NMR studies of nonplanar porphyrins. Part 1. Axial ligand orientations in highly nonplanar porphyrins

Abstract

The ligand orientations in the nonplanar porphyrin complexes 1a–e and 2a–e have been investigated using molecular mechanics calculations and proton NMR spectroscopy. The minimum energy structures calculated for complexes 1a–e show that the planes of the axial pyridine or imidazole ligands are orientated approximately parallel to the Co^III–N_porphyrin bonds, with the ligand ring planes being perpendicular to each other. For complexes 2a–e, the planes of the axial ligands in the calculated minimum energy structures are orientated along the porphyrin meso carbon axis and the ligand ring planes are perpendicular to each other. Thus, for both series of complexes the planes of the axial ligands are orientated parallel to cavities formed by these very nonplanar porphyrins. Proton NMR studies suggest that structures similar to those obtained from the molecular mechanics calculations are retained in solution. In some complexes, hindered rotation of the axial ligands is also observed. Complexes 1a–e and 2a–e are unusual examples of the porphyrin conformation influencing the orientations of axial ligands and, as such, may be useful as models for studying ligand orientation effects in relation to biological systems.