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 using H-1 NMR spectroscopy, density functional theory (DFT) calculations, and X-ray crystallography. The H-1 NMR titration studies suggest that the receptor forms a 1:1 complex with an anion, showing a binding trend in the order of fluoride > chloride > bromide > iodide. The interaction of a fluoride anion with the receptor was further confirmed by 2D NOESY and F-19 NMR spectroscopy in DMSO-d(6). DFT calculations indicate that the internal halide anion is held by six NH center dot center dot center dot 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.