Synthesis, crystal structures and physicochemical performance of the triethylamine salt and copper (II) complex based on 3, 5-dinitropyrazole

: Two novel energetic ionic and coordination frameworks based on 3, 5-dinitropyrazole (3, 5-DNP) were prepared under the facile reaction conditions. They were characterized by FT-IR spectroscopy, Raman spectrum, single-crystal X-ray analysis and differential scanning calorimetry thermal analysis (DSC). Hirshfeld analysis showed that they both presented strong hydrogen-bond intermolecular interactions. Thermal properties and the nonisothermal thermokinetic parameters were obtained by differential scanning calorimeter (DSC) as well as Kissinger, Ozawa and Starink methods. The two energetic materials exhibit excellent thermal stabilities ([DNP] NH (Et) 3 : T m =251℃, T d =377℃; Cu [DNP] 2 [H 2 O] 2 : T d =350℃). They can be potential candidates for heat-resistant energetic materials for military uses.


X-ray diffraction crystallography
Crystals of compound 2 and 3 were obtained using slow evaporation of ethanol for 24 h in the room temperature(CCDC number：1452000 and 1452001). All crystal parameters including bond lengths, bond angles, torsion angles and other relative data can be found in the Supporting Information. The optical microscopies of 2 and 3 are captured in Figure 1, which present the different appearance of the two compounds in sheets or blocks. As shown in Figure 2a, compound 2 contains one crystallographically independent anion and cation. The geometric structure of anion is the same with those of the units in compound 3. Due to the hydrogen bonding and electrovalent bonding, the nitrogen atoms between triethylamine and 3,5-DNP in compound 2 can form the stable molecular structure. The torsion angles suggesting that the two nitro groups are coplanar with the pyrazole ring. Figure 3a shows the crystal structure of coordination compound 3. For 3, the coplanar characteristics also exist. This indicates that the structures between copper and each 3,5-DNP molecule are nearly symmetrical. Comparing with 3,5-dinitropyrazole (1.430 and 1.452 Å) [9] , the bond length of C-NO2 in compound 2 and compound 3 are little different, which are 1.427 and 1.447 Å in compound 2 as well as 1.430 and 1.450 Å in compound 3.
The crystal packing of 2 and 3 are shown in Figure 2b and Figure 3b, 3c. It can be described as a layered arrangement in compound 2, and a similar like metal-organic framework (MOF) structure for compound 3 while along the (100) crystal-facet.

Structure description
The sum of molecular interactions in compound 2 and 3 are depicted in fingerprint plots and their contributions for intermolcular close contacts are also summerized in Figure 4. The similarity of the two compounds is that the O· · · H and H· · · O close contacts are dominating interaction in the crystal packing, H· · · H contacts of 2 are more than those of 3 while O· · · O contacts of 2 are less than those of 3. However, the hydrogen-bond (HB) interactions are distinct in the two crystals. In compound 2, triethylamine cations provide more H atoms which can form sufficient HB interactions. But in compound 3, the HB interactions are mostly from the two H2O molecules and O· · · O contacts can attribute to the nitro groups from two DNP molecules. Meanwhile, the distributions of Cu· · · N and Cu· · · O contacts in 3 are close to those of C· · · H and H· · · C contacts.
Here, kB is the Boltzmann constant and h is the Planck constant, T=Tp0, Ea=Ek and A=AK (adopted from Kissinger's method).

Heat of formation and energetic performance
The heat of formation (HOF) of two energetic salts were predicted using density functional theory (DFT) method by Guassian 09 package. [15] The heats of formation (HOFs) for cation and anion of compound 2 were computed using isodesmic reaction (Scheme 2) at B3LYP/6-311G(d,p) level.

Scheme 2: Isodesmic reactions for anion and cation of [DNP]NH(Et)3 (a) and Born-Habor Cycle for
predicting the heat of formation of the energetic salts (b).
Based on a Born-Haber energy cycle, the solid HOF of organic energetic salt [DNP]NH(Et)3 can be simplified as the following equation: [ Where ∆HL is the lattice energy of the salts that can be predicted by the following formula suggested by H. D. B. Jenkins et al.: [17] RT n q n p U H Where nm and nx depend on the numbers of the ions Mp+ and Xq-respectively, which are equal to 3 for monatomic ions, 5 for linear polyatomic ions, and 6 for nonlinear polyatomic ions. The equation for lattice potential energy UPOT is shown as follows: Here, ρ is the density and M is the formula mass of the ionic materials. For 1:1 (charge ratio) salts, the coefficients γ and δ are 1981.2 KJ· mol -1 · cm -1 and 103.8 kJ· mol -1 , respectively. For 1:2 salts, the coefficients γ and δ are 8375.6 kJ· mol -1 · cm -1 and -178.8 kJ· mol -1 , respectively. HOF for coordination compound 3 was calcualted at Lanl2dz level using the atomization method (shown in Scheme 3) due to the exist of metal atoms. [18] Scheme 3: The atomization method.

Conclusion
In this study, two novel energetic materials based on 3, 5-dinitropyrazole (3, 5-DNP) (2 and 3) were synthesized and their structures were confirmed by X-ray diffraction for the first time. Hirshfeld analysis showed that they both presented strong hydrogen-bond intermolecular interactions as shown in dominating H· · · H, O· · · H and H· · · O contacts. Moreover, thermal-dynamic performance were evaluated by non-isothermal kinetic methods based on the results of differential scanning calorimeter (DSC), indicating that the two energetic materials possess excellent thermal properties (2: Tm=251℃, Td=377℃; 3 : Td=350℃). For 2, the apparent activation energy (Ea) evaluated by Kissinger's, Ozawa's, and Starink's methods are 275.07, 272.90 and 274.68 kJ· mol -1 , respectively. And for 3, Ea estimated by three methods are 319.89, 315.72 and 318.71 kJ· mol -1 , respectively. Remarkably, 2 and 3 possess excellent thermal stability since the exothermic temperatures of them exceed 300 ℃, which are superior to those of reported copper-based nitrogen-rich salts or coordination compounds. Considering these above, it is worthwile to further explore the electronic properties of metal complexes of the DNP based energetic frameworks.