The effect of structural modifications on the thermal stability, melting points and ion interactions for a series of tetraaryl-phosphonium-based mesothermal ionic liquids

Abstract

A family of mesothermal ionic liquids comprised of tetraarylphosphonium cations and the bis(trifluoromethanesulfonyl)amidate anion are shown to be materials of exceptional thermal stability, enduring (without decomposition) heating in air at 300 °C for three months. It is further established that three specific structural elements - phenoxy, phenacyl, and phenyl sulfonyl - can be present in the cation structures without compromising their thermal stability, and that their incorporation has specific impacts on the melting points of the salts. Most importantly, it is shown that the ability of such a structural component to lower a salt melting point is tied to its ability to lower cation-cation repulsions in the material.

Ionic liquids of superior thermal stability. Validation of PPh4+ as an organic cation of impressive thermodynamic durability

“…Per Scheme 5, thermolysis of the ethyl-substituted salt 12 appears to proceed in a fashion closely related to that involved in the tolyl-methyl oxidation. At all temperatures, MS analysis shows the emergence of an unambiguous peak for the same carboxylic-acid functionalized salt (5) as that formed by the pyrolysis of 3, indicating that the -CH 3 of the ethyl is altogether shed, with the -CH 2 group being oxidized to -COOH. Even so, it is equally apparent, especially from MS and NMR of the 200 C experiments, that the reaction proceeds by the formation of a ketone, from which the -CH 3 group is subsequently cleaved and replaced by -OH, forming the carboxylic acid.…”

“…Since 2013 we have sought to design, prepare, and study ionic liquids that have degrees of thermal stability superior to those exhibited by commonplace imidazolium, quaternary ammonium, and tetraalkylphosphonium-type ILs. [1][2][3][4][5][6][7][8] And, while high thermal stability is important to ILs slated to be used as solvents in synthetic applications, 9 their development is especially important to their practical utility as additives to highperformance polymers, advanced lubricants, and as liquid-or 'liquiable' materials for energy-related uses such as energy storage via phase change and heat transfer uids in solar thermal arrays. 10 We note that the latter applications are those in which we especially envision future practical roles for the present class of ILs.…”

Thermal decomposition of phosphonium salicylate and phosphonium benzoate ionic liquids

“…Previous studies attributed the reactivity of ILs primarily to the anions. 42,56,93 Further, some of these studies correlated thermal stability to the anion basicity/nucleophilicity in phosphonium-based ILs 42 and other IL systems. 94,95 This correlation was investigated here using the MD simulations of phosphonium salicylate and benzoate.…”

“…54 These methods were used to investigate various parameters and phenomena, including interactions between anions and cations, spatial distribution functions, reaction pathways, energy barriers for decomposition reactions, decomposition products, and reaction mechanisms. 52,55,56 Previous studies have suggested that high nucleophilicity 45,52,55 and basicity 45,57 of anions affects the thermal stability of phosphonium-based ILs. However, previous studies used either density functional theory (DFT) calculations, which can not model the complex interactions between many anions and cations in large model systems over long durations, or classical MD simulations, which do not model the chemical reactions that are necessarily part of thermal decomposition.…”

Tetraarylphosphonium perfluorocyclobutyl polyelectrolyte with low critical surface energy, high thermal stability, and high alkaline resistance

“…Tetraarylphosphonium salts are among the most stable ionic liquids known, and their incorporation into polymer backbones has been targeted as a strategy to likewise access polymers with high thermal stability . The resistance of tetraarylphosphonium moieties to alkali degradation has also garnered interest in utilizing tetraarylphosphonium polyelectrolytes (TPELs, Scheme ) as membrane components of alkaline fuel cells and related electrochemical energy technologies.…”