DFT Study of the Active Intermediate in the Fenton Reaction

Abstract

Density functional theory has been used to investigate the nature of the oxidizing agent in the Fenton reaction. Starting from the primary intermediate [FeII(H2O)5H2O2]2+, we show that the oxygen-oxygen bond breaking mechanism has a small activation energy and could therefore demonstrate the catalytic effect of the metal complex. The O-O bond cleavage of the coordinated H2O2, however, does not lead to a free hydroxyl radical. Instead, the leaving hydroxyl radical abstracts a hydrogen from an adjacent coordinated water leading to the formation of a second Fe-OH bond and of a water molecule. Along this reaction path the primary intermediate transforms into the [FeIV(H2O)4(OH)2]2+ complex and in a second step into a more stable high valent ferryl-oxo complex [FeIV(H2O)5O]2+. We show that the energy profile along the reaction path is strongly affected by the presence of an extra water molecule located near the iron complex. The alternative intermediate [FeII(H2O)4(OOH-)(H3O+)]2+ suggested in the literature has been also investigated, but it is found to be unstable against the primary intermediate. Our results support a picture in which an FeIV-oxo complex is the most likely candidate as the active intermediate in the Fenton reaction, as indeed first proposed by Bray and Gorin already in 1932.

Interfacial mechanisms of heterogeneous Fenton reactions catalyzed by iron-based materials: A review

“…(6) (Hu et al, 2011;Luo et al, 2010) is sometimes used for illustration. FeO 2+ might also be generated from the leached iron-induced homogeneous Fenton reaction, most likely through heterolytic O-O bond cleavage by the inner-sphere reaction of FeOH + and H 2 O 2 (Buda et al, 2001;Remucal and Sedlak, 2011;Li et al, 2010). High-valent iron is a weaker and more selective oxidant compared to UOH (Table 1).…”

Bispidine Coordination Chemistry

“…OH radicals may be involved in substrate oxidation. For iron(II) aqua ions, this has also been proposed on the basis of DFT calculations (323), and a similar mechanism is confirmed by DFT calculations with the bispidine complexes (174). No high-valent iron complexes have been trapped and spectroscopically characterized with the iron complex based on the tetradentate ligand 14, although DFT calculations indicate that ferryl complexes may be formed (173).…”

The Ruff degradation: a review of previously proposed mechanisms with evidence that the reaction proceeds by a Hofer–Moest-type reaction

“…The likely competition of mechanisms and the transience of intermediate species further complicate the determination of what occurs.Buda et al83 and Ensing et al84 offer strong evidence that Fenton chemistry is essentially ionic in vacuo.Chart 7. Proposed mechanism of the Ruff degradation by Buchanan et al74 …”