Ferrous-Porphyrin

Electrocatalytic reduction of both O2 and H2O2 starts at potentials close to that of the Fe couple in the absence of a substrate (which for most porphyrins is about 0.2-0 V with respect to a normal hydrogen electrode (NHE) at pH < 6 the exception being Fe(TMPyP), E k 0.5 V). Catalytic reduction of H2O2 by simple/erne porphyrins is too slow to be detectable in typical electrocatalytic experiments whereas ferrous porphyrins catalyze rapid reduction of H2O2,... 

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). 

Only three steps of the proposed mechanism (Fig. 18.20) could not be carried out individually under stoichiometric conditions. At pH 7 and the potential-dependent part of the catalytic wave (>150 mV vs. NHE), the —30 mV/pH dependence of the turnover frequency was observed for both Ee/Cu and Cu-free (Fe-only) forms of catalysts 2, and therefore it requires two reversible electron transfer steps prior to the turnover-determining step (TDS) and one proton transfer step either prior to the TDS or as the TDS. Under these conditions, the resting state of the catalyst was determined to be ferric-aqua/Cu which was in a rapid equilibrium with the fully reduced ferrous-aqua/Cu form (the Fe - and potentials were measured to be within < 20 mV of each other, as they are in cytochrome c oxidase, resulting in a two-electron redox equilibrium). This first redox equilibrium is biased toward the catalytically inactive fully oxidized state at potentials >0.1 V, and therefore it controls the molar fraction of the catalytically active metalloporphyrin. The fully reduced ferrous-aqua/Cu form is also in a rapid equilibrium with the catalytically active 5-coordinate ferrous porphyrin. As a result of these two equilibria, at 150 mV (vs. NHE), only <0.1%... 

Literature reports of NO disproportionation reactions with Fe(II) porphyrins contain many mutually inconsistent observations. Although facile NO disproportionation is promoted by Ru(II) and Os(II) (88) porphyrins to yield N20 and the respective M(Por)(NO)(ONO) complexes, the reactivity appears to be quite different with analogous Fe(II) complexes. Ferrous porphyrins such as Fen(TPP) undergo NO addition in ambient temperature solution to give the relatively stable... 

D. S. Bohle, C. H. Hung, Ligand-Promoted Rapid Nitric Oxide Dissociation from Ferrous Porphyrin Nitrosyls , J. Am. Chem. Soc. 1995,117, 9584-9585. 

We have examined the effects of electron donors on the CO binding rate for a pentacoordinated ferrous porphyrin (27). Comparison of the rates measured in toluene with those in a nonaromatic solvent indicate the maximal effect obtainable from D/A interactions with the phenyl ring of phenylalanine. We have also looked for effects from the addition of N N N -t j pjig l()Jyg gg lenediamine (TMPD), a... 

Anaerobic samples were prepared by degassing the solvent by three freeze-thaw cycles while solids and NMR tubes were degassed by placing under vacuum, then storing in a nitrogen atmosphere. If oxygen is not carefully excluded, the ferrous porphyrin obtained by the autoreduction may be rapidly oxidized to the oxo-bridged dimer. 

The ferrous porphyrins were reoxidized by introducing oxygen into the NMR tube of the autoreduced sample. To detect water as a product of the reoxidation of TPPFen(CN)22", it was necessary to completely exclude the possibility of atmospheric contact (dry DMSO rapidly absorbs water from the atmosphere). These experiments were done in an NMR tube fitted with a ground glass stopcock. This allowed the addition of oxygen into the NMR tube by vacuum line techniques, completely eliminating atmospheric contact. Experiments done on blanks of dry DMSO showed no water peak. 

Oxygen uptake studies show that the conversion of 39 to 40 requires 0.6 (1) mol of 02, indicating that four ferrous atoms are oxidized per consumed 02 (63). This stoichiometry is similar to that reported for the autoxidation of ferrous porphyrins. The proposed mechanism, shown in Scheme 5, suggests that the autoxidation is initiated by the interaction of 02 with monomeric units of 39 that are either coordinatively unsaturated or contain a labile ligand such as solvent. The reaction of a second monomeric unit with the transient superoxo ferric complex gives rise to a (fi-peroxo)diferric species similar to that reported for [Fe HB(3,5-iPr2pz)3 (OBz)(CH3CN)] (67). This species is then somehow reduced by two electrons from 39 to yield two molecules of 40. No intermediates in the autoxidation of 39 have yet been detected even at low temperatures (—80 °C), unlike the porphyrin systems in which intermediate species such as [(Por)Fe2+02], [(Por)Fe3+-00-Fe3+(Por)], and [(Por)-Fe4+ = 0] have been spectroscopically identified (68). 

Calculations in which the metal orbitals are explicitly considered will be discussed in Section HID. The molecular orbital theory as outlined here has been quite successful in dealing with the spectra of non-ferric porphyrins, with some exceptions. Ferrous porphyrins and haemopro-teins with small unsaturated ligands such as NO and O2 have additional bands (45) and Mn(III) porphyrins exhibit some quite unusual features (46). Distortion of the haem group may also give rise to anomalous effects. We shall discuss these further in Section V. 

Hemiglobin (Hi) is the hemoglobin derivative in which the ferrous porphyrin complex is converted to the ferric form by oxidation. This trans-... 

Model heme systems The mechanisms of heme and hemoprotein reactions with small molecules such as O2, CO and NO has attracted considerable experimental attention owing to the importance of such processes in biological systems. Flash photolysis studies [87] have indicated that the photolabilization of L from simple heme complexes and kinetics of the resulting back reaction (Eq. 6.40) can be modeled by the intermediacy of solvent caged contact pair . Equation (6.41) illustrates this mechanism for the thermal back reaction for the photochemically generated intermediates for a ferrous porphyrin (Por)Fe L (For = porphyrin)... 

Ferrous porphyrin is a good dioxygen binder, and this leads to the binding of molecular oxygen to produce the LS ferrous-dioxygen complex,... 

The Pentacoordinate Ferric-Porphyrin (2) and Ferrous-Porphyrin (3) Compiexes... 

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