EPR study of a novel [Fe-porphyrin] catalyst

This paper reports on an EPR and catalytic study of a novel [Fe-porphyrin] catalyst, [FeR4P], used for the catalytic decomposition of pentacholorophenol (PCP). The porphyrin complex [FeR4P] bears 2,6-di-tert-butylphenols(where R = di-Bu(t)Phen) at each meso-aryl position of the porphyrin ring. The FeR4P-SiO2 catalyst was found to be very efficient for the degradation of PCP, using NaIO4 as oxidant plus imidazole as cocatalyst. EPR spectroscopy shows that in the presence of the oxidant NaIO4, at a redox potential of E-h = 250 mV, [FeR4P] forms a high valent [Fe-IV= O Por(+center dot)] state with an EPR spectrum characterized by effective g-values g(eff) = 3.41 and g(parallel to)(eff) = 2, similar to Compound I of Horseradish Peroxidase. The [Fe-IV= O Por(+center dot)] state can described by a S = 1 state of the oxoferryl Fe-IV = O weakly coupled by exchange interaction J to a S' = 1/2 porphyrin cation radical. The zero field splitting of the Fe-IV = O is D = +16.5K and the coupling parameters are J/D 0.31, J = +5.2 K. We suggest that the formation of [Fe-IV = O Por(+center dot)] specie to be the key intermediate responsible for the observed efficient catalytic decomposition of PCP.