Asymmetric mixed-valence binuclear ruthenium complexes containing benzotriazolate or benzimidazolate bridging ligands

Asymmetric binuclear ruthenium complexes of the type [(NH3)5Ru–L–Ru(edta)]n (where L=benzotriazolate, benzimidazolate; edta=ethylenediaminetetraacetate; n=1, 0, −1) have been prepared and investigated by means of electrochemical and spectroelectrochemical methods. Special emphasis has been given to the intramolecular electron transfer phenomena. The corresponding mixed-valence species display contrasting electronic properties: while the benzimidazolate derivative behaves as a moderately coupled system, the analogous benzotriazolate exhibits a rather large electronic coupling and significant delocalization, as deduced from their quite different intervalence transfer band profiles, in addition to their distinct redox and metal-to-ligand and ligand-to-metal charge transfer behavior. The special behavior of the benzotriazolate species has been attributed to the π-acid and the fluxional coordination properties of the ligand, improving the geometry approximation in the mixed-valent species. In the attempt to exploit the limits of Hushʹs theory, a set of elucidative electrochemical and spectroelectrochemical measurements for the (NH3)5RuII(bta)RuIII(edta) complex have been carried out in organic solvents, confirming its borderline class II/class III nature in the Robin–Day categorization. Efforts have also been directed to the evaluation of electronic and thermodynamic parameters for unsymmetrical systems.