Large Crown Ethers Research Articles

Molecular dynamics simulation of the fractionation of lithium and magnesium-ions on crown ether-grafted cellulose acetate polyelectrolyte membranes

Considerable amounts of lithium and magnesium are present in the sea. When properly harnessed, they could complement the existing extraction technologies and serve as a sustainable source for meeting up the rapidly growing demands. In the present study, we report using classical molecular dynamics simulations the formulation of new polyelectrolyte membranes comprising of cellulose acetate (CA) backbone and severally grafted crown ether macrocyclic pendant groups for the segregation and recovery of Li+ and Mg2+ ions. Highly compact hydrophilic polymeric membranes stabilized by both intra- and inter-molecular hydrogen bonds were successfully achieved with sufficient fractional free volumes for the diffusion of the ions. The mean square displacement of the ions revealed that Li+ ions exhibit superior diffusion on the 12-crown-4 grafted CA membrane, while the 15-crown-5 and 18-crown-6 grafted systems demonstrated stronger attraction for Mg2+ ions raising the diffusion energy barrier. The analysis of the interference of co-existing counterions in binary solutions revealed that the electronegative oxygen atoms on the macrocyclic rings on 12-crown-4 grafted CA consistently shielded the interfering ions resulting in the rapid diffusion of Li+ ions. The larger crown ethers in contrast exerted a minimal attraction on the divalent Ca2+ ions, enabling the diffusion of Mg2+ ions. This study demonstrates the inherent potential of the crown ether-grafted polymeric membranes in the recovery of lithium and magnesium from seawater, a prospect that could be further explored in the stride toward effective brine mining.

Gas-phase organocatalysis with crown ethers

Catalytic cycles can be investigated in detail in the gas phase in a step-by-step manner by tandem mass spectrometric experiments that allow for the mass-selection of each of the intermediates prior to the subsequent step. In the present article, we describe large crown ethers (dibenzo-24-crown-8 to dibenzo-30-crown-10) to mediate the E2-elimination of propene from crown ether-propylammonium complexes giving rise to NH4+/crown ether complexes. A back exchange of the ammonia against propyl amine completes the catalytic cycle. To the best of our knowledge, this reaction cycle represents the first example of organocatalysis in the gas-phase. In addition, the larger crown ethers significantly accelerate the H/D-exchange reaction of the five NH hydrogen atoms in singly protonated ethylenediamine as compared to the analogous smaller crown ether complexes. This effect, which is not observed for the propylammonium complexes, can be rationalized by a favorable pre-organization of the protonated ethylenediamine ion within the cavity of the crown ether through hydrogen bonding to both the ammonium and the amine groups. Within this complex, the H/D-exchange can proceed through an efficient “relay” mechanism. A comparison of the H/D-exchange behavior of ammonium complexes with differently sized crown ethers reveals a distinct reactivity pattern that strongly depends on the crown ether size. While 18-crown-6 acts as a non-covalent protective group suppressing the H/D-exchange almost completely, smaller and larger crown ethers exhibit higher exchange rates.

Complexes of large crown ethers with the lithium cation studied by electrospray ionization mass spectrometry

Complexes of large crown ethers with the lithium cation studied by electrospray ionization mass spectrometry

Regioselective synthesis of syn disubstituted dibenzo-30-crown-10 ethers

A practical and regioselective synthetic method for the synthesis of syn substituted dibenzo-30-crown-10 ethers is reported. This novel methodology is reported with the syntheses of dibenzo crown ethers bearing nitro, formyl and carbomethoxy groups. The synthesis of macrocyclization precursors was accomplished in three steps and featured an application of para-methoxybenzyl group (PMB) as protecting group of phenol moiety that is orthogonal to NO 2, CHO and COOMe groups. General non-high dilution macrocyclization conditions have been developed that allow for the effective preparation of substituted large crown ethers.

Unusual complex between 18-Crown-6 and tetramethylammonium cation—detection by electrospray ionization mass spectrometry

Complexes between crown ethers and quaternary ammonium cations have been studied by electrospray ionisation mass spectrometry (ESI-MS). The ESI-MS method has been shown to allow observation of not only stable inclusion complexes between large crown ethers and tetramethylammonium cation (e.g. [DB30C10 + (CH3)4N]+ ion) but also of unstable inclusion complexes between smaller crown ethers and quaternary ammonium cations which are difficult to observe by other methods, namely [18C6 + (CH3)4N]+ ion. Stability of the complexes between crown ethers containing aromatic ring and tetramethylammonium cation is enhanced by cation-Π interactions. The molecule of 18C6 does not contain aromatic rings, thus [18C6 + (CH3)4N]+ ion exists due to the formation of C–H···O hydrogen bonds. Such a complex is quite unusual, since C–H···O hydrogen bonds are very weak and usually coexist with other strong interactions.

Formation of stoichiometric complexes between dibenzo-30-crown-10 and guanidinium moiety containing compounds

Inclusion complexes between protonated guanidinium moiety and a large crown ether, dibenzo-30-crown-10, have been studied by electrospray ionization mass spectrometry (ESI-MS). In order to determine which solvent is the most suitable for generation of these complexes, methanol, water, acetonitrile and their mixtures (most common solvents for ESI) have been tested. Acetonitrile/water (1/1) has been found the best choice. In acetonitrile/water solution, the formation of stoichiometric complexes has been established (stoichiometric means that a peptide attaches the number of crown ether molecules equal to the number of guanidinium moieties it contains). For example, for the peptide containing three arginine residues the peak of the respective complex with three crown ether molecules is clearly seen.

Resorcinarenes as Templates

AbstractFor Abstract see ChemInform Abstract in Full Text.

Spectroscopic properties of uranyl crown ether complexes in non-aqueous solvents

The spectroscopic properties of the complexes of uranyl with oxacrowns, azacrowns, thiacrowns of different ring size, polyethyleneglycols (PEG-200, PEG-300, PEG-400), and [2.2.1] and [2.2.2] cryptands were studied in acetonitrile and propylene carbonate solutions by absorption and luminescence spectroscopy. Depending on the ligand, marked differences in the fine structure of the spectra were observed. The crown ethers with six oxygen atoms (18-crown-6, dicyclohexano-18-crown-6, benzo-18-crown-6, dibenzo-18-crown-6 and [2,4]-dibenzo-18-crown-6) form inclusion complexes with the uranyl ion in the central cavity. In the case of 18-crown-6 and dibenzo-18-crown-6, a complex with a high coordination symmetry is formed. Outer-sphere complexes are formed with the crown ethers with small ring size (12-crown-4 and 15-crown-5) and with PEG-200. Only minor changes in the spectra were observed when changing the ligand. The spectra of the large crown ethers ([3,4]-dibenzo-21-crown-7, dibenzo-24-crown-8 and dibenzo-30-crown-10), together with those of PEG-300 and PEG-400 show that these ligands coordinate in the equatorial plane to the uranyl ion. In the presence of competing inorganic ligands like chloride and nitrate ions, the crown ether is replaced in the first coordination sphere. Azacrowns and cryptands coordinate externally to the uranyl ion, and the corresponding spectra do not show fine structure. In the case of the thiacrowns, no coordination of the ligand to the uranyl ion was observed.

Resorcinarenes as templates.

Metal ions are superb at templating the synthesis of small macrocycles that are decorated with Lewis basic sites. However, for the synthesis of larger macrocycles, or the synthesis of macrocycles devoid of an array of Lewis basic sites, metal ions are less useful. Here we demonstrate that resorcinarenes can be used as templates to engender the efficient formation of large crown ethers. A three step process of 1). tethering moieties to the template, 2). linking those moieties, and 3). then cleaving off the template leads to the efficient formation of a family of aromatic crown ethers. If adaptable, this approach will prove useful for the construction of macrocycles that are hard to obtain from a step-wise synthesis.

Solubility of large crown ethers in supercritical carbon dioxide

The equilibrium solubilities of dibenzo-24-crown-8 (DB24C8), dibenzo-27-crown-9 (DB27C9), and dibenzo-30-crown-10 (DB30C10) have been measured in supercritical carbon dioxide. The measurements were performed in the pressure range from 120 to 440 atm at temperatures 35, 45, 55 and 65°C for DB24C8 and in the pressure range from 120 to 480 atm at temperatures 35, 45, 55, 65 and 75°C for DB27C9 and DB30C10. The solubility of the crown ethers in supercritical carbon dioxide was found to vary in the order DB24C8>DB27C9⪢DB30C10. The measured solubilities were correlated using a semi-empirical model. The calculated results show satisfactory agreement with the experimental data.

Silver complexes with large crown ethers in propylene carbonate

The complex formation between large crown ethers – e.g. dibenzo-30-crown-10 – and Ag + in propylene carbonate has been studied using potentiometric and calorimetric titrations. The values of the reaction enthalpies with larger crown ethers than 18-crown-6 especially with dibenzo-30-crown-10 indicate that more than six donor atoms are taking part in the interaction with the complexed cation. The values of the reaction entropies of the largest crown ethers examined indicate the high sterical requirements of the complex formation. Obviously large crown ethers are able to encapsulate the complexed cation. The effectiveness of the encapsulation however depends upon the ring size and flexibility of the crown ethers.

Crown ether complexation of p-block metal halides: synthesis and structural characterisation of [InI2(dibenzo-24-crown-8)(H2O)][InI4], [(SnBr4)2(dibenzo-24-crown-8)]·MeCN, [(SbCl3)2(dibenzo-24-crown-8)]·MeCN, [(BiCl3)2(dibenzo-24-crown-8)]·MeCN and [(SbBr3)2(dibenzo-24-crown-8)

The reactions of the large crown ether 6,7,9,10,12,13,20,21,23,24,26,27-dodecahydrodibenzo[b,n][1,4,7,10,13,16,19,22]octaoxycyclotetracosine (dibenzo-24-crown-8) with p-block metal halides have been studied, leading to the isolation and structural characterisation of [InI2(dibenzo-24-crown-8)(H2O)][InI4] (I), [(SnBr4)2(dibenzo-24-crown-8)]·MeCN (II), [(SbCl3)2(dibenzo-24-crown-8)] (III), [(BiCl3)2(dibenzo-24-crown-8)]·MeCN (IV) and [(SbBr3)2(dibenzo-24-crown-8)]·MeCN (V). Compound I was isolated as colourless block crystals; crystals are triclinic, space group P1̄, Z=2, R=0.0372. The structure consists of [InI2(dibenzo-24-crown-8)(H2O)]+ cations and [InI4]− anions. For the cation, the threaded near-linear InI2+ unit (In-I 2.6977(5), 2.6919(5) Å) is located off-centre within the cavity and is bonded to four ring oxygen atoms (In-Ocrown 2.344(4)–2.560(4) Å) and a single water molecule (In-Owater 2.271(4) Å). The latter forms intracavity hydrogen bonds of the type M-OH2⋯Ocrown to two more of the oxygen atoms of the crown (Owater⋯Ocrown 2.89, 2.94 Å). Compound II was isolated as orange block crystals; crystals are monoclinic, space group I2/m, Z=2, R=0.0391. The structure features the two metal centres in exocyclic locations each bound to two oxygens of the ring (Sn-Ocrown 2.262(3) Å). The metal geometry is distorted octahedral (Sn-Br 2.5235(7), 2.5187(6) Å). Compound III was isolated as colourless block crystals; crystals are monoclinic, space group C2/c, Z=4, R=0.0311. The two SbCl3 moieties (Sb-Cl 2.3807(8), 2.3874(8), 2.4015(9) Å) are coordinated to opposite sides of the crown which adopts a sigmoidal conformation to create two O5 donor cavities. Each Sb(III) centre is bound to five oxygens of the ring (Sb-Ocrown 2.852(2)–3.153(2) Å). Crystals of IV are monoclinic, space group C2/c, Z=4, R=0.0325. The structure is directly similar to that of III with two BiCl3 units (Bi-Cl 2.5034(12), 2.5054(13), 2.5389(12) Å) coordinated to opposite sides of the ring which exhibits a sigmoidal conformation. Each Bi(III) centre is bonded to five crown oxygen atoms (Bi-Ocrown 2.733(3)–3.221(4) Å). Compound V crystallises in the triclinic space group P1̄, Z=2, R=0.0693. The two SbBr3 groups are located on opposite sides of the ring (Sb-Br 2.5268(15), 2.5478(15), 2.5558(14) Å) and each one is bound to four of the ring oxygen atoms (Sb-Ocrown 2.893(7)–3.183(8) Å).

Novel dual-host approach in ion pair extraction: a simple tripodal nitrate host facilitates CsNO3 transfer to 1,2-dichloroethane by a large crown ether

The readily available benzene-1,3,5-tricarboxamide 1 enhances CsNO3 extraction by tetrabenzo-24-crown-8 via hydrogen bonding to the nitrate anion, as demonstrated by NMR and electrospray ionization mass spectrometry.

Conductance Study of Binding of Some Rb+ and Cs+ Ions by Macrocyclic Polyethers in Acetonitrile Solution

A conductance study of the interaction between Rb+ and Cs+ ions and18-crown-6 (18C6), dicyclohexyl-18-crown-6 (DC18C6), dibenzo-18-crown-6 (DB18C6),dibenzo-24-crown-8 (DB24C8), and dibenzo-30-crown-10 (DB30C10) inacetonitrile solution has been carried out at various temperatures. The formationconstants of the resulting 1:1 complexes were determined from the molarconductance-mole ratio data and found to vary in the orderDC18C6 > 18C6 > DB30C10 > DB18C6 ∼ DB24C8for Rb+ ion andDC18C6 > 18C6 > DB30C10 ∼ DB24C8 > DB18C6for Cs+ ion. The enthalpy and entropy of complexation were determined fromthe temperature dependence of the formation constants. The complexes with the18-crowns are both enthalpy and entropy stabilized while, in the case of largecrown ethers, the corresponding complexes are enthalpy stabilized but entropydestabilized.

Polarographic Study of Thallium(I) Complexes with Large Crown Ethers in Binary Acetonitrile‐Water Mixtures

AbstractThe complexation reactions between the Tl+ ion and large crown ethers dibenzo‐30‐crown‐10 (DB30C10), dibenzo‐27‐crown‐9 (DB27C9), dibenzo‐24‐crown‐8 (DB24C8) and dibenzo‐21‐crown‐7 (DB21C7) were studied in different acetonitrile‐water mixtures at 25°C using an a.c. polarographic technique. The stoichiometry and stability of the complexes were determined by monitoring the shift in peak potential of the polarographic waves of the metal ion against the crown concentration. In all solvent mixtures used, the stability of the resulting 1:1 complexes was found to vary in the order DC24C8 » DB30C10 > DB21C7 > DB27C9 > DB24C8. There is an inverse relationship between the complex formation constants and the amount of water in the mixed solvent. In all cases, a linear relation was observed between log Kf and the mole fraction of acetonitrile in its mixtures with water.

Synthesis of poly[(styrene)- rotaxa-(crown ether)]s via free radical polymerization

Poly[(styrene)- rotaxa-(crown ether)]s were synthesized by free radical polymerizations of styrene in the presence of crown ethers using bulky `blocking group' initiators to ensure end-capping for prevention of dethreading. These statistical processes produced only low threading yields, but up to 26% cyclic by mass with large crown ethers. The poly[(styrene)- rotaxa-(crown ether)]s `emulsified' in protic solvents such as methanol and water. The dispersion polymerization of styrene in methanol or water using `42-crown-14' (`42C14') indicated that threading provided stability of the polyrotaxane micelles in such solvents through hydrophilic–hydrophobic phase separation. DSC investigations indicated that the threading prevents the crystallization of the crown ether and allows phase mixing to some extent, since two altered T g's were observed.

Synthesis of 4′-Carboxy-benzo-30-crown-10 and a Thermodynamic Study of Its Complexes with Thallium and Alkali Cations in Acetonitrile Solution

A synthetic procedure has been developed for the preparation of 4‘-carboxy-benzo-30-crown-10. The formation of Na+, K+, Rb+, Cs+ and Tl+ complexes with the large crown ether was investigated conductometrically in acetonitrile solution at various temperatures. The formation constants of the resulting 1:1 complexes were determined from the molar conductance-mole ratio data. It was found that the stability of the complexes vary in the order Tl+ > K+ > Rb+ > Cs+ > Na+. The data obtained in this study support the existence of a ‘wrap around’ structure for the above complexes in solution. The enthalpy and entropy of complexation reactions were determined from the temperature dependence of the formation constants. In all cases, the complexes were enthalpy stabilized but entropy destabilized. The resulting TΔS° vs. ΔH°plot showed a fairly good linear correlation, indicating the existence of an entropy-enthalpy compensation in the large crown complexation reactions.

X-ray Crystallographic Studies on the Noncovalent Syntheses of Supermolecules

An approach to the supramolecular syntheses of discrete multicomponent aggregates of noncovalently bound molecules, i.e., supermolecules, is described. This approach involved the careful analysis of X-ray crystal structures so as to permit a gradual increase in superstructural complexity. Many elaborate supermolecules were synthesized noncovalently from dialkylammonium-containing cations and crown ethers, following the initial observation that the dibenzylammonium ion threads through dibenzo[24]crown-8 to generate a singly stranded, singly encircled [2]pseudorotaxane, principally as a result of \([{\text{N}}^{\text{ + }} - {\text{H}} \cdot \cdot \cdot {\text{O}}]\) and \([{\text{C}} - {\text{H}} \cdot \cdot \cdot {\text{O}}]\) hydrogen bond formation. The scope of the fundamental recognition motif obtained from this initial observation was then broadened, through the use of thread-like ions with multiple dialkylammonium centers and/or larger crown ethers, so that multiply stranded and/or multiply encircled pseudorotaxanes could be prepared. Cations bearing both dialkylammonium and crown ether recognition sites were also used for the nocovalent synthesis of a discrete daisy chain supermacrocycle and the basic recognition motif was combined with other motifs for the production of a wide range of novel superarchitectures. As a greater understanding of the noncovalent interactions governing the self-assembly of the complex superarchitectures was acquired, new protocols for the noncovalent syntheses of doubly docked pseudorotaxanes and interwoven supramolecular bundles, including a supramolecular analogue of the photosynthetic special pair, were developed. The discovery that anions can play a prominent role in the solid-state self-assembly of some of the supermolecules was a valuable spinoff of the research.

Chain folding in cyclic oligo(oxyethylene)s Crystallinity of large unsubstituted crown ethers by X-ray scattering and differential scanning calorimetry

Uniform cyclic oligo(oxyethylene)s with chain lengths E15 to E18 (unsubstituted 45-crown-15 to 54-crown-18) have been prepared by reaction of corresponding linear glycols with tosyl chloride under alkaline conditions, purified by preparative gel permeation chromatography (GPC), and characterised by analytical GPC, NMR spectroscopy, and other methods. Wide-angle and small-angle X-ray scattering were used to investigate crystal structure and layer spacing. Comparison was made with scattering from linear oligomers crystallised in unfolded conformation. It was found that cyclo-E18 crystallised into the poly(oxyethylene) structure (monoclinic sub-cell), while the lower oligomers crystallised into different structures. In confirmation of this result, the layer spacing of cyclo-E18 was that expected for the twice-folded conformation. Melting temperatures and enthalpies of fusion of cyclo-E18 and cyclo-E27 (from previous work), measured by differential scanning calorimetry, were compared with those of linear oligomers to obtain values of the enthalpy and entropy of fold formation in the crystallisation of uniform oligo(oxyethylene).

Crown ether molecular complexes with urea and thiourea

Crystalline complexes of urea and thiourea with crown ethers, have been prepared, viz., 18-crown-6 (18C6), benzo-18-crown-6 (B18C6), dibenzo-18-crown-6 (DB18C6), dicyclohexano-18-crown-6 (DC 18C6) and dibenzo-24-crown-8 (DB24C8). While the complexes of the large ring size crown ether, DB24C8, have high ether to (thio)urea ratios, the stoichiometry of the others lies between one molecule of crown ether and from one to six molecules of (thio)urea. An IR spectral study of the urea and thiourea complexes showed that the behavior of thiourea in these complexes is clearly different from that of urea, indicating the role of sulphur in the interaction of thiourea with crown ethers. The urea and thiourea complexes were classified according to their stoichiometries and their IR spectral behavior into three classes for which credible structures were proposed.