Valence-tautomerism in high-valent iron and manganese porphyrins

Iron and manganese hemes are "high-valent" when the valence state of the me tal exceeds III. Redox chemistry of the high valent metal complexes involve s redistribution of holes and electrons over the metal ion and the porphyri n and axial ligands, defined as valence tautomerism. Thus, catalytic pathwa ys of heme-containing biomolecules such as peroxidases, catalases and cytoc hromes P450 involve valence tautomerism, as do pathways of biomimetic oxyge n transfer catalysis by manganese porphyrins, robust catalysts with potenti al commercial value. Determinants of the site of electron abstraction are k ey to understanding valence tautomerism. In model systems, metal-centered o xidation is supported by hard anionic axial ligands that are also strongly pi -donating, such as oxo, aryl, bix-methoxy and bis-fluoro groups. Mangane se(IV) is more stable than iron(IV) and metal-centered one-electron oxidati ons occur with weaker pi -donating axial ligands such as bisazido, -isocyan ato, -hypochlorito and bis chloro groups. Virtually all known high-valent i ron porphyrin complexes oxidized by two-electrons above the ferric state ar e coordinated by the strongly pi -donating oxo or nitrido ligands. In all w ell-characterized oxo complexes, iron is in the ferryl state and the second oxidizing equivalent resides on the porphyrin. Complexes with iron(V) have not been definitively characterized. One-electron oxidation of oxomanganes e(IV) porphyrin complexes gives the oxomanganese(IV) porphyrin pi -cation r edicals. In aqueous solution, oxidation of Mn(III) complexes of tetra catio nic N-methylpyridiniumylporphyrin isomers by monooxygen donors yields a tra nsient oxomanganese(V) species.