Calix[4]pyrrole bis-crowns as ion pair receptors: cation selectivity modulated by counter anions

Abstract

A series of doubly strapped calix[4]pyrroles 2-4 with different sizes of crown ethers have been synthesized by the reactions of the meso-tetra-4-hydroxyphenylcalix[4]pyrrole (6) with tri-, tetra-, and penta-ethylene glycol ditosylates, respectively, in the presence of K2CO3 as a base. Receptors 2-4 were found to bind relatively small anions such as F- and HCO3- selectively over other various test anions in CDCl3. Calix[4]pyrrole biscrown-5 and -6 (3 and 4) were also proved capable of binding only the sodium cation (Na+) among the alkali metal cations (as their perchlorate salt forms) in 10% methanol-d4 in CDCl3 while receptor 2 has no affinity for those cations. The selectivity of receptors 3 and 4 for Na+ is completely reversed to Cs+ when the counter anions are changed to F- or Cl-. It was demonstrated by 1H NMR spectroscopic analyses that receptors 2 and 3 complex CsF and CsCl with different binding modes. Solid-liquid extraction experiments revealed that receptors 2-4 enable solubilization of the otherwise insoluble CsF salt into chloroform more effectively than does the parent calix[4]pyrrole (1). A series of doubly strapped calix[4]pyrrole bis-crowns were synthesized as ion pair receptors. Their cation selectivity could be reversed by counter anions.