Ruthenium(II) Complex of a Tridentate Azoaromatic Pincer Ligand and its Use in Catalytic Transfer Hydrogenation of Aldehydes and Ketones with Isopropanol

Abstract

In this work, a new Ru(II) complex with the redox‐active pincer 2,6‐bis(phenylazo)pyridine ligand (L) is reported which acts as a metal‐ligand bifunctional catalyst for transfer hydrogenation reactions. The isolated complex [(L)Ru(PMe2Ph)2(CH3CN)](ClO4)2; [1](ClO4)2 is characterized by a host of spectroscopic measurements and X‐ray structure determination. It is diamagnetic and single‐crystal X‐ray structure analysis reveals that [1]2+ adopts a distorted octahedral geometry where L binds Ru center in meridional fashion. The observed elongation in the coordinated azo bond length (1.29 Å) is attributed to the extensive π‐back bonding, dπ(RuII)→π*(azo)L. The complex [1](ClO4)2 acts as an efficient catalyst, which brings about catalytic transfer hydrogenation reactions of a broad array of aldehydes and ketones in isopropanol and in inert conditions. The selectivity of the catalyst for aldehyde reduction over the other reducible functional groups such as nitro, nitrile, ester etc was also investigated. Mechanistic studies, examined by suitable control reactions and isotope labelling experiments, indicate synergistic participation of both ligand and metal centres via the formation of a fleeting Ru−H intermediate and hydrogen walking to the coordinated azo function of L.