Structure and Properties of High Stability Geminal Dicationic Ionic Liquids

Abstract

Thirty-nine geminal dicationic ILs were synthesized and characterized in terms of their surface tensions, densities, melting points, refractive indices, viscosities, and miscibilities with a polar and nonpolar solvent. Two imidazolium or pyrrolidinium cations were joined via different length hydrocarbon linkage chains (from 3 to 12 carbons long). The various geminal dications were paired with up to four different anions. The effect of the dication type, linkage chain, alkyl substituents, and anion type on the physicochemical properties of these compounds was examined. Among the more interesting findings for this class of compounds was that their liquid and thermal stability ranges generally exceeded those of the more conventional, better known ILs. Indeed, this range was from −4 to >400 °C for one of the pyrrolidinium-based geminal dicationic liquids. Xray crystallography of the smaller solid ionic compounds indicated that there may be a correlation between the configurational degrees of freedom of the ILs and their melting points/glass transition temperatures. In one case, the crystal structure showed that a dicationic moiety had three distinct conformations in an asymmetric unit cell. The solvation properties of the geminal dicationic ILs tend to be similar to those of their monocationic analogues. KeywordsCrystal structure, glass transition, hydrocarbons, liquids, melting, negative ions, refractive index, solubility, solvents, surface tension, thermodynamic stability, viscosity, x ray crystallography, pyrrolidinium cations Abstract: Thirty-nine geminal dicationic ILs were synthesized and characterized in terms of their surface tensions, densities, melting points, refractive indices, viscosities, and miscibilities with a polar and nonpolar solvent. Two imidazolium or pyrrolidinium cations were joined via different length hydrocarbon linkage chains (from 3 to 12 carbons long). The various geminal dications were paired with up to four different anions. The effect of the dication type, linkage chain, alkyl substituents, and anion type on the physicochemical properties of these compounds was examined. Among the more interesting findings for this class of compounds was that their liquid and thermal stability ranges generally exceeded those of the more conventional, better known ILs. Indeed, this range was from -4 to >400°C for one of the pyrrolidiniumbased geminal dicationic liquids. X-ray crystallography of the smaller solid ionic compounds indicated that there may be a correlation between the configurational degrees of freedom of the ILs and their melting points/glass transition temperatures. In one case, the crystal structure showed that a dicationic moiety had three distinct conformations in an asymmetric unit cell. The solvation properties of the geminal dicationic ILs tend to be similar to those of their monocationic analogues.

Solution and thermal behaviour of novel dicationic imidazolium ionic liquids

“…This result, according to a previous report of Armstrong and co-workers, could be a consequence of the high viscosity of the liquid phases obtained from the melting process. 3 However, as stated above on the basis of NMR measurements and according to previous reports in the literature, 51,52 it could be also complicated by the high flexibility of the cation that can adopt different conformations giving rise to different energies of the crystal lattice and multistep transitions.…”

“…Similar results have been previously obtained by analysing the thermal stability of some unsymmetrical dicationic ionic liquids. 3,57,58 Perusal of the data reported in Table 2 evidences how, differently from the melting process (see above), the decomposition process is significantly affected by the anion nature. Reversible dealkylations of imidazolium salts via a nucleophilic attack by the anion could account for the decomposition process.…”

“…Wider ranges have been previously reported for dicationic imidazolium salts having an aliphatic spacer. 3 Analysis of T m values evidences how the different nature of the anion induces only small differences. The collected data do not allow us to make a clear distinction between mono-and dianions.…”

“…In this light, the operational limit for the dicationic ionic liquids ranges from 228.8°C for [2C 8 31,56 In general, our ionic liquids show a lower thermal stability with respect to dicationic pyrrolidinium and imidazolium salts having aliphatic spacers previously reported by Armstrong and co-workers. 3 The very different thermal stability could be a result of the different nature of the spacer used. However, alternatively in our case, it could be also ascribed to the higher substitution degree induced by the presence of the basic functionality in the cation structure that gives rise to the different conformers (see above).…”

“…1,2 Room temperature ionic liquids with dipositive cations are rare, hence their physical behaviour is not well understood. [3][4][5][6][7][8][9] Tripositive cations are even scarcer. [10][11][12] The interest in dicationic ionic liquids is largely due to their high thermal stability, higher than that normally characterising the corresponding monocationic ionic liquids.…”