Catalysis activity and chemoselectivity control with the trans ligand in Ru-H pincer complexes.

(PhPNHP)Ru(H)(Cl)(CO) serves as a precatalyst to a variety of important catalytic transformations but most improvements have been restricted to the replacement of the CO ligand cis to the hydride or changing the Ph groups of the pincer for other aryl or alkyl groups. The ligand trans to the hydride is often another hydride and studies that utilize other trans ligands in catalysis are limited. In this work, we synthesized a series of [(PhPNHP)Ru(H)(CO)(L)][BPh4] complexes bearing isonitrile, PMe3, or a N-heterocyclic ligand trans to the Ru-H. We compared the new complexes abilities to catalyze the transfer hydrogenation of ketones. We found that all the trans ligands improved the chemoselectivity and stability of the catalysts; and strong π-accepting ligands resulted in poor catalytic activities whereas strong σ-donating ligands accelerated the catalysis.