1798 Inorganic Chemistry, Vol. 39, No. 8, 2000
Kanth and Brown
generation experiments using NaBH4 in triglyme or tetraglyme and BF3
adducts of ethers such as tert-butyl methyl ether, di-n-butyl ether,
tetrahydropyran, monoglyme, and dioxane, addition of further amounts
of the corresponding ether (BF3-carrying ether) after the diborane
generation is complete, precipitates unsolvated NaBF4 from the glyme-
ether mixture. In the diborane generation experiments using NaBH4 in
triglyme or tetraglyme and BF3 adducts of the same glyme, unsolvated
NaBF4 from glyme was obtained by heating the generation flask to
100-120 °C after the diborane generation is complete. Fortunately,
addition of volatile solvents such as dichloromethane, n-pentane,
n-hexane, tert-butyl methyl ether, dioxane, and diethyl ether to the
glyme-NaBF4 mixture also precipitates unsolvated NaBF4 at room
temperature. The precipitation of unsolvated NaBF4 from triglyme is
more efficient using dichloromethane and n-pentane. However, in the
case of tetraglyme, n-pentane is not effective because of miscibility
problems. The procedure followed for the separation of unsolvated
NaBF4 from glyme is same for all the experiments, and the following
procedure used for the separation of NaBF4 from triglyme is representa-
tive.
The usual diborane generation was carried out using slow addition
of NaBH4 in triglyme (15.0 mL, 2.00 M, 30 mmol) to BF3 in triglyme
(10.0 mL, 4.00 M, 40 mmol) at room temperature. The diborane
generated was recovered and measured by the usual methods. Into the
resulting reaction mixture in the generation flask, dichloromethane (25.0
mL) was added. The white precipitate formed was separated by filtration
under an inert atmosphere, and the filtrate was again treated with
dichloromethane (20.0 mL). This resulted in further precipitation of
NaBF4 from the filtrate, which was separated as mentioned earlier. The
combined precipitate was washed with dichloromethane (5.0 mL) to
remove traces of triglyme and was dried under reduced pressure to
yield NaBF4 in 92% yield (2.85 g). The NaBF4 thus obtained did not
melt up to 300 °C (solvated NaBF4 in triglyme melts at 58-60 °C),
indicating the formation of unsolvated NaBF4. Simple distillation of
the combined filtrate gave dichloromethane-free triglyme. The residual
triglyme contains up to 5% of solvated NaBF4, which can be separated
by distillation under reduced pressure or recycled as such for the
process.
Generation of Diborane Using NaBH4 in Triglyme (or Tetra-
glyme) and BF3 in Triglyme (or Tetraglyme) in the Presence of
AlCl3. The procedures followed using triglyme or tetraglyme are the
same, and the procedure followed for triglyme is representative. An
oven-dried apparatus was assembled in a hood as mentioned earlier.
The generation flask was charged with the BF3 in triglyme (10.0 mL,
4.00 M, 40 mmol). A solution of NaBH4 in triglyme (15.0 mL, 2.00
M, 30 mmol) placed in the pressure-equalizing funnel was added slowly
to the generation flask for 2-3 h at room temperature. The diborane
gas generated was immediately bubbled into tetrahydrofuran (80.0 mL)
contained in the absorption flask through a coarse-sintered-glass
dispersion tube at ice-salt temperature. The completion of diborane
generation was confirmed by 11B NMR, which showed the absence of
peaks due to BF3-triglymate and NaBH4 in triglyme, and the formation
of a new peak assigned to NaBF4 at -1.2 ppm. Into the generation
flask was introduced AlCl3 (2.66 g, 20 mmol) in triglyme (10.0 mL)
using a double-ended needle. (Caution! AlCl3 and triglyme must be
mixed at 0 °C). The contents were further stirred for 2 h at room
temperature. Into the generation flask further amounts of NaBH4 (45.0
mL, 2.00 M, 90 mmol) were introduced in portions using a pressure-
equalizing funnel for 3 h at room temperature. The contents were further
stirred for another 3 h at room temperature. The amount of diborane
generated was estimated by the standard hydride analysis using
hydrolysis of an aliquot. The adduct THF:BH3 was 1.88 M in BH3,
indicating a 94% yield of diborane.
NaBH4 in tetraglyme (15.0 mL, 3.00 M, 45 mmol), and this was added
slowly to the generation flask for 2-3 h at room temperature. The
diborane gas generated was bubbled into tetrahydrofuran (30.0 mL)
contained in the absorption flask through a coarse-sintered-glass
dispersion tube at ice-salt temperature. The completion of diborane
generation was confirmed by 11B NMR, which showed the absence of
the peak due to NaBH4. The 11B NMR also showed the presence of
trace amounts of NaBF4 (-1.2 ppm) and NaBH4:BH3 (-24.5 ppm),
which persisted even after 24 h, indicating a very slow reaction toward
the end. The amount of diborane generated was estimated by the
standard hydride analysis using hydrolysis of an aliquot. The adduct
THF:BH3 was 1.80 M in BH3, indicating a 90% yield of diborane.
Generation of Diborane Using NaBH4 in Triglyme (or Tetra-
glyme) and BF3 in Triglyme (or Tetraglyme) with Regeneration of
the BF3 Adduct of Triglyme (or Tetraglyme) with BCl3. The
procedures followed using triglyme or tetraglyme are the same, and
the procedure followed for triglyme is representative. An oven-dried
apparatus was assembled in a hood as mentioned earlier. The generation
flask was charged with the BF3 in triglyme (5.0 mL, 4.00 M, 20 mmol).
A solution of NaBH4 in triglyme (7.5 mL, 2.00 M, 15 mmol) placed in
the pressure-equalizing funnel was added slowly to the generation flask
for 1 h at room temperature. The diborane gas generated was
immediately bubbled into tetrahydrofuran (40.0 mL) contained in the
absorption flask through a coarse-sintered-glass dispersion tube at ice-
salt temperature. The completion of diborane generation was confirmed
by 11B NMR, which showed the absence of peaks due to BF3-
triglymate and NaBH4 in triglyme, and the formation of a new peak
assigned to NaBF4 at -1.0 ppm. Into the generation flask was
introduced BCl3 in triglyme (2.5 mL, 2.00 M, 5 mmol) using a double-
ended needle. The contents were further stirred for 1 h at room
temperature, by which time 11B NMR showed the complete formation
of BF3-triglymate (+0.2 ppm). Into the generation flask a further
amount of NaBH4 (7.5 mL, 2.00 M, 15 mmol) was introduced slowly
for 1 h at room temperature. Diborane gas was generated rapidly and
quantitatively, further confirming the complete regeneration of BF3-
triglymate. The total amount of diborane generated after the second
cycle was estimated by the standard hydride analysis using hydrolysis
of an aliquot. The adduct THF:BH3 was 0.98 M in BH3, indicating a
98% yield of diborane. The NaBF4 thus obtained can be again converted
to BF3-triglymate by the addition of stoichiometric amounts of BCl3
in triglyme.
Results and Discussion
To minimize the volatility of the ether used for the boron
trifluoride-etherate, we avoided diethyl ether and tested the
less volatile ethers, tert-butyl methyl ether, di-n-butyl ether,
tetrahydrofuran, tetrahydropyran, monoglyme, and dioxane, as
the boron trifluoride-etherate to be introduced into the reaction
flask.
To avoid possible problems arising from the formation of a
mixture of solvents, the reactions of sodium borohydride in
triglyme (or tetraglyme) with the BF3-triglymate (or -tetra-
glymate) were also carried out. These two reactions proceed
rapidly, providing an essentially quantitative yield of pure
diborane with easy recovery of sodium tetrafluoroborate and
pure triglyme (or tetraglyme) for recycling.
The following systems were examined: (1) the addition of
each of the six boron trifluoride-etherates to suspensions of
sodium borohydride in the corresponding ether; (2) the addition
of a solution of sodium borohydride in diglyme, triglyme, or
tetraglyme to each of the boron trifluoride adducts of various
ethers such as tert-butyl methyl ether, di-n-butyl ether, tetrahy-
drofuran, tetrahydropyran, monoglyme, and dioxane; (3) the
addition of a solution of sodium borohydride in diglyme,
triglyme, or tetraglyme to the boron trifluoride adduct of the
same glyme; (4) a detailed examination of the sodium borohy-
dride in the triglyme (or tetraglyme) system added to boron
trifluoride-triglymate (or -tetraglymate); (5) a detailed ex-
Generation of Diborane Using NaBH4 in Triglyme (or Tetra-
glyme) and NaBF4 in Triglyme (or Tetraglyme) in the Presence of
AlCl3. The procedures followed using triglyme or tetraglyme are the
same. The procedure followed for tetraglyme is representative. An oven-
dried apparatus was assembled in a hood as mentioned earlier. The
generation flask was charged with the NaBF4 (1.65 g, 15 mmol) and
AlCl3 (2.66 g, 20 mmol) at 0 °C. Tetraglyme (15.0 mL) was added
slowly with stirring at 0 °C, and the contents were further stirred at
room temperature for 2 h. Into the pressure-equalizing funnel was placed