Fe TMPyP

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,... 

Photocatalytic cleavage of 1,2-dioIs [117] or l,2-diphenylethane-l,2-diols [118] with Fe" porphyrin (Fe " (tmpyp)) leads to aldehydes and small quantities of the corresponding acids (eq. (16)). 

In similar conditions with KHSO5, Fe-TMPyP is less active compared to the manganese derivative such a difference is not fully understood at present. Electrochemical methods were also used to activate Mn and Fe complexes of TMPyP to bring about DNA cleavage (281). 

Mn- or Fe-TMPyP were also activated with ascorbate, KO2 or PhIO (187). In the presence of ascorbate and molecular oxygen the iron porphyrins appeared to be more efficient whereas manganese porphyrins were better activated with PhIO (187,191). However, PhIO is poorly soluble in water, and consequently, the next association of Mn-TMPyP with KHSO5 (182,192,193), or... 

Schoder (9) has determined the oxidation potential of the couple (47), and it can be deduced that in solutions containing Fe + and Fe(tir) at pH 3, both Fe iP and Fe P are formed. On the other hand, in 1 M HCl, the complexes FeClm ( =0,1,2) oxidize Fen(TMPyP) completely and rapidly to the extremely stable Fe (TMPyP)CH+. Hence, the insertion of Fe(III) from intensely coloured tiron complexes can be stopped by quenching in acid solution whereby the tiron complexes are decomposed. 

When the potential of the bulk reduction was shifted to that of the third reduction wave, Fe (OEP) generation increased with a corresponding loss of the fx-oxo complex, in addition to a new Fe (OEP)NO" band at 538 nm. The formation of the nitrosyl adduct by reduction of Fe(OEP)Cl in the presence of nitrite was also characterized by spectroelectrochemical IR measurements. The intermediacy of /x-oxo dimer reduction was also observed by Ikeda et al. during the reduction of NO by Fe(TMPyP). ... 

Electrocatalytic nitric oxide reductions were noted in the previously described studies by Meyer s group on nitrite reduction by water-soluble porphyrins. As NO is a proposed intermediate, it was used independently as a substrate to verify the assigned potential for the ferrous nitfosyl to nitroxyl transformation. Bettelheim et al. also investigated NO reduction by Fe(TMPyP) immobilized onto a GCE with a Nafion coating that repelled anions like nitrite from the electrode surface. Differential pulse voltammetry of this system obtained three distinct reduction waves at potentials ca. 0.5, -0.6, and —0.8 V, assigned using the mechanistic scheme of Meyer " to sequential reductions of Ee (NO" ), Ee (NO), and Ee (NO ). 

Similarly, Bedioui et al. used Ee(TPPS) immobilized in polyCpyrrole-alkylammonium) films to investigate the reversible reduction of [Ee (NO)] to [Fe (NO )] . They also examined similar reactivity with electropolymer-ized hemin, hematin, Fe(TMPyP), and Fe(TSPP). They found that a polymeric... 

Su and coworkers examined the effect of different metals like Mn " and Co as well as Fe " on this reactivity. For Fe porphyrins, e.g., Fe(TMPyP), an EC mechanism, encompassing Fe reduction preceding NO coordination, is pro-posed ". Water soluble Mn(TMPyP), Mn(2-TMPyP), Mn(TSS), and Mn(TSPP) illustrate that porphyrins with electron withdrawing groups exhibit higher rates in electrocatalytic NO reduction activity. While the reduction products are similar, the mechanism for Mn is proposed to be via a ECE mechanism, i.e., initial reduction of the Mn to Mn° precedes NO coordination and reduction. The Co(2-TMPyP) illustrated that a two electron couple, Co /, is more efficient than a one electron couple, Fe /, in yielding the N-N coupled product N2O. 

Barley, Meyer and coworkers showed that Fe TMPyP and Fe TSPP were effective electrocatalysts for the reduction of NOj to N2O, NH3, and NH20H . Current efficiencies as high as 97% were obtained for the formation of ammonia in some cases, Table 7.4. Under acidic conditions where nitrite disproportionates into mainly NO, a nitrosyl intermediate was implicated during... 

Outer-sphere reduction of [Fe ii(TMpyP)(OH2)] and [Fe (TMpyP)-(OHa)OH] by [Ru(NH3)e] +, with second-order rate constants 1.6x10 and 5.1 X 10 M s at 25 °C, 7=0.5 M, allows calculation of the Marcus selfexchange rate constants for the two species. The five-co-ordinate high-spin aquo-complex has a self-exchange rate of 1.2 x 10 M S whereas for the low-spin hydroxy-species the exchange rate exceeds 10 M s, consistent with a lower Franck-Condon barrier in reduction to low-spin iron(ii) for the latter complex. 

M. The data do not, however, allow a distinction to be drawn between electron transfer at the bridging oxygen, terminal water, or porphyrin periphery. In the presence of histidine, the major species is [Fe (TMpyP)(his)a], which is reduced by ascorbate in an inner-sphere pathway involving protonation and dissociation of one of the axial histidine ligands. The rate constant for histidine dissociation, 25.2 s, is remarkably similar to the rate of dissociation of methionine-80, 60 s, in the ascorbate reduction of cytochrome c. Autoreduction of [Fe TPP] in the presence of piperidine proceeds by proton abstraction from co-ordinated piperidine as shown by a large isotope effect in the reaction. Electron transfer produces iron(ii) and a nitrogen radical. 

Zaworotko and coworkers also studied the epoxidation property of porph MOM-4, containing ship-in-a-bottle encapsulated metalloporphyrins. Porph MOM-4 efficiently oxidizes 85% styrene substrate into 30% styrene oxide and 57% benzaldehyde, whereas the homogeneous catalyst Fe -TMPyP only let 35% conversion under the same conditions. To prove porph MOM-4 has size selectivity, they studied the oxidation efficiency of larger-sized substrates such as trans-stilbene and triph-enylethylene. Accompanying the increase in substrate size, there was a decrease in conversion. These results are consistent with the pore windows ( 9 x 9 A ) of porph MOM-4 that can select small substrate to enter the cages. 

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