A highly selective
recognition of fluoride was achieved through
the design of a small hemicryptophane cage (3) 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 [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 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 3 allows
for an exclusive selectivity for fluoride over other competing halides.