Potassium (3-Methyl-2-oxido-1,2,5-oxadiazol-4-yl)dinitromethanide

Abstract

Furoxan derivatives enriched with explosophoric functionalities are promising compounds in the preparation of novel energetic materials. Herein, a previously unknown potassium (3-methyl-2-oxido-1,2,5-oxadiazol-4-yl)dinitromethanide (also referred to as potassium 4-dinitromethyl-3-methylfuroxanate) was synthesized via tandem nitration-reduction reactions of an available (furoxanyl)chloroxime. The structure of the synthesized compound was established by elemental analysis, IR, ¹H, ¹³C and ¹⁴N NMR spectroscopy. Thermal stability and mechanical sensitivity of the prepared compound toward impact and friction were experimentally determined.

1. Introduction

High-energy materials (HEMs) are among the most important functional materials and occupy a dominant position in the development of dual-use technologies [1,2,3,4,5,6,7,8,9]. Commonly used cyclic nitramines-1,3,5-trinitro-1,3,5-triazacyclohexane (RDX) and 1,3,5,7-tetranitro-1,3,5,7-tetraazacyclooctane (HMX) synthesized over a century ago are still considered the “golden standard” in the field of energetic materials due to their high detonation performance and facile synthesis. By the end of the 20th century, next-generation HEMs were obtained: 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL-20) [10,11] and octanitrocubane (ONC) [12]. Their functional properties are significantly higher than those of RDX and HMX; however, their sensitivity to mechanical stress is also much higher. Therefore, the search for new and effective, but less sensitive to mechanical stimuli, HEMs among polynitrogen–oxygen and nitrogen–oxygen heterocyclic compounds remain especially relevant. The use of heterocyclic scaffolds as a synthetic platform for the construction of high-energy structures has a number of advantages: such compounds usually have high enthalpies of formation, higher densities, and high nitrogen content, which allows for reducing the amount of oxidant used in energetic formulations. In a recent decade, new synthetic methodologies for the construction of heterocycle-based energetic materials were developed, and nowadays, a search of new HEMs is mainly performed in the field of heterocyclic chemistry [13,14,15,16,17]. A special interest in the energetic materials’ chemistry is focused on energy-rich substances containing the furoxan (1,2,5-oxadiazole 2-oxide) ring. Compared with other azoles, furoxan derivatives have a number of advantages, including high density, high enthalpy of formation and low volatility (Figure 1) [18,19]. Due to these properties, the furoxan ring became a key structural fragment in the synthesis of a number of high-energy substances [20,21,22,23].

Keeping in mind the desired synthesis of high-energy materials with an increased oxygen content, it seems promising to combine the furoxan cycle and the dinitromethyl functionality in one molecule. Recently, energetic dinitromethylfuroxan potassium salts 1 [24] and 2 [25] exhibiting high densities and thermal stability, as well as a positive oxygen balance to CO, were synthesized (Figure 2). However, both of these substances are highly sensitive to mechanical stress. Therefore, the goal of this work includes the synthesis of energy-rich potassium 4-dinitromethyl-3-methylfuroxanate 3, which will also have a high oxygen content, but due to the presence of a methyl group, it may turn out to be less sensitive to impact and friction.

2. Results and Discussion

A synthetic route to the target compound 3 is based on one-pot transformations of the readily available (furoxanyl)chloroxime 4, which was previously synthesized by our research group from ethyl acetoacetate [26]. We found that nitration of substrate 4 using an excess of N₂O₄ followed by a one-pot reduction in the intermediate gem-chlorodinitrofuroxan 5 upon its treatment with KI afforded potassium salt 3 in an overall yield of 58% (Scheme 1). The structure of the furoxanate 3 was confirmed by elemental analysis, IR, ¹H, ¹³C and ¹⁴N NMR spectroscopy. The ¹³C NMR spectrum showed four signals of carbon atoms: Me group (8.0 ppm), characteristic signals of C-3 and C-4 carbons of the furoxan ring (113.7 and 152.6 ppm, respectively) and a broadened signal of the C(NO₂)₂ functionality (123.3 ppm). The presence of the C(NO₂)₂ moiety was additionally confirmed by ¹⁴N NMR spectroscopy (characteristic signal at −23.5 ppm).

A set of important physicochemical properties of the potassium salt 3 was also established (

Table 1. Comparison of physicochemical parameters of the compound 3 with known energetic materials.

Compound	Td 1 [°C]	ρ 2 [g cm−3]	ΩCO 3 [%]	N+O 4 [%]	IS 5 [J]	FS 6 [N]
3	207	1.91	0	62.8	6	90
1	218	2.13	+21.3	65.9	2	5
2	186	2.14	+12.2	65.0	3	60
RDX	204 7	1.80	0	81.1	8 7	140 7
PETN	181 7	1.78	+15.2	78.5	3 7	70 7

¹ Decomposition temperature (DSC, 5 K min⁻¹). ² Density measured by gas pycnometry (298 K). ³ Oxygen balance (based on CO) for C_aH_bO_cN_d, 1600(c-a-b/2)/MW. ⁴ Combined nitrogen and oxygen content. ⁵ Impact sensitivity. ⁶ Friction sensitivity. ⁷ Ref. [27].

). It was found that compound 3 has a remarkably high density (1.91 g cm⁻³), zero oxygen balance (to CO) and quite high combined nitrogen-oxygen content (62.8%). Under linear heating at 5 K min⁻¹ rate, sample 3 first shows a phase transition at 133 °C with the following thermal decomposition after 207 °C (Figure 3). Hence, with this preliminary estimate of thermal stability, compound 3 could be placed in between known dinitromethylfuroxan derivatives 1 and 2. At the same time, the mechanical sensitivity of synthesized 3 is beneficially less than for previous synthesized salts 1 and 2 (Table 1). Impact sensitivity of 3 is on the level of nitramines (see RDX, Table 1), whereas the friction sensitivity of 3 approaches the nitroester’s level (see PETN, Table 1).

In conclusion, a new high-energy potassium 4-dinitromethyl-3-methylfuroxanate was synthesized through tandem nitration-reduction reactions of the readily available 3-methyl-4- (chloroximino)furoxan. Thermal stability and mechanical sensitivity of the synthesized compound were experimentally determined. It was found that thus-prepared potassium salt has a high decomposition temperature (207 °C), high density (1.91 g cm⁻³) and moderate sensitivity to mechanical stress, which enable its potential application in energetic materials science.

3. Materials and Methods

CAUTION! Although we have encountered no difficulties during preparation and handling of compounds described in this paper, they are potentially explosive energetic materials which are sensitive to impact and friction. Mechanical actions of these energetic materials, involving scratching or scraping, must be avoided. Any manipulations must be carried out by using appropriate standard safety precautions.

All reactions were carried out in well-cleaned oven-dried glassware with magnetic stirring. ¹H and ¹³C NMR spectra were recorded on a Bruker (Billerica, MA, USA) AM-300 (300 and 75.5 MHz, respectively) spectrometer and referenced to residual solvent peak. The ¹⁴N NMR spectrum was measured on a Bruker AM-300 (21.7 MHz) spectrometer using MeNO₂ (δ_14N = 0.0 ppm) as an external standard. The chemical shifts are reported in ppm (δ). The IR spectrum was recorded on a Bruker “Alpha” spectrometer in the range 400–4000 cm⁻¹ (resolution 2 cm⁻¹). Elemental analysis was performed by the CHN analyzer EuroVector EA (Pavia, Italy). All solvents were purified and dried using standard methods prior to use. All standard reagents were purchased from Aldrich(Burlington, MA, USA) or Acros Organics (Geel, Belgium) and used without further purification. Thermal analysis was performed using Netzsch DSC 204 HP (Selb, Germany) apparatus. Sample weighting 1.1 mg was poured in an aluminum pan, covered with a pierced lid and heated at 5 K min⁻¹ rate up to 300 °C under nitrogen flow (50 mL min⁻¹). Impact and friction sensitivity were determined with standard procedures; the details can be found elsewhere [27]. IR, ¹H, ¹³C and ¹⁴N NMR spectra are available in Supplementary Materials.

Synthesis of potassium (3-methyl-2-oxido-1,2,5-oxadiazol-4-yl)dinitromethanide 3. Chloroxime 4 (355 mg, 2 mmol) was placed in a 10 mL round-bottom flask and then N₂O₄ (2.6 mL, 40 mmol) was added in one portion at 20 °C. The resulting mixture was stirred until the complete dissolution of initial compound 4 and then for additional 24 h at 20 °C;. The thus-formed dark green solution was concentrated on a rotary evaporator and the residue was passed through a short pad of SiO₂ (eluent–CH₂Cl₂). Then, the solvent was evaporated and thus obtained chlorodinitrofuroxan 5 (340 mg) was dissolved in MeOH (4.5 mL) followed by an addition of KI (522 mg, 3.15 mmol) in one portion. The reaction mixture was cooled to 0 °C, stirred for 4 h and left in a refrigerator at 4 °C; for 12 h to initiate crystallization. The solid formed was filtered off, washed with cold MeOH (1 × 1 mL) and hexanes (2 × 5 mL) and dried in air. Yield 280 mg (58%), pale yellow solid. IR spectrum (KBr), ν, cm⁻¹: 2937, 2836, 1618, 1537, 1475, 1373, 1233, 1143, 1021, 818, 749. ¹H NMR (300 MHz, DMSO-d₆, 300K), δ, ppm: 2.00 (3H, s, Me). ¹³C NMR (75.5 MHz, DMSO-d₆, 300K), δ, ppm: 8.0 (Me), 113.7 (C-3 furoxan), 123.3 (br, C(NO₂)₂), 152.6 (C-4 furoxan). ¹⁴N NMR (21.7 MHz, DMSO-d₆, 300K), δ, ppm: −23.5 (C(NO₂)₂). Anal. calcd for C₄H₃KN₄O₆: C, 19.84; H, 1.25; N, 23.13; found: C, 20.12; H, 1.43; N, 22.79%.