The anti-crystal engineering principles of imidazolium cations for ionic liquids

Crystallography provides a powerful framework for identifying, characterizing, and designing new ionic liquids (ILs) with targeted thermal and structural properties. While the design of imidazolium-based ILs has historically relied on empirical modification of alkyl chain length, cation symmetry, and electronic or steric effects, crystallography reveals how these molecular parameters dictate lattice packing, intermolecular interactions, and ultimately melting behavior. Despite extensive study, critical structure-property relationships remain unresolved, including the impact of C4 and C5 methylation, odd-numbered alkyl chains, and conformational polymorphism. From a crystal engineering perspective, the design of low-melting ionic compounds can be viewed as a deliberate inversion of traditional crystal design principles. Rather than promoting long-range order, researchers aim to disrupt specific noncovalent synthons and reduce lattice enthalpy to favor fluidity. This tutorial review unifies these perspectives by examining how crystallography has helped steer structural design to control interactions, torsion angles, molecular descriptors, and hydrogen-bond networks to modulate the behavior of dialkylated imidazolium salts. The discussion highlights how crystallography transforms the empirical art of IL synthesis into a rational, structure-guided design strategy for next-generation materials.