Oxo-iron complexes

The second example of a high-spin iron-oxo complex, [Fe (0)(TMG3tren)] (see Table 8.4) has been published only recently. In this compound, the HS state is afforded by the trigonal bipyramidal symmetry of the TMG3tren ligand, causing... 

Extensive studies have established that the catalytic cycle for the reduction of hydroperoxides by horseradish peroxidase is the one depicted in Figure 6 (38). The resting enzyme interacts with the peroxide to form an enzyme-substrate complex that decomposes to alcohol and an iron-oxo complex that is two oxidizing equivalents above the resting state of the enzyme. For catalytic turnover to occur the iron-oxo complex must be reduced. The two electrons are furnished by reducing substrates either by electron transfer from substrate to enzyme or by oxygen transfer from enzyme to substrate. Substrate oxidation by the iron-oxo complex supports continuous hydroperoxide reduction. When either reducing substrate or hydroperoxide is exhausted, the catalytic cycle stops. 

Oxidation of benzyl alcohol catalysed by chloroperoxidase exhibits a very high prochiral selectivity involving only the cleavage of the pro-S C-H bond. The reaction mechanism involved the transfer of a hydrogen atom to the ferryl oxygen of the iron-oxo complex. An a-hydroxy-carbon radical and the iron-hydroxy complex P-Fe -OH form. They may lead to the hydrated benzaldehyde or stepwise with the formation of the intermediate a-hydroxy cation. 

Fe catalytically decomposes H2O2, imfortimately much less attention has been paid to this reaction and the possible mechanism varies according to the authors. Those that support the free radical OH approach, agree with the classical radical chain mechanism represented by equations (9, 10, 11 and 12). However, on the other side, high valent iron-oxo complex has also been proposed in Fe -H202 interaetions Kremer 1985 Gozzo 2001). 

Finally, iron catalysts based on salen-type ligands have been used. These iron(III)-salen complexes were regarded as enzyme models, using PhIO as oxidant (Scheme 3.52) [162]. Initially, the corresponding active iron-oxo complexes were formed by reaction with PhIO and isolated before use. A stoichiometric amount of the iron-oxo complex allowed the efficient oxidation of a variety of aryl methyl sulfides in moderate to good yields. 

In analogy with the corresponding known iron-oxo complexes, an iron nitrene complex (L Fe=NR) has been proposed as a reactive intermediate in these processes for both catalytic systems described above [176]. 

Yoshizawa, K.. (2002) Theoretical study on kinetic isotope effects in the C-H bond activation of alkanes by iron-oxo complexes, Coordination Chemistry Reviews 226, 251-259. 

One motivation for the characterization of the above compounds has been to more fully understand the involvement of such higher valent manganese porphyrin complexes in model systems which imitate the catalytic activity of monooxygenase cytochrome P-450 and related enzymes. The catalytic cycle of cytochrome P-450 appears to involve the binding and reduction of molecular oxygen at a haem centre followed by the ultimate formation of a reactive iron oxo complex which is responsible for oxidation of the substrate. For example, cytochrome P-450 is able to catalyse alkane hydroxylation with great selectivity. 

Witt and Chan, 1987 Rao et al., 1988) described as ferryl states. The Fe(IV) state has been designated as an iron-oxo complex (Fe02+) the Fe(V) state is a similar complex, the extra electron being lost from the porphyrin jr-system (P +-Fe02+) (Felton et al., 1976 Rakshit et al., 1976). 

These reactivity studies, and the observation of the peroxide shunt described above, indicate that Fe (P )(0) + or Fe (P )(0) is the most likely eandi-date for active oxygen. These two formulations are, of course, isoelectronie, and it is tempting to conclude that the latter is the more likely formulation of the enzymatic intermediate. However, it is important to remember that the model systems lack the axial cysteinyl ligand present in cytochrome P-450. The effect of the relatively easily oxidized sulfur ligand on the electron distribution within that intermediate is not known, since model systems for high-valent iron-oxo complexes containing axial thiolate ligands have not been synthesized. 

So what is the mechanism of HAS The simplest mechanism that is consistent with the available data is that HAS oxidizes heme O to heme A via an electron transfer mechanism. In this scenario, HAS would use O2 to generate a high valent iron-oxo complex (compound I), but then oxidize heme O via... 

The only relatively stable high-valent iron-oxo complexes reported to date are ferrate(VI, V,... 

It has been widely accepted that the high-valent iron oxo complex 1 is the active oxidant in metalloporphyrin systems and in non-porphyrin iron complex systems as well. A high-valent iron oxo species can be formed via heterolytic 0-0 bond cleavage of the iron-hydroperoxide 2 or of the iron-peroxyacid species 4 (Figure 5, pathway A). Recently, elegant proof of this heterolytic 0-0 bond cleavage of hydrogen peroxide, MCPBA, and f-butyl hydroperoxide has been provided by Traylor et al in iron porphyrin... 

The non-heme enzyme methane monooxygenase (MMO) from methanotropic bacteria catalyzes the hydroxylation of methane to methanol. Methane is most difficult to hydroxylate and cytochrome P-450 cannot perform this reaction. MMO consists of three components. Component A is a dimer with subunits of dinuclear iron with monooxygenase activity. Components B and C are electron donor and transfer sites. Like cytochrome P-450, a high valent iron-oxo complex is proposed for component A in MMO. This species abstracts a H atom from CH4 to generate a CHs" radical. 

Among all the published redox potentials of iron(IV)-oxo compounds so far, those of the bispidine complexes are the highest. An important and not often appreciated point is that the very high potentials indicate that the ferryl complexes are unstable. Obviously, this not only involves the reduction to Fe but also the stability of the bispidine-iron-oxo complexes that is, it emerges that these very efficient oxidation catalysts have a high propensity to decay (e.g., by decomplexa-tion) and this could be one of the reasons why only a limited number of turnovers is observed in various of the catalytic reactions (see Section 6.6). 

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