Stabilization of the Less Common (dxzdyz)⁴(dxy)¹Iron(III) Porphyrin Ground Electronic State: ¹H NMR Investigations of Iron(III) 5,10,15,20-Tetracyclohexylporphyrin

High-spin and low-spin complexes of the iron(III) tetrakis( meso -cyclohexyl)porphyrin ((TCHP)Fe III ) have been studied by means of 1-D and 2-D 1 H NMR. The complete assignment of porphyrin and R-imidazole 1 H 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)Fe III Cl complex, leading to the formation of the low-spin [(TCHP)Fe III (CN) 2 ] -, with the rare (d xz d yz ) 4 (d xy ) 1 ground electronic state (the pyrrole β-H resonance at 12.01 ppm at 293 K in CD 3 OD). A contribution of two electronic configurations, (d xy ) 2 (d xz d yz ) 3 and (d xz d yz ) 4 (d xy ) 1, to the ground state of the metal ion has been invoked for the low-spin [(TCHP)Fe III (R-Im) 2 ] + complexes. Characteristic 1 H 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 (d xz d yz ) 4 (d xy ) 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)Fe III (1,2-diMeIm) 2 ] + complex is slow on the 1 H 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.