Applications of transition metal complexes containing 3,3′-bis(diphenylphosphinoamine)-2,2′-bipyridine ligand to transfer hydrogenation of ketones

Abstract

Hydrogen transfer reduction processes are attracting increasing interest from synthetic chemists in view of their operational simplicity. 3,3′-bis(diphenylphosphinoamine)-2,2′-bipyridine, (Ph2PNH)2C10H6N2, was prepared through a single step reaction of 3,3′-diamino-2,2′-bipyridine with diphenylchlorophosphine. Reaction of (Ph2PNH)2C10H6N2 with [Ru(η6-benzene)(μ-Cl)Cl]2, [Rh(μ-Cl)(cod)]2 or [Ir(η5-C5Me5)(μ-Cl)Cl]2 gave a range of new bridged dinuclear complexes [C10H6N2{NHPPh2Ru(η6-benzene)Cl2}2], 1, [C10H6N2{PPh2NHRh(cod)Cl}2], 2 and [C10H6N2{NHPPh2Ir(η5-C5Me5)Cl2}2], 3, respectively. All new complexes have been fully characterized by analytical and spectroscopic methods. 1H31P-{1H} NMR, 1H13C HETCOR or 1H1H COSY correlation experiments were used to confirm the spectral assignments. 1, 2 and 3 are suitable catalyst precursors for the transfer hydrogenation of acetophenone derivatives. Notably [Ru((Ph2PNH)2C10H6N2)(η6-benzene)Cl2], 1 acts as an excellent catalyst, giving the corresponding alcohols in 98–99% yields in 10 min at 82 °C (TOF ≤600 h−1) for the transfer hydrogenation reaction in comparison to analogous rhodium or iridium complexes. This transfer hydrogenation is characterized by low reversibility under these conditions.