Synthesis, characterization and reactivity of transition metal complexes with multiple NHC donor ligands

Abstract

NHC ligand precursors, 2,6-Bis(1-methylimidazolium pyridine)dibromide (CNCMe·2HBr), 2,6-Bis(1-t-butylimidazolium pyridine)dibromide (CNCtBu·2HBr) and 2,6-Bis(1-cyclohexylimidazolium pyridine)dibromide (CNCCy·2HBr) were prepared by following literature procedures. Metal precursor [Ru(CN)MeCl3OH2] was synthesized by using a protocol recently developed in our group, starting from RuCl3·3H2O. Ru(II) complexes, [Ru(CNC)Me(CN)MeI]PF6 (1), [Ru(CNC)tBu(CN)MeI]PF6 (2) and [Ru(CNC)Cy(CN)MeI]PF6 (3) have been synthesized by reaction of NHC ligand precursors and metal precursor under thermal condition. All the synthesized ligands and complexes have been characterized by mass spectrometry and multinuclear NMR spectroscopic techniques. The molecular structure of complex 3 was confirmed by single crystal X-ray diffraction technique. The crystal structure reveals that Ru(II) centre is coordinated by one bidentate, one tridentate NHC donor ligands and acetonitrile molecule, forming a distorted octahedron. Catalytic activities of complexes 1, 2 and 3 have been checked and it was found that complex 2 is an excellent catalyst for transfer hydrogenation and 1 for acceptor-less dehydrogenation reactions.