Cooperativity and Complexity in the Binding of Anions and Cations to a Tetratopic Ion-Pair Host

Unusual binding selectivity with non-selective homoditopic pillar[5]arene oxime: serendipitous discovery of a unique approach to heterobinuclear metalation in solution

A heterobinuclear complexation strategy on homoditopic pillar[5]arene was developed by using a pillar[5]arene with two rims decorated with benzaldehyde oximes. The unique selective recognition process was found to result from vesicular formation based on a controllable self-assembly, leading to binding of thorium(iv) only onto one rim, with the other rim being unoccupied for subsequent complexation of a second different metal ion.

The death of the Job plot, transparency, open science and online tools, uncertainty estimation methods and other developments in supramolecular chemistry data analysis

Data analysis is central to understanding phenomena in host-guest chemistry. We describe here recent developments in this field starting with the revelation that the popular Job plot method is inappropriate for most problems in host-guest chemistry and that the focus should instead be on systematically fitting data and testing all reasonable binding models. We then discuss approaches for estimating uncertainties in binding studies using case studies and simulations to highlight key issues. Related to this is the need for ready access to data and transparency in the methodology or software used, and we demonstrate an example a webportal () that aims to address this issue. We conclude with a list of best-practice protocols for data analysis in supramolecular chemistry that could easily be translated to other related problems in chemistry including measuring rate constants or drug IC₅₀ values.

Supramolecular anion recognition by β-HCH

The formation of highly ordered supramolecular architectures via cooperative C(aliphatic)-H·anion contacts between β-HCH and various anions (Cl^-, Br^-, I^- and HSO₄^-) was investigated by single crystal X-ray diffractometry, molecular modelling, ESI-MS and ¹H-NMR titrations.

Aromatic oligureas as hosts for anions and cations

Aromatic oligoureas 3 and 4 have urea moieties engaging in weak intramolecular H-bonding that constrains their backbones. The shorter 3a and 3b are able to bind chloride and acetate but not their corresponding counterion. The longer 4 binds both an anion and its counterion with the same affinity.

Allosteric cooperativity in ternary complexes with low symmetry

Strong positive allosteric cooperativity was achieved in a low-symmetry ternary complex with two different hosts on one guest.

Interplay of Lewis acidity, intramolecular O→Sn interactions and selectivity: organotin-functionalized crown ethers as ditopic hosts for sodium and potassium halides

Not only ditopic complexation, but also selectivity has been realized by increasing the Lewis acidity of tin atom in organotin-functionalized crown ethers.

An ion-pair receptor comprising urea groups and N-benzyl-aza-18-crown-6: effective recognition and liquid–liquid extraction of KCl salt

Heteroditopic ion-pair receptor that contains two urea groups and N-benzyl-18-crown-6 is shown to effectively recognizes and extracts KCl salt.

Tritopic ion-pair receptors based on anion–π interactions for selective CaX2 binding

Selective ion-pair binding of CaX₂ (X = Br⁻ and I⁻) was realized by a tritopic receptor incorporating two homoditopic anion–π binding sites and a pentaethylene glycol moiety.

Achieving Strong Positive Cooperativity through Activating Weak Non-Covalent Interactions

Positive cooperativity achieved through activating weak non-covalent interactions is common in biological assemblies but is rarely observed in synthetic complexes. Two new molecular tubes have been synthesized and the syn isomer binds DABCO-based organic cations with high orientational selectivity. Surprisingly, the ternary complex with two hosts and one guest shows a high cooperativity factor (α=580), which is the highest reported for synthetic systems without involving ion-pairing interactions. The X-ray single-crystal structure revealed that the strong positive cooperativity likely originates from eight C-H⋅⋅⋅O hydrogen bonds between the two head-to-head-arranged syn tube molecules. These relatively weak hydrogen bonds were not observed in the free hosts and only emerged in the complex. Furthermore, this complex was used as a basic motif to construct a robust [2+2] cyclic assembly, thus demonstrating its potential in molecular self-assembly.

Indole-based perenosins as highly potent HCl transporters and potential anti-cancer agents

Prodigiosin is one of the most potent anion transporters in lipid bilayer membranes reported to date. Inspired by the structure of this natural product, we have recently designed and synthesised a new class of H⁺/Cl⁻ cotransporters named ‘perenosins’. Here we report a new library of indole-based perenosins and their anion transport properties. The new transporters demonstrated superior transmembrane transport efficiency when compared to other indole-based transporters, due to favourable encapsulating effects from the substituents on the perenosin backbone. Anion transport assays were used to determine the mechanism of chloride transport revealing that the compounds function as ‘strict’ HCl cotransporters. Cell viability studies showed that some compounds specifically trigger late-onset cell death after 72 h with a unique correlation to the position of alkyl chains on the perenosins. Further investigations of cell death mechanism showed a mixture of cell cycle arrest and apoptosis was responsible for the observed decrease in cell viability.

Tuning Sensory Properties of Triazole-Conjugated Spiropyrans: Metal-Ion Selectivity and Paper-Based Colorimetric Detection of Cyanide

Tuning the sensing properties of spiropyrans (SPs), which are one of the photochromic molecules useful for colorimetric sensing, is important for efficient analysis, but their synthetic modification is not always simple. Herein, we introduce an alkyne-functionalized SP, the modification of which would be easily achieved via Cu-catalyzed azide-alkyne cycloaddition ("click reaction"). The alkyne-SP was conjugated with a bis(triethylene glycol)-benzyl group (EG-BtSP) or a simple benzyl group (BtSP), forming a triazole linkage from the click reaction. The effects of auxiliary groups to SP were tested on metal-ion sensing and cyanide detection. We found that EG-BtSP was more Ca²⁺-sensitive than BtSP in acetonitrile, which were thoroughly examined by a continuous variation method (Job plot) and UV-VIS titrations, followed by non-linear regression analysis. Although both SPs showed similar, selective responses to cyanide in a water/acetonitrile co-solvent, only EG-BtSP showed a dramatic color change when fabricated on paper, highlighting the important contributions of the auxiliary groups.

Tetra-(meta-butylcarbamoyl)azobenzene: A Rationally Designed Photoswitch with Binding Affinity for Oxoanions in a Long-Lived Z-State

A new photoswitchable anion receptor 1 based on a tetra-meta-substituted azobenzene skeleton has been readily synthesized in two steps. Titration studies (¹H NMR) and theoretical predictions (DFT/M06-2X/6-31G(d)/DMSO-SM8) revealed that nonplanar Z-1 is a better host for anions than E-1, which results from the greater ability of four amide NH protons in the Z-state to cooperatively bind oxoanions, in particular tetrahedral H₂PO₄^- and H₂AsO₄^-. Furthermore, the thermal decay of Z-1 (τ_1/2 = 11 days) is not accelerated by anion binding.

Cu2+ recognition by N,N'-benzylated bis(amino amides)

Two new C₂-symmetric N,N'-benzylated bis(amino amides) have been synthesised and their interaction with different transition metals studied using a variety of techniques including UV-Vis and CD spectroscopy or ESI-MS. The determination of the corresponding stability constants with Cu²⁺ has been possible, in H₂O/CH₃CN 7/3 v/v, for one of these ligands (4) using potentiometric titrations. The results obtained reveal that N-benzylation affords significant changes to their properties and is accompanied by an appreciable decrease in the corresponding complexation stability constants. However, this, along with the low kinetics associated to Ni²⁺, facilitates the recognition of Cu²⁺ by 4 that can be followed by the naked-eye up to the submillimolar range. Very interestingly, the chiral nature of this ligand provides an intense and well defined CD curve for the corresponding Cu²⁺ complex, very sensitive to the coordination geometry, facilitating the analysis of this interaction even at the μM range. The formation by both ligands (3 and 4) of square planar complexes with Cu²⁺ and Ni²⁺ displaying a 1 : 1 stoichiometry was confirmed by their X-ray crystal structures.

Triamide macrocyclic chloride receptors via a one-pot tandem reduction–condensation–cyclization reaction

A chloride binding triamide macrocycle has been developed in one pot from the corresponding monomer via tandem reduction–condensation–cyclization reactions.

Assessing cooperativity in supramolecular systems

In this tutorial review, different aspects of cooperativity in supramolecular chemistry and their thermodynamic analysis are discussed.

Supramolecular Chemistry of Cucurbiturils: Tuning Cooperativity with Multiple Noncovalent Interactions from Positive to Negative

Rational control of the cooperativity of multiple noncovalent interactions often plays an important role in the design and construction of supramolecular self-assemblies and materials, especially in precision supramolecular engineering. However, it still remains a challenge to control the cooperativity of multiple noncovalent interactions through tuning the hydrophobic effect. In this work, we demonstrate that the binding cooperativity of cucurbit[8]uril(CB[8])-mediated homoternary complexes is strongly influenced by the amphiphilicity of guest molecule side groups on account of an interplay between both classical (entropy-driven) and nonclassical (enthalpy-driven) hydrophobic effects. To this end, we rationally designed and prepared a series of guest molecules bearing a benzyl group as the CB[8] homoternary binding motif with various hydrophilic and hydrophobic side groups for cooperative control. By gradually tuning side groups of the guest molecules from hydrophilic to hydrophobic, we are able to control the binding from positive to negative cooperativity. An advanced molecular recognition process and self-assembling system can be developed by adjusting the positive and negative cooperativity. The ability to regulate and control the binding cooperativity will enrich the field of supramolecular chemistry, and employing cooperativity-controlled multiple noncovalent interactions in precision supramolecular engineering is highly anticipated.

Macrocyclic shape-persistency of cyclo[6]aramide results in enhanced multipoint recognition for the highly efficient template-directed synthesis of rotaxanes

Examples of using two-dimensional shape-persistent macrocycles, i.e. those having noncollapsible and geometrically well-defined skeletons, for constructing mechanically interlocked molecules are scarce, which contrasts the many applications of these macrocycles in molecular recognition and functional self-assembly. Herein, we report the crucial role played by macrocyclic shape-persistency in enhancing multipoint recognition for the highly efficient template-directed synthesis of rotaxanes. Cyclo[6]aramides, with a near-planar conformation, are found to act as powerful hosts that bind bipyridinium salts with high affinities. This unique recognition module, composed of two macrocyclic molecules with one bipyridinium ion thread through the cavity, is observed both in the solid state and in solution, with unusually high binding constants ranging from ∼10¹³ M^-2 to ∼10¹⁵ M^-2 in acetone. The high efficacy of this recognition motif is embodied by the formation of compact [3]rotaxanes in excellent yields based on either a "click-capping" (91%) or "facile one-pot" (85%) approach, underscoring the great advantage of using H-bonded aromatic amide macrocycles for the highly efficient template-directed synthesis of mechanically interlocked structures. Furthermore, three cyclo[6]aramides bearing different peripheral chains 1-3 demonstrate high specificity in the synthesis of a [3]rotaxane from 1 and 2, and a [2]rotaxane from 3via a "facile one-pot" approach, in each case as the only isolated product. Analysis of the crystal structure of the [3]rotaxane reveals a highly compact binding mode that would be difficult to access using other macrocycles with a flexible backbone. Leveraging this unique recognition motif, resulting from the shape-persistency of these oligoamide macrocycles, in the template-directed synthesis of compact rotaxanes may open up new opportunities for the development of higher order interlocked molecules and artificial molecular machines.

Ion-Pair Complexation with Dibenzo[21]Crown-7 and Dibenzo[24]Crown-8 bis-Urea Receptors

Synthesis and ion-pair complexation properties of novel ditopic bis-urea receptors based on dibenzo[21]crown-7 (R(1) ) and dibenzo[24]crown-8 (R(2) ) scaffolds have been studied in the solid state, solution, and gas phase. In a 4:1 CDCl3 /[D6 ]DMSO solution, both receptors clearly show positive heterotropic cooperativity toward halide anions when complexed with Rb(+) or Cs(+) , with the halide affinity increasing in order I(-) <Br(-) <Cl(-) . In solution, the rubidium complexes of both receptors have higher halide affinities compared to the caesium complexes. However, Rb(+) and Cs(+) complexes of R(2) show stronger affinities toward all the studied anions compared to the corresponding cationic complexes of R(1) . Similar selectivity of the receptors toward the studied ion pairs was also observed also in the gas phase by competition experiments with mass spectrometry. A total of eight crystal structures with different rubidium and caesium halides and oxyanions were obtained in addition to the crystal structure of R(2) ⋅BaCl2 . The selectivity observed in solution and in the gas phase is explainable by the conformational differences observed in the crystal structures of ion-pair complexes with R(1) and R(2) . In the solid state, R(1) has an open conformation due to the asymmetric crown-ether scaffold, whereas R(2) has a compact, folded conformation. Computational studies of the ion-pair complexes of R(2) show that the interaction energies of the complexes increase in the order CsI<CsBr<CsCl<RbCl, supporting the selectivity observed in solution and the gas-phase.

pH-Regulated Nonelectrogenic Anion Transport by Phenylthiosemicarbazones

Gated ion transport across biological membranes is an intrinsic process regulated by protein channels. Synthetic anion carriers (anionophores) have potential applications in biological research; however, previously reported examples are mostly nonspecific, capable of mediating both electrogenic and electroneutral (nonelectrogenic) transport processes. Here we show the transmembrane Cl(-) transport studies of synthetic phenylthiosemicarbazones mimicking the function of acid-sensing (proton-gated) ion channels. These anionophores have remarkable pH-switchable transport properties with up to 640-fold increase in transport efficacy on going from pH 7.2 to 4.0. This "gated" process is triggered by protonation of the imino nitrogen and concomitant conformational change of the anion-binding thiourea moiety from anti to syn. By using a combination of two cationophore-coupled transport assays, with either monensin or valinomycin, we have elucidated the fundamental transport mechanism of phenylthiosemicarbazones which is shown to be nonelectrogenic, inseparable H(+)/Cl(-) cotransport. This study demonstrates the first examples of pH-switchable nonelectrogenic anion transporters.

Recognizing the Limited Applicability of Job Plots in Studying Host-Guest Interactions in Supramolecular Chemistry

Continuous variation method, known as Job plot, is the most commonly applied method for the determination of stoichiometry of complex chemical entities for over 100 years. Although, the method was proven successful in the analysis of very stable metal-ligand complexes, we demonstrate that its use in supramolecular chemistry often provides false results. We support this statement with multiple simulations as well as cases studies of several real host-guest systems. We propose an alternative, general method relying on the analysis of residual distribution in titration data fitting. The latter method is more convenient compared to the Job plot and unlike it gives correct results in all real cases studied.

Multiresponsive Dynamic Covalent Assemblies for the Selective Sensing of Both Cu(2+) and CN(-) in Water

The use of dynamic assembly for molecular sensing is an intensive area of research in supramolecular chemistry. However, the development of self-assembly architectures for the detection of multiple signals remains challenging. Here, we present dynamic covalent assemblies with multiresponsive properties that also show unique selectivity profiles in water. The receptors were generated in a single step with modularly designed building blocks through acylhydrazone linkages, and their orthogonal assembly with a series of external stimuli was investigated. Notably, the system exhibits responses toward cations, anions, solvents, pH, and amphiphilicity. The discrimination of Cu(2+) from other divalent metal ions was achieved by simply changing the solvent and was evidenced by a Cu(2+)-induced pKa shift. The selective recognition of CN(-) in pure aqueous media was also accomplished through a cooperative effect in conjunction with Zn(2+). Furthermore, the assembly and its responsiveness are functional both in solution and the solid state. The aggregation ability of these dynamic covalent systems supports their binding and sensing properties.

Halogen bonding influences perylene-core twists in non-core substituted perylene tetraesters

CO⋯X halogen bonding of perylene tetraesters causes perylene-core torsion which affects their spectral properties.

Multicavity macrocyclic hosts

Multicavity macrocyclic hosts are host molecules comprising more than one macrocyclic guest binding components connected through multipoint linkages.

Cooperatively enhanced ion pair binding with a hybrid receptor

A simple 18-crown-6-based bis-urea receptor R(1) was synthesized in three steps from a commercial starting material. The receptor's behavior toward anions, cations, and ion pairs was studied in solution with (1)H NMR, in solid state with single-crystal X-ray diffraction, and in gas phase with mass spectrometry. In 4:1 CDCl3/dimethyl sulfoxide solution the receptor's binding preference of halide anions is I(-) < Br(-) < Cl(-) following the trend of the hydrogen-bonding acceptor ability of the anions. The receptor shows a remarkable positive cooperativity toward halide anions Cl(-), Br(-), and I(-) when complexed with Na(+), K(+), or Rb(+). The solid-state binding modes of R(1) with alkali and ammonium halides or oxyanions were confirmed by the X-ray structures of R(1) with KF, KCl, KBr, KI, RbCl, NH4Cl, NH4Br, KAcO, K2CO3, and K2SO4. They clearly present two different binding modes, either as separated or contact ion pairs depending on the nature and size of the bound cation and anion. Complexation capability of R(1) in the gas phase was studied with competition experiments with electrospray ionization mass spectrometry showing preference of KCl complexation over NaCl, KBr, or KI supporting the results obtained in solution.

Pyrrole-pyridine and pyrrole-naphthyridine hosts for anion recognition

The association constants of the complexes formed by two hosts containing pyrrole, amide and azine (pyridine and 1,8-naphthyridine) groups and six guests, all monoanions (Cl⁻, CH₃CO₂⁻, NO₃⁻, H₂PO₄⁻, BF₄⁻, PF₆⁻), have been determined using NMR titrations. The X-ray crystal structure of the host N²,N⁵-bis(6-methylpyridin-2-yl)-3,4-diphenyl-1H-pyrrole-2,5-dicarboxamide (1) has been solved (P2₁/c monoclinic space group). B3LYP/6-31G(d,p) and calculations were carried out in an attempt to rationalize the trends observed in the experimental association constants.

Cooperative ion pair recognition by multitopic l-ornithine based salt receptors

The development of l-ornithine based multitopic receptors allowed us to obtain an effective and selective salt receptor.

Selective recognition and extraction of KBr via cooperative interactions with a urea functionalized crown ether dual-host

Selective solid–liquid extraction of KBr is demonstrated for the first time with a crown ether based pentafluorophenyl urea functionalised dual-host receptor.

Remarkably selective, non-linear allosteric regulation of anion binding by a tetracationic calix[4]pyrrole homodimer

A covalently coupled, dimeric tetra-cationic calix[4]pyrrole homodimer bearing anthracene linkers displayed distinctive cooperativity and fluoride selectivity with large positive allosterism.

A simple and highly selective 2,2-diferrocenylpropane-based multi-channel ion pair receptor for Pb2+ and HSO4−

A 2,2-diferrocenylpropane-based multi-channel ion pair receptor 1 was designed and structurally characterized. It was a “naked-eye-detectable” chemosensor towards Pb²⁺ and HSO₄⁻ with excellent selectivity and sensitivity.