(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.     

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.     

N‐heterocyclic carbenes as organocatalysts in controlled/living ring‐opening polymerization of O‐carboxyanhydrides derived from l‐lactic acid and l‐mandelic acid

An organocatalytic approach to controlled/living ring‐opening polymerizations (ROPs) of O‐carboxyanhydrides (OCAs) using N‐heterocyclic carbenes (NHCs) as nucleophilic catalysts has been investigated. NHCs with different structures were used in order to compare the catalytic performances in the ROP of OCA of l ‐lactic acid. ¹H NMR, SEC, and MALDI‐TOF MS measurements of the products clearly indicated a controlled/living manner of the polymerization. The controlled/living nature was further confirmed by kinetic and chain extension experiments. Additionally, polylol initiators were used to produce α,ω‐dihydroxy telechelic, 3‐, and 4‐armed star‐shaped polymers. Moreover, star‐shaped diblock copolymer, bearing methyl and phenyl side groups, has been successfully synthesized with OCA/NHC system. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 . 52, 2306–2315     

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 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.     

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.     

Group 4 complexes bearing alkoxide functionalized N‐heterocyclic carbene ligands as catalysts in the polymerization of olefins

Novel octahedral zirconium complexes bearing alkoxide functionalized N‐heterocyclic carbene ligands have been synthesized and characterized. NMR analysis showed that more than one species was obtained during synthesis. The synthesized complexes were able to polymerize ethylene and propylene giving rise to a linear polyethylene with high M_w and polydispersity index often bimodal with molecular weight distribution (MDI) > 2, and highly isotactic polypropylene together with an atactic fraction. Density functional theory (DFT) studies on the complex stability indicate the possible species active during the polymerization reaction. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

The effect of molecular weight and catalyst concentration on catalytic activity in mechanochemically activated transesterification using silver(I)‐N‐heterocyclic carbene latent catalysts

We have investigated the effect of the concentration and molecular weight on the activity of polymeric silver(I)‐NHC (NHC = N‐heterocyclic carbene) catalyst complexes in ultrasound‐induced mechanochemical catalyst activation. A strong dependence of the turnover number (TON) on initial catalyst concentration was observed in the transesterification of vinyl acetate with benzyl alcohol. The main findings of this study are that the concentration and molecular weight effects on TON are caused by competition between mechanochemical catalyst activation and deactivation, most likely by reactive species produced during the sonication process. Performing the transesterification reaction under radical‐suppressing conditions resulted in a significant increase of TON. This result clearly demonstrates the increased catalyst lifetime when reducing the amount of sonochemical impurities, and it highlights the importance of controlling and suppressing secondary, sonochemical processes when using ultrasound‐induced mechanochemical generation of reactive species such as catalysts. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

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.     

Synthesis and reactivity of allyl nickel(II) N‐heterocyclic carbene enolate complexes

Two new N‐heterocyclic carbene enolate nickel(II) allyl complexes have been prepared and their activity towards ethylene polymerization was investigated. It was found that in the presence of diethyl zinc, the carbene enolate complex bearing a nitro substituent produces highly linear polyethylene of modest molecular weight and high polydispersity. The influence of the reaction parameters on catalytic activity and the characteristics of the resulting polymer were investigated through systematic variation of the time, temperature, and diethyl zinc concentration. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45:3637–3647, 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 and properties of high‐molecular‐weight stereo di‐block polylactides with nonequivalent D/L ratios

Di‐stereoblock polylactides (di‐sb‐PLA: PLLA‐b‐PDLA) having high molecular weight (M_n > 100 kDa) were successfully synthesized by two‐step ring‐opening polymerization (ROP) of L ‐ and D ‐lactides using tin(2‐ethylhexanoate) as a catalyst. By optimizing the polymerization conditions, the block sequences were well regulated at non‐equivalent feed ratios of PLLA and PDLA. This synthetic method consisted of three stages: (1) polymerization of either L ‐ or D ‐lactide to obtain a PLLA or PDLA prepolymer with a molecular weight less than 50 kDa, (2) purification of the obtained prepolymer to remove residual lactide, and (3) polymerization of the enantiomeric lactide in the presence of the purified prepolymer. Their ¹³C and ³¹P NMR spectra of the resultant di‐sb‐PLAs strongly supported their di‐stereo block structure. These di‐sb‐PLAs, having weight‐average molecular weights higher than 150 kDa, were fabricated into polymer films by solution casting and showed exclusive stereocomplexation. The thermomechanical analysis of the films revealed that their heat deformation temperature was limited probably because of their low crystallinity owing to the non‐equivalent PLLA/PDLA ratio. The blend systems of the di‐sb‐PLAs having complementary stereo‐sequences (the one with a long PLLA block and the other with long PDLA block) were also prepared and characterized to enhance the sc crystallinity. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 794–801, 2010     

Synthesis and characterization of polymer linked copper(I) bis(N‐heterocyclic carbene) mechanocatalysts

In this paper, the synthesis and characterization of a series of latent polymeric bis(N‐heterocyclic carbene) (NHC) copper(I) complexes is reported, which can be activated for the copper(I)‐catalyzed azide/alkyne cycloaddition (CuAAC) via ultrasound. To prove the influence of chain length and nature of the polymer towards the activation, poly(isobutylene) (PIB), poly(styrene) (PS) and poly(tetrahydrofuran) (PTHF) are synthesized via living polymerization techniques (LCCP, ATRP, CROP) obtaining different chain lengths (from 2500 to 9000 g/mol), followed by quaternization with N‐methylimidazole, generating the corresponding N‐methylimidazolium‐telechelic polymers. The deprotonation of these macroligands via strong bases like sodium tert‐butoxide (NaO^tBu) or potassium hexamethyldisilazide (KHMDS) yields the free N‐heterocyclic carbenes (NHCs), which are used to coordinate to tetrakis(acetonitrile)copper(I) hexafluorophosphate, forming the final polymer‐based mono‐ and bis(N‐methylimidazole‐2‐ylidene) copper(I)X complexes. The structural proof of these complexes is accomplished via ¹H‐NMR spectroscopy, MALDI‐TOF‐MS and GPC‐techniques. The activation of the copper(I) biscarbene catalysts by ultrasound is studied by GPC, revealing the cleavage of one shielding NHC‐ligand. The initial catalytic latency and the via ultrasound introduced catalytic activation is successfully demonstrated monitoring a CuAAC “click” reaction of benzyl azide and phenylacetylene by in situ ¹H‐NMR spectroscopy introducing thus “click” conversions up to 97%. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 3893–3907     

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.     

Synthesis and structural characterization of iridium(I) complexes of 20‐membered macrocyclic rings bearing chelating bis(N‐heterocyclic carbene) ligands

The reactivities of two 20‐membered macrocyclic ligands, each containing two N‐heterocyclic carbene (NHC) and two amine groups, towards [IrCl(COD)]₂ (COD is cycloocta‐1,5‐diene) were investigated. Macrocycles containing imidazolin‐2‐ylidene groups formed the monometallic complex [(1,2,5,6‐η)‐cycloocta‐1,5‐diene](5,16‐dibenzyl‐1,5,9,12,16,20‐hexaazatricyclo[18.2.1.1^9,12]tetracosa‐10,21‐dien‐21,22‐diylidene)iridium(I) bromide dichloromethane monosolvate, [Ir(C₈H₁₂)(C₃₂H₄₂N₆)]Br·CH₂Cl₂, 2a . The structure of iridium complex 2a at 100 K has triclinic P symmetry. The ligand in 2a coordinates to the Ir center through the NHC moieties in a cis fashion. Additionally, the ligand adopts an umbrella‐like structure that appears to envelope the Ir center. The structure displays C—H…Br interactions. Macrocycles containing benzimidazolin‐2‐ylidene groups formed the bimetallic complex [μ‐5,20‐dibenzyl‐1,5,9,16,20,24‐hexaazapentacyclo[22.6.1.1^9,16.0^10,15.0^25,30]dotriaconta‐10(15),11,13,25(30),26,28‐hexaene‐31,32‐diylidene]bis{bromido[(1,2,5,6‐η)‐cycloocta‐1,5‐diene]iridium(I)}, [Ir₂Br₂(C₈H₁₂)₂(C₄₀H₄₆N₆)], 2b . The structure of complex 2b at 100 K has orthorhombic Pbca symmetry. Each NHC moiety in 2b coordinates in a monodentate fashion to an Ir(COD) fragment. The structure exhibits disorder of the main molecule. This disorder is found in the portion of the macrocycle containing an amine group. This structure also displays C—H…Br interactions. Finally, the structure of the hexafluorophosphate salt of the imidazolin‐2‐ylidene‐containing macrocycle, namely 5,16‐dibenzyl‐1λ⁵,5,9,12λ⁵,16,20‐hexaazatricyclo[18.2.1.1^9,12]tetracosa‐1(23),10,12(24),21‐tetraene‐1,12‐diium bis(hexafluorophosphate), C₃₂H₄₄N₆²⁺·2PF₆^?, 1c , was determined. The structure of macrocycle 1c at 100 K has triclinic P symmetry and was found to contain C—H…F interactions.     

Synthesis,reactivity and preliminary biological activity of iron(0) complexes with cyclopentadienone and amino‐appended N‐heterocyclic carbene ligands

Neutral and cationic cyclopentadienone (CpO) N‐heterocyclic carbene (NHC) bis‐carbonyl iron(0) complexes bearing, appended to the NHC ligand, either a terminal amino group on the lateral chain, [Fe(η⁴‐CpO)(CO)₂(κC‐NHC(CH₂)_nNH₂)] with n = 2 ( 2a ) and 3 ( 2b ), or a cationic NMe₃⁺ fragment, [Fe(η⁴‐CpO)(CO)₂(κC‐NHC(CH₂)₂NMe₃)](I) ( 3 ), were prepared and characterized in terms of their structure, stability and reactivity. The photochemical properties of 2a and 2b were examined both in organic solvents and in water, revealing the photoactivated release of one CO ligand followed by the formation of the chelated complex [Fe(η⁴‐CpO)(CO)(κ²C,N‐NHC(CH₂)₂NH₂)] ( 4 ), whose molecular structure was confirmed by single crystal X‐ray diffraction studies. This metallacyclization occurs only in the case of 2a , with the ethylene spacer between NHC ring and NH₂ group in the lateral chain, allowing the formation of a stable 6‐membered ring. On the other hand, 2b undergoes decomposition upon irradiation. The reactivity in aqueous solutions revealed the chemical speciation of the complexes at different pH and especially under physiological conditions (phosphate buffer solution at pH 7.4 and 37 °C). The lack of data on the biological properties of iron(0) complexes prompted us to preliminarily investigate their cytotoxicity against model cancer cells (AsPC‐1 and HPAF‐II), along with a determination of their lipophilicity.     

Synthesis,characterization and catalytic activity in Suzuki–Miyaura coupling of palladacycle complexes with n‐butyl‐substituted N‐heterocyclic carbene ligands

A series of monomeric palladacycle complexes bearing n‐butyl‐substituted N‐heterocyclic carbenes, namely [Pd(NHC)X(dmba)] (dmba: dimethylbenzylamine and [Pd(NHC)X(ppy)]; NHC: 1‐n‐butyl‐3‐substituted benzylimidazol‐2‐ylidene; ppy: 2‐phenylpyridine), were prepared either by transmetallation from the corresponding silver carbene complexes or by the reaction of the corresponding acetate‐bridged palladacycle dimer with N‐heterocyclic carbene ligands in high yields. The palladium(II) complexes were characterized using elemental analyses, APCI‐MS, ¹H NMR and ¹³C NMR spectroscopies. These complexes are efficient in the Suzuki–Miyaura coupling reaction between phenylboronic acid and aryl bromides.     

Controlled ring‐opening polymerization of trimethylene carbonate and access to PTMC‐PLA block copolymers mediated by well‐defined N‐heterocyclic carbene zinc alkoxides

Four novel Zinc–NHC alkyl/alkoxide/chloride complexes ( 4 , 5 , 9 and 9′ ) were readily prepared and fully characterized, including X‐ray diffraction crystallography for 5 and 9′ . The reaction of N‐methyl‐N′‐butyl imidazolium chloride ( 3.HCl ) with ZnEt₂ (2 equiv.) afforded the corresponding [(C_NHC)ZnCl(Et)] complex ( 4 ) via a protonolysis reaction, as deduced from NMR data. The alcoholysis of 4 with BnOH led to quantitative formation of the dinuclear Zn(II) alkoxide species [(C_NHC)ZnCl(OBn)]₂ ( 5 ), as confirmed by X‐ray diffraction analysis. The NMR data are in agreement with species 5 retaining its dimeric structure in solution at room temperature. The protonolysis reaction of N‐(2,6‐diisopropylphenyl)‐N′‐ethyl methyl ether imidazolium chloride ( 8.HCl ) with ZnEt₂ (2 equiv.) yielded the [(C_NHC)ZnCl(Et)] species 9 . The latter was found to be reactive with CH₂Cl₂ in solution and to cleanly convert to the corresponding Zn(II) dichloride [(C_NHC)ZnCl₂]₂ ( 9′ ), whose molecular structure was also elucidated using X‐ray diffractometry. Unlike Zn(II)–NHC alkoxide species 1 and 2 , which contain a NHC flanked with an additional N‐functional group (i.e. thioether and ether, respectively), the Zn(II) alkoxide species 5 incorporates a monodentate NHC ligand. The Zn(II) complexes 1 , 2 and 5 were tested in the ring‐opening polymerization (ROP) of trimethylene carbonate (TMC). All three species are effective initiators for the controlled ROP of trimethylene carbonate, resulting in the production of narrow disperse PTMC material. Initiator 1 (incorporating a thioether moiety) was found to perform best in the ROP of TMC. Notably, the latter also readily undergoes the sequential ROP of TMC and rac‐LA in the presence of a chain‐transfer agent, leading to well‐defined and high‐molecular‐weight PTMC/PLA block copolymers. Copyright © 2014 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.     

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.