Cationic half-sandwich ruthenium (II) complexes ligated by pyridyl-triazole ligands: Transfer hydrogenation and mechanistic studies

Abstract

A series of novel cationic ruthenium half-sandwich complexes, bearing 1-substituted-4-pyridyl-1H-1,2,3-triazole ligands, were synthesized and fully characterized by a range of analytical techniques. The complexes were found to be efficient catalyst precursors for transfer hydrogenation reactions using ketones as substrates. We demonstrated that the complexes could hydrogenate acetophenone in excellent conversions (∼75 %) within 3 h, employing a low concentration of base of only 2 mol %. Extending the reaction time to 6 h gave near quantitative conversions for both catalysts employed. In addition to this, the transfer hydrogenation of acetophenone was also found to proceed even at low catalyst loadings (0.025–0.05 mol%) and low base concentrations (0.25–1.0 mol%). Under these conditions substrate conversion was moderate (22–60 %). The catalytic efficiency of the most effective catalyst was then evaluated in the transfer hydrogenation of a small library of aromatic and aliphatic ketones. It was shown that more challenging substrates such as benzophenone and 2-octanone could be hydrogenated to the corresponding secondary alcohols in conversions > 90 %. Finally, based on several experimental observations, a possible mechanism for the process is proposed.