Iridium complexes with N-allyl-substituted benzimidazol-2-ylidene ligands and their application in catalytic transfer hydrogenation

The reactions of [Ir(mu-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)(eta(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)(eta(2):eta(2)-C-NHC)]BF4 (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 AgBF4. The reaction between [Ir(mu-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.