Cocrystallization of photosensitive energetic copper(II) perchlorate complexes with the nitrogen-rich ligand 1,2-Di(1 H -tetrazol-5-yl)ethane

Article abstract of DOI:10.1021/ic5020175

Two recently introduced concepts in the design of new energetic materials, namely complexation and cocrystallization, have been applied in the synthesis and characterization of the energetic copper(II) compound [Cu(dt-5-e)₂(H₂O)](ClO₄)₂, which consists of two different complex cations and can be described as a model energetic ionic cocrystal. The presence of both the N-rich 1,2-di(1H-tetrazol-5-yl)ethane ligand and oxidizing perchlorate counterion results in a new type of energetic material. The ionic complex cocrystal consists of a mononuclear and a trinuclear complex unit. It can be obtained by precipitation from perchloric acid or by dehydration of the related mononuclear coordination compound [Cu(dt-5-e)₂(H₂O)₂](ClO₄)₂·2H₂O at 70 °C in the solid state. The transformation starting at 60 °C was monitored by X-ray powder diffraction and thermal analysis. The energetic ionic cocrystal was shown to be a new primary explosive suitable for laser ignition. The different coordination spheres within the ionic cocrystal (octahedral and square pyramidal) were shown by UV/vis/NIR spectroscopy to result in excellent light absorption.

Full text of DOI:10.1021/ic5020175

Article
pubs.acs.org/IC
Cocrystallization of Photosensitive Energetic Copper(II) Perchlorate
Complexes with the Nitrogen-rich Ligand 1,2-Di(1H‑tetrazol-5-
yl)ethane
Jurgen Evers, Ivan Gospodinov, Manuel Joas, Thomas M. Klapotke,* and Jorg Stierstorfer
̈
̈
̈
Department of Chemistry, Energetic Materials Research, Ludwig-Maximilian University of Munich, Butenandtstr. 5−13, 81377
Munich, Germany
S
* Supporting Information
ABSTRACT: Two recently introduced concepts in the design of new
energetic materials, namely complexation and cocrystallization, have been
applied in the synthesis and characterization of the energetic copper(II)
compound “[Cu(dt-5-e)2(H2O)](ClO4)2,” which consists of two different
complex cations and can be described as a model energetic ionic cocrystal.
The presence of both the N-rich 1,2-di(1H-tetrazol-5-yl)ethane ligand and
oxidizing perchlorate counterion results in a new type of energetic material.
The ionic complex cocrystal consists of a mononuclear and a trinuclear
complex unit. It can be obtained by precipitation from perchloric acid or by
dehydration of the related mononuclear coordination compound [Cu(dt-5-
e)2(H2O)2](ClO4)2·2H2O at 70 °C in the solid state. The transformation
starting at 60 °C was monitored by X-ray powder diffraction and thermal
analysis. The energetic ionic cocrystal was shown to be a new primary
explosive suitable for laser ignition. The different coordination spheres
within the ionic cocrystal (octahedral and square pyramidal) were shown by UV/vis/NIR spectroscopy to result in excellent light
absorption.
Another trend in the improvement of EMs is the synthesis
and investigation of energetic cocrystals. In such compounds,
usually secondary explosives such as 2,4,6-trinitrotoluene
(TNT),12 octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine
(HMX),13 or 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaiso-
wurtzitane (CL-20)12,13 are used for cocrystallization. Inves-
tigations have shown that the cocrystallization of two energetic
compounds can decrease the sensitivity but (crucially) maintain
the performance. An important example of this are 2:1
cocrystals of Cl-20/HMX.13
While cocrystals of (nonenergetic) coordination compounds
are well-known in inorganic chemistry,14 to the best of our
knowledge, cocrystals of compounds containing energetic
complexes have not yet been described. The concept of
combing both the formation of a coordination compound and
the introduction of an ionic cocrystal to develop a new class of
EMs has been used in this work, and the synthesis and
characterization of the energetic ionic complex cocrystal
abbreviated as “[Cu(dt-5-e)2(H2O)](ClO4)2” (2) containing
1,2-di(1H-tetrazol-5-yl)ethane (dt-5-e, 1)15−17 as a neutral
coordinating ligand and the perchlorate anion as a counterion
has been achieved. The ionic cocrystal (2) consists of a
mononuclear ([Cu(dt-5-e)2(H2O)2](ClO4)2) and a trinuclear
([(dt-5-e)(H2O)Cu(μ-dt-5-e)Cu(dt-5-e)2(μ-dt-5-e)Cu(dt-5-
INTRODUCTION
■
The improvement of the performance, safety, and environ-
mental compatibility of energetic materials (EMs) is a major
goal for energetic material researchers around the world.1−4
Not only due to the European REACH regulations from 2007,
the replacement of toxic, carcinogenic, and hazardous energetic
materials like 1,3,5-trinitroperhydro-1,3,5-triazine (RDX),
monomethylhydrazine (MMH), or lead azide (LA) by
“green” alternatives is a main object.5−7 Strategies for the
development of new EMs include increasing the nitrogen
content, introducing ring strain, or oxidation of the molecule
backbone.8 An interesting and recent approach is the synthesis
of energetic coordination compounds (ECCs), which offers the
advantage of allowing selective design, since the metal center,
energetic ligand, and oxidizing anion can be varied.9 This is an
extremely important possibility since it allows the development
of EMs with specific, defined, and targeted properties. Two of
the most powerful coordination compounds of this type which
have been reported are tris(hydrazine)cobalt(II) perchlorate
(CHP) and tris(hydrazine)nickel(II) perchlorate (NHP),
which show detonation energies in the range of those of
secondary explosives.10 Unfortunately, both of these com-
pounds are too sensitive and not suitable for any application.
Probably the best investigated and most promising ECC is
tetraammine-cis-bis(5-nitro-2H-tetrazolato)cobalt(III) perchlo-
rate (BNCP).11
Received: August 19, 2014
© XXXX American Chemical Society
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Scheme 1. (a) Literature Synthesis of the Ligand 1,2-di(1H-tetrazol-5-yl)ethane,15 (b) Reaction of dt-5-e Forming the
Copper(II) Compounds 2 and 3, (c) Reaction of 1 Forming the Nitrato Compound 4, (d) Methylation of 1 According to
Literature,18 (e) Further Reaction of 5 Forming Compound 6
Figure 1. Crystal structures of the ionic cocrystal 2 showing the mononuclear (A) and trinuclear (B) copper units. Thermal ellipsoids of non-
hydrogen atoms are drawn at the 50% probability level. The perchlorate counterions were omitted for clarity. Selected bond lengths [Å] and angles
[deg]: Cu1−N4 2.006(3), Cu1−N8 2.009(3), Cu1−N12 2.002(3), Cu1−N16 2.022(3), Cu1−O1 2.172(3), Cu2−N10 2.5674(31), Cu2−N20
2.013(3), Cu2−N24 2.014(3), Cu3−N28 1.997(3), Cu3−N32 2.049(3), Cu3−O2 2.411(3), N25−C13 1.328(5), Cl1−O3 1.411(4), N4−Cu1−N8
87.58(12), N12−Cu1−N8 91.89(12), N8−Cu1−N16 176.15(12), N4−Cu1−O1 98.35(13), N8−Cu1−O1 91.84(13), N12−Cu1−N4 149.41(13),
N20−Cu1−N24 88.09(12), N10−Cu2−N20 85.94(11), N10−Cu2−N10i 180.00, N20−Cu2−N20i 180.00, N24−Cu2−N24i 180.00, N28−Cu3−
N32 89.97(12), N28−Cu3−O2 83.94(12), N28−Cu3−N28ii 180.00, N32−Cu3−N32ii 180.00, O2−Cu3−O2ii 180.00, C13−N25−N26−N27
0.1(4), C1−C2−C3−C4 57.2(4), C13−C14−C15−C16 67.1(4). Symmetry code: (i) 1 − x, 1 − y, 1 − z; (ii) 1 − x, 2 − y, −z.
e)(H2O)](ClO4)6) complex unit. Additionally, two mono-
nuclear coordination compounds with dt-5-e as a ligand and
perchlorate (3) and nitrate (4) as counterions as well as the
mononuclear perchlorate coordination compound with 1,2-
bis(1-methyltetrazol-5-yl)ethane as a ligand (6) were prepared,
characterized, and compared to the ionic cocrystal 2.
2H2O (3) in the solid state at 70 °C. The mononuclear
coordination compound 3 is obtained within a few days by
precipitation of the product due to the addition of ethanol to
the reaction mixture (diluted perchloric acid). Treating 1 with
copper(II) nitrate in nitric acid yields the nitrato compound 4
with the formula [Cu(NO3)2(dt-5-e)2] (Scheme 1c). Methyl-
ation of 1 using the method of Ivashkevich et al. resulted in the
formation of 1,2-bis(1-methyltetrazol-5-yl)ethane (bmte, 5),18
which can react with Cu(ClO4)2·6H2O to form the
mononuclear coordination compound 6 (Scheme 1d,e).
Crystal Structures. The structures of 2, 3, 4, and 6 were
determined using single crystal X-ray diffraction. In contrast to
3, compound 2 does not only show a mononuclear octahedral
coordination sphere. Instead, an atypical and rare energetic
ionic cocrystal consisting of a mononuclear and a trinuclear
RESULTS AND DISCUSSION
■
Synthesis. Preparation of ligand 1 (dt-5-e) was similar to
the method of Demko and Sharpless in yields of 30%.15
Compound 2 can be obtained two ways (Scheme 1a,b): (i) by
extremely slow evaporation (three months) of a concentrated
solution of Cu(ClO4)2·6H2O and 1 in perchloric acid or (ii) by
dehydration of the similarbut more accessiblemononu-
clear coordination compound [Cu(dt-5-e)2(H2O)2](ClO4)2·
B
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Figure 2. Selected hydrogen bonds in 2. Thermal ellipsoids of non-hydrogen atoms are drawn at the 50% probability level. Symmetry code: (i) 1 −
x, 1 − y, 1 − z; (ii) 1 − x, 2 − y, −z; (iii) x, 1 + y, z; (iv) 1 − x, −y, 1 − z; (v) −x, 2 − y, −z; (vi) 1 + x, −1 + y, z.
complex unit is formed (Figure 1). Compound 2 crystallized as
values)14,18 of two bidentate dt-5-e ligands (N12−Cu1−N4
149.41(13)° and N8−Cu1−N16 176.15(12)°). The apical
position is occupied by the oxygen atom of an aqua ligand. The
Cu−O bond (Cu1−O1 2.172(3) Å) is slightly elongated
compared to the equatorial Cu−N bonds but is still shorter
than the values reported for Cu−O bonds in similar square-
pyramidal copper complexes in the literature.14 The two dt-5-e
ligands differ in their coordination mode. One is bidentate only,
whereas the other is bidentate and additionally μ-bridging at
the N10 position to Cu2. The copper center Cu2 is surrounded
by six nitrogen atoms arranged in a Jahn−Teller distorted
octahedron (Cu2−N20 2.013(3), Cu2−N24 2.014(3), and
Cu2−N10 2.5674(31) Å). Two dt-5-e ligands coordinate in a
terminal bidentate mode (equatorial positions), while the axial
positions are occupied by N atoms of μ-bridging dt-5-e ligands
of neighboring copper units. The bond lengths and angles of
the dt-5-e ligand in 2 are similar to those observed for the two
blue rod (20 × 2 × 1 mm3) in the triclinic space group P1 with
̅
a density of 1.908 cm−3 at 173 K. The mononuclear unit shows
the expected Jahn−Teller distorted octahedral coordination
sphere, as is also observed for compound 3.
The angles and Cu−N and Cu−O bond lengths of the
mononuclear unit are in the expected range for typical Jahn−
Teller distorted octahedral copper(II) complexes.19 The
trinuclear copper complex consists of two pentacoordinated
copper centers which are connected through a six-coordinated
octahedral unit. The coordination sphere of the pentacoordi-
nated copper center is between those of a trigonal-bipyramid
and a square-pyramid (Addison parameter τ = 0.45).20 Due to
the similar N positions and an Addison parameter τ < 0.5, the
structure is described as square-pyramidal. All of the equatorial
positions of the two symmetric square-pyramids are occupied
by nitrogen atoms (Cu−N 2.00−2.02 Å similar to literature
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polymorphs of the free ligand 1 and other 5-substituted
tetrazoles.16−19
In compound 2, numerous hydrogen bonds are formed
between the neutral dt-5-e ligands of the mononclear and
trinuclear unit, the aqua ligands, and the perchlorate counter-
ions (Figure 2). The distances and angles of selected hydrogen
bonds are given in Table 1. Most notable are the hydrogen
Coordination compound 4 crystallizes in the triclinic space
group P1 with a calculated density of 1.948 g cm−3 (at 100 K).
̅
Compound 4 is the only compound of the herein investigated
ones which crystallizes in a water-free form. As expected, the
copper center exhibits a Jahn−Teller distorted octahedral
coordination sphere with nitrato anions in the axial position
and bidentate dt-5-e ligands in the equatorial (Figure 4). The
Table 1. Distances and Angles of Selected Hydrogen Bonds
in 2
a
D−H···A
D−H/Å
H···A/Å
D···A/Å
D−H···A/deg
N1−H1···N27vi
O2−H2C···N4v
O2−H2D···N4
N5−H5···O14iv
N17−H17···O7
N21i−H21i···N26
N21i−H21i···O9iii
N25−H25···N22i
0.76(5)
0.79(7)
0.87(7)
0.73(4)
0.89(4)
0.75(4)
0.75(4)
0.84(4)
2.12(5)
2.06(7)
2.19(6)
2.12(4)
1.97(4)
2.49(4)
2.34(4)
2.46(4)
2.878(5)
2.829(5)
2.922(5)
2.824(5)
2.855(6)
2.969(5)
3.022(7)
3.000(4)
175(6)
166(7)
141(5)
161(4)
170(4)
123(4)
152(4)
123(3)
a
Symmetry codes: (i) 1 − x, 1 − y, 1 − z; (iii) x, 1 + y, z; (iv) 1 − x,
−y, 1 − z; (v) −x, 2 − y, −z; (vi) 1 + x, −1 + y, z.
Figure 4. Crystal structure of the mononuclear coordination
compound 4. Thermal ellipsoids of non-hydrogen atoms are drawn
at the 50% probability level. Selected bond lengths [Å] and angles
[deg]: Cu1−N8 1.9672(16), Cu1−N4 2.0856(16), Cu1−O1
2.4180(14), N1−C1 1.332(2), O1−N9 1.262(2), N8−Cu1−N4
90.75(6), N8−Cu1−O1 88.84(6), O1−Cu1−O1i 180.00, N4−Cu1−
N4i 180.00, N8−Cu1−N8i 180.00(7). Symmetry code: (i) −x, 1 − y, 1
− z.
bonds between N1 and N27vi, N21i and N26, as well as N22i
and N25, which clearly show the connections between the
mononuclear and the trinuclear complex units (symmetry
codes: (i) 1 − x, 1 − y, 1 − z; (vi) 1 + x, −1 + y, z).
The mononuclear coordination compound 3 crystallizes in
the orthorhombic space group Pbc21 with four formula units
per unit cell and a calculated density of 1.881 g cm−3 at 173 K.
The copper(II) center is coordinated by two bidentate dt-5-e
ligands at the equatorial positions and by two aqua ligands at
the axial positions (Figure 3). The coordination sphere is
octahedral and similar to the mononuclear complex unit of
compound 2. Jahn−Teller distortion can be observed along the
O1−Cu1−O2 axis. Further, the structure contains two crystal
water molecules which are disordered. The bond lengths and
angles are in a typical range.
equatorial positions are occupied by the N4 atoms of the two
bidentate dt-5-e ligands with typical Cu−N bond lengths of
1.9672(16) and 2.0856(16) Å. The bidentate dt-5-e forms a
seven-membered ring with the copper center. The axial
positions are occupied by the O1 atoms of the two nitrato
ligands. The Cu−O bond is strongly elongated with 2.4180(14)
Å.
The obtained copper(II) perchlorate coordination com-
pound 6 with 5 as a ligand forms the expected mononuclear
octahedron with two bidentate bmte ligands in the equatorial
and two aqua ligands in the axial positions (Figure 5) showing
the typical Jahn−Teller distortion (Cu1−O1 2.4387(13)). The
bond lengths and angles are very similar to the mononuclear
complex unit of compound 2. The ECC 6 crystallizes in the
monoclinic space group P21/c with two formula units per unit
cell and a calculated density of 1.790 g cm−3 (at 100 K).
Powder Diffraction. Due to the mixed coordination mode,
compound 2 shows superior laser absorption. Therefore, the
formation of 2 by thermal transition of the mononuclear
coordination compound 3 was investigated by X-ray powder
diffraction. Gunier diffractograms (Figures S1 and S2) with Mo
Kα1 radiation (λ = 0.7093 Å) were performed in the 2θ range
between 4 and 34° with 3 (orthorhombic) and 2 (triclinic). In
addition, a series of Gunier diffractograms was obtained for
orthorhombic 3, which was heated for 2 h at 50, 60, 70, and 80
°C. At 50 °C the diffractogram showed only the phase-pure
diffraction pattern of 3. At 60 °C, equal amounts of 3 and 2
were observed, whereas at 70 °C, a diffraction pattern
containing only phase-pure 2 was obtained. In addition to
infrared spectroscopy and elemental analysis, powder diffrac-
tion proved that the mononuclear, octahedral compound 3
transforms into the mono- and trinuclear coordination
Figure 3. Crystal structure of the mononuclear coordination
compound 3. Thermal ellipsoids of non-hydrogen atoms are drawn
at the 50% probability level. Selected bond lengths [Å] and angles
[deg]: Cu1−N4 2.016(3), Cu1−N8 1.992(3), Cu1−N12 2.009(3),
Cu1−N16 1.993(3), Cu1−O1 2.7471(28), Cu1−O2 2.6453(29),
N1−C1 1.320(5), Cl1−O3 1.432(3), N8−Cu1−N4 90.86(11), N8−
Cu1−N12 89.45(13), N12−Cu1−N4 179.68(14), N8−Cu1−N16
178.01(14), O2−Cu1−O1 178.76(9), O3−Cl1−O4 108.43(17), N1−
C1−N4 107.4(3), C4−C3−C2−C1−62.5(4).
D
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Table 2. Energetic Properties of 2, 3, 4, and 6
2
3
4
6
a
IS/J
1
5
3
4
4
a
FS/N
14
160
80
grain size/μm
100−500
500−1000
defl.
<100
500−1000
b
c
d
e
d
hot needle
det.
decomp.
defl.
f
c
d
laser init.
det.
defl.
function
67
292
time/μs
g
F(R)@940nm
1.35
0.04
0.09
a
b
According to BAM standard methods. The hot needle test was
performed by penetrating the sample, which is confined by adhesive
c
d
tape, with the hot tip of a needle. Detonation. Deflagration.
e
f
Decomposition. Wavelength of the diode laser λ = 940 nm at
g
constant power density on the order of 105 W cm−2. Relative light
absorption [arbitrary units] as a function of the reflectance at 940 nm.
Figure 5. Crystal structure of the mononuclear coordination
compound 6. Thermal ellipsoids of non-hydrogen atoms are drawn
at the 50% probability level. Selected bond lengths [Å] and angles
[deg]: Cu1−N4 1.9998(14), Cu1−N8 2.0294(14), Cu1−O1
2.4387(13), N1−C1 1.335(2), N4−Cu1−N8 89.66(6), N4−Cu1−
O1 85.91(5), O1−Cu1−O1i 180.00, N4−Cu1−N4i 180.00(6), N8−
Cu1−N8i 180.00, C1−N1−N2−N3−0.40(18), C1−C2−C3−C4
62.7(2). Symmetry code: (i) −x, −y, −z.
show an increased sensitivity toward mechanical stimuli. The
coordination compound 2 in particular shows sensitivities and
performance corresponding to that of a primary explosive.
Despite this, however, it can be handled appropriately. The
thermal stabilities of the compounds were determined by
differential thermal analysis using a heating rate of 5 °C min−1.
The mononuclear compound 3 can be dehydrated at about 70
°C and is transformed to 2 in the solid state. Compound 2 is
thermally stable up to 188 °C at which temperature
dehydration occurs followed by exothermic decomposition.
The dehydration of 6 occurs in a similar range, although the
resulting compound is stable up to 278 °C. However, only
compound 2 shows energetic properties comparable with those
of a primary explosive. Without confinement, 2 only deflagrates
by thermal ignition while with confinement it detonates if
exposed to shock, thermal, or laser ignition. Due to the high
light absorption over a broad range (500 to 1200 nm; Figure 6)
compound 2 shows an outstanding response toward laser
radiation.
compound 2 by a solid state thermal dehydration and migration
process.
Due to the dehydration which occurs, the copper centers
rearrange within the unit cell to saturate the free coordination
sites which arise and which were originally occupied by aqua
ligands. Saturation of the copper coordination sites is achieved
by the new coordination modes of the dt-5-e ligand which are
observed (μ2, κ3N2,N4,N4′ instead of only κ2N4,N4′) and the
new coordination spheres which are formed (square-pyramidal
instead of only octahedral). The cell parameters (see Table S1)
change slightly due to the loss of water (e.g., 3, V = 2355.3 Å →
2, V = 2132.9 Å).
Infrared Spectroscopy. In addition to the powder
diffractions of 2 and 3, infrared spectra were recorded for all
compounds 1−6. A comparison of the infrared spectra of the
free ligand 1 with the coordination compounds 2 and 3 shows
that both coordination compounds exhibit the ligand vibrations
of 1, although in 2 and 3 they are shifted owing to coordination
to the Cu2+ center (Figure S3 and S4). For example, the CN
stretching vibration of the free ligand 1 at 1583 cm−1 is shifted
to 1564 cm−1 for 3 and to 1571, 1562, and 1557 cm−1 for 2.
The three CN vibration bands in the spectra of 2 also
indicate that different coordination modes of ligand 1 are
present in compound 2, which is in agreement with the crystal
structure. The IR spectra of the mononuclear octahedral
coordination compounds 3 and 6 show similar vibration bands
and agree with the crystal structures of [CuL2(H2O)2](ClO4)2.
For compound 3, an additional broad vibration band at
approximately 3490 cm−1 is observed which can be assigned to
uncoordinated crystal water. The spectra of compounds 2, 3,
and 6 also show the typical perchlorate vibration bands (2,
1071 cm−1; 3, 1073 cm−1; 6, 1087 cm−1), while in the spectrum
of 4 split nitrate vibrations are observed, which is due to the
coordination of the nitrate ion (4, 1333 and 1323 cm−1).
Energetic Properties and Laser Ignition. The metal
coordination compounds were also investigated for their
potential use as primary explosives, and selected parameters
are summarized in Table 2. As expected (in contrast to the
nitrato coordination compound 4), the perchlorate compounds
Figure 6. UV/vis/NIR spectra of compounds 2, 3, and 6.
The UV/vis/NIR spectra of 3 and 6 are very similar,
indicating the same electronic transitions occur (from the xy
orbital to the x2 − y2 and from xz/yz to x2 − y2), which are
typical for a Jahn−Teller distorted d9 complex. Owing to the
different coordination spheres present in compound 2
(octahedral and square-pyramidal), 2 absorbs in a much larger
range than the mononuclear, octahedral coordination com-
pounds 3 and 6. Compound 2 is therefore more suitable for
laser ignition because the use of radiation sources emitting in
the near-infrared (∼800−1100 nm) is preferred for technical
reasons. Confined samples of 2 and 6 were irradiated with a
monopulsed laser beam at 940 nm and a pulse length of 400 μs.
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Compound 2 detonated, while the less powerful compound 6
only deflagrated. The energy output was measured qualitatively
by an aluminum witness plate (Figure 7) and indicated
UV/vis/NIR reflectance of solid samples (powders) was
determined with a Varian Cary 500 spectrometer in the
wavelength range of 350−1300 cm−1.28 The relative reflectance
R [%] was transformed by the Kubelka−Munk equation to the
absorption intensity F(R) [arbitrary units]. A reflectance of R >
100% is possible because the values are relative to the
background reflection of blank samples.29
The crystal structures were determined on an Oxford
Diffraction Xcalibur 3 diffractometer with a Sapphire CCD
detector, four circle kappa platform, enhanced molybdenum Kα
radiation source (λ = 71.073 pm), and Oxford Cryosystems
Cryostream cooling unit.28 Data collection and reduction were
performed with the CrysAlisPro software.30,31 The structures
were solved with SIR97 or SIR2004.32,33 The refinement was
performed with SHELXL-97.34 The CIF files were checked at
the checkCIF Web site.35 The non-hydrogen atoms were
refined anisotropically and the hydrogen atoms isotropically if
not calculated.
Figure 7. (a) Laser ignition setup. (b) Witness plates after laser
ignition of coordination compounds 2 (left) and 6 (right).
X-ray powder experiments were performed on a Gunier
diffractometer (Huber G644) with Mo Kα1 radiation (λ =
0.7093 Å; quartz monochromator) in Lindemann capillaries
(0.7 mm diameter). The angle calibration was performed with
electronic grade germanium. In the 2θ range between 4 and 34°
with an increment of 0.04°, 750 data points were collected with
a counting rate of 10 s for each increment. The Rietveld
parameters were analyzed with the program FullProf.36 The
Rietveld data set contained 106 crystallographic independent
atoms for the triclinic compound 2 and 59 crystallographic
independent atoms for the orthorhombic compound 3. Due to
excessive peak overlapping, calculating diffractograms with
FullProf was only possible in the 2θ range between 4 and 16°.
All chemicals were used as purchased. The ligands 1,2-di(1H-
tetrazol-5-yl)ethane (dt-5-e, 1) and 1,2-bis(1-methyltetrazol-5-
yl)ethane (bmte, 5) were synthesized similar to the literature
procedures.15,18
detonation for 2. For compound 6, no dent was observed in the
witness plate. Furthermore, the function times tf (time delay
between beginning irradiation and complete reaction) were
determined and showed a 4 times faster value for 2 (tf = 67 μs)
than for 6 (tf = 292 μs). This is in agreement with the higher
absorption by 2 at 940 nm (Table 2). Values for the function
time in the low-microsecond range are essential for potential
alternatives for fast-functioning electric type detonators such as
exploding-bridge wires.21
CONCLUSION
■
In conclusion, the successful combination of the synthesis of
energetic coordination compounds with the concept of
cocrystallization was achieved in the synthesis and character-
ization of the mono- and trinuclear copper(II) perchlorate
coordination compound 2 using dt-5-e as a flexible ligand. This
represents a new concept in the design of energetic materials
and offers the possibility to prepare highly photosensitive
explosives. Furthermore, it was shown that the mono- and
trinuclear coordination compound 2 can be obtained in a facile
method by the dehydration of its mononuclear homologue 3 in
the solid state. Due to the three different copper(II)
coordination spheres in 2, light is absorbed over a broad
range with high intensity making 2 suitable for advanced laser
initiated explosive devices.
{[Cu(dt-5-e)2(H2O)2](ClO4)2}{[(dt-5-e)(H2O)Cu(μ-dt-5-e)-
Cu(dt-5-e)2(μ-dt-5-e)Cu(dt-5-e)(H2O)](ClO4)6} (2) Abbre-
viated as “[Cu(dt-5-e)2(H2O)](ClO4)2”. (a) The nitrogen-rich
compound 1 (166 mg, 1.00 mmol) was dissolved in perchloric
acid (30%, 5.0 mL) at 60 °C. Under stirring, a solution of
copper(II) perchlorate hexahydrate (186 mg, 0.5 mmol) in
perchloric acid (30%, 1.0 mL) was added, and the resulting blue
reaction mixture cooled to room temperature and left for
crystallization. After three months, one single crystal in the
form of a blue rod (20 × 2 × 1 mm3) suitable for X-ray
diffraction was obtained. The crystal was filtered off and dried
under ambient conditions for 1 h. Yield: 76 mg (0.04 mmol,
12%). (b) Drying of compound 3 (333 mg, 0.50 mmol) at 70
°C in the oven for 2 h yielded 306 mg (0.13 mmol, 100%) of 2.
DTA (5 °C min−1) onset: 188 °C (dehydration), 206 °C
EXPERIMENTAL SECTION
■
Caution! The prepared copper(II) compounds are energetic
materials with increased sensitivity to various stimuli (e.g., friction,
impact, or heat). Proper protective measures (face shield, ear
protection, leather coat, earthed equipment, conductive floor and
shoes, Kevlar gloves) should be used at all time. Only small
amounts of the compounds 2 and 3 should be handled.
(decomposition); IR (ATR): v = 3527 (w), 3476 (vw), 3317
̃
(w), 3153 (w), 1571 (w), 1562 (m), 1557 (m), 1415 (w), 1394
(w), 1101 (s), 1091 (s), 1071 (s), 1051 cm−1 (vs). Elem anal.
(C32H56Cl8Cu4N64O36, 2451.17 g mol−1) calcd.: C 15.68, H
2.30, N 36.57%. Found: C 15.98, H 2.44, N 36.29%. BAM
impact: 1 J. BAM friction: ≤ 5 N (at grain size 100−500 μm).
[Cu(dt-5-e)2(H2O)2](ClO4)2·2H2O (3). Compound 1 (330
mg, 2.00 mmol) was dissolved in diluted perchloric acid (10%,
6.0 mL) at 60 °C. A solution of copper(II) perchlorate
hexahydrate (372 mg, 1.00 mmol) in water was added under
stirring and the blue reaction mixture cooled to room
temperature. After 2 days, ethanol (3.0 mL) was added and
the solution left for crystallization for further 3 days. Single
The impact and friction sensitivity tests were performed
according to standard methods by using a BAM (Bundesanstalt
fur Materialforschung and -prufung) drop hammer and a BAM
̈
̈
friction tester.22,23 Decomposition temperatures were measured
via differential thermal analysis (DTA) with an OZM Research
DTA 552-Ex instrument at a heating rate of 5 °C min−1 and in
a range of room temperature to 400 °C.24,25 The determination
of the carbon, hydrogen, and nitrogen contents was carried out
by combustion analysis using an Elementar Vario EL.26 Infrared
(IR) spectra were recorded on a PerkinElmer BXII FT-IR
system with a Smith DuraSamplIR II diamond ATR unit.27 The
F
dx.doi.org/10.1021/ic5020175 | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
crystals in the form of violet blocks were obtained, filtered off,
washed with a small amount of ether, and dried under ambient
conditions. Yield: 454 mg (0.68 mmol, 68%). DTA (5 °C
12-1-0468, and the Office of Naval Research (ONR) under
grant nos. ONR.N00014-10-1-0535 and ONR.N00014-12-1-
0538 is gratefully acknowledged. The authors acknowledge
collaborations with Dr. Mila Krupka (OZM Research, Czech
Republic) in the development of new testing and evaluation
methods for energetic materials and with Dr. Muhamed
min−1) onset: 77 °C (dehydration). IR (ATR): v = 3490 (m),
̃
3153 (w), 1564 (m), 1417 (w), 1403 (w), 1071 (vs), 1054
cm−1 (vs). Elem anal. (C8H20Cl2CuN16O12, 666.80 g mol−1)
calcd.: C 14.41, H 3.02, N 33.61%. Found: C 14.68, H 3.17, N
33.22%. BAM impact: 3 J. BAM friction: 14 N (at grain size
500−1000 μm).
́
Suceska (Brodarski Institute, Croatia) in the development of
new computational codes to predict the detonation and
propulsion parameters of novel explosives. We are indebted
to and thank Drs. Betsy M. Rice and Brad Forch (ARL,
Aberdeen, Proving Ground, MD) for many inspired
discussions. The authors thank Stefan Huber for assistance
while performing the sensitivity and laser ignition tests.
[Cu(NO3)2(dt-5-e)2] (4). Compound 1 (330 mg, 2.00
mmol) was dissolved in 0.5 M nitric acid (7.0 mL) at 60 °C.
A solution of copper(II) nitrate trihydrate (247 mg, 1.00
mmol) in water (1.0 mL) was added under stirring. The blue
solution was cooled to room temperature and left for
crystallization. After 1 week, X-ray suitable single crystals in
the form of blue blocks were obtained. The crystals were
filtered off, washed with a small amount of ethanol, and air-
dried. Yield: 495 mg (0.95 mmol, 95%). DTA (5 °C min−1)
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ASSOCIATED CONTENT
* Supporting Information
■
S
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AUTHOR INFORMATION
Corresponding Author
* tmk@cup.uni-muenchen.de.
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Notes
The authors declare no competing financial interests.
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ACKNOWLEDGMENTS
■
Financial support of this work by the Ludwig-Maximilian
University of Munich (LMU), the U.S. Army Research
Laboratory (ARL) under grant no. W911NF-09-2-0018, the
Armament Research, Development and Engineering Center
(ARDEC) under grant nos. W911NF-12-1-0467 and W911NF-
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