Selective Binding of Spherical and Linear Anions by Tetraphenyl(thio)urea-Based Dihomooxacalix[4]arene Receptors

Three novel tetra­(thio)­ureido
dihomo­oxa­calix­[4]­arene
anion receptors (phenylurea 4a, phenylthiourea 4b, and tert-butylurea 4c) were
synthesized and obtained in the cone conformation in solution, as
shown by NMR studies. The X-ray crystal structure of 4c is reported. The host–guest properties of these receptors
toward several anions were investigated by 1H NMR titrations.
Phenylurea 4a displayed a very efficient binding toward
the spherical F– and Cl– anions,
and the linear CN– (log Kass = 3.46, 3.50, and 4.02, respectively). In comparison to related
bidentate phenylurea dihomooxacalix[4]­arenes, tetraphenylurea 4a is more preorganized and the higher number of hydrogen
bond donor sites provides a remarkable enhancement of its binding
efficiency.