(N‐heterocyclic carbene)PdCl(N‐heterocyclic carboxylate) complexes: Synthesis and catalytic activities towards arylation of benzoxazoles with aryl halides

A series of N‐heterocyclic carboxylate‐stabilized N‐heterocyclic carbene palladium complexes have been synthesized and fully characterized. The solid‐state structures indicate that each of the palladium centers is coordinated by an N‐heterocyclic carbene, a chloride and a bidentate N,O‐donor N‐heterocyclic carboxylate ligand. The catalytic performance of the complexes was screened and the results revealed that the complexes exhibit moderate to high catalytic activities for the direct C─H bond arylation of benzoxazoles with aryl bromides.     

In situ generation of highly active bis(N‐heterocyclic)carbene palladium as an efficient catalyst in direct S‐arylation of methylphenyl sulfoxide and the Heck reaction: Ligand steric effects in product selectivity

The use of 1,3‐bis(N‐heterocyclic)carbene ligands with different alkyl wingtip groups (alkyl = methyl, isopropyl and tert ‐butyl) is an effective method for the palladium‐catalysed direct S ‐arylation of methylphenyl sulfoxide and C–C coupling of various of aryl halides with alkenes. The reactions proceed in moderate to good yields. Interestingly, it is shown experimentally that, by using bulkier bidentate N‐heterocyclic carbene ligands, more selective catalytic systems towards cis products in Heck coupling reactions can be achieved.     

Piperazine‐ and DABCO‐bridged dinuclear N‐heterocyclic carbene palladium complexes: synthesis,structure and application to Hiyama coupling reaction

Four dinuclear N ‐heterocyclic carbene (NHC) palladium complexes were prepared by reaction of imidazolinium salts, PdCl₂ and bridging ligands (piperazine and DABCO) in one pot or by direct cleavage of the chloro‐bridged dimeric compounds [Pd(μ ‐Cl)(Cl)(NHC)]₂ with bridging ligands. All of the complexes were fully characterized using ¹H NMR, ¹³C NMR, high‐resolution mass and infrared spectroscopies, elemental analysis and single‐crystal X‐ray diffraction. The catalytic activities of the obtained palladium catalysts towards Hiyama coupling of aryl chlorides with phenyltrimethoxysilane were investigated and the results showed that the dinuclear palladium complexes were considerably active for the coupling reaction. Copyright © 2016 John Wiley & Sons, Ltd.     

Synthesis of novel rhodium‐xylyl linked N‐heterocyclic carbene complexes as hydrosilylation catalysts

Reaction of ortho‐xylylbis(N‐2,4,6‐trimethylbenzylimidazolinium); xylylbis(N‐butylimidazolinium) and para‐xylylbis(N‐2,4,6‐trimethylbenzylimidazolinium); xylylbis(N‐butylimidazolinium) salts with KOBu^t and [RhCl(COD)]₂ yields ortho‐ and para‐xylylbis{(N‐alkylimidazolidin‐2‐ylidene)chloro(η⁴‐1,5‐cyclooctadiene) rho dium(I)} complexes (2a–d). All compounds synthesized were characterized by elemental analysis and NMR spectroscopy, and the molecular structures of the 2a and 2d were determined by X‐ray crystallography. Triethylsilane reacts with acetophenone derivatives in the presence of catalytic amount of the new rhodium(I)–carbene complexes (2a–d), to give the corresponding silylethers in good yields (83–99%). Copyright © 2008 John Wiley & Sons, Ltd.     

Synthesis of novel palladium N‐heterocyclic‐carbene complexes as catalysts for Heck and Suzuki cross‐coupling reactions

Novel palladium‐1,3‐dialkylperhydrobenzimidazolin‐2‐ylidene (2a–c) and palladium‐1,3‐dialkylimidazolin‐2‐ylidene complexes (4a,b) have been prepared and characterized by C, H, N analysis, ¹H‐NMR and ¹³C‐NMR. Styrene or phenylboronic acid reacts with aryl halide derivatives in the presence of catalytic amounts of the new palladium‐carbene complexes, PdCl₂(1,3‐dialkylperhydrobenzimidazolin‐2‐ylidene) or PdCl₂(1,3‐dialkylimidazolin‐2‐ylidene) to give the corresponding C? C coupling products in good yields. Copyright © 2006 John Wiley & Sons, Ltd.     

Synthesis and antimicrobial activity of Ag(I)‐N‐heterocyclic carbene complexes derived from benzimidazol‐2‐ylidene

Novel benzimidazol‐2‐ylidene carbene complexes of Ag(I) were prepared by interaction of the corresponding benzimidazolium salt with Ag₂O in dichloromethane. Their structures were characterized by elemental analyses, ¹H‐NMR, ¹³C‐NMR and IR spectroscopy techniques. All compounds studied in this work were screened for their in vitro antimicrobial activities against the standard strains: Enterococcus faecalis (ATCC 29212), Staphylococcus aureus (ATCC 29213), Escherichia coli (ATCC 25922), Pseudomonas aeruginosa (ATCC 27853) and the fungi Candida albicans and Candida tropicalis. The new complexes were found to be effective antimicrobial activity against a series of bacteria and fungi. Copyright © 2010 John Wiley & Sons, Ltd.     

Synthesis,structural characterization and catalytic activity of benzimidazole‐functionalized Pd(II) N‐heterocyclic carbene complexes

Two Pd(II)–NHC complexes bearing benzimidazole and pyridine groups have been successfully prepared and fully characterized by NMR and X‐ray diffraction analysis. The structure of palladium complexes are a typical square‐planar with palladium surrounded by two pairs of trans‐arranged benzimidazole and carbene ligands. The Pd–NHC complexes have been proved to be a highly efficient catalyst for the Mizoroki–Heck coupling reaction of aryl halides with various substituted acrylates under mild conditions in excellent yields. Copyright © 2013 John Wiley & Sons, Ltd.     

Active ruthenium‐(N‐heterocyclic carbene) complexes for hydrogenation of ketones

Four ruthenium‐N‐heterocyclic carbene complexes ( 3–6 ) have been prepared and the new compounds characterized by C, H, N analyses, ¹H‐NMR and ¹³C‐NMR. The reduction of ketones to alcohols via transfer hydrogenation was achieved with catalytic amounts of complexes 3–6 in the presence of t‐BuOK. Copyright © 2006 John Wiley & Sons, Ltd.     

New palladium(II)–N‐heterocyclic carbene complexes containing benzimidazole‐2‐ylidene ligand derived from menthol: synthesis,characterization and catalytic activities

A new series of sterically hindered ligands containing (1R,2S,4R)‐(+)‐menthoxymethyl group attached to benzimidazole‐based N‐heterocyclic carbene (NHC), palladium–bis‐NHC complexes and (κ²‐C,N)‐palladacyclic NHC complexes have been synthesized and characterized using appropriate spectroscopic techniques. Catalytic performance of the palladium complexes has been investigated for allylic alkylation, Suzuki and Heck carbon–carbon coupling reactions. These complexes smoothly catalyse the carbon–carbon bond formation reactions. Copyright © 2016 John Wiley & Sons, Ltd.     

Symmetric and dissymmetric N‐heterocyclic carbene rhodium(I) complexes: a comparative study of their catalytic activities in transfer hydrogenation reaction

Six new [RhBr(NHC)(cod)] (NHC = N‐heterocyclic carbene; cod = 1,5‐cyclooctadiene) type rhodium complexes ( 4–6 ) have been prepared by the reaction of [Rh(μ‐OMe)(cod)]₂ with a series of corresponding imidazoli(in)ium bromides ( 1–3 ) bearing mesityl (Mes) or 2,4,6‐trimethylbenzyl (CH₂Mes) substituents at N¹ and N³ positions. They have been fully characterized by ¹ H, ¹³ C and heteronuclear multiple quantum correlation NMR analyses, elemental analysis and mass spectroscopy. Complexes of type [(NHC)RhBr(CO)₂] (NHC = imidazol‐2‐ylidene) ( 7b–9b ) were also synthesized to compare σ‐donor/π‐acceptor strength of NHC ligands. Transfer hydrogenation (TH) reaction of acetophenone has been comparatively studied by using complexes 4–6 as catalysts. The symmetrically CH₂Mes‐substituted rhodium complex bearing a saturated NHC ligand ( 5a ) showed the highest catalytic activity for TH reaction. Copyright © 2012 John Wiley & Sons, Ltd.     

Hydrogen‐transfer reduction of α,β‐unsaturated carbonyl compounds catalyzed by naphthyridine‐functionalized N‐heterocyclic carbene complexes

Substitution of silver complex of 2‐chloro‐7‐(mesitylimidazolylidenylmethyl)naphthyridine (NpNHC) with palladium(II), rhodium(I) and iridium(I) metal precursors provided [Pd(C ,N ‐NpNHC)(η³‐allyl)](BF₄) ( 5 ), RhCl(COD)(C ‐NpNHC) ( 6a ) and IrCl(COD)(C ‐NpNHC) ( 6b ), respectively. Abstraction of chloride from 6a and 6b with AgBF₄ provided the chelation complexes [Rh(COD)(C ,N ‐NpNHC)](BF₄) ( 7a ) and Ir(COD)(C ,N ‐NpNHC)(BF₄) ( 7b ), respectively. All complexes were characterized using NMR and elemental analyses and the structural details of 5 and 6a were further confirmed using X‐ray crystallography. In catalytic activity studies, complex 5 was found to be an effective catalyst in the hydrogen‐transfer reduction of α,β‐unsaturated carbonyl compounds into the corresponding saturated carbonyl compounds.     

N‐Heterocyclic carbene/phosphite synergistically assisted Pd/C‐catalyzed Suzuki coupling of aryl chlorides

An N‐heterocyclic carbene and phosphite synergistically enhanced Pd/C catalyst system has been developed for Suzuki coupling of aryl chlorides and aryl boronic acids from commercially available Pd/C with sterically demanding N,N′‐bis(2,6‐diisopropylphenyl)imidazolylidene and trimethylphosphite. A remarkable increase in catalytic activity of Pd/C was observed when used along with 1 equiv. N,N′‐bis(2,6‐diisopropylphenyl)imidazolium chloride and 2 equiv. phosphite with respect to palladium in appropriate solvents that were found to play a crucial role in Pd/C‐NHC‐P(OR)₃‐catalyzed Suzuki coupling. A dramatic ortho‐substitution effect of carbonyl and nitrile groups in aryl chlorides was observed and explained by a modified quasi‐heterogeneous catalysis mechanism. The Pd/C catalyst could be easily recovered from reaction mixtures by simple filtration and only low palladium contamination was detected in the biparyl products. A practical process for the synthesis of 4‐biphenylcarbonitrile has therefore been developed using the N‐heterocyclic carbene/phosphite‐assisted Pd/C‐catalyzed Suzuki coupling. Copyright © 2013 John Wiley & Sons, Ltd.     

Well-defined PEPPSI-themed palladium–NHC complexes: synthesis,and catalytic application in the direct arylation of heteroarenes

In this study, a series of benzimidazolium salts were synthesized as unsymmetrical N-heterocyclic carbene (NHC) precursors. Benzimidazolium salts were used for synthesis of the PEPPSI (pyridine enhanced precatalyst preparation stabilization and initiation)-themed, six new Pd-complexes with the general formula [PdX₂(NHC)(pyridine)]. The structures of all compounds were characterized by various spectroscopic techniques such as ¹H NMR, ¹³C NMR and FT-IR. The more detailed structural characterization of four of the complexes was determined by single-crystal X-ray diffraction study. The catalytic activities of all Pd-complexes were evaluated in the direct arylation of the 2-acetylfuran and 2-acetylthiophene with aryl bromides in the presence of 1 mol% catalyst loading.     

N‐Phenyl‐substituted carbene precursors and their silver complexes: synthesis,characterization and antimicrobial activities

A series of unsymmetrically substituted N‐heterocyclic carbene (NHC) precursors ( 1a , 1b , 1c , 1d , 1e ) were synthesized from the reaction of N‐phenylbenzimidazole with various alkyl halides. These compounds were used to synthesize NHC–silver(I) complexes ( 2a , 2b , 2c , 2d , 2e ). The five new 1‐phenyl‐3‐alkylbenzimidazolium salts ( 1a , 1b , 1c , 1d , 1e ) and their NHC–silver complexes ( 2a , 2b , 2c , 2d , 2e ) were characterized by the ¹H NMR, ¹³C NMR and FT‐IR spectroscopic methods and elemental analysis techniques. Also, the two NHC–silver complexes 2b and 2c were characterized by single‐crystal X‐ray crystallography, which confirmed the linear C―Ag―Cl arrangements. The antibacterial activities of the NHC precursor and NHC–silver complexes were tested against three Gram‐positive bacterial strains (Bacillus subtilis, Listeria monocytogenes and Staphylococcus aureus) and three Gram‐negative bacterial strains (Escherichia coli, Klebsiella pneumoniae and Pseudomonas aeruginosa) using the microdilution broth method. The NHC–silver complexes showed higher antibacterial activity than the NHC precursors. In addition, silver complexes 2a , 2b , 2c , 2d showed high antibacterial activity against the Gram‐positive bacteria L. monocytogenes and S. aureus compared to the standard, tetracycline. Copyright © 2014 John Wiley & Sons, Ltd.     

Platinum (II) N‐heterocyclic carbene complexes: Synthesis,characterization and cytotoxic properties

Platinum (II) complexes bearing N‐heterocyclic carbene (NHC) ligands have been widely used in catalytic chemistry, but there are very few reports of biological properties of this type of complexes. A series of [PtCl₂(NHC)(PEt₃)] complexes were synthesized. The structures of all compounds were characterized by ¹H‐NMR, ¹³C‐NMR, IR and elemental analysis techniques, which supported the proposed structures. The single crystal structures of complexes 1a and 1e were determined. The title complexes show slightly distorted square‐planar coordination around the platinum (II) metal center. The cytotoxic properties of the platinum (II)–NHC complexes have been assessed in various human cancer lines, including cisplatin‐sensitive and resistant cells. IC₅₀ values of these four complexes were determined by the MTS‐based assay on three human cell lines—brain (SHSY5Y), colon (HTC116) and liver (HEP3B). These complexes have been highlighted cancer therapeutic agent with unique structures and functions.     

New bis(N‐heterocyclic carbene) palladium complex immobilized on magnetic nanoparticles: as a magnetic reusable catalyst in Suzuki‐Miyaura cross coupling reaction

A new bis(N ‐heterocyclic carbene) (NHC) palladium complex supported on silica coated magnetic nanoparticles (MNPs) was prepared using the reaction of synthesized Pd‐NHC complex with MNPs. The Pd‐NHC complex was prepared using the reaction of a hydroxyl‐functionalized bis‐imidazolium ionic liquid. The Pd‐NHC organometallic complex was used as a heterogeneous recyclable and active catalyst in the Suzuki‐Miyaura reaction and various aryl halides were coupled with arylboronic acids in order to synthesize diverse biaryls in good to excellent yields. The prepared catalyst was characterized by use of some different microscopic and spectroscopic techniques including elemental analysis, FT‐IR spectroscopy, diffuse reflectance UV–Vis spectrophotometery, scanning electron microscopy (SEM), transmission electron microscopy (TEM), vibrating sample magnetometer (VSM) and X‐ray diffraction (XRD). The Pd‐NHC catalyst system is a magnetic reusable catalyst and it can be separated from the reaction mixture using an external magnetic field. The catalyst was reusable in the Suzuki‐Miyaura coupling reaction at least for 6 times without significant decreasing in its catalytic activity.     

Well‐defined NHC?Pd complex‐mediated intermolecular direct annulations for synthesis of functionalized indoles (NHC = N‐hetero‐cyclic carbene)

As alternatives to the common tertiary phosphine/Pd systems, well‐defined N‐heterocyclic carbene–Pd complexes have been proven to be highly efficient precatalysts for intermolecular direct annalution of o‐haloanilines and ketones at lower catalyst loadings. A highly efficient and practical protocol for synthesis of functionalized indoles was developed using (IPr)Pd(acac)Cl as catalyst. Both o‐bromoanilines and o‐chloroanilines gave rise to efficient coupling under the reaction conditions. Related to acyclic ones, cyclic ketones coupled more effectively with o‐haloanilines. With [Pd(IPr)₂] as catalyst, the base‐sensitive groups including OH and CO₂H groups could be tolerated. Copyright © 2011 John Wiley & Sons, Ltd.     

Sulfonated N‐heterocyclic carbine‐silver (I) complexes: Synthesis,characterisation and biological evaluation

The synthesis, characterisation and biological activity of water‐soluble Ag(I)‐NHC complexes, general formula Na[(NHC)AgCl] where NHC is a sulfonated and sterically hindered N‐heterocyclic carbene, is reported. The Ag‐NHC complexes (2a–e) were synthesised by reacting the corresponding sulfonated NHC ligands with Ag₂O in the presence of NaCl or NaBr in methanol/water (1:1) solution. Synthesised silver (I)‐N‐heterocyclic carbene complexes have been characterised by NMR, micro‐analysis and HRMS spectroscopic methods. The IC₅₀ values of these complexes were determined by a proliferation BrdU enzyme‐linked immunosorbent assay (ELISA) against HeLa (human cervix carcinoma), HT29 (human adenocarcinoma) and L929 (mouse fibroblast) cell lines. These complexes have been highlighted as promising and original platforms for building new types of metalodrug. All new water‐soluble Ag(I) complexes demonstrated remarkable cytotoxic activity against HeLa, HT29 and L929 cell lines.     

Palladium complexes of bidentate pyridine N‐heterocyclic carbenes: Optical resolution,antimicrobial and cytotoxicity studies

A series of bidentate pyridine‐functionalized palladium N‐heterocyclic carbene (Pd NHC) complexes with various wingtip substituents (R = methyl, phenyl and tert‐butyl) have been synthesized and evaluated for their potential biomedical applications as antimicrobials and antiproliferative drug candidates. The obtained Pd NHC complexes were applied in a standard broth microdilution assay for determination of their antimicrobial activities against thirteen strains of pathogenic microorganisms. In addition to that, cytotoxic activities of the Pd NHC complexes were also evaluated against three human cancer cell lines, namely breast (MCF‐7), colon (HCT116) and oral (H103) cancer cells, using a standard MTT assay. Upon coordination to palladium, the Pd NHC complexes show significant antimicrobial activities with minimum inhibitory concentrations in the micromolar range, and they are cytotoxic to the tested carcinomas with IC₅₀ ranging from 13 to 38 μM. Evidences for influence of both wingtip substituents and optical isomerism on the biological activities of the complexes have been found.     

Facile Chan‐Lam coupling using ferrocene tethered N‐heterocyclic carbene‐copper complex anchored on graphene

Ferrocene tethered N‐heterocyclic carbene‐copper complex anchored on graphene ([GrFemImi]NHC@Cu complex) has been synthesized by covalent grafting of ferrocenyl ionic liquid in the matrix of graphene followed by metallation with copper (I) iodide. The [GrFemImi]NHC@Cu complex has been characterized by fourier transform infrared (FT‐IR), fourier transform Raman (FT‐Raman), CP‐MAS ¹³C NMR spectroscopy, transmission electron microscopy (TEM), thermogravimetric analysis (TGA), energy dispersive X‐ray (EDX) analysis, X‐ray photoelectron spectroscopy (XPS), Brunauer–Emmett–Teller (BET) surface area analysis and X‐ray diffractometer (XRD) analysis. This novel complex served as a robust heterogeneous catalyst for the synthesis of bioactive N‐aryl sulfonamides from variety of aryl boronic acids and sulfonyl azides in ethanol by Chan‐Lam coupling. Recyclability experiments were executed successfully for six consecutive runs.