COMPLEXES OF CU(II) WITH ELECTROACTIVE CHELATING LIGANDS ADSORBED ON GRAPHITE-ELECTRODES - SURFACE COORDINATION CHEMISTRY AND ELECTROCATALYSIS

Three electroactive chelating ligands, 3-(2-pyridyl)-5,6-diphenyl-1,2,4-triazine,3-(2-pyridyl)-5,6-di-phenyl-p,p'disulfonate-1,2,4-triazine and 2,3-bis(2-pyridyl)pyrazine, were irreversibly adsorbed on pyrolytic graphite electrodes where their voltammetric responses were used to measure the quantities adsorbed. The adsorbed ligands reacted with Cu2+ cations in solution to produce complexes adsorbed on the electrode surface. The stoichiometric compositions of the adsorbed complexes were determined by means of coulometric assays of the quantities of ligand and Cu2+ present on the surface. The complex of Cu2+ with 2,3-bis(2-pyridyl)pyrazine involved only one ligand per Cu2+ center and was catalytically active towards the reduction of O2 and H2O2. The complexes formed with the triazine ligands contained two ligands per Cu2+ center and were not catalytically active. The pH dependencies of the formal potentials of the adsorbed complexes and of the half-wave potentials for the reduction of the substrates provided some insight into the mechanisms of the catalysed reductions.