Imidazole complexes of low-spin iron(III) porphyrin .pi.-cation radical species. Models for the compound I .pi.-cation radical state of peroxidases

Goff, Harold M.; Phillippi, Martin A.

doi:10.1021/ja00364a017

Cited by 38 publications

(36 citation statements)

References 2 publications

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“…It is also found to have a strongly saddled porphyrinate ring. 73 Yet another Fe(III) porphyrinate cation radical complex is that of the low-spin Fe(III) complex [TPPFe(ImH) 2 ] 2+ , which has a pyrrole-H shift of -40.1 ppm at -38°C, and the meso-phenyl-H shift differences, δ m -δ p , and δ m -δ o , are both quite large and positiVe 76 (Table 1), indicating positiVe spin density on the porphyrinate ring. Consistent with this, the magnetic moment of the complex (µ eff ) 2.8 ( 0.2 µ B ) 76 indicates two unpaired electrons (one on the metal and one on the ring).…”

Section: The Interesting Cases Of Macrocycle Radicalsmentioning

confidence: 99%

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 Interesting Cases Of Macrocycle Radicalsmentioning

confidence: 99%

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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“…It is now well established that there are two types of electronic ground states in low-spin Fe(III) porphyrins depending on which d orbital has unpaired electron, i.e. (d xy ) 2 (d xz , d yz ) 3 (d p -type) and (d xz , d yz ) 4 (d xy ) 1 (d xy -type). 1,2 One electron oxidation of Fe(III) porphyrins usually produces Fe(III) porphyrin radical cations, where an electron is removed from the porphyrin a 2u HOMO in the case of meso-substituted complexes.…”

mentioning

confidence: 99%

“…1,2 One electron oxidation of Fe(III) porphyrins usually produces Fe(III) porphyrin radical cations, where an electron is removed from the porphyrin a 2u HOMO in the case of meso-substituted complexes. [3][4][5] Because the Fe(III) ion has various spin-states and electron configurations, the Fe(III) porphyrin radical cations should have a wide variety of electronic ground states. If the Fe(III) ion is a low-spin ion, two types of electronic ground states are expected, i.e.…”

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confidence: 99%

Equilibrium between Fe(iv) porphyrin and Fe(iii) porphyrin radical cation: new insight into the electronic structure of high-valent iron porphyrin complexes

2013

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“…The fact that there are optical bands at long wavelength (between 600 and 750 nm) raises the possibility that these spectral features arise from a form of HRP at the Fe(IV) oxidation level in which an electron has been removed from the porphyrin ring and passed to the metal, producing a m-cation radical containing ferric iron [20,21].…”

Section: Resultsmentioning

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A comparison by magnetic circular dichroism of compound X and compound II of horseradish peroxidase

et al. 1987

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The chlorite product of horseradish peroxidase, compound X, is shown by magnetic circular dichroism (MCD) spectroscopy in the temperature range 1.6–50 K to have a very similar haem structure to compound II under the same conditions (pH 10.7). Both are concluded to contain the Fe(IV)=0 group. The MCD spectrum also detects an unusual species, absorbing at wavelengths between 600 and 750 nm, that has magnetic properties different from those of the ferryl haem group. It is suggested that this is a species at the same oxidation level as ferryl haem but with the porphyrin ring having suffered a one‐electron oxidation, i.e. [Fe(III) P.+].

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Imidazole complexes of low-spin iron(III) porphyrin .pi.-cation radical species. Models for the compound I .pi.-cation radical state of peroxidases

Cited by 38 publications

References 2 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

Equilibrium between Fe(iv) porphyrin and Fe(iii) porphyrin radical cation: new insight into the electronic structure of high-valent iron porphyrin complexes

A comparison by magnetic circular dichroism of compound X and compound II of horseradish peroxidase

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