Generation of ferryl species through dioxygen activation in iron/EDTA systems: a computational study

The ferryl species (oxidoiron(IV), FeO(2+)) is a ubiquitous, highly oxidative intermediate in oxidation catalysis. We study theoretically its abiotic generation, in the form of the singularly active complex of FeO(2+) with the EDTAH(n)(-4+n), n = 0-4 ligands, from O(2) and Fe(2+)-EDTA complexes. The calculations are for the gas phase using generalized gradient corrected (BLYP and OPBE) Density Functional Theory (DFT). We examine the effects of ligand protonation on the coordination geometry and electronic structure of the chelated Fe(2+) ion, on its affinity to bind dioxygen, and on the generation of dinuclear Fe/EDTA/O(2) complexes, whose formation has been hypothesized on the basis of kinetic measurements of Fe(II)/Fe(III) autoxidation reactions in aqueous solution. We also consider the homolytic cleavage of the O-O bond within one such complex, [Fe x EDTAH x O(2) x EDTAH x Fe](2-), and we show that this reaction leads to a pair of Fe(IV)O/EDTA systems with an energetic barrier comparable to those computed for model systems of active sites of enzymes involved in dioxygen activation, such as methane monooxygenase. Our study supports the recently advanced hypothesis that high valent iron compounds capable of oxidizing organic substrates may be produced as a byproduct of the Fe(II)/Fe(III) autoxidation in aqueous Fe/EDTA/O(2) solutions at ambient conditions. We also identify the origin of the enhanced O(2) activation ability in the monoprotonated [Fe x EDTAH](-) complex, compared to other ligand protonation states, which has been observed in kinetic measurements.