Ruthenium complexes bearing an unsymmetrical pincer ligand with a 2-hydroxypyridylmethylene fragment: active catalysts for transfer hydrogenation of ketones

Five ruthenium(II) complexes were synthesized, including (HO-C5H3N-CH2-C5H3N-C5H4N)Ru(PPh3)Cl-2 (3), [(HO-C5H3N-CH2-C5H3N-C5H4N) Ru(PPh3)(2)Cl][PF6] (4) and [(HO-C5H3N-CH2-C5H3N-C5H4N) Ru(PPh3)(2)OH][PF6] (5) bearing an unsymmetrical pincer NNN ligand with a 2-hydroxypyridylmethylene fragment, and [(CH3O-C5H3N-CH2-C5H3N-C5H4N)(2)Ru][Cl](2) (6) and [(CH3O-C5H3N-CH2C5H3N- C5H4N)(2)Ru][PF6](2) (7) containing 2-methoxypyridylmethylene moieties. 4 reacts with H2O at room temperature to give 5 whose crystal structure reveals the existence of intramolecular hydrogen-bonding between its two-OH groups. 3 exhibits high catalytic activity for transfer hydrogenation of ketones.