The EDTA Complex of Oxidoiron(IV) as Realisation of an Optimal Ligand Environment for High Activity of FeO²⁺

Abstract

A prerequisite for the high activity of the FeO2+ moiety as a hydroxylation agent is that its ligand environment stabilizes the 3σ*↑LUMO, which dominates the reactivity of this system. Features in the ligand environment that promote the reactivity of FeO2+ are: weak equatorial ligand field to obtain a quintet ground state that stabilizes the unoccupied 3σ*↑; weak axial ligand field to stabilize the 3σ*↑; a positive overall charge to lower the 3σ*↑. Generalised gradient‐corrected Density Functional Theory (DFT) calculations for the series of oxidoiron compounds of composition [FeO·EDTAHn](n–2)+, with n = 0, 1, 2, 3, 4, show that in particular the complex with n = 4 (charge +2) realises such an environment. Hypothetically, these species may appear as intermediates in the degradation of EDTA and related organics in aerated aqueous FeII/EDTA solutions. A strong dependence of the C–H activation properties in the hydroxylation of methane on the overall charge of yielding the lowest C–H dissociation barriers. In the n = 4 case, C–H dissociation occurs with anactivation energy of ca. 7 kJ mol–1, which is below the value computed for the corresponding reaction catalysed by [FeO(H2O)5]2+ (23 kJ mol–1). This enhanced catalytic activity is explained by EDTAHn(2–n)– satisfying the listed requirements for an effective ligand, in particular by the very weak axial coordination by the EDTA nitrogen atoms due to large Fe–N distances.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

Axial and equatorial ligand effects on biomimetic cysteine dioxygenase model complexes

“…On the other hand, occupation of the σ * z2 orbital in the quintet spin state would have elongated the Fe–O and Fe-axial ligand distances considerably, as observed before. 35 The geometries, therefore, support the assignment of a singly occupied σ * x2–y2 orbital and virtual σ * z2 orbital in the quintet spin states.…”

On how the binding cavity of AsqJ dioxygenase controls the desaturation reaction regioselectivity: a QM/MM study

“…S3F). Such close contacts are expected to facilitate hydrogen atom transfer (HAT) from both C10 ( TS-1b ) and C3 ( TS-1a ); however, values of the Fe–O–H angle (146° for C3-bound H, 90° for C10-bound H) suggest that HAT from C3 may be preferred, at least for the sigma channel, which usually requires angles larger than 120° [44–46]. …”

Oxygen Atom Transfer Using an Iron(IV)‐Oxo Embedded in a Tetracyclic N‐Heterocyclic Carbene System: How Does the Reactivity Compare to Cytochrome P450 Compound I?

“…Thus, the electrophilicp athway via TS E resultsi na ne lectront ransfer from substrate to iron(IV)-oxo speciest hat occupiest he virtual s* z 2 orbitalw ith one electron. This so-called 3 s-pathway [25] gives an iron(III) intermediate ( 3 I E )w ith an orbitalo ccupationo ft hree unpaired electrons, with an up-spin on the metal (p* xz 1 p* yz 1 s* z2 1 )a nd ad own-spin radical on the carbon atom of the substrate (f Sub 1 ). Group spin densities (see the Supporting Information) confirm the assigned pathway.…”