Manganese Porphyrin Heterodimers and -trimers in Aqueous Solution

Noncovalent face-to-face heterodimers and -trimers between β-tetracationic and meso-tetraanionic manganese(III) porphyrins have been prepared in bulk water at pH 12. They are held together by Coulomb interactions between four β-methylpyridinium and meso-phenylsulfonate or meso-phenylcarboxylate ion pairs in eclipsed orientations. Spectroelectrochemistry has been used to characterize the redox products and to establish reversibility. UV-visible titrations indicate quantitative trimerization at concentrations > 10^-5 M. Cyclic voltammetry shows that all three Mn(III) ions were oxidized simultaneously to Mn(IV) at potentials close to 300 mV at pH 12. Electroreduction to Mn(II) was often not observed in the trimers, although the monomers reacted readily under the same conditions. Quantitative chemical reduction of Mn(III) to Mn(II) porphyrin trimers was, however, achieved with dithionite. Trimers containing three paramagnetic Mn(II) or Mn(IV) ions are thus easily accessible. The heterodimers and -trimers and homodimers also catalyzed the formation of dioxygen by electrooxidation of Mn(III) to Mn(IV) between 0.6 and 2.0 V while at pH 12.