Stable Ion Pairs Research Articles

Complexity-Building Annulations of Strained Cycloalkanes and C═O π Bonds

This Perspective details a developing research program that emerged from simple plans for achieving the synthesis of tetrahydrofurans from cyclopropanes and C═O π bonds. Lewis acid catalyzed annulations of malonate-derived donor-acceptor cyclopropanes with aldehydes are unusually broad in scope and lead to the synthesis of structurally diverse tetrahydrofurans. The reactions are stereospecific, with inversion observed at the cyclopropane donor site. Substituent effects on the aldehyde suggest that it acts as a nucleophile in the reaction. An unusual mechanism emerges in which the aldehyde traps a configurationally stable intimate ion pair to stereospecifically construct the C-O bond. In addition to the stereospecific conversion of enantiomerically enriched cyclopropanes into nonracemic heterocycles, we have also demonstrated that racemic cyclopropane 1,1-diesters can undergo dynamic kinetic asymmetric annulations catalyzed by (pybox)MgI(2) complexes. Asymmetric syntheses of (+)-polyanthellin A and (+)-virgatusin have been achieved; both rely upon cyclopropane/aldehyde annulation for construction of the core tetrahydrofurans.

Features of ion hydration and association in aqueous rubidium fluoride solutions at ambient conditions

Aqueous RbF solutions have been investigated at ambient conditions by dielectric relaxation spectroscopy (DRS) over the concentration range of 0 ≤ c/M ≤ 5 and by statistical mechanics (RISM integral equation theory) in the concentration range of 2.15–9.18 M. The behavior of the dielectric spectra and their characteristic parameters (dispersion amplitudes and relaxation times) is discussed, as well as the pair correlation functions and partial coordination numbers derived from statistical mechanics. The paper focuses on the influence of salt concentration on the features of ion hydration and association. The observed changes in the dielectric properties indicate a decrease of the effective hydration number from ∼ 15–18 at infinite dilution to ∼ 3.5 at 5 M and possibly weak association involving stable ion pairs at c ≤ 0.5 M. A major finding is that both ions, Rb + and F −, slow down surrounding water molecules, leading to the emergence of a separate “slow water” relaxation in the spectra. According to the analysis of RISM equilibrium properties, an average number of ∼ 6 water molecules coordinates both Rb + and F − at c = 2.15 M. This number decreases by ∼ 19% for the cation and by ∼ 10% for the anion as c growths up to 9.18 M. The RISM-data indicate a significant increase in the number of contact and solvent-separated ion pairs with increasing c. The possibility of ion clustering is discussed for concentrated RbF(aq) solutions.

Surface behavior of a model surfactant: A theoretical simulation study

A quantum mechanics (QM) method has been used to calculate molecular properties of sodium dodecylbenzenesulfonate (SDBS) in vacuum and in solution. Furthermore, molecular dynamics (MD) simulations have been used to determine the dynamic behavior of SDBS moving from the bulk solution to the air/water interface. QM calculations suggest that two head-group oxygen atoms on each surfactant molecule interact with a Na + ion, despite the availability of three oxygen atoms in the head group. MD simulations showed that the Na + ion must overcome the energy barrier between two solvent layers around the head group to form stable ion pair in solution, which is consistent with experimental results. In the simulation, in moving from the bulk to the interface, SDBS can aggregate in a short time, and the adsorption adopts a preferred orientation. The results indicate that formation of favorable hydrophobic interactions of the surfactant alkyl chains is the origin of interfacial adsorption of SDBS.

Benzimidazole and imidazole lithium salts for battery electrolytes

The intrinsic anion oxidation potential (Δ E v) and lithium ion pair dissociation energy (Δ E d) are two important properties for predicting the potential use of new lithium salts for battery electrolytes. In this work several cyano substituted fluoroalkylated benzimidazole and imidazole anions have been investigated computationally to obtain Δ E v and Δ E d. Varying the number and position of cyano substituents results in large effects on the electrochemical stability of the anion and on the possible lithium ion pair configurations. The lengthening of the fluoroalkyl group introduces several new stable ion pair configurations and a small increase in anion oxidation stability. The most promising fluoroalkylated anions in the present work are the 4,5,6,7-tetracyano-2-fluoroalkylated benzimidazolides (TTB and PTB), with oxidation potentials suitable for high voltage Li-ion battery applications (<4.2 V) and much improved Δ E d compared to PF 6 −—a benchmark for commercially available anions. Further improvements in Δ E d, with maintained stability towards oxidation, are obtainable by replacing the fluoroalkyl group by an additional cyano group, but possibly demanding increased synthesis efforts.

Stability of Ion Pairs of Bis(trifluoromethanesulfonyl)amide-Based Ionic Liquids in Dichloromethane

The ion pair formation constants at infinite dilution (KIP0) of bis(trifluoromethanesulfonyl)amide ([NTf2]−) salts in dichloromethane were determined conductometrically at 298.2 K. The cations used were nine monovalent organic ions having similar molecular size, i.e., 1-butyl-3-methylimidazolium ([BuMeIm]+), 1-butyl-2,3-dimethylimidazolium ([BuMe2Im]+), 1-butylpyridinium ([BuPy]+), 1-propyl-2,3,5-trimethylpyrazolium ([Me3PrPyra]+), 1-butyl-1-methylpyrrolidinium ([BuMePyrr]+), 1-methyl-1-propylpiperidinium ([MePrPip]+), methyltripropylammonium ([MePr3N]+), propyltrimethylammonium ([Me3PrN]+), and tetraethylammonium ([Et4N]+), where the results for [BuMeIm]+, [BuMe2Im]+, and [Et4N]+ were cited from our previous study. It was found from the KIP0 values that the ion pair stability varies with the cation in the order [BuMeIm]+ > [Me3PrN]+ > [BuPy]+ ≥ [BuMePyrr]+ ≥ [MePrPip]+ > [MePr3N]+ ≥ [Et4N]+ ≈ [BuMe2Im]+ ≥ [Me3PrPyra]+, which was explained on the basis of the structures of the cations. For the salts used,...

Spectrophotometric determination of peptic ulcer sulfur‐containing drugs in bulk form and in tablets

Two spectrophotometric procedures are suggested for the determination of three irreversible proton pump inhibitors, rabeprazole (RAB), omeprazole (OMP) and pantoprazole (PAN) in pure form and in different pharmaceutical formulations. The first method is based on the oxidation of RAB and PAN with potassium iodate in an acidic medium followed by extracting the liberated iodine with cyclohexane and measurement at λ = 520 nm. Beer's law is valid in the concentration ranges from 10-400 and 5-400 µg ml(-1) for RAB and PAN, respectively. The apparent molar absorptivities of the resulting coloured product were found to be 1.34 × 10(3) and 1.64 × 10(3) l.mol(-1). cm(-1) for RAB and PAN, respectively. The second method is based on the interaction of the basic drugs, OMP, RAB and PAN, in 1,2-dichloroethane with bromophenol blue (BPB), bromocresol green (BCG) and bromocresol purple (BCP) in the same solvent to produce stable coloured ion pairs with maximum absorbance at 385-405 nm. Regression analysis of Beer's plots showed good correlation in the concentration ranges 10-60, 10-60 and 5-40 µg ml(-1) for OMP, 10-150, 10-150 and 10-60 µg ml(-1) for RAB and 10-250, 10-150 and 10-100 µg ml(-1) for PAN with BPB, BCG and BCP reagents, respectively. The limits of detection are 0.46-7.69 µg ml(-1) and limits of quantitation range between 1.52-8.53 µg ml(-1). The optimum assay conditions were investigated and the recovery of the drugs from their dosage forms ranged from 99.33% to 100.5%. Intraday relative standard deviations (RSD) were 0.029-1.397% and the correlation coefficients ranged from 0.9992 to 1. The two methods can be applied successfully for the determination of these drugs in tablets. The results of analysis were validated statistically through recovery studies.

Determination of Basic Strength of Aliphatic Amines through Ion Pair Formation in Some Ionic Liquid Solutions

To have an evaluation of the basic strength of aliphatic amines in ionic liquid solution, the stability constants relevant to the formation of amine/p-nitrophenol ion pairs were determined in different ionic liquids at 298 K. In particular, aliphatic (pyrrolidinium) and aromatic (imidazolium) ionic liquids were used. Imidazolium ions, bmim(+) and bm(2)im(+), having different hydrogen bond donor abilities were taken into account. Anions were chosen ([BF(4)(-)], [PF(6)(-)], and [NTf(2)(-)]; where NTf(2) = bis(trifluoromethansulfonyl)imide) showing different shape, size, and coordination ability. Several primary, secondary (cyclic or not), and tertiary amines were used to study the effect of amine structure on the ion pair stability. The comparison between data collected in this work and those previously obtained in ordinary molecular solvents evidence that ionic liquids are solvent media able to exalt the amine basicity. Furthermore, the experimental trends obtained in different ionic liquids can only be rationalized considering the whole of parameters affecting their tridimensional structure.

Cation specific binding with protein surface charges

Biological organization depends on a sensitive balance of noncovalent interactions, in particular also those involving interactions between ions. Ion-pairing is qualitatively described by the law of "matching water affinities." This law predicts that cations and anions (with equal valence) form stable contact ion pairs if their sizes match. We show that this simple physical model fails to describe the interaction of cations with (molecular) anions of weak carboxylic acids, which are present on the surfaces of many intra- and extracellular proteins. We performed molecular simulations with quantitatively accurate models and observed that the order K(+) < Na(+) < Li(+) of increasing binding affinity with carboxylate ions is caused by a stronger preference for forming weak solvent-shared ion pairs. The relative insignificance of contact pair interactions with protein surfaces indicates that thermodynamic stability and interactions between proteins in alkali salt solutions is governed by interactions mediated through hydration water molecules.

Alkyl Chain Size Effects on Liquid Phase Properties of 1-Alkyl-3-Methylimidazolium Tetrachloroaluminate Ionic Liquids—A Microscopic Point of View from Computational Chemistry

Microscopic insights for effects of the alkyl chain length on macroscopic physical properties of 1-alkyl-3-methylimidazolium ionic liquids are provided from molecular dynamics computer simulation and quantum mechanical calculations. In particular, the liquid densities, internal energies, and hydrogen bonding behavior of the ions were studied. The effect of the alkyl chain size on the hydrogen bonding behavior of these liquids, in terms of both radial distribution functions and pair energetics, is demonstrated. Finally, studies of the ion pair stabilization were performed at the quantum mechanical level by means of the Kitaura-Morokuma energy partition. The fitting of atomic charges to the ion pairs has been performed to illustrate the charge transfer effect between cation and anion.

Consequences of Acid Strength for Isomerization and Elimination Catalysis on Solid Acids

We address here the manner in which acid catalysis senses the strength of solid acids. Acid strengths for Keggin polyoxometalate (POM) clusters and zeolites, chosen because of their accurately known structures, are described rigorously by their deprotonation energies (DPE). Mechanistic interpretations of the measured dynamics of alkane isomerization and alkanol dehydration are used to obtain rate and equilibrium constants and energies for intermediates and transition states and to relate them to acid strength. n-Hexane isomerization rates were limited by isomerization of alkoxide intermediates on bifunctional metal-acid mixtures designed to maintain alkane-alkene equilibrium. Isomerization rate constants were normalized by the number of accessible protons, measured by titration with 2,6-di-tert-butylpyridine during catalysis. Equilibrium constants for alkoxides formed by protonation of n-hexene increased slightly with deprotonation energies (DPE), while isomerization rate constants decreased and activation barriers increased with increasing DPE, as also shown for alkanol dehydration reactions. These trends are consistent with thermochemical analyses of the transition states involved in isomerization and elimination steps. For all reactions, barriers increased by less than the concomitant increase in DPE upon changes in composition, because electrostatic stabilization of ion-pairs at the relevant transition states becomes more effective for weaker acids, as a result of their higher charge density at the anionic conjugate base. Alkoxide isomerization barriers were more sensitive to DPE than for elimination from H-bonded alkanols, the step that limits 2-butanol and 1-butanol dehydration rates; the latter two reactions showed similar DPE sensitivities, despite significant differences in their rates and activation barriers, indicating that slower reactions are not necessarily more sensitive to acid strength, but instead reflect the involvement of more unstable organic cations at their transition states. These compensating effects from electrostatic stabilization depend on how similar the charge density in these organic cations is to that in the proton removed. Cations with more localized charge favor strong electrostatic interactions with anions and form more stable ionic structures than do cations with more diffuse charges. Ion-pairs at elimination transition states contain cations with higher local charge density at the sp(2) carbon than for isomerization transition states; as a result, these ion-pairs recover a larger fraction of the deprotonation energy, and, consequently, their reactions become less sensitive to acid strength. These concepts lead us to conclude that the energetic difficulty of a catalytic reaction, imposed by gas-phase reactant proton affinities in transition state analogues, does not determine its sensitivity to the acid strength of solid catalysts.

Electrochemical and Spectroscopic Investigation of Counterions Exchange in Polyelectrolyte Brushes

Scanning electrochemical microscopy (SECM) is employed to characterize the transport of redox-active probe ions through quenched polyelectrolyte brushes. The counterion exchange through polyelectrolyte brushes is also investigated by infrared spectroscopy in attenuated total reflection (FTIR-ATR), X-ray photolectron spectroscopy (XPS), and cyclic voltammetry (CV). The synthesis of poly(methacryloyloxy)ethyl trimethylammonium chloride (PMETAC) brushes is performed using surface-initiated atom transfer radical polymerization followed by in situ quaternization reaction. The chloride (Cl(-)) counterions of the positively charged polymer brush are exchanged by ferrocyanide (Fe(CN)(6)(4-)) and ferricyanide (Fe(CN)(6)(3-)) ions that are both detectable by spectroscopy and electrochemically active. A good agreement is found when comparing the results obtained by spectroscopic (FTIR-ATR and XPS) and electrochemical (SECM and CV) methods. The counterions exchange is completely reversible and reproducible. We show that (Fe(CN)(6)(4-)) and (Fe(CN)(6)(3-)) species form stable ion pairs with the quaternary ammonium groups of the polymer brush. The transport of iodide (I(-)) redox-active ions is also investigated. In all cases (ferrocyanide, ferricyanide, or iodide), we find that chloride counterions are partially replaced by electroactive ions. This partial exchange may be attributed to an osmotic effect, since the external salt concentration for the exchange is much lower than the counterion concentration inside the brush.

Properties of Radical Anions of Triptindanones and Indanones: Electronic Communication and Stability of Ion Pairs Containing Lithium Cations

The properties of radical anions and dianions of three related spatial ketones, 9-triptindanone (T1), 9,10-triptindanedione (T2), and 9,10,11-triptindanetrione (T3), in DMF solutions containing tetraalkylammonium and lithium cations were investigated by electrochemical methods and compared with the properties of radical anions of planar 1-indanone (I1), 1,3-indanedione (I2), and 1,2,3-indanetrione (I3). Electronic communication between carbonyl groups in T2 in the presence of tetraethyl and tetrabutyl cations was found to be two times smaller than for I3, but communication decreases substantially in the presence of Li+. Radical anions T1•− and T2•− and dianion T22− that form ion pairs with Li+ cations and corresponding association constants Kass are equal to 493 ± 38, 418 ± 32 and (2.6 ± 0.9) × 107 dm3 mol−1, respectively. The chelating structure of the last ion pair is supported by theoretical calculations of the distribution of the negative charge. A linear relationship between log Kass and maximum valu...

Automated potentiometric titrations in KCl/water-saturated octanol: method for quantifying factors influencing ion-pair partitioning.

The knowledge base of factors influencing ion pair partitioning is very sparse, primarily because of the difficulty in determining accurate log PI values of desirable low molecular weight (MW) reference compounds. We have developed a potentiometric titration procedure in KCl/water-saturated octanol that provides a link to log PI through the thermodynamic cycle of ionization and partitioning. These titrations have the advantage of being independent of the magnitude of log P, while maintaining a reproducibility of a few hundredths of a log P in the calculated difference between log P neutral and log P ion pair (diff (log PN − I)). Simple model compounds can be used. The titration procedure is described in detail, along with a program for calculating pKa′′ values incorporating the ionization of water in octanol. Hydrogen bonding and steric factors have a greater influence on ion pairs than they do on neutral species, yet these factors are missing from current programs used to calculate log PI and log D. In contrast to the common assumption that diff (log PN − I) is the same for all amines, they can actually vary more than 3 log units, as in our examples. A major factor affecting log PI is the ability of water and the counterion to approach the charge center. Bulky substituents near the charge center have a negative influence on log PI. On the other hand, hydrogen bonding groups near the charge center have the opposite effect by lowering the free energy of the ion pair. The use of this titration method to determine substituent ion pair stabilization values (IPS) should bring about more accurate log D calculations and encourage species-specific QSAR involving log DN and log DI. This work also brings attention to the fascinating world of nature’s highly stabilized ion pairs.

Hydrophobic Effects in the Critical Destabilization and Release Dynamics of Degradable Multilayer Films.

Recent research has highlighted the ability of hydrolytically degradable electrostatic layer-by-layer films to act as versatile drug delivery systems capable of multi-agent release. A key element of these films is the potential to gain precise control of release by evoking a surface-erosion mechanism. Here we sought to determine the extent to which manipulation of chemical structure could be used to control release from hydrolytically degradable layer-by-layer films through modification of the degradable polycation. Toward this goal, films composed of poly(β-amino ester)s, varying only in the choice of diacrylate monomer, and the model biological drug, dextran sulfate, were used to ascertain the role of alkyl chain length, steric hindrance, and hydrophobicity on release dynamics. Above a critical polycation hydrophobicity, as determined using octanol:water coefficients, the film becomes rapidly destabilized and quickly released its contents. These findings indicate that in these unique electrostatic assemblies, hydrolytic susceptibility is dependent not only on hydrophobicity, but a complex balance between hydrophobic composition, charge density, and stability of electrostatic ion pairs. Computational determination of octanol:water coefficients allowed for the reliable prediction of release dynamics. The determination of a correlation between octanol:water coefficient and release duration will enables advanced engineering to produce custom drug delivery systems.

Geometrical and electronic structures of the dication and ion pair in the geminal dicationic ionic liquid 1,3-bis[3-methylimidazolium-yl]propane bromide

Geminal dicationic ionic liquids (ILs), a new category of IL family, have been developed recently and found to possess unique properties compared to conventional monocationic ILs. To establish a basis for understanding their novel properties, we studied the geometrical and electronic structures of the dication ([(mim)C 3(mim)] 2+) and the ion pair ([(mim)C 3(mim)] 2+-2Br −) in the geminal dicationic IL 1,3-bis[3-methylimidazolium-yl]propane bromide by performing density functional theory calculations. The geometrical structures and relative stabilities for the dication and the ion pair are discussed, and their electronic properties are analyzed in detail. The intrinsic interaction between the dication and Br anions in the most stable conformer was investigated by performing the natural bond orbital analyses. Results for the dication and the ion pair are compared with those of the corresponding monocation ([C 4mim] +) and ion pair ([C 4mim] +-Br −). 1H NMR spectroscopy for the most stable ion pair has been calculated and the general trend is found to be in fairly agreement with the experimental data.

Fast Substitution Reactions of Pt(II) in Different Ionic Liquids. Reactivity Control by Anionic Components

The effect of several imidazolium-based ionic liquids on the rate and mechanism of the substitution reaction of [Pt(terpyridine)Cl](+) with thiocyanate was investigated as a function of thiocyanate concentration and temperature under pseudo-first-order conditions using stopped-flow and other kinetic techniques. The obtained rate constants and activation parameters showed good agreement with the ion-pair stabilization energies between the anions of the ionic liquids and the cationic Pt(II) complex derived from density-functional theory calculations (RB3LYP/LANL2DZp) and with parameters derived from the linear solvation energy relationship set by the Kamlet-Taft beta parameter, which is a measure of a solvent's hydrogen bonding acceptor ability. In general, the substitution reactions followed an associative mechanism as found for conventional solvents, but the observed rate constants showed a significant dependence on the nature of the anionic component of the ionic liquid used as solvent. The second order rate constant measured in [emim][NTf(2)] is 2000 times higher than the one measured in [emim][EtOSO(3)]. This difference is much larger than observed for a neutral entering nucleophile studied before.

Mechanistic Studies on Fast Ligand Substitution Reactions of Pt(II) in Different Ionic Liquids: Role of Solvent Polarity and Ion-Pair Formation

The effect of several imidazolium-based ionic liquids on the mechanism of a classical ligand substitution reaction of [Pt(terpyridine)Cl] (+) with thiourea was investigated. A detailed kinetic study as a function of the nucleophile concentration and temperature was undertaken under pseudo-first-order conditions using stopped-flow techniques. Polarity measurements were performed for the employed ionic liquids on the basis of solvatochromic effects, and they show similarities with conventional polar solvents. Density-functional theory calculations (RB3LYP/LANL2DZp) were employed to predict the ion-pair stabilization energy between the ionic components of the ionic liquids and/or between the anions of the ionic liquids and the cationic Pt (II) complex. These data illustrate how the anions of the ionic liquids can affect the investigated substitution reaction. In general, the substitution mechanism in ionic liquids was found to have an associative character similar to that in conventional solvents. The observed deviations reflect the influence of the ionic liquid on the interaction between the anionic component of the liquid and the positively charged complex.

Electrical conductivity and translational diffusion in the 1-butyl-3-methylimidazolium tetrafluoroborate ionic liquid

Broadband dielectric and terahertz spectroscopy (10(-2)-10(+12) Hz) are combined with pulsed field gradient nuclear magnetic resonance (PFG-NMR) to explore charge transport and translational diffusion in the 1-butyl-3-methylimidazolium tetrafluoroborate ionic liquid. The dielectric spectra are interpreted as superposition of high-frequency relaxation processes associated with dipolar librations and a conductivity contribution. The latter originates from hopping of charge carriers on a random spatially varying potential landscape and quantitatively fits the observed frequency and temperature dependence of the spectra. A further analysis delivers the hopping rate and enables one to deduce--using the Einstein-Smoluchowski equation--the translational diffusion coefficient of the charge carriers in quantitative agreement with PFG-NMR measurements. By that, the mobility is determined and separated from the charge carrier density; for the former, a Vogel-Fulcher-Tammann and for the latter, an Arrhenius temperature dependence is obtained. There is no indication of a mode arising from the reorientation of stable ion pairs.

Pyrene-tetrathiafulvalene supramolecular assembly with different types of carbon nanotubes

Pyrene-tetrathiafulvalenes reveal strong interactive forces with different types of carbon nanotube (CNT) samples—ranging from single-walled carbon nanotubes (SWNTs) to multi-walled carbon nanotubes (MWNTs). Notable, although subtle, are the differences that emerged while preparing, handling, and characterizing these composite materials. Stronger are the differences seen in the context of photoinduced charge transfer. In particular, charge injection into the conduction band of CNT probes afforded stable radical ion pair states (i.e., on the time scale of our femto/picosecond investigation) only for MWNTs, while the lifetimes for SWNTs are shorter. A likely rationale involves the multiple electron acceptor levels in MWNTs, when compared to SWNTs or double-walled carbon nanotubes (DWNTs).

Formation of Chelated Counteranions Using Lewis Acidic Diboranes: Relevance to Isobutene Polymerization

The reactions of chelating diboranes with PhCMe2Cl and related initiators were studied by variable-temperature NMR spectroscopy. Although thermally stable ion-pairs featuring weakly coodinating anions (WCA) are formed, isobutene polymerization is complicated by the tendency of these WCA to act as hindered bases toward Brønsted acidic chain ends.