Tripodal, Squaramide-Based Ion Pair Receptor for Effective Extraction of Sulfate Salt

Abstract

Combining three features—the high affinity of squaramides toward anions, cooperation in ion pair binding and preorganization of the binding domains in the tripodal platform—led to the effective receptor 2. The lack of at least one of these key elements in the structures of reference receptors 3 and 4 caused a lower affinity towards ion pairs. Receptor 2 was found to form an intramolecular network in wet chloroform, which changed into inorganic–organic associates after contact with ions and allowed salts to be extracted from an aqueous to an organic phase. The disparity in the binding mode of 2 with sulfates and with other monovalent anions led to the selective extraction of extremely hydrated sulfate anions in the presence of more lipophilic salts, thus overcoming the Hofmeister series.

Tritopic Bis-Urea Receptors for Anion and Ion-Pair Recognition

“…These results suggest that the acetate is bound through hydrogen bonds with the N–H hydrogens of both ureas; this behavior has been widely described in the literature. ,− , Additionally, the signals of the naphthyl hydrogens adjacent to ureidic oxygen, which are influenced by hydrogen bonding, also shift downfield. The rest of the signals of the naphthyl hydrogens experienced an upfield shift because of a higher electron density due to the recognition of the anion by urea, possibly accompanied by an anion−π interaction. , It was observed that the phenyl protons of the receptors closest to the polyether chain experienced two behaviors in the cases of AcO – , BzO – , and H 2 SO 4 – titrations: first an upfield chemical shift change and then a downfield chemical shift change, thus indicating two equilibria in the complexation process. On the other hand, there were no significant changes for the methylene hydrogens of the polyether chain.…”

“…The rest of the signals of the naphthyl hydrogens experienced an upfield shift because of a higher electron density due to the recognition of the anion by urea, possibly accompanied by an anion−π interaction. 33 , 34 It was observed that the phenyl protons of the receptors closest to the polyether chain experienced two behaviors in the cases of AcO – , BzO – , and H 2 SO 4 – titrations: first an upfield chemical shift change and then a downfield chemical shift change, thus indicating two equilibria in the complexation process. On the other hand, there were no significant changes for the methylene hydrogens of the polyether chain.…”

“…Therefore, molecular recognition of cationic and anionic species has been a topic of great interest during the past few decades. − In particular, anion recognition remains challenging due to the intrinsic properties of anions, such as their diverse geometry, pH dependence, and high solvation energies. − The design of cyclic and acyclic neutral receptors for anion recognition usually includes amide, urea, or thiourea groups as recognition sites, since they can form strong and directional hydrogen bonds with anions. ,− On the other hand, the inclusion in the structure of the receptors of an additional site capable of binding cations has been a strategy to improve the affinity toward their anionic targets. In this sense, there has been a significant increase in the last decade of reports related to heteroditopic and multitopic receptors for ion-pair recognition. − The correct design of this type of compounds can result in a higher affinity and selectivity due to a cooperative effect regulated by a balance of enthalpic and entropic contributions . However, heteroditopic and multitopic compounds are generally based on macrocycles and thus often difficult to synthesize.…”

“…For this reason, it is important to design efficient and easily obtainable systems for different purposes. Taking this into account, ion-pair recognition is a challenging topic in modern supramolecular chemistry. − ,,− …”

Fluorescence Recognition of Anions Using a Heteroditopic Receptor: Homogenous and Two-Phase Sensing

“…This may reflect the intrinsic difficulties associated with the design and synthesis of receptors that extract salts from the aqueous layer to the organic phase, including selective extraction of extremely hydrophilic sulfate salts [35]. It has recently been found that a solution to this problem may lie in the use of ion pair receptors and certain squaramide derivatives have been shown to efficiently extract potassium sulfate from the aqueous to organic phase [36][37][38][39]. To the best of our knowledge, a combination of these two features, leading to molecules capable of selectively extracting sulfates and their fluorescent detection, has not yet been reported.…”