## 3 High Energy Density Materials (HEDMs)
#### 1,2,4,5-Tetrazine Derivatives as HEDMs
The product NBAT (**47**) was obtained with a 32% yield and an enthalpy of formation \(\Delta_{\text{H}}H\) = +546 kJ/mol, which is higher than that of its precursor, **46**. The introduction of two nitro groups also enhanced the compound's oxidizing properties [97]. 3,6-Diamino-1,2,4,5-tetrazine (**45**) can also be directly nitrated to produce \(N\),\(N^{\prime}\)-dinitro-1,2,4,5-tetrazine-3,6-diamine (**48**, DNAT, Scheme 22). This compound was synthesized by reacting 3,6-diamino-1,2,4,5-tetrazine (**45**) with fuming nitric acid at 0 \({}^{\circ}\)C for 1 h with a yield of 65%. The resulting yellow DNAT precipitate has a density of 1.915 g/cm\({}^{3}\) and a detonation temperature of 106.4 \({}^{\circ}\)C. DNAT was further converted into the corresponding salts (**49**) in satisfactory yields (67-75%) using bases such as LiOH, KOH, NaOH, Rb\({}_{2}\)CO\({}_{3}\), and Cs\({}_{2}\)CO\({}_{3}\) under mild reaction conditions (30 min., rt). The resulting salts exhibit significantly higher detonation temperatures (241-260 \({}^{\circ}\)C) compared to the starting dinitroamine **48** and have a favorable oxygen balance. They are also characterized by low impact sensitivity, comparable to primary explosives (IS \(\leq\) 4 J), but much higher friction sensitivity (FS = 108-360 N), in contrast to DNAT (12 N) [98,99]. Tetrazine derivatives connected with guanidine fragments have also garnered considerable scientific interest. 3,6-Diguanidino-1,2,4,5-tetrazine (**50**) was synthesized from 3,6-bis(3,5-dimethylpyrazol-1-yl)-1,2,4,5-tetrazine (**23**) and two molar equivalents of guanidine in methanol as the solvent (Scheme 23). The main reaction product was the disubstituted derivative **50**, although a monoguanidine derivative, 6-methoxy-1,2,4,5-tetrazine (**51**), was also formed as a side product. Both compounds were subsequently converted into corresponding high-energy salts bearing inorganic and organic anions (Figure 6). These salts are characterized by high densities and excellent detonation parameters [100, 101, 102, 103]. Similar to the guanidine derivatives of 1,2,4,5-tetrazine described above, substitution with a 2-nitroquanidine moiety was also performed (Scheme 24). The synthesis of the sodium salt of bis(nitroquanidyl)tetrazine (**52**) was carried out using sodium methoxide in methanol at 45-50 \({}^{\circ}\)C, achieving a yield of 90%. [PERSON] et al. also investigated the reaction of 1 equivalent of nitroguanidine with sodium hydride in \(N\),\(N\)-dimethylformamide to synthesize a monosubstituted nitroguanidyl-1,2,4,5-tetrazine [104]. 3,6-Bis(nitrogenquanidylyl)-1,2,4,5-tetrazine (**52**) was converted into high-energy salts using alkaline earth metal nitrates (Mg, Ca, Sr, Ba), achieving high yields (86-93%) [73]. The resulting tetrazine-alkali metal salts (**53**, Scheme 25) are characterized by high densities (\(\rho\) = 1.75-2.30 g/cm\({}^{3}\)) and high sensitivity to impact (IS > 40 J). The barium salt derived from **52** was identified as the most energetic compound, with a density of 2.30 g/cm\({}^{3}\) and high thermal stability (T\({}_{\rm d}\) = 195 \({}^{\circ}\)C). Organic salts of 3,6-bis(nitrogenquanidylyl)-1,2,4,5-tetrazine (**54**) were also synthesized using organic cations derived from the corresponding chlorides (RCI). According to the authors, these compounds are insensitive to impact below 24 J due to the existence of hydrogen and ionic bonds [105]. Figure 6: 3,6-Diguanidino-1,2,4,5-tetrazine salts derived from compound **50** as HEDMs. Figure 3: 3,6-bis(nitrogenquanidyl)-1,2,4,5-tetrazine salts derived from compound **50** as HEDMs. [PERSON] and his team reported notable results by synthesizing a derivative of 1,2,4,5-tetrazine (**56**, BHDT, Scheme 26) with remarkable energetic properties (IS = 30 J, FS = 300 N, T\({}_{\rm d}\) = 240 \({}^{\circ}\)C).