Linking the Thermal and Electronic Properties of Functional Dicationic Salts with Their Molecular Structures

Abstract

The two major properties that underpin ionic liquids are tunability and the potential to create task-specific media. Together, these properties allow ionic liquids to surpass the roles long held by traditional molecular solvents. However, at elevated temperatures or under prolonged heating, the structural components that impart such properties decompose or degrade. Dicationic pyridine salts present new opportunities to extend functionality and tunability to high temperatures because they are coordinating and thermally robust. In this work, we present three structurally related series of dicationic pyridine salts, which have been characterized by a wide array of techniques to link thermal and electronic properties to structural variation. The phase transitions and thermal stabilities of the salts were significantly influenced by small structural changes, and several new candidates for high-temperature-based applications were identified. The electron density, and therefore the electron donating ability, of the pyridine functional group could also be controlled by structural variation of cations and anions. Therefore, dicationic pyridine salts are highly tunable choices for task-specific solvents at elevated temperatures. Importantly, thermally robust solvents not only extend operational ranges but also reduce the need to replace or replenish solvents that degrade over time at temperatures commonly employed in industrial settings (i.e., 150−200 °C); solvent lifetimes are extended, and production is reduced. This is a critical requirement for complex media such as ionic liquids, which have high economic and environmental production costs.

Halometallate ionic liquids: thermal properties, decomposition pathways, and life cycle considerations

“…Importantly, TGA-MS favours volatile species because low volatility decomposition products can condense in transfer lines or exhaust ports. In this work, we observed mainly hydrocarbon fragments and the reverse Menshutkin product MeCl, as expected from previous reports on metal free ionic liquids 27,55. Interestingly, [C 8 C 1 Im]Cl, [C 8 C 1 Im]Cl 0.33 ZnCl 2 , and [C 8 C 1 Im]Cl 0.67 ZnCl 2 were all found to release MeCl on heating.…”

“…In this work, we observed mainly hydrocarbon fragments and the reverse Menshutkin product MeCl, as expected from previous reports on metal free ionic liquids. 27,55 Interestingly, [C 8 C 1 Im]Cl, [C 8 C 1 Im]Cl 0.33 ZnCl 2 , and [C 8 C 1 Im]Cl 0.67 ZnCl 2 were all found to release MeCl on heating. This supported the hypothesis that free chloride was produced as a decomposition product in metal containing [C 8 C 1 Im]Cl χ ZnCl 2 ionic liquids when they were heated to high temperatures.…”

“…In this work, we observed mainly hydrocarbon fragments and the reverse Menshutkin product MeCl, as expected from previous reports on metal free ionic liquids. 27,55 Interestingly, [ Interestingly, this data suggested that decomposition did proceed through reverse Menshutkin chemistry, and as volatile products vaporised, the remaining ZnCl 2 concentrated in the ionic liquid so that subsequent decompositions matched that of high ZnCl 2 content ionic liquids. This process would ultimately give ZnCl 2 which has a higher T 1% than any of the ionic liquids measured in this work.…”

Expanding the Chemical Space of Benzimidazole Dicationic Ionic Liquids

“…DILs are composed of two cations connected by a suitable linker and paired with two counteranions [24]. Although linear spacers are the most commonly found in literature, linkers of different nature have also been reported, such as branched alkyl [25], polyethers [26], aromatic chains [27][28][29], and fragments derived from natural sources [30]. Compared to the parent monocationic ILs, DILs exhibit superior tunability and present different structural organizations [31].…”

“…Compared to the parent monocationic ILs, DILs exhibit superior tunability and present different structural organizations [31]. These peculiar features often result in lower polarity, higher thermal stability [28,32], higher glass transition temperatures and melting points [33][34][35], larger surface tensions, higher shear viscosities [36], and lower ecotoxicity when compared to monocationic ILs [37]. These properties allowed DILs to be used effectively in a wider portfolio of applications, including catalysis [38,39], high-temperature batteries [40], high-temperature lubricants [41], stationary phases for chromatography [42], and dye-sensitized solar cells [43].…”