Cyclometalated Ruthenium(II) Complexes as Highly Active Transfer Hydrogenation Catalysts

The cyclometalated complex 2 is easily obtained in high yield from the 14-electron complex [RuCl2{(2,6-Me2C6H3)PPh2}2] (1) by reaction with formaldehyde in the presence of triethylamine. The molecular structure of 2 has been determined by an X-ray investigation and reveals a five-coordinate complex with a δ-agostic interaction of one ortho-methyl group. By displacement of one phosphane from 2, a series of new cyclometalated complexes can be obtained. The derivative 4 bearing 2-(aminomethyl)pyridine is a highly active catalyst for hydrogen transfer reactions. Thus, a large number of ketones can be quantitatively converted to alcohols with very high TOF values (up to 63000 h-1).