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Functionalised N-heterocyclic carbene complexes  

CERN Multimedia

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

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Luminescent organoplatinum(II) complexes containing bis(N-heterocyclic carbene) ligands selectively target the endoplasmic reticulum and induce potent photo-toxicity.  

UK PubMed Central (United Kingdom)

A panel of luminescent platinum(II) complexes containing bidentate N-heterocyclic carbene ligands selectively localize to the endoplasmic reticulum (ER) domain, induce ER stress and cell apoptosis. Some of them show potent photo-toxicity to cancer cells.

Zou T; Lok CN; Fung YM; Che CM

2013-06-01

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

Directory of Open Access Journals (Sweden)

Full Text Available 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 evaluated for their in vitro antimicrobial activity against a variety of Gram-positive and Gram-negative bacteria and fungal species.

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

2010-01-01

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Accessible gold clusters using calix[4]arene N-heterocyclic carbene and phosphine ligands.  

UK PubMed Central (United Kingdom)

We investigate the synthesis of accessible calix[4]arene-bound gold clusters consisting of open "coordinatively unsaturated" active sites, using a comparative approach that relies on calix[4]arene ligands with various upper- and lower-rim substituents. In contrast with a reported Au(i)-tert-butyl-calixarene phosphine complex, which exhibits a single cone conformer in solution, the H upper-rim analog exhibits multiple conformers in solution. This contrasts with observations of the tert-butyl upper-rim analog, which exhibits a single cone conformer in solution under similar conditions. In the solid state, as determined by single-crystal X-ray diffraction, both H and tert-butyl upper-rim analogs exhibit exclusively cone conformer. A detailed structural analysis of these two solid-state structures highlights a CH-? interaction involving a methoxy lower-rim substituent and phenyl substituent on P as the key feature that enforces a tight configuration of Au(i) atoms on the same side of the calix[4]arene lower-rim plane. We hypothesize that such a configuration promotes chelation of the ligand to a gold surface and facilitates the synthesis of small Au11-sized clusters after reduction of both complexes. The new cluster, like the one reported with the tert-butyl analog, has an extraordinary 25% of surface atoms that are open and accessible to a 2-NT (2-naphthalenethiol) probe in solution. We also investigated the effect of calix[4]arene lower-rim substituents that coordinate to the metal, by using N-heterocyclic carbene (NHC) functional groups rather than phosphines. Four small (<1.6 nm diameter) calix[4]arene NHC-bound gold clusters were synthesized, including three using novel calix[4]arene NHC ligands. The smallest calix[4]arene NHC-bound Au cluster consisted of a 1.2 nm gold core, and its number density of accessible and open surface sites was measured. This required development of a new titration method for open sites on gold clusters, using a SAMSA fluorescein dye molecule, which excites and emits at lower energy relative to the previously used 2-NT probe. The number density of open sites on the new calix[4]arene NHC-bound gold cluster measured by the SAMSA fluorescein probe strongly supports the generality of a mechanical model of accessibility, which does not depend on the functional group involved in binding to the gold surface and rather depends on the relative radii of curvature of bound ligands and the gold cluster core.

Nigra MM; Yeh AJ; Okrut A; Dipasquale AG; Yeh SW; Solovyov A; Katz A

2013-08-01

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Accessible gold clusters using calix[4]arene N-heterocyclic carbene and phosphine ligands.  

Science.gov (United States)

We investigate the synthesis of accessible calix[4]arene-bound gold clusters consisting of open "coordinatively unsaturated" active sites, using a comparative approach that relies on calix[4]arene ligands with various upper- and lower-rim substituents. In contrast with a reported Au(i)-tert-butyl-calixarene phosphine complex, which exhibits a single cone conformer in solution, the H upper-rim analog exhibits multiple conformers in solution. This contrasts with observations of the tert-butyl upper-rim analog, which exhibits a single cone conformer in solution under similar conditions. In the solid state, as determined by single-crystal X-ray diffraction, both H and tert-butyl upper-rim analogs exhibit exclusively cone conformer. A detailed structural analysis of these two solid-state structures highlights a CH-? interaction involving a methoxy lower-rim substituent and phenyl substituent on P as the key feature that enforces a tight configuration of Au(i) atoms on the same side of the calix[4]arene lower-rim plane. We hypothesize that such a configuration promotes chelation of the ligand to a gold surface and facilitates the synthesis of small Au11-sized clusters after reduction of both complexes. The new cluster, like the one reported with the tert-butyl analog, has an extraordinary 25% of surface atoms that are open and accessible to a 2-NT (2-naphthalenethiol) probe in solution. We also investigated the effect of calix[4]arene lower-rim substituents that coordinate to the metal, by using N-heterocyclic carbene (NHC) functional groups rather than phosphines. Four small (<1.6 nm diameter) calix[4]arene NHC-bound gold clusters were synthesized, including three using novel calix[4]arene NHC ligands. The smallest calix[4]arene NHC-bound Au cluster consisted of a 1.2 nm gold core, and its number density of accessible and open surface sites was measured. This required development of a new titration method for open sites on gold clusters, using a SAMSA fluorescein dye molecule, which excites and emits at lower energy relative to the previously used 2-NT probe. The number density of open sites on the new calix[4]arene NHC-bound gold cluster measured by the SAMSA fluorescein probe strongly supports the generality of a mechanical model of accessibility, which does not depend on the functional group involved in binding to the gold surface and rather depends on the relative radii of curvature of bound ligands and the gold cluster core. PMID:23917776

Nigra, Michael M; Yeh, Alexander J; Okrut, Alexander; Dipasquale, Antonio G; Yeh, Sheila W; Solovyov, Andrew; Katz, Alexander

2013-08-13

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Planar tris-N-heterocyclic carbenes.  

Science.gov (United States)

To further explore the coordination chemistry of poly-carbene complexes, two triphenylene-based planar tridentate N-heterocyclic carbenes and their corresponding tri-gold complexes were synthesized. Molecular structures of the tert-butyl substituted tritopic free carbene and the tri-nuclear gold complex were determined experimentally. A silver-dicarbene organometallic polymer was also prepared with the newly synthesized tris-NHC. PMID:23846191

Wang, Yun-Ting; Chang, Meng-Ting; Lee, Gene-Hsiang; Peng, Shie-Ming; Chiu, Ching-Wen

2013-07-11

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Planar tris-N-heterocyclic carbenes.  

UK PubMed Central (United Kingdom)

To further explore the coordination chemistry of poly-carbene complexes, two triphenylene-based planar tridentate N-heterocyclic carbenes and their corresponding tri-gold complexes were synthesized. Molecular structures of the tert-butyl substituted tritopic free carbene and the tri-nuclear gold complex were determined experimentally. A silver-dicarbene organometallic polymer was also prepared with the newly synthesized tris-NHC.

Wang YT; Chang MT; Lee GH; Peng SM; Chiu CW

2013-08-01

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Synthesis of palladium(0) and -(II) complexes with chelating bis(N-heterocyclic carbene) ligands and their application in semihydrogenation.  

UK PubMed Central (United Kingdom)

A transmetallation route, using silver(I) precursors, to several zero- and di-valent palladium complexes with chelating bis(N-heterocyclic carbene) ligands bearing various N-substituents has been established. The resulting complexes have been characterized by NMR and mass spectroscopy. In addition, the structure of a representative compound, [Pd(0)(bis-(Mes)NHC)(?(2)-ma)] (3a), was confirmed by X-ray crystal structure determination. In contrast to the transfer semihydrogenation, in which only low activity was observed, complex 3a showed activity (TOF = 49 mol(sub) mol(cat)(-1) h(-1)) and selectivity comparable to its monodentate counterparts in the semihydrogenation of 1-phenyl-1-propyne with molecular hydrogen.

Sluijter SN; Warsink S; Lutz M; Elsevier CJ

2013-05-01

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Iridium metal complexes containing N-heterocyclic carbene ligands for blue-light-emitting electrochemical cells.  

UK PubMed Central (United Kingdom)

A new series of cationic blue-emitting, heteroleptic iridium(III)-based metal complexes were systematically synthesized using two 4,6-difluorophenylpyridine ligands as well as one methyl- or n-butyl-substituted bisimidazolium salt carbene-type ligand. In degassed CH(2)Cl(2), all complexes display highly efficient, blue phosphorescence (?(max) ? 452 nm; emission quantum yield ? 0.30) at room temperature and also show blue emission in a thin film. The measured photophysical properties of the complexes have been rationalized with the help of quantum-chemical calculations. Because of the high solubility of the complexes, solution-processed devices, light-emitting electrochemical cells (LEECs), were made. The results showed that true blue emission and short turn-on time is achieved when an ionic conductor, tetrabutylammonium trifluoromethanesulfonate, was used as the matrix for the film containing the emitters. These iridium complexes and the described devices are the bluest materials ever reported and the first case of LEECs based on carbene ligands.

Yang CH; Beltran J; Lemaur V; Cornil J; Hartmann D; Sarfert W; Fröhlich R; Bizzarri C; De Cola L

2010-11-01

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Iridium metal complexes containing N-heterocyclic carbene ligands for blue-light-emitting electrochemical cells.  

Science.gov (United States)

A new series of cationic blue-emitting, heteroleptic iridium(III)-based metal complexes were systematically synthesized using two 4,6-difluorophenylpyridine ligands as well as one methyl- or n-butyl-substituted bisimidazolium salt carbene-type ligand. In degassed CH(2)Cl(2), all complexes display highly efficient, blue phosphorescence (?(max) ? 452 nm; emission quantum yield ? 0.30) at room temperature and also show blue emission in a thin film. The measured photophysical properties of the complexes have been rationalized with the help of quantum-chemical calculations. Because of the high solubility of the complexes, solution-processed devices, light-emitting electrochemical cells (LEECs), were made. The results showed that true blue emission and short turn-on time is achieved when an ionic conductor, tetrabutylammonium trifluoromethanesulfonate, was used as the matrix for the film containing the emitters. These iridium complexes and the described devices are the bluest materials ever reported and the first case of LEECs based on carbene ligands. PMID:20932024

Yang, Cheng-Han; Beltran, Juan; Lemaur, Vincent; Cornil, Jérôme; Hartmann, David; Sarfert, Wiebke; Fröhlich, Roland; Bizzarri, Claudia; De Cola, Luisa

2010-11-01

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Synthesis, photo-, and electrochemistry of ruthenium bis(bipyridine) complexes comprising a N-heterocyclic carbene ligand.  

UK PubMed Central (United Kingdom)

Analogues of [Ru(bpy)3](2+) were prepared in which one pyridine ligand site is substituted by a N-heterocyclic carbene (NHC) ligand, that is, either by an imidazolylidene with a variable wingtip group R (R = Me, 3a; R = Et, 3b; R = iPr, 3c), or by a benzimidazolylidene (Me wingtip group, 3d), or by a 1,2,3-triazolylidene (Me wingtip group, 3e). All complexes were characterized spectroscopically, photophysically, and electrochemically. An increase of the size of the wingtip groups from Me to Et or iPr groups distorts the octahedral geometry (NMR spectroscopy) and curtails the reversibility of the ruthenium oxidation. NHC ligands with methyl wingtip groups display reversible ruthenium oxidation at a potential that reflects the donor properties of the NHC ligand (triazolylidene > imidazolylidene > benzimidazolylidene). The most attractive properties were measured for the triazolylidene ruthenium complex 3e, featuring the smallest gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) in the series (2.41 eV), a slightly red-shifted absorption profile, and reasonable excited-state lifetime (188 ns) when compared to [Ru(bpy)3](2+). These features demonstrate the potential utility of triazolylidene ruthenium complexes as photosensitizers for solar energy conversion.

Leigh V; Ghattas W; Lalrempuia R; Müller-Bunz H; Pryce MT; Albrecht M

2013-05-01

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Electronic and Ligand Properties of Annelated Normal and Abnormal (Mesoionic) N-Heterocyclic Carbenes: A Theoretical Study.  

UK PubMed Central (United Kingdom)

The effect of annelation and carbonylation on the electronic and ligating properties of N-heterocyclic carbenes (NHCs) has been studied quantum chemically. The thermodynamic and kinetic stability of these NHCs have been assessed on the basis of their singlet-triplet and HOMO-LUMO gaps respectively. Both annelation and carbonylation have been found to decrease the stability of NHCs. Compared to non-annelated carbenes, annelated and carbonylated carbenes are found to be weaker sigma donors but better pi acceptors. However, the effect of carbonylation is more pronounced than annelation towards increasing the pi acidity of the NHCs. The reactivity of these carbenes has been discussed in terms of nucleophilicity and electrophilicity indices. The calculated values of the relative redox potential and 31P NMR chemical shifts of corresponding carbene-phosphinidene adducts have been found to correlate well with the pi acidity of the NHCs.

Phukan AK; Guha AK; Sarmah S; Dewhurst RD

2013-10-01

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Iridium(I) complexes with anionic N-heterocyclic carbene ligands as catalysts for the hydrogenation of alkenes in nonpolar media.  

UK PubMed Central (United Kingdom)

A series of lithium complexes of anionic N-heterocyclic carbenes that contain a weakly coordinating borate moiety (WCA-NHC) was prepared in one step from free N-heterocyclic carbenes by deprotonation with n-butyl lithium followed by borane addition. The reaction of the resulting lithium-carbene adducts with [M(COD)Cl]2 (M = Rh, Ir; COD = 1,5-cyclooctadiene) afforded zwitterionic rhodium(I) and iridium(I) complexes of the type [(WCA-NHC)M(COD)], in which the metal atoms exhibit an intramolecular interaction with the N-aryl groups of the carbene ligands. For M = Rh, the neutral complex [(WCA-NHC)Rh(CO)2] and the ate complex (NEt4)[(WCA-NHC)Rh(CO)2Cl] were prepared, with the latter allowing an assessment of the donor ability of the ligand by IR spectroscopy. The zwitterionic iridium-COD complexes were tested as catalysts for the homogeneous hydrogenation of alkenes, which can be performed in the presence of nonpolar solvents or in the neat alkene substrate. Thereby, the most active complex showed excellent stability and activity in hydrogenation of alkenes at low catalyst loadings (down to 10 ppm).

Kolychev EL; Kronig S; Brandhorst K; Freytag M; Jones PG; Tamm M

2013-08-01

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Iridium(I) complexes with anionic N-heterocyclic carbene ligands as catalysts for the hydrogenation of alkenes in nonpolar media.  

Science.gov (United States)

A series of lithium complexes of anionic N-heterocyclic carbenes that contain a weakly coordinating borate moiety (WCA-NHC) was prepared in one step from free N-heterocyclic carbenes by deprotonation with n-butyl lithium followed by borane addition. The reaction of the resulting lithium-carbene adducts with [M(COD)Cl]2 (M = Rh, Ir; COD = 1,5-cyclooctadiene) afforded zwitterionic rhodium(I) and iridium(I) complexes of the type [(WCA-NHC)M(COD)], in which the metal atoms exhibit an intramolecular interaction with the N-aryl groups of the carbene ligands. For M = Rh, the neutral complex [(WCA-NHC)Rh(CO)2] and the ate complex (NEt4)[(WCA-NHC)Rh(CO)2Cl] were prepared, with the latter allowing an assessment of the donor ability of the ligand by IR spectroscopy. The zwitterionic iridium-COD complexes were tested as catalysts for the homogeneous hydrogenation of alkenes, which can be performed in the presence of nonpolar solvents or in the neat alkene substrate. Thereby, the most active complex showed excellent stability and activity in hydrogenation of alkenes at low catalyst loadings (down to 10 ppm). PMID:23883399

Kolychev, Eugene L; Kronig, Sabrina; Brandhorst, Kai; Freytag, Matthias; Jones, Peter G; Tamm, Matthias

2013-08-08

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N-Heterocyclic Carbenes in FLP Chemistry.  

UK PubMed Central (United Kingdom)

The use of N-heterocyclic carbenes (NHCs) for the design and construction of frustrated Lewis pairs (FLPs) is outlined in this review. Stable carbene-borane adducts are briefly discussed, followed by a detailed survey of the reactivity of NHCs, in particular Arduengo-type imidazolin-2-ylidenes, towards B(C6F5)3. Structural and electronic NHC modification by variation of substituents and ring-size affords either stable normal adducts or FLPs, which undergo manifold deactivation reactions in the absence of substrates, e.g., by abnormal adduct formation, self-dehydrogenation, and other types of C-H and C-F bond activation. The degree of frustration is correlated with the calculated energies for the formation of the normal adducts NHC·B(C6F5)3, allowing one to predict and rationalize their reactivity towards small molecules such as dihydrogen, ethers, alkynes, main group elements, carbon dioxide, and nitrous oxide. Other carbene-based FLP systems with a variety of alternative Lewis acids such as boranes, alanes, and carbon-based Lewis acids are also covered, whenever "frustration" is observed. A brief introduction of structurally related bifunctional FLPs based on borylated N-heterocycles is also included.

Kolychev EL; Theuergarten E; Tamm M

2013-01-01

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Ruthenium-based complexes containing a benzimidazolium tag covalently connected to N-heterocyclic carbene ligands: environmentally friendly catalysts for olefin metathesis transformations.  

UK PubMed Central (United Kingdom)

Two ruthenium-based pre-catalysts containing an ionic tag covalently connected to a N-heterocyclic carbene (NHC) ligand are reported. These novel complexes, bearing a polar benzimidazolium group, are air-stable and can be easily prepared from commercially available reagents. The quaternary benzimidazolium tag allows the efficient separation of ruthenium waste from the metathesis product after reaction. Application in several olefin metathesis transformations leads to desired products of high purity, which exhibit ruthenium contamination levels as low as 1 ppm after simple filtration through a pad of silica gel.

Klu?iar M; Grela K; Mauduit M

2013-05-01

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Beyond catalysis: N-heterocyclic carbene complexes as components for medicinal, luminescent, and functional materials applications  

Digital Repository Infrastructure Vision for European Research (DRIVER)

This tutorial review compiles the advances that have been achieved in using transition metal complexes containing N-heterocyclic carbene ligands as components for materials. Applications of metal carbene complexes in fields different from catalysis are remarkably scarce. During the last few years, p...

Mercs, Laszlo; Albrecht, Martin

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N-heterocyclic carbene catalyzed synthesis of oxime esters.  

UK PubMed Central (United Kingdom)

A triazolium salt derived N-heterocyclic carbene catalyzes the redox esterification reaction between ?-?-unsaturated aldehydes and oximes. The resulting saturated oxime esters were obtained in very good yields for a broad range of aliphatic, aromatic and heteroaromatic substrates.

Enders D; Grossmann A; Van Craen D

2013-01-01

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Dioxygen adducts of rhodium N-heterocyclic carbene complexes.  

UK PubMed Central (United Kingdom)

Rhodium complexes functionalized by N-heterocyclic carbene ligands react with dioxygen to form adducts. Depending on the specifics of the ancillary ligands, oxygen binds to Rh either as a peroxide to form a fully oxidized Rh(III) complex, or as singlet dioxygen in a Rh(I) square planar complex. We have shown through analysis of a series of compounds, some previously published and some novel, that the presence of additional ligands that would support the formation of an octahedral geometry, as typically found with Rh(III) complexes, is critical for formation of the peroxide. In addition, we have demonstrated through DFT studies, that the potential energy surface with regard to the O-O bond length is relatively shallow, which provides a rationale for the distribution of bond lengths observed for apparently similar complexes analyzed by crystallography.

Keske EC; Zenkina OV; Asadi A; Sun H; Praetorius JM; Allen DP; Covelli D; Patrick BO; Wang R; Kennepohl P; James BR; Crudden CM

2013-05-01

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Catalisadores contendo carbenos n-heterocíclicos como ligantes: propriedades, sínteses, aplicações e comparação com outros ligantes/ Catalysts containing n-heterocyclic carbenes as ligands: properties, syntheses, applications and comparison with other catalysts  

Scientific Electronic Library Online (English)

Full Text Available Abstract in english N-heterocyclic carbenes (NHCs) have become of considerable importance in modern organometallic chemistry and homogeneous catalysis. There are several advantages in the use NHCs over their phosphorus analogues, which explains the enormous development of NHC ligands in the field of organometallic catalysis in the past few years. In this article, we present an overview of the importance of the catalysts containing NHC ligands, their synthesis, some pertinent synthetic applications, and a brief comparison with other catalysts.

Pastre, Julio Cezar; Correia, Carlos Roque Duarte

2008-01-01

 
 
 
 
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Catalisadores contendo carbenos n-heterocíclicos como ligantes: propriedades, sínteses, aplicações e comparação com outros ligantes Catalysts containing n-heterocyclic carbenes as ligands: properties, syntheses, applications and comparison with other catalysts  

Directory of Open Access Journals (Sweden)

Full Text Available N-heterocyclic carbenes (NHCs) have become of considerable importance in modern organometallic chemistry and homogeneous catalysis. There are several advantages in the use NHCs over their phosphorus analogues, which explains the enormous development of NHC ligands in the field of organometallic catalysis in the past few years. In this article, we present an overview of the importance of the catalysts containing NHC ligands, their synthesis, some pertinent synthetic applications, and a brief comparison with other catalysts.

Julio Cezar Pastre; Carlos Roque Duarte Correia

2008-01-01

22

Metal N-heterocyclic carbene complexes as potential antitumor metallodrugs.  

UK PubMed Central (United Kingdom)

The discovery of cisplatin's antitumor activity in 1969 prompted the search for novel metal-containing complexes as potential anticancer drugs. Among these novel complexes, metal N-heterocyclic carbene (NHC) complexes have recently gained considerable attention because they perfectly fit prerequisites for efficient drug design and fast optimization. Moreover, most of them have shown higher cytotoxicity than cisplatin. This review describes the advances that have been achieved in using transition metal (Ag, Au, Pt, Pd, Cu, Ni, and Ru) complexes containing NHC ligands as antitumor agents. Their modes of action at the cellular lever are further discussed. All these initial achievements clearly demonstrate the great potential of metal-NHC complexes as antitumor agents.

Liu W; Gust R

2013-01-01

23

Alkyne hydroarylation with Au N-heterocyclic carbene catalysts.  

UK PubMed Central (United Kingdom)

Mono- and dinuclear gold complexes with N-heterocyclic carbene (NHC) ligands have been employed as catalysts in the intermolecular hydroarylation of alkynes with simple unfunctionalised arenes. Both mono- and dinuclear gold(III) complexes were able to catalyze the reaction; however, the best results were obtained with the mononuclear gold(I) complex IPrAuCl. This complex, activated with one equivalent of silver tetrafluoroborate, exhibited under acidic conditions at room temperature much higher catalytic activity and selectivity compared to more commonly employed palladium(II) catalysts. Moreover, the complex was active, albeit to a minor extent, even under neutral conditions, and exhibited lower activity but higher selectivity compared to the previously published complex AuCl(PPh(3)). Preliminary results on intramolecular hydroarylations using this catalytic system indicate, however, that alkyne hydration by traces of water may become a serious competing reaction.

Tubaro C; Baron M; Biffis A; Basato M

2013-01-01

24

N-Heterocyclic carbene metal complexes in medicinal chemistry.  

UK PubMed Central (United Kingdom)

Metal complexes with N-heterocyclic carbene (NHC) ligands are widely used in chemistry due to their catalytic properties and applied for olefin metathesis among other reactions. The enhanced application of this type of organometallics has over the last few years also triggered a steadily increasing number of studies in the fields of medicinal chemistry, which take advantage of the fascinating chemical properties of these complexes. In fact it has been demonstrated that metal NHC complexes can be used to develop highly efficient metal based drugs with possible applications in the treatment of cancer or infectious diseases. Complexes of silver and gold have been biologically evaluated most frequently but also platinum or other transition metals have demonstrated promising biological properties.

Oehninger L; Rubbiani R; Ott I

2013-03-01

25

N-Heterocyclic carbene metal complexes in medicinal chemistry.  

Science.gov (United States)

Metal complexes with N-heterocyclic carbene (NHC) ligands are widely used in chemistry due to their catalytic properties and applied for olefin metathesis among other reactions. The enhanced application of this type of organometallics has over the last few years also triggered a steadily increasing number of studies in the fields of medicinal chemistry, which take advantage of the fascinating chemical properties of these complexes. In fact it has been demonstrated that metal NHC complexes can be used to develop highly efficient metal based drugs with possible applications in the treatment of cancer or infectious diseases. Complexes of silver and gold have been biologically evaluated most frequently but also platinum or other transition metals have demonstrated promising biological properties. PMID:23223752

Oehninger, Luciano; Rubbiani, Riccardo; Ott, Ingo

2012-12-07

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N-Heterocyclic Carbene Complexes in Reactions Involving Carbon Monoxide  

Science.gov (United States)

This chapter focuses on carbon monoxide as a reagent in M-NHC catalysed reactions. The most important and popular of these reactions is hydroformylation. Unfortunately, uncertainty exists as to the identity of the active catalyst and whether the NHC is bound to the catalyst in a number of the reported reactions. Mixed bidentate NHC complexes and cobalt-based complexes provide for better stability of the catalyst. Catalysts used for hydroaminomethylation and carbonylation reactions show promise to rival traditional phosphine-based catalysts. Reports of decarbonylation are scarce, but the potential strength of the M-NHC bond is conducive to the harsh conditions required. This report will highlight, where appropriate, the potential benefits of exchanging traditional phosphorous ligands with N-heterocyclic carbenes as well as cases where the role of the NHC might need re-evaluation. A review by the author on this topic has recently appeared [1].

Jeletic, Matthew; Veige, Adam

27

An Electronic Rationale for Observed Initiation Rates in Ruthenium-Mediated Olefin Metathesis: Charge Donation in Phosphine And N-Heterocyclic Carbene Ligands  

Energy Technology Data Exchange (ETDEWEB)

Ru K-edge XAS data indicate that second generation ruthenium-based olefin metathesis precatalysts (L = N-heterocyclic carbene) possess a more electron-deficient metal center than in the corresponding first generation species (L = tricyclohexylphosphine). This surprising effect is also observed from DFT calculations and provides a simple rationale for the slow phosphine dissociation kinetics previously noted for second-generation metathesis precatalysts.

Getty, K.; Delgado-Jaime, M.U.; Kennepohl, P.

2009-06-01

28

Synthesis, structures, and antimalarial activities of some silver(I), gold(I) and gold(III) complexes involving N-heterocyclic carbene ligands.  

UK PubMed Central (United Kingdom)

A series of mono-and dinuclear silver(I) and mononuclear gold(I) complexes containing bis(N-heterocyclic carbene) (NHC) or N-functionalized NHC ligands were synthesized and fully characterized by spectroscopic methods and, in some cases, by single crystal X-ray diffraction. The in vitro antiplasmodial and antifungal activities of a previously described family of N-functionalized bis(imidazolium) proligands and their corresponding silver(I), gold(I) and gold(III) complexes but also the new here described compounds were investigated in a chloroquine-resistant strain of Plasmodium falciparum, and against two Candida strains, respectively. For the first family, interesting antiplasmodial and antifungal activities were found for the dinuclear silver(I) species but they also showed strong hemolytic properties. Pharmaco-modulations leading to the second series of complexes allowed notably increase in the antiplasmodial activity, in particular of the mononuclear gold(I) complexes with IC(50) values up to 330 nM, without any hemolysis.

Hemmert C; Fabié A; Fabre A; Benoit-Vical F; Gornitzka H

2013-02-01

29

Use of molecular electrostatic potential at the carbene carbon as a simple and efficient electronic parameter of N-heterocyclic carbenes.  

UK PubMed Central (United Kingdom)

Topographical analysis of the molecular electrostatic potential (MESP) has been carried out for a variety of N-heterocyclic carbenes at the B3LYP, BP86, and M05 levels of density functional theory (DFT) using a 6-311++G** basis set. The electron rich character of the carbene carbon is assessed in terms of the absolute minimum of the MESP at the carbene lone pair region (V(min)), as well as the MESP at the carbene nucleus (V(C)). A linear relationship is established between the V(C) and Tolman electronic parameter (TEP) which suggested the use of the former as a simple and efficient descriptor for the electron donating power of N-heterocyclic carbene ligands toward metal coordination. The V(min) of the carbene also showed good correlation with TEP. However, the deviation from linearity was higher than V(C)-TEP correlation, and the reason for this was attributed to the steric effect of N-substituents at the lone pair region. The greater coordinating power of N-heterocyclic carbenes over phosphines is explained on the basis of deeper V(min) values obtained for the former, and in fact even the V(min) of the least electron rich N-heterocyclic carbene is comparable to the highly electron rich phosphine ligands. Thus the MESP topographical approach presented herein offers quantification of the inherent electron donating power of a free N-heterocyclic carbene ligand.

Mathew J; Suresh CH

2010-05-01

30

Use of molecular electrostatic potential at the carbene carbon as a simple and efficient electronic parameter of N-heterocyclic carbenes.  

Science.gov (United States)

Topographical analysis of the molecular electrostatic potential (MESP) has been carried out for a variety of N-heterocyclic carbenes at the B3LYP, BP86, and M05 levels of density functional theory (DFT) using a 6-311++G** basis set. The electron rich character of the carbene carbon is assessed in terms of the absolute minimum of the MESP at the carbene lone pair region (V(min)), as well as the MESP at the carbene nucleus (V(C)). A linear relationship is established between the V(C) and Tolman electronic parameter (TEP) which suggested the use of the former as a simple and efficient descriptor for the electron donating power of N-heterocyclic carbene ligands toward metal coordination. The V(min) of the carbene also showed good correlation with TEP. However, the deviation from linearity was higher than V(C)-TEP correlation, and the reason for this was attributed to the steric effect of N-substituents at the lone pair region. The greater coordinating power of N-heterocyclic carbenes over phosphines is explained on the basis of deeper V(min) values obtained for the former, and in fact even the V(min) of the least electron rich N-heterocyclic carbene is comparable to the highly electron rich phosphine ligands. Thus the MESP topographical approach presented herein offers quantification of the inherent electron donating power of a free N-heterocyclic carbene ligand. PMID:20384355

Mathew, Jomon; Suresh, Cherumuttathu H

2010-05-17

31

Synthesis and Properties of Chelating N-Heterocyclic Carbene Rhodium(I) Complexes: Synthetic Experiments in Current Organometallic Chemistry  

Science.gov (United States)

The preparation and characterization of two air-stable Rh(I) complexes bearing a chelating N-heterocyclic carbene (NHC) ligand is described. The synthesis involves the preparation of a Ag(I)-NHC complex and its use as carbene transfer agent to a Rh(I) precursor. The so obtained complex can be further reacted with carbon monoxide to give the…

Mata, Jose A.; Poyatos, Macarena; Mas-Marza, Elena

2011-01-01

32

DFT prediction of multitopic N-heterocyclic carbenes using Clar's aromatic sextet theory.  

UK PubMed Central (United Kingdom)

Existence of several multitopic N-heterocyclic carbene (NHC) ligands with up to four carbene centers have been predicted on the basis of Clar's aromatic sextet theory. Assessment on stability and reactivity of NHCs was made by quantifying aromaticity, aromatic stabilization energy (E(aroma)), strength of carbene lone pair, proton affinity, and CuCl binding energy. On NICS(0) and HOMA scales of aromaticity, several NHCs showed high aromaticity, while E(aroma) (17.2-19.4 kcal/mol) indicated substantial stability for the N-heterocycle. Homodesmotic reactions suggested that heat of formation of most of the newly designed carbenes is very close to that of the existing bis-NHCs. Designing a multitopic ligand through branching via C(sp3) linkage was very effective as it improved the stability of the carbene. Electrostatic potential minimum (V(min)) at the carbene lone pair suggested that annelation of heterocycle to a benzenoid ring or branching through C(sp3) linkage can only marginally influence the electron donating power of the ligand. Hence, all multitopic NHCs showed proton affinity (252.3-267.4 kcal/mol) and CuCl binding energy (62.9-66.6 kcal/mol) very close to those of 1,3-dimethylimidazolidine-2-ylidene (1). It has also been demonstrated that branched multitopic 3-dimensional NHCs are attractive for designing metal-organic framework with narrow (1-1.5 nm) cage/pore size.

Suresh CH; Ajitha MJ

2013-04-01

33

N-heterocyclic carbenes--catalysts for the preparation of polyhedral silsesquioxanes.  

UK PubMed Central (United Kingdom)

N-Heterocyclic carbenes could be used as powerful catalysts for the preparation of various polyhedral silsesquioxanes. NHCs also catalyze a rearrangement of existing cages and a scrambling between two different cages at a concentration as low as 1 mol%.

Koželj M; Orel B

2013-07-01

34

Group 7 transition metal complexes with N-heterocyclic carbenes.  

Science.gov (United States)

This tutorial review summarizes all works and highlights recent advances published in the growing field of group 7 N-heterocyclic carbene (NHC) complexes. It provides a valuable source for all scientists that are interested in conducting research with new compounds bearing these metals. The article provides an overview of all manganese NHC complexes that are known to date. There are a lot of examples where manganese NHC complexes show unpredicted behaviour during synthesis, very different from other transition metal NHC complexes and their higher homologues rhenium and technetium. These differences are depicted and discussed. Furthermore, the chemistry of technetium NHC compounds and their chemical behaviour are discussed. To date published work on technetium NHC chemistry has been restricted to the oxidation state +v. It was found that such compounds are very reactive but show great stability in dry air. Their radioactivity makes such compounds interesting candidates for radiochemical applications. Since most group 7 NHC chemistry was conducted on rhenium NHC complexes, this tutorial review highlights the chemical behaviour of such compounds. Rhenium(i) complexes reveal luminescent properties, making them interesting candidates for applications ranging from biological markers to organic light-emitting diodes (OLEDs). Another interesting feature is the radioactivity of some compounds, which makes them excellent candidates for radiopharmaceutical research; hence their synthesis and reactivity are discussed. PMID:23475206

Hock, Sebastian J; Schaper, Lars-Arne; Herrmann, Wolfgang A; Kühn, Fritz E

2013-06-21

35

Preparation of a N-Heterocyclic Carbene Nickel(II) Complex: Synthetic Experiments in Current Organic and Organometallic Chemistry  

Science.gov (United States)

A four-part experiment that leads to the synthesis of a cyclopentadienyl chloro-nickel(II) complex bearing a N-heterocyclic carbene (NHC) ligand is presented. In the first part, the preparation of 1,3-bis-(2,4,6-trimethylphenyl)imidazolium chloride (IMes[middle dot]HCl) in a one-pot procedure by reaction of 2,4,6-trimethylaniline with…

Ritleng, Vincent; Brenner, Eric; Chetcuti, Michael J.

2008-01-01

36

Antimicrobial Properties of Some Bis(Iminoacenaphthene (BIAN)-Supported N-Heterocyclic Carbene Complexes of Silver and Gold  

Directory of Open Access Journals (Sweden)

Full Text Available The AgCl, AgOAc, AuCl, and AuOAc complexes of the new bis(imino)acenaphthene(BIAN)-supported N-heterocyclic carbene ligand and the precursor imidazolium salt have been investigated with respect to their antimicrobial activities against Staphylococcus aureus, Bacillus subtilis, Escherichia coli and Psudomonas aeruginosa. The most active antimicrobial is the precursor imidazolium salt, which has a minimum inhibitory concentration (MIC) value of

Rachel R. Butorac; Salem S. Al-Deyab; Alan H. Cowley

2011-01-01

37

Silver(I)-carbene complexes/ionic liquids: novel N-heterocyclic carbene delivery agents for organocatalytic transformations.  

UK PubMed Central (United Kingdom)

[reaction: see text] N-Heterocyclic carbene (NHC) complexes with silver were investigated as sources of unsaturated NHC carbene catalysts via thermal decomposition. The NHC complex (1-ethyl-3-methylimidazol-2-ylidene)silver(I) chloride is an ionic liquid, and was found to catalyze the ring-opening polymerization of lactide at elevated temperatures to give narrowly dispersed polylactide of predictable molecular weight. Silver-carbene complexes can also be used for the catalysis of small molecule transesterification reactions. Thermolysis of the silver complexes in the presence of CS(2) yielded the zwitterionic CS(2) adducts of the carbene, implicating the intermediacy of the free carbene in these reactions.

Sentman AC; Csihony S; Waymouth RM; Hedrick JL

2005-03-01

38

Tautomerisation of 2-substituted pyridines to N-heterocyclic carbene ligands induced by the 16?e- unsaturated [Tp(Me2)Ir(III)(C6H5)2] moiety.  

UK PubMed Central (United Kingdom)

The complex [Tp(Me2)Ir(C(6)H(5))(2)(N(2))] reacts with several 2-substituted pyridines to generate N-heterocyclic carbenes resulting from a formal 1,2-hydrogen shift from C(6) to N. In this paper we provide a detailed report of the scope and the mechanistic aspects (both experimental and theoretical) of the tautomerisation of 2-substituted pyridines.

Conejero S; López-Serrano J; Paneque M; Petronilho A; Poveda ML; Vattier F; Álvarez E; Carmona E

2012-04-01

39

Mono-N-heterocyclic carbene amido and alkyl complexes. Cobalt-mediated C-H activation and C-C coupling reactions involving benzyl ligands on a putative 3-coordinate intermediate.  

UK PubMed Central (United Kingdom)

Aminolysis of [Co(NHC){N(SiMe3)2}Cl] with N-H acidic compounds such as substituted amidines and anilines leads to novel Co(II) complexes with monodentate N-heterocyclic carbene ligation. Attempts to prepare Co(II) dibenzyl complexes with bulky IPr coligands led to metal reduction to Co(I) and aromatic C-H activation and C-C coupling between the benzyl groups.

Danopoulos AA; Braunstein P

2013-05-01

40

Quantifying and understanding the electronic properties of N-heterocyclic carbenes.  

Science.gov (United States)

The use of N-heterocyclic carbenes (NHCs) in chemistry has developed rapidly over the past twenty years. These interesting compounds are predominantly employed in organometallic chemistry as ligands for various metal centres, and as organocatalysts able to mediate an exciting range of reactions. However, the sheer number of NHCs known in the literature can make the appropriate choice of NHC for a given application difficult. A number of metrics have been explored that allow the electronic properties of NHCs to be quantified and compared. In this review, we discuss these various metrics and what they can teach about the electronic properties of NHCs. Data for approximately three hundred NHCs are presented, obtained from a detailed survey of the literature. PMID:23788114

Nelson, David J; Nolan, Steven P

2013-08-21

 
 
 
 
41

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.  

UK PubMed Central (United Kingdom)

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.

Celik MA; Dash C; Adiraju VA; Das A; Yousufuddin M; Frenking G; Dias HV

2013-01-01

42

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

2012-12-28

43

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; Matthias D’hooghe

2012-01-01

44

N-heterocyclic carbene Pt(II) complexes from caffeine: synthesis, structures and photoluminescent properties.  

UK PubMed Central (United Kingdom)

The caffeine-derived N-heterocyclic carbene (NHC) complex [Pt(II)(C^N)(NHC)Cl] (C^N = 2-phenylpyridine), 4 has the opposite stereochemistry and a shorter Pt-C(carbene) bond compared to that of an analogous benzimidazole-derived N,N-heterocyclic carbene (NNHC) Pt complex 2. These suggest a lower trans influence of pyridyl N compared to cyclometallated carbon and an increased Pt-NHC ?-backbonding because of decreased ?-donation resulting from conjugation to the electron deficient pyrimidine of caffeine. Complex 4 has a lower emission quantum yield (?) and is blue-shifted into the green region of the visible spectrum relative to non-carbene Pt(II) cyclometalated complex 5.

Hu JJ; Bai SQ; Yeh HH; Young DJ; Chi Y; Hor TS

2011-05-01

45

Ruthenium(II) and osmium(II) complexes bearing bipyridine and the N-heterocyclic carbene-based C^N^C pincer ligand: an experimental and density functional theory study.  

Science.gov (United States)

Ruthenium(II) and osmium(II) complexes [M(C^N^C)(N^N)L](n+) (L = Cl(-), n = 1; L = CH3CN, t-BuNC, n = 2) containing a neutral tridentate N-heterocyclic carbene (NHC)-based pincer ligand, either 2,6-bis(1-butylimidazol-2-ylidene)pyridine (C(1)^N^C(1)) or 2,6-bis(3-butylbenzimidazol-2-ylidene)pyridine (C(2)^N^C(2)), and a neutral 2,2'-bipyridine-type aromatic diimine have been prepared. Investigations into the effects of varying M (Ru and Os), C^N^C, N^N, and L on the structural, electrochemical, absorption, and emission characteristics associated with [M(C^N^C)(N^N)L](n+) are presented. Interestingly, spectroscopic findings and time-dependent density functional theory (TD-DFT) calculations in this work support a d?(Ru(II)/Os(II)) ? ?*(N^N) metal-to-ligand charge transfer (MLCT) assignment for the lowest-energy transition in [M(C^N^C)(N^N)L](n+) and not a d?(Ru(II)/Os(II)) ? ?*(C^N^C) MLCT assignment. This is in stark contrast to [Ru(tpy)(bpy)Cl](+) and [Os(tpy)(bpy)Cl](+) (tpy = 2,2':6',2?-terpyridine, bpy = 2,2'-bipyridine) for which the lowest-energy transitions are assigned as d?(Ru/Os) ? ?*(tpy) MLCT transitions. [Ru(II)(C^N^C)(N^N)L](n+) is emissive with emission maxima of around 600-700 nm observed upon photoexcitation of their d?(Ru(II)) ? ?*(N^N) MLCT bands. The electronic structures for [Ru(C^N^C)(N^N)Cl](0) have also been probed by spectroelectrochemistry, electron paramagnetic resonance (EPR) spectroscopy, and DFT calculations, which reveal that the lowest unoccupied molecular orbitals (LUMOs) for [Ru(C^N^C)(N^N)Cl](+) are N^N-based. PMID:23952294

Chung, Lai-Hon; Cho, Ka-Sin; England, Jason; Chan, Siu-Chung; Wieghardt, Karl; Wong, Chun-Yuen

2013-08-16

46

Ruthenium(II) and osmium(II) complexes bearing bipyridine and the N-heterocyclic carbene-based C^N^C pincer ligand: an experimental and density functional theory study.  

UK PubMed Central (United Kingdom)

Ruthenium(II) and osmium(II) complexes [M(C^N^C)(N^N)L](n+) (L = Cl(-), n = 1; L = CH3CN, t-BuNC, n = 2) containing a neutral tridentate N-heterocyclic carbene (NHC)-based pincer ligand, either 2,6-bis(1-butylimidazol-2-ylidene)pyridine (C(1)^N^C(1)) or 2,6-bis(3-butylbenzimidazol-2-ylidene)pyridine (C(2)^N^C(2)), and a neutral 2,2'-bipyridine-type aromatic diimine have been prepared. Investigations into the effects of varying M (Ru and Os), C^N^C, N^N, and L on the structural, electrochemical, absorption, and emission characteristics associated with [M(C^N^C)(N^N)L](n+) are presented. Interestingly, spectroscopic findings and time-dependent density functional theory (TD-DFT) calculations in this work support a d?(Ru(II)/Os(II)) ? ?*(N^N) metal-to-ligand charge transfer (MLCT) assignment for the lowest-energy transition in [M(C^N^C)(N^N)L](n+) and not a d?(Ru(II)/Os(II)) ? ?*(C^N^C) MLCT assignment. This is in stark contrast to [Ru(tpy)(bpy)Cl](+) and [Os(tpy)(bpy)Cl](+) (tpy = 2,2':6',2?-terpyridine, bpy = 2,2'-bipyridine) for which the lowest-energy transitions are assigned as d?(Ru/Os) ? ?*(tpy) MLCT transitions. [Ru(II)(C^N^C)(N^N)L](n+) is emissive with emission maxima of around 600-700 nm observed upon photoexcitation of their d?(Ru(II)) ? ?*(N^N) MLCT bands. The electronic structures for [Ru(C^N^C)(N^N)Cl](0) have also been probed by spectroelectrochemistry, electron paramagnetic resonance (EPR) spectroscopy, and DFT calculations, which reveal that the lowest unoccupied molecular orbitals (LUMOs) for [Ru(C^N^C)(N^N)Cl](+) are N^N-based.

Chung LH; Cho KS; England J; Chan SC; Wieghardt K; Wong CY

2013-09-01

47

Piano-stool iron(ii) complexes as probes for the bonding of n-heterocyclic carbenes: indications for -acceptor ability  

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A series of new piano-stool iron(II) complexes comprising mono- and bidentate chelating N-heterocyclic carbene ligands [Fe(cp)(CO)(NHC)(L)]X have been prepared and analyzed by spectroscopic, electrochemical, crystallographic, and theoretical methods. Selectively substituting the L site with a...

Mercs, Laszlo; Labat, Gaël; Neels, Antonia; Ehlers, Andreas; Albrecht, Martin

48

Solid-State Covalent Capture of CO2 by Using N-Heterocyclic Carbenes.  

Science.gov (United States)

Capture me! The first report of an N-heterocyclic carbene (NHC) as a solid-state carbon capture reagent is presented. Experimental and theoretical measurements demonstrate the ability of the NHC to react rapidly and stoichiometrically with CO2 at low partial pressures. PMID:23893356

Vogt, Monika; Bennett, Joshua E; Huang, Yong; Wu, Chao; Schneider, William F; Brennecke, Joan F; Ashfeld, Brandon L

2013-07-26

49

Synthesis and reactivity of alkyl-palladium N-heterocyclic carbene complexes.  

Science.gov (United States)

The transamination of alkyl-palladium halide N-heterocyclic carbene complexes has enabled the isolation of products that reveal interesting insights into the factors which might be barriers to the development of a palladium-catalysed alkyl-amination reaction. PMID:17347724

Esposito, Oriana; Lewis, Alexandra K de K; Hitchcock, Peter B; Caddick, Stephen; Cloke, F Geoffrey N

2007-02-14

50

Direct ?-functionalization of simple aldehydes via oxidative N-heterocyclic carbene catalysis.  

UK PubMed Central (United Kingdom)

A direct ?-functionalization of simple aldehydes under N-Heterocyclic Carbene (NHC) catalysis and direct generation of ester enolate equivalents from nonfunctionalized aldehydes are disclosed. The catalysis involves selective enolate generation from an oxidatively generated NHC-bounded ester intermediate as a key step. The ester enolate intermediates undergo stereoselective reactions with enones and trifluoromethyl ketones.

Mo J; Yang R; Chen X; Tiwari B; Chi YR

2013-01-01

51

Pericyclic rearrangements of N-heterocyclic carbenes of indazole to substituted 9-aminoacridines.  

UK PubMed Central (United Kingdom)

On deprotonation, 1-arylindazolium salts form 1-arylindazol-3-ylidenes which rearrange spontaneously via ring cleavage, ring closure and subsequent proton transfer to substituted 9-aminoacridines. By contrast, the N-heterocyclic carbene of 2-phenylindazolium cannot rearrange similarly and was trapped by sulfur.

Guan Z; Wiechmann S; Drafz M; Hübner E; Schmidt A

2013-06-01

52

Highly efficient catalytic hydrosilylation of carbon dioxide by an N-heterocyclic carbene copper catalyst.  

UK PubMed Central (United Kingdom)

The N-heterocyclic carbene-copper alkoxide complex [(IPr)Cu(OtBu)] acts as a highly efficient catalyst for the hydrosilylation of carbon dioxide with a hydrosilane. The desired product, silyl formate could be isolated in multigram quantity under solvent free conditions. A copper formate catalyst intermediate was successfully isolated and structurally characterized, thus offering important insight into the mechanistic details.

Zhang L; Cheng J; Hou Z

2013-05-01

53

N-Heterocyclic carbene catalyzed C-glycosylation: a concise approach from Stetter reaction.  

UK PubMed Central (United Kingdom)

Described herein is the first example of an organocatalytic approach for acylanion addition to the anomeric carbon of 2-nitroglucal using an N-heterocyclic carbene catalyst. Control over the reaction conditions gives ?-selective and nitro-eliminated C-glycosides, providing opportunities to produce new classes of C-glycoside.

Vedachalam S; Tan SM; Teo HP; Cai S; Liu XW

2012-01-01

54

N-Heterocyclic carbene catalyzed C-glycosylation: a concise approach from Stetter reaction.  

Science.gov (United States)

Described herein is the first example of an organocatalytic approach for acylanion addition to the anomeric carbon of 2-nitroglucal using an N-heterocyclic carbene catalyst. Control over the reaction conditions gives ?-selective and nitro-eliminated C-glycosides, providing opportunities to produce new classes of C-glycoside. PMID:22185609

Vedachalam, Seenuvasan; Tan, Shi Min; Teo, Hui Ping; Cai, Shuting; Liu, Xue-Wei

2011-12-20

55

Catalysis with N-heterocyclic carbenes under oxidative conditions.  

UK PubMed Central (United Kingdom)

This Concept article discusses the potential of oxidative carbene catalysis in synthesis and comprehensively covers pioneering studies as well as recent developments. Oxidative carbene catalysis can be conducted by using inorganic and organic oxidants. Applications in cascade processes, in enantioselective catalysis, and also in natural product synthesis are discussed.

De Sarkar S; Biswas A; Samanta RC; Studer A

2013-04-01

56

Catalysis with N-heterocyclic carbenes under oxidative conditions.  

Science.gov (United States)

This Concept article discusses the potential of oxidative carbene catalysis in synthesis and comprehensively covers pioneering studies as well as recent developments. Oxidative carbene catalysis can be conducted by using inorganic and organic oxidants. Applications in cascade processes, in enantioselective catalysis, and also in natural product synthesis are discussed. PMID:23436489

De Sarkar, Suman; Biswas, Anup; Samanta, Ramesh C; Studer, Armido

2013-02-21

57

The photochemistry of rhenium(i) tricarbonyl N-heterocyclic carbene complexes.  

UK PubMed Central (United Kingdom)

The photophysical and photochemical properties of the new tricarbonyl rhenium(i) complexes bound to N-heterocyclic carbene ligands (NHC), fac-[Re(CO)3(N^C)X] (N^C = 1-phenyl-3-(2-pyridyl)imidazole or 1-quinolinyl-3-(2-pyridyl)imidazole; X = Cl or Br), are reported. The photophysics of these complexes highlight phosphorescent emission from triplet metal-to-ligand ((3)MLCT) excited states, typical of tricarbonyl rhenium(i) complexes, with the pyridyl-bound species displaying a ten-fold shorter excited state lifetime. On the other hand, these pyridyl-bound species display solvent-dependent photochemical CO dissociation following what appear to be two different mechanisms, with a key step being the formation of cationic tricarbonyl solvato-complexes, being themselves photochemically active. The photochemical mechanisms are illustrated with a combination of NMR, IR, UV-Vis, emission and X-ray structural characterization techniques, clearly demonstrating that the presence of the NHC ligand is responsible for the previously unobserved photochemical behavior in other photoactive tricarbonyl rhenium(i) species. The complexes bound to the quinolinyl-NHC ligand (which possess a lower-energy (3)MLCT) are photostable, suggesting that the photoreactive excited state is not any longer thermally accessible. The photochemistry of the pyridyl complexes was investigated in acetonitrile solutions and also in the presence of triethylphosphite, showing a competing and bifurcated photoreactivity promoted by the trans effect of both the NHC and phosphite ligands.

Vaughan JG; Reid BL; Ramchandani S; Wright PJ; Muzzioli S; Skelton BW; Raiteri P; Brown DH; Stagni S; Massi M

2013-08-01

58

The photochemistry of rhenium(i) tricarbonyl N-heterocyclic carbene complexes.  

Science.gov (United States)

The photophysical and photochemical properties of the new tricarbonyl rhenium(i) complexes bound to N-heterocyclic carbene ligands (NHC), fac-[Re(CO)3(N^C)X] (N^C = 1-phenyl-3-(2-pyridyl)imidazole or 1-quinolinyl-3-(2-pyridyl)imidazole; X = Cl or Br), are reported. The photophysics of these complexes highlight phosphorescent emission from triplet metal-to-ligand ((3)MLCT) excited states, typical of tricarbonyl rhenium(i) complexes, with the pyridyl-bound species displaying a ten-fold shorter excited state lifetime. On the other hand, these pyridyl-bound species display solvent-dependent photochemical CO dissociation following what appear to be two different mechanisms, with a key step being the formation of cationic tricarbonyl solvato-complexes, being themselves photochemically active. The photochemical mechanisms are illustrated with a combination of NMR, IR, UV-Vis, emission and X-ray structural characterization techniques, clearly demonstrating that the presence of the NHC ligand is responsible for the previously unobserved photochemical behavior in other photoactive tricarbonyl rhenium(i) species. The complexes bound to the quinolinyl-NHC ligand (which possess a lower-energy (3)MLCT) are photostable, suggesting that the photoreactive excited state is not any longer thermally accessible. The photochemistry of the pyridyl complexes was investigated in acetonitrile solutions and also in the presence of triethylphosphite, showing a competing and bifurcated photoreactivity promoted by the trans effect of both the NHC and phosphite ligands. PMID:23939232

Vaughan, Jamila G; Reid, Brodie L; Ramchandani, Sushil; Wright, Phillip J; Muzzioli, Sara; Skelton, Brian W; Raiteri, Paolo; Brown, David H; Stagni, Stefano; Massi, Massimiliano

2013-09-18

59

The photochemistry of rhenium(i) tricarbonyl N-heterocyclic carbene complexes.  

UK PubMed Central (United Kingdom)

The photophysical and photochemical properties of the new tricarbonyl rhenium(i) complexes bound to N-heterocyclic carbene ligands (NHC), fac-[Re(CO)3(N^C)X] (N^C = 1-phenyl-3-(2-pyridyl)imidazole or 1-quinolinyl-3-(2-pyridyl)imidazole; X = Cl or Br), are reported. The photophysics of these complexes highlight phosphorescent emission from triplet metal-to-ligand ((3)MLCT) excited states, typical of tricarbonyl rhenium(i) complexes, with the pyridyl-bound species displaying a ten-fold shorter excited state lifetime. On the other hand, these pyridyl-bound species display solvent-dependent photochemical CO dissociation following what appear to be two different mechanisms, with a key step being the formation of cationic tricarbonyl solvato-complexes, being themselves photochemically active. The photochemical mechanisms are illustrated with a combination of NMR, IR, UV-Vis, emission and X-ray structural characterization techniques, clearly demonstrating that the presence of the NHC ligand is responsible for the previously unobserved photochemical behavior in other photoactive tricarbonyl rhenium(i) species. The complexes bound to the quinolinyl-NHC ligand (which possess a lower-energy (3)MLCT) are photostable, suggesting that the photoreactive excited state is not any longer thermally accessible. The photochemistry of the pyridyl complexes was investigated in acetonitrile solutions and also in the presence of triethylphosphite, showing a competing and bifurcated photoreactivity promoted by the trans effect of both the NHC and phosphite ligands.

Vaughan JG; Reid BL; Ramchandani S; Wright PJ; Muzzioli S; Skelton BW; Raiteri P; Brown DH; Stagni S; Massi M

2013-09-01

60

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.  

UK PubMed Central (United Kingdom)

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.

Lefebvre JF; Lo M; Leclercq D; Richeter S

2011-03-01

 
 
 
 
61

New chiral ligands bearing two N-heterocyclic carbene moieties at a dioxolane backbone. Gold, palladium and rhodium complexes as enantioselective catalysts.  

UK PubMed Central (United Kingdom)

Biscarbene ligands with two imidazolin-2-ylidene moieties at a chiral dioxolane backbone were used as ligands for gold, rhodium and palladium complexes. All new complexes showed varying degrees of enantioselectivity toward hydrogenation of prochiral alkenes with ees up to 95%.

Arnanz A; González-Arellano C; Juan A; Villaverde G; Corma A; Iglesias M; Sánchez F

2010-05-01

62

Evaluation of arene ruthenium(II) N-heterocyclic carbene complexes as organometallics interacting with thiol and selenol containing biomolecules.  

Science.gov (United States)

Metal complexes with N-heterocyclic carbene (NHC) ligands have been widely used in catalytic chemistry and are now increasingly considered for the development of new chemical tools and metal based drugs. Ruthenium complexes of the type (p-cymene)(NHC)RuCl(2) interacted with biologically relevant thiols and selenols, which resulted in the inhibition of enzymes such as thioredoxin reductase or cathepsin B. Pronounced antiproliferative effects could be obtained provided that an appropriate cellular uptake was achieved. Inhibition of tumor cell growth was accompanied by a perturbation of metabolic parameters such as cellular respiration. PMID:23149817

Oehninger, Luciano; Stefanopoulou, Maria; Alborzinia, Hamed; Schur, Julia; Ludewig, Stephanie; Namikawa, Kazuhiko; Muñoz-Castro, Alvaro; Köster, Reinhard W; Baumann, Knut; Wölfl, Stefan; Sheldrick, William S; Ott, Ingo

2012-11-14

63

Evaluation of arene ruthenium(II) N-heterocyclic carbene complexes as organometallics interacting with thiol and selenol containing biomolecules.  

UK PubMed Central (United Kingdom)

Metal complexes with N-heterocyclic carbene (NHC) ligands have been widely used in catalytic chemistry and are now increasingly considered for the development of new chemical tools and metal based drugs. Ruthenium complexes of the type (p-cymene)(NHC)RuCl(2) interacted with biologically relevant thiols and selenols, which resulted in the inhibition of enzymes such as thioredoxin reductase or cathepsin B. Pronounced antiproliferative effects could be obtained provided that an appropriate cellular uptake was achieved. Inhibition of tumor cell growth was accompanied by a perturbation of metabolic parameters such as cellular respiration.

Oehninger L; Stefanopoulou M; Alborzinia H; Schur J; Ludewig S; Namikawa K; Muñoz-Castro A; Köster RW; Baumann K; Wölfl S; Sheldrick WS; Ott I

2013-02-01

64

Towards longer-lived metal-to-ligand charge transfer states of iron(II) complexes: an N-heterocyclic carbene approach.  

UK PubMed Central (United Kingdom)

A 9 ps (3)MLCT lifetime was achieved by a Fe(II) complex based on C(NHC)^N(py)^C(NHC) pincer ligands. This is the longest known so far for any kind of complexes of this abundant metal, and increased by almost two orders of magnitude compared to the reference Fe(II) bis-terpyridine complex.

Liu Y; Harlang T; Canton SE; Chábera P; Suárez-Alcántara K; Fleckhaus A; Vithanage DA; Göransson E; Corani A; Lomoth R; Sundström V; Wärnmark K

2013-07-01

65

Towards longer-lived metal-to-ligand charge transfer states of iron(II) complexes: an N-heterocyclic carbene approach.  

Science.gov (United States)

A 9 ps (3)MLCT lifetime was achieved by a Fe(II) complex based on C(NHC)^N(py)^C(NHC) pincer ligands. This is the longest known so far for any kind of complexes of this abundant metal, and increased by almost two orders of magnitude compared to the reference Fe(II) bis-terpyridine complex. PMID:23752944

Liu, Yizhu; Harlang, Tobias; Canton, Sophie E; Chábera, Pavel; Suárez-Alcántara, Karina; Fleckhaus, André; Vithanage, Dimali A; Göransson, Erik; Corani, Alice; Lomoth, Reiner; Sundström, Villy; Wärnmark, Kenneth

2013-07-21

66

N-heterocyclic carbene organocatalysts for dehydrogenative coupling of silanes and hydroxyl compounds.  

UK PubMed Central (United Kingdom)

Go organic! N-Heterocyclic carbene (NHC) 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene (IiPr) has been found to be an efficient and selective catalyst for the dehydrogenative coupling of a wide range of silanes and hydroxyl groups to form Si?O bonds under mild and solvent-free conditions. Mechanistic studies indicated that the activation of hydroxyl groups by the NHC is the most plausible initial step for the process.

Gao D; Cui C

2013-08-01

67

Polyaminoborane main chain scission using N-heterocyclic carbenes; formation of donor-stabilised monomeric aminoboranes.  

UK PubMed Central (United Kingdom)

The reaction of N-heterocyclic carbenes with polyaminoboranes [MeNH-BH2]n or [NH2-BH2]n at 20 °C led to depolymerisation and the formation of labile, monomeric aminoborane-NHC adducts, RNH-BH2-NHC (R = Me or H); a similar NHC adduct of Ph2N[double bond, length as m-dash]BCl2 was characterized by single crystal X-ray diffraction.

Stubbs NE; Jurca T; Leitao EM; Woodall CH; Manners I

2013-08-01

68

Linking borane with N-heterocyclic carbenes: effective hydrogen-atom donors for radical reactions.  

Science.gov (United States)

Undue influence: N-heterocyclic carbenes (NHCs) were found to reduce the strength of the B-H bonds of borane by a surprisingly large amount upon the formation of NHC-BH(3) complexes. This property was exploited in the development of a suite of NHC-borane complexes for the reduction of xanthates in radical-mediated Barton-McCombie-type deoxygenation reactions (see scheme). AIBN = azobisisobutyronitrile, Bn = benzyl. PMID:19173269

Walton, John C

2009-01-01

69

Linking borane with N-heterocyclic carbenes: effective hydrogen-atom donors for radical reactions.  

UK PubMed Central (United Kingdom)

Undue influence: N-heterocyclic carbenes (NHCs) were found to reduce the strength of the B-H bonds of borane by a surprisingly large amount upon the formation of NHC-BH(3) complexes. This property was exploited in the development of a suite of NHC-borane complexes for the reduction of xanthates in radical-mediated Barton-McCombie-type deoxygenation reactions (see scheme). AIBN = azobisisobutyronitrile, Bn = benzyl.

Walton JC

2009-01-01

70

Highly efficient catalytic hydrosilylation of carbon dioxide by an N-heterocyclic carbene copper catalyst.  

Science.gov (United States)

The N-heterocyclic carbene-copper alkoxide complex [(IPr)Cu(OtBu)] acts as a highly efficient catalyst for the hydrosilylation of carbon dioxide with a hydrosilane. The desired product, silyl formate could be isolated in multigram quantity under solvent free conditions. A copper formate catalyst intermediate was successfully isolated and structurally characterized, thus offering important insight into the mechanistic details. PMID:23598425

Zhang, Liang; Cheng, Jianhua; Hou, Zhaomin

2013-05-25

71

N-Heterocyclic carbenes (NHCs) as organocatalysts and structural components in metal-free polymer synthesis.  

UK PubMed Central (United Kingdom)

The chemistry of N-heterocyclic carbenes (NHCs) has witnessed tremendous development in the past two decades: NHCs have not only become versatile ligands for transition metals, but have also emerged as powerful organic catalysts in molecular chemistry and, more recently, in metal-free polymer synthesis. To understand the success of NHCs, this review first presents the electronic properties of NHCs, their main synthetic methods, their handling, and their reactivity. Their ability to activate key functional groups (e.g. aldehydes, esters, heterocycles, silyl ketene acetals, alcohols) is then discussed in the context of molecular chemistry. Focus has been placed on the activation of substrates finding analogies with monomers (e.g. bis-aldehydes, multi-isocyanates, cyclic esters, epoxides, N-carboxyanhydrides, etc.) and/or initiators (e.g. hydroxy- or trimethylsilyl-containing reagents) employed in such "organopolymerisation" reactions utilizing NHCs. A variety of metal-free polymers, including aliphatic polyesters and polyethers, poly(?-peptoid)s, poly(meth)acrylates, polyurethanes, or polysiloxanes can be obtained in this way. The last section covers the use of NHCs as structural components of the polymer chain. Indeed, NHC-based photoinitiators, chain transfer agents or functionalizing agents, as well as bifunctional NHC monomer substrates, can also serve for metal-free polymer synthesis.

Fèvre M; Pinaud J; Gnanou Y; Vignolle J; Taton D

2013-03-01

72

N-Heterocyclic carbenes (NHCs) as organocatalysts and structural components in metal-free polymer synthesis.  

Science.gov (United States)

The chemistry of N-heterocyclic carbenes (NHCs) has witnessed tremendous development in the past two decades: NHCs have not only become versatile ligands for transition metals, but have also emerged as powerful organic catalysts in molecular chemistry and, more recently, in metal-free polymer synthesis. To understand the success of NHCs, this review first presents the electronic properties of NHCs, their main synthetic methods, their handling, and their reactivity. Their ability to activate key functional groups (e.g. aldehydes, esters, heterocycles, silyl ketene acetals, alcohols) is then discussed in the context of molecular chemistry. Focus has been placed on the activation of substrates finding analogies with monomers (e.g. bis-aldehydes, multi-isocyanates, cyclic esters, epoxides, N-carboxyanhydrides, etc.) and/or initiators (e.g. hydroxy- or trimethylsilyl-containing reagents) employed in such "organopolymerisation" reactions utilizing NHCs. A variety of metal-free polymers, including aliphatic polyesters and polyethers, poly(?-peptoid)s, poly(meth)acrylates, polyurethanes, or polysiloxanes can be obtained in this way. The last section covers the use of NHCs as structural components of the polymer chain. Indeed, NHC-based photoinitiators, chain transfer agents or functionalizing agents, as well as bifunctional NHC monomer substrates, can also serve for metal-free polymer synthesis. PMID:23288304

Fèvre, Maréva; Pinaud, Julien; Gnanou, Yves; Vignolle, Joan; Taton, Daniel

2013-01-04

73

Fascinating frontiers of N/O-functionalized N-heterocyclic carbene chemistry: from chemical catalysis to biomedical applications.  

UK PubMed Central (United Kingdom)

The in vogue N-heterocyclic carbenes (NHCs) have attracted attention largely for its new found popularity in chemical catalysis and also for displaying promising traits in biomedical applications. The current perspective provides an account of our efforts in exploring the utility of N/O-functionalized N-heterocyclic carbenes in these two areas. On the catalysis front, we have employed the N/O-functionalized N-heterocyclic carbene based precatalysts for the C-C and C-N bond forming reactions like the Suzuki-Miyaura, Sonogashira and Hiyama cross-couplings, the base-free Michael addition, the alkene and alkyne hydroamination reactions and the ring-opening polymerization (ROP) of L-lactides that produce biodegradable polylactide polymers while on the biomedical application front, the anticancer and antimicrobial properties of these N/O-functionalized N-heterocyclic carbene complexes were evaluated. Towards this objective, the N-heterocyclic carbene chemistry of a variety of transition metals like Ag, Au, Ni and Pd has been investigated.

John A; Ghosh P

2010-08-01

74

Abnormal N-heterocyclic carbene main group organometallic chemistry: a debut to the homogeneous catalysis.  

Science.gov (United States)

Abnormal N-heterocyclic carbene (aNHC) adducts of zinc(ii) () and aluminum(iii) () were synthesized. The compounds were characterized by NMR spectroscopy and elemental analysis. The solid state structures of these complexes ( and ) were determined by single crystal X-ray study. Furthermore, these organozinc and organoaluminum adducts ( and ) were tested for the ring opening polymerization of cyclic esters. These adducts were found to be quite efficient catalysts for the polymerization of cyclicesters such as rac-lactide (rac-LA), ?-caprolactone (?-CL), and ?-valerolactone (?-VL). Furthermore, aNHC zinc adduct has been used as catalyst for the synthesis of a tri-block copolymer. PMID:23945705

Sen, Tamal K; Sau, Samaresh Chandra; Mukherjee, Arup; Hota, Pradip Kumar; Mandal, Swadhin K; Maity, Bholanath; Koley, Debasis

2013-09-18

75

Molecular iodine initiates hydroborations of alkenes with N-heterocyclic carbene boranes.  

UK PubMed Central (United Kingdom)

The hydroboration of alkenes of diverse structural types by assorted N-heterocyclic carbene boranes can be accomplished by addition of 5-10% diiodine. For example, reaction of 1,3-dimethylimidazol-2-ylidene borane (diMe-Imd-BH3) with 10% I2 followed by addition of 2,3-dimethyl-2-butene provided the corresponding thexyl NHC-borane (diMe-Imd-BH2thexyl) in 75% yield. This and related mono-hydroboration products are stable to chromatography and storage. The scope of the new reaction is described and the mechanism is probed by 11B NMR experiments.

Pan X; Boussonnière A; Curran DP

2013-08-01

76

Theoretical characterization of a Ru N-heterocyclic carbene derivative of a polyoxometalate. Enhanced ?-interaction in oxide supported TM-organic linkages.  

UK PubMed Central (United Kingdom)

The electronic structure of the N-heterocyclic carbene (NHC) functionalized polyoxometalate [K(PW(9)O(34))(2)(cis-WO(2))(cis-RuLMe(2))](12-) (1) is studied computationally; and its Ru-NHC bonding situation is compared with other monometallic Ru-NHC complexes, as well as other representative carbene complexes. The DFT analysis further supports the low spin configuration, S = ½, as the ground state, as the unoccupied d ruthenium orbitals are high in energy and mixed with the d-type orbitals of W. We gauge the TM-carbene bonding nature by means of atoms in molecules (AIM) quantum theory and a modified version of energy decomposition analysis (EDA) based on orbital deletion. According to the calculations, the NCH ligand in POM derivative 1 is more tightly bonded than in its analogous neutral monometallic Ru-NHC complexes. This is manifested in a higher electron density ?(r) at the bond critical point (bcp) between Ru and the carbene carbon atoms. The energetic contribution of ?-donation is similar for all Ru-NHC complexes, and significantly higher than for Fischer-type carbene complex, in agreement with the description of the N-heterocyclic carbene ligand as a strong ?-donor. Among the studied Ru-NHC complexes, the POM derivative 1 shows the largest ?-back-donation ability, explaining the observed short Ru-C(carbene) bond distance and the calculated high interaction energy. The POM framework acting as ligand makes the ruthenium d electrons more loose via orbital mixing and promotes the back-donation to the ligand.

Antonova NS; Carbó JJ; Poblet JM

2011-03-01

77

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; Han Vinh Huynh

2012-01-01

78

Dehydrogenative Synthesis of Imines from Alcohols and Amines Catalyzed by a Ruthenium N-Heterocyclic Carbene Complex  

DEFF Research Database (Denmark)

A new method for the direct synthesis of imines from alcohols and amines is described where hydrogen gas is liberated. The reaction is catalyzed by the ruthenium N-heterocyclic carbene complex [RuCl2(IiPr)(p-cymene)] in the presence of the ligand DABCO and molecular sieves. The imination can be applied to a variety of primary alcohols and amines and can be combined with a subsequent addition reaction. A deuterium labeling experiment indicates that the catalytically active species is a ruthenium dihydride. The reaction is believed to proceed by initial dehydrogenation of the alcohol to the aldehyde, which stays coordinated to ruthenium. Nucleophilic attack of the amine affords the hemiaminal, which is released from ruthenium and converted into the imine.

Maggi, Agnese; Madsen, Robert

2012-01-01

79

Synthesis, structural characterisation and stereochemical investigation of chiral sulfur-functionalised N-heterocyclic carbene complexes of palladium and platinum.  

UK PubMed Central (United Kingdom)

Palladium and platinum complexes containing a sulfur-functionalised N-heterocyclic carbene (S-NHC) chelate ligand have been synthesised. The absolute conformations of these novel organometallic S-NHC chelates were determined by X-ray structural analyses and solution-phase 2D (1)H-(1)H ROESY NMR spectroscopy. The structural studies revealed that the phenyl substituents on the stereogenic carbon atoms invariably take up the axial positions on the Pd-C-S coordination plane to afford a skewed five-membered ring structure. All of the chiral complexes are structurally rigid and stereochemically locked in a chiral ring conformation that is either (Rs ,S,R)-? or (Ss ,R,R)-? in both the solid state and solution.

Krishnan D; Pullarkat SA; Wu M; Li Y; Leung PH

2013-04-01

80

Fluorescent silver(i) and gold(i)-N-heterocyclic carbene complexes with cytotoxic properties: mechanistic insights.  

UK PubMed Central (United Kingdom)

Silver(i) and gold(i)-N-heterocyclic carbene (NHC) complexes bearing a fluorescent anthracenyl ligand were examined for cytotoxicity in normal and tumor cells. The silver(i) complex exhibits greater cytotoxicity in tumor cells compared with normal cells. Notably, in cell extracts, this complex determines a more pronounced inhibition of thioredoxin reductase (TrxR), but it is ineffective towards glutathione reductase (GR). Both gold and silver complexes lead to oxidation of the thioredoxin system, the silver(i) derivative being particularly effective. In addition, the dimerization of peroxiredoxin 3 (Prx3) was also observed, demonstrating the ability of these compounds to reach the mitochondrial target. The fluorescence microscopy visualization of the subcellular distribution of the complexes shows a larger diffusion of these molecules in tumor cells with respect to normal cells.

Citta A; Schuh E; Mohr F; Folda A; Massimino ML; Bindoli A; Casini A; Rigobello MP

2013-07-01

 
 
 
 
81

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

82

Metal free catalytic hydroboration of multiple bonds in methanol using N-heterocyclic carbenes under open atmosphere.  

UK PubMed Central (United Kingdom)

An easy to operate method of catalytic hydroboration of unsaturated compounds has been developed with wide substrate scope. Reactions of various aldimines, ketimines, ?,?-unsaturated carbonyl compounds, and alkynes were successfully executed with bis(pinacolato)diboron and N-heterocyclic carbenes in methanol without requiring a transition metal or inert atmosphere.

Wen K; Chen J; Gao F; Bhadury PS; Fan E; Sun Z

2013-10-01

83

A short and modular synthesis of bulky and electron-rich N-phosphinomethyl-functionalised N-heterocyclic carbene complexes.  

UK PubMed Central (United Kingdom)

Bulky and electron-rich N-phosphinomethyl-substituted N-heterocyclic carbene transition metal complexes have been prepared in a short and efficient protocol. The modular synthesis allows one to convert borane-protected phosphino-functionalized imidazolium salts into their corresponding metal chelate complexes in a one-pot procedure.

Brill M; Kühnel E; Scriban C; Rominger F; Hofmann P

2013-08-01

84

Efficient synthesis of benzofuranones: N-heterocyclic carbene (NHC)/base-catalyzed hydroacylation-Stetter-rearrangement cascade.  

UK PubMed Central (United Kingdom)

A N-heterocyclic carbene/base-catalyzed cascade reaction leading to the formation of functionalized benzofuranones is reported. The reaction proceeds via an intramolecular hydroacylation of unactivated alkynes followed by an intermolecular Stetter reaction and a base-catalyzed chromanone to benzofuranone rearrangement.

Padmanaban M; Biju AT; Glorius F

2011-10-01

85

Efficient synthesis of benzofuranones: N-heterocyclic carbene (NHC)/base-catalyzed hydroacylation-Stetter-rearrangement cascade.  

Science.gov (United States)

A N-heterocyclic carbene/base-catalyzed cascade reaction leading to the formation of functionalized benzofuranones is reported. The reaction proceeds via an intramolecular hydroacylation of unactivated alkynes followed by an intermolecular Stetter reaction and a base-catalyzed chromanone to benzofuranone rearrangement. PMID:21919536

Padmanaban, Mohan; Biju, Akkattu T; Glorius, Frank

2011-09-16

86

Toward molecular rotors: tetra-N-heterocyclic carbene Ag(I)-halide cubane-type clusters.  

UK PubMed Central (United Kingdom)

1,3-Bis(3'-butylimidazol-1'-yl)benzene diiodide (2a), 1,3-bis(3'-but-3''-enyl-imidazolium-3'-yl)benzene diiodide (2b), 1,3-bis(3'-pent-4''-enyl-imidazolium-3'-yl)benzene diiodide (2c), 1,3-bis(4'-butyl-1',2',4'-triazolium-1'-yl)benzene diiodide (2d), or 1,3-bis(4'-butyl-1',2',4'-triazolium-1'-yl)benzene dibromide (2e) was reacted with Ag2O yielding unprecedented tetra-N-heterocyclic carbene-Ag(I)-X cubane-type clusters. These were characterized by (1)H and (13)C NMR spectroscopy, ESI-TOF MS, elemental analysis, and X-ray crystallography. Results from VT (13)C NMR spectroscopy and cross-over experiments are consistent with intramolecular exchange suggesting that the Ag(I) complexes are molecular rotors.

Clark WD; Tyson GE; Hollis TK; Valle HU; Valente EJ; Oliver AG; Dukes MP

2013-05-01

87

Photoswitchable organocatalysis: using light to modulate the catalytic activities of N-heterocyclic carbenes.  

Science.gov (United States)

A 4,5-dithienylimidazolium salt was found to undergo electrocyclic isomerization upon exposure to UV radiation (?(irr) = 313 nm) under neutral and basic conditions; subsequent exposure to visible light reversed the reaction. Under ambient light and in the presence of base, the imidazolium species catalyzed transesterifications as well as amidations in a manner similar to those of previously reported N-heterocyclic carbene precatalysts. However, upon UV irradiation to effect the aforementioned photocyclization, the rate of the transesterification reaction between vinyl acetate and allyl alcohol was significantly attenuated (k(vis/UV) = 12.5), as was the rate of the condensation of ethyl acetate with aminoethanol (k(vis/UV) = 100). The rates of these reactions were successfully toggled between fast and slow states by alternating exposure to visible and UV light, respectively, thus demonstrating a rare example of a photoswitchable catalyst that operates via photomodulation of its electronic structure. PMID:22809240

Neilson, Bethany M; Bielawski, Christopher W

2012-07-19

88

Photoswitchable organocatalysis: using light to modulate the catalytic activities of N-heterocyclic carbenes.  

UK PubMed Central (United Kingdom)

A 4,5-dithienylimidazolium salt was found to undergo electrocyclic isomerization upon exposure to UV radiation (?(irr) = 313 nm) under neutral and basic conditions; subsequent exposure to visible light reversed the reaction. Under ambient light and in the presence of base, the imidazolium species catalyzed transesterifications as well as amidations in a manner similar to those of previously reported N-heterocyclic carbene precatalysts. However, upon UV irradiation to effect the aforementioned photocyclization, the rate of the transesterification reaction between vinyl acetate and allyl alcohol was significantly attenuated (k(vis/UV) = 12.5), as was the rate of the condensation of ethyl acetate with aminoethanol (k(vis/UV) = 100). The rates of these reactions were successfully toggled between fast and slow states by alternating exposure to visible and UV light, respectively, thus demonstrating a rare example of a photoswitchable catalyst that operates via photomodulation of its electronic structure.

Neilson BM; Bielawski CW

2012-08-01

89

The Crystallographic and Quantum Mechanical Analysis of some Pd(II) N-heterocyclic carbene Complexes  

International Nuclear Information System (INIS)

Because of their extraordinary properties, N-heterocyclic carbenes (NHC) have found access to a great variety of catalytic processes which include C-C coupling reactions, formation of furans, cyclopropanation, olefin metathesis, hydroformylation, polymerization and hydrosilylation reactions. In this study, molecular and crystal structures of Pd(II) NHC complexes have been determined by single crystal x-ray diffraction technique. In addition, molecular geometries of all complexes under study were optimized at the B3LYP level of density functional theory (DFT) and the effective core potentials of Hay and Wadt with LanL2DZ basis set were used. In order to investigate binding orbitals of metal and charge transfer mechanism occurred in NHC ring, natural bond orbital (NBO) analyses were performed at the B3LYP/LanL2DZ level on the basis of the optimized ground state for complexes.

2008-01-01

90

The development and catalytic uses of N-heterocyclic carbene gold complexes.  

UK PubMed Central (United Kingdom)

Gold has emerged as a powerful synthetic tool in the chemist's arsenal. From the early use of inorganic salts such as AuCl and AuCl(3) as catalysts, the field has evolved to explore ligands that fine-tune reactivity, stability, and, more recently, selectivity in gold-mediated processes. Substrates generally contain alkenes or alkynes, and they usually involve straightforward protocols in air with solvents that can often times be of technical grade. The actual catalytic species is the putative cationic gold(I) complex [Au(L)](+) (where L is a phosphorus-based species or N-heterocyclic carbene, NHC). The early gold systems bearing phosphine and phosphite ligands provided important transformations and served as useful mechanistic probes. More recently, the use of NHCs as ligands for gold has rapidly gained in popularity. These two-electron donor ligands combine strong ?-donating properties with a steric profile that allows for both stabilization of the metal center and enhancement of its catalytic activity. As a result, the gold-NHC complexes have been used as well-defined precatalysts and have permitted the isolation of reactive single-component systems that are now used instead of the initial [Au(L)Cl]/silver salt method. Because some are now commercially available, NHC-containing gold(I) complexes are gathering increasing interest. In this Account, we describe the chronological development of this chemistry in our laboratories, highlighting the advantages of this family of gold complexes and reviewing their synthesis and applications in catalysis. We first outline the syntheses, which are straightforward. The complexes generally exhibit high stability, allowing for indefinite storage and easy handling. We next consider catalysis, particularly examining efficacy in cycloisomerization, other skeletal rearrangements, addition of water to alkynes and nitriles, and C-H bond activation. These processes are quite atom-economical, and in the most recent C-H reactions the only byproduct is water. State-of-the-art methodology now involves single-component catalysts, precluding the need for costly silver co-catalysts. Remarkably, the use of an NHC as a supporting ligand has permitted the isolation of [Au(L)(S)](+) species (where S is a solvent molecule such as a nitrile), which can act as single-component catalysts. Some improvements are still needed, as the single components are most often synthesized with a silver reagent. Owing to the stabilizing effect of NHC coordination, some NHC-containing systems can catalyze extremely challenging reactions (at temperatures as high as 140 °C) and react at very low loadings of gold (ppm levels). Our latest developments deal with C-H bond functionalization and hold great promise. We close with a selection of important developments by the community with gold-NHC complexes. As demonstrated by the turns and twists encountered during our own journey in the gold-NHC venture, the chemistry described here, combining fundamental organometallic, catalytic, and organic methodology, remains rich in opportunities, especially considering that only a handful of gold(I) architectures has been studied. We hope this Account will encourage young researchers to explore this emerging area, as the adage "the more you do, the more you have to do" surely holds true in gold-mediated catalysis.

Nolan SP

2011-02-01

91

Adducts of nitrous oxide and N-heterocyclic carbenes: syntheses, structures, and reactivity.  

UK PubMed Central (United Kingdom)

N-Heterocyclic carbenes (NHCs) react at ambient conditions with nitrous oxide to give covalent adducts. In the crystal, all compounds show a bent N2O group connected via the N-atom to the former carbene carbon atom. Most adducts are stable at room temperature, but heating induces decomposition into the corresponding ureas. Kinetic experiments show that the thermal stability of the NHC-N2O adducts depends on steric as well as electronic effects. The coordination of N2O to NHCs weakens the N-N bond substantially, and facile N-N bond rupture was observed in reactions with acid or acetyl chloride. On the other hand, reaction with tritylium tetrafluoroborate resulted in a covalent modification of the terminal O-atom, and cleavage of the C-N2O bond was observed in a reaction with thionyl chloride. The coordination chemistry of IMes-N2O (IMes = 1,3-dimesitylimidazol-2-ylidene) was explored in reactions with the complexes CuOTf, Fe(OTf)2, PhSnCl3, CuCl2, and Zn(C6F5)2. Structural analyses show that IMes-N2O is able to act as a N-donor, as an O-donor, or as a chelating N,O-donor. The different coordination modes go along with pronounced electronic changes as evidenced by a bond length analysis.

Tskhovrebov AG; Vuichoud B; Solari E; Scopelliti R; Severin K

2013-06-01

92

Copolymer-incarcerated nickel nanoparticles with N-heterocyclic carbene precursors as active cross-linking agents for Corriu-Kumada-Tamao reaction.  

UK PubMed Central (United Kingdom)

We have developed heterogeneous polymer-incarcerated nickel nanoparticles (NPs), which catalyze cross-coupling reactions. The matrix structure of these catalysts incorporates both N-heterocyclic carbenes (NHCs) as ligands and Ni-NPs, thanks to a new design of cross-linking agents in polymer supports. These embedded NHCs were detected by field gradient swollen-resin magic angle spinning NMR analysis. They were successfully applied to Corriu-Kumada-Tamao reactions with a broad substrate scope including functional group tolerance, and the catalyst could be recovered and reused several times without loss of activity.

Soulé JF; Miyamura H; Kobayashi S

2013-07-01

93

Cross-coupling of ArX with ArMgBr catalyzed by N-heterocyclic carbene-based nickel complexes.  

UK PubMed Central (United Kingdom)

N-Heterocyclic carbene-based pincer nickel complexes were synthesized and characterized. These complexes efficiently catalyze cross-coupling of aryl Grignard reagents with aryl chlorides or fluorides under mild conditions.

Guo WJ; Wang ZX

2013-02-01

94

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

UK PubMed Central (United Kingdom)

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.

Jian TY; Chen XY; Sun LH; Ye S

2013-01-01

95

Cationic gold(I) heteroleptic complexes bearing a pyrazole-derived N-heterocyclic carbene: syntheses, characterizations, and cytotoxic activities.  

Science.gov (United States)

A series of cationic gold(I) heteroleptic complexes bearing the pyrazole-derived N-heterocyclic carbene (NHC) FPyr (1,2,3,4,6,7,8,9-octahydropyridazino[1,2-a]indazolin-11-ylidene), and either a 1,3-disubstituted benzimidazole-derived NHC of the type RR'-bimy (3: R = R' = CHPh2; 4: R = CHPh2, R' = (i)Pr; 5: R = R' = CH2Ph; 6: R = R' = (i)Bu; 7: R = R' = n-Pr; 8: R = R' = Et; 9: R = R' = 2-propenyl) or a non-NHC co-ligand L (10: L = PPh3; 11: L = P(OPh)3; 12: L = DMAP) (DMAP = 4-dimethylaminopyridine) have been synthesized from [AuCl(FPyr)] (1). Complexes 3-12 have been characterized using multinuclei NMR spectroscopies, ESI mass spectrometry, and elemental analysis. X-ray diffraction analyses have been performed on complexes 5, 6, and 9-11. To the best of our knowledge, 11 represents the first gold-NHC complex to bear the P(OPh)3 ligand. The cytotoxic activities of complexes 3-12 have been studied in vitro with the NCI-H1666 non-small cell lung cancer cell line. PMID:23860905

Sivaram, Haresh; Tan, Jackie; Huynh, Han Vinh

2013-07-17

96

[3+2] Fragmentation of an [RP5 Cl](+) Cage Cation Induced by a N-Heterocyclic Carbene.  

UK PubMed Central (United Kingdom)

The cage compound [DippP5 Cl][GaCl4 ] (Dipp=2,6-diisopropylphenyl) reacts with a 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.

Holthausen MH; Surmiak SK; Jerabek P; Frenking G; Weigand JJ

2013-08-01

97

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

Directory of Open Access Journals (Sweden)

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; Ai-Ping Jin; Li-Xiong Shao

2012-01-01

98

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

UK PubMed Central (United Kingdom)

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 TT; Jin AP; Shao LX

2012-01-01

99

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

2012-11-12

100

Novel platinum-N-heterocyclic carbene complex is more cardiotoxic than cis-platin in rats.  

UK PubMed Central (United Kingdom)

Cis-platin and other platinum complexes are important chemotherapeutic agents useful in the treatment of several cancers. However, therapeutic usage of cis-platin and other platinum complex are limited by their undesirable side effects including cardiotoxicity. In this context, we aimed to compare the damage caused in heart by cis-platin and novel platinum-N-heterocyclic carbene (Pt-NHC) complex. For this purpose, 35 Sprague-Dawley rats were divided randomly into five equal groups (n = 7 for each group). Cis-platin and novel Pt-NHC complex were intraperitoneally administered at a single dose of 5 mg/kg and 10 mg/kg and then sacrificed 10 days after this treatment. The heart tissues were taken from all rats for determination of oxidative and myocardial damage. Cis-platin and novel Pt-NHC complex caused oxidative and histological damage in the heart tissue in a dose-dependent manner (p < 0.05). On the other hand, at the same dose levels, cis-platin caused lower oxidative and histological damage in heart tissue compared to novel Pt-NHC complex. These results suggest that novel Pt-NHC complex is more cardiotoxic than cis-platin.

Ciftci O; Ozdemir I; Vardi N; Gurbuz N

2011-09-01

 
 
 
 
101

Novel platinum-N-heterocyclic carbene complex is more cardiotoxic than cis-platin in rats.  

Science.gov (United States)

Cis-platin and other platinum complexes are important chemotherapeutic agents useful in the treatment of several cancers. However, therapeutic usage of cis-platin and other platinum complex are limited by their undesirable side effects including cardiotoxicity. In this context, we aimed to compare the damage caused in heart by cis-platin and novel platinum-N-heterocyclic carbene (Pt-NHC) complex. For this purpose, 35 Sprague-Dawley rats were divided randomly into five equal groups (n = 7 for each group). Cis-platin and novel Pt-NHC complex were intraperitoneally administered at a single dose of 5 mg/kg and 10 mg/kg and then sacrificed 10 days after this treatment. The heart tissues were taken from all rats for determination of oxidative and myocardial damage. Cis-platin and novel Pt-NHC complex caused oxidative and histological damage in the heart tissue in a dose-dependent manner (p cis-platin caused lower oxidative and histological damage in heart tissue compared to novel Pt-NHC complex. These results suggest that novel Pt-NHC complex is more cardiotoxic than cis-platin. PMID:21075807

Ciftci, Osman; Ozdemir, Ilknur; Vardi, Nigar; Gurbuz, Nevin

2010-11-12

102

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

103

Theoretical study of N-heterocyclic carbenes-catalyzed cascade annulation of benzodienones and enals.  

UK PubMed Central (United Kingdom)

Growing attention in developing new N-heterocyclic carbene (NHC)-mediated reactions involving homoenolate intermediates has prompted our interest in exploring the mechanistic details of the related reactions. In this work, we carried out a detailed theoretical study for the NHC-catalyzed annulation reaction of cinnamaldehyde () and benzodi(enone) () in the presence of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU). By performing density functional theory calculations, we show clearly the detailed reaction mechanism and rationalize the experimental observation. The reaction of and falls into two stages: the formation of homoenolate intermediate and the annulation of homoenolate with . In the homoenolate formation stage, three possible paths are characterized. The pathway involving the DBU-assisted 1,2-proton transfer with a stepwise mechanism is kinetically more favorable, and the DBU-assisted C1 proton departure is the rate-determining step of the total reaction. The annulation of homoenolate with involves four elementary steps. The conformational difference of homoenolate (cis and trans) leads to two slightly different reaction processes. In the total reaction, the process involving cis-conformation of is kinetically more feasible. This can be clearly understood through the frontier molecular orbital analysis and the electronic inductive effect. The calculated results are expected to offer valuable information for further design and development of NHC-mediated reactions. Chirality 25:521-528, 2013. © 2013 Wiley Periodicals, Inc.

Sun H; Fang X; Chi YR; Li G

2013-09-01

104

N-Heterocyclic carbene stabilized adducts of alkyl magnesium amide, bisalkyl magnesium and Grignard reagents: trapping oligomeric organo s-block fragments with NHCs.  

Science.gov (United States)

Developing N-heterocyclic carbene (NHC) chemistry of simple organomagnesium compounds, this study reports the synthesis, X-ray crystallographic, and NMR spectroscopic characterization of three such new carbene complexes. The 1?:?1 alkyl magnesium amide?:?carbene complexes nBuMg(TMP)·IPr 1 and nBuMg(HMDS)·IPr 2 both exist as mononuclear complexes in the crystal but differ in solution as 2 remains intact whereas 1 undergoes a dynamic exchange involving partial decoordination of IPr [TMP is 2,2,6,6-tetramethylpiperidide; IPr is 1,3-bis-(2,6-diisopropylphenyl)imidazol-2-ylidene); HMDS is 1,1,1,3,3,3-hexamethyldisilazide]. Reaction of commercial nBu(2)Mg with IPr surprisingly produced the organoaluminium carbene complex nBu(3)Al·IPr, 3, which also forms a simple mononuclear structure in the crystal. The presence of the Al could be traced to the deliberate addition of a small quantity of Et(3)Al as a stabilizing agent in the commercial nBu(2)Mg reagent. Repeating this reaction with Al-free nBu(2)Mg afforded the hemisolvated carbene complex nBu(8)Mg(4)·2IPr, 4, the stoichiometry of which is dictated by its structure rather than by that used in the initial reaction mixture. The molecular structure of 4 is tetranuclear with a linear chain of 4 Mg centres bridged by nBu ligands and capped at each end by terminal nBu and IPr ligands. Synthesized by treating the Grignard reagent nBuMgCl with IPr, nBuMgCl·IPr, 5, forms a cyclodimer structure with chloro bridges and terminal nBu and IPr ligands. PMID:20733980

Kennedy, Alan R; Mulvey, Robert E; Robertson, Stuart D

2010-08-23

105

N-Heterocyclic carbene stabilized adducts of alkyl magnesium amide, bisalkyl magnesium and Grignard reagents: trapping oligomeric organo s-block fragments with NHCs.  

UK PubMed Central (United Kingdom)

Developing N-heterocyclic carbene (NHC) chemistry of simple organomagnesium compounds, this study reports the synthesis, X-ray crystallographic, and NMR spectroscopic characterization of three such new carbene complexes. The 1?:?1 alkyl magnesium amide?:?carbene complexes nBuMg(TMP)·IPr 1 and nBuMg(HMDS)·IPr 2 both exist as mononuclear complexes in the crystal but differ in solution as 2 remains intact whereas 1 undergoes a dynamic exchange involving partial decoordination of IPr [TMP is 2,2,6,6-tetramethylpiperidide; IPr is 1,3-bis-(2,6-diisopropylphenyl)imidazol-2-ylidene); HMDS is 1,1,1,3,3,3-hexamethyldisilazide]. Reaction of commercial nBu(2)Mg with IPr surprisingly produced the organoaluminium carbene complex nBu(3)Al·IPr, 3, which also forms a simple mononuclear structure in the crystal. The presence of the Al could be traced to the deliberate addition of a small quantity of Et(3)Al as a stabilizing agent in the commercial nBu(2)Mg reagent. Repeating this reaction with Al-free nBu(2)Mg afforded the hemisolvated carbene complex nBu(8)Mg(4)·2IPr, 4, the stoichiometry of which is dictated by its structure rather than by that used in the initial reaction mixture. The molecular structure of 4 is tetranuclear with a linear chain of 4 Mg centres bridged by nBu ligands and capped at each end by terminal nBu and IPr ligands. Synthesized by treating the Grignard reagent nBuMgCl with IPr, nBuMgCl·IPr, 5, forms a cyclodimer structure with chloro bridges and terminal nBu and IPr ligands.

Kennedy AR; Mulvey RE; Robertson SD

2010-10-01

106

N-heterocyclic carbene catalyzed addition of aldehydes to diazo compounds: stereoselective synthesis of N-acylhydrazones.  

UK PubMed Central (United Kingdom)

An innovative stereoselective synthesis of N-acylhydrazones via an unprecedented N-heterocyclic carbene catalyzed addition of aldehydes to diazo compounds is presented. Enals exclusively afforded N-acylhydrazones, in yields up to 91%. The observed regioselectivity was traced back to the reaction of the vinylogous Breslow intermediate via the acyl anion pathway over competing homoenolate, enol, and acyl azolium pathways. This unusual reaction profile was studied based on DFT calculations, which revealed that the reaction is under orbital control, rather than being ruled by charge.

Santos FM; Rosa JN; André V; Duarte MT; Veiros LF; Gois PM

2013-04-01

107

Reductions of aldehydes and ketones with a readily available N-heterocyclic carbene borane and acetic acid.  

UK PubMed Central (United Kingdom)

Acetic acid promotes the reduction of aldehydes and ketones by the readily available N-heterocyclic carbene borane, 1,3-dimethylimidazol-2-ylidene borane. Aldehydes are reduced over 1-24 h at room temperature with 1 equiv of acetic acid and 0.5 equiv of the NHC-borane. Ketone reductions are slower but can be accelerated by using 5 equiv of acetic acid. Aldehydes can be selectively reduced in the presence of ketones. On a small scale, products are isolated by evaporation of the reaction mixture and direct chromatography.

Lamm V; Pan X; Taniguchi T; Curran DP

2013-01-01

108

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

Directory of Open Access Journals (Sweden)

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

2011-01-01

109

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

2013-05-16

110

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

UK PubMed Central (United Kingdom)

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.

Yuan D; Huynh HV

2013-06-01

111

Synthesis of Pd-N-heterocyclic carbene Pd-catalyst and its application in MW-assisted Heck and Suzuki reaction  

Science.gov (United States)

The first Pd-N-heterocyclic carbene (NHC) complex in the form of organic silica is prepared using sol-gel method and its application in Heck and Suzuki reactions are demonstrated. These C-C coupling reactions proceeded efficiently under the influence of microwave irradiation, wit...

112

N-Heterocyclic Carbene Catalyzed [4+3] Annulation of Enals and o-Quinone Methides: Highly Enantioselective Synthesis of Benzo-?-Lactones.  

UK PubMed Central (United Kingdom)

Enantioselectivity through H bonding: An unprecedented [4+3] annulation of enals with o-quinone methides catalyzed by N-heterocyclic carbenes (NHCs) to give benzo-?-lactones is described. High to excellent enantioselectivity was achieved by using a chiral triazolium NHC having a free OH group, which participates in a hydrogen-bonding interaction with the substrate.

Lv H; Jia WQ; Sun LH; Ye S

2013-08-01

113

H2 Cleavage, Hydride Formation, and Catalytic Hydrogenation of Imines with Zinc Complexes of C5 Me5 and N-Heterocyclic Carbenes.  

UK PubMed Central (United Kingdom)

Decamethylzincocene, [ZnCp*2 ], reacts with H2 to give the reduced Zn(I) compound [Zn2 Cp*2 ]. In the presence of coordinating and (more efficiently) of non-coordinating N-heterocyclic carbenes (NHCs), the catalytic hydrogenation of imines with H2 is achieved. The monomeric hydride [Zn(Cp*)(H)(SIMes)] is presented and its mechanistic implications are considered.

Jochmann P; Stephan DW

2013-07-01

114

Pd(II) immobilized on mesoporous silica by N-heterocyclic carbene ionic liquids and catalysis for hydrogenation.  

UK PubMed Central (United Kingdom)

In this work we synthesized Pd(II) immobilized on mesoporous silica by N-heterocyclic carbene (NHC) ionic liquids (ILs) with different alkyl chain lengths. The catalysts were characterized by Fourier transform infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA), low-angle X-ray powder diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and nitrogen sorption. The catalysts were used for the hydrogenation of alkenes and allyl alcohol. The results indicated that the catalysts were very active, selective, and stable. The selectivity for the hydrogenation of allyl alcohol to 1-propanol increased with the increase of the alkyl chain length of the ILs. The effect of supercritical CO(2) (scCO(2)) on the hydrogenation of allyl alcohol was also studied, and it was demonstrated that scCO(2) could enhance the selectivity of the reaction considerably. The XPS study showed that the valence of Pd(II) remained unchanged under hydrogenation conditions.

Liu G; Hou M; Wu T; Jiang T; Fan H; Yang G; Han B

2011-02-01

115

Cross-coupling and dehalogenation reactions catalyzed by (N-heterocyclic carbene)Pd(allyl)Cl complexes.  

Science.gov (United States)

A series of well-defined, air- and moisture-stable (NHC)Pd(allyl)Cl (NHC = N-heterocyclic carbene) complexes has been used in several catalytic reactions: Suzuki-Miyaura cross-coupling, catalytic dehalogenation of aryl halides, and aryl amination. The scope of the three processes using various substrates was examined. A general system involving the use of (IPr)Pd(allyl)Cl as catalyst and NaO(t)Bu as base has proven to be highly active for the Suzuki-Miyaura cross-coupling of activated and unactivated aryl chlorides and bromides, for the catalytic dehalogenation of aryl chlorides, and for the catalytic aryl amination of aryl triflates. All reactions proceed in short reaction times and at mild temperatures. The system has also proven to be compatible with the microwave-assisted Suzuki-Miyaura cross-coupling and catalytic dehalogenation processes, affording yields similar to those of the conventionally heated analogous reactions. PMID:15104459

Navarro, Oscar; Kaur, Harneet; Mahjoor, Parisa; Nolan, Steven P

2004-04-30

116

Insertion of reactive rhodium carbenes into boron-hydrogen bonds of stable N-heterocyclic carbene boranes.  

UK PubMed Central (United Kingdom)

Readily available rhodium(II) salts catalyze reactions between NHC-boranes (NHC-BH3) and diazocarbonyl compounds (N2CRCOR'). Stable ?-NHC-boryl carbonyl compounds (NHC-BH2-CHRCOR') are isolated in good yields. The reaction is a reliable way to make boron-carbon bonds with good tolerance for variation in both the NHC-borane and diazocarbonyl components. It presumably occurs by insertion of a transient rhodium carbene into a boron-hydrogen bond of the NHC-borane. Competitive experiments show that a typical NHC-borane is highly reactive toward rhodium carbenes.

Li X; Curran DP

2013-08-01

117

Insertion of reactive rhodium carbenes into boron-hydrogen bonds of stable N-heterocyclic carbene boranes.  

Science.gov (United States)

Readily available rhodium(II) salts catalyze reactions between NHC-boranes (NHC-BH3) and diazocarbonyl compounds (N2CRCOR'). Stable ?-NHC-boryl carbonyl compounds (NHC-BH2-CHRCOR') are isolated in good yields. The reaction is a reliable way to make boron-carbon bonds with good tolerance for variation in both the NHC-borane and diazocarbonyl components. It presumably occurs by insertion of a transient rhodium carbene into a boron-hydrogen bond of the NHC-borane. Competitive experiments show that a typical NHC-borane is highly reactive toward rhodium carbenes. PMID:23865527

Li, Xiben; Curran, Dennis P

2013-07-31

118

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 (Ecat = -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

2013-09-13

119

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

UK PubMed Central (United Kingdom)

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 (Ecat = -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.

Thoi VS; Kornienko N; Margarit CG; Yang P; Chang CJ

2013-09-01

120

Combining oxidative N-heterocyclic carbene catalysis with click chemistry: a facile one-pot approach to 1,2,3-triazole derivatives.  

Science.gov (United States)

A combination of the oxidative N-heterocyclic carbene catalysis and click chemistry has been explored for the direct, one-pot synthesis of 1,2,3-triazole derivatives from aromatic aldehydes. This procedure was found to be very efficient and a variety of 1,2,3-triazole derivatives could be accessed through their corresponding propargyl esters in moderate-to-good yields under mild conditions. PMID:23606655

Ramanjaneyulu, B T; Reddy, Virsinha; Arde, Panjab; Mahesh, Sriram; Anand, R Vijaya

2013-04-19

 
 
 
 
121

Combining oxidative N-heterocyclic carbene catalysis with click chemistry: a facile one-pot approach to 1,2,3-triazole derivatives.  

UK PubMed Central (United Kingdom)

A combination of the oxidative N-heterocyclic carbene catalysis and click chemistry has been explored for the direct, one-pot synthesis of 1,2,3-triazole derivatives from aromatic aldehydes. This procedure was found to be very efficient and a variety of 1,2,3-triazole derivatives could be accessed through their corresponding propargyl esters in moderate-to-good yields under mild conditions.

Ramanjaneyulu BT; Reddy V; Arde P; Mahesh S; Anand RV

2013-07-01

122

Asymmetric N-heterocyclic carbene catalyzed addition of enals to nitroalkenes: controlling stereochemistry via the homoenolate reactivity pathway to access ?-lactams.  

UK PubMed Central (United Kingdom)

An asymmetric intermolecular reaction between enals and nitroalkenes to yield ?-nitroesters has been developed, catalyzed by a novel chiral N-heterocyclic carbene. Key to this work was the development of a catalyst that favors the ?-nitroester pathway over the established Stetter pathway. The reaction proceeds in high stereoselectivity and affords the previously unreported syn diastereomer. We also report an operationally facile two-step, one-pot procedure for the synthesis of ?-lactams.

White NA; DiRocco DA; Rovis T

2013-06-01

123

Asymmetric N-heterocyclic carbene catalyzed addition of enals to nitroalkenes: controlling stereochemistry via the homoenolate reactivity pathway to access ?-lactams.  

Science.gov (United States)

An asymmetric intermolecular reaction between enals and nitroalkenes to yield ?-nitroesters has been developed, catalyzed by a novel chiral N-heterocyclic carbene. Key to this work was the development of a catalyst that favors the ?-nitroester pathway over the established Stetter pathway. The reaction proceeds in high stereoselectivity and affords the previously unreported syn diastereomer. We also report an operationally facile two-step, one-pot procedure for the synthesis of ?-lactams. PMID:23713683

White, Nicholas A; DiRocco, Daniel A; Rovis, Tomislav

2013-05-28

124

N-heterocyclic carbene (NHC)-stabilized silanechalcogenones: NHC-->Si(R2)=E (E=O, S, Se, Te).  

UK PubMed Central (United Kingdom)

A series of N-heterocyclic carbene-stabilized silanechalcogenones 2 a,b (Si=O), 3 a,b (Si=S), 4 a,b (Si=Se), and 5 a,b (Si=Te) are described. The silanone complexes 2 a,b were prepared by facile oxygenation of the carbene-silylene adducts 1 a,b with N(2)O, whereas their heavier congeners were synthesized by gentle chalcogenation of 1 a,b with equimolar amounts of elemental sulfur, selenium, and tellurium, respectively. These novel compounds have been isolated in a crystalline form in high yields and have been fully characterized by a variety of techniques including IR spectroscopy, ESIMS, and multinuclear NMR spectroscopy. The structures of 2 b, 3 a, 4 a, 4 b, and 5 b have been confirmed by single-crystal X-ray crystallography. Due to the NHC-->Si donor-acceptor electronic interaction, the Si=E (E=O, S, Se, Te) moieties within these compounds are well stabilized and thus the compounds possess several ylide-like resonance structures. Nevertheless, these species also exhibit considerable Si=E double-bond character, presumably through a nonclassical Si=E pi-bonding interaction between the chalcogen lone-pair electrons and two antibonding Si-N sigma* orbitals, as evidenced by their high stretching vibration modes and the shortening of the Si-E distances (between 5.4 and 6.3%) compared with the corresponding Si-E single-bond lengths.

Yao S; Xiong Y; Driess M

2010-01-01

125

N-heterocyclic carbene (NHC)-stabilized silanechalcogenones: NHC-->Si(R2)=E (E=O, S, Se, Te).  

Science.gov (United States)

A series of N-heterocyclic carbene-stabilized silanechalcogenones 2 a,b (Si=O), 3 a,b (Si=S), 4 a,b (Si=Se), and 5 a,b (Si=Te) are described. The silanone complexes 2 a,b were prepared by facile oxygenation of the carbene-silylene adducts 1 a,b with N(2)O, whereas their heavier congeners were synthesized by gentle chalcogenation of 1 a,b with equimolar amounts of elemental sulfur, selenium, and tellurium, respectively. These novel compounds have been isolated in a crystalline form in high yields and have been fully characterized by a variety of techniques including IR spectroscopy, ESIMS, and multinuclear NMR spectroscopy. The structures of 2 b, 3 a, 4 a, 4 b, and 5 b have been confirmed by single-crystal X-ray crystallography. Due to the NHC-->Si donor-acceptor electronic interaction, the Si=E (E=O, S, Se, Te) moieties within these compounds are well stabilized and thus the compounds possess several ylide-like resonance structures. Nevertheless, these species also exhibit considerable Si=E double-bond character, presumably through a nonclassical Si=E pi-bonding interaction between the chalcogen lone-pair electrons and two antibonding Si-N sigma* orbitals, as evidenced by their high stretching vibration modes and the shortening of the Si-E distances (between 5.4 and 6.3%) compared with the corresponding Si-E single-bond lengths. PMID:20013763

Yao, Shenglai; Xiong, Yun; Driess, Matthias

2010-01-25

126

Electrogenerated N-heterocyclic carbene: N-acylation of chiral oxazolidin-2-ones in ionic liquids  

Energy Technology Data Exchange (ETDEWEB)

An electrochemical procedure for the N-acylation of chiral oxazolidin-2-ones, in the absence of volatile molecular organic solvents, has been set up via electrolyses of ionic liquid [bmim]BF{sub 4} containing oxazolidin-2-ones followed by addition of saturated or unsaturated anhydrides. N-acyloxazolidin-2-ones were isolated in good to elevated yields. The electrochemically induced N-acylation of chiral oxazolidin-2-ones occurs with total retention of the absolute configuration of all the chiral atoms. The electrogenerated carbene (1-butyl-3-methyl-1H-imidazol-2-ylidene) has been indicated as the base involved in the deprotonation of chiral oxazolidin-2-ones.

Chiarotto, Isabella [Dipartimento Ingegneria Chimica Materiali Ambiente, Universita ' La Sapienza' , via Castro Laurenziano, 7, 00161 Roma (Italy)], E-mail: isabella.chiarotto@uniroma1.it; Feeney, Michelle M.M.; Feroci, Marta [Dipartimento Ingegneria Chimica Materiali Ambiente, Universita ' La Sapienza' , via Castro Laurenziano, 7, 00161 Roma (Italy); Inesi, Achille [Dipartimento Ingegneria Chimica Materiali Ambiente, Universita ' La Sapienza' , via Castro Laurenziano, 7, 00161 Roma (Italy)], E-mail: achille.inesi@uniroma1.it

2009-02-01

127

Electrogenerated N-heterocyclic carbene: N-acylation of chiral oxazolidin-2-ones in ionic liquids  

International Nuclear Information System (INIS)

An electrochemical procedure for the N-acylation of chiral oxazolidin-2-ones, in the absence of volatile molecular organic solvents, has been set up via electrolyses of ionic liquid [bmim]BF4 containing oxazolidin-2-ones followed by addition of saturated or unsaturated anhydrides. N-acyloxazolidin-2-ones were isolated in good to elevated yields. The electrochemically induced N-acylation of chiral oxazolidin-2-ones occurs with total retention of the absolute configuration of all the chiral atoms. The electrogenerated carbene (1-butyl-3-methyl-1H-imidazol-2-ylidene) has been indicated as the base involved in the deprotonation of chiral oxazolidin-2-ones

2009-02-01

128

Trans [O=Re(V)-OH] core stabilised by chelating N-heterocyclic dicarbene ligands.  

UK PubMed Central (United Kingdom)

Three Re(v) N-heterocyclic carbene complexes [ReO(OH)(L(Me))(2)][PF(6)](1.4)[ReO(4)](0.6), [ReO(OH)(L(iPr))(2)][PF(6)](1.4)[ReO(4)](0.6) and [ReO(OH)(L(Bn))(2)][PF(6)](1.3)[ReO(4)](0.7) (L(Me) = 1,1'-methylene-bis(3-methylimidazole-2-ylidene); L(iPr) = 1,1'-methylene-bis(3-isopropylimidazole-2-ylidene) and L(Bn) = 1,1'-methylene-bis(3-benzylimidazole-2-ylidene)) with trans oxo and hydroxo at axial positions have been synthesized.

Lum R; Zhang H; Zhang W; Bai SQ; Zhao J; Hor TS

2013-01-01

129

N-alkyl functionalised expanded ring N-heterocyclic carbene complexes of rhodium(I) and iridium(I): structural investigations and preliminary catalytic evaluation.  

UK PubMed Central (United Kingdom)

A series of new N-alkyl functionalised 6- and 7-membered expanded ring N-heterocyclic carbene (NHC) pro-ligands 3-6 and their corresponding complexes of rhodium(I) and iridium(I), [M(NHC)(COD)Cl] 7-14 and [M(NHC)(CO)2Cl] 15-22 are described. The complexes have been characterised by (1)H and (13)C{(1)H} NMR, mass spectrometry, IR and X-ray diffraction. It is noted from X-ray diffraction studies that the N-alkyl substituents are found to orientate themselves away from the metal centre due to unfavourable steric interactions resulting in low percent buried volume (%V(bur)) values in the solid state. The heterocycle ring size is also found to dictate the spatial orientation of the N-alkyl substituents in the neopentyl functionalised derivatives 10 and 14. The 7-membered derivative 14 allows for a conformational 'twist' of the heterocycle ring with the N-alkyl substituents adopting a mutually trans configuration with respect to each other, while the more rigid 6-membered system 10 does not allow for this conformational 'twist' and consequently the N-alkyl substituents adopt a mutually cis configuration. The ?-donor function of this new class of expanded ring NHC ligand has also been probed by measured IR stretching frequencies of the [M(NHC)(CO)2Cl] complexes 15-22. A preliminary catalytic survey of the hydrogenation of functionalised alkenes with molecular hydrogen under mild conditions has also been undertaken with complex , affording an insight into the application of large ring NHC ancillary ligands bearing N-alkyl substituents in hydrogenation transformations.

Dunsford JJ; Tromp DS; Cavell KJ; Elsevier CJ; Kariuki BM

2013-05-01

130

Tautomerism-dependent ring construction of N-heterocyclic compounds from the reactions of 1-alkynyl Fischer carbene complexes and substituted pyrazolinones.  

UK PubMed Central (United Kingdom)

Four types of N-heterocyclic ring systems were successfully constructed from the reactions of 1-alkynyl Fischer carbene complexes (OC)5M=C(OEt)C[triple bond]CPh (1) (M = Cr, W) and substituted pyrazolinones (2). Reactions of 1 with 3-methyl-2-pyrazolin-5-one (2a), 3-n-propyl-2-pyrazolin-5-one (2b), 3,4-dimethyl-2-pyrazolin-5-one (2c), 3,4-trimethylene-2-pyrazolin-5-one (2d), or 3,4-tetramethylene-2-pyrazolin-5-one (2e) generated three kinds of Fischer aminocarbene complexes (3-5), and reactions of 1 with phenyl-substituted pyrazolinones, i.e., 3-phenyl-2-pyrazolin-5-one (2f) and its tautomer 3-phenyl-3-pyrazolin-5-one (2g), gave Fischer alkoxycarbene complexes (6) as the major products and aminocarbene complexes of types 3-5 as the minor products. Multiple tautomerism of pyrazolinones is attributed to the versatile formation of N-heterocyclic Fischer carbene complexes. Oxidative demetalation of complexes 3-6 with pyridine N-oxide or m-chloroperoxybenzoic acid efficiently afforded organic carbonyl products, and thus, strongly fluorescent syn-mixed-bimanes were prepared. The present findings constitute an alternative new method to synthesize mixed bimanes and other novel N-heterocyclic compounds.

Zheng Z; Yu Z; Luo N; Han X

2006-12-01

131

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).

1989-01-01

132

ortho-Phenylene bridged palladium bis-N-heterocyclic carbene complexes: synthesis, structure and catalysis.  

UK PubMed Central (United Kingdom)

A series of ortho-phenylene bridged palladium bis-NHC complexes has been synthesized. Complexes with imidazolium and benzimidazolium derived NHCs and methyl-/benzyl-wingtips are reported. Bis(benz)imidazoles with a doubly brominated ortho-phenylene bridge could be obtained by an electrophilic substitution reaction. The structure of the complexes could be confirmed by three solid-state structures. All catalysts have been tested in the catalytic functionalisation of propane. The catalytic activity is highly dependent on the ligand, whereas ligand effects on the regioselectivity (n/iso) are much smaller.

Munz D; Poethig A; Tronnier A; Strassner T

2013-05-01

133

Electronic Structure Trends in N-Heterocyclic Carbenes (NHCs) with Varying Number of Nitrogen Atoms and NHC?Transition-Metal Bond Properties.  

Science.gov (United States)

Carbenes derived from five-membered heterocycles with different numbers of nitrogen atoms ranging from two to four lead formally either to normal N-heterocyclic or mesoionic carbenes with, in some cases, the same skeletal structure. The electronic structures of fourteen of these compounds were examined by means of DFT calculations at the B3LYP/aug-cc-pVTZ level. The examined parameters include the energies of the ?-lone pair at Ccarbene and the ?-HOMO of the protonated form, which are correlated to the first and second proton affinities. The singlet-triplet energy gap was used as a measure of the stability of the N-heterocyclic carbene (NHC) towards dimerisation. Natural population analysis provided insight into the variation of the p? population and the natural charge at Ccarbene with NHC structure. Additionally, the transition metal?NHC bond in L-AuCl and L-TiCl4 and the nature of the orbital interactions between the NHC and the transition-metal fragment were analysed in detail by the extended transition state-natural orbitals for chemical valence (ETS-NOCV) approach at the BP86/TZ2P level. Similarities and differences between the NHC?gold and the NHC?titanium bond are discussed, and trends in key bonding properties can be traced back to the variation of the electronic parameters of the NHC. PMID:23955586

Bernhammer, Jan C; Frison, Gilles; Huynh, Han Vinh

2013-08-16

134

Unveiling the stereoelectronic properties of a triphenylene-based tris N-heterocyclic carbene.  

UK PubMed Central (United Kingdom)

Two rhodium complexes with a triphenylene-based tris-NHC have been fully characterized. DFT and electrochemical studies suggest that the electronic communication between the metals is very weak, and that the electron donating properties of the ligand are very similar to those shown by its benzimidazolylidene analogue.

Gonell S; Alabau RG; Poyatos M; Peris E

2013-08-01

135

Synthesis of Silver (I) Complexes of Iminoalkyl Functionalised N-Heterocyclic Carbenes  

Directory of Open Access Journals (Sweden)

Full Text Available A range of silver iminoalkyl imidazol-2-ylidene complexes have been isolated in good yield (50%–85%) and characterised by 1H and 13C NMR spectroscopy. A single crystal X-ray diffraction structure determination of 1-{2-(benzylhydrylidene-amino)-ethyl}-3-benzyl imidazol-2-ylidene silver bromide indicated monodentate coordination of the ligand.

Neil A. Williams; Mahboub Merzouk; Peter B. Hitchcock

2010-01-01

136

When phosphorus and NHC (N-heterocyclic carbene) meet each other.  

UK PubMed Central (United Kingdom)

Since the discovery of the first air stable NHC, this class of compounds has been intensively studied and applied in organometallic chemistry and homogeneous catalysis due to the stability brought to the metal centre. Alternatively, phosphorus based family ligands have already a very long history in these fields of chemistry. It was naturally obvious that the NHC and the phosphorus ligands had to be mixed for the best of organometallic chemistry and homogeneous catalysis. This perspective will highlight the new synthetic routes for NHC bearing a phosphorus moiety such as NHC-phosphenium salt, N-phosphorylated-imidazolium salt, 4-phosphino or 4,5-diphosphino-imidazolium salt and tethered phosphino-imidazolium salt. Then, the recent applications in organometallic chemistry and homogeneous catalysis will be presented.

Gaillard S; Renaud JL

2013-05-01

137

Synthesis, electronic structure, and magnetism of [ni(6-mes)2](+): a two-coordinate nickel(i) complex stabilized by bulky N-heterocyclic carbenes.  

Science.gov (United States)

The two-coordinate cationic Ni(I) bis-N-heterocyclic carbene complex [Ni(6-Mes)2]Br (1) [6-Mes =1,3-bis(2,4,6-trimethylphenyl)-3,4,5,6-tetrahydropyrimidin-2-ylidene] has been structurally characterized and displays a highly linear geometry with a C-Ni-C angle of 179.27(13)°. Density functional theory calculations revealed that the five occupied metal-based orbitals are split in an approximate 2:1:2 pattern. Significant magnetic anisotropy results from this orbital degeneracy, leading to single-ion magnet (SIM) behavior. PMID:23971827

Poulten, Rebecca C; Page, Michael J; Algarra, Andrés G; Le Roy, Jennifer J; López, Isidoro; Carter, Emma; Llobet, Antoni; Macgregor, Stuart A; Mahon, Mary F; Murphy, Damien M; Murugesu, Muralee; Whittlesey, Michael K

2013-09-04

138

Synthesis, electronic structure, and magnetism of [ni(6-mes)2](+): a two-coordinate nickel(i) complex stabilized by bulky N-heterocyclic carbenes.  

UK PubMed Central (United Kingdom)

The two-coordinate cationic Ni(I) bis-N-heterocyclic carbene complex [Ni(6-Mes)2]Br (1) [6-Mes =1,3-bis(2,4,6-trimethylphenyl)-3,4,5,6-tetrahydropyrimidin-2-ylidene] has been structurally characterized and displays a highly linear geometry with a C-Ni-C angle of 179.27(13)°. Density functional theory calculations revealed that the five occupied metal-based orbitals are split in an approximate 2:1:2 pattern. Significant magnetic anisotropy results from this orbital degeneracy, leading to single-ion magnet (SIM) behavior.

Poulten RC; Page MJ; Algarra AG; Le Roy JJ; López I; Carter E; Llobet A; Macgregor SA; Mahon MF; Murphy DM; Murugesu M; Whittlesey MK

2013-09-01

139

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

140

A new class of remote N-heterocyclic carbenes with exceptionally strong ?-donor properties: introducing benzo[c]quinolin-6-ylidene.  

UK PubMed Central (United Kingdom)

We make the case for benzo[c]quinolin-6-ylidene (1) as a strongly electron-donating carbene ligand. The facile synthesis of 6-trifluoromethanesulfonylbenzo[c]quinolizinium trifluoromethanesulfonate (2) gives straightforward access to a useful precursor for oxidative addition to low-valent metals, to yield the desired carbene complexes. This concept has been achieved in the case of [Mn(benzo[c]quinolin-6-ylidene)(CO)5](+) (15) and [Pd(benzo[c]quinolin-6-ylidene)(PPh3)2(L)](2+) L = THF (21), OTf (22) or pyridine (23). Attempts to coordinate to nickel result in coupling products from two carbene precursor fragments. The CO IR-stretching-frequency data for the manganese compound suggests benzo[c]quinolin-6-ylidene is at least as strong a donor as any heteroatom-stabilised carbene ligand reported.

Mayer UF; Murphy E; Haddow MF; Green M; Alder RW; Wass DF

2013-03-01

 
 
 
 
141

Palladium complexes of abnormal N-heterocyclic carbenes as precatalysts for the much preferred Cu-free and amine-free Sonogashira coupling in air in a mixed-aqueous medium.  

UK PubMed Central (United Kingdom)

A series of new PEPPSI (Pyridine Enhanced Precatalyst Preparation Stabilization and Initiation) themed precatalysts of abnormal N-heterocyclic carbenes for the highly desirable Cu-free and amine-free Sonogashira coupling in air in a mixed-aqueous medium is reported. Specifically, the PEPPSI themed (NHC)PdI2(pyridine) type precatalysts, 1b-4b, efficiently carried out the highly convenient Cu-free and amine-free Sonogashira coupling of aryl bromides and iodides with terminal acetylenes in air in a mixed aqueous medium. Complexes, 1b-4b, were synthesized by the direct reaction of the corresponding imidazo[1,2-a]pyridinium iodide salts, 1a-4a, with PdCl2 in pyridine in the presence of K2CO3 as a base while the imidazo[1,2-a]pyridinium iodide salts, 1a-4a, were in turn synthesized by the alkylation reactions of the respective imidazo[1,2-a]pyridine derivatives with alkyl iodides. The density functional theory (DFT) studies revealed that these imidazol-3-ylidene[1,2-a]pyridine derived abnormal carbenes are strongly sigma-donating and consequently significantly weaken the catalytically important labile trans pyridine ligand in 1b-4b.

John A; Shaikh MM; Ghosh P

2009-12-01

142

Anionic and zwitterionic copper(I) complexes incorporating an anionic N-heterocyclic carbene decorated with a malonate backbone: synthesis, structure and catalytic applications.  

UK PubMed Central (United Kingdom)

The anionic malonate-derived N-heterocyclic carbenes (maloNHCs) react cleanly and rapidly with copper chloride to generate the anionic complexes of type [(maloNHC)CuCl]·Li, which crystallize in the solid state either in an oligomeric trimer arrangement or in polymeric helixes depending on the substitution pattern and the solvent. Ten zwitterionic heteroleptic Cu(I) complexes combining the anionic maloNHC and a neutral imidazol-2-ylidene are also obtained in a very selective manner and fully characterized. Whereas the anionic complexes are relatively active catalysts for the hydrosilylation of carbonyl compounds, the zwitterionic complexes reveal to be efficient and extremely robust pre-catalysts for the intramolecular cyclopropanation reaction of a diazo ester and outperform the corresponding cationic Cu(i) complexes with classical imidazol-2-ylidenes.

César V; Barthes C; Farré YC; Cuisiat SV; Vacher BY; Brousses R; Lugan N; Lavigne G

2013-05-01

143

Palladium carbene complexes for selective alkene di- and oligomerization  

Digital Repository Infrastructure Vision for European Research (DRIVER)

A series of palladium complexes were synthesized that comprise three sterically different C,N-bidentate coordinating NHC-pyridine ligands (NHC = N-heterocyclic carbene). In one set, the pyridine and the carbene are linked by a flexible CH2 group (a), in the other two sets, the two ligand units are d...

Khlebnikov, Vsevolod; Meduri, Angelo; Müller-Bunz, Helge; Montini, Tiziano; Fornasiero, Paolo; Zangrando, Ennio

144

Bromide ion binding by a dinuclear gold(I) N-heterocyclic carbene complex: a spectrofluorescence and X-ray absorption spectroscopic study.  

UK PubMed Central (United Kingdom)

Fluorescence and X-ray absorption spectroscopy were used to investigate the anion binding properties of a luminescent, dinuclear Au(I) N-heterocyclic carbene (NHC) complex ([1](2+)) with a short Au(I)···Au(I) contact. The addition of Br(-) ions to a DMSO solution of [1](PF(6))(2) caused a red-shift in the fluorescence emission band from 396 nm to 496 nm. Similarly, the addition of Br(-) ions to [1](PF(6))(2) caused a decrease in the energy of the Au L(3)-edge in the X-ray absorption spectrum, consistent with the formation of an association complex between the cation [1](2+) and Br(-) ions. Solution-based structural studies of the association complex were carried out using extended X-ray absorption fine structure (EXAFS) modelling of the Au(I)···Au(I) core of the cation. These studies indicate that the association complex results from Au(I)···Br(-) interactions, with the Br(-) ions occupying two partially occupied sites at ~2.9 and 3.9 Å from the Au(I) atoms.

Wedlock LE; Aitken JB; Berners-Price SJ; Barnard PJ

2013-01-01

145

N-Heterocyclic carbene-mediated zwitterionic polymerization of N-substituted N-carboxyanhydrides toward poly(?-peptoid)s: kinetic, mechanism, and architectural control.  

UK PubMed Central (United Kingdom)

N-Heterocyclic carbene (NHC)-mediated polymerizations of N-butyl N-carboxyanhydride (Bu-NCA) to produce cyclic poly(N-butyl glycine)s (c-NHC-PNBGs) have been investigated in various solvents with NHCs having differing steric and electronic properties. Control over the polymer molecular weight (MW) and polymerization rate is strongly dependent on the solvent and the NHC structure. Kinetic studies reveal that the propagating intermediates for the polymerization in low dielectric solvents (e.g., THF or toluene) maintain cyclic architectures with two chain ends in close contact through Coulombic interaction. The NHCs not only initiate the polymerization, but also mediate the chain propagation as intramolecular counterions. Side reactions are significantly suppressed in low dielectric solvents due to the reduced basicity and nucleophilicity of the negatively charged chain ends of the zwitterions, resulting in quasi-living polymerization behavior. By contrast, the two charged chain ends of the zwitterionic species are fully dissociated in high dielectric solvents. The chain propagation proceeds as in conventional anionic polymerizations, wherein side reactions (e.g., transamidation) compete with chain propagation, resulting in significantly diminished control over polymer MW. The cyclic zwitterionic propagating species can be converted into their linear polymeric analogues (l-NHC-PNBGs) by end-capping with electrophiles (e.g., acetyl chloride) or the NHC-free cyclic analogues (c-PNBGs) by treatment with NaN(TMS)(2), as evidenced by MALDI-TOF MS, NMR, and SEC analysis.

Guo L; Lahasky SH; Ghale K; Zhang D

2012-06-01

146

A highly active bifunctional iridium complex with an alcohol/alkoxide-tethered N-heterocyclic carbene for alkylation of amines with alcohols.  

Science.gov (United States)

A series of new iridium(III) complexes containing bidentate N-heterocyclic carbenes (NHC) functionalized with an alcohol or ether group (NHC-OR, R = H, Me) were prepared. The complexes catalyzed the alkylation of anilines with alcohols as latent electrophiles. In particular, biscationic Ir(III) complexes of the type [Cp*(NHC-OH)Ir(MeCN)](2+)2[BF(4)(-)] afforded higher-order amine products with very high efficiency; up to >99% yield using a 1:1 ratio of reactants and 1-2.5?mol?% of Ir, in short reaction times (2-16?h) and under base-free conditions. Quantitative yields were also obtained at 50?°C, although longer reaction times (48-60?h) were needed. A large variety of aromatic amines have been alkylated with primary and secondary alcohols. The reactivity of structurally related iridium(III) complexes was also compared to obtain insights into the mechanism and into the structure of possible catalytic intermediates. The Ir(III) complexes were stable towards oxygen and moisture, and were characterized by NMR, HRMS, single-crystal X-ray diffraction, and elemental analyses. PMID:23008192

Bartoszewicz, Agnieszka; Marcos, Rocío; Sahoo, Suman; Inge, A Ken; Zou, Xiaodong; Martín-Matute, Belén

2012-09-24

147

Mono-, di-, and trinuclear luminescent silver(I) and gold(I) N-heterocyclic carbene complexes derived from the picolyl-substituted methylimidazolium salt: 1-methyl-3-(2-pyridinylmethyl)-1H-imidazolium tetrafluoroborate.  

UK PubMed Central (United Kingdom)

The N-heterocyclic carbene (NHC) precursor, 1-methyl-3-(2-pyridinylmethyl)-1H-imidazolium tetrafluoroborate, [HCH3im(CH2py)]BF4, reacted with AgBF4 in the presence of aqueous NaOH to produce the silver complex [Ag(CH3im(CH2py))2]BF4 (1) which was then reacted with Au(tht)Cl to form the corresponding gold(I) complex, [Au(CH3im(CH2py))2]BF4 (2). Complex 2 reacted with 1 equiv of AgBF4 to produce the mixed-metal species [AuAg(CH3im(CH2py))2](BF4)2 (3). The reaction of 2 with 1 equiv of Au(tht)Cl followed by metathesis with NaBF4 produces the dimetallic gold complex [Au2(CH3im(CH2py))2](BF4)2 (4). The reaction of [Ag(CH3im(CH2py))2]BF4 (1) with 1 equiv of AgBF4 produces the trinuclear [Ag3(CH3im(CH2py))3(NCCH3)2](BF4)3 (5) complex, which appears to dissociate into a dimetallic complex in solution. Complexes 1-5 were characterized by 1H NMR, 13C NMR, UV-vis, luminescence spectroscopy, elemental analysis, mass spectrometry, and X-ray crystallography. The CH3im(CH2py) ligands in 3 are arranged in a head-to-head fashion spanning a Au-Ag separation of 3.0318(5) A with the carbene portion of the ligand remaining coordinated to the Au(I) center. In 4, the ligands are arranged in a head-to-tail fashion with an Au-Au separation of 3.1730(5) A. In 5, the ligands bridge the nearly symmetrical Ag3 triangular core with short Ag-Ag separations of 2.7765(8), 2.7832(8), and 2.7598(8) A. All of these complexes, including the ligand precursor, are intensely luminescent in solution and the solid state.

Catalano VJ; Moore AL

2005-09-01

148

Mono-, di-, and trinuclear luminescent silver(I) and gold(I) N-heterocyclic carbene complexes derived from the picolyl-substituted methylimidazolium salt: 1-methyl-3-(2-pyridinylmethyl)-1H-imidazolium tetrafluoroborate.  

Science.gov (United States)

The N-heterocyclic carbene (NHC) precursor, 1-methyl-3-(2-pyridinylmethyl)-1H-imidazolium tetrafluoroborate, [HCH3im(CH2py)]BF4, reacted with AgBF4 in the presence of aqueous NaOH to produce the silver complex [Ag(CH3im(CH2py))2]BF4 (1) which was then reacted with Au(tht)Cl to form the corresponding gold(I) complex, [Au(CH3im(CH2py))2]BF4 (2). Complex 2 reacted with 1 equiv of AgBF4 to produce the mixed-metal species [AuAg(CH3im(CH2py))2](BF4)2 (3). The reaction of 2 with 1 equiv of Au(tht)Cl followed by metathesis with NaBF4 produces the dimetallic gold complex [Au2(CH3im(CH2py))2](BF4)2 (4). The reaction of [Ag(CH3im(CH2py))2]BF4 (1) with 1 equiv of AgBF4 produces the trinuclear [Ag3(CH3im(CH2py))3(NCCH3)2](BF4)3 (5) complex, which appears to dissociate into a dimetallic complex in solution. Complexes 1-5 were characterized by 1H NMR, 13C NMR, UV-vis, luminescence spectroscopy, elemental analysis, mass spectrometry, and X-ray crystallography. The CH3im(CH2py) ligands in 3 are arranged in a head-to-head fashion spanning a Au-Ag separation of 3.0318(5) A with the carbene portion of the ligand remaining coordinated to the Au(I) center. In 4, the ligands are arranged in a head-to-tail fashion with an Au-Au separation of 3.1730(5) A. In 5, the ligands bridge the nearly symmetrical Ag3 triangular core with short Ag-Ag separations of 2.7765(8), 2.7832(8), and 2.7598(8) A. All of these complexes, including the ligand precursor, are intensely luminescent in solution and the solid state. PMID:16156614

Catalano, Vincent J; Moore, Adam L

2005-09-19

149

Highly emissive platinum(II) complexes bearing carbene and cyclometalated ligands.  

UK PubMed Central (United Kingdom)

A series of heteroleptic platinum(II) complexes (1a-3a and 1b-3b) bearing a dicarbene ligand and a cyclometalated bidentate ligand have been prepared. Two of the complexes (1a and 3a) were characterized by X-ray diffraction analyses, confirming the square-planar structures. Detailed spectroscopic properties of the complexes have been studied, and two complexes, 3a and 3b bearing a carbene chelate, which has a trimethylene group between the two N-heterocyclic carbene ligating groups, show intense luminescence (quantum yields >50%) in fluid solution at room temperature. Non-radiative rate constants of the emissive excited states of the complexes vary markedly among the 6 complexes, although radiative constants of them lie in a narrow range of values, so that the difference in the photophysical properties is ascribed to the difference in the non-radiative decay rates. The photophysical parameters have been discussed with the results of the quantum mechanical calculations. The DFT and TDDFT calculations show that the emissive excited states have mainly ligand-centered character with a slight contribution of MLCT.

Uesugi H; Tsukuda T; Takao K; Tsubomura T

2013-05-01

150

Multimetallic complexes featuring a bridging N-heterocyclic phosphido/phosphenium ligand: synthesis, structure, and theoretical investigation.  

UK PubMed Central (United Kingdom)

By incorporating an N-heterocyclic phosphenium/phosphide (NHP) ligand into a chelating pincer ligand framework (PPP(+)/PPP(-)), we have elucidated several different and unprecedented binding modes of NHP ligands in homobimetallic, heterobimetallic, and trimetallic metal complexes. One-electron reduction of the previously reported (PPP)(-)/M(II) complexes (PPP)M-Cl (M = Pd (1), Pt (2)) results in clean formation of the symmetric homobimetallic M(I)/M(I) complexes [(?-PPP)Pd]2 (5) and [(?-PPP)Pt]2 (6). The tridentate NHP ligand has also been utilized as a bridging linker in the M/Co heterobimetallic compounds (OC)3Co(u-PPP)M(CO) (M = Pd (7), Pt (8)), synthesized via salt elimination from mixtures of 1 and 2 and Na[Co(CO)4]. Furthermore, an NHP-bridged trimetallic complex (PPP)2Pd3Cl2 (9) can be synthesized in a manner similar to precursor 1 (Pd(PPh3)4 + (PPP)Cl) via careful adjustment of reaction stoichiometry. Examination of the interatomic distances and angles in complexes 5-9, in tandem with density functional theory calculations have been used to evaluate and characterize the bonding interactions in these complexes.

Pan B; Evers-McGregor DA; Bezpalko MW; Foxman BM; Thomas CM

2013-08-01

151

Multimetallic complexes featuring a bridging N-heterocyclic phosphido/phosphenium ligand: synthesis, structure, and theoretical investigation.  

Science.gov (United States)

By incorporating an N-heterocyclic phosphenium/phosphide (NHP) ligand into a chelating pincer ligand framework (PPP(+)/PPP(-)), we have elucidated several different and unprecedented binding modes of NHP ligands in homobimetallic, heterobimetallic, and trimetallic metal complexes. One-electron reduction of the previously reported (PPP)(-)/M(II) complexes (PPP)M-Cl (M = Pd (1), Pt (2)) results in clean formation of the symmetric homobimetallic M(I)/M(I) complexes [(?-PPP)Pd]2 (5) and [(?-PPP)Pt]2 (6). The tridentate NHP ligand has also been utilized as a bridging linker in the M/Co heterobimetallic compounds (OC)3Co(u-PPP)M(CO) (M = Pd (7), Pt (8)), synthesized via salt elimination from mixtures of 1 and 2 and Na[Co(CO)4]. Furthermore, an NHP-bridged trimetallic complex (PPP)2Pd3Cl2 (9) can be synthesized in a manner similar to precursor 1 (Pd(PPh3)4 + (PPP)Cl) via careful adjustment of reaction stoichiometry. Examination of the interatomic distances and angles in complexes 5-9, in tandem with density functional theory calculations have been used to evaluate and characterize the bonding interactions in these complexes. PMID:23923983

Pan, Baofei; Evers-McGregor, Deirdra A; Bezpalko, Mark W; Foxman, Bruce M; Thomas, Christine M

2013-08-07

152

Syntheses, characterizations and photoluminescent properties of two novel coordination polymers constructed by poly-carboxylate and N-heterocyclic ligands  

Science.gov (United States)

Two novel coordination compounds constructed from aromatic acid and N-heterocyclic ligands, namely, [Mn(dipt)2(n-BDC)]n (1) [dipt = 2-(2,4-dichlorophenyl)-1H-imidazo[4,5-f][1,10]phenanthroline, n-BDC = 5-nitrobenzene-1,3-dioic acid] and [Cu2(bip)2(m-BDC)2(H2O)3]?2H2O (2) [bip = 2-(4-bromophenyl)-1H-imidazo[4,5-f][1,10]phenanthroline, m-BDC = isophthalic acid] have been synthesized by hydrothermal reaction: compounds 1 and 2 were characterized by elemental analysis, infrared spectrum (IR), single crystal X-ray diffraction and thermogravimetric analysis (TGA). In 1, n-BDC anions link the adjacent Mn(II) centers to generate a one-dimensional (1D) zigzag chain. Furthermore, unprecedented intersections of C-H\\ctdot O bonds lead 1D chain into 2D sheet supra-molecular architecture. Differed from 1, 2 exhibits 0D structure, and displays a 2D topology via strong H-bond and ?-? stacking interactions. TG analysis shows clear courses of weight loss, which corresponds to the decomposition of different ligands. The structure-related solid-state fluorescence spectra of compounds 1 and 2 have been determined, and the result displays that compounds 1 and 2 are potential luminescent material.

Yan, Li; Li, Chuanbi; Wang, Yifei

2013-03-01

153

Luminescent square-planar platinum(II) complexes with tridentate 3-bis(2-pyridylimino)isoindoline and monodentate N-heterocyclic ligands.  

UK PubMed Central (United Kingdom)

A series of platinum(II) complexes with 1,3-bis(2-pyridylimino)isoindoline (BPI) derivatives were prepared by substitution of the coordinated Cl in the precursor complex Pt(BPI)Cl with a N-heterocyclic ligand such as pyridine, phthalazine or phenanthridine. These complexes display orange to red luminescence in fluid dichloromethane solutions and in the solid states at room temperature. The photophysical properties were tuned by introducing electron-withdrawing -NO(2) or electron-donating -NH(2) to the BPI ligand. The DFT computational studies suggest that the emission in the N-heterocyclic ligand substituted platinum(II) complexes originates mainly from the (3)[???*(BPI)] (3)IL triplet excited state, mixed with some (3)[d?(Pt)??*(BPI)] (3)MLCT character. Compared with the precursor Pt(BPI)Cl, both the low-energy absorption and the emission in the N-heterocyclic ligand substituted platinum(II) complexes exhibits a distinct blue-shift due to an obviously enhanced contribution from the (3)IL state and a reduced (3)MLCT character.

Wen HM; Wu YH; Xu LJ; Zhang LY; Chen CN; Chen ZN

2011-07-01

154

Luminescent square-planar platinum(II) complexes with tridentate 3-bis(2-pyridylimino)isoindoline and monodentate N-heterocyclic ligands.  

Science.gov (United States)

A series of platinum(II) complexes with 1,3-bis(2-pyridylimino)isoindoline (BPI) derivatives were prepared by substitution of the coordinated Cl in the precursor complex Pt(BPI)Cl with a N-heterocyclic ligand such as pyridine, phthalazine or phenanthridine. These complexes display orange to red luminescence in fluid dichloromethane solutions and in the solid states at room temperature. The photophysical properties were tuned by introducing electron-withdrawing -NO(2) or electron-donating -NH(2) to the BPI ligand. The DFT computational studies suggest that the emission in the N-heterocyclic ligand substituted platinum(II) complexes originates mainly from the (3)[???*(BPI)] (3)IL triplet excited state, mixed with some (3)[d?(Pt)??*(BPI)] (3)MLCT character. Compared with the precursor Pt(BPI)Cl, both the low-energy absorption and the emission in the N-heterocyclic ligand substituted platinum(II) complexes exhibits a distinct blue-shift due to an obviously enhanced contribution from the (3)IL state and a reduced (3)MLCT character. PMID:21643611

Wen, Hui-Min; Wu, Yu-Hui; Xu, Liang-Jin; Zhang, Li-Yi; Chen, Chang-Neng; Chen, Zhong-Ning

2011-06-06

155

Anchoring of Silyl Donors on a N-Heterocyclic Carbene through the Cobalt-Mediated Silylation of Benzylic C?H Bonds.  

UK PubMed Central (United Kingdom)

Pep up your carbene catalyst: A sequential cobalt-mediated C?H activation and silylation protocol has been developed for the preparation of novel silyl-donor-functionalized NHC complexes (see scheme; Mes=mesityl; R=H, Me, Ph). The resulting cobalt complexes exhibited high activity and selectivity as catalysts for the hydrosilylation of olefins.

Mo Z; Liu Y; Deng L

2013-08-01

156

Facile Thermal W-W Bond Homolysis in the N-Heterocyclic Carbene-Containing Tungsten Dimer [CpW(CO)2(IMe)]2  

Energy Technology Data Exchange (ETDEWEB)

The thermal W-W bond homolysis in [CpW(CO)2(IMe)]2 (IMe = 1,3-dimethylimidazol-2-ylidene) was investigated and was found to occur to a large extent compared to other tungsten dimers such as [CpW(CO)3]2. CpW(CO)2(IMe)H was prepared by heating a solution of [IMeH]+[CpW(CO)2(PMe3)]?, and exists in solution as a mixture of interconverting cis and trans isomers. The carbene rotation in CpW(CO)2(IMe)H was explored by DFT calculations, and low enthalpic barriers (< 3.5 kcal mol?1) are predicted. CpW(CO)2(IMe)H has pKaMeCN = 31.5(3) and deprotonation with KH gives K+[CpW(CO)2(IMe)]? (• MeCN). Hydride abstraction from CpW(CO)2(IMe)H with Ph3C+PF6? in the presence of a coordinating ligand L (MeCN or THF) gives [CpW(CO)2(IMe)(L)]+PF6?. Electrochemical measurements on the anion [CpW(CO)2(IMe)]? in MeCN, together with digital simulations, give an E1/2 of ?1.54(2) V vs Cp2Fe+/0 for the [CpW(CO)2(IMe)]•/? couple. A thermochemical cycle provides the solution bond dissociation free energy of the W-H bond of CpW(CO)2(IMe)H as 61.3(6) kcal mol?1. In the electrochemical oxidation of [CpW(CO)2(IMe)]?, reversible dimerization of the electrogenerated radical CpW(CO)2(IMe)• occurs, and digital simulation provides kinetic and thermodynamic parameters for the monomer-dimer equilibrium: kdimerization ~ 2.5 ? 104 M?1 s?1, khomolysis ~ 0.5 s?1 (i.e., Kdim ~ 5 ? 104 M?1). Reduction of [CpW(CO)2(IMe)(MeCN)]+PF6? with cobaltocene gives the dimer [CpW(CO)2(IMe)]2, which in solution exists as a mixture of anti and gauche rotomers. As expected from the electrochemical experiments, the dimer is in equilibrium with detectable amounts of CpW(CO)2(IMe)•. This species was observed by IR spectroscopy, and its presence in solution is also in accordance with the observed reactivity toward 2,6-di-tert-butyl-1,4-benzoquinone, chloroform and dihydrogen. This work was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Biosciences and Geosciences. Pacific Northwest National Laboratory is a multiprogram national laboratory operated for DOE by Battelle. The EPR studies were performed at EMSL, a national scientific user facility sponsored by the Department of Energy’s Office of Biological and Environmental Research located at Pacific Northwest National Laboratory.

van der Eide, Edwin F.; Liu, Tianbiao L.; Camaioni, Donald M.; Walter, Eric D.; Bullock, R. Morris

2012-03-05

157

Electronic bond tuning with heterocyclic carbenes.  

UK PubMed Central (United Kingdom)

We discuss the impact of the nature of the heterocyclic carbene ring, when used as a complex forming ligand, on the relative stability of key intermediates in three typical Ru, Pd and Au promoted reactions. Results show that P-heterocyclic carbenes have a propensity to increase the bonding of the labile ligand and of the substrate in Ru-promoted olefin metathesis, whereas negligible impact is expected on the stability of the ruthenacycle intermediate. In the case of Pd cross-coupling reactions, dissociation of a P-heterocyclic carbene is easier than dissociation of the N-heterocyclic analogue. In the case of the Au-OH synthon, the Au-OH bond is weakened with the P-heterocyclic carbene ligands. A detailed energy decomposition analysis is performed to rationalize these results.

Falivene L; Caporaso L; Cavallo L; Jacobsen H

2013-05-01

158

Metal-Free Catalytic Enantioselective C-B Bond Formation: (Pinacolato)boron Conjugate Additions to ?,?-Unsaturated Ketones, Esters, Weinreb Amides and Aldehydes Promoted by Chiral N-Heterocyclic Carbenes  

Science.gov (United States)

The first broadly applicable metal-free enantioselective method for boron conjugate addition (BCA) to ?,?-unsaturated carbonyls is presented. The C–B bond forming reactions are promoted in the presence of 2.5–7.5 mol % of a readily accessible C1-symmetric chiral imidazolinium salt, which is converted, in situ, to the catalytically active diastereo- and enantiomerically pure N-heterocyclic carbene (NHC) by the common organic base 1,8-diazabicyclo[5.4.0]undec-7-ene (dbu). In addition to the commercially available bis(pinacolato)diboron [B2(pin)2], and in contrast to reactions with the less sterically demanding achiral NHCs, the presence of MeOH is required for high efficiency. Acyclic and cyclic ?,?-unsaturated ketones, as well as acyclic esters, Weinreb amides and aldehydes can serve as suitable substrates; the desired ?-boryl carbonyls are isolated in up to 94% yield and >98:2 enantiomer ratio (er). Transformations are often carried out at ambient temperature. In certain cases, such as when the relatively less reactive unsaturated amides are used, elevated temperatures are required (50–66 °C); nonetheless, reactions remain highly enantioselective. The utility of the NHC-catalyzed method is demonstrated through comparison with the alternative Cu-catalyzed protocols; in cases involving a polyfunctional substrate, unique profiles in chemoselectivity are exhibited by the metal-free approach (e.g., conjugate addition vs reaction with an alkyne, allene or aldehyde).

Wu, Hao; Radomkit, Suttipol; O'Brien, Jeannette M.; Hoveyda, Amir H.

2012-01-01

159

Gold(I)-Phosphine-N-Heterocycles: Biological Activity and Specific (Ligand) Interactions on the C-Terminal HIVNCp7 Zinc Finger.  

UK PubMed Central (United Kingdom)

The syntheses and the characterization by chemical analysis, (1)H and (31)P NMR spectroscopy, and mass spectrometry of a series of linear triphenylphosphine gold(I) complexes with substituted N-heterocycle ligands (L), [(PPh3)Au(I)(L)](+), is reported. The reaction of [(PPh3)Au(L)](+) (L = Cl(-) or substituted N- heterocyclic pyridine) with the C-terminal (Cys3His) finger of HIVNCp7 shows evidence by mass spectrometry (ESI-MS) and (31)P NMR spectroscopy of a long-lived {(PPh3)Au}-S-peptide species resulting from displacement of the chloride or pyridine ligand by zinc-bound cysteine with concomitant displacement of Zn(2+). In contrast, reactions with the Cys2His2 finger-3 of the Sp1 transcription factor shows significantly reduced intensities of {(PPh3)Au} adducts. The results suggest the possibility of systematic (electronic, steric) variations of "carrier" group PR3 and "leaving" group L as well as the nature of the zinc finger in modulation of biological activity. The cytotoxicity, cell cycle signaling effects, and cellular accumulation of the series are also reported. All compounds display cytotoxicity in the micromolar range upon 96 h continuous exposure to human tumor cells. The results may have relevance for the reported inhibition of viral load in simian virus by the gold(I) drug auranofin.

Abbehausen C; Peterson EJ; de Paiva RE; Corbi PP; Formiga AL; Qu Y; Farrell NP

2013-09-01

160

Easy abstraction of a hydride anion from an alkyl C-H bond of a coordinated bis(N-heterocyclic carbene).  

UK PubMed Central (United Kingdom)

The high basicity of a trimethylene-linked bis(NHC), acting as a chelating ligand in a ruthenium(0) complex, is responsible for its involvement in a room-temperature reaction in which the metal atom to which this bis(NHC) ligand is coordinated replaces a hydride anion of the ligand trimethylene linker, which can be taken by a hydride abstractor as unusual, in that role, as [Ru3(CO)12].

Cabeza JA; Damonte M; García-Álvarez P; Pérez-Carreño E

2013-04-01

 
 
 
 
161

Ruthenium catalysts for hydrogenation of aromatic and aliphatic esters: make use of bidentate carbene ligands.  

UK PubMed Central (United Kingdom)

Committed carbenes: The convenient application of bidentate carbene ligands is described for the hydrogenation of carboxylic acid esters. The ligand precursors are easily synthesized through the dimerization of N-substituted imidazoles with diiodomethane. The catalyst is generated in situ and exhibits good activity and functional group tolerance for the hydrogenation of aromatic and aliphatic carboxylic acid esters.

Westerhaus FA; Wendt B; Dumrath A; Wienhöfer G; Junge K; Beller M

2013-06-01

162

A C3v-symmetrical tribenzotriquinacene-based threefold N-heterocyclic carbene. Coordination to rhodium(i) and stereoelectronic properties.  

UK PubMed Central (United Kingdom)

A novel tribenzotriquinacene-based tris-NHC has been obtained and coordinated to rhodium. The new ligand displays a unique rigid C3v symmetry. The electrochemical analysis of the tri-rhodium complex reveals that the three metals are essentially disconnected.

Segarra C; Linke J; Mas-Marzá E; Kuck D; Peris E

2013-10-01

163

Density functional theory investigation of the alkyl-alkyl Negishi cross-coupling reaction catalyzed by N-heterocyclic carbene (NHC)-Pd complexes.  

Science.gov (United States)

A novel mechanism is proposed for the Pd-1,3-(2,6-diisopropylphenyl)imidazolyl-2-ylidene (1) catalyzed Negishi reaction. DFT computations supported by atoms-in-molecules (AIM) analyses of non-truncated models show that a "steric wall" created by the N-substituent on the ligand guides reactants to and from the Pd center. Notably, transmetalation and not oxidative addition is found to be rate-limiting. Additionally, a key Pd-Zn interaction (approximately = 2.4 A, rho(b) approximately = 0.0600 au) is identified in the mechanism. This interaction persists beyond reductive elimination and, in combination with the ligand, facilitates reductive elimination by creating a highly sterically crowded environment in the coordination sphere of the Pd center. PMID:19288489

Chass, Gregory A; O'Brien, Christopher J; Hadei, Niloufar; Kantchev, Eric Assen B; Mu, Wei-Hua; Fang, De-Cai; Hopkinson, Alan C; Csizmadia, Imre G; Organ, Michael G

2009-01-01

164

Density functional theory investigation of the alkyl-alkyl Negishi cross-coupling reaction catalyzed by N-heterocyclic carbene (NHC)-Pd complexes.  

UK PubMed Central (United Kingdom)

A novel mechanism is proposed for the Pd-1,3-(2,6-diisopropylphenyl)imidazolyl-2-ylidene (1) catalyzed Negishi reaction. DFT computations supported by atoms-in-molecules (AIM) analyses of non-truncated models show that a "steric wall" created by the N-substituent on the ligand guides reactants to and from the Pd center. Notably, transmetalation and not oxidative addition is found to be rate-limiting. Additionally, a key Pd-Zn interaction (approximately = 2.4 A, rho(b) approximately = 0.0600 au) is identified in the mechanism. This interaction persists beyond reductive elimination and, in combination with the ligand, facilitates reductive elimination by creating a highly sterically crowded environment in the coordination sphere of the Pd center.

Chass GA; O'Brien CJ; Hadei N; Kantchev EA; Mu WH; Fang DC; Hopkinson AC; Csizmadia IG; Organ MG

2009-01-01

165

Water-soluble tetrapodal N, O ligands incorporating soft N-heterocycles for the selective complexation of Am(III) over Ln(III)  

International Nuclear Information System (INIS)

A series of four water-soluble N, O-tetrapodal ligands derived from ethylenediamine, bearing hard acetate groups and soft N-heterocycles, either pyridine or pyrazine, was developed to study the impact of the softness of N-donors on the complexation properties with trivalent f ions. Two novel ligands of enhanced soft character, bearing three pyridines (L3py) or three pyrazines (L3pz), were synthesized and the related lanthanide complexes were studied in solution. The ligand containing three pyridyl-methyl moieties L3py gives complexes with a coordination similar to EDTA, i.e. a hexa-dentate coordination mode as indicated by NMR and luminescence decays (q = 3) and stability constants in the range log ?110 = 6.99-9.3 (La-Lu). On the other hand, the softest molecule L3pz forms much less stable complexes with log ?110 = 4.0-4.4 (La-Eu). The selective back-extraction of Am(III) from organic solutions containing 4f and 5f elements was tested with the four water-soluble complexing agents. The ligand L3pz demonstrates poor stripping ability and selectivity. In contrast, the three ligands Lpy, Lpz and L3py give interesting back-extraction results with Eu/Am separation factors ranging from 36 to 46, which are significantly higher than with HEDTA. This exemplifies the role of the N-hetero-cycle softness in enhancing the separation between Am(III) and Eu(III). Interestingly, the pyrazine-based ligand, Lpz, demonstrates the best stripping properties, with a distribution factor that approaches that of HEDTA in the same conditions (DAm?0.3). This molecule is a good compromise between softness and hardness and forms complexes still stable at pH 3 due to its low basicity. (authors)

2010-01-01

166

Azobenzene-functionalized N-heterocyclic carbenes as photochromic ligands in silver(i) and gold(i) complexes.  

Science.gov (United States)

The reaction of meta- and para-bromomethylene-azobenzenes with 1-methyl-imidazole yields the respective meta-/para-functionalized azobenzenes tagged with an imidazolium group. Similar reactions of ortho- and para-bromo-azobenzene with imidazole and successive quaternation with benzylbromide give the analogues, with an imidazolium group in ortho/para substituted azobenzenes. With the exception of the ortho derivative, all imidazolium salts could be transformed into their respective silver(i) complexes by reaction with Ag2O. Transmetallation of these silver(i) complexes with (Me2S)AuCl gives the azobenzene-containing complexes (NHC)AuCl. Two of these formed crystals suitable for X-ray diffraction, which revealed the typical linear coordination geometry of the NHC-Au-Cl moiety. All gold complexes feature E?Z photo-isomerisation upon irradiation with UV light. The thermal back reaction to the E-isomers is relatively slow and comparable to that of other azobenzene compounds. PMID:23922028

Kaiser, Manuel; Leitner, Sebastian P; Hirtenlehner, Christa; List, Manuela; Gerisch, Alexander; Monkowius, Uwe

2013-10-01

167

Heavier carbene analogues and their derivatives as ?-El, ?2-El,N, and ?2-N,N' ligands: different reactivity patterns for acyclic and cyclic diamidogermylenes and -stannylenes.  

UK PubMed Central (United Kingdom)

Because of the increasing importance of N-heterocyclic carbenes in organometallic chemistry we investigated the ligand properties of structurally-related acyclic and cyclic heavier carbene analogues with transition metal chlorides. Acyclic {(Me(3)Si)(2)N}(2)El, El = Ge and Sn, react with CuCl with transfer of one (Me(3)Si)(2)N ligand to yield the known copper tetramer {(Me(3)Si)(2)NCu}(4). The cyclic Me(2)Si(?-N(t)Bu)(2)Ge, by contrast, binds copper through germanium only, furnishing a tetranuclear ladder structure with both terminal and bridging germylenes. The tin homologue, however, inserts into the CuCl bond, and the ensuing {Me(2)Si(?-N(t)Bu)(2)SnCl}(-) ions then coordinate one copper ion via their tin atoms while sandwiching the remaining three copper ions in an unprecedented ?(2)-N,N' fashion. Chemically-harder Cr(II)--created in a redox reaction of Me(2)Si(?-N(t)Bu)(2)Sn with CrCl(3)(THF)(3)--is not coordinated by tin, but chelated by both nitrogen atoms of one {Me(2)Si(?-N(t)Bu)(2)SnCl}(-) ion and more weakly through the tin-bound chloride.

West JK; Fondong GL; Noll BC; Stahl L

2013-03-01

168

Synthesis and reactivity of metal complexes with acyclic (amino)(ylide)carbene ligands.  

UK PubMed Central (United Kingdom)

No cycle required: The straightforward synthesis of acyclic (amino)(ylide)carbene gold complexes was achieved by reaction of isocyanide gold complexes with phosphorus and arsenic ylides as well as electron-rich olefins. Their ability to form bimetallic species and to act as ligand-transfer reagents has also been established.

González-Fernández E; Rust J; Alcarazo M

2013-10-01

169

Ligand effects of NHC-iridium catalysts for signal amplification by reversible exchange (SABRE).  

Science.gov (United States)

SABRE hyperpolarizes substrates by polarization transfer from para-hydrogen in a metal complex. We have measured the signal enhancement of pyridine and its exchange rate in various [Ir(NHC)(Py)3(H)2](+) complexes to gain insight into their dependence on the N-Heterocyclic Carbene (NHC) ligand's steric and electronic properties. PMID:23862181

van Weerdenburg, Bram J A; Glöggler, Stefan; Eshuis, Nan; Engwerda, A H J Ton; Smits, Jan M M; de Gelder, René; Appelt, Stephan; Wymenga, Sybren S; Tessari, Marco; Feiters, Martin C; Blümich, Bernhard; Rutjes, Floris P J T

2013-07-25

170

Ligand effects of NHC-iridium catalysts for signal amplification by reversible exchange (SABRE).  

UK PubMed Central (United Kingdom)

SABRE hyperpolarizes substrates by polarization transfer from para-hydrogen in a metal complex. We have measured the signal enhancement of pyridine and its exchange rate in various [Ir(NHC)(Py)3(H)2](+) complexes to gain insight into their dependence on the N-Heterocyclic Carbene (NHC) ligand's steric and electronic properties.

van Weerdenburg BJ; Glöggler S; Eshuis N; Engwerda AH; Smits JM; de Gelder R; Appelt S; Wymenga SS; Tessari M; Feiters MC; Blümich B; Rutjes FP

2013-07-01

171

Lewis-base stabilized diiodine adducts with N-heterocyclic chalcogenamides.  

UK PubMed Central (United Kingdom)

Oxidation reactions of stable chalcogenamides with iodine are intriguing due to their broad application in various organic syntheses. In the present study we report on the utilization of N-heterocyclic carbene and cyclic-alkyl-amino carbenes L(1-3): (L(1): = :C[N(2,6-iPr2-C6H3)CH]2, L(2): = :C(CH2)(CMe2)(C6H10)N-2,6-iPr2-C6H3, L(3): = :C(CH2)(CMe2)2N-2,6-iPr2-C6H3) for the syntheses of chalcogenamides L(1-3)[double bond, length as m-dash]E (E = S, Se, Te) 1-4 and zwitterionic adducts L(1-3)[double bond, length as m-dash]E-I-I 5-8. The synthesis of compounds 1-4 involved the addition reaction of ligand L(1-3): and elemental chalcogen. Treatment of 1-4 with iodine resulted in the formation of adducts 5-8. Compounds 5-8 are well characterized with various spectroscopic methods and single-crystal X-ray structural analysis.

Tretiakov M; Shermolovich YG; Singh AP; Samuel PP; Roesky HW; Niepötter B; Visscher A; Stalke D

2013-08-01

172

Addition of a phosphine ligand switches an N-heterocyclic carbene-zirconium catalyst from oligomerization to polymerization of 1-hexene.  

UK PubMed Central (United Kingdom)

A catalyst for the oligomerization of 1-hexene, generated by the activation of a benzimidazolylidene zirconium dibenzyl complex, switches to a polymerization catalyst on addition of a trialkylphosphine.

Despagnet-Ayoub E; Henling LM; Labinger JA; Bercaw JE

2013-09-01

173

X-ray fluorescence imaging of single human cancer cells reveals that the N-heterocyclic ligands of iodinated analogues of ruthenium anticancer drugs remain coordinated after cellular uptake.  

Science.gov (United States)

Analogues of KP1019 containing iodinated indazole ligands were prepared to investigate the biological fate of the Ru-N-heterocycle bond in this class of anticancer agents. The new complexes, 5-iodoindazolium trans-tetrachloridobis(5-iodoindazole)ruthen(III)ate (1) and 5-iodoindazolium trans-tetrachlorido(dimethyl sulfoxide)(5-iodoindazole)ruthen(III)ate (3), were characterized by elemental analysis, mass spectrometry and UV-vis spectrophotometry. Tetramethylammonium salts of these complexes (2 and 4) were synthesized and characterized in a similar manner. Half-maximum inhibitory concentrations of 2 and 4 with regard to A549 cells at 24 h were determined on the basis of the dose-response curves derived from real-time cell adhesion impedance measurements and were shown to be in the same range as those determined for KP1019 and NAMI-A using the same method. X-ray fluorescence imaging of single cultured A549 cells treated with 2 or 4 showed that, in both cases, the distribution of ruthenium and iodine was identical, indicating that the Ru-N bonds in the anionic complexes remained intact after incubation in culture medium and subsequent cellular uptake and processing. PMID:23943098

Antony, Sumy; Aitken, Jade B; Vogt, Stefan; Lai, Barry; Brown, Tracey; Spiccia, Leone; Harris, Hugh H

2013-08-14

174

Carbene-stabilized beryllium borohydride.  

UK PubMed Central (United Kingdom)

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).

Gilliard RJ Jr; Abraham MY; Wang Y; Wei P; Xie Y; Quillian B; Schaefer HF 3rd; Schleyer PV; Robinson GH

2012-06-01

175

Molecular versus ionic structures in adducts of GaX3 with monodentate carbon-based ligands.  

UK PubMed Central (United Kingdom)

A molecular donor-acceptor adduct has been isolated by the reaction of the N-heterocyclic carbene 1,3-dimethyl imidazol-2-ylidene (diMe-IMD) with GaCl(3). In contrast, the structurally related, yet much more nucleophilic, 1,3-dimethyl-2-methylene-2,3-dihydro-1H-imidazole (diMe-MDI) gave rise to ion pairs of type [L(2)GaX(2)][GaX(4)], where X = Cl, Br, or I. With IBioxMe(4), a N-heterocyclic carbene that is more nucleophilic than diMe-IMD, the outcome of the reaction was dependent on the nature of the halide. Ionic 1:1 adducts between monodentate ligands and GaX(3) salts have only one precedent in the literature. The peculiar behavior of carbon-based ligands was explained on the basis of their electronic properties and reaction kinetics.

El-Hellani A; Monot J; Guillot R; Bour C; Gandon V

2013-01-01

176

Rhodium and iridium complexes of a new ferrocene-derived chelating bis(NHC) ligand.  

UK PubMed Central (United Kingdom)

Rhodium and iridium complexes of a new ferrocene-derived bis(N-heterocyclic carbene) ligand, [M(cod){1,2-(MeNCHCHNCCH(2))(2)C(5)H(3)}Fe(C(5)H(5))]BF(4) (M = Rh, 8a; M = Ir, 8b; cod = 1,5-cyclooctadiene), were synthesized from the corresponding bis(imidazolium) salt 6. The molecular structure of 8a was determined by single-crystal X-ray diffraction. Complexes 8a and 8b smoothly react with CO with displacement of the chelating cod ligand to give the corresponding dicarbonyl derivatives 9a and 9b.

Panov DM; Petrovskii PV; Ezernitskaya MG; Smol'yakov AF; Dolgushin FM; Koridze AA

2012-08-01

177

Rhodium and iridium complexes of a new ferrocene-derived chelating bis(NHC) ligand.  

Science.gov (United States)

Rhodium and iridium complexes of a new ferrocene-derived bis(N-heterocyclic carbene) ligand, [M(cod){1,2-(MeNCHCHNCCH(2))(2)C(5)H(3)}Fe(C(5)H(5))]BF(4) (M = Rh, 8a; M = Ir, 8b; cod = 1,5-cyclooctadiene), were synthesized from the corresponding bis(imidazolium) salt 6. The molecular structure of 8a was determined by single-crystal X-ray diffraction. Complexes 8a and 8b smoothly react with CO with displacement of the chelating cod ligand to give the corresponding dicarbonyl derivatives 9a and 9b. PMID:22772354

Panov, Dmitriy M; Petrovskii, Pavel V; Ezernitskaya, Mariam G; Smol'yakov, Alexander F; Dolgushin, Fedor M; Koridze, Avthandil A

2012-07-09

178

A dual lewis base activation strategy for enantioselective carbene-catalyzed annulations.  

UK PubMed Central (United Kingdom)

A dual activation strategy integrating N-heterocyclic carbene (NHC) catalysis and a second Lewis base has been developed. NHC-bound homoenolate equivalents derived from ?,?-unsaturated aldehydes combine with transient reactive o-quinone methides in an enantioselective formal [4 + 3] fashion to access 2-benzoxopinones. The overall approach provides a general blueprint for the integration of carbene catalysis with additional Lewis base activation modes.

Izquierdo J; Orue A; Scheidt KA

2013-07-01

179

Complex N-heterocycle synthesis via iron-catalyzed, direct C-H bond amination.  

UK PubMed Central (United Kingdom)

The manipulation of traditionally unreactive functional groups is of paramount importance in modern chemical synthesis. We have developed an iron-dipyrrinato catalyst that leverages the reactivity of iron-borne metal-ligand multiple bonds to promote the direct amination of aliphatic C-H bonds. Exposure of organic azides to the iron dipyrrinato catalyst furnishes saturated, cyclic amine products (N-heterocycles) bearing complex core-substitution patterns. This study highlights the development of C-H bond functionalization chemistry for the formation of saturated, cyclic amine products and should find broad application in the context of both pharmaceuticals and natural product synthesis.

Hennessy ET; Betley TA

2013-05-01

180

The active role of NHC ligands in platinum-mediated tandem hydroboration-cross coupling reactions.  

UK PubMed Central (United Kingdom)

Stable N-heterocyclic platinum-carbene complexes are the first example of platinum-mediated regioselective H-B addition to vinylarenes and alkynes, allowing consecutive cross coupling reactions with the same catalytic system.

Lillo V; Mata JA; Segarra AM; Peris E; Fernandez E

2007-06-01

 
 
 
 
181

Tris(carbene)borate ligands featuring imidazole-2-ylidene, benzimidazol-2-ylidene, and 1,3,4-triazol-2-ylidene donors. Evaluation of donor properties in four-coordinate {NiNO}10 complexes.  

UK PubMed Central (United Kingdom)

The synthesis and characterization of new tris(carbene)borate ligand precursors containing substituted benzimidazol-2-ylidene and 1,3,4-triazol-2-ylidene donor groups, as well as a new tris(imidazol-2-ylidene)borate ligand precursor are reported. The relative donor strengths of the tris(carbene)borate ligands have been evaluated by the position of ?(NO) in four-coordinate {NiNO}(10) complexes, and follow the order: imidazol-2-ylidene > benzimidazol-2-ylidene > 1,3,4-triazol-2-ylidene. There is a large variation in ?(NO), suggesting these ligands to have a wide range of donor strengths while maintaining a consistent ligand topology. All ligands are stronger donors than Tp* and Cp*.

Muñoz SB 3rd; Foster WK; Lin HJ; Margarit CG; Dickie DA; Smith JM

2012-12-01

182

3-COORDINATE COPPER(I)-CARBENE COMPLEXES  

UK PubMed Central (United Kingdom)

Novel phosphorescent trigonal copper carbene complexes are provided. The complex comprise a carbene ligand coordinated to a three coordinate copper atom. The complex may be used in organic light emitting devices. In particular, the complexes may be especially useful in OLEDs used for lighting applications.

THOMPSON MARK; DJUROVICH PETER; KRYLOVA VALENTINA

183

Ruthenium(II) carbonyl complexes bearing CCC-pincer bis-(carbene) ligands: synthesis, structures and activities toward recycle transfer hydrogenation reactions.  

Science.gov (United States)

A new series of ruthenium(II) carbonyl complexes with benzene-based CCC-pincer bis-(carbene) ligands, [((R)CCC(R))Ru(CO)2(X)](0/+) and [((R)CCC(R))Ru(CO)(NN)](+) ((R)CCC(R) = 2,6-bis-(1-alkylimidazolylidene)benzene, R = Me or (n)Bu; X = I, Br, CH3CN, or 6-(aminomethyl)pyridine (ampy); NN = 2·CH3CN, or chelating ampy or bipyridine), was synthesized and fully characterized. X-Ray structure determinations revealed that these eight complexes have pseudo-octahedral configurations around the ruthenium center with the pincer ligand occupying three meridional sites. These complexes prove to be efficient precatalysts demonstrating very good activity and reusability for the transfer hydrogenation of ketones. PMID:23880878

Naziruddin, Abbas Raja; Huang, Zhao-Jiunn; Lai, Wei-Chih; Lin, Wan-Jung; Hwang, Wen-Shu

2013-07-24

184

Ruthenium(II) carbonyl complexes bearing CCC-pincer bis-(carbene) ligands: synthesis, structures and activities toward recycle transfer hydrogenation reactions.  

UK PubMed Central (United Kingdom)

A new series of ruthenium(II) carbonyl complexes with benzene-based CCC-pincer bis-(carbene) ligands, [((R)CCC(R))Ru(CO)2(X)](0/+) and [((R)CCC(R))Ru(CO)(NN)](+) ((R)CCC(R) = 2,6-bis-(1-alkylimidazolylidene)benzene, R = Me or (n)Bu; X = I, Br, CH3CN, or 6-(aminomethyl)pyridine (ampy); NN = 2·CH3CN, or chelating ampy or bipyridine), was synthesized and fully characterized. X-Ray structure determinations revealed that these eight complexes have pseudo-octahedral configurations around the ruthenium center with the pincer ligand occupying three meridional sites. These complexes prove to be efficient precatalysts demonstrating very good activity and reusability for the transfer hydrogenation of ketones.

Naziruddin AR; Huang ZJ; Lai WC; Lin WJ; Hwang WS

2013-09-01

185

Synthetic and structural studies on amine coordination to Pd-N-heterocyclic carbene complexes.  

UK PubMed Central (United Kingdom)

The reaction between 2-chlorobenzylamine or 2-chlorobenzylalcohol and Pd(I(t)Bu)(2) (I(t)Bu = 1,3-di-tert-butylimidazol-2-ylidene) in benzene affords the dimeric complexes [Pd(I(t)Bu)(mu-NH{2-CH(2)C(6)H(4)})](2) and [Pd(I(t)Bu)(mu-O{2-CH(2)C(6)H(4)})](2); the latter has been structurally characterised. The syntheses, structural characterisation and reactivity of the Pd-NHC amine complexes [(I(t)Bu)Pd(R-4-C(6)H(4))(morpholine)Cl] (R = Me, OMe, CO(2)Me), intermediates in the Buchwald-Hartwig aryl amination reaction, are also reported.

de K Lewis AK; Caddick S; Esposito O; Cloke FG; Hitchcock PB

2009-09-01

186

Synthetic and structural studies on amine coordination to Pd-N-heterocyclic carbene complexes.  

Science.gov (United States)

The reaction between 2-chlorobenzylamine or 2-chlorobenzylalcohol and Pd(I(t)Bu)(2) (I(t)Bu = 1,3-di-tert-butylimidazol-2-ylidene) in benzene affords the dimeric complexes [Pd(I(t)Bu)(mu-NH{2-CH(2)C(6)H(4)})](2) and [Pd(I(t)Bu)(mu-O{2-CH(2)C(6)H(4)})](2); the latter has been structurally characterised. The syntheses, structural characterisation and reactivity of the Pd-NHC amine complexes [(I(t)Bu)Pd(R-4-C(6)H(4))(morpholine)Cl] (R = Me, OMe, CO(2)Me), intermediates in the Buchwald-Hartwig aryl amination reaction, are also reported. PMID:20449153

de K Lewis, Alexandra K; Caddick, Stephen; Esposito, Oriana; Cloke, F Geoffrey N; Hitchcock, Peter B

2009-07-21

187

N-Heterocyclic-Carbene-Catalyzed One-Pot Synthesis of Hydroxamic Esters.  

UK PubMed Central (United Kingdom)

The honey pot: The NHC-catalyzed reaction between nitrosobenzenes, aromatic/aliphatic aldehydes, and enals proceeded through an aza-benzoin-type reaction and an internal redox esterification to afford hydroxamic esters in good yields.

Song X; Ni Q; Grossmann A; Enders D

2013-09-01

188

N-heterocyclic carbene-initiated hydrosilylation of styryl alcohols with dihydrosilanes: a mechanistic investigation.  

UK PubMed Central (United Kingdom)

Investigation of the mechanism of the NHC-initiated hydrosilylation of styryl alcohols in the presence of a dihydrosilane suggests a general base catalysis mechanism and not the activation of the dihydrosilane by the NHC.

Gatineau D; Zhao Q; Curran DP; Malacria M; Lacôte E; Fensterbank L; Goddard JP

2013-05-01

189

Tetrahydrofuran ring opening and related reactions with an N-heterocyclic carbene-boryl trifluoromethanesulfonate.  

UK PubMed Central (United Kingdom)

Reaction of the triflate group in dipp-Imd-BH(2)OTf with nucleophiles generally occurs by direct substitution to give products dipp-Imd-BH(2)Nu. In contrast, reaction of this boryl triflate with aryloxides (ArO(-)) in THF resulted in insertion of THF with ring opening in between the NHC-boryl electrophile and the aryloxide to give products dipp-Imd-BH(2)O(CH(2))(4)OAr. The incorporation of THF was observed with other nucleophiles such as ethylthiolate and trimethylsilanolate whose conjugate acids have pK(a) values similar to that of phenol. The cleavage of ethyl acetate and oxetane is reported as well. A possible mechanism is proposed.

Solovyev A; Lacôte E; Curran DP

2013-01-01

190

Theory of the formation and decomposition of N-heterocyclic aminooxycarbenes through metal-assisted [2+3]-dipolar cycloaddition/retro-cycloaddition.  

UK PubMed Central (United Kingdom)

The theoretical background of the formation of N-heterocyclic oxadiazoline carbenes through a metal-assisted [2+3]-dipolar cycloaddition (CA) reaction of nitrones R(1)CH=N(R(2))O to isocyanides C?NR and the decomposition of these carbenes to imines R(1)CH=NR(2) and isocyanates O=C=NR is discussed. Furthermore, the reaction mechanisms and factors that govern these processes are analyzed in detail. In the absence of a metal, oxadiazoline carbenes should not be accessible due to the high activation energy of their formation and their low thermodynamic stability. The most efficient promotors that could assist the synthesis of these species should be "carbenophilic" metals that form a strong bond with the oxadiazoline heterocycle, but without significant involvement of ?-back donation, namely, Au(I), Au(III), Pt(II), Pt(IV), Re(V), and Pd(II) metal centers. These metals, on the one hand, significantly facilitate the coupling of nitrones with isocyanides and, on the other hand, stabilize the derived carbene heterocycles toward decomposition. The energy of the LUMO(CNR) and the charge on the N atom of the C?N group are principal factors that control the cycloaddition of nitrones to isocyanides. The alkyl-substituted nitrones and isocyanides are predicted to be more active in the CA reaction than the aryl-substituted species, and the N,N,C-alkyloxadiazolines are more stable toward decomposition relative to the aryl derivatives.

Novikov AS; Kuznetsov ML; Pombeiro AJ

2013-02-01

191

Application of 1,2,3-triazolylidenes as versatile NHC-type ligands: synthesis, properties, and application in catalysis and beyond.  

UK PubMed Central (United Kingdom)

Triazolylidenes have rapidly emerged as a powerful subclass of N-heterocyclic carbene ligands for transition metals. They are readily available through regioselective [2 + 3] cycloaddition of alkynes and azides and subsequent metallation according to procedures established for related carbenes. Due to their mesoionic character, triazolylidenes are stronger donors than Arduengo-type imidazol-2-ylidenes. Spurred by these attractive attributes and despite their only recent emergence, triazolylidenes have shown major implications in catalysis. This feature article summarises the synthetic accessibility of triazolylidene metal complexes and their electronic and structural characteristics, and it compiles their applications, in particular, as catalyst precursors for various bond forming and redox reactions, as well as first approaches into photophysical and biochemical domains.

Donnelly KF; Petronilho A; Albrecht M

2013-02-01

192

Application of 1,2,3-triazolylidenes as versatile NHC-type ligands: synthesis, properties, and application in catalysis and beyond.  

Science.gov (United States)

Triazolylidenes have rapidly emerged as a powerful subclass of N-heterocyclic carbene ligands for transition metals. They are readily available through regioselective [2 + 3] cycloaddition of alkynes and azides and subsequent metallation according to procedures established for related carbenes. Due to their mesoionic character, triazolylidenes are stronger donors than Arduengo-type imidazol-2-ylidenes. Spurred by these attractive attributes and despite their only recent emergence, triazolylidenes have shown major implications in catalysis. This feature article summarises the synthetic accessibility of triazolylidene metal complexes and their electronic and structural characteristics, and it compiles their applications, in particular, as catalyst precursors for various bond forming and redox reactions, as well as first approaches into photophysical and biochemical domains. PMID:23235474

Donnelly, Kate F; Petronilho, Ana; Albrecht, Martin

2013-02-11

193

Technetium(I) tricarbonyl complexed with the N-heterocyclic aldehyde thiosemicarbazones: potential precursors of the radiopharmaceuticals  

International Nuclear Information System (INIS)

Technetium(I) tricarbonyl complexes may form with the pyridine aldehyde thiosemicarbazones (TSCs), in which chelating ligand is bound tri- or bidentately. Intend of the presented work was to check, if labeling the N-heterocyclic TSCs with tricarbonyl [99mTc]-technetium(I) may lead to formation of the complexes suitable for the radiopharmaceutical purposes. Syntheses of the complexes were provided in the conditions analogous to those performed in the nuclear medicine laboratories. Main physicochemical properties of the complexes important in the medicinal chemistry were studied. Relevant results of the numerical calculations remain in fair agreement with these properties. (author)

2012-01-01

194

Comprehensive experimental study of N-heterocyclic ?-stacking interactions of neutral and cationic pyridines.  

UK PubMed Central (United Kingdom)

A comprehensive experimental study was carried out by measuring the relative strengths of parallel ?-stacking interactions of N-heterocycles with nonheterocycles. A versatile and rigid model system was developed, which was in equilibrium between a "closed" conformation that forms an intramolecular ?-stacking interaction and an "open" conformation that cannot form the interaction. First, the formation and geometries of the intramolecular N-heterocyclic ?-stacking interactions were verified by X-ray crystallography. Next, the closed/open ratios were measured in solution via integration of the (1)H NMR spectra, providing an accurate comparison of the N-heterocyclic ?-stacking interactions. The synthetic versatility of this model system enabled the systematic and comprehensive comparison of the influences of position, charge, and substituent effects of the nitrogen atom of the N-heterocycles within a single model system. The ?-stacking interactions of the neutral N-heterocyclic rings were slightly stronger than that of nonheterocyclic rings. Cationic N-heterocycles formed significantly stronger ?-stacking interactions than neutral N-heterocycles. The position of the nitrogen atom also had a strong influence on the stability of N-heterocyclic ?-stacking complexes. Interestingly, opposite stability trends were observed for neutral and cationic N-heterocycles. For neural N-heterocycles, geometries with the nitrogen away from the ?-face of the opposing ring were the more stable. For cationic N-heterocycles, geometries with the nitrogen close to the ?-face of the opposing ring were the more stable. Finally, N-methylated heterocycles consistently formed stronger ?-stacking interactions than N-protonated heterocycles.

Li P; Zhao C; Smith MD; Shimizu KD

2013-06-01

195

Organosulphur and related ligands in Suzuki-Miyaura C-C coupling.  

UK PubMed Central (United Kingdom)

Suzuki-Miyaura C-C cross coupling (SMC), an important synthetic strategy for many organic molecules, has several advantages such as mild reaction conditions, high tolerance toward various functional groups and ease in isolation of the product. Palladium(II) ligated with phosphines (particularly bulky and electron-rich) and N-heterocyclic carbenes (NHCs) has been found to be efficient in the catalysis of SMC. The drawback with many of these catalysts is their air/moisture sensitivity. Since 2000, palladium(II) complexes of organosulphur and related ligands have emerged as viable alternatives to palladium-phosphine/carbene complexes as they have sufficient thermal stability, air and moisture insensitivity. Moreover synthesis of complexes of such ligands is easy. In this perspective Suzuki-Miyaura C-C coupling reactions catalyzed with palladium(II)-complexes of organosulphur ligands have been reviewed. Catalysis of SMC with palladium(II) complexes of organoselenium and tellurium ligands, studied much less in comparison to those of organosulphur ligands, is also included.

Kumar A; Rao GK; Kumar S; Singh AK

2013-04-01

196

Gold(I) carbene complexes causing thioredoxin 1 and thioredoxin 2 oxidation as potential anticancer agents.  

UK PubMed Central (United Kingdom)

Gold(I) complexes with 1,3-substituted imidazole-2-ylidene and benzimidazole-2-ylidene ligands of the type NHC-Au-L (NHC = N-heterocyclic carbene L = Cl or 2-mercapto-pyrimidine) have been synthesized and structurally characterized. The compounds were evaluated for their antiproliferative properties in human ovarian cancer cells sensitive and resistant to cisplatin (A2780S/R), as well in the nontumorigenic human embryonic kidney cell line (HEK-293T), showing in some cases important cytotoxic effects. Some of the complexes were comparatively tested as thioredoxin reductase (TrxR) and glutathione reductase (GR) inhibitors, directly against the purified proteins or in cell extracts. The compounds showed potent and selective TrxR inhibition properties in particular in cancer cell lines. Remarkably, the most effective TrxR inhibitors induced extensive oxidation of thioredoxins (Trxs), which was more relevant in the cancerous cells than in HEK-293T cells. Additional biochemical assays on glutathione systems and reactive oxygen species formation evidenced important differences with respect to the classical cytotoxic Au(I)-phosphine compound auranofin.

Schuh E; Pflüger C; Citta A; Folda A; Rigobello MP; Bindoli A; Casini A; Mohr F

2012-06-01

197

Synthesis, Characterization and Catalytic Applications of Water-Soluble Carbene Complexes for the Hydrogenation of Acetophenone Synthese, Charakterisierung und Katalytische Anwendungen von Wasserlöslichen Carbenkomplexe für die Hydrierung von Acetophenon  

Digital Repository Infrastructure Vision for European Research (DRIVER)

The synthesis of water-soluble Rh(I), Ir(I), and Ru(II) N-heterocyclic carbene complexes is described. These complexes are applied as catalysts for aqueous phase hydrogenation reactions. Good hydrogenation activities under ca. 40 atm pressure H2 at room temperature are observed. , Die Synthese von wass...

Syska, Hitrisia

198

Azido, triazolyl, and alkynyl complexes of gold(I): syntheses, structures, and ligand effects.  

UK PubMed Central (United Kingdom)

Gold(I) triazolyl complexes are prepared in [3 + 2] cycloaddition reactions of (tertiary phosphine)gold(I) azides with terminal alkynes. Seven such triazolyl complexes, not previously prepared, are described. Reducible functional groups are accommodated. In addition, two new (N-heterocyclic carbene)gold(I) azides and two new gold(I) alkynyls are described. Eight complexes are crystallographically authenticated; aurophilic interactions appear in one structure only. The packing diagrams of gold(I) triazolyls all show intermolecular hydrogen bonding between N-1 of one molecule and N-3 of a neighbor. This hydrogen bonding permeates the crystal lattice. Density-functional theory calculations of (triphenylphosphine)gold(I) triazolyls and the corresponding alkynyls indicate that the triazolyl is a stronger trans-influencer than is the alkynyl, but the alkynyl is more electron-releasing. These results suggest that trans-influences in two-coordinate gold(I) complexes can be more than a simple matter of ligand donicity.

Robilotto TJ; Deligonul N; Updegraff JB 3rd; Gray TG

2013-08-01

199

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

UK PubMed Central (United Kingdom)

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.

Payne PR; Garcia P; Eisenberger P; Yim JC; Schafer LL

2013-05-01

200

N-heterocyclic phosphenium cations: syntheses and cycloaddition reactions.  

Science.gov (United States)

A series of trifluoromethanesulfonate (OTf) salts of N-heterocyclic phospheniums (NHP) bearing phenyl (1a), para-methoxyphenyl (1b), 2,6-diisopropylphenyl (1c) and mesityl (1d) substituents is reported. The compounds are made by a modification to a literature procedure that improves the overall yields for and by 15 and 23%, respectively. Two unwanted side-products in the synthesis of , the diammonium salt, [(2,6-iPr-C6H3)N(H)2CH2CH2N(H)2(2,6-iPr-C6H3)]Cl2 (4) and the bisphosphine (2,6-iPr-C6H3)N(PCl2)CH2CH2N(PCl2)(2,6-iPr-C6H3) (5), are crystallographically characterized, as is the intermediate cyclic chlorophosphine, C1PN(4-OMe-C6H4)CH2CH2N(4-OMe-C6H4) (3b). The phenyl-substituted NHP is fully characterized, including by X-ray crystallography, for the first time; this compound contains a short P-O contact of 2.1850(14) A. Cycloaddition reactions of with 2,3-dimethyl-1,3-butadiene give the expected spirocyclic phospholeniums, 7,8-dimethyl-1,4-diaryl-1,4-diaza-5-phopshoniaspiro[4.4]non-7-ene, as their OTf salts (6a-d), while reactions with N,N'-dimesityl-1,4-diaza-1,3-butadiene give, except in the case of , which is too bulky to react, the aza analogues, 1,4-dimesityl-6,9-diaryl-1,4,6,9-tetraaza-5-phosphoniaspiro[4.4]non-2-ene (7a, 7b and 7d). The sterically congested is in thermal equilibrium with and free diazadiene, and undergoes a substitution reaction with 2,3-dimethyl-1,3-butadiene to give . PMID:18580983

Caputo, Christine A; Price, Jacquelyn T; Jennings, Michael C; McDonald, Robert; Jones, Nathan D

2008-04-22

 
 
 
 
201

Axial coordination of NHC ligands on dirhodium(II) complexes: generation of a new family of catalysts.  

Science.gov (United States)

An efficient new methodology for the arylation of aldehydes is disclosed which uses dirhodium(II) catalysts and N-heterocyclic carbene (NHC) ligands. Complexes of Rh 2(OAc) 4 with one and two NHCs attached on the axial positions were successfully isolated, fully characterized, and used as catalysts in the reaction. The saturated monocomplex ((NHC 5)Rh 2(OAc) 4) 31 was shown to be the most active catalyst and was particularly efficient in the arylation of alkyl aldehydes. DFT calculations support participation of complexes with one axial NHC in the reaction as the catalysts active species and indicate that hydrogen bonds involving dirhodium unit, reactants, and solvent (alcohol) play an important role on the reaction mechanism. PMID:18457453

Trindade, Alexandre F; Gois, Pedro M P; Veiros, Luís F; André, Vânia; Duarte, M Teresa; Afonso, Carlos A M; Caddick, Stephen; Cloke, F Geoffrey N

2008-05-06

202

Impact of terminal dimethylation on the resistance profile of ?-N-heterocyclic thiosemicarbazones.  

Science.gov (United States)

Triapine is an ?-N-heterocyclic thiosemicarbazone with promising anticancer activity against hematologic malignancies but widely ineffective against solid tumor types in clinical trials. The anticancer activity of thiosemicarbazones can be dramatically increased by terminal dimethylation. KP1089 is a gallium compound containing two terminal dimethylated thiosemicarbazone ligands. To gain insights on the vulnerability of this highly active terminal dimethylated thiosemicarbazone to drug resistance mechanisms, a new cell model with acquired resistance against the lead compound KP1089 was established. Subsequent genomic analyses (arrayCGH and FISH) revealed amplification of the ABCC1 gene on double minute chromosomal DNA in KP1089-resistant cells as well as overexpression of ABCC1 and ABCG2 on the protein level. KP1089 was further confirmed as a substrate of ABCC1 and ABCG2 but not of ABCB1 using a panel of ABC transporter-overexpressing cell models as well as ABC transporter inhibitors. Moreover, glutathione depletion strongly enhanced KP1089 activity, although no glutathione conjugate formation by glutathione-S-transferase was observed. Thus, a co-transport of KP1089 together with glutathione is suggested. Finally, a panel of thiosemicarbazone derivatives was tested on the new KP1089-resistant cell line. Notably, KP1089-resistant cells were not cross-resistant against thiosemicarbazones lacking terminal dimethylation (e.g. Triapine) which are less active than KP1089. This suggests that terminal dimethylation of thiosemicarbazones - linked with distinctly enhanced anticancer activity - leads to altered resistance profiles compared to classical thiosemicarbazones making this compound class of interest for further (pre)clinical evaluation. PMID:22426010

Heffeter, Petra; Pirker, Christine; Kowol, Christian R; Herrman, Gerrit; Dornetshuber, Rita; Miklos, Walter; Jungwirth, Ute; Koellensperger, Gunda; Keppler, Bernhard K; Berger, Walter

2012-03-15

203

Impact of terminal dimethylation on the resistance profile of ?-N-heterocyclic thiosemicarbazones  

Science.gov (United States)

Triapine is an ?-N-heterocyclic thiosemicarbazone with promising anticancer activity against hematologic malignancies but widely ineffective against solid tumor types in clinical trials. The anticancer activity of thiosemicarbazones can be dramatically increased by terminal dimethylation. KP1089 is a gallium compound containing two terminal dimethylated thiosemicarbazone ligands. To gain insights on the vulnerability of this highly active terminal dimethylated thiosemicarbazone to drug resistance mechanisms, a new cell model with acquired resistance against the lead compound KP1089 was established. Subsequent genomic analyses (arrayCGH and FISH) revealed amplification of the ABCC1 gene on double minute chromosomal DNA in KP1089-resistant cells as well as overexpression of ABCC1 and ABCG2 on the protein level. KP1089 was further confirmed as a substrate of ABCC1 and ABCG2 but not of ABCB1 using a panel of ABC transporter-overexpressing cell models as well as ABC transporter inhibitors. Moreover, glutathione depletion strongly enhanced KP1089 activity, although no glutathione conjugate formation by glutathione-S-transferase was observed. Thus, a co-transport of KP1089 together with glutathione is suggested. Finally, a panel of thiosemicarbazone derivatives was tested on the new KP1089-resistant cell line. Notably, KP1089-resistant cells were not cross-resistant against thiosemicarbazones lacking terminal dimethylation (e.g. Triapine) which are less active than KP1089. This suggests that terminal dimethylation of thiosemicarbazones – linked with distinctly enhanced anticancer activity – leads to altered resistance profiles compared to classical thiosemicarbazones making this compound class of interest for further (pre)clinical evaluation.

Heffeter, Petra; Pirker, Christine; Kowol, Christian R.; Herrman, Gerrit; Dornetshuber, Rita; Miklos, Walter; Jungwirth, Ute; Koellensperger, Gunda; Keppler, Bernhard K.; Berger, Walter

2012-01-01

204

Impact of terminal dimethylation on the resistance profile of ?-N-heterocyclic thiosemicarbazones.  

UK PubMed Central (United Kingdom)

Triapine is an ?-N-heterocyclic thiosemicarbazone with promising anticancer activity against hematologic malignancies but widely ineffective against solid tumor types in clinical trials. The anticancer activity of thiosemicarbazones can be dramatically increased by terminal dimethylation. KP1089 is a gallium compound containing two terminal dimethylated thiosemicarbazone ligands. To gain insights on the vulnerability of this highly active terminal dimethylated thiosemicarbazone to drug resistance mechanisms, a new cell model with acquired resistance against the lead compound KP1089 was established. Subsequent genomic analyses (arrayCGH and FISH) revealed amplification of the ABCC1 gene on double minute chromosomal DNA in KP1089-resistant cells as well as overexpression of ABCC1 and ABCG2 on the protein level. KP1089 was further confirmed as a substrate of ABCC1 and ABCG2 but not of ABCB1 using a panel of ABC transporter-overexpressing cell models as well as ABC transporter inhibitors. Moreover, glutathione depletion strongly enhanced KP1089 activity, although no glutathione conjugate formation by glutathione-S-transferase was observed. Thus, a co-transport of KP1089 together with glutathione is suggested. Finally, a panel of thiosemicarbazone derivatives was tested on the new KP1089-resistant cell line. Notably, KP1089-resistant cells were not cross-resistant against thiosemicarbazones lacking terminal dimethylation (e.g. Triapine) which are less active than KP1089. This suggests that terminal dimethylation of thiosemicarbazones - linked with distinctly enhanced anticancer activity - leads to altered resistance profiles compared to classical thiosemicarbazones making this compound class of interest for further (pre)clinical evaluation.

Heffeter P; Pirker C; Kowol CR; Herrman G; Dornetshuber R; Miklos W; Jungwirth U; Koellensperger G; Keppler BK; Berger W

2012-06-01

205

'Carbene radicals' in Co(II)(por)-catalyzed olefin cyclopropanation.  

Science.gov (United States)

The mechanism of cobalt(II)-porphyrin-mediated cyclopropanation of olefins with diazoesters was studied. The first step--reaction of cobalt(II)-porphyrin with ethyl diazoacetate (EDA)--was examined using EPR and ESI-MS techniques. EDA reacts with cobalt(II)-porphyrin to form a 1:1 Co(por)(CHCOOEt) adduct that exists as two isomers: the 'bridging carbene' C' in which the 'carbene' is bound to the metal and the pyrrolic nitrogen of the porphyrin that has a d(7) configuration on the metal, and the 'terminal carbene' C in which the 'carbene' behaves as a redox noninnocent ligand having a d(6) cobalt center and the unpaired electron residing on the 'carbene' carbon atom. The subsequent reactivities of the thus formed 'cobalt carbene radical' with propene, styrene, and methyl acrylate were studied using DFT calculations. The calculations suggest that the formation of the carbene is the rate-limiting step for the unfunctionalized Co(II)(por) and that the cyclopropane ring formation proceeds via a stepwise radical process: Radical addition of the 'carbene radical' C to the C=C double bonds of the olefins results in formation of the gamma-alkyl radical intermediates D. Species D then easily collapse in almost barrierless ring-closure reactions (TS3) to form the cyclopropanes. This radical mechanism readily explains the high activity of Co(II)(por) species in the cyclopropanation of electron-deficient olefins such as methyl acrylate. PMID:20681723

Dzik, Wojciech I; Xu, Xue; Zhang, X Peter; Reek, Joost N H; de Bruin, Bas

2010-08-11

206

'Carbene radicals' in Co(II)(por)-catalyzed olefin cyclopropanation.  

UK PubMed Central (United Kingdom)

The mechanism of cobalt(II)-porphyrin-mediated cyclopropanation of olefins with diazoesters was studied. The first step--reaction of cobalt(II)-porphyrin with ethyl diazoacetate (EDA)--was examined using EPR and ESI-MS techniques. EDA reacts with cobalt(II)-porphyrin to form a 1:1 Co(por)(CHCOOEt) adduct that exists as two isomers: the 'bridging carbene' C' in which the 'carbene' is bound to the metal and the pyrrolic nitrogen of the porphyrin that has a d(7) configuration on the metal, and the 'terminal carbene' C in which the 'carbene' behaves as a redox noninnocent ligand having a d(6) cobalt center and the unpaired electron residing on the 'carbene' carbon atom. The subsequent reactivities of the thus formed 'cobalt carbene radical' with propene, styrene, and methyl acrylate were studied using DFT calculations. The calculations suggest that the formation of the carbene is the rate-limiting step for the unfunctionalized Co(II)(por) and that the cyclopropane ring formation proceeds via a stepwise radical process: Radical addition of the 'carbene radical' C to the C=C double bonds of the olefins results in formation of the gamma-alkyl radical intermediates D. Species D then easily collapse in almost barrierless ring-closure reactions (TS3) to form the cyclopropanes. This radical mechanism readily explains the high activity of Co(II)(por) species in the cyclopropanation of electron-deficient olefins such as methyl acrylate.

Dzik WI; Xu X; Zhang XP; Reek JN; de Bruin B

2010-08-01

207

Al-H ?-bond coordination: expanded ring carbene adducts of AlH3 as neutral bi- and tri-functional donor ligands.  

UK PubMed Central (United Kingdom)

Thermally robust expanded ring carbene adducts of AlH3 have been synthesized with a view to probing their ligating abilities via Al-H ?-bond coordination. While ?(2) binding to the 14-electron [Mo(CO)4] fragment is readily demonstrated, interaction with [Mo(CO)3] results in ?:?(1),?(1) and ?:?(2),?(2) bridging linkages rather than terminal ?(3) binding.

Abdalla JA; Riddlestone IM; Tirfoin R; Phillips N; Bates JI; Aldridge S

2013-06-01

208

Al-H ?-bond coordination: expanded ring carbene adducts of AlH3 as neutral bi- and tri-functional donor ligands.  

Science.gov (United States)

Thermally robust expanded ring carbene adducts of AlH3 have been synthesized with a view to probing their ligating abilities via Al-H ?-bond coordination. While ?(2) binding to the 14-electron [Mo(CO)4] fragment is readily demonstrated, interaction with [Mo(CO)3] results in ?:?(1),?(1) and ?:?(2),?(2) bridging linkages rather than terminal ?(3) binding. PMID:23673812

Abdalla, Joseph A B; Riddlestone, Ian M; Tirfoin, Remi; Phillips, Nicholas; Bates, Joshua I; Aldridge, Simon

2013-06-21

209

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

UK PubMed Central (United Kingdom)

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.

Ozdemir I; Gürbüz N; Kalo?lu N; Do?an O; Kalo?lu M; Bruneau C; Doucet H

2013-01-01

210

Heterogenised N-heterocyclic carbene complexes: synthesis, characterisation and application for hydroformylation and C-C bond formation reactions.  

UK PubMed Central (United Kingdom)

The imidazolium salts: 1-mesityl-3-(3-trimethoxysilylpropyl)imidazolium iodide and 1-tert-butyl-3-(3-trimethoxysilylpropyl)imidazolium iodide, abbreviated as (tmpMes)HI (3a) and (tmp(t)Bu)HI (3b), respectively, have been synthesised. The palladium(ii) complexes (?(3)-C(3)H(5)) (tmpMes)PdCl (5a) and (?(3)-C(3)H(5))(tmp(t)Bu)PdCl (5b), rhodium(i) and iridium(i) complexes (?(4)-1,5-COD) (tmpMes)MCl, M = Rh (6a), Ir (7a) and (?(4)-1,5-COD)(tmp(t)Bu)MCl, where M = Rh (6b), Ir (7b), were synthesised by silver transmetallation reactions using the silver(i) complexes (tmpMes)AgI (4a) and (tmp(t)Bu)AgI (4b). The iridium complex 7b has been structurally characterised. The Pd(ii) and Rh(i) complexes have been immobilised by attachment to chemically modified MCM-41. The immobilised palladium(ii) materials have been tested as recyclable catalysts for Suzuki type C-C bond formation reactions in water and the immobilised rhodium(i) materials have been examined for their catalytic ability for the hydroformylation of 1-octene.

Dastgir S; Coleman KS; Green ML

2011-01-01

211

Heterogenised N-heterocyclic carbene complexes: synthesis, characterisation and application for hydroformylation and C-C bond formation reactions.  

Science.gov (United States)

The imidazolium salts: 1-mesityl-3-(3-trimethoxysilylpropyl)imidazolium iodide and 1-tert-butyl-3-(3-trimethoxysilylpropyl)imidazolium iodide, abbreviated as (tmpMes)HI (3a) and (tmp(t)Bu)HI (3b), respectively, have been synthesised. The palladium(ii) complexes (?(3)-C(3)H(5)) (tmpMes)PdCl (5a) and (?(3)-C(3)H(5))(tmp(t)Bu)PdCl (5b), rhodium(i) and iridium(i) complexes (?(4)-1,5-COD) (tmpMes)MCl, M = Rh (6a), Ir (7a) and (?(4)-1,5-COD)(tmp(t)Bu)MCl, where M = Rh (6b), Ir (7b), were synthesised by silver transmetallation reactions using the silver(i) complexes (tmpMes)AgI (4a) and (tmp(t)Bu)AgI (4b). The iridium complex 7b has been structurally characterised. The Pd(ii) and Rh(i) complexes have been immobilised by attachment to chemically modified MCM-41. The immobilised palladium(ii) materials have been tested as recyclable catalysts for Suzuki type C-C bond formation reactions in water and the immobilised rhodium(i) materials have been examined for their catalytic ability for the hydroformylation of 1-octene. PMID:21116572

Dastgir, Sarim; Coleman, Karl S; Green, Malcolm L H

2010-11-30

212

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; Nevin Gürbüz; Nazan Kalo?lu; Öznur Do?an; Murat Kalo?lu; Christian Bruneau; Henri Doucet

2013-01-01

213

Mechanistic and preparative studies of radical chain homolytic substitution reactions of N-heterocyclic carbene boranes and disulfides.  

UK PubMed Central (United Kingdom)

Reactions of 1,3-dimethylimidazol-2-ylidene-borane (diMe-Imd-BH3) and related NHC-boranes with diaryl and diheteroaryl disulfides provide diverse NHC-boryl monosulfides (diMe-Imd-BH2SAr) and NHC-boryl disulfides (diMe-Imd-BH(SAr)2). Heating in the dark with 1 equiv of disulfide favors monosulfide formation, while irradiation with 2 equiv disulfide favors disulfide formation. With heteroaryl disulfides, the NHC-borane in the primary NHC-boryl sulfide product migrates from sulfur to nitrogen to give new products with a thioamide substructure. Most substitution reactions are thought to proceed through radical chains in which homolytic substitution of a disulfide by an NHC-boryl radical is a key step. However, with electrophilic disulfides under dark conditions, a competing ionic path may also be possible.

Pan X; Vallet AL; Schweizer S; Dahbi K; Delpech B; Blanchard N; Graff B; Geib SJ; Curran DP; Lalevée J; Lacôte E

2013-07-01

214

Mechanistic and preparative studies of radical chain homolytic substitution reactions of N-heterocyclic carbene boranes and disulfides.  

Science.gov (United States)

Reactions of 1,3-dimethylimidazol-2-ylidene-borane (diMe-Imd-BH3) and related NHC-boranes with diaryl and diheteroaryl disulfides provide diverse NHC-boryl monosulfides (diMe-Imd-BH2SAr) and NHC-boryl disulfides (diMe-Imd-BH(SAr)2). Heating in the dark with 1 equiv of disulfide favors monosulfide formation, while irradiation with 2 equiv disulfide favors disulfide formation. With heteroaryl disulfides, the NHC-borane in the primary NHC-boryl sulfide product migrates from sulfur to nitrogen to give new products with a thioamide substructure. Most substitution reactions are thought to proceed through radical chains in which homolytic substitution of a disulfide by an NHC-boryl radical is a key step. However, with electrophilic disulfides under dark conditions, a competing ionic path may also be possible. PMID:23718209

Pan, Xiangcheng; Vallet, Anne-Laure; Schweizer, Stéphane; Dahbi, Karim; Delpech, Bernard; Blanchard, Nicolas; Graff, Bernadette; Geib, Steven J; Curran, Dennis P; Lalevée, Jacques; Lacôte, Emmanuel

2013-07-03

215

Correlations between electrochemical and spectrochemical parameters of ruthenium sulfoxides series with N-heterocyclic  

International Nuclear Information System (INIS)

A systematic study of Ru Cl sub(2) (DMSO) sub(2) L sub(2) derivates, where L = N-heterocyclic base is described, contributing for a best understanding of chemical behaviour and electronic structure of the ruthenium sulfoxides. The correlations between the electrochemical and the spectroscopical parameters of the serie are presented with more emphasis. (author)

1990-01-01

216

Activation of water, ammonia, and other small molecules by PC(carbene)P nickel pincer complexes.  

UK PubMed Central (United Kingdom)

Nickel complexes of a PC(carbene)P pincer ligand framework are described. Dehydrobromination of the precursor (PC(sp)(3)P)NiBr in the presence of a donor (PPh3 or NC(t)Bu) leads to the title complexes, which feature a rare nickel-carbene linkage as the pincer ligand anchor point. This strongly donating, nucleophilic carbene center engages in a variety of E-H bond activations (E = H, C, N, O), some of which are reversible. This represents a new mode of bond activation by ligand cooperativity in nickel pincer complexes.

Gutsulyak DV; Piers WE; Borau-Garcia J; Parvez M

2013-08-01

217

Activation of water, ammonia, and other small molecules by PC(carbene)P nickel pincer complexes.  

Science.gov (United States)

Nickel complexes of a PC(carbene)P pincer ligand framework are described. Dehydrobromination of the precursor (PC(sp)(3)P)NiBr in the presence of a donor (PPh3 or NC(t)Bu) leads to the title complexes, which feature a rare nickel-carbene linkage as the pincer ligand anchor point. This strongly donating, nucleophilic carbene center engages in a variety of E-H bond activations (E = H, C, N, O), some of which are reversible. This represents a new mode of bond activation by ligand cooperativity in nickel pincer complexes. PMID:23906261

Gutsulyak, Dmitry V; Piers, Warren E; Borau-Garcia, Javier; Parvez, Masood

2013-08-01

218

Enantioselective construction of quaternary N-heterocycles by palladium-catalysed decarboxylative allylic alkylation of lactams.  

UK PubMed Central (United Kingdom)

The enantioselective synthesis of nitrogen-containing heterocycles (N-heterocycles) represents a substantial chemical research effort and resonates across numerous disciplines, including the total synthesis of natural products and medicinal chemistry. In this Article, we describe the highly enantioselective palladium-catalysed decarboxylative allylic alkylation of readily available lactams to form 3,3-disubstituted pyrrolidinones, piperidinones, caprolactams and structurally related lactams. Given the prevalence of quaternary N-heterocycles in biologically active alkaloids and pharmaceutical agents, we envisage that our method will provide a synthetic entry into the de novo asymmetric synthesis of such structures. As an entry for these investigations we demonstrate how the described catalysis affords enantiopure quaternary lactams that intercept synthetic intermediates previously used in the synthesis of the Aspidosperma alkaloids quebrachamine and rhazinilam, but that were previously only available by chiral auxiliary approaches or as racemic mixtures.

Behenna DC; Liu Y; Yurino T; Kim J; White DE; Virgil SC; Stoltz BM

2012-02-01

219

Carbene generation by cytochromes and electronic structure of heme-iron-porphyrin-carbene complex: a quantum chemical study.  

UK PubMed Central (United Kingdom)

Carbene-heme-iron-porphyrin complexes generated from cytochrome P450 (CYP450)-mediated metabolism of compounds containing methylenedioxyphenyl (MDP) moiety lead to the mechanism-based inhibition (MBI) of CYPs. This coordination complex is termed as the metabolic-intermediate complex (MIC). The bioinorganic chemistry of MDP carbenes has been studied using quantum chemical methods employing density functional theory (B3LYP functional with implicit solvent corrections) to (i) analyze the characteristics of MDP-carbene in terms of singlet-triplet energy difference, protonation, and dimerization energies, etc.; (ii) determine the electronic structure and analyze the Fe-carbene interactions; and (iii) elucidate the potential reaction pathways for the generation of carbene, using Cpd I (iron(IV)-oxo-porphine with SH(-) as the axial ligand) as the model oxidant to mimic the activity of CYP450. The results show that MDP-carbenes are sufficiently stable and nucleophilic, leading to the formation of stable MIC (-40.35 kcal/mol) on the doublet spin state, formed via interaction between ?(LP) of carbene and empty dz(2) orbital of heme-iron. This was aided by the back-bonding between filled d(xz) orbital of heme-iron and the empty p orbital of carbene. The mechanistic pathway proposed in the literature for the generation of MDP-carbene (CH hydroxylation followed by water elimination) was studied, and observed to be unfavorable, owing to the formation of highly stable hydroxylated product (-57.12 kcal/mol). An intriguing pathway involving hydride ion abstraction and proton transfer followed by water elimination step was observed to be the most probable pathway.

Taxak N; Patel B; Bharatam PV

2013-05-01

220

Carbene generation by cytochromes and electronic structure of heme-iron-porphyrin-carbene complex: a quantum chemical study.  

Science.gov (United States)

Carbene-heme-iron-porphyrin complexes generated from cytochrome P450 (CYP450)-mediated metabolism of compounds containing methylenedioxyphenyl (MDP) moiety lead to the mechanism-based inhibition (MBI) of CYPs. This coordination complex is termed as the metabolic-intermediate complex (MIC). The bioinorganic chemistry of MDP carbenes has been studied using quantum chemical methods employing density functional theory (B3LYP functional with implicit solvent corrections) to (i) analyze the characteristics of MDP-carbene in terms of singlet-triplet energy difference, protonation, and dimerization energies, etc.; (ii) determine the electronic structure and analyze the Fe-carbene interactions; and (iii) elucidate the potential reaction pathways for the generation of carbene, using Cpd I (iron(IV)-oxo-porphine with SH(-) as the axial ligand) as the model oxidant to mimic the activity of CYP450. The results show that MDP-carbenes are sufficiently stable and nucleophilic, leading to the formation of stable MIC (-40.35 kcal/mol) on the doublet spin state, formed via interaction between ?(LP) of carbene and empty dz(2) orbital of heme-iron. This was aided by the back-bonding between filled d(xz) orbital of heme-iron and the empty p orbital of carbene. The mechanistic pathway proposed in the literature for the generation of MDP-carbene (CH hydroxylation followed by water elimination) was studied, and observed to be unfavorable, owing to the formation of highly stable hydroxylated product (-57.12 kcal/mol). An intriguing pathway involving hydride ion abstraction and proton transfer followed by water elimination step was observed to be the most probable pathway. PMID:23560646

Taxak, Nikhil; Patel, Bhargav; Bharatam, Prasad V

2013-04-05

 
 
 
 
221

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

UK PubMed Central (United Kingdom)

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.

Schranck J; Tlili A; Beller M

2013-07-01

222

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

2013-06-17

223

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

Digital Repository Infrastructure Vision for European Research (DRIVER)

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, we...

Franca M. Cordero; Carolina Vurchio; Stefano Cicchi; Armin de Meijere; Alberto Brandi

224

Near-UV phosphorescent emitters: N-heterocyclic platinum(ii) tetracarbene complexes.  

UK PubMed Central (United Kingdom)

Although examples of nickel(ii), palladium(ii) and platinum(ii) N-heterocyclic tetracarbene complexes are known in the literature, particularly platinum(ii) tetracarbene complexes are rare. We developed a new synthetic route via biscarbene acetate complexes, which make homoleptic as well as heteroleptic platinum(ii) tetracarbene complexes accessible. The reported photoluminescence data show that these complexes have good quantum yields and photostability and are a promising class of emitters for PhOLEDs. Characterization of the compounds includes a solid-state structure of the homoleptic complex bis(1,1'-diisopropyl-3,3'-methylenediimidazoline-2,2'-diylidene)platinum(ii) dibromide.

Unger Y; Zeller A; Taige MA; Strassner T

2009-06-01

225

Medium and high oxidation state metal/non-metal fluoride and oxide-fluoride complexes with neutral donor ligands.  

UK PubMed Central (United Kingdom)

While most high and medium oxidation state (O.S. ? 3) metal and non-metal fluorides and oxide fluorides are strong Lewis acids, exploration of their coordination chemistry with neutral ligands has been limited and mostly non-systematic. This is despite the very different properties conferred on the acceptor centre by the small electronegative fluoride ligands compared to the heavier halides. This article sets out these key differences, discusses possible synthetic routes, the key characterisation techniques, and appropriate bonding models. Current knowledge of the coordination chemistry of d, f and p-block fluorides and oxide fluorides with neutral ligands (with donor atoms drawn from Groups 15 and 16 and including N-heterocyclic carbenes) is then presented and discussed, and the differences in properties compared to complexes containing the heavier halides are illustrated. The emphasis is on work published post 1990, but earlier work is also included as essential background and where no more recent information exists. Attention is drawn to unexplored areas meriting investigation and to possible applications of these complexes.

Benjamin SL; Levason W; Reid G

2013-02-01

226

Facile access to silyl-functionalized N-heterocyclic olefins with HSiCl3.  

Science.gov (United States)

N-heterocyclic olefins (NHOs), IPrCH2 () and SIPrCH2 () (IPrCH2 = {N(2,6-iPr2C6H3)CH}2CCH2 and SIPrCH2 = {N(2,6-iPr2C6H3)CH2}2CCH2), react with HSiCl3 and afford IPrCH(SiHCl2) () and SIPrCH(SiHCl2) (), respectively. Compounds and have been isolated in almost quantitative yield. Interestingly, treatment of the silylene IPr·SiCl2 with also affords , where silylene insertion into a C-H bond is observed. Computational analysis shows a high energy barrier for silylene insertion, therefore a protonation-deprotonation mechanism is more likely. PMID:24005446

Ghadwal, Rajendra S; Reichmann, Sven O; Engelhardt, Felix; Andrada, Diego M; Frenking, Gernot

2013-10-21

227

Fast CO2 Sequestration, Activation, and Catalytic Transformation Using N-Heterocyclic Olefins.  

UK PubMed Central (United Kingdom)

N-Heterocyclic Olefin (NHO) with high electronegativity at the terminal carbon atom was found to show a strong tendency for CO2 sequestration, affording a CO2 adduct (NHO-CO2). X-ray single crystal analysis revealed the bent geometry of the binding CO2 in the NHO-CO2 adducts with an O-C-O angle of 127.7-129.9°, dependent on the substitute groups of N-heterocyclic ring. The length of the Ccarboxylate-CNHO bond is in the range of 1.55-1.57 Å, significantly longer than that of the Ccarboxylate-CNHC bond (1.52-1.53 Å) of the previously reported NHC-CO2 adducts. The FTIR study by monitoring the ?(CO2) region of transmittance change demonstrated that the decarboxylation of NHO-CO2 adducts is easier than that of the corresponding NHC-CO2 adducts. Notably, the NHO-CO2 adducts were found to be highly active in catalyzing the carboxylative cyclization of CO2 and propargylic alcohols at mild conditions (even at ambient temperature and 0.1 MPa CO2 pressure), selectively giving ?-alkylidene cyclic carbonates in good yields. The catalytic activity is about 10-200 times that of the corresponding NHC-CO2 adducts at the same conditions. Two reaction paths regarding the hydrogen at the alkenyl position of cyclic carbonates coming from substrate (path A) or both substrate and catalyst (path B) were proposed on the basis of deuterium labeling experiments. The high activity of NHO-CO2 adduct was tentatively ascribed to its low stability for easily releasing the CO2 moiety and/or the desired product, a possible rate-limiting step in the catalytic cycle.

Wang YB; Wang YM; Zhang WZ; Lu XB

2013-08-01

228

Fast CO2 Sequestration, Activation, and Catalytic Transformation Using N-Heterocyclic Olefins.  

Science.gov (United States)

N-Heterocyclic Olefin (NHO) with high electronegativity at the terminal carbon atom was found to show a strong tendency for CO2 sequestration, affording a CO2 adduct (NHO-CO2). X-ray single crystal analysis revealed the bent geometry of the binding CO2 in the NHO-CO2 adducts with an O-C-O angle of 127.7-129.9°, dependent on the substitute groups of N-heterocyclic ring. The length of the Ccarboxylate-CNHO bond is in the range of 1.55-1.57 Å, significantly longer than that of the Ccarboxylate-CNHC bond (1.52-1.53 Å) of the previously reported NHC-CO2 adducts. The FTIR study by monitoring the ?(CO2) region of transmittance change demonstrated that the decarboxylation of NHO-CO2 adducts is easier than that of the corresponding NHC-CO2 adducts. Notably, the NHO-CO2 adducts were found to be highly active in catalyzing the carboxylative cyclization of CO2 and propargylic alcohols at mild conditions (even at ambient temperature and 0.1 MPa CO2 pressure), selectively giving ?-alkylidene cyclic carbonates in good yields. The catalytic activity is about 10-200 times that of the corresponding NHC-CO2 adducts at the same conditions. Two reaction paths regarding the hydrogen at the alkenyl position of cyclic carbonates coming from substrate (path A) or both substrate and catalyst (path B) were proposed on the basis of deuterium labeling experiments. The high activity of NHO-CO2 adduct was tentatively ascribed to its low stability for easily releasing the CO2 moiety and/or the desired product, a possible rate-limiting step in the catalytic cycle. PMID:23865980

Wang, Yan-Bo; Wang, Yi-Ming; Zhang, Wen-Zhen; Lu, Xiao-Bing

2013-07-31

229

Robust cyclometallated Ir(III) catalysts for the homogeneous hydrogenation of N-heterocycles under mild conditions.  

Science.gov (United States)

Cyclometallated Cp*Ir(N^C)Cl complexes derived from N-aryl ketimines are highly active catalysts for the reduction of N-heterocycles under ambient conditions and 1 atm H2 pressure. The reaction tolerates a broad range of other potentially reducible functionalities and does not require the use of specialised equipment, additives or purified solvent. PMID:23812043

Wu, Jianjun; Barnard, Jonathan H; Zhang, Yi; Talwar, Dinesh; Robertson, Craig M; Xiao, Jianliang

2013-08-14

230

Robust cyclometallated Ir(III) catalysts for the homogeneous hydrogenation of N-heterocycles under mild conditions.  

UK PubMed Central (United Kingdom)

Cyclometallated Cp*Ir(N^C)Cl complexes derived from N-aryl ketimines are highly active catalysts for the reduction of N-heterocycles under ambient conditions and 1 atm H2 pressure. The reaction tolerates a broad range of other potentially reducible functionalities and does not require the use of specialised equipment, additives or purified solvent.

Wu J; Barnard JH; Zhang Y; Talwar D; Robertson CM; Xiao J

2013-08-01

231

Observation of a stable carbene at the active site of a thiamin enzyme.  

UK PubMed Central (United Kingdom)

Carbenes are highly reactive chemical compounds that are exploited as ligands in organometallic chemistry and are powerful organic catalysts. They were postulated to occur as transient intermediates in enzymes, yet their existence in a biological system could never be demonstrated directly. We present spectroscopic and structural data of a thiamin enzyme in a noncovalent complex with substrate, which implicate accumulation of a stable carbene as a major resonance contributor to deprotonated thiamin.

Meyer D; Neumann P; Ficner R; Tittmann K

2013-08-01

232

Conjugate reduction of alpha,beta-unsaturated carbonyl compounds catalyzed by a copper carbene complex.  

Science.gov (United States)

[reaction: see text] An N-heterocyclic carbene copper chloride (NHC-CuCl) complex (2) has been prepared and used to catalyze the conjugate reduction of alpha,beta-unsaturated carbonyl compounds. The combination of catalytic amounts of 2 and NaOt-Bu with poly(methylhydrosiloxane) (PMHS) as the stoichiometric reductant generates an active catalyst for the 1,4-reduction of tri- and tetrasubstituted alpha,beta-unsaturated esters and cyclic enones. The active catalytic species can also be generated in situ from 1,3-bis(2,6-di-isopropylphenyl)-imidazolium chloride (1) CuCl(2).2H(2)O in the presence of NaOt-Bu and PMHS. PMID:12841744

Jurkauskas, Valdas; Sadighi, Joseph P; Buchwald, Stephen L

2003-07-10

233

New thiocyanatocadmates templated by multi-dentate N-heterocyclic/diamine molecules.  

UK PubMed Central (United Kingdom)

With diverse multi-dentate N-heterocyclic/diamine molecules as the cations, the reactions between Cd(2+) and SCN(-) in C2H5OH/H2O solution acidified by H2SO4 created five new thiocyanatocadmates [H2(bim)][Cd(SCN)2(H2O)2]SO4 (bim = 2,2'-biimidazole) 1, [H2(pympip)]2[Cd2(SCN)4(SO4)2(H2O)4]·2H2O (pympip = 1-(2-pyrimidyl)piperazine) 2, [H2(tdpy)]2[Cd3(SCN)10] (tdpy = 4,4'-thiodipyridine) 3, [H2(badpm)][Cd(SCN)4] (badpm = 4,4'-biaminodiphenylmethane) 4 and [H2(pdma)][Cd2(SCN)4(SO4)] (pdma = 1,4-phenylenedimethanamine) 5. It is noteworthy that (i) in compounds 1, 2 and 5, SO4(2-) was incorporated into the organically templated thiocyanatocadmate frameworks; (ii) compound is a rare example, in which two types of chained thiocyanatocadmates are found, and (iii) compound shows an interesting layer structure, constructed up from the ribboned thiocyanatocadmates by ?2-mode SO4(2-). Photoluminescence analysis revealed that compounds 1, 2 and 5 in solid state emit blue light with the maximum at 425 nm for 1, 405 nm for 2 and 445 nm for 5, respectively.

Jia HL; Jia MJ; Li GH; Wang YN; Yu JH; Xu JQ

2013-05-01

234

New thiocyanatocadmates templated by multi-dentate N-heterocyclic/diamine molecules.  

Science.gov (United States)

With diverse multi-dentate N-heterocyclic/diamine molecules as the cations, the reactions between Cd(2+) and SCN(-) in C2H5OH/H2O solution acidified by H2SO4 created five new thiocyanatocadmates [H2(bim)][Cd(SCN)2(H2O)2]SO4 (bim = 2,2'-biimidazole) 1, [H2(pympip)]2[Cd2(SCN)4(SO4)2(H2O)4]·2H2O (pympip = 1-(2-pyrimidyl)piperazine) 2, [H2(tdpy)]2[Cd3(SCN)10] (tdpy = 4,4'-thiodipyridine) 3, [H2(badpm)][Cd(SCN)4] (badpm = 4,4'-biaminodiphenylmethane) 4 and [H2(pdma)][Cd2(SCN)4(SO4)] (pdma = 1,4-phenylenedimethanamine) 5. It is noteworthy that (i) in compounds 1, 2 and 5, SO4(2-) was incorporated into the organically templated thiocyanatocadmate frameworks; (ii) compound is a rare example, in which two types of chained thiocyanatocadmates are found, and (iii) compound shows an interesting layer structure, constructed up from the ribboned thiocyanatocadmates by ?2-mode SO4(2-). Photoluminescence analysis revealed that compounds 1, 2 and 5 in solid state emit blue light with the maximum at 425 nm for 1, 405 nm for 2 and 445 nm for 5, respectively. PMID:23467616

Jia, Hong-Li; Jia, Ming-Jun; Li, Guang-Hua; Wang, Yan-Ning; Yu, Jie-Hui; Xu, Ji-Qing

2013-05-14

235

Chromium-53 nuclear magnetic resonance studies of pentacarbonylchromium-carbene complexes  

International Nuclear Information System (INIS)

The 53Cr NMR spectra of 46 chromium-carbene complexes were recorded, and chemical shift and line width data are reported, along with associated infrared, uv-visible, and 13C NMR spectroscopic data. The 53Cr chemical shifts correlated very well with the donor-acceptor properties of the carbene ligand and were quite sensitive to steric influences on efficiency of ?-overlap of the heteroatom with the carbene carbon. There was no correlation of ?(53Cr) with reactivity toward imines to form ?-lactams, but line width appeared to be related to reactivity. Preliminary attempts to utilize 53Cr NMR data to direct synthetic utilization of chromium-carbene complexes are reported. 5 refs., 49 figs., 1 tab.

1988-01-01

236

Direct synthesis of cis-dihalido-bis(NHC) complex of nickel(ii) and catalytic application in olefin addition polymerization: Effect of halogen co-ligands and density functional theory study.  

Science.gov (United States)

Two novel amine-containing N-heterocyclic carbene ligand precursors [H()]Br have been prepared in good yield and fully characterized. Direct syntheses of cis- and trans-dihalido-bis(NHC) nickel complexes [Ni(NHC)2X2] (X = Cl, Br) are reported. The solid structures of trans-[Ni()2Br2] () and cis-[Ni()2Cl2] () were determined by single-crystal X-ray analysis and was found to be the first example of cis-configuration coordination of monodentate NHC ligands to a metal center for dihalido-bis(NHC) nickel complexes. DFT calculations were conducted to determine the energy difference between cis- and trans-isomers of complexes and bearing bromide and chloride co-ligands. The cis-[Ni()2Cl2] (cis-) is 1.77-1.55 kcal mol(-1) lower in energy than its trans-isomer in polar solvents including CH2Cl2 and THF, while the trans-[Ni()2Br2] (trans-) is more stable than the cis-isomer similarly in the gas phase. The cis nickel complex with two coordinated monodentate NHCs was tested for olefin addition polymerization at standard conditions. It was found that cis- was inactive in ethylene polymerization but showed moderate catalytic activities (0.5-3.0 × 10(6) g of PNB (mol of Ni)(-1) h(-1)) in the addition polymerization of norbornene in the presence of methylaluminoxane (MAO) as cocatalyst. PMID:23838704

Zhang, Dao; Zhou, Sen; Li, Zhiming; Wang, Quanrui; Weng, Linhong

2013-07-31

237

Probing the Relevance of NHC Ligand Conformations in the Ru-Catalysed Ring-Closing Metathesis Reaction.  

UK PubMed Central (United Kingdom)

Cat power: Judicious backbone substitution of N-heterocyclic carbenes (NHCs) leads to stable Ru metathesis catalysts with frozen NHC conformations. This finding not only permits the isolation of complexes that are among the most active catalysts in the ring-closing metathesis of hindered olefins (see graphic; Ts=p-toluenesulfonyl), but also provides fundamental mechanistic insights on the role of N-aryl substituent conformations on catalyst activity.

Perfetto A; Costabile C; Longo P; Bertolasi V; Grisi F

2013-08-01

238

Amides of gold(I) diphosphines prepared from N-heterocyclic sources and their in vitro and in vivo screening for anticancer activity.  

UK PubMed Central (United Kingdom)

A series of new neutral mononuclear or dinuclear gold(I) complexes and a cyclic cationic tetranuclear amidogold(I) complex comprising of the phosphines 1,2-bis(dimethylphosphino)ethane (dmpe), ?-1,2-bis(diphenylphosphino)ethane (dppe), ?-1,3-bis(diphenylphosphino)propane (dppp), ?-1,5-bis(diphenylphosphino)pentane (dpppe), ?-1,6-bis(diphenylphosphino)hexane (dpph) or trimethylphosphine, and several N-heterocyclic ring systems (imidazolate, pyrazolate, 1,2,3-triazolate, 1,2,4-triazolate, pyrrolate, 9H-purine-9-ate or 9H-purine-6-amine-9-ate) as ligands, reveal intermolecular aurophilic interactions and 2D channels available for solvent molecules in some of their crystal structures. The antitumour activity of the acyclic gold(I) compounds is highly dependent on the substituents on the phosphorus atoms being highest for phenyl groups and lower for methyl groups. The activity of these compounds against selected cell lines is linked to the length of the carbon bridge between the two phosphorus atoms being highest with a bridge consisting of 5 or 6 carbons. Two compounds with the highest tumour specifities that contain dpppe and pyrazolate (a lipophilic compound) or 1,2,4-triazolate (a hydrophilic compound) induce an apoptotic cell death pathway and a maximum dose to Balb/C mice is tolerated.

Horvath UE; Dobrza?ska L; Strasser CE; Bouwer Neé Potgieter W; Joone G; van Rensburg CE; Cronje S; Raubenheimer HG

2012-06-01

239

N-heterocyclic carbene (NHC)-catalyzed/Lewis acid mediated conjugate umpolung of alkynyl aldehydes for the synthesis of butenolides: a formal [3+2] annulation.  

UK PubMed Central (United Kingdom)

Reverse to success! A new formal [3+2] annulation reaction combining alkynyl aldehydes and ?,?-unsaturated ?-ketoesters has been disclosed by using a NHC-catalyzed/Lewis acid mediated strategy. This cooperative catalysis strategy first allows the "allenolate" intermediate as a nucleophilic synthon at the ?-position to react with activated electrophilic reagents by an addition reaction as the key C-C bond-forming step.

Qi J; Xie X; Han R; Ma D; Yang J; She X

2013-03-01

240

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.

Lian-jun Liu; Feijun Wang; Wenfeng Wang; Mei-xin Zhao; Min Shi

2011-01-01

 
 
 
 
241

Carbene transfer from triazolylidene gold complexes as a potent strategy for inducing high catalytic activity.  

Science.gov (United States)

A series of gold(I) complexes [AuCl(trz)] were synthesized that contain 1,2,3-triazolylidene (trz) ligands with variable wingtip groups. In the presence of AgBF4, these complexes undergo ligand redistribution to yield cationic complexes [Au(trz)2]BF4 in high yields as a result of efficient carbene transfer. Identical reactivity patterns were detected for carbene gold complexes comprised of Arduengo-type IMes ligands (IMes = N,N'-dimesityl-imidazol-2-ylidene). Reaction of cationic complexes [Au(trz)2](+) with [AuCl(trz')] afforded the heteroleptic complex [Au(trz)(trz')](+) and [AuCl(trz)] (trz, trz' = triazolylidene ligands with different wingtip groups). Carbene transfer occurs spontaneously, yet is markeldy rate-enhanced in the presence of Ag(+). The facile carbene transfer was exploited as a catalyst activation process to form active gold species for the aldol condensation of isocyanides and aldehydes to form oxazolines. The catalytic activity is strongly dependent on the presence of Ag(+) ions to initiate catalyst activation. High turnovers (10(5)) and turnover frequencies (10(4) h(-1)) were accomplished. Structural analysis at early stages of the reaction support the critical role of triazolylidene dissociation to activate the precatalyst and dynamic light scattering revealed the presence of nanoparticles (±100 nm diameter) as potential catalytically active species. Furthermore, the triazolylidene scaffold had no impact on the diastereoselectivity of the oxazoline formation, and chiral triazolylidenes did not induce any asymmetry in the product. The facile dissociation of carbenes from [AuCl(carbene)] in the presence of Ag(+) ions suggests a less stable Au-Ccarbene interaction than often assumed, with potential implications for gold-catalyzed reactions that employ a silver salt as (putative) halide scavenger. PMID:23902160

Canseco-Gonzalez, Daniel; Petronilho, Ana; Mueller-Bunz, Helge; Ohmatsu, Kohsuke; Ooi, Takashi; Albrecht, Martin

2013-08-21

242

N-heterocyclic carbazole-based hosts for simplified single-layer phosphorescent OLEDs with high efficiencies.  

UK PubMed Central (United Kingdom)

Highly efficient single-layer organic light-emitting diodes (OLEDs) are demonstrated using new N-heterocyclic carbazole-based host materials. Phosphorescent OLEDs with a structure of ITO/MoO(3) /host/host:dopant/host/Cs(2) CO(3) /Al are fabricated in which the new materials act simultaneously as electron-transport, hole-transport, and host layer. Devices with maximum current and external quantum efficiencies of 92.2 cd A(-1) and 26.8% are achieved, the highest reported to date for a single-layer OLED.

Hudson ZM; Wang Z; Helander MG; Lu ZH; Wang S

2012-06-01

243

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

Directory of Open Access Journals (Sweden)

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%).

Franca M. Cordero; Carolina Vurchio; Stefano Cicchi; Armin de Meijere; Alberto Brandi

2011-01-01

244

Luminescent biscarbene iridium(III) complexes as living cell imaging reagents.  

UK PubMed Central (United Kingdom)

Five iridium(III) complexes with two N-heterocyclic carbene (NHC) ligands and an ancillary ligand have been designed and successfully synthesized. With multicolor photoluminescence and low toxicity, these carbene complexes were tested, for the first time, as living cell imaging reagents and showed promise for application beyond the OLED (organic light emitting diode) area.

Zhou Y; Jia J; Li W; Fei H; Zhou M

2013-04-01

245

Syntheses, Structural, Computational and Thermal Analysis of Acid-Base Complexes of Picric Acid with N-Heterocyclic Bases.  

UK PubMed Central (United Kingdom)

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 non-covalent 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. Keywords: X-ray crystallography, Hydrogen bond interaction energy, Isothermal TG, Kinetics, Ignition delay.

Goel N; Singh UP

2013-09-01

246

Transition metal free hydrolysis/cyclization strategy in a single pot: synthesis of fused furo N-heterocycles of pharmacological interest.  

UK PubMed Central (United Kingdom)

A transition metal free tandem two-step strategy has been developed involving hydrolysis of 2-chloro-3-alkynyl quinoxalines/pyrazines followed by in situ cyclization of the corresponding 2-hydroxy-3-alkynyl intermediates in a single pot leading to fused furo N-heterocycles as potential inhibitors of sirtuins. A representative compound showed promising pharmacological properties in vitro and in vivo.

Nakhi A; Rahman MS; Seerapu GP; Banote RK; Kumar KL; Kulkarni P; Haldar D; Pal M

2013-08-01

247

Hydroformylation by Pt-Sn compounds from N-heterocyclic stannylenes.  

UK PubMed Central (United Kingdom)

[(Me(2)Si{NAr}(2))-?(2)N,N']Sn, reacts with PtCl(2)(L(2)) and [PtCl(?-Cl)(L)](2) to afford products containing Pt-Sn bonds. In the absence of supporting ligands L, coordination of the stannylene and rearrangement to a structurally unique PtSn(2)N(2)Si metallacycle occurs. The hydroformylation activity of a representative Pt-Sn compound is investigated.

Day BM; Dyer PW; Coles MP

2012-07-01

248

Hydroformylation by Pt-Sn compounds from N-heterocyclic stannylenes.  

Science.gov (United States)

[(Me(2)Si{NAr}(2))-?(2)N,N']Sn, reacts with PtCl(2)(L(2)) and [PtCl(?-Cl)(L)](2) to afford products containing Pt-Sn bonds. In the absence of supporting ligands L, coordination of the stannylene and rearrangement to a structurally unique PtSn(2)N(2)Si metallacycle occurs. The hydroformylation activity of a representative Pt-Sn compound is investigated. PMID:22622769

Day, Benjamin M; Dyer, Philip W; Coles, Martyn P

2012-05-24

249

Palladium-catalyzed carbene migratory insertion using conjugated ene-yne-ketones as carbene precursors.  

Science.gov (United States)

Palladium-catalyzed cross-coupling reactions between benzyl, aryl, or allyl bromides and conjugated ene-yne-ketones lead to the formation of 2-alkenyl-substituted furans. This novel coupling reaction involves oxidative addition, alkyne activation-cyclization, palladium carbene migratory insertion, ?-hydride elimination, and catalyst regeneration. Palladium (2-furyl)carbene is proposed as the key intermediate, which is supported by DFT calculations. The palladium carbene character of the key intermediate is validated by three aspects, including bond lengths, Wiberg bond order indices, and molecular orbitals, by comparison to those reported for stable palladium carbene species. Computational studies also revealed that the rate-limiting step is ene-yne-ketone cyclization, which leads to the formation of the palladium (2-furyl)carbene, while the subsequent carbene migratory insertion is a facile process with a low energy barrier (<5 kcal/mol). PMID:23947689

Xia, Ying; Qu, Shuanglin; Xiao, Qing; Wang, Zhi-Xiang; Qu, Peiyuan; Chen, Li; Liu, Zhen; Tian, Leiming; Huang, Zhongxing; Zhang, Yan; Wang, Jianbo

2013-09-03

250

Palladium-catalyzed carbene migratory insertion using conjugated ene-yne-ketones as carbene precursors.  

UK PubMed Central (United Kingdom)

Palladium-catalyzed cross-coupling reactions between benzyl, aryl, or allyl bromides and conjugated ene-yne-ketones lead to the formation of 2-alkenyl-substituted furans. This novel coupling reaction involves oxidative addition, alkyne activation-cyclization, palladium carbene migratory insertion, ?-hydride elimination, and catalyst regeneration. Palladium (2-furyl)carbene is proposed as the key intermediate, which is supported by DFT calculations. The palladium carbene character of the key intermediate is validated by three aspects, including bond lengths, Wiberg bond order indices, and molecular orbitals, by comparison to those reported for stable palladium carbene species. Computational studies also revealed that the rate-limiting step is ene-yne-ketone cyclization, which leads to the formation of the palladium (2-furyl)carbene, while the subsequent carbene migratory insertion is a facile process with a low energy barrier (<5 kcal/mol).

Xia Y; Qu S; Xiao Q; Wang ZX; Qu P; Chen L; Liu Z; Tian L; Huang Z; Zhang Y; Wang J

2013-09-01

251

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

UK PubMed Central (United Kingdom)

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.

Mourgas G; Nieger M; Förster D; Gudat D

2013-04-01

252

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

2013-03-21

253

Preparation of second generation ionic liquids by efficient solvent-free alkylation of N-heterocycles with chloroalkanes.  

UK PubMed Central (United Kingdom)

Non-conventional techniques, such as microwave (MW) and power ultrasound (US) as well as combined MW/US irradiation, have been used to promote one-pot synthesis of second-generation ionic liquids (ILs), cutting down reaction times and improving yields. However, the use of chloroalkanes in the alkylation of N-heterocycles requires more drastic conditions if results are to match those obtained with more reactive alkyl halides. The present paper describes a series of MW- or MW/US-promoted IL preparations starting from chloroalkanes and classic heterocycles (1-methylimidazole, pyridine and 1-methylpyrrolidine). When reactions were carried out under conventional heating in an oil bath they required longer reaction times and gave poorer yields. (1)H-NMR analysis and ion-exchange chromatography showed that the present solventless procedure afforded ILs of satisfactory purity. The observed high yields (usually 70-98% isolated), and short reaction times showed that a straightforward access to ILs can be also achieved with the use of alkyl chlorides, resulting in a considerable reduction of costs.

Cravotto G; Gaudino EC; Boffa L; Lévêque JM; Estager J; Bonrath W

2008-01-01

254

Direct synthesis of cis-dihalido-bis(NHC) complex of nickel(II) and catalytic application in olefin addition polymerization: effect of halogen co-ligands and density functional theory study.  

UK PubMed Central (United Kingdom)

Two novel amine-containing N-heterocyclic carbene ligand precursors [H(1a-b)]Br have been prepared in good yield and fully characterized. Direct syntheses of cis- and trans-dihalido-bis(NHC) nickel complexes [Ni(NHC)2X2] (X = Cl, Br) are reported. The solid structures of trans-[Ni(1a-b)2Br2] (2a-b) and cis-[Ni(1a)2Cl2] (3) were determined by single-crystal X-ray analysis and 3 was found to be the first example of cis-configuration coordination of monodentate NHC ligands to a metal center for dihalido-bis(NHC) nickel complexes. DFT calculations were conducted to determine the energy difference between cis- and trans-isomers of complexes 2a and 3 bearing bromide and chloride co-ligands. The cis-[Ni(1a)2Cl2] (cis-3) is 1.77-1.55 kcal mol(-1) lower in energy than its trans-isomer in polar solvents including CH2Cl2 and THF, while the trans-[Ni(1a)2Br2] (trans-2a) is more stable than the cis-isomer similarly in the gas phase. The cis nickel complex 3 with two coordinated monodentate NHCs was tested for olefin addition polymerization at standard conditions. It was found that cis-3 was inactive in ethylene polymerization but showed moderate catalytic activities (0.5-3.0 × 10(6) g of PNB (mol of Ni)(-1) h(-1)) in the addition polymerization of norbornene in the presence of methylaluminoxane (MAO) as cocatalyst.

Zhang D; Zhou S; Li Z; Wang Q; Weng L

2013-09-01

255

A study of the resistance of some N-heterocycles (BTPS) to radiolysis  

International Nuclear Information System (INIS)

Full text of publication follows. One option for the selective extraction of trivalent minor actinides within highly active effluents coming from the PUREX process could be as follows: firstly, to co-extract trivalent actinides [An(III)Am(III) and Cm(III)] and also lanthanides [Ln(III)] from the PUREX raffinates; secondly, to separate An(III) from Ln(III) by means of selective extractants. Although efficient extractants for the co-extraction of An(III) and Ln(III) have been developed, selective extractants for An(III) are still required. The various tetra-alkyl substituted BTP molecules, which have been studied within the FP5 European integrated project PARTNEW have been shown to be too sensitive towards acidic hydrolysis and alpha-radiolysis for the development of an An(III)/Ln(III) separation process. Other extracting molecules have been developed within the framework of the FP6 European Integrated Project EUROPART. These molecules include some nitrogen heterocycles. This paper presents some of the basic research carried out in order to improve the resistance of the chosen molecules towards radiolytic degradation. Attempts to improve their extraction properties by the addition of synergists, which usually avoid third-phase formation and improve the kinetics of extraction, will be outlined. It appears that some reagents are not only more selective towards An(III) than tetra-alkyl substituted BTPs, but also more resistant to radiolysis. The carbon atoms on the alpha positions of their triazinyl rings are fully substituted, thus preventing oxidation and nitration of the alkyl groups. Although none of the presently studied ligands has yet met the requirements of a process development strategy, huge improvements have been made in the field of nitrogen heterocycles since the first reference molecule, tetra-n-propyl-bis-triazinyl-pyridine (nPr-BTP), was first chosen in 1998 to demonstrate the scientific feasibility of An(III)/Ln(III) separation from highly active waste of medium acidity ([HNO3] > 0.5 mol/L). Many of these developments have taken place within European research programmes. (authors)

2007-01-01

256

Transition metal free hydrolysis/cyclization strategy in a single pot: synthesis of fused furo N-heterocycles of pharmacological interest.  

Science.gov (United States)

A transition metal free tandem two-step strategy has been developed involving hydrolysis of 2-chloro-3-alkynyl quinoxalines/pyrazines followed by in situ cyclization of the corresponding 2-hydroxy-3-alkynyl intermediates in a single pot leading to fused furo N-heterocycles as potential inhibitors of sirtuins. A representative compound showed promising pharmacological properties in vitro and in vivo. PMID:23824158

Nakhi, Ali; Rahman, Md Shafiqur; Seerapu, Guru Pavan Kumar; Banote, Rakesh Kumar; Kumar, Kummari Lalith; Kulkarni, Pushkar; Haldar, Devyani; Pal, Manojit

2013-07-03

257

Process for the preparation of transition metal carbene complexes  

UK PubMed Central (United Kingdom)

Preparation of cyclometalated transition metal-carbene complex (I) comprises reacting a ligand precursor (III) with a base and an auxiliary reagent comprising at least a metal salt of silver, mercury, antimony, magnesium, boron or aluminum (step A), or with a basic auxiliary reagent comprising at least a metal such as silver, mercury, antimony, magnesium, boron or aluminum (step B). Preparation of cyclometalated transition metal-carbene complex (I) of formula (((L) m)((K 1>) o-M 1>-(carbene) n)) comprises reacting a ligand precursor of formula (III) with a base and an auxiliary reagent comprising at least a metal salt of silver, mercury, antimony, magnesium, boron or aluminum (step A), or with a basic auxiliary reagent comprising at least a metal such as silver, mercury, antimony, magnesium, boron or aluminum (step B), where (III) reacts with a base, an auxiliary reagent and a metal complex comprising at least a metal (M 1>) (step A) or (III) reacts with a basic auxiliary reagent and a metal complex containing at least a metal (M 1>) (step B). M 1>metal atom such as Ir, Pt, Rh, Os (all preferred), Co, Ni, Pd, Fe, Ru, Cr, Mo, W, Mn, Tc, Re, Cu or Au L : mono- or di- anionic ligand, which is optionally mono- or bi- dentate K 1>neutral mono- or bi- dentate ligand carbene : carbene ligand of formula (II) n : greater than 1, preferably 2 (and the carbene ligand in (I) is n with greater than 1 or different) m : 0 or >=1 and the ligand (L) in m is greater than 1 or different o : 0 or greater than 1, where the number of ligand K 1> in o is greater than 1 or different Do 1>donor atoms such as P, N, O, S (all preferred) C or Si Do 2>donor atoms such as C, N, P, O or S r : 2 (if Do 1> is Si), 1 (if Do 1> is N or P) or 0 (if Do 1> is O or S) s : 2 (if Do 2> is C), 1 (if Do 2> is N or P) or 0 (if Do 2> is O or S) X : spacer such as silylene, alkylene, (hetero)arylene, alkynylene, alkenylene, NR 13>, PR 14>, B 1>R 15>, O, S, SO, SO 2, CO, CO-O, O-CO or (CR 16>R 17>) w, where one or more adjacent groups of (CR 16>R 17>) is replaced by NR 13>, PR 14>, B 1>R 15>, O, S, SO, SO 2, CO, CO-O or O-CO w : 2-10 R 13>-R 17>H, alkyl, (hetero)aryl, alkenyl or alkynyl p, q : 0 or 1 either Y 1>, Y 2>H or C such as alkyl-, (hetero)aryl-, alkynyl- or alkenyl or Y 1>, Y 2>form a bridge between Do 1> and nitrogen atom and exhibits at least two atoms, in which at least one is a carbon atom Y 3>H, alkyl, (hetero)aryl, alkynyl, alkenyl or H-(((R 3>) s-Do 2>) q)(R 2>)-C=C(R 1>)(X) p-CH 3 (where Y 3> and Y 2> are connected together over a bridge, which is alkylene, arylene, heteroarylene, alkynylene, alkenylene, NR 25>, PR 26>, BR 27>, O, S, SO, SO 2, SiR 32>R 33>, CO, CO-O, O-CO or (CR 28>R 29>) y) y : 2-10 R 25>-R 29>, R 32>, R 33>H, alkyl, aryl, heteroaryl, alkenyl or alkynyl either R 1>, R 2>H, alkyl-, aryl-, heteroaryl-, alkynyl or alkenyl or R 1>R 2>a bridge with 3-5 atoms, where one or two atoms are heteroatoms and the remaining atoms are carbon atoms, so that the group forms a 5-6 membered ring (optionally exhibits one double bond in addition to already available double bonds- or two further double bonds in 6 or 7 membered ring, optionally substituted with alkyl- or aryl and/or groups with donor- or acceptor effect, and optionally contains a heteroatom), where Y 1> and R 1> are connected together over a bridge, which is alkylene, arylene, heteroarylene, alkynylene, alkenylene, NR 18>, PR 19>, BR 20>, O, S, SO, SO 2, SiR 30>R 31>, CO, CO-O, O-CO or (CR 21>R 22>) x, where one or more non-adjacent group (CR 21>R 22>) is replaced by NR 18>, PR 19>, BR 20>, O, S, SO, SO 2, SiR 30>R 31>, CO, CO-O or O-CO x : 2-10 R 3>, R 18>-R 22>, R 30>, R 31>H, alkyl, aryl, heteroaryl, alkenyl or alkynyl Q ->monoanionic counter ion, preferably halogen, pseudohalogen, BF 4 ->, BPh 4 ->, PF 6 ->, AsF 6 -> or SbF 6 -> and G :

MOLT OLIVER; FUCHS EVELYN; EGEN MARTINA; KAHLE KLAUS; LENNARTZ CHRISTIAN

258

Retention or inversion in stereospecific nickel-catalyzed cross-coupling of benzylic carbamates with arylboronic esters: control of absolute stereochemistry with an achiral catalyst.  

UK PubMed Central (United Kingdom)

Stereospecific coupling of benzylic carbamates and pivalates with aryl- and heteroarylboronic esters has been developed. The reaction proceeds with selective inversion or retention at the electrophilic carbon, depending on the nature of the ligand. Tricyclohexylphosphine ligand provides the product with retention, while an N-heterocyclic carbene ligand provides the product with inversion.

Harris MR; Hanna LE; Greene MA; Moore CE; Jarvo ER

2013-03-01

259

Retention or inversion in stereospecific nickel-catalyzed cross-coupling of benzylic carbamates with arylboronic esters: control of absolute stereochemistry with an achiral catalyst.  

Science.gov (United States)

Stereospecific coupling of benzylic carbamates and pivalates with aryl- and heteroarylboronic esters has been developed. The reaction proceeds with selective inversion or retention at the electrophilic carbon, depending on the nature of the ligand. Tricyclohexylphosphine ligand provides the product with retention, while an N-heterocyclic carbene ligand provides the product with inversion. PMID:23414579

Harris, Michael R; Hanna, Luke E; Greene, Margaret A; Moore, Curtis E; Jarvo, Elizabeth R

2013-02-22

260

Enantioselective synthesis of pyrrolidine-, piperidine-, and azepane-type N-heterocycles with ?-alkenyl substitution: the CpRu-catalyzed dehydrative intramolecular N-allylation approach.  

UK PubMed Central (United Kingdom)

A cationic CpRu complex of chiral picolinic acid derivatives [(R)- or (S)-Cl-Naph-PyCOOCH(2)CH?CH(2)] catalyzes asymmetric intramolecular dehydrative N-allylation of N-substituted ?-amino- and -aminocarbonyl allylic alcohols with a substrate/catalyst ratio of up to 2000 to give ?-alkenyl pyrrolidine-, piperidine-, and azepane-type N-heterocycles with an enantiomer ratio of up to >99:1. The wide range of applicable N-substitutions, including Boc, Cbz, Ac, Bz, acryloyl, crotonoyl, formyl, and Ts, significantly facilitates further manipulation toward natural product synthesis.

Seki T; Tanaka S; Kitamura M

2012-01-01

 
 
 
 
261

Studies of coal structure using carbene chemistry  

Energy Technology Data Exchange (ETDEWEB)

The object of this grant was to react coal, derivatized forms of coal, and solvent swelled coal with carbenes (divalent carbon species) under mild conditions. These carbenes were to be prepared by treating the coal with several diazo compounds and then thermally decomposing them at relatively low temperatures (80--130{degree}C). The carbenes were to be chosen to show varying selectively toward aromatic rings containing heteroatom functionalities and toward polynuclear aromatic systems. In some instances, where selectivities toward aromatic and heteroaromatic ring systems were not known, model studies were to be carried out. Because of the generally mild conditions employed and the good selectivity anticipated, and actually observed with one particular system, it was expected that this methodology would provide structural information about the coal, along with data on the extent of occurrence and type of aromatic systems. After carbene reactions, treatment of the coal samples was to include extractions and thermolysis. Physical studies included thermogravimetric analysis, diffuse reflectance FT-IR spectroscopy, NMR ({sup 1}H and {sup 13}C) spectroscopy, gas chromatography, GC/MS and GC/FT-IR. 7 figs., 10 tabs.

1990-01-01

262

[3,3?-Dimesityl-1,1?-(4,5:16,17-dibenzo-3,6,9,12,15,18-hexaoxaeicosane-1,20-diyl)diimidazolin-2-ylidene]dithiocyanatopalladium(II)  

Directory of Open Access Journals (Sweden)

Full Text Available The coordination geometry of the Pd atom in the title compound, [Pd(SCN)2(C46H54N4O6)], is approximately square-planar. The N-heterocyclic carbene (NHC) metallacrown ether ligand binds to the Pd atom in a trans orientation through the carbene C atoms of the two imidazole rings and generates a 25-membered chelate ring. Two mutually trans S-bound thiocyanate ligands complete the coordination.

Xiao-Qin Zhang; Mei-Ming Luo

2009-01-01

263

Oxygenation of Ruthenium Carbene Complexes Containing Naphthothiophene or Naphthofuran: Spectroscopic and DFT Studies.  

UK PubMed Central (United Kingdom)

The aryl propargylic alcohol 1-[2-(thiophen-3-yl)phenyl]prop-2-yn-1-ol (1?a) is readily prepared from 2-(thiophen-3-yl)benzaldehyde. In the presence of visible light, treatment of 1?a with one-half mole equivalent of [Ru]Cl ([Ru]?Cp(dppe)Ru) (dppe=1,2-bis(diphenylphosphino)ethane) and NH4 PF6 in O2 affords the naphtha[2,1-b]thiophene-4-carbaldehyde (4?a) in high yields. The cyclization reaction of 1?a proceeds through the formation of the carbene complex 2?a 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 2?a using single-crystal X-ray diffraction analysis. Facile oxygenation of 2?a by O2 yields the aldehyde product 4?a 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 4?a. Presumably the resulting complex with the half-oxidized dppe ligand continuously promotes cyclization/oxygenation of 1?a to yield the second aldehyde molecule. In alcohol such as MeOH or EtOH, the oxygenation reaction affords a mixture of 4?a and the corresponding esters 5?a or 5?a'. Four other aryl propargylic alcohols 1?b-e, which contain thiophen-2-yl, isopropenyl, fur-3-yl, and fur-2-yl, respectively, on the aryl ring are also prepared. Analogous aldehydes 4?b-e are similarly prepared from 1?b-e, respectively. For oxygenations of 1?b, 1?d, and 1?e in alcohol, mixtures of aldehyde 4, ester 5, and acetal 8 are obtained. The carbene complex 2?b obtained from 1?b was also characterized by single-crystal X-ray diffraction analysis. The UV/Vis spectra of 2?a and 2?b consist of absorption bands with a high extinction coefficient. From DFT calculations on 2?a and 2?b, 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.

Tsai FY; Lo JX; Hsu HT; Lin YC; Huang SL; Wang JC; Liu YH

2013-08-01

264

Divergent pathways in the reaction of Fischer carbenes and palladium.  

UK PubMed Central (United Kingdom)

[reaction: see text] The Pd-catalyzed reaction of beta-arylaminochromium(0) carbene complexes produces by transmetalation the first isolated and X-ray structurally characterized bis-Pd(II) carbene complex, as well as other alternative reaction pathways, such as the oxidative addition-transmetalation sequence, not seen before in this chemistry.

López-Alberca MP; Mancheño MJ; Fernandez I; Gómez-Gallego M; Sierra MA; Torres R

2007-04-01

265

Electron-rich N-heterocyclic silylene (NHSi)-iron complexes: synthesis, structures, and catalytic ability of an isolable hydridosilylene-iron complex.  

Science.gov (United States)

The first electron-rich N-heterocyclic silylene (NHSi)-iron(0) complexes are reported. The synthesis of the starting complex is accomplished by reaction of the electron-rich Fe(0) precursor [(dmpe)2Fe(PMe3)] 1 (dmpe =1,2-bis(dimethylphosphino)ethane) with the N-heterocyclic chlorosilylene LSiCl (L = PhC(N(t)Bu)2) 2 to give, via Me3P elimination, the corresponding iron complex [(dmpe)2Fe(?:Si(Cl)L)] 3. Reaction of in situ generated 3 with MeLi afforded [(dmpe)2Fe(?:Si(Me)L)] 4 under salt metathesis reaction, while its reaction with Li[BHEt3] yielded [(dmpe)2Fe(?:Si(H)L)] 5, a rare example of an isolable Si(II) hydride complex and the first such example for iron. All complexes were fully characterized by spectroscopic means and by single-crystal X-ray diffraction analyses. DFT calculations further characterizing the bonding situation between the Si(II) and Fe(0) centers were also carried out, whereby multiple bonding character is detected in all cases (Wiberg Bond Index >1). For the first time, the catalytic activity of a Si(II) hydride complex was investigated. Complex 5 was used as a precatalyst for the hydrosilylation of a variety of ketones in the presence of (EtO)3SiH as a hydridosilane source. In most cases excellent conversions to the corresponding alcohols were obtained after workup. The reaction pathway presumably involves a ketone-assisted 1,2-hydride transfer from the Si(II) to Fe(0) center, as a key elementary step, resulting in a betaine-like silyliumylidene intermediate. The appearance of the latter intermediate is supported by DFT calculations, and a mechanistic proposal for the catalytic process is presented. PMID:23570308

Blom, Burgert; Enthaler, Stephan; Inoue, Shigeyoshi; Irran, Elisabeth; Driess, Matthias

2013-04-22

266

Electron-rich N-heterocyclic silylene (NHSi)-iron complexes: synthesis, structures, and catalytic ability of an isolable hydridosilylene-iron complex.  

UK PubMed Central (United Kingdom)

The first electron-rich N-heterocyclic silylene (NHSi)-iron(0) complexes are reported. The synthesis of the starting complex is accomplished by reaction of the electron-rich Fe(0) precursor [(dmpe)2Fe(PMe3)] 1 (dmpe =1,2-bis(dimethylphosphino)ethane) with the N-heterocyclic chlorosilylene LSiCl (L = PhC(N(t)Bu)2) 2 to give, via Me3P elimination, the corresponding iron complex [(dmpe)2Fe(?:Si(Cl)L)] 3. Reaction of in situ generated 3 with MeLi afforded [(dmpe)2Fe(?:Si(Me)L)] 4 under salt metathesis reaction, while its reaction with Li[BHEt3] yielded [(dmpe)2Fe(?:Si(H)L)] 5, a rare example of an isolable Si(II) hydride complex and the first such example for iron. All complexes were fully characterized by spectroscopic means and by single-crystal X-ray diffraction analyses. DFT calculations further characterizing the bonding situation between the Si(II) and Fe(0) centers were also carried out, whereby multiple bonding character is detected in all cases (Wiberg Bond Index >1). For the first time, the catalytic activity of a Si(II) hydride complex was investigated. Complex 5 was used as a precatalyst for the hydrosilylation of a variety of ketones in the presence of (EtO)3SiH as a hydridosilane source. In most cases excellent conversions to the corresponding alcohols were obtained after workup. The reaction pathway presumably involves a ketone-assisted 1,2-hydride transfer from the Si(II) to Fe(0) center, as a key elementary step, resulting in a betaine-like silyliumylidene intermediate. The appearance of the latter intermediate is supported by DFT calculations, and a mechanistic proposal for the catalytic process is presented.

Blom B; Enthaler S; Inoue S; Irran E; Driess M

2013-05-01

267

Pyridine-derived N-heterocyclic carbenes: an experimental and theoretical evaluation of the bonding in and reactivity of selected normal and abnormal complexes of nickel(II) and palladium(II)  

Digital Repository Infrastructure Vision for European Research (DRIVER)

We report a thorough investigation of a series of isomeric complexes with the general formula trans-(pyridylidene)M(PPh?)?Cl (M = Pd, Ni). For the first time, a systematic comparison of normal, abnormal, and remote bonding modes is presented. X-ray structural and ¹³C NMR ...

Stander-Grobler, Elzet; Schuster, Oliver; Heydenrych, Greta; Cronje, Stephanie; Tosh, Evangeline; Albrecht, Martin

268

Pyridine-derived N-heterocyclic carbenes: An experimental and theoretical evaluation of the bonding in and reactivity of selected normal and abnormal complexes of Ni(II) and Pd(II)  

Digital Repository Infrastructure Vision for European Research (DRIVER)

We report a thorough investigation of a series of isomeric complexes with the general formula trans-(pyridylidene)M(PPh3)2Cl (M = Pd, Ni). For the first time, a systematic comparison of normal, abnormal and remote bonding modes is presented. X-ray structural and 13C NMR data indicate the importance ...

Stander-Gobler, Elzet; Schuster, Oliver; Cronje, Stephanie; Tosh, Evangeline; Albrecht, Martin; Frenking, Gernot

269

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)

[en] 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

1997-01-01

270

Synthesis, structure, and electronic properties of extended ?-conjugated Group?6 Fischer alkoxy-bis(carbene) complexes.  

UK PubMed Central (United Kingdom)

The synthesis, structure and electronic properties of novel Group?6 Fischer alkoxy-bis(carbene) complexes are reported. The UV/Vis spectra of these species display two main absorptions at approximately 350 and 550?nm attributable to a ligand-field (LF) and metal-to-ligand charge-transfer (MLCT) transitions, respectively. The planarity of the system and the cooperative effect of both pentacarbonyl metal moieties greatly enhance the conjugation between the group at the end of the spacer and the metal carbene fragment provoking dramatic changes in the LF and MLCT absorptions. This is in contrast to related push-pull Fischer monocarbenes, where the position of the MLCT band remains mostly unaltered regardless the substituent attached to the donor fragment. In addition, the MLCT maxima can be tuned with subtle modifications of the electronic nature of the central aryl fragment in the novel A-?-D-?-A (A = acceptor, D = donor) systems. DFT and time-dependent (TD) DFT quantum chemical calculations at the B3LYP/def2-SVP level have also been performed to determine the minimum-energy molecular structure of this family of compounds and to analyse the nature of the vertical one-electron excitations associated to the observed UV/Vis absorptions as well as to rationalise their electrochemical behaviour. The ability of tuning up the electronic properties of the compounds studied herein may be of future use in material chemistry.

Chu GM; Fernández I; Sierra MA

2013-05-01

271

Synthesis, structure, and electronic properties of extended ?-conjugated Group?6 Fischer alkoxy-bis(carbene) complexes.  

Science.gov (United States)

The synthesis, structure and electronic properties of novel Group?6 Fischer alkoxy-bis(carbene) complexes are reported. The UV/Vis spectra of these species display two main absorptions at approximately 350 and 550?nm attributable to a ligand-field (LF) and metal-to-ligand charge-transfer (MLCT) transitions, respectively. The planarity of the system and the cooperative effect of both pentacarbonyl metal moieties greatly enhance the conjugation between the group at the end of the spacer and the metal carbene fragment provoking dramatic changes in the LF and MLCT absorptions. This is in contrast to related push-pull Fischer monocarbenes, where the position of the MLCT band remains mostly unaltered regardless the substituent attached to the donor fragment. In addition, the MLCT maxima can be tuned with subtle modifications of the electronic nature of the central aryl fragment in the novel A-?-D-?-A (A = acceptor, D = donor) systems. DFT and time-dependent (TD) DFT quantum chemical calculations at the B3LYP/def2-SVP level have also been performed to determine the minimum-energy molecular structure of this family of compounds and to analyse the nature of the vertical one-electron excitations associated to the observed UV/Vis absorptions as well as to rationalise their electrochemical behaviour. The ability of tuning up the electronic properties of the compounds studied herein may be of future use in material chemistry. PMID:23504973

Chu, Gong M; Fernández, Israel; Sierra, Miguel A

2013-03-15

272

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

2004-02-16

273

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

Energy Technology Data Exchange (ETDEWEB)

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 pK{sub HB} values for the three ligands. Curve fitting of the isotherm for pyridine binding to this polymer to the Langmuir model gave an approximate K{sub d} of 1.1{+-}0.1 mM and a binding site concentration of 57{+-}2 mmol g{sup -1}. Acridine binding did not show saturation behaviour and was non-specific and cooperative in nature.

Kirsch, N.; Alexander, C.; Davies, S.; Whitcombe, M.J

2004-02-16

274

(Bis{2-[3-(2,4,6-trimethylbenzyl)imidazolin-2-yliden-1-yl-?C2]-4-methylphenyl}amido-?N)chloridopalladium(II)  

Directory of Open Access Journals (Sweden)

Full Text Available The coordination geometry about the Pd centre in the title compound, [Pd(C40H42N5)Cl], is approximately square-planar. The CNC pincer-type N-heterocyclic carbene ligand binds to the Pd atom in a tridentate fashion by the amido N atom and the two carbene atoms and generates two six-membered chelate rings, completing the coordination.

Guan-Jun Cheng; Wei Wei; Chuang Zhou; Mei-Ming Luo

2010-01-01

275

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

UK PubMed Central (United Kingdom)

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.

Gangopadhyay S; Mistri T; Dolai M; Alam R; Ali M

2013-01-01

276

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

2012-10-17

277

Palladium-promoted [2 + 2 + 2] cocyclization of arynes and unsymmetrical conjugated dienes: synthesis of justicidin B and retrojusticidin B.  

Science.gov (United States)

A facile synthesis of natural and unnatural arylnaphthalenes has been demonstrated via the unprecedented palladium-promoted [2 + 2 + 2] cocyclization of arynes and unsymmetrical conjugated dienes using the N-heterocyclic carbene as a ligand. The unsymmetrical dienes used herein are actually ?-coupled acrylate-cinnamate combinations bearing two ?-positive carbons and follow the key [2 + 2 + 2] cocyclization pathway. PMID:23237050

Patel, Ramesh M; Argade, Narshinha P

2012-12-13

278

Carbene Radicals in Cobalt(II)-Porphyrin-Catalysed Carbene Carbonylation Reactions; A Catalytic Approach to Ketenes.  

UK PubMed Central (United Kingdom)

One-pot radicals: Cobalt(III)-carbene radicals, generated by metallo-radical activation of diazo compounds and N-tosylhydrazone sodium salts with cobalt(II) complexes of porphyrins, readily undergo radical addition to carbon monoxide, allowing the catalytic production of ketenes. These ketenes subsequently react with various amines, alcohols and imines in one-pot tandem transformations to produce differently substituted amides, esters and ?-lactams in good isolated yields.

Paul ND; Chirila A; Lu H; Zhang XP; de Bruin B

2013-09-01

279

Synthesis of Endohedral Metallofullerene Glycoconjugates by Carbene Addition  

Directory of Open Access Journals (Sweden)

Full Text Available Endohedral metallofullerene glycoconjugates were synthesized under mild conditions by carbene addition using appropriate glycosylidene-derived diazirine with La2@Ih-C80. NMR spectroscopic studies revealed that the glycoconjugate consists of two diastereomers of [6,6]-open mono-adducts. The electronic properties were characterized using Vis/NIR absorption spectroscopy and electrochemical measurements. This study demonstrates that glycosylidene carbene is useful to incorporate carbohydrate moieties onto endohedral metallofullerene surfaces.

Michio Yamada; Chika I. Someya; Tsukasa Nakahodo; Yutaka Maeda; Takahiro Tsuchiya; Takeshi Akasaka

2011-01-01

280

Synthesis of endohedral metallofullerene glycoconjugates by carbene addition.  

Science.gov (United States)

Endohedral metallofullerene glycoconjugates were synthesized under mild conditions by carbene addition using appropriate glycosylidene-derived diazirine with La(2)@I(h)-C(80). NMR spectroscopic studies revealed that the glycoconjugate consists of two diastereomers of [6,6]-open mono-adducts. The electronic properties were characterized using Vis/NIR absorption spectroscopy and electrochemical measurements. This study demonstrates that glycosylidene carbene is useful to incorporate carbohydrate moieties onto endohedral metallofullerene surfaces. PMID:22083236

Yamada, Michio; Someya, Chika I; Nakahodo, Tsukasa; Maeda, Yutaka; Tsuchiya, Takahiro; Akasaka, Takeshi

2011-11-14

 
 
 
 
281

Synthesis of endohedral metallofullerene glycoconjugates by carbene addition.  

UK PubMed Central (United Kingdom)

Endohedral metallofullerene glycoconjugates were synthesized under mild conditions by carbene addition using appropriate glycosylidene-derived diazirine with La(2)@I(h)-C(80). NMR spectroscopic studies revealed that the glycoconjugate consists of two diastereomers of [6,6]-open mono-adducts. The electronic properties were characterized using Vis/NIR absorption spectroscopy and electrochemical measurements. This study demonstrates that glycosylidene carbene is useful to incorporate carbohydrate moieties onto endohedral metallofullerene surfaces.

Yamada M; Someya CI; Nakahodo T; Maeda Y; Tsuchiya T; Akasaka T

2011-01-01

282

Adsorbed States of phosphonate derivatives of N-heterocyclic aromatic compounds, imidazole, thiazole, and pyridine on colloidal silver: comparison with a silver electrode.  

UK PubMed Central (United Kingdom)

Here, we report a systematic surface-enhanced Raman spectroscopy (SERS) study of the structures of phosphonate derivatives of the N-heterocyclic aromatic compounds imidazole (ImMeP ([hydroxy(1H-imidazol-5-yl)methyl]phosphonic acid) and (ImMe)(2)P (bis[hydroxy-(1H-imidazol-4-yl)-methyl]phosphinic acid)), thiazole (BAThMeP (butylaminothiazol-2-yl-methyl)phosphonic acid) and BzAThMeP (benzylaminothiazol-2-yl-methyl)phosphonic acid)), and pyridine ((PyMe)(2)P (bis[(hydroxypyridin-3-yl-methyl)]phosphinic acid)) adsorbed on nanometer-sized colloidal particles. We compared these structures to those on a roughened silver electrode surface to determine the relationship between the adsorption strength and the geometry. For example, we showed that all of these biomolecules interact with the colloidal surface through aromatic rings. However, for BzAThMeP, a preferential interaction between the benzene ring and the colloidal silver surface is observed more so than that between the thiazole ring and this substrate. The PC(OH)C fragment does not take part in the adsorption process, and the phosphonate moiety of ImMeP and (ImMe)(2)P, being removed from the surface, only assists in this process.

Podstawka E; Olszewski TK; Boduszek B; Proniewicz LM

2009-09-01

283

Versatile conversion of N-heterocyclic silylene to silyl metal compounds by insertion of divalent silicon into metal-carbon and metal-hydrogen bonds.  

UK PubMed Central (United Kingdom)

The facile one-pot reaction of the stable N-heterocyclic silylene LSi: 1 (L=(ArN)C(=CH(2))CH=C(Me)(NAr), Ar=2,6-iPr(2)C(6)H(3)) with Me(2)Zn, Me(3)Al, H(3)Al-NMe(3), and MeLi has been investigated. The silicon(II) atom in 1 is capable of insertion into the corresponding M-C and Al-H bonds under very mild reaction conditions. Thus, Me(2)Zn furnishes the bis(silyl) zinc complex LSi(Me)ZnSi(Me)L 2 as the sole product, irrespective of the molar ratio of the starting materials applied. Moreover, the reactions of 1 with Me(3)Al, H(3)Al-NMe(3), and MeLi lead directly to the 1,1-addition products LSi(Me)(Al(thf)Me(2)) 3, LSi(H)(AlH(2)(NMe(3))) 4, and LSi(Me)Li(thf)(3) 5, respectively. All new compounds 2-5 were fully characterized by multinuclear NMR spectroscopy, mass spectrometry, elemental analyses, and single-crystal X-ray diffraction analyses.

Xiong Y; Yao S; Driess M

2012-03-01

284

Versatile conversion of N-heterocyclic silylene to silyl metal compounds by insertion of divalent silicon into metal-carbon and metal-hydrogen bonds.  

Science.gov (United States)

The facile one-pot reaction of the stable N-heterocyclic silylene LSi: 1 (L=(ArN)C(=CH(2))CH=C(Me)(NAr), Ar=2,6-iPr(2)C(6)H(3)) with Me(2)Zn, Me(3)Al, H(3)Al-NMe(3), and MeLi has been investigated. The silicon(II) atom in 1 is capable of insertion into the corresponding M-C and Al-H bonds under very mild reaction conditions. Thus, Me(2)Zn furnishes the bis(silyl) zinc complex LSi(Me)ZnSi(Me)L 2 as the sole product, irrespective of the molar ratio of the starting materials applied. Moreover, the reactions of 1 with Me(3)Al, H(3)Al-NMe(3), and MeLi lead directly to the 1,1-addition products LSi(Me)(Al(thf)Me(2)) 3, LSi(H)(AlH(2)(NMe(3))) 4, and LSi(Me)Li(thf)(3) 5, respectively. All new compounds 2-5 were fully characterized by multinuclear NMR spectroscopy, mass spectrometry, elemental analyses, and single-crystal X-ray diffraction analyses. PMID:22331750

Xiong, Yun; Yao, Shenglai; Driess, Matthias

2012-02-13

285

Carbene formation in ionic liquids: spontaneous, induced, or prohibited?  

UK PubMed Central (United Kingdom)

We present a theoretical study of carbene formation from the 1-ethyl-3-methylimidazolium acetate ionic liquid in the absence and presence of CO2 in gas and liquid phase. Although CO2 physisorption constitutes a precursory step of chemisorption (the CO2's reaction with carbenes, which forms from cations via proton abstraction by anions), it also enables a very stable CO2-anion associate. However, this counteracts the chemical absorption by reducing the basicity of the anion and the electrophilicity of the CO2, which is reflected by charge transfer. Accordingly, the observable carbene formation in the gas phase is hindered in the presence of CO2. In the neat liquid, the carbene formation is also suppressed by the charge screening compared to the case of the gas phase; nevertheless, indications for carbene incidents appear. Interestingly, in the CO2-containing liquid we detect more carbene-like incidents than in the neat one, which is caused by the way CO2 is solvated. Despite the weakness of the CO2-cation interaction, the CO2-anion associate is distorted by cations, which can be seen in longer associate distances and reduced "binding" energies. While the single solvating anion is shifted away from CO2, many more solvating cations approach it compared to the case of the gas phase. This leads to the conclusion that while the ionic liquid effect stabilizes charged species, introducing neutral species such as CO2 provides an opposite trend, leading to an inverse ionic liquid effect with the facilitation of carbene formation and thus of chemical absorption.

Hollóczki O; Firaha DS; Friedrich J; Brehm M; Cybik R; Wild M; Stark A; Kirchner B

2013-05-01

286

Nonempirical investigation of electronic structure of simple model alkyl and carbene complexes of metals of the first transition period beginning  

International Nuclear Information System (INIS)

The nonempirical LCAO MO SCF method in small bases of groupped Gauss functions is used to calculate electronic structure of model complexes of the Hsub(n)MCHsub(3) and Hsub(m)MCHsub(2) composition, where M=Ti, V, Cr. The character of the metal-carbon bond depending on the nature of metal and hydrocarbon ligand is analyzed. The energetics of some processes accompanied by the breaking of ?- or ?-component of the metal-carbon bond is considered. The growth of relative ability to the formation of carbene complex from initial alkyl in the process od ?-elimination in the Ti

1985-01-01

287

Synthesis and comparison of transition metal complexes of abnormal and normal tetrazolylidenes: a neglected ligand species.  

UK PubMed Central (United Kingdom)

To date, only few examples of tetrazolylidene carbenes coordinated to transition metal complexes have been described. A direct comparison of "normal" tetrazolylidenes (1,4-substitution pattern) and "abnormal" tetrazolylidenes (also referred to as "mesoionic carbenes"; 1,3-substitution pattern) has not been performed at all. In this work, we describe coordination of both ligand types to a row of transition metals. For example, the first examples of tetrazolylidene Mo, Ag, or Ir complexes have been isolated and fully characterized. The mesoionic tetrazolylidene Ag compound 2d is shown to be a viable carbene transfer reagent. By means of NMR, IR, and Raman spectroscopy, as well as X-ray crystallography and computational results, the pronounced differences between the two ligand substitution patterns are highlighted. It is shown that simply changing the position of a methyl group from N3 to N4 in the heterocycle can cause enormous differences in ligand characteristics.

Schaper LA; Wei X; Altmann PJ; Öfele K; Pöthig A; Drees M; Mink J; Herdtweck E; Bechlars B; Herrmann WA; Kühn FE

2013-06-01

288

A highly conjugated benzimidazole carbene-based ruthenium sensitizer for dye-sensitized solar cells.  

UK PubMed Central (United Kingdom)

A new type of carbene-based ruthenium sensitizer, CB104, with a highly conjugated ancillary ligand, diphenylvinylthiophene-substituted benzimidazolepyridine, was designed and developed for dye-sensitized solar cell applications. The influence of the thiophene antenna on the performance of the cell anchored with CB104 was investigated. Compared with the dye CBTR, the conjugated thiophene in the ancillary ligand of CB104 enhanced the molar extinction coefficient of the intraligand ?-?* transition and the intensity of the lower energy metal-to-ligand charge-transfer band. However, the incident photon-to-current conversion efficiency spectrum of the cell anchored with CB104 (0.15?mM) showed a maximum of 63?% at 420?nm. The cell sensitized with the dye CB104 attained a power conversion efficiency of 7.30?%, which was lower than that of the cell with nonconjugated sensitizer CBTR (8.92?%) under the same fabrication conditions. The variation in the performance of these two dyes demonstrated that elongating the conjugated light-harvesting antenna resulted in the reduction of short-circuit photocurrent density, which might have been due to the aggregation of dye molecules. In the presence of a coabsorbate, chenodeoxycholic acid, the CB104-sensitized cell exhibited an enhanced photocurrent density and achieved a photovoltaic efficiency of 8.36?%.

Ho SY; Su C; Li CY; Prabakaran K; Shen MT; Chen YF; Chang WC; Tingare YS; Akula S; Tsai SH; Li WR

2013-09-01

289

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

290

Recent developments in actinide-ligand multiple bonding.  

UK PubMed Central (United Kingdom)

In the last few years, considerable progress has been made in the synthesis and characterization of complexes containing actinide-ligand multiple bonds, especially in regards to isolation of terminal oxo, nitrido and carbene-containing complexes. This review summarizes the synthesis, structure, and reactivity of these complexes, from 2010 until present. These complexes are of interest for a variety of reasons, including their potential use in novel catalytic transformations and their ability to engage the 5f orbitals in metal-ligand bonding. Of particular note are the recent syntheses of the first isolable complex containing the nitride-substituted uranyl ion, [NUO](+), and the first report of an isolable terminal uranium nitride. Considerable progress has also been made toward the syntheses of actinide carbenes and thorium complexes containing metal-ligand multiple bonds.

Hayton TW

2013-04-01

291

Anionic 1,2,3-triazole-4,5-diylidene: a 1,2-dihapto ligand for the construction of bimetallic complexes.  

Science.gov (United States)

A super pyrazolate: deprotonation of the flanking hydrogen of metal complexes of mesoionic carbenes (MICs) offers a simple and general route for the preparation of bimetallic complexes of a 1,2-dihapto anionic dicarbene ligand that is isoelectronic with widely used pyrazolate ligands, while conferring greater electron donation and stronger M-L bonds. PMID:23042507

Yan, Xiaoyu; Bouffard, Jean; Guisado-Barrios, Gregorio; Donnadieu, Bruno; Bertrand, Guy

2012-10-05

292

Anionic 1,2,3-triazole-4,5-diylidene: a 1,2-dihapto ligand for the construction of bimetallic complexes.  

UK PubMed Central (United Kingdom)

A super pyrazolate: deprotonation of the flanking hydrogen of metal complexes of mesoionic carbenes (MICs) offers a simple and general route for the preparation of bimetallic complexes of a 1,2-dihapto anionic dicarbene ligand that is isoelectronic with widely used pyrazolate ligands, while conferring greater electron donation and stronger M-L bonds.

Yan X; Bouffard J; Guisado-Barrios G; Donnadieu B; Bertrand G

2012-11-01

293

Singlet 2-adamantylidene--an ambiphilic foiled carbene stabilized by hyperconjugation.  

UK PubMed Central (United Kingdom)

The EDA-NOCV analysis shows that the singlet 2-adamantylidene is a foiled-type carbene stabilized by the hyperconjugative interaction of one pair of the vicinal C-C ?-MOs with the empty p-orbital on the carbene carbon atom. The high proton affinity and hydride affinity indicate the ambiphilic nature of 2-adamantylidene.

Rojisha VC; Nijesh K; De S; Parameswaran P

2013-10-01

294

Theoretical study of catalytic reactions of carbenes: haptotropic rearrangements and the Dötz reaction  

Digital Repository Infrastructure Vision for European Research (DRIVER)

En aquesta tesi s'han estudiat mecanismes de reaccions de cicloanulació en carbens de Fischer a través de mètodes teòrics, concretament fent servir el nivell de teoria B3LYP/(Wachters' basis / 6-31G**). Els alcoxi- i amino carbens de pentacarbonil crom, ja siguin vinílics o aromàtics, reaccionen amb...

Jiménez Halla, José Óscar Carlos

295

Regioselective and stepwise [8 + 2] cycloaddition reaction between alkynyl-Fischer carbene complexes and tropothione.  

UK PubMed Central (United Kingdom)

The formal [8 + 2] cycloaddition reaction between alkynyl Fischer carbene complexes and tropothione leads to the regioselective formation of novel 3aH-cyclohepta[b]thiophene carbene complexes. Computational DFT calculations indicate that the process proceeds stepwise via antiaromatic zwitterionic intermediates.

Rivero AR; Fernández I; Sierra MA

2012-08-01

296

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

UK PubMed Central (United Kingdom)

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.

Ruiz DA; Ung G; Melaimi M; Bertrand G

2013-07-01

297

Carbynes and carbenes in coordination chemistry: A new class of pentaammine and tetraammine complexes of osmium(II)  

Energy Technology Data Exchange (ETDEWEB)

Since their discovery by Fischer and co-workers in 1973, the study of transition-metal carbyne complexes and their role in alkyne metathesis has rapidly developed into a mature field. Although carbyne complexes are known for a diverse set of early- and mid-transition metals, the vast majority of these complexes contain carbon or phosphine [pi]-acids, or bulky alkoxide ligands, which limit the coordination number. We wish to report the first example of a carbyne species, as well as several heteroatom-carbene derivatives, in which the metal fragment, Os[sup II](NH[sub 3])[sub 5], provides a classical octahedral coordination environment. The carbyne [Os(NH[sub 3])[sub 5]([equivalent to]CPh)](OTf)[sub 3] (2) is synthesized in two steps from Os(NH[sub 3])[sub 5](OTf)[sub 3] and benzaledhyde dimethyl acetal.

Hodges, L.M.; Sabat, M.; Harman, W.D. (Univ. of Virginia, Charlottesville (United States))

1993-02-17

298

Production method for metal aza ring carbene complex  

UK PubMed Central (United Kingdom)

The invention discloses preparation methods of a metal nitrogen heterocyclic-carbene complex. No.1 preparation method comprises the steps: acetonitrile is taken as a solvent and added with imidazole salt and silver oxide with the mol ratio of which is 1: 0.5 to 3, then stirred at the temperature of 40 DEG C to 60 DEG C for light-resistant reaction for 2h to 12h and next added with active metal powder (manganese, iron, cobalt, nickel or copper) to react at the temperature of 40 DEG C to 80 DEG C for 2h to 24h, wherein, the mol ratio of the active metal powder and the nitrogen heterocyclic-carbene complex is 2 to 10:1, for reaction and No.2 preparation method comprises the stpes: acetonitrile is taken as a solvent and added with imidazole salt and active metal powder the mol ratio of which is 2 to 10: 1 to be heated to 40 DEG C to 80 DEG C in the air for reaction for 12h to 48h the reaction solution is filtrated and the obtained filtrate is condensed and added with ether to separate solid products out and the solids are then washed for two times to three times with alcohol and ether, dissolved with the acetonitrile, slowly added with the ether and crystallized to obtain the metal nitrogen heterocyclic-carbene complex. The preparation methods are simple to operate, avoid the synthesis of complex metal substrates with higher yield and have a broad application prospect in the preparation of catalysts with carbine as complex.

WANZHI CHEN; BIN LIU

299

Copper-Catalyzed Borylation of ?-Alkoxy Allenes with Bis(pinacolato)diboron: Efficient Synthesis of 2-Boryl 1,3-Butadienes.  

UK PubMed Central (United Kingdom)

Something solid to build on: 2-Boryl 1,3-butadienes with various substitution patterns were formed in good to high yields in a copper-catalyzed borylation of ?-alkoxy allenes with bis(pinacolato)diboron (see scheme; Bn=benzyl, pin=pinacolate, L is an N-heterocyclic carbene ligand). The products were found to be useful intermediates for the synthesis of cyclic vinyl boranes, ?,?-unsaturated ketones, and functionalized multisubstituted dienes.

Semba K; Fujihara T; Terao J; Tsuji Y

2013-10-01

300

Isolation of Neutral Mono- and Dinuclear Gold Complexes of Cyclic (Alkyl)(amino)carbenes.  

UK PubMed Central (United Kingdom)

The smallest pieces of gold! Thanks to the presence of two ?-accepting cyclic (alkyl)(amino)carbenes (CAACs),complexes featuring one and two atoms of gold in the formal oxidation state of zero can be isolated.

Weinberger DS; Melaimi M; Moore CE; Rheingold AL; Frenking G; Jerabek P; Bertrand G

2013-08-01

 
 
 
 
301

Reaction of trisubstituted alkenes with iron porphyrin carbenes: facile synthesis of tetrasubstituted dienes and cyclopentadienes.  

UK PubMed Central (United Kingdom)

The unprecedented reaction of trisubstituted alkenes with iron porphyrin carbenes has been successfully developed. Both multiply substituted 1,3-butadiene and cyclopentadiene products are readily accessible with high efficiency and selectivity in good yields.

Wang P; Liao S; Wang SR; Gao RD; Tang Y

2013-08-01

302

Reaction of trisubstituted alkenes with iron porphyrin carbenes: facile synthesis of tetrasubstituted dienes and cyclopentadienes.  

Science.gov (United States)

The unprecedented reaction of trisubstituted alkenes with iron porphyrin carbenes has been successfully developed. Both multiply substituted 1,3-butadiene and cyclopentadiene products are readily accessible with high efficiency and selectivity in good yields. PMID:23857008

Wang, Peng; Liao, Saihu; Wang, Sunewang R; Gao, Run-Duo; Tang, Yong

2013-08-28

303

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.

Barbara A. Wilson; Ramaiyer Venkatraman; Cedrick Whitaker; Quintell Tillison

2005-01-01

304

Facile displacement of ?5-cyclopentadienyl ligands from half-sandwich alkyl,NHC-nickel complexes: an original route to robust cis-C,C-nickel square planar complexes.  

UK PubMed Central (United Kingdom)

The ?(5)-cyclopentadienyl (Cp) ligands of 18-electron half-sandwich alkyl,NHC-nickel complexes are readily displaced under acidic conditions to afford a novel class of cis-C,C-nickel square planar complexes. Remarkably, the nickel-alkyl and nickel-carbene bonds are not ruptured in these unprecedented Cp acidolysis reactions.

Henrion M; Oertel AM; Ritleng V; Chetcuti MJ

2013-07-01

305

Donor-free phosphenium-metal(0)-halides with unsymmetrically bridging phosphenium ligands.  

UK PubMed Central (United Kingdom)

Reactions of (cod)MCl2 (cod = 1,5 cyclooctadiene, M = Pd, Pt) with N-heterocyclic secondary phosphines or diphosphines produced complexes [(NHP)MCl]2 (NHP = N-heterocyclic phosphenium). The Pd complex was also accessible from a chlorophosphine precursor and Pd2(dba)3. Single-crystal X-ray diffraction studies established the presence of dinuclear complexes that contain ?-bridging NHP ligands in an unsymmetrical binding mode and display a surprising change in metal coordination geometry from distorted trigonal (M = Pd) to T-shaped (M = Pt). DFT calculations on model compounds reproduced these structural features for the Pt complex but predicted an unusual C2v-symmetric molecular structure with two different metal coordination environments for the Pd species. The deviation between this structure and the actual centrosymmetric geometry is accounted for by the prediction of a flat energy hypersurface, which permits large distortions in the orientation of the NHP ligands at very low energetic cost. The DFT results and spectroscopic studies suggest that the title compounds should be described as phosphenium-metal(0)-halides rather than conventional phosphido complexes of divalent metal cations and indicate that the NHP ligands receive net charge donation from the metals but retain a distinct cationic character. The unsymmetric NHP binding mode is associated with an unequal distribution of ?-donor/?-acceptor contributions in the two M-P bonds. Preliminary studies indicate that reactions of the Pd complex with phosphine donors provide a viable source of ligand-stabilized, zerovalent metal atoms and metal(0)-halide fragments.

Förster D; Nickolaus J; Nieger M; Benk? Z; Ehlers AW; Gudat D

2013-07-01

306

Donor-Free Phosphenium-Metal(0)-Halides with Unsymmetrically Bridging Phosphenium Ligands.  

UK PubMed Central (United Kingdom)

Reactions of (cod)MCl2 (cod = 1,5 cyclooctadiene, M = Pd, Pt) with N-heterocyclic secondary phosphines or diphosphines produced complexes [(NHP)MCl]2 (NHP = N-heterocyclic phosphenium). The Pd complex was also accessible from a chlorophosphine precursor and Pd2(dba)3. Single-crystal X-ray diffraction studies established the presence of dinuclear complexes that contain ?-bridging NHP ligands in an unsymmetrical binding mode and display a surprising change in metal coordination geometry from distorted trigonal (M = Pd) to T-shaped (M = Pt). DFT calculations on model compounds reproduced these structural features for the Pt complex but predicted an unusual C2v-symmetric molecular structure with two different metal coordination environments for the Pd species. The deviation between this structure and the actual centrosymmetric geometry is accounted for by the prediction of a flat energy hypersurface, which permits large distortions in the orientation of the NHP ligands at very low energetic cost. The DFT results and spectroscopic studies suggest that the title compounds should be described as phosphenium-metal(0)-halides rather than conventional phosphido complexes of divalent metal cations and indicate that the NHP ligands receive net charge donation from the metals but retain a distinct cationic character. The unsymmetric NHP binding mode is associated with an unequal distribution of ?-donor/?-acceptor contributions in the two M-P bonds. Preliminary studies indicate that reactions of the Pd complex with phosphine donors provide a viable source of ligand-stabilized, zerovalent metal atoms and metal(0)-halide fragments.

Förster D; Nickolaus J; Nieger M; Benk? Z; Ehlers AW; Gudat D

2013-06-01

307

A click approach to polymetallic chromium(0) and tungsten(0) Fischer-carbene complexes and their use in the synthesis of functionalized polymetallic metal-carbene complexes.  

UK PubMed Central (United Kingdom)

Alkynylamino Cr(0) and W(0) Fischer carbenes undergo a CuAAC reaction with a diverse range of di-, tri-, and tetra-azides to produce polymetallic chromium(0) and tungsten(0) (Fischer)-carbene complexes in good-to-excellent yields. This method is simple, versatile, and is suitable for the preparation of a diverse range of structures with a high level of symmetry. Moreover, the resulting polymetallic carbene complexes are suitable partners for the peripheral functionalization of the metal nuclei, whilst retaining the metal fragment. This fact has been demonstrated in a simultaneous Pauson-Khand reaction, which, in some cases, allows for the generation of four bicyclic [5,5] rings on the periphery of a tetrametallic molecule in a process that involves the formation of 12 new C-C bonds, with four simultaneous CO-insertion processes. The electrochemistry of the polymetallic Fischer carbenes show completely independent behavior for each nucleus, as well as an anomalous observation of the reversible oxidation of the allyl substituents, which has not been reported before in this class of chemistry.

Baeza B; Casarrubios L; Sierra MA

2013-01-01

308

A click approach to polymetallic chromium(0) and tungsten(0) Fischer-carbene complexes and their use in the synthesis of functionalized polymetallic metal-carbene complexes.  

Science.gov (United States)

Alkynylamino Cr(0) and W(0) Fischer carbenes undergo a CuAAC reaction with a diverse range of di-, tri-, and tetra-azides to produce polymetallic chromium(0) and tungsten(0) (Fischer)-carbene complexes in good-to-excellent yields. This method is simple, versatile, and is suitable for the preparation of a diverse range of structures with a high level of symmetry. Moreover, the resulting polymetallic carbene complexes are suitable partners for the peripheral functionalization of the metal nuclei, whilst retaining the metal fragment. This fact has been demonstrated in a simultaneous Pauson-Khand reaction, which, in some cases, allows for the generation of four bicyclic [5,5] rings on the periphery of a tetrametallic molecule in a process that involves the formation of 12 new C-C bonds, with four simultaneous CO-insertion processes. The electrochemistry of the polymetallic Fischer carbenes show completely independent behavior for each nucleus, as well as an anomalous observation of the reversible oxidation of the allyl substituents, which has not been reported before in this class of chemistry. PMID:23203658

Baeza, Beatriz; Casarrubios, Luis; Sierra, Miguel A

2012-11-30

309

3,3?-Dimethyl-1,1?-(methylenedi-p-phenylene)diimidazolium bis(hexafluorophosphate)  

Digital Repository Infrastructure Vision for European Research (DRIVER)

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 bein...

Kun Huang; Ping Zhang; Da-Bin Qin

310

Ring contraction of six-membered metallabenzynes to five-membered metal-carbene complexes: a comparison with organic analogues.  

UK PubMed Central (United Kingdom)

The ring contraction of six-membered metallabenzynes to five-membered metal-carbene complexes in comparison with the corresponding rearrangement of benzyne to cyclopentadienylidene carbene have been studied by computational quantum mechanical calculations. The rearrangement of benzyne to a cyclopentadienylidene carbene is endothermic by 27.8 kcal mol(-1). The instability of the cyclopentadienylidene carbene is attributed to the electron deficient singlet carbene carbon atom. However, the isolobal replacement of a sp-hybridized carbon atom in benzyne by a 14 valence electron transition metal fragment (M(PH3)2Cl2, M = Fe, Ru and Os) makes the corresponding rearrangement feasible. The first row transition metal iron shows thermodynamic (exothermic by 19.1 kcal mol(-1)) and kinetic preference (energy barrier of 1.8 kcal mol(-1)) towards the carbene complex. However, the preference reduces down the group. The coordination number of the metal as well as the low-lying in-plane M-C ?*-MO play a crucial role for this rearrangement. The conversion of metallabenzynes to metal-carbene complexes is also associated with a ring plane rotation to attain an effective overlap between the metal d-orbital and the p-orbital on the carbene carbon atom.

Anusha C; De S; Parameswaran P

2013-08-01

311

Study of the ESI-mass spectrometry ionization mechanism of Fischer carbene complexes.  

Science.gov (United States)

By means of deuterium-labeling experiments, we have carried out a systematic ESI-MS study to determine the mechanism of ESI ionization of alkenyl and alkynyl group 6 Fischer carbene complexes. These compounds can be ionized under ESI conditions only in the presence of additives such as hydroquinone (HQ) or tetrathiafulvalene (TTF). Our results demonstrate that in the ESI source an anion-radical is formed after the initial HQ- or TTF-mediated electron transfer to the metallic carbene complex. For alkenyl carbene complexes, this species evolves by extrusion of a hydrogen radical to form an allenylchromium anion that is detected as the [M - H](-) ion in the mass spectrum. The preference for this mechanistic pathway could be rationalized by DFT calculations. In the case of alkynyl carbene complexes, experiments combining deuterated substrate, additive, and solvent demonstrate that the previously proposed allene-anion carbene complex is not formed. Instead, the H transfer from the ethoxy group in the anion radical, followed by extrusion of a hydrogen radical, leads to an allenyl anion that is detected in the ESI-MS as [M - H - CO](-). PMID:15960531

Wulff, William D; Korthals, Keith A; Martínez-Alvarez, Roberto; Gómez-Gallego, Mar; Fernández, Israel; Sierra, Miguel A

2005-06-24

312

Study of the ESI-mass spectrometry ionization mechanism of Fischer carbene complexes.  

UK PubMed Central (United Kingdom)

By means of deuterium-labeling experiments, we have carried out a systematic ESI-MS study to determine the mechanism of ESI ionization of alkenyl and alkynyl group 6 Fischer carbene complexes. These compounds can be ionized under ESI conditions only in the presence of additives such as hydroquinone (HQ) or tetrathiafulvalene (TTF). Our results demonstrate that in the ESI source an anion-radical is formed after the initial HQ- or TTF-mediated electron transfer to the metallic carbene complex. For alkenyl carbene complexes, this species evolves by extrusion of a hydrogen radical to form an allenylchromium anion that is detected as the [M - H](-) ion in the mass spectrum. The preference for this mechanistic pathway could be rationalized by DFT calculations. In the case of alkynyl carbene complexes, experiments combining deuterated substrate, additive, and solvent demonstrate that the previously proposed allene-anion carbene complex is not formed. Instead, the H transfer from the ethoxy group in the anion radical, followed by extrusion of a hydrogen radical, leads to an allenyl anion that is detected in the ESI-MS as [M - H - CO](-).

Wulff WD; Korthals KA; Martínez-Alvarez R; Gómez-Gallego M; Fernández I; Sierra MA

2005-06-01

313

Probing the nature and extent of stabilization within foiled carbenes: homoallylic participation by a neighboring cyclopropane ring.  

UK PubMed Central (United Kingdom)

Oxadiazoline 6 was synthesized to generate endo-tricyclo[3.2.1.0(2,4)]octan-8-ylidene (3) by either photolysis or thermolysis. Diastereomer 6a thermally decomposed twice as fast as 6b. Carbene 3 was trapped stereoselectively by acrylonitrile and diethylamine in high yields. It behaved as a nucleophilic carbene with electron-poor alkenes, like acrylonitrile, but as an electrophile with very electron-rich species, such as diethylamine. However, when the reactions were performed in cyclohexane and cyclohexene, isomerization of 3 was favored. Replacement of the double bond in 7-norbornenylidene (1) by the single bond in the endo-fused cyclopropane unit of carbene 3 led to similar outcomes. Carbene 3 rightfully belongs to the family of foiled carbenes.

Apeland IM; Kählig H; Lorbeer E; Brinker UH

2013-05-01

314

Cobalt azide complexes with a tris(carbene)borate ligand scaffold.  

UK PubMed Central (United Kingdom)

The four-coordinate Co(II) complex, (azido-?N)[1,1,',1''-(phenylboranetriyl)tris(3-tert-butyl-1H-imidazol-2-ylidene)]cobalt(II), [Co(C27H38BN6)(N3)], (1), denoted PhB(t-BuIm)3CoN3, was prepared by the reaction of the corresponding chloride complex with NaN3. One-electron oxidation results in the isolation of the five-coordinate Co(III) complex, bis(azido-?N)[1,1,',1''-(phenylboranetriyl)tris(3-tert-butyl-1H-imidazol-2-ylidene)]cobalt(III), [Co(C27H38BN6)(N3)2], (2), denoted PhB(t-BuIm)3Co(N3)2. Attempts to prepare cobalt nitrides by thermolysis or photolysis of these complexes were unsuccessful.

Scepaniak JJ; Margarit CG; Bontchev RP; Smith JM

2013-09-01

315

Identification of carbene particles forming during decomposition of molybdenum neopentyl derivatives  

International Nuclear Information System (INIS)

Method of chromatomass-spectrometry was used to study MoCl5 interaction with neopentylmagnesium bromide at -78 and 20 deg C in the medium of dibutyl ether with successive addition of benzaldehyde to the system. It is shown that the main products of reactions are tetramethylhexane with 45-47 and 3-7% yield respectively, as well as ?-neopentylstyrene and ?-neopentylbutyl ether. If reaction is conducted at -78 deg C the major part of formed carbenes is used for reaction with benzaldehyde. The yield of ?-neopentylstyrene decreases at 20 degC. This is conditioned by carbene reactions with the solvent or other system components. The obtained data are the direct experimental corroboration of carbene particle formation during decomposition of molybdenum neopentyl derivatives.

1989-01-01

316

Olefin cyclopropanation via carbene transfer catalyzed by engineered cytochrome P450 enzymes.  

Science.gov (United States)

Transition metal-catalyzed transfers of carbenes, nitrenes, and oxenes are powerful methods for functionalizing C=C and C-H bonds. Nature has evolved a diverse toolbox for oxene transfers, as exemplified by the myriad monooxygenation reactions catalyzed by cytochrome P450 enzymes. The isoelectronic carbene transfer to olefins, a widely used C-C bond-forming reaction in organic synthesis, has no biological counterpart. Here we report engineered variants of cytochrome P450(BM3) that catalyze highly diastereo- and enantioselective cyclopropanation of styrenes from diazoester reagents via putative carbene transfer. This work highlights the capacity to adapt existing enzymes for the catalysis of synthetically important reactions not previously observed in nature. PMID:23258409

Coelho, Pedro S; Brustad, Eric M; Kannan, Arvind; Arnold, Frances H

2012-12-20

317

Olefin cyclopropanation via carbene transfer catalyzed by engineered cytochrome P450 enzymes.  

UK PubMed Central (United Kingdom)

Transition metal-catalyzed transfers of carbenes, nitrenes, and oxenes are powerful methods for functionalizing C=C and C-H bonds. Nature has evolved a diverse toolbox for oxene transfers, as exemplified by the myriad monooxygenation reactions catalyzed by cytochrome P450 enzymes. The isoelectronic carbene transfer to olefins, a widely used C-C bond-forming reaction in organic synthesis, has no biological counterpart. Here we report engineered variants of cytochrome P450(BM3) that catalyze highly diastereo- and enantioselective cyclopropanation of styrenes from diazoester reagents via putative carbene transfer. This work highlights the capacity to adapt existing enzymes for the catalysis of synthetically important reactions not previously observed in nature.

Coelho PS; Brustad EM; Kannan A; Arnold FH

2013-01-01

318

Complexes of Ge(IV)- and Sn(IV)-fluorides with cyclic and acyclic carbenes: bis(dialkylamino)-difluoromethylenes as carbene sources.  

UK PubMed Central (United Kingdom)

Carbene complexes of Ge(IV)- and Sn(IV)-fluorides have been synthesized by oxidative addition of 2,2-difluoro-1,3-dimethylimidazolidine and bis(dimethylamino)difluoromethane to GeCl(2)•dioxane and SnF(2). Chloride analogs of the Ge(IV) complexes were also isolated. All compounds were characterized in the solid state by single-crystal X-ray diffraction.

Böttcher T; Bassil BS; Röschenthaler GV

2012-01-01

319

Pd-catalyzed cyclization and carbene migratory insertion: new approach to 3-vinylindoles and 3-vinylbenzofurans.  

Science.gov (United States)

A Pd-catalyzed stereoselective synthesis of 3-vinylindoles and 3-vinylbenzofurans has been developed. The reaction merges the alkyne-based Pd-catalyzed cyclization and Pd carbene migratory insertion in a single catalytic cycle, generating a C-C single bond and a C?C double bond in one operation. PMID:24066705

Liu, Zhenxing; Xia, Ying; Zhou, Shiyi; Wang, Long; Zhang, Yan; Wang, Jianbo

2013-09-25

320

Pd-Catalyzed Cyclization and Carbene Migratory Insertion: New Approach to 3-Vinylindoles and 3-Vinylbenzofurans.  

UK PubMed Central (United Kingdom)

A Pd-catalyzed stereoselective synthesis of 3-vinylindoles and 3-vinylbenzofurans has been developed. The reaction merges the alkyne-based Pd-catalyzed cyclization and Pd carbene migratory insertion in a single catalytic cycle, generating a C-C single bond and a C?C double bond in one operation.

Liu Z; Xia Y; Zhou S; Wang L; Zhang Y; Wang J

2013-09-01

 
 
 
 
321

Cyclopropylmethyl palladium species from carbene migratory insertion: new routes to 1,3-butadienes.  

UK PubMed Central (United Kingdom)

Cyclopropylmethyl palladium species can be accessed by Pd-catalyzed reaction of either cyclopropyl N-tosylhydrazone with halide or N-tosylhydrazone with cyclopropyl halide. In both approaches migratory insertion of Pd carbene is the key process. These transformations constitute new approaches toward 1,3-butadiene derivatives.

Zhou L; Ye F; Zhang Y; Wang J

2012-02-01

322

Carbodiphosphorane analogues E(PPh3)2 with E=C-Pb: a theoretical study with implications for ligand design.  

UK PubMed Central (United Kingdom)

Quantum-chemical calculations at the BP86/TZVPP level have been carried out for the heavy Group 14 homologues of carbodiphosphorane E(PPh(3))(2), where E=Si, Ge, Sn, Pb, which are experimentally unknown so far. The results of the theoretical investigation suggest that the tetrelediphosphoranes E(PPh(3))(2) (1E) are stable compounds that could become isolated in a condensed phase. The molecules possess donor-acceptor bonds Ph(3)P?E?PPh(3) to a bare tetrele atom E, which retains its four valence electrons as two electron lone pairs. The analysis of the bonding situation and the calculation of the chemical reactivity indicate that the molecules 1E belong to the class of divalent E(0) compounds (ylidones). All molecules 1C-1Pb have very large first but also very large second proton affinities, which distinguishes them from the N-heterocyclic carbene homologues, in which the donor atom is a divalent E(II) species that possesses only one electron lone pair. Compounds 1E are powerful double donors that strongly bind Lewis acids such as BH(3) and AuCl in the complexes 1E(BH(3))(n) and 1E(AuCl)(n) (n=1, 2). The bond dissociation energies (BDEs) of the second BH(3) and AuCl molecules are only slightly less than the BDE of the first BH(3) and AuCl. The results of this work are a challenge for experimentalists.

Takagi N; Tonner R; Frenking G

2012-02-01

323

PPAR LIGAND  

UK PubMed Central (United Kingdom)

Disclosed is a ligand for peroxisome proliferator-activated receptor (PPAR) which originates in a safe food material. Specifically disclosed is a PPAR ligand which comprises, as the active ingredient, a Luffa acutangula L. extract or a novel compound contained in the Luffa acutangula L. extract. This PPAR ligand is efficacious in preventing and/or ameliorating obesity and obesity-related disorders such as insulin resistance, hyperlipemia, hypertension, diabetes, etc.

MAEDA MITSURU; OGURA KYOICHI; MAEJIMA KAZUHIRO

324

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)

2004-01-01

325

Sterically encumbered pyrrolyl ligands and their incorporation into the cycloheptatrienyl zirconium coordination sphere.  

Science.gov (United States)

An improved synthesis of 2,5-di(tert-butyl)-3,4-dimethylpyrrole (2-H, HPyr(tBu(2)Me(2))) and a subsequent reaction with KH yield K(Pyr(tBu(2)Me(2))) (2-K) in multi-gram quantities. Four different pyrrolyl (Pyr) and imidazolyl (Im) ligands were used in salt metathesis reactions with [(?(7)-C(7)H(7))ZrCl(tmeda)] (7) to afford a series of azatrozircenes: [(?(7)-C(7)H(7))Zr(?(5)-Pyr(tBu(2)))] (1-Zr), [(?(7)-C(7)H(7))Zr(?(5)-Pyr(tBu(2)Me(2)))] (2-Zr), [(?(7)-C(7)H(7))Zr(?(5)-Pyr(tBu(3)))] (3-Zr) and [(?(7)-C(7)H(7))Zr(?(5)-Im(tBu(3)))] (4-Zr), which were characterized by NMR spectroscopy and elemental analysis. In addition, the molecular structures of 2-H, 2-K·18-crown-6, 1-Zr, 2-Zr and 4-Zr were determined by X-ray diffraction analysis, revealing ?(5)- rather than ?(1)-N-coordination of the N-heterocyclic ligands. Cone angle measurements on the sandwich complexes 1-Zr–4-Zr showed that their nitrogencontaining ligands belong to the class of very sterically encumbered ?-ligands, but DFT calculations suggest lower stabilities compared to their all-carbon analogues. PMID:23192438

Kreye, Markus; Glöckner, Andreas; Daniliuc, Constantin G; Freytag, Matthias; Jones, Peter G; Tamm, Matthias; Walter, Marc D

2013-02-14

326

Sterically encumbered pyrrolyl ligands and their incorporation into the cycloheptatrienyl zirconium coordination sphere.  

UK PubMed Central (United Kingdom)

An improved synthesis of 2,5-di(tert-butyl)-3,4-dimethylpyrrole (2-H, HPyr(tBu(2)Me(2))) and a subsequent reaction with KH yield K(Pyr(tBu(2)Me(2))) (2-K) in multi-gram quantities. Four different pyrrolyl (Pyr) and imidazolyl (Im) ligands were used in salt metathesis reactions with [(?(7)-C(7)H(7))ZrCl(tmeda)] (7) to afford a series of azatrozircenes: [(?(7)-C(7)H(7))Zr(?(5)-Pyr(tBu(2)))] (1-Zr), [(?(7)-C(7)H(7))Zr(?(5)-Pyr(tBu(2)Me(2)))] (2-Zr), [(?(7)-C(7)H(7))Zr(?(5)-Pyr(tBu(3)))] (3-Zr) and [(?(7)-C(7)H(7))Zr(?(5)-Im(tBu(3)))] (4-Zr), which were characterized by NMR spectroscopy and elemental analysis. In addition, the molecular structures of 2-H, 2-K·18-crown-6, 1-Zr, 2-Zr and 4-Zr were determined by X-ray diffraction analysis, revealing ?(5)- rather than ?(1)-N-coordination of the N-heterocyclic ligands. Cone angle measurements on the sandwich complexes 1-Zr–4-Zr showed that their nitrogencontaining ligands belong to the class of very sterically encumbered ?-ligands, but DFT calculations suggest lower stabilities compared to their all-carbon analogues.

Kreye M; Glöckner A; Daniliuc CG; Freytag M; Jones PG; Tamm M; Walter MD

2013-02-01

327

Selective olefination of carbonyl compounds via metal-catalyzed carbene transfer from diazo reagents.  

UK PubMed Central (United Kingdom)

A number of transition metal complexes are capable of catalyzing selective olefination of carbonyl compounds, including aldehydes, activated and unactivated ketones, with diazo reagents in the presence of triphenylphosphine or related tertiary phosphines. These catalytic olefination reactions can be carried out in a one-pot fashion under neutral conditions with the use of different diazo reagents as carbene sources, typically affording olefins in high yields and high stereoselectivity.

Hu Y; Zhang XP

2012-01-01

328

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

Scientific Electronic Library Online (English)

Full Text Available 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 there 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 (more) (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.

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

2011-01-01

329

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; PING LI; WEIHUA WANG; GENGXIU ZHENG; QIUFEN WANG

2010-01-01

330

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

Directory of Open Access Journals (Sweden)

Full Text Available 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 there 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

2011-01-01

331

Abnormal carbenes derived from the 1,5-cycloaddition product between azides and alkynes: structural characterization of Pd(II) complexes and their catalytic properties.  

UK PubMed Central (United Kingdom)

The exclusive formation of the 1,5-cycloaddition product between azides and alkynes is taken advantage of in generating the first examples of abnormal carbenes from these precursors. This new route provides unprecedented post-functionalization possibilities for such abnormal carbenes.

Hohloch S; Frey W; Su CY; Sarkar B

2013-08-01

332

Abnormal carbenes derived from the 1,5-cycloaddition product between azides and alkynes: structural characterization of Pd(II) complexes and their catalytic properties.  

Science.gov (United States)

The exclusive formation of the 1,5-cycloaddition product between azides and alkynes is taken advantage of in generating the first examples of abnormal carbenes from these precursors. This new route provides unprecedented post-functionalization possibilities for such abnormal carbenes. PMID:23727705

Hohloch, Stephan; Frey, Wolfgang; Su, Cheng-Yong; Sarkar, Biprajit

2013-05-31

333

Well-defined NHC-rhodium hydroxide complexes as alkene hydrosilylation and dehydrogenative silylation catalysts.  

UK PubMed Central (United Kingdom)

Alkene hydrosilylation and dehydrogenative silylation reactions, mediated by [Rh(cod)(NHC)(OH)] complexes (cod = 1,5-cyclooctadiene; NHC = N-heterocyclic carbene) are described. The study details a comparison of the catalytic activity and steric characteristics of four rhodium complexes bearing different NHC ligands. The novel [Rh(cod)(Ii-PrMe)(OH)] complex (Ii-PrMe = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidine) was designed to improve the reactivity of Rh(I)-hydroxides and proved to be a successful promoter of hydrosilylation and dehydrogenative silylation, displaying good stereo- and regiocontrol.

Truscott BJ; Slawin AM; Nolan SP

2013-01-01

334

Highly efficient ruthenium catalysts for the formation of tetrasubstituted olefins via ring-closing metathesis.  

Science.gov (United States)

[reaction: see text] A series of ruthenium-based metathesis catalysts with N-heterocyclic carbene (NHC) ligands have been prepared in which the N-aryl groups have been changed from mesityl to mono-ortho-substituted phenyl (e.g., tolyl). These new catalysts offer an exceptional increase in activity for the formation of tetrasubstituted olefins via ring-closing metathesis (RCM), while maintaining high levels of activity in ring-closing metathesis (RCM) reactions that generate di- and trisubstituted olefins. PMID:17378575

Stewart, Ian C; Ung, Thay; Pletnev, Alexandre A; Berlin, Jacob M; Grubbs, Robert H; Schrodi, Yann

2007-03-23

335

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

UK PubMed Central (United Kingdom)

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.

Labande A; Debono N; Sournia-Saquet A; Daran JC; Poli R

2013-05-01

336

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

337

Formation of imidates, amides, amines, carbamates, and ureas from the 3-NPh ligands of Fe3( 3-NPh)2(CO)9  

Energy Technology Data Exchange (ETDEWEB)

The bis(nitrene) cluster Fe3( 3-NPh)2(CO)9 (1) reacts with Li(HBEt3), MeLi, PhLi, and NaOMe to form the formyl and acyl clusters (Fe3( 3-NPh)2(CO)8C)O)R) (4, R = H; 6, R = Ph; 7, R = Me; 8, R = OMe). Formyl cluster 4 is unstable at room temperature and slowly loses CO to form the hydride cluster (HFe3( 3-NPh)2(CO)8) . The benzoyl cluster 7 reacts with EtOTf to yield the nitrene-carbene cluster Fe3( 3-NPh)2(CO)8(C)OEt)Ph)(9) which has been structurally characterized. The carbene and nitrene ligands in 9 couple to form the imidate PhN double bond C(OEt)Ph when 9 is exposed to air or allowed to stand in solution for prolonged periods under CO or N2 atmospheres. Similar nitrene-benzoyl coupling from 7 gives benzanilide, and the methoxycarbonyl and nitrene ligands in 8 couple to give methyl N-phenylcarbamate when the clusters are oxidatively degraded with (FeCp2) . The bis(phosphinidene) cluster Fe3( 3-PPh)2(CO)9 (2) also reacts with PhLi to yield a benzoyl derivative, (Fe3( 3-PPh)2(CO)8(C)O)Ph)) . However, addition of EtOTf to this species does not result in a carbene cluster analogous to 9, but instead phosphinidene-carbene coupling occurs to give Fe3( 2-PPh)( 3-PhPC)OEt)Ph)(CO)9 which has been structurally characterized. The phosphorus atom of the 3-PhPC)OEt)Ph ligand bridges two iron atoms, and the ethoxy-substituted carbon is attached to the third iron atom.

Williams, G.D.; Whittle, R.R.; Geoffroy, G.L.; Rheingold, A.L.

1987-06-24

338

2-Ferrocenyl-2-thiazoline as a building block of novel phosphine-free ligands.  

Science.gov (United States)

New 1,2-disubstituted ferrocenes [, in which R = -SMe, -SPh, -SiPr, -SiMe3, -SePh, -SnBu3, -B(OH)2, -Me, -I] with a thiazoline ring in the ferrocene backbone using as key intermediate a ferrocenyl Fischer carbene complex were synthesized. The capability of the 2-thiazoline moiety as an ortho-directed metalation group was demonstrated by subsequent quenching of lithium intermediate with several electrophiles, proving to be an excellent method for synthesizing bidentate ligands. The catalytic scope of the [N,S] ligand as the corresponding palladium complex in a microwave-promoted Heck reaction was also tested. Results obtained showed better catalytic activity of compared to other catalytic systems based on a [N,S] ferrocenyl ligand. PMID:23824245

Corona-Sánchez, Ricardo; Toscano, Rubén A; Ortega-Alfaro, M Carmen; Sandoval-Chávez, César; López-Cortés, José G

2013-07-31

339

Synthesis, structure, and characterization of dinuclear copper(I) halide complexes with P^N ligands featuring exciting photoluminescence properties.  

UK PubMed Central (United Kingdom)

A series of highly luminescent dinuclear copper(I) complexes has been synthesized in good yields using a modular ligand system of easily accessible diphenylphosphinopyridine-type P^N ligands. Characterization of these complexes via X-ray crystallographic studies and elemental analysis revealed a dinuclear complex structure with a butterfly-shaped metal-halide core. The complexes feature emission covering the visible spectrum from blue to red together with high quantum yields up to 96%. Density functional theory calculations show that the HOMO consists mainly of orbitals of both the metal core and the bridging halides, while the LUMO resides dominantly on the heterocyclic part of the P^N ligands. Therefore, modification of the heterocyclic moiety of the bridging ligand allows for systematic tuning of the luminescence wavelength. By increasing the aromatic system of the N-heterocycle or through functionalization of the pyridyl moiety, complexes with emission maxima from 481 to 713 nm are obtained. For a representative compound, it is shown that the ambient-temperature emission can be assigned as a thermally activated delayed fluorescence, featuring an attractively short emission decay time of only 6.5 ?s at ?PL = 0.8. It is proposed to apply these compounds for singlet harvesting in OLEDs.

Zink DM; Bächle M; Baumann T; Nieger M; Kühn M; Wang C; Klopper W; Monkowius U; Hofbeck T; Yersin H; Bräse S

2013-03-01

340

Theoretical studies of mechanisms of cycloaddition reaction between difluoromethylene carbene and acetone  

Science.gov (United States)

Mechanisms of the cycloaddition reaction between singlet difluoromethylene carbene and acetone have been investigated with the second-order Møller-Plesset (MP2)/6-31G* method, including geometry optimization and vibrational analysis. Energies for the involved stationary points on the potential energy surface (PES) are corrected by zero-point energy (ZPE) and CCSD(T)/6-31G* single-point calculations. From the PES obtained with the CCSD(T)//MP2/6-31G* method for the cycloaddition reaction between singlet difluoromethylene carbene and acetone, it can be predicted that path B of reactions 2 and 3 should be two competitive leading channels of the cycloaddition reaction between difluoromethylene carbene and acetone. The former consists of two steps: (i) the two reactants first form a four-membered ring intermediate, INT2, which is a barrier-free exothermic reaction of 97.8 kJ/mol; (ii) the intermediate INT2 isomerizes to a four-membered product P2b via a transition state TS2b with an energy barrier of 24.9 kJ/mol, which results from the methyl group transfer. The latter proceeds in three steps: (i) the two reactants first form an intermediate, INT1c, through a barrier-free exothermic reaction of 199.4 kJ/mol; (ii) the intermediate INT1c further reacts with acetone to form a polycyclic intermediate, INT3, which is also a barrier-free exothermic reaction of 27.4 kJ/mol; and (iii) INT3 isomerizes to a polycyclic product P3 via a transition state TS3 with an energy barrier of 25.8 kJ/mol.

Lu, Xiu Hui; Yu, Hai Bin; Wu, Wei Rong; Xu, Yue Hua

 
 
 
 
341

Propagation and termination steps in Rh-mediated carbene polymerisation using diazomethane.  

UK PubMed Central (United Kingdom)

Rh-mediated carbene (co)polymerisation of diazomethane works best in the presence of Rh(III) catalyst precursors, the use of which leads to a significant increase in polymer yield and molecular weight. Chain termination via ?-hydride elimination is severely suppressed for these species, although this process does still occur leading to unsaturated chain ends. Subsequent chain walking leading to the formation of branched polymers seems not to occur. Computational studies describing pathways for both chain propagation and chain termination using a (cycloocta-2,5-dien-1-yl)Rh(III)(alkyl) species as a representative model for the active species revealed that chain propagation is favoured for these species, although ?-hydride elimination is still viable at the applied reaction temperatures. The computational studies are in excellent agreement with all experimental results, and further reveal that chain propagation via carbene insertion (leading to linear poly-methylene) occurs with a much lower energy barrier than insertion of 1-alkenes into either the Rh-H bond after ?-hydride elimination or into the Rh-C bond of the growing polymer chain (leading to branched polymers). These energetic differences conveniently explain why experimentally the formation of branches is not observed in (co)polymerisation reactions employing diazomethane. The formation of substantial amounts of low-M(w) oligomers and dimers in the experimental reactions can be ascribed to the presence of (1,5-cyclooctadiene)Rh(I) species in the reaction mixture, for which chain termination via ?-hydride elimination is clearly favoured over chain propagation. These two species stem from a non-selective catalyst activation process during which the catalyst precursors are in situ activated towards carbene polymerisation, and as such the results in this paper might contribute to further improvements of this reaction.

Franssen NM; Finger M; Reek JN; de Bruin B

2013-03-01

342

Laboratory detection of a second carbon chain carbene - Butatrienylidene, H2CCCC  

International Nuclear Information System (INIS)

From laboratory measurement of 45 lines in its rotational spectrum, butatrienylidene, H2C4, has been identified, the second carbene with a known microwave spectrum in the sequence formed by the addition of successive carbon atoms to a linear cumulene backbone. With a dipole moment computed to be high (4.5 D), H2C4 is a good candidate for interstellar and circumstellar detection. This paper presents a determination of the rotational and leading centrifugal distortion constants and a tabulation of possible astronomical lines in the radio and far-IR. 7 refs

1990-01-01

343

Laboratory detection of a second carbon chain carbene - Butatrienylidene, H2CCCC  

Energy Technology Data Exchange (ETDEWEB)

From laboratory measurement of 45 lines in its rotational spectrum, butatrienylidene, H2C4, has been identified, the second carbene with a known microwave spectrum in the sequence formed by the addition of successive carbon atoms to a linear cumulene backbone. With a dipole moment computed to be high (4.5 D), H2C4 is a good candidate for interstellar and circumstellar detection. This paper presents a determination of the rotational and leading centrifugal distortion constants and a tabulation of possible astronomical lines in the radio and far-IR. 7 refs.

Killian, T.C.; Gottlieb, C.A.; Gottlieb, E.W.; Vrtilek, J.M.; Thaddeus, P. (Harvard Univ., Cambridge, MA (USA) Harvard-Smithsonian Center for Astrophysics, Cambridge, MA (USA))

1990-12-01

344

Highly Enantioselective Dearomatizing Formal [3+3] Cycloaddition Reactions of N-Acyliminopyridinium Ylides with Electrophilic Enol Carbene Intermediates.  

UK PubMed Central (United Kingdom)

Extrusion of dinitrogen from enol diazoacetates with a Rh(II) catalyst generates metal enol carbenes. Subsequent vinylogous addition of these to N-acyliminopyridinium ylides results in an effective formal [3+3] cycloaddition to give highly substituted 1,2,3,6-tetrahydropyridazines in up to 98?% ee and high yield.

Xu X; Zavalij PY; Doyle MP

2013-10-01

345

One-pot three-component synthesis of quinoxaline and phenazine ring systems using Fischer carbene complexes  

Digital Repository Infrastructure Vision for European Research (DRIVER)

One-pot three-component coupling of o-alkynylheteroaryl carbonyl derivatives with Fischer carbene complexes and dienophiles leading to the synthesis of quinoxaline and phenazine ring systems has been investigated. This involves the generation of furo[3,4-b]pyrazine and furo[3,4-b

Priyabrata Roy; Binay Krishna Ghorai

346

Beyond click chemistry: spontaneous C-triazolyl transfer from copper to rhenium and transformation into mesoionic C-triazolylidene carbene.  

UK PubMed Central (United Kingdom)

The exceptional versatility of the 1,2,3-triazole ring in tridentate carbonyl rhenium complexes has been explored starting from a "click chemistry" unexpected product obtained from the spontaneous copper to rhenium transmetalation, which opens the way to successive transformation to C-triazolyl and triazolylidene mesoionic carbene.

Álvarez CM; García-Escudero LA; García-Rodríguez R; Miguel D

2012-07-01

347

Synthesis of phenanthrene derivatives through the net [5+5]-cycloaddition of prenylated carbene complexes with 2-alkynylbenzaldehyde derivatives  

Science.gov (United States)

The reaction of prenylated carbene complexes and 2-alkynylbenzoyl derivatives has been investigated. Phenanthrene derivatives are produced if iodine is added prior to product isolation. Under these conditions alkyl migration reactions occur to form the observed products. The product yields are considerably higher using bis(prenylated) species owing to an increase in the effective molarity of dienophilic entities.

Menon, Suneetha; Sinha-Mahapatra, Dilip

2007-01-01

348

Bis(carbene)pyridine complexes of Cr(III): exceptionally active catalysts for the oligomerization of ethylene.  

UK PubMed Central (United Kingdom)

Pincer-heterocyclic carbene complexes of Cr(III), of the form [2,6-(1-alkylimidazol-2-ylidene)pyridine]CrCl3, have been prepared and evaluated as catalysts for the oligomerization of ethylene to alpha-olefins. In combination with methylaluminoxane cocatalyst, exceptionally high activities are obtained, ranging up to ca. 40 000 g mmol-1 bar-1 h-1.

McGuinness DS; Gibson VC; Wass DF; Steed JW

2003-10-01

349

One-pot three-component synthesis of quinoxaline and phenazine ring systems using Fischer carbene complexes  

Digital Repository Infrastructure Vision for European Research (DRIVER)

One-pot three-component coupling of o-alkynylheteroaryl carbonyl derivatives with Fischer carbene complexes and dienophiles leading to the synthesis of quinoxaline and phenazine ring systems has been investigated. This involves the generation of furo[3,4-b]pyrazine and furo[3,4-b]quinoxaline as tran...

Roy, Priyabrata; Ghorai, Binay Krishna

350

On the incidence of non-covalent intramolecular interligand interactions on the conformation of carbene complexes: a case study.  

UK PubMed Central (United Kingdom)

The prevalence of a counterintuitive carbene conformation in a series of piano-stool Mn(I) alkylalkoxycarbene complexes was rationalized by means of DFT calculations and high-resolution XRD analysis in terms of stabilizing non-covalent C-H···C?O interligand interactions.

Lugan N; Fernández I; Brousses R; Valyaev DA; Lavigne G; Ustynyuk NA

2013-01-01

351

Neutral carbene analogues of group 13 elements: the dimerization reaction to a biradicaloid.  

UK PubMed Central (United Kingdom)

The dimerization reactions of the neutral carbene analogues with the group 13 elements boron, aluminum, gallium, and indium are studied. Besides boron, all monomeric species possess singlet ground states. For Al, bulky substituted cases were investigated; they reveal no essential changes in the singlet-triplet energy separations compared with the parent species. The dimerization energies increase with an increase in the bulk of the substituents; this is a consequence of an enhancement of van der Waals forces for association. The latter is opposed by entropic forces, which facilitate dissociation. An equilibrium between monomeric and dimeric structures is predicted because of enthalpy versus entropy control. The low-temperature domain association should prevail in the formation of a dimer with Al (Ga) within the formal oxidation state I+. The Al-Al bond refers to a chelated biradicaloid species with an energetically low-lying triplet state. It emerges from the metal-metal contacts in the dimer. The biradical character of the dimer decreases in the order E = Al ? Ga > In. The carbene analogue of In forms upon dimerization of only weak coordinative metal-metal interactions.

Schoeller WW

2011-03-01

352

Insights into the strength and nature of carbene···halogen bond interactions: a theoretical perspective.  

UK PubMed Central (United Kingdom)

Halogen-bonding, a noncovalent interaction between a halogen atom X in one molecule and a negative site in another, plays critical roles in fields as diverse as molecular biology, drug design and material engineering. In this work, we have examined the strength and origin of halogen bonds between carbene CH? and XCCY molecules, where X?=?Cl, Br, I, and Y?=?H, F, COF, COOH, CF?, NO?, CN, NH?, CH?, OH. These calculations have been carried out using M06-2X, MP2 and CCSD(T) methods, through analyses of surface electrostatic potentials V S(r) and intermolecular interaction energies. Not surprisingly, the strength of the halogen bonds in the CH?···XCCY complexes depend on the polarizability of the halogen X and the electron-withdrawing power of the Y group. It is revealed that for a given carbene···X interaction, the electrostatic term is slightly larger (i.e., more negative) than the dispersion term. Comparing the data for the chlorine, bromine and iodine substituted CH?···XCCY systems, it can be seen that both the polarization and dispersion components of the interaction energy increase with increasing halogen size. One can see that increasing the size and positive nature of a halogen's ?-hole markedly enhances the electrostatic contribution of the halogen-bonding interaction.

Esrafili MD; Mohammadirad N

2013-06-01

353

Synthesis of iridium and rhodium complexes with new chiral phosphine-NHC ligands based on 1,1'-binaphthyl framework and their application in asymmetric hydrogenation.  

UK PubMed Central (United Kingdom)

The first series of chiral phosphine-imidazole carbene ligands based on a 1,1'-binaphthyl framework were synthesized from (R)-2-amine-2'-(diphenylphosphino)-1,1'-binaphthyl (1) in a four-step pathway. After deprotonation of these phosphine-imidazolium salts with LiO(t)Bu, and subsequent complexation with [Ir(COD)Cl]2 and anion exchange with NaBArF, phosphine-carbene chelated iridium complexes (R)-6a and (R)-6b were obtained. Their structures have been characterized by NMR and X-ray diffraction analysis. The NHC-phosphine rhodium complex (R)-6c has been also obtained by a similar synthetic method. These iridium complexes have been applied to catalyze the asymmetric hydrogenation of alkenes to give the corresponding products in moderate to excellent conversion (up to 99%) and moderate enantioselectivities under mild conditions (up to 61% ee).

Gu P; Zhang J; Xu Q; Shi M

2013-10-01

354

Synthesis of iridium and rhodium complexes with new chiral phosphine-NHC ligands based on 1,1'-binaphthyl framework and their application in asymmetric hydrogenation.  

Science.gov (United States)

The first series of chiral phosphine-imidazole carbene ligands based on a 1,1'-binaphthyl framework were synthesized from (R)-2-amine-2'-(diphenylphosphino)-1,1'-binaphthyl (1) in a four-step pathway. After deprotonation of these phosphine-imidazolium salts with LiO(t)Bu, and subsequent complexation with [Ir(COD)Cl]2 and anion exchange with NaBArF, phosphine-carbene chelated iridium complexes (R)-6a and (R)-6b were obtained. Their structures have been characterized by NMR and X-ray diffraction analysis. The NHC-phosphine rhodium complex (R)-6c has been also obtained by a similar synthetic method. These iridium complexes have been applied to catalyze the asymmetric hydrogenation of alkenes to give the corresponding products in moderate to excellent conversion (up to 99%) and moderate enantioselectivities under mild conditions (up to 61% ee). PMID:23900600

Gu, Peng; Zhang, Jun; Xu, Qin; Shi, Min

2013-07-30

355

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

Scientific Electronic Library Online (English)

Full Text Available 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 (more) 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 of 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 d (more) iazo 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.

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

2009-09-01

356

{3,3?-Bis[(anthracen-9-yl)methyl]-1,1?-[(ethane-1,2-diyldioxy)bis(ethane-1,2-diyl)]bis(imidazol-2-ylidene)}mercury(II) bis(hexafluoridophosphate) acetonitrile disolvate  

Directory of Open Access Journals (Sweden)

Full Text Available In the title compound, [Hg(C42H38N4O2)](PF6)2·2CH3CN, the HgII cation lies on a twofold axis which is also the internal symmetry element of the complete cationic complex. The HgII cation is coordinated by two symmetry-related C(carbene) atoms [Hg—C = 2.058?(9)?Å] in a nearly linear geometry, with a C—Hg—C angle of 175.8?(5)°. There are weak intermolecular C—H...F interactions in the crystal packing between an F atom of a hexafluoridophosphate anion and a –CH2– group of the bis-N-heterocyclic carbene ligand.

Jun-Wen Wang; Yue Guo; Gui-Ying Dong; Yu Gu; Di-Si Bai

2012-01-01

357

?3-Chlorido-tris(bis{1-[2-(dimethylamino)ethyl]-3-methylimidazol-2-ylidene}silver(I)) dichloride  

Directory of Open Access Journals (Sweden)

Full Text Available In the crystal structure of the title compound, [Ag3Cl(C8H15N3)6]Cl2, the AgI ion, which is located on a twofold rotation axis, exists in a T-shape coordination environment. Two carbene C atoms of the N-heterocyclic carbene (NHC) ligands are bonded tightly forming a slightly bent [Ag(NHC)2]+ cation [C—Ag—C angle = 162.80?(18)°]. Three of these complex cations are further aggregated by one bridging chloride anion, which is lying on a threefold rotoinversion axis and is only loosely binding to the Ag+ ions. The N atom of the amine group is not engaged in any coordinative bond.

Christoph Topf; Sebastian Leitner; Uwe Monkowius

2012-01-01

358

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(OAc)2 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.

Sedat Ya?ar; Emine Özge Özcan; Nevin Gürbüz; Bekir Çetinkaya; ?smail Özdemir

2010-01-01

359

Carbodiphosphorane analogues E(PPh3)2 with E=C-Pb: a theoretical study with implications for ligand design.  

Science.gov (United States)

Quantum-chemical calculations at the BP86/TZVPP level have been carried out for the heavy Group 14 homologues of carbodiphosphorane E(PPh(3))(2), where E=Si, Ge, Sn, Pb, which are experimentally unknown so far. The results of the theoretical investigation suggest that the tetrelediphosphoranes E(PPh(3))(2) (1E) are stable compounds that could become isolated in a condensed phase. The molecules possess donor-acceptor bonds Ph(3)P?E?PPh(3) to a bare tetrele atom E, which retains its four valence electrons as two electron lone pairs. The analysis of the bonding situation and the calculation of the chemical reactivity indicate that the molecules 1E belong to the class of divalent E(0) compounds (ylidones). All molecules 1C-1Pb have very large first but also very large second proton affinities, which distinguishes them from the N-heterocyclic carbene homologues, in which the donor atom is a divalent E(II) species that possesses only one electron lone pair. Compounds 1E are powerful double donors that strongly bind Lewis acids such as BH(3) and AuCl in the complexes 1E(BH(3))(n) and 1E(AuCl)(n) (n=1, 2). The bond dissociation energies (BDEs) of the second BH(3) and AuCl molecules are only slightly less than the BDE of the first BH(3) and AuCl. The results of this work are a challenge for experimentalists. PMID:22238129

Takagi, Nozomi; Tonner, Ralf; Frenking, Gernot

2012-01-11

360

Electronic structure of the carbyne C-Li and the carbene, Li/sub 2/C  

Energy Technology Data Exchange (ETDEWEB)

The electronic structure of the carbyne C-Li and the carbene Li/sub 2/C has been investigated using ab initio GVB, SCF, and CI techniques. We find that C-Li has a /sup 4/..sigma../sup -/ ground state with the companion /sup 2/II/sub r/ state approximately 34 kcal/mol higher in energy. This should be contrasted with C-H in which the /sup 2/II/sub r/ is the ground state and the /sup 4/..sigma../sup -/ state is approximately 17 kcal/mol higher. In addition we find a /sup 2/II/sub i/ state some 49 kcal/mol above the ground /sup 4/..sigma../sup -/ state. An analysis of the bonding indicates that these states are highly polar. Li/sub 2/C is an unusual carbene having three triplets below the first singlet: /sup 3/..sigma../sub g//sup -/, R = 3.717 bohrs; /sup 3/II/sub g/ (3), R = 3.507 bohrs; /sup 3/A/sub 2/, THETA = 88.1/sup 0/, R = 3.815 bohrs. At the SCF + 1 + 2 level the /sup 3/..sigma../sub g//sup -/ is the ground state with the /sup 3/A/sub 2/ 6.2 kcal/mol higher. Correcting for unlinked clusters inverts this order and makes the /sup 3/A/sub 2/ the ground state 0.2 kcal/mol below the /sup 3/..sigma../sub g//sup -/ state. An analysis of the bonding in the /sup 3/A/sub 2/ states reveals a dative bond between the doubly occupied Li/sub 2/ /sup 2/sigma/sub g/ orbital and an empty p sigma orbital on C. The implications of this mode of bonding for the dilithiomethanes are discussed. 10 figures, 7 tables.

Mavridis, A. (Michigan State Univ., East Lansing); Harrison, J.F.

1982-07-14

 
 
 
 
361

Modulating Robo: Ligand Interactions.  

Science.gov (United States)

Disclosed are methods and compositions for identifying agents which modulate the interaction of Robo and a Robo ligand and for modulating the interaction of Robo and a Robo ligand. The methods for identifying Robo:ligand modulators find particular applica...

C. S. Goodman K. Brose M. Tessier-Lavigne T. Kidd

2004-01-01

362

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

UK PubMed Central (United Kingdom)

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.

Forshaw AP; Smith JM; Ozarowski A; Krzystek J; Smirnov D; Zvyagin SA; Harris TD; Karunadasa HI; Zadrozny JM; Schnegg A; Holldack K; Jackson TA; Alamiri A; Barnes DM; Telser J

2013-01-01

363

A search for thermal isomerization of olefins to carbenes: Thermal generations of the silicon-nitrogen double bond  

Energy Technology Data Exchange (ETDEWEB)

The first part of this thesis will search for the thermal isomerization of olefins to carbenes which is predicted to be a high energy process by calculations and has only been observed in a few strained olefins. The possibility of thermal isomerization of simple olefins to carbenes will be explored. Substitution of a silyl group on the double bond of an olefin allows a potential intermediate which has a {beta}-radical to the silyl group during the cis-trans isomerization. The effects of a trimethylsilyl group on this isomerization are the subject of this study. The second part of this thesis will include the generation and chemistry of intermediates containing a silicon-nitrogen double bond. The isomerization of parent silanimine to the aminosilylene was calculated to be a high energy process. New approaches to the silicon-nitrogen double bond will also be presented. 92 refs., 12 figs., 11 tabs.

Zhang, Xianping.

1990-09-21

364

One-pot three-component synthesis of quinoxaline and phenazine ring systems using Fischer carbene complexes  

Directory of Open Access Journals (Sweden)

Full Text Available One-pot three-component coupling of o-alkynylheteroaryl carbonyl derivatives with Fischer carbene complexes and dienophiles leading to the synthesis of quinoxaline and phenazine ring systems has been investigated. This involves the generation of furo[3,4-b]pyrazine and furo[3,4-b]quinoxaline as transient intermediates, which were trapped with Diels–Alder dienophiles. This is the first report on furo[3,4-b]pyrazine intermediates.

Priyabrata Roy; Binay Krishna Ghorai

2010-01-01

365

Control over the E/Z selectivity of the catalytic dimerization of group 6 (Fischer) metal carbene complexes.  

UK PubMed Central (United Kingdom)

The systematic investigation of the effect of different catalysts and additives in the reaction of self-dimerization of alkoxychromium(0) (Fischer) carbene complexes resulted in the selection of Pd(P(t)Bu(3))(2) to effect this transformation with good to excellent E selectivities and acceptable to excellent chemical yields. This catalyst will allow the control of the geometry in the synthesis of polyconjugated olefins, one emerging application of these catalytic reactions.

Chu GM; Fernández I; Sierra MA

2013-02-01

366

Difluoroboroxymolybdenum Fischer carbene complexes as precursors of acyl radicals: dimerization and trapping with electron-deficient alkenes  

UK PubMed Central (United Kingdom)

Pentacarbonyl acyl molybdates 1 react with boron trifluoride to give difluoroboroxy Fischer carbene complexes 2, which undergo loss of the metal fragment at room temperature to form 1,2-diketones 3, 1,2-hydroxy ketones 4, or dimers 5 through a dimerization or decarbonylation-dimerization process of acyl radicals. Decomposition of 2 in the presence of electron-deficient alkenes 11 and 18 furnishes the two-, three-, and four-component coupling products 12, 13, 19, 20, and 21.

Barluenga J; Rodriguez F; Fananas FJ

2000-06-01

367

Chemistry of Polynuclear Metal Complexes with Bridging Carbene or Carbyne Ligands. Part 89. (1) Tetra- and Penta-nuclear Tungsten-Rhodium Complexes: Crystal Structures of (W3Rh2(mu-CO)2(mu-CMe)(mu-C(Me)C(O))(Mu-PPh2)2-(Mu3-CMe)(CO)2(eta- C5H5)3) and (W3Rh2(mu-CO)3(mu-CMe)(mu-C(Me)PPh2)(mu3-CMe)-(CO)2(mu-C5H5)3).  

Science.gov (United States)

Several mixed-metal tungsten-rhodium cluster compounds have been prepared in which the metal-metal bonds are bridged by alkylidyne, ketenyl, phosphido, or phosphaalkyne groups. These ligands undergo novel migratory reactions, a feature established by X-ra...

S. J. Davies F. G. Stone J. A. Howard M. U. Pilotti

1989-01-01

368

C-H activation by multiply bonded complexes with potentially noninnocent ligands: a computational study.  

UK PubMed Central (United Kingdom)

Second- and third-row (typically precious metals) transition metal complexes are known to possess certain electronic features that define their structure and reactivity and are usually not observed in their first-row (base metal) congeners. Can these electronic features be conferred onto first-row transition metals with the aid of noninnocent and/or very high-field ligands? In this research, the impact upon methane C-H bond activation was modeled using the dipyridylazaallyl (smif) supporting ligand for late, first-row transition metal (M) imide, oxo, and carbene complexes (M = Fe, Co, Ni, or Cu; E = O, NMe, or CMe2). Density functional theory calculations suggest that the combination of smif with iron and the oxo activating ligand is the most energetically favorable complex for methane C-H activation. A change in the preferred transition state for methane C-H activation from [2+2] addition to hydrogen atom abstraction was observed upon going from Fe to Cu and for Fe as compared to precious metals. Contrary to expectations, it was the imide ligand rather than the dipyridylazaallyl ligand that was found to possess redox "noninnocent" characteristics.

Olatunji-Ojo OA; Cundari TR

2013-07-01

369

C-H activation by multiply bonded complexes with potentially noninnocent ligands: a computational study.  

Science.gov (United States)

Second- and third-row (typically precious metals) transition metal complexes are known to possess certain electronic features that define their structure and reactivity and are usually not observed in their first-row (base metal) congeners. Can these electronic features be conferred onto first-row transition metals with the aid of noninnocent and/or very high-field ligands? In this research, the impact upon methane C-H bond activation was modeled using the dipyridylazaallyl (smif) supporting ligand for late, first-row transition metal (M) imide, oxo, and carbene complexes (M = Fe, Co, Ni, or Cu; E = O, NMe, or CMe2). Density functional theory calculations suggest that the combination of smif with iron and the oxo activating ligand is the most energetically favorable complex for methane C-H activation. A change in the preferred transition state for methane C-H activation from [2+2] addition to hydrogen atom abstraction was observed upon going from Fe to Cu and for Fe as compared to precious metals. Contrary to expectations, it was the imide ligand rather than the dipyridylazaallyl ligand that was found to possess redox "noninnocent" characteristics. PMID:23802880

Olatunji-Ojo, Olayinka A; Cundari, Thomas R

2013-06-26

370

Alkene cyclopropanation catalyzed by Halterman iron porphyrin: participation of organic bases as axial ligands.  

Science.gov (United States)

With the iron(III) complex of the Halterman iron porphyrin [P*Fe(Cl)] and ethyl diazoacetate (EDA) as catalyst and carbene source, respectively, styrene-type substrates were converted to cyclopropyl esters with high trans/cis ratio (not less than 12) and high enantioselectivity for the trans-isomers (74-86% ee). The isomeric distribution of the cyclopropyl esters so obtained is akin to that obtained from the previously reported Ru(II) counterpart [P*Ru(CO)]. A linear Hammett correlation log(k(X)/k(H)) = sigma(+)rho was observed with rho = -0.57 suggesting the involvement of an electrophilic cyclopropanating species derived from the iron(II) center as the reactive intermediate in the catalytic cycle. This is further supported by a dramatic decrease in the enantioselectivity and trans/cis ratio observed in an experiment of styrene cyclopropanation when the reaction mixture was deliberately exposed to air. Axial ligand effects on the selectivities was also investigated. Substantial improvement in trans/cis ratios could be achieved by addition of organic bases such as pyridine (py) and 1-methylimidazole (MeIm) to the catalytic reaction. The existence of axially ligated iron carbene moieties, [P*Fe(CHCO(2)Et)(py)] and [P*Fe(CHCO(2)Et)(MeIm)], was established by electrospray mass spectrometry. Study of secondary kinetic isotope effect indicated that a more product-like transition state was generated by addition of MeIm. PMID:17033710

Lai, Tat-Shing; Chan, Fung-Yi; So, Pui-Kin; Ma, Dik-Lung; Wong, Kwok-Yin; Che, Chi-Ming

2006-08-22

371

Alkene cyclopropanation catalyzed by Halterman iron porphyrin: participation of organic bases as axial ligands.  

UK PubMed Central (United Kingdom)

With the iron(III) complex of the Halterman iron porphyrin [P*Fe(Cl)] and ethyl diazoacetate (EDA) as catalyst and carbene source, respectively, styrene-type substrates were converted to cyclopropyl esters with high trans/cis ratio (not less than 12) and high enantioselectivity for the trans-isomers (74-86% ee). The isomeric distribution of the cyclopropyl esters so obtained is akin to that obtained from the previously reported Ru(II) counterpart [P*Ru(CO)]. A linear Hammett correlation log(k(X)/k(H)) = sigma(+)rho was observed with rho = -0.57 suggesting the involvement of an electrophilic cyclopropanating species derived from the iron(II) center as the reactive intermediate in the catalytic cycle. This is further supported by a dramatic decrease in the enantioselectivity and trans/cis ratio observed in an experiment of styrene cyclopropanation when the reaction mixture was deliberately exposed to air. Axial ligand effects on the selectivities was also investigated. Substantial improvement in trans/cis ratios could be achieved by addition of organic bases such as pyridine (py) and 1-methylimidazole (MeIm) to the catalytic reaction. The existence of axially ligated iron carbene moieties, [P*Fe(CHCO(2)Et)(py)] and [P*Fe(CHCO(2)Et)(MeIm)], was established by electrospray mass spectrometry. Study of secondary kinetic isotope effect indicated that a more product-like transition state was generated by addition of MeIm.

Lai TS; Chan FY; So PK; Ma DL; Wong KY; Che CM

2006-10-01

372

N, N'-Olefin functionalized bis-imidazolium gold(I) salt is an efficient candidate to control keratitis-associated eye infection.  

UK PubMed Central (United Kingdom)

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.

Samanta T; Roymahapatra G; Porto WF; Seth S; Ghorai S; Saha S; Sengupta J; Franco OL; Dinda J; Mandal SM

2013-01-01

373

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.

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

2013-01-01

374

The use of benzil to obtain functionalized N-heterocycles  

Scientific Electronic Library Online (English)

Full Text Available Abstract in portuguese Neste trabalho foi estudada a reatividade da benzila, empregando-se C, N- e N, N-nucleófilos tais como 2-butenoatos de etila 3-amino substituidos 4a-c, (S, Z)-3-(1-etóxi-3-hidróxi-1-oxo-2-propanilamina)-2-butenoato de etila 6, semicarbazida 8 e tiossemicarbazida 10, para avaliar os centros eletrofílicos na formação de compostos heterocíclicos polifuncionalizados, obtendo-se pirrolinonas 5a-c, pirrol 7, triazinona 9 e triazintiona 11. A dibenzoila 3 empregada foi ob (more) tida pela oxidação da benzoina na ausência de solvente em um estudo comparativo entre diferentes protocolos de oxidação, utilizando suportes sólidos associados ou não a energia de microondas com bons rendimentos sem empregar reagentes agressivos. Abstract in english In this work, the reactivity of benzil was studied, employing C, N- and N, N-nucleophiles, such as ethyl(3-amino substituted) 2-butenoates 4a-c , (S, Z)-ethyl 3-(1-ethoxy-3-hydroxy-1-oxopropan-2-ylamino)but-2-enoate 6, semicarbazide 8 or thiosemicarbazide 10, to evaluate their electrophilic centers as building blocks for the synthesis of the polyfunctionalized heterocyclic compounds, resulting in pyrrolinone 5a-c, pyrrole 7, triazinone 9 and triazinethione 11. The employe (more) d benzil 3 was obtained by the oxidation of the benzoin under solvent free conditions in a comparative study between different protocols of oxidation, using the methodology under mild reaction conditions and supported reagent associated to the microwave irradiation, with good results and without aggressive reagents.

Braibante, Mara E. F.; Braibante, Hugo T. S.; Uliana, Marciana P.; Costa, Carla C.; Spenazzatto, Marcelo

2008-01-01

375

Thiadiazines, N,N-Heterocycles of Biological Relevance  

Directory of Open Access Journals (Sweden)

Full Text Available The 3,5-disubstituted tetrahydro-2H-1,3,5-thiadiazine-2-thione scaffold has found many applications in recent years. This review is aimed at highlighting the most important aspects of these compounds: Synthesis, spectroscopic characterization and biological activities. How the chemical nature of N-substituents influences the overall activity and cytotoxicity profile will also be discussed.

Hortensia Rodríguez; Margarita Suárez; Fernando Albericio

2012-01-01

376

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

UK PubMed Central (United Kingdom)

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.

Xu X; Zavalij PY; Doyle MP

2013-08-01

377

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

2013-08-09

378

Efficient Near-UV Emitters Based on Cationic Bis-Pincer Iridium(III) Carbene Complexes.  

UK PubMed Central (United Kingdom)

We report on the photophysical studies of two cationic near-UV emitters based on bis-pincer Ir(III) carbene complexes: [Ir(nBu)(CNHC(Me)CCNHC)2]X, where Ir(nBu)(CNHC(Me)CCNHC) is (4,6-dimethyl-1,3-phenylene-?C(2))bis(1-butylimidazol-2-ylidene) and X = I(-) or PF6(-)). The compounds are highly emitting in deaerated CH3CN solution with emission maxima at 384 and 406 nm, and photoluminescence quantum yields of 0.41 and 0.38, for [Ir(nBu)(CNHC(Me)CCNHC)2]I and Ir(nBu)(CNHC(Me)CCNHC)2]PF6, respectively. In order to gain deeper understandings into their structural and electronic features, as well as to ascertain the nature of the excited states involved into the electronic absorption processes, density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations have been performed on the ground and excited states of the closely related complex [Ir(Me)(CNHC(Me)CCNHC)2](+). In the solid state, an emission at low energy is observed (?max = 500 nm) for both complexes. However, the intensity of the emission at high energy versus the intensity of the new emission at low energy is dependent on the nature of counterions. The origin of this emission is not completely clear, but the experimental data point to the formation of trapping sites induced by aggregation processes involving the interaction between the cationic emitter and the counterion.

Darmawan N; Yang CH; Mauro M; Raynal M; Heun S; Pan J; Buchholz H; Braunstein P; De Cola L

2013-10-01

379

GEOMETRY AND ELECTRONIC STRUCTURE OF (CO)3N1CH2. A MODEL TRANSITION METAL CARBENE  

Energy Technology Data Exchange (ETDEWEB)

The first application of nonempirical molecular electronic structure theory to a realistic transition metal carbene complex is reported. The system chosen was (CO){sub 3}NiCH{sub 2}, methylene (tricarbonyl) nickel(0). All studies were carried out at the self-consistent-field (SCF) level. A large and flexibly contracted basis set was chosen, labeled Ni(15s 11p 6d/11s 8p 3d); C,O(9s 5p/4s 2p); H(5s/3s). In addition, the importance of methylene carbon d functions was investigated. The critical predicted equilibrium geometrical parameters were R [Ni-C (methylene)]=1.83 {Angstrom}, {theta}(HCH)=108°. The sixfold barrier to rotation about the Ni-C (methylene) axis is small, ~o.2 kcal. The electronic structure of (CO){sub 3}NiCH{sub 2} is discussed and compared with those of the "naked" complex NiCH{sub 2} and the stable Ni(CO){sub 4} molecule.

Spangler, Dale; Wendoloski, John J.; Dupuis, Michel; Chen, Maynard M.L.; Schaefer III, Henry F.

1980-04-01

380

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 SVP level) is consistent with electrochemical results in showing that electrochemically generated chromium(I) species may be further electrochemically irreversibly oxidised to chromium(II) at Epa > 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

2013-02-15

 
 
 
 
381

Magnetic properties of 1 : 4 complexes of CoCl2 and pyridines carrying carbenes (S(0) = 4/2, 6/2, and 8/2) in diluted frozen solution; influence of carbene multiplicity on heterospin single-molecule magnets.  

UK PubMed Central (United Kingdom)

The microcrystalline sample of a parent complex, [CoCl(2)(py)(4)], showed a single-molecule magnet (SMM) behavior with an effective activation barrier, U(eff)/k(B), of 16 K for reversal of the magnetism in the presence of a dc field of 3 kOe. Pyridine ligands having 2-4 diazo moieties, DYpy; Y = 2, 3l, 3b, and 4, were prepared and confirmed to be quintet, septet, septet, and nonet in the ground state, respectively, after irradiation. The 1 : 4 complexes, CoCl(2)(DYpy)(4); Y = 2, 3l, 3b, and 4 in frozen solutions after irradiation showed the magnetic behaviors of SMMs with total spin multiplicity, S(total) = 17/2, 25/2, 25/2, and 33/2, respectively. Hysteresis loops depending on the temperature were observed and the values of coercive force, H(c), at 1.9 K were 12, 8.4, 11, and 8.1 kOe for CoCl(2)(CYpy)(4); Y = 2, 3l, 3b, and 4, respectively. In dynamic magnetic susceptibility experiments, ac magnetic susceptibility data obeyed the Arrhenius law to give U(eff)/k(B) values of 94, 92, 93, and 87 K for CoCl(2)(CYpy)(4); Y = 2, 3l, 3b, and 4, respectively, while the relaxation times for CoCl(2)(CYpy)(4); Y = 2 and 3l, obtained by dc magnetization decay in the range of 3.5-1.9 K slightly deviated downward from Arrhenius plots on cooling. The dynamic magnetic behaviors for CoCl(2)(CYpy)(4) including [CoCl(2)(py)(4)] and CoCl(2)(C1py)(4) suggested that the generated carbenes interacted with the cobalt ion to increase the relaxation time, ?(q), due to the spin quantum tunneling magnetization, which became larger with increasing S(total) of the complex.

Karasawa S; Nakano K; Tanokashira J; Yamamoto N; Yoshizaki T; Koga N

2012-11-01

382

Magnetic properties of 1 : 4 complexes of CoCl2 and pyridines carrying carbenes (S(0) = 4/2, 6/2, and 8/2) in diluted frozen solution; influence of carbene multiplicity on heterospin single-molecule magnets.  

Science.gov (United States)

The microcrystalline sample of a parent complex, [CoCl(2)(py)(4)], showed a single-molecule magnet (SMM) behavior with an effective activation barrier, U(eff)/k(B), of 16 K for reversal of the magnetism in the presence of a dc field of 3 kOe. Pyridine ligands having 2-4 diazo moieties, DYpy; Y = 2, 3l, 3b, and 4, were prepared and confirmed to be quintet, septet, septet, and nonet in the ground state, respectively, after irradiation. The 1 : 4 complexes, CoCl(2)(DYpy)(4); Y = 2, 3l, 3b, and 4 in frozen solutions after irradiation showed the magnetic behaviors of SMMs with total spin multiplicity, S(total) = 17/2, 25/2, 25/2, and 33/2, respectively. Hysteresis loops depending on the temperature were observed and the values of coercive force, H(c), at 1.9 K were 12, 8.4, 11, and 8.1 kOe for CoCl(2)(CYpy)(4); Y = 2, 3l, 3b, and 4, respectively. In dynamic magnetic susceptibility experiments, ac magnetic susceptibility data obeyed the Arrhenius law to give U(eff)/k(B) values of 94, 92, 93, and 87 K for CoCl(2)(CYpy)(4); Y = 2, 3l, 3b, and 4, respectively, while the relaxation times for CoCl(2)(CYpy)(4); Y = 2 and 3l, obtained by dc magnetization decay in the range of 3.5-1.9 K slightly deviated downward from Arrhenius plots on cooling. The dynamic magnetic behaviors for CoCl(2)(CYpy)(4) including [CoCl(2)(py)(4)] and CoCl(2)(C1py)(4) suggested that the generated carbenes interacted with the cobalt ion to increase the relaxation time, ?(q), due to the spin quantum tunneling magnetization, which became larger with increasing S(total) of the complex. PMID:22898723

Karasawa, Satoru; Nakano, Kimihiro; Tanokashira, Jun-ichi; Yamamoto, Noriko; Yoshizaki, Takahito; Koga, Noboru

2012-08-17

383

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(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

2007-01-01

384

Highly selective intramolecular carbene insertion into primary C-H bond of ?-diazoacetamides mediated by a (p-cymene)ruthenium(II) carboxylate complex.  

UK PubMed Central (United Kingdom)

Complex [(p-cymene)Ru(?(1)-O(2)CCF(3))(2)(OH(2))] mediated transformation of ?-diazoacetamides ArCH(2)N(C(CH(3))(3))C(O)CHN(2) to result in carbene insertion into the primary C-H bond exclusively, with the ?-lactam products being isolated in up to 98% yield. This unexpected reaction is striking in view of the presence of usually more reactive sites such as secondary C-H bonds in the substrates. DFT calculations based on proposed Ru-carbene species provide insight into this unique selectivity.

Lo VK; Guo Z; Choi MK; Yu WY; Huang JS; Che CM

2012-05-01

385

Handling ligands with Coot.  

UK PubMed Central (United Kingdom)

Coot is a molecular-graphics application primarily aimed to assist in model building and validation of biological macromolecules. Recently, tools have been added to work with small molecules. The newly incorporated tools for the manipulation and validation of ligands include interaction with PRODRG, subgraph isomorphism-based tools, representation of ligand chemistry, ligand fitting and analysis, and are described here.

Debreczeni JÉ; Emsley P

2012-04-01

386

Ancillary ligand effects upon dithiolene redox noninnocence in tungsten bis(dithiolene) complexes.  

UK PubMed Central (United Kingdom)

An expanded set of compounds of the type [W(S2C2Me2)2L1L2](n) (n = 0: L1 = L2 = CO, 1; L1 = L2 = CN(t)Bu, 2; L1 = CO, L2 = carbene, 3; L1 = CO, L2 = phosphine, 4; L1 = L2 = phosphine, 5. n = 2-: L1 = L2 = CN(-), [6](2-)) has been synthesized and characterized. Despite isoelectronic formulations, the compound set reveals gradations in the dithiolene ligand redox level as revealed by intraligand bond lengths, ?(CCchelate), and rising edge energies in the sulfur K-edge X-ray absorption spectra (XAS). Differences among the terminal series members, 1 and [6](2-), are comparable to differences seen in homoleptic dithiolene complexes related by full electron transfer to/from a dithiolene-based MO. The key feature governing these differences is the favorable energy of the CO ?* orbitals, which are suitably positioned to overlap with tungsten d orbitals and exert an oxidizing effect on both metal and dithiolene ligand via ?-backbonding. The CN(-) ?* orbitals are too high in energy to mix effectively with tungsten and thus leave the filled dithiolene ?* orbitals unperturbed. This work shows how, and the degree to which, the redox level of a noninnocent ligand can be modulated by the choice of ancillary ligands(s).

Yan Y; Keating C; Chandrasekaran P; Jayarathne U; Mague JT; DeBeer S; Lancaster KM; Sproules S; Rubtsov IV; Donahue JP

2013-06-01

387

Ancillary ligand effects upon dithiolene redox noninnocence in tungsten bis(dithiolene) complexes.  

Science.gov (United States)

An expanded set of compounds of the type [W(S2C2Me2)2L1L2](n) (n = 0: L1 = L2 = CO, 1; L1 = L2 = CN(t)Bu, 2; L1 = CO, L2 = carbene, 3; L1 = CO, L2 = phosphine, 4; L1 = L2 = phosphine, 5. n = 2-: L1 = L2 = CN(-), [6](2-)) has been synthesized and characterized. Despite isoelectronic formulations, the compound set reveals gradations in the dithiolene ligand redox level as revealed by intraligand bond lengths, ?(CCchelate), and rising edge energies in the sulfur K-edge X-ray absorption spectra (XAS). Differences among the terminal series members, 1 and [6](2-), are comparable to differences seen in homoleptic dithiolene complexes related by full electron transfer to/from a dithiolene-based MO. The key feature governing these differences is the favorable energy of the CO ?* orbitals, which are suitably positioned to overlap with tungsten d orbitals and exert an oxidizing effect on both metal and dithiolene ligand via ?-backbonding. The CN(-) ?* orbitals are too high in energy to mix effectively with tungsten and thus leave the filled dithiolene ?* orbitals unperturbed. This work shows how, and the degree to which, the redox level of a noninnocent ligand can be modulated by the choice of ancillary ligands(s). PMID:23675834

Yan, Yong; Keating, Christopher; Chandrasekaran, Perumalreddy; Jayarathne, Upul; Mague, Joel T; DeBeer, Serena; Lancaster, Kyle M; Sproules, Stephen; Rubtsov, Igor V; Donahue, James P

2013-05-15

388

Theoretical insight into electronic spectra of carbon chain carbenes H2Cn (n = 3-10).  

UK PubMed Central (United Kingdom)

Ground-state geometries of carbenes H2Cn (n = 3-10) have been fully optimized with the C2?-symmetry constraint at the density functional theory and restricted-spin coupled-cluster single-double plus perturbative triple excitation levels of theory, respectively. Comparison of structures corresponding to the X(1)A1 and B(1)B1 electronic states has been made by the complete active space self-consistent field calculations. Parity alternation effect on various properties of the ground-state geometries has been discovered in the present study, which generally gives illustration for the relative stabilities of the titled carbon chains. Further calculations on their electronic spectra have been carried out by means of the complete active space second-order perturbation theory method along with the cc-pVTZ basis set. It is found that the vertical excitation energies of the dipole-allowed B(1)B1 ? X(1)A1 transition in the gas phase are 2.28, 4.75, 1.69, 3.66, 1.30, 2.94, 1.12, and 2.49 eV, respectively, which agree very well with the available experimental result for H2C3 (2.27 eV). In addition, the vertical excitation energies for both transitions B(1)B1 ? X(1)A1 and A(1)A2 ? X(1)A1 are found to obey a nonlinear ?E-n relationship as a function of chain size by performing curves fitting.

Zhang Y; Wang L; Li Y; Zhang J

2013-05-01

389

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

UK PubMed Central (United Kingdom)

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 <-1.7 V vs. FcH/FcH(+). A computational study on 1, 3 and 4 (B3LYP/def2-SVP level) is consistent with electrochemical results in showing that electrochemically generated chromium(I) species may be further electrochemically irreversibly oxidised to chromium(II) at Epa > 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.

van der Westhuizen B; Swarts PJ; Strydom I; Liles DC; Fernández I; Swarts JC; Bezuidenhout DI

2013-04-01

390

Efficient Near-UV Emitters Based on Cationic Bis-Pincer Iridium(III) Carbene Complexes.  

Science.gov (United States)

We report on the photophysical studies of two cationic near-UV emitters based on bis-pincer Ir(III) carbene complexes: [Ir(nBu)(CNHC(Me)CCNHC)2]X, where Ir(nBu)(CNHC(Me)CCNHC) is (4,6-dimethyl-1,3-phenylene-?C(2))bis(1-butylimidazol-2-ylidene) and X = I(-) or PF6(-)). The compounds are highly emitting in deaerated CH3CN solution with emission maxima at 384 and 406 nm, and photoluminescence quantum yields of 0.41 and 0.38, for [Ir(nBu)(CNHC(Me)CCNHC)2]I and Ir(nBu)(CNHC(Me)CCNHC)2]PF6, respectively. In order to gain deeper understandings into their structural and electronic features, as well as to ascertain the nature of the excited states involved into the electronic absorption processes, density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations have been performed on the ground and excited states of the closely related complex [Ir(Me)(CNHC(Me)CCNHC)2](+). In the solid state, an emission at low energy is observed (?max = 500 nm) for both complexes. However, the intensity of the emission at high energy versus the intensity of the new emission at low energy is dependent on the nature of counterions. The origin of this emission is not completely clear, but the experimental data point to the formation of trapping sites induced by aggregation processes involving the interaction between the cationic emitter and the counterion. PMID:24093480

Darmawan, Noviyan; Yang, Cheng-Han; Mauro, Matteo; Raynal, Matthieu; Heun, Susanne; Pan, Junyou; Buchholz, Herwig; Braunstein, Pierre; De Cola, Luisa

2013-09-09

391

Electroabsorption spectroscopy of charge-transfer states of transition-metal complexes. 2. Metal-to-ligand and ligand-to-metal charge-transfer excited states of pentaammineruthenium complexes  

Energy Technology Data Exchange (ETDEWEB)

The absorption spectra of Ru{sup II}(NH{sub 3}){sub 5}L and Ru{sup III}(NH{sub 3}){sub 5}L (L is an aromatic N-heterocycle or nitrile) complexes in 50:50 glycerol-water glasses at 77 K (D{sub s} = 3.9) are a function of the applied field in the 10{sup 6}-10{sup 7} V/m range. Analysis of the spectra in terms of the Liptay equations yields ground-excited state dipole-moment differences ranging from 4 to 37 D, depending upon the nature of L. The measured dipole moment differences, particularly those for the MLCT transitions, are much smaller than the values estimated from a simple consideration of the electron-transfer distances. The discrepancy between the observed and naive dipole moment estimates arises mainly from the multielectron nature of the response to excitation. Good agreement is obtained with the predictions of a model which includes refinement of the effective electron-transfer distance, the shift in the valence electron distribution in the excited state, and the effects of electron delocalization ({pi}-backbonding for Ru(II) and {pi}-bonding for the Ru(III) complexes). Other contributions, namely the dipole moment induced by the NH{sub 3} ligands and by the surrounding solvent molecules, are also considered. 55 refs., 11 figs., 4 tabs.

Shin, Y.G.K.; Brunschwig, B.S.; Creutz, C.; Sutin, N. [Brookhaven National Lab., Upton, NY (United States)

1996-05-16

392

The nature of the U=C double bond: pushing the stability of high-oxidation-state uranium carbenes to the limit.  

UK PubMed Central (United Kingdom)

Treatment of [K(BIPM(Mes)H)] (BIPM(Mes)={C(PPh2NMes)2}(2?); Mes=C6H2-2,4,6-Me3) with [UCl4(thf)3] (1 equiv) afforded [U(BIPM(Mes)H)(Cl)3(thf)] (1), which generated [U(BIPM(Mes))(Cl)2(thf)2] (2), following treatment with benzyl potassium. Attempts to oxidise 2 resulted in intractable mixtures, ligand scrambling to give [U(BIPM(Mes))2] or the formation of [U(BIPM(Mes)H)(O)2(Cl)(thf)] (3). The complex [U(BIPM(Dipp))(?-Cl)4(Li)2(OEt2)(tmeda)] (4) (BIPM(Dipp)={C(PPh2NDipp)2}(2?); Dipp=C6H3-2,6-iPr2; tmeda=N,N,N?,N?-tetramethylethylenediamine) was prepared from [Li2(BIPM(Dipp))(tmeda)] and [UCl4(thf)3] and, following reflux in toluene, could be isolated as [U(BIPM(Dipp))(Cl)2(thf)2] (5). Treatment of 4 with iodine (0.5 equiv) afforded [U(BIPM(Dipp))(Cl)2(?-Cl)2(Li)(thf)2] (6). Complex 6 resists oxidation, and treating 4 or 5 with N-oxides gives [{U(BIPM(Dipp)H)(O)2- (?-Cl)2Li(tmeda)] (7) and [{U(BIPM(Dipp)H)(O)2(?-Cl)}2] (8). Treatment of 4 with tBuOLi (3 equiv) and I2 (1 equiv) gives [U(BIPM(Dipp))(OtBu)3(I)] (9), which represents an exceptionally rare example of a crystallographically authenticated uranium(VI)–carbon ? bond. Although 9 appears sterically saturated, it decomposes over time to give [U(BIPM(Dipp))(OtBu)3]. Complex 4 reacts with PhCOtBu and Ph2CO to form [U(BIPM(Dipp))(?-Cl)4(Li)2(tmeda)(OCPhtBu)] (10) and [U(BIPM(Dipp))(Cl)(?-Cl)2(Li)(tmeda)(OCPh2)] (11). In contrast, complex 5 does not react with PhCOtBu and Ph2CO, which we attribute to steric blocking. However, complexes 5 and 6 react with PhCHO to afford (DippNPPh2)2C=C(H)Ph (12). Complex 9 does not react with PhCOtBu, Ph2CO or PhCHO; this is attributed to steric blocking. Theoretical calculations have enabled a qualitative bracketing of the extent of covalency in early-metal carbenes as a function of metal, oxidation state and the number of phosphanyl substituents, revealing modest covalent contributions to U=C double bonds.

Cooper OJ; Mills DP; McMaster J; Tuna F; McInnes EJ; Lewis W; Blake AJ; Liddle ST

2013-05-01

393

The nature of the U=C double bond: pushing the stability of high-oxidation-state uranium carbenes to the limit.  

Science.gov (United States)

Treatment of [K(BIPM(Mes)H)] (BIPM(Mes)={C(PPh2NMes)2}(2?); Mes=C6H2-2,4,6-Me3) with [UCl4(thf)3] (1 equiv) afforded [U(BIPM(Mes)H)(Cl)3(thf)] (1), which generated [U(BIPM(Mes))(Cl)2(thf)2] (2), following treatment with benzyl potassium. Attempts to oxidise 2 resulted in intractable mixtures, ligand scrambling to give [U(BIPM(Mes))2] or the formation of [U(BIPM(Mes)H)(O)2(Cl)(thf)] (3). The complex [U(BIPM(Dipp))(?-Cl)4(Li)2(OEt2)(tmeda)] (4) (BIPM(Dipp)={C(PPh2NDipp)2}(2?); Dipp=C6H3-2,6-iPr2; tmeda=N,N,N?,N?-tetramethylethylenediamine) was prepared from [Li2(BIPM(Dipp))(tmeda)] and [UCl4(thf)3] and, following reflux in toluene, could be isolated as [U(BIPM(Dipp))(Cl)2(thf)2] (5). Treatment of 4 with iodine (0.5 equiv) afforded [U(BIPM(Dipp))(Cl)2(?-Cl)2(Li)(thf)2] (6). Complex 6 resists oxidation, and treating 4 or 5 with N-oxides gives [{U(BIPM(Dipp)H)(O)2- (?-Cl)2Li(tmeda)] (7) and [{U(BIPM(Dipp)H)(O)2(?-Cl)}2] (8). Treatment of 4 with tBuOLi (3 equiv) and I2 (1 equiv) gives [U(BIPM(Dipp))(OtBu)3(I)] (9), which represents an exceptionally rare example of a crystallographically authenticated uranium(VI)–carbon ? bond. Although 9 appears sterically saturated, it decomposes over time to give [U(BIPM(Dipp))(OtBu)3]. Complex 4 reacts with PhCOtBu and Ph2CO to form [U(BIPM(Dipp))(?-Cl)4(Li)2(tmeda)(OCPhtBu)] (10) and [U(BIPM(Dipp))(Cl)(?-Cl)2(Li)(tmeda)(OCPh2)] (11). In contrast, complex 5 does not react with PhCOtBu and Ph2CO, which we attribute to steric blocking. However, complexes 5 and 6 react with PhCHO to afford (DippNPPh2)2C=C(H)Ph (12). Complex 9 does not react with PhCOtBu, Ph2CO or PhCHO; this is attributed to steric blocking. Theoretical calculations have enabled a qualitative bracketing of the extent of covalency in early-metal carbenes as a function of metal, oxidation state and the number of phosphanyl substituents, revealing modest covalent contributions to U=C double bonds. PMID:23559370

Cooper, Oliver J; Mills, David P; McMaster, Jonathan; Tuna, Floriana; McInnes, Eric J L; Lewis, William; Blake, Alexander J; Liddle, Stephen T

2013-04-04

394

rac-cis-Dicarbonyl-chlorido{1-[2-(diphenyl-phosphanyl-?P)benz-yl]-3-(phenyl-?C(1))imidazol-2-yl-idene-?C(2)}ruthenium(II) dichloro-methane monosolvate.  

UK PubMed Central (United Kingdom)

In the title compound, [Ru(C(28)H(22)N(2)P)Cl(CO)(2)]·CH(2)Cl(2), the Ru(II) atom exhibits a distorted octa-hedral 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 inter-est with respect to transfer hydrogenation catalysis and also provides an example of C-H activation behavior in late transition metal complexes. The dichloro-methane solvent mol-ecule is disordered over two sets of sites with an occupancy ratio of 0.873?(14):0.127?(14).

Domski GJ; Pecak WH; Swenson DC

2012-09-01

395

rac-cis-Dicarbonylchlorido{1-[2-(diphenylphosphanyl-?P)benzyl]-3-(phenyl-?C1)imidazol-2-ylidene-?C2}ruthenium(II) dichloromethane monosolvate  

Directory of Open Access Journals (Sweden)

Full Text Available In the title compound, [Ru(C28H22N2P)Cl(CO)2]·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; Wiktoria H. Pecak; Dale C. Swenson

2012-01-01

396

Iridium-catalyzed addition of aroyl chlorides and aliphatic acid chlorides to terminal alkynes.  

Science.gov (United States)

Iridium complexes show high catalytic activity in intermolecular additions of acid chlorides to terminal alkynes to afford valuable (Z)-?-chloro-?,?-unsaturated ketones. Ligands in the catalytic system play a crucial role in this reaction. An N-heterocyclic carbene (NHC) is an efficient ligand for the addition of aroyl chlorides, while dicyclohexyl(2-methylphenyl)phosphine (PCy(2)(o-Tol)) is indispensable for the reaction of aliphatic acid chlorides. The addition reactions proceed regio- and stereoselectively with suppression of decarbonylation and ?-hydrogen elimination, which have been two major intrinsic problems in transition-metal-catalyzed reactions. Stoichiometric reactions of active iridium catalysts with aroyl chlorides and aliphatic acid chlorides are carried out to gain insights into the reaction mechanisms. PMID:22148675

Iwai, Tomohiro; Fujihara, Tetsuaki; Terao, Jun; Tsuji, Yasushi

2011-12-28

397

Iridium-catalyzed addition of aroyl chlorides and aliphatic acid chlorides to terminal alkynes.  

UK PubMed Central (United Kingdom)

Iridium complexes show high catalytic activity in intermolecular additions of acid chlorides to terminal alkynes to afford valuable (Z)-?-chloro-?,?-unsaturated ketones. Ligands in the catalytic system play a crucial role in this reaction. An N-heterocyclic carbene (NHC) is an efficient ligand for the addition of aroyl chlorides, while dicyclohexyl(2-methylphenyl)phosphine (PCy(2)(o-Tol)) is indispensable for the reaction of aliphatic acid chlorides. The addition reactions proceed regio- and stereoselectively with suppression of decarbonylation and ?-hydrogen elimination, which have been two major intrinsic problems in transition-metal-catalyzed reactions. Stoichiometric reactions of active iridium catalysts with aroyl chlorides and aliphatic acid chlorides are carried out to gain insights into the reaction mechanisms.

Iwai T; Fujihara T; Terao J; Tsuji Y

2012-01-01

398

Imidazolium-Salt-Functionalized Ionic-CNT-Supported Ru?Carbene/Palladium Nanoparticles for Recyclable Tandem Metathesis/Hydrogenation Reactions in Ionic Liquids.  

UK PubMed Central (United Kingdom)

NP and tuck: Two different catalysts, a Ru?carbene complex and palladium nanoparticles, were immobilized onto the same imidazolium-salt-functionalized ionic CNTs. These supported dual-function catalysts showed excellent catalytic activity in tandem metathesis/hydrogenation reactions in an ionic liquid and could be recovered and reused four times. RCM=ring-closing metathesis.

Lee S; Shin JY; Lee SG

2013-09-01

399

Mechanistic switch in dual gold catalysis of diynes: C(sp(3))-H activation through bifurcation--vinylidene versus carbene pathways.  

UK PubMed Central (United Kingdom)

The other side of the mountain: Changing the framework of diyne systems opens up new cyclization modes for dual gold catalysis. Instead of a 5-endo cyclization and gold vinylidenes a 6-endo cyclization gives rise to gold-stabilized carbenes as key intermediates for selective C-H insertions.

Hansmann MM; Rudolph M; Rominger F; Hashmi AS

2013-02-01

400

Copper-catalyzed C-O bond formation: an efficient one-pot highly regioselective synthesis of furans from (2-furyl)carbene complexes.  

UK PubMed Central (United Kingdom)

A convenient one-pot Cu(I)-catalyzed strategy for regioselective synthesis of trisubstituted furan derivatives has been developed via (2-furyl) carbene complexes. This process has opened a new synthetic route to a variety of ?-carbonyl furans using air as the oxidant affording furans in good yields.

Cao H; Zhan H; Cen J; Lin J; Lin Y; Zhu Q; Fu M; Jiang H

2013-03-01

 
 
 
 
401

Transition metal-induced activation of alkynes leading to metal carbene species: synthetic application to new ?-conjugated molecules  

Science.gov (United States)

The in situ generation of furylcarbene complexes by means of transition metals was applied to catalytic carbene reactions, such as Wittig-type olefination and cycloisomerization reactions. In the presence of [Rh(OAc)2]2 as a catalyst and triphenylphosphine as a carbene accepter, carbonyl-ene-ynes 1a reacted with benzaldehyde to give a 2-styrylfuran derivative 3a. The reaction involving phosphine-ylide formation followed by Wittig-type olefination with aldehydes was applied to synthesis of furfurylidene-containing p-extended conjugated molecules. A Ruthenium catalyst underwent cycloisomerization of 1,2-bis(carbonylenyl)acetylene 10a,b to afford 2,2'-bifuran structures. The crystal structure of 5,5'-diphenyl-2,2'-bifuran 11b was determined by X-ray crystallography. The ORTEP drawings of 11b exhibited transoid and highly planar structure. The emission bands of the bisfuran 11b were observed at 414 and 440 nm, its quantum yield (84%) was higher than that of a similar 5,5'-diphenyl-2,2'-bithiophene.

Abo, T.; Ohe, K.

2008-03-01

402

BeCH2: The Simplest Metal Carbene. High Levels of Theory.  

Science.gov (United States)

The simplest metal carbene, BeCH2, is experimentally unknown. Its isomer, HBeCH, lies higher in energy, but has been detected by the infrared matrix isolation [J. Am. Chem. Soc. 1998, 120, 6097]. In the present study the ground and low-lying excited states of the BeCH2 and HBeCH isomers were investigated using state-of-the-art ab initio methods, including coupled-cluster theory with up to full quadruple excitations (CCSDTQ), and complete active space self-consistent field (CASSCF) with multireference configuration interaction with single and double excitations (MRCISD). The relative energies were obtained using the focal point analysis combined with large correlation-consistent cc-pCVXZ basis sets (X = D, T, Q, 5) and were extrapolated to the complete basis set (CBS) limit. The (3)B1 state of BeCH2 (C2v symmetry) is the global minimum on the ground triplet potential energy surface (PES). The (3)?(-) state of the linear isomer HBeCH is located 4.9 kcal mol(-1) above the global minimum, at the CCSDTQ/CBS level of theory. The BeCH2 and HBeCH isomers are connected through the (3)A? transition state lying 46.1 kcal mol(-1) above the global minimum. The higher-lying energy HBeCH structure has much larger Be-C bond dissociation energy (126.6 kcal mol(-1), cf. BDE(BeCH2) = 62.1 kcal mol(-1)). The lowest excited state of BeCH2 is the open-shell (1)B1 state, with a relative energy of only 4.9 kcal mol(-1) above the global minimum, followed by (1)A1 state (16.8 kcal mol(-1)) at the MRCISD/cc-pCVQZ level of theory. For the HBeCH isomer the lowest-energy excited states are (1)? and (1)?(+), lying about 30 kcal mol(-1) above the global minimum. For the ground state of BeCH2 the fundamental vibrational frequencies computed using second-order vibrational perturbation theory (VPT2) at the CCSD(T)/cc-pCVQZ level are reported. We hope that our highly accurate theoretical results will assist in the experimental identification of BeCH2. PMID:23972228

Qiu, Yudong; Sokolov, Alexander Yu; Yamaguchi, Yukio; Schaefer, Henry F

2013-09-12

403

BeCH2: The Simplest Metal Carbene. High Levels of Theory.  

UK PubMed Central (United Kingdom)

The simplest metal carbene, BeCH2, is experimentally unknown. Its isomer, HBeCH, lies higher in energy, but has been detected by the infrared matrix isolation [J. Am. Chem. Soc. 1998, 120, 6097]. In the present study the ground and low-lying excited states of the BeCH2 and HBeCH isomers were investigated using state-of-the-art ab initio methods, including coupled-cluster theory with up to full quadruple excitations (CCSDTQ), and complete active space self-consistent field (CASSCF) with multireference configuration interaction with single and double excitations (MRCISD). The relative energies were obtained using the focal point analysis combined with large correlation-consistent cc-pCVXZ basis sets (X = D, T, Q, 5) and were extrapolated to the complete basis set (CBS) limit. The (3)B1 state of BeCH2 (C2v symmetry) is the global minimum on the ground triplet potential energy surface (PES). The (3)?(-) state of the linear isomer HBeCH is located 4.9 kcal mol(-1) above the global minimum, at the CCSDTQ/CBS level of theory. The BeCH2 and HBeCH isomers are connected through the (3)A? transition state lying 46.1 kcal mol(-1) above the global minimum. The higher-lying energy HBeCH structure has much larger Be-C bond dissociation energy (126.6 kcal mol(-1), cf. BDE(BeCH2) = 62.1 kcal mol(-1)). The lowest excited state of BeCH2 is the open-shell (1)B1 state, with a relative energy of only 4.9 kcal mol(-1) above the global minimum, followed by (1)A1 state (16.8 kcal mol(-1)) at the MRCISD/cc-pCVQZ level of theory. For the HBeCH isomer the lowest-energy excited states are (1)? and (1)?(+), lying about 30 kcal mol(-1) above the global minimum. For the ground state of BeCH2 the fundamental vibrational frequencies computed using second-order vibrational perturbation theory (VPT2) at the CCSD(T)/cc-pCVQZ level are reported. We hope that our highly accurate theoretical results will assist in the experimental identification of BeCH2.

Qiu Y; Sokolov AY; Yamaguchi Y; Schaefer HF 3rd

2013-09-01

404

BeCH2: The Simplest Metal Carbene. High Levels of Theory.  

UK PubMed Central (United Kingdom)

The simplest metal carbene, BeCH2, is experimentally unknown. Its isomer, HBeCH, lies higher in energy, but has been detected by the infrared matrix isolation [J. Am. Chem. Soc. 1998, 120, 6097]. In the present study the ground and low-lying excited states of the BeCH2 and HBeCH isomers were investigated using state-of-the-art ab initio methods, including coupled-cluster theory with up to full quadruple excitations (CCSDTQ), and complete active space self-consistent field (CASSCF) with multireference configuration interaction with single and double excitations (MRCISD). The relative energies were obtained using the focal point analysis combined with large correlation-consistent cc-pCVXZ basis sets (X = D, T, Q, 5) and were extrapolated to the complete basis set (CBS) limit. The (3)B1 state of BeCH2 (C2v symmetry) is the global minimum on the ground triplet potential energy surface (PES). The (3)?(-) state of the linear isomer HBeCH is located 4.9 kcal mol(-1) above the global minimum, at the CCSDTQ/CBS level of theory. The BeCH2 and HBeCH isomers are connected through the (3)A'' transition state lying 46.1 kcal mol(-1) above the global minimum. The higher-lying energy HBeCH structure has much larger Be-C bond dissociation energy (126.6 kcal mol(-1), cf. BDE(BeCH2) = 62.1 kcal mol(-1)). The lowest excited state of BeCH2 is the open-shell (1)B1 state, with a relative energy of only 4.9 kcal mol(-1) above the global minimum, followed by (1)A1 state (16.8 kcal mol(-1)) at the MRCISD/cc-pCVQZ level of theory. For the HBeCH isomer the lowest-energy excited states are (1)? and (1)?(+), lying about 30 kcal mol(-1) above the global minimum. For the ground state of BeCH2 the fundamental vibrational frequencies computed using second-order vibrational perturbation theory (VPT2) at the CCSD(T)/cc-pCVQZ level are reported. We hope that our highly accurate theoretical results will assist in the experimental identification of BeCH2.

Qiu Y; Sokolov AY; Yamaguchi Y; Schaefer HF

2013-08-01

405

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; Louis Fensterbank; Max Malacria; Emmanuel Lacôte; Steven J. Geib; Dennis P. Curran

2010-01-01

406

Ligand modeling and design  

International Nuclear Information System (INIS)

[en] The purpose of this work is to develop and implement a molecular design basis for selecting organic ligands that would be used in the cost-effective removal of specific radionuclides from nuclear waste streams. Organic ligands with metal ion specificity are critical components in the development of solvent extraction and ion exchange processes that are highly selective for targeted radionuclides. The traditional approach to the development of such ligands involves lengthy programs of organic synthesis and testing, which in the absence of reliable methods for screening compounds before synthesis, results in wasted research effort. The author's approach breaks down and simplifies this costly process with the aid of computer-based molecular modeling techniques. Commercial software for organic molecular modeling is being configured to examine the interactions between organic ligands and metal ions, yielding an inexpensive, commercially or readily available computational tool that can be used to predict the structures and energies of ligand-metal complexes. Users will be able to correlate the large body of existing experimental data on structure, solution binding affinity, and metal ion selectivity to develop structural design criteria. These criteria will provide a basis for selecting ligands that can be implemented in separations technologies through collaboration with other DOE national laboratories and private industry. The initial focus will be to select ether-based ligands that can be applied to the recovery and concentration of the alkali and alkaline earth metal ions including cesium, strontium, and radium

1997-01-01

407

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 in the cost-effective removal of specific radionuclides from nuclear waste streams. Organic ligands with metal ion specificity are critical components in the development of solvent extraction and ion exchange processes that are highly selective for targeted radionuclides. The traditional approach to the development of such ligands involves lengthy programs of organic synthesis and testing, which in the absence of reliable methods for screening compounds before synthesis, results in wasted research effort. The author`s approach breaks down and simplifies this costly process with the aid of computer-based molecular modeling techniques. Commercial software for organic molecular modeling is being configured to examine the interactions between organic ligands and metal ions, yielding an inexpensive, commercially or readily available computational tool that can be used to predict the structures and energies of ligand-metal complexes. Users will be able to correlate the large body of existing experimental data on structure, solution binding affinity, and metal ion selectivity to develop structural design criteria. These criteria will provide a basis for selecting ligands that can be implemented in separations technologies through collaboration with other DOE national laboratories and private industry. The initial focus will be to select ether-based ligands that can be applied to the recovery and concentration of the alkali and alkaline earth metal ions including cesium, strontium, and radium.

Hay, B.P. [Pacific Northwest National Lab., Richland, WA (United States)

1997-10-01

408

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

409

Ligand modeling and design  

International Nuclear Information System (INIS)

[en] 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

1996-01-01

410

Molecular electrocatalysts for the hydrogen production from iron based hydrogenases  

International Nuclear Information System (INIS)

[en] The complex type [Fe2(CO)6(?-SRS)] are structural analogue of the active site of iron hydrogenases constituted of a dinuclear entity and diatomic ligands CO and CN. The today knowledge on the electrocatalytic activity of some of these organometallic complexes towards the proton reduction in hydrogen1,2,3, makes possible the elaboration of bio inspired electrocatalysts. Studies must now be realized to better understand the reduction processes of the iron dinuclear complexes with and without acids. In this framework the authors synthesized series of complexes type [Fe2(CO)6-n Ln(?-E-CH2-X-CH2-E)] (n=0,1 or 2; L=carbene N-heterocyclic; E=S or P(Ph); X= CH2, C6H4 or NR). (A.L.B.)

2005-01-01

411

Molecular electrocatalysts for the hydrogen production from iron based hydrogenases; Electrocatalyseurs moleculaires pour la production d'hydrogene inspires des hydrogenases a fer  

Energy Technology Data Exchange (ETDEWEB)

The complex type [Fe{sub 2}(CO){sub 6}({mu}-SRS)] are structural analogue of the active site of iron hydrogenases constituted of a dinuclear entity and diatomic ligands CO and CN. The today knowledge on the electrocatalytic activity of some of these organometallic complexes towards the proton reduction in hydrogen{sup 1,2,3}, makes possible the elaboration of bio inspired electrocatalysts. Studies must now be realized to better understand the reduction processes of the iron dinuclear complexes with and without acids. In this framework the authors synthesized series of complexes type [Fe{sub 2}(CO){sub 6-n} L{sub n}({mu}-E-CH{sub 2}-X-CH{sub 2}-E)] (n=0,1 or 2; L=carbene N-heterocyclic; E=S or P(Ph); X= CH{sub 2}, C{sub 6}H{sub 4} or NR). (A.L.B.)

Gloaguen, F.; Capon, J.F.; Schollhammer, Ph.; Talarmin, J. [Laboratoire de Chimie, Electrochimie Moleculaires et Chimie Analytique, UMR CNRS 6521 UBO, 29 - Brest (France)

2005-07-01

412

Benzimidazol-2-ylidene gold(I) complexes are thioredoxin reductase inhibitors with multiple antitumor properties.  

UK PubMed Central (United Kingdom)

Gold(I) complexes such as auranofin have been used for decades to treat symptoms of rheumatoid arthritis and have also demonstrated a considerable potential as new anticancer drugs. The enzyme thioredoxin reductase (TrxR) is considered as the most relevant molecular target for these species. The here investigated gold(I) complexes with benzimidazole derived N-heterocyclic carbene (NHC) ligands represent a promising class of gold coordination compounds with a good stability against the thiol glutathione. TrxR was selectively inhibited by in comparison to the closely related enzyme glutathione reductase, and all complexes triggered significant antiproliferative effects in cultured tumor cells. More detailed studies on a selected complex revealed a distinct pharmacodynamic profile including the high increase of reactive oxygen species formation, apoptosis induction, strong effects on cellular metabolism (related to cell surface properties, respiration, and glycolysis), inhibition of mitochondrial respiration and activity against resistant cell lines.

Rubbiani R; Kitanovic I; Alborzinia H; Can S; Kitanovic A; Onambele LA; Stefanopoulou M; Geldmacher Y; Sheldrick WS; Wolber G; Prokop A; Wölfl S; Ott I

2010-12-01

413

Haloaurate and halopalladate imidazolium salts: structures, properties, and use as precursors for catalytic metal nanoparticles.  

UK PubMed Central (United Kingdom)

The synthesis and characterisation of a series of new gold- and palladium-containing symmetrical imidazolium salts are described which display significant cation-dependent effects determined by the structure of the alkyl chains of the imidazolium motifs. Whereas direct reduction of the Pd salts can produce stable nanoparticles (NPs) coated by imidazolium salts, the addition of strong base to the Pd or Au salts before reduction gives stable NPs, potentially pacified by N-heterocyclic carbene units. The possibility of NP surface protection by metal-carbon bonds in these systems is investigated by spectroscopic, synthetic, and catalytic investigations, providing support for the hypothesis. Significantly, the catalytic activity of the NPs is not inhibited by the continued presence of the ligands.

Serpell CJ; Cookson J; Thompson AL; Brown CM; Beer PD

2013-02-01

414

[2-Butyl-4-(4-tert-butyl-benz-yl)-1,2,4-triazol-3-yl-idene]chlorido[(1,2,5,6-?)-cyclo-octa-1,5-diene]iridium(I).  

UK PubMed Central (United Kingdom)

In the title compound, [IrCl(C(8)H(12))(C(17)H(25)N(3))], the Ir(I) ion has a distorted square-planar coordination geometry. The N-heterocyclic carbene ligand has an extended S-shaped conformation. The butyl group was refined using a two-part 1:1 disorder model. In the crystal, three unique weak C-H?Cl contacts are present. Two of these form a motif described as R(2) (1)(6) in graph-set notation, while a third forms an R(2) (2)(8) motif about a crystallographic inversion center. The result is a chain structure which extends parallel to the crystallographic a axis.

Nichol GS; Walton DP; Anna LJ; Rajaseelan E

2012-02-01

415

[2-Butyl-4-(4-tert-butylbenzyl)-1,2,4-triazol-3-ylidene]chlorido[(1,2,5,6-?)-cycloocta-1,5-diene]iridium(I)  

Directory of Open Access Journals (Sweden)

Full Text Available In the title compound, [IrCl(C8H12)(C17H25N3)], the IrI ion has a distorted square-planar coordination geometry. The N-heterocyclic carbene ligand has an extended S-shaped conformation. The butyl group was refined using a two-part 1:1 disorder model. In the crystal, three unique weak C—H...Cl contacts are present. Two of these form a motif described as R21(6) in graph-set notation, while a third forms an R22(8) motif about a crystallographic inversion center. The result is a chain structure which extends parallel to the crystallographic a axis.

Gary S. Nichol; David P. Walton; Laura J. Anna; Edward Rajaseelan

2012-01-01

416

Dehydrocoupling Reactions of Dimethylamine-Borane by Pt(II) Complexes: A New Mechanism Involving Deprotonation of Boronium Cations.  

UK PubMed Central (United Kingdom)

Coordinatively unsaturated Pt(II) complex [Pt(I(t)Bu')(I(t)Bu)](+) stabilized by N-heterocyclic carbene (NHC) ligands dehydrogenates N,N-dimethylamineborane through a mechanism that involves hydride abstraction, assisted by an amine, to yield a platinum-hydride complex [PtH(I(t)Bu')(I(t)Bu)] with concomitant formation of the boronium cation [(NHMe2)2BH2](+). This latter species is very likely in equilibrium with the THF stabilized borenium cation [(NHMe2)(THF)BH2](+), bearing an acidic NH group that is able to protonate the platinum hydride [PtH(I(t)Bu')(I(t)Bu)] releasing H2, the amino borane H2B-NMe2 and regenerating the catalytic [Pt](+) species.

Roselló-Merino M; López-Serrano J; Conejero S

2013-07-01

417

3,5-Bis(3-methylimidazolium-1-ylmethyl)toluene bis(hexafluorophosphate)  

Directory of Open Access Journals (Sweden)

Full Text Available The asymmetric unit of the title N-heterocyclic carbene compound, C17H22N42+·2PF6?, consists of one N-heterocyclic carbene dication and two hexafluorophosphate anions. The two imidazole rings are twisted away from but to the same side of the central toluene ring, making dihedral angles of 76.69?(7) and 78.03?(7)° with the central ring. In the crystal, the components are linked by C—H...F interactions, generating a three-dimensional network.

Abbas Washeel; Rosenani A. Haque; Siang Guan Teoh; Chin Sing Yeap; Hoong-Kun Fun

2010-01-01

418

3,3?-Di-n-butyl-1,1?-(p-phenylenedimethylene)diimidazolium 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; Abbas Washeel; S. Fatimah Nasri; Chin Sing Yeap; Hoong-Kun Fun

2010-01-01

419

Binding mode characterization of 6?- and 6?-N-heterocyclic substituted naltrexamine derivatives via docking in opioid receptor crystal structures and site-directed mutagenesis studies: Application of the 'message-address' concept in development of mu opioid receptor selective antagonists.  

Science.gov (United States)

Highly selective opioid receptor antagonists are essential pharmacological probes in opioid receptor structural characterization and opioid agonist functional studies. Currently, there is no highly selective, nonpeptidyl and reversible mu opioid receptor antagonist available. Among a series of naltrexamine derivatives that have been designed and synthesized, two compounds, NAP and NAQ, were previously identified as novel leads for this purpose based on their in vitro and in vivo pharmacological profiles. Both compounds displayed high binding affinity and selectivity to the mu opioid receptor. To further study the interaction of these two ligands with the three opioid receptors, the recently released opioid receptor crystal structures were employed in docking studies to further test our original hypothesis that the ligands recognize a unique 'address' domain in the mu opioid receptor involving Trp318 that facilitates their selectivity. These modeling results were supported by site-directed mutagenesis studies on the mu opioid receptor, where the mutants Y210A and W318A confirmed the role of the latter in binding. Such work not only enriched the 'message-address' concept, also facilitated our next generation ligand design and development. PMID:24055076

Zaidi, Saheem A; Arnatt, Christopher K; He, Hengjun; Selley, Dana E; Mosier, Philip D; Kellogg, Glen E; Zhang, Yan

2013-09-04

420

Ligand and metal complex  

UK PubMed Central (United Kingdom)

A ligand of Formula (I) is provided: wherein A4 represents a hydrogen atom, a nitro group, an amino group, a thiocyanato group, or -Z-Y, in which Z is a divalent linking group and Y is a group derived from a biocompatible molecule, with the proviso that when X is methylene, A4 cannot be a hydrogen atom or a nitro group. A metal complex having the ligand is also provided and is useful as a blood pool contrast agent or a targeting contrast agent.

WANG YUN-MING; CHEN TING-JUNG

 
 
 
 
421

Ligand-gated channels.  

Science.gov (United States)

Ligand-gated ion channels (LGICs) are fast-responding channels in which the receptor, which binds the activating molecule (the ligand), and the ion channel are part of the same nanomolecular protein complex. This paper will describe the properties and functions of the nicotinic acetylcholine LGIC superfamily, which plays a critical role in the fast chemical transmission of electrical signals between nerve cells and between nerve and muscle cells. The superfamily will mainly be exemplified by the excitatory nicotinic acetylcholine receptor (nAChR) and the inhibitory glycine receptor (GlyR) channels. PMID:15816173

Barry, Peter H; Lynch, Joseph W

2005-03-01

422

Ligand-gated channels.  

UK PubMed Central (United Kingdom)

Ligand-gated ion channels (LGICs) are fast-responding channels in which the receptor, which binds the activating molecule (the ligand), and the ion channel are part of the same nanomolecular protein complex. This paper will describe the properties and functions of the nicotinic acetylcholine LGIC superfamily, which plays a critical role in the fast chemical transmission of electrical signals between nerve cells and between nerve and muscle cells. The superfamily will mainly be exemplified by the excitatory nicotinic acetylcholine receptor (nAChR) and the inhibitory glycine receptor (GlyR) channels.

Barry PH; Lynch JW

2005-03-01

423

Exact ligand cone angles.  

Science.gov (United States)

Many properties of transition-metal complexes depend on the steric bulk of bound ligands, usually quantified by the Tolman (?) and solid (?) cone angles, which have proven utility but suffer from various limitations and coarse approximations. Here, we present an improved, mathematically rigorous method to determine an exact cone angle (?°) by solving for the most acute right circular cone that contains the entire ligand. The procedure is applicable to any ligand, planar or nonplanar, monodentate or polydentate, bound to any metal center in any environment, and it is ideal for analyzing structures from quantum chemical computations as well as X-ray crystallography experiments. Exact cone angles were evaluated for a wide array of phosphine and amine ligands bound to palladium, nickel, or platinum by optimizing structures using B3LYP/6-31G* density functional theory with effective core potentials for the transition metals. The mean absolute deviations of the standard ? and ? parameters from the exact cone angles were 15-25°, mostly caused by distortions from the assumed idealized structures. PMID:23408559

Bilbrey, Jenna A; Kazez, Arianna H; Locklin, Jason; Allen, Wesley D

2013-02-13

424

Exact ligand cone angles.  

UK PubMed Central (United Kingdom)

Many properties of transition-metal complexes depend on the steric bulk of bound ligands, usually quantified by the Tolman (?) and solid (?) cone angles, which have proven utility but suffer from various limitations and coarse approximations. Here, we present an improved, mathematically rigorous method to determine an exact cone angle (?°) by solving for the most acute right circular cone that contains the entire ligand. The procedure is applicable to any ligand, planar or nonplanar, monodentate or polydentate, bound to any metal center in any environment, and it is ideal for analyzing structures from quantum chemical computations as well as X-ray crystallography experiments. Exact cone angles were evaluated for a wide array of phosphine and amine ligands bound to palladium, nickel, or platinum by optimizing structures using B3LYP/6-31G* density functional theory with effective core potentials for the transition metals. The mean absolute deviations of the standard ? and ? parameters from the exact cone angles were 15-25°, mostly caused by distortions from the assumed idealized structures.

Bilbrey JA; Kazez AH; Locklin J; Allen WD

2013-05-01

425

Identification of enantiomeric ligands  

UK PubMed Central (United Kingdom)

A method of identifying macromolecules (peptides, oligonucleotides, sugars and macromolecular complexes, such as RNA-protein complexes, protein-lipid complexes), which are not of the natural handedness (not of the chirality as they occur in nature or as a wild type molecule) and which are ligands for other chiral macromolecules.

SCHUMACHER ANTONIUS NICOLASS MARIA; KIM PETER S

426

Phosphorus(V) complexes with acyclic monoaminocarbene ligands via oxidative addition.  

UK PubMed Central (United Kingdom)

(Difluoroorganyl)dimethylamines, RCF2NMe2 (R = H, Ph, tBu), can be used as carbene precursors for phosphorus trifluoride in an oxidative addition reaction. By this method, complexes of sterically nondemanding asymmetric and acyclic carbenes were obtained that are otherwise not accessible.

Böttcher T; Bassil BS; Zhechkov L; Röschenthaler GV

2013-05-01

427

Peptide-tethered monodentate and chelating histidylidene metal complexes: synthesis and application in catalytic hydrosilylation.  

UK PubMed Central (United Kingdom)

The N?,N?-dimethylated histidinium salt (His*) was tethered to oligopeptides and metallated to form Ir(III) and Rh(I) NHC complexes. Peptide-based histidylidene complexes containing only alanine, Ala-Ala-His*-[M] and Ala-Ala-Ala-His*-[M] were synthesised ([M] = Rh(cod)Cl, Ir(Cp*)Cl2), as well as oligopeptide complexes featuring a potentially chelating methionine and tyrosine residue, Met-Ala-Ala-His*-Rh(cod)Cl and Tyr-Ala-Ala-His*-Rh(cod)Cl. Chelation of the methionine-containing histidylidene ligand was induced by halide abstraction from the rhodium centre, while tyrosine remained non-coordinating under identical conditions. High catalytic activities in hydrosilylation were achieved with all peptide-based rhodium complexes. The cationic S(Met),C(His*)-bidentate peptide rhodium catalyst outperformed the monodentate neutral peptide complexes and constitutes one of the most efficient rhodium carbene catalysts for hydrosilylation, providing new opportunities for the use of peptides as N-heterocyclic carbene ligands in catalysis.

Monney A; Nastri F; Albrecht M

2013-04-01

428

Unimolecular reaction mechanism of an imidazolin-2-ylidene: an iPEPICO study on the complex dissociation of an Arduengo-type carbene.  

UK PubMed Central (United Kingdom)

The photoionization and dissociative photoionization of Im(iPr)2, 1,3-diisopropylimidazolin-2-ylidene, was investigated by imaging photoelectron photoion coincidence (iPEPICO) with vacuum ultraviolet (VUV) synchrotron radiation. A lone-pair electron of the carbene carbon atom is removed upon ionization and the molecular geometry changes significantly. Only 0.5 eV above the adiabatic ionization energy, IEad =7.52±0.1 eV, the carbene cation fragments, yielding propene or a methyl radical in parallel dissociation reactions with appearance energies of 8.22 and 8.17 eV, respectively. Both reaction channels appear at almost the same photon energy, suggesting a shared transition state. This is confirmed by calculations, which reveal the rate-determining step as hydrogen-atom migration from the isopropyl group to the carbene carbon center forming a resonance-stabilized imidazolium ion. Above 10.5 eV, analogous sequential dissociation channels open up. The first propene-loss fragment ion dissociates further and another methyl or propene is abstracted. Again, a resonance-stabilized imidazolium ion acts as intermediate. The aromaticity of the system is enhanced even in vertical ionization. Indeed, the coincidence technique confirms that a real imidazolium ion is produced by hydrogen transfer over a small barrier. The simple analysis of the breakdown diagram yields all the clues to disentangle the complex dissociative photoionization mechanism of this intermediate-sized molecule. Photoelectron photoion coincidence is a promising tool to unveil the fragmentation mechanism of larger molecules in mass spectrometry.

Hemberger P; Bodi A; Gerber T; Würtemberger M; Radius U

2013-05-01

429

Radiobiology with DNA ligands  

International Nuclear Information System (INIS)

[en] 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

1997-01-01

430

1. Medicinal chemistry of a small molecule drug lead: Tamoxilog 2. Electronic communication through ruthenium nanoparticles: Synthesis of custom ligands and nanoparticles  

Science.gov (United States)

1. Compound NSC-670224, previously shown to be toxic to Saccharomyces cerevisiae at low micromolar concentrations, potentially acts via a mechanism of action related to that of tamoxifen (NSC 180973), a widely utilized breast cancer drug. The structure of NSC-670224, previously thought to be a 2,4-dichloro arene, was established as the 3,4-dichloro arene, and a focused library of analogues were synthesized and biologically evaluated in conjunction with the UCSC Chemical Screening Center. The synthesis of a biotinylated affinity probe was also completed in order to extract the protein target(s) of NSC-670224 from yeast and human cell lines in collaboration with the Hartzog lab (UCSC MCD Biology) 2. Stabilization of ruthenium nanoparticles (Ru NPs) through carbene bound ligands has led to a simple and effective means to generate new materials with unique optoelectronic properties. The affinity of freshly prepared Ru NPs to diazo compounds, specifically octyl diazoacetate (ODA), provides a robust nanostructure that can be further functionalized via metathesis of terminal olefins to generate these unique materials. Carbene-stabilized Ru NPs have provided insights into the nature of extended conjugation and intraparticle charge delocalization through covalently bound probes (e.g., ferrocene and pyrene). The growing interest to study electronic communication through Ru NPs has lead to collaborative, multidisciplinary efforts between analytical (Shaowei Chen lab, UCSC), theoretical (Haobin Wang Lab, NMSU), and synthetic organic chemists (Konopelski Lab, UCSC). With this powerful collaboration, new methods to generate stabilized Ru NPs, testing theory with experiment, and efficient means to functionalize NPs have been investigated. The syntheses of custom ligands and their applications to nanoparticle-mediated electronic communication are reported.

Zuckerman, Nathaniel Benjamin

431

A new class of photoactivatable and carbene generating reagents with extremely high specific radioactivity. Synthesis, characterization and applications  

International Nuclear Information System (INIS)

[en] The main objective of this work was the development of new photocrosslinking and labeling reagents which show favourable photochemical properties and can be synthesized in an extremely high specific radioactivity. A key compound in the synthesis of these reagents was 2-tributyltin-4-(3-trifluormethyl-3H-diazirin-3-yl)-benzyl alcohol. Esters of this alcohol can be easily radioiodinated at a specific radioactivity of >2000 Ci/mmol under mild conditions. By experiments with a model compound it was shown that 2-iodo-4-(3-trifluormethyl-3H-diazirin-3-yl)-benzyl esters, upon photolysis, generate highly reactive (singlet) carbenes capable of inserting into CH-bonds. Equally important, there is no indication of loss of iodine under the photolysis conditions applied. Therefore two key requirements for photolabeling reagents are fulfilled. Several photoactivatable radioiodinated phospholipids have been synthesized. Their properties have been evaluated by labeling of the the membranes of intact erythrocytes and influenza viruses. Currently these lipids are used to study the interaction of various proteins (shown are experiments with MARCKS) with membranes. Furthermore a new class of actual label transfer crosslinkers (two thiol specific and one amino specific) have been developed. The range of potentialities of these reagents is currently