## 3 High Energy Density Materials (HEDMs)
#### 1,2,4,5-Tetrazine Derivatives as HEDMs
The orange product was obtained with a 67% yield and exhibited the following detonation parameters: \(\rho\) = 1.85 g/cm\({}^{3}\); \(D\) = 9.2 km/s; \(P\) = 36.3 GPa; IS = 1 J. Compound **96** was further transformed into a series of salts. Among these, the hydroxylammonium salt **97** displayed the best parameters (Yield = 72%; \(\rho\) = 1.79 g/cm\({}^{3}\); \(D\) = 9.3 km/s; \(P\) = 36.8 GPa; IS = 2 J). The authors concluded that the salt **97** could serve as a primary explosive due to its low impact sensitivity (IS) [115]. In 2022, [PERSON] et al. synthesized a fused derivative of 1,2,4,5-tetrazine and triazole (**98**) consisting of three rings (Scheme 39). The transformation, carried out in ethanol, used 3,6-dihydroxy-1,2,4,5-tetrazine and cyanogen bromide as starting reagents. The product **98** was obtained as a yellow solid with a 75% yield and was characterized by a high enthalpy of formation \(\Delta_{H}\) = 4.27 kJ/g and favorable detonation parameters (\(\rho\) = 1.76 g/cm\({}^{3}\); \(D\) = 8.5 km/s; \(P\) = 27.3 GPa; IS > 40 J). Compound **98** was then nitrated with fuming nitric acid, yielding the corresponding nitro derivative **99** as a brown solid (yield = 57%; \(\rho\) = 1.88 g/cm\({}^{3}\); \(D\) = 9.1 km/s; \(P\) = 34.0 GPa; IS > 40 J). The latter was subsequently reacted with bases to produce its corresponding salts with excellent yields, between 93% and 95% (M = K\({}^{+}\), NH\({}_{3}\)OH\({}^{+}\), NH\({}_{4}\)\({}^{+}\), N\({}_{2}\)H\({}_{5}\)\({}^{+}\)). Among the obtained products, the hydrazinium salt **99**-N\({}_{2}\)H\({}_{4}\) was identified as the most energetic 1,2,4,5-tetrazine derivative (\(\rho\) = 1.84 g/cm\({}^{3}\); \(D\) = 9.1 km/s; \(P\) = 32.3; GPa; IS > 40 J; \(\Delta_{H}\)= 2.72 kJ/g) [116]. [PERSON] and his team also undertook the synthesis of a fused tricyclic 1,2,4,5-tetrazine-1,2,4-triazole hybrid (**100**) and its derivatives [117]. Heating \(N^{3}\),\(N^{6}\)-di(1\(H\)-tetrazol-5-yl)-1,2,4,5-tetrazine-3,6-diamine (**41**, BTATz) in polyphosphoric acid (PPA) for 3-4 h at 180 \({}^{\circ}\)C resulted in the formation of bis([1,2,4]triazolo)[1,5-\(b\):5,\({}^{\prime}\)!/\({}^{\prime}\)f][1,2,4,5]tetrazin-2,7-diamine (**100**, Scheme 40). The product was obtained as a light red solid with a yield of 40% and exhibited the following detonation parameters: \(\rho\) = 1.71 g/cm\({}^{3}\); \(D\) = 8.3 km/s; \(P\) = 29.2 GPa;IS > 45 J. Compound **100** was subsequently subjected to oxidation reactions, yielding other high-energy derivatives depicted in Scheme 41. In 2020, [PERSON] et al. also synthesized bicyclic fused 1,2,4,5-tetrazines connected to the 1,2,5-oxadiazole ring via an amino group [118]. The \(N\)-heteroarylation of aminofrazans was performed by refluxing 6-(3,5-dimethyl-1\(H\)-pyrazol-1-yl)-[1,2,4]triazoleol[4,3-\(b\)][1,2,4,5]tetrazine (**81**, Scheme 42) in a mixture of acetonitrile and cesium carbonate as the base for 0.3-2 h. This reaction yielded three yellow products (**101-103**), all of which were characterized by favorable detonation parameters. In 2016, [PERSON] et al. heated 6-(3,5-dimethyl-1\(H\)-pyrazol-1-yl)-[1,2,4]triazoleol[4,3-\(b\)][1,2,4,5]tetrazine (**81**, Scheme 43) with 2-amino-2-(hydroxymethyl) propane-1,3-diol in a mixture of acetonitrile and \(N\),\(N\)-dimethylformamide to produce the corresponding substitution product. The resulting intermediate was then nitrated with nitronium acetate, generated in situ from glacial acetic acid, acetic anhydride, and nitric acid at 5 \({}^{\circ}\)C, to produce a yellow trinitro derivative **104** with a 76% yield (\(\rho\) = 1.79 g/cm\({}^{3}\); \(D\) = 8.3 km/s; \(P\) = 29.4 GPa; IS \(>\) 3.9 J) [119].