13C-NMR relaxation studies of alkali metal cryptate complexes

The ¹³C spin-lattice relaxation times (T₁'s) of cryptands [2.1.1], [2.2.1] and [2.2.2] as well as those of the corresponding cryptate complexes with Li⁺, Na⁺, and K⁺ in CDCl₃ and CH₃OH:D₂O (90:10) were measured and the results are interpreted in terms of molecular compression and desolvation effects.     

Synthesis and alkali metal complexation studies of novel cage-functionalized cryptands

The synthesis of novel cage-functionalized cryptands 1–5 containing adamantane-, 2-oxaadamantane- or noradamantane-moiety [i.e., 1,3-diethyladamantano[2.2.0]cryptand (1), 1,3-diethoxyadamantano[2.2.2]cryptand (2), 1,3-di[(ethyloxy)methyl]adamantano[2.2.2]-cryptand (3), 1,3-di[(ethyloxy)methyl]-2-oxaadamantano[2.2.3]cryptand (4), and 1,2-diethyloxynoradamantano[2.2.2]cryptand (5)] and their alkali metal binding properties are reported. The results obtained by extraction experiments showed that all the cryptands displayed lower extraction capabilities than the parent [2.2.2]cryptand. However, cryptands 1 and 2 showed much higher selectivity toward K⁺ than the reference [2.2.2]cryptand. When the third bridge is enlarged by two additional CH₂-groups as well as by two oxygen atoms, as in cryptands 3 and 4, the complexational abilities for bigger cations (K⁺, Rb⁺ and Cs⁺) are enhanced. Cryptand 5 displayed very good extraction capabilities of all cations, but showed practically no selectivity towards any of the alkali metal cation. The experimental findings are corroborated by calculation studies consisting of force field based conformational search using Monte Carlo method followed by investigation of the stabilities of the complexes of cryptands with Na⁺ and K⁺ metal ions in chloroform by means of quantum chemical calculations at the density functional theory level.     

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.     

Reversible organogels triggered by dynamic K+ binding and release

In this study, a new lipophilic guanosine derivative was synthesized as an organogelator. The self-aggregation behavior of this organogelator was investigated by NMR, XRD and AFM. In solution, the lipophilic guanosine derivative can form a stable ribbon-like structure through NH(1)–N(7) and NH(2)–O(6) hydrogen bonds. However, gelation would occur in some aprotic solvents after the concentration reached a definite value. More interesting, the ribbon-like structure was able to change to G-quartets in the presence of K⁺, which led to the transformation from a gel to a sol. Upon the addition of the cryptand [2.2.2], which can efficiently complex with K⁺, G-quartets reverted to the original ribbon-like structure and the gel recovered. Subsequently, upon the addition of acids, K⁺ was released from the cryptate with the transformation of gel-to-sol simultaneously. Finally, upon the addition of bases which deprotonated [H⁺ ? 2.2.2], the liberated cryptand [2.2.2] recaptured K⁺ and the gel was regenerated again. This process of interconversion between G-ribbon 1_n and octamer 1₈·K⁺ was well monitored by circular dichroism spectra.     

Gas-Phase studies of valinomycin-alkali metal cation complexes: attachment rates and cation affinities

Reactions of laser-desorbed Na⁺, K⁺, Rb⁺, and Cs⁺ with thermally vaporized valinomycin generate metal-ligand complexes in a Fourier transform ion cyclotron resonance trapping cell, proving that complexes can form via gas-phase ion-molecule reactions. Although desorption of intact pre-formed complexes cannot be ruled out, this route appears minor. Relative rate constants for the complexation reactions show strong dependence on the charge densities of the cations. Competition experiments between valinomycin and the synthetic ionophores 18-crown-6 (18C6) and [2.2.2]-cryptand ([2.2.2]) show that valinomycin has a higher intrinsic alkali metal cation affinity than either 18C6 or [2.2.2], in contrast to the complex formation constants observed in methanol, where K+ affinities are in the order [2.2.2] > 18C6 > valinomycin.     

Synthesis and Characterization of Cationic Low‐Valent Gallium Complexes of Cryptand[2.2.2]

The synthesis and characterization of two bimetallic, cationic low‐valent gallium–cryptand[2.2.2] complexes is reported. The reaction of cryptand[2.2.2] with Ga₂Cl₄ gave two different cations, [Ga₃Cl₄(crypt‐222)]⁺ ( 1 ) or [Ga₂Cl₂(crypt‐222)]²⁺ ( 2 ), depending on whether or not trimethylsilyl triflate (Me₃SiOTf) was added as a co‐reagent. Complexes 1 and 2 are the first examples of bimetallic cryptand[2.2.2] complexes, as well as the first low‐valent gallium–cryptand[2.2.2] complexes. Computational methods were used to evaluate the bonding in the gallium cores.     

Novel Methods of Synthesis of Dibenzo[3n]crown-n and Their Cation Binding Studied by Fluorescence Spectroscopy. Part V

The dibenzo[3n]crown-n were synthesised starting from bis[2-(o-hydroxyphenoxy)ethyl]ether obtained from bis[2-(o-formylphenoxy)ethyl]ether via Baeyer-Villiger oxidation in H₂O₂/CH₃COOH in a good yield. The cyclic condensation ofbis[2-(o-hydroxyphenoxy)ethyl]etherwith tri- and tetraethylene glycol bisdichlorides andthe bisditosylate of pentaethylene glycol in DMF/Me₂CO₃ afforded the large cyclic ethers of dibenzo[21]crown-7, dibenzo[24]crown-8 and dibenzo[27]crown-9. The structures were analysed with IR, ¹H NMR, ¹³C NMR and low-resolution mass spectroscopy methods. The Na⁺, K⁺, Rb⁺ and Cs⁺ cations' recognition of the molecules were conducted withsteady-state fluorescence spectroscopy. The 1:1 association constants, K_a, in acetonitrile were estimated. Dibenzo[21]crown-7 was the best both for K⁺ and Rb⁺ binding but showed too small an effect on Cs⁺. Dibenzo[24]crown-8 exhibited the binding power in the order of Rb⁺ > K⁺ > Na⁺ > Cs⁺. However, dibenzo[27]crown-9 displayed marked binding with only K⁺ but not with Rb⁺ or with Cs⁺ cations probably due to the heavy atom effect of fluorescence quenching.     

Host‐Guest Complexes of [TriPip222], the Piperazine Analogue of [2.2.2]: Prediction of Ion Selectivity by Quantum Chemical Calculations VIII[#]

The selectivity of the cryptand [TriPip222], a per‐aza analogue of cryptand [2.2.2], in which each of the linking arms contains a piperazine ring for the endohedral complexation of metal cations of the I, II, and III main groups and group 12 of the periodic table of elements, was predicted on the basis of DFT [B3LYP/LANL2DZp (LANL2DZp = LANL2DZ augmented with polarization functions on non‐hydrogen atoms)] calculated structures and complex‐formation energies. The cavity size of the studied cryptand is similar to that of [bpy.bpy.bpy], [2.bpy.bpy] and [2.phen.phen], such that the complexation of K⁺ > Na⁺ and of Sr²⁺ ≈ Ca²⁺ > Ba²⁺ are most favorable. The essential flexibility for achieving the selectivity of the cryptand is mainly associated with a twist of the CH₂–N_bridgehead ··· N_bridgehead–CH₂ angle and not with the piperazine moiety.     

Calix[4]pyrrole‐Based Heteroditopic Ion‐Pair Receptor That Displays Anion‐Modulated,Cation‐Binding Behavior

A new ditopic ion‐pair receptor 1 was designed, synthesized, and characterized. Detailed binding studies served to confirm that this receptor binds fluoride and chloride ions (studied as their tetraalkylammonium salts) and forms stable 1:1 complexes in CDCl₃. Treatment of the halide‐ion complexes of 1 with Group I and II metal ions (Li⁺, Na⁺, K⁺, Cs⁺, Mg²⁺, and Ca²⁺; studied as their perchlorate salts in CD₃CN) revealed unique interactions that were found to depend on both the choice of the added cation and the precomplexed anion. In the case of the fluoride complex [ 1? F]^? (preformed as the tetrabutylammonium (TBA⁺) complex), little evidence of interaction with the K⁺ ion was seen. In contrast, when this same complex (i.e., [ 1? F]^? as the TBA⁺ salt) was treated with the Li⁺ or Na⁺ ions, complete decomplexation of the receptor‐bound fluoride ion was observed. In sharp contrast to what was seen with Li⁺, Na⁺, and K⁺, treating complex [ 1? F]^? with the Cs⁺ ion gave rise to a stable, receptor‐bound ion‐pair complex [Cs ?1? F] that contains the Cs⁺ ion complexed within the cup‐like cavity of the calix[4]pyrrole, which in turn was stabilized in its cone conformation. Different complexation behavior was observed in the case of the chloride complex [ 1? Cl]^?. In this case, no appreciable interaction was observed with Na⁺ or K⁺. In addition, treating [ 1? Cl]^? with Li⁺ produces a tightly hydrated dimeric ion‐pair complex [ 1? LiCl(H₂O)]₂ in which two Li⁺ ions are bound to the crown moiety of the two receptors. In analogy to what was seen in the case of [ 1? F]^?, exposure of [ 1? Cl]^? to the Cs⁺ ion gives rise to an ion‐pair complex [Cs ?1? Cl] in which the cation is bound within the cup of the calix[4]pyrrole. Different complexation modes were also observed when the binding of the fluoride ion was studied by using the tetramethylammonium and tetraethylammonium salts.     

Conductometric Determination of the Stability Constants of the Inclusion Complexes of Alkali Cations with [22-DD] Diaza Crown Ether, [211] and [221] Cryptands in Acetonitrile

An equilibrium study concerning the association of Na⁺, K⁺, Rb⁺ and Cs⁺ with 4, 7, 13, 18-tetraoxa-1,10-diazabicyclo [8, 5, 5]-eicosane [211], 4, 7, 13, 16, 21-pentaoxa-1, 10-diazabicyclo [8, 8, 5]-tricosane [221] and 4, 13-didecyl-1, 7, 10, 16-tetraoxa-4, 13-diazacyclooctadecane [22-DD] in acetonitrile has been carried out at 25 °C by using a conductometric technique. The observed molar conductivity, Λ, of a test solution was found to decrease significantly for mole ratios less than 1:1 upon the addition of the complexing ligand. A model based on 1:1 stoichiometry has been used to analyze the conductivity data. The data have been fitted according to a non-linear least-squares analysis that provides the stability constant, K, and the molar conductivity, Λ_c, for each cation – ligand inclusion complex. The binding sequences were found to follow the order: Na⁺ > K⁺ > Rb⁺ ≫ Cs⁺ (K ≈ 0) for [211], Na⁺ > K⁺ > Rb⁺ > Cs⁺ for [221] and K⁺ > Na⁺ > Rb⁺ > Cs⁺ for [22-DD] complexes. Trends in ionic conductivities of complexed ions are also discussed.     

Facilitated Transfer of Alkali Metal Ions by a Tetraester Derivative of Thiacalix[4]arene at the Liquid–Liquid Interface

The facilitated transfer of alkali metal ions (Na⁺, K⁺, Rb⁺, and Cs⁺) by 25,26,27,28‐tetraethoxycarbonylmethoxy‐thiacalix[4]arene across the water/1,2‐dichloroethane interface was investigated by cyclic voltammetry. The dependence of the half‐wave transfer potential on the metal and ligand concentrations was used to formulate the stoichiometric ratio and to evaluate the association constants of the complexes formed between ionophore and metal ions. While the facilitated transfer of Li⁺ ion was not observed across the water/1,2‐dichloroethane interface, the facilitated transfers were observed by formation of 1 : 1 (metal:ionophore) complex for Na⁺, K⁺, and Rb⁺ ions except for Cs⁺ ion. In the case of Cs⁺ a 1 : 2 (metal:ionophore) complex was obtained from its special electrochemical response to the variation of ligand concentrations in the organic phase. The logarithms of the complex association constants, for facilitated transfer of Na⁺, K⁺, Rb⁺, and Cs⁺, were estimated as 6.52, 7.75, 7.91 (log β₁°), and 8.36 (log β₂°), respectively.     

Host–guest complexes of mixed glycol-phenanthroline cryptands: prediction of ion selectivity by quantum chemical calculations IV

Abstract Structures and complex-formation energies, calculated with DFT (B3LYP/LANL2DZp) for the cryptands [2.2.phen] and [2.phen.phen] with endohedrally complexed alkali and alkaline earth metal ions, were utilized to predict their ion selectivity. Both cryptands [2.2.phen] and [2.phen.phen] have a cavity size smaller than [2.2.2], [phen.phen.phen] and [bpy.bpy.bpy], and prefer to bind K⁺ and Sr²⁺, whereas [2.2.phen] that is larger than [2.phen.phen], has a preference for Ba²⁺, and [2.phen.phen] favours Na⁺ and Ca²⁺. The cryptand flexibility is mainly attributed to the presence of CH₂–N_SP3···N_SP3–CH₂ groups. Graphical abstract Host–Guest Complexes of mixed Glycol-Phenanthroline Cryptands—Prediction of Ion Selectivity by Quantum Chemical Calculations III Ralph Puchta* and Rudi van Eldik Keywords Cation selectivity Host–guest DFT DFT-studies allow a sensitive analysis of selectivity and cage size. Calculations predict a favourable binding of K⁺, Sr²⁺ and Ba²⁺ by [2.2.phen], and binding of K⁺, Na⁺, Ca²⁺ and Sr²⁺ by [2.phen.phen]. The cryptands fold around the ions by twisting their torsion angles in order to reach the best coordination mode for each cation. For “Prediction of ion selectivity by quantum chemical calculations III” see, R. Puchta, R. van Eldik. Aust. J. Chem. 60, 889–897 (2007).     

Complexation of alkali metal cations by calix[4]Arene-bis-crown-6 in methanol,acetonitrile and propylene carbonate

Résumé The interactions of Li⁺, Na⁺, K⁺, Rb⁺, and Cs⁺ with the double-crown calix, calix[4]arene-bis-crown-6, have been studied in methanol, acetonitrile, and propylene carbonate at 25°C using precise conductivity measurements. For Li⁺ and Na⁺ in solutions containing this calix[4]arene, only 1:1 cation:ligand complexes are formed which permit the determination of the thermodynamic complexation formation constants,K _f. The conductivity data strongly suggest that 2:1 cationcalixarene complexes form with K⁺, Rb⁺, and Cs⁺. The existence of 2:1 complexes was experimentally confirmed for the potassium systems by a mass spectroscopic method.     

Complex formation of alkali and alkaline earth metal ions with cryptands containing thiourea units as a part of macrocyclic framework in aqueous methanol

The stability constants (K_s) of the complexes of alkali and alkaline earth metal ions with new type of the cryptands containing one or two thiourea moieties in one of the bridges were determined by means of pH-metric measurements in 95% aqueous methanol at 25 °C. Cryptands studied do not show any regular alteration of complexes stability depending on the mutual relation of cryptand cavity and cation sizes. In all cases, they form the most stable complexes with K⁺ along the series of alkali metal ions and with cations of Ba²⁺ or Sr²⁺ in the series of alkaline earth ions independently of variations of their structure. The log K_s values for K⁺, Sr²⁺ and Ba²⁺ vary in limits 3.51-5.90, 2.29-7.05 and 2.35-7.51, respectively, depending on the cryptands structure. The complexes stability of the studied cryptands increases in the order Li⁺ < Na⁺ (Cs⁺) < Cs⁺ (Na⁺) < Rb⁺ < K⁺ and Mg²⁺ < Ca²⁺ < Sr²⁺ (Ba²⁺) < Ba²⁺ (Sr²⁺). However, cryptands containing at least one oxygen atom between the nitrogen bridgehead and group of thiourea form considerably more stable complexes with respect to cryptands in which thiourea group connected with nitrogen bridgeheads via ethylenic chain. The origins of the cryptands complexation behavior are discussed in terms of ligands and complexes structural features.     

Etude de la protolyse du cryptate d'iode positif

The diprotonated cryptand ∣2.2.2 H₂∣²⁺ is formed by the addition in nitromethane 1 of two equivalent of trifluoroacetic acid (or hydroiodic acid) to the cryptate ∣2.2.2 I∣⁺, I^?; the reaction is accompanied by the release of molecular iodine. Upon adding only one equivalent of trifluoroacetic acid, no release of iodine is observed and the formation of complex ∣H - 2.2.2- I∣²⁺ is deduced from NMR, UV and conductance measurements.     

First Principles Investigation of Noncovalent Complexation: A [2.2.2]‐Cryptand Ion‐Binding Selectivity Study

A first principles methodology, aimed at understanding the roles of molecular conformation and energetics in host–guest binding interactions, is developed and tested on a system that pushes the practical limits of ab initio methods. The binding behavior between the [2.2.2]‐cryptand host (4,7,13,16,21,24‐hexaoxa‐1,10‐diaza‐bicyclo[8.8.8]hexacosane) and alkali metal cations (Li⁺, Na⁺, and K⁺) in gas, water, methanol, and acetonitrile is characterized. Hartree–Fock and density functional theory methods are used in concert with crystallographic information to identify gas phase, energy‐minimized conformations. Gas phase free energies of binding, with vibrational contributions, are compared to solution‐state binding constants through relative binding selectivity analysis. Calculated relative binding free energies qualitatively correlated with solution state experiments only after gas phase metal desolvation is considered. The B3LYP exchange–correlation functional improves theoretical correlations with experimental relative binding free energies. The relevance of gas phase calculations towards understanding binding in condensed phases is discussed. Natural bond orbital methods highlights previously unidentified intramolecular and intermolecular M⁺(222) chemistries, such as an intramolecular n′_O→σ*_CH hydrogen bond.     

Disordered Crystal Structure of Bis[(2.2.2-Cryptand)Sodium] Bis[Aqua(Isothiocyanato)(μ-Isothiocyanato)Sodium] 2[Na(2.2.2-Crypt)]+ · [Na2(NCS)2(μ-NCS)2(H2O)2]2–

Crystals of bis[(2.2.2-cryptand)sodium] bis[aqua(isothiocyanato)(-isothiocyanato)sodium]: 2[Na(C₁₈H₃₆N₂O₆)]⁺ · [Na₂(NCS)₂(-NCS)₂(H₂O)₂]^2– (I) were synthesized and studied by X-ray diffraction analysis. The disordered structure of I (a = 12.715 Å, b = 10.458 Å, c = 21.767 Å, = 102.56°, space group P2₁/n) was solved by the direct method and refined by the full-matrix least-squares method in anisotropic approximation to R = 0.058 from 3896 independent reflections (CAD4 automated diffractometer, MoK ). The crystal consists of two complex ions [I¹]⁺ and [I²]^2– (molar ratio 2 : 1). The Na⁺ cation of the host–guest cation I¹ is coordinated by all eight heteroatoms (6O + 2N) of the cryptand ligand. The coordination polyhedron of this Na⁺ cation is a distorted cube. The atoms of two groups (CH₂–CH₂ and CH₂–O–CH₂–CH₂) in the cryptand ligand are disordered over two positions. The independent cation Na⁺ of the centrosymmetric binuclear complex anion I² is coordinated by one bifurcated O atom of the disordered water molecule and by three N atoms of the SCN^– ligands (including two bridging ligands). The coordination polyhedron of this Na⁺ caiotn is a distorted tetrahedron. The complex ions in the crystal structure of I are united by hydrogen bonds.     

Free energies and entropies of transfer from water to methanol of cations complexed by 18-crown-6

Free energies and entropies of transfer from water to methanol have been obtained for [M⁺18C6] complexes, where M⁺ = Na⁺, K⁺, Rb⁺, Cs⁺, and Ag⁺. The variation of ΔG_t° and ΔS_t° with the central metal cation is smaller than with the [M⁺222] complexes and it is concluded that 18-crown-6 shields the metal cation from the solvent more effectively than crystal structure determinations would suggest.     

Complexation Properties of a Carbon-Bridged Cryptand with Metal Ions in Aqueous Solution at 25°C

Abstract

Thermodynamic quantities (log K, ΔH, and ΔS) for the interactions of a carbon-bridged cryptand with Li⁺, Na⁺, K⁺, Ca²⁺, Sr²⁺, Ba²⁺, and Pb²⁺ were determined at 25° C by calorimetric titration in aqueous solution. The cryptand forms complexes with Na⁺, Sr²⁺, Ba²⁺, and Pb²⁺ with log K ≤ 2. Complexation was not detected for Li⁺, K⁺, and Ca²⁺. Weak interactions with Li⁺ and K⁺ and a log K value of 2.4 for Na⁺ suggest that the cavity size of the cryptand is close to that of Na⁺ but too small for K⁺ and too large for Li⁺. The carbon-bridged cryptand selectively binds Sr²⁺ (log K = 3.2) over Ca²⁺ and Ba²⁺ by more than one order of magnitude.     

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.