Anion Recognition by a Macrobicycle Based on a Tetraoxadiaza Macrocycle and an Isophthalamide Head Unit

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 center dot center dot center dot O hydrogen bonds) and the other two (O-H center dot center dot center dot N) involving the amine groups as hydrogen bonding acceptors. H-1 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 H-1 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 H-1 NMR titrations in CDCl3 (and DMSO-d(6)) 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 center dot center dot center dot Cl- hydrogen bonds and electrostatic interactions. The binding trend follows the order Cl- > Br- > 1(-) approximate to 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.