Structure-property relationship in energetic cationic metal-organic frameworks: New insight for design of advanced energetic materials

Understanding the structure-property relationship in a material is of great importance in materials science. To study the effect of ligand backbones and anionic groups on the properties of energetic cationic metal-organic frameworks (CMOFs) and to disclose their structure- property relationships, we designed and synthesized a series of CMOFs based on either 4,4′-bi-1,2,4-triazole (btrz) or its azo analogous, 4,4′-azo-1,2,4-triazole (atrz) as ligand, and either perchlorate [ClO₄^-] or nitroformate [C(NO₂)₃^-, NF^-] anion as extra-framework anion. Surprisingly, the effect of ligand backbones on the CMOFs is inverse that of the backbones on traditional energetic compounds, while the effect of the anionic groups follows the traditional group law. We found that btrz-based CMOFs exhibit higher densities and better chemical and thermal stabilities than those of their corresponding atrz-based CMOFs, although btrz has a lower density and a lower stability than atrz. In particular, the density of btrz-Fe is more than 0.11 g cm^-3 higher than that of its atrz-based analogue (atrz-Fe). Moreover, the decomposition temperature of btrz-Zn (363 °C) is 80 °C higher than that of atrz-Zn, even higher than that of 1,3,5-triamino-2,4,6-trinitrobenzene (TATB), making it a potential heat-resistant explosive. The effect mechanisms were also discussed according to the experimental results. This investigation is significant for understanding the structure-property relationship in energetic CMOFs. Moreover, it also brings about new design rules for future high-performance energetic materials.