Utilisation of Silver Cryptate [222] /Silver System as a Potential Reference for the Determination of the Solvation Transfer Coefficients of Ions. Stability of Cryptates in a Solvent Medium

Abstract

The formal potential of Ag [222] /Ag(s) system is supposed to be independent of the solvent, taking in account the [222] ligand structure and the corresponding cryptate. This extrathermodynamic hypothesis is confronted to the one usually considered, water, methanol, D. M. S. O., acetonitrile and tetramethylurea being the used solvents. It seems possible to generalize this hypothesis to other cations and cryptands. The stability constants of cryptate [222] of the evaluation of p^K _Ag[222]⁺ constants, Nernst's law being respected. The argentometric titration of ligand [222] in presence of Na⁺, Li⁺, Tl⁺, Cd²⁺, Ni²⁺ ions allows to evaluate the p^K of corresponding cryptates in D. M. S. O. and methanol. Tl [222]⁺/Tl system follows Nernst's law in methanol and D. M. S. O. The dissociation constants have been evaluated from polarographic measurements in acetonitrile. The electrochemical systems are not rigorously fast, which does not allow an accurate determination of those constants by that method. Still it gives an order of magnitude.     

Réduction du cryptate de thallium(I) par polarographie impulsionnelle normale: Constante de réduction électrochimique (hétérogène) et constante de vitesse (homogène) de décomplexation

In propylene carbonate as solvent (+(n-hex)₄NClO₄ 0.1 M) the cryptate [222 Tl]⁺ and the cation Tl⁺ are electroducible. The reduction step is monoelectronic and slowed down for the cryptate compared to the uncomplexed cation. For the cryptate [222 Tl]⁺, by correlation of n.m.r. results on homogeneous ion exchange and combination with the electrochemical heterogeneous data, the standard redox potential of the couple [222 Tl]⁺/[222 Tl]⁰ is estimated to be ?0.73±0.02 V/SCE. This value is in agreement with the electrochemical results. The stability constant of [222 Tl]⁺ is calculated by two independent procedures, from electrochemical results and from n.m.r. data. The results are concordant: log K_s=9.0±0.3 at 25°C, in propylene carbonate (+(n-hex)₄NClO₄ 0.1 M).     

Electrochemistry of macrocyclic complexes of alkaline cations: Part IV. Reduction of (mnp,K)+ cryptates

The reductions of five macrobicyclic potassium cryptates (ligands (322), (222), (221), (22 C₈), (211)) and two macromonocyclic potassium complexes (ligands (22) and (18 crown 6)) are studied by polarography, coulometry and cyclic voltammetry in propylene carbonate (PC). The mechanism and the parameters of the electrochemical reduction are discussed in terms of dependence upon three important features of the macrocyclic ligands: the size of the intramolecular cavity, the number of oxygen heteroatoms in the ligand, and the number of “branches” in the ligand (cryptate effect). The electrochemical results are consistent with the other known physicochemical parameters of these complexes. The electroreduction behaviour might allow the structure and the stability constant of alkaline cryptates in a given medium (PC) to be approximated. Reciprocally, these physicochemical characteristics may lead to a rather precise prediction of the electrochemical behaviour of the corresponding macrocyclic complexes.     

Complexes macrocycliques des lanthanides: Stabilité et comportement électrochimique dans le méthanol et le carbonate de propylène

Macrocyclic Complexes of Lanthanides: Stability and Electrochemical Behaviour in Methanol and Propylene Carbonate The stabilities of the 1:1 complexes of the trivalent lanthanides with the diazapolyoxamacrocycles (2.1.) and (2.2.1.) in anhydrous methanol and propylene carbonate have been determined at 25°, by competitive potentiometric methods using H⁺ or Ag⁺ as auxiliary cations, with Et₄NClO₄ as supporting-electrolyte. Additional data are also reported for the crown ethers 15C5 and 18C6 in propylene carbonate. It is shown that the diazapolyoxamacrocycles are much stronger complexing agents for trivalent lanthanides than macrocyclic polyethers, and that the bicyclic (2.2.1.) cryptates are more stable than the monocyclic (2.1.) complexes. With increasing atomic number of the lanthanides, the stability increases with diazapolyoxamacrocycles and decreases with cyclic polyethers. The electrochemical reduction of the trivalent samarium and europium cryptates has been investigated by polarography on a dropping Hg-electrode, in water and methanol. In both solvating solvents, the +2 oxidation states of the cations are stabilized by complexation.     

Stability of Ag<Superscript>+</Superscript> Complexes with Cryptand 222 in Ionic Liquids

Stability constants of silver(I) complexes with cryptand 222 were measured in a number of ionic liquids, applying potentiometric titration. The ionic liquids were based on 1-butyl-3-methylimidazolium, 1-ethyl-3-methylimidazolium, 1-butyl-1-methyl-pyrrolidinium and 1-methyl-1-propyl-pyrrolidinium cations, as well as on tetrafluoroborate, triflate and bis(trifluoromethane sulfonyl) imide. The stability constants, expressed in log K scale, were within the broad range of 8.4–17.2. The formation of the Ag⁺222 cryptates was not detected in ionic liquids based on halide anions. Free enthalpy of silver(I) transfer from dimethylsulfoxide as a reference molecular solvent to ionic liquids was calculated applying the cryptate assumption. The results were discussed in terms of the competition between silver(I) complexation by ion forming ionic liquid and its complexation by cryptand 222.in final form: 6 December 2004This revised version was published online in July 2005 with a corrected issue number.     

Electrochemical study of oxaferrocene cryptands and their complexation with barium and sodium ions

The electrochemical behaviour and cation recognition properties of two oxaferrocene cryptand ligands, 1,1′-[(1,4,10-trioxa-7,13-diazacyclopentadecane-7,13-diyl)diethoxy]-3,3′,4,4′-tetraphenylferrocene and 1,1′-[(1,4,10,13-tetraoxa-7,16-diazacyclooctadecane-7,16-diyl)diethoxy]-3,3′,4,4′-tetraphenylferrocene, have been characterized in acetonitrile in the presence of Ba²⁺ and Na⁺ by cyclic voltammetry, square wave voltammetry and a rotating disc electrode. The changes in the redox signals for the cryptates at varying concentrations of the target cations are used as a direct measure of the electronic coupling between the two units, leading to the conclusion that the cryptate formation process proceeds in multiple stages and the ligand offers several binding sites in the complex.     

Adsorption of the lanthanum and thorium cryptates at mercury electrode

The adsorption behavior of cryptates of La³⁺ and Th⁴⁺ cations at the Hg/aqueous solution interface is studied by using the electrochemical impedance spectroscopy method. It was shown that the studied complexes have high surface activity at the interface. The lanthanum cryptate adsorption parameters are found using the regression analysis based on the two parallel capacitor model supplemented by the Frumkin adsorption isotherm. The differential capacitance curves calculated by using these parameters agree satisfactorily with the experimental ones. The more complicated adsorption behavior of lanthanum cryptate, as compared with the complexes of lower valence cations, is interpreted with taking into consideration the strong interaction of the La³⁺ cation contained in the [2.2.2.] cryptand intramolecular cavity with the supporting electrolyte anions.     

Enthalpy and Entropy of Formation of Alkali and Alkaline-Earth Macrobicyclic Cryptate Complexes [1]

The enthalpies and entropies of complexation of alkali and alkaline-earth metal cations by several macrobicyclic ligands have been obtained from calorimetric measurements and from the previously determined stability constants [2]. Both enthalpy and entropy changes play an important role in the stability and selectivity of the complexes. Particularly noteworthy are the large enthalpies and the negative entropies of complexation obtained for the alkali cation complexes (Na⁺, K⁺, Rb⁺ and Cs⁺ cryptates). The Sr²⁺ and Ba²⁺ as well as [Li⁺ ? 2.1.1]

1 For use of the symbols see [2].

and [Na⁺ ? 2.2.1] cryptates are of the enthalpy dominant type with also a favourable entropy change. The Ca²⁺ and [Li⁺ ? 2.2.1] cryptates are entirely entropy stabilized with about zero heat of reaction. The high stability of the macrobicyclic complexes as compared to the macromonocylcic ones, the cryptate effect, is of enthalpic origin. The enthalpies of complexation display selectivity peaks, as do the stabilities, whereas the entropy changes do not. The high M²⁺/M⁺ selectivities found in terms of free energy, may be reversed when enthalpy is considered in view of the very different role played by the entropy term for M²⁺ and M⁺ cations. The enthalpies and entropies of ligation show that whereas the cryptate anions are similar in terms of entropy irrespective of which cation is included, the ligands, despite being more rigid than the hydration shell, are nevertheless able to adjust to some extent to the cation. This conclusion agrees with published X-rays data. The origin of the enthalpies and entropies of complexation is discussed in terms of structural features of the ligands and of solvation effects.     

Complexation of sodium and silver ions by the cryptand 4,7,13-trioxa-1,10-diazabicyclo[8.5.5]eicosane in a range of solvents

The apparent stability constants (K) of the cryptates [AgβC21C₅]⁺ and [Na·C21C₅]⁺ (C21C₅ = 4,7,13-trioxa-1,10-diazabicyclo[8.5.5]eicosane) have been determined in a range of solvents. For [Na·C21C₅]⁺ log (K/dm³ mol⁻¹) = 2.87, 3.72, 3.76, 3.98, 5.08 and 5.12 at 298.2 K in dimethylformamide, pyridine, methanol, acetone, acetonitrile and propylene carbonate, respectively, in the presence of 0.05 mol dm⁻³ tetraethylammonium perchlorate supporting electrolyte. These values are substantially lower than those observed for [Na·C211]⁺ (C211 = 4,7,13,18-tetraoxa-1,10-diazabicyclo[8.5.5]eicosane) and this is attributed to the presence of one less oxygen atom in C21C₅ than in C211. A similar relationship is observed for [Ag·C211]⁺ and [Ag·C21C₅]⁺, where, for the latter cryptate, log (K/dm³ mol⁻¹) = 5.19, <2, 7.62, 8.58 and 4.55 at 298.2 K in dimethylformamide, pyridine, methanol, acetone and acetonitrile, respectively.     

Conductance study of the thermodynamics of some transition and heavy metal cryptates in binary acetonitrile-dimethylsulfoxide mixtures

A conductance study of the interaction between Co²⁺, Ni²⁺, Cu²⁺, Cd²⁺, Zn²⁺ and Pb²⁺ ions with cryptands C211, C221 and C222 in different acetonitrile-dimethylsulfoxide mixtures has been carried out at various temperatures. The formation constants of the resulting metal cryptates were determined from the molar conductance-mole ratio data. It was found that the stability of Co²⁺, Ni²⁺, Cu²⁺ and Zn²⁺ cryptates vary in the order C211>C221>C222, while for Cd²⁺ cryptates the stability order is C221>C222>C211. A linear relationship is observed between logK _f of different metal cryptates and the mole fraction of acetonitrile in the solvent mixture. The enthalpy and entropy of cryptate formation reactions were determined from the temperature dependence of the formation constants. The enthalpy and entropy changes are quite sensitive to the solvent composition and the resultingTS ⁰–H ⁰ plot shows a fairly good linear correlation, indicating the existence of an entropy-enthalpy compensation in the cryptate formation reactions. Supplementary Data relating to this article are deposited with the British Library as Supplementary Publication No. SUP 82181 (30 pages).     

Cryptates Sphériques. Synthèse et Complexes d'Inclusion de Ligands Macrotricycliques Sphériques

Spherical Cryptates. Synthesis and Inclusion-Complexes of Spherical Macrotricyclic Ligands A general strategy for the synthesis of spherical macrotricyclic ligands has been developed. Four spherical cryptands, SC - 24 , SC - 25 , SC - 26 and SC - 27 have been obtained by this route. The synthesis and cation-complexing properties of these compounds are described in detail. Stability constants and cation exchange rates of the spherical cryptates obtained with alkali and alkaline-earth cations have been determined. Highly stable complexes are formed by SC - 24 ; the Rb⁺ and Cs⁺ cryptates of SC - 24 are the most stable complexes of these cations known to date. The size of the intramolecular cavity affects the complexation selectivity. The cation exchange rates are very slow, and the corresponding free energies of activation are even larger than, for macrobicyclic cryptates of similar stability. Both the high complex stabilities and the high activation energies required for cation exchange indicate a marked ‘spherical cryptate effect’ resulting from the highly connected nature of the molecular architecture of spherical macrotricyclic ligands.     

Conductometric study of cationic and anionic complexes in propylene carbonate

Conductivity data are used to determine thermodynamic complex formation constants for cases in which both the initial electrolyte and the complexed electrolyte form ion pairs. Using the method described in the text, the complex formation constants of Li⁺, Na⁺ and K⁺ with the crown ether 18-crown-6 and of Li⁺ with the ligand triphenylphosphine oxide in propylene carbonate have been evaluated from conductance data. The complexation of AgBr in propylene carbonate solutions of n-etrabutylammonium bromide has also been studied by the measurement of molar conductivities. The results of these studies indicate that ion pairing should not be neglected, even in high permittivity solvents such as propylene carbonate, and that the ion pair association constants correlate well with structural studies on cation-crown ether molecular conformations.     

Spectrophotometric determination of trace amounts of lead(II) by ion-pair extraction with cryptand (2.2.2) and eosin

The solvent extraction of ion-pair complexes of calcium, strontium and lead (2.2.2) cryptates is described. The extraction equilibrium constants (D_C, K_ex and K_D) at room temperature are reported. The formation of the positively-charged lead cryptate ion and its extraction into chlorobenzene as an ion-pair with eosin are the basis of the proposed spectrophotometric determination of traces of lead. The high molar absorptivity of the ion-pair complex (? = 1.1 × 10⁵ l mol^-1 cm^-1) and the linearity of the calibration graph over the range 0–10^-5 M, allow even 0.1 ppm lead to be determined. The selectivity is high; there is no interference from cations often occurring with lead, such as Bi³⁺, Sn²⁺, Hg²⁺, Zn²⁺, Cd²⁺, Cu²⁺, Ca²⁺, Pd²⁺, Ag⁺ and Tl⁺.     

A Cryptate Reference Electrode for Ionic Liquids

The cryptate electrode (Ag/Ag⁺222), prepared by immersing silver wire in a solution of silver(I) salt and the cryptand 222 (4,7,13,16,21,24‐hexaoxa‐1,10‐diazabicyclo[8.8.8]hexacosane) in ionic liquids have been studied. The potential of the electrode is stabilized by the equilibrium of the Ag⁺ ion complexation by the cryptand, similarly to the potential stabilization by the ionic product of slightly soluble salts, used in aqueous electrodes of the second kind. The Ag/Ag⁺222 cryptate electrode (concentration of the cryptate was much higher than the silver(I) cation concentration, [222]>[Ag⁺]) may be used as a reference electrode in room temperature ionic liquids. The potential of the Ag/Ag⁺222 electrode is less sensitive to the presence of impurities, such as halides or water, in comparison to the Ag/Ag⁺ electrode. After anodic or cathodic polarization, the potential of the Ag/Ag⁺222 electrode comes back to the initial open circuit potential quickly. Preparation of the Ag/Ag⁺222 reference electrode is very easy: a silver wire is immersed in a solution of Ag⁺ salt and cryptand 222 (both available commercially) in the ionic liquid under study.     

Metathetical Reactions in the System Na(K)PF6 – LiBF4 – Aprotic Media

¹⁹F NMR spectroscopy, X‐ray powder diffraction, elemental analysis, and ab initio quantum chemical calculations were used to study metathetical reactions between potassium or sodium hexafluorophosphate and lithium tetrafluoroborate in a mixture of propylene carbonate (PC) – dimethyl carbonate (DMC). It was shown that the increase in size of the cations in the second coordination sphere from Na⁺ to K⁺ results in an increase of the equilibrium conversion. This is in agreement with the influence of the cation size on the solubility of tetrafluoroborates in the media investigated.     

Ferrocene as a Reference Redox Couple for Aprotic Ionic Liquids

The electrochemical behavior of ferrocene has been studied in a number of room temperature ionic liquids. Diffusion‐controlled, well‐defined anodic and cathodic peaks were found for the Fc/Fc⁺ (ferrocene/ferrocenium) oxidation/reduction on the gold electrode. Ohmic resistance R between working and auxiliary electrodes was deduced from impedance measurements. Cyclic voltammograms were corrected for the base line current as well as for the ohmic drop (IR). The formal potential 1/2(E_pa+E_pc) for ferrocene reduction/oxidation in aprotic ionic liquids tested is within a relatively narrow range and may be approximated by the value of 0.527±0.018 V (against the cryptate Ag/Ag⁺222 in acetonitrile reference). Ferrocene diffusion coefficients, calculated from the peak current dependence on the sweep rate, were of the order of 10^?7 cm² s^?1.     

Rubidium-87 NMR studies of rubidium salts and complexes with 18-crown-6 and cryptand-222 in solutions

Rubidium-87 NMR measurements were used to study the behavior of the Rb⁺ ion in water, methanol, and propylene carbonate solutions. In aqueous solutions the ⁸⁷Rb chemical shift varies linearly with the mean activity of the salt. In methanol and propylene carbonate solutions the relationship is linear only at high salt concentrations. The resonance lines are broad and vary from ∼ 150 Hz (at half height) in water to ∼ 1000 Hz in propylene carbonate. Additions of macrocyclic ligands ¹⁸C6 and C222 to Rb⁺ solutions in the three solvents result in further broadening of the resonance line so that variations in the resonance frequency cannot be measured with a reasonable precision.     

An equilibrium and kinetic study of the complexation of lithium and sodium ions by the cryptand 4, 7, 13-trioxa-1, 10-diazabicyclo-[8. 5. 5]-eicosane (C21C5)

In the solid and solution state Li⁺ and Na⁺ form inclusive and exclusive cryptates respectively with C21C₅, in which Li⁺ resides inside and Na⁺ resides outside the C21C₅ cavity. Similar inclusive and exclusive structures are observed for [Li.C211]⁺ and [Na. C211]⁺. The logarithms of the stability constants in dimethylformamide for [Li.C21C₅]⁺, [Li.C211]⁺, [Na.C21C₅]⁺ and [Na.C211]⁺ are: 2.80, 6.99, 2.87 and 5.20; and^o the corresponding decomplexation rate constants are: 107, 0.013, 28800 and 12 s^–1 at 298.2 K. The relationships between cryptate structure, stability and lability are considered, as are solvent influences.     

Estimation of parameters characterizing the adsorption of sodium,potassium, and rubidium cryptates at the mercury electrode/solution interface

The adsorption of complexes formed by sodium, potassium, and rubidium cations with macrobicyclic ligand (kryptofix 222 with C₁₈H₃₆N₂O₆ composition) is studied as a function of the ligand concentration on a stationary mercury drop in 0.1 M solutions of corresponding sulfates and chlorides by using the differential capacitance technique. Based on the model of two parallel capacitors supplemented by the Frumkin isotherm, the adsorption parameters of studied cryptates are estimated by using the regression analysis technique. Differential capacitance curves calculated with the parameters found are compared with experimental data. The comparison of the found adsorption parameters makes it possible to reveal the effects of the nature of included cations and specifically adsorbed supporting-electrolyte anions on the adsorption behavior of cryptates under study.__________Translated from Elektrokhimiya, Vol. 41, No. 4, 2005, pp. 475–481.Original Russian Text Copyright © 2005 by Stenina, Sviridova.     

Spectrophotometric studies of alkali and alkali earth metal ions complexes of mono amino derivative of calix[4]arene

The synthesis and complexive abilities of 5,11,17-tris(tert-butyl)-23 amino-25,26,27,28-tetra-propoxycalix[4]arene towards alkali cations Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺ and alkali earth cations Mg²⁺, Ca²⁺, Sr²⁺ and Ba²⁺ in methanol-chloroform mixture have been evaluated at 25°C, using UV-Vis spectrophotometric techniques. The results showed that the ligand is capable to complex with all the cations by 1: 1 metal to ligand ratios. The selectivity presented considering the calculated formation constants are in the order Li⁺ > Na⁺ > K⁺ > Rb⁺ > Cs⁺ and Mg²⁺ > Ca²⁺ > Sr²⁺ > Ba²⁺ with the ligand.