Crown Ethers for Chemical Analysis: A Review

Three series of heterocycle substituted calixarenes, derivatized at lower and upper rim, were synthesized and successfully evaluated for metal extraction towards alkali, alkaline, transition and heavy metal ions. The presence and placement of sulfur, heterocycle functionality at upper/lower rim played a crucial role toward the extractability and selectivity of metal ions. The lower rim substituted calixarenes have shown high extractability and poor selectivity. In contrast to this, upper rim substituted calixarenes exhibited good selectivity. Moreover, sulfur functionalized calixarenes have shown better selectivity for heavy metal ions than alkali and alkaline metal ions. Among upper rim substituted calixarenes, 17 and 18 were found to be suitable for Na+, K+ and Ag+, 19,13 for heavy metal ions i.e., Pb2+, Hg+, Hg2+ and Ag+, and 11,12 for Pb2+ and Ag+ only.

The formation constants (KML+) for 1: 1 complexes of dibenzo-24-crown-8 (DB24C8) with alkali metal ions (Na+, K+, Rb+, and Cs+) in propylene carbonate (PC) and methanol have been determined conductometrically at different temperatures in order to obtain the values of the enthalpy (ΔH) and entropy changes (ΔS). The complexation reactions between DB24C8 and the alkali metal ions in PC and methanol are all exothermic, and the ΔS values of these systems are all negative. In the case of the PC system, there is no considerable difference in ΔS among the alkali metal ions (Na+, K+, Rb+, and Cs+); thus, the KML+ sequences depend entirely on the −ΔH sequences. Since, in methanol, the −ΔH and −ΔS values for K+ are nearly equal to those for Cs+, and since the largest −ΔH value for Rb+ is balanced by the largest −ΔS value, DB24C8 shows no selectivity for the alkali metal ions (K+, Rb+, and Cs+) in methanol.

The traditional approach to utilizing an ion-relay mechanism for ion transport requires three or more ion-relay stations. Herein, we describe a novel strategy, incorporating a swing action to reali...

Stabilities in nitromethane of alkali metal ion complexes with dibenzo-18-crown-6 and dibenzo-24-crown-8 and their transfer from nitromethane to other polar solvents

Cation binding by thallium(I) selective crown ethers

Voltammetric studies for cation recognition with thiacalix[4]crown-6s

The ion association potential of non-UV-absorbing crown ether [12-crown-4 (12C4), 15-crown-5 (15C5) and 18-crown-6 (18C6)]–metal complexes with picrate ion was established through the change in mobility of picrate ion by capillary zone electrophoresis. The apparent electrophoretic mobility of picrate ion was found to decrease considerably with increasing concentration of 18C6 in the migrating solution in the presence of alkali metal ion. However, on increasing the alkali metal concentration in the presence of 18C6, the apparent electrophoretic mobility of picrate ion also decreased at concentrations up to 15 mM and then started to increase gradually. The decrease was independent of the kind of counter anion of the alkali metal ion, and was attributed to the interaction between the cationic complex and the picrate ion, whereas the increase was attributed as being due to both concentrated salt effects and competitive ion association towards the cations between picrate ion and the counter anions. No remarkable change in the mobility of picrate ion was observed with cationic12C4 or 15C5, which is attributed to weak complexation capability. The ion association constants (KML·X) between the complexes and picrate ion were determined using such mobility changes. 18-Crown-6 was found to provide the best ion associability whereas the ion association constants for 12C4 and 15C5 metal complexes with picrate ion could not be determined. Ion association constants obtained in this study were compared with those of other crown ethers; they decreased in the order 18C6 < benzo-18-crown-6 (B18C6) < dibenzo-18-crown-6 (DB18C6) and hydrophobic interaction was suggested. In all cases, Na+ complexes showed higher ion associability than K+ complexes.

In this study, calix[4]arene derivatives (11–14) bearing a single nucleobase (adenine, thymine, cytosine or guanine) were synthesised via click chemistry. The complexation ability of the synthesised derivatives with alkali metal ions was measured using MALDI-TOF mass spectrometry, and their molecular assembly in CDCl3 was determined using 1H NMR. Calix[4]arene derivatives (11–14) formed 1:1 complexes with all alkali metal ions and the rank order for the complexation selectivity was Rb+ > Cs+ > K+ ≅ Na+ > Li+. The attachment of nucleobase at the upper rim of calix[4]arene had little effect on its complexation selectivity for alkali metal ions. Thymine-, adenine- and guanine-calix[4]arenes formed self-assembled structures in CDCl3 via base–base interactions. In addition, adenine-calix[4]arene (11) bound to thymine-calix[4]arene (12) to form a discrete species via Hoogsteen hydrogen bonding.

The utility of electrospray ionization mass spectrometry (ESI-MS) for characterizing dissolved metal species has generated considerable interest in the use of this technique for metal speciation. However, the development of accurate speciation methods based on ESI-MS requires a detailed understanding of the mechanisms by which dissolved metal species are ionized during electrospray. We report how the analysis of alkali and alkaline earth metal species provides new information about some of the processes that affect electrospray ion yield. Selected metal ions and organic ligands were combined in 50 : 50 water-acetonitrile buffered with acetic acid or ammonium acetate and analyzed by flow injection ESI-MS using mild electrospray conditions. Species formed by alkali metal ions with thiol and oxygen-donating ligands were detected in acidic and neutral pH solutions. Electrochemical oxidation of N, N-diethyldithiocarbamate and glutathione during electrospray was indicated by detection of the corresponding disulfides as protonated or alkali metal species. The extent of ligand oxidation depended on solution pH and the dissociation constant of the thiol group. Tandem mass spectrometric experiments suggested that radical cations such as [NaL](+.) (where L=N,N-diethyldithiocarbamate) can be generated by in-source fragmentation of disulfide species. Greater complexation of alkali metals at neutral pH was indicated by a corresponding decrease in the relative abundance of the free metal ion. The number of alkali metal ions bound by glutathione and phthalic acid also increased with increasing pH, in accordance with thermodynamic equilibrium theory. Alkaline earth metal species were detected only in acidic solutions, the absence of 8-hydroxyquinoline complexes being attributed to their relative instability and subsequent dissociation during electrospray. Hence, accurate speciation by ESI-MS depends on experimental conditions and the intrinsic properties of each analyte. Copyright 2000 John Wiley & Sons, Ltd.

In the present work, we describe four previously unreported cucurbituril‐based coordination supramolecular self‐assemblies, formed in the presence of alkali metal and cadmium ions. Unexpectedly, in the presence of potassium or rubidium and cadmium cations, cucurbituril prefers to coordinate with cadmium to form three dimensional frameworks. The alkali metal ions seem to play a role of structure inducer in HCl medium and are not present in the final structures. In neutral aqueous solution, cadmium cations also show stronger coordination to cucurbituril and form one‐dimensional supramolecular chains through static electronic interactions. When the alkali metal ion is sodium in HCl solution, sodium ions show stronger coordination to cucurbituril and form one‐dimensional coordination polymers. [CdCl4]2– anions link the linear polymers through static electronic interactions between coordinated chloride anions and sodium cations, which results in the formation of two‐dimensional networks.

The graphene quantum dot based fluorescent probe community needs unambiguous evidence about the control on the ion selectivity. In this paper, polyethylene glycol modified N-doped graphene quantum dots (PN-GQDs) were synthesized by alkylation reaction between graphene quantum dots and organic halides. We demonstrate the tunable selectivity and sensitivity by controlling the supramolecular recognition through the length and the end group size of the polyether chain on PN-GQDs. The relationship formulae between the selectivity/detection limit and polyether chains are experimentally deduced. The polyether chain length determines the interaction between the PN-GQDs and ions with different ratios of charge to radius, which in turn leads to a good selectivity control. Meanwhile the detection limit shows an exponential growth with the size of end groups of the polyether chain. The PN-GQDs can be used as ultrasensitive and selective fluorescent probes for Li(+), Na(+), K(+), Mg(2+), Ca(2+) and Sr(2+), respectively.

A physical model of selective “ion binding” in the L-type calcium channel is constructed, and consequences of the model are compared with experimental data. This reduced model treats only ions and the carboxylate oxygens of the EEEE locus explicitly and restricts interactions to hard-core repulsion and ion–ion and ion–dielectric electrostatic forces. The structural atoms provide a flexible environment for passing cations, thus resulting in a self-organized induced-fit model of the selectivity filter. Experimental conditions involving binary mixtures of alkali and/or alkaline earth metal ions are computed using equilibrium Monte Carlo simulations in the grand canonical ensemble. The model pore rejects alkali metal ions in the presence of biological concentrations of Ca2+ and predicts the blockade of alkali metal ion currents by micromolar Ca2+. Conductance patterns observed in varied mixtures containing Na+ and Li+, or Ba2+ and Ca2+, are predicted. Ca2+ is substantially more potent in blocking Na+ current than Ba2+. In apparent contrast to experiments using buffered Ca2+ solutions, the predicted potency of Ca2+ in blocking alkali metal ion currents depends on the species and concentration of the alkali metal ion, as is expected if these ions compete with Ca2+ for the pore. These experiments depend on the problematic estimation of Ca2+ activity in solutions buffered for Ca2+ and pH in a varying background of bulk salt. Simulations of Ca2+ distribution with the model pore bathed in solutions containing a varied amount of Li+ reveal a “barrier and well” pattern. The entry/exit barrier for Ca2+ is strongly modulated by the Li+ concentration of the bath, suggesting a physical explanation for observed kinetic phenomena. Our simulations show that the selectivity of L-type calcium channels can arise from an interplay of electrostatic and hard-core repulsion forces among ions and a few crucial channel atoms. The reduced system selects for the cation that delivers the largest charge in the smallest ion volume.

Crown ethers have been used for over twenty years as selective binding agents for alkali metal and alkaline earth ions. Although there have been many modifications to the crown ether core, none is so dramatic as to substitute heteroatoms into the positions normally occupied by carbon atoms. In this article we describe a new class of metal clusters which incorporate trivalent metal ions and nitrogen atoms into molecules that structurally and functionally mimic the behavior of crown ethers. These metallacrown ethers can be prepared in high yield in only a couple of steps and lead to molecules that may have utility in analytical chemistry, catalysis and new polymeric materials.

Complex formation reaction of some crown ethers with alkali metal ions was analyzed by measuring the electrophoretic mobility of analytes in capillary zone electrophoresis. For UV-absorbing crown ethers, the crown ethers were used as the analytes, whereas for less UV-absorbing crown ether, alkali metal ions were used as the analytes and detected indirectly. The apparent electrophoretic mobility of the analytes changed with increasing the concentrations of reacting agents in the migrating solution. The complex formation constants were obtained by analyzing the mobility change by a nonlinear least-squared method. Also ion association reaction of the metal complexes with some hydrophobic anions were analyzed. ©2000 John Wiley & Sons, Inc. J Micro Sep 12: 113–119, 2000

Cation-permselective membranes enabled by synergistic ion channels of crown ethers and anionic sites