A Density Functional Study of O−O Bond Cleavage for a Biomimetic Non-Heme Iron Complex Demonstrating an Fe^V-Intermediate

Abstract

Density functional theory using the B3LYP hybrid functional has been employed to investigate the reactivity of Fe(TPA) complexes (TPA = tris(2-pyridylmethyl)amine), which are known to catalyze stereospecific hydrocarbon oxidation when H(2)O(2) is used as oxidant. The reaction pathway leading to O-O bond heterolysis in the active catalytic species Fe(III)(TPA)-OOH has been explored, and it is shown that a high-valent iron-oxo intermediate is formed, where an Fe(V) oxidation state is attained, in agreement with previous suggestions based on experiments. In contrast to the analogous intermediate [(Por.)Fe(IV)=O](+1) in P450, the TPA ligand is not oxidized, and the electrons are extracted almost exclusively from the mononuclear iron center. The corresponding homolytic O-O bond cleavage, yielding the two oxidants Fe(IV)=O and the OH. radical, has also been considered, and it is shown that this pathway is inaccessible in the hydrocarbon oxidation reaction with Fe(TPA) and hydrogen peroxide. Investigations have also been performed for the O-O cleavage in the Fe(III)(TPA)-alkylperoxide species. In this case, the barrier for O-O homolysis is found to be slightly lower, leading to loss of stereospecificity and supporting the experimental conclusion that this is the preferred pathway for alkylperoxide oxidants. The difference between hydroperoxide and alkylperoxide as oxidant derives from the higher O-O bond strength for hydrogen peroxide (by 8.0 kcal/mol).

Two Faces of a Biomimetic Non‐Heme HOFeVO Oxidant: Olefin Epoxidation versus cis‐Dihydroxylation

“…[4] As previously suggested, [1,2,6] it is likely that a high-valent iron-oxo species is formed along the reaction pathway, and our DFT calculations have provided insight into the mechanism of olefin oxidation by the exceptional HOÀFe V = O oxidant. It is possible that epoxidation may also occur by direct attack of the precursor Fe À OOH on the olefin, and a preliminary study in this direction has suggested that such a pathway is slightly disfavored over the competing OÀ O bond cleavage to generate the HOÀFe V = O oxidant.…”

“…[6] The three unpaired electrons are mainly distributed on the iron atom (1.58), the oxo oxygen atom (1.0), and the hydroxo oxygen atom (0.44): a spin distribution not unlike that found for compounds I of heme peroxidases and cytochrome P450 and related model complexes. [7,8] The corresponding sextet and doublet states of HOÀFe V =O lie more than 10 kcal mol À1 higher in energy than the quartet ground state.…”

Non-heme iron enzymes: Contrasts to heme catalysis

“…However, in the cis-hydroxylating dioxygenases, the Rieske center can only provide one electron; thus, an Fe III OOOH, Fe V AO, or a redox equivalent species must be invoked in this reaction (37). From the calculations in Table 2 for the non-heme ligand set of PAH, the energetics are reasonable for OOO heterolysis (because of the negatively charged carboxylate and hydroxide ligands), as has recently been calculated for a low-spin Fe III model complex (38). However for the high-spin enzyme case, there is an additional energy barrier (Ϸ10 kcal͞mol), at least for OOO bond homolysis, because of an allowed crossing of energy levels (34).…”

Direct Detection of Fe(IV)=O Intermediates in the Cytochrome aa3 Oxidase from Paracoccus denitrificans/H2O2 Reaction

“…However, we demonstrate that the 607-nm form produces the 804 cm Ϫ1 species and suggest that it arises from a Fe(IV)ϭO/Cu B 3ϩ species. A Fe(V)ϭO/Cu B 2ϩ structure has been proposed for the 804 cm Ϫ1 mode in the CcO/O 2 reaction, and very recently, evidence supporting the ϩ5 valence state in a non-heme Fe has been reported (58). As suggested by Kitagawa and co-workers (59), in the presence of an electron-withdrawing group attached to heme a 3 (formyl), the a 1u and a 2u level could be lower than the d xz and d yz orbitals.…”