## 3 High Energy Density Materials (HEDMs)
#### 1,2,4,5-Tetrazine Derivatives as HEDMs
Figure 15: Conversion of 3,6-dihydrazino-1,2,4,5-tetrazine (DHT) into corresponding salts (**38**) exhibiting very good energetic characteristics. The previously mentioned 3,6-bis(3,5-dimethyl-1\(H\)-pyrazol-1-yl)-1,2,4,5-tetrazine (**23**) was also used as a substrate in the synthesis of two other high-energy derivatives reported by [PERSON] and co-workers: 3,6-bis(1\(H\)-1,2,3,4-tetrazol-5-ylamino)-1,2,4,5-tetrazine (**41**, BTATz) and 3-(1\(H\)-1,2,3,4-tetrazol-5-ylamino)-6-(3,5-dimethylpyrazol-1-yl)-1,2,4,5-tetrazine (**42**, TADPTz, Scheme 18). BTATz is a solid with low sensitivity to impact. After heat initiation, it decomposes rapidly and flamelessly, generating a large amount of nitrogen gas (0.7 dm\({}^{3}\)/g g\({}_{\text{{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{ \rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{       }}}}}}}}}}}}}}}}}}\)). This property makes BTATz useful for applications such as fire extinguishing systems and airbags. The compound decomposes at 270 \({}^{\circ}\)C, has a heat of formation of 883 kJ/mol, and a density of 1.78 g/cm\({}^{3}\) at room temperature. Anhydrous 5-amino-1\(H\)-1,2,3,4-tetrazole was used in the reaction at an approximately two-and-a-half-fold molar excess compared to the tetrazine substrate (**23**) with sulfolane as the solvent. The mixture was heated at 135 \({}^{\circ}\)C for 16 h. BTATz was purified by dissolving it in DMSO and precipitating it with methanol, yielding the product in 70%. The second derivative, 3-(1\(H\)-1,2,3,4-tetrazol-5-ylamino)-6-(3,5-dimethyl-pyrazol-1-yl)- 1,2,4,5-tetrazine (**42**, TADPTz), was synthesized by heating 3,6-bis(3,5-dimethylpyrazol-1-yl)-1,2,4,5-tetrazine with a two-fold molar excess of 5-amino-1\(H\)-1,2,3,4-tetrazole in \(N\),\(N\)-dimethylformamide (DMF). The TADPTz was obtained as a red solid with a decomposition temperature of 250 \({}^{\circ}\)C and a 65% yield [94]. [PERSON] et al. synthesized two other high-energy 3,6-disubstituted polynitroalkoxy-1,2,4,5-tetrazines via nucleophilic substitution involving 3,6-dichloro-1,2,4,5-tetrazine (**25**) (Scheme 19). The transformations were conducted using two molar equivalents of polynitro alcohols (2,2,2-trinitroethanol or 2-fluoro-2,2-dinitroethanol) in dry dichloromethane, with 2,4,6-collidine as a base, at room temperature for 48 h. The products obtained were 3,6-bis(2,2,2-trinitroethoxy)-1,2,4,5-tetrazine (**43**) in 54% yield and 3,6-bis(2-fluoro-2,2-dinitroethoxy)-1,2,4,5-tetrazine (**44**) in 60% yield. These compounds exhibit good detonation parameters, with detonation velocities (\(D\)) of 8.1-8.7 km/s and detonation  
Figure 17: Transformation of 3,6-dihydrozino-1,2,4,5-tetrazine (**24**) into DIAT (**40**). pressures (\(P\)) of 33-35 GPa. According to the authors, these derivatives can be utilized as melt-castable explosives [95]. Polynitroalcohols can also react with 3,6-diamino-1,2,4,5-tetrazine (**45**) to produce \(N^{3}\),\(N^{6}\)-bis(2,2,2-trinitroethyl)-1,2,4,5-tetrazine-3,6-diamine (**46**, BTAT, Scheme 20). The reaction was performed with the addition of hydrochloric acid in water at 70 \({}^{\circ}\)C, yielding the product in 63%. This compound exhibits a favorable oxygen balance (\(\Omega\) = \(-\)10.9%), a high enthalpy of formation (\(\Delta_{\text{H}}H\) = +336 kJ/mol), and a lower decomposition temperature (T\({}_{\text{dec}}\) = 184 \({}^{\circ}\)C) compared to 1,3,5-trinitro-1,3,5-triazine (RDX) (\(\Omega\) = \(-\)21.6%; \(\Delta_{\text{H}}H\) = +85 kJ/mol; T\({}_{\text{dec}}\) = 202 \({}^{\circ}\)C). Additionally, BTAT demonstrates excellent detonation parameters (\(D\) = 9261 m/s; \(P\) = 389 kbar) and a density of 1.886 g/cm\({}^{3}\)[96; 97]. [PERSON] and co-workers nitrated the resulting BTAT (**46**), introducing nitro groups onto the exocyclic nitrogen atoms (Scheme 21). The reaction was conducted stepwise: in the first step, nitration was carried out using fuming nitric acid, followed by the addition of trifluoroacetic acid anhydride in the second step.