Synthesis, structure and spectroscopic properties of branched oligosilanes

Article abstract of DOI:10.1016/S0022-328X(00)00518-0

A number of various branched heptasilanes [(Me₃Si)₂MeSi]₂MeSiX (2: X = Ph; 3: X = Me; 6: X = Br; 7: X = I; 8: X = Cl; 9: X = F; 10: X = OH) bearing two (Me₃Si)₂MeSi groups as well as branched decasilanes [(Me₃Si)₂MeSi]₃Si-X (12: X = H; 13: X = Br; 14: X = Cl; 15: X = F; 16: X = OH) bearing three (Me3Si)₂MeSi groups at one silicon centre were synthesized. The structures of the compounds prepared were elucidated on the basis of comprehensive NMR and MS studies. Additionally, the molecular structures of the decasilanes 12 and 14-16 were obtained from X-ray diffraction data, which verify a ca. spherical shape with the core silicon atom at the centre of the sphere. The spatial demand of three (Me₃Si)₂MeSi groups forces a widening of the Si-Si-Si angles of the XSiSi₃ tetrahedra toward the group X (X = H, Cl, F, OH) and a remarkable elongation of the central Si-X bonds. The analysis of the Si-Si-Si-Si dihedral angles of the tetrasilane subunits in the decasilanes 12 and 14-16 indicates the existence of three conformations denoted as anti, ortho and gauche and four different arrangements of pentasilane subunits, denoted as anti-gauche, anti-ortho, ortho-gauche and ortho-ortho. The absorption spectra of the heptasilanes 2-10 exhibit broadened absorption maxima, shifted strongly to the red relative to those of the decasilanes 11-16. As a result of strongly limited conformational flexibility due to steric overcrowding, the singlet excitation in 11-16 is at much higher energy than in linear pentasilanes. A comparison of NMR chemical shifts of 2-16 with those of branched silanes of the type (Me₃Si)₃SiX and (Me₃Si)₂MeSiX reveals that the replacement of methyl groups by additional SiMe₃ groups in the β-position leads to a strong low-field shift of the signal of the central nucleus.

Full text of DOI:10.1016/S0022-328X(00)00518-0

www.elsevier.nl/locate/jorganchem
Journal of Organometallic Chemistry 613 (2000) 208–219
Synthesis, structure and spectroscopic properties of branched
oligosilanes
Silke Chtchian a, Rhett Kempe b, Clemens Krempner a,*
a Fachbereich Chemie der Uni6ersita¨t Rostock, Buchbinderstrasse 9, D-18051 Rostock, Germany
b Institut fu¨r Organische Katalyseforschung an der Uni6ersita¨t Rostock e. V., D-18055 Rostock, Germany
Received 3 April 2000; received in revised form 17 July 2000
Abstract
A number of various branched heptasilanes [(Me3Si)2MeSi]2MeSiX (2: X=Ph; 3: X=Me; 6: X=Br; 7: X=I; 8: X=Cl; 9:
X=F; 10: X=OH) bearing two (Me3Si)2MeSi groups as well as branched decasilanes [(Me3Si)2MeSi]3SiꢀX (12: X=H; 13:
X=Br; 14: X=Cl; 15: X=F; 16: X=OH) bearing three (Me3Si)2MeSi groups at one silicon centre were synthesized. The
structures of the compounds prepared were elucidated on the basis of comprehensive NMR and MS studies. Additionally, the
molecular structures of the decasilanes 12 and 14–16 were obtained from X-ray diffraction data, which verify a ca. spherical shape
with the core silicon atom at the centre of the sphere. The spatial demand of three (Me3Si)2MeSi groups forces a widening of the
SiꢀSiꢀSi angles of the XSiSi3 tetrahedra toward the group X (X=H, Cl, F, OH) and a remarkable elongation of the central SiꢀX
bonds. The analysis of the SiꢀSiꢀSiꢀSi dihedral angles of the tetrasilane subunits in the decasilanes 12 and 14–16 indicates the
existence of three conformations denoted as anti, ortho and gauche and four different arrangements of pentasilane subunits,
denoted as anti–gauche, anti–ortho, ortho–gauche and ortho–ortho. The absorption spectra of the heptasilanes 2–10 exhibit
broadened absorption maxima, shifted strongly to the red relative to those of the decasilanes 11–16. As a result of strongly limited
conformational flexibility due to steric overcrowding, the singlet excitation in 11–16 is at much higher energy than in linear
pentasilanes. A comparison of NMR chemical shifts of 2–16 with those of branched silanes of the type (Me3Si)3SiX and
(Me3Si)2MeSiX reveals that the replacement of methyl groups by additional SiMe3 groups in the b-position leads to a strong
low-field shift of the signal of the central nucleus. © 2000 Elsevier Science B.V. All rights reserved.
Keywords: Silicon; Silanes; Branched oligosilanes; Structure elucidation; UV–vis-spectroscopy
electronic spectra of heptamethyltrisil-2-yl derivatives
substituted by electronegative donor groups and by
West et al. [4] in the course of ESR studies of the
phenyl-tris(trimethylsilyl)silane radical anion. Further-
more, the remarkable steric requirements of branched
oligosilyl groups, especially of the space-filling
(Me3Si)3Si group, made these substituents versatile lig-
ands in main group [5] and in transition metal chem-
istry [6].
Of special interest in the search for sterically more
demanding ligands for the kinetic stabilization of low-
valent intermediates are compounds in which two or
three bulky (Me3Si)2RSi substituents are fixed at the
same central atom. Some of those derivatives, mainly of
heavier main Group IV elements, have been prepared
and for a few of those compounds the X-ray crystal
structures were reported [7]. Recently, Apeloig, [8]
1. Introduction
Compounds containing branched oligosilyl sub-
stituents of type (Me3Si)2RSi — (R=oligosilyl,
trimethylsilyl, aryl, alkyl, hydrogen) have been shown
to display unusual chemical behaviour as well as novel
electronic properties [1]. For example, Bock et al.
demonstrated that radical anions as well as radical
cations of tris(trimethylsilyl)silyl derivatives are stabi-
lized by delocalizing the negative or positive charge,
respectively, into the oligosilyl groups [2]. The ability of
branched oligosilyl substituents to act as a p-acceptor
was discussed by Pitt [3] in the course of studies of the
* Corresponding author. Tel.: +49-381-4981841; fax: +49-381-
4981763.
E-mail address: clemens.krempner@chemie.uni.rostock.de (C.
Krempner).
0022-328X/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved.
PII: S0022-328X(00)00518-0

S. Chtchian et al. / Journal of Organometallic Chemistry 613 (2000) 208–219
209
Marschner [9] and Oehme [10] succeeded in synthesiz-
2. Results and discussion
ing compounds containing two hypersilyl [11] groups at
carbon or silicon atoms, respectively, starting with
(Me3Si)3SiLi×3THF [12]. Moreover, Klinkhammmer
[13] reported the preparation and X-ray structure of a
tris(hypersilyl)stannyl anion, whereas the silicon and
carbon analogues of the compound had not been syn-
thesized before. Reducing the bulkiness of the oligosilyl
substituents by going from the (Me3Si)3Si group to the
(Me3Si)2MeSi group, Lambert succeeded in the fixation
of three of those units at one central silicon atom in
synthesizing a highly branched decasilane [14].
These recent developments prompted us to design
branched oligosilane structures by SiꢀSi linkage of
methyl-bis(trimethylsilyl)silylmetal-derivatives [15] with
substituted dichlorosilanes and trichlorosilanes via salt
elimination reactions. This way it should be possible to
get stable, hemispherical and space-filling molecules
that might be used as ligands with exceptional steric
properties [16]. In this paper, we report the synthesis of
various halosilanes and hydroxysilanes substituted on
the central silicon atom by two or three (Me3Si)2MeSi
groups. In addition, we discuss the results of the X-ray
structure analysis of some selected compounds and
report the spectroscopic properties of the synthesized
oligosilanes.
2.1. Synthesis of heptasilanes 2–10
As mentioned above, our strategy for preparing
branched oligosilanes involved the selective formation
of SiꢀSi bonds by salt elimination reactions of methyl-
bis(trimethylsilyl)silyllithium (1) with easily available
dichlorosilanes and trichlorosilanes. Normally, the re-
quired starting material 1 can be prepared by a cleavage
of methyl-tris(trimethylsilyl)silane with methyllithium
in tetrahydrofuran and can be used in situ for coupling
reactions as described previously [16]. But the outcome
of those coupling reactions strongly depends on the
temperature and the solvent applied. Particularly, the
metal–halogen exchange and the SiꢀSi bond cleavage
were observed in the case of polar solvents such as
tetrahydrofuran. Therefore, after changing the solvent
from tetrahydrofuran to pentane, the silanide 1 was
allowed to react with the appropriate chlorosilanes at
low temperatures. In fact, the reaction of two equiva-
lents of 1 with dichloromethylphenylsilane, dichloro-
dimethylsilane and dichloromethylsilane at −78°C
gave the heptasilanes 2, 3 and 4, respectively, in excel-
lent yields (Scheme 1).
Starting with the hydridosilane 4 [17], bromosilane 6
and iodosilane 7 were easily prepared by reaction with
tribromomethane or triiodomethane at 120°C, whereas
the chlorosilane 8 could not be synthesized in this way.
Nevertheless, compound 8 was obtained by selective
cleavage of the SiꢀPh bond of phenylsilane 2 with
trifluoromethanesulfonic acid giving the silyl triflate 5.
Without isolation of 5, the triflate group was replaced
by chloride in the presence of LiCl in THF to give pure
8. The corresponding fluorosilane 9 was synthesized by
fluorination of the bromosilane 6 with TASF [18] in
toluene. On the other hand, hydrolysis of 6 led to the
formation of hydroxysilane 10. It should be emphasised
that the isolated heptasilanes 2, 3 and 6–10 are easily
available, air-stable compounds, which could be
purified by Kugelrohr distillation in vacuum and were
obtained in high yields. The structures proposed for
compounds 2, 3 and 6–10 are in full agreement with
the straightforward NMR spectra and the MS data.
2.2. Synthesis of decasilanes 12–16
As mentioned above, Lambert [14] showed that the
treatment of three equivalents of 1 with methyl-
trichlorosilane led to the formation of the highly
branched decasilane 11 (Scheme 2). In view of the
relative bulkiness of the three (Me3Si)2MeSi groups
bonded at one silicon centre, the ease of formation of
11 is really surprising. Therefore, it should be possible
to synthesize comparable structures, substituted by
groups such as hydrido, chloro or phenyl at the central
silicon atom.
Scheme 1. Synthesis of heptasilanes 2–10.

210
S. Chtchian et al. / Journal of Organometallic Chemistry 613 (2000) 208–219
and tetrachlorosilane, respectively. Unfortunately, nei-
ther the desired phenylsilane nor chlorosilane, bearing
three (Me3Si)2MeSi substituents, could be detected. In
contrast, the reaction of 1 with trichloromethylsilane
and trichlorosilane at −78°C afforded the decasilanes
11 (yield 76%) and 12 (yield 67%), respectively, as
colourless solids. The proposed structure of 12 was
confirmed by NMR, MS and IR data and was in full
agreement with the results of an X-ray crystal struc-
ture analysis [19]. The NMR spectra of 12 are rather
1
straightforward. In the H-NMR spectra as well as in
the 13C-NMR spectra only one signal was found for
the SiMe3 groups, and also the 29Si-NMR spectra
showed only one signal for the SiMe3 silicon atoms.
The results of the X-ray analysis of 12 (Fig. 1)
confirm the expected extensive shielding of the hydro-
gen atom by the three (Me3Si)2MeSi substituents.
The extreme steric congestion gives rise to a chemi-
cal inertness of 12. Thus, it is not unexpected that
attempts to replace the hydrogen atom by a chlorine
atom under the formation of the chlorosilane 14 by
heating a solution of 12 in trichloromethane and te-
trachloroethene, respectively, failed. Even after
3
weeks under reflux no exchange had occurred. In or-
der to replace the hydrogen atom with an iodine
atom we used triiodomethane, N-iodosuccinimide and
iodine, respectively, in the reaction with 12 instead of
trichloromethane and tetrachloroethene. However, al-
though the chromatographic control of the progress
of the reactions indicated that 12 was completely con-
sumed, no iodosilane could be detected. Only frag-
mentation and decomposition of the starting material
were observed. Nevertheless, by analogy with the
preparation of 6, the hydridosilane 12 was treated
with an excess of tribromomethane at 80°C for 10
days. Crystallization of the crude product from dry
acetone gave the bromosilane 13 in a yield of 30%. In
comparison with 12, bromosilane 13, which contains
a good leaving group, should be the ideal starting
material for substitution reactions at the central sili-
con atom. In fact, 13 could be substituted very easily
by hydrolysis in a mixture of THF and 10% sulfuric
acid, yielding the hydroxysilane 16 almost quantita-
tively.
The treatment of 13 with concentrated hydrochloric
acid in ether led to full exchange of the halogen
groups under formation of the chlorosilane 14. The
reaction behaviour of 13 is unusual insofar as steri-
cally less congested bromosilanes hydrolyse to silanols
or siloxanes. Nevertheless, besides 14 only traces of
the corresponding hydroxysilane 16 were observed. In
a similar manner, 13 could be converted into the
fluorosilane 15 almost quantitatively by the reaction
with 40% hydrofluoric acid.
Scheme 2. Synthesis of decasilanes 11–16.
Fig. 1. Molecular structure of 12 in the crystal (ORTEP, 30% probabil-
ity level, H-atoms omitted for clarity). Selected bond lengths (A) and
,
angles (°): Si1ꢀH1, 1.48(2); Si1ꢀSi, 2 2.365(2); Si1ꢀSi3, 2.3660(13);
Si1ꢀSi, 4 2.3654(13); Si2ꢀSi, 5 2.3458(14); Si2ꢀSi6, 2.3516(13); Si2ꢀC1,
1.901(3); Si2ꢀSi1ꢀH1, 102.3(10); Si2ꢀSi1ꢀSi3, 115.51(5); Si2ꢀSi1ꢀSi4,
116.03(5); Si3ꢀSi1ꢀSi4, 115.84(5); Si1ꢀSi2ꢀSi5, 108.68(5); Si1ꢀSi2ꢀSi6,
119.55(5); Si5ꢀSi2ꢀSi6, 109.24(5).
With the intention to confirm the concept, the pro-
cedure used successfully for the generation of the hep-
tasilanes 2 and 4 was modified by treatment of a
threefold molar excess of 1 with phenyltrichlorosilane

S. Chtchian et al. / Journal of Organometallic Chemistry 613 (2000) 208–219
211
2.3. Molecular structures and conformations of the
decasilanes 12 and 14–16
The synthesized branched decasilanes 12–16 (Scheme
2) are all solid, air-stable compounds, which were
purified by crystallization and characterized on the
basis of NMR, MS and IR data (see Section 3). In
addition, the molecular structures of the decasilanes 12
[19] and 14–16 have been derived from X-ray diffrac-
tion data. The results of the X-ray analyses give a good
idea of the steric congestion in the molecules. As we see
in Figs. 1–4 the presented molecules have a threefold
axis along the central SiꢀX bond (X=H, Cl, F, OH)
and adopt a ca. spherical shape, with the core silicon
atom at the centre of the sphere. The functional group
X is fully enclosed by the three (Me3Si)2MeSi sub-
stituents, filling the hole between these groups.
Fig. 4. Molecular structure of 16 in the crystal (ORTEP, 30% probabil-
,
ity level, H-atoms omitted for clarity). Selected bond lengths (A) and
angles (°): Si1ꢀO1, 1.689(4); Si1ꢀSi2, 2.3773(15); Si1ꢀSi3, 2.3814(11);
Si1ꢀSi4, 2.3770(11); Si2ꢀSi5, 2.3603(13); Si2ꢀSi6, 2.3569(14); Si2ꢀC1,
1.905(3); O1ꢀSi1ꢀSi2, 102.84(14); Si2ꢀSi1ꢀSi3, 114.40(4); Si2ꢀSi1ꢀSi4,
114.88(4); Si3ꢀSi1ꢀSi4, 114.76(4); Si1ꢀSi2ꢀSi5, 108.11(5); Si1ꢀSi2ꢀSi6,
120.65(5); Si5ꢀSi2ꢀSi6, 108.59(5).
The intramolecular distances between the group X
(F, Cl, OH) and the three next neighbouring Me3Si
carbon atoms are relatively short. For example, the
,
average CꢀX distances of 15 (CꢀF1, 3.45 A) and 16
,
(CꢀO1, 3.50 A) correspond ca. with the sum of the van
,
der Waals radii [20] of a methyl group (2.0 A) and a
fluorine (1.47 A) or a hydroxy group (1.52 A), respec-
tively. Only the average distance CꢀCl1 (3.58 A) in 14 is
slightly smaller than the sum of the van der Waals radii
of a methyl group and chlorine atom amounting to
,
,
,
,
about 3.76 A. The central SiꢀX bonds are remarkably
elongated (Si1ꢀH1 1.482; Si1ꢀF1 1.637; Si1ꢀCl1 2.137;
Fig. 2. Molecular structure of 14 in the crystal (ORTEP, 30% probabil-
,
Si1ꢀO1 1.689 A). The spatial demand of the three
,
ity level, H-atoms omitted for clarity). Selected bond lengths (A) and
(Me3Si)2MeSi groups forces a widening of the SiꢀSiꢀSi
angles of the XSiSi3 tetrahedra toward the group X
within the range of 115–117°. The sum of the three
SiꢀSiꢀSi bond angles around the central silicon atoms
are 347.55 (12), 346.26 (14), 348.14 (15) and 344.04°
(16). Most of the SiꢀSi bond lengths lie unremarkably
angles (°): Si1ꢀCl1, 2.1370(11); Si1ꢀSi2, 2.3799(12); Si1ꢀSi3,
2.3771(12); Si1ꢀSi4, 2.3829(12); Si2ꢀSi5, 2.3663(13); Si2ꢀSi6,
2.3705(13); Si2ꢀC1, 1.900(3); Cl1ꢀSi1ꢀSi2, 101.59(5); Si2ꢀSi1ꢀSi3,
115.24(4); Si2ꢀSi1ꢀSi4, 117.35(4); Si3ꢀSi1ꢀSi4, 115.55(4); Si1ꢀSi2ꢀSi5,
110.43(5); Si1ꢀSi2ꢀSi6, 118.78(5); Si5ꢀSi2ꢀSi6, 108.43(5).
,
within the range of 2.360–2.370 A. Only the three
bonds emanating from the central silicon are slightly
,
elongated within the range 2.375–2.385 A. At the posi-
tions where the trimethylsilyl groups of the three
(Me3Si)2Me Si substituents contact each other, the an-
gles Si1ꢀSi2ꢀSi6 are significantly widened within the
range 118.8–120.7°, reducing the steric interactions.
Thus, the average distances of neighboring methyl
,
groups are 4.0 (12, 14, 16) and 3.9 A (15) and are
comparable with the sum of the van der Waals radii of
,
two methyl groups amounting to about 4.0 A. These
results are in good agreement with the structural data
obtained for the corresponding decasilane 11 reported
by Lambert et al. [14].
Very instructive is a view along the Si2ꢀSi1 axis of
the molecules, shown for the decasilane 14 (Fig. 5),
which reveals that the conformation is predominantly
determined by the steric repulsion of the three
Fig. 3. Molecular structure of 15 in the crystal (ORTEP, 30% probabil-
,
ity level, H-atoms omitted for clarity). Selected bond lengths (A) and
angles (°): Si1ꢀF1, 1.637(2); Si1ꢀSi2, 2.375(2); Si1ꢀSi3, 2.377(2);
Si1ꢀSi4, 2.368(2); Si2ꢀSi5, 2.357(2); Si2ꢀSi6, 2.361(2); Si2ꢀC1,
1.890(4); F1ꢀSi1ꢀSi2, 102.39(10); Si2ꢀSiꢀSi3, 115.04(6); Si2ꢀSi1ꢀSi4,
115.70(6); Si3ꢀSi1ꢀSi4, 115.52(6); Si1ꢀSi2ꢀSi5, 107.97(6); Si1ꢀSi2ꢀSi6,
119.58(7); Si5ꢀSi2ꢀSi6, 109.21(7).

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S. Chtchian et al. / Journal of Organometallic Chemistry 613 (2000) 208–219
91 (ortho) and 162° (anti ). Furthermore, Michl pointed
out that these values are nearly the same as in the
conformers of linear dodecamethylpentasilane and that
the all-anti forms seem to be the most stable of all
conformers, as it was shown in n-Si4Me10 [21]. The
results of our conformational analyses of the decasi-
lanes 12 and 14–16 reveal four tetrasilane subunits with
different dihedral angles (Fig. 5) and 12 possible pen-
tasilane subunits, each defined by two dihedral angles.
Following the denomination of conformers by Michl et
al. [21], we termed the dihedral angles obtained for 12
and 14–16 as gauche (30–38), ortho (88–107) and anti
(127–134°). In addition, Table 1 lists the dihedral an-
gles for the 12 pentasilane subunits in the decasilanes 12
and 14–16. We found a total of 12 ortho, six anti and
six gauche arrangements of the tetrasilane subunits and
four combinations of pentasilane subunits, denoted as
anti–gauche, anti–ortho, ortho–gauche and ortho–ortho
[14].
In conclusion, it should be stated that the arrange-
ment of the tetrasilane as well as of the pentasilane
chains is strongly determined by the space filling
(Me3Si)2MeSi groups. Thus, it is not unexpected that in
the pentasilane subunits of the compounds 12 and
14–16 no all-anti conformers were observed, which has
been noted to be optimal for delocalization or s-conju-
gation [22]. Moreover, the conformational parameters
of 14 as well as of 11 reported by Lambert et al. [14] are
comparable to those for 12, 15 and 16, indicating that
in these classes of compounds the space demand of the
central group X (X=H, Me, Cl, F, OH) is ca. the
same.
Fig. 5. Molecular structure of 14 in the crystal (ORTEP, 30% probabil-
ity level, H-atoms and C-atoms omitted for clarity). Selected dihedral
angles (°): Cl1ꢀSi1ꢀSi2ꢀSi5, 21.71(6); Cl1ꢀSi1ꢀSi2ꢀSi6, 147.84(5);
Si3ꢀSi1ꢀSi2ꢀSi5, 87.65(6); Si4ꢀSi1ꢀSi2ꢀSi5, 131.03(5); Si3ꢀSi1ꢀSi2ꢀ
Si6, 38.48(7); Si4ꢀSi1ꢀSi2ꢀSi6, 102.84(6).
(Me3Si)2MeSi groups, while the SiꢀCl1 group is playing
a minor role. For example, neither is the trimethylsilyl
group (Si5) in a gauche conformation nor is the
trimethylsilyl group (Si6) in an anti conformation with
Cl1, since this would lead to an extreme approach of
the neighbouring SiMe3 groups. Therefore, both
trimethylsilyl groups evade the steric strain by rotating
the (Me3Si)2MeSi group around the Si2ꢀSi1 axis, de-
creasing the dihedral angles Si5ꢀSi2ꢀSi1ꢀCl1 (21.71)
and Si6ꢀSi2ꢀSi1ꢀCl1(147.84°). Obviously due to the
steric stress in the molecule the conformation of the
tetrasilane subunits also differ strongly from the usual
gauche or anti conformation of linear permethyloligosi-
lanes. Thus, the dihedral angles in 14 were found to be
38.48 (Si6ꢀSi2ꢀSi1ꢀSi3), 102.84 (Si6ꢀSi2ꢀSi1ꢀSi4), 87.65
(Si5ꢀSi2ꢀSi1ꢀSi3) and 131.03° (Si5ꢀSi2ꢀSi1ꢀSi4).
2.4. UV spectra — conformational effects
Interestingly, calculations by Michl and coworkers
[21] have shown that the relatively unstrained linear
decamethyltetrasilane has three conformational min-
ima, whose energies lie within 1 kcal mol−1 of each
other, and whose dihedral angles are near 53 (gauche)
The absorption spectra of typical linear permethy-
loligosilanes strongly depend on the degree of catena-
tion as well as on the conformation of the silicon-
backbone [22]. Thus, the absorption maxima of oligo-
silane chains move to longer wavelengths as the number
Table 1
Tetrasilane dihedral angles (°) for compounds 12 and 14–16
Pentasilane subunits
12
88–127
14
88–128
15
89–128
16
90–129
Conformers
Si5ꢀSi2ꢀSi1ꢀSi3ꢀSi7
Si5ꢀSi2ꢀSi1ꢀSi3ꢀSi8
Si5ꢀSi2ꢀSi1ꢀSi4ꢀSi9
Si5ꢀSi2ꢀSi1ꢀSi4ꢀSi10
Si6ꢀSi2ꢀSi1ꢀSi3ꢀSi7
Si6ꢀSi2ꢀSi1ꢀSi3ꢀSi8
Si6ꢀSi2ꢀSi1ꢀSi4ꢀSi9
Si6ꢀSi2ꢀSi1ꢀSi4ꢀSi10
Si7ꢀSi3ꢀSi1ꢀSi4ꢀSi9
Si7ꢀSi3ꢀSi1ꢀSi4ꢀSi10
Si8ꢀSi3ꢀSi1ꢀSi4ꢀSi9
Si8ꢀSi3ꢀSi1ꢀSi4ꢀSi10
ortho–anti
ortho–ortho
anti–ortho
88–108
132–93
132–33
39–127
39–108
102–93
102–33
93–127
93–107
33–127
33–107
88–106
131–90
131–36
38–128
38–106
103–90
103–36
90–129
90–105
36–129
36–105
89–107
133–94
133–32
37–128
37–107
102–94
102–32
93–128
93–107
31–128
31–107
90–106
134–95
134–31
35–129
35–106
101–95
101–31
95–129
95–105
30–129
30–105
anti–gauche
gauche–anti
gauche–ortho
ortho–ortho
ortho–gauche
ortho–anti
ortho–ortho
gauche–anti
gauche–ortho

S. Chtchian et al. / Journal of Organometallic Chemistry 613 (2000) 208–219
213
Table 2
UV absorption spectral data for compounds 3, 4, 6 and 8–16
Heptasilanes
umax (nm)
m
Decasilanes
umax (nm)
m
3
4
8
6
9
254.8
253.4
247.5
245.5
259.0
262.2
2.27×104
1.72×104
2.10×104
2.40×104
1.55×104
1.49×104
11
12
14
13
15
16
239.5
3.40×104
3.42×104
5.70×104
4.08×104
5.38×104
4.25×104
238.5 a
230.6
233.8
219.5
10
ꢀ225 a
a Shoulder.
of silicon atoms increases, with a limiting wavelength
near 300 nm. Oligosilanes with more than three silicon
atoms exhibit broad absorption maxima with multiple
peaks attributable to the presence of conformational
isomers [23]. Consequently, the degree of s-conjugation
in the oligosilane chain is a very sensitive function of
the chain conformation.
In order to examine the conformational dependence
of electronic properties of highly branched oligosilanes
more closely, near-UV spectra of the silanes 2–16 were
measured in heptane as solvent. Absorption spectral
data of the compounds 3, 4, 6 and 8–16 at 293 K are
summarized in Table 2. UV spectra of the decasilanes
11–16 are shown in Fig. 6. The UV spectra of the
heptasilanes 3, 4, 6 and 8–10 are shown in Fig. 7.
The absorption curves of the decasilanes 11–16 show
multiple peaks, however they exhibit very broad ab-
sorption maxima within the range of 210–240 nm.
Interestingly, although the conformational parameters
of these class of compounds are quite similar, the
positions of their first intense peaks in the UV spectra
are markedly different. Especially the absorption max-
ima of the methyl substituted silane 11 (umax=239.5
nm) are definitely at longer wavelengths than those of
silanes 13–16. Moreover, the absorption of 11 is clearly
at a shorter wavelength than those of the linear dode-
camethylpentasilane (umax=250 nm) [24]. These re-
markable differences in excitation energies between 11
and the linear dodecamethylpentasilane should also
apply to the functionalized silanes 12–16 compared to
their linear analogues, which however had not yet been
synthesized.
In contrast to these results, the absorption curves of
the heptasilanes 3, 4, 6 and 8–10 show a more homoge-
neous picture (Fig. 7). They exhibit broad absorption
maxima with a significant shift to the red relative to
those of the former compounds, within the range of
245–262 nm. Especially the observed value of the ab-
sorption maximum of 3 (umax=254.8 nm) is slightly
longer than that of the linear dodecamethylpentasilane
(umax=250 nm) [24].
Fig. 6. UV absorption spectra of the decasilanes 11–16 (1×10−5 M)
in heptane.
Fig. 7. UV absorption spectra of the heptasilanes 3, 4, 6 and 8–10
(1×10−5 M) in heptane.

214
S. Chtchian et al. / Journal of Organometallic Chemistry 613 (2000) 208–219
As mentioned above the space demand of the
maximum moves clearly to shorter wavelengths relative
to the methyl substituted silane 11, suggesting that the
elongated SiꢀX bonds render the non-bonding electron
interactions with the silicon backbone negligible.
(Me3Si)2MeSi group in the branched decasilanes 11–16
leads to strong limitations in the conformational flexi-
bility in the silicon backbones, i.e. interconversion of
conformers through rotations around SiꢀSi single
bonds are restricted. As a result, only four combina-
tions of conformers anti–gauche, anti–ortho, ortho–
gauche and ortho–ortho, but no anti–anti conformers
were observed in the pentasilane subunits, for which it
is commonly believed that the anti–anti-form has the
lowest singlet excitation energy of all conformers [23].
Furthermore, it is well known, that the excitation en-
ergy rapidly increases upon twisting from trans to ortho
or gauche conformation. Thus, the obtained broadened
absorption maxima of 11–16 shifted strongly to the
blue, can be explained as a statistical distribution of
various conformers and, hence, absorb at a higher
energy that depends on twisting in the pentasilane
chains. In contrast, the corresponding broadened ab-
sorption maxima of the heptasilanes 3, 4, 6 and 8–10
are significantly shifted to the red. This suggests, due to
the minor steric stress in the molecules, a considerably
higher conformational flexibility in the silicon back-
bones. Consequently, it is reasonable to assume that in
the four possible pentasilane pathways of 3, 4, 6 and
8–10 besides a statistical distribution of other conform-
ers also anti–anti conformers exist, causing a noticeably
lower singlet excitation energy.
2.5. IR and NMR spectra — trends of chemical shifts
The 29Si-NMR chemical shift data for the central
silicon atom of the oligosilanes 2–16 are summarised in
Table 3 together with the data for selected silanes,
trisilanes and tetrasilanes reported in the literature. As
already reported by Radeglia et al. [25] in the case of
substituted siloxanes and phenylsilanes, it can be seen,
that the low-field shift value of the central silicon atom
increases with the electronegativity of the group X.
Furthermore, the replacement of methyl groups in
Me3SiX by one or more trimethylsilyl groups in the
a-position leads to a strong high-field shift, which can
be attributed mainly to the electronegativity difference
between these groups. For example, the observed value
for the central silicon nucleus of tetrakis(trimethylsi-
lyl)silane (l= −135.9), a compound in which four
trimethylsilyl groups are fixed at the same silicon atom,
is the lowest so far described for comparable oligosi-
lanes. Contrary, the additional replacement of methyl
groups of compounds (Me3Si)2MeSiX and (Me3Si)3SiX
by trimethylsilyl groups in the b-position leads to a
low-field shift of the signal for the central silicon nu-
cleus. The observed influences on the chemical shift are
in full agreement with the data for a series of branched
as well as linear permethylated oligosilanes, reported by
Interestingly, we found that the fluorosilane 9 and
hydroxysilane 10 show a significant bathochromic shift
of the absorption, coupled with some reduction in the
intensity relative to the methyl substituted silane 3. This
can be attributed to a weak interaction of the nonbond-
ing electrons with the heptasilane skeleton [3]. These
results are completely different from those obtained for
the bulkier analogues 12–16, in which such a significant
interaction is not evident. Although sharp maxima of
absorptions could not be determined in all cases, the
1
Ishikawa and West [26]. H-, 13C- and 19F-NMR chem-
ical shifts (Table 4) observed for the central group X
indicate the same effect, a strong high-field shift by
trimethylsilyl substitution in the a-position and a strong
low-field shift as a result of b-substitution of additional
SiMe3 groups.
Table 3
Comparison of 29Si-NMR shift values (ppm) of the heptasilanes and decasilanes 2–16 with the corresponding trisilanes and tetrasilanes measured
in benzene-d6
lSi*X=CH3 lSi*X=H
lSi*X=F
lSi*X=Cl
lSi*X=Br
lSi*X=I
lSi*X=OH lSi*X=Ph
a
d
Me3Si*X
(Me3Si)2MeSi* X
(Me3Si)3Si*X
[(Me3Si)2MeSi]2MeSi*X
[(Me3Si)2MeSi]3Si*X
0.0
−17.5
32.0
36.3 f
34.2
54.3
74.8
30.2 b
9.9 f
−12.6
26.7
26.4 b
2.4 f
8.7 b
16.0 c,
8.8
−9.7
26.1
37.6
−4.6
−48.5 e
−88.3
−33.0
−55.4
−72.7 f
−115.6
−67.1
−22.8 f
−56.8 g
−14.2
−46.2 f
−77.0
−34.4
−24.8 g
17.2
−111.4
27.9
12.3
a Ref. [27].
b Ref. [28].
c Ref. [29].
d Measured in CDCl3.
e Ref. [26a].
f Ref. [30].
g Ref. [31].

S. Chtchian et al. / Journal of Organometallic Chemistry 613 (2000) 208–219
215
Table 4
Selected spectroscopic data of the silanes 4, 9, 11, 12 and 15 compared with those reported in the literature
Compounds
lH (ppm) X=H
lF (ppm) X=F
lC (ppm) X=CH3
1JSiꢀH (Hz)
w˜SiꢀH (cm−1
)
Me3SiX
4.04 a
3.37 e
3.95
2.52
3.53
−158.0 b
−208.9
−182.2
−255.1
−204.4
0.0
182 c
163 g
160
2121 d
2075 g
2058
2051
2017
(Me3Si)2MeSiX
[(Me3Si)2MeSi]2MeSiX
(Me3Si)3SiX
−6.9 f
0.5
−13.3
154 h
147
[(Me3Si)2MeSi]3SiX
0.4 i
a Ref. [34].
b Ref. [35].
c Ref. [36].
d Ref. [37].
e Ref. [38].
f Ref. [26b].
g Ref. [39].
h Ref. [33].
i Ref. [14].
Interestingly, the values of coupling constants
1J(SiꢀH) of the selected hydridosilanes (Table 4) in-
crease as substituents on the central silicon atoms be-
come bulkier. It is well known that in substituted
hydridosilanes the s-character of the Si hybrid orbital is
correlated with spin–spin coupling constants [32]. The
Absorption spectra of the heptasilanes 2–10 exhibit
broadened absorption maxima, shifted strongly to the
red relative to those of the highly branched decasilanes
11–16. These results support the suggestion that if the
conformational flexibility is strongly limited by steric
overcrowding, the singlet excitation in higher oligosi-
lane dendrimers tends to be localized at a relatively
small number of silicon atoms in a way dictated by
chain conformation. Finally, spectroscopic investiga-
tions have shown that the properties of the SiꢀX func-
tion are strongly influenced by electronic effects of the
pentasilanes chains as well as by the space demand of
two or three (Me3Si)2MeSi groups. Despite the exten-
sive shielding of the SiꢀX function, the oligosilanes
2–15 are proven to be reactive, and their chemical
behaviour is the subject of current studies.
1
trend of J(SiꢀH) indicates that the s-character of the
SiꢀH orbitals becomes smaller as the substituents be-
1
come bulkier. Especially the value of J(SiꢀH) of [(i-
Pr)3Si]3SiH [33], a molecule with a nearly planar
structure of the central SiSi3 skeleton, is the lowest
among those of hydridosilanes so far reported.
Moreover, the values of the SiꢀH stretching frequen-
cies, obtained for the hydridosilanes summarised in
Table 4, indicate that the SiꢀH bonds are dramatically
weakened by successive substitution of trimethylsilyl
groups in the a-position and the b-positions. Thus, the
observed value for the hydridosilane 12 (2017 cm−1) is
the lowest so far described for comparable oligosilanes.
Although the origin of the effect is unclear, the assump-
tion of steric repulsion alone leading to a flattening of
the central SiSi3 skeleton of 12 and making the SiꢀH
bond quite weak is not valid, since the stretching fre-
quency of [(i-Pr)3Si]3SiH was found to be 2050 cm−1
[33]. However, it is expected that mainly s-conjugation
into the pentasilane chains together with inductive ef-
fects of the trimethylsilyl groups decrease the SiꢀH
bond strength.
3. Experimental
3.1. General procedures and material
All reactions involving organometallic reagents were
carried out under an atmosphere of argon using stan-
dard Schlenk techniques. Methyl-tris(trimethylsi-
lyl)silane [31], methyl-bis(trimethylsilyl)silyllithium (1)
[15] and 1,1,1,2,3,4,5,5,5-nonamethyl-2,4-bis(trimethyl-
silyl)pentasilane (4) [17] were prepared as previously
described. NMR: Bruker AC 250, Bruker ARX 300,
Bruker ARX 400 (250 MHz, 62.9 MHz, 79.5 MHz and
2.6. Conclusions
1
235.3 MHz, for H, 13C, 29Si and 19F, respectively). For
We have prepared various branched halosilanes and
hydroxysilanes substituted at the central silicon atom
by two or three (Me3Si)2MeSi groups. X-ray analyses
revealed the branched decasilanes 12 and 14–16 to be
hemispherical and space-filling molecules, in which the
group X (X=H, Cl, F, OH) is strongly shielded and
the SiꢀSiꢀSiꢀSi dihedral angles are remarkably twisted.
1H-, 13C- and 29Si-NMR, benzene-d6 as solvent, TMS as
internal standard; for 19F-NMR, benzene-d6 as solvent,
CFCl3 as internal standard. IR: Nicolet 205 FT-IR.
MS: Intectra AMD 402, chemical ionization with
isobutane as the reactant gas. UV–vis: Perkin–Elmer
Lambda 2, quartz cells of 1.0 cm path length and
spectral grade n-heptane.

216
S. Chtchian et al. / Journal of Organometallic Chemistry 613 (2000) 208–219
3.2. 1,1,1,2,3,4,5,5,5-Nonamethyl-3-phenyl-2,4-
bis(trimethylsilyl)pentasilane (2)
C15H45BrSi7 (502.03): C, 35.89; H, 9.04; Br, 15.92.
Found: C, 34.30; H, 8.85; Br, 13.56%.
Dichloromethylphenylsilane (2.44 ml, 15 mmol) was
3.5. 3-Iodo-1,1,1,2,3,4,5,5,5-nonamethyl-2,4-bis-
(trimethylsilyl)pentasilane (7)
added at −78°C to
a
solution of methyl-
bis(trimethylsilyl)silyllithium (30 mmol) in pentane (100
ml). Stirring was continued for 1 h and the mixture was
allowed to warm to room temperature (r.t.) within 2 h.
After filtration and removal of the solvent under re-
duced pressure the residue was distilled (170°C, 0.03
A
mixture of 1.1.1.2.3.4.5.5.5-nonamethyl-2.4-
bis(trimethylsilyl)-pentasilane (4) (1.6 g, 3.8 mmol),
CHI3 (1.6 g, 4 mmol) and toluene (20 ml) was heated to
110°C for 20 h. After removal of the volatiles, the
residue was distilled (175°C, 0.03 mbar) to give 7 (1.41
1
mbar) to give 2 (5.99 g, 80%). M.p. 50°C. H-NMR: l
1
7.49–7.53, 7.06–7.2 (2m, phenyl, 5H), 0.71 (s, SiMePh,
3H), 0.39 (s, SiMe, 6H), 0.11, 0.18 (2s, SiMe3, 2×18H).
13C-NMR: l 140.2, 135.0, 128.3, 128.1 (C-aromat.),
−2.9 (SiMePh), −10.1 (SiMe), 0.8, 1.1 (SiMe3). 29Si-
NMR: l −34.4 (SiMePh), −80.5 (SiMe), −11.6,
−11.8 (SiMe3). MS: (70 eV) m/z (%)=498 (5) [M+],
483 (3) [M+−Me], 309 (100) [M+−SiMe(SiMe3)2].
HRMS: Calc. for C21H50Si7 498.22387. Found:
498.22974. Anal. Calc. for C21H50Si7 (499.235): C,
50.52; H, 10.10. Found: C, 48.56; H, 9.99%.
g, 68%). H-NMR: l 1.20 (s, SiMeI, 3H), 0.30 (s, SiMe,
6H), 0.33, 0.26 (2s, SiMe3, 2×18H). 13C-NMR: l 2.9
(SiMeI), −9.2 (SiMe), 1.0, 1.1 (SiMe3). 29Si-NMR: l
−14.2 (SiMeI), −75.0 (SiMe), −11.5, −10.6
(SiMe3). MS: (70 eV) m/z (%)=548 (1) [M+], 475 (51)
[M+−SiMe3]. Anal. Calc. for C15H45ISi7 (549.03): C,
32.82; H, 8.26. Found: C, 32.50; H, 8.17%.
3.6. 3-Chloro-1,1,1,2,3,4,5,5,5-nonamethyl-2,4-bis-
(trimethylsilyl)pentasilane (8)
3.3. 1,1,1,2,3,3,4,5,5,5-Decamethyl-2,4-
bis(trimethylsilyl)pentasilane (3)
Trifluoromethansulfonic acid (0.1 ml, 1.1 mmol) was
added at r.t. to a stirred solution of compound 2 (0.5 g,
1.0 mmol) in CH2Cl2. After removal of the solvent
under reduced pressure, the solid residue was dissolved
in 10 ml THF and 50 mg (1.2 mmol) of anhydrous LiCl
were added. Stirring was continued for 1 day. After
filtration and removal of the solvent under reduced
pressure, the residue was distilled (165°C, 0.02 mbar) to
Dichlorodimethylsilane (1.82 ml, 15 mmol) was
added at −78°C to
a
solution of methyl-
bis(trimethylsilyl)silyllithium (30 mmol) in pentane (100
ml). Stirring was continued for 1 h and the mixture was
allowed to warm to r.t. within 2 h. After filtration and
removal of the solvent under reduced pressure the
residue was distilled (120°C, 0.01 mbar) to give 3 (5.82
1
give 8 (0.41 g, 90%). M.p. 47–48°C. H-NMR: l 0.84
(s, SiMeCl, 3H), 0.30 (s, SiMe, 6H), 0.32, 0.23 (2s,
1
g, 87%). M.p. 41°C. H-NMR: l 0.44 (s, SiMe2, 6H),
SiMe3, 2×18H). 13C-NMR: l 6.3 (SiMeCl), −11.0
0.28 (s, SiMe, 6H), 0.25 (s, SiMe3, 36H). 13C-NMR: l
0.5 (SiMe2), −10.7 (SiMe), 1.1 (SiMe3). 29Si-NMR: l
−33.0 (SiMe2), −81.6 (SiMe), −12.0 (SiMe3). MS:
(70 eV) m/z (%)=436 (14) [M+], 421 (6) [M+−Me].
Anal. Calc. for C16H48Si7 (437.162): C, 43.96; H, 11.07.
Found: C, 43.97; H, 10.92%.
(SiMe), 0.8, 0.83 (SiMe3). 29Si-NMR:
l
26.7
(SiMeCl),−75.4 (SiMe), −11.3, −11.2 (SiMe3). MS:
(CI) m/z (%)=457 (4) [M++H], 441 (30) [M+−Me],
421 (40) [M+−Cl], 348 (100) [M+−ClSiMe3]. Anal.
Calc. for C15H45ClSi7 (457.57): C, 39.37; H, 9.91; Cl,
7.75. Found: C, 39.30; H, 9.83; Cl, 7.54%.
3.4. 3-Bromo-1,1,1,2,3,4,5,5,5-nonamethyl-2,4-
bis(trimethylsilyl)pentasilane (6)
3.7. 3-Fluoro-1,1,1,2,3,4,5,5,5-nonamethyl-2,4-bis-
(trimethylsilyl)pentasilane (9)
A
mixture of 1.1.1.2.3.4.5.5.5-nonamethyl-2.4-
A mixture of compound 6 (0.3 g, 0.6 mmol), tris(-
dimethylamino)-sulfonium-difluorotrimethylsilicate
(TASF) (0.181 g, 0.65 mmol) and toluene (10 ml) was
stirred at r.t. for 1 day. After filtration and removal of
the volatiles, the residue was distilled (115°C, 0.02
bis(trimethylsilyl)-pentasilane (4) (5.3 g, 12 mmol) and
CHBr3 (2.1 ml, 23 mmol) was heated to 120°C for 15 h.
After removal of the volatiles, the residue was distilled
(145°C, 0.005 mbar) to give 6 (5.76 g, 92%). M.p. 62°C.
1H-NMR: l 0.99 (s, SiMeBr, 3H), 0.31 (s, SiMe, 6H),
0.33, 0.24 (2s, SiMe3, 2×18H). 13C-NMR: l 5.2
(SiMeBr), −10.4 (SiMe), 0.9, 0.87 (SiMe3). 29Si-NMR:
l 17.2 (SiMeBr), −75.0 (SiMe), −12.0, −10.9
(SiMe3). MS: (70 eV) m/z (%)=502 (0.4) [M+], 487 (9)
1
mbar) to give 9 (0.195 g, 74%). H-NMR: l 0.75 (d,
3J=9.5 Hz, SiMeF, 3H), 0.31 (s, SiMe, 6H), 0.31, 0.22
2
(2s, SiMe3, 2×18H). 13C-NMR: l 5.8 (d, J=12.4 Hz,
SiMeF), −11.9 (SiMe), 0.7, 0.5 (SiMe3). 29Si-NMR: l
1
54.3 (d, J=331.5 Hz, SiMeF), −80.9 (SiMe), −11.5,
[M+−Me], 429 (15) [M+−SiMe3], 348 (65) [M+−
SiMe3−HBr]. HRMS: Calc. for C15H BrSi7
500.10923. Found: 500.10895. Anal. Calc. for
−12.8 (SiMe3). 19F-NMR: l −182.2 (SiMeF). MS:
(CI) m/z (%)=441 (10) [M++1], 425 (50) [M+−Me].
HRMS: Calc. for C15H45FSi7 440.18428. Found:
79
45

S. Chtchian et al. / Journal of Organometallic Chemistry 613 (2000) 208–219
217
440.18903. Anal. Calc. for C15H45FSi7 (441.120): C,
40.84; H, 10.28. Found: C, 40.26; H, 10.39%.
3.11. 3-Bromo-1,1,1,2,4,5,5,5-octamethyl-3-[1%,2%,2%,2%-
tetramethyl-1%-(trimethylsilyl)disilanyl]-2,4-
bis(trimethylsilyl)pentasilane (13)
3.8. 3-Hydroxy-1,1,1,2,3,4,5,5,5-nonamethyl-2,4-
bis(trimethylsilyl)pentasilane (10)
A mixture of compound 12 (3 g, 5.02 mmol) and
CHBr3 (20 ml) was heated to 80°C for 10 days. After
removal of the volatiles, the solid residue was recrystal-
lized twice from acetone to give 13 (1.02 g, 30%). M.p.
Sulfuric acid (15 ml of 10%) was added to a solution
of compound 6 (0.3 g, 0.6 mmol) in tetrahydrofuran (30
ml) and the mixture was vigorously stirred for 1 day at
r.t. After addition of diethylether the organic phase was
separated, dried with MgSO4, and evaporated. The oily
residue was distilled (120°C, 0.005 mbar) to give 10
1
164°C. H-NMR: l 0.51 (s, SiMe, 9H), 0.36 (s, SiMe3,
54H). 13C-NMR: l 1.9 (SiMe3), −5.9 (SiMe). 29Si-
NMR: l 12.3 (SiBr), −9.8 (SiMe3), −68.7 (SiMe).
MS: (CI) m/z (%)=660 (30) [M+−Me], 595 (100)
(0.241 g, 92%). IR (Nujol) w˜ =3672 cm−1 (OH). H-
[M+−Br], 406 (95) [M+−BrMeSi(SiMe3)2]. HRMS:
1
79
60
NMR: l 0.65 (s, SiMeOH, 3H), 0.53 (s, OH, 1H), 0.28
(s, SiMe, 6H), 0.29, 0.24 (2s, SiMe3, 2×18H). 13C-
NMR: l 6.7 (SiMeOH), −11.6 (SiMe), 0.9 (SiMe3).
29Si-NMR: l 26.1 (SiMeOH), −81.5 (SiMe), −11.9,
−12.8 (SiMe3). MS: (70 eV) m/z (%)=438 (2) [M+],
365 (10) [M+−SiMe3], 348 (100) [M+−HOSiMe3].
Anal. Calc. for C15H46OSi7 (439.134): C, 41.03; H,
10.56. Found: C, 40.73; H, 10.38%.
Calc. for C20H BrSi10 659.1466. 659.1571. Anal. Calc.
for C21H63BrSi10 (676.49): C, 37.29; H, 9.38. Found: C,
35.64; H, 9.05%.
3.12. 3-Chloro-1,1,1,2,4,5,5,5-octamethyl-3-[1%,2%,2%,2%-
tetramethyl-1%-(trimethylsilyl)disilanyl]-2,4-
bis(trimethylsilyl)pentasilane (14)
Concentrated HCl (20 ml) was added to a solution of
compound 13 (0.1 g, 0.16 mmol) in diethylether (30 ml).
The mixture was vigorously stirred for 7 days at r.t.
The organic phase was separated, dried with MgSO4,
and evaporated. The solid residue was recrystallized
from acetone to give 14 (0.085 g, 90%). M.p. 189°C.
1H-NMR: l 0.5 (s, SiMe, 9H), 0.35 (s, SiMe3, 54H).
13C-NMR: l 1.6 (SiMe3), −6.6 (SiMe). 29Si-NMR: l
27.9 (SiCl), −10.6 (SiMe3), −67.1 (SiMe). MS: (CI)
m/z (%)=629 (5) [M+−H], 615 (30) [M+−Me], 595
3.9. 1,1,1,2,3,4,5,5,5-Nonamethyl-3-[1%,2%,2%,2%-
tetramethyl-1%-(trimethylsilyl)disilanyl]-2,4-
bis(trimethylsilyl)pentasilane (11)
Trichloromethylsilane (1.18 ml, 10 mmol) was added
at −78°C to a solution of methyl-bis(trimethylsi-
lyl)silyllithium (33 mmol) in pentane (100 ml). Stirring
was continued for 1 h, and the mixture was allowed to
warm to r.t. within 2 h. After filtration and removal of
the solvent under reduced pressure, the solid residue
was recrystallized from acetone to give 11 (4.64 g, 76%).
The spectroscopic data obtained were in full agreement
with those reported [14].
(54) [M+−Cl], 406 (100) [M+−ClMeSi(SiMe3)2].
35
60
HRMS: Calc. for C20H ClSi10 615.1972. Found:
615.20764. Anal. Calc. for C21H63ClSi10 (632.04): C,
39.91; H, 10.05. Found: C, 37.13; H, 9.85%.
3.10. 1,1,1,2,4,5,5,5-Octamethyl-3-[1%,2%,2%,2%-
tetramethyl-1%-(trimethylsilyl)disilanyl]-2,4-
bis(trimethylsilyl)pentasilane (12)
3.13. 3-Fluoro-1,1,1,2,4,5,5,5-octamethyl-3-[1%,2%,2%,2%-
tetramethyl-1%-(trimethylsilyl)disilanyl]-2,4-
bis(trimethylsilyl)pentasilane (15)
Trichlorosilane (1.01 ml, 10 mmol) was added at
HF (20 ml of 40%) was added to a solution of
compound 13 (0.1 g, 0.16 mmol) in ether (30 ml). The
mixture was vigorously stirred for 4 days at r.t. After
addition of 100 ml of water the organic phase was
separated, dried with MgSO4, and evaporated. The
solid residue was recrystallized from acetone to give 14
−78°C to
a
solution of methyl-bis(trimethylsi-
lyl)silyllithium (33 mmol) in pentane (100 ml). Stirring
was continued for 1 h, and the mixture was allowed to
warm to r.t. within 2 h. After filtration and removal of
the solvent under reduced pressure at 20°C, the solid
residue was suspended with cold acetone and filtered
off, yield 4.01 g (67%). M.p. 165–167°C. IR (Nujol)
1
(0.082, g 90%). M.p. 184–185°C. H-NMR: l 0.50 (s,
SiMe, 9H), 0.32 (s, SiMe3, 54H). 13C-NMR: l 1.2
1
1
w˜ =2016.7 cm−1 (SiH). H-NMR: l 3.53 (s, SiH, 1H),
(SiMe3), −7.3 (SiMe). 29Si-NMR: l 74.8 (d, J=339.4
0.39 (s, SiMe, 9H), 0.31 (s, SiMe3, 54H). 13C-NMR: l
−6.1 (SiMe), −0.7 (SiMe3). 29Si-NMR: l −11.7
Hz, SiF), −11.4 (SiMe3), −73.2 (SiMe). 19F-NMR: l
−204.4 (SiMeF). MS: (CI) m/z (%)=613 (4) [M+−
H], 599 (55) [M+−Me], 425 (100) [M+−
MeSi(SiMe3)2], 406 (40) [M+−MeFSi(SiMe3)2].
HRMS: Calc. for C20H60FSi10 599.24451. Found:
599.23718. Anal. Calc. for C21H63FSi10 (615.59): C,
40.97; H, 10.32. Found: C, 37.76; H, 10.14%.
1
(SiMe3), −80.8 (SiMe),−111.4 (d, J=147 Hz, SiH).
MS: (CI) m/z (%)=597 (15) [M++1], 523 (30) [M+−
SiMe3], 507 (55) [M+−Me−HSiMe3], 406 (100) [M+
−HMeSi(SiMe3)2]. Anal. Calc. for C21H64Si10 (597.60):
C, 42.21; H, 10.79. Found: C, 41.89; H, 10.59%.

218
S. Chtchian et al. / Journal of Organometallic Chemistry 613 (2000) 208–219
3.14. 3-Hydroxy-1,1,1,2,4,5,5,5-octamethyl-3-
[1%,2%,2%,2%-tetramethyl-1%-(trimethylsilyl)disilanyl]-2,4-
bis(trimethylsilyl)pentasilane (16)
the difference map. XP (Siemens Analytical X-ray In-
struments) was used for structure representations (for
full crystallographic data see Table 5).
Sulfuric acid (15 ml of 10%) was added to a solution
of compound 13 (0.1 g, 0.15 mmol) in tetrahydrofuran
(30 ml) and the mixture was vigorously stirred for 1 day
at r.t. After the usual workup, the solid residue was
recrystallized from acetone to give 16 (0.083 g, 92%).
M.p.180°C. IR (Nujol) w˜ =3654 cm−1 (OH). 1H-NMR:
l 0.49 (s, SiMe, 9H), 0.33 (s, SiMe3, 54H). 13C-NMR: l
1.5 (SiMe3), −7.1 (SiMe). 29Si-NMR: l 37.6 (SiOH),
−11.7 (SiMe3), −73.2 (SiMe), MS: (CI) m/z (%)=612
(6) [M+], 595 (99) [M+−OH], 423 (100) [M+−
MeSi(SiMe3)2]. HRMS: Calc. for C21H64OSi10 612.2666.
Found: 612.26501. Anal. Calc. for C21H64OSi10
(613.60): C, 41.11; H, 10.51. Found: C, 39.47; H,
10.38%.
4. Supplementary material
Crystallographic data (excluding structure factors)
for the structures reported in this paper have been
deposited with the Cambridge Crystallographic Data
Centre, CCDC no. 134653 for compound 12, no.
134651 for compound 14, no. 134650 for compound 15
and no. 134652 for 16. Copies of this information may
be obtained free of charge from: The Director, CCDC,
12 Union Road, Cambridge, CB2 1EZ, UK (Fax:
+44-1223-336-033; e-mail: deposit@ccdc.cam.ac.uk or
www: http://www.ccdc.cam.ac.uk).
Acknowledgements
3.15. Crystal structure determination of 12, 14, 15 and
16
We thank Professor Dr H. Oehme for his generous
support and T. Groß for carrying out the X-ray diffrac-
tion analyses. We gratefully acknowledge the support
of our research by the Deutsche Forschungsgemein-
schaft and the Fonds der Chemischen Industrie. We
thank Dr M. Michalik, Dr W. Baumann and Professor
N. Stoll for recording the NMR and MS spectra,
respectively.
The crystal structure determinations were performed
on
a
STOE-IPDS diffractometer with graphite
monochromated Mo–Ka radiation. The structures were
solved by direct methods (SHELXS-86) [40] and refined
by full matrix least-squares techniques against F2
(SHELXL-93) [41]. The position of the central hydrogen
atom (H1) in compound 12 could be elucidated from
Table 5
Summary of crystal and structure solution data of 12, 14, 15 and 16
12
14
15
16
Formula
C21H64Si10
597.62
20.236(4)
12.012(2)
19.174(4)
90.00
117.43(3)
90.00
4136.7(14)
0.960
C21H63ClSi10
632.065
18.154(4)
12.101(2)
19.061(4)
90.00
90.42(3)
90.00
4187.2(14)
1.003
C21H63FSi10
615.61
20.283(4)
12.015(2)
19.139(4)
90.00
117.32(3)
90.00
4143.9(14)
0.987
C21H64OSi10
613.62
20.339(4)
12.053(2)
19.064(4)
90.00
117.31(3)
90.00
4152.5(14)
0.982
Formula weight (g mol−1
)
,
a (A)
,
b (A)
,
c (A)
h (°)
i (°)
k (°)
3
,
V (A )
D (g cm−3
)
Z
4
4
4
4
Crystal system
Monoclinic
P2(1)/c
0.327
0.5×0.3×0.2
293(2)
Monoclinic
P2(1)/c
0.388
0.3×0.3×0.2
293(2)
Monoclinic
P2(1)/c
0.331
0.2×0.3×0.4
293(2)
Monoclinic
P2(1)/c
0.329
0.3×0.3×0.2
293(2)
Space group
v (Mo–Ka) (mm−1
Crystal size (mm)
Temperature (°C)
)
Scan range (2q) (°)
hkl Range
2.04–24.38
−23/0, −13/13,−19/21
12167
6555
3556
1.99–21.99
−20/20, −13/13, 0/21
9504
5131
3568
2.20–20.00
0/23, −13/13,−22/19
11964
6567
2352
2.03-24.26
−23/0, −13/13,−18/20
12078
6253
3595
Measured reflections
Unique reflections
Observed reflections
Refined parameters
R1 [I\2|(I)]
284
0.044
289
0.036
289
0.043
293
0.039
R2 for all
0.105
0.060
0.155
0.079
Goodness-of-fit
0.855/1.024
0.859/0.983
0.601/0.864
0.794/0.985

S. Chtchian et al. / Journal of Organometallic Chemistry 613 (2000) 208–219
219
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1999, Abstract 4B13.
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