Dicationic bis-imidazoliums as a platform for ionic liquids: Long tails and short spacers

Abstract

A broad set of bis-imidazolium salts with long tails and short spacers (LTSS BIS) with odd and even number of carbons was synthesized and characterized with a number of experimental and computational methods, such as 1H and 13C NMR (2D techniques included), optical and electron microscopy, water content determinations, solubility in water assessments, melting points measurements, geometry and thermochemistry evaluation for model dications in PM6. Theoretically substantiated improvements for alkylimidazoles and bis-imidazolium dihalides synthesis have been proposed and experimentally verified. Good yields of LTSS BIS pure enough for physicochemical studies can be achieved in a fast and efficient manner, and factors essential for getting good results have been unveiled. The majority of LTSS BIS are prone to absorb water and retain it under usual conditions, but easily lose at heating. Large variety in crystal forms and thermal behavior of LTSS BIS subfamilies were found, depending mainly from spacer and anion. High thermal stability (more than 200°C) is typical for all LTSS BIS under study, but only a small part may be considered as a true ionic liquid. Fully stretched conformations were supposed for all alkyl fragments (tails and spacers) in solution, in accordance with experimental and computational results. Rotation around ordinary bonds is restricted only in 1,3-di(imidazolium-1-yl)-2-hydroxypropane dication, leading to unique four sorts of protons in three sp³-carbon chain. Due to specific structural features, bis-imidazolium salts cannot be represented as rod-like molecules. The concept of three principal structural motifs for LTSS BIS (U-shape, V-shape, and Z-shape) is proposed.