Capture and displacement-based release of the bicarbonate anion by calix[4]pyrroles with small rigid straps

Two-phenoxy walled calix[4]pyrroles 1 and 2 strapped with small rigid linkers containing pyridine and benzene, respectively, have been synthesized. ¹H NMR spectroscopic analyses carried out in CDCl₃ revealed that both of receptors 1 and 2 recognize only F^- and HCO₃ ^- among various test anions with high preference for HCO₃ ^- (as the tetraethylammonium, TEA⁺ salt) relative to F^- (as the TBA⁺ salt). The bound HCO₃ ^- anion was completely released out of the receptors upon the addition of F^- (as the tetrabutylammonium, TBA⁺ salt) as a result of significantly enhanced affinities and selectivities of the receptors for F^- once converted to the TEAHCO₃ complexes. Consequently, relatively stable TEAF complexes of receptors 1 and 2 were formed via anion metathesis occurring within the receptor cavities. By contrast, the direct addition of TEAF to receptors 1 and 2 produces different complexation products initially, although eventually the same TEAF complexes are produced as via sequential TEAHCO₃ and TBAF addition. These findings are rationalized in terms of the formation of different ion pair complexes involving interactions both inside and outside of the core receptor framework.