Synthesis and characterization of new imidazolium based protic ionic liquids obtained by nitro- and cyano-functionalization

Journal article, 2024

The prevailing strategies for synthesizing Brønsted acidic ionic liquids have primarily relied on [Formula presented] functionalization and the use of acidic anions (e.g. [Formula presented] and [Formula presented]), which limit the range of viable new compounds that can be developed. In this work, we propose enhancing the intrinsic acidity of imidazolium through functionalization with electron-withdrawing groups, a strategy that resulted in room-temperature Brønsted acidic protic ionic liquids. Specifically, nitro- and cyano-functionalized room temperature protic ionic liquids were successfully synthesised and characterised. Compared to a prototypical protic ionic liquid such as [Formula presented], the addition of a nitro- or cyano-group led to increased acidity, as demonstrated by converging experimental results obtained by spectroscopic methods and molecular modelling. This functionalization significantly impacted thermal and transport properties, with the nitro- and cyano-functionalized protic ionic liquids displaying reduced thermal stability, higher viscosity and lower ionic conductivity. In addition, this functionalization affects the temperature dependence of the ionic conductivity with a supposed influence also on fragility, a parameter that, in the case of ionic liquids, requires more dedicated studies for a more comprehensive understanding.