Two Crystalline Forms of Low-Spin [Fe(TMP)(5-MeHIm)2]ClO4. Relative Parallel and Perpendicular Axial Ligand Orientations  [J. Am. Chem. Soc. 1999, 121, 11144−11155].

Munro, Orde Q.; Serth-Guzzo, Judith A.; Turowska‐Tyrk, Ilona; Mohanrao, K.; Shokhireva, Tatjana Kh.; Walker, F. Ann; Debrunner, and Peter G.; Scheidt, W. Robert

doi:10.1021/ja004643q

Cited by 5 publications

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“…There is good evidence based on the X-ray and single-crystal EPR studies, that for almost all complexes exhibiting g tensors similar to that of [(TPP)Fe(Im-h 4 ) 2 ] + the ligand planes are nearly parallel (the angle between the planes is less than 30°). 18,[47][48][49][50][51] In fact, for single crystals of [(TPP)Fe(Im-h 4 ) 2 ] + itself, two types of molecules exist in the unit cell, both with the imidazole planes being parallel. 48 For the first type the plane orientation was close to the meso positions (41°) of the porphyrin ring, and for the second site the ligand planes were close to the N-Fe-N axis (6°).…”

Section: Resultsmentioning

confidence: 99%

¹H Pulsed ENDOR and ESEEM Evidence That the Bis-imidazole Complexes of Iron(III) Tetraphenylchlorin and Tetraphenylporphyrin Have the Same Order of g Values, and the Same Electronic Ground State

2001

J. Am. Chem. Soc.

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The electronic structures of the bis-imidazole complexes of iron(III) tetraphenylporphyrin ([(TPP)Fe(ImH)(2)](+)) and iron(III) tetraphenylchlorin ([(TPC)Fe(ImH)(2)](+)) in frozen glassy solutions have been studied by the pulsed electron nuclear double resonance (ENDOR) technique of Mims and by electron spin-echo envelope modulation (ESEEM) spectroscopy. ESEEM spectra have been used to determine the orientation of the imidazole ligand planes with respect to the g tensor axes. In the ENDOR spectra, the manifestations of the implicit TRIPLE effect described and explained earlier by Doan et al. (J. Am. Chem. Soc. 1996, 118, 7014) were seen. In this work, the explicit expressions describing this effect were derived for the first time and used to successfully simulate the proton ENDOR spectra at the low- (LF) and high-field (HF) edges of the EPR spectrum. Using pulsed ENDOR, we have been able to determine the spin density distributions in the pi-systems of both tetrapyrroles and show that [(TPC)Fe(ImH)(2)](+) has the electronic orbital ground state (d(xy)())(2)(d(xz)(),d(yz)())(3), the same as that known for [(TPP)Fe(ImH)(2)](+), and the largest principal g value corresponds to the g tensor axis 3, which is normal to the heme plane. For the TPP complex, the g tensor axis 1, corresponding to the smallest principal g value, was found to be at an angle phi(1) of 30-35 degrees from the N-Fe-N axis, with the ligand planes rotated by the angle of 20-25 degrees in the opposite direction. For the TPC complex, phi(1) was found to be about 25 degrees from the direction N(I)-Fe-N(III), where N(I) corresponds to the nitrogen of the saturated pyrrole ring. The ligand planes in this complex were found to be oriented at an angle of about 10 degrees in the opposite direction.

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Section: Resultsmentioning

confidence: 99%

¹H Pulsed ENDOR and ESEEM Evidence That the Bis-imidazole Complexes of Iron(III) Tetraphenylchlorin and Tetraphenylporphyrin Have the Same Order of g Values, and the Same Electronic Ground State

2001

J. Am. Chem. Soc.

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“…Subsequently, a number of additional iron(III) species were shown to have the two planar ligands oriented perpendicular to each other. [17][18][19]22 Most of these species display a large g max EPR spectrum 17,18,20,23 and an unusually small value of the Mo ¨ssbauer quadrupole splitting constant. 16,17,19,21,22 A final case is found for strong π-accepting ligands, such as 3-and 4-cyanopyridine, where the interaction with axial ligands lowers the energy of iron d π orbitals below d xy so that the ground state changes to (d xz ,d yz ) 4 (d xy ) 1 .…”

Section: Introductionmentioning

confidence: 99%

“…In addition to the above-mentioned [Fe(OEP)(3-ClPy) 2 ]-ClO 4 , 8 several other examples of iron(III) porphyrins with identical axial ligands that have different structure and/or physical properties have been studied. 18,24,25 Both [Fe(TPP)-(HIm) 2 ] + and [Fe(TMP)(5-MeIm) 2 ] + have been shown to form two different low-spin crystalline species. They are structurally distinguished by the relative orientation of the two axial ligands.…”

Section: Introductionmentioning

confidence: 99%

Low-Spin Bis(2-methylimidazole)(octaethylporphyrinato)iron(III) Chloride (perp-[Fe(OEP)(2-MeHIm)₂]Cl): A Consequence of Hydrogen Bonding?

Noll

Schulz

et al. 2006

Inorg. Chem.

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“…EPR spectroscopy, 6 and in some cases Mo ¨ssbauer spectroscopy, 7 of well-defined low-spin heme model compounds with high basicity pyridines, imidazoles, or cyanides as the axial ligands has shown that their "large g max " EPR signal is indicative of the (d xy ) 2 (d xz ,d yz ) 3 electronic ground state and near degeneracy of d xz and d yz . For complexes with planar axial ligands this correlates with perpendicular alignment of these ligands, [7][8][9][10] 12 also reveals properties such as low-intensity visible 13 and near-IR 14 MCD bands, as well as EPR g values that resemble those of iron chlorins 15 and proteins that contain them. 15,16 A Mo ¨ssbauer, EPR, and NMR study of the tert-butyl isocyanide complexes, [(TPP)Fe(t-BuNC) 2 ]ClO 4 and [(OEP)-Fe(t-BuNC) 2 ]ClO 4 , 17 has shown that their g values (g ⊥ ) 2.20-2.28, g || ) 1.94-1.83) are consistent with the purest (d xz ,d yz ) 4 -(d xy ) 1 ground-state systems observed thus far, with the most complete quenching of orbital momentum (∑g 2 as small as 13.5).…”

Section: Introductionmentioning

confidence: 99%

Structural, Magnetic, and Dynamic Characterization of the (d_xz,d_y_z)⁴(d_x_y)¹ Ground-State Low-Spin Iron(III) Tetraphenylporphyrinate Complex [(p-TTP)Fe(2,6-XylylNC)₂]CF₃SO₃

et al. 2000

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The synthesis and characterization of the trifluoromethanesulfonate salt of bis(2,6-xylyl isocyanide)tetrakis(p-tolyl)porphyrinatoiron(III), 2 ]CF 3 SO 3 (1), is reported. The crystal structure shows that the porphyrinate ring is strongly ruffled. The equatorial Fe-N bond distances average to 1.961(7) Å for 1, which is quite short for low-spin iron(III) porphyrinate derivatives. Two additional complexes, having TPP (2) and m-TTP (3) as the porphyrinates, were also synthesized and studied by NMR, EPR, and Mo ¨ssbauer spectroscopy. All physical properties are consistent with a low-spin iron(III) porphyrinate with the less-common ground-state configuration (d xz ,d yz ) 4 (d xy ) 1 . The 1 H NMR chemical shift of the pyrrole protons at 297 K is +10.7 ppm for 1. The EPR spectrum of 1 in solution is axial, with g ⊥ ) 2.15 and g || ) 1.94, ∑g 2 ) 13.0, while in the solid state g ⊥ ) 2.2 and g || ) 1.94, ∑g 2 ) 13.4. The Mo ¨ssbauer spectrum of 1 at 190 K has an isomer shift of 0.14 mm/s and quadrupole splitting of 1.81 mm/s. Magnetic Mo ¨ssbauer spectra analyzed in the intermediate spin-spin relaxation regime by the dynamic line-shape formalism of Blume and Clauser confirm this electron configuration and yield large negative quadrupole splittings, ∆E Q ) -1.8 to -2.0 mm/s for the three complexes. To our knowledge, this is the first case in which the Mo ¨ssbauer spectra of low-spin ferriheme systems have been analyzed in terms of the effect of intermediate rates of spin fluctuations on the appearance of the spectra. Analysis of the temperature dependence of the quadrupole splitting, ∆E Q , for 2 yielded a different estimate of the energy separation between the (d xz ,d yz ) 4 (d xy ) 1 ground state and an excited state than did the temperature dependence of the NMR isotropic shifts. It is postulated that the excited state is actually the planar transition state between the two ruffled conformations of the porphyrinate that are related by "inversion". To explain the temperature dependence of both NMR isotropic shifts and Mo ¨ssbauer quadupole splittings, the planar transition state must have the (d xy ) 2 (d xz ,d yz ) 3 electron configuration. The energy barrier appears to be smaller in homogeneous solution than in the solid state and is considerably lower than that predicted for the (d xz ,d yz ) 3 (d xy ) 2 excited electronic state of the ruffled conformation on the basis of the EPR g values.

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Two Crystalline Forms of Low-Spin [Fe(TMP)(5-MeHIm)₂]ClO₄. Relative Parallel and Perpendicular Axial Ligand Orientations [J. Am. Chem. Soc. 1999, 121, 11144−11155].

Cited by 5 publications

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¹H Pulsed ENDOR and ESEEM Evidence That the Bis-imidazole Complexes of Iron(III) Tetraphenylchlorin and Tetraphenylporphyrin Have the Same Order of g Values, and the Same Electronic Ground State

¹H Pulsed ENDOR and ESEEM Evidence That the Bis-imidazole Complexes of Iron(III) Tetraphenylchlorin and Tetraphenylporphyrin Have the Same Order of g Values, and the Same Electronic Ground State

Low-Spin Bis(2-methylimidazole)(octaethylporphyrinato)iron(III) Chloride (perp-[Fe(OEP)(2-MeHIm)₂]Cl): A Consequence of Hydrogen Bonding?

Structural, Magnetic, and Dynamic Characterization of the (d_xz,d_y_z)⁴(d_x_y)¹ Ground-State Low-Spin Iron(III) Tetraphenylporphyrinate Complex [(p-TTP)Fe(2,6-XylylNC)₂]CF₃SO₃

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Two Crystalline Forms of Low-Spin [Fe(TMP)(5-MeHIm)2]ClO4. Relative Parallel and Perpendicular Axial Ligand Orientations [J. Am. Chem. Soc. 1999, 121, 11144−11155].

Cited by 5 publications

References 0 publications

1H Pulsed ENDOR and ESEEM Evidence That the Bis-imidazole Complexes of Iron(III) Tetraphenylchlorin and Tetraphenylporphyrin Have the Same Order of g Values, and the Same Electronic Ground State

1H Pulsed ENDOR and ESEEM Evidence That the Bis-imidazole Complexes of Iron(III) Tetraphenylchlorin and Tetraphenylporphyrin Have the Same Order of g Values, and the Same Electronic Ground State

Low-Spin Bis(2-methylimidazole)(octaethylporphyrinato)iron(III) Chloride (perp-[Fe(OEP)(2-MeHIm)2]Cl): A Consequence of Hydrogen Bonding?

Structural, Magnetic, and Dynamic Characterization of the (dxz,dyz)4(dxy)1 Ground-State Low-Spin Iron(III) Tetraphenylporphyrinate Complex [(p-TTP)Fe(2,6-XylylNC)2]CF3SO3

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Two Crystalline Forms of Low-Spin [Fe(TMP)(5-MeHIm)₂]ClO₄. Relative Parallel and Perpendicular Axial Ligand Orientations [J. Am. Chem. Soc. 1999, 121, 11144−11155].

¹H Pulsed ENDOR and ESEEM Evidence That the Bis-imidazole Complexes of Iron(III) Tetraphenylchlorin and Tetraphenylporphyrin Have the Same Order of g Values, and the Same Electronic Ground State

¹H Pulsed ENDOR and ESEEM Evidence That the Bis-imidazole Complexes of Iron(III) Tetraphenylchlorin and Tetraphenylporphyrin Have the Same Order of g Values, and the Same Electronic Ground State

Low-Spin Bis(2-methylimidazole)(octaethylporphyrinato)iron(III) Chloride (perp-[Fe(OEP)(2-MeHIm)₂]Cl): A Consequence of Hydrogen Bonding?

Structural, Magnetic, and Dynamic Characterization of the (d_xz,d_y_z)⁴(d_x_y)¹ Ground-State Low-Spin Iron(III) Tetraphenylporphyrinate Complex [(p-TTP)Fe(2,6-XylylNC)₂]CF₃SO₃