Ion-pair Binding Research Articles

Binding of ion pairs in a thiourea-functionalized self-folding cavitand

A bioinspired supramolecular container with complementary binding sites displays cooperative binding of ion pairs.

Multiply Reduced Oligofluorenes: Their Nature and Pairing with THF-Solvated Sodium Ions

Conjugated oligofluorenes are chemically reduced up to five charges in tetrahydrofuran solvent and confirmed with clear spectroscopic evidence. Stimulated by these experimental results, we have conducted a comprehensive computational study of the electronic structure and the solvation structure of representative oligofluorene anions with a focus on the pairing between sodium ions and these multianions. Using density functional theory (DFT) methods and a solvation model of both explicit solvent molecules and implicit polarizable continuum, we first elucidate the structure of tightly solvated free sodium ions, and then explore the pairing of sodium ions either in contact with reduced oligofluorenes or as solvent-separated ion pairs. Computed time-dependent-DFT absorption spectra are compared with experiments to assign the dominant ion pairing structure for each multianion. Computed ion pair binding energies further support our assignment. The availability of different length and reducing level of oligofluorenes enables us to investigate the effects of total charge and charge density on the binding with sodium ions, and our results suggest both factors play important roles in ion pairing for small molecules. However, as the oligofluorene size grows, its charge density determines the binding strength with the sodium ion.

Ion Pair Binding in the Solid-State with Ditopic Crown Ether Uranyl Salophen Receptors.

Two ditopic uranyl salophen receptors with benzo-15-crown-5 and benzo-18-crown-6 units (R(1) and R(2), respectively) have been synthesized from commercially available starting materials. Comprehensive studies on the solid-state ion pair complexation with various alkali and ammonium halides have been conducted. From the 19 obtained solid-state structures (6 structures with R(1), 13 structures with R(2)), three general interaction motifs I-III have been observed. Interaction motif I has a separated ion pair with the cation coordinated to the crown ether unit, and the anion or oxygen containing solvent molecule coordinated to the uranyl center. The interaction motif II manifests a polymeric structure with a contact ion pair between the uranyl-coordinated anion and cation coordinated in the crown ether in the adjacent receptor. Interaction motif III consists of a more general stacked packing structure of the receptors with or without ion pairs. From the obtained solid-state structures, the complex R(2)·NaI shows an interesting formation of infinite coordination polymeric structure where the crown ether complexed sodium cations and the O═U═O units of the adjacent receptors form a nearly linear 1-D chains. In the course of this work also the first solid-state structure of uranyl salophen acetate complex was obtained (R(2)·KAcO).

Molecular dynamics simulations of K+–Cl− ion pair in polar mixtures of acetone and water: Preferential solvation and structural studies

Molecular dynamics simulation studies have been performed for a series of polar mixtures of acetone and water containing a K+–Cl− ion pair. We have obtained the potentials of mean force (PMFs) for 15 mixtures spanning the whole composition range from 0% to 100% acetone. The PMFs indicate the presence of a stable contact ion pair (CIP), a solvent assisted ion pair (SAIP) and a solvent separated ion pair (SSIP) in all the compositions with acetone mole fraction (xacetone)≤0.50. However, for xacetone>0.50 and xacetone≤0.90, PMFs suggest the presence of only a stable CIP and SAIP. For xacetone>0.90, PMFs show the existence of only a stable CIP. While the stability of CIPs in mixtures increases with xacetone, the reverse trend is observed for SAIPs. The determination of thermodynamic properties suggests that entropy favours the CIPs and SAIPs in all the mixtures. The analysis of radial distribution functions using Kirkwood–Buff (KB) integrals explains the preferential solvation of the ion pair. We observed that in all the mixtures, K+–Cl− ion pair is preferentially solvated by water. Running coordination numbers of solvent molecules around the ion pair and preferential binding parameters also support the above observations. The dynamical aspects have been explored by calculating self-diffusion constants and hydrogen bond dynamics.

Cooperatively enhanced ion pair binding with a hybrid receptor.

A simple 18-crown-6-based bis-urea receptor R(1) was synthesized in three steps from a commercial starting material. The receptor's behavior toward anions, cations, and ion pairs was studied in solution with (1)H NMR, in solid state with single-crystal X-ray diffraction, and in gas phase with mass spectrometry. In 4:1 CDCl3/dimethyl sulfoxide solution the receptor's binding preference of halide anions is I(-) < Br(-) < Cl(-) following the trend of the hydrogen-bonding acceptor ability of the anions. The receptor shows a remarkable positive cooperativity toward halide anions Cl(-), Br(-), and I(-) when complexed with Na(+), K(+), or Rb(+). The solid-state binding modes of R(1) with alkali and ammonium halides or oxyanions were confirmed by the X-ray structures of R(1) with KF, KCl, KBr, KI, RbCl, NH4Cl, NH4Br, KAcO, K2CO3, and K2SO4. They clearly present two different binding modes, either as separated or contact ion pairs depending on the nature and size of the bound cation and anion. Complexation capability of R(1) in the gas phase was studied with competition experiments with electrospray ionization mass spectrometry showing preference of KCl complexation over NaCl, KBr, or KI supporting the results obtained in solution.

DFT study of 1-butyl-3-methylimidazolium salicylate: a third-generation ionic liquid.

A detailed theoretical investigation of the third-generation ionic liquid (IL) 1-butyl-3-methylimidazolium salicylate ([BMIM][Sal]), performed within the framework of density functional theory (DFT), is presented in this paper. The B3LYP-D3, M06-2X, and M06-2X-D3 functionals were used to obtain the equilibrium geometries of the two ions [BMIM](+) and [Sal](-). It is shown that the equilibrium ion geometries obtained with the dispersion-corrected B3LYP functional are very close to the ion geometries obtained with the M06-2X and M06-2X-D3 functionals. Global reactivity was assessed using molecular orbital theory and quantum molecular descriptors. Molecular electrostatic potential (MEP) surfaces and average local ionization energy (ALIE) surfaces were created in order to elucidate the charge distribution and reactivity of the investigated IL. Ion-pair binding energies were calculated with all three functionals, and the results confirmed the presence of a strong electrostatic interaction between the ions, while further insight into the interactions between the two ions was obtained by analyzing noncovalent interactions based on the reduced density gradient (RDG) surface, which revealed a total of nine interactions between the ions. Finally, the aromaticity of each ion was investigated by calculating the nucleus-independent chemical shift (NICS) parameter, which indicated that significant changes in the charge delocalization on each ion occur when the two ions interact.

Electrostatic and Allosteric Cooperativity in Ion-Pair Binding: A Quantitative and Coupled Experiment-Theory Study with Aryl-Triazole-Ether Macrocycles.

Cooperative binding of ion pairs to receptors is crucial for the manipulation of salts, but a comprehensive understanding of cooperativity has been elusive. To this end, we combine experiment and theory to quantify ion-pair binding and to separate allostery from electrostatics to understand their relative contributions. We designed aryl-triazole-ether macrocycles (MC) to be semiflexible, which allows ion pairs (NaX; X = anion) to make contact, and to be monocyclic to simplify analyses. A multiequilibrium model allows us to quantify, for the first time, the experimental cooperativity, α, for the equilibrium MC·Na(+) + MC·X(-) ⇌ MC·NaX + MC, which is associated with contact ion-pair binding of NaI (α = 1300, ΔGα = -18 kJ mol(-1)) and NaClO4 (α = 400, ΔGα = -15 kJ mol(-1)) in 4:1 dichloromethane-acetonitrile. We used accurate energies from density functional theory to deconvolute how the electrostatic effects and the allosteric changes in receptor geometry individually contribute to cooperativity. Computations, using a continuum solvation model (dichloromethane), show that allostery contributes ∼30% to overall positive cooperativity. The calculated trend of electrostatic cooperativity using pairs of spherical ions (NaCl > NaBr > NaI) correlates to experimental observations (NaI > NaClO4). We show that intrinsic ionic size, which dictates charge separation distance in contact ion pairs, controls electrostatic cooperativity. This finding supports the design principle that semiflexible receptors can facilitate optimal electrostatic cooperativity. While Coulomb's law predicts the size-dependent trend, it overestimates electrostatic cooperativity; we suggest that binding of the individual anion and cation to their respective binding sites dilutes their effective charge. This comprehensive understanding is critical for rational designs of ion-pair receptors for the manipulation of salts.

Molecular insights accompanying aggregation in amino acid ionic liquids

Molecular interactions between 1-Ethyl-3-methylimidazolium cation ([Emim]) and hydrophobic amino acids anions (AA) viz., glycine [Gly], alanine [Ala], valine [Val], leucine [Leu], and isoleucine [Ile] have been studied employing the M05–2X based Density Functional Theory (DFT). It has been shown that the emergent hierarchy in the binding energy, viz., [Emim]-[Ile]∼[Emim]-[Leu]>[Emim]-[Val]>[Emim]-[Ala]>[Emim]-[Gly]; is attributable both to the number of hydrogen bonds and to the CH---π interactions. Molecular electrostatic potential (MESP) serves as a tool for building of [Emim]x[AA]y (x, y=1, 2) molecular aggregates. Molecular Electron Density (MED) topographic investigations further revealed an interesting correlation between the electron density at the bond critical point of the hydrogen bond from the acidic proton and the ion-pair binding energies. Normal coordinate-analysis brings forth that the [Emim][AA] ion-pair binding accompanied by the frequency up-shift (blue-shift) of the NH stretching relative to the isolated AA anion; with the largest shift of 54cm−1 occurring for [Emim][Leu]. On the other hand, frequency shifts ranging from 49cm−1 in [Leu] to 34cm−1 in [Ile], in the opposite direction have been noticed for the CO stretching mode. The direction of frequency shifts of characteristic CO, NH and CH vibrations are justified using the ‘Quantum Theory of Atoms in Molecules’ approach in conjunction with Natural-Bond-Orbital analysis. Frontier orbital analysis and hardness parameters together validate the trends in the binding strengths of ion-pairs as well as in aggregates underlying amino acid based ILs.

Ion pair binding by an l-tyrosine-based polymerizable molecular receptor

A polymerizable molecular receptor able to bind ion pairs and new functional polymers containing the receptor units were synthesized and characterized.

Probing the origin of opposite ion-pair binding behavior for two new calix[4]pyrrole bis-phosphonate receptors

The origin of opposite ion-pair binding behavior for two new calix[4]pyrrole bis-phosphonate receptors has been explored.

Exploring the mechanism of ion-pair recognition by new calix[4]pyrrole bis-phosphonate receptors: insights from quantum mechanics study

Three diastereomeric bis-phosphonate cavitands based on an aryl extended calix[4]pyrrole can act as ion-pair receptors for alkylammonium/phosphonium chloride salts. In this contribution, the ion-pair binding mode and binding affinity were investigated using density functional theory (DFT) calculations and new nonconvalent weak interaction analysis method. One of the three receptors was chosen as the host and two guests were chosen to model quaternary phosphonium chloride salts and primary ammonium chloride salts respectively, and two types of arrangements – separated and contact – were taken into account for each guest. The binding energy suggests that contact arrangement is the favorable binding mode for this receptor and it prefers to bind primary ammonium chloride salts rather than quaternary phosphonium chloride salts both in the gas-phase and in solution, in agreement with the experiment. Moreover, geometry analysis and charge transfer based on natural bond orbital (NBO) analysis suggest that the binding modes and binding affinities of the receptor towards different ion-pairs are determined by the amount and strength of hydrogen bonds. Furthermore, nonconvalent weak interactions between the host and the guests have been explored to unravel the driving forces responsible for the ion-pair recognition. There are hydrogen bonding, van der Waals, cation–π, ion-induced dipole and charge–charge interactions in ion-pair recognition, and hydrogen bonding interactions play the dominant role in this process. This work unveils the mechanism of ion-pair recognition by new calix[4]pyrrole bis-phosphonate receptors, while opening exciting perspectives for the design of novel calix[4]pyrrole-based ion-pair receptors.

Frustrated ion-pair binding by heteroditopic macrocyclic receptors

A series of Ag+ complexes of the macrocyclic heteroditopic receptor, 4-phenylurea-benzo-15-crown-5 (1) have been prepared from hydrophobic- (,) and oxo- (, ) anion salts. These complexes contain the Ag+ cation, partially interacting with the macrocyclic cavity and the anions H-bonded to urea moieties. Additional interactions of the Ag+ cation with a –C = O moiety of a neighbouring receptor 1 molecule or with the oxo-anions (, ) have been observed. Kass values calculated for the resulted ion-pair macrocyclic complexes , , , were fitted with 1:1 model and correspond to the values of 140, 255, 660 and 980 M− 1, respectively. The cooperative anion/urea and cation/macrocycle binding and their interaction favour the whole recognition process. This synergism is amplified by self-assembly and depends on the structural features of the anion. In the combined system, the cation–anion interactions are favourable for the stability of the whole system.

Salt‐Solubilization and Ion‐Pair Recognition by a Quinoline‐Substituted Crown Ether

AbstractA novel quinoline‐substituted crown ether has been synthesized as a receptor for ion pairs and its binding behavior was investigated in various solvents. The binding of ion pairs in [D6]DMSO and in mixed [D6]DMSO/CD3CN/CDCl3 solvents was studied and the results revealed positive cooperativity effects. As a novel solubilizing agent, the crown ether can extract halide salts into organic media, especially chloride salts. ESI MS provided evidence for the formation of salt‐receptor complexes. A recycling experiment showed that the receptor can solubilize salts and can be recovered. This process can be performed as a cycle. This study validates the idea that a straightforward combination of a common anion receptor and a well‐studied cation complexation moiety enables the formation of ion‐pair receptors and solubilizers of salts in organic solvents.

A computational approach to design energetic ionic liquids

The present work deals with the theoretical estimation of ion-pair binding energies and the energetic properties of four ion pairs formed by combining the 1-butyl-2,4-dinitro-3-methyl imidazolium ion with nitrate (I), perchlorate (II), dinitramide (III), or 3,5-dinitro-1,2,4-triazolate (IV) anions. The counterpoise-corrected ion-pair binding energies were calculated for each ion pair at the B3LYP/6-311+G(d,p) level of theory. Results show that the cation-anion interaction is strongest for ion pair I and weakest for IV, indicating that the nitrate (I) has a greater tendency to exist as a stable ionic salt whereas the 3,5-dinitro-1,2,4-triazolate (IV) may exist as an ionic liquid. Natural bond orbital (NBO) analysis and electrostatic potential (ESP) mapping revealed that charge transfer occurs in all of the ion pairs, but is greatest (0.25e) for ion pair I and smallest (0.03e) for IV, resulting in ion pair I being the least polarized. A nucleus-independent chemical shift (NICS) study revealed that the aromaticity of the 1-butyl-2,4-dinitro-3-methyl imidazolium ion significantly increases in ion pair IV, indicating that this has the greatest charge delocalization among all of the four ion pairs considered. Studies of thermodynamic and detonation properties showed that ion pair II is the most energetic ion pair in terms of its detonation velocity (D = 7.5 km s(-1)) and detonation pressure (P = 23.1 GPa). It is also envisaged that ion pair IV would exist as an energetic azolium azolate type ionic liquid that could be conveniently used as a secondary explosive characterized by detonation parameters D and P of 6.9 km s(-1) and 19.3 GPa, respectively. These values are comparable to those of conventional explosives such as TNT.

Foldamer-based chiral supramolecular alternate block copolymers tuned by ion-pair binding

A chiral supramolecular alternate block copolymer has been self-assembled from a ureidopyrimidinone (UPy)-terminated arylamide-based hydrogen bonded foldamer and a structurally flexible pentamer, which is driven by the ion-pair binding of the two arylamide segments toward chiral ammonium and carboxylate guests.

Switching from Separated to Contact Ion-Pair Binding Modes with Diastereomeric Calix[4]pyrrole Bis-phosphonate Receptors

We describe the design, synthesis and conformational assignment of three diasteromeric bis-phosphonate cavitands based on an aryl extended calix[4]pyrrole tetrol scaffold. The diastereoisomers differ in the relative spatial orientation of the P═O groups installed at their upper rims. We demonstrate that these compounds act as heteroditopic receptors for ion pairs forming ion-paired 1:1 complexes with alkylammonium (quaternary and primary) chloride salts in dichloromethane (DCM) solution and in the solid-state. (1)H NMR titrations indicate that the complexes are highly stable thermodynamically and kinetically. In the case of tetraalkyl-phosphonium/ammonium chloride guests, the host featuring the two P═O groups directed outwardly with respect to the aromatic cavity, 4oo, produces the most thermodynamically stable complexes. Conversely, for the primary alkyl ammonium chloride, the most effective receptor is the diastereoisomer 4ii with the two P═O groups converging on top of the aromatic cavity. In the nonpolar DCM solvent, the size of the quaternary cation has a strong impact in the thermodynamic stability of the complexes and their binding geometry. We use 2D-ROESY experiments to map out the binding geometries of the 1:1 complexes formed in solution. The 1:1 complexes of the 4oo host with the chloride salts have a separated arrangement of the bound ion-pair. In contrast, those of the 4ii host display a close-contact arrangement. We also investigate the same complexation processes in acetonitrile (ACN) solution. Both the salt and the initially formed anionic complex are fully dissociated in this more polar solvent. The receptors show an analogous trend in their binding affinities for quaternary phosphonium/ammonium chloride salts to the one seen in DCM solution. However, in ACN solution, the magnitudes of the binding affinities are reduced significantly and the size of the cation does not play a role. In addition, the inversion in the trend of relative binding affinities of the complexes, which was revealed in DCM solution, is eradicated in ACN when changing the cation substitution from quaternary to primary.

ChemInform Abstract: Heteroditopic Receptors for Ion‐Pair Recognition.

AbstractReview: 46 refs.

Heteroditopic Receptors for Ion‐Pair Recognition

Ion-pair recognition is a new field of research emerging from cation and anion coordination chemistry. Specific types of heteroditopic receptor designs for ion pairs and the complexity of ion-pair binding are discussed to illustrate key concepts such as cooperativity. The importance of this area of research is reflected by the wide variety of potential applications of ion-pair receptors, including applications as membrane transport and salt solubilization agents and sensors.

Extending the family of heteroditopic calix[4]diquinone receptors for cooperative AND ion-pair recognition

New heteroditopic calix[4]diquinone receptors with different anion binding units and calix[4]diquinone scaffolds have been synthesised. Ion-pair binding studies using UV/visible and 1H NMR spectroscopies reveal that receptors 1, 2 and 4 are cooperative AND ion-pair receptors, which display little affinity for ‘free’ ions but enhanced binding of the ion-pairs NaCl, NH4Cl and KCl. The more preorganised receptor 2 binds the ion-pairs more weakly than receptor 1. Varying the nature of the calix[4]diquinone scaffold appears to have little effect on ion-pair binding, although the calix[4]diquinone framework is more conformationally flexible than the tert-butylcalix[4]diquinone one and can be synthesised using the milder oxidant chlorine dioxide.

Predicting properties of new ionic liquids: density functional theory and experimental studies of tetra-alkylammonium salts of (thio)carboxylate anions, RCO2−, RCOS− and RCS2−

Density functional theory calculations of alkyl-carboxylate anions and their sulfur substituted variants are presented here as an aid for the development of new ionic liquids. Electron transfer both within the anion, and between the anion and cation of an ion pair, are described using natural bond order analysis, using tetraethylammonium as a common cation. The overall stabilising effect of this electron transfer is quantified for the series of anions, and is found to correlate with clear trends in ion-pair binding energy. These and other electronic properties determine which compounds are synthesised, and experimental results validate the computational results. In combination with tetraethylammonium, a carboxylate with an unsaturated alkyl chain produces an ionic liquid at room temperature. However, computations suggest that sulfur substituted anions will produce a lower melting point and perhaps more fluid ionic liquid, but one which would be less stable against oxidation.