Cesium Halide Ion Pair Recognition by a Pyrrole Strapped Calix[4]pyrrole

The binding properties of the pyrrole-strapped calix[4]pyrrole 2 for cesium halide ion pairs were studied via ¹H NMR spectroscopic and single crystal X-ray diffraction analyses. Receptor 2 was found to bind CsF, CsCl, and CsBr in the solid state and in chloroform/methanol (4/1, v/v) solution with relatively high affinity as compared with the parent calix[4]pyrrole 1. It was also revealed by solid-liquid extraction experiments that receptor 2 was capable of solubilizing CsF in CDCl₃, a medium in which this salt is otherwise insoluble. Single crystal X-ray diffraction analyses and ¹H NMR spectroscopic data recorded in 20% CD₃OD in CDCl₃ provide support for the suggestion that the strap pyrrolic NH proton of 2, as well as those of the calix[4]pyrrole framework, contribute to anion recognition, thus increasing affinity for cesium halide salts relative to the parent system 1. In the solid state, receptor 2 interacts with CsF to form a two dimensional coordination polymer in the presence of methanol. A linear coordination polymer is observed in the case of CsCl and CsBr. Receptor 2 was also found to form a complex with CsF in chloroform/methanol (4/1, v/v) solution, albeit with a different binding mode than is seen in the solid state.