Carboxamide carbonyl-ruthenium(ii) complexes: detailed structural and mechanistic studies in the transfer hydrogenation of ketones

Abstract

Reactions of N-(benzo[d]thiazol-2-yl)pyrazine-2-carboxamide (HL1) and N-(1H-benzo[d]imidazol-2-yl)pyrazine-2-carboxamide (HL2) ligands with Ru(PPh₃)₃ClH(CO) and Ru(PPh₃)₃(CO)₂H₂ precursors afforded the respective organo-carboxamide ruthenium(II) complexes [Ru^(II)(CO)Cl(PPh₃)₂L1] (Ru1), [Ru^(II)(CO)H(PPh₃)₂L1] (Ru2), [Ru^(II)(CO)Cl(PPh₃)₂L2] (Ru3), and [Ru^(II)(CO)H(PPh₃)₂L2] (Ru4). The Ru(II) complexes were characterised by NMR, FT-IR spectroscopies, mass spectrometry, micro-analyses, and single X-ray crystallography. The solid-state structures of complexes Ru1, Ru2, and Ru4 confirm distorted octahedral geometries around the Ru(II) atoms, containing one bidentate anionic carboxamidate ligand and four auxiliary ligands (PPh₃/CO/H/Cl). All the complexes (Ru1–Ru4) displayed moderate catalytic activities in the transfer hydrogenation of a broad spectrum of ketones, giving a maximum turnover number (TON) of 990 within 6 h. The catalytic activities of the Ru(II) complexes were dependent on both the carboxamidate and auxiliary ligands. ³¹P{¹H) NMR spectroscopy studies aided in proposing a monohydride pathway for the transfer hydrogenation reaction of ketones.