STABILIZATION OF THE LESS COMMON (D(XZ)D(YZ))(4)(D(XY))(1) IRON(III) PORPHYRIN GROUND ELECTRONIC-STATE - H-1-NMR INVESTIGATIONS OF IRON(III) 5,10,15,20-TETRACYCLOHEXYLPORPHYRIN

High-spin and low-spin complexes of the iron(III) tetrakis(meso-cycloh exyl)porphyrin (TCHP)Fe-III) have been studied by means of 1-D and 2-D H-1 NMR. The complete assignment of porphyrin and R-imidazole H-1 res onances 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 beta-Hs have been used as the probe of the electronic state o f an iron(III) metal ion. It has been found that cyanide coordinates t o the high-spin (TCHP)(FeCl)-Cl-III complex, leading to the formation of the low-spin [(TCHP)Fe-III(CN2)](-), with the rare (d(xz)d(yz))(4)( d(xy))(1) ground electronic state (the pyrrole beta-H resonance at 12. 01 ppm at 293 K in CD3OD). A contribution of two electronic configurat ions, (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 H-1 NMR shifts for these complexes include the pyrrole resonance at 2.81 ppm accompanied by th e 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 s tate increases in the order imidazole (ImH) < 1-methylimidazole (1-MeI m) < 1,2-dimethylimidazole (1,2-diMeIm), following an enlargement of t he axial ligand steric hindrance. The rotation of the 1,2-diMeIm aroun d Fe-N bond in the low-spin [(TCHP)Fe-III(1,2-diMeIm)(2)](+) complex i s slow on the H-1 NMR time scale even at 293 K. Consequently four beta -H resonances and the diastereotopy of the cyclohexyl meso-substituent s have been observed. The meso-cyclohexyl groups rotate freely at temp erature above 243 K, whereas the frozen rotation below 233 K leads to the formation of additional rotational isomers as demonstrated by mult iplicity of beta-H resonances for high-spin and low-spin complexes stu died.