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

The electrochemistry of a μ-oxo dimanganese(III) octaethylporphyrin (II) and those of the corresponding mononuclear complexes Mn^III(OEP)Cl (I), Mn^III(OEP)ClO₄ (III) and Mn^III(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 Mn^II(OEP) to Mn^III(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 Å) 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.