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Synlett 2024; 35(17): 1985-1988
DOI: 10.1055/a-2283-5829
letter
Energetic Molecules

Electrochemical Efficient Synthesis of Two Azo Energetic Compounds

Jinhao Zhang
,
Yulan Song
,
Wenjia Hao
,
Rufang Peng∗
,
Bo Jin∗

This work was supported financially by the National Natural Science Foundation of China (22275151) and as a project of the State Key Laboratory of Environment-Friendly Energy Materials, Southwest University of Science and Technology (project no. 22fksy18).
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Abstract

Azo compounds with a high density, high enthalpy, and excellent detonation performance have received increasing research attention. The conventional method of chemical dehydrogenation that is used to form azo compounds involves the use of strong oxidants, resulting in environmental pollution. Electrochemical organic synthesis is considered an old method and a new technology. In this work, azofurazan tetrazole {H2AzFT; 5,5′-[diazene-1,2-diylbis(1,2,5-oxadiazole-4,3-diyl)]bis-1H-tetrazole} and azofurazan hydroxytetrazole (H2AzFTO) were synthesized by a green and efficient electrochemical dehydrogenation coupling of 5-(4-aminofurazan-3-yl)-1H-tetrazole and 5-(4-aminofurazan-3-yl)-1-hydroxytetrazole, respectively. The structures of H2AzFT and (NH4)2AzFTO were fully characterized by infrared spectroscopy, nuclear magnetic resonance, and elemental analysis, and their thermal stabilities were determined by differential thermal analysis.


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Key words

azo compounds - energetic materials - electrochemistry - green chemistry - dehydrogenation - coupling reaction

Energetic materials are a class of metastable substances that, on stimulation by external energy, can undergo intense redox reactions and quickly release their internal energy (usually accompanied by large amounts of gas and heat), resulting in combustion or explosion.[1] Energetic materials, such as explosives, propellants, initiators, smoke agents, and other substances, are widely used in military and civilian fields, and they occupy a pivotal position in national defense science and technology and national economy. In recent years, high-energy-density materials (HEDMs) have received extensive attention and have become a research hotspot.[2] Modern military requirements for explosives and propellants are stringent and must fulfill the following criteria: good thermal stability, high density, positive enthalpy of formation, high detonation velocity and pressure, high nitrogen content, near or above zero oxygen balance, and insensitivity.[3] However, high energy and low sensitivity are usually mutually exclusive properties, which is a major challenge for research on new HEMDs. Therefore, a rational design strategy to guide synthesis is crucial.[4]

Azo-linked azoheterocyclic energetic compounds have high enthalpies of formation. In addition, two or more azo-connected azoheterocyclic compounds can form a large conjugated system that increases the density of the compound.[5] The introduction of a N=N group increases the enthalpy of formation and improves the energy density, but reduces the hydrogen atom count of the compound and increases its oxygen balance.[6] The formation of azo bonds by conventional chemical methods involves the use of strong oxidants such as KMnO4/H+, K2Cr2O7/H+, or NaOCl/NaOBr.[7] [8] These methods require the use of hazardous and corrosive reagents, and excessive amounts of oxides or acids are likely to pollute the environment.

Electrochemical organic synthesis is considered a green synthesis technology. As an effective chemical synthesis method, electrochemical organic synthesis, which is sometimes referred to as an ‘old method, new technology’, has recently received increasing research attention.[9]Compared with conventional organic synthesis, electrochemical organic synthesis uses clean electrons as a reagent. It eliminates the need for additional reducing agents or oxidants, and it can be conducted at normal temperatures and pressures. Consequently, electrochemical organic synthesis is environmentally friendly. The voltage or current (density) can also be adjusted to control the progress of the reaction in an electrochemical reaction cell.[10]

Electrochemical synthesis, which provides unique benefits in synthetic chemistry, is undergoing a renaissance.[11] In recent years, the frequency of reports on the use of electrochemical methods to synthesize energetic materials has been gradually increasing.[9] [12] However, the intentional synthesis of new high-energy materials by electrochemistry remains a largely unexplored field.[13] Azofurazan tetrazole {H2AzFT; 5,5′-[diazene-1,2-diylbis(1,2,5-oxadiazole-4,3-diyl)]bis-1H-tetrazole} and azofurazan hydroxytetrazole (H2AzFTO), synthesized by oxidation with KMnO4, are high-energy azo-energetic compounds that display a high stability.[14] [15] In the present work, H2AzFT and H2AzFTO were synthesized by a green electrochemical method. Their structures were confirmed by a series of characterizations, and their thermal stabilities were tested by differential thermal analysis (DTA).

5-(4-Aminofurazan-3-yl)-1H-tetrazole [HAFT; 4-(1H-tetrazol-5-yl)-1,2,5-oxadiazol-3-amine] and 5-(4-aminofurazan-3-yl)-1-hydroxytetrazole (HAFTO) were synthesized by the method reported in the literature [see the experimental section in the Supporting Information (SI)].[16] [17] The compounds HAFTH and HAFTO were examined by cyclic voltammetry (CV). As shown in Figures [1b] and 1c, each compound showed only one independent oxidation process: an irreversible oxidation reaction. Their irreversible oxidations occurred at peak currents (E p) of 2.071 and 1.897 V versus Ag/AgCl, respectively. HAFT and HAFTO were electrolyzed in an undivided cell at room temperature at a constant potential of 1.8 V (Figures [1d–e] and SI, S1). The aqueous electrolyte solution was acidified with 2 M hydrochloric acid and extracted with ethyl acetate, and the organic solvent was removed to obtain H2AzFT (solid) or H2AzFTO (oil). (NH4)2AzFTO powder was subsequently obtained by adding a methanolic ammonia solution to H2AzFTO.

Zoom Image
Figure 1 (a) The electrochemical synthesis route. (b) and (c) CV curves (scan rate: 0.1 V/s) of HAFT and HAFTO. (c) Electrochemical synthesis product H2AzFT and (c) H2AzFTO

We first studied the coupling of HAFTO by –NH2 dehydrogenation. After an extensive evaluation of the reaction conditions, the desired product H2AzFTO was isolated as its ammonium salt (NH4)2AzFTO in 98% yield in a nonseparated cell by using a carbon rod anode and a platinum plate cathode, with Et4NCl as the supporting electrolyte and water as the solvent, at a constant voltage of 1.8 V at room temperature (Table [1], entry 1). On removal of (NH4)2CO3, the reaction hardly occurred (entry 2). This result showed that the electrochemical dehydrogenation of –NH2 on the HAFTO structure did not occur under acidic conditions. After confirming the importance of a base to the reaction, we chose K2CO3 and KOH to replace (NH4)2CO3 to explore their influence on the reaction (entries 3 and 4). In the presence of the strong alkali KOH, the yield decreased markedly, showing that a strongly alkaline environment is not conducive to the oxidative coupling of HAFTO. When (NH4)2CO3 was replaced with K2CO3, the yield was 84%, indicating that the NH4 + ion promotes the oxidative coupling of HAFTO. In addition, Et4NCl and several other electrolytes were studied. When Et4NAcO (entry 5) or Bu4NBr (entry 6) was used, the yield dropped significantly to 48 and 33%, respectively. In this electrochemical dehydrogenation coupling reaction, solvent molecules play an important role. When ethanol and acetonitrile were used as solvents, the reaction barely occurred (entries 7 and 8). Finally, no reaction occurred in the absence of an electric current, indicating that the reaction was caused by the electric current (entry 9).

Table 1 Optimization of the Reaction Conditionsa

Entry

Variation from standard conditions

Yieldb (%)

1

–

98

2

no (NH4)2CO3

trace

3

K2CO3 instead of (NH4)2CO3

84

4

KOH instead of (NH4)2CO3

25

5

Et4NAcO instead of Et4NCl

48

6

Bu4NBr instead of Et4NCl

33

7

EtOH instead of H2O

–

8

MeCN instead of H2O

trace

9

no electric voltage

–

a Standard reaction conditions: HAFTO (1.0 mmol), (NH4)2CO3 (0.75 mmol), Et4NCl (2.0 mmol), H2O (30 mL), C anode, Pt cathode, undivided cell, 1.8 V constant voltage, air atmosphere, r.t., 6 h.

b Isolated yield of the diammonium salt.

After determining the optimal reaction conditions, we carried out an electrocatalytic coupling reaction of HAFT. Under the optimized conditions, HAFT also had high reactivity and the yield reached 96%. The electrocatalytic coupling reaction conditions of HAFT were then examined (SI; Table S1). As with HAFTO, both the alkaline conditions and the electrolyte had a marked influence on the reaction, and NH4 + displayed a promotional effect on the reaction. To further verify the universality of the reaction conditions, we subjected 5-amino-1-oxytetrazolium hydroxylamine salt to electrolysis under the optimized conditions. Although we could observe progress in the reaction, decomposition occurred on the electrode due to the excessively high static inductance of the product (NH4)2AzTO,[7] and the electrode showed obvious vibrations and emitted a smell of smoke. Therefore, the electrochemical reaction is an efficient green synthesis method, but it is not suitable for the synthesis of energetic compounds with high static inductance. Subsequently, the thermal stability of (NH4)2AzFTO, synthesized by electrochemical dehydrogenation coupling, was tested by DTA (SI; Figure S2). The results showed that the decomposition of (NH4)2AzFTO involves only one intense exothermic stage, with no endothermic stage for loss of crystal water, compared with (NH4)2AzFTO tetrahydrate reported in the literature.[15] Crystals of (NH4)2AzFTO were obtained and tested by single-crystal X-ray diffraction. The single-crystal structure and crystal packing of (NH4)2AzFTO are shown in Figure [2]. The crystal data, structure refinement, and selected bond lengths and angles are listed in the SI, Tables S2 and S3. As expected, (NH4)2AzFTO consists of one AzFTO2– anion and two NH4 + cations, with no solvent molecules (Figure [2a]). Compared with (NH4)2AzFTO·4H2O (SI; Figure S3), solvent-free (NH4)2AzFTO exhibited a more regular face-to-face π–π stacking and a closer layer spacing (3.122 Å) than that of (NH4)2AzFTO·4H2O (4.318 Å). Therefore, (NH4)2AzFTO has a higher density and, possibly, better detonation properties than the other compounds.[18]

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Figure 2 Single-crystal structure (a) and crystal packing (b) of (NH4)2AzFTO

On the basis of these experimental observations and findings reported in the literature,[12] a possible mechanism for the electrochemical dehydrogenation coupling of the amino groups of HAFTO is proposed (Scheme [1]). First, two electrons are transferred from the C rod anode to the NH4AFTO salt to form stable free-radical intermediates. Secondly, the two free-radical intermediates couple to form the final product (NH4)2AzFTO.

In summary, we utilized the advantage of electrochemical synthesis to successfully synthesize H2AzFT and H2AzFTO in a weakly alkaline aqueous solution of (NH4)2CO3, using Et4NCl as electrolyte, carbon rod as anode, Pt sheet as cathode, and Ag/AgCl as reference, at a constant pressure of 1.8V. They all have very high yields. Their structures were confirmed by a series of characterization. Compared with conventional chemical synthesis, organic electrochemical synthesis of azo compounds is a safe, efficient and green method, which has a good prospect for the development of azo energetic compounds.

Zoom Image
Scheme 1 Proposed reaction mechanism

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Conflict of Interest

The authors declare no conflict of interest.

Supporting Information

    Supporting information for this article is available online at https://doi-org.accesdistant.sorbonne-universite.fr/10.1055/a-2283-5829.
  • Supporting Information
  • CIF File

Corresponding Authors

Rufang Peng
State Key Laboratory for Environment-Friendly Energy Materials, School of Materials Science and Engineering, Southwest University of Science and Technology
Mianyang, 621010
P. R. of China   

Bo Jin
State Key Laboratory for Environment-Friendly Energy Materials, School of Materials Science and Engineering, Southwest University of Science and Technology
Mianyang, 621010
P. R. of China   

Publication History

Received: 05 January 2024

Accepted after revision: 07 March 2024

Accepted Manuscript online:
08 March 2024

Article published online:
27 March 2024

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Zoom Image
Figure 1 (a) The electrochemical synthesis route. (b) and (c) CV curves (scan rate: 0.1 V/s) of HAFT and HAFTO. (c) Electrochemical synthesis product H2AzFT and (c) H2AzFTO
Zoom Image
Figure 2 Single-crystal structure (a) and crystal packing (b) of (NH4)2AzFTO
Zoom Image
Scheme 1 Proposed reaction mechanism