Spectroscopic studies on the reaction of superoxide ion with non-redox metalloporphyrins

Abstract

The reactions of superoxide ion, O − 2, with some non-redox metalloporphyrins were investigated at room temperature by spectroscopic methods (UV/vis, ESR) in dimethyl sulfoxide (DMSO). Addition of the O − 2 solutions prepared from KO 2 to DMSO solutions of the meso-tetraphenylporphyrin which is complexed with group II metals such as Zn(II), Mg(II) and Cd(II), caused the visible spectra to shift to the red and made the g ∥ component of the ESR spectrum due to O − 2 at 77 K shift to the higher field. Addition of methanol decomposed the superoxo complexes to the corresponding metalloporphyrins. These results suggest that the reaction products between these metalloporphyrins and O − 2 are the O − 2 adducts. Other non-redox metalloporphyrins containing group VIII metals such as Ni(II) and Pd(II) do not produce the O − 2 adducts under the experimental conditions. The reasons for the red shifts caused by the formation of the O − 2 adducts and for the difference in the reactivities of metalloporphyrins towards O − 2 are discussed.