The EDTA complex of oxidoiron(IV) as realisation of an optimal ligand environment for high activity of FeO2+

A prerequisite for the high activity of the FeO2+ moiety as a hydroxylation agent is that its ligand environment stabilizes the 3 sigma* up arrow 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 3 sigma* up arrow; weak axial ligand field to stabilize the 3 sigma* up arrow; a positive overall charge to lower the 3 sigma* up arrow. Generalised gradient-corrected Density Functional Theory (DFT) calculations for the series of oxidoiron compounds of composition [FeO center dot EDTAH(n)]((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 Fe-II/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 an activation energy of ca. 7 kJmol(-1), which is below the value computed for the corresponding reaction catalysed by [FeO(H2O)(5) ](2+) (23 kJmol(-1)). This enhanced catalytic activity is explained by EDTAH(n)((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. (C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008).