O2 Activation in a Dinuclear Fe(II)/EDTA Complex: Spin Surface Crossing As a Route to Highly Reactive Fe(IV)oxo Species
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- 作者：Paola Belanzoni ; Leonardo Bernasconi ; Evert Jan Baerends
- 刊名：Journal of Physical Chemistry A
- 出版年：2009
- 出版时间：October 29, 2009
- 年：2009
- 卷：113
- 期：43
- 页码：11926-11937
- 全文大小：475K
- 年卷期：v.113,no.43(October 29, 2009)
- ISSN：1520-5215
文摘
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.