Highly Selective Fluoride Recognition by a Small Tris-Urea Covalent Cage

Abstract

A highly selective recognition of fluoride was achieved through the design of a small hemicryptophane cage (<b>3</b>) presenting a southern tris-urea hosting moiety. The resulting host-guest complex has been characterized by electrospray ionization-high-resolution mass spectrometry, ¹H and ¹⁹F NMR, and X-ray diffraction techniques. In particular, X-ray diffraction analysis of [<b>3</b>·F^-] reveals that the encapsulation of one fluoride, within <b>3</b>, occurs through NH···F^- H-bonding with the six NH residues of the tris-urea ligand. An association constant of 1200 M^-1 was extracted from ¹H NMR titration experiments, indicating that efficient fluoride binding also occurs in solution. Finally, in sharp contrast with previously reported urea-based hemicryptophane hosts, the small preorganized cavity found in <b>3</b> allows for an exclusive selectivity for fluoride over other competing halides.