  A urea-based tripodal receptor L substituted with p-cyanophenyl groups has
been studied for halide anions by $^1$H NMR spectroscopy, density functional
theory (DFT) calculations and X-ray crystallography. The $^1$H NMR titration
studies suggest that the receptor forms a 1:1 complex with an anion, showing
the binding trend in the order of fluoride > chloride > bromide > iodide. The
interaction of fluoride anion with the receptor was further confirmed by 2D
NOESY and $^{19}$F NMR spectroscopy in DMSO-d$_6$. DFT calculations indicate
that the internal halide anion is held by six NH...X interactions with L,
showing the highest binding energy for the fluoride complex. Structural
characterization of the chloride, bromide, and silicon hexafluoride complexes
of $[LH^+]$ reveals that the anion is externally located via hydrogen bonding
interactions. For the bromide or chloride complex, two anions are bridged with
two receptors to form a centrosymmetric dimer, while for the silicon
hexafluoride complex, the anion is located within a cage formed by six ligands
and two water molecules.
