Synthesis and properties of symmetrical N,N′-bis(alkyl)imidazolium bromotrichloroferrate(III) paramagnetic, room temperature ionic liquids with high short-term thermal stability

A series of paramagnetic ionic liquids (PILs) based on N,N′-bis(alkyl)imidazolium bromotrichloroferrate(III), [(Cn)2Im][FeCl3Br] (Im = imidazole; Cn = ethyl, butyl, hexyl, octyl, decyl, dodecyl), was prepared and compared against the more traditional [CnC1Im][FeCl3Br] (C1 = methyl) analogs. Both series were extensively characterized by physical, chemical, and computational methods including differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), spectroscopy (FTIR and UV‐vis-NIR), magnetic susceptibility, surface tensiometry, density, and viscosity. The X-ray crystal structure of [(C12)2Im][FeCl3Br] was also obtained. Notably, all ionic liquids prepared in this study show short-term thermal stabilities exceeding 300 °C in air and have melting points equal to or lower than 25 °C, which is surprisingly low for ionic liquids containing symmetrical cations. In addition, all presented ionic liquids have relatively low viscosities allowing for easier transitioning into applications. Physical property trends are discussed.