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

Luca Guglielmero,Cinzia Chiappe,Lorenzo Guazzelli,Christian S Pomelli,Andrea Mezzetta,Felicia D'Andrea

doi:10.3389/fchem.2018.00612

Abstract

Dicationic ionic liquids (DILs), a subset of the ionic liquid (IL) family, have attracted growing interest in recent years, and the range of applications within which they are investigated is constantly expanding. However, data which allows structure to property correlation of a DIL is still limited, and thus selecting an appropriate salt to address a specific challenge can be problematic. In comparison to traditional ILs, DILs physico-chemical properties can be tuned by changing the length and type of spacer which connects the cationic heads as well as the type of cation. This in turn could give rise to symmetrical or asymmetrical DILs. In this work, a systematic study of a homogeneous class of 12 dibromide DILs and 12 di-carboxylate salts has been performed. The latter class of DILs were also compared to mono cation derivatives. The different traditional exchange methods to prepare carboxylate DILs have been evaluated and an insight into the drawbacks encountered is also presented. Prepared DILs were characterized (NMR, TGA, DSC) allowing the influence of the structure on their thermal stability to be understood. Most DILs were obtained as solid salts after careful drying. For some of these compounds, a new possible application was studied, namely their use as hydrogen bond acceptors (HBA) of deep eutectic mixtures, showing again some significant structural related effects.

Highlights

Conventional room temperature ionic liquids (ILs) have been extensively studied starting from the beginning of this century by several authors due to their unique physico-chemical properties, which have allowed for their use in numerous applications (Welton, 2018)
The four 1-methylimidazolium bromides dicationic (or geminal) ionic liquids (DILs) 1-4, differing for the linker length, were transformed into the corresponding dicarboxylate salts (Figure 2), namely malonate, succinate, and glutarate whereas salts 6 and 10 were converted into the corresponding succinates DILs (25 and 26). 1-Methyl-3butylimidazolium methyl carbonate was used as starting material to prepare the corresponding malonate, succinate and glutarate salts (27-29) following known procedures (Mezzetta et al, 2017b)
A homogeneous class of 12 dibromide DILs has been prepared by performing the Menshutkin reaction in MIBK, a greener solvent than the frequently used toluene, between alkyl imidazoles and 1,n-dibromo alkanes

Summary

Introduction

Conventional room temperature ionic liquids (ILs) have been extensively studied starting from the beginning of this century by several authors due to their unique physico-chemical properties, which have allowed for their use in numerous applications (Welton, 2018). More recently a new branch of ILs has been proposed: the generally defined dicationic (or geminal) ionic liquids (DILs) These ILs seem to differ from the traditional monocationic ones in many ways offering a growing portfolio of possible applications, ranging from the “classical” use as solvents, catalysts or catalytic supports in organic reactions, to more specific applications as high temperature lubricants/heat transfer fluids, including a variety of roles in analytical sciences (Patil et al, 2016). Further development of these interesting materials in terms of improved performances or new applications are expected, especially if the impact that structural changes can bring upon physical, optical and chemical properties of these unusual material will be understood. The studies performed up to now showed promising toxicity profiles for DILs when compared with the corresponding monocationic ILs (Pretti et al, 2009; Frizzo et al, 2018; Montalbána et al, 2018)

Methods

Results

Conclusion