Solvent isotope effects on the complexation and exchange kinetics of Tl(I), K and Na with 18-crown-6 by competitive NMR spectroscopy: Competition between homo- and heterobimolecular cations exchange

The thermodynamic properties of complexation and exchange kinetics of thallium by 18-crown-6 have been studied by thallium NMR spectroscopy. Effects of solvent isotope, counterion (ClO₄⁻ and NO₃⁻) and presence of competitive cations, such as Na⁺ and K⁺, on the exchange characteristics of the system have been considered. The obvious relationships between the effects of D₂O-H₂O solvent isotope on the thermodynamic properties and activation parameters of complexation have been investigated. In the absence of competitor cations, the mechanism of thallium exchange is unimolecular decomplexation and in the presence of competitor cations, homobimolecular cation exchange is the predominant mechanism at low concentrations of the ligand. At higher concentrations of the ligand, the measured rate constants show that the complexation/decomplexation process obeys a heterobimolecular cation interchange mechanism. The rate constants ratios (k_D2O/k_H2O < 1) for unimolecular mechanisms also show an inverse solvent isotope effect.     

Cation transport at 25°c from binary Na+Mn+, Cs+Mn+ and Sr2+Mn+ nitrate mixtures in a H2OCHCl3H2O liquid membrane system containing a series of macrocyclic carriers

Cation fluxes from binary mixtures of either Na⁺, Cs⁺ or Sr²⁺ with other alkali metal cations, alkaline earth metal cations, and Pb²⁺ through a H₂OCHCl³H₂O bulk liquid membrane system containing one of several macrocyclic carriers have been determined Nitrate salts were used in all cases. The most selective transport of Na⁺ over all other cations studied was found with the carrier cryptand [2.2.1]. Selective transport of Na⁺ relative to Li⁺, Cs⁺ and the alkaline earth cations was found with cryptand [2.2.2B] and cryptand [2.2.2D]. The ligands 21-crown-7 and dibenzo-24-crown-8 showed selective transport of Cs⁺ over the second cation in all cases. Several macrocycles showed selectivity for Sr²⁺ over the second cation with the macrocycle 1,10-diaza-18-crown-6 showing the highest selectivity for this cation of all ligands studied. Relative fluxes from binary cation mixtures are rationalized in terms of macrocycle cavity size, donor atom type and ring substituents.     

Selective separation of alkali metal cations by bulk chloroform membranes containing lipophilic crown ether phosphonic acid monoethyl esters

A series of crown ether phosphonic acid monoethyl esters with crown ether ring size variation from 12-crown-4 to 24-crown-8 is used in bulk chloroform membranes to separate alkali metal cations from mixtures. Selective proton-coupled transport of alkali metal cations from weakly alkaline aqueous phases is achieved. With individual ionizable crown ether carriers, transport selectivity for Li⁺, Na⁺, K⁺, and Rb⁺-Cs⁺ is achieved. A closely related lipophilic phosphonic acid monoethyl ester derivative with a cyclohexyl unit in place of the crown ether exhibits transport selectivity for Li⁺. However, the corresponding phosphonic acid diethyl ester is devoid of transport activity. Effects of structural variation within the carrier upon the selectivity and efficiency of competitive alkali metal cation transport are assessed.     

Studies on the macrocycle mediated transport of some metal cations through a bulk liquid membrane system using kryptofix 22

Competitive transport experiments involving Fe⁺³, Cr⁺³, Ni⁺², Co⁺², Ca⁺², Mg⁺² and K⁺ metal cations from an aqueous source phase through some organic membranes into an aqueous receiving phase have been carried out using 4,13-diaza-18-crown-6 (kryptofix 22) as an ionophore present in the organic membrane phase. Fluxes and selectivities for competitive of the metal cations transport across bulk liquid membranes have been determined. A good selectivity was observed for K⁺ cation by kryptofix 22 in 1,2-dichloroethane (1,2-DCE) membrane system. The sequence of selectivity for potassium ion in the organic solvents was found to be: 1,2-DCE > DCM (dichloromethane) >CHCl₃. The transport of K⁺ cation was also studied in the DCM-1,2-DCE, CHCl₃-1,2-DCE and CHCl₃-DCM binary mixed solvents as membrane phase. A non-linear relationship was observed between the transport rate of K⁺ ion and the composition of these binary mixed solvents. The amount of K⁺ transported follows the trend: DCM-DCE > CHCl₃-DCE > CHCl₃-DCM in the bulk liquid membrane studies. Then, the selective transport of K⁺ cation through a DCM-1,2-DCE bulk liquid membrane was studied by kryptofix 22 as an efficient carrier. The highest transport efficiency was obtained by investigating the influence of different parameters such as the concentration of kryptofix 22 in the membrane phase, pH of the source and the receiving phases and the equilibrium time of the transport process. Maximum transport value of 71.62 ± 1.61% was observed for K⁺ ion after 4 hours, when its concentration was 4 × 10^–3 M.     

Nitro derivatives of <Emphasis Type="Italic">N</Emphasis>-alkylbenzoaza-15-crown-5: synthesis,structures, and complexation with metal and ammonium cations

A number of N-alkylnitrobenzoaza-15-crown-5 with the macrocycle N atom conjugated with the benzene ring were obtained. The structural and complexing properties of these compounds were compared with those of model nitrobenzo- and N-(4-nitrophenyl)aza-15-crown-5 using X-ray diffraction, ¹H NMR spectroscopy, and DFT calculations. The macrocyclic N atom of benzoazacrown ethers are characterized by a considerable contribution of the sp3-hybridized state and a pronounced pyramidal geometry; the crownlike conformation of the macrocycle is preorganized for cation binding, which facilitates complexation. The stability constants of the complexes of crown ethers with the NH₄ ⁺, EtNH₃ ⁺, Na⁺, K⁺, Ca²⁺, and Ba²⁺ ions were determined by 1H NMR titration in MeCN-d₃. The most stable complexes were obtained with alkaline-earth metal cations, which is due to the higher charge density at these cations. The characteristics of the complexing ability of N-alkylnitrobenzoaza-15-crown-5 toward alkaline earth metal cations are comparable with analogous characteristics of nitrobenzo-15-crown-5 and are much better than those of N-(4-nitrophenyl)aza-15-crown-5.     

The effect of the counter anion on the transport of thiourea in a PVC-based polymer inclusion membrane using Capriquat as carrier

This paper describes the effect of four counter anions (CH₃COO⁻, Cl⁻, NO₃⁻, ClO₄⁻) of the trioctylmethylammonium (TOMA) cation on the rate of solvent extraction of thiourea and its transport across poly(vinyl chloride) (PVC)-based polymer inclusion membranes (PIMs). The membranes also contained 2-nitrophenyl octyl ether (NPOE) as the plasticizer while chloroform was used as diluent in the solvent extraction studies. It is demonstrated that the counter anion affects substantially the rate of membrane transport and the degree of extraction follows the order: CH₃COO⁻ > Cl⁻ > NO₃⁻ ? ClO₄⁻. The transport rate is negligible for the perchlorate anion. This order is consistent with thiourea interacting with the counter anion through hydrogen bonding to form a heteroconjugate anion.     

Synthesis and crystal structure of hexakis(isothiocyanato)erbium(III) thiocyanate bis(18-crown-6)dipotassium bis[(18-crown-6)ethanolpotassium]

A new complex compound, [K₂(18-crown-6)₂[K(18-crown-6)(EtOH)]₂[Er(NCS)₆](SCN) (I), was synthesized and its crystal structure was studied by X-ray diffraction. In this work, the synthes and X-ray difraction stady of the crystals of a new complex, hexakis (isothiocyanato) erbiu(III) thiocyanate bis(18-crown-6) dipotassium bis(18-crown-6) ethanolpotassium], [K₂(18-crown-6)₂][K(18-crown-6)(ETON)]₂[Er(NCS)₆(SCN)(I)] are described. In crystal I, the alternating [Er(NCS)₆]^3? anions and binuclear complex cation [K(18-crown-6)₂]²⁺ from infinite chains via the F-S bonds, while two complex cations [K(18-crown-6)(ETON)]⁺ and the statistically disordered SCN^? anion between them are linked by the hydragen bonds O-H…S and O-H…N. Complex I contains the host-guest complex cations [K₂(18-crown-6)₂)]²⁺ and [K(18-crown-6)(ETON)]⁺ [1]. The alternating octabedral [Er(NCS)₆]^3? anions and binuclear complex cations [K₂(18-crown-6)₂]²⁺of crystal I form infinite chains via the K-S bonds, while two complex cations [K(18-crown-6)(EtOH)]⁺ and the statistically disordered SCN^? anion lying between them are linked by interionic hydrogen bonds O-H…S and O-H…N. Complex I contains the host-guest complex cations [K₂(18-crown-6)₂]²⁺ and [K(18-crown-6)(EtOH)]⁺ [1].     

Synthesis,binding properties and electrochemistry of nickel(II) macrocyclic complexes containing crown ethers

Macrotetracyclic complexes of nickel(II) containing crown ethers as pendant arms, [Ni(B)](ClO₄)₂ and [Ni(C)](ClO₄)₂, were prepared and characterized. The binding constants of the complexes toward alkali metal ions are relatively small compared with those of free 15-crown-5 or 18-crown-6 and the reduction potentials of the [Ni(B)](ClO₄)₂ and [Ni(C)](ClO₄)₂ in the presence of alkali metal ions shift to the positive direction in the order Li⁺>Na⁺>K⁺ and K⁺>Na⁺>Li⁺, respectively.     

Ion transport numbers in crown-containing polymers

The predominant participation of anions of sorbed electrolytes in electrical charge transfer in polymers was demonstrated based on measurement of the transport numbers of Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺, Tl⁺, and NO₃ ^–ions through homogeneous polymer membranes containing dibenzo-18-crown-6 or dibenzo-24-crown-8. The coordination reaction of the cations with the crown ethers in the polymer phase is the cause of the decrease in the proportion of cations in electrical charge transfer.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 2, pp. 310–314, February, 1990.     

Proton-ionizable crown compounds. 12. Proton-Coupled selective membrane transport of Li+ using a proton-ionizable pyridono macrocycle

The macrocycle-mediated fluxes of several alkali metal cations have been determined in a H₂O-CH₂Cl₂-H₂O liquid membrane system. Water-insoluble proton-ionizable macrocycles of the pyridono type were used. The proton-ionizable feature allows the coupling of cation transport to reverse H⁺ transport. This feature offers promise for the effective separation and/or concentration of alkali metal ions with the metal transport being driven by a pH gradient. A counter anion in the source phase is not co-transported. The desired separation of a particular metal ion involves its selective complexation with the macrocycle, subsequent extraction from the aqueous phase to the organic phase, and exchange for H⁺ at the organic phase-receiving phase interface. Factors affecting transport which were studied include ring size, source phase pH, and receiving phase pH. Lithium was transported at a rate higher than that of the other alkali metals in both single and competitive systems using a 15-crown-5 pyridono carrier.     

Stabilization of a K+-(bis-Cage-Annulated 20-Crown-6) Complex by Bidentate Picrate

Bis-cage-annulated 18-crown-6 and 20-crown-6 macrocyclic ethers (i.e., 1 and 2, respectively) have been synthesized, and their alkali metal picrate extraction profiles have been determined. Host system 1 proved to be a significantly more avid alkali metal cation complexant than 2 and somewhat more avid than 18-crown-6. Both 1 and 18-crown-6 display modest selectivity toward K⁺ and Rb⁺. A stable host–guest complex was prepared by slow evaporation of a CH₂Cl₂–hexane solution of an equimolar mixture of 2 and potassium picrate. The X-ray crystal structure of this complex reveals that picrate anion functions as a bidentate ligand therein. The gas-phase interaction energy between the 2 K⁺ complex and picrate anion was calculated to be ca. –64.9 kcal mol^–1, thereby indicating that participation of picrate anion as an additional bidentate ligand results in significant stabilization of complex 10.     

Preparation and crystal structure of bis[(dibenzo-18-crown-6)potassium]bis(μ<Subscript>2</Subscript>-chloro)-tetrachlorodicuprate(II)

A new complex compound bis[(dibenzo-18-crown-6)potassium]bis(μ₂-chloro)-tetrachlorodicuprate( II), {[K(Db18c6)]₂Cu₂Cl₆} (I) was prepared and its crystal structure was investigated by XRD analysis. Complex molecule I consist of anion [Cu₂Cl₆]²⁻ located in a crystallographic center of inversion, and two centrosymmetrical to each other complex cations [K(Db18c6)]⁺ of “guest-host” type: the cation K⁺ is located in the cavity of the crown-ligand Db18c6 and is coordinated by all its six O atoms, and also by one Cl atom of anion [Cu₂Cl₆]²⁻. The coordination of this cation K⁺ is enlarged up to hexagonal-bipyramidal due to the formation of unusual coordination bond K⁺ → π(      

DFT calculation of benzoazacrown ethers and their complexes with calcium perchlorate

The complexation constants of several azacrown ethers with Ca(ClO₄)₂ were determined and turned out to be the higher, the large the macrocycle. The structures of free ligands and their complexes and the complexation energies were calculated by the DFT method. In the aza-12(15)-crown-4(5) ether complexes with Ca(ClO₄)₂, the metal cations lie outside the averaged plane of heteroatoms of the macrocycle, and the coordination of both counterions is V-like. In the complexes of aza-18-crown-6 ethers, the counterions are in the axial position relatively to the macrocycle in the center of which the Ca²⁺ ion is localized. The complexation energies increase with an increase in the size of the azacrown ether macrocycle. The involvement of the nitrogen atom in binding with the Ca²⁺ ion decreases with the expansion of the macrocycle. Two methods for quantitative estimation of the degree of pre-organization of ligands to complexation were considered: geometric and energetic methods. Benzoaza-15-crown-5 ether is a ligand which is more pre-organized to complexation than N-phenylaza-15-crown-5 ether.     

Preparation and crystal structure of bis(dibenzo-18-crown-6)-(tetrachlorocuprato(II)-Cl,Cl′, Cl″, Cl‴)dipotassium diaqua(dibenzo-18-crown-6)potassium dichlorocuprate(I)-dibenzo-18-crown-6

New mixed complex compound bis(dibenzo-18-crown-6)(tetrachlorocuprato(II)-Cl, Cl′, Cl″, Cl?) dipotassium diaqua(dibenzo-18-crown-6)potassium dichlorocuprate(I)dibenzo-18-crown-6 [(CuCl₄)[K(Db18C6)]₂]·[K(Db18C6)(H₂O)₂]⁺·[CuCl₂]^?·Db18C6 was prepared and its structure was studied by the X-ray structural analysis. The structure was found to be disordered. The asymmetric part of its unit cell contains 1/4 of each of its four components. For a given [CuCl₄]^2? anion its Cu²⁺ cation is disordered over two equally probable positions and its independent Cl atom is disordered over three positions differing by occupancy. In this structure two [K(Db18C6)]⁺ fragment of the complex molecule and the complex cation [K(Db18C6)(H₂O)₂]⁺ are of guest-host type with K⁺ cation as the guest. In this structure the statistically disordered alternating cations and Db18C6 molecules form infinite chains. The statistically disordered [CuCl₂]^? anions also form infinite chains.     

Applications of Macrocyclic Polyethers in Analytical Sensors and Ion Separation

Artificial macrocyclic polyethers were synthesized and applied as neutral carriers for ion-selective PVC membrane electrodes, ion-chromatographic packing materials, extractants and adsorbents for ion separation, coating materials for piezoeletrical membrane sensors for organic species, and ion-transport carriers through liquid membranes. Ion-selective electrodes such as those for K⁺ Na⁺, UO₂²⁺, Cs⁺, Pb²⁺, Fe³⁺, Hg²⁺ and Ag⁺ ions based on crown ether-phosphotungstic acid (PW) precipitates and dithio crown ethers respectively were prepared and showed good sensitivity and selectivity. Crown ether-PW precipitates were applied as adsorbents of rare-earth ions and some common heavy-metal ions. Some rare-earth ions were easily extracted with crown ethers, especially 15-crown-5. Poly(stytene/divinyl benzene) cryptand-22 resin was synthesized and applied as a bifunctional stationary phase of ion chromatography to separate bom cations and anions, even some organic carboxylate geometric isomers. Crown ethers such as mono-benzo-15-crown-5 was successfully applied as a coating material on piezoelectric quartz membrane sensors for some organic species. The oscillation frequency of the crown-ether quartz-membrane sensor was sensitive to organic vapours such as amines and alcohols. Upon adsorption of organic species on the crown-ether quartz membrane, the oscillation frequency of the sensor decreased obviously. Special crown ether such as dibenzo-16-crown-5-oxyacetic acid, decyl-cryptand-22 and 1, 4-dihydro-pyridine-18-crown-5 were synthesized and successfully applied as ion-transport carriers (ionophores) for transport of Na⁺ K⁺ and Mg²⁺ ions through liquid membranes.     

Kinetics of partition between aqueous solutions of salts and bulk liquid membranes containing neutral carriers

The relationship between the rates of transport of alkali metal cations through a bulk chloroform liquid membrane containing polynactin or dibenzo-18-crown-6 as neutral carrier and the rates of uptake and release of cation at the interfaces between aqueous phase and membrane phase were investigated. The fluxes of cations through the membranes and cation-distribution ratios between aqueous solution and membrane were strongly dependent on the anions present. The distribution ratio increased in the following order: Cl^? < NO₃^? < SCN^? < ClO₄^?, and the flux increased in the same order as the distribution ratio, except for the fluxes of KSCN and KClO₄ with polynactin. In the case of polynactin, the flux of KSCN was comparable to that of KClO₄ in spite of the fact that KSCN was less soluble in the membrane than was KClO₄. In order to clarify the cause of this apparently contradictory behavior, the apparent rate constants of uptake and release of potassium were determined independently using an equation derived from Fick's first law of diffusion. From the rates of uptake and release, it was suggested that the overall rate of cation transport through the membrane was dependent on the rate of release rather than that of uptake.     

Proton-driven cation transport through polyamic acid membranes incorporating crown ether moiety

Proton-driven cation transport against cation concentration gradient has been investigated using films of polyamic acid 18-crown-6 (1) and polyamide 18-crown-6 (3)/polyamic acid (5) mixtures as the polymeric membrane. Both membrane systems containing the crown ethers were found to act as efficient alkali metal ion pumps. The ion-transportability of the polyamic acid 18-crown-6 membrane decreased in the order K⁺ > Cs⁺ > Na⁺ > Li⁺, which is reflected in the cation-complexing ability of the 18-crown-6 moiety. The transport selectivity, however, was varied remarkably by the combined use of polyvinylpyrrolidone with (1) and, therefore, by the resulting increase in hydrophilicity of the membrane. The ion-selectivity in the transport through mixed membranes of (3) and (5) was also dependent on the membrane composition. For the proton-driven cation transport two mechanisms are proposed; in one of the transport mechanisms, the carboxylic group cooperates with the crown ether moiety and in the other the carboxylic group participates independently.     

Cation-complexation-induced aggregation and specific ion conduction of lipophilic crowned azobenzenes

Lipophilic azobenzene derivatives incorporating a monoazacrown or oligooxyethylene moiety were employed as a component of ion-conducting composite films containing a polyester elastomer and an alkali metal (Li⁺, Na⁺, K^–) perchlorate. Composite films of monoaza-15-crown-5-containing azobenzene1 exhibited ionic conductivities following the order of Na > Li > K, reflecting the cation-binding selectivity of the 15-crown-5 ring. The ion-conducting behavior of composite films of1 is quite different from that of composite films containing a 1 : 1 mixture of an azobenzene derivative without any crown moiety andN-phenyl-monoaza-15-crown-5 (ionic conductivity order of Li > Na > K >). It was suggested that cation and anion migration is predominant in the LiClO₄ and NaClO₄ systems, respectively. The specific ion conduction can be attributed to ordered aggregation of1 induced by cation complex formation of its crown moiety. Azobenzene derivatives incorporating a monoaza-12-crown-4, monaza-18-crown-6, or oligooxyethylene moiety cannot afford such aggregate formation and specific ion-conduction as is seen in the1 system.     

Synthesis and crystal structure of aqua(dibenzo-18-crown-6)potassium (dibenzo-18-crown-6)(perchlorato-O)potassium perchlorate

A new compound, aqua(dibenzo-18-crown-6)potassium (dibenzo-18-crown-6)(perchlorato-O)potassium perchlorate ([K(DB18C6)(H₂O)]⁺ · [K(ClO₄)(DB18C6)] · ClO ₄ ^? ; compound I) is synthesized and studied by X-ray crystallography. The crystals are triclinic: a = 9.050 Å, b = 9.848 Å, c = 26.484 Å, α = 82.87°, β = 84.16°, γ = 77.93°, Z = 2, space group P $\bar 1$ . The structure is solved by a direct method and refined by the full-matrix least-squares method in the anisotropic approximation to R = 0.058 for 5960 independent reflections (CAD4 diffractometer, λMoK _α radiation). A complex cation [K(DB18C6)(H₂O)]⁺ and a complex molecule [K(ClO₄)(DB18C6)] are of the host-guest type; they are linked into a dimer through two K⁺ → π(C) bonds formed by one of the two K⁺ cations with two C atoms of the benzene ring of the DB18C6 ligand from the adjacent complex. Both DB18C6 ligands in I have a butterfly conformation with approximate symmetry C _2v .     

Solvent extraction of sodium perrhenate by 3m-crown-m ethers (m = 5, 6) and their mono-benzo-derivatives into 1,2-dichloroethane: Elucidation of an overall extraction equilibrium based on component equilibria containing an ion-pair formation in water

Equilibrium constants () for the ion-pair formation of a complex ion NaL⁺ with ReO₄⁻ in water were determined potentiometrically at 25 °C and the ionic strength (I) of 0 mol dm⁻³ using a Na⁺-selective electrode. Here, crown ethers, L, were 15-crown-5 ether (15C5), benzo-15C5, 18-crown-6 ether (18C6) and benzo-18C6. Also, NaReO₄ was extracted by the L into 1,2-dichloroethane and then extraction constants (K_ex/mol⁻² dm⁶) for the species, NaLReO₄, were determined at 25 °C by AAS. These K_ex values were resolved into four component equilibrium constants containing K_MLA calculated at given I values. Based on these data, extraction-abilities of the L against the perrhenate were discussed in comparison with those of sodium picrate-L systems reported previously.