## 3 High Energy Density Materials (HEDMs)
#### 1,2,4,5-Tetrazine Derivatives as HEDMs
The product 5,5'-(1,4-dihydro-1,2,4,5-tetrazine-3,6-diyl)bis(1\(H\)-1,2,4-triazoleol-3-amine) (**64**) was obtained with a 66% yield and subjected to nitration using a nitrating mixture consisting of concentrated nitric and sulfuric acids at \(-10\)\({}^{\circ}\)C. The transformation led to the corresponding dinitro derivative **65** with a 69% yield. Isolated as a red solid, this compound is characterized by high density (\(\rho=1.88\) g/cm\({}^{3}\)), high detonation velocity and pressure (\(D=9.1\) km/s; \(P=31.7\) GPa), and other notable properties, including impact sensitivity IS = 20 J, friction sensitivity FS = 270 N, decomposition temperature T\({}_{\rm d}=162\)\({}^{\circ}\)C, and enthalpy of formation \(\Delta_{t}H=1336\) kJ/mol [110]. **Scheme 29. Synthesis of \(N\),\(N^{\prime}\)-((1,2,4,5-tetrazine-3,6-diyl)bis(4\(H\)-1,2,4-triazoleole-5,3-diyl))dinitramide (65) and its salts. **The dinitro compound **65** was transformed into a series of salts (**66**) (Scheme 29, Figure 7) exhibiting interesting properties as high-energy materials, including high impact and friction sensitivities (IS \(>\) 40 J; FS \(>\) 360 N). These salts were synthesized by reacting compound **65** with appropriate bases in methanol for 2 h at room temperature [110]. [PERSON] et al. synthesized three distinct triazole hybrids in which 1,2,4,5-tetrazine served as a bridge (Scheme 30). The derivative **67** was formed in a substitution reaction between 3,6-bis(3,5-dimethyl-1\(H\)-pyrazol-1-yl)-1,2,4,5-tetrazine (**23**, BPT) and 1\(H\)-1,2,4-triazole-3,5-diamine. This reaction involved the nitrogen atom at position 1 of the triazole ring and the nitrogen of the amino group at position 6. The transformation was conducted in sulfolane at 140 \({}^{\circ}\)C for 24 h. The product was precipitated with DMSO, washed with methanol, and dried. This compound (**67**), obtained in 82% yield, exhibited a high decomposition temperature (T\({}_{\rm d}\) = 313 \({}^{\circ}\)C), density (\(\rho\) = 1.87 g/cm\({}^{3}\)), and the following detonation properties: \(D\) = 8.6 km/s; \(P\) = 269 kbar. Another symmetrical substitution product of BPT with 1\(H\)-1,2,4-triazole-3,5-diamine was obtained by performing the reaction in a similar manner at a temperature of 210 \({}^{\circ}\)C. This compound (**68**) had the highest decomposition temperature (T\({}_{\rm d}\) = 357 \({}^{\circ}\)C) among the three derivatives, high impact sensitivity (IS \(>\) 98 J), typical density, and strong detonation properties (\(\rho\) = 1.81 g/cm\({}^{3}\); \(D\) = 8.2 km/s; \(P\) = 239 kbar). Finally, the reaction of 1\(H\)-1,2,4-triazole-3,5-diamine with 3,6-dichloro-1,2,4,5-tetrazine (**25**) in a mixture of acetonitrile and \(N\),\(N\)-dimethylformamide at room temperature resulted in substitution involving the nitrogen atom from the triazole ring. The resulting red-brown solid product (**69**) was obtained with an 85% yield and exhibited notable detonation properties (T\({}_{\rm d}\) = 370 \({}^{\circ}\)C; \(\rho\) = 1.75 g/cm\({}^{3}\); \(D\) = 7.7 km/s; \(P\) = 227 kbar; IS \(>\) 40 J; FS \(>\) 360 N) [111]. Figure 7: Characteristic of \(N\),\(N^{\prime}\)-((1,2,4,5-tetrazine-3,6-diyl)bis(4\(H\)-1,2,4-triazole-5,3-diyl))dinitramide salts. The presence of amino groups attached to the triazole ring in the resulting products (**70**-**75**) enabled the synthesis of other potential high-energy materials, specifically the corresponding azides and nitro derivatives. The typical synthetic procedures involved dissolving the appropriate triazole-1,2,4,5-tetrazine derivative (**67**-**69**) in sulfuric acid at 0 \({}^{\circ}\)C, introducing an aqueous solution of sodium nitrite, and then adding sodium azide solution at 0 \({}^{\circ}\)C (for azides **70**-**72**) or sodium nitrite solution at 40 \({}^{\circ}\)C (for nitro compounds **73**-**75**). The detonation properties of the obtained compounds are presented in Figure 8 [111].