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-d4 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-d5 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.