(Thio)ureido Anion Receptors Based on a 1,3-Alternate Oxacalix[2]arene[2]pyrimidine Scaffold

In pursuit of highly preorganized macrocyclic host molecules
for
the complexation of anions, a series of oxacalix[2]­arene[2]­pyrimidine-based
bis­(thio)­ureido receptors were synthesized and fully characterized.
The pincer-like 1,3-alternate conformation of the oxacalix[4]­arene
scaffold, essential for an efficient host–guest interaction,
was visualized by single-crystal X-ray analysis and supported by variable-temperature
NMR studies. The anion binding properties of the receptors were evaluated
via 1H NMR titration experiments, showing intermolecular
interactions with H2PO4–,
AcO–, BzO–, and Cl– ions. The host molecule bearing 4-nitrophenyl substituents on the
bisurea binding pocket showed association constants in the range of
200–400 M–1 in the strongly competitive solvent
mixture of DMSO/0.5% H2O.