Entropy contribution to the relative solution stability of copper(III) and nickel(III) tetraazamacrocyclic complexes in aqueous perchloric acid

Abstract

Solutions containing equimolecular amounts of [Ni^II(cyclam)][ClO₄]₂ and [Cu^II(cyclam)][ClO₄]₂(cyclam = 1,4,8,11-tetraazacyclotetradecane) of varying concentration of HClO₄(4–11.56 mol dm^–3) were investigated through voltammetric techniques and the potentials associated with the Ni^II–Ni^III and Cu^II–Cu^III redox changes determined. The potential difference, E_Cu–E_Ni, which is related to the metathetic equilibrium [Cu^IIIL]³⁺+[Ni^IIL]²⁺⇌[Cu^IIL]²⁺+[Ni^IIIL]³⁺, becomes less positive with increasing HClO₄ concentration, indicating a stabilizing effect on the copper(III) complex, compared to Ni^III. This effects is related to the competition by the ions of the electrolyte for the water molecules co-ordinated to the electroactive species. The entropy change ΔS^⊖ associated with the metathetic equilibrium was determined through voltammetric investigations performed at varying temperatures (–15 to +40°C). On the basis of ΔS^⊖ values, it is suggested that, below an HClO₄ concentration of 8 mol dm^–3, water molecules occupy the available co-ordination sites in the electroactive metal complexes; at a higher concentration the solvent molecules are replaced by ClO₄^– anions.