Electron Configuration and Spin Distribution in Low-Spin (meso-Tetraalkylporphyrinato)iron(III) Complexes Carrying One or Two Orientationally Fixed Imidazole Ligands

Mikio Nakamura, Takahisa Ikeue, Hiroshi Fujii, Tetsuhiko Yoshimura, Kunihiko Tajima

Research output: Contribution to journalArticle

29 Citations (Scopus)

Abstract

To understand the orientation effect of coordinated imidazole ligands, a series of low-spin (tetraalkylporphyrinato)-iron(III) complexes, [Fe(TRP)(L)2J+ and [Fe(TRP)(L)(CN)], carrying at least one orientationally fixed imidazole (L) have been prepared. The 1H NMR pyrrole signals of a series of [Fe(TRP)(2-MeIm)2]+ have shown considerable downfield shifts as the meso substituent becomes bulkier, from -30.4 (R = H) to +5.6 ppm (R = iPr) at -71 °C. These complexes have exhibited four pyrrole signals at lower temperature due to the hindered ligand rotation. The spread of the pyrrole signals decreases from 9.4 (Me) to 8.2 (Et) and then to 5.7 (iPr) ppm. The downfield pyrrole signals together with the small spread in [Fe(TiPrP)(2-MeIm)2]+ are in sharp contrast to the other low-spin complexes with orientationally fixed imidazole ligands; the chemical shifts and spreads of the pyrrole signals in [tetrakis(2,4.6-trialkylphenyl)porphyrinato]iron(III) complexes [Fe(R-TPP)(2-MeIm)2]+ (R = Me. Et, iPr) are ca. -20 and ca. 9 ppm, respectively, at -71 °C. The EPR spectra of a series of [Fe(TRP)(2-MeIm)2]+ were then taken at 4.2 K. While the R = H, Me, and Et complexes have shown so-called "large gmax type" spectra as in the case of [Fe(R-TPP)(2-MeIm)2]+, the iPr complex has exhibited an "axial type" spectrum. The result indicates that the electron configuration of the ferric ion of [Fe(TiPrP)(2-MeIm)2]+ is presented by the unusual (dxz, dyz)4-(dxy)1 in contrast to the other low-spin complexes where ferric ions have the (dxy)2(dxz, dyz)3 configuration. When one of the 2-MeIm ligands in [Fe(TRP)(2-MeIm)2]+ is replaced by CN-, not only the iPr but also the Me and Et complexes have shown the (dxz, dyz)4(dxy)1 configuration as revealed from the EPR spectra. The pyrrole signals of the iPr complex [Fe(TiPrP)(2-MeIm)(CN)] have been observed at 12.2, 14.1, 14.8, and 16.2 ppm at -71 °C. Thus, the spread is only 4.0 ppm. The value is quite different from that of the corresponding [Fe(Me-TPP)(2-iPrIm)(CN)] where the spread reaches as much as 11.4 ppm. On the basis of these results, it is concluded that the spin distribution on the pyrrole β-carbons in the complexes with (dxz, dyz)4(dxy)1 is rather homogeneous even if the coordinated imidazole is orientationally fixed. On the contrary, the fixation induces a larger asymmetric spin distribution on these carbons in the complexes with (dxy)2(dxz, dyz)3 configuration.

Original languageEnglish
Pages (from-to)2405-2414
Number of pages10
JournalInorganic Chemistry
Volume37
Issue number10
Publication statusPublished - 1998 May 18

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Pyrroles
pyrroles
imidazoles
Iron
Ligands
iron
ligands
Electrons
configurations
electrons
ferric ions
Paramagnetic resonance
Carbon
Ions
carbon
Chemical shift
imidazole
chemical equilibrium
Nuclear magnetic resonance
nuclear magnetic resonance

ASJC Scopus subject areas

  • Inorganic Chemistry

Cite this

Electron Configuration and Spin Distribution in Low-Spin (meso-Tetraalkylporphyrinato)iron(III) Complexes Carrying One or Two Orientationally Fixed Imidazole Ligands. / Nakamura, Mikio; Ikeue, Takahisa; Fujii, Hiroshi; Yoshimura, Tetsuhiko; Tajima, Kunihiko.

In: Inorganic Chemistry, Vol. 37, No. 10, 18.05.1998, p. 2405-2414.

Research output: Contribution to journalArticle

Nakamura, Mikio ; Ikeue, Takahisa ; Fujii, Hiroshi ; Yoshimura, Tetsuhiko ; Tajima, Kunihiko. / Electron Configuration and Spin Distribution in Low-Spin (meso-Tetraalkylporphyrinato)iron(III) Complexes Carrying One or Two Orientationally Fixed Imidazole Ligands. In: Inorganic Chemistry. 1998 ; Vol. 37, No. 10. pp. 2405-2414.
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title = "Electron Configuration and Spin Distribution in Low-Spin (meso-Tetraalkylporphyrinato)iron(III) Complexes Carrying One or Two Orientationally Fixed Imidazole Ligands",
abstract = "To understand the orientation effect of coordinated imidazole ligands, a series of low-spin (tetraalkylporphyrinato)-iron(III) complexes, [Fe(TRP)(L)2J+ and [Fe(TRP)(L)(CN)], carrying at least one orientationally fixed imidazole (L) have been prepared. The 1H NMR pyrrole signals of a series of [Fe(TRP)(2-MeIm)2]+ have shown considerable downfield shifts as the meso substituent becomes bulkier, from -30.4 (R = H) to +5.6 ppm (R = iPr) at -71 °C. These complexes have exhibited four pyrrole signals at lower temperature due to the hindered ligand rotation. The spread of the pyrrole signals decreases from 9.4 (Me) to 8.2 (Et) and then to 5.7 (iPr) ppm. The downfield pyrrole signals together with the small spread in [Fe(TiPrP)(2-MeIm)2]+ are in sharp contrast to the other low-spin complexes with orientationally fixed imidazole ligands; the chemical shifts and spreads of the pyrrole signals in [tetrakis(2,4.6-trialkylphenyl)porphyrinato]iron(III) complexes [Fe(R-TPP)(2-MeIm)2]+ (R = Me. Et, iPr) are ca. -20 and ca. 9 ppm, respectively, at -71 °C. The EPR spectra of a series of [Fe(TRP)(2-MeIm)2]+ were then taken at 4.2 K. While the R = H, Me, and Et complexes have shown so-called {"}large gmax type{"} spectra as in the case of [Fe(R-TPP)(2-MeIm)2]+, the iPr complex has exhibited an {"}axial type{"} spectrum. The result indicates that the electron configuration of the ferric ion of [Fe(TiPrP)(2-MeIm)2]+ is presented by the unusual (dxz, dyz)4-(dxy)1 in contrast to the other low-spin complexes where ferric ions have the (dxy)2(dxz, dyz)3 configuration. When one of the 2-MeIm ligands in [Fe(TRP)(2-MeIm)2]+ is replaced by CN-, not only the iPr but also the Me and Et complexes have shown the (dxz, dyz)4(dxy)1 configuration as revealed from the EPR spectra. The pyrrole signals of the iPr complex [Fe(TiPrP)(2-MeIm)(CN)] have been observed at 12.2, 14.1, 14.8, and 16.2 ppm at -71 °C. Thus, the spread is only 4.0 ppm. The value is quite different from that of the corresponding [Fe(Me-TPP)(2-iPrIm)(CN)] where the spread reaches as much as 11.4 ppm. On the basis of these results, it is concluded that the spin distribution on the pyrrole β-carbons in the complexes with (dxz, dyz)4(dxy)1 is rather homogeneous even if the coordinated imidazole is orientationally fixed. On the contrary, the fixation induces a larger asymmetric spin distribution on these carbons in the complexes with (dxy)2(dxz, dyz)3 configuration.",
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TY - JOUR

T1 - Electron Configuration and Spin Distribution in Low-Spin (meso-Tetraalkylporphyrinato)iron(III) Complexes Carrying One or Two Orientationally Fixed Imidazole Ligands

AU - Nakamura, Mikio

AU - Ikeue, Takahisa

AU - Fujii, Hiroshi

AU - Yoshimura, Tetsuhiko

AU - Tajima, Kunihiko

PY - 1998/5/18

Y1 - 1998/5/18

N2 - To understand the orientation effect of coordinated imidazole ligands, a series of low-spin (tetraalkylporphyrinato)-iron(III) complexes, [Fe(TRP)(L)2J+ and [Fe(TRP)(L)(CN)], carrying at least one orientationally fixed imidazole (L) have been prepared. The 1H NMR pyrrole signals of a series of [Fe(TRP)(2-MeIm)2]+ have shown considerable downfield shifts as the meso substituent becomes bulkier, from -30.4 (R = H) to +5.6 ppm (R = iPr) at -71 °C. These complexes have exhibited four pyrrole signals at lower temperature due to the hindered ligand rotation. The spread of the pyrrole signals decreases from 9.4 (Me) to 8.2 (Et) and then to 5.7 (iPr) ppm. The downfield pyrrole signals together with the small spread in [Fe(TiPrP)(2-MeIm)2]+ are in sharp contrast to the other low-spin complexes with orientationally fixed imidazole ligands; the chemical shifts and spreads of the pyrrole signals in [tetrakis(2,4.6-trialkylphenyl)porphyrinato]iron(III) complexes [Fe(R-TPP)(2-MeIm)2]+ (R = Me. Et, iPr) are ca. -20 and ca. 9 ppm, respectively, at -71 °C. The EPR spectra of a series of [Fe(TRP)(2-MeIm)2]+ were then taken at 4.2 K. While the R = H, Me, and Et complexes have shown so-called "large gmax type" spectra as in the case of [Fe(R-TPP)(2-MeIm)2]+, the iPr complex has exhibited an "axial type" spectrum. The result indicates that the electron configuration of the ferric ion of [Fe(TiPrP)(2-MeIm)2]+ is presented by the unusual (dxz, dyz)4-(dxy)1 in contrast to the other low-spin complexes where ferric ions have the (dxy)2(dxz, dyz)3 configuration. When one of the 2-MeIm ligands in [Fe(TRP)(2-MeIm)2]+ is replaced by CN-, not only the iPr but also the Me and Et complexes have shown the (dxz, dyz)4(dxy)1 configuration as revealed from the EPR spectra. The pyrrole signals of the iPr complex [Fe(TiPrP)(2-MeIm)(CN)] have been observed at 12.2, 14.1, 14.8, and 16.2 ppm at -71 °C. Thus, the spread is only 4.0 ppm. The value is quite different from that of the corresponding [Fe(Me-TPP)(2-iPrIm)(CN)] where the spread reaches as much as 11.4 ppm. On the basis of these results, it is concluded that the spin distribution on the pyrrole β-carbons in the complexes with (dxz, dyz)4(dxy)1 is rather homogeneous even if the coordinated imidazole is orientationally fixed. On the contrary, the fixation induces a larger asymmetric spin distribution on these carbons in the complexes with (dxy)2(dxz, dyz)3 configuration.

AB - To understand the orientation effect of coordinated imidazole ligands, a series of low-spin (tetraalkylporphyrinato)-iron(III) complexes, [Fe(TRP)(L)2J+ and [Fe(TRP)(L)(CN)], carrying at least one orientationally fixed imidazole (L) have been prepared. The 1H NMR pyrrole signals of a series of [Fe(TRP)(2-MeIm)2]+ have shown considerable downfield shifts as the meso substituent becomes bulkier, from -30.4 (R = H) to +5.6 ppm (R = iPr) at -71 °C. These complexes have exhibited four pyrrole signals at lower temperature due to the hindered ligand rotation. The spread of the pyrrole signals decreases from 9.4 (Me) to 8.2 (Et) and then to 5.7 (iPr) ppm. The downfield pyrrole signals together with the small spread in [Fe(TiPrP)(2-MeIm)2]+ are in sharp contrast to the other low-spin complexes with orientationally fixed imidazole ligands; the chemical shifts and spreads of the pyrrole signals in [tetrakis(2,4.6-trialkylphenyl)porphyrinato]iron(III) complexes [Fe(R-TPP)(2-MeIm)2]+ (R = Me. Et, iPr) are ca. -20 and ca. 9 ppm, respectively, at -71 °C. The EPR spectra of a series of [Fe(TRP)(2-MeIm)2]+ were then taken at 4.2 K. While the R = H, Me, and Et complexes have shown so-called "large gmax type" spectra as in the case of [Fe(R-TPP)(2-MeIm)2]+, the iPr complex has exhibited an "axial type" spectrum. The result indicates that the electron configuration of the ferric ion of [Fe(TiPrP)(2-MeIm)2]+ is presented by the unusual (dxz, dyz)4-(dxy)1 in contrast to the other low-spin complexes where ferric ions have the (dxy)2(dxz, dyz)3 configuration. When one of the 2-MeIm ligands in [Fe(TRP)(2-MeIm)2]+ is replaced by CN-, not only the iPr but also the Me and Et complexes have shown the (dxz, dyz)4(dxy)1 configuration as revealed from the EPR spectra. The pyrrole signals of the iPr complex [Fe(TiPrP)(2-MeIm)(CN)] have been observed at 12.2, 14.1, 14.8, and 16.2 ppm at -71 °C. Thus, the spread is only 4.0 ppm. The value is quite different from that of the corresponding [Fe(Me-TPP)(2-iPrIm)(CN)] where the spread reaches as much as 11.4 ppm. On the basis of these results, it is concluded that the spin distribution on the pyrrole β-carbons in the complexes with (dxz, dyz)4(dxy)1 is rather homogeneous even if the coordinated imidazole is orientationally fixed. On the contrary, the fixation induces a larger asymmetric spin distribution on these carbons in the complexes with (dxy)2(dxz, dyz)3 configuration.

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