Systematic Selection of Metalloporphyrin-Based Catalysts for Oxygen Reduction by Modulation of the Donor-Acceptor Intermolecular Hardness

Incisive modulation of the intermolecular hardness between metalloporphyrins and O-2 can lead to the identification of promising catalysts for oxygen reduction. The dependency of the electrocatalytic reduction of O-2 by metalloporphyrins on the nature of the central metal yields a volcano-type curve, which is rationalized to be in accordance with the Sabatier principle by using an approximation of the electrophilicity of the complexes. By using electrochemical and UV/Vis data, the influence of a selection of meso-substituents on the change in the energy for the * excitation of manganese porphyrins was evaluated allowing one to quantitatively correlate the influence of the various ligands on the electrocatalysis of O-2 reduction by the complexes. A manganese porphyrin was identified that electrocatalyzes the reduction of oxygen at low overpotentials without generating hydrogen peroxide. The activity of the complex became remarkably enhanced upon its pyrolysis at 650 degrees C.