Determining Ion‐Pair Binding Affinities of Heteroditopic Receptor Systems

Determining ion‐pair affinities in heteroditopic receptor systems presents a persistent and significant challenge. The plethora of technical and experimental problems implicated in measuring ion‐pair affinities have encouraged the use of several expedient experimental practices as a means of characterising ion‐pair recognition behaviour. Exploiting a model heteroditopic receptor system, we interrogate the reliability of these methods and demonstrate that these commonly used techniques can be highly questionable and without extreme care can lead to incorrect conclusions

Bis‐Squaramide‐Based [2]Rotaxane Hosts for Anion Recognition

The first examples of bis‐squaramide axle containing [2]rotaxanes linked via rigid aryl and flexible alkyl spacers synthesised using copper(I) catalysed active metal template methodology are reported. The halide and oxoanion binding properties of the [2]rotaxanes in aqueous‐organic solvent media are examined through extensive 1H‐NMR titration experiments to investigate the impact of integrating multiple squaramide motifs on the anion binding capabilities of the interlocked receptors. These studies reveal that the bis‐squaramide rotaxane host systems exhibit enhanced halide anion binding capabilities relative to a mono‐squaramide axle functionalised rotaxane, demonstrating a rare anti‐Hofmeister bias halide anion selectivity trend in aqueous‐organic mixtures and highlighting the efficacy of the potent solvent shielded hydrophobic interlocked binding pocket created upon mechanical bond formation. Notably, employing a rigid aryl linker between the two squaramide motifs in the axle component enables the rotaxane host to exhibit strong and selective binding of tetrahedral oxoanions. Conversely, a flexible alkyl spacer facilitates trigonal oxoanion selective recognition by the bis‐squaramide [2]rotaxane

Stabilisation of Bromenium Ions in Macrocyclic Halogen Bond Complexes

Halenium ions (X+) are highly reactive electron deficient species that are prevalent transient intermediates in halogenation reactions. The stabilisation of these species is especially challenging, with the most common approach to sequester reactivity through the formation of bis‐pyridine (Py) complexes; [(Py)2X]+. Herein, we present the first example of a macrocyclic stabilisation effect for halenium species. Exploiting a series of bis‐pyridine macrocycles, we demonstrate that preorganised macrocyclic ligands stabilise bromenium cations via endotopic complexation, impressively facilitating the isolation of a bench stable ‘Br+ NO3−’ species. Solid state X‐ray crystallographic structural comparison of macrocyclic Br(I) complexes with Ag(I) and Au(I) analogues provides insightful information concerning similarities and stark contrasts in halenium/metal cation coordination behaviors. Furthermore, the first chemical ligand exchange reactions of Br(I) complexes are reported between acyclic [(Py)2Br]+ species and a bis‐pyridine macrocyclic donor ligand which importantly highlights a macrocycle effect for halenium cation stabilisation in the solution phase

Stabilisation of Bromenium Ions in Macrocyclic Halogen Bond Complexes

Halenium ions (X+) are highly reactive electron deficient species that are prevalent transient intermediates in halogenation reactions. The stabilisation of these species is especially challenging, with the most common approach to sequester reactivity through the formation of bis‐pyridine (Py) complexes; [(Py)2X]+. Herein, we present the first example of a macrocyclic stabilisation effect for halenium species. Exploiting a series of bis‐pyridine macrocycles, we demonstrate that preorganised macrocyclic ligands stabilise bromenium cations via endotopic complexation, impressively facilitating the isolation of a bench stable ‘Br+ NO3−’ species. Solid state X‐ray crystallographic structural comparison of macrocyclic Br(I) complexes with Ag(I) and Au(I) analogues provides insightful information concerning similarities and stark contrasts in halenium/metal cation coordination behaviors. Furthermore, the first chemical ligand exchange reactions of Br(I) complexes are reported between acyclic [(Py)2Br]+ species and a bis‐pyridine macrocyclic donor ligand which importantly highlights a macrocycle effect for halenium cation stabilisation in the solution phase

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

A Halogen Bonding [2]Rotaxane Shuttle for Chloride‐Selective Optical Sensing

The first example of a [2]rotaxane shuttle capable of selective optical sensing of chloride anions over other halides is reported. The rotaxane was synthesised via a chloride ion template‐directed cyclisation of an isophthalamide macrocycle around a multi‐station axle containing peripheral naphthalene diimide (NDI) stations and a halogen bonding (XB) bis(iodotriazole) based station. Proton NMR studies indicate the macrocycle resides preferentially at the NDI stations in the free rotaxane, where it is stabilised by aromatic donor‐acceptor charge transfer interactions between the axle NDI and macrocycle hydroquinone moieties. Addition of chloride ions in an aqueous‐acetone solvent mixture induces macrocycle translocation to the XB anion binding station to facilitate the formation of convergent XB⋅⋅⋅Cl− and hydrogen bonding HB⋅⋅⋅Cl− interactions, which is accompanied by a reduction of the charge‐transfer absorption band. Importantly, little to no optical response was induced by addition of bromide or iodide to the rotaxane, indicative of the size discriminative steric inaccessibility of the interlocked cavity to the larger halides, demonstrating the potential of using the mechanical bond effect as a potent strategy and tool in chloride‐selective chemo‐sensing applications in aqueous containing solvent environments

Selective sodium halide over potassium halide binding and extraction by a heteroditopic halogen bonding [2]catenane †

The synthesis and ion-pair binding properties of a heteroditopic [2]catenane receptor exhibiting highly potent and selective recognition of sodium halide salts are described. The receptor design consists of a bidentate halogen bonding donor motif for anion binding, as well as a di(ethylene glycol)-derived cation binding pocket which dramatically enhances metal cation affinity over previously reported homo[2]catenane analogues. 1H NMR cation, anion and ion-pair binding studies reveal significant positive cooperativity between the cation and anion binding events in which cation pre-complexation to the catenane subsequently ‘switches-on’ anion binding. Notably, the heteroditopic catenane displayed impressive selectivity for sodium halide recognition over the corresponding potassium halides. We further demonstrate that the catenane is capable of extracting solid alkali metal salts into organic media. Crucially, the observed solution phase binding selectivity for sodium halides translates to superior functional extraction capabilities of these salts relative to potassium halides, overcoming the comparatively higher lattice enthalpies NaX > KX dictated by the smaller alkali metal sodium cation. This is further exemplified in competitive solid–liquid experiments which revealed the exclusive extraction of sodium halide salts from solid mixtures of sodium and potassium halide salts

Amphoteric chalcogen-bonding and halogen-bonding rotaxanes for anion or cation recognition

The ever-increasing demand in the development of host molecules for the recognition of charged species is stimulated by their fundamental roles in numerous biological and environmental processes. Here, capitalizing on the inherent amphoteric nature of anisotropically polarized tellurium or iodine atoms, we demonstrate a proof of concept in charged guest recognition, where the same neutral host structure binds both cations or anions solely through its chalcogen or halogen donor atoms. Through extensive 1H nuclear magnetic resonance titration experiments and computational density functional theory studies, a library of chalcogen-bonding (ChB) and halogen-bonding (XB) mechanically interlocked [2]rotaxane molecules, including seminal examples of all-ChB and mixed ChB/XB [2]rotaxanes, are shown to function as either Lewis-acidic or Lewis-basic multidentate hosts for selective halide anion and metal cation binding. Notably, the exploitation of the inherent amphoteric character of an atom for the strategic purpose of either cation or anion recognition constitutes the inception of a previously unexplored area of supramolecular host–guest chemistry

Ammonium halide selective ion pair recognition and extraction with a chalcogen bonding heteroditopic receptor †

The first example of a heteroditopic receptor capable of cooperative recognition and extraction of ammonium salt (NH4X) ion-pairs is described. Consisting of a bidentate 3,5-bis-tellurotriazole chalcogen bond donor binding cleft, the appendage of benzo-15-crown-5 (B15C5) substituents to the tellurium centres facilitates binding of the ammonium cation via a co-facial bis-B15C5 sandwich complex, which serves to switch on chalcogen bonding-mediated anion binding potency. Extensive quantitative ion-pair recognition 1H NMR titration studies in CD3CN/CDCl3 (1 : 1, v/v) solvent media reveal impressive ion-pair binding affinities towards a variety of ammonium halide, nitrate and thiocyanate salts, with the heteroditopic receptor displaying notable ammonium halide salt selectivity. The prodigious solution phase NH4X recognition also translates to efficient solid–liquid and liquid–liquid extraction capabilities

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.publishe