Chemistry of osmium phenolates. Synthesis, structure and redox properties

Abstract

The reaction of three phenolic ligands (viz. salicylaldehyde, Hsal; 2-hydroxyacetophenone, Hacp and 2-hydroxynaphthaldehyde, Hnap; generally abbreviated as HL, where H stands for the phenolic proton) with [Os(bpy)₂Br₂] has afforded complexes of type [Os(bpy)₂(L)]⁺, which have been isolated as the perchlorate salts. The complexes are diamagnetic (low-spin d⁶, S=O and in acetonitrile solution shows several MLCT transitions in the visible region. Structure of the [Os^II(bpy)₂(sal)]ClO₄ complex has been determined by X-ray crystallography. The salicylaldehyde anion is coordinated to osmium as a bidentate O,O-donor and the OsN₄O₂ coordination sphere is distorted octahedral. Cyclic voltammetry shows a reversible osmium(II)-osmonium(III) oxidation in the range of 0.17-0.25 V vs SCE followed by an irreversible osmium(III)-osmium(IV) oxidation in the range of 1.38-1.48 V vs SCE. Three one-electron reductions of the coordinated bpy ligands are also observed on the negative side of SCE (-1.49- -2.08 V). Chemical or electrochemical oxidation of the [Os^II(bpy)₂(L)]ClO₄ complex affords green [Os^III(bpy)₂(L)]²⁺ species, which have been isolated as the perchlorate salts. These complexes are one-electron paramagnetic (low-spin d⁵, S = 0) and in acetonitrile solution show LMCT transitions in the visible region. Reduction of the green [Os^III(bpy)₂(L)](ClO₄)₂ complexes gives back the respective brown [Os^II(bpy)₂(L)]ClO₄ complexes.