Synthesis and characterization of N-(arylmethylene)-benzimidazole/imidazole-borane compounds

Article abstract of DOI:10.1016/j.jorganchem.2020.121544

N-(arylmethylene)-benzimidazole/imidazole-borane compounds (7-10) based on ferrocene/benzene were synthesized in a cost-effective way by reacting N-(arylmethylene)-benzimidazole/imidazoles with ammonium sulfate ((NH₄)₂SO₄) and sodium borohydride (NaBH₄). The structures of compounds 7-9 were determined by X-ray crystallography. Four compounds displayed high thermal decomposition temperatures (182-256°C) according to the thermogravimetry (TG) measurements. Interestingly, these compounds showed promising applications as potential propellant due to their spontaneous combustion upon contact concentrated HNO₃ (≥ 95 %) and catalytic effect on the thermal degradation of ammonium perchlorate (AP).

Full text of DOI:10.1016/j.jorganchem.2020.121544

Journal of Organometallic Chemistry 927 (2020) 121544
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Synthesis and characterization of
N-(arylmethylene)-benzimidazole/imidazole-borane compounds
Wei-Ming Liu, Min Zhang, Jian-Feng Yan, Cai-Xia Lin∗, Yao-Feng Yuan∗
Key Laboratory of Molecule Synthesis and Function Discovery (Fujian Province University), Department of Chemistry, Fuzhou University, Fuzhou 350108,
People’s Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history:
N-(arylmethylene)-benzimidazole/imidazole-borane compounds (7-10) based on ferrocene/benzene were
synthesized in a cost-effective way by reacting N-(arylmethylene)-benzimidazole/imidazoles with am-
monium sulfate ((NH4)2SO4) and sodium borohydride (NaBH4). The structures of compounds 7-9 were
determined by X-ray crystallography. Four compounds displayed high thermal decomposition tempera-
tures (182-256°C) according to the thermogravimetry (TG) measurements. Interestingly, these compounds
showed promising applications as potential propellant due to their spontaneous combustion upon contact
concentrated HNO3 (≥ 95 %) and catalytic effect on the thermal degradation of ammonium perchlorate
(AP).
Received 25 February 2020
Revised 15 September 2020
Accepted 21 September 2020
Available online 2 October 2020
Keywords:
Ferrocene
Imidazole
Borane
© 2020 Elsevier B.V. All rights reserved.
Thermal properties
Combustion test;Catalytic effect on
ammonium perchlorate
1. Introduction
imidazole-borane (10) were also prepared for comparation. In ad-
diton, compounds 7-9 were characterized by X-ray crystallography.
It is well known that ferrocene derivatives are a kind of impor-
tant burning rate catalysts (BRCs) of composite solid rocket propel-
lants [1,2]. Most of them have catalytic effects on ammonium per-
chlorate (AP), which is known as one of the most common oxidizer
in composite solid rocket propellants. Ferrocenyl nitrogen hetero-
cyclic compounds [3–6] are representative type of these deriva-
tives due to their low volatility [7], microscopic homogeneities
in distribution [8], higher compatibility with organic binders [9],
broad range of burning rate adjustments [10–12] and excellent ig-
nition performance. In the past decade, it had been found that im-
idazolium borohydride [13–16], N-alkylimidazole-borane adducts
[17] and imidazolyl-amine-BH2 [18] were promising green pro-
pellants for future space propulsion. Imidazole-borane compounds
that contained B-H bonds showed excellent integrated proper-
ties and ignition performances [19]. Thus, it would be an inter-
esting research topic to study the burning rate catalytic perfor-
mance of compounds containing both ferrocene and imidazole-
borane groups. Nevertheless, ferrocene derivatives of imidazole-
borane as potentially BRCs have been rarely reported so far.
Herein, in this work N-(ferrocenylmethyl)-benzimidazole-borane
(7) and N-(ferrocenylmethyl)- imidazole-borane (8) were synthe-
sized and N-(benzyl)-benzimidazole-borane (9) and N-(benzyl)-
The compounds showed promising applications as potential pro-
pellant due to their spontaneous combustion upon contact con-
centrated HNO3 (≥ 95 %) and effectively accelerated the thermal
degradation of AP.
2. Results and discussion
2.1. Synthesis
As depicted in Scheme 1, using ferrocenyl quaternary ammo-
nium salt (1) or benzyl bromide (2) as raw materials, the inter-
mediate compounds 3-6 were obtained by reacted with imida-
zole/benzimidazole respectively. Then these intermediates were re-
acted with NaBH4 and (NH4)2SO4 in anhydrous THF to give prod-
ucts 7-10. Among them, compound 10 had been reported in litera-
ture without the relevant thermal property experiments [25].
2.2. Crystal structures
To gain insight into the structural characteristics of imidazole-
borane adducts, the crystals of compounds 7-9 were obtained by
slow diffusion of petroleum ether into the dichloromethane solu-
tion of them. The spatial structures of them were confirmed by X-
ray crystallography. The crystallographic data and collection were
summarized in Table 1. Selected bonds distances and angles were
given in Tables S1-S6.
∗
Correspondence author.
E-mail address: yaofeng_yuan@fzu.edu.cn (Y.-F. Yuan).
https://doi.org/10.1016/j.jorganchem.2020.121544
0022-328X/© 2020 Elsevier B.V. All rights reserved.

W.-M. Liu, M. Zhang, J.-F. Yan et al.
Journal of Organometallic Chemistry 927 (2020) 121544
Scheme 1. Synthetic procedure of compounds 7-10.
Table 1
Crystal data and structure refinement for 7, 8, 9.
Compound
7
C
8
9
Chemical formula
Formula weight
Temperature (K)
Crystal system
Space group
18H19BFeN2
C14H17BFeN2
279.96
C14H15BN2
222.13
330.01
296(2)
296(2)
296(2)
Monoclinic
P212121
8.2008(14)
9.9847(15)
19.067(3)
90.00
Monoclinic
P212121
10.0167(7)
11.4232(7)
11.7815(8)
90.00
Monoclinic
P21/ c
˚
a (A)
9.4606(5)
15.4485(8)
8.5232(4)
90.00
˚
b (A)
˚
c (A)
α (°)
β (°)
γ (°)
90.00
90.00
98.003(2)
90.00
90.00
90.00
3
˚
V (A )
1561.2(4)
4
1348.07(16)
4
1233.55(11)
26
Z
Crystal size(mm3)
0.50 × 0.40 × 0.20
1.404
0.50 × 0.40 × 0.20
1.379
0.50 × 0.40 × 0.20
1.196
Dc (mg m−3
)
Abs coeff (mm−1
)
0.962
1.100
0.070
F(000)
688
584
472
θ range (°)
2.30-25.03
5886
2.48 -25.02
16044
2.17-25.03
19800
Reflections collected
Independent reflections
Goodness-of-fit on F2
R(int)
2706
2374
2176
1.055
1.051
1.092
0.0355
0.0671
0.0480
R1/wR2[I > 2σ(I)]
R1/wR2 (all data)
0.0323/0.0543
0.0403/0.0562
0.0271/0.0536
0.0335/0.0554
0.0394/0.0913
0.0508/0.0972
The single crystal structures of compounds 7-9 were illustrated
in Fig. 1. In order to facilitate the description of the hydrogen
atoms on the boron, the whole single crystal structures didn’t
shield the hydrogen atoms. In the single crystal structure of com-
pound 7, two cyclopentadiene (Cp) rings of a ferrocenyl group
were nearly eclipsed. The angle between the substituted Cp ring
(Cp ring: C6-C10) plane of ferrocenyl moieties with the benzimida-
zolium ring was 71.06° (Fig. 1a), owing to a free methylene group
between them. There were not intermolecular π-π interaction or
hydrogen bonding in 7. In addition, there was only a {BH3} not a
borane dimer, which can be supported by 11 B NMR (Fig. S3). The
boron spectra of the four compounds were split into quartet or re-
folded doublet with hydrogen coupling. The {BH3} moiety was co-
ordinated with N(2) atom in the benzimidazolium framework. The
˚
length of N(2)-B(1) was 1.579 A and the bond angles of C12-N2-B1
and C13-N2-B1 bonds were up to 127.4(3)°and 126.2(3)°, respec-
˚
tively, which was consistent with the bond length (1.579 A) of the
nitrogen-boron coordination bond in a similar structure reported
in the literature [17]. The single crystal structures of compounds 8
and 9 were similar to 7. It was worth pointing out that the angle
between the substituted Cp ring and the imidazole ring in the sin-
gle crystal structure of compound 8 was 66.08° (Fig. 1b) and the
2

W.-M. Liu, M. Zhang, J.-F. Yan et al.
Journal of Organometallic Chemistry 927 (2020) 121544
Fig. 1. Molecular structures of (a) 7, (b) 8, (c) 9 (thermal ellipsoids were set at 50 % probability).
3

W.-M. Liu, M. Zhang, J.-F. Yan et al.
Journal of Organometallic Chemistry 927 (2020) 121544
Fig. 3. Combustion changes of compound 8 under concentrated HNO3 at different
times.
Fig. 2. TG curve of compounds 7-10.
Table 3
Table 2
Ignition time (Ti) of borane com-
pounds 7-10.
Thermal properties of borane compounds 7-10.
Compound
7
8
9
10
Compound
7
7
8
5
9
10
67
Tm /°C
Td /°C
Wls /%
189-190
210
163-164
222
131-132
182
93-94
256
Ti /ms
37
70.8
70.7
89.1
73.4
[a] Tm: melting point; [b] Td: thermal decomposition temper-
ature; [c] Wls: weightlessness ratio.
Fig. 3 as an example. The ignition time (Ti) of compounds 7-10
were collected in Table 3. It could be found that compounds 7 and
8 with ferrocenyl groups took less time than compounds 9 and
10 to start combustion with concentrated HNO3 and compound 8
was ignited in the shortest time (5 ms). The ferrocene-containing
stereo aromatic structure may play a role in accelerating combus-
tion. Moreover, intermediate 4 without {BH3} moiety couldn’t be
ignited when the same operation was performed. It can be specu-
lated that the vigorous combustion of compound was result from
a violent oxidation-reduction stimulated by the B-H bonds in com-
pound with the oxidant HNO3. However, if the HNO3 was dripped
onto compounds 7-10 with burettes, the phenomenon shown in
the Fig. 3 couldn’t be observed, which was caused by the insuf-
ficient contact between HNO3 and products. Besides, the concen-
tration of HNO3 had a great influence on the ignition experiment,
that HNO3 with a concentration lower than 95% couldn’t ignite the
compounds.
angle between the benzene ring and the benzimidazole ring in the
single crystal structure of compound 9 was 79.02° (Fig. 1c).
2.3. Thermal stability of compounds 7-10
As shown in Fig. 2, compounds 7-10 showed no obvious
weightlessness at the beginning. As the temperature continued to
rise, the structures of compounds collapsed and they began to
lose weight quickly. The specific decomposition temperatures and
weightlessness ratios of four compounds were shown in Table 2.
The decomposition temperatures of compounds 7-10 were ranged
from 182 to 256°C. Ferrocene-based derivatives (7 and 8) had
lower weightlessness ratios than benzene-based derivatives (9 and
10), which may be due to the conversion of ferrocene into ferric
oxide at high temperature [20,21]. In addition, compounds 8 and
10 containing imidazolyl were more stable than compounds 7 and
9 containing benzimidazolyl from the perspective of the decom-
position temperatures. Based on the TG curves and the melting
points, the compounds had a liquefaction process. Among them,
compound 10 showed the best stability with the widest tempera-
ture range (94-256 °C) in the liquid state. Ferrocene-based deriva-
tives (7 and 8) also had high thermal stability, but the stable tem-
perature range of them in the liquid state was shorter than that of
compound 10.
2.4. Catalytic effect on the decomposition of ammonium perchlorate
(AP)
Ferrocene derivatives are widely used catalysts and their po-
tential as propellants can usually be estimated by studying their
catalytic effects on AP [1]. The catalytic effects of the products on
AP were evaluated used TG analysis with a weight percentage of
BRCs in AP were generally 2–5 wt.% [5,26]. Hence, we prepared
four samples of pure AP and mixtures of AP with 2 wt.%, 3 wt.%,
5 wt.% of compound 7 and compared their thermal decomposition
process by TG. As shown in Fig. 4, the final thermal decomposi-
tion temperature of AP was 443 °C. After adding 2 wt.%, 3 wt.%,
5 wt.% of compound 7, the final thermal decomposition tempera-
tures of AP decreased by 42 °C, 52 °C, 65 °C, respectively. Further-
more, TG curves of pure AP and mixtures of AP with 5 wt.% of
four products were investigated. As illustrated in Fig. 5, with the
addition of 7 and 8 the final thermal decomposition temperatures
of AP decreased to 378 °C and 398 °C, while with the addition of
9 and 10, the final thermal decomposition temperatures only re-
duced to 411 °C and 419 °C, which indicated that benzene-based
2.3. Combustion test
To verify the reduction and combustion behavior of the com-
pounds, ignition tests were conducted according reported proce-
dures [32,33]. Each sample (about 10 mg) was dropped into a
10 mL glass beaker containing concentrated HNO3 (2 mL) and
recorded with a 1000 fps high-speed camera at the same time. Vi-
sual analysis of four compounds showed that all compounds could
burn rapidly with high flame height. A series of images of com-
pound 8 collected with the high-speed camera were shown in
4

W.-M. Liu, M. Zhang, J.-F. Yan et al.
Journal of Organometallic Chemistry 927 (2020) 121544
celerated the decomposition of AP. Consequently, we can explain
why the benzene-based derivatives (9 and 10) had catalytic effects
on AP. We believed that the catalytic effects of 7 and 8 on AP also
benefited from B-H bonds. But due to the influence of ferrocene,
the weight loss of AP in the first process was not obvious, espe-
cially after increasing the content of ferrocene derivatives as shown
in Fig. 4.
3. Conclusion
In this paper, we have synthesized four energetic small molec-
ular compounds, N-(arylmethylene)- benzimidazole/imidazole-
borane compounds (7-10) with relatively cheap raw materials
NaBH4 and (NH4)2SO4. These products possessed some unique
properties including the high stability, outstanding ignition per-
formance with HNO3 and catalytic effect for AP. Ferrocene-based
derivatives (7 and 8) took less times than benzene-based deriva-
tives (9 and 10) to start combustion with HNO3. Compounds 7-10
all had catalytic effects on AP as burning rate catalysts in propel-
lants. Compound 7 had the best catalytic effect on AP with the
final thermal decomposition temperature of AP decreased to 378
°C. In summary, we believed that ferrocene-based derivatives (7
and 8) had good ignition and combustion-supporting performance
at once and would have great value in propellants.
Fig. 4. TG curve of AP, AP + 2 wt.% 7, AP + 3 wt.% 7, AP + 5 wt.% 7.
4. Experimental
4.1. General methods
All chemicals were commercially available. All reactions
were performed under an atmosphere of dry nitrogen. Melt-
ing points (M.p.) were determined with a 4 × micro melting
point apparatus. Nuclear magnetic resonance spectra (1H and
13C and 11 B NMR) were measured in CDCl3, with tetramethyl-
silane (TMS) as the internal standard, with
a Bruker AV400
spectrometer at ambient temperature. High resolution mass
spectrometer was obtained on an Accurate-Mass Q-TOFLC/MS.
The intermediate 1-(ferrocenylmethyl)benzimidazole (3) [22], 1-
(ferrocenylmethyl)imidazole (4) [23], 1-(benzyl)benzimidazole (5)
[24], 1-(benzyl)imidazole (6) [24] were synthesized by the proce-
dures according to the literatures.
Fig. 5. TG curve of AP, AP + 5 wt.% compound (7-10).
X-ray structural measurements were carried out on a Rigaku
RAXISIV CCD diffract meter with a graphite-monochromator Mo
Kα radiation (λ = 0.071073 nm) at 296(2) K. All diffraction data
were collected by scanning in a certain mode and refined in Lp fac-
tor. All data were corrected by semi-empirical method using SAD-
ABS program, the SAINT program was used for integration of the
diffraction profiles. The structure was solved by the direct meth-
ods using SHELXS program of the SHELXL-97 [31]. The position
coordinates and each anisotropic thermal parameter of nonhydro-
gen atoms were refined by full-matrix least-squares on F2 through
confirmation. All hydrogen atoms were generated geometrically as-
signed appropriated isotropic thermal parameters and included in
the final calculations.
derivatives 9 and 10 had poorer catalytic effects on AP compared
with ferrocenyl-based derivatives 7 and 8. However, the effect of
benzimidazole or imidazole groups in compound on the catalytic
thermal decomposition of AP was not obvious.
Specifically, researchers generally agreed that the thermal de-
composition of AP was divided into two parts as shown in Fig. 4.
Firstly, AP would decompose to produce NH3 and HClO4 at the
lower temperature (294 °C - 332 °C). Then NH3 would adhere to
the surface of AP and HClO4 further decomposed to produce var-
ious oxidizing products. Secondly, as NH3 desorbed on the sur-
face of AP at the higher temperature (332 °C - 443 °C), AP con-
tinued to react and decompose. [27,28]. As mentioned above, the
decomposition temperatures of four additives (7-10) were all be-
low 256 °C (Table 2), which was much lower than the thermal de-
composition temperature of AP. And compounds 7-10 released a
certain amount of hydrogen when they decomposed [29,30]. Hy-
drogen reacted with the oxidizing products produced by HClO4,
which would further increase the reaction rate of HClO4. We can
find evidence to support the above view from Fig. 5. After adding
additives 9 and 10, TG curve of AP would lose weight quickly in
the first part compared with the thermal decomposition of pure
AP. Therefore, the thermal decomposition process of AP proceeded
in the atmosphere of some hydrogen, which gave out heat and ac-
Thermogravimetry (TG) analysis on compounds 7-10 and AP
were dried under 0.1 MPa at 20 °C, then recorded with a TG Ther-
mal Analyzer SDT Q600 instrument. The experiments were con-
ducted at heating rate of 10 °C/min.
4.2. General procedure for compounds 7-10
4.2.1. Synthesis of N-(ferrocenylmethyl)-benzimidazole- borane (7)
A solution of NaBH4 (114 mg, 3 mmol) and (NH4)2SO4 (198 mg,
1.5 mmol) was added to THF (5 mL) under vigorous stirring. 1-
(ferrocenylmethyl)benzimidazole (100 mg, 0.30 mmol,) which was
dissolved with THF (30 mL) was added dropwise to the reaction
5

W.-M. Liu, M. Zhang, J.-F. Yan et al.
Journal of Organometallic Chemistry 927 (2020) 121544
system and refluxed at 66°C for 0.5 h. After cooling to room tem-
perature, the mixture was filtered with diatomite and washed with
dichloromethane. The residue was purified by column chromatog-
raphy to give compound as a yellow solid. Yield 93.3 % (94 mg).
M.p. 189-190 °C. 1H NMR (400 MHz, Chloroform-d) δ: 8.05 (s,
1H, NCHN), 8.00 – 7.94 (m, 1H, PhH), 7.56 – 7.50 (m, 1H, PhH),
7.46 (t, J= 7.1 Hz, 2H, PhH), 5.12 (s, 2H, CH2), 4.29 (s, 2H, CpH),
4.28 (s, 2H, CpH), 4.24 (s, 5H, CpH), 2.64 – 2.10 (m, 3H, BH3). 13C
NMR (101 MHz, Chloroform-d) δ: 141.03, 137.37, 132.20, 124.94,
124.73, 117.63, 110.52, 70.06, 69.66, 69.33, 69.03, 45.99. 11 B NMR
(128 MHz, Chloroform-d) δ: -21.32 (q, JBH = 91.7 Hz). HRMS (ESI+):
C18 H19 BFeN2, [M+Na]+ requires 353.0883, found 353.0877.
Appendix A. Supplementary material
1H NMR, 13C NMR and 11 B NMR of compounds 7-10 were
given in Figs. S1-S12. HRMS spectra of compounds 7-10 were given
˚
in Figs. S13-S16. Selected bond lengths (A) and angles for com-
pounds 7-9 were given in Tables S1-S6. CCDC 1978238, 1978239
and 1978240 contained the supplementary crystallographic data
for this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
Supplementary materials
Supplementary material associated with this article can be
found, in the online version, at doi:10.1016/j.jorganchem.2020.
121544.
4.2.2. Synthesis of N-(ferrocenylmethyl)-imidazole- borane (8)
Compound 8 was prepared by adopting the same procedure as
that of 7 by using NaBH4 (137 mg, 3.60 mmol) and (NH4)2SO4
(238 mg, 1.8 mmol) and 1-(ferrocenylmethyl)imidazole (100 mg,
0.36 mmol). The pure product obtained was a yellow solid. Yield
77.5 % (87.7 mg). M.p.163-164 °C. 1H NMR (400 MHz, Chloroform-
d) δ: 7.67 (s, 1H, NCHN), 7.01 (s, 1H, NCH), 6.85 (s, 1H, NCH), 4.88
(s, 2H, CH2), 4.25 (m, 2H, CpH), 4.23 (m, 2H, CpH), 4.19 (s, 5H,
CpH), 2.48 – 1.92 (m, 3H, BH3). 13C NMR (101 MHz, Chloroform-d)
δ: 135.61, 127.57, 119.38, 79.58, 69.59, 69.03, 68.96, 48.39. 11 B NMR
(128 MHz, Chloroform-d) δ: -19.47 (q, JBH= 92.7 Hz). HRMS (ESI+):
C14 H17 BFeN2, [M+Na]+ requires 303.0726, found 303.0723.
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Compound 9 was prepared in a manner analogous to that of
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The procedure adopted for the preparation of 10 the same as
that of 7 using NaBH4 (221 mg, 5.80 mmol) and (NH4)2SO4 (383
mg, 2.90 mmol) and 1-(benzyl)imidazole (100 mg, 0.58 mmol).
The pure product obtained was a white solid. Yield 81.3 % (87.8
mg). M.p.93-94 °C. 1H NMR (400 MHz, Chloroform-d) δ: 7.76 (s,
1H, NCHN), 7.43 – 7.35 (m, 3H, PhH), 7.24 – 7.17 (m, 2H, PhH),
7.06 (s, 1H, NCH), 6.89 (s, 1H, NCH), 5.11 (s, 2H, CH2), 2.63 – 1.90
(m, 3H, BH3).13C NMR (101 MHz, Chloroform-d) δ:136.36, 133.64,
129.41, 129.23, 128.01, 52.32. 11 B NMR (128 MHz, Chloroform-d) δ:
-19.32 (q, JBH = 91.2 Hz). HRMS (ESI+): C10H13BN2, [M+Na]+ re-
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Declaration of Competing Interest
None.
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2 (2019)
This work was supported financially by the National Natural
Science Foundation of China (No. 21642009 and 21772023) and
the Valuable Instrument and Equipment Open Test Fund of Fuzhou
University (No. 2019T004). We also thank You-Ren Huang for his
help in crystal analysis.
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