Spectrophotometric and Voltammetric Characterization of Complexes of Bis(2,2’-bipyridine)(2,2’-bibenzimidazole)ruthenium and -osmium in Oxidation States II, III, and IV in Acetonitrile-Water Mixtures

A combination of spectrophotometric and voltammetric studies has enabled a complete description of the acid-base and redox chemistry of [M(bpy)₂(BiBzImH₂)]²⁺ complexes to be provided in a 50% acetonitrile-water medium in each of the oxidation states II, III, and IV (M = Ru, Os; bpy = 2,2’-bipyridine; BiBzImH₂ = 2,2’-bibenzimidazole), Over the pH range 0–12 nonprotonated, monoprotonated and diprotonated M(II) and M(III) complexes can be identified (protonation is associated with the imidazole ligand) whereas for M(IV) complexes protonation does not occur even at pH 0. Calculated pK_avalues show that the acid strength increases markedly with increase in oxidation state. In the redox sense, osmium(III) and osmium(IV) complexes are more accessible via oxidation of osmium(II) than are their ruthenium analogues. At no pH value in the range 0–12 does a crossover of the order of reversible half-wave potentials for the M(III)/M(II) and M(IV)/M(III) redox couples occur so that stabilization of oxidation state III complexes with respect to disproportionation is achieved. The osmium(III) complexes are readily prepared electrochemically as stable entities in acetonitrile/water mixtures whereas the ruthenium(III) complexes are comparatively reactive. The presence of the imidazole ligand stabilizes the oxidation state IV complexes of ruthenium and osmium so that they can be observed under fast scan rate conditions (>5 V s^-1) with cyclic voltammetry or with short pulse widths (<10 ms) with differential pulse voltammetry at glassy carbon electrodes. With longer time scale voltammetric experiments a chemically irreversible catalytic process is observed. The potential range available at glassy-carbon electrodes, but not at platinum, is adequate to observe the M(III)/M(IV) oxidation process prior to onset of oxidation of water. Comparison with the acid-base and redox behavior of related systems reveals interesting differences. For example the similarity in pH dependence of the two metals leads to the conclusion that both oxidation processes are metal based, in contrast to other studies on [M(bpy)₃]²⁺ where it has been suggested that the second process is metal based for osmium but ligand based for ruthenium.