A criterion for proper cosolvents used for ionic liquids: The Lewis acidic and basic dual nature of propylene carbonate

Probe of the Lewis acidic property of propylene carbonate (PC) has been initiated by temperature-dependent 19F chemical shifts recorded for 1-butyl-3-methylimidazolium hexafluorophosphate/PC (BMIPF6/PC) mixtures. Investigation of hyperconjugative interactions by natural bond orbital method reveals the trend of electron-accepting capabilities from PF6- anion, PC > dimethyl sulfoxide (DMSO) > N,N-dimethyl formamide (DMF) > acetonitrile (AN). Comparison of solvent-induced ionic association of BMIPF6 in the four basic aprotic organic solvents, made by monitoring concentration- and temperature-dependent-ratios of solvent diffusion coefficients to BMI+-containing components, Dsol/DBMI, shows that the extent of original ionic association can be reduced by solvent dilution in the same decreasing order, PC ∼ DMSO > DMF > AN. In DMF and AN, two basic solvents having similar dielectric constants, the greater Lewis acid of DMF accounts for its better ability to dissociate aggregates of BMIPF6. By replacing ethanol with 2,2,2-trifluoroethanol (TFE), the poor miscibility of BMIPF6 with ethanol can be greatly enhanced due to the CF3 group, capable of adapting electron density from anions via hyperconjugative interactions. In BMIPF6/TFE mixtures, values of DTFEF/DPF6 measured on 19F resonances show larger aggregates than DTFEH/DBMImeasured on 1H resonances, indicating charged clusters prefer anionic to cationic states. This observation can be explained by varying ionic states concerning with unbalanced charged aggregates between positive and negative ones.

Knowing that propylene carbonate (PC) plays the role of both a Lewis base by H-bonding accepting with the cation via the carbonyl group and a Lewis acid by H-bonding with PF6- anion via the propylene framework in hyperconjugative interaction manner, we have utilized the fluorinated ethanol to improve the miscibility of BMIPF6 with CF3CH2OH.Figure optionsDownload as PowerPoint slide