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.

Enhanced anion recognition by ammonium [2]catenane functionalisation of a halogen bonding acyclic receptor

Ammonium-dibenzo[24]crown-8 [2]catenane functionalisation of a 3,5-bis-iodotriazole-pyridine motif produces a potent halogen bonding (XB) receptor capable of binding anions in aqueous-acetone solvent mixtures of up to 20% water. Exploiting the kinetically inert nature of the mechanically bonded cationic ammonium [2]catenane substituents, the XB receptor is demonstrated to exhibit superior anion recognition behaviour in comparison to labile sodium cation complexed bis-benzo[15]crown-5 XB and HB triazole-pyridine heteroditopic receptor analogues.

Halogen-Bonding Heteroditopic [2]Catenanes for Recognition of Alkali Metal/Halide Ion Pairs

The first examples of halogen bonding (XB) heteroditopic homo[2]catenanes were prepared by discrete Na⁺ template-directed assembly of oligo(ethylene glycol) units derived from XB donor-containing macrocycles and acyclic bis-azide precursors, followed by a Cu^I -mediated azide-alkyne cycloaddition macrocyclisation reaction. Extensive ¹ H NMR spectroscopic studies show the [2]catenane hosts exhibit positive cooperative ion-pair recognition behaviour, wherein XB-mediated halide recognition is enhanced by alkali metal cation pre-complexation. Notably, subtle changes in the catenanes' oligo(ethylene glycol) chain length dramatically alters their ion-binding affinity, stoichiometry, complexation mode, and conformational dynamics. Solution-phase and single-crystal X-ray diffraction studies provide evidence for competing host-separated and direct-contact ion-pair binding modes. We further demonstrate the [2]catenanes are capable of extracting solid alkali-metal halide salts into organic media.

The effect of outer-sphere anions on the spectroscopic response of metal-binding chemosensors

Ion pair receptors typically contain two separate binding sites, for the metal and the anion respectively. Here we report a less synthetically demanding approach, whereby we prepared a family of ion pair sensors based on a rhodamine fluorescent scaffold containing a tunable cation binding motif. When exposed to ion pairs, a competition for the metal ion is established between these ligands and anions. Structural and spectroscopic evidence showed that anions bind through weaker secondary interactions in the metal's outer coordination sphere and their presence influences the optical spectroscopic properties of the coordination complex in distinctive ways. The relationship between the binding site's metal affinity and its tunable properties, and the sensors' discriminatory power for anions was explained as a function of the metal ion's binding preferences. These effects were also exploited to discriminate cations and anions concurrently through multivariate data analysis methods.

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

This work reports on the synthesis and characterization of three tritopic receptors and their binding properties toward various anions, as their tetrabutylammonium salts, and three alkali metal-acetate salts by UV-vis, fluorescence, ¹H, ⁷Li, ²³Na, and ³⁹K NMR in MeCN/dimethyl sulfoxide (DMSO) 9:1 (v/v). Molecular recognition studies showed that the receptors have good affinity for oxyanions. Furthermore, these compounds are capable of ion-pair recognition of the alkali metal-acetate salts studied through a cooperative mechanism. Additionally, molecular modeling at the density functional theory (DFT) level of some lithium and sodium acetate complexes illustrates the ion-pair binding capacity of receptors. The anion is recognized through strong hydrogen bonds of the NH- groups from the two urea sites, while the cation interacts with the oxygen atoms of the polyether spacer. This work demonstrates that these compounds are good receptors for anions and ion pairs.

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.

Enhancement of Ion Pairing of Sr(II) and Ba(II) Salts by a Tritopic Ion-Pair Receptor in Solution

Tritopic ion-pair receptors can bind bivalent salts in solution; yet, these salts have a tendency to form ion-pairs even in the absence of receptors. The extent to which such receptors can enhance ion pairing has however remained elusive. Here, we study ion pairing of M2+ (Ba2+ , Sr2+ ) and X- (I- , ClO4- ) in acetonitrile with and without a dichlorooxacalix[2]arene[2]triazine-related receptor containing a pentaethylene-glycol moiety. We find marked ion association already in receptor-free solutions. When present, most of the MX+ ion-pairs are bound to the receptor and the overall degree of ion association is enhanced due to coordinative, hydrogen-bonding, and anion-π interactions. The receptor shows higher selectivity for iodides but also stabilizes perchlorates, despite the latter are often considered as weakly coordinating anions. Our results show that ion-pair binding is strongly correlated to ion pairing in these solutions, thereby highlighting the importance of taking ion association in organic solvents into account.

Chalcogen Bond Mediated Enhancement of Cooperative Ion-Pair Recognition

A series of heteroditopic receptors containing halogen bond (XB) and unprecedented chalcogen bond (ChB) donors integrated into a 3,5-bis-triazole pyridine structure covalently linked to benzo-15-crown-5 ether motifs exhibit remarkable cooperative recognition of halide anions. Multi-nuclear 1 H, 13 C, 125 Te and 19 F NMR, ion pair binding investigations reveal sodium cation-benzo-crown ether binding dramatically enhances the recognition of bromide and iodide halide anions, with the chalcogen bonding heteroditopic receptor notably displaying the largest enhancement of halide binding strength of over two hundred-fold, in comparison to the halogen bonding and hydrogen bonding heteroditopic receptor analogues. DFT calculations suggest crown ether sodium cation complexation induces a polarisation of the sigma hole of ChB and XB heteroditopic receptor donors as a significant contribution to the origin of the unique cooperativity exhibited by these systems.

Ditopic binuclear copper(II) complexes for DNA cleavage

Herein we present the synthesis of two ligands containing two di(2-picolyl)amine (DPA) units linked by either a 1,1'-(pyridine-2,6-diyl)bis(3-ethylurea) (L1) or a 1,1'-(1,3-phenylene)bis(3-ethylurea) (L2) spacer. The corresponding binuclear Cu^II and Zn^II complexes were prepared and isolated. The X-ray structures of the L1 ligand and the [Cu₂L1Cl₂]²⁺ complex evidence an unusual cis/trans conformation of one of the urea groups stabilized by an intramolecular hydrogen bond with the nitrogen atom of the pyridyl spacer. The Cu^II complexes form rather strong ternary complexes with phosphorylated anions. The [Cu₂L1]⁴⁺ complex presents a rather high affinity for pyrophosphate (logK₁₁ = 8.19 at pH 7, 25 °C), while [Cu₂L2]⁴⁺ stands out because of its strong binding to AMP^2- (logK₁₁ = 9.3 at pH 7, 25 °C). The interaction of the Cu^II complexes with deoxyribonucleic acid from calf thymus (ct-DNA) was monitored using circular dichroism (CD) and luminescence spectroscopies. These studies revealed a quite strong interaction of the complexes with ct-DNA (K_b = (6.4 ± 0.7) × 10³ for [Cu₂L1]⁴⁺ and K_b = (6.3 ± 1.0) × 10³ for [Cu₂L2]⁴⁺). Competition experiments carried out in the presence of methyl green and BAPPA (N¹,N³-Bis(4-amidinophenyl)propane-1,3-diamine) as major and minor groove competitors, respectively, confirm that the interaction of both complexes with DNA takes place through the minor groove, in agreement with docking studies. The [Cu₂L2]⁴⁺ complex is quite efficient in promoting the cleavage of the double-stranded pUC19 plasmid DNA, by favoring the conversion of the supercoiled form to the nicked form following a hydrolytic mechanism.

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.

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

Prepared in only three synthetic steps, a non-multimacrocyclic heteroditopic receptor binds potassium salts of halides and carboxylates with unusually high cooperativity, suggesting salt binding as associated ion-pairs. Unprecedented extraction of highly hydrophilic KAcO salt from water to organic solution is also demonstrated.