High-density materials have attracted extensive attention because of their broad applications. However, strategies for improving the densities of MOFs and preparing denser MOFs remain almost unexplored. Herein, we propose a tandem anion-ligand exchange strategy for synthesizing denser MOFs by using three-dimensional cationic MOFs (3D CMOFs) with pillared layered structures as precursors and high-density anions and small monotopic ligands as exogenous guests. By means of this strategy, we choose the high-density nitroformate ion [C(NO)] as an exogenous anion and water as an exogenous ligand to successfully synthesize two layered CMOFs. Single-crystal X-ray diffraction showed that after this transformation, the extra-framework anions are replaced with the C(NO) anions, and the distances between adjacent layers in the two-dimensional (2D) networks are more than 3.70 Å shorter than those of their 3D precursors. The resultant materials exhibit higher densities, higher heats of detonation, higher nitrogen and oxygen contents, and lower metal contents. In particular, the density of {Cu(atrz)[C(NO)](HO)·atrz·2HO} (2b, ρ = 1.76 g cm, atrz = 4,4'-azo-1,2,4-triazole) is increased by 0.12 g cm compared to its 3D precursor {2a, [Cu(atrz)(NO)·2HO], ρ = 1.64 g cm}, and its heat of detonation is also enhanced to more than 1900 kJ kg. The resultant 2D layered CMOFs are also new potential high-energy density materials. This work may provide new insights into the design and synthesis of high-density MOFs. Moreover, we anticipate that the approach reported here would be useful for the preparation of new MOFs, in particular, which are otherwise difficult or unfeasible through traditional synthetic routes.

The investigation of high-nitrogen compounds has been significant for the evolution of energetic materials. Azo-bis-1,2,4-triazole (aTRz) can be an excellent energetic backbone, owing to its characteristics: high heat of formation, high nitrogen content, and plane structure. Nevertheless, aTRz-based energetic compounds have been rarely synthesized using the covalent modification method, owning to the decomposition of aTRz under harsh reaction conditions. Cocrystallization has been widely used as a mild and efficient method for modulating the properties of energetic compounds. In this study, electrostatic potential (ESP) maps were used for theoretical guidance, and four aTRz-based energetic cocrystals have been obtained via cocrystallization. The single-crystal structures of these cocrystals indicated that the N···H–N hydrogen bonds between the side nitrogen atoms of aTRz and the amino groups of the nitro azole compounds were the driving force for the assembly of multimers with aTRz and polynitroazole compounds. Consequently, the formation of cocrystals via the self-assembly of these multimers was driven by other weak hydrogen bonds and van der Waals forces. The detonation performance of aTRz-based cocrystals was increased by appropriately selecting the coformers. Particularly, when 4-amino-3,5-dinitro-pyrazole (ADNP) was used as coformer, resultant cocrystal 3 was a potential high-energy density material that exhibited high density, high detonation velocity (8329 m s–1) and detonation pressure (28.6 GPa). Thus, in this study, cocrystallization has been demonstrated to be an effective method for the noncovalent modification of aTRz-based energetic materials.

High-energy metal–organic frameworks (MOFs) based on nitrogen-rich ligands are an emerging class of explosives, and density is one of the positive factors that can influence the performance of energetic materials. Thus, it is important to design and synthesize high-density energetic MOFs. In the present work, hydrothermal reactions of Cu(II) with the rigid polynitro heterocyclic ligands 5,5′-dinitro-2H,2′H-3,3′-bi-1,2,4-triazole (DNBT) and 5,5′-dinitro-3,3′-bis-1,2,4-triazole-1-diol (DNBTO) gave two high-density MOFs: [Cu(DNBT)(ATRZ)3]n (1) and [Cu(DNBTO)(ATRZ)2(H2O)2]n (2), where ATRZ represents 4,4′-azo-1,2,4-triazole. The structures were characterized by infrared spectroscopy, elemental analysis, ultraviolet-visible (UV) absorption spectroscopy and single-crystal X-ray diffraction. Their thermal stabilities were also determined by thermogravimetric/differential scanning calorimetry analysis (TG/DSC). The results revealed that complex 1 has a two-dimensional porous framework that possesses the most stable chair conformations (like cyclohexane), whereas complex 2 has a one-dimensional polymeric structure. Compared with previously reported MOFs based on copper ions, the complexes have higher density (ρ = 1.93 g cm−3 for complex 1 and ρ = 1.96 g cm−3 for complex 2) and high thermal stability (decomposition temperatures of 323 °C for complex 1 and 333.3 °C for complex 2), especially because of the introduction of an N–O bond in complex 2. We anticipate that these two complexes would be potential high-energy density materials.

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Energetic metal organic frameworks (MOFs) with energetic anions as ligands can be used as new-generation explosives. Many powerful anions have been introduced into energetic MOFs to improve the properties; however, the hydroxyl as a common group for energetic MOFs has rarely been studied. In this article, we present two examples of energetic MOFs ([Cu(atz)(NO 3 )(OH)] n ) and [Zn(ata)(OH)] (atz = 4-amino-1,2,4-triazole; ata = 5-amino-1H-tetrazole) with the hydroxyl group as the ligand. Crystal structure analyses reveal that the two compounds possess compact two-dimensional (2-D) structures with densities up to 2.41 g cm À3 and 2.54 g cm À3 , respectively. These two compounds have excellent physicochemical properties. The results demonstrate that a hydroxyl group as the ligands could commendably increase the densities of energetic MOFs, thereby enhancing the detonation performance. It is anticipated this work will open a new direction for the development of energetic MOFs.

The modification of the properties of energetic materials is significant not only for gaining insight into the correlation between the structure and properties but also for various applications. In this study, three bis-1,2,4-triazole derivatives containing different energetic groups such as NH2, NO2, and NHNO2 (5,5′-diamino-3,3′-bis-1,2,4-triazole (H2DABT), 5,5′-dinitro-3,3′-bis-1,2,4-triazole (H2DNBT) and 5,5′-dinitroimino-3,3′-bis-1,2,4-triazole (H2DNABT)) were chosen as ligands to tune the structures and energetic properties of their complexes. Single-crystal X-ray diffraction studies show that as the modifying groups on the bis-1,2,4-triazole ligand were varied from NH2 to NO2 and NHNO2, the densities of the resultant complexes gradually increased from 1.838 g cm–3 to 1.978 g cm–3 and 2.049 g cm–3 for Cu-based complexes, while for Zn-based complexes, the densities also showed a gradual increase from 1.818 g cm–3 to 2.044 g cm–3 and 2.056 g cm–3. Additionally, the thermal stabilities and sensitivities of these complexes were also determined, and their detonation properties were calculated by the modified Kamlet method. As a result, the energetic performances of these resultant complexes were successfully modulated. This work may provide an effective approach to modulate the energetic properties of complexes via introduction of different energetic groups into the ligands.