Crown Ether Complexes Research Articles

Nontransition metal–crown ether complexes as hydroperoxide decomposition catalysts

The decomposition of tert-butyl hydroperoxide in a chlorobenzene medium in the presence of complexes of dibenzo-18-crown-6 with calcium, strontium, and barium chlorides has been studied. It has been found and kinetically proven that the decomposition of tert-butyl hydroperoxide is preceded by the formation of an intermediate hydroperoxide–catalyst complex. Kinetic and thermodynamic parameters of the complex formation have been determined.

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.

Ab initio molecular dynamics investigation of proton delocalization in crown ether complexes with H3O+ and NH4 +

The simple structure of crown ethers has found a wide range of applications in macrocyclic chemistry, largely due to its ability to capture positively charged ions either in atomic or molecular state. In the present work, ab initio molecular dynamics calculations are performed to characterize the host–guest interactions and conformational dynamics of the complexes formed by the H3O+ and NH4 + cations with the native crown ethers 15-crown-5 and 18-crown-6. A particular focus is set on the proton delocalization effects unveiled in recent spectroscopy experiments for some of these complexes under well-defined isolated (solvent-less) conditions. The calculations confirm that the infrared signature of these benchmark crown ether complexes provides a sensitive probe for proton delocalization. The ab initio study also shows that proton delocalization occurs for the H3O+ host (and not for NH4 +), provided that the size of the crown ring allows for tight H-bonding interactions between the cationic guest and the crown ether, leading short H2O···H+···O (crown) bonds.

Laser Spectroscopic Study of Cold Gas-Phase Host-Guest Complexes of Crown Ethers.

The structure, molecular recognition, and inclusion effect on the photophysics of guest species are investigated for neutral and ionic cold host-guest complexes of crown ethers (CEs) in the gas phase. Here, the cold neutral host-guest complexes are produced by a supersonic expansion technique and the cold ionic complexes are generated by the combination of electrospray ionization (ESI) and a cryogenically cooled ion trap. The host species are 3n-crown-n (3nCn; n = 4, 5, 6, 8) and (di)benzo-3n-crown-n ((D)B3nCn; n = 4, 5, 6, 8). For neutral guests, we have chosen water and aromatic molecules, such as phenol and benzenediols, and as ionic species we have chosen alkali-metal ions (M(+) ). The electronic spectra and isomer-specific vibrational spectra for the complexes are observed with various laser spectroscopic methods: laser-induced fluorescence (LIF); ultraviolet-ultraviolet hole-burning (UV-UV HB); and IR-UV double resonance (IR-UV DR) spectroscopy. The obtained spectra are analyzed with the aid of quantum chemical calculations. We will discuss how the host and guest species change their flexible structures for forming best-fit stable complexes (induced fitting) and what kinds of interactions are operating for the stabilization of the complexes. For the alkali metal ion•CE complexes, we investigate the solvation effect by attaching water molecules. In addition to the ground-state stabilization problem, we will show that the complexation leads to a drastic effect on the excited-state electronic structure and dynamics of the guest species, which we call a "cage-like effect".

Probing Supramolecular Interactions between a Crown Ether Appended Zinc Phthalocyanine and an Ammonium Group Appended to a C60 Derivative.

Self-assembly driven by crown ether complexation of zinc phthalocyanines equipped with one 18-crown-6 moiety and fullerenes bearing an ammonium head group afforded a novel donor-acceptor hybrid. In reference experiments, fullerenes containing a Boc-protected amine functionality have been probed. The circumvention of zinc phthalocyanine aggregation is important for the self-assembly, which required the addition of pyridine. From absorption and fluorescence titration assays, which provided sound and unambiguous evidence for mutual interactions between the electron donor and the electron acceptor within the hybrids, association constants in the order of 8.0×105 m-1 have been derived. The aforementioned is based on 1:1 stoichiometries, which have been independently confirmed by Job's plot measurements. In the excited state, which has been examined by transient absorption experiments, intermolecular charge separation evolves from the photoexcited zinc phthalocyanine to the fullerene subunit and leads to short-lived charge-separated states. Interestingly, photoexcitation of zinc phthalocyanine dimers/aggregates can also be followed by an intermolecular charge separation between vicinal phthalocyanines. These multicomponent supramolecular ensembles have also been shown by in-depth electrospray ionization mass spectrometry (ESI-MS) studies, giving rise to the formation and detection of a variety of non-covalently linked species.

Syntheses, structures and magnetic properties of a series of mono- and di-nuclear dysprosium(iii)-crown-ether complexes: effects of a weak ligand-field and flexible cyclic coordination modes

Six Dy(iii)-crown-ether complexes show the effects of coordination anions and geometries on slow magnetic relaxation behaviours.

Synthesis and Characterization of Some Crown Ether Complexes of Tl(I) Ion

The affinity of crown ethers for metal ion is strongly dependent on the size of the ring of the crown ether. The crown ether-metal ion binding is enhanced in the absence of non-coplanar donor oxygen atoms and electron withdrawing substituents in the crown ether skeleton. The study of absorption of radio frequency radiation by a magnetic nucleus provides useful information regarding structure of a number of organic and inorganic compounds. Small changes were observed in the chemical shift of 1-H in 15C5, 18C6 and, 1-H as well as 2-H, in dibenzo18C6. After formation of the [Metal-Crown ether]Ligand complex, the proton chemical shift δ(−CH2-O-) shows significant downfield shifts [Δδ(−CH2-O-)= 0.08–0.25 ppm], indicating metal-ligand bond formation. The degree of downfield shift shows the relative strength of the complexes. The 1H-NMR spectrum of dibenzo18C6 shows peaks at, δ1=3.9–4.1 ppm, (16H, 8-CH2O-), δ2=6.8–7.0 ppm, (8H, aryl-CH-), in CDCl3. The shift of-CH2-signals in complexes from free crown ether unambiguously suggested the coordination of crown ether oxygen of 15C5, 18C6 as well as dibenzo18C6 with thallium ion.

Lewis acidic ionic liquids of crown ether complex cations: preparation and applications in organic reactions

New Lewis acidic ionic liquids composed of crown ether complex cations were designed, synthesised and characterised by Raman, MS, FT-IR, TGA-DSC and elemental analysis. They have been successfully used in various organic reactions as catalysts.

Gas-phase microsolvation of ubiquitin: investigation of crown ether complexation sites using ion mobility-mass spectrometry.

In this study the gas-phase structure of ubiquitin and its lysine-to-arginine mutants was investigated using ion mobility-mass spectrometry (IM-MS) and electron transfer dissociation-mass spectrometry (ETD-MS). Crown ether molecules were attached to positive charge sites of the proteins and the resulting non-covalent complexes were analysed. Collision induced dissociation (CID) experiments revealed relative energy differences between the wild type and the mutant crown-ether complexes. ETD-MS experiments were performed to identify the crown ether binding sites. Although not all of the binding sites could be revealed, the data confirm that the first crown ether is able to bind to the N-terminus. IM-MS experiments show a more compact structure for specific charge states of wild type ubiquitin when crown ethers are attached. However, data on ubiquitin mutants reveal that only specific lysine residues contribute to the effect of charge microsolvation. A compaction is only observed for one of the investigated mutants, in which the lysine has no proximate interaction partner. On the other hand when the lysine residues are involved in salt bridges, attachment of crown ethers has little effect on the structure.

First-Principles Study of Crown Ether and Crown Ether-Li Complex Interactions with Graphene

Adsorption of molecules on graphene is a promising route to achieve novel functionalizations, which can lead to new devices. Density functional theory is used to calculate stabilities, electronic structures, charge transfer, and work function for a crown-4 ether (CE) molecule and a CE–Li (or CE–Li+) complex adsorbed on graphene. For a single CE on graphene, the adsorption distance is large with small adsorption energies, regardless of the relative lateral location of the CE. Because CE interacts weakly with graphene, the charge transfer between the CE and graphene is negligibly small. When Li and Li+ are incorporated, the adsorption energies significantly increase. Simultaneously, an n-type doping of graphene is introduced by a considerable amount of charge transfer in CE–Li adsorbed system. In all of the investigated systems, the linear dispersion of the pz band in graphene at the Dirac point is well-preserved; however, the work function of graphene is effectively modulated in the range of 3.69 to 5.09 e...

Ion‐Transfer Voltammetry at the Interface between Organic and Salt‐Doped Ice Phases

AbstractIons play critical roles in reactions of biological, environmental, and industrial importance under frozen conditions. Transfer voltammetry of tetramethylammonium (TMA) and K+ ions is measured at the interface between frozen aqueous NaCl and dichloroethane to give an insight into chemistry in frozen aqueous phases. The ion‐transfer current of TMA initially increases as the temperature becomes lower from the freezing point, but the further decrease in temperature reduces the transfer current. The half‐wave potential of the TMA transfer shows a negative shift when the liquid phase volume is smaller than 10 % of the total volume of the frozen phase. Similarly, the half‐wave potential of the facilitated transfer of K+ by crown ether complexation also becomes negative with decreasing temperature, due to the freeze concentration of K+ in the liquid phase.

Structure and stability of supramolecular crown ether complexes.

Despite the fact that the complexation of ammonium cations with ionophores like crown ethers plays an important role in biological and industrial processes, there is still a lack of theoretical methods to reproduce or even predict the host-guest complex structures or their thermodynamic stabilities in an accurate manner. Hence, the development of ionophores has often relied on a trial-and-error approach and the synthetic efforts associated with this have been enormous, so far. Therefore, theoretical methods for the reliable prediction of binding affinities of crown ether derivatives with ammonium ions would be an indispensable tool for the rational design of new receptors with tailored properties. Here, we suggest a computationally efficient but still accurate theoretical approach. It is tested for a model system consisting of 18-crown-6 ether and an ammonium cation, but is invented for application to much larger complexes. The accuracy of various approximate quantum-chemical methods, based on density functional theory (DFT) and many-body perturbation theory, is evaluated against the gold standard CCSD(T) in the basis set limit as internal reference. An important aspect is the consideration of dispersion interactions in DFT methods, for which the dispersion-correction by Grimme was employed. For all selected methods, the basis-set dependence of calculated interaction energies was investigated.

Synthesis, reactivity, electrochemical behaviour, and molecular structure of crown ether cyrhetrene complexes

Cyrhetrenyl crown ether complexes [(η5-C5H4CHN-M)Re(CO)3] (where M = 4-benzo-15-crown-5 (3a), 4-benzo-18-crown-6 (3b), 2-methyl-15-crown-5 (3c), or 2-methyl-18-crown-6 (3d)) were synthesised from cyrhetrenylcarboxaldehyde (1) and the corresponding crown ether amines 2a–d. All the complexes were characterised by IR spectroscopy, 1H and 13C NMR spectroscopies, and mass spectrometry. The stereochemistry for imine compounds 3a–d were determined using the 1H and 13C NMR spectroscopy data, which indicated that these complexes have the anti-(E) conformation. This was also confirmed by the X-ray crystal structures of 3b and 3c in the solid state. Additionally, the electrochemical behaviours of 1 and 3a–d were studied.

Iso-selective ring-opening polymerization of rac-lactide catalyzed by crown ether complexes of sodium and potassium naphthalenolates.

Two crown ether complexes of sodium and potassium naphthalenolates were synthesized and entirely characterized. The two complexes can iso-selectively catalyze the ring-opening polymerization (ROP) of rac-lactide at room temperature and afford polylactides with desired molecular weights and narrow PDIs; the best isotacticity (Pm) achieved was 0.73.

New insights into metal ion–crown ether complexes revealed by SEIRA spectroscopy

We demonstrate the SEIRA spectroscopy of crown ether complexes for examining the relationship between the guest selectivity, structure, and solvent effect.

Supramolecular complexes: Determination of stability constants on the basis of various experimental methods

Stability constants and formation enthalpies of supramolecular complexes of crown ethers and their cyclic and acyclic analogues are determined on the basis of experimental data obtained by different physicochemical methods in the terms of a general approach developed and implemented in the ChemEqui software package. The established regularities of variation of stability of complexes are discussed as dependent on the ligand structure, nature of the cation, solvent, and anion. The applicability of the suggested method of determining complexation selectivity is shown for multicomponent equilibria in solutions.

ChemInform Abstract: Coordination and Organometallic Compounds in the Functionalization of Carbon Nanotubes

AbstractReview: 83 refs.

Crown ether complex cation ionic liquids (CECILs) as environmentally benign catalysts for three-component synthesis of 4,5-dihydropyrano[3,2-c]chromene and 4,5-dihydropyrano[4,3-b]pyran derivatives

Crown ether complex cation ionic liquids (CECILs) catalyze three-component reaction of aromatic aldehydes, malononitrile, and 4-hydroxycoumarin or 4-hydroxy-6-methylpyrone, in EtOH under reflux, for synthesis of 4,5-dihydropyrano[3,2-c]chromene and 4,5-dihydropyrano[4,3-b]pyran derivatives. CECILs are environmentally benign, easily obtained, stable catalysts. High conversion, short reaction times, and cleaner reaction profiles are some of the advantages of this method.

Supramolecular Stark Effect in Host-Guest Complexes via Charge Density Analysis

The effect of an electric field on the vibrational spectra, the Vibrational Stark Effect (VSE), has been utilized extensively to probe the local electric field in the active sites of enzymes [1, 2]. For this reason, the electric field and consequent polarization effects induced by a supramolecular host system upon its guest molecules attain special interest due to the implications for various biological processes. Although the host-guest chemistry of crown ether complexes and clathrates is of fundamental importance in supramolecular chemistry, many of these multicomponent systems have yet to be explored in detail using modern techniques [3]. In this direction, the electrostatic features associated with the host-guest interactions in the inclusion complexes of halogenated acetonitriles and formamide with 18-crown-6 host molecules have been analyzed in terms of their experimental charge density distribution. The charge density models provide estimates of the molecular dipole moment enhancements which correlate with the simulated values of dipole moments under electric field. The accurate electron density mapping using the multipole formalism also enable the estimation of the electric field experienced by the guest molecules. The electric field vectors thus obtained were utilized to estimate the vibrational stark effect in the nitrile (-C≡N) and carbonyl (C=O) stretching frequencies of the guest molecules via quantum chemical calculations in gas phase. The results of these calculations indicate remarkable elongation of C≡N and C=O bonds due to the electric fields. The electronic polarization in these covalent bonds induced by the field manifests as notable red shifts in their characteristic vibrational frequencies. These results derived from the charge densities are further supported by FT-IR experiments and thus establish the significance of a phenomenon that could be termed as the "supramolecular Stark effect" in crystal environment.

Crown ether complexes of uranyl, neptunyl, and plutonyl: hydration differentiates inclusion versus outer coordination.

The structures of actinyl-crown ether complexes are key to their extraction behavior in actinide partitioning. Only UO2(18C6)(2+) and NpO2(18C6)(+) (18C6 = 18-Crown-6) have been structurally characterized. We report a series of complexes of uranyl, neptunyl, and plutonyl with 18-Crown-6, 15-Crown-5 (15C5), and 12-Crown-4 (12C4) produced in the gas phase by electrospray ionization (ESI) of methanol solutions of AnO2(ClO4)2 (An = U, Np, or Pu) and crown ethers. The structures of 1:1 actinyl-crown ether complexes were deduced on the basis of their propensities to hydrate. Hydration of a coordinated metal ion requires that it be adequately exposed to allow further coordination by a water molecule; the result is that hydrates form for outer-coordination isomers but not for inclusion isomers. It is demonstrated that all the actinyl 18C6 complexes exhibit fully coordinated inclusion structures, while partially coordinated outer-coordination structures are formed with 12C4. Both inclusion and outer-coordination isomers were observed for actinyl-15C5 complexes, depending on whether they resulted from ESI or from collision-induced dissociation. Evidence for the formation of 1:2 complexes of actinyls with 15C5 and 12C4, which evidently exhibit bis-outer-coordination structures, is presented.