Anion recognition based on halogen bonding: a case study of macrocyclic imidazoliophane receptors

The structures and properties of noncovalent interactions involving three imidazoliophane receptors 1-3 and halide anions have been investigated by means of density functional theory calculations. To account for the influence of the solvent environment, the implicit polarized continuum model was also employed. For the halogenated cyclophane receptors 1 and 2, the halide ions are held by a bidentate array of halogen bonds (C-Br/C-I...X-), while multiple hydrogen-bonding interactions (C-H...X-) are present in the complexes of the nonhalogenated macrocyclic receptor 3. To accommodate the negatively charged guest anions, the structures of 1 and 2 fully reorganize into a calix-like shape, while both the imidazole and benzene rings in 3 tend to point towards the anions and thus rotate to form a cage-like shape. In both the gas phase and aqueous solution, the binding affinities of the anions for halogen-bonding receptors 1 and 2 become stronger than those for hydrogen-bonding receptor 3. The results reported here should prove to be of great value in the design and synthesis of effective and selective anion receptors based on halogen bonding.