O2 activation in a dinuclear Fe(II)/EDTA complex: spin surface crossing as a route to highly reactive Fe(IV)oxo species

We study the cleavage of O2 in gas phase [(EDTAH)Fe(O2)Fe(EDTAH)]2-, a proposed intermediate in the aqueous Fe(II)-to-Fe(III) autoxidation reaction in the presence of atmospheric dioxygen and EDTA ligand. The role of the exchange coupling between the locally high-spin Fe centers in the O-O dissociation is investigated. Using results from broken symmetry (BS) density functional theory (DFT) calculations, we show that the system can be modeled as two high-spin (HS) S=5/2 Fe(III) d5 centers coupled through a bridging peroxo O22- ligand, consistent with hypotheses advanced in the literature. We show that in this electronic configuration the O-O cleavage reaction is forbidden by (spin) symmetry. Dissociation of the O22- group to the product ground state may only take place if the system is allowed to undergo a transition to a state of lower spin multiplicity (S=4) as the O-O bond is stretched. We show that the exchange coupling between the two Fe ions in [(EDTAH)Fe(O2)Fe(EDTAH)]2- plays only a minor role in defining the chemistry of O2 activation in this system. The peroxo/oxo interconversion involves a state outside the Heisenberg spin ladder of the initial S = 5 state. In this S = 4 state, the dinuclear complex evolves to two oxo complexes, [EDTAH·Fe(IV)O]- , with an overall energy barrier of only ∼86 kJ mol-1 . According to recent theoretical work, the latter species are exceptionally strong oxidants, making them ideal candidate catalysts for organic oxidations (including C-H bond hydroxylation). We highlight the (spin) symmetry forbidden nature of the reaction on the S =5 surface and its symmetry allowed character in the electronic configuration with S =4.