Iron also peroxo complexes

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). 

Several diverse metal centres are involved in the catalysis of monooxygenation or hydroxylation reactions. The most important of these is cytochrome P-450, a hemoprotein with a cysteine residue as an axial ligand. Tyrosinase involves a coupled binuclear copper site, while dopamine jS-hydroxylase is also a copper protein but probably involves four binuclear copper sites, which are different from the tyrosinase sites. Putidamonooxin involves an iron-sulfur protein and a non-heme iron. In all cases a peroxo complex appears to be the active species. 

Despite detailed and repeated measurements by various techniques, almost no evidence can be found for the formation of (LFe=0) in the cyclidene systems, despite the fact that peroxo complexes form readily and by one route that constitutes the reverse of the reaction of equation 6. The results reported here also stand in contrast to recent studies based on non-heme oxygenase model systems that indicate the possibility of multiple pathways, implying that (hydroperoxo)iron(III) species might have catalytic activity e.g., a) Y.-D. Wu, K. N. Houk, J. S. Valentine, and W. Nam, Inorg, Chem, 31 718 (1992) b) W. Nam, R. Ho, and J. S. Valentine, J, Am,... 

The formation of [X-peroxo complexes upon exposure to dioxygen is well known for iron(II) porphyrin complexes, but p.-peroxo complexes discussed here are only transiently observed at low temperature. Thus, the formation of a stable dinuclear peroxide from the highly sterically demanding iron(II) precursor is surprising. It is also noteworthy that the carboxylate iron(II) complex is ineffective for CO binding [64]. Since nonheme iron proteins do not react with CO either, whereas iron(II) porphyrin complexes and heme proteins exhibit a high affinity toward CO, the reactivity pattern of the model complexes... 

Fig. 3. Proposed routes for conversion of the peroxo intermediate to intermediate Q, one involving loss of water (left-hand side) and one not (right-hand side). In the former case the resulting diiron(IV) oxo species could bind an oxygen atom with one iron, or the oxygen could be bound symmetrically by both iron atoms. Although written as an iron(IV) oxo species, Q can also be formulated as an iron(III) oxyl radical complex (35,51).

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