Iron porphyrin peroxo complexes

The porphyrin-iron(III)-peroxo complex [Fe(TPP)02] (163) was prepared by the reaction of K02 with Fen(TPP) in the presence of a crown ether, and characterized by spectroscopic methods [p(0—O) = 806 cm-1]542. This peroxo complex (163) was found to be inactive toward hydrocarbons. However, addition of excess acetic anhydride to (163) dissolved in a benzene-cyclohexane mixture results in the formation of cyclohexanol and cyclohexanone. This reaction is thought to proceed via acylation of the peroxo group, giving iron percarboxylate (164), which decomposes to an Fev-oxo compound (165) capable of hydroxylating alkanes.543 Such a mechanism has been suggested for the hydroxylation of camphor by Pseudomonas cytochrome P-450.544... 

Titanium(IV)-porphyrin [TiO(TPP)] as well as molybdenum(V)-porphyrin [MoO(OMe)(TPP)] complexes were found to be active for the catalytic epoxidation of alkenes by alkyl hydroperoxides, whereas their peroxo derivatives are inactive.632 Iron(III)-porphyrin-peroxo complexes such as Fe(TPP)02NMe4 did not react with hydrocarbons, but form sulfato complexes upon reaction with S02.632 Manganese(III)-porphyrin-peroxo complexes Mn(02)(TPP) K+ were recently characterized by X-ray crystallography.634... 

Deprotonated peroxide coordinates to metals in low- and intermediate-oxidation states tend to bind side-on and display nucleophilic properties. For example, iron(III) porphyrin-peroxo complexes are nucleophilic they did not transfer an oxygen atom to electron-rich substrates (such as electron-rich olefins), but brought about epoxidation of electron-poor olefins or oxidative deformylation of aldehydes.130 Dinuclear... 

The use of porphyrin complexes for the electrocatalytic detection of 8O2 is limited. Iron and titanium porphyrin peroxo complexes (TPPFe-O-FeTPP or TiP(02)) interact with 8O2 to give sulfato complexes of the porphyrins or to give free sulfate Carballo et al. showed that NiPPIX polymerized on GCE catalyzed the oxidation of 8O2, and the modified electrode was stable under hydro-dynamic conditions of a flow cell . ... 

Copper(I) complexes similarly react with dioxygen to form peroxo-bridged binuclear complexes.Such complexes do not readily undergo O—O bond cleavage, apparently because the copper(III) oxidation state is not as readily attainable as the Fe(IV) oxidation state in an iron-porphyrin complex. Nevertheless, stable peroxo complexes of copper(II) have been difficult to obtain, because, as soon as it is formed, the peroxo complex either is protonated to give free hydrogen peroxide or is itself reduced by more copper(I) (Reactions 5.42 to 5.46). 

Fig. 3. Push-pull mechanism for the heterolysis of the 0-0 bond of an iron-peroxo complex. The oval indicates an electron-rich ligand, such as a porphyrin L is a heme-proximal ligand (cysteine or histidine).

Low-spin iron -oxo-porphyrin complexes such as (NMeIm)(TPP)Fe =0 can be obtained from the homolytic dissociation of the dinuclear Fe -/u.-peroxo complex in the presence of bases (B) (equation However, this iron(IV)-oxo complex was only found to oxidize phosphines... 

The postulated formation of the polynuclear peroxo intermediate has precedence in the literature of Fe(II)-porphyrin systems. In addition, the only structurally characterized non-heme iron(ni) peroxide complex, [Fe6(02)C>2(0Bz)i2] (5), has a structural motif similar to that in the key intermediate, namely, a p,p- n2, q2-peroxide spedes in which four ferric ions are coordinated to the peroxide dianion (I3 in Scheme 1). Because 5 has all ferric ions, this peroxide-bridged complex is stable. The species in Scheme 1, on the other hand, is mixed-valent Fe 2F 2/ and can therefore decompose by redox chemistry to form the stable (p-oxo)diiron(ni) product. 

Miksztal, A.R. and J.S. Valentine (1984). Reactivity of the peroxo hgand in metal-loporphyrin complxes Reaction of sulfur dioxide with iron and titanium porphyrin peroxo complxes to give sulfato complexes or sulfate. Inorg. Chem. 23(22), 3548-3552. 

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... 

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