Synthesis, Stability and Electrochemical Characterization of New Ionic Liquids Based on Asymmetrical Borates

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It is well known that lack of symmetry entails a drop in the fusion point of nearly equal substances. E.g., ethylene carbonate shows a fusion point of about +36°C whereas the liquid range of propylene carbonate extends to roughly -49°C despite its higher molar mass and may be easily supercooled to about -80°C. Thus larger liquid ranges may be obtained for ionic liquids (ILs) based on asymmetric borates when compared to known ILs which are based on symmetric borates such, e.g. EMIBF4. Some asymmetric borates with lithium counter ions are already known. However they show a lack of chemical stability. Exchange reactions entail mixtures of the symmetric and asymmetric borates. We have assumed that this exchange reaction is mainly assisted by the very small lithium ion and tried to synthesize some salts with asymmetric borates and inert cations such as imidazolium ions. Our promising preliminary work showed that our assumption is correct. The first thermally stable member of a new class of ILs was synthesised, tetraethylammoniumdifluoromono1, 2-oxalato(2-)-O,O¿]borate (1-). Therefore we propose a project including synthesis and electrochemical characterisation of new ILs based on asymmetric borates with one- and bidentate ligands and cations of imidazolium, sulfonium and ammonium types. In addition, a long time stability investigation is planned at moderate temperatures, where the composition of the solution of above mentioned borates is studied by NMR-measurements over a long period.