Synthesis of bis-benzimidazolium cyclic receptors and their anion binding properties

A series of cyclic receptors 1-5 based on bis-benzimidazolium group were designed and synthesized. Their anion recognition properties were investigated by UV-vis, fluorescent spectroscopy, ¹H NMR and MALDI-TOF mass spectrometry. The UV-vis and fluorescent spectroscopic studies showed that the receptors 1-5 had different binding abilities for various anions such as fluoride, chloride, bromide, iodide and acetate. The high selectivity of the receptors for fluoride and acetate anions is attributed to their basicity. Job plot and MALDI-TOF mass spectrometry demonstrated that the receptors form 1:1 complex with fluoride. DFT calculation and ¹H NMR studies indicated that the receptors bind anions through (CH) ⁺···X^- hydrogen bond.