Controlling Cesium Cation Recognition via Cation Metathesis within an Ion Pair Receptor

Ion pair receptor 3 bearing an anion binding site and multiple cation binding sites has been synthesized and shown to function in a novel binding–release cycle that does not necessarily require displacement to effect release. The receptor forms stable complexes with the test cesium salts, CsCl and CsNO3, in solution (10% methanol-d 4 in chloroform-d) as inferred from 1H NMR spectroscopic analyses. The addition of KClO4 to these cesium salt complexes leads to a novel type of cation metathesis in which the “exchanged” cations occupy different binding sites. Specifically, K+ becomes bound at the expense of the Cs+ cation initially present in the complex. Under liquid–liquid conditions, receptor 3 is able to extract CsNO3 and CsCl from an aqueous D2O layer into nitrobenzene-d 5 as inferred from 1H NMR spectroscopic analyses and radiotracer measurements. The Cs+ cation of the CsNO3 extracted into the nitrobenzene phase by receptor 3 may be released into the aqueous phase by contacting the loaded nitrobenzene phase with an aqueous KClO4 solution. Additional exposure of the nitrobenzene layer to chloroform and water gives 3 in its uncomplexed, ion-free form. This allows receptor 3 to be recovered for subsequent use. Support for the underlying complexation chemistry came from single-crystal X-ray diffraction analyses and gas-phase energy-minimization studies.