A Halogen‐Bonding Bis‐triazolium Rotaxane for Halide‐Selective Anion Recognition

A systematic study on the anion‐binding properties of acyclic halogen‐ and hydrogen‐bonding bis‐triazolium carbazole receptors is described. The halide‐binding potency of halogen‐bonding bis‐iodotriazolium carbazole receptors was found to be far superior to their hydrogen‐bonding bis‐triazolium‐based analogues. This led to the synthesis of a mixed halogen‐ and hydrogen‐bonding rotaxane host containing a bis‐iodotriazolium carbazole axle component. The rotaxane’s anion recognition properties, determined by ¹H NMR titration experiments in a competitive aqueous solvent mixture, demonstrated the preorganised halogen‐bonding interlocked host cavity to be halide‐selective, with a strong binding affinity for bromide.     

Active‐Metal Template Synthesis of a Halogen‐Bonding Rotaxane for Anion Recognition

The synthesis of an all‐halogen‐bonding rotaxane for anion recognition is achieved by using active‐metal templation. A flexible bis‐iodotriazole‐containing macrocycle is exploited for the metal‐directed rotaxane synthesis. Endotopic binding of a Cu^I template facilitates an active‐metal CuAAC iodotriazole axle formation reaction that captures the interlocked rotaxane product. Following copper‐template removal, exotopic coordination of a more sterically demanding rhenium(I) complex induces an inversion in the conformation of the macrocycle component, directing the iodotriazole halogen‐bond donors into the rotaxane’s interlocked binding cavity to facilitate anion recognition.     

Halotriazolium Axle Functionalised [2]Rotaxanes for Anion Recognition: Investigating the Effects of Halogen‐Bond Donor and Preorganisation

The anion‐templated synthesis of three novel halogen‐bonding 5‐halo‐1,2,3‐triazolium axle containing [2]rotaxanes is described, and the effects of altering the nature of the halogen‐bond donor atom together with the degree of inter‐component preorganisation on the anion‐recognition properties of the interlocked host investigated. The ability of the bromotriazolium motif to direct the halide‐anion‐templated assembly of interpenetrated [2]pseudorotaxanes was studied initially; bromide was found to be the most effective template. As a consequence, bromide anion templation was used to synthesise the first bromotriazolium axle containing [2]rotaxane, the anion‐binding properties of which, determined by ¹H NMR spectroscopic titration experiments, revealed enhanced bromide and iodide recognition relative to a hydrogen‐bonding protic triazolium rotaxane analogue. Two halogen‐bonding [2]rotaxanes with bromo‐ and iodotriazolium motifs integrated into shortened axles designed to increase inter‐component preorganisation were also synthesised. Anion ¹H NMR spectroscopic titration experiments demonstrated that these rotaxanes were able to bind halide anions even more strongly, with the iodotriazolium axle integrated rotaxane capable of recognising halides in aqueous solvent media. Importantly, these observations suggest that a halogen‐bonding interlocked host binding domain, in combination with increased inter‐component preorganisation, are requisite design features for a potent anion receptor.     

Halogen‐ and Hydrogen‐Bonding Catenanes for Halide‐Anion Recognition

Halogen‐bonding (XB) interactions were exploited in the solution‐phase assembly of anion‐templated pseudorotaxanes between an isophthalamide‐containing macrocycle and bromo‐ or iodo‐functionalised pyridinium threading components. ¹H NMR spectroscopic titration investigations demonstrated that such XB interpenetrated assemblies are more stable than analogous hydrogen bonding (HB) pseudorotaxanes. The stability of the anion‐templated halogen‐bonded pseudorotaxane architectures was exploited in the preparation of new halogen‐bonding interlocked catenane species through a Grubbs’ ring‐closing metathesis (RCM) clipping methodology. The catenanes’ anion recognition properties in the competitive CDCl₃/CD₃OD 1:1 solvent mixture revealed selectivity for the heavier halides iodide and bromide over chloride and acetate.     

Polymeric Halogen‐Bond‐Based Donor Systems Showing Self‐Healing Behavior in Thin Films

The synthesis and comprehensive characterization of a systematic series of cleft‐type anion receptors imbedded into a polymeric architecture is presented. For the first time, isothermal calorimetric titrations on polymeric halogen‐bond‐based donors were exploited to evaluate the dependence of the anion affinity on different key parameters (i.e. monomeric versus polymeric receptor, halogen versus hydrogen bonding, charge assistance). The combination of these donor systems with a copolymer bearing accepting carboxylate groups led to supramolecular cross‐linked polymer networks showing excellent intrinsic self‐healing behavior. FT‐Raman spectroscopy and nano‐indentation measurements were utilized to clarify the thermally induced self‐healing mechanism based on the formation of halogen bonds. These first self‐healing materials based on halogen bonds pave the way for new applications of halogen‐bond donors in polymer and material science.     

Anion Recognition in Water: Recent Advances from a Supramolecular and Macromolecular Perspective

The recognition of anions in water remains a key challenge in modern supramolecular chemistry, and is essential if proposed applications in biological, medical, and environmental arenas that typically require aqueous conditions are to be achieved. However, synthetic anion receptors that operate in water have, in general, been the exception rather than the norm to date. Nevertheless, a significant step change towards routinely conducting anion recognition in water has been achieved in the past few years, and this Review highlights these approaches, with particular focus on controlling and using the hydrophobic effect, as well as more exotic interactions such as C?H hydrogen bonding and halogen bonding. We also look beyond the field of small‐molecule recognition into the macromolecular domain, covering recent advances in anion recognition based on biomolecules, polymers, and nanoparticles.     

Self‐Assembly of an Anion‐Binding Cryptand for the Selective Encapsulation,Sequestration, and Precipitation of Phosphate from Aqueous Systems

The self‐assembled trimetallic species [L₂Cu₃]⁶⁺ contains a cavity that acts as a host to many different anions. By using X‐ray crystallography, ESI‐MS, and UV/Vis spectroscopy we show that these anions are encapsulated both in the solid state and aqueous systems. Upon encapsulation, the anions Br⁻, I⁻, CO₃²⁻, SiF₆²⁻, IO₆³⁻, VO₄³⁻, WO₄²⁻, CrO₄²⁻, SO₄²⁻, AsO₄³⁻, and PO₄³⁻ are all precipitated from aqueous solution and can be removed by filtration. Furthermore, the cavity can be tuned to be selective to either phosphate or sulfate anions by variation of the pH. Phosphate anions can be removed from water, even in the presence of other common anions, reducing the concentration from 1000 to <0.1 ppm and recovering approximately 99 % of the phosphate anions.     

Halogen Bonding Between Anions: Association of Anion Radicals of Tetraiodo‐p‐benzoquinone with Iodide Anions

Halogen bonding between two negatively charged species, tetraiodo‐p‐benzoquinone anion radicals (I₄Q^?.) and iodide anions, was observed and characterized for the first time. X‐ray structural and EPR/UV–Vis spectral studies revealed that the anion–anion bonding led to the formation of crystals comprising 2D layers of I₄Q^?. anion radicals linked by iodides and separated by Et₄N⁺ counter‐ions. Computational analysis suggested that the seemingly antielectrostatic halogen bonds in these systems were formed via a combination of several factors. First, an attenuation of the interionic repulsion by the solvent facilitated close approach of the anions leading to their mutual polarization. This resulted in the appearance of positively charged areas (σ‐holes) on the surface of the iodine substituents in I₄Q^?. responsible for the attractive interaction. Finally, the solid‐state associations were also stabilized by multicenter (4:4) halogen bonding between I₄Q^?. and iodide.     

Halogen Bonding Between Anions: Association of Anion Radicals of Tetraiodo-p-benzoquinone with Iodide Anions

Halogen bonding between two negatively charged species, tetraiodo-p-benzoquinone anion radicals (I₄Q^−.) and iodide anions, was observed and characterized for the first time. X-ray structural and EPR/UV–Vis spectral studies revealed that the anion–anion bonding led to the formation of crystals comprising 2D layers of I₄Q^−. anion radicals linked by iodides and separated by Et₄N⁺ counter-ions. Computational analysis suggested that the seemingly antielectrostatic halogen bonds in these systems were formed via a combination of several factors. First, an attenuation of the interionic repulsion by the solvent facilitated close approach of the anions leading to their mutual polarization. This resulted in the appearance of positively charged areas (σ-holes) on the surface of the iodine substituents in I₄Q^−. responsible for the attractive interaction. Finally, the solid-state associations were also stabilized by multicenter (4:4) halogen bonding between I₄Q^−. and iodide.     

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/H₂O). 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.     

Iodide Discrimination by Tetra-Iodotriazole Halogen Bonding Interlocked Hosts

Whilst the exploitation of interlocked host frameworks for anion recognition is widely established, examples incorporating halogen bond donor groups are still relatively rare. Through the integration of a novel tetra(iodotriazole)-pyridinium motif into macrocycle and axle components, a family of halogen bonding catenane and rotaxanes are constructed for anion recognition studies in a competitive aqueous-organic solvent mixture. Importantly, the degree of anion selectivity displayed is dictated by the topological nature and charged state of the respective interlocked host cavity. All the interlocked hosts exhibit iodide anion selectivity over other halides and sulfate, with the level of discrimination being the greatest with the mono-cationic rotaxane. Arising from greater electrostatic interactions working in tandem with halogen bonding and hydrogen bonding, the di-cationic rotaxane displays stronger anion association at the expense of a relatively lower degree of iodide selectivity.     

Allyl‐Functionalized Dioxynaphthalene[38]Crown‐10 Macrocycles: Synthesis,Self‐Assembly,and Thiol‐ene Functionalization

Five dioxynaphthalene[38]‐crown‐10 ( DNP38C10 ) macrocycles bearing one, two, three, or four allyl moieties have been synthesized and their ability to spontaneously self‐assemble with methyl viologen to form [2]pseudorotaxanes has been evaluated. Association constants between methyl viologen and several of the allyl‐functionalized DNP38C10 macrocycles are found to be comparable to that of methyl viologen and unfunctionalized DNP38C10 , however, the enthalpic and entropic factors that underlie overall binding free energy vary systematically with increasing allyl substitution. These variations are explained through a combination of solution phase and solid‐state analysis of the macrocycles and their complexes. The utility of endowing DNP38C10 macrocycles with allyl moieties is further demonstrated by the ease with which they can be functionalized through thiol‐ene click chemistry.     

Host–Guest Binding‐Site‐Tunable Self‐Assembly of Stimuli‐Responsive Supramolecular Polymers

Despite the remarkable progress made in controllable self‐assembly of stimuli‐responsive supramolecular polymers (SSPs), a basic issue that has not been consideration to date is the essential binding site. The noncovalent binding sites, which connect the building blocks and endow supramolecular polymers with their ability to respond to stimuli, are expected to strongly affect the self‐assembly of SSPs. Herein, the design and synthesis of a dual‐stimuli thermo‐ and photoresponsive Y‐shaped supramolecular polymer (SSP2) with two adjacent β‐cyclodextrin/azobenzene (β‐CD/Azo) binding sites, and another SSP (SSP1) with similar building blocks, but only one β‐CD/Azo binding site as a control, are described. Upon gradually increasing the polymer solution temperature or irradiating with UV light, SSP2 self‐assemblies with a higher binding‐site distribution density; exhibits a flower‐like morphology, smaller size, and more stable dynamic aggregation process; and greater controllability for drug‐release behavior than those observed with SSP1 self‐assemblies. The host–guest binding‐site‐tunable self‐assembly was attributed to the positive cooperativity generated among adjacent binding sites on the surfaces of SSP2 self‐assemblies. This work is beneficial for precisely controlling the structural parameters and controlled release function of SSP self‐assemblies.     

Palladium‐Templated Subcomponent Self‐Assembly of Macrocycles,Catenanes, and Rotaxanes

The reaction of 2,6‐diformylpyridine with diverse amines and Pd^II ions gave rise to a variety of metallosupramolecular species, in which the Pd^II ion is observed to template a tridentate bis(imino)pyridine ligand. These species included a mononuclear complex as well as [2+2] and [3+3] macrocycles. The addition of pyridine‐containing macrocyclic capping ligands allows for topological complexity to arise, thereby enabling the straightforward preparation of structures that include a [2]catenane, a [2]rotaxane, and a doubly threaded [3]rotaxane.     

A Halogen‐Bond‐Induced Triple Helicate Encapsulates Iodide

The self‐assembly of higher‐order anion helicates in solution remains an elusive goal. Herein, we present the first triple helicate to encapsulate iodide in organic and aqueous media as well as the solid state. The triple helicate self‐assembles from three tricationic arylethynyl strands and resembles a tubular anion channel lined with nine halogen bond donors. Eight strong iodine???iodide halogen bonds and numerous buried π‐surfaces endow the triplex with remarkable stability, even at elevated temperatures. We suggest that the natural rise of a single‐strand helix renders its linear halogen‐bond donors non‐convergent. Thus, the stringent linearity of halogen bonding is a powerful tool for the synthesis of multi‐strand anion helicates.     

An All‐Halogen Bonding Rotaxane for Selective Sensing of Halides in Aqueous Media

The synthesis and anion binding properties of the first rotaxane host system to bind and sense anions purely through halogen bonding, is described. Through a combination of polarized iodotriazole and iodotriazolium halogen bond donors, a three‐dimensional cavity is created for anion binding. This rotaxane incorporates a luminescent rhenium(I) bipyridyl metal sensor motif within the macrocycle component, thus enabling optical study of the anion binding properties. The rotaxane topology was confirmed by single‐crystal X‐ray structural analysis, demonstrating halogen bonding between the electrophilic iodine atoms and chloride anions. In 50 % H₂O/CH₃CN solvent mixtures the rotaxane host exhibits strong binding affinity and selectivity for chloride, bromide, and iodide over a range of oxoanions.     

Desymmetrization of an Octahedral Coordination Complex Inside a Self‐Assembled Exoskeleton

The synthesis of a centrally functionalized, ribbon‐shaped [6]polynorbornane ligand L that self‐assembles with Pd^II cations into a {Pd₂ L ₄} coordination cage is reported. The shape‐persistent {Pd₂ L ₄} cage contains two axial cationic centers and an array of four equatorial H‐bond donors pointing directly towards the center of the cavity. This precisely defined supramolecular environment is complementary to the geometry of classic octahedral complexes [M(XY)₆] with six diatomic ligands. Very strong binding of [Pt(CN)₆]^2? to the cage was observed, with the structure of the host–guest complex {[Pt(CN)₆]@Pd₂L₄} supported by NMR spectroscopy, MS, and X‐ray data. The self‐assembled shell imprints its geometry on the encapsulated guest, and desymmetrization of the octahedral platinum species by the influence of the D_4h‐symmetric second coordination sphere was evidenced by IR spectroscopy. [Fe(CN)₆]^3? and square‐planar [Pt(CN)₄]^2? were strongly bound. Smaller octahedral anions such as [SiF₆]^2?, neutral carbonyl complexes ([M(CO)₆]; M=Cr, Mo, W) and the linear [Ag(CN)₂]^? anion were only weakly bound, showing that both size and charge match are key factors for high‐affinity binding.