Crystal Solvates of Energetic 2,4,6,8,10,12-Hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane Molecule with [bmim]-Based Ionic Liquids

Crystallization of different biological targets and pharmaceuticals from ionic liquids (ILs) could result in previously unknown polymorph modifications. Crystallization of high-energy materials (HEMs) from ILs has not been studied so far. This paper is devoted to the first attempts to crystallize a highly energetic compound2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (HNIW or CL-20) from different ILs. However, instead of the possible polymorphs these investigations unexpectedly resulted in multicomponent crystal solvates containing an HNIW molecule and anions and cations of two ILs1-butyl-3-methylimidazolium hexafluorophosphate ([bmim]­[PF6]) and 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim]­[BF4]). Their structure was determined by single-crystal X-ray diffraction analysis. The HNIW molecule in both structures adopts different but the most common conformations, similar to the β- and γ-polymorphs of the pure HNIW. Crystal packing was analyzed in terms of extended Hirshfeld surface analysis that allowed distinguishing the contacts between different solvate components. In HNIW·[bmim]­[PF6] intermolecular bonding interactions were also analyzed by topological analysis of theoretical (B3LYP-D3/POB-TZVP) electron density distribution. It was found that, despite small difference in the ionic liquid, the arrangement of crystal components in two solvates differs significantly. In HNIW·[bmim]­[PF6] with a chainlike packing pattern of the HNIW molecules the HNIW···HNIW interactions play a much lesser role than in HNIW·[bmim]­[BF4] with layerlike arrangement of the energetic molecules. The presence of the anion and cation of the ionic liquid significantly decreases the melting point and density of the materials, relative to ε-HNIW. Importantly, the sensitivity to impact (18 J) measured for HNIW·[bmim]­[BF4] is also significantly lower as compared to ε-HNIW (4 J).