Preparation and crystal structures of two salts with the 5-nitrotetrazolate anion

Article abstract of DOI:10.1002/hc.20509

Tetraphenylphosphonium 5-nitrotetrazolate (2) was prepared by metathesis of sodium 5-nitrotetrazolate dihydrate (1; NaNT) with tetraphenylphosphonium chloride in acetone. The new compound was fully characterized by vibrational (IR, Raman) and NMR ('H,

Full text of DOI:10.1002/hc.20509

Heteroatom Chemistry
Volume 20, Number 1, 2009
Preparation and Crystal Structures of Two
Salts with the 5-Nitrotetrazolate Anion
Thomas M. Klapo¨tke, Carles Miro´ Sabate´, and Jan M. Welch
Department of Chemistry and Biochemistry, Energetic Materials Research, Ludwig-Maximilian
University, Butenandtstr. 5-13 (D), 81377 Munich, Germany
Received 24 October 2008; revised 16 November 2008
˚
˚
˚
A, b= 9.908(4) A, c = 8.940(3) A, and V = 1311.4(1)
ABSTRACT: Tetraphenylphosphonium 5-nitrotetra-
zolate (2) was prepared by metathesis of sodium
5-nitrotetrazolate dihydrate (1; NaNT) with tetra-
phenylphosphonium chloride in acetone. The new
compound was fully characterized by vibrational (IR,
Raman) and NMR (1H, 13C, and 14N) spectroscopies,
elemental analysis, and mass spectrometry. Attempted
synthesis of 2-methyl-5-nitrotetrazole (2-MeNT) by
methylation of 1 with dimethylsulfate at reflux
from acetonitrile failed, and crystals of an explosive
compound with the formula (NaNT)2(H2O)2CH3CN
(3), NT = 5-nitrotetrazolate, formed. X-ray diffrac-
tion techniques were used to determine the crys-
tal structure of 2 and 3. Compound 2 crystallizes
in the orthorhombic space group P212121 with four
molecules in the unit cell and unit cell parameters
3
˚
ꢀ
C
A .
2009 Wiley Periodicals, Inc. Heteroatom Chem
20:35–44, 2009; Published online in Wiley InterScience
(www.interscience.wiley.com). DOI 10.1002/hc.20509
INTRODUCTION
The synthesis of nitrogen-rich compounds for use as
highly energetic materials is the goal of our research
group [1–4]. 5-Amino-1H-tetrazole (A) and 5-nitro-
2H-tetrazole (B) are nitrogen-rich energetic com-
pounds, which are readily deprotonated by bases
[5,6]. Nitrogen-rich salts of B with easily protonated
nitrogen bases [7] have shown interesting proper-
ties in terms of low sensitivities and high perfor-
mances. We recently synthesized metal salts of A,
which are prospective candidates for use in new en-
vironmentally friendly pyrotechnic compositions [5]
and metal salts of B, which show an increased sensi-
tivity to shock and friction [6]. The presence of crys-
tal water in the solid-state structure of metal salts of
B increases the distances between the metal center
and the anion, and the sensitivity of the compounds
toward classical stimuli is markedly reduced.
˚
˚
˚
a = 7.7413(4) A, b= 13.624(1) A, c = 21.252(1) A, and
3
˚
V = 2241.5(2) A , whereas 3 crystallizes in the or-
thorhombic space group Ama2 with four formula unit
in the unit cell and unit cell parameters a = 14.805(6)
Correspondence to: Thomas M. Klapo¨tke; e-mail: tmk@cup.uni-
muenchen.de.
Contract grant sponsor: Ludwig-Maximilian University of
Munich.
Contract grant sponsor: Fonds der Chemischen Industrie.
Contract grant sponsor: the European Research Office (ERO)
of the U.S. Army Research Laboratory (ARL).
Contract grant sponsor: ARDEC (Armament Research, Devel-
opment and Engineering Center).
Contract grant numbers: N 62558-05-C-0027, R&D 1284-CH-01,
R&D 1285-CH-01, 9939-AN-01, and W911NF-07-1-0569.
Contract grant sponsor: Bundeswehr Research Institute for Ma-
terials, Explosives, Fuels and Lubricants.
To investigate 5-nitrotetrazole moieties, which
do not interact in the solid state we synthesized
the tetraphenylphosphonium salt of B (2). In addi-
tion, synthesis of neutral 2-methyl-5-nitrotetrazole
(2-MeNT) was attempted and bis-(sodium 5-
nitrotetrazolate) dihydrate acetonitrile adduct (3)
formed instead. Here, we would like to report on the
synthesis and characterization of 2 and 3 as well as
their crystal structures. To our knowledge, together
Contract grant numbers: E/E210/4D004/X5143 and E/E210/
7D002/4F088.
c
ꢀ
2009 Wiley Periodicals, Inc.
35

36 Klapo¨tke, Sabate´, and Welch
with our reports [6,7] and that of the nickel salt [8],
these are the only papers where the structure of the
anion of B has been determined by X-ray analysis.
5 mL dry acetone was added under a stream of nitro-
gen. Immediate precipitation of sodium chloride was
observed, and the reaction mixture was stirred for
further 20 min at room temperature. The insoluble
solid was filtered and discarded, and the acetone so-
lution was left to slowly evaporate yielding a slightly
yellow residue of the product, which could be puri-
fied by dissolving it in minimal acetone and letting
ether to diffuse into it overnight. The crystals formed
were suitable for X-ray analysis (301 mg, 79%).
Analytical Data of 2. Raman v˜/cm−1 (rel.
int.): 3149(4), 3146(5), 3068(45), 2958(3), 1586(49),
1576(20), 1527(12), 1441(6), 1403(100), 1307(4),
1187(11), 1166(13), 1109(13), 1099(22), 1040(37),
1029(24), 1014(50), 1002(69), 834(7), 726(4),
679(14), 617(8), 531(5), 287(9), 254(21), 205(12),
143(6); IR v˜/cm−1 (KBr, rel. int.): 3437(w), 3091(w),
3051(w), 3024(w), 2820(vw), 2691(vw), 2204(vw),
1991(vw), 1833(vw), 1700(vw), 1585(w), 1572(w),
1558(w), 1526(s), 1482(m), 1440(s), 1435(s), 1428(s),
1404(s), 1340(w), 1307(s), 1182(w), 1162(m),
1152(w), 1107(vs), 1038(w), 1020(w), 997(m),
853(vw), 835(s), 757(s), 723(s), 691(s), 678(m),
EXPERIMENTAL SECTION
Materials and Methods
CAUTION: Tetrazoles are energetic materials.
Sodium 5-nitrotetrazolate dihydrate (1) has low
sensitivity against friction and electrostatic dis-
charge; however upon loss of water and/or rapid
heating, it becomes highly sensitive and explosive.
Safety equipment such as leather gloves, face shield,
earplugs, and use of Teflon spatulas are recom-
mended. It is not recommended the synthesis and
manipulation of compound 1 on scales greater than
200 mg.
All chemicals and solvents were used as supplied
by Sigma-Aldrich Fine Chemicals Inc. (Munich,
Germany) without any further purification. Sodium
5-nitrotetrazolate dihydrate (1) was synthesized ac-
cording to the literature [9,10]. 1H, 13C, 14N, and
31P NMR spectra were recorded on a JEOL Eclipse
400 instrument in DMSO-d6 at room temperature.
The infrared (IR) spectrum of 2 was recorded on a
Perkin–Elmer spectrum one FT-IR instrument [11]
(vs = very strong, s = strong, m = medium, w = weak,
and vw = very weak). A Perkin–Elmer spectrum
2000R NIR FT-Raman instrument equipped with a
Nd: YAG laser (1064 nm) was used for the record-
ing of the Raman spectra (intensity percentages are
given in brackets). A Netsch simultaneous thermal
analyzer STA 429 was used to perform the ele-
mental analyses. A differential scanning calorimetry
(Linseis DSC PT-10 instrument [12], calibrated with
standard pure indium and zinc) was used to deter-
mine the melting point and decomposition tempera-
ture of 2 (2◦C min−1). Closed aluminum sample pans
with a 1-μm hole in the top for gas release under a
nitrogen flow of 20 mL min−1 were used.
1
672(m), 614(w), 478(vs), 456(m); H NMR (DMSO-
d6, 400.18 MHz, 25◦C, TMS) δ/ppm: 7.98–7.94 (4H,
1
m, p-H), 7.84–7.71 (16H, m, o−H/m-H); 13C{ H}
NMR (DMSO-d6, 100.6 MHz, 25◦C, TMS) δ/ppm:
4
165.2 (1C, s, C1), 135.3 (4C, d, JC-P = 2.7 Hz,
2
p-C), 134.5 (8C, d, JC-P = 10.8 Hz, o-C), 130.4 (8C,
3
1
d, JC-P = 12.7 Hz, m-C), 117.7 (4C, d, JC-P = 89.2
Hz, i-C); 31P NMR (DMSO-d6, 162.00 MHz, 25◦C,
35% H3PO4) δ/ppm: +23.0 (P); 14N NMR (DMSO-
d6, 28.92 MHz, 25◦C, CH3NO2) δ/ppm: +21 (2N, ν1/2
∼420 Hz, N2/3), −21 (1N, ν1/2 ∼80 Hz, -NO2), −58
(2N, ν1/2 ∼400 Hz, N1/4); C25H20N5O2P (453.44 g
mol−1, calc/found): C 66.22/66.11, H 4.45/4.44, N
15.45/15.37; m/z (FAB+, xenon, 6 keV, m-NBA ma-
trix): 339.34 [PPh4]+; (FAB−, xenon, 6 keV, m-NBA
matrix): 114.0 [NT]−, 228.8 [H(NT)2]−; DSC (2◦C
min−1): 125.7◦C (mp), ∼188◦C (dec); Sensitivity data:
friction >360 N; shock >30 J; electrost disch insensi-
tive.
Bis(sodium 5-nitrotetrazolate) Dihydrate
Acetonitrile Adduct (3)
Sodium 5-nitrotetrazolate dihydrate (0.294 g,
1.70 mmol) was refluxed in 10 mL acetonitrile for
2 h after which time the heating was switched off,
and the solvent was rotavaporated down at 60◦C to
∼4 mL. Immediate precipitation of the product was
observed, and the yield was increased by putting the
mother liquors in the fridge overnight. The solid was
filtered and washed with ether, rendering 0.382 g of
Tetraphenylphosphonium 5-Nitrotetrazolate (2)
Tetraphenylphosphonium chloride (0.314 g, 0.84
mmol) was dissolved in 15 mL dry acetone under
an stream of nitrogen before a solution of sodium
5-nitrotetrazolate dihydrate (0.145 g, 0.84 mmol) in
Heteroatom Chemistry DOI 10.1002/hc

Preparation and Crystal Structures of Two Salts with the 5-Nitrotetrazolate Anion 37
a colorless solid. Some of the crystals precipitated
in the fridge were measured by X-ray analysis, con-
RESULTS AND DISCUSSION
Tetraphenylphosphonium 5-nitrotetrazolate (2) was
prepared in high yield and purity by a metathesis re-
action between sodium 5-nitrotetrazolate dihydrate
(1) [9,10] and tetraphenylphosphonium chloride in
acetone according to Eq. (1). After filtering the pre-
cipitated sodium chloride the solvent was stripped,
and crystals were grown by storage of a saturated
solution of the compound in acetonitrile in the
refrigerator. Attempted synthesis of highly ener-
getic neutral 2-methyl-5-nitrotetrazole (2-MeNT) by
boiling 1 with an excess dimethylsulfate in ace-
tonitrile failed, and an adduct with the formula
(NaNT)2(H2O)2CH3CN (3) was formed together with
unreacted 1. The result was reproducible when
this reaction was repeated without the presence of
dimethylsulfate (Eq. (2)).
firming the formation of 3.
Analytical Data of 3. Raman v˜/cm−1 (rel.
int.): 3015(3), 2950(5), 1548(7), 1424(100), 1363(4),
1319(5), 1180(4), 1056(47), 1044(41), 921(2), 841(4),
774(2), 546(3), 459(2), 392(2), 248(2), 145(2); 1H
NMR (DMSO-d6, 400.18 MHz, 25◦C, TMS) δ/ppm:
4.57 (∼4H, s, H2O), 1.97 (3H, s, CH3); 13C{ H} NMR
1
(DMSO-d6, 100.63 MHz, 25◦C, TMS) δ/ppm: 165.2
(2C, C/C1), 118.2 (1C, CN), 1.4 (1C, CH3); 14N NMR
(DMSO-d6, 28.92 MHz, 25◦C, CH3NO2) δ/ppm: +22
(2N, ν1/2 ∼400 Hz, N2/4), -19 (1N, ν1/2 ∼70 Hz, -NO2),
-60 (2N, ν1/2 ∼370 Hz, N1/3); m/z (FAB−, xenon, 6
keV, m-NBA matrix): 114.0 [NT]−, 228.8 [H(NT)2]−;
DSC (5◦C min−1): ∼200 ◦C (dec), Sensitivity data: fric-
tion >360 N; shock >30 J; electrost disch insensitive.
Acetone
Na+NT− + PPh+4 CI− −→ PPh+4 (cation)NT− (1)
1
−NaCl
X-ray Structure Determinations of 2 and 3
2
CH CN
3
Na+NT− −→ (NaNT)2 · 2H2O · CH3CN.
(2)
Compounds 2 and 3 were characterized by X-ray
structure determination (Table 1). Crystals were
grown as described in the Preparation Section. The
X-ray crystallographic data were collected on an Ox-
ford Diffraction Xcalibur 3 diffractometer equipped
with a CCD detector at a temperature of 200 K (2)
and 100 K (3) and using the CrysAlis CCD soft-
ware [13]. All data were collected using graphite-
monochromated Mo Kα radiation. No absorption
correction was applied to the data sets collected. The
data reduction was performed with the CrysAlis RED
software [14].
1
reflux
3
Compounds 2 and 3 were characterized by
vibrational spectroscopy. The shifts for the cation
in 2 are well established, and thus a descrip-
tion is omitted. Much more interesting are the
bands corresponding to the 5-nitrotetrazolate
anion in 2 and 3 that could be assigned by com-
parison with the calculated shifts [7,21]: ∼1530
[νasymm(NO2)], ∼1400 [νasymm(N C N)], ∼1380
[νsymm(NO2) + νsymm(N C N), “in phase”], ∼1300
[νsymm(NO2) + νsymm(N C N), “out of phase”],
∼1180 [νasymm(tetrazole)], ∼1160 [νsymm(Tetrazole)
Structure Analysis and Refinement. The crystal
structures of 2 and 3 were solved by direct methods
using the corresponding programs available in the
WinGX package [15–18], and the structures were fi-
nally checked using the program PLATON [19]. All
non-hydrogen atoms were refined anisotropically,
whereas the hydrogen atoms were located from
difference Fourier electron density maps and re-
fined isotropically. A SCALE3 ABSPACK multiscan
method was used for the absorption correction [20].
Further information on the crystal–structure de-
terminations (excluding structure factors) has been
deposited at the Cambridge Crystallographic Data
Centre (CCDC) under numbers CCDC 673959 (2) and
CCDC 673958 (3) and is available free of charge by
request from the CCDC. A copy of the cif file for com-
pounds 2 and 3 can be obtained free of charge on
application to The Director, CCDC, 12 Union Road,
Cambridge CB2 1EZ, UK (Fax: int. code (1223)336–
033. E-mail for inquiry: fileserv@ccdc.cam.ac.uk.
E-mail for deposition: deposit-@ccdc.cam.ac.uk).
+
νsymm(NO2), “in phase”], ∼1020 [δ(N C N)
+ νsymm(NO2), “in phase”], ∼1000 [ν(N N) +
δsymm(tetrazole), “out of phase”], ∼835 [δ(NO2)
+ δ(N C N), “in of phase”], ∼765 [γ (NO2) +
γ (N C N), “out of phase”], ∼720 [γ (tetrazole)
“in phase”], ∼670 [γ (tetrazole) “out of phase”],
522 [ω(NO2) + ω(tetrazole), “out of phase”], ∼455
[ν(C–N) + δ(NO2)], ∼250 [ω(NO2) + ω(tetrazole),
“in phase”], and ∼205 [γ (N C N)] cm−1. The most
significant fact in the Raman spectrum of 2 is that
the asymmetric nitro-group stretching is shifted and
split in two signals at 1586 and 1576 cm−1 and the
symmetric nitro-group stretching is strongly shifted
(∼15 cm−1). This is indicative of little interaction
between the nitro-group and the cation as confirmed
by the crystal structure of the compound (see X-Ray
Structure Discussion). On the other hand, for
compound 3 the nitro-group stretches are observed
much closer to the calculated values, indicative of
strong interaction by the nitro-group oxygen atoms
Heteroatom Chemistry DOI 10.1002/hc

38 Klapo¨tke, Sabate´, and Welch
TABLE 1 Crystal Data and Refinement for Compounds 2 and 3
2
3
Chemical formula
Molecular weight
Crystal size
C25H20N5O2P
453.43
0.3 × 0.15 × 0.1
Orthorhombic
P212121
7.7413(4)
13.624(1)
21.252(1)
90
90
90
2241.5(2)
4
1.342
0.156
944
3.84–28.99
200(2)
−10 ≤ h ≤ 10
−18 ≤ k ≤ 18
−28 ≤ l ≤ 28
28111
5949
3421
0.0767
C4H7N11O6Na2
351.19
0.15 × 0.15 × 0.05
Orthorhombic
Ama2
14.805(6)
9.908(4)
8.940(3)
90
90
90
1311.4(1)
4
1.778
Crystal system
Space group
˚
a (A)
˚
b (A)
˚
c (A)
α (◦)
β (◦)
γ (◦)
3
˚
V(A )
Z
ρcalc. (g/cm3)
μ (mm−1
F(000)
)
0.211
712
θ range (◦)
4.11–29.97
100(2)
−20 ≤ h ≤ 20
−13 ≤ k ≤ 13
−12 ≤ l ≤ 12
8563
1045
787
0.0597
Temperature (K)
Index range
Reflections collected
Reflections unique
Reflections observed (4σ)
R (int.)
Data/restraints/parameters
GOF
R1, wR2 [I > 4σ(I )]
R1, wR2 (all data)
5949/0/298
1.051
0.0419, 0.0937
0.0726, 0.0988
1045/1/128
0.997
0.0345, 0.0801
0.0498, 0.0847
as seen in the coordination of the Na cations by the
nitro-groups in the molecular structure.
The H NMR spectrum of 2 (Fig. 1) shows two
multiplets in the aromatic region corresponding to
the phenyl-group hydrogen atoms in the cation,
whereas 3 shows a broad and a sharp resonances
at 4.57 and 1.97 ppm corresponding to the coor-
dination water and acetonitrile, respectively. The
1
FIGURE 1 (a) 1H NMR and (b) 13C NMR spectra of 2 in DMSO-d6.
Heteroatom Chemistry DOI 10.1002/hc

Preparation and Crystal Structures of Two Salts with the 5-Nitrotetrazolate Anion 39
◦
˚
TABLE 2 Distances (A) and Angles ( ) for the 5-Nitrotetrazolate Anion in Compounds 2 and 3
2
3 (A)
3 (B)
N5–O1
N5–O2
N5–C1
N1–N2
N2–N3
N3–N4
C1–N1
C1–N4
1.205(3)
1.212(3)
1.436(3)
1.343(3)
1.299(3)
1.341(3)
1.298(3)
1.287(3)
1.226(1)
1.226(1)
1.441(1)
1.354(1)
1.319(1)
1.354(1)
1.325(1)
1.325(1)
1.227(1)
1.227(1)
1.441(1)
1.338(1)
1.342(1)
1.338(1)
1.328(1)
1.328(1)
2
3 (A)
3 (B)
O1–N5–O2
O1–N5–C1
O2–N5–C1
C1–N1–N2
N1–N2–N3
N2–N3–N4
N3–N4–C1
N4–C1–N1
N4–C1–N5
N1–C1–N5
123.0(3)
118.2(2)
118.8(2)
103.7(2)
108.6(2)
109.6(2)
103.4(2)
114.7(2)
122.8(2)
122.5(2)
124.8(3)
117.6(2)
117.6(2)
102.0(2)
110.0(2)
110.0(2)
102.0(2)
114.6(3)
122.7(2)
122.7(2)
125.0(2)
117.5(2)
117.5(2)
103.2(2)
109.5(2)
109.5(2)
103.2(2)
115.9(2)
122.0(2)
122.0(2)
A and B denote two crystallographically independent anions in the crystal structure of 3.
resonance of the anion carbon atom in the 13C NMR
of 2 and 3 has very low intensity and is found at
165.2 ppm in both cases. In addition, 3 shows the
expected resonances for the coordinated acetonitrile
(see Analytical Data of 3) and 2 shows the resonances
corresponding to the aromatic carbon atoms in the
cation in the range between ∼118 and ∼135 ppm.
In addition, Fig. 1 shows the 13C NMR spectra of
compound 2. The coupling constants between the
carbon and the phosphorous atoms have the follow-
ing values: 1 JC-P = 89.2, 2 JC-P = 10.8, 3 JC-P = 12.7, and
4 JC-P = 2.7 Hz and are useful to assign the carbon
atoms resonances by comparison with the literature
[22]. Finally, the 14N NMR shows two broad (ꢀν1/2
∼400 Hz) resonances at ∼+20 and ∼ −60 ppm cor-
responding to the tetrazole-ring nitrogen atoms and
the nitro-group resonance is much sharper (ꢀν1/2
∼70 Hz) and found at ∼–20 ppm.
there are no interactions between cations and an-
ions due to the great bulk of the tetraphenylphos-
phonium cations and the lack of hydrogen atoms
linked to an electronegative atom to form classical
hydrogen bonds. The coordination around the phos-
phorus atoms is tetrahedral, and the phenyl groups
arrange themselves in such a way as to diminish the
clashes between the aromatic protons as seen in the
structure of compounds, containing the same cation
[23]. A further discussion of the bonds and angles
in the cation (Table 3) of the cation is omitted here.
The solid-state structures of 2 and 3 were deter-
mined experimentally by X-ray diffraction analysis.
In Table 2, there are tabulated the distances and an-
gles for the anion in both compounds. Whereas the
crystal structure of 2 is composed of one only crys-
tallographically independent anion, that of 3 is com-
posed of two (A and B). The distances and angles
in 3 are in agreement with previous reports [6–7],
whereas 2 shows, in general, comparatively shorter
bonds, indicating a better conjugation of the nega-
tive charge throughout the tetrazole ring as expected
from the lack of interaction to the cation.
FIGURE 2 Asymmetric unit of 2, showing the labeling
scheme (diamond ellipsoids at the 50% probability). The hy-
drogen atoms have been omitted for the sake of simplicity.
Figure 2 shows the asymmetric unit of 2 with
the labeling scheme. As expected in the solid state,
Heteroatom Chemistry DOI 10.1002/hc

40 Klapo¨tke, Sabate´, and Welch
◦
˚
TABLE 3 Distances (A) and Angles ( ) for the Tetraphenylphosphonium Cation in 2
˚
Distances (A)
P–C1
P–C7
P–C13
P–19
C1–C2
C3–C2
C4–C3
C4–C5
C6–C5
C6–C1
C7–C8
C8–C9
C9–C10
C10–C11
1.786(2)
1.793(2)
1.793(2)
1.791(2)
1.396(3)
1.374(3)
1.386(4)
1.368(3)
1.383(3)
1.392(3)
1.393(3)
1.377(3)
1.378(3)
1.384(3)
C11–C12
C12–C13
C13–C14
C14–C15
C15–C16
C16–C17
C17–C18
C18–C13
C19–C20
C20–C21
C21–C22
C22–C23
C23–C24
C24–C19
1.392(3)
1.387(3)
1.388(3)
1.382(3)
1.376(3)
1.382(3)
1.387(3)
1.394(3)
1.393(3)
1.374(3)
1.387(3)
1.373(3)
1.373(3)
1.393(3)
Angles (◦)
C1–P–C7
110.8(1)
109.9(1)
108.9(1)
109.9(1)
108.3(1)
108.7(1)
119.3(2)
119.8(1)
120.7(1)
119.8(2)
120.1(2)
119.8(2)
120.8(1)
119.3(2)
119.1(2)
120.2(2)
120.4(2)
119.7(2)
119.9(2)
C9–C8–C7
120.0(2)
120.3(2)
120.2(2)
120.1(2)
119.5(2)
119.8(2)
120.4(2)
120.0(2)
120.8(2)
120.3(2)
120.0(2)
120.2(2)
119.6(2)
119.9(2)
120.4(2)
120.2(2)
120.3(2)
120.2(2)
C1–P–C13
C1–P–C19
C19–P–C13
C19–P–C7
C13–P–C7
C24–C19–C20
C24–C19–P
C20–C19–P
C10–C11–C12
C5–C6–C1
C12–C7–C8
C12–C7–P
C8–C7–P
C16–C15–C14
C21–C20–C19
C15–C14–C13
C17–C18–C13
C7–C12–C11
C16–C17–C18
C15–C16–C17
C5–C4–C3
C23–C22–C21
C4–C5–C6
C8–C9–C10
C2–C3–C4
C20–C21–C22
C9–C10–C11
C22–C23–C24
C18–C13–P
C6–C1–C2
C6–C1–P
C2–C1–P
C14–C13–C18
C14–C13–P
C23–C24–C19
Regardless of the bulk of the cations, there still exists
a certain planarity among the 5-nitrotetrazolate an-
ions, which sit on a layer with the nitro-groups point-
ing toward the opposite direction in respect to the
nitro-groups of the anions on the next layer (Fig. 3).
The distances between anions layers are approxi-
Fig. 5, there is represented a view along the z axis
of the unit cell of the compound. There exists lines
of (type A) anions along the y axis that interact
through π-stacking, and the acetonitrile molecules
are coplanar to the plane formed by this type of an-
ions as mentioned above. N3 and O2 (see Fig. 7 for
labeling scheme) chelate the Na cations, and these
are linked to other cations by coordination to water
molecules forming the squares shown in Fig. 5, with
lines of anions (type B) perpendicular to it with the
axis created by C1 and N5 cutting through two Na
atoms.
As shown in Fig. 6, every Na cation coordi-
nates to seven electronegative atoms (four oxygen
+ three nitrogen atoms), forming a distorted pen-
tagonal bipyramid where the axial positions are oc-
cupied by a water molecule and a 5-nitrotetrazolate
anion (∧O1–Na1–N1 = 151.84(1)◦). The rest of the co-
ordination around the metal center is completed
on the pentagonal base equatorial “plane” by a sec-
ond water molecule, two single contacts to two ni-
trotetrazolates (one to the nitro-group oxygen and
˚
mately ∼4 A, which together with the big cations
make for a very inefficient packing. This is reflected
in the low-density value (ρcalc = 1.342 g/cm3).
Figure 4 shows a view of the unit cell of 3 along
the y axis. The two crystallographically independent
5-nitrotetrazolate anions can be distinguished eas-
ily: type A anions are found on the vertices of the
unit cell, on two of the faces, on four edges and in
the center, and they are roughly coplanar to one an-
other and approximately perpendicular to the type
B anions, which are found within the unit cell face-
on to acetonitrile molecules. The water molecules,
which are also found within the unit cell, are linked
together through hydrogen bonds to the anions
˚
(dO-N = 2.899(1) A) and the nitrogen corresponding
˚
to an acetonitrile molecule dO-N = 3.037(1) A). In
Heteroatom Chemistry DOI 10.1002/hc

Preparation and Crystal Structures of Two Salts with the 5-Nitrotetrazolate Anion 41
FIGURE 3 View of the unit cell of 2 along the x axis (diamond ellipsoids at the 50% probability). The hydrogen atoms have
been omitted for the sake of simplicity.
one to a nitrogen atoms) and one nitrotetrazolate,
which chelates the Na cation. On the pentagonal
base the angles vary between ∧N2–Na1–N6 = 91.60(1)◦
and ∧O3−Na1−O5 = 60.30(1)◦ (Table 4).
Fig. 7. One of them chelates sodium through N3 and
one of the nitro-group oxygen atoms with distances
of 2.588(1) and 2.762(1) A, respectively. A similar
˚
chelating effect is also observed for 1, but is stronger
˚
˚
The two types of crystallographically indepen-
dent 5-nitrotetrazolate anions are represented in
(dN–Na = 2.467(2) A, dO–Na = 2.641(2) A) [6]. It can be
argued that the insertion of acetonitrile molecules
FIGURE 4 Diamond representation (ellipsoids at 50% probability) of a view of the unit cell along the y axis in 3 (a) and a
schematic representation of the unit cell and (b) 1, 2, 3, and 4 represent 5-nitrotetrazolate anions placed at the edges, in the
middle of a face, in the middle of a vertex or in the middle of the cell, respectively.
Heteroatom Chemistry DOI 10.1002/hc

42 Klapo¨tke, Sabate´, and Welch
FIGURE 5 View of the unit cell of 3 along the z axis, showing the stacks of anions along the y axis (diamond ellipsoids at the
50% probability) and the coordination around the Na atoms (dotted lines).
FIGURE 6 Coordination of the sodium atoms in 3 (a) and simplified coordination (b). Diamond ellipsoids at the 50% probability.
in the crystal structure of 2 forces the sodium atoms
to place themselves more distant to the nitrotetrazo-
late anions than when there is no solvent molecules
“disturbing” the packing; however, surprisingly, the
presence of acetonitrile molecules in the structure of
2 (ρcalc. = 1.778 g/cm3) seems to make for a more ef-
ficient overall packing than in the case of 1 where
the density calculated from the crystal structure
measurement is slightly lower (ρcalc. = 1.731 g/cm3).
The other type of anion is characterized by the
Heteroatom Chemistry DOI 10.1002/hc

Preparation and Crystal Structures of Two Salts with the 5-Nitrotetrazolate Anion 43
◦
˚
TABLE 4 Distances (A) and Angles ( ) for the Coordination Around the Metal Center in 3
˚
Distances (A)
Na–N2
Na–O1
Na–O3
Na–O5
2.527(1)
2.391(1)
2.734(1)
2.763(1)
Na–N6
Na–O1
Na–N1
2.588(1)
2.454(1)
2.471(1)
Angles (◦)
N2–Na–N6
N6–Na–O5
O5–Na–O3
91.60(1)
60.77(1)
60.30(1)
O3–Na–O1
O1–Na–N2
O1–Na–N1
74.02(1)
76.01(1)
151.84(1)
FIGURE 7 Coordination of the 5-nitrotetrazolate anions to cations type A (a) and type B (b) in 3 with the labeling scheme.
coordination to one sodium cation of all electroneg-
ative atoms except for the nitro-group nitrogen atom
(N5). The strongest coordination is observed, as ex-
also insensitive to an electrostatic discharge of ∼20
kV, ESD testing using a HF-vacuum-tester type VP
24. Rapid heating of 2 in the flame results in loud
explosion, whereas compound 3 burns normally.
In conclusion, we synthesized a salt of 5-
nitrotetrazole (B) with the bulky tetraphenylphos-
phonium cation (2) and fully characterized it by an-
alytical and spectroscopic methods. The synthesis
of highly energetic neutral 2-methyl-5-nitrotetrazole
failed, and single crystals of explosive bis-(sodium
5-nitrotetrazolate) dihydrate acetonitrile adduct (3)
were obtained. Both compounds showed decreased
sensitivity to classical stimuli. Finally, the crystal
structures of 2 and 3 were determined by X-ray
diffraction techniques and are discussed in detail.
˚
pected, for the nitrogen atoms (dN1–Na = 2.471(1) A,
˚
dN2–Na = 2.527(1) A), whereas the oxygen atoms co-
˚
ordinate more loosely with dO–Na = 2.734(1) A. In
comparison to the other crystallographically inde-
pendent nitrotetrazolate anions, this second type
has a much closer bonding to the metal (type
˚
˚
A: dN1–Na = 2.471(1) A, dN2–Na = 2.527(1) A; type B:
˚
dN–Na = 2.588(1) A).
To assess the energetic properties of 2 and 3, we
measured its sensitivity to shock, friction, and elec-
trostatic discharge by standard BAM tests [24,25]
and the thermal stabilities by differential scanning
calorimetry (DSC) measurements [12]. Impact: In-
sensitive >40 J, less sensitive ≥35 J, sensitive ≥4 J,
very sensitive ≤3 J; friction: Insensitive >360 N, less
sensitive = 360 N, sensitive <360 N a. >80 N, very
sensitive ≤80 N, extreme sensitive ≤10 N; According
to the UN Recommendations on the Transport of Dan-
gerous Goods, (+) indicates: not safe for transport.
At a heating rate of 2◦C min−1, 2 melts at ∼126◦C
and decomposes at ∼188◦C, whereas 3 decomposes
at ∼200◦C. Both compounds showed no sensitiv-
ity toward shock (>30 J) nor in the friction test
(>360 N). Grinding the compounds in a mortar did
not result in explosion either. Both compounds were
ACKNOWLEDGMENTS
The authors acknowledge collaborations Dr.
M. Krupka (OZM Research, Czech Republic) in the
development of new testing and evaluation methods
for energetic materials and with Dr. M. Sucesca
(Brodarski Institute, Croatia) in the development of
new computational codes to predict the detonation
parameters of high-nitrogen explosives. We are
indebted to and thank Dr. Betsy M. Rice (ARL,
Aberdeen, Proving Ground, MD) for many helpful
and inspired discussions and support of our work.
Heteroatom Chemistry DOI 10.1002/hc

44 Klapo¨tke, Sabate´, and Welch
[12] http://www.linseis.net/html en/thermal/dsc/dsc pt10.
php.
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