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

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.