A Potent Halogen-Bonding Donor Motif for Anion Recognition and Anion Template Mechanical Bond Synthesis

Hypervalent Iodine(III) Mediated Halogen Bonded Supramolecular Chiral System with Cholesteryl Naphthalimides

Halogen bonding acknowledged as a noteworthy weak interaction, has gained growing recognition in the field of supramolecular chemistry. In this study, we selected structurally rigid diaryliodonium ions (I(III)) with two biaxial σ-holes as halogen-bond donors, to bind with three chiral acceptor molecules bearing cholesteryl and naphthalimides with distinct geometries. The abundant carbonyl oxygen atoms in side-arm substituents function as multiple acceptors for halogen bonding. The self-aggregation of chiral acceptor molecules demonstrates adaptiveness to solvent media, evidenced by the inversion of the Cotton effect and the morphological evolution from spherical to rod-like nanoarchitectures in different solvent systems. The distinct geometries of the acceptor molecules conferred various binding modes with I(III). The introduction of I(III) as a halogen-bond donor regulates the aggregation of the donors, achieving amplification of chiroptical signals and inheriting solvent responsiveness from the self-aggregated assembly. This study successfully utilized rational structural design and multimodal control strategies to achieve regulation of supramolecular chirality.

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.

Halogen Bonding Tripodal Metallo-Receptors for Phosphate Recognition and Sensing in Aqueous-Containing Organic Media

The anion recognition and electrochemical anion-sensing properties of halogen-bonding (XB) tripodal zinc(II) receptors strategically designed and constructed for tetrahedral anion guest binding are described. The XB tris(iodotriazole)-containing hosts exhibit high affinities and selectivities for inorganic phosphate over other more basic, mono-charged oxoanions such as acetate and the halides in a competitive CD3 CN/D2 O (9 : 1 v/v) aqueous solvent mixture. 1 H NMR anion binding and electrochemical voltammetric anion sensing studies with redox-active ferrocene functionalised metallo-tripodal receptor analogues, reveal each of the XB tripods as superior anion complexants when compared to their tris(prototriazole)-containing, hydrogen bonding (HB) counterparts, not only exemplifying the halogen bond as a strong alternative interaction to the traditional hydrogen bond for molecular recognition but also providing rare evidence of the ability of XB receptors to preferentially bind the "harder" phosphate oxoanion over the "softer" and less hydrated halides in aqueous containing media.

From construction to application of a new generation of interlocked molecules composed of heteroditopic wheels

Over the last few decades, research on mechanically interlocked molecules has significantly evolved owing to their unique structural features and interesting properties. A substantial percentage of the reported works have focused on the synthetic strategies, leading to the preparation of functional MIMs for their applications in the chemical, materials, and biomedical sciences. Importantly, various macrocyclic wheels with specific heteroditopicity (including phenanthroline, amide, amine, oxy-ether, isophthalamide, calixarene and triazole) and threading axles (bipyridine, phenanthroline, pyridinium, triazolium, etc.) have been designed to synthesize targeted multifunctional mononuclear/multinuclear pseudorotaxanes, rotaxanes and catenanes. The structural uniqueness of these interlocked systems is advantageous owing to the presence of mechanical bonds with specific three-dimensional cavities. Furthermore, their multi-functionalities and preorganised structural entities exhibit a high potential for versatile applications, like switching, shuttling, dynamic properties, recognition and sensing. In this feature article, we describe some of the most recent advances in the construction and chemical behaviour of a new generation of interlocked molecules, primarily focusing on heteroditopic wheels and their applications in different directions of the modern research area. Furthermore, we outline the future prospects and significant perspectives of the new generation heteroditopic wheel based interlocked molecules in different emerging areas of science.

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.

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.

Site-Selective Aryl Diazonium Installation onto Protein Surfaces at Neutral pH using a Maleimide-Functionalized Triazabutadiene

Aryl diazonium cations are versatile bioconjugation reagents due to their reactivity towards electron-rich aryl residues and secondary amines, but historically their usage has been hampered by both their short lifespan in aqueous solution and the harsh conditions required to generate them in situ. Triazabutadienes address many of these issues as they are stable enough to endure multiple-step chemical syntheses and can persist for several hours in aqueous solution, yet upon UV-exposure rapidly release aryl diazonium cations under biologically-relevant conditions. This paper describes the synthesis of a novel maleimide-functionalized triazabutadiene suitable for site-selectively installing aryl diazonium cations into proteins at neutral pH; we show reaction with this molecule and a surface-cysteine of a thiol disulfide oxidoreductase. Through photoactivation of the site-selectively installed triazabutadiene motifs, we generate aryl diazonium functionality, which we further derivatize via azo-bond formation to electron-rich aryl species, showcasing the potential utility of this strategy for the generation of photoswitches or protein-drug conjugates.

Dynamic Metalloporphyrin-Based [2]Rotaxane Molecular Shuttles Stimulated by Neutral Lewis Base and Anion Coordination

A series of dynamic metalloporphyrin [2]rotaxane molecular shuttles comprising of bis-functionalised Zn(II) porphyrin axle and pyridyl functionalised macrocycle components are prepared in high yield via active metal template synthetic methodology. Extensive variable temperature 1 H NMR and quantitative UV-Vis spectroscopic titration studies demonstrate dynamic macrocycle translocation is governed by an inter-component co-ordination interaction between the macrocycle pyridyl and axle Zn(II) metalloporphyrin, which serves to bias a 'resting state' co-conformation. The dynamic shuttling behaviour of the interlocked structures is dramatically inhibited by the addition of a neutral Lewis base such as pyridine, but can also be tuned via post-synthetic rotaxane demetallation of the porphyrin axle core to give free-base, or upon subsequent metallation, Ni(II) [2]rotaxane analogues. Importantly, the Lewis acidic Zn(II) porphyrin axle component is also capable of coordinating anions which induces mechanical bond shuttling behaviour resulting in a novel optical sensing response.

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.

Photo-switchable anion binding and catalysis with a visible light responsive halogen bonding receptor

Photo-switchable receptors allow for photo-control over guest binding and release with spatial and temporal precision. Here we report the first halogen bonding photo-switchable anion receptors in which chloride binding may be reversibly modulated by irradiation with red and blue light, with over a 50-fold enhancement in chloride binding affinity observed for the Z isomer. We demonstrate that this switchable binding enables unprecedented photo-controlled catalysis of XB-mediated halide abstractions and a Mukaiyama Aldol reaction.

A Theoretical Study of the Halogen Bond between Heteronuclear Halogen and Benzene

Halogen bonds play an important role in many fields, such as biological systems, drug design and crystal engineering. In this work, the structural characteristics of the halogen bond between heteronuclear halogen XD (ClF, BrCl, IBr, ICl, BrF and IF) and benzene were studied using density functional theory. The structures of the complexes between heteronuclear halogen and benzene have Cs symmetry. The interaction energies of the complexes between heteronuclear halogen XD (ClF, BrCl, IBr, ICl, BrF and IF) and benzene range from -27.80 to -37.18 kJ/mol, increasing with the increases in the polarity between the atoms of X and D, and are proportional to the angles of a between the Z axis and the covalent bond of heteronuclear halogen. The electron density (ρ) and corresponding Laplacian (∇²ρ) values indicate that the interaction of the heteronuclear halogen and benzene is a typical long-range weak interaction similar to a hydrogen bond. Independent gradient model analysis suggests that the van der Waals is the main interaction between the complexes of heteronuclear halogen and benzene. Symmetry-adapted perturbation theory analysis suggests that the electrostatic interaction is the dominant part in the complexes of C₆H₆⋯ClF, C₆H₆⋯ICl, C₆H₆⋯BrF and C₆H₆⋯IF, and the dispersion interaction is the main part in the complexes of C₆H₆⋯BrCl, C₆H₆⋯IBr.

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.

Development of fluorophoric [2]pseudorotaxanes and [2]rotaxane: selective sensing of Zn(II)

Fluorophoric [2]pseudorotaxanes {NiPR1(ClO4)2-NiPR3(ClO4)2} are synthesized by utilizing newly designed fluorophoric bidentate ligands (L1-L3) and a heteroditopic naphthalene containing macrocycle (NaphMC) with high yields via Ni(II) templation and π-π stacking interactions. Subsequently, a fluorophoric [2]rotaxane (NAPRTX) is established through a Cu(I) catalysed click reaction between an azide terminated pseudorotaxane, {NiPR4(ClO4)2}, which contains the newly designed fluorophoric ligand L4, and alkyne terminated bulky stopper units. All these fluorophoric [2]pseudorotaxanes and the [2]rotaxane were characterized using numerous techniques such as mass spectrometry, NMR, UV/Vis, PL, and elemental analysis, wherever applicable. Furthermore, to investigate the effect of the fluorophoric moieties, the coordinating ability of chelating units, and size and shape of the three dimensional cavity generated by the mechanical bond in the interlocked [2]rotaxane (NAPRTX), we have performed a sensing study of various metal ions. Thus, the interlocked [2]rotaxane is found to have potential as a selective fluorescent sensor for Zn(II) metal ions over other transition, alkali and alkaline earth metal ions, where the 2,2'-bipyridyl arylvinylene moiety of the axle acts as a fluorescence signalling unit.

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.

Hydroxy Groups Enhance [2]Rotaxane Anion Binding Selectivity

We report the synthesis of two [2]rotaxanes containing an interlocked three dimensional binding cavity formed from a pyridinium bis(amide) axle component containing two phenol donors, and an isophthalamide based macrocycle. In the competitive solvent mixture 1 : 1 CDCl₃ : CD₃OD, one of the receptors exhibits a much higher selectivity preference for chloride than an analogous rotaxane without the hydroxy groups. X‐ray crystal structures reveal the chloride anion guest encapsulated within the interlocked binding cavity, though not all of the hydrogen bond donors are utilised. Computational semi‐empirical simulations indicate that secondary intermolecular interactions occur between the axle hydroxy hydrogen bond donors and the [2]rotaxane macrocycle components, contributing to a more preorganised binding pocket, which may be responsible for the observed enhanced selectivity.

Molecular Probes, Chemosensors, and Nanosensors for Optical Detection of Biorelevant Molecules and Ions in Aqueous Media and Biofluids

Synthetic molecular probes, chemosensors, and nanosensors used in combination with innovative assay protocols hold great potential for the development of robust, low-cost, and fast-responding sensors that are applicable in biofluids (urine, blood, and saliva). Particularly, the development of sensors for metabolites, neurotransmitters, drugs, and inorganic ions is highly desirable due to a lack of suitable biosensors. In addition, the monitoring and analysis of metabolic and signaling networks in cells and organisms by optical probes and chemosensors is becoming increasingly important in molecular biology and medicine. Thus, new perspectives for personalized diagnostics, theranostics, and biochemical/medical research will be unlocked when standing limitations of artificial binders and receptors are overcome. In this review, we survey synthetic sensing systems that have promising (future) application potential for the detection of small molecules, cations, and anions in aqueous media and biofluids. Special attention was given to sensing systems that provide a readily measurable optical signal through dynamic covalent chemistry, supramolecular host-guest interactions, or nanoparticles featuring plasmonic effects. This review shall also enable the reader to evaluate the current performance of molecular probes, chemosensors, and nanosensors in terms of sensitivity and selectivity with respect to practical requirement, and thereby inspiring new ideas for the development of further advanced systems.

I⋯N halogen bonding in 1 : 1 co-crystals formed between 1,4-diiodotetrafluorobenzene and the isomeric n-pyridinealdazines (n = 2, 3 and 4): assessment of supramolecular association and influence upon solid-state photoluminescence properties

1 : 1 co-crystals formed between 1,4-diiodotetrafluorobenzene and each of the three isomeric n-pyridinealdazines (n = 2, 3 and 4), featuring I⋯N halogen bonding contacts within one-dimensional chains, are described.

Towards hydrogen and halogen bonded frameworks based on 3,5-bis(triazolyl)pyridinium motifs

Construction of supramolecular assemblies using hydrogen and halogen bonding between anions and the 3,5-bis(triazolyl)pyridinium motif was investigated.

Charge neutral halogen bonding tetradentate-iodotriazole macrocycles capable of anion recognition and sensing in highly competitive aqueous media

A series of neutral tetradentate halogen bonding (XB) macrocycles, comprising of two bis-iodotriazole XB donors were synthesised in 60-70% yields via a stepwise CuAAC-mediated cyclisation strategy. Extensive ¹H NMR anion titration experiments reveal halide binding affinities are critically dependent on the substitution pattern of the xylyl spacer unit. The meta-substituted macrocycle remarkably displays cooperative tetradentate XB-halide anion recognition in highly competitive 40% aqueous-organic D₂O/acetone-d₆ (40 : 60, v/v) solvent mixtures. Integration of para-xylyl and naphthyl spacer units generates extended macrocyclic cavities, capable of selective oxalate recognition. Furthermore, preliminary fluorescence exeperiments reveal dicarboxylate specific sensing can be achieved through monitoring of the naphthylene centred emission.

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.

Self-Assembly of Stimuli-Responsive [2]Rotaxanes by Amidinium Exchange

Advances in supramolecular chemistry are often underpinned by the development of fundamental building blocks and methods enabling their interconversion. In this work, we report the use of an underexplored dynamic covalent reaction for the synthesis of stimuli-responsive [2]rotaxanes. The formamidinium moiety lies at the heart of these mechanically interlocked architectures, because it enables both dynamic covalent exchange and the binding of simple crown ethers. We demonstrated that the rotaxane self-assembly follows a unique reaction pathway and that the complex interplay between crown ether and thread can be controlled in a transient fashion by addition of base and fuel acid. Dynamic combinatorial libraries, when exposed to diverse nucleophiles, revealed a profound stabilizing effect of the mechanical bond as well as intriguing reactivity differences between seemingly similar [2]rotaxanes.

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

A novel halogen bonding (XB) bis(iodotriazole)-imidazolium motif is incorporated into the axle component of a [2]rotaxane via a discrete chloride anion template directed clipping methodology. ¹H NMR anion titration experiments reveal the interlocked host is capable of strong halide and sulfate oxoanion binding in competitive aqueous-organic CDCl₃/CD₃OD/D₂O (45 : 45 : 10 v/v) solvent mixtures. In comparison to a hydrogen bonding rotaxane analogue, which exhibited no pronounced selectivity between Cl^-, I^- and SO₄^2-, the axle iodo-triazole donor motifs of the XB rotaxane modulate the anion recognition preference towards the lighter halides Cl^- ≈ Br^- > SO₄^2- > I^-.

Modulating Chalcogen Bonding and Halogen Bonding Sigma-Hole Donor Atom Potency and Selectivity for Halide Anion Recognition

A series of acyclic anion receptors containing chalcogen bond (ChB) and halogen bond (XB) donors integrated into a neutral 3,5‐bis‐triazole pyridine scaffold are described, in which systematic variation of the electronic‐withdrawing nature of the aryl substituents reveal a dramatic modulation in sigma‐hole donor atom potency for anion recognition. Incorporation of strongly electron‐withdrawing perfluorophenyl units appended to the triazole heterocycle telluro‐ or iodo‐ donor atoms, or directly linked to the tellurium donor atom dramatically enhances the anion binding potency of the sigma‐hole receptors, most notably for the ChB and XB receptors displaying over thirty‐fold and eight‐fold increase in chloride anion affinity, respectively, relative to unfluorinated analogues. Linear free energy relationships for a series of ChB based receptors reveal the halide anion recognition behaviour of the tellurium donor is highly sensitive to local electronic environments. This is especially the case for those directly appended to the Te centre (3⋅ChB), where a remarkable enhancement of strength of binding and selectivity for the lighter halides is observed as the electron‐withdrawing ability of the Te‐bonded aryl group increases, highlighting the exciting opportunity to fine‐tune anion affinity and selectivity in ChB‐based receptor systems.

Halogen Bonding Tetraphenylethene Anion Receptors: Anion-Induced Emissive Aggregates and Photoswitchable Recognition

A series of tetraphenylethene (TPE) derivatives functionalized with highly potent electron-deficient perfluoroaryl iodo-triazole halogen bond (XB) donors for anion recognition are reported. 1 H NMR titration experiments, fluorescence spectroscopy, dynamic light scattering measurements, TEM imaging and X-ray crystal structure analysis reveal that the tetra-substituted halogen bonding receptor forms luminescent nanoscale aggregates, the formation of which is driven by XB-mediated anion coordination. This anion-coordination-induced aggregation effect serves as a powerful sensory mechanism, capable of luminescence chloride sensing at parts per billion concentration. Furthermore, the doubly substituted geometric isomers act as unprecedented photoswitchable XB donor anion receptors, where the composition of the photostationary state can be modulated by the presence of a coordinating halide anion.

The Chloroazaphosphatrane Motif for Halogen Bonding in Solution

Chloroazaphosphatranes, the corresponding halogenophosphonium cations of the Verkade superbases, were evaluated as a new motif for halogen bonding (XB). Their modulable synthesis allowed for synthetizing chloroazaphosphatranes with various substituents on the nitrogen atoms. The binding constants determined from NMR titration experiments for Cl^-, Br^-, I^-, AcO^-, and CN^- anions are comparable to those obtained with conventional iodine-based monodentate XB receptors. Remarkably, the protonated azaphosphatrane counterparts display no affinity for anions under the same conditions. The strength of the XB interaction is, to some extent, related to the basicity of the corresponding Verkade superbase. The halogen bonding abilities of this new class of halogen donor motif were also revealed by the Δδ(³¹P) NMR shift observed in CD₂Cl₂ solution in the presence of triethylphosphine oxide (TEPO). Thus, chloroazaphosphatranes constitute a new class of halogen bond donors, expanding the repertory of XB motifs mainly based on C_Ar-I bonds.

Highly Active Halogen Bonding and Chalcogen Bonding Chloride Transporters with Non-Protonophoric Activity

Synthetic anion transporters show much promise as potential anti-cancer agents and therapeutics for diseases associated with mis-regulation of protein anion channels. In such applications high activity and anion selectivity are crucial to overcome competing proton or hydroxide transport which dissipates cellular pH gradients. Here, highly active bidentate halogen bonding and chalcogen bonding anion carriers based on electron deficient iodo- and telluromethyl-triazole derivatives are reported. Anion transport experiments in lipid bilayer vesicles reveal record nanomolar chloride transport activity for the bidentate halogen bonding anion carrier, and remarkably high chloride over proton/hydroxide selectivity for the chalcogen bonding anionophore. Computational studies provide further insight into the role of sigma-hole mediated anion recognition and desolvation at the membrane interface. Comparison with hydrogen bonding analogues demonstrates the importance of employing sigma-hole donor motifs in synthetic anionophores for achieving both high transport activity and selectivity.

Dissection of the Origin of π-Holes and the Noncovalent Bonds in Which They Engage

Accompanying the rapidly growing list of σ-hole bonds has come the acknowledgment of parallel sorts of noncovalent bonds which owe their stability in large part to a deficiency of electron density in the area above the molecular plane, known as a π-hole. The origins of these π-holes are probed for a wide series of molecules, comprising halogen, chalcogen, pnicogen, tetrel, aerogen, and spodium bonds. Much like in the case of their σ-hole counterparts, formation of the internal covalent π-bond in the Lewis acid molecule pulls density toward the bond midpoint and away from its extremities. This depletion of density above the central atom is amplified by an electron-withdrawing substituent. At the same time, the amplitude of the π*-orbital is enhanced in the region of the density-depleted π-hole, facilitating a better overlap with the nucleophile's lone pair orbital and a stabilizing n → π* charge transfer. The presence of lone pairs on the central atom acts to attenuate the π-hole and shift its position somewhat, resulting in an overall weakening of the π-hole bond. There is a tendency for π-hole bonds to include a higher fraction of induction energy than σ-bonds with proportionately smaller electrostatic and dispersion components, but this distinction is less a product of the σ- or π-character and more a function of the overall bond strength.

Solvent Effects in Halogen and Hydrogen Bonding Mediated Electrochemical Anion Sensing in Aqueous Solution and at Interfaces

Sensing anionic species in competitive aqueous media is a well-recognised challenge to long-term applications across a multitude of fields. Herein, we report a comprehensive investigation of the electrochemical anion sensing performance of novel halogen bonding (XB) and hydrogen bonding (HB) bis-ferrocene-(iodo)triazole receptors in solution and at self-assembled monolayers (SAMs), in a range of increasingly competitive aqueous organic solvent media (ACN/H2 O). In solution, the XB sensor notably outperforms the HB sensor, with substantial anion recognition induced cathodic voltammetric responses of the ferrocene/ferrocenium redox couple persisting even in highly competitive aqueous solvent media of 20 % water content. The response to halides, in particular, shows a markedly lower sensitivity to increasing water content associated with a unique halide selectivity at unprecedented levels of solvent polarity. The HB sensor, in contrast, generally displayed a preference towards oxoanions. A significant surface-enhancement effect was observed for both XB/HB receptive films in all solvent systems, whereby the HB sensor generally displayed larger responses towards oxoanions than its halogen bonding analogue.

Proximity Effects of Substituents on Halogen Bond Strength

It is well known that the presence of an electron-withdrawing substituent (EWS) placed near the halogen (X) atom on a Lewis acid molecule amplifies the ability of this unit to engage in a halogen bond with a base. Quantum calculations are applied to examine how quickly these effects fade as the EWS is moved further and further from the X atom. Conjugated alkene and alkyne chains of varying lengths with a terminal C-I first facilitate analysis as to how the number of these multiple bonds affects the strength of CI··N XB to NH₃. Then, electron-withdrawing F and C≡N substituents are placed on the opposite end of the chain, and their effects on the XB properties are monitored as a function of their distance from I. These same EWSs are added to the ortho, meta, and para positions of aromatic iodobenzene. It is found that the XB grows in strength as more triple bonds are added to the alkyne, but there is little change caused by elongating an alkene. The cyano group has a much stronger effect than does F. While F strengthens the XB, its effects are quickly attenuated as it is moved further from I. The consequences of C≡N substitution are stronger and extend over a longer distance. Placement of an EWS on the phenyl ring diminishes with distance: o > m > p, and the effects of disubstitution are nearly additive. These trends apply not only to energetics but also to geometries, properties of the wave function, σ-hole depth, and NMR shielding.

Solution and Solid State Studies of Urea Derivatives of DITIPIRAM Acting as Powerful Anion Receptors

Herein, we present the synthesis and anion binding studies of a family of homologous molecular receptors 4–7 based on a DITIPIRAM (8-propyldithieno-[3,2-b:2′,3′-e]-pyridine-3,5-di-amine) platform decorated with various urea para-phenyl substituents (NO₂, F, CF₃, and Me). Solution, X-ray, and DFT studies reveal that the presented host–guest system offers a convergent array of four urea NH hydrogen bond donors to anions allowing the formation of remarkably stable complexes with carboxylates (acetate, benzoate) and chloride anions in solution, even in competitive solvent mixtures such as DMSO-d₆/H₂O 99.5/0.5 (v/v) and DMSO-d₃/MeOH-d₃ 9:1 (v/v). The most effective derivatives among the series turned out to be receptors 5 and 6 containing electron-withdrawing F- and -CF₃para-substituents, respectively.

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.

Anion Templated Supramolecular Structures Assembled using 1,2,3-Triazole and Triazolium motifs

The use of 1,2,3-triazole and triazolium motifs to construct anion templated supramolecular structures has grown rapidly over the past decade and has enabled a range of complex structures to be synthesised. In this Minireview we highlight the significant advances that have been made in areas such as foldamers, polymers and interlocked systems.

Lithium halide ion-pair recognition with halogen bonding and chalcogen bonding heteroditopic macrocycles

A series of halogen bonding and chalcogen bonding phenanthroline containing heteroditopic macrocyclic receptors exhibit cooperative recognition of lithium halide (LiX) ion-pairs.

A Paramagnetic NMR Spectroscopy Toolbox for the Characterisation of Paramagnetic/Spin-Crossover Coordination Complexes and Metal-Organic Cages

The large paramagnetic shifts and short relaxation times resulting from the presence of a paramagnetic centre complicate NMR data acquisition and interpretation in solution. As a result, NMR analysis of paramagnetic complexes is limited in comparison to diamagnetic compounds and often relies on theoretical models. We report a toolbox of 1D (1H, proton-coupled 13C, selective 1H-decoupling 13C, steady-state NOE) and 2D (COSY, NOESY, HMQC) paramagnetic NMR methods that enables unprecedented structural characterisation and in some cases, provides more structural information than would be observable for a diamagnetic analogue. We demonstrate the toolbox's broad versatility for fields from coordination chemistry and spin-crossover complexes to supramolecular chemistry through the characterisation of CoII and high-spin FeII mononuclear complexes as well as a Co4L6 cage.

Facile and Reliable Emission-Based Nanomolar Anion Sensing by Luminescent Iridium Receptors Featuring Chelating Halogen-Bonding Sites

An anion sensor is presented that combines a bidentate hydrogen‐ (HB) or halogen‐bonding (XB) site with a luminescent monocationic Ir fragment for strong binding of common anions (K_a up to 6×10⁴  m⁻¹) with diagnostic emission changes. A new emission‐based protocol for fast and reliable detection was derived on the basis of correction for systematic but unspecific background effects. Such a simple correction routine circumvents the hitherto practical limitations of systematic emission‐based analysis of anion binding with validated open‐source software (BindFit). The anticipated order of K_a values was obeyed according to size and basicity of the anions (Cl>Br=OAc) as well as the donor atom of the receptor (XB: 6×10⁴  m⁻¹ > HB: 5×10³  m⁻¹), and led to submicromolar limits of detection within minutes. The results were further validated by advanced NMR techniques, and corroborated by X‐ray crystallographic data and DFT analysis, which reproduced the structural and electronic features in excellent agreement. The results suggest that corrected emission‐based sensing may become a complementary, reliable, and fast tool to promote the use of XB in various application fields, due to the simple and fast optical determination at high dilution.

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.

Functionalized Chiral Bambusurils: Synthesis and Host-Guest Interactions with Chiral Carboxylates

Bambusurils are a class of macrocyclic anion receptors that exhibit notable anion recognition properties, able to bind various inorganic anions as well the carboxylates or sulfonates. Recently, we reported enantioselective recognition of chiral carboxylates using non-functionalized chiral bambusuril derivatives. Herein, we report the synthesis and host-guest properties of two new representatives of chiral bambusuril macrocycles bearing ester functional groups, differing by the substituents attached to their portals. Their supramolecular properties in terms of carboxylate binding were studied by means of NMR in DMSO-d₆ . The reported bambusurils bind selected chiral carboxylates with enantioselectivity factors up to 3.1. The results indicated that the selectivity towards different carboxylates is governed by the steric constraint of the substituents surrounding bambusuril portals. No clear trend in the binding affinities and their enantioselectivities was found.

Hypervalent Iodine(III) Compounds as Biaxial Halogen Bond Donors

“Hypervalent” iodine(III) derivatives have been established as powerful reagents in organic transformations, but so far only a handful of studies have addressed their potential use as halogen-bonding noncovalent Lewis acids. In contrast to “classical” halogen-bond donors based on iodine(I) compounds, iodine(III) salts feature two directional electrophilic axes perpendicular to each other. Herein we present the first systematic investigation on biaxial binding to such Lewis acids in solution. To this end, hindered and unhindered iodolium species were titrated with various substrates, including diesters and diamides, via ¹H NMR spectroscopy and isothermal titration calorimetry. Clear evidence for biaxial binding was obtained in two model systems, and the association strengths increased by 2 orders of magnitude. These findings were corroborated by density functional theory calculations (which reproduced the trend well but underestimated the absolute binding constants) and a cocrystal featuring biaxial coordination of a diamide to the unhindered iodolium compound.

Luminescent Anion Sensing by Transition-Metal Dipyridylbenzene Complexes Incorporated into Acyclic, Macrocyclic and Interlocked Hosts

A series of novel acyclic, macrocyclic and mechanically interlocked luminescent anion sensors have been prepared by incorporation of the isophthalamide motif into dipyridylbenzene to obtain cyclometallated complexes of platinum(II) and ruthenium(II). Both the acyclic and macrocyclic derivatives 7⋅Pt, 7⋅Ru⋅PF6 , 10⋅Pt and 10⋅Ru⋅PF6 are effective sensors for a range of halides and oxoanions. The near-infra red emitting ruthenium congeners exhibited an increased binding strength compared to platinum due to the cationic charge and thus additional electrostatic interactions. Intramolecular hydrogen-bonding between the dipyridylbenzene ligand and the amide carbonyls increases the preorganisation of both acyclic and macrocyclic metal derivatives resulting in no discernible macrocyclic effect. Interlocked analogues were also prepared, and preliminary luminescent chloride anion spectrometric titrations with 12⋅Ru⋅(PF6 )2 demonstrate a marked increase in halide binding affinity due to the complementary chloride binding pocket of the [2]rotaxane. 1 H NMR binding titrations indicate the interlocked dicationic receptor is capable of chloride recognition even in competitive 30 % aqueous mixtures.

Transmembrane anion transport mediated by halogen bonding and hydrogen bonding triazole anionophores

Transmembrane ion transport by synthetic anionophores is typically achieved using polar hydrogen bonding anion receptors. Here we show that readily accessible halogen and hydrogen bonding 1,2,3-triazole derivatives can efficiently mediate anion transport across lipid bilayer membranes with unusual anti-Hofmeister selectivity. Importantly, the results demonstrate that the iodo-triazole systems exhibit the highest reported activity to date for halogen bonding anionophores, and enhanced transport efficiency relative to the hydrogen bonding analogues. In contrast, the analogous fluoro-triazole systems, which are unable to form intermolecular interactions with anions, are inactive. The halogen bonding anionophores also exhibit a remarkable intrinsic chloride over hydroxide selectivity, which is usually observed only in more complex anionophore designs, in contrast to the readily accessible acyclic systems reported here. This highlights the potential of iodo-triazoles as synthetically accessible and versatile motifs for developing more efficient anion transport systems. Computational studies provide further insight into the nature of the anion-triazole intermolecular interactions, examining the origins of the observed transport activity and selectivity of the systems, and revealing the role of enhanced charge delocalisation in the halogen bonding anion complexes.

Halogen and hydrogen bonding 1,2,3-triazole derivatives efficiently mediate anion transport across lipid bilayer membranes with unusual anion selectivity profiles.

Regulating the Structures of Self-Assembled Mechanically Interlocked Moleculecular Constructs via Dianion Precursor Substituent Effects

Substituent effects play critical roles in both modulating reaction chemistry and supramolecular self-assembly processes. Using substituted terephthalate dianions (p-phthalic acid dianions; PTADAs), the effect of varying the type, number, and position of the substituents was explored in terms of their ability to regulate the inherent anion complexation features of a tetracationic macrocycle, cyclo[2](2,6-di(1H-imidazol-1-yl)pyridine)[2](1,4-dimethylenebenzene) (referred to as the Texas-sized molecular box; 1⁴⁺), in the form of its tetrakis-PF₆^- salt in DMSO. Several of the tested substituents, including 2-OH, 2,5-di(OH), 2,5-di(NH₂), 2,5-di(Me), 2,5-di(Cl), 2,5-di(Br), and 2,5-di(I), were found to promote pseudorotaxane formation in contrast to what was seen for the parent PTADA system. Other derivatives of PTADA, including those with 2,3-di(OH), 2,6-di(OH), 2,5-di(OMe), 2,3,5,6-tetra(Cl), and 2,3,5,6-tetra(F) substituents, led only to so-called outside binding, where the anion interacts with 1⁴⁺ on the outside of the macrocyclic cavity. The differing binding modes produced by the choice of PTADA derivative were found to regulate further supramolecular self-assembly when the reaction components included additional metal cations (M). Depending on the specific choice of PTADA derivatives and metal cations (M = Co²⁺, Ni²⁺, Zn²⁺, Cd²⁺, Gd³⁺, Nd³⁺, Eu³⁺, Sm³⁺, Tb³⁺), constructs involving one-dimensional polyrotaxanes, outside-type rotaxanated supramolecular organic frameworks (RSOFs), or two-dimensional metal-organic rotaxane frameworks (MORFs) could be stabilized. The presence and nature of the substituent were found to dictate which specific higher order self-assembled structure was obtained using a given cation. In the specific case of the 2,5-di(OH), 2,5-di(Cl), and 2,5-di(Br) PTADA derivatives and Eu³⁺, so-called MORFs with distinct fluorescence emission properties could be produced. The present work serves to illustrate how small changes in guest substitution patterns may be used to control structure well beyond the first interaction sphere.