Diiron peroxide

Of the diiron peroxide complexes discussed thus far, only the HPTB and HXTA complexes have been reported to effect oxidation chemistry. The HPTB complex is claimed to convert 2,4-di-tert-butylphenol to 3,5-di-tert-butylbenzoquinone (4 days, 5% yield) (160), while the 5-Me-HXTA complex can disproportionate H2O2, epoxidize olefins, and hydroxylate aromatics (62). The mechanistic details for these reactions are not well developed and require further study. 

Fig. 2. Possible structures for a diiron(III) peroxide unit in the peroxo intermediate consistent with available Raman and Mossbauer spectroscopic data. The symbols N and 0 designate nitrogen and oxygen donor atoms of histidine and glutamate residues, respectively. Some of the latter must be bidentate to fill the coordination spheres.

The detection of a peroxodiferric intermediate in the ferritin ferroxidase reaction establishes the ferritin ferroxidase site as being very similar to the sites in the 02-activating (/x-carboxylato)diiron enzymes. However, in ferritins, the peroxodiferric intermediate forms diferric oxo or hydroxo precursors, which are transferred to biomineralization sites with release of hydrogen peroxide. 

Progress continues in uncovering the mechanism of oxygen activation at nonheme diiron active sites. A diiron(III) peroxide intermediate of the mutant D84E R2 protein has been isolated and shown to have similar visible and Moss-... 

Moenne-Loccoz, P., Krebs, C., Herlihy, K., Edmondson, D. E., Theil, E. C., Huynh, B. H., and Loehr, T., 1999, The ferroxidase reaction of ferritin reveals a diferric p-1,2 bridging peroxide intermediate in common with other 02-activating non-heme diiron proteins,... 

The mechanism of MMO including the different states of the iron dimer complex has been reviewed several times [66, 67, 68]. The lowest oxidation state of the diiron complex is Fe2(II,II) which is a loosely bound, ferro-magnetically coupled dimer with a long Fe-Fe distance. This complex, termed O, reacts with O2 to form another complex P, which is normally assigned to an Fe2(III,III) peroxide complex. One or more intermediates in between O and P have been postulated [70]. In the next step, the dioxygen bond is cleaved and an unprecedented Fe2(IV,IV) complex termed Q is formed. The oxidation state assignment was made based on Mbssbauer spectroscopy [71]. Compoimd Q has been suggested to be the active oxidant that attacks methane. 

N. Mizumo, C. Nozaki, 1. Kiyoto, M. Misono, Highly efficient utilization of hydrogen peroxide for selective oxygenation of alkanes catalyzed by Diiron-substituted polyoxometalate precursor, J. Am. Chem. Soc. 120 (1998) 9267. 

Peroxo-diiron(III) complexes can undergo not only redox but also ligand substitution reactions. Liberation of H202 was observed in the reactions with phenols and carboxylic acids leading also to the respective phenolate or carboxylate iron(III) complexes.86 Hydrolysis of a peroxo-diiron(III) complex results in an oxo-diiron(III) species and hydrogen peroxide. Such reaction is responsible for the autoxidation of hemerythrin, but is very slow for the native protein due to hydrophobic shielding of the active site (Section 4.2.3).20 The hydrolysis of iron(III) peroxides is reversible, and the reverse reaction, the formation of peroxo intermediates from H202 and the (di)iron(III), is often referred to as peroxide shunt and is much better studied for model complexes. 

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