Chemistry of osmium phenolates. Synthesis, structure and redox properties

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)(2)Br-2] has afforded complexes of type [Os(bpy)(2)(L)](+), 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-II(bpy)(2)(sal)]ClO4 complex has been determined by X-ray crystallography. The salicylaldehydate 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)-osmium(III) oxidation in the range of 0.17-0.25 V vs SCE follwed 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 d(5), S=1/2) 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. (C) 1998 Elsevier Science Ltd. All rights reserved.