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

Abstract

Reaction of five 2- (arylazo)phenol ligands (abbreviated in general as Hap-R, where H stands for the phenolic proton) with [Os (bpy)₂Br₂] has afforded complexes of type [Os^II (bpy)₂ (ap-R)]⁺, which have been isolated as the perchlorate salts. The complexes are diamagnetic (low-spin d ⁶, S=0) and in acetonitrile solution shows several MLCT transitions in the visible region. Structure of the [Os (bpy)₂ (ap-Me))ClO₄ 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 OsN₅O 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^II (bpy)₂ (ap-R)]ClO₄ complexes affords brownish-yellow [Os^III (bpy)₂ (ap-R)]fn₂ species, which have been isolated as the perchlorate salts. These complexes are one-electron paramagnetic (low-spin d ⁵, S=½) and in acetonitrile solution show LMCT transitions in the visible region. Reduction of the brownish-yellow [Os^III (bpy)₂ (ap-R)] (ClO₄)₂ complexes gives back the respective brown [Os^II (bpy)₂ (ap-R)]ClO₄ complexes.