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.