Synthesis and selective anion recognition of imidazolium cyclophanes

Cyclophanes based on imidazolium or benzimidazolium groups were synthesized as anion recognition motifs by quaternarization of the bridged imidazole or benzimidazole compounds with dibromides in acetonitrile under reflux with high dilution. X-Ray analysis showed that C–H⋯Br− hydrogen bonds connected the hydrogen atoms of the cationic imidazolium rings, the m-xylene ring and the spacer of the macrocycle with bromide anions. 1H NMR study in DMSO-d6 showed that the H-2 of imidazolium rings and the proton of the benzene ring were shifted downfield upon addition of Br−, suggesting the formation of the C–H⋯Br−hydrogen bonds between the cyclophane and bromide anion in solution. UV spectroscopic titration in acetonitrile at 25°C showed 1:1 complexes between the cyclophanes and halide anions, and the binding constants (Ka) and Gibbs free energy changes (−ΔG°) were calculated according to the modified Benesi–Hildebrand equation. Cyclophane 1·2PF6− exhibits selective recognition for F−, Cl−, Br− and I− in acetonitrile. The binding constant of 1·2PF6− with Cl− is 4.06×104M−1, 2, 5 and 2000 times those of 1·2PF6− with Br−, F− and I−, respectively. Binding experiments indicate that the electrostatic interactions, hydrogen bonding and preorganization of the binding sites of the hosts play essential roles in the anion recognition by imidazolium cyclophanes.