Interaction of cis-diastereomers of dicyclohexano-18-crown-6 with maleic anhydride and some “small” polar molecules

The cis-anti-cis diastereomer of dicyclohexano-18-crown-6 with maleic anhydride or formamide gives a crystalline complex of ratio 12. The three-dimensional structure of the former was found from x-ray studies. The formation of complexes was used for the separation of the individual cis-syn-cis and cis-anti-cis diastereomers from a mixture. Crystallization of a cis-anti-cis diastereomer from DMFA, DMSO, and N-methylpyrrolidine gave a high melting form.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 10, pp. 1329–1332, October, 1989.     

2,4-dithiouracil: the reproducible H-bonded structural motifs in the complexes with 18-membered crown ethers

2,4-dithiouracil (DTU) forms in the crystals the H-bonded monohydrates of a 1:1:1 ratio with 18-crown-6 (18C6) 1, cis,syn,cis-isomer of dicyclohexano-18-crown-6 (DCH6A) 2, and benzo-18-crown-6 (B18C6) 3, while the anhydrous adduct with cis,anti,cis-isomer of dicyclohexano-18-crown-6 (DCH6B) 4 is of a 2:1 ratio. In 1-3 the components reproducibly alternate in the chains, while in 4 the chains are built of the alternative centrosymmetric dimers of 2,4-dithiouracil and the molecules of the cis,anti,cis-isomer of dicyclohexano-18-crown-6.     

Study of gas-phase molecular recognition using Fourier transform ion cyclotron resonance mass spectrometry (FTICR/MS)

The application of Fourier transform ion cyclotron resonance (FTICR) mass spectrometry to the quantitative study of molecular recognition in the gas phase is reviewed. Because most quantitative measurements are dependent on accurate determination of the pressure of a neutral reagent, methods for accurate pressure measurement in FTICR, including gauge calibration using a reaction with known rate constants (the traditional method), exothermic proton transfer rate measurement (often the best method when accurate neutral pressures in the trapping cell are desired), and linewidth measurement (a little-used, but generally applicable method) are discussed. The use of rate constant measurements in molecular recognition is illustrated with examples employing natural abundance isotopic labeling to study self-exchange and 2 : 1 ligand:metal complex formation kinetics in crown ether-alkali cation systems. Self-exchange rates do not correlate with alkali cation/crown cavity size relationships, whereas 2 : 1 complex formation kinetics correlate strongly with size relationships. The use of exchange equilibrium constant measurements to characterize molecular recognition is illustrated by alkali cation exchanges between 18-crown-6 and the isomers of dicyclohexano-18-crown-6. These experiments show that the alkyl-substituted ligand binds alkali cations better than unsubstituted 18-crown-6 in the gas phase, in accordance with expectations based on the higher polarizability of the alkyl-substituted ligand. Further, the metal binding thermochemistry differs for the two dicyclohexano-18-crown-6 isomers, with the bowl-shaped cis-syn-cis isomer binding all the alkali cations more strongly than the cis-anti-cis isomer. The measurement of entropies and enthalpies associated with one of the most subtle forms of molecular recognition, enantiomeric discrimination, is illustrated by studies of the discrimination between enantiomers of chiral amines by dimethyldiketopyridino-18-crown-6. This chiral ligand binds chiral primary ammonium cations that have the opposite absolute configuration at their stereocenter more strongly than the enantiomer with the same absolute configuration. Gas-phase studies show that this enantiomeric discrimination is enthalpic in origin, likely related to more favorable pi-pi stacking for the preferred enantiomer. Entropy disfavors binding of the preferred enantiomer.     

Crown ether complexes of potassium and thallium(I) 2- and 4-nitrophenoxides

18-Crown-6 and dicyclohexano-18-crown-6 complexes of potassium 2- and 4-nitrophenoxide, and 18-crown-6 complexes of thallium(I) 2- and 4-nitrophenoxide have been synthesized. Solvent effects on the visible spectra of the nitrophenoxide anions are independent of the nature of the cation and the nature of the complexing agent. The 18-crown-6 complex of thallium(I) 2-nitrophenoxide is a 1:2 complex. All the other complexes are 1:1. X-ray crystallographic examination of the potassium dicyclohexano-18-crown-6 complexes showed the potassium ion is octacoordinated in the 2-nitrophenoxide and heptacoordinated in the 4-nitrophenoxide.     

Synergistic extraction of americium(III) with 2-thenoyltrifluoroacetone and crown ethers

Extracton, of Am³⁺ in benzene with 2-thenoyltrifluoroacetone (HTTA) and crown ethers (CEs) such as 15-crown-5, 18-crown-6, dicyclohexano-18-crown-6, dibenzo-18-crown-6, dicyclohexano-24-crown-8, and dibenzo-24-crown-8 was investigated. Synergistic effect by CE was observed regardless of the kind of CE examined. The extracted species was found to be Am(TTA)₃(CE), and adduct formation constants between Am(TTA)₃ and CE in the organic phase were determined. The sequence of constant could not be explained only by basicity of CE and the steric effect of CE should be taken into account to elucidate the adduct complex formation.     

Synthesis of highly functionalized macrocycles by the peripheral functionalization of macrocyclic diimines

The easily available macrocyclic diimines 4-7 can be stereoselectively transformed to macrocyclic bis-beta-amino acids 13-17, macrocyclic bisazetidines 18-20, and macrocyclic bisamides 21 and 22 by means of the corresponding bis-beta-lactam scaffolds 8-12. These key intermediates are available through standard Staudinger reaction and obtained as the cis-cis diastereomers, exclusively. An interesting relation between the proximity of the reactive C=N bonds and the selectivity in the formation of the bis-beta-lactams 8-12 is observed. Thus, diimine 4 leads to low selectivities, producing a 1:1 mixture of cis-syn-cis and cis-anti-cis diastereomers, while diimines 5-7 having the diimine sites more separated lead almost exclusively to the cis-anti-cis diastereomers. The stereochemistry of all the products was unambiguously assigned by X-ray diffraction analysis of compounds cis-syn-cis 8 and cis-anti-cis 12-Co2CO6 complex.     

Host: Guest chemistry of diaza-18-crown-6: Selective precipitation from mixtures of oligohydroxy phenols and the structures of five host: Guest complexes

Diaza-18-crown-6 (1,4,10,13-tetraoxa-7,16-diaza-cyclo-octadecane) selectively precipitates as a 1,4-dihydroxybenzene-complex from a mixture of isomeric phenols and as a 2,6-dihydroxynapthalene-complex from mixtures of isomeric diols. These selective precipitations are discussed in terms of structure and solubility of the host-guest complexes and phenol acidity. The crystal structures of diaza-18-crown-6 with guestsp-nitrophenol (2: 1), 2,4-dinitroaniline (2: 1), 5,5-diethylbarbituric acid (2: 1), salicylaldoxime (2: 1) and 1,4-dihydroxybut-2-yne (1: 1) are reported. Supplementary Data relating to this article are deposited with the British Library as Supplementary Publication No. SUP 82065 (74 pages).     

Ion-pair extraction of tetravalent plutonium from hydrochloric acid medium using crown ethers

Ion-pair extraction behaviour of plutonium (IV) from varying concentrations of HCl solution was studied employing crown ethers (benzo-l5-crown-5 (B15C5), 18-crown-6, (18C6), dibenzo-18-crown-6 (DB18C6), dicyclohexano-18-crown-6, (DC18C6), dibenzo-24-crown-8 (DB24C8) and dicyclohexano-24-crown-8 (DCH24C8)) in nitrobenzene as the extractant. Ammonium metavanidate was used as the holding oxidant in the aqueous phase and the conditions necessary for the quantitative extraction of the tetravalent ion were found. The co-extraction of species of the type [HL⁺].[HPu(Cl) ₆ ^– ] and [HL⁺]2·[Pu(Cl) ₆ ^2– ] as ion-pairs (where L represents the crown ether) is suggested.     

Extraction of tetrafluoroborate with crown ethers and its application to silicate rock analysis

Summary The extraction behaviour of tetrafluoroborate with crown ethers was studied. A high distribution ratio of tetrafluoroborate is obtained by extraction with dicyclohexano-18-crown-6 (DC18C6) in an organic solvent of high dielectric constant from potassium fluoride solution. The molar ratios of crown ether to KBF₄ in the extracted species are probably 1:1 for DC18C6, dibenzo-18-crown-6 and 18-crown-6, and 2:1 for benzo-15-crown-5 and 15-crown-5. The flow-injection extraction-spectrophotometric determination of tetrafluoroborate with Brilliant Green was worked out. Many rock reference samples were analyzed for boron (1–150 ppm).     

Reaction of lead halides with 18-crown-6 and dicyclohexano-18-crown-6: Solvent extraction,synthesis and crystal structure of [Pb(18-crown-6)I2]

Abstract

Solvent extraction of lead halides with 18-crown-6 (18C6), dicyclohexano-18-crown-6 (DC18C6, cis-syn-cis and cis-anti-cis isomers) in chloroform was studied, and the extraction constants corrected for side reactions and ionic strength effects were obtained. The compounds of the same composition as those being extracted were also isolated in crystal form. The molecular structure of the [Pb(18C6)I₂] complex has been determined. Crystals are monoclinic, P2₁/n, a = 11.237(2), b = 10.992(2), c = 8.139(2)Å, β = 97.32(3)°, V = 997.1(7)ų, D_calc = 2.416(2)gcm^?3, Z = 2 for the composition C₁₂H₂₄O₆PbI₂. The final R-factor is 0.043 for 558 unique reflections. The lead atom is coordinated to six oxygen atoms of the crown ether and two iodine atoms forming a hexagonal bipyramidal coordination polyhedron. The 18C6 molecule and the two halogen atoms form a hydrophobic coating for the lead atom which may be assumed to be the main reason of high extraction constants of the iodine complexes. For 10-coordinate lead ion (bidentate counter ions) the cis-syn-cis isomer of DC18C6 appears to be the best extraction reagent, while for 8-coordinate lead ion (monodentate halide anion) no difference between isomers was observed.     

Development of a dispersive liquid–liquid microextraction method for determination of palladium in water samples using dicyclohexano-18- crown-6 as extracting agent

A new separation procedure for determination of palladium using dispersive liquid–liquid microextraction with dicyclohexano-18-crown-6 as complexing reagent was developed. In this method, potassium–dicyclohexano-18-crown-6 was used as a hydrophobic complex for the microextraction of palladium as PdCl₄ ^2? complex ion. The main factors affecting DLLME efficiency, such as type and volume of extractant and disperser solvent, concentration of chelating reagent, concentration of KCl and pH were optimized. Under the optimal conditions, the limit of detection for palladium was 16.0 ng mL^?1 with enrichment factor of 138. The present method was applied to the determination of palladium in water samples with satisfactory analytical results. The method was simple, rapid, cost efficient and sensitive for the extraction and preconcentration of palladium.     

Rates of dediazoniations of arenediazonium ions complexed with 18-, 21- and 24-membered crown ethers

The evaluation of the kinetics of dediazoniation of benzenediazonium tetrafluoroborate and $\text{p}$-chlorobenzenediazonium tetrafluoroborate in 1,2-dichloroethane at 50°C in the presence of 18-crown-6, 21-crown-7 and dicyclohexano-24-crown-8 demonstrates that the rate constant for the dediazoniation within the complex is smallest, and the equilibrium constant for complex formation is largest for the complex with 21-crown-7.     

Spectroscopic Study of Charge-Transfer Complexes of Some Dicyclohexano-Crown Ethers with Π-acceptors in Dichloromethane Solution

A spectrophotometric study was conducted on solutions of dicyclohexano-18-crown-6 and dicyclohexano-24-crown-8 with some -acceptors in methylene choride at 25 °C. The spectroscopic data indicate the formation of a charge-transfer complex. In contrast to previous results, our study shows that the Ph-O-CH2 structure is not essential for the formation of charge-transfer complexes. The formation constants (Kf) were calculated and the effect of KCl and NaCl salts on the formation and stability of the complexes is discussed.     

Separation of rhenium by extraction with crown ethers and flow-injection extraction—spectrophotometric determination with Brilliant Green

The extraction behavior of perrhenate with crown ethers was studied and methods for the separation and determination of rhenium were developed. The distribution ratio of perrhenate with dicyclohexano-18-crown-6 (DC18C6) increases with increases in the dielectric constant of organic solvents and in the potassium ion concentration of aqueous solution. The molar ratios of crown ether to KReO₄ in the extracted species are probably 1:1 for DC18C6, dibenzo-18-crown-6 and 18-crown-6 and 2:1 for benzo-15-crown-5 and 15-crown-5. Microgram amounts of rhenium were satisfactorily separated from large amounts of molbdenum(VI) by extraction with DC18C6 in 1,2-dichloroethane from 2 M potassium hydroxide solution containing tartrate and by back-extraction with sodium phosphate buffer solution after the addition of a twofold volume of hexane to the organic phase. Rhenium was determined by the flow-injection extraction-photometric method with Brilliant Green. Rhenium was satisfactory determined in molybdenite and other ore samples.     

A stereoselective reduction of dibenzo-18-crown-6 ether to dicyclohexyl-18-crown-6 ether

Dibenzo-18-crown-6 ether is reduced to dicyclohexano-18-crown-6 ether under very mild conditions (room temperature and atmospheric hydrogen pressure) in the presence of rhodium salt and phase-transfer reagents. Control over the stereoselectivity of the reduction can be achieved through the proper choice of the phase-transfer agent. Moreover, at higher pressure, the stereoselectivity can be increased up to a 95/5 ratio of the syn/anti isomers of the dicyclohexano-18-crown-6 ether.     

Further study of extraction equilibrium of uranium(VI) with dicyclohexano-18-crown-6 and its application to separating uranium and thorium

In our publication (1), the extraction of uranium with dicyclohexano-18-crown-6 (mixed isomers) has been described. The extraction equilibrium of uranium(VI) from aqueous hydrochloric acid solution with dicyclohexano-18-crown-6 isomer A (Ia) and isomer B (Ib) in 1,2-dichloroethane is presented in this paper. The extracted species are found to be 1:2 (metal/crown) for Ia and 2:3 for Ib from slope analysis and direct determination of extracted complexes. The extraction equilibrium constants (Kex) have been determined at 25°C, and equal 29.5 for the former and 0.208 for the latter. It is concluded that Ia has stronger coordinate ability for uranium than Ib. The different orientation of the lone pairs of the oxygen atoms in both isomers will be taken into account for interpreting above results. The extraction of uranium(VI) with dicyclohexano-18-crown-6 (mixed isomers) or Ia from aqueous hydrochloric acid solution is effective and selective. In 0.1M crown ether-1,2-dichloroethane-6N HCl system, the separation factor U(VI)/Th(IV) exceeds 1000. The result can be taken in separating uranium and thorium.     

Interfacial tension studies of crown ethers at air/water and hexane/water interfaces

The adsorption of phase transfer catalysts, 18-crown-6 and dicyclohexano-18-crown-6, at the air/water and the hexane/water interfaces were investigated. Interfacial tension sigma decreased by increasing concentrations of these compounds and therefore both of these crown ethers are accumulated at interfaces. The variation of sigma with concentration for both compounds follows the Szyszkowski equation very well, from which the values of saturated surface densities and interaction parameters have been evaluated. On the basis of occupied surface area of each molecule, the orientation of each of molecules at the air/water and the hexane/water interfaces have been proposed. The present results show that dicyclohexano-18-crown-6 has the higher tendency not only to dissolve into the hexane phase but also to adsorb at the hexane/water interface than 18-crown-6 and that the Starks extraction mechanism was suggested for the present phase transfer catalysis systems.     

Preparation and decomposition of potassium alkalide-lipophilic crown ether complexes in tetrahydrofuran

Cyclohexano-15-crown-5, cyclohexano-18-crown-6, dicyclohexano-15-crown-5, and dicyclohexano-18-crown-6, but not dicylohexano-16-crown-5, in THF dissolve potassium metal to form dark blue potassium alkalide solutions at ambient temperature. On standing, the potassium alkalide complexes decompose and the solutions turn colorless at differing rates. Identification of the products provides insight into the decomposition mechanism.     

Crystal Structure and Properties of a New Copper(II) Complex of Dioxocyclam Appended with 8-Methylquinoline (Dioxocyclam = 1,4,8,11-tetraazacyclotetradecane-5,7-dione)

The IR spectra of the crystalline complexes of 3-and 4-nitrophenol with crown ethers were studied, viz.,18-crown-6 (18C6), benzo-18-crown-6 (B18C6),dibenzo-18-crown-6 (DB18C6), dicyclohexano-18-crown-6 (DC18C6) and dibenzo-24-crown-8 (DB24C8). The spectra of uncomplexed crown ethers showed water absorption bands which indicate the presence of two types of bound water molecules, viz., cavitant water enclosed by the strong ether-cavity field and outer-layer hydrogen-bonded water molecules. Upon complexation with 3- and 4-nitrophenol, the bands attributed to cavitant water disappeared, leaving the outer layer water to act as a bridge between the host crown ether and the guest phenols. The results further showed that of the crown ethers and of the phenols, B18C6 and DC18C6 and 3-nitrophenol, have the strongest interaction. The behaviour of the phenols was explained by the increased contribution of the inductive-moment over the resonance -moment in thecomplexes.     

Conductance Study of the Thermodynamics of Tl+ ion Complexes with Different 18-Membered Crown Ethers in Binary Dimethylformamide-Acetonitrile Mixtures

A conductance study of the interactionbetween Tl⁺ ion and 18-crown-6 (18C6),dicyclohexano-18-crown-6 (DC18C6), benzo-18-crown-6(B18C6), diaza-18-crown-6 (DA18C6),dibenzyldiaza-18-crown-6 (DBzDA18C6) andhexaaza-18-crown-6 (HA18C6) indimethylformamide-acetonitrile mixtures was carriedout at various temperatures. The formation constantsof the resulting 1 : 1 complexes were determined fromthe molar conductance-mole ratio data and found tovary in the order HA18C6 > DA18C6 > DBzDA18C6 >18C6 > DC18C6 > B18C6. The enthalpy and entropy ofthe complexation reactions were determined from thetemperature dependence of the formation constants.