Synthesis and Properties of Two Energetic Salts Based on 1-Amino-2-nitroguanidine

1-amino-2-nitroguanidinum chloride, 2,4,5-trinitroimidazole and 5-nitrotetrazole were prepared according to References 1, 2 and 3, respectively. Silver salts were prepared as follows: to a solution of 2,4,5-trinitroimidazole (2.02 g, 10 mmol), or 5-nitrotetrazole (1.14 g, 10 mmol) in water (30 mL) were added dropwise a solution of silver nitrate in water and many solids were formed immediately. Then the suspension were stirred for 2 h at room temperature and filtered, then washed with ice water. The silver salts were obtained in excellent yield. The titled energetic salts were prepared as follows: to a suspension of the silver salts in water (30 mL) were added slowly a solution of 1-amino-2-nitroguanidinum chloride (1.55 g, 1 mmol) in water (15 mL). The resulting reaction mixture was stirred at 40 °C for 6 h and filtered. The filtrate was concentrated under reduced pressure and the collected residue was recrystallized from methanol/water to afford the corresponding product in excellent yield. 1-amino-2-nitroguanidinum 2,4,5-trinitroimidazole salt: yield: 84.6%; H NMR (500 MHz, DMSO-d6) dH 9.7 (s, 1H, NH), 8.4 (s, 2H, NH2), 6.7 (s, 3H, NH3 ); C NMR (125 MHz, DMSO-d6) dC 138.4, 146.9, 159.7; IR (KBr) vmax/ cm -1 3318, 2999, 1632, 1540, 1474, 1398, 1392, 1280, 1222, 1191, 1110, 1026, 909, 870, 834, 783, 587; ESI-MS m/z 202 [M H]-, 120 [M + H]; elemental analysis calcd. for C4H12N18O8: C 14.91, H 1.88, N 43.48; found: C 14.83, H 1.97, N 43.53. 1-amino-2-nitroguanidinum 5-nitrotetrazole salt: yield 86.1%; H NMR (500 MHz, DMSO-d6) dH 9.4 (s, 1H, NH), 8.4 (s, 3H, NH3 ), 8.0 (s, 2H, NH2); C NMR (125 MHz, DMSO-d6) dC 159.6, 169.2; IR (KBr) vmax/ cm -1 3468, 3359, 2949, 2700, 2162, 2070, 1645, 1508, 1481, 1453, 1396, 1321, 1276, 1239, 1107, 904, 834, 782, 666, 545, 486; ESI-MS m/z 114 [M – H], 120 [M + H]; elemental analysis: calcd. for C4H12N18O8: C 10.26, H 2.58, N 59.82; found: C 10.19, H 2.62, N 59.91.


Introduction
2][3][4] However, people found that there appear sharp contradictions among these energetic materials regarding to the high detonation performances and stabilities such as sensitivity to impact, friction, thermal shock and so on.In order to solve these problems, a focus of recent interests in energetic compounds has been the synthesis of nitrogen-rich energetic salts which exhibit a combination of high positive heats of formation, satisfactory detonation performances and low sensitivity.This is because these salt-based energetic materials often possess advantages over non-ionic molecules since these salts tend to exhibit lower vapor pressures and higher densities than their atomically similar non-ionic analogues. 5uanidine and its derivatives such as aminoguanidine, diaminoguanidine and triaminoguanidine, were often Vol.26, No. 1, 2015   selected as the nitrogen-rich cations in the previous research. 6,7However, little investigation was reported on 1-amino-2-nitroguanidine, which has superior detonation velocity and detonation pressure to those of guanidine derivatives. 8It is predicted that the 1-amino-2-nitroguanidine-based energetic salts may have potentially broad application in the fields of explosives.On the other hand, poly-nitro anion containing a large number of inherently energetic C-N and N-N bonds, have been the preferred selections for scientists since these types of compounds will help improve the heat of formation during the decomposition process.Obviously, 2,4,5-trinitroimidazole 9 and 5-nitrotetrazole 10 are the compounds of this type with N-heterocyclic ring and a large number of C-N or N-N bonds in the molecular.For the above reasons, two 1-amino-2-nitroguanidinebased nitrogen-rich energetic salts were designed and synthesized (Scheme 1).The thermal behavior, electron structure and detonation properties were also investigated to give a better understanding of their physical and chemical properties.
Experimental 1 H and 13 C nuclear magnetic ressonance (NMR) spectra of the 1-amino-2-nitroguanidine-based salts were obtained by a Bruker Avance III 500 MHz spectrometer; infrared (IR) spectra were performed on a Thermo Nicolte IS10 IR instrument; electrospray ionization mass spectrometry (ESI-MS) results were obtained from a Finnigan TSQ Quantum Mass Spectrometer and all the thermodynamic tests were performed on a NETZSCH STA 409 PC/PG system with an innitial mass of 3.0 mg were placed in alumina crucibles with high-purity nitrogen.
Computations were performed with the Gaussian 03 suite of programs 11 using B3LYP functional with 6-31+G(d,p) basis set.Input geometric structures of these salts were based on the optimum configuration that obtained by Hyperchem software.All of the optimized structures were characterized to be true local energy minima on the potential energy surface without imaginary frequencies.
From TG curves, it is obviously seen that the decomposition of the two salts were a two-stage process with approximately 90% weight loss.Accordingly, some evident or faint peaks were also found, which were corresponding to the two-stage decomposition process in the TG curves.In view of the DSC curves, both of salts 1 and 2 have an evident sharp exothermic peak.On the other hand, salt 1 has the melting point which indicate that the 1-amino-2nitroguanidinium 2,4,5-trinitroimidazole salt has a potential to be cast explosives.Based on the curves, it is predicted that all the salts have potential application as primary explosive.
Kissinger 12 and Ozawa's 13 method are two important ways to investigate the thermodynamic properties of an energetic material.
Scheme 1. Synthetic route of the energetic salts.
Synthesis and Properties of Two Energetic Salts Based on 1-Amino-2-nitroguanidine J. Braz.Chem.Soc.126 Kissinger's method: 12 (1) Ozawa's method: 13 (2) where, β is the heating rate; T pi is the maximum peak temperature; R is the gas constant; E k and E o are the activation energy calculated by the Kissinger and Ozawa's methods, respectively.To obtain the relative kinetic parameters such as activation energy (E), pre-exponential constant (A), entropy of activation (ΔS ≠ ), enthalpy of activation (ΔH ≠ ), free energy of activation (ΔG ≠ ) the critical temperature of thermal explosion (T b ), equation 3-7 were employed: 14 (3) where T = T p0 , the peak temperature (T pi ) corresponding to β → 0; E k , calculated by Kissinger's method; A = A k , calculated by Kissinger's method; k B , the Boltzmann constant, 1.3807 × 10 -23 J K -1 ; h, the Plank constant, 6.626 × 10 -34 J s -1 .
Values of the relative kinetic parameters were tabulated in Table 1.It is obviously seen the calculated apparent activation energy (E) by Kissinger and Ozawa's methods are approximately the same and the linear correlation coefficients (r) are very close to 1, which indicates that the results are credible enough.The measured thermodynamic parameters such as activation energy (E k , salt 1, 282.1 kJ mol −1 ; salt 2, 116.2 kJ mol −1 ; E o , salt 1,275.0kJ mol −1 ; salt 2, 116.9 kJ mol -1 ), entropy of activation (ΔS ≠ , salt 1, 395.7 J mol -1 K -1 ; salt 2, 5.8 J mol −1 K -1 ), enthalpy of activation (ΔH ≠ , salt 1, 278.7 kJ mol -1 ; salt 2, 113.1 kJ mol -1 ), free energy of activation (ΔG ≠ , salt 1, 117.6 kJ mol -1 ; salt 2, 110.9 kJ mol −1 ) involves that salt 2 is less stable compared with the salt 1, since it has the lowest activation energy and enthalpy of activation.In view of the critical temperature of thermal explosion (T b ), salt 1 (412.3K) is higher than that of salt 2 (388.9K), which also demonstrated that salt 1 is much more stable than salt 2. Besides, the values of T b of the two salts are relative high and meet the security requirements during storage or use.Table 1.Calculated data of the kinetic parameters for the main exothermic decomposition reaction of the title salts Salt No.

Kissinger method
Ozawa method Thermodynamic parameters The molecular orbital and the electronic structure of the two salts were investigated based on the B3LYP/6-31G(d,p) level-optimized structure.The energy gap (ΔE), which can be used for predicting the reactivity of a molecular were investigated in this paper.The calculated energy gap value of the title compounds are 5.58 eV and 4.39 eV, respectively, indicating that salt 1 may have a lower reactivity, while salt 2 has a higher reactivity.To give a better understanding of the chemical and physical properties of the title compounds, the 3D plots of the highest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital (LUMO) and the molecular electrostatic potentials (MEP) were illustrated in Figure 2. It is obviously seen that most of the HOMO and LUMO levels are located in the anion and the orbitals are 2-fold degenerated, which indicates that the removal of an electron from the HOMO level or addition of an electron to the LUMO level could weaken the skeleton framework.As for the MEPs, it is seen that most of the negative potentials appear to be distributed on the O atoms in the −NO 2 groups, while the positive potentials appear to be at C, N or H atoms.This may attribute to the stabilization of the molecular structure according to the law proposed by Klapötke et al.. 15 In order to give a better understanding of the thermodynamic properties of the title compounds, the standard molar heat capacity C 0 p,m , standard molar entropy S 0 m and standard molar enthalpy H 0 m from 200 to 700 K were evaluated by quantum chemistry and presented in Figure 3.
Obviously, all the thermodynamic parameters increase with the increasing of the temperature.The main reasons were as follows: when the temperature is low, the main contributions to the thermodynamic functions are from the translation and rotation of molecules while the main contributions to the thermodynamic functions are from the intensified vibrations at a higher temperature.Besides, the correlation equations between the standard molar heat capacity C 0 p,m , standard molar entropy S 0 m , standard molar enthalpy H 0 m and the temperature were also presented as follows (where R 2 is the correlation coefficients): The semi-empirical Kamlet-Jacobs equations 16 were often employed when the detonation properties, (detonation velocity, D and detonation pressure, P) were predicted and has been demonstrated as a reliable way.It was written as follows: (8) (9)  where D, the detonation velocity (km s −1 ); P, the detonation pressure (GPa); N, the moles of detonation gases per gram explosive; -M, the average molecular weight of these gases; Q, the heat of detonation (cal g −1 ); r, the density of explosives (g cm −3 ) that obtained according to the reference method; 17 and N, -M, and Q were calculated by the equations that summarized in Table 2.
Based on the equations in Table 2, the heat of formation (ΔH f 0 ) of the salts should be known first to calculate the detonation velocity and detonation pressure.Then the Born-Haber energy cycle (Figure 4), isodesmic reactions (Scheme 2) and equations 10-12 18 were joined together to calculate the accurate values of heat of formation.However, it also should be pointed out that the enthalpy of the isodesmic reaction (ΔH f 0 ) is obtained by combining the MP2/6-311++G** energy difference, the zero-point energies (B3LYP/6-31+G**), and other thermal factors (B3LYP/6-31+G**).
where ΔH L is the lattice energy of the salts; U POT is the lattice potential energy; p and q are the charge number of the cation and anion; n M and n X are equal to 6, according to reference 18.
The values for the calculated detonation were summarized and tabulated in Table 3.It is seen that all the salts have high positive heat of formation (salt 1, 264.7 kJ mol −1 ; salt 2, 487.6 kJ mol −1 ) which are benefit to the detonation velocity and detonation pressure.The calculated P (salt 1, 30.2 GPa; salt 2, 29.1 GPa) and D (salt 1, 8398 m s -1 ; salt 2, 8334 m s ) indicates that the detonation performance of the salts are superior to those of trinitrotoluene (TNT) and are equal to those of 2,4,6-triamino-1,3,5-trinitrobenzene (TATB).Besides, the   oxygen balance is closer to zero and the nitrogen content is also higher compared with those of TNT and TATB.Based on the above-described data, it is predicted that the two 1-amino-2-nitroguanidinium-based salts have the potential to be useful energetic material in primary explosives and gas generating agent.
The calculated energy gap value of the title compounds are 5.58 eV and 4.39 eV, respectively, and salt 1 may have a lower reactivity while salt 2 has a higher reactivity.The molecular orbital and the electronic structure of the salts also meet the requirements of stability.

Figure 1 .
Figure 1.TG-DTG-DSC curves of the title salts at a heating rate of 10 °C min -1 .

Figure 3 .
Figure 3. Relationships between the thermodynamic functions and temperature (T).

Figure 2 .
Figure 2. HOMO, LUMO, and MEP of the title compounds.

Figure 4 .Scheme 2 .
Figure 4. Born-Haber cycle for the formation of energetic salts.a, b, c and d are the number of moles of the respective products.

Table 3 .
Detonation values of the title salts ) which were obtained according to reference 19; b calculated molar enthalpy of formation of the cation (kJ mol −1 ); c calculated molar enthalpy of formation of the anion (kJ mol −1 ); d calculated molar lattice energy (kJ mol −1 ); e calculated molar enthalpy of formation of the salt (kJ mol −1 ); f detonation pressure (GPa); g detonation velocity (m s −1 ); h oxygen balance (OB), an index of the deficiency or excess of oxygen in a compound required to convert all C into CO 2 and all H into H 2 O; for the compound with molecular formula C a H b N c O d (without crystal water), OB (%) = 1600 (d − 2a − b/2)/M w (%); data from reference 20; k the melting temperature (°C); l thermal decomposition temperature (°C) under nitrogen gas (DSC, 5 °C min −1 ).
i nitrogen content; j