Ion-pair Receptor Research Articles

Modulating fluorescent sensing of the pyrene-based ion pair receptor by docking on graphene quantum dots

We successfully synthesized squaramide-based ion pair receptor 3, possessing a cation binding domain linked to the receptor scaffold so as act as an electron-withdrawing group and enhance anion binding. This compound was able to efficiently and cooperatively bind anions in the presence of sodium cations. Using reference compounds 1 and 2, we demonstrated that the presence of a carbonyl group on the aromatic ring significantly enhances the efficiency of anion binding. The incorporation of this group in the structure of receptor 3, combined with other strongly electron-withdrawing groups located on another aryl subunit, enabled the receptor to bind salts even in the presence of water. Furthermore, the capacity for modular synthesis of diarylsquaramides allowed to obtain sensor 4, which features a pyrene moiety directly attached to the anion-binding domain, selectively sensing dihydrogen phosphate anions. Besides the signaling feature, the presence of pyrene rings facilitated the docking of sensor 4 onto the surface of graphene quantum dots, resulting in a conjugate with improved fluorescent properties.

Calix[4]pyrrole bis-crowns as ion pair receptors: cation selectivity modulated by counter anions

A series of doubly strapped calix[4]pyrrole bis-crowns were synthesized as ion pair receptors. Their cation selectivity could be reversed by counter anions.

Ion pair extractant selective for LiCl and LiBr.

Improved methods for achieving the selective extraction of lithium salts from lithium sources, including rocky ores, salt-lake brines, and end-of-life lithium-ion batteries, could help address projected increases in the demand for lithium. Here, we report an ion pair receptor (2) capable of extracting LiCl and LiBr into an organic receiving phase both from the solid state and from aqueous solutions. Ion pair receptor 2 consists of a calix[4]pyrrole framework, which acts as an anion binding site, linked to a phenanthroline cation binding motif via ether linkages. Receptor 2 binds MgBr2 and CaCl2 with high selectivity over the corresponding lithium salts in a nonpolar aprotic solvent. The preference for Mg2+ and Ca2+ salts is reversed in polar protic media, allowing receptor 2 to complex LiCl and LiBr with high selectivity and affinity in organic media containing methanol or water. The effectiveness of receptor 2 as an extractant for LiCl and LiBr under liquid-liquid extraction (LLE) conditions was found to be enhanced by the presence of other potentially competitive salts in the aqueous source phase.

Mechanically interlocked host systems for ion-pair recognition.

The ever-increasing interest directed towards the construction of host architectures capable of the strong and selective recognition of various ionic species of biological, medical and environmental importance has identified mechanically interlocked molecules (MIMs), such as rotaxanes and catenanes, as potent host systems, owing to their unique three-dimensional topologically preorganised cavity recognition environments. Ion-pair receptors are steadily gaining prominence over monotopic receptor analogues due to their enhanced binding strength and selectivity, demonstrated primarily through acyclic and macrocyclic heteroditopic host systems. Exploiting the mechanical bond for ion-pair recognition through the strategic design of neutral heteroditopic MIMs offers exciting opportunities to accomplish potent and effective binding while mitigating competing interactions from the bulk solvent and counter-ions. This review details the design and ion-pair recognition capabilities of rotaxanes and catenanes employing hydrogen bonding (HB) and halogen bonding (XB) motifs, providing valuable insight into the burgeoning field and inspiration for future research.

Fullerene-Functionalized Halogen-Bonding Heteroditopic Hosts for Ion-Pair Recognition.

Despite their hydrophobic surfaces with localized π-holes and rigid well-defined architectures providing a scaffold for preorganizing binding motifs, fullerenes remain unexplored as potential supramolecular host platforms for the recognition of anions. Herein, we present the first example of the rational design, synthesis, and unique recognition properties of novel fullerene-functionalized halogen-bonding (XB) heteroditopic ion-pair receptors containing cation and anion binding domains spatially separated by C60. Fullerene spatial separation of the XB donors and the crown ether complexed potassium cation resulted in a rare example of an artificial receptor containing two anion binding sites with opposing preferences for hard and soft halides. Importantly, the incorporation of the C60 motif into the heteroditopic receptor structure has a significant effect on the halide binding selectivity, which is further amplified upon K+ cation binding. The potassium cation complexed fullerene-based receptors exhibit enhanced selectivity for the soft polarizable iodide ion which is assisted by the C60 scaffold preorganizing the potent XB-based binding domains, anion-π interactions, and the exceptional polarizability of the fullerene moiety, as evidenced from DFT calculations. These observations serve to highlight the unique properties of fullerene surfaces for proximal charged guest binding with potential applications in construction of selective molecular sensors and modulating the properties of solar cell devices.

Highly Selective Transport and Enrichment of Lithium Ions through Bionic Ion Pair Receptor Nanochannels.

Inspired by ion pair cotransport channels in biological systems, a bionic nanochannel modified with lithium ion pair receptors is constructed for selective transport and enrichment of lithium ions (Li+). NH2-pillar[5]arene (NP5) is chosen as ion pair receptors, and the theoretical simulation and NMR titration experiments illustrate that NP5 has good affinity for the ion pair of LiCl through a strong host-guest interaction at the molecular level. Due to the confinement effect and ion pair cooperation recognition, an NP5-based receptor was introduced into an artificial PET nanochannel. An I-V test indicated that the NP5 channel realized the highly selective recognition for Li+. Meanwhile, transmembrane transport and COMSOL simulation experiments proved that the NP5 channel achieved the transport and enrichment of Li+ through the cooperative interaction between NP5 and LiCl. Moreover, the receptor solution of transmembrane transport LiCl in the NP5 channel was used to cultivate wheat seedlings, which obviously promoted their growth. This nanochannel based on the ion pair recognition will be much useful for practical applications like metal ion extraction, enrichment, and recycle.

Alkali Metal Halide Ion-Pair Binding in Conformationally Dynamic Halogen Bonding Heteroditopic [2]Rotaxanes.

A series of heteroditopic halogen bonding (XB) [2]rotaxanes were prepared via a combination of passive and active metal template-directed strategies. The ability of the [2]rotaxanes to bind alkali metal halide ion-pairs was investigated by extensive 1H NMR titration studies, wherein detailed analysis of cation, anion and ion-pair affinity measurements indicate dramatic positive cooperative enhancements in halide anion association upon either Na+ or K+ pre-complexation. We demonstrate that careful consideration of multiple, parallel and competing binding equilibria is essential when interpreting observed 1H NMR spectral changes in ion-pair receptor systems, especially those which exhibit dynamic behaviour. Importantly, in comparison to XB [2]catenane analogues, these neutral XB heteroditopic [2]rotaxane host systems demonstrated that despite their relatively weaker cation and anion binding abilities, they exhibit a notably higher level of positive cooperativity for alkali metal halide ion-pair binding, highlighting the role of greater co-conformational adaptive behaviour in mechanically-bonded hosts for the purposes of charged species recognition.

Direct Extraction of Sodium Hydroxide by Calix[4]pyrrole-Based Ion-Pair Receptors.

Described in this work are calix[4]pyrrole-based ion-pair receptors, cis/trans-1 and cis/trans-2, designed for the extraction of sodium hydroxide. An X-ray diffraction analysis of a single crystal of the cis-1·NaOH isomer isolated from a mixture of cis/trans-1 revealed a unique dimeric supramolecular structure. An average dimer in toluene-d8 solution was inferred on the basis of diffusion-ordered spectroscopy (DOSY). Support for the proposed stoichiometry came from density functional theory (DFT) calculations. The structural stability of the dimeric cis-1·NaOH complex in toluene solution was further confirmed by ab initio molecular dynamics (AIMD) simulation with explicit representation of solvent. Under conditions of liquid-liquid extraction (LLE), purified receptors cis- and trans-2 were both found to remove NaOH from a pH 11.01 aqueous source phase into toluene with extraction efficiencies (E%) of 50-60% when used equimolar to NaOH. However, in all cases, precipitation was observed. Complexities associated with precipitation could be avoided by immobilization of the receptors onto a chemically inert poly(styrene) resin by means of solvent impregnation. The use of solvent-impregnated resins (SIRs) eliminated precipitation in solution while retaining the extraction efficiency toward NaOH. This allowed both the pH and salinity of the alkaline source phase to be lowered.

A porphyrin-based ion pair receptor constructed through click chemistry

A novel heteroditopic porphyrin-based ion pair receptor with four ureas and four triazoles on the same side of the porphyrin ring was synthesized through click chemistry. This receptor showed high affinity to complex with the ion pairs KCl, NaCl and NH4Cl in 1:1 binding stoichiometries with association constants of 2.48 (± 0.43) × 104, 1.06 (± 0.02) × 105, and 1.97 (± 0.04) × 105 M[Formula: see text], respectively. After complexation with the ion pairs, fluorescence quenching of the porphyrin was observed, which enables its potential applications in ion detection.

Synthesis and Structural Characterization of some 3-oxapentane Podand Derivatives

A series of eight 3-oxapentane podand derivatives was successfully prepared based on the croton condensation reaction from 1,5-bis(aryl)-1,5-dithia-3-dioxadecane and 1,5-bis(aryl)-1,3,5-trioxadecane with bezaldehyde or acetophenone derivatives in acidic condition. These synthesized derivatives have structure of acyclic crown rings inserted by polyether or polythiaether chain and vinyl ketone chain conjugated with aryl group. Therefore, they are important sources for host-guest chemistry. They can wrap around a central alkali metal cations as well as organic cations and anions or even molecules to make interesting ion pair receptors. Structure of new podands was determined by spectral analyses including IR, 1H NMR, MS spectrocopies.

Novel class of crown ether functionalized ionic liquids with multiple binding sites for efficient separation of lithium isotopes

Lithium isotopes (6Li, 7Li) separation is an industrially important but challenging process due to their very similar chemical and physical properties. Crown ethers (CEs) have great potential in the isotopes separation for their typical cavity structures. However, the low distribution of Li+ has restricted their industrialuse. In this work, a novel class of functionalized ionic liquids with grafted crown ether-moieties (CE-FILs) for efficient separation of 6Li and 7Li are first reported. The enhanced distribution of Li+ could be achieved by the synergistic effect of ion–dipole interaction, hydrogen bonding and coordination interaction. As a result, an exceptionally high distribution coefficient for Li+ (DLi, up to 136) is obtained using 0.3 M [C6(B15C5)mim][NTf2] with a satisfied separation factor of 1.034 (at 273 K) for 6Li/7Li selectivity from an extremely hydrophilic chloride solutions, which strikes an excellent balance between Li distribution and 6Li/7Li separation. Theoretical calculations and spectroscopic characterization confirm that [C6(B15C5)mim][NTf2] supplies as an ion pair receptor to bind Li+, forming a 1:1 ion-pair complex [C6(B15C5)mim][NTf2]…Li+FeCl4- with double cations and anions. The separation performance of 6Li/7Li mainly depends on the weak interactions associated with Li+. The CE-FILs pave a new way for Li isotopes separation.

Dissipative crystallization of ion-pair receptors

•Crystal presence is dependent on the transient presence of chemical stimuli •Single crystals undergo dissipative crystal growth that is visually observable •Partially degraded single crystals can be rescued back to their original state •This dissipative system is highly modular Temporally controlled assemblies exhibit behaviors associated with living systems. Thus far, these transient assemblies have been studied primarily in soft materials, such as gels, vesicles, and colloidal assemblies. Here, we describe transient single crystals of supramolecular assemblies based on calix[4]pyrrole as an ion-pair receptor. The crystals form transiently upon the addition of a chemical stimulus, terephthalic acid, and disassemble when the chemical stimulus is consumed by alkylation using bromoethane. Single crystals observable by optical microscopy were grown and allowed to degrade repetitively by adding the chemical stimulus multiple times to a solution of the monomers. Partially degraded crystals were rescued and healed by adding more chemical stimulus to the solution before they dissolved completely. These results demonstrate that host-guest chemistry can be leveraged to create high-quality yet dynamic crystals that exhibit dissipative behavior. The modular nature of calix[4]pyrrole ion-pair recognition shows promise for obtaining diverse molecular materials capable of transient assembly. Temporally controlled assemblies exhibit behaviors associated with living systems. Thus far, these transient assemblies have been studied primarily in soft materials, such as gels, vesicles, and colloidal assemblies. Here, we describe transient single crystals of supramolecular assemblies based on calix[4]pyrrole as an ion-pair receptor. The crystals form transiently upon the addition of a chemical stimulus, terephthalic acid, and disassemble when the chemical stimulus is consumed by alkylation using bromoethane. Single crystals observable by optical microscopy were grown and allowed to degrade repetitively by adding the chemical stimulus multiple times to a solution of the monomers. Partially degraded crystals were rescued and healed by adding more chemical stimulus to the solution before they dissolved completely. These results demonstrate that host-guest chemistry can be leveraged to create high-quality yet dynamic crystals that exhibit dissipative behavior. The modular nature of calix[4]pyrrole ion-pair recognition shows promise for obtaining diverse molecular materials capable of transient assembly.

The use of microelectrodes to study ion recognition by a squaramide-based ion pair receptor consisting of a ferrocene reporter

The use of microelectrodes to study ion recognition by a squaramide-based ion pair receptor consisting of a ferrocene reporter

Allosteric and Electrostatic Cooperativity in a Heteroditopic Halogen Bonding Receptor System.

A halogen bonding (XB) heteroditopic receptor, consisting of a 1,3-bis-iodo-triazole benzene XB anion binding site covalently appended via a flexible methylene group with two benzo-15-crown-5 (B15C5) cation binding moieties, and its hydrogen bonding receptor analogue, are used to delineate the mechanisms of cooperativity for alkali metal halide ion-pair recognition. Extensive cation, anion and ion-pair 1 H NMR titration investigations demonstrate the combination of allosteric pre-organisation, via 1 : 1 stoichiometric intramolecular potassium and rubidium metal cation bis-B15C5 sandwich complexation, in concert with favourable electrostatics and XB potency, results in a remarkable enhancement of halide anion binding affinity by a factor of at least 700. By contrast, a notably diminished halide anion affinity enhancement factor of just 15 is observed with the corresponding 1 : 1 stoichiometric sodium cation complexed receptor system, where the smaller sized cation singly occupies one B15C5 unit and only an electrostatic contribution to cooperativity is possible. These observations serve to illustrate that allosteric pre-organisation capability, electrostatic attraction and XB mediated anion recognition are important strategic design features to incorporate in future high-fidelity heteroditopic ion-pair receptor development.

Recent Advancements in Ion-Pair Receptors.

Over the past two decades, non-covalent chemistry has introduced various promising artificial receptors and revolutionized the host-guest chemistry. These versatile receptors have particularly been entertained in sensing and recognizing of diverse neutral molecules and/or ionic entities (e. g. anions, cations and ion-pair) of particular interest. Notably, supramolecular chemistry had given birth to a plethora of important molecules, explored in the chemical, biological, environmental, and pharmacological world to resolve the critical issues related to the human health while keeping environmental concerns in mind. Amongst the various types of supramolecular monotopic receptors (anions, cations, and neutral molecules), heteroditopic receptors (ion-pair receptors) consisting of distinct binding sites in one system for both cation and anion, have gained much interest from the scientific community in recent past because of their unique binding abilities. Interestingly, these promising artificial receptors have shown potential applications in sensing, recognition, transport and extraction processes besides their uses in salt/waste purification. Bearing the importance of these systems in mind, we intended to report the recent developments in ion-pair chemistry. Herein, we divided the whole document into three main sections; first one describes the introduction and history of the ion-pairs receptors. The second portion highlights the synthesis and applications of ion-pair receptors in sensing, recognition, molecular machines, photoswitching behaviour, extraction and transport properties, whereas the last part of this manuscript provides concluding remarks as well as future prospects of ion-pair receptors. We hope that this manuscript will be helpful to stimulating researchers around the globe to find out the hidden opportunities in this and related areas.

Tris(pyridin‐2‐ylmethyl)amine‐Based Ion Pair Receptors for Selective Lithium Salt Recognition

AbstractTripodal ion pair receptors 1 and 2 consisting of tris(pyridine‐2‐ylmethyl)amine as a cation binding site linked with nitrophenyl urea or amidonitroindole groups as anion binding motifs have been synthesized. 1H NMR spectroscopic analyses carried out in 10 % DMSO in acetonitrile revealed that both ion pair receptors 1 and 2 are able to bind the LiCl ion pair with high selectivity over NaCl, KCl, RbCl, and CsCl. 1H NMR spectroscopic analyses in combination with molecular mechanics calculations proved that the lithium cation is bound to the tris(pyridin‐2‐ylmethyl)amine subunit while the chloride anion interacts with the NHs of urea or amido indole groups via hydrogen bonds. The addition of fluoride to the lithium cation complexes of receptors 1 and 2 leads to the release of the lithium cation from the receptors whereas chloride and bromide are co‐bound with lithium forming LiCl and LiBr ion pair complexes, respectively. The affinity of receptor 1 for the lithium cation was found to be improved by 5.0‐fold when the counter anion of Cl− is co‐bound within the receptor.

Selective Potassium Chloride Recognition, Sensing, Extraction, and Transport Using a Chalcogen-Bonding Heteroditopic Receptor.

Chalcogen bonding (ChB) is rapidly rising to prominence in supramolecular chemistry as a powerful sigma (σ)-hole-based noncovalent interaction, especially for applications in the field of molecular recognition. Recent studies have demonstrated ChB donor strength and potency to be remarkably sensitive to local electronic environments, including redox-switchable on/off anion binding and sensing capability. Influencing the unique electronic and geometric environment sensitivity of ChB interactions through simultaneous cobound metal cation recognition, herein, we present the first potassium chloride-selective heteroditopic ion-pair receptor. The direct conjugation of benzo-15-crown-5 ether (B15C5) appendages to Te centers in a bis-tellurotriazole framework facilitates alkali metal halide (MX) ion-pair binding through the formation of a cofacial intramolecular bis-B15C5 M+ (M+ = K+, Rb+, Cs+) sandwich complex and bidentate ChB···X– formation. Extensive quantitative 1H NMR ion-pair affinity titration experiments, solid–liquid and liquid–liquid extraction, and U-tube transport studies all demonstrate unprecedented KCl selectivity over all other group 1 metal chlorides. It is demonstrated that the origin of the receptor’s ion-pair binding cooperativity and KCl selectivity arises from an electronic polarization of the ChB donors induced by the cobound alkali metal cation. Importantly, the magnitude of this switch on Te-centered electrophilicity, and therefore anion-binding affinity, is shown to correlate with the inherent Lewis acidity of the alkali metal cation. Extensive computational DFT investigations corroborated the experimental alkali metal cation–anion ion-pair binding observations for halides and oxoanions.

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.

A TD-DFT study on photoswitchable chloride salts receptor based on acylhydrazone and crown ether embedded Macrocyclic molecule

The physical method of remediating saline-alkali soil, flushing soil drainage, and extracting lithium ions from salt lakes require tons of ion pair receptors with high selectivity to salts and reusability for the purpose of water conservation. The photoswitchable ion pair receptor is much fascinating if the ion pair receptor possesses two controllable states, namely capture state and release state. Using LiCl, NaCl, KCl as target ion pairs, two crown ether embedded cyclic receptors were computationally designed by introducing ether chain into a acylhydrazone photoswitch. The affinities to XCl of the designed receptors were observed by calculation binding energy and molecular topologic analysis. The photochemical properties of the receptors were observed by simulating UV–Vis absorption spectra according to the TD-DFT calculation results. Only the receptor 1 shows obviously different affinity to XCl ion pairs. The binding energies of Z-1 and XCl are reduced by about 7 kcal•mol-1, or about 50%. The binding of XCl has almost no effect on the photochemical properties of E-1. The designed 1 is therefore a XCl ion pair receptor worthy of further study to meet the great needs in the field of chloride salts extraction.

Utilizing a polymer containing squaramide-based ion pair receptors for salt extraction

Herein, we report a novel polymer containing synthetic squaramide-based ion pair receptors, which effectively capture and remove sulfate salts from aqueous source phases during liquid–liquid extraction. The monomer M1 was synthesized in a modular fashion, assuring acidity of the anion binding domain and allowing the receptor to be incorporated into the polymer chain and then tested in ion pair binding studies using UV–vis and NMR spectroscopy. After the incorporation of the monomer M1 into the polystyrene polymer chain, its solubility was improved, and a system was developed to selectively remove sulfate ions from systems similar to those naturally occurring in the environment, i.e. among mixtures with other salts. To identify the polymer's salt binding mechanism, additional solid–liquid extractions (SLE) were carried out, which were characterized using 1H NMR, thermal analyses (DSC and TGA) and Scanning Electron Microscopy (SEM).