Chloride Selective Macrocyclic Bisurea Derivatives with 2,2′‑Binaphthalene Moieties as Spacers

A cyclic bisurea derivative 2a has been successfully prepared from the corresponding diamine and diisocyanate in the presence of tetrabutylammonium chloride as a template. A more soluble cyclic bisurea 2b has also been prepared by introduction of sterically bulky tert-butyl groups. X-ray crystal analyses of [2a·Cl]− and [2b·Cl]− revealed that overall structure was saddle like and the chloride anion was located in the center of the cavity. The bound chloride anion was hydrogen bonded by four N–H of urea groups and weakly hydrogen bonded by four 1-C–H of naphthyl groups, respectively. After removal of the bound chloride anions of [2b·Cl]− with silver nitrate, two different X-ray crystals of free 2b were obtained; one was intermolecular hydrogen bonded shrunken structure and the other was extended structure. Receptor 2b showed large binding ability for Cl–, however, the selectivity for Cl– against basic anions, such as AcO– and F–, has been insufficient. In aqueous MeCN, the association constant of 2b for Cl– was reduced but still large, and the selectivity for hydrophobic Cl– was greatly improved. In this solvent, 2b also selectively recognized alkaline metal chloride salts. Therefore, cyclic bisurea 2b is highly selective and effective Cl– selective receptor.