Two isostructural explosive cocrystals with significantly different thermodynamic stabilities.

Cocrystallization is currently having a tremendous impact on pharmaceuticals and energetic materials (energetics) and is poised to make a significant mark on other fields such as non-linear optics, ferroelectrics, and organic electronics. In the energetics field the ability of cocrystallization to combine two known explosive compounds into a novel material with distinct properties presents an elegant means of generating improved explosives from existing compounds, and several such cocrystals have recently been reported. Unfortunately, with the chemistries of energetic compounds being defined primarily by weakly interacting nitro groups, energetic cocrystals have proven difficult to design; this contrasts with their hydrogen-bonded counterparts in the pharmaceutical cocrystal field. As such, many reported energetic cocrystals were discovered through screening different stoichiometries of existing compounds. Needed in the field are reliable design strategies for generating novel energetic–energetic cocrystals. One supramolecular synthon that has proven reliable in previous energetic cocrystal work involves interactions of the electron-deficient aromatic ring common to energetic compounds such as 2,4,6-trinitrotoluene (TNT) and 1,3,5-triamino-2,4,6-trinitrobenzene (TATB). Such molecules have been observed to p-stack with the electron-rich rings of many aromatic compounds such as naphthalenes, anthracenes, and perylene. Unfortunately, electron-rich rings are rare among energetic compounds, which often feature significant electron-withdrawing nitro substitution. Thus, in order for this synthon to be used to design energetic–energetic cocrystals, an electron-rich energetic compound with favorable geometry for interacting with electron-deficient rings must be found. Diacetone diperoxide (DADP) is a member of the acetone peroxide family of energetic compounds and features a conformation and electrostatic potential that make it ideal for cocrystallizing with electron-deficient aromatic compounds (Figure 1a). Acetone peroxides are the cyclic products of the acid-catalyzed nucleophilic addition of hydrogen peroxide to acetone. The dimer product, DADP, and the trimer product, triacetone triperoxide (TATP, Figure 1b), are the two most common of these compounds, and their formation can be dictated by the strength of the acid used in the reaction. Though TATP is the more common of the two, DADP has an important feature that makes it more useful for energetic cocrystal design: the peroxide oxygen atoms of DADP are accessible (Figure 1c) while those of TATP are largely shielded by methyl groups (Figure 1d). Thus, DADP is better positioned to interact with electropositive moieties in the solid state. With this in mind, DADP was expected to cocrystallize with some common energetic compounds that feature electron-deficient nitro-substituted rings such as TNT, TATB, and the TATB precursor 1,3,5trichloro-2,4,6-trinitrobenzene (TCTNB, Figure 2). The last of those compounds also features halogens, which make it especially well-suited for cocrystallization as halogens are reported to participate with nitro groups in supramolecular synthons. Noting these potential interactions, DADP and Figure 1. Chemical structures of a) diacetone diperoxide (DADP) and b) triacetone triperoxide (TATP). Electrostatic potential maps of c) DADP and d) TATP by AM1 semi-empirical calculation.