Oxo iron(iv) as an oxidative active intermediate of p-chlorophenol in the Fenton reaction: a DFT study

Abstract

Debate continues over which active species plays the role of oxidative agent during the Fenton reaction—the HO˙ radical or oxo iron [Fe^IVO]²⁺. In this context, the present study investigates the oxidation of p-chlorophenol by [Fe^IVO(H₂O)₅]²⁺ using DFT calculations, within gas-phase and micro-solvated models, in order to explore the possible role of oxo iron as a reactant. The results show that the chlorine atom substitution of p-chlorophenol by oxo iron is a highly stabilising step (ΔH = −83 kcal mol⁻¹) with a free energy barrier of 5.8 kcal mol⁻¹ in the micro-solvated model. This illustrates the high oxidising power of the [Fe^IVO(H₂O)₅]²⁺ complex. On the other hand, the breaking of the Fe–O bond, leading to the formation of hydroquinone, is observed to be the rate-determining step of the reaction. The rather large free energy barrier corresponding to this bond cleavage amounts to 10.2 and 9.3 kcal mol⁻¹ in the gas-phase and micro-solvated models, respectively. Elsewhere, the lifetime of the HO˙ radical has previously been shown to be extremely small. These facts, combined with observations of oxo iron under certain experimental conditions, suggest that oxo iron is a highly plausible oxidative species of the reaction. In addition, a trigonal bipyramidal iron complex, coordinated either by hydroxyl groups and/or by water molecules, has been found in all described mechanisms. This structure appears to be a stable intermediate; and to our knowledge, it has not been characterised by previous studies.