Thermodynamics of Complexation of 18-Crown-6 with NH4+ Ion in Water-Dioxane Mixtures

The stability constants of 1 : 1 complexes of ammonium ion with 18-crown-6 in water and aqueous dioxane (dioxane weight fraction 0.2, 0.4, 0.6, and 0.8) in the range 283-318 K were determined electrometrically, and the thermodynamic parameters of the complexation were calculated. The stability of the complexes is determined by the enthalpy factor. The contributions from the Gibbs energy of solvation of NH₄ ⁺ ion, 18-crown-6·NH₄ ⁺ complex, and free 18-crown-6 to stabilization of the complex with increasing content of dioxane in the mixed solvent were estimated. The thermodynamics of complexation of ammonium, sodium, and potassium ions with 18-crown-6 in aqueous-organic solvents, such as water-2-propanol, water-acetone, and water-dioxane, were compared considering the effects of reactant solvation. The variations of the conformational component of the Gibbs energy of solvation of 18-crown-6 and the parameters of selective solvation of the reactants were evaluated. The influence of the dielectric permittivity and donor-acceptor properties of mixed aqueous-organic solvents on the Gibbs energy of complexation and solvation of the cations and 18-crown-6 was subjected to correlation analysis.     

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.     

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.     

Potentiometric study of complexation of phenylaza-15-crown-5, 4-nitrobenzo-15-crown-5 and dibenzopyridino-18-crown-6 and other derivative of 18-crowns-6 with Na+ ion in methanol

The complexation reaction of phenylaza-15-crown-5, and 4-nitrobenzo-15-crown-5, benzo-15-crown-5 and dibenzopyrdino-18-crwon-6, dibenzo-18-crown-6,dicyclohexyl-18-crown-6(cis and trans), and 18-crown-6 with Na⁺ ion in methanol have been studied by potentiometric method. The Na⁺ ion-selective electrode has been used both as indicator and reference electrode. The stoichiometry and stability constants of complexes of these crown ethers with sodium ion were evaluated by MINIQUAD program. The major trend of stability of resulting complexes of these macrocycle with Na⁺ ion varied in the order DCY18C6 > DB18C6 > 18C6 > DBPY18C6 > phenylaza-15C5 > benzo-15C5 > 4-nitrobenzo-15C5. The obtained results in particular stability constant of complexes of DBPY18C6, phenylaza-15C5 and 4-nitrobenzo-15C5 with sodium ion in comparison with other crowns ether are novel, and interesting.     

A Conductometric Study of Complexation Reactions Between Dibenzo-18-Crown-6 (DB18C6) with Cu2+, Zn2+, Tl+ and Cd2+ Metal Cations in Dimethylsulfoxide–Ethylacetate Binary Mixtures

The complex formation between Cu²⁺, Zn²⁺, Tl⁺ and Cd²⁺ metal cations with macrocyclic ligand, dibenzo- 18-crown-6 (DB18C6) was studied in dimethylsulfoxide (DMSO)–ethylacetate (EtOAc) binary systems at different temperatures using conductometric method. In all cases, DB18C6 forms 1:1 complexes with these metal cations. The stability constants of the complexes were obtained from fitting of molar conductivity curves using a computer program, Genplot. The non-linear behaviour which was observed for variations of log K _f of the complexes versus the composition of the mixed solvent was discussed in terms of 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 order of DB18C6 for the metal cations in pure ethylacetate and pure dimethylsulfoxide is: Tl⁺ > Cu²⁺ > Zn²⁺ > Cd²⁺ but the selectivity order is changed with the composition of the mixed solvents. The values of enthalpy changes (ΔH°_C) for complexation reactions were obtained from the slope of the van’t Hoff plots and the changes in standard enthalpy (ΔS°_C) were calculated from the relationship: ΔG°_C,298.15=ΔH°_C − 298.15 ΔS°_C. The obtained results show that in most cases, the complexes are enthalpy stabilized, but entropy destabilized and the values of ΔH°_C and ΔS°_C depend strongly on the nature of the medium.     

Complexation of Ba2+, Pb2+, Cd2+, and UO22+ Ions with 18-Crown-6 and Dicyclohexyl-18-Crown-6 in Nitromethane and Acetonitrile Solutions by a Competitive NMR Technique Using the 7Li Nucleus as a Probe

[⁷Li] NMR measurements were used to determine the stoichiometry and stability of Li⁺ complexes with 18-crown-6 and dicyclohexyl-18-crown-6 in nitromethane and acetonitrile solutions. A competitive [⁷Li] NMR technique was also employed to probe the complexation of Ba²⁺, Pb²⁺, Cd²⁺, and UO₂²⁺ ions with the same crown ethers–solvent systems. All the resulting 1 : 1 complexes were more stable in nitromethane than acetonitrile solution. In all cases, the stability of both crown complexes in nitromethane and acetonitrile varied in the order Pb²⁺ > Ba²⁺ > Li⁺ > Cd²⁺ > UO₂²⁺.     

Sodium and potassium benzeneazophosphonate complexes with crown ethers: Solid-state microwave synthesis, characterization and biological activity

The eco-friendly synthesis, spectroscopic (IR, MS, ¹H and ¹³C NMR) study and biological (cytostatic, antiviral) activity of sodium and potassium benzeneazophosphonate complexes, obtained by reaction in the solid state under microwave irradiation of the alkali salts of ethyl [α-(4-benzeneazoanilino)-N-benzyl]phosphonic acid and [α-(4-benzeneazoanilino)-N-4-methoxybenzyl]phosphonic acid with crown ethers containing 18-membered (dibenzo-18-crown-6 and bis(4′-di-tert-butylbenzo)-18-crown-6), 24-membered (dibenzo-24-crown-8) and 30-membered (dibenzo-30-crown-10) macrocyclic rings, have been described. The simple work-up solvent free reaction is an efficient green procedure for the formation of mononuclear crown ether complexes in which the sodium/potassium ion is bound to oxygen atoms of the macrocycle and the phosphonic acid oxygen. The free crown ethers, alkali benzeneazophosphonate salts and their complexes were evaluated for their cytostatic activity in vitro against murine leukemia L1210, murine mammary carcinoma FM3A and human T-lymphocyte CEM and MT-4 cell lines, as well as for their antiviral activity against a wide variety of DNA and RNA viruses. The investigated compounds showed no specific antiviral activity, whereas all the free crown ethers and their complexes demonstrated cytostatic activity, which was especially pronounced in the case of bis(4′-di-tert-butylbenzo)-18-crown-6 and its complexes.     

Influence of ligand structure and solvent effects on complexation of neutral guests by several crown ethers

The formation of complexes between crown ethers and acetonitrile, chloroform, and nitromethane were investigated in carbon tetrachloride at 25°C. A significant influence of the ring size on the selectivity of the host is evident. The host 18-crown-6 forms complexes for which the reaction enthalpy and entropy are quite high. Host molecules with benzene side groups form complexes of lower reaction enthalpy and entropy and therefore the complexes formed are less stable than that of the analogous crown ethers without aromatic groups. Solvent effects on the stability constant K, the reaction enthalpy H, and the reaction entropy S were studied for the complexation of malonitrile by 18-crown-6. The reaction enthalpy and entropy values change in accordance with the dielectric constant of the solvent used, but no overall effect on complex stability with change in solvent dielectric constant was observed.     

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.     

A Polarographic Study of Tl+, Pb2+ and Cd2+ Complexes with Aza-18-crown-6 and Dibenzopyridino-18-crown-6 in some Binary Mixed Non-aqueous Solvents

The complexation of Tl⁺, Pb²⁺and Cd²⁺ cations by macrocyclic ligands, aza-18-crown-6 (L1) and dibenzopyridino-18-crown-6 (L2) was studied in some binary mixtures of methanol (MeOH), n-propanol (n-PrOH), nitromethane (NM) and acetonitrile (AN) with dimethylformamide (DMF) at 22 °C using DC (direct current) and differential pulse polarographic techniques (DPP). The stoichiometry and stability constants of the complexes were determined by monitoring the shifts in half-waves or peak potentials of the polarographic waves of metal ions against the ligand concentration. In all of the solvent systems, the stability of the resulting 1:1 complexes was found to be L1 > L2. The selectivity order of the L2 ligand for the cations was found to be Pb²⁺ > Tl⁺ > Cd²⁺ and the selectivity of the L1 ligand for Pb²⁺ ion was greater than that of Tl⁺ ion. The results show that the stability of the complexes depends on the nature and composition of the mixed solvents. There is an inverse relationship between the stability constants of the complexes and the amount of dimethylformamide in the mixed solvent systems.     

Side arm participation in lariat ether carboxylate-alkali metal cation complexes in solution

Lariat ether carboxylic acids of structure CECH₂OCH₂C₆H₄-2-CO₂H with crown ether (CE) ring sizes of 12-crown-4, 15-crown-5 and 18-crown-6 are prepared and converted into alkali metal-lariat ether carboxylate complexes. Absorptions for the diastereotopic benzylic protons in the ¹H NMR spectra of the complexes in CDCl₃ are utilized to probe the extent of side arm interaction with the crown ether-complexed metal ion as a function of the crown ether ring size and identity of the alkali metal cation.     

The influence of solvation on the stability constants of Ag<Superscript>+</Superscript>-18-crown-6 complexes in acetonitrile-dimethylsulfoxide mixtures at 298.15 K

The influence of the composition of acetonitrile-dimethylsulfoxide solvents on the stability of silver(I) complexes with 18-crown-6 ether was studied potentiometrically. An increase in the concentration of dimethylsulfoxide decreased the stability of the coordination compound. It was shown on the basis of the thermodynamic characteristics of solvation of the reagents that a determining factor of complex formation equilibrium shifts was the solvation effect of the Ag⁺ ion. An equation was suggested for predicting the stability of silver(I) coordination compounds with crown ethers and pyridine-type ligands in binary mixtures of aprotic solvents from changes in the solvation state of the central ion.     

Crown Ether As The Phase-Transfer Catalyst for Free Radical Polymerization of Hydrophobic Vinyl Monomers

Various crown ethers were used as phase-transfer catalysts for free radical polymerizations of some water-insoluble vinyl monomers such as acrylonitrile, methylmethacrylate and styrene with persulfate as initiator. The catalytic abilities of these crown ethers for free radical polymerization of acrylonitrile with S₂O₈^2?ion as an initiator were in the order: 18-crown-6 > 15-crown-4 > 12-crown-4 > benzo-15-crown-5 > dibenzo-18-crown-6. Among various persulfates such as Na₂S₂O₈ K₂S₂O₈ and (NH₄)₂S₂O₈, ammonium persulfate was the optimum initiator for the polymerization of acrylonitrile catalyzed by 18-crown-6 or 15-crown-5. Among the organic solvents used, chloroform seems to be the best solvent for the catalytic polymerization of acrylonitrile. An apparent activation energy of 72.9 kJ mol^?1 was observed for the polymerization of acrylonitrile. The catalytic reaction rates of free radical polymerization for these hydrophobic vinyl monomers were in the order: acrylonitrile > methylmethacrylate > styrene > isoprene. Effects of concentrations of crown ether, initiator, and nitrogen on the polymerization of these vinyl monomers were investigated.     

Inhibition of Na+, K+-ATPase in the presence of crown ethers: modulation of ionic composition or pharmacological effects

It is believed that the biological effects of chelating agents such as crown ethers are largely related to their ability to form complexes with ions and/or to facilitate ion transport across membranes. Specific influences are rarely related. Here we present the evidence that even one of the simplest representatives of the crown ether super-family, 1,4,7,10,13,16-hexaoxacyclooctane (18-crown-6), is able to affect the activity of Na⁺, K⁺-ATPase directly. Using nonlinear regression fitting to kinetic data we have found that the crown ether diminishes the apparent Michaelis constant, K _m , and the maximal rate of ATP hydrolysis, V _m , acting as noncompetitive inhibitors. The apparent dissociation constants, K _i , for the crown interaction with the free ATPase and with the enzyme-substrate complex were established to be of 77 ± 3 mM and 21 ± 2 mM, respectively. So 18-crown-6 possesses weak but “direct” pharmacological activity on Na⁺, K⁺-ATPase hinders the formation of enzyme–substrate complex and detains the enzyme in this state.     

A competitive polarographic study of complexation of ammonium,anilinium, hydrazinium and pyridinium ions with some macrocyclic ligands in binary ethanol-water mixtures using a Pb(II)/Pb(Hg) couple as an electrochemical probe

The formation of ammonium, anilinium, hydrazinium and pyridinium ion complexes with the crown ethers 18-crown-6 (18C6) and 1,10-diaza-18-crown-6 (C22) and the cryptand C222 in different binary ethanol-water mixtures has been studied by a competitive polarographic method using a Pb²⁺/Pb(Hg) couple as a sensitive electro-chemical probe. Lead ion was found to form very stable complexes with the ligands used, in all solvent mixtures studied; Pb²⁺–C222 cryptate revealed a pronounced cryptate effect compared to the corresponding complexes with the monocyclic crown ethers used. In all solvent mixtures studied, the stability of the resulting 11 complexes between the protonated amines and macrocyclic ligands used vary in the order C22>C222>18C6. The observed selectivity order of each macrocyclic ligand used for different protonated amines is discussed based on the chemical and structural features of the host-guest partners in solution. In all cases studied there is an inverse linear relationship between the complex formation constants and the mole fraction of water in the mixed solvent.     

Effect of solvation on the thermodynamics of the formation of molecular complexes of 18-crown-6 ether with glycine in water-dimethylsulfoxide solutions

Complexation of the 18-crown-6 ether (18C6) with glycine (Gly) in mixed H₂O-DMSO solvents with the composition of 0.1, 0.2, and 0.25 mole fraction of DMSO (T = 298.15 K) was studied calorimetrically. Thermodynamic characteristics of the reaction of the formation of the molecular Gly18C6 complex (Δ_r G°, Δ_r H°, TΔ_r S°) were calculated from the calorimetric data. It was established that the change in the stability of the Gly18C6 complex is mainly determined by the predominance of the enthalpy component of the Gibbs energy over the entropy component. It was shown during the analysis of the enthalpy contributions of the reagents to the enthalpy of the reaction of the formation of Gly18C6 that the change in the enthalpy of the reaction upon a change the solvent composition was due to changes in the solvation state of 18C6.     

Studies on the syntheses and properties of Schiff base type crown ethers

Eight bis-,two tris-and two mono-crown ethers of Schiff base type have been pre-parcd by the reaction of 4'-formylbenzo-15-crown-5 with diamincs.Conductivity measurementsshow that the bis-crown ethers form 1:1 complexes(crown cther unit:metal ion)with sodiumion and 2:1 complexes with potassium,rubidium and ammonium ion,respectively.     

The complexation of the ammonium ion by 18-crown-6 in different solvents and by noncyclic ligands,crown ethers and cryptands in methanol

The complexation of the ammonium ion with the macrocyclic ligand 18-crown-6 was studied using calorimetric and potentiometric titrations in different solvents. In water and dimethyl sulphoxide the stability constants had the lowest values compared with all other solvents examined. No specific interactions between the ammonium ion and solvent molecules were observed. Crown ethers formed more stable complexes in methanol with the ammonium ion than diaza crown ethers. The most stable complexes were formed with cryptands. The highest values of the stability constants for the reaction with macrocyclic and macrobicyclic ligands were measured if the dimensions of the ammonium ion and of the cavities were nearly identical.     

Ion selectivity of crown ethers investigated by UV and IR spectroscopy in a cold ion trap

Electronic and vibrational spectra of benzo-15-crown-5 (B15C5) and benzo-18-crown-6 (B18C6) complexes with alkali metal ions, M(+)?B15C5 and M(+)?B18C6 (M = Li, Na, K, Rb, and Cs), are measured using UV photodissociation (UVPD) and IR-UV double resonance spectroscopy in a cold, 22-pole ion trap. We determine the structure of conformers with the aid of density functional theory calculations. In the Na(+)?B15C5 and K(+)?B18C6 complexes, the crown ethers open the most and hold the metal ions at the center of the ether ring, demonstrating an optimum matching in size between the cavity of the crown ethers and the metal ions. For smaller ions, the crown ethers deform the ether ring to decrease the distance and increase the interaction between the metal ions and oxygen atoms; the metal ions are completely surrounded by the ether ring. In the case of larger ions, the metal ions are too large to enter the crown cavity and are positioned on it, leaving one of its sides open for further solvation. Thermochemistry data calculated on the basis of the stable conformers of the complexes suggest that the ion selectivity of crown ethers is controlled primarily by the enthalpy change for the complex formation in solution, which depends strongly on the complex structure.     

Complex Formation of Crown Ethers with Cations in Water–Organic Solvent Mixtures. Part IV. Thermodynamics of Interaction of Na+ Ion with Benzo-15-crown-5 Ether in the Mixtures of Water with Acetonitrile at 298.15 K

The equilibrium constants of complex formation of benzo-15-crown-5 ether with sodium ion have been determined by molar conductance at various molar ratios of benzo- 15-crown-5 ether and sodium iodide in mixtures of water with acetonitrile at 298.15 K. The thermodynamic quantities of complex formation of benzo-15-crown-5 ether with sodium cation are calculated. The enthalpy of solvation of benzo-15-crown-5 ether and sodium ion complex is discussed together with solvation enthalpies of the cation and ligand. The contribution of the benzene ring to the thermodynamic properties of complex formation and to the enthalpy of solvation of the crown ether/ Na⁺ complex in the mixtures of water with acetonitrile are analyzed and discussed.