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

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Abstract

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 calix[4]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 calix[4]pyrrole. Different complexation modes were also observed when the binding of the fluoride ion was studied by using the tetramethylammonium and tetraethylammonium salts.

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