Crown Ether Research Articles

Perfect confinement of crown ethers in MOF membrane for complete dehydration and fast transport of monovalent ions.

Fast transport of monovalent ions is imperative in selective monovalent ion separation based on membranes. Here, we report the in situ growth of crown ether@UiO-66 membranes at a mild condition, where dibenzo-18-crown-6 (DB18C6) or dibenzo-15-crown-5 is perfectly confined in the UiO-66 cavity. Crown ether@UiO-66 membranes exhibit enhanced monovalent ion transport rates and mono-/divalent ion selectivity, due to the combination of size sieving and interaction screening effects toward the complete monovalent ion dehydration. Specifically, the DB18C6@UiO-66 membrane shows a permeation rate (e.g., K+) of 1.2 mol per square meter per hour and a mono-/divalent ion selectivity (e.g., K+/Mg2+) of 57. Theoretical calculations and simulations illustrate that, presumably, ions are completely dehydrated while transporting through the DB18C6@UiO-66 cavity with a lower energy barrier than that of the UiO-66 cavity. This work provides a strategy to develop efficient ion separation membranes via integrating size sieving and interaction screening and to illuminate the effect of ion dehydration on fast ion transport.

Supramolecular channels via crown ether functionalized polyaniline for proton-self-doped cathode in aqueous zinc-ion battery

Polyaniline (PANI) has been widely used as a cathode in aqueous zinc-ion batteries (AZIBs) because of its attractive conductivity and energy storage capability. However, the extensive application of PANI is limited by spontaneous deprotonation and slow diffusion kinetics. Herein, an 18-crown-6-functionalised PANI pseudorotaxane (18C6@PANI) cathode is successfully developed through a facile template-directed polymerisation reaction. The 18C6@PANI cathode exhibits a high specific capacity of 256 mAh g−1 at 0.2 A/g, excellent rate performance of 134 mAh g−1 at 6 A/g and outstanding cycle stability at a high current density of 3 A/g over 10,000 cycles. Experimental and theoretical analyses demonstrate the formation of the –N–Zn–O– structure. The abundant supramolecular channels in pseudorotaxane, induced by crown ether functional groups, are beneficial for achieving superior cyclability and rate capability. These encouraging results highlight the potential for designing more efficient PANI-based cathodes for high-performance AZIBs.

Ni(I) and Ni(II) Bis(trimethylsilyl)amides Obtained in Pursuit of the Elusive Structure of Ni{N(SiMe3)2}2.

Salt metathesis routes to five new -N(SiMe3)2 nickel derivatives were studied to illuminate their mode of formation, structures, and spectroscopy. The reaction between NiI2 and K{N(SiMe3)2} afforded the Ni(II) and Ni(I) complexes [K][Ni{N(SiMe3)2}3] (1) and [K][Ni{N(SiMe3)2}2] (2). Dissolving 1 in tetrahydrofuran (THF) gave the Ni(II) species [K(THF)2][Ni{N(SiMe3)2}3] (3). The Ni(I) salt [K(DME)][Ni2{N(SiMe3)2}3] (4) was obtained by using NiCl2(DME) (DME = 1,2-dimethoxyethane) as the nickel source rather than NiI2. The isolation of the Ni(I) complexes 2 and 4 highlights the tendency for K{N(SiMe3)2} to function as a reducing agent. Introduction of adventitious O2 to solutions of [K][Ni{N(SiMe3)2}2] (2) gave the nickel inverse crown ether (ICE) species [K2][O(Ni{N(SiMe3)2}2)2] (5). Complex 5 is the first ICE complex of nickel and is one of four known ICE complexes for the 3d metals. The experimental results indicate that the reduced Ni(I) bis(trimethylsilyl)amides are relatively easily generated, whereas Ni(III) derivatives that might be expected from a disproportionation of a Ni(II) derivative are apparently not yet isolable by the above routes. Overall, the new species crystallize readily from the reaction mixtures, but under ambient conditions, they begin to decompose as solids within ca. 24 h, which hinders their characterization.

Cs+ Extraction from Chloride-Rich Brine Solutions Using the Calixarene Crown Ether MAXCalix

ABSTRACT We have developed an extraction method for the retrieval of Cs+ from chloridecontaining salt brine solutions. We employ the calixarene crown ether known as MAXCalix as the extractant, which exhibits a remarkable selectivity for Cs+ over Na+, K+, Mg2+ and Ca2+, with separation factors ranging from 100 to 104. The efficiency of the extraction strongly depends on the chloride concentration in the aqueous phase. Notably, when the chloride concentration reaches 4 mol L−1, we achieve a Cs+ distribution ratio of 7.9. Accordingly, the retrieval of Cs+ from the organic phase is achieved when the HCl concentration is maintained at a low level. In multicomponent chloride brine solutions, the distribution ratios for Cs+ are somewhat lower, yet still fall within the range of 4–6. Nevertheless, these ratios remain favorable for separation purposes. Additionally, the distribution ratios of Na+, K+, Mg2+, and Ca2+ offer insights into the coordination chemistry of MAXCalix. Our experimental data indicate a correlation between ionic size and distribution ratio. Cs+ forms the strongest complex, followed by K+ and Na+, while Mg2+ and Ca2+ exhibit minimal extraction, potentially due to interactions with the diluent. These findings may open new approaches for the development of a decontamination strategy tailored to address salts and brine solutions contaminated with Cs-137.

Theoretical simulation of ion transport in main-chain poly(arylene piperidinium) anion exchange membranes with crown ether groups

ABSTRACT Experimental studies have confirmed that introduction of the proper content of dibenzo-18-crown-6-ether (DE) into main-chain poly(arylene piperidinium) anion exchange membranes (AEMs) improved the efficiency of OH− transport. In this study, models of the AEMs (PDTP-x) were constructed with varying molar contents of DE groups (x = 0, 0.05, 0.10, 0.15, 0.20 and 0.25). Molecular dynamics simulations were employed to investigate the microscopic effects of the DE groups on OH− transport. The analysis examined the impact of the volume of the overlapping hydration shells around piperidinium groups and the continuities and uniformities of the ion transport channels. Furthermore, the stabilities of the hydrogen bonds between water molecules explained the effect of x on OH− transport. The results showed that the total overlap volume initially increased and then decreased as x increased. When the content of DE groups was appropriate, the hydration shells of the cations exhibited considerable overlap, which facilitated the formation of continuous and uniform ion transport channels. Additionally, the stabilities of the water hydrogen bonds initially decreased and then increased as x increased. The stabilities of the hydrogen bonds were lowest when the DE content was optimal, which facilitated the transport of OH−.

Studying Cation Exchange in {Cr7Co} Pseudorotaxanes: Preparatory Studies for Hybrid Molecular Machines.

In the design of dynamic supramolecular systems used in molecular machines, it is important to understand the binding preferences between the macrocycle and stations along the thread. Here, we apply1H NMR spectroscopy to investigate the relative stabilities of a series of linear alkylammonium templated pseudorotaxanes with the general formula [H2NRR'][Cr7CoF8(O2CCH2tBu)16] by exchanging the cation in solution. Our results show that the pseudorotaxanes are able to exchange threads via a dissociative mechanism. The position of equilibrium is dependent upon the ammonium cation and solvent used. Short chain primary ammonium cations are shown to be far less favourable macrocycle stations than secondary ammonium cations. Collision-induced dissociation mass spectrometry (CID-MS) has been used to look at disassembly of the pseudorotaxanes in a solvent-free environment and stability trends compared to those in acetone-d6. The energy needed to induce 50% of the precursor ion loss (E50) is used shows a similar trend to the equilibria measured by NMR. The relative stabilities of these hybrid inorganic-organic pseudo-rotaxanes are different to those of host-guest compounds involving crown ethers and this may be valuable for the design of molecular machines.

Crown Ether Copolymerized Polyimide Film: Enhanced Mechanical, Thermal Properties and Low Dielectric Constant under High Frequency.

Polymer materials with a low dielectric constant and low dielectric loss have the potential to be applied to high-frequency signal transmissions, such as mobile phone antennas and millimeter wave radars. Two types of diamines, 4,4'-diamino-p-tetraphenyl (DPT) and crown ether diamine (CED), were prepared for ternary copolymerization with BPDA in this study. Cross-links with molecular chains were formed, increasing molecular chain distance by utilizing rings of CED. The MPI films exhibit a good thermal performance with the increase in CED addition, with Tg > 380 °C and CTE from -4 × 10-6 K-1 to 5 × 10-6 K-1. The Young's modulus can reach 8.6 GPa, and the tensile strength is above 200 MPa when 5% and 7% CED are introduced. These MPI films exhibit good mechanical performances. The dielectric constant of PI-10% film can go as low as 3.17. Meanwhile, the relationship between dielectric properties and molecular structure has been demonstrated by Molecular Simulation (MS). PI molecules are separated by low dielectric groups, resulting in a decrease in the dielectric constant.

Crown Ether Electrolyte Induced Li2O2 Amorphization for Low Polarization and Long Lifespan Li‐O2 batteries

Lithium‐oxygen batteries possess an extremely high theoretical energy density, rendering them a prime candidate for next‐generation secondary batteries. However, they still face multiple problems such as huge charge polarization and poor cycle life, which lay a significant gap between laboratory research and commercial applications. In this work, we adapt 15‐crown‐5 ether (C15) as solvent to regulate the generation of discharge products in lithium‐oxygen batteries. The coronal structure endows C15 with strong affinity to Li+, firmly stabilizes the intermediate LiO2 and discharge product Li2O2. Thus, the crystalline Li2O2 is amorphized into easily decomposable amorphous products. The lithium‐oxygen batteries assembled with 0.5 M C15 electrolyte show an increased discharge capacity from 4.0 mAh cm−2 to 5.7 mAh cm−2 and a low charge overpotential of 0.88 V during the whole lifespan at 0.05 mA cm−2. The batteries with 1 M C15 electrolyte can cycle stably for 140 cycles. Furthermore, the amorphous characteristic of Li2O2 product is preserved when matched with redox mediators such as LiI, with the charge polarization further decreasing to 0.74 V over a cycle life of 190 cycles. This provides new possibilities for electrolyte design to promote Li2O2 amorphization and reduce charge overpotential in lithium‐oxygen batteries.

Crown Ether Electrolyte Induced Li2O2 Amorphization for Low Polarization and Long Lifespan Li-O2 batteries.

Lithium-oxygen batteries possess an extremely high theoretical energy density, rendering them a prime candidate for next-generation secondary batteries. However, they still face multiple problems such as huge charge polarization and poor cycle life, which lay a significant gap between laboratory research and commercial applications. In this work, we adapt 15-crown-5 ether (C15) as solvent to regulate the generation of discharge products in lithium-oxygen batteries. The coronal structure endows C15 with strong affinity to Li+, firmly stabilizes the intermediate LiO2 and discharge product Li2O2. Thus, the crystalline Li2O2 is amorphized into easily decomposable amorphous products. The lithium-oxygen batteries assembled with 0.5 M C15 electrolyte show an increased discharge capacity from 4.0 mAh cm-2 to 5.7 mAh cm-2 and a low charge overpotential of 0.88 V during the whole lifespan at 0.05 mA cm-2. The batteries with 1 M C15 electrolyte can cycle stably for 140 cycles. Furthermore, the amorphous characteristic of Li2O2 product is preserved when matched with redox mediators such as LiI, with the charge polarization further decreasing to 0.74 V over a cycle life of 190 cycles. This provides new possibilities for electrolyte design to promote Li2O2 amorphization and reduce charge overpotential in lithium-oxygen batteries.

Ion-pair chromatographic attribute in the retention of ammonium ion on a crown ether-based and a polysaccharide-based chiral stationary phases. Some mobile phase additives compatible with liquid chromatography-electrospray ionization-mass spectrometry

The design of the mobile phase additive useful in HPLC analysis on two types of chiral stationary phases, one based on chiral crown ether, and another based on polysaccharide derivative, was developed. CROWNPAK® CR-I, the chiral stationary phase bearing (R)- or (S)-(3,30-diphenyl-1,10-binaphtyl)-20-crown-6 (1) moiety is a useful CSP to separate primary amine enantiomers. While perchloric acid had been recommended as the mobile phase additive to attain a better separation, we have reexamined the effect of mobile phase additives and revealed that the retention of an ammonium ion is enhanced by a chaotropic or a hydrophobic anion in a water-based mobile phase like well-known ion pair chromatography in an RPLC. However, the retention was enhanced in reverse by a rather cosmotropic or a hydrophilic anion in an acetonitrile (ACN)-based mobile phase. These results looking contradictory to each other may be rationalized both in the basis of ion pair chromatography. A similar but smaller effect of pairing anion species was also observed on a polysaccharide-based CSP. Based on these findings, some types of mobile phase have been proposed and some of them were demonstrated to be compatible with liquid chromatography-electrospray ionization-mass spectrometry (ESI LC/MS).

Self-assembled crown ether (15CR5) with alkaline earth metals to stimulate nonlinear optical properties: The comprehensive study by silico technique

Crown Ether (15CR5) molecule is a worthwhile organic material exhibiting excellent NLO properties and characteristics. In this research, density functional theory (DFT) was extensively employed to study the nonlinear optical (NLO) properties of alkaline earth metal doped 15CR5 by using WB97XD/6-31G+(d, p). Binding energies, interaction energies, vertical ionization energies and reactivity parameters unveiled the thermodynamic stability of these systems. These molecules show a lower value of HUMO-LUMO energy gap indicating their superior electronic properties. Moreover, these complexes possess high oscillator strength and lower excitation energies displaying their better electrical and optical characteristics than pure 15CR5. Non-covalent analysis was carried out for all the designed systems forecasted the van der wall interactions between 15CR5 and dopants. These complexes in the visible region have excess electrons that are highly absorbed in the range of 405.73 to 608.27 compared to 15CR5. The Hyperpolarizability (βtot) of these designed materials was in the range of 9097.21–21627.85 au which is significantly higher than its pure counterpart. Results based on the density of state (DOS), natural bond orbital analysis (NBO) and transition density matrix (TDM) were also strongly supported the enhancement of the NLO properties of these systems. These newly designed alkaline earth metal doped 15CR5 systems with promising NLO characteristics can be used for optoelectronic applications.

Variation in the relationship between odd isotopes of tin in mass-independent fractionation induced by the magnetic isotope effect

Experimental results are presented showing the variation in the relationship between odd isotopes of tin (Sn) in mass-independent fractionation caused by the magnetic isotope effect (MIE), which has previously only been observed for mercury. These results are consistent with the trend predicted from the difference between the magnitudes of nuclear magnetic moments of odd isotopes with a nuclear spin. However, the correlation between odd isotopes in fractionation induced by the MIE for the reaction system used in this study (solvent extraction using a crown ether) was different from that reported for the photochemical reaction of methyltin. This difference between the two reaction systems is consistent with a theoretical prediction that the correlation between odd isotopes in fractionation induced by the MIE is controlled by the relationship between the spin conversion time and radical lifetime. The characteristic changes in the correlation between odd isotopes in fractionation induced by the MIE observed for Sn in this study provide a guideline for quantitatively determining fractionation patterns caused by the MIE for elements that have multiple isotopes with a nuclear spin. These results improve our understanding of the potential impact of the MIE on mass-independent fractionation observed in natural samples, such as meteorites, and analytical artifacts of high-precision isotope analysis for heavy elements.

Crown-Assisted CsCu2I3 Growth and Trap Passivation for Perovskite Light-Emitting Diodes.

Copper (Cu)-based perovskites are promising for lead-free perovskite light-emitting diodes (PeLEDs). However, it remains a significant challenge to achieve high performance devices due to the nonradiative loss caused by the disordered crystallization and lack of passivation. Crown ethers are known to form host-guest complexes by the interaction between C-O-C groups and certain cations, and 18-crown-6 (18C6) with an appropriate complementary size can interact with Cs+ and Cu+ cations. Herein, we studied the interaction between CsCu2I3 and two crowns with the same cyclic size, 18C6 and dibenzo-18-crown-6 (D18C6). Particularly, D18C6 can reduce the nonradiative recombination rate of CsCu2I3 film by passivating the defects and optimizing the film morphology effectively. The room mean square (RMS) decreased from 5.06 to 2.95 nm, and the PLQY was promoted from 4.71% to 19.9%. Besides, D18C6 can also decrease the barrier of hole injection. The PeLEDs based on D18C6-modified CsCu2I3 realized noticeable improvement with a maximum luminance and EQE of 583 cd/m2 and 0.662%, respectively.

Regulating Access to Active Sites via Hydrogen Bonding and Cation-Dipole Interactions: A Dual Cofactor Approach to Switchable Catalysis.

Hydrogen bonding networks are ubiquitous in biological systems and play a key role in controlling the conformational dynamics and allosteric interactions of enzymes. Yet in small organometallic catalysts, hydrogen bonding rarely controls ligand binding to the metal center. In this work, a hydrogen bonding network within a well-defined organometallic catalyst works in concert with cation-dipole interactions to gate substrate access to the active site. An ammine ligand acts as one cofactor, templating a hydrogen bonding network within a pendent crown ether and preventing the binding of strong donor ligands, such as nitriles, to the nickel center. Sodium ions are the second cofactor, disrupting hydrogen bonding to enable switchable ligand substitution reactions. Thermodynamic analyses provide insight into the energetic requirements of the different supramolecular interactions that enable substrate gating. The dual cofactor approach enables switchable catalytic hydroamination of crotononitrile. Systematic comparisons of catalysts with varying structural features provide support for the critical role of the dual cofactors in achieving on/off catalysis with substrates containing strongly donating functional groups that might otherwise interfere with switchable catalysts.

Proximity Effects on the Reactivity of a Nonheme Iron (IV) Oxo Complex in C-H Oxidation.

Precise control of substrate positioning and orientation (its proximity to the reactive unit) is often invoked to rationalize the superior enzymatic reaction rates and selectivities when compared to synthetic models. Artificial nonheme iron (IV) oxo (Fe(IV)=O) complexes react with C(sp3)-H bonds via a biomimetic Hydrogen Atom Transfer/Hydroxyl Rebound mechanism, but rates, site-selectivity and even hydroxyl rebound efficiency (ligand rebound versus substrate radical diffusion) are smaller than in oxygenases. Herein, we quantitatively analyze how substrate binding modulates nonheme Fe(IV)=O reactivity by comparing rates and outcomes of C-H oxidation by a pair of Fe(IV)=O complexes that share the same first coordination sphere but only one contains a crown ether receptor that recognizes the substrate. Substrate binding makes the reaction intramolecular, exhibiting Michaelis-Menten kinetics and increased reaction rates. In addition, C-H oxidation occurs with high site selectivity for remote sites. Analysis of Effective Molarity reveals that the system operates at its maximal theoretical capability for the oxidation of these remote sites. Remarkably, substrate positioning also affects Hydroxyl Rebound, whose efficiency only increases on the sites placed in proximity by recognition. Overall, these observations provide evidence that supramolecular control of substrate positioning can effectively modulate the reactivity of oxygenases and its models.

Proximity Effects on the Reactivity of a Nonheme Iron (IV) Oxo Complex in C−H Oxidation

AbstractPrecise control of substrate positioning and orientation (its proximity to the reactive unit) is often invoked to rationalize the superior enzymatic reaction rates and selectivities when compared to synthetic models. Artificial nonheme iron (IV) oxo (Fe(IV)=O) complexes react with C(sp3)−H bonds via a biomimetic Hydrogen Atom Transfer/Hydroxyl Rebound mechanism, but rates, site‐selectivity and even hydroxyl rebound efficiency (ligand rebound versus substrate radical diffusion) are smaller than in oxygenases. Herein, we quantitatively analyze how substrate binding modulates nonheme Fe(IV)=O reactivity by comparing rates and outcomes of C−H oxidation by a pair of Fe(IV)=O complexes that share the same first coordination sphere but only one contains a crown ether receptor that recognizes the substrate. Substrate binding makes the reaction intramolecular, exhibiting Michaelis–Menten kinetics and increased reaction rates. In addition, C−H oxidation occurs with high site selectivity for remote sites. Analysis of Effective Molarity reveals that the system operates at its maximal theoretical capability for the oxidation of these remote sites. Remarkably, substrate positioning also affects Hydroxyl Rebound, whose efficiency only increases on the sites placed in proximity by recognition. Overall, these observations provide evidence that supramolecular control of substrate positioning can effectively modulate the reactivity of oxygenases and its models.

Porous organic polymers with heterocyclic crown ethers for selective lithium-ion capture

Porous organic polymers with heterocyclic crown ethers for selective lithium-ion capture

Synthesis, Isolation, and Study of Heterobimetallic Uranyl Crown Ether Complexes.

Although crown ethers can selectively bind many metal cations, little is known regarding the solution properties of crown ether complexes of the uranyl dication, UO22+. Here, the synthesis and characterization of isolable complexes in which the uranyl dication is bound in an 18-crown-6-like moiety are reported. A tailored macrocyclic ligand, templated with a Pt(II) center, captures UO22+ in the crown moiety, as demonstrated by results from single-crystal X-ray diffraction analysis. The U(V) oxidation state becomes accessible at a quite positive potential (E1/2) of -0.18 V vs Fc+/0 upon complexation, representing the most positive UVI/UV potential yet reported for the UO2n+ core. Isolation and characterization of the U(V) form of the crown complex are also reported here; there are no prior reports of reduced uranyl crown ether complexes, but U(V) is clearly stabilized by crown chelation. Joint computational studies show that the electronic structure of the U(V) form results in significant weakening of U-Ooxo bonding despite the quite positive reduction potential at which this species can be accessed, underscoring that crown-ligated uranyl species could demonstrate unique reactivity under only modestly reducing conditions.

Nitrogen-doped substrate material ion imprinting-capacitive deionization selective recovery of lithium ions from acidic solutions.

With the continuous development of global industry and the increasing demand for lithium resources, recycling valuable lithium from industrial solid waste is necessary for sustainable development and environmental friendliness. Herein, we employed ion imprinting and capacitive deionization to prepare a new electrode material for lithium-ion selective recovery. The material morphology and structure were characterized using scanning electron microscopy, Fourier-transform infrared spectroscopy, and other characterization methods, and the adsorption mechanism and water clusters were correlated using the density functional theory. The electrode material exhibited a maximum adsorption capacity of 36.94mg/g at a Li+ concentration of 600mg/L. The selective separation factors for Na+, K+, Mg2+, and Al3+ in complex solution environments were 2.07, 9.82, 1.80, and 8.45, respectively. After undergoing five regeneration cycles, the material retained 91.81% of the initial Li+ adsorption capacity. Meanwhile, the electrochemical adsorption capacity for Li+ was more than twice the corresponding conventional physical adsorption capacity because electrochemical adsorption provides the energy needed for deprotonation, enabling exposure of the cavities of the crown ether molecules to enrich the active sites. The proposed environment-friendly separation approach offers excellent selectivity for Li+ recovery and addresses the growing demand for Li+ resources.

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.