J. Scheerder et al., COMPLEXATION OF HALIDE ANIONS AND TRICARBOXYLATE ANIONS BY NEUTRAL UREA-DERIVATIZED P-TERT-BUTYLCALIX[6]ARENES, Journal of organic chemistry, 60(20), 1995, pp. 6448-6454
Two neutral receptors for halide anions and tricarboxylate anions have
been synthesized on the basis of p-tert-butylcalix[6]arene, symmetric
ally functionalized with three butyl(thio)urea groups at the 1,3,5-phe
nolic positions. The anion complexation has been studied by H-1 NMR ti
tration experiments, FTIR spectroscopy, and FAB mass spectrometry. The
receptors bind halide and tricarboxylate anions exclusively through h
ydrogen bonding in a 1:1 fashion in CDCl3. For halide anions, a prefer
ence for bromide over chloride ions is observed, with a highest bindin
g constant K-a of 1.4 x 10(3) M(-1) with receptor 4 containing the ure
a moieties. Thiourea receptor 5 most strongly binds 1,3,5-benzenetrica
rboxylate anions (K-a = 2.9 x 10(5) M(-1)) whereas 1,2,4- and 1,2,3-be
nzenetricarboxylate 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 th
e difference in the binding of halide and tricarboxylate anions by 4 a
nd 5 is given. The mo de of binding in the complex of 5 with 1,3,5-ben
zenetricarboxylate was elucidated by low-temperature NOESY spectroscop
y.