Chemistry of 2-(arylazo)phenolate complexes of ruthenium. Synthesis, characterization and redox properties

A family of five 2-(arylazo)phenolate (ap-R) complexes of ruthenium of type [RuII(bpy)(ap-R)<SUB>2</SUB>] (bpy = 2,2&#8242;-bipyridine) has been synthesized and characterized. The complexes are diamagnetic (low-spin d<SUP>6</SUP>, S = 0) and in acetonitrile solution show intense MLCT transitions in the visible region. Cyclic voltammetry of the complexes in acetonitrile solution shows a reversible ruthenium (II)-ruthenium (III) oxidation in the range 0.04-0.26 V vs SCE followed by an irreversible ruthenium(III)-ruthenium(IV) oxidation in the range 1.18-1.32 V vs SCE. The potential of these two oxidations is sensitive to the nature of substituent R in the apR ligand. A one-electron reduction of the coordinated bpy is also observed near -1.6 V vs SCE. The five [Ru<SUP>III</SUP>(bpy)(ap-R)<SUB>2</SUB>]<SUP>+</SUP> complexes have been synthesized by chemical oxidation of their respective ruthenium(II) precursors by iodine and isolated as the iodide salts. These oxidized complexes are one-electron paramagnetic low-spin, d<SUP>5</SUP>, S = &#189;) and show rhombic ESR spectra at 77 K. In acetonitrile solution they show intense LMCT transitions in the visible region together with weak ligand-field transitions at lower energies. Chemical reduction of these ruthenium(III) complexes by hydrazine gives back the parent ruthenium(II) complexes