NMR Studies of Nonplanar Porphyrins. Part 1. Axial Ligand Orientations in Highly Nonplanar Porphyrins
2010
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 CoIII–Nporphyrin 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.
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