N-Functionalization of 5-Aminotetrazoles: Balancing Energetic Performance and Molecular Stability by Introducing ADNP

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 (vD = 8610 m s−1), detonation pressure (P = 30.2 GPa), and impact sensitivity of 30 J.


Introduction
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. Scheme 1. Energetic derivatives of AT from modifying the backbone and functionalized group.
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 2016, 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 .
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.

Single-Crystal X-ray Analysis
The suitable crystals of compounds DMPT-1 and DMPT-2 were obtained by slow evaporation from a mixture of ethyl acetate and petroleum ether. Compound DMPT-1 crystallizes in the monoclinic space group P21/c with a crystal density of 1.806 g cm −3 at 296 K (Z = 4). The crystal structure of compound DMPT-1 is given in Figure 1a. The C-NO 2 groups with C-N bonds lengths are 1.394(3) Å (C12-N2), 1.434(3) Å (C7-N1); compared to the other C-N bonds of N,N'-methylene bridge C4-N6, 1.441(3) Å and C4-N5, 1.452(2) Å, the C-N bonds lengths are shorter. Crystal packing of DMPT-1 exhibits a face-to-face stacking with a packing index of 74.8%. The distance between the pyrazoles (purple part) and pyrazoles (purple part) faces is 2.991 Å. The distance between the tetrazoles (blue part) and tetrazoles (blue part) faces is 3.145 Å. The N-methylene-N bridge dihedral angle between tetrazoles (blue part) and pyrazoles (purple part) ring is 87.47 • for compound DMPT-1. Compound DMPT-2 belongs to the monoclinic P2 1 /n space group with a crystal density of 1.770 g cm −3 at 296 K. The crystal structure of compound DMPT-2 is given in Figure 2d

Results and Discussion
The thermostability of energetic materials is one of the most significant parameters in assessing their safety level under severe environments. The decomposition temperature was determined using differential scanning calorimetric (DSC) measurements at a heating rate of 10 • C min −1 under a dry nitrogen atmosphere. Compound DMPT-1 exhibits the lowest decomposition temperature of 191 • C, whereas DMPT-2 has a higher thermal stability of 206 • C. In general, for polynitro energetic compounds, the C-NO 2 bonds are always regarded as the weakest bonds, which can trigger the decomposition of the compound [29]. The Mayer bond orders of C-NO 2 bonds for DMPT-1 and DMPT-2 were also calculated to evaluate their molecular stabilities, as shown in Figure 2a,b. The weakest Mayer bond order of C-NO 2 bond for compound DMPT-1 (BN = 0.794) is shorter than that of DMPT-2 (BN = 0.797), showing that the bond strength of C-NO 2 bonds in DMPT-1 is stronger than that of DMPT-2, which could explain in part the difference in thermal stability. Generally, aromaticity is considered an important parameter for the determination of molecular stability [30,31]. Therefore, the aromaticity of each heterocycle atmosphere could also be quantitatively described through the multicenter bond orders analysis using Multiwfn3.5 [32]. As shown in Figure 2c,d, the multicenter bond order of the tetrazole ring in DMPT-1 (B1 = 0.5589) is shorter than that of DMPT-2 (B1 = 0.5702), which indicates that the tetrazole ring of DMPT-2 exhibits better aromaticity than that of DMPT-1, which is consistent with their thermal behaviors.
To better explain the bond strength differences between compounds DMPT-1 and DMPT-2, their electrostatic potential (ESP) surfaces were analyzed. As shown in Figure 3, the red and blue regions represent the positive and negative potentials, respectively. In comparison to DMPT-1, the difference between the maxima and minima of ESP for the branching DMPT-2 molecule is smaller (DMPT-1: +49.83, −39.23 kcal/mol; DMPT-2: +44.96, −30.59 kcal/mol), so that DMPT-2 exhibits a more uniform charge distribution. The C-NO 2 and tetrazole ring on DMPT-2 moiety exhibit minimum potential parts, which are much lower than the C-NO 2 and tetrazole ring on DMPT-1, thus indicating the distinct stability of regioisomers as may be related to the ESP values (DMPT-1, T d , 191 • C; FS, 108 N; DMPT-2, T d , 209 • C; FS, 192N) [33]. In principle, the lower negative ESP values and more significant electronegative regions (blue areas) on the molecular surface often give rise to lower sensitivities, which rationalize the different mechanical sensitivities. To gain insight into understanding the relationship between weak interactions and the performance in energetic materials, the two-dimensional (2D) fingerprint spectra and Hirshfeld surfaces of DMPT-1 and DMPT-2 were studied using CrystalExplorer [34]. Generally, the blue and red regions on the Hirshfeld surfaces represent low and high close contact populations, respectively. As shown in Figure 4, The Hirshfeld surfaces of DMPT-1 were near "L"-shaped structures, and the Hirshfeld surfaces of DMPT-2 were slightly distorted. In comparison to DMPT-1, the reddest spots are more evenly distributed for the DMPT-2 molecule, showing that compound DMPT-2 can form hydrogen bonds with more surrounding molecules, which suggests that the sensitivity of compound DMPT-2 (FS = 192 N) is better than that of compound DMPT-1 (FS = 108 N). At the same time, the weak interactions in these molecules are given in the 2D fingerprint plots.

Safety Precaution
In this work, compounds DMPT-1 and DMPT-2 are potential energetic materials that tend to explode under certain external stimuli. Therefore, the whole experimental process should be carried out using proper safety equipment, such as safety shields, eye protection, and leather gloves.

General Methods
All of the reactions were carried out in the air. Ammonium 4-amino-3,5-dinitropyrazolate monohydrate [37] was prepared following procedures found in the literature. Other commercial reagents and solvents were obtained from commercial providers and used without further purification. 1 H NMR and 13 C NMR spectra were recorded at 25 • C on a Bruker 400 MHz and 125 MHz, respectively, and TMS as the internal standard. Chemical shifts were reported in parts per million (ppm). The onset decomposition temperature was measured using a TA Instruments DSC25 differential scanning calorimeter at a heating rate of 10 • C min −1 under a dry nitrogen atmosphere. Infrared spectra (IR) were obtained on a PerkinElmer Spectrum BX FT-IR instrument equipped with an ATR unit at 25 • C. Elemental analyses of C/H/N were investigated on a Thermo Scientific Flash 2000 Elemental Analyzer. A BAM fallhammer and friction tester tested impact and friction sensitivities. Densities were determined at room temperature by employing a Micromeritics AccuPyc 1340 gas pycnometer. The crystal structures were produced employing Mercury 2021.1.0 software.

Computational Details
The heats of formation of compounds DMPT-1 and DMPT-2 were performed by using the Gaussian 09 suite of programs [38,39]. Gas phase heats of formation of the title compounds were computed based on an isodesmic reaction (Scheme S1). The enthalpy of reaction was carried out by combining the M062X/6-311++G** energy dif-ference for the reactions, the scaled zero-point energies (ZPE), values of thermal cor-rection (HT), and other thermal factors. The solid state heats of formation were further obtained by employing Trouton's rule according to Equation (1) (T represents either melting point or decomposition temperature when no melting occurs prior to decom-position) [40] (Scheme S1).

Conclusions
In summary, N,N-methylene bridged 5-aminotetrazole and pyrazole were synthesized according to the methylene bridge strategy, using 5-aminotetrazole as a skeleton. A simple synthetic route, including the reaction of ammonium 4-amino-3,5-dinitropyrazolate with chloroiodomethane, followed by a reaction with 5-aminotetrazoles, was proposed to form two high-density compounds, DMPT-1 and DMPT-2. The two compounds were fully characterized using IR and NMR spectroscopic data and elemental analysis. Compared to similar N,N -ethylene-bridged asymmetric compounds, the two methylene-bridged asymmetric compounds possess higher density and enhanced detonation performance. Among them, DMPT-1 possesses a promising overall performance (d = 1.806 g cm −3 , v D = 8610 m s −1 ; P = 30.2 GPa). The detailed comparative properties of methylene bridged isomers indicate the pivotal role the bridging strategy plays in the energetic molecular design of novel energetic materials.