Carbene Transfer Reagents Research Articles

A Fluoroponytailed NHC-Silver Complex Formed from Vinylimidazolium/AgNO3 under Aqueous-Ammoniacal Conditions.

3-(1H,1H,2H,2H-Perfluorooctyl)-1-vinylimidazolium chloride [2126844–17–3], a strong fluorosurfactant with remarkably high solubility in water, was expediently converted into the respective doubly NHC-complexed silver salt with nitrate as counter ion in quantitative yield. Due to its vinyl substituents, [bis(3-(1H,1H,2H,2H-perfluorooctyl)-1-vinylimidazol-2-ylidene)silver(I)] nitrate, Ag(FNHC)2NO3, represents a polymerizable N-heterocyclic carbene transfer reagent, thus potentially offering simple and robust access to coordination polymers with crosslinking metal bridges. The compound was characterized by infrared and NMR spectroscopy, mass spectrometry as well as elemental analysis, and supplemented by X-ray single-crystal structure determination. It crystallizes in the monoclinic crystal system in the space group P21/c. With 173.3°, the geometry of the Ag-carbene bridge deviates slightly from linearity. The disordered perfluoroalkyl side chains exhibit a helical conformation.

The carbene transfer to strong Lewis acids: copper is better than silver.

A new convenient approach to the synthesis of useful N-heterocyclic carbene complexes of group 13 metals was successfully developed. We demonstrate that air-stable copper(i) diaminocarbene complexes are excellent carbene transfer reagents for the synthesis of (IMes or IPr)MCl3 with high yields (M = Al, Ga). Use of this simple method allowed for the first time to obtain (IPr)AlCl3, inaccessible via a free carbene route. In contrast to copper complexes, under the same conditions the more commonly used silver analog (IPr)AgCl reacts with MCl3 (M = Al, Ga, In) yielding only the homoleptic cationic complexes [(IPr)2Ag]+MCl4-.

Synthesis and structural characterization of 20-membered macrocyclic rings bearing trans-chelating bis(N-heterocyclic carbene) ligands and the catalytic activity of their palladium(ii) complexes.

Macrocycles consisting of a 20-membered ring containing two imidazolium salt functionalities and of the formula [PhCH2N(CH2CH2CH2)Im(CH2CH2CH2)2][Br]2 (Im = imidazole = 3a, benzimidazole = 3b) were synthesized in 70-75% yields. These salts serve as precursors to macrocycles containing two N-heterocyclic carbene (NHC) moieties. Reaction of the macrocyclic salts 3a and 3b with silver oxide afforded macrocyclic-bis(NHC)silver(i) complexes 4a and 4b. Single-crystal X-ray diffraction studies of macrocyclic-bis(NHC)silver(i) complex 4a revealed a tetranuclear silver core with a short Ag-Ag distance (2.9328 Å). Complexes 4a and 4b serve as carbene transfer reagents to Pd. The treatment of macrocyclic-bis(NHC)silver(i) complexes 4a and 4b with one equivalent of PdCl2(MeCN)2 in methylene chloride afforded square-planar trans-macrocyclic-bis(NHC)Pd(ii)X2 complexes 5a and 5b. Preliminary screening of these palladium complexes showed they are competent precatalysts for Heck and Suzuki coupling reactions.

Copper(i)-NHC complexes as NHC transfer agents.

This report presents the recent advances in the use of copper(i)-NHC (NHC = N-heterocyclic carbene) complexes as transmetallating agents enabling the transfer of NHCs to a wide range of transition metals. The synthesis of these carbene transfer reagents is also discussed.

Alkali Metal Carbenoids: A Case of Higher Stability of the Heavier Congeners.

As a result of the increased polarity of the metal-carbon bond when going down the group of the periodic table, the heavier alkali metal organyl compounds are generally more reactive and less stable than their lithium congeners. We now report a reverse trend for alkali metal carbenoids. Simple substitution of lithium by the heavier metals (Na, K) results in a significant stabilization of these usually highly reactive compounds. This allows their isolation and handling at room temperature and the first structure elucidation of sodium and potassium carbenoids. The control of stability was used to control reactivity and selectivity. Hence, the Na and K carbenoids act as selective carbene-transfer reagents, whereas the more labile lithium systems give rise to product mixtures. Additional fine tuning of the M-C interaction by means of crown ether addition further allows for control of the stability and reactivity.

Homoleptic and heteroleptic bis-NHC Cu(I) complexes as carbene transfer reagents.

Homo- and heteroleptic bis-NHC copper(I) complexes have been efficiently used as carbene transfer reagents to Au and Pd. The simple and straightforward procedure allows for the synthesis of well-known gold complexes as well as novel cationic bis-NHC Pd(II) species.

Preparations of a series of coinage metal complexes with pyridine-based bis(N-heterocyclic carbene) ligands including transmetalation to palladium complexes

A silver complex bearing 2,6-bis(3-tert-butylimidazol-2-ylidene)pyridine (CNC) ligands was easily prepared from the reaction of the CNC ligand precursor with Ag2O. The CNC ligand contains relatively bulky substituents, tert-butyl, on nitrogen atoms. Use of the silver complex as a carbene transfer reagent gave rise to a copper and a gold complex, completing a series of coinage metal complexes [M2(CNC)2](BF4)2 (M = Cu, Ag, Au). In all the coinage metal complexes, the X-ray crystallographic analyses showed that the two CNC ligand strands are intertwined each other and surround the metal ions in a double helical fashion. Moreover, the carbene transfer reactions afforded a monomeric palladium complex [Pd(CNC)Cl]BF4 and a CNC-bridged dipalladium complex [{Pd(η3-C3H5)Cl}2(μ-CNC)]. Facile chloride ligand abstraction from the dipalladium complex yielded a chlorido-bridged dipalladium complex [{Pd(η3-C3H5)}2(μ-CNC)(μ-Cl)]BF4.

Ruthenium(II) complexes incorporating salicylaldiminato-functionalized N-heterocyclic carbene ligands as efficient and versatile catalysts for hydration of organonitriles

We describe a new synthetic procedure for synthesis of ruthenium(II) complexes containing salicylaldiminato functionalized mixed N-heterocyclic carbene (NHC) ligand and phosphine co-ligand. The complexes (3a–3d) have been obtained in good to excellent yields by transmetalation from the corresponding Ag–NHC complexes (2a–2d) as carbene transfer reagents. All the [Ru–NHC] complexes have been characterized by elemental analyses, spectroscopic methods as well as ESI mass spectrometry. The ligands 1a–1d show their versatility by switching to be O,N,C-chelating in these ruthenium(II) complexes. The resulting complexes have been evaluated as potential catalysts for the selective hydration of nitriles to primary amides, and related amide bond forming reactions, in environmentally friendly medium. The reaction tolerated ether, hydroxyl, nitro, bromo, formyl, pyridyl, benzyl and alkyl functional groups. The catalyst was stable for weeks and could be recovered and reused more than six times without significant loss of activity.

A simple access to transition metal cyclopropenylidene complexes.

We report the first example of BAC-Cu complex (BAC = bis(diisopropylamino)cyclopropenylidene) and its use as a carbene-transfer reagent, allowing access to Au-, Pd-, Ir- and Rh-BAC compounds. Catalytic experiments show the high activity of the [CuCl(BAC)] complex in Click chemistry.

NHC→SiCl4 : an ambivalent carbene-transfer reagent.

The addition of BCl3 to the carbene-transfer reagent NHC→SiCl4 (NHC=1,3-dimethylimidazolidin-2-ylidene) gave the tetra- and pentacoordinate trichlorosilicon(IV) cations [(NHC)SiCl3 ](+) and [(NHC)2 SiCl3 ](+) with tetrachloroborate as counterion. This is in contrast to previous reactions, in which NHC→SiCl4 served as a transfer reagent for the NHC ligand. The addition of BF3 ⋅OEt2 , on the other hand, gave NHC→BF3 as the product of NHC transfer. In addition, the highly Lewis acidic bis(pentafluoroethyl)silane (C2 F5 )2 SiCl2 was treated with NHC→SiCl4 . In acetonitrile, the cationic silicon(IV) complexes [(NHC)SiCl3 ](+) and [(NHC)2 SiCl3 ](+) were detected with [(C2 F5 )SiCl3 ](-) as counterion. A similar result was already reported for the reaction of NHC→SiCl4 with (C2 F5 )2 SiH2 , which gave [(NHC)2 SiCl2 H][(C2 F5 )SiCl3 ]. If the reaction medium was changed to dichloromethane, the products of carbene transfer, NHC→Si(C2 F5 )2 Cl2 and NHC→Si(C2 F5 )2 ClH, respectively, were obtained instead. The formation of the latter species is a result of chloride/hydride metathesis. These compounds may serve as valuable precursors for electron-poor silylenes. Furthermore, the reactivity of NHC→SiCl4 towards phosphines is discussed. The carbene complex NHC→PCl3 shows similar reactivity to NHC→SiCl4 , and may even serve as a carbene-transfer reagent as well.

Synthesis and Comparison of Transition Metal Complexes of Abnormal and Normal Tetrazolylidenes: A Neglected Ligand Species

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.

(NHCMe)SiCl4: a versatile carbene transfer reagent – synthesis from silicochloroform

A new synthetic pathway for the N-heterocyclic carbene adduct (NHCMe)SiCl4 (2) (NHCMe = 1,3-dimethylimidazolidin-2-ylidene) using silicochloroform is presented. Supported by DFT calculations, the energy for dissociation of 2 into the carbene and the SiCl4 fragment was found to be comparable to carbene transfer reagents based on silver(I) chloride. Compound 2 was used to transfer the NHC ligand to three different phosphorus(III) chloro compounds, resulting in the neutral complexes (NHCMe)PCl3 (3a), (NHCMe)PCl2Ph (3b) and (NHCMe)PCl2Me (3c). The sterically non-demanding NHC ligand allowed the phosphorus(III) in complex 3a to be oxidized to phosphorus(V) without loss of the NHC ligand, and afford (NHCMe)PF4H (4). Furthermore, bis-carbene complexes of Ni(II) (5) and Pd(II) (6) were obtained by reacting 2 with the respective metal chlorides.

Carbene Transfer Reactivities of Nickel(II)–N-Heterocyclic Carbene Complexes and Their Applications in the Synthesis of Metal–NHC Complexes

We describe a new synthetic procedure for transition-metal N-heterocyclic carbene complexes (NHCs). A number of Pd(II), Pt(II), Co(III), and Ru(II) complexes containing functionalized NHCs have been obtained in good to excellent yields using NiII–NHC complexes as carbene transfer reagents. NiII–NHC complexes are easily prepared from the direct reactions of the corresponding imidazolium salts with commercially available Raney nickel powder. The byproduct of transmetalation reactions, NiCl2, can be easily removed by simple filtration. This study offers a new, economical, and practical synthetic method for metal–NHC complexes.

Trinuclear copper(I) complex of 1,3-bis(2-pyridinylmethyl)imidazolylidene as a carbene-transfer reagent for the preparation of catalytically active nickel(II) and palladium(II) complexes

Reactions of 1,3-bis(pyridin-2-ylmethyl)-1H-imidazol-3-ium hexafluorophosphate, ([HL1](PF 6), L1 = 1,3-bis(pyridin-2-ylmethyl)imidazolylidene) and 1,3-bis(pyridin-2-ylmethyl)-1H-benzimidazol-3-ium hexafluorophosphate ([HL2](PF 6), L2 = 1,3-bis(pyridin-2-ylmethyl)benzoimidazolylidene) with cuprous oxide in acetonitrile readily yielded trinuclear complexes [Cu 3(L1) 3(PF 6) 3] ( 1) and [Cu 3(L2) 3(PF 6) 3] ( 2). Treatment of 1 with Ni(PPh 3) 2Cl 2 and Pd(cod)Cl 2 gave [Ni(L1)Cl](PF 6) ( 3) and [Pd(L1)Cl](PF 6) ( 4), respectively, due to transmetalation. [Ni(L1) 2](PF 6) 2 ( 5) was obtained from the reaction of [Cu 3(L1) 3(PF 6) 3] and Raney nickel in acetonitrile. All these complexes have been fully characterized. Both 1 and 2 consist of a triangular Cu 3 core with each Cu–Cu bond capped by an imidazolylidene group. Each imidazolylidene acts as a bridging ligand in a μ 2 mode and is bonded equally to two Cu(I) ions. The pincer nickel and palladium complexes are square-planar and contain a tridentate NCN ligand. Complexes 3 and 4 are efficient catalyst precursors for Kumada–Corriu and Suzuki–Miyaura coupling reactions of aryl halides with organometallic reagents.

Distinct Reactivity of Mono- and Bis-NHC Silver Complexes: Carbene Donors versus Carbene–Halide Exchange Reagents

The synthesis of N-heterocyclic carbene (NHC) complexes of silver is an established technique for their use as carbene transfer reagents. While it is known that both mono-NHC Ag(I) and bis-NHC Ag(I) complexes are accessible, different reactivities of these species have not been explored synthetically. Whereas the commonly used mono-NHC Ag(I) complexes act as NHC donors only, we found that a bis-NHC Ag(I) complex was capable of transferring one carbene under halide abstraction. This is exemplified in the transformation of P–Cl compounds into the respective imidazolium–phosphines by a bis-NHC Ag(I) complex but not by its corresponding mono-NHC Ag(I) complex. When the bis-NHC Ag(I) complex was allowed to react with [M(cod)Cl]2 (M = Rh, Ir), both NHCs were transferred under chloride abstraction to give [M(cod)(NHC)2]+ exclusively, a reaction which is reported to stop at [M(cod)Cl(NHC)] even with an excess of mono-NHC Ag(I) complex in the presence of AgPF6.

Copper N-heterocyclic carbene (NHC) complexes as carbene transfer reagents

A method is described that makes use of easily prepared, inexpensive copper synthons as N-heterocyclic carbene (NHC) transfer agents to generate catalytically relevant gold and palladium complexes.

N-Heterocyclic Carbene Complexes of Mercury, Silver, Iridium, Platinum, Ruthenium, and Palladium Based on the Calix[4]arene Skeleton

Two independent pathways to Ag(I)−N-heterocyclic-calix[4]arene complexes (5a,b and 6−8) starting from three calix[4]arenes bearing one or two (distal or proximal) imidazolium substituents attached at the upper rim via a methylene linker are described. Complexes 5 were synthesized using the free carbene route, and complexes 6−8 were obtained via in situ formation of the N-heterocyclic carbene by deprotonation and reaction with Ag2O. The structure of 5a could be verified by an X-ray single-crystal structure determination. 1H NMR studies of the compounds 5a,b showed dynamic behavior in solution, similar to a conformational change observed for known phosphine-based calixarene ligands. Using the imidazolium−calixarenes as precursors, NHC complexes of mercury, iridium, platinum, and ruthenium have been prepared and characterized. Additionally, cis and trans Pd(II)−N-heterocyclic-calix[4]arene complexes (3 and 13) were synthesized independently by in situ deprotonation of 2 with Pd(OAc)2 and by transmetalation using Ag(I) complex 6 as a supramolecular carbene transfer reagent, respectively. Both isomeric complexes 3 and 13 were tested as catalysts in the Suzuki−Miyaura reaction and compared to previously reported in situ systems. Both isolated complexes exhibit, independently of their geometry, the same catalytic activity, which is superior to the in situ system used for comparison.

Palladium(II)−N-Heterocyclic Carbene Metallacrown Ether Complexes: Synthesis, Structure, and Catalytic Activity in the Suzuki−Miyaura Reaction

Pd(II)−N-heterocyclic carbene (NHC) metallacrown ether complexes have been synthesized for the first time. Reaction of bis(imidazolium) salts with a long polyether chain with Ag2O afforded silver−NHC complexes [Ag2(L)Cl2] 5, which reacted as carbene transfer reagents with palladium salts to give Pd(II)−NHC metallacrown ether complexes 6a,b. The Ag(I)−NHC complex 5b possesses a one-dimensional chain configuration by each Ag(I) center coordinated to one carbene and two bridging chlorides. Each of Pd(II)−NHC metallacrown ether complexes 6a,b contains a 25-membered macrocyclic NHC metallacrown ether formed by a bidentate chelate bis(carbene) ligand with a long flexible linkage and a Pd(II). The coordination geometry on the Pd(II) is approximately square planar. Pd(II)−NHC metallacrown ether complexes 6a and 6b 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 and Characterization of Nitrile-Functionalized N-Heterocyclic Carbenes and Their Complexes of Silver(I) and Rhodium(I)

A series of unsymmetrical nitrile-functionalized N-heterocyclic carbene precursors were synthesized by N-alkylation and N-arylation of 2-cyanophenylimidazole. These imidazolium salts were used to synthesize N-heterocyclic carbene complexes of silver(I), all of which were characterized by NMR and Fourier transform infrared (FT-IR) spectroscopies and elemental analyses. The compound bis[1-(2-cyanophenyl)-3-methylimidazol-2-ylidene]silver(I) tetrafluoroborate ([Ag(m-CN)2]BF4, 2a) was structurally characterized by single-crystal X-ray diffraction. Interestingly, the preparation of the silver(I) carbene complex of 3-(cyanomethyl)-1-(2-cyanophenyl)imidazolium (1b) with excess silver(I) oxide leads to the formation of the desired bis-carbene complex, and the bis-carbene complex with hydrolyzed cyanomethyl groups on the ligands. The selectivity in hydrolysis of the cyanomethyl group over the cyanophenyl group on the ligand, as evident from the NMR data, suggests that the process must be mediated by silver(I) centers. The use of an N-heterocyclic carbene complex of silver(I) as a carbene transfer reagent was demonstrated by the reaction of 2a with [Rh(cod)Cl]2. The crystal structure of bis[(1-(2-cyanophenyl)-3-methylimidazol-2-ylidene)-(η4-1,5-cycloctadiene)rhodium(I) tetrafluoroborate ([Rh(m-CN)(cod)]2(BF4)2, 3a) revealed the dimeric nature of the complex, of which the nitrile nitrogen and the carbene carbon of the bridging ligand were coordinated to two different rhodium(I) centers, with a slight distortion of the Rh−N≡C bond angles. This is the first dimeric N-functionalized N-heterocyclic carbene complex of rhodium(I) that has been structurally characterized.

Novel scorpionate-type triscarbene ligands and their silver and gold complexes

New silver(I) carbene complexes were obtained starting from the N-heterocyclic carbene ligand precursors {[HB(RImH) 3]Br 2} (R = Bn, Mes and t-Bu) and {[HC(MeBImH) 3](BF 4) 3}, by treatment of the imidazolium salt with Ag 2O. Use of the tris-imidazolylborate precursors resulted in stable, well-characterized trimetallic complexes of general formula {Ag 3[HB(RIm) 3] 2}Br, which were successfully employed as carbene transfer reagents in the synthesis of related gold(I) complexes by transmetallation. The silver complexes also proved to be active catalysts of the coupling of aryl iodides with terminal alkynes (the Sonogashira reaction), although related bimetallic silver complexes were found to exhibit enhanced reactivity.