Reaction of alkyl radicals with chromium porphyrins

Abstract

The reactions of chromium(III) and chromium(II) mesoporphyrin-IX (MSP) with methyl and several substituted alkyl radicals (R) in aqueous solutions have been studied by radiolysis techniques. Transient spectra and kinetics were obtained by pulse radiolysis and stable products were determined after γ-radiolysis. The alkyl radicals were produced in solutions containing Cr^IIIMSP under two different conditions: (a) where the radicals are allowed to react directly with Cr^IIIMSP, and (b) where the porphyrin is reduced to Cr^IIMSP during production of the radicals, which subsequently react with the reduced porphyrin. We find that both Cr^IIMSP and Cr^IIIMSP react rapidly with carbon-centred radicals to form relatively stable Cr—C bonds. The products of these reactions are formulated as R—Cr^IIIMSP and R—Cr^IVMSP, respectively. Whereas Cr^IIMSP is oxidized by O₂ very rapidly to Cr^IIIMSP, R—Cr^IIIMSP is oxidized more slowly (t_1/2≈ 30–60 min) to give the same product, Cr^IIIMSP, and R—Cr^IVMSP is oxidized very slowly (t_1/2 of several h) to give a product, oxidized on the prophyrin ring. The observed stability of R—Cr^IVMSP is in contrast with the reported very short lifetime of other R—Cr^IV species.