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.