Crystal structure analysis of twisted cucurbit [14]uril conformations

Double-cavity cucurbiturils: synthesis, structures, properties, and applications

Double-cavity Q[n]s are relatively new members of the Q[n] family and have garnered significant interest due to their distinctive structures and novel properties. While they incorporate n glycoluril units, akin to their single-cavity counterparts, their geometry can best be described as resembling a figure-of-eight or a handcuff, distinguishing them from single-cavity Q[n]s. Despite retaining the core molecular recognition traits of single-cavity Q[n]s, these double-cavity variants introduce fascinating new attributes rooted in their distinct configurations. This overview delves into the synthesis, structural attributes, properties, and intriguing applications of double-cavity Q[n]s. Some of the applications explored include their role in supramolecular polymers, molecular machinery, supra-amphiphiles, sensors, artificial light-harvesting systems, and adsorptive separation materials. Upon concluding this review, we discuss potential challenges and avenues for future development and offer valuable insights for other scholars working in this area with the aim of stimulating further exploration and interest.

Cucurbit[n]uril-Based Supramolecular Frameworks Assembled through Outer-Surface Interactions

Porous materials, especially metal-organic frameworks, covalent organic frameworks, and supramolecular organic frameworks, are widely used in heterogeneous catalysis, adsorption, and ion exchange. Cucurbit[n]urils (Q[n]s) suitable building units for porous materials because they possess cavities with neutral electrostatic potential, portal carbonyls with negative electrostatic potential, and outer surfaces with positive electrostatic potential, which may result in the formation of Q[n]-based supramolecular frameworks (QSFs) assembled through the interaction of guests within Q[n]s, the coordination of Q[n]s with metal ions, and outer-surface interaction of Q[n]s (OSIQ). This review summarizes the various QSFs assembled via OSIQs. The QSFs can be classified as being assembled by 1) self-induced OSIQ, 2) anion-induced OSIQ, and 3) aromatic-induced OSIQ. The design and construction of QSFs with novel structures and specific functional properties may establish a new research direction in Q[n] chemistry.

Cucurbituril-Oriented Nanoplatforms in Biomedical Applications

Cucucrbituril (CB) belongs to a family of macrocycles that are easily accessible. Their structural specificity provides excellent molecular recognition capabilities, with the ability to be readily chemically modified. Because of these properties, researchers have found CB to be a useful molecular carrier for delivering drug molecules and therapeutic biomolecules. Their significance lies in the fact that CB not only increases the solubility and stability of an encapsulated guest but also provides the possibility to achieve targeted delivery of the guest molecule. Therefore, the emergence of CB undoubtedly provides opportunities for the development of targeted drug delivery in an era where intelligent drugs have attracted considerable attention. It has also been found that CB can enhance fluorescent dyes, allowing the preparation of biosensors with enhanced sensitivity for use in clinical settings. In the present review, the acquisition, properties, and structural modifications of CB are first comprehensively described, and then the value of this macrocycle in applications within the medical field is discussed. In addition, we have also summarized patent applications of CB in this field over recent years, aiming to illustrate the current status of developments of this molecule. Finally, we discuss the challenges faced by CB in the medical field and future trends in its development.

Thorium(iv) and uranium(vi) compounds of cucurbit[10]uril: from a one-dimensional nanotube to a supramolecular framework

Cucurbit[10]uril {Q[10]} has the largest portal size and cavity in the series of Q[n] (n = 5-10) molecules. In contrast to its rich host-guest chemistry, its coordination chemistry is underdeveloped with only limited metal ions being investigated so far. In this work, we initiated the study of Q[10] complexes with Th(iv) and U(vi) ions in HCl solutions via a self-assembly approach. The coordination of Th(iv) ions with Q[10] led to the formation of a compound, {Th4(Cl)16(H2O)20(Q[10])}·nH2O (Q[10]-Th), with a unique nano-tubular structure, while U(vi) ions facilitated the formation of a compound, [(UO2)2(Cl)4(H2O)6]·(Q[10])2·HCl·nH2O (Q[10]-U), with a Q[10]-based supramolecular framework structure via intermolecular outer-surface and second-shell interactions. The structural and spectroscopic aspects of the two compounds together with their optical and thermal properties have been investigated. The successful preparation and characterization of the first two Q[10] compounds with Th(iv)/U(vi) ions highlighted the potential for further exploration of Q[10] coordination chemistry with actinide ions.

Yttrium and lanthanide (Ln = La and Gd) complexes with cucurbit[10]uril: crystals transforming from supramolecular frameworks to coordination nanotubes

Yttrium and lanthanide complexes with cucurbit[10]uril have been investigated with crystals transforming from supramolecular structures to coordination nanotubes being revealed.

Supramolecular Fluorescence Probe Based on Twisted Cucurbit[14]uril for Sensing Fungicide Flusilazole

The host-guest complex of the common dye, thioflavin T (ThT), and twisted cucurbit[14]uril (tQ[14]) was selected as a fluorescent probe to determine non-fluorescent triazole fungicides, including flusilazole, azaconazole, triadimefon, tebuconazole, tricyclazole, flutriafol, penconazole, and triadimenol isomer A, in an aqueous solution. The experimental results reveal that the ThT@tQ[14] probe selectively responded to flusilazole with significant fluorescence quenching and a detection limit of 1.27 × 10^-8 mol/L. In addition, the response mechanism involves not only a cooperation interaction-ThT occupies a side-cavity of the tQ[14] host and the triazole fungicide occupies another side-cavity of the tQ[14] host-but also a competition interaction in which both ThT and the triazole fungicide occupy the side-cavities of the tQ[14] host.

Lanthanoid Heteroleptic Complexes with Cucurbit[5]uril and Dicarboxylate Ligands: From Discrete Structures to One-Dimensional and Two-Dimensional Polymers

Lanthanoid heteroleptic complexes with cucurbit[5]uril {Q[5]} and two dicarboxylate ligands, e.g., diglycolic acid (H₂DGC) and glutaric acid (H₂GT), have been investigated with six new compounds featuring a tetrametallic and dimetallic discrete structures, a one-dimensional (1D) polymer, and three two-dimensional (2D) polymers with a unique honeycomb-type topology being synthesized and structurally characterized. [La₄(Q[5])₃(DGC)₂(NO₃)₂(H₂O)₁₂][La(DGC)(H₂O)₆]·7NO₃· nH₂O (1) has a tetrametallic structure constructed with three bis-bidentate Q[5] ligands linking two [La(DGC)(H₂O)₂]⁺ species in the middle and two [La(H₂O)₄(NO₃)]²⁺ species at both ends. [Ce₂(Q[5])(DGC)(NO₃)(H₂O)₁₀]·3NO₃·4H₂O (2) has a dimetallic structure built up with a bis-bidentate Q[5] ligand linking [Ce(DGC)(H₂O)₃(NO₃)] and [Ce(H₂O)₇]³⁺ on each side of the Q[5] portals. [Ce₃(Q[5])₃(DGC)₂(H₂O)₉][Ce(DGC)(H₂O)₆]₂·7NO₃· nH₂O (3) has a 1D polymeric structure built up with bis-bidentate Q[5] ligands in-turn linking one [Ce(H₂O)₆]³⁺ and two [Ce(DGC)(H₂O)₆]¹⁺ cationic species. [Ln₂(Q[5])₂(GT)(H₂O)₆]·4NO₃· nH₂O [Ln = La (4), Ce (5) and Nd (6)] have similar 2D polymeric structures built up with two types of 9-fold coordinated Ln polyhedra linked by Q[5] via bis-bidentate carbonyl groups on both sides forming 1D chains which are further connected by bridging GT^2- ligands to form 2D polymers with a unique honeycomb-type topology. Their vibrational modes and electronic structures have also been investigated.

Thorium(IV) and Uranium(IV) Complexes with Cucurbit[5]uril

Tetravalent thorium and uranium complexes with cucurbit[5]uril (Q[5]) were investigated with eight new complexes being synthesized and structurally characterized. [Th(Q[5])(OH)(H₂O)₂]₆·18NO₃· nH₂O (1) has a hexagonal nanowheel structure with each of the six Th⁴⁺ ions being cap-coordinated by a Q[5] and monodentate-coordinated to the nearby Q[5]. [Th(Q[5])(HCOO)(H₂O)₄][Th(NO₃)₅(H₂O)₂]₂[Th(NO₃)₃(HCOO)(H₂O)₂]_0.5·NO₃· nH₂O (2) has a heteroleptic mononuclear structure with a Th⁴⁺ ion cap-coordinated on one side of the Q[5] portal and monodentate-coordinated to a formate anion inside the Q[5] cavity. [KTh_1.5(Q[5])Cl(NO₃)₃][Th(NO₃)₅(H₂O)₂]·2NO₃·2.5H₂O (3) has a heterometallic structure with both Th⁴⁺ and K⁺ ions each occupying one side of the two Q[5] portals forming a capsule. [CsTh(Q[5])Cl(NO₃)₂(H₂O)₃]·2NO₃· nH₂O (4) has a heterometallic 1D polymeric structure with both Th⁴⁺ and Cs⁺ ions each occupying one side of the two Q[5] portals, forming monomers which are linked together by sharing two water molecules and one carbonyl oxygen atom between Th⁴⁺ and Cs⁺ ions. [Th(Q[5])Cl(H₂O)][CdCl₃][CdCl₄]·0.5HCl·4H₂O (5), [Th(Q[5])Cl(H₂O)][Ru₂OCl₉(H₂O)]·0.5HCl·9.5H₂O (6), [Th(Q[5])Cl(H₂O)][IrCl₆]_1.5·3H₂O (7), and [U(Q[5])Cl(H₂O)][ZnCl₃(H₂O)][(ZnCl₄)]·8H₂O (8) have similar 1D polymeric structures with Th⁴⁺/U⁴⁺ ions cap-coordinated on one side of a Q[5] and bidentate coordinated to the nearby Q[5]. The transition metal chlorides act as anions for charge compensation as well as structure inducers via cation-anion interactions forming various anion patterns around the 1D polymers. Actinide contraction has been observed in the early actinide series.

4-Sulfocalix[4]arene/Cucurbit[7]uril-Based Supramolecular Assemblies through the Outer Surface Interactions of Cucurbit[n]uril

Upon mixing of aqueous solutions of the freely soluble building blocks cucurbit[7]uril (Q[7]) and 4-sulfocalix[4]arene (SC[4]A), white microcrystals instantly separate in near-quantitative yield. The driving force for this assembly is suggested to be the outer-surface interaction of the Q[n]. Dynamic light scattering, scanning electron microscopy, and NMR (diffusion-ordered NMR spectroscopy) analyses have confirmed the supramolecular aggregation of Q[7] and SC[4]A. Titration ¹H NMR spectroscopy and isothermal titration calorimetry have shown that the interaction ratio of Q[7] and SC[4]A is close to 3:1. Moreover, the Q[7]/SC[4]A-based supramolecular assembly can accommodate molecules of some volatile compounds or luminescent dyes. Thus, this work offers a simple and highly efficient means of preparing adsorbent or solid fluorescent materials.

Coordination and Supramolecular Assemblies of Fully Substituted Cyclopentanocucurbit[6]uril with Lanthanide Cations in the Presence of Tetrachlorozincate Anions, and Their Potential Applications

Coordination and supramolecular assemblies of a fully substituted cyclopentanocucurbit[6]uril (CyP₆Q[6]) with a series of lanthanide cations (Ln³⁺) have been investigated in the presence of tetrachlorozincate anion ([ZnCl₄]^2-). X-ray single-crystal diffraction analysis has revealed that the interaction of CyP₆Q[6] and a series of Ln³⁺cations unexpectedly results in the formation of at least seven different CyP₆Q[6]-based coordination complex adduct and supramolecular assemblies groups, including with (1) La³⁺, Ce³⁺cations; (2) Pr³⁺, Nd³⁺cations; (3) Eu³⁺, Gd³⁺, Tb³⁺, Dy³⁺ with P1̅ or P1 space group, in which CyP₆Q[6] molecules coordinate alternatively with Ln³⁺cations and form linear coordination polymers; (4) CyP₆Q[6] molecules interact alternatively with [Ho(H₂O)₈]³⁺ aqueous complexes and form linear supramolecular chains; CyP₆Q[6] molecules can assemble two different Ln³⁺ free porous supramolecular assemblies from CyP₆Q[6]-Ln(NO₃)₃-ZnCl₂-HCl systems, Ln = Tm, Yb, and Lu; however, no solid crystals were obtained from system containing Er³⁺cation. Thus, these differences could lead CyP₆Q[6] to be useful in not only the isolation of lighter lanthanides from their heavier lanthanides but also special selectivity for different volatile organic compounds.

Coordination supramolecular assemblies of a monohydroxycucurbit[7]uril and their potential applications in gas sorption

Coordination supramolecular assemblies of monohydroxycucurbit[7]uril ((HO)Q[7]) with alkaline earth metal ions (AE²⁺) have been formed in aqueous HCl solution in the presence of tetrachloride cadmium anions ([CdCl₄]^2-) as a structure directing agent. The driving force for the assembly could be attributed to the interaction of the positive electro-potential outer-surface of (HO)Q[7] molecules with [CdCl₄]^2- anions and ionic dipole interaction of the hydroxyl of (HO)Q[7] molecules with [CdCl₄]^2- anions. Moreover, the porous structure of the (HO)Q[7]/AE²⁺-based coordination supramolecular assemblies could result in potential applications in the selective sorption of polar volatile organic molecules, which may be useful in molecular sieves, sensors, absorption and separation.

Alkaline earth cation-mediated photoluminescent complexes of thioflavin T with twisted cucurbit[14]uril

The introduction of alkaline earth metal cations into the tQ[14]–ThT interaction system leds to the precipitation of solid tQ[14]/ThT/AE²⁺ interaction products, which emitted strong blue fluorescence.