Molecular Recognition in the Axial Ligation of Pyridines to Zinc Complexes of Tetraarylporphyrin Atropisomers

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Various tetraarylporphyrin atropisomers of the (o-substituted-phenyl)_m-(phenyl)_nporphyrin (m+n=4) type have been synthesized, separated chromatographically, and purified by the recrystallization in order to investigate molecular recognition of the rotational isomers systematically. In the cases of o-nitro- and o-methoxo- substituents, separation of atropisomers could be performed as is showing that these substituents have large bulk enough to hinder the rotation of aryl ring around the porphyrin plane. This situation affords unsymmetrical molecular spaces for the axial ligand on each side of the metalloporphyrin plane. During two years of this research project, we obtained nine pure species of the zinc complex of m=0 to m=3 fraction of o-nitro substituent compounds. This report (in Japanese) deals with a systematic study of the axial coordination of pyridine and nicotine to these nine zincporphyrins. The equilibrium constants determined by the spectral titration methods become larger accordinng to the increase of number of the hindered nitro substitution. The crystal and molecular structures of some Zn complexes have been determined by the X-ray method.The stereochemistry of the complexes shows 5-coordinate zincporphyrins with an axial pyridine coordinating to the "hindered" side of the porphyrin plane. It is concluded that the axial coordination space surrounded with peripheral nitro substituents of tetraarylporphyrins works affinitively toward the pyridine ligation, thus the ligand molecules recognize the shape of the coordination space developed on the porphyrin plane.