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 ( 4–6 ) have been prepared by the reaction of [Rh(μ‐OMe)(cod)] 2 with a series of corresponding imidazoli(in)ium bromides ( 1–3 ) bearing mesityl (Mes) or 2,4,6‐trimethylbenzyl (CH 2 Mes) substituents at N 1 and N 3 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) ( 7b–9b ) were also synthesized to compare σ‐donor/π‐acceptor strength of NHC ligands. Transfer hydrogenation (TH) reaction of acetophenone has been comparatively studied by using complexes 4–6 as catalysts. The symmetrically CH 2 Mes‐substituted rhodium complex bearing a saturated NHC ligand ( 5a ) showed the highest catalytic activity for TH reaction. Copyright © 2012 John Wiley & Sons, Ltd.