Carbene Ligand Precursors Research Articles

Anodic Desulfurization of Heterocyclic Thiones - A Synthesis to Imidazoles and Analogues.

An electrochemical desulfurization of 2-mercapto-imidazoles to the corresponding imidazole is established. This novel anodic transformation is bromide-mediated and easy to conduct in the simplest electrochemical setup, consisting of an undivided cell, carbon electrodes, and constant current electrolysis. The method proved successful in 14 diverse examples of imidazoles and triazoles with up to a 97% yield. The scalability was proven in the multigram synthesis of a technically relevant N-heterocyclic carbene (NHC) ligand precursor.

A new highly efficient NHC/Cu(I) catalyst system for the Ullmann-type N-arylation reactions

A novel N-heterocyclic carbene (NHC) ligand precursor, o-Phen-L, based on 1,10-phenanthroline was designed and synthesized. This Imidazole salt was characterized by NMR spectroscopy and elemental analysis. The performance of o-Phen-L as a ligand has been evaluated. o-Phen-L coexisted with CuI to form a highly efficient catalytic system for the Ullmann type N-arylation reactions of (hetero)aryl chlorides and azoles. This catalytic system shows broad functional group tolerance and can provide the desired coupling products with high to excellent yields (80–98%) under mild conditions.

Nickel-Catalyzed Cross-Coupling Reaction of Aryl Bromides/Nitriles with Imidazolium Salts Involving Inert C-N Bond Cleavage.

We herein present a nickel-catalyzed cross-coupling reaction of aryl halides and nitriles with imidazolium salts. A series of 2-arylated imidazoles could be obtained in moderate to good yields through inert C-N bond cleavage. The imidazolium salt in this reaction acts as both a coupling partner and N-heterocyclic carbene (NHC) ligand precursor. Mechanistic studies reveal that consecutive steps of migratory insertion of the NHC into the aryl C-Ni bond and β-C elimination might be involved in the proposed reaction mechanism.

Coordination Chemistry of Ir with Chelating Ligands Containing a Xanthine‐Derived, Protic N‐Heterocyclic Carbene (NHC) Moiety

AbstractA series of protic N‐heterocyclic carbene ligand precursors based on theophylline or adenine has been prepared, and the reactivity and coordination chemistry of these proligands towards iridium(III) centers has been investigated. Complex formation appears to be highly dependent on the proligand structure. In one case, a particularly stable complex is formed that enables fully reversible protonation/deprotonation of the coordinated NHC moiety. The acidity constant of the protonated complex was consequently determined.

N,S-heterocyclic carbene containing benzothiazol-2-ylidene-Ru(II) and Pd(II) new complexes functionalized with butyl linked carbazole moiety: Synthesis, characterization and their catalytic efficiency and electropolymerizations

In contrast to the large number of polymeric N-heterocyclic carbenes (NHCs) there are relatively few examples of NHCs-containing conductive polymers. However, conducting polymers (CPs) could be excellent hosts for NHC compounds, as they can easily form polymer films electrochemically, and their optical & electrical properties can be easily tuned by electrochemically or changing bonding groups in their structure. In this work, the N,S-heterocyclic carbene (NHC) ligand precursor, N-(4-butylcarbazole)benzothiazolium bromide (1) was prepared by solvent-free alkylation of benzothiazole by 9-(4-bromobutyl)carbazole with high efficiency. The air- and moisture-stable new piano-stool Ru(II)-N,S-NHC (2) and PEPPSI type Pd(II)-N,S-NHC (3) complexes, were synthesized and characterized by 1H- and 13C NMR, FT-IR, UV–vis and ESI-MS spectroscopic methods and elemental analysis. Further confirmations of structural details for benzothiazolium salt (1) and its Ru(II)-N,S-NHC complex (2) were determined by X-ray single-crystal diffraction. The catalytic performance of the complexes (2 and 3) in transfer hydrogenation (TH) of acetophenone and Suzuki–Miyaura cross-coupling reactions of aryl bromides were also investigated, respectively. All compounds (1–3) were electrochemically polymerized and conducting polymer films with pendant carbene complex moieties were obtained. The electrical and optical properties of the resulting polymers were characterized via spectroelectrochemical studies.

One pot synthesis of propargylamines by three component amine-aldehyde-acetylene (A3) coupling catalyzed by neutral Ag(I) and Au(I) and cationic Pd(II) and Ni(II) complexes of a pincer N-heterocyclic carbene

The amine−aldehyde−acetylene (A3) coupling yielding propargylamines was successfully catalyzed by a series of neutral silver and gold complexes, [2,6-bis-(1-(1S)-menthyl-3-methylene-imidazol-2-ylidene)-pyridine]M2Cl2 [M = Ag (2), Au (3)], and cationic palladium and nickel complexes {[2,6-bis-(1-(1S)-menthyl-3-methylene-imidazol-2-ylidene)-pyridine]MCl}X [M = Pd, X = Cl (4), BF4 (5); M = Ni, X = Cl (6), BF4 (7)], stabilized over a new (1R)-(−)-menthyl substituted bis(imidazole) based [CcarbeneNCcarbene] type pincer ligand. The Au(I) (3), Pd(II) (4) and Ni(II) (6) complexes were conveniently synthesized by transmetallation reaction from the Ag(I) analog (2), which was obtained from the N-heterocyclic carbene ligand precursor (1) by treatment with Ag2O. The anion exchange reaction of Pd(II) (4) and Ni(II) (6) with AgBF4 yielded the corresponding Pd(II) (5) and Ni(II) (7) complexes respectively. Mass spectrometry studies suggested the formation of a catalytically active metal acetylide species namely, [NHC]M2(CCPh)Cl [M = Ag (2a), Au (3a)] and {[NHC]M(CCPh)}X [M = Pd, X = Cl (4a), BF4 (5a); M = Ni, X = Cl (6a), BF4 (7a)] (where NHC = 2,6-bis-(1-(1S)-menthyl-3-methylene-imidazol-2-ylidene)-pyridine). which mediates the formation of the propargylamine with a protonated Schiff imine salt produced in situ from an aldehyde and an amine in the single pot reaction.

A New Class of Task-Specific Imidazolium Salts and Ionic Liquids and Their Corresponding Transition-Metal Complexes for Immobilization on Electrochemically Active Surfaces.

Adding to the versatile class of ionic liquids, we report the detailed structure and property analysis of a new class of asymmetrically substituted imidazolium salts, offering interesting thermal characteristics, such as liquid crystalline behavior, polymorphism or glass transitions. A scalable general synthetic procedure for N‐polyaryl‐N’‐alkyl‐functionalized imidazolium salts with para‐substituted linker (L) moieties at the aryl chain, namely [LPh m ImHR]+ (L=Br, CN, SMe, CO2Et, OH; m=2, 3; R=C12, PEG n ; n=2, 3, 4), was developed. These imidazolium salts were studied by single‐crystal X‐ray diffraction (SC‐XRD), NMR spectroscopy and thermochemical methods (DSC, TGA). Furthermore, these imidazolium salts were used as N‐heterocyclic carbene (NHC) ligand precursors for mononuclear, first‐row transition metal complexes (MnII, FeII, CoII, NiII, ZnII, CuI, AgI, AuI) and for the dinuclear Ti‐supported Fe‐NHC complex [(OPy)2Ti(OPh2ImC12)2(FeI2)] (OPy=pyridin‐2‐ylmethanolate). The complexes were studied concerning their structural and magnetic behavior via multi‐nuclear NMR spectroscopy, SC‐XRD analyses, variable temperature and field‐dependent (VT‐VF) SQUID magnetization methods, X‐band EPR spectroscopy and, where appropriate, zero‐field 57Fe Mössbauer spectroscopy.

Synthesis and catalytic application of new [{IrCl(cod)}2(μ2-diNHC)] and [{Ir(cod)(sulfonated phosphine)}2(μ2-diNHC)] complexes

Four new dinuclear iridium(I) complexes were synthesized from the di(N-heterocyclic carbene) ligand precursors 1,1′-methylene-bis(3-benzyl-imidazolium)dichloride and 1,1′-methylene-bis(3-(2,4,6-trimethylbenzyl)imidazolium)dichloride. The complexes were fully characterized by spectroscopic methods, HR-MS (ESI) spectrometry, furthermore, the structure of complexes 1 and 2 were determined by single-crystal X-ray diffraction. Complexes 3a and 3b which have sulfonated triphenylphosphine ligands, were successfully used under mild conditions as catalysts in reduction reaction of ketones via hydrogen transfer from aqueous 2-propanol, and for hydrogenation of alkynes in aqueous-organic biphasic systems.

Synthesis and characterization of the N-heterocyclic carbene ligand (NHC) precursors bearing long-alkyl chain substituent and the corresponding Ru(II)-carbene complexes for transfer hydrogenation catalysts

A new series of Ru(II)-NHC complexes [RuCl2(NHC)(p-cymene)] (4a-e) were obtained through a transmetallation reaction of the corresponding Ag(I)-NHC complexes with [RuCl2(p-cymene)]2 in dichloromethane (DCM) under Argon atmosphere. All new compounds were characterized by NMR spectroscopy, elemental analysis, FT-IR, MS, and the molecular structure of 4a was determined by X-ray crystallography. The complex of 4a adopts three-legged piano stool geometry with the facial planar p-cymene representing the “seat”, and the two chloro and one NHC ligands form the three “legs”. All complexes have been tested as catalysts for the transfer hydrogenation of aromatic ketones, showing moderate to high activity in this reaction.

Larger scale Stahl oxidation with instant Cu removal in convenient synthesis of chiral bidentate N–heterocyclic carbene precursor

Commercially available N-Boc protected l-proline can be efficiently converted into a chiral, bidentate, aminoalkyl N-heterocyclic carbene ligand precursor in high yield (50% total after 4 steps) without column chromatography purification at any moment. The developed synthetic path includes: (1) redox step leading to an aldehyde, (2) imine condensation and in situ reduction to 1,2-diamine; (3) heterocyclization; (3) removal of the protecting group. Instant separation of Cu traces after the key Stahl oxidation at gram-scale was facilitated by the use of bis(isocyanide) scavenger, SnatchCat, forming insoluble, easy to remove RNC:→[Cu] complexes that allowed usto obtain crude intermediate suitably pure for next steps without tedious purification.

Synthesis, characterization, and coordination chemistry of a phenanthridine-containing N-heterocyclic carbene ligand

An N-heterocyclic carbene ligand precursor bearing a π-extended phenanthridine (3,4-benzoquinoline) unit is presented. The proligand was isolated as the imidazolium salt of chloride (1•HCl), bromide (1•HBr), or hexafluorophosphate (1•HPF6) counterions. These salts can be deprotonated and the carbene installed on silver centres using Ag2O as both a base and a source of metal ion. The resulting Ag(I) complex (1)AgCl can be used in a transmetalation reaction to generate a Pd(II) coordination complex (1)Pd(CH3CN)Cl2. The characterization and photophysical properties of these complexes is presented, along with a demonstration of the utility of (1)Pd(CH3CN)Cl2 in mediating a catalytic C-N cross-coupling reaction for the preparation of the pharmaceutical Piribedil.

Mercury(II) Complexes of Anionic N-Heterocyclic Carbene Ligands: Steric Effects of the Backbone Substituent.

Mercury(II) complexes (Me-maloNHCDipp)HgCl (1b), (t-Bu-maloNHCDipp)HgCl (2b) and (t-Bu-maloNHCDipp)HgMe (2c) supported by anionic N-heterocyclic carbenes have been obtained in good yields from the reaction of the potassium salt of N-heterocyclic carbene ligand precursors and mercury(II) salts, HgCl2 and MeHgI. These molecules have been characterized by 1H-NMR, 13C-NMR and IR spectroscopy and elemental analysis. X-ray crystal structures of 1b and 2b are also presented. Interestingly, complex 1b is polymeric {(Me-maloNHCDipp)HgCl}n in the solid state, as a result of inter-molecular Hg-O contacts, and features rare three coordinate mercury sites with a T-shaped arrangement, whereas the (t-Bu-maloNHCDipp)HgCl (2b) is monomeric and has a linear, two-coordinate mercury center. The formation of T-shaped structure and the aggregation of complex 1b is attributable to the reduced steric demand of the N-heterocyclic carbene ligand backbone substituent.

Crystal structure of zwitterionic 3,3'-[1,1'-(butane-1,4-di-yl)bis-(1H-imidazol-3-ium-3,1-di-yl)]bis-(propane-1-sulfonate) dihydrate.

The crystal structure of the title compound, C16H26N4O6S2·2H2O, a water-soluble di-N-heterocyclic carbene ligand precursor was determined using a single crystal grown by the slow cooling of a hot N,N-di-methyl-formamide solution of the compound. The dihydrate crystallizes in the monoclinic space group P21/c, with half of the zwitterionic mol-ecule and one water mol-ecule of crystallization in the asymmetric unit. The remaining part of the mol-ecule is completed by inversion symmetry. In the mol-ecule, the imidazole ring planes are parallel with a plane-to-plane distance of 2.741 (2) Å. The supra-molecular network is consolidated by hydrogen bonds of medium strength between the zwitterionic mol-ecules and the water mol-ecules of crystallization, as well as by π-π stacking inter-actions between the imidazole rings of neighbouring mol-ecules and C-H⋯O hydrogen-bonding inter-actions.

Silver(I) N-heterocyclic carbene complexes: Synthesis, characterization and cytotoxic properties

In this study, sulfonate functionalized N-heterocyclic carbene ligand precursors were obtained from the compounds 1-alkylbenzimidazole, 1,3-propane sultone, and 1,4-butane sultone. These NHC ligand precursors were metallated with Ag2O on moderate conditions to give novel silver-NHC complexes. All new compounds were characterized by by 1H NMR, 13C NMR, FT-IR and elemental analysis techniques. The cytotoxic properties of the silver(I) 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 MTT based assay on three human cell lines brain (SHSY5Y) and liver (HEP3B). These silver N-heterocyclic carbene complexes have been highlighted metal-based cancer therapeutic agents with unique structures and functions. These strategies provide exciting opportunities for discovering new types of metallodrugs.

The Natural Product Lepidiline A as an N-Heterocyclic Carbene Ligand Precursor in Complexes of the Type [Ir(cod)(NHC)PPh3)]X: Synthesis, Characterisation, and Application in Hydrogen Isotope Exchange Catalysis

A range of iridium(I) complexes of the type [Ir(cod)(NHC)PPh3)]X are reported, where the N-heterocyclic carbene (NHC) is derived from the naturally-occurring imidaozlium salt, Lepidiline A (1,3-dibenzyl-4,5-dimethylimidazolium chloride). A range of complexes were prepared, with a number of NHC ligands and counter-ions, and various steric and electronic parameters of these complexes were evaluated. The activity of the [Ir(cod)(NHC)PPh3)]X complexes in hydrogen isotope exchange reactions was then studied, and compared to established iridium(I) complexes.

Coinage metal complexes of N‐heterocyclic carbene bearing nitrile functionalization: Synthesis and photophysical properties

A new series of N‐heterocyclic carbene (NHC) ligand precursors (1 and 2) with their [Ag(I)(NHC)2]PF6 complexes (3 and 4) and [ClAu(I)(NHC)] complexes (5 and 6) are reported. Complexes 5 and 6 were synthesized via transmetalation reaction using either 3 or 4 and AuCl(SMe2) as reactants, respectively. All the synthesized compounds were fully characterized using elemental analyses and Fourier transform infrared, 1H NMR and 13C NMR spectroscopies. In the crystal structures of 3, 5 and 6, the Ag(I) and Au(I) ions are in a linear geometry. The entire structure of 3 is stabilized by significant π–π interactions, while the structures of 5 and 6 are stabilized with the presence of aurophilic interactions between the adjacent Au(I) ions as well as CH–π or π–π interactions. From photoluminescence studies, complexes 5 and 6 show dual‐emission characteristics. The higher‐energy fluorescence originates from 1XLCT mixed with 1MLCT, while the lower‐energy phosphorescence is ascribed to 3XLCT and 3MLCT with small contribution of 3ILCT, as evidenced by density functional theory (DFT) and time‐dependent DFT calculations of the modelled molecules.

Transfer hydrogenation of ketones catalyzed by a trinuclear Ni(II) complex of a Schiff base functionalized N-heterocyclic carbene ligand

A new Schiff base-functionalized N-heterocyclic carbene ligand precursor 3-benzyl-1-[2-((2-hydroxy-benzylidene)-amino]-ethyl-3H-imidazol-1-ium bromide (3), and its trinuclear Ni(II) complex [LNiL-Ni-LNiL].2Br (4) where L = 2-[2-(3-benzylimidazol-1-yl) ethyliminomethyl]phenol, were synthesized via the solventless and free carbene routes respectively. Both compounds were characterized by spectroscopic and X-ray diffraction techniques. Single crystal XRD analysis of 4 showed that it is composed of a central square planar Ni(II) ion symmetrically linked to two distorted square planar Ni(II) ions via two bridging ligands. The central Ni(II) ion is only bound to the Schiff base moieties of the bridging ligands via the phenolate oxygen donor (O1) and imine nitrogen donor (N1) atoms in a trans [N^O^(Ni2+)^N^O] mode, whilst the carbene moieties of each bridging ligand and a tridentate L are coordinated in a distorted square planar CNHC-(Ni2+)^N^O^CNHC mode to stabilise each of the terminal Ni(II) ions. Complex 4 showed significant activity as a catalyst in the transfer hydrogenation of a range of aliphatic and aromatic ketones, at a catalyst concentration of 0.1 mol%. An excellent conversion up to 100% was achieved for aromatic ketones after 4 h.

A Sustainable, User‐Friendly Protocol for the Pd‐Free Sonogashira Coupling Reaction

We herein present a new catalytic system for the palladium‐free Sonogashira coupling reaction. The catalytically active moiety is formed in situ, in a straightforward and user‐friendly manner, by combining a widely available low‐cost copper salt and an N‐heterocyclic carbene precursor. A series of N‐heterocyclic carbene ligand precursors with variable structural features, some of which are novel, were tested and the reaction conditions were optimized. Using the catalytic system with the optimum performance, the scopes of the alkyne and the aryl halide were probed. Aryl iodides readily react with terminal alkynes, providing the coupling products in high to excellent yields. The protocol is highly efficient with alkynes bearing either alkyl or aryl substituents, the latter having either electron‐donating or electron‐withdrawing groups.

Synthesis, Structures, and Catalytic Properties of Dinuclear Iridium(I) Complexes with a Hexadentate Macrocyclic Diamine‐Tetracarbene Ligand

Two dinuclear macrocyclic IrI NHC complexes, [Ir2(L1)(COD)2](PF6)2 (1, COD = 1,5‐cyclooctadiene) and [Ir2Cl(L1)(COD)]Cl (2), were prepared by the reaction between [{IrCl(COD)}2] and macrocyclic N‐heterocyclic carbene (NHC) ligand precursor (H4L1)(PF6)4 containing four benzimidazolium and two secondary amino groups. Furthermore, 1 and 2 were carbonylated to give [Ir2(L1)(CO)4](PF6)2 (3) and [Ir2Cl(L1)(CO)2]Cl (4), respectively. Notably, 1 can capture the chloride ion to turn itself into 2 with accompanying transformation from cis to trans configuration. Complexes 2 and 4 have unusual five‐coordinate IrI centers, which are rarely reported for IrI NHC complexes. The catalytic properties of 1, 2, and related mononuclear IrI complex [Ir(L2)(COD)](PF6) (5, L2 = bis‐NHC ligand) were evaluated in 1,4 conjugate addition reactions of arylboronic acids to cyclohex‐1‐en‐2‐one. These three complexes are the first examples of IrI NHC species showing such catalytic properties in 1,4‐addition reactions, and have higher catalytic activities than the reported IrI complex [Ir(hydroquinone)(COD)]BF4.

Metal Complexes with a Hexadentate Macrocyclic Diamine-Tetracarbene Ligand.

A hexadentate macrocyclic N-heterocyclic carbene (NHC) ligand precursor (H4L)(PF6)4 containing four benzimidazolium and two secondary amine groups, has been synthesized and characterized. Coordination chemistry of this new macrocyclic diamine-tetracarbene ligand has been studied by the synthesis of its Ag(I), Au(I), Ni(II), and Pd(II) complexes. Reactions of (H4L)(PF6)4 with different equiv of Ag2O result in Ag(I) complexes [Ag(H2L)](PF6)3 (1) and [Ag2(H2L)](PF6)4 (2). A mononuclear Au(I) complex [Au(H2L)](PF6)3 (3) and a trinuclear Au(I) complex [Au3(H2L)(Cl)2](PF6) (4) are obtained by transmetalation of 1 and 2 with AuCl(SMe2), respectively. Reactions of (H4L)(PF6)4 with Ni(OAc)2 and Pd(OAc)2 in the presence of NaOAc yield [Ni(L)](PF6)2 (5) and [Pd(L)](PF6)2 (6), respectively, containing one Ni(II) and Pd(II) ion with distorted square-planar geometry. Using more NaOAc results in the formation of unusual dinuclear complexes [Ni2(L-2H)](PF6)2 (7) and [Pd2(L-2H)](PF6)2 (8) (L-2H = deprotonated ligand after removing two H+ ions from two secondary amine groups in L), respectively, featuring a rare M2N2 core formed by two bridging amides. 7 is also formed by the reaction of 5 with 1.0 equiv of Ni(OAc)2·4H2O in the presence of NaOAc. Transmetalation of 2 with 2.0 equiv of Ni(PPh3)2Cl2 gives [Ni2(L)(μ-O)](PF6)2 (9), the first example of a dinuclear Ni(II) complex with a singly bridging oxo group. 9 is converted to 7 in good yield through the treatment with NaOAc.