Preparation and Characterization of a Dialkoxyiron(IV) Porphyrin

Groves, John T.; Quinn, Robert E.; McMurry, Thomas J.; Nakamura, Mikio; Lang, Georgina; Boso, Brian

doi:10.1021/ja00288a014

Cited by 120 publications

(93 citation statements)

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“…Table 1 summarizes the values in the literature and our results on a series of spectra from various complexes with different porphyrins, counter ions and solvents. The ferryl oxidation state of iron in these complexes is suggested from the isomer shifts which are consistent with published data on other complexes containing the [Fe0I3+ unit [l8, 27,32,[34][35][36][37][38][39].…”

Section: Mosshauer and Epr Spectroscopysupporting

confidence: 89%

Evidence for variable metal‐radical spin coupling in oxoferrylporphyrin cation radical complexes

Bill

Ding

Bominaar

et al. 1990

European Journal of Biochemistry

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Oxoferrylporphyrin cation radical complexes were generated by m-chloroperoxybenzoic acid oxidation of the chloro and trifluoromethanesulfonato complexes of tetramesitylporphyrinatoiron(II1) [(TMP)Fe] and the trifluoromethanesulfonato complex of tetra(2,6-dichlorophenyl)porphyrinatoiron(IlI) [TPP(2,. Coupling between ferry1 iron (S = 1) and porphyrin radical (S' = 1/2) spin systems was investigated by Mossbauer and EPR spectroscopy. The oxoferrylporphyrin cation radical systems generated from the TMP complexes show strong ferromagnetic coupling. Analysis of the magnetic Mossbauer spectra, using a spin Hamiltonian explicitly including a coupling tensor J , suggests an exchange-coupling constant J > 80 cm-'. The EPR spectra show non-zero rhombicity, the origin of which is discussed in terms of contributions from the usual zero-field effects of iron and from iron-radical spin-dipolar interaction. A consistent estimate of zero-field splitting parameter D = + 6 cm-' was obtained by EPR and Mossbauer measurements. EPR and Mossbauer parameters are shown to be slightly dependent on solvent, but not on the axial ligand in the starting (TMP)Fe complex. In contrast to the TMP complex, the oxoferrylporphyrin cation radical system generated from [TPP(2,6-C1)FeOS02CF3] exhibits Mossbauer and EPR spectra consistent with weak iron-porphyrin radical coupling of IJI = 1 cm-'.High-valency oxoheme complexes, formally two-electron oxidized above the ferric state, have been of considerable interest because of their importance in the chemistry of catalase and peroxidase enzymes [l, 21 and because they play a putative role in monooxygen transfer to organic substrates by cytochrome P450 [3, 41. Efforts to model biochemical function and to characterize better the electronic structure of these intermediates have resulted in the synthesis of oxoheme complexes in the same oxidation state as the enzymatic transients [5, 61 and in mimicking some of the biochemical functions of cytochrome P450 [5 -151. The sum of the physicochemical evidence marshalled to date best describes the complexes as oxoferrylporphyrin cation radical complexes [5, 16 -291. Despite the potential importance of the synthetic complexes for the elucidation of the electronic structures of the biological transients, only the oxoferrylporphyrin cation radical system [(TMP)Fe = 0 1 +Cl-, derived from tetramesitylporphyrinatoiron(II1) chloride [(TMP)FeCl], has been the object of serious efforts at characterization (complexes like [(TMP)Fe = O](Cl-), the counter ions are written in parentheses to indicate uncertainty as to the exact nature of their association with the iron(1V) center}. A large variety of spectroscopic techniques has been used for investigation of this species [6,16,18, 231. Strong ferromagnetic coupling between the electron spins of iron and the tetramesitylporphyrin radical has been demonCorrespondenre to A.

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Section: Mosshauer and Epr Spectroscopysupporting

confidence: 89%

Evidence for variable metal‐radical spin coupling in oxoferrylporphyrin cation radical complexes

Bill

Ding

Bominaar

et al. 1990

European Journal of Biochemistry

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“…Controlled-potential coulometry on the first oxidation peak of 1-OCH3 resulted in the removal of0.5 electron per equivalent ofiron porphyrin and the formation of the porphyrin ir-cation radical of the 1L-oxo dimer as the sole oxidation product (Fig. 4, trace B by NaOCH3 addition to the 7r-cation radical of 2-Cl has a Xmax at 545 nm with a shoulder at 575 nm as reported (25) for a similar species generated chemically. …”

Section: Resultssupporting

confidence: 67%

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

Lee¹,

Calderwood²,

Bruice³

1985

Proc. Natl. Acad. Sci. U.S.A.

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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...

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“…It is an Fe(V) equivalent. Furthermore, a six-coordinate Fe(IV) heme species with two axial amino acid ligands has also not been described in a protein, but analogous bis-ligated high-valent inorganic porphyrin model compounds have been generated, and in all cases a small isomer shift (␦) and large quadruple splitting (⌬E Q ) parameters are observed from those model compounds (25,26). An antiferromagnetic coupled diamagnetic Fe(III) radical has been observed in some model complexes by NMR spectroscopy (27).…”

Section: Identification Of Two Distinct Fe(iv) Heme Species In the Maugmentioning

confidence: 99%

A catalytic di-heme bis -Fe(IV) intermediate, alternative to an Fe(IV)=O porphyrin radical

Lee

et al. 2008

Proc. Natl. Acad. Sci. U.S.A.

207

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Preparation and Characterization of a Dialkoxyiron(IV) Porphyrin

Cited by 120 publications

References 1 publication

Evidence for variable metal‐radical spin coupling in oxoferrylporphyrin cation radical complexes

Evidence for variable metal‐radical spin coupling in oxoferrylporphyrin cation radical complexes

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

A catalytic di-heme bis -Fe(IV) intermediate, alternative to an Fe(IV)=O porphyrin radical

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