An electrochemical study of hydroxide-and methoxide-ligated iron(II) tetraphenylporphyrins possessing ortho-phenyl substituents that block ,L-oxo dimer formation has been carried out. Ligation by these strongly basic oxyanions promotes the formation of iron(IV)-oxo porphyrins upon one-electron oxidation. Further one-electron oxidation of the latter provides the iron(IV)-oxo porphyrin ir-cation radical. (6), Mossbauer (7), ESR (8), and electron nuclear double resonance (ENDOR) (9) spectral data, compound I has been assigned the structure of a low-spin oxo-ligated iron(IV) porphyrin ir-cation radical. This same entity is suggested to serve as the oxygen transfer agent formed at the active site of the cytochrome P-450 enzymes (10, 11). One-electron reduction of the ir-cation radical of compound I results in the formation of an iron(IV)-oxo species termed compound 11 (12). Interestingly, compounds I and II of the peroxidases exhibit almost identical redox potentials (13).Only a few well-characterized model analogs of compounds I and II exist. These iron(IV) porphyrins have all been generated chemically at low temperatures. Until now, an iron(IV)-oxo porphyrin species has not been generated electrochemically. In the cyclic voltammogram of simple iron porphyrins, anodic sweeps beyond the iron(II)/iron(III) couple yield two reversible one-electron oxidation waves (32). Originally it was proposed that the first wave pertained to the iron(III)/iron(IV) couple and the second wave was due to porphyrin 1r-cation radical formation (14,15). This was shown to be incorrect in a report (16) which demonstrated that the redox potential of the first oxidation was independent of the many axial ligands employed (16) and that the physical properties of the oxidized species were consistent with an iron(III) porphyrin ir-cation radical structure (17). The second wave was attributed to an iron(III) porphyrin dication.In the literature, there is no mention of electrochemical investigations involving HO-, CH30-, etc. ligated iron porphyrins. This does not of course indicate an oversight (17) but simply reflects the instability, due to ,u-oxo dimer formation, of oxo-ligated iron(III) porphyrins. The inability to electrochemically generate a compound I or compound II analog could be due to the importance of oxo ligation to the stabilization of high-valent iron porphyrin species. It is well established that metals in their higher-valent oxidation states are generally stabilized by oxo ligation (18). Using sterically hindered porphyrins that are capable of hydroxide ligation, we have now been able to observe the electrochemical generation ofiron(IV)-oxo porphyrin species. In addition, the similarity of the iron(IV)-oxo and the porphyrin ir-cation redox potentials provides a rationale for the redox potentials of compounds I and II of the peroxidases. In the present study we describe the electrochemistry of porphyrins 1 to 5 when ligated to Cl-, HO-, and CH30 and discuss these results as they relate to studies involving the chemical ge...