Oxidation of chromium(III) porphyrins to their π-radical cations or to oxochromium(IV) porphyrins

Abstract

One-electron oxidation of several chromium(III) porphyrins has been studied by steady-state and pulse radiolysis techniques under different conditions. Two types of products are observed: those exhibiting intense broad absorptions at λ 600 nm, ascribed to the π-radical cations, and others exhibiting minor shifts of the porphyrin bands, ascribed to chromium(IV) porphyrins. Cr^IIIOEP (octaethylporphyrin), Cr^IIITPP (tetraphenylporphyrin), and Cr^IIITMP (tetramesitylporphyrin) in CH₂Cl₂ undergo one-electron oxidation on the ligand to give π-radical cations, which are stable after ligation, and further oxidation gives the dications. In the presence of KOH, however, they form Cr^IV-porphyrins and further radiolysis gives Cr^v-porphyrins. Cr^IIITSPP [tetrakis (4-sulfonatophenyl)porphyrin], Cr^IIIT3PyP [tetrakis(3-pyridyl)porphyrin], and Cr^IIIMSP (mesoporphyrin-IX), oxidized in 1 mol dm^–3 aqueous HCl, form unstable π-radical cations which decay by disproportionation. The pulse radiolysis results indicate that the initial step in the oxidation of all Cr^IIIP and OCr^IVP species occurs at the porphyrin π-system and that, under certain conditions, the initial π-radical cation may undergo intramolecular electron transfer from the metal centre to the ligand to form the higher oxidation state Cr-porphyrin.