Crystal structure and supramolecular assembly of the ionic liquid-like salt tributylmethylammonium acyclovir ([N4,4,4,1][Acy])

In search of low melting, ionic liquid-like forms of the active pharmaceutical ingredient, acyclovir, we isolated tributylmethylammonium acyclovir ([N 4,4,4,1 ][Acy]) and structurally characterized it by single crystal X-ray diffraction. The crystal structure reveals discrete tributylmethylammonium cations and acyclovir anions arranged into well-defined polar and nonpolar domains. The acyclovir anions form an extended supramolecular network stabilized by multiple N–H···O and O–H···O hydrogen bonds, while the cations engage in weaker C–H···O interactions that reinforce lattice cohesion. The longer butyl substituents of the cation segregate into hydrophobic regions, leading to a layered packing arrangement with clear separation between hydrogen-bonded acyclovir frameworks and alkyl-rich domains. This amphiphilic organization highlights the dual role of directional hydrogen bonding and dispersive alkyl interactions in defining the three-dimensional packing of this pharmaceutically relevant ionic liquid-like salt.