Construction of Highly Active Ruthenium(II) NNN Complex Catalysts Bearing a Pyridyl-Supported Pyrazolyl-Imidazolyl Ligand for Transfer Hydrogenation of Ketones

A family of hemilabile ruthenium(II) NNN complexes bearing a unsymmetrical 2-(benzoimidazol-2-yl)-6-(pyrazol-1-yl)pyridine ligand has been synthesized and exhibited good to excellent catalytic activity in transfer hydrogenation of ketones in refluxing 2-propanol, reaching final TOFs up to 7.2 × 105 h−1 with 0.05 mol % loading. The γ-NH effect of the benzoimidazol-2-yl moiety in the ligand and coordination modes of the metal center in a Ru(II) NNN complex has great influence on the catalytic activity of the complex catalyst in transfer hydrogenation of ketones. It has been demonstrated that one of the structural prerequisites for an active Ru(II) complex catalyst is the coordinatively unsaturated environment around the metal center in the complex or the precatalyst, and the catalytic activity of a complex catalyst can be enhanced by making its metal center cationic. This paper presents a methodology to construct new types of efficient Ru(II) complex catalysts for transfer hydrogenation of ketones.