Selective Synthesis and Characterization of the Highly Energetic Materials 1-Hydroxy-5H-tetrazole (CHN4O), its Anion 1-Oxido-5H-tetrazolate (CN4O?) and Bis(1-hydroxytetrazol-5-yl)triazene

CHEMISTRY

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www.chemasianj.org

Accepted Article

Title: Selective Synthesis and Characterization of the Highly Energetic

Materials 1-Hydroxy-tetrazole (CHN4O), its Anion 1-Oxido-

tetrazolate (CN4O−) and Bis(1-hydroxytetrazol)triazene

Authors: Joerg Stierstorfer, Thomas M. Klapötke, Moritz Kofen, Laszlo

Schmidt, and Maximilian H. H. Wurzenberger

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the VoR from the journal website shown below when it is published

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content of this Accepted Article.

To be cited as: Chem. Asian J. 10.1002/asia.202100714

Link to VoR: https://doi.org/10.1002/asia.202100714

A Journal of

A sister journal of Angewandte Chemie

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Selective Synthesis and Characterization of the Highly Energetic

Materials 1-Hydroxy-5H-tetrazole (CHN₄O), its Anion 1-Oxido-5H-

tetrazolate (CN₄O⁻) and Bis(1-hydroxytetrazol-5-yl)triazene

Thomas M. Klapötke*^[a], Moritz Kofen^[a], Laszlo Schmidt^[a], Jörg Stierstorfer^[a], and Maximilian H. H.

Wurzenberger^[a]

[a]

Prof. Dr. Thomas M., Klapötke, Moritz Kofen, Laszlo Schmidt, Dr. Jörg Stierstorfer, Dr. Maximilian H. H. Wurzenberger

Department of Chemistry

Ludwig-Maximilian University of Munich

D-81377 Munich

Email: tmk@cup.uni-muenchen.de

Internet: www.hedm.cup.uni-muenchen.de

Supporting information for this article is given via a link at the end of the document.

Abstract: For the first time, an adequate selective synthesis,

circumventing the formation of the 2-hydroxy-5H-tetrazole, of

1-hydroxy-5H-tetrazole (HTO), as well as the synthesis of bis(1-

hydroxytetrazol-5-yl)triazene (H₃T) are reported. Several salts thereof

were synthesized and characterized which resulted in the formation

of new primary and secondary explosives containing the 1-

oxidotetrazolate unit. Molecular structures are characterized by single

crystal X-ray diffraction, ¹H and ¹³C NMR, IR, and elemental analysis.

Calculation of the detonation performance using the EXPLO5 code

confirmed the energetic properties of 1-hydroxy-5H-tetrazole. The

detonation properties can be adjusted to the requirements for those

of a secondary explosive by forming the hydroxylammonium (6) or

hydrazinium (7) salts, or to meet the requirements of a primary

explosive by forming the silver salt 4, which shows a fast DDT on

contact with a flame. The sensitivities of all compounds towards

external stimuli such as impact, friction, and electrostatic discharge

were measured.

high-performing secondary explosive. TKX-50 surpasses

properties of 1,3,5-Trinitro-1,3,5-triazinan (RDX) and 1,3,5,7-

Tetranitro-1,3,5,7-tetrazocan (HMX), which have long been used

as the main charge in detonation devices. Even though the 5,5’-

bistetrazole has been known since 1913,^[11]it took nearly 100

years until its potential for use as an explosive was discovered in

the compound TKX-50. It is therefore surprising that 1,5H-

tetrazole has been known since 1892,^[12]but complete

characterization of the corresponding N-oxide, namely 1-hydroxy-

5H-tetrazole, is missing.

Introduction

Tetrazoles are a class of heterocycles that exhibit a wide variety

of possible applications. A prominent representative is Losartan,

which is included in the WHO’s List of Essential Medicines^[1]as a

treatment for hypertension.^[2]In addition to pharmaceutical uses,

tetrazoles show great potential for application as energetic

materials with high nitrogen contents like in copper(I)

nitrotetrazolate (DBX-1).^[3-4]The high endothermic heat of

formation of tetrazoles (e.g. +236 kJ mol⁻¹for 1,5H-tetrazole) is

advantageous for possible applications as energetic materials,

since a large amount of energy is released upon detonation.^[5]

General protocols to further increase the enthalpy of formation of

[8]

tetrazoles include N-oxidation by Oxone^®,^[6]HOF,^[7]or H₂O₂

,

which also has the advantage of increasing the oxygen balance

and crystal density.^[9]In 2012, Klapötke et al.^[10]obtained

Figure 1. Top: Increasing the density and enhancing the performance of

energetic compounds can be achieved by N-oxidation. Bottom: Previous work

concerned with the synthesis of 1-hydroxy-5H-tetrazole and bis(tetrazol-5-

yl)triazene.

dihydroxylammonium-5,5’-bistetrazolyl-1,1’-diolat

(TKX-50)

through the N-oxidation of 5,5’-bistetrazole followed by formation

of the hydroxylammonium salt. This compound possesses

excellent detonation properties while being thermally stable

(221 °C) and moderately sensitive towards impact (20 J) and

friction (120 N), thus being stable enough to be safely used as a

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The first synthesis was reported by Pallazzo in 1910,^[13]however,

the addition of hydrazoic acid to sodium fulminate is not suitable

nor desired for a synthesis on gram-scale due to the high risks

involved when working with fulminates. In 1956, Bettinetti et al.

successfully synthesized 1-hydroxy-5H-tetrazole by the addition

of hydrazoic acid to nitrolic acids, which too is not desired for the

work on gram-scale.^[14]Bettinetti et al. also described the silver

salt of 1-hydroxytetrazole, yet an extensive characterization is

missing. In 1995, Begtrup et al.^[15]used Oxone^®to oxidize 1,5H-

tetrazole. This idea was then followed by Giles et al. ^[16]in 1999

for oxidizing ethyl tetrazole-5-carboxylate. Both procedures result

in an isomeric mixture of the 1-hydroxy- and 2-hydroxy-tetrazole

derivatives, requiring further protection (for the oxidized 1H-

tetrazole), as well as separation of the isomers and subsequent

secession of the protecting group. In a previous paper in 2013,

we showed that the addition of hydroxylamine to a solution of

cyanogen azide results in a cyclization forming 5-amino-1-

hydroxytetrazole (5-ATO).^[17]In 1954, Henry et al. showed that

1,5H-tetrazole can be obtained by the elimination of the amino

group of 5-amino-1H-tetrazole through diazotization followed by

subsequent reduction of the diazonium cation. These so-called

hydro-dediazonizations are performed by boiling a diazonium

cation in acidic ethanol. However, this procedure usually results

in ethoxy-dediazonization.^[18]Kornblum et al. later replaced the

step in which reduction with acidic ethanol occurs by using

hypophosphorous acid as the reducing agent instead.^[19]

Independently from the applied reduction procedure, the

dediazonization always starts with a diazotization of the amine,

which, for several azoles, can instantly explode and therefore

should never be isolated. It can also couple with other compounds

and this well-known behavior is exploited to produce azo dyes,

such as Methyl red (2-(N,N-dimethyl-4-aminophenyl)^[20]or

Tartrazine

(trisodium

1-(4-sulfonatophenyl)-4-(4-

Lesser

sulfonatophenylazo)-5-pyrazolone-3-carboxylate).^[21]

known are triazenes, which result from the coupling of a

diazonium cation with an amine. In 1910 Hofmann et al. published

the first synthesis of sodium bis(tetrazol-5-yl)triazene by

diazotization of amino guanidinium nitrate with sodium nitrite,

intermediately producing 5-aminotetrazole which then further

reacts to the triazene species.^[22]

When applying the systems analogously to 5-ATO, the

dediazonization selectively produces 1-hydroxy-5H-tetrazole,

thus representing the first adequate selective synthesis for this

long over-due compound, whereas following Hofmann et al.

results in the formation of bis(1-hydroxytetrazol-5-yl)triazene, the

twice N-oxidized bis(tetrazol-5-yl)triazene. Therefore, we report

the first adequate procedure to obtain 1-hydroxy-5H-tetrazole

without the need of protection groups and elaborate workups.

Additionally, the first synthesis of bis(1-hydroxytetrazol-5-

yl)triazene is reported. Both acids as well as salts thereof are

prepared and are characterized by single crystal X-ray diffraction

experiments, complemented by NMR and IR spectrometry as well

as thermal and physical analysis

Results and Discussion

Synthesis:

Warning! The synthetic work described in this section involves the

handling of very sensitive intermediates (diazotetrazole-1N-oxide)

and products (e.g., silver salt 4). Proper protective measurements

and equipment must be used!

ether. The silver salt (4) can be precipitated by adding silver

nitrate to a solution of 1 in water and filtering off the solid. The

hydroxylammonium (6) and hydrazinium (7) salts were obtained

by adding the corresponding base to a solution of 1 in ethanol and

precipitating with diethyl ether. Due to coextraction of sulfuric acid

after dediazonization pure 1 is obtained by dissolving 3 in 2 M

hydrochloric acid, extracting with ethyl acetate, and removing the

solvent in vacuo. Bis-(1-hydroxytetrazol-5-yl)triazene 8 was

obtained as a monohydrate by dissolving 5-ATO in concentrated

hydrochloric acid followed by diazotization with half an equivalent

of sodium nitrite, adjusting to pH>10 with sodium hydroxide, and

extraction into ethyl acetate. Compound 8 crystallized by slow

evaporation of the solvent. Salts 9–14 were precipitated by adding

the corresponding hydroxide (9, 10), carbonate (11–13), or free

base (14) dissolved in the minimal volume methanol to a solution

of 8 in ethyl acetate. The copper ammonium salt 15 was obtained

by precipitating the copper salt of 8 using copper sulfate followed

by recrystallization from concentrated ammonia.

The starting material 5-ATO is readily available from the reaction

of cyanogen azide and hydroxylamine.^[17]1-Hydroxy-5H-tetrazole

(1) can be obtained by dissolving 5-ATO in semi-concentrated

sulfuric acid (40%) and diazotizing with sodium nitrite, while

keeping the temperature below 5 °C (Scheme 1). The

diazotization solution is then added to a mixture of ethanol and

elemental copper and stirred at 55 °C for 2 hours. Due to the

intermediate formation of 5-diazonium-1-hydroxytetrazole, the

ratio of 5-ATO to sulfuric acid (40%) should be quite low. A ratio

of 1:10 was used for the synthesis of all herein investigated

compounds, as

a higher concentration of 5-diazonium-1-

hydroxytetrazole leads to micro-detonations within the reaction

solution. An attempt to reduce the amount of acid to a ratio of 1:2

resulted in a violent detonation of the whole reaction solution,

destroying the round-bottom flask. Compound 1 was extracted

into DCM, and the ammonium salt (5) was subsequently

precipitated by passing gaseous ammonia through the organic

phase. The sodium (2) and potassium (3) salts were obtained by

refluxing a solution of the corresponding carbonate and 5 in water,

followed by evaporating to complete dryness, extracting the

residue with ethanol, and precipitating the salts by adding diethyl

When preparing compound 8, it is important to make sure any

trace amounts of the diazonium cation are removed by stirring

under basic conditions (pH>10), since it is possible to co-extract

it into the organic solvent. When removing the solvent in vacuo, a

detonation occurred on slightly touching the solid with a plastic

spatula, leading to the person performing the experiment being

injured.

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Scheme 1. Synthesis of 1-hydroxy-5H-tetrazole (1) and its salts thereof (2–7), as well as the synthesis of bis(1-hydroxytetrazol-5-yl)-triazene (8) and its salts thereof

(9–15). Hx⁺: hydroxylammonium; Hz⁺: hydrazinium; G⁺: guanidinium.

bond lengths [Å] and angles [°] (D–H···A, d(D–H), d(H···A), d(D···A), <(D–

Crystal structures:

The solid-state crystal structure of all of the compounds

H···A)): O1–H2···N3ⁱⁱⁱ: 1.02(2), 1.57(2), 2.5878(16), 178(2); O1–H2···N4ⁱⁱⁱ:

1.02(2), 2.53(2), 3.4431(18), 150.1(17); C1–H1···O1ⁱ: 0.913(16), 2.600(15),

3.2504(19), 128.8(12); C1ⁱⁱ–H1ⁱⁱ···N3: 0.913(16), 2.543(16), 3.325(2), 144.1(13).

synthesized in this work, except compound 14, were determined

using low-temperature single-crystal X-ray diffraction. All of the

data and parameters of the measurements, as well as of the

refinements are given in the Supporting Information Table S1. All

crystal densities are recalculated to their respective room

temperature crystal density. Crystal datasets were deposited in

the CSD database and can be obtained free of charge with the

following codes: CCDC 2088975 (1), 2088974 (2a), 2088980 (2b),

2088977 (3), 2088979 (4), 2088985 (5), 2088982 (6), 2088986 (7),

2088988 (8), 2088983 (9), 2088978 (10), 2088976 (11), 2088987

(12), 2088981 (13) and 2088984 (15). 1-Hydroxy-5H-tetrazole (1)

crystallizes in the monoclinic space group P2₁/n. The density of

(1) (1.63 g cm⁻³) is comparable to that of the parent molecule 5-

amino-1-hydroxytetrazole (1.66 g cm⁻³)^[17]and is significantly

higher than that of 1-aminotetrazole (1-AT, 1.48 g cm⁻³).^[23]

Figure 2 shows the crystal structure of 1, which forms wave-like

layers in an ABA layering (along the b and c-axis) due to the two

H atoms present per molecule participating in a total of three

hydrogen bonds to two different 1-hydroxy-5H-tetrazole

molecules – one of which lies within the same wave-like layer (A),

whereas the other is from a different layer (B) (Figure 2B).

Symmetry codes: i) 5/2−x, ½+y, ½−z; ii) 1/2+x, ½−y, ½+z; iii) ½+x, ½−y, −1/2+z.

Interestingly, the respective C-N and N-N bond lengths within the

tetrazole ring are nearly equivalent for compound 1 compared to

5-ATO and 1-AT. The bond length of between the hydroxy group

and the tetrazole ring in 1 is the same as that reported for 5-ATO,

but is slightly shorter than the corresponding bond of the amine in

1-AT, thus explaining the higher density. All of the bond lengths

are in the typical range for N–N, C–N, and N–O single bonds, as

well as N–N and C–N double bonds. The tetrazole ring in 1 is

planar, with the oxygen atom slightly protruding from the plane

(O1 – N1 – N2 – N3 = 5°). In all of the salts of 1 investigated in

this work, a planar arrangement of the oxygen atom with respect

to the tetrazole-plane was observed for the 1-oxido-5H-tetrazolate

anions.

Sodium 1-oxido-5H-tetrazolate (2) crystallizes as a monohydrate

in two polymorphs, one in the triclinic space group P1 (2a) with a

density of 1.81 g cm⁻³and one in the orthorhombic space group

P2₁2₁2₁(2b) with a slightly lower density of 1.78 g cm⁻³. The

triclinic form is obtained by crystallization from water. whereas the

orthorhombic polymorph from ethanol. Deprotonation of 1 results

in marginal changes in the bond lengths of the tetrazole ring, with

the N1–O1 bond length being generally shorter by 0.03–0.04 Å

for all investigated salts in comparison with the neutral compound

1. Potassium (3) and silver 1-oxido-5H-tetrazolate (4) both

crystallize in the orthorhombic space group Pna2₁with densities

of 1.88 g cm⁻³(3) and 3.48 g cm⁻³(4), respectively. Figure 3

shows the crystal structure of 4, where all silver cations are

threefold coordinated by individual anions bridging between the

cations.

Figure 2. Crystal structure of compound 1 (A) as well as crystal packing (B) due

to hydrogen bonds between four symmetrically distinct 1-hydroxy-5H-tetrazole

molecules; ellipsoids in all structures are shown with a probability of 50%;

Selected bond lengths [Å]: O1–N1 1.354(2), N1–N2 1.337(2), N2–N3 1.299(2),

N3–N4 1.353(2), N4–C1 1.316(2), C1–N1 1.324(2); Angles [°]: O1–N1–N2

121.17(11), O1–N1–C1 127.85(12), N1–N2–N3 105.07(11), N2–N3–N4

110.51(12), N3–N4–C1 106.81(12), N4–C1–N1 106.91(12). Selected hydrogen

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Hydroxylammonium 1-oxido-5H-tetrazolate (6) crystallizes in the

triclinic space group P1 with a density of 1.67 g cm⁻³(@ 298 K)

with one molecular unit in the unit cell as shown in Figure 5. Even

although all of the protons participate in hydrogen bonds, the

density of 6 is significantly lower than that of comparable

molecules such as dihydroxylammonium dinitro-bis-1,2,4-

triazole-1,1’-diol (MAD-X1) (1.90 g cm⁻³at 298 K)^[6]or TKX-50

(1.92 g cm⁻³at 298 K).^[10]

Figure 5. A) Crystal structure of hydroxylammonium 1-oxidotetrazolate (6). B)

Packing due to the presence of strong hydrogen bonds involving of all the

protons. Selected bond lengths [Å]: O1–N1 1.319(2), N1–N2 1.343(3), N2–N3

1.304(3), N3–N4 1.350(3), N4–C1 1.328(3), C1–N1 1.332(3), N5–O2 1.411(2);

Angles [°] O1–N1–N2 122.13(2), N1–N2–N3 106.19(18), N2–N3–N4

110.74(18), N3–N4–C1 105.86(18), N4–C1–N1 108.36(18), C1–N1–O1

129.02(18); Selected hydrogen bond lengths [Å] and angles [°] (D–H···A, d(D–

H), d(H···A), d(D···A), <(D–H···A)): O2–H2···O1: 0.82(4), 1.76(4), 2.576(2),

172(4); C1ⁱ–H1ⁱ···O2: 0.92(3), 2.57(3), 3.274(2), 134(2); N5–H5A···N3^iv: 0.81(3),

2.18(3), 2.913(3), 151(3); N5–H5B···N4ⁱⁱⁱ: 0.84(3), 2.05(3), 2.883(3), 170(3);

N5–5C···O1ⁱⁱ: 0.88(3), 1.93(3), 2.766(2), 160(3). Symmetry codes: i)−1+x, 1+y,

z ii) x,1+y, z iii) −1+x, 1+y, −1+z iv) x, y, −1+z.

Figure 3. Crystal structure of silver 1-oxido-5H-tetrazolate (4) showing the

coordination environment of the silver cations. Selected bond distances [Å]: N4–

Ag1ⁱ: 2.235(3), N3–Ag1ⁱⁱ: 2.275(3), O1–Ag1ⁱⁱⁱ: 2.628(8), O1ⁱⁱ–Ag1^iv: 2.406(7);

angles [°]: O1ⁱⁱ–Ag1–N4 124.97(12), N3ⁱ–Ag1–N4 122.14(11), O1ⁱⁱ–Ag1–N3ⁱ

81.73(12); Symmetry codes: i) −x, 1−y, ½+z; ii) ½−x, ½+y, ½+z; iii) −1/2+x, ½−y,

z.

Ammonium 1-oxido-5H-tetrazolate (5) also crystallizes in the

orthorhombic space group Pna2₁with four molecular units in the

unit cell and a density of 1.40 g cm⁻³. The crystal packing shows

the presence of strong hydrogen bonds involving all of the protons

of the ammonium cations (Figure 4), with bonds towards three

oxygen atoms and one nitrogen atom of four different 1-

oxidotetrazolate anions. The observed density is the lowest of all

of the herein investigated compounds, and remarkably, it is even

lower than that of comparable compounds such as ammonium 1-

oxido-5-aminotetrazolate (1.50 g cm⁻³)^[17]and ammonium 5-

azidotetrazolate (1.61 g cm⁻³).^[24]

The lower density is also indicated by a lower packing

coefficient^[25](Figure 6) of 6 (0.752) compared to TKX-50 (0.811)

and MAD-X1 (0.797). Interestingly, the packing coefficient of 6 is

comparable to the hydroxylammonium salt of 5-ATO (0.762) but

is significantly higher than that for 1,5H-tetrazole (0.675). This

confirms the positive effect of N-oxidation on the density as stated

by Fischer et al.,^[17]whereas amination at the carbon position does

not lead to significant changes in density. Generally, it can be

assumed that the introduction of a bicycle (TKX-50, MAD-X1) is

beneficial as it increases the packing coefficient, and thus

increases the density.

Figure 4. Packing of ammonium 1-oxido-5H-tetrazolate (5) showing layering

due to the presence of hydrogen bonds between the hydrogen atoms of an

ammonium cation with four distinct tetrazolate anions. Selected hydrogen bond

lengths [Å] and angles [°] (D–H···A, d(D–H), d(H···A), d(D···A), <(D–H···A)):

N5–H3···O1: 0.93(2), 1.86(2), 2.7762(18), 172(2); N5–H2···O1ⁱ: 0.91(2),

1.90(2), 2.8036(19), 171(2); N5–H5···O1ⁱⁱ: 0.90(3), 2.04(3), 2.9063(18), 162(2);

N5–H4···N3ⁱⁱⁱ: 0.88(3), 2.18(2), 2.976(2), 151(2). Symmetry codes: i)

1/2+x,1/2−y, z; ii) x, −1+y, z; iii) 1−x, 1−y, −1/2+z.

Figure 6. Packing coefficient and crystal densities of compound 6 (HxTO) and

comparable compounds such as MAD-X1, TKX-50, tetrazole and

hydroxylammonium 5-amino-1-oxidotetrazolate (HxATO).

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Hydrazinium 1-oxido-5H-tetrazolate (7) crystallizes in the

monoclinic space group Pc with two molecular units in the unit cell

and a density of 1.6 g cm⁻³, following the trend observed of

hydrazinium salts having lower densities compared to the

corresponding hydroxylammonium salts. The crystal structure of

7 is shown in Figure 7A, in which all hydrogen atoms are

units which are oriented in the same way towards the triazenide

bridge.

+

participating in hydrogen bonds resulting in one N₂H₅cation

being linked to four anions (Figure 7B).

Figure 7. A) Crystal structure of hydrazinium 1-oxido-5H-tetrazolate (7). B)

Packing due to strong hydrogen bonds of all protons, linking four distinct anions

by one hydrazinium cation. Selected bond lengths [Å]: O1–N1 1.329(3), N1–N2

1.3336(3), N2–N3 1.316(4), N3–N4 1.344(4), N4–C1 1.329(4), C1–N1 1.329(4),

N5–N6 1.443(4); Angles [°] O1–N1–N2 120.8(2), N1–N2–N3 105.7(2), N2–N3–

N4 111.0(3), N3–N4–C1 105.6(3), N4–C1–N1 108.6(3) , C1–N1–O1 130.1(3);

Selected hydrogen bond lengths [Å] and angles [°] (D–H···A, d(D–H), d(H···A),

d(D···A), <(D–H···A)): C1ⁱ–H1ⁱ···N2: 0.88(4), 2.59(4), 3.453(4), 165(3); N5–

H5A···O1ⁱ: 0.97(4), 1.80(5), 2.742(4), 165(4); N5–H5B···O1ⁱⁱ: 0.99(4), 1.94(4),

2.826(4), 148(4), N5–H5C···N3: 0.89(4), 2.45(4), 3.047(4), 125(3); N6–

H6A···N3: 0.91(3), 2.50(3), 3.147(4), 128(3), N6–H6B···O1ⁱⁱⁱ: 0.81(4), 2.36(4),

3.112(4), 156(3). Symmetry codes: i)−1+x, 1−y, −1/2+z ii) −1+x,−1+y, z iii) x,

−1+y, z.

Figure 8. Crystal structure of bis(1-hydroxytetrazol-5-yl)triazene monohydrate

(8), showing the zwitterionic form. Selected bond lengths [Å]: O1–N1 1.321(2),

N8–O2 1.349(2), N3–N4 1.338(2), N4–C1 1.337(2), C1–N5 1.382(2), N5–N6

1.269(2), N6–N7 1.317(2), N7–C2 1.368(2), N11–N10 1.361(2), N11–C2

1.318(2); Angles [°] O1–N1–N2 121.8(1), O1–N1–C1 128.3(1), O2–N8–N9

123.5(1), O2–N8–C2 126.6(1), N3–N4–C1 109.4(2), N10–N11–C2 104.7(1),

C1–N5–N6 107.9(1), C2–N7–N6 114.8(1).

Compounds 12 and 13 both crystallize as trihydrates. It was not

possible to obtain crystals of compound 14 suitable for single

crystal X-ray diffraction experiments. The crystal structures of

compounds 9, 10, 12 and 13 are shown in the Supporting

Information (Figures S1-S4).

Bis-(1-hydroxytetrazol-5-yl)triazene (8) crystallizes in the

monoclinic space group P2₁/n as a monohydrate with a density of

1.80 g cm⁻³and four molecular units in the unit cell (Figure 8).

Due to the high acidity of the hydroxy group next to the azo-moiety,

8 crystallizes in a zwitterionic form where one tetrazole ring (N4)

is protonated by its own hydroxy group (O1). While the self-

protonation only leads to small differences in bond lengths

between the two, changes in bond angles between the two

moieties of up to 4.7° (N3–N4–C1 = 109.4(2), N10–N11–C2 =

104.7(1)) are observed. The largest difference in bond lengths

and angles is observed for the triazene bridge. Within the triazene

bridge, the N5–N6 bond (1.269(2) Å) is significantly shorter than

the N6–N7 bond (1.327(2) Å), suggesting a higher bond order and

therefore more double bond character for the N5–N6 bond.

Additionally, the zwitterionic ring is more strongly bent towards the

triazene bridge (N6–N5–C1 = 107.9(1)°) than its counterpart (N6–

N7–C2 = 114.8(1)°). Compounds 9 and 10 both crystallize as

hexahydrates in the triclinic space group P–1 with a density of

1.67 g cm^–3and 1.71 g cm^–3, respectively. Tripotassium bis(1-

oxidotetrazol-5-yl)triazenide (11) crystallizes as a trihydrate in the

orthorhombic space group P2₁2₁2₁with a density of 1.98 g cm^–3

(Figure 9) While complete deprotonation of 8 doesn’t appear to

lead to significant changes in bond lengths within the tetrazole

rings, the double bond character of the N5-N6 bond in the

triazene-bridge has now been more delocalized, which is

indicated by the equality of the N5–N6 and N6–N7 bond lengths

which are both 1.309(3) Å in 11. The most drastic change in the

structure of the anion resulting from complete deprotonation of 8

is the arrangement of the 1-oxidotetrazolate moieties with respect

to the triazenide bridge. While the two moieties are not equally

bent towards the triazene bridge in 8, after complete

deprotonation, the anion in 11 now shows two 1-oxidotetrazolate

Figure 9. Crystal structure of tripotassium bis(1-oxidotetrazol-5-yl)triazenide

trihydrate (11). Selected bond lengths of the anion [Å]: O1–N1 1.330(3), N1–N2

1.352(4), N2–N3 1.311(3), N3–N4 1.359(3), N4–C1 1.340(4), C1–N1 1.339(4),

C1–N5 1.384(4), N5–N6 1.309(3), N6–N7 1.309(3), N7–C2 1.375(4), C2–N8

1.346(4), N8–O2 1.330(3), N8–N9 1.346(2), N9–N10 1.316(4), N10–N11

1.359(3), N11–C2 1.340(4); Angles [°]: O1–N1–N2 122.0(2), O1–N1–C1

129.1(3), O2–N8–N9 122.8(2), O2–N8–C2 127.7(2), C1–N4–N3 105.5(2), C2–

N11–N10 105.9(3), N6–N5–C1 111.4(3), N6–N7–C2 111.1(2).

Ammonium copper bis(1-oxidotetrazol-5-yl)triazenide (15)

crystallizes in the triclinic space group P–1 with two molecular

units in the unit cell and a density of 1.96 g cm^–3(Figure 10). As

was observed for compound 11, the bond lengths and angles of

the anion are not significantly influenced by deprotonation. The

drastic conformational change observed in 11 is a result of the

unequal coordination of the copper(II)-cation with the two 1-

oxidotetrazolate moieties. Whereas the most favored

conformation for compounds 8––14 is apparently a triazene

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bridge with a double (E-) arrangement along N5–N6 and N6–N7,

threefold coordination of the copper(II) cation by the anion in 15

results in rotation along the N6–N7 bond, resulting in a E/Z-

conformation along N5–N6 and N6–N7, respectively. Interestingly,

the copper(II) cation shows a nearly perfectly square pyramidal

coordination by the anion and two aqua ligands. The angles

involving O3 and atoms forming the equatorial plane (O1, N8, N11,

O4) range from 89.13° (O1–Cu1–O3) to 104.42° (O3–Cu1–N11).

Figure 11. ¹³C NMR spectrum of compound 8 in DMSO-d₆, showing only one

carbon atom.

Physicochemical properties:

Thermal behavior. The thermal behaviors of compounds 1–7 are

shown in Figure 12. Compound 1 shows a decomposition

temperature of 186°C (Figure 12), and two endothermic events at

80 °C and 96 °C, which TGA indicates as corresponding to a

phase transition and/or melting point. Compound 1, 1-hydroxy-

5H-tetrazole, exhibits a higher thermal stability compared to the

similar compound 5-amino-1-hydroxytetrazole^[17], and is almost

identical with that of 1-aminotetrazole (182°C).^[23]Compound 2

shows loss of water at 110°C and a decomposition temperature

of 273°C, which is the highest of all of the 1-hydroxy-5H-tetrazole

salts reported in this work (Figure 12). The potassium (3) and the

silver (4) salts, which are both free of water, show decomposition

temperatures of 236°C and 211°C, respectively. Both of these

salts detonate violently on reaching their critical temperatures.

The silver salt (4) also immediately detonates on contact with a

flame. Compound 5 shows an endothermic event at 180°C, which

corresponds to the loss of ammonia due to evaporation, as

indicated by the onset of mass loss in the TG. The endothermic

event seamlessly evolves into an exothermic decomposition at

188°C, at which temperature a significant mass loss of 85.5 wt.%

occurs as shown by the TG (Figure 12). The same behavior is

observed for the hydroxylammonium salt 6. Figure 12 shows an

endothermic event at 115°C, which evolves into the first of two

exothermic decomposition events at 159°C. TGA measurements

revealed the first decomposition to correspond to the

decomposition of hydroxylamine accompanied by a weight loss of

24.9 wt.%. The second exothermic event with onset at 203°C is

attributed to the decomposition of the residual 1-hydroxy-5H-

tetrazole.

Figure 10. Crystal structure of ammonium copper bis(1-oxidotetrazol-5-

yl)triazenide trihydrate (15). Selected bond lengths [Å]: O1–N1 1.347(6), N1–N2

1.341(6), N2–N3 1.321(6), N3–N4 1.374(6), N4–C1 1.323(7), C1–N1 1.338(7),

C1–N5 1.379(6), N5–N6 1.343(6), N6–N7 1.284(6), N7–C2 1.386(6), C2–N8

1.354(7), N8–O2 1.305, N8–N9 1.352(6), N9–N10 1.304(6), N10–N11 1.351(6),

N11–C2 1.336(7), Cu1–O1 1.982(4), Cu1–N11 1.993(4), Cu1–N8 1.938(4),

Cu1–O3 2.332(4), Cu1–O4 1.977(5); Angles [°]: O1–N1–C1 125.1(4), O2–N8–

C2 128.4(4), C1–N5–N6 113.8(4), N6–N7–C2 121.3(4), O1–Cu1–O3 89.19(14),

O1–Cu1–O4 93.85(17), O1–Cu1–N8 169.87(16), O1–Cu1–N11 85.17(15), O3–

Cu1–N8 95.84(16), O3–Cu1–N11 104.42(16), O3–Cu1–O4 91.82(17).

NMR spectroscopy:

NMR spectroscopy was performed in DMSO-d₆, D₂O or acetone-

d₆and spectra are depicted in the Supporting Information. The ¹H

NMR spectrum of compound 1 in DMSO-d₆shows one signal at

δ = 9.48 ppm for the proton attached to the carbon atom of the

tetrazole ring. The ¹³C NMR of compound 1 in DMSO-d₆shows

one signal at δ = 137.7 ppm, which is shifted upfield compared to

the starting material 5-ATO (δ = 150.5 ppm). Deprotonation of 1

1

leads to a shift in the signals observed in the H as well as ¹³C

1

NMR spectra. For example, the H NMR of potassium 1-oxido-

5H-tetrazolate (3) in D₂O shows one signal at δ = 8.54 ppm, which

is shifted upfield compared to the free acid 1. Additionally, the

signal in the ¹³C NMR of 3 in D₂O is shifted downfield to

δ = 164.5 ppm. The ¹³C NMR of compound 8 in DMSO-d₆shows

one signal at δ = 150.0 attributing both carbon atoms (Figure 11).

Compared to the parent molecule, 5-ATO (δ = 150.5 ppm), the

carbon atoms of 8 are, contrary to 1, not drastically shifted.

Deprotonation of 8 results in a downfield shift of the signal. The

¹³C NMR of the tripotassium salt (11) in D₂O shows one signal at

δ = 153.2 ppm. The influence of deprotonation of 8 on the ¹³C

NMR signal is not as prominent as that observed for compound 1.

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which evolves into an exothermic decomposition, in behavior

similar to that of compound 5 (Figure 12). This can be explained

by the evaporation of hydroxylamine occurring, which results in

the formation of an unstable residue that immediately

decomposes at 148°C. Interestingly, compound 15 shows no loss

of water until the onset of decomposition at 198°C, indicating

strong coordinative bonds of the aqua ligands to the copper(II)

cation as well as a high stability of the crystal water.

Figure 12. Thermal analytic measurements (DTA) of HTO (1), NaTO • H₂O (2),

KTO (3), AgTO (4), NH₄TO (5), HxTO (6), HzTO (7). For compounds 1, 5, 6,

and 7, additional TGA measurements were performed to analyze the

endothermic events observed in the DTA.

Compound 7 shows an exothermic decomposition temperature of

213°C, with two endothermic events in addition observed at 80°C

and 103°C. The TG measurement of 7 shows a mass loss of

13 wt.% with onset at 103°C, which corresponds to the

evaporation of hydrazine. Therefore, it can be assumed that the

exothermic event actually corresponds to thermal decomposition

of residual neutral compound 1, rather than the hydrazinium salt

itself. Figure 13 shows that bis(1-hydroxytetrazol-5-yl)triazene

monohydrate (8) is thermally stable up to 95°C. Despite being a

monohydrate, no endothermic event which could be attributed to

a loss of water was observed in the DTA spectrum of 8. Due to

stabilization by crystal water, 8 decomposes immediately on

reaching a temperature (95°C) high enough to remove the crystal

water molecule. The alkaline salts 9 – 12 all show the presence

of endothermic events between 85 – 135°C corresponding to the

loss of water in each compound (Figure 13). In general, these

compounds are very thermally stable, with decomposition

temperatures of between 292°C (12) and 335°C (10). Interestingly,

the triguanidinium salt 13 shows no endothermic event in the DTA,

which confirms loss of water already occurring at room

temperature. This agrees with the measured elemental analysis,

which fits perfectly with the values calculated for the anhydrous

salt, which shows a thermal stability of up to 222 °C. The

hydroxylammonium salt 14 shows an endothermic event at 104°C

Figure 13. DTA measurements of bis-(1-hydroxytetrazol-5-yl)triazene

monohydrate (8), Li₃T • 6 H₂O (9), Na₃T • 6 H₂O (10), K₃T • 3 H₂O (11), Rb₃T •

3 H₂O (12), Gua₃T (13), Hx₃T (14), Cu(NH₄)T (15).

Heats of formation, sensitivity, and detonation parameters. The

calculated and measured explosive properties of all compounds

are listed in Tables 1 and 2. Heat of formations were calculated

by applying the atomization method using room temperature

CBS-4M enthalpies.^[24-28]A detailed explanation of the calculation

can be found in the Supporting Information. Compound 1 shows

a comparable heat of formation (252 kJ mol^–1) to its parent

molecule 5-amino-1-hydroxytetrazole (256 kJ mol^–1). However, 1

possesses a slightly lower density than its parent compound,

which results in a minor reduction of the detonation velocity

(8405 m s⁻¹) and detonation pressure (269 kbar) compared to 5-

ATO (8609 m s^–1, 298 kbar). While the impact sensitivity of 1

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(10 J) is comparable to that of 5-ATO (10 J), the friction sensitivity

is higher for 1 (28 N) than for 5-ATO (108 N), meaning 1 is more

sensitive towards external stimuli than 5-ATO. In contrast, the

hydroxylammonium (6) salt of 1-hydroxy-5H-tetrazole shows a

higher heat of formation (259 kJ mol^–1(6)) compared to the

corresponding salt of 5-ATO (Hx⁺: 227 kJ mol^–1). Compound 6

shows a very high detonation velocity (9284 m s⁻¹) compared to

compound 5 (8013 m s⁻¹), which is due to the higher density of 6

1.67 g cm⁻³(5: 1.40 g cm⁻³). Although compound 7 shows a

slightly lower density (1.60 g cm⁻³) than compound 6, the

significantly higher enthalpy of formation of 7 (346 kJ mol⁻¹)

results in its even higher detonation velocity of 9437 m s⁻¹.

Compounds 6 and 7 both outperform even HMX in terms of

detonation velocity. While compound 5 is completely insensitive

(IS >40 J, FS >360 N), compounds 6 (IS = 6 J, FS 240 N) and 7

(IS = 26 J, FS >360 N) show higher impact sensitivities, but

crucially, are comparable to (6) or less sensitive (7) than HMX.

The monohydrate sodium salt (2) is completely insensitive

towards external stimuli, whereas the potassium (3) salt is more

sensitive (IS = 4 J, FS = d54 N). The silver salt (4) is the most

sensitive out of all of the compounds, and has to be classified as

extremely sensitive, with an impact sensitivity below 1 J and a

friction sensitivity of 1 N. The silver salt shows the characteristics

of a primary explosive, immediately detonating on contact with a

flame (Figure 14).

Figure 14. Initiation of silver 1-oxido-5H-tetrazolate (4) by a lighter. t₀) right

before the flame is in contact with the compound; t₁) immediate detonation of 4

on contact with the flame; t₂) smoke residue after detonation.

Due to this behavior, the initiation capability of 4 towards

Pentaerythrityltetranitrat (PETN) was tested, whereby it was

found that 50 mg of 4 were able to initiate 200 mg of pressed

PETN (Figure 15). Compound 2a shows a low detonation velocity

of 7555 m s⁻¹, which is attributed to its low heat of formation as

well as being a monohydrate. The orthorhombic form 2b exhibits

a slightly lower detonation velocity (7428 m s⁻¹) than triclinic 2a,

due to the lower density of 2b. Interestingly, the potassium salt 3

exhibits an even lower detonation velocity than 2a/b, even though

it has a higher density (1.88 g cm⁻³). Due to the high heat of

formation of compound 8 (734 kJ mol⁻¹), it shows an outstanding

detonation velocity of 9433 m s⁻¹, as well as a very high

detonation temperature (3940 K), which even outperforms HMX.

However, in addition to its low thermal stability (decomposition

temperature of only 100°C), compound 8 is also very sensitive

towards impact (<1 J) and friction (4 N), preventing application of

8 as a high-performance secondary explosive.

Table 1. Energetic properties of 1-hydroxy-5H-tetrazole (1) and its salts 2–7.

1

2a

2b

3

4

5

6

7

HMX^[q]

CH₂N₄O

CH₃N₄O₂Na

CHN₄OK

CHN₄OAg

CH₅N₅O

CH₅N₅O₂

CH₆N₆O

C₄H₈N₈O₈

Formula

M [g mol^–1

IS [J]^[a]

]

86.05

10

126.05

>40

126.05

>40

124.14

4

192.91

<1

103.09

>40

119.08

6

118.10

26

296.16

7^[31]

FS [N]^[b]

ESD [mJ]^[c]

ρ [g cm^{–3 [d]}

28

>360

1080

1.81

>360

1080

1.78

54

1

>360

>1500

1.40

240

>360

740

112^[31]

200^[31]

1.91

37.84

0.0

960

63

<0.28

3.47

29.04

−8.29

−16.59

–

>1500

1.67

]

1.63

65.11

−18.59

−37.19

80, 96

186

1.88

45.13

−32.22

−45.11

–

1.60

N [%]^[e]

44.45

−12.36

−25.39

110

44.45

−12.36

−25.39

110

67.94

−38.80

−54.33

180

58.81

−20.15

−33.59

115

71.16

−40.64

−54.19

80, 103

213

Ω_CO[%]^[f]

Ω_CO2[%]^[g]

T_endo[°C]^[h]

T_exo[°C]^[i]

Δ_fH⁰[kJ mol^{–1 [j]}

Δ_fH⁰[kJ kg^{–1 [k]}

EXPLO5 V6.05.04

−Δ_ExU⁰[kJ kg^{–1 [l]}

T_det[K]^[m]

−21.61

–

273

236

47

211

–

188

159, 203

259

275

]

252

−366

−2803

−364

−2780

207

346

75

]

3042

441

–

2170

2325

3100

369

]

5333

3744

469

2792

2070

451

2817

2088

456

3265

2669

426

–

4663

2978

549

6137

3666

461

5409

3114

485

5699

3622

401

V₀[L kg^{–1 [n]}

P_CJ[kbar]^[o]

]

269

189

182

187

213

331

319

378

–1

[p]

V_det[m s

]

8405

7555

7428

7056

8013

9284

9437

9192

[a] Impact sensitivity (BAM drophammer (1 of 6)). [b] Friction sensitivity (BAM friction tester (1 of 6)). [c] Electrostatic discharge device (OZM research). [d] From X-

ray diffraction analysis recalculated to 298 K. [e] Nitrogen content. [f] Oxygen balance towards CO formation [g] Oxygen balance towards CO₂formation [h]

Temperature of endothermic event (DTA; β = 5 °C min⁻¹). [i] Temperature of exothermic event (DTA; β = 5 °C min⁻¹). [j] Calculated enthalpy of formation. [k]

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Calculated mass related enthalpy of formation. [l] Energy of explosion. [m] Detonation temperature. [n] Volume of detonation products (assuming only gaseous

products). [o] Detonation pressure at Chapman-Jouguet point. [p] Detonation velocity. [q] Values based on the EXPLO5 Database.

.

Table 2. Energetic properties of bis-(1-hydroxytetrazol-5-yl)triazene monohydrate (8) and its salts 9–15.

8

C₂H₅N₁₁O₃

234.14

<1

9

C₂H₁₂N₁₁O₈Li₃

339.01

>40

10

C₂H₁₂N₁₁O₈Na₃

387.16

>40

11

C₂H₆N₁₁O₅K₃

381.45

>40

12

C₂H₆N₁₁O₅Rb₃

520.55

>40

13

C₅H₁₈N₂₀O₂

390.34

>40

14

C₂H₁₂N₁₄O₅

312.22

4

15

C₂H₁₀N₁₂O₅Cu

345.73

11

Formula

M [g mol^–1

IS [J]^[a]

]

FS [N]^[b]

ρ [g cm^{–3 [c]}

N [%]^[d]

Ω_CO[%]^[e]

Ω_CO2[%]^[f]

T_endo[°C]^[g]

T_exo[°C]^[h]

4

>360

288

>360

1.59^[p]

71.77

−49.19

−69.69

–

128

288

]

1.80

1.67

1.71

1.97

2.55

1.79^[p]

62.81

−15.37

−25.62

123

1.96

66.66

−10.38

−24.23

−

45.45

−7.08

−16.52

122

39.80

40.39

−25.17

−33.56

135

29.60

−9.22

−15.37

99, 121

292

48.62

−13.88

−23.14

–

−6.20

−14.46

85

100

311

339

294

222

146

205

Δ_fH° [kJ mol^{–1 [i]}

Δ_fH° [kJ kg^{–1 [j]}

EXPLO5 V6.05.04

−Δ_ExU⁰[kJ kg^{–1 [k]}

T_det[K]^[l]

]

734

−1505

−4303

−1480

−3705

−1475

−3786

–

158

464

−53

]

3248

–

559

1633

−38

]

5896

3940

429

3568

2208

476

3003

2150

474

1239

1316

420

–

2106

1736

501

5330

3299

437

3173

2654

402

V₀[L kg^{–1 [m]}

]

P_CJ[kbar]^[n]

362

213

173

143

201

364

249

–1

[o]

V_det[m s

]

9433

7941

7352

6546

7870

9663

7725

[a] Impact sensitivity (BAM drophammer (1 of 6)). [b] Friction sensitivity (BAM friction tester (1 of 6)). [c] From X-ray diffraction analysis recalculated to 298 K. [d]

Nitrogen content. [e] Oxygen balance towards CO formation [f] Oxygen balance towards CO₂formation [g] Temperature of endothermic event (DTA; β = 5 °C min⁻¹).

[h] Temperature of exothermic event (DTA; β = 5 °C min⁻¹). [i] Calculated enthalpy of formation. [j] Calculated mass related enthalpy of formation. [k] Energy of

explosion. [l] Detonation temperature. [m] Volume of detonation products (assuming only gaseous products). [n] Detonation pressure at Chapman-Jouguet point.

[o] Detonation velocity. [p] Density measured pycnometrically.

Complete deprotonation of 8 forming the alkali salts 9 (Li⁺), 10

(Na⁺), and 11 (K⁺) results in a drastic decrease in energetic

performance. This is due to the presence of a large amount of

crystal water which results in highly exothermic enthalpy of

formations (−1475 – −1505 kJ mol⁻¹). Additionally, the absence of

protons, not included as crystal water, leads to a decrease in

detonation velocity within the series of increasing cation weight.

Compound 13, crystallizes as a trihydrate, but completely loses

its crystal water by drying in air at room temperature, as confirmed

by elemental analysis as well as IR spectroscopy. Therefore, the

heat of formation and detonation properties were calculated for

the anhydrous compound, showing an endothermic heat of

formation (158 kJ mol⁻¹). The rather low density for 13 of 1.59

g cm⁻³, detonation velocity of 7870 m s⁻¹and detonation pressure

of 201 kbar means it lies in the range of the other alkali salts.

Compound 14 was obtained as an amorphous solid which is

thermally stable up to 146°C. It has an endothermic heat of

formation of 464 kJ mol⁻¹and density of 1.79 g cm⁻³, with a

calculated detonation velocity of 9663 m s⁻¹, which surpasses

that of HMX. Compound 14 has a calculated detonation pressure

of 364 kbar as well as moderate sensitivities to external sources

(IS = 4 J, FS = 128 N), which are comparable to those of HMX.

Conclusion

Figure 15. Positive initiation of PETN (200 mg) by 4 (50 mg) indicated by the

perforated witness plate.

For the first time, an adequate selective synthesis, circumventing

the formation of 2-hydroxy-5H-tetrazole, of 1-hydroxy-5H-

tetrazole is reported by the dediazonization of the readily available

precursor, 5-amino-1-hydroxytetrazole. Diazotization was

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Sodium 1-oxido-5H-tetrazolate monohydrate (2): Compound 1 (500 mg,

5.81 mmol) was dissolved in methanol (10 mL) and sodium hydroxide

(232 mg, 5.81 mmol) in methanol was added. After stirring for 5 minutes,

the precipitate was filtered off and recrystallized from ethanol. Sodium 1-

oxido-5H-tetrazolate monohydrate (2) was obtained as yellow crystals in

quantitative yield (625 mg, 5.78 mmol, 99%).

followed by reduction of the diazonium cation using either

hypophosphorous acid or a mixture of ethanol/copper(0). While

ethanol/copper(0) was inferior to hypophosphorous acid in terms

of yields, this procedure was chosen for the synthesis of 1-

hydroxy-5H-tetrazole (1), as it is far easier to workup.

Diazotization of 5-ATO with half an equivalent of sodium nitrite

resulted in the formation of bis(1-hydroxytetrazol-5-yl)triazene

monohydrate (8). Compound 1 was converted to the sodium (2),

potassium (3), silver (4), ammonium (5), hydroxylammonium (6)

and hydrazinium (7) salts. Compound 8 was converted to the

lithium (9), sodium (10), potassium (11), rubidium (12),

guanidinium (13), hydroxylammonium (14), and ammonium

copper(II) (15) salts. All compounds, except 14, were analyzed

using single crystal X-ray diffraction, as well as NMR, IR, and EA

analysis. The thermal stabilities were investigated, sensitivities

towards external stimuli were determined, and the detonation

performances were calculated using the EXPLO5 code.

Compound 1 shows a good detonation performance (8405 m s⁻¹)

which can compete with that of RDX. The detonation performance,

thermal stability and sensitivities of 1 could be adjusted to meet

the requirements of a primary explosive by formation of the silver

salt (4), or to those of a highly energetic secondary explosive by

formation of 6 (VoD = 9284 m s⁻¹) or 7 (VoD = 9437 m s⁻¹), which

surpass the detonation velocity of even HMX (9192 m s⁻¹). Bis(1-

¹H NMR (DMSO-d₆, 25 °C): δ = 8.07 ppm (s, 1H, CH); ¹³C NMR (DMSO-

d₆, 25 °C): δ = 133.1 ppm (N4-C); IR (ATR, cm⁻¹): ṽ = 3465 (m), 3427 (m),

3281 (w), 3150 (m), 1652 (m), 1472 (m), 1433 (vw), 1407 (m), 1384 (w),

1350 (vw), 1309 (w), 1247 (s), 1192 (m), 1158 (w), 1114 (w), 1065 (w), 982

(w), 824 (m), 738 (m), 715 (w), 670 (m), 529 (s), 481 (vs); DTA

(5 °C min⁻¹): 273 °C (T_exo); elemental analysis calcd. [%] for CH₃N₄NaO₂

(126.05): C 9.53, H 2.40, N 44.45; found: C 9.71, H 2.45, N 43.54; BAM

drophammer: >40 J, BAM friction tester: >360 N, ESD: 1080 mJ.

Potassium 1-oxido-5H-tetrazolate (3): Compound

5

(500 mg,

4.85 mmol) was dissolved in water (10 mL) and potassium carbonate

(670 mg, 4.85 mmol) was added. The solvent was completely dried off in

air, and the solid obtained was extracted with hot ethanol and filtered. After

adding diethyl ether, pure potassium 1-oxido-5H-tetrazolate (3) was

filtered off and obtained as a yellow powder in good yield (518 mg, 4.17

mmol, 86%). Crystals suitable for single crystal X-Ray measurements were

obtained by recrystallization from water.

¹H NMR (DMSO-d₆, 25°C): δ = 8.45 ppm (s, 1H, CH); ¹³C NMR (DMSO-

d₆, 25 °C): δ = 135.1 ppm (N4-C); IR (ATR, cm⁻¹): ṽ = 1632 (m), 1622 (m),

1475 (m), 1435 (w), 1402 (m), 1386 (m), 1369 (m), 1352 (w), 1290 (s),

1234 (vs), 1187 (s), 1137 (m), 1124 (w), 1098 (m), 1067 (m), 1012 (m),

968 (m), 923 (w), 874 (w), 836 (m), 817 (s), 781 (w), 731 (m), 718 (m), 690

(s), 675 (m), 645 (m), 625 (w), 585 (w), 477 (m); DTA (5 °C min^–1): 236 °C

(T_exo); elemental analysis calcd. [%] for CHKN₄O (124.14): C 9.68, H 0.81,

N 45.13; found: C 9.93, H 1.03, N 44.96; BAM drophammer: 4 J, BAM

friction tester: 54 N, ESD: 63 mJ.

hydroxytetrazol-5-yl)triazene monohydrate (8) is

a

highly

sensitive (IS <1 J, FS = 4 N) material, with the performance of a

high energetic secondary explosive (VoD = 9433 m s⁻¹). Salts of

8 are completely insensitive due to the incorporation of up to six

equivalents of crystal water (for the lithium salt), which is

accompanied by

a

drastic loss of performance. The

hydroxylammonium salt (14) is an anhydrous compound (as

proven by elemental analysis), which shows a very high

detonation velocity of 9663 m s^–1but with impact and friction

sensitivities comparable to those of PETN (IS = 4 J, FS = 80 N)

Silver 1-oxido-5H-tetrazolate (4): Compound 1 (150 mg, 1.74 mmol) was

dissolved in ethanol (10 mL) and silver nitrate (296 mg, 1.74 mmol) was

added. After stirring for several minutes, the precipitate was filtered off and

washed with ethanol (10 mL) and diethyl ether (10 mL). Silver 1-oxido-5H-

tetrazolate (4) was obtained as a white powder in quantitative yield

(332 mg, 1.72 mmol, 99%). Crystals suitable for single crystal X-Ray

measurements were obtained by a three-layer diffusion crystallization over

several days.

Experimental Section

1-Hydroxy-5H-tetrazole (1): 5-Amino-1-hydroxytetrazole (1.00 g,

10 mmol) was dissolved in sulfuric acid (16 mL, 40%) and cooled below

5 °C with an ice-bath. Sodium nitrite (684 mg, 10 mmol) dissolved in water

(20 mL) was slowly added keeping the temperature below 5 °C. After

complete addition the reaction solution was stirred for 5 minutes, and was

then added to a flask containing a mixture of elemental copper (628 mg,

10 mmol) and ethanol (20 mL) which was warmed to 55 °C. After stirring

for 2 hours, the reaction mixture was extracted by DCM/Et₂O (1:1, 200 mL),

the organic phase was dried over magnesium sulfate and the solvent was

removed in vacuo. The yellow oil which was obtained contained 1-hydroxy-

5H-tetrazole and residual sulfuric acid as a by-product. Pure 1-hydroxy-

5H-tetrazole was obtained by dissolving the potassium salt (3) in

hydrochloric acid (2 M), followed by extraction into ethyl acetate. After

drying the organic phase over MgSO₄, the solvent was removed by a

stream of nitrogen gas, and 1-hydroxy-5H-tetrazole (1) was obtained as a

colorless crystalline solid in moderate yield (400 mg, 4.65 mmol, 46%).

IR (ATR, cm⁻¹): ṽ = 3100 (m), 1454 (s), 1414 (vs), 1404 (s), 1371 (w), 1340

(w), 1301 (w), 1246 (vs), 1206 (s), 1159 (m), 1108 (s), 1058 (w), 995 (w),

859 (s), 810 (vw), 732 (s), 701 (m), 667 (m), 495 (s); DTA (5 °C min^–1):

211 °C (T_exo); elemental analysis calcd [%] for CHAgN₄O (192.91):

C 6.23, H 0.52, N 29.04; found: C 6.11, H 0.63, N 28.38; BAM

drophammer: <1 J, BAM friction tester: 1 N, ESD: <0.28 mJ.

Ammonium 1-oxido-5H-tetrazolate (5): Compound 5 was obtained by

passing gaseous ammonia through a solution of 1 in DCM/Et₂O. The

precipitate in the DCM/Et₂O solution was filtered off, extracted with

methanol (40 mL), prior to diethyl ether being added. The precipitate was

filtered off and dried in air. Ammonium 1-oxidotetrazolate (5) was obtained

as an off-white powder in moderate yield (629 mg, 6.11 mmol, 61%).

Crystals suitable for single crystal X-Ray measurements were obtained by

recrystallization from water.

¹H NMR (acetone-d₆, 25 °C): δ = 9.23 ppm (s, 1H, CH); ¹³C NMR

(acetone-d₆, 25 °C): δ = 137.5 ppm (N4-C); IR (ATR, cm⁻¹): ṽ = 3001 (w),

2952 (w), 2924 (w), 1652 (vs), 1575 (s), 1416 (s), 1351 (s), 1284 (vs), 1244

(s), 1214 (vs), 1106 (w), 1059 (s), 1004 (m), 976 (s), 952 (s), 902 (s),

841 (vs), 750 (vs), 737 (s), 695 (vs), 667 (s), 635 (s), 588 (s); DTA

(5 °C min⁻¹): 186 °C (T_exo); elemental analysis calcd. [%] for CH₂N₄O

(86.05): C 13.96, H 2.34, N 65.11; found: C 14.15, H 2.31, N 64.96; BAM

drophammer: 10 J, BAM friction tester: 28 N, ESD: 960 mJ.

¹H NMR (DMSO-d₆, 25°C): δ = 8.44 ppm (s, 1H, CH); ¹³C NMR (DMSO-

d₆, 25 °C): δ = 135.1 ppm (N4-C); IR (ATR, cm⁻¹): ṽ = 3465 (m), 3427 (m),

3281 (w), 3150 (m), 1652 (m), 1472 (m), 1433 (vw), 1407 (m), 1384 (w),

1350 (vw), 1309 (w), 1247 (s), 1192 (m), 1158 (w), 1114 (w), 1065 (w), 982

(w), 824 (m), 738 (m), 715 (w), 670 (m), 529 (s), 481 (vs); DTA (5 °C min^–

¹): 188 °C (T_exo); elemental analysis calcd [%] for CH₅N₅O (103.09):

C 11.65, H 4.89, N 67.94; found: C 11.26, H 4.89, N 68.58; BAM

drophammer: >40 J, BAM friction tester: >360 N, ESD: >1500 mJ.

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Hydroxylammonium 1-oxido-5H-tetrazolate (6): Compound 1 (500 mg,

5.81 mmol) was dissolved in ethanol (10 mL) and aqueous hydroxylamine

solution (50%, 479 mg, 7.20 mmol) was added. After stirring for 5 minutes

at 50 °C, the precipitate was filtered off, washed with diethyl ether and

dried in air. Hydroxylammonium 1-oxido-5H-tetrazolate (6) was obtained

as a yellowish powder in good yield (546 mg, 4.59 mmol, 79%). Crystals

suitable for single crystal X-Ray measurements were obtained by

recrystallization from water.

¹³C NMR (D₂O, 25 °C): δ = 153.0 ppm; IR (ATR, cm⁻¹): 휈

̃ = 3338 (s), 3085

(s), 1650 (m), 1579 (m), 1544 (s), 1518 (s), 1455 (m), 1435 (m), 1354 (s),

1325 (s), 1251 (s), 1243 (s), 1227 (s), 1176 (s), 1152 (s), 1125 (m), 1035

(m), 1011 (m), 963 (m), 826 (s), 753 (s), 732 (s), 649 (vs), 544 (vs), 489

(vs), 481 (vs), 465 (s), 457 (vs), 419(vs), 410 (vs); DTA (5 °C min⁻¹):

311 °C (T_exo); elemental analysis calcd [%] for C₂H₁₂Li₃N₁₁O₈(339.01):

C 7.09, H 3.57, N 45.45; found: C 7.38, H 3.59, N 45.24; BAM

drophammer: > 40 J; BAM friction tester: > 360 N; ESD: ≥ 1500 mJ.

¹H NMR (DMSO-d₆, 25°C): δ = 8.51 ppm (s, 1H, CH); ¹³C NMR (DMSO-

d₆, 25 °C): δ = 135.0 ppm (N4-C); IR (ATR, cm⁻¹): ṽ = 3160 (m), 2889 (s),

2791 (s), 1864 (w), 1687 (w), 1643 (w), 1597 (w), 1468 (s), 1433 (s), 1399

(vs), 1379 (s), 1333 (m), 1233 (vs), 1182 (s), 1147 (m), 1098 (s), 1046 (w),

967 (s), 824 (s), 717 (vs), 666 (s), 485 (m), 421 (w); DTA (5 °C min^–1): 159

(T_exo), 203 (T_exo); elemental analysis calcd [%] for CH₅N₅O₂(119.08):

C 10.09, H 4.23, N 58.81; found: C 10.26, H 3.86, N 58.01; BAM

drophammer: 6 J, BAM friction tester: 240 N, ESD: >1500 mJ.

Trisodium bis(1-oxidotetrazol-5-yl)triazenide hexahydrate (10):

Compound 10 was obtained as yellow-brown needles in moderate yield

(283 mg, 0.73 mmol, 59%).

¹³C NMR (D₂O, 25 °C): δ = 153.1 ppm; IR (ATR, cm⁻¹): 휈

̃ = 3334 (m), 1664

(m), 1639 (m), 1518 (m), 1497 (m), 1444 (m), 1426 (m), 1357 (s), 1302 (s),

1256 (s), 1242 (s), 1210 (s), 1168 (m), 1147 (m), 1114 (m), 955 (m), 817

(m), 753 (m), 735 (m), 653 (m), 645 (m), 582 (s), 561 (vs), 561 (vs), 512

(vs), 491 (vs), 412 (s); DTA (5 °C min⁻¹): 338 °C (T_exo); elemental

analysis calcd. [%] for C₂H₁₂N₁₁Na₃O₈(387.16): C 6.20, H 3.12, N 39.80;

found: C 6.19 , H 2.83, N 39.02; BAM drophammer: > 40 J; BAM friction

tester: > 360 N; ESD: 1060 mJ.

Hydrazinium 1-oxido-5H-tetrazolate (7): Compound

1

(500 mg,

5.81 mmol) was dissolved in ethanol (10 mL) and hydrazinium hydrate

(100%, 291 mg, 5.81 mmol) was added. After stirring for 5 minutes at

50 °C, the precipitate was filtered off, washed with diethyl ether and dried

in air. Hydrazinium 1-oxido-5H-tetrazolate (7) was obtained as a white

powder in quantitative yield (685 mg, 5.80 mmol, 99%). Crystals suitable

for single crystal X-Ray measurements were obtained by recrystallization

from water.

Tripotassium bis(1-oxidotetrazol-5-yl)triazenide trihydrate (11):

Compound 11 was obtained as yellow needles in moderate yield (248 mg,

0.65 mmol, 53%).

¹³C NMR (D₂O, 25 °C): δ = 153.2 ppm; IR (ATR, cm⁻¹): 휈

̃ = 3489 (m), 3478

¹H NMR (DMSO-d₆, 25°C): δ = 8.52 ppm (s, 1H, CH); ¹³C NMR (DMSO-

d₆, 25 °C): δ = 135.1 ppm (N4-C); IR (ATR, cm⁻¹): ṽ = 3297 (m), 3139 (s),

2754 (s), 2628 (s), 1587 (s), 1532 (s), 1455 (m), 1400 (s), 1379 (s), 1336

(w), 1286 (w), 1267 (w), 1238 (s), 1217 (s), 1185 (m), 1134 (s), 1106 (s),

1077 (s), 1042 (m), 957 (vs), 832 (m), 779 (w), 760 (vw), 732 (s), 717 (s),

667 (m), 483 (m), 417 (m), 404 (m); DTA (5 °C min^–1): 213 °C (T_exo);

elemental analysis calcd [%] for CH₆N₆O (118.10): C 10.17, H 5.12,

N 71.16; found: C 10.11, H 5.25, N 71.33; BAM drophammer: 26 J, BAM

friction tester: >360 N, ESD: 740 mJ.

(m), 3262 (m), 3230 (m), 3084 (m), 1688 (w), 1654 (w), 1521 (m), 1505 (s),

1444 (m), 1419 (m), 1320 (vs), 1254 (s), 1238 (s), 1224 (s), 1213 (s), 1159

(m), 1134 (m), 1023 (w), 999 (w), 952 (w), 808 (s), 747 (m), 734 (s), 716

(m), 678 (m), 620 (s), 561 (s), 480 (m), 458 (s); DTA (5°C min⁻¹): 292 °C

(T_exo); elemental analysis calcd [%] for C₂H₆K₃N₁₁O₅(381.44): C 6.30,

H 1.59, N 40.39; found: C 6.32, H 1.46, N 40.43; BAM drophammer:

>40 J; BAM friction tester: > 360 N; ESD: >1500 mJ.

Trirubidium bis(1-oxidotetrazol-5-yl)triazenide trihydrate (12):

Compound 12 was obtained as yellow-brown needles in moderate yield

(392 mg, 0.75 mmol, 61%).

Bis(1-hydroxytetrazol-5-yl)triazene monohydrate (8): Aqueous sodium

nitrite (85.3 mg, 1.24 mmol, 5 mL) was added to a solution of 5-amino-1-

hydroxytetrazole (250 mg, 2.47 mmol) in hydrochloric acid (2 M, 5 mL) and

cooled to 0 – 5 °C. After stirring for 30 minutes, the pH of the solution was

adjusted to pH ≥ 10 by adding sodium hydroxide solution (32%) and stirred

at 35 °C for another 30 minutes. The pH of the solution was then adjusted

to pH ≤ 2 using hydrochloric acid (37%) and extracted into ethyl acetate (4

x 50 mL). After slowly evaporating the solvent in air, bis(1-hydroxytetrazol-

5-yl)triazene monohydrate (8) was obtained as a brown crystalline solid.

Due to the immediate onset of slow decomposition of 8, elemental analysis

and IR spectroscopic analysis were not performed. Sensitivity

measurements were performed immediately after synthesis was complete

without determining the purity of the sample.

¹³C NMR (D₂O, 25 °C): δ = 153.2 ppm; IR (ATR, cm⁻¹): 휈

̃ = 3486 (m), 3158

(s), 2159 (w), 1668 (w), 1562 (vw), 1516 (m), 1498 (s), 1428 (m), 1409 (m),

1309 (vs), 1249 (s), 1227 (vs), 1208 (s), 1163 (m), 1146 (s), 1027 (m),

1002 (w), 952 (w), 811 (s), 744 (s), 733 (s), 675 (m), 667 (m), 624 (s), 595

(s), 572 (s), 518 (s), 501 (s), 492 (s), 435 (m); DTA (5 °C min⁻¹): 292 °C

(T_exo); elemental analysis calcd [%] for C₂H₆N₁₁O₅Rb₃(520.55): C 4.61,

H 1.16, N 29.60; found: C 4.75, H 1.04, N 29.53; BAM drophammer:

> 40 J; BAM friction tester: 288 N; ESD: ≥ 750 mJ.

Triguanidinium bis(1-oxidotetrazol-5-yl)triazenide (13): Compound 13

was obtained as yellow plates in good yield (328 mg, 0.84 mmol, 68%).

¹³C NMR (DMSO-d₆, 25 °C): δ = 150.0 ppm; DTA (5 °C min^–1): 100 °C

(T_exo); BAM drophammer: <1 J, BAM friction tester: 4 N.

¹³C NMR (D₂O, 25 °C): δ = 157.9, 153.0 ppm; IR (ATR, cm⁻¹): 휈

̃ = 3046

(vw), 3349 (vw), 3404 (w) , 2815 (w), 1644 (w), 1519 (vw), 1490 (w), 1434

(vw), 1412 (m), 1340 (vw), 1299 (w), 1242 (w), 1229 (w), 1213 (w), 1158

(w), 1140 (w), 1121 (w), 999 (vw), 954 (vw), 880 (vw), 812 (w), 771 (w),

738 (w), 721 (w), 680 (w), 654 (w), 589 (w), 554 (w), 526 (w), 504 (w), 491

(w), 430 (w), 405 (w); DTA (5 °C min⁻¹): 222 °C (T_exo); elemental analysis

calcd [%] for C₅H₁₈N₂₀O₂(390.24): C 15.39, H 4.65, N 71.77; found:

C 15.33, H 4.65, N 70.31; BAM drophammer: >40 J; BAM friction

tester: >360 N; ESD: >1500 mJ.

General procedure for the synthesis of bis(1-oxidotetrazol-5-

yl)triazenide salts:

Stoichiometric equivalents of hydroxides, carbonates or free bases (14),

dissolved in methanol/water were added to a solution of 8 in ethyl acetate.

The resulting solution was stirred for 5 minutes or until precipitation was

complete. The precipitate was filtered off and recrystallized from water.

Tri(hydroxylammonium)

Compound 14 was obtained as a pale yellow solid in moderate yield

(228 mg, 0.73 mmol, 59%).

bis(1-oxidotetrazol-5-yl)triazenide

(14):

Trilithium

bis(1-oxidotetrazol-5-yl)triazenide

hexahydrate

(9):

Compound 9 was obtained as brown needles in good yield (324 mg,

0.96 mmol, 77%).

IR (ATR, cm⁻¹): 휈

2858 (m), 2724 (s), 2445 (s), 1606 (s), 1558 (m), 1519 (m), 1497 (m), 1472

̃ = 3597 (m), 3511 (w), 3157 (w), 3059 (m), 3048 (m),

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(s), 1460 (s), 1440 (s), 1415 (s), 1338 (w), 1286 (m), 1263 (s), 1245 (s),

1234 (s), 1199 (vs), 1146 (s), 1126 (s), 1029 (m), 1016 (m), 1004 (m), 991

(m), 933 (s), 886 (m), 810 (s), 742 (m), 727 (m), 710 (m), 692 (m), 668 (m),

645 (s), 576 (m), 483 (m), 455 (m), 440 (s), 426 (m), 403 (s); DTA

(5 °C min⁻¹): 148 °C (T_exo); elemental analysis calcd [%] for C₂H₁₂N₁₄O₅

(312.21): C 7.69, H 3.87, N 62.81; found: C 8.05 , H 3.64, N 61.28; BAM

drophammer: 4 J; BAM friction tester: 128 N; ESD: ≥ 750 mJ.

[20] T. S. Basu Baul, D. Dutta, A. Duthie, M. F. C. Guedes da Silva, Inorg.

Chim. Acta 2017, 455, 627-637.

[21] J. W. Staples, J. M. Stine, E. Mäki-Lohiluoma, E. Steed, K. M. George,

C. M. Thompson, E. L. Woodahl, Food Chem. Toxicol. 2020, 146,

111785.

[22] K. A. Hofmann, H. Hock, Ber. Dtsch. Chem. Ges. 1910, 43, 1866-1871.

[23] N. Szimhardt, M. H. H. Wurzenberger, L. Zeisel, M. S. Gruhne, M.

Lommel, J. Stierstorfer, J. Mater. Chem. A 2018, 6, 16257-16272.

[24] T. M. Klapötke, J. Stierstorfer, J. Am. Chem. Soc. 2009, 131, 1122-1134.

[25] A. L. Spek, (Ed.: T. N. Utrecht University), 1999.

[26] J. A. Montgomery Jr., M. J. Frisch, J. W. Ochterski, G. A. Petersson, J.

Chem. Phys. 2000, 112, 6532-6542.

Ammonium copper(II) bis(1-oxidotetrazol-5-yl)triazenide (15):

Aqueous sodium nitrite (85.3 mg, 1.24 mmol, 5 mL) was added to a

solution of 5-amino-1-hydroxytetrazole (250 mg, 2.47 mmol) in

hydrochloric acid (2 M, 5 mL) and cooled to 0 – 5 °C. After stirring for 30

minutes, the pH of the solution was adjusted to pH ≥ 10 by adding aqueous

sodium hydroxide solution (32%), and stirred at 35 °C for another

30 minutes. Copper sulfate pentahydrate (926.4 mg, 3.71 mmol) dissolved

in water (5 mL) was added to the reaction solution, and after stirring for

5 minutes, the precipitate was filtered off and washed with water.

Recrystallization from aqueous ammonia resulted in compound 15 as

green crystalline plates in moderate yield (205 mg, 0.52 mmol, 42%).

[27] J. W. Ochterski, G. A. Petersson, J. A. Montgomery Jr., J. Chem. Phys.

1996, 104, 2598-2619.

[28] L. A. Curtiss, K. Raghavachari, P. C. Redfern, J. A. Pople, J. Chem. Phys.

1997, 106, 1063-1079.

[29] B. M. Rice, S. V. Pai, J. Hare, Combust. Flame 1999, 118, 445-458.

[30] E. F. C. Byrd, B. M. Rice, J. Phys. Chem. A 2006, 110, 1005-1013.

[31] R. Mayer, J. Köhler, A. Homburg, Explosives, 5th ed., Wiley-VCH,

Weinheim, 2002.

IR (ATR, cm⁻¹): 휈

̃ = 3323 (m), 3259 (m), 3176 (m), 1609 (m), 1536 (m),

1466 (m), 1433 (w), 1374 (vs), 1319 (s), 1298 (s), 1250 (vs), 1237 (vs),

1219 (vs), 1122 (m), 1052 (m), 1005 (m), 976 (m), 833 (m), 750 (s), 715

(s), 679 (s), 659 (s), 608 (s), 574 (s), 536 (s), 499 (s), 476 (s), 457 (s), 445

(s), 433 (s); DTA (5 °C min⁻¹): 212 °C (T_exo); elemental analysis calcd [%]

for C₂H₁₀CuN₁₂O₅(345.73): C 6.95, H 2.92, N 48.62; found: C 6.68, H 2.98,

N 48.63; BAM drophammer: 11 J; BAM friction tester: 288 N; ESD:

380 mJ.

Acknowledgements

Financial support of this work by the Ludwig-Maximilian University of

Munich (LMU), the Office of Naval Research (ONR) under grant no.

N00014-19-1-2078 as well as the Strategic Environmental Research and

Development Program (SERDP) under grant no. WP19-1287 is gratefully

acknowledged.

Keywords: tetrazoles

•

explosives

•

triazenes

•

diazotation

•

dediazonization

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Article Doi

Selective Synthesis and Characterization of the Highly Energetic Materials 1-Hydroxy-5H-tetrazole (CHN4O), its Anion 1-Oxido-5H-tetrazolate (CN4O?) and Bis(1-hydroxytetrazol-5-yl)triazene

DOI: 10.1002/asia.202100714

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