Synthesis, structure and catalytic activity of cycloruthenated carbonyl complexes containing arylazo phenolate ligands

A simple and convenient synthesis of a new class of air-stable mononuclear cyclometallated ruthenium(II) carbonyl 2-(arylazo)phenolate complexes bearing triphenylarsine [Ru(ap-R)(AsPh3)2(CO)] has been described. The 2-(arylazo)phenolate ligands behave as dianionic tridentate ligands and are coordinated to ruthenium through C, N and O by dissociation of the phenolic proton and the phenyl proton at the ortho position of the phenyl ring forming two five-membered chelate rings. These complexes have been characterized by elemental analysis, and FT-IR, 1H NMR and UV–vis spectroscopies. In dichloromethane solution all the metal complexes exhibit characteristic metal-to-ligand charge transfer (MLCT) absorption and emission bands in the visible region. One of the complexes [Ru(ap-Cl)(AsPh3)2(CO)] was successfully characterized by X-ray crystallography. Cyclic voltammetric data of all the complexes show RuIII/RuII oxidation and RuII/RuI reduction within the range of +0.75 to +0.86 V and −0.50 to −0.57 V respect to Ag/AgCl, respectively. The potentials are observed with respect to the electronic nature of substituents (R) in the 2-(arylazo)phenolate ligands. Further, a complex (2) was tested as a new catalyst in the oxidation of primary and secondary alcohols in the presence of NMO as a more viable oxidant with moderate to high conversion. The formation of high valent RuIV = O species as a catalytic intermediate is proposed for the catalytic process.

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