EPR study of a novel [Fe–porphyrin] catalyst

Abstract

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^tPhen) 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^+· ] state with an EPR spectrum characterized by effective g-values g ⊥ ^eff  = 3.41 and g _|| ^eff  = 2, similar to Compound I of Horseradish Peroxidase. The [Fe^IV = O Por^+· ] 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 = +16.5 K and the coupling parameters are J/D = 0.31, J = +5.2 K. We suggest that the formation of [Fe^IV = O Por^+· ] specie to be the key intermediate responsible for the observed efficient catalytic decomposition of PCP.

Keywords:

• Porphyrin
• Catalysis
• Ferryl
• FeO(IV)
• High Valent Iron
• Catalysis
• chlorophenol
• PCP
• imidazole

Acknowledgement

This work has been supported by a Greek-Russia bilateral collaboration project funded by the General Secretariat for Research and Technology (GGET).