Ionic Guests Research Articles

Probing Ion-Receptor Interactions in Halide Complexes of Octamethyl Calix[4]Pyrrole.

Ion receptors are molecular hosts that bind ionic guests, often with great selectivity. The interplay of solvation and ion binding in anion host-guest complexes in solution governs the binding efficiency and selectivity of such ion receptors. To gain molecular-level insight into the intrinsic binding properties of octamethyl calix[4]pyrrole (omC4P) host molecules with halide guest ions, we performed cryogenic ion vibrational spectroscopy (CIVS) of omC4P in complexes with fluoride, chloride, and bromide ions. We interpret the spectra using density functional theory, describing the infrared spectra of these complexes with both harmonic and anharmonic second-order vibrational perturbation theory (VPT2) calculations. The NH stretching modes of the pyrrole moieties serve as sensitive probes of the ion binding properties, as their frequencies encode the ion-receptor interactions. While scaled harmonic spectra reproduce the experimental NH stretching modes of the chloride and bromide complexes in broad strokes, the high proton affinity of fluoride introduces strong anharmonic effects. As a result, the spectrum of F-·omC4P is not even qualitatively captured by harmonic calculations, but it is recovered very well by VPT2 calculations. In addition, the VPT2 calculations recover the intricate coupling of the NH stretching modes with overtones and combination bands of CH stretching and NH bending modes and with low-frequency vibrations of the omC4P macrocycle, which are apparent for all of the halide ion complexes investigated here. A comparison of the CIVS spectra with infrared spectra of solutions of the same ion-receptor complexes in d3-acetonitrile and d6-acetone shows how ion solvation changes the ion-receptor interactions for the different halide ions.

Dynamic mechanostereochemical switching of a co-conformationally flexible [2]catenane controlled by specific ionic guests

Responsive synthetic receptors for adaptive recognition of different ionic guests in a competitive environment are valuable molecular tools for not only ion sensing and transport, but also the development of ion-responsive smart materials and related technologies. By virtue of the mechanical chelation and ability to undergo large-amplitude co-conformational changes, described herein is the discovery of a chameleon-like [2]catenane that selectively binds copper(I) or sulfate ions and its associated co-conformational mechanostereochemical switching. This work highlights not only the advantages and versatility of catenane as a molecular skeleton in receptor design, but also its potential in constructing complex responsive systems with multiple inputs and outputs.

Isomerism of Cyclodextrin Tetramer Induced by Alkali Halide Cluster Ions Observed by Ion Mobility Spectrometry-Mass Spectrometry.

Cyclodextrins (CDs) exhibit versatile self-assembly properties due to their hydrophilic and hydrophobic components, with applications such as drug delivery and selective binding. While research on CD self-assembly is extensive, limited studies have explored their aggregation behavior, particularly in interactions with small ionic guests. The present work investigates the structure of β-CD tetramers aggregated with alkali metal chloride clusters using ion mobility spectrometry-mass spectrometry (IMS-MS). The results revealed that diverse structures emerge in the tetramer depending on the alkali metal cluster size. Notably, the doubly charged tetramer exhibits distinct aggregation trends with specific numbers of MCl clusters for Na+ and K+ ions. After initially adopting a bucket-wheel structure with two internal cations, the structure transforms into a new isomer with a tetrahedral configuration upon cluster addition. The formation of the new isomer structure is closely linked to filling the cavity volume with MCl clusters and ionic interactions, which possibly compensate for the weakened hydrogen bonds between CDs. Theoretical calculations further support the structures, showing well-matched collision cross-section (CCS) values compared with the experimental CCS values. This study highlights the role of alkali metal chloride clusters as potential templates, leading to the formation of novel CD assemblies.

Guest size effects on a robust structure of semiclathrate hydrates and their thermophysical properties.

The ability to tune the pore size, shape, and functionality of semiclathrate hydrates, host-guest materials formed from aqueous solutions of ionic guest materials and water, makes them attractive materials for thermal storage and gas storage applications. The flexibility of semi-clathrate hydrates and their guest-molecule-dependent reactions produce these unexpected and desirable properties. As an ionic guest, tetra-n-butylammonium cation is known for best-fit in hydrogen-bonded water structures. Few investigations have been conducted for other cations, while there are numerous candidates. Relationships between the molecular structures of ionic guest substances and their hydrate structure and relevant thermodynamic properties are yet to be understood. In this study, the semiclathrate hydrates formed with two variations of tetra-n-butylammonium chloride (N4444Cl) that are n-propyl, tri-n-butylammonium chloride (N3444Cl) and tri-n-butyl, n-pentylammonium chloride (N4445Cl) were investigated. Structure analyses found that both salts formed Jeffrey's type III tetragonal hydrate structure which is the same as that of tetra-n-butylammonium chloride hydrate, although their lattice parameters are significantly different. The present data found that this hydrate structure can cover a wide range of melting temperature compared to the other two main semiclathrate structures. The present N4445Cl hydrate is an example in which its melting temperature was adjusted to be suitable for air conditioning, i.e., ∼282 K, compared to that of the N4444Cl hydrate, the melting temperature of which is slightly too high for this purpose. The results provide insight that the thermal properties of the tetragonal P42/m hydrate structure can be widely tuned by ionic guests for various practical requirements.

Biogas separation by semiclathrate hydrates formed with tetra-n-butylammonium and tetra-n-butylphosphonium salts

CH4/CO2 gas separation which frequently appears in biomass energy remains as a kind of challenge for gas hydrates, since these two molecules are similarly preferred by the hydrate phases. Semiclathrate hydrates can improve gas capture preference from the canonical gas hydrates by unique phase behavior. In this study, we perform parametric gas separation tests by the semiclathrate hydrates formed from major four ionic guests, i.e., TBAB, TBAC, TBPB and TBPC with pressure conditions of 1, 3 and 5 MPa and aqueous salt concentrations of 0.10, 0.20, 0.30 in mass fraction. The captured CO2 amounts basically increased with aqueous concentrations. The highest CO2 capture amount was obtained by the TBAB solution with 0.30 in salt mass fraction. TBAC showed the highest CO2 selectivity, while interestingly, the lower the pressure, the higher the CO2 composition in the TBAC hydrate phase, which means that the low-cost operation without high compression of biogas could be feasible and reasonable with this material. TBAB showed relatively simple behavior compared to N2 + CO2 and H2 + CO2 systems. The present results showed that it is possible to eliminate CH4 and enrich CO2 in the hydrate phase when the ionic guest and thermodynamic conditions are appropriately chosen.

“Tai Chi” philosophy driven dual-network ionogel membrane comprised of ionic guests in a multifunctional host for propylene/propane separation

Facilitated transport ionogel membranes have shown great prospects in propylene/propane separation due to the stability of carrier media, no leakage, and promising thermal/physical properties. In the present study, inspired by the Tai Chi philosophy of adjusting inorganic and organic features, a dual-network ionogel was made of multi-functional organosilane, [EMIM][TFSI] ionic liquid, AgBF4, and catalysts. An effective strategy, including solvent-free solvolytic sol-gel and epoxide ring-opening reaction, was proposed to keep the inorganic/organic balance and obtain ionogel films with high IL loading (up to 82%) and self-standing. The ionogels were characterized by a phase diagram, formation mechanism, FTIR, N2 adsorption-desorption isotherms, TGA, SEM, mechanical properties, and transport properties. The Multifunctional host induced the breaking of bondage between the components of IL electrolyte (AgBF4 and IL); thereby more free Ag+ ions become available for facilitating the olefin transport. A computational molecular approach was applied to better understand the possible interactions between the ionic guest and multifunctional host. Results indicate that the optimized ionogel not only has excellent transport properties (C3H6 permeability of 243.1 Barrer and C3H6/C3H8 selectivity of 20.2) but also has desirable mechanical and thermal properties. The synthesis of ionogel based on Tai Chi philosophy opens a new window for facilitated transport of gas separation membranes.

Crownphyrins: Metal‐Mediated Transformations of the Porphyrin‐Crown Ether Hybrids

AbstractCrownphyrins are hybrid macrocycles combining structural features of porphyrin and crown ethers. The molecular architecture renders them an intriguing class of hosts capable of binding neutral, and ionic guests. The presence of dynamic covalent imine linkages connecting the dipyrrin segment with the ether chain enables unusual coordination behavior of crownphyrins, as demonstrated by the formation of two classes of strikingly different complexes. The remarkable metal‐mediated expansion to the helical [2+2] macrocyclic complex is reversible. The reaction of the figure‐eight mercury(II) assembly with [2.2.2]cryptand results in ring contraction providing the metal‐free crownphyrin macrocycle.

Crownphyrins: Metal-Mediated Transformations of the Porphyrin-Crown Ether Hybrids.

Crownphyrins are hybrid macrocycles combining structural features of porphyrin and crown ethers. The molecular architecture renders them an intriguing class of hosts capable of binding neutral, and ionic guests. The presence of dynamic covalent imine linkages connecting the dipyrrin segment with the ether chain enables unusual coordination behavior of crownphyrins, as demonstrated by the formation of two classes of strikingly different complexes. The remarkable metal-mediated expansion to the helical [2+2] macrocyclic complex is reversible. The reaction of the figure-eight mercury(II) assembly with [2.2.2]cryptand results in ring contraction providing the metal-free crownphyrin macrocycle.

Ion Binding Site Structure and the Role of Water in Alkaline Earth EDTA Complexes.

The interactions between molecular hosts and ionic guests and their dependence on the chemical environment are challenging to disentangle from solution data alone. The vibrational spectra of cold complexes of ethylenediaminetetraacetic acid (EDTA) chelating alkaline earth dications in vacuo encode structural characteristics of these complexes and their dependence on the size of the bound ion. The correlation between metal binding geometry and the relative intensities of vibrational bands of the carboxylate groups forming the binding pocket allows us to characterize water-induced changes in molecular geometry. The evolution of these structural markers from bare ions to water adducts to aqueous solution illustrates the role of water for the structure of ion binding sites in chelators. The binding pocket of EDTA opens up in aqueous solution, bringing the bound ion closer to the mouth of the binding site and leading to an increased exposure of the ion to the chemical environment.

Anion-Afforded Functions of Ionic Metal-Organic Frameworks: Ionochromism, Anion Conduction, and Catalysis.

The exchangeable counterions in ionic metal-organic frameworks (IMOFs) provide facile and versatile handles to manipulate functions associated with the ionic guests themselves and host-guest interactions. However, anion-exchangeable stable IMOFs combining multiple anion-related functions are still undeveloped. In this work, a novel porous IMOF featuring unique self-penetration was constructed from an electron-deficient tris(pyridinium)-tricarboxylate zwitterionic ligand. The water-stable IMOF undergoes reversible and single-crystal-to-single-crystal anion exchange and shows selective and discriminative ionochromic behaviors toward electron-rich anions owing to donor-acceptor interactions. The IMOFs with different anions are good ionic conductors with low activation energy, the highest conductivity being observed with chloride. Furthermore, integrating Lewis acidic sites and nucleophilic guest anions in solid state, the IMOFs act as heterogeneous and recyclable catalysts to efficiently catalyze the cycloaddition of CO2 to epoxides without needing the use of halide cocatalysts. The catalytic activity is strongly dependent upon the guest anions, and the iodide shows the highest activity. The results demonstrate the great potential of developing IMOFs with various functions related to the guest ions included in the porous matrices.

Designing the structure and relevant properties of semiclathrate hydrates by partly asymmetric alkylammonium salts.

Semiclathrate hydrates are host-guest materials that form from ionic guests and water. There are numerous options for ionic guests, such as quaternary ammonium salts, to tune the functional properties of these materials such as melting temperature, fusion heat, and gas capacity and selectivity. To design these materials, the stabilization mechanism of the side chains of quaternary ammonium salts must be understood based on both thermodynamic and crystallographic properties and relevant host-guest dynamics. In this paper, we studied semiclathrate hydrates formed from n-propyl, tri-n-butylammonium bromide (N3444Br) and tri-n-butyl, n-pentylammonium bromide (N4445Br). Their cation side chains are decremented or incremented from tetra-n-butylammonium (N4444 or TBA), which is one of the best fits for semiclathrate hydrate structures. The use of the widely used tetra-n-butylammonium bromide (N4444Br or TBAB) as an ionic guest, an increment of the carbon chain, i.e., N4445Br, caused disorders in its hydrate structure due to the oversizing of the cation. This suitably oversized cation selectively stabilized the orthorhombic structure, whose hydration number is relatively high. As a result, the fusion heat at the congruent composition of the hydrate phase was higher than that of the widely used N4444Br (TBAB) hydrates. The N3444Br hydrate showed both significantly decreased melting temperature and fusion heat compared to the N4444Br (TBAB) hydrates. The phase behaviour of the N3444Br hydrate was found to be analogous to that of the N4444Br (TBAB) hydrates. It was demonstrated that the semiclathrate hydrate structures and relevant properties can be modified by adjusting the alkyl side chain length of quaternary ammonium salts.

Selective sensing and transport in bionic nanochannel based on macrocyclic host-guest chemistry

Imitating the signal transduction and transmembrane transport controlled by biological channels in the cell membrane, artificial nanochannels with a similar capability of sensing and transport are constructed as bionic nanochannels. To accomplish selective sensing and transport of biological analyte (as “guest”), the bionic nanochannels are modified with the artificial receptor (as “host”). Based on selective recognition between host and guest, bionic nanochannels translate the stimulus of the guest to electrochemical signal as sensors, and further regulate the transmission of guest as transporters. However, throughout all kinds of guests, the selective sensing and transport of ions and chiral molecules is a challenging problem. And throughout all hosts of ions and chiral molecules, the macrocyclic hosts with multisite of recognition show better selectivity, such as crown ethers, cyclodextrins, calixarenes, and pillararenes. In this article, we highlight recent advances in the macrocyclic host-based nanochannels for the selective sensing and transport of ionic and chiral guests, summarize the similarities and differences of different kinds of macrocyclic host-based nanochannels, and expect the research direction and application prospect.

Light-driven coordination anions-directed regulation of chromism in three metal complexes assembled by cyano-equipped dithienylethene ligand

Three coordination compounds were self-assembled successfully with Ag(I) ions and photo-responsive ligands featuring a cyano-equipped dithienylethene unit. Anions with different coordinating abilities were selected to construct distinct topologies, which are evidenced by X-ray crystallographic analysis. CF3 COO− and CF3 SO3− are involved into coordination in 1 and 3, respectively, whereas in 2 BF4− present merel y as ionic guests counterbalancing the framework charge. Anions also show their subtle effects on coordinated atoms, interactions, and ligand configurations. The quite different structures of 1–3 indicate an interesting anion-directed structural self-assembly.Followed examinations on photochromism illustrated that the reversible photo-isomerizations of three complexes are all retained after complexzation with Ag(I) center. Various degrees of bathochromic shifts in absorptions were observed as compared with that of the metal-free ligand. Therefore, the photo-switching of these compounds could be modulated conveniently and finely by varying simply the coordination anions and then triggering by light irradiation. The discussions of structure-properties relationship concluded that anions exert their effects on coordination structure through adjusting the configurations of ligands in complexes. Strong coordinating anions facilitate a pronounced conformational change of ligand whereas un-coordinating anion leads to smaller variation. The distinct structural changes of ligand result finally the different perturbation of photochromic behavior.

Li[LiCs2 Cl][Ga3 S6 ]: A Nanoporous Framework of GaS4 Tetrahedra with Excellent Nonlinear Optical Performance.

A large nonlinear optical (NLO) coefficient and a wide band gap are two crucial but contradictory parameters that are difficult to achieve simultaneously in a single infrared (IR) NLO compound. A salt-inclusion chalcogenide (SIC), Li[LiCs2 Cl][Ga3 S6 ] (1), was prepared that presents a nanosized tunnel framework constructed from monotype chalcogenide tetrahedra. Highly oriented covalent GaS4 tetrahedra in the host lead to a moderate second harmonic generation response (0.7 AgGaS2 ), and ionic guests effectively broaden the band gap to the widest value (4.18 eV) among all IR NLO chalcogenides, thereby achieving a remarkable balance between NLO efficiency and band gap.

Li[LiCs2Cl][Ga3S6]: A Nanoporous Framework of GaS4 Tetrahedra with Excellent Nonlinear Optical Performance

AbstractA large nonlinear optical (NLO) coefficient and a wide band gap are two crucial but contradictory parameters that are difficult to achieve simultaneously in a single infrared (IR) NLO compound. A salt‐inclusion chalcogenide (SIC), Li[LiCs2Cl][Ga3S6] (1), was prepared that presents a nanosized tunnel framework constructed from monotype chalcogenide tetrahedra. Highly oriented covalent GaS4 tetrahedra in the host lead to a moderate second harmonic generation response (0.7 AgGaS2), and ionic guests effectively broaden the band gap to the widest value (4.18 eV) among all IR NLO chalcogenides, thereby achieving a remarkable balance between NLO efficiency and band gap.

Competitive Binding of Organic Dyes between Cucurbiturils and Octa Acid.

Employing six cationic water-soluble organic dye molecules as probes, we have attempted to qualitatively understand the factors that govern the attraction between such molecules and the anionic water-soluble host, octa acid (OA). Examination of the competitive host–guest complexation between cucurbit[8]uril (CB[8]) and OA using absorption and emission spectroscopy revealed that the dye molecules included within CB[8] could be “pulled out” by OA. However, an order of magnitude higher concentration of OA was required to shift the equilibrium toward OA, suggesting that attraction between the anionic host OA and the cationic dye molecules such as cresyl violet perchlorate and methylene blue is weaker than the hydrophobic and cation–dipolar interaction between these dye molecules and CB[8]. The importance of Coulombic attraction between OA and dye molecules is also revealed by monomer-to-dimer conversion upon addition of OA to an aqueous solution of monomeric dye molecules. Under conditions where the dye-to-OA ratio is high, freely dissolved monomeric dye molecules are attracted to the exterior of OA and aggregate as dimers on the exterior wall of OA. On the other hand, at high ratios of OA to dye molecules, the dye molecules adsorb as monomers on the exterior of OA. Thus, the monomer-to-dimer ratio in aqueous solution can be controlled by adjusting the ratio of dye to OA molecules. The results presented are of value in qualitatively understanding the relative binding properties of ionic guests with ionic hosts. Studies are qualitative in nature, and further detailed quantitative studies planned for the future are likely to provide deeper understanding of the interaction between water-soluble dye molecules, OA, and CB.

Control over multiple molecular states with directional changes driven by molecular recognition

Recently, ligand–metal coordination, stimuli-responsive covalent bonds, and mechanically interlinked molecular constructs have been used to create systems with a large number of accessible structural states. However, accessing a multiplicity of states in sequence from more than one direction and doing so without the need for external energetic inputs remain as unmet challenges, as does the use of relatively weak noncovalent interactions to stabilize the underlying forms. Here we report a system based on a bispyridine-substituted calix[4]pyrrole that allows access to six different discrete states with directional control via the combined use of metal-based self-assembly and molecular recognition. Switching can be induced by the selective addition or removal of appropriately chosen ionic guests. No light or redox changes are required. The tunable nature of the system has been established through a combination of spectroscopic techniques and single crystal X-ray diffraction analyses. The findings illustrate a new approach to creating information-rich functional materials.

Alkaline Anion-Exchange Membranes Containing Mobile Ion Shuttles.

A new class of alkaline anion-exchange membranes containing mobile ion shuttles is developed. It is achieved by threading ionic linear guests into poly(crown ether) hosts via host-guest molecular interaction. The thermal- and pH-triggered shuttling of ionic linear guests remarkably increases the solvation-shell fluctuations in inactive hydrated hydroxide ion complexes (OH(-) (H2 O)4 ) and accelerates the OH(-) transport.

Small Is (also) Beautiful: Dynamic Covalent Self-Assembly of Cryptates

Cryptands can be seen as the smallest three-dimensional molecular cages still capable of accommodating (ionic) guests. We have recently realized the first dynamic covalent self-assembly of a monometallic cryptate, whose template synthesis and unique structure crucially rest on orthoester bridgeheads [Nat. Commun. 2015, 6, 7129]. In this Synpacts article, I discuss selected aspects that led to this discovery and I attempt to put the study into the context of recent trends in the self-assembly of larger molecular cages. 1 Introduction 2 Dynamic Covalent Self-Assembly of Orthoester Cryptates 3 From Covalent to Metallosupramolecular to Dynamic Covalent Synthesis 4 Orthoester Cryptates as Tiny Self-Assembled Cages 5 Conclusions

ChemInform Abstract: Linear Aromatic Amide Foldamer‐Derived Supramolecular Architectures and Materials

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