Molecular structures and magnetic resonance spectroscopic investigations of highly distorted six-coordinate low-spin iron(III) porphyrinate complexes

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Three bis-axially ligated complexes of iron(III) octaethyltetraphenylporphyrin, (OETPP)FeIII, have been prepared, which are low-spin complexes, each with two axial nitrogen-donor ligands (N-methylimidazole (N-MeIm), 4-(dimethylamino)pyridine (4-NMe2Py), and 2-methylimidazole (2-MeImH)). The crystal and molecular structure of the bis-(2-MeImH) complex shows the macrocycle to be in a saddled conformation, with the ligands in perpendicular planes aligned at 14° to the porphyrin nitrogens so as to relieve the steric interaction between the 2-methyl groups and the porphyrin. The Fe-N(por) bond lengths are typical of nonplanar six-coordinate low-spin FeIII complexes, while the axial Fe-N(ax) bond lengths are substantially longer than those of [(TPP)Fe(2-MeImH)2]+ (2.09(2) Å as compared to 2.015(4) and 2.010(4) Å). The crystal and molecular structure of the bis-(4-NMe2Py) complex also shows the macrocycle to be in a mainly saddled conformation, but with a significant ruffled component. As a result, the average Fe-N(por) bonds are significantly shorter (1.951 Å as compared to 1.974 Å) than those of the bis-(2-MeImH) complex. One ligand is aligned at 9° to two trans porphyrin nitrogens, while the other is at 79° to the same porphyrin nitrogens, producing a dihedral angle of 70° between the ligand planes. The EPR spectrum of this complex, like that of the bis-(2-MeImH) complex, is of the "large gmax" type, with gmax = 3.29 and 3.26, respectively. However, in frozen CD2Cl2, [(OETPP)Fe(N-MeIm)2]+ exhibits both "large gmax" and normal rhombic signals, suggesting the presence of both "perpendicular" and "parallel" ligand orientations. The 1- and 2D 1H NMR spectra of each of these complexes, as well as the chloroiron(III) starting material, were investigated as a function of temperature. The COSY and NOESY/EXSY spectra of the chloride complex are consistent with the expected J-coupling and saddle inversion dynamics, respectively. Complete spectral assignments for the bis-(N-MeIm) and -(4-NMe2-Py) complexes have been made using 2D 1H NMR techniques. In each case, the number of resonances due to methylene (two) and phenyl protons (one each) is consistent with D2d symmetry, and therefore an effective perpendicular orientation of the axial ligands on the time scale of the NMR experiments. The temperature dependences of the 1H resonances of these complexes show significant deviations from Curie behavior, and also evidence of extensive ligand exchange and rotation. Spectral assignment of the eight methylene resonances of the bis-(2-MeImH) complex to the four ethyl groups was possible through the use of 2D 1H NMR techniques. The complex is fluxional, even at -90 °C, and ROESY data suggest that the predominant process is saddle inversion accompanied by simultaneous rotation of the axial ligands. Saddle inversion becomes slow on the 2D NMR time scale as the temperature is lowered in the ligand order of N-MeIm > 4-NMe2Py > 2-MeImH, probably due mainly to progressive destabilization of the ground state rather than progressive stabilization of the transition state of the increasingly "hindered" bis-ligand complexes.

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Journal of the American Chemical Society

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