Porphyrins radicals

The electronic absorption spectra of the products of one-electron electrochemical reduction of the iron(III) phenyl porphyrin complexes have characteristics of both iron(II) porphyrin and iron(III) porphyrin radical anion species, and an electronic structure involving both re.sonance forms Fe"(Por)Ph] and tFe "(Por—)Ph has been propo.sed. Chemical reduction of Fe(TPP)R to the iron(II) anion Fe(TPP)R) (R = Et or /7-Pr) was achieved using Li BHEt3 or K(BH(i-Bu)3 as the reductant in benzene/THF solution at room temperature in the dark. The resonances of the -propyl group in the F NMR spectrum of Fe(TPP)(rt-Pr) appear in the upfield positions (—0.5 to —6.0 ppm) expected for a diamagnetic porphyrin complex. This contrasts with the paramagnetic, 5 = 2 spin state observed... 

In classical mechanisms for C—H bond activation either C—H rr-bond donation or cyclic. 4-centered transition states are important, but these are precluded in the porphyrin systems and the mechanism proposed for activation of CH4, toluene, and Hy by the Rh porphyrin radicals is a new mechanistic possibility. 

However, it was pointed out that two other observations are out of line with the iron(I) formulation and more consistent with an iron(II)-porphyrin radical anion [290] (1) the low-intensity red-shifted Soret band in the UV-VIS spectrum with broad maxima in the a,(3-region compared to, for instance, Fe(TPP) in THF, is typical of a porphyrin radical, and (2) the bond lengths of the porphyrin core indicate population of the (antibonding) LUMO of the ligand (i.e., the presence of an extra electron in the re-system). The presence of porphyrin radical character in the electronic ground state was also inferred from the paramagnetic NMR-shifts of the pyrrole protons at the meso and p-carbon atoms [291]. 

The oxidation of peroxidases by hydroperoxide leads to a ferryl iron-oxo species as well as a radical cation on the porphyrin ring, which is sometimes transferred to an adjacent amino acid. This species is referred to as compound I. Compound I can oxidize substrates directly by a two-electron process to regenerate the native peroxidase, but, more commonly, it oxidizes substrates by an one-electron process to form compound II where the porphyrin radical cation has been reduced. Compound II, in turn, can perform a second one-electron... 

IV, F and Fig. 7). In catalatic mode, HPI exhibits no significant spectral change, snggesting that compoimd I has an oxidation equivalent in the form of a protein radical rather than a porphyrin radical. On the other hand, the W105F variant of HPI, which operates only in peroxidatic mode, has a porphyrin radical clearly evident in the absorbance and EPR spectra 101). 

The composition may then be described as Ln (OEP) (OEP ). This is easily seen from the paramagnetism of Lu(0EP)2 which has the magnetic moment of a radical, i. e. 1,7 B. M, in the solid state (4) As lutetium ions other than diamagnetic fare not known, only a porphyrin radical state is left to explain the properties, A similar radical state has been found for another doubledecker molecule, lutetium bis(phthalocyaninate), Lu(Pc)2 which is hence to be described as Lu (Pc) (Pc ) (11,12). All the... 

The ease of formation of porphyrin radicals and the existence of ground state charge transfer into the axial ligands (3 ) makes it necessary to keep in mind the possibility of ligand ejection accompanied by oxidation-reduction during the excited states of metal1oporphyri ns. 

Resonance Raman Studies of Porphyrin Radical Cations, Excited States, and Ligation Photodynamics... 

Metal(V) species derived from the complexes in Table I are rare. In fact, only one such species, L1Cr(V) (presumably a dioxo or hydro-oxo species), has been observed and characterized by ESR and UV-visible spectroscopies (45,69), Figs. 5 and 6. This Cr(V) species, which has a lifetime of several seconds at room temperature, was generated from a hydroperoxo precursor by an intramolecular transformation that closely resembles the proposed, but so far unobserved step in the chemistry of cytochrome P450, whereby the hydroperoxoiron(III) is transformed to the FeIV(P + ) form also known as oxene (P += porphyrin radical cation). All the steps in Scheme 1 for the L1Cr(H20)2+/02 reaction have been observed directly (45,69). 

Peroxidases are haem proteins that are activated from the ferric state to one-electron oxidants by H202. They play a significant role in the generation of radicals from xenobiotics. The compound I state contains one oxidising equivalent as an oxoferryl-haem entity and the second as a porphyrin -radical cation. Upon the oxidation of a substrate the porphyrin radical is repaired, giving the compound II. Reduction of the oxoferryl haem back to the ferric state by a second substrate molecule completes the enzyme cycle. In addition to the classical peroxidases, several other haem proteins display pseudo-peroxidase activity. The plant enzyme horseradish peroxidase (HRP) is often employed in model systems. 

The magnesium porphyrin radical, Mg(tetraphenylporphyrin)C104 (136), has been used as a model for the structural and stereochemical consequences of loss of an electron in photosynthetic chromophores.521 The primary photosynthetic reaction in plants and bacteria consists of a transfer of an electron, in the picosecond time domain, from the chlorophyll phototrap to nearby acceptors yielding chlorophyll -cation radicals. The structure of (136) shows a five-coordinated Mg2+ cation which is not quite symmetrically sited in the porphyrin ring but has metal-ligand distances similar to those found in the previous structures. The perchlorate anion is tightly bound (Mg—O = 2.01 A) in a monodentate mode. It was concluded that the porphyrin can act as an electron sink and that no major effects are found in the bond lengths, or on the stereochemistry, of the macrocycle. 

Fig. 5. Preparation of an iron(IV)oxo porphyrin radical cation bold lines, refer to meso-tetra mesityl porphine...

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