Redox Chemistry of Metalloporphyrins in Aqueous Solution

Article abstract of DOI:10.1021/j150642a038

A series of water-soluble metalloporphyrins has been prepared and the redox chemistry investigated by electrochemical and pulse radiolytic techniques.All of the metalloporphyrins exhibit reasonably intense absorption transitions (ε = ca. 2 X 10⁴ dm³ mol^-1 cm^-1) in the visible region and a strong absorption (ε = (2-7) X 10⁵ dm³ mol^-1 cm^-1) around 430 nm.Cyclic voltammetry showed that the compounds undrgo well-defined reduction and oxidation steps, although the oxidation processes were not readily reversible.For diamagnetic metalloporphyrins, the difference in E_1/2 between addition and removal of an electron for a particular compound was 2.05 +/- 0.20 V while the difference in E_1/2 between addition of one and two electrons was 0.28 +/- 0.12 V.Similarly, E_1/2 for the removal of a second electron from the porphyrin ? system was some 0.25 +/- 0.10 V higher than that for removal of the first electron.These findings are consistent with the central metal ion exerting only an inductive effect upon the porphyrin ? levels.The absorption spectra of the one-electron reduction and oxidation products were recorded by pulse radiolysis methods.Both products exhibit broad absorption transitions stretching across the entire visible and near-IR regions.The reduction products were identified as ?-radical anions and, in many cases these were unstable with respect to disproportionation.The oxidation products were identified as ?-radical cations and these were also unstable in aqueous solution but the decay route remains obscure.