Catalytic behavior of a μ-oxo dimanganese(III) octaethylporphyrin in O2 reduction

Abstract

The electrochemistry of a µ-oxo dimanganese(III) octaethylporphyrin (II) and those of the corresponding mononuclear complexes MnIII(OEP)Cl (I), MnIII(OEP)ClO4 (III) and MnIII(OEP)OH (IV) (where OEP represents octaethylporphyrin) were studied. When complex II was adsorbed on an electrode and placed in aqueous acidic electrolyte solutions, efficient catalysis of the four-electron reduction of O2 to H2O was accomplished. A control experiment using an electrode modified with the mononuclear manganese(III) complex I resulted in a significant decrease in the selectivity for the four-electron reduction. The hydroxomanganese(III) complex IV was produced in the course of the oxidation of MnII(OEP) to MnIII(OEP)+ with O2 under acidic conditions. Complex I was isolated as single crystals and characterized by X-ray crystallography. Although complex II was not isolated as single crystals, the long manganese-axial ligand distance (2.385 A) in I provided an estimate that a similar geometry would be true for II, allowing accommodation of an O2 molecule between the two manganese atoms in the adsorbed cofacial porphyrins.