Efficient and recyclable Ru(II) arene thioamide catalysts for transfer hydrogenation of ketones: Influence of substituent on catalytic outcome

Six cationic ruthenium(II) arene thioamide complexes with the general molecular formula [Ru(eta(6)-p-cymene)(PPh3)(L)](+) [where, L = pyridine-2-thioamide and its derivatives] have been successfully synthesized from the reaction of [Ru(eta 6-p-cymene)Cl-2](2) with chelating thioamide ligands and PPh3 in methanol in 1:2 M ratio respectively. All the complexes were isolated as their BPh4- salts and were fully characterized by analytical and spectral (FT-IR, UV-Vis and H-1-NMR) methods. The solid-state structure of one of the complexes, [Ru(eta(6)-p-cymene)(PPh3)(L4)]BPh4 (4) (L4 = N-(2,4,6-Trimethylphenyl)pyridine-2-thiocarboxamide) has been established by X-ray single crystal diffraction which indicates a pseudooctahedral (piano-stool) coordination geometry is present in the complex. The ruthenium(II) complexes have been examined for the transfer hydrogenation of various aromatic, heterocycle and cyclic ketones. The formation of ruthenium(II) hydride is confirmed by H-1-NMR and is proposed as the catalytic intermediate in this reaction. Under the optimized conditions, these ruthenium complexes served as excellent catalyst precursors which smoothly reduce the ketones with conversion up to 100%. The influence of other variables on the transfer hydrogenation reaction such as solvent, base, temperature, time, catalyst loading and substrate scope is also reported. Furthermore, the catalyst could be easily recovered and reused at least three times without obvious loss of conversions. (C) 2016 Elsevier B.V. All rights reserved.