## 3 High Energy Density Materials (HEDMs)
#### 1,2,4,5-Tetrazine Derivatives as HEDMs
After dissolving the reaction mixture and washing the product, 3-nitroamino-(1\(H\)-tetrazol-5-yl)-[1,2,4]triazoleol[4,3-\(b\)][1,2,4,5]tetrazine (**87**) was obtained as a yellow solid with a high yield. This compound **87** exhibited the following detonation parameters: \(\rho\) = 1.80 g/cm\({}^{3}\); Td = 167 \({}^{\circ}\)C; \(D\) = 8.6 km/s; \(P\) = 29.0 GPa; IS = 6 J. Another interesting reaction involving compound **86** was the synthesis of 3,3\({}^{\prime}\)-(daizene-1,2-diyl)bis(\(N\)-1\(H\)-tetrazol-5-yl)-[1,2,4]triazoleol[4,3-\(b\)][1,2,4,5]tetrazin-6-amine (**88**, Scheme 36). The transformation was carried out in concentrated hydrochloric acid by dropwise addition of an aqueous solution of potassium permanganate at \(-\)5 \({}^{\circ}\)C, followed by heating at 45 \({}^{\circ}\)C for 4 h. Afterworkup, a yellow solid product **88** was obtained with a yield of 92% (\(\rho\) = 1.79 g/cm\({}^{3}\); T\({}_{\rm d}\) = 240 \({}^{\circ}\)C; \(D\) = 8.5 km/s; \(P\) = 26.9 GPa; IS = 10 J). The resulting compound **88** has the highest enthalpy of formation among all derivatives synthesized by [PERSON] and co-authors (\(\Delta_{\rm f}H\) = 2175.3 kJ/mol) [114]. **Scheme 34. Transformations of (a)** [1,2,4]triazoleol[4,3-b][1,2,4,5]tetrazin-6-ol (**82**) and (**b**) 3-amino-[1,2,4]triazoleol[4,3-b][1,2,4,5]tetrazin-6-ol (**84**) into the corresponding salts. **Scheme 35. Synthesis of 3-amino-6-(1\(H\)-tetrazol-5-yl)-[1,2,4]triazoleol[4,3-b][1,2,4,5]tetrazin (**86**). [PERSON] and co-workers used 3-hydrazinyl-1,2,4,5-tetrazine (**89**) to synthesize other fused 1,2,4,5-tetrazine derivatives. Cyanogen bromide and hydrochloric acid served as co- reagents, and the mixture was stirred for 24 h to produce [1,2,4]triazole[4,3-_b_][1,2,4,5]tetrazin-3-amine (**90**) as a purple solid with a 71% yield (Scheme 37). This compound was treated with potassium permanganate, resulting in 1,2-bis([1,2,4]triazole[4,3-_b_][1,2,4,5]tetrazin-3-yl)diazene (**91**) with an 82% yield. The symmetrical product exhibited a high detonation temperature T\({}_{\rm d}\) = 305 \({}^{\circ}\)C and density (\(\rho\) = 1.91 g/cm\({}^{3}\)), which influences its detonation properties (D = 9.2 km/s; P = 34.8 GPa). It also has a high enthalpy of formation \(\Delta_{\rm f}H\) = 1525.2 kJ/mol and friction sensitivity FS > 360 N. The authors highlighted compound **91** as a potential heat-resistant explosive. The intermediate product **90** was also nitrated using funtime nitric acid at 0 \({}^{\circ}\)C. After several hours of agitation, trifluoroacetic acid was added, leading to the precipitation of \(N\)-([1,2,4]triazole[4,3-_b_][1,2,4,5]tetrazin-3-yl)nitramide (**92**) as an orange solid with a 74% yield (\(\rho\) = 1.81 g/cm\({}^{3}\); T\({}_{\rm d}\) = 132 \({}^{\circ}\)C; \(D\) = 8.8 km/s; \(P\) = 33.2 GPa; IS = 31 J). The intermediate **92** was further utilized to synthesize a series of high-energy salts through reactions with bases. Among them, the salt **93**, formed in the reaction with ammonia, demonstrated the best properties (\(\rho\) = 1.83 g/cm\({}^{3}\); \(D\) = 9.4 km/s; \(P\) = 37.8 GPa; IS = 43 J) [115]. In their synthesis of high-energy compounds, [PERSON] and co-workers used the fused tetrazine **83** (Scheme 38). This compound was converted to 6-hydrazinyl-[1,2,4]triazole[4,3-_b_][1,2,4,5]tetrazin-3-amine (**94**) by reacting it with hydrazine hydrate in acetonitrile, followed by treatment with nitrous acid generated in situ from hydrochloric acid and sodium nitrite. A colored product **95**, substituted at position 6 with an azide group, was obtained with a 68% yield (\(\rho\) = 1.74 g/cm\({}^{3}\); \(D\) = 8.6 km/s; \(P\) = 28.5 GPa; IS = 30 J). The next transformations focused on modifying the amino group attached to the triazole ring. A nitration reaction using concentrated nitric acid produced \(N\)-(6-azid-[1,2,4]triazolo[4,3-\(b\)][1,2,4,5]tetrazin-3-yl)nitramide (**96**), which was precipitated using trifluoroacetic acid.