Arene-ruthenium complexes with 2-(arylazo)phenol as ancillary ligand: Synthesis, characterization, and utilization in catalytic transfer-hydrogenation

Abstract

Reaction of 2-(arylazo)phenols (HL-R, where H represents the phenolic proton and, R = CH3, H and Cl) with [{Ru(p-cymene)Cl2}2] in the presence of triethylamine affords a group of three reddish-brown complexes of type [Ru(p-cymene)(L-R)Cl] in good yields. Structure of [Ru(p-cymene)(L-CH3)Cl] has been determined by X-ray crystallography. The 2-(arylazo)phenolate ligand (L-R) is coordinated to the metal center as a mono-anionic bidentate N,O-donor forming five-membered chelate ring. All the complexes are diamagnetic, and show characteristic 1H NMR signals. They also show intense absorptions in the visible and ultraviolet regions, which have been analyzed by TDDFT calculations. Cyclic voltammetry on the complexes shows two successive irreversible oxidations within 1.10–1.40 V versus SCE. The [Ru(p-cymene)(L-R)Cl] complexes are found to serve as efficient catalyst-precursor for the transfer-hydrogenation of aldehydes.