Trapping Anions within Stacks of Tetra-Urea Macrocycles

Designing molecular receptors that bind anions in water is a significant challenge, and an even greater difficulty lies in using these receptors to remove anions from water without resorting to the hazardous liquid-liquid extraction approach. We here demonstrate an effective and synthetically simple strategy toward these goals by exploiting ion-pair assembly of macrocycles. Our anion binding ensemble consists of an octa-chloro tetra-urea macrocyclic anion receptor (ClTU), which forms water-dispersible aggregates, and a tetra-cationic fluorescent dye 5,10,15,20-tetrakis(1-methyl-4-pyridinio)porphyrin (TMPyP4), which provides Coulombic stabilization and fluorescence reporting of anion binding in an ion-pair assembly. The ability of ClTU to form oligomeric/polymeric stacks with chloride ions and countercations was demonstrated by X-ray crystallographic and ¹H NMR studies in DMSO. In water, the addition of anions to a mixture of ClTU and TMPyP4 led to the formation of ion-pair assemblies showing quenched fluorescence from TMPyP4 and apparent selectivity in the order of CrO₄^2- ≈ SO₄^2- > Cl^- > ClO₄^- ≈ NO₃^- > AcO^- > F^- > H₂PO₄^-. The assembly can be applied to remove traces of toxic CrO₄^2- anions from tap water simply by using syringe filters to separate large aggregates from water. Molecular dynamics studies support the formation of columnar stacks of ClTU in solution, which allow for insertion of both anionic guests and cationic TMPyP4 dyes.