(Pyridyl)benzoazole ruthenium(ii) and ruthenium(iii) complexes: role of heteroatom and ancillary phosphine ligand in the transfer hydrogenation of ketones

Abstract

The synthesis and structural characterization of ruthenium complexes supported by 2-(2-pyridyl)benzoazole ligands and their evaluation as catalysts in the transfer hydrogenation of ketones are reported. Reactions of 2-(2-pyridyl)benzoimidazole (L1), 2-(2-pyridyl)benzothiazole (L2) and 2-(2-pyridyl)benzoxazole (L3) with RuCl₃·3H₂O produced the corresponding complexes [RuCl₃(L1)] (1), [RuCl₃(L2)] (2) and [RuCl₃(L3)] (3), respectively. Similarly, treatment of L1–L3 with RuCl₂(PPh₃)₂ afforded the corresponding Ru(II) complexes [RuCl₂(PPh₃)₂(L1)] (4), [RuCl₂(PPh₃)₂(L2)] (5) and [RuCl₂(PPh₃)₂(L3)] (6), respectively. Solid state structures of 1 and 2 confirmed the bidentate coordination mode of L1 and L2 to ruthenium. ³¹P{¹H} NMR spectroscopy revealed coordination of two PPh₃ ligands trans to each other in an octahedral environment in 4–6 as confirmed by the solid state structure of 6. Complexes 1–6 produced active catalysts in the transfer hydrogenation of ketones in 2-propanol at 82 °C. Ruthenium(II) complexes 4–6, containing the PPh₃ ligand, exhibited higher catalytic activities than the corresponding ruthenium(III) compounds 1–3. Complexes 1 and 4 of L1 were more active than the corresponding complexes of L2 and L3. Density functional theoretical calculations showed that dipole moments of 1–6 control their catalytic activities.