Anion recognition by a macrobicycle based on a tetraoxadiaza macrocycle and an isophthalamide head unit

(Chemical Equation Presented) A macrobicycle formed by a tetraoxadiaza macrocycle containing a dibenzofuran (DBF) spacer and an isophthalamide head unit, named DBF-bz, was used as receptor for anion recognition. The molecular structure of DBF-bz was established in solution by NMR and ESI-MS spectroscopies and in single crystal by X-ray diffraction analysis. The X-ray structure showed a water molecule encapsulated into the macrobicyclic cavity by four hydrogen bonds, two of them involving the two N-H amide binding sites and the oxygen of the water molecule (N-H⋯O hydrogen bonds) and the other two (O-H⋯N) involving the amine groups as hydrogen bonding acceptors. 1H NMR temperature dependence studies demonstrated that the same structure exists in solution. The ability of this ditopic receptor to recognize alkali halide salts was evaluated by extraction studies followed by 1H NMR and ESI-MS spectroscopies. The macrobicycle showed a capacity to extract halide salts from aqueous solutions into organic phases. The binding ability of this macrobicycle for halides was also quantitatively investigated using 1H NMR titrations in CDCl3 (and DMSO-d6) solution, and in acidic D2O solution. The largest binding association constant was found for the chloride anion and the completely protonated receptor. The results suggest that the diammonium-diamide unit of the receptor strongly bind the anionic substrate via multiple N-H⋯Cl- hydrogen bonds and electrostatic interactions. The binding trend follows the order Cl- > Br- > I- ≈ F- established from the best fit between the size of the anion and the cavity size of the protonated macrobicycle. Molecular dynamics (MD) simulations of the DBF-bz in CHCl3 solution allowed a detailed insight into the structural and binding properties of the receptor