Silaborates with an Unprecedented Cluster Geometry

Article abstract of DOI:10.1021/om9903994

The reaction of o-silaborane with a series of carbanions is presented. Adducts of o-silaborane were prepared in reaction with Grignard reagents (RMgBr R = Me, Ph, benzyl, allyl, vinyl, and ethinyl) (3a-f). The salts were isolated in high yield and charact

Full text of DOI:10.1021/om9903994

4654
Organometallics 1999, 18, 4654-4659
Sila bor a tes w ith a n Un p r eced en ted Clu ster Geom etr y
Lars Wesemann,*,§ Michael Trinkaus,† Ulli Englert,† and J ens Mu¨ller†
Institut fu¨r Anorganische Chemie, Universita¨t zu Ko¨ln, Greinstrasse 6,
D-50939 Ko¨ln, Germany, and Institut fu¨r Anorganische Chemie, Rheinisch Westfa¨lische
Technische Hochschule Aachen, Prof.-Pirlet-Strasse 1, D-52056 Aachen, Germany
Received May 25, 1999
The reaction of o-silaborane with a series of carbanions is presented. Adducts of o-silaborane
were prepared in reaction with Grignard reagents (RMgBr R ) Me, Ph, benzyl, allyl, vinyl,
and ethinyl) (3a -f). The salts were isolated in high yield and characterized by elemental
analyses and NMR spectroscopy. In the case of the benzylide adduct the structure in the
solid state was determined by X-ray single-crystal structure analysis. Ab initio calculations
were carried out on the methylide derivative [Me3Si2B10H10]-, and results of the geometry
optimizations together with calculated NMR chemical shifts are discussed. The first
substitution reaction at a silicon vertex of the o-silaborane skeleton is presented with the
synthesis of [MeSiPhSiB10H10], which is characterized by X-ray crystal structure analysis.
In tr od u ction
The geometry of a polyborane or heteroborane cluster
is usually elucidated with respect to Williams1 geo-
metrical systematics and the skeletal electron counting
rules invented by Wade2 and elaborated by Mingos.3 By
counting the cluster with respect to the number of
electrons and skeleton atoms, the geometry of a cluster
can be predicted. Following these systematics, an in-
crease in the number of cluster electrons is tantamount
to a cluster-opening reaction. Thus, the reduction of the
most prominent heteroborane, o-carborane [C2B10H12],
results in a dramatic change of the icosahedral geom-
-
etry.4 The reduction product [C2B10H13
]
exhibits a
F igu r e 1. closo and nido cluster geometries for 12 and 13
geometry that is derived from the deltahedral shape of
the 13-vertex closo cluster by removal of one vertex
(geometry A in Figure 1).5 Another cluster-opening
reaction of an icosahedral cluster skeleton is known for
aza-closo-dodecaborane(12). The icosahedral azaborane
[RNB11H11] reacts with a variety of nucleophiles to yield
adducts comprising a 12-vertex nido cluster geometry
(geometry B in Figure 1).6 Both geometries A and B
were described by Williams as primary fragments
resulting from removal of one vertex from the 13-vertex
deltahedron.1
vertices.
cially in the case of the tetracarborane R4C4B8H8
a
variety of cluster geometries were determined either by
crystal structure analysis or by NMR spectroscopical
investigations.7 A relationship between these geometries
and the primary fragments was shown by Williams after
breaking and reintroduction of various edge connec-
tions.1
Another unexpected cluster geometry is realized in
the case of the dialkylamide adducts of 1,2-dimethyl-
1,2-disila-closo-dodecaborane(12)8 (o-silaborane) (anion
[Et2N(MeSi)2B10H10]-, 2, in Figure 2).9 Whereas Wade’s
cluster rules predict an arachno cluster, the closo
geometry of o-silaborane remains almost unchanged
after adduct formation. Ab initio calculations of o-
silaborane and the amide adduct gave further insight
Twelve-vertex nido clusters with other than the
primary fragment geometries are very common. Espe-
* Corresponding author. Fax: + 49 221 470 5083. E-mail: lars.
wesemann@uni-koeln.de.
§ Universita¨t zu Ko¨ln.
† Rheinisch Westfa¨lische Technische Hochschule Aachen.
(1) Williams, R. E. In Electron Defecient Boron and Carbon Clusters;
Olah, G. A., Wade, K., Williams, R. E., Eds.; Wiley: New York, 1990;
pp 11-93.
(7) (a) Grimes, R. N. Adv. Inorg. Chem. Radiochem. 1983, 26, 55-
117. (b) Grimes, R. N. Coord. Chem. Rev. 1995, 143, 71-96.
(8) (a) Seyferth, D.; Bu¨chner, K.; Rees, W. S., J r.; Davis, W. M.
Angew. Chem., Int. Ed. Engl. 1990, 29, 918-920. (b) Seyferth, D.;
Bu¨chner, K.; Rees, W. S., J r.; Wesemann, L.; Davis, W. M.; Bukalov,
S. S.; Leites, L. A.; Bock, H.; Solouki, B. J . Am. Chem. Soc. 1993, 115,
3586-3594.
(9) (a) Wesemann, L.; Trinkaus, M.; Ramjoie, Y.; Ganter, B.; Mu¨ller,
J . Angew. Chem., Int. Ed. Engl. 1998, 37, 1412-1415. (b) Wesemann,
L.; Ramjoie, Y.; Trinkaus, M.; Ganter, B. Z. Allg. Anorg. Chem. 1998,
624, 1573-1576.
(2) Wade, K. J . Chem. Soc., Chem Commun. 1971, 792. Wade, K.
Adv. Inorg. Chem. Radiochem. 1976, 18, 1-66.
(3) Mingos, D. M. P.; Wales, D. J . Introduction to Cluster Chemistry;
Prentice Hall: London, 1990.
(4) Getman, T. D.; Knobler, C. B.; Hawthorne, M. F. Inorg. Chem.
1990, 29, 158-160.
(5) Venable, T. L.; Maynard, R. B.; Grimes, R. N. J . Am. Chem. Soc.
1984, 106, 6187-6193.
(6) Meyer, F.; Mu¨ller, J .; Paetzold, P.; Boese, R. Angew. Chem., Int.
Ed. Engl. 1992, 31, 1227-1228.
10.1021/om9903994 CCC: $18.00 © 1999 American Chemical Society
Publication on Web 10/07/1999

Silaborates with an Unprecedented Cluster Geometry
Organometallics, Vol. 18, No. 22, 1999 4655
X-r a y Str u ctu r a l An a lysis of th e Ben zylid e Ad -
d u ct. To further elucidate the cluster geometry of the
carbanion adducts, an X-ray structure analysis was
carried out in the case of the benzylide adduct 3c.
Crystal, data collection, and refinement parameters are
given in Table 2. Atomic coordinates and equivalent
isotropic displacement parameters are given in the
Supporting Information. The molecular structure of the
silaborate 3c is depicted in Figure 4.
Selected interatomic distances are compiled in Table
1. On the basis of the interatomic distances between Si2
and the boron atoms the incorporation of Si2 in the
cluster skeleton remains nearly unchanged with respect
to o-silaborane. As expected, for the attacked cluster
vertex Si1 the Si1-B distances exhibit a substantial
geometrical change. Surprisingly the distances toward
B3 and B6 are symmetrically elongated, resulting in the
formation of two quadrangles in the cluster skeleton.
This skeleton can be compared with that of the R4C4B8H8
cluster geometries determined by crystal structure
analysis.10 As in 3c, the carborane skeleton features
(Figure 5) two neighboring quadrangles.
Adducts of the closo clusters MC2R2B4H4 and
MC2R2B9H9 (M ) RGa, RAl, Ge, Sn) exhibit opened
cluster geometries that were described as highly slip
distorted with respect to the central atom.11 Interest-
ingly the commo-cluster Si(C2B9H11)2 reacts with pyri-
dine under almost complete removal of the silicon atom
from the cluster framework.12 Obviously the nido 12-
vertex compounds can adopt a variety of structures.
F igu r e 2. Diethylamide adduct of o-silaborane.
into the acceptor abilities of the icosahedral silaborane
and proved the LUMO to be essentially concentrated
on the two silicon centers.
The interesting structure of the amide adduct
prompted us to study carbanions as another type of
nucleophile in this reaction.
Resu lts a n d Discu ssion
The carbanion adducts of o-silaborane (1) can be
obtained straightforwardly in high yield from the reac-
tion with the respective Grignard reagent (Scheme 2).
Crown ether (12-C-4) was added in stoichiometric
amounts to a thf solution of the magnesium salts, and
crystalline substances result from slow diffusion of a less
polar solvent into these solutions. The very moisture-
sensitive colorless crystalline adducts were character-
ized by heteronuclear and two-dimensional NMR spec-
troscopy as well as by elemental analyses. However, due
to the high sensitivity toward small amounts of mois-
ture, the results of the elemental analyses for 3a and
3d show relatively large deviations from theory. In the
case of the benzylide adduct the structure in the solid
state was determined by X-ray single-crystal structure
analysis.
NMR Sp ectr oscop ica l In vestiga tion s. The num-
ber of 11B NMR resonances for the cluster skeleton is a
good indicator of the symmetry of the anionic adducts
in solution. Six signals with an intensity ratio of 2:2:2:
2:1:1 for the methylide adduct 3a give evidence for Cs
symmetry of these species in solution. For the other also
Cs symmetrical clusters 3b-f only five signals are
visible. The resonances for B9 and B12 are accidentally
isochronic (Figure 3). Due to the different chemical
environments for the MeSi groups, two resonances were
detected in the 1H NMR as well as in the 29Si NMR
spectra.
In the special case of o-silaborane two different
nucleophiles react under the formation of adducts with
different cluster geometries. In the case of the sym-
metrically bridged dialkylamide almost no change of the
cluster geometry is detectable (Table 1), and for car-
banion adducts an opening of the skeleton takes place.
Ab In itio Ca lcu la tion . To achieve a better under-
standing of the chemistry of the silaboranes, we con-
ducted ab initio calculations13 on the methylide adduct
[Me3Si2B10H10]-. The results of the geometry optimiza-
tion are compiled in Table 3, and the calculated NMR
chemical shifts are listed in Table 4. Due to the lack of
an X-ray crystal structure analysis of the methylide
adduct, we compare the results of the geometry opti-
mization with the structure of the benzylide derivative.
An indicator of the good correspondence between ex-
periment and theory is the largest variation between
the geometrical data of only 0.043 Å (Si1-B4). The NMR
chemical shifts were calculated with the GIAO method14
at the B3LYP/6-311+G(d, p) level15 of theory based on
the B3LYP/6-31G(d) geometries.16 In contrast to the
relatively small deviations between the calculated and
(10) (a) Freyberg, D. P.; Weiss, R.; Sinn, E.; Grimes, R. N. Inorg.
Chem. 1977, 16, 1847-1851. (b) Grimes, R. N.; Maxwell, W. M.;
Maynard, R. B.; Sinn, E. Inorg. Chem. 1980, 19, 2981-2985.
(11) Saxena, A. K.; Maguire, J . A.; Hosmane, N. S. Chem. Rev. 1997,
97, 2421-2461.
In contrast to the Cs symmetrical carbanion adducts,
the amide adduct shows C2v symmetry in solution
(Figure 2). For the anions of type 2 one resonance for
(12) Schubert, D. M.; Rees, W. S., J r.; Knobler, C. B.; Hawthorne,
M. F. Organometallics 1990, 9, 2938-2944.
1
the MeSi groups in the H NMR as well as in the 29Si
(13) See also: J emmis, E. D.; Kiran, B. J . Am.Chem. Soc. 1997, 119,
4076-4077.
NMR and only four signals in the 11B NMR spectrum
for 10 boron atoms were detected. Obviously the number
of electron pairs at the donor atom of the attacking
nucleophile determines the symmetry and structure of
the adduct.
(14) For application in borane chemistry see: Yang, X. Y.; J iao, H.;
Schleyer, P. v. R. Inorg. Chem. 1997, 36, 4897-4899, and references
therein.
(15) (a) Becke, A. D. Phys. Rev. 1988, A38, 3098-3100. (b) Becke,
A. D. J . Chem. Phys. 1993, 98, 5648-5652. (c) Lee, C.; Yong, W.; Parr,
R. G. Phys. Rev. 1988, B37, 785-789.

4656 Organometallics, Vol. 18, No. 22, 1999
Wesemann et al.
Sch em e 1
Ta ble 2. Cr ysta l Da ta , Da ta Collection
P a r a m eter s, a n d Con ver gen ce Resu lts for 3c a n d 5
3c
5
formula
fw
system
space group
a, Å
C
42H94B20MgO10Si4
C7H18B10Si2
266.5
1112.1
monoclinic
C2/c (No. 15)
16.122(6)
16.968(6)
23.43(1)
106.55(4)
6144(1)
4
1.202
1.51
26.0
203
monoclinic
P21/n (No. 14)
7.356(1)
23.692(3)
8.942(2)
97.10(1)
1546.4(4)
4
1.146
18.33
70
293
b, Å
c, Å
â, deg
U, Å3
Z
d
calc, g cm-3
µ, cm-1
max, deg
θ
temperature, K
λ, Å,
0.71073
0.6 × 0.4 × 0.15
6421
1.54184
0.6 × 0.2 × 0.2
4156
crystal dimens, mm3
no reflns
indep obs reflns
I > nσ(I)
3872
(n ) 1.5)
337
0.091
0.084
1.770
0.53
1859
(n ) 1)
203
0.093
0.078
1.281
0.42
no vars
Ra
F igu r e 3. 11B{1H} NMR spectra of the carbanion adducts
of o-silaborane.
b
Rw
GOFc
res el dens, e Å-3
Ta ble 1. Selected In ter a tom ic Dista n ces in
o-Sila bor a n e a n d th e Ad d u cts of o-Sila bor a n e ([Å],
Sta n d a r d Devia tion s in P a r en th eses)
a
b
2
R ) ∑||Fo| - |Fc||/∑|Fo|. Rw ) [∑w(|Fo| - |Fc|)2/∑w|Fo| ]1/2. w-1
) σ2(Fo). c GOF ) [∑w(|Fo| - |Fc|)2/nobs - nvar
observations, nvar no. of variables refined.
]
1/2, nobs no. of
be understood as an interaction of the incoming anions
with the LUMO, which is mainly concentrated on the
silicon atoms.
Su bstitu tion Rea ction a t Dip h en yl-o-sila bor a n e.
The substitution of an alkyl or phenyl group at a silicon
atom in a common organosilicon compound is carried
out by electrophilic catalysis. So far all standard pro-
cedures for the substitution of the organic substituent
at the silaborane clusters [Me2Si2B10H10, Ph2Si2B10H10
]
failed. Nevertheless in this publication we present the
first example of the successful substitution of a phenyl
group in place of a methyl group at the 1,2-diphenyl-
1,2-disila-closo-dodecaborane(12) cluster. We took ad-
vantage of the carbanion adduct formation of diphenyl-
disilaborane with methylgrignard (MeMgBr). In a one-
pot procedure the methylide adduct of the silaborane 4
was formed by reaction with a small excess of methyl-
magesium bromide. Without purification the silaborate
-
[(MePhSi)(PhSi)B10H10
]
was treated with the trityl
(16) All calculations were conducted with Gaussian 94: Frisch, M.
J .; Trucks, G. W.; Schlegel, H. B.; Gill, P. M. W.; J ohnson, P. G.; Robb,
M. A.; Cheeseman, J . R.; Keith, T.; Petersson, G. A.; Montgomery, J .
A.; Raghavachari, K.; Al-Laham, M. A.; Zakrzewski, V. G.; Ortiz, J .
V.; Foresman, J . B.; Cioslowski, J .; Stefanov, B. B.; Nanayakkara, A.;
Challacombe, M.; Peng, C. Y.; Ayala, P. Y.; Chen, W.; Wong, M. W.;
Andres, J . L.; Replogle, E. S.; Gomperts, R.; Martin, R. L.; Fox, D. J .;
Binkley, J . S.; Defrees, D. J .; Baker, J .; Stewart, J . P.; Head-Gordon,
M.; Gonzalez, C.; Pople, J . A.. Gaussian 94; Gaussian, Inc.: Pittsburgh,
PA, 1995.
measured 11B NMR chemical shifts the values for the
silicon nucleus show only the same tendency but not a
satisfactory correspondence.
So far adducts of o-silaborane were isolated in the case
of dialkylamides and carbanions (Table 1). In both cases
the attack of the nucleophile at the silicon vertices can

Silaborates with an Unprecedented Cluster Geometry
Organometallics, Vol. 18, No. 22, 1999 4657
F igu r e 4. Structure of the anion of 3c in the crystal
(PLATON representation,17 ellipsoids at 30% probability
level). Selected angles [deg], standard deviations in paren-
theses: Si2-Si1-C1 123.4(2), Si2-Si1-C3 109.9(1), C1-
Si1-C3 95.8(2), Si1-Si2-C2 127.9(2), Si1-Si2-B3 64.1(1),
Si1-Si2-B6 63.5(2).
F igu r e 6. Structure of 5 in the crystal (PLATON repre-
sentation,17 ellipsoids at 30% probability level). Selected
interatomic distances [Å] and angles [deg], standard devia-
tions in parentheses: Si1-Si2 2.304(2), Si1-C1 1.828(5),
Si2-C2 1.842(6), Si1-B3 2.108(7), Si1-B4 2.019(7), Si1-
B5 2.019(7), Si1-B6 2.099(7), Si2-B3 2.105(6), Si2-B6
2.125(7), Si2-B7 2.026(8), Si2-B11 2.015(7), Si2-Si1-C1
121.9(2), Si1-Si2-C2 126.4(2), Si2-Si1-B3 56.8(2), Si2-
Si1-B6 57.5(2).
F igu r e 5. Top view on the Me4C4B8H8 cluster and car-
banion adduct of o-silaborane.
Sch em e 2
Ta ble 3. Selected In ter a tom ic Dista n ces [Å] of th e
An ion of 3a Ca lcu la ted a t B3LYP /6-31G(d ) a n d
HF /6-31G(d ) (in p a r en th eses)
Si1-Si2
Si1-B3
Si1-B6
Si2-B3
Si2-B6
Si1-B4
Si1-B5
2.365 (2.362)
2.398 (2.397)
2.409 (2.397)
2.097 (2.107)
2.093 (2.107)
2.148 (2.154)
2.147 (2.154)
Ta ble 4. Ca lcu la ted (B3LYP -GIAO/6-311+G(d ,p )//
B3LYP /6-31G(d ))8-10 a n d Obser ved NMR Ch em ica l
-
Sh ifts [p p m ] of [Me3Si2B10H10
]
An ion of 3a
calcd
exptl
Si1
Si2
-105.4
-73.6
-85.3
-61.2
-8.3
-16.0
-18.5
-11.4
-21.2
-19.4
Ta ble 5. 11B NMR Ch em ica l Sh ifts of o-Sila bor a n es
B3/B6
B4/B5
B7/B11
B8/B10
B9
-5.9/-6.0
1
4
5
-18.3/-18.3
-20.3/-19.3
-12.8/-13.1
-22.1
B3/6
B4/5/7/11
B8/10
-13.9
-14.8
-11.5
-12.9
-14.7
-14.7
-11.8
-12.8
-14.1
-14.8
-11.5
-12.9
B9/12
B12
-19.2
ment parameters are given in Table 2. Atomic coordi-
nates and equivalents isotropic displacement coefficients
are given in the Supporting Information. The geometri-
cal data of the icosahedral cluster reveal almost no
difference in comparison to the structures of 18 and 4.9b
This nucleophilic substitution runs presumably via a
bimolecular two-step mechanism. The carbanion adduct
of the silaborane is the intermediate in this reaction.
In summary, we have demonstrated that o-silaborane
forms adducts with carbanions. In the case of diphenyl-
o-silaborane we were able to use the adduct formation
in order to substitute a phenyl group by a methyl group.
electrophile [Ph3C][BF4] (Scheme 1). Work up by sub-
limation resulted in the isolation of a new silaborane 5
in 35% yield.
The new silaborane was characterized by NMR spec-
troscopy, mass spectrometry, and X-ray single-crystal
structure analysis. Six signals were expected to appear
in the 11B NMR spectrum for the Cs symmetrical closo-
borane 5. Due to the small substituent effect on the
boron atoms of the cluster cage, the 11B NMR spectrum
indicates with four resonances a higher symmetry
(Table 5). The molecular structure of the silaborane is
depicted in Figure 6. Crystal, data collection, and refine-

4658 Organometallics, Vol. 18, No. 22, 1999
Wesemann et al.
1
or 7/11), -19.2 (d, J ) 134, B9 and B12). 13C{1H} NMR (thf-
d8, 125 MHz, TMS): -9.2 (CH3Si1), 1.4 (CH3Si2), 67.0 (12-K-
4), 126.0 (p- Ph), 127.4 (o- Ph), 131.8 (m- Ph), 157.0 (i-Ph).
29Si NMR (thf-d8, 100 MHz): -90.0 (Si1), -57.7 (Si2). C, H
([%], calcd in parenth): C 40.36 (40.90), H 8.03 (7.94).
Exp er im en ta l Section
Gen er a l Meth od s. All manipulations were carried out
under an atmosphere of nitrogen using standard Schlenk
techniques. Solvents were distilled from sodium and benzophe-
none (THF, diethyl ether) or from potassium (hexane) and
stored under nitrogen. Elemental analyses were done by
Mikroanalytisches Labor Pascher in Remagen, Analytischen
Laboratorien in Lindlar. Mass spectrometry was done with a
Varian MAT-CH-%, EI 70 eV. Due to very high sensitivity
toward moisture in the case of 3a and 3d , insufficient C,H
analyses were obtained.
P r ep a r a tion of [Mg(12-C-4)2][B10H10Si2Me2(CH2P h )]2‚
2th f (3c). To a solution of 354.1 mg (1.73 mmol) of o-silaborane
in 20 mL of thf was added 1.90 mL of a solution of benzyl-
magnesium bromide in Et2O (1 M, 1.10 equiv). The solution
yellowed and clouded instantly. After 1 h 55 µL of Me3SiCl
(1/4 equiv) was added. After 1 h 1.8 mL of a 1 M solution of
12-crown-4 in thf was added. The same procedure as for 3a
gave yellowish crystals that were recrystallized to gain fine
colorless needles that showed poor solubility even in thf. 4c
crystallizes with one molecule of thf per asymmetric unit. Yield
Ab In itio Ca lcu la tion s. The Gaussian 94 package,16 run
on a cluster of workstations (Rechenzentrum der RWTH
Aachen), was applied for all ab initio calculations. The
-
geometries for [Me3Si2B10H10
]
(anion of 3a ) were optimized
(before recryst): 0.781 g (M ) 1112.07 g mol-1, 81%). H{11B}
1
at the HF/6-31G(d) and B3LYP/6-31G(d) levels of theory; the
calculation of the vibrational frequencies at the HF/6-31G(d)
level showed no imaginary frequency (ZPVE ) 703.97 kJ /mol;
the total energies in hartrees are -949.426462 (HF) and
-953.501904 (B3LYP)). The 11B and 29Si NMR shifts are
reported relative to BF3‚OEt2 and TMS, respectively. It is
inconvenient to compute the magnetic shielding of BF3‚OEt2;
therefore B2H6 was calculated instead and set to 16.6 ppm.18
The absolute chemical shieldings at the B3LYP-GIAO/6-
311+G(d, p)//B3LYP/6-31G(d) level are 84.11 ppm (11B of B2H6)
and 338.80 ppm (29Si of TMS).
NMR (thf-d8, 500 MHz, TMS, J (Hz)): 0.28 (s, 3H, Si2CH3),
0.67 (s, 3H, Si1CH3), 0.79 (s, 1H, H9 or H12), 1.21 (s, 2H, H4/5
or H7/11), 1.45 (s, 2H, H4/5 or H7/11), 1.69 (s, 2H, H8/10), 1.77
(mult, thf), 1.93 (s, 2H, H3/6), 3.60 (mult, thf), 4.07 (mult, 16H,
12-K-4), 6.92 (trtr, 3J ) 7.3, 1H, p- Ph), 7.11 (mult, 2H, m-
Ph), 7.21 (mult, 2H, o- Ph). 11B NMR (thf-d8, 160 MHz, Et2O‚
BF3, J (Hz)): -7.0 (d, J ) 134, B3/6), -10.8 (d, J ) 134, B8/
10), -15.7 (d, 1J ) 134, B4/5 or 7/11), -18.3 (d, 1J ) 134, B4/5
or 7/11), -21.1 (d, 1J ) 134, B9 and 12). 13C{1H} NMR (thf-d8,
125 MHz, TMS): -11.3 (MeSi1), 10.0 (MeSi2), 23.9 (thf), 51.4
(CH2Ph), 66.0 (thf), 67.0 (12-K-4), 127.2, 127.9 (p-Ph, o-Ph),
131.8 (m-Ph), 143.4 (i-Ph). 29Si NMR (thf-d8, 100 MHz): -90.2
(Si1), -64.0 (Si2). C, H ([%], calcd in parenth): C 44.70 (45.36),
H 8.60 (8.52).
1
1
P r ep a r a tion of [Mg(12-C-4)2][B10H10Si2Me3]2 (3a ). A 0.7
mL sample of a solution of methylmagnesium chloride in thf
(3 M, thf, 1.07 equiv) was added to a solution of o-silaborane
(431.2 mg, 2.11 mmol) in 25 mL of the same solvent at room
temperature. The reaction mixture yellowed and thickened
instantly. It was stirred for 1 h. Then 60 µL of Me3SiCl (1/4
equiv) was added in order to dispose of any excess of the
Grignard reagent, the mixture was stirred for another 15 min,
and 2.0 mL of a 1 M solution of 12-crown-4 in thf was added.
Fine colorless crystal needles and leafs formed upon filtration
and slow diffusion of hexane into the solution at 4 °C and were
recrystallized from thf/Et2O for removal of remaining magne-
P r ep a r a tion of [Mg(12-C-4)2][B10H10Si2Me2C3H5]2‚th f
(3d ). To a solution of 214.7 mg (1.05 mmol) of o-silaborane in
20 mL of thf was added 1.05 mL of a solution of allylmagne-
sium bromide in Et2O (1 M, 1,1 equiv). The solution yellowed
instantly and clouded after 15 min. One hour later 1.8 mL of
a 1 M solution of 12-crown-4 in thf was added. The same
procedure as for 3a gave slightly yellowish crystals in good
yield that contained 1 equiv of thf. Yield: 0.436 g (M ) 939.85
1
g mol-1, 88%). H{11B} NMR (thf-d8, 500 MHz, TMS, J (Hz)):
1
sium chloride. Yield: 0,765 g (M ) 815.67 g mol-1, 89%). H-
0.56 (s, 3H, SiCH3), 0.71 (s, 3H, SiCH3), 1.28 (s, 1H, H9 or
H12), 1.43 (s, 2H, H4/5 or H7/11 a. H9 or 12), 1.76 (s, 2H, 8/10),
1.76 (mult, thf), 1.84 (s, 2H, H4/5 or H7/11), 1.95 (d, 3J ) 8.52,
2H, CH2CHCH2), 3.61 (mult, thf), 2.02 (s, 2H, H3/6), 4.07
{11B} NMR (thf-d8, 500 MHz, TMS, J (Hz)): 0.56 (s, 3H, SiCH3),
0.60 (s, 6H, SiCH3), 0.87 (s, 1H, H9), 1.27 (s, 2H, H7/11), 1.40
(s, 1H, H12), 1.42 (s, 2H, H4/5), 1.75 (s, 2H, H8/10), 1.92 (s,
2H, H3/6), 4.07 (mult, 16H, 12-K-4). 11B NMR (thf-d8, 160 MHz,
3
2
4
(mult, 16H, 12-K-4), 4.69 (ddtr, J ) 10.1, J ) 2.8, J ≈ 0.9,
1
1
Et2O‚BF3, J (Hz) -8.3 (d, J ) 134, B3/6), -11.4 (d, J ) 147,
B8/10), -16.0 (d, 1J ) 134, B4/5), -18.5 (d, 1J ) 140, 7/11),
-19.4 (d, 1J ) 140, B12), -21.2 (d, 1J ) 134, B9). 3C{1H} NMR
(thf-d8, 125 MHz): -10.0 (MeSi), 1.1 (MeSi), 25.1 (thf), 66.9
(12-K-4), 67.2 (thf). 29Si NMR (thf-d8, 100 MHz): -61.2 (Si2),
-85.3 (Si1). C, H ([%], calcd in parenth): C 31.18 (32.40), H
8.65 (8.65).
3
2
4
1H, SiCH2CHCH2 (Z)), 4.79 (ddtr, J ) 17.05, J )2.78, J )
3
3
1.39, 1H, SiCH2CHCH2 (E)), 6.12 (ddtr, J (Z) ) 10.11, J (E)
)
17.05, 3J ) 8.52, 1H, SiCH2CHCH2). 11B NMR (thf-d8, 160
1
MHz, Et2O‚BF3, J (Hz)): -5.8 (d, J ) 134 Hz, B3/6), -9.3 (d,
1J ) 135, B8/10), -14.1 (d, J ) 134, B4/5 or 7/11), -16.6 (d,
1
1J ) 140, B4/5 or 7/11 and B9 or B12), -19.3 (d, J ) 134, B9
1
or 12). 13C{1H} NMR (thf-d8, 125 MHz): -9.8 (MeSi1), -21.6
(MeSi2), 51.3 (SiCH2CHCH2), 67.0 (12-K-4), 110.4 (SiCH2-
CHCH2), 139.3 (SiCH2CHCH2). 29Si NMR (thf-d8, 100 MHz):
-62.5 (Si2), -90.6 (Si1). C, H ([%], calcd in parenth): C 39.35
(38.34), H 8.14 (8.79).
P r ep a r a tion of [Mg(12-C-4)2][B10H10Si2Me2P h ]2 (3b). To
a solution of 221.2 mg (1.08 mmol) of o-silaborane in 20 mL of
thf was added 370 µL of a solution of phenylmagnesium
bromide in Et2O (3 M, 1.03 equiv). The solution yellowed
instantly and clouded after 15 min. After 1 h 35 µL of Me3-
SiBr (1/4 equiv) and again 15 min later 1.2 mL of a 1 M
solution of 12-crown-4 in thf were added. The same procedure
as for 3a gave beautiful colorless crystals in excellent yield.
P r ep a r a tion of [Mg(12-C-4)2][B10H10Si2Me2C2H3]2 (3e).
To a solution of 285.3 mg (1.40 mmol) of o-silaborane in 20
mL of thf was added 1.40 mL of a solution of vinylmagnesium
bromide in Et2O (1 M, 1.00 equiv). The solution yellowed and
clouded in the course of 15 min. After 1 h and subsequent
addition of 1.6 mL of a 1 M solution of 12-crown-4 in thf the
reaction mixture was filtrated. Slow diffusion of hexane and
refrigerating gave yellowish leaf-shaped crystals in acceptable
yield. Yield: 0.385 g (M ) 839.69 g mol-1, 66%). 1H{11B} NMR
(thf-d8, 500 MHz, TMS, J (Hz)): 0.54 (s, 3H, SiCH3), 0.79 (s,
3H, SiCH3), 0.93 (s, 1H, H9 or H12), 1.30(s, 3H, H4/5 or H7/
11 a. H9 or 12), 1.43 (s, 2H, H4/5 or H7/11), 1.78 (s, 2H, H8/
10), 1.77 (mult, thf), 2.00 (s, 2H, H3/6), 3.62 (mult, thf), 4.07
(mult, 16H, 12-K-4), 5.32 (dd, 3J ) 19.8, 2J ) 3.1, 1H, SiCHCH2
(Z)), 5.51 (dd, 3J ) 13.7, 2J ) 3.1, 1H, SiCHCH2 (E)), 6.09 (dd,
1
Yield: 0.462 g (M ) 939.81 g mol-1, 91%). H{11B} NMR (thf-
d8, 500 MHz, TMS, J (Hz)): 0.57 (s, 3H, Si2CH3), 0.78 (s, 3H,
Si1CH3), 1.20 (s, 1H, H9 or H12), 1,40 (s, 2H, H4/5 or H7/11),
1,43 (s, 1H, H9 or 12), 1.65 (s, 2H, H4/5 or H7/11), 1.71 (s, 2H,
H8/10), 1.76 (mult, thf), 1.88 (s, 2H, H3/6), 3.61 (mult, thf),
3.99 (mult, 16H, 12-K-4), 7.01 (trtr, 1H, p-Ph), 7.11 (mult, 2H,
o-Ph), 7.51 (mult, 2H, m-Ph). 11B NMR (thf-d8, 160 MHz, Et2O‚
1
1
BF3, J (Hz)): -8.6 (d, J ) 134, B3/6), -12.1 (d, J ) 135, B8/
10), -16.2 (d, 1J ) 134, B4/5 or 7/11), -17.5 (d, 1J ) 140, B4/5
(17) Spek, A. L. Acta Crystallogr. Sect. A 1990, 46, C34.
(18) Onak, T. P.; Landesman, H. L.; Williams, R. E.; Shapiro, I. J .
Phys. Chem. 1959, 63, 1533-1535.
3
3J (Z(H)) ) 13.7, J (E(H)) ) 19.8, 1H, SiCHCH2). 11B NMR (thf-d8,

Silaborates with an Unprecedented Cluster Geometry
Organometallics, Vol. 18, No. 22, 1999 4659
1
160 MHz, Et2O‚BF3, J (Hz)): -9.0 (d, J ) 134, B3/6), -11.4
X-r a y Str u ctu r e Deter m in a tion of 3c a n d 5. Geometry
and intensity data were collected on Enraf-Nonius CAD4
diffractometers equipped with graphite monochromators. A
summary of crystal data, data collection parameters, and
convergence results is compiled in Table 2. In the case of 5 an
empirical absorption correction based on azimuthal scans19 was
applied before averaging over symmetry equivalent reflections.
The structures were solved by direct methods20 and refined
on structure factors with the SDP program suite.21 The quality
of the structure determination of 3c suffers from disorder in
the crown ether moiety and highly anisotropic displacement
parameters in the cocrystallized thf molecule. In the refine-
ment of 3c the oxygen and carbon atoms of the crown ether
group were refined isotropically, remaining non-hydrogen
atoms were assigned anisotropic displacement parameters, and
hydrogen atoms were included as riding in calculated positions
with B-H ) 1.1 Å, C-H ) 0.98 Å, a common refined
displacement parameter for the hydrogen atoms bonded to
boron, and Uiso(H) ) 1.3Ueq(C) for the H atoms bonded to
carbon. In 5 all non-hydrogen atoms were refined with
anisotropic displacement parameters, and the positional pa-
rameters and a common isotropic displacement parameter
were refined for the hydrogen atoms bonded to boron; carbon-
bonded H atoms were treated as riding with C-H ) 0.98 Å,
1
1
(d, J ) 134, B8/10), -16.1 (d, J ) 122, B4/5 or 7/11), -18.4
1
1
(d, J ) 134, B4/5 or 7/11 and B9 or B12), -20.8 (d, J ) 147,
B9 or 12). 13C{1H} NMR (thf-d8, 125 MHz): -9.9 (CH3Si1), 20.5
(CH3Si2), 66.9 (12-K-4), -125.5 (SiCHCH2), 158.7 (SiCHCH2).
29Si NMR (thf-d8, 100 MHz): -59.1 (Si2), -95.4 (Si1). C, H
([%], calcd in parenth): C 34.54 (34.33), H 7.93 (8.40).
P r ep a r a tion of [Mg(12-C-4)2][B10H10Si2Me2(C2H)]2‚th f
(3f). To a solution of 334.3 mg (1.64 mmol) of o-silaborane in
20 mL of thf was added 3.50 mL of a solution of ethynylmag-
nesium bromide in the same solvent (0.5 M, 1.07 equiv). The
solution turned slightly pinkish and clouded instantly. After
1 h 22 µL of Me3SiBr (1/10 equiv) and 4.2 mL of a 0.8 M
solution of 12-crown-4 in thf were added. The same procedure
as for 3a gave colorless needles that lost thf upon evacuation.
Yield: 0.395 g (M ) 835.66 g mol-1, 58%). H{11B} NMR (thf-
1
d8, 500 MHz, TMS, J (Hz)): 0.68 (s, 3H, SiCH3), 0.87 (s, 3H,
SiCH3), 1.18 (s, 1H, H9 or H12), 1.34(s, 2H, H4/5 or H7/11),
1.45 (s, 1H, H9 or 12), 1.53 (s, 2H, H4/5 or H7/11), 1.71(s, 2H,
H8/10), 1.76 (mult, thf), 2.05 (s, 2H, H3/6), 2.36 (s, 1H, CCH),
3.61 (mult, thf), 4.07 (mult, 16H, 12-K-4). 11B NMR (thf-d8,
1
160 MHz, Et2O‚BF3, J (Hz)): -8.6 (d, J ) 134, B3/6), -11.9
1
1
(d, J ) 134, B8/10), -15.6 (d, J ) 122, B4/5 or 7/11), -17.6
(d, B4/5 or 7/11), -18.9 (d, B9 a. B12). 13C{1H} NMR (thf-d8,
125 MHz): -9.9 (CH3Si1), 1.2 (CH3Si2), 25.1 (thf), 66.9 (12-
K-4), 67.2 (thf), 70.3 (Si1CCH), 82.3 (Si1CCH). 29Si NMR (thf-
d8, 100 MHz): -52.2 (Si2), -112.3 (Si1). C, H ([%], calcd in
parenth): C 34,78 (34,50), H 7,96 (7,96).
U
iso(H) ) 1.3Ueq(C). Further details of the crystal structure
investigations are available on request from the Fachinfor-
mationszentrum Karslruhe, Gesellschaft fu¨r wissenschaftlich-
technische Information mbh, 76344 Eggenstein-Leopoldshafen,
on quoting the depository numbers CCDC-133530 (3c) and
CSD-133529 (for 5).
P r epar ation of 1-Meth yl-2-ph en yl-1,2-disila-closo-dode-
ca bor a n e(12) (5). To a solution of 1.1469 g (3.49 mmol) of
1,2-diphenyl-1,2-disila-closo-dodecaborane(12) in 35 mL of thf
was added 1.25 mL of a solution of methylmagnesium bromide
in Et2O (3 M, 1.07 equiv). The solution yellowed slightly. After
1 h a freshly prepared suspension of 1.3260 g (4.02 mmol, 1.15
equiv) of Ph3CBF4 (4.02 mmol, 1.15 equiv) in thf was added.
The trityl salt dissolved instantly, while the solution turned
deep yellow. After 35 min the solvent was removed under
reduced pressure, and the oily residue was sublimated over-
night at 160 °C/10-3 mbar. The product was dissolved in very
little diethyl ether and cooled to -30 °C. Beautiful colorless
needles resulted that were suitable for X-ray structure analy-
Ack n ow led gm en t. We wish to acknowledge the
Deutsche Forschungsgemeinschaft Schwerpunktpro-
gramm Polyeder for financial support. We are grateful
to the Rechenzentrum der RWTH Aachen for providing
generous computer time. In particular, we thank Th.
Eifert and J . Risch for their support.
Su p p or tin g In for m a tion Ava ila ble: Listings of all frac-
tional atomic coordinates and equivalent isotropic displace-
ment parameters, interatomic distances and angles, and
anisotropic displacement parameters. Listings of the Cartesian
coordinates of the calculated geometry for the anion of 3a . This
material is available free of charge via the Internet at
http://pubs.acs.org.
sis. Yield: 0.330 g (35%, M ) 266.50 g mol-1). H{11B} NMR
1
(acetone-d6, 500 MHz, TMS, J (Hz)): 1.47 (s, 3H, SiCH3), 1.93
(s, 2H, H8/10), 2.04 (s, 2H, H4/5/7/11), 2.11(s, 2H, H9/12), 2.19
(s, 2H, H3/6), 7.66 (mult, 2H, m-Ph), 7.81 (mult, 1H, p-Ph),
7.98 (mult, 2H, o-Ph). 11B NMR (acetone-d6, 160 MHz, Et2O‚
BF3, J (Hz)): -11.5 (d, B8/10), -12.9 (d, B9/12), -14.1 (d, B3/
6), -14.8 (d, B4/5/7/11). 13C{1H} NMR (acetone-d6, 125 MHz):
-10.9 (CH3MeSi), 130.6 (o- or m-Ph), 135.0 (p-Ph), 137.7 (o-
or m-Ph), i-Ph was not observed. 29Si NMR (acetone-d6, 100
OM9903994
(19) North, A. C. T.; Phillips, D. C.; Mathews, F. S. Acta Crystallogr.
Sect. A 1968, 24, 351.
(20) Sheldrick, G. M. SHELXS86, Program for Structure Solution;
University of Go¨ttingen, 1986.
1
11
28
MHz): -35.1 (MeSi), -42.9 (PhSi). HRMS for 12C7 H18
B
-
10
Si2: 268.188032 (calcd 268.187762).
(21) SDP Version 5.0; ENRAF-Nonius, 1989.