Towards Safer Rocket Fuels: Hypergolic Imidazolylidene-Borane Compounds as Replacements for Hydrazine Derivatives

Article abstract of DOI:10.1002/chem.201601343

Currently, toxic and volatile hydrazine derivatives are still the main fuel choices for liquid bipropellants, especially in some traditional rocket propulsion systems. Therefore, the search for safer hypergolic fuels as replacements for hydrazine derivatives has been one of the most challenging tasks. In this study, six imidazolylidene-borane compounds with zwitterionic structure have been synthesized and characterized, and their hypergolic reactivity has been studied. As expected, these compounds exhibited fast spontaneous combustion upon contact with white fuming nitric acid (WFNA). Among them, compound 5 showed excellent integrated properties including wide liquid operating range (?70–160 °C), superior loading density (0.99 g cm^?3), ultrafast ignition delay times with WFNA (15 ms), and high specific impulse (303.5 s), suggesting promising application potential as safer hypergolic fuels in liquid bipropellant formulations.

Full text of DOI:10.1002/chem.201601343

DOI: 10.1002/chem.201601343
Full Paper
&
Cleaner Propellants
Towards Safer Rocket Fuels: Hypergolic Imidazolylidene-Borane
Compounds as Replacements for Hydrazine Derivatives
Shi Huang,[a] Xiujuan Qi,[b] Tianlin Liu,[a] Kangcai Wang,[a] Wenquan Zhang,[a] Jianlin Li,[c] and
Qinghua Zhang*[a]
Abstract: Currently, toxic and volatile hydrazine derivatives
are still the main fuel choices for liquid bipropellants, espe-
cially in some traditional rocket propulsion systems. There-
fore, the search for safer hypergolic fuels as replacements
for hydrazine derivatives has been one of the most challeng-
ing tasks. In this study, six imidazolylidene-borane com-
pounds with zwitterionic structure have been synthesized
and characterized, and their hypergolic reactivity has been
studied. As expected, these compounds exhibited fast spon-
taneous combustion upon contact with white fuming nitric
acid (WFNA). Among them, compound 5 showed excellent
integrated properties including wide liquid operating range
(ꢀ70–1608C), superior loading density (0.99 gcmꢀ3), ultrafast
ignition delay times with WFNA (15 ms), and high specific
impulse (303.5 s), suggesting promising application potential
as safer hypergolic fuels in liquid bipropellant formulations.
Introduction
satisfy the requirement of developing safer liquid hypergolic
fuels. For instance, Schneider et al. and Shreeve et al. described
several imidazolium dicyanamide (DCA) and nitrocyanamide
(NCA) HILs and studied their hypergolic reactivity with the
white fuming nitric acid (WFNA).[12,13] These hypergolic ionic
liquids exhibited relatively routine ignition performance, for ex-
ample, their ID times were commonly longer than 30 ms. In
order to achieve superior hypergolic reactivity, a wide variety
With the rapid development of aerospace technologies, vari-
ous chemically-propelled rockets have been invented and ap-
plied for space exploration over the past decades.[1–3] In some
liquid rocket engines that are still in use today, hydrazine and
its derivatives are still widely used as the chemical propulsion
fuels in some bipropellant systems. However, these hydrazine
derivatives are acutely toxic, highly volatile, carcinogenic sub-
stances and consequently have high handling costs. Against
such a context, there is increasing demand in developing
greener and safer rocket fuels as replacements for hydrazine
derivatives.[4,5] Thus, exploring safer hypergolic fuels has
become an area of intense research.
of
borohydride-rich
anions
including
[BH2(CN)2]ꢀ,[14]
[BH3CN]ꢀ,[15] [BH4]ꢀ,[16] [BH3(CN)BH2(CN)]ꢀ,[17] and [Al(BH4)4]ꢀ,[18]
have been developed and used for the construction of new
high-performance HILs (Figure 1). As expected, these borohy-
dride-rich HILs showed significantly enhanced ID performance
and most examples gave the ID times of <20 ms with WFNA
as the oxidizer. However, the majority of them suffered from
the problem of poor water stability due to the high reactivity
of their anions. Although it has been demonstrated that intro-
ducing two cyano groups into the borohydride structure can
In recent years, a new concept of hypergolic liquids (HILs)
has emerged and shown great promise as relatively greener
rocket fuels owing to their unique properties including low
vapor pressure, high thermal stability, high density, and ultra-
fast ignition delay (ID) times, etc.[6–11] These properties can well
[a] Dr. S. Huang, Dr. T. Liu, Dr. K. Wang, Dr. W. Zhang, Prof. Q. Zhang
Research Center of Energetic Material Genome Science
Institute of Chemical Materials
China Academy of Engineering Physics (CAEP)
Mianyang, 621900 (China)
E-mail: qinghuazhang@caep.cn
[b] Dr. X. Qi
School of Material Science and Engineering
Southwest University of Science and Technology
Mianyang, 621900 (China)
[c] Dr. J. Li
State Key Laboratory of Multiphase Flows in Power Engineering
Xi’an Jiaotong University
Xi’an, 710049 (China)
Supporting information for this article can be found under http://
dx.doi.org/10.1002/chem.201601343.
Figure 1. Pioneering studies in the field of HILs.
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significantly improve anionic water stability (e.g. [BH2(CN)2]ꢀ
and [BH3(CN)BH2(CN)]ꢀ), the difficulty in synthesizing these
cyano-rich borohydride salts decreases their practical applica-
tion values.
Results and Discussion
Except for compound 1 with a melting point of 1418C, the
other five imidazolylidene-borane compounds (2–6, Scheme 1)
showed relatively low melting points of <508C and three
compounds with unsymmetrical substituents (3, 4, and 5) are
liquids at room temperature (Tg =ꢀ708C). All these new NHC-
borane compounds were fully characterized by 1H and
13C NMR, IR, elemental analysis, and ESI-HRMS, etc. The charac-
terization data support the expected structure and their high
purities (see the characterization data and NMR spectra in the
Supporting Information).
Against this background, we tried to develop new hypergo-
lic compounds with low ID times and good water stability as
replacements for toxic hydrazine-based fuels. It is known that
N-heterocyclic carbenes (NHCs) have received significant atten-
tion in the fields of both organometallic chemistry and cataly-
sis.[19–22] Due to the lone pair of electrons on the carbon atom
in the structure, NHCs are capable of combining the unoccu-
pied orbital of borane (BH3) to form stable NHC-borane ad-
ducts. These NHC-borane compounds represent a new “clean”
class of reagents with moderate reductive ability.[23] Due to
their intrinsic reducing properties, violent exothermic redox re-
actions will occur when the NHC-boranes come into contact
with strong oxidizers such as WFNA. In this violent process,
gaseous reactive species (e.g., radicals, carbenes, etc.) may be
produced, which easily initiate a spontaneous combustion. To
the best of our knowledge, so far there are no examples of
using NHC-borane compounds as safer hypergolic fuels in
liquid bipropellants.
Structural analysis
The X-ray crystallographic analysis shows that 1 crystallizes in
the monoclinic space group P21/m (no. 11) with lattice para-
meters:
a=7.1421(7),
b=7.0068(10),
c=7.2312(7) ꢁ;
b=106.516(12)8; V=349.86(4) ꢁ3; Z=2; 1=1.044 gcm3. In its
structural data, the bond length of C2ꢀB1 is 1.596 ꢁ. No typical
hydrogen bonds are present in the molecule[26] (the crystallo-
graphic data are summarized in the Supporting Information).
In addition, the structure of 1 was also simulated by theoretical
computations. Electrostatic potential analysis (Figure 2a)
In our continuous efforts to develop safer hypergolic fuels,
in this study six imidazolylidene-borane compounds were syn-
thesized in moderate yields, and their hypergolic reactivity
with the oxidizer of WFNA was investigated. As shown in
Scheme 1, the imidazolium halides can be readily synthesized
Figure 2. a) Electrostatic potential surfaces of compound 1 at the B3LYP/6-
31+G (d,p) level, 0.004 electron/bohr isosurface, energy values ꢀ0.06172 to
+0.06172H. Color coding: red, negative; green, neutral; blue, positive.
b) Hirshfeld charges for 1 at the B3LYP/6-31+G(d,p) level. The red and
green of the ruler (from left to right) indicate regions of more negative and
positive Hirshfeld charges, respectively.
Scheme 1. Synthesis of imidazolylidene-borane compounds.
reveals that there is obvious charge separation in this mole-
cule, that is, the negative charge is assembled in the {BH3}
moiety, whereas the positive region is delocalized around the
imidazolium ring. The Hirshfield charge on the site of molecule
was calculated by B3LYP/6-31+G (d, p).[27] From Figure 2b and
Table S2 (Supporting Information), it has been clearly demon-
strated that in the structure of 1, the {BH3} moiety possesses
more negative charge (ꢀ0.456 e) and the imidazolium ring
shows a more positive charge (0.456 e). In combination with
the analysis of the electrostatic potential and Hirshfeld charge,
it can be concluded that this class of NHC-boranes has typical
characteristics of zwitterionic structures.
according to literature methods,[24,25] followed by reactions
with sodium borohydride in toluene. Six imidazolylidene-
borane compounds were obtained (1–6) in moderate yields
ranging from 63–76%. In these NHC-borane compounds, the
{BH3} moiety is directly connected to the 2-position of imidazo-
lylidene. More importantly, all these NHC-borane compounds
showed expected hypergolic reactivity upon contact with
WFNA as the oxidizer, demonstrating their potential applica-
tion as safer hypergolic fuels or additives in liquid bipropellant
formulations.
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Thermal properties, density, and viscosity
Density is an important indicator for evaluating the energy
level of propellant fuels. In general, a fuel with higher density
means higher loading capacity in the fuel tank and, therefore,
higher energy contribution in the propulsion process. The den-
sities of the six imidazolylidene-borane compounds ranged
from 0.93 to 1.06 gcmꢀ3, which are much higher than the
0.79 gcmꢀ3 of UDMH; this suggests that these imidazolylidene-
borane compounds as potential fuels have higher loading
capacities than hydrazine-based fuels in propellant tanks.
Among them, 1 had the highest density of 1.06 gcm3, whereas
4 had the lowest density of 0.93 gcm3. On the other hand, the
low viscosity of hypergolic fuels is highly desirable
Physicochemical properties of six imidazolylidene-boranes
(1–6) were measured and studied. First, their thermal proper-
ties including melting point, glass transition temperature and
thermal stability were measured and comparatively studied.
From the viewpoint of practical applications, low melting
points and high thermal decomposition temperatures make
for ideal liquid fuels, while they can be safely stored and
handled over a wide range of liquid operating temperatures.
From Table 1, three compounds 3–5 are liquids at room tem-
for the mixing and mass-transfer operation with the
Table 1. Physicochemical properties of imidazolylidene-borane.
oxidizer in the thrust chamber. Among these NHC-
borane compounds, three liquid compounds 3–5 ex-
hibited relatively low viscosities ranging from 26–
[g]
Entry
Tg[a][8C] Td[b][8C] 1[c][gcmꢀ3
]
N
[d][mPa] DfH[e][kJmolꢀ1
]
ID[f][ms] Isp [s]
1
2
3
4
5
6
141
45
188
190
172
208
160
186
1.0560
1.0274
0.9604
0.9273
0.9887
1.0280
0.79
–
–
61.61
29.06
2
100
35
38
15
304.6
200.5
308.1
304.0
303.5
305.1
313.6
31 MPa, in which
(26 MPa).
5 had the lowest viscosity
ꢀ70
ꢀ70
ꢀ70
41
29
31
26
–
8.30
ꢀ14.93
145.21
ꢀ5.25
53.28
Water stability
79
4.8
UDMH ꢀ57
64[h]
0.51
For rocket fuels, water stability is very important for
actual applications, including manufacture, storage,
transportation, and ignition processes.[28,29] To our
knowledge, the majority of known HILs with ultrafast
ID times (e.g., <20 ms) have shown very poor water
stability and typical examples include the [Al(BH4)4]ꢀ-
based HILs[18] and the [BH4]ꢀ-based HILs.[16] With sev-
[a] Melting point/glass transition temperature. [b] Thermal decomposition tempera-
ture. [c] Density at 258C. [d] Viscosity at 258C. [e] Heat of formation. [f] Ignition-delay
time with WFNA, 1 was tested as a solid. [g] Vacuum specific impulse (CEA400); pres-
sure 0.95 MPa; area expansion ratio of nozzle 70; oxidizer N2O4 (equivalence ratio=
1.0). [h] Boiling point.
perature with glass-transition temperatures (Tg) of ꢀ708C,
while compounds 1, 2, and 6 are solids at room temperature.
The high melting points may be partly due to their structural
symmetry or relatively short alkyl substituents on the imidazo-
lium ring. Furthermore, their thermal stabilities were also eval-
uated. As shown in Table 1, compounds 1–6 showed high
decomposition temperatures (Td) ranging from 160 to 2088C,
in which 4 gave the highest Td value of 2088C (Figure 3), while
5 showed the lowest Td value of 1608C. When compared to
unsymmetrical dimethylhydrazine (UDMH, Tg =ꢀ578C and
b.p.=648C), it is obvious that these imidazolylidene-borane
compounds have a much wider liquid operating range and are
therefore safe to operate.
eral exceptions, some [BH2CN2]ꢀ-based HILs reported by the
Shreeve’ group[14] showed both good hydrolytic stability and
very short ID times. In this study, the water stability of these
imidazolylidene-borane compounds was also evaluated. The
NMR experiments have demonstrated that these compounds
showed good water stability. For example, when 1 was dis-
solved and stored in deuterium oxide at room temperature for
a month, no obvious chemical degradation was observed from
1
the H NMR spectra (Figure 4), providing convincing evidence
for the high water stability of these compounds. This has also
demonstrated that this imidazolylidene-borane framework can
1
Figure 4. Hydrolytic experiment of 1 by H NMR: a) stored in deuterium
oxide at room temperature for 30 min; b) stored in deuterium oxide at room
temperature for a week; c) stored in deuterium oxide at room temperature
for a month.
Figure 3. TGA graph of 4.
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play a role in stabilizing the {BH3} moiety like the cyano group,
thereby improving the water stability of {BH3}-containing struc-
tures.
engine.[33,34] It is highly desirable for the Isp value of a propellant
fuel to be as high as possible. The Isp values of new hypergolic
fuels were also calculated by professional CEA400 software at
e=70, considering the variation of specific heat ratio with tem-
perature.[35,36] As listed in Table 1, the vacuum Isp values of
NHC-borane adducts vary from 200.5–308.1 s. Among the com-
pounds 1–6, 1 and 3–6 exhibited the relatively high vacuum Isp
values of >300 s, demonstrating their promising potentials as
high-performance green fuels in formulating the future hyper-
golic liquid bipropellants.
Theoretical study
The enthalpy calculations were performed with the Gaussi-
an 09 (Revision D.01) suite of programs.[30] The geometric opti-
mization and frequency analyses of the structures are based
on available single-crystal structures and using the B3LYP func-
tional with the 6-31+G (d,p) basis set.[31] Single-point energies
were calculated at the MP2/6-311+ +G (d, p) level. Atomiza-
tion energies for the molecules were obtained by the G2 ab
initio method. All of the optimized structures were character-
ized to be true local energy minima on the potential energy
surface without imaginary frequencies. The isodesmic reactions
(Scheme 2) were carried out to obtain the gas-phase heats of
Hypergolic test
As potential hypergolic fuels, evaluating the hypergolic reactiv-
ity with bipropellant oxidizers is very important since it deter-
mines whether these fuels are suitable for practical propulsion
applications. In general, hypergolic reactivity is estimated by
recording the ID times. Ultra-short ID times usually indicate
very fast ignition rates of hypergolic fuels with the oxidizer. In
this study, a droplet test was performed to evaluate the hyper-
golic reactivity of compounds 2–5. Each sample was measured
three times and averaged so the degree of variability was in
a reasonable range (ꢂ3 ms). A high-speed camera was used to
record the ID times as show in Figure 5 (the time between the
initial contact between fuels and oxidant and the flame
appears).
Scheme 2. Isodesmic reactions for the HOFs calculation of imidazolylidene-
borane compounds.
formation of these boranes compounds. The solid-state enthal-
py of formation for six NHC-borane compounds was calculated
by subtracting the heat of sublimation from the calculated
gas-phase heat of formation. Employing Trouton’s rule, the
heat of sublimation was calculated based on the decomposi-
tion temperature according to Equation (1).[32]
Figure 5. Drop test setup used to measure the ID times.
DHsub ¼ 188 J molꢀ1 Kꢀ1 ꢁ T
ð1Þ
A series of high-speed camera (2000 fps) photos of 5 is
shown in Figure 6. When the liquid drop of 5 contacted the
pool surface of WFNA, an intense reaction occurred and spon-
taneous ignition was observed. The drop tests showed that
these compounds exhibited very short ID times ranging from
2–100 ms. When compared with the widely used UDMH
(4.7 ms), 1 showed a shorter ID time. However, the high melt-
ing point of 1 limits its direct application as a liquid fuel. Fortu-
nately, liquids 3–5 also showed very short ID times ranging
from 15–38 ms, which can well meet the requirements of
liquid hypergolic fuels with desired ID time of <30 ms.
Among them, 5 showed the shortest ID time of 15 ms, which
From Table 1, the DHf values of six NHC-borane compounds
ranged from ꢀ14.93 to 145.21 kJmolꢀ1, in which 5 gave the
highest DfH value which is much higher than that of UDMH
(53.28 kJmolꢀ1). Analyzing the influences of different substitu-
ents on heats of formation, the allyl of imidazolylidene in 5
shows a more positive influence than other alkyl substituents
such as the methyl, ethyl, and butyl, etc., perhaps due to the
fuel-rich characteristic of the allyl group, which is in agreement
with the results reported in the literature.[15]
Specific impulse (Isp) is very indicative of the efficiency of
a chemical propellant and helps in the design of the rocket
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into it. The flask was fitted with a cold water condenser and placed
in an oil bath at 1258C for 18–24 h. After which time, the reaction
solvent was cautiously decanted from the insoluble mixture, and
the remaining residue was extracted with hot toluene. The solvent
was then filtered through a pad of silica gel. After evaporation of
the solvent, crude compounds were purified by silica gel column
chromatography or recrystallization.
Synthesis of compound 1
As a general procedure, 22.4 g (100 mmol) 1,3-dimethylimidazoli-
um iodide and 4.68 g (120 mmol) sodium borohydride was stirred
for 18 h at 1258C to afford crude product. The crude product was
recrystallized from water to give the pure product 1 as a white
1
solid. Yield: 70%; m.p. 1448C; H NMR (400 MHz, CDCl3): d=0.69–
1.33 (m, 3H, BH3), 3.74 (s, 6H, CH3), 6.81 ppm (s, 2H, CH); 13C NMR
(101 MHz, CDCl3): d=119.95, 35.96 ppm; IR (KBr): n˜ =3133, 3167,
3051, 2985, 2953, 2275, 1574, 1479, 1445, 1266, 1232, 1187, 1122,
876, 736, 702, 655 cmꢀ1; HRMS (ESI): m/z: calcd for C5H10BN2:
109.0937 [M]+; found: 109.0935; elemental analysis calcd (%) for
C5H11BN2: C 54.61, H 10.08, N 25.47; found: C 54.56, H 10.25,
N 25.58.
Figure 6. High-speed camera photos that show a spatially resolved ignition
event for a droplet of 5 falling into 100% HNO3.
may be attributed to the fuel-rich allyl substituent in the struc-
ture. For the liquid compound 5, its low Tg (ꢀ708C), high ther-
mal stability (Td =1608C), short ID time (15 ms), high specific
impulse (303.5 s), and low volatility makes it a potentially safer
fuel to replace hydrazine derivatives in liquid bipropellants.
Synthesis of compound 2
As a general procedure, 23.8 g (100 mmol) 1-methy-3-ethyl imida-
zolium iodide and 4.68 g (120 mmol) sodium borohydride was
stirred for 18 h at 1258C to afford the crude product, which was
purified by column chromatography (20% EtOAc/hexane) to afford
1
2 as a white solid. Yield: 68%; H NMR (400 MHz, CDCl3): d=0.70–
Conclusions
1.40 (m, 6H, CH3, BH3), 3.73 (s, 3H, CH3), 4.16 (q, 2H, CH2),
6.85 ppm (d, 2H, CH); 13C NMR (101 MHz, CDCl3): d=170.33–
172.02,120.18, 118.17, 43.72, 35.79, 15.46 ppm; IR (KBr): n˜ =3161,
3123, 2973, 2933, 2873, 2275, 1574, 1479, 1441, 1273, 1229, 1131,
753 cmꢀ1; HRMS (ESI): m/z: calcd for C6H12BN2:123.1094 [M]+;
found: 123.1089; elemental analysis calcd (%) for C6H13BN2:
C 58.12, H 10.57, N 22.59; found: C 57.98, H 10.68, N 23.12.
In summary, six imidazolylidene-borane compounds were syn-
thesized and characterized. These compounds exhibited good
water-stability and hypergolic reactivity with WFNA as the oxi-
dizer. As a potential hypergolic fuel, compound 5 showed
excellent properties including a low melting point, high ther-
mal stability, low viscosity, and a very short ID time, and there-
fore has potential as a hypergolic fuel in bipropellant formula-
tions.
Synthesis of compound 3
As a general procedure, 25.2 g (100 mmol) 1-methy-3-propyl imi-
dazolium iodide and 4.68 g (120 mmol) sodium borohydride was
stirred for 18 h at 125 8C to afford the crude product, which was
purified by column chromatography (20% EtOAc/hexane) to afford
3 as a colorless liquid. Yield: 64%; 1H NMR (400 MHz, CDCl3): d=
0.75–1.35 (m, 6H, CH3, BH3), 1.80 (m, 2H, CH2), 3.73 (s, 3H, CH3),
4.06 (t, 2H, CH2), 6.86 ppm (s, 2H, CH), 13C NMR (101 MHz, CDCl3):
d=170.45–171.97, 120.04, 118.92 50.26, 35.74, 23.40, 10.93 ppm; IR
(KBr): n˜ =3125, 2965, 2877, 2295, 1572, 1474, 1412, 1384, 1251,
1221, 1127, 864, 728, 636, 577 cmꢀ1; HRMS (ESI): m/z: calcd for
C7H14BN2: 137.1250 [M]+; found: 137.1246; elemental analysis calcd
for C7H15BN2: C 60.92, H 10.95, N 20.30; found: C 60.75, H 11.03,
N, 20.19.
Experimental Section
Chemicals
The organic solvents were of analytical grade. 1-Methylimidazole
(99%), 1-ethylimidazole (99%), 1-propylimidazol, 1-butylimidazole
(99%), and 1-allylimidazole (99%) were purchased form Sigma–
Aldrich. Iodomethane (99%), iodoethane (99%), and sodium boro-
hydride (97%) were purchased from J&K Scientific. All of the chem-
icals were used without further purification.
Synthesis of imidazolylidene-borane compounds
Synthesis of compound 4
Iodomethane or iodoethane (1.2 equiv) was added slowly in por-
tions to a dichloromethane solution of the appropriate imidazole
(5m) over the course of 15–30 min. The reaction mixture was
allowed to stir for 1 h, after which time the mixture was concen-
trated by rotary evaporation. Then, the remaining residue was
washed by ethyl acetate three times and dried under vacuum to
give the salt sample. The sample was used directly. Then a 250 mL
flask was charged with 100 mmol of imidazolium salt and 100 mL
of toluene, and then 120 mmol of sodium borohydride was added
As a general procedure, 26.6 g (100 mmol) 1-methy-3-butyl imida-
zolium iodide and 4.68 g (120 mmol) sodium borohydride was
stirred for 18 h at 125 8C to afford the crude product, which was
purified by column chromatography (20% EtOAc/hexane) to afford
4 as a colorless liquid. Yield: 65%; 1H NMR (400 MHz, CDCl3): d=
0.70–1.31 (m, 6H, CH3, BH3), 1.37 (m, 2H, CH2), 1.76 (m, 2H, CH2),
3.73 (s, 2H, CH3), 4.10 (d, 2H, CH2), 6.81 ppm (d, 2H, CH); 13C NMR
(101 MHz, CDCl3): d=120.01, 118.81, 48.55, 35.81, 32.19, 19.68,
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13.66 ppm; IR (KBr): n˜ =3164, 3125, 2958, 2934, 2871, 2339, 2281,
1572, 1474, 1412, 1238, 1180, 1126, 864, 728, 636, 575 cmꢀ1; HRMS
(ESI): m/z: calcd for C8H16BN2: 151.1407 [M]+; found: 151.1401;
elemental analysis calcd (%) for C8H17BN2: C 60.92, H 10.95,
N 20.30; found: C 60.75, H 11.03, N 20.19.
Keywords: boranes
hypergolicity · propellants
·
green chemistry
·
hydrazines
·
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Synthesis of compound 5
As a general procedure, 25.0 g (100 mmol) 1-methy-3-allyl imidazo-
lium iodide and 4.68 g (120 mmol) sodium borohydride was stirred
for 18 h at 125 8C to afford the crude product, which was purified
by column chromatography (20% EtOAc/hexane) to afford 5 as
1
a colorless liquid. Yield: 60%; H NMR (400 MHz, CDCl3): d=0.71–
1.35 (m, 3H, BH3), 3.75 (s, 3H, CH3), 4.74 (d, 2H, CH2), 5.27 (dd, 2H,
CH2), 5.93 (m, 1H, CH), 6.86 ppm (d, 2H, CH), 13C NMR (101 MHz,
CDCl3): d=170.85–172.39, 132.42, 120.36, 118.97, 118.63, 50.98,
35.89 ppm; IR (KBr): n˜ =3166, 3129, 2946, 2340, 2281, 1646, 1572,
1472, 1411, 1253, 1216, 1122, 994, 862, 729, 634, 606 cmꢀ1; HRMS
(ESI): m/z: calcd for C7H12BN2: 135.1094 [M]+; found: 135.1094;
elemental analysis calcd (%) for C7H13BN2: C 61.82, H 9.63, N 20.60;
found: C 61.75, H 9.71, N 20.45.
Synthesis of compound 6
As a general procedure, 25.2 g (100 mmol) 1, 3-diethyl imidazolium
iodide and 4.68 g (120 mmol) sodium borohydride was stirred for
18 h at 125 8C to afford the crude product, which was purified by
column chromatography (20% EtOAc/hexane) to afford 6 as
1
a white solid. M.p. 418C; yield: 67%; H NMR (400 MHz, CDCl3): d=
0.72–1.41 (m, 9H, CH3, BH3), 4.16 (q, 4H, CH2), 6.87 ppm (s, 2H, CH),
13C NMR (101 MHz, CDCl3): d=170.14–171.16, 118.35, 43.55,
15.42 ppm; IR (KBr): n˜ =3126, 2975, 2931, 2307, 1568, 1472, 1432,
1378, 1349, 1266, 1206, 1120, 1044, 803, 761, 740, 649 cmꢀ1; HRMS
(ESI): m/z: calcd for C7H14BN2:: 137.1250 [M]+; found: 137.1246;
elemental analysis calcd (%) for C7H15BN2: C 60.92, H 10.95,
N 20.30; found: C 60.75, H 11.03, N 20.19.
Instrumentation and analysis methods
1H and 13C NMR spectra were recorded on Bruker 400 AVANCE
spectrometer (400 and 101 MHz, respectively) with internal stan-
dard (1H NMR: CDCl3 at d=7.28 ppm; 13C NMR: CDCl3 at d=
77.16 ppm). IR spectra were performed on Perkin–Elmer Spectrum
Two IR Spectrometers. HRMS were performed on Shimadzu LCMS-
IT-TOF mass spectrometer using electrospray ionization (ESI).
Elemental analysis was performed on Flash EA-1112 elemental ana-
lyzer. Thermal property measurements were performed on TGA/
DSC1 and DSC3 Mettler Toledo calorimeter equipped with auto
cool accessory. Densities were measured on a Micromeritics Accu-
pyc II1340 gas pycnometer at 258C. Viscosity measurements were
performed on a Brookfield Rheometer DV3T at 258C. Ignition pho-
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This work was supported by the National Natural Science
Foundation of China (No. 11472251, 11572258), China Postdoc-
toral Science Foundation (2015M570796), Development Foun-
dation of CAEP (No. 2015B0302056, 2014B0302039), and the
Talent Startup Foundation of ICM (ST2014006).
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Published online on && &&, 0000
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FULL PAPER
Making a bang! Imidazolylidene-borane
compounds with a zwitterionic structure
exhibited fast spontaneous combustion
on contact with 100% HNO3 (see pic-
ture). Among these hypergolic liquids,
compound 5 has potential application
as safer hypergolic fuels in liquid bipro-
pellant formulations.
&
Cleaner Propellants
S. Huang, X. Qi, T. Liu, K. Wang,
W. Zhang, J. Li, Q. Zhang*
&& – &&
Towards Safer Rocket Fuels:
Hypergolic Imidazolylidene-Borane
Compounds as Replacements for
Hydrazine Derivatives
&
&
Chem. Eur. J. 2016, 22, 1 – 8
www.chemeurj.org
8
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!