3-Methylpiperidinium ionic liquids

Abstract

A wide range of room temperature ionic liquids based on the 3-methylpiperdinium cation core were produced from 3-methylpiperidine, which is a derivative of DYTEK® A amine. First, reaction with 1-bromoalkanes or 1-bromoalkoxyalkanes generated the corresponding tertiary amines (Rm_βpip, R = alkyl or alkoxyalkyl); further quaternisation reactions with the appropriate methylating agents yielded the quaternary [Rmm_βpip]X salts (X⁻ = I⁻, [CF₃CO₂]⁻ or [OTf]⁻; Tf = –SO₂CF₃), and [Rmm_βpip][NTf₂] were prepared by anion metathesis from the corresponding iodides. All [NTf₂]⁻ salts are liquids at room temperature. [Rmm_βpip]X (X⁻ = I⁻, [CF₃CO₂]⁻ or [OTf]⁻) are low-melting solids when R = alkyl, but room temperature liquids upon introduction of ether functionalities on R. Neither of the 3-methylpiperdinium ionic liquids showed any signs of crystallisation, even well below 0 °C. Some related non-C-substituted piperidinium and pyrrolidinium analogues were prepared and studied for comparison. Crystal structures of 1-hexyl-1,3-dimethylpiperidinium tetraphenylborate, 1-butyl-3-methylpiperidinium bromide, 1-(2-methoxyethyl)-1-methylpiperidinium chloride and 1-(2-methoxyethyl)-1-methylpyrrolidinium bromide are reported. Extensive structural and physical data are collected and compared to literature data, with special emphasis on the systematic study of the cation ring size and/or asymmetry effects on density, viscosity and ionic conductivity, allowing general trends to be outlined. Cyclic voltammetry shows that 3-methylpiperidinium ionic liquids, similarly to azepanium, piperidinium or pyrrolidinium counterparts, are extremely electrochemically stable; the portfolio of useful alternatives for safe and high-performing electrolytes is thus greatly extended.