Kinetic studies on the interactions of manganese-porphyrins with peraceticacid. 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 pi-radical cation (PcZn+.) with peracetic acid in acetonitrile-ac etic acid has been studied, in the presence of Mn-III complexes of meso-tet ra(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 "ca talyst-oxidant" type (adduct 'A') with rate constant k(1) is the first stag e of the reaction. The balance of the electron donating properties of the p orphyrin ring and electron accepting properties of Mn-III determines the fo rmation of 'A', as shown by the independence of k(1) of catalyst structure. The influence of added acetic acid on the UV-Vis spectra of RTDCPPMnX in C H3CN and on the oxidation rate of DPPH and PcZn evidenced that a molecule o f AcOH is included in the internal co-ordination sphere of RTDCPPMnX and of the reaction intermediates. The rate constant k(2) for the irreversible tr ansformation of 'A' into an equilibrium mixture of Mn-V and Mn-IV oxo-speci es (established in Part 1) must thus refer to the acetic acid adducts of Mn -porphyrins {[RTDCPPMn(AcOH)](X)}. A non-linear dependence of log k(2)(rel) on sigma(para) of the porphyrin substituents R has also been found. Howeve r, electron-withdrawing substituents are particularly effective for enhance ment of the rate of formation of the high-valent oxomanganese species.