Comparison of 1-Ethyl-5H-tetrazole and 1-Azidoethyl-5H-tetrazole as Ligands in Energetic Transition Metal Complexes

Article abstract of DOI:10.1002/asia.201900269

Energetic coordination compounds (ECC) based on 3d or 4d transition metals show promising characteristics to be used as potential replacements for highly toxic lead-containing primary explosives. Herein we report the synthesis of 12 new ECC based on 1-azi

Full text of DOI:10.1002/asia.201900269

CHEMISTRY
AN ASIAN JOURNAL
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Accepted Article
Title: Comparison of 1-Ethyl-5H-tetrazole and 1-Azidoethyl-5H-
tetrazole as Ligands in Energetic Transition Metal Complexes
Authors: Joerg Stierstorfer, Maximilian H. H. Wurzenberger, Thomas
M. Klapötke, Michael S. Gruhne, Marcus Lommel, and
Norbert Szimhardt
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Comparison of 1-Ethyl-5H-tetrazole and 1-Azidoethyl-5H-tetrazole
as Ligands in Energetic Transition Metal Complexes
Maximilian H. H. Wurzenberger[a], Michael S. Gruhne[a], Marcus Lommel[a], Norbert Szimhardt[a], Thomas
M. Klapötke[a], and Jörg Stierstorfer*[a]
Abstract: Energetic coordination compounds (ECC) based on 3d or
4d transition metals show promising characteristics to be used as
potential replacements for highly toxic lead-containing primary
explosives. Herein we report the synthesis of 12 new ECC based on
1-azidoethyl-5H-tetrazole (AET) or 1-ethyl-5H-tetrazole (1-ETZ) as
azides, are known as thermally decomposable and, in part,
explosive classes of compounds. LA is one of the most commonly
used primary explosives in both industrial and military
applications. However, alternatives, so-called green primary
explosives, are in great demand to prevent the considerable,
nitrogen-rich ligands as well as various central metals (Cu2+, Fe2+, Zn2+, highly toxic lead contamination in military training grounds. Over
Ag+) and anions such as perchlorate and nitrate. The influence of the
increased endothermicity by adding an additional azide group was
studied by comparing analogous ECC based on AET and 1-ETZ.
Furthermore, the compounds were extensively analyzed by XRD, IR,
EA, solid-state UV/Vis, and DTA as well as their sensitivities toward
impact and friction were determined with BAM standard techniques,
together with their sensitivity against electrostatic discharge. The
sensitivities were compared with the one toward ball drop impact
measurements. Classical initiation tests (nitropenta filled detonators)
and ignition by laser irradiation highly prove the potential use of the
most promising compounds in lead-free initiation systems.
95 % of all missile launches, shooting, and explosions within the
police forces or military are done exclusively for training purposes
in “friendly” areas,[4] leading to an ongoing research for new and
more eco-friendly energetic materials.[5–10]
Since the discovery of phenyl azide in 1864,[11] azido organic
compounds have attracted the attention of chemists due to their
unique characteristics. The initial postulation of Curtius and
Hantzsch for the structural determination of organic azides
suggested a cyclic 1H-triazirine structure. Later, however, this
hypothesis was rapidly revised in favor of the linear structure
(Chart 1).[1,12–14] The chemical diversity of many organic azides
can be explained on the basis of the physicochemical properties
by a consideration of the polar mesomeric structures.[15,16]
Introduction
Ever since the discovery of the azide anion by Curtius in 1890,[1]
the ongoing azide chemistry has fascinated several generations
of chemists. Today almost every single element from the periodic
table has been combined to build up covalent and ionic azides.
The combination of simple main-group and transition metals with
the azide anion has provided one high point in this chemistry
together with the well-deserved reputation for the explosive
behavior.[2] Among easily available and industrial useful ionic
azides, lead azide (LA) and sodium azide are the most prominent
examples. Nowadays, NaN3, as the cheapest azide source, is
indispensable in many chemical transformations like substitutions
or addition reactions in synthetic organic chemistry. Additionally,
sodium azide was also found in previous airbag systems.[3]
Covalently bonded and ionic azides, in particular, heavy metal
Chart 1. Mesomeric structures of covalent bonded azide.
The mesomeric structures c and d, proposed by Pauling,[17,18]
compellingly explain the reactivity in 1,3-dipolar cycloadditions
and the decomposition of azides to form nitrenes under the
elimination of nitrogen gas. The reactivity of the azide functionality
in terms of nucleophilicity (nucleophilic attack of N1) and
electrophilicity (nucleophilic attack on N3) can be deduced from
the basis of structure d. From a thermodynamic standpoint, the
azide group is a structural fragment, which shows a high positive
heat of formation and adding about 364 kJ mol−1 of
endothermicity to a hydrocarbon compound.[19] Additionally, the
azide groups exhibit an environmentally friendly balance, as
nitrogen gas is produced as an exclusive smokeless combustion
product. Therefore, organic azides are potential candidates for
the use in high energetic materials like binders, hypergolic ionic
liquids, plasticizers and additives (Chart 2).[2,20–23]
[a]
M. H. H. Wurzenberger, M. S. Gruhne, Dr. N. Szimhardt, M.
Lommel, Prof. Dr. T. M. Klapötke, Dr. J. Stierstorfer
Department of Chemistry, Ludwig-Maximilian-University of Munich
Butenandtstr. 5–13, 81377 Munich (Germany)
E-mail: jstch@cup.uni-muenchen.de
Homepage: http://www.hedm.cup.uni-muenchen.de
Supporting information, including X-ray data, IR spectra, DTA plots,
column diagrams of the complexes, hot plate and hot needle tests,
initiation capability tests, laser initiation tests, UV/Vis spectra,
experimental part and general methods for this article is given via a
link at the end of the document.
Chart 2. Representative examples for energetic organic compounds: hypergolic
ionic liquids (bis(2-azidoethyl)dimethylammonium dicyanamide (A) and 1-(2-
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azidoethyl)-4-amino-1,2,4-triazolium azide (B)), energetic plasticizers (3,3’,5,5’-
tetra(azidomethyl)-4,4’-azo-1,2,4-triazole (C) and 4-amino-3,5-di(azidomethyl)-
1,2,4-triazole (D)), or an energetic additive in nanotubes (2,4,6-
triazidopyrimidine (E)).
closure reaction again is performed, using triethyl orthoformate,
sodium azide, and glacial acetic acid at 80 °C for 12 h. After
filtration and extraction using ethyl acetate, the product was
received in a satisfying yield of 75 %. Both tetrazole derivatives
can easily be characterized by IR spectroscopy (Fig. S1), proton
and carbon NMR spectroscopy. Additional, two dimensional H,
15N NMR HMBC or proton-coupled 15N NMR spectroscopy can be
performed (Fig. 1 and 2).
1
Due to the smokeless gas evolution, the major application
nowadays is found in gas generators. Besides these utilizations
of organic azides, the use in primary explosives is especially
promising. Nitrogen-rich molecules are considered as prime
candidates for “green” energetic materials[24–27] since the main
combustion product is molecular dinitrogen and the materials
exhibit desirable performance characteristics in high explosives
(HE) due to their high positive heat of formation.
Scheme 2. Selective route for the synthesis of 1-ethyl-5H-tetrazole (2).
In this paper, we demonstrate the increasing performance of
ECC by solely substituting one proton of the alkyl rest of 1-ethyl-
5H-tetrazole with an azide group. The AET based transition metal
complexes were both investigated toward their usability as lead-
free primary and laser ignitable explosives. In addition, they were
compared to their analogous ECC with unsubstituted 1-ETZ as
nitrogen-rich ligand.
Results and Discussion
Synthesis
Over three decades ago, the synthesis of 1-azidoethyl-5H-
tetrazole was published for the first time by Gaponik et al..[28] The
only reported complex is based on copper(II) chloride as metal
salt and was only investigated for its crystal structure.[29] For
synthesizing ligand 1 a two-step reaction, starting from a simple
and cheap feedstock was performed (Scheme 1).
Scheme 1. Synthesis of 1-azidoethyl-5H-tetrazole (1) starting from
2-chloroethylamine hydrochloride.
Figure 1. Proton coupled 15N NMR spectra and two dimensional 1H, 15N-HMBC
NMR spectra of 1. 15N NMR (MeCN-d3, 25 °C, ppm) δ: 11.4 (N3, d, 2JN-H = 3.0),
−13.9 (N2), −52.2 (N4, d, 2JN-H = 12.1 Hz), −135.2 (N6, t, 2JN-H = 3.3 Hz), −147.4
(N1, d, 2JN-H = 7.3 Hz), −171.1 (N7).
The first target molecule was 2-azidoethylamine
hydrochloride, which is less volatile than the neutral compound
and for which several syntheses are already reported in
literature.[30,31] 2-Chloroethylamine hydrochloride was reacted
with sodium azide in water to form the intermediate in quantitative
yield. The crude hydrochloride salt was converted in a [3+1+1]
cyclization to the corresponding substituted alkyl tetrazole 1 by
applying an excess of triethyl orthoformate and sodium azide in
glacial acetic acid at elevated temperature. For the isolation of the
nitrogen-rich ligand, the crude was filtrated, extracted with ethyl
acetate and subjected to column chromatography leading to a
yellowish oil in acceptable 33 % yield over two-steps. The
physical state of 1 can be explained by previous reports showing
the liquefying effect of the azidoethyl functionality.[2]
As mentioned, a large number of complexes based on various
mono-tetrazoles (e.g. 1-methyl- or 1-amino-5H-tetrazole) have
been described in literature, either having insufficient power or are
way too sensitive for practical use.[32,33] In order to maximize the
performance, ligand 1 is applied in the formation of new transition
metal complexes, whereas the use of ligand 2 is a promising
opportunity for reaching the middle course between both
demands. The absence of acidic protons in both ligands allows
their inclusion in neutral metal coordination compounds with
oxidizing anions like nitrates, perchlorates, picrates (PA),
styphnates (TNR) or 2,4,6-trinitro-3,5-dihydroxyphenolates
(H2TNPG). The selective integration of these anions in
With 1-ethyl-5H-tetrazole (2), another ethyl-substituted 5H-
tetrazole derivative is used for the synthesis of nitrogen-rich ECC
in this work. The ligand can be obtained via the same selective
pathway (Scheme 2) as mentioned above.[28] The [3+1+1] ring
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combination with one of the two ligands and different metal(I) or
metal(II) cations, results in energetic coordination compounds
with adjustable sensitivities and performance parameters. The
formation of the coordination compounds 3–9 was achieved
through the combination of ethanolic solutions of the relevant
metal(I) or metal(II) salts and the ligand in stoichiometric amounts
at r.t. or elevated temperatures in case of compound 3 (Scheme
3 and 4). Water and ethanol as solvent were chosen because of
their non-toxic character and the good solubility of the ligand and
the metal salts. Except coordination compound 3, every complex
crystallized without inclusion of water molecules. Complex 4 has
only been obtained as a side species of 3, making a further
characterization impossible. Copper(II) chlorate complexes based
on ligand 1 and 2 have been synthesized analogously to our
recently published work (Scheme 5).[34]
Scheme 4. Synthesis of the copper(II) perchlorate complexes 8 and 9.
Scheme 5. Synthesis of the chlorate complexes 10 and 11.
Due to the commercial unavailability of the copper(II) starting
materials of the compounds 12–14, an acid-base reaction had to
be applied, starting from the respective 2,4,6-trinitrophenol
derivatives and copper(II) carbonate in water at 70 °C. Filtration
and treating with ligand 1 yielded the respective coordination
compound (Scheme 6).
Figure 2. Proton coupled 15N NMR spectra and two dimensional 1H, 15N-HMBC
NMR spectra of 2. 15N NMR (MeCN-d3, 25 °C, ppm) δ: 10.4 (N3), −14.1 (N2),
−52.8 (N4, d, 2JN-H = 12.2 Hz), −139.5 (N1).
Scheme 6. Synthesis of the nitroaromatic coordination compounds 12–14.
Potential precipitates during the addition of the ligand are
dissolved by dropwise addition of water. Single crystals suitable
for X-Ray diffraction were obtained directly from the mother liquor
within a day. The filtration step after the acid-base reaction toward
12–14 turned out to be indispensable for elemental analysis pure
Scheme 3. Synthesis of the nitrato complexes 3 and 4 together with the
perchlorate complexes 5–7.
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products because traces of unreacted copper(II) carbonate
cannot be removed by washing with cold ethanol after
crystallization. All coordination compounds were obtained directly
from the mother liquor and crystallized in satisfactory yields (38–
87 %). The crystalline materials were filtered off, washed with
small amounts of ice-cold ethanol and dried in air overnight.
Single crystals suitable for X-ray diffraction of every compound
except 5, 8, 9 and 10 were also obtained directly from the mother
liquor. Crystals of the remaining coordination compounds were
obtained after recrystallization from water.
The copper(II) nitrato complex (3) crystallizes in the form of
blue plates in the triclinic space group P–1 with two formula units
per unit cell and a calculated density of 1.834 g cm−3 at 143 K.
The sevenfold coordination environment of the copper(II) center
consists of two chelating nitrato, one aqua, and two AET ligands.
The nitrates are functioning as bidentate nitrato-ligands, resulting
in a longer (2.734 Å) and a shorter (1.971 Å) Cu–O bond. Overall,
the unusual amount of five ligands together with the extended
sevenfolded coordination sphere results in three clearly longer
Cu–O bonds (O2–Cu1 2.734(5) Å, O5–Cu1 2.734(5) Å O7–Cu1
2.337(5) Å), including the aqua ligand. The anhydrous nitrato
complex 4 crystallizes in the form of blue blocks in the monoclinic
space group P21/c with two formula units per unit cell. The
calculated density is 1.895 g cm−3 at 173 K. The closed
octahedral coordination sphere again includes two chelating
nitrato and two AET ligands (Fig. 3). The nitrato ligands are
located in the equatorial and axial positions, whereas ligand 1
exclusively occupies equatorial positions. In consequence of the
bidentate character of the nitrate, the bond angles are highly
distorted from the ideal bond angle of 90°. A comparison of the
complexes 3 and 4 shows, that the addition of a water ligand leads
to a simple extension of the coordination sphere from a sixfold
highly symmetric complex to a sevenfolded coordinated complex
without any symmetry. In case of an extended coordination
sphere, it is not surprising that in complex 4 the bond distances
between the coordinating ligands and the copper(II) center are
longer than the bond distances reported for the water-free
compound 3. Additionally, the torsion angle between the oxygens
of one nitrate ligand is shortened from 53.54° (4) to 52.15° (3).
The formation of a sevenfolded coordination sphere is a behavior,
already observed for 1H-substituted tetrazole derivatives in
copper(II) nitrato complexes with similar bond lengths around the
central metal.[33]
Crystal Structures
The crystal structures of the ligands 1-azidoethyl-5H-tetrazole (1)
and 1-ethyl-5H-tetrazole (2) are not described in literature. Due to
their liquid state at r.t. and very low melting points, single crystals
of those molecules were not obtained.
Therefore, single-crystal X-ray diffraction studies of all
complexes were performed. Details on the crystal structures of
compound 9 and 11 together with the measurement and
refinement data of all complexes are given in the Supporting
Information (Table S1–3 and Fig. S5–7). The crystal datasets
were uploaded to the CSD database[35] and can be obtained free
of charge with the CCDC no. 1898402 (3), 1898401 (4), 1898397
(5), 1898400 (7), 1898399 (9), 1898541 (11), 1898395 (12),
1898398 (13), 1898396 (14).
All metal(II) (per)chlorate
complexes show octahedral coordination spheres around the
central metals built up by six molecules of ligand 1 or 2 (Fig. 4, S5
and S6). Unfortunately, compound 6, 8 and 10 could only be
measured at r.t. and are therefore highly disordered. Finalization
of the datasets was impossible but the measurements allowed an
insight into the complexes’ composition, which were confirmed by
elemental analysis and IR spectroscopy (Fig. S2–4). In all
complexes, except compound 7, both tetrazole derivatives
coordinate to the central metals exclusively through their
heterocyclic N4 nitrogen atoms. ECC 3 shows a rather uncommon
sevenfold-bonded copper(II) cation (Fig. 3) and complex 7, with
Ag–N bond lengths between 2.323(3) and 2.538(3) Å, possesses
distorted tetrahedrally coordinated silver ions.
Figure 3. Molecular units of [Cu(NO3)2(H2O)(AET)2] (3) and [Cu(NO3)2(AET)2]
(4). Selected bond lengths (Å) of 3: Cu1–N6 1.997(5), Cu1–N13 1.981(5);
selected bond angles (°) of 3: O1–Cu1–O2 52.15(15), O1–Cu1–O4 178.70(16),
O1–Cu1–O5 127.78(15), O1–Cu1–N6 91.55(19), O1–Cu1–N13 88.86(19), O2–
Cu1–O4 127.13(15), O2–Cu1–O5 78.29(14), O4–Cu1–O5 51.98(15), O4–Cu1–
N13 92.32(19), O5–Cu1–O7 137.21(17), O5–Cu1–N6 98.24(17), O5–Cu1-N13
79.86(17), O7–Cu1–N6 92.5(2), O7–Cu1–N13 89.6(2), N6–Cu1–N13
177.85(19). Selected bond lengths (Å) of 4: Cu–O1 1.9837(14), Cu1–O3
2.6497(18), Cu1–N4 1.971(2); selected bond angles (°) of 4: O1–Cu1–O3
53.54(6), O1–Cu1–N4 89.96(7), O1–Cu1–O1i 180.00, O1–Cu1–O3i 126.46(6);
symmetry code of 4: (i) 1−x, 2−y, 1−z.
Figure 4. Molecular unit of [Fe(AET)6](ClO4)2 (5). Selected bond lengths (Å):
Fe1–N4 1.990(4), Fe1–N11 1.996(4), Fe–N18 1.977(5); selected bond angles
(°): N11–Fe1–N11i 180.00, N4–Fe1–N11 89.17(16), N18–Fe1–N18i 180.00,
N4–Fe1–N18i 89.96(16), N4–Fe1–N4i 180.00, N11–Fe1–N18 89.41(18);
symmetry code: −x, −y, −z.
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The metal(II) perchlorate complexes 5, 6 and 8 all crystallize
isotypically in the monoclinic space group P21/c with similar cell
parameters and densities. Unfortunately, compounds 6 and 8
could only be measured at r.t. and therefore the ethyl azide
moieties are highly disordered. The unit cell of 5 is built up by two
formula units and possesses a calculated density of 1.648 g cm−3
at 128 K. The iron(II) central atom is surrounded by six molecules
of ligand 1 (Fig. 4) with similar Fe1–N bond lengths. Both
perchlorate anions are non-coordinating. The silver(I) complex 7
crystallizes in the form of colorless needles in the triclinic space
group P–1 and consists of two formula units per unit cell. With a
calculated density of 2.470 g cm−3 at 173 K, the complex
possesses the highest one of all compounds (Fig. 5). In contrast
to all other complexes based on AET, the ligand coordinates with
four different coordination sides, in particular, the nitrogen atoms
N2, N3, N4, and N5. This is resulting in the formation of one-
dimensional polymeric chains (Fig. 6). For the first time, the azide
functionality of the ligand 1 is involved in the coordination bonding,
with an astonishing long bond of Ag1–N5 (2.538 Å). With its bond
angles (N3ii–Ag–N4i = 122.84°, N2–Ag1–N5 = 77.31°), the
tetrahedral formed by compound 7 strongly differs from the
perfect angles of 109.47°. This deviation can be explained by the
perchlorate anions located above and underneath the layers of
silver ions and their interaction with these.
The coordination compound 12 crystallizes in the form of
green plates in the monoclinic space group P21/c with two formula
units per unit cell. The complex possesses a calculated density of
1.815 g cm−3 at 143 K. The coordination sphere is built up by two
equatorial arranged AET ligands and two molecules of picrate (Fig.
7). The anions are acting as bidental ligands, coordinating over
the deprotonated hydroxy group and by another oxygen atom of
one of the nitro groups. The latter occupy axial positions leading
to a typical Jahn-Teller distortion along the O2–Cu1–O2i axis.
Figure 7. Molecular unit of [Cu(PA)2(AET)2] (12). Selected bond lengths (Å):
Cu1–O1 1.931(4), Cu1–O2 2.334(3), Cu1–N4 1.997(4); selected bond angles
(°): O1–Cu1–O1i 180.00, O1–Cu1–O2 79.35(14), O1–Cu1–N4 91.62(17);
symmetry code: (i) −x, 1−y, −z.
The styphnate complex 13 shows
a similar density
(1.803 g cm−3 at 143 K) to compound 12 and crystallizes in the
triclinic space group P−1 with two formula units per unit cell (Fig.
8).
Figure 5. Coordination environment of [Ag(AET)](ClO4) (7). Selected bond
lengths (Å): Ag1–N2 2.420(3), Ag1–N4i 2.323(3), Ag1–N3ii 2.331(3); selected
bond angles (°): N2–Ag1–N4i 122.84(10), N2–Ag1–N3ii 111.68(11), N4i–Ag1–
N5 86.06(11), N3ii–Ag1–N5 133.85(10); symmetry codes: (i) −x, 1−y,1−z; (ii)
1+x, y, z.
Figure 8. Coordination environment of [Cu(TNR)(AET)2] (13). Selected bond
lengths (Å): Cu1–O1 1.9266(16), Cu1–O2 2.3431(17), Cu1–N4 2.0115(19);
selected bond angles (°): O1–Cu1–O1i 180.00, O1–Cu1–O2 80.65(7), O1–
Cu1–N4 88.92(7); symmetry code: (i) 2−x, −y, 2−z.
Figure 6. Segment of the polymeric chain of 7, formed by tetrazole rings linking
between three different silver atoms.
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Both hydroxy groups of styphnic acid are deprotonated
leading to the formation of a trinitroresorcinate anion, which is
coordinating to two different copper(II) cations by chelating each
with one nitro and one phenolate oxygen. The Jahn-Teller-
distortion along the O2–Cu1–O2i is formed by two weaker bonded
nitro groups of two different anions. Similar to silver complex 7,
13 forms one-dimensional polymeric chains, which are however
not caused by bridging ligands but by the fourfold coordinating
anion (Figure S7). Coordination compound 14 crystallizes in the
triclinic space group P−1 with only one formula unit per unit cell
and a calculated density of 1.766 g cm−3 at 143 K. In contrast to
nitroaromatic compound 13, the anions are only single
deprotonated and solely coordinating with the phenolate group
(Fig. 9). The remaining equatorial positions are occupied by four
instead of two molecules of ligand 1. Again, the Jahn-Teller-
distortion can be observed along the O–Cu–O axis.
based on and is already reported in literature.[32,33] Besides its low
point of exothermic decomposition, the nitrato complex (3) is the
only compound showing two endothermic signals (Tendo1 = 94 °C,
Tendo2 = 121 °C), indicating melting, followed by the loss of
coordinating aqua ligand (proofed by TGA). Other endothermic
events of compounds 6 and 8-11 can also be assigned to their
respective melting points, which were also observed during
heating of the respective compounds in hot plate tests. In order to
examine the endothermic events more closely, some of the
compounds were further investigated by thermal gravimetric
analysis (Fig. S15). Except 3, none of the compounds based on
ligand 1 showed significant loss of mass until their exothermic
decomposition points. Copper(II) complexes comprising 1-ETZ
(2) show similar to the methyl-substituted analogous compounds
evaporation of the ligand before their exothermic
decomposition.[32] In the row of 2,4,6-trinitroaromatic based
copper(II) complexes, the thermal stability decreases with
increasing number of hydroxy groups, with 14 (Texo = 121 °C)
showing the lowest stability of all investigated compounds.
The highest exothermic decomposition temperature was
observed for the copper(II) perchlorate complex 9 (Texo = 210 °C),
based on ligand 2, showing an exothermic signal 52 °C higher
than the analogous coordination compound based on ligand 1 (8:
Texo = 158 °C). The same can be observed for the two comparable
chlorate complexes (Fig. 10). This trend is confirmed, looking at
the temperatures evaluated for the respective ligand (1: Texo
=
193 °C; 2: Texo = 208 °C) and is guessed to be caused by the
azido function of ligand 1.
Figure 9. Unit cell of coordination compound [Cu(H2TNPG)2(AET)4] (14).
Selected bond lengths (Å): Cu1–O1 2.345(2), Cu1–N4 2.027(3), Cu1–N11
1.983(3); selected bond angles (°): O1–Cu1–N4 89.97(11), O1–Cu1–N11
95.31(11), O1–Cu1–O1i 180.00, N4–Cu1–N11 89.59(11); symmetry code: (i)
1−x, −y, 1−z.
Figure 10. Comparison of the sensitivities and thermal stability of the chlorate
and perchlorate complexes based on the ligands 1 and 2, confirming the lower
stability of chlorate compounds toward external stimuli.
Sensitivities and thermal stability
The behavior of all compounds, except the side-product 4, was
investigated in differential thermal analysis (DTA). The
measurements were performed in the range from 25–400 °C at a
heating rate of β = 5 °C min−1. Critical points are given as onset
temperatures. The observed endothermic events (melting,
dehydration or loss of coordinating ligand), as well as exothermic
events, are listed in Table 1. DTA-plots of the compounds 1–3 and
5–14 are available in Fig. S12–15 in the SI.
A general trend within the row of the investigated 3d
perchlorate coordination compounds shows higher exothermic
decomposition temperatures with an increasing atomic number
(Fe2+ < Cu2+ < Zn2+), which is in conflict (Cu2+-based complexes
being less stable than Fe2+) with our prior findings for 1H-
substiuted tetrazoles (Fig 11).[32] Interestingly, compounds 6 and
9
even show slightly higher exothermic decomposition
The exothermic decomposition temperature reported for
ligand 1 is in accordance with our findings, whereas for ligand 2
no data is available.[28] The exothermic decomposition
temperatures of most of the investigated compounds are above
150 °C, except 10 (Texo. = 146 °C) and 14 (Texo. = 121 °C). The
relatively low thermal stabilities of the nitrato compound 3 (Texo. =
152 °C), as well as the chlorate complexes, are a known issue,
resulting from the metal salt, the coordination compounds are
temperatures (Texo = 196 °C and 210 °C, respectively) than the
corresponding ligand.
The sensitivities toward impact (IS) and friction (FS) were
assessed according to BAM standard methods (1 of 6) together
with the electrostatic discharge sensitivity (ESD) for all
compounds. In addition, all complexes have been categorized in
accordance with the “UN Recommendations on the Transport of
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Dangerous Goods” using the determined sensitivities.[36] An
overview of the sensitivities is given in Table 1. Because
sensitivity data are highly affected by the crystal morphology, the
compounds’ grain size and habitus were investigated using light
microscopy. The data are represented in the supplementary
information (Fig. S8–11).
The free ligand 2 is the only investigated compound to be
ranked as insensitive with its high stability toward friction and
impact. In contrast to that, ligand 1 has to be classified as
sensitive because of its sensitivity of 9 J toward impact, whereas
it is insensitive toward friction. No measurements regarding
electrostatic discharge could be performed due to the physical
state of both ligands. The highest stability toward impact of all
ECC (15 J, sensitive) is possessed by the only zinc coordination
compound 6, which has to be considered as very sensitive
because of its friction sensitivity. Comparing the perchlorate
complexes reveals, that all have to be considered as very
sensitive (6, 8) or even extremely sensitive (5, 7), except
compound 9, which is stated as sensitive, also possessing the
highest stability toward friction of all investigated perchlorates.
The sensitivity toward impact of the ECC based on perchlorate
and AET increases in the following order: Zn2+ (15 J) < Fe2+ (3 J)
< Cu2+ (<1 J) ≈ Ag+ (<1 J) and in the following order against
friction: Zn2+ (40 N) < Cu2+ (15 N) < Fe2+ (3.75 N) < Ag+ (0.6 N)
(Fig. 11). Again these results contradict earlier observations.[32,33]
Figure 11. Comparison of the thermal stabilities together with the sensitivities
toward various stimuli of the perchlorate coordination compounds 5–8, showing
that silver compound is the most sensitive and the zinc one is the most
insensitive one.
Table 1. Overview of the compounds’ thermal stability[a], sensitivities toward various external stimuli and results of hot plate (HP) and hot needle (HN) tests
compared to lead azide.
Compound
Tendo. (°C)[b]
Texo. (°C)[c]
IS[d] (J)
FS[e] (N)
ESD[f] (mJ)
BDIS[g] (mJ)
HP[h]
HN[h]
AET (1)
-
193
208
9
> 40
10
> 360
> 360
108
3.75
40
n.d.
n.d.
840
65-0
368
65.0
368
960
226
608
226
123
608
7.00
n.d.
n.d.
n.d.
< 4
n.d.
n.d.
def.
det.
def.
det.
def.
def.
def.
def.
def.
def.
def.
n.d
n.d.
n.d.
def.
det.
def.
det.
det.
def.
det.
def.
def.
def.
def.
n.d
1-ETZ (2)
-
[Cu(NO3)2(H2O)(AET)2] (3)
[Fe(AET)6](ClO4)2 (5)
[Zn(AET)6](ClO4)2 (6)
[Ag(AET)]ClO4 (7)
94, 121
152
-
159
-
151
3
196
15
20
165
< 1
< 1
10
0.6
< 4
[Cu(AET)6](ClO4)2 (8)
[Cu(1-ETZ)6](ClO4)2 (9)
[Cu(AET)6](ClO3)2 (10)
[Cu(1-ETZ)6](ClO3)2 (11)
[Cu(PA)2(AET)2] (12)
[Cu(TNR)(AET)2] (13)
[Cu(H2TNPG)2(AET)4] (14)
135
172
77
111
-
158
15
12
210
120
4
n.d.
29
146
2.5
7
158
60
n.d.
> 200
> 200
> 200
n.d.
183
3
252
240
84
-
177
< 1
1.5
2.5-4
-
121
[37]
Pb(N3)2
-
320-360
0.1-1.0
[a] Onset temperature at a heating rate of 5 °C min−1 measured by DTA; [b] Endothermic peak, which indicates melting, vaporization, dehydration, or loss of aqua
[c]
ligands;
Exothermic peak, which indicates decomposition. [d] Impact sensitivity according to the BAM drophammer (method 1 of 6). [e] Friction sensitivity
according to the BAM friction tester (method 1 of 6). [f] Electrostatic discharge sensitivity (OZM Electric Spark XSpark10). [g] Ball drop impact sensitivity determined
with the 1 of 6 method in accordance with the MIL-STD 1751A (method 1016). [h] def.: deflagration; det.: detonation.
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Compounds 8–11 show comparable trends to external stimuli
as in thermal experiments. Complexes based on ligand 1 are
more sensitive (8 and 10 are both ranked as extremely sensitive)
than those of compound 2 (both sensitive) and the copper(II)
chlorate complexes are less stable (11) or at least show similar
sensitivities (10) compared to the analogous perchlorate ones
(Fig. 10), which is in accordance to our previous findings.[34] The
nitroaromatic derivatives 12–14 possess one of the highest
stabilities of all complexes toward friction, but due to their
sensitivities toward impact, they are all ranked as very sensitive
(Fig. S17).
concerning the copper(II) and zinc(II) perchlorate compound 6
and 8 did in our test not result in a positive DDT.
Because of the realistic terms of testing, sensitivity toward ball
drop impact (BDIS) was also determined for the most sensitive
complexes 5–8, and 10 (Fig. S18). Due to the high discrepancy
of 12–14 regarding the sensitivities toward impact and friction
according to BAM standard methods, ball drop experiments were
carried out for those compounds too. In the case of the
perchlorate complexes it becomes clear that low stability to friction
(7 < 5 < 8 < 6) is accompanied by a higher sensitivity to BDIS (7
≈ 5 < 8 < 6) and that the impact sensitivity in the drophammer
experiments (7 ≈ 8 < 5 <<< 6) is less significant. The same trend
can be observed for the nitroaromatic compounds 12–14.
Figure 13. Positive results of the initiation capability tests toward PETN of
compounds 5 and 7.
Nowadays used primary explosives like LA or LS suffer,
besides their toxicity, from their high sensitivity toward mechanical
stimuli.[25] In classical initiation devices, which are based on
mechanical stimuli, these properties are mandatory, but with the
ability to be initiated by laser irradiation they are no longer
required. This circumstance is responsible for the great interest
that the field has gained in recent years.[38,39] Possible industrial
applications like optical detonators or laser ignitable ammunition
are already under current research.[40,41] Based on latest results
concerning the laser ignition of ECC, this work focused only on
the colored compounds 3, 5 and 8–14.[33] Further information for
the applied setup can be found in the Supporting Information. All
complexes showed reaction toward laser irradiation (Table 2),
which strongly differed by the applied ligand and anion system.
Primary explosive suitability evaluation and laser initiation
For an insight into the behavior toward fast heating with and
without confinement and for gaining an overall insight in their
applicability as a primary explosive hot plate and hot needle tests
were performed for every investigated coordination compound
(Table 1, Fig. 12 and S19–30). Except the chlorate and
perchlorate complexes 5, 7, 8 and 10, which at least showed
detonations during one of the tests, every other compound only
deflagrated.
Table 2. Results of the laser initiation experiments.[a]
Compound
0.17 mJ
—
25.5 mJ
—
30 mJ
deflag.
det.
—
111 mJ
—
3
5
—
deflag.
det.
—
—
8
deflag.
—
—
9
dec.
—
—
10
11
12
13
14
det.
—
det.
—
—
Figure 12. Hot plate test (top) and hot needle test (bottom) of the iron(II)
perchlorate complex 5, shown as a sequence.
dec.
—
—
—
—
dec.
—
—
—
dec.
—
The most promising compounds were tested in initiation
capability tests with nitropenta (PETN) as the main charge.
Further information on the test setup can be found in the General
Methods of the Supporting Information. Positive tests, indicated
by a hole in the copper witness plate and fragmentation of the
shell, caused by a positive deflagration-to-detonation transition
(DDT) toward the secondary explosive, were observed for 5 and
7. [Ag(AET)]ClO4 (7) is representing one of the rare ECC with
silver as central metal being able to initiate PETN (Fig. 13). Tests
—
dec.
—
[a]
(—: not tested, dec.: decomposition, deflag.: deflagration, det.:
detonation). Operating parameters: current I = 7–12 A; voltage U = 4 V;
theoretical maximal output power Pmax = 45 W; theoretical energy Emax
0.17–111 mJ; wavelength λ = 915 nm; pulse length τ = 0.1–20 ms.
=
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As already recognized during hot plate and hot needle tests,
coordination compounds based on the more energetic ligand 1
tend way more likely to show detonations than compounds based
on ligand 2. The use of nitrate or aromatic anions is leading to
deflagrations only, whereas, in case of ligand 1, the use of
chlorates and perchlorates leads to detonations (Fig. 14 and S31–
37).
in the range of secondary explosives, most of the ECC based on
ligand 1 are primary explosives. The iron (5) and silver (7)
perchlorate compounds were both positive tested to initiate PETN,
Especially compound 5 with its manageable sensitivities and
nontoxic central metal could be a candidate of future interest.
During laser ignition experiments coordination compound 5 as
well as the copper compounds 8 and 10 showed detonations at
energies between 0.17 mJ and 30.0 mJ making them promising
candidates as green laser ignitable explosives. The comparison
of the impact sensitivities determined by drophammer and ball
drop reveals a higher similarity of the BDIS to FS than IS.
Acknowledgements
Financial support of this work by the Ludwig-Maximilian University
of Munich (LMU). The authors would like to thank Prof. Dr.
Konstantin Karaghiosoff for the measurement of various 15N NMR
spectra, Antonia Stadler for proofreading and Valentin Bockmair
as well as Philipp Spieß for their great contribution to this work.
Figure 14. Detonation of the copper(II) perchlorate complex 8 during laser
initiation test.
UV-Vis spectroscopy
Regarding the mechanism of laser initiation, UV-Vis spectra (Fig.
S38 and S39) of all tested compounds were recorded in the solid-
state and analyzed in detail at the laser operating wavelength of
915 nm. A summary of the optical properties of the measured
compounds is given in Table S4. The observed absorption in the
near infrared, visible and ultra-violet region results from the d-d
transitions, based on the respective central metal and its
interaction with the ligand and anion. Nevertheless, the process
of initiation via laser irradiation (e.g. thermal, electrochemical) has
still not been understood completely and is therefore of high
importance and strongly investigated.[42,43] All complexes show
moderate absorption behavior at the laser wavelength of 915 nm.
This could be a possible explanation for the ignitability via laser
irradiation since the colorless zinc and silver perchlorate
compounds 6 and 7 could not be ignited. Besides the absorption
in the desired area, other factors are important concerning laser
initiation, like the corresponding metal or its electron configuration.
Therefore, future studies should aim at the initiation process, in
particular, the influence of the complexes’ structure and the
mechanism behind the initiation.
Conflict of interest
The authors declare no conflict of interest.
Keywords: energetic coordination compounds • tetrazoles •
copper(II) • explosives • laser ignition
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Entry for the Table of Contents
FULL PAPER
The performance of energetic
coordination compounds (ECC) can
be easily adjusted to the desired
properties by varying one of their
building blocks. In that study the
substitution of one proton of
1-ethyltetrazole with an azide group
and the comparison of the analog
complexes by applying both ligands
is discussed. ECC, consisting of the
Maximilian H. H. Wurzenberger,
Michael S. Gruhne, Marcus Lommel,
Norbert Szimhardt, Thomas M.
Klapötke, and Jörg Stierstorfer*
Page No. – Page No.
Comparison of 1-Ethyl-5H-tetrazole
and 1-Azidoethyl-5H-tetrazole as
Ligands in Energetic Transition
Metal Complexes
azide-ligand,
show
promising
performances making them potential
lead azide replacements.
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