A non-multimacrocyclic heteroditopic receptor that cooperatively binds and effectively extracts KAcO salt

Supramolecular chemistry of liquid-liquid extraction

Liquid-Liquid Extraction (LLE) is a venerable and widely used method for the separation of a targeted solute between two immiscible liquids. In recent years, this method has gained popularity in the supramolecular chemistry community due to the development of various types of synthetic receptors that effectively and selectively bind specific guests in an aqueous medium through different supramolecular interactions. This has eventually led to the development of state-of-the-art extraction technologies for the removal and purification of anions, cations, ion pairs, and small molecules from one liquid phase to another liquid phase, which is an industrially viable method. The focus of this perspective is to furnish a vivid picture of the current understanding of supramolecular interaction-based LLE chemistry. This will not only help to improve separation technology in the chemical, mining, nuclear waste treatment, and medicinal chemistry sectors but is also useful to address the purity issue of the extractable species, which is otherwise difficult. Thus, up-to-date knowledge on this subject will eventually provide opportunities to develop large-scale waste remediation processes and metallurgy applications that can address important real-life problems.

A poly-pseudorotaxane constructed by threading pillar[5]arene onto an ion-pair recognition-based calix[4]pyrrole supramolecular polymer

The ion-pair recognition ability of calix[4]pyrrole was utilized to form a multicomponent monomeric assembly and a linear supramolecular polymer via concurrent anion and bis-cation complexation. The inherent dynamic interactions of these assemblies were further used to construct pseudorotaxanes in monomeric and supramolecular polymer forms with pillar[5]arene.

Halogen Bonding Heteroditopic Materials for Cooperative Sodium Iodide Binding and Extraction

A series of novel heteroditopic halogen bonding (XB) receptor functionalised silica based materials, containing mono- and bis-iodotriazole benzo-15-crown-5 groups are investigated for the cooperative binding and extraction of sodium halide ion-pair species from aqueous solution. Characterisation of the XB materials by CHN elemental analysis, 13 C CP/MAS NMR and ATR-FTIR spectroscopies confirms and quantifies the successful incorporation of the ion-pair receptor frameworks to the silica material. ICP-MS solid-liquid extraction studies demonstrate the bidentate XB functionalised material is capable of NaI extraction from water. Importantly, cooperative XB-mediated sodium halide ion-pair binding is determined to be crucial to the material's extraction capabilities, impressively demonstrating a two-fold enhancement in sodium iodide extraction efficiency relative to a heteroditopic hydrogen bonding receptor functionalised silica material analogue.

Utilizing an Amino Acid Scaffold to Construct Heteroditopic Receptors Capable of Interacting with Salts under Interfacial 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.

Development of effective potassium acetate extractant

Carboxylates are commonly used in the food and pharmaceutical industry and due to their extensive use, carboxylates present a significant environmental burden. In this context, valine based, heteroditopic receptor 1 was prepared and its ability to bind simultaneously potassium cation and acetate anion in water containing CH₃CN solutions was demonstrated. Under liquid–liquid extraction conditions the receptor 1 was capable of extracting hydrophilic AcOK salt from aqueous solution and was proved to be nearly ten times more effective than the equimolar mixture of monotopic receptors. Furthermore, compound 1 could extract one of the most popular nonsteroidal anti-inflammatory drugs, ibuprofen (IbuOK), from relatively dilute aqueous solutions.

Structurally simple, heteroditopic receptor is capable of extracting hydrophilic potassium acetate and other carboxylate salts from water to organic phase.

From Heteroditopic to Multitopic Receptors for Ion-Pair Recognition: Advances in Receptor Design and Applications

Ion-pair recognition has emerged from cation and anion recognition and become a diverse and active field in its own right. The last decade has seen significant advances in receptor design in terms of the types of binding motifs, understanding of cooperativity and increase in complexity from heteroditopic to multitopic receptors. As a result, attention has turned to applying this knowledge to the rational design of ion-pair receptors for applications in salt solubilisation and extraction, membrane transport and sensing. This Review highlights recent progress and developments in the design and applications of heteroditopic and multitopic receptors for ion-pair recognition.

Heteroditopic receptor flexibility – an important design principle for effective ion pair extractants based on carboxylate studies

Potassium carboxylate salt binding and extraction experiments of a heteroditopic receptor series reveal that conformational freedom is an important factor affecting salt extraction efficiency.

Monofunctionalized Bambus[6]urils and Their Conjugates with Crown Ethers for Liquid-Liquid Extraction of Inorganic Salts

Bambusurils are a growing family of macrocyclic anion receptors. In this Letter, we present the first syntheses of monofunctionalized bambusurils and their use for the preparation of heteroditopic bambusuril-crown ether conjugates suitable for the extraction of ion pairs from water to chloroform.

The Effect of Substitution Pattern on Binding Ability in Regioisomeric Ion Pair Receptors Based on an Aminobenzoic Platform

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·NaPF₆ complex showed that binding was hampered due to the formation of an intramolecular hydrogen bond. Analysis of the crystal structures of 2·NaBr and 3·NaBr complexes revealed that proper mutual orientation of binding domains was responsible for the improved binding of the sodium salts.

Macrocycles as Ion Pair Receptors

Cation and anion recognition have both played central roles in the development of supramolecular chemistry. Much of the associated research has focused on the development of receptors for individual cations or anions, as well as their applications in different areas. Rarely is complexation of the counterions considered. In contrast, ion pair recognition chemistry, emerging from cation and anion coordination chemistry, is a specific research field where co-complexation of both anions and cations, so-called ion pairs, is the center of focus. Systems used for the purpose, known as ion pair receptors, are typically di- or polytopic hosts that contain recognition sites for both cations and anions and which permit the concurrent binding of multiple ions. The field of ion pair recognition has blossomed during the past decades. Several smaller reviews on the topic were published roughly 5 years ago. They provided a summary of synthetic progress and detailed the various limiting ion recognition modes displayed by both acyclic and macrocyclic ion pair receptors known at the time. The present review is designed to provide a comprehensive and up-to-date overview of the chemistry of macrocycle-based ion pair receptors. We specifically focus on the relationship between structure and ion pair recognition, as well as applications of ion pair receptors in sensor development, cation and anion extraction, ion transport, and logic gate construction.

Convergent Ditopic Receptors Enhance Anion Binding upon Alkali Metal Complexation for Catalyzing the Ritter Reaction

A supramolecular approach to catalyzing the Ritter reaction by utilizing enhanced anion-binding affinity in the presence of alkali metal cations was developed with ditopic hydrogen-bonded amide macrocycles. With prebound cations in the macrocycle, particularly Li+ ion, their metal complexes exhibit greatly enhanced catalytic activities. The catalysis is switchable by removal or addition of the bound cation. The method described in this work may be generalized for use in other anion-triggered organic reactions involving heteroditopic receptors capable of ion pairing.

Constructing bridged multifunctional calixarenes by intramolecular indole coupling

2,2′-Bisindole bridges can be easily created at (thia)calixarene cores providing the molecules with multiple functionalities for application in supramolecular chemistry.