Electrochemical and Electronic Absorption Spectroscopic Studies of Substituent Effects in Iron(IV) and Manganese(IV) Corroles. Do the Compounds Feature High-Valent Metal Centers or Noninnocent Corrole Ligands? Implications for Peroxidase Compound I and II Intermediates

Abstract

We report here an electrochemical and optical spectroscopic study of new Fe(IV) and Mn(IV) meso -triarylcorrole complexes. The complexes studied are three Fe(IV)Cl, three Mn(IV)Cl, and three dimeric Fe(IV)OFe(IV) meso -tris( p -X-phenyl)corrole complexes, where X = CH 3, H, and CF 3 . The first oxidation potentials of the Fe(IV)Cl and Mn(IV)Cl corrole complexes are considerably higher than those of the corresponding Fe(IV) corrole μ -oxo dimers, suggesting that the corrole ligands in the chloride complexes are already oxidized to a radical-like state. This is consistent with the suggestion by Walker and co-workers (ref 12) that the iron center in an (octaalkylcorrolato)Fe IV Cl complex is best described as intermediate spin ( S = 3/2) and that it is antiferromagnetically coupled to a corrole π-radical. We have attempted to clarify the nature of this antiferromagnetic coupling by means of DFT calculations and propose that it results from an metal(d z 2 )-corrole(“b 1 ”) orbital interaction. In contrast, the corrole ligand in the Fe(IV) corrole μ -oxo dimers does not seem to have radical character. The optical spectra of the Fe(IV)Cl and Mn(IV)Cl corrole derivatives exhibit distinctive split Soret bands, one arm of which is strongly substituent sensitive. This behavior contrasts with that of free-base corroles and porphyrins and of typical metalloporphyrins whose optical spectra are relatively substituent-insensitive. We qualitatively assign this substituent-sensitive feature to a transition with significant ligand-to-metal charge-transfer character.