Two carbazole disulfonamide-diamide macrocycles with semi-flexible meta-xylyl linkages for anion recognition

Abstract

Two novel carbazole disulfonamide-diamide macrocycles 1 and 2 with semi-flexible meta-xylyl linkages were designed, synthesized, and assessed for their anion binding properties, via¹H NMR and UV-vis titration studies. The single-crystal X-ray diffraction analysis indicated that all five of the NH groups in macrocycle 1 pointed into its inner cavity, with a favorable geometrical conformation for the complexation of anionic guests. The ¹H NMR titration results demonstrated that both macrocycles bound strongly and selectively to fluoride ions in DMSO-d₆ solution with the affinities in the order of magnitude of 10³ M⁻¹ through the formation of 1 : 1 complexes, which was higher than other ten anions tested (including its competitors acetate and dihydrogen phosphate ions). The binding affinities of the two macrocycles were found to follow the order F⁻ ≫ AcO⁻ > PhCOO⁻ > H₂PO₄⁻ > Cl⁻ > NO₂⁻/N₃⁻/Br⁻/HSO₄⁻/NO₃⁻/ClO₄⁻ (no measurable binding), which was not in accordance with the basicity order of these anions. In particular, macrocycle 1 containing the pyridine-2,6-diamide moiety displayed a preference for F⁻ over AcO⁻ and H₂PO₄⁻ (having the selectivity factors of 30 and 182, respectively), which was improved by nearly 3-fold after comparison with those for macrocycle 2 bearing the isophthalamide moiety.