Ionic liquids: "normal" solvents or nanostructured fluids?

Ionic liquids (ILs) are a class of non-conventional solvents, which, for almost two decades, have continued to generate burgeoning interest in different fields of present-day chemical research with few similar precedents. Among the various aspects related to ILs, a topic worthy of in-depth analysis is their influence on organic reactivity and reaction rates. In light of this, the present short review aims to provide an overview of the literature from 2010 to the present day that addresses this issue. In particular, we herein present two main different viewpoints by which the solvent effect of ILs is explained: the first is mainly based on considering the bulk polarity of ILs and linear solvation energy relationships, while the other treats ILs as nanostructured fluids. In both cases, studies dealing with IL mixtures are also covered. Finally, literature addressing the area of supramolecular catalysis "by" or "in" ILs is also reported. This is one of the few reviews covering these specific aspects, aiming to provide a useful framework to guide future research into the effects of ILs on organic reactivity.

Machine learning approaches to understand and predict rate constants for organic processes in mixtures containing ionic liquids

The ability to tailor the constituent ions in ionic liquids (ILs) is highly advantageous as it provides access to solvents with a range of physicochemical properties. However, this benefit also leads to large compositional spaces that need to be explored to optimise systems, often involving time consuming experimental work. The use of machine learning methods is an effective way to gain insight based on existing data, to develop structure-property relationships and to allow the prediction of ionic liquid properties. Here we have applied machine learning models to experimentally determined rate constants of a representative organic process (the reaction of pyridine with benzyl bromide) in IL-acetonitrile mixtures. Multiple linear regression (MLREM) and artificial neural networks (BRANNLP) were both able to model the data well. The MLREM model was able to identify the structural features on the cations and anions that had the greatest effect on the rate constant. Secondly, predictive MLREM and BRANNLP models were developed from the full initial set of rate constant data. From these models, a large number of predictions (>9000) of rate constant were made for mixtures of different ionic liquids, at different proportions of ionic liquid and molecular solvent, at different temperatures. A selection of these predictions were tested experimentally, including through the preparation of novel ionic liquids, with overall good agreement between the predicted and experimental data. This study highlights the benefits of using machine learning methods on kinetic data in ionic liquid mixtures to enable the development of rigorous structure-property relationships across multiple variables simultaneously, and to predict properties of new ILs and experimental conditions.

The effect of bisimidazolium-based ionic liquids on a bimolecular substitution process. Are two head(group)s better than one?

A homologous series of biscationic ionic liquids based on two imidazolium centres, separated by alkyl chains of varying length, were examined as solvents for a bimolecular substitution reaction across a range of proportions of ionic liquid in the reaction mixture. Their effects on the rate constant of the process were compared to monocationic ionic liquids, with generally a greater rate constant increase observed. Importantly, it was observed that the magnitude of the effect was shown to vary with the length of the linking chain. To investigate the origins of these solvent effects, temperature dependent kinetic studies were performed to obtain activation parameters at high and low mole fractions of ionic liquid. The observed activation parameters showed the rate constant enhancement was due to interaction of the ionic liquid with the starting materials, consistent with previous results. Significantly, however, these data also showed that the balance of enthalpic and entropic effects varied dramatically with the length of the alkyl chain between the cationic centres.

Ion-Reagent Interactions Contributing to Ionic Liquid Solvent Effects on a Condensation Reaction

Molecular dynamics simulations of solutions of hexan-1-amine or 4-methoxybenzaldehyde in acetonitrile, an ionic liquid/acetonitrile mixture (χIL =0.2), and a number of different (neat) ionic liquids were performed, to further understand the solvent effects on the condensation reaction of these species. This work indicates that, in the presence of an ionic liquid, the amine group of hexan-1-amine is exclusively solvated by the components of the ionic liquid, and not by acetonitrile, and that the anion interacts with the aldehyde group of 4-methoxybenzaldehyde. These interactions showed little dependence on the proportion of the ionic liquid present. When varying the cation of the ionic liquid there were changes in the cation-amine interaction, and 1-butyl-2,3-dimethylimidazolium bis(trifluoromethanesulfonyl)imide ([Bm2 im][N(CF3 SO2 )2 ]) was found to order more than expected about the amine. This ordering is likely the origin of the large rate constant values determined in [Bm2 im][N(CF3 SO2 )2 ] for this condensation reaction and explains an anomaly seen previously. When changing the anion, changes were seen in the interactions between both the cation and anion with hexan-1-amine, and the anion with 4-methoxybenzaldehyde. The differing magnitude of these interactions likely causes subtle changes in the activation parameters for this condensation reaction, and provides an explanation for the anomalous rate constant values previously determined when varying the anion.

Ionic liquid solvents: the importance of microscopic interactions in predicting organic reaction outcomes

Ionic liquids are attractive alternatives to molecular solvents as they have many favourable physical properties and can produce different organic reaction outcomes compared to molecular solvents. Thus far, interactions between the ionic liquid components and specific sites (such as charged centres, lone pairs and π systems) on the reagents and transition state have been identified as affecting reaction outcome; a comprehensive understanding of these interactions is necessary to allow prediction of ionic liquid solvent effects. This manuscript summarises our recent progress in the development of a framework for predicting the effect of an ionic liquid solvent on the outcome of organic processes. There will be a particular focus on the importance of the different interactions between the ionic liquid components and the species along the reaction coordinate that are responsible for the changes in reaction outcome observed in the cases described.

Understanding the Effect of Solvent Structure on Organic Reaction Outcomes When Using Ionic Liquid/Acetonitrile Mixtures

The rate constant for the reaction between hexan-1-amine and 4-methoxybenzaldehyde was determined in ionic liquids containing an imidazolium cation. The effect on the rate constant of increasing the length of the alkyl substituent on the cation was examined in a number of ionic liquid/acetonitrile mixtures. In general it was found that there was no significant effect of changing the alkyl substituent on the rate constant of this process, suggesting that any nanodomains in these mixtures do not have a significant effect on the outcome of this process. A series of small-angle X-ray scattering and wide-angle X-ray scattering experiments were performed on mixtures of the ionic liquid 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([Bmim][N(CF₃SO₂)₂]) and acetonitrile; this work indicated that the main structural changes in the mixtures occur by about a 0.2 mole fraction of ionic liquid in the mixture (χ_IL). This region at which the main changes in the solvent structuring occurs corresponds to the region at which the main changes in the rate constant and activation parameters occur for S_N2 and condensation reactions examined previously; this is the first time that such a correlation has been observed. To examine the ordering of the solvent about the nucleophile hexan-1-amine, WAXS experiments were performed on a number of [Bmim][N(CF₃SO₂)₂]/acetonitrile/hexan-1-amine mixtures, where it was found that some of the patterns featured asymmetric peaks as well as additional peaks not observed in the [Bmim][N(CF₃SO₂)₂]/acetonitrile mixtures; this suggests that the addition of hexan-1-amine to the mixture affects the bulk structure of the liquid. The SAXS/WAXS patterns of mixtures of 1-butyl-2,3-dimethylimidazolium bis(trifluoromethanesulfonyl)imide ([Bm₂im][N(CF₃SO₂)₂]) and acetonitrile were also determined, with the results suggesting that [Bm₂im][N(CF₃SO₂)₂] is more ordered than [Bmim][N(CF₃SO₂)₂] due to an enhancement in the short-range interactions.

Reaction mechanisms in ionic liquids: the kinetics and mechanism of the reaction of O,O-diethyl (2,4-dinitrophenyl) phosphate triester with secondary alicyclic amines

In the title reaction, the ionic liquids used stabilized the zwitterionic pentacoordinate intermediate (P±), leading to a change in the mechanism from concerted to stepwise.

The effects of an ionic liquid on unimolecular substitution processes: the importance of the extent of transition state solvation

An ionic liquid significantly increases benzylic carbocation formation due to favourable ionic liquid–transition state interactions. The magnitude of transition state solvation was shown to be critical, explaining the difference between this and previous cases.

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.