Synthesis and characterization of O-alkylated amidium ionic liquids

In the present study, a new series of ionic liquids (ILs) derived from low-cost amides and lactams (cyclic amides), such as N,N-dimethylformamide (DMF), were synthesized and characterized. Unlike other nucleophiles like amines, the alkylation reaction of the amides with alkyl triflates to form cationic species takes place at the carbonyl oxygen atom, instead of the nitrogen atom. For these O-alkylated amidium ILs, the basic physicochemical properties, such as melting point, glass transition temperature, plastic crystal phase transition, thermal stability, density, surface tension, viscosity, ionic conductivity and electrochemical window, were investigated and studied. Generally, these ILs are distinguished by low viscosity and high conductivity, in particular the DMF-derived ILs with viscosity as low as 21.6 cP and conductivity up to 15.45 mS cm^-1 at 25 °C. This result should be attributed to the cationic DMF structures: planar geometry, low symmetry, C2-proton and ether moiety, resulting in much lower viscosity and higher conductivity than the best-known imidazolium ILs. Meanwhile, these amidium ILs also possess wide electrochemical windows (∼4.5 V) comparable to imidazolium ILs, implying their potential in electrochemical applications. Furthermore, several of the amidium ILs can form plastic crystal with a maximum enthalpy gain of -35.7 J g^-1 at the temperature range of -10-90 °C.