Isostructural Cocrystals of 1,3,5-Trinitrobenzene Assembled by Halogen Bonding

Two isostructural energetic cocrystals containing 1,3,5-trinitrobenzene (TNB) were obtained with the energetic materials 1,3,5-triiodo-2,4,6-trinitrobenzene (TITNB) and 1,3,5-tribromo-2,4,6-trinitrobenzene (TBTNB) in ratios of 2:1 TNB/TITNB (1) and 2:1 TNB/TBTNB (2). These materials contain the shortest nitro-iodo and second shortest nitro-bromo interactions seen in the Cambridge Structural Database for organohalides. Computational studies indicate that the cocrystals are more stable than their respective component crystals by 11.5 kJ/mol for 1 and 8.2 kJ/mol for 2. While the formation of an isostructural 2:1 cocrystal with 1,3,5-trichloro-2,4,6-trinitrobenzene (TCTNB) was calculated to be favorable by 8.5 kJ/mol, only a physical mixture of the two coformers was obtained experimentally. Both 1 and 2 possess high crystallographic densities (2.203 and 1.980 g/cm³, respectively) and were found to be insensitive in small-scale impact drop tests, possessing sensitivity between that of TNB and TXTNB (X = I or Br). The halogen content of the two cocrystals suggests application as insensitive biocidal energetics. Halogen bonding, facilitated by the strong polarization induced by aromatic ring nitration, plays a critical role in the formation of these cocrystals and offers a promising route to the development of new energetic cocrystals.