Polyvalent Ionic Energetic Salts Based on 4-Amino-3-hydrazino-5-methyl-1,2,4-triazole

The synthesis of the new energetic material 4-amino-3-hydrazino-5-methyl-1,2,4-triazole, which shows excellent performance and reliable safety, has drawn attention recently. To fully characterize this material, a comprehensive analysis was performed using various techniques, including differential scanning calorimetry (DSC), infrared spectroscopy (IR), elemental analysis, and 1H and 13C NMR spectroscopy. Additionally, three compounds, 3, 5 and 9, were further characterized using single X-ray diffraction. The X-ray data suggested that extensive hydrogen bonds affect molecular structure by means of intermolecular interactions. In order to evaluate the explosive properties of these synthesized compounds, detonation pressures and velocities were calculated using EXPLO5 (V6.01). These calculations were carried out utilizing experimental data, including density and heat of formation. Among the explosives tested, compounds 7 and 8 exhibited zero oxygen balance and demonstrated exceptional detonation properties. Compound 7 achieved the highest recorded detonation pressure, at 34.2 GPa, while compound 8 displayed the highest detonation velocity, at 8887 m s−1.


Introduction
The synthesis and design of new energetic materials has been ongoing since the discovery of 2,4,6-trinitrotoluene (TNT) in 1863 [1]. Accompanying their growing demand in military and civilian applications, researchers have made significant progress by developing compounds such as Hexogeon (RDX) [2], Cyclotetramethylene tetranitramine (HMX) [2], and Hexanitrohexaazaisowurtzitane (CL-20) [3]. However, these explosives still face several limitations that hinder their widespread use. In recent decades, many researchers have directed their efforts towards the development of nitrogen-rich energetic materials. Because of the existence of large numbers of C-N and N-N bonds, nitrogen-rich energetic materials have a higher heat of formation [4]. Molecular structures with high nitrogen and low carbon content can enable energetic materials to have a higher density, which is also beneficial for achieving oxygen balance [5]. Triazole is the representative of high nitrogen-containing compounds and increasingly attracts considerable interest. In energetic skeletons such as those of imidazole [6], triazole [7], tetrazole [8], triazine [9], and tetrazine [10], 1,2,4-triazole shows unique advantages.
In order to achieve a balance between detonation performance and safety in energetic materials, researchers have employed various methods, including the synthesis of dense ring compounds, the development of eutectic materials, and the preparation of energetic ionic salts. In recent years, energetic salts have drawn widespread attention as a new type of energetic material [11]. Compared to similar molecular compounds, these synthetic products possess more advantages in terms of their basic properties. Energetic salts composed of anions and cations exhibit the characteristics of ionic compounds. These include a low vapor pressure, excellent thermal stability, a high density, and minimal environmental impacts, therefore representing extensive application potential [12]. The studies available in the literature have revealed that the incorporation of methyl groups into the molecular structure of these salts can prove effective in reducing sensitivity and enhancing safety, as well as elevating the decomposition temperature of energetic materials [13,14].
As a nitrogen-rich compound, 4-amino-3-hydrazino-5-methyl-1,2,4-triazole, which brings methyl and 1,2,4-triazole into a single material, has multiple modification sites. This compound can be chemically modified with different groups, leading to enhanced physical and chemical properties when incorporated into ionic salts. Furthermore, the triazole framework allows for the selection of various anions, including those containing nitro groups, which can improve oxygen balance. In contrast to the precursors reported in the literature, which usually have one monovalent anion, the 4-amino-3-hydrazino-5-methyl-1,2,4-triazole described in this article serves as a precursor for polyvalent ion salts, and can be paired with multiple-ion ligands. Polyvalent ions have higher formation heat compared to those of monovalent ion salts with a similar structure. Therefore, the polyvalent state of this invention is related to significant detonation. Taking into consideration the aforementioned advantages, energetic salts with good explosive performances based on 3-hydrazino-4-amino-5-methyl-1,2,4-triazole are reported and comprehensively characterized.

X-ray Crystallography
The crystal structure of compound 3 reveals that it belongs to the space group Pbca, with a crystal density of 1.435 g cm −3 at 296 K. It can be seen from Figure 1a that one unit of compound 3 consists of two chloridions and one hetercocylic cation, with two protonating hydrogen atoms on N1 and N5. There is one intramolecular interaction between the cation and chloridion (N1-H1A. . .Cl1), with a distance of 3.066(2) Å and an angle of 177(3) • . Notably, it can surprisingly be found that all of the atoms except the H of the cation are in nearly the same plane, which can be verified by the torsion angles N6-N3-C1-N4 = 5.866 • , C3-C2-N3-C1 = 178.976 • , and N3-C1-N2-N1 = −0.149 • . However, there is a slight deviation in the torsion angle of the hydrazine moiety, with the angle of N5-N4-C1-N3 being −165.121 • . In terms of bond lengths, the distance of N3-NH 2 (1.4013 Å) is shorter than that of N4-NH 2 (1.4145 Å), and the distance of C2-CH 3 is the longest among those of all of the bonds in compound 3, at 1.4639 Å. Additionally, the packing diagram of compound 3 is shown in Figure 1b, from which it can be found that the stacked structures are symmetrical and the different molecules are connected by multiple hydrogen bonds (N4-H4A. . .Cl2, N5-H5A. . .Cl2, N5-H5B. . .Cl1, N5-H5C. . .Cl1, N6-H6A. . .Cl1, N6-H6B. . .Cl2, and N6-H6C. . .Cl2) to form a stable network structure [15].
composed of anions and cations exhibit the characteristics of ionic compounds. These include a low vapor pressure, excellent thermal stability, a high density, and minimal environmental impacts, therefore representing extensive application potential [12]. The studies available in the literature have revealed that the incorporation of methyl groups into the molecular structure of these salts can prove effective in reducing sensitivity and enhancing safety, as well as elevating the decomposition temperature of energetic materials [13,14].
As a nitrogen-rich compound, 4-amino-3-hydrazino-5-methyl-1,2,4-triazole, which brings methyl and 1,2,4-triazole into a single material, has multiple modification sites. This compound can be chemically modified with different groups, leading to enhanced physical and chemical properties when incorporated into ionic salts. Furthermore, the triazole framework allows for the selection of various anions, including those containing nitro groups, which can improve oxygen balance. In contrast to the precursors reported in the literature, which usually have one monovalent anion, the 4-amino-3-hydrazino-5-methyl-1,2,4-triazole described in this article serves as a precursor for polyvalent ion salts, and can be paired with multiple-ion ligands. Polyvalent ions have higher formation heat compared to those of monovalent ion salts with a similar structure. Therefore, the polyvalent state of this invention is related to significant detonation. Taking into consideration the aforementioned advantages, energetic salts with good explosive performances based on 3-hydrazino-4-amino-5-methyl-1,2,4-triazole are reported and comprehensively characterized.

Physicochemical and Energetic Properties
The thermal stabilities of the above energetic salts were determined using DSC measurements with a scanning rate of 5 • C min −1 . As shown in Table 1, most of new materials decompose at temperatures below 200 • C varying in the range between 126.8 and 213.7 • C. Among these energetic salts, compound 3 exhibits the highest thermal stability, with a decomposition temperature of 213.7 • C. Compounds 5 and 9 also show high thermal decomposition temperatures of 145.7 • C and 173.4 • C, respectively. The observed high thermal stability can be attributed to the extensive hydrogen bonding interactions between the amino groups of the cation and the oxygen atoms of the anion within these compounds. These strong intermolecular interactions contribute to the overall stability of the materials.

Physicochemical and Energetic Properties
The thermal stabilities of the above energetic salts were determined using DSC measurements with a scanning rate of 5 °C min −1 . As shown in Table 1, most of new materials decompose at temperatures below 200 °C varying in the range between 126.8 and 213.7 °C. Among these energetic salts, compound 3 exhibits the highest thermal stability, with a decomposition temperature of 213.7 °C. Compounds 5 and 9 also show high thermal decomposition temperatures of 145.7 °C and 173.4 °C, respectively. The observed high thermal stability can be attributed to the extensive hydrogen bonding interactions between the amino groups of the cation and the oxygen atoms of the anion within these compounds. These strong intermolecular interactions contribute to the overall stability of the materials.
Density is one of the most important evaluation parameters used to measure the performance of energetic materials, because it has a huge effect on their detonation performances [16]. The densities of these salts were measured using Automatic Density Analyzer. The densities of the synthesized salts 3-11 are in the range of 1.435-1.989 g cm −3 , that most of them are higher than that of TNT (1.65 g cm −3 ).
Oxygen balance is a measure of the degree of oxidation in an explosive. Currently, most common explosives have a negative oxygen balance, which means they do not have enough oxidizing agents to completely oxidize the combustible elements. As a result, their detonation performances do not reach their maximum potential [17]. The CO oxygen balance of the energetic salts that were synthesized are listed in Table 1. The oxygen balance of salts 6 (−12.6%), 7 (0%), 8 (0%) and 10 (−16.4%) are much better than that of TNT (−24.7%) and RDX (−21.6%). Compound 7 and 8 are the best in these salts, where the oxygen balance approaches zero, which means that the combustible element of the energetic materials 7 and 8 can be completely oxidized and release maximum heat.
The impact sensitivities of the synthesized energetic salts were measured by using a BAM drop hammer [18]. Compound 3 exhibited excellent sensitivity parameters, with an impact sensitivity (IS) exceeding 40 J, meaning it can be classified as an insensitive energetic material. On the other hand, 5-nitrotetrazolate-2N-oxide salt 10 demonstrated high sensitivity to impact, with a recorded value of 6.1 J. In comparison to RDX, it was found that all the salts, except for 5-nitrotetrazolate salt 9 and 5-nitrotetrazolate-2N-oxide salt 10, displayed lower sensitivities. This suggests that the majority of the synthesized salts possess reduced sensitivity to impact when compared to that of RDX, which is a desirable characteristic in terms of safety and stability. EXPLO5 V6.01 was used to calculate the detonation velocities and pressures with the data of heats of formation, formula and density [19,20].   Density is one of the most important evaluation parameters used to measure the performance of energetic materials, because it has a huge effect on their detonation performances [16]. The densities of these salts were measured using Automatic Density Analyzer. The densities of the synthesized salts 3-11 are in the range of 1.435-1.989 g cm −3 , that most of them are higher than that of TNT (1.65 g cm −3 ).
Oxygen balance is a measure of the degree of oxidation in an explosive. Currently, most common explosives have a negative oxygen balance, which means they do not have enough oxidizing agents to completely oxidize the combustible elements. As a result, their detonation performances do not reach their maximum potential [17]. The CO oxygen balance of the energetic salts that were synthesized are listed in Table 1. The oxygen balance of salts 6 (−12.6%), 7 (0%), 8 (0%) and 10 (−16.4%) are much better than that of TNT (−24.7%) and RDX (−21.6%). Compound 7 and 8 are the best in these salts, where the oxygen balance approaches zero, which means that the combustible element of the energetic materials 7 and 8 can be completely oxidized and release maximum heat.
The impact sensitivities of the synthesized energetic salts were measured by using a BAM drop hammer [18]. Compound 3 exhibited excellent sensitivity parameters, with an impact sensitivity (IS) exceeding 40 J, meaning it can be classified as an insensitive energetic material. On the other hand, 5-nitrotetrazolate-2N-oxide salt 10 demonstrated high sensitivity to impact, with a recorded value of 6.1 J. In comparison to RDX, it was found that all the salts, except for 5-nitrotetrazolate salt 9 and 5-nitrotetrazolate-2N-oxide salt 10, displayed lower sensitivities. This suggests that the majority of the synthesized salts possess reduced sensitivity to impact when compared to that of RDX, which is a desirable characteristic in terms of safety and stability. EXPLO5 V6.01 was used to calculate the detonation velocities and pressures with the data of heats of formation, formula and density [19,20]. The detonation velocities fall in the range of 6600-8887 m s −1 and the detonation pressures are in the range of 17.0-34.2 GPa. These values indicate significantly improved performance compared to that of TNT (6881 m s −1 , 19.5 GPa) with the exception of compound 3. The oxygen-balanced explosives 7 and 8 show the best performance; 7 has the highest detonation pressure (34.2 GPa) and 8 has the highest detonation velocity (8887 m s −1 ), which is comparable to that of RDX.

2D Fingerprints and Hirshfeld Surface Analysis
To investigate the relationship between the molecular structure of the compounds and their sensitivity, Hirschfeld surfaces and 2D fingerprint plots are used to analyze the intraand intermolecular interactions of compounds 5 and 9 [21,22]. There are some red dots observed in the Hirshfeld surfaces that can be seen from Figure 4a

Noncovalent Interaction (NCI) Analysis
The noncovalent interactions result of the compounds 3, 5 and 9 are presented in Figure 5 [23][24][25]. In the figure, we can see that there are three different colors; blue represents a strong interaction, green represents a weak interaction and red represents strong repulsion. It is easy to see the blue round ellipsoids presented in Figure 5a-c which mean that 3, 5 and 9 have the hydrogen bonds, and which is consistent with the results of the 2D fingerprints and Hirshfeld surface analysis. Additionally, we find that there are many green planes in the figure, that means the compounds have π-π interactions. Notably, we find that there are more green planes representing π-π accumulations in Figure 5c than in Figure 5b, which corresponds with the results of the percentage contribution of the single atom making contact with the Hirshfeld surfaces in Figure 4c. In addition, we find a few red ellipsoids near 1,2,4-triazole and C-CH 3 .

Electrostatic Potential Surface (ESP) Analysis
To gain further insights into the relationship between different substituents and molecular sensitivity, the electrostatic potential surface analysis (ESP) of compounds 3, 5 and 9 were conducted [24][25][26]. In ESP analysis, the minimum and maximum values are represented by blue and red points, respectively. The red and blue areas indicate electropositive and electronegative regions. As shown in the following Figure 6a, the minimum value (−77.98 kcal/mol) of 3 arises from the atom of Cl and the maximum value (95.77 kcal/mol) of 3 arises from hydrazine. Moving on to compound 5, we can find that the minimum value (−96.81 kcal/mol) is attributed to the nitrate group and the maximum value of it arises from triazole which is 88.30 kcal/mol. We find that the electrostatic value of C=O in the cation is negative at −26.69 kcal/mol, as shown in Figure 6b. In contrast, the values for compound 9 are much smaller than those of compounds 3 and 5, with a minimum value of −4.43 kcal/mol and a maximum value of 5.57 kcal/mol. As commonly known, the lower the maximum ESP values and the greater the negative charge accumulation in the nitro-substituent, the more stable the compound.

Noncovalent Interaction (NCI) Analysis
The noncovalent interactions result of the compounds 3, 5 and 9 are presented in Figure 5 [23][24][25]. In the figure, we can see that there are three different colors; blue represents a strong interaction, green represents a weak interaction and red represents strong repulsion. It is easy to see the blue round ellipsoids presented in Figure 5a-c which mean that 3, 5 and 9 have the hydrogen bonds, and which is consistent with the results of the 2D fingerprints and Hirshfeld surface analysis. Additionally, we find that there are many green planes in the figure, that means the compounds have π-π interactions. Notably, we find that there are more green planes representing π-π accumulations in Figure 5c than in Figure 5b, which corresponds with the results of the percentage contribution of the single atom making contact with the Hirshfeld surfaces in Figure 4c. In addition, we find a few red ellipsoids near 1,2,4-triazole and C-CH3.

Electrostatic Potential Surface (ESP) Analysis
To gain further insights into the relationship between different substituents and molecular sensitivity, the electrostatic potential surface analysis (ESP) of compounds 3, 5 and value (−96.81 kcal/mol) is attributed to the nitrate group and the maximum value of it arises from triazole which is 88.30 kcal/mol. We find that the electrostatic value of C=O in the cation is negative at −26.69 kcal/mol, as shown in Figure 6b. In contrast, the values for compound 9 are much smaller than those of compounds 3 and 5, with a minimum value of −4.43 kcal/mol and a maximum value of 5.57 kcal/mol. As commonly known, the lower the maximum ESP values and the greater the negative charge accumulation in the nitrosubstituent, the more stable the compound.

Materials and Methods
The synthesis route for 4-amino-3-hydrazino-5-methyl-1,2,4-triazole and its salts, those described in this study, is illustrated in Scheme 1. Guanidine hydrochloride was added into 3 equivalents of hydrazine hydrate, refluxing at 110 • C for 2 h, yielding triaminoguanidine hydrochloride 1 (TAG·HCl). After reacting with acetic acid, the solvent was removed and produced a viscous, resinous residue. The reaction mixture was then treated with acetic acid, and the solvent was evaporated to yield a viscous, resinous residue. From this residue, a moderate yield of 4-acetamido-3-acethydrazino-5-methyl-1,2,4-triazole hydrochloride 2 was isolated. added into 3 equivalents of hydrazine hydrate, refluxing at 110 °C for 2 h, yielding triaminoguanidine hydrochloride 1 (TAG·HCl). After reacting with acetic acid, the solvent was removed and produced a viscous, resinous residue. The reaction mixture was then treated with acetic acid, and the solvent was evaporated to yield a viscous, resinous residue. From this residue, a moderate yield of 4-acetamido-3-acethydrazino-5-methyl-1,2,4triazole hydrochloride 2 was isolated.
As depicted in Scheme 1, the combination of the nitrogen-rich divalent cation (AHMT 2+ ) with nitrogen-rich anions forms a class of energetic materials, whose energy is derived from their positive heats of formation as well as the combustion of the carbon atoms. Compounds 6−11 were readily synthesized via anion exchange with silver nitrate, silver perchlorate, silver dinitramide, silver 5-nitrotetrazolate, silver 5-nitrotetrazolate-2N-oxide, and silver 5-nitroiminotetrazolate, respectively, with 3 in methanol or water. 4amino-3-hydrazino-5-methyl-1,2,4-triazolium 4 was isolated by treating 3 with stoichiometric amounts of NaHCO3. Additionally, salt 6 could be synthesized by refluxing TAG·HNO3 in acetic acid, followed by hydrolysis with dilute nitric acid.

Experimental Section
The instruments and setting parameters used in the preparation and analysis are available in the general methods included in the Supporting Information.

Caution
Although we did not meet any danger in the process of these experiments, some necessary safeguard procedures were carried out such as the use of a hood, a pair of goggle and leather gloves. The risk of making contact with any of these materials directly should be avoided. Due to the positive heats of formation, extreme caution should be exercised in the performance of synthesis and characterization. As depicted in Scheme 1, the combination of the nitrogen-rich divalent cation (AHMT 2+ ) with nitrogen-rich anions forms a class of energetic materials, whose energy is derived from their positive heats of formation as well as the combustion of the carbon atoms. Compounds 6-11 were readily synthesized via anion exchange with silver nitrate, silver perchlorate, silver dinitramide, silver 5-nitrotetrazolate, silver 5-nitrotetrazolate-2N-oxide, and silver 5-nitroiminotetrazolate, respectively, with 3 in methanol or water. 4-amino-3-hydrazino-5-methyl-1,2,4-triazolium 4 was isolated by treating 3 with stoichiometric amounts of NaHCO 3 . Additionally, salt 6 could be synthesized by refluxing TAG·HNO 3 in acetic acid, followed by hydrolysis with dilute nitric acid.

Experimental Section
The instruments and setting parameters used in the preparation and analysis are available in the general methods included in the Supporting Information.

Caution
Although we did not meet any danger in the process of these experiments, some necessary safeguard procedures were carried out such as the use of a hood, a pair of goggle and leather gloves. The risk of making contact with any of these materials directly should be avoided. Due to the positive heats of formation, extreme caution should be exercised in the performance of synthesis and characterization.

Conclusions
In this study, we successfully synthesized a new precursor, 4-amino-3-hydrazino-5methyl-1,2,4-triazolium hydrochloride 3 (AHMT·2HCl). The nitrogen-rich compound 3 was synthesized via the reaction of triaminoguanidine hydrochloride and acetic acid and then hydrolyzed with diluted hydrochloric acid. Via Brønsted acid-base reactions, a series of energetic salts based on compound 3 were synthesized and fully characterized. The structures of 3, 5 and 9 were confirmed via X-ray single-crystal diffraction. Cations can be either polyvalent, monovalent or bivalent. Among all energetic salts based on 4-amino-3hydrazino-5-methyl-1,2,4-triazole, compound 3 shows the best stability, and dinitramide salt 8 shows excellent properties with a detonation pressure of 33.9 GPa, a detonation velocity of 8887 m s −1 and an optimal oxygen-balance state. Therefore, considering its excellent performance and desirable properties, compound 8 holds great potential for utilization in various areas within the energetic materials field.