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Stabilization of the less common (dxzdyz)4(dxy)1 iron(III) porphyrin ground electronic state: 1H NMR investigations of iron(III) 5,10,15,20-tetracyclohexylporphyrin.

Autorzy

Stanisław Wołowiec

Lechosław Latos-Grażyński

D. Toronto

J. C. Marchon

Rok wydania

1998

Czasopismo

Inorganic Chemistry

Numer woluminu

37

Strony

724-732

DOI

10.1021/ic9707927

Kolekcja

Naukowa

Język

Angielski

Typ publikacji

Artykuł

Streszczenie

High-spin and low-spin complexes of the iron(III) tetrakis(meso-cyclohexyl)porphyrin ((TCHP)FeIII) have been studied by means of 1-D and 2-D 1H NMR. The complete assignment of porphyrin and R-imidazole 1H resonances has been done on the basis of 2-D COSY and NOESY techniques as well as by selective deuteration of imidazole. The chemical shifts of pyrrole β-Hs have been used as the probe of the electronic state of an iron(III) metal ion. It has been found that cyanide coordinates to the high-spin (TCHP)FeIIICl complex, leading to the formation of the low-spin [(TCHP)FeIII(CN)2]-, with the rare (dxzdyz)4(dxy)1 ground electronic state (the pyrrole β-H resonance at 12.01 ppm at 293 K in CD3OD). A contribution of two electronic configurations, (dxy)2(dxzdyz)3 and (dxzdyz)4(dxy)1, to the ground state of the metal ion has been invoked for the low-spin [(TCHP)FeIII(R-Im)2]+ complexes. Characteristic 1H NMR shifts for these complexes include the pyrrole resonance at 2.81 ppm accompanied by the markedly upfield shifted imidazole resonances at −19.67 ppm (2-H), −10.58 ppm (4-H), −4.05 ppm (5-H), and 0.97 ppm (1-H). An admixture of a (dxzdyz)4(dxy)1 configuration into the ground electronic state increases in the order imidazole (ImH) < 1-methylimidazole (1-MeIm) < 1,2-dimethylimidazole (1,2-diMeIm), following an enlargement of the axial ligand steric hindrance. The rotation of the 1,2-diMeIm around Fe−N bond in the low-spin [(TCHP)FeIII(1,2-diMeIm)2]+ complex is slow on the 1H NMR time scale even at 293 K. Consequently four β-H resonances and the diastereotopy of the cyclohexyl meso-substituents have been observed. The meso-cyclohexyl groups rotate freely at temperature above 243 K, whereas the frozen rotation below 233 K leads to the formation of additional rotational isomers as demonstrated by multiplicity of β-H resonances for high-spin and low-spin complexes studied.

Słowa kluczowe

Imidazoles, Ligands, Nuclear magnetic resonance spectroscopy, Pyrroles, Resonance structures

Adres publiczny

https://doi.org/10.1021/ic9707927

Strona internetowa wydawcy

https://www.acs.org/content/acs/en.html

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