Analysis of the Frequency and Diversity of 1,3-Dialkylimidazolium Ionic Liquids Appearing in the Literature

Abstract

The hypothetical assertion that there are potentially millions of ionic liquids (ILs) can be validated using basic knowledge of IL precursors and simple calculations. While it is an interesting thought exercise to ponder the possibilities of ILs, it is important to investigate how many discrete ILs (i.e., unique cation and anion pairings) have actually been synthesized and reported in the literature. A complete analysis of all ILs is a daunting task, especially if the definition of what constitutes an IL is broad. Nonetheless, insight can be gained into the use of ILs over the past ∼20 years from a focused analysis of systematically varied 1,3dialkylimidazolium-based ILs (i.e., [R 1 R 2 im][X], where R 1 and R 2 are n-alkyl chains and X is either a halide or molecular anion). This family of ILs includes many of the most widely used and most thoroughly studied IL substances, including 1-butyl-3-methylimidazolium hexafluorophosphate ([C 4 mim][PF 6 ]) and 1-ethyl-3-methylimidazolium bistriflimide ([C 2 mim][Tf 2 N]). Using data retrieved from SciFinder searches, we analyzed the variety and frequency of 1,3-dialkylimidazolium-based ILs appearing in the literature. Our analysis reveals that there is a relatively large number of simple 1,3-dialkylimidazolium cations which have not yet been synthesized. Our findings indicate that despite the broad possibilities of IL structures available, there are but a handful of ILs that are commonly used and studied. Within the 1248 [R 1 R 2 im][X] ILs studied, there were only 12 cations extensively used, which included [R 1 R 2 im] + (R 1 = C 1 and R 2 = C 1 −C 12 ). Of the 16 anions studied, only 5, including [BF 4 ] − , [Cl] − , [PF 6 ] − , [Tf 2 N] − , and [Br] − , were used significantly. Thus, it appears that although ILs can be used as designer solvents with task-specific properties, there is a small default subset of ILs, which are "go-to" or "one-size-fits-all." We rationalize the current state of the IL landscape in terms of the historical perceptions (and misconceptions) around ILs, familiarity, availability and cost of starting materials, commercial availability of ILs, and the motivations from which researchers choose to synthesize and characterize new ILs.

Electrodeposition of neodymium and dysprosium from organic electrolytes

“…23,24 Secondly, bistriflimide salts are synthesized via a multi-step procedure and are only produced on a relatively small scale, mainly for the battery and catalyst markets. 25,26 Therefore, bistriflimide salts and the corresponding ionic liquids are expensive, which makes these ionic liquids less suitable for large-scale applications such as electrowinning. Thirdly, the environmental impact of bistriflimide is considerable because it is strongly fluorinated and thus persistent in the environment.…”

Systematic Synthesis and Properties Evaluation of Dicationic Ionic Liquids, and a Glance Into a Potential New Field

“…Furthermore, the wide electrochemical window and the high conductivity enabled their use as electrolytes in solar cells and batteries (Watanabe et al, 2017). Generally, they are constituted by an asymmetric organic cation, imidazolium based ILs are the most investigated, associated to a polyatomic inorganic or organic anion (Scalfani et al, 2018). Nonetheless, more recently a new branch of ILs has been proposed: the generally defined dicationic (or geminal) ionic liquids (DILs).…”

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

“…Although imidazolium moieties are among the most widely used cationic cores to obtain ILCs, the majority of studies have focused on N-substituted derivatives due to the ease of their synthesis [5][6][7][28][29][30].…”

“…For comparison, 1,3-dimethylimidazolium n-dodecylsulfonate self-organizes into an enantiotropic SmA phase upon heating and displays a considerably higher clearing point (Cr • 90 • SmA • 177 • Iso ( • C)) whereas the corresponding 1,2,3-trimethylimidazolium (Cr • 202 • Iso ( • C)) and 1,3,4,5-tetramethylimidazolium (Cr • 109 • Iso ( • C)) salts are not LC [32]. Even in the realm of ILs, limited physical property data are available for tri-, tetra-, and pentasubstituted imidazolium salts (with the additional substituents often being limited to methyl groups) [29,30,[41][42][43][44][45][46][47][48]. Beyond differences in thermal properties, highly substituted imidazolium salts may be expected to show an increased stability toward base, particularly if they are to be used in, for instance, alkaline fuel cell membranes [49].…”

“…Collectively, the results that will be described below show how the disposition of long alkyl chains around five-membered, heteroaromatic, cationic cores affects the thermal characteristics of the resulting salts. Although imidazolium moieties are among the most widely used cationic cores to obtain ILCs, the majority of studies have focused on N-substituted derivatives due to the ease of their synthesis [5][6][7][28][29][30]. The influence of installing alkyl or aryl substituents in the 2-position of imidazolium-based ILCs on thermal phase characteristics has also been investigated [31][32][33][34][35][36][37][38][39][40].…”