Ruthenium Complex Catalysts Supported by a Bis(trifluoromethyl)pyrazolyl-Pyridyl-Based NNN Ligand for Transfer Hydrogenation of Ketones

Ru(III) and Ru(II) complexes bearing a tridentate 2-(3',5'-dimethylpyrazol-1'-yl)-6-(3,5-bis(trifluoromethyl)pyrazol-1-yl)pyridine or 2-(benzimidazol-2'-yl)-6-(3,5-bis(trifluoromethyl)pyrazol-1-yl)-pyridine ligand were synthesized and applied to the transfer hydrogenation of ketones. The Ru(II) complex was structurally confirmed by the X-ray crystallographic analysis and achieved up to 2150 turnover numbers and final TOFs up to 29700 h(-1) in the transfer hydrogenation of ketones. The benzimidazolyl moiety with an unprotected NH functionality in the ligand exhibited an enhancement effect on the catalytic activity of its RuCl3 complex in the ketone reduction reactions, reaching a final TOF value up to 35640 h(-1). The controlled experiments have revealed that the compatibility of the trifluoromethylated pyrazolyl and unprotected benzimidazolyl is crucial for the establishment of the highly active catalytic system.