Bambusuril Anion Receptors

Cycloglycolurils: Hybrid Glycoluril-Cyclobenzil Macrocycles

Two novel glycoluril macrocycles have been synthesized from cyclotetrabenzil and cyclotribenzoin precursors using solvent-free condensations with urea. The crystal structure of the cyclotetra(p-phenylene)glycoluril macrocycle shows a twisted ring conformation, while that of the cyclotri(m-phenylene)glycoluril hybrid exhibits a distinct tubular supramolecular packing. These structures establish a potentially broad new class of macrocycles with intriguing guest binding properties owing to their available N-H motifs.

Merging Bambus[6]uril and Biotin[6]uril into an Enantiomerically Pure Monofunctionalized Hybrid Macrocycle

Bambus[6]urils and biotin[6]urils are macrocycles with an exceptional affinity for inorganic anions. Here, we investigated statistical condensation of 2,4-dibenzylglycoluril and d-biotin, monomers of the corresponding macrocycles, to prepare the enantiomerically pure macrocycle 1 containing a single d-biotin and five glycoluril units. Host-guest properties of 1 in chloroform solution and solid state were investigated. The macrocycle 1 bearing a single functional group was employed in the formation of [1]rotaxane utilizing reversible covalent bonds.

Selective Perchlorate Sensing Using Electrochemical Impedance Spectroscopy with Self-Assembled Monolayers of semiaza-Bambusurils

In the last two decades, perchlorate salts have been identified as environmental pollutants and recognized as potential substances affecting human health. We describe self-assembled monolayers (SAMs) of novel semiaza-bambus[6]urils (semiaza-BUs) equipped with thioethers or disulfide (dithiolane) functionalities as surface-anchoring groups on gold electrodes. Cyclic voltammetry (CV) with Fe(CN)6 3-/4- as a redox probe, together with X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and ellipsometry, were employed to characterize the interactions at the interface between the anchoring groups and the metal substrate. Data showed that the anion receptors' packing on the gold strongly depends on the anchoring group. As a result, SAMs of BUs with lipoic amide side chains show a concentration-dependent layer thickness. The BU SAMs are extremely stable on repeated electrochemical potential scans and can selectively recognize perchlorate anions. Our electrochemical impedance spectroscopy (EIS) studies indicated that semiaza-BU equipped with the lipoic amide side chains binds perchlorate (2-100 mM) preferentially over other anions such as F- , Cl- , I- , AcO- , H2 PO4 - , HPO4 2- , SO4 2- , NO2 - , NO3 - , or CO3 2- . The resistance performance is 10 to 100 times more efficient than SAMs containing all other tested anions.

Tuning CH Hydrogen Bond-Based Receptors toward Picomolar Anion Affinity via the Inductive Effect of Distant Substituents

Inspired by nature, artificial hydrogen bond-based anion receptors have been developed to achieve high anion selectivity; however, their binding affinity is usually low. The potency of these receptors is usually increased by the introduction of aryl substituents, which withdraw electrons from their binding site through the resonance effect. Here, we show that the polarization of the C(sp3 )-H binding site of bambusuril receptors, and thus their potency to bind anions, can be modulated by the inductive effect. The presence of electron-withdrawing groups on benzyl substituents of bambusurils significantly increases their binding affinities to halides, resulting in the strongest iodide receptor reported to date with an association constant greater than 1013  M-1 in acetonitrile. A Hammett plot showed that while the bambusuril affinity toward halides linearly increases with the electron-withdrawing power of their substituents, their binding selectivity remains essentially unchanged.

Synthesis of Novel Phosphorus-Containing Derivatives of 1,3,4-Trimethylglycoluril via the Birum-Oleksyszyn Reaction

This work presents the synthesis of a new compound, 1-[aryl-(diphenylphosphono)methyl]-3,4,6-trimethylglycolurils, via the interaction of benzaldehyde and its mononitro- and monohydroxyderivatives with 1,3,4-trimethylglycoluril and triphenylphosphite. By varying the reaction conditions and the catalysts, the obtained product yields ranged from satisfactory to good. The diastereomers formed during the reaction were separated by semipreparative HPLC on the C18 stationary phase. The isolated diastereomers were characterized by 1H, 13C, and 31P NMR, and the structures of the diastereomers were confirmed using a single-crystal X-ray crystal structure analysis and quantum chemical calculations.

Nanoparticles Based on Silver Chloride and Bambusuril[6] for the Fine-Tuning of Biological Activity

The prevalence of numerous infectious diseases has emerged as a grave concern within the realm of healthcare. Currently, the issue of antibiotic resistance is compelling scientists to explore novel treatment approaches. To combat these infectious diseases, various treatment methods have been developed, harnessing cutting-edge disinfecting nanomaterials. Among the range of metallic nanoparticles employed in medicine, silver nanoparticles (AgNPs) stand out as both highly popular and well-suited for the task. They find extensive utility in cancer diagnosis and therapies and as effective antibacterial agents. The interaction between silver and bacterial cells induces significant structural and morphological alterations, ultimately leading to cell demise. In this study, nanoparticles based on silver and bambusuril[6] (BU[6]) were developed for the first time. These NPs can be used for different biomedical purposes. A simple, single-step, and effective synthesis method was employed to produce bambusuril[6]-protected silver chloride nanoparticles (BU[6]-Ag/AgCl NPs) through the complexation of BU[6] with silver nitrate. The NPs were characterized using X-ray phase analysis (XPS), infrared spectroscopy (IR), thermogravimetric analysis (TGA), scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS). When the SEM images were examined, it was seen that the synthesized BU[6]-Ag/AgCl NPs were distributed with homogeneous sizes, and the synthesized NPs were mostly spherical and cubic. The EDS spectra of BU[6]-Ag/AgCl NPs demonstrated the presence of Ag, Cl, and all expected elements. BU[6]-Ag/AgCl NPs showed high antibacterial activity against both E. coli and S. aureus bacteria.

Large anion binding in water

Large water-soluble anions with chaotropic character display surprisingly strong supramolecular interactions in water, for example, with macrocyclic receptors, polymers, biomembranes, and other hydrophobic cavities and interfaces. The high affinity is traced back to a hitherto underestimated driving force, the chaotropic effect, which is orthogonal to the common hydrophobic effect. This review focuses on the binding of large anions with water-soluble macrocyclic hosts, including cyclodextrins, cucurbiturils, bambusurils, biotinurils, and other organic receptors. The high affinity of large anions to molecular receptors has been implemented in several lines of new applications, which are highlighted herein.

Synthesis of Enantiomerically Pure Bambus[6]urils Utilizing Orthogonal Protection of Glycolurils

A general strategy for the synthesis of 2N,4N'-disubstituted glycoluril enantiomers on a multigram scale using orthogonal protection is reported. The use of these glycolurils is demonstrated in the synthesis of enantiomerically pure bambus[6]uril macrocycles. Moreover, the deprotection of (S)-1-phenylethyl substituents on the macrocycle was achieved, opening access to various chiral bambus[6]urils via post-macrocyclization modification strategy.

Engineered Binding Microenvironments in Halogen Bonding Polymers for Enhanced Anion Sensing

Mimicking Nature's polymeric protein architectures by designing hosts with binding cavities screened from bulk solvent is a promising approach to achieving anion recognition in competitive media. Accomplishing this, however, can be synthetically demanding. Herein we present a synthetically tractable approach, by directly incorporating potent supramolecular anion-receptive motifs into a polymeric scaffold, tuneable through a judicious selection of the co-monomer. A comprehensive analysis of anion recognition and sensing is demonstrated with redox-active, halogen bonding polymeric hosts. Notably, the polymeric hosts consistently outperform their monomeric analogues, with especially large halide binding enhancements of ca. 50-fold observed in aqueous-organic solvent mixtures. These binding enhancements are rationalised by the generation and presentation of low dielectric constant binding microenvironments from which there is appreciable solvent exclusion.

Orthogonal, modular anion-cation and cation-anion self-assembly using pre-programmed anion binding sites

Subcomponent self-assembly relies on cation coordination whereas the roles of anions often only emerge during the assembly process. When sites for anions are instead pre-programmed, they have the potential to be used as orthogonal elements to build up structure in a predictable and modular way. We explore this idea by combining cation (M⁺) and anion (X^-) binding sites together and show the orthogonal and modular build up of structure in a multi-ion assembly. Cation binding is based on a ligand (L) made by subcomponent metal-imine chemistry (M⁺ = Cu⁺, Au⁺) while the site for anion binding (X^- = BF₄ ^-, ClO₄ ^-) derives from the inner cavity of cyanostar (CS) macrocycles. The two sites are connected by imine condensation between a pyridyl-aldehyde and an aniline-modified cyanostar. The target assembly [LM-CS-X-CS-ML],⁺ generates two terminal metal complexation sites (LM and ML) with one central anion-bridging site (X) defined by cyanostar dimerization. We showcase modular assembly by isolating intermediates when the primary structure-directing ions are paired with weakly coordinating counter ions. Cation-directed (Cu⁺) or anion-bridged (BF₄ ^-) intermediates can be isolated along either cation-anion or anion-cation pathways. Different products can also be prepared in a modular way using Au⁺ and ClO₄ ^-. This is also the first use of gold(i) in subcomponent self-assembly. Pre-programmed cation and anion binding sites combine with judicious selection of spectator ions to provide modular noncovalent syntheses of multi-component architectures.

Synthetic Receptors with Micromolar Affinity for Chloride in Water

A water-soluble coordination cage was obtained by reaction of Pd(NO₃ )₂ with a 1,3-di(pyridin-3-yl)benzene ligand featuring a short PEG chain. The cavity of the metal-organic cage contains one nitrate anion, which is readily replaced by chloride. The apparent association constant for chloride binding in buffered aqueous solution is K_a =1.8(±0.1)×10⁵  M^-1 . This value is significantly higher than what has been reported for other macrocyclic chloride receptors. The heavier halides Br^- and I^- compete with binding or self-assembly, but the receptor displays very good selectivity over common anions such as phosphate, acetate, carbonate, and sulfate. A further increase of the chloride binding affinity by a factor of 3 was achieved using a fluorinated dipyridyl ligand.

Functionalization of the Surface of Porous Nickel-Titanium Alloy with Macrocyclic Compounds

For the first time, we performed functionalization of the surface of porous titanium nickelide alloys with bambusuril[6]-based macrocyclic compounds by different methods in order to provide the basis for saturation with therapeutic agents to impart antibacterial activity and accelerate its osteogenesis. It has been shown for the first time that the vacuum modification method is preferable for bambusuril deposition, since it provides a uniform deposition of organic matter on both the outer and inner surfaces of the pores. The effect of bambusuril deposition methods on the continuity, structure, and cytocompatibility of the porous titanium nickelide surface was evaluated. In vitro tests proved high biocompatibility and low toxicity of porous TiNi treated with BU[6] under vacuum. The SEM study of the structure of the surface layer of TiNi modified with BU[6] under the vacuum method showed that BU[6] agglomerates are uniformly deposited on the inner and outer surfaces of TiNi pores, which will provide an even saturation of BU[6] cavities with various pharmaceuticals, including antibiotics and inhibitors.

Synthesis of Glycoluril Dimers with the Ability to Form Polymeric Self-Associates in Water

Supramolecular self-assembly in water resulting in polymeric structures is emerging because of its potential in the preparation of adaptive materials with applications in biology and medicine. Here, we report the first example of host molecules based on glycoluril dimers, which self-associate into linear oligomers in water. The degree of polymerization for the resulting supramolecular aggregates was calculated using the isodesmic model and the Carothers equation. The model compound was prepared to enable a deeper understanding of the forces responsible for the self-association of the glycoluril dimer-based monomers in water.

Bambus[4,6]urils as Dual Scaffolds for Multivalent Iminosugar Presentation and Ion Transport: Access to Unprecedented Glycosidase-Directed Anion Caging Agents

Bambusurils, BU[4] and BU[6], were used for the first time as multivalent scaffolds to link glycosidases inhibitors derived from 1-deoxynojirimycin (DNJ). Two linear DNJ ligands having six or nine carbon alkyl azido linkers or a trivalent DNJ dendron were grafted onto octapropargylated BU[4] and dodecapropargylated BU[6] using copper-catalyzed cycloaddition (CuAAC) to yield corresponding neoglycobambus[4] and neoglycobambus[6]urils bearing 8 to 24 iminosugars. The inhibition potencies of neoglycoBU[4], neoglycoBU[6] and neoglycoBU[6] caging anions were evaluated against Jack Bean α-mannosidase and compared to monovalent DNJ derivatives. Strong affinity enhancements per inhibitory head were obtained for the clusters holding trivalent dendrons with inhibitory constants in the nanomolar range (K_i = 24 nM for BU[4] with 24 DNJ units). Interestingly, the anion (bromide or iodide) encapsulated inside the cavity of BU[6] does not modify the inhibition potency of neoglycoBU[6], opening the way to water-soluble glycosidase-directed anion caging agents that may find applications in important fields such as bio(in)organic chemistry or oncology.

Monofunctionalized Fluorinated Bambusurils and Their Conjugates for Anion Transport and Extraction

Bambusurils are macrocyclic molecules that are known for their high binding affinity and selectivity toward anions. Here, we present the preparation of two bambusurils bearing fluorinated substituents and one carboxylic function. These monofunctionalized bambusurils were conjugated with crown ether and cholesterol units. The resulting conjugates were successfully tested in liquid-liquid extraction of inorganic salts and chloride/bicarbonate transport across lipid bilayers.

When Molecules Meet in Water-Recent Contributions of Supramolecular Chemistry to the Understanding of Molecular Recognition Processes in Water

Molecular recognition processes in water differ from those in organic solvents in that they are mediated to a much greater extent by solvent effects. The hydrophobic effect, for example, causes molecules that only weakly interact in organic solvents to stay together in water. Such water‐mediated interactions can be very efficient as demonstrated by many of the synthetic receptors discussed in this review, some of which have substrate affinities matching or even surpassing those of natural binders. However, in spite of considerable success in designing such receptors, not all factors determining their binding properties in water are fully understood. Existing concepts still provide plausible explanations why the reorganization of water molecules often causes receptor‐substrate interactions in water to be strongly exothermic rather than entropically favored as predicted by the classical view of the hydrophobic effect.

Dynamic [1]rotaxanes via a reversible covalent bond and host-guest anion recognition

Bambus[6]uril-based [1]rotaxanes were formed quantitatively, utilizing a bis(acyloxy)iodate(I) reversible covalent bond and host-guest anion recognition. These novel [1]rotaxanes exhibited a dynamic nature facilitating carboxylate component exchange reactions in acetonitrile.

Perchlorate Solid-Contact Ion-Selective Electrode Based on Dodecabenzylbambus[6]uril

Dodecabenzylbambus[6]uril (Bn12BU[6]) is an anion receptor that binds the perchlorate ion the most tightly (stability constant ~1010 M−1) of all anions due to the excellent match between the ion size in relation to the receptor cavity. This new bambusuril compound was used as an ionophore in the ion-selective membrane (ISM) to develop ion selective electrodes (ISEs) for determination of perchlorate concentration utilizing the poly(3,4-ethylenedioxythiophene) (PEDOT) polymer film as a solid-contact material. Variation of the content of Bn12BU[6] and tridodecylmethylammonium chloride (TDMACl) in the plasticized poly(vinyl chloride)-based ISM was also tested. All the prepared solid-contact ISEs and their analytical performance were characterized by potentiometry, cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and chronopotentiometry. The ISEs showed rapid response and a sub-Nernstian slope (~57 mV/decade) during potentiometric measurements in perchlorate solutions in the concentration range from 10−1 to 10−6 M simultaneously with their high stability and sufficient selectivity to other common inorganic anions like bromide, chloride, nitrate and sulphate. The function of the ISE was further verified by analysis of real water samples (lake, sea, and mineral water), which gave accurate and precise results.

Nonspherical anion sequestration by C-H hydrogen bonding

Macrocyclic arenes laid the foundations of supramolecular chemistry and their study established the fundamentals of noncovalent interactions. Advancing their frontier, here we designed rigidified resorcin[4]arenes that serve as hosts for large nonspherical anions. In one synthetic step, we vary the host's anion affinity properties by more than seven orders of magnitude. This is possible by engineering electropositive aromatic C–H bond donors in an idealized square planar geometry embedded within the host's inner cavity. The hydrogen atom's electropositivity is tuned by introducing fluorine atoms as electron withdrawing groups. These novel macrocycles, termed fluorocages, are engineered to sequester large anions. Indeed, experimental data shows an increase in the anion association constant (K_a) as the number of F atoms increase. The observed trend is rationalized by DFT calculations of Hirshfeld Charges (HCs). Most importantly, fluorocages in solution showed weak-to-medium binding affinity for large anions like [PF₆]⁻ (10²< K_a <10⁴ M⁻¹), and high affinity for [MeSO₃]⁻ (K_a >10⁶).

Fluorocages: new class of rigidified host utilizing nontraditional C–H hydrogen bonds to capture the nonspherical anions.

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.

Self-Assembly of Chiral Cyclohexanohemicucurbit[n]urils with Bis(Zn Porphyrin): Size, Shape, and Time-Dependent Binding

In order to investigate the ability of bis(zinc octaethylporphyrin) (bis–ZnOEP) to discriminate cyclohexanohemicucurbit[n]urils (cycHC[n]) of different shapes and sizes, the self-assembly of barrel-shaped chiral cycHC[n] with bis–ZnOEP was studied by various spectroscopic methods (absorption, fluorescence, circular dichroism (CD), and NMR). While the binding of 6-membered cycHC[6] induced a tweezer-like conformation followed by the formation of anti-form of bis–ZnOEP upon further addition of cycHC[6], the interaction of 8-membered cycHC[8] is more complex and proceeds through the featured syn-to-anti conformational change of bis–ZnOEP and further intermolecular self-assembly via multiple noncovalent associations between cycHC[8] and bis–ZnOEP. Whilst bis–porphyrins are known to be effective chemical sensors able to differentiate various guests based on their chirality via induced CD, their ability to sense small differences in the shape and size of relatively large macrocycles, such as chiral cycHC[6] and cycHC[8], is scarcely examined. Both studied complexes exhibited characteristic induced CD signals in the region of porphyrin absorption upon complexation.

Synthesis of a novel aminobenzene-containing hemicucurbituril and its fluorescence spectral properties with ions

A novel hemicucurbituril-based macrocycle, alternately consisting of amidobenzene and 2-imidazolidione moieties was designed and synthesized. Based on the fragment coupling strategy, nitrobenzene-containing hemicucurbituril was firstly prepared facilely under alkaline environment, and reduction of the nitro groups produced the desired amidobenzene-containing hemicucurbituril. As an original fluorescent chemosensor, it exhibited strong interactions with Fe³⁺ over other metal cations. The experimental evidence of fluorescence spectra suggested that a 1:1 complex was formed between this macrocycle and Fe³⁺ with an association constant up to (2.1 ± 0.3) × 10⁴ M⁻¹. Meanwhile, this macrocycle showed no obvious or only slight enhancement of the fluorescence intensity with selected anions.

Cyclohexanohemicucurbit[8]uril Inclusion Complexes With Heterocycles and Selective Extraction of Sulfur Compounds From Water

Solid-phase extraction that utilizes selective macrocyclic receptors can serve as a useful tool for removal of chemical wastes. Hemicucurbiturils are known to form inclusion complexes with suitably sized anions; however, their use in selective binding of non-charged species is still very limited. In this study, we found that cyclohexanohemicucurbit[8]uril encapsulates five- and six-membered sulfur- and oxygen-containing unsubstituted heterocycles, which is investigated by single-crystal X-ray diffraction, NMR spectroscopy, isothermal titration calorimetry, and thermogravimetry. The macrocycle acts as a promising selective sorption material for the extraction of sulfur heterocycles, such as 1,3-dithiolane and α-lipoic acid, from water.

Solution NMR of synthetic cavity containing supramolecular systems: what have we learned on and from?

NMR has been instrumental in studies of both the structure and dynamics of molecular systems for decades, so it is not surprising that NMR has played a pivotal role in the study of host-guest complexes and supramolecular systems. In this mini-review, selected examples will be used to demonstrate the added value of using (multiparametric) NMR for studying macrocycle-based host-guest and supramolecular systems. We will restrict the discussion to synthetic host systems having a cavity that can engulf their guests thus restricting them into confined spaces. So discussion of selected examples of cavitands, cages, capsules and their complexes, aggregates and polymers as well as organic cages and porous liquids and other porous materials will be used to demonstrate the insights that have been gathered from the extracted NMR parameters when studying such systems emphasizing the information obtained from somewhat less routine NMR methods such as diffusion NMR, diffusion ordered spectroscopy (DOSY) and chemical exchange saturation transfer (CEST) and their variants. These selected examples demonstrate the impact that the results and findings from these NMR studies have had on our understanding of such systems and on the developments in various research fields.

Binding Between Cyclohexanohemicucurbit[n]urils and Polar Organic Guests

Inherently chiral, barrel-shaped, macrocyclic hosts such as cyclohexanohemicucurbit[n]urils (cycHC[n]) bind zinc porphyrins and trifluoroacetic acid externally in halogenated solvents. In the current study, we tested a set of eighteen organic guests with various functional groups and polarity, namely, thiophenols, phenols, and carboxylic and sulfonic acids, to identify a preference toward hydrogen bond–donating molecules for homologous cycHC[6] and cycHC[8]. Guests were characterized by Hirshfeld partial charges on acidic hydrogens and their binding by ¹H and ¹⁹F NMR titrations. Evaluation of association constants revealed the complexity of the system and indirectly proved an external binding with stoichiometry over 2:1 for both homologs. It was found that overall binding strength is influenced by the stoichiometry of the formed complexes, the partial atomic charge on the hydrogen atom of the hydrogen bond donor, and the bulkiness of the guest. Additionally, a study on the formation of complexes with halogen anions (Cl⁻ and Br⁻) in methanol and chloroform, analyzed by ¹H NMR, did not confirm complexation. The current study widens the scope of potential applications for host molecules by demonstrating the formation of hydrogen-bonded complexes with multisite hydrogen bond acceptors such as cycHC[6] and cycHC[8].

Transmembrane Transport of Bicarbonate Unravelled*

Anion receptors can be used to transport ions across lipid bilayers, which has potential for therapeutic applications. Synthetic bicarbonate transporters are of particular interest, as defects in transmembrane transport of bicarbonate are associated with various diseases. However, no convenient method exists to directly observe bicarbonate transport and study the mechanisms involved. Here, an assay is presented that allows the kinetics of bicarbonate transport into liposomes to be monitored directly and with great sensitivity. The assay utilises an encapsulated europium(III) complex, which exhibits a large increase in emission intensity upon binding bicarbonate. Mechanisms involving CO2 diffusion and the dissipation of a pH gradient are shown to be able to lead to an increase in bicarbonate concentration within liposomes, without transport of the anion occurring at all. By distinguishing these alternative mechanisms from actual bicarbonate transport, this assay will inform the future development of bicarbonate transporters.

Bambusuril Macrocycles as Mediators of Supramolecular Interactions: Application to the Europium Cage Helicate

Herein, it is shown how bambusurils can be used for tuning and/or characterizing supramolecular systems. Indeed, the addition of bambusurils as anion scavengers to metal-mediated self-assemblies allows manipulation of the subtle equilibria in the given system. This is demonstrated for the case of the tetranuclear europium helical cage, which is well suited to different applications. Among the reported results, experimental evidence is provided showing that perchlorate and triflate anions act as a molecular template for the cage assembly. The complexation of inorganic anions with neutral bambusurils resulted in bulky non-coordinating counterions that may trigger the self-assembly process or stimulate specific interactions between components. Moreover, bambusuril was able to selectively remove coordinating nitrates from the mixture with non-coordinating anions, enabling the regeneration of the helical cage.

Dynamic chiral cyclohexanohemicucurbit[12]uril

NMR spectroscopy and DFT modeling studies of chiral cyclohexanohemicucurbit[12]uril indicate that the macrocycle adopts a concave octagonal shape with two distinct conformational flexibilities in solution. Methylene bridge flipping occurs at temperatures above 265 K, while urea monomers rotate at temperatures above 308 K, resulting in the loss of confined space within the macrocycle.

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.

Clickable Bambusurils to Access Multivalent Architectures

Propargylated bambus[4,6]urils were prepared by an efficient one-step condensation of dipropargylglycoluril with formaldehyde under microwave irradiation. Their functionalization by click chemistry (CuAAC) afforded new multivalent architectures decorated with 8 or 12 ligands. Grafting of glycosides provided water-soluble glycobambus[4,6]uril platforms with glucosyl₁₂BU[6] showing good affinity toward iodide anion in aqueous medium.

Monofunctionalized Bambus[6]urils and Their Conjugates with Crown Ethers for Liquid-Liquid Extraction of Inorganic Salts

Bambusurils are a growing family of macrocyclic anion receptors. In this Letter, we present the first syntheses of monofunctionalized bambusurils and their use for the preparation of heteroditopic bambusuril-crown ether conjugates suitable for the extraction of ion pairs from water to chloroform.

Equatorial Sulfur Atoms in Bambusurils Spawn Cavity Collapse

This study confirms the hypothesis that bambusurils (BUs) with equatorial sulfur atoms cannot assume an anion-binding jigger conformation due to strong intramolecular van der Waals attractive interactions. NMR, X-ray crystallography, and computation with newly synthesized eq-semithio-BU[4] and ax-semiaza-eq-semithio-BU[4]s indicate that they all assume compact conformations. Intramolecular distances and a torsional angle serve as reliable indicators of the conformation of any BU. Chemoselective alkylation at the peripheral (equatorial) thiourea functions provides a convenient entry to novel hetero-BUs.

Glycoluril derived cucurbituril analogues and the emergence of the most recent example: tiarauril

Cucurbituril analogues can bear some of the chemical and physical characteristics of their parental origin and are derived wholly or in part from glycolurils (including homologues).

Batista AP, Demets GJF. Formation of the non-classical interhalide anion [I2Cl]− in methyl-bambus[6]uril cavity

Methyl-bambus[6]uril may form a variety of inclusion compounds, especially with anionic species. Uncommon interhalide species may be produced in its cavity.

Hypervalent Iodine Based Reversible Covalent Bond in Rotaxane Synthesis

Reversible covalent bonds play a significant role in achieving the high-yielding synthesis of mechanically interlocked molecules. Still, only a handful of such bonds have been successfully employed in synthetic procedures. Herein, we introduce a novel approach for the fast and simple preparation of interlocked molecules, combining the dynamic bond character of bis(acyloxy)iodate(I) anions with macrocyclic bambusuril anion receptors. The proof of principle was demonstrated on rotaxane synthesis, with near-quantitative yields observed in both the classical and "in situ" approach. The rotaxane formation was confirmed in the solid-state and solution by the X-ray and NMR studies. Our novel approach could be utilized in the fields of dynamic combinatorial chemistry, supramolecular polymers, or molecular machines, as well inspire further research on molecules that exhibit dynamic behavior, but owing to their high reactivity, have not been considered as constituents of more elaborate supramolecular structures.

Bambusurils as a mechanistic tool for probing anion effects

Bambusuril macrocycles have high affinity towards anions (X⁻) such as PF₆⁻ and SbF₆⁻ or BF₄⁻ and ClO₄⁻. Therefore, addition of bambusurils to reaction mixtures containing these anions effectively removes the free anions from the reaction process. Hence, comparing reactions with and without addition of bambusurils can demonstrate whether the anions actively participate in the reaction mechanism or not. We show this approach for gold(i) mediated addition of methanol to an alkyne. The reaction mechanism can proceed via monoaurated intermediates (e.g., in catalysis with [(IPr)AuX]) or via diaurated intermediates (e.g., in catalysis with [(PPh₃)AuX]). We show that anions X⁻ slightly affect the reaction rates, however the effect stays almost the same even after their encapsulation in the cavity of bambusurils. We also demonstrate that X⁻ affects the overall reaction rate in the very same way as the reaction rate of the protodeauration step. All results are consistent with the indirect effect of X⁻ by the acidity of the conjugated acid HX on the rate-determining step. There is no evidence that a direct involvement of X⁻ would affect the reaction rate.

We present bambusurils, macrocycles which strongly bind anions of different sizes, as a mechanistic tool to probe counter ion effects in metal catalysis.

Intramolecular van der Waals Interactions Challenge Anion Binding in perthio-Bambusurils

Bambusurils (BUs) are known to be rigid cavitands that feature an extended, jigger-like conformation, and the BU[6]s strongly bind anions within their hydrophobic cavity. These features are not necessarily shared by the family of perthio-BUs. This study reveals that perthio-BUs assume a compact conformation and perthio-BU[6]s are poor anion binders, crystallizing as anion-free species from solutions containing halide salts. Computational studies show that the equatorial sulfur atoms compete against guest anions for binding with the glycoluril methine groups via strong van der Waals (vdW) attractive interactions. These competitive contacts not only account for the diminished anion-binding of perthio-BUs, but also explain their compact conformation. The semithio- and perthio-BU[4]s form linear coordination polymers with Hg^II in the solid-state regardless of their intrinsic molecular conformation. The strong involvement of sulfur atoms in intramolecular interactions differentiates the equatorial from the axial (peripheral) heteroatoms, thus offering chemoselective and regioselective transformations.

Bambusuril analogs based on alternating glycoluril and xylylene units

The glycoluril monomer is a popular building block in supramolecular chemistry as it is used for the synthesis of versatile host molecules which can interact with cationic, anionic or neutral guest molecules. Here we present the design and synthesis of a new hybrid macrocycle containing glycoluril and aromatic units. The reaction afforded a mixture of macrocyclic homologues from which a two-membered macrocycle was isolated as the main product. Two disastereomers of the macrocycle were separated and characterized by means of NMR spectroscopy and X-ray crystallography. Conformational changes of these diastereomers were investigated using DFT models and variable-temperature NMR.

Triazole functionalized acyclic cucurbit[n]uril-type receptors: host·guest recognition properties

We report the synthesis of three new triazole functionalized acyclic CB[n]-type receptors (2-4) by click chemistry. The compounds have good solubility in water (≥8 mM) and do not undergo strong self-association (Ks ≤ 903 M-1). We measured the binding constants of 2-4 toward guests 9-24 and compared the results to those obtained for the prototypical acyclic CB[n]-type receptor 1. The X-ray crystal structure of 4 is also described.

Tiara[ n]uril: A Glycoluril-Based Macrocyclic Host with Cationic Walls

The synthesis of new cationic macrocyclic host molecules is described. These macrocycles are comprised of glycoluril oligomers linked to two pyrazolium groups, which form part of a cationic wall facing into their cavities. A number of derivatives have been prepared with an objective to increasing the cavity size, and each new product has been fully characterized. Preliminary investigations of p K_as of Me₁₀Tu[3]²⁺ and an interaction of L-glutamine indicate a potential for binding anionic molecules that also carry H-bond donor groups.

Size-Control by Anion Templating in Mechanochemical Synthesis of Hemicucurbiturils in the Solid State

Self-organization is one of the most intriguing phenomena of chemical matter. While the self-assembly of macrocycles and cages in dilute solutions has been extensively studied, it remains poorly understood in solvent-free environments. Provided here is the first example of using anionic templates to achieve selective assembly of differently-sized macrocycles in a solvent-free system. Using acid-catalyzed synthesis of cyclohexanohemicucurbiturils as a model, size-controlled, quantitative synthesis of 6- or 8-membered macrocycles by spontaneous anion-directed reorganization of mechanochemically-made oligomers in the solid state is demonstrated.

19F-GEST NMR: studying dynamic interactions in host–guest systems

GEST NMR provides dynamic information on host–guest systems. It allows signal amplification of low concentrated complexes, detection of intermolecular interactions and quantification of guest exchange rates.