The reaction of Ni(OAc) 2 with the benzimidazolium salts 1,3-dimethylbenzimidazolium iodide (1), 1,3-bis(2-propenyl)benzimidazolium bromide (2), 1,3-dipropylbenzimidazolium bromide (3), 1-(2propenyl)-3-methylbenzimidazolium bromide ( 6), and 1-propyl-3-methylbenzimidazolium iodide (7) in molten [Bu 4 N]X (X ) Br, I, BF 4 ) as ionic liquid afforded novel square-planar nickel(II) bis(benzimidazolin-2-ylidene) complexes of the general formula trans-[NiX 2 (NHC) 2 ] (X ) I, NHC ) 1,3-dimethylbenzimidazolin-2-ylidene, 8; X ) Br, NHC ) 1,3-bis(2-propenyl)benzimidazolin-2-ylidene, 9; X ) Br, NHC ) 1,3-dipropylbenzimidazolin-2-ylidene, 10; X ) Br, NHC ) 1-(2-propenyl)-3-methylbenzimidazolin-2-ylidene, 11; X ) I, NHC ) 1-propyl-3-methylbenzimidazolin-2-ylidene, 12). X-ray diffraction studies of 8-12 reveal a square-planar coordination geometry for all complexes with the carbene ligands arranged in a trans fashion.

The reactions of [Ir(μ-OMe)(cod)]2 with the N-allyl-substituted benzimidazolium salts 1-methyl-3-(2-propenyl)benzimidazolium iodide (2) and 1,3-di(2-propenyl)benzimidazolium bromide (3) have been found to afford the five-coordinated Ir(I) complexes [IrX(cod)(η2-C-NHC)] (X = I, NHC = 1-methyl-3-(2-propenyl)benzimidazol-2-ylidene, 4; X = Br, NHC = 1,3-di(2-propenyl)benzimidazol-2-ylidene, 5), respectively. The cationic derivative [Ir(cod)(η2:η2-C-NHC)]BF4 (NHC = 1,3-di(2-propenyl)benzimidazol-2-ylidene, 6), which contains an iridium center exclusively coordinated by sp2 carbon atoms, has been prepared by treatment of 5 with AgBF4. The reaction between [Ir(μ-Cl)(cod)]2 and 3 in ethanol, in the presence of excess NaOEt, has allowed the synthesis of the four-coordinate complex [IrBr(cod)(C-NHC)] (NHC = 1,3-di(propyl)benzimidazol-2-ylidene, 7) after deprotonation of 3 and hydrogenation of both N-allyl substituents. The compounds 5, 6, and 7 have been characterized by X-ray diffraction. The neutral complexes 4, 5, and 7 have been tested as catalysts in the transfer hydrogenation of cyclohexanone using 2-propanol as hydrogen source. The catalytic reactions using 4 and 5 have been observed to progress without hydrogenation of the allyl substituents.

The palladium(II) dicarbene complex trans-[PdBr2(L)2], 2 (L = 1,3-di-(2-propenyl)-benzimidazol- 2-ylidene) was synthesized from 1,3-di-(2-propenyl)-benzimidazolium bromide 1 and Pd(OAc)2 by in situ deprotonation. The X-ray structure analysis revealed a mononuclear complex with a palladium( II) center coordinated in a square-planar fashion by the two carbene functions and two bromo ligands.