Thermodynamic Study of Alkylsilane and Alkylsiloxane-Based Ionic Liquids

The thermodynamic properties of ionic liquids (ILs) bearing alkylsilane and alkylsiloxane chains, as well as their carbon-based analogs, were investigated. Effects such as the replacement of carbon atoms by silicon atoms, the introduction of a siloxane linkage, and the length of the alkylsilane chain were explored. Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) were used to study the thermal and phase behavior (glass transition temperature, melting point, enthalpy and entropy of fusion, and thermal stability). Heat capacity was obtained by high-precision drop calorimetry and differential scanning microcalorimetry. The volatility and cohesive energy of these ILs were investigated via the Knudsen effusion method coupled with a quartz crystal microbalance (KEQCM). Gas phase energetics and structure were also studied to obtain the gas phase heat capacity as well as the energy profile associated with the rotation of the IL side chain. The computational study suggested the existence of an intramolecular interaction in the alkylsiloxane-based IL. The obtained glass transition temperatures seem to follow the trend of chain flexibility. An increase of the alkylsilane chain leads to a seemingly linear increase in molar heat capacity. A regular increment of 30 J·K-1·mol-1 in the molar heat capacity was found for the replacement of carbon by silicon in the IL alkyl chain. The alkylsilane series was revealed to be slightly more volatile than its carbon-based analogs. A further increase in volatility was found for the alkylsiloxane-based IL, which is likely related to the decrease of the cohesive energy due to the existence of an intramolecular interaction between the siloxane linkage and the imidazolium headgroup. The use of Si in the IL structure is a suitable way to significantly reduce the IL's viscosity while preserving its large liquid range (low melting point and high thermal stability) and low volatilities.

Attraction between Like Charged Ions in Ionic Liquids: Unveiling the Enigma of Tetracyanoborate Anions

Intermolecular interactions in ionic liquids are mainly governed by Coulombic forces. Attraction between cations has been previously observed and was attributed to dispersion interactions between nonpolar moieties, hydrogen bonding, or π stacking. In this study, we present the intriguing behavior of tetracyanoborate anions in ionic liquids that, unlike their dicyanamide and tricyanomethanide counterparts, form dimers in both solid and liquid phases. A joint simulation and experimental study uncovers the origin of such anion-anion attraction: stabilization by induction and dispersion forces between several cyano groups, which is strong enough to overcome electrostatic repulsion. These findings open up new opportunities in the rational design of ionic liquids, where interactions between ions of the same charge can be controlled and fine-tuned by the presence of cyano groups.

A kinetics study of copper-catalysed click reactions in ionic liquids

Copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC) reactions are of extensive interest in chemical synthesis. While the use of ionic liquids (ILs) as solvents for synthesis has been widely explored in recent years, the understanding of their influence on the mechanism and reactivity of CuAAC reactions remains poorly understood. Here, we investigate the kinetics of a phenylacetylene-benzylazide and acetylene-benzylazide CuAAC reaction to probe the influence of IL structure, including the role of the base used to promote the reaction and the importance of water content. The use of 'wet' ILs led to remarkable changes in the kinetic profile of the reaction by eliminating the initial induction period. The reaction rate was found to be dependent on the copper(I) source. The effect of an added base was also studied, with the use of a tertiary amine-bearing IL leading to high conversions in under 5 min at ambient temperature. The results of this study highlight the nature and complexity of CuAAC reactions in ILs. As more ILs are getting involved in industrial processes, the data obtained from this study are valuable for better understanding processes that affect the CuAAC reaction in IL media and for creating customized systems for organic synthesis, thus improving the efficiency and sustainability of such processes.

Alkylphosphonium carboxylate ionic liquids with tuned microscopic structures and properties

Eleven pure alkylphosphonium carboxylate ionic liquids (ILs) were synthesised following a reliable and accessible route. Tetrabutylphosphonium and tetradecyltrihexylphosphonium cations were associated to a variety of [R-COO]^- anions with R varying from shorter to longer linear alkyl chains; smaller to bulkier branched alkyl chains; cyclic saturated aliphatic and aromatic moieties; and one heterocyclic aromatic ring containing nitrogen. A combined experimental and molecular simulation study allowed the full characterization of the physico-chemical properties, the structure and the thermal stability of the synthesized ILs. Although slightly more viscous than their imidazolium counterparts, the viscosities of the prepared salts decrease dramatically with temperature and are comparable to other ILs above 50 °C, a manageable temperature as they are thermally stable up to temperatures above 250 °C, even under an oxidizing atmosphere. The microscopic structure of the phophonium ILs is rich and has been studied both experimentally using SAXS and by molecular dynamics simulation using state of the art polarizable force fields whose parameters were determined when necessary. Unique and surprising anion-anion correlations were found for the tetrazolate-based IL allowing to explain some of the unique physical-chemical properties of this phosphonium salt.

Unravelling free volume in branched-cation ionic liquids based on silicon

The branching of ionic liquid cation sidechains utilizing silicon as the backbone was explored and it was found that this structural feature leads to fluids with remarkably low density and viscosity. The relatively low liquid densities suggest a large free volume in these liquids. Argon solubility was measured using a precise saturation method to probe the relative free volumes. Argon molar solubilities were slightly higher in ionic liquids with alkylsilane and siloxane groups within the cation, compared to carbon-based branched groups. The anion size, however, showed by far the dominant effect on argon solubility. Thermodynamic solvation parameters were derived from the solubility data and the argon solvation environment was modelled utilizing the polarizable CL&Pol force field. Semiquantitative analysis was in agreement with trends established from the experimental data. The results of this investigation demonstrate design principles for targeted ionic liquids when optimisation for the free volume is required, and demonstrate the utility of argon as a simple, noninteracting probe. As more ionic liquids find their way into industrial processes of scale, these findings are important for their utilisation in the capture of any gaseous solute, gas separation, or in processes involving the transformation of gases or small molecules.

The branching of ionic liquid cation sidechains utilizing silicon as the backbone was explored and it was found that this structural feature leads to fluids with remarkably low density and viscosity.

Mixing divalent ionic liquids: effects of charge and side-chains

We have prepared novel divalent ionic liquids (ILs) based on the bis(trifluoromethylsulfonyl)imide anion where two charged imidazolium groups in the cations are either directly bound to each other or linked by a single atom. We assessed the influence of the side-chain functionality and divalency on their physical properties and on the thermodynamics of mixing. The results indicate that shortening the spacer of a divalent IL reduces its thermal stability and increases its viscosity. Mixtures of divalent and monovalent ILs show small but significant deviations from ideality upon mixing. These deviations appear to depend primarily on the (mis)match of the nature and length of the cation side-chain. The non-ideality imposed by mixing ILs with different side-chains appears to be enhanced by the increase in formal charge of the cations in the mixture.

Evidence for the spontaneous formation of N-heterocyclic carbenes in imidazolium based ionic liquids

We present a study of the reactions of aldehydes in ionic liquids which gives evidence for the spontaneous formation of N-heterocyclic carbenes in ionic liquids based on 1,3-dialkyl substituted imidazolium cations from the lack of a deuterium isotope effect on the reaction of these ionic liquids with aldehydes.

The impact of ionic liquids on the coordination of anions with solvatochromic copper complexes

Traditionally weakly associating anions demonstrate a stronger than expected level of coordinating ability within ILs.

Ionic liquids as solvents for the Knoevenagel condensation: understanding the role of solvent–solute interactions

The hydrogen bond basicityβof ionic liquids, as demonstrated by the NMR studies and the Kamlet–Taft linear solvation energy relationship, was confirmed to be the dominant solvent descriptor determining the rate of the Knoevenagel condensation.