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

Abstract

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 (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) (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 CH2Mes‐substituted rhodium complex bearing a saturated NHC ligand (5a) showed the highest catalytic activity for TH reaction. Copyright © 2012 John Wiley & Sons, Ltd.