How to Engineer the Best Possible Ionic Liquid?─Scrutinizing Structure–Property Relationships in Ammonium Ionic Liquids for Anti-Crystal Engineering

A set of IL-forming ion combinations has been studied to gain a deeper understanding of how, aside from obvious electrostatic interactions and ion size effects, secondary bonding such as hydrogen as well as halogen bonding and van der Waals interactions along with conformational and structural flexibility influence the crystallization behavior of potentially IL forming salts. The scrutinized ions have been specifically chosen to allow for unraveling preferential interactions of functional groups that may favor or disfavor crystallization with respect to secondary bonding interactions, i.e., primary and quaternary ammonium cations of variable alkyl chain lengths, which were also endowed with hydroxy groups, combined with formate and bis(trifluoromethanesulfonyl)amide anions. The background is to provide a deeper fundamental understanding of how to intentionally pair cations and anions that will not support the formation of a crystalline solid but rather IL formation, an approach described as “anti-crystal engineering”. This concept is based on the idea to avoid combining ions that are strong supramolecular synthons for crystallization. To this avail, the crystallization behavior of salts constituted of combinations of selected ions bearing different structural, supramolecular crystallization motifs has been studied in detail by low-temperature differential scanning calorimetry (DSC). Single crystal X-ray structure analysis has been used to elucidate ion packing and preferential interactions whenever crystalline solid formation is observed. The study reveals that the lowest melting points are supported by cation–anion combinations that have the least hydrogen bonding. However, if there are multiple possibilities of H-bonding for an ion with its counteranion, this bonding frustration leads as well to low melting points–albeit they are still higher compared to ion combinations with no H-bonding capacity. Through a careful balance of primary and secondary, directional and nondirectional interactions, it was possible to rationally identify a record class of ionic liquids, which combine exceptionally high decomposition points (440–450 °C) with an enormously high liquid range around of more than 500 °C and no tendency for solidification down to well below ambient temperature (−90 °C). These ILs are formed by bis(trifluoromethane)sulfonylamides with quaternary ammonium ions that bear an −OH group in the side chain.