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).