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 1 H, 13 C NMR, and MALDI‐TOF‐MS analyses. The host–guest complex formation between 1 and halides was investigated by using 1 H 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 10 8 , 1.79×10 7 , and 1.84×10 5   M −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 CH⋅⋅⋅X hydrogen bond interactions resulted in the strong binding affinity of 1 to halides.