Strong binding affinity of a zinc-porphyrin-based receptor for halides through the cooperative effects of quadruple C-H hydrogen bonds and axial ligation

A new type of host compound (1), tetraphenyl zinc-porphyrin (ZnTPP) that contains four triazole groups at the ortho-position of each phenyl group, has been synthesized and characterized by using ¹H, ¹³C NMR, and MALDI-TOF-MS analyses. The host-guest complex formation between 1 and halides was investigated by using ¹H NMR spectroscopy in [D ₆]DMSO. The triazole, benzyl, and phenylene proton signals were shifted upfield by the addition of halides in the form of tetrabutylammonium salts, which implies that the triazole protons in 1 are allocated very closely to the porphyrin ring and are directed toward the binding pocket over the porphyrin ring during the formation of hydrogen bonds. The UV/Vis absorption spectra showed that both the Soret and Q band absorptions of 1 underwent a strong redshift due to the addition of halides. Compound 1 exhibited surprisingly strong binding affinities for the halides, where the association constants for Cl^-, Br^-, and I^- binding were estimated to be larger than 10⁸, 1.79× 10⁷, and 1.84× 10⁵M^-1, respectively. The UV/Vis absorption changes and the result of competitive titration using 4-tert-butylpyridine indicated that the cooperative effects of axial coordination and C-H⋯X hydrogen bond interactions resulted in the strong binding affinity of 1 to halides.