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

Abstract

Ion pair receptor <b>3</b> 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-<i>d</i>₄ in chloroform-<i>d</i>) 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 <b>3</b> is able to extract CsNO₃ and CsCl from an aqueous D₂O layer into nitrobenzene-<i>d</i>₅ as inferred from ¹H NMR spectroscopic analyses and radiotracer measurements. The Cs⁺ cation of the CsNO₃ extracted into the nitrobenzene phase by receptor <b>3</b> 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 <b>3</b> in its uncomplexed, ion-free form. This allows receptor <b>3</b> 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