Complexation of Halide Anions and Tricarboxylate Anions by Neutral Urea-Derivatized p-tert-Butylcalix[6]arenes

Two neutral receptors for halide anions and tricarboxylate anions have been synthesized on the basis of p-tert-butylcalix[6]arene, symmetrically functionalized with three butyl(thio)urea groups at the 1,3,5-phenolic positions. The anion complexation has been studied by H-1 NMR titration experiments, FTIR spectroscopy, and FAB mass spectrometry. The receptors bind halide and tricarboxylate anions exclusively through hydrogen bonding in a 1:1 fashion in CDCl3. For halide anions, a preference for bromide over chloride ions is observed, with a highest binding constant K-a of 1.4 x 10(3) M(-1) with receptor 4 containing the urea moieties. Thiourea receptor 5 most strongly binds 1,3,5-benzenetricarboxylate anions (K-a = 2.9 x 10(5) M(-1)) whereas 1,2,4- and 1,2,3-benzenetricarboxylate anions are complexed better by receptor 4 (K-a = 2.3 x 10(4) and 4.7 x 10(4) M(-1), respectively). An explanation for the difference in the binding of halide and tricarboxylate anions by 4 and 5 is given. The mo de of binding in the complex of 5 with 1,3,5-benzenetricarboxylate was elucidated by low-temperature NOESY spectroscopy.