H. Miyaji et al., Mono halogen substituted calix[4]pyrroles: Fine-tuning the anion binding properties of calix[4]pyrrole, SUPRAMOL CH, 13(6), 2001, pp. 661-669
Single halogen atom (i.e. I, Br, Cl and F) substituted calix[4]pyrroles, co
mpounds 2, 3, 4 and 5, were synthesized. Studies of these systems reveal th
at replacement of a single P-pyrrolic hydrogen atom can increase the anion
binding ability of calix[4]pyrroles for a variety of anions (e.g. Cl-, Br-,
H2PO4- and 4 HSO4- relative to normal non-halogen substituted calix[4]pyrr
ole 1. In the case of chloride anion, the expected relative affinity sequen
ce of 5 > 4 > 3 > 2 was observed. This was not found to be true for Br-, H2
PO4-, and HSO4-. Here, the chlorine substituted calix[4]pyrrole 4 was found
to display a slightly higher affinity in the case of each anion than the f
luorine-bearing derivative 5. This was rationalized in terms of intermolecu
lar NH . . . F hydrogen bonding interactions being present in CD2Cl2 soluti
ons of 5. Support for this latter conclusion came from concentration and te
mperature-dependent NMR spectroscopic studies.
A matched set of mono halogen substituted calix[4]pyrroles was used to stud
y in detail, the extent to which halogen substituents may be used to fine-t
une the anion binding properties of calix[4]-pyrroles.