Binuclear meta‐xylyl‐linked Ag(I)‐N‐heterocyclic carbene complexes of N‐alkyl/aryl‐alkyl‐substituted bis‐benzimidazolium salts: synthesis,crystal structures and in vitro anticancer studies

Salts of meta‐xylyl‐linked N‐ethyl/n‐butyl/benzyl‐substituted bis‐benzimidazolium having hexafluorophosphate counterions have been synthesized. The corresponding binuclear Ag(I)‐N‐heterocyclic carbene complexes were prepared by the reaction of Ag₂O. The N‐heterocyclic carbene (NHC) ligand precursor 7 and Ag(I)–NHC complexes 10 and 11 have been structurally characterized by single‐crystal X‐ray diffraction technique. All of the reported compounds have been tested for their anticancer activity using human colorectal (HCT 116) cancer cell lines. Sterically varied benzimidazolium salts displayed significant activity against HCT 116 cell line, yielding IC₅₀ values in the range 0.1–19.4 µ m , while Ag(I)–carbene complexes showed exceptionally good activity (0.2–1.3 µ m ) against tested cancer cell lines. Copyright © 2013 John Wiley & Sons, Ltd.     

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 palladium complexes derived from imidazolidin‐2‐ylidene ligands and used for catalytic amination reactions

N‐Aryl amination and the Buchwald–Hartwig reaction are of great synthetic and industrial interest and scientists accept their usefulness and versatility for obtaining arylamines. In this study Ag–N‐heterocyclic carbene complexes were used as transmetallation reagents for the synthesis of Pd–N‐heterocyclic carbene complexes. The new Pd–N‐heterocyclic carbene complexes were characterized using elemental analysis and ¹H NMR, ¹³C NMR and infrared spectroscopies. The crystal structure of one, namely dichlorobis[1,3‐bis(2‐methylbenzyl)imidazolidin‐2‐yliden]palladium(II), is presented. The activity of the Pd(II) complexes in the coupling reaction of anilines or amines with bromobenzene was investigated. These complexes exhibited high catalytic activities in the direct synthesis of triarylamines and secondary amines in a single step. Copyright © 2016 John Wiley & Sons, Ltd.     

Synthesis of 3,3‐Disubstituted Oxindoles by Palladium‐Catalyzed Asymmetric Intramolecular α‐Arylation of Amides: Reaction Development and Mechanistic Studies

Palladium complexes incorporating chiral N‐heterocyclic carbene (NHC) ligands catalyze the asymmetric intramolecular α‐arylation of amides producing 3,3‐disubstituted oxindoles. Comprehensive DFT studies have been performed to gain insight into the mechanism of this transformation. Oxidative addition is shown to be rate‐determining and reductive elimination to be enantioselectivity‐determining. The synthesis of seven new NHC ligands is detailed and their performance is compared. One of them, L8 , containing a tBu and a 1‐naphthyl group at the stereogenic centre, proved superior and was very efficient in the asymmetric synthesis of fifteen new spiro‐oxindoles and three azaspiro‐oxindoles often in high yields (up to 99 %) and enantioselectivities (up to 97 % ee; ee=enantiomeric excess). Three palladacycle intermediates resulting from the oxidative addition of [Pd(NHC)] into the aryl halide bond were isolated and structurally characterized (X‐ray). Using these intermediates as catalysts showed alkene additives to play an important role in increasing turnover number and frequency.     

Acyclic Palladium(II)‐N‐heterocyclic Carbene Metallacrown Ether Complexes: Synthesis,Structure and Catalytic Activity

Novel acyclic Pd(II)‐N‐heterocyclic carbene (NHC) metallacrown ethers 5a , 5b have been synthesized. Reaction of the imidazolium salts bearing a long polyether chain with Ag₂O afforded Ag‐NHC complexes, which then reacted as carbene transfer agent with PdCl₂(MeCN)₂ to give the desired acyclic Pd(II)‐NHC metallacrown ether complexes 5a and 5b . The ¹H NMR and ¹³C NMR spectra show 5a and 5b exist as mixtures of cis and trans isomers in solution. The trans isomer of 5a was characterized by X‐ray diffraction, which clearly demonstrated two pseudo‐crown ether cavities in trans‐ 5a . Pd(II)‐NHC complexes 5a and 5b have been shown to be highly effective in the Suzuki‐Miyaura reactions of a variety of aryl bromides in neat water without the need of inert gas protection.     

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.     

Catalytic activities in direct arylation of novel palladium N‐heterocyclic carbene complexes

Eight novel palladium N‐heterocyclic carbene (Pd‐NHC) complexes were synthesized by the reaction of chloro 1,3‐dialkylbenzimidazolin‐2‐ylidene silver(I) complexes with bis(benzonitrile)palladium(II) chloride in dichloromethane. These eight Pd‐NHC complexes are as follows: bis[1‐phenyl‐3‐(2,4,6‐trimethylbenzyl)benzimidazol‐2‐ylidene]dichloropalladium(II), bis[1‐phenyl‐3‐(2,3,5,6‐tetramethylbenzyl)benzimidazol‐2‐ylidene]dichloropalladium(II), bis[1‐phenyl‐3‐(2,3,4,5,6‐pentamethylbenzyl)benzimidazol‐2‐ylidene]dichloropalladium(II), bis[1‐phenyl‐3‐(3,4,5‐trimethoxybenzyl)benzimidazol‐2‐ylidene]dichloropalladium(II), bis[1‐(2‐diethylaminoethyl)‐3‐(3‐methylbenzyl)benzimidazol‐2‐ylidene]dichloropalladium(II), bis[1‐(2‐diethylaminoethyl)‐3‐(2,3,5,6‐tetramethylbenzyl)benzimidazol‐2‐ylidene]dichloropalladium(II), bis[1‐(2‐morpholinoethyl)‐3‐naphthalenomethylbenzimidazol‐2‐ylidene]dichloropalladium(II) and bis[1‐(2‐morpholinoethyl)‐3‐(2‐methylbenzyl)benzimidazol‐2‐ylidene]dichloropalladium(II). Also, these synthesized complexes were fully characterized using Fourier transform infrared, ¹H NMR and ¹³C NMR spectroscopic methods and elemental analysis techniques. These synthesized novel Pd‐NHC complexes were tested as catalysts in the direct arylation of 2‐n‐butylthiophene, 2‐n‐butylfuran and 2‐isopropylthiazole with various aryl bromides at 130°C for 1 h. The complexes showed very good catalytic activities in these reactions. Copyright © 2014 John Wiley & Sons, Ltd.     

Well‐Defined N‐Heterocyclic Carbene‐Palladium Complexes as Efficient Catalysts for Domino Sonogashira Coupling/Cyclization Reaction and C‐H bond Arylation of Benzothiazole

Well‐defined and air‐stable PEPPSI (Pyridine Enhanced Precatalyst Preparation Stabilization and Initiation) themed palladium bis‐N‐heterocyclic carbene complexes have been developed for the domino Sonogashira coupling/cyclization reaction of 2‐iodophenol with a variety of terminal alkynes and C‐H bond arylation of benzothiazole with aryl iodides. The PEPPSI themed palladium complexes, 2a and 2b were synthesized in good yields from the reaction of corresponding imidazolium salts with PdCl₂ and K₂CO₃ in pyridine. The new air‐stable palladium‐NHC complexes were characterized by NMR spectroscopy, X‐ray crystallography, elemental analysis, and mass spectroscopy studies. The PEPPSI themed palladium(II) bis‐N‐heterocyclic carbene complexes 2a and 2b exhibited excellent catalytic activities for domino Sonogashira coupling/cyclization reaction of 2‐iodophenol with terminal alkynes yielding benzofuran derivatives. In addition, the palladium complexes, 2a and 2b successfully catalyzed the direct C‐H bond arylation of benzothiazole with aryl iodides as coupling partners in presence of CuI as co‐catalyst.     

Preparation of novel axially chiral NHC–Pd(II) complexes and their application in oxidative kinetic resolution of secondary alcohols

Novel axially chiral N‐heterocyclic carbene (NHC) Pd(II) complexes were prepared from optically active 1,1′‐binaphthalenyl‐2,2′‐diamine (BINAM) and H₈‐BINAM and their crystal structures were unambiguously determined by X‐ray diffraction. These chiral N‐heterocyclic carbene (NHC) Pd(II) complexes were applied in the oxidative kinetic resolution of secondary alcohols using molecular oxygen as a terminal oxidant or under aerobic conditions, affording the corresponding sec‐alcohols in good yields with moderate to good enantioselectivities. Copyright © 2009 John Wiley & Sons, Ltd.     

Synthesis and antimicrobial activity of bulky 3,5‐di‐tert‐butyl substituent‐containing silver–N‐heterocyclic carbene complexes

A series of novel benzimidazolium bromides containing bulky 3,5‐di‐tert ‐butyl group were synthesized in high yields as N‐heterocyclic carbene (NHC) ligands. These NHC ligands were metallated with Ag₂O under moderate conditions to give novel silver–NHC complexes. The structures of all compounds were characterized using ¹H NMR, ¹³CNMR, infrared and elemental analysis techniques, which supported the proposed structures. The silver–NHC complexes were screened for their in vitro antimicrobial activities against the standard bacterial strains Enterococcus faecalis , Staphylococcus aureus , Escherichia coli and Pseudomonas aeruginosa and the fungal strains Candida albicans and C. tropicalis . The results showed that most of the silver–NHC complexes inhibited the growth of all bacterial strains and fungal strains and were found to display effective antimicrobial activity against different microorganisms.     

Potential N‐Heterocyclic Carbene Precursors in the Palladium‐Catalyzed Heck Reaction

A novel 1‐(cyclobutylmethyl)‐substi‐tuted imidazolidinium/benzimidazolium salts as N‐heterocyclic carbene (NHC) precursors were successfully synthesized and characterized by ¹H NMR, ¹³C NMR, IR, and elemental analysis techniques. These compounds were easily prepared from the reaction of N‐alkyl imidazoline/N‐alkyl benzimidazole with bromomethylcyclobutane in high yields. The in situ formed catalytic system derived from the NHC precursor and Pd(OAc)₂ was used in the Heck reaction between aryl halides and styrene with potassium hydroxide in water. The corresponding Heck products were obtained in good yields. © 2012 Wiley Periodicals, Inc. Heteroatom Chem 24:77–83, 2013; View this article online at wileyonlinelibrary.com . DOI 10.1002/hc.21065     

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.     

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.     

Poly(ethylene glycol)‐ and glucopyranoside‐substituted N‐heterocyclic carbene precursors for the synthesis of arylfluorene derivatives using efficient palladium‐catalyzed aqueous Suzuki reaction

This paper reports an environmentally friendly and highly efficient synthesis of organic semiconductor materials via a Pd/N‐heterocyclic carbene (NHC)‐catalyzed Suzuki reaction in aqueous ethanol with high isolated yields (86–98%). Firstly, four glucopyranoside‐substituted NHC precursors with poly(ethylene glycol) (PEG) chains were synthesized and characterized. The NHC precursor with the longest PEG chain (n = 16) was found to be the most efficient ligand in the reactions of a wide range of aryl halides and arylboronic acids. The best catalyst system obtained in this work could be recycled five times without significant loss of catalytic activity.l Copyright © 2016 John Wiley & Sons, Ltd.     

Polymerization of carboxylic ester functionalized norbornenes catalyzed by (η3‐allyl)palladium complexes bearing N‐heterocyclic carbene ligands

An in situ generated cationic allylpalladium complex bearing N‐heterocyclic carbene (NHC) ligands, derived from the reaction of [(η³‐C₃H₅)Pd(NHC)Cl] with AgX (X = BF₄ or SbF₆), is an active catalyst for the addition polymerization of norbornene and norbornene derivatives [5‐norbornene‐2‐carboxylic acid methyl ester ( b ) and 5‐norbornene‐2‐carboxylic acid n‐butyl ester ( c )] with an ester group containing a large portion of endo‐isomers. The catalytic activities, polymer yields, molecular weights, and molecular weight distributions of polynorbornenes were investigated under various reaction conditions: the catalytic activity was highly dependent on the counteranion, the reaction solvent, and the reaction temperature. For b , as the portion of an endo‐isomer increased, the activity of 13 (SbF) was much higher than those of 14 and 15 , and for c (exo/endo = 95:5), the maximum turn over number (TON) was observed with 15 (SbF). © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 3042–3052, 2007     

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.     

Synthesis,Structural Characterisation and Stereochemical Investigation of Chiral Sulfur‐Functionalised N‐Heterocyclic Carbene Complexes of Palladium and Platinum

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 ¹H–¹H 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 (R_s,S,R)‐λ or (S_s,R,R)‐δ in both the solid state and solution.     

Oxorhenium(V) Complexes with 1,3,4‐Triphenyl‐1,2,4‐triazol‐5‐ylidene

Reactions of the oxorhenium(V) complexes [ReOX₃(PPh₃)₂] (X = Cl, Br) with the N‐heterocyclic carbene (NHC) 1,3,4‐triphenyl‐1,2,4‐triazol‐5‐ylidene (L^Ph) under mild conditions and in the presence of MeOH or water give [ReOX₂(Y)(PPh₃)(L^Ph)] complexes (X = Cl, Br; Y = OMe, OH). Attempted reactions of the carbene precursor 5‐methoxy‐1,3,4‐triphenyl‐4,5‐dihydro‐1H‐1,2,4‐triazole ( 1 ) with [ReOCl₃(PPh₃)₂] or [NBu₄][ReOCl₄] in boiling xylene resulted in protonation of the intermediately formed carbene and decomposition products such as [HL^Ph][ReOCl₄(OPPh₃)], [HL^Ph][ReOCl₄(OH₂)] or [HL^Ph][ReO₄] were isolated. The neutral [ReOX₂(Y)(PPh₃)(HL^Ph)] complexes are purple, airstable solids. The bulky NHC ligands coordinate monodentate and in cis‐position to PPh₃. The relatively long Re–C bond lengths of approximate 2.1Å indicate metal‐carbon single bonds.     

Synthesis,characterization, and reactivity of palladium(II) complexes containing piperidoimidazolin‐2‐ylidene

A series of Pd–N‐heterocyclic carbene (Pd‐NHC) complexes with pyrazine ( 1 ) or pyridine ( 2 ) and NHC ( 3 ) were synthesized and characterized by elemental analysis and spectroscopic methods. In addition, the molecular structure of 3 was determined by X‐ray diffraction studies. The effects of these ligands on catalyst activation and the performance of complexes 1 , 2 , 3 were studied on Suzuki–Miyaura reactions of phenylboronic acid with aryl chlorides. Finally, we demonstrated that complex 1 is very adept at re‐forming the Kumada–Tamao–Corriu cross‐coupling reaction. Copyright © 2013 John Wiley & Sons, Ltd.     

Photofunctional Platinum Complexes Featuring N‐heterocyclic Carbene‐Based Pincer Ligands

Photoactive platinum complexes of stoichiometry [Pt(^RCCC^R)L]^0/+ (R=Me, nBu and L=? CN, ? C≡CPh, ? N≡CCH₃, ? Py, ? CO) featuring pincer‐type bis N‐heterocyclic carbene (NHC) ligands (^RCCC^R) were synthesized. Organometallic syntheses of these complexes are facile and achievable through standard laboratory procedures. Control of intermolecular Pt???Pt interaction, π–π stacking, and emission tuning is achieved through suitable choice of the NHC‐wingtip substituent (R) and the auxiliary ligand (L). Exposure to specific volatile organic compounds (VOCs) or mechanical grinding triggers changes in emission colors, which render these complexes photofunctional. Solid‐state structures and photoluminescence results are described herein.