Efficient Transfer Hydrogenation Using Iridium and Rhodium Complexes of Benzannulated N-Heterocyclic Carbenes

Benzimidazol-2-ylidene(NHC) derivatives of iridium(I) and rhodium(I) hearing electron-rich and/or functional substituents R and R' at the nitrogen atoms of the heterocycle [IrBr(cod)(NHC)] (a) or [RbBr(cod)(NHC)] (b) (R = 2,3,5,6-tetramethylbenzyl; R' = methoxyethyl, 1a,b; R = R' = 2,3,5,6-tetramethylbenzyl, 2a,b; R = pentamethylbenzyl, R' = methoxyethyl, 3a,b; R = R' = pentamethylbenzyl, 4a,b) were prepared. All compounds were fully characterized by H-1 and C-13 NMR spectroscopy, mass spectrometry (MALDI), and elemental analysis. In addition, the molecular structures of la, 2a, and 3a were determined by single-crystal X-ray diffraction studies. All of: the new benzimidazol-2-ylidene iridium(I) and rhodium(I) complexes were found to be effective catalysts for transfer hydrogenation with iridium(I) derivative la showing the highest catalytic activity, ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)