Nonaborane and decaborane cluster anions can enhance the ignition delay in hypergolic ionic liquids and induce hypergolicity in molecular solvents

Article abstract of DOI:10.1021/ic500622f

The dissolution of nido-decaborane, B₁₀H₁₄, in ionic liquids that are hypergolic (fuels that spontaneously ignite upon contact with an appropriate oxidizer), 1-butyl-3-methylimidazolium dicyanamide, 1-methyl-4-amino-1,2,4-triazolium dicyanamide, and 1-allyl-3-methylimidazolium dicyanamide, led to the in situ generation of a nonaborane cluster anion, [B₉H₁₄]^-, and reductions in ignition delays for the ionic liquids suggesting salts of borane anions could enhance hypergolic properties of ionic liquids. To explore these results, four salts based on [B₁₀H₁₃]^- and [B₉H ₁₄]^-, triethylammonium nido-decaborane, tetraethylammonium nido-decaborane, 1-ethyl-3-methylimidazolium arachno-nonaborane, and N-butyl-N-methyl-pyrrolidinium arachano-nonaborane were synthesized from nido-decaborane by reaction of triethylamine or tetraethylammonium hydroxide with nido-decaborane in the case of salts containing the decaborane anion or via metathesis reactions between sodium nonaborane (Na[B₉H ₁₄]) and the corresponding organic chloride in the case of the salts containing the nonaborane anion. These borane cluster anion salts form stable solutions in some combustible polar aprotic solvents such as tetrahydrofuran and ethyl acetate and trigger hypergolic reactivity of these solutions. Solutions of these salts in polar protic solvents are not hypergolic.

Full text of DOI:10.1021/ic500622f

Article
pubs.acs.org/IC
Nonaborane and Decaborane Cluster Anions Can Enhance the
Ignition Delay in Hypergolic Ionic Liquids and Induce Hypergolicity
in Molecular Solvents
Parker D. McCrary, Patrick S. Barber, Steven P. Kelley, and Robin D. Rogers*
Center for Green Manufacturing and Department of Chemistry, The University of Alabama, Box 870336, Tuscaloosa, Alabama
35487, United States
S
* Supporting Information
ABSTRACT: The dissolution of nido-decaborane, B10H14, in ionic liquids that are hypergolic
(fuels that spontaneously ignite upon contact with an appropriate oxidizer), 1-butyl-3-
methylimidazolium dicyanamide, 1-methyl-4-amino-1,2,4-triazolium dicyanamide, and 1-allyl-
3-methylimidazolium dicyanamide, led to the in situ generation of a nonaborane cluster anion,
[B9H14]−, and reductions in ignition delays for the ionic liquids suggesting salts of borane
anions could enhance hypergolic properties of ionic liquids. To explore these results, four salts
based on [B10H13]− and [B9H14]−, triethylammonium nido-decaborane, tetraethylammonium
nido-decaborane, 1-ethyl-3-methylimidazolium arachno-nonaborane, and N-butyl-N-methyl-
pyrrolidinium arachano-nonaborane were synthesized from nido-decaborane by reaction of
triethylamine or tetraethylammonium hydroxide with nido-decaborane in the case of salts
containing the decaborane anion or via metathesis reactions between sodium nonaborane
(Na[B9H14]) and the corresponding organic chloride in the case of the salts containing the
nonaborane anion. These borane cluster anion salts form stable solutions in some combustible
polar aprotic solvents such as tetrahydrofuran and ethyl acetate and trigger hypergolic reactivity of these solutions. Solutions of
these salts in polar protic solvents are not hypergolic.
manuscript, it was also demonstrated that a triethylammonia
borane adduct (N(Et)3·BH3) can act as an ignition delay
enhancer if added to known hypergolic ILs.13
INTRODUCTION
■
Hypergolic ionic liquids (ILs), a class of salts with melting
points below 100 °C that spontaneously ignite upon contact
with an oxidizer,1−3 have been previously proposed as
replacements for hydrazine as a rocket bipropellant; however,
key properties, such as heat of combustion,4 ignition delay
(ID),2 and low temperature viscosity,5 need improvement to
match the performance of hydrazine.6 In our own research to
understand the chemistry of hypergolic ILs and their analogues,
instead of focusing entirely on the ions within the IL, we have
sought to improve the performance of hypergolic ILs by
incorporation of catalytic nanoparticle additives, such as B(0)7,8
or Ti(0)9 to improve overall energetic content or graphene10 to
improve low-temperature viscosity. One key challenge with
these nanoparticulate systems has been colloidal stability (<2−
3 days in many cases).7−10 We hypothesized that the stability of
energetic additives could be improved by reducing their size
from suspended nanoparticles to solutions of molecular clusters
on the order of angstroms rather than nanometers.
Given the increased flame height that we obtained by
incorporating B(0) nanoparticles in hypergolic ILs,7 we
wondered whether neutral or charged borane clusters could
be dissolved in ILs in a catalytic ratio to enhance the energetic
properties. Recently, several molecules containing the B−H
functional group have been demonstrated as hypergolic
salts,3,11 and BH3 hydrazine adducts have been utilized in
high concentrations as ignition delay enhancers by dissolving
them in hypergolic ILs.12 During the preparation of this
Boranes are known for their potential to form air stable
clusters14 (including anionic species15) and to be oxidized
readily16 in hypergolic ignition reactions. Indeed, boranes have
been explored in the past as potential fuels,17,18 but interest
waned due to issues with high volatility, toxicity, reactivity with
many organic solvents, and solid product formation (B2O3)
upon oxidation.16 However, many of the properties inherent to
many ILs, such as improved stability and low volatility, might
be exploited by transforming neutral boranes into salts of
borane anions.
While our initial interest in utilizing molecular borane
clusters as energetic additives stemmed from their high
reported reactivity with oxidizers,16 we were also interested in
exploring how the dissolved borane clusters would interact with
complex hypergolic IL solvents, much like our previous
explorations of the surface reactivity of nanomaterials with
hypergolic ILs.7−10 We selected nido-decaborane (B10H14) as
our molecular borane source because it is air stable as a
crystalline solid19 and reacts to form anionic species by acting
as a Brønsted acid (pKa 2.720) in the presence of various
bases.21 This has previously been utilized to synthesize
22,23
[B10H13]−
24,25
and [B9H14]−
(Figure 1). Decaborane has
Received: March 17, 2014
Published: April 9, 2014
© 2014 American Chemical Society
4770
dx.doi.org/10.1021/ic500622f | Inorg. Chem. 2014, 53, 4770−4776

Inorganic Chemistry
Article
a Teflon stirbar. The flask was slowly heated to 35 °C during the
addition. Once the 4-amino-1,2,4-triazole was completely dissolved
(∼3 h), the flask was covered to minimize light exposure. While at 35
°C, iodomethane (422 g, 2.98 mol) was added slowly dropwise over
∼2 h, keeping the temperature of the solution under 40 °C during the
addition. At the end of the iodomethane addition, the heat was turned
off, and the reaction mixture was allowed to cool while stirring
overnight. When the reaction was complete as determined by 1H
NMR, the flask was cooled to 0 °C, and a solid precipitated. The crude
solids were recrystallized from absolute ethanol to yield a white
crystalline solid. Melting point: 98 °C.7,29 1H NMR (360 MHz,
DMSO-d6) δ ppm: 10.12 (1H, s), 9.16 (1H, s), 6.93 (2H, s), 4.02 (3H,
s). 13C NMR (125 MHz, DMSO-d6) δ ppm: 145.1, 143.0, 39.1.
1-Allyl-3-methylimidazolium Chloride ([Amim]Cl). [Amim]Cl was
prepared following the synthesis in ref 28 and was only modified in
scale. Allyl chloride (150 mmol, 11.478 g) was dropwise added to
freshly distilled 1-methylimidazole (120 mmol, 9.852 g) at room
temperature, and the solution was stirred at 55 °C for 18 h. The excess
allyl chloride was removed under reduced pressure to obtain an amber,
Figure 1. Diagrams of stable borane cluster anions generated from
crystallographic coordinates in the Cambridge Structural Database for
entries SATDOG ([PhCH2(CH3)3N][B10H13])23 and MOTGIL
(K[B9H14]).24
also been reported to react with the slightly basic chloride anion
in 1-butyl-3-methylimidazolium chloride ([C4mim]Cl) via an
acid−base reaction to form [B10H13]− as an intermediate in the
synthesis of o-carboboranes.26,27
Here we report our efforts to explore and understand the
reactivity between the borane cluster B10H14 and dicyanamide-
based hypergolic ILs and the composition and energetic role of
the generated species. We also discuss our findings that salts of
borane cluster anions not only can reduce the ignition delay of
hypergolic ionic liquids in which they are dissolved but also can
act as hypergolic triggers leading to combustion of non-
hypergolic aprotic solvents in which they can be solubilized.
1
viscous liquid. H NMR (360 MHz, DMSO-d6) δ ppm: 9.65 (1H, s),
7.90 (1H, s), 7.88 (1H, s), 6.04 (1H, tdd), 5.31 (1H, dd), 5.28 (1H,
dd), 4.94 (2H, d), 3.91 (3H, s). 13C NMR (125 MHz, DMSO-d6) δ
ppm: 137.3, 132.4, 124.2, 122.8, 120.5, 51.1, 36.2.
Silver Dicyanamide (Ag[DCA]). Sodium dicyanamide (13.35 g, 150
mmol) was dissolved in 20 mL of DI water and added dropwise to a
saturated aqueous solution of silver nitrate (25.48 g, 150 mmol) in a
100 mL round-bottom flask with a Teflon-coated magnetic stirbar.
The mixture was stirred overnight in darkness at room temperature,
and the resulting white solid was vacuum filtered and washed with DI
water followed by methanol. The solid was dried in a vacuum oven at
70 °C for 24 h.
EXPERIMENTAL SECTION
Metathesis Reactions and Drying for [DCA]− ILs.2,7 1-Butyl-3-
methylimidazolium Dicyanamide ([C4mim][DCA]). The method for
the synthesis of all [DCA]− ILs was obtained from ref 2 and was only
modified for the scale of the synthesis as presented in ref 7. In a
metathesis procedure common to all of the [DCA]− ILs prepared,
[C4mim]Cl (8.734 g, 50.0 mmol) was dissolved in 25 mL of dry
methanol and transferred to a 250 mL round-bottom flask containing a
1.1 molar excess of Ag[DCA] (11.193 g, 55.0 mmol) containing 175
mL of anhydrous methanol.2 An excess of Ag[DCA] was used to
ensure complete conversion due to the limited solubility in methanol.
The flask was covered with aluminum foil and stirred for 3 days. AgCl
and remaining Ag[DCA] were filtered, and the filtrate was evaporated
under reduced pressure. The resulting clear oil was dissolved in a
minimal amount of methanol and filtered again to remove trace
amounts of AgCl dissolved in the IL. The methanol was removed
under reduced pressure.
■
Caution! Although no accidents were experienced in the studies reported
here, special care is needed prior to and during the handling of all borane
hydride compounds. Toxicity, volatility, shock sensitivity, and impact
sensitivity are common in molecular boranes and borohydrides; thus
extreme caution should be exercised when handling these materials.
Materials. 1-Chlorobutane, triethylamine, allyl chloride, iodo-
methane, tetraethylammonium hydroxide, and chloroethane were
purchased from Sigma-Aldrich (St. Louis, MO) and used as received.
1-Methylimidazole was purchased from Sigma-Aldrich (St. Louis,
MO) and distilled prior to use. nido-Decaborane, 4-NH2-1,2,4-triazole,
sodium dicyanamide, silver nitrate, and sodium hydroxide were
purchased from Alfa-Aesar (Ward Hill, MA) and used as received. 1-
Butyl-1-methylpyrrolidinium chloride ([Pyrr14]Cl) was purchased
from EMD Chemicals (Darmstadt, Germany) and utilized as received.
1-Ethyl-3-methylimidazolium chloride ([C2mim]Cl) was purchased
from BASF (Ludwigshafen, Germany) and was recrystallized from
dichloromethane prior to use. Absolute ethanol, acetonitrile,
anhydrous methanol, and anhydrous diethyl ether were purchased
from Fischer Scientific (Hampton, NH) and used without any further
purification.
The IL was dried using high vacuum (1 × 10−4 Torr) at 70 °C and
then degassed using three separate freeze−thaw cycles with liquid
nitrogen under high vacuum. A clear, nonviscous liquid was obtained.
The water content was less than 500 ppm as determined by
Coulometric Karl Fischer titration utilizing a Mettler-Toledo
(Columbus, OH) C20 Coulometric KF Titrator. All other ILs were
dried in the same manner.
Synthesis of Ionic Liquid Precursors.7,28 1-Butyl-3-methyl-
imidazolium Chloride ([C4mim]Cl). [C4mim]Cl was prepared
following the synthesis in ref 7. 1-Methylimidazole (150.00 g, 1.83
mol) was distilled into a flask containing a 1.1 molar excess of 1-
chlorobutane (204.64 g, 2.00 mol), and the resulting mixture was
refluxed under argon gas through Schlenk-line techniques. The
reaction was monitored by NMR until all traces of 1-methylimidazole
were no longer visible (96 h). The excess 1-chlorobutane was removed
under reduced pressure, and the product was slowly cooled forming a
1H NMR (500 MHz, neat liquid) δ ppm: 9.08 (1H, s), 7.74 (1H, s),
7.67 (1H, s), 4.16 (2H, s), 3.85 (3H, s), 1.78 (2H, quintet (triplet of
triplets)), 1.27 (2H, sextet), 0.90 (3H, t). 13C NMR (125 MHz, neat
liquid) δ ppm: 137.0, 124.0, 122.7, 119.6, 49.1, 36.2, 31.8, 19.3, 13.6.
1-Methyl-4-amino-1,2,4-triazolium Dicyanamide ([MAT][DCA]).
The iodide anion in [MAT]I (22.602 g, 100.0 mmol) was exchanged
for the [DCA]− anion with Ag[DCA] (22.385 g, 110 mmol) and dried
1
white, crystalline solid. Melting point: 68 °C. H NMR (360 MHz,
1
as described above. H NMR (500 MHz, neat liquid) δ ppm: 10.67
DMSO-d6) δ ppm: 9.12 (1H, s), 7.93 (1H, s), 7.85 (1H, s), 4.22 (2H,
t), 3.90 (3H, s), 1.77 (2H, quintet), 1.25 (2H, sextet), 0.890 (3H, t).
13C NMR (125 MHz, DMSO-d6) δ ppm: 137.6, 124.5, 123.1, 49.3,
36.6, 32.3, 19.6, 14.2.
(1H, s), 9.74 (1H, s), 7.39 (2H, s, NH2), 5.00 (3H, s). 13C NMR (125
MHz, neat liquid) δ ppm: 146.2, 144.0, 120.1, 40.4.
1-Allyl-3-methylimdazolium Dicyanamide ([Amim][DCA]). The
chloride anion in [Amim]Cl (7.932 g, 50.0 mmol) was exchanged for
the [DCA]− anion with Ag[DCA] (11.193 g, 55.0 mmol) and dried as
1-Methyl-4-amino-1,2,4-triazolium Iodide ([MAT]I). [MAT]I was
prepared following the synthesis in ref 7. 4-Amino-1,2,4-triazole (100
g, 1.19 mol) was weighed out and added piecewise into a 1 L reaction
flask containing 0.5 L of reagent grade acetonitrile under agitation with
1
described above. H NMR (500 MHz, neat liquid) δ ppm: 9.09 (1H,
s), 7.22 (1H, s), 7.20 (1H, s), 6.03 (1H, tdd), 5.36 (1H, dd), 5.31 (1H,
4771
dx.doi.org/10.1021/ic500622f | Inorg. Chem. 2014, 53, 4770−4776

Inorganic Chemistry
Article
dd), 4.31 (2H, d). 13C NMR (125 MHz, neat liquid) δ ppm: 137.1,
132.2, 124.3, 122.8, 120.6, 119.6, 51.3, 36.3.
NMR spectra were collected using DMSO-d6 or CD3CN as the lock
solvents with TMS and BF3·Et2O as the internal standards. All shifts
are reported in δ (ppm) relative to the internal standards. Infrared
(IR) spectra were collected using a Bruker AlphaFTIR (Madison, WI)
by direct measurement via attenuated total reflectance of the neat
samples (or ILs loaded with borane clusters) on a diamond crystal.
Differential Scanning Calorimetry. Solid and melting transitions
were evaluated with a Mettler Toledo (Columbus, OH) DSC 1. The
calorimeter was calibrated for temperature and cell constants using In,
Zn, H2O, and n-octane. Samples were weighed and sealed in aluminum
pans (5−15 mg) and heated at a rate of 5 °C/min to 75 °C. Following
the initial heating cycle, the samples were cooled to −50 °C via a
recirculating chiller followed by a heating cycle to 75 °C at a rate of 5
°C/min. After each dynamic temperature ramp, a 15 min isotherm was
employed to ensure equilibration of the temperature in the cell. The
entire cycle was repeated three times, and the values for phase changes
were analyzed. Each sample was referenced to an empty aluminum
pan.
Hypergolic Ignition Tests. Liquid hypergol ignition tests were
conducted as in ref 2 by the addition of a single drop (10 μL) of fuel
(either neat IL or IL with added borane clusters) via Hamiltonian
syringe to a vial containing 500 μL of 99.5% white fuming nitric acid
(WFNA). In the case of the 1:10 sample of [B9H14]− in
[MAT][DCA], the viscosity of the sample was so high that, instead,
the sample of [MAT][DCA] was placed on a 10 cm watch glass and a
single drop of WFNA was added to the watch glass.
In the hypergolic ignition tests of the solid borane clusters and the
borane clusters dissolved in molecular solvents, 2−3 drops of WFNA
was dropped on a 10 cm watch glass containing the solid borane
clusters (∼10 mg) or the borane clusters dissolved in molecular
solvents (∼10 mg/mL). A Redlake MotionPro Y4 (Tallahassee, FL)
high speed CCD camera at 1000 frames/s was utilized to follow the
ignition drop test in each case. The ignition delay was measured as the
time in milliseconds for ignition to occur after the initial contact of fuel
and oxidizer. Samples were replicated 3 times, the values for ignition
delay were averaged, and the standard deviation was calculated. In the
case of the 1:10 sample of [B9H14]− in [C4mim][DCA], only a single
run was conducted because the walls of the Hamiltonian syringe were
inadvertently shattered while setting up the second run.
Loading of B10H14 into Hypergolic ILs. Decaborane (0.01 mmol,
0.0012 g to 0.1 mmol, 0.0122 g) was added to a 2 dram vial containing
the IL (0.1 to 10 mmol) corresponding to the desired molar ratio of
decaborane/IL from 1:1 to 1:1000. The vials were immediately capped
and mixed with a vortex mixer for 5 s. Upon mixing, the vial warmed
and a gas was produced. The vial was opened in a hood to prevent the
gas from pressurizing the vial. The clear to yellow IL started to turn
bright orange, and the cap was placed back on the vial. On standing, a
black precipitate was produced and separated after sedimentation.
Synthesis of Borane Salts.30 1-Ethyl-3-methylimidazolium
arachno-Nonaborane ([C2mim][B9H14]). Decaborane (3 mmol,
0.3666 g) dissolved in dry ethanol was added dropwise to an aqueous
solution of NaOH (3 mmol, 0.1200 g). Upon addition, the solution
turned bright yellow and emitted a small amount of gas. [C2mim]Cl (3
mmol, 0.440 g) dissolved in a minimum amount of water was added
dropwise to the solution containing the newly generated Na[B9H14].
[C2mim][B9H14] immediately precipitated out of solution as a yellow
1
powder. H NMR (CD3CN): 8.41 (s, 1H, C2−H), 7.40 (s, 1H, C4−
H), 7.35 (s, 1H, C−H), 4.18 (q, 2H, N-CH2CH3), 3.84 (s, 3H, N−
CH3), 1.48 (t, 3 H, −CH2CH3), 0.50−2.50 (q, 9H, B−H terminal as
overlapping 1:1:1:1 quartets), −1.50 (m, 5H, exchanging B−H). 11B
NMR (CD3CN): −8.7 (d, 3B), −21.0 (d, 3B), −24.2 (d, 3B).
N-Butyl-N-methyl-pyrrolidinium arachno-Nonaborane ([Pyrr14]-
[B9H14]). Decaborane (3 mmol, 0.3666 g) dissolved in dry ethanol was
added dropwise to an aqueous solution of NaOH (3 mmol, 0.1200 g).
Upon addition, the solution turned bright yellow and bubbled. [Pyrr14]
Cl (3 mmol, 0.533 g) dissolved in a minimal amount of water was
added dropwise to the solution containing the newly generated
Na[B9H14]. [Pyrr14][B9H14] immediately precipitated out of solution
1
as a yellow powder. H NMR (CD3CN): 3.42 (m, 4H), 3.24 (t, 2H),
2.96 (s, 3H), 2.18 (m, 4H), 1.73 (quintet, 2H), 1.40 (sextet, 2H), 0.99
(t, 3H), 0.50−2.50 (q, 9H, B−H terminal as overlapping 1:1:1:1
quartets), −1.49 (m, 5H, exchanging B−H. 11B NMR (CD3CN): −8.9
(d, 3B), −21.3 (d, 3B), −24.6 (d, 3B).
Triethylammonium nido-Decaborane ([H−N222][B10H13]). Deca-
borane (3 mmol, 0.3666 g) dissolved in diethyl ether was added
dropwise to a solution of triethylamine (3 mmol, 0.3036 g) in diethyl
ether. Immediately upon addition, a yellow solid precipitated. The
solid was separated by vacuum filtration and washed with 3 equiv of
anhydrous diethyl ether to yield a bright yellow crystalline powder.
The resulting powder was recrystallized by dissolving the yellow solid
into EtOAc followed by the slow addition of diethyl ether until
precipitation was observed, which resulted in a pale yellow crystalline
RESULTS AND DISCUSSION
■
Reactivity of B10H14 with Hypergolic ILs. Our
investigations began with attempts to dissolve B10H14 in the
known hypergolic ILs,2 1-butyl-3-methylimidazolium dicyan-
amide ([C4mim][DCA]), 1-allyl-3-methylimidazolium dicyan-
amide ([Amim][DCA]), and 1-methyl-4-amino-1,2,4-triazo-
lium dicyanamide ([MAT][DCA]). The ILs were synthesized
through metathesis reactions between the respective halide
precursors and silver dicyanamide in methanol,2,7 and each IL
was dried to <500 ppm H2O. The ILs were added dropwise to a
corresponding amount of solid B10H14 in a preweighed 2 dram
vial followed by vortex mixing for 15 s to give 1:1000, 1:100,
and 1:10 B10H14−IL molar compositions. Upon addition, a
black precipitate and gaseous product were formed, and
[C4mim][DCA] and [Amim][DCA] turned bright orange,
while [MAT][DCA] obtained a cloudy orange hue. In all three
ILs, the more concentrated solutions had darker orange hues,
higher viscosities, and larger amounts of gaseous product and
black precipitate. Apart from the observation of the black
precipitates, which were easily separated after sedimentation to
the bottom of the vial, the orange solutions (Figure 2) did not
show any signs of further degradation or precipitation on the
time scale of publication (>12 months).
1
powder. H NMR (CD3CN): 6.62 (t, 1H, H-N(Et)3), 3.18 (dq, 6H,
H−N(-CH2-CH3)3), 1.5−3 (q, 8H, B−H Terminal), 1.28 (t, 9H, H−
N(−CH2−CH3)3), 0.05 (q, 2H, B−H Terminal), −2.86 (broad, 1H,
B−H Bridging), −3.76 (broad, 2H, B−H Bridging). 11B NMR
(CD3CN): 7.2 (d), 2.8 (d), −4.8 (d), −35.9 (d) (neutral B10H14 and a
small amount of [B9H14]− were observed in the 11B NMR).
Tetraethylammonium nido-Decaborane ([N2222][B10H13]). Deca-
borane (3 mmol, 0.3666 g) dissolved in diethyl ether was dropwise
added to a solution of [N2222]OH (3 mmol, 0.7784 g) in methanol.
Immediately upon addition, a yellow solid precipitated. The solid was
separated by vacuum filtration and washed with 3 equiv of anhydrous
diethyl ether to yield a bright yellow crystalline powder. The resulting
powder was recrystallized by dissolving the yellow solid into EtOAc
followed by the slow addition of diethyl ether until precipitation was
observed, which resulted in a pale yellow crystalline powder. 1H NMR
(CD3CN): 6.62 (t, 1H, H-N(Et)3), 3.18 (dq, 6H, H−N(-CH2-CH3)3),
1.5−3 (q, 8H, B−H Terminal), 1.28 (t, 9H, H−N(−CH2−CH3)3),
0.05 (q, 2H, B−H Terminal), −2.86 (broad, 1H, B−H Bridging),
−3.76 (broad, 2H, B−H Bridging). 11B NMR (CD3CN): 6.2 (d), 2.0
(d), −5.6 (d), −35.9 (d) (neutral B10H14 and a small amount of
[B9H14]− were observed in the 11B NMR).
The black precipitates suggested an acid−base reaction
between B10H14 and the [DCA]− anion, which would result in
the in situ formation of a reduced borane species19 and the
protonation of [DCA]−, which would lead to formation of
Characterization and Testing. Spectroscopy. All NMR spectra
were recorded utilizing a Bruker Avance spectrometer, Bruker/Magnex
UltraShield 500 MHz (Madison, WI), or a Bruker Spectrospin DRX
1
360 MHz Ultrashield spectrometer (Madison, WI). H, 13C, and 11B
4772
dx.doi.org/10.1021/ic500622f | Inorg. Chem. 2014, 53, 4770−4776

Inorganic Chemistry
Article
fuel (IL) was added via Hamiltonian syringe to a vial containing
500 μL of 99.5% white fuming nitric acid (WFNA), and the
resulting ignition was monitored with a Redlake MotionPro Y4
high speed camera at 1000 frames/s. Ignition delays (IDs) were
measured as the time from when the oxidizer first came into
contact with the fuel until ignition was observed. Due to the
high viscosity of the most concentrated (1:10) [MAT][DCA]
solution, testing of this system was conducted by placing ∼10−
20 mg onto a 10 cm watch glass and directly dropping a 2−3
drop aliquot of WFNA onto the sample (Supporting
Information, Figure S14). Flames with a light green hue,
indicative of the oxidation of B−H bonds to boron oxide,16
were observed upon ignition in all cases.
While the viscosity of these IL solutions was not directly
measured due to previous reports of shock sensitivity of similar
compounds,16 the viscosity of the ILs dramatically increased
visually upon the addition of [B9H14]−. Thus, the slightly longer
IDs observed in the 1:1000 samples might be expected due to
mixing effects;10 however, at the 1:100 loadings the IDs were
shortened 15−20% despite the even higher viscosities. At the
1:10 loadings of [C4mim][DCA] and [Amim][DCA], even
greater reductions (89−90%) in the ignition delay were
observed with respect to the neat ILs. The viscosity was high
enough to limit the ability to use a syringe to conduct the IL
drop test for the 1:10 loadings of [MAT][DCA], but the
mixture still led to a reduction of the ignition delay by 72%
compared with the neat IL (Figure S13−S21, Supporting
Information).
Figure 2. Stable orange solution formed when B10H14 was added to
[C4mim][DCA] at a 1:100 ratio.
melamine.20 In order to determine the structural identity of the
borane cluster, infrared (IR) spectra of the IL solutions were
compared to a solid sample of neat B10H14. A single, very weak
B−H stretch was observed in each spectra at a lower
wavenumber (2500−2520 cm−1) relative to the several B−H
stretches observed in neat B10H14 (2530−2600 cm−1). A highly
concentrated sample was then prepared by combining [MAT]-
[DCA] with an equimolar portion of B10H14, which resulted in
a dark orange gel after a large amount of bubbling and
precipitation. The isolated gel yielded a single B−H stretch at
2488 cm−1 in the IR spectrum indicative of the generation of a
more reduced31 or negatively charged species (Figure S12,
Supporting Information). Additionally, the stretches corre-
sponding to the symmetric and asymmetric −CN stretches
from the [DCA]− were shifted to a higher wavenumber, which
indicated the generation of a more neutral species.
Synthesis of Borane Salts. To understand the composi-
tion, mechanism of formation, and reactivity of the reduced
borane species, we prepared salts containing [B10H13]− and
[B9H14]− anions. Our initial strategy was to maximize the
oxidation potential of B10H14 by abstracting a single proton in a
direct acid−base reaction with either a soft nitrogen base or a
hard oxygen-containing base (Scheme 2a). Triethylammonium
([H−N222]+) and tetraethylammonium ([N2222]+) were
selected based on previous reports utilizing these cations to
isolate borane anions from B10H14.20,21
Each of the 1:100 loaded ILs were further characterized neat
by 11B, 1H, and 13C NMR spectroscopy using a sealed capillary
tube of the sample surrounded by an external locking solvent
(CDCl3) and compared with each pure IL. 11Boron NMR
revealed three doublets at −8.4, −21.1, and −24.2 ppm, which
were identified to be the [B9H14]− anion (Supporting
Information, Figure S9) based on a spectral match to the
peaks present in the known compound K[B9H14].24 No shifts
were observed in any of the signals associated with the neat ILs
1
in either the H or 13C NMR spectra upon the addition of
First, a solution of B10H14 in diethyl ether was added
dropwise to a stirred solution of triethylamine in an equimolar
ratio, whereupon a pale yellow powder precipitated. The
precipitate was recrystallized by dissolution in EtOAc followed
by the slow addition of diethyl ether, which resulted in pale
B10H14. However, a new peak at 0.09 ppm in the 1H NMR in all
three solutions integrated to be approximately 14 hydrogen
atoms at a concentration of 1:100, which corresponds to the
presence of the [B9H14]− anion. Importantly, no differences in
the borane cluster were observed based on which IL cation was
studied, and key IR stretches, such as the shift in the B−H
stretch to lower wavenumbers and the shift of the −CN
stretches to higher wavenumbers, suggested that the reactivity
was likely between B10H14 and the [DCA]− anion. Additionally,
the [B10H13]− anion has also been previously reported to be
susceptible to nucleophilic attack32 and to form the [B9H14]−
anion.33 Taken together, these observations suggest the overall
reaction shown in Scheme 1.
1
yellow crystalline needles. H and 11B NMR of the isolated
needles dissolved in CD3CN confirmed formation of
[H−N222][B10H13] (1H, N−H triplet 6.7 ppm; 11B, four unique
doublets 7.2, 2.8, −4.8, −14.8 ppm; 2:1:5:2 matching previous
11B results for this anion;34,35 Supporting Information). The 11
B
NMR also suggested the presence of small amounts of B10H14
and [B9H14]−, which could not be removed by further
purification.
[B9H14]− in IL Solution. Hypergolic ignition tests (Table 1)
for the nine [B9H14]− IL solutions were conducted utilizing a
standard IL hypergolic drop test.2 A single aliquot (10 μL) of
In a second reaction, an aprotic salt was prepared by reaction
of [N2222]OH in methanol with B10H14 dissolved in absolute
ethanol. Upon the dropwise addition of the B10H14 solution, a
light yellow crystalline solid immediately precipitated. After
recrystallization of the solid through the slow addition of
diethyl ether to a dissolved sample in EtOAc, a yellow powder,
composed of very small needles, was isolated. 11B NMR of the
isolated salt dissolved in CD3CN displayed four unique
doublets at 6.2, 2.0, −5.6, and −15.9 ppm (2:1:5:2 intensity)
confirming formation of [N2222][B10H13] (See Supporting
Information). Again, small amounts of B10H14 and [B9H14]−
Scheme 1. Proposed Reaction Pathway for the in Situ
Generation of [B9H14]−
4773
dx.doi.org/10.1021/ic500622f | Inorg. Chem. 2014, 53, 4770−4776

Inorganic Chemistry
Article
Table 1. Ignition Delays Observed for Three Hypergolic ILs and their [B9H14]− Solutions
a
b
Only one ignition delay obtained as a result of high viscosity. The drop test was performed in a watch glass instead of a vial as a result of high
viscosity. Values in parentheses denote the standard deviation of three averaged tests.
Scheme 2. Synthesis of Hypergolic Borane Salts
were identified in the 11B NMR spectrum but could not be
removed by further purification.
the 11B NMR matched previous experimental24 and theoreti-
cal36 literature values (see Supporting Information) for the
[B9H14]− anion.
To prepare [B9H14]− salts with organic cations that are
typically paired with hypergolic ILs,2,7 a sodium salt was first
made by adding a solution of B10H14 in anhydrous ethanol
dropwise to a stirred, equimolar solution of NaOH33 in
aqueous ethanol (Scheme 2b). Upon addition, the solution
turned a bright yellow indicative of the formation of
Na[B10H13], but after stirring for 12 h, the solution became
clear. Solutions of 1-ethyl-3-methylimdazolium chloride
([C2mim]Cl) or N-butyl-N-methylpyrrolidinium chloride
([Pyrr14]Cl) in ethanol were then added dropwise yielding
immediate light yellow precipitates. Each water insoluble
1
In the H NMR spectra of [C2mim][B9H14] and [Pyrr14]-
[B9H14], a broad peak at −1.50 ppm was observed that
corresponded to five hydrogen atoms, which are rapidly
exchanging in solution. These five fluxional hydrogen atoms
correspond to the hydrogen atoms along the open face of the
borane cluster whose exact orientation and location in the solid
state are still in debate today.24,25,36 The remaining nine
terminal B−H hydrogen atoms along the base of the borane
“bowl” were observed between 2.5 and 0.5 ppm as overlapping
1:1:1:1 quartets based on the isotopic abundance and nuclear
spin of 11B.
1
powder was collected by vacuum filtration. H and 11B NMR
on solutions of the isolated powders in CD3CN confirmed
them to be [C2mim][B9H14] and [Pyrr14][B9H14]. Three
doublets observed at 8.7, −21.0, and −24.2 ppm ([C2mim]-
[B9H14]) and 8.9, −21.3, and −24.6 ppm (Pyrr14][B9H14]) in
Reactivity of Borane Salts. All four of the new salts,
[H−N222][B10H13], [N2222][B10H13], [C2mim][B9H14], and
[Pyrr14][B9H14], were tested for hypergolic reactivity with a
variety of oxidizers. In each case, a small portion (∼10−20 mg)
4774
dx.doi.org/10.1021/ic500622f | Inorg. Chem. 2014, 53, 4770−4776

Inorganic Chemistry
Article
Table 2. Reactivity with WFNA and Solubility in Molecular Solvents
compound
B10H14
THF
EtOAc
MeOH
EtOH
soluble, not hypergolic
soluble, hypergolic
soluble, hypergolic
soluble, hypergolic
soluble, hypergolic
soluble, not hypergolic
soluble, hypergolic
soluble, hypergolic
soluble, hypergolic
soluble, hypergolic
N/A
N/A
[H−N222][B10H13]
[N2222][B10H13]
[C2mim][B9H14]
[Pyrr14][B9H14]
soluble, not hypergolic
soluble, not hypergolic
not soluble
soluble, not hypergolic
soluble, not hypergolic
not soluble
not soluble
not soluble
Figure 3. [H−N222][B10H13] as a trigger additive induces hypergolicity in solutions of EtOAc: (left) 0 ms, initial contact with oxidizer; (center) 2 ms,
hypergolic ignition; (right) dilute green flame visible.
of the solid salt was placed on a 10 cm watch glass. An aliquot
(2−3 drops) of each oxidizer (WFNA, inhibited red fuming
nitric acid (IRFNA); 70% nitric acid (NA)) was dropped
directly onto the samples, and the resulting ignition was
monitored. Upon contact with the oxidizers, all four salts
exhibited essentially instantaneous ignition (ID < 3 ms) with a
green flame. The small impurities present in [H−N222][B10H13]
and [N2222][B10H13] did not seem to result in decreased
reactivity, suggesting that extensive purification may not be
required. Other more dilute oxidizers such as 1 M nitric acid,
30% H2O2, and 50% H2O2, were tried but did not result in
ignition, perhaps due in part to the low water solubility of the
borane salts.
[Pyrr14][B9H14] in THF or EtOAc were all hypergolic with
WFNA, igniting instantaneously on contact. For example,
[H−N222][B10H13] in EtOAc ignited instantaneously (Figure 3
and Table S2, Supporting Information) and burned for several
hundred milliseconds until all of the solvent was consumed.
Lower concentrations of nitric acid (e.g., IFNA, NA) did not
result in hypergolic ignition.
Interestingly, even though the [B9H14]− or [B10H13]− salts
prepared were insoluble in water, the presence of water did not
prevent ignition. Floating any of the four salts on water
followed by addition of the oxidizer (WFNA) resulted in
ignition. This might allow some design flexibility if these salts
were used in current propellant formulations, such as RP-1.38
Given the rather dramatic reduction in ID of hypergolic ILs
when [B9H14]− was formed in situ simply by adding B10H14, the
four borane salts prepared here were tested to determine
whether they could induce hypergolicity in nonhypergolic
molecular solvents. Small portions (∼10−20 mg) of each salt
and of B10H14 were placed on a 10 cm watch glass and dissolved
in approximately 10 mL of molecular solvents that could serve
as potential fuels. The solvents screened were chosen based on
known energetic properties and potential for reactivity with the
anion. Chlorinated solvents were avoided due to the potential
hazard of generation of shock sensitive primary explosives16 and
low combustibility due to high chlorine contents. DMSO and
acetonitrile were also not utilized due to known reactivity with
neutral B10H14 and similar anions.37 Water, diethyl ether, and
hydrocarbon based nonpolar solvents did not dissolve any of
the four isolated salts. Polar protic solvents (e.g., MeOH and
EtOH) were able to dissolve [H−N222][B10H13] and [N2222]-
[B10H13] but neither of the [B9H14]− salts. Only polar aprotic
solvents (e.g., THF and EtOAc) dissolved all four borane salts,
which led to a total of 14 testable fuel−borane solutions (Table
2).
CONCLUSIONS
■
The results provided here demonstrate that dissolution of nido-
decaborane, B10H14, in ionic liquids containing the dicyanamide
anion leads to in situ generation of a nonaborane cluster anion,
[B9H14]−, through a direct acid−base reaction. Additionally,
borane cluster anions, [B10H13]− and [B9H14]−, either
generated in situ from B10H14 or synthesized as pure salts,
can form stable solutions in ILs and polar molecular solvents
leading to enhanced ignition in known hypergols, such as
dicyanamide ionic liquids, or induced hypergolicity in certain
nonhypergolic polar aprotic molecular solvents. This new class
of hypergolic trigger additives, on the order of angstroms rather
than nanometers, can easily be customized for specific
requirements of solubility, reactivity, and physical properties
by the choice of cation. This could provide control over the
energetic content of virtually any fuel.
ASSOCIATED CONTENT
■
S
* Supporting Information
Additional characterization and hypergolic data. This material is
available free of charge via the Internet at http://pubs.acs.org.
Several drops of oxidizer (WFNA, IR, and NA) were
dropped directly onto each of the six prepared solutions on the
watch glasses, and the resulting ignition was monitored with the
high speed camera as previously discussed. Neutral B10H14 was
not hypergolic either as a neat solid or in THF or EtOAc.
Similarly, none of the salts were hypergolic when dissolved in
the protic polar solvents. However, freshly prepared solutions
of [H−N222][B10H13], [N2222][B10H13], [C2mim][B9H14], and
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: RDRogers@ua.edu.
4775
dx.doi.org/10.1021/ic500622f | Inorg. Chem. 2014, 53, 4770−4776

Inorganic Chemistry
Article
(26) Li, Y.; Carroll, P. J.; Sneddon, L. G. Inorg. Chem. 2008, 47,
9193−9202.
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
(27) Kusari, U.; Li, Y.; Bradley, M. G.; Sneddon, L. G. J. Am. Chem.
Soc. 2004, 126, 8662−8663.
(28) McCrary, P. D.; Chatel, G.; Alaniz, S. A.; Cojocaru, O. A.;
Beasley, P. A.; Flores, L. A.; Kelley, S. P.; Barber, P. S.; Rogers, R. D.
Energy Fuels 2014, DOI: 10.1021/ef500264z.
Notes
The authors declare no competing financial interest.
(29) Drake, G.; Hawkins, T.; Tollison, K.; Hall, L.; Vij, A.; Sobaski, S.
1R-4-Amino-1,2,4-triazolium Salts: New Families of Ionic Liquids. In
Ionic Liquids IIB: Fundamentals, Progress, Challenges, and Opportunities;
Rogers, R. D., Seddon, K. R., Eds.; American Chemical Society:
Washington, DC, 2005; pp 259−302.
(30) McCrary, P. D.; Rogers, R. D. Hypergolic Boronate Salts. US
Provisional Patent Application 61837404, June 20, 2013.
(31) Chin, T. T.; Lovelace, S. R.; Geiger, W. E.; Davis, C. M.;
Grimes, R. N. J. Am. Chem. Soc. 1994, 116, 9359−9360.
(32) Fitch, S. J.; Laubengayer, A. W. J. Am. Chem. Soc. 1958, 80,
5911.
ACKNOWLEDGMENTS
■
P.D.M. is supported by the Department of Defense (DoD)
through the National Defense Science & Engineering Graduate
Fellowship (NDSEG) Program. This work was supported by
the Air Force Office of Scientific Research under AFOSR
Award No. FA9550-10-1-0521.
REFERENCES
■
(1) Clark, J. D. Ignition: An Informal History of Liquid Rocket
Propellants; Rutgers University Press: New Brunswick, NJ, 1972.
(2) Schneider, S.; Hawkins, T.; Rosander, M.; Vaghjiani, G.;
Chambreau, S.; Drake, G. Energy Fuels 2008, 22, 2871−2872.
(3) Zhang, Y.; Gao, H.; Joo, Y.-H.; Shreeve, J. M. Angew. Chem., Int.
Ed. 2011, 50, 9554−9562.
(4) Emel’yanenko, V. N.; Verevkin, S. P.; Heintz, A. J. Am. Chem. Soc.
2007, 129, 3930−3937.
(5) MacFarlane, D. R.; Golding, J.; Forsyth, S.; Forsyth, M.; Deacon,
G. B. Chem. Commun. 2001, 1430−1431.
(6) Spengler, V. G.; Bauer, J. Brennst.-Chem. 1965, 4, 43−50.
(7) McCrary, P. D.; Beasley, P. A.; Cojocaru, O. A.; Schneider, S.;
Hawkins, T. W.; Perez, J. P. L.; McMahon, B. W.; Pfeil, M.; Boatz, J.
A.; Anderson, S. L.; Son, S. F.; Rogers, R. D. Chem. Commun. 2012, 48,
4311−4313.
(33) Benjamin, L. E.; Stafiej, S. F.; Takacs, E. A. J. Am. Chem. Soc.
1963, 85, 2674−2675.
(34) Hofmann, M.; Schleyer, P. v R. Inorg. Chem. 1998, 37, 5557−
5565.
(35) Siedle, A. R.; Bodner, G. M.; Todd, L. J. J. Inorg. Nucl. Chem.
1971, 33, 3671−3676.
(36) Hofmann, M.; Schleyer, P. v R. Inorg. Chem. 1998, 37, 652−660.
(37) Bould, J.; Laromaine, A.; Bullen, N. J.; Vinas, C.; Thornton-Pett,
M.; Sillanapaa, R.; Kivekas, R.; Kennedy, J. D.; Teixidor, F. Dalton
Trans. 2008, 1552−1563.
(38) Ott, L. S.; Hadler, A. B.; Bruno, T. J. Ind. Eng. Chem. Res. 2008,
47, 9225−9233.
(8) Perez, J. P. P.; McMahon, B. W.; Schneider, S.; Boatz, J. A.;
Hawkins, T. W.; McCrary, P. D.; Beasley, P. A.; Rogers, R. D.;
Anderson, S. L. J. Phys. Chem. C 2013, 117, 5693−5707.
(9) McCrary, P. D.; Beasley, P. A.; Kelley, S. P.; Schneider, S.; Boatz,
J. A.; Hawkins, T. W.; Perez, J. P. L.; McMahon, B. W.; Pfeil, M.; Son,
S. F.; Anderson, S. L.; Rogers, R. D. Phys. Chem. Chem. Phys. 2012, 14,
13194−13198.
(10) McCrary, P. D.; Beasley, P. A.; Alaniz, S. A.; Griggs, C. S.;
Frazier, R. M.; Rogers, R. D. Angew. Chem., Int. Ed. 2012, 51, 9784−
9787.
(11) Schneider, S.; Hawkins, T. W.; Ahmed, Y.; Rosander, M.;
Hudgens, L.; Mills, J. Angew. Chem., Int. Ed. 2011, 50, 5886−5888.
(12) Gao, H.; Shreeve, J. M. J. Mater. Chem. 2012, 22, 11022−11025.
(13) Li, S.; Gao, H.; Shreeve, J. M. Angew. Chem., Int. Ed. 2014, 53,
2969−2972.
(14) Fox, M. A.; Wade, K. Pure Appl. Chem. 2003, 75, 1315−1323.
(15) Guter, G. A.; Schaeffer, G. W. J. Am. Chem. Soc. 1956, 78, 3546.
(16) Dequasie, A. The Green Flame: Surviving Government Secrecy;
American Chemical Society: Washington, DC, 1991.
(17) Martin, D. R. J. Chem. Educ. 1959, 36, 208−214.
(18) Rastogi, R. P.; Baijal, S. K. AIAA J. 1968, 6, 1815−1816.
(19) Kasper, J. S.; Lucht, C. M.; Harker, D. Acta Crystallogr. 1950, 3,
436−456.
(20) Li, Y.; Carroll, P. J.; Sneddon, L. G. Inorg. Chem. 2008, 47,
9193−9202.
(21) Hawthorne, M. F.; Miller, J. J. J. Am. Chem. Soc. 1958, 80, 754−
754.
(22) Sneddon, L. G.; Huffman, J. C.; Schaeffer, R. O.; Strieb, W. E. J.
Chem. Soc., Chem. Commun. 1972, 474−475.
(23) Wynd, A. J.; Welch, A. J. Acta Crystallogr., Sect. C: Cryst. Struct.
Commun. 1989, 45, 615.
(24) Bould, J.; Greatex, R.; Kennedy, J. D.; Ormsby, D. L.;
Londesborough, M. G. S.; Callaghan, K. L. F.; Thornton-Pett, M.;
Spalding, T. R.; Teat, S. J.; Clegg, W.; Fang, H.; Rath, N. P.; Barton, L.
J. Am. Chem. Soc. 2002, 124, 7429−7439.
(25) Greenwood, N. N.; McGinnety, J. A.; Owen, J. D. J. Chem. Soc.,
Dalton Trans. 1972, 986−989.
4776
dx.doi.org/10.1021/ic500622f | Inorg. Chem. 2014, 53, 4770−4776