Ionic liquids (ILs) based on molecular cations and halogen anions were among the first to be synthetized, characterized and used in large-scale applications. On the other hand, salts of alkali cations coupled with molecular anions usually exhibit melting point temperatures well above 100 °C even when combined with large anions such as bis(trifluoromethylsulfonyl)imide. Such conceptual disparity was studied by classical molecular dynamics simulations using two different families of ILs: 1-alkyl-3-methylcyclopentadienyl anions, [CnC1Cp]– (n = 2, 4, 6, 8, 10), with potassium cations and 1-alkyl-3-methylimdazolium chloride family, [CnC1im]Cl (n = 2, 4, 6, 8, 10). The salts based on potassium 1-alkyl-3-methylcyclopentadienyl can be considered as the mirror images of ionic liquids of the 1-alkyl-3-methylimdazolium chloride family, in respect of charge: the potassium cation is the isoelectronic counterpart of the chloride anion; 1,3-dialkylcyclopentadienyl anions are the isoelectronic and isostructural counterparts of 1,3-dialkylimidazolium cations. The members of this new charge-inverted series are not as stable as their imidazolium-based counterparts due to the extreme sensitivity of their Cp moiety towards air and moisture. However, the proof-of-concept that inverted-IL salts can also be liquid at relatively low temperatures opens new possibilities for the preparation of novel, paradigm-shifting IL families. The comparison between the properties of inverted-ILs and their ILs counterparts can provide important insights into the nature of the interactions and structure of different ionic compounds. A systematic analysis comprising 1,3-dialkylimidazolium halide ILs, alkali 1,3-dialkylcyclopentadienyl ILs, and 1,3-dialkylimidazolium bis(trifluoromethylsulfonyl)imide ILs was also carried out using different correlations and molecular dynamics simulation techniques. The results show that the electrostatic interactions are correlated to the size of the intervening ions (regardless of their charge), while ionic packing and mobility are also defined by factors such as the relative size and polarizability of the ions, the anisotropy of electronic density, or the presence of non-polar groups. Charge asymmetry among charge-inverted ion pairs can cause deep structural differences such as those observed via MD simulations between the K[CnC1Cp] and [CnC1Im]Cl series.

Structure in conventional and not so conventional aromatic ionic liquids

255th National Meeting (2018)

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