sup 2+}, the inorganic anion [Cu{sub 2}I{sub 6}]{sup 2?} of 1 possesses a dinuclear structure, whereas templated by tmbp{sup 2+}, the inorganic anion [Cu{sub 2}I{sub 4}]{sup 2?} of 2 exhibits a one-dimensional (1D) chain structure; (iii) templated by tmbpp{sup 2+}, the inorganic anion [Cu{sub 4}I{sub 8}]{sup 4?} of 3 shows a cubane-like structure modified by four terminal I{sup ?} ions. The photoluminescence analysis indicates that compounds 1 and 2 emit blue light, while compound 3 emits green light. - Graphical abstract: By employing hydrothermal in situ N-alkylation of bp/bpp with CH{sub 3}OH, three new organically templated iodocuprates(I) were obtained. Display Omitted - Highlights: • Three new organically templated iodocuprates(I) were reported. • Cations tmbp{sup 2+} and tmbpp{sup 2+} originated from in situ alkylation of bp/bpp with CH{sub 3}OH. • H{sup +} and I{sup ?} play a key role in alkylation of bp/bpp with CH{sub 3}OH. • Photoluminescence emission for iodocuprates(I) is related to Cu···Cu interaction.

Zhao, Jin-Jing [College of Chemistry and State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, Jilin University, Changchun, Jilin 130023 (China); Zhang, Xiao [State Key Lab of Urban Water Resource and Environment (SKLUWRE) and Academy of Fundamental and Interdisciplinary Science, Harbin Institute of Technology, Harbin, Heilongjiang 150080 (China); Wang, Yan-Ning; Jia, Hong-Li [College of Chemistry and State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, Jilin University, Changchun, Jilin 130023 (China); Yu, Jie-Hui, E-mail: jhyu@jlu.edu.cn [College of Chemistry and State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, Jilin University, Changchun, Jilin 130023 (China); Xu, Ji-Qing, E-mail: xjq@mail.jlu.edu.cn [College of Chemistry and State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, Jilin University, Changchun, Jilin 130023 (China)

2013-11-15

373

New iodocuprates(I) with N-heterocyclic molecules as the cations  

International Nuclear Information System (INIS)

Under the hydrothermal conditions, the reactions between CuI, KI and bp/bpp (bp=4,4?-bipiperidine, bpp=1,3-bis(4-piperidyl)propane) in an acidic alcohol solution produced three new organically templated iodocuprates(I) as [H2bp]2[Cu2I6] 1, [tmbp][Cu2I4] 2 and [tmbpp] 2 [Cu4I8]·2H2O 3 (tmbp2+=N,N,N?,N?-tetramethyl-4,4?-bipiperidinium; tmbpp2+=N,N,N?,N?-tetramethyl-1,3-bis(4-piperidyl)propane dication). X-ray analysis revealed that (i) tmbp2+ and tmbpp2+ in compounds 2 and 3 originated from the complete N-alklation of bp/bpp with CH3OH; (ii) templated by H2bp2+, the inorganic anion [Cu2I6]2? of 1 possesses a dinuclear structure, whereas templated by tmbp2+, the inorganic anion [Cu2I4]2? of 2 exhibits a one-dimensional (1D) chain structure; (iii) templated by tmbpp2+, the inorganic anion [Cu4I8]4? of 3 shows a cubane-like structure modified by four terminal I? ions. The photoluminescence analysis indicates that compounds 1 and 2 emit blue light, while compound 3 emits green light. - Graphical abstract: By employing hydrothermal in situ N-alkylation of bp/bpp with CH3OH, three new organically templated iodocuprates(I) were obtained. Display Omitted - Highlights: • Three new organically templated iodocuprates(I) were reported. • Cations tmbp2+ and tmbpp2+ originated from in situ alkylation of bp/bpp with CH3OH. • H+ and I? play a key role in alkylation of bp/bpp with CH3OH. • Photoluminescence emission for iodocuprates(I) is related to Cu···Cu interaction

374

Ligand recognition by the vitamin D receptor.  

Science.gov (United States)

Three-dimensional structure of the ligand binding domain (LBD) of the vitamin D receptor (VDR) docked with the natural ligand 1 alpha,25-dihydroxyvitamin D(3) [1,25-(OH)(2)D(3)] has been mostly solved by the X-ray crystallographic analysis of the deletion mutant (VDR-LBD Delta 165-215). The important focus, from now on, is how the VDR recognizes and interacts with potent synthetic ligands. We now report the docking models of the VDR with three functionally and structurally interesting ligands, 22-oxa-1,25-(OH)(2)D(3) (OCT), 20-epi-1,25-(OH)(2)D(3) and 20-epi-22-oxa-24,26,27-trihomo-1,25-(OH)(2)D(3). In parallel with the computational docking studies, we prepared twelve one-point mutants of amino acid residues lining the ligand binding pocket of the VDR and examined their transactivation potency induced by 1,25-(OH)(2)D(3) and these synthetic ligands. The results indicate that L233, R274, W286, H397 and Y401 are essential for holding the all ligands tested, S278 and Q400 are not important at all, and the importance of S237, V234, S275, C288 and H305 is variable depending on the side-chain structure of the ligands. Based on these studies, we suggested key structural factors to bestow the selective action on OCT and the augmented activities on 20-epi-ligands. Furthermore, the docking models coincided well with our proposed active space-region theory of vitamin D based on the conformational analyses of ligands. PMID:11425573

Choi, M; Yamamoto, K; Masuno, H; Nakashima, K; Taga, T; Yamada, S

2001-07-01

375

Modulating NHC catalysis with fluorine  

Science.gov (United States)

Summary Fluorination often confers a range of advantages in modulating the conformation and reactivity of small molecule organocatalysts. By strategically introducing fluorine substituents, as part of a ?-fluoroamine motif, in a triazolium pre-catalyst, it was possible to modulate the behaviour of the corresponding N-heterocyclic carbene (NHC) with minimal steric alterations to the catalyst core. In this study, the effect of hydrogen to fluorine substitution was evaluated as part of a molecular editing study. X-ray crystallographic analyses of a number of derivatives are presented and the conformations are discussed. Upon deprotonation, the fluorinated triazolium salts generate catalytically active N-heterocyclic carbenes, which can then participate in the enantioselective Steglich rearrangement of oxazolyl carbonates to C-carboxyazlactones (e.r. up to 87.0:13.0). PMID:24367445

Rey, Yannick P

2013-01-01

376

3,3?-Dimethyl-1,1?-(methylenedi-p-phenylenediimidazolium bis(hexafluorophosphate  

Directory of Open Access Journals (Sweden)

Full Text Available The title N-heterocyclic carbene compound, C21H22N42+·2PF6?, crystallizes as an inversion twin. There are two independent N-heterocyclic carbene dications (A and B and four independent hexafluorophosphate anions in the asymmetric unit. The cations are L-shaped with the benzene rings being inclined to one another by 88.82?(16° in cation A and 87.03?(16° in cation B. The imidazole rings make dihedral angles of 35.7?(2 and 32.83?(18° with the attached benzene rings in cation A, and 30.14?(19 and 31.96?(18° in cation B. In the crystal, the cations are linked via C—H...F hydrogen bonds, forming a three-dimensional network. ?–? interactions involving the benzene and imidazole rings [centroid–centroid distances = 3.602?(2 and 3.723?(2?Å] and C—H...? interactions are also present.

Kun Huang

2010-09-01

377

Structural requirements for ligands of the ?-opioid receptor  

OpenAIRE

The ?-opioid receptor is sensitive to ligand geometry. In order to assist the synthesis of new ?-selective opioid ligands, the structure elements of ?-selective opioid ligands necessary for their effective binding were investigated. The automated docking procedure with a flexible ligand was used to simulate the binding of 17 ?-selective ligands to the ?-receptor. It was found that voluminous N-alkyl groups reduce the binding potency of naltrindole derivatives by preventing the ligands fr...

Mic?ovic?, Vuk I.; Ivanovic?, Milovan D.; LJILJANA DOŠEN-MI?OVI?

2009-01-01

378

Dichlorosilylene: a high temperature transient species to an indispensable building block.  

Science.gov (United States)

Isolating stable compounds with low-valent main group elements have long been an attractive research topic, because several of these compounds can mimic transition metals in activating small molecules. In addition, compounds with heavier low-valent main group elements have fundamentally different electronic properties when compared with their lighter congeners. Among group 14 elements, the heavier analogues of carbenes (R(2)C:) such as silylenes (R(2)Si:), germylenes (R(2)Ge:), stannylenes (R(2)Sn:), and plumbylenes (R(2)Pb:) are the most studied species with low-valent elements. The first stable carbene and silylene species were isolated as N-heterocycles. Among the dichlorides of group 14 elements, CCl(2) and SiCl(2) are highly reactive intermediates and play an important role in many chemical transformations. GeCl(2) can be stabilized as a dioxane adduct, whereas SnCl(2) and PbCl(2) are available as stable compounds. In the Siemens process, which produces electronic grade silicon by thermal decomposition of HSiCl(3) at 1150 °C, chemists proposed dichlorosilylene (SiCl(2)) as an intermediate, which further dissociates to Si and SiCl(4). Similarly, base induced disproportionation of HSiCl(3) or Si(2)Cl(6) to SiCl(2) is a known reaction. Trapping these products in situ with organic substrates suggested the mechanism for this reaction. In addition, West and co-workers reported a polymeric trans-chain like perchloropolysilane (SiCl(2))(n). However, the isolation of a stable free monomeric dichlorosilylene remained a challenge. The first successful attempt of taming SiCl(2) was the isolation of monochlorosilylene PhC(NtBu)(2)SiCl supported by an amidinate ligand in 2006. In 2009, we succeeded in isolating N-heterocyclic carbene (NHC) stabilized dichlorosilylene (NHC)SiCl(2) with a three coordinate silicon atom. (The NHC is 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr) or 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene (IMes).) Notably, this method allows for the almost quantitative synthesis of (NHC)SiCl(2) without using any hazardous reducing agents. Dehydrochlorination of HSiCl(3) with NHC under mild reaction conditions produces (NHC)SiCl(2). We can separate the insoluble side product (NHC)HCl readily and recycle it to form NHC. The high yield and facile access to dichlorosilylene allow us to explore its chemistry to a greater extent. In this Account, we describe the results using (NHC)SiCl(2) primarily from our laboratory, including findings by other researchers. We emphasize the novel silicon compounds, which supposedly existed only as short-lived species. We also discuss silaoxirane, silaimine with tricoordinate silicon atom, silaisonitrile, and silaformyl chloride. In analogy with N-heterocyclic silylenes (NHSis), oxidative addition reactions of organic substrates with (NHC)SiCl(2) produce Si(IV) compounds. The presence of the chloro-substituents both on (NHC)SiCl(2) and its products allows metathesis reactions to produce novel silicon compounds with new functionality. These substituents also offer the possibility to synthesize interesting compounds with low-valent silicon by further reduction. Coordination of NHC to the silicon increases the acidity of the backbone protons on the imidazole ring, and therefore (NHC)SiCl(2) can functionalize NHC at the C-4 or C-5 position. PMID:23181482

Ghadwal, Rajendra S; Azhakar, Ramachandran; Roesky, Herbert W

2013-02-19

379

THEORETICAL STUDY OF THE MECHANISM OF CYCLOADDITION REACTION BETWEEN DICHLORO-SILYLENE CARBENE(CL2SI=C:) AND ACETONE  

Scientific Electronic Library Online (English)

Full Text Available SciELO Chile | Language: English Abstract in english The mechanism of the cycloaddition reaction between singlet dichloro-silylene carbene and acetone has been investigated with DFT, including geometry optimization and vibrational analysis for the involved stationary points on the potential energy surface. The energies of the different conformations a [...] re calculated by CCSD(T)//B3LYP/6-31G* method. From the potential energy profile, it can be predicted that the reaction has two competitive dominant reaction pathways. The channel (I) consists of two steps: (1) the two reactants firstly form a four-membered ring intermediate through a barrier-free exothermic reaction of 307.1 kJ/mol; (2) four-membered ring intermediate then isomerizes to a CH3-transfer product via a transition state with energy barrier of 11.3 kJ/mol. The process of channel (II) is as following: on the basis of four-membered ring intermediate created between the two reactants, four-membered ring intermediate further reacts with acetone to form the intermediate through a barrier-free exothermic reaction of 165.8 kJ/mol; Then, intermediate isomerizes to a silicic bis-heterocyclic product via a transition state, for which the barrier is 57.6 kJ/mol.

XIUHUI, LU; PINGPING, XIANG; LEYI, SHI; JUNFENG, HAN; ZHENXIA, LIAN.

380

Mixed ligand complexes of Cd(II) with NTA, EDTA and HEDTA as primary ligands and glutamic acid as secondary ligand  

International Nuclear Information System (INIS)

The formation of mixed ligand complexes of Cd(II) with nitrilotriacetic acid (NTA), ethylenediaminetetraacetic acid (EDTA) and hexaethylenediaminetetraacetic acid (HEDTA) as primary ligands and glumatic acid as the secondary ligand has been studied potentiometrically. Stability constants of mixed ligand complexes, KMLA, are found to lie between K1 and K2 values of simple Cd(II)-glutamate chelates, the stability sequence with respect to primary ligands being NTA< EDTA< HEDTA. (auth.)

381

A benzobisimidazolium-based fluorescent and colorimetric chemosensor for CO2.  

Science.gov (United States)

A new sensor for the fluorescent and colorimetric detection of CO(2) is described. The system utilizes fluoride to activate a tetrapropyl benzobisimidazolium salt and operates in the absence of an exogenous base. On the basis of spectroscopic and theoretical analyses, the mode of action of the present system is ascribed to the fluoride-induced formation of an N-heterocyclic carbene intermediate that reacts with CO(2) to form an imidazolium carboxylate. PMID:22931227

Guo, Zhiqian; Song, Na Ri; Moon, Jong Hun; Kim, Myounwoo; Jun, Eun Jin; Choi, Jiyoung; Lee, Jin Yong; Bielawski, Christopher W; Sessler, Jonathan L; Yoon, Juyoung

2012-10-31

382

A 4,5-diphosphino-substituted imidazolium salt: a building block for the modular synthesis of mixed diphosphine-NHC heterometallic complexes.  

Science.gov (United States)

Fused in the same molecule! An N-heterocyclic carbene and a diphosphine are generated and used in the modular synthesis of a variety of target heterometallic complexes. The experimental approach involves formation of a unique 4,5-bis(diphenylphosphino)-1,3-dimethyl-imidazolium salt, simply by starting from 1-methylimidazole (see scheme; M(1) = Cr, Mn; M(2) = Au, Rh, Pd; LiHMDS = LiN(SiMe(3))(2)). PMID:22407582

Ruiz, Javier; Mesa, Alejandro F

2012-04-10

383

Frustrated lewis pair activation of an N-sulfinylamine: a source of sulfur monoxide.  

Science.gov (United States)

Inter- and intramolecular P/B frustrated Lewis pairs are shown to react with an N-sulfinylamine to form PNSOB linakages. These species can be regarded as phosphinimine-borane-stabilized sulfur monoxide complexes, and indeed these species act as sources of SO, effecting the oxidation of PPh3 and delivering SO to [RhCl(PPh3 )3 ] and an N-heterocyclic carbene. PMID:25376102

Longobardi, Lauren E; Wolter, Vanessa; Stephan, Douglas W

2015-01-12

384

1-(1H-Benzimidazol-1-ylmeth­yl)-3-[2-(di­isopropyl­amino)eth­yl]-1H-benzimid­azolium bromide 0.25-hydrate  

OpenAIRE

The title N-heterocyclic carbene derivative, C23H30N5+·Br?·0.25H2O, was synthesized using microwave heating and was characterized by 1H and 13C NMR spectroscopy and a single-crystal X-ray diffraction study. The structure of the title compound are stabilized by a network of intra- and intermolecular C—H...Br hydrogen-bonding interactions. The crystal structure is further stabilized by ?–? stacking interactions between benzene an...

Hakan Arslan; Don VanDerveer; Serpil Demir; smail Özdemir; Bekir Çetinkaya

2009-01-01

385

Gold(I)-catalyzed synthesis of ?-vinylbutyrolactones by intramolecular oxaallylic alkylation with alcohols  

OpenAIRE

Gold(I)-N-heterocyclic carbene (NHC) complexes proved to be a reliable catalytic system for the direct synthesis of functionalized ?-vinylbutyrolactones by intramolecular oxaallylic alkylation with primary alcohols. Good isolated chemical yields were obtained for a range of malonyl and acetate derivatives. The good performance in reagent-grade solvents and the functional group/moisture tolerance make this catalytic process a promising route for the synthesis of architecturally com...

Michel Chiarucci; Mirko Locritani; Gianpiero Cera; Marco Bandini

2011-01-01

386

Alkenyl-functionalized NHC iridium-based catalysts for hydrosilylation  

OpenAIRE

A family of alkenyl-functionalized N-heterocyclic-carbene–iridium(I) complexes has been synthesized, providing a series of mono-coordinated, bis-chelate and pincer alkenyl-NHC species. Olefin coordination is highly influenced by the nature of the substituents on the NHC ring, and on the length of the alkenyl branch. A fluxional process involving coordination/decoordination of the olefin in bis-allyl-NHC complexes has been studied, and the activation parameters have been deter...

Zanardi, Alessandro; Peris Fajarne?s, Eduardo; Mata Marti?nez, Jose? Antonio

2008-01-01

387

Synthesis of azoimidazolium dyes with nitrous oxide.  

Science.gov (United States)

A new method for the synthesis of industrially important azoimidazolium dyes is presented. The procedure is based on a reagent which is rarely used in the context of synthetic organic chemistry: nitrous oxide ("laughing gas"). N2 O is first coupled to N-heterocyclic carbenes. Subsequent reaction with aromatic compounds through an AlCl3 -induced C?H activation process provides azoimidazolium dyes in good yields. PMID:25420599

Tskhovrebov, Alexander G; Naested, Lara C E; Solari, Euro; Scopelliti, Rosario; Severin, Kay

2015-01-19

388

A Facile Route to C2-Substituted Imidazolium Ionic Liquids  

OpenAIRE

A convenient route for the preparation of C2-substituted imidazolium ionic liquids is reported. This method involves the alkylation of N-heterocyclic carbenes, which are readily generated from the C2-unsubstituted imidazolium ionic liquids. It works well for non-functionalized alkyl chlorides, and less well for alkyl bromides and iodides, likely due to competing elimination reactions. The resulting C2-substituted salts can be transformed into ionic liquids via standard anion metathesis reacti...

Handy, Scott T.; Elliot Ennis

2009-01-01

389

Formation and properties of a bicyclic silylated digermene.  

Science.gov (United States)

In the presence of PMe3 or N-heterocyclic carbenes, the reaction of oligosilanylene dianions with GeCl2?dioxane gives germylene-base adducts. After base abstraction, the free germylenes can dimerize by formation of a digermene. An electrochemical and theoretical study of a bicyclic tetrasilylated digermene revealed formation of a comparably stable radical anion and a more reactive radical cation, which were characterized further by UV/Vis and ESR spectroscopy. PMID:24981992

Hlina, Johann; Baumgartner, Judith; Marschner, Christoph; Albers, Lena; Müller, Thomas; Jouikov, Viatcheslav V

2014-07-21

390

Asymmetric NHC-catalyzed aza-Diels-Alder reactions: highly enantioselective route to ?-amino acid derivatives and DFT calculations.  

Science.gov (United States)

A facile N-heterocyclic carbene catalytic enantioselective aza-Diels-Alder reaction of oxodiazenes with ?-chloroaldehydes as dienophile precursors is reported, with excellent enantioselectivity (ee > 99%) and excellent yield (up to 93%). DFT study showed that cis-TSa, formed from a top face approach of oxodiazene to cis-IIa, is the most favorable transition state and is consistent with the experimental observations. PMID:24987938

Yang, Limin; Wang, Fei; Lee, Richmond; Lv, Yunbo; Huang, Kuo-Wei; Zhong, Guofu

2014-08-01

391

1,3-Bis(pyren-1-yl)imidazolium chloride (IPyr·HCl)  

OpenAIRE

A novel UV-A-light absorbing N-heterocyclic carbene precursor imidazolium salt 2 was synthesized by reaction of 1,4-bis(pyren-1-yl)-1,4-diazadiene 1 with paraformaldehyde in the presence of chlorotrimethylsilane. The title compound was characterized by 1H-NMR, 13C-NMR, 2D-NMR, mass spectrometry, IR and UV/VIS spectroscopy.

Peter Schroll; Burkhard König

2011-01-01

392

The fluorine-NHC gauche effect: a structural and computational study  

OpenAIRE

Herein, we report the synthesis and X-ray structural analysis of a collection of fluorinated metal N-heterocyclic carbenes (Ag, Au, Pd, Rh, Ir) and their precursor salts. The common structural feature of these species is a flanking fluoroethyl group which is either freely rotating or embedded within a bicyclic framework. Solid state analysis confirmed a gauche conformational preference in all cases with the fluorine adopting a syn clinal arrangement (?[NCCF] ~ 60°) with respe...

Paul, S.; Schweizer, W. B.; Rugg, G.; Senn, H. M.; Gilmour, R.

2013-01-01

393

Synthesis, characterization and luminescence studies of gold(I)–NHC amide complexes  

OpenAIRE

A flexible, efficient and straightforward methodology for the synthesis of N-heterocyclic carbene gold(I)–amide complexes is reported. Reaction of the versatile building block [Au(OH)(IPr)] (1) (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) with a series of commercially available (hetero)aromatic amines leads to the synthesis of several [Au(NRR’)(IPr)] complexes in good yields and with water as the sole byproduct. Interestingly, these complexes present luminescence properties. U...

Go?mez-sua?rez, Adria?n; Nelson, David J.; Thompson, David G.; Cordes, David B.; Graham, Duncan; Slawin, Alexandra M. Z.; Nolan, Steven P.

2013-01-01

394

The first bismuth-NHC complexes.  

Science.gov (United States)

The synthesis, isolation and crystallographic characterization of the first N-heterocyclic carbene adducts of bismuth is reported, by direct reaction of the Dipp2NHC (Dipp = 2,6-diisopropylphenyl) or (i)Pr2(Me2)NHC with BiCl3. This represents the last non-radioactive element from groups 13-17 for which an NHC-element fragment remained unreported. PMID:24150033

Aprile, Antonino; Corbo, Robert; Vin Tan, Kel; Wilson, David J D; Dutton, Jason L

2014-01-14

395

Gold-catalyzed Intermolecular [4C + 3C] Cycloaddition Reactions  

OpenAIRE

In the presence of the N-heterocyclic carbene gold catalyst (NHC-AuIPr, 7), propargyl esters 1a–f and 13 undergo a [4C + 3C] cycloaddition reaction with cyclopentadiene and furan under mild conditions. The evidence suggests the formation of the seven-membered ring occurs by a direct cycloaddition process, rather than a stepwise cyclopropanation/Cope rearrangement sequence.

Gung, Benjamin W.; Bailey, Lauren N.; Wonser, Josh

2010-01-01

396

1,3-Bis(2,4,6-trimethylphenyl-3H-imidazol-1-ium tetraoxidorhenate(VII  

Directory of Open Access Journals (Sweden)

Full Text Available The title compound, (C21H25N2[ReO4], was formed as the unexpected product in an attempted synthesis of a rhenium(I–N-heterocyclic carbene (NHC complex. The compound has crystallographic mirror symmetry with both the cation and the tetrahedral anion located across a mirror plane. The cation and anion are linked by a C—H...O hydrogen bond.

Marilé Landman

2011-12-01

397

Intermolecular inter-ligand interaction in mixed ligand complexes of lanthanides(III) involving histidine and some amino acids  

International Nuclear Information System (INIS)

The formation constant studies on binary and mixed ligand complexes of Pr (III) and Nd (III) involving histidine (hist) as primary ligand and ?-alanine (?-ala), valine (val) or leucine (leu) as secondary ligands have been undertaken to evaluate the extra-stabilization in the mixed ligand complexes due to possible occurrence of intramolecular inter-ligand interaction. Stability quantifying parameters (? log k and ?? log k) indicate significant inter-ligand interaction. (author)

398

A rigid trans-spanning dinitrile ligand.  

OpenAIRE

A rigid dinitrile ligand was synthesized from two xanthene units condensed to a naphthalene-1,4,5,8-diimide spacer. The rigidity and C shape of the ligand gave exclusively trans complexes with Pd(II), Ag(I), and Au(I). Evidence for complexation, coordination geometry, and stoichiometry was provided by a combination of 1H NMR, 19F NMR, and IR spectroscopy. The AuBF4 and PdCl2 complexes were shown to have a 1:1 (metal-to-ligand) stoichiometry and the AgBF4 complex was shown to have a 1:2 stoich...

Shimizu, K. D.; Rebek, J.

1996-01-01

399

Di-, tri-, and tetranuclear copper(I) complexes of phenanthroline-linked dicarbene ligands.  

Science.gov (United States)

A family of 2,9-di(3-R-1H-imidazolium-1-yl)-1,10-phenanthroline iodides and hexafluorophosphates (R = allyl, benzyl, mesityl, picolyl) were synthesized from 2,9-diiodophenanthroline and imidazole or N-substituted imidazoles. Simple reactions of these diimidazolium salts with copper powder at room temperature have afforded a series of multinuclear copper(I)-NHC complexes in good yields. The structures vary depending on the N substituents and counterions. [Cu4(L1)2(MeCN)4](PF6)4 (R = allyl) exhibits a zigzag Cu4 chain with two terminal [Cu(NHC)(MeCN)2] and two internal [Cu(phen)(NHC)] moieties. [Cu3(L2)2](PF6)3 (R = benzyl) contains a strictly linear Cu3 framework with two [Cu(NHC)2] units and a [Cu(phen)2] located at the center. Both complexes [Cu3(L4)2](PF6)3 (R = mesityl) and [Cu3(L5)2](PF6)3 (R = picolyl) consist of a triangular Cu3 core in which two copper(I) ions are surrounded by a phen and a NHC group and the third copper(I) is coordinated by two NHC groups. [Cu3(L3)2](PF6)3 derived from 2,9-di(3-benzyl-1H-benzimidazolium-1-yl)-1,10-phenanthroline hexafluorophosphate can undergo transannulation of the benzimidazolylidene ring giving [Cu2(L3')2](PF6)2. The decomposition process might involve solvent-induced rearrangement of the ligand and hydrolysis of carbene moieties. Treatment of 2,9-di(3-mesityl-1H-benzimidazolium-1-yl)-1,10-phenanthroline iodide with copper powder generated dinuclear complex [Cu2(L4)2][Cu2(?-I)2I2] (R = mesityl) possessing a macrocyclic cation and [Cu2(?-I)2I2](2-) anion. Tetranuclear complex [Cu4(L5)2(?-I)2](CuI3) (R = picolyl) has been obtained from its diimidazolium iodide and copper powder. All Cu(I) complexes have been characterized by X-ray single diffraction analysis, elemental analysis, and NMR spectra. Their redox behavior and fluorescent properties have also been studied. PMID:25216090

Liu, Bo; Pan, Shanfei; Liu, Bin; Chen, Wanzhi

2014-10-01

400

Metrical oxidation states of 2-amidophenoxide and catecholate ligands: structural signatures of metal-ligand ? bonding in potentially noninnocent ligands.  

Science.gov (United States)

Catecholates and 2-amidophenoxides are prototypical "noninnocent" ligands which can form metal complexes where the ligands are best described as being in the monoanionic (imino)semiquinone or neutral (imino)quinone oxidation state instead of their closed-shell dianionic form. Through a comprehensive analysis of structural data available for compounds with these ligands in unambiguous oxidation states (109 amidophenolates, 259 catecholates), the well-known structural changes in the ligands with oxidation state can be quantified. Using these correlations, an empirical "metrical oxidation state" (MOS) which gives a continuous measure of the apparent oxidation state of the ligand can be determined based on least-squares fitting of its C-C, C-O, and C-N bond lengths to this single parameter (a simple procedure for doing so is provided via a spreadsheet in the Supporting Information). High-valent d(0) metal complexes, particularly those of vanadium(V) and molybdenum(VI), have ligands with unexpectedly positive, and generally nonintegral, MOS values. The structural effects in these complexes are attributed not to electron transfer, but rather to amidophenoxide- or catecholate-to-metal ? bonding, an interpretation supported by the systematic variation of the MOS values as a function of the degree of competition with the other ?-donating groups in the structures. PMID:22260321

Brown, Seth N

2012-02-01

401

A rigid trans-spanning dinitrile ligand.  

Science.gov (United States)

A rigid dinitrile ligand was synthesized from two xanthene units condensed to a naphthalene-1,4,5,8-diimide spacer. The rigidity and C shape of the ligand gave exclusively trans complexes with Pd(II), Ag(I), and Au(I). Evidence for complexation, coordination geometry, and stoichiometry was provided by a combination of 1H NMR, 19F NMR, and IR spectroscopy. The AuBF4 and PdCl2 complexes were shown to have a 1:1 (metal-to-ligand) stoichiometry and the AgBF4 complex was shown to have a 1:2 stoichiometry in solution. The preorganization of the dinitrile ligand resulted in complexes much more stable than their monodentate counterparts. Images p4257-a PMID:8633051

Shimizu, K D; Rebek, J

1996-01-01

402

Automated design of ligands to polypharmacological profiles  

Science.gov (United States)

The clinical efficacy and safety of a drug is determined by its activity profile across multiple proteins in the proteome. However, designing drugs with a specific multi-target profile is both complex and difficult. Therefore methods to rationally design drugs a priori against profiles of multiple proteins would have immense value in drug discovery. We describe a new approach for the automated design of ligands against profiles of multiple drug targets. The method is demonstrated by the evolution of an approved acetylcholinesterase inhibitor drug into brain penetrable ligands with either specific polypharmacology or exquisite selectivity profiles for G-protein coupled receptors. Overall, 800 ligand-target predictions of prospectively designed ligands were tested experimentally, of which 75% were confirmed correct. We also demonstrate target engagement in vivo. The approach can be a useful source of drug leads where multi-target profiles are required to achieve either selectivity over other drug targets or a desired polypharmacology. PMID:23235874

Besnard, Jérémy; Ruda, Gian Filippo; Setola, Vincent; Abecassis, Keren; Rodriguiz, Ramona M.; Huang, Xi-Ping; Norval, Suzanne; Sassano, Maria F.; Shin, Antony I.; Webster, Lauren A.; Simeons, Frederick R.C.; Stojanovski, Laste; Prat, Annik; Seidah, Nabil G.; Constam, Daniel B.; Bickerton, G. Richard; Read, Kevin D.; Wetsel, William C.; Gilbert, Ian H.; Roth, Bryan L.; Hopkins, Andrew L.

2012-01-01

403

Ligand-Induced Conformational Change in the ?7 Nicotinic Receptor Ligand Binding Domain  

OpenAIRE

Molecular dynamics simulations of a homology model of the ligand binding domain of the ?7 nicotinic receptor are conducted with a range of bound ligands to induce different conformational states. Four simulations of 15 ns each are run with no ligand, antagonist d-tubocurarine (dTC), agonist acetylcholine (ACh), and agonist ACh with potentiator Ca2+, to give insight into the conformations of the active and inactive states of the receptor and suggest the mechanism for conformational change. Th...

Henchman, Richard H.; Wang, Hai-long; Sine, Steven M.; Taylor, Palmer; Mccammon, J. Andrew

2005-01-01

404

Supported ligands for metal catalyzed reactions  

OpenAIRE

Synthesis of homogeneous ligands and their use in catalysis constitutes one of the most important subjects in organic chemistry originating thousands of published works every year. From an industrial point of view, the application of such methodologies is rather limited due to the high cost of chiral ligands and noble metals used in such transformations. Moreover, sometimes the final products contain high levels of metal contamination, which can become a serious drawback if the metal is toxic...

Marcos Escarti?n, Rocio

2011-01-01

405

New functionalized ?-diketiminate ligands and f elements  

OpenAIRE

?-Diketiminate ligands have received increased interest in coordination chemistry, especially for homogeneous catalysis. Their successful applications arise from an easy and fine tuning of the ligand electronic and geometric properties. However, these modifications are limited to the introduction of neutral donors (ethers or amines), on the nitrogen substituents of the ?-diketiminate skeleton. The main focus of this research project is to overcome this limitation by synthesizing new ?-dike...

Dulong, Florian

2013-01-01

406

Screening protein-ligand interactions using 1H NMR techniques for detecting the ligand  

International Nuclear Information System (INIS)

NMR is a valuable screening tool for the binding of ligands to proteins providing structural information on both protein and ligands and is thus largely applied to drug-discovery. Among the recent NMR techniques to probe weak binding protein-ligand complexes we have critically evaluated the advantages and disadvantages of STD (Saturation Transfer Difference), WaterLOGSY (Water Ligand Observation with Gradient Spectroscopy), NOE pumping and DOSY-NOESY (Diffusion-Ordered NOESY) using a mixture of BSA (bovine serum albumin) plus salicylic acid, caffeine, citric acid, adipic acid and D-glucose. (author)

407

Energy Transfer Ligands of the GluR2 Ligand Binding Core†  

OpenAIRE

Ionotropic glutamate receptors (iGluRs) are ligand-gated ion channels that mediate excitatory signaling in the central nervous system. Upon ligand binding to the extracellular domain of iGluRs local conformational changes ensue and this motion is translated to the transmembrane domain inducing channel opening. We have used an isolated ligand binding domain, GluR2-S1S2J (GluR2), as a model system to study the protein-ligand complex by steady-state and time-resolved intrinsic tryptophan fluores...

Petrik, Amy F.; Strub, Marie-paule; Lee, Jennifer C.

2010-01-01

408

LigandRFs: random forest ensemble to identify ligand-binding residues from sequence information alone  

KAUST Repository

Background Protein-ligand binding is important for some proteins to perform their functions. Protein-ligand binding sites are the residues of proteins that physically bind to ligands. Despite of the recent advances in computational prediction for protein-ligand binding sites, the state-of-the-art methods search for similar, known structures of the query and predict the binding sites based on the solved structures. However, such structural information is not commonly available. Results In this paper, we propose a sequence-based approach to identify protein-ligand binding residues. We propose a combination technique to reduce the effects of different sliding residue windows in the process of encoding input feature vectors. Moreover, due to the highly imbalanced samples between the ligand-binding sites and non ligand-binding sites, we construct several balanced data sets, for each of which a random forest (RF)-based classifier is trained. The ensemble of these RF classifiers forms a sequence-based protein-ligand binding site predictor. Conclusions Experimental results on CASP9 and CASP8 data sets demonstrate that our method compares favorably with the state-of-the-art protein-ligand binding site prediction methods.

Chen, Peng

2014-12-03

409

Study of the factors supporting the selective complexation of the trivalent lanthanide and actinide ions; Etude des facteurs favorisant la complexation selective des ions lanthanides et actinides trivalents  

Energy Technology Data Exchange (ETDEWEB)

In order to obtain clear-cut information on the factors which favour the discrimination between trivalent actinides and lanthanides, we investigated the complexation of the tris(cyclopentadienyl) Ce(III) and U(III) compounds, (RCp)3M (R = tBu, SiMe3), with a series of monocyclic azines with distinct Lewis basicity and reduction potential. Coordination of pyrazine and 4,4' and 2,2'-bipyridines on the (RCp)3M complexes has also been studied. Of major interest is the reversible oxidation of the (RCp)3U species into the uranium(IV) [(RCp)3U]2(pyz) complexes by pyrazine. The presence of cooperativity in the binding of the cyclopentadienyl groups by U(III), due to late appearance of back-bonding, leads to a greater stabilization of the uranium(III) complexes. Complexation of the species Cp*2MI (M = Ce, U) by 2,2'-bipyridine, phenanthroline and ter-pyridine affords the adducts [Cp*2M(L)]I. For L = bipy and terpy, these compounds are reduced into Cp*2M(L). The magnetic data for [Cp*2M(terpy)]I and Cp*2M(terpy) are consistent with Ce(III) and U(III) species, with the formulation Cp*2MIII(terpy). An electron transfer reaction between these species was observed in NMR. Reactions of the [Cp*2M(terpy)]I and Cp*2M(terpy) complexes with H and H{sup +} donor reagents lead to a clear differentiation of these trivalent ions. We studied the coordination of the stable N-heterocyclic carbene and isonitrile molecules on (RCp)3M and Cp*2MI; competition reactions and comparison of the crystal structures of the carbene compounds reveal the much better affinity of the NHC and tBuNC ligands for the 5f rather than for the 4f ion. (authors)

Mehdoui, T

2005-09-15

410

Study of the factors supporting the selective complexation of the trivalent lanthanide and actinide ions  

International Nuclear Information System (INIS)

In order to obtain clear-cut information on the factors which favour the discrimination between trivalent actinides and lanthanides, we investigated the complexation of the tris(cyclopentadienyl) Ce(III) and U(III) compounds, (RCp)3M (R = tBu, SiMe3), with a series of monocyclic azines with distinct Lewis basicity and reduction potential. Coordination of pyrazine and 4,4' and 2,2'-bipyridines on the (RCp)3M complexes has also been studied. Of major interest is the reversible oxidation of the (RCp)3U species into the uranium(IV) [(RCp)3U]2(pyz) complexes by pyrazine. The presence of cooperativity in the binding of the cyclopentadienyl groups by U(III), due to late appearance of back-bonding, leads to a greater stabilization of the uranium(III) complexes. Complexation of the species Cp*2MI (M = Ce, U) by 2,2'-bipyridine, phenanthroline and ter-pyridine affords the adducts [Cp*2M(L)]I. For L = bipy and terpy, these compounds are reduced into Cp*2M(L). The magnetic data for [Cp*2M(terpy)]I and Cp*2M(terpy) are consistent with Ce(III) and U(III) species, with the formulation Cp*2MIII(terpy). An electron transfer reaction between these species was observed in NMR. Reactions of the [Cp*2M(terpy)]I and Cp*2M(terpy) complexes with H and H+ donor reagents lead to a clear differentiation of these trivalent ions. We studied the coordination of the stable N-heterocyclic carbene and isonitrile molecules on (RCp)3M and Cp*2MI; competition reactions and comparison of the crystal structures of the carbene compounds reveal the much better affinity of the NHC and tBuNC ligands for the 5f rather than for the 4f ion. (authors)

411

Gold(I) biscarbene complexes derived from vascular-disrupting combretastatin A-4 address different targets and show antimetastatic potential.  

Science.gov (United States)

Gold N-heterocyclic carbene (NHC) complexes are an emerging class of anticancer drugs. We present a series of gold(I) biscarbene complexes with NHC ligands derived from the plant metabolite combretastatin A-4 (CA-4) that retain its vascular-disrupting effect, yet address different cellular and protein targets. Unlike CA-4, these complexes did not interfere with tubulin, but with the actin cytoskeleton of endothelial and cancer cells. For the highly metastatic 518A2 melanoma cell line this effect was accompanied by a marked accumulation of cells in the G1 phase of the cell cycle and a suppression of active prometastatic matrix metalloproteinase-2. Despite these mechanistic differences the complexes were as strongly antivascular as CA-4 both in vitro in tube formation assays with human umbilical vein endothelial cells, and in vivo as to blood vessel disruption in the chorioallantoic membrane of chicken eggs. The antiproliferative effect of the new gold biscarbene complexes in a panel of six human cancer cell lines was impressive, with low sub-micromolar IC50 values (72 h) even against CA-4-refractory HT-29 colon and multidrug-resistant MCF-7 breast carcinoma cells. In preliminary studies with a mouse melanoma xenograft model the complexes led to significant decreases in tumor volume while being very well tolerated. PMID:24648184

Muenzner, Julienne K; Biersack, Bernhard; Kalie, Hussein; Andronache, Ion C; Kaps, Leonard; Schuppan, Detlef; Sasse, Florenz; Schobert, Rainer

2014-06-01

412

Ru-?(6) -arene cations [{(Ph2 PC6 H4 )2 B(?(6) -Ph)}RuX](+) (X=Cl, H) as Lewis acids.  

Science.gov (United States)

The reactivity of [{(Ph2 PC6 H4 )2 B(?(6) -Ph)}RuCl][B(C6 F5 )4 ] (1) as a Lewis acid was investigated. Treatment of 1 with mono and multidentate phosphorus Lewis bases afforded the Lewis acid-base adducts with the ortho-carbon atom of the coordinated arene ring. Similar reactivity was observed upon treatment with N-heterocyclic carbenes; however, adduct formation occurred at both ortho- and para-carbon atoms of the bound arene with the para-position being favoured by increased steric demands. Interestingly treatment with isocyanides resulted in adduct formation with the B-centre of the ligand framework. The hydride-cation [{(Ph2 PC6 H4 )2 B(?(6) -Ph)}RuH] [B(C6 F5 )4 ] was prepared via reaction of 1 with silane. This species in the presence of a bulky phosphine behaves as a frustrated Lewis pair (FLP) to activate H2 between the phosphorus centre and the ortho-carbon atom of the ?(6) -arene ring. PMID:24616079

Boone, Michael P; Stephan, Douglas W

2014-03-17

413

PDBToSDF: Create ligand structure files from PDB file  

OpenAIRE

Protein Data Bank (PDB) file contains atomic data for protein and ligand in protein-ligand complexes. Structure data file (SDF) contains data for atoms, bonds, connectivity and coordinates of molecule for ligands. We describe PDBToSDF as a tool to separate the ligand data from pdb file for the calculation of ligand properties like molecular weight, number of hydrogen bond acceptors, hydrogen bond receptors easily.

Muppalaneni, Naresh Babu; Rao, Allam Appa

2011-01-01

414

Towards Ligand Docking Including Explicit Interface Water Molecules  

OpenAIRE

Small molecule docking predicts the interaction of a small molecule ligand with a protein at atomic-detail accuracy including position and conformation the ligand but also conformational changes of the protein upon ligand binding. While successful in the majority of cases, docking algorithms including RosettaLigand fail in some cases to predict the correct protein/ligand complex structure. In this study we show that simultaneous docking of explicit interface water molecules greatly improves R...

Lemmon, Gordon; Meiler, Jens

2013-01-01

415

A conformation-equilibrium model captures ligand-ligand interactions and ligand-biased signalling by G-protein coupled receptors.  

Science.gov (United States)

G-protein coupled receptors (GPCRs) are a versatile, important class of cell-surface receptors. GPCRs occur in different conformations that exist in a dynamic ligand-sensitive equilibrium. These conformations vary in their affinities for intracellular signalling proteins and initiate signalling via different intracellular routes. The binding of extracellular ligands and allosteric ligand-ligand interactions shift conformation equilibria to cause biased signalling. Here, we present a mathematical model that describes the effects of ligands on the conformation equilibria of GPCRs. Our extended Monod-Wyman-Changeux model describes the receptor as shifting between active and inactive receptor conformations under the influence of extracellular ligands. For each receptor conformation, the intracellular domain of the receptor can attain alternative domain conformations that differ in their affinity for intracellular signalling proteins. At the extracellular domain, the model can accommodate different mechanisms for allosteric ligand-ligand interactions that induce shifts in receptor and domain conformation equilibria. We use the model to study ligand-biased signalling and how ligand affinity, ligand sensitivity and maximal signalling output depend on allosteric ligand-ligand interactions. PMID:25145284

Roth, Susanne; Bruggeman, Frank J

2014-10-01

416

Supramolecular architectures constructed using angular bipyridyl ligands  

International Nuclear Information System (INIS)

This work details the synthesis and characterization of a series of coordination frameworks that are formed using bidentate angular N-donor ligands. Pyrimidine was reacted with metal(ll) nitrate salts. Reactions using Cd(NO3)2 receive particular focus and the analogous reactions using the linear ligand, pyrazine, were studied for comparison. In all cases, two-dimensional coordination networks were prepared. Structural diversity is observed for the Cd(ll) centres including metal-nitrate bridging. In contrast, first row transition metal nitrates form isostructural one-dimensional chains with only the bridging N-donor ligands generating polymeric propagation. The angular ligand, 2,4-bis(4-pyridyl)-1,3,5-triazine (dpt), was reacted with Cd(NO3)2 and Zn(NO3)2. Whereas Zn(NO3)2 compounds exhibit solvent mediated polymorphism, a range of structures were obtained for the reactions with Cd(NO3)2, including the first example of a doubly parallel interpenetrated 4.82 net. 4,7-phenanthroline, was reacted with various metal(ll) nitrates as well as cobalt(ll) and copper(ll) halides. The ability of 4,7-phenanthroline to act as both a N-donor ligand and a hydrogen bond acceptor has been discussed. Reactions of CuSCN with pyrimidine yield an unusual three-dimensional structure in which polymeric propagation is not a result of ligand bridging. The reaction of CuSCN wf ligand bridging. The reaction of CuSCN with dpt yielded structural supramolecular isomers. (author)

417

Posttranslational regulation of Fas ligand function  

Directory of Open Access Journals (Sweden)

Full Text Available Abstract The TNF superfamily member Fas ligand acts as a prototypic death factor. Due to its ability to induce apoptosis in Fas (APO-1, CD95 expressing cells, Fas ligand participates in essential effector functions of the immune system. It is involved in natural killer cell- and T cell-mediated cytotoxicity, the establishment of immune privilege, and in termination of immune responses by induction of activation-induced cell death. In addition, Fas ligand-positive tumours may evade immune surveillance by killing Fas-positive tumour-infiltrating cells. Given these strong cytotoxic capabilities of Fas ligand, it is obvious that its function has to be strictly regulated to avoid uncontrolled damage. In hematopoietic cells, the death factor is stored in secretory lysosomes and is mobilised to the immunological synapse only upon activation. The selective sorting to and the release from this specific lysosomal compartment requires interactions of the Fas ligand cytosolic moiety, which mediates binding to various adapter proteins involved in trafficking and cytoskeletal reorganisation. In addition, Fas ligand surface expression is further regulated by posttranslational ectodomain shedding and subsequent regulated intramembrane proteolysis, releasing a soluble ectodomain cytokine into the extracellular space and an N-terminal fragment with a potential role in intracellular signalling processes. Moreover, other posttranslational modifications of the cytosolic domain, including phosphorylation and ubiquitylation, have been described to affect various aspects of Fas ligand biology. Since FasL is regarded as a potential target for immunotherapy, the further characterisation of its biological regulation and function will be of great importance for the development and evaluation of future therapeutic strategies.

Paulsen Maren

2008-12-01

418

Immobilisation of ligands by radio-derivatized polymers; Immobilisering av ligander med radioderiverte polymerer  

Energy Technology Data Exchange (ETDEWEB)

The invention relates to radio-derivatized polymers and a method of producing them by contacting non-polymerizable conjugands with radiolysable polymers in the presence of irradiation. The resulting radio-derivatized polymers can be further linked with ligand of organic or inorganic nature to immobilize such ligands. 2 figs., 5 tabs.

Varga, J.M.; Fritsch, P.

1995-01-30

419

Survey of long d10-d10 metallophilic contacts in four-membered rings of Ag(I) and Au(I) supported by carbene-pyrazole mixed ligands.  

Science.gov (United States)

The interesting case of long intramolecular d(10)-d(10) contacts has been studied through [Ag(4)L(2)](2+) and [Au(4)L(2)](2+) (L = 3,5-bis((N-methylimidazolyl)methyl)pyrazole) systems, showing interesting features gained by analysis of the electronic structure and the overall shielding tensor in the molecular domain, in terms of its components. The long intramolecular closed-shell separations are attributed to the population of the bonding, nonbonding, and antibonding combinations of the ns atomic shells in the [M(4)](4+) core, contrasting with that observed in systems with shorter d(10)-d(10) distances. This point allows to concludeb that separations shorter then the sum of the van der Waals radii (3.4 Å for Ag-Ag, and 3.2 Å for Au-Au) of the nucleus involved requires a net bonding population between ns and np atomic shells of the d(10) closed-shell centers. Moreover, [Au(4)L(2)](2+) exhibits an increased covalency observed for the enhanced charge-donation due to the stabilization of the ns and destabilization of the (n - 1)d driven by the relativistic effects. The magnetic response denotes a slight interaction between the closed-shell centers at distances in the range of their sum of van der Waals radii because the observed remote effect (or anisotropic effect) caused by each d(10) nucleus does not influence considerably the neighbor center. The analysis of ? in terms of its components allows to conclude that the [Au(4)L(2)](2+) system exhibits an increased magnetic response due to the increase in the number of the inner-electrons in comparison to [Ag(4)L(2)](2+). PMID:22889434

Maturana, Raul Guajardo; Vargas, Miguel Ponce; Muñoz-Castro, Alvaro

2012-08-30

420

Modeling of the Aryl Hydrocarbon Receptor (AhR) ligand binding domain and its utility in virtual ligand screening to predict new AhR ligands  

OpenAIRE

The Aryl Hydrocarbon Receptor (AhR) is a ligand-activated transcription factor; the AhR Per-AhR/Arnt-Sim (PAS) domain binds ligands. We developed homology models of the AhR PAS domain to characterize previously observed intra- and inter-species differences in ligand binding using Molecular Docking. In silico structure-based virtual ligand screening using our model resulted in the identification of pinocembrin and 5-hydroxy-7-methoxyflavone, which promoted nuclear translocation and transcripti...

Bisson, William; Koch, Daniel; O’donnell, Edmond; Khalil, Sammy M.; Kerkvliet, Nancy; Tanguay, Robert; Abagyan, Ruben; Kolluri, Siva Kumar

2009-01-01

421

Dockomatic - automated ligand creation and docking  

Directory of Open Access Journals (Sweden)

Full Text Available Abstract Background The application of computational modeling to rationally design drugs and characterize macro biomolecular receptors has proven increasingly useful due to the accessibility of computing clusters and clouds. AutoDock is a well-known and powerful software program used to model ligand to receptor binding interactions. In its current version, AutoDock requires significant amounts of user time to setup and run jobs, and collect results. This paper presents DockoMatic, a user friendly Graphical User Interface (GUI application that eases and automates the creation and management of AutoDock jobs for high throughput screening of ligand to receptor interactions. Results DockoMatic allows the user to invoke and manage AutoDock jobs on a single computer or cluster, including jobs for evaluating secondary ligand interactions. It also automates the process of collecting, summarizing, and viewing results. In addition, DockoMatic automates creation of peptide ligand .pdb files from strings of single-letter amino acid abbreviations. Conclusions DockoMatic significantly reduces the complexity of managing multiple AutoDock jobs by facilitating ligand and AutoDock job creation and management.

Hampikian Greg

2010-11-01

422

Designed multiple ligands for cancer therapy.  

Science.gov (United States)

The concept of a single chemical entity with desirable activity at more than one biological target is an attractive one. Increasingly, multiple complex biochemical pathways are implicated in a variety of diseases including cancer. Successful treatment of these conditions often depends on pharmaceutical intervention at multiple pathways, with a combination of different drugs. Designed multiple ligands (DMLs) are drugs which act at multiple biomolecular targets. Numerous terms have been used to describe such ligands, including multiple-target directed ligands, heterodimers, promiscuous drugs and pan-agonists. However, although there are many reported examples of multiple-targeting anti-cancer agents, no review of these has been presented to date. A huge variety of biological signalling-pathways, proteins and enzymes are currently targeted and implicated in the pathogenesis of cancer. This review will provide an overview of reported designed multiple ligands for cancer and an exploration of the advantages and drawbacks of such compounds. The review also provides brief commentaries on the biological processes and proteins that are currently targeted in cancer therapy and the potential for dual or triple targeting of these with designed multiple ligands. PMID:21919848

O'Boyle, N M; Meegan, M J

2011-01-01

423

Tightly bound ligands for hafnium nanoparticle EUV resists  

Science.gov (United States)

Relative ligand binding energies were determined for a series of common ligand types with hafnium oxide nanoparticles, and from these results a series of novel strong binding ligands were developed. The relative equilibrium concentrations of two competing ligands bound to the nanoparticles were measured using nuclear magnetic resonance spectroscopy (NMR). For each ligand type, equilibrium constants and relative binding energies were then calculated and compared. Methane sulfonic acid was found to have the strongest binding energy, 2.0 Kcal/mol stronger than acetic acid. A group of three sulfonate ligands capable of freeradical crosslinking were made, along with three sulfonate ligands capable of creating aqueous developable nanoparticles. One of these ligands resulted in insoluble nanoparticles, however, the other two ligands resulted in nanoparticles that coated well on a silicon substrate and had dissolution rates greater than 100 nm per second.

Cardineau, Brian; Krysak, Marie; Trikeriotis, Markos; Giannelis, Emmanuel; Ober, Christopher K.; Cho, Kyoungyong; Brainard, Robert

2012-03-01

424

Supramolecular architectures constructed using angular bipyridyl ligands  

Energy Technology Data Exchange (ETDEWEB)

This work details the synthesis and characterization of a series of coordination frameworks that are formed using bidentate angular N-donor ligands. Pyrimidine was reacted with metal(ll) nitrate salts. Reactions using Cd(NO{sub 3}){sub 2} receive particular focus and the analogous reactions using the linear ligand, pyrazine, were studied for comparison. In all cases, two-dimensional coordination networks were prepared. Structural diversity is observed for the Cd(ll) centres including metal-nitrate bridging. In contrast, first row transition metal nitrates form isostructural one-dimensional chains with only the bridging N-donor ligands generating polymeric propagation. The angular ligand, 2,4-bis(4-pyridyl)-1,3,5-triazine (dpt), was reacted with Cd(NO{sub 3}){sub 2} and Zn(NO{sub 3}){sub 2}. Whereas Zn(NO{sub 3}){sub 2} compounds exhibit solvent mediated polymorphism, a range of structures were obtained for the reactions with Cd(NO{sub 3}){sub 2}, including the first example of a doubly parallel interpenetrated 4.8{sup 2} net. 4,7-phenanthroline, was reacted with various metal(ll) nitrates as well as cobalt(ll) and copper(ll) halides. The ability of 4,7-phenanthroline to act as both a N-donor ligand and a hydrogen bond acceptor has been discussed. Reactions of CuSCN with pyrimidine yield an unusual three-dimensional structure in which polymeric propagation is not a result of ligand bridging. The reaction of CuSCN with dpt yielded structural supramolecular isomers. (author)

Barnett, Sarah Ann

2003-07-01

425

A versatile dinucleating ligand containing sulfonamide groups  

DEFF Research Database (Denmark)

Copper, iron, and gallium coordination chemistries of the new pentadentate bis-sulfonamide ligand 2,6-bis(N-2-pyridylmethylsulfonamido)-4-methylphenol (psmpH3) were investigated. PsmpH3 is capable of varying degrees of deprotonation, and notably, complexes containing the fully trideprotonated ligand can be prepared in aqueous solutions using only divalent metal ions. Two of the copper(II) complexes, [Cu2(psmp)(OH)] and [Cu2(psmp)(OAc)2]-, demonstrate the anticipated 1:2 ligand/metal stoichiometry and show that the dimetallic binding site created for exogenous ligands possesses high inherent flexibility since additional one- and three-atom bridging ligands bridge the two copper(II) ions in each complex, respectively. This gives rise to a difference of 0.4 Å in the Cu···Cu distances. Complexes with 2:3 and 2:1 ligand/metal stoichiometries for the divalent and trivalent metal ions, respectively, were observed in [Cu3(psmp)2(H 2O)] and [M(psmpH)(psmpH2)], where M = GaIII, FeIII. The deprotonated tridentate N-2-pyridylsulfonylmethylphenolato moieties chelate the metal ions in a meridional fashion, whereas in [Cu 3(psmp)2(H2O)] the rare ?2-N- sulfonamido bridging coordination mode is observed. In the bis-ligand mononuclear complexes, one picolyl arm of each ligand is protonated and uncoordinated. Magnetic susceptibility measurements on the doubly and triply bridged dicopper(II) complexes indicate strong and medium strength antiferromagnetic coupling interactions, with J = 234 cm-1 and 115 cm-1 for [Cu2(psmp)(OH)] and [Cu2(psmp)(OAc) 2]-, respectively (in HHDvV =...+JS 1S2 convention). The trinuclear [Cu3(psmp) 2(H2O)], in which the central copper ion is linked to two flanking copper atoms by two ?2-N-sulfonamido bridges and two phenoxide bridges shows an overall magnetic behavior of antiferromagnetic coupling. This is corroborated computationally by broken-symmetry density functional theory, which for isotropic modeling of the coupling predicts an antiferromagnetic coupling strength of J = 70.5 cm-1. © 2014 American Chemical Society.

Sundberg, Jonas; Witt, Hannes

2014-01-01

426

Ligand Intermediates in Metal-Catalyzed Reactions  

Energy Technology Data Exchange (ETDEWEB)

The longest-running goal of this project has been the synthesis, isolation, and physical chemical characterization of homogeneous transition metal complexes containing ligand types believed to be intermediates in the metal-catalyzed conversion of CO/H{sub 2}, CO{sub 2}, CH{sub 4}, and similar raw materials to organic fuels, feedstocks, etc. In the current project period, complexes that contain unusual new types of C{sub x}(carbide) and C{sub x}O{sub y} (carbon oxide) ligands have been emphasized. A new program in homogeneous fluorous phase catalysis has been launched as described in the final report.

Gladysz, John A.

1999-07-31

427

Tuning metal-metal distances through ligands  

OpenAIRE

Metal-metal distances in polynuclear metal complexes are often controlled by factors other than favourable metal-metal (M-M) interactions. Recently metal cluster topology has been identified as a major factor in controlling Cu-Cu distances in tetrahedral copper clusters. The more obvious ligand control in M--M distances was shown to play a secondary role. In order to probe ways to tune metal-metal distances in clusters, a study of clusters with similar ligands around the metal has been taken ...

Abraham, Santosh P.; Samuelson, Ashoka G.

1996-01-01

428

Ligand iron catalysts for selective hydrogenation  

Science.gov (United States)

Disclosed are iron ligand catalysts for selective hydrogenation of aldehydes, ketones and imines. A catalyst such as dicarbonyl iron hydride hydroxycyclopentadiene) complex uses the OH on the five member ring and hydrogen linked to the iron to facilitate hydrogenation reactions, particularly in the presence of hydrogen gas.

Casey, Charles P. (Madison, WI); Guan, Hairong (Cincinnati, OH)

2010-11-16

429

Supramolecular architectures constructed using angular bipyridyl ligands  

CERN Document Server

This work details the synthesis and characterization of a series of coordination frameworks that are formed using bidentate angular N-donor ligands. Pyrimidine was reacted with metal(ll) nitrate salts. Reactions using Cd(NO sub 3) sub 2 receive particular focus and the analogous reactions using the linear ligand, pyrazine, were studied for comparison. In all cases, two-dimensional coordination networks were prepared. Structural diversity is observed for the Cd(ll) centres including metal-nitrate bridging. In contrast, first row transition metal nitrates form isostructural one-dimensional chains with only the bridging N-donor ligands generating polymeric propagation. The angular ligand, 2,4-bis(4-pyridyl)-1,3,5-triazine (dpt), was reacted with Cd(NO sub 3) sub 2 and Zn(NO sub 3) sub 2. Whereas Zn(NO sub 3) sub 2 compounds exhibit solvent mediated polymorphism, a range of structures were obtained for the reactions with Cd(NO sub 3) sub 2 , including the first example of a doubly parallel interpenetrated 4.8 sup...

Barnett, S A

2003-01-01

430

Substrate coated with receptor and labelled ligand for assays  

International Nuclear Information System (INIS)

Improvements in the procedures for assaying ligands are described. The assay consists of a polystyrene tube on which receptors are present for both the ligand to be assayed and a radioactively labelled form of the ligand. The receptors on the bottom portion of the tube are also coated with labelled ligands, thus eliminating the necessity for separate addition of the labelled ligand and sample during an assay. Examples of ligands to which this method is applicable include polypeptides, nucleotides, nucleosides and proteins. Specific examples are given in which the ligand to be assayed is digoxin, the labelled form of the ligand is 3-0-succinyl digoxyigenin tyrosine (125I) and the receptor is digoxin antibody. (U.K.)

431

Structural requirements for ligands of the ?-opioid receptor  

Directory of Open Access Journals (Sweden)

Full Text Available The ?-opioid receptor is sensitive to ligand geometry. In order to assist the synthesis of new ?-selective opioid ligands, the structure elements of ?-selective opioid ligands necessary for their effective binding were investigated. The automated docking procedure with a flexible ligand was used to simulate the binding of 17 ?-selective ligands to the ?-receptor. It was found that voluminous N-alkyl groups reduce the binding potency of naltrindole derivatives by preventing the ligands from adopting the preferred conformation in the receptor. This was confirmed by enantiospecific binding of chiral compounds where only one enantiomer adopts the naltrindole-like preferred conformation in the binding pocket. Voluminous groups replacing the hydroxyl group in the 3-hydroxybenzyl fragment of naltrindole analogs reduce the binding potency due to unfavorable steric interactions with the receptor. The two diastereoisomers of the potent ?-opioid ligand SNC80 confirmed the preferred binding conformation and the major receptor–ligand interactions.

VUK I. MI?OVI?

2009-11-01

432

Ammonia formation by metal-ligand cooperative hydrogenolysis of a nitrido ligand  

Science.gov (United States)

Bioinspired hydrogenation of N2 to ammonia at ambient conditions by stepwise nitrogen protonation/reduction with metal complexes in solution has experienced remarkable progress. In contrast, the highly desirable direct hydrogenation with H2 remains difficult. In analogy to the heterogeneously catalysed Haber-Bosch process, such a reaction is conceivable via metal-centred N2 splitting and unprecedented hydrogenolysis of the nitrido ligands to ammonia. We report the synthesis of a ruthenium(IV) nitrido complex. The high nucleophilicity of the nitrido ligand is demonstrated by unusual N-C coupling with ?-acidic CO. Furthermore, the terminal nitrido ligand undergoes facile hydrogenolysis with H2 at ambient conditions to produce ammonia in high yield. Kinetic and quantum chemical examinations of this reaction suggest cooperative behaviour of a phosphorus-nitrogen-phosphorus pincer ligand in rate-determining heterolytic hydrogen splitting.

Askevold, Bjorn; Nieto, Jorge Torres; Tussupbayev, Samat; Diefenbach, Martin; Herdtweck, Eberhardt; Holthausen, Max C.; Schneider, Sven

2011-07-01

433

Controlling Ligand Surface Density Optimizes Nanoparticle Binding to ICAM-1  

OpenAIRE

During infection, pathogens utilize surface receptors to gain entry into intracellular compartments. Multiple receptor-ligand interactions that lead to pathogen internalization have been identified and the importance of multivalent ligand binding as a means to facilitate internalization has emerged. The effect of ligand density, however; is less well known. In this study, ligand density was examined using poly(dl-lactic-co-glycolic acid) nanoparticles (PLGA NPs). A cyclic peptide, cLABL, was ...

Fakhari, Amir; Baoum, Abdulgader; Le, Khoi Ba; Siahaan, Teruna J.; Berkland, Cory

2010-01-01

434

Chiral polytopic ligands : self-assembly and catalysis  

OpenAIRE

Chiral C2-symmetric bis(oxazolines), bidentate ligands formed by two oxazoline rings separated by a carbon atom with two identical substituents, are one of the most popular types of chiral ligands which have already been successfully used in coordination chemistry and asymmetric catalysis (heterogeneous and homogeneous). This work describes the synthesis of thirteen polytopic chiral ligands bearing bis(oxazolines) moieties which have then been used, on the one hand, as chiral ligands for the ...

Torres-werle?, Maria Lourdes

2013-01-01

435

Protein-ligand cocrystal structures: we can do better.  

Science.gov (United States)

There is a large body of evidence that many protein-ligand cocrystal structures contain poorly refined ligand geometries. These errors result in bound structures that have nonideal bond lengths and angles, are strained, contain improbable conformations, and have bad protein-ligand contacts. Many of these problems can be greatly reduced with better refinement models. PMID:25050154

Reynolds, Charles H

2014-07-10

436

Evolution game: which came first, the receptor or the ligand?  

Directory of Open Access Journals (Sweden)

Full Text Available On the basis of a bioinformatic approach, we suggest that in invertebrates many ligands interact with a single, ancestral and generalized receptor driving ligand evolution. In vertebrates, on the other hand, the occurrence of gene/genome duplications induced the shift to a ligand-directed evolution of receptors.

M Mandrioli

2007-04-01

437

CHEMOMETRIC ANALYSIS OF LIGAND RECEPTOR COMPLEMENTARITY: IDENTIFYING COMPLEMENTARY LIGANDS BASED ON RECEPTOR INFORMATION (CoLiBRI)  

OpenAIRE

We have developed a novel structure-based approach to search for Complimentary Ligands Based on Receptor Information (CoLiBRI). CoLiBRI is based on the representation of both receptor binding sites and their respective ligands in a space of universal chemical descriptors. The binding site atoms involved in the interaction with ligands are identified by the means of computational geometry technique known as Delaunay tessellation as applied to x-ray characterized ligand-receptor complexes. TAE/...

Oloff, Scott; Zhang, Shuxing; Sukumar, Nagamani; Breneman, Curt; Tropsha, Alexander

2006-01-01

438

Optical ligand-to-ligand charge transfer of Zn(2,2'-bipyridyl)(3,4-toluenedithiolate)  

OpenAIRE

The electronic structure and the optical ligand-to-ligand charge transfer (LLCT) of the pseudotetrahedral complex Zn (2,2?-bipyridyl) (3,4-toluenedithiolate) is interpreted on the basis of EHT calculations. By comparison with square-planar complexes, it is shown that the extent of electron delocalization between donor and acceptor ligand and charge separation by LLCT excitation depends on the mutual orientation of both ligands.

Benedix, R.; Hennig, H.; Kunkely, Horst; Vogler, Arnd

1990-01-01

439

Rational Optimization of a Bispecific Ligand Trap Targeting EGF Receptor Family Ligands  

Science.gov (United States)

The human epidermal growth factor (EGF) receptor (HER) family members cooperate in malignancy. Of this family, HER2 does not bind growth factors and HER3 does not encode an active tyrosine kinase. This diversity creates difficulty in creating pan-specific therapeutic HER family inhibitors. We have identified single amino acid changes in epidermal growth factor receptor (EGFR) and HER3 which create high affinity sequestration of the cognate ligands, and may be used as receptor decoys to downregulate aberrant HER family activity. In silico modeling and high throughput mutagenesis were utilized to identify receptor mutants with very high ligand binding activity. A single mutation (T15S; EGFR subdomain I) enhanced affinity for EGF (two-fold), TGF-? (twenty-six-fold), and heparin-binding (HB)-EGF (six-fold). This indicates that T15 is an important, previously undescribed, negative regulatory amino acid for EGFR ligand binding. Another mutation (Y246A; HER 3 subdomain II) enhanced neuregulin (NRG)1-? binding eight-fold, probably by interfering with subdomain II-IV interactions. Further work revealed that the HER3 subunit of an EGFR:HER3 heterodimer suppresses EGFR ligand binding. Optimization required reversing this suppression by mutation of the EGFR tether domain (G564A; subdomain IV). This mutation resulted in enhanced ligand binding (EGF, ten-fold; TGF-?, thirty-four-fold; HB-EGF, seventeen-fold; NRG1-?, thirty-one-fold). This increased ligand binding was reflected in improved inhibition of in vitro tumor cell proliferation and tumor suppression in a human non-small cell lung cancer xenograft model. In conclusion, amino acid substitutions were identified in the EGFR and HER3 ECDs that enhance ligand affinity, potentially enabling a pan-specific therapeutic approach for downregulating the HER family in cancer. PMID:19048033

Jin, Pei; Zhang, Juan; Beryt, Malgorzata; Turin, Lisa; Brdlik, Cathleen; Feng, Ying; Bai, Xiaomei; Liu, Jim; Jorgensen, Brett; Shepard, H Michael

2009-01-01

440

Ligand-specific conformational changes in the alpha1 glycine receptor ligand-binding domain  

DEFF Research Database (Denmark)

Understanding the activation mechanism of Cys loop ion channel receptors is key to understanding their physiological and pharmacological properties under normal and pathological conditions. The ligand-binding domains of these receptors comprise inner and outer beta-sheets and structural studies indicate that channel opening is accompanied by conformational rearrangements in both beta-sheets. In an attempt to resolve ligand-dependent movements in the ligand-binding domain, we employed voltage-clamp fluorometry on alpha1 glycine receptors to compare changes mediated by the agonist, glycine, and by the antagonist, strychnine. Voltage-clamp fluorometry involves labeling introduced cysteines with environmentally sensitive fluorophores and inferring structural rearrangements from ligand-induced fluorescence changes. In the inner beta-sheet, we labeled residues in loop 2 and in binding domain loops D and E. At each position, strychnine and glycine induced distinct maximal fluorescence responses. The pre-M1 domain responded similarly; at each of four labeled positions glycine produced a strong fluorescence signal, whereas strychnine did not. This suggests that glycine induces conformational changes in the inner beta-sheet and pre-M1 domain that may be important for activation, desensitization, or both. In contrast, most labeled residues in loops C and F yielded fluorescence changes identical in magnitude for glycine and strychnine. A notable exception was H201C in loop C. This labeled residue responded differently to glycine and strychnine, thus underlining the importance of loop C in ligand discrimination. These results provide an important step toward mapping the domains crucial for ligand discrimination in the ligand-binding domain of glycine receptors and possibly other Cys loop receptors.

Pless, Stephan Alexander; Lynch, Joseph W

2009-01-01

441

Mixed ligand complexes of some lanthanides with EDTA as primary ligand and malic or N,N-hydrazinediacetic acids as secondary ligands  

International Nuclear Information System (INIS)

The acid dissociation constants of malic acid (H2L) and N,N-hydrazinediacetic acid (H2Z) and the formation constants of their mixed ligand complexes with lanthanides and EDTA have been determined potentiometrically in 0.1M KNO3 and 30 ± 0.1degC. Malic acid is found to act as a monodentate ligand in the ternary complexes while N,N-hydrazinediacetic acid acts as a bidentate ligand. (author)

442

Olefin metathesis catalysts bearing a pH-responsive NHC ligand: a feasible approach to catalyst separation from RCM products.  

Science.gov (United States)

Two novel ruthenium-based olefin metathesis catalysts, H(2)ITap(PCy(3))Cl(2)Ru[double bond, length as m-dash]CH-Ph and H(2)ITapCl(2)Ru[double bond, length as m-dash]CH-(C(6)H(4)-O-iPr) (H(2)ITap = 1,3-bis(2',6'-dimethyl-4'-dimethylaminophenyl)-4,5-dihydroimidazol-2-ylidene), were synthesized bearing a pH-responsive NHC ligand with two aromatic NMe(2) groups. The crystal structures of complexes and were determined via X-ray crystallography. Both catalysts perform ring opening metathesis polymerization (ROMP) of cyclooctene (COE) at faster rates than their commercially available counterparts H(2)IMes(PCy(3))Cl(2)Ru[double bond, length as m-dash]CH-Ph and H(2)IMesCl(2)Ru[double bond, length as m-dash]CH-(C(6)H(4)-O-iPr) (H(2)IMes = 1,3-bis(2',4',6'-trimethylphenyl)-4,5-dihydroimidazol-2-ylidene) and perform at similar rates during ring closing metathesis (RCM) of diethyldiallylmalonate (DEDAM). Upon addition of 2 equiv. of HCl, catalyst is converted into a mixture of several mono and diprotonated Ru-carbene species 12' which are soluble in methanol but degrade within a few hours at room temperature. Catalyst can be protonated with 2 equiv. of HCl and the resulting complex is moderately water-soluble. The complex is stable in aqueous solution in air for >4 h, but over prolonged periods of time shows degradation in acidic media due to hydrolysis of the NHC-Ru bond. Catalysts and perform RCM of diallylmalonic acid in acidic protic media with only moderate activity at 50 degrees C and do not produce polymer in the ROMP of cationic 7-oxanorbornene derivative under the same conditions. Catalyst was used for Ru-seperation studies when RCM of DEDAM or 3,3-diallypentadione (DAP) was conducted in low-polar organic solution and the Ru-species was subsequently precipitated by addition of strong acid. The Ru-species were removed by (1) filtration and (2) filtration and subsequent extraction with water. The residual Ru-levels could be reduced to as far as 11 ppm (method 2) and 24 ppm (method 1) without the use of chromatography or other scavenging methods. PMID:18941667

Balof, Shawna L; P'pool, Steven J; Berger, Nancy J; Valente, Edward J; Shiller, Alan M; Schanz, Hans-Jörg

2008-11-14

443

Measurement of Ligand-Receptor Interactions  

Science.gov (United States)

One distinguishing feature of "life" is that the physical forces between biological molecules and membrane surfaces are often highly specific, in contrast to nonspecific interactions such as van der Waals, hydrophobic, and electrostatic (Coulombic) forces. We have used the surface-forces-apparatus technique to study the specific "lock and key" or "ligand-receptor" interaction between two model biomem-brane surfaces in aqueous solution. The membranes were lipid bilayers supported on mica surfaces; one carrying streptavidin receptors, the other exposing biotin ligand groups. We found that, although no unusual or specific interaction occurs between two avidin or two biotin surfaces, an avidin and a biotin surface exhibit a very strong, very short-range (rearrangements. The results also show that highly specific biological interactions such as are involved in immunological recognition and cell-cell contacts may be studied at the molecular level and in real time by the surface-forces-apparatus technique.

Helm, Christiane A.; Knoll, Wolfgang; Israelachvili, Jacob N.

1991-09-01

444

Peptide-receptor ligands and multivalent approach.  

Science.gov (United States)

The overexpression of peptide receptors in human tumours makes peptide-ligands attractive agents for the development of specific diagnostic imaging and/or therapy of cancers. Solid-phase peptide synthesis, modern phage display technology and combinatorial peptide chemistry have profoundly affected the pool of available targeting peptides for efficient and specific delivery of imaging or therapeutic label molecules. Additionally, the availability of a wide range of bifunctional chelating agents for the radiolabelling of bioactive peptides with radionuclides has produced a wide variety of useful radiopharmaceutical molecules. This review article examines the principal receptors-binding peptides and their overexpression on tumour cells. We discuss the advantage and the challenges in developing multivalent peptide-based ligands summarising their design strategies and applications. PMID:22292761

Ruzza, Paolo; Marchiani, Anna; Antolini, Nicola; Calderan, Andrea

2012-06-01

445

[Opioid receptors and their selective ligands].  

Science.gov (United States)

Opioid receptors (micro, delta, and kappa) belong to a large family of G protein-coupled receptors and play an important physiological role. Stimulation of these receptors triggers analgesic effects and affects the function of gastrointestinal tract. The discovery of opioid peptides, which are endogenous ligands of opioid receptors, including delta-selective enkephalins, kappa-selective dynorphins, and micro-selective endomorphins, initiated their structure-activity relationship studies. For the last 30 years, hundreds of analogs of opioid peptides have been synthesized in an effort to obtain the compounds more active, selective, and resistant to biodegradation than the endogenous ligands. Different unnatural amino acids, as well as cyclisation procedures, leading to conformationaly restricted analogs, were employed. All these modifications resulted in obtaining very selective agonists and antagonists with high affinity at micro-, dlta-, and kappa-opioid receptors, which are extremely useful tools in further studies on the pharmacology of opioid receptors in a mammalian organism. PMID:17201067

Piestrzeniewicz, Mariola Katarzyna; Fichna, Jakub; Michna, Jakub; Janecka, Anna

2006-01-01

446

A New Synthetic Method for the Preparation of Protonated-NHCs and Related Compounds.  

Science.gov (United States)

Protonated versions of N-heterocyclic carbenes (NHC,H(+)) are classically prepared by closing the ring through the introduction of the CH+ fragment. He we report a totally different synthetic approach, which can be viewed as the addition of a 1,3-diazaallyl anion to a compound featuring two leaving groups (hereafter named "di-electrophile"). Using 1,3- and 1,4-dibromides, six and seven membered NHC,H(+)s have been prepared in good yields. Similarly, with 1,3,2-dioxathiolane-2,2-dioxide as a di-electrophile, imidazolidinium salts were obtained. To illustrate its broad scope of application, this synthetic route has been expanded to the preparation of protonated cyclic amino alkyl carbenes (CAACs) and amino thio carbenes, using 1-aza-allyl and 1,3-azathio-allyl anions, respectively. PMID:19543436

Jazzar, Rodolphe; Liang, Hongze; Donnadieu, Bruno; Bertrand, Guy

2006-01-01

447

Protein Dynamics, Ligand Binding, and Biological Function  

Science.gov (United States)

Dynamics is essential for protein function. To demonstrate this point, this talk presents three studies. (1) For a ligand-gated ion channel, ligand binding leads to channel activation by modulating the dynamics of the channel protein. A common theme that emerges from different families of ligand-gated ion channels is that agonist binding closes the ligand-bidning domain (LBD), leading to pore opening in the transmembrane domain (TMD); in contrast, antagonist binding opens the LBD, leading to pore closing in the TMD [1]. (2) When the structure [2] and gating dynamics [3] of the influenza M2 proton channel are accounted for, the calculated rate of ion transport is in quantitative agreement with experimental data [4]. (3) In enzymes, gating dynamics afford substrate selectivity [5]. [4pt] [1] M. Yi, H. Tjong, and H.-X. Zhou (2008). Spontaneous conformational change and toxin binding in ?7 nicotinic acetylcholine receptor: insight into channel activation and inhibition. Proc. Natl. Acad. Sci. 105, 8280-8285. [0pt] [2] M. Sharma, M. Yi, H. Dong, H. Qin, E. Peterson, D. D. Busath, H.-X. Zhou, and T. A. Cross (2010). Insight into the mechanism of the influenza A proton channel from a structure in a lipid bilayer. Science 330, 509-512. [0pt] [3] M. Yi, T. A. Cross, and H.-X. Zhou (2009). Conformational heterogeneity of the M2 proton channel and a structural model for channel activation. Proc. Natl. Acad. Sci. USA 106, 13311-13316. [0pt] [4] H.-X. Zhou (2010). Diffusion-influenced transport of ions across a transmembrane channel with an internal binding site. J. Phys. Chem. Lett. 1, 1973-1976. [0pt] [5] H.-X. Zhou, S. T. Wlodek, and J. A. McCammon (1998). Conformation gating as a mechanism for enzyme specificity. Proc. Natl. Acad. Sci. USA 95, 9280-9283.

Zhou, Huan-Xiang

2011-03-01

448

Micropatterned Surfaces with Controlled Ligand Tethering  

OpenAIRE

Microcontact printing (?-CP) is a facile, cost-effective, and versatile soft-lithography technique to create 2-dimensional patterns of domains with distinct functionalities that provides a robust platform to generate micropatterned biotechnological arrays and cell culture substrates. Current ?-CP approaches rely on non-specific immobilization of biological ligands, either by direct printing or adsorption from solution, onto micropatterned domains surrounded by a non-fouling background. This...

Petrie, Timothy A.; Stanley, Brandon T.; Garci?a, Andre?s J.

2009-01-01

449

Applications of surface ligand design to flotation  

OpenAIRE

This thesis involves the design, synthesis and testing of organic hydrophobic ligands. They would act as co-collectors in froth flotation processes to enhance the recovery of sulfidic minerals which have undergone some oxidation on processing and are not efficiently collected by the commercial reagents used in froth flotation. Strong and selective binding to iron(III) oxide/hydroxide surfaces, e.g. goethite, over unwanted silicaceous material was considered essential criteria f...

Rio Echevarria, Iria M.

2007-01-01

450

Ligand diffusion in globins: Simulations versus Experiment  

OpenAIRE

Computer simulations in molecular biophysics describe in atomic detail structure, dynamics, and function of biological macromolecules. To assess the quality of these models and to pick up new mechanisms, comparisons with experimental measurements are made. Most comparisons examine thermodynamic and average structural properties. Here we discuss studies of dynamics and fluctuations in a protein. The diffusion of a small ligand between internal cavities in myoglobin, and its escape to solvent a...

Elber, Ron

2010-01-01

451

Discovering High-Affinity Ligands for Proteins  

Science.gov (United States)

This article reports on development of a method for producing high-affinity ligands in which small molecules that bind to proximal subsites of a protein are identified in an NMR-based screen and then linked together in their experimentally determined bound orientations. The method is called âÂÂSAR by NMR,â which stands for âÂÂstructure-activity relationships by nuclear magnetic resonance.âÂÂ

Philip J Hajduk (Abbott Laboratories;); Robert P. Meadows (Abbott Laboratories;); Stephen Fesik (Abbott Laboratories;)

1997-10-17

452

Automated design of ligands to polypharmacological profiles  

OpenAIRE

The clinical efficacy and safety of a drug is determined by its activity profile across multiple proteins in the proteome. However, designing drugs with a specific multi-target profile is both complex and difficult. Therefore methods to rationally design drugs a priori against profiles of multiple proteins would have immense value in drug discovery. We describe a new approach for the automated design of ligands against profiles of multiple drug targets. The method is demonstrated by the evolu...

Besnard, Je?re?my; Ruda, Gian Filippo; Setola, Vincent; Abecassis, Keren; Rodriguiz, Ramona M.; Huang, Xi-ping; Norval, Suzanne; Sassano, Maria F.; Shin, Antony I.; Webster, Lauren A.; Simeons, Frederick R. C.; Stojanovski, Laste; Prat, Annik; Seidah, Nabil G.; Constam, Daniel B.

2012-01-01

453

Combinatorial Ligand-directed Lung Targeting  

OpenAIRE

Phage display of random peptide libraries is a powerful, unbiased method frequently used to discover ligands for virtually any protein of interest and to reveal functional protein–protein interaction partners. Moreover, in vivo phage display permits selection of peptides that bind specifically to different vascular beds without any previous knowledge pertaining to the nature of their corresponding receptors. Vascular targeting exploits molecular differences inherent in blood vessels within ...

Giordano, Ricardo J.; Edwards, Julianna K.; Tuder, Rubin M.; Arap, Wadih; Pasqualini, Renata

2009-01-01

454

Novel fenofibrate derivatives as cannabinoid receptor ligands  

OpenAIRE

Fenofibrate is a PPAR? agonist, used to treat dyslipidemia. Unpublished work that has been previously carried out in our group has identified that fenofibrate also displays affinity for the cannabinoid receptors, CB1 and CB2. A dual receptor ligand, with the PPAR? agonist activity of fenofibrate, combined with antagonist activity at the CB1 receptor, or agonist activity at the CB2 receptor, could reduce appetite, decrease plasma triglyceride levels, increase high density lipoprotein (HD...

Spencer, Sarah Jane

2011-01-01

455

Separation of Am(III) by nitrogen-donor ligand  

International Nuclear Information System (INIS)

Soft-donor extractants containing sulfur- or nitrogen-donor are applicable to the separation of trivalent actinides (An(III)) from lanthanides (Ln(III)) by solvent extraction method. We designed two ligand systems which are expected to efficiently separate An(III) from Ln(III). The one is 'Oligo-pyridine' ligand containing multiple pyridine moieties. The other is 'Chirality-controlled' ligand containing chiral centers in the ligand. We synthesized more than thirty ligands and found out the ability on the separation of An(III) from Ln(III). In this presentation, we will discuss about the comprehensive results on the separation ability of An(III) from Ln(III) by means of 'Oligo-pyridine' and 'Chirality-controlled' ligands, and the extraction equilibrium of An(III) with selected tripodal ligands in detail. (author)

456

A Annealing Algorithm for Designing Ligands from Receptor Structures.  

Science.gov (United States)

DEenspace NOVO, a simulated annealing method for designing ligands is described. At a given temperature, ligand fragments are randomly selected and randomly placed within the given receptor cavity, often replacing existing ligand fragments. For each new ligand fragment combination, bonded, nonbonded, polarization and solvation energies of the new ligand-receptor system are compared to the previous system. Acceptance or rejection of the new system is decided using the Boltzmann distribution. Thus, energetically unfavorable fragment switches are sometimes accepted, sacrificing immediate energy gains in the interest of finding the system with the globally minimum energy. By lowering the temperature, the rate of unfavo