Thinking Outside the Energetic Box: Stabilizing and Greening High-Energy Materials with Reticular Chemistry

ConspectusReticular chemistry has provided intriguing opportunities for systematically designing porous materials with different pores by adjusting the building blocks. Among them, framework materials have demonstrated outstanding performance for the design of new functional materials used in a broad range of fields, including energetic materials. Energetic materials are widely used for rockets, satellites, mining, and tunneling. In terms of energetic materials, explosophores and nitrogen-rich heterocycles are fundamental building blocks for high-energy compounds. However, the traditional strategy of synthesizing HEDMs (high energy density materials) at the molecular level has faced the long-term challenge of balancing energy and stability. Inspired by reticular chemistry, nitrogen-rich heterocycles offer diverse nitrogen sites for designing diversified coordination interactions. Ionic bond interactions exist in a wide range of energetic salts. Furthermore, most metastable explosophores, e.g., nitro, nitramino, and amino groups, can form strong hydrogen-bonding networks. Based on these noncovalent interactions (such as coordination, ionic, and/or hydrogen bonds (HBs)) and/or covalent interactions can determine intermolecular packing/linkage of the energetic fuel and oxidizer components, reticular chemistry provides a new platform evolving from single-molecular design to various energetic frameworks (E of the energetic frameworks with superior comprehensive properties. For example, to achieve coordination with metals or introduce sufficient hydrogen bond donor/acceptor structural units, the host structure of energetic framework materials usually contains less oxygen-rich substituents such as nitro, so the host molecules of the framework, F) at the crystal level, which can enhance the integrated stabilities of EFs.Along with growing concerns about the environment and safety issues, considerable effort has been devoted to pursuing environmentally friendly and insensitive energetic materials. The newly emerging EFs are conducive to introducing explosophores into a green chemical pathway. Benefiting from these cross-disciplinary achievements, taming metastable energetic molecules in specific porous frameworks is a green strategy to desensitize energetic materials while concomitantly retaining excellent energetic properties, which has become one of the most forward and promising investigations. In the past decade, EFs have achieved further results in stabilizing and greening energetic materials using HBs, covalent bonds, and alkaline earth metal-involving coordination bonds to avoid heavy metal toxicity and to employ halogen-free oxidizers. Because this field is still expanding rapidly, it is of great value for researchers and possible users of the work to be able to view all the progress.Through this Account, we intend that more readers will become knowledgeable about EFs, including their definition, history, synthesis, properties, and possible applications. The aim of this Account is to present the latest advances in EFs in recent years and to offer a perspective on the future direction of this field.

Tuning Green Explosives through SNAr Chemistry

Reactivity and regioselectivity of SNAr-type fluorine substitution with azide in polyfluorosubstituted nitrobenzenes was studied both theoretically and experimentally. The obtained polyazido-substituted nitrobenzene derivatives were extensively characterized by NMR, IR, HPLC, X-ray, and DFT methods. It was found that the substitution with the azide nucleophile occurs first at the para- and the ortho-positions to the NO2-group and that transazidation reactions also occur here. Thermal decomposition of prepared azidonitrobenzenes was studied both in controlled (kinetic decay) and uncontrolled (explosion) modes. In case of the controlled thermal decomposition of ortho-azidonitrobenzenes, benzofuroxans were found as major products of the reaction unless another azido group was adjacent to the furoxan moiety. The bursting power of azidonitrobenzenes was found to rise gradually with the number of the azide substituents in the aromatic ring.

Construction of Adaptive Deformation Block: Rational Molecular Editing of the N-Rich Host Molecule to Remove Water from the Energetic Hydrogen-Bonded Organic Frameworks

Energetic hydrogen-bonded organic frameworks (E-HOFs), as a type of energetic material, spark fresh vitality to the creation of high energy density materials (HEDMs). However, E-HOFs containing cations and anions face challenges such as reduced energy density due to the inclusion of crystal water. In this work, the modification of amino groups in N-rich organic units could form a smart building block of hydrogen-bonded frameworks capable of changing the volume of the void space in the molecule through adaptive deformation of E-MOF blocks, thus enabling the replacement of water. Based on the above strategy, we report an interesting example of a series of hydrogen-bonded organic frameworks (E-HOF 2a and 3a) synthesized using a facile method. The crystal structure data of all of the compounds were also obtained in this work. Anhydrous 2a and 3a exhibit higher density, good thermal stability, and low mechanical sensitivity. The strategy of covalent bond modification for the host molecules of energetic frameworks shows enormous potential in eliminating the crystalline H2O of hydration and exploring high energy density materials.

Size-matched hydrogen bonded hydroxylammonium frameworks for regulation of energetic materials

Size matching molecular design utilizing host-guest chemistry is a general, promising strategy for seeking new functional materials. With the growing trend of multidisciplinary investigations, taming the metastable high-energy guest moiety in well-matched frameworks is a new pathway leading to innovative energetic materials. Presented is a selective encapsulation in hydrogen-bonded hydroxylammonium frameworks (HHF) by screening different sized nitrogen-rich azoles. The size-match between a sensitive high-energy guest and an HHF not only gives rise to higher energetic performance by dense packing, but also reinforces the layer-by-layer structure which can stabilize the resulting materials towards external mechanic stimuli. Preliminary assessment based on calculated detonation properties and mechanical sensitivity indicates that HHF competed well with the energetic performance and molecular stability (detonation velocity = 9286 m s-1, impact sensitivity = 50 J). This work highlights the size-matched phenomenon of HHF and may serve as an alternative strategy for exploring next generation advanced energetic materials.

Hypergolic zeolitic imidazolate frameworks (ZIFs) as next-generation solid fuels: Unlocking the latent energetic behavior of ZIFs

Hypergolic materials, capable of spontaneous ignition upon contact with an external oxidizer, are of critical importance as fuels and propellants in aerospace applications (e.g., rockets and spacecraft). Currently used hypergolic fuels are highly energetic, toxic, and carcinogenic hydrazine derivatives, inspiring the search for cleaner and safer hypergols. Here, we demonstrate the first strategy to design hypergolic behavior within a metal-organic framework (MOF) platform, by using simple "trigger" functionalities to unlock the latent and generally not recognized energetic properties of zeolitic imidazolate frameworks, a popular class of MOFs. The herein presented six hypergolic MOFs, based on zinc, cobalt, and cadmium, illustrate a uniquely modular platform to develop hypergols free of highly energetic or carcinogenic components, in which varying the metal and linker components enables the modulation of ignition and combustion properties, resulting in excellent hypergolic response evident by ultrashort ignition delays as low as 2 ms.

Salt loading in MOFs: solvent-free and solvent-assisted loading of NH4NO3 and LiNO3 in UiO-66

The salts NH₄NO₃ and LiNO₃ were loaded in the MOF UiO-66 by a solvent-free and solvent-assisted method, respectively.

Adsorption of tetranitromethane in zeolitic imidazolate frameworks yields energetic materials

Absorption of tetranitromethane in the zeolitic imidazolate frameworks ZIF-8 and ZIF-70 is a facile route to borderline primary/secondary explosives that contain no toxic heavy metals.

Optimizing the oxygen balance by changing the A-site cations in molecular perovskite high-energetic materials

Two new members of molecular perovskite high-energetic materials exhibit optimized oxygen balances by changing the A-site cations.

Resolving synthetic challenges faced in the syntheses of asymmetric N,N′-ethylene-bridged energetic compounds

Synthetic challenges faced during the syntheses of asymmetric N,N′-ethylene-bridged energetic compounds due to the differences in the reactivity and stability of various types of energetic rings are addressed.