Models of the cytochromes b. 8. Two-dimensional nuclear Overhauser and exchange spectroscopy studies of paramagnetic "cavity" type (tetraphenylporphinato)iron(III) complexes of planar ligands

Abstract

The paramagnetic bis-2-methylimidazole complex of (tetramesitylporphinato)iron(III) chloride, with short TI and T, relaxation times, was investigated in CD2C12 by NOESY (nuclear Overhauser and exchange spectroscopy) experiments as a function of temperature (-29, -54, -74 C). The two-dimensional (2-D) maps obtained at -29 and -54 C revealed cross peaks due to both chemical exchange and the nuclear Overhauser effect (NOE), while the map obtained at -74 C showed only NOE cross peaks. The NOE cross correlations are consistent with the 2-MeImH ligands being positioned over or near the meso carbon atoms, rather than over the porphyrin nitrogens, as previously suggested by Nakamura and Groves (Tetrahedron 1988, 44, 3225). The nonbonded interactions between the methyl groups of the 2-methylimidazole (2-MeImH) ligands and the mesityl groups of the porphyrin phenyl rings for this orientation of the axial ligands produce a strongly ruffled porphyrin core having two oblong cavities at right angles to each other, one above and one below the plane of the porphyrin ring, which hold the hindered axial ligands in perpendicular planes over the meso positions. The chemical exchange cross peaks are indicative of interconversion of all resonances of a particular type, due to rotation of the axial ligands rather than axial ligand exchange. This interpretation is confirmed by the fact that there are no cross peaks observed between free and coordinated 2-MelmH resonances at any of the temperatures investigated. Rotation of the 2-MeImH ligands requires that alternate phenyl rings flap up and down as the ligands rotate. The coupling pattern between the pyrrole-H protons is consistent with that expected on the basis of the extrapolated intercepts of the Curie plot.