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

Abstract

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 Mn^III 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 Mn^III 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 Mn^V and Mn^IV 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 k₂^rel on σ_para 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.