## 3 High Energy Density Materials (HEDMs)
#### 1,2,4,5-Tetrazine Derivatives as HEDMs
[PERSON] and his co-workers utilized the primary amine group-containing compound **83**, specifically 6-(3,5-dimethyl-1\(H\)-pyrazol-1-yl)-[1,2,4]triazoleol[4,3-\(b\)][1,2,4,5]tetrazine-3-amine, in a diazotization reaction with sodium nitrite, followed by displacement with 65% nitric acid, to synthesize the nitro derivative: 3-(3,5-dimethyl-1\(H\)-pyrazol-1-yl)-6-nitro-[1,2,4]triazoleol[4,3-\(b\)][1,2,4,5]tetrazine **105** (Scheme 44) [120]. This compound was then treated with an aqueous ammonia solution under reduced pressure, resulting in the substitution of the 3,5-dimethylpyrazole group with an amine group to yield a yellow compound **106** (TTNA) with a 72% yield. [PERSON] et al. synthesized a series of high-energy fused 1,2,4,5-tetrazine derivatives using the symmetrical reagent, the adequate 1,2-bis(6-(3,5-dimethyl-1\(H\)-pyrazol-1-yl)-[1,2,4]triazoleol[4,3-\(b\)][1,2,4,5]tetrazin-3-yl)diazene (**107**, Scheme 45) [121]. When compound **107** reacted with ammonia in acetonitrile, it produced product **108**, in which the 3,5-dimethylpyrazole rings were replaced by ammonia. Nitration of this compound yielded the corresponding symmetrical \(N\)-nitro derivative **109** with a yield of 72%, and notable detonation parameters (\(\rho\) = 1.85 g/cm\({}^{3}\); \(D\) = 9.5 km/s; \(P\) = 39.8 GPa; IS = 14 J). The yellow solid product **109** was then transformed into a silver salt, which served as a precursor for the synthesis of other high-energy salts (NH\({}_{4}\)+, NH\({}_{3}\)OH\({}^{+}\)). These salts exhibited excellent detonation parameters (for NH\({}_{4}\)+ salt: Yield = 82%; \(\rho\) = 1.84 g/cm\({}^{3}\); \(D\) = 9.6 km/s; \(P\) = 38.8 GPa; IS = 10 J, and for NH\({}_{3}\)OH\({}^{+}\) salt: Yield = 82%; \(\rho\) = 1.99 g/cm\({}^{3}\); \(D\) = 10.2 km/s; \(P\) = 48.6 GPa; IS = 14 J, respectively). The same authors also utilized [1,2,4]triazolo[4,3-\(b\)][1,2,4,5]tetrazine-3,6-diamine (**110**) to synthesize a series of high-energy salts [121]. In the first step, both amino groups in compound **110** were nitrated using 100% nitric acid, yielding an orange product: 3,6-dinitramino-1,2,4-triazoleol[4,3-\(b\)][1,2,4,5]tetrazine (**111**, Scheme 46) with a yield of 70%. The resulting compound, which exhibited excellent detonation parameters (\(\rho\) = 1.91 g/cm\({}^{3}\); \(D\) = 9.3 km/s; \(P\) = 38.3 GPa; IS = 3 J), was then transformed into a silver salt. This salt served as the precursor for the synthesis of a series of other high-energy salts, as shown in Scheme 46. Among these derivatives, the hydroxyammonium salt derived from **111** demonstrated the best detonation properties (\(\rho\) = 1.92 g/cm\({}^{3}\); \(D\) = 9.7 km/s; \(P\) = 42.9 GPa; IS = 25 J). Corresponding \(N\)-oxides derived from 1,2,4,5-tetrazines also constitute an intriguing class of high-energy materials due to their potential applications in material science and industry. Generally, introducing an oxygen atom into the structure of a high-energy material enhances the oxygen balance, increases density, and consequently improves the detonation parameters of such compounds. [PERSON] and co-workers investigated the synthesis of \(N\)-oxides using selected 1,2,4,5-tetrazines and popular oxidizing agents, including hypofluorous acid HOF, peroxysulfuric acid, also known as Caro's acid (H\({}_{2}\)SO\({}_{5}\)), peroxytrifluoroacetic acid (PTFA), or oxone (KHSO\({}_{5}\):KHSO\({}_{4}\):K\({}_{2}\)SO\({}_{4}\)) [122]. Each of these agents presents distinct advantages and disadvantages: while hypofluuric acid HOF is effective, its synthesis requires fluorine, which is harmful to the environment. Conversely, oxone is relatively less effective, whereas Caro's acid (H\({}_{2}\)SO\({}_{5}\)) is overly strong as an oxidizing agent. The literature indicates that peroxytrifluoroacetic acid is the most commonly employed oxidant for converting 1,2,4,5-tetrazines into their corresponding \(N\)-oxides (Figure 9).