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

A highly selective recognition of fluoride was achieved through the design of a small hemicryptophane cage (3) presenting a southern tris-urea host-ing moiety. The resulting host-guest complex has been characterized by ESI-HRMS, 1H and 19F NMR, and X-ray diffraction techniques. In particular, X-ray diffraction analysis of [3.F-] reveals that the encapsulation of one fluoride, within 3, 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 ex-periments indicating that the efficient fluoride binding also occurs in solution. Finally, in sharp contrast with previously reported urea-based hemicryptophane hosts, the small pre-organized cavity found in 3 allows for an exclusive selectivity for fluoride over other competing halides.