Paul D. Beer

The first orthogonal template strategy for the synthesis of higher order interlocked structures is described. Orthogonal cation and anion templates were employed in a stepwise fashion to construct a hetero[3]catenane composed of structurally distinct macrocyclic components. Importantly, the [3]catenane contains two interlocked binding cavities tailored for cation and anion binding, enabling it to function as a heteroditopic ion-pair receptor. The co-conformational dynamics and cation/anion binding properties of the [3]catenane were investigated by ¹H NMR studies, which displayed temperature-dependent and guest-induced changes in chemical shift and linewidth. The potent ion-pair binding capability of the [3]catenane was confirmed by single-crystal X-ray diffraction studies, which showed the [3]catenane binding to a sodium chloride ion-pair in a receptor-separated binding mode, wherein the two guest ions are encapsulated within their respective mechanically bonded interlocked binding cavities.

Glass substrates, functionalised with BODIPY-(iodo)triazole host monolayers, facilitate anion detection via fluorescence emission changes in both organic solution and pure water circumventing solubility constraints and enabling sensor reuse.

A series of macrocyclic heteroditopic receptors was synthesized to investigate cooperative recognition of alkali-metal halide ion pairs. The receptors combine either a 1,3-bis-iodotriazole (XB) or 1,3-bis-prototriazole (HB) benzene scaffold for halide binding with poly(ethylene glycol)-based macrocyclic moieties for cation coordination. Intensive ¹H NMR binding studies revealed that XB-functionalized macrocycles exhibit significantly higher halide affinities than their HB analogues, while increased macrocycle size enhances alkali-metal cation binding strength. Notably, the halide-bound 1·XB macrocycle induced strong positive cooperativity in lithium-ion recognition, with up to a 7-fold increase in binding affinity. Density functional theory (DFT) calculations suggest that electrostatic stabilization between cobound ions underlies this effect, with the most pronounced enhancement observed for the 1·XB@LiI complex. Solid-liquid extraction experiments further demonstrated the practical potential of the XB system, achieving efficient transfer of lithium halide salts into organic solution. These findings establish halogen-bonded macrocycles as effective platforms for cooperative ion-pair recognition and highlight their promise for applications in lithium salt recovery and recycling.

Two families of Ni( ii ) salen complexes, with halogen bonding and crown ether binding moieties, are reported as neutral receptor molecules capable of binding catalytically relevant anion and ion-pair species in proximity to the Ni( ii ) centre.

A novel ion-pair receptor, consisting of a diaza-18-crown-6 cation binding motif and a BODIPY-appended halogen bonding optical anion-sensing motif, is prepared and shown to be a potent receptor for inorganic and organic ion-pairs. Comprehensive optical emission and ¹H NMR spectroscopic titration studies, in combination with computational investigations, demonstrate that the receptor exhibits significant selectivity for the ion-pair guest species K⁺/Cl⁻ over other alkali metal halide ion-pairs, and the neurotransmitter salt dopamine hydrochloride over other phenethylamine hydrochloride derivatives. Notably, the specific host–guest three-dimensional binding mode, resulting from the simultaneous participation of the heteroditopic receptor's multiple proximal convergent binding motifs, dictates the observed inorganic and organic ion-pair selectivity trends, representing a potent biomimetic strategy for the design of optical sensors for a range of biologically-relevant ion-pair analytes.

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.

A Zn(ii) metalloporphyrin-containing [2]rotaxane molecular shuttle is reported. A significant inter-component interaction inhibits anion sensing in the absence of the ion-pair. Li⁺/X⁻ binding induces macrocycle shuttling, enabling anion sensing.

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.

This review details utilising mechanically interlocked host molecules containing cation and anion binding motifs to achieve ion-pair recognition.