Determination of the thermodynamic parameters,stoichiometry, and stability constants for complex formation of dicyclohexyl-18-crown-6 and 15-crown-5 with Y3+, La3+, and Hg2+ cations in binary solutions

The complexation of Y³⁺, La³⁺, and nd Hg²⁺ cations with macrocyclic ligands, dicyclohexyl-18-crown-6 (DCH18C6) and 15-crown-5 (15C5) have been studied in acetonitrile (AN)-N,N-dimethylformamide (DMF) binary solutions at different temperatures using conductometric method. The conductance data revealed 1: 1 [ML] stoichiometry for most complexes in pure DMF and AN-DMF binary solutions, except for the (DCH18C6-Y³⁺) complex in pure AN (1: 2, [ML₂]). The stability constants of DCH18C6-La³⁺ and 15C5-La³⁺ in pure AN were higher than in pure DMF at all temperatures. Nonlinear behavior was observed for the stability constants of complexes against the composition of AN-DMF binary solutions at all temperatures. The minimum log K _f value for the 15C5-La³⁺ complex in AN-DMF binary solutions was obtained at χ_AN = 0.5, which may be due to negative excess viscosities η^E of AN-DMF mixtures over the whole composition range with a minimum value of χ_AN = 0.5. Moreover, the selectivity order of DCH18C6 and 15C5 for Y³⁺, La³⁺, and Hg²⁺ cations 25°C depended on the AN-DMF ratio. The thermodynamic parameters (ΔH _C ⁰ ) for complex formation were obtained from the temperature dependences of the stability constants of the complexes using the van’t Hoff plots, and the standard entropy (ΔS _C ⁰ ) was calculated from the relationship: ΔG _{C, 298.15} ⁰ = ΔH _C ⁰ ? 298.15ΔS _C ⁰ .     

Separation of uranium(VI) as chloride complex by solvent extraction with dicyclohexyl-18-crown-6

Uranium(VI) was quantitatively extracted from 6 to 8M hydrochloric acid with 0.02M DC-18-crown-6 in chloroform. It was stripped from the organic phase with 0.5M hydrochloric acid and determined as its Arsenazo-III complex at 665 nm. Uranium(VI) was separated from several elements such as thorium, zirconium, scandium, yttrium, thallium and tin in complex mixtures. The method was extended for analysis of uranium in monazite and rock sample.     

Radiopolarographic determination of the stability constant of 18-crown-6 complex for Ra2+

The interaction of Ra²⁺, Sr²⁺ and Ba²⁺ with 1,4,7,10,13,16-hexaoxacyclooctadenace /18-crown-6/ in an aqueous solution was investigated by radio- and dc polarography. It was found that all these ions form 11 complexes with 18-crown-6 and their stability constants, lg K, are 3.43 for Ra²⁺, 2.40 for Sr²⁺ and 3.67 for Ba²⁺, respectively. The stability constant for Am²⁺ was estimated from the relationship between lg K for Sr²⁺, Ba²⁺, Ra²⁺, Eu²⁺, Yb²⁺ and Cf²⁺ and the ionic radii.     

Thermodynamic study of complex formation of benzo-18-crown-6 with K+, Tl+, and Pb2+ in water

H ⁰ and S ⁰ values of the complex formation in water of benzo-18-crown-6 (B18C6) with K⁺, Tl⁺, and Pb²⁺ were determined and compared with those of 18-crown-6. The H⁰ values of B18C6 are negative. The stability in water of the B18C6-metal ion complex at 25°C is governed largely by the magnitude of the H ⁰ value. The B18C6-metal ion complex is less stable in water than the corresponding 18C6-metal ion complex. This is due largely to a less favorable enthalpic contribution of the B18C6-metal ion complex compared with the corresponding 18C6-metal ion complex. The two aromatic ether oxygen atoms of B18C6 are responsible for the larger H ⁰ value of the B18C6-metal ion complex compared with the corresponding 18C6-metal ion complex.     

Stability constant of the sodium complex with 18-crown-6 in the mixed water-ionic liquid solvent

The stability constant of the Na⁺ complex with dibenzo-18-crown-6, [Na(DB18C6)]⁺, was measured by the ²³Na NMR method in the mixed water-ionic liquid solvent at 23°C. N-Butyl-4-methylpyridinium tetrafluoroborate (BMPTB) was used as room temperature ionic liquid. It was found that for the solvent containing 0.8 mol fraction of water and 0.2 mol fraction of BMPTB, the value log K = 0.77 ± 0.05. The addition of BMPTB to water was found to increase the solubility of the crown ether but to decrease the sodium nitrate solubility.     

Effect of solvent on complexation between Y3+ cation and 4,13-diaza-18-crown-6 in some binary mixed non-aqueous solvents

The complexation reaction of 4,13-diaza-18-crown-6 (DA18C6) with Y³⁺ cation was studied in some binary mixed solvent solutions of acetonitrile (AN) with methanol (MeOH), ethanol (EtOH), 2-propanol (2-PrOH) and methyl acetate (MeOAc) at different temperatures by conductometric method. The obtained data show that in all studied solutions the stoichiometry of the complex formed between DA18C6 and Y³⁺ cation is 1: 1 [ML], but in the case of pure MeOAc, a 2: 1 [ML₂] complex is formed in solution upon addition of the ligand to the metal salt solution, and further addition of the ligand results in formation of a M₂L₂ complex in solution. This results show that the stoichiometry of the composition of the macrocyclic complexes may be affected by the nature of the solvent system. The results obtained in this study show that the stability constant of the resulting 1: 1 [ML] complex in the binary solvent solutions decreases in the order: AN-MeOAc > AN-2PrOH > AN-MeOH > AN-EtOH. A non-linear relationship was observed between the stability constant (logK _f ) of [Y(DA18C6)]³⁺ complex with the composition of the binary mixed solvent solutions. The corresponding standard thermodynamic parameters (H° _c , Δ S° _c ) for 1: 1 [ML] complexation reaction between DA18C6 and Y³⁺ cation were obtained from temperature dependence of the stability constant of the complex. The results show that, in all solvent systems, the (DAI8C6.Y)³⁺ complex is entropy stabilized, but from enthalpy point of view, depending on the solvent system, it is stabilized or destabilized and the result show that the values of both thermodynamic quantities change with the nature and composition of the binary mixed solvent solutions.     

Study of complex formation between dicyclohexyl-18-crown-6 and UO2 2+ cation in some binary mixed non-aqueous solvents using conductometric method

In the present work, the complexation process between UO₂ ²⁺ cation and the macrocyclic ligand, dicyclohexyl-18-crown-6 (DCH18C6) was studied in ethyl acetate/1,2-dichloroethane (EtOAc/DCE), acetonitrile/1,2-dichloroethane (AN/DCE), methanol/1,2-dichloroethane (MeOH/DCE) and ethanol/1,2-dichloroethane (EtOH/DCE) binary solutions at different temperatures using the conductometric method. The conductance data show that in most cases, the stoichiometry of the complex formed between DCH18C6 and UO₂ ²⁺ cation is 1:1 [M:L], but in some solvent systems also a 1:2 [M:L₂] complex is formed in solutions. The values of stability constant of (DCH18C6·UO₂)²⁺ complex which were obtained from conductometric data, show that the stability of the complex is affected by the nature and also the composition of the solvent system and in all cases, a non-linear behavior is observed for the variation of (log?K _f) of the (DCH18C6·UO₂)²⁺ complex versus the composition of the binary mixed solvents. The values of thermodynamic quantities $ \Updelta H_{c}^{\circ} $ and $ \Updelta S_{c}^{\circ} $ for formation of (DCH18C6·UO₂)²⁺ complex were obtained from temperature dependence of the stability constant using the van’t Hoff plots. The experimental results show that depending on the nature and composition of the solvent systems, the complex is enthalpy stabilized or destabilized, but in most cases, it is stabilized from entropy view point and both thermodynamic parameters are affected by the nature and composition of the binary mixed solutions.     

Influence of polar solvents upon the complex formation of 18-crown-6 with cations in chloroform

The complex formation of crown ethers with cations in nonpolar medium with small amounts of polar solvents added has been studied. The goal has been to get deeper insight into the influence of solvation (hydration) of the salts for the formation of complexes with the macrocyclic ligand 18-crown-6 (18C6). A linear dependence of the reaction enthalpy for complex formation between 18C6 and alkali metal cations in chloroform in the presence of water or methanol has been observed. The presence of acetonitrile or acetone has had no influence upon the measured reaction enthalpies. The influence of methanol on the complex formation between 18C6 and alkali metal cations in chloroform is weaker than in the case of water. This underpins the selective solvation of alkali cations in chloroform after the addition of small amounts of water or methanol. The experiments have been performed using calorimetric titrations.     

The influence of the composition of an aqueous-acetone solvent on the thermodynamic characteristics of complex formation of 18-crown-6 ether with glycine

The heat effects of solution of 18-crown-6 ether were determined calorimetrically over a wide range of water-acetone solvent compositions (298.15 K). The influence of the composition of water-acetone solvents on the enthalpies of complex formation between 18-crown-6 and glycine and solvation of chemical equilibrium participants was studied. It was shown that the factor determining changes in the enthalpy of the reaction was changes in the solvation state of the crown ether.     

A thermodynamic study of the association of alkali metal cations with dicyclohexano-18-crown-6

A thermodynamic study of the association of Na⁺, K⁺, Rb⁺, and Cs⁺ with dicyclohexano-18-crown-6 in acetonitrile has been carried out at 308, 303, 298, 293, and 288 K using a conductometric technique. The observed molar conductivities, A, were found to decrease significantly for mole ratios less than unity. A model involving 11 stoichiometry has been used to analyze the conductivity data. The stability constant,K, and the limiting molar conductivity, A _c , for each 11 complex were determined from the conductivity data by using a nonlinear least squares curve fitting procedure. The binding sequence, based on the value of logK at 298 K, as derived from this study is K⁺>Na⁺>Rb⁺>Cs⁺. Values of H ^o and S ^o are reported and their significance is discussed.     

Complex formation of Ag+ with polyether 18-crown-6

The complex formation of Ag⁺ with polyether 18-crown-6 (18C6) and their solvation have been studied using calorimetric and potentiometric methods in H₂O-EtOH solvents in wide range of ethanol concentration. The standard enthalpies of dissolution AgNO₃, AgClO₄ and 18C6 in aqueous-ethanol solvents are obtained. The stability of a complex [Ag18C6]⁺ grows with increasing the EtOH content a solvent. Using the method based on the thermodynamic characteristics of solvation of metal-ion, ligand and complex-ion the interpretation of the results has been given. This revised version was published online in August 2006 with corrections to the Cover Date.     

Thermodynamic behavior of complexation process between dibenzo-18-crown-6 and K+, Ag+, NH 4 + , and Hg2+ cations in ethylacetate-dimethylformamide binary media

The complexation reactions between K⁺, Ag⁺, NH₄⁺, and Hg²⁺ cations and the macrocyclic ligand, dibenzo-18-crown-6 (DB18C6), were studied in ethylacetate (EtOAc)-dimethylformamide (DMF) binary mixtures at different temperatures using the conductometric method. The conductance data show that the stochiometry of all the complexes is 1:1. A non-linear behavior was observed for the variation of log K _f of the complexes versus the composition of binary mixed solvents, which was discussed in terms of heteroselective solvation and solvent-solvent interactions in binary solutions. It was found that the stability order of the complexes changes with changing the composition of the mixed solvents. The sequence of stabilities for the K⁺, Ag⁺, NH₄⁺, and Hg²⁺ complexes with DB18C6 in EtOAc-DMF binary solutions (mol. % DMF 25.0) and (mol. % DMF 50.0) at 25°C is (DB18C6-Ag)⁺ > (DB18C6-K)⁺ > (DB18C6-Hg)²⁺ > (DB18C6-NH₄)⁺, but in the cases of pure DMF and a binary solution of EtOAc-DMF (mol. % DMF 75.0) is (DB18C6-K)⁺ > (DB18C6-Hg)²⁺ > (DB18C6-Ag)⁺ ≈ (DB18C6-NH₄)⁺. The values of thermodynamic quantities (ΔH _c ^o, ΔS _c ^o) for these complexation reactions have been determined from the temperature dependence of the stability constants, and the results show that the thermodynamics of the complexation reactions is affected by the nature and composition of the mixed solvents and, in all cases, positive values of ΔS _c ^o characterize the formation of these complexes. In addition, the experimental results show that the values of entropies for the complexation reactions between K⁺, Ag⁺, NH₄⁺, and Hg²⁺ cations and DB18C6 in EtOAc-DMF binary solutions do not change monotonically with the solvent composition. The text was submitted by the authors in English.     

A thermodynamic study of complexation of 18-crown-6 with Zn2+, Tl+, Hg2+ and {\text{UO}}^{{{\text{2 + }}}}_{{\text{2}}} cations in acetonitrile-dimethylformamide binary media and study the effect of anion on the stability constant of (18C6-Na+) complex in methanol solutions

The equilibrium constants and thermodynamic parameters for complex formation of 18-crown-6(18C6) with Zn²⁺, Tl⁺, Hg²⁺ and $ {\text{UO}}^{{{\text{2 + }}}}_{{\text{2}}} $ cations have been determined by conductivity measurements in acetonitrile(AN)-dimethylformamide(DMF) binary solutions. 18-crown-6 forms 1:1 complexes [M:L] with Zn²⁺, Hg²⁺ and $ {\text{UO}}^{{{\text{2 + }}}}_{{\text{2}}} $ cations, but in the case of Tl⁺ cation, a 1:2 [M:L₂] complex is formed in most binary solutions. The thermodynamic parameters ( $ \Delta {\text{H}}^{ \circ }_{{\text{c}}} $ and $ \Delta {\text{S}}^{ \circ }_{{\text{c}}} $ ) which were obtained from temperature dependence of the equilibrium constants show that in most cases, the complexes are enthalpy destabilized but entropy stabilized and a non-monotonic behaviour is observed for variations of standard enthalpy and entropy changes versus the composition of AN/DMF binary mixed solvents. The obtained results show that the order of selectivity of 18C6 ligand for these cations changes with the composition of the mixed solvent. A non-linear relationship was observed between the stability constants (logK_f) of these complexes with the composition of AN/DMF binary solutions. The influence of the $ {\text{ClO}}^{ - }_{{\text{4}}} $ , $ {\text{NO}}^{ - }_{{\text{3}}} $ and $ {\text{Cl}}^{ - } $ anions on the stability constant of (18C6-Na⁺) complex in methanol (MeOH) solutions was also studied by potentiometry method. The results show that the stability of (18C6-Na⁺) complex in the presence of the anions increases in order: $ {\text{ClO}}^{ - }_{{\text{4}}} $  >  $ {\text{NO}}^{ - }_{{\text{3}}} $  >  $ {\text{Cl}}^{ - } $ .     

Potentiometric study of the mixed-metal complex formation of tetracarboxylate-18-crown-6 with aluminum and alkali and alkaline earth cations

We give evidence for the formation of mixed-metal complexes of tetracarboxylate-18-crown-6 in the form of Al³⁺-macrocycle-M^Z+, with M^Z+ being an alkali or an alkaline earth cation. These binuclear systems are characterized by potentiometric p[H] titrations and the equilibrium constants for all major complexes formed are described. The results are presented in the form of distribution diagrams revealing the concentrations of individual complex species as a function of p[H]. The stability of the resulting mixed-metal complexes with Al³⁺ and alkali and alkaline earth cations vary in the order Na⁺>K⁺>Cs⁺ and Ca²⁺>Sr²⁺>Ba²⁺. The complex behavior can be rationalized in terms of electrostatic interactions and direct coordination of the cations by the carboxylate groups from the crown ether periphery.     

Crystal structures of the dicyclohexyl-(18-crown-6) uranyl perchlorate complex and of its hydrolysis product

The structures of dicyclohexyl-(18-crown-6) uranyl perchlorate, [(C₂₀H₃₆O₆)UO₂] (ClO₄)₂ (complex I) and of dicyclohexyl-(18-crown-6) uranyl hydroxyperchlorate [C₂₀H₃₆O₆]₃ [(UO₂)₂(H₂O)₆] · (ClO₄)₂, CH₃CN, (complex II) have been determined from three dimension X-ray diffraction data.The uranyl group is directly coordinated to the oxygen atoms of the polyether ring in complex I; its hydrolysis (complex II) leads to a dimerization of the uranyl ions by sharing two OH groups with an U-U distance of only 3.827(8) Å. The polyether molecules are connected by hydrogen bonds with the dimeric ion [(UO₂)₂ (OH)₂ (H₂O)₆]²⁺.     

Liquid-liquid extraction of zirconium from hafnium and other elements with dicyclohexyl-18-crown-6

Zirconium was quantitatively extracted with 2.5 × 10^?2 M dicyclohexyl-18-crown-6 in dichloromethane from 8.5 M hydrochloric acid. It was stripped with 0.5 M hydrochloric acid and was determined spectrophotometrically as its complex with Arsenazo III. Hafnium was not extracted under these conditions, but from the residual aqueous phase it was extracted with 7.0 × 10^?2 M dicyclohexyl-18-crown-6 in dichloromethane from 9.0 M hydrochloric acid. It was stripped with 0.1 M perchloric acid and determined spectrophotometrically at 540 nm as its complex with xylenol orange. The separation of zirconium and hafnium from other metals is also described.     

A thermodynamic study of complex formation between dicyclohexyl-18-crown-6 (DCH18C6) and La3+, UO 22+ , Ag+, and NH 4+ cations in acetonitrile-tetrahydrofuran binary media using conductometric method

The complex formation between La³⁺, UO₂²⁺ Ag⁺, and NH₄⁺ cations and macrocyclic ligand, dicyclohexyl-18-crown-6 (DCH18C6), was studied in acetonitrile-tetrahydrofuran (AN-THF) binary mixtures at different temperatures using the conductometric method. The results show that with the exception of complexation of the NH₄⁺ cation with DCH18C6 in pure acetonitrile, the stoichiometry of all the complexes is being 1: 1 (M: L). The stability constants of the complexes were determined using a GENPLOT computer program. The nonlinear behavior which was observed for changes of log K _f of the complexes versus the composition of the mixed solvent was discussed in terms of solvent-solvent interaction in their binary solution, which results in changing the chemical and physical properties of the constituent solvents when they mix with one another and, therefore, changing the solvation capacities of the metal cations, crown ether molecules, and even the resulting complexes with changing the mixed solvent composition. The results show that the selectivity of DCH18C6 for the studied cations changes with the composition of the AN-THF binary system. The sequence of stabilities of complexes in an AN-THF binary solution (mol. % AN = 75.0) at 25°C is [(DCH18C6)La)]³⁺ > [(DCH18C6)UO₂]²⁺ > [(DCH18C6)Ag]⁺ ∼ [(DCH18C6)NH₄]⁺, but in the case of other binary systems of AN/THF (mol. % AN = 25.0 and 50.0) is [(DCH18C6)La]⁺ > [(DCH18C6)NH₄]⁺ ∼ [DCH18C6)UO₂]²⁺ > [(DCH18C6)Ag]⁺. The text was submitted by the authors in English.     

Synthesis of a crystalline molecular complex of Y2+, [(18-crown-6)K][(C5H4SiMe3)3Y

The La(2+) complex [K(18-crown-6)(OEt(2))][Cp″(3)La] (1) [Cp″ = C(5)H(3)(SiMe(3))(2)-1,3], can be synthesized under N(2), but in the presence of KC(5)Me(5), 1 reduces N(2) to the (N═N)(2-) product [(C(5)Me(5))(2)(THF)La](2)(μ-η(2):η(2)-N(2)). This suggests a dichotomy in terms of ligands that optimize isolation of reduced dinitrogen complexes versus isolation of divalent complexes of the rare earths. To determine whether the first crystalline molecular Y(2+) complex could be isolated using this logic, Cp'(3)Y (2) (Cp' = C(5)H(4)SiMe(3)) was synthesized from YCl(3) and KCp' and reduced with KC(8) in the presence of 18-crown-6 in Et(2)O at -45 °C under argon. EPR evidence was consistent with Y(2+) and crystallization provided the first structurally characterizable molecular Y(2+) complex, dark-maroon-purple [(18-crown-6)K][Cp'(3)Y] (3).     

Effect of the 18-crown-6 and benzo-18-crown-6 on the solvent extraction and separation of lanthanide(III) ions with 8-hydroxyquinoline

The synergistic solvent extraction of 13 lanthanides with mixtures of 8-hydroxyquinoline (HQ) and the crown ethers (S) 18-crown-6 (18C6) or benzo-18-crown-6 (B18C6) in 1,2-dichloroethane has been studied. The composition of the extracted species has been determined as LnQ₃ · S. The values of the equilibrium constant and separation factor have been calculated. Here, the effect of the synergistic agent (18C6 or B18C6) on the extraction process is discussed.     

Cation-crown ether complex formation in water. II. Alkali and alkaline earth cations and 12-crown-4, 15-crown-5, and 18-crown-6

The stability constants and the partial molal volume and isentropic partial molal compressibility changes of complex formation between cations and crown ethers in water at 25°C are presented. The cations involved are Na⁺, K⁺, Rb⁺, Cs⁺, Ca²⁺, and Ba²⁺, and the crown ethers are 12-crown-4, 15-crown-5, and 18-crown-6. Values of V of complex formation have been discussed in terms of two simple models, one based on the scaled particle theory, and the others on the Drude-Nernst continuum model. The results indicate that the charge of the potassium cation in 18-crown-6 is especially well screened from the water. On this basis hydration numbers of complexed cations have been calculated. This shows that the size of the cation compared to the crown ether hole is important for the contacts between complexed cations and water.