The Pyridazine Scaffold as a Building Block for Energetic Materials: Synthesis, Characterization, and Properties

Article abstract of DOI:10.1002/zaac.201900146

In the present studies, the synthesis of new energetic materials based on the pyridazine scaffold and their characterization is the main subject. For this purpose, desired 3,5-dimethoxy-4,6-dinitropyridazine-1-oxide (7) was synthesized in the first instance. The persubstituted pyridazine precursor laid the groundwork for further preparative modification. The targeted functionalization through the regioselective introduction of various smaller amine nucleophiles such as methylamine or 2-aminoethanol gave several new energetic materials. Among them are 3,5-bis(methylamino)-4,6-dinitropyridazine-1-oxide (8), 3,5-bis(methylnitramino)-4,6-dinitropyridazine-1-oxide (9), 3,5-bis(dimethylamino)-4,6-dinitropyridazine-1-oxide (10), and 3,5-bis((2-hydroxyethyl)amino)-4,6-dinitropyridazine-1-oxide (11). With the aim of increasing the detonation performance, compound 8 was additionally nitrated and 3,5-bis(methylnitramino)-4,6-dinitropyridazine-1-oxide (9) was obtained. These new energetic materials were characterized and identified by multinuclear NMR (¹H, ¹³C, ¹⁴N, ¹⁵N) and IR spectroscopy, elemental analysis and mass spectrometry. In addition, their sensitivities toward impact, friction and electrostatic discharge were thoroughly examined. Furthermore, obtained single-crystals of the substances were characterized by low-temperature single-crystal X-ray diffraction.

Full text of DOI:10.1002/zaac.201900146

Title: The Pyridazine Scaffold as a Building Block for Energetic
Materials: Synthesis, Characterization and Properties
Authors: Thomas M. Klapötke, Joerg Stierstorfer, Ivan Gospodinov,
and Johannes Singer
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To be cited as: Z. anorg. allg. Chem. 10.1002/zaac.201900146
Link to VoR: http://dx.doi.org/10.1002/zaac.201900146

10.1002/zaac.201900146
Zeitschrift für anorganische und allgemeine Chemie
ARTICLE
The Pyridazine Scaffold as a Building Block for Energetic
Materials: Synthesis, Characterization and Properties
Ivan Gospodinov,[a] Johannes Singer,[a] Thomas M. Klapötke*[a] and Jörg Stierstorfer[a]
Abstract: In the present studies, the synthesis of new energetic
materials based on the pyridazine scaffold and their
characterization is the main subject. For this purpose, desired
3,5-dimethoxy-4,6-dinitropyirdazine-1-oxide (7) was synthesized
in the first instance. The persubstituted pyridazine precursor laid
moiety results in the formation of thermally stable, insensitive
materials with good detonation parameters. This can be
explained with the formation of intra- and intermolecular
hydrogen bonding throughout the explosive material, which on
other hand results in the stabilization of the molecular structure
(e.g. LLM-105, TANPyO and DADNP). The introduction of an
N+–O− moiety into the energetic materials results in an increase
of the density,[6] improves the oxygen balance[7], increases the
stability in the molecular structure[8] and in addition improves
the detonation properties.[9] Good examples for this class of N-
the groundwork for further preparative modification.
The
targeted functionalization through the regioselective introduction
of various smaller amine nucleophiles such as methylamine or
2-aminoethanol gave several new energetic materials. Among
them are 3,5-bis(methylamino)-4,6-dinitropyridazine-1-oxide (8),
3,5-bis(methylnitramino)-4,6-dinitropyridazine-1-oxide (9),
3,5-
oxidized energetic materials are dihydroxylammonium 5,5
bistetrazole-1,1 -dioxide (TKX-50) and 2,6-diamino-3,5-
dinitropyrazine-1-oxide (LLM-105). In comparison to its O-free
analogue, dihydroxylammonium 5,5 -bistetrazolate, the density
of TKX-50 increases from 1.74 to 1.88 g cm−3,and its calculated
-
bis(dimethylamino)-4,6-dinitropyridazine-1-oxide (10) and 3,5-
bis((2-hydroxyethyl)amino)-4,6-dinitro-pyridazine-1-oxide (11).
With the aim of increasing the detonation performance,


compound
8
was
additionally
nitrated
and
3,5-
was
bis(methylnitramino)-4,6-dinitropyridazine-1-oxide
(9)
detonation velocity increases from 8854 m
s−1 to 9698
obtained. These new energetic materials were characterized
and identified by multi-nuclear NMR (1H, 13C, 14N, 15N) and IR
spectroscopy, elemental analysis and mass spectrometry. In
addition, their sensitivities toward impact, friction and
electrostatic discharge were thoroughly examined. Furthermore,
obtained single-crystals of the substances were characterized by
low-temperature single-crystal X-ray diffraction.
m s−1.[10,11] The same effect is observed for LLM-105 (ρ = 1.92
g cm−3, DC-J = 8516 m s−1) and its precursor 2,6-diamino-3,5-
dinitropyrazine (ANPZ, ρ = 1.84 g cm−3, DC-J = 7892 m s−1).[7,12]
Introduction
Figure 1. Literature known N-oxidized energetic materials.
Recently, in our research group we managed to functionalize the
During the last decades, extensive work and efforts have been
made to discover highly efficient and performing new energetic
materials with high thermal stability and good sensitivities toward
accidental stimuli.[1] In addition, the production of new
environmental benign energetic materials is desired process,
due to the fact that mostly of the nowadays synthesized HEDM
(RDX, TATB and TNT) are either carcinogenic or toxic.[2] For
this purpose scientists worldwide have been developing modern
approaches for the synthesis of new energetic materials. An
emerging approach is the use of polyfunctionalized nitrogen-rich
heterocycles as a building block for new materials. In several
instances, azole and diazine based energetic materials have
been found to have promising physico-chemical properties e.g.
1,2-diazine
dinitropyridazine-1-oxide and its precursor 3,5-dimethoxy-4,6-
dinitropyridazine-1-oxide.[13] For this purpose, selective
scaffold
by
synthesizing
3,5-diamino-4,6-
a
functionalization of the pyridazine scaffold was carried out by
introducing explosophore NO2 groups and selectively introducing
a N+–O− moiety. In this present work we reacted 3,5-dimethoxy-
4,6-dinitropyridazine-1-oxide (7) with different amines and
investigated the physico-chemical properties of the newly
synthesized derivatives 8–11. The detonation properties and the
sensitivities of the energetic pyridazines can be adjusted
depending on the used amine for the reaction.
3,4,5-trinitropyrazole,[3]
bis(4-amino-3,5-dinitropyrazolyl)-
Results and Discussion
methane[4] and 2,6-diamino-3,5-dinitropyrazine-1-oxide (LLM-
105).[5] It appears that the combination of nitrogen-rich
heterocycles with an alternating C–NH2/C–NO2 and N+–O−
Synthesis
Herein, we report the synthesis of new pyridazine based
energetic materials. The target molecules 3,5-bis(methylamino)-
4,6-dinitropyridazine-1-oxide (8), 3,5-bis(methylnitramino)-4,6-
[a] Prof. Dr. Thomas M. Klapötke, Ivan Gospodinov, Dr. Jörg
Stierstorfer.
Ludwig-Maximilian University Munich, Department of Chemistry
Butenandtstr. 5-13 (D), 81377 München, Germany
Fax: (+) 49 (0) 89 - 2180 77492
dinitropyridazine-1-oxide
dinitroypridazine-1-oxide
(9)
(10)
3,5-bis(dimethylamino)-4,6-
and 3,5-bis((2-
E-mail: tmk@cup.uni-muenchen.de
Homepage: http://www.hedm.cup.uni-muenchen.de
(hydroxyethyl)amino)-4,6-dinitropyridazine-1-oxide (11) were
synthesized by using 3,5-dimethoxy-4,6-dinitropyridazine-1-
oxide (7) as starting material. Compound 7 was prepared
according the literature known five step procedure and will not
Supporting information for this article is given via a link at the end of the
document.
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10.1002/zaac.201900146
Zeitschrift für anorganische und allgemeine Chemie
ARTICLE
be discussed in this work.[13] The exact synthetic path for 7 is
shown in Scheme 1.
free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif. Single-crystals
of compound 8 were obtained by evaporating a solution of 8 in
acetone and water. Compound 8 crystallizes in the orthorhombic
space group P bca (no. 61) with eight formula units per cell and
has a cell volume of 1941.40(7) Å3. The cell constants are a =
9.7734(2) Å, b = 13.5583(3) Å and c = 14.6509(3) Å. The
calculated density at 143(2) K is 1.67 g cm−3.
Looking at the determined bond lengths 1.3276(18) Å (N1–N2),
1.3506(19) Å (C4–N1), 1.352(2) Å (N2–C1), 1.432(2) Å (C1–C2),
1.433(2) Å (C2–C3) and 1.412(2) Å (C3–C4), the existing
aromaticity inside the ring structure can be comprehended
(Figure 2). They are all between the typical values for single and
double bonds concerning these involved elements.[14,15,16]
The interatomic distances of N3–C1 (1.329 Å) and N5–C3
(1.332(2) Å) also display a participation of these bonds in
aromatic resonance. The bond in the N+–O− moiety is
1.2635(17) Å and herewith slightly shorter than that of its
precursor 3,5-dimethoxy-4,6-dinitropyridazine-1-oxide (7) with
1.268(2) Å.[13] The slight deformation of the planar ring
structure can be recognized by the observed divergent dihedral
angles 4.5(2)° (N1–N2–C1–C2) and −6.6(2)° (N2–C1–C2–C3).
The C4-connected nitro group is twisted against the ring plane
according to the O5–N6–C4–N1 torsions angle of −74.69(18)°.
Its significant rotation can be understood by both the high spatial
demand of the voluminous methylamine neighbor, which even
shows only a moderate displacement regarding the ring plane,
and especially the high electrostatic repulsion between the
vicinal-located N+–O− moiety and the nitro-oxygens. As a result
of its noticeable turn, the C4–N6 bond length of 1.4637(19) Å is
elongated compared that of the other intramolecular nitro group
with 1.4157(19) Å.
Scheme 1. Literature known synthesis for 3,5-dimethoxy-4,6-dinitropyridazine-
1-oxide.
Compound 7 was used as the starting material for further
functionalization of the pyridazine scaffold. Further nucleophilic
substitution of the methoxy groups allows regioselectiv
functionalization of compound 7. As appropriate nucleophiles
various smaller amines e.g. methylamine, dimethylamine and 2-
aminoethanol were reacted with compound 7. Compounds 8–11
were obtained in good yields 76–91 %. To enhance the
performance of the newly synthesized pyridazine based
energetic materials compounds 8 and 11 were further nitrated.
Nitration of compound 8 resulted in the formation of 3,5-
bis(methylnitramino)-4,6-dinitropyridazine-1-oxide (9) in 91 %
yield. Nitration of compound 11 resulted in decomposition of the
starting material and the polynitrated compound 12 was not
obtained. All obtained energetic materials were thoroughly
characterized and further examined regarding their sensitivities
toward mechanical and electrostatic stimuli. All reactions are
displayed in Scheme 2.
Figure 2. Molecular unit of compound 8 in the crystalline state. Ellipsoids
correspond to 50% probability levels. Hydrogen radii are arbitrary. Selected
bond lengths reported in [Å] and angles reported in [°]: N1–N2 1.3276(18),
C4–N1 1.3506(19), N2–C1 1.352(2), C1–C2 1.432(2), C2–C3 1.433(2), C3–
C4 1.412(2), O1–N1 1.2635(17), N3–C1 1.329(2), N5–C3 1.332(2), N4–C2
1.4157(19), O2–N4 1.2401(19), N6–C4 1.4637(19), O4–N6 1.2229(18), N2–
N1–C4 123.90(13), N1–C4–N6 111.94(13), O1–N1–N2–C1 −177.94(14) N1–
N2–C1–C2 4.5(2), N2–C1–C2–C3 −6.6(2), C2–C3–C4–N1 2.7(2), C5–N3–
C1–N2 −2.6(2), C6–N5–C3–C4 −9.1(3), O2–N4–C2–C1 12.6(2), N5–C3–C4–
N6 −0.2(2), O5–N6–C4–N1 −74.69(18).
Scheme 2. Synthesis of new energetic pyridazine derivatives by
functionalizing the 3,5-dimethoxy-4,6-dinitropyridazine-1-oxide scaffold (7).
Crystal structures
During this work the crystal structures of all four synthesized
compounds (8–11) were obtained. Selected data and
parameters from the low-temperature X-ray data collection are
given in the Supporting Information (Tables S1 and S2). CCDC-
1917212 (8), CCDC-1917211 (9), CCDC-1917213 (10) and
Single-crystals of 9 were obtained by evaporating a solution of
compound 9 in a mixture of acetone and water. Compound 9
crystallizes in the orthorhombic space group P 2 2 2 (no. 16)
with four formula units per cell and has a cell volume of
1287.01(10) Å3. The calculated density at 143(2) K is
CCDC-19172110
(11)
contain
the
supplementary
crystallographic data for this paper. These data can be obtained
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10.1002/zaac.201900146
Zeitschrift für anorganische und allgemeine Chemie
ARTICLE
1.73 g cm−3, which is higher than the density of its precursor 8
group is 1.4157(19) Å with a twist of −74.69(18)°. In comparison,
this bond in pyridazine derivative 9 is 1.459(2) Å and thus
slightly elongated. The dihedral angles for the tertiary amino
substituents are −58.0(2)° (N7–N6–C3–C2) und −26.0(2)° (C5–
N3–C1–N2). The distances of the N3–N4 and N5–N6 bonds in
the nitramine moieties are 1.408(3) Å and 1.385(2) Å,
respectively. The bond angle 123.21(17)° included from C2–C1–
N3 is somewhat widened, while the adjacent N2–C1–N3 bond
angle with 112.77(16)° is noticeably compressed (Figure 3).
Single-crystals from compound 10 were obtained by evaporating
a solution of 10 in acetone and water. Compound 10 crystallizes
in the monoclinic space group P 21/c (no. 14) with four formula
units per cell and has a cell volume of 1144.00(8) Å3. The cell
constants are a = 9.7162(4) Å, b = 13.6057(5) Å and c =
8.7037(4) Å with β = 96.137(4)°. The calculated density at
143(2) K is 1.58 g cm−3.
with 1.67 g cm−3.
Figure 3. Molecular unit of compound 9 in the crystalline state. Ellipsoids
correspond…to 50% probability levels. Hydrogen radii are arbitrary. Selected
bond lengths reported in [Å] and angles reported in [°]: N1–N2 1.335(2), C4–
N1 1.358(2), N2–C1 1.325(2), C1–C2 1.399(3), C2–C3 1.384(3), C4–C3
1.372(2), O1–N1 1.2561(17), N3–C1 1.403(2), N3–N4 1.408(3), N4–O2
1.215(2), N3–C5 1.462(3), N5–C2 1.465(2), O4–N5 1.221(2), N6–C3 1.417(2),
N6–N7 1.385(2), C4–N8 1.459(2), O1–N1–N2 118.26(15), N2–C1–N3
112.77(16), N2–C1–C2 124.01(18), N4–N3–C5 116.68(18), N1–N2–C1–C2
−0.1(3), N2–C1–C2–C3 −2.7(3), C1–C2–C3–C4 3.4(2), C5–N3–C1–N2
−26.0(2), C5–N3–N4–O2 −17.0(3), N3–C1–C2–N5 −5.4(3), O4–N5–C2–C1
−40.3(2), N5–C2–C3–N6 9.4(3), N7–N6–C3–C2 −58.0(2), C3–N6–N7–O6
−17.2(2), N8–C4–N1–O1 1.9(2), C3–C4–N8–O8 81.8(2).
The N–O distance 1.2561(17) Å in the N+–O− moiety is distinctly
shortened compared to those values found in precursor 8
(1.2635(17) Å) and related 3,5-diamino-4,6-dinitropyridazine-1-
oxide (DADNP, 1.2681(19) Å).[13] These differences in length
can be understood by comparing the mesomeric impacts of the
different amino substituents to the aromatic system in each
compound. With increasing the degree of substitution regarding
the amino substituents their +M character is diminished. In
consequence, the less electron density inside the aromatic ring
causes the observed bond contractions. In molecule 9 this effect
attains an extra dimension due to the nitration of the respective
amino substituents because of the strong electron-withdrawing
nitro groups. In contrast, the N–N distances in the pyridazine
ring structure show an opposing trend. The N–N bond lengths in
DADNP with 1.323(2) Å and compound 8.1.3276(18) Å are
shorter than that found in pyridazine 9 with 1.335(2) Å (N1–
N2).[13] All remaining N–C and C–C bonds in the ring are with
slightly divergence to each other in the aromatic
range.[14,15,16] The ring-internal dihedral angles such as C1–
C2–C3–C4 with 3.4(2)° and N2–C1–C2–C3 with −2.7(3)° show
only slight aberration. This reflects the high planarity of the
aromatic backbone. The nitro groups connected to the backbone
exhibit different amounts of rotation against the ring plane
according to the torsions angles O4–N5–C2–C1 with −40.3(2)°
and C3–C4–N8–O8 with even 81.8(2)°. The eminently twist of
latter substituent, which is surprisingly near to 90°, restricts the
pz-orbital of the N8-nitrogen to participate in aromaticity because
of the slim π-overlap. In further consequence, the considerable
elongation of the interatomic distance between N8 and C4
results. In its precursor 8 the bond length of the analogous nitro
Figure 4. Molecular unit of compound 10 in the crystalline state. Ellipsoids
correspond to 50% probability levels. Hydrogen radii are arbitrary. Selected
bond lengths reported in [Å] and angles reported in [°]: N1–N2 1.3289(17),
N1–C4 1.3645(18), N2–C1 1.3553(18), C1–C2 1.4408(19), C2–C3 1.432(2),
C3–C4 1.4057(19), O1–N1 1.2577(15), N3–C1 1.3238(19), N3–C5 1.4717(19),
N4–C2 1.4234(18), O2–N4 1.2403(17), O4–N6 1.2215(17), N5–C7 1.4611(19),
N1–N2–C1 116.76(12), C4–C3–C2 113.49(12), O1–N1–N2 117.12(11), C1–
N3–C5 120.74(13), C6–N3–C5 115.07(13), C7–N5–C8 113.46(12), N1–N2–
C1–C2 −14.7(2), N2–C1–C2–C3, C1–C2–C3–C4 −19.07(19), N2–C1–C2–C3
28.2(2), N1–N2–C1–N3 166.97(13), C5–N3–C1–N2 5.0(2), N3–C1–C2–N4
50.1(2), O2–N4–C2–C1 −17.9(2), C7–N5–C3–C2 −21.6(2), N5–C3–C4–N6
−2.5(2), O4–N6–C4–C3 −71.22(19).
Compared
to
3,5-diamino-4,6-dinitropyridazine-1-oxide
(DADNP) with 1.89 g cm−3 and related 3,5-bis(methylamino)-4,6-
dinitropyridazine-1-oxide (8) with 1.67 g cm−3 its crystal-structure
is clearly less dense.[13] The N–N bond length in the pyridazine
ring is 1.3289(17) Å (Figure 4). The ring-internal N–C bonds are
1.3645(18) Å and 1.3553(18) Å and the remaining C–C
distances are 1.426 Å on average. These length values are all in
the aromatic range and show no significant difference to the
bonds inside the pyridazine ring of 8.[13,14,15,16] Indeed,
despite the existing aromaticity the expected flat pyridazine ring
is drastically deformed. The C1–C2–C3–C4 and N2–C1–C2–C3
torsions angles aberrate from the ring plane with −19.07(19)°
and even 28.2(2)°, respectively. Compared to the dihedral
angles C1–C2–C3–C4 with 2.8(2)° and N2–C1–C2–C3 −6.6(2)°,
the aberration of planarity found for related 8 is eminently
smaller. Also, the ring-internal bond angles diverge from the
regular angle of 120° for sp2-hybridized bond centers. Among
them is the C4–C3–C2 bond angle with the divergence of 6.5°.
The bond length between the nitrogen and oxygen inside the
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10.1002/zaac.201900146
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ARTICLE
N+–O− function is 1.2577(15) Å (N1–O1). In comparison, this N–
O distance is markedly shorter than those found in precursor 7
the N1–N2 distance is 1.328(2) Å and the N2–C4 distance is
1.358(2) Å. Also, the internal bond angles such as N2–N1–C1
of 122.74(14)°), C2–C3–C4 of 119.07(15)° and N2–C4–C3 of
122.14(15)° are all near to the characteristic 120° for sp2
hybridized bond centers. All of these bond lengths and angles
indicate the existing aromaticity.[14] The N–O bond lengths in
the N+–O− moiety is 1.2603(17) Å, which is somewhat shorter
(1.268(2)
Å),
3,5-diamino-4,6-dinitropyridazine-1-oxide
(1.2681(19) Å, DADNP) and even 3,5-bis(methylamino)-4,6-
dinitropyridazine-1-oxide (8, 1.2635(17) Å).[13] Besides, the N–
C bonds connecting the spatially demanding dimethylamino
substituents to the molecular backbone of 10 have no significant
difference. Against this, the amount of rotation noticeably differs.
The dimethylamino substituent connected to the C1-carbon
shows only a slight turn against the ring plane, whereas the
other one in between of both nitro groups is significantly more
rotated against the plane according to the C7–N5–C3–C2
torsions angle of −21.6(2)°. The medium length of the four bonds
between the methyl groups and the tertiary substituted amino
groups is 1.463 Å, which is in the typical range for N–C single
bonds.[14] Despite the relatively high spatial demand of
dimethylamino substituents, the nitro group in between is slightly
rotated against the molecule plane (O2–N4–C2–C1 −17.9(2)°).
In contrast, the torsions angle of O4–N6–C4–C3 with
−71.22(19)° displays the very high aberration of the other nitro
group. In spite of the higher spatial need concerning the vicinal
tertiary amino substituent, the amount of rotation for this
substituent is even lower than that found in compound 8
(−74.69(18)°).
than that found in precursor
7 (1.268(2) Å) and related
compound (1.2635(17) Å).[13] The interatomic distances
8
between the nitrogen atoms of the (2-(hydroxyl)ethyl)amino
substituents and the pyridazine backbone also show an aromatic
participation. All torsions angles between the ring-internal atoms
show only slight deformation (|Θout of plane| ≤ 2.1(3)°) and is lower
than that in precursor 7 (|Θout of plane| ≤ 6.6°)) and even 10 (|Θout of
plane| ≤ 28.2(2)°). The nitro group next to the N-oxide shows the
significant twist of 70.1(2)° (O2–N3–C1–N1). This amount of
rotation is in the range of those found for the analogous
substituents in 8 (−74.69(18)°) and 10 (−71.22(19)°).
15N NMR spectroscopy
In addition, 15N NMR spectra of compounds 8 and 11 were
recorded (Figure 6). Compound 8 exhibits six resonances in the
15N spectrum; both NO2 groups are observed at −17.1 and 22.2
ppm, the pyridazine nitrogen atoms at −69.8 (N+–O−) and −113.2
ppm. Both methylamine nitrogen atoms show a low intensity and
can be observed at −317.9 and −323.1 ppm as singlet.
Single crystals of 11 were obtained by evaporating a solution of
compound 11 in acetone. Compound 11 crystallizes in the
monoclinic space group P 21/c (no. 14) with four formula units
per cell and has a cell volume of 1165.20(11) Å3. The cell
constants are a = 6.6388(4) Å, b = 22.0275(12) Å and c =
8.0977(4) Å with β = 100.271(5)°. The calculated density at
416(2) K is 1.73 g cm−3.
Figure 6. 15N NMR spectra of compounds 8 and 11.
Six resonances can be observed in the 15N spectrum of
compound 11. The nitro groups appear at −17.0 and −22.6 ppm
and the pyridazine nitrogen atoms exhibit similar shift as
compound 8 −69.5 (N-oxide) and 113.5 ppm. Both duplets at
−282.4 (JNH = 95.4 Hz) and −287.4 (JNH = 92.8 Hz) ppm can be
assigned to both NH atoms.
Figure 5. Molecular unit of compound 11 in the crystalline state. Ellipsoids
correspond to 50% probability levels. Hydrogen radii are arbitrary. Selected
bond lengths / [Å] and angles / [°]: N1–N2 1.328(2), N1–C1 1.353(2), N2–C4
1.358(2), C1–C2 1.411(2), C2–C3 1.428(2), C3–C4 1.433(2), O1–N1
1.2603(17), N6–C4 1.329(2), N6–C8 1.462(2), C8–C9 1.513(3), C9–O7
1.422(2), N5–C3 1.420(2), O4–N5 1.2423(18), N2–N1–C1 122.74(14), C2–
C3–C4 119.07(15), N2–C4–C3 122.14(15), C4–N6–C8 122.87(15), N6–C8–
C9 111.10(15), O7–C9–C8 112.11(16), C2–N4–C5 129.24(15), O6–C6–C5
109.28(15), C1–N1–N2–C4 1.7(2), N1–N2–C4–C3 −0.6(2), C1–C2–C3–C4
−0.1(2), O1–N1–N2–C4 179.54(14), N2–N1–C1–N3 174.94(14), O2–N3–C1–
N1 70.1(2), N1–C1–C2–N4 −179.50(16), C5–N4–C2–C1 14.6(3), C2–N4–C5–
C6 150.46(18), N4–C2–C3–N5 3.6(3), O4–N5–C3–C2 2.5(2), N1–N2–C4–N6
179.21(15), C8–N6–C4–N2 6.5(2), C4–N6–C8–C9 82.1(2).
Detonation properties
Compounds 8–11 can be classified as energetic materials,
therefore their energetic properties were investigated. All
theoretically and experimentally determined values for all four
compounds are reported in Table 1. In addition, the thermal
behavior was investigated with an OZM Research DTA 552-Ex
instrument with a heating rate of 5 °C min−1. All compounds
show sharp decomposition in the DTA plots and decompose
without melting prior. Compound 8 decomposes at 250 °C,
The N–N, N–C and C–C distances inside the pyridazine ring are
all in the range for typical single and double bond lengths
concerning these elements (Figure 5).[13,14,15,16] Among them,
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whereas the nitrated derivative 9 shows sharp decomposition at
120 °C. The reason for the drastic change in the thermal
behavior from compound 8 to 9 can be explained with the
formation of the thermally sensitive −NHNO2 moiety in 3,5-
bis(methylnitramino)-4,6-dinitropyridazine-1-oxide (9). DTA plots
of compounds 8, 9 and 11 are shown in Figure 7. Compounds
10 and 11 decompose at 217 and 170 °C, respectively.
compound 9 shows the best performance with detonation
pressure and detonation velocity of 291 kbar and 8276 m∙s−1,
respectively. Compounds 8 and 11 have similar properties with
pC-J =204 kbar and DC-J = 7365 m∙s−1 for 8 and pC-J =202 kbar and
DC-J = 7389 m∙s−1 for 11. The calculated detonation properties for
9 in comparison to its precursor 8 and compound 11 can be
explained again with the introduced nitramino groups, which
result in a good performance but decreases the stability of the
molecule.
Table 1. Physico-chemical properties of compounds 8–11.
8
9
10
11
IS[a] [J]
FS[b] [N]
10
5
30
18
360
360
120
360
ESD[c] [J]
Ω[d] [%]
Tdec[e] [°C]
ρ[f] [g∙cm−3
ΔfH°[g] [kJ∙kg−1
ΔfH°[h] [kJ∙mol−1
0.203
−72
0.033
−29
0.37
−100
217
0.25
−79
250
120
170
]
1.63
420.6
102.7
1.68
892.7
298.3
1.54
574.6
156.4
1.69
]
−824.4
−250.8
]
EXPLO5 6.03
−ΔEU°[i] [kJ∙kg−1
TC-J[j] [K]
pC-J [k] [kbar]
]
4403
3036
204
5844
4164
291
4267
2790
176
3828
2632
202
DC-J[l] [m∙s−1
V [m] [L3∙kg−1
]
7365
756
8276
753
6994
767
7389
757
]
[a] Impact sensitivity (BAM drophammer, method 1 of 6); [b] friction sensitivity
(BAM drophammer, method 1 of 6); [c] electrostatic discharge device (OZM
research); [d] oxygen balance with respect to CO2; [e] temperature of
decomposition (DTA, β = 5°C∙min−1); [f] density at 298 K; [g] standard weight
enthalpy of formation; [h] standard molar enthalpy of formation; [i] detonation
energy; [j] detonation temperature; [k] detonation pressure; [l] detonation
velocity; [m] volume of detonation gases at standard temperature and pressure
conditions.
Figure 7. DTA plots of compounds 8, 9 and 11.
Further, the sensitivities of all compounds were determined
toward external stimuli (friction and impact) by using the BAM
standards and the theoretical detonation properties were
calculated by using the EXPLO5_V6.03 computer code.[17] For
all computer calculations with the EXPLO5 code the room
temperature densities of all compounds were calculated by
using the obtained X-ray structures as reported in the
literature.[18]
The highest room temperature density was measured for
compound 11 with 1.69 g cm−3 and the lowest was determined
for 10 with 1.54 g cm−3. The sensitivity values of all four
compounds toward impact, friction and electrostatic discharge
were determined. Compounds 8, 10 and 11 exhibit impact
sensitivity of 10, 30 and 18 J, respectively. The determined
friction sensitivity for all three compounds is 360 N. Compound 9
is the most sensitive compound with values of IS = 5 J, FS = 120
N and ESD = 0.033 J. Compound 9 contains the nitramino
moiety (−NHNO2), which leads to lower thermal stability of the
molecule and higher sensitivity toward external stimuli. This is
supported with the obtained physic-chemical properties for the
pyridazine derivative 9. According to the EXPLO5 calculations
Conclusions
In conclusion, we report on the synthesis of four new N-oxidized
pyridazine based energetic materials (8–11). For this purpose,
3,5-dimethoxy-4,6-dinitropyridazine-1-oxide (7) was reacted with
methylamine, dimethylamine and 2-aminoethanol to result in the
formation of 3,5-bis(methylamino)-4,6-dinitropyridazine-1-oxide
(8), 3,5-bis(methylnitramino)-4,6-dinitropyridazine-1-oxide (10)
and 3,5-bis((2-hydroxyethyl)amino)-4,6-dinitropyridazine-1-oxide
(11). Further, the acidity of the protons in the –NHMe moiety of
compound 8 was used and it was nitrated in 100% nitric acid to
give 3,5-bis(methylnitramino)-4,6-dinitropyridazine-1-oxide (9) in
excellent yields. The synthesized energetic pyridazine
derivatives (8–11) were characterized extensively using
multinuclear NMR spectroscopy, vibrational spectroscopy, DTA,
elemental analysis and BAM sensitivity methods. In addition, the
crystal structures of all four compounds were obtained an
extensively discussed. The experimentally and theoretically
determined energetic properties were well investigated.
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244.17 g∙mol-1): Calc.: C 29.56, H 3.30, N 34.42%. Found: C 29.56, H
3.20, N 34.42%.
Compounds 8 (IS = 10 J; FS = 360 N), 10 (IS = 30 J; FS =
360 N) and 11 (IS = 18 J; FS = 360 N) exhibit similar properties
toward impact and friction, whereas compound 9 with the
nitramino moiety exhibits higher sensitivity toward external
stimuli (IS = 5 J; FS = 120 N).
3,5-Bis(methylnitramino)-4,6-dinitropyridazine-1-oxide (9)
3,5-Bis(methylamino)-4,6-dinitropyridazine-1-oxide
(8,
500
mg,
2.05 mmol, 1.0 eq.) was added in small portions to 100% HNO3
(3.33 mL) at 0 °C and stirred for 3.5 h. After pouring onto crushed ice and
stirring until the ice had melted, the resulting suspension was filtered and
washed with ice-water until the filtrate was acid free. Drying on air gave
compound 9 as yellow solid (625 mg, 1.87 mmol, 91%).
Experimental Section
CAUTION! All investigated compounds are potentially explosive
materials, although no hazards were observed during preparation and
handling these compounds. Nevertheless, safety precautions such as
(wearing leather coat, face shield, Kevlar sleeves, Kevlar gloves, earthed
equipment and ear plugs) should be drawn.
DTA (5 °C min−1): 120 °C (dec.); BAM: drop hammer: 5 J (100–500 μm);
friction tester: 120 N (100–500 μm); ESD: 33 mJ (100–500 μm). IR (ATR),
ṽ (cm−1) = 1573 (s), 1557 (s), 1453 (w), 1426 (w), 1386 (vw), 1336 (m),
1301 (m), 1260 (vs), 1150 (vw), 1094 (w), 1064 (w), 999 (m), 911 (w),
799 (m). 1H NMR (CDCl3, 400 MHz, [ppm]): δ = 3.94 (s, 3H), 3.74 (s, 3H).
13C NMR (CDCl3, 101 MHz, [ppm]): δ = 162.6, 148.0, 132.9, 121.8, 41.7,
39.4. 14N NMR (CDCl3, 29 MHz, [ppm]): δ = –30, –38, –40, –42, –71.
Elem. Anal. (C6H6N8O9, 334.16 g∙mol-1): Calc.: C 21.57, H 1.81, N
33.53%. Found: C 21.83, H 1.98, N 33.29%.
1H, 13C, 14N and 15N NMR spectra were recorded on JEOL 270 and
BRUKER AMX 400 instruments. The samples were measured at room
temperature in standard NMR tubes (Ø 5 mm). Chemical shifts are
reported as δ values in ppm relative to the residual solvent peaks of d6-
DMSO (δH: 2.50, δC: 39.5). Solvent residual signals and chemical shifts
for NMR solvents were referenced against tetramethylsilane (TMS, δ = 0
ppm) and nitromethane. Unless stated otherwise, coupling constants
were reported in hertz (Hz) and for the characterization of the observed
signal multiplicities the following abbreviations were used: s (singlet), d
(doublet), t (triplet), q (quartet), quint (quintet), sept (septet), m (multiplet)
and br (broad). Infrared spectra (IR) were recorded from 4500 cm−1 to
650 cm−1 on a PERKIN ELMER Spectrum BX-59343 instrument with a
SMITHS DETECTION DuraSamplIR II Diamond ATR sensor. The
absorption bands are reported in wavenumbers (cm−1). Elemental
analysis was carried on a Elementar Vario el by pyrolysis of the sample
and subsequent analysis of the formed gases. 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. All sensitivities toward
impact (IS) and friction (FS) were determined according to BAM
(German: Bundesanstalt für Materialforschung und Prüfung) standards
using a BAM drop hammer and a BAM friction apparatus.[19] All
energetic compounds were tested for sensitivity towards electrical
discharge using an Electric Spark Tester ESD 2010 EN from OZM.
3,5-Bis(dimethylamino)-4,6-dinitropyridazine-1-oxide (10)
3,5-Dimethoxy-4,6-dinitropyridazine-1-oxide (7, 500 mg, 2.03 mmol,
1.0 eq.) was dissolved in acetonitrile (25 mL) and an aqueous
dimethylamine solution (40%, 0.600 mL, 4.74 mmol, 2.3 eq.) was added
dropwise. The reaction was stirred at room temperature overnight. The
resulting suspension was filtered and the solvent was removed under
reduced pressure. The residue was resuspended in water, filtered and
washed with ice-water. After drying on air compound 10 was obtained as
orange solid (503 mg, 1.85 mmol, 91%).
DTA (5 °C min−1): 217 °C (dec.); BAM: drop hammer: 30 J (100–500 μm);
friction tester: 360 N (100–500 μm); ESD: 370 mJ; IR (ATR), ṽ (cm−1) =
1573 (s), 1521 (m), 1484 (m), 1464 (w), 1431 (w), 1378 (m), 1334 (w),
1250 (vs), 1210 (s), 1184 (s), 1135 (m), 1072 (s), 933 (vw), 906 (vw), 841
(w), 823 (w), 798 (w), 760 (m). 1H NMR (d6-DMSO, 400 MHz, [ppm]): δ =
3.15 (s, 6H), 3.07 (s, 6H). 13C NMR (d6-DMSO, 101 MHz, [ppm]): δ =
155.1, 145.5, 137.7, 111.6, 43.8, 41.1. 14N NMR (d6-DMSO, 29 MHz,
[ppm]): δ = –20, –69. Elem. Anal. (C8H12N6O5, 272.22 g∙mol-1): Calc.: C
35.30, H 4.44, N 30.87%. Found: C 34.78, H 3.99, N 30.24%. m/z (DEI+):
272(100) [M]+.
3,5-Bis(methylamino)-4,6-dinitropyridazine-1-oxide (8)
3,5-Dimethoxy-4,6-dinitropyridazine-1-oxide (7, 1.00 g, 4.06 mmol,
1.0 eq.) was dissolved in acetonitrile (50 ml) and an aqueous
methylamine solution (40%, 2.5 mL, 29 mmol, 7.1 eq.) was added
dropwise. The reaction mixture was stirred at room temperature
overnight. The solvent was removed in vacuo. The residue was
suspended in a small amount of water, filtered and washed with ice-water.
Drying on air yielded compound 8 as yellow solid (841 mg, 3.44 mmol,
85%).
3,5-Bis((2-hydroxyethyl)amino)-4,6-dinitropyridazine-1-oxide (11)
3,5-Dimethoxy-4,6-dinitropyridazine-1-oxide (7, 632 mg, 2.57 mmol,
1.0 eq.) was dissolved in acetonitrile (30 mL) and 2-aminoethanol
(0.330 mL, 5.39 mmol, 2.1 eq.) was added dropwise. The solution was
stirred at room temperature overnight. The solvent was removed in vacuo.
Subsequently, the residue was suspended in a small amount of water,
filtered and thoroughly washed with ice-water. Drying on air yielded
compound 11 as yellow solid (709 mg, 2.33 mmol, 76%).
DTA (5 °C min−1): 250 °C (dec.); BAM: drop hammer: 10 J (100–500 μm);
friction tester: 360 N (100–500 μm); ESD: 203 mJ (100–500 μm). IR
(ATR), ṽ (cm−1) = 3351 (m), 3236 (m), 2955 (vw), 1573 (s), 1537 (s),
1403 (s), 1327 (s), 1244 (s), 1189 (s), 1135 (s), 1037 (s), 986 (m), 799
(m), 768 (s), 715 (s), 649 (s), 625 (s), 510 (m). 1H NMR (d6-DMSO,
400 MHz, [ppm]): δ = 9.71 (s, 1H), 9.27 (d, 3J = 4.8 Hz, 1H), 2.95 (d, 3J =
4.9 Hz, 3H), 2.86 (s, 3H). 13C NMR (d6-DMSO, 101 MHz, [ppm]): δ =
152.2, 142.5, 133.9, 112.1, 30.0, 29.1. 14N NMR (d6-DMSO, 29 MHz,
[ppm]): δ = –24, –71. 15N NMR (d6-DMSO, 41 MHz, [ppm]): δ = –17.1, –
22.2, –69.7, –113.0, –288.7, –291.1. Elem. Anal. (C6H8N6O5,
DTA (5 °C min−1): 170 °C (dec.); BAM: drop hammer: 15 J (100–500 μm);
friction tester: 360 N (100–500 μm); ESD: 250 mJ (100–500 μm). IR
(ATR), ṽ (cm−1) = 3411 (w), 3327 (m), 3209 (w), 2950 (vw), 2887 (vw),
1593 (m), 1525 (s), 1427 (m), 1411 (m), 1337 (s), 1281 (m), 1236 (s),
1180 (s), 1123 (m), 1047 (s), 943 (w), 866 (m), 767 (s), 715 (s), 632 (s),
517 (m). 1H NMR (d6-DMSO, 400 MHz, [ppm]): δ = 9.90 (t, 3J = 4.8 Hz,
1H), 9.35 (t, 3J = 5.3 Hz, 1H), 5.25 (t, 3J = 5.0 Hz, 1H), 4.96 (t, 3J = 5.2 Hz,
1H), 3.63–3.56 (m, 4H), 3.53 (q, 3J = 5.3 Hz, 2H), 3.15 (q, 3J = 5.0 Hz,
2H). 13C NMR (d6-DMSO, 101 MHz, [ppm]): δ = 152.1, 142.2, 133.9,
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112.2, 58.5, 58.4, 45.4, 44.1. 14N NMR (d6-DMSO, 29 MHz, [ppm]): δ = –
23, –71. 15N NMR (d6-DMSO, 41 MHz, [ppm]): δ = –17.0, –22.6, –69.5, –
113.5, –282.4, –287.4. Elem. Anal. (C8H12N6O7, 304.22 g∙mol-1): Calc.: C
31.59, H 3.98, N 27.63%. Found: C 31.58, H 3.87, N 27.43%. m/z (DEI+):
305(100) [M+H]+.
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The financial support of this work by the Ludwig-Maximilian
University of Munich (LMU), the Office of Naval Research (ONR)
under grant no. ONR.N00014-16-1-2062 and the Strategic
Environmental Research and Development Program (SERDP)
under contact no. WP19-1287 are gratefully acknowledged. We
thank Stefan Huber for his help with the sensitivity testing.
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Keywords: Nitrogen-rich heterocycle • Pyridazines • Crystal
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Synthesis and characterization
of new pyridazine based
energetic materials.
I. Gospodinov, J. Singer, T. M.
Klapötke* and J. Stierstorfer
Page No. – Page No.
The Pyridazine Scaffold as a
Building Block for Energetic
Materials:
Synthesis,
Characterization and Properties
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