molecules Article Energetic Di- and Trinitromethylpyridines: Synthesis and Characterization Yiying Zhang, Xiaoyu Sun, Shannan Yu, Lingxiang Bao, Chenghui Sun * ID and Siping Pang * School of Material Science & Engineering, Beijing Institute of Technology, Beijing 100081, China; [email protected] (Y.Z.); [email protected] (X.S.); [email protected] (S.Y.); [email protected] (L.B.) * Correspondence: [email protected] (C.S.); [email protected] (S.P.); Tel.: +86-010-6891-8049 (C.S.); +86-010-6891-3038 (S.P.) Received: 6 November 2017; Accepted: 18 December 2017; Published: 21 December 2017 Abstract: Pyridine derivatives based on the addition of trinitromethyl functional groups were synthesized by the reaction of N2O4 with the corresponding pyridinecarboxaldoximes, then they were converted into dinitromethylide hydrazinium salts. These energetic compounds were fully characterized by IR and NMR spectroscopy, elemental analysis, differential scanning calorimetry (DSC), and X-ray crystallography. These pyridine derivatives have good densities, positive enthalpies of formation, and acceptable sensitivity values. Theoretical calculations carried out using Gaussian 03 and EXPLO5 programs demonstrated good to excellent detonation velocities and pressures. Each of these compounds is superior in performance to TNT, while 2,6-bis(trinitromethyl)pyridine (D = 8700 m·s−1, P = 33.2 GPa) shows comparable detonation performance to that of RDX, but its thermal stability is too low, making it inferior to RDX. Keywords: energetic materials; trinitromethyl; dinitromethyl; pyridine; detonation properties 1. Introduction There are currently ongoing efforts to synthesize novel energetic materials with improved performance and decreased sensitivity [1–6]. However, these requirements are quite often mutually exclusive, such that the desired combination of high detonation performance with maximum possible chemical stability is a major challenge. Over the last few decades, several strategies for the design of energetic materials that have a high energy content in conjunction with good thermal and mechanical stabilities have been developed [7,8], one of which is to replace benzene rings with nitrogen-containing heterocycles as fundamental components of molecular structures [2,9]. Compared with the benzene ring, nitrogen-containing heterocycles typically possess a greater enthalpy of formation as well as a higher density and more positive oxygen balance [10,11], all of which are desirable characteristics for new energetic compounds. In recent years, the application of azine as a new energetic skeleton has attracted considerable interest, and many polynitro-azine derivatives have been designed and synthesized [12]. However, due to the electron deficient character of the azine ring, it is difficult to perform electrophilic aromatic substitution on the ring, so the introduction of nitro groups into azine rings remains a challenging task in practice. Nitro-based moieties represent the most common energetic functional group, and contribute markedly to the overall performance of an energetic compound by enhancing both the oxygen balance (OB) and density [13–17]. Among these groups, the trinitromethyl group has the highest oxygen content, and its introduction to a molecule is essentially equivalent to adding two nitro groups (since one of the nitro groups is necessary for the complete oxidation of the carbon atom in the methyl group). The incorporation of trinitromethyl moieties into an azine ring is thus a potential means of increasing the OB so as to improve the detonation performance, and some trinitromethylazines have been synthesized using various methods. As an example, 2,4,6-tris(trinitromethyl)-1,3,5-triazine Molecules 2018, 23, 2; doi:10.3390/molecules23010002 www.mdpi.com/journal/molecules Molecules 2018, 23, 2 2 of 13 Molecules 2018, 23, 0002 2 of 13 wastrinitromethylazines obtained by the have destructive been synthesized nitration of using the carboxyl various groups methods. to trinitromethylAs an example, entities 2,4,6- in 2,4,6-tris(dicarboxymethylene)-1,3,5-triazinetris(trinitromethyl)-1,3,5-triazine was obtained by by action the destructive of concentrated nitration HNO of3 [the18]. carboxyl This compound groups to is antrinitromethyl oxidant and hasentities a high indensity 2,4,6-tris(dicarboxymethylene)-1,3,5-triaz (2.00 g/cm3), but it is unstable in theine air by and action very easyof concentrated to hydrolyze. Trinitromethyl-substitutedHNO3 [18]. This compound pyrazineis an oxidant was and also has obtained a high asdensity an unexpected (2.00 g/cm product3), but it byis unstable the treatment in the ofair 2-alkoxy-3-methylpyrazineand very easy to hydrolyze. Trinitromethyl-substit with an HNO3/H2SOuted4 mixture pyrazine [19]. was The also attachment obtained ofas trinitromethylan unexpected groupsproduct with by athe pyridine treatment ring of has 2-alko also beenxy-3-methylpyrazine attempted by different with an techniques. HNO3/H2 GakhSO4 mixture and Khutoretskii [19]. The reportedattachment the of synthesis trinitromethyl of 2-trinitromethylpyridine groups with a pyri bydine the reactionring has of alsoN-fluoropyridinium been attempted fluorideby different with trinitromethanetechniques. Gakh [ 20and]. PyridineKhutoretskii with reported methyl groupsthe synt inhesis the of 2, 2-trinitromethylpyridine 3, and 4 positions behaved by inthe the reaction same wayof N-fluoropyridinium and the corresponding fluoride trinitromethyl-substituted with trinitromethane methylpyridines[20]. Pyridine with were methyl obtained. groups Subsequently, in the 2, 3, theyand also4 positions studied behaved the behavior in the of thissame series way of and trinitromethylpyridines the corresponding upontrinitromethyl-substituted electron impact [21]. Anothermethylpyridines synthesis were method obtained. for 2-trinitromethylpyridine Subsequently, they also was reportedstudied the in 1993 behavior by the of ozone-mediated this series of Ctrinitromethylpyridines-nitration of methylpyridines upon withelectron nitrogen impa dioxidect [21]. [22]. MoreAnother recently, synthesis Katritzky method et al. obtainedfor 2- 5-methyl-2-trinitromethylpyridinetrinitromethylpyridine was reported on in the 1993 nitration by the ofozone-mediated 2,5-lutidine with C-nitration nitric acid of inmethylpyridines trifluoroacetic anhydridewith nitrogen [23 ].dioxide However, [22]. these More methods recently, shared Katritzky an obvious et al. obtained disadvantage 5-methyl-2-trinitromethylpyridine in that the yields were rather lowon the (below nitration 20%). of 2,5-lutidine with nitric acid in trifluoroacetic anhydride [23]. However, these methodsIn contrast, shared an the obvious synthesis disadvantage route of 2-trinitromethylpyridinein that the yields were rather through low (below the reaction 20%). of N2O4 withIn pyridine-2-carboxaldoxime, contrast, the synthesis route which of 2-trinit was developedromethylpyridine at the N.D.through Zelinsky the reaction Institute of ofN2 OrganicO4 with Chemistry,pyridine-2-carboxaldoxime, appeared to be a favorablewhich was method developed for high at yieldthe (moreN.D. Zelinsky than 60%) Institute [24]. In fact,of Organic a series ofChemistry, trinitromethyl-substituted appeared to be a favorable arenes and method hetarenes for high have yield been (more prepared than 60%) through [24]. theIn fact, interaction a series of Ntrinitromethyl-substituted2O4 with corresponding arenes carboxaldehyde and hetarenes oximes have [ 25been,26 ].prepared Although through tris(trinitromethyl)benzene the interaction of N2O4 haswith been corresponding synthesized carboxaldehyde by the reaction oximes [25], to [25,26]. the best Although of our knowledge, tris(trinitromethyl)benzene heterocyclic compounds has been withsynthesized more thanby the one reaction trinitromethyl [25], to the group best synthesizedof our knowledge, in this heterocyclic way have notcompounds yet been with reported. more Hence,than one introducing trinitromethyl more group trinitromethyl synthesized groups in intothis theway pyridine have not ring yet becomes been reported. an attractive Hence, but challengingintroducing more objective, trinitromethyl as it requires groups overcoming into the py electronicridine ring and becomes stereo-hindrance an attractive effects but challenging (Figure1). Atobjective, the same as it time, requires in consideration overcoming electronic of the well-known and stereo-hindrance instability of effects the trinitromethyl (Figure 1). At moietythe same at highertime, in temperatures, consideration the ionizationof the well-known of trinitromethyl-substituted instability of the pyridines trinitromethyl is also worthmoiety exploring, at higher as ionictemperatures, compounds the usuallyionization display of trinitromethyl-substituted better stabilities than their pyridines nonionic is precursors also worth [ 27exploring,]. An interesting as ionic denitrationcompounds reactionusually ofdisplay trinitromethyl better stabilities groups intothan dinitromethyltheir nonionic groups precursors in heterocycle [27]. An substratesinteresting hasdenitration been studied. reaction For of trinitromethyl example, The groups denitration into dinitromethyl of 2-trinitromethylpyridine groups in heterocycle with substrates potassium has iodide been instudied. methanol For example, allowed The for thedenitration obtainment of 2-trinitromethylpyridine of the potassium salt with of 2-(dinitromethyl)pyridine potassium iodide in methanol [24]. Theallowed reaction for the of mono(trinitromethyl)-1,3,5-triazinesobtainment