Comparison of Functionalized Lithium Dihydrobis(azolyl)borates with Their Corresponding Azolates as Environmentally Friendly Red Pyrotechnic Coloring Agents

Article abstract of DOI:10.1002/cplu.202000427

The recent awareness of the impact of strontium on health has stimulated research efforts on lithium-based red pyrotechnic colorants. We have previously shown lithium dihydrobis(azolyl)borates to be promising candidates due to their favorable adjustment to a reductive and low-temperature flame atmosphere. These compounds are assumed to be sufficiently stable only if the pK_a values of the heterocycles are between 5 and 20. Apart from their acidities, functionalization of 1H-tetrazole and 1H-pyrazole with nitro or amino groups, respectively, tailors the oxygen balances of the resulting Lewis acid base adducts to enhance the fuel-rich flame environment or to make them oxidizing agents. This work determines whether the lithium salts of dihydrobis(3-nitropyrazol-1-yl)borate and dihydrobis(5-aminotetrazol-1-yl)borate are suitable replacements for strontium-containing color imparters. Furthermore, the influence of potentially green-light-producing boron is evaluated by comparing the emissions of the lithium borates and the corresponding lithium azolates.

Full text of DOI:10.1002/cplu.202000427

A Multidisciplinary Journal Centering on Chemistry
Accepted Article
Title: Comparison of Functionalized Lithium Dihydrobis(azolyl)borates
with Their Corresponding Azolates as Environmentally Friendly
Red Pyrotechnic Coloring Agents
Authors: Thomas M. Klapötke, Alicia M. W. Dufter, Magdalena Rusan,
Alexander Schweiger, and Jörg Stierstorfer
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To be cited as: ChemPlusChem 10.1002/cplu.202000427
Link to VoR: https://doi.org/10.1002/cplu.202000427
01/2020

10.1002/cplu.202000427
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Comparison of Functionalized Lithium Dihydrobis(azolyl)borates
with Their Corresponding Azolates as Environmentally Friendly
Red Pyrotechnic Coloring Agents
Alicia M. W. Dufter,[a] Thomas M. Klapötke,*[a] Magdalena Rusan,[a] Alexander Schweiger,[a] and Jörg
Stierstorfer[a]
Dedication
[a]
A. M. W. Dufter, Prof. Dr. T. M. Klapötke, Dr. M. Rusan, A. Schweiger, Dr. J. Stierstorfer
Department of Chemistry
Ludwig-Maximilian University of Munich
Butenandtstr. 5-13, 81377 Munich (Germany)
E-mail: tmk@cup.uni-muenchen.de
Supporting information for this article is given via a link at the end of the document.
Abstract: The recent awareness of the impact of strontium on health
set off a wave of international research efforts on lithium-based red
pyrotechnic
colorants.
Our
previous
study
on
lithium
dihydrobis(azolyl)borates revealed such moieties to be promising
candidates due to their favorable adjustment of a reductive and low-
temperature flame atmosphere. Sufficient stability of the former is
assumed to be ensured only, if the pKa values of the heterocycles are
higher than 5 and lower than 20. Apart from their acidities,
functionalization of 1H-tetrazole and 1H-pyrazole with nitro or amino
groups, respectively, tailors the oxygen balances of the resulting
Lewis acid base adducts to enhance the fuel-rich flame environment
or to make them oxidizing agents. This work determines, whether the
lithium salts of dihydrobis(3-nitropyrazol-1-yl)borate and dihydrobis(5-
aminotetrazol-1-yl)borate are suitable replacements for strontium-
containing color imparters. Furthermore, the influence of potentially
green light-producing boron is evaluated by comparing the emissions
of the lithium borates and the corresponding lithium azolates.
Figure 1. Stability of lithium dihydrobis(azolyl)borates as a function of the pKa
value of the ligands.
Introduction
When first concerns were expressed about the substitution of
calcium by strontium in the human bone,[1] the search for other
elements coloring a pyrotechnic flame red began. A quick survey
of the literature on flame theory suggests that an astonishingly
large number of elements is capable of emitting in the red region
of the electromagnetic spectrum.[2] However, the radioactivity of
radium as well as the high cost of the rare earth elements
praseodymium, neodymium, samarium, yttrium, and scandium is
prohibitive for their application.[2a] Red emissions from metastable
copper(II) oxide were only observed as a marginal phenomenon
in blue pyrotechnics,[2b] whereas the flame color of calcium has a
dominant yellowish aspect.[2c] Lithium is well known for its
production of carmine red light and is not currently considered
toxic as demonstrated by its use in the treatment of mental
diseases and LD50 values of 422−1165 mg kg−1 for oral
administration of several compounds to different animal
species.[3]
In contrast to all other elements mentioned above, in the case of
lithium it is the metastable atomic species, which provokes two
emission lines of high intensity at 671 and 610 nm.[2a,4] Regarding
that chlorine donors have fallen into disrepute due to their
combustion to highly carcinogenic polychlorinated aromatics,[4,5]
this is a further advantage of lithium. The presence of chlorine is
even considered to have an adverse effect on the light output of
a lithium-containing pyrotechnic formulation.[2a] A fuel-rich flame
atmosphere is suggested in order to convert incandescent lithium
hydroxide back to gaseous atomic lithium and water vapor in the
flame and thus to avoid paling of the flame color. Furthermore, the
probability of higher energetic electron transitions leading to a
blue shift of the dominant wavelength should be low, if cool flame
temperatures are adjusted.[2a,6] Despite all this theoretical
knowledge on lithium flame chemistry the selection of chlorine-
free lithium-based pyrotechnic compositions is very narrow.[4,7]
1
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Additionally, a significant percentage of these show a burning
behavior rather comparable to a strobe than to a flare,[7] which
might be due to the strong tendency of lithium to attract water in
some form.
However, 3-nitro-1H-pyrazole has a mid-range pKa value of 9.8[16]
so that, given a reasonable solubility of this azole in the chosen
solvent, the hydrogen formation upon encounter of the two
starting materials was assumed to be relatively exothermic,
especially at elevated temperature and thus reaction rate.
Therefore, the reaction mixture was prepared under ice-cooling
and was only heated, when the brisk hydrogen evolution had
eased up. Apart from being safer, this approach also provides a
higher degree of control over the stoichiometry of the reagents
due to isolation of the ligand beforehand. Since acetonitrile has a
lower boiling point than anisole and thus is more easily removable
and since the acidity of 3-nitro-1H-pyrazole only slightly differs
from that of 1H-1,2,4-triazole, our previously established
synthesis protocol for potassium dihydrobis(1,2,4-triazol-1-
yl)borate[7b] using this reaction medium was attempted to be
transferred to the target compound. Indeed, refluxing a solution of
potassium tetrahydroborate and two equivalents of 3-nitro-1H-
pyrazole in acetonitrile for six days yielded potassium
dihydrobis(3-nitropyrazol-1-yl)borate (see Scheme 1). Small
impurities of boric acid or tetrahydroxyborate and of 3-
nitropyrazolate, which might result from the hydrolysis of
unconverted borohydride and the intermediate trihydro(3-
nitropyrazol-1-yl)borate upon contact with moisture,[8a] can be
removed by recrystallization from methanol and by washing with
ethanol, respectively. In this way, the treatment with toxic n-
hexane proposed by Pellei et al. is avoided. Even the yield
achieved by this procedure of 69 % overcomes the literature value
of 60 %.[14]
The hygroscopicity of the lightest alkaline metal might be
delimited by saltification of bulky molecules offering a large
number of coordination sites. Bis(azolyl)borates, especially with
1H-pyrazole as substituent, are not only chelating ligands,[8]
adjust a reductive environment and release high volumes of
nitrogen to cool the flame,[9] but have extensively been
investigated in the past.[8,10] A mechanism based on the principles
of nucleophilic substitution was postulated for the reaction of the
borohydride anion with an azole, whereby the nucleophilic
heterocycle and electrophilic tetrahydroborate form a cyclic five-
membered transition state and hydrogen leaves concertedly.[8]
While previous studies proved lithium dihydrobis(1,2,4-triazol-1-
yl)borate sesquihydrate to be a suitable red pyrotechnic color
agent, their lack of stability renders the corresponding salts
carrying 1H-pyrazole or 1H-tetrazole as ligands not useful for
practical applications.[7b] Also the Lewis acid-base reaction toward
lithium dihydrobis(tetrazol-1-yl)borate is accompanied by a highly
exothermic formation of hydrogen and the dissociation of
unconverted azole so that it bears a certain risk and affords
unsatisfying yields. In regard of the acidities of the different
substituents (1H-pyrazole: pKa = 20.4[11], 1H-1,2,4-triazole: pKa =
10.0[12], 1H-tetrazole: pKa = 4.9) this means that a pKa value
higher than 5 and lower than 20 allows for convenient preparation
and sufficient stability of the resulting dihydrobis(azolyl)borate
(see Figure 1). By functionalization of the pyrazole or tetrazole
rings with nitro or amino groups, respectively, their pKa values are
shifted into this range and additionally the oxygen balances of the
compounds can be tuned toward oxidizers or fuels to reduce the
number of components of a pyrotechnic mixture. This work
concentrates on the lithium salts of dihydrobis(3-nitropyrazol-1-
yl)borate and dihydrobis(5-aminotetrazol-1-yl)borate, whereby
particular attention is paid to dehydration.
Present boron may potentially form metastable green light-
emitting boron dioxide in the flame even, if this process should
require an oxygen-rich atmosphere to be observable.[13] Its
influence on the color performance is evaluated by comparing the
emissive properties of the lithium borates with those of the
respective azolates.
Scheme 1. Synthesis of lithium dihydrobis(3-nitropyrazol-1-yl)borate (1).
Results and Discussion
Synthesis
The metathesis reaction of the potassium precursor with lithium
perchlorate leads to the corresponding lithium salt, which can be
dehydrated at 100 °C over night. Interestingly, potassium
dihydrobis(3-nitropyrazol-1-yl)borate as well as its lithium
analogue (1) change their color from yellowish or brownish,
respectively, to green when exposed to air for one week.
Nevertheless, no explanation for this behavior was found so far
(see Supporting Information).
In the case of potassium dihydrobis(5-aminotetrazol-1-yl)borate a
stoichiometric mixture of potassium borohydride and 5-amino-1H-
tetrazole in acetonitrile is refluxed for four days (see Scheme 2).
The literature also describes the acidification of the potassium salt
by glacial acetic acid.[15] (5-amino-1H-tetrazole)dihydro(5-
aminotetrazol-1-yl)borane hemihydrate in contrast to potential
impurities such as residual boric acid or tetrahydroxyborate and
In general, bis(azolyl)borate salts are accessible by the Lewis
acid-base reaction between borohydride and depending on its
acidity at least two equivalents of the heterocycle under inert gas
atmosphere, whereby hydrogen evolves.[7b,8a,10a,10c] If such a
reaction is carried out in solvent, the latter is distilled beforehand.
A water-free reaction environment needs to be ensured due to the
low resistance of tetrahydroborate to hydrolysis.
Procedures for both the potassium salts of dihydrobis(3-
nitropyrazol-1-yl)borate[14] and dihydrobis(5-aminotetrazol-1-
yl)borate[15] have already been reported. The former is
synthesized by thermal rearrangement of 1-N-nitropyrazole in
anisole, addition of a stoichiometric amount of potassium
borohydride at 80 °C and gradual heating of the resulting solution.
2
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unconsumed 5-amino-1H-tetrazole has a poor solubility in water.
The saltification of the free acid with lithium hydroxide leads to
lithium dihydrobis(5-aminotetrazol-1-yl)borate monohydrate in
satisfying purity and yield. The crystal water can be removed by
drying at 100 °C over night. The water-free moiety (2) did not give
any hints at ageing.
Scheme 2. Synthesis of lithium dihydrobis(5-aminotetrazol-1-yl)borate (2).
Figure 2. Dimer of lithium dihydrobis(3-nitropyrazol-1-yl)borate monohydrate.
Selected bond lengths (Å): B1-H1A 1.035(14), B1-H1B 1.043(12), N1-B1
1.560(2), N4-B1 1.559(2), N5i-Li1 2.157(3), O3i-Li1 2.272(3), O1-Li1 2.229(3),
N2-Li1 2.179(3), O5-Li1 2.044(3), O5i-Li1 2.077(3); selected bond angles (°):
H1A-B1-H1B 112.7(11), N1-B1-N4 109.66(11), O3i-Li1-N5i 73.51(9), O1-Li1-N5i
94.52(11), O5-Li1-N5i 98.63(14), O5i-Li1-N5i 94.04(13), O1-Li1-N2 74.22(11),
O3i-Li1-N2 93.10(12), O5i-Li1-N2 95.53(12), O5-Li1-N2 94.12(12), O1-Li1-O3i
80.50(9), O3i-Li1-O5i 93.43(12), O5-Li1-O5i 90.42(10), O1-Li1-O5 97.08(13),
O5-Li1-O5i 90.42(10), Li1-O5-Li1i 89.58(12); symmetry code: (i) 1−x, 1−y, 1−z.
While lithium 5-aminotetrazolate[17] (4) and its tetrazolate
analogue[18] are the only two water-free lithium salts known to
literature, lithium 3-nitropyrazolate (3) has not been mentioned
until now. The latter was prepared in 76 % yield by the
deprotonation of 3-nitro-1H-pyrazole by lithium hydroxide and
subsequent drying at 100 °C over night. Nevertheless, this
compound proved to be hygroscopic.
Crystal Structures
Crystalline material of lithium dihydrobis(3-nitropyrazol-1-
yl)borate monohydrate and lithium dihydrobis(5-aminotetrazol-1-
yl)borate monohydrate was obtained by evaporation of an
ethanolic solution or of a hot solution of 2-propanol, respectively,
before single crystals were prepared and the latter were analyzed
by low-temperature X-ray diffraction. Both compounds crystallize
in the monoclinic space group P21/n. The more carbon atoms in
the unsubstituted heterocycle are replaced by smaller and heavier
nitrogen, the higher the density seems to be (potassium
ρ = ;
1.41 g cm−3 at 150 K[8b]
Figure 3. Advanced molecular unit of lithium dihydrobis(5-aminotetrazol-1-
yl)borate monohydrate. Selected bond lengths (Å): B1-H1A 1.121(19), B1-H1B
1.14(2), N1-B1 1.569(3), N6-B1 1.558(3), N3iii-Li1 2.045(4), N4-Li1 2.018(4),
O1i-Li1 1.967(3), O1ii-Li1 1.959(3); selected bond angles (°): H1A-B1-H1B
114.9(14), N1-B1-N6 108.50(17), N3iii-Li1-N4 113.74(16), O1i-Li1-N3iii
107.29(16), O1ii-Li1-N3iii 109.17(16), O1i-Li1-N4 114.30(17), O1i-Li1-O1ii
90.92(14), O1ii-Li1-N4 118.88(17), Li1-O1i-Li1i 89.08(15); symmetry codes: (i) x,
y, 1+z; (ii) −x, −y, 1−z; (iii) 1−x, −y, 2−z.
dihydrobis(pyrazol-1-yl)borate:
potassium dihydrobis(tetrazol-1-yl)borate: ρ = 1.47 g cm−3 at
283−303 K[10c]). Nitro and amino substituents contribute
approximately equally to a closer packing, since the oxygen
atoms of nitro functionalities offer additional coordination sites for
metal cations and amino groups push electron density into the
ring and thus enhance its σ-donation toward metal centers
(potassium dihydrobis(3-nitropyrazol-1-yl)borate: ρ = 1.69 g cm−3
at 100 K[14], potassium dihydrobis(5-aminotetrazol-1-yl)borate: ρ
= 1.70 g cm−3 at 296 K[15]). Therefore, the density of lithium
dihydrobis(5-aminotetrazol-1-yl)borate monohydrate of 1.60 g
cm−3 at 113 K exceeds that of its 3-nitropyrazole analogue of 1.52
g cm−3 at 143 K. Additionally, the former forms a polymeric
structure (see Figure 3) as opposed to the dimers of lithium
dihydrobis(3-nitropyrazol-1-yl)borate monohydrate (see Figure 2).
In both molecular units the bis(azolyl)borate anion has a half-boat
conformation. In lithium dihydrobis(3-nitropyrazol-1-yl)borate
monohydrate the cis-oriented nitro groups are coplanar with the
heterocycles, whereas the protons of the amino functionalities in
the corresponding 5-aminotetrazole compound are in trans
conformation and slightly bend out of the ring planes. While in the
bis(nitropyrazolyl)borate the lithium cation has
a distorted
octahedral coordination sphere of one oxygen and one nitrogen
each of two adjacent anions and two water molecules, in the
corresponding borate carrying 5-aminotetrazole as substituent
lithium is distortedly tetrahedrally coordinated by one nitrogen
atom each of two neighboring dihydrobis(5-aminotetrazol-1-
3
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yl)borate units and by two water ligands. Interestingly, in both
cases the lithium atoms of two monomers are connected by their
two water molecules in a slightly distorted square planar manner.
The B-N bond distances in lithium dihydrobis(3-nitropyrazol-1-
yl)borate are very similar to those of the potassium
representative[14] (B-H 1.102−1.140 Å, B-N 1.554−1.567 Å, H-B-H
114.6 °, N-B-N 108.2 °), whereas the B-H bonds are shorter, the
protons on the boron center are more strongly and the nitrogen
atoms less angled. The B-H bonds in lithium dihydrobis(5-
aminotetrazol-1-yl)borate monohydrate are elongated compared
to the corresponding 3-nitropyrazole material, because the higher
number of electron-withdrawing nitrogen atoms in the azole
compensates for the amino ligands pushing electron density to
the boron atom. The B-N bond distances as well as the N-B-N
angle in lithium dihydrobis(5-aminotetrazol-1-yl)borate are almost
identical to those determined for amino(2-(propan-2-
ylidene)hydrazinyl)methaniminium dihydrobis(5-aminotetrazol-1-
yl)borate (B-N 1.559−1.566 Å, N-B-N 108.5 °).[10b]
[a] Oxygen balance with respect to formation of CO2, Li2O, and B2O3. [b] Onset
decomposition temperature observed as exothermic peak during differential
thermal analysis using a heating rate of β = 5 °C min−1. [c] Grain size. [d] Friction
sensitivity. [e] Impact sensitivity. [f] Electrostatic discharge sensitivity. [g] The
impact sensitivity of lithium dihydrobis(3-nitropyrazol-1-yl)borate was
determined for a grain size < 100 μm.
Additionally, the oxygen balances of the latter were calculated in
order to evaluate their capability of serving as oxidizers or fuels.
The oxygen balance reflects, if the full burn of a compound with
the sum formula CaHbNcOd to CO2, H2O and N2 produces or
requires extra oxygen (see Equation 1).[20b]
푏
2
(푑−2푎−
푀
)
훺퐶푂2
=
× 1600
(1)
The respective values of the two lithium borates and the lithium
azolate were determined assuming that lithium combusts to
Li2O[21] and boron to B2O3[22]. Although in moiety 1 the two
heterocycles carry one nitro group each, its oxygen balance is
even more highly negative than those of the other two compounds
containing amino-functionalized azoles. This is because in
materials 2 and 4 more carbon atoms in the ring are substituted
by nitrogen. However, all three salts are thermally stable. While
the impact sensitivity of 1 is in the range of a primary explosive,[20a]
2 as well as 4 are insensitive toward all tested ignition stimuli. This
means that in contrast to the amination of the tetrazole units the
nitration of the pyrazole rings only partially stabilizes the
respective bis(azolyl)borate and that moieties 2 and 4 are worth
considering application.
Physico-Chemical Properties
Apart from being storable, a lithium-based red pyrotechnic
formulation should at least be as safe as conventional mixtures.[19]
Therefore, the thermal stabilities and resistances toward friction,
impact and electrostatic discharge of the stable lithium salts 1, 2,
and 4 were measured (see Figure 4 and Table 1).
Pyrotechnic Formulations
In order to compare the emissive properties of 2 and 4 to a
strontium-based coloring agent and amongst each other, drop-in
formulations of
a
chlorine-free strontium-containing red
pyrotechnic composition[9] were prepared (see Table 2) and
spectrometrically analysed. Instead of strontium nitrate, its
ammonium analogue was used as oxidizer. Magnesium is known
to increase the flame temperature and to wash out the red flame
color imparted by lithium due to its incandescent oxidation
product[7a] so that the content of 2 or 4, respectively, with their
highly negative oxygen balances was increased at the expense of
the metallic fuel. Since these mixtures, especially that employing
the azolate, were hygroscopic, the percentage of binder was
raised. However, this only marginally curbed the hygroscopicity of
the mixtures.
Figure 4. DTA plots from top to bottom of A – lithium dihydrobis(3-nitropyrazol-
1-yl)borate (1) before and after drying, B – lithium dihydrobis(5-aminotetrazol-
1-yl)borate (2) before and after drying, C – lithium 3-nitropyrazolate (3) after
synthesis and exposure to air for one day, D – lithium 5-aminotetrazolate (4).
Table 2. Composition of a chlorine-free strontium-based control[9] and of test
formulations A and B containing lithium dihydrobis(5-aminotetrazol-1-yl)borate
(2) or lithium 5-aminotetrazolate (4), respectively.
Table 1. Physico-chemical properties of lithium dihydrobis(3-nitropyrazol-1-
yl)borate (1), lithium dihydrobis(5-aminotetrazol-1-yl)borate (2), and lithium 5-
aminotetrazolate (4).
Mg
Epon 813/
Versamid 140
[wt%][b]
Sr(NO3)2
[wt%]
NH4NO3
[wt%]
CH3N5
[wt%]
2
4
ΩCO2
[%][a]
Tdec(onset)
[°C][b]
g.s.
[μm][c]
FS
[N][d]
IS
[J][e]
ESD
[J][f]
50/100
[wt%][a]
[wt%]
[wt%]
1
2
4
−72.16
−59.61
−52.75
200
280
369
100−500
100−500
< 100
144
2[g]
0.42
> 1.5
> 1.5
Control
48
33
12
12
12
7
> 360
> 360
> 40
> 40
A
B
48
48
30
10
10
30
4
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safety and yield. In order to avoid an incontinuous burning
behavior of pyrotechnic formulations employing these moieties as
red colorants, the latter were dehydrated where necessary. The
enhanced resistance of lithium dihydrobis(3-nitropyrazol-1-
yl)borate to hydrolysis compared to its unnitrated analogue and
the insensitiveness of lithium dihydrobis(5-aminotetrazol-1-
yl)borate seem to support our theory that heterocycles with pKa
values higher than 5 and lower than 20 build stable lithium
bis(azolyl)borates. However, lithium dihydrobis(3-nitropyrazol-1-
yl)borate is very sensitive to impact and lithium 3-nitropyrazolate
is hygroscopic so that only the other two salts were investigated
further. Although the color purities of mixtures containing lithium
[a] Magnesium with a mesh size 50/100 (300 μm > grain size > 150 μm). [b]
Epon 813 and Versamid 140 in a weight percent ratio of 80:20.
Both mixtures exhibit dominant wavelengths well below that of the
control and also significantly weaker luminosities, which most
probably originates from their reduced magnesium contents. Still,
the spectral purities of A and B greatly exceed that of the
reference despite their magnesium content. Although, in contrast
to A, test composition B does not sustain combustion, the carmine
color aspect of the flame is more dominant in this case (see Figure
5).
dihydrobis(5-aminotetrazol-1-yl)borate
or
lithium
5-
aminotetrazolate, respectively, do not support this theory, green
light-emitting boron seems to cause a blue shift of the dominant
wavelength.
Experimental Section
CAUTION! The reactions between potassium borohydride and azoles
described herein, especially that with 3-nitro-1H-pyrazole, are exothermic
and proceed with the release of high volumes of hydrogen gas. All reaction
products are potentially explosive energetic materials, which are partly
sensitive toward various ignition stimuli (e.g. heat, friction, impact or
electrostatic discharge). Therefore, proper safety precautions (safety
glasses, face shield, leather coat, earthed equipment and shoes, Kevlar
gloves, Kevlar sleeves and ear protectors) have to be taken, when
synthesizing and manipulating these compounds.
Figure 5. Combustion of test formulations A and B, respectively.
The slightly higher dominant wavelength of B compared to A
according to Table 3 indeed suggests that lithium borate 2 forms
metastable boron dioxide in the flame causing a blue shift of the
value. In this context, the extremely high spectral purity of A is
contradictory. Beyond the less efficient burn-down of B resulting
in a longer burn time and a lower light output, the enhanced
hygroscopicity of this formulation also shows in a strobe-like
burning behavior.
3-Nitro-1H-pyrazole:[23] 4.999 g 1-N-nitropyrazole (44 mmol) were
thermally rearranged by heating a solution in 50 mL of benzonitrile at
180 °C for three hours. The colorless needles, which precipitated from
solution upon cooling to 0 °C, were filtered off and dried at 100 °C
overnight. This procedure afforded 3.564 g of the title compound (71 %).
IR (ATR): 휈̃ = 3142(s), 3022(m), 2926(m), 2884(m), 2630(w), 1676(vw),
1556(s), 1511(s), 1483(s), 1438(m), 1423(m), 1379(vs), 1351(vs),
1286(m), 1249(m), 1209(s), 1090(m), 1058(m), 1047(s), 990(s), 928(m),
903(m), 862(m), 862(m), 820(vs), 794(s), 783(vs), 752(vs), 640(w), 613(m),
Table 3. Physico-chemical properties of the control and test formulations A and
B.
474(vw), 442(s) cm−1 14N{H} NMR (DMSO-d6, 25 °C): δ = −170.2 (br s,
.
NH), −18.8 (br s, NO2) ppm. 13C{H} NMR (DMSO-d6, 25 °C): δ = 156.3 (s,
C-NO2), 132.3 (s, CH), 101.8 (s, CH) ppm. 1H{/} NMR (DMSO-d6, 25 °C):
δ = 13.94 (br s, 1H, NH), 8.03 (d, 1H, CH), 7.03 (d, 1H, CH) ppm. EA
(C3H3N3O2, 113.08): calcd. N 37.16, C 31.86, H 2.67 %; found N 37.29, C
Tdec(onset)
[°C][a]
FS
IS
ESD
[J][d]
BT
λd
Ʃ
LI
[N][b]
[J][c]
[s][e]
[nm][f]
[%][g]
[cd][h]
Control
244
218
204
192
10
10
20
1
1
1
3
606
591
593
75
99
87
14083
1002
763
31.73,
H
2.62 %. DTA (5 °C min−1): 176 (endothermic), 316
(endothermic) °C.
A
B
> 360
> 360
10
15[i]
Potassium dihydrobis(3-nitropyrazol-1-yl)borate: A reaction mixture
consisting of 0.625 g potassium borohydride (12 mmol), 2.623 g 3-nitro-
1H-pyrazole (23 mmol) and 3 mL of anhydrous acetonitrile was prepared
under ice-cooling. When the release of hydrogen had eased up under
stirring at room temperature, the suspension was refluxed for six days. The
resulting solid was washed twice with approximately 5 mL of ethanol each
and recrystallized from 44 mL of methanol, before the crystals were dried
at 100 °C overnight. The product (2.201 g, 69 %) was obtained in the form
of yellowish crystals.
[a] Onset decomposition temperature observed as exothermic peak during
differential thermal analysis using a heating rate of β = 5 °C min−1. [b] Friction
sensitivity. [c] Impact sensitivity. [d] Electrostatic discharge sensitivity. [e] Burn
time. [f] Dominant wavelength. [g] Spectral purity. [h] Luminous intensity. [i] Burn
time determined taking only one sample into account; all other combustion
parameters were averaged over two measurements each.
IR (ATR): 휈̃ = 3146(w), 3129(w), 2452(w), 2428(m), 2385(w), 2269(w),
1551(w), 1528(m), 1521(m), 1490(m), 1478(s), 1440(m), 1392(m), 1381(s),
1362(s), 1301(m), 1231(s), 1206(m), 1142(vs), 1055(s), 1006(m), 989(m),
913(w), 913(w), 899(m), 886(w), 830(s), 822(m), 785(s), 754(vs), 728(m),
Conclusion
The new lithium dihydrobis(azolyl)borates with 5-aminotetrazole
or 3-nitropyrazole as ligands as well as their corresponding
azolates were prepared, whereby the synthesis of potassium
dihydrobis(3-nitropyrazol-1-yl)borate was improved in regard of
677(w), 662(m), 643(m), 616(m), 540(w), 451(w), 440(w) cm−1 11B{/} NMR
.
(DMSO-d6, 25 °C): δ = −5.5 (br s, BH2) ppm. 14N{H} NMR (DMSO-d6,
25 °C): δ = −19.3 (br s, NO2) ppm. 13C{H} NMR (DMSO-d6, 25 °C): δ =
156.6 (s, C-NO2), 137.1 (s, CH), 101.4 (s, CH) ppm. 1H{/} NMR (DMSO-d6,
5
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10.1002/cplu.202000427
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25 °C): δ = 7.62 (d, 2H, CH), 6.78 (d, 2H, CH), 3.61 (br s, 2H, BH2) ppm.
EA (KBC6H6N6O4, 276.06): calcd. N 30.44, C 26.11, H 2.19 %; found N
30.36, C 25.90, H 2.11 %. DTA (5 °C min−1): 248 (exothermic) °C.
ppm. 11B{/} NMR (DMSO-d6, 25 °C): δ = −13.8 (br s, BH2) ppm. 13C{H}
NMR (DMSO-d6, 25 °C): δ = 159.2 (s, C-NH2) ppm. 1H{/} NMR (DMSO-d6,
25 °C): δ = 5.60 (br s, 4H, NH2), 3.57 (br s, 2H, H2O), 3.30 (br s, 2H, BH2)
ppm. EA (LiBC2H8N10O, 205.91): calcd. N 68.02, C 11.67, H 3.91 %; found
N 67.35, C 12.21, H 3.69 %. DTA (5 °C min−1): 117 (endothermic), 290
(exothermic) °C.
Lithium dihydrobis(3-nitropyrazol-1-yl)borate monohydrate: The
metathesis reaction between 0.998 g potassium dihydrobis(3-nitropyrazol-
1-yl)borate (4 mmol) and 0.384 g lithium perchlorate (4 mmol) in 50 mL of
methanol was allowed to proceed for 24 hours at room temperature. After
potassium perchlorate had been filtered off, the solvent was removed from
the filtrate in vacuo. Recrystallization of the residue from roughly 5 mL of
ethyl acetate gave 0.583 g of a light brown powder (62 %).
Lithium dihydrobis(5-aminotetrazol-1-yl)borate (2): Drying 2.443 g
lithium dihydrobis(5-aminotetrazol-1-yl)borate monohydrate (12 mmol) at
100 °C overnight yielded 2.140 g of the water-free compound (96 %).
IR (ATR): 휈̃ = 3474(w), 3442(w), 3409(w), 3379(m), 3291(w), 3213(w),
3169(w), 2457(w), 2417(w), 1621(vs), 1560(s), 1465(m), 1303(m),
1238(m), 1221(w), 1205(w), 1183(m), 1146(s), 1099(s), 1062(m), 1034(w),
876(w), 780(m), 780(m), 762(w), 751(w), 674(m), 660(s), 631(m), 501(m),
460(w), 442(w), 431(w), 418(w) cm−1. 7Li{H} NMR (DMSO-d6, 25 °C): δ =
−1.0 (s, Li) ppm. 11B{/} NMR (DMSO-d6, 25 °C): δ = −14.5 (br s, BH2) ppm.
13C{H} NMR (DMSO-d6, 25 °C): δ = 159.2 (s, C-NH2) ppm. 1H{/} NMR
(DMSO-d6, 25 °C): δ = 5.60 (br s, 4H, NH2), 3.34 (br s, 2H, BH2) ppm. EA
(LiBC2H6N10, 187.89): calcd. N 74.55, C 12.78, H 3.22 %; found N 73.64,
IR (ATR): 휈̃ = 3608(w), 3497(w), 3164(w), 3151(w), 3131(w), 2469(w),
2419(w), 1602(w), 1558(w), 1533(m), 1494(vs), 1456(w), 1447(w),
1392(m), 1359(vs), 1299(m), 1237(s), 1206(w), 1193(w), 1154(s),
1134(vs), 1068(s), 1059(s), 1059(s), 1010(m), 1001(m), 987(w), 976(w),
882(m), 828(s), 797(m), 786(s), 753(s), 723(m), 658(m), 639(w), 613(w),
548(w), 481(m), 453(m), 418(w) cm−1. 7Li{H} NMR (DMSO-d6, 25 °C): δ =
−1.0 (s, Li) ppm. 11B{/} NMR (DMSO-d6, 25 °C): δ = −6.0 (br s, BH2) ppm.
14N{H} NMR (DMSO-d6, 25 °C): δ = −16.4 (br s, NO2) ppm. 13C{H} NMR
(DMSO-d6, 25 °C): δ = 156.6 (s, C-NO2), 137.1 (s, CH), 101.4 (s, CH) ppm.
1H{/} NMR (DMSO-d6, 25 °C): δ = 7.62 (d, 2H, CH), 6.78 (d, 2H, CH), 3.60
(br s, 2H, BH2), 3.37 (br s, 2H, H2O) ppm. EA (LiBC6H8N6O5, 261.92): calcd.
N 32.09, C 27.52, H 3.08 %; found N 32.15, C 27.69, H 2.82 %. DTA
(5 °C min−1): 121 (endothermic), 189 (exothermic) °C.
C
13.06, H
2.87 %. DTA (5 °C min−1): 280 (exothermic) °C. FS
(100−500 μm): > 360 N. IS (100−500 μm): > 40 J. ESD (100−500 μm): >
1.5 J.
Lithium 3-nitropyrazolate (3): A suspension of 1.000 g 3-nitro-1H-
pyrazole (9 mmol) and 0.212 g lithium hydroxide (9 mmol) in roughly
13 mL of water was stirred at room temperature for one hour, before
unreacted azole was filtered off and the solvent was removed from the
filtrate in vacuo. Drying of the residue at 100 °C overnight gave 0.802 g of
a yellow powder (76 %).
Lithium dihydrobis(3-nitropyrazol-1-yl)borate (1): The target moiety
(0.503 g, 93 %) was obtained by dehydration of lithium dihydrobis(3-
nitropyrazol-1-yl)borate monohydrate (0.583 g, 2 mmol) at 100 °C over
night.
IR (ATR): 휈̃ = 3182(vw), 3169(vw), 3149(w), 3126(w), 2480(w), 2421(w),
1561(w), 1541(m), 1529(m), 1495(s), 1486(s), 1448(w), 1360(vs), 1299(w),
1231(s), 1187(w), 1179(m), 1168(m), 1159(m), 1151(s), 1136(vs),
1124(vs), 1063(s), 1063(s), 1011(m), 982(w), 903(w), 884(w), 871(w),
842(m), 827(s), 800(m), 787(s), 753(vs), 720(m), 710(m), 666(w), 656(m),
IR (ATR): 휈̃ = 3152(w), 3128(vw), 2568(vw), 2285(vw), 1559(vw), 1516(m),
1510(m), 1488(s), 1437(w), 1423(m), 1364(m), 1329(vs), 1241(m),
1133(vs), 1108(vs), 1056(s), 1039(s), 1003(vs), 937(s), 897(w), 881(w),
825(s), 779(s), 779(s), 753(s), 613(m), 571(m), 503(m), 446(w), 414(w)
cm−1 7Li{H} NMR (DMSO-d6, 25 °C): δ = −0.8 (s, Li) ppm. 14N{H} NMR
.
634(m), 622(w), 611(w), 550(w), 466(w), 450(w) cm−1
.
7Li{H} NMR
(DMSO-d6, 25 °C): δ = −14.2 (br s, NO2) ppm. 13C{H} NMR (DMSO-d6,
25 °C): δ = 157.4 (s, C-NO2), 138.1 (s, CH), 101.0 (s, CH) ppm. 1H{/} NMR
(DMSO-d6, 25 °C): δ = 7.39 (d, 1H, CH), 6.64 (d, 1H, CH) ppm. EA
(LiC3H2N3O2, 119.01): calcd. N 35.31, C 30.27, H 1.70 %; found N 35.38,
C 30.22, H 1.75 %. DTA (5 °C min−1): 332 (exothermic) °C.
(DMSO-d6, 25 °C): δ = −1.1 (s, Li) ppm. 11B{/} NMR (DMSO-d6, 25 °C): δ
= −6.0 (br s, BH2) ppm. 14N{H} NMR (DMSO-d6, 25 °C): δ = −18.8 (br s,
NO2) ppm. 13C{H} NMR (DMSO-d6, 25 °C): δ = 156.6 (s, C-NO2), 137.1 (s,
CH), 101.4 (s, CH) ppm. 1H{/} NMR (DMSO-d6, 25 °C): δ = 7.62 (d, 2H,
CH), 6.77 (d, 2H, CH), 3.60 (br s, 2H, BH2) ppm. EA (LiBC6H6N6O4,
243.90): calcd. N 34.46, C 29.55, H 2.48 %; found N 34.22, C 29.12, H
2.49 %. DTA (5 °C min−1): 200 (exothermic) °C. FS (100−500 μm): 144 N.
IS (< 100 μm): 2 J. ESD (100−500 μm): 0.42 J.
Acknowledgements
Lithium dihydrobis(5-aminotetrazol-1-yl)borate monohydrate:[15]
10 mL of anhydrous acetonitrile were added to 1.116 g potassium
borohydride (21 mmol) and 3.518 g 5-amino-1H-tetrazole (41 mmol) and
the reaction mixture was first left undisturbed and then stirred at room
temperature, until the release of hydrogen had dropped. Subsequently, the
Lewis acid-base reaction between the two starting materials was allowed
to proceed for four days under reflux, whereby a precipitate was formed.
The latter (4.561 g) was filtered off, suspended in roughly 9 mL of
bidistilled water and acidified with the same volume of glacial acetic acid.
After the suspension had been stirred at room temperature for 30 minutes,
the resulting solid was filtered off and washed three times with
approximately 9 mL of water each. Finally, the crude free acid (2.835 g)
was suspended in roughly 50 mL water, neutralized with lithium hydroxide
(0.358 g, 15 mmol) and the suspension was stirred at room temperature
for one hour. Filtering off unreacted intermediate and removing the solvent
from the filtrate in vacuo afforded 2.443 g of a white powder (79 %).
For financial support of this work by Ludwig-Maximilian University
(LMU), the Office of Naval Research (ONR) under grant no. ONR
N00014-19-1-2078 and the Strategic Environmental Research
and Development Program (SERDP) under contract no.
W912HQ19C0033 are gratefully acknowledged.
Keywords: boron • lithium azolates • lithium bis(azolyl)borates •
red pyrotechnic colorant • stability
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6
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Entry for the Table of Contents
Our previous study on lithium bis(azolyl)borates suggests that these are stable in case the ligands have pKa values higher than 5 and
lower than 20. This hypothesis was tested at the example of the 3-nitropyrazole and 5-aminotetrazole representatives, whose potential
to serve as environmentally benign red pyrotechnic colorants was determined. The impact of boron was evaluated by comparing their
emissions to those of the corresponding azolates.
8
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