Electrochemically driven reversible solid state metal exchange processes in polynuclear copper complexes

The electrochemical characteristics of polynuclear di-copper and tetra-copper complexes of an expanded Robson-type macrocyclic ligand are explored by solid, state voltammetry in aqueous media. When adhered to a graphite electrode surface in the form of microcrystalline powders and immersed in aqueous buffer solution, these water-insoluble polynuclear copper complexes show well-defined voltammetric reduction and re-oxidation responses. The di-copper metal complexes [Cu-2(H3L)(OH)][BF4](2) and the tetra-copper complexes [Cu-4(L)(OH)][NO3](3) with an O4N4 octadentate macrocyclic ligand L are shown to exhibit interrelated and proton concentration sensitive solid state voltammetric characteristics. At sufficiently negative potential, copper is extracted from the complexes to form a solid copper deposit and the neutral form of the insoluble free ligand. Upon re-oxidation of the copper deposit, Cu2+ undergoes facile re-insertion into the ligand sphere to re-form solid di- and tetra-copper complexes at the electrode surface. The reduction process occurs in two stages, with two Cu2+ cations being extracted in each step. The ability of the macrocyclic ligand to efficiently release and accumulate copper is demonstrated.