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We successfully synthesized a novel hydrogel based on derivatives of natural α-amino acids: ornithine and cystine. To make ornithine attachable to the polymer chain, the δ-amino group was modified with an acryloyl group and the main monomer was obtained. From cystine, the cross-linker

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.

We synthesized ion pair receptors 1 and 2 consisting of a benzocrown ether cation binding site and a ferrocene-supported squaramide anion binding domain and compared their binding ability with anion receptor 3, lacking a crown ether unit, using spectroscopic, spectrophotometric, and electrochemical measurements in acetonitrile solution. All receptors were obtained in a modular fashion by sequential amidation of dimethyl squarate with corresponding amines, which allows for the simple introduction of a ferrocene unit and enables them to act as sensors. We found that ion pair receptors 1 and 2 recognized the tested anions more strongly in the presence of cations, whereas the homotopic anion receptor 3 was unable to bind sodium or potassium salts more strongly than tetrabutylammonium salts. This enhancement in anion binding was attributed to a cation complexation induced alteration in the nature of the phenyl ring substituents, directly linked to the squaramide anion binding site, changing from electron-donating to more withdrawing. Solid state X-ray measurements support this design principle of the receptors and reveal strong coordination of anions and cations to the corresponding binding domains. In both 1·NaCl and 2·KCl systems, organic moieties linked by the cations form 1-D polymeric structures in the crystal lattice. However, in the case of the 2·NaCl complex the polymers’ shape and orientation result in the formation of porous crystals with approximately 12% of the space unoccupied. Electrochemical measurements showed when the ion pair receptors (but not the anion receptor) were pretreated with sodium or potassium cations, the addition of anions resulted in greater changes in oxidation and reduction potentials compared to the addition of anions to the same receptors in the absence of cations. Overall, the study demonstrates that squaramides offer a convenient platform for constructing ion pair sensors in modular fashion by varying the cation binding site and reporters. This opens up the possibility of recognizing salts in real life scenarios where simultaneous binding of anions and cations is needed.

The formation of a supramolecular core–shell like assembly upon interaction of the receptor with potassium sulfate enables its selective extraction., Selective extraction of sulfates in the form of alkali metal salts using charge-neutral molecular receptors is one of the holy grails of supramolecular chemistry. Herein we describe, for the first time, a squaramide-based ion pair receptor equipped with a crown ether site that is able to extract potassium sulfate from the aqueous to the organic phase (an analogous monotopic anion receptor lacking the crown ether unit lacks this ability). 1H NMR, UV-vis, DOSY-NMR, DLS, and MS experiments and the solid-state single crystal structure provided evidence of the formation of a supramolecular core–shell like assembly upon interaction of the receptor with potassium sulfate. The presence of monovalent potassium salts, in contrast, promoted the formation of simple 1 : 1 complexes. Unlike the 4 : 1 assembly, the 1 : 1 complexes are poorly soluble in organic media. This feature was utilized to overcome the Hofmeister bias and allow for selective extraction of extremely hydrophilic sulfates over lipophilic nitrate anions, which was unambiguously proved by quantitative AES and ion chromatography measurements. A simple modification of the receptor structure led to a “naked eye” optical sensor able to selectively detect sulfates under both SLE and LLE conditions.

A 4-nitro-L-phenylalanine scaffold was used to construct effective ion pair receptors capable of binding anions in an enhanced manner with the assistance of alkali metal cations. A benzocrown ether was linked to a receptor platform via the amide function so as to support the squaramide function in anion binding and to allow all three NHs to act simultaneously. The binding properties of the receptors were determined using UV-vis, 1H NMR, 2D NMR, and DOSY spectroscopy in MeCN and in the solid state by X-ray measurements. Ion pair receptor 2 was found to interact with the most strongly with salts, and the removal of its key structural elements was shown to hinder the receptor action. The amide proton was recognized to switch from having involvement in an intramolecular hydrogen bond to interacting with anions upon complexation. Apart from carboxylates, which promote deprotonation, and other monovalent salts creating 1:1 complexes with the receptor, more complex equilibria were established upon the complexation of 2 with sulfates. Receptor 2 was shown to be capable of the extraction of ion pairs from the aqueous to organic phase and of the cation-enhanced transport chloride and sulfate anions across a bulk chloroform membrane. These features may open the door for its use in regulating ion concertation under interfacial conditions and acting as a potential drug to treat channelopathies.

A series of ditopic ion pair receptors equipped with 4-nitrophenylurea and 1-aza-18-crown-6-ether linked by ortho-(1), meta-(2), and para-(3) substituted benzoic acid were readily synthesized in three steps from commercially available materials. The binding properties of these regioisomeric receptors were determined using UV-vis and 1H NMR spectroscopy in MeCN and in the solid state by single-crystal X-ray diffraction crystallography. The solution studies revealed that, apart from carboxylates, all the anions tested formed stronger complexes in the presence of sodium cations. Receptors 2 and 3 were found to interact with ion pairs with remarkably higher affinity than ortho-substituted 1. 1H NMR titration experiments showed that both urea NH protons interacted with anions with comparable strength in the case of receptors 2 and 3, but only one of the NHs was effective in anion binding in the case of receptor 1. X-ray analysis of the crystal structure of receptor 1 and 1&middot, NaPF6 complex showed that binding was hampered due to the formation of an intramolecular hydrogen bond. Analysis of the crystal structures of 2&middot, NaBr and 3&middot, NaBr complexes revealed that proper mutual orientation of binding domains was responsible for the improved binding of the sodium salts.

We synthesized an ion pair receptor 1 consisting of a crown ether cation binding site and a squaramide anion binding domain and compared its binding properties to those of its analogous urea counterpart 2. We studied their salt binding properties using spectrophotometric and spectroscopic measurements in an acetonitrile solution and in acetonitrile/water mixtures. Apart from carboxylate anions, all of the anions tested were found to associate with receptor 1 and 2 more strongly in the presence of sodium cations. A homotopic anion receptor 3, lacking a crown ether unit, was unable to bind sodium salt more strongly than tetrabutylammonium salts. Solution and solid-state X-ray measurements revealed strong sodium chloride coordination to receptor 1, which is able to bind this salt even in the presence of 10% water. In contrast to the urea-based ion pair receptor 2 or anion receptor 3, ditopic receptor 1 is capable of extracting sodium chloride from aqueous media to the organic phase, as was evidenced unambiguously by