Chemistry of 2-(arylazo) phenolate complexes of osmium. Synthesis, structure and redox properties

Reaction of five 2- (arylazo)phenol ligands (abbreviated in general as Hap-R, where H stands for the phenolic proton) with [Os (bpy)<SUB>2</SUB>Br<SUB>2</SUB>] has afforded complexes of type [Os<SUP>II</SUP> (bpy)<SUB>2</SUB> (ap-R)]<SUP>+</SUP>, which have been isolated as the perchlorate salts. The complexes are diamagnetic (low-spin d <SUP>6</SUP>, S=0) and in acetonitrile solution shows several MLCT transitions in the visible region. Structure of the [Os (bpy)<SUB>2</SUB> (ap-Me))ClO<SUB>4</SUB> complex has been determined by X-ray crystallography. The 2- (arylazo)phenolate anion is coordinated to osmium as a bidentate N,O-donor forming a five-membered chelate ring and the OsN<SUB>5</SUB>O coordination sphere is distorted octahedral. Cyclic voltammetry shows a reversible osmium (II)-osmium (III) oxidation in the range of 0.37-0.51 V vs SCE followed by an irreversible osmium (III)-osmium (IV) oxidation in the range of 1.36-1.50 V vs SCE. These oxidation potentials are sensitive to the electronic nature of the substituent R in the 2- (arylazo)phenolate ligands. Three one-electron reductions of the coordinated bpy ligands are also displayed on the negative side of SCE below -1.0 V. Chemical or electrochemical oxidation of the [Os<SUP>II </SUP>(bpy)<SUB>2 </SUB>(ap-R)]ClO<SUB>4</SUB> complexes affords brownish-yellow [Os<SUP>III </SUP>(bpy)<SUB>2</SUB> (ap-R)]fn<SUB>2</SUB> species, which have been isolated as the perchlorate salts. These complexes are one-electron paramagnetic (low-spin d <SUP>5</SUP>, S=&#189;) and in acetonitrile solution show LMCT transitions in the visible region. Reduction of the brownish-yellow [Os<SUP>III</SUP> (bpy)<SUB>2</SUB> (ap-R)] (ClO<SUB>4</SUB>)<SUB>2</SUB> complexes gives back the respective brown [Os<SUP>II</SUP> (bpy)<SUB>2</SUB> (ap-R)]ClO<SUB>4</SUB> complexes