Symmetric and dissymmetric N-heterocyclic carbene rhodium(I) complexes: a comparative study of their catalytic activities in transfer hydrogenation reaction

Six new [RhBr(NHC)(cod)] (NHC?=?N-heterocyclic carbene; cod?=?1,5-cyclooctadiene) type rhodium complexes (46) have been prepared by the reaction of [Rh(mu-OMe)(cod)]2 with a series of corresponding imidazoli(in)ium bromides (13) bearing mesityl (Mes) or 2,4,6-trimethylbenzyl (CH2Mes) substituents at N1 and N3 positions. They have been fully characterized by 1?H, 13?C and heteronuclear multiple quantum correlation NMR analyses, elemental analysis and mass spectroscopy. Complexes of type [(NHC)RhBr(CO)2] (NHC?=?imidazol-2-ylidene) (7b9b) were also synthesized to compare s-donor/p-acceptor strength of NHC ligands. Transfer hydrogenation (TH) reaction of acetophenone has been comparatively studied by using complexes 46 as catalysts. The symmetrically CH2Mes-substituted rhodium complex bearing a saturated NHC ligand (5a) showed the highest catalytic activity for TH reaction. Copyright (c) 2012 John Wiley & Sons, Ltd.