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 CsNO ₃, in solution (10% methanol-d ₄ in chloroform-d) as inferred from ¹H NMR spectroscopic analyses. The addition of KClO ₄ 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 CsNO ₃ and CsCl from an aqueous D ₂O layer into nitrobenzene-d ₅ as inferred from ¹H NMR spectroscopic analyses and radiotracer measurements. The Cs ⁺ cation of the CsNO ₃ extracted into the nitrobenzene phase by receptor 3 may be released into the aqueous phase by contacting the loaded nitrobenzene phase with an aqueous KClO ₄ 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.