Improved Stability and Smart‐Material Functionality Realized in an Energetic Cocrystal

Though energetic compounds, and explosives in particular, represent some of the most influential materials in human history, their modern evolution has been relatively slow. Aside from being hindered by the inherent dangers of the field, improving on the state-of-the-art is difficult as novel energetic materials (energetics) must achieve a challenging combination of properties including high explosive power, high stability, and low cost. Moreover, explosive power is highly dependent on solid-state density, and though chemical structures with high energies can be designed, engineering their crystal structures is not possible. In fact, crystal structure prediction, a problem closely related to crystal engineering, is fraught with challenges. This is especially so for compounds with nitro groups, a common moiety among energetic compounds. Though an admirable amount of work has gone into the discovery of novel compounds with high chemical potential energy, few of these materials have proven viable for explosives applications. Work to improve the material properties of existing energetics has focused primarily on exploring polymorphism in hopes to find more dense or less sensitive forms. This approach is attractive because it allows one to improve energetic compounds without the daunting task of implementing new, safe, and scalable chemistry. This advantage is present in another approach that is only starting to garner interest in solid-state energetics research: cocrystallization. Until recently, cocrystallization had been absent from the literature as a method for energetic solid form engineering despite its current success in engineering solid forms of pharmaceuticals. This imbalance is perhaps due to the chemical differences distinguishing energetics and pharmaceuticals. Most active pharmaceutical ingredients feature polar groups that are rich in predictable interactions conducive to cocrystal formation: primarily hydrogen bonding. Energetics, in contrast, are defined primarily by nitro groups, a solitary moiety that offers very few predictable interactions sufficiently strong or versatile to be useful in cocrystal design. Recently, we demonstrated that cocrystallization can be used to generate novel solid forms of energetic materials by presenting seventeen cocrystals containing 2,4,6-trinitrotoluene (TNT). Though the work proved the viability of cocrystallization to increase density and improve thermal stability in energetic materials, these improvements were achieved to the detriment of explosive power. When cocrystallized with a non-energetic compound, as was the case with these TNT cocrystals, the energetic component inevitably sees its explosive power diluted. Several measureable material properties are improved, but the resultant cocrystals are not viable explosives. Moreover, all of these cocrystals were formed by p–p stacking, a synthon available to the aromatic class of energetics, but not to the broader and more powerful non-aromatic class. Therefore, it remains unclear if energeticenergetic cocrystals can be realized based solely upon C H and nitro-group interactions. Presented here is an energetic–energetic cocrystal composed of 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL-20) and TNT in a 1:1 molar ratio (1, Figure 1). CL-20 is a relatively new energetic compound