Coordination chemistry with 1-methyl-5: H -tetrazole: Cocrystallization, laser-ignition, lead-free primary explosives-one ligand, three goals

Article abstract of DOI:10.1039/c7ta07780g

The synthesis and characterization of 1-methyl-5H-tetrazole (1, MTZ) from 1,5H-tetrazole and methyl iodide is achieved. This neutral ligand was evaluated for its use in energetic transition metal complexes. The formation of complexes is a valuable concept because of their easy synthesis and the large number of possible combinations: (i) metal, (ii) ligand, and (iii) anion. A series of 31 new complexes based on MTZ as the ligand was prepared in order to tune their optical properties and sensitivity values by using seven different metals (Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, and Ag⁺) and six different anions (chloride, nitrate, perchlorate, cyanodinitromethanide, picrate, and styphnate). This variation allows tailoring of the energetic properties of the desired molecule toward e.g. suitable primary explosives or laser-ignitable materials. Obtained compounds were characterized by low temperature single crystal X-ray diffraction, IR spectroscopy, elemental analysis and thermal analysis (DTA, DSC and TGA). The sensitivities toward external stimuli (impact, friction and electrostatic discharge) were determined according to the Bundesamt für Materialforschung und -prüfung (BAM) standard methods. These complexes have been characterized in laser ignition experiments and as new primary explosives. UV-vis measurements of selected complexes were carried out to get a possible insight into the laser initiation mechanism. For the first time a large number of compounds with sensitivities ranging from insensitive to very sensitive were synthesized to give a wide range of new materials for different possible applications.

Full text of DOI:10.1039/c7ta07780g

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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS2 nanocages as a lithium ion battery anode material
rsc.li/materials-a

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Coordination chemistry with 1-methyl-5H-tetrazole:
Cocrystallization, laser-ignition, lead-free primary explosives –
One Ligand, three Goals
Received 00th January 20xx,
Accepted 00th January 20xx
Norbert Szimhardt,[a] Maximilian H. H. Wurzenberger[a], Andreas Beringer,[a] Lena J. Daumann,[a]
and Jörg Stierstorfer*[a]
DOI: 10.1039/x0xx00000x
www.rsc.org/
The synthesis and characterization of 1-methyl-5H-tetrazole (1, MTZ) from 1,5H-tetrazole and methyl iodide is achieved.
This neutral ligand was evaluated for its use in energetic transition metal complexes. The formation of complexes is a
valuable concept because of the easy synthesis and the large number of possible combinations: i) metal, ii) ligand, and iii)
anion. A series of 31 new complexes based on MTZ as the ligand was prepared in order to tune optical properties and
sensitivity values by using seven different metals (Mn2+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+, Ag+) and six different anions (chloride,
nitrate, perchlorate, cyanodinitromethanide, picrate, styphnate). This variation allows to tailor the energetic properties of
the desired molecule toward e.g. suitable primary explosives or laser-ignitable materials. Obtained compounds were
characterized by low temperature single crystal X-ray diffraction, IR spectroscopy, elemental analysis and thermal analysis
(DTA, DSC and TGA). The sensitivities toward external stimuli (impact, friction and electrostatic discharge) were
determined according to the Bundesamt für Materialforschung und -prüfung (BAM) standard methods. These complexes
have been characterized in laser ignition experiments and as new primary explosives. UV-Vis measurements of selected
complexes were carried out to get a possible insight into the laser initiation mechanism. For the first time a large number
of compounds with sensitivities ranging from insensitive to very sensitive were synthesized to give a wide range of new
materials for different possible applications.
Crystalline compounds containing two or more different
species – including hydrates, solvates, coordinating and non-
coordinating ligands – are defined as cocrystals and have an
Introduction
Lead styphnate and lead azide were put on the candidate list
of authorization (substances of very high concern, Annex XIV)
for the REACH (Registration, Evaluation, Authorization and
Restriction of Chemicals)1 regulations in 2011 due to their high
toxicity. Almost certainly, the two compounds will be strictly
regulated or banned. Therefore; research into possible lead-
free replacements is strongly needed.
increasing
number
of
applications
as
molecular
semiconductors, advanced pharmaceuticals and optical
materials.7 In the field of energetic materials, cocrystallization
can be used for enhancing the density and the
detonation/combustion performance or to lower the
mechanical sensitivities.8 Regarding energetic complexes, this
concept has recently been described with 1,2-di(1H-tetrazol-5-
yl)ethane as the ligand.9 In this work, the neutral ligand itself
has been used as a cocrystallized species for the first time to
increase the stability of the energetic complexes.
The use of endothermic ligands, such as tetrazoles and
triazoles, in coordination compounds has been growing over
the past few years with multiple uses in the field of inorganic
chemistry.2,3 Specifically, these complexes have applications as
metal-organic frameworks (MOFs),4 spin crossover systems5
and energetic materials.6
Coordination compounds of iron(II), in particular bearing
alkylated tetrazole ligands like 1-methyl-5H-tetrazole (1), are
one of the most investigated compounds in spin crossover
systems.10 The perchlorate and tetrafluoroborate iron(II)
alkyltetrazole coordination compounds exhibited two different
lattice sites for the central metals with a spin transition from
high spin (hs) to low spin (ls) at a specific temperature.11
Metastable long-lived hs and ls states on both sides can be
induced through irradiation of green or red light, which makes
spin crossover compounds interesting for uses in liquid
crystals, gel phases and molecular switches.12
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In terms of energetic materials, transition metal complexes
have been proposed for numerous applications.13 Ordinary 1-Methyl-5H-tetrazole
(
1)
was first synthesized 1910 by
used primary explosives, such as lead(II) azide, lead(II) reaction of methylisocyanide with hydrazoic acid by Oliveri-
styphnate and, in former times, mercury(II) fulminate (Chart Nandalá.26 The ligand has a high nitrogen content of 66.63 %
1), have led to significant contamination of the and can be easily synthesized at low cost. The resulting driving
environment.14,15 Energetic coordination compounds (ECCs) force of producing nitrogen gas out of N–N single or N=N
containing nitrogen-rich tetrazole ligands with high positive double bonds, makes it promising as a ligand in combination
heats of formation and low or non-toxic metals like copper, with oxidizing anions for ECCs. Until now, the compound has
zinc or iron can be used as substitutes for currently used only been described in relation to EPR studies,27 spin transition
primary explosives containing toxic heavy metals (Chart complexes10-12,28, and pyrotechnic29/propellant compositions.30
2a/b).6,16,17,18
Therefore, in this paper, efforts have been made to utilize MTZ
as a neutral ligand to generate new energetic complexes with
promising properties as (laser-ignitable) primary explosives or
cocrystallized coordination compounds.
RESULTS AND DISCUSSION
Synthesis
Chart 1 Commonly used primary explosives: lead(II) azide, lead(II) styphnate and
The nitrogen-rich heterocycle 1-methyl-5H-tetrazole (1, MTZ)
mercury(II) fulminate.
can be synthesized in various ways. One route is the
cycloaddition reaction of sodium azide with triethyl
orthoformate and methylamine in acetic acid under reflux.31
Another route is the nucleophilic methylation of 1H-tetrazole
The use of laser initiation systems is also becoming more
attractive as
a strategy for replacing sensitive primary
explosives.19 Instead of using very sensitive compounds for
igniting secondary explosives, relatively insensitive energetic
materials are ignited directly by laser irradiation. The ignition
can be triggered by using high power lasers or with lower
energies by adding small amounts of light absorbing materials
to the energetic materials, like carbon black or chemical
dyes.20 However, promising explosive coordination complexes
(ECCs), using transition metals as the central atom (for
example Cu2+, Co2+, Ni2+ or Fe2+) and tetrazole ligands have
been reported in literature that are ignitable by laser
irradiation without the addition of dyes (Chart 2d).21,22 The
most prominent examples include pentaammine(5-cyano-2H-
tetrazolate)cobalt(III) perchlorate (CP)23, tetraammine-cis-
bis(5-nitro-2H-tetrazolato-N2)cobalt(III) perchlorate (BNCP)24
and 5-hydrazino-1H-tetrazolemercury(II) perchlorate (HTMP,
Chart 2c)25. While they were initiated by laser ignition, they
proved to be too sensitive for any application. The research of
insensitive replacements is still of interest.
with
a methylating agent such as methyl iodide. The
methylation reaction reported here was carried out in acetone
at 40 °C under reflux using potassium carbonate as base,
according to a modified literature procedure (Scheme 1).32 The
methylation leads to an isomeric mixture of 1-methyl-5H-
tetrazole (1) and 2-methyl-5H-tetrazole. The desired product
can be isolated from this mixture either by flash column
chromatography or distillation. Filtration of the reaction
mixture followed by distillation (MTZ: 90 °C, 0.01 mbar, 2-MTZ:
40 °C, 12 mbar) of the organic residue resulted in the isolation
of the product
1 with a yield of 48 %. The side product 2-
methyl-5H-tetrazole was obtained with a yield of 23 %.
Scheme 1.
Synthesis of 1-methyl-5H-tetrazole (1, MTZ).
The divalent transition metal complexes
2–13 and 15–26 were
prepared by combining solutions of 1-methyl-5H-tetrazole (
1)
and metal(II) salts with the respective stoichiometries at room
temperature (Scheme 2–4). Water was chosen as a non-
flammable, low-volatile and cheap reaction medium because
of the good solubility of both the metal(II) salts and the
tetrazole ligand in this solvent. In order to get crystalline
materials, a minimal amount of solvent was used. An iron(II)
Chart 2 Energetic coordination compounds (ECC´s): a) [Zn(N2H4)2(N3)2]16 and b)
17
[Ni(N2H4)3](NO3)2 as possible replacements for lead containing primaries; c)
5-hydrazino-1H-tetrazolemercury(II) perchlorate (old generation of laser ignitable
25
explosives)
and d) [Fe(TriTzPyr)3](ClO4)2 (new generation of laser ignitable
tetrachloridoferrate(III) coordination compound
2 with iron in
explosives)22
.
two different oxidation states was synthesized analogous to
the literature known spin crossover tetrabromidoferrate(II)
2 | J. Name., 2012, 00, 1-3
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complex of 1-methyl-5H-tetrazole.12 Ethanol was used for for crystallization until a precipitate appeared. The majority of
recrystallization of and as co-solvent in the synthesis of the the complexes were obtained in satisfying yields directly from
perchlorate complexes 15 20 to get better crystallization. the mother liquor as single crystals suitable for X-ray
Complexes and have already been described in the diffraction within few hours or days. Unfortunately
2
–
4
5
a
literature and were synthesized to fill the gaps in the physico- compound 14 precipitated as an amorphous powder in a low
chemical analytics.33,34 The cobalt(II) and nickel(II) chlorido yield of 18 % and despite considerable recrystallization efforts
coordination compounds
and the literature reported CoCl2, NiCl2 complexes, with four The complex was synthesized by treatment of
coordinating and two cocrystallized non-coordinating MTZ carbonatotetraaminecobalt(III) perchlorate with two
molecules. The nickel(II) chlorido complex precipitated as a equivalents of in 15 % perchloric acid for 4 h. under reflux
powder and single crystals could not be obtained by according to modified procedure of an analogous
recrystallization. On comparison of the infrared spectra of
compound.35 The compound was filtered off, washed with a
and and elemental analysis complex formula of small amount of ethanol and dried in air.
[NiCl2(MTZ)4] • 2 MTZ is proposed for the coordination
6
and
7
crystallized, in contrast to
3–
single crystals could not be obtained.
5
7
1
a
3, 6
7,
a
L) [CoCO3(NH3)4]ClO4
L) [Co(NH3)4(MTZ)2](ClO4)3 (14)
compound 7 (Figure S20).
15 % HClO4, reflux, 4 h
2 equiv. MTZ
M) Mn(ClO4)2 6 H2O
N) Fe(ClO4)2 6 H2O
O) Co(ClO4)2 6 H2O
P) Ni(ClO4)2 6 H2O
Q) Cu(ClO4)2 6 H2O
R) Zn(ClO4)2 6 H2O
M) [Mn(MTZ)6](ClO4)2 (15)
N) [Fe(MTZ)6](ClO4)2 (16)
O) [Co(MTZ)6](ClO4)2 (17)
P) [Ni(MTZ)6](ClO4)2 (18)
Q) [Cu(MTZ)6](ClO4)2 (19)
R) [Zn(MTZ)6](ClO4)2 (20)
N
N
H2O/EtOH, rt
6 equiv. MTZ
N
N
S) Mn(ClO4)2 6 H2O
T) Fe(ClO4)2 6 H2O
U) Co(ClO4)2 6 H2O
V) Ni(ClO4)2 6 H2O
W) Cu(ClO4)2 6 H2O
X) Zn(ClO4)2 6 H2O
MTZ (1)
S) [Mn(MTZ)6](ClO4)2 2 MTZ (21)
T) [Fe(MTZ)6](ClO4)2 2 MTZ (22)
U) [Co(MTZ)6](ClO4)2 2 MTZ (23)
V) [Ni(MTZ)6](ClO4)2 2 MTZ (24)
W) [Cu(MTZ)6](ClO4)2 2 MTZ (25)
X) [Zn(MTZ)6](ClO4)2 2 MTZ (26)
H2O, rt
Scheme 2.
Synthesis of the metal(II) chloride based MTZ complexes 2–7 with
ex. 10 equiv. MTZ
V): H2O, 60 °C
ex. 16 equiv. MTZ
different metals centers.
The ligand has no acidic proton, which enables the integration
of the neutral tetrazole ligand to assemble cationic or neutral
complexes with oxidizing anions like nitrates, perchlorates,
Scheme 4.
Synthesis of the energetic perchlorate coordination compounds 14–26.
All perchlorate complexes were obtained without inclusion of
aqua ligands or water solvent molecules, which results in a
higher performance. The synthesis of the perchlorate
complexes is strongly dependent on the quantity of the ligand
added and yields either a structure where all MTZ ligands are
cyanodinitromethanides
(CDNM),
picrates
(PA)
or
trinitroresorcinates (HTNR). The incorporation of these
oxidizing anions leads to more sensitive and powerful
energetic materials and are possible alternatives for lead free
laser ignitable primary explosives.
bound to the metal ions (15
–
20) or a complex unit with two
26) when an excess of
additional cocrystallized tetrazoles (21
–
the ligand is used. Both species can be clearly distinguished
through infrared spectroscopy.32 This is illustrated exemplarily
for the cobalt(II) perchlorate complexes 17 and 23 in Figure
S22. In addition to an intense stretching vibration around
1106 cm−1 stemming from the perchlorate counter ions, a
characteristic ring vibration of the bound MTZ ligand around
965 cm−1 is observed in the spectra of both 17 and 23. The free
ligand as well as complex 23 exhibits a stretch in the spectra at
around 997 cm−1 stemming from in plane ring vibrations in free
MTZ.32 The C-H vibration can be detected at 3100 cm −1 in the
free ligand and complex 23 spectra, respectively, while the
bound MTZ ligand in 23 and 17 does occur at slightly higher
wavenumbers (3147 cm−1).
Scheme 3.
Synthesis of selected metal nitrate based MTZ complexes 8–13 with
different metal centers.
Compound 11 lost the crystal water molecules and its
crystallinity overnight, in contrast to 8–10, and was analyzed in
its anhydrous form 12. The colored reaction mixtures were left
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database36 and can be obtained free of charge with the CCDC
nos. 1566368 ( ), 1566362 ( ), 1566371 ( ), 1566381 (10),
Y) CuSO4 5 H2O/ 2 KCDNM
Y) [Cu(CDNM)2(MTZ)4] (27)
H2O, rt
4 equiv. MTZ
2
3
6
1566384 (11), 1566379 (13), 1566370 (15), 1566366 (17),
1566363 (18), 1566364 (19), 1566361 (20), 1566359 (21),
1566380 (22), 1566382 (23), 1566385 (24), 1566386 (25),
1566360 (26), 1566378 (27), 1566377 (28), 1566376 (29),
1566372 (30), 1566375 (31), 1566383 (32), 1566373 (33),
1566367 (34), 1566374 (35), 1566365 (36), 1566369 (37). The
bond lengths and angles of the coordinating and non-
coordinating MTZ rings in the investigated complexes are in
the typical range of tetrazole ligands and similar to the non-
N
N
Z) MnCO3/ 2 HPA
a) CoCO3/ 2 HPA
b) NiCO3/ 2 HPA
c) CuCO3/ 2 HPA
d) ZnCO3/ 2 HPA
N
N
Z) [Mn(H2O)4(MTZ)2](PA)2 2 H2O (28)
29
a) [Co(H2O)4(MTZ)2](PA)2 2 H2O (
b) [Ni(H2O)4(MTZ)2](PA)2 (30)
c) [Cu(PA)2(MTZ)2] (31)
)
MTZ
1
( )
H2O, 70 °C
4 equiv. MTZ
d) 6 equiv. MTZ
32
d) [Zn(PA)2(MTZ)4] (
)
Scheme 5.
Formation of the complexes 27–32 with varying metal centers and
anions (CDNM= cyanodinitromethanide; HPA = picric acid, PA = picrate).
coordinating ligand 1 37,38
The tetrazole ligand is therefore not
.
The corresponding metal(II) precursors for the complexes 27
–
discussed in detail in any of the following compounds.
Coordination to the metal centers always takes place by the
37 were prepared and used in situ by simple acid-base
reactions between metal(II) carbonates and picric/styphnic
acid or metathesis reactions. The aqueous solutions of the
N4 nitrogen atom of 1-methyl-5H-tetrazole (1). All complexes
with MTZ show an octahedral coordination sphere. This can be
explained by the low steric hindrance of the ligand.
corresponding precursors were then reacted with ligand
1
(Schemes 5 and 6). In some cases non-stoichiometric amounts
of the ligand were used to increase the yield or purity of the
respective compounds. All complexes crystallized within
several minutes (copper(II) picrate and styphnate compounds)
or days and were obtained directly from the mother liquor in
reasonable yields (60–86 %). Crystals were filtered off, washed
with cold ethanol when necessary to remove unreacted
starting materials and dried in air, resulting in suitable samples
for X-ray diffraction. Due to the high solubility of compound 33
in ethanol, no purification step was performed. In case of the
manganese picrate complex 28 unfortunately only a few X-ray
suitable single crystals could be picked between unreacted
starting material and powder bulk material with unknown
composition. The attempted synthesis of the coordination
compound in sufficient amounts failed and only the crystal
structure could be determined which is shown in the SI (Figure
S12).
[Fe(MTZ)6](FeCl4)2 (2) contains iron in the oxidation states +II
and +III. It crystallizes in the form of yellow blocks in the
trigonal space group P−3 with one formula unit per unit cell
and
a
calculated density of 1.747 g cm−3 at 123 K. The
molecular unit consists of six asymmetric unit cells with a
octahedrally coordinated iron(II) center, which is bonded to six
MTZ ligands (Fe1–N1 = 2.010(13) Å, < (N1—Fe1—N1i) =
88.68(6)°). The second iron(III) atom is tetrahedrally
coordinated by four chlorido-ligands (Fe2–Cl1 = 2.203(5) Å, <
(Cl2—Fe2—Cl1vii) = 109.49(2)°, (Cl1vi—Fe2—Cl1vii) = 109.46(2)°)
forming the tetrachloridoferrate(III) counteranion (Figure 1).
e) MnCO3/ 2 H2TNR
f) CoCO3/ 2 H2TNR
g) NiCO3/ 2 H2TNR
h) CuCO3/ 2 H2TNR
i) ZnCO3/ 2 H2TNR
33
e) [Mn(HTNR)2(MTZ)4] (
f) [Co(H2O)4(MTZ)2](HTNR)2 2 H2O (34)
)
N
35
g) [Ni(H2O)4(MTZ)2](HTNR)2 2 H2O (
36
)
N
H2O, 70 °C
2 equiv. MTZ
e) 4 equiv. MTZ
h) [Cu(HTNR)2(MTZ)2] (
i) [Zn(HTNR)2(MTZ)2] (37)
)
N
N
MTZ
1
)
(
Figure 1 Molecular unit of [Fe(MTZ)6](FeCl4)2 (2). Thermal ellipsoids of non-hydrogen
atoms in all structures are set to the 50 % probability level. Selected bond lengths (Å):
Fe1–N1 2.010(13), Fe2–Cl2 2.185(8), Fe2–Cl1 2.202(5); selected bond angles (°): N1–
Fe1–N1i 88.68(6), N1–Fe1–N1ii 91.32(6), Cl2–Fe2–Cl1vi 109.49(2), Cl1vi–Fe2–Cl1vii
109.46(2). Symmetry codes: (i) y, −x+y, 1−z; (ii) −y, x−y, z; (iii) x−y, x, 1−z; (iv) −x+y, −x, z;
(v) −x, −y, 1−z; (vi) −x+y, 1−x, z; (vii) 1−y, 1+x−y, z.
Scheme 6.
Synthesis of the metal(II) based styphnate complexes 33–37.
Crystal structures
All complexes (except
4, 5, 7–9, 12, 14 and 16) were
characterized using low temperature single crystal X-ray
diffraction. Details on the crystal structures of complexes 11
15 17 18 20 21 23 26 28 30 32 and 34 37 are given in the
The cobalt(II) chlorido complex [CoCl2(MTZ)4]
• 2 MTZ (6)
3,
,
crystallizes in the form of yellow blocks in the monoclinic space
group P21/c with two formula units per unit cell and a
calculated density of 1.617 g cm−3 at 123 K. In comparison to
literature tetrazole containing complexes, most of the
investigated compounds, show low densities, which can be
attributed to the low density of MTZ itself.39 The molecular
unit contains two cocrystallized non-coordinating MTZ
molecules, which are stabilized by non-covalent interactions
and an octahedrally coordinated cobalt(II) cation. The metal(II)
center is coordinated (Figure 2) by two chlorido ligands in axial
,
,
,
,
,
–
,
–
,
–
SI together with the measurement and refinement data. The
single crystals of the cobalt(II) and nickel(II) nitrate complexes
8
and 9 were measured to give an indication of the most likely
structures. Finalization of the data set was not possible
because of the strongly disordered moieties and the related
weak diffractions. The complexes
8 and 9 exhibit the same
coordination environment and were therefore compared with
10 by infrared spectroscopy to confirm its structure (Figure
S21). The crystal structures were uploaded to the CSD
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positions and by four MTZ molecules in a plane (Co1–N1 = copper(II) ion in 11 shows Jahn-Teller distortion along its axial
2.143(6) Å, Co1–N5 = 2.161(6) Å, Co1–Cl1 = 2.443(2) Å). The coordination sphere. In comparison, the respective zinc(II)
deviation of the ideal coordination sphere is marginal (< (N1— complex exhibits a nearly undistorted coordination sphere and
Co1—N1) = 89.52(2)°, < (Cl11—Co1—N5) = 90.48(2)°, < (N1— the six surrounding MTZ´s is nearly undistorted (Zn1–N5 =
Co1—N5) = 87.7(2)°).
2.139(2) Å, Zn1–N1 = 2.166(2) Å, < (N5i—Zn1—N9i) = 88.23(8)°,
< (N1—Zn1—N9i) = 89.51(8)°).
The silver(I) complex 13 crystallizes in the monoclinic space
group P21/c with four formula units per unit cell and a
calculated density of 2.112 g cm−3 at 123 K. Every silver(I)
center is penta-coordinated by two MTZ ligands and two
bridging nitrates (Figure 4).
Figure 2 Molecular unit of [CoCl2(MTZ)4] • 2 MTZ (6). Selected bond lengths (Å): Co1–
N1 2.143(6), Co1–N5 2.161(6), Co1–Cl1 2.443(2); selected bond angles (°): Cl1–Co1–N1
89.52(2), Cl1–Co1–N5 90.48(2), N1–Co1–N5 87.7(2). Symmetry code: (i) −x, 1−y, −z.
The complexes 10 (colorless block) and 11 (blue block) with
nitrate counterions are crystallizing isotypically in the triclinic
space group P−1 with two formula units per unit cell showing
almost the same metric (see Table S1 in the SI) and densities:
10: 1.608 g cm−3 and 11: 1.626 g cm−3 at 123 K.
Figure 4 Molecular unit of [AgNO3(MTZ)2] (13). The fifth ligand from an adjacent
nitrate was omitted for clarity. Selected bond lengths (Å): Ag1–N1 2.233(3), Ag1–N5
2.255(3), Ag1–O1 2.544(3), Ag1–O3i 2.545(3), Ag1–O2 2.719(3), O1–N9 1.251(4), O2–
N9 1.257(5), O3–N9 1.252(4); selected bond angles (°): N1–Ag1–N5 137.76(12), N1–
Ag1–O1 111.85(11), N5–Ag1–O1 95.09(11), N1–Ag1–O3i 119.23(11), N5–Ag1–O3i
95.82(11), O1–Ag1–O3i 81.30(10), O1–N9–O3 120.7(3), O1–N9–O2 119.6 (3), O3–N9–
O2 119.7(3). Symmetry codes: (i) 1+x, y, z; (ii) −1+x, y, z.
The nitrato ligand binds through all of its three oxygen atoms.
In addition to a bidentate coordination mode to one silver(I)
cation, the nitrato ligand is also bound in a monodentate
fashion to a second silver(I) ion, bridging the two cations and
forming polymeric chains along the a axis (Figure 5). The
distances between silver(I) and oxygen (Ag1–O1 = 2.544(3) Å,
Ag1–O3i 2.545(3) Å, Ag1–O2 2.719(3) Å) are longer than the
silver(I) nitrogen distances (Ag1–N1 = 2.233(3) Å, Ag1–N5 =
2.255(3) Å).
Figure 3 Molecular unit of [Zn(MTZ)6](NO3)2 • 2 H2O (10). Selected bond lengths (Å):
Zn1–N5 2.139(2), Zn1–N1 2.166(2), Zn1–N9 2.185(2), O2–N25 1.257(3), O3–N25
1.250(3), O4–N26 1.223(4); selected bond angles (°): N5–Zn1–N1 88.91(8), N5i–Zn1–N1
91.09(8), N5–Zn1–N9i 91.77(8), N5i–Zn1–N9i 88.23(8), N1–Zn1–N9i 89.51(8), N1i–Zn1–
N9i 90.49(8), O3–N25–O2 120.0(2), O1–N25–O2 119.8(2), O4–N26–O5 120.6(3), O4–
N26–O6 119.8(3), O5–N26–O6 119.5(3). Symmetry codes: (i) 2−x, 2−y, −z; (ii) 2−x, 1−y,
1−z.
Figure 5 View on the structure of 13 along the a axis (Ag–O bonds = cyan; Ag–N bonds
= green). Stabilization through non-classical hydrogen bonds (C–HꢀꢀN and C–HꢀꢀO; C–
HꢀꢀO hydrogen bonds = purple) and hydrophobic interactions between the methyl
groups.
The complexes with perchlorate counterions 15/20 and 17/18
crystallize isotypically in monoclinic and trigonal space groups
15: P21/n, 1.582 g cm−3 (Figure S3); 17: R−3, 1.614 g cm−3
Both metal(II) centers have an octahedral coordination sphere
with six MTZ ligands (Figure 3 and S2). As expected, the d9-ion
(
(Figure S4); 18: R−3, 1.629 g cm−3 (Figure S5); 20: P21/n,
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1.635 g cm−3 at 123 K (Figure S6)). The blue copper(II)
perchlorate compound 19 is the only complex that crystallizes
in the triclinic space group P−1 with two formula units per unit
cell and a calculated density of 1.645 g cm−3 at 123 K, which is
the highest of all perchlorates. Its molecular motif is illustrated
in Figure 6. Each metal(II) center contains an octahedral
surrounding and is connected to six tetrazole ligands. As
already observed in the coordination environment of the
cation in 11, the copper(II) center shows a Jahn-Teller
distortion caused by the d9-configuration (Cu1−N1 2.035(2) Å,
Cu1−N5 2.014(2) Å, Cu1−N9 2.401(2) Å).
Figure 7 Complex unit of [Fe(MTZ)6](ClO4)2 • 2 MTZ (22). Selected bond lengths (Å):
Fe1–N12 2.181(2), Fe1–N4 2.183(2), Fe1–N8 2.194(2), Cl1–O4 1.405(4); selected bond
angles (°): N12–Fe1–N4 89.53(8), N12i–Fe1–N4 90.48(8), O1–Cl1–O4 108.1(2), N12–
Fe1–N8i 91.61(8), N12i–Fe1–N8i 88.39(8), N4–Fe1–N8i 89.63(8), N4i–Fe1–N8i 90.37(8),
N12i–Fe1–N8 91.61(8). Symmetry codes: (i) −x, −y, 1−z; (ii) 1−x, 1−y, −z; (iii) 1−x, 1−y,
1−z.
Again, two additional non-coordinating MTZ molecules could
be cocrystallized. Six regular tetrazole ligands are surrounding
the transition metal(II) centers in octahedral arrangements
and interact hydrophobically and through non-classical
hydrogen bonds with the cocrystallized compounds.
The cyanodinitromethanide complex 27 crystallizes in the form
of blue blocks in the triclinic space group P−1 with one formula
unit per unit cell and a calculated density of 1.746 g cm−3 at
123 K.
The molecular unit is built up of two asymmetric unit cells. The
octahedral complex (Figure 8), with four MTZ ligands in a plane
(Cu1–N8 = 2.002(16) Å, Cu1–N1 = 2.019(19) Å, < (N8i–Cu1–N7i)
= 88.54(6)°, < (N1i–Cu1–N7i) = 90.72(7)°), shows, elongation
along the axial positions, which are occupied by two
cyanodinitromethanide anions (Cu1–N7 = 2.412(18) Å). The
structure of the CDNM anion is comparable to literature
values.40
Figure 6 Complex unit of [Cu(MTZ)6](ClO4)2 (19). Selected bond lengths (Å): Cu1–N1
2.035(2), Cu1–N5 2.014(2), Cu1–N9 2.401(2), Cl2–O5 1.437(2); selected bond angles (°):
N1–Cu1–N5 89.83(2), N1–Cu1–N9 91.04(2), N5–Cu1–N9 89.54(2), O5–Cl2–O6
109.32(2). Symmetry codes: (i) −x, 2−y, −z; (ii) 1−x, −y, 1−z; (iii) 1−x, 1−y, 1−z; (iv) −x,
1−y, −z.
New perchlorate species 21–26 of the investigated metals(II)
were obtained after using an excess of the tetrazole ligand
during the synthesis. The complexes with two cocrystallized
tetrazoles crystallize isotypically in the triclinic space group
P−1 with one formula unit per unit cell and similar densities.
However, their densities increase in the row of Mn2+
(1.590 g cm−3; 123 K) < Fe2+ (1.592 g cm−3; 173 K) < Co2+
(1.612 g cm−3, 143 K) < Ni2+ (1.621 g cm−3; 143 K) < Cu2+
(1.623 g cm−3; 143 K)
<
Zn2+ (1.630 g cm−3; 123 K). The
molecular structure of complex 22 is shown in Figure 7 and
used exemplarily for all complexes.
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almost the same density (1.732 g cm−3 at 123 K) but with
different anionic ligands, and is depicted in Figure 10.
Figure 8 Molecular unit of [Cu(CDNM)2(MTZ)4] (27). Selected bond lengths (Å): Cu1–
N8 2.002(16), N6–C3 1.399(2), N7–C4 1.151(3), Cu1–N1 2.019(19), C3–C4 1.407(3),
Cu1–N7 2.412(18), N5–C3 1.394(3), O2–N5 1.238 (2), O4–N6 1.243(3); selected bond
angles (°): N8–Cu1–N1i 91.31(7), N8i–Cu1–N1i 88.69(7), N8–Cu1–N7i 91.46(6), N1i–Cu1–
N7i 90.72(7), N1–Cu1–N7i 89.28(7), N8–Cu1–N7 88.54(6), O3–N6–O4 122.53(17), O3–
N6–C3 122.02(18), O4–N6–C3 115.45(16), C1–N1–N2 106.45(18), N5–C3–N6
124.47(17), N5–C3–C4 117.23(16), N7–C4–C3 178.0(2). Symmetry code: (i) 1−x, −y, −z.
The manganese(II) 28 and cobalt(II) 29 picrate complexes
crystallize isotypically in the monoclinic space group P21/c with
two formula units per unit cell and calculated densities of
1.746 g cm-3 at 123 K for 28 and 1.786 g cm-3 at 173 K for 29
.
Their molecular units consist of two asymmetric units,
containing one MTZ ligand, two aqua ligands, one crystal water
and one non-coordinating picrate counteranion (Figure
S12/13). The same monoclinic space group P21/c could be
observed for the nickel(II) picrate complex 30 (1.757 g cm-3 at
123 K), which shows a different molecular composition with no
crystallized water molecules, two aqua and four MTZ ligands
(Figure S14). Among the metal(II) picrate complexes, only the
copper(II) 31 and zinc(II) 32 complexes crystallize water-free in
monoclinic space groups (31: P21/c; 32: P21/n) with two
formula units per unit cell and calculated densities of
Figure 10 Molecular unit of [Mn(HTNR)2(MTZ)4] (33). Selected bond lengths (Å): Mn1–
O3 2.156(2), Mn1–N4 2.250(3), Mn1–N8 2.239(3); selected bond angles (°): O3–Mn1–
N4 93.04(9), O1–Mn1–N8 85.42(11), N4–Mn1–N8 92.18(10). Symmetry code: (i) −x,
2−y, −z.
1.828 g cm−3 at 123 K for 31 and 1.724 g cm−3 at 173 K for 32
.
The cobalt(II) 34 and nickel(II) 35 styphnate complexes (Figures
S16 and 17) crystallize isotypically to each other and to the
picrate complexes 28/29. A similar situation can be observed
for complex 36, which exhibits a similar coordination sphere
like 31 with analogous cell parameters but with styphnates
instead of picrates (Figure S18). The zinc(II) complex 37 has the
same sum formula as compound 36, but crystallizes triclinic in
the space group P−1 and a different metal center (Figure S19).
Sensitivities and thermal stability
Thermal measurements of highly energetic and sensitive
complexes are very challenging since larger quantities
(> 2 mg) can damage the instrument seriously. On the other
hand endothermic signals may not be detected properly when
using an insufficient amount. Endothermic events like
dehydration, melting or loss of cocrystallized/coordinating
Figure 9 Molecular unit of the copper(II) picrate complex 31. Selected bond lengths MTZ molecules, as well as other critical temperatures of the
(Å): Cu1–N4 2.001(3), Cu1–O4 2.354(2), Cu1–O7 1.929(2); selected bond angles (°): O4–
described compounds were measured with a heating rate of
β = 5 °C min−1 via differential thermal analysis, differential
Cu1–N4 91.63(9), O7–Cu1–N4 91.07(9), O4–Cu1–O7 79.32(9). Symmetry code: (i) 1−x,
1−y, −z.
scanning calorimetry or by thermal gravimetric analysis. The
measured onset temperatures are summarized in Tables 1 and
In comparison to the other picrate compounds discussed so
2. Details on the DTA plots of complexes
1–10, 12–14, 27 and
far, the trinitrophenolate counteranions coordinate
bidentately to the copper(II) (via the nitro- and deprotonated
phenolate-groups) and monodentately to the zinc(II) metal
29–37 are given in the Supporting Information. Most of the
complexes have exothermic decomposition events higher or
close to 200 °C, which is in the range of the uncoordinated
cations in 31/32. The remaining coordination sites of the
ligand
1 (Texo 206 °C). However, endothermic decompositions
:
octahedron are occupied by two methyltetrazole ligands in 31
(Figure 9) and by four tetrazoles in 32 (Figure S15). The
octahedral geometry of 31 shows deviations to the ideal 90°
angle caused by the fixed structure of the picrate anion and its
Jahn-Teller distortion along the O4–Cu1–O4i axis (< (O4–Cu1–
N4) = 91.63(9)°, < (O7–Cu1–N4) = 91.07(9)°, < (O4–Cu1–O7) =
79.32(9)°).
e.g. loss of water or ligand have been observed partly at lower
temperatures. Both nitrate complexes 12 and 13, which
decompose exothermically at relatively low temperatures of
160 °C (12) and 119 °C (13), showed the lowest thermal
stabilities of all investigated compounds. Comparing them to
the other coordination compounds bearing the same metal
ion, but different anions, no general trend regarding their
decomposition temperatures could be observed.
The manganese(II) styphnate complex 33 interestingly
crystallizes isotypically to the zinc(II) picrate complex 32 with
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possible explanation for the observed endothermic peaks in
the DTA plots for the above-mentioned complexes.
Comparable phenomena have been reported in the
literature.41,42 The endothermic loss of two coordinating MTZ
molecules in the complexes 19 and 32 leads most likely to an
intermediate species, which decomposes at a later stage
exothermically (similar to compounds 20 and 33). Various
decomposition paths and mechanisms are possible. The
tendency of vaporization of the ligands and the relating bond
strength of the coordinating tetrazoles is strongly dependant
Table 1. Thermal stability measurements of 1–10 and 11–20 by DTA.
Tendo1. (°C)[a]
Tendo2. (°C)[a]
Texo. (°C)[b]
MTZ (1)
39
—
206
[Fe(MTZ)6](FeCl4)2 (2)
[MnCl2(MTZ)2] (3)
—
—
—
—
—
—
—
—
—
166
—
—
—
—
—
—
—
—
—
—
236
234
199
204
206
199
199
194
196
160
119
201
223
213
253
271
211
221
[CuCl2(MTZ)2] (4)
185
184
66
[ZnCl2(MTZ)2] (5)
[CoCl2(MTZ)4] • 2 MTZ (6)
[NiCl2(MTZ)4] • 2 MTZ (7)
[Co(MTZ)6](NO3)2 • 2 H2O (8)
[Ni(MTZ)6](NO3)2 • 2 H2O (9)
[Zn(MTZ)6](NO3)2 • 2 H2O (10)
[Cu(MTZ)6](NO3)2 (12)
[AgNO3(MTZ)2] (13)
106
—
on the metal (MnCl2
(
3
) vs. CuCl2
(4)/ ZnCl2 (5)), anion
86
50–75[c]
(Ni(NO3)2 ( vs Ni(ClO4)2 (
9
)
18)) and the corresponding
coordination environment of the complexes. For example, the
monodentate picrate complex 32 disintegrates in two stages
(loss of two ligands followed by exothermic event) whereas
the bidentate styphnate complex 36 decomposes in only one
single stage.
—
—
[Co(NH3)4(MTZ)2](ClO4)3 (14)
[Mn(MTZ)6](ClO4)2 (15)
[Fe(MTZ)6](ClO4)2 (16)
[Co(MTZ)6](ClO4)2 (17)
[Ni(MTZ)6](ClO4)2 (18)
[Cu(MTZ)6](ClO4)2 (19)
[Zn(MTZ)6](ClO4)2 (20)
—
—
—
—
—
178
196
Onset temperatures at a heating rate of 5 ° min−1 [a] endothermic peak, which
indicates melting, dehydration or loss of coordinating/cocrystallized MTZ
molecules; [b] exothermic peak, which indicates decomposition; [c] weight loss
detected by TGA.
Table 2. Thermal stability measurements of 21–27 and 29–37 by DTA.
Tendo1. (°C)[a]
Tendo2. (°C)[a]
Texo. (°C)[b]
[Mn(MTZ)6](ClO4)2 • 2 MTZ (21)
105
—
216
[Fe(MTZ)6](ClO4)2 • 2 MTZ (22)
[Co(MTZ)6](ClO4)2 • 2 MTZ (23)
[Ni(MTZ)6](ClO4)2 • 2 MTZ (24)
[Cu(MTZ)6](ClO4)2 • 2 MTZ (25)
[Zn(MTZ)6](ClO4)2 • 2 MTZ (26)
[Cu(CDNM)2(MTZ)4] (27)
111
107
93
—
—
215
210
207
206
212
201
248
211
226
212
202
200
227
231
214
—
78
—
108
—
61[c]
106
—
—
Figure 11 TGA measurements (5 ° min−1) of the selected compounds.
—
79[c]
[Co(H2O)4(MTZ)2](PA)2 • 2 H2O (29)
[Ni(H2O)4(MTZ)2](PA)2 (30)
[Cu(PA)2(MTZ)2] (31)
The thermal stability of the metal(II) MTZ perchlorate
complexes (Figure 12) increases in the following order: Cu2+
(211 °C) < Fe2+ (213 °C) < Zn2+ (221 °C) < Mn2+ (223 °C) < Co2+
(253 °C) < Ni2+ (271 °C). For previous compounds a similar
trend has been reported in literature.43 The cocrystal
—
—
[Zn(PA)2(MTZ)4] (32)
92
—
[Mn(HTNR)2(MTZ)4] (33)
141
—
[Co(H2O)4(MTZ)2](HTNR)2 • 2 H2O (34)
[Ni(H2O)4(MTZ)2](HTNR)2 • 2 H2O (35)
[Cu(HTNR)2(MTZ)2] (36)
70
86
—
—
80
105
—
perchlorate complexes 21
chlorides lose their cocrystallized MTZ molecules in the
range between 66–111 °C and are slightly less temperature
stable than their corresponding perchlorate compounds 15
–26 (Figure 13) as well as the
6/7
[Zn(HTNR)2(MTZ)2] (37)
—
Onset temperatures at a heating rate of 5 ° min−1 [a] endothermic peak, which
indicates melting, dehydration or loss of coordinating/cocrystallized MTZ
molecules; [b] exothermic peak, which indicates decomposition; [c] measured by
DSC.
–
20. The aqua ligand and crystal water containing complexes
can by dehydrated after heating demonstrated by an
endothermic peak in the DTA/DSC plots or by weight loss
during the TGA measurements.
Exceptional endothermic events during the DTA
measurements could be observed for the complexes (
19 20 32 33) and were further investigated by thermal
gravimetric analysis (Figure 11 and Figure S27). The free ligand
, which has a relatively low melting point of 39 °C, shows a
4, 5, 9,
,
,
,
1
continuous weight loss at elevated temperatures until its
exothermic decomposition temperature is reached. This fact
can be attributed to its high vapour pressure and could be a
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diaminotetrazole perchlorate complexes.44 Cocrystallization of
the nitrogen-rich ligand (21 26) leads in most cases to a
–
decrease in sensitivities toward mechanical stimuli in
comparison to the related perchlorate complexes (Figure S29)
while maintaining its highly energetic character and is
therefore a great concept for the desensitization of energetic
complexes (Figure 14).
Table 3. Sensitivities toward impact, friction and ESD of 1, 3–10, 12–27 and 29–37.
grain size
IS (J) [a]
FS (N) [b]
ESD (J) [c]
(μm)
1
> 40
> 360
1.50
< 100
Figure 12 DTA plots (5 ° min−1) of the perchlorate complexes 15–20.
3
> 40
> 40
> 40
> 40
> 40
> 40
> 40
> 40
> 40
40
9
> 360
> 360
> 360
> 360
> 360
> 360
324
1.50
1.30
1.50
1.00
0.80
1.50
1.50
0.40
1.50
0.15
0.036
0.08
0.10
0.50
0.10
0.08
0.25
0.46
0.40
0.13
0.20
0.20
0.60
1.50
1.00
1.50
0.30
0.25
0.20
0.23
0.15
1.50
0.80
100–500
500–1000
< 100
4
5
6
< 100
7
< 100
8
< 100
9
100–500
100–500
100–500
< 100
10
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
29
30
31
32
33
34
35
36
37
> 360
> 360
160
160
< 100
7
120
< 100
1
60
< 100
3
60
< 100
5
60
< 100
2.5
10
7
54
< 100
120
< 100
216
100–500
100–500
500–1000
< 100
Figure 13 DTA plots (5 ° min−1) of the perchlorate cocrystal compounds 21–26.
10
10
6
120
80
The sensitivities toward impact, friction and electrostatic
discharge for the compounds were determined according to
BAM standards. In addition, the compounds have been
classified in accordance to the “UN Recommendations on the
Transport of Dangerous Goods” using the measured values. An
overview of the sensitivities is given in Table 3. The
192
7
72
500–1000
100–500
100–500
100–500
100–500
500–1000
100–500
100–500
100–500
< 100
10
4
144
324
> 40
> 40
4
> 360
> 360
> 360
> 360
288
uncoordinated ligand
1 has sensitivities greater than 40 J,
15
15
25
25
2
greater than 360 N for friction and an ESD value of 1.50 J and
can therefore be classified as insensitive. The chloride
> 360
> 360
160
complexes
3
–
8
are insensitive toward impact, friction and ESD.
of oxidizing anions like nitrates,
Introduction
100–500
500–1000
cyanodinitromethanides, picrates or styphnates increases the
sensitivity against impact, friction and ESD under most
circumstances (impact: 2–40 J; friction: 160–360 N; ESD: 0.13–
1.50 J). All water-free complexes with trinitrobenzene-
derivatives as anions are more sensitive toward mechanical
stimuli compared with their crystal water or aqua ligand
4
240
[a] Impact sensitivity according to the BAM drophammer (method 1 of 6); [b]
friction sensitivity according to the BAM friction tester (method 1 of 6); [c]
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. According to the UN Recommendations on the
Transport of Dangerous Goods, (+) indicates not safe for transport.
containing analogs. The perchlorate complexes 14–26 are
sensitive toward impact (1–10 J) and vary from sensitive to
very sensitive against friction (54–216 N). The stability toward
impact of the [MII(MTZ)6](ClO4)2 complexes (Figure S28)
decreases in the order: Zn2+ (10 J) < Mn2+ (7 J) < Ni2+ (5 J) < Co2+
(3 J) < Cu2+ (2.5 J) < Fe2+ (1 J) and in the following order toward
friction: Zn2+ (120 N) ≈ Mn2+ (120 N) < Ni2+ (60 N) ≈ Co2+ (60 N)
≈ Fe2+ (60 N) < Cu2+ (54 N). Similar trends have been observed
in literature in correlation to energetic metal(II)
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device when it comes to groundwater contamination. The half
maximal effective concentration EC50 of these compounds was
determined after an incubation time of 30 min.. This
toxicological parameter is defined as concentration of the
investigated compound, at which the bioluminescence of the
bacteria strain is halved after a certain exposure time (e.g 15
or 30 min) and then compared to the original bioluminescence
of the sample before being treated. With EC50 (30 min) values
of 5.45 g/L for the free tetrazole
1 and 1.66 g/L for the iron(II)
perchlorate complex 16, both compounds can be considered
as non-toxic (toxicity level after 30 min incubation: very toxic <
0.10 g/L; toxic 0.10-1.00 g/L; non-toxic
>
1.00 g/L).46 As
expected, the copper(II) perchlorate compounds showed half
maximum effective concentrations of 0.13 g/L for 19 and
0.26 g/L for 25. Therefore, both complexes have to be
classified as toxic, which is not surprising, due to the known
toxicity of copper compounds toward microorganisms.47
Laser initiation
Chavez et al. and our group showed that iron(II) and copper(II)
perchlorate complexes of suitable ligands can be initiated in
laser ignition experiments.22,48 These compounds could be
potential candidates for laser ignition systems in the future.
About 15 mg of the carefully pestled complex to be
investigated were filled into a transparent plastic cap, pressed
with a pressure force of 1 kN and sealed by a UV-curing
adhesive. The laser initiation experiments were carried out
with a 45 W InGaAs laser diode operating in the single-pulsed
mode. The diode was connected to an optical fiber with a core
diameter of 400 μm and a cladding diameter of 480 μm. The
optical fiber was coupled via a SMA type connecter directly to
the laser and to a collimator. This collimator was linked to an
optical lens, which was positioned in its focal distance (f =
29.9 mm) to the sample. The lens was protected from the
explosive by a sapphire glass. Only complexes with sensitivities
in the range of PETN (IS: ≥ 2.5 J, FS: ≥ 54 N) were tested in laser
initiation experiments. These compounds show slightly
increased sensitivities toward mechanical stimuli but are still
safe to handle. The confined samples were irradiated at a
wavelength of 915 nm, a pulse length of 15 ms, a varying
current of 7–10 A and a voltage of 4 V, which combined results
in an approximately energy output of 26–60 mJ. The
Figure 14 Comparison of the impact and friction sensitivities of both the metal
perchlorates with and without cocrystallization of 1-methyl-5H-tetrazole (1).
Hot needle and hot plate tests of the investigated compounds
16, 19 and 25 showed deflagrations (Figure 15 and Figure S30–
33). The samples were fixed on an copper plate using adhesive
tape. Detonation of the compound typically indicates a
valuable primary explosive. The safe and practicable hot plate
test shows only the behavior of the unconfined sample toward
fast heating on a copper plate. It does not necessarily allow
any conclusions on a compound`s capability as a primary
explosive.
investigated compounds (14–20, 22, 24, 25, 27, 31, 36, 37)
showed different responses to the laser irradiation depending
on the used metal(II) and anion.
Figure 15 Moment of deflagration of compound 16 during the hot needle tests (left)
and hot plate test (right).
Toxicity
The toxicity of the uncoordinated free ligand 1-methyl-5H-
was determined together with the most
19 and 25 using the
tetrazole
(
1)
promising perchlorate complexes 16
,
known luminescence bacteria inhibition test in aqueous
media.45 The luminescent marine bacterium Vibrio fischeri
NRRL-B-11177, which is part of the commercially available
bioassay system LUMIStox, is a representative species for
other aquatic organisms and therefore a valuable indicating
10 | J. Name., 2012, 00, 1-3
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ARTICLE
Table 4
Results of the laser ignition tests.
7 A
8 A
9 A
10 A
14
–
–
deflag.
–
15
16
17
18
19
20
22
24
25
27
31
36
37
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
x
deflag.
x
dec.
–
–
–
–
–
deflag.
det.
x
det.
–
–
–
–
–
deflag.
deflag.
–
–
–
deflag.
deflag.
–
–
–
–
dec.
–
–
–
–
dec.
deflag.
x
(–: not tested, x: no ignition, dec.: decomposition, deflag.: deflagration, det.:
detonation). Operating parameters: current 7–10 A; voltage 4 V;
I
=
U =
theoretical maximal output power Pmax = 45 W; theoretical energy Emax = 26–
60 mJ; wavelength λ = 915 nm; pulse length τ = 15 ms.
Figure 16 Moment of deflagration of compound 22 shown as sequence.
The results of these tests are summarized in Table 4. No
UV-Vis spectroscopy
reaction was observed for the complexes 15
under the employed laser parameters. The other compounds
could be initiated showing decompositions for 27 31
deflagrations for 14 16 19 22 24 25 and 36 (Figure 16 and
, 17, 19 and 37
The solid state UV-Vis spectra for the complexes 14–27, 31 and
36 were collected in the wavelength range of 350–1000 nm in
order to get a better understanding of the laser ignition
process. Observed optical properties are summarized in Table
5. The step in the absorption intensity at 800 nm in the spectra
is caused by a detector change. The UV-Vis spectra exhibit only
qualitative character. Selected complexes of different metal
centers are exemplarily shown in Figure 18.
/
,
,
,
,
,
,
Figure S34–38) and detonation of compound 19 (Figure 17).
The complexes exhibit different laser initiation thresholds at
comparable sensitivities toward mechanical stimuli. The two
cocrystallized MTZ molecules in 25 cause a higher initiation
threshold in comparison to 19, which is expressed in a higher
required energy input. These results correlate with their
observed sensitivities and the conclusion of a higher stability
of the cocrystals. Consequently, copper(II) complex 19
represents
a
promising compound with manageable
sensitivities, which can be initiated easily by laser at low
initiation energies.
Figure 18 UV-Vis spectra in the solid state of selected complexes.
Figure 17 Moment of detonation of complex [Cu(MTZ)6](ClO4)2 (19).
The coordination compounds had absorptions in the UV,
visible and near-infrared region typical for the corresponding
transition metals. These transitions can be assigned to the
characteristic d-d-transitions and correlate with the
complementary colors. As expected, the d5 manganese(II) and
d10 zinc(II) complexes showed no absorptions in the measured
range and are illustrated together with the cocrystallized
perchlorate compounds in the Supporting Information (Figure
S39). The cocrystals diverged only minimally from their regular
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ARTICLE
Journal Name
analogues. In addition, the copper(II) cyanodinitromethanide
and trinitrobenzene derivative complexes 27 31 and 36 also
Conclusions
,
The nitrogen-rich heterocycle 1-methyl-5H-tetrazole (1, MTZ)
have strong absorptions ranging from the UV to the visible
region of the electromagnetic spectrum (Figure S40). These
observations indicated the expected presence of additional
electronic transitions in these compounds caused by the
coordinating anions (n–π*: 358 (27); 380 (31); 404 (36) nm).
was synthesized in a one-step synthesis by methylation of
1,5H-tetrazole. The resulting isomeric mixture was purified by
distillation and yielded the desired product in a yield of 48 %.
In order to develop new laser-ignitable and environmentally
benign primary explosives, diverse energetic coordination
compounds were synthesized and characterized. Present
energetic transition metal complexes are built up by the
nitrogen-rich ligand 1-methyl-5-tetrazole, a transition metal
(Mn, Fe, Co, Ni, Cu, Zn and Ag) and oxidizing anions such as
perchlorate, nitrate, cyanodinitromethanide, picrate and
styphnate. The synthetic route to these complexes is simple,
low-cost and fulfills environmental requirements. 31 new
coordination compounds, based on MTZ as the monodentate
ligand, were prepared and comprehensively characterized.
Thermal analysis by DTA showed thermal stabilities with
exothermic decomposition temperatures higher or equal to
200 °C for most of the complexes (except 12 and 13).
Extraordinary endothermic events such as loss of MTZ and
water during the DTA measurements were further investigated
by thermal gravimetric analysis. Interesting results in terms of
the decomposition pathways of selected compounds were
achieved. In addition, X-ray crystal structures were determined
for 28 complexes to get better insight into the interesting and
challenging complex units showing (except for 13) octahedral
coordination spheres. To explore the structural diversity and to
tailor the optical and energetic properties of these complexes,
seven different metals and seven different ligands/counter-
anions (Cl, NH3, NO3, ClO4, CDNM, PA, HTNR) were used.
Table 5
Optical properties of 14-27, 31 and 36.
[a]
[b]
M
CoIII (ls)
MnII
FeII (hs)
CoII
color
orange
colorless
colorless
orange
purple
blue
λd-d
λ915/λd-d
0.14
–
14
463
–
15
16
17
18
19
20
21
22
23
24
25
26
27
31
36
887
0.99
0.64
0.75
0.46
–
464, 960
536, 883
663
NiII
CuII
ZnII
colorless
colorless
colorless
orange
purple
blue
–
MnII
FeII (hs)
CoII
–
–
883
0.99
0.59
0.84
0.42
–
467, 961
547, 904
676
NiII
CuII
ZnII
colorless
blue
–
CuII
358[c], 612
380[c], 630
404[c], 657
0.53
0.74
0.69
CuII
green
CuII
green
[a] absorption intensity maximum wavelength, which can be assigned to electron
d-d excitations in the measured range of 350–1000 nm; [b] quotient of the
absorption intensity at the laser wavelength and the intensity at the d-d
absorption wavelength; [c] n–π* transitions.
The laser initiation mechanism is still not fully understood.49
Many different reaction pathways (e.g. electronically or
thermally) are imaginable. Simply regarding the absorption of
the complexes at the laser wavelength of 915 nm as shown in
Figure 18 and Table 5, one might derive a direct correlation to
the performed laser ignition. In contrast to that, the examined
Preparation of the metal perchlorate complexes 15
strongly dependent on the stoichiometry of the ligand and
yielded complex units without and with (21 26) two co-
–20 was
–
crystallized tetrazoles. Cocrystallization of the endothermic
MTZ molecule leads in most cases to a decrease in sensitivities
toward mechanical stimuli while maintaining its highly
energetic character and is therefore an excellent concept for
the desensitization of energetic coordination compounds.
Co(II) compound 17
,
which clearly absorbs in the
corresponding region, showed no reaction after exposure to
laser irradiation. For MTZ complexes of iron(II) it is well known
that the structure in the crystal as well as the counterion (e.g.
Environmentally benign iron and copper complexes 16, 19 and
−
−
−
ClO4 , BF4 , PF6− or CF3SO3 ) can have an profound effect on for
example the spin crossover properties.50 It is possible that not
only the absorption properties but also subtle changes in the
geometry or crystal packing influence the susceptibility of the
complexes to laser ignition. The underlying initiation processes
are likely to depend on multiple different parameters. Based
on the presented results, a general tendency has been
observed, that the examined compounds are more likely to
deflagrate/detonate by applying laser irradiation if they exhibit
high mechanical sensitivities. Due to the fact that the ligands´
influence on the laser initiation process, as well as the process
itself has not been clarified yet, future investigation will be
necessary to fully understand the laser initiation of energetic
materials.
25 were also evaluated for use as primary explosives by hot
needle tests, hot plate tests and toxicity assessments. Laser
ignition experiments, which were carried out at a wavelength
of 915 nm and a pulse length of 15 ms showed detonation of
compound 16 (IS: 2.5 J, FS: 54 N, Texo: 211 °C). The successful
initiation makes 16 to a promising candidate for future laser
initiation systems.
Conflicts of interest
“There are no conflicts to declare”.
Acknowledgements
Financial support of this work by the Ludwig-Maximilian
University of Munich (LMU. The authors want to thank Mr.
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Journal of Materials Chemistry A
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DOI: 10.1039/C7TA07780G
Graphical Abstract
An extensive study on 31 new energetic coordination complexes was carried out using 1-methyl-5H-
tetrazole as the nitrogen-rich ligand. Energetic coordination compounds show potential use in
mechanical or optical initiation systems. Their energetic character can be adjusted by using different
metal centers (MnII, FeII, CoII, NiII, CuII, ZnII, AgI) and counter anions (e.g. NO3, ClO4, picrate,
styphnate). A consistent series of cocrystallized complexes was achieved by using an excess of the
ligand and compared to the classical complexes.