Fe’-porphyrins

Complexes III and IV have Fe-porphyrin prosthetic groups (hemes), complex IV also contains copper atoms which are involved in electron transport. Complexes I, III, and IV use the energy of electron transport to pump protons out of the matrix so as to maintain a pH gradient and an electrical potential difference across the inner membrane required for ATP synthesis (see below and Appendix 3). It is important to remember that all dehydrogenations of metabolic substrates remove two protons as well as two electrons and that a corresponding number of protons are consumed in the final reduction of dioxygen (Figures 5, 6). 

Scheme 22 Oxidation of pyrene with Fe-porphyrin catalysts...

Scheme 1 Schematic structures of iron meio-tetraarylporphyrins and preparation of Fe-porphyrin carbene complexes...

Hydroxyl radical is a strong indiscriminate outer-sphere oxidant (generating OH ) and H-atom abstractor (generating H2O) [Huie and Neta, 1999]. Simple Fe porphyrins are known to promote 0-0 bond homolysis in reaction with H2O2 [Watanabe, 2000]. Because of its high reactivity, once generated, "OH probably reacts with the... 

The simple porphyrin category includes macrocycles that are accessible synthetically in one or few steps and are often available commercially. In such metallopor-phyrins, one or both axial coordinahon sites of the metal are occupied by ligands whose identity is often unknown and cannot be controlled, which complicates mechanistic interpretation of the electrocatalytic results. Metal complexes of simple porphyrins and porphyrinoids (phthalocyanines, corroles, etc.) have been studied extensively as electrocatalysts for the ORR since the inihal report by Jasinsky on catalysis of O2 reduction in 25% KOH by Co phthalocyanine [Jasinsky, 1964]. Complexes of all hrst-row transition metals and many from the second and third rows have been examined for ORR catalysis. Of aU simple metalloporphyrins, Ir(OEP) (OEP = octaethylporphyrin Fig. 18.9) appears to be the best catalyst, but it has been little studied and its catalytic behavior appears to be quite distinct from that other metaUoporphyrins [CoUman et al., 1994]. Among the first-row transition metals, Fe and Co porphyrins appear to be most active, followed by Mn [Deronzier and Moutet, 2003] and Cr. Because of the importance of hemes in aerobic metabolism, the mechanism of ORR catalysis by Fe porphyrins is probably understood best among all metalloporphyrin catalysts. 

Simple Fe porphyrins whose catalytic behavior in the ORR has been smdied fairly extensively are shown in Fig. 18.9. Literature reports disagree substantially in quantitative characterization of the catalytic behavior overpotential, stability of the catalysts, pH dependence, etc.). It seems plausible that in different studies the same Fe porphyrin possesses different axial hgation, which depends on the electrolyte and possibly specific residues on the electrode surface the thicknesses and morphologies of catalytic films may also differ among studies. AU of these factors may contribute to the variabUity of quantitative characteristics. The effect of the supporting surface on... 

Figure 18.9 Chemical structures of simple Fe porphyrins whose catal3ftic properties in the ORR have heen studied extensively. These properties are tabulated in CoUman et al. [2004a].

The apparent redox stoichiometry of O2 reduction catalysis [ av. Reaction (18.8)] is pH-independent, but for many catalysts depends strongly on the applied potential (Fig. 18.10). The apparent selectivity of Fe porphyrins deposited on the electrode surface typically increases with the amount of deposited catalyst. 

Usually, simple Fe porphyrins degrade rapidly during catalytic reduction of O2 or of H2O2. 

Anaerobic cyclic voltammetry suggests that simple Fe porphyrins deposited on an electrode in contact with an aqueous buffer contain two axial water molecules (or an... 

Figure 18.11 Plausible catalytic cycle for the ORR by simple Fe porphyrins adsorbed on the electrode surface and side Reactions (18.15)-(18.18). At pH < 3, the resting state of the catalyst is assumed to be ferric-aqua.

The second mechanism often invoked to explain the increase in n y of simple Fe porphyrins at potentials more reducing than that of the Fe couple (under anaerobic conditions) is based on the fact that at such potentials the fraction of the catalyst in the 5 -coordinate ferrous state is maximal because (i) the equilibrium (18.9) is shifted completely to the ferrous form and (ii) the concentration of O2 in the catalytic film is low owing to mass transport limitations. The higher the concentration of the 5-coor-dinate ferrous porphyrin in the catalytic film, the greater the probability that any released H2O2 will re-enter the catalytic cycle by coordinating to a molecule of ferrous porphyrin and decay according to (18.13b) instead of (18.17). 

Within the mechanism in Fig. 18.11, it seems implausible that simple Fe porphyrins can be effective ORR catalysts, since large overpotentials are required to access intermediates in which 0-0 bond heterolysis is facile. The only strategy discovered so far to facilitate this 0-0 bond heterolysis in the ferric-hydroperoxo intermediate is to control both the distal and the proximal environments of Fe porphyrins. In those cases, the overpotential of ORR reduction appears to be controlled by the potential of the (por)Fe / couple (see Section 18.6). 

Stepwise oxidation of [(FTF4)Co202] to the mono- and dications was reported to proceed reversibly in benzonitrile [LeMest et al., 1997] such reversible redox chemistry has not been reported for any O2 adducts of monomeric Co or Fe porphyrins. 

The equilibrium constant K for (por)Fe(OH2) (por)Fe, which determines the molar fraction of the 5-coordinate redox-active Fe catalyst. This constant was estimated from analysis of the catalytic turnover frequencies in the presence of varying concentrations of an inhibitor, CN, which competes with both O2 and H2O for the 5-coordinate Fe porphyrin. 

Collman JP, Shiryaeva IM, Boulatov R. 2003b. Effect of electron availability on selectivity of O2 reduction by synthetic monometallic Fe porphyrins. Inorg Chem 42 4807. 

Two dendrimers based on Fe-porphyrin core carrying peptide-like branches of different sizes have been synthesized in order to have more open and a more densely packed (23) structures [43]. The electrochemical behavior has been examined in CH2C12 and in aqueous solution. In the less polar solvent, the two dendrimers show similar potentials for the Fem/Fen couple, suggesting that the iron porphyrins in both the more open and the more densely packed dendrimers experience similar microenvironments. On the contrary, in water the behavior of the two dendrimers is very different since the reduction from Fem to Fe11 is much easier for the densely packed dendrimer. This result can be explained considering that in the dendrimer with the relatively open structure the aqueous solvation of the iron porphyrin is still possible, whereas in the densely packed one the contact between the heme and the external solvent is signifi-... 

Flowever, Mansuy el al. reported that treatment of olefins with [A-(p-toluenesulfonyl)imino]-phenyliodinane, PhI=NTs, in the presence of a Mn or Fe porphyrin, provided the corresponding aziridine or allylic sulfonamide, depending on the substrate and the catalyst used (Scheme 33).145-147 The reaction has been considered to proceed through a metal-A-toluenesulfonylnitrenoid species. Similarly to epoxidation via the oxo-Mn species, this aziridination is nonstereospecific, and the participation of a radical intermediate has been suggested.146... 

Table 18. Electronic absorption data for several Mn and Fe porphyrins of the type M(P)X (for abbreviations, see Table 2 solvents used CHCI3 for Mn(Etio)X, CgHg for Fe(Deut-DME)X, CHjClj for Fe(OEP)X X in nm)...

The water-soluble Fe porphyrin, 3Na+ [Fe(III)(TPPS)] -12H20 [H2TPPS4- = tetra-anionic form of meso-tetrakis(7r-sulfonatophenyl)porphine], has recently been shown to be an effective catalyst for the electroreduction of nitrite to ammonia [419]. The Fe meso-tetrakis(A -methyl-4-pyridyl) porphyrin and/or the Fe meso-tetrakis (jr -sulfophenyl) porphyrin complex shows a catalytic activity for the reduction of dioxygen in aqueous solutions, leading to hydrogen peroxide [420]. 

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