Liquid-Crystalline Ionic Liquids as Ordered Reaction Media for the Diels-Alder Reaction

Small-Angle X-ray Scattering Study of the Amphiphilic Bulk Nanostructure of Tetraalkylammonium Deep Eutectic Solvents

Deep eutectic solvents (DESs) are low-melting mixtures, often prepared from a salt and a molecular hydrogen bond donor. Like ionic liquids, DESs that contain at least one sufficiently amphiphilic component can form bicontinuous nanostructures consisting of polar and nonpolar domains, although this has not been widely explored for many DES combinations. Here, the bulk nanostructures of DESs comprising tetraalkylammonium bromide salts (tetrabutylammonium bromide, tetraoctylammonium bromide, and methyltrioctylammonium bromide) with alkanols and alkanoic acids of systematically varied chain lengths (C2, C6, C8, and C10) as hydrogen bond donors have been studied. Small-angle X-ray scattering techniques were used to identify the relationship between the alkyl chain length and functionality of the hydrogen bond donor on the nature of the amphiphilic nanostructures formed. These findings demonstrated that the amphiphilic nanostructures of the DESs were not affected by the functional group on the hydrogen bond donor, with these nanostructures influenced primarily by both the absolute and relative alkyl chain lengths of the salt and hydrogen bond donor.

The amphiphilic nanostructure of ionic liquids affects the dehydration of alcohols

The effect of the amphiphilic nanostructure of ionic liquids on the dehydration of secondary alcohols to alkenes has been investigated. The influence of these nanostructures was inverted when an acid catalyst was added to the reaction. This phenomenon was ascribed to a balance between ion-solute interactions and the formation of solute-catalyst hydrogen bonds, highlighting the complex interplay between interactions and reaction outcomes in these nanostructured solvent systems.

Copper-Catalyzed Regioselective Olefination and Trifluoromethylation of Carboxylic Acids To Give (Z)-Trifluoromethyl Enol Esters

Herein, we describe a method to produce (Z)-trifluoromethyl enol esters via the olefination and trifluoromethylation of carboxylic acids with TMSCF₃. This synthetic method uses inexpensive and easy-to-handle TMSCF₃. It employs a commercially available CuCl catalyst to transform a broad range of carboxylic acids into versatile (Z)-trifluoromethyl enol esters with good regio- and stereoselectivity. This protocol allows the concise synthesis of highly functionalized (Z)-trifluoromethyl enol esters directly from carboxylic acids.

Small-angle neutron scattering from mixtures of long- and short-chain 3-alkyl-1-methyl imidazolium bistriflimides

The preparation of mixtures of ionic liquids (ILs) represents an attractive strategy to tune their properties, an important aspect of which is to understand how the structure of the bulk varies with composition. In this study, small-angle neutron scattering (SANS) was used to probe mixtures of methylimidazolium-based ionic liquids [C_nmim][Tf₂N] with [C₂mim][Tf₂N]) (n = 4, 6, 8 and 10) and of [C_mmim][Tf₂N] with [C₁₂mim][Tf₂N] (m = 2, 4, 6 and 8). Mixtures were prepared in both contrasts, which is to say that one component would be fully hydrogenated while the other was fully deuterated, and vice versa. Data were fitted using a range of appropriate models, of which the Teubner-Strey model provided most useful information and the pure materials showed a nascent Polar Non-polar Peak (PNPP) for n = 6, which became more evident as n increased. In the mixtures [C_nmim]_x[C₂mim]_1-x[Tf₂N], the PNPP was evident for n = 10 and 8, nascent for n = 6 and absent for n = 4, with percolation showing a very strong dependence on the chain length of the added IL, [C_nmim][Tf₂N]. In contrast, while the ability of [C₁₂mim][Tf₂N] to form percolated structures was damped when mixed with [C_mmim][Tf₂N], as m increased from 2 to 6, this effect was less strong. However, data obtained for mixtures of [C₁₂mim][Tf₂N] and [C₈mim][Tf₂N], both of which percolate as pure materials, did not fit easily in any of the models applied to the previous systems and gave results that depended on the contrast used. Complementary small-angle X-ray scattering (SAXS) data, however, showed the expected evolution and behaviour of the PNPP, COP and CP, revealing that the unexpected observations were due to an adventitious matching out of isotopic contrasts. As well as revealing details of the structures of these IL mixtures, the results also point to complementary strategies for generating bulk percolated structures as a function of cation chain length.

Ionic Liquids and Water: Hydrophobicity vs. Hydrophilicity

Many chemical processes rely extensively on organic solvents posing safety and environmental concerns. For a successful transfer of some of those chemical processes and reactions to aqueous media, agents acting as solubilizers, or phase-modifiers, are of central importance. In the present work, the structure of aqueous solutions of several ionic liquid systems capable of forming multiple solubilizing environments were modeled by molecular dynamics simulations. The effect of small aliphatic chains on solutions of hydrophobic 1-alkyl-3-methylimidazolium bis(trifluoromethyl)sulfonylimide ionic liquids (with alkyl = propyl [C₃C₁im][NTf₂], butyl [C₄C₁im][NTf₂] and isobutyl [iC₄C₁im][NTf₂]) are covered first. Next, we focus on the interactions of sulphonate- and carboxylate-based anions with different hydrogenated and perfluorinated alkyl side chains in solutions of [C₂C₁im][C_nF_2n+1SO₃], [C₂C₁im][C_nH_2n+1SO₃], [C₂C₁im][CF₃CO₂] and [C₂C₁im][CH₃CO₂] (n = 1, 4, 8). The last system considered is an ionic liquid completely miscible with water that combines the cation N-methyl-N,N,N-tris(2-hydroxyethyl)ammonium [N_{1 2OH 2OH 2OH}]⁺, with high hydrogen-bonding capability, and the hydrophobic anion [NTf₂]^-. The interplay between short- and long-range interactions, clustering of alkyl and perfluoroalkyl tails, and hydrogen bonding enables a wealth of possibilities in tailoring an ionic liquid solution according to the needs.

Functional Ionic Liquid Crystals

Ionic liquid crystals have emerged as a new class of functional soft materials in the last two decades, and they exhibit synergistic characteristics of ionic liquids and liquid crystals such as macroscopic orientability, miscibility with various species, phase stability, nanostructural tunability, and polar nanochannel formation. Owing to these characteristics, the structures, properties, and functions of ionic liquid crystals have been a hot topic in materials chemistry, finding various applications including host frameworks for guest binding, separation membranes, ion-/proton-conducting membranes, reaction media, and optoelectronic materials. Although several excellent review articles of ionic liquid crystals have been published recently, they mainly focused on the fundamental aspects, structures, and specific properties of ionic liquid crystals, while these applications of ionic liquid crystals have not yet been discussed at one time. The aim of this feature article is to provide an overview of the applications of ionic liquid crystals in a comprehensive manner.

Low temperature glass/crystal transition in ionic liquids determined by H-bond vs. coulombic strength

Self-assembled ionic liquid crystals are anisotropic ionic conductors, with potential applications in areas as important as solar cells, battery electrolytes and catalysis. However, many of these applications are still limited by the lack of precise control over the variety of phases that can be formed (nematic, smectic, or semi/fully crystalline), determined by a complex pattern of different intermolecular interactions. Here we report the results of a systematic study of crystallization of several imidazolium salts in which the relative contribution of isotropic coulombic and directional H-bond interactions is carefully tuned. Our results demonstrate that the relative strength of directional H-bonds with respect to the isotropic Coulomb interaction determines the formation of a crystalline, semi-crystalline or glassy phase at low temperature. The possibility of pinpointing H-bonding directionality in ionic liquids make them model systems to study the crystallization of an ionic solid under a perturbed Coulomb potential.

Ordered Solvents and Ionic Liquids Can Be Harnessed for Electrostatic Catalysis

Herein, we employ classical molecular dynamics simulations using the Drude oscillator-based polarizable force field, quantum chemical calculations, and ONIOM multiscale calculations to study (a) how an external field orders the solvent environment in a chemical reaction and then (b) whether in the absence of this same applied field the ordered solvent environment alone can electrostatically catalyze a chemical reaction when compared with the corresponding disordered solvent. Our results show that a 0.2 V/Å external electric field, which is below the threshold for bond breaking of solvent molecules, leads to significant ordering of bulk methanol solvent and the ionic liquid [EMIM][BF₄]. Importantly, in the absence of this same field, the ordered solvent lowers the activation energy of the hydrogen-transfer reaction of o-alkylphenyl ketones in excess of 20 kcal/mol when the solvent is methanol and by over 30 kcal/mol for [EMIM][BF₄]. Even a 0.1 V/Å external field has effects of ca. 10 and 20 kcal/mol, respectively. This work suggests a possible strategy for scaling electrostatic catalysis by applying a pulsed external field to the reaction medium to maintain solvent ordering while allowing the reaction to proceed largely in the absence of an external field.

MAS NMR Investigation of Molecular Order in an Ionic Liquid Crystal

The structure and molecular order in the thermotropic ionic liquid crystal (ILC), [choline][geranate(H)octanoate], an analogue of Choline And GEranate (CAGE), which has potential for use as a broad-spectrum antimicrobial and transdermal and oral delivery agent, were investigated by magic-angle spinning (MAS) nuclear magnetic resonance (NMR), polarizing optical microscopy, small-angle X-ray scattering (SAXS), and mass spectrometry. Mass spectrometry and the ¹H NMR chemical shift reveal that CAGE-oct is a dynamic system, with metathesis (the exchange of interacting ions) and hydrogen exchange occurring between hydrogen-bonded/ionic complexes such as [(choline)(geranate)(H)(octanoate)], [(choline)(octanoate)₂(H)], and [(choline)(geranate)₂(H)]. These clusters, which are shown by mass spectrometry to be significantly more stable than expected for typical electrostatic ion clusters, involve hydrogen bonding between the carboxylic acid, carboxylate, and hydroxyl groups, with rapid hydrogen bond breaking and re-formation observed to average the ¹H chemical shifts. The formation of a partial bilayer liquid crystal (LC) phase was identified by SAXS and polarizing optical microscopy at temperatures below ∼293 K. The occurrence of this transition close to room temperature could be utilized as a potential temperature-induced "switch" of the anisotropic properties for particular applications. The presence of an isotropic component of approximately 23% was observed to coexist with the LC phase, as detected by polarizing optical microscopy and quantified by both ¹H-¹³C dipolar-chemical shift correlation (DIPSHIFT) and ¹H double-quantum (DQ) MAS NMR experiments. At temperatures above the LC-to-isotropic transition, intermediate-range order (clustering of polar and nonpolar domains), a feature of many ILs, persists. Site-specific order parameters for the LC phase of CAGE-oct were obtained from the MAS NMR measurement of the partially averaged ¹³C-¹H dipolar couplings (D_CH) by cross-polarization (CP) build-up curves and DIPSHIFT experiments, and ¹H-¹H dipolar couplings (D_HH) by double-quantum (DQ) build-up curves. The corresponding order parameters, S_CH and S_HH, are in the range 0-0.2 and are lower compared to those for smectic (i.e., layered) phases of conventional nonionic liquid crystals, resembling those of lamellar phases formed by lyotropic surfactant-solvent systems.

Structural investigations of molecular solutes within nanostructured ionic liquids

Ionic liquids (ILs) containing sufficiently long alkyl chains form amphiphilic nanostructures with well-defined polar and non-polar domains. Here we have explored the robustness of these amphiphilic nanostructures to added solutes and gained insight into how the nature of the solute and IL ions affect the partitioning of these solutes within the nanostructured domains of ILs. To achieve this, small angle X-ray scattering (SAXS) investigations were performed and discussed for mixtures of 9 different molecular compounds with 6 different ILs containing imidazolium cations. The amphiphilic nanostructure of ILs persisted to high solute concentrations, over 50 mol% of added solute for most 1-butyl-3-methylimidazolium ILs and above 80 mol% for most 1-decyl-3-methylimidazolium ILs. Solute partitioning within these domains was found to be controlled by the inherent polarity and size of the solute, as well as specific interactions between the solute and IL ions, with SAXS results corroborated with IR spectroscopy and molecular dynamics simulations. Molecular dynamics simulations also revealed the ability to induce π⁺-π⁺ stacking between imidazolium cations through the use of these added molecular compounds. Collectively, these results provide scope for the selection of IL ions to rationally influence and control the partitioning behaviour of given solutes within the amphiphilic nanostructure of ILs.

Thermotropic Liquid-Crystalline and Light-Emitting Properties of Bis(4-aalkoxyphenyl) Viologen Bis(triflimide) Salts

A series of bis(4-alkoxyphenyl) viologen bis(triflimide) salts with alkoxy chains of different lengths were synthesized by the metathesis reaction of respective bis(4-alkoxyphenyl) viologen dichloride salts, which were in turn prepared from the reaction of Zincke salt with the corresponding 4-n-alkoxyanilines, with lithium triflimide in methanol. Their chemical structures were characterized by 1H and 13C nuclear magnetic resonance spectra and elemental analysis. Their thermotropic liquid-crystalline (LC) properties were examined by differential scanning calorimetry, polarizing optical microscopy, and variable temperature X-ray diffraction. Salts with short length alkoxy chains had crystal-to-liquid transitions. Salts of intermediate length alkoxy chains showed both crystal-to-smectic A (SmA) transitions, Tms, and SmA-to-isotropic transitions, Tis. Those with longer length of alkoxy chains had relatively low Tms at which they formed the SmA phases that persisted up to the decomposition at high temperatures. As expected, all of them had excellent thermal stabilities in the temperature range of 330-370 °C. Their light-emitting properties in methanol were also included.

Surface crystallization of ionic liquid crystals

The evidence for surface crystallization in ionic liquids is scarce. The existing reports seem to be contradictory as for its driving forces, since in the two compounds investigated in the literature, the contribution of coloumbic and van der Waals forces is very different. In this work 1-dodecyl-3-methylimidazolium tetrafluoroborate was studied and its surface crystallization characterized by surface tension, ellipsometry and optical microscopy. The results obtained seem to reconcile previous observations, and it was further shown, using the same techniques, that this phenomenon is prevalent in other ionic liquids. MD simulation results illustrate the different possibilities of organization, providing reasonable models to rationalize the experimental observations.

Ionic N-phenylpyridinium tetracatenar mesogens: competing driving forces in mesophase formation and unprecedented difference in phase stabilisation within a homologous series

Ionic, tetracatenar liquid crystals containing an N-phenylpyridinum core are described; many of these compounds display a SmA phase, something extremely rare in tetracatenar materials. The competing forces driving mesophase formation lead to an unprecedented difference in phase stabilities for SmA and Colh phases.

On the structural origin of free volume in 1-alkyl-3-methylimidazolium ionic liquid mixtures: a SAXS and 129Xe NMR study

Ionic liquid (IL) mixtures enable the design of fluids with finely tuned structural and physicochemical properties for myriad applications. In order to rationally develop and design IL mixtures with the desired properties, a thorough understanding of the structural origins of their physicochemical properties and the thermodynamics of mixing needs to be developed. To elucidate the structural origins of the excess molar volume within IL mixtures containing ions with different alkyl chain lengths, 3 IL mixtures containing 1-alkyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ILs have been explored in a joint small angle X-ray scattering (SAXS) and 129Xe NMR study. The apolar domains of the IL mixtures were shown to possess similar dimensions to the largest alkyl chain of the mixture with the size evolution determined by whether the shorter alkyl chain was able to interact with the apolar domain. 129Xe NMR results illustrated that the origin of excess molar volume in these mixtures was due to fluctuations within these apolar domains arising from alkyl chain mismatch, with the formation of a greater number of smaller voids within the IL structure. These results indicate that free volume effects for these types of mixtures can be predicted from simple considerations of IL structure and that the structural basis for the formation of excess molar volume in these mixtures is substantially different to IL mixtures formed of different types of ions.

Design and Synthesis of a Novel Banana-Shaped Functional Molecule via Double Cross-Coupling

A novel banana-shaped molecule using 2,8-Dimethyl-6H,12H-5,11-methanodibenzo [b,f] [1,5]diazocine (Tröger's base) as bent-core was synthesized via double Carbon-Carbon cross-coupling reaction. The double Sonogashira cross-coupling reaction was optimized by using Pd(PPh₃)₂Cl₂ as catalyst, CuI as cocatalyst and diisopropylamine as base in place of triethylamine. The structure of this compound was confirmed by ¹H-NMR, ¹³C-NMR, Fourier transform infrared (FT-IR) spectroscopy and mass spectrometry.

Substituted Azolium Disposition: Examining the Effects of Alkyl Placement on Thermal Properties

We describe the thermal phase characteristics of a series of 4,5-bis(n-alkyl)azolium salts that were studied using thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), polarized-light optical microscopy (POM), and synchrotron-based small- to wide-angle X-ray scattering (SWAXS) measurements. Key results were obtained for 1,3-dimethyl-4,5-bis(n-undecyl)imidazolium iodide (1-11), 1,3-dimethyl-4,5-bis(n-pentadecyl)- imidazolium iodide (1-15), and 1,2,3-trimethyl-4,5-bis(n-pentadecyl)imidazolium iodide (2), which were found to adopt enantiotropic smectic A mesophases. Liquid-crystalline mesophases were not observed for 1,3-dimethyl-4,5-bis(n-heptyl)imidazolium iodide (1-7), 3-methyl-4,5-bis(n-penta-decyl)thiazolium iodide (3), and 2-amino-4,5-bis(n-pentadecyl)imidazolium chloride (4). Installing substituents in the 4- and 5-positions of the imidazolium salts appears to increase melting points while lowering clearing points when compared to data reported for 1,3-disubstituted analogues.

Understanding the effects of ionic liquids on a unimolecular substitution process: correlating solvent parameters with reaction outcome

The polarisability of an ionic liquid is key in determining the rate constant of a unimolecular substitution process.

Chiral ionic liquid crystals based on thiourea

Chiral ionic liquid crystals based on the S-methyl-N,N′-α-methylarylthiouronium cation have been prepared and studied.

1,2,4-Triazolium ions as flexible scaffolds for the construction of polyphilic ionic liquid crystals

A novel scaffold for the construction of self-organised ionic liquids and ionic liquid crystals bearing both perfluorocarbon and hydrocarbon moieties has been developed. The phase behaviour and physical properties of these materials can be tuned as a function of chain length and fluorine content and significant structural elaboration is possible, giving a highly flexible system.

Self-assembly and biological activities of ionic liquid crystals derived from aromatic amino acids

The self-assembly of amino acid-derived ionic liquid crystals (ILCs) into lamellar or micellar-like aggregates suggests that they might interact with biological membranes. To get some insight, guanidinium chlorides derived from the natural l-amino acids phenylalanine (Phe), tyrosine (Tyr) and 3,4-dihydroxyphenylalanine (DOPA) were synthesized and their mesomorphic properties were investigated via polarizing optical microscopy (POM), differential scanning calorimetry (DSC) and X-ray diffraction (SAXS, WAXS). Mesophase types depended on the number of alkoxy side chains. Phe- and Tyr-based ILCs with one and two side chains, respectively, self-assembled into smectic A bilayers (SmA2), while Dopa-derived ILCs with three side chains formed columnar (Colh) mesophases. The mesophase ranges for Phe ILCs increased steadily with side chain length, for Tyr- and Dopa-based ILCs, however, size matching effects were observed. To clarify whether the mesomorphic behaviour has an impact on biological properties, cytotoxic and antibacterial activities of the ILCs were studied. Phe and Tyr ILCs exhibited much higher cytotoxicities (against the L-929 mouse fibroblast cell line) and/or antibacterial activities (against Staphylococcus aureus) than Dopa ILCs, which were mostly inactive. Furthermore, within each series, the side chain length largely influenced the biological activity. Thus, the bulk mesophase behaviour appeared to correlate with the biological properties, in particular, the interactions with membranes, as shown by measuring the intracellular Ca2+ concentration in human monocytic U937 cells after treatment with the amino acid-based ILCs.

Thermodynamic properties of selenoether-functionalized ionic liquids and their use for the synthesis of zinc selenide nanoparticles

Three selenoether-functionalized ionic liquids (ILs) of N-[(phenylseleno)methylene]pyridinium (1), N-(methyl)- (2) and N-(butyl)-N'-[(phenylseleno)methylene]imidazolium (3) with bis(trifluoromethanesulfonyl)imide anions ([NTf2]) were prepared from pyridine, N-methylimidazole and N-butylimidazole with in situ obtained phenylselenomethyl chloride, followed by ion exchange to give the desired compounds. The crystal structures of the bromide and tetraphenylborate salts of the above cations (1-Br, 2-BPh4 and 3-BPh4) confirm the formation of the desired cations and indicate a multitude of different supramolecular interactions besides the dominating Coulomb interactions between the cations and anions. The vaporization enthalpies of the synthesized [NTf2]-containing ILs were determined by means of a quartz-crystal microbalance method (QCM) and their densities were measured with an oscillating U-tube. These thermodynamic data have been used to develop a method for assessment of miscibility of conventional solvents in the selenium-containing ILs by using Hildebrandt solubility parameters, as well as for modeling with the electrolyte perturbed-chain statistical associating fluid theory (ePC-SAFT) method. Furthermore, structure-property relations between selenoether-functionalized and similarly shaped corresponding aryl-substituted imidazolium- and pyridinium-based ILs were analyzed and showed that the contribution of the selenium moiety to the enthalpy of vaporization of an IL is equal to the contribution of a methylene (CH2) group. An incremental approach to predict vaporization enthalpies of ILs by a group contribution method has been developed. The reaction of these ILs with zinc acetate dihydrate under microwave irradiation led to ZnSe nanoparticles of an average diameter between 4 and 10 nm, depending on the reaction conditions.

Theoretical Investigation of the Kinetic Effect of the 1-Ethylpyridinium Trifluoroacetate Ionic Liquid in the Acceleration of Diels–Alder Reactions of Isoprene with Acrylic Acid and Acrylonitrile

The mechanism, the regioselectivity, the stereoselectivity and the kinetics of Diels–Alder reactions of isoprene with acrylic acid and acrylonitrile have been studied, at the B3PW91/6-31G(d,p) level of theory, both in the gas phase and in the presence of organic [dichloromethane (DCM)] and ionic liquid [1-ethylpyridinium trifluoroacetate (EPTFA)] solvents. Intrinsic reaction coordinate calculations show that these reactions take place through an asynchronous concerted mechanism leading to the endo para cycloadducts as the major products in the gas phase and to the exo para cycloadducts as the major products both in organic and in ionic liquid solvents. The explicit solvation model involving the coordination of one molecule of the solvent with the dienophiles shows a considerable decrease of the activation energy when passing from DCM to EPTFA. The enhancement of these cycloaddition reactions can be explained by the strong hydrogen bonding created between the ion pair of the ionic liquid and the oxygen atom of the dienophile reagents. Moreover, density functional theory-based reactivity indices also show an increase of the reaction polarity and consequently of the reaction rate, when replacing DCM solvent by EPTFA solvent. The results obtained give evidence that the ionic liquid EPTFA is an excellent solvent for Diels–Alder reactions in comparison with conventional organic solvents.

Tuning Coulombic interactions to stabilize nematic and smectic ionic liquid crystal phases in mixtures of charged soft ellipsoids and spheres

We have investigated the effect of electrostatic interactions in mixtures of soft ellipsoids and spheres based on the well-known Gay-Berne (GB) and Lennard-Jones (LJ) potential, respectively. These model systems, in their original version, that is without any electrostatic charge, have been thoroughly investigated in the literature both as pure components and mixtures. In particular, mixtures of particles of different shapes, such as spheres and ellipsoids, tend to phase separate because of the excluded volume effects. Common ionic liquid crystals, based on imidazolium or other quaternary ammonium salts, are usually composed of roughly elongated (although flexible) cations and roughly spherical anions, that is, particles with a similar shape such as the GB and LJ models. Therefore, in this work, we present the results of molecular dynamics simulations of mixtures of positively charged GB and negatively charged LJ particles as models of ionic liquid crystals. Interestingly, by modulating the charge of the particles it is possible to stabilize isotropic, nematic, smectic and crystalline ionic phases. The relative weight of Coulomb (a radial, therefore isotropic interaction) and van der Waals (an anisotropic interaction) contributions is a key parameter to tune the stability of various mesophases.