Energetic Cocrystal, Ionic Salt, and Coordination Polymer of a Perchlorate Free High Energy Density Oxidizer: Influence of p K_a Modulation on Their Formation

Cocrystal, ionic salt, and coordination polymer had prompted the development of propellants, explosives, and pyrotechnics. However, the difference between their formation based on the same coformer has rarely been studied. 5,5′-Bis(trinitromethyl)-3,3′-bi-1H-1,2,4-triazole (BTNMBT) is perchlorate-free, favorable to scale-up, and a green energetic oxidizer with high physical performance (oxygen balance: +18.4%, IS: 22.5 J, FS: 252 N, D: 9073 m s^-1, P: 36.2 GPa). To investigate the influence of different coformers on the formation of BTNMBT's cocrystal, ionic salt, and metal-organic framework, organic acid as well as organic and inorganic bases with different dissociation constants (pK_a) were theoretically studied. In this work, two energetic cocrystals, energetic ionic salt, and energetic metal-organic framework were synthesized based on BTNMBT. For their formation, the basic principle is found as follows: (i) when the pK_a value of organic base is far lower than both pK_a values of hydrogen-protons in organic acid, the reaction systems are prone to forming cocrystals; (ii) if the pK_a value of the selected organic base is higher than that of one of the hydrogen-protons in organic acid but lower than that of the other one, a 1:1 energetic ionic salt appears; (iii) the 1:2 type of energetic ionic salt (or coordination polymer) can form when the pK_a value of corresponding base is higher than values of both hydrogen-protons in organic acid. Among these shapes of derivatives, the coordination polymer form of BTNMBT not only exhibits good detonation performance (D: 8872 m s^-1), but also shows positive oxygen balance (+18.2%) and high thermal stability (T_d: 180 °C) comparable to those of AP and superior to those of ADN. These discoveries can assist the design and preparation of other promising energetic materials toward future high-performing energy applications.