Calix[4]pyrrole‐Based Heteroditopic Ion‐Pair Receptor That Displays Anion‐Modulated,Cation‐Binding Behavior

A new ditopic ion‐pair receptor 1 was designed, synthesized, and characterized. Detailed binding studies served to confirm that this receptor binds fluoride and chloride ions (studied as their tetraalkylammonium salts) and forms stable 1:1 complexes in CDCl₃. Treatment of the halide‐ion complexes of 1 with Group I and II metal ions (Li⁺, Na⁺, K⁺, Cs⁺, Mg²⁺, and Ca²⁺; studied as their perchlorate salts in CD₃CN) revealed unique interactions that were found to depend on both the choice of the added cation and the precomplexed anion. In the case of the fluoride complex 1? F]^? (preformed as the tetrabutylammonium (TBA⁺) complex), little evidence of interaction with the K⁺ ion was seen. In contrast, when this same complex (i.e., 1? F]^? as the TBA⁺ salt) was treated with the Li⁺ or Na⁺ ions, complete decomplexation of the receptor‐bound fluoride ion was observed. In sharp contrast to what was seen with Li⁺, Na⁺, and K⁺, treating complex 1? F]^? with the Cs⁺ ion gave rise to a stable, receptor‐bound ion‐pair complex Cs ?1? F] that contains the Cs⁺ ion complexed within the cup‐like cavity of the calix4]pyrrole, which in turn was stabilized in its cone conformation. Different complexation behavior was observed in the case of the chloride complex 1? Cl]^?. In this case, no appreciable interaction was observed with Na⁺ or K⁺. In addition, treating 1? Cl]^? with Li⁺ produces a tightly hydrated dimeric ion‐pair complex 1? LiCl(H₂O)]₂ in which two Li⁺ ions are bound to the crown moiety of the two receptors. In analogy to what was seen in the case of 1? F]^?, exposure of 1? Cl]^? to the Cs⁺ ion gives rise to an ion‐pair complex Cs ?1? Cl] in which the cation is bound within the cup of the calix4]pyrrole. Different complexation modes were also observed when the binding of the fluoride ion was studied by using the tetramethylammonium and tetraethylammonium salts.