A density functional study of possible intermediates of the reaction of dioxygen molecule with nonheme iron complexes. 2. "Water-Assisted" model studies

Maricel Torrent, Djamaladdin G. Musaev, Keiji Morokuma, Harold Basch

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Abstract

The structure and stabilities of various intermediates of the "water-assisted" O-O bond activation reaction on the MMOHred and R2red were studied using the B3LYP hybrid density functional method, and are compared with those for the "water-free" model studies. It was shown that the first step of the reaction is coordination of the O2 molecule to one of the Fe atoms of complex I_H2 ("water-assisted" model complex) leading to formation of the mixed-valence superoxo species, [FeII(O2)FeIII], II_H2. Then, the second electron transfer from the other Fe atom to the O2 moiety leads to formation of the peroxo complex [FeIII(O2)2- FeIII], which is found to have four different isomers, III_H2O, IV_H2O, V_H2O, and VI_H2O, corresponding to the cis-μ-1,2, end-on μ-1,1 distorted trans-μ-1,2, and μ-η22 coordination modes of O2, respectively. The stability (relative to I_H2O + O2) of these isomers increases via III_H2O (34.5 kcal/mol) < VI_H2O (36.4 kcal/mol) < V_H2O (44.7 kcal/mol) < IV_H2O (57.4 kcal/mol). On the basis of the structural analysis and calculated O-O bond distances, we have predicted that isomers III_H2O, V_H2O, and VI_H2O are intermediates on the potential energy surface of the homolytic O-O activation process, while isomers III_H2O and IV_H2O are intermediates of the heterolytic O-O activation reaction. The homolytic O-O activation by MMOHred is predicted to occur via the following intermediates: I_H2O + O2 → II_H2O (superoxo species) → III_H2O (cis-μ-1,2-peroxo) → V_H2O (distorted trans-μ-1,2-peroxo) → VI_H2O (μ-η22 peroxo) → VII_H2 (compound Q with water) → VIII (compound Q). Having an additional water molecule around the active site of MMOHred (and R2red) facilitates the superoxo → peroxo conversion process because of the opening of one of the "legs" of μ-1,2-bridged carboxylate leading to formation of Fe1-HOH⋯OCHO-Fe2 network.

Original languageEnglish
Pages (from-to)4453-4463
Number of pages11
JournalJournal of Physical Chemistry B
Volume105
Issue number19
DOIs
StatePublished - 17 May 2001

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