Electrochemical Synthesis and Characterization of the Single-Electron Oxidation Products of Ferric Porphyrins

Article abstract of DOI:10.1021/ja00386a031

High-spin iron(III) porphyrins are oxidized by electrochemical and selected chemical methods.Both monomeric chloro complexes and μ-oxo dimeric species have been examined.Products are isolated as perchlorate salts in analytically pure form in favorable cases.Contrary to previous interpretations, the singly oxidized species are better formulated as iron(III) porphyrin ?-cation radicals rather than as iron(IV) complexes.This finding is based on electrochemical measurements, electronic absorption spectra, NMR spectra, infrared spectra, and Moessbauer measurements.Oxidation potentials for diverse anionic complexes of iron(III) tetraphenylporphyrin are essentially constant at 1.10+/-0.02 V (SCE), suggestive of porphyrin-centered rather than metal-centered oxidation.Electronic absorption spectra exhibit broad bands at long wavelength, not unlike those for known ? radicals.Unusually large NMR paramagnetic shifts for phenyl protons of oxidized iron tetraphenylporphyrin derivatives are rationalized by proton-radical couplings previously measured for zinc a_2u porphyrin radicals.The oxidized iron octaethylporphyrin complex is better described as an a_1u species.Efficient relaxation of spin by the paramagnetic iron center permits well-resolved NMR spectra.Infrared spectra serve to demonstrate integrity of the oxo linkage in oxidized μ-oxo dimers.A new absorption band near 1280 cm^-1 is seemingly diagnostic of porphyrin-centered oxidation of tetraphenylporphyrin metal complexes.Moessbauer spectra reveal little perturbation of charge at the iron center associated with oxidation.Isomer shift values near 0.4 mm/s (iron metal) resemble those for parent iron(III) complexes.Solution magnetic moment measurements yield an unusual 5.5-μ_B value for oxidized iron tetraphenylporphyrin and iron octaethylporphyrin derivatives.Absence of metal-centered oxidation for weak-field anionic complexes indirectly supports the necessity of an oxo or hydroxo ligand in iron(IV) states of hemoproteins.