Iron porphyrins electrochemical oxidation

Iron porphyrin complexes with axial (7-alkyl and (7-aryl groups have been prepared and fully characterized by several groups (17,18). Addition of a chemical oxidant to (19, 20), or electrochemical oxidation of (21), the low-spin iron(III)-alkyl (-aryl) porphyrins results in transient formation of an iron(IV) (7-alkyl (a-aryl) complex that undergoes reductive elimination to give the iron(II) N-substituted product as shown in Scheme 2. The iron(IV) intermediate has been directly observed by low temperature lH NMR spectroscopy (22) and spectroelectrochemistry (21). 

For the insertion of the iron ion into the porphyrin a variety of general procednres have been described and reviewed. In most cases, these methods lead to the formation of Fe complexes, which are then nsed to prepare Fe Fe, Fe, and Fe porphyrins. The most commonly employed methods for synthesizing Fe° porphyrins are described below. The preparation of the Fe° and Fe complexes from the iron(III) porphyrins by chemical or electrochemical means and the oxidized iron porphyrins (Fe° itt-cation radicals, Fe, Fe 7T-cation radicals, and Fe ) by chemically or electrochemically oxidizing the iron(III) porphyrins is described in more detail in the sections on the corresponding iron porphyrins below. Whereas Fe porphyrins can be photochemically rednced to Fe porphyrins, only a few examples of photooxidations of the iron center are known, which include laser photolysis of the co-condensation products of PFe at 15 K to produce... 

As already mentioned above, iron(I) porphyrins can undergo various reactions. They are of synthetic value for preparation of a-alkyl iron porphyrins, which can be obtained by direct alkylation of the corresponding electrochemically generated iron(I) porphyrins with alkyl halides. The highly reduced species can also be oxidized by molecular oxygen or hydrogen peroxide to yield the iron oxyporphyrin (oxophlorin), which is further oxidized to verdoheme by molecular oxygen, " as discussed in Section 6.1.5. 

Mitochondrial cytochrome c is perhaps the most widely studied of all metalloproteins with respect to its electrochemical properties. It is located in the inner-membrane space of mitochondria and transfers electrons between membrane-bound complex III and complex IV. The active site is an iron porphyrin with a redox potential (7) of -1-260 mV vs. NHE. The crystal structures of cytochrome c from tuna have been determined (8, 9) in both oxidation states at atomic resolution. It is found that the heme group is covalently linked to the protein via two thioether bridges, and part of its edge is exposed at the protein surface. Cytochrome c is a very basic protein, with an overall charge of -1-7/-l-8 at neutral pH. Furthermore, many of the excess basic lysine residues are clustered around the mouth of the heme crevice, giving rise to a pronounced charge asymmetry. 

The rich low-valent chemistry of iron porphyrins has been studied over many years. For example, [Fc°TPP]2 generated electrochemically reacts with R4N+ electrolyte cation to give [RFe TPP]. 79 The reactions of reduced porphyrins have also been studied by DVCA 80 The oxidation of iron(III) complexes of novel pentadentate pendant macrocyclic ligands were investigated and shown to polymerize in the presence of superoxide (02-).81... 

Oxoiron(IV) porphyrins, one oxidizing equivalent above the resting ferric state, are known as compound II in the catalytic cycle of peroxidases and catalases. The (Porp)Fe =0 complexes can be generated by (i) the homolytic O—O bond cleavage of (Porp)Fe -0-0-Fe (Porp), which is formed by the addition of dioxygen to iron(II) porphyrins in the presence of a nitrogen base, (ii) the chemical oxidation of iron(III) porphyrins by m-CPBA and PhIO under certain circumstances, (iii) the electrochemical oxidation of hydroxoiron(III) porphyrins, and (iv) the reactions of iron(III) porphyrins with hydroperoxides (ROOH) in aqueous or organic solvents. More detailed preparation methods and physical properties of various oxoiron(IV) porphyrin complexes are summarized in recent reviews. ... 

Also, with iron porphyrin deposited at the cathode, alkanes have been catalytically oxidized to ketones and alcohols in an electrochemical cell in the presence of oxygen An oxidation mechanism similar to that of a P-450 oxidation is assumed in this case. 

The direct electrochemical oxidation of an iron(III) porphyrin to give an iron(IV) complex has been observed on a number of occasions, but the high oxidation state product is often quite reactive [23, 24, 26, 189, 214-218]. Thus, the oxidations... 

Iron(IV) porphyrins can be generated upon oxidation of a-bonded iron(III) porphyrins [7, 12, 62, 184, 188, 219] with the stability of the electrooxidized product, depending in large part on the porphyrin macrocycle and the type of cr-bonded axial ligand [12]. The chemical or electrochemical oxidation of a Hs-fluoro Fe(III) complex, [(T(p, m-F2)PP)Fe(F)2] , has been examined by Nanthakumar and Goff [220,221] and the iron oxidation state was assigned as Fe(III) or Fe(IV), on the basis of NMR and UV-visible spectroscopy. 

In 1971, Felton etal examined one electrochemical oxidation of Fe(TPP)Cl, Fe(OEP)Cl, [Fe(TPP)]20, and [Fe(OEP)]20, and showed the formation of Fe(III) porphyrin TC-cation radicals and Fe(IV) porphyrin complexes [108]. Later, correlation between the half-wave potentials and a constants of phenyl and P-pyrrole substituents for Fe (TPP)Cl was reported, while oxidation potentials are insensitive to the nature of the axial anionic ligands [113]. These results are indicative of one electron abstract from the porphyrin rings for iron porphyrins with rather weak-field axial anionic ligands. Phillippi and Goff isolated these products as perchlorate salts and characterized them to be porphyrin rc-cation radicals by UV-vis, NMR, IR, and Mossbauer measurements [109]. 

The porphyrin ligand can support oxidation states of iron other than II and III. [Fe(I)Por] complexes are obtained by electrochemical or chemical reduction of iron(II) or iron(III) porphyrins. The anionic complexes react with alkyl hahdes to afford alkyl—iron (III) porphyrin complexes. Iron(IV) porphyrins are formally present in the carbene, RR C—Fe(IV)Por p.-carbido, PorFe(IV)—Fe(IV)Por nitrene, RN—Fe(IV)Por and p.-nittido, PorFe(IV)... 

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