Thermodynamics of Complex Formation and Solvation in the System NaCl-18-crown-6-Water-Dioxane

Stability constants of Na⁺ complexes with 18-crown-6-ether and thermodynamic characteristics of the complex formation in water and mixed water-dioxane solvents (0.2, 0.4, 0.6, and 0.8 wt. fraction of dioxane, 283-318 K) were determined by the method of EMF of galvanic circuits without transfer. Comparative thermodynamic analysis of the complex 18-crown-6Na⁺ formation reactions in water-dioxane, water-acetonitrile, water-acetone, water-methanol, and water-2-propanol mixtures was carried out. Contributions of the Gibbs energies of transfer (G _t) of 18-crown-6Na⁺, Na⁺, and the ligand to the increase in the stability of the complexes on replacement of water by mixed water-dioxane solvents were estimated. It was shown that the increase in the stability of sodium crown ether complexes primarily depends on solvation of the complex cation and desolvation of the central cation. Changes in the conformational Gibbs energy of the ligand and quantitative parameters of selective solvation of the reagents were estimated.     

Equilibrium Constant Studies for Complexation between Ammonium Ions and 18-Crown-6 in Aqueous Solutions at 298.15 K

Osmotic vapor pressure and density measurements have been carried out for binary aqueous and ternary aqueous solutions containing a fixed concentration of 18-crown-6 (0.2 mol⋅kg⁻¹) and ammonium chloride or ammonium bromide at 298.15 K. The concentration of the ammonium salts was varied between 0.02 to 0.5 mol⋅kg⁻¹. The measured water activities were used to obtain the activity coefficient of water and the mean molal activity coefficient of the ions in binary as well as ternary solutions. Using the method developed by Patil and Dagade reported earlier in this journal and the McMillan-Meyer pair and triplet Gibbs energy interaction parameters, the thermodynamic equilibrium constant (K) for the 18-crown-6:NH₄ ⁺ complexes were determined. It is observed that the nature and polarizability of anions play important roles in imparting stability to the complexed species. The log₁₀ K values for the 18-crown-6:NH₄ ⁺ complexed species are lower than for the complexes involving alkali metal ions such as K⁺. The volume of complexation for the studied systems obtained from the apparent molar volumes of ammonium halides in ternary solutions are positive and of smaller magnitude than those reported for complexation with alkali ions. The results are further discussed in terms of water structural effects, complex formation, the role of counter anions and hydrophobic interactions.     

Solvation-thermodynamic effects of the water-methanol solvent on the coordination of Na+, K+, NH 4 + , and Ag+ ions with 18-crown-6: Gibbs energies of complexation and resolvation of reagents

The results of solvation-thermodynamic monitoring of aqueous-methanol solutions of electrolytes (NaCl, KCl, NH₄Cl, AgNO₃) and 18-crown-6 ether (L) in the mole fraction scale are summarized and systematized. The stability of sodium mono(crown ether) complexes in water-methanol solvents is due to both enthalpy and entropy contributions, and the stability of the ammonium and silver complexes, to the enthalpy contribution. The solvent effects in formation of crown ether complexes of sodium, potassium, ammonium, and silver are subjected to solvation-thermodynamic and correlation analyses. An equation is suggested for estimating the ion selectivity of crown ethers, and the contributions of energy constituents (the Gibbs energy of transfer of reagents) to varation of the ion selectivity of 18-crown-6 toward M-Na⁺ pairs in the water-methanol solvent are revealed.     

The influence of solvation on the formation of Ag<Superscript>+</Superscript> complexes with 18-crown-6 ether in water-dimethyl sulfoxide solvents

The Gibbs energies of transfer of 18-crown-6 ether from water into water-dimethyl sulfoxide (DMSO) solvents (χ_DMSO = 0.0–0.97 mole fractions) at 298.15 K were determined by the interphase distribution method. Changes in the composition of the aqueous-organic solvent did not cause noticeable changes in the stability of 18-crown-6 ether solvato complexes. Reagent solvation contributions to shifts of complex formation equilibrium between silver(I) and 18-crown-6 ether when water was replaced with dimethyl sulfoxide were analyzed.     

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.     

Complex formation of crown ethers with α-amino acids: Estimation by NMR spectroscopy

Complex formation of 18-crown-6 and dibenzo crowns with glycine, leucine, and norleucine was studied by NMR spectroscopy. The efficiency of non-valence interactions with participation of different active centers of the host and guest molecules is determined by solvation effects, mutual arrangement of benzene rings in dibenzo crowns, and the presence of bulky aliphatic substituents in the α-amino acid. The complexation of dibenzo crowns with α-amino acids in acid medium involves a system of different non-valence interactions, the most efficient of which are NH ₃ ⁺ ... O hydrogen bond between the ammonium group in the guest molecule and ether oxygen atoms in the host molecule and dipole-dipole interaction between the guest ammonium group and host benzene ring (NH ₃ ⁺ ... Ar). The efficiency of NH ₃ ⁺ ... O hydrogen bonding decreases in going from 18-crown-6 to dibenzo crowns due to distortion of symmetry of the macroring cavity and violation of geometric complementarity of some ether oxygen atoms. The integral efficiency of non-valence interactions in the system dibenzo crown-α-amino acid was estimated on a quantitative level by ¹H NMR relaxation technique.     

Gibbs energies of transferring triglycine from water into H2O-DMSO solvent

The Gibbs energies of transferring triglycine (3Gly, glycyl-glycyl-glycine) from water into mixtures of water with dimethyl sulfoxide (χ_DMSO = 0.05, 0.10, and 0.15 mole fractions) at 298.15 K are determined from the interphase distribution. An increased dimethyl sulfoxide (DMSO) concentration in the solvent slightly raises the positive values of Δ_tr G ^○(3Gly), possibly indicating the formation of more stable 3Gly-H₂O solvated complexes than ones of 3Gly-DMSO. It is shown that the change in the Gibbs energy of transfer of 3Gly is determined by the enthalpy component. The relationship of 3Gly and 18-crown-6 ether (18C6) solvation’s contributions to the change in the Gibbs energy of [3Gly18C6] molecular complex formation in H₂O-DMSO solvents is analyzed, and the key role of 3Gly solvation’s contribution to the change in the stability of [3Gly18C6] upon moving from H₂O to mixtures with DMSO is revealed.     

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.     

Anion dependence of crown ether selectivities for alkali picrates and sulfonates in dioxane and toluene. A synergistic effect

The binding constants,K _N, of sodium and potassium 8-anilinonaphthalene-1-sulfonate (ANS) and of sodium 5-dimethylamino-1-naphthalenesulfonate (DNS) to benzo-18-crown-6 bound to a 2% cross-linked polystyrene network (RN18C6) were measured spectrophotometrically in dioxane and the results compared with those obtained for picrate salts. The network RN18C6 was then used to measure in dioxane and toluene by a competition method the equilibrium constant,K, of the reaction A^–M⁺N+CrA^–M⁺Cr+N.A^–M⁺N denotes the ionic solute (ANS, DNS, methyl orange or picrate salt) bound to the network RN18C6 (N) and A^–M⁺Cr is the solute bound to a soluble ligand Cr, where Cr represents a series of 18-crown-6 and 15-crown-5 compounds. Combining theK _N andK values the formation constants,K _L, of the crown ether complexes of the respective salts were obtained in dioxane. The data show a reversal in the complexation strength of the 18-crown-6 compounds in dioxane when sodium picrate is replaced by sodium ANS. The results were rationalized in terms of a synergistic effect exerted by dioxane, with dioxane forming a 1:1 dioxanate with the crown ion pair complex. This effect is especially strong with ANS and with a rigid planar crown ether like dibenzo-18-crown-6. The binding constants,K _N, of NaANS and NaDNS to RN18C6 in dioxane are nearly three times larger than for sodium picrate, and the same holds for the potassium salts. Differences in anion interactions with the network appear to be a plausible cause for the anion dependence ofK _N.     

A thermodynamic study of reactions of amino acids with crown ethers in nonaqueous media as examples of guest—host molecular complex formation

Our findings and the literature data on the effect of binary aqueous organic solvents on the enthalpy and entropy of molecular complexation reactions of 18-crown-6 with glycine, D, L-alanine, and L-phenylalanine and on the stability of the resulting complexes were reviewed. The relationships between the thermodynamic parameters of the complexation reactions and the reactant solvation were analyzed to reveal the key factors that are crucial for the stability of the complexes and the increasing exothermicity of the processes under study in water—ethanol, water—DMSO, and water—acetone. The changes in the stability of 18-crown-6 complexes with amino acids were assumed to be predicted from the Gibbs energy changes in the transsolvation of the amino acids. This criterion was proposed as a basic thermodynamic parameter for estimating the changes in the stability of 18-crown-6 complexes with peptides and ammoniumtype cations (structurally related to amino acids) when the composition of the binary aqueous organic solvent is varied.     

The thermochemistry of complexation of silver(I) with nicotinamide in water-ethanol solvents

The changes in the enthalpy of formation of complexes of nicotinamide with Ag⁺ ion at 25 ± 0.1^°C and ionic strength of 0.25 (NaClO₄) were obtained in the composition range of water-ethanol solvent from 0.1 to 0.9 mole fractions using precise calorimetry. We ascertained the stabilization of nicotinamide-silver(I) complexes in water-ethanol solvents and the maximum increase in exothermicity of the reaction in ethanol concentration range from 0 to 0.3 mole fractions. The enthalpy component of the change in the Gibbs energy of the complexation of Ag⁺ ions with NicNH₂ was shown to dominate over the entropy component; the changes in thermodynamic argue for the prevailing contribution of the solvation state of a ligand to the exothermicity of complexation process at low ethanol concentrations.     

Effect of the Composition of Ethanol–DMSO Solvents on the Stability of Silver(I) Complexes with 18-Crown-6

The effect of composition of ethanol–dimethyl sulfoxide (EtOH–DMSO) solvents (χ_DMSO = 0.0–1.0 mole fractions) on the stability of silver(I) complexes with 18-crown-6 ether (18C6) has been studied potentiometrically at 298.15 K. The increasing of DMSO concentrations in mixed solvents are shown to considerably reduce the stability of 18C6 complexes with silver(I) ion ([Ag18C6]⁺). A change in the solvation state of the central ion is suggested to be the key factor in shifting complexing equilibrium.     

Stability of Complexes of NH4 + with 18-Crown-6, Dicyclohexyl-18-Crown-6, Dibenzo-18-Crown-6 and Dibenzo-24-Crown-8 in Water Saturated Nitrobenzene

From extraction experiments with ²²Na tracer, the exchange extraction constants corresponding to the NH₄ ⁺(aq) + NaL⁺ (nb)NH₄L⁺(nb) + Na⁺ (aq) equilibrium taking place in the two-phase water-nitrobenzene system (L = 18-crown-6, dicyclohexyl-18-crown-6, dibenzo-18-crown-6 and dibenzo-24-crown-8; aq = aqueous phase, nb = nitrobenzene phase) were evaluated. Furthermore, the stability constants of the NH₄L⁺ complexes in nitrobenzene saturated with water were calculated; they were found to increase in the order dibenzo-24-crown-8 (DB24C8) < dibenzo-18-crown-6 (DB18C6) < dicyclohexyl-18-crown-6 (DCH18C6) < 18-crown-6 (18C6).     

Effects of Solvent Composition and Temperature on K+–18-Crown-6 Complexation in Acetonitrile–Water Mixed Solvents

Complexation of K⁺ by 18-crown-6 ether (18C6) in pure water and in acetonitrile–water mixed solvents containing 0.1 mol-dm^{? 3} (C₂H₅)₄NCl has been systematically studied by isothermal titration calorimetry (ITC) at 293, 298, and 303 K. The formation constant K of the 1:1 [K(18C6)]⁺ complex and the complexation enthalpy Δ _rH were simultaneously determined from the titration data. The logK and Δ _rH(kJ-mol^{? 1}) values at 298 K are 2.04, ?26.2 in pure water and 2.23, ?25.0; 2.61, ?24.2; 2.95, ?23.8; 3.48, ?21.0; 3.85, ?19.4; 4.36, ?18.7; and 5.73, ?17.0 in the mixed solvents at x _AN (mole fraction of acetonitrile) of 0.043, 0.135, 0.258, 0.448, 0.578, 0.759, and 1.0, respectively. The change in heat capacity for the complex formation, Δ C _p °, was also determined by the temperature dependence of Δ _rH. Whereas the Δ C _p ° is (57 ± 11) and (63 ± 20) J-mol^{? 1}-K^{? 1} in pure water and in the solvent mixture at x _AN = 0.043, respectively, it decreases with increasing x _AN. The Δ C _p ° values are ?(48 ± 11), ?(110 ± 25), ?(354 ± 40), ?(359 ± 24), and ?(304 ± 30) J-mol^{? 1}-K^{? 1} at x _AN = 0.135, 0.258, 0.448, 0.578, and 0.759, respectively. The changes in complexation thermodynamics (Δ Δ _rG, Δ Δ_rH, and Δ Δ _r S) are discussed in terms of the corresponding transfer thermodynamics of K⁺, 18-crown-6, and [K(18C6)]⁺ upon transferring from water to acetonitrile–water mixed solvents. It was found that hydrophobic solvation of the complex [K(18C6)]⁺ plays an important role in complex formation occurring in water and in the water-rich mixed solvent. Moreover, changes in solvent structure significantly affect the transfer enthalpy and entropy of each species, i.e., K⁺, 18-crown-6, and [K(18C6)]⁺. The observed monotonous changes in the complexation Gibbs energy, enthalpy, and entropy with solvent composition are due to the effective compensation of the Δ _trG, Δ _trH, and Δ _tr S for K⁺ with those for 18-crown-6 and [K(18C6)]⁺.     

Stabilities in Water of Alkali Metal Ion Complexes with Dibenzo-24-crown-8 and Dibenzo-18-crown-6 and Their Transfer Activity Coefficients from Water to Nonaqueous Solvents

Stability constants K _ML for the 1:1 complexes of Na⁺, K⁺, Rb⁺, and Cs⁺ with dibenzo-24-crown-8 (DB24C8) and dibenzo-18-crown-6 (DB18C6) in water have been determined by a capillary electrophoretic technique at 25°C. The K _ML sequence is Na⁺ < K⁺ < Rb⁺ < Cs⁺ for DB24C8 and Na⁺ < K⁺ > Rb⁺ > Cs⁺ for DB18C6. Compared with DB18C6, DB24C8 exhibits higher selectivity for K⁺ over Na⁺, but lower selectivity for K⁺, Rb⁺, and Cs⁺. To evaluate the solvation of the complexes in water, their transfer activity coefficients sH₂O between polar nonaqueous solvents and water have been calculated. The sH₂O values provide the following information: interactions with water of the metal ions and of the crown-ether oxygens are greatly reduced upon complexation and the complexes undergo hydrophobic hydration in water; the character of each alkali metal ion in solvation is more effectively masked by DB24C8 than by DB18C6, because of the larger and more flexible ring structure of DB24C8. Solvent effects on the complex stabilities are discussed on the basis of the sH₂O values.     

Stability of H‐complexes of nicotinamide nitrogen heteroatom with water and ethanol in mixed solvents by 13C NMR probing

The formation constants of the nicotinamide H‐complexes with protonic solvents such as water and ethanol in aqueous dimethyl sulfoxide and aqueous ethanol were determined using ¹³C NMR data. Free Gibbs energy of nicotinamide donor center (nitrogen heteroatom) solvation was calculated. Gibbs energy of entire nicotinamide molecule solvation was shown to be antibate towards Gibbs energy of a pyridine nitrogen solvation. The solvation state of this molecule fragment must be taken into consideration when analyzing the reagents contributions in the thermodynamics of complexation. Copyright © 2013 John Wiley & Sons, Ltd.     

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.     

Conductometric studies of thermodynamics of complexation of Li+, Na+ and K+ ions with 4′,4″(5″)-di-tert-butyldibenzo-18-crown-6 in binary acetonitrile–nitromethane mixtures

The complexation reactions between 4′,4″(5″)-di-tert-butyldibenzo-18-crown-6 (DTBDB18C6) and Li⁺, Na⁺ and K⁺ ions were studied conductometrically in different acetonitrile–nitromethane mixtures at various temperatures. The formation constants of the resulting 1:1 complexes were calculated from the computer fitting of the molar conductance-mole ratio data at different temperatures. At 20 °C and in nitromethane solvent, the stability of the resulting complexes varied in the order K⁺ > Na⁺ > Li⁺. The enthalpy and entropy changes of the complexation reactions were evaluated from the temperature dependence of formation constants. It was found that the stability of the resulting complexes increased with increasing nitromethane in the solvent mixture. The TΔS° versus ΔH° plot of thermodynamic data obtained shows a fairly good linear correlation indicating the existence of enthalpy–entropy compensation in the complexation reactions. The ab initio studies calculated at B3LYP/6-31G level of theory, indicate the binding energy of complexes decreases with increasing cation size in the gas phase. In the solution phase, DTBDB18C6 preferentially forms complexes with the larger ions rather than the smaller ions because the solvation energies of the smaller ions are large enough to overcome and reverse the trends in gas phase complexation. The findings of this study suggest that the current understanding of the factors influencing the selectivity of metal ion complexation by crown ethers may be in need of revision.     

Study of Complex Formation of Dibenzo-18-Crown-6 with Ce3+, Y3+, $\mathrm{UO}_{2}^{2 +}$ and Sr2+ Cations in Acetonitrile–Dioxane Binary Solvent Mixtures

The complexation reactions of dibenzo-18-crown-6 (DB18C6) with Ce³⁺, Y³⁺, UO₂^{2 +}\mathrm{UO}_{2}^{2 +} and Sr²⁺ cations were studied in acetonitrile–dioxane (AN–dioxane) binary solvent solutions at different temperatures by the conductometric method. The stability constants of the resulting 1:1 complexes were determined from computer fitting of the conductance–mole ratio data. The results show that dibenzo-18-crown-6 does not exhibit selectivity for the cation whose ionic size is closest to the cavity size of this macrocyclic ligand in AN–dioxane binary solvent solutions. A nonlinear relationship was observed between the stability constants (log ₁₀ K _f) of these complexes with the composition of the AN–dioxane binary solvent. Values of thermodynamic parameters (DH_c^°, DS_c^°\Delta H_{\mathrm{c}}^{\circ}, \Delta S_{\mathrm{c}}^{\circ}) for complexation reactions were obtained from the temperature dependence of the stability constants. The results show that the values along with the sign of these parameters are influenced by the nature and composition of the mixed solvent.     

Conductance Studies on Complex Formation Between Aza-18-Crown-6 with Ag+, Hg2+ and Pb2+ Cations in DMSO–H2O Binary Solutions

The complexation reactions between Ag⁺, Hg²⁺ and Pb²⁺ metal cations with aza-18-crown-6 (A18C6) were studied in dimethylsulfoxide (DMSO)–water (H₂O) binary mixtures at different temperatures using the conductometric method. The conductance data show that the stoichiometry of the complexes in most cases is 1:1(ML), but in some cases 1:2 (ML₂) complexes are formed in solutions. A non-linear behaviour was observed for the variation of log K _f of the complexes vs. the composition of the binary mixed solvents. Selectivity of A18C6 for Ag⁺, Hg²⁺ and Pb²⁺ cations is sensitive to the solvent composition and in some cases and in certain compositions of the mixed solvent systems, the selectivity order is changed. The values of thermodynamic parameters (ΔH _c^o, ΔS _c^o) for formation of A18C6–Ag⁺, A18C6–Hg²⁺ and A18C6–Pb²⁺ complexes in DMSO–H₂O binary systems were obtained from temperature dependence of stability constants and the results show that the thermodynamics of complexation reactions is affected by the nature and composition of the mixed solvents.