Stabilization of higher-valent states of iron porphyrin by hydroxide and methoxide ligands: electrochemical generation of iron(IV)-oxo porphyrins.

Abstract

An electrochemical study of hydroxide-and methoxide-ligated iron(II) tetraphenylporphyrins possessing ortho-phenyl substituents that block ,L-oxo dimer formation has been carried out. Ligation by these strongly basic oxyanions promotes the formation of iron(IV)-oxo porphyrins upon one-electron oxidation. Further one-electron oxidation of the latter provides the iron(IV)-oxo porphyrin ir-cation radical. (6), Mossbauer (7), ESR (8), and electron nuclear double resonance (ENDOR) (9) spectral data, compound I has been assigned the structure of a low-spin oxo-ligated iron(IV) porphyrin ir-cation radical. This same entity is suggested to serve as the oxygen transfer agent formed at the active site of the cytochrome P-450 enzymes (10, 11). One-electron reduction of the ir-cation radical of compound I results in the formation of an iron(IV)-oxo species termed compound 11 (12). Interestingly, compounds I and II of the peroxidases exhibit almost identical redox potentials (13).Only a few well-characterized model analogs of compounds I and II exist. These iron(IV) porphyrins have all been generated chemically at low temperatures. Until now, an iron(IV)-oxo porphyrin species has not been generated electrochemically. In the cyclic voltammogram of simple iron porphyrins, anodic sweeps beyond the iron(II)/iron(III) couple yield two reversible one-electron oxidation waves (32). Originally it was proposed that the first wave pertained to the iron(III)/iron(IV) couple and the second wave was due to porphyrin 1r-cation radical formation (14,15). This was shown to be incorrect in a report (16) which demonstrated that the redox potential of the first oxidation was independent of the many axial ligands employed (16) and that the physical properties of the oxidized species were consistent with an iron(III) porphyrin ir-cation radical structure (17). The second wave was attributed to an iron(III) porphyrin dication.In the literature, there is no mention of electrochemical investigations involving HO-, CH30-, etc. ligated iron porphyrins. This does not of course indicate an oversight (17) but simply reflects the instability, due to ,u-oxo dimer formation, of oxo-ligated iron(III) porphyrins. The inability to electrochemically generate a compound I or compound II analog could be due to the importance of oxo ligation to the stabilization of high-valent iron porphyrin species. It is well established that metals in their higher-valent oxidation states are generally stabilized by oxo ligation (18). Using sterically hindered porphyrins that are capable of hydroxide ligation, we have now been able to observe the electrochemical generation ofiron(IV)-oxo porphyrin species. In addition, the similarity of the iron(IV)-oxo and the porphyrin ir-cation redox potentials provides a rationale for the redox potentials of compounds I and II of the peroxidases. In the present study we describe the electrochemistry of porphyrins 1 to 5 when ligated to Cl-, HO-, and CH30 and discuss these results as they relate to studies involving the chemical ge...

Ionically Tagged Iron Complex‐Catalyzed Epoxidation of Olefins in Imidazolium‐Based Ionic Liquids

“…Then, complex 4 was submitted to anodic and cathodic sweeps (Figure 9). A small cathodic shift was noted when comparing the values with those reported in the literature, [26] mostly because we used a different ligand (not a porphyrin-based ligand). However, in the anodic sweep, three interesting peaks are noted (a, b, and c in Figure 9) at +1.25, +1.01, and +0.80 V. Based on the elegant and classical study of Lee et al, [26] we could attribute those peaks to Fe III L 3 /Fe III L 3 (for c), Fe III L 3 /Fe IV L 3 (for b), and Fe IV L 3 /Fe IV L 3 (for a) cation radicals, in which L represents the ionophilic ligand 3.…”

“…First, ligand 3 was submitted to the same conditions and displayed an irreversible oxidation peak at +1.4 V (versus an Ag/AgCl electrode, data not shown). However, in the anodic sweep, three interesting peaks are noted (a, b, and c in Figure 9) at +1.25, +1.01, and +0.80 V. Based on the elegant and classical study of Lee et al, [26] we could attribute those peaks to Fe III L 3 /Fe III L 3 (for c), Fe III L 3 /Fe IV L 3 (for b), and Fe IV L 3 /Fe IV L 3 (for a) cation radicals, in which L represents the ionophilic ligand 3. It can be seen from Figure 9 that the complex displays two sets of reversible reduction processes at À0.57 (and À0.48) and À0.89 V (and À0.76).…”

Oxygen transfer involving nonheme iron: the influence of leaving group ability on the rate constant for oxygen transfer to (EDTA)Fe(III) from peroxycarboxylic acids and hydroperoxides.

“…Since a homolytic mechanism is not observed with (EDTA)Fe(III) or (TPP)Cr(III)Cl, the intersections of their linear free-energy plots must occur at a pKa of YOH >17 (essentially beyond the limit of pKa values of alcohol leaving groups). The 2e-oxidation potentials for the meso-tetrakis-(2,4,6-trimethylphenyl)porphinato iron(III) hydroxide and meso-tetrakis(2,4,6-trimethylphenyl)porphinato iron(III) methoxide to give the iron(IV) porphyrin ir-cation radical is +1.14 V (SCE), whereas the 2e-oxidation potential for (TPP)Fe(III)OMe is +1.16 V suggesting that the methyl substituents on the phenyl rings do not influence the oxidation potentials to yield the iron(IV) porphyrin r-cation radical (19,20). If one compares this 2e-oxidation potential to generate the iron(IV)-oxo porphyrin ir-cation radical with the 2e-oxidation potential to generate (TPP)Cr(V)O from (TPP)- (19,20).…”

“…The 2e-oxidation potentials for the meso-tetrakis-(2,4,6-trimethylphenyl)porphinato iron(III) hydroxide and meso-tetrakis(2,4,6-trimethylphenyl)porphinato iron(III) methoxide to give the iron(IV) porphyrin ir-cation radical is +1.14 V (SCE), whereas the 2e-oxidation potential for (TPP)Fe(III)OMe is +1.16 V suggesting that the methyl substituents on the phenyl rings do not influence the oxidation potentials to yield the iron(IV) porphyrin r-cation radical (19,20). If one compares this 2e-oxidation potential to generate the iron(IV)-oxo porphyrin ir-cation radical with the 2e-oxidation potential to generate (TPP)Cr(V)O from (TPP)- (19,20). Therefore, the le-oxidation is thermodynamically favored over the 2e-oxidation by -2.3 kCal M-1 so that a homolytic le-oxidation by alkyl hydroperoxides is observed with the iron(III)porphyrin.…”

Cytochrome P450 Enzymes

“…The conclusions about peroxy intermediates and stepwise reaction also have evidence against them (Auclair et al, 2002;Austin et al, 2006). Such reactions have considerable precedent in the case of peroxidases (Chapter 4.09), and it can be argued that the E m,7 for the (FeO) 3+ /(FeO) 2+ pair is at least as high in P450 as in horseradish peroxidase (Hayashi and Yamazaki, 1979;Lee et al, 1985;Macdonald et al, 1989). This mechanism (abstraction) could be applied to the dealkylation reactions as well, because the products are unstable and would degrade to the observed carbonyls.…”