KF and CsF Recognition and Extraction by a Calix[4]crown‑5 Strapped Calix[4]pyrrole Multitopic Receptor

On the basis of ¹H NMR spectroscopic analyses and single crystal X-ray crystal structural data, the ion-pair receptor 1, bearing a calix[4]­pyrrole for anion binding and calix[4]­arene crown-5 for cation recognition, was found to act as a receptor for both CsF and KF ion-pairs. Both substrates are bound strongly but via different binding modes and with different complexation dynamics. Specifically, exposure to KF in 10% CD₃OD in CDCl₃ leads first to complexation of the K⁺ cation by the calix[4]­arene crown-5 moiety. As the relative concentration of KF increases, then the calix[4]­pyrrole subunit binds the F^– anion. Once bound, the K⁺ cation and the F^– anion give rise to a stable 1:1 ion-pair complex that generally precipitates from solution. In contrast to what is seen with KF, the CsF ion-pair interacts with receptor 1 in two different modes in 10% CD₃OD in CDCl₃. In the first of these, the Cs⁺ cation interacts with the calix[4]­arene crown-5 ring weakly. In the second interaction mode, which is thermodynamically more stable, the Cs⁺ cation and the counteranion, F^–, are simultaneously bound to the receptor framework. Further proof that system 1 acts as a viable ion-pair receptor came from the finding that receptor 1 could extract KF from an aqueous phase into nitrobenzene, overcoming the high hydration energies of the K⁺ and F^– ions. It was more effective in this regard than a 1:1 mixture of the constituent cation and anion receptors (4 and 5).