Synthetic zinc tetrapyrroles complexing with pyridine as a single axial ligand

Zinc chlorins were prepared from chlorophyll-a. Visible spectra in benzene showed that synthetic zinc chlorins complexed with pyridine as an axial ligand to form the monopyridine adducts. The equilibrium constants for the complexation were dependent upon the chlorin structure: substitution of electron-withdrawing groups at the peripheral position enhanced the coordinated ability of the central zinc. 1H NMR spectra in benzene-d6 also indicated that single pyridine coordinated to the central zinc. Comparison of the equilibrium constant in a zinc chlorin with those of the corresponding zinc bacteriochlorin (7,8-dihydrochlorin) and porphyrin (17,18-dedihydrochlorin) led to an increase in the saturation and flexibility of the tetrapyrrole pi-plane ligands making the central zinc more axial-ligated. All the zinc tetrapyrroles in benzene complexed with pyridine to form 5-coordinated (1:1) complexes, not 6-coordinated bisadducts. The observed equilibrium constants were consistent with the energy changes of the complexation calculated from molecular modeling.