Electrochemical synthesis and characterization of the single-electron oxidation products of ferric porphyrins

Phillippi, Martin A.; Goff, Harold M.

doi:10.1021/ja00386a031

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“…Rather, the meso-H or mesophenyl-H shifts are by far the most diagnostic, because they provide evidence for the presence or absence of negative spin density at the meso-carbons. Similarly, for the antiferromagnetically coupled chloroiron porphyrinate radicals discussed in the previous section, the pyrrole-H shifts are not all that much different from those of other high-spin Fe-(III)-containing porphyrinate systems, whereas the mesophenyl shift differences are much larger and negative (+5 to +6 ppm for chloroiron porphyrinates 48,50,52,53 as compared to -42 to -53 ppm for antiferromagnetically coupled chloroiron porphyrinate radicals 74,75 ).…”

Section: The Special Case Of Iron Corrolatesmentioning

confidence: 70%

Pulsed EPR and NMR Spectroscopy of Paramagnetic Iron Porphyrinates and Related Iron Macrocycles: How To Understand Patterns of Spin Delocalization and Recognize Macrocycle Radicals

2003

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Pulsed EPR spectroscopic techniques, including ESEEM (electron spin echo envelope modulation) and pulsed ENDOR (electron−nuclear double resonance), are extremely useful for determining the magnitudes of the hyperfine couplings of macrocycle and axial ligand nuclei to the unpaired electron(s) on the metal as a function of magnetic field orientation relative to the complex. These data can frequently be used to determine the orientation of the g-tensor and the distribution of spin density over the macrocycle, and to determine the metal orbital(s) containing unpaired electrons and the macrocycle orbital(s) involved in spin delocalization. However, these studies cannot be carried out on metal complexes that do not have resolved EPR signals, as in the case of paramagnetic evenelectron metal complexes. In addition, the signs of the hyperfine couplings, which are not determined directly in either ESEEM or pulsed ENDOR experiments, are often needed in order to translate hyperfine couplings into spin densities. In these cases, NMR isotropic (hyperfine) shifts are extremely useful in determining the amount and sign of the spin density at each nucleus probed. For metal complexes of aromatic macrocycles such as porphyrins, chlorins, or corroles, simple rules allow prediction of whether spin delocalization occurs through σ or π bonds, and whether spin density on the ligands is of the same or opposite sign as that on the metal. In cases where the amount of spin density on the macrocycle and axial ligands is found to be too large for simple metal−ligand spin delocalization, a macrocycle radical may be suspected. Large spin density on the macrocycle that is of the same sign as that on the metal provides clear evidence of either no coupling or weak ferromagnetic coupling of a macrocycle radical to the unpaired electron(s) on the metal, while large spin density on the macrocycle that is of opposite sign to that on the metal provides clear evidence of antiferromagnetic coupling. The latter is found in a few iron porphyrinates and in most iron corrolates that have been reported thus far. It is now clear that iron corrolates are remarkably noninnocent complexes, with both negative and positive spin density on the macrocycle: for all chloroiron corrolates reported thus far, the balance of positive and negative spin density yields −0.65 to −0.79 spin on the macrocycle. On the other hand, for phenyliron corrolates, the balance of spin density on the macrocycle is zero, to within the accuracy of the calculations (Zakharieva, O.; Schünemann, V.; Gerdan, M.; Licoccia, S.; Cai, S.; Walker, F. A.; Trautwein, A. X. J. Am. Chem. Soc. 2002, 124, 6636−6648), although both negative and positive spin densities are found on the individual atoms. DFT calculations are invaluable in providing calculated spin densities at positions that can be probed by 1 H NMR spectroscopy, and the good agreement between calculated spin densities and measured hyperfine shifts at these positions leads to increased confidence in the calculated spin densities at position...

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Section: The Special Case Of Iron Corrolatesmentioning

confidence: 70%

Pulsed EPR and NMR Spectroscopy of Paramagnetic Iron Porphyrinates and Related Iron Macrocycles: How To Understand Patterns of Spin Delocalization and Recognize Macrocycle Radicals

2003

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“…results in a porphyrin-centered oxidation and not a metal-centered oxidation (17). Extensive physical characterization of the oneelectron oxidation product suggests it is best formulated as an iron(III) porphyrin ir-cation radical.…”

Section: Discussionmentioning

confidence: 99%

“…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.…”

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confidence: 94%

“…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.…”

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confidence: 99%

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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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“…If an oxygen transfer reaction occurred under the conditions of this experiment to form Fe(OEP)(NO) and Fe(OEP)(NO3), the latter complex should be observed in the proton NMR [5]. In addition, the experiments were completed rapidly enough so that there was no evidence for the μ-oxo complex (CH2: 6.04 and 5.08 ppm [19]), which was the long term decomposition product observed in our work by visible spectroscopy.…”

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Electrochemistry and spectroelectrochemistry of iron porphyrins in the presence of nitrite

Wei

2001

Inorganica Chimica Acta

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Abstract:The reaction of nitrite with ferric and ferrous porphyrins was examined using visible, infrared and NMR spectroscopy. Solutions of either ferric or ferrous porphyrin were stable in the presence of nitrite, with only complexation reactions being observed. Under voltammetric conditions, though, a rapid reaction was observed between nitrite and iron porphyrins to form the nitrosyl complex, Fe(P)(NO), where P = porphyrin. The products of the reduction of ferric porphyrins in the presence of nitrite were confirmed by visible spectroelectrochemistry to be Fe(P)(NO) and [Fe(P)]2O. Visible, NMR and infrared spectroscopy were used to rule out the formation of Fe(P)(NO) by the iron catalyzed disproportionation of nitrite.NOT THE PUBLISHED VERSION; this is the author's final, peer-reviewed manuscript. The published version may be accessed by following the link in the citation at the bottom of the page.Inorganica Chimica Acta, Vol. 314, No. 1-2 (March 2001): pg. 49-57. Publisher's link. This article is © Elsevier and permission has been granted for this version to appear in e-Publications@Marquette. Elsevier does not grant permission for this article to be further copied/distributed or hosted elsewhere without the express permission from Elsevier. 2A reaction between iron porphyrins and nitrite only occurred by the presence of both oxidation states (ferric/ferrous). The kinetics of the reaction was monitored by visible spectroscopy, and the reaction was found to be firstorder with respect to Fe(OEP)(Cl) and Fe(OEP). The products were the same as those observed in the spectroelectrochemical experiment. The rate was not strongly dependent upon the concentration of nitrite, indicating that the coordinated, not the free nitrite, was the reaction species. The observed kinetics was consistent with a mixed oxidation state nitrite bridged intermediate, which carried out the oxygen transfer reaction from nitrite to the iron porphyrin. The effect of nitrite coordination on the reaction rate was examined.

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Electrochemical synthesis and characterization of the single-electron oxidation products of ferric porphyrins

Cited by 82 publications

References 0 publications

Pulsed EPR and NMR Spectroscopy of Paramagnetic Iron Porphyrinates and Related Iron Macrocycles: How To Understand Patterns of Spin Delocalization and Recognize Macrocycle Radicals

Pulsed EPR and NMR Spectroscopy of Paramagnetic Iron Porphyrinates and Related Iron Macrocycles: How To Understand Patterns of Spin Delocalization and Recognize Macrocycle Radicals

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

Electrochemistry and spectroelectrochemistry of iron porphyrins in the presence of nitrite

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