Iridium(I) N-heterocyclic carbene complexes of benzimidazol-2-ylidene: effect of electron donating groups on the catalytic transfer hydrogenation reaction

Two new [Ir(NHC)(COD)Cl] (NHC = N-heterocyclic carbene; COD = 1,5-cyclooctadiene) iridium complexes (2a,b) have been prepared by the reaction of [Ir(COD) Cl](2) with in situ prepared NHC-Ag carbene transfer agents in dichloromethane at ambient temperature. They have been fully characterized by H-1, C-13 NMR, and elemental analysis. X-ray diffraction studies on single crystals of 2a and 2b confirm the square planar geometry. Complexes of type [Ir(NHC)(CO)(2)Cl] [NHC = 1,3-diisopropyl(5,6-dimethyl)benzimidazol-2-ylidene] 3 were also synthesized to compare sigma-donor/pi-acceptor strength of NHC ligands. Transfer hydrogenation (TH) reactions of various aldehydes and ketones have been studied using complexes 2a and 2b as catalysts. The 5,6-dimethyl substituted iridium complex (2b) showed the highest catalytic activity for the TH reaction.