Perimidin-2-ylidene rhodium(I) complexes; unexpected halogen exchange and catalytic activities in transfer hydrogenation reaction

Four new [(NHC)RhCl(COD)] (NHC = perimidin-2-ylidene; COD = 1,5-cyclooctadiene) rhodium complexes (4a–c and 5a) have been prepared by the reaction of [Rh(μ-OMe)(COD)]2 with perimidinium bromides (2a–c and 3a). The halogen exchange, abstracted from 1,2-dichloroethane was observed during the reaction and [(NHC)RhCl(COD)] complexes were obtained instead of [(NHC)RhBr(COD)]. All ligand precursors and complexes have been fully characterized by 1H, 13C NMR, IR spectroscopy, elemental analysis and mass spectroscopy. X-ray diffraction studies on single crystals of 4a and 5a confirm the square planar geometry. The υ(CO) values of [(NHC)RhCl(CO)2] complexes (6a–c and 7a) revealed that the NHC ligands are not good σ-donors. Transfer hydrogenation (TH) reaction of aliphatic and aromatic ketones has been comparatively studied by using complexes 4a–c and 5a as catalysts.