OXIDATION OF CHROMIUM(III) PORPHYRINS TO THEIR PI-RADICAL CATIONS OR TO OXOCHROMIUM(IV) PORPHYRINS

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 lambda greater-than-or-equal-to 600 nm, ascribed to the pi-radical cations, and others exhibiting minor shifts of the porphyrin bands, ascribed to chromium(IV) porphyrins. Cr(III)OEP (octaethylporphyrin), Cr(III)TPP (tetraphenylporphyrin), and Cr(III)TMP (tetramesitylporphyrin) in CH2Cl2 undergo one-electron oxidation on the ligand to give pi-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(III)TSPP [tetrakis (4-sulfonatophenyl)porphyrin], Cr(III)T3PyP [tetrakis(3-pyridyl)porphyrin], and Cr(III)MSP (mesoporphyrin-IX), oxidized in 1 mol dm-3 aqueous HCl, form unstable pi-radical cations which decay by disproportionation. The pulse radiolysis results indicate that the initial step in the oxidation of all Cr(III)P and O-Cr(IV)P species occurs at the porphyrin pi-system and that, under certain conditions, the initial pi-radical cation may undergo intramolecular electron transfer from the metal centre to the ligand to form the higher oxidation state Cr-porphyrin.