Axial Ligand Effects on the Redox Reactions of Manganese Porphyrins

A systematic study for the effect of axially coordinated monovalent anions on the electrode reactions of several manganese porphyrins in acetonitrile is presented. Potential shifts of the metal-centered reduction with changes in counterion were related to the degree of Mn(III)-counterion interaction. In the electrochemically induced ligand exchange, perchlorate anion replaces the other anions as axial ligand coordinated to Mn(III) at oxidation potential less than the first oxidation of manganese porphyrins. Formation constants for axial ligation of OH^? are calculated. One-electron oxidation of dihydroxide coordinated manganese porphyrins generate oxomanganese(IV) porphyrin complexes electrochemically. O=Mn^IVOEP(OH) is more thermodynamically stable than O=Mn^IVTPP(OH), while O=Mn^IVTpFPP(OH) cannot be generated electrochemically. In the presence of styrene or cyclohexene, the absorption spectra of oxomanganese(IV) porphyrins are changed to form manganese(III) porphyrins gradually, which indicates the oxygen atom transfer from oxomanganese(IV) porphyrins to the substrates.