## 3 High Energy Density Materials (HEDMs)
#### 1,2,4,5-Tetrazine Derivatives as HEDMs
The compound was synthesized by heating a suspension of 1,1-diamino-2-nitro-2-(1\(H\)-triazol-5-yl)ethene (**55**, tri-FOX) with 85% hydrazine hydrate at 100 \({}^{\circ}\)C for 5 h. The red solid product **56** was isolated in a 79% yield [106]. An equally intriguing group of high-energy 1,2,4,5-tetrazines includes oxadiazole and triazole derivatives, where the tetrazine core serves as a linker between two azole rings (Scheme 27). The substrate for these reactions was 3,6-bis(3,5-dimethylpyrazol-1-yl)-1,2,4,5-tetrazine (**23**), which reacted with a 1,2,5-oxadiazole-3-amine derivative in the presence of a base (NaH, K\({}_{2}\)CO\({}_{3}\), Cs\({}_{2}\)CO\({}_{3}\)). The transformations proceeded via different pathways, yielding monosubstitution, disubstitution, and tetrazine-to-oxadiazole system disubstitution products. Monosubstitution (**57**) was performed with 1.1 molar equivalents of the 1,2,5-oxadiazol-3-amine derivative in refluxing acetonitrile for 0.8-4 h, yielding the desired products at 73-92%. When twice the molar amount of tetrazine relative to 1,2,5-oxadiazol was used, disubstitution products **58**, consisting of two 1,2,4,5-tetrazine units bonded via amino groups to 1,2,5-oxadiazol, were obtained in yields ranging from 42% to 97% [107; 108]. Using a four-fold excess of 3,4-diamino-1,2,5-oxadiazol relative to the tetrazine reagent enabled the preparation of 3,6-bis(3-amino-1,2,5-oxadiazol-4-ylamino)-1,2,4,5-tetrazine (**59**). In this case, sodium hydride in \(N\),\(N\)-dimethylformamide was employed as the base, and the reaction mixture was stirred at room temperature for 1 h. After workup, the product was obtained as a red solid with a satisfactory yield of 91%. [PERSON] and co-workers investigated the reaction of 3,6-bis(3-amino-1,2,5-oxadiazol-4-ylamino)-1,2,4,5-tetrazine (**59**, BOAT) with 2,2,2-trinitroethanol in the presence of hydrochloric acid at 70 \({}^{\circ}\)C (Scheme 28). The symmetrical product of the disubstitution at both terminal amino groups (**60**) was obtained with a yield of 89%. This compound exhibits a high decomposition temperature (T\({}_{\rm d}\) = 204 \({}^{\circ}\)C) and density (\(\rho\) = 1.86 g/cm\({}^{3}\)), resulting in excellent detonation properties (D = 9.04 km/s; \(P\) = 37 GPa). Derivative **60** was then nitrated with 100% nitric acid at \(-\)5 \({}^{\circ}\)C, and after 4 h of agitation, the mixture was poured into water, yielding product **61** as an orange solid (yield = 88%). This compound features a positive oxygen balance (\(\Omega\) = 11.5%) and high density (\(\rho\) = 1.92 g/cm\({}^{3}\)), and the following detonation parameters: \(D\) = 9.6 km/s and \(P\) = 42 GPa. Another efficient route to high-energy salts is the direct nitration of 3,6-bis(3-amino-1,2,5-oxadiazol-4-ylamino)-1,2,4,5-tetrazine (**59**, BOAT) with fumig nitric acid at \(-\)10 \({}^{\circ}\)C. The resulting dinitration product **62** has a high heat of formation (\(\Delta_{t}H=3.00\) kJ/g), high density (\(\rho=1.85\) g/cm\({}^{3}\)) and detonation parameters: \(D=9.1\) km/s, \(P=36\) GPa. This compound was further converted into a series of high-energy salts **63** using appropriate bases in acetonitrile as the solvent. The red-brown solid products were obtained with yields of 89-91% and exhibited notable energetic properties (T\({}_{\rm d}=211\)-\(225\)\({}^{\circ}\)C; \(\rho=1.76\)-\(1.87\) g/cm\({}^{3}\); \(D=8.8\)-\(9.4\) km/s; \(P=31\)-\(39\) GPa; \(\Delta_{t}H=2.45\)-\(4.37\) kJ/g; IS = 28-40 J; FS > 360 N) [109]. In 2017, [PERSON] et al. synthesized high-energy 1,2,4,5-tetrazine-1,2,4-triazole hybrids by reacting dimethyl 1,4-dihydro-1,2,4,5-tetrazine-3,6-dicarboxylate (**30**) with aminoguanidine sulfate (VI) in the presence of sodium throwoxide at 0 \({}^{\circ}\)C (Scheme 29).