CONFORMATION CHANGES OF DIMERIC ZINC(II) PORPHYRINS INDUCED BY BINDING OF BIDENTATE LIGANDS

Three zinc(II) porphyrin dimers have been prepared and characterized by H-1 NMR spectroscopy. Their conformations in solution were evaluated on the basis of a calculation of the ring current shifts. The conformation of Zn2-1, in which the linking groups between two meso-tetrakis(2-aminophenyl)porphyrins are propanediamido groups, was found to be a "screwed down" conformation in CDCl3. The ligand binding properties of the Zn(II) porphyrins were analyzed by UV-vis and NMR spectroscopy. The ligands employed were 4,4'-bipyridyl (bpy), 1,2-bis(4-pyridyl)ethane (bpyEt), and 1,3-bis(4-pyridyl)propane(bpyPr). The formation constants and the thermodynamic parameters for the ligand adducts were calculated and compared with those for the corresponding monomers. The formation constant for the bpy adducts of Zn2-1 was small compared with that of Zn2-2, in which the linking groups between two meso-tetrakis(2-aminophenyl)porphyrins were phenylenediamido groups. The small formation constant for the bpy adduct of Zn2-1 can be ascribed to the free energy costs for breaking the screwed-down conformation of Zn2-1. On the other hand, Zn2-2 has a complementary structure for bpy binding and binds bpy with small conformational changes.