Peroxo-iron complex

A suitable model for the oxygen carrier protein hemerythrin is [Fe2(Et-HPTB)(OBz)](BF4)2 (Et-HPTB = AWAT,iV -tetrakis[(N-ethyl-2-benzimidazolyl)methyl]-2-hydroxy-l,3-diaminopropane, OBz = benzoate). It can mimic the formation of a binuclear peroxo iron complex in the natural system (101). The measured value of -12.8 cm3 mol1 for the activation volume of the oxidation reaction together with the negative value of the activation entropy confirm the highly structured nature of the transition state. 

Vaz, A.D.N., D.F. McGinnity, H.M. Peng, G.M. Raner, C.L. Kuo, K.P. Vatsis et al. (1997). Mechanism of oxidations involving the cytochrome P450 peroxo-iron complex as probed by site-specific mutagenesis. FASEBJ. 11, P32. 

Because there exist a number of reviews which deals with the structural and mechanistic aspects of high-valent iron-oxo and peroxo complexes [6,7], we focus in this report on the application and catalysis of iron complexes in selected important oxidation reactions. When appropriate we will discuss the involvement and characterization of Fe-oxo intermediates in these reactions. 

The 0-0 stretch for the hydroperoxo complexes (60,86,88,90,94,95,99- ) falls in the range 780 900 cm-1. In the case of a nonheme iron complex it was found that the low-spin hydroperoxo form absorbs at a lower frequency than the high-spin peroxo complex (89,102). This trend also holds for some iron alkylperoxo complexes, and DFT calculations have shown the high-spin alkylperoxide form to have a larger activation energy for the 0-0 bond cleavage than the corresponding low-spin alkylperoxo form (104). 

Lippard and co-workers (58, 59) proposed a route (Scheme 4) for the production of 37 from 36 based on the stoichiometry of iron complex to dioxygen. Binding of dioxygen to 36 might yield a reactive superoxo or peroxo adduct that might oxidatively react with solvent, ligand, or adventitious protons to yield 37 directly, using one mole of 02 per mole of 36. 

G. Roelfes, V. Vrajmasu, K. Chen, R. Y. N. Ho, J.-U. Rohde, C. Zondervan, R. M. la Crois, E. P. Schudde, M. Lutz, A. L. Spek, R. Hage, B. L. Feringa, E. Miinck, L. Que, Jr., End-on and side-on peroxo derivatives of non-heme iron complexes with pentadentate ligands Models for putative intermediates in biological iron/dioxygen chemistry, Inorg. Chem. 42 (2003) 2639. 

Wertz DL, Valentine JS (2000) Nucleophilicity of Iron-Peroxo Porphyrin Complexes. 97 37-60... 

While peroxo dimer formation is sometimes reversible for cobalt complexes, it inevitably leads to further oxidations in case of iron complexes. Steric hindrance about the metal center would prevent dimerization processes. 

Wertz, D. L. Valentine, J. S. Nucleophilicity of iron-peroxo porphyrin complexes. Struct. Bond. 2000, 97, 38-60. 

McCandlish et al. have isolated a peroxo-iron(III) complex (9) (Fig. 4) in the reaction of Fe(III)TPP(Cl) and KO2 according to Eq. (4) (37). The Soret band of 9 appears at 437 nm with unusually red-shifted a- and -b-bands (565 and 609 nm in DMSO). The EPR spectmm of 9 at 77K showed a relatively narrow, sharp resonance at g = 4.2 and weak resonances at g = 2 and g = 8, typical of rhombic high-spin ferric complexes such as Fe" EDTA (JS). Such a spectrum is not typical of high-spin ferric porphyrin complexes, which usually show resonance at g = 2 and 6, indicative of axial symmetry. An IR band at 806 cm" was observed to shift to 759 cm when K 02 rather than K 02 was used to prepare solutions of 9 these observations suggested the side-on bonding formulation illustrated in Fig. 4. Extended X-ray absorption fine structure (EXAFS) studies of 9 also... 

However, several questions remain to be answered (1) Is the postulated formation of an acylperoxo-iron(III) complex correct (2) If the answer is yes, how can it be proved (3) Does the peroxo-iron(III) porphyrin complex afford an oxoferryl porphyrin cation radical Recently such an acylperoxo-iron(III) porphyrin complex (16) in solution at low temperature has been directly observed and characterized (78). It was found that 16 did indeed react to yield the oxoferryl porphyrin cation radical 14. 

O—O bond cleaves and two equivalents of an iron(IV) oxo complex are formed (Reaction 5.38). Subsequent reactions between the peroxo-bridged complex and the Fe oxo complex produce the /x-oxo dimer (see Reactions 5.39-5.40). 

Simaan, A.J., F. Banse, J.-J. Girerd, K. Wieghardt, and E. Bill (2001). The electronic structure of nonheme iron(llI)-hydroperoxo and iron(llI)-peroxo model complexes studied by Moessbauer and electron paramagnetic resonance spectroscopies. Inorg. Chem. 40, 6538-6540. 

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