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Persistent URL http://purl.org/net/epubs/work/51363
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Record Id 51363
Title 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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Abstract We study the cleavage of O$_2$ in gas phase [(EDTAH)Fe(O$_2$)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 modelled as two high-spin (HS) $S=5/2$ Fe(III) $d\mspace{1mu}^5$ centers coupled through a bridging peroxo O$_2^{2-}$ 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 O$_2^{2-}$ 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(O$_2$)Fe(EDTAH)]$^{2-}$ plays only a minor role in defining the chemistry of O$_2$ 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 $\sim 86$~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 hydroxilation). 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 $S=4$.
Organisation CSE , STFC
Keywords Chemistry , SSTD 2009-2010
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Language English (EN)
Type Details URI(s) Local file(s) Year
Journal Article J Phys Chem A 113 (2009): 11926-11937. doi:10.1021/jp9033672 2009