Improved synthetic route to n-B18H22

Article abstract of DOI:10.1021/ic051712z

Simple iodine oxidation of the B₉H₁₂^- anion in toluene at room temperature reliably gives excellent yields (～80%) of n-B₁₈H₂₂ (anti-B₁₈H₂₂) and thus provides a convenient, large-scale, safe route to this important polyborane cluster.

Full text of DOI:10.1021/ic051712z

Inorg. Chem. 2006, 45, 470−471
Improved Synthetic Route to n-B18H22
Yuqi Li and Larry G. Sneddon*
Department of Chemistry, UniVersity of PennsylVania, Philadelphia, PennsylVania 19104-6323
Received October 4, 2005
-
Me2SB9H13 9
∆8
Simple iodine oxidation of the B9H12 anion in toluene at room
temperature reliably gives excellent yields (
18H22) and thus provides a convenient, large-scale, safe route to
this important polyborane cluster.
∼80%) of n-B18H22 (anti-
n-B18H22 + B10H14 + B16H20 + B5H9 + MeSBH3 + H2 (1)
B
K+[B9H14-] + HCl + Bu2O H , KCl8
2
Bu2OB9H13 9
∆8 n-B18H22 + Bu2O + H2 (2)
Octadecaborane has been isolated in two isomeric forms,
n-B18H22 (also known as anti-B18H22) and i-B18H22 (sym-
B18H22). Crystallographic determinations1 have shown that
both compounds have fused-cage structures in which two
10-vertex frameworks share a common edge. The n-B18H22
isomer has a structure (Figure 1) containing a center of
symmetry, while i-B18H22 has the two fragments related by
C2 symmetry. Their high boron content and air stability make
these compounds particularly attractive for many potential
medicinal2,3 and materials2,4 applications.
More recently, we have found8 (eq 3) that when the
4-Me2S-arachno-B9H13 pyrolysis is carried out in the pres-
ence of the inert ionic liquid solvent 1-butyl-3-methylimid-
azolium chloride (BmimCl) under biphasic conditions, the
yield of n-B18H22 increased to ∼50%.
Me2SB9H13 BmimCl/toluene8 n-B18H22 + B10H14 + H2 (3)
120 °C
The best previously reported route9 to n-B18H22 has been
via oxidation of the B9H12- anion with mercuric ion, which
gave a 68% yield according to eq 4.
The n-B18H22 isomer was originally prepared by degrada-
2-
tion of the B20H18 anion,5 but better yield syntheses have
3HgBr2 + 4[Me4N+][B9H12-] f
been developed starting with 4-L-arachno-B9H13 (L ) Lewis
base) adducts. As shown in eq 1, pyrolysis of 4-Me2S-
arachno-B9H13 either in vacuo or in xylene solution produced
n-B18H22 in ∼20-28% yields along with a mixture of other
polyboranes,6 while pyrolysis of in situ generated 4-Bu2O-
arachno-B9H13 (eq 2) was reported to give a 34% yield.7
2n-B18H22 + Hg + Hg2Br2 + 4Me4NBr + 2H2 (4)
Although this method has proven useful for laboratory-
scale syntheses of n-B18H22, the cost, safety concerns, and
waste disposal problems associated with the use of the HgBr2
oxidant limit the usefulness of this route for the large-scale
production of n-B18H22.
* To whom correspondence should be addressed. E-mail: lsneddon@
sas.upenn.edu.
The oxidation of polyborane anions with iodine has been
previously employed in a number of reactions to effect
arachno to nido or nido to closo polyhedral conversions.
Examples include the synthesis of nido-SB9H11 from arachno-
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1515-1562.
-
SB9H12- 10 and the synthesis of [closo-1-CB8H9 ] from nido-
1-CB8H12 (in the presence of Et3N).11 In other cases, such
as in the formation of B3H8- from BH4-,12 iodine oxidation
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J.; Larson, L.; Ling, P. Proc. Electrochem. Soc. 2001, 2001-2009,
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402.
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National Meeting of the American Chemical Society, Washington, DC,
Aug 2005; American Chemical Society: Washington, DC, 2005;
INOR-063.
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106, 7266-7267.
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470 Inorganic Chemistry, Vol. 45, No. 2, 2006
10.1021/ic051712z CCC: $33.50
© 2006 American Chemical Society
Published on Web 12/15/2005

COMMUNICATION
Figure 1. Structure of n-B18H22.
has been reported to result in oxidative dehydrocondensation
to produce fused polyborane fragments. We report here that
-
simple iodine oxidation of the B9H12 anion in toluene at
room temperature (eq 5) likewise results in an intermolecular
dehydrocondensation reaction that reliably gives excellent
yields (∼80%) of n-B18H22. This reaction thus provides a
convenient, large-scale, safe route to this important poly-
borane cluster.
2[Me4N+][B9H12-] + I2 toluene8 n-B18H22 + 2Me4NI + H2
Figure 2. (a) 11B{1H} NMR spectrum of the crude reaction product: (*)
n-B18H22; (b) B10H14; (#) unidentified product. (b) 11B{1H} NMR spectrum
of the final product obtained after pumping in vacuo.
(5)
In a typical reaction, a 250-mL two-necked, round-
bottomed flask equipped with a stir bar was charged, under
an argon atmosphere, with 10.11 g (55.0 mmol) of
- 13
[Me4N+][B9H12
]
and ∼60 mL of dry toluene. After the
mixture was sonicated to form a slurry, 7.39 g (29.1 mmol)
of I2 and an additional ∼85 mL of toluene were added. This
mixture was then vigorously stirred at room temperature until
(∼40 min) the iodine color disappeared and a precipitate had
formed. The precipitate was removed from the light-yellow
solution by filtration. Rotovaporization of the filtrate solvent
gave a yellow solid, which 11B{1H} NMR analysis (Figure
2a) showed to be predominantly n-B18H22 along with minor
amounts of B10H14 and another unidentified product. The
impurities were then easily removed by pumping the material
overnight at room temperature on the high vacuum line to
leave behind 4.87 g (22.5 mmol, 81.6% yield) of n-B18H22
as a light-yellow solid. Only trace impurities in the final
product could be detected by thin-layer chromatography,
Figure 3. 1H{11B} NMR spectrum of the final product obtained after
pumping in vacuo.
be readily achieved by sublimation in vacuo at 115 °C onto
a -78 °C coldfinger.
Because of its combination of mild conditions, high yields,
simple workup, innocuous side products, and the use of an
economical and safe oxidation reagent (iodine), the method
described above has clear advantages over the other reported
procedures. This method will now allow the production of
n-B18H22 on much larger scales than previously possible.
These results also suggest that analogous iodine oxidations
of other polyborane anions may provide simple routes to
other fused-cage polyboranes.
1
11B{1H} NMR (Figure 2b), H{11B} NMR (Figure 3), or
mass spectrometry. The spectroscopic data and observed
melting point (178-181 °C) of this powdery solid agree with
the literature values.14 If necessary, further purification can
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1785-1793.
Acknowledgment. We thank the National Science Foun-
dation for support of this research.
IC051712Z
Inorganic Chemistry, Vol. 45, No. 2, 2006 471