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 H-1 NMR titrations. Phenylurea 4a displayed a very efficient binding toward the spherical F- and Cl- anions, and the linear CN- (log K-ass = 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.
