Developing a highly scalable synthetic strategy for 5-amino-4-nitrobenzo[1,2-c:3,4-c']bis([1,2,5]oxadiazole)1,b-dioxide (CL-18) and investigating the influence of crystal engineering and positional isomerization on its safety and laser ignition performance

Abstract

5-amino-4-nitrobenzo[1,2-c:3,4-c']bis([1,2,5]oxadiazole)1,6-dioxide (CL-18) exhibits significant potential as an initiating explosive. However, its current synthesis process remains non-scalable due to low yields and safety risks. In this study, we have developed a simple and safe synthetic route for CL-18. It was synthesized from 3,5-dihaloanisole in a four-step reaction with an overall yield exceeding 60%, surpassing all reported yields in the literature. Subsequently, recrystallization of CL-18 was successfully achieved by carefully selecting appropriate solvents and antisolvents to reduce its mechanical sensitivity. Ultimately, when DMF-ethanol was employed as the recrystallization solvent system, satisfactory product yield (>90%) and reduced mechanical sensitivity (IS = 15 J; FS = 216 N) were obtained. Additionally, CL-18 is derived from the rearrangement of oxygen atoms on i-CL-18 furoxan, and a comparative analysis of their physicochemical properties was conducted. The thermal stability of both compounds is similar, with onset decomposition temperatures recorded at 186 and 182 °C respectively. Similarly, they exhibit 5 s breaking point temperatures of 236 and 237 °C. Additionally, we present novel insights into the positional-isomerization-laser-ignition performance of CL-18 and its isomer i-CL-18 using laser irradiation for the first time. Remarkably, our findings demonstrate that i-CL-18 exhibits enhanced laser sensitivity, as it can be directly ignited by a 1064 nm wavelength laser, whereas CL-18 lacks this characteristic.