Monomolecular films of a nickel(II) pentaazamacrocyclic complex for the electrocatalytic oxidation of hydrogen peroxide at gold electrodes

Self-assembled monolayers (SAMs) of a redox active nickel(II) pentaazamacrocyclic complex 1 and mixed monolayers of 1 with ethyl disulfide have been fabricated on a gold electrode, and their electrochemical behavior has been studied by cyclic voltammetry in aqueous solutions of Na2SO4 and NaNO3. The results demonstrate that the redox behavior as well as the electrocatalytic activity towards the oxidation of H2O2 of the SAM and mixed monolayers of 1 largely depend on the electrolyte anions, i.e. SO42- and NO3-: the formal potential in the SO42- electrolyte is about 220 mV less positive than that in the NO3- electrolyte, and the SAM and mixed monolayers of 1 possess an efficient electrocatalysis for the oxidation of H2O2 in the NO3- electrolyte, but not in the SO42- electrolyte. In addition, a unique cyclic voltammogram with a sharp peak of inverted 'V' shape has been observed in the cathodic scan for the electrocatalytic oxidation of H2O2, largely depending on the concentration of the NO3- electrolyte anion and the solution pH. These electrolyte anion-dependent redox behaviors have been discussed on the basis of different coordinating tendencies of SO42- and NO3- to the nickel(III) centre of the complex and a possible reaction mechanism for the observed electrocatalytic reaction. (C) 1998 Elsevier Science S.A. All rights reserved.