Iridium Complexes withN-Allyl-Substituted Benzimidazol-2-ylidene Ligands and Their Application in Catalytic Transfer Hydrogenation

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)(η 2 - 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)(η 2:η 2 - C -NHC)]BF 4 (NHC = 1,3-di(2-propenyl)benzimidazol-2-ylidene, 6 ), which contains an iridium center exclusively coordinated by sp 2 carbon atoms, has been prepared by treatment of 5 with AgBF 4 . 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.