Mono halogen substituted calix[4]pyrroles: Fine-tuning the anion binding properties of calix[4]pyrrole

Single halogen atom (i.e. I, Br, Cl and F) substituted calix[4]pyrroles, compounds 2, 3, 4 and 5, were synthesized. Studies of these systems reveal that 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]pyrrole 1. In the case of chloride anion, the expected relative affinity sequence of 5 > 4 > 3 > 2 was observed. This was not found to be true for Br-, H2PO4-, 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 fluorine-bearing derivative 5. This was rationalized in terms of intermolecular NH . . . F hydrogen bonding interactions being present in CD2Cl2 solutions of 5. Support for this latter conclusion came from concentration and temperature-dependent NMR spectroscopic studies. A matched set of mono halogen substituted calix[4]pyrroles was used to study in detail, the extent to which halogen substituents may be used to fine-tune the anion binding properties of calix[4]-pyrroles.