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 center dot center dot center dot O hydrogen bond with an amide oxygen of a tripodal side arm, and binding of anions is primarily governed by N-H center dot center dot anion and C-H center dot center dot center dot 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 center dot F)]2DMSO center dot 4H(2)O (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 pi-acidic receptor. H-1 NMR titration experiments further provide evidence for the formation of the F encapsulated receptor capsule from the charged complexes.