Kinetic studies on the interactions of manganese–porphyrins with peracetic acid. Part 2. The influence of acetic acid and porphyrin substituents

The one-electron oxidation of 1,1-diphenyl-2-picrylhydrazine (DPPH) and Zn–tetra-tert-butylphthalocyanine (PcZn) to the corresponding stable radical (DPP·) and π-radical cation (PcZn+·) with peracetic acid in acetonitrile–acetic acid has been studied, in the presence of MnIII complexes of meso-tetra(2,6-dichloro-4-R-phenyl)porphyrins (RTDCPPMnCl; R = CH3O, H, Br, Cl or NO2) as catalysts. The formation of a two-centre donor–acceptor complex of 'catalyst–oxidant' type (adduct ‘A’) with rate constant k1 is the first stage of the reaction. The balance of the electron donating properties of the porphyrin ring and electron accepting properties of MnIII determines the formation of ‘A’, as shown by the independence of k1 of catalyst structure. The influence of added acetic acid on the UV–Vis spectra of RTDCPPMnX in CH3CN and on the oxidation rate of DPPH and PcZn evidenced that a molecule of AcOH is included in the internal co-ordination sphere of RTDCPPMnX and of the reaction intermediates. The rate constant k2 for the irreversible transformation of ‘A’ into an equilibrium mixture of MnV and MnIV oxo-species (established in Part 1) must thus refer to the acetic acid adducts of Mn–porphyrins {[RTDCPPMn(AcOH)](X)}. A non-linear dependence of log k2rel on σpara of the porphyrin substituents R has also been found. However, electron-withdrawing substituents are particularly effective for enhancement of the rate of formation of the high-valent oxomanganese species.