Squaramide-based Receptors for Anion Recognition

Abstract

A series of squaramide based receptors bearing benzene and pyridine linkers was developed, ranging from simple alkyl-substituted mono-squaramides to macrocyclic tri-squaramides by altering the number of squaramide binding sites, adjusting the geometries of compounds and appending different solubilizing groups. These receptors were studied via host-guest 1H NMR titrations with a range of anions in DMSO-d6 and water. In this work, the successful trial of aqueous soluble, macrocyclic squaramide receptors (MSQs) were synthesized that bind sulfate with high affinity and selectivity over other tetrahedral divalent anions in aqueous media up to 50% v/v H2O/DMSO-d6. In mixtures of anions mimicking the composition of either nuclear waste or plasma, the MSQs were still able to bind sulfate ions with high affinity. Encouraged by these results, another library of MSQs was synthesized for sulfate extraction and transport by appending alkyl chains to the macrocycles. According to the sulfate concentration monitored by ion chromatography, MSQs are able to extract approximately 15% sulfate from aqueous solutions under the experimental conditions used. Sulfate transport experiments monitored by ICP-OES suggests that approximately 0.4% of sulfate was carried across a bulk chloroform membrane after 15 days. Furthermore, in order to investigate more complicated structures for anion recognition, a small family of (thio)urea and (thio)squaramide based anion receptors and their corresponding rotaxanes were developed in collaboration with Prof. Steve Goldup at the University of Southampton. According to anion titrations experiments with a range of anions in CDCl3, CD3CN, the thread and rotaxane behave differently, as the thread is an anion binder whereas the rotaxane switch binds ion-pairs with a charge separated binding mode (i.e., a heteroditopic receptor). Meanwhile, the thread and rotaxane also exhibit different selectivities for anions.