Biomimetic iron oxide

P. Calvert and A. Broad, Biomimetic routes to magnetic iron oxide-polymer composites. In K.L. Mittal (Ed.), Polymers in Information Storage Technology, Plenum Press, New York, 1991, p. 257. 

A mild aerobic palladium-catalyzed 1,4-diacetoxylation of conjugated dienes has been developed and is based on a multistep electron transfer46. The hydroquinone produced in each cycle of the palladium-catalyzed oxidation is reoxidized by air or molecular oxygen. The latter reoxidation requires a metal macrocycle as catalyst. In the aerobic process there are no side products formed except water, and the stoichiometry of the reaction is given in equation 19. Thus 1,3-cyclohexadiene is oxidized by molecular oxygen to diacetate 39 with the aid of the triple catalytic system Pd(II)—BQ—MLm where MLm is a metal macrocyclic complex such as cobalt tetraphenylporphyrin (Co(TPP)), cobalt salophen (Co(Salophen) or iron phthalocyanine (Fe(Pc)). The principle of this biomimetic aerobic oxidation is outlined in Scheme 8. 

Iron-only hydrogenase, dithiolate-bridged compounds as biomimetic models, 6, 239 Iron oxide films, synthesis, 12, 51 Iron-palladium nanoparticles, preparation, 12, 74 Iron-platinum bimetallic clusters, with isocyanide clustes,... 

Smejkalova D, Piccolo A. Rates of oxidative coupling of humic phenolic monomers catalyzed by a biomimetic iron-porphyrin. Environ Sci Technol 2006 40 1644-9. 

Traylor (38) has also shown that biomimetic iron N-alkylporphyrins themselves are competent catalysts for epoxidation of alkenes with a rate constant of about 104 M-1 s-1. On the basis of these observations and rearrangement reactions of specific alkenes, Traylor has proposed the reaction sequence outlined in Scheme 3 as representative of the oxidation and N-alkylation reactions of the P-450 model systems. In this scheme, the epoxide and the N-alkylated heme are derived from a common, electron-transfer intermediate (caged ferrylporphyrin-alkene cation radical). Collman and co-workers (28, 29) prefer a concerted mechanism (or a short-lived, acyclic intermediate) for epoxidation and N-alkylation reactions. Both authors note that the reactions catalyzed by cytochrome P-450 (and biomimetic reactions) probably can not be ascribed to any single mechanism. 

Analogously, Douglas et al. have used a small 24 subunit heat shock protein isolated from the hyperthermophilhc archaeon, Methanococcus jannaschii, as a biomimetic template for the oxidative mineralization of iron. Both the wild type and a mutant (glycine at position 41 replaced for cysteine) exhibited ferrihydrite mineralization as determined by an increase in the ligand to metal charge transfer absorbance at 400 nm and the presence of discrete iron oxide particles of... 

Ren, Y., Rivera, J.G., He, L., Kulkarai, H., Lee, D.-K., Messersmith, P.B., 2011. Facile, high efficiency immobilization of lipase enzyme on magnetic iron oxide nanoparticles via a biomimetic coating. BMC Biotechnology 11. 

Scheme 14 Biomimetic ligands used in iron-catalyzed alkane oxidation...

In recent years, several model complexes have been synthesized and studied to understand the properties of these complexes, for example, the influence of S- or N-ligands or NO-releasing abilities [119]. It is not always easy to determine the electronic character of the NO-ligands in nitrosyliron complexes thus, forms of NO [120], neutral NO, or NO [121] have been postulated depending on each complex. Similarly, it is difficult to determine the oxidation state of Fe therefore, these complexes are categorized in the Enemark-Feltham notation [122], where the number of rf-electrons of Fe is indicated. In studies on the nitrosylation pathway of thiolate complexes, Liaw et al. could show that the nitrosylation of complexes [Fe(SR)4] (R = Ph, Et) led to the formation of air- and light-sensitive mono-nitrosyl complexes [Fe(NO)(SR)3] in which tetrathiolate iron(+3) complexes were reduced to Fe(+2) under formation of (SR)2. Further nitrosylation by NO yields the dinitrosyl complexes [(SR)2Fe(NO)2], while nitrosylation by NO forms the neutral complex [Fe(NO)2(SR)2] and subsequently Roussin s red ester [Fe2(p-SR)2(NO)4] under reductive elimination forming (SR)2. Thus, nitrosylation of biomimetic oxidized- and reduced-form rubredoxin was mimicked [121]. Lip-pard et al. showed that dinuclear Fe-clusters are susceptible to disassembly in the presence of NO [123]. 

One-step hydroxylation of aromatic nucleus with nitrous oxide (N2O) is among recently discovered organic reactions. A high eflSciency of FeZSM-5 zeolites in this reaction relates to a pronounced biomimetic-type activity of iron complexes stabilized in ZSM-5 matrix. N2O decomposition on these complexes produces particular atomic oj gen form (a-oxygen), whose chemistry is similar to that performed by the active oxygen of enzyme monooxygenases. Room temperature oxidation reactions of a-oxygen as well as the data on the kinetic isotope effect and Moessbauer spectroscopy show FeZSM-5 zeolite to be a successfiil biomimetic model. 

Results discussed above show in several lines a distinct biomimetic-type activity of iron complexes stabilized in the ZSM-S matrix. The most important feature is their unique ability to coordinate a very reactive a-oxygen form which is similar to the active oxygen species of MMO. At room temperature a-oxygen provides various oxidation reactions including selective hydroxylation of methane to methanol. Like in biological oxidation, the rate determining step of this reaction involves the cleavage of C-H bond. 

Selective Conversion of Hydrocarbons with H202 Using Biomimetic Non-heme Iron and Manganese Oxidation Catalysts... 

Kruger, H.-J. Iron-containing models of catechol dioxygenases, Biomimetic Oxidations Catalyzed by Transition Metal Complexes , Ed. Meunier, B. Imperial College Press London, 2000, pp. 363—413. 

---