## 3 High Energy Density Materials (HEDMs)
#### 1,2,4,5-Tetrazine Derivatives as HEDMs
In 2017, [PERSON] and co-workers converted 3-(3,5-dimethyl-1\(H\)-pyrazol-1-yl)-1,2,4,5-tetrazine (**76**) into novel high-energy derivatives using aminotetrazines (Scheme 31). The reaction of the pyrazole-1,2,4,5-tetrazine moiety (**76**) with 3,6-diamino-1,2,4,5-tetrazine was conducted in a 2:1 molar ratio in acetonitrile with the addition of potassium carbonate. After workup, the corresponding high-energy \(N\),\(N^{\prime}\)-di(1,2,4,5-tetrazin-3-yl)-1,2,4,5-tetrazine-3,6-diamine (**77**) was obtained with a high yield as a red solid (T\({}_{\text{d}}\) = 279 \({}^{\circ}\)C; \(\rho\) = 1.69 g/cm\({}^{3}\); \(D\) = 8.1 km/s; \(P\) = 22.9 GPa; Q\({}_{\text{exp}}\) = 3782 kJ/kg). When 1,2,4,5-tetrazin-3-amine was used in a similar reaction with compound **76**, the adequate \(N\)-(1,2,4,5-tetrazin-3-yl)-1,2,4,5-tetrazin-3-amine (**78**) was obtained as a dark-pink solid with a 55% yield. This compound exhibited slightly lower energetic parameters (T\({}_{\text{d}}\) = 210 \({}^{\circ}\)C; \(\rho\) = 1.63 g/cm\({}^{3}\); \(D\) = 7.9 km/s; \(P\) = 21.0 GPa; Q\({}_{\text{exp}}\) = 3778 kJ/kg) compared to the precursor **76**. However, compound **78** was further transformed in a reaction with 3-chloro-1,2,4,5-tetrazine to produce a more energetic derivative, \(N\),\(N\)-di(1,2,4,5-tetrazin-3-yl)-1,2,4,5-tetrazin-3-amine (**79**) (T\({}_{\text{d}}\) = 231\({}^{\circ}\)C; \(\rho\) = 1.88 g/cm\({}^{3}\); \(D\) = 9.1 km/s; \(P\) = 30.5 GPa; Q\({}_{\text{exp}}\) = 4067 kJ/kg). This transformation was carried out using sodium hydride in THF under reflux for 2 h, yielding a brown product with a 58% yield [112]. One of the subgroups of tetrazine derivatives that has recently gained attention is fused systems. [PERSON] et al. synthesized 6-(3,5-dimethyl-1\(H\)-pyrazol-1-yl)-[1,2,4]triazoleol[4,3-b][1,2,4,5]tetrazine (**81**) by reacting 3-(3,5-dimethyl-1\(H\)-pyrazol-1-yl)-6-hydrazinyl-1,2,4,5-tetrazine (**80**) with triethyl orthoformate. The intermediate **81** was then treated with potassium hydroxide followed by hydrochloric acid, resulting in the transformation into [1,2,4]triazoleol[4,3-b][1,2,4,5]tetrazin-6-ol (**82**, Scheme 32). The solid product **82**, obtained with a 93% yield, exhibits good detonation parameters (\(\rho\) = 1.84 g/cm\({}^{3}\); T\({}_{\rm d}\) = 232 \({}^{\circ}\)C; \(D\) = 8.5 km/s; \(P\) = 28.6 GPa; IS = 30 J) [113]. Figure 8: Characteristics of azides and nitro derivatives obtained from triazole-1,2,4,5-tetrazine hybrids 67, 68, 69. Compound **80** was also reacted with cyanogen bromide to synthesize 6-(3,5-dimethyl-1\(H\)-pyrazol-1-yl)-[1,2,4]triazoleol[4,3-\(b\)][1,2,4,5]tetrazin-3-amine (**83**). Subsequent treatment of the resulting product **83** with potassium hydroxide, followed by the work-up with hydrochloric acid, gave the corresponding 3-amino-[1,2,4]triazoleolol[4,3-\(b\)][1,2,4,5]tetrazin-6-ol (**84**, Scheme 33) with a high yield (\(\rho\) = 1.86 g/cm\({}^{3}\); Td = 227 \({}^{\circ}\)C; \(D\) = 8.6 km/s; \(P\) = 28.0 GPa; IS = 34 J) [113]. The solid fused tetrazine products **82** and **84** were further converted into their corresponding salts by reacting with various bases under mild conditions in methanol (Scheme 34). Another compound used to prepare fused 1,2,4,5-tetrazine derivative is 6-hydrazinyl-N-(1\(H\)-tetrazol-5-yl)-1,2,4,5-tetrazin-3-amine (**85**, HTATz). In 2022, [PERSON] et al. heated this compound with cyanogen bromide in 1\(M\) hydrochloric acid for 4 h to obtain the corresponding 3-amino-6-(1\(H\)-tetrazol-5-yl)-[1,2,4]triazoleolol[4,3-\(b\)][1,2,4,5]tetrazine (**86**, Scheme 35). The product was isolated as a brown solid with a 60% yield (\(\rho\) = 1.76 g/cm\({}^{3}\); Td = 189 \({}^{\circ}\)C; \(D\) = 8.5 km/s; \(P\) = 26.1 GPa; IS = 36 J) [114]. Compound **86** was later utilized in nitration and oxidation reactions to synthesize other valuable energetic derivatives (Scheme 36). Nitration was performed using fumig nitric acid at \(-\)15 \({}^{\circ}\)C for 30 min., followed by an additional 6 h at room temperature.