Anion recognition by halogen bonding and hydrogen bonding bis(triazole)-imidazolium [2]rotaxanes

Exploiting the Mechanical Bond Effect for Enhanced Molecular Recognition and Sensing

The ubiquity of charged species in biological and industrial processes has resulted in ever-increasing interest in their selective recognition, detection, and environmental remediation. Building on the established coordination chemistry principles of the chelate and macrocyclic effects, and host preorganization, supramolecular chemists seek to construct specific 3D binding cavities reminiscent of biotic systems to enhance host-guest binding affinity and selectivity. Mechanically interlocked molecules (MIMs) present a wholly unique platform for synthetic host design, wherein topologies afforded by the mechanical bond enable the decoration of 3D cavities for non-covalent interactions with a range of target guest geometries. Notably, MIM host systems exhibit mechanical bond effect augmented affinities and selectivities for a variety of charged guest species, compared to non-interlocked acyclic and macrocycle host analogs. Furthermore, the modular nature of MIM synthesis facilitates incorporation of optical and electrochemical reporter groups, enabling fabrication of highly sensitive and specific molecular sensors. This review discusses the development of recognition and sensing MIMs, from the first reports in the late 20th century through to the present day, delineating how their topologically preorganized and dynamic host cavities enhance charged guest recognition and sensing, demonstrating the mechanical bond effect as a potent tool in future chemosensing materials.

Anion-templated synthesis of a switchable fluorescent [2]catenane with sulfate sensing capability

Anion templation strategies have facilitated the synthesis of various catenane and rotaxane hosts capable of strong and selective binding of anions in competitive solvents. However, this approach has primarily relied on positively charged precursors, limiting the structural diversity and the range of potential applications of the anion-templated mechanically interlocked molecules. Here we demonstrate the synthesis of a rare electroneutral [2]catenane using a powerful, doubly charged sulfate template and a complementary diamidocarbazole-based hydrogen bonding precursor. Owing to the unique three-dimensional hydrogen bonding cavity and the embedded carbazole fluorophores, the resulting catenane receptor functions as a sensitive fluorescent turn-ON sensor for the highly hydrophilic sulfate, even in the presence of a large excess of water. Importantly, the [2]catenane exhibits enhanced binding affinity and selectivity for sulfate over its parent macrocycle and other acyclic diamidocarbazole-based receptors. We demonstrate also, for the first time, that the co-conformation of the catenane may be controlled by reversible acid/base induced protonation and deprotonation of the anionic template, SO42-. This approach pioneers a new strategy to induce molecular motion of interlocked components using switchable anionic templates.

Exploiting the Catenane Mechanical Bond Effect for Selective Halide Anion Transmembrane Transport

The first examples of [2]catenanes capable of selective anion transport across a lipid bilayer are reported. The neutral halogen bonding (XB) [2]catenanes were prepared via a chloride template-directed strategy in an unprecedented demonstration of using XB⋅⋅⋅anion interactions to direct catenane assembly from all-neutral components. Anion binding experiments in aqueous-organic solvent media revealed strong halide over oxoanion selectivity, and a marked enhancement in the chloride and bromide affinities of the catenanes relative to their constituent macrocycles. The catenanes additionally displayed an anti-Hofmeister binding preference for bromide over the larger iodide anion, illustrating the efficacy of employing sigma-hole interactions in conjunction with the mechanical bond effect to tune receptor selectivity. Transmembrane anion transport studies conducted in POPC LUVs revealed that the catenanes were more effective anion transporters than the constituent macrocycles, with high chloride over hydroxide selectivity, which is critical to potential therapeutic applications of anionophores. Remarkably these outperform existing acyclic halogen bonding anionophores with regards to this selectivity. Record chloride over nitrate anion transport selectivity was also observed. This represents a rare example of the direct translation of intrinsic anion binding affinities to anion transport behaviour, and demonstrates the key role of the catenane mechanical bond effect for enhanced anion transport selectivity.

Squaramide-Based Heteroditopic [2]Rotaxanes for Sodium Halide Ion-Pair Recognition

A series of squaramide-based heteroditopic [2]rotaxanes consisting of isophthalamide macrocycle and squaramide axle components are synthesized using an alkali metal cation template-directed stoppering methodology. This work highlights the unprecedented sodium cation template coordination of the Lewis basic squaramide carbonyls for interlocked structure synthesis. Extensive quantitative 1 H NMR spectroscopic anion and ion-pair recognition studies reveal the [2]rotaxane hosts are capable of cooperative sodium halide ion-pair mechanical bond axle-macrocycle component recognition, eliciting up to 20-fold enhancements in binding strengths for bromide and iodide, wherein the Lewis basic carbonyls and Lewis acidic NH hydrogen bond donors of the squaramide axle motif operate as cation and anion receptive sites simultaneously in an ambidentate fashion. Notably, varying the length and nature of the polyether cation binding unit of the macrocycle component dramatically influences the ion-pair binding affinities of the [2]rotaxanes, even overcoming direct contact NaCl ion-pair binding modes in polar organic solvents. Furthermore, the cooperative ion-pair binding properties of the squaramide-based heteroditopic [2]rotaxanes are exploited to successfully extract solid sodium halide salts into organic media.

Solution Studies of a Water-Stable, Trivalent Antimony Pnictogen Bonding Anion Receptor with High Binding Affinities for CN-, OCN-, and OAc. Inorg Chem 2023. [PMID: 37499143 DOI: 10.1021/acs.inorgchem.3c01887] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

The solution phase anion binding behavior of a water-stable bidentate pnictogen bond donor was studied. A modest change in the visible absorption spectrum allowed for the determination of the binding constants. High binding constants were observed with cyanide, cyanate, and acetate, and these were corroborated with density functional theory (DFT) calculations. The receptor could be recovered free from the anion following treatment with methyl triflate, confirming that it remains intact. The tight binding of cyanide and water stability were exploited to use this system as a supramolecular catalyst in a phase-transfer Strecker reaction, further demonstrating the utility of pnictogen bonding as a tool in noncovalent catalysis.

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.

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.

Anti‐Hofmeister Anion Selectivity via a Mechanical Bond Effect in Neutral Halogen‐Bonding [2]Rotaxanes

Exceptionally strong halogen bonding (XB) donor-chloride interactions are exploited for the chloride anion template synthesis of neutral XB [2]rotaxane host systems which contain perfluoroaryl-functionalised axle components, including a remarkably potent novel 4,6-dinitro-1,3-bis-iodotriazole motif. Halide anion recognition properties in aqueous-organic media, determined via extensive ¹ H NMR halide anion titration experiments, reveal the rotaxane host systems exhibit dramatically enhanced affinities for hydrophilic Cl^- and Br^- , but conversely diminished affinities for hydrophobic I^- , relative to their non-interlocked axle counterparts. Crucially, this mechanical bond effect induces a binding selectivity which directly opposes Hofmeister bias. Free-energy analysis of this mechanical bond enhancement demonstrates anion recognition by neutral XB interlocked host systems as a rare and general strategy to engineer anti-Hofmeister bias anion selectivity in synthetic receptor design.

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.

Halogen-Bonded Driven Tetra-Substituted Benzene Dimers and Trimers: Potential Hosts for Metal Ions

Cyclic dimers and trimers of tetra-substituted benzenes, ((HOOC)2-C6H2-(NHI)2), are selected as convenient model systems for investigating NI…O=C halogen bond strength and cooperativity. The four substituents in benzene are chosen so that two of them act as halogen bond acceptors (COOH) and two act as halogen bond donors (NHI), as shown in the graphical abstract below. The potential for metal ion binding by each of the halogen-bonded aggregates is also investigated using the monoatomic sodium ion, Na+. Density functional theory calculations performed using the wB97XD functional and the DGDZVP basis set confirmed the ability of halogen bonding to drive the formation of the cyclic dimers and trimers of the model system chosen for this study. Evidence of halogen bond cooperativity is seen, for example, in a 9% shortening of each NI…O=C halogen bond distance with a corresponding 53% increase in the respective critical point density value, ρNI…O=C. Cooperativity also results in a 36% increase in the magnitude of the complexation energy per halogen-bond of the trimer relative to that of the dimer. The results of this study confirm the potential for binding a single Na+ ion by either the dimer or the trimer through their respective halogen-bond networks. Binding of two metal ions was shown to be possible by the dimer. Likewise, the trimer was also found to bind three metal ions. Lastly, the overall structure of the halogen-bonded dimer or trimer endured after complexation of the Na+ ions.

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.

Exploring cyclohexane/piperazine-urea motifs for spherical halide (X = Cl−/Br−) recognition: effects on anion coordination, photoluminescence, and morphological tunability

Two bis-urea receptors bearing aromatic meta-substituted electron-withdrawing groups demonstrated  halide-coordination-triggered self-assemblies with contrasting coordination numbers and variation in morphology.