Ether Cavity Research Articles

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.

Alkali metal cation complexation by 1,3-alternate, mono-ionisable calix[4]arene-benzocrown-6 compounds

Alkali metal cation extraction behaviour for two series of 1,3-alternate, mono-ionisable calix[4]arene-benzocrown-6 compounds is examined. In Series 1, the proton-ionisable group (PIG) is a substituent on the benzo group of the polyether ring that directs it away from the crown ether cavity. In Series 2, the PIG is attached to one para position in the calixarene framework, thereby positioning it over the crown ether ring. Competitive solvent extraction of alkali metal cations from aqueous solutions into chloroform shows high Cs+ efficiency and selectivity. Single-species extraction pH profiles of Cs+ for Series 1 and 2 ligands with the same PIG are very similar. Thus, association of Cs+ with the calixcrown ring is more important than the position of the PIG relative to the crown ether cavity. Solid-state structures of two unionised ligands from Series 2 are presented. Also described is a crystal containing two different ionised ligand–Cs+ complexes.

Novel active carbon/crown ether derivative hybrid material for the selective removal of Cu(II) ions: The crucial role of the surface chemical functions

The selective elimination of metal ions from solution is a subject of large interest from both theoretical and practical points of view. For this purpose it is desirable to have solid materials having specific functions able of capturing the ions. This paper reports the preparation of a crown ether derivative/active carbon hybrid material and the application for the selective elimination of Cu(II). The hybrid material has been prepared by adsorption of the crown ether derivative N-2-(4-amino-1,6-dihydro-1-methyl-5-nitroso-6-oxopyrimidinyl)-15-crown-5 (HL) onto an activated carbon (Merck). This adsorption process is not dependent on the pH and it is highly irreversible due to strong π–π interactions between the arene centers of the activated carbon and the pyrimidine moiety of HL. This means that the pyrimidine groups of HL are blocked on the surface of the activated carbon due to the adsorption while the 15-crown-5 ether functions remain ready for coordination of metal ions. Thus, the Merck/HL hybrid has a much larger adsorption capacity of Cu(II) than this of the unfunctionalized activated carbon. In addition, the adsorption of Cu(II) ions in the presence of K(I) and Ca(II) is highly selective. It is shown that this selectivity for Cu(II) is related to the size of the 15-crown-5 ether cavity is very appropriate to house this ion. The desorption of Cu(II) from the loaded Merck/HL hybrid material in the presence of EDTA allows the regeneration of most of the actives sites of the hybrid due to the formation of the more stable Cu(II)–EDTA complex. This paper also reports the preparation of HL, its behavior in aqueous solution, its complexing behavior and its adsorption on the active carbon. These studies are the cornerstones to explain the behavior of the Merck/HL hybrid material in the adsorption of Cu(II).

Thermodynamics of alkali metal cation complexation by ionised calixcrown ligands

Binding of alkali metal cations (AMCs) by 1,3-alternate, ionised calix[4]arene-benzocrown-6 ligands was investigated by isothermal calorimetric titration in methanol. The groups of –C(O)NHSO2CH3 and more acidic –C(O)NHSO2CF3 were attached to the 1,3-alternate calix[4]arene-benzocrown-6 skeleton in two locations. In one series, the acidic group was attached to the para position of one aromatic ring in the calixarene framework, thereby positioning it over the polyether cavity. In the other series, the ionisable group was a substituent on the benzo group in the polyether ring. This oriented the acidic group away from the crown ether cavity. For these calixcrown ligands, the effects of the location of the ionisable group and its acidity and the identity of the AMC on the binding constant, enthalpy and entropy of complexation are assessed.

Development of Chemically Bonded Crown Ether Stationary Phases in Capillary Ion Chromatography

Three types of crown ethers (i.e. 1-aza-15-crown-5-ether, 1-aza-18-crown-6-ether and 2-aminomethyl-18-crown-6-ether) were chemically bonded to silica gels via a simple 2-step reaction and the resulted stationary phases were evaluated for the separation of both anions and cations in capillary ion chromatography. It was found that the chemically bonded stationary phases were most stable and relatively sharper peaks were obtained when a mixture of toluene:acetonitrile = 7:3 was used as the reaction solvent in the second step reaction. The size of crown ether cavity as well as the presence of nitrogen atom within the cavity affected the retention behaviors of the ions. As for the separation of anions, it was found that the eluent cations were trapped on the stationary phases and acted as the anion-exchange site, viz. the anions were separated in the ion-exchange mode. Without the obstruction of nitrogen atom, 2-aminomethyl-18-crown-6-ether could easily form complexes with metal cations, and the retention as well as elution order were dependant on the complex formation constant between crown ether and the metal cations. Eluent with higher pH favored the retention and separation of the metal cations.

Lipophilic, Mono-ionizable, Calix[4]arene-bis(benzocrown-6) Compounds for Solvent Extraction of Cesium from Nuclear Wastes: Synthesis and Evaluation

As potential radiocesium extractants from nuclear waste solutions, a series of lipophilic, proton-ionizable calix[4]arene-bis(benzocrown-6) compounds with their calixarene units locked in the 1,3-alternate conformation was prepared. In these ligands, the acidic side arm is positioned over one crown ether cavity to enhance metal ion extraction. Upon ionization of the acidic group, the ligand provides the requisite anion for electroneutral extraction complex formation. To enhance the lipophilicity of the extractants, a 2-ethylhexyl group was attached to the benzo group of each crown ether fragment. Using radiotracer techniques and ICAP spectrometry, the ligands were evaluated in terms of the efficiency and selectivity with which they extract Cs+ from aqueous solutions into chloroform.

Synthesis and application of novel magnetite nanoparticle based azacrown ether for protein recognition

This article is the first report showing the human serum albumin (HSA) binding studies by using a new magnetite nanoparticle containing azacrown ether moieties via solid-liquid extraction process. The structure of the newly prepared magnetite nanoparticle was clarified by using attenuated total reflectance (ATR) infrared spectro- scopy, scanning electron microscopy (SEM), UV-vis and elemental analysis. Analytic results indicated that modifi- cation of the surface of the magnetite nanoparticles with azacrown ether derivative was successfully carried out. The protein binding studies exhibited that the modified magnetite nanoparticles could be efficiently used for the binding of the human serum albumin (HSA) in aqueous solutions via non-covalent interaction between crown ether cavity and amino group of the protein.

ChemInform Abstract: Crown and Lariat Ethers

AbstractReview: 103 refs.

Electrochemical investigation of nano-baskets of calix[4]-1,3-crowns-5,6 complexes

Four nano-baskets of calixarene including cone 25,27-di(carboxymethoxy)calix[4]arene-crown-5, 1,3-alternate 25,27-di[carboxymethoxy]-calix[4]arene-crown-5, cone 25,27-bis[carboxymethoxy]calix[4]arene-crown-6 and 1,3-alternate 25,27-di[carboxymethoxy]-calix[4]arene-crown-6 were synthesized and their binding abilities towards alkali and alkaline earth metals as well as some lanthanides were studied using differential pulse voltammetry. The novelty of this study was investigation of those macrocyclic complexes by voltammetric behaviors of two acidic moieties in each scaffold during complexation of crown ether ring. Their voltammetric behaviors were closely related to the complex formation by entrapment of cation into crown ether cavity and ion–dipole interaction between cation and acidic moieties in calixcrowns. The results revealed the selective changes in voltammetric behavior of synthesized scaffolds toward the cations. Moreover, the position of crown ether in 1,3-alternate instead of cone enhanced the domain of binding ability to more cations. Furthermore, it was shown that those carboxylic acid moieties, which were far from the crown ether ring in the 1,3-alternate, did not affected by encapsulated cations in the coordination space of crown ether and showed no voltammetric behavior.

Voltammetric Study of Nano-baskets of Calix[4]-1,3-crowns-5, -crowns-6 Complexes

Four nano-baskets of calixarene including di(carboxymethoxy)calix[4]arene-crown-5 and -crown-6 in both cone and 1,3-alternate conformations, were synthesized and their binding abilities towards alkali and alkaline earth metals and some lanthanides were studied using differential pulse voltammetry. The novelty of this study is investigation of calixarene complexes by voltammetric behaviors of two acidic moieties during complexation with crown ether ring. Their voltammetric behaviors not only were related to complex formation (by entrapment of cation into crown ether cavity) but also were controlled by ion–dipole interaction between cation and carboxylic acid moieties. The results revealed the selective changes in voltammetric behavior of synthesized scaffolds toward cations. Moreover, the position of crown ether in 1,3-alternate conformations enhanced the domain of binding ability to more cations. Furthermore, carboxylic acid moieties, which were far from the crown ether ring in 1,3-alternate, were not affected by encapsulated cations in the coordination space of crown ether and hence, they showed no voltammetric behavior.

Acyclic Palladium(II)‐N‐heterocyclic Carbene Metallacrown Ether Complexes: Synthesis, Structure and Catalytic Activity

AbstractNovel acyclic Pd(II)‐N‐heterocyclic carbene (NHC) metallacrown ethers 5a, 5b have been synthesized. Reaction of the imidazolium salts bearing a long polyether chain with Ag2O afforded Ag‐NHC complexes, which then reacted as carbene transfer agent with PdCl2(MeCN)2 to give the desired acyclic Pd(II)‐NHC metallacrown ether complexes 5a and 5b. The 1H NMR and 13C NMR spectra show 5a and 5b exist as mixtures of cis and trans isomers in solution. The trans isomer of 5a was characterized by X‐ray diffraction, which clearly demonstrated two pseudo‐crown ether cavities in trans‐5a. Pd(II)‐NHC complexes 5a and 5b have been shown to be highly effective in the Suzuki‐Miyaura reactions of a variety of aryl bromides in neat water without the need of inert gas protection.

Response of Crown Ether Functionalized Polythiophenes to Alkaline Ions

The sensing response of 15-crown-5-ether functionalized polythiophene to Li+, Na+, and K+ has been investigated at the atomistic level using molecular dynamics simulations. The stability associated with all the identified binding sites has been corroborated by quantum mechanical calculations. Although the cavity of the macrocycle is not the most visited binding site, such receptor is responsible of the selective sensing response of this polythiophene derivative. PF6(-) counterions reduce the mobility of the alkaline cations, which do not occupy the crown ether cavity of consecutive repeating units due to electrostatic repulsions. Furthermore, the relative entropy for the “free state → bound state” has been estimated using a procedure based on the covariance matrix atom-positional fluctuations. Evaluation of the entropic contributions allow us to complete the thermodynamics scenario of binding process, which was recently initiated by calculating the enthalpies at quantum mechanical level [ Chem. Eur. J. 2009, 15, 4676]. Results indicate an entropycally driven binding preference.

Mono- and ditopic models of binding of a photochromic chromene annelated with an 18-crown-6ether with protonated amino acids

In this work, the interaction of protonated amino acids with a chromene bearing a fused 18-crown-6 ether moiety was studied by UV-vis and NMR spectroscopy. Initial closed forms of the chromene form monotopic 1 : 1 complexes, the ammonium group being localized inside the crown ether cavity. UV-irradiation leads to transformation of the ring-closed species into the ring-opened form. Depending on the amino acid length, either ditopic or monotopic 1 : 1 complexes are formed. Such complexes are stabilized by the additional H-bonding between the carboxylic group of the acid and the carbonyl oxygen atom of the ring-opened form. Cessation of the irradiation results in ring-closure to the chromene with concomitant change of the complexation mode.

Binding Study of Ionizable Calix[4]-1,3-Crowns-5,6 Nano-baskets by Differential Pulse Voltammetry

Four proton di-ionizable diacid conformers of cone 25,27-di(carboxymethoxy)calix[4]arene-crown-5, 1,3-alternate 25,27-di[carboxymethoxy]-calix[4]arene-crown-5, cone 25,27-bis[carboxymethoxy]calix[4]arene-crown-6 and 1,3-alternate 25,27-di[carboxymethoxy]-calix[4]arene-crown-6 were synthesized and their binding abilities toward alkali and alkaline earth metals as well as some lanthanides were studied by means of differential pulse voltammetry. The novelty of this study was investigation of those nano-baskets by voltammetric behavior of two acidic moieties in each scaffold during complexation of the crown ether ring. Their voltammetric behavior was closely related to the complex formation by entrapment of the cation into the crown ether cavity and ion-dipole interaction between cation and acidic moieties in calixcrowns. The results revealed the selective changes in voltammetric behavior of synthesized scaffolds toward the cations. Moreover, the position of crown ether in 1,3-alternate instead of cone enhanced the domain of binding ability to more cations. Furthermore, it was shown that those carboxylic acid moieties, which were far from the crown ether ring in the 1,3-alternate, were not affected by encapsulated cations in the coordination space of crown ether and showed no voltammetric response.

Binding Survey of Ionizable Calix[4]‐1,2‐crown‐3 Nano‐baskets by Differential Pulse Voltammetry

AbstractThree nano‐baskets of p‐tert‐butylcalix[4]arene‐1,2‐crown‐3 were synthesized and their binding abilities towards alkali and alkaline earth metals as well as some lanthanides were studied using differential pulse voltammetry. The novelty of this study was the investigation of macrocyclic complexes by voltammetric behaviors of two acidic moieties in each scaffold. Their voltammetric behaviors were closely related to the complex formation by entrapment of cation into crown ether cavity and ion‐dipole interaction between cation and acidic moieties in calixcrowns. The results revealed that the sulfur atom in the crown ring as well as the position of crown‐3 ether in 1,2‐alternate instead of cone conformer changed the binding ability of the scaffold to more cations.

Intraparticle Charge Delocalization of Carbene-Functionalized Ruthenium Nanoparticles Manipulated by Selective Ion Binding

Olefin metathesis reactions of carbene-stabilized ruthenium nanoparticles were exploited for the incorporation of multiple functional moieties onto the nanoparticle surface. When the nanoparticles were cofunctionalized with 4-vinylbenzo-18-crown-6 and 1-vinylpyrene, the resulting particles exhibited fluorescence characteristics that were consistent with dimeric pyrene with a conjugated chemical bridge, with three peaks observed in the emission spectra at 391, 410, and 485 nm. The behaviors were ascribed to intraparticle charge delocalization between the pyrene moieties afforded by the conjugated Ru═carbene interfacial linkages. Notably, upon the binding of metal ions in the crown ether cavity, the emission intensity of the nanoparticle fluorescence was found to diminish at 485 nm and concurrently increase at 391 and 410 nm rather markedly, with the most significant effects observed with K(+). This was accounted for by the selective binding of 18-crown-6 to potassium ions, where the positively charged ions led to the polarization of the nanoparticle core electrons that was facililated by the conjugated linkage to the metal surface and hence impeded intraparticle charge delocalization. Control experiments with a pyrene-crown ether conjugate (2) and with ruthenium nanoparticles cofunctionalized with 4-vinylbenzo-18-crown-6 and 1-allylpyrene suggested that the through-bond pathway played a predominant role in the manipulation of intraparticle electronic communication whereas the contributions from simple electrostatic interactions (i.e., through-space pathway) were minimal.

Crown Ether Cavity-Containing Copolymers via Controlled Alternating Cyclocopolymerization

The cyclocopolymerization of difunctional styrenic flexible monomers with maleic anhydride via reversible addition–fragmentation chain transfer (RAFT) has been investigated. The reaction proceeded to yield gel-free copolymers soluble in organic solvents. The kinetic study indicated that the cyclocopolymerization of difunctional styrenic flexible monomer with maleic anhydride proceeded in a controlled manner. The structures of resulting crown ether cavity-containing copolymers were characterized by the use of NMR spectroscopy and MALDI-TOF mass spectrometry. The crown ether cavities in copolymers exhibited a selective recognition for certain dialkylammonium ions.

Lower-rim versus upper-rim functionalization in di-ionizable calix[4]arene-crown-5 isomers. Synthesis and divalent metal ion extraction

Two series of di-ionizable calix[4]arene-crown-5 isomers in the cone conformation are synthesized to probe the effect of the pendant acidic group location on their metal ion extraction properties. In one series, the ionizable groups are attached to the lower rim of the calix[4]arene scaffold, which orients them near the crown ether cavity. In the second series, the ionizable groups are connected to the upper rim positioning them away from the crown ether ring, but close to the hydrophobic pocket of the calix[4]arene unit. The metal ion extraction behaviors of the two series of ligands are compared in extractions of alkaline earth metal cations, Hg 2+, and Pb 2+.

Chirality‐Sensing Binaphthocrown Ether–Polythiophene Conjugate

An enantiomeric binaphthyl unit was tethered to adjacent thiophenes with oxyethylene linkers to give a chiral polythiophene with binaphthocrown ether cavities. Upon inclusion of a chiral cationic guest in the cavity of the chiral crown ether-polythiophene conjugate, the bithiophene unit was twisted to shorten the effective conjugation length of polythiophene backbone, enabling us to sense the guest binding by reading out the amplified optical signal gains arising from the backbone structure change. This strategy allowed us to discriminate the guest chirality without using chiroptical signals or a circular dichroism spectrometer to achieve the highest enantioselectivity of 7.3 for valine methyl ester with a 40-fold enhanced sensitivity compared with the corresponding monomeric sensor.