Impact of distortion of porphyrins on axial coordination in (porphyrinato)zinc(II) complexes with aminopyridines as axial ligands

A series of (porphyrinato)zinc(II) compounds were synthesized with use of 5,10,15,20-tetraphenylporphyrin (H₂TPP), 2,3,5,10,12,13,15,20- octaphenylporphyrin (H₂OPP), and 2,3,5,7,8,10,12,13,15,17,18,20- dodecaphenylporphyrin (H2DPP). Those compounds form complexes with aniline, pyridine, and 3- and 4-aminopyridines as axial ligands. X-ray crystallography was performed on the complexes with 3-aminopyridine (3-AP) and 4-aminopyridine (4-AP) as axial ligands. 3-Aminopyridine was revealed to bind through the amino group to the Zn(OPP), exhibiting intermolecular π-π interaction between 3-AP and one of the pyrrole rings and intermolecular NH-π interactions of the coordinated amino group with two β-phenyl groups of an adjacent molecule. In solu-tion, the aminopyridines form a single species at ambient temperature and are assumed to have pyridine coordination through the aromatic pyridine nitrogen atom. Variable-temperature NMR spectroscopy in CD₂Cl ₂ indicates that two different species exist at lower temperatures, suggesting that amino-bound complexes of 3-AP can be formed as a metastable species in solution, which is stabilized in the crystal as a result of noncovalent interactions. The binding constants of aminopyridines to the three kinds of (porphyrinato)zinc complexes reveal enhancement of the axial ligation by virtue of the distortion of the porphyrin ring.