Investigating the effects of structure on sulfate recognition by neutral dipeptide receptors

Squaramide-based receptors in anion supramolecular chemistry: insights into anion binding, sensing, transport and extraction

Over the last 15 years, squaramide-based receptors have attracted the attention of supramolecular chemists working in the field of anion recognition. Herein, we highlight examples of squaramide-based receptors that are able to bind, sense, extract and transport anions.

Potentiometric Sensing of Nonsteroidal Painkillers by Acyclic Squaramide Ionophores

We report here a small library of a new type of acyclic squaramide receptors (L1-L5) as selective ionophores for the detection of ketoprofen and naproxen anions (KF- and NS-, respectively) in aqueous media. 1H NMR binding studies show a high affinity of these squaramide receptors toward KF- and NS-, suggesting the formation of H-bonds between the two guests and the receptors through indole and -NH groups. Compounds L1-L5 have been tested as ionophores for the detection of KF- and NS- inside solvent PVC-based polymeric membranes. The optimal membrane compositions were established through the careful variation of the ligand/tridodecylmethylammonium chloride (TDMACl) anion-exchanger ratio. All of the tested acyclic squaramide receptors L1-L5 have high affinity toward KF- and NS- and anti-Hofmeister selectivity, with L4 and L5 showing the highest sensitivity and selectivity to NS-. The utility of the developed sensors for a high precision detection of KF- in pharmaceutical compositions with low relative errors of analysis (RSD, 0.99-1.4%) and recoveries, R%, in the range 95.1-111.8% has been demonstrated. Additionally, the chemometric approach has been involved to effectively discriminate between the structurally very similar KF- and NS-, and the possibility of detecting these analytes at concentrations as low as 0.07 μM with R2 of 0.947 and at 0.15 μM with R2 of 0.919 for NS- and KF-, respectively, was shown.

Protonation and anion-binding properties of aromatic sulfonylurea derivatives

In this work the anion-binding properties of three aromatic sulfonylurea derivatives in acetonitrile and dimethyl sulfoxide were explored by means of NMR titrations. It was found that the studied receptors effectively bind anions of low basicity (Cl^-, Br^-, I^-, NO₃ ^- and HSO₄ ^-). The stoichiometry of the complexes with receptors containing one binding site was 1 : 1 exclusively, whereas in the case of the receptor containing two sulfonylurea groups 1 : 2 (receptor : anion) complexes were also detected in some cases. The presence of strongly basic anions (acetate and dihydrogen phosphate) led to the deprotonation of the sulfonylurea moiety. This completely hindered its anion-binding properties in DMSO and only proton transfer occurred upon the addition of basic anions to the studied receptors. In MeCN, a complex system of equilibria including both ligand deprotonation and anion binding was established. Since ionisation of receptors was proven to be a decisive factor defining the behaviour of the sulfonylurea receptors, their pK _a values were determined using several deprotonation agents in both solvents. The results were interpreted in the context of receptor structures and solvent properties and applied for the identification of the interactions with basic anions.

Macrocyclic squaramides as ion pair receptors and fluorescent sensors selective towards sulfates

Through the high dilution technique, we obtained macrocyclic ion pair receptors R1 and R2, an anion receptor R3, and a fluorescent sensor R4 using a combination of particular members of simple libraries consisting of synthesized diamines and methyl squarates, respectively. The receptors were investigated in terms of anion and ion pair binding using the ¹H NMR titration method in DMSO-d₆. We found that the major contribution to the anion binding comes from the interaction with the squaramide protons rather than with the amide functions of the receptors. The receptors demonstrated the highest affinity towards benzoates and sulfates over the anions tested, and in the case of sulfate binding more complex equilibria in solution were observed. Unlike the anion receptor R3, the ion pair receptor R1 was found to recognize anions in an enhanced manner with the assistance of sodium or potassium cations. Tethering of a simple fluorophore in close proximity to the amide function of receptor R4 resulted in an optical ion pair sensor selective towards sulfates. DFT calculations carried out for the 1 : 1 complexes of R3 with the anions helped clarify this selectivity, showing more effective participation of tetrahedral sulfate anions in binding with the amide function than in the case of benzoates or chlorides.

Selective Recognition of Chloride Anion in Water

A tricationic cage was synthesized via click chemistry. The cage has a preorganized cavity, which is surrounded by six relatively acidic C-H bonds. In organic solvent, the cage can recognize a variety of anions, among which the Cl^- anion has the highest binding affinity due to size selectivity. The cage is also able to selectively recognize the Cl^- anion in water, even though the binding affinity is just moderate.

Extraction and transport of sulfate using macrocyclic squaramide receptors

Lipophilic macrocycles efficiently extract sulfate ions from water into chloroform and transport this ion across a bulk liquid membrane in the presence of competing anions (chloride, nitrate and dihydrogenphosphate).