UREA-DERIVATIZED P-TERT-BUTYLCALIX[4]ARENES - NEUTRAL LIGANDS FOR SELECTIVE ANION COMPLEXATION

Functionalization of the lower rim of p-tert-butylcalix[4]arene with f our (thio)urea groups, results in a class of receptors selective for s pherical onions that-are bound exclusively through hydrogen bonding. H -1 NMR spectroscopy in CDCl3 reveals a selectivity for Cl- over Br- an d I-. The stoichiometry is 1:1 in all cases as was confirmed by Job pl ots. The association constants are strongly dependent on the nature of the substituent at the urea moiety. The bidentate phenylurea derivati ve 9 shows the strongest complexation (K-ass. Cl- = 7.1 x 10(3) M(-1)) and the largest selectivity for Cl-.