Maximization of the energy capability level in transition metal complexes through application of 1-amino- and 2-amino-5: H -tetrazole ligands

Article abstract of DOI:10.1039/c8ta06326e

Toxic lead compounds (especially lead azide and lead styphnate), reliable but highly harmful compounds, still dominate the field of primary explosives used in most ignition and initiation systems to this day. Therefore, an extensive investigation into energetic materials with similar performance and minimized toxicity led to the very promising strategy of developing energetic coordination compounds (ECC). For the first time in literature, two isomeric amino-5H-tetrazoles with sensitivities already in the range of primaries, which could be obtained via practicable, easily scalable synthesis, proved to be very promising new energetic ECC ligands. By variation of the respective isomer (1-AT, 2-AT), metal(ii) center (Mn^II, Fe^II, Cu^II, Zn^II) and the corresponding counter-anion (Cl^-, NO₃^-, ClO₃^-, ClO₄^-, styphnate, trinitrophloroglucinate) desired properties could be tailored gradually, yielding 20 new, highly energetic and easily accessible transition metal(ii) complexes. In addition to comprehensive characterization by XRD, IR, EA, solid-state UV/Vis, DTA and TGA, BAM standard sensitivity measurements as well as ESD and ball drop impact sensitivities were determined. Classical initiation tests (nitropenta filled detonator) validated the present approach to be extraordinarily promising for potential use in advanced initiation systems. Beyond successfully achieving set goals in terms of sensitivity and toxicity, investigated complexes match or even outperform lead-based primaries' high performance and, more importantly, do not necessarily depend on initiation by mechanical stimuli. Laser irradiation of the received primary explosives with near-infrared light (NIR) yielded initiation reliably and safely, further extending their potential for future applications.

Full text of DOI:10.1039/c8ta06326e

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Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
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This comprehensive study features the synthesis, characterization and evaluationDOoIf: 10n.1e0w39/Ce8nTAe0r6g3e26tiEc
coordination compounds (ECC) based on two of the most powerful neutral tetrazoles, which have great
potential as lead-free primary explosives. In addition to its extensive physico-chemical characterization,
obtained complexes were tested in laser ignition and PETN initiation experiments to explore its
suitability as primary explosives. The properties of the very promising candidates can be fine-tuned by
variation of the implemented metal(II) salt and corresponding isomeric ligand.
1

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Maximization of the Energy Capability Level in Transition Metal
Complexes through Application of 1-Amino- and 2-Amino-5H-
tetrazole Ligands†
Received 00th January 20xx,
Accepted 00th January 20xx
Norbert Szimhardt, Maximilian H. H. Wurzenberger, Lukas Zeisel, Michael S. Gruhne, Marcus
Lommel, and Jörg Stierstorfer*
DOI: 10.1039/x0xx00000x
www.rsc.org/
Toxic lead compounds (especially lead azide and lead styphnate), reliable but highly harmful compounds, still dominate
the field of primary explosives used in most ignition and initiation systems to this day. Therefore, an extensive
investigation into energetic materials with similar performance and minimized toxicity led to the very promising strategy
of developing energetic coordination compounds (ECC). For the first time in literature, two isomeric amino-5H-tetrazoles
with sensitivities already in the range of primaries, which could be obtained via practicable, easily scalable synthesis,
proved to be very promising new energetic ECC ligands. By variation of the respective isomer (1-AT, 2-AT), metal(II) center
(MnII, FeII, CuII, ZnII) and the corresponding counter-anion (Cl–, NO3–, ClO3–, ClO4–, styphnate, trinitrophloroglucinate)
desired properties could be tailored gradually, yielding 20 new, highly energetic and easily accessible transition metal(II)
complexes. In addition to comprehensive characterization by XRD, IR, EA, solid-state UV/Vis, DTA and TGA, BAM standard
sensitivity measurements as well as ESD and ball drop impact sensitivities were determined. Classical initiation tests
(nitropenta filled detonator) validated the present approach to be extraordinarily promising for potential use in advanced
initiation systems. Beyond successfully achieving set goals in terms of sensitivity and toxicity, investigated complexes
match or even outperform lead-based primaries’ high performance and, more importantly, do not necessarily depend on
initiation by mechanical stimuli. Laser irradiation of the received primary explosives with near infrared light (NIR), yielded
initiation
reliably
and
safely,
further
extending
their
potential
for
future
applications.
In recent years, the demand for “green” chemicals, with re- and Restriction of Chemicals) regulations.3
spect to the invention, design and application of chemical The REACH regulations were established in June 2007 as a
products and processes, has grown continuously with the European Union-wide project to reduce the amount of harmful
objective of reducing or eliminating the use and generation of or at least suspected-to-be-harmful chemicals to which Euro-
hazardous substances.1 The wide and fast expanding field of pean citizens were exposed to in their everyday life. In fact,
“green” chemicals comprises biodegradable plastics, bio-based REACH is not restricted to industrial processes, and also covers
polymers and environmentally friendly explosives from differ- the chemicals used in everyday products such as paint, clean-
ent portfolios of sustainable, eco-sufficient materials feasible ing products or personal hygiene articels.4 ECHA (European
of outclassing products based on petroleum feedstock or toxic Chemicals Agency) uses this decree to hold the chemical indus-
heavy metals such as lead, mercury, barium and others.2 Inter- try responsible for collecting information about their manufac-
action between the chemical industry and academic research tured chemicals, while also assessing their hazards and poten-
plays a key part in this social and economic evolution and the tial risk to the environment. Based on the simple “one sub-
eventual success of green and sustainable materials. A guide- stance, one registration” principle manufacturers and import-
line relevant to applied research in this area is the candidate ers register their chemicals in conjunction with each other for
list of substances of very high concern for authorization (Annex later evaluation to clarify if these registered chemicals bear
XIV) from the REACH (Registration, Evaluation, Authorization any risk to the environment or health.5 The subsequent au-
thorization process ensures that substances of very high con-
cern (SVHCs) are gradually replaced with other less dangerous
and less harmful alternatives – if possible and readily available.
As soon as the committee or the commission reaches an
agreement that the addition of a SVHC to the candidate list is
reasonable, the substance is added to the database.6
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Figure 1. List of requirements to be met for new potential primary explosives, which are compiled in our suitability evaluation diagram.
One of the many candidates mentioned on this list are lead coordination compounds are well-known with different te-
azide and lead styphnate, which are widely used as primary trazole ligands but surprisingly no complexes have been pre-
explosives. A total of 31 lead containing compounds appear on pared up to date with two of the most energetic and powerful
the REACH list, with all of them being suspected to be toxic for neutral ligands, 1-amino-5H-tetrazole (
1, 1-AT) and 2-amino-
reproduction and some also to be carcinogenic (e.g. lead 5H-tetrazole (
2
, 2-AT, Figure 2).25–31 These two isomers, which
chromate since 2009).7 As lead azide and lead styphnate are proved to be non-toxic toward aquatic organisms (see 2.5) and
mainly used in small arm ammunition for military and com- have an incorporated hydrazine function (NTetrazole–NH2) exhibit
mercial applications (mining, deconstruction works, etc.), an outstandingly high nitrogen content of 82.33% and were
there is a strong drive for substitution of these currently used- described almost 50 years ago for the first time in 1966 and
primary explosives. Scientists in many countries from all over 1969 respectively.32,33 Several recently published energetic
the world are therefore looking for possible alternatives and coordination compounds based on 1-methyl-5H-tetrazole as
replacements.8–14 Thought must be given to many different the ligand show a deficit in energetic performance.15 Substitu-
factors (fast deflagration-to-detonation transition (DDT), com- tion of the methyl group by an amino group tremendously
patibility, energetic properties etc.) during the design, the increases the energy content of the molecule due to the addi-
synthesis and evaluation of potential new primary explosives. tional N–N single bond (Figure 2). The highly endothermic
The most important characteristics for suitable candidates character and the resulting driving force of nitrogen gas for-
include the sensitivities, the chemical and physical stability, mation, makes these tetrazoles exceptional great ligands,
low toxicity, low price, high performance and practicabilities which show potential in combination with oxygen-rich anions
according to industrial needs (Figure 1). The concept of ener- as primary explosives.
getic coordination compounds (ECC), containing nitrogen rich The obtained compounds easily outperform commonly used-
ligands with low or non-toxic metals (e.g. Mn, Fe, Cu, Zn), primaries like lead azide or lead styphnate through the maxi-
stands out from other competing strategies and fulfills all of mization of the energy content in the resulting transition metal
the desired criteria.15–17 One of the biggest advantages of this complexes.
promising approach is that the simple adjustment of one of
the three building blocks (metal, ligand, anion) allows exact
tailoring of the complexes in regard to the physico-chemical
properties. Another important benefit of energetic coordina-
tion compounds, making them highly promising and suitable
candidates for potential future applications, is their ignitability
by optical methods like laser irradiation.18,19 This approach
offers various advantages in comparison to conventional initia-
tion methods like short function times or the high isolation
toward other stimuli.20
In particular endothermic tetrazoles have proven themselves
in the past as very promising ligands in energetic complexes.21–
23
Their energetic character can easily be modified by various
Figure 2. Comparison of the per unit mass calculated enthalpies of formation for
methods such as alteration of the functional groups (alkyl
various monotetrazoles shows the highest values for 1- and 2-amino-5H-tetrazole. Gas
chains, nitro groups, nitramine units, hydrazinyl derivatives,
phase enthalpies of formation were calculated using the atomization method (∆fH0
(g, M)
attached tetrazoles etc.) or their substitution pattern (N1-
substituted vs. N2-substituted vs. C5-substituted).15,24 Many
= H(M) – ΣH0(A) + Σ∆fH0(A)) using Gaussian09 computed CBS-4M electronic enthalpies.
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In this comprehensive study, considerable efforts have been
made to implement the highly endothermic ligands 1-amino-
5H-tetrazole (1) and 2-amino-5H-tetrazole (2) into a large
number of promising coordination compounds, which show
great potential for possible future applications (optical and
classical initiation systems).
Results and Discussion
Synthesis
CAUTION! All investigated compounds are potentially explosive
energetic materials (the majority of the compounds lie in the
range of primary explosives), which show partly increased
sensitivities toward various stimuli (e.g. elevated temperatures,
impact, friction or electrostatic discharge). Therefore, proper
security precautions (safety glasses, face shield, earthed
equipment and shoes, leather jacket, Kevlar gloves, Kevlar
sleeves and ear plugs) have to be worn while synthesizing and
handling the described compounds. Especially the very sensi-
Scheme 1. a.) Synthesis of 1-amino-5H-tetrazole (1) and 2-amino-5H-tetrazole (2) by
amination of 1,5H-tetrazole by hydroxylamine-O-sulfonic acid (HOSA). b.) Synthesis of
2-amino-5H-tetrazolium perchlorate (3).
1-Amino-5H-tetrazole (
1) can be prepared (overall yield: 32 %),
in contrast to compound
2, directly and isomerically pure via a
simple and upscalable three-step synthesis (Scheme 2). In the
first step, one amino group of the hydrazine molecule is pro-
tected as benzhydrazone by treatment with benzaldehyde in a
condensation reaction. Particular attention should be given to
the low reaction temperature, the sequence of starting mate-
rials added and the reaction time to prevent formation of a
further reaction to the bisbenzhydrazone. The next step entails
the ring closure of the remaining free amino group of benzhy-
drazone in a [3+1+1] cyclization with triethyl orthoformate and
sodium azide carried out at 80 °C for 2.5 h under acidic condi-
tions. The resulting tetrazole immediately precipitates in the
form of a yellow-orange solid right after pouring into water.
The benzyl protection group is cleaved almost quantitatively in
the last reaction step under acidic conditions with hydrochloric
acid and the resulting benzaldehyde was removed using the
rotary evaporator at elevated temperatures under vacuum.
This reaction step has been greatly improved and simplified in
comparison to literature-reported procedures where water
steam distillation and distillation under vacuum are applied to
remove the emerging benzaldehyde.32,35
tive compounds
4–
7
and 18–23 must be handled with great
care! An excess of ligand
2
has to be applied during the synthe-
sis of complex 18 in order to prevent the formation of unisola-
ble side species (most likely a diperchlorato or hexacoordinated
tetrazole complex), which tends to spontaneous violent de-
composition during crystallization.
In principle, the preparation of 1-amino-5H-tetrazole (1) can
be achieved via two different synthetic protocols. One way is
the amination of 1,5H-tetrazole, reported in 1969 for the first
time, which leads to an isomeric mixture of compound
2-amino-5H-tetrazole (
) as a side product.33 As of today, un-
fortunately no direct synthetic path has been developed for
this isomer, which in comparison to compound , is surprising-
1 and
2
1
ly more sensitive. The amination reaction of the in situ gener-
ated sodium tetrazolate proceeds via a nucleophilic substitu-
tion mechanism and has been performed with the well-known
amination reagent hydroxylamine-O-sulfonic acid (HOSA) in
water under reflux and a constantly readjusted pH value of 7–
8 according to a modified literature procedure (Scheme 1).33,34
Instead of the very dangerous separation by distillation of the
isomeric mixture obtained, a much safer and more efficient
purification by flash column chromatography (EtOAc/DCM 5:1)
on silica gel was developed. The two isomers could be isolated
Scheme 2. Selective and isomerically pure preparation of 1-amino-5H-tetrazole (1) in a
in yields of 27% for compound
1 and 19% for the nitrogen-rich
three-step reaction starting from benzaldehyde and hydrazine monohydrate.
ligand 2. The relatively low yields can most likely be attributed
to the electron-poor character of the aromatic heterocycle and
its related low nucleophilicity. Treatment of the two amino-
Both isomers can easily be differed by infrared spectroscopy
(Figure S1) or proton and carbon NMR spectroscopy. Further-
5H-tetrazoles
1 and 2 with aqueous diluted acids (HNO3 and
1
more, two dimensional H, 15N NMR HMBC or proton coupled
HClO4) resulted in ionic liquids and were therefore not been
investigated in more detail. The only exception was the proto-
15N NMR spectroscopy can be performed (Figure 3).
A large number of complexes based on various monotetrazoles
like 1,5-diaminotetrazole or 1-methyl-5H-tetrazole have been
described in literature but have insufficient power for any
possible application as a primary explosive.16,30 Owing to this
lack of power and to maximize the energetic performance,
new transition metal complexes were synthesized with even
more powerful endothermic ligands.
nated 2-amino-5H-tetrazolium perchlorate salt
(3), which
crystallized within 5 days and has been comprehensively char-
acterized (Scheme 1).
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accomplished through combination of aqueous solutions of
the relevant metal(II) salts and the ligand in the correspond-
1
ing stoichiometries at room temperature (Scheme 3).
Scheme 3. Synthesis of the transition metal(II) complexes 4–9 and 12 based on 1-
amino-5H-tetrazole (1) as ligand.
Water has been selected as the green solvent for all reaction
mixtures to ensure slow crystallization of the desired complex-
es and because of the good solubilities of the ligands and the
transition metal(II) salt starting materials in this solvent sys-
tem. All coordination compounds crystallized with decent
yields (32–79%) without inclusion of aqua ligands or crystal
water molecules resulting in a higher performance of the com-
plexes. The exclusive use of ethanol as solvent instead of water
leads to precipitation of the complexes in most cases in the
form of amorphous powders.
Scheme 4. Formation of complexes 10, 11 and 13–15 with varying metal(II) centers,
anions and ligands.
Figure 3. Proton coupled 15N NMR spectra and two dimensional 1H, 15N-HMBC NMR
spectra of 1 and 2.
Analogous to our recently published work, an ethanolic solu-
tion of copper(II) chlorate was reacted with 1-amino-5H-
tetrazole (1) in order to synthesize the corresponding cop-
Our synthetic strategy comprises the preparation of cationic or
neutral complex units to increase the energetic character of
the coordination compounds and to ensure the integration of
oxidizing anions, facilitated through the application of neutral
basic nitrogen-rich ligands. In addition, the influence on the
coordination sphere and the physico-chemical properties were
investigated by alteration of the tetrazole isomers while main-
taining the metal(II) and anion building blocks. The synthesis of
per(II) chlorate coordination compound (Scheme 4).36 After a
few days and evaporation of the solvent, two different crystal-
line species appeared from the reaction mixture, which could
be picked and handled while still wet. X-ray diffraction analysis
revealed a complex species with six coordinating 1-AT ligands
(
10, Figure S9) and a species with four 1-AT ligands and two
coordinating chlorato anions (11, Figure S10). All attempts to
dry the compounds for further characterization failed due to
the divalent transition metal(II) complexes
4–9 and 12 was
4 | J. Name., 2012, 00, 1-3
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the extreme sensitivity and resulted in violent detonations. small amount of ice-cold ethanol when necessary in order to
Unfortunately, only a few single crystals suitable for X-ray remove unreacted starting materials and were then dried in air
diffraction of the manganese(II) complexes 13–15 (Scheme 4) overnight.
could be isolated in between amorphous bulk material with
indefinite composition and unreacted starting materials.
Crystal structures
Therefore, only their X-ray crystal structures, which are shown The nitrogen-rich ligand
in the SI can be presented. All attempted syntheses to achieve plexes were characterized by low temperature single crystal X-
a pure elemental analysis of the complexes were unsuccessful. ray diffraction. In contrast to ligand , 1-amino-5H-tetrazole (
The 2-amino-5H-tetrazole ( ) based coordination compounds could be solidified through controlled freezing with liquid
16
23 (Scheme 5) were synthesized in analogy to their 1- nitrogen. Its low melting point of 5 °C35 allowed the picking of
substituted isomers (easy distinguishable by IR, Figure S3) a single crystal at 0 °C for X-ray determination. The measure-
through a straightforward reaction of the corresponding met- ments of compound and 11 showed considerable structural
al(II) salts in the necessary stoichiometries in water or ethanol disorders within the tetrazole rings as well as in the perchlo-
at room temperature. An excess of ligand had to be applied rate ( ) and chlorate (11) anions. Nevertheless, they allow the
1, the perchlorate salt 3 and all com-
2
1)
2
–
4
2
4
during the synthesis of complex 18 in order to prevent the identification of the complexes’ composition. Elemental analy-
formation of an unisolable side species (most likely a diper- sis and infrared spectra comparison (Figure S2) verified the
chlorato or hexacoordinated tetrazole complex), which tended analogous structure of
to spontaneous violent decomposition during crystallization. It amino-5H-tetrazole) metal(II) perchlorate complexes
should also be noted, that the use of six equivalents of in- high degree of disorder observed for the copper(II) chlorate
stead of five during the synthesis of the complexes 19 20 complex 11 unfortunately allowed only an isotropic illustration
leads to the same monoaqua species and has no influence on (Figure S10) of the molecular structure. All complexes, except
the composition of the resulting coordination compounds. The compound , show an octahedral coordination sphere of the
preparation of the metal(II) perchlorate complexes 18 23 metal(II) center. Complexation of the ligands and always
yields for each metal(II) salt two different species depending takes place through the N4 nitrogen atom of the tetrazole ring,
largely on the applied stoichiometry and solvent system. In except for compounds and 19 which show bidentate amino-
addition to the aqua ligand containing coordination com- tetrazole ligands. The crystal datasets were uploaded to the
pounds 18
20, complex units with two additional cocrystallized CSD database37 and can be obtained free of charge with the
aminotetrazoles 21 23 can be isolated when an excess of lig- CCDC no. 1834855 ( ), 1834849 ( ), 1834847 ( ), 1834859 ( ),
and is used. Both species can easily be detected and distin- 1834851 ( ), 1834845 ( ), 1834844 ( ), 1834853 (10), 1834852
guished from each other by IR spectroscopy (Figure S4/5). In 12), 1834858 (13), 1834857 (14), 1834854 (15), 1834842 (16),
the case of the copper(II) perchlorate complexes 18 23, the 1834843 (17), 1834841 (18), 1834856 (19), 1834846 (20),
solvent system plays an important role in the formation of the 1834848 (21), 1834850 (22), 1838980 (23).
relevant complexes. Water leads to the diaqua species 18 Compound crystallizes in the form of colorless rods in the
whereas ethanol as solvent results in the formation of the orthorhombic space group P212121 with four formula units per
4
as in the other isotypic hexakis(1-
5–7
. The
2
–
9
–
1
2
8
-
-
1
3
5
6
7
8
9
(
/
,
1
cocrystallized compound 23
.
unit cell and a calculated density of 1.515 g cm−3 at 143 K. All
non-hydrogen atoms lie within a plane with torsion angles
close to 0° (C1–N1–N2–N3 0.31(15)°, N5–N2–C1–N1 1.59(1)°
and N2–N1–C1–N4 0.12(16)°) and show bond lengths as well
as angles comparable to those of other 1-substituted tetrazole
compounds (Figure 4).38
Figure 4. Molecular unit (left) and unit cell (right) of 1-AT (1). Thermal ellipsoids of non-
hydrogen atoms in all structures are set to the 50 % probability level. Selected bond
lengths (Å): N1–C1 1.3256(18), N1–N5 1.4000(17), N1–N2 1.3441(15), N2–N3
1.2986(19), N3–N4 1.370(2), N4–C1 1.3128(19); selected bond angles (°): N2–N1–N5
123.21(11), N2–N1–C1 109.17(11), N5–N1–C1 127.54(12), N1–N2–N3 105.71(11), N2–
N3–N4 110.82(12), N3–N4–C1 105.18(12).
Scheme 5. Synthesis of selected energetic coordination compounds 16–23 based on 2-
amino-5H-tetrazole (2) as ligand in combination with different metal(II) salts.
All coordination compounds were directly obtained from the
mother liquor and crystallized in satisfactory yields with excep-
tion of the cocrystallized species. The crystalline materials, all
suitable for X-ray diffraction, were filtered off, washed with a
The perchlorate salt
3, crystallizes in the form of colorless
blocks in the monoclinic space group P21/n with four formula
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units per unit cell and a calculated density of 1.963 g cm−3 at rich ligands are bound to each metal(II) center and the per-
173 K. The distances between the ring atoms lie within the chlorate counter-anions are non-coordinating (Figures 6 and
range of partial double bonds and the torsion angle C1–N1– S6–8).
N2–N3 is very close to 0° (C1–N1–N2–N3 −0.5(2)°). Moreover, In each case, the three dimensional stability is guaranteed by
every bond length besides the N1–N2 distance is slightly several hydrogen bonds represented in Table S6, exemplarily
shorter in the non-coordinating tetrazole ring of 3. In addition illustrated for complex [Fe(1-AT)6](ClO4)2 (5). The unit cell of 5
to these results, the aromaticity of the heterocycle is proved is depicted in Figure 7 illustrating the two-dimensional packing
by the bond angles amongst the ring atoms. The bond distance in view along the crystallographic c-axis. The comparison of
between the amine's and the adjacent ring nitrogen is close to the investigated perchlorate complexes with 1-amino-5H-
those of the tetrazole's π-system. In regard to the N2's sp2- tetrazole as the coordinating ligand shows no major differ-
hybridization as part of an aromatic heterocycle, the bond ences besides the Jahn-Teller distorted Cu2 center in
6 (Figure
angles between N5, N1 and the neighboring ring atoms are S7) and the previously mentioned densities. Notwithstanding,
close to be the ideal 120°. The interesting conclusion, that the one might recognize the slight differences regarding the bond
whole aminotetrazolium cation appears as one plane molecule coordination distances between the ligands and the respective
when including the amino group at N5, can be made based on metal(II) center. While all complexes show disordered perchlo-
the torsion angle N5–N2–N3–N4 which almost amounts to rate anions, the manganese(II) compound
4 additionally pos-
180° (N5–N2–N3–N4 = 175.03(15)°). Due to the previously sesses strong disorders in the heterocyclic ligands (Figure S6).
mentioned sp2-hybridization of the atoms of this aromatic
heterocycle, the hydrogen atoms at N4 and C1 are part of the
molecule's plane. On the other hand, the hydrogen atoms of
the sp3-hybridized N5 are both orientated toward in the same
direction. The perchlorate counter-ion of the tetrazolium cati-
on is only marginally distorted exhibiting typical Cl–O bond
distances and bond angles based on their tetrahedral molecu-
lar structure (Figure 5). In summary, the resulting structural
parameters almost exclusively match data quoted in literature
of similar tetrazolium perchlorate salts.39
Figure 6. Molecular unit of complex [Fe(1-AT)6](ClO4)2 (5). Selected bond length (Å):
Fe1–N14 2.185(3); selected bond angles (°): N14–Fe1–N14vii 88.88(11), N14–Fe1–N14iii
91.12(12). Symmetry codes: (i) 1−y, x−y, z; (ii) 1−x+y, 1−x, z; (iii) −x, −y, −z; (iv) x−y, x, −z;
(v) −x+y, −x, z; (vi) −y, x−y, z; (vii) y, −x+y, −z.
Figure 5. Molecular unit of 2-ATH ClO4 (3). Selected bond lengths (Å): Cl1–O1
1.4369(14), N1–C1 1.318(2), N1–N2 1.333(2), N2–N3 1.289(2), N2–N5 1.369(2), N3–N4
1.331(2), N4–C1 1.322(2); selected bond angles (°): O1–Cl1–O3 109.82(8), O2–Cl1–O4
108.65(8), N2–N1–C1 101.68(14), N1–N2–N3 116.77(15), N1–N2–N5 121.17(15), N2–
N3–N4 101.33(15), N3–N4–C1 110.89(15), N1–C1–N4 109.33(15).
The bond lengths and angles of the coordinating ligands in the
complexes analyzed are in the same range as in the non-
coordinating ligand
sion in any of the following coordination compounds.
The metal(II) perchlorate compounds of 1-AT ( ) all crystal-
lize isotypically in the trigonal space group P−3 with densities
between 1.736 (
) and 1.918 g cm–3 ) (Mn: 1.736 g cm–3
(143 K); Fe: 1.766 g cm–3 (173 K); Cu: 1.918 g cm–3 (173 K); Zn:
1.783 g cm–3 (173 K)). All complexes consist of two distinguish-
able metal centers with one exhibiting an ideal coordination
polyhedron and the other being slightly distorted. Six nitrogen-
1 and are therefore not part of the discus-
Figure 7. Unit cell of [Fe(1-AT)6](ClO4)2 (5) in the direction of view along the c-axis.
4–7
The copper(II) chloride coordination compounds of 1-AT and 2-
AT both crystallize in the form of green blocks in similar mono-
4
(6
clinic space groups P21/n (
formula units per unit cell respectively. The calculated densi-
ties at 173 K vary only slightly (2.146 g cm−3
and
2.177 g cm−3
16) respectively). The two complexes show com-
8) and P21/c (16) with four and two
(8)
(
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pletely different coordination spheres, which are highly de- copper(II) coordination sphere with significant Jahn-Teller
pendent on the inserted ligand. 2-Amino-5H-tetrazole acts as a distortion, compound consists of a sevenfold coordinated
9
monodentate ligand in complex 16 with bridging chlorido metal(II) center leading to the coordination polyhedron of an
ligands, which are responsible for the assembly of 1D polymer- unusual pentagonal-like bipyramid (Figure 9). The equatorial
ic chains. The 1-AT ligand instead shows a bidentate behavior positions are occupied by two chelating nitrate anions and one
in compound
8
leading to the formation of 2D polymeric lay- equivalent of
1 forming a distinctly deformed plane. The ideal
ers. The copper(II) center of
8
is octahedrally coordinated and properties of a pentagonal bipyramid prescribe the bond an-
bridged to polymeric planes over two chlorido and two 1- gles between the equatorial ligands to be 72° – a requirement,
amino-5H-tetrazole ligands (Figure 8). The equatorial positions which is definitely not fulfilled for the investigated complex
are occupied by one bridging and one terminal equivalent of due to the nitrate anion's fixed structure. In addition, the axis
each, the chlorido and the 1-amino-5H-tetrazole ligand, gener- through the two-remaining amino-5H-tetrazole ligands in axial
ating a plane. The coordination octahedron exhibits a Jahn- positions is crooked, differing considerably from the ideal 180°.
Teller distortion along the Cl2ii–Cu1–N5 axis with N5 being part Another deviation from the perfect pentagonal bipyramid can
of the second bridging equivalent of 8. Moreover, the struc- be observed regarding the bond angles between axial and
ture of the complex is close to an ideal octahedron with the equatorial ligands partially varying by up to 10° from the ex-
maximum deviation of the bond angles between two cis- pected 90°. The nitrate complex 17 crystallizes in the form of
ligands from 90° being 5.76° and of two trans-ligands from blue plates in the monoclinic space group P21/n with two for-
180° being 4.41°. In contrast the molecular unit of 16 consists mula units per unit cell and
a calculated density of
of four bridging chlorido and two terminal 2-amino-5H- 2.169 g cm−3 at 173 K. This coordination compound consists of
tetrazole ligands forming an octahedral coordination sphere an octahedrally coordinated copper(II) center including a
(Figure 8). Once again, the d9-copper center shows a significant straight, Jahn-Teller distorted N5ii–Cu1–N5iv axis between the
Jahn-Teller distortion along the Cl1–Cu–Cl1iv axis. The equato- axial 2-AT ligands (Figure 9). The equatorial positions of 17 are
rial coordination sites are occupied by two bridging chlorido occupied by the remaining two equivalents of
2 and the coor-
ligands and the remaining two equivalents of 2-AT building up dinating nitrate anions building up an almost ideal plane. Every
an almost ideal plane. Considering the findings that the bond bond angle of cis coordinating axial or equatorial ligands in 17
angles between ligands in cis positions are close to the ideal slightly differs from the ideal 90°.
90° and angles amongst molecules coordinating trans to each
other are exclusively 180°, the coordination polyhedron shows
multiple signs of being an ideal octahedron, despite the previ-
ously discovered distortions in regard to the bond lengths.
Figure 9. Molecular unit of [Cu(NO3)2(1-AT)3] (9) (left) and the copper(II) coordination
environment of [Cu(NO3)2(μ-2-AT)2] (17) (right). Selected bond lengths (Å) of 9: Cu–O1
1.987(3), Cu–O2 2.637(3), Cu–O4 1.990(3), Cu–O6 2.742(4), Cu–N13 2.287(4), Cu–N4
2.003(4), Cu–N9 1.999(4); selected bond angles (°) of (9): N4–Cu–N9 162.97(13), O1–
Cu–O2 54.03(11), O1–Cu–N13 84.56(13), O2–Cu–O6 80.11(10), O4–Cu–O6 51.81(11),
O4–Cu–N13 89.95(13), O1–Cu–N4 92.57(15), O1–Cu–N9 88.03(14), O2–Cu–N4
82.95(12), O2–Cu–N9 83.65(12), O4–Cu–N4 90.25(14), O4–Cu–N9 90.85(14), O6–Cu–
N4 87.65(14), O6–Cu–N9 79.72(13). Selected bond lengths (Å) of (17): Cu1–N5ii
2.4208(16), Cu1–O1 2.0339(12), Cu1–N4 1.9831(16); selected bond angles (°) of (17):
O1–Cu1–N4 87.42(6), O1–Cu1–N5ii 81.45(5), O1–Cu1–N5iv 98.55(5). Symmetry codes of
(17): (i) −x, −y, −z; (ii) 0.5+x, 0.5−y, 0.5+z; (iii) 0.5−x, 0.5+y, 0.5−z; (iv) 0.5−x, −0.5+y,
0.5−z.
Figure 8. Copper(II) coordination environments of [CuCl(μ-Cl)(1-AT)(μ-1-AT)] (8) (left)
and [Cu(μ-Cl)2(2-AT)2] (16) (right). Selected bond lengths (Å) of 8: Cu1–Cl1 2.2854(6),
Cu1–Cl2 2.3122(6), Cu1–N9 2.0113(19), Cu1–N5 2.725(2); selected bond angles (°) of 8:
Cl1–Cu1–Cl2 177.96(2), Cl2–Cu1–N5 93.90(5), N5–Cu1–Cl1 84.55(5), N9–Cu1–Cl1
90.78(5), Cl2–Cu1–N9 90.64(5), . Symmetry codes of 8: (i) 0.5+x, 1.5−y, 0.5+z; (ii) 1.5+x,
1.5−y, 1.5+z. Selected bond lengths (Å) of 16: Cu–Cl1 2.8124(6), Cu–N4 2.0422(19);
selected bond angles (°) of 16: Cl1–Cu–N4 90.25(5), Cl1–Cu–Cl1ii 91.30(2), Cl1iv–Cu–N4
89.76(5), Cl1ii–Cu–N4 88.84(6). Symmetry codes of 16: (i) 2−x, 2−y, −z; (ii) x, 1+y, z; (iii)
2−x, 1−y, −z; (iv) x, −1+y, z.
The spatial order of the investigated copper(II) nitrate complex
(
17) reveals some very interesting observations: Each of the
Analogous to the chlorido complexes, the copper(II) nitrate
coordinating 2-amino-5H-tetrazole ligands possesses bridging
characteristics and thus causes cross-linking of adjacent cop-
per(II) centers.
coordination compounds
9
and 17 show a completely different
and 16, 1-
metal(II) coordination environment. In contrast to
8
AT now behaves monodentately, a fact leading to the for-
mation of closed coordination spheres, whereas the bidentate
behavior of 2-AT causes the building-up of 2D polymeric layers.
The extremely sensitive copper(II) perchlorate complex 18 is
the only compound which crystallizes in the tetragonal space
group P42/n with two formula units per unit cell and a calcu-
lated density of 1.947 g cm−3 at 173 K. Once again, the Jahn-
Teller distortion along the O1–Cu–O1iii axis induces significant
deviations between the coordination distances of axial and
equatorial ligands (Figure 10). The four equatorial 2-AT ligands
The very interesting copper(II) nitrate complex
9 crystallizes in
the form of blue rods in the monoclinic space group Cc with
four formula units per unit cell and a calculated density of
1.968 g cm−3 at 173 K. Despite the usually observed octahedral
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form a crooked plane, a fact that causes the bond angles be- the structural parameters, besides the substitution of the aqua
tween axial and equatorial ligands to slightly differ from the ligand by one additional equivalent of
ideal 90°. On the other hand, the angles between cis amino-
5H-tetrazole ligands in the plane almost perfectly conform to
90°. The angles amongst two equatorial ligands coordinating
trans to each other are also influenced by the obliqueness of
the N1–N1iii–N1ii–N1i plane (N1–Cu–N1ii 177.77(12)°). The
analogous manganese(II) complex 15 crystallizes in the form of
colorless blocks in the triclinic space group P−1 with two for-
2, could be observed.
mula units per unit cell and
a calculated density of
1.897 g cm−3 at 173 K. Compared to compound 18 the Mn–N
bonds are slightly longer than the Cu–N ones with simultane-
ous shorter Mn–O bonds (Figure S14).
Figure 11. Molecular unit of [Zn(H2O)(2-AT)5](ClO4)2 (20). Selected bond lengths (Å): Zn–
O1 2.083(3), Zn–N4 2.146(3), Zn–N9 2.197(3), Zn–N14 2.147(3), Zn–N19 2.166(3), Zn–
N21 2.160(3); selected bond angles (°): O1–Zn–N21 176.80(11), N4–Zn–N14 177.92(11),
O1–Zn–N4 87.56(12), O1–Zn–N9 86.13(11), O1–Zn–N14 91.08(12), O1–Zn–N19
89.52(11), N4–Zn–N9 90.45(11), N4–Zn–N19 90.29(11), N4–Zn–N21 89.63(11), N9–Zn–
N14 87.89(11).
The cocrystallized 2-AT molecules show no considerable devia-
tions from the previously examined coordinating
2 referring to
the bond lengths and bond distances. The octahedral coordi-
nation spheres of the d10 and d6 metal centers zinc(II) and
iron(II) show no Jahn-Teller distortion but differ slightly from
the ideal octahedron (Figure 12 and S18). Nevertheless, the d9
copper(II) center possesses a typical Jahn-Teller distortion
along the axial Cu–N bonds (Figure S19). The coordination
distances between the metal(II) center and the coordinating
ligand are roughly within the same range as for the com-
pounds 19 and 20 but appear to be somewhat longer than in
the previous cases. All perchlorate counter-anions show no
significant distortion.
Figure 10. Molecular unit of [Cu(H2O)2(2-AT)4](ClO4)2 (18). Selected bond lengths (Å):
Cu–O1 2.311(4), Cu–N1 2.025(2); selected bond angles (°): N1–Cu–N1ii 177.77(12), O1–
Cu–N1 91.11(9), N1–Cu–N1iii 90.02(8). Symmetry codes: (i) y, 0.5−x, 0.5−z; (ii) 0.5−x,
0.5−y, z; (iii) 0.5−y, x, 0.5−z; (iv) 1−y, −0.5+x, −0.5+z.
The coordination polyhedrons built up by five equivalents of
2
are completed by one aqua ligand forming an octahedral coor-
dination sphere around the respective metal center (Figure 11
and S17). In both complexes, the M2+–N distances are signifi-
cantly longer than the particular M2+–O coordination distances
between the metal(II) center and the aqua ligands. Regarding
the bond angles of the metal(II) centers, it becomes clear that
there is no major distortion to be observed. Nevertheless,
none of the bond angles of either of these perchlorate com-
pounds conform the ideal values of 90° for cis coordinating
ligands and 180° for molecules coordinating trans to each
other. The isotypic complexes 19 and 20, two coordination
isomers containing an iron(II) (19) or alternatively a zinc(II) (20
)
metal center both crystallize in the form of colorless blocks in
the triclinic space group P−1 with two formula units per unit
cell and calculated densities of 1.855 g cm−3
(
19
20) at 173 K. On an additional note, the perchlo-
rate counter-anions containing Cl2 are significantly distorted.
The investigated complexes 21 23, all including cocrystallized
) and
Figure 12. Molecular unit of [Fe(2-AT)6](ClO4)2 ·2 2-AT (21). Selected bond lengths (Å):
Fe1–N4 2.2191(16), Fe1–N9 2.1618(16), Fe1–N11 2.2135(16), Fe1–N19 2.1868(16),
Fe1–N24 2.2157(16), Fe1–N29 2.1629(16); selected bond angles (°): N4–Fe1–N19
175.41(6), N4–Fe1–N9 89.38(6), N4–Fe1–N11 86.35(6), N4–Fe1–N24 91.91(6), N24–
Fe1–N29 90.62(6).
1.884 g cm−3
(
–
2-amino-5H-tetrazole, crystallize in the triclinic space group
P−1 with two formula units per unit cell and calculated densi-
( (23) (Fe:
ties between 1.784 g cm−3 21) and 1.815 g cm−3
Powder Diffraction
1.784 g cm−3 (173 K); Cu: 1.815 g cm−3 (143 K); Zn: 1.812 g cm−3
(173 K)). In comparison to the very similar coordination com-
pounds 19 and 20, once again, no major differences regarding
Due to the possible formation of different species or poly-
morphs, X-ray powder diffraction measurements were carried
out in addition to elemental analysis to ensure the purity of
the bulk material. Figure 13 shows the powder diffraction
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patterns for the compounds 5, 6, 9 and 17. The results prove
the purity of the nitrogen-rich coordination compounds. Small
deviations between the measured and the calculated data sets
can be observed in all diffractograms and are caused by the
temperature differences between the single crystal and pow-
der diffraction experiments. The larger differences in the in-
Figure 14. SEM image (350 x magnitude) and EDX spectrum of compound 5.
tensities for the iron(II) and copper(II) complexes
5 and 6 can
be attributed to the heavy disorders in the perchlorate moie-
ties (Figure 13A) and B)).
Figure 15. SEM images (250 x magnitude (left) and 800 x magnitude(right)) and EDX
spectrum of compound 6.
Figure 13. Powder diffraction experiments (comparison of the measured and calculated
data) for the complexes 5 (A), 6 (B), 9 (C) and 17 (D).
Scanning electron microscopy (SEM) and energy dispersive X-ray
spectroscopy (EDX)
Since sensitivity data are strongly dependent on the grain size
and crystal habitus (microscope images S20–25), scanning
electron microscopy and energy-dispersive X-ray spectroscopy
were
Figure 16. SEM image (4500 x magnitude) and EDX spectrum of compound 9.
performed to explore the compounds’ morphology but also
the chemical composition. The results of the measurements
for compounds
5
,
6
,
9
and 17 are shown in Figures 14–17.
forms up to 200 µm-sized
The iron(II) perchlorate complex
5
blocks beside plate-like crystals. Aside from insignificant trace
amounts of silicon, the EDX spectrum did not show any other
atoms present in the complex.
Figure 17. SEM image (650 x magnitude) and EDX spectrum of compound 17.
Sensitivities and thermal stability
Complex
the block-like crystals except for small spikes. Compared to
compound , no other elements, which are not included in the
6, illustrated in Figure 15 reveals the consistency of
The behavior of all compounds toward heating in a certain
range was investigated by differential thermal analysis (DTA) at
a heating rate of β = 5 °C min−1. The observed endothermic
events (melting, dehydration or loss of coordinat-
ing/cocrystallized AT ligands) as well as other critical tempera-
5
complex composition, can be observed in the EDX spectrum.
The SEM image of complex 15 exposes an amorphous constitu-
tion in comparison to its microscope images. The phase-purity
is confirmed by the EDX spectrum.
tures are listed in Table 1. DTA plots of the compounds
1–3, 8,
Compound 17 shows a plate-like morphology and compared to
9,
12, 16–23, and additional information regarding the thermal
nitrate complex
9 a lower content of carbon and nitrogen,
stabilities can be found in the Figures S27–30 in the Supporting
Information.
which is consistent with the chemical compositions of the two
coordination compounds.
In accordance with our previous findings and literature re-
34
ports, higher vaporization temperatures Tvap.
,
(and melting
temperatures) in combination with lower exothermic decom-
position temperatures can be observed for 1-AT and 2-AT, and
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the more N1 substitution pattern is existent at the te- possible and leads to an unstable intermediate, which deto-
trazole.40,43 Both tetrazoles are liquids under ambient condi- nates immediately without significant endothermic signal.
tions and start to vaporize slowly at certain temperatures until An exceptional endothermic peak could be detected during
both compounds reach their respective exothermic decompo- the DTA measurement of the zinc(II) nitrate coordination com-
sition stages and deflagrate, as the TGA plot of both ligands pound 12 and was therefore investigated into more detail by
clearly illustrates (Figure S26). The thermal stabilities of most thermal gravimetric analysis together with complex
of the investigated compounds lie in the satisfying range of ence sample (Figure 19).
4 as refer-
175–180 °C (except
tion temperatures can be observed for the 2-amino-5H-
tetrazolium perchlorate salt ( , Texo. = 122 °C) and for the cop-
per(II) nitrato complexes (Texo. = 161 °C) and 17 (Texo.
157 °C). Notably the poor thermal stabilities for the nitrato
coordination compounds and 17 are not surprising and a
3). Nevertheless, the lowest decomposi-
3
9
=
9
known issue in the literature.15
Table 1. Thermal stability measurements of 1–9, 12, and 16–23 by DTA.a
Tendo.b (°C)
Texo.c (°C)
1-AT (1)
—
—
182
197
122
195
187
174
189
175
161
180
171
157
173
159
200
169
192
157
2-AT (2)
2-ATH ClO4 (3)
122
—
[Mn(1-AT)6](ClO4)2 (4)
[Fe(1-AT)6](ClO4)2 (5)
[Cu(1-AT)6](ClO4)2 (6)
[Zn(1-AT)6](ClO4)2 (7)
[CuCl(μ-Cl)(1-AT)(μ-1-AT)] (8)
[Cu(NO3)2(1-AT)3] (9)
—
—
Figure 18.
containing metal(II) perchlorate complexes 4–7.
Comparison of the thermal stabilities (β = 5 °C min−1
) of the 1-AT
—
—
—
[Zn(1-AT)6](NO3)2 (12)
[Cu(μ-Cl)2(2-AT)2] (16)
[Cu(NO3)2(μ-2-AT)2] (17)
[Cu(H2O)2(2-AT)4](ClO4)2 (18)
[Fe(H2O)(2-AT)5](ClO4)2 (19)
[Zn(H2O)(2-AT)5](ClO4)2 (20)
[Fe(2-AT)6](ClO4)2 ∙ 2-AT (21)
[Zn(2-AT)6](ClO4)2 ∙ 2-AT (22)
[Cu(2-AT)6](ClO4)2 ∙ 2-AT (23)
115
—
—
110
—
Figure 19.
TGA measurements (β = 5 °C min−1) of selected compounds 4 and 12.
132
106
96
While no change in weight after heating has been detected for
the manganese(II) complex until it decomposes, a gradual
4
108
loss of coordinating 1-AT molecules beginning at 110 °C up to
its decomposition point for coordination complex 12 can be
observed. It can be assumed that this endothermic loss leads
to the formation of an intermediate species, which later de-
composes exothermically.41
a
Onset temperatures at a heating rate of 5 °C min–1 b Endothermic peak, which
.
indicates melting, dehydration, or loss of coordinating/cocrystallized AT ligands. c
Exothermic peak, which indicates decomposition.
Relatively high thermal stabilities have been determined for
the water-free 1-AT based metal(II) perchlorate complexes
By comparison of complexes with the same transition metal(II)
but with varying AT isomers or anions (except
9 and 17), no
(Figure 18), which increase in the following order CuII (Texo.
174 °C) < FeII (Texo. = 187 °C) < ZnII (Texo. = 189 °C) < MnII (Texo.
=
=
general trend regarding their exothermic decomposition tem-
peratures can be drawn.
195 °C). Analogical complexes with 1-methyl-5H-tetrazole
instead of 1-amino-5H-tetrazole (methyl vs. amino group)
showed, interestingly exactly the same tendency, which could
possibly be a useful indicator during the planning and design of
future high-temperature stable energetic complexes.15 Dehy-
dration (complex 18 and 20) or loss of co-crystallized 2-amino-
The sensitivities toward impact and friction were assessed
according to BAM standard methods (1 of 6) together with the
electrostatic discharge sensitivity for all compounds. In addi-
tion, the compounds have been categorized in accordance
with the “UN Recommendations on the Transport of Danger-
ous Goods” using the determined sensitivities. An overview of
the sensitivities is given in Table 2. The ligands as well as all
complexes have to be considered as very sensitive to all ap-
plied stimuli (impact, friction and ESD) and should therefore be
categorized as primary explosives. The majority of the synthe-
5H-tetrazoles during heating of 21–23 indicated by endother-
mic signals strongly agree with the identified X-ray crystal
structures. Drying of the iron(II) monoaqua complex 19 is not
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sized coordination compounds can even be described as “ex-
tremely sensitive” with values smaller than 10 N against fric-
tion. Implementation of non-oxidizing chlorido anions into the
complexes lowered the performance on the one hand but on
the other hand led to a desensitization of the coordination
compounds.
Figure 20.
Moment of detonation of compounds 6 and 9 during the ball drop
Table 2. Sensitivities toward impact, friction, and ESD of 1–9, 12 and 16–23 compared
impact test.
to lead styphnate and lead azide42
.
compound
ISa (J)
< 1
< 1
< 1
< 1
< 1
< 1
< 1
3
FSb (N)
64
ESDc (mJ)
—
The sensitivity (Figure 21) of the 1-AT perchlorate complexes
against ball drop impact rises in the following order Fe2+
(25 mJ) < Zn2+ (16 mJ) < Mn2+ (14 mJ) < Cu2+ (< 4 mJ) and in the
following order toward friction Zn2+ (0.5 N) ~ Mn2+ (0.5 N) <
Fe2+ (< 0.1 N) ~ Cu2+ (< 0.1 N).
1d
2d
36
—
3
0.2
10
4
0.5
11e
Table 3. Ball drop impact sensitivities for 4–7 and 9 a
.
5
< 0.1
< 0.1
0.5
12
6
5
4
5
6
7
9
7
12
IS (mJ)
14
25
< 4
16
6
8
60
50
a
ball drop impact sensitivity determined with the 1 of 6 method in accordance
9
3
2.25
6
10
with the MIL-STD 1751A (method 1016). For the testing a 30 mg sample size, a
drop weight of a 0.500-inch diameter steel ball (m = 8.35 g) and a steel ball
guidance track, which rose 32-inches vertically from a steel base were used.
12
< 1
< 2
< 1
1
620e
16
40
10
17
2
10
The implementation of oxidizing anions like nitrates or per-
chlorates into the nitrogen-rich coordination compounds, in
comparison to halides, generally leads to increased sensitivi-
ties toward mechanical stimuli (Figure 22).
18
0.4
10
19
1.5
1.5
< 1
2
0.15
0.5
10
20
10
21
22
0.1
25
0.2
30
23
1
< 0.2
1.5
1.2e
0.02–1.0
6–12
lead styphnatef
lead azidef
2.5–5
2.5–4
0.1–1
Determined at a grain size of < 100 μm were
3
,
6
,
8
,
9,
16
,
17, and 20. The
sensitivities of the remaining complexes were determined at grain sizes
a
between 100 and 500 μm. impact sensitivity according to the BAM drop-
b
hammer test (method 1 of 6); friction sensitivity according to the BAM
c
friction tester (method 1 of 6); electrostatic discharge sensitivity (OZM
ESD tester); Impact: insensitive > 40 J, less sensitive ≥ 35 J, sensitive ≥ 4 J,
very sensitive ≤ 3 J; Friction: insensitive > 360 N, less sensitive = 360 N,
sensitive < 360 N and > 80 N, very sensitive ≤ 80 N, extremely sensitive
≤ 10 N. As per the UN Recommendations on the Transport of Dangerous
d
e
Goods. from reference.34 measured on our new OZM Electric Spark
XSpark10 device. f from reference.42
The sensitivity against ball drop impact is a very realistic and
safety-relevant parameter for compounds with a potential
industrial application. Therefore, and in order to get a better
understanding of their stability, the sensitivity toward ball
drop impact has been determined for the very promising wa-
Figure 21.
Comparison of the ball drop impact and friction sensitivities of the
metal(II) 1-AT perchlorates 4–7.
ter-free [MII(1-AT)6](ClO4)2 coordination compounds
the copper(II) nitrato complex (Figure 20). An overview of
4–7 and
9
the experimental results is given in Table 3.
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ure 25). Additional details on the test procedures can be found
in the Experimental Section of the SI.
Figure 23.
Moment of detonation of compound 22 during the hot needle (HN)
initiation shown in sequence.
Figure 24.
Moment of detonation of complex 22 during the hot plate (HP) test
shown in sequence.
Figure 22.
Stabilities of the copper(II) chlorido and nitrato complexes 8/9 and
Positive initiation tests caused by a DDT of PETN indicated by a
hole in the witness plate and fragmentation of the copper shell
16/17 based on the isomeric amino-5H-tetrazoles.
(Figure 26 and Figure S49–52) were observed for 3, 5, 6, 9, 17–
The same trend can be observed when
2 is applied as a ligand
19, and 21–23.
instead of 1. Therefore, exact tailoring of the desired optical
and energetic properties can be easily accomplished by varia-
tion of the corresponding coordination environment, facilitat-
ing the synthesis of a wide range of different materials (Figure
S 31/32).
Primary explosive suitability evaluation
Hot plate and hot needle tests of the most promising com-
pounds were carried out in order to evaluate their applicability
as suitable primary explosives (Figure 23/24 and S33–48) and
to observe their behavior toward fast heating with and with-
out confinement. An outline of the experimental results is
given in Table 4. All compounds showed strong detonations
during the tests, which was indicated by the heavy defor-
mations of the used copper plate and needles which were
Figure 25.
PETN initiation capability test setup.
used in the analysis (except
only sharply).
9, 12, and 17, which deflagrated
Table 4. Overview of the hot plate and hot needle test results for 3–7, 9, 12, and 17–
23.a
Figure 26.
Positive PETN initiation tests for the coordination compounds 5 and 6.
compound
HP
HN
det.
def.
det.
det.
det.
def.
def.
compound
HP
HN
def.
det.
det.
det.
def.
det.
det.
3
4
det.
det.
det.
det.
def.
def.
def.
17
18
19
20
21
22
23
def.
det.
det.
det.
det.
det.
det.
Energetic coordination compounds bearing nitrates as anions
are usually unable to initiate common secondary explosives,
due to their lack of performance. Thus, it is even more
5
6
astounding that both copper(II) nitrate complexes
9 and 17,
which show great promise for the future, were able to initiate
PETN. Negative test results, indicated by intact copper shells
and plates were surprisingly obtained for the perchlorate
7
9
12
complexes
4,
7
and 20. An overview of the test results can be
a def.: deflagration; det.: detonation.
found in Table 5.
On the basis of the positive results of the hot needle and hot
plate tests, which gave a first indication of its suitability as a
primary explosive, the initiation capability for most of the
promising compounds toward pentaerythritol tetranitrate
(PETN) as the main charge in copper shells was explored (Fig-
12 | J. Name., 2012, 00, 1-3
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ARTICLE
a
tive copper(II) nitrate complexes
9
and 17 were successfully be
Table 5. Results of PETN initiation tests of 3–7, 9 and 17–23.
initiated and are therefore one of the rare examples of laser-
ignitable nitrate complexes in literature.47,48
compound
initiation
compound
initiation
3
4
pos.
neg.
pos.
pos.
neg.
pos.
pos.
18
19
20
21
22
23
pos.
pos.
neg.
pos.
pos.
pos.
Table 6. Laser initiation test results.
5
compound
0.15 mJ
0.17 mJ
26 mJ
6
5
det.
det.
det.
—
—
—
det.
det.
—
7
6
9
9
det.
—
17
16
17
18
19
21
23
x
a pos.: positive; neg.: negative.
x
det.
det.
—
—
x
—
Toxicity
det.
det.
x
—
In order to evaluate the toxicity of the synthesized complexes,
the toxicological impact on aquatic organisms of the two un-
—
—
coordinated nitrogen-rich ligands
1 and 2 was measured using
det.
—
the commonly known luminescence marine bacterium Vibrio
Fischeri NRRL-B-11177 strain.43 Both tetrazoles can be classi-
fied as non-toxic (toxicity level after 30 min exposure: very
toxic < 0.10 g/L; toxic 0.10–1.00 g/L; non-toxic > 1.00 g/L) with
half the maximal effective concentration values EC50 of
(–: not tested, x: no ignition, dec.: decomposition, deflag.: deflagration,
det.: detonation). Operating parameters: current I = 6–7 A; voltage U = 4 V;
theoretical maximal output power Pmax = 45 W; theoretical energy Emax
0.15–26 mJ; wavelength λ = 915 nm; pulse length τ = 0.1–15 ms.
=
4.58 g/L for
1 and 3.87 g/L for 2 determined after an incuba-
tion time of 30 minutes. The values obtained lie within the
range of 1-methyl-5H-tetrazole, the methylated analogue of
1,5H-tetrazole15, and confirm the recently observed trend of
higher toxicities for 2-subsituted tetrazoles in comparison to
their 1-subsituted isomers.44
Figure 27.
Moment of detonation of complex 18 (left) and destroyed lens and
Laser initiation
sample holder (right) caused by the tremendous shockwave.
The optical initiation of energetic materials has received con-
siderable attention in the last decades and a lot of research
has been done from many scientists all over the world.18,44–46
Laser initiation has several advantages over classical ignition
methods (thermal or mechanical) and could therefore be of
great interest for industrial applications (e.g. laser ignitable
ammunition or optical detonators).20 All here investigated
compounds suffer from their increased sensitivities toward
mechanical stimuli making them highly suitable for application
as lead-free primaries but unfortunately not for any use in
laser ignition systems. Therefore, the focus of this contribution
was placed on more academic questions like the laser ignitibil-
ity of the complexes at different energy thresholds. Only se-
lected colored coordination compounds showed a response to
laser irradiation differing in the required energy input depend-
ing on the isomer/metal(II)/anion combination. A summary of
the performed experiments is given in Table 6. More details on
the experimental setup can be found in the Supporting Infor-
mation. All complexes investigated except the copper(II) chlo-
ride complex 16 detonated heavily under the employed set-
tings resulting in deformations of the sample holder and de-
struction of the optical lens and sapphire glass due to the
extreme powerful nature of the complexes (Figure 27, Figure
S53–57). The complexes exhibited similar initiation thresholds
and could be ignited at the lowest applicable initiation energy
settable in the parameters of the lasers. Even the very sensi-
UV-Vis spectroscopy
The UV-Vis spectra in the solid state, illustrated in Figure 28
and S58, were measured for selected laser-ignitable complexes
in the range of 350–1000 nm with regard to any possible cor-
relation between the laser ignitability and the absorption of
the compounds at the corresponding wavelength. The investi-
gated optical properties of the complexes are compiled and
summarized in Table 7. The examined energetic coordination
compounds showed typical absorptions in the NIR, visible and
ultra-violet region resulting from the corresponding d-d transi-
tions, which are highly dependent on the implemented 3d
metal(II), anion and isomeric ligand. The exact mechanism of
the underlying laser initiation process (e.g. thermal, electroni-
cal) is still questionable and has not been understood com-
pletely.49,50
All investigated complexes show a moderate absorption in the
laser wavelength region of 915 nm, which could be a possible
explanation for the laser ignitibility of the coordination com-
pounds since none of the colorless complexes (MnII or ZnII)
could be initiated. The laser ignitibility is not only linked to its
absorption behavior but also to several other parameters like
the coordination sphere of the corresponding transition met-
al(II) or its electronic configuration. One plausible explanation
of the laser ignitibility of the complexes and imaginable initia-
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tion pathway could be the formation of several hot spots in sulfonic acid (HOSA). The obtained isomeric mixture was puri-
the material caused by photothermal excitations after irradia- fied by column chromatography, preventing the highly risky
tion. It should be emphasized that contrary to literature as- distillation mentioned in the literature and yielded the prod-
sumings, complexes bearing the nitrate anion could be ignit- ucts in satisfying yields. Due to the great potential of 1-AT as
ed.47
ligand in powerful energetic transition metal(II) complexes, a
selective, upscalable and isomerically pure route toward this
highly endothermic compound has been developed. The re-
ceived nitrogen-rich aminoazoles were comprehensively char-
acterized by diverse analytical methods enclosing heteronu-
clear 15N NMR spectroscopy, X-ray studies and toxicity deter-
minations. The toxicity assessments toward aquatic organisms
revealed the non-toxicity of both heterocycles. Transition
metal(II) complexes based on 1-methyl-5H-tetrazole as ligand
have shown great promise in the past but had not enough
power to be appropriate candidates as lead-free primary ex-
plosives. As obvious and logical consequence, N-aminoazoles
with a higher nitrogen content exhibiting the powerful outer
N–N single bond have been incorporated into the equivalent
complexes resulting in a tremendous performance increase
releasing several of the most energetic coordination com-
pounds synthesized so far. The present study features the
preparation, analysis and evaluation of 20 new lead, nickel and
cobalt-free energetic complexes, which are built-up by two
isomeric aminotetrazoles, four different metals and six varying
anions. The synthesized complexes, which show a great coor-
dination sphere diversity depending on the inserted compo-
nents, can be prepared by an easy upscalable and low-cost
synthetic route. The strategy of energetic coordination com-
pounds (ECC), which enables the fine-tuning of the optical and
energetic properties through easy change of the building
blocks (metal, anion and ligand) has proved once again its
suitability in the difficult and challenging research area of lead-
free primary explosives. The synthesis of the metal(II) perchlo-
rate coordination compounds based on 2-amino-5H-tetrazole
were highly dependent on the reaction conditions like stoichi-
ometry or solvent system and yielded either aqua complexes
or co-crystallized species. Single X-ray studies in combination
with powder diffraction and electron microscopy of the auspi-
cious and mostly water-free complexes gave interesting in-
sights and allowed correlations between the observed struc-
ture and their sensitivity against mechanical stimuli. After
comparing all compounds, the following trends can be formu-
lated: i.) iron(II) and copper(II) complexes are more sensitive
than manganese(II) and zinc(II) coordination compounds; ii.)
the more oxidizing the anion, the more sensitive the com-
pound; iii.) complexes with 1-AT are more sensitive than those
with 2-AT, which can be attributed to the anhydrous nature of
those. Laser initiation experiments executed with NIR light at a
wavelength of 915 nm showed fast deflagration-to-detonation
transitions at extremely low energy thresholds for most of the
coordination compounds and destroyed large parts of the laser
setup due to the destructive power of the complexes. The
initiation capability toward pentaerythritol tetranitrate (PETN)
filled copper shell detonators was tested for several pre-
selected complexes (hot plate and hot needle experiments)
and demonstrated the enormous potential of the synthesized
complexes as lead-free primary explosive replacements. Be-
Table 7. Measured optical properties of complexes 5, 6, 8, 9, 16, 17–19, 21 and 23.
a
b
compound
M
color
λd-d
λ915/λd-d
5
FeII (hs)
CuII
yellowish
blue
871
677
768
686
796
689
647
868
795
655
0.97
0.27
0.92
0.68
0.63
0.50
0.37
0.95
0.93
0.35
6
8
CuII
green
blue
9
CuII
16
17
18
19
21
23
CuII
green
blue
CuII
CuII
blue
FeII (hs)
FeII (hs)
CuII
yellowish
yellowish
blue
a
Absorption intensity maximum wavelength, which can be assigned to electron
d-d transitions in the measured range of 350–1000 nm; b quotient of the absorp-
tion intensity at the laser wavelength and the intensity at the d-d absorption
wavelength
Additional studies in the future should be carried out to obtain
further information regarding the initiation process, which
should cover the influence of the complexes´ structure and the
type of the mechanism.
Figure 28.
Solid state UV-Vis spectra of selected copper(II) and iron(II) com-
plexes. The step in the absorption intensity at 800 nm is caused by a detector
change. The UV-Vis spectra have only qualitative character.
Conclusions
Two isomeric amino-5H-tetrazoles (1-amino-5H-tetrazole (
1, 1-
AT) and 2-amino-5H-tetrazole (2, 2-AT)) with sensitivities al-
ready in the range of primary explosives were synthesized in a
simple one step reaction starting from 1,5H-tetrazole and the
commercially available amination reagent hydroxylamine-O-
14 | J. Name., 2012, 00, 1-3
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11 D. Chen, H. Yang, Z. Yi, H. Xiong, L. Zhang, S. Zhu and G.
Cheng, Angew. Chem., Int. Ed., 2018, 57, 2081.
12 J.-G. Xu, C. Sun, M.-J. Zhang, B.-W. Liu, X.-Z. Li, J. Lu, S.-H.
sides the lead-free, but not completely environmentally friend-
ly perchlorate compounds, the nitrato complexes show excel-
lent properties and can therefore be considered as promising
primary explosive candidates.
Wang, F.-K. Zheng and G.-C. Guo, Chem. Mater., 2017, 29
9725.
,
13 M. H. V. Huynh, M. A. Hiskey, D. E. Chavez, D. L. Naud and R.
D. Gilardi, J. Am. Chem. Soc., 2005, 127, 12537.
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Associated Content
Supporting Information
Compounds overview; IR spectroscopy of
DTA plots of 12 and 16 23; Column diagrams of the
complexes 18 23; Hot plate and hot needle tests; Initiation
1–9, 12, and 16–23;
1–
3,
8,
9,
–
–
capability tests; Laser ignition tests; UV-Vis spectra of 18
, 19,
21, and 23; Experimental part and general methods.
The Supporting Information is available free of charge on the
RSC Publications website.
19 I. A. Ugryumov, M. A. Ilyushin, I. V. Tselinskii and A. S. Ko-
zlov, Russ. J. Appl. Chem., 2003, 76, 439.
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Conflicts of interest
The authors declare no conflict of interest.
23 M. A. Ilyushin, I. V. Bachurina, A. V. Smirnov, I. V. Tselinsky
and I. V. Shugalei, Cent. Eur. J. Energ. Mater., 2009, 7, 33.
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facilities but also for his scientific support. In addition, the
authors would like to thank Prof. Dr. Konstantin Karaghiosoff
for the measurement of various 15N NMR spectra, Prof. Dr.
Jürgen Evers for the recording of several X-ray powder diffrac-
tions, Mrs. Cornelia Unger for toxicity assessments as well as
Jasmin Lechner, Alexander Kremsmair and Philipp Spiess for
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