Ligand Effects on the Overpotential for Dioxygen Reduction by Tris(2-pyridylmethyl)amine Derivatives

A series of copper complexes based on the tris(2-pyridylmethyl)amine (TPA) ligand are examined for their oxygen reduction reaction (ORR) activity. Increasing the potential of the Cu-I/II couple from 0.23 V vs RHE for [Cu(TPA)(L)](2+) to 0.52 V for [Cu(TEPA)(L)](2+) (tris(2-pyridylethyl)amine) at pH 7 or adding a hydrogen-bonding secondary coordination sphere does not increase the onset potential from 0.69 V vs RHE for the ORR. The underlying mechanism for the ORR is determined to be first-order in O-2 and second-order in Cu. The rate-determining step is found to not be Cu-II to Cu-I reduction, as seen in other systems. The rate-determining step is also not the protonation of an intermediate, but may be the reduction of a hydroperoxo intermediate. Pyrolysis of the Cu complex of TPA affords an inactive material; activity is recovered through addition of intact TPA to the electrode surface.