The quest for silylhydroboranes: (Me3Si)3SiBH 2

Article abstract of DOI:10.1039/c0dt00096e

Reaction of BH₃.NEt₃ with tris(trimethylsilyl) silylpotassium yielded the respective silylboranate. Abstraction of a hydride with B(C₆F₅)₃ led to the neutral silylborane, which could be derivatised as an Et₃N adduct. Treatment of the boranate with Cp₂Ti(btmsa) resulted in exchange of THF coordinating to potassium by Cp₂Ti(btmsa). The Royal Society of Chemistry.

Full text of DOI:10.1039/c0dt00096e

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Dalton Trans., 2010, DOI: 10.1039/C0DT00287A
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www.rsc.org/dalton | Dalton Transactions
The quest for silylhydroboranes: (Me Si) SiBH †‡
3
3
2
Henning Arp, Christoph Marschner* and Judith Baumgartner*
Received 9th March 2010, Accepted 12th May 2010
DOI: 10.1039/c0dt00096e
Reaction of BH
Abstraction of a hydride with B(C
Et N adduct. Treatment of the boranate with Cp
3
·NEt
3
with tris(trimethylsilyl)silylpotassium yielded the respective silylboranate.
led to the neutral silylborane, which could be derivatised as an
Ti(btmsa) resulted in exchange of THF coordinating
6
F
)
5 3
3
2
to potassium by Cp Ti(btmsa).
2
Introduction
(Scheme 1). Multinuclear NMR spectroscopy showed the for-
mation of potassium tris(trimethylsilyl)silylboranate 1 with one
coordinating THF molecule. Already from the spectroscopic
data an unusual coordination motif was expected as potassium
ions commonly require more than one THF to saturate their
coordination sphere.
While the syntheses of the first substances containing silicon–
boron bonds were reported in the early 1960s, the field remained
largely neglected for a long time. Only in recent years has the
synthetic potential of silylboranes been recognised and they have
now become popular reagents for organic synthesis. Early on,
1
2
however, studies on silylboranes concentrated on compounds with
p-donor substituents, namely alkoxy and dialkylamino groups.
In contrast to the situation for silylboranes, research on
alkylboranes is a very well established field of research. While many
alkylboranes are easily accessible by hydroboration of alkenes,
the treatment of suitable boranes with alkyllithium compounds
is another synthetic approach. In the context of the current
Scheme 1 Formation of potassium tris(trimethylsilyl)silylboranate 1.
3
study reports of the research groups of Eaborn, Smith and
4
Paetzold are notable. They describe the synthesis of boranes and
Crystal structure analysis of the obtained compound re-
vealed a tetrameric arrangement (Fig. 1). To convert silylbo-
ranate 1 to a neutral borosilane, it was treated with the reli-
boranates bearing the tris(trimethylsilyl)methyl substituent, which
is structurally related to the tris(trimethylsilyl)silyl group employed
in our study. The synthesis of [(Me
by hydrogenation of (Me Si) CBCl
abstraction or oxidation from the boranate Li[(Me
3
Si)
3
CBH
with LiBH
2
]
2
was achieved either
able hydride abstraction reagent B(C
6
F ) . NMR spectroscopic
5
3
3
3
3
2
4
or by hydride
Si) CBH ] with
observation provided evidence for a clean transformation to
tris(trimethylsilyl)silylborane 2 (Scheme 2).
3
3
3
3,4
4
acid or iodine.
In contrast to alkylhydroboranes, compounds of the type
SiBH are virtually unknown. Nonetheless, base adducts with
R
3
2
5
6,7
8
amines, phosphanes or isocycanides thereof have been de-
scribed and also boranates of the type: Li[R
from a seminal contribution by N o¨ th and coworkers.
3
SiBH
3
] are known
9
Results and discussion
Oligosilanyl substituents have been proven useful to stabilise a
number of main group and transition metal compounds with
interesting bonding situations, with the number of reported
10
oligosilanylboranes still being small. Our own studies in this
field have concentrated on the use of oligosilanylpotassium
11
compounds. For this reason, it was decided to start our
endeavour in the area of silyboranes with the most easily ac-
12
Fig. 1 Tetrameric arrangement of 1. Molecular structure in the crystal
with methyl groups and hydrogen atoms omitted for clarity.
cessible oligosilanylpotassium reagent: (Me
3
Si)
3
SiK. Reaction
9
with BH
3
·NEt
3
, following N o¨ th’s procedure, proceeded cleanly
While the structure of 2 was later unambiguously confirmed
as a dimer by means of single crystal structure analysis (Fig. 2),
first attempts to obtain additional evidence for its formation were
sought by reaction with triethylamine (Scheme 3). Clean adduct
formation was observed spectroscopically by NMR analysis and
also by X-ray single crystal structure analysis (Fig. 3).
Institut f u¨ r Anorganische Chemie, Technische Universit a¨ t Graz, Stremayr-
gasse 16, 8010 Graz, Austria. E-mail: christoph.marschner@tugraz.at,
baumgartner@tugraz.at; Fax: 0043 316 873 8701; Tel: 0043 316 873 8209
†
CCDC reference numbers 767902–767904. For crystallographic data in
CIF or other electronic format see DOI: 10.1039/c0dt00096e
‡
Dedicated to Prof. Uwe Rosenthal on the occasion of his 60th birthday.
9
270 | Dalton Trans., 2010, 39, 9270–9274
This journal is © The Royal Society of Chemistry 2010

Scheme 3 Adduct formation of 2 with triethylamine to form 3.
Scheme 2 Hydride abstraction of 1 to form silylborane 2.
Inspired by Berry’s work on silylboranate complexes of
7
tantalocene and by several other reports on boranate com-
13
plexes of early transition metals we were interested to react 1
with Rosenthal’s titanocene precursor: Cp
2
Ti(btmsa) (btmsa =
14
bis(trimethylsilyl)acetylene). Replacement of the btmsa ligand
by silyborohydride 1 was assumed. However, the facile reaction
occurring did not bring about substituent exchange on titanium
but only substitution of the THF coordinating to potassium by
Cp
2
Ti(btmsa) (Scheme 4, Fig. 4).
Fig. 2 Molecular structure of 2, as a dimer in the solid state. Hydrogen
atoms on methyl groups omitted for clarity. The molecule lies about an
inversion centre, thus atoms labels containing an underscore correspond
to crystallographically equivalent positions (1 - x, 1 - y, -z).
2
Scheme 4 Lewis base exchange of THF against Cp Ti(btmsa).
NMR and IR spectroscopy
Compounds 1–4 were characterised using multinuclear NMR
1
1
spectroscopy. Among the nuclei observed B and to some extent
1
H are the most useful ones as they provide the most accurate
picture of the situation at the boron atom.
1
1
For the silylboranate 1 the B resonance was observed at
-
43.4 ppm, which is almost identical to the values reported by
9
N o¨ th and coworkers for their respective compounds. The fact
that the chemical shift for THF complexed LiBH
15
4
is -42.0 ppm
indicates the close electronic resemblance of silicon and hydrogen.
Related lithium alkylboranates were found in the region between
-20 and -30 ppm. The proton-coupled signal of 1 featured the
Fig. 3 Molecular structure of 3 in the solid state. Only one of two
independent molecules shown and hydrogen atoms on methyl and ethyl
groups omitted for clarity.
16
2
Fig. 4 Molecular structure of 4 in the solid state with only half of the oligomeric structure shown. Methyl groups and one molecule of Cp Ti(btmsa)
omitted for reasons of clarity. The supramolecular arrangement includes an inversion centre, thus atoms labels containing an underscore correspond to
crystallographically equivalent positions (1 - x, 1 - y, 1 - z).
This journal is © The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 9270–9274 | 9271

1
expected quadruplet with a JBH coupling constant of 84 Hz.
being less broad and shifted slightly to higher wavenumbers
at 2233 cm .
2
9
-1
The Si spectrum showed a resonance at -10.0 ppm for the
trimethylsilyl groups and also the boron substituted silicon atom
could be observed with a shift of -125.2 ppm as a rather weak
Crystal structures
1
signal featuring a quadruplet with a JSiB coupling constant of
5
1 Hz. The difficulty to observe this signal is associated with
Single crystal structure analysis of 1 revealed its tetrameric
structure (Fig. 1). Four molecular units form a distorted cube
with the potassium ions coordinating to two hydride atoms of
the strong quadrupolar line broadening caused by boron. For
this reason no values for this signal were provided by N o¨ th and
coworkers. In the proton NMR spectrum of 1 the signals for THF
9
each neighbouring BH unit. In addition one THF molecule
3
and the trimethylsilyl group show expected resonance behaviour.
coordinates to potassium from the outside of the cube. No other
boranate with this structural motif was found in the Cambridge
Crystallographic Database. Hydride atoms on boron could be
The hydride signals were found at -0.51 ppm as a quadruplet with
1
1
22
8
4 Hz coupling to B. The IR spectrum of 1 shows a very broad
-
1
and strong BH stretching band at 2223 cm , again consistent with
N o¨ th’s observations.
Typical B NMR shifts for alkylboranes of the type RBH
in the range between +22 and +24 ppm.
located and were used in the refinement. The observed B–H bond
9
˚
lengths vary in range from 1.07 to 1.16 A. A similar behaviour
1
1
9
2
are
was also found by N o¨ th for structures of Li[Ph
3
SiBH ] and
3
17
t
R
2
BH compounds are
Li[ BuPh
2
SiBH ]. One of the tris(trimethylsilyl)silyl groups of 1
3
usually observed in a similar range as long as they are dimeric.
Monomeric forms, with very bulky alkyl substituents are reflected
shows strong disorder. The Si–Si bond lengths in the remaining
˚
tris(trimethylsilyl)silyl units are on average only 2.32 A, which
17c,18
in a strong down-field shift to values around 81 ppm.
The
is unusually short. In contrast to this are the Si–B distances
˚
observed resonance for silylborane 2 at +34.4 ppm is between
which were found between 2.02 and 2.04 A, longer than the
19
9
˚
the value for neat BH
3
: (+57.1 ppm) and the mentioned
1.99 A reported by N o¨ th. The cube’s distortion is caused by
its construction of rhomboids with BKB angles close to 95 and
◦
alkylhydroboranes thus reflecting the more electropositive silyl
substituent and in addition indicating a dimeric arrangement. In
◦
KBK angles near 85 .
1
1
the proton coupled B spectrum a broad resonance was observed
with no resolved coupling to the hydride substituents. This is in
line with the proton spectrum, where only the trimethylsilyl signal
was observed and no hydride resonances could be found. These
observations together with the fact that no signal was observed in
For the silylborane 2 the crystal structure analysis reveals a
dimeric arrangement. Very similar to the reported structures of
3,4
[(Me
3
Si)
3
CBH
2
]
2
a distorted square planar arrangement of the
B–H–B–H unit was found with the bulky silyl groups in trans-
position. It is somewhat puzzling that the number of reported
crystallographically characterised dimeric boranes of the type
2
9
the Si spectrum for the boron substituted silicon atom are again
caused by the quadrupole moment of boron. In contrast to 1 the
stretching B–H band of 2 is sharp and is located at a wavenumber
23
(RBH
[(Me Si)
astonishing differences in the description of the diborane part.
2
) is limited to m-terphenylborane and the mentioned
2
3,4
3
3
CBH
2
]
2
of which two structures were reported with
-
1
of 2489 cm .
The Et N adduct 3 exhibits a B resonance at -12.7 ppm, which
is again close to the parent compound Et (-13.6 ppm),
1
1
˚
3
The Si–Si bond lengths of 2 are with an average value of 2.34 A
still comparably short but noticeably longer than for 1, while the
Si–B distance of 1.991(10) A is shorter than the corresponding
20
3
N·BH
3
˚
whereas the analogous adducts of alkylhydroboranes are found
in a range between +2 and -7 ppm. The proton NMR spectrum
exhibits sharp signals for the trimethylsilyl resonance at 0.42 ppm
and the ethyl signals. No NMR signals for the silicon and
hydrogen atoms directly connected to boron could be observed.
This behaviour is quite typical also for other boranes and can
be associated with the quadrupolar moment of boron and the
lower local symmetry compared to 1. The IR spectrum shows
bond in 1.
The structure contains the expected rhomboidal arrangement
◦
with bridging hydrides. The acute H
b
BH
b
angle is with 78
Si) CBH
◦
substantially smaller than the 85/88 for [(Me
3
3
2
]
˚
2
and the
◦
24
96 for diborane. The B–B¢ distance in 2 is with 1.80 A slightly
elongated compared to diborane or m-terphenylborane.
Crystals of 3, which were subjected to X-ray crystal structure
analysis were found to be pseudo merohedral twinned. The partial
-
1
the BH stretching band at 2388 cm comparing well to related
21
25
triethylamine adducts.
As expected were the spectroscopic properties of compound
solution of the structure confirmed the monomeric arrangement
of 3 (Fig. 3).
1
1
26
4
very similar to those of 1. The B resonance was found with
The crystal structure of 4 could also only be partially solved
the same chemical shift (-43.4 ppm) and the same B–H coupling
constant (84 Hz) for the quadruplet. In the proton spectrum singlet
(Fig. 4). As already deduced from the NMR spectroscopic analysis
4 consists of the tris(trimethylsilyl)silylboranate anion with the
signals for the Cp, the SiSiMe
3
and the CSiMe
3
fragments were
Cp Ti(btmsa) complex acting as ligand for the potassium via
2
observed with reasonable chemical shifts. And also the quadruplet
for the hydride signal is only slightly shifted upfield to -0.68 ppm
exhibiting 85 Hz coupling constant. The fact that only one set of
the Cp rings. However, the whole arrangement is a bit more
complicated than for 1.
The cubic arrangement of 1 is only retained to some extent.
Two of the potassium atoms in the cube do not possess an
additional donor now. To the remaining two potassium ions in
resonances was observed for the Cp
2
Ti(btmsa) complex indicates
a fast exchange of molecules coordinating to potassium. The
2
9
Si spectrum eventually shows two trimethylsilyl resonances and
the cube Cp
serving as base. Of the two coordinating Cp
2
Ti(btmsa) units are coordinating with the Cp rings
Ti(btmsa) molecules
again a rather weak signal for the boron substituted central
silicon atom as a quadruplet with 52 Hz coupling constant
and a chemical shift of -125.3 ppm. In accordance with 1 the
IR spectrum of 4 shows a very strong BH stretching band,
2
one serves as a bridging unit with both Cp rings coordinating.
The Cp ring not coordinating to the cube, however, does not
27
coordinate to another tetramer but to a dimer. The centre of this
9
272 | Dalton Trans., 2010, 39, 9270–9274
This journal is © The Royal Society of Chemistry 2010

dimer is the inversion center of the supramolecular arrangement,
which can be described as CpTi(btmsa)Cp-[KH BSi(SiMe
CpTi(btmsa)Cp-[KH BSi(SiMe -CpTi(btmsa)Cp-[KH
SiMe -CpTi(btmsa)Cp (with bold Cp units coordinating). In
addition the asymmetric unit contains two other Cp Ti(btmsa)
molecules which do not interact substantially with the potassium
ions. One of these units exhibits a strong disorder of the btmsa
ligand, which could not be resolved in a satisfactory way.
Potassium tris(trimethylsilyl)silylboranate THF (1)
3
3
) ] -
BSi-
3
4
To a solution of tris(trimethylsilyl)silylpotassium [starting from
tetrakis(trimethylsilyl)silane (321 mg, 1.0 mmol) and KO Bu
3
3
)
3
]
2
3
t
(
3
)
]
3 4
(
118 mg, 1.05 mmol)] in 5 mL THF a solution of BH
3
·NEt
3
2
[
prepared from 1 mL BH
3
·THF (1 molar solution) and 0.01 mL
triethylamine] was added. After heating to reflux for 24 h the
solvent was removed in vacuum and the precipitate treated
three times with 5 mL pentane. The pentane extract was reduced to
about 4 mL and cooled to -60 C for 16 h. A colourless crystalline
product (1) was obtained (324 mg, 87%). NMR H (C
◦
1
6
D , d ppm):
6
Conclusions
1
3
3
.52 (s, 4H), 1.41 (s, 4H), 0.41 (s, 27H), -0.51 (q, J11B-1H = 84 Hz,
1
1
1
13
H). B (C D ,
6
D
6
, d ppm): -43.3 (q, J11B-1H = 84 Hz). C (C
6
6
The fact that compounds of the type R
unknown prompted us to attempt the synthesis of (Me
This was accomplished by initial reaction of (Me Si)
BH to obtain K[(Me Si) SiBH ] followed by hydride ab-
·NEt
straction with B(C
3
SiBH
2
are virtually
Si) SiBH
SiK with
2
9
d ppm): 67.9, 25.6, 2.9. Si (C
(
6
D
6
, d ppm): -10.0 (TMS), -125.2
3
3
2
.
1
-1
q, J29Si-11B = 51 Hz). IR (Nujol) n: 2223 (br, vs) cm .
3
3
3
3
3
3
3
F
5
3
) .
Tris(trimethylsilyl)silylborane (2)
To a solution of compound 1 (100 mg, 0.27 mmol) in pentane
6
Preliminary reactivity studies concerning the hydrobora-
tion/silylboration behaviour are currently under way. The simple
access to the silylhydroborane will facilitate additional silylation
(
0
3 mL) a solution of tris(pentafluorophenyl)borane (138 mg,
.27 mmol) in pentane (5 mL) was added. After 15 min the
steps to obtain compounds like K[(R
3
Si)
2
BH
2
] and (R
3
Si) BH
2
colourless precipitate was removed by centrifugation and the
solvent removed. The residue was dissolved in a very small amount
which most likely will also have interesting addition properties.
1
of pentane from which colourless plates of 2 crystallised. NMR H
1
1
(
C
6
D
6
, d ppm): 0.28 (s, 18H). B (C
FWHM = 276 Hz} which sharpens upon proton decoupling
FWHM = 124 Hz}). C (C , d ppm): 2.0. Si (C , d ppm):
10.5. IR (Nujol) n: 2489 (sh) cm .
6
D
6
, d ppm): 34.3 (broad signal
Experimental
{
1
3
29
{
6
D
6
6
D
6
General remarks
-
1
-
All reactions involving air-sensitive compounds were carried out
under an atmosphere of dry nitrogen or argon using either
Schlenk techniques or a glove box. All solvents were dried
over sodium/potassium alloy under nitrogen and were freshly
distilled prior to use. Potassium tert-butanolate was purchased
from MERCK. All other chemical were bought from different
suppliers and were used without further purification.
Tris(trimethylsilyl)silylborane Et N (3)
3
Triethylamine (0.1 mL) was added to a solution of 2 in benzene
and colourless crystals were obtained in quantitative yield. NMR
1
3
3
H (C
6
D
6
, d ppm): 2.35 (q, JHH = 7.2 Hz, 6H), 0.70 (t, JHH
=
1
1
7
.2 Hz, 9H), 0.42 (s, 27H), no BH
2
resonances observed. B (C D ,
6 6
1
13
1
13
11
29
d ppm): -12.7 (t, JBH = 105 Hz). C (C
6
D , d ppm): 52.4, 8.7,
6
H (300 MHz), C (75.4 MHz), B (96 MHz), and Si
59.3 MHz) NMR spectra were recorded on a Varian INOVA
00 spectrometer. If not noted otherwise for all samples C was
used or in case of reaction samples they were measured with a D
capillary in order to provide an external lock frequency signal. To
2
9
-1
3
.5. Si (C , d ppm): -9.8. IR (Nujol) n: 2388 (sh) cm .
6
D
6
(
3
6
D
6
2
O
Potassium tris(trimethylsilyl)silylboranate·Cp Ti(btmsa) (4)
2
2
9
Compound 1 (186 mg, 0.5 mmol) and Cp Ti(btmsa) (174 mg,
0.5 mmol) were dissolved in pentane (5 mL). After stirring for 3 h
at r.t. the solvent was reduced and cooled to -60 C for 16 h. A
2
compensate for the low isotopic abundance of Si the INEPT
28
pulse sequence was used for the amplification of the signal.
◦
yellow crystalline product (4) was obtained (355 mg, 78%). NMR
1
H (C
6
D , d ppm): 6.41 (s, 10 H), 0.43 (s, 27 H), -0.32 (s, 18
6
X-Ray structure determination†
1
1
1
H); -0.68 (q, J11B-1H = 85 Hz, 3H). B (C
6
D , d ppm): -43.4 (q,
6
For X-ray structure analyses the crystals were mounted onto
the tip of glass fibres, and data collection was performed with
a BRUKER-AXS SMART APEX CCD diffractometer using
1
13
29
J
BH = 84 Hz). C (C
6
D
6
, d ppm): 117.9, 2.9, 0.6. Si (C
6
6
D ,
1
d ppm): -10.0, -14.8, -125.3 (q, J29Si-11B = 52 Hz). IR (Nujol) n:
-1
2
233 (vs) cm .
˚
graphite-monochromated Mo-Ka radiation (0.71073 A). The
2
data were reduced to F
o
and corrected for absorption effects
Crystallographic data†
29
30
with SAINT and SADABS, respectively. The structures were
solved by direct methods and refined by full-matrix least-squares
method (SHELXL97). All non-hydrogen atoms were refined
with anisotropic displacement parameters. All hydrogen atoms
which could not be detected were located at calculated positions
to correspond to standard bond lengths and angles.
C
52
H
152
B
4
K
4
O
4
Si16 1: orthorhombic, Pbca (no. 61), a = 14.291(3),
31
3
˚
˚
b = 26.470(5), c = 51.562(10) A, V = 19505(7) A , Z = 8, D
c
=
-
3
-1
˚
1.015 g cm , m = 0.411 mm (Mo-Ka, l = 0.71073 A), T =
100 K; R = 0.2082 and 0.2212) for
= 0.1045 and 0.1301 (wR
17 161 unique measured reflections. CCDC 767902.
1
2
Ortep plots of structures 1, 2, 3 and 4 at the 30% probability
C
18
H
58
B
2
Si
8
2: monoclinic, P2
1
/n (no. 14), a = 9.2013(18), b =
32
◦
3
˚
˚
level were generated using Ortep-3 for Windows (Version 2.02)
12.502(3), c = 15.972(3) A, b = 99.60(3) , V = 1811.6(6) A , Z =
33
-3
-1
˚
and POV-ray for Windows 3.6.
2, D
c
= 0.955 g cm , m = 0.302 mm (Mo-Ka, l = 0.71073 A),
Dalton Trans., 2010, 39, 9270–9274 | 9273
This journal is © The Royal Society of Chemistry 2010

T = 100 K; R
1
= 0.0957 and 0.2020 (wR
2
= 0.1492 and 0.1839)
11 For a review concerning some recent chemistry with oligosilanylpotas-
sium compounds see: C. Marschner, Organometallics, 2006, 25, 2110.
for 3179 unique measured reflections. CCDC 767904.
1
1
2 C. Marschner, Eur. J. Inorg. Chem., 1998, 221.
3 U. Welling, P. Paetzold and U. Englert, Inorg. Chim. Acta, 1995, 231,
175; F.-C. Liu, K.-Y. Chen, J.-H. Chen, G.-H. Lee and S.-M. Peng,
Inorg. Chem., 2003, 42, 1758; F-C. Liu, S.-C. Chen, G.-H. Lee and S.-
M. Peng, J. Organomet. Chem., 2007, 692, 2375; F.-C. Liu, J.-H. Chen,
S.-C. Chen, K.-Y. Chen, G.-H. Lee and S.-M. Peng, J. Organomet.
Chem., 2005, 690, 291.
Acknowledgements
This study was supported by the Austrian Fonds zur F o¨ rderung der
Wissenschaften (FWF) via the project P-21346 (J. B.).
1
4 V. V. Burlakov, U. Rosenthal, P. V. Petrovskii, V. B Shur and M. E.
Vol’pin, Organomet. Chem. USSR, 1988, 1, 526.
Notes and references
1
1
1
5 H. C. Brown and T. P. Stocky, J. Am. Chem. Soc., 1977, 99, 8218.
6 B. Singaram, T. E. Cole and H. C. Brown, Oganometallics, 1984, 3, 774.
7 H. C. Brown, B. Nazer, J. S. Cha and J. A. Sikorski, J. Org. Chem.,
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274 | Dalton Trans., 2010, 39, 9270–9274
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