Fluoride Selectivity Induced Transformation of Charged Anion Complexes into Unimolecular Capsule of a π-Acidic Triamide Receptor Stabilized by Strong N–H···F– and C–H···F– Hydrogen Bonds

The structural aspects of binding of anions such as halides (chloride, 1, and bromide, 2) and oxyanions (perchlorate, 3, and hydrogen sulfate, 4) with the protonated tris(amide) receptor L were carried out in detail. In all of these complexes, the hydrogen of protonated bridgehead nitrogen of the ligand is endo-oriented, forming a strong N–H···O hydrogen bond with an amide oxygen of a tripodal side arm, and binding of anions is primarily governed by N–H···anion and C–H···anion interactions involving multiple receptor cations. Furthermore, transformation of the charged complexes into a unimolecular capsule of L has also been accomplished in the presence of excess fluoride ion in dimethyl sulfoxide (DMSO) solution. Crystallographic analysis of fluoride complex [TBA(L·F)]2DMSO·4H2O (5; TBA = tetrabutylammonium) shows that F– is fully encapsulated within the tripodal cleft governed by six strong hydrogen bonds from the amide -NH and aryl -CH protons of the π-acidic receptor. 1H NMR titration experiments further provide evidence for the formation of the F– encapsulated receptor capsule from the charged complexes.