Azepanium ionic liquids

The seven-member alicyclic secondary amine, azepane, has been used as starting material to synthesise a new family of room temperature ionic liquids. Such useful transformations of this coproduct of diamine production processes, generated in large amounts in the polyamide industry, would mitigate its disposal, which usually involves combustion. Reaction of azepane with 1-bromoalkanes or 1-bromoalkoxyalkanes produced the corresponding tertiary amines with good selectivity; further quaternisation reactions with the appropriate methylating agents yielded quaternary azepanium salts, [Rmazp]X (R = alkyl or alkoxyalkyl; X− = I−, [CF3CO2]− or [OTf]−; Tf = (trifluoromethyl)sulfonyl). Analogous [NTf2]− salts have also been produced by metathetic reactions. Liquid temperature ranges are significantly affected by the nature of the anion and the substituents on the azepanium cation core; for example, [CF3CO2]− or [OTf]− salts based on cations with alkyl substitution are solids, whereas those with alkoxyalkyl substitution are liquids at ambient temperature. The crystal structures of [C4mazp][CF3CO2], [C4mazp]I and [C6mazp][NTf2] (C4 = butyl, C6 = hexyl) are reported. The effects of the structural features of cations and anions on density, viscosity and conductivity data are discussed. The presence of ether linkages in the cationic side chains causes a marked decrease in viscosities and an increase in conductivities. Cyclic voltammetry showed that azepanium ionic liquids exhibit extremely wide electrochemical windows, thus becoming promising and safe alternatives to electrolytes based on volatile organic compounds.