Analysis of the Frequency and Diversity of 1,3-Dialkylimidazolium Ionic Liquids Appearing in the Literature
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 1,3-dialkylimidazolium-based ILs (i.e., [R1R2im][X], where R1 and R2 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 ([C4mim][PF6]) and 1-ethyl-3-methylimidazolium bistriflimide ([C2mim][Tf2N]). 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 [R1R2im][X] ILs studied, there were only 12 cations extensively used, which included [R1R2im]+ (R1 = C1 and R2 = C1−C12). Of the 16 anions studied, only 5, including [BF4] −, [Cl]−, [PF6] −, [Tf2N]−, 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.