Crown Ether Cavity Research Articles

Synthesis, Structure, and Ion Selective Complexation of Trans and Cis Isomers of Photochromic Dithia-18-crown-6 Ethers

Photochromic styryl dyes containing benzodithia-18-crown-6 and N-substituted benzothiazolium moieties are synthesized and characterized spectroscopically and, for a specific case, by X-ray crystallography. The effects of the N-substituent and of sulfur atom substitution in the crown ether cavity on metal cation complex formation and on photochromism due to reversible trans−cis photoisomerization are evaluated. High selectivities for heavy metal ions, such as Ag+ and Hg2+, are observed, in contrast to the benzo-18-crown-6 dyes which selectively bind alkali and alkali earth cations. Relative complex formation stability constants, determined spectrophotometrically, are converted to absolute values by polarographic measurements. The stability constant for Hg2+ complex formation by the trans dye is enhanced 47-fold upon substitution of the 4-butylsulfonate group for methyl at the nitrogen of the benzothiazolium moiety. Photoisomerization to the cis dye is accompanied by a further 11-fold stability constant increase and by spectral changes that are consistent with formation of an ion-“capped” complex, involving intramolecular coordination of the sulfonate anion to Hg2+. Addition of a large excess of Mg2+ ions disrupts this intramolecular coordination bond without displacing the Hg2+ ion from the dithiacrown ether cavity.

Structural and conformational studies of alcohol, diol and methyl ether derivatives of dibenzo-14-crown-4. Implications for ligand and tecton design

Molecular structures are determined for six dibenzo-14-crown-4 derivatives that have one or two substituents on the central carbon(s) of the three-carbon bridge(s). The series of compounds includes three crown ether alcohols, one crown ether trans-diol, and two methoxy crown ether compounds. The crystal structures for these six crown ethers reveal that due to hydrogen-bonding and steric interactions, a hydroxy substituent is directed, at least partially, toward the crown ether cavity and an unusual intra- and intermolecular hydrogen bond network is formed between the hydroxy group protons and the ether oxygens of the crown ether ring. On the other hand, an ether group or a substituent with carbon as the first atom is oriented away from the polyether ring. The structure of sym-(methoxy)(methyl)dibenzo-14-crown-4 is markedly different from that of sym-(hydroxy)(methyl)dibenzo-14-crown-4 both in terms of the substituent orientation and very significant distortion from planarity of the four crown ether oxygens in the former. Support for an unusual conformation for sym-(methoxy)(methyl)dibenzo-14-crown-4 in solution is derived from 13C NMR measurements. Crown ether alcohols are hydrogen-bonding “tectons” that participate in strong, specific and directional intermolecular interactions.

Supramolecular chemistry with macromolecules: Macromolecular knitting, reversible formation of branched polyrotaxanes by self-assembly

Mechanically linked branched polymer 10 was prepared via rotaxane formation by self assembly (“knitting”) of poly[bis(5-methylene-1,3-phenylene)-32-crown-10 sebacate] (4) and preformed polyurethane 9 containing N,N′-bis(β-oxyethyl)-4,4′-bipyridinium 2PF6− (“paraquat”) moieties in tetrahydrofuran (THF). The interpenetrating structure resulting from “knitting” the bipyridinium units of the polyurethane through the crown ether cavities of the polyester structure was proved by its color, NMR studies and gel permeation chromatography (GPC) measurements. In accord with its equilibrium nature, the branching process (rotaxane formation) was reversible depending on the solvent and temperature. The branched material was soluble because the two polymeric components were designed to have relativley low molecular weights and the polyurethane to contain only about two paraquat units, so as to be able to carry out solution characterization. However, extension of this concept to higher molecular weight and/or more highly functionalized systems provides interesting possibilities for reversible processing of thermoset-like materials and production of new types of polymeric supermolecules with potentially interesting and controllable properties.

Supramolecular chemistry with macromolecules: Macromolecular knitting, reversible formation of branched polyrotaxanes by self-assembly

Mechanically linked branched polymer 10 was prepared via rotaxane formation by self assembly (“knitting”) of poly[bis(5-methylene-1,3-phenylene)-32-crown-10 sebacate] (4) and preformed polyurethane 9 containing N,N′-bis(β-oxyethyl)-4,4′-bipyridinium 2PF6− (“paraquat”) moieties in tetrahydrofuran (THF). The interpenetrating structure resulting from “knitting” the bipyridinium units of the polyurethane through the crown ether cavities of the polyester structure was proved by its color, NMR studies and gel permeation chromatography (GPC) measurements. In accord with its equilibrium nature, the branching process (rotaxane formation) was reversible depending on the solvent and temperature. The branched material was soluble because the two polymeric components were designed to have relativley low molecular weights and the polyurethane to contain only about two paraquat units, so as to be able to carry out solution characterization. However, extension of this concept to higher molecular weight and/or more highly functionalized systems provides interesting possibilities for reversible processing of thermoset-like materials and production of new types of polymeric supermolecules with potentially interesting and controllable properties.

Supramolecular chemistry with macromolecules: Macromolecular knitting, reversible formation of branched polyrotaxanes by self-assembly

Mechanically linked branched polymer 10 was prepared via rotaxane formation by self assembly (knitting) of poly[bis(5-methylene-1,3.-phenylene)-32-crown-1 sebacate] (4) and preformed polyurethane 9 containing N,N'.-bis(β.-oxyethyl)-4,4'-bipyridinium 2PF 6 (paraquat ) moieties in tetrahydrofuran (THF). The interpenetrating structure resulting from knitting the bipyridinium units of the polyurethane through the crown ether cavities of the polyester structure was proved by its color, NMR studies and gel permeation chromatography (GPC) measurements. In accord with its equilibrium nature, the branching process (rotaxane formation) was reversible depending on the solvent and temperature. The branched material was soluble because the two polymeric components were designed to have relativley low molecular weights and the polyurethane to contain only about two paraquat units, so as to be able to carry out solution characterization. However, extension of this concept to higher molecular weight and/or more highly functionalized systems provides interesting possibilities for reversible processing of thermoset-like materials and production of new types of polymeric supermolecules with potentially interesting and controllable properties.

Organization of the Cavity in a Bipyridyl Crown Ether Through Coordination with PdCl2

In the crystal structure of dichloro[3,6,9,12,15,18-hexaoxa-24,27-diazatricyclo[24.4.0.020,25]- triaconta-1(26),20,22,24,27,29-hexaene-N,N']palladium(II), [PdCl2(C22H30N2O6)], the Pd atom is tetracoordinated by the two pyridyl N atoms and two Cl atoms. The planes formed by atoms Pd1, N21, N31 and atoms Pd1, Cl1, Cl2 make an angle of 12.12 (9)° with one another. The twist of the two pyridyl rings of 31.59 (9)° is associated with the ortho substitution, and the complexation of PdCl2 seems to alter the shape of the crown ether cavity, so that it is less suitable for complexing metal cations.

Synthesis and crystal structure of rubidium hydrogen oximate complexes with 18-crown-6: is it possible to reach a perfect fit of the rubidium atom inside the crown ether cavity?

Complexes of composition Rb(18-crown-6) {H(X) 2} · nH 2O (X = isonitrosocyanoacetamide, n = 3 ( 1) and isonitrosobenzoylacetonitrile, n = 0 ( 2) have been prepared and studied by means of X-ray diffraction ( 1: triclinic, space group P 1 , with a = 10.143(2), b = 11.912(2), c = 13.518(2) A ̊ ; α = 114.40 (1), β = 91.61(1), γ = 103.92(1)°, V = 1428.6(4) A ̊ 3, Z = 2 ; final R1 = 0.032 based on 3660 independent reflections with 1 > 2 σ (1); 2: monoclinic, space group P2 1/ n, with a = 9.522(2), b = 16.469(3), c = 10.794(2) A ̊ ; β = 95.13(3)° , V = 1685.9(6) A ̊ 3, Z = 2 ; final R1 = 0.038 based on 1547 independent reflections with I > 2 σ ( I). Both the structures consist of Rb(18-crown-6) + cations and complex hydrogen oximate anions {H(X) 2}, formed via strong hydrogen bonding between the oxygen atoms of nitroso groups (O···O ca. 2.455(6)–2.473(3) Å). In 1 the rubidium atom is situated 1.012(1) Å above the mean plane of macrocyclic oxygen atoms (RbO 2.921(2)–3.129(2) Å) and adopts a typical ‘sunrise coordination’, while the structure of complex 2 demonstrates an unprecedented example of a perfect fit of the rubidium atom inside the crown ether cavity. Metal atom occupies a center of inversion and resides exactly in the center of the crown ether (RbO 2.822(5)–2.872(4) Å).

Crystal Structures of 1,3-Calix[4]-bis-crown-6·3CH3CN and 1,3-Calix[4]-bis-(benzo-crown-6)·3 CH3CN

The crystal structures of a new solvate of the ditopic receptor 1,3-calix[4]-bis-crown-6, Bis-C6, and of 1,3-calix[4]-bis-(benzo-crown-6), Bis-benzoC6, are reported. Bis-C6.3 CH3CN (1) crystallizes in the monoclinic space group P21/n, a = 14.388(3), b = 26.947(8), c = 14.707(4) A, β = 113.19(3)°, V = 5241(5) A3, Z = 4. Refinement led to a final conventional R value of 0.092 for 2723 reflections. The structure of (1) differs from the previously reported structure of Bis-C6.4 CH3CN by the conformation of one crown either chain. Two acetonitrile molecules are in the close neighbourhood of the crown ether cavities. Bis-benzoC6.3 CH3CN (2) crystallizes in the monoclinic space group P21/c, a = 10.391(4), b = 17.264(11), c = 30.426(9) A, β = 94.62(3)°, V = 5440(7) A3, Z = 4. Refinement led to a final conventional R value of 0.106 for 2965 reflections. Two acetonitrile molecules are located near the crown ether cavities, as in (1). One of the crown ether conformations is the same as in the binuclear caesium complex of Bis-benzoC6, supporting the hypothesis of a preorganization of this ligand towards the complexation of this ion; the second crown ether chain is partially disordered.

Ion-Pair Sorption of Alkali Metal Chlorides by Crown Ether Carboxamide Resins

Abstract Ten crown ether carboxamide resins are prepared by condensation polymerization of N,N-dialkyl sym-(R)dibenzo-16-crown-5-oxyacetamide monomers with formaldehyde in formic acid. Competitive ion-pair sorption of alkali metal chlorides from aqueous methanol solutions by these novel resins reveals that both sorption selectivity and efficiency are influenced by: 1) the conformational positioning of the pendant carboxamide group with respect to the crown ether cavity; 2) the length of the N,N-dialkyl chains on the pendant carboxamide group; 3) the methanol content of the aqueous methanol solution; 4) the concentration of alkali metal chlorides in the sample solution; and 5) the temperature of the sample solution. The highest sorption efficiency and Na+ selectivity are obtained for a resin prepared from N.N-dibutyl sym-(propyl)dibenzo-16-crown-5-oxacetamide monomer in which the geminal propyl group orients the carboxamide-containing side arm over the crown ether cavity. Lengthening the alkyl chains in th...

Immobilization of Crown Ether Carboxylic Acids on Silica Gel and Their Use in Column Concentration of Alkali Metal Cations from Dilute Aqueous Solutions

Eight crown ethers with pendent carboxylic acid groups are immobilized on silica gel and utilized for column concentration of alkali metal cations from dilute aqueous solutions. The column concentration selectivity and efficiency are found to be strongly influenced by (1) the cavity size of the crown ether unit, (2) conformational positioning of the proton-ionizable side arm with respect to the crown ether cavity, and (3) capping of residual silanol surface groups with trimethylsilyl functions. By use of a chromatographic stripping technique, selective column concentration of Na(+), K(+), (Rb(+) and Cs(+)), and Cs(+) by different functionalized silica gels has been achieved.

X-Ray Structures and Spectroscopic Studies of Diaqua- and Dichlorocopper(II) Complexes of 15-Crown-5 and 4′-Substituted Benzo-15-Crown-5 with a 3dz2 Ground-State Doublet

Abstract Crystallographic and spectroscopic investigations were carried out for Cu(II) complexes of 15-crown-5 (15C5) and 4′-substituted benzo-15-crown-5 (B15C5) derivatives. X-Ray diffraction measurements were performed for two single crystals of [Cu(15C5)(H2O)2](ClO4)2 and [Cu(B15C5)(H2O)2](ClO4)2, obtained from 60% HClO4 solutions. The results of the X-ray analysis demonstrated that these complexes take a seven-coordinate pentagonal bipyramidal geometry, in which the copper ion is located in the center of the crown ether cavity with two water molecules at the axial positions. The ESR spectra observed for these complexes in 60% HClO4 exhibited the characteristic g-anisotropy (gz < gx, gy), leading to an electronic configuration of a 3dz2 ground-state doublet. ESR and electronic absorption measurements were made for Cu(II) complexes of 4′-substituted B15C5 in systems of BF3–ether–water and of dry CHCl3 in which complexes having two chloride ions at the axial positions were produced. Although these systems showed the same remarkable substituent effects, clear differences were detected between them, attributable to the different axial ligands. The results of an extended Hückel MO calculation performed for the models of these complexes provide qualitative explanations for the experimental results.

Synthesis and crystal structure of a thallium (I) hydrogen benzothiazolylcyanoximate complex with 18-crown-6: the thallium atom is in the center of the crown ether cavity

Complexes of composition [Tl(18-crown-6){BTCO}] and [Tl(18-crown-6){H(BTCO) 2}] (BTCO={ONC(CN)-(benzothiazolyl-2)}su−) have been prepared. The crystal and molecular structure of the [Tl(18-crown-6){H(BTCO) 2}] complex has been determined from X-ray diffraction data (orthorhommbic, space group Pbca, with a=15.520(3), b=11.668(2), c=19.661(4) A ̊ , V=3560.4(12) A ̊ 3, Z=4 ). The structure was refined by least-squares methods to a final value of R=0.029 based on 2525 independent reflections with 1>2 σ(1). The metal atom occupies a center of symmetry. The lattice consists of centrosymmetric cations Tl(18-crown-6) + and anions [H(BTCO) 2} −, which are bridges between neighboring cations to give a one-dimensional polymeric structure. The thallium atoms adopt the eight-fold coordination of a slightly distorted hexagonal-bipyramidal geometry with axial nitrile nitrogen atoms and reside exactly in the center of the crown ether (TlO ∼ 2.820–2.876 Å, TlN 2.987 Å). No stereochemical activity of a lone pair electron density of Tl was observed.

Unusual Complexation Behavior of Metallomacrocycles Based on Isothiosemicarbazides with Respect to Alkali and Alkaline Earth Metal Ions: Novel 2:1 Associates

AbstractThe reactions between alkali and alkaline earth metal ions and nickel(II) macrocycles based on S‐alkylated isothiosemi‐carbazides with different crown ether cavity size were studied in propylene carbonate by spectrophotometric and calorimetric titrations. Metallomacrocycles 1 and 2 exhibit normal behavior on 1:1 complexation with alkali‐ and alkaline earth metal ions and resemble in this respect 15C5 and 18C6, respectively. The most stable complexes are formed by these “ligands” when the diameter of the cation and the crown ether hole have approximately the same size. The most striking feature of the complexation processes studied is the formation of 1:2 metal‐ligand associates even in the case of the smallest cations. These associates are very different from “normal” crown ether sandwich complexes. In reality, the particle formed is an associate between a 1:1 complex, in which the corresponding metal ion is well accommdated inside the ligand cavity, and a second metallomacrocyclic ligand. Their formation is disfavored by enthalpic contributions. A special kind of “switch” from these associates to normal sandwich complexes takes place in the case of 1, when the cation diameter compared to hole size increases. The macrocycle 2 forms this kind of associates with all akali and alkaline earth ions.

Mechanism and Thermodynamics of Ion Selectivity in Aqueous Solutions of 18-Crown-6 Ether: A Molecular Dynamics Study

We have performed extensive classical molecular dynamics simulations to examine the mechanism and thermodynamics for ion selectivity of 18-crown-6 ether in aqueous solutions. We have computed the free energy profiles or potentials of mean force and the corresponding binding free energies for M{sup +}:18-crown-6 (M{sup +} = K{sup +}, Na{sup +}, Rb{sup +}, Cs{sup +}) complexation in water. To the best of our knowledge, the present work is the first to employ the potential of mean force approach to the evaluation of crown ether selectivity in an aqueous solution. The resultant potentials of mean force indicate that minima free energy surfaces for K{sup +} and Na{sup +} are located at the crown ether center-of-mass. A well-defined second minimum is also observed in the potential of mean force of the Na{sup +}:18-crown-6 complex in water. It appears that K{sup +} is selected over Na{sup +} because of the greater free energy penalty associated with displacing water molecules from Na{sup +} as it approaches the crown ether. This leads to a substantial increase in the activation free energy and a decrease in the corresponding binding free energy for Na{sup +}:18-crown-6 complexation as compared to K{sup +}:18-crown-6. The selection of K{sup +} overmore » Rb{sup +} and Cs{sup +} is due to the size of the cation relative to that of the crown ether cavity. 26 refs., 11 figs., 2 tabs.« less

Nuclear magnetic resonance studies and molecular modelling of the solution structure of some dibenzo crown ethers and their complexes

The 1H and 13C NMR spectra of the structurally related crown ethers 1– 7 with further potential coordination groups (OH, COOH) and the alkali metal complexes of the latter were recorded. Structural information was deduced especially from 13C NMR chemical shifts, the salt-induced 13C chemical shifts (as a measure of both the in-plane interactions HC 2C 1OC 8H 2 and HC 5C 6OC 7H 2), vicinal H,H coupling constants and spin-lattice relaxation times, T 1, of protonated 13C nuclei. The complex stability constants ( K A) for the complexes of 1– 7 with LiI were determined by 7Li NMR spectroscopy. Accompanying quantum-mechanical calculations corroborate the spectroscopical conclusions about the preferred conformers of the studied crown ethers both in the free and in the complexed state. New insights into the complexation behaviour of the crown ether carboxylic acids 3– 5 were obtained which are in contrast to the generally discussed “cavity effects”. The results of NMR spectroscopy and molecular modelling prove the complexation of the crown ether carboxylic acid to take place first at the carboxyl group. In a second step, the bound cation is folded over the cavity of the same crown ether or over one of another crown ether ring system.

Interactions of some crown ethers, cyclam and its tetrathia analogue with alkali, alkali earth and other metal ions: an electrospray mass spectrometric study

Electrospray mass spectrometry (ESMS) has been used to investigate the interactions in solution between three crown ethers (12-crown-4, 18-crown-6, dibenzyl-18-crown-6), 1,4,8,11-tetraazacyclotetradecane (cyclam) and its tetrathia analogue (S-crown) and a variety of metal ions. These include the alkali and alkali earth metal ions, various divalent transition metal ions and main group metal ions such as Cd 2+ and Pb 2+. The previously proposed relationships between the cation radius and the radius of the crown ether cavity which determine the stoichiometry of the complexes formed are dominant in solution. Although the cavity size of DB-18-C-6 is similar to that of 18-C-6, it behaves as though the cavity is effectively smaller as a result of its less flexible structure. Cyclam gave no complexes containing alkali and alkali earth cations, but ionic species were observed with the transition metal ions. S-crown did form ionic complexes with the alkali metal ions and transition metal ions, but not the alkali earth cations.

Control of Highly Selective Ag+ Transport by Redox Reactions between Thiol and Disulfide Located inside a Cavity of Crown Ether

Control of Highly Selective Ag+ Transport by Redox Reactions between Thiol and Disulfide Located inside a Cavity of Crown Ether

Conduction of alkali metal cations in poly(diaza-crown ether)s having hydroxyethyl side arms

Poly(diaza-3n-crown-n)s having hydroxyethyl side arms (PDCns: n=5-7) were prepared, and their effects on the ionic conductivity of both inorganic salts and cation conductive polymers were analysed. A series of alkali metal perchlorates (MClO4: M=Li, Na, K, Rb, and Cs) were mixed with the PDCn materials. However, although ionic conductivity better than 10−6 S cm−1 above 60°C was achieved, no effect was seen of any specific coupling of the cation and the crown cavity size on the ionic conductivity. This was probably due to the contribution of the anion conduction. In order to neglect the effect of anion migration, cation conductive polymers were mixed with the PDCn materials. K+ and Na+ showed the lowest ionic conductivity in PDC6 and PDC5, respectively, reflecting stronger interactions with crown ethers having more suitable cavity sizes. The presence of crown ether units in the polymer was also suggested as being responsible for accelerating the dissociation of salts with cations which were smaller than the size of the crown ether cavity.

Effect of Ring-Size Variation within Dibenzocrown Ether Resins upon Ion-Pair Sorption of Alkali-Metal Cations from Aqueous and Aqueous Methanol Solutions

Abstract A batch analysis method has been developed for evaluation of metal salt sorption by crown ether polymers. Selectivity in competitive alkali-metal chloride sorption by a series of formaldehyde condensation polymers of dibenzocrown ethers is influenced by the relationship between the crown ether cavity size and metal ion diameter, as well as the degree of hydration of the metal salt. Effective and selective sorption of KCl from the other alkali-metal chlorides was obtained with a dibenzo-18-crown-6 resin. Excellent sorption selectivity for the monovalent metal chlorides was noted for competitive ion-pair sorption of NaCl, KCl, MgCl2, and CaCl2 by this resin. This resin was examined as a stationary phase for selective column separation of KCl from alkali-metal chlorides and of KCl and NaCl from alkali-metal and alkaline-earth chloride mixtures in 80% methanol—20% water.

Effect of structural variations of dibenzocrown ether resins and their acyclic analogues upon ion-pair sorption of alkali metal salts from aqueous and aqueous methanol solutions

Abstract Ion-pair sorption of alkali metal salts from aqueous and aqueous methanol solutions by acyclic and cyclic dibenzopolyether resins possessing different side arm groups such as hydroxy, methoxy and carboxy has been investigated. The results reveal that both sorption selectivity and efficiency are influenced by: (1) the methanol content of the aqueous sample solution; (2) the acyclic or cyclic nature of the polyether unit; (3) the conformational positioning of the side arm group with respect to the crown ether cavity; and (4) the identity of the counteranion species of the alkali metal salt. For sym-(C3H7)(R′)dibenzo-16-crown-5 resins, the sorption selectivity and efficiency increased as the R′ group was varied: -OCH3 < -OH < -OCH2CO2H. The highest sorption efficiency and Na+ selectivity was obtained for sym-(propyl)dibenzo-16-crown-5-oxyacetic acid resin (7) in which the pendent carboxylic acid group is oriented over the crown ether cavity. The use of a less hydrated anion in the alkali metal salt ...