Versatile N-Functionalization Strategies for - American Chemical Society

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Dancing with Energetic Nitrogen Atoms: Versatile N‑Functionalization Strategies for N‑Heterocyclic Frameworks in High Energy Density Materials Ping Yin,† Qinghua Zhang,*,‡ and Jean’ne M. Shreeve*,† †

Department of Chemistry, University of Idaho, Moscow, Idaho 83844-2343, United States Institute of Chemical Materials, China Academy of Engineering Physics, Mianyang 621900, China

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CONSPECTUS: Nitrogen-rich heterocycles represent a unique class of energetic frameworks featuring high heats of formation and high nitrogen content, which have generated considerable research interest in the field of high energy density materials (HEDMs). Although traditional C-functionalization methodology of aromatic hydrocarbons has been fully established, studies on N-functionalization strategies of nitrogen-containing heterocycles still have great potential to be exploited by virtue of forming diverse N−X bonds (X = C, N, O, B, halogen, etc.), which are capable of regulating energy performance and the stability of the resulting energetic compounds. In this sense, versatile N-functionalization of N-heterocyclic frameworks offers a flexible strategy to meet the requirements of developing new-generation HEDMs. In this Account, the role of strategic N-functionalization in designing new energetic frameworks, including the formation of N−C, N−N, N−O, N−B and N−halogen bonds, is emphasized. In the family of N-functionalized HEDMs, energetic derivatives, by virtue of forming N−C bonds, are the most widely used type due to the good nucleophilic capacity of most heterocyclic backbones. Although introduction of carbon tends to decrease energetic performance, significant improvement in material sensitivity makes this strategy attractive for safety concerns. More importantly, most “explosophores” can be readily introduced into the N−C linkage, thus providing a promising route to various HEDMs. Formation of additional N−N bonds typically gives rise to higher heats of formation, implying the potential enhancement in detonation performance. In many cases, the increased hydrogen bonding interactions within N−N functionalized heterocycles also improve thermal stability accordingly. Introduction of a single N,N′-azo bridge into several azole moieties leads to an extended nitrogen chain, demonstrating a new strategy for designing high-nitrogen compounds. The strategy of N−O functionalization has become an increasingly efficient tool for exploring new HEDMs with both high energy and low sensitivity. As a highly dense building block, introduction of oxygen not only improves density significantly but also gives rise to a better oxygen balance. Furthermore, the N−O functionalized strategy is highly suitable for a broad variety of N-heterocycles including five-membered azoles and six-membered azines. Newly explored N−halogen and N−B functionalization strategies have endowed the resulting HEDMs with some new energetic characteristics. Typical examples include the N-halogenated fused triazole and FOX-7 as potential hypergolic oxidizers with very short ignition delay times. In addition, some exploratory studies of N−B functionalized heterocycles have expanded energetic applications as hypergolic ionic liquids, green pyrotechnic colorants, and high-oxygen carriers. Overall, flexible N-functionalization methodologies involving different N−X bond formation have not only provided an efficient approach to diverse energetic ingredients but also expanded the application scope of energetic materials. Discussion and perspectives of N-functionalized protocols are given to summarize possible structure−property correlations, thus providing efficient guidelines for future design of new HEDMs.

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INTRODUCTION For an extended period, energetic materials have been one of the most important functional materials in the worldwide competition for the dominant position in areas of advanced weapons and equipment. As the source of energy in weapons, the superiority of advanced military equipment is greatly determined by the performance level of energetic materials. Due to their special importance to national defense of countries around the world, exploitation of novel energetic materials is a long-term strategic task. Examination of the research and development history of © XXXX American Chemical Society

energetic materials indicates that the evolution of high-energy density materials (HEDMs) has undergone several important stages where every conceptual breakthrough greatly promotes the research and development of energetic materials. First generation HEDMs are exemplified by the well-known 2,4,6trinitrotoluene (TNT), which was first synthesized in 1863 and has found wide use in melt-pour explosives.1 In the early 20th Received: October 22, 2015

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DOI: 10.1021/acs.accounts.5b00477 Acc. Chem. Res. XXXX, XXX, XXX−XXX

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Accounts of Chemical Research

Figure 1. Versatile N-functionalized HEDMs.

Scheme 1. Synthesis of N-Alkyl Functionalized Tetrazoles and Representative Compounds

age of second generation HEDMs.2 In 1987−2000, two caged high explosives with higher energy level, 2,4,6,8,10,12-hexanitro2,4,6,8,10,12-hexaazaisowurtzitane (CL-20) and octanitrocubane

century, cyclic nitramine-based explosives such as 1,3,5-trinitro1,3,5-triazacyclohexane (RDX) and 1,3,5,7-tetranitro-13,5,7tetraazacyclooctane (HMX) were synthesized, marking a new B


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Accounts of Chemical Research Scheme 2. Comparison of N-Methyl and N,N′-Ethylene Functionalized 5-Nitroimino Tetrazoles

Scheme 3. Synthesis of N,N′-Ethylene Bridged 1,2,4-Triazoles and Pyrazoles

(ONC), were discovered as third generation HEDMs.3,4 Because of these discoveries, the energy level of explosives and propellant ingredients has been greatly improved. Accompanying the significant enhancement in their detonation performance, the safety issue has become a more important concern for all countries. Against this background, recent research in energetic materials has focused on the design and synthesis of new HEDMs with both high energy and low sensitivity, with the aim of meeting the changing demands for specific military applications. With the development of energetic materials in the new century, numerous new materials, new concepts, and new technologies are emerging. In particular, the design and synthesis of new HEDMs based on nitrogen-rich heterocycles have become a hotspot in the field of energetic materials. Typical nitrogen-rich heterocycles include tetrazole, triazole, furazan, and tetrazine, which have a large number of inherently energetic N−N and C−N bonds, and a planar conjugated structure with electronic delocalization in these frameworks, thereby endowing them with some intrinsic benefits including high density, high heats of formation, high molecular stability, environmentally friendly explosion products, etc. Hence, the combination of these promising nitrogen-rich frameworks with versatile N-functionalization strategies that can efficiently introduce energetic groups into structures should provide new opportunities for the research and development of new-generation HEDMs. Since 2000, our

lab has been interested in the development of new high energy density frameworks through diversified N-functionalization strategies of nitrogen-rich heterocycles. An array of pyrrole, triazole, tetrazole, imidazole, tetrazine, and even fused polycyclic scaffolds participate as high enthalpimetric nucleophiles in the synthetic methodology of forming diverse N−X bonds (X = C, N, O, B, halogen, etc.). This followed by smartly introducing highly energetic “explosophores,” such as nitro, nitramine, nitrate ester, and trinitroethyl moieties into the newly formed N−X bond functional backbones, provides efficient protocols to synthesize new HEDMs including highly energetic salts. In this Account, recent contributions to this field by our group are described, especially focusing on those N-functionalization studies of nitrogen-rich heterocycles and the synthetic methods used to introduce “explosophore” groups into heterocyclic frameworks. A comprehensive comparison and discussion of synthetic chemistry of the N−X bond (X = C, N, O, B, halogen, etc.) formation and the suitability of N-functionalization methodology of nitrogen-rich heterocycles is emphasized (Figure 1). Through selected examples, the significance of versatile N-functionalization strategies and their potential as new architectural methodologies in the design and synthesis of new generation HEDMs are described. In addition, the aim of this Account is not only to present some fundamental methodological principles used for the design and synthesis of new N-heterocyclic HEDMs but also to provide some insights and C


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Accounts of Chemical Research Scheme 4. Synthesis of N,N′-2-Nitrazapropyl and Ether Bridged Azoles

Scheme 5. Introduction of the N-Tetrazolyl Group into 1,2,4-Triazoles Using Cyanogen Azide

N-substituted isomers. For example, N-methylation of sodium 5-aminotetrazolate using dimethyl sulfate yields a mixture of 1-methyl 5-aminotetrazole and 2-methyl 5-aminotetrazole in 35% and 30% yields, respectively.5 However, despite relatively poor selectivity, the alkylated products always exhibit a promising low sensitivity feature, thereby offering a fresh architectural tool for developing safer HEDMs. Earlier we demonstrated a highly efficient synthetic strategy for N-alkyl functionalized 5-aminotetrazole. When various primary amines were reacted with cyanogen azide generated in situ from sodium azide and cyanogen bromide, the resulting

inspiration for future research and development of energetic materials. In a few cases, the work of others has been referenced.

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SYNTHESIS OF HIGH-ENERGY DENSITY MATERIALS INVOLVING AN N−C FUNCTIONALIZATION STRATEGY The N−C functionalization strategy relies mainly on nucleophilic N-alkylation of nitrogen-containing heterocycles with alkyl halides. The challenges in this field have been concentrated on product selectivity because in many cases of nitrogen-rich azoles, N-alkylations of N-heterocycles only give a mixture of D


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Accounts of Chemical Research Scheme 6. Synthesis of N-Trinitromethyl and N-Dinitromethyl Azoles

Scheme 7. N-Amino Functionalization of Azoles

ethylene bis(5-nitroiminotetrazolate) (1-11) exceed those of the corresponding neutral precursors 1-7 (1-7, Td, 194 °C, vD, 9329 m s−1; 1-11, Td, 223 °C, vD, 9478 m s−1). Trinitroethylation of versatile N-substituted 5-aminotetrazoles was attempted by incorporating the oxygen-rich trinitroethyl moiety into nitrogen-rich tetrazole frameworks. The energetic compounds were found to have a performance level comparable to RDX with respect to both detonation properties and sensitivities.9 In the C−N functionalization strategy of new HEDMs, the ethylene moiety was a promising linker to bridge two energetic azolate functionalities. For example, 5-nitraminotetrazole (1-16) exhibits good density and detonation performance (d, 1.87 g cm−3; P, 36.3 GPa; vD, 9173 ms−1), but it has poor thermal stability and impact sensitivity (Td, 122 °C; IS, 1.5 J) (Scheme 2). N-Methylation of 5-nitriminotetrazole gives rise to a less sensitive product, 1-methyl 5-nitriminotetrazole (1-15) with decreased detonation performance (d, 1.76 g cm−3; P, 29.5 GPa; vD, 8433 ms−1; Td, 125 °C; IS, 12.5 J).10 In contrast, N-ethylenebridged 5-nitraminotetrazole (1-7) has very competitive properties including density, detonation values, thermal stability, and sensitivity (d, 1.86 g cm−3; P, 38.2 GPa; vD, 9329 m s−1; Td, 194 °C; IS, 10 J). In this design strategy, the ethylene linkage of bis(tetrazole) has become one of the most significant N-functionalities to tailor the integrated performance of energetic compounds. Due to the advantages of ethylene-bridged functionality for improving both energy level and safety issues, energetic analogues based on 1,2,4-triazole and pyrazole frameworks were synthesized, and their structure−property relationships were preliminarily studied.11 Unlike the N-alkylated nitraminotetrazoles, the bridging reaction of two energetic 3-amino-5nitro-1,2,4-triazole (ANTA) molecules by an ethylene linkage gives high selectivity due to electronic effect and steric hindrance (Scheme 3a). Further nitration of N,N′-ethylene bridged 3-amino-5-nitro-1,2,4-triazole resulted in a series of energetic salts. In the presence of tetraethylammonium bromide as phase transfer catalyst, N,N′-ethylene bridged 4-amino-3,5-dinitropyrazole (1-21) was obtained using dibromoethane and ammonium 4-amino-3,5-dintropyrazolate (Scheme 3b).12 Subsequent transformations of 1-21 involving nitration, oxidation, azidation, and diazotization gave rise to diversified HEDMs (1-22−1-25) with excellent energetic performance. When compared with our previous study of N,N′-ethylene tetrazoles, these 1,2,4-triazole- and pyrazole-based energetic analogues

Scheme 8. N-Trinitroethylamino Functionalized Energetic Azoles

nucleophilic substitution and cyclization gave rise to mono-, bisand tris(N-substituted 5-aminotetrazole) derivatives in good yields (Scheme 1).6 On the basis of nitrogen-rich N-alkylated 5-aminotetrazoles, energetic derivatizations were attempted using 100% nitric acid with N-alkyl substituted 5-aminotetrazoles, with the aim of enhancing their energetic performance.7 Further studies of their energetic salts were carried out by introducing nitrogen-rich cations.8 Of these energetic salts, the detonation velocity and thermal stability of bis(hydrazinium) E


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Scheme 9. Synthesis of N,N′-Azo Bridged Nitroazoles via Oxidative (a) Intermolecular and (b) Intramolecular Coupling Reaction

Scheme 10. Representative N-Nitroamino Functionalized Azoles

1-33) with reduced acidity and significantly improved thermal stability (1-32, P, 35.1 GPa; vD, 8909 m s−1; Td, 130 °C; 1-33, P, 34.6 GPa; vD, 8892 m s−1; Td, 203 °C). In continuing efforts to explore the diversified chemistry of cyanogen azide, this valuable methodology enables chemists to introduce the N-tetrazolyl functionality efficiently into frameworks of nitrogen-rich azoles, for example, 3,5-diamino-1,2,4triazole (DAT) and ANTA (Scheme 5a). Accordingly, a bicyclic triazole−tetrazole analogue was readily obtained via a one-pot cascade reaction. The possible mechanism was verified by a comparative reaction using 3,4,5-triamino-1,2,4-triazole with cyanogen azide as shown in Scheme 5b. Due to the excellent thermal stability (Td, 324 °C) and low impact sensitivity, the nitrogen-rich compound 1-40 can be used as a potential heatresistant explosive and propellant additive.

exhibit lower detonation performance, but feature better thermal and shock stability and versatile molecular functionalities. In addition to N,N′-ethylene nitramino azoles, continuing efforts have been focused on an energetic linker to connect two nitrogen-rich azoles. 1,3-Dichloro-2-nitro-2-aza-propane was employed as a building block to synthesize a family of new N,N′-2-nitrazapropyl bridged azoles (Scheme 4a).13,14 Compared with the ethylene moiety, the extra nitramine unit of the 2-nitrazapropyl linker leads to higher density, positive oxygen balance, and better detonation performance. Similarly, the facile syntheses of N,N′-ether bridged energetic azole compounds were also reported (Scheme 4b).15 Ether linkages can also be employed successfully to bridge highly energetic tetrazoles, e.g., 5-trinitromethyltetrazole and 5-nitrotetrazole moieties, thereby leading to high performance energetic compounds (1-32 and F


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energetic moieties such as −N−NO2, −NN−, and −NH− CH2−C(NO2)3 was achieved by first forming an −N−NH2 functionality in N-heterocycle structures. In general, energetic functionalization of −N−NH2 results in HEDMs with high heats of formation and excellent detonation performance. The main approach to N−N bond formation relies on introduction of an N-amino group using aminating reagents, for example, O-tosylhydroxylamine (THA), 2,4,6-trinitrophenylO-hydroxylamine (PHA), or hydroxylamine-O-sulfonic acid (HOSA). Although various N-amination methodologies were explored for organic synthesis for a long time, new efforts in constructing HEDMs and in-depth studies of their energetic properties are limited. In recent years, the N-amino functionality has been introduced into a variety of azole-based energetic frameworks. As seen in Scheme 7, N-amination of nitrogen-rich azoles leads to a family of new HEDMs (2-1−2-8).22−25 Among them, 2-amino-4,5-dinitro-1,2,3-2H-triazole (2-2) exhibits the most promising energetic properties (d, 1.83 g cm−3; P, 36.2 GPa; vD, 8843 m s−1; Td, 190 °C; IS, 24 J), which are comparable to those of RDX.23 The low melting point (94 °C) of 2-2 also highlights its promise as a potential replacement for TNT in melt-pour explosives. After successful N-amination of highly energetic N-heterocycles, Mannich-type reactions of N−NH2 are also useful for introducing the oxygen-rich trinitroethyl functionality. A two-step protocol of N-trinitroethylamination, involving N-amination followed by trinitroethyl functionalization, was reported recently (Scheme 8).22,26,27 Based on this strategy, a series of new N-trinitroethylamino functionalized azoles containing diversified explosophores, including nitro, azido, and nitro groups, was synthesized. Of these, 2-17 is a potential energetic oxidizer with a high density of 1.869 g cm−3 and positive oxygen balance (ΩCO = +14.3%). Recent intensive studies associated with the oxidative coupling of the N−NH2 moiety have illustrated a favorable pathway to synthesis of new high-nitrogen scaffolds, that is, HEDMs with long catenated nitrogen-atom chains.28 Here, however, the contradiction between high energy and molecular stability remains a challenge. For most azole-derived compounds with structures containing such nitrogen chains, the high energy level relies mainly on high nitrogen content and heat of formation arising from these growing nitrogenatom chains. Hence, continuous efforts were concentrated on

Scheme 11. Synthesis of 1,5-Di(nitramino)tetrazole and Its Energetic Salts

In addition to the search for replacements of ammonium perchlorate, chlorine-free HEDMs with positive oxygen balances have become an important goal for developing green energetic oxidizers. Trinitromethyl and dinitromethyl functionalities contribute positively to improving the oxygen balance of HEDMs. N-Propanonyl or N-butanonyl azoles can be readily converted to N-trinitromethyl functionalized energetic compounds in a mixture of sulfuric acid and nitric acid (Scheme 6).19Highly selective syntheses of N-dinitromethyl azoles were achieved by using water as the additive.20 A series of N-trinitromethyl pyrazoles was synthesized among which the polynitro compound 1-49 exhibited a positive oxygen balance (ΩCO2 = +7.8%) and a high density of 1.94 g cm−3.21 Extensive studies of these HEDMs involving heats of formation, detonation properties, and sensitivities indicate their likely application potential as green high energy oxidizers.

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SYNTHESIS OF HIGH-ENERGY DENSITY MATERIALS INVOLVING AN N−N FUNCTIONALIZATION STRATEGY Formation of N−N bonds is a widely used strategy for constructing new functional frameworks in organic methodology. In HEDM design, incorporating N-functionalized

Scheme 12. N-Oxidation of Azoles and Some Representative Compounds

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Accounts of Chemical Research Scheme 13. Synthesis of Tetrazole-1-olates and Representative Compounds

Scheme 14. N-Alkoxyl Functionalized Tetrazoles and Representative Compounds

Scheme 15. Synthesis of Tetrazine N-Oxides

HEDMs with long catenated nitrogen atom chains, these interesting energetic compounds possess superior overall energetic properties with respect to density, detonation performance, and impact and friction sensitivities. In addition, oxidative intramolecular coupling reaction of 5,5′-dinitro-2H,2′H-3,3′bi(1,2,4-triazole)-2,2′-diamine gives rise to a fused, tricyclic 1,2,3,4-tetrazine ring structure (2-24) (Scheme 9b).32 The crystal structure shows that 2-24 has high densities for both polymorphs (α polymorph, 1.907 g cm−3 at 21 °C; β polymorph, 1.901 g cm−3 at 21 °C) and a nearly planar structure. Additionally 2-24 has excellent detonation properties (P, 38 GPa; vD, 9.4 km s−1) comparable to HMX.

other approaches that can improve the energy level by introducing various energetic functionalities, for example, nitro, azido, and amino groups.29−31 A series of N,N′-azo-bridged nitroazoles containing four to six catenated nitrogen atom linkages was synthesized via sodium dichloroisocyanurate (SDIC) oxidation of N-amino nitropyrazoles, nitrimidazoles, and nitrotriazoles (Scheme 9a).31 Structure confirmation by X-ray diffraction analysis shows that the N,N′-azo moiety is coplanar with azole rings giving good thermal stability. In comparison with previously reported H


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Accounts of Chemical Research Scheme 16. N-Nitration and N-Chlorination of Tris(triazolo)benzene

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SYNTHESIS OF HIGH-ENERGY DENSITY MATERIALS INVOLVING AN N−O FUNCTIONALIZATION STRATEGY In the energetic materials field, N−O functionalities including N−OH and N→O units are among the most significant building blocks for use in constructing energetic moieties, such as nitro, azoxy, furazan, and furoxan. The N-oxidation methodology of nitrogen-rich heterocycles, in particular, for five-membered azoles and six-membered azines, has enabled a straightforward strategic approach for introducing additional N−O moieties. In addition to direct oxidation strategy, 1-hydroxyl and 1-alkoxyl functionalized tetrazoles with both high energy and low sensitivity can be readily constructed by cyclization of N-hydroximidoyl azides. In comparison to their precursors, the N−O functionalized heterocycles exhibit increased oxygen balance and higher density, thereby resulting in enhanced detonation performance. Using Oxone as the oxidant, a series of N-hydroxyl azoles was prepared, and the following reactions with nitrogen-rich bases gave a variety of energetic salts (Scheme 12).38−41 Due to their excellent detonation performance and high nitrogen and oxygen content, some monocyclic azolates (3-1−3-4) are potential propellant ingredients. In addition, various bis(azoles), for example, bis(1,2,4-triazole), bis(1,2,3-triazole), and bis(tetrazole) can be oxidized efficiently with Oxone with certain pH requirements. Compared with monocyclic analogues, some N-hydroxyl bis(azole)-based energetic salts (3-5−3-7) exhibit both high detonation performance and low impact sensitivity.25,42,43 In the family of N−O functionalized azoles, tetrazole-based frameworks always possess promising energetic properties. In general, direct N-oxidation of tetrazoles with Oxone give 2-hydroxyltetrazoles as the main product. Hence, preparation of 1-hydroxyl tetrazoles relies on cyclization of N-hydroximidoyl azides.44−48 Furthermore, tandem cyclization of aqueous hydroxylamine with cyanogen azide gives 1-hydroxy-5-aminotetrazole, which can be further oxidized to azo-coupled derivatives.49 The combination of 1-hydroxyl tetrazoles with various nitrogen-rich building blocks, for example, furazan, tetrazole, and 1,2,4-triazole, forms diversified new HEDMs with favorable energetic properties. In Scheme 13, some representative compounds (3-8−3-11) exhibit good thermal stabilities and excellent detonation performance as well as low sensitivities, thereby showing their application potential as promising RDX replacements. Cyanogen azide induced cyclization of alkoxyamine results in 5-aminotetrazole derivatives with an N−O functionality. By analogous nitration and salt formation, N-alkoxyl nitraminotetrazoles and their energetic salts are readily obtained (Scheme 14).50,51 In contrast to monocyclic tetrazole 3-12, N,N′-alkoxyl bridged tetrazoles exhibit high densities and superior detonation properties, for example, 3-12, d, 1.66 g cm−3, P, 31.5 GPa, vD, 8660 m s−1; 3-13, d, 1.90 g cm−3, P, 46.7 GPa, vD, 9867 m s−1;

Scheme 17. N-Halogen Functionalized FOX-7

While C-nitramino functionalized azoles have been widely studied, N-nitramino functionalized azoles are the new hot spot in the exploration of high performance HEDMs (Scheme 10). Using nitronium tetrafluoroborate as a mild nitrating reagent, N-nitramino functionalized tetrazoles and 1,2,4-triazoles (2-25−2-28) were prepared from various N-amino nitrogenrich precursors. 33,34 Theoretical studies of the 1,3-bis(nitrimido)-1,2,3-triazolate anion demonstrated the efficiency of the design strategy of alternating positive and negative charges (APNC) toward novel high explosives. A comparative study of N-nitro and N-nitramino functionalities also illustrated the superiority of the N-nitramination strategy.35,36 Energetic salts 2-29 and 2-31 behave as promising candidates for new HEDMs. Furthermore, N-nitramino functionalized bis(imidazole) 2-32 has an excellent density (d, 1.94 g cm−3) and detonation properties (P, 40.1 GPa; vD, 9350 m s−1), which are superior to those of HMX (d, 1.90 g cm−3; P, 39.5 GPa; vD, 9320 m s−1). Cyanogen azide-induced cyclization has been utilized in the synthesis of highly energetic 1,5-di(nitramino)tetrazole (Scheme 11).37 In the first experimental attempt, methyl carbazate was readily prepared from hydrazine hydrate and commercially available dimethylcarbonate. Its subsequent cyclization with cyanogen azide forms the tetrazole ring, giving rise to N-methoxycarbonyl protected 1,5-diaminotetrazole 2-33. Nitration using nitrogen pentoxide and deprotection in aqueous KOH gave potassium 1,5-di(nitramino)tetrazole (2-35), which can be acidified to the neutral molecule, 2-36, with dilute hydrochloric acid. Treating 2-36 with nitrogen-rich bases gives various energetic salts (2-37−2-39) in good yields. Despite high sensitivities, these new nitrogen- and oxygen-rich compounds show detonation performances competitive with that of CL-20, demonstrating their application potential as new primary explosives. Additionally, with both C- and Nnitramino functionalities, 2-36 has a very high N + O content of 92.62%. I


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Accounts of Chemical Research Scheme 18. Representative N-Boron Functionalized Energetic Compounds

3-15, d, 1.81 g cm−3, P, 38.4 GPa, vD, 9200 m s−1). Considering the high impact sensitive characteristics of 3-13 and 3-15 (IS 3-13, 1 J; 3-15, 1.5 J), these high performance HEDMs might be used with some desensitized ingredients. While more effort toward N-oxidation was dedicated to versatile five-membered azoles, N−O functionalization of sixmembered azines mainly focused on the N→O strategy of 1,2,4,5-tetrazine derivatives because of their high nitrogen content and good thermal stability. More importantly, different from most azole-containing acidic protons, N→O functionalized tetrazines are rarely acidic, a favorable property for practical applications. In the presence of trifluoroacetic anhydride and 50% hydrogen peroxide, a series of tetrazine-1-N-oxides and tetrazine-2,4-di-N-oxides was prepared (Scheme 15).52,53 In a single case, the N-oxidation process was accompanied by the transformation of a C-amino group into a C-nitro group (3-21), that is, N-oxidation of 6-azido-1,2,4,5-tetrazin-3-amine gave rise to a fused tetrazine−tetrazole structure (3-23) with an N→O functionality, with a high density of 1.874 g cm−3 and relatively low sensitivities (IS, 10 J; FS, 80 N).

Earlier the synthesis of N-nitro and N-chloro functionalized tris(triazolo)benzene derivatives as high-density energetic materials was initiated (Scheme 16).54 The N-nitration of tris(triazolo)benzene, using concentrated nitric acid and acetic anhydride, formed N-nitro functionalized tris(triazolo)benzene with good energetic performance (4-1). With sodium hypochlorite and acetic acid, the N-chloro functionalized product (4-2) was obtained in good yield. Different from traditional N-nitro functionalization that aims at improving density and detonation properties, N-chloro functionalization often endows the target energetic frameworks with a new role as hypergolic oxidizers. Compound 4-2 not only exhibits very promising oxidizing properties with alcohols but also displays excellent hypergolicity with reducing hydrazine-type fuels with short ignition delay times (ID) (e.g., hydrazine hydrate, ID, 8 ms; methylhydrazine, ID, 1 ms). With our interest in seeking novel hypergolic oxidizers, halogenated 1,1-diamino-2,2-dinitroethene (FOX-7) derivatives were synthesized (Scheme 17)55,56 with trichloroisocyanuric acid (TCICA). Initial attempts to oxidize FOX-7 resulted in a mixture of dichloro (4-3) and tetrachloro products (4-4). In the presence of excess TCICA, 4-3 undergoes oxidative coupling to form the azo-bridged compound 4-4. A more effective oxidative coupling of FOX-7 was achieved with sodium hypochlorite, resulting in formation of 4-4 and its isomer 4-5 in higher yields.57 Additionally, reactions of 1-bromo-1,1-dinitro-2-(N-bromoamidino)ethane with TCICA give two analogous azo-bridged FOX-7 compounds (4-7 and 4-8). Given their high density, good detonation performance, and hypergolic properties, these FOX-7 derivatives with N−halogen functionalities demonstrate a wide range of potential applications for both high-performance explosives and propellants.

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SYNTHESIS OF HIGH-ENERGY DENSITY MATERIALS INVOLVING AN N−HALOGEN AND N−B FUNCTIONALIZATION STRATEGY N−Halogen compounds are known as mild oxidizers and halogenating reagents in synthetic chemistry. In the field of energetic materials, some N−halogen functionalized HEDMs are hypergolic, which may arise from the combination of highly reactive N−halogen moieties with highly energetic nitrogen-rich backbones. When these energetic oxidizers containing N−halogen bonds contact a fuel, for example, hydrazine, spontaneous ignition is frequently observed with short ignition delay (ID) time. J


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seeking new HEDMs. From the view of synthetic chemistry, future directions of research will focus on the development of highly efficient and environmentally friendly energetic frameworks, which will also contribute greatly in promoting research and development of energetic materials. We believe that the four major categories of N-functionalization strategies described in this Account are very efficient tools to grow new HEDMs based on nitrogen-rich heterocycles that are far from being fully exploited. The research and development of new HEDMs will provide new opportunities, just as they have contributed to energetic material science over the past decade.

Energetic boron-containing compounds are widely used in pyrotechnic and propellant ingredients because of their unique combustion characteristics. With versatile structural design, N−boron-functionalized HEDMs show potential applications as energetic ionic liquids, green pyrotechnic colorants, and high oxygen carriers. Earlier we reported an effective approach to azolium poly(1,2,4-triazolyl)borate salts (Scheme 18a)58 including some nitrogen-rich compounds exhibiting favorable stabilities, densities, viscosities, and ignition delay times, which might be potential propellant fuels for replacement of toxic hydrazine.59,60 A novel family of N−boron functionalized nitroazolebased energetic salts was studied for pyrotechnic applications (Scheme 18b),61 which illustrate their promising performance as green colorants. In contrast to the environmentally hazardous barium(II) nitrate-based pyrotechnic formulations, these energetic azole borates are potential candidates for less-toxic green pyrotechnic colorants. In a solid propellant system, a highly dense oxidant is a determining factor for overall performance. Various boronderived compounds have been demonstrated recently as green energetic oxidizers.62−64 For some representative HEDMs (Scheme 18c), nitrogen−boron bond formation is the key strategy to link oxygen-rich units, which highlights a general concept of synthesizing high-oxygen carriers.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest. Biographies Ping Yin was born in Anhui (China) in 1986. He received his Ph.D. from Sichuan University in 2012, under the guidance of Professor Ling He. He then joined the research group of Professor Jean’ne M. Shreeve at the University of Idaho, United States. His current research interests are focused on the synthesis of nitrogen-rich heterocycles and their applications as energetic materials.

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CONCLUSIONS AND OUTLOOK Development of efficient and environmentally friendly N-functionalization methodology plays an important role in design and synthesis of new HEDMs based on heterocyclic frameworks. In this Account, we outline the major advances of the recently emerging energetic materials synthesized by various N-functionalization strategies of nitrogen-rich heterocycles and their properties, as well as potential applications in the energetics field. In the design of new energetic materials, these N-functionalization studies of nitrogen-rich heterocycles exhibit some favorable features: (1) reactions of primary amines with cyanogen azide generated in situ to construct 1-substituted 5-aminotetrazole frameworks; effective C−N bond formation achieved through N-functionalization reactions between nitrated azoles and energetic haloalkanes, and the advantage of high selectivity due to electronic effects and steric hindrance of nitro groups in the substrates; (2) selective N-amination approach, that is, the direct amination of an −NH group in energetic nitrogen-rich heterocycles such as C-fully nitrated azoles to form diversified derivatives of N-amino energetic compounds, thereby offering a wider platform of developing N-nitramino functionalized HEDMs and the N-nitramino and N,N′-azo functionalization strategy of N−NH2 in energetic azoles to improve the energy and sensitivity of energetic molecules simultaneously; (3) N-oxidation strategy of nitrogen-rich heterocycles, for example, introducing the N−OH or N→O moieties into fully C-nitrated azoles to improve the oxygen balance and density of frames straightforwardly and efficiently to achieve an enhanced level in detonation performance; and (4) introduction of N−halogen and N−B bonds into nitrogen-rich backbones to endow the resultant energetic compounds with versatile new applications, such as hypergolic oxidizers, pyrotechnic colorants, and high oxygen carriers. There is no doubt that this versatile N-functionalization of nitrogen-rich heterocycles methodology provides great opportunities to develop new HEDMs. Overall, appreciation of the significance of new functionalization strategies and methodologies is very helpful to researchers

Qinghua Zhang was born in Shandong (China) in 1979. He received his Ph.D. at the Lanzhou Institute of Chemical Physics (LICP) in 2008, under the supervision of Professor Youquan Deng. In 2010−2012, he worked in the group of Professor Francois Jérôme as a CNRS associate researcher at the University of Poitiers, France. In 2012−2013, he joined the group of Professor Jean’ne M. Shreeve as a postdoctoral fellow at the University of Idaho, USA. In 2014, he became a professor of chemistry at the Institute of Chemical Materials, China Academy of Engineering Physics. His research interests mainly focus on ionic liquids chemistry and energetic materials. Jean’ne M. Shreeve is a Montana native and received a Ph.D. in inorganic chemistry at the University of Washington, Seattle. She has been at the University of Idaho since 1961 where she served as chemistry department head and vice president for research and graduate studies. In 2011, Shreeve was named a University Distinguished Professor. Her research interests include the design, syntheses, characterization, and reactions of energetic materials, fluorine-containing compounds, and energetic ionic liquids published in 560 papers in refereed journals.

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ACKNOWLEDGMENTS We are grateful to ONR (Grant NOOO14-12-1-0536) and DTRA (Grant HDTRA 1-11-1-0034) for support of this work.

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