Spectroscopic, Structural, and Theoretical Studies of Halide Complexes with a Urea-based Tripodal Receptor

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.