The role of ammonia in promoting ammonia borane synthesis

Article abstract of DOI:10.1039/c6dt02925f

Ammonia promotes the synthesis of pure ammonia borane (AB) in excellent yields from sodium borohydride and ammonium sulfate in tetrahydrofuran under ambient conditions. An examination of the influence of added ammonia reveals that it is incorporated into the product AB, contrary to its perceived function as a catalyst or a co-solvent. Mechanistic studies point to a nucleophilic attack by ammonia on ammonium borohydride with concurrent dehydrogenation to yield AB.

Full text of DOI:10.1039/c6dt02925f

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The Role of Ammonia in Promoting Ammonia Borane Synthesis
P. Veeraraghavan Ramachandran* and Ameya S. Kulkarni
Received 00th January 20xx,
Accepted 00th January 20xx
Ammonia promotes the synthesis of pure ammonia borane (AB) in excellent yields from sodium borohydride and
ammonium sulfate in tetrahydrofuran under ambient conditions. Examination of the influence of added ammonia reveals
that it is incorporated into the product AB, contrary to its perceived function as a catalyst or co-solvent. Mechanistic
studies point to a nucleophilic attack by ammonia on ammonium borohydride with concurrent dehydrogenation to yield
AB.
DOI: 10.1039/x0xx00000x
www.rsc.org/
Shore, Zhao, and coworkers, through an experimental and
computational study, elucidated the mechanism of DADB and AB
formation via the displacement route.17 They employed a non-polar
Introduction
The ever-growing demand for energy to power the modern world
with minimal environmental impact has brought increased
attention to renewable energy sources as alternatives to fossil
fuels.1 A robust “hydrogen economy” has been proposed as a viable
long-term solution.2 To realize its full potential, the development
and safe transportation of efficient, high-density hydrogen carriers
is vital. Ammonia borane (AB, 1), a solid with excellent stability,
high gravimetric hydrogen content (19.6 wt% H2), and relatively
facile hydrogen release has emerged as one of the promising on-
board hydrogen-storage materials for automobile applications.3
Recent reports on the regeneration of AB from spent fuel
(dehydrogenation products) have further enhanced its appeal.4
Moreover, due to its unique reactivity, AB has been successfully
utilized as a green alternative to traditional reagents for reduction,5
reductive amination,6 and transfer hydrogenation.7 It has also found
application in materials chemistry for the synthesis of metal
nanoparticles,8 as precursor to polyaminoboranes9 and boron
nitride,10 and as a green hypergolic propellant.11
solvent (toluene) and a strong Lewis base (dimethyl sulfide (DMS)
or dimethyl aniline (DMA) to provide 93-95% pure AB in 91-94%
yields (Scheme 2(i)).18 Kim and coworkers followed up with a low
temperature synthesis of AB from ammonia and diborane in 92%
yields, containing 5-10% DADB as impurity.19 Nonetheless, an
inherent issue with the displacement approach is the use of
pyrophoric reagents and strictly anhydrous conditions.
THF
THF
-H2
NaBH4 + (NH4)2SO4
NH4BH4
NH3BH3
-H2
H
N
HB
HN
BH
THF
-H2
((NH3)2BH2)BH4
+ NH3BH3
NH3 + B2H6
NH
B
DADB
AB, 1
H
CTB
Scheme 1. DADB formation in AB synthesis.
Salt metathesis of metal borohydrides with ammonium salts
remains a preferred approach to AB due to the stability, safety, and
handling convenience of the raw materials. Shore and Parry
described the preparation of AB, in 45% yield, by adding LiBH4 to a
slurry of NH4Cl or (NH4)2SO4 in diethyl ether (Scheme 2(ii)).13
Building on Parry’s work, Geanangel and co-workers20 described the
synthesis of AB from NaBH4 and (NH4)2CO3 in 80% yield in 24 h in
anhydrous tetrahydrofuran (THF) at 45 oC (Scheme 2(iii)). However,
NaBH4 and ammonium salts are sparingly soluble in common
ethereal solvents such as THF and diethyl ether. As a result, the
primary metathesis product ammonium borohydride (ABH) reacts
with the nascent AB to form notable amounts of DADB,21 which
decomposes to cyclotriborazane (CTB) impurity (Scheme 1).22
Utilizing dilute reaction conditions (0.165 M NaBH4 in THF), we
improved upon the Geanangel protocol to provide >98% pure AB in
96% isolated yield (Scheme 2(iv)).23 Despite the reusability of the
solvent after the reaction, the low dilution rendered the process
impractical to scale-up. This was overcome by utilizing anhydrous
The synthesis of AB remained an elusive endeavor through the
first half of the 20th century. The straightforward approach,
involving the direct combination of ammonia and diborane,
unexpectedly, provided diammoniate of diborane (DADB), an ionic
dimer of AB (Scheme 1).12 Six decades ago, Parry and Shore
described in a series of seminal publications that AB could be
formed by decomposing DADB in diethyl ether in the presence of
catalytic NH3.13 During the subsequent decades, efforts from the
laboratories of Shore,14 Mayer,15 and Manners16 provided more
insight into the synthesis of AB via the displacement of borane-
Lewis base complexes with NH3. Yet, a large-scale synthesis of pure
AB directly from NH3 did not materialize. In a major breakthrough,
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dioxane as the solvent.23 Although the reaction could now be
conducted at higher concentrations (1 M NaBH4 in dioxane), the
carcinogenicity of dioxane remained a major concern (Scheme 2(v)).
We then developed a one-pot synthesis of AB in 90% yield and
>99% purity from trimethyl borate and NH4Cl (Scheme 2(vi))
involving in situ formation of LiBH4.24 The use of pyrophoric LiAlH4
as the hydride source and the need for strictly anhydrous reaction
conditions were notable disadvantages. Autrey and coworkers
synthesized ABH in liquid NH3 at -78 oC from NaBH4 and NH4Cl,
followed by an anhydrous THF-mediated decomposition (Scheme
2(vii)).25 Despite the quantitative AB yields, energy-intensive
condensation of large quantities of liquid NH3, coupled with cooling
for the reaction (-78 oC) can make scale-up arduous.
Scheme 2. Approaches to AB.
All of the above described routes to AB require anhydrous reaction
conditions and heating or cooling, making scale-up inconvenient
and adding to the process cost. A detailed comparison of all the
known routes to AB is presented in Table 1. With a cost-effective,
scalable and high-yielding AB synthesis being a pre-requisite to its
success as an effective hydrogen carrier, development in this regard
is vital. To this effect, we had recently communicated a facile, large-
scale synthesis of high-purity AB from NaBH4 and (NH4)2SO4 in
ammoniated reagent-grade THF under ambient conditions.26
Described herein is a full account of our study, including a
mechanistic investigation to identify the role of added NH3.
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Table 1 Detailed comparison of all known routes to AB.
Reagents
Reaction Conditions
Temperature, time
Yield
(%)
Purity
Ref
Entry Reaction Type
Solvent (molarity)a
(%)
NH3 Source
BH3 source
Anhydrous Et2O
(0.46 M)
1
2
Metathesis
Displacement
Decomposition
(NH4)2SO4
LiBH4
NaBH4
NaBH4
NaBH4
NaBH4
rtb
45
80
--
--
13
20
23
23
25
24
Anhydrous THF
(0.67 M)
(NH4)2CO3
(NH4)2SO4
NH4HCO2
NH4Cl
NH4Cl
(NH4)2SO4
NH3
40-45 ˚C, 24 h
40 ˚C, 2 h
Anhydrous THF
(0.17 M)
3
96
>98
>98
99
Anhydrous Dioxane
(1 M)
4
40 ˚C, 2 h
95
Liquid NH3 and
anhydrous THF
(0.74-1.9 M)
-78 ˚C, 2 h
rt, 1 h
5
99
B(OMe)3
LiAlH4
Anhydrous THF
(0.13 M)
6
0 ˚C, 3 h
90
>99
>98
--
Reagent-grade THF
containing 5% NH3
(1 M)
0 ˚C, 2 h
rt, 8 h
Current
work
7
NaBH4
(CH3)2O:BH3
THF:BH3
B2H6
92
8
-78 ˚C
-78 ˚C
Dimethyl ether
70
13
14
THF
(1 M)
9
NH3
50
--
Monoglyme
(0.05 M)
10
11
12
13
14
15
16
NH3
-63 ˚C, 1 h
68-76
86
--
15(a)
16
-20 ˚C, 0.3 h
rt, overnight
NH3
(CH3)2S:BH3
(CH3)2S:BH3
DMA:BH3
B2H6
Anhydrous Et2O
--
Anhydrous toluene
(2 M)
NH3
rt
94
>93
>95
90-95
--
18
Anhydrous toluene
(3 M)
NH3
rt
-78 ˚C to rt, 20 h
rt
91
18
Anhydrous THF
(0.36 M)
NH3
92
19
Anhydrous Et2O
containing NH4Cl
and catalytic NH3
Diglyme containing
catalytic diborane
(0.12 M)
[H2B(NH3)2]BH4
80
13
[H2B(NH3)2]BH4
25 ˚C, 40 h
80-91
--
15(b)
aWith respect to the BH3 source. brt = room temperature.
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through celite, and washed with THF. The filtrate was
Experimental Section
concentrated in vacuo to obtain ammonia borane.
General Information
Optimization of reaction conditions with NaBH4 and
ammonium sulfate with NH3 as the promoter (Table 2):
Unless otherwise noted, all manipulations were carried out in an
open-flask. 11B and 1H NMR spectra were recorded at room
Condensed liquid ammonia was transferred via cannula to
reagent-grade THF contained in an indented 250 mL three-neck
round bottom flask fitted with an overhead stirrer, stopper, and
a cold finger. The flask was cooled in an ice-water bath. Sodium
borohydride (3.78 g, 100 mmol) and ammonium sulfate (13.21 g,
100 mmol) were transferred to the reaction flask after addition
of ammonia. The mixture was stirred carefully at 0 °C and then
at room temperature for the desired period of time to complete
the reaction, as monitored by 11B NMR spectroscopy (prior to
conducting 11B NMR experiments, a drop of DMSO was added to
the reaction aliquot to solubilize all of the NaBH4). In workup A,
THF (50 mL) was added to the reaction mixture, whereas, in
workup B, 1 M ammoniated THF (50 mL) was added to the
reaction mixture and stirred for 30 min, filtered through celite,
and washed with THF. The filtrate was concentrated in vacuo to
obtain ammonia borane.
temperature, on
a
Varian INOVA 300 MHz NMR
spectrophotometer with a Nalorac quad probe. Chemical shifts
(δ values) are reported in parts per million (ppm) relative to
BF3.Et2O for 11B NMR respectively. Differential scanning
calorimetry (DSC) was conducted using a TA instruments DSC
Q20 calorimeter. AB was weighed (0.5 mg) in an aluminum pan
that was then crimped closed. Ramp rates of 5 oC/min were
used with an N2 purge gas. Thermogravimetric analysis (TGA)
was conducted using the TA Instruments TGA Q500 with a ramp
rate of 5 oC/min with 2 mg of AB.
Tetrahydrofuran (THF, ACS grade containing 0.025% BHT) was
purchased from Mallinckrodt chemicals and used without
purification. Sodium borohydride (NaBH4, powder, 99% pure by
hydride estimation27) was purchased in bulk from Dow Chemical
Co. (Rohm and Haas). Ammonium acetate (ACS reagent, Sigma-
Aldrich), ammonium bicarbonate (>99%, Sigma-Aldrich),
ammonium fluoride (98%, Sigma-Aldrich), ammonium chloride
(ACS reagent, Mallinckrodt), ammonium formate (97%, Sigma-
Aldrich), ammonium hydrogen fluoride (95%, Sigma-Aldrich),
ammonium phosphate (>98.5%, Sigma-Aldrich), ammonium
hydrogen sulfate (98%, Sigma-Aldrich), ammonium sulfate (ACS
reagent, Mallinckrodt), and ammonium carbonate (ACS reagent,
Sigma-Aldrich) were purchased from the respective commercial
sources and powdered prior to use. Prior to conducting 11B NMR
experiments, a drop of DMSO was added to the reaction aliquot
to solubilize all of the SBH. The purity of AB was determined by
both 11B NMR spectroscopy and hydride estimation (hydrolysis
reaction).
Optimized large-scale synthesis of AB with NH3 as the
promoter:
Condensed liquid ammonia (500 mL) was transferred via cannula
to 10 L reagent-grade THF contained in an indented 22 L three-
neck round bottom flask fitted with an overhead stirrer, stopper,
and a cold finger. The flask was cooled in an ice-water bath.
Sodium borohydride (378.3 g, 10 mol) and powdered
ammonium sulfate (1.32 Kg, 10 mol) were transferred to the
reaction flask after addition of ammonia. The mixture was
stirred carefully for 2 h at 0 °C and then at room temperature for
8 h. Upon completion of the reaction, as monitored by 11B NMR
spectroscopy, 1 M ammoniated THF solution (5 L) was added to
the reaction mixture, stirred for 30 min, filtered through celite,
and washed with THF. The filtrate was concentrated in vacuo to
obtain ammonia borane (283.9 g, 92%) at >98% chemical purity,
as was determined by both 11B NMR spectroscopy (δ -22.37 (q, J
= 94.9 Hz)) and hydride analysis. Most of the solvent THF was
recovered and re-used.
Synthetic Methods
Caution: All of the reactions were carried out in a well-ventilated
hood with the reaction vessel outlet directly leading into the
hood exhaust due to the hazards associated with escaping toxic
and corrosive ammonia and the liberation of large quantities of
highly flammable hydrogen.
Deuterium labeling study:
Preparation of AB using NaBH4 and ammonium salts with NH3
as the promoter (Table 1):
Sodium borohydride (0.19 g, 5 mmol) and ammonium chloride-
d4 (0.57 g, 10 mmol) were added to a 50 mL round bottom flask
with a magnetic stir-bar. The reaction contents were cooled in
an ice-water bath, followed by addition of THF (5 mL).
Condensed liquid ammonia (0.25 mL) was then transferred via
cannula to the reaction mixture and reaction careful stirred at 0
oC for 2 h and room temperature (rt) for 8 h. The contents were
then filtered through celite, washed with THF, and the filtrate
concentrated in vacuo to obtain the product.
Condensed liquid ammonia (2.5 mL) was transferred via cannula
to reagent-grade THF (97.5 mL) contained in an indented 250 mL
three-neck round bottom flask fitted with an overhead stirrer,
stopper, and a cold finger. The flask was cooled in an ice-water
bath. Sodium borohydride (3.78 g, 100 mmol) and ammonium
salt were transferred to the reaction flask after addition of
ammonia. The mixture was stirred carefully at 0 °C and then at
room temperature for desired period of time. Upon completion
of the reaction, as monitored by 11B NMR spectroscopy (prior to
conducting 11B NMR experiments, a drop of DMSO was added to
the reaction aliquot to solubilize all of the NaBH4), THF (50 mL)
was added to the reaction mixture, stirred for 30 min, filtered
The effect of amines on salt metathesis (Table 3):
Sodium borohydride (0.19 g, 5 mmol) and ammonium sulfate
(0.66 g, 5 mmol) were added to a 50 mL round bottom flask with
a magnetic stir-bar. The corresponding amine was then added to
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the reaction mixture, followed by THF (5 mL). The
heterogeneous reaction contents were stirred vigorously at rt till
all of the sodium borohydride was consumed, following which a
reaction aliquot was analyzed by 11B NMR spectroscopy to gauge
the ratio of the amine-borane to ammonia borane.
c)
Preparation of ammonium borohydride (ABH):
ABH was synthesized as described in the literature28 with minor
modifications. NaBH4 (0.19 g, 5 mmol) and powdered NH4F (0.2
g, 5.5 mmol) were transferred to a 50 mL round bottom flask.
The reaction flask was purged with N2 gas to maintain an inert
atmosphere and cooled to -60 oC using a chloroform/dry ice
bath. Liquid NH3 (3 mL) was added to the flask followed by
b)
a)
o
vigorous stirring for ~6 h at -60 C. NH3 was then evaporated in
vacuo at -60 oC to yield a mixture of ABH and NaF, that was used
as is for further experiments.
Figure 1. (a) 11B NMR spectrum of NaBH4 and (NH4)2SO4 (1:1) at 0 oC after
2 h in THF. (b) 11B NMR spectrum of AB prepared from NaBH4 and
(NH4)2SO4 (1:1) at rt after 16 h in THF. (c) 11B NMR spectrum of AB
prepared from NaBH4 and (NH4)2SO4 (1:1) in THF containing 2.5% NH3.
Representative procedure for hydride analysis (Hydrolysis
reaction) of AB:
An aqueous solution of AB (1 mmol in 1 mL H2O) was
transferred to a glass vial with a septum inlet fitted with a
connecting tube. The connecting tube was attached to an
analytical gas burette filled with CuSO4 solution. 3 M HCl
was syringed into the vial, dropwise, till no further gas
evolution was observed. The gas evolved was measured
using the analytical gas burette maintained at 25˚C.
Delightfully, when the NH3-promoted AB synthesis was repeated
in reagent-grade THF, no drop in yield or chemical purity was
observed. We then proceeded to screen a variety of ammonium
salts with the hope of improving the reaction yield (Table 2).
Ammonium acetate, fluoride, and formate provided AB in
respectable yields, but lower chemical purity than with
ammonium sulfate (entries 2-4). Switching to ammonium
chloride resulted in marginally faster reactions, but lower yields
of AB (entry 5). Amongst the other salts screened (entries 6-10),
only ammonium bisulfate gave high purity AB, but in
significantly lower, 58% yield. Reactions with ammonium
bicarbonate and carbonate led to excessive frothing, rendering
stirring and filtration difficult. Overall, none of the screened
ammonium salts provided results superior to (NH4)2SO4.
Notably, utilizing lower equiv. of (NH4)2SO4 led to the formation
of small amounts of CTB byproduct. Substituting THF with
diethyl ether resulted in slower reactions with considerable
amounts of unreacted NaBH4 present, even after 12 h.
Results and Discussion
Our aim was to achieve a convenient synthesis of high purity AB
in near quantitative yields at high concentration of reactants
and, if possible, in reagent-grade solvents under open-flask and
ambient conditions. We envisioned that achieving facile salt
metathesis would necessitate a reaction environment wherein
the ionic reagents (NaBH4 and ammonium salts) are soluble. The
29
high solubility of NaBH4 in NH3 and the reported facile
decomposition of DADB by NH3 in ether13b prompted the use of
NH3 as an additive to THF. Indeed, a reaction with an equivalent
each of NaBH4 and (NH4)2SO4 in anhydrous THF at
1 M
concentration containing 2.5% NH3 provided AB under ambient
conditions in 77% isolated yield and >99% purity by 11B NMR
spectroscopy (Figure 1(c); Table 2, Entry 1, highlighted in
boldface). Computational studies from the Dixon laboratory
have described the role of NH3 as a Lewis base catalyst for AB
dehydrogenation.30 Gratifyingly, no AB dehydrogenation
products were observed in our protocol and a hydride analysis
revealed >98% chemical purity. A reaction without added NH3
under similar conditions was found to be extremely sluggish
(Figure 1(a): 11B NMR spectra of aliquot at 0 oC after 2 h) and
provides impure AB only after 16 h at rt (Figure 1(b)).
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Table 2: Preparation of AB using NaBH4 and ammonium salts with NH3 as
Table 3: Optimization of reaction conditions with (NH4)2SO4 for AB
synthesis with NH3 as the promoter.
the promoter.a
Ammonium
salt
Reaction
Condition
NH3
Entry Molaritya
(%)
Reaction
condition
Entry
Yield (%)b
Purity (%)c
Workupb Yieldc
1
2
3
4
5
6
1
1
2.5
2.5
2.5
5
0 °C, 4 h, rt, 5 h
0 °C, 4 h, rt, 5 h
rt, 15 h
A
B
B
B
B
B
77%
87%
70%
92%
69%
43%
1
2
(NH4)2SO4
NH4OAc
NH4F
0 °C, 4 h; rt, 5 h
0 °C, 2 h; rt, 2 h
0 °C, 2 h; rt, 9 h
0 °C, 2 h; rt, 2.5 h
0 °C, 2 h; rt, 2 h
0 °C, 2 h; rt, 2 h
0 °C, 2 h; rt, 3 h
0 °C, 2 h; rt, >16 h
0 °C, 2 h; rt, 8 h
0 °C, 4 h; rt, 6 h
77
98
94
96
97
98
96
95
NDd
98
97
70
1
3
74
1
0 °C, 2 h, rt, 8 h
0°C, 2 h, rt, 10 h
0°C, 2 h, rt, 18 h
1.5
2
5
4
NH4HCO2
NH4Cl
74
5
5
71
aMolarity of NaBH4 in THF containing NH3. bA: No addition of
ammoniated THF prior to isolation. B: Additional 1 M solution of NH3 in
THF was added to the reaction mixture prior to isolation. Isolated yield
with respect to NaBH4.
6
NH4HCO3
NH4HF2
75
c
7
73
Incomplete
58
8
(NH4)3PO4
NH4HSO4
(NH4)2CO3
The success of NH3 as an additive to accelerate AB synthesis
(Scheme 3) posed a fundamental question on its role in the
reaction. Interestingly, Parry and Shore had identified
anhydrous NH3 as a “catalyst” for the decomposition of
DADB in diethyl ether during the preparation of AB.13b To
delineate whether NH3 is a catalyst in our AB synthesis, a
deuterium labeling study was conducted aiming to access
ND3BH3 from ND4Cl and NaBH4 in THF with 5% NH3 as the
promoter. Ideally, if NH3 is accelerating the reaction by
catalysis or solvation, it should not be incorporated into the
9
10
76
aAll reactions were performed in THF (1 M with respect to NaBH4)
containing 2.5% NH3. bIsolated yields with respect to NaBH4.
cDetermined by hydride analysis. dND = Not determined (AB was not
isolated due to slow reaction).
Surprisingly, the best protocol (Table 3, entry 1) provided
AB in varying yields (70-85%) with different batches. To
understand this discrepancy, the filter cake was dissolved in
DMSO and analysed by 11B NMR spectroscopy, which
revealed the presence of residual AB. To avoid this loss of
AB, THF containing 1 M NH3 was added to the reaction
mixture prior to filtration, improving the isolated yield of
AB to a consistent 87% (Table 3, entry 2, Workup B).
Unsatisfied with the less than quantitative yields of AB, we
further optimized the reaction conditions for molarity,
ammonia content, and reaction temperature (Table 3).
Conducting the entire protocol at room temperature (rt)
led to longer reaction times and lower AB yields,
presumably due to the loss of dissolved NH3 (entry 3).
Increasing the quantity of added NH3 to 5% led to a rise in
AB yields (92%, entry 4, highlighted in boldface). Raising the
reaction concentration beyond 1 M provided inferior
outcomes (entries 5-6). Thus, the conditions in Table 3,
entry 4 was selected as optimal. The robustness and
scalability of this optimized NH3-promoted AB synthesis
was demonstrated with a 10-mole scale reaction with no
loss in yield or purity.
product. When such
a
reaction was performed,
unexpectedly, NH3BH3 was the major product with ND3BH3
as a minor constituent,32 pointing to the probable role of
NH3 as a reagent.
The observed NH3BH3 formation could alternately be
rationalized either by a NH3-ND3 or H-D exchange from
ND3BH3. However, the NH3-ND3 exchange was ruled out
since previous work from this laboratory33 has shown that
amine exchange with NH3BH3 occurs extremely slowly at
room temperature (rt). To evaluate the possibility of a H-D
exchange, a control experiment involving the addition of
7b
NH3 to preformed ND3BH3 in THF at
0
oC/rt was
conducted. Indeed NH3BH3 was obtained as the major
product corroborating a fast H-D exchange. Thus, NH3BH3
formation from ND4Cl and NaBH4 in ammoniated THF could
be either due to NH3 being the reagent or a H-D exchange,
rendering the isotope labeling study inconclusive and
further experiments to delineate the role of ammonia were
sought.
Thermal analysis of the product AB by DSC displayed
melting with exothermic decomposition with an
extrapolated onset temperature of 110 oC and a peak
melting temperature of 112 oC. TG analysis revealed an
Scheme 3. AB synthesis in THF with NH3 as the promoter.
o
onset dehydrogenation temperature of 117 C, which is in
excellent agreement with the literature value.31
Amines were considered as an appropriate surrogate for
NH3 to ascertain its role due to their similar NaBH4 solvation
capabilities.29 If NH3 was indeed acting as a reagent in AB
synthesis, the amines also should act as reagents and be
incorporated in the product providing the corresponding
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amine-boranes. Thus, we embarked upon an investigation
on the influence of amines on the metathesis between
NaBH4 and (NH4)2SO4 in THF at rt at 1 M concentration
(Table 4).
nucleophilic
amines,
such
as
triisobutylamine,
diisopropylethylamine, and 2,2,6,6-tetramethylpiperidine
was revealing (entries 8-10). They provided AB as the
predominant product but with concurrent formation of CTB
and no enhancement of the reaction rate, similar to the
control experiment. Summarizing the above results, it is
clear that the presence of a stoichiometric nucleophilic
amine is essential to promote the reaction with its
concurrent incorporation into the product and suppression
of CTB. To further support this theory, dimethyl sulfide, a
Table 4: Effect of added nucleophiles on salt metathesis.
Product ratioa (%)
Entry
Nucleophile (Nu)
Time (h)
Nu-BH3
AB
CTB
1
2
None
16
2
00
94
06
Cyclohexylamine
Piperidine
82
93
32
85
84
62
09
03
00
00
70
18
07
67
15
16
38
85
96
97
98
30
00
00
01
00
00
00
06
01
03
02
00
poor nucleophile, and triphenylphosphine,
a strong
nucleophile, were utilized as additives. As expected, the
reaction with dimethyl sulfide provided no rate acceleration
and AB was formed along with CTB, whereas that with
triphenylphosphine was complete within 2 h yielding a
mixture consisting only of triphenylphosphine-borane and
AB in 7:3 ratio (entries 11-12).
3
2
4
Piperidine (20 mol%)
Triethylamine
16
2
5
6
Pyridine
2
With the knowledge that nucleophilic attack on AB occurs
extremely slowly at ambient conditions,33 the incorporation
of nucleophiles in the final product could be rationalized on
the basis of two distinct reaction pathways (Scheme 4).
Path A describes a tandem nucleophile-ammonium salt
equilibration-metathesis sequence. Although this justifies
the synthesis of amine- and phosphine-boranes at rt from
NaBH4,34 it does not explain the observed rate acceleration
and complete suppression of CTB. In addition, the facility of
AB synthesis with NH3 as the nucleophile is not explained
since the exchange will provide an identical ammonium salt
and offer no rate enhancement. As a result, the possibility
of a tandem nucleophile-ammonium salt equilibration-
metathesis sequence can be discounted.
7
Diisopropylamine
Triisobutylamine
Diisopropylethylamine
2
8
16
24
16
16
2
9
2,2,6,6-
Tetramethylpiperidine
10
11
12
Dimethyl sulfide
Triphenylphosphine
aAscertained by 11B NMR spectroscopy.
A control experiment, without an amine, had yielded a
product mixture containing 94% AB and 6% CTB after 16 h
(vide supra, Figure 1(b); Table 4, entry 1). Repeating the
above reaction in the presence of an equiv. of
cyclohexylamine, however, led to significant rate
enhancement providing a mixture of cyclohexylamine-
borane and AB in 82:18 ratio within 2 h (entry 2). The
incorporation of cyclohexylamine in the final product
corroborated, indirectly, the role of NH3 as a reagent. The
formation of minor quantities of AB could be explained via
a THF-mediated ABH decomposition.25 Notably, no CTB was
detected similar to the results obtained when NH3 was the
additive. Piperidine, a secondary amine, also provided rate
acceleration and a mixture of piperidine-borane and AB in
93:7 ratio, within 2 h, with no CTB (entry 3). The rate
acceleration was neutralized when only catalytic amounts
of piperidine were used (entry 4), stressing the necessity for
stoichiometric quantities of additive, further ruling out the
role of NH3 (indirectly) as a catalyst. Screening tertiary and
heteroaryl amines, such as triethylamine and pyridine,
respectively, also provided the corresponding amine-
boranes predominantly, within 2 h (entries 5-6).
Scheme 4. Possible rationale for nucleophile incorporation into the
product
Path
B illustrates an in situ synthesis of ABH from
ammonium salts and NaBH4 via salt metathesis, followed by
a tandem nucleophilic attack-dehydrogenation sequence,
resulting in the incorporation of the nucleophile in the final
product. This could be envisioned via an attack of the
nucleophile on the boron center of ABH with concurrent
dehydrogenation and ammonia release. The rate
enhancement observed with strong nucleophiles may be
explained via a nucleophile accelerated dehydrogenation of
ABH. Furthermore, the byproduct CTB, formed by the
reaction between ABH and nascent AB, is circumvented due
to the direct nucleophilic attack on ABH. Thus, all of the
experimental observations could be reasoned by the
mechanistic rationale in Path B.35 For additional support of
To gauge the effect of the nucleophilicity of the amines on
reaction outcome, a hindered amine, diisopropylamine,
was employed when the reaction was complete within 2 h
at rt; however the ratio of the diisopropylamine-borane to
AB altered to 62:38 (entry 7). Utilizing even bulkier non-
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DOI: 10.1039/C6DT02925F
ARTICLE
Journal Name
4124. (d) N. C. Smythe and J. C. Gordon, Eur. J. Inorg.
Chem., 2010, 509-521. (e) B. Peng and J. Chen, Energy
Environ. Sci., 2008, 1, 479-483. (f) F. H. Stephens, V. Pons
and R. T. Baker, Dalton Trans., 2007, 2613-2626. (g) T. B.
Marder, Angew. Chem., Int. Ed., 2007, 46, 8116-8118.
our premise, ABH isolated from a reaction of NaBH4 and
NH4F in liquid NH3,28 was treated with THF containing 1
equiv. of piperidine at -40 oC. Upon warming to rt and
stirring for 2 h, the reaction mixture revealed the presence
of 77% piperidine-borane and 23% AB, substantiating the
occurrence of a nucleophilic attack on ABH (Scheme 5).
Increasing the equiv. of piperidine (2 equiv.) improved the
proportion of piperidine-borane to 86%.
4
For
Summerscales and J. C. Gordon, Dalton Trans., 2013, 42
a review on AB regeneration, see: (a) O. T.
,
10075-10084. For selected independent studies, see: (b)
Y. Tan, L. Zhang, X. Chen and X. Yu, Dalton Trans., 2015,
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Chem., Int. Ed., 2012, 51, 11731-11735. (d) A. D. Sutton,
A. K. Burrell, D. A. Dixon, E. B. Garner III, J. C. Gordon, T.
Nakagawa, K. C. Ott, J. P. Robinson and M. Vasiliu,
Science, 2011, 331, 1426-1429. (e) B. L. Davis, D. A. Dixon,
E. B. Garner, J. C. Gordon, M. H. Matus, B. Scott and F. H.
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(a) E. Vasilikogiannaki, C. Gryparis, V. Kotzabasaki, I. N.
Scheme 5. Reaction of ABH with piperidine in THF.
5
Lykakis and M. Stratakis, Adv. Synth. Catal., 2013, 355
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Conclusions
In conclusion, a convenient and scalable synthesis of pure
AB from sodium borohydride and ammonium sulfate in
reagent-grade THF with added NH3 has been developed.
The presence of ammonia is critical for the reaction to
proceed at ambient temperature and pressure and under
open-flask conditions. The role of ammonia in accelerating
AB synthesis has been examined. On the basis of amines as
surrogates, we hypothesize that, contrary to its presumed
function as catalyst or co-solvent, the added NH3 is
incorporated into the product AB. Ammonium borohydride,
the primary metathesis product of NaBH4 and ammonium
salts, is believed to undergo a tandem nucleophilic attack-
dehydrogenation sequence to furnish the product. We
believe that this convenient synthesis of AB should aid in its
development as a reliable hydrogen carrier.
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Acknowledgements
Financial assistance from the Department of Energy (DOE)
and U.S. Army/General Atomics is gratefully acknowledged.
We thank Dr. Hitesh Mistry and Dr. Pravin Gagare for initial
experiments. We are also grateful to Dr. John Harwood for
assistance with NMR experiments.
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DOI: 10.1039/C6DT02925F
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35 We believe that our earlier reported amine-borane
synthesis,
wherein
an
amine-ammonium
salt
equilibration metathesis sequence was invoked could
simultaneously progress through a nucleophilic attack of
amines on ABH. See Ref. 34 for further details.
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