Chemistry of 2-(arylazo)phenolate complexes of ruthenium. Synthesis, structure and reactivities

A group of six ruthenium(III) complexes of type [Ru(acac)(L)(2)], where acac=acetylacetonate anion and L=2-(arylazo)-4-methylphenolate anion or 1-(phenylazo)-2-naphtholate anion, have been synthesized and characterized. Structural characterization of a representative complex, where L=1-(phenylazo)-2-naphtholate anion, shows that the azophenolate ligands are coordinated as N,O-donor ligands forming six-membered chelate rings. The complexes are paramagnetic (low-spin d(5), S=1/2) and show rhombic ESR spectra in 1:1 dichloromethane-toluene solution at 77 K. In carbon tetrachloride solution, these complexes show intense LMCT transitions in the visible region together with weak ligand-field transitions in the near-IR region. All the complexes display two cyclic voltammetric responses, a ruthenium(III)-ruthenium(IV) oxidation in the range of 0.83 to 1.03V vs SCE and a ruthenium(III)ruthenium(II) reduction in the range of -0.24 to -0.52 V vs SCE. Formal potentials of both the couples correlate linearly with the Hammett constant of the para substituent in the arylazo fragment of the 2-(arylazo)-4-methylphenolate ligand. The ruthenimn(IV) and ruthenium(II) congeners of the [Ru-III(acac)(L)(2)] complexes have been generated by chemical or electrochemical methods and they have been characterized by electronic spectroscopy and cyclic voltammetry. (C) 1999 Elsevier Science Ltd. All rights reserved.