Stereochemistry of low-spin cobalt porphyrins. 9. Molecular stereochemistry of two picket fence cobalt(II) derivatives.

The preparation and molecular structures of two five-coordinate cobalt(II) picket fence porphyrinates with imidazole ligands are described, [Co(TpivPP)(L)] (TpivPP, dianion of picket fence porphyrin). The ligands are the unhindered imidazole, 1-ethylimidazole, and the sterically hindered imidazole, 1,2-dimethylimidazole. Although the equatorial aspects of the geometry are quite equivalent, the axial coordination group geometry strongly reflects the differing steric requirements of the axial ligand. The hindering methyl group in 1,2-dimethylimidazole, adjacent to the coordinated imidazole nitrogen atom, leads to an increased CoN bond distance, a tilt of the CoN bond and unequal CoNC bond angles, all of which serve to reduce the steric strain when compared with the unhindered derivative.