5-aminotetrazole is one of the most marked high-nitrogen tetrazole compounds. However, the structural modification of 5-aminotetrazole with nitro groups often leads to dramatically decreased molecular stability, while the N-bridging functionalization does not efficiently improve the density and performance. In this paper, we report on a straightforward approach for improving the density of 5-aminotetrazole by introducing 4-amino-3,5-dinitropyrazole. The following experimental and calculated properties show that nitropyrazole functionalization competes well with energetic performance and mechanic sensitivity. All compounds were thoroughly characterized using IR and NMR spectroscopy, elemental analysis, and differential scanning calorimetry (DSC). Two energetic compounds ( DMPT-1 and DMPT-2 ) were further confirmed by implementing single-crystal X-ray diffraction studies. Compound DMPT-1 featured a high crystal density of 1.806 g cm &#;3 , excellent detonation velocity (v D = m s &#;1 ), detonation pressure (P = 30.2 GPa), and impact sensitivity of 30 J.
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High energy density materials (HEDMs) remain a significant class of materials chemistry because energetic material has been widely applied in the military and civilian fields [1]. However, with the development of science and technology, higher requirements are put forward for new energetic materials, including high energy levels, low sensitivities, excellent thermal stability, and facile preparation. High positive heats of formation and densities constitute two significant parameters for the energetic properties of energetic compounds, where detonation pressures and velocities are proportional to the square of the densities [2]. Thus, developing effective strategies for increasing the densities of energetic materials is highly desirable.
In recent years, nitrogen-rich heterocycles have emerged as a new class of high energy density materials (HEDMs), which have been developed to meet the needs of national defense and environmental protection. Owing to their high thermal stability, high heat of formation, and enormous ring tension of azoles with C-N and N-N bonds, they have been the main building blocks for the design and preparation of nitrogen-rich materials [3,4,5,6]. Compared with other azole units, tetrazole is a highly stable heterocyclic structure with an extremely high nitrogen content and heat of formation, which may endow tetrazole-based energetic compounds with high performance and environmentally friendly properties [7,8,9,10,11].
Furthermore, 5-aminotetrazole (AT) provides a platform for a variety of functionalized tetrazoles, which have broad applications in almost all areas of energetic materials [12]. The functionalization of AT was mainly focused on modifying the tetrazolyl backbone and the substituted group, respectively. As can be seen in Scheme 1, the introduction of explosophores, e.g., azido, nitro, and nitramino groups, enhances the energetic performance remarkably. Nevertheless, balancing high energy with stability is still a highly challenging task. Based on the literature, very few outstanding compounds possess superior overall performance compared to the benchmark energetic materials [13,14,15]. For most high-energy derivatives, further practical applications are impeded by relatively poor thermal stability and sensitivity.
Energetic derivatives of AT from modifying the backbone and functionalized group.
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Inspired by the attractive chemistry of high-nitrogen materials, a synthetic method for efficiently preparing 1-substituted 5-aminotetrazoles from cyanogen azide and amine was developed (Scheme 2a) [16]. The excellent reaction scope enables this approach to access various monocyclic and bicyclic AT derivatives [17]. In , a new approach for various pyrazole derivatives with 2-haloethylamines, followed by a reaction with cyanogen azide, resulted in ethylene-bridged 5-aminotetrazole and nitropyrazole (Scheme 2b) [18]. Most of these compounds have good thermal stability and high heat of formation, however, the density of aminotetrazole-based derivatives could meet the standard of high-density compounds. As shown in Scheme 2d, most AT-based compounds display a density below 1.70 g cm&#;3.
(a) Monocyclic derivatives of 5-aminotetrazoles (A9: determined by X-ray diffraction and converted into room-temperature values); (b) alkyl-bridged bicyclic 5-aminotetrazoles; (c) other bridged bicyclic 5-aminotetrazoles; (d) bar graph of density and impact sensitivity for AT-based energetic compounds.
Five-membered azoles are commonly used as the framework for the construction of nitrogen-rich heterocyclic energetic materials, such as imidazole, pyrazole, triazole, and tetrazole [19,20,21,22]. Pyrazole energetic compounds have good thermal stabilities and low sensitivities, which have been considered a beneficial building block for energetic materials [23,24]. For example, 4-amino-3,5-dinitropyrazole (ADNP) has been widely used in developing energetic materials due to their good density, low friction and impact sensitivities [25,26,27]. Herein, we reported our latest progress on the energetic derivatives of AT. By incorporating ADNP, the bicyclic AT-based compounds DMPT-1 and DMPT-2 are successfully accessed. Compared to previously reported AT derivatives, these compounds exhibit higher densities and detonation properties while retaining thermal stability and sensitivity.
5-Aminotetrazole monohydrate is a useful reagent in condensation reactions to form pyrimidines.
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