Azoniaspiro salts: towards bridging the gap between room-temperature ionic liquids and molten salts

Abstract

In a continued effort to improve the suitability of ionic liquids in applications operating at raised temperatures, novel spirocyclic 'azoniaspiro' salts (with cations derived from five-, six-, seven- and eight-membered rings) are prepared and characterised. The structural and thermal properties of these salts are compared against those of established analogues. The stable geometries and ion pairing behaviour of these species are investigated via a combined experimental/computational approach, employing X-ray crystallography and Density Functional Theory (DFT) methods. Subsequently, the thermal stabilities of these organic salts are characterised and compared using a broad range of techniques. Hyphenated Thermogravimetry-Mass Spectrometry investigations enable complex mechanisms underlying thermal decomposition to be elucidated. Lastly, transition state structures are optimised, corresponding to plausible decomposition mechanisms of the azoniaspiro salt, 6-azoniaspiro[6.5]dodecanium chloride, and one prototypical monocyclic species 1-butyl-1-methylpiperidinium chloride, using DFT. The observed improved thermal stabilities of the azoniaspiro salts, and their potential higher-temperature stable-liquid ranges, render them promising candidates for future ionic liquid applications.

Superbase ionic liquids for effective cellulose processing from dissolution to carbonisation

“…Tonset values collected in Table 1 give the first approximation of the thermal stability of ILs, but tend to overestimate their longterm thermal stability. Therefore, the T0.01/10 already reported for [C2C1im][OAc], and found to be 102 °C, 45 was also calculated for [C2C1im][DEP] and the SILs. The T0.01/10 parameter corresponds to the temperature at which 1% weight loss will occur in 10h.…”

Thermally-Stable Imidazolium Dicationic Ionic Liquids with Pyridine Functional Groups

“…However, lengthy production routes make ILs unlikely candidates for large-scale use. Despite this, many applications can benefit from thermally-stable ILs and many researchers are pursuing a structure driven design to achieve more robust ILs. − Unfortunately, thermally-stable ILs require noncoordinating anions, which eliminates the tuneability that is usually a key selling point for ILs as “designer solvents”. Furthermore, the presence of common functional groups introduces bonds susceptible to thermolysis, compromising the thermal stability of the IL. − Therefore, functional and thermally-stable ILs could help reclaim the tunable nature of ILs at elevated temperatures, providing task-specific functions to suit the desired application.…”

Alcohol‐ and Amine‐Tolerant Synthesis of Six‐Membered Cyclic Quaternary Ammonium Salts by Using a Triazine‐Based Reagent

“…The dimethylaminog roup in 2b was selectively activated to afford 6b at room temperature after 1.5 h( Ta ble 2, entry 2). The spiro ammoniums alt 6c, which has recently been reported as at hermally stable ionic liquid, [30] wasa lso prepared in an excellent yield ( Table 2, entry 3). Moreover,i tw as possible to prepares terically hindered N,N-diisopropylpiperidinium salts 6d,w hich would be difficult to synthesize by using the conventional N-alkylation methods hown in Scheme 1(a) ( Table 2, entry 4).…”

“…Namely,the solvent should be less nucleophilic than methanol to avoid solvolysis and more polar than acetonitrile to inhibitd emethylation by the chloride anion (Table 1, entries 3-5). Such polar solvents were chosen on the basis of the Dimroth-Reichardt solvent parameters [E T (30), [24] Ta ble1].…”