6⁃（（2H⁃tetrazol⁃5⁃yl）⁃amino）⁃1，2，4，5⁃tetrazin⁃3（2H）⁃one：High⁃nitrogen Insensitive Energetic Compound Stabilized by π⁃stacking and Hydrogen⁃bonding Interactions

ZHANG Cong，CHEN Xiang，BAI Yang，GUO Zhao⁃qi，SONG Ji⁃rong，MA Hai⁃xia

（1.School of Chemical Engineering，Northwest University，Xi'an，710069，China；2.Conservation Technology Department，the Palace Museum，Beijing 100009，China.）

1 Introduction

The energetic materials（EMs） used at present are mainly nitro（—NO2），nitrate（—ONO2），and nitramine（—NHNO2）containing compounds，such as 2，4，6-trinitrotoluene（TNT），Hexogeon（RDX）and Octagon （HMX）.These materials，however，have drawbacks of sensitive to external stimulus，high toxicity and high environmental contamination during production.Herein，it is essential to exploit alternative environmentally friendly and insensitive energetic materials［1］.High-nitrogen heterocycle compounds，including tetrazine，tetrazole and triazole，exhibit excellent properties such as high positive heats of formation（HOF），high density，low sensitivity，good thermal stability，and releasing environment-friendly decomposition products［2-5］.The 1，2，4，5-tetrazine heterocycle with four nitrogen atoms in the six-member ring，is electron-deficient at positions 3 and 6，at which nucleophilic aromatic substitution is extremely easy to take place［6-7］.Therefore，the oxygen balance and density of tetrazine derivatives could be further improved by introducing various explosophores at these two positions.

As high nitrogen content heterocycles，1，2，4，5-tetrazine derivatives have shown great potential in the design of EMs and several related compounds have been reported.However，to the best of our knowledge，most of these studies focused on the synthesis and characterization of symmetrical tetrazine compounds［4-6］.The unsymmetrical 1，2，4，5-tetrazine derivatives，such as 3-amino-6-nitroamino-tetrazine（ANAT）and 3-（3，3'-dinitroazetidine）-6-（3，5-dimethylpyrazole）-tetrazine（DNAZTzDMP），also have high positive HOF，good thermal stability，excellent detonation performance and mechanical stability for the applications of EMs.Probably due to the need for more synthetic steps，the derivatives of unsymmetrical 1，2，4，5-tetrazine are seldom studied［8-9］.Therefore，it is worthy of taking effort to explore novel unsymmetrical 1，2，4，5-tetrazine based materials.

Hydrogen bonding interaction plays an important role in reducing sensitivity and increasing density of EMs.The introduction of hydrogen bonding donor and accepter to a main moiety is good strategy for design EMs.In addition，the crystal packing within a lattice has been shown a great influence on the sensitivity of EMs.Explosives that have parallel layers are generally low-sensitive to mechanical stimuli due to the ability of the layers to freely slide and disperse energy upon external stimuli［10-11］.A co-planar molecular structure combining with multiple hydrogen bonding interactions may result in insensitive EMs.

In view of the above considerations，we designed and synthesized a novel unsymmetrical tetrazine derivative 6-（（2H-tetrazol-5-yl）amino）-1，2，4，5-tetrazin-3（2H）-one（TATzO）using BTATz（3，6-Bis（1H-1，2，3，4-tetrazol-5-yl-amino）-1，2，4，5-tetrazine）as a raw material.In the structure，three-NH act as hydrogen donors while unprotonated N atoms and ketone O atom act as hydrogen acceptors.The crystal structure and thermal behavior of TATzO were investigated by single crystal X-ray diffraction，DSC and TG-DTG.The thermal explosion critical temperature（Tbp）was also estimated to evaluate the thermal safety performance.Besides，the detonation properties，including the detonation velocity（D）and pressure（p），were calculated based on Kamlet-Jacobs（K-J） equation，and the impact sensitivity（IS）was determined.

2 Experimental Part

2.1 Experimental Measurements

3，6-bis（3，5-dimethylpyrazolyl）-1，2，4，5-tetrazine（BT）and 3，6-bis（1H-1，2，3，4-tetrazol-5-ylamino）-1，2，4，5-tetrazine was prepared according to the Ref［7］.Sodium hydroxide and hydrochloric acid were purchased from Aladdin and used without further purification.

IR spectra were determined using an IRAffinity-1S spectrometer with ATR （Shimadzu）.Elemental analysis was performed on a VarioEL Ш analyzer（Elementar Co）.1H and13C NMR spectra were recorded on 600 MHz NMR spectrometer（Bruker） at 25℃，respectively.TG-DTG and DSC measurements were performed on SDT-Q600（TA） and Q2000（TA）apparatus under nitrogen atmosphere at a heating rate of 10℃min-1with the nitrogen flowing rate of 50 ml·min-1.The HOF of the title compound was theoretically calculated using Gaussian 03 program package［12］.

The impact sensitivity test was performed on a ZBL-B impact sensitivity instrument（Nachcn）.The mass of drop hammer was 2.0 kg，and the sample mass for test was 30 mg.

2.2 Synthesis of TATzO

BTATz（0.9 g，4 mmol）was immersed in 1.0 mol/L NaOH solution（5mL）for 10 h at 25 ℃with stirring.After that，the obtained solution was neutralized with 3 mol/L HCl aqueous solution until the bright orange precipitate formed.The precipitate was filtered，washed with H2O and dried under vacuum to give TATzO as orange solid or powder（Scheme1）.Elementalanalysis： calcd（% ） for C3H3N9O：C 19.82，H 1.84，N 70.21；found：C 19.89，H 1.67，N 69.90.IR（ATR，ν/cm-1）3498（w），3433（w），3252（m），3184（m），1693（s），1573（s），1514（s），1406（s），1357（m），1155（m），1103（s），1043（s），993（s）.1H NMR（600 MHz，DMSO-d6）：δ 12.13（s，1H）；13C NMR（600 MHz，DMSO-d6）：δ 151.2，151.7，155.6.

2.3 Crystal Structure Determination

The crystal of TATzO was chosen for X-ray de-termination.The data were collected on a Bruker SMART APEX CCD X-ray diffractometer（Bruker，Germany） with graphite-monochromatized Mo-Kαradiation（λ=0.071073 nm）.The structure was solved by the direct methods and refined by the fullmatrix least-squares method on F2with anisotropic the rmalparameters for allnonhydrogen atoms（SHELXS-97 and SHELXL2014）［13-14］.The hydrogen positions for water molecules were calculated by Fourier syntheses and hydrogen atoms of TATzO were added to their geometrically ideal positions and refined isotropically using a riding model.Crystal data and refinement results were summarized in Table 1（CCDC 1873209）.

Scheme 1 Synthesis of TATzO

3 Results and Discussion

3.1 Synthesis of the Title Compound

We initially tried to prepare the target compound through path 1（Scheme 1），and successfully obtained intermediate compound 6-（3，5-dimethylpyrazol-1-yl）-1，2，4，5-tetrazin-3（2H）-one（DPTzO）［15］.Then we attempted to synthesize TATzO involved the reaction of DPTzO and 5-amino-tetrazole（5-AT）in refluxing methanol.However，no product was obtained even after long reaction time.The reason was speculated to be that the strong electron-withdrawing effect of the ketone group reduces nucleophilic substitution ability of the para-carbon position of the tetrazine.After considering the reactivity of the tetrazine moiety，we switched the synthetic route to path 2 and successfully obtained the target compound.

Table 1 X-ray diffraction data collection and refinement parameters for TATzO·H2O

Fig.1 The asymmetric unit of TATzO·H2O，with displacement ellipsoids drawn at the 30%probability level.

3.2 Structural Characterization

Itcrystallizes with a calculated density of 1.730 g·cm-3at 296 K in the orthorhombic Pnma space group.A water molecule co-crystallize in the single crystal so that the structure can be described as TATzO·H2O（Fig.1）.For the tetrazine and tetrazole rings，the N—C bond lengths are within the range between 1.276 Å to 1.410 Å and an average value of 1.351 Å，which is longer than normal N═C bond（1.22 Å）and shorter than normal N—C bond（1.47 Å）（Table 2）.The N—N bond lengths have a minimum of 1.276 Å for N（1）—N（2）and a maximum of 1.355 Å for N（3）—N（4），which is longer than that of the normal N ═N bond（1.20 Å）and shorter than that of the normal N—N bond（1.41 Å）［16］.The tetrazine ring and the tetrazole moiety connected by amino group located on a crystallographic mirror plane.Thus，the whole molecule is a plane structure.The TATzO and solvent molecules bring about six types of hydrogen bonds in the crystal（Table 3），including five intermolecular hydrogen bonds（O（2）—H（2）…N（9）ⅰ，O（2）—H（2）A…N1ⅱ，N（3）—H（3）…N（8）ⅰ，N（5）—H（5）…O（1）ⅲ，N（6）—H（6）…O（2））and one intramolecular hydrogen bond（N（6）—H（6）…N（4））.

Table 2 Selected bond lengths and bond angles of TATzO·H2O

Table 3 Hydrogen-bond geometry of TATzO·H2O

Besides，those hydrogen bonds also form hydrogen-bond graph-set motifs（(8)，(10)and R(6)）， according to the definition of Bernstein［17］.The extensive inter-and intramolecular hydrogen bonds link the molecules into two-dimensional layer like structure（Fig.2）.It is reported that the intensive hydrogen-bonding interactions make an important contribution to improving the thermal stability and decreasing the sensitivity of EMs.On the other hand，the crystal structure of TATzO·H2O features face-toface geometrieswith the interlayerdistance of 3.104 Å and the π -stacked sheets are arranged in a layer-like structure（Fig.3）.In the case of external mechanical stimulation，the π -stacked structures can cause slippage between the layers，thereby counteracting external stimuli and reducing sensitivity.［18］

Fig.2 The two-dimensional structure of TATzO·H2O along the b axis hydrogen-bond interactions are shown as dashed lines.

To further understand the interactions between the molecules contained in TATzO·H2O，the Hirshfeld surface and the 2D fingerprint spectrum were analyzed［19-21］.Fig.4 shows an approximately platelike surface，indicating that the planar molecule of TATzO·H2O.Besides，the red dots showing the intermolecular close contacts are located on the side faces of the plate.It suggests that the intermolecular interactions in the crystal occur through the oxygen and nitrogen atoms encompassing the molecules.From fingerprint spectra of TATzO·H2O in Fig.5，it was observed that two sharp spikes exist in the bottom left of the spectra，indicating the interactions of O…H and N…H.Among them，the interaction of N…H（44.5%）was much higher than that of O…H（16.8%），showing the main interactions of the energetic molecules were N…H interactions.

Fig.3 Crystal packing of TATzO·H2O along the c axis.

Fig.4 Hirshfeld surface of TATzO·H2O.

Fig.5 Fingerprint spectra of TATzO·H2O.

3.3 Thermal Behavior Analysis

The typical DSC and TG-DTG curves of TATzO·H2O measured at heating rate of 10.0 K·min-1are shown in Fig.6 and Fig.7.Both DSC and TG-DTG curves show two reaction processes.On the DSC curve，the first stage is an endothermic decomposition process with the peak temperature of 136.34℃due to the dehydration of crystalline water.The second one is an intense exothermic decomposition process with the peak temperature of 230.46℃，which is higher than 1，1-diamino-2，2-dinitroethylene FOX-7（221.9 ℃）and lower than BTATz（320.5 ℃）［7，22］.Meanwhile，the first stage of decomposition process on TG curve between 129.18 to 138.94℃with a mass loss of 9.14%corresponds to the releasing of crystalline water（cal：9.05%）.The second stage ranges from 213.59℃to 234.05℃with a mass loss of 35.49%.

Fig.6 DSC curve for TATzO·H2O at 10 ℃·min−1

Fig.7 TG-DTG curves for TATzO·H2O at 10 ℃·min−1

3.4 Non⁃isothermal Kinetics and Thermal Explosion Critical Temperature

In order to estimate the thermal kinetics of the intense exothermic process of TATzO·H2O during the decomposition process，the kinetic parameters（the apparent activation energy（E）and pre-exponential constant（A））have been calculated through Kissinger method［23］and Ozawa method［24］at different Tp（maximum peak temperature）and Te（the extrapolated onset temperature）.All the data are listed in Table 4.The value of EOobtained by Ozawa method is in agreement with EKobtained by Kissinger method，and the linear correlation coefficient shows that the result is convincible.

Kissinger method：

Table 4 The kinetic parameters for TATzO·H2O at various heating rates.

The thermal stability of the EMs can be evaluated through the thermal ignition temperature（Tbe） and the thermal explosion critical temperature（Tbp）.Tbeand Tbpwere obtained by third order polynomial fit of the extrapolated onset decomposition temperature（Te）and the extrapolated peak temperature（Tp）corresponding to β →0 based on Eqs.（3） and（4）［26-27］.The parameters a，b and c are coefficients.The values of Tbeand Tbpof the compound are 213.75℃and 223.03℃，respectively，which are higher than the typical explosive FOX-7（206.0，207.1 ℃）and lower than BTATz（262.5，272.1 ℃），indicating good thermal stability of the title compound［25］.We speculate that high thermal stability is caused by the presence of a conjugation in the molecular structure and the introduction of amino groups to the structure.

3.5 Heats of Formation and Detonation Properties

The HOF of TATzO was calculated by designing an isodesmic reaction（Scheme 2）.For the involved compounds，geometric optimization and frequency were calculated using the B3LYP functional with the 6-311+G**basis set and the HOF of C2HN4O（1，2，4，5-tetrazin-3（2H）-one） was obtained byG3 method.Allofthe optimized structureswere characterized to be local energy minima on the potential-energy surface without any imaginary frequency.On the basis of the energy properties of the reference compounds（Table 5），the HOFof C2HN4O calculated by G3 theory is 459.92 kJ·mol-1，and the HOF of TATzO was predicted to be 777.09 kJ·mol-1.

Scheme 2 Designed isodesmic reaction for the prediction of the HOF of TATzO.

Table 5 Total energies（E0），zero-point energies（ZPE），thermal corrections（HT），and HOFs

Then，the heat of formation of TATzO（677.8 kJ·mol-1） in solid state was estimated with Trouton's rule（Eq.5），in which Tdrepresents the decomposition temperature［30］.

The performance of EMs is evaluated by its detonation velocity（D） and detonation pressure（p）.And the empirical Kamlet-Jacobs（K-J） equation（Eqs.（6）-（8）） are widely applied to estimate the values of D and p for the explosives containing C，where D is detonation velocity（m·s-1），p is detonation pressure（GPa），N is moles of gaseous detonation products per gram of explosives，Maveis average molecular weights of gaseous products，Q is chemical energy of detonation（J·g-1），and ρ is the density of explosive（g·cm-3）.

When using K-J equation，for the compound of CaHbNcOd，TATzO （C3H3N9O） suites b/2≥c，thus，the equation of explosion reaction is Equation（9）：

The calculated density of TATzO is 1.71 g·cm-3by the Monte Carlo method，which is almost equal with the density of single crystal of TATzO·H2O.Consequently， thecalculated Q，D and p are 5082 J·g-1，7757 m·s-1and 25.74 GPa，respectively.The detonation parameters of TATzO are slightly lower than those of BTATz（8055 m·s-1，25.39 GPa）［7］.

3.6 Impact Sensitivity

Sensitivity is an important property for EMs in terms of storage and practical application.The impact sensitivity（IS）was determined by fall hammer method using 2-kg drop weight with maximum height of 120 cm.No explosion was detected after 10 time strikes which means the compound has the IS value larger than 24 J.The IS of TATzO is slightly lower than BTATZ（22 J）.The reason for the low sensitivity is that there are hydrogen bonds and π stacking in the crystal.The π stacking in crystal makes it impossible to slide the molecules during impact resistance，resulting in counteracting external stimuli and reducing sensitivity［32］.

4 Conclusions

（1）The unsymmetrical tetrazine derivative 6-（（2H-tetrazol-5-yl）-amino）-1，2，4，5-tetrazin-3（2H）-one（TATzO）was synthesized and characterized by IR，EA and NMR.The hydrate with composition of TATzO·H2O was confirmed by single crystal analysis.The adjacent molecules form face-to-face molecular packing diagram with the interlayer distance of 3.104 Å.The π -stacked sheets are further expanding into layer-like structure through intensive hydrogen-bonding interactions.

（2）The thermal decomposition peak temperature is determined to be 230.46℃，indicating a better thermal stability than the traditional explosive FOX-7.The detonation velocity（D）and pressure（p） estimated by Eq.of Kamlet-Jacobs（K-J） are 7757m·s-1and 25.74 GPa，respectively.