Compressive Symbol: A New Way to Evaluate the High-Pressure Behaviors of Energetic Tetrazole Materials

Energetic materials may transit to different phases or decompose directly under compression. Their reactivity in the explosions can be evaluated by their high-pressure induced behaviors, including polymorphism or phase transition. Here, we applied DFT methods to understand high-pressure behaviors of four typical tetrazole derivate crystals, including 5-aminotetrazole (ATZ), 1,5-aminotetrazole (DAT), 5-hydrazinotetrazole (HTZ), and 5-azidotetrazole (ADT), under the gradually increased pressure from ambient pressure to 200 GPa. In response to the extreme-high pressures, the performances are dominated by compressibility of crystals, reflected by compressive symbols on the basis of the molecular orientation in crystals. The crystal with weak compressibility (large symbol) generally dissociates, triggered by cleavage of weak bonds. However, the crystal with low compressive symbol is generally corresponding to a pressure-induced structural transformation or phase transition.

Coordination polymerization of nitrogen-rich linkers and dicyanamide anions toward energetic coordination polymers with low sensitivities

The design and synthesis of energetic materials (EMs) with high energy and reliable stabilities has attracted much attention in the field of EMs. In this work, we employed a strategy of the coordination polymerization of mild dicyanamide ions (DCA^-), two isomeric ligands 1-methyl-5-aminotetrazole (1-MAT) and 2-methyl-5-aminotetrazole (2-MAT) to construct energetic coordination polymers (ECPs). Four new ECPs {[Co(DCA)₂(1-MAT)₂]·H₂O}_n1, [Cu(DCA)₂(1-MAT)]_n2, [Cd(DCA)₂(1-MAT)₂]_n3 and [Cd(DCA)₂(2-MAT)₂]_n4 were successfully synthesized through solvent evaporation routes. Compounds 1 and 4 display 1D chains, while 2 and 3 exhibit 2D-layered structures. Compounds 1-3 with the 1-MAT ligand all exhibit reliable thermal stabilities (> 200 °C). The calculated heats of detonation (ΔH_det) of 1-3 are all higher than 1.4 kJ g^-1, which are higher than traditional explosive TNT (1.22 kJ g^-1) and the reported ECP AgMtta (HMtta = 5-methyl-1H-tetrazole, ΔH_det = 1.32 kJ g^-1). Furthermore, sensitivity testing demonstrates that 1-4 features low mechanical sensitivity to external mechanical action in contrast with the extremely sensitive azide-based ECPs [Cu₃(2-MAT)₂(N₃)₆]_n. In addition, compound 2 shows hypergolic properties via an 'oxidizer-fuel' drop experiment, demonstrating its application prospects in the field of propellants. This work details an approach of synthesizing multipurpose ECPs with reliable stabilities by introducing mild dicyanamide anions into nitrogen-rich skeletons.

Exoergic pathways triggered by O/H radicals in different metallic carbohydrazide perchlorates (M2+ = Mn2+, Fe2+, Co2+, Ni2+, Zn2+ and Cd2+)

Metallic carbohydrazide perchlorates (M[(N₂H₃)₂C = O](ClO₄)₂, M²⁺ = Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺, Zn²⁺ and Cd²⁺, simplified as MCPs) are a series of energetic primary explosives, among which ZnCP and CdCP are already applied in civilian/military fields. The six MCPs possess similar structures but demonstrate different energetic performances in their decomposition, which are obviously determined by their different central metals. Here, we apply DFT and Car-Parrinello molecular dynamics (CPMD) to understand the electronic structures and decomposition pathways of the MCPs. Based on the results, the crystal MCPs with larger electronic band gaps show lower impact sensitivity. However, the friction sensitivity of MCPs is dominated by the strength of their intermolecular O⋯H interactions. In the CPMD simulations, we obtained a different conclusion from the traditional viewpoint, where the decomposition is spontaneous from the cleavage of M-N bonds. Indeed, there are two stages in the decomposition of the MCPs, based on our calculations: (I) nonspontaneous 3-step departure of the CHZ groups and (II) spontaneous exoergic decomposition pathways of the CHZ groups triggered by the transfer of O/H radicals. Our study provides a systematic study of the MCP family, which also affords a new route for understanding the relationship between the energetic properties and electronic structures of energetic metal complexes.

Chelate Coordination Compounds as a New Class of High-Energy Materials: The Case of Nitro-Bis(Acetylacetonato) Complexes

The existence of areas of strongly positive electrostatic potential in the central regions of the molecular surface of high-energy molecules is a strong indicator that these compounds are very sensitive towards detonation. Development of high-energy compounds with reduced sensitivity towards detonation and high efficiency is hard to achieve since the energetic molecules with high performance are usually very sensitive. Here we used Density Functional Theory (DFT) calculations to study a series of bis(acetylacetonato) and nitro-bis(acetylacetonato) complexes and to elucidate their potential application as energy compounds with moderate sensitivities. We calculated electrostatic potential maps for these molecules and analyzed values of positive potential in the central portions of molecular surfaces in the context of their sensitivity towards detonation. Results of the analysis of the electrostatic potential demonstrated that nitro-bis(acetylacetonato) complexes of Cu and Zn have similar values of electrostatic potential in the central regions (25.25 and 25.06 kcal/mol, respectively) as conventional explosives like TNT (23.76 kcal/mol). Results of analysis of electrostatic potentials and bond dissociation energies for the C-NO₂ bond indicate that nitro-bis(acetylacetonato) complexes could be used as potential energetic compounds with satisfactory sensitivity and performance.

High-Energy Metal-Organic Frameworks with a Dicyanamide Linker for Hypergolic Fuels

In this study, a hypergolic linker (dicyanamide, DCA) and a high-energy nitrogen-rich ligand (1,5-diaminotetrazole, DAT) were applied to construct high-energy metal-organic frameworks (HEMOFs) with hypergolic property. Three novel metal-organic frameworks (MOFs) were synthesized via a mild method with fascinating 2D polymeric architectures, and they could ignite spontaneously upon contact with white fuming nitric acid (WFNA). The gravimetric energy densities of the three HEMOFs all exceeded 26.2 kJ·g^-1. The cupric MOF exhibits the highest gravimetric and volumetric energy density of 27.5 kJ·g^-1 and 51.3 kJ·cm^-3, respectively. By adjusting the metal cations, high-energy ligands and hypergolic linkers can improve the performance of hypergolic MOFs. This work provides a strategy for manufacturing MOFs as potential high-energy hypergolic fuels.

High performance 5-aminotetrazole-based energetic MOF and its catalytic effect on decomposition of RDX

The energetic compound [Ag₂(5-ATZ)(N₃)] (1) features a compacted 3D framework structure, remarkable thermostability above 300 °C and perfect detonation performance. Remarkably, 1 can accelerate effectively the thermal decomposition of RDX.