& Energetic Materials | Very Important Paper |

Energetic Salts Based on Tetrazole N-Oxide

Piao He, Jian-Guo Zhang,* Xin Yin, Jin-Ting Wu, Le Wu, Zun-Ning Zhou, andTong-Lai Zhang[a]

Chem. Eur. J. 2016, 22, 7670 – 7685 Ó 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim7670

ReviewDOI: 10.1002/chem.201600257

Abstract: Energetic materials (explosives, propellants, and

pyrotechnics) are used extensively for both civilian and mili-tary applications and the development of such materials,

particularly in the case of energetic salts, is subject to con-tinuous research efforts all over the world. This Review con-cerns recent advances in the syntheses, properties, and po-tential applications of ionic salts based on tetrazole N-oxide.

Most of these salts exhibit excellent characteristics and can

be classified as a new family of highly energetic materialswith increased density and performance, alongside de-

creased mechanical sensitivity. Additionally, novel tetrazoleN-oxide salts are proposed based on a diverse array of func-tional groups and ions pairs, which may be promising candi-dates for new energetic materials.

Introduction

Developing modern energetic materials is a significant area ofmaterials science research. However, achieving a workable bal-

ance between high detonation performance and low sensitivi-

ty remains a huge challenge. In addition to energetic co-crys-tals and metal–organic frameworks, another straightforward

and powerful approach is through the formation of energeticsalts.[1–4]

In comparison to conventional nonionic molecules, energet-ic salts have proven advantages, such as low vapor pressures,

which eliminate the risk of exposure through inhalation. More-

over, their properties can be readily optimized and improvedthrough judicious combination and independent modification

of different cations and anions, significantly increasing thenumber of energetic compounds available. Such novel proper-

ties contribute to a variety of unique applications, includingexplosives, gas generators, smoke-free pyrotechnic fuels, solid

fuels in micropropulsion systems, and effective precursors for

carbon nitride nanomaterials and carbon nanospheres.[5]

Among the most recent and exciting developments of high

energy density materials (HEDMs), energetic salts based on tet-razole continue to attract considerable research attention. The

first tetrazole was prepared in 1885 by the Swedish chemistJ. A. Bladin and knowledge of the tetrazoles has been consider-

ably increased since that time.[6] The tetrazole is characterized

by a five-membered ring consisting of one carbon and four ni-trogen atoms and, as the position of the hydrogen atom at-tached to nitrogen is indeterminate, may exist in tautomericforms, whereas the tetrazolate anion is formed through elimi-

nation of a proton from the NH moiety of tetrazole. Tetrazole-based salts are an important core of energetic materials be-

cause of the practical and theoretical significance and diversityof their properties: 1) because of the high nitrogen contentand ring strain, tetrazole compounds may be of high density,

have high heats of formation, and release considerable energyand gases upon decomposition or explosion; 2) owing to its

aromaticity, the tetrazole ring is thermodynamically stable inthe presence of acids and alkalis and during long periods of

boiling and heating; 3) the physical and explosive properties

of tetrazole derivatives can be easily modified by the replace-

ment of ring substituents with various functional groups;4) these tetrazole salts are more environmentally benign, since

their decomposition products predominantly consist of dinitro-gen.[7–9]

The combination of interesting energetic properties and un-

usual chemical structures has attracted numerous researchersto this unique class of compounds. Major advances have been

reported in the chemistry of 1H-tetrazole,[10–13] 5-aminotetra-zole,[14–24] 5-nitroiminotetrazole,[25–32] 5-nitrotetrazole,[33–38] 1,5-

diaminotetrazole,[33–43] bistetrazoles [5,5’-bistetrazole,[44–47] 5,5’-bis(1H-tetrazolyl)amine][48–50] and 5,5’-azotetrazole[51–55] and re-

lated energetic salts (Scheme 1). These tetrazole salts have de-

sirable properties that make them very promising candidatesfor highly energetic materials for industrial or military applica-

tions.The desirable characteristics of modern HEDMs include high

density, positive heat of formation, favorable oxygen balance,high thermal stability, remarkable detonation performance, low

sensitivity to external forces, and environmental friendliness.[56]

Among these properties, oxygen balance (OB) is used to repre-sent the degree to which an explosive can be oxidized, and

(OB) % for an explosive with the general formula CaHbNcOd andmolecular mass Mr can be calculated as: (OB) % = 1600 Õ

[d¢2 a¢b2]/Mr.

[57] Additionally, the sensitivity, strength, and bris-ance of an explosive are functions of the oxygen balance andtend to reach their maxima as the oxygen balance approaches

zero.[58]

To achieve a favorable oxygen balance, the traditional

method is to introduce nitro groups, such as thoe found in1,3,5-trinitro-1,3,5-triazinane (RDX), 1,3,5,7-tetranitro-1,3,5,7-tet-

razocane (HMX), and 1,1-diamino-2,2-dinitroethene (FOX-7).However, HEDMs, such as tetrazoles, with multiple nitro

groups may cause unexpected danger during both their syn-thesis and performance testing.[59] New trends in the researchand development of tetrazole-based energetic materials in-

clude introducing N-oxides onto the tetrazole ring to improvethe oxygen balance without losing energy or stability.[60]

In fact, there have been many publications devoted to thedetailed investigation of N-oxides. For instance, 1,2,3,4-tetra-

zine-1,3-dioxides show remarkably high stabilities and many

derivatives of this compound decompose at above 200 8C.[61]

2,4,6-Trinitropyridine-1-oxide simultaneously possesses both

higher density and higher energetic performance.[62] Amine-N-oxides show increasing density, stability, and performance of

energetic materials and have been used in insensitive explo-sives.[63] However, tetrazole N-oxides[64] are relatively uncom-

[a] P. He, Prof. J.-G. Zhang, X. Yin, J.-T. Wu, L. Wu, Z.-N. Zhou, T.-L. ZhangState Key Laboratory of Explosion Science and TechnologyBeijing Institute of Technology, Beijing 100081 (P. R. China)Fax: (+ 86) 10-68918091E-mail : zjgbit@bit.edu.cn

Chem. Eur. J. 2016, 22, 7670 – 7685 www.chemeurj.org Ó 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim7671

Review

mon in the literature and there is a pressing need for a system-

atic and comprehensive review of these interesting energeticcandidates with unique properties.

In this Review, several new families of energetic salts basedon tetrazole N-oxide that have been designed and synthesized

recently are discussed. Most of them exhibit suitable character-istics to be classified as new highly energetic members of the

well-known class of ionic salts with increased performance and

thermal stability, but decreased mechanical sensitivity. Recentdevelopments in the design, synthesis, and the available prop-

erty data including density (1), decomposition temperature(Td), heat of formation (HOF), detonation pressure (P), detona-

tion velocity (D), impact sensitivity (IS), friction sensitivity (FS),and electrostatic discharge (ESD) sensitivity of these tetrazole

N-oxide salts are provided. In addition, we have also designed

novel tetrazole N-oxide salts involving the 5-substituted tetra-zole N-oxide salts and bistetrazolate N-oxide salts, which may

be promising candidates for new energetic materials.

Tetrazole N-Oxide-Based Energetic Salts

Most non-oxidized tetrazoles suffer from a low oxygen balanceand a useful recent strategy for tailoring tetrazole-based ener-getic materials involved the oxidation of the tetrazole ringsinto their corresponding tetrazole N-oxides. Essentially, the N-

substituent of the tetrazole ring is replaced by a hydroxygroup. There are two (1N & 2N) possible different positions for

oxidation, namely, tetrazole 1N-oxides, and tetrazole 2N-oxides(Scheme 2). Tetrazole 1N-oxides are slightly more common inthe literature and have been prepared by rearrangement of 1-

alkoxy tetrazoles at high temperature or by use of trifluoroace-tic acid,[65] as well as through the reaction of toxic HN3 with ni-

trolic acids. Tetrazole 2N-oxides were not known until 2010,when 1,5-disubstituted tetrazoles were successfully oxidized

with hypofluorous acid in acetonitrile to give 1,5-disubstituted

tetrazole 2N-oxides. Transferring an oxygen atom to the 1- or2-substituted 5-alkyl or aryl tetrazole ring resulted in the corre-

sponding 2N-oxides by using the acetonitrile complex of thehypofluorous acid HOF·CH3CN.[66] Of the tetrazole N-oxides dis-

cussed herein, we have detailed the energetic salts of alkyl oraryl tetrazole N-oxides, nitrotetrazolate 2N-oxide, azidotetrazo-

late 2N-oxide, aminotetrazole 1N-oxide, cyanotetrazolate 2N-oxide, bistetrazolate N-oxide, and azotetrazole N-oxide.

Jian-Guo Zhang was born in 1974 in Hebei, P.R. China. He received a B.S. in chemistry atthe Hebei Nomal Univer, an M. S. in physicalchemistry and a Ph. D. in applied chemistryat the Beijing Institute of Technology. Since2000, he has been a faculty member at Bei-jing Institute of Technology, where he hasserved as head of the Energetic MaterialsDepartment and vice president of theSchool of Mechatronical Engineering. In2012, he was named a University Distin-guished Young Professor. His research inter-ests include the molecular design, synthesis,characterization, properties, and applicationof energetic materials and hydrogen storage materials.

Piao He earned a bachelor’s degree inChemistry at Hubei Institute for Nationalitiesin 2012 after 4 years’ study. In 2012, she wasa postgraduate student in Beijing Institute ofTechnology with Professor Jian-Guo Zhang.Since 2014, she has been a Ph.D. student atBeijing Institute of Technology under the su-pervision of Professor Zhang. Her main scien-tific interests include theoretical calculationsand the design and synthesis of organic com-pounds and ionic salts as promising energeticmaterials.

Scheme 1. Structural formulae of selected tetrazoles

Scheme 2. Tetrazole 1N-oxides and tetrazole 2N-oxides

Chem. Eur. J. 2016, 22, 7670 – 7685 www.chemeurj.org Ó 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim7672

Review

Alkyl or aryl tetrazole N-oxides

In the past, it was widely thought that the tetrazole ring resistsoxidation, even when strong oxidizing agents were used, be-

cause of the low HOMO,[67] so that there was a limited scopeof possible starting materials for formation of the N¢O bond in

this family of compounds. Fortunately, the first preparation ofa tetrazole oxide by oxidation of the tetrazole ring appeared in2010, when Harel and Rozen reported the synthesis of tetra-

zole N-oxides by using HOF·CH3CN (Scheme 3).[66] This is a pow-erful oxygen transfer reagent and incorporates a strongly elec-trophilic oxygen bonded to the electronegative fluorine atom.This reagent was previously employed in several unique pro-

cesses,[68–72] besides the useful transformation of tetrazole ringsinto their corresponding N-oxide derivatives.

Various 1- or 2-substituted 5-alkyl or aryl tetrazoles were suc-

cessfully oxidized by the HOF·CH3CN complex to give the pre-viously unknown tetrazole 3N-oxides including 1,5-pentame-

thylenetetrazole 3N-oxide (1 a’), 5-chloro-1-phenyltetrazole 3N-oxide (1 b’), 1-cyclohexyl-5-(4-chlorobutyl)tetrazole 3N-oxide

(1 c’), and 1-carboethoxymethyl-5-phenyltetrazole 3N-oxide(1 d’). Additionally the selective N-oxidation processes have

been summarized in two reviews, which stated the following:

1) the formation of tetrazole N-oxide derivatives is unique toHOF·CH3CN; 2) the oxygen’s position is exclusively at N3, re-

gardless of the group that is attached to the carbon of the tet-razole ring. This method of transferring oxygen may be a good

choice for first or difficult transformations, but there is a needto further form salts based on these alkyl or aryl tetrazole N-

oxides, which may improve their energetic performances.

Nitrotetrazolate 2N-oxide salts

5-Nitrotetrazole and its salts have high detonation velocities

and excellent combined oxygen and nitrogen contents,making them more powerful and environmentally friendly pro-

spective replacements for commonly used primary explo-sives.[33, 35] Gçbel et al.[73] developed a facile procedure for aque-

ous oxidation of nitrotetrazole to the corresponding nitrotetra-

zolate 2N-oxide and its ionic salts (Scheme 4).Ammonium nitrotetrazolate 2N-oxide (2-1) was mildly syn-

thesized by the oxidation of ammonium 5-nitrotetrazolatehemihydrate in a saturated potassium peroxymonopersulfate

(Oxone) solution at 40 8C and then the free acid nitrotetrazole2N-oxide (2) was easily separated after protonation reaction.

The hydroxylammonium salt 2-2 was prepared by simple ion

exchange with the ammonium salt. Salts 2-1, 2-3, and 2-4 withnitrotetrazolate 2N-oxide anion were prepared from the am-

monium salt by acid–base chemistry and metathesis reactionswith either acid or amino guanidinium bicarbonate, whereas

salts 2-6 and 2-7 were prepared from the silver salt 2-5 andthe corresponding substituted guanidinium halides. The com-

pounds prepared were characterized by X-ray diffraction, infra-

red and Raman spectroscopy, multinuclear NMR spectroscopy,elemental analysis, and differential scanning calorimetry (DSC)

measurements, and energetic performance properties werealso predicted. The physicochemical and detonation properties

of all 5-nitrotetrazolate 2N-oxide salts are given in Table 1.The substituted guanidinium salts show decreased thermal

stability with an increased number of amino substituents on

the guanidinium cation. The triaminoguanidinium salt 2-7 hasthe lowest decomposition temperature of 153 8C, and the gua-Scheme 3. Oxygenation of 1,5-disubstituted tetrazole

Scheme 4. Synthesis of nitrotetrazolate 2N-oxide salts

Chem. Eur. J. 2016, 22, 7670 – 7685 www.chemeurj.org Ó 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim7673

Review

nidinium salt 2-3 has the highest decomposition temperature

of 211 8C, which indicates that the nitrotetrazole 2N-oxide

anion is capable of forming highly thermally stable salts by ap-propriate cation pairing. Unlike the silver nitrotetrazolate, the

silver salt 2-5 is not a sensitive primary explosive and can besafely handled, whereas other salts with the nitrotetrazolate

2N-oxide anion are proven to have lower thermal stabilities.The remarkably high density of 1.94 g cm¢3 for nitrotetrazo-

late 2N-oxide 2 can be rationalized in terms of intermolecular

interactions. All nitrotetrazolate 2N-oxides have lower heats offormation compared with the free nitrotetrazole, except acid 2,

which has an increased heat of formation of 308.6 kJ mol¢1 asa result of the proton lying on oxygen as opposed to nitrogen.

Owing to their high experimentally determined densities, thesalts of nitrotetrazolate 2N-oxides have high performances.

Ammonium (2-1), diaminoguanidinium (2-6), and triaminogua-

nidinium (2-7) salts of nitrotetrazolate 2N-oxide show detona-tion properties (detonation velocities and pressures) similar to

those of RDX, making these compounds potential green alter-natives to RDX. The impact sensitivities of ammonium (2-1)

and hydroxylammonium (2-2) salts are 7 J and 4 J, respectively,which are slightly lower than that of RDX, whereas the substi-

tuted guanidinium salts (2-3, 2-4, 2-6, and 2-7) have impact

sensitivities much safer than that of RDX, ranging from 20 J to>40 J. In all cases, with the exception of the aminoguanidini-

um salt (2-4), the nitrotetrazolate 2N-oxide salts are less sensi-tive than the corresponding nitrotetrazolates.

The addition of the single oxygen atom to the ring simulta-neously decreases the heat of formation while allowing moreintermolecular interactions, both of which are factors that

reduce the sensitivity of tetrazole-based energetic materials to-wards mechanical stimuli. Even though there were slightly low-

ered decomposition temperatures, the increased density com-bined with the increased oxygen balance make tetrazole N-oxides a useful strategy for increasing energetic performance.The field of tetrazole chemistry may be expanded by develop-ing the first synthesis of an anionic tetrazole 2N-oxide in

a high-yielding, aqueous preparation starting from the parenttetrazole and it is possible that this new class of compoundswill form excellent energetic materials based on nitrogen-richsalts of the nitrotetrazole 2N-oxide moiety.

Azidotetrazolate 2N-oxide salts

5-Azidotetrazole and its salts are comparable or only slightlymore sensitive than most tetrazoles and alkali metal salts ofazidotetrazole often explode during crystallization.[75] The goodenergetic performance of azidotetrazole-based derivatives and

their unacceptable sensitivities made them interesting candi-dates for oxidation with anticipated lowering of sensitivities

and increase in performances. Klapçtke et al. synthesized the

5-azidotetrazolate 2N-oxide anion through oxidation of the 5-azidotetrazolate anion under mild aqueous conditions,[76] and

various nitrogen-rich and alkali metal salts were prepared bymeans of metathesis reactions (Scheme 5).

Oxidation of the 5-azidotetrazolate anion was performed ina saturated, buffered aqueous Oxone (2 KHSO5·KHSO4·K2SO4

triple salt) solution and the ammonium azidotetrazolate 2N-

oxide (3-1) was yielded over three days at 40 8C. Free acid 5-azido-2-hydroxytetrazole (3) and its sodium (3-2), potassium

(3-3), aminoguanidinium (3-4), and silver (3-5) salts were pre-pared from the ammonium salt by simple acid–base chemistry

and metathesis reactions with acid, alkali hydroxide, silver ni-trate, or aminoguanidine bicarbonate. The hydroxylammonium

azidotetrazolate (3-6) was prepared from 5-azidotetrazole and

hydroxylamine in aqueous solution. Detailed characterizationincluded X-ray diffraction, IR and NMR spectroscopy, differen-

tial scanning calorimetry (DSC), and impact, friction, and elec-trostatic discharge. The properties of azidotetrazolate 2N-oxide

salts are summarized in Table 2.The density of ammonium azidotetrazolate 2N-oxide (3-1;

1.69 g cm¢3) is the highest among the metal-free salts and is

also higher than that of its isomer 3 a (1.65 g cm¢3), which cor-roborates the theory of increased density in N-oxide com-

pounds. The most stable ammonium salt (3-1) decomposes at151 8C with no prior melting. However, the overall trend of the

azidotetrazolate 2N-oxide salts is that they possess slightly low-ered decomposition temperatures than non-N-oxide counter-

parts. This trend is comparable to that for the heats of forma-

tion, namely, the solid-state heats of formation of the ammoni-um (3-1) and aminoguanidinium (3-4) salts are lower by

6 kJ mol¢1 and 20 kJ mol¢1, respectively. As expected, N-oxideincorporation into tetrazole-based energetic materials leads to

increased performance. The ammonium (3-1) and aminoguani-dinium (3-4) salts have higher detonation velocities than the

Table 1. Properties of nitrotetrazolate 2N-oxide salts.

Species[a] 1 [g cm¢3] Td [8C] DfHq [kJ mol¢1] D [m s¢1] P [GPa] IS [J] IF [N] ESD (+ /-)[J]

2 1.94 ca. 120 308.6 9447 40.4 – – –2-1 1.73 173 152.0 8885 32.2 7 120 0.252-2 1.85 157 218.7 9499 39.0 4 60 0.052-3 1.70 211 136.7 8201 26.6 >40 252 0.202-4 1.70 185 256.4 8514 28.5 20 112 0.202-6 1.69 174 361.0 8686 29.2 40 120 0.202-7 1.64 153 471.5 8768 29.4 25 72 0.20RDX[74] 1.82 230 93 8977 35.2 7.4 120 0.2

[a] Density (1), decomposition temperature (Td), heat of formation (DfHq), detonation velocity (D), detonation pressure (P), impact sensitivity (IS), friction

sensitivity (FS), and electrostatic discharge (ESD) sensitivity.

Chem. Eur. J. 2016, 22, 7670 – 7685 www.chemeurj.org Ó 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim7674

Review

corresponding azidotetrazolates that lack an N-oxide. Sodium

azidotetrazolate 2N-oxide (3-2) is completely insensitive to-wards impact (>40 J) and has low sensitivities to friction

(120 N) and ESD (0.5 J). The most sensitive silver salt (3-5) ex-ploded repeatedly on attempts to handle and measure sensi-

tivities, so it can be concluded that the impact sensitivity ismuch less than 1 J and the friction sensitivity is much less than

5 N. In all cases, the azidotetrazolate 2N-oxides were found to

be of lower sensitivity than the analogous azidotetrazolatesalts while still being highly sensitive towards mechanical stim-

uli.Again, the lower heats of formation, higher performances,

and lower sensitivities of azidotetrazolate 2N-oxide salts aremanifested and the same trend that was observed for nitrote-

trazolate 2N-oxide and nitrotetrazolate. However, several ofthese compounds exhibit extensive nonexplosive decomposi-tion during synthesis. Therefore, only crystal structures wereobtained. An interesting sodium azidotetrazolate 2N-oxidemonohydrate is insensitive towards impact, thus rendering it

the safest known compound that contains the azidotetrazolemoiety.

Aminotetrazole 1N-oxide salts

5-Aminotetrazole[14] is a commercially available nitrogen-rich(82 %) compound and has also been utilized as a valuable in-

termediate in the synthesis of tetrazole compounds and ener-getic salts, owing to its versatility and ease of preparation. The

attractive properties of 5-aminotetrazole derivatives and their

salts combine with reasonably high hydrolytic and thermal sta-bilities to make them promising candidates for future applica-

tions as high-explosive compounds, gas generators, or compo-nents of propellants.[16, 20, 24] A useful strategy for tailoring tetra-

zoles is the oxidation of the tetrazole rings into their corre-sponding tetrazole N-oxides. However, the methods of aque-ous Oxone oxidation that work for 5-nitro- or 5-azidotetrazoles

fail with 5-aminotetrazole because of the presence of other ox-idizable nitrogenous species in the molecules. Fischer et al.proposed the cyclization of azido-oximes,[77] namely, 1-hydroxy-5-aminotetrazole (4), which is readily and economically avail-

able from the reaction of aqueous hydroxylamine with cyano-gen azide. Several energetic salts of 1-hydroxy-5-aminotetra-

zole with high performances have also been successfully syn-thesized (Scheme 6).

1-Hydroxy-5-aminotetrazole (4) was accessible from the reac-

tion of one equivalent of hydroxylamine with a solution of cya-nogen azide in MeCN solution, albeit in very low yield (about

10 %). The yield can be increased to about 90 % by using twoequivalents of hydroxylamine, which precipitates the hydroxy-

lammonium salt of 1-hydroxy-5-aminotetrazole (4-1). Pure an-

hydrous compound 4 can be crystallized from the aqueous so-lution after acidification, then the ammonium salt (4-2) and hy-

droxylammonium salt (4-1) of 1-hydroxy-5-aminotetrazolewere obtained from compound 4 after the addition of ammo-

nia and 50 % hydroxylamine, respectively. The prepared com-pounds were characterized by X-ray diffraction, IR, Raman, and

Scheme 5. Synthesis of azidotetrazolate 2N-oxide salts

Table 2. Properties of azidotetrazolate 2N-oxide salts

Species[a] 1 [g cm¢3] Td [8C] DfHq [kJ mol¢1] D [m s¢1] P [GPa] IS [J] IF [N] ESD (+ /-)[J]

3-1 1.69 151 534.0 8926 32.5 1 20 0.033-2 1.61 – – – – >40 120 0.503-3 2.07 – – – – – – –3-4 1.61 – 623.3 8332 26.2 – – –3 a 1.65 96 608.8 8959 32.4 <1 <5 0.10TNT[74] 1.65 295 ¢67 6881 19.5 15 353 3.5RDX[74] 1.82 230 93 8977 35.2 7.4 120 0.2

Chem. Eur. J. 2016, 22, 7670 – 7685 www.chemeurj.org Ó 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim7675

Review

NMR spectroscopy, elemental analysis, and DSC. The important

properties and behaviors of aminotetrazole 1N-oxide and its

salts are listed in Table 3.It is worth noting that hydroxylammonium 1-oxido-5-amino-

tetrazolate (4-2) crystallizes in two different polymorphs, an or-thorhombic one (4-1 a) with a density of 1.66 g cm¢3 and

a monoclinic one (4-1 b) with a higher density of 1.74 g cm¢3.The monoclinic form was obtained by crystallization from

MeOH and the orthorhombic form from water, whereas the

original precipitate from MeCN was orthorhombic. Compound4 decomposes at 105 8C which is quite lower than the decom-

position point of 5-aminotetrazole (199 8C).[78] With regard to 1-oxido-5-aminotetrazolate salts, low-density polymorph 4-1 amelts (with decomposition) at 155 8C, whereas the ammoniumsalt (4-2) has a decomposition temperature of 195 8C. 1-Hy-

droxy-5-aminotetrazole (4) and its hydroxylammonium salt (4-1 a) show high detonation velocities of 8609 m s¢1 and9056 m s¢1, respectively, and were moderately sensitive, with

an impact sensitivity of 10 J and friction sensitivities of 108 Nand 360 N, respectively. Additionally the monoclinic form 4-1 bhas an even higher velocity of detonation of 9312 m s¢1, owingto its higher density.

1-Hydroxy-5-aminotetrazole, as a unique precursor for the

preparation of several energetic compounds, was synthesizedfor the first time by a novel approach, namely, the reaction ofaqueous hydroxylamine and cyanogen azide. The attractiveproperties of hydroxylammonium salts again confirm the high

performance of tetrazole N-oxides as energetic materials.

Cyanotetrazolate N-oxide salts

5-Cyanotetrazole would be a good starting material for the

synthesis of C-H-N-only compounds and a new family of ener-

getic salts containing the 5-cyanotetrazolate anion and nitro-gen-rich cations.[79] The combination of high detonation veloci-

ties (comparable to common explosives, such as TNT or RDX)with lower sensitivities and excellent thermal stabilities affords

them prospective interest for energetic applications. Withinthe field of tetrazole N-oxides, researchers were previously lim-

ited by the lack of comparison between 1- and 2-tetrazoleoxides as nitrotetrazole and azidotetrazole were known as

their 2-oxides, whereas aminotetrazole was known as its 1-

oxide. Boneberg et al. detailed the preparation and characteri-zation of salts of cyanotetrazole 1- and 2-oxides,[80] and also re-ported tetrazole oxides functionalized with carboxamide,1,2,4,5-tetrazine, and 1,4-dihydro-1,2,4,5-tetrazine (Scheme-

s 7and 8).Silver cyanotetrazolate 2N-oxide (5 a-1) was first prepared by

oxidation of an aqueous solution of sodium 5-cyanotetrazolate

with Oxone overnight at 40 8C followed by precipitation withsilver nitrate. The ammonium salt (5 a-2) was obtained from

the ion exchange of the tributylammonium salt, which was iso-lated by extraction of the crude reaction mixture with ethyl

acetate and the addition of aqueous sodium tributylammoni-um sulfate. The cyanotetrazolate 1-oxide anion can be ob-

tained from the sodium salt by the reaction of azidoaminofura-

zan and sodium nitrite. This sodium salt was precipitated asthe silver salt (5 b-1) by addition of silver nitrate, or trans-

formed into the ammonium salt (5 b-2) by ion-exchange chro-matography. From the silver salts 5 a-1 and 5 b-1, the nitrogen-

rich aminoguanidinium (5 a-3), diaminoguanidinium (5 a-4),and triaminoguanidinium (5 a-5) salts of cyanotetrazolate 2N-

oxide and aminoguanidinium (5 b-3), and triaminoguanidinium

(5 b-5) salts of cyanotetrazolate 1N-oxide were easily preparedby metathesis reactions with the corresponding halides. Two

interesting copper salts of the carboxamidotetrazole oxides, asdehydrates 5 c-1 and 5 d-1, were yielded by the hydrolysis of

cyanoazoles in aqueous media.When cyanotetrazole 1N-oxide and 2N-oxide were protonat-

ed with nitric acid yielding 5-cyano-2-hydroxytetrazole (5 a)

and 5-cyano-1-hydroxytetrazole (5 b), subsequent treatmentwith hydrazine formed the corresponding dihydrotetrazines as

their hydrazinium salts (5 e-1, 5 f-1). The protonated products(5 e, 5 f) were then deprotonated with ammonia, yielding am-

monium salts (5 e-2, 5 f-2), respectively and the correspondingtetrazines (5 g, 5 h) were obtained after oxidation of the freedihydrotetrazines. Additionally the ammonium salt (5 g-1) was

also obtained by further reaction with aqueous ammonia andwas fully characterized. the chemical (NMR, IR, and Raman

spectroscopies, mass spectrometry, etc.) and explosive (detona-

Scheme 6. Synthesis of aminotetrazole 1N-oxide salts

Table 3. Properties of aminotetrazole 1N-oxide salts

Species[a] 1 [g cm¢3] Td [8C] DfHq [kJ mol¢1] D [m s¢1] P [GPa] IS [J] IF [N] ESD (+ /-)[J]

4 1.70 105 255.7 8609 29.8 10 108 0.64-1 a 1.66 155 284.8 9056 32.7 10 >360 0.34-1 b 1.74 – 278.3 9312 35.7 – – –4-2 1.53 195 226.7 8225 24.5 >40 >360 1.5RDX[74] 1.82 230 93 8977 35.2 7.4 120 0.2HMX[74] 1.91 287 105 9320 39.6 7.4 112 0.2

Chem. Eur. J. 2016, 22, 7670 – 7685 www.chemeurj.org Ó 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim7676

Review

tion and sensitivity) properties of these energetic salts are

summarized in Table 4.The density of 5 g-1 (1.63 g cm¢3) is the highest observed for

the crystalline CHNO compounds within this work. Again thecyanotetrazolate 1-oxide has a slightly higher density than that

of the cyanotetrazolate 2-oxide in cases of ammonium salts

5 a-2 (1.53 g cm¢3) and 5 b-2 (1.55 g cm¢3), triaminoguanidini-um salts 5 a-5 (1.55 g cm¢3) and 5 a-5 (1.58 g cm¢3). With re-

spect to thermal stabilities, only ammonium salt 5 a-2 and ami-noguanidinium salt 5 a-3 of cyanotetrazolate 2-oxide show the

Scheme 7. Synthesis of cyanotetrazolate N-oxide salts

Scheme 8. Synthesis of cyanotetrazolate N-oxide derivatives

Chem. Eur. J. 2016, 22, 7670 – 7685 www.chemeurj.org Ó 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim7677

Review

decomposition temperature above 180 8C. These nitrogen-rich

salts are of comparable or higher performances than TNT andthe best values were obtained for triaminoguanidinium salts,

5 a-5 and 5 b-5, with detonation velocities of 8044 km s¢1 and

8214 km s¢1, respectively, thus indicating higher performancefor the 1-oxides. However, there is no appreciable difference

between the two ammonium salts, 5 a-2 and 5 b-2, with deto-nation velocities of 7749 km s¢1 and 7730 km s¢1, respectively.

With the exception of highly sensitive tetrazine 5 g, the pre-pared compounds show lower sensitivities, generally being

classified as “less sensitive” to “sensitive” (Classification system

according to the UN Recommendations on the Transport ofDangerous Goods: [a] Impact: Insensitive, >40 J; less sensitive,

�35 J; sensitive, �4 J; very sensitive, �3 J. Friction: Insensi-tive, >360 N; less sensitive = 360 N; sensitive, <360 N and

>80 N; very sensitive, �80 N; extremely sensitive, �10 N) en-ergetic materials. The specific impulse, related to potential use

as a propellant ingredient, was also calculated and the highest

value was computed for compounds 5 b-2, 5 a-5, and 5 b-5,with the values of 228 s, 227 s, and 229 s, respectively.

5-Cyanotetrazolate 2-oxide was synthesized from the 5-cya-notetrazolate anion with Oxone, whereas 5-cyanotetrazolate 1-

oxide was obtained by rearrangement of azidoaminofurazanand this offers a unique opportunity to compare the effects of

tetrazole 1- versus 2-oxidation. The preparations and character-

izations of energetic salts of cyanotetrazolate 1- and 2-oxides,as well as tetrazine oxides thereof, may provide base materials

or synthetic strategies to afford new energetic materials.

5,5’-Bistetrazolate N-oxide salts

5,5’-Bistetrazole, which has a very high nitrogen content of

83 %, and in particular its nitrogen-rich salts show excellentthermal stabilities, while being comparatively insensitive,which has resulted in a wide variety of applications.[81] For in-stance, owing to its high nitrogen content and high decompo-

sition temperature, ammonium 5,5’-bistetrazolate was investi-gated as a gas-generating component in fire-resistant resins

and fire-proof adhesives in airbags. The suitability of several ni-

trogen- rich 5,5’-bistetrazolates have been reported for use aslow-smoke pyrotechnic fuels.[82] Hydrazinium 5,5’-azotetrazo-

late and triaminoguanidinium 5,5’-azotetrazolate are currentlybeing developed for use in composite modified double-base

propellant formulations.[83] A further improvement of the ener-getic character of tetrazole derivatives is the introduction of N-

oxides, which not only leads to a better oxygen balance, thus

ensuring maximum energy output on decomposition/explo-sion, but also affords compounds with higher densities and

lower sensitivities.

5,5’-Bis(tetrazole 1N-oxide) salts

Fischer et al. recently systematically described the preparations

and characterizations of the energetic nitrogen-rich salts of

5,5’-bis-(tetrazole 1N-oxide),[84, 85] which show great promise foruse as an explosive filler in the future, and compared these to

their 5,5’-bis-(tetrazole 2N-oxide) analogues.[86] A promisingcandidate that fulfils various desirable properties is 1H,1’H-5,5’-bistetrazole-1,1’-diol (BTO). This dihydroxylated bistetrazole isstrongly acidic and bears two protons, which can be easily re-

moved by nitrogen-rich bases or other tetrazole derivatives.

There were three major routes to the BTO moiety(Scheme 9). Oxidation of the parent heterocycle 5,5’-bistetra-

zole with aqueous Oxone yielded mixtures of the 1,1’- and1,2’-dihydroxy species and favored the 2,2’ isomer as the major

product, with only low amounts (11 %) of the 1,1’ isomer,which crystallized upon addition of aqueous hydroxylamine

(method 1).[84] In 2001, Tselinskii et al. reported the formation

of the aforementioned precursor BTO by cyclization of diazido-glyoxime,[87] which itself was synthesized by starting from com-

mercially available glyoxal (method 2). Diazidoglyoxime wasprepared in DMF by the chloro–azido exchange reaction of di-chloroglyoxime, which was prepared by chlorination of glyox-ime with Cl2 gas in ethanol. The prepared solution of diazido-glyoxime in DMF/N-methyl pyrrolidone (with sodium chlorideimpurity) was cyclized under acidic conditions (HCl gas in di-

ethyl ether) and gave the 1H,1’H-5,5’-bistetrazole-1,1’-diol(method 3).[84]

An overview is depicted in Scheme 10. Starting from 1H,1’H-

5,5’-bistetrazole-1,1’-diol (6 a), most of the salts were synthe-sized by treating 6 a with stoichiometric amounts of the re-

spective free bases, such as hydroxylamine, ammonia, hydra-zine hydrate, and other nitrogen-rich bases or, in the cases of

guanidinium and aminoguanidinium salts 6 a-4 and 6 a-5, the

respective carbonate and hydrogencarbonate. The diamino-guanidinium salt 6 a-6 was prepared from a reaction mixture

with the barium salt of 6 a and diaminoguanidinium sulfateafter removing the barium sulfate precipitate by filtration. The

triaminoguanidinium salt 6 a-7 was obtained from a mixture ofthe free acid 6 a and triaminoguanidinium chloride. For 6 a-7

Table 4. Properties of cyanotetrazolate N-oxide salts

Species[a] 1 [g cm¢3] Td [8C] DfHq [kJ mol¢1] D [m s¢1] P [GPa] IS [J] IF [N] ESD (+ /-)[J]

5 a-2 1.55 184 325.9 7749 22.2 15 216 0.305 b-2 1.53 172 354.9 7730 22.0 35 360 0.755 a-3 1.50 228 426.0 7451 19.6 40 324 1.505 a-4 1.53 152 537.2 7790 21.9 40 324 0.505 a-5 1.55 160 649.2 8044 23.7 40 324 0.555 b-5 1.58 150 672.9 8214 25.1 40 216 0.505 g-1 1.63 189 ¢40.8 7799 22.7 40 240 0.50TNT[74] 1.65 295 ¢67 6881 19.5 15 353 3.5

Chem. Eur. J. 2016, 22, 7670 – 7685 www.chemeurj.org Ó 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim7678

Review

and also for the 3,6-dihydrazinium 1,2,4,5-tetrazine salt 6 a-12,

the respective chloride salts were treated with 6 a. 2:1 stoichio-metries were used in all respective reactions, except for 6 a-8,

owing to its low water solubility, and 6 a-12, since the tetrazinederivative was used as its dihydrochloride. Unexpectedly, the

crystallization of compounds only occurred with a 2:1 stoichi-

ometry in 6 a, 6 a-1 to 6 a-4, 6 a-10, and 6 a-15, whereas thestoichiometry was 1:1 in the cases of 6 a-5 to 6 a-8, 6 a-11 to

6 a-14, and 6 a-16, with even doubly protonated cations in thecases of 6 a-7, 6 a-9, 6 a-11, and 6 a-12. All compounds were

isolated and fully characterized by X-ray diffraction, IR, Raman,and multinuclear NMR spectroscopy, elemental analysis, and

DSC. Characterization data and various critical properties arelisted in Table 5.

The densities of the energetic salts ranged from 1.60 (amino-guanidinium salt 6 a-5) to 1.88 g cm¢3 (dihydroxylammoniumsalt 6 a-1). With the exception of the 1-amino-3-nitroguanidini-

um salt 6 a-8, the nonionic compound 6 a-15, and the 1,5-dia-

minotetrazolium salt 6 a-16, all remaining salts decomposed attemperatures above 180 8C; the highest decomposition tem-

perature was observed for the ammonium salt 6 a-2 (290 8C),which is known as ammonium BTO explosive (ABTOX). The

heats of formation range from highly positive values (>3000 kJ kg¢1) for the hydrazinium salt 6 a-3, as well as the bis(h-

Scheme 9. Synthesis of 5,5’-bis(tetrazole 1N-oxide)

Scheme 10. Synthesis of 5,5’-bis(tetrazole 1N-oxide) salts

Chem. Eur. J. 2016, 22, 7670 – 7685 www.chemeurj.org Ó 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim7679

Review

ydrazinotetrazinium) (6 a-12) and diaminouronium (6 a-9) salts,

even the tetrazolium derivatives (6 a-13 to 6 a-16) to negative

values for the oxalyldihydrazidinium salt 6 a-11 (¢1363 kJ kg¢1).The detonation velocities range from 7917 m s¢1 (6 a-4) to

9698 m s¢1 (6 a-1), whereas only four compounds (6 a-1, 6 a-3,6 a-13, 6 a-16) attain the detonation velocity calculated for

RDX (8977 m s¢1), whereas the detonation pressures rangefrom 23.3 GPa (6 a-4) to 42.4 GPa (6 a-1), and only three com-

pounds (6 a-1, 6 a-13, 6 a-16) slightly exceed the value for RDX

(35.2 GPa). Another restriction of their utility as RDX replace-ments is their high sensitivities (6 a-3 : IS = 9 J, FS = 252 N; 6 a-13 : IS = 4 J, FS = 72 N; 16 a-3 : IS = 2 J, FS = 160 N). However, itis worth noting that dihydroxylammonium 5,5’-bistetrazole-

1,1’-diolate (TKX-50, 6 a-1)[88–90] has exemplified the utility ofthe tetrazole N-oxide chemistry by providing a new explosivematerial with exceedingly powerful performance (D =

9698 m s¢1, P = 42.4 GPa), sufficiently low thermal sensitivity,low toxicity, and safe handling. All of these characteristics ofTKX-50 make this excellent material an appropriate candidateto replace the most commonly used military explosive RDX.

1H,1’H-5,5’-bistetrazole-1,1’-diol can be easily deprotonatedin aqueous media by using different nitrogen-rich bases or

metathesis reactions. The highest heats of formation of solids

are observed for the tetrazolium derivatives, owing to thelarge number of inherently energetic nitrogen–nitrogen bonds

in combination with solvent-free crystallization. Unfortunately,the compounds with the highest heats of formation do not

have the highest densities, resulting in the undesirable per-formances. However, TKX-50 shows potential to fulfil the long-

standing goal of a green RDX replacement.

2.6.2 5,5’-Bis(tetrazole 2N-oxide) salts

After discovering the outstanding characteristics of TKX-50 as

a high explosive, more and more attention has been paid tothis approach for the formation of N-oxides, which not only re-

sults in a better oxygen balance but also affords compounds

with higher densities and lower sensitivities. Fischer et al. gave

an overview of the preparation of a 5,5’-bis-(2-hydroxytetra-zole) species,[86] namely 5,5’-bis(tetrazole 2N-oxide) and its en-

ergetic nitrogen-rich salts, as well as a comparison to their 5,5’-bis-(tetrazole 1N-oxide) analogues (Scheme 11).

Similar to other recently reported tetrazole N-oxides, the5,5’-bis(tetrazole 2N-oxide) 6 b was synthesized by oxidation

using commercially available Oxone in high purities and mod-

erate yields in a potassium acetate-buffered aqueous suspen-sion overnight. However, the successful preparation of 5,5’-bis(-

tetrazole 2N-oxide) from 5,5’-bistetrazole depends on severalparameters, such as the purity of starting material 5,5’-bistetra-

zole, an excess of Oxone, the reaction temperature, and thebuffer system. For example, different isomers including 5,5’-bis-(1-hydroxytetrazole) or the mixed 2-hydroxytetrazol-5-yl-1-

hydroxytetrazole were obtained at a reaction temperature of35 8C and the system favored the formation of 5,5’-bis-(2-hy-

droxytetrazole) when the temperature was increased to 45 8C.5,5’-Bis(2-hydroxytetrazole) (6 b), with two highly acidic pro-tons, can be easily deprotonated with various free bases togive a series of nitrogen-rich salts, including ammonium (6 b-1), hydroxylammonium (6 b-2), guanidinium (6 b-3), aminogua-

nidinium (6 b-4), triaminoguanidinium (6 b-5), aminonitroguani-dinium (6 b-6) and 5-aminotetrazolium (6 b-7). All of thesecompounds were characterized by XRD, NMR and vibrationalspectroscopy, DSC, and mass spectrometry, and compared totheir 5,5’-bis(tetrazole 1N-oxide) analogues. The importantproperties of 5,5’-bis(tetrazole 2N-oxide) and its energetic saltsare listed in Table 6.

Besides the highest density for the free acid 6 b(1.95 g cm¢3), hydroxylammonium salt 6 b-2 (1.83 g cm¢3) and

aminonitroguanidinium salt 6 b-6 (1.82 g cm¢3) showed thehighest densities among the energetic salts, whereas the

lowest was that for the triaminoguanidinium salt 6 b-5(1.60 g cm¢3). All ionic salts except 3-amino-1-nitroguanidinium

Table 5. Properties of 5,5’-bis(tetrazole 1N-oxide) salts

Species[a] 1 [g cm¢3] Td [8C] DfHq [kJ mol¢1] D [m s¢1] P [GPa] IS [J] IF [N] ESD (+ /-)[J]

6 a 1.81 214 114.3 8764 33.1 >40 216 0.506 a-1 1.88 221 446.6 9698 42.4 20 120 0.106 a-2 1.80 290 300.5 8817 31.6 35 360 0.256 a-3 1.73 220 677.7 9159 34.0 9 252 0.16 a-4 1.64 274 408.0 7917 23.3 >40 >360 0.506 a-5 1.60 228 668.6 8111 24.3 40 324 0.256 a-6 1.73 204 409.6 8598 29.4 12 168 0.256 a-7 1.75 210 316.7 8028 24.6 15 120 0.256 a-8 1.78 140 222.1 8796 32.5 10 192 0.086 a-9 1.80 220 ¢20.7 8306 27.7 20 240 0.406 a-10 1.85 224 472.9 8878 33.3 20 360 0.406 a-11 1.89 222 ¢479.2 8203 27.5 40 288 0.356 a-12 1.79 180 987.9 8788 32.0 40 80 0.086 a-13 1.84 224 911.1 9097 35.8 4 72 0.306 a-14 1.76 192 903.8 8718 31.3 6 240 0.506 a-15 1.61 155 1164.2 8090 24.9 8 360 0.406 a-16 1.83 170 990.6 9160 36.1 2 160 0.35RDX[74] 1.82 230 93 8977 35.2 7.4 120 0.2HMX[74] 1.91 287 105 9320 39.6 7.4 112 0.2

Chem. Eur. J. 2016, 22, 7670 – 7685 www.chemeurj.org Ó 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim7680

Review

salt 6 b-6 (163 8C), free acid 6 b (165 8C), and hydroxylammoni-

um salt 6 b-2 (172 8C) are thermally stable (>180 8C), with thehighest decomposition temperature of 331 8C for the guanidi-

nium salt 6 b-3. The highest heat of formation was calculated

for the 3-amino-1-nitroguanidinium salt 6 b-6 (1043.8 kJ mol¢1)followed by the aminoguanidinium salt 6 b-4 (608.1 kJ mol¢1)

and neutral 5,5’-bis(2-hydroxytetrazole) 6 b (559.7 kJ mol¢1).With respect to the explosive performance of the energetic

salts, hydroxylammonium salt 6 b-2 and 3-amino-1-nitroguani-dinium salt 6 b-6 were the most promising replacements for

RDX, with detonation velocities of 9264 m s¢1 (6 b-2) and

9350 m s¢1 (6 b-6) and detonation pressures of 37.2 GPa (6 b-2)and 38.2 GPa (6 b-6). The sensitivities of 6 b to 6 b-7 vary from

very sensitive towards impact (6 b-1, 6 b-3 : IS = 3 J) to insensi-tive (6 b-4 : IS>40 J). Friction sensitivities range from extremely

sensitive (free acid 6 b : IF = 5 N) to insensitive (6 b-3, 6 b-4 : IF>360 N). Therefore the utilities of 6 b-2 and 6 b-6 may be re-

stricted as their thermal stabilities and sensitivities do not satis-

fy the corresponding values of RDX (Td = 230 8C, IS = 7.4 J).There are also some very interesting comparisons between

5,5’-bis(2-hydroxytetrazole) and 5,5’-bis(1-hydroxytetrazole) andtheir nitrogen-rich salts. Firstly, the decomposition tempera-

tures for the nitrogen-rich salts of 5,5’-bis (2-hydroxytetrazole)are not always decreased, although the thermal stability of

neutral 5,5’-bis(2-hydroxytetrazole) (165 8C) is much lower than

that of 5,5’-bis(1-hydroxytetrazole) (214 8C). Secondly, both theneutral and deprotonated forms of the 1N-oxide derivative

have lower heats of formation than that of the corresponding

forms of 2N-oxide derivative. Thirdly, there is an observabletrend that the nitrogen-rich salts of 5,5’-bis(2-hydroxytetrazole)

are more sensitive than those of 5,5’-bis(1-hydroxytetrazole).

Azotetrazole 1N-oxide salts

The properties of 5,5’-azobistetrazolate salts make them attrac-

tive for further study as a new class of high-performance, in-sensitive, thermally stable, and environmentally friendly ener-

getic materials. Many 5,5’-azobistetrazolate salts have foundpractical applications, for example, the guanidinium, triamino-

guanidinium, and hydrazinium salts in gas generators for fire-extinguishing systems or airbags.[51, 53, 55] Fischer et al. synthe-sized 5,5’-azotetrazolate-1,1’-dioxide and its salts as new high-

performance energetic materials from the precursor hydroxy-lammonium aminotetrazole 1N-oxide (Scheme 12).[77]

Azotetrazole-1,1’-dioxide dipotassium salt (7-1) was preparedby oxidation of hydroxylammonium aminotetrazole 1-oxide in

a solution of KMnO4 at 70 8C. The free dihydroxyazotetrazole(7) can be protonated by hydrochloric acid and isolated by ex-

Scheme 11. Synthesis of 5,5’-bis(tetrazole 2N-oxide) salts

Table 6. Properties of 5,5’-bis(tetrazole 2N-oxide) salts

Species[a] 1 [g cm¢3] Td [8C] DfHq [kJ mol¢1] D [m s¢1] P [GPa] IS [J] IF [N] ESD (+ /-)[J]

6 b 1.95 165 559.7 9364 40.9 3 5 0.036 b-1 1.66 265 257.3 8212 25.8 10 360 0.756 b-2 1.82 172 390.7 9264 37.2 3 60 0.266 b-3 1.63 331 322.0 7752 22.1 >40 >360 0.156 b-4 1.64 255 568.7 8137 24.7 30 >360 0.206 b-5 1.60 217 454.9 8256 25.1 15 324 0.56 b-6 1.83 163 1043.8 9350 38.2 10 48 156 b-7 1.63 188 608.1 8094 25.2 5 360 1.0RDX[74] 1.82 230 93 8977 35.2 7.4 120 0.2HMX[74] 1.91 287 105 9320 39.6 7.4 112 0.2

Chem. Eur. J. 2016, 22, 7670 – 7685 www.chemeurj.org Ó 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim7681

Review

traction into EtOAc and evaporation of the solvent. The addi-

tion of hydroxylamine, ammonia, and hydrazine to solutions ofthe free acid yielded the desired 5,5’-azotetrazolate-1,1’-dioxide

salts, including the dihydroxylammonium salt (7-2), diammoni-

um salt (7-3) and dihydrazinium salt (7-4), respectively. Anaqueous solution of 7-1 can be reduced with magnesium

powder into its hydrazine derivative and a new compound,1,1’-dihydroxy-5,5’-bistetrazolylhydrazine (8), was obtained

after removal of the magnesium, acidification, and extractionwith EtOAc. 5,5’-Azobis(1-N-oxidotetrazole) and its salts were

intensively characterized by XRD, IR, Raman, and multinuclear

NMR spectroscopy, elemental analysis, and DSC and importantproperties are summarized in Table 7.

It is quite unusual that the dihydroxylammonium salt 7-2crystallizes with a lower density (1.78 g cm¢3) than the dia-

mmonium salt 7-3 (1.80 g cm¢3). Hydrazinium 5,5’-azobis(1-oxi-dotetrazolate) 7-4 forms two polymorphs: low-density poly-morph 7-4 a (1.67 g cm¢3) and high-density polymorph 7-4 b(1.73 g cm¢3). The nitrogen-rich salts of 7 have decompositiontemperatures of 180–250 8C and the two polymorphs of 7-4have different decomposition temperatures of 190 8C (7-4 a)and 180 8C (7-4 b). Owing to the impressively high heat of for-

mation (883 kJ mol¢1) and density (1.90 g cm¢3) of neutral 7, itperforms very well with a detonation velocity of 9548 m s¢1

and a detonation pressure of 42.4 GPa, albeit with high sensi-

tivity towards friction and impact. Compound 7-1 has impact

and friction sensitivities of 20 J and >360 N and quite high cal-culated performance values (D = 9753 m s¢1, P = 41.0 GPa). The

nitrogen-rich salts of 7 all have velocities of detonation in the

range 9032–9753 m s¢1 and detonation pressures in the range32.9–42.4 GPa.

It is worth noting that the 5,5’-azobis(1N-oxidotetrazolate)dianion is even more stable than azotetrazolate, which is mani-

fested in the cases of ammonium salt 7-3, with a decomposi-tion temperature of 250 8C, and (NH4)2AzT (ammonium azote-

trazolate), with a value of 190 8C, respectively. Additionally, oxi-

dation resulted in a significant decrease in the friction sensitivi-ty of such potassium salts. In contrast to the azoxybis(tetra-

zole), azotetrazole dioxide does not decompose in acidicaqueous solution, confirming the stabilizing effect of the hy-

droxy moieties. The comparatively favorable properties of azo-tetrazole dioxide salts could qualify them to be used as high-

performance explosive or propellant ingredients, without the

disadvantages of the azotetrazolate anion.

Newly Designed Tetrazole N-Oxide EnergeticSalts

The often restricted time cost and danger associated with the

synthesis and testing of an energetic material have driven the

Scheme 12. Synthesis of azotetrazole 1N-oxide salts

Table 7. Properties of azotetrazole 1N-oxide salts

Species[a] 1 [g cm¢3] Td [8C] DfHq [kJ mol¢1] D [m s¢1] P [GPa] IS [J] IF [N] ESD (+ /-)[J]

7 1.90 170 883.2 9548 42.4 <1 <5 0.017-1 2.20 285 240.2 9753 41.0 20 >360 0.357-2 1.78 190 730.9 9348 37.5 15 54 0.27-3 1.80 250 551.7 9032 33.8 3 160 0.27-4 a 1.67 190 932.5 9066 32.9 3 20 0.77-4 b 1.73 180 916.7 9246 35.0 3 20 0.78 1.71 120 390.1 8711 31.2 1 <5 0.007RDX[74] 1.82 230 93 8977 35.2 7.4 120 0.2HMX[74] 1.91 287 105 9320 39.6 7.4 112 0.2

Chem. Eur. J. 2016, 22, 7670 – 7685 www.chemeurj.org Ó 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim7682

Review

advancement of computationally assisted design and predic-tion of new HEDMs.[91, 92] The improved properties confirm that

oxidation of tetrazoles to tetrazole oxides is indeed an effectivemethod of improving energetic performance. To expand the

field of tetrazole N-oxides, we also designed extensive energet-ic salts based on several new oxides, including 5-hydrazinote-

trazole N-oxide (3 a), 5-nitroiminotetrazole N-oxide (3 b), 1,5-di-aminotetrazole N-oxide (3 c), 5-nitroguanidyltetrazole N-oxide

(3 d), 4-amino-3-(5-tetrazolyl)furazan-N-oxide (3 e), 5-dinitrome-

thyltetrazole N-oxide (3 f), 5-trinitromethyltetrazole N-oxide(3 g), and bridged bistetrazole N-oxide (3 h). By appropriately

regulating the substituents and anion–cation pairings, a largenumber of nitrogen-rich salts have also been proposed

(Scheme 13).

Conclusion

Recently, intensification and improvement in design and syn-

thesis of modern HEDMs and most specifically of energeticsalts have been growing in interest among energetic materials

researchers. The molecular design, preparation and characteri-

zation, and even the development of potential applications ofenergetic materials with good physicochemical properties and

high performance is not just inherently attractive and challeng-ing work, but also has significant potential environmental and

economic impact.Tetrazole is an exceedingly important precursor to prepare

a wide range of energetic salts, from insensitive secondary ex-plosives to sensitive primary explosives. However, many of

non-oxidized tetrazoles suffer from low oxygen balance. Theoxidation of tetrazoles to tetrazole N-oxides has been shown

to promote this new class of compounds to form energetic

materials with high performance. Recently, publications devot-ed to tetrazole N-oxides have demonstrated the oxidation and

transformation of tetrazoles into corresponding salts throughneutralization reactions combined with subsequent metathe-

ses.In this Review, we have summarized the syntheses and prop-

erties of a series of tetrazole N-oxides, including alkyl- or aryl-,

nitro-, azido-, amino-, and cyanotetrazoles, bistetrazoles, andazotetrazole oxides, as well as their nitrogen-rich energetic

salts. Herein we used the acceptable criteria that new HEDMsshould meet to be considered possible replacements for com-

monly used energetic compounds, such as RDX or HMX (D>8500 m s¢1; Td>200 8C; IS>7 J; IF>120 N; clean decomposi-

tion products; high-yield; economic synthesis).[93] Among all of

Scheme 13. Novel tetrazolate N-oxide salts

Chem. Eur. J. 2016, 22, 7670 – 7685 www.chemeurj.org Ó 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim7683

Review

the tetrazole N-oxide-based salts discussed herein, many saltshave physical and detonation properties that are competitive

with TNT or TATB, and several compounds have similar or evenbetter detonation properties compared to RDX (2-1, 2-2, 2-4,

2-6, 2-7, 4, 4-1, 6 a, 6 a-1, 6 a-2, 6 a-3, 6 a-6, 6 a-8, 6 a-10, 6 a-12, 6 a-13, 6 a-14, 6 b, 6 b-2, 6 b-6, 7-1, 7-2, 7-3, 7-4). When

considering the thermal stabilities and the sensitivities of thecompounds, 6 a, 6 a-1, 6 a-2, 6 a-3, 6 a-6, 6 a-10, and 7-1 could

be possible candidates to replace RDX and 6 a-1 might even

be a possible candidate to replace HMX. It is surprising and ex-citing that a new high-performing explosive, dihydroxylammo-

nium 5,5’-bistetrazole-1,1’-diolate (TKX-50), has great potentialto replace the most commonly used explosive RDX, not only

because it is easily prepared and exceedingly powerful, butalso because it possesses the required thermal insensitivityand low toxicity, as well as safety of handling.

From recent work in this field, further improvements of theenergetic character and property of tetrazole compounds are

undoubtedly demonstrated and confirmed by the introductionof N-oxides. The general trends for N-oxides are that the densi-ty is increased as a result of forming hydrogen bonds in thesolid state, and the oxygen content of the molecule is also

better balanced, thereby ensuring a maximum energy output

during decomposition or explosion. Moreover, the N-oxidescan be partially stabilized by removing lone pair electron den-

sity (increasing s–p separation), as that would otherwise desta-bilize the nitrogen system by donating electron density into

antibonding orbitals.[94, 95] In addition, the increased intermolec-ular interactions often help decrease their sensitivities towards

mechanical stimuli, thus making them safer to handle in both

preparation and application. However, a major limit of thesetetrazole N-oxides may be that the available synthetic methods

are not general in scope. For instance, the cyclization of azido-oximes (for synthesizing the 5,5’-bistetrazole-1,1’-diol) is usually

limited in both the scope of available azido-oximes and theirhigh sensitivities under manipulation. Another drawback is

that N-oxidation at either the 1- or 2-positions of the tetrazoles

lowers the thermal stability of the protonated species, particu-larly for 5-nitrotetrazole, 5,5’-bistetrazole, and 5-azidotetrazole.

A possible solution to the problem of thermal stabilities is themodification of tetrazole N-oxides with either a mono- or mul-tifunctional group as well as the right cation pairing [e.g. , 5,5’-azobis(1N-oxidotetrazolate) dianion is much more stable thanazotetrazolate].

This indicates that these unique tetrazole N-oxide anions,through combination with appropriate cations, can afford val-uable high-performance energetic salts to replace traditionalenergetic materials. Firstly, better understanding of the rela-tionship between molecular structure and energetic propertiesis critical, not only for the design of a new molecule but also

for the synthesis of the corresponding energetic salts. Second-ly, we must attach importance to theoretical predictions ofdensity, heat of formation, and detonation parameters, to mini-mize expense in both time and cost, as well as to avoid thecertain risk in practice. Thirdly, the successful N-oxidation of

tetrazoles and subsequent formation of the corresponding en-ergetic salts depends on many criteria in practical experiments,

such as the purification of starting materials, selection of effec-tive oxidation agents, suitable buffer systems, appropriate re-

action temperatures and times (to avoid formation of byprod-ucts), and convenient separation and purification of the prod-

ucts.Additionally, we have also designed a large number of new

tetrazole N-oxides and exclusive energetic salts for the purposeof further expanding the newly exploited field of tetrazole

chemistry to achieve novel energetic materials simultaneously

with high performances and good stabilities. We believe thatthese unique compounds might represent promising replace-

ments for the currently used secondary explosives, with poten-tially excellent properties. Of course, there is a need for com-

bined theoretical and experimental study of either the func-tional groups or the ions (high nitrogen content, high density,

high energy, high thermal stability, or a combination thereof)

that are selected for the modification of the salts so as to satis-fy the requirements of next-generation green HEDMs in the

future.

Acknowledgements

We gratefully acknowledge the support of the National NaturalScience Foundation of China (no. 10776002) and the State Key

Laboratory of Science and Technology (project nos. ZDKT12-03and YBT16-04).

Keywords: energetic materials · explosives · heterocycles ·propellants · pyrotechnics

[1] J. Zhang, J. M. Shreeve, J Am Chem. Soc 2014, 136, 4437 – 4445.[2] S. Li, Y. Wang, C. Qi, X. Zhao, J. Zhang, S. Zhang, S. Pang, Angew. Chem.

Int. Ed. 2013, 52, 14031 – 14035; Angew. Chem. 2013, 125, 14281 – 14285.[3] K. B. Landenberger, O. Bolton, A. J. Matzger, Angew. Chem. Int. Ed. 2013,

52, 6468 – 6471; Angew. Chem. 2013, 125, 6596 – 6599.[4] J. Zhang, Q. Zhang, T. T. Vo, D. A. Parrish, J. M. Shreeve, J Am Chem. Soc

2015, 137, 1697 – 1704.[5] H. Gao, J. M. Shreeve, Chem. Rev. 2011, 111, 7377 – 7436.[6] F. R. Benson, Chem. Rev. 1947, 41, 1 – 61.[7] J. C. G�lvez-Ruiz, G. Holl, K. Karaghiosoff, T. M. Klapçtke, K. Lçhnwitz, P.

Mayer, H. Nçth, K. Polborn, C. J. Rohbogner, M. Suter, J. J. Weigand,Inorg. Chem. 2005, 44, 4237 – 4253.

[8] R. S. Damse, A. K. Sikder, J. Hazard. Mater. 2009, 166, 967 – 971.[9] H. Delalu, K. Karaghiosoff, T. M. Klapotke, C. M. Sabate, Cent. Eur. J.

Energ. Mater. 2010, 7, 197 – 215.[10] T. M. Klapçtke, J. Stierstorfer, Dalton Trans. 2009, 643 – 653.[11] J. S. Mihina, R. M. Herbst, J. Org. Chem. 1950, 15, 1082 – 1092.[12] T. M. Klapçtke, M. Stein, J. Stierstorfer, Z. Anorg. Allg. Chem. 2008, 634,

1711 – 1723.[13] T. M. Klapçtke, H. Radies, J. Stierstorfer, K. R. Tarantik, G. Chen, A. Nagori,

Propellants Explos. Pyrotech. 2010, 35, 213 – 219.[14] T. M. Klapçtke, C. Mirû Sabat¦, Eur. J. Inorg. Chem. 2008, 5350 – 5366.[15] T. M. Klapçtke, C. Mirû Sabat¦, Z. Anorg. Allg. Chem. 2009, 635, 1812 –

1822.[16] V. Ernst, T. M. Klapçtke, J. Stierstorfer, Z. Anorg. Allg. Chem. 2007, 633,

879 – 887.[17] T. M. Klapçtke, C. Mirû Sabat¦, A. Penger, M. Rusan, J. M. Welch, Eur. J.

Inorg. Chem. 2009, 880 – 896.[18] K. Karaghiosoff, T. M. Klapçtke, C. Mirû Sabat¦, Chem. Eur. J. 2009, 15,

1164 – 1176.[19] R. Wang, H. Gao, C. Ye, B. Twamley, J. M. Shreeve, Inorg. Chem. 2007, 46,

932 – 938.

Chem. Eur. J. 2016, 22, 7670 – 7685 www.chemeurj.org Ó 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim7684

Review

[20] M. von Denffer, T. M. Klapçtke, C. Mirû Sabat¦, Z. Anorg. Allg. Chem.2008, 634, 2575 – 2582.

[21] T. M. Klapçtke, C. M. Sabat¦, J. Stierstorfer, Z. Anorg. Allg. Chem. 2008,634, 1867 – 1874.

[22] J. Stierstorfer, K. R. Tarantik, T. M. Klapçtke, Chem. Eur. J. 2009, 15, 5775 –5792.

[23] K. Karaghiosoff, T. M. Klapotke, P. Mayer, C. M. Sabate, A. Penger, J. M.Welch, Inorg. Chem. 2008, 47, 1007 – 1019.

[24] G.-H. Tao, Y. Guo, Y.-H. Joo, B. Twamley, J. M. Shreeve, J. Mater. Chem.2008, 18, 5524 – 5530.

[25] S. N. Semenov, A. Y. Rogachev, S. V. Eliseeva, Y. A. Belousov, A. A. Droz-dov, S. I. Troyanov, Polyhedron 2007, 26, 4899 – 4907.

[26] H. Xue, H. Gao, B. Twamley, J. M. Shreeve, Chem. Mater. 2007, 19, 1731 –1739.

[27] T. M. Klapçtke, J. Stierstorfer, B. Weber, Inorg. Chim. Acta 2009, 362,2311 – 2320.

[28] T. M. Klapçtke, J. Stierstorfer, Helvetica Chimica Acta 2007, 90, 2132 –2150.

[29] T. M. Klapçtke, J. Stierstorfer, K. R. Tarantik, I. D. Thoma, Z. Anorg. Allg.Chem. 2008, 634, 2777 – 2784.

[30] H. Gao, Y. Huang, C. Ye, B. Twamley, J. M. Shreeve, Chem. Eur. J. 2008,14, 5596 – 5603.

[31] B. C. Tappan, R. W. Beal, T. B. Brill, Thermochim. Acta 2002, 388, 227 –232.

[32] G. Geisberger, T. M. Klapçtke, J. Stierstorfer, Eur. J. Inorg. Chem. 2007,4743 – 4750.

[33] T. M. Klapotke, C. M. Sabate, J. M. Welch, Dalton Trans. 2008, 45, 6372 –6380.

[34] T. M. Klapçtke, C. M. Sabat¦, J. M. Welch, Eur. J. Inorg. Chem. 2009, 769 –776.

[35] T. M. Klapçtke, C. M. Sabate, J. Stierstorfer, New J. Chem. 2009, 33, 136 –147.

[36] T. M. Klapçtke, C. M. Sabate, Dalton Trans. 2009, 1835 – 1841.[37] T. M. Klapçtke, P. Mayer, C. Mirû Sabat¦, J. M. Welch, N. Wiegand, Inorg.

Chem. 2008, 47, 6014 – 6027.[38] T. M. Klapotke, C. Miro Sabate, M. Rasp, J. Mater. Chem. 2009, 19, 2240 –

2252.[39] T. M. Klapçtke, C. Mirû Sabat¦, J. M. Welch, Z. Anorg. Allg. Chem. 2008,

634, 857 – 866.[40] J. C. Galvez-Ruiz, G. Holl, K. Karaghiosoff, T. M. Klapotke, K. Loehnwitz, P.

Mayer, H. Noeth, K. Polborn, C. J. Rohbogner, M. Suter, J. J. Weigand,Inorg. Chem. 2005, 44, 5192 – 5192.

[41] T. M. Klapçtke, J. Stierstorfer, Eur. J. Inorg. Chem. 2008, 4055 – 4062.[42] Z.-M. Li, J.-G. Zhang, Y. Cui, T.-L. Zhang, Y.-J. Shu, V. P. Sinditskii, V. V. Ser-

ushkin, V. Y. Egorshin, J. Chem. Eng. Data 2010, 55, 3109 – 3116.[43] F. D. Marsh, J. Org. Chem. 1972, 37, 2966 – 2969.[44] H. Gao, Y. Huang, B. Twamley, C. Ye, J. M. Shreeve, ChemSusChem 2008,

1, 222 – 227.[45] E. E. Baroody, G. A. Carpenter, J. Chem. Eng. Data 1979, 24, 3 – 6.[46] P. J. Eulgem, A. Klein, N. Maggiarosa, D. Naumann, R. W. H. Pohl, Chem.

Eur. J. 2008, 14, 3727 – 3736.[47] Y. Guo, G.-H. Tao, Z. Zeng, H. Gao, D. A. Parrish, J. M. Shreeve, Chem. Eur.

J. 2010, 16, 3753 – 3762.[48] T. M. Klapçtke, P. Mayer, J. Stierstorfer, J. J. Weigand, J. Mater. Chem.

2008, 18, 5248 – 5258.[49] W. P. Norris, R. A. Henry, J. Org. Chem. 1964, 29, 650 – 660.[50] T. M. Klapçtke, C. Kuffer, P. Mayer, K. Polborn, A. Schulz, J. J. Weigand,

Inorg. Chem. 2005, 44, 5949 – 5958.[51] T. M. Klapçtke, C. M. Sabat¦, Z. Anorg. Allg. Chem. 2007, 633, 2671 –

2677.[52] Y. Guo, H. Gao, B. Twamley, J. M. Shreeve, Adv. Mater. 2007, 19, 2884 –

2888.[53] G.-H. Tao, B. Twamley, J. M. Shreeve, Inorg. Chem. 2009, 48, 9918 – 9923.[54] C. Ye, J.-C. Xiao, B. Twamley, J. M. Shreeve, Chem. Commun. 2005, 2750 –

2752.[55] T. M. Klapçtke, C. M. Sabat¦, New J. Chem. 2009, 33, 1605 – 1617.[56] H. Gao, J. M. Shreeve, Angew. Chem. Int. Ed. 2015, 54, 6335 – 6338;

Angew. Chem. 2015, 127, 6433 – 6436.[57] V. Thottempudi, J. M. Shreeve, J Am Chem. Soc 2011, 133, 19982 –

19992.[58] Q. Wu, W. Zhu, H. Xiao, J. Mater. Chem. A 2014, 2, 13006 – 13015.

[59] X. Jin, B. Hu, W. Lu, S. Gao, Z. Liu, C. Lv, RSC Adv. 2014, 4, 6471 – 6477.[60] H. Wei, J. Zhang, C. He, J. M. Shreeve, Chem. Eur. J. 2015, 21, 8607 –

8612.[61] A. M. Churakov, V. A. Tartakovsky, Chem. Rev. 2004, 104, 2601 – 2616.[62] L. Jia-Rong, Z. Jian-Min, D. Hai-Shan, J. Chem. Crystallogr. 2005, 35, 943 –

948.[63] C. Zhang, X. Wang, H. Huang, J Am Chem. Soc 2008, 130, 8359 – 8365.[64] T. M. Klapçtke, J. Stierstorfer in Green Energetic Materials (Ed. : T. Brinck),

Wiley, Chichester, 2014, pp. 133 – 178.[65] A. J. Liepa, D. A. Jones, T. D. McCarthy, R. H. Nearn, Aust J Chem. 2000,

53, 619 – 622.[66] T. Harel, S. Rozen, J. Org. Chem. 2010, 75, 3141 – 3143.[67] T. Eicher,S. Hauptman, The Chemistry of Heterocycles, 2nd ed. , Wiley,

Hoboken, 2003.[68] T. Harel, E. Amir, S. Rozen, Org Lett. 2006, 8, 1213 – 1216.[69] M. Carmeli, S. Rozen, J. Org. Chem. 2006, 71, 5761 – 5765.[70] E. Amir, S. Rozen, Angew. Chem. Int. Ed. 2005, 44, 7374 – 7378; Angew.

Chem. 2005, 117, 7540 – 7544.[71] N. Shefer, T. Harel, S. Rozen, J. Org. Chem. 2009, 74, 6993 – 6998.[72] T. Harel, S. Rozen, J. Org. Chem. 2007, 72, 6500 – 6503.[73] M. Gçbel, K. Karaghiosoff, T. M. Klapçtke, D. G. Piercey, J. Stierstorfer, J

Am Chem. Soc 2010, 132, 17216 – 17226.[74] R. Wang, H. Xu, Y. Guo, R. Sa, J. M. Shreeve, J Am Chem. Soc 2010, 132,

11904 – 11905.[75] T. M. Klapçtke, J. Stierstorfer, J Am Chem. Soc 2008, 131, 1122 – 1134.[76] T. M. Klapçtke, D. G. Piercey, J. Stierstorfer, Chem. Eur. J. 2011, 17,

13068 – 13077.[77] D. Fischer, T. M. Klapçtke, D. G. Piercey, J. Stierstorfer, Chem. Eur. J. 2013,

19, 4602 – 4613.[78] P. K. Swain, H. Singh, S. P. Tewari, J. Mol. Liq. 2010, 151, 87 – 96.[79] M.-J. Crawford, T. M. Klapçtke, F. A. Martin, C. M. Sabat¦, M. Rusan,

Chem. Eur. J. 2011, 17, 1683 – 1695.[80] F. Boneberg, A. Kirchner, T. M. Klapçtke, D. G. Piercey, M. J. Poller, J.

Stierstorfer, Chem. Asian J. 2013, 8, 148 – 159.[81] N. Fischer, D. Izs�k, T. M. Klapçtke, S. Rappenglìck, J. Stierstorfer, Chem.

Eur. J. 2012, 18, 4051 – 4062.[82] D. E. Chavez;, M. A. Hiskey, D. L. Naud, J. Pyrotech. 1999, 17 – 36.[83] R. Sivabalan, M. B. Talawar, N. Senthilkumar, B. Kavitha, S. N. Asthana, J.

Therm. Anal. Calorim. 2004, 78, 781 – 792.[84] N. Fischer, D. Fischer, T. M. Klapçtke, D. G. Piercey, J. Stierstorfer, J.

Mater. Chem. 2012, 22, 20418 – 20422.[85] N. Fischer, T. M. Klapçtke, M. Reymann, J. Stierstorfer, Eur. J. Inorg. Chem.

2013, 2167 – 2180.[86] N. Fischer, L. Gao, T. M. Klapçtke, J. Stierstorfer, Polyhedron 2013, 51,

201 – 210.[87] I. V. Tselinskii, S. F. Mel’nikova, T. V. Romanova, Russ J Org Chem. 2001,

37, 430 – 436.[88] V. K. Golubev,T. M. Klapçtke, Comparative analysis of TKX-50, MAD-X1,

RDX and HMX blasting performance in one-, two- and three-dimensionalgeometry, Proceedings of the 17th Seminar on New Trends in Researchof Energetic Materials, Pardubice, Czech Republic, Apr. 9 – 11, 2014, 1,20.

[89] V. Bohanek, M. Dobrilovic, D. Fischer, V. K. Golubev, T. M. Klapçtke, J.Stierstorfer, Synthesis, Chemical and Physical Characterization of TKX-50,19th Biennial Conference on Shock Compression of Condensed Matter(SCCM-2015), Tampa, FL, Jun. 14 – 19, 2015.

[90] V. K. Golubev, T. M. Klapçtke, Comparative analysis of shock wave actionof TKX-50 and some other explosives on various barriers, Proceedings ofthe 17th Seminar on New Trends in Research of Energetic Materials,Pardubice, Czech Republic, Apr. 9 – 11, 2014, 1, 529.

[91] X. Song, J. Li, H. Hou, B. Wang, J. Comput. Chem. 2009, 30, 1816 – 1820.[92] P. He, J.-G. Zhang, K. Wang, X. Yin, T.-L. Zhang, J. Org. Chem. 2015, 80,

5643 – 5651.[93] T. M. Klapçtke, C. M. Sabat¦, Chem. Mater. 2008, 20, 3629 – 3637.[94] S. Inagaki, N. Goto, J. Am. Chem. Soc. 1987, 109, 3234 – 3240.[95] M. Noyman, S. Zilberg, Y. Haas, J. Phys. Chem. A 2009, 113, 7376 – 7382.

Received: January 19, 2016

Published online on April 8, 2016

Chem. Eur. J. 2016, 22, 7670 – 7685 www.chemeurj.org Ó 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim7685

Review