Crown Ether Cavity Research Articles

Pressure‐Induced Encapsulation of Cs+ Cations within an 18‐Crown‐6 Cavity: Insights from X‐ray Diffraction, Raman Spectroscopy, and DFT Calculations

AbstractThis study utilizes single‐crystal X‐ray diffraction in a diamond anvil cell (DAC) to explore the structural response of [Cs(18‐Crown‐6)][SbCl6] under high pressure (HP). Notably, the pressure drives large Cs atoms closer to the center of the crown ether cavity, ultimately requiring pressures of 3 GPa for its complete encapsulation. Remarkably, the absence of short contacts in the crystal classifies the material as a “loose crystal” offering a unique model for understanding compression mechanisms. The crystal exhibits highly anisotropic compression behavior with an impressive volume reduction and non‐linear pressure – unit‐cell parameters relationships facilitated by the absence of short CH⋅⋅⋅Cl contacts up to 0.9 GPa. Beyond this pressure, steric repulsion due to shortening H⋅⋅⋅Cl and H⋅⋅⋅H interactions hinders further effective compression, so pressure dependence on unit cell parameters and volume becomes more linear. The behavior of [Cs(18‐Crown‐6)][SbCl6] at different conditions was studied using Raman spectroscopy and supplemented by DFT calculations.

Molecular recognition of dicyanoalkane guest in a cyclic dimeric silver(I) complex based on pillar[5]-bis-crown

We herein report the formation of the cyclic dimeric silver(I) complexes and the selective organic guest recognition. The reaction of pillar[5]-bis-crown with AgPF6 afforded a cyclic dimeric supramolecular complex [Ag4(L)2(H2O)2(CH3CN)2](PF6)4 in which the silver(I) atoms exist inside the crown ether cavities. The cyclic dimeric silver(I) complex reacts with 1,2-dicyanoethane (C2) to form a guest-inclusion complex [Ag4(C2@L)2(CH3CN)2](PF6)4, exhibiting its formation being controlled by the length of the guest molecules used. Additionally, the silver(I) supramolecular complex demonstrated molecular recognition for C2 in both solution and solid states.

Attractive and repulsive forces in a crystal of [Rb(18-crown-6)][SbCl6] under high pressure.

The compression behavior of [Rb(18-crown-6)][SbCl6] crystal under pressure up to 2.16 (3) GPa was investigated in a diamond anvil cell (DAC) using a mixture of pentane-isopentane (1:4) as the pressure-transmitting fluid. The compound crystallizes in trigonal space group R3 and no phase transition was observed in the indicated pressure range. The low value of pressure bulk modulus [9.1 (5) GPa] found in this crystal is a characteristic of soft materials with predominant dispersive and electrostatic interaction forces. The nonlinear relationship between unit-cell parameters under high pressure is attributed to the influence of reduced intermolecular H...Cl contacts under pressure over 0.73 GPa. It also explains the high compression efficiency of [Rb(18-crown-6)][SbCl6] crystals at relatively low pressures, resulting in a significant shift of the Rb atom to the center of the crown ether cavity. At pressures above 0.9 GPa, steric repulsion forces begin to play a remarkable role, since an increasing number of interatomic H...Cl and H...H contacts become shorter than the sum of their van der Waals (vdW) radii. Below 0.9 GPa, both unit-cell parameter dependences (P-a and P-c) exhibit hysteresis upon pressure release, demonstrating their influence on the disordered model of Rb atoms. The void reduction under pressure also demonstrates two linear sections with the inflection point at 0.9 GPa. Compression of the crystal is accompanied by a significant decrease in the volume of the voids, leading to the rapid approach of Rb atoms to the center of the crown ether cavity. For the Rb atom to penetrate into the center of the crown ether cavity in [Rb(18-crown-6)][SbCl6], it is necessary to apply a pressure of about 2.5 GPa to disrupt the balance of atomic forces in the crystal. This sample serves as a compression model demonstrating the influence of both attractive and repulsive forces on the change in unit-cell parameters under pressure.

Effects of the crown ether cavity on the performance of anion exchange membranes

Anion exchange membrane fuel cells (AEMFCs) have emerged as a promising alternative to proton exchange membrane fuel cells (PEMFCs) due to their adaptability to low-cost stack components and non-noble-metals catalysts. However, the poor alkaline resistance and low OH− conductivity of anion exchange membranes (AEMs) have impeded the large-scale implementation of AEMFCs. Herein, the preparation of a new type of AEMs with crown ether macrocycles in their main chains via a one-pot superacid catalyzed reaction was reported. The study aimed to examine the influence of crown ether cavity size on the phase separation structure, ionic conductivity and alkali resistance of anion exchange membranes. Attributed to the self-assembly of crown ethers, the poly (crown ether) (PCE) AEMs with dibenzo-18-crown-6-ether (QAPCE-18-6) exhibit an obvious phase separated structure and a maximum OH− conductivity of 122.5 mS cm−1 at 80 °C (ionic exchange capacity is 1.51 meq g−1). QAPCE-18-6 shows a good alkali resistance with the OH− conductivity retention of 94.5% albeit being treated in a harsh alkali condition. Moreover, the hydrogen/oxygen single cell equipped with QAPCE-18-6 can achieve a peak power density (PPD) of 574 mW cm−2 at a current density of 1.39 A cm−2.

Study on Viscosity and density of Crown ether (15C5) Complexes in aqueous Solutions at 25oC

This paper reports on the viscosity and density study of the 1,4,7,10,13 pentaoxcyclopentadecane (15C5) as binary system and 15C5 in present of KBr (the ternary system) in aqueous solutions at 25oC. The results of the viscosity and density depend on type of system, and concentration rang. The relative viscosity obtain was fitted well by the use of Jones-Dole and Falkenhagen equations to the viscosity parameters i.e. the apparent molar volume and the effective rigid molar volume at infinite dilution. The results indicate that 4.2 mole of water are engaged in one mole of 15C5 and the complexation volume 0.015 dm3mol-1 K+ with 15C5 has been determine. The results are interpreted in terms of molecular and occupation of crown ether cavity by solvent molecules.

Porous Liquids as Electrolyte: A Case Study of Li+ and Mg2+ Ion Transport in Crown Ether-Based Type-II Porous Liquids

We demonstrated an enhanced ion transport properties of type-II porous liquid-based electrolyte in which crown ether provides internal porosity and are capable of coordinating and transporting Li+ and Mg2+ ions. We investigated the solvation structure and transport properties of the porous liquid electrolytes with different functional groups and cavity sizes. Among the electrolytes studied, the 12-crown-4 (12C4)-based porous liquid electrolyte exhibited the most enhanced ionic conductivity due to size match between the crown ether cavity and the Li+ ion. Nuclear magnetic resonance and Fourier transform infrared analyses revealed that addition of crown ethers reduces the Li+ ion–solvent interaction, resulting in the formation of Li-crown ether complexes. Molecular simulations complement such observations by describing the detailed solvation environment at molecular scale, including complexation between Li+ and Mg2+ ions and 12C4. The strategy described here using crown ether-based porous liquids should be applicable to design versatile electrolytes for various metal-ion batteries.

Discretization and Encapsulation of Palladium inside the Cavity of Crown Ether within the Interlayer of Layered Double Hydroxide for Enhanced Activity: A Case Study with Hydrogenation Reaction

AbstractThe activity of the noble metal‐based heterogeneous catalysts is limited by weak metal–support interactions, aggregation, and low surface to volume (S/V) ratio. The activity can be augmented in many ways. Among them, the discretization of the active sites and redistribution of electron density around the metal atom is an important one. In this work, these two phenomena are studied concerning a model reaction, hydrogenation of p‐nitrophenol (p‐NP). Herein, 1,4,7,10,13‐pentaoxacyclopentadecane ether is introduced in the basal space of layered double hydroxide (LDH) to encapsulate noble Pd0 atom inside the cavity of the crown molecule strategically. The modified LDH (Pda‐ECC‐L0.10@in situ CoAl LDH) augments the properties, like, high S/V ratio and nonaggregation of active sites by forming nonagglomerative discrete catalytic (DNSC) sites within the cavity of crown ether in the basal space. The developed catalyst exhibits higher turnover frequency demonstrating the improved activity due to the formed DNSC sites and redistributes electron density around the Pd atoms by LDH layers and crown molecules. Thus, the present material synthesis route can be considered as a stand‐alone method for preparation of the supported sub‐nanometer noble metal catalyst with higher activity and can be exploited for reactions where noble metal catalyst are used.

Multiresponsive Cyclometalated Crown Ether Bearing a Platinum(II) Metal Center.

Multiresponsive materials can adapt to numerous changes in their local environment, which makes them highly valuable for various applications. Although nanostructured and polymeric multiresponsive materials are plentiful, small-molecule analogues are scarce. This work presents a compact cyclometalated platinum(II) complex that bears a crown ether cavity (18C6-PtII); the intimate ring/emitter connectivity is key to unlocking multiresponsiveness. Complex 18C6-PtII responds to (i) cationic guests, producing changes in luminescence in both solution and the solid state, (ii) solvent molecules, which perturb the packing of the complex in the solid state and cause reversible color changes, and (iii) solvent polarity, which leads to controlled aggregation. These responses may enable 18C6-PtII to function as a sensor for ions and solvents, or as a functional unit for the fabrication of hybrid supramolecular polymers and metallogels.

A Synthetic Phospholipid Derivative Mediates Ion Transport Across Lipid Bilayers

Inspired by the natural phospholipid structures for cell membrane, a synthetic phospholipid LC with an ion recognition group benzo-18-crown-6 (B18C6) moiety was prepared which has been demonstrated to be able to transport ions across the lipid bilayers. Fluorescent vesicle assay shows that LC has an excellent transport activity, and the EC50 value for K+ is 11.2 μM. The voltage clamp measurement exhibits regular square-like current signals with considerably long opening times, which indicates that LC achieves efficient ion transport through a channel mechanism and its single channel conductivity is 17 pS. Both of the vesicle assay and patch clamp tests indicate that LC has selectivity for Rb+, whose ionic radius is larger than the cavity of crown ether. It suggests that the sandwich interaction may play a key role in the ion transport across lipid bilayers. All these results help us to speculate that LC transports ions via a channel mechanism with a tetrameric aggregate as the active structure. In addition, LC had obvious toxicity to HeLa cells, and the IC50 was 100.0 μM after coculture for 36 h. We hope that this simple synthetic phospholipid will offer novel perspectives in the development of more efficient and selective ion transporters.

Switching Ion Binding Selectivity of Thiacalix[4]arene Monocrowns at Liquid-Liquid and 2D-Confined Interfaces.

Understanding the interaction of ions with organic receptors in confined space is of fundamental importance and could advance nanoelectronics and sensor design. In this work, metal ion complexation of conformationally varied thiacalix[4]monocrowns bearing lower-rim hydroxy (type I), dodecyloxy (type II), or methoxy (type III) fragments was evaluated. At the liquid–liquid interface, alkylated thiacalixcrowns-5(6) selectively extract alkali metal ions according to the induced-fit concept, whereas crown-4 receptors were ineffective due to distortion of the crown-ether cavity, as predicted by quantum-chemical calculations. In type-I ligands, alkali-metal ion extraction by the solvent-accessible crown-ether cavity was prevented, which resulted in competitive Ag+ extraction by sulfide bridges. Surprisingly, amphiphilic type-I/II conjugates moderately extracted other metal ions, which was attributed to calixarene aggregation in salt aqueous phase and supported by dynamic light scattering measurements. Cation–monolayer interactions at the air–water interface were monitored by surface pressure/potential measurements and UV/visible reflection–absorption spectroscopy. Topology-varied selectivity was evidenced, towards Sr2+ (crown-4), K+ (crown-5), and Ag+ (crown-6) in type-I receptors and Na+ (crown-4), Ca2+ (crown-5), and Cs+ (crown-6) in type-II receptors. Nuclear magnetic resonance and electronic absorption spectroscopy revealed exocyclic coordination in type-I ligands and cation–π interactions in type-II ligands.

Dynamics of coordination of H3O+ and NH4+ in crown ether cavities.

Crown ethers stand out for their ability to form inclusion complexes with metal cations and positively charged molecular moieties. Hydronium and ammonium interact strongly with crown ethers and potentially modulate their ionophoric activity in protic solvents and physiological environments commonly involved in (bio)technological applications. In this work, Born-Oppenheimer molecular dynamics (BOMD) computations are employed to gain insights into the coordination arrangements of H3O+ and NH4+ in the complexes with the native crown ethers 15-crown-5 (15c5) and 18-crown-6 (18c6). Both cations display dynamic changes in coordination inside the cavities of the crown ethers. On the one hand, hydronium explores different coordination arrangements, through rotation around its C3 axis in the 15c5 complex, and through breathing motions, involving rapid inversions of the O atom along the C3 axis in the 18c6 complex. On the other hand, ammonium undergoes a facile rotation in three dimensional space, leading to frequent changes in the NH bonds involved in the coordination with the crown ether. The reduced host-guest symmetry matching of the 15c5 macrocycle enhances the reorientation dynamics and, in the case of H3O+, it promotes short H-bonding distances yielding events of proton transfer to the crown ether. The infrared vibrational spectra predicted by the BOMD computations within this dynamic framework reproduce with remarkable accuracy the action spectra of the isolated complexes obtained in previous infrared laser spectroscopy experiments. The experimentally observed band positions and broadening can then be rationalized in terms of orientational diffusion of the cations, changes in the coordinating H-bonding pairs sustaining the complex and eventual proton bridge formation.

Multiple Stimuli-Responsiveness Fluorescent Probe Derived from Cyclopolymers and Pyrene-Ended Ammonium Salts

A multiple stimuli-responsiveness fluorescent probe was fabricated by host-guest recognition between cyclopolymers with larger pseudo crown ether cavities in backbones and pyrene-ended ammonium sal...

Photophysical and antimicrobial properties of new double-armed benzo-15-crown-5 ligands and complexes

New double-armed crown ether ligands linked to pyridine derivatives have been synthesized and characterized. These macrocyclic ligands (1–5) have been synthesized by the reactions of 4′,5′-bis(bromomethyl)benzo-15-crown-5 with 3-hydroxy pyridine derivatives. A series of Na+, K+ and Ag+ complexes (1a–5a, 1b–5b and 1c–5c) of the macrocyclic ligands have been prepared from sodium perchlorate, sodium picrate, potassium iodide, potassium picrate and silver nitrate salts, respectively. The most suitable cation Na+ is bound to the 15-crown-5 cavity and 1:1 “filling complexes” are formed (1a–5a) while the K+ cation interacts with the crown ether cavity and forms sandwich-type complexes (1b–5b). The Ag+ complexes (1c–5c) have been obtained with a pyridine moiety of the new crown ethers. New ligands undergo photophysical changes when bonding the cation. The influence of metal cations such as Na+, Li+, K+, Fe3+, Cu2+, Ca2+, Ba2+ and Al3+ on the spectroscopic properties of the pyridine linked to the double-armed crown ether moiety was investigated in EtOH solution by means of absorption and emission spectrometry. The prepared compounds (1–5, 1a–5a, 1b–5b and 1c–5c) were evaluated for antibacterial and antifungal activities against pathogenic microorganisms. The results show that the antimicrobial activity of the synthesized compounds varying a degree of inhibitory effects on the growth of different tested pathogenic strains.

Synthesis, X-ray crystal structures and thermal behavior of calcium β-diketonate complexes [Ca(fod)2(15-crown-5)] and [Ca(fod)(C3F7COO)(15-crown-5)] (Hfod = 1,1,1,2,2,3,3-heptafluoro-7,7-dimethyl-4,6-octanedione)

The synthesis of the Ca-β-diketonate complexes with 15-crown-5 [Ca(fod)2(15-crown-5)] (1), [Ca(fod)(C3F7COO)(15-crown-5)] (2), (Hfod = 1,1,1,2,2,3,3-heptafluoro-7,7-dimethyloctane-4,6-dione; 15-crown-5 = 1,4,7,10,13-pentaoxacyclopentadecane) is described. The complex 1 has been prepared by reaction of metallic Ca with 1 equiv of 15-crown-5 and 2 equiv of Hfod in ethanol. The reaction of calcium covered by surface calcium hydroxide with excess of Hfod in presence of 15-crown-5 in boiling toluene results in the complex 2 as main product and complex 1 as by-product. The solvated complex [Ca(fod)(C3F7COO)(15-crown-5)](CH2Cl2) (3) was obtained by recrystallization from CH2Cl2-hexane solution of 2. The complexes 1, 2 were characterized by elemental analyses, IR-spectroscopy, NMR-spectroscopy. The molecular structures of 1, 3 were characterized by single-crystal X-ray diffraction method. Complexes 1, 3 have monomolecular structures. In 1 the calcium cation Ca2+ is outside the crown-ether cavity and coordinated by five O atoms of 15-crown-5 and four O atoms of two fod-ligands; the average Ca-Ofod distance is 2.38(2) Å and the average Ca-Ocrown distance is 2.64(2) Å. In the compound 3 the coordination environment of the Ca is set up by two O atoms of chelating fod ligand, one O atom of C3F7COO ligand and five O atoms of 15-crown-5 ligand. Ca atom is outside 15-crown-5 plane, β-diketonate and carboxylate ligands are in cis-position relative to 15-crown-5. The average Ca-Ofod bond distance 2.318(2) Å and Ca-OC3F7COO bond distance 2.311(2) Å are practically equal. The 1 and 2 are thermal stable in 50–227 °C and in 50–180 °C temperature ranges, respectively. In dynamic vacuum (residual pressure 10−2 Torr) complexes 1 and 2 sublimed in 70–110 °C temperature range congruently.

Theoretical study on the binding selectivity of 18-membered azacrown ethers with alkaline earth metal species

The binding selectivity of 18-membered azacrown ethers (monoaza- N1, diaza- N2, triaza- N3, tetraaza- N4, pentaaza- N5, and hexaaza-18-crown-6 N6) with Ca2+, Sr2+, Ba2+ have been studied by density functional theory (DFT) calculations. The complex binding selectivity was analyzed in term of interaction energies, thermodynamic properties, second order interaction energies, and charge transfer effects. The geometrical study shows that Ca2+ and azacrown complexes acquire envelope like structure, leading to shorter bond lengths. As a result, these complex systems have the highest interaction energies. Theoretical study also showed that N6 complex with alkaline earth metal ion were shown to be more stable complex than those ligand with lower nitrogen number. The interaction energy order is N0 < N1 < N2 < N3 < N4 < N5 < N6. This trend shows that the presence of more nitrogen on the crown ether cavity increases the interaction energies by approx. 7.3 % in going from N0 to N6. It is clearly showed that the contribution of the number of nitrogen play a dominant role in the binding selectivity of these systems.

Self-Assembly of a [2]Pseudorotaxane by an Inchworm-Motion Mechanism.

The threading of biomolecules through pores or channels in membranes is important to validate the physiological activities of cells. To aid understanding of the controlling factors required for the translocation in space with confined size and distorted conformation, it is desirable to identify experimental systems with minimized complexity. We demonstrate the mechanism of a linear guest L1 threading into a tris(crown ether) host TC with a combinational distorted cavity to form a triply interlocked [2]pseudorotaxane 3in-[L1⊂TC]. An inchworm-motion mechanism is proposed for the process. For the forward-threading steps that lead to the formation of higher-order interlocked species, guest L1 must adopt a bent conformation to find the next crown ether cavity. Two simplified models are applied to investigate the self-assembly dynamic of 3in-[L1⊂TC]. Kinetic NMR spectroscopic and molecular dynamics (MD) studies show that formation of the singly penetrated species is fast, whereas formation of the doubly and triply threaded species is several orders of magnitude slower. During threading the freedom of both the guest L1 and host TC gradually decrease due to their interactions. This results in a significant entropy effect for the threading dynamic, which is also observed for the threading of a biomolecular chain through a channel.

Combination of size selective binding ability of 18-crown-6 dissolved in aqueous phase and extractive properties of an amic acid; toward enhancement of rare earths separation

The separation of La(III), Eu(III) and Er(III) ions by an amic acid, N,N-dioctyldiglycolamic acid (HL), dissolved in carbon tetrachloride has been improved in the presence of 18-crown-6 (18C6) in aqueous phase as a selective masking agent. The interaction between the studied metal ions and 18C6 resulted a shift in the extraction curve of the studied metal ions versus pH toward higher pH region. The displacement of the extraction curves was more pronounced for lanthanum ions and was varied as La(III) > Eu(III) > Er(III). This order of complexing ability of 18C6 toward the studied ions was attributed to the size adaptation of the ions and that of the crown ether cavity. The stability constants of the lanthanide–crown ether complexes in aqueous phase were evaluated. The influence of temperature on the extraction of studied metal ions from aqueous phase in the absence and the presence of 18C6 was tested in the range 298–308 K. This investigation allowed evaluating the thermodynamic parameters associated with the extraction process and those of the complexation of cations by 18C6 in the aqueous phase.

The Influence of Alkali Metal Cation Size on the Formation and Dissociation of Crown Ether Fullerene Dimers in Electrospray Mass Spectrometry

Crown ether fullerenes, C60H(cr – H), form two different dimers in electrospray ionization mass spectrometry, a noncovalently bound, metal-bridged dimer [C60H(cr – H)]2M+, and a singly bonded fullerene dimer of the type (cr – H)–C60–C60–(cr – H)Mnn+, with n = 1, 2. In this work, the dependence of both the formation and the dissociation of these dimers on the respective sizes of the crown ether moiety (12cr4, 15cr5, 18cr6) and of the alkali metal ion (M+ = Li+, Na+, K+, Rb+, Cs+) is studied. The size ratio of crown ether cavity to metal cation size has a profound influence on the formation of the noncovalent dimers, [C60H(cr – H)]2M+. These are only formed if the cation is larger than the crown ether cavity. Also, the doubly charged, covalent dimers [C60(cr – H)]2M22+ show a dependence on the size ratio. The proposed formation mechanism involves the recombination of two C60(cr – H)M•+ radical cations, which can only occur if the repulsion between the two charges is sufficiently reduced by encapsulating the...

Insights into the effects of 2:1 "sandwich-type" crown-ether/metal-ion complexes in responsive host-guest systems.

In-depth investigations of the specific ion-responsive characteristics based on 2:1 "sandwich" structures and effects of crown ether cavity sizes on the metal-ion/crown-ether complexation are systematically performed with a series of PNIPAM-based responsive copolymers containing similar contents of crown ether units with different cavity dimensions (12-crown-4 (12C4), 15-crown-5 (15C5), 18-crown-6 (18C6)). The lower critical solution temperature (LCST) values of copolymers in deionized water shift to lower temperatures gradually when the crown ether contents increase or the ring sizes decrease from 18C6 to 12C4. With increasing the concentrations of alkali metal ions (Na(+), K(+), Cs(+)) or the contents of pendent crown ether groups, the copolymers with different crown ether cavity sizes exhibit higher selectivity and sensitivity to corresponding cations. Importantly, the ion sensitivities of the copolymers in response to corresponding alkali metal ions increase dramatically with an increase in the crown ether cavity size. Interestingly, a linear relationship between the crown ether cavity size and the diameter of corresponding cation for the formation of stable 2:1 "sandwich" complexes is found for the first time, from which the size of metal ions or other guests that able to form 2:1 "sandwich" complexes with crown ethers can be deduced. The results in this work are valuable and useful for further developments and practical applications of various crown-ether-based smart materials.

Fine-tuning of electromembrane extraction selectivity using 18-crown-6 ethers as supported liquid membrane modifiers.

Selectivity of electromembrane extractions (EMEs) was fine-tuned by modifications of supported liquid membrane (SLM) composition using additions of various 18-crown-6 ethers into 1-ethyl-2-nitrobenzene. Gradually increased transfer of K(+) , the cation that perfectly fits the cavity of 18-crown-6 ethers, was observed for EMEs across SLMs modified with increasing concentrations of 18-crown-6 ethers. A SLM containing 1% w/v of dibenzo-18-crown-6 in 1-ethyl-2-nitrobenzene exhibited excellent selectivity for EMEs of K(+) . The established host-guest interactions between crown ether cavities in the SLM and potassium ions in donor solution ensured their almost exhaustive transfer into acceptor solution (extraction recovery ∼92%) within 30 min of EME at 50 V. Other inorganic cations were not transferred across the SLM (Ca(2+) and Mg(2+) ) or were transferred negligibly (NH4 (+) , Na(+) ; extraction recovery < 2%) and had only subtle effect on EMEs of K(+) . The high selectivity of the tailor-made SLM holds a great promise for future applications in EMEs since the range of similar selective modifiers is very broad and may be applied in various fields of analytical chemistry.