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.