Aza crown ether calix[4]arenes containing cation and anion binding sites: effects of metal ions towards anion binding ability

Tripodal aza crown ether calix[4]arenes containing both cation and anion binding sites (5a and 5b) have been synthesized. The X-ray analysis shows that 5a forms a self-threaded rotaxane-like structure in the solid state. 1H NMR titrations of the two ligands with various halide anions indicate that 5a and 5b can form complexes with Br− and I− but not F−. However, both compounds form more stable complexes with I− than with Br− in the presence of Bu4N+. The presence of K+ enhances the binding ability of 5a towards Br−.