## 3 High Energy Density Materials (HEDMs)
#### 1,2,4,5-Tetrazine Derivatives as HEDMs  
The high nitrogen content, positive enthalpy of formation, and thermal stability of 1,2,4,5-tetrazine make its derivatives excellent candidates for HEDM materials. Introducing explosion-enhancing groups (-NO\({}_{2}\), -N=N-, -N\({}_{3}\)) into the 1,2,4,5-tetrazine core is challenging due to the formation of unstable compounds. For instance, 3,6-dinitroamino-1,2,4,5-tetrazine (DNAT) readily hydrolyzes in the presence of trace amounts of water to form 3,6-diamino-1,2,4,5-tetrazine. The first synthesized derivative of the HEDM family was 3,6-dihydrazino-1,2,4,5-tetrazine (**24**, DHT). This compound is produced with 100% yield via the substitution reaction of 3,6-bis-(3,5-dimethylpyrazol-1-yl)-1,2,4,5-tetrazine (**23**) with hydrazine in acetonitrile (Scheme 9). DHT is thermally stable, decomposing above 160 \({}^{\circ}\)C, and exhibits an enthalpy of formation of 124.3 kcal/mol, an enthalpy of combustion \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Entry** & **Compound** & \begin{tabular}{c} **Oxygen Balance** \\ **\(\Omega\) [\%]** \\ \end{tabular} & \begin{tabular}{c} **Density \(\rho\)** \\ **[g/cm\({}^{3}\)]** \\ \end{tabular} & \
\begin{tabular}{c} **Detonation Velocity** \\ **\(D\) [km/s]** \\ \end{tabular} &
\begin{tabular}{c} **Impact Sensitivity** \\ **IS [J]** \\ \end{tabular} \\ \hline
1 & TNG (**32**) & +3.5 & 1.59 & 7.6 & 0.2 \\ \hline
2 & TNT (**33**) & \(-\)73.9 & 1.65 & 6.9 & 15 \\ \hline
3 & RDX (**34**) & \(-\)21.6 & 1.82 & 8.8 & 7.5 \\ \hline
4 & Lead azide (**35**) & \(-\)5.5 & 3.60 & 4.5 & 2.5-4.0 \\ \hline
5 & PA (**36**) & \(-\)45.4 & 1.78 & 7.4 & 7.4 \\ \hline
6 & Tetryl (**37**) & \(-\)47.4 & 1.73 & 7.6 & 3 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Characteristics of some representatives of the HEDM family [91]. \begin{table}
\begin{tabular}{c c} \hline \hline
**Performance** & \
\begin{tabular}{c} Detonation velocity \\ Detonation pressure \\ Heat explosion \\ \end{tabular} &
\begin{tabular}{c} \(D\) \textgreater{} 8500 m/s \\ \(P\) \textgreater{} 340 kbar (34 GPa) \\ Q \textgreater{} 6000 kJ/kg \\ \end{tabular} \\ \hline
**Stability** & \
\begin{tabular}{c} Thermal stability \\ Impact sensitivity \\ Friction sensitivity \\ Electrostatic sensitivity \\ \end{tabular} &
\begin{tabular}{c} T\({}_{\text{dec}}\) \textgreater{} 180 \({}^{\circ}\)C \\ IS \textgreater{} 7 J \\ \end{tabular} \\ \hline
**Chemical properties** &
\begin{tabular}{c} Hydrolytically stable \\ Compatible with binder and plasticizer \\ Low water solubility \\ Non-toxic \\ Smoke-free combustion \\ \end{tabular} \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Desired properties of new nitrogen-rich, highly energetic compounds [88]. of \(-\)517.3 kcal/mol, and a density of 1.73 g/cm\({}^{3}\)[92]. 3,6-Dihydrazino-1,2,4,5-tetrazine can react with organic anions to form more energetic salts (**38**) (Scheme 15). These salts are more stable, exhibit superior energetic properties, and have an improved oxygen balance compared to DHT [74]. Another compound exhibiting high thermal stability (252 \({}^{\circ}\)C), a high density of 1.84 g/cm\({}^{3}\), and an enthalpy of formation of 862 kJ/mol is 3,3\({}^{\prime}\)-azobis(6-amino-1,2,4,5-tetrazine) (**39**, DAAT). [PERSON] and [PERSON] synthesized this compound using a multi-step methodology, starting with 3,6-bis(3,5-dimethyllypyrazol-1-yl)-1,2,4,5-tetrazine (**23**) as the substrate (Scheme 16). The reaction sequence involved substituting the pyrazole ring in the substrate with hydrazine, followed by oxidation of the hydrazine fragment with NBS and final substitution with ammonia [93]. In 2004, [PERSON] et al. synthesized 3,6-diazido-1,2,4,5-tetrazine (**40**, DIAT), a compound sensitive to sparks, friction, and impact, with a decomposition temperature lower than that of the previously discussed compounds (130 \({}^{\circ}\)C). The synthesis of DIAT involved diazotizing 3,6-dihydrazino-1,2,4,5-tetrazine (**24**) using sodium nitrite and hydrochloric acid at 0 \({}^{\circ}\)C (Scheme 17) [79].