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.

Potential Applications of Cucurbit[n]urils and Their Derivatives in the Capture of Hazardous Chemicals

Cucurbit[n]urils (Q[n]s) are a relatively young family of macrocycles, consisting of glycoluril units bridged by methylene groups, and their unique structures have attracted extensive attention from chemists in recent decades. Due to the presence of a rigid hydrophobic inner cavity and two polar outer portals lined with carbonyl groups, Q[n]s not only encapsulate guest species into the cavity, but also coordinate with metal ions/clusters. Considerable achievements have been obtained in the fields of Q[n]s-based host-guest chemistry, coordination chemistry, as well as the combination of host-guest and coordination chemistry. Furthermore, the outer surface of Q[n]s has been demonstrated to be capable of interacting with definite species to generate supramolecular architectures in recent years. With more in-depth research into Q[n]s, their application studies have also emerged as a hot topic. This Minireview focuses on recent advances in the potential applications of solid-state materials based on Q[n]s and their derivatives for the capture and adsorption of hazardous chemicals from a solution or a gas mixture.

Supramolecular chemistry of substituted cucurbit[n]urils

This review covers important advances in the field of substituted cucurbit[n]urils.

Specific Recognition of Methanol Using a Symmetric Tetramethylcucurbit[6]uril-Based Porous Supramolecular Assembly Incorporating Adsorbed Dyes

A symmetric tetramethylcucurbit[6]uril-based porous supramolecular assembly was prepared in an aqueous H2SO4 solution (5M). The driving force for the formation of this assembly is mainly the outer surface interaction of Q[n], which includes the ion-dipole interaction of SO42− anions and the positive electrostatic potential of the outer surface of the symmetric tetramethylcucurbit[6]uril (TMeQ[6]), the dipole-dipole interactions between the positive electrostatic potential of the outer surface of TMeQ[6] and portal carbonyl oxygens of TMeQ[6], and the hydrogen bonding between lattice water molecules and portal carbonyl oxygen atoms in TMeQ[6]. The TMeQ[6]-based porous supramolecular assembly exhibits the characteristics of absorbed fluorophore guests (FGs), such as dyes and polycyclic compounds with different fluorescence characteristics. Moreover, the resulting luminescent assemblies (FG@As) can respond to certain volatile organic compounds; in particular, the luminescent assemblies of rhodamine B or pyrene display a unique fluorescence enhancement in response to methanol.

Porous supramolecular assemblies and functional properties of perhydroxylated cucurbit[6]uril and polyoxometallates

(HO)₁₂Q[6] and POMs were used as basic building blocks, and interactions yielded two novel types of supramolecular assemblies at different HCl concentrations.

The Chaotropic Effect as an Assembly Motif in Chemistry

Following up on scattered reports on interactions of conventional chaotropic ions (for example, I- , SCN- , ClO4- ) with macrocyclic host molecules, biomolecules, and hydrophobic neutral surfaces in aqueous solution, the chaotropic effect has recently emerged as a generic driving force for supramolecular assembly, orthogonal to the hydrophobic effect. The chaotropic effect becomes most effective for very large ions that extend beyond the classical Hofmeister scale and that can be referred to as superchaotropic ions (for example, borate clusters and polyoxometalates). In this Minireview, we present a continuous scale of water-solute interactions that includes the solvation of kosmotropic, chaotropic, and hydrophobic solutes, as well as the creation of void space (cavitation). Recent examples for the association of chaotropic anions to hydrophobic synthetic and biological binding sites, lipid bilayers, and surfaces are discussed.

Cucurbit[6]uril-based multifunctional supramolecular assemblies: synthesis, removal of Ba(ii) and fluorescence sensing of Fe(iii)

Supramolecular assemblies formed of cucurbit[6]uril and flexible aromatic compounds were achieved via outer-surface interactions, and show selective adsorption and sensing properties.

Highly selective absorption of polychloromethanes in perhydroxylated cucurbit[6]uril-based supramolecular assemblies

Two perhydroxylated cucurbit[6]uril-based porous assemblies have been obtained by the crystallization of perhydroxylated cucurbit[6]uril {(HO)₁₂Q[6]} from aqueous HCl solutions of different concentrations, which exhibit highly selective absorption of polychloromethanes.

Synthesis and Structure of the Inclusion Complex {NdQ[5]K@Q[10](H₂O)₄}·4NO₃·20H₂O

Heating a mixture of Nd(NO₃)₃·6H₂O, KCl, Q[10] and Q[5] in HCl for 10 min affords the inclusion complex {NdQ[5]K@Q[10](H₂O)₄}·4NO₃·20H₂O. The structure of the inclusion complex has been investigated by single crystal X-ray diffraction and by X-ray Photoelectron spectroscopy (XPS).

Coordination and supramolecular assembly of {Cd2Ge8V12O48} building block and cucurbit[6] to form rotaxane-shaped hybrids

Assembly of cucurbit[6] and a {Cd2Ge8V12O48} cluster produced two rotaxane-shaped and polyrotaxane-shaped solids by changing the ratio of starting precursors in the system. The high oxygen density of the polyoxoanion surface provides active sites to extend a single rotaxane-shaped hybrid 1 to a 1D polyrotaxane-shaped hybrid 2. This construction strategy may afford an entirely new methodology for polyoxometalate-based hybrid chemistry.

Adducts of aqua complexes of Ln3+with hexahydroxyhexamethylcucurbit[6]uril: potential application in the isolation of heavy lanthanides

The interactions between a series of Ln³⁺and HHQ[6] were investigated. X-ray crystallography analysis revealed that HHQ[6] selectively interacts with certain Ln ions. The experimental results point to a possible means of isolating lighter or heavier Ln cations.

Interaction of a symmetrical α,α′,δ,δ′-tetramethyl-cucurbit[6]uril with Ln3+: potential applications for isolation of lanthanides

Interaction of a symmetrical α,α′,δ,δ′-tetramethyl-cucurbit[6]uril (TMeQ[6]) with a series of lanthanide cations (Ln³⁺) was investigated in neutral water and acidic solution.

Self-assemblies based on the "outer-surface interactions" of cucurbit[n]urils: new opportunities for supramolecular architectures and materials

Supramolecular architectures and materials have attracted immense attention during the last decades because they not only open the possibility of obtaining a large variety of aesthetically interesting structures but also have applications in gas storage, sensors, separation, catalysis, and so on. On the other hand, cucurbit[n]urils (Q[n]s), a relatively new class of macrocyclic hosts with a rigid hydrophobic cavity and two identical carbonyl fringed portals, have attracted much attention in supramolecular chemistry. Because of the strong charge-dipole and hydrogen bonding interactions, as well as hydrophobic and hydrophilic effect derived from the negative portals and rigid cavities of Q[n]s, nearly all research in Q[n]s has been focused on utilizing the portals and cavities to construct supramolecular assemblies similar to other macrocyclic receptors such as cyclodextrin and calixarenes. Interestingly, a recent study revealed that other weak noncovalent interactions such as hydrogen bonding and π···π stacking, as well as C-H···π and ion-dipole interactions, could also be defined as "outer-surface interactions", which are derived from the electrostatically positive outer surface of Q[n]s. These interactions could be the driving forces in the formation of various novel Q[n]-based supramolecular architectures and functional materials. In this Account, we provide a comprehensive overview of supramolecular self-assemblies based on the outer-surface interactions of Q[n]s. These outer-surface interactions include those between Q[n]s, Q[n]s and aromatic molecules, Q[n]s and calixarenes, Q[n]s and inorganic complex ions, and Q[n]s and polyoxometalates. Pioneering work has shown that such weak noncovalent interactions play very important roles in the formation of various Q[n]-based functional materials and supramolecular architectures. For example, hydrogen bonds in outer-surface interactions between Q[n] molecules not only function as the sole driving force in the formation of one-dimensional Q[n] porous channels but also assist the bonding forces of the channels in capturing and accommodating acetylene molecules and carbon dioxide in the channel cavities. Moreover, upon introduction of a third species such as an aromatic molecule or inorganic anion into the Q[n]/metal system, "outer-surface interactions" could lead to Q[n]/metal-based self-assemblies from simple finite supramolecular coordination complexes to infinite polydimensional supramolecular architectures and other structures. Overall, this Account focuses on the novel self-assembly driving force derived from Q[n]s including (i) concepts of the outer-surface interactions of Q[n]s, (ii) providing plausible explanations of the mechanisms of the outer-surface interactions of Q[n]s, and (iii) introduction of an overview of the developments and practical applications of outer-surface interactions of Q[n]s in supramolecular chemistry. It is hoped that this study based on the outer-surface interactions of Q[n]s can enrich the field of molecular engineering of functional supramolecular systems and provide new opportunities for the construction of functional materials and architectures.