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

The structure and stabilities of various intermediates of the "water-assisted" O-O bond activation reaction on the MMOH_red and R2_red 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, [Fe^II(O₂)Fe^III], II_H₂. Then, the second electron transfer from the other Fe atom to the O₂ moiety leads to formation of the peroxo complex [Fe^III(O₂)²- Fe^III], 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-η²-η² 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 MMOH_red 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-η²-η² peroxo) → VII_H₂ (compound Q with water) → VIII (compound Q). Having an additional water molecule around the active site of MMOH_red (and R2_red) 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.