Oxidation of iron porphyrins

As a possible in-vitro model for one-electron transfer in photosynthesis, the photochemical reaction between hemin and Chla in pyridine solution has been studied, and it has been shown that the relatively slow reduction and oxidation of iron porphyrins can be accelerated by the presence of Chla by an order of magnitude and that light further increases the rate or reaction. Chla probably forms a rather stable complex with hemin, as is shown by fluorescence quenching experiments [Brody (17)]. 

Thianthrene radical cation is also an excellent one-electron oxidant of iron porphyrin complexes. Such oxidation of Fem(0Cl03)(TPP), where TPP is meso-tetraphenylporphyrin, provides the corresponding porphyrin 7r-cation radical analytically pure [32]. Similar oxidation of the AT-methyl porphyrin complex (N-MeTPP)FenCl, where AT-MeTPP is AT-methyl-meso-tetraphenylporphyrin, afforded [N-MeTPPFemCl]+ which was not further oxidized [33]. Thus thianthrene radical cation selectively oxidized the aromatic porphyrin ligand in one case and the metal center in the other. Ligand oxidation at a phenolic moiety has also been reported [34] on treatment of a 1,4,7-triazacyclononane appended with one or two phenol moieties ligated to Cu(II) complex with thianthrene radical cation. 

The electrochemistry of iron porphyrins in nonaqueous media has been discussed in several reviews [2, 7, 10, 12], and only a few of the major trends of iron porphyrin electrochemistry will be summarized in the current paper. Both high and low oxidation states of the metal ion can be accessed upon reduction or oxidation of iron porphyrins and the overall electron-transfer mechanism of these metalloporphyrins is shown in Sch. 3, where [(P)Fe "]" represents the initial compound in the absence of an associated anionic ligand. 

The reduction ofsec-, and /-butyl bromide, of tnins-1,2-dibromocyclohexane and other vicinal dibromides by low oxidation state iron porphyrins has been used as a mechanistic probe for investigating specific details of electron transfer I .v. 5n2 mechanisms, redox catalysis v.v chemical catalysis and inner sphere v.v outer sphere electron transfer processes7 The reaction of reduced iron porphyrins with alkyl-containing supporting electrolytes used in electrochemistry has also been observed, in which the electrolyte (tetraalkyl ammonium ions) can act as the source of the R group in electrogenerated Fe(Por)R. ... 

Wade RS, CE Castro (1973) Oxidation of iron(ll) porphyrins by alkyl halides. J Am Chem Soc 95 226-230. 

Dioxo-ruthenium porphyrin (19) undergoes epoxidation.69 Alternatively, the complex (19) serves as the catalyst for epoxidation in the presence of pyridine A-oxide derivatives.61 It has been proposed that, under these conditions, a nms-A-oxide-coordinated (TMP)Ru(O) intermediate (20) is generated, and it rapidly epoxidizes olefins prior to its conversion to (19) (Scheme 8).61 In accordance with this proposal, the enantioselectivity of chiral dioxo ruthenium-catalyzed epoxidation is dependent on the oxidant used.55,61 In the iron porphyrin-catalyzed oxidation, an iron porphyrin-iodosylbenzene adduct has also been suggested as the active species.70... 

In the case of iron-containing small molecule analogs of Mb and Hb a much rockier road to successful model compounds was encountered. Even though the syntheses of iron porphyrin complexes were carried out in analogous manner to the cobalt species described above, their irreversible oxidation to the p-oxo dimer upon... 

Denaturation of hemoproteins in cooked meats leads to liberation of the heme and oxidation of the porphyrin ring. Nonheme iron is less available nutritionally than heme iron and affects lipid oxidation more. In methemoglo-bin and metmyoglobin solutions heated for one hour at 78°C and 100°C the degradation of heme was about 22 to 26%, while after two hours at 120°C it increased to about 85 to 95% (Oellingrath, 1988). In meat cookery, however, such severe conditions do not apply. 

Castro, C.E. The rapid oxidation of iron(II) porphyrins by alkyl halides. A possible mode of intoxication of organisms by alkyl... 

The large number of electronic configuration of iron porphyrins with oxidative states of 2+ or 3+, high and low spin forms, and charge transfer states with different axial ligands offer the possibility of a number of non-radiative decay pathways ( ). In... 

Interactions and reactions of iron porphyrins with dioxygen species,with nitric oxide (see Section 5.4.3.8), with nitrite and with nitrate, and also of complexes containing carbon-bonded ligands, have continued to attract considerable interest and be reviewed. 

There are three different approaches which can be employed to prevent irreversible oxidation of iron(II) porphyrins and promote dioxygen adduct formation (1) low temperatures to reduce the rate of dimerization (2) steric hindrance to prevent dimerization and (3) immobilization of the heme to prevent dimerization. 

Steric hindrance. An obvious way of preventing p-oxo dimer formation, i.e. irreversible oxidation of iron(II) porphyrins, is to introduce bulky substituents at the porphyrin such that the resultant dioxygen adducts are unable to approach each other closely enough for dimer formation to occur. 

As observed from reaction (6.19) and experimental data [41,120,121], ROOH satisfactorily replaces molecular oxygen and the reducer. When oxidized with hydroperoxides in the presence of iron porphyrin catalysts (cytochrome P-450 analogs), olefins mostly convert to allyl oxidation products, namely unsaturated alcohols and ketones, whereas the quantity of epoxides does not exceed 1% [122], According to current suggestions [121] such behavior of iron porphyrin catalysts is explained by olefin epoxidation with the cata-lyst-ROOH complex by the heterolytical mechanism according to the following equation ... 

Treatment of TPPFe(III)Cl or raeso-tetra-o-tolylporphinato-iron(III) chloride [TTPFe(III)Cl (10)] with iodosylbenzene caused rapid oxidation of the porphyrin and loss of catalytic activity for hydrocarbon oxidation. Figure 1 shows changes in the visible absorption spectrum upon treatment of 10 with iodosylbenzene. These data indicate that shortly after the addition of iodosylbenzene (Scan b, Figure 1) a new porphyrin species (11) is formed, which then rapidly decays to oxidized porphyrin products. The kinetics of this decay process are approximately first order (Figure 2). 

Loss of water from the FeIV(OH-)2 species would yield Fe02+, a compound formed in the oxidation of iron(III) haems by hydrogen peroxide (as well as an oxidised porphyrin ligand). One might ask the question whether copper can form a cupryl, or oxocopper(III), species. It has been argued that this is not... 

NMR spectroscopy is uniquely effective in probing the oxidation state, spin state, and ligation state of iron porphyrins (47, 48) and iron N-alkylporphyrins (22). For iron-tetraarylporphyrins the /3-pyrrole proton resonance is most indicative of the electronic structure of the metal... 

Oxidation of the porphyrin 7r-system of iron(IH) (45) and iron(IV) tet-raarylporphyrins (8) leads to large chemical shifts for the meso-aryl protons. This is a result of spin density on the raeso-carbon, which is delocalized into the aryl rings via a 7r-spin delocalization mechanism (50). Accordingly, the chemical shifts of ortho-, meta-, and para-protons have alternating signs. Coupling of the porphyrin spin to the metal spin in both a ferro- and antiferromagnetic manner has been demonstrated (45). 

The iron porphyrins and related compounds constitute an extremely important class of coordination complex due to their chemical behaviour and involvement in a number of vital biological systems. Over recent years a vast amount of work on them has been published. Chapter 21.1 deals with the general coordination chemistry of metal porphyrins, hydroporphyrins, azaporphyrins, phthalocyanines, corroles, and corrins. Low oxidation state iron porphyrin complexes are discussed in Section 44.1.4.5 and those containing nitric oxide in Section 44.1.4.7, while a later section in this chapter (44.2.9.2) is mainly concerned with iron(III) and higher oxidation state porphyrin complexes. Inevitably however, a considerable amount of information on iron(II) complexes is contained in that section as well as in Chapter 21.1. Therefore in order to prevent excessive duplication, the present section is restricted to highlighting some of the more important aspects of the coordination chemistry of the iron(II) porphyrins while the related unusually stable phthalocyanine complexes are discussed in the previous section. 

Figure 2 Possible oxidation and spin states of iron porphyrins and the d orbital configurations expected in each case. AU of these states (excluding high-spin d, which is not included in the figure) have been observed for iron porphyrins, and are discussed in this article. In addition, the spin-admixed 5 = 3/2,5/2 state of Fe " and the alternative orbital configuration for low-spin Fe , with dxy higher in energy than dxi,dyz are also discussed...

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