Synthesis and anion binding properties of carbazole-based macrocycles with bis-sulfonamide and bis-amide groups

Two carbazole-based macrocycles 1 and 2 with bis-sulfonamide and bis-amide groups were facilely synthesized and carefully evaluated for their anion binding properties, via 1H NMR, UV–vis, and X-ray diffraction studies. Both macrocycles exhibited strong and selective binding to CH3COO−, PhCOO−, and F− ions (Ka > 6500 M−1 for the resultant 1:1 complexes) in the strongly polar solvent DMSO-d6, over the other anions tested. Interestingly, pyridine-2,6-diamide-containing macrocycle 2 showed a nearly 6-fold enhancement in affinity for Cl−, relative to the isophthalamide-containing analogous macrocycle 1. Additionally, the complexation between Cl− and macrocycle 2 was about one order of magnitude stronger than its acyclic analogue 3. More interestingly, given unique chemical shift changes experienced by two macrocycles upon addition of Cl−, this allowed them to serve as useful receptors for discriminating chloride from other anions in DMSO-d6. Finally, the X-ray crystallographic analysis provided convincing evidence for the binding of F− to macrocycle 2 (via three N–H⋯F− hydrogen bonds from one carbazole NH and two sulfonamide NHs groups, if excluding the bridging water molecules), fully consistent with the solution-phase 1H NMR titration experiments.