Halogen Bonding Heteroditopic [2]Catenanes for Recognition of Alkali Metal/Halide Ion Pairs.

The first examples of halogen bonding (XB) heteroditopic homo[2]catenanes were prepared by discrete Na+ template-directed assembly of oligo(ethylene glycol) units derived from XB donor-containing macrocycles and acyclic bis-azide precursors, followed by a Cu(I) mediated azide-alkyne cycloaddition macrocyclisation reaction. Extensive 1H NMR spectroscopic studies show that the [2]catenane hosts exhibit positive cooperative ion-pair recognition behaviour, wherein XB-mediated halide recognition is enhanced by alkali metal cation pre-complexation. Notably, subtle changes in the catenanes' oligo(ethylene glycol) chain length dramatically alters ion binding affinity, stoichiometry, complexation mode and conformational dynamics. Solution phase and single crystal X-ray diffraction studies provide evidence for competing host-separated and direct contact ion-pair binding modes. We further demonstrate the [2]catenanes are capable of extracting solid alkali metal halide salts into organic media.