Synthesis of Decaborane by the Reaction of Sodium Undecaborate with Mild Organic Oxidants

Article abstract of DOI:10.1134/S107042721711012X

New organic oxidants (aldehydes and ketones) allowing efficient synthesis of decaborane in a high yield via intermediate alkali metal salt were found. The sodium undecaborate oxidation process was refined, and new reaction stoichiometry was suggested.

Full text of DOI:10.1134/S107042721711012X

ISSN 1070-4272, Russian Journal of Applied Chemistry, 2017, Vol. 90, No. 11, pp. 1804−1809. © Pleiades Publishing, Ltd., 2017.
Original Russian Text © P.P. Belov, P.A. Storozhenko, N.S. Voloshina, M.G. Kuznetsova, 2017, published in Zhurnal Prikladnoi Khimii, 2017, Vol. 90, No. 11,
pp. 1498−1503.
SPECIFIC
TECHNOLOGICAL PROCESSES
Synthesis of Decaborane by the Reaction
of Sodium Undecaborate with Mild Organic Oxidants
P. P. Belov*, P. A. Storozhenko, N. S. Voloshina, and M. G. Kuznetsova
State Research Institute of Chemistry and Technology of Organoelement Compounds,
sh. Entuziastov 38, Moscow, 111123 Russia
*
e-mail: ppbeloff@mail.ru
Received November 17, 2017
Abstract—New organic oxidants (aldehydes and ketones) allowing efficient synthesis of decaborane in a high
yield via intermediate alkali metal salt were found. The sodium undecaborate oxidation process was refined, and
new reaction stoichiometry was suggested.
DOI: 10.1134/S107042721711012X
The ability of boron compounds to form stable
framework structures that are similar to aromatic
compounds and incorporate other elements is of much
interestfordifferentbranchesofchemistry. Diversesteric
structure and strong chemical bonds of such derivatives
of boron hydrides as carboranes and metallocarboranes
not only determine their unique properties, but also allow
their use in catalysis [1], in organometallic chemistry,
and also in synthesis of luminescent materials [2], of
polymers resistant to high temperatures, oxidation, and
radiation [3, 4], of nanomaterials, and of liquid crystals.
Analysis of the published data shows that the route
to decaborane via intermediate alkali metal tetradeca-
hydroundecaborates (hereinafter, undecaborates) is the
most promising for further development and improve-
ment.
Previously reported procedures for decaborane
synthesisarebasedonthereactionofsodiumborohydride
NaBH4 with either boron trifluoride etherates [9, 10]
or alkyl halides (C H Br etc.) [11] as Lewis acids in
5
11
diglyme at 100–110°С with the formation of a solution
of sodium undecaborate NaB H [Eqs. (1), (2)]:
11 14
Since recently, carboranes became particularly de-
manded by medicine, where they are used as absorbents
of epithermal neutrons in boron neutron capture therapy
1
7NaBH + 20BF ·O(C H )
4 3 2 5 2
→ 2NaB11H14 + 15NaBF4 + 20H2 + 20O(C2H5)2,
1NaBH + 10C H Br
(1)
(2)
(BNCT) [5–7].
1
4
5
11
Increased interest in framework boron derivatives
→
NaB H +10C H + 10NaBr + 10H2.
11 14 5 12
stimulated the development of procedures for preparing
the starting compound for their synthesis, decaborane.
This framework boron hydride was synthesized for the
first time by Alfred Stock in 1912 [8]. Since that time,
the decaborane synthesis process underwent major
changes. However, because of the synthesis complexity
and relatively low yield of the target product, decaborane
remains expensive and is in short supply. Therefore,
the development of efficient, simple, and economically
advantageous procedure for its synthesis is a topical
problem.
Then, diglyme was replaced by water, and the
sodium undecaborate solution was oxidized with
various oxidants in the presence of sulfuric acid. The
following oxidants were used: potassium permanganate
(
KMnO ), sodium bichromate (Na Cr O ), hydrogen
4 2 2 7
peroxide (H O ) in the presence of a bivalent iron salt
2
2
[
12, 13], manganese dioxide (MnO ), chromic anhydride
2
(
CrO ), and a trivalent iron salt (Fe (SO ) ·5H O) [9].
3
2
4 3
2
The assumed reaction stoichiometry was described by
Eq. (3) [13], according to which 1 mol of decaborane
1
804

SYNTHESIS OF DECABORANE BY THE REACTION OF SODIUM UNDECABORATE
1805
and 12 mol of boric acid are formed from 2 mol of
sodium undecaborate:
and foaming were controllable. To the resulting acidic
solution of sodium undecaborate, 1.36 mol of acetone
was added dropwise over a period of 105 min. Then,
after the phase separation, the lower aqueous layer was
discarded, and the upper organic layer was washed with
distilled water (7 × 300 mL). The resulting toluene
solution of decaborane was evaporated to the required
–
2
B H + 36H O → B H
11
14
2
10 14
+
+
12B(OН) + 18H + 14H + 16e–.
(3)
3
2
Thus, in the reaction with various inorganic oxidants
morethanahalfofboronfromthepreviouslysynthesized
undecaborate ion is irretrievably lost. Furthermore,
the process involves formation of a large amount of
harmful waste (manganese, chromium, fluorine, boron
compounds). Introduction of strong inorganic oxidants
into the reaction mixture in the step of oxidation of
undecaborate ion significantly restricts the choice of
the extractant for decaborane, thus complicating its
isolation. As for the solvent replacement step, each of
the suggested procedures (azeotropic distillation of
diglyme with water [13] or treatment with hexane [9])
implies an additional step of diglyme regeneration,
which strongly complicates the process and makes it
much more expensive.
11
concentration and analyzed by В NMR spectroscopy.
To separate the impurity, a 20% solution of
decaborane in toluene (321 g) was charged into a
nitrogen-flushed four-necked flask equipped with a
turbine stirrer, a thermometer, a dropping funnel, and
a reflux condenser. After the temperature of 70°С was
reached, dimethylaniline (140 g) was slowly added
dropwise, with the subsequent gradual heating to 100°С.
The reaction mixture was continuously stirred at this
temperature until the gas evolution ceased. The cooled
reaction mixture was transferred onto a glass frit and
washed with three portions of pure toluene. In so doing,
the impurity passed into the toluene solution. This
solution was evaporated in a water-jet-pump vacuum.
The dry residue was dissolved in n-hexane and analyzed
To eliminate these drawbacks and increase the
yield of the target product by finding an appropriate
oxidant, we performed a thorough detailed chemical
and technological analysis of the decaborane synthesis
by oxidation of alkali metal undecaborates.
11
by В NMR spectroscopy.
The 11B NMR spectra were recorded on a Bruker
АМ-360 spectrometer operating at 115.526 MHz. The
chemical shifts are given relative to boron trifluoride
etherate. Alkyl halides (n-bromobutane, -pentane,
-hexane, 98+% purity) and diglyme (99% purity, water
EXPERIMENTAL
content <0.5%) were purchased from Alfa Aesar.
Also, we used chemically pure grade toluene [TU
All the steps of the decaborane synthesis were
performed in one vessel: a 2-L flask with a jacket,
equipped with a stirrer, a thermometer, and a descending
condenser. The reactants were fed with a Masterflex
L/S peristaltic pump (7524-55). The temperature
was controlled with a Huber Unistat 125 thermostat.
Synthesis of sodium undecaborate in diglyme solution
was performed for 8 h at 100–105°С. The diborane
vapor was trapped in a Drechsel bottle filled with
triethylamine and in a U-shaped alcoholic trap. The
synthesis was based on reaction (2); 3.8 mol of alkyl
halide (30% excess) was taken per 3.2 mol of sodium
borohydride. The diglyme volume was 800 mL. After
the 8-h reaction, the mixture was cooled to room
temperature with continuous stirring in an inert gas
stream. Vacuum distillation of diglyme and unchanged
Lewis acid was performed at 85°C/5–7 mm Hg. After
cooling the residue to 20–25°С, 320 mL of toluene
was added, and then 40% sulfuric acid (1600 g) was
carefully added at a rate at which the gas evolution
(
Technical Specification) 2631-020-44493179–98 with
revisions 1, 2, 3], chemically pure grade sulfuric acid
TU 2631-020-44493179–98 with revisions 1, 2, 3),
(
pure grade acetone [GOST (State Standard) 2603–79],
analytically pure grade dimethylaniline (GOST 5855–
78), and analytically pure grade hexane (TU 2631-003-
05807999–98).
RESULTS AND DISCUSSION
Comparative analysis of the above procedures for
preparing sodium undecaborate shows that the reaction
of sodium borohydride with alkyl bromide (С H2n+1Br,
n
where n = 4–6) in diglyme is preferable from the process
viewpoint. We reached the best results with n-butyl
bromide. In addition, this bromide is many times cheaper
than other bromoalkanes. We decided to perform solvent
replacement by vacuum distillation, which allowed us to
recycle the distilled-off diglyme without any additional
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 90 No. 11 2017

1806
BELOV et al.
m , g
one molecule of sodium undecaborate is required for the
formation of one decaborane molecule:
o.l
2
NaB H + 4CH COCH + H SO + 6H O
11 14 3 3 2 4 2
→
2B H + 2H BO + 4C H OH + Na SO .
(4)
1
0
14
3
3
3
7
2
4
In accordance with Eq. (4), the reaction occurs
without hydrogen evolution. However, actually the
decaborane synthesis is accompanied by the formation
of a large amount of this gas, which contradicts the
suggested stoichiometry. Therefore, to understand the
reaction mechanism better, we divided the process
into two steps: preparation of aqueous acid solution of
NaB H and the redox reaction proper.
m , g
a
Fig. 1. Organic layer weight mo.l as a function of the amount
of the acid added ma.
11
14
regeneration. As a result, the sodium undecaborate
synthesis was considerably simplified and was made
appreciably less labor-consuming.
We found that the major fraction of hydrogen is
released specifically during preparation of the aqueous
acid solution. Also, the reaction mixture underwent
phase separation. The weight of the upper organic layer
first increased with the addition of the acid and then
reached a certain limit (Fig. 1).
However, we paid particular attention to the search
for new effective oxidants for the sodium undecaborate
oxidation step. We have found in the course of our
studies that some organic compounds containing
carbonyl group, such as acetone, benzaldehyde, methyl
ethyl ketone, acetylacetone, formaldehyde, etc., are
In the 11В NMR spectra (С D ) of the diglyme
6
6
solution, along with the signals (δ, ppm) at –14.1 (d, 1B),
–16.0 (d, 5B), and –16.7 (d, 5B), belonging to the boron
atoms in NaB H , we identified the signals at –8.19 (d,
–
capable to oxidize the B H ion to decaborane with
11
14
the formation of the corresponding alcohols [14]. The
use of such oxidants also allowed us to use aromatic
solvents.
11
14
3B), –20.5 (d, 3B), and –23.8 (d, 3B) (intensity ratio 1 :
: 1), corresponding to the boron atoms in B H14–, and
1
9
–
However, in the course of developing the process
for sodium undecaborate oxidation with acetone, we
reached the decaborane yield higher than 100% relative
to the stoichiometry of Eq. (3) (run nos. 3 and 4 in the
table).
a characteristic multiplet of B H at –30.1 ppm (nonet,
3
8
1JBH = 33.3 Hz). The 11B{ H} NMR spectrum of the
upper organic layer formed (Fig. 2b) contains virtually
no signals apart from those of B H14–.
1
11
Analysis of the 11В{ H} NMR spectra (Figs. 2a,
1
Therefore, we suggested refined stoichiometry of
this redox reaction [Eq. (4)], according to which only
2b) shows that treatment with sulfuric acid does not
alter the boron framework of sodium undecaborate
Data on the theoretical [Eqs. (1)–(4)] and practically reached yield of decaborane synthesized by different procedures
Theoretical
Theoretical
yield of B H
yield of B H
Actual yield
of B H , g
1
0
14
10 14
Run no.
NaBH , g
Lewis acid
Oxidant
4
according
to Eq. (3), g
according to Eq.
(4), g
1
0
14
1
2
3
4
5
60
60
60
60
60
BF ·O(C H )
H О /Fe(II)
11.4
11.4
8.8
–
8.9a
9.1a
9.9
9.6
8.7
3
2
5 2
2
2
BF ·O(C H )
5 2
KMnO4
CO(CH3)2
CO(CH3)2
CO(CH3)2
–
3
2
C H Br
17.6
17.6
17.6
4
9
C H Br
8.8
5
11
C H Br
8.8
6
13
a
Experimental data given in [13].
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 90 No. 11 2017

SYNTHESIS OF DECABORANE BY THE REACTION OF SODIUM UNDECABORATE
1807
(a)
δ, ppm
(b)
δ, ppm
Fig. 2. 11В{ H} NMR spectra of the (a) diglyme solution of sodium undecaborate and (b) upper organic layer obtained after treatment
1
of sodium undecaborate with an aqueous solution of sulfuric acid. (δ) Chemical shift.
–
–
but leads to the disappearance of В Н , B H , and
НB H + CH COCH + 3H O
11 14 3 3 2
9
14
3
8
→
B H + H BO + C H OH + H .
(6)
other impurities formed in the step of synthesizing the
diglyme solution of sodium undercaborate, as these
impurities are readily broken down to boric acid with
the evolution of hydrogen in the step of preparation of
the acid solution. The gas evolution is also accompanied
by the decomposition of the unchanged NaBH4.
10 14
3
3
3
7
2
In accordance with Eqs. (5) and (6), 1 mol of sodium
undecaborate is still required for obtaining 1 mol of
decaborane, but 1 mol of hydrogen is released in the
process, which well agrees with the experimental data.
Thus, oxidation of sodium undecaborate can be
described by overall Eq. (7):
Thus, treatment of sodium undecaborate with an
aqueous sulfuric acid solution leads to exchange reac-
tion (5):
2
NaB H + H SO + 2CH COCH + 6H O
11 14 2 4 3 3 2
2
NaB H + H SO → 2НB H + Na SO .
(5)
11
14
2
4
11 14
2
4
→
2B H + 2H BO + 2C H OH + Na SO + 2H . (7)
10 14 3 3 3 7 2 4 2
In the second step, the species reacting with
acetone is not sodium undecaborate but its acid form,
It should be noted that the decaborane thus obtained,
irrespective of particular oxidant containing a carbonyl
“
undecaboric” acid [Eq. (6)]:
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 90 No. 11 2017

1808
BELOV et al.
δ, ppm
Fig. 3. 11В{ H} NMR spectrum of decaborane containing impurities.
1
δ, ppm
Fig. 4. 11В{ H} NMR spectrum of the boron impurity of type 5-RB H .
1
10
13
group, always contains similar boron impurity in an
amount of 3–5% and more (Fig. 3).
efficient, but also technologically and environmentally
safer and less resource-consuming.
The 11В{ H} NMR spectrum of the solution of the
separated impurity (Fig. 4) contains nine signals (δ,
ppm) at 21.9 (s, 1B), 13.1 (d, 1B), 10.6 (d, 1B), 3.2 (d,
1
CONCLUSIONS
(
1) A highly efficient procedure for oxidation of
1
B), –3.6 (d, 1B), –6.9 (d, 1B), –11.4 (d, 1B), and –38.5
sodium undecaborate was developed. It allows the
process to be performed with considerably lower loss of
“hydride” boron, which led to a more than 25% increase
in the yield of the target product.
(
d, 2B), which correspond to chemical shifts of boron
5
atoms in decaborane with a substituent at the B atom,
5
-RB H [15].
10 13
The results obtained allowed us to patent a new
procedure for decaborane synthesis [14]. As compared
to the known analogs, this procedure is not only more
(2) A comprehensive study of the sodium undecabo-
rate oxidation process allowed suggesting a new reaction
stoichiometry describing the process most adequately.
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SYNTHESIS OF DECABORANE BY THE REACTION OF SODIUM UNDECABORATE
1809
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