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

Abstract

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.