2-Methyl-substituted monotetrazoles in copper(ii) perchlorate complexes: manipulating coordination chemistry and derived energetic properties

Article abstract of DOI:10.1039/C8NJ05375H

A proposed correlation between coordination chemistry and deduced energetic properties (thermal behaviour, and sensitivities towards mechanical and optical stimuli) of copper(ii) complexes is investigated. Starting from a system comprising Cu(ClO₄)₂ and either of the ligands 2-methyl-5-aminotetrazole (1, 2-MAT) or 2-methyl-5H-tetrazole (2, 2-MTZ), typically altered parameters like the metal(ii) centre, ligand, or counterion were predefined. Instead, solely slight changes in ligand concentration and the solvent system were implemented in order to provide an insight into structure-property relationships of energetic coordination compounds (ECC) of this type. As a result, five highly energetic complexes [Cu(H₂O)₂(2-MAT)₄](ClO₄)₂·H₂O (3), [Cu(H₂O)₂(2-MAT)₄](ClO₄)₂ (4), [Cu(H₂O)₂(2-MAT)₄](ClO₄)₂·2 2-MAT (5), [Cu(ClO₄)₂(H₂O)₂(2-MAT)₂] (6), and [Cu(H₂O)₂(2-MTZ)₄](ClO₄)₂ (7) were synthesized and, except for 5, elaborately characterized. Besides structural elucidation via X-ray diffraction, NIR-spectroscopy, differential thermal analysis (DTA), standard sensitivity measurements (impact, friction, and electrostatic discharge), UV/vis-spectroscopy, and optical initiation experiments were conducted to deduce a precise relationship between coordination chemistry and the consequential energetic characteristics of these complexes.

Full text of DOI:10.1039/C8NJ05375H

NJC
View Article Online
View Journal
PAPER
2-Methyl-substituted monotetrazoles in copper(II)
perchlorate complexes: manipulating coordination
Cite this:DOI: 10.1039/c8nj05375h
chemistry and derived energetic properties†‡
Lukas Zeisel, Norbert Szimhardt,
Maximilian H. H. Wurzenberger,
Thomas M. Klapotke and Jorg Stierstorfer
*
¨
¨
A proposed correlation between coordination chemistry and deduced energetic properties (thermal
behaviour, and sensitivities towards mechanical and optical stimuli) of copper(II) complexes is
4 2
investigated. Starting from a system comprising Cu(ClO ) and either of the ligands 2-methyl-5-
aminotetrazole (1, 2-MAT) or 2-methyl-5H-tetrazole (2, 2-MTZ), typically altered parameters like the
metal(II) centre, ligand, or counterion were predefined. Instead, solely slight changes in ligand
concentration and the solvent system were implemented in order to provide an insight into structure–
property relationships of energetic coordination compounds (ECC) of this type. As a result, five highly
energetic complexes [Cu(H
2
O)
2
(2-MAT)
4
](ClO
4
)
2
ÁH
2
O (3), [Cu(H
2
O)
2
(2-MAT)
4
](ClO
4
)
2
(4), [Cu(H
2 2
O) -
(
2-MAT) ](ClO
4
4
2
) Á2 2-MAT (5), [Cu(ClO
4
)
2
(H O)
2
2
(2-MAT)
2
] (6), and [Cu(H
2
O) (2-MTZ)
2
4
](ClO (7) were
4 2
)
Received 23rd October 2018,
Accepted 26th November 2018
synthesized and, except for 5, elaborately characterized. Besides structural elucidation via X-ray
diffraction, NIR-spectroscopy, differential thermal analysis (DTA), standard sensitivity measurements (impact,
friction, and electrostatic discharge), UV/vis-spectroscopy, and optical initiation experiments were conducted
to deduce a precise relationship between coordination chemistry and the consequential energetic
characteristics of these complexes.
DOI: 10.1039/c8nj05375h
rsc.li/njc
7
8
In recent years, seminal reports from Shreeve, Myers, and
Klap o¨ tke set the stage for future applications of ECC.
Introduction
9
Dominated by the oxidation states +I and +II, copper exhibits In targeting novel transition metal complexes with auspicious
9
II
diverse coordination chemistry. Especially the d Cu cation is properties (convenient accessibility, thermal stability, and safe
commonly applied in many areas of chemistry as the metal initiation), a consistently implemented general approach has
1
2,3
4
center of coloring agents, insecticides,
catalysts, and emerged: starting from a desired transition metal cation with
5
metalloproteins. In most of these cases, copper(II)’s coordination the corresponding counter-anion, energetic ligands, as well as
chemistry is influenced by oxygen- and nitrogen-donating ligands, the applied reaction conditions, offer precise tailoring of the
typically forming square planar, tetrahedral and, predominantly, desired characteristics. Whereas variation of parameters like
7
–9
3
octahedral coordination environments.
the ligand, metal centre, and anion tend to distinctly influence
In recent years, the growing field of energetic coordination the coordination chemistry and the complexes’ properties,
compounds (ECC) has greatly benefited from promising additional fine tuning towards desired attributes might be
II
Cu complexes. Due to the urgent necessity of substituting achieved by accurately controlling the applied reaction
commonly applied, toxic primary explosives like lead azide and conditions. While initial experiments in this direction have
lead styphnate, an approach using significantly less toxic already led to sought results, a comprehensive report is still
6
9b–12
transition metal complexes appeared to be highly encouraging.
anticipated until today.
In regard to previously applied ligand systems, mostly
7
–16
nitrogen-rich, endothermic heterocycles were considered.
Among others, mono-substituted tetrazoles like the highly
Department of Chemistry, Ludwig Maximilian University Munich, Butenandtstrasse 5–13,
Haus D, D-81377 Munich, Germany. E-mail: jstch@cup.uni-muenchen.de
†
‡
Dedicated to Prof. Dr Ingo-Peter Lorenz on the occasion of his 75th birthday.
Electronic supplementary information (ESI) available: Experimental and
energetic 1,5-diaminotetrazole (1,5-DAT), 1-amino-5H-tetrazole
(1-AT) and 2-amino-5H-tetrazole (2-AT) as well as 1-methyl-5H-
general methods, X-ray diffraction, IR spectroscopy, DTA plots, TGA plots of 1–4,
hot plate and hot needle tests, and laser ignition tests. CCDC 1850124 (3), 1850121
tetrazole (1-MTZ) and 1-methyl-5-aminotetrazole (1-MAT) were
10,13,14
investigated as ligands in ECC (Chart 1).
Hereby, the obtained
(4), 1850122 (5), 1850120 (6) 1850123 (7). For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c8nj05375h
transition metal complexes proved to be highly energetic, readily
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2018
New J. Chem.

View Article Online
Paper
NJC
Chart 1 Precedents and current work using energetic monotetrazoles in
ECC.
Scheme 1 Synthesis of complexes 3–6 using 2-methyl-5-amino-
tetrazole (1).
synthesized and partially exhibited auspicious properties as laser-
10,13
ignitable primary explosives.
Based on these results, the present work aims for a deeper frequently observed hexacoordinate copper(II) center.10,21
insight into the possibility of precisely manipulating an ECC’s In contrast to that and regardless of the applied ligand concen-
coordination chemistry and the resulting energetic properties tration, the present reaction provided [Cu(H O) (2-MAT) ]-
2
2
4
(
sensitivities, thermal properties, and laser initiation) by solely (ClO ) ÁH O (3) in 52% yield after four days of crystallization.
4
2
2
modifying the reaction conditions (ligand concentration, Proceeding from these preliminary results, the utilization of
solvent, and additives) in hand. Predefinitions for the applied a solvent with decreased polarity was tested. Using ethanol as
systems included copper(II) as metal species, perchlorate counter- the reaction medium and 4 equiv. of the ligand indeed led
anions and, as an extension to previously published reports, to the isolation of a new complex 4. Nevertheless and despite
0
(g)
À1 17
2
-methyl-5-aminotetrazole (1, 2-MAT, D H = 2802 kJ kg
)
successfully preventing the incorporation of cocrystallized
f
0
and 2-methyl-5H-tetrazole (2, 2-MTZ, D H
3
(2-MTZ)
=
water, [Cu(H O) (2-MAT) ](ClO ) (4) simultaneously retained
f
(g)
2
2
4
4 2
À1 17
347 kJ kg
)
as endothermic ligands. Both nitrogen-rich the actual coordination sphere with two coordinating aqua
heterocycles are expected to allow comparison to the respective ligands. To further extend this condition screening, an
-substituted tetrazoles and their energetic metal complexes. increased 2-MAT concentration (8 equivalents) was introduced
1
While recently published results from Szimhardt et al. provide while maintaining ethanol as the solvent. As a result and
a comprehensive study of 1-MTZ and its transition metal immediately after combining the ligand and copper(II) perchlorate,
1
0
complexes, only a few examples of 1-MAT ECC have been a light green precipitate occurred. After filtration, preliminary
reported alongside a range of corresponding tetrazolium characterization of the residue by IR spectroscopy confirmed the
1
8–20
salts.
Moreover, and in relation to related, tremendously formation of a previously unknown complex [Cu(H O) (2-MAT) ]-
2 2 4
1
3
sensitive 2-aminotetrazole complexes, 2-MAT and 2-MTZ (ClO ) Á2 2-MAT (5) alongside ECC 4. Since conducted attempts
4
2
incorporating ECC are proposed to combine efficient optical of clean isolation or purification of 5 proved to be unsuccessful,
initiation with enhanced stability towards mechanical stimuli, further characterization was impeded. Interestingly, the earlier
thus allowing safe handling of these materials.
collected filtrate of the present reaction once again provided 4
after crystallization for six days. A final attempt of interfering
with the previously observed coordination chemistry of this
system was to apply concentrated perchloric acid. According
to reported syntheses, this approach promised a water-free
Results and discussion
Synthesis
Initially, nitrogen-rich ligand 1 was synthesized via a convenient, ECC with enhanced energetic characteristics by substituting
2
2
single step reaction using commercially available 5-amino- aqua ligands with coordinating perchlorate counter-anions.
tetrazole as a starting material (see Scheme S1, ESI‡). Due to the Unfortunately, charging the aqueous reaction mixture with
tautomerization of 1,5H-tetrazole, the dimethylsulfate-mediated 30 equivalents of HClO led to complex 6, which still exhibited
4
methylation step provided the desired ligand alongside 1-methyl- coordinating aqua ligands (yield: 76%). In contrast to previously
5
-aminotetrazole in a ratio of 3 : 4. With the targeted 2-MAT in obtained ECC, the coordination sphere of [Cu(ClO
hand, initial investigations concerning its coordination chemistry (2-MAT) ] indeed comprises two copper(II) associated perchlorato
were accessed (Scheme 1). anions while solely exhibiting two equivalents of the nitrogen-
4 2 2 2
) (H O) -
2
The performed reactions were carried out at ambient rich ligand 1. A possible and reoccurring explanation for a
temperature by slowly adding pre-dissolved ligand to a solution decrease of coordinating tetrazole ligands might be the proto-
of copper(II) perchlorate. By solely applying a minimal amount nation and simultaneous elimination of an N-coordination
9f,12
of solvent, the crystallization process at room temperature site.
In comparison to that, the corresponding 1-methyl-5-
could be accelerated. Thereby received solids were filtered off, aminotetrazole exhibited low solubility in water, thus leading to
1
9
washed with cold ethanol and dried in air, not requiring further the reoccurring precipitation of the ligand. Moreover, and
purification. whenever ligand crystallization could be prevented, collected
As a starting point, 6 and 4 equivalents of 2-MAT were X-ray data of isolated crystals repeatedly proved the simultaneous
19
4 2
applied to an aqueous solution of Cu(ClO ) aiming for a presence of multiple ECC species.
New J. Chem.
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2018

View Article Online
NJC
Paper
Scheme 2 Formation of the copper(II) perchlorate complex 7.
Conclusively, 2-methyl-5H-tetrazole (2-MTZ) was investi-
gated as a comparative example for additional 2-substituted
monotetrazoles (Scheme 2). The ligand itself was accessed via a Fig. 1 Calculated (B3LYP/6-31G(d,p)) electrostatic potentials (3D isosur-
face of electron density) of 2-MAT (1, left) and 2-MTZ (2, right).
straightforward two-step reaction starting from basic building
blocks (see Scheme S2, ESI‡). Once again, tautomerization
led to a significant decrease in the yield of 2-MTZ due to the
formation of isomeric 1-methyl-5H-tetrazole (1-MTZ) (1-MTZ/
The first of the obtained complexes, [Cu(H O) (2-MAT) ](ClO ) ÁH O
2
2
4
4 2
2
(
3), crystallizes in the form of blue blocks in the monoclinic space
2-MTZ = 2 : 1).
group C2/c with 16 formula units per unit cell and a calculated
When being applied to an aqueous solution of copper(II)
À3
density of 1.669 g cm at 173 K. The Jahn–Teller distorted,
perchlorate, 2-MTZ exhibits remarkable similarity to the
previously investigated 2-MAT. In comparison to complexes 3
and 4, [Cu(H O) (2-MTZ) ](ClO ) (7), which was isolated from
octahedral copper(II) coordination sphere comprises two axial aqua
and four equatorial 2-MAT ligands (Fig. 2) forming a slightly tilted
O–Cu–O axis and a marginally distorted plane, respectively.
In addition to these observations in regard to the structure
of a single complex unit, the three-dimensional topology of this
compound reveals interesting stacking patterns (Fig. 3).
When viewing along the crystallographic b-axis, a chain
pattern with alternating components can be observed. In addition
to this ABAB pattern, whose ECC-layers are aligned along the
crystallographic c-axis, equally oriented planes of perchlorate
anions and cocrystallized water function as H-bond-mediating
linkers between adjacent complex layers (Table 1). The aggregation
of complexes to said layers equally benefits from intermolecular
hydrogen bonding. In addition to that, viewing along the c-axis
reveals a similar, but even more complex picture. While exhibiting
the same ABAB pattern in between distinguishable chains, B lines
further reveal an intrinsic CDCD motif along b.
2
2
4
4 2
the mother liquor after one day (yield: 73%), features almost
equivalent coordination chemistry under identical reaction
conditions. Moreover, and in contrast to the 2-MAT system, a
change of solvent (ethanol) or stoichiometry (x = 4, 6, and 10)
exclusively led to the same product 7. Whenever the metal(II)
2
+
2+
2+
center was altered (Fe , Zn , and Mn ) no crystallization of a
potential ECC was achieved. Interestingly, and in accordance
with previous reports, 2-substituted tetrazoles as ligands appear
1
8b,21
to consistently promote the incorporation of aqua ligands.
Reasons for this could be the steric hindrance of six tetrazoles
coordinating to the transition metal or the thermodynamically
and kinetically preferred formation of more inert complexes.
However, further studies including thermodynamic calcula-
tions are needed for the future.
Crystals of 4 have been isolated in the form of blue blocks in
Crystal structures
the monoclinic space group P2 with two formula units per unit
1
Since each of the obtained complexes was obtained as single
crystals, structural characterization was conducted using low
temperature X-ray diffraction. Measurement and refinement
data are provided in the ESI.‡ Additionally, details of the
obtained crystal structures were deposited in the CSD database
and can be obtained free of charge from the CCDC: 1850124 (3),
2
3
1850121 (4), 1850122 (5), 1850120 (6), and 1850123 (7).‡
In terms of the observed coordination geometry, the examined
copper(II) centres provided Jahn–Teller distorted octahedra,
which consistently exhibited N4-coordinating ligands. Consid-
ering calculated electrostatic potentials at the respective
ligand’s binding sites (Fig. 1), the highest electron density of
both 2-MAT and 2-MTZ occurs at the tetrazole’s carbon–
adjacent nitrogen N4, thus explaining the observed coordina-
tion behaviour observed by X-ray diffraction and additionally
conducted IR spectroscopy (see ESI,‡ Fig. S1).
Fig. 2 Molecular unit of complex 3. Thermal ellipsoids of non-hydrogen
atoms in all structures are set to the 50% probability level. Selected bond
lengths (Å): O1–Cu1 2.430(6), O2–Cu1 2.363(5), Cu1–N14 2.014(5), Cu1–
N19 2.004(4), Cu1–N4 2.003(4), and Cu1–N9 2.008(5); selected bond
angles (1): O1–Cu1–O2 177.2(2), N4–Cu1–O1 86.95(19), N4–Cu1–O2
Ahead of the comprehensive analysis of received crystal
structures it should be stated that structural data of coordinating
2-MAT and 2-MTZ consistently resemble corresponding, literature-
known X-ray data of the respective non-coordinating ligand or
comparable ECC and are therefore not discussed any further.
9
3.91(18), N4–Cu1–N9 91.07(19), N4–Cu1–N14 88.89(19), N14–Cu1–O1
24,25
92.9(2), and N9–Cu1–O1 87.2(2).
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2018
New J. Chem.

View Article Online
Paper
NJC
to be observed along an axis that connects the copper(II) center
with the two aqua ligands in axial positions. Considering these
first observations it becomes clear that the structural parameters
of 4 are almost identical to those of the previously discussed
compound 3 with four 2-MAT ligands in the plane. The only
significant differences such as the space group and the cell volume
are caused by the absence of crystal water molecules. Another
slight alteration can be observed regarding the bond angles
between the coordinating ligands and copper(II), especially among
the aqua and 2-MAT ligands. In the case of 4, the bond angles
approach the 901 of an ideal octahedron.
Crystallizing in the space group P1%, ECC 5 comprises two
formula units per unit cell and has a calculated density of
À3
1
.699 g cm at 173 K. The observed octahedral coordination
sphere makes up the molecular unit including four 2-MAT and
two aqua ligands. In addition, two equivalents of ligand 1 have
been found as cocrystallized species (Fig. 5). Due to the
reoccurring Jahn–Teller effect, the bond lengths along the tilted
distortion axis O1–Cu1–O2 are significantly longer than those
of the metal and the equatorial ligands. Since the equatorial
Fig. 3 Structure of 3 with the direction of view along the crystallographic
b- (top) and c-axis (bottom).
2-MAT-organized plane is not situated perpendicular on the
O1–Cu1 and O2–Cu1 axes, a noticeable deviation from ideal
flatness is observed.
Table 1 Lengths and angles of selected hydrogen bonds in 3
As the final representative of the 2-MAT derived complexes,
crystallizes in the form of blue rods in the monoclinic
D–HÁ Á ÁA
d(D–H) [Å] d(HÁ Á ÁA) [Å] d(DÁ Á ÁA) [Å] +D–HÁ Á ÁA [1]
6
N5–H5Á Á ÁN16
0.88
0.83
2.27
1.89
2.57
2.17
3.03
2.71
2.96
3.02
144.0
173.6
107.0
163.0
space group P2
/n with two formula units per unit cell and a
1
O3–H31Á Á ÁO6
À3
remarkably high calculated density of 1.997 g cm at 173 K.
Two asymmetric unit cells make up the molecular unit.
The latter consists of a copper(II) center, and two aqua, two
perchlorato, and two 2-MAT ligands forming a Jahn–Teller
N15–H15BÁ Á ÁO9 0.88
N10–H10AÁ Á ÁO9 0.88
À3
cell and a calculated density of 1.788 g cm at 173 K. The distorted, octahedral coordination sphere (Fig. 6).
molecular unit consists of four 2-MAT and two aqua ligands The resulting polyhedron is made up of three straight axes
once again resulting in an octahedral coordination sphere (Fig. 4). which are caused by the fact that the two ligands of each kind
The corresponding perchlorate anions are non-coordinating. coordinate trans to one another. The remaining four ligands are
Regarding the appearance of the coordination polyhedron it
becomes obvious that there is yet again a Jahn–Teller distortion
Fig. 5 Molecular unit of [Cu(H
2
O)
2
(2-MAT)
4
](ClO
)
4 2
Á2 2-MAT (5). Selected
Fig. 4 Molecular unit of [Cu(H
2
O)
2
(2-MAT)
4
](ClO
4
)
2
(4). Selected bond
bond lengths (Å): O1–Cu1 2.385(2), O2–Cu1 2.501(2), N1–Cu1 2.006(2),
N7–Cu1 2.004(2), N11–Cu1 2.020(2), and N16–Cu1 2.000(2); selected
bond angles (1): O1–Cu1–O2 173.44(7), N1–Cu1–O1 93.38(8), N1–Cu1–O2
92.11(8), N7–Cu1–O2 85.43(8), N16–Cu1–O2 85.14(8), N7–Cu1–N16
88.68(9), N11–Cu1–N16 86.82(9), N1–Cu1–N16 176.71(9), and N7–Cu1–N11
175.13(9); symmetry codes: (i) 1 À x, 1 À y, 1 À z, and (ii) Àx, 2 À y, 1 À z.
lengths (Å): O1–Cu1 2.417(5), O2–Cu1 2.397(5), N1–Cu1 1.999(5), N6–
Cu1 2.049(6), N11–Cu1 2.000(5), and N16–Cu1 2.065(6); selected bond
angles (1): O1–Cu1–O2 179.34(19), N1–Cu1–O1 87.6(2), N1–Cu1–O2
9
9
1.9(2), N6–Cu1–O1 88.2(2), N6–Cu1–O2 92.2(2), N16–Cu1–O1
0.85(19), and N16–Cu1–O2 88.8(2).
New J. Chem.
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2018

View Article Online
NJC
Paper
Thereby conducted differential thermal analysis (DTA, heating
À1
rate: b = 5 1C min , see Table 2, Fig. S5, ESI‡) provided a
distinct differentiation between uncoordinated ligands 1 and 2
and the obtained ECC. While the nitrogen-rich ligands 2-MAT
(1) and 2-MTZ (2) both exhibit high thermal stability in the
measured range of 25–400 1C, the corresponding copper(II)
complexes each show exothermic decomposition temperatures
from 171 1C (4) to 240 1C (7). Endothermic events observed for
both ligands are assumed to be caused either by melting (Tendo1
(1)) or evaporation of the compound (Tendo2 (1), Tendo1 (2), see
corresponding TGA plots, Fig. S6, ESI‡). Whereas the loss
of cocrystallized water in the DTA measurement of 4 solely
Fig. 6 Molecular unit of [Cu(ClO
)
4 2
(H
2
O)
2
(2-MAT)
2
] (6). Selected bond marginally influences the determined exothermic decomposition
lengths (Å): Cu1–O5 2.443(3), Cu1–N4 1.998(3), and Cu1–O1 1.960(3);
temperature, it seems possible that in 3 a slow decomposition
starts before the onset temperature of 176 1C. In the case of the
structurally analogous complexes 3, 4, and 7, endothermic events
which are most likely caused by evaporation of cocrystallized or
coordinated water were detected in an expectedly narrow range
i
selected bond angles (1): O1–Cu1–O5 96.32(10), O1–Cu1–O5 83.69(10),
i
i
O1–Cu1–N4 90.59(12), O1–Cu1–N4 89.41(12), and O5–Cu1–O5 180.00;
symmetry code: (i) 2 À x, Ày, Àz.
located in an ideal plane with bond lengths between the (125 1C (3), 110 1C (4), and 115 1C (7)). To gain a better under-
metal(II) center and the respective ligand being expectedly standing of this partial ECC decomposition prior to the actual
i
shorter than along the O5–Cu1–O5 axis. Another significant exothermic event, thermal gravimetric analysis was implemented
2 2 4 4 2
deviation from an ideal octahedron can be observed regarding for ECC 3, 4 (see ESI,‡ Fig. S6), and [Cu(H O) (2-MTZ) ](ClO ) (7)
the bond angles between axial ligands and coordinating mole- (Fig. 8).
cules in the equatorial plane. Lastly, the molecular structure
of 7 was investigated. In analogy to complex 6, [Cu(H O)
2-MTZ) ](ClO
P2
Whereas TGA plots of 3 and 4 (see ESI,‡ Fig. S6) show
an almost continuous loss of the compounds’ mass between
) crystallizes in the monoclinic space group 100 and 250 1C and no sharp mass decrease at the decomposi-
4 2
/n (Z = 2), while exhibiting a significantly lower calculated tion temperature, the corresponding plot of 7 exhibits distinct
2
2
-
(
4
1
À3
density at 173 K (1.702 g cm ). As stated beforehand, the disintegration events. Affirmatively, the observed mass loss
obtained crystal structure displays a remarkable resemblance starting below 100 1C (5.67%) can be assigned to the loss of
to the molecular structures of 3 and 4 (Fig. 7). Nevertheless, present aqua ligands and refers to the first endothermic event
copper(II)–aqua distances along the Jahn–Teller distorted (115 1C) during the DTA measurement. Moreover, Tendo2 (DTA:
i
O1–Cu–O1 axis proved to be distinguishably shorter. Despite 208 1C) was confirmed by a matching 26.5% mass loss during
this deviation, measured torsion angles consistently exhibit
values close to those of an ideal octahedron.
a
Table 2 Thermal stability measurements of 1–4, 6, and 7 by DTA
b
b
c
Thermal properties and sensitivities
Compound
T
endo1 [1C]
T
endo2 [1C]
Texo [1C]
Following the structural characterization of the received
complexes, energetic properties in terms of thermal stability
and sensitivities towards mechanical stimuli were investigated.
1
2
3
4
6
7
104
152
125
110
—
252
—
—
—
—
—
—
176
171
192
240
115
208
a
À1
b
Onset temperatures at a heating rate of b = 5 1C min
.
Endothermic
peak, which indicates melting, dehydration, or loss of coordinating or
c
cocrystallized molecules. Exothermic peak, indicating decomposition.
2 2 4 4 2
Fig. 7 Molecular unit of [Cu(H O) (2-MTZ) ](ClO ) (7). Selected bond
lengths (Å): Cu1–O1 2.300(3), Cu1–N4 2.023(2), and Cu1–N8 2.029(2);
selected bond angles (1): O1–Cu1–N4 93.15(11), O1–Cu1–N8 88.31(11),
i
i
O1–Cu1–O1 180.00, N4–Cu1–N8 88.11(9), O1 –Cu1–N8 91.69(11), and
i
À1
N4–Cu1–N4 180.00; symmetry code: (i) 1 À x, Ày, 2 À z.
Fig. 8 TGA measurements (5 1C min ) of 6 and 7.
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2018
New J. Chem.

View Article Online
Paper
NJC
the TGA measurement, a fact that presumably is effected by to 0 represents the overall most sensitive compound in terms
ligand evaporation. The final step of decomposition is identi- of impact and friction and can be compared to the analogous
18b
fied by an exothermic event at the remarkably high temperature chlorate complex. ECC 4, which, in comparison to structurally
of 240 1C. Lastly, complex 6, which exhibited two copper(II)– equivalent 7, incorporates the less energetic ligand, displays a
perchlorato bonds, proved to similarly disintegrate at high, but higher nitrogen content and exhibits the highest sensitivities
slightly lower temperatures without indicating an event of towards impact and electrostatic discharge. Conducted hot plate
endothermic decomposition (Table 2 and Fig. 8). In order to and hot needle tests for each of the characterized ECC showed
explain this rather unusual observation, one might consult the sharp deflagration events (see ESI,‡ Fig. S9–S14).
complex’s coordination chemistry: contrary to complexes 3, 4,
Optical initiation and UV-vis spectroscopy
5, and 7, coordinating aqua ligands are in the plane and
exhibit significantly shortened (E0.4 Å) copper–aqua bond In addition to the well-established initiation of energetic
lengths which presumably lead to aggravated dissociation. compounds via mechanical and thermal stimuli, recent devel-
Additionally, the sharp mass loss observed during TGA opments in this field dealt with the neglected sensitivities
1
0,21,31
measurements further proves ECC 6 remains in its distinct of these species towards optical initiation.
Until today,
composition until being exposed to its decomposition tempera- a broad range (Mn, Fe, Co, Ni, Cu, and Zn) of 3d transition
ture (Fig. 8). Resulting from the observed thermal properties, the metal complexes and energetic materials have been success-
incorporation of cocrystallized water and the introduction of fully initiated via laser irradiation while an unambiguous
coordinating perchlorato anions cause a measurable increase in mechanistic proposal is still awaited. On one hand, common
1
0,22
thermal stability.
suggestions discuss a photochemical initiation mechanism
Regarding the investigated compounds’ sensitivities towards through an excitation–relaxation process during which
mechanical stimuli, 1 and 2 both can be classified as insensitive photonic energy is translated to the decomposition-initiating
9c,32
according to UN standards (impact: 440 J, friction: 4360 N). vibrational energy.
Additionally, a reoccurring proposal
In contrast to that, introducing the highly oxidizing perchlorate deals with a photothermal mechanism, during which photonic
anion and a coordination site like copper(II) once more proves energy is directly converted into lattice heat and vibrational
the potential of the ECC approach due to the formation of energy, which according to the ‘‘hot-spot’’-theory further leads to
10,32b
distinctly more sensitive compounds (Table 3). Considering decomposition of the material.
To investigate the optical
the series of 2-MAT based copper(II) complexes, one might sensitivities of 3–7 and a possible coherence with either of the
recognize the tendency that with decreasing content of water, proposed initiation mechanisms, laser initiation experiments
more specifically the loss of the cocrystallized species, the were conducted along with UV-vis spectroscopy (Table 4 and
10
resulting complexes show increased sensitivities (3 vs. 4 and 6).
Fig. 9). The distinction between deflagration and detonation
Unfortunately, proceeding from this initial proposition, was made by means of acoustic output and high-speed recordings
ECC 4, 6, and 7 each exhibit sensitivities at the lower end of (Fig. S7, ESI‡).
the measuring range. As a result, a quantitative evaluation of
While each of the investigated ECC showed a reaction
sought structure–property relationships most likely is pre- resulting from the applied conditions, solely perchlorato ligand
vented by possibly varying fractal characteristics and particle incorporating complex 6, as well as the 2-MTZ complex 7,
2
6
sizes. Moreover, the correlation between sensitivities and detonated in the course of the experiment.
molecular structure is proposed to be dependent on multiple
While 3 exhibited sharp deflagration upon laser irradiation,
4 solely decomposed without any sign of an exothermic
molecular electrostatic potentials, and dissociation energies reaction. Resulting from that, an unambiguous confirmation of
2
7
28
factors like oxygen balance (OB), the electronic band gap,
2
9
3
0
of critical bonds. Nevertheless, complex 6, which exhibits the either of the proposed initiation mechanisms (photochemical or
lowest nitrogen (‘‘propellant’’) content and the OB closest thermal) remains anticipated. In contrast to the literature, the
comparably strong response of the least sensitive compound 3
Table 3 Sensitivities towards the external stimuli impact, friction, and ESD
a
Table 4 Results of laser ignition tests and UV-vis spectroscopy
a
b
c
Compound
IS [J]
FS [N]
ESD [mJ]
Grain size [mm]
b
c
Compound
Reaction
Color
l
d–d
OD(l915)/OD(ld–d)
1
2
3
4
6
7
440
440
5
4360
4360
30
24
2.2
1500
—
260
20
150
50
100–500
100–500
500–1000
o100
3
4
6
7
deflag.
dec.
det.
Blue
Blue
Blue
Blue
613
620
658
681
0.39
0.37
0.46
0.34
o1
2
o100
det.
5
5
100–500
a
dec.: decomposition, deflag.: deflagration, det.: detonation. Operating
a
Impact sensitivity according to the BAM drophammer (method 1 of 6). parameters: current I = 7 A; voltage U = 4 V; theoretical maximal output
Friction sensitivity according to the BAM friction tester (method 1 of 6). power Pmax = 45 W; theoretical energy Emax = 0.17 mJ; wavelength
Electrostatic discharge sensitivity (OZM ESD tester); impact: insen- l = 915 nm; pulse length t = 0.1 ms. Absorption intensity maximum
b
c
b
sitive 440 J, less sensitive Z35 J, sensitive Z4 J, very sensitive r3 J; wavelength, which can be assigned to electron d–d excitations in the
c
friction: insensitive 4360 N, less sensitive = 360 N, sensitive o360 N measured range of 350–1000 nm. Quotient of the absorption intensity
and 480 N, very sensitive r80 N, extremely sensitive r10 N. According at the laser wavelength and the intensity at the d–d absorption
to the UN Recommendations on the Transport of Dangerous Goods.
wavelength.
New J. Chem.
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2018

View Article Online
NJC
Paper
perchlorato ligands led to a rough increase of thermal stability
and overall sensitivity. Inconsistencies, especially in regard to
measured sensitivities and laser ignitability, might be attri-
buted to fractal characteristics and particle sizes. Introducing
a new nitrogen-rich ligand with increased energetic character
(
2-MTZ, 2) results in novel patterns of the examined properties
despite identical coordination chemistry (7 vs. 4). In the
present case, higher thermal stability was accompanied by
an increase in optical and friction sensitivity and a simulta-
neous decrease of ESD and impact sensitivities. Consequently,
the present report provides lead-off insights into the correla-
tion of an ECC’s coordination chemistry and derived energetic
properties, while future investigations might further develop
observed patterns. Moreover, the obtained complexes, especially
6
and 7, indeed meet targeted properties including exceptionally
Fig. 9 Solid state UV/vis spectra of selected copper(II) and iron(II) com- high thermal stability, decreased mechanical sensitivities in
plexes. The step in the absorption intensity at 800 nm is caused by a
detector change. The UV-vis spectra have only qualitative character.
comparison to the 2-aminotetrazole ECC and retained laser
ignitability. Therefore, both conveniently synthesized ECC repre-
sent promising candidates for being applied in future ignition
(
optical or mechanical) systems.
towards optical stimuli suggests no distinct correlation between
mechanical and optical sensitivites, further being proved by the
1
0,12,21
observed difference between 4 and 7.
Furthermore, an
Conflicts of interest
obvious coherence between thermal properties (see Table 2) and
optical initiation is equally prohibited since 6 and 7, which There are no conflicts to declare.
proved to be highly thermally stable, gave the most violent
responses upon laser irradiation (see ESI,‡ Fig. S7). Regarding
the densities of the complexes and their outcome in the initia-
Acknowledgements
tion tests, no general trend can be observed. Whereas compound
Financial support of this work by the Ludwig Maximilian University
of Munich (LMU) and the Office of Naval Research (ONR) under
grant no. ONR.N00014-16-1-2062 is acknowledged. Furthermore,
the authors want to thank Mr Michael Gruhne and Mr Marcus
Lommel for their great help with the sensitivity determinations.
6
shows the highest density and a positive result in the experi-
ments, complex 3 with the lowest one gives a stronger response
to irradiation than compound 4. Conducted solid state UV/vis
measurements, which solely provide qualitative information,
equally lack unequivocal trends (Fig. 9). To begin with, the
spectra of 3 and 4, both reacting noticably differently during
laser experiments, are almost identical in terms of the observed Notes and references
d–d-transitions (613 and 620 nm) and curve shape. Moreover,
1
(a) E. Platania, N. L. W. Streeton, H. Kutzke, A. Karlsson,
E. Uggerud and N. H. Andersen, Vib. Spectrosc., 2018, 97,
6–74; (b) S. Bette, R. K. Kremer, G. Eggert and R. E.
the detected absorbance of the applied radiation (915 nm) solely
corresponds to a fraction of the d–d-transition’s maximum (see
Table 4). Resulting from these conclusions, a desired correlation
between coordination chemistry and optical sensitivity once
again remained unfulfilled. Based on the previously noted, vague
but highly relevant impact of fractal characteristics and the
overall inconclusive current state of research in the field of
laser-ignitable ECC, further investigations remain indispensable.
6
Dinnebier, Dalton Trans., 2018, 47, 8209–8220.
2
(a) F. Brunel, N. E. El Gueddari and B. M. Moerschbacher,
Carbohydr. Polym., 2013, 92, 1348–1356; (b) S. Chandraleka
and G. Chandramohan, Int. J. ChemTech Res., 2014, 6,
5450–5457.
3
4
R. R. Conry, Copper: Inorganic & Coordination Chemistry,
Encyclopedia of Inorganic Chemistry, John Wiley & Sons, Ltd.,
1st edn, 2006.
Conclusions
(a) H.-J. Zhang, A. W. Schuppe, S.-T. Pan, J.-X. Chen,
B.-R. Wang, T. R. Newhouse and L. Yin, J. Am. Chem. Soc.,
2018, 140, 5300–5310; (b) S. B. Thorpe, J. A. Calderone and
W. L. Santos, Org. Lett., 2012, 14, 1918–1921; (c) P. C. Roest,
N. W. M. Michel and R. A. Batey, J. Org. Chem., 2016, 81,
6774–6778.
In summary, a series of energetic copper(II) perchlorate complexes
were successfully synthesized and characterized by manipulating
the present coordination chemistry through adjusting the para-
meters of the solvent, stoichiometry, acidity, and the applied
2-substituted tetrazole ligand. In regard to the 2-MAT complexes
[
(
Cu(H
4) and [Cu(ClO
2
O)
2
(2-MAT)
4
](ClO
(H
4
)
2
ÁH
2
O (3), [Cu(H
] (6), a decrease of the
2
O)
2
(2-MAT)
4
](ClO
4
)
2
5 (a) P. L. Holland and W. B. Tolman, Coord. Chem. Rev., 1999,
190–192, 855–869; (b) W. L. Nettles, H. Song, E. R. Farquhar,
N. C. Fitzkee and J. P. Emerson, Inorg. Chem., 2015, 54(12),
4
)
2
2
O) (2-MAT)
2
2
compound’s water content as well as the incorporation of
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2018
New J. Chem.

View Article Online
Paper
NJC
5
2
671–5680; (c) C. A. Blasie and J. M. Berg, J. Am. Chem. Soc., 18 (a) A. Y. Zhilin, M. A. Ilyushin, I. V. Tselinskii, A. S. Kozlov
003, 125, 6866–6867.
and N. E. Kuz’mina, Russ. J. Appl. Chem., 2002, 75,
1849–1851; (b) M. H. H. Wurzenberger, N. Szimhardt and
J. Stierstorfer, J. Am. Chem. Soc., 2018, 140, 3206–3209.
6
7
T. M. Klap ¨o tke, Chemistry of High-Energy Materials, Walter
de Gruyter, Berlin, Boston, 2nd edn, 2012.
(a) G.-H. Tao, D. A. Parrish and J. M. Shreeve, Inorg. Chem., 19 H. Radies, Synthesis and Characterization of Energetic
2012, 51, 5305–5312; (b) R. P. Singh, R. D. Verma, D. T.
Transition Metal–Tetrazolate Complexes, Diploma thesis,
Meshri and J. M. Shreeve, Angew. Chem., Int. Ed., 2006, 45,
Ludwig Maximilian University of Munich, 2006.
3
584–3601; (c) Q. Zhang and J. M. Shreeve, Angew. Chem., 20 (a) K. Karaghiosoff, T. M. Klap o¨ tke, P. Mayer, C. M. Sabat ´e ,
Int. Ed., 2014, 53, 2540–2542.
(a) T. W. Myers, D. E. Chavez, S. K. Hanson, R. J. Scharff,
B. L. Scott, J. M. Veauthier and R. Wu, Inorg. Chem., 2015, 54,
A. Penger and J. M. Welch, Inorg. Chem., 2008, 47,
1007–1019; (b) T. M. Klap o¨ tke and J. Stierstorfer, Eur.
J. Inorg. Chem., 2008, 4055–4062; (c) T. M. Klap ¨o tke and
C. M. Sabat ´e , Eur. J. Inorg. Chem., 2008, 5350–5366.
8
9
8077–8086; (b) T. W. Myers, K. E. Brown, D. E. Chavez,
R. J. Scharff and J. M. Veauthier, Inorg. Chem., 2017, 56, 21 N. Szimhardt, M. H. H. Wurzenberger, T. M. Klap o¨ tke,
2297–2303; (c) T. W. Myers, J. A. Bjorgaard, K. E. Brown,
J. T. Lechner, H. Reichherzer, C. C. Unger and
J. Stierstorfer, J. Mater. Chem. A, 2018, 6, 6565–6577.
22 M. Freis, T. M. Klap o¨ tke, J. Stierstorfer and N. Szimhardt,
Inorg. Chem., 2017, 56, 7936–7947.
D. E. Chavez, S. K. Hanson, R. J. Scharff, S. Tretiak and
J. M. Veauthier, J. Am. Chem. Soc., 2016, 138, 4685–4692.
(a) M. Friedrich, J. C. G ´a lvez-Ruiz, T. M. Klap ¨o tke, P. Mayer,
B. Weber and J. J. Weigand, Inorg. Chem., 2005, 44, 8044–8052; 23 Crystallographic data for the structures have been deposited.
b) G. Geisberger, T. M. Klap ¨o tke and J. Stierstorfer, Eur.
with the Cambridge Crystallographic Data Centre.‡
(
J. Inorg. Chem., 2007, 4743–4750; (c) M. Joas, T. M. Klap o¨ tke, 24 J. H. Bryden, Acta Crystallogr., 1956, 9, 874–878.
J. Stierstorfer and N. Szimhardt, Chem. – Eur. J., 2013, 19, 25 (a) P. N. Gaponik, M. M. Degtyarik, A. S. Lyakhov, V. E.
9
995–10003; (d) M. Joas, T. M. Klap ¨o tke and N. Szimhardt,
Eur. J. Inorg. Chem., 2014, 493–498; (e) M. Joas and
T. M. Klap o¨ tke, Z. Anorg. Allg. Chem., 2014, 640, 1886–1891;
Matulis, O. A. Ivashkevich, M. Quesada and J. Reedijk, Inorg.
Chim. Acta, 2005, 358, 3949–3957; (b) E. J. van den Huevel,
P. L. Franke, G. C. Verschoor and A. P. Zuur, Acta Crystallogr.,
1983, 39, 337–339.
(
f ) T. M. Klap ¨o tke, P. C. Schmid, J. Stierstorfer and
N. Szimhardt, Z. Anorg. Allg. Chem., 2016, 642, 383–389.
26 (a) X. Song, Y. Wang, C. An, X. Guo and F. Li, J. Hazard.
Mater., 2008, 159, 222–229; (b) Y. Wang, X. Song, D. Song,
W. Jiang, H. Liu and F. Li, Propellants, Explos., Pyrotech.,
2011, 36, 505–512.
27 C. B. Storm, J. R. Stine and J. F. Kramer, Chem. Phys. Energ.
Mater., 1990, 309, 605–639.
1
1
1
1
0 N. Szimhardt, M. H. H. Wurzenberger, A. Beringer, L. J. Daumann
and J. Stierstorfer, J. Mater. Chem. A, 2017, 5, 23753–23765.
1 J. Evers, I. Gospodinov, M. Joas, T. M. Klap o¨ tke and
J. Stierstorfer, Inorg. Chem., 2014, 53, 11749–11756.
2 N. Szimhardt and J. Stierstorfer, Chem. – Eur. J., 2018, 24,
2
687–2698.
28 W. Zhu and H. Xiao, Struct. Chem., 2010, 21, 657–665.
3 N. Szimhardt, M. H. H. Wurzenberger, L. Zeisel, M. S. 29 P. Politzer and J. S. Murray, J. Mol. Model., 2015, 21, 25.
Gruhne, M. Lommel and J. Stierstorfer, J. Mater. Chem. A, 30 D. Mathieu, Ind. Eng. Chem. Res., 2017, 56, 8191–8201.
2
018, 6, 16257–16272.
31 (a) M. T. Greenfield, S. D. McGrane, C. A. Bolme,
J. A. Bjorgaard, T. R. Nelson, S. Tretiak and R. J. Scharff,
J. Phys. Chem. A, 2015, 119, 4846–4855; (b) A. E. Sifain,
J. A. Bjorgaard, T. W. Myers, J. M. Veauthier, D. E. Chavez,
O. V. Prezhdo, R. J. Scharff and S. Tretiak, J. Phys. Chem. C,
2016, 120, 28762–28773; (c) E. D. Aluker, A. G. Krechetov,
A. Y. Mitrofanov, D. R. Nurmukhametov and M. M. Kuklja,
J. Phys. Chem. C, 2011, 115, 6893–6901.
1
4 (a) Y. Cui, T.-L. Zhang, J.-G. Zhang, L. Yang, X.-C. Hu and
J. Zhang, J. Mol. Struct., 2008, 889, 177–185; (b) Y. Cui,
J. Zhang, T. Zhang, L. Yang, J. Zhang and X. Hu, J. Hazard.
Mater., 2008, 160, 45–50.
5 A. V. Smirnov, M. A. Ilyushin and I. V. Tselinskii, Russ.
J. Appl. Chem., 2004, 77, 794–796.
1
1
1
6 Y. Xu, W. Liu, D. Li, H. Chen and M. Lu, Dalton Trans., 2017,
4
6, 11046–11052.
32 (a) D. D. Dlott and M. D. Fayer, J. Chem. Phys., 1990, 92,
3798–3812; (b) E. D. Aluker, A. S. Zverev, A. G. Krechetov,
A. Yu. Mitrofanov, A. O. Terentyeva and A. V. Tupitsyn, Russ.
J. Phys. Chem. B, 2014, 8, 687–691.
7 Gas phase enthalpies of formation were calculated using the
atomization method (D H
using Gaussian09 computed CBS-4M electronic enthalpies.
0
0
(A)
0
= H(M) À SH
+ SD H
)
(A)
f
(g,M)
f
New J. Chem.
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2018