Framework-Interpenetrated Nitrogen-Rich Zn(II) Metal–Organic Frameworks for Energetic Materials

Abstract

Nitrogen-rich high density metal–organic frameworks (MOFs) can be used as a new type of energetic materials. The energetic MOFs provide a new method to reconcile contradiction between high energy and reliable safety in energetic materials. Framework interpenetration would be an effective approach to reduce the pore volume and meanwhile to enhance the structural/chemical stabilities of the target energetic MOFs. In this work, mixed ligands of 5-aminotetrazole (HATZ) and tetrazole (HTZ) were chosen to assemble with Zn(II) ions with the purpose to prepare interpenetrated MOF materials with insensitivity. Ultimately, a high-density 3D energetic compound, [Zn2(ATZ)2(TZ)2]n (1) was in situ isolated under simple hydrothermal conditions. In the crystal structure of 1, there existed two independent 3D diamond networks, which were formed by Zn1(II) ions/TZ– ligands and Zn2(II) ions/ATZ– ligands, respectively. These two independent 3D diamond networks were interpenetrated to each other to construct a 3D condensed high-density framework. The standard molar enthalpy of formation (ΔfHo) of 1 was deduced as 1786.45 kJ/mol (4.09 kJ/g), which to date still occupies the status of the top high ΔfHo value among the reported energetic MOF materials. Conducted sensitivity measurements demonstrated the insensitivity of 1 to external mechanical stimuli. TGA showed 1 had good thermal stability up to 332 °C, whereas the decomposition temperature of pure HATZ and HTZ ligands are 207 and 174 °C, respectively. This shows that the greater stability of 1 could be attributed to the structural reinforcement induced by the effect of coordination polymerization and framework interpenetration. Compound 1 can serve as a promising energetic material with a high level of safety because of its good energetic properties, insensitivity, and high thermal stability.