Catalysts ferryl

N-substituted iron porphyrins form upon treatment of heme enzymes with many xenobiotics. The formation of these modified hemes is directly related to the mechanism of their enzymatic reactivity. N-alkyl porphyrins may be formed from organometallic iron porphyrin complexes, PFe-R (a-alkyl, o-aryl) or PFe = CR2 (carbene). They are also formed via a branching in the reaction path used in the epoxidation of alkenes. Biomimetic N-alkyl porphyrins are competent catalysts for the epoxidation of olefins, and it has been shown that iron N-alkylporphyrins can form highly oxidized species such as an iron(IV) ferryl, (N-R P)Fe v=0, and porphyrin ir-radicals at the iron(III) or iron(IV) level of metal oxidation. The N-alkylation reaction has been used as a low resolution probe of heme protein active site structure. Modified porphyrins may be used as synthetic catalysts and as models for nonheme and noniron metalloenzymes. 

In addition to the cationic, radical, and non-synchronous concerted mechanisms outlined above, many other proposals have been offered [9, 56], In a recent provocative paper, an organometallic mechanism was postulated for the activation of alkanes by a ferryl porphyrin model species [79]. Less reactive substrates such as H2, D2 and CH4 were observed to inhibit the reaction between the synthetic catalyst and cyclohexane. In the proposed mechanism, a 2 -t- 2 C-H addition across the Fe-O bond is preceded by coordination of the alkane to the metal center to form an intermolecular <7-adduct. Inhibition arises from preferential binding of the smaller substrates to the congested metal site. Attempts to identify a similar effect with sMMO have been unsuccessful the presence of H2 had no effect on the rate of reaction between methane and Q (A. M. Valentine, S. S. Stahl, S. J. Lippard, unpublished results). 

As non-toxic chiral Fe complexes have recently been used as catalysts [118-120], increased knowledge of their structure-reactivity relationships becomes pertinent. X-band CW-EPR spectra of [Fe °Cl(l)], reported by Bryliakov et al. [121], were found to be typical of high-spin S = 5/2 Fe complexes with EID K, 0.15. Using this complex, the conversion and selectivity of the asymmetric sulphide oxidation reaction was investigated in a variety of solvents. In previous studies [122], the active site was proposed to be the [Fe =0(l)] species. However an alternative active species was proposed [121]. Oxo-ferryl 7i-cation radicals are expected to have typical S = 3/2 spectra with resonances at geff 4... 

The functional importance of the alkaline transition in Compound II is intriguing. Hayashi and Yamazaki [153] have shown that the ferryl/ferric redox potential is dramatically decreased at high pH. Temer and co-workers [154] recently observed that the resonance Raman oxidation state marker frequency (V4 [155, 156]) of alkaline pH ferric horseradish peroxidase was quite similar to that of Fe hemes. They proposed that oxidation of alkaline ferric horseradish peroxidase (Fe" -OH) to the Fe =0 state is promoted by the distal histidine acting as a base catalyst [154]. Likewise, the higher ferryl/ferric redox potential at lower pH [153] is consistent with the idea that hydrogen bonding to the 0x0... 

A conceptual model has been proposed (Fig. 53), in which the redox potential of the Fe + center was modified to suppress the irreversible conversion to the Fe +—O—Fe + /jl-oko complex in favor of the /u.-peroxo species Fe " "—O—O—Fe " ". It has been suggested that via this route the formation of the ferryl species, 0=Fe, would be facilitated. Four catalysts have been prepared on these principles iron perhaloporphyrins complexes Keggin structures with iron in the framework and with proximate iron centers and crystalline iron zeolite materials. 

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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