Stewart A. Forsyth

Ionic Liquids—An Overview

Ionic liquids have become an increasingly popular class of solvent in the last decade as the potential applications of these materials become more diverse. Rather than being viewed simply as replacement for conventional organic solvent media, research into ionic liquids has progressed to the deliberate choice and design of these materials for reasons of improved rate, specificity, and yield. Design of ionic liquids centres on the development of novel cations and anions to impart the specific physical properties required for each application. Therefore, the materials being synthesized and studied are also becoming increasingly complex and diverse. Here we provide an overview of ionic liquids generally, and some of their current applications, as well as an introduction to some of the new cations and anions that have been developed for specific properties.

Ionic liquids based on imidazolium, ammonium and pyrrolidinium salts of the dicyanamide anion

New families of salts, based on quaternary ammonium, 1-methyl-3-alkylimidazolium or N-methyl-N-alkylpyrrolidinium organic cations together with the dicyanamide (dca, N(CN)2−) anion are reported. The salts are low melting compounds, all those reported are liquid at room temperature, for example 1-methyl-3-ethylimidazolium dicyanamide (mp −21 °C) and N-methyl-N-ethylpyrrolidinium dicyanamide (mp −10 °C). Some of the salts exhibit multiple crystalline phases below their melting points. Above their melting points they are stable to at least 200 °C. Many of the salts were found to be glass forming when cooled rapidly to −100 °C. The room-temperature liquids exhibit very low viscosities, for example ethylmethylimidazolium dicyanamide: η = 21 cP at ambient temperature (25 °C).

Low viscosity ionic liquids based on organic salts of the dicyanamide anion

New families of salts viz. quaternary ammonium, N-alkyl-N-methylpyrrolidinium or 1-alkyl-3-methylimidazolium dicyanamides, Cat+N(CN)2−, are low melting compounds, most being liquid at rt, water-miscible and have low (for ionic liquids) viscosity at rt, e.g.η = 21 cP for 1-ethyl-3-methylimidazolium dicyanamide.

N-methyl-N-alkylpyrrolidinium tetrafluoroborate salts : ionic solvents and solid electrolytes

A series of N-methyl-N-alkylpyrrolidinium tetrafluoroborate salts were synthesised. The spectroscopic, physical and electrochemical characteristics of this family of salts have been investigated with respect to potential usage as ionic solvents and electrolytes. The lowest melting point among the family is 64°C for the N-methyl-N-propylpyrrolidinium tetrafluoroborate (P13BF4). This is sufficiently low to enable this salt to be useful as an ionic liquid in chemical synthesis involving reactions above 70°C. Most of the compounds exhibit one or more solid–solid transitions below the melting point, this behaviour is thought to indicate the existence of plastic crystal phases.

Lewis base ionic liquids.

Ionic liquids which are (weak) Lewis bases have a number of interesting and useful properties different to those of traditional ionic liquids, including volatility and the possibility of being distillable in some cases, a base catalysis effect in others and enhancement of the acidity of dissolved acids.

Ionic Liquids Based on Imidazolium and Pyrrolidinium Salts of the Tricyanomethanide Anion

A novel series of tricyanomethanide ionic liquids have been prepared and characterized for potential use as ionic liquid solvents. Full thermal analyses of all salts at ambient and sub-ambient temperatures are reported (melting points –17° to 160°C). The thermal stability and decomposition temperatures are also presented (Tdecomp ≈ 300°C). An electrochemical window of ~3 V has been established and the conductivity measured over a range of temperatures (20 mS cm–1 at 25°C).

Methanesulfonate and p-toluenesulfonate salts of the N-methyl-N-alkylpyrrolidinium and quaternary ammonium cations: novel low cost ionic liquids

The preparation and characterization of a series of novel salts, based on the N-methyl-N-alkylpyrrolidinium or quaternary ammonium organic cations coupled with sulfonate type anions, namely the mesylate (CH3SO3−) and tosylate (CH3C6H4SO3−) anions are reported. These salts are analogues of the previously described organic cation bis(trifluoromethanesulfonyl)amide (TFSA) salts that form useful ionic liquids of interest in “Green” synthesis. Several of the salts are liquid below 50 °C, e.g. tributylhexylammonium tosylate and ethylmethylpyrrolidinium mesylate and one is liquid at and below room temperature (tributylhexylammonium mesylate). These new salts have a cost advantage over salts of the TFSA−, PF6− and CF3SO3− anions. Electrochemical and thermal properties have been investigated. The salts are stable to beyond 100 °C and exhibit electrochemical potential windows of at least ±2 V vs. Ag/Ag+. Some of the salts exhibit multiple crystalline phases below their melting points, potentially indicative of plastic crystal behaviour, whilst others showed more simple solid–liquid behaviour. Many of the salts were found to be glass forming.

1-Alkyl-3-methylbenzotriazolium salts: ionic solvents and electrolytes

A series of 1-alkyl-3-methylbenzotriazolium halides were synthesised and purified by recrystallisation. Novel salts of these benzotriazolium cations were obtained by metathesis with a number of diverse anions including: dicyanamide N(CN)2, mesylate CH3SO3, tosylate CH3C6H4SO3 and bis(trifluoromethane)sulfonamide N(SO2CF3)2. Thermal analyses of these very stable salts included the determination of glass transition temperatures (−65 to −12 °C), melting points (29 to 143 °C), entropies of fusion (14 to 69 J K−1 mol−1) and decomposition temperatures (up to 315 °C). Electrochemical analyses show electrochemical windows of up to ≈3.5 V, the cation producing electrochemical reduction reactions below −1 V (Ag/Ag+). Solubility data for a range of organic compounds dissolved in the new ionic liquids and for comparison in some more common ionic liquids are also provided.

Conductivity, NMR and crystallographic study of N,N,N,N-tetramethylammonium dicyanamide plastic crystal phases : an archetypal ambient temperature plastic electrolyte material

N,N,N,N-Tetramethylammonium dicyanamide (Me4NDCA) has been examined via differential scanning calorimetry (DSC), thermogravimetric analysis, conductivity, single crystal X-ray diffraction and 1H nuclear magnetic resonance (NMR) analyses, and was found to be highly conductive in the solid state (σ=10−3 S cm−2 at 420 K) and to also exhibit unusual plastic crystal behaviour. To investigate the correlation between such behaviour and the occurrence of molecular rotations in the crystal, 1H NMR second moment measurements are compared with calculated values predicted from the crystal structure. While DSC analysis indicates a number of solid–solid transitions at ambient temperatures, subsequent 1H NMR analysis of the Me4N+ cation shows that a variety of rotational motions become active at low (<240 K) temperatures, and that such transitions in rotational states occur over a range of temperatures rather than in a sharp transition. Conductivity analysis reveals that between 320 K and 420 K the conductivity increases by more than six orders of magnitude in the solid state, in line with the transition of the Me4N+ cation to a diffusive state, and that other phase transitions observed in this temperature range have no marked effect on the conductivity. Conduction in this solid state is therefore envisaged to involve a vacancy-diffusion model, involving Me4N+ cation vacancies.

New ionic liquids from azepane and 3-methylpiperidine exhibiting wide electrochemical windows

New ionic liquids based on azepanium and 3-methylpiperidinium cations have been synthesised; they exhibit moderate viscosities and remarkably wide electrochemical windows, thereby showing promise, inter alia, as electrolytes and battery materials, and as synthetic media.