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2,3,5,6-Tetra(1H-tetrazol-5-yl)pyrazine: A Thermally Stable Nitrogen-rich Energetic Material Tomasz Grzegorz Witkowski, Elena Sebastiao, Bulat Gabidullin, Anguang Hu, Fan Zhang, and Muralee Murugesu ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.7b00138 • Publication Date (Web): 10 Jan 2018 Downloaded from http://pubs.acs.org on January 12, 2018

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2,3,5,6-Tetra(1H-tetrazol-5-yl)pyrazine: A Thermally Stable Nitrogen-rich Energetic Material Tomasz G. Witkowski,† Elena Sebastiao,† Bulat Gabidullin,† Anguang Hu,*‡ Fan Zhang,‡ Muralee Murugesu*† †

Department of Chemistry and Biomolecular Sciences, University of Ottawa, 10 Marie Curie, Ottawa, ON, Canada K1N 6N5 ‡ Defence Research and Development Canada-Suffield, PO Box 4000, Stn Main, Medicine Hat, AB, Canada T1A 8K6

ABSTRACT: A synthetic protocol was developed to create a unique nitrogen-rich (N = 71.58%) energetic material, namely 2,3,5,6-tetra(1H-tetrazol-5-yl)pyrazine (H4TTP). This compound was prepared using a convenient and straightforward method of synthesis and shows useful energetic properties such as detonation velocity (VC−1 J = 8655 m s ), detonation pressure (pC-J = 28.0 GPa) and high temperature of decomposition (Tdec = 260 °C). H4TTP was isolated and characterized using mass spectrometry, 1 13 multinuclear ( H, C) NMR spectroscopy, and vibrational spectroscopy (IR, Raman). The molecular structure was determined via single-crystal X-ray diffraction. High-nitrogen content and relative planarity of the molecule, along with the attained properties such as hydrolytic stability, high thermal stability, density, detonation velocity, and its facile synthesis make H4TTP a highly interesting energetic material.

Keywords: pyrazines, nitrogen-rich molecules, energetic materials, detonation properties, crystal structure High performance, stability, safety, and facile synthesis are the key aspects in the development of new energetic materi1 als (EMs) for future defense and space applications. In an effort to create evermore powerful and effective EMs, researchers use different strategies to increase the energy con1-2 tent of the desired molecule. The foremost challenge in the development of EMs is the combination of targeted performance properties with chemical and mechanical stability, features that tend to be counterproductive (e.g., the higher the energy content of the molecule the lower the thermal stability and/or higher the sensitivity towards external stimu1 li). One of the most promising approaches towards powerful EMs is carefully designed synthesis of high-nitrogen content 3-11 molecules (e.g. 1,2,4,5-tetrazines - 3,3'-azobis(6-amino10, 11 12-18 18 1,2,4,5-tetrazine), and tetrazoles - 5,5'-bistetrazole ; Chart 1). The significant differences in the bond energies for −1 −1 singly (160 kJ mol ), doubly (418 kJ mol ), and triply −1 (954 kJ mol ) bonded nitrogen atoms, as well as their strength, enables nitrogen-rich molecules to store relatively large amounts of energy which can be released upon its de1 composition. Moreover, nitrogen-rich EMs can be classified as environmentally friendly materials, since after decomposition the main product is non-toxic dinitrogen (N2) gas.

Chart 1. Examples of nitrogen-rich energetic materials: 3,3′azobis(6-amino-1,2,4,5-tetrazine) (DAAT) and 5,5'bistetrazole (H2BT). Highly sought-after EMs that operate at high-temperature regimes can be envisioned via the synthesis of covalent conjugated compounds with incorporated aromatic heterocy19-20 cles. In our on-going efforts to pursue powerful, nitrogenrich and eco-friendly EMs, we have focused our attention towards the synthesis of aromatic systems containing tetrazole moieties connected to a pyrazine molecule, two heterocyclic moieties that are of particular interest in the devel12-17, 21-23 opment of novel EMs. These heteroaromatic compounds are known to act as essential building blocks for the development of novel EMs with high enthalpy of formation, high thermal stability and dependable safety features; however, until now there has been no reported EM which combine these moieties within one powerful energetic molecule. To date, only one example of a tetrakis homosubstituted pyrazine with a heterocyclic system containing nitrogen atoms has been reported (e.g., 2,3,5,6-tetrakis(424 (trifluoromethyl)thiazol-5-yl)pyrazine). Other examples of tetrakis heterocycle-substituted pyrazine derivatives include 25-27 2,3,5,6-tetra(furan-2-yl)pyrazine and 2,3,5,628 tetra(thiophen-2-yl)pyrazine. However, in the recent literature, there are no reports related to the synthesis of highnitrogen content pyrazine-based EMs and their utility as possible EMs has not been explored. Nitrogen-rich covalent compounds can be used as EMs themselves, or their properties can be further tuned in order to meet the requirements necessary to be employed as primary explosives (e.g.: synthesis of energetic coordination compounds of d-block ele-

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ments ), or high-performing secondary explosives (e.g.: 32 2 synthesis of N-oxides and/or energetic salts ).

DMSO-d6. The C{ H} NMR spectrum of H4TTP showed two resonance bands at 154.0 (CN4) and 140.8 (C4N2) in DMSO-d6.

Herein, we report the first example of a facile synthesis of 2,3,5,6-tetra(1H-tetrazol-5-yl)pyrazine (H4TTP) from readily available starting materials and its comprehensive characterization as an EM. H4TTP is distinguished by high-nitrogen content (N = 71.58%) as a tetrazolo-pyrazine based EM and exhibits very promising performance characteristics (e.g., VC-J, pC-J). The presented results expand EMs research into a new and relatively unexplored area of nitrogen-rich molecules.

The molecular structure of H4TTP in the crystalline state was determined using low-temperature single-crystal X-ray 38 diffraction. Selected crystallographic data, measurement parameters and refinement details are given in the Supporting Information, while the molecular structure is shown in Figure 1. H4TTP crystallizes from tetrahydrofuran with two solvent and two water molecules per H4TTP molecule in the −3 monoclinic space group P 21/c, with a density of 1.424 g cm at 200 K and two molecules in the unit cell (Figures S1, S2; Table S1).

The present synthetic protocol towards pyrazine-2,3,5,6tetracarbonitrile (TCP, 3; Scheme 1) has been optimized 33 based on the initial reports by Webster, et al. and Begland, 34, 35 et al. Knowing that diiminosuccinonitrile readily reacts with water and hydroxylic solvents with the evolution 36 of hydrogen cyanide, further studies were performed in order to achieve the synthesis of TCP in one pot without isolation of diiminosuccinonitrile. The best results were obtained for trifluoroacetic acid (yield: 70%) as a solvent for both steps. In the first step, diaminomaleonitrile (1) is oxidized to diiminosuccinonitrile (2) using 2,3-dichloro-5,6dicyano-1,4-benzoquinone (DDQ) at ambient temperature. Subsequently, further addition of diaminomaleonitrile to the reaction mixture was performed to give TCP (3) as the main product. The now reduced 2,3-dichloro-5,6dicyanohydroquinone can be easily oxidized back to DDQ 37 using nitric acid. The final step involves the reaction of TCP (3) with sodium azide at 100 °C in dimethyl sulfoxide to give H4TTP (4), which is readily soluble in tetrahydrofuran, dimethylformamide, dimethyl sulfoxide, sulfolane, and sulfuric acid. As such, X-ray quality single crystals could be isolated using the aforementioned solvents/acid; however this leads to the inclusion of these guest solvent molecules in the crystal lattice (Figures S1-S4; Table S1). With that said, some of these reagents are commonly utilized in the production of commercial EMs, thus does not hinder any future large-scale production nor application.

The carbon-carbon bonds connecting the tetrazole and pyrazine rings are involved in a conjugated π electron system and therefore their lengths are shorter (C1-C5: 1.482(3) Å; C2C6: 1.472(3) Å) than the standard carbon-carbon single bond 39 length (dC−C = 1.54 Å). All five aromatic rings are essentially coplanar (∡N1-C1-C5-N3, 176.05(23)°; ∡C1-C2-C6-N7 −175.40(21)°) due to intramolecular hydrogen bonding between the tetrazole moieties (d(H1···N10) = 1.914(25) Å; ∡N3H1···N10 = 142.61(186)°). Such planar arrangement positively impacts the overall density of the molecule by providing an optimal spatial arrangement of the tetrazole rings.

Figure 1. Molecular structure of 2,3,5,6-tetra(1H-tetrazol-5yl)pyrazine (H4TTP). Perpendicular view of the plane encompassing H4TTP atoms (a) and parallel view of the plane encompassing H4TTP atoms (b). Thermal ellipsoids are drawn at the 50% probability level, and H atoms are shown as spheres of arbitrary radii. Co-crystallized tetrahydrofuran and water molecules are removed for clarity. Scheme 1. Synthetic route for the isolation of 2,3,5,6tetra(1H-tetrazol-5-yl)pyrazine (H4TTP). All isolated materials were initially investigated through high-resolution mass spectrometry (HRMS), multinuclear 1 13 ( H, C) NMR spectroscopy (Figures S5-S7), and vibrational 1 spectroscopy (IR, Raman, Figures S8, S9). The H NMR spectrum of H4TTP revealed one broad peak at 7.16 (4 H, NH) in

H4TTP is a hydrolytically stable energetic material. The thermal stability of H4TTP was measured using differential scanning calorimetry (DSC) where a single sharp peak at 260 °C is observed (Figure 2), which is indicative of a clean and complete decomposition. It should be noted that the decomposition temperature of H4TTP is higher than some of the state-of-the art systems such as 1,3,5-trinitro-1,3,540 triazinane (RDX) (Tdec(RDX) = 237 °C), making H4TTP a

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sure (pC-J = 28.0 GPa) are higher than the corresponding −1 values for H2BT (VC-J = 8109 m s , pC-J = 23.5 GPa) and are in −1 the range of DAAT (VC-J = 8800 m s , pC-J = 28.5 GPa; Table 1). Finally, the sensitivity to impact of a molecule can be connected with the electrostatic potential (ESP) of an isolated 1, 52-54 – molecule. The ESP of H4TTP on the 0.001 electron·bohr 3 hypersurface, with the color range from red (V(r) ≤ –0.075 Hartree) for electron-rich regions to blue (V(r) ≥ +0.075 Hartree) for electron-deficient, is given in Figure 2 (Fig55 ure S14). H4TTP possesses weaker and larger positive electrostatic potential regions than negative ones, which is an 1, 52-54 attribute of low-impact sensitive covalent species. Details on the measurements and computations are given in the Supporting Information.

Figure 2. Differential scanning calorimetry curve of H4TTP −1 (β = 5 °C min ) and the calculated electrostatic potential of H4TTP (view perpendicular to the plane encompassing H4TTP atoms). Color range: red for electron-rich regions, blue for electron-deficient regions. The molecular structure of H4TTP was fully optimized without symmetry constraints to C1 symmetry. The gas phase absolute molar enthalpy of H4TTP at 298 K and 1 atm were calculated theoretically using the modified complete basis set method (CBS-4M) with the Gaussian 09 (Revision E.01) 41 software. Gas phase standard molar enthalpy of formation at 298 K of H4TTP was attained using the atomization energy 42-44 Standard molar enthalpies of formation were method. calculated using the gas phase standard molar enthalpy and the standard molar enthalpies of sublimation (applying Trou45 ton's rule) - details are given in the Supporting Information. To put into perspective of the obtained results, the calculated −1 enthalpy of formation for H4TTP (3926 kJ kg ) is comparable with these given for 3,3′-azobis(6-amino-1,2,4,5-tetrazine) −1 (DAAT) and 5,5'-bistetrazole (H2BT) (3915, 3850 kJ kg , re10, 11, 46 spectively). To determine the molecular crystal structure of H4TTP for calculating the theoretical density, molecular packing simulations were carried out using the multistep simulation ap41 47, 48 proach with the Gaussian G09, modified MOLPAK and 49, 50 SIESTA program packages (details are given in the Supporting Information, Figures S10-S13; Tables S2, S3). The −3 calculated density of 1.945 g cm for H4TTP is higher than 10, 11, 51 densities reported of both DAAT and H2BT. Detonation parameters based on the calculated ΔfH° values and densities were computed using the EXPLO5 V6.04 thermochemical 38 computer code. The Becker-Kistiakowsky-Wilson equation of state (BKW EOS, with the following sets of constants: α = 0.5, β = 0.38, κ = 9.32, and Θ = 4120 for gaseous detonation products) and the Murnaghan equation of state for condensed products (compressible solids and liquids) were applied. The high density and a high heat of formation results in notable detonation parameters for H4TTP. Calculated −1 detonation velocity (VC-J = 8655 m s ) and detonation pres-

Table 1. Physico-chemical properties of DAAT, H2BT and H4TTP. DAAT H2BT H4TTP Formula C4H4N12 C2H2N8 C8H4N18 a N (%) 76.35 81.14 71.58 b −Ω (%) 72.68 57.93 81.75 c 10, 11 56 Tdec (°C) 252 255 260 d −3 10, 11 51 ρ (g cm ) 1.84 1.693 1.945 e −1 10, 11 46 ΔfH° (kJ mol ) 862 531.70 1383 −1 3915 3850 3926 ΔfH° (kJ kg ) 38 EXPLO5 V6.04 f −1 −ΔEU° (kJ kg ) 3767 3746 3530 g TC-J (K) 2751 2928 2692 h pC-J (GPa) 28.5 23.5 28.0 i −1 VC-J (m s ) 8800 8109 8655 j Gas vol. 750 781 678 3 −1 (dm kg ) a b c Nitrogen content. Oxygen balance. Temperature d e of decomposition. Density at 298 K. Standard molar f g enthalpy of formation. Heat of detonation. Detonation h i temperature. Detonation pressure. Detonation velocij ty. Volume of detonation gases at standard temperature and pressure conditions. Details are given in the Supporting Information. In summary, 2,3,5,6-tetra(1H-tetrazol-5-yl)pyrazine (H4TTP, 4) was synthesized by employing a facile and straightforward method. H4TTP was comprehensively characterized including single-crystal X-ray diffraction. H4TTP has highly desirable energetic properties, specifically, a high –3 calculated density of 1.945 g cm , great standard molar en−1 thalpy of formation of 1383 kJ mol , hydrolytic stability and high decomposition temperature of 260 °C, which are superior to those of RDX, H2BT and DAAT. This nitrogen-rich molecule (71.58%) has calculated detonation properties, which are comparable to high nitrogen content DAAT and H2BT molecules suggesting that H4TTP has a viable potential application as an environmentally friendly energetic material. The versatile structural features based on tetrazole/pyrazine motif opens a new avenue for the future exploration of H4TTP as a high-energy content building block for new primary explosives (e.g.: synthesis of energetic coordination compounds of d-block elements) and highperforming secondary explosives (e.g.: synthesis of N-oxides and/or energetic salts).

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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Experimental procedures and characterization, X-ray diffraction details, NMR spectra, calculation details (PDF)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected]

ACKNOWLEDGMENT This project was solely funded by Defense Research and Development Canada. The authors acknowledge collaborations with Dr. Mila Krupka (OZM Research, Czech Republic), in the development of new testing and evaluation methods for energetic materials and with Professor Dr. Muhamed Sućeska (University of Zagreb, Croatia), in the development of new computational codes for prediction of the detonation and propulsion parameters of energetic materials. Dr. Pierre Faudot dit Bel is gratefully thanked for many inspired and fruitful discussions.

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