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).