σ-Bond electron delocalization in oligosilanes as function of substitution pattern, chain length, and spatial orientation

Article abstract of DOI:10.3390/molecules21081079

Polysilanes are known to exhibit the interesting property of σ-bond electron delocalization. By employing optical spectroscopy (UV-vis), it is possible to judge the degree of delocalization and also differentiate parts of the molecules which are conjugated or not. The current study compares oligosilanes of similar chain length but different substitution pattern. The size of the substituents determines the spatial orientation of the main chain and also controls the conformational flexibility. The chemical nature of the substituents affects the orbital energies of the molecules and thus the positions of the absorption bands.

Full text of DOI:10.3390/molecules21081079

molecules
Article
σ-Bond Electron Delocalization in Oligosilanes as
Function of Substitution Pattern, Chain Length,
and Spatial Orientation
Johann Hlina 1, Filippo Stella 1, Mohammad Aghazadeh Meshgi 1, Christoph Marschner 1,
*
and Judith Baumgartner 2,
*
1
Institut für Anorganische Chemie, Technische Universität Graz, Stremayrgasse 9, A-8010 Graz, Austria;
johann.hlina@tugraz.at (J.H.); filippo.stella@unipd.it (F.S.); m.meshgi@ucc.ie (M.A.M.)
Institut für Chemie, Karl Franzens Universität Graz, Stremayrgasse 9, 8010 Graz, Austria
Correspondence: christoph.marschner@tugraz.at (C.M.); baumgartner@tugraz.at (J.B.);
Tel.: +43-316-873-32112 (C.M.); +43-316-873-32107 (J.B.); Fax: +43-316-873-1032112 (J.B.)
2
*
Academic Editor: Mitsuo Kira
Received: 21 July 2016; Accepted: 4 August 2016; Published: 18 August 2016
Abstract: Polysilanes are known to exhibit the interesting property of σ-bond electron delocalization.
By employing optical spectroscopy (UV-vis), it is possible to judge the degree of delocalization and
also differentiate parts of the molecules which are conjugated or not. The current study compares
oligosilanes of similar chain length but different substitution pattern. The size of the substituents
determines the spatial orientation of the main chain and also controls the conformational flexibility.
The chemical nature of the substituents affects the orbital energies of the molecules and thus the
positions of the absorption bands.
Keywords: oligosilanes; σ-bond electron delocalization; UV-spectroscopy; single crystal diffraction
analysis
1. Introduction
Polysilanes, that is compounds containing Si-Si bonds, are known to exhibit optical properties
similar to conjugated organic molecules [ ]. However, in these compounds, it is not -electrons that
are distributed over a system of multiple bonds, but -electrons that are delocalized over a number
of -bonds. For both types of delocalization it is important that the involved orbitals are spatially
aligned in a way that they can overlap. For conjugated -systems, this means that all involved atoms
are located in one plane and for delocalized -electrons this requirement is associated with anti- or
1
π
σ
σ
π
σ
transoid conformations of the involved molecular chain [2,3].
The σ electron delocalization effect was initially discovered by the research groups of Kumada [4–6]
and Gilman [7,8] in the 1960s, in the course of studying UV/vis spectra of small oligosilanes. Later,
longer polysilanes were found to exhibit thermochromism, where the length of aligned chain segments
which absorb light of a certain wave length, changes as a function of temperature [9].
Our own studies in this field [10–17] are strongly connected to investigations concerned with the
chemistry of silanides [18]. The availability of polysilanyl anions allows the construction of oligosilanes
chains with defined conformational properties.
We were thus interested in studying whether it is possible to impose preferred conformations on
short oligosilanes by the deliberate choice of substitution pattern. By careful variation of substituent
type and shape, it should be possible to manipulate the σ-electron delocalization.
Molecules 2016, 21, 1079; doi:10.3390/molecules21081079
www.mdpi.com/journal/molecules

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2. Results
In our previous studies we learned that, while a rotational process in permethylated oligosilanes
is energetically very facile, the introduction of bulky end groups such as the tris(trimethylsilyl)silyl
unit (Figure 1) substantially suppresses these rotations if the groups are not too far apart [10]. This is
consistent with the appearance of only one low energy UV-absorption band, which can be associated
with an all-transoid conformer. With increasing chain length between two tris(trimethylsilyl)silyl
units, their steric interaction diminishes and thus the force that drives the molecule to engage only
in one conformer diminishes. This effect is reflected in the UV-absorption spectra by the appearance
of a second low energy absorption band of higher energy for internal
α,ω-silanylene chain units of
-(SiMe2)5- and longer, which corresponds to a conformer with a shorter transoid aligned segment [10].
SiMe
SiMe
3
3
Me Si
Si
SiMe
(SiMe ) Si
SiMe
3
3
2 n
SiMe
3
3
n = 1, 2 (8), 3 (25), 4, 5 (20), 6
Figure 1. Oligosilanes with bulky tris(trimethylsilyl)silyl end groups.
In a subsequent study, we found that for longer chains with (Me3Si)3Si end groups, the formal
substitution of two geminal methyl groups on internal -(SiMe2)- units for trimethylsilyl groups enforces
the ability of the molecule to exist as a single conformer again by suppressing rotational processes.
However, the thus favored conformers were found to not be all-transoid ones but consist of several
transoid aligned segments which are separated by the introduced trimethylsilyl groups [13]. This
behavior is exemplified in Figure 2. Additional information about conformational preferences of
oligosilanes was obtained by single crystal X-ray diffraction analysis. Of course, the solid state
structure of a polysilane is not necessarily identical with the structure in solution. For the cases of
longer chains with a preference for only one conformer, Raman spectroscopic measurements of solution
and solid state samples suggest identical conformational properties [10].
Figure 2. Schematic representation of the preferentially existing conformer of a permethylated
tetradecasilane with geminal trimethylsilyl substitution at the 2, 5, 10, and 13 positions. Terminal
transoid aligned hexasilane segments marked on the left side, a transoid aligned octasilane segment on
the right side.
The current study can be considered as a continuation of previous work [10,13–17]. In a first
part, the comparison of the UV absorption spectra of a set of hexasilanes with different substitution
patterns allows a judgement of substituent influence on orbital energies and conformational properties.

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A second part deals with longer oligosilanes which are variations of the previously investigated
compounds where trimethylsilyl groups are either exchanged for methyl or for triisopropylsilyl
groups. A final small part is dedicated to UV-spectroscopic analysis of some cyclosilanes.
2.1. Synthesis of Oligosilanes
Compound 1 was obtained by simple reaction of isopropylbis(trimethylsilyl)silylpotassium [19]
with isopropylchloride (Scheme 1). It is interesting to note that
tert-butoxide to the respective 1,1-diisopropyltrimethyldisilanylpotassium. The attempt to achieve
similar chemistry with octamethyltrisilane led to complicated oligomerization chemistry [18 20].
A likely reason for the clean conversion of seems to be the increased steric demand of
1 reacts cleanly with potassium
,
1
1,1-diisopropyltrimethylsilanylpotassium compared to pentamethyldisilanylpotassium. Further
reaction with 1,2-dichlorotetramethydisilane gave compound 2 (Scheme 1).
K
t
i
1. 2 BuOK
PrCl
- 2 KCl
Me Si
3
Si
SiMe
3
2
2
Me Si
Me Si
3
Si
Si
Me
Si
Me
Si
SiMe
3
Si
1
SiMe
3
3
2. Cl(Me Si) Cl
2
2
2
2
2
Scheme 1. Synthesis of a hexasilane with bulky isopropyl groups in the 2- and 5-positions.
Reaction of 2,5-bis(trimethylsilyl)decamethylhexasilane (
silanide in a clean reaction (Scheme 2). The latter can be used as building block for the construction
of a series of oligosilane chains. Coupling of 2 equiv. of with 1,2-dichlorotetramethyldisilane,
1,3-dichlorohexamethyltrisilane, or 1,4-dichlorooctamethyltetrasilane gave compounds , and with
12 13, and 14 catenated silicon atoms in the main chain (Scheme 2). The strategy to prepare , and
is similar to what we have reported earlier for the synthesis of 5a, 6a, and 7a (Figure 3) [13].
3) with potassium tert-butoxide gives the
4
4
5
,
6
7
,
5,
6
7
Me3Si
SiMe3
SiMe
Me3Si
MeSi
Me3Si
K
2 tBuOK
2
Si
Si
Si
Si
2
MeSi
SiMe
-2 tBuOSiMe3
Me2 Me2
Me2 Me2
Me3Si
SiMe3
SiMe3
3
4
SiMe3
SiMe3
Me3Si
MeSi
Me3Si
SiMe3
SiMe
Cl-(SiMe2)n-Cl
-2 KCl
Si
Si
Si (SiMe2)n
Me
Si
Si
Si
Me2 Me2
Me2 Me2
Me
SiMe3
n = 2: 5
n = 3: 6
n = 4: 7
Scheme 2. Synthesis of elongated oligosilanes with trimethylsilyl substituents in the core and at the
periphery of the chains.
SiMe3
Si
SiMe3
Si (SiMe2)n Si
SiMe3
SiMe3
SiMe3
Me3Si
Si
Si
Si
Si
Si
SiMe3
Me2 Me2
Me2 Me2
SiMe3
SiMe3
SiMe3
n = 2: 5a
n = 3: 6a
n = 4: 7a
Figure 3. Compounds 5a, 6a, and 7a serve as reference standards for the judgement of the optical
properties of the two series of oligosilanes 5, 6, and 7 as well as 11, 12, and 13.

Molecules 2016, 21, 1079
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The latter were prepared from 2,2,5,5-tetrakis(trimethylsilyl)decamethylhexasilane (
8
) and contain
tris(trimethylsilyl)silyl end groups and bis(trimethylsilyl)silylene units in the main chain, which were
found to cause a turn in the main chain, interrupting an all-transoid conformation.
Another strategy for the manipulation of conformational properties of oligosilanes chains aims
in the opposite direction. Instead of replacing trimethylsilyl units by methyl groups and thus
creating more flexible chains, it is possible to increase the steric influence of substituents by replacing
trimethylsilyl groups with triisopropylsilyl groups. Starting from 2,5-bis(triisopropylsilyl)-2,5-
bis(trimethylsilyl)decamethylhexasilane (9) reaction with potassium tert-butoxide gives the respective
silanide 10 in a clean reaction (Scheme 3). Two equiv. of 10 can be coupled to the elongated oligosilanes
11, 12, and 13 (Scheme 3).
SiMe
Si
SiMe
Si
SiMe
K
3
3
3
t
BuOK
i
i
i
i
2
2
Pr Si
3
Si
Si
Si Pr
Pr Si
3
Si
Si
Me Me
2
Si
Si
Si Pr
3
3
t
- BuOSiMe
Me Me
3
2
2
2
SiMe
SiMe
SiMe
SiMe
3
3
3
3
10
9
i
i
SiMe
Si Pr
Si Pr
SiMe
Si
3
3
3
3
Cl-(SiMe ) -Cl
2 n
i
i
Si
Si
Si
Si
Pr Si
3
Si
Si
(SiMe )
Si
Si Pr
3
2 n
Me Me
Me Me
2
2
2
2
-2 KCl
n = 2: 11
n = 3: 12
n = 4: 13
SiMe
SiMe
SiMe
SiMe
3
3
3
3
Scheme 3. Synthesis of elongated oligosilanes with trimethylsilyl and triisopropylsilyl substituents in
the core and at the periphery of the chains.
Compound 14 represents an oligosilanes chain which contains bis(trimethylsilyl)silylene units in the
core of the chain as in the compounds of the previous study but features bis(trimethylsilyl)methylsilyl
end groups instead of tris(trimethylsilyl)silyl groups. The compound was prepared by reaction of
2,5-bis(trimethylsilyl)undecamethylhexasilanyl-2-potassium with 1,2-dichlorotetramethyldisilane
(Scheme 4).
Me Si
3
Me Si
3
SiMe
SiMe
3
Me Si
3
SiMe
3
3
t
1. 2 BuOK
Me
Si
Si
Si
Si
Si
Si
Si
Si
Me Me
2
Si
Si
Me
Si
Si
2
Me
Si
Si
SiMe
3
Me Me
Me Me
2. Cl(Me Si) Cl
Me Me
2
2
2
2
2
2
2
2
2
Me Si
3
Me Si
3
SiMe
SiMe
3
Me Si
3
SiMe
3
3
14
Scheme 4. Synthesis of a dodecasilane chain with two internal bis(trimethylsilyl)silylene units and
bis(trimethylsilyl)methylsilyl end groups.
In our studies, we found that the simply accessible 2,2,5,5-tetrakis(trimethylsilyl)
decamethylhexasilane ( ) is a conformationally rather rigid molecule. Since analogous compounds
are easily available, it was tempting to prepare a number of compounds of molecules with different
end groups (15 16 17) (Scheme 5). Compound 18 with phenyl groups on the 1,2-disilanylene unit
8
,
,
(Figure 4) was synthesized in a similar way by reaction of tris(trimethylsilyl)silyl potassium with
the 1,2-ditriflate obtained from 1,2-dimethyltetraphenyldisilane. Also, compound 19 was prepared
in a similar way (Scheme 5) and was then converted further to compound 20 by AlCl3 catalyzed
isomerization (Figure 4) [21].
Compound
9 was also converted into a 1,4-disilandiide by reaction with 2 equiv. potassium
tert-butoxide. Its reaction with 1,2-dichlorotetramethyldisilane leads to a mixture of the two isomers
21a and 21b, where the triisopropylsilyl groups are either cis- or trans-oriented to each other (Scheme 6).
In a related way, compound 18 was converted to the 1,4-disilanide, which was further reacted with
dimethyldichlorosilane to give the cyclopentasilane 22 (Figure 4).

Molecules 2016, 21, 1079
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15 n = 2, R = SiMe (norbornyl)
SiMe
Si
SiMe
Si
SiMe
2
3
3
3
16 n = 2, R = Si(OMe)
3
17 n = 2, R = SiPh tBu
2
R
Si
Me
R
+
Cl(SiMe ) Cl
2 n
R
Si
K
2
-2 KCl
2
19 n = 3, R = SiMe -SiMe
2 3
n
SiMe
SiMe
SiMe
3
3
3
R = Si(OMe) (16a)
3
Scheme 5. Synthesis of a number of oligosilanes with derivatized tris(trimethylsilyl)silyl groups and
α,ω-oligosilanylene units of different lengths.
SiMe
SiMe
3
SiMe
Si
SiMe
Si
3
3
3
i
Si
Me
Si
Ph
Pr Si
3
Si
Si
SiMe
3
Me Si
3
Si
Me
SiMe
3
2
2
2
5
SiMe
SiMe
SiMe
SiMe
3
3
3
3
20
23
24
Me
SiMe
Si
SiMe
3
2
3
Me Si
SiMe
3
3
Si
Si
PhMeSi
Si
SiMePh
Me Si
3
Si
Si
Si
R
SiMe
3
PhMe PhMe
Me Si
SiMe
3
3
R
22
R = SiMe (18)
3
K (18a)
Figure 4. Some new compounds prepared in this study either as starting materials or as subjects for
UV-spectroscopic studies.
Me Si
3
Si(iPr)
3
SiMe
Si
SiMe
Si
3
3
Si
t
1. 2 BuOK
Me Si
SiMe
SiMe
2
2
18-cr-6
i
i
Pr Si
3
Si
Si
Si Pr
3
Me Me
2
2
2. Cl-(SiMe ) -Cl
2 2
Me Si
2
2
SiMe
SiMe
3
3
Si
9
Me Si
3
Si(iPr)
3
21a,b
Scheme 6. Use of 2,5-bis(trimethylsilyl)-2,5-bis(triisopropylsilyl)decamethylhexasilane for the synthesis
of a mixture of cis- and trans-1,4-bis(triisopropylsilyl)-1,4-bis(trimethylsilyl)cyclohexasilane.
Compound 23 (Figure 4) was prepared as starting material for the synthesis of 17 and compound
24 was used for structural comparison purposes.
2.2. UV-Spectroscopy Studies
UV/vis spectroscopy is a good tool to achieve some insight into the
polysilanes. The position of the lowest energy absorption band allows making an estimate about the
extension of delocalization. Effective -electron delocalization requires a transoid orientation of the
polysilane chain [ 22]. If the transoid arrangement is broken by bending the chain into another direction,
σ-electron delocalization in
σ
3,
electron delocalization is interrupted. This was experimentally verified by Tsuji and Tamao in a series
of elegant publications, where they were able to lock polysilane conformations by introducing a rigid
backbone [23
–
27]. Our own studies [10
,
13
,
17] have shown that it is possible to prepare polysilanes with
-electron
restricted rotational properties, which contain two or three different segments which exhibit
σ
delocalization but are separated by a rotational twist. In the current study, we aim to investigate the
steric and electronic influence of substituents on conformational and delocalization properties.
Scheme 6 and Figure 4 show a number of hexasilanes with different substitution pattern. While
it is known that the permethylated n-hexasilane (n-Si6Me14) exhibits a
λ
max
at 260 nm [
6
,28], the
respective band for
8, which is an n-hexasilane with four trimethylsilyl groups in the 2- and 5-positions,
is slightly blue-shifted to 257 nm (Figure 5, Table 1) [10]. There are two possible explanations for this
slight shift. On one hand, the bulk of the tris(trimethylsilyl)silyl groups largely suppresses rotation

Molecules 2016, 21, 1079
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around internals Si-Si bonds and thus locks the conformation of
8
to an all–transoid arrangement. This
enhances the absorption ability of the chain because many of the energetically accessible conformations
of n-Si6Me14 contain segments that are not fully delocalized. On the other hand, it needs to be taken
into consideration that unfavorable 1,4-interactions of the bulky tris(trimethylsilyl)silyl groups also
prevent the molecule from engaging in an all-anti conformation. It seems, however, that this special
conformation, which corresponds to a perfect
Apart from this steric explanation model, also the different electronic properties of silyl and alkyl
groups should to be taken into consideration. As the -electron delocalization process involves
σ-electron delocalization is accessible for n-Si6Me14.
σ
σ*-orbitals of the Si-substituents bonds the energy of these is relevant.
100000
80000
60000
40000
20000
0
220
230
240
250
8
260
9
270
λ / nm
280
16
290
17
300
18
310
320
2
3
15
Figure 5. Comparison of the UV spectra of differently substituted hexasilanes.
Table 1. UV spectroscopic data of some selected oligosilanes.
Oligosilane
Chain Length
λ
ε
λ
ε
λ
ε
3
1
1
2
2
3
Compound
−1
−1
−1
−1
−1
−1
]
[nm]
[M ·cm
]
[nm]
[M ·cm
]
[nm]
[M ·cm
1.4 × 104
6.6 × 104
2.4 × 104
3.0 × 104
1.4 × 104
7.1 × 104
8.4 × 104
2.4 × 104
6.8 × 104
6.2 × 104
n-Si6Me14 [6]
6
6
6
6
6
6
6
6
6
260
257
260
264
264
258
253
262
270
269
290
~289 (sh)
268
264
8 [10]
2
3 [29]
9
15
16
18
17
-
-
-
-
3.2 × 104
240
-
25 [10]
n-Si12Me26 [28]
7
-
-
-
12
12
12
12
12
13
13
13
14
14
14
9
2.3 × 104
1.6 × 104
5.1 × 104
5.0 × 104
1.8 × 104
1.3 × 104
3.1 × 103
1.8 × 104
2.4 × 104
1.5 × 104
1.2 × 104
3.6 × 104
4.0 × 104
7.4 × 104
3.2 × 104
5a [13]
5
11
14
6a [13]
6
12
7a [13]
7
13
27 [13]
19
259
218
232
3.1 × 104
216
265
5.6 × 104
1.7 × 104
2.6 × 104
7.8 × 104
3.6 × 104
3.8 × 104
9.2 × 104
1.9 × 104
2.7 × 104
7.3 × 104
2.7 × 104
4.6 × 104
9.5 × 104
~295 (sh)
~287 (sh)
~307 (sh)
~303 (sh)
285
~311 (sh)
~284 (sh)
273
278
272
~283 (sh)
286
222
293
262
230
271
258
212
267
259
4.5 × 104
268
2.1 × 104
9
9
226
246
1.7 × 104
20
288
To estimate which effect is more pronounced compounds
2, 3, and 9 are of interest. Compound 2,
with four bulky isopropyl groups in the 2- and 4-positions, can be considered electronically equivalent

Molecules 2016, 21, 1079
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to n-Si6Me14 and sterically similar to
8. The fact that the absorption maximum of
2
is identical with
that of n-Si6Me14 seems to indicate that conformational properties of these two compounds are similar.
Compound is similar to but two of the trimethylsilyl groups are replaced by triisopropylsilyl
groups. However, the crystal structure of
shows a first torsional angle of w1 = 168◦ compared to
159◦ found for
. This seems to indicate a better transoid alignment of the chain, which results in
a bathochromic shift of 7 nm to a low energy absorption maximum of 264 nm (Table 1, Figure 5).
Interestingly enough, compound where the same trimethylsilyl groups of are replaced by methyl
groups also exhibits its low energy absorption maximum at 264 nm (Table 1, Figure 5). It almost
seems as if the tris(trimethylsilyl)silyl groups of are particularly ill-suited for a transoid alignment
of the chain. Even compound 15, where two of the trimethylsilyl groups of are replaced by
9
8
9
8
3
8
8
8
dimethyl-exo-2-norbornylgroups, shows a slight bathochromic shift of the low energy absorption
maximum to 258 nm (Table 1, Figure 5).
Compound 16 represents another example of formal exchange of two of the trimethylsilyl groups
of 8. This time Si(OMe)3 are introduced. Despite the fact that there are conformations for 16, where
the transoid aligned chain contains trimethylsilyl end-groups, a hypsochromic shift of the low energy
absorption maximum is observed (Table 1, Figure 5). It seems evident that this is an electronic effect,
caused by the less electron donating Si(OMe)3 groups. In contrast to this, compound 17, where the
t
two trimethylsilyl groups are replaced by SiPh2 Bu units, exhibits the most bathochromicly shifted
absorption maximum at 270 nm (Table 1, Figure 5). Although the crystal structure of 17 (Table 2) does
not indicate a substantially better aligned chain, it can be assumed that the phenyl groups of 17 are
responsible for an extension of the delocalization. The introduction of phenyl groups into the spacer
unit as can be seen in compound 18 has only a minor effect on the location of the absorption maximum
(Table 1, Figure 5).
Figures 6 and 7 provide insight to judge the effectivity of internal substituents of longer oligosilanes
to control the conformation by their UV-spectra. Figure 6 shows a comparison of compounds
and with the respective compounds 5a 6a, and 7a 13] which possess tris(trimethylsilyl)silyl end
groups and internal bis(trimethylsilyl)silylene units. The higher degree of these elements compared to
bis(trimethylsilyl)methylsilyl end groups and internal trimethylsilylmethylsilylene units render 5a
6a, and 7a more rigid, whereas , and are more flexible and thus are able to engage in different
5, 6,
7
,
[
,
5
,
6
7
conformations. This difference is nicely reflected in the UV spectra. The spectra of 5a and, in particular,
6a and 7a display relatively sharp bands, which can be assigned to defined delocalized segments of
the molecules. For 5, 6, and 7, the corresponding bands are much broader indicating contributions of
conformations consisting of more than two separated extended delocalized segments.
100000
90000
80000
70000
60000
50000
40000
30000
20000
10000
0
210
230
3
250
270
290
310
330
λ / nm
5
6
7
5a
6a
7a
Figure 6. Comparison of the UV spectra of different dodeca-, trideca-, and tetradecasilanes with either
methyl or trimethylsilyl groups in terminal and internal positions.

Molecules 2016, 21, 1079
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Interestingly, the picture of Figure 7 is quite different from that in Figure 6. While the latter
shows the effect of increased conformational flexibility, Figure 7 features the impact of even stronger
enforced conformational locking which is accomplished by exchanging some of the trimethylsilyl for
triisopropylsilyl groups. A first consequence of this exchange was already visible in the comparison of
8
with 9 (Figure 5). In this case, the exchange of peripheral trimethylsilyl groups for triisopropylsilyl
units resulted in a bathochromic shift of the low energy absorption maximum of 7 nm. Related red-shift
behavior is also visible for 11, 12, and 13.
100000
90000
80000
70000
60000
50000
40000
30000
20000
10000
0
210
230
9
250
270
290
5a
310
330
λ / nm
11
12
13
6a
7a
Figure 7. Comparison of the UV spectra of different dodeca-, trideca-, and tetradecasilanes with either
triisopropylsilyl or trimethylsilyl groups in terminal and internal positions.
Figure 8 shows the UV spectra of compounds 19, 20, and 25. Compounds 19 and 20 are not
only isomers but the main chains of both compounds are nonasilanes. Compound 19 contains
two pentamethyldisilanyl units and an internal 1,3-trisilanylene unit, whereas Compound 20 has
two tris(trimethylsilyl)silyl units and an internal 1,5-pentasilanylene unit. The UV-spectra of 19
and 20 suggest that the conformations of these two oligosilanes at ambient temperature are quite
different. The low energy absorption maximum of 19 is at 273 nm corresponding to an aligned
heptasilane fragment. This seems to indicate that the pentamethyldisilanyl units do not participate in
the delocalized segment. One reason for this is a likely easy rotation around the Si-Si bonds. Another
reason might be that the pentamethyldisilanyl is not inclined to stretch out but to bend back toward
the main body of the molecule. Such behavior had been observed by Apeloig and co-workers for the
tris(pentamethyldisilanyl)silyl unit and was described as an umbrella effect [30].
Compound 20 on the other hand exhibits two distinct absorption bands at 271 and at 288 nm.
The latter can be considered as the low energy absorption maximum of an aligned nonasilane [13
]
whereas a band around 270 nm hints at a aligned heptasilane segment. The existence of a second
conformer in tris(trimethylsilyl)silyl terminated silanes was observed before for 2,2,9,9-tetrakis
(trimethylsilyl)octadecamethyldecasilane, where a decasilane band was accompanied by an octasilane
band [10]. The reason for the effect of two distinct conformers is that, for spacer lengths of more
than four dimethylsilylene units, the steric bulk of the two tris(trimethylsilyl)silyl end groups
is not sufficient to align the whole molecule. The UV spectrum of 2,2,6,6-tetrakis(trimethylsilyl)
dodecamethylheptasilane (25), which is also seen in Figure 8 tells us that for 19 at least some degree of
improved alignment is existent as evidence of a slight bathochromic shift of the low energy absorption
band from 269 to 273 nm.

Molecules 2016, 21, 1079
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70000
60000
50000
40000
30000
20000
10000
0
210
230
250
270
19
290
310
330
350
λ / nm
20
25
Figure 8. Comparison of the UV spectra of the isomeric compounds 19 and 20 and compound 25
which can be regarded as the core of compound 19.
,
The nonasilane band of 20 can also be compared to 2,2,5,5,8,8-hexakis(trimethylsilyl)
decamethylnonasilane (27) (Table 1) [13], where the two methyl groups at the central silicon atom of
the compound are replaced by trimethylsilyl groups. The nonasilane absorption band associated with
27 is only visible as a shoulder and the spectrum is dominated by a prominent hexasilane band [13].
The UV spectra shown in Figure 9 are those of
discussed the difference between and 5a already above. Compound 14 contains the end-groups of
and the core of 5a. Compared to 5a, the UV spectrum of 14 shows a bathochromic shift associated
with a higher conformational flexibility but the degree of this shift is not as distinct as observed for
This is also emphasized by the shape of the absorption band, which suggests still the presence of a
conformational preference. Again, the band of compound tells us that the predominant conformations
5, 5a, and 14 in addition to compound 8. We have
5
5
5
.
8
of 5, 5a, and 14 include segments longer or better aligned than the hexasilane unit in 8.
120000
100000
80000
60000
40000
20000
0
210
230
250
5
270
290
310
330
λ / nm
5a
8
14
Figure 9. Comparison of the UV spectra of different dodecasilanes (5, 5a, 14) and compound 8.

Molecules 2016, 21, 1079
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The last UV comparison of this study is between cyclosilanes 21a
,
21b
,
22, and 26 (Figure 10). From
seminal work by West and co-workers, it is known that the UV absorption behavior of permethylated
cyclosilanes exhibits a trend of hypsochromic shifts for compounds with increasing ring sizes up
to the cyclononasilane [31]. Our own studies, however, have shown that UV absorption behavior
of trimethylsilyl-substituted cyclosilanes is different from that of permethylated cyclosilanes of the
same ring size. Spectroscopic analysis suggested that
segments in the ring reaching from one trimethylsilyl group to the other [32]. Using this model for
1,1,4,4,-tetrakis(trimethylsilyl)octamethylcyclohexasilane (26) [33 34], a band around 255 nm should be
σ-electron delocalization occurs along chain
,
observed. This is indeed true as an absorption maximum is observed at 260 nm (Figure 10).
40000
35000
30000
25000
20000
15000
10000
5000
0
210
220
230
240
250
260
270
280
290
300
310
λ / nm
21a
21b
22
26
Figure 10. Comparison of the UV spectra of cyclosilanes 21a, 21b, 22, 23, 24, and 26.
The spectra for 21a and 21b hexasilane bands should be similar but the associated bands are
shifted to 251 nm for 21a and to at 250 nm for 21b. Both compounds show shoulders that tail into the
more bathochromic region. The UV spectrum of 22 is meaningful (Figure 10). Despite some absorption
resembling that of 26, the absorption features are less pronounced. It is not exactly clear whether this
absorption behavior is that of the cyclosilane core or includes interaction with its phenyl substituents.
2.3. X-ray Crystallography
Compounds
9 (Figure 11), 14 (Figure 12), 15 (Figure 13), 16a (Figure 14), 17 (Figure 15), 18
(Figure 16), 18a (Figure 17), 21a (Figure 18), 21b (Figure 19), 22 (Figure 20), 23 (Figure 21), and 24
(Figure 22) of this study were characterized by X-ray single-crystal structure analysis (for tables
concerning crystallographic information see: Supplementary Information). As numerous related
polysilanes structures are already known, these compounds provide an excellent opportunity to
compare structural properties of organooligosilanes.
Compounds 9, 14, 18, 15, and 17 are oligosilanes with longer chains and their structure in the
solid state can provide insight into the effectiveness of σ-electron delocalization.
The structures of compounds with a 1,2-disilanylene spacer between the tris(trimethylsilyl)silyl
groups were found to crystallize in monoclinic space groups for
9 and 18 and the triclinic space
group P-1 for 15 and 17. For all compounds, an inversion center between the two central silicon
atoms was found. The asymmetric units of 15 and 17 are containing only two half molecules. The
structures of three other compounds with a 1,2-disilanylene spacers between the tris(trimethylsilyl)silyl
or -germyl groups were reported to be triclinic (P-1), namely 1,1,1,4,4,4-hexakis(trimethylsilyl)-2,2,3,3-
tetramethyltetrasilane [35], 1,2-bis[tris(trimethylsilyl)germyl]tetramethyldisilane [36], and 1,2-bis[tris
(trimethylsilyl)germyl]tetramethyldigermane [36].

Molecules 2016, 21, 1079
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Figure 11. Crystal structure of 9. Thermal ellipsoids are represented at the 30% level and hydrogen
atoms have been omitted for clarity (bond lengths in Å, angles in deg). Si(1)-C(1) 1.9106(17), Si(1)-Si(2)
2.3945(7), Si(2)-Si(4) 2.3764(7), Si(2)-Si(3) 2.4043(9), Si(3)-Si(3A) 2.3943(9), Si(4)-Si(2)-Si(5) 103.71(3),
Si(4)-Si(2)-Si(1) 112.86(3), Si(5)-Si(2)-Si(1) 108.66(3), Si(4)-Si(2)-Si(3) 108.10(3), Si(5)-Si(2)-Si(3) 103.58(2),
Si(1)-Si(2)-Si(3) 118.52(3), Si(3A)-Si(3)-Si(2) 121.66(3).
The longer polysilane 14 also crystallizes in the triclinic space group P-1 again with an inversion
center in the silicon backbone. Its main chain is divided into three segments which was observed
earlier with similar structures [13]. After the bis(trimethylsilyl)silylene unit, the skeleton makes a turn
adopting a torsional angle of 83◦. Table 2 reports the conformational properties of the oligosilanes
by listing the torsional angles along the main chain. The conformations of 9, 18, 15, and 17 can be
described as transoid-anti-transoid (TAT) [2].
Figure 12. Crystal structure of 14. Thermal ellipsoids are represented at the 30% level and hydrogen
atoms have been omitted for clarity (bond lengths in Å, angles in deg). Si(1)-C(1) 1.875(11), Si(1)-Si(2)
2.358(5), Si(2)-Si(7) 2.333(5), Si(2)-Si(3) 2.357(4), Si(3)-Si(4) 2.342(4), Si(4)-Si(5) 2.379(3), Si(5)-Si(8)
2.354(3), Si(5)-Si(9) 2.371(3), Si(5)-Si(6) 2.376(3), Si(7)-Si(2)-Si(3) 108.05(16), Si(7)-Si(2)-Si(1) 109.16(18),
Si(3)-Si(2)-Si(1) 116.28(17), Si(4)-Si(3)-Si(2) 111.55(15), Si(3)-Si(4)-Si(5) 117.83(13), Si(8)-Si(5)-Si(9)
107.50(13), Si(8)-Si(5)-Si(6) 116.91(12), Si(9)-Si(5)-Si(6) 109.33(14), Si(8)-Si(5)-Si(4) 110.18(12),
Si(9)-Si(5)-Si(4) 103.60(13), Si(6)-Si(5)-Si(4) 108.51(13).

Molecules 2016, 21, 1079
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Figure 13. Crystal structure of 15. Thermal ellipsoids are represented at the 30% level and hydrogen
atoms have been omitted for clarity (bond lengths in Å, angles in deg). Si(6)-Si(9) 2.3357(19), Si(6)-Si(8)
2.3460(17), Si(6)-Si(7) 2.357(2), Si(6)-Si(10) 2.3694(17), Si(7)-C(18) 1.869(6), Si(9)-Si(6)-Si(8) 107.60(7),
Si(9)-Si(6)-Si(7) 109.89(7), Si(8)-Si(6)-Si(7) 105.08(7), Si(9)-Si(6)-Si(10) 114.57(6), Si(8)-Si(6)-Si(10) 112.08(7),
Si(7)-Si(6)-Si(10) 107.21(6).
Table 2. Conformational properties of oligosilanes described as the torsional angles of the main chain.
Compound
ω
ω
ω
ω
ω
5
Conformation
1
2
3
4
8 [10,37]
159
168
178
162
160/161
160/160
180
180
158
180
180
180
159
168
TAT
TAT
ATOTATOTA
TAT
9
14
18
15
17
−83
159
180
162
160/161
160/160
TAT
TAT
It should be noted that the first torsional angle of
15 and 17. Given the fact that exhibits the most bathochromicly shifted absorption band of the
9 ,
is with 168◦ more transoid than those of 18
9
structurally related permethylated compounds, this might be in part caused by a more effective
delocalization. The structure of 14 is particularly interesting. Judging the torsional angles, an
ATOTATOTA conformation can be assigned. Compared to the structure of 5a
tris(trimethylsilyl)silyl end groups, 14 features more aligned chain end segments.
[13], which has
Figure 14. Crystal structure of 16a. Thermal ellipsoids are represented at the 30% level and hydrogen
atoms have been omitted for clarity (bond lengths in Å, angles in deg). K(1)-Si(1) 3.5694(19), Si(2)-Si(1)
2.313(2), Si(2)-Si(3) 2.3386(19), Si(2)-Si(4) 2.350(2), Si(3)-C(18) 1.883(5), Si(1)-O(7) 1.658(4), Si(1)-Si(2)-Si(4)
103.97(7), Si(3)-Si(2)-Si(4) 104.33(7), Si(2)-Si(1)-K(1) 158.14(7).

Molecules 2016, 21, 1079
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Figure 15. Crystal structure of 17. Thermal ellipsoids are represented at the 30% level and hydrogen
atoms have been omitted for clarity (bond lengths in Å, angles in deg). Si(1)-Si(5) 2.3726(9), Si(1)-Si(3)
2.3806(9), Si(1)-Si(2) 2.4011(8), Si(1)-Si(4) 2.4184(9), Si(2)-C(1) 1.887(2), Si(5)-Si(1)-Si(3) 105.73(3),
Si(5)-Si(1)-Si(2) 107.99(3), Si(3)-Si(1)-Si(2) 114.99(3), Si(5)-Si(1)-Si(4) 107.83(3), Si(3)-Si(1)-Si(4) 100.65(3),
Si(2)-Si(1)-Si(4) 118.74(3).
For 1,4-substituted cyclohexasilanes, a chair conformation with the large substituents in equatorial
positions is typical [14,33,38–40]. However, 1,1,4,4,-tetrakis(trimethylsilyl)octamethylcyclohexasilane
(26) crystallizing in the monoclinic space group C2/c is an exception as all four trimethylsilyl groups
have the same steric demand allowing the system to adopt a twist conformation [34]. Changing
two trimethylsilyl against triisopropylsilyl groups as in 21 led to two isomers whose crystals differ
in shape so that they can be separated under the microscope. Both isomers crystallize in the
space group C2/c. In the cis-isomer (21a) the two triisopropylsilyl groups occupy axial positions
and the ring adopts a twist conformation whereas in 21b the substituents are both in equatorial
position and the ring exhibits a chair conformation. A chair conformation was also reported for the
related 1,4-bis(tert-butyldimethylsilyl)-1,4-bis(trimethylsilyl)octamethylcyclohexasilane with the two
tert-butyldimethylsilyl groups in equatorial positions [33]. The cyclopentasilane ring in 22 exhibits an
envelope conformation with one MePhSi unit on the flap and the phenyl groups in cis orientation.
Figure 16. Crystal structure of 18. Thermal ellipsoids are represented at the 30% level and hydrogen
atoms have been omitted for clarity (bond lengths in Å, angles in deg). Si(1)-Si(3) 2.3543(12), Si(1)-Si(4)
2.3592(12), Si(1)-Si(5) 2.3681(13), Si(1)-Si(2) 2.3762(11), Si(2)-C(1) 1.886(3), Si(3)-Si(1)-Si(4) 106.64(4),
Si(3)-Si(1)-Si(5) 108.04(4), Si(4)-Si(1)-Si(5) 105.41(5), Si(3)-Si(1)-Si(2) 118.47(4), Si(4)-Si(1)-Si(2) 111.25(4),
Si(5)-Si(1)-Si(2) 106.28(4).

Molecules 2016, 21, 1079
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Figure 17. Crystal structure of 18a. Thermal ellipsoids are represented at the 30% level and hydrogen
atoms have been omitted for clarity (bond lengths in Å, angles in deg). Si(1)-Si(5) 2.3449(19),
Si(1)-Si(6) 2.350(2), Si(1)-Si(2) 2.360(2), Si(1)-K(1) 3.5079(19), Si(2)-C(1) 1.908(5), Si(2)-Si(3) 2.3734(17),
Si(5)-Si(1)-Si(6) 99.17(7), Si(5)-Si(1)-Si(2) 106.41(7), Si(6)-Si(1)-Si(2) 98.25(7), Si(1)-Si(2)-Si(3) 117.41(7),
Si(2)-Si(3)-Si(4) 115.99(7), Si(7)-Si(4)-Si(8) 96.39(7), Si(7)-Si(4)-Si(3) 99.23(8), Si(8)-Si(4)-Si(3) 104.83(7).
The two neopentasilanes 23 and 24 crystallize both in the monoclinic space group P2(1)/c. The
structures are unexceptional with the Si-Si distances of 2.340 Å in tetrakis(trimethylsilyl)silane [41
]
being elongated to 2.364–2.385 Å in 24 and 2.360–2.383 Å in 23.
Figure 18. Crystal structure of 21a
. Thermal ellipsoids are represented at the 30% level and
hydrogen atoms have been omitted for clarity (bond lengths in Å, angles in deg). Si(1)-Si(5) 2.380(2),
Si(1)-Si(3) 2.381(2), Si(1)-Si(2) 2.382(2), Si(1)-Si(4) 2.408(2), Si(2)-C(2) 1.882(6), Si(5)-Si(1)-Si(3) 105.57(7),
Si(5)-Si(1)-Si(2) 104.19(8), Si(3)-Si(1)-Si(2) 106.93(7), Si(5)-Si(1)-Si(4) 113.14(8), Si(3)-Si(1)-Si(4) 112.16(8),
Si(2)-Si(1)-Si(4) 114.13(8).
The structures of the silyl anions 16a and 18a show both unexpected motives. While it is
typical that the two potassium atoms of the silandiide 18a are coordinated by 18-crown-6 ethers,
a torsional angle of 48.0◦ for K1-Si1-Si4-K2, which leads to a gauche conformation is rather unusual.
For the few known structures of other crown ether potassium silyl dianions [33,42,43], the respective
torsion angles for K-Si-Si-K are ranging from 180.0◦ to 147.7◦ and therefore consequently show
trans conformation. The only exception with a smaller angle was 1,1,5,5-tetrakis(trimethylsilyl)
hexamethylpentasilanyl-1,5-dipotassium [42] with an ortho conformation (84.8◦).

Molecules 2016, 21, 1079
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Figure 19. Crystal structure of 21b
. Thermal ellipsoids are represented at the 30% level and
hydrogen atoms have been omitted for clarity (bond lengths in Å, angles in deg). Si(1)-Si(2)
2.3613(7), Si(1)-Si(6) 2.3714(7), Si(1)-Si(8) 2.3734(8), Si(1)-Si(7) 2.4033(7), Si(2)-C(1) 1.8900(17), Si(2)-Si(3)
2.3603(7), Si(3)-Si(4) 2.3842(7), Si(4)-Si(5) 2.3899(8), Si(4)-Si(9) 2.3987(7), Si(4)-Si(10) 2.4169(8), Si(5)-Si(6)
2.3957(7), Si(2)-Si(1)-Si(6) 102.27(3), Si(2)-Si(1)-Si(8) 110.43(3), Si(6)-Si(1)-Si(8) 106.77(3), Si(2)-Si(1)-Si(7)
110.93(3), Si(6)-Si(1)-Si(7) 115.81(4), Si(8)-Si(1)-Si(7) 110.29(3), Si(3)-Si(2)-Si(1) 117.55(3), Si(2)-Si(3)-Si(4)
117.96(4), Si(3)-Si(4)-Si(5) 109.23(3), Si(3)-Si(4)-Si(9) 109.41(3), Si(5)-Si(4)-Si(9) 103.01(3), Si(3)-Si(4)-Si(10)
110.47(4), Si(5)-Si(4)-Si(10) 114.69(3), Si(9)-Si(4)-Si(10) 109.71(3), Si(4)-Si(5)-Si(6) 120.74(3), Si(1)-Si(6)-Si(5)
119.81(3).
Typically, the position of the potassium atom in solid state structures of potassium silanides
is that of a tetrahedral substituent. Recently, some 18-crown-6 potassium oligosilanylsilatranes
were published which show a distortion of the potassium atom in order to coordinate to both the
anionic silicon atom and to an oxygen atom of remaining molecular scaffold [44]. In the silanides
(MeOMe2Si)3SiK [45] and (MeOCH2CH2OMe2Si)3SiK [46], the potassium atom coordinates solely
to the alkoxy groups. In 16a, a similar pattern can be observed where the potassium is coordinating
exclusively to two oxygen atoms of the (MeO)3Si substituent.
Figure 20. Crystal structure of 22. Thermal ellipsoids are represented at the 30% level and hydrogen
atoms have been omitted for clarity (bond lengths in Å, angles in deg). Si(1)-Si(5) 2.3632(8),
Si(1)-Si(7) 2.3643(10), Si(1)-Si(2) 2.3684(10), Si(1)-Si(6) 2.3696(9), Si(2)-Si(3) 2.3796(10), Si(3)-Si(9) 2.3617(8),
Si(3)-Si(8) 2.3637(9), Si(3)-Si(4) 2.3667(9), Si(4)-Si(5) 2.3570(9), Si(5)-Si(1)-Si(7) 107.82(4), Si(5)-Si(1)-Si(2)
104.09(3), Si(7)-Si(1)-Si(2) 112.73(3), Si(5)-Si(1)-Si(6) 112.97(3), Si(7)-Si(1)-Si(6) 105.35(3), Si(2)-Si(1)-Si(6)
113.90(3), Si(1)-Si(2)-Si(3) 107.73(3), Si(9)-Si(3)-Si(8) 106.82(3), Si(9)-Si(3)-Si(4) 107.26(3), Si(8)-Si(3)-Si(4)
113.59(3), Si(9)-Si(3)-Si(2) 111.69(3), Si(8)-Si(3)-Si(2) 112.20(3), Si(4)-Si(3)-Si(2) 105.27(3), Si(5)-Si(4)-Si(3)
107.18(4), Si(4)-Si(5)-Si(1) 104.61(3).

Molecules 2016, 21, 1079
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Figure 21. Crystal structure of 23. Thermal ellipsoids are represented at the 30% level and hydrogen
atoms have been omitted for clarity (bond lengths in Å, angles in deg). Si(1)-Si(3) 2.3600(10), Si(1)-Si(5)
2.3654(10), Si(1)-Si(4) 2.3727(10), Si(1)-Si(2) 2.3827(10), Si(3)-Si(1)-Si(5) 107.02(4), Si(3)-Si(1)-Si(4)
105.31(4), Si(5)-Si(1)-Si(4) 103.48(4), Si(3)-Si(1)-Si(2) 108.01(3), Si(5)-Si(1)-Si(2) 119.85(3), Si(4)-Si(1)-Si(2)
112.18(3).
Figure 22. Crystal structure of 24. Thermal ellipsoids are represented at the 30% level and hydrogen
atoms have been omitted for clarity (bond lengths in Å, angles in deg). C(1)-C(2) 1.543(5), C(1)-Si(2)
1.914(3), Si(1)-Si(4) 2.3649(11), Si(4)-Si(1)-Si(3) 104.76(4), Si(4)-Si(1)-Si(2) 106.26(4), Si(3)-Si(1)-Si(2)
105.44(4).
3. Experimental Section
General Remarks: 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. Solvents
were dried using a column solvent purification system [47]. All chemicals were obtained from
different suppliers and were used without further purification. 1,2-Dichlorotetramethyldisilane [48,49],
1,3-dichlorohexamethyltrisilane [48,49], 1,4-dichlorooctamethyltetrasilane [48,49], tetrakis(trimethylsilyl)
silane [50], isopropylbis(trimethylsilyl)silylpotassium [19], 2,5-bis(trimethylsilyl)dodecamethylhexasilane
(3) [51], bis(trimethylsilyl)triisopropylsilylsilylpotassium [19], tris(trimethylsilyl)triisopropylsilyl
silane [19], 2,2,5-tris(trimethylsilyl)undecamethylhexasilane [21], 2,2,6,6-tetrakis(trimethylsilyl)dodecameth
ylheptasilane (25) [52], 2,2,5-tris(trimethylsilyl)undecamethylhexasilane [21], tris(trimethylsilyl)]
trimethoxysilylsilane [53], 1,2-dimethyltetraphenyldisilane [54] and 1,1,4,4,-tetrakis(trimethylsilyl)
octamethylcyclohexasilane (26) [33] were prepared according to literature procedures.
1H- (300 MHz), 13C- (75.4 MHz), and 29Si- (59.3 MHz) NMR spectra were recorded on a Varian
Unity INOVA 300 (Varian, Palo Alto, CA, USA). Samples for 29Si spectra were either dissolved in

Molecules 2016, 21, 1079
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deuterated solvents or in cases of reaction samples measured with a D2O capillary in order to provide
an external lock frequency signal. To compensate for the low isotopic abundance of 29Si, the INEPT
pulse sequence [55,56] was used for the amplification of the signal. If not noted, otherwise the used
solvent was C6D6 and samples were measured at rt. Elementary analysis was carried using a Heraeus
VARIO ELEMENTAR EL apparatus (Hanau, Germany). UV spectra were measured on a Perkin Elmer
Lambda 35 spectrometer (Perkin-Elmer Corp., Waltham, MA, USA) using spectroscopy grade pentane
as solvent.
For X-ray structure analyses, the crystals were mounted onto the tip of glass fibers, and data
collection was performed with a BRUKER-AXS SMART APEX CCD diffractometer (Bruker-AXS:
Madison, WI, USA) using graphite-monochromated Mo Kα radiation (0.71073 Å). The data were
2
reduced to Fo and corrected for absorption effects with SAINT [57] and SADABS [58
,59], respectively.
Structures were solved by direct methods and refined by full-matrix least-squares method (SHELXL97
and SHELX2013) [60]. All non-hydrogen atoms were refined with anisotropic displacement parameters.
Hydrogen atoms were placed in calculated positions to correspond to standard bond lengths and
angles. Crystallographic data (excluding structure factors) for the structures of compounds
15 16a 17 18 18a 21a 21b 22 23 and 24 reported in this paper have been deposited with the
Cambridge Crystallographic Data Center as supplementary publication no. CCDC 670864 ( ), 975429
14), 1486947 (15), 1486948 (16a), 1486950 (17), 1486951 (18), 1027155 (18a), 974732 (21a), 974731
21b), 1486946 (22), 1486949 (23), and 1486945 (24). Copies of data can be obtained free of charge at:
9, 14,
,
,
,
,
,
,
,
,
9
(
(
http://www.ccdc.cam.ac.uk/products/csd/request/. Figures of solid state molecular structures were
generated using Ortep-3 as implemented in WINGX [61] and rendered with POV-Ray 3.6 [62].
Bis(trimethylsilyl)diisopropylsilane (
1
). A solution of isopropylbis(trimethylsilyl)silyl potassium (1.272 g,
◦
2.431 mmol) in benzene (20 mL) was added dropwise at 0 C to a solution of 2-chloropropane (0.191 g,
1.00 equiv) in benzene (10 mL). The reaction mixture remained colorless during the whole addition
process and a white precipitate was formed. After 2 h at 0 ◦C, the suspension was allowed to come
to rt followed by an aqueous workup (diethyl ether). Compound
a colorless liquid. 1H-NMR (
in ppm): 1.18 (m, 2H, CH), 1.11 (d, J = 6.3 Hz, 12H, CH3CH), 0.20
(s, 18H, Me3Si). 13C-NMR ( in ppm): 21.2 (CH3CH), 12.5 (CH), 1.4 (Me3Si). 29Si-NMR (
in ppm):
17.0 (Me3Si), Me], 217 (27.4)
1 (484 mg, 74%) was obtained as
δ
δ
δ
−
−
27.4 (Me3SiSi). MS (70 eV) m/z (%): 260 (33.7) [M+], 245 (6.5) [M+
−
[M+ − CHMe2], 175 (61.5) [M+ − (CHMe2)2], 73 (100) [SiMe3+].
2,2,5,5-Tetraisopropyldecamethylhexasilane (
1,2-dichlorotetramethyldisilane (54 mg, 0.289 mmol) and for the preparation of the silanide:
0.577 mmol), 18-crown-6 (152 mg, 1.00 equiv), and KOtBu (65 mg, 1.00 equiv) (29Si-NMR (
D2O capillary): 14.4 (Me3SiSiK)), 54.0 (Me3SiSiK)). Compound
colorless crystalline solid. Mp: 199–206 ◦C. 1H-NMR (
in ppm): 1.33 (m, 4H, CH), 1.18 (d, J = 7.8 Hz,
12H, CH3), 1.15 (d, J = 7.8 Hz, 12H, CH3), 0.47 (s, 12H, Me2Si), 0.28 (s, 18H, Me3Si). 13C-NMR (
in ppm):
21.7 (CH3), 21.5 (CH3), 12.0 (CH), 2.2 (Me3Si), in ppm): 15.7 (Me3Si), 18.3
0.3 (Me2Si). 29Si-NMR (
(Me3SiSi), Si2Me3(CH)2(CH3)4],
38.6 (Me2Si). MS (70 eV) m/z (%): 490 (4.0) [M+], 303 (9.4) [M+
245 (26.3) [M+
2).
Same procedure as described for 1 using
1
δ
(150 mg,
in ppm,
−
−
2 (360 mg, 90%) was obtained as a
δ
δ
−
δ
−
−
−
−
−
Si2Me4(CH)3(CH3)6], 203 (44.2) [Si4Me6H+], 187 (24.8) [Si4Me5+], 172 (57.8) [Si4Me4+],
73 (100) [SiMe3+]. Anal. calcd. for C22H58Si6 (491.22): C 53.79, H 11.90. Found: C−513.42, H 11.85. UV
−1
absorption: λ1 = 209 nm (ε1 = 1.7
×
104 M
·
cm−1), λ2 = 260 nm (ε2 = 2.4
×
104 M
·
cm−1), shoulder:
227 nm.
2,5,8,11-Tetrakis(trimethylsilyl)docosamethyldodecasilane (5). To a solution of 1,2-dichlorotetramethyldisilane
(38 mg, 0.202 mmol) in THF (30 mL) at 0 ◦C a solution of 5-trimethylsilyldodecamethylhexasilanyl-
2-potassium (
0.404 mmol), KOtBu (48 mg, 0.424 mmol) in THF (20 mL);
(Me3SiSiK), 12.5 (Me3SiSiMe), 26.5 (Me2Si), 35.7 (Me2Si),
4
) (freshly prepared from: 2,5-bis(trimethylsilyl)dodecamethylhexasilane (
3
) (200 mg,
in ppm, D2O capillary): 6.8
129.5 (MeSiK)) was
4
:
29Si-NMR (
δ
−
−
−
−
−
88.1 (MeSi),
−
added dropwise. After 14 h stirring at rt it was poured into a mixture of H2SO4 (0.5 M), diethylether,

Molecules 2016, 21, 1079
18 of 25
and ice. The layers were separated and the organic layer dried with Na2SO4. After removal of the
solvent the two isomers of in ppm,
were obtained as a colorless oil (136 mg, 70%). 1H-NMR (
CDCl3): 0.31–0.23 (m, 36H), 0.18 (s, 36H), 0.16 (s, 12H), 0.13 (s, 18H). 13C-NMR (
in ppm, CDCl3): 1.4,
in ppm, CDCl3): 11.6
33.7 (Me2Si), 33.7 (Me2Si),
81.6 (MeSi). UV absorption: λ1 = 218 nm
5
δ
δ
1.0, 0.4,
(Me3Si),
−
1.0,
11.7 (Me3Si),
34.0 (Me2Si),
−
1.1,
−
1.1,
−
−
1.2,
11.8 (Me3Si),
−
1.2,
−
1.5,
−
1.6,
−
9.3,
−
11.4. 29Si-NMR (
33.1 (Me2Si),
74.3 (MeSi),
δ
−
−
−
11.8 (Me3Si),
−
−
−
−
33.9 (Me2Si),
−
−
73.8 (MeSi),
−
−
(ε1 = 3.2 × 104 M−1·cm−1), λ2 = 268 nm (ε2 = 1.6 × 104 M−1·cm−1).
2,5,9,12-Tetrakis(trimethylsilyl)tetracosamethyltridecasilane (
6
). Same procedure as described for
was obtained
in ppm, CDCl3): 0.30–0.22 (m, 36H),
in ppm, CDCl3): 1.4, 1.0, 0.4, 1.1, 1.2, 1.3,
in ppm, CDCl3): 11.7 (Me3Si), 11.8 (Me3Si), 11.9
33.8 (Me2Si), 34.0 (Me2Si), 34.2 (Me2Si), 34.7 (Me2Si),
74.4 (MeSi), 75.3 (MeSi), 81.6 (MeSi), 82.0 (MeSi). UV absorption:
5 using:
4
(0.404 mmol) and 1,3-dichlorohexamethyltrisilane (0.202 mmol, 50 mg). Compound
6
1
as colorless oil (191 mg, 93%, mixture of both isomers). H-NMR (
0.18 (s, 36H), 0.16 (s, 12H), 0.13 (s, 24H). 13C-NMR (
−1.5,
(Me3Si),
36.3 (Me2Si),
δ
δ
−
−
−
−
1.6,
−
2.9,
−
9.4,
−
−
11.4. 29Si-NMR (
33.7 (Me2Si),
δ
−
−
−
−
33.2 (Me2Si),
−
−
−
−
−
−
36.4 (Me2Si),
−
−
−
−
λ1 = 212 nm (ε1 = 2.7 × 104 M−1·cm−1), λ2 = 272 nm (ε2 = 1.7 × 104 M−1·cm−1), shoulder: 287 nm.
2,5,10,13-Tetrakis(trimethylsilyl)hexacosamethyltetradecasilane (
using: (0.404 mmol) and 1,4-dichlorooctamethyltetrasilane (0.202 mmol, 62 mg). Compound
obtained as colorless oil (193 mg, 89%, mixture of both isomers). H-NMR (
(m, 36H), 0.18 (s, 36H), 0.16 (s, 12H), 0.13 (s, 18H), 0.13 (s, 12H). 13C-NMR (
7
). Same procedure as described for
was
in ppm, CDCl3): 0.29–0.22
in ppm, CDCl3): 1.4,
in ppm, CDCl3): 11.7
34.0 (Me2Si), 36.2 (Me2Si),
82.0 (MeSi). UV absorption: λ1 = 222 nm
5
4
7
1
δ
δ
1.0, 0.5,
(Me3Si),
−
1.1,
11.8 (Me3Si),
74.5 (MeSi),
−
1.2,
−
1.3,
−
−
1.5,
11.9 (Me3Si),
75.5 (MeSi),
−
1.6,
−
3.2,
33.7 (Me2Si),
81.6 (MeSi),
−
3.2,
−
9.5,
−
−
11.4. 29Si-NMR (
33.8 (Me2Si),
δ
−
−
−
−
−
36.4 (Me2Si),
−
−
−
−
(ε1 = 3.6 × 104 M−1·cm−1), λ2 = 285 nm (ε2 = 2.4 × 104 M−1·cm−1).
2,5-Bis(triisopropylsilyl)-2,5-bis(trimethylsilyl)decamethylhexasilan
(
9).
To
a
solution of
1,2-dichlorotetramethyldisilane (1.30 mmol, 243 mg) in THF (20 mL) a solution of bis(trimethylsilyl)
triisopropylsilylsilyl potassium (2.47 mmol, 916 mg) in THF (10 mL) was added dropwise. After
6 h same the work up procedure as for
yielded colorless crystalline
5
was applied and ◦crystallization from pentane/acetone
1
9
(908 mg, 91%). Mp.: 221–225 C. H-NMR (
δ
in ppm, CDCl3): 1.32
(sept, 6H, J = 7 Hz, CH(CH3)2), 1.22 (d, 36H, J = 7 Hz, CH(CH3)2), 0.50 (s, 12H, SiMe2), 0.33 (s, 36H,
SiMe3). 13C-NMR (
in ppm, CDCl3): 20.6 (CH(CH3)2), 15.2 (SiCH(CH3)2), 5.0 (SiMe3), 3.1 (SiMe2).
29Si-NMR ( in ppm, CDCl3): 11.1 (iPr3Si),
9.1 (Me3Si), 24.5 (Me2Si), 120.3 (Me3SiSi). Anal. calcd.
δ
δ
−
−
−
for C34H90Si10 (779.94): C 52.36, H 11.63. Found: C 52.09, H 11.58. UV Absorption: λ1 = 217 nm
(ε1 = 2.7 × 104 M−1·cm−1), λ2 = 264 nm (ε2 = 1.4 × 104 M−1·cm−1), shoulder: 230 nm.
2,5,8,11-Tetrakis(triisopropylsilyl)-2,5,8,11-tetrakis(trimethylsilyl)docosamethyl-dodeca-silane (11). Same procedure
as described for
5
using: 2,5-bis(triisopropylsilyl)-5-tris(trimethylsilyl)decamethylhexasilyl-2-potassium
(200 mg, 0.256 mmol) and KOtBu (30 mg, 0.269 mmol) in THF (10 mL))
in ppm): 1.42 (m, 36H), 1.34 (s, 12H), 1.32 (s, 6H), 0.83 (s, 6H), 0.67 (s, 6H), 0.55 (s, 12H),
0.53 (s, 18H). 29Si-NMR ( in ppm): 22.4 (iPr3SiSiK), 13.5 (iPr3SiSiq),
7.2 (Me3SiSiK), 9.6 (Me3Si),
14.2 (Me2SiSiK), 35.2 (Me2Si), 126.0 (Siq), 189.7 (SiK).] and 1,2-dichlorotetramethyldisilane
(
10) (freshly prepared from
9
1
10: H-NMR (
δ
δ
−
−
−
−
−
−
(0.202 mmol, 62 mg). After 3 h same work up procedure as for
5 yielded colorless crystalline 11
in ppm): 1.10–1.37 (m, 50H), 0.43 (s, 36H), 0.33
1
(220 mg, 87%, mixture of both isomers). H-NMR (
δ
(s, 18H), 0.31 (s, 36H). 29Si-NMR (
δ
in ppm): 15.9 (iPr3Si), 15.9 (iPr3Si), 13.9 (iPr3Si), 13.5 (iPr3Si),
9.2 (Me3Si), 9.3 (Me3Si), 9.4 (Me3Si), 24.1 (Me2Si), 24.5 (Me2Si),
32.0 (Me2Si), 113.2 (Siq), 120.0 (Siq), 125.0 (Siq), 125.2 (Siq). UV absorption:
−
9.1 (Me3Si),
28.2 (Me2Si),
−
9.2 (Me3Si),
−
−
−
−
−
−
−
−
−
−
−
λ1 = 216 nm (ε1 = 3.1 × 104 M−1·cm−1), λ2 = 264 nm (ε2 = 5.1 × 104 M−1·cm−1), shoulder: 232 nm.

Molecules 2016, 21, 1079
19 of 25
2,5,9,12-Tetrakis(triisopropylsilyl)-2,5,9,12-tetrakis(triisopropylsilyl)tetracosamethyl-tri-deca-silane (12). Same
procedure as described for 11 using: 10 (0.152 mmol) and 1,3-dichlorhexamethyltrisilane (40 mg,
0.161 mmol). After 5 h same work up procedure as for
5 yielded 12 colorless oil (271 mg, 71%,
1
mixture of both isomers). H-NMR (
δ
in ppm): 1.10–1.39 (m, 50H), 0.48 (s, 42H), 0.32 (s, 18H), 0.30
9.1 (Me3Si), 9.2 (Me3Si),
24.2 (Me2Si), 28.1 (Me2Si),
33.0 (Me2Si), 108.3 (Siq), 119.7
104 M−1 cm−1), λ2 = 267 nm
(s, 36H). 29Si-NMR (
δ
in ppm): 16.0 (iPr3Si), 14.1 (iPr3Si), 13.6 (iPr3Si),
9.4 (Me3Si), 9.4 (Me3Si), 10.7 (Me3Si), 23.7 (Me2Si),
32,7 (Me2Si), 32.8 (Me2Si), 33.0 (Me2Si),
−
−
−
9.3 (Me3Si),
32.0 (Me2Si),
−
−
−
−
−
−
−
−
−
−
−
−
−
(Siq),
−
124.9 (Siq), 125.1 (Siq). UV absorption: λ1 = 219 nm (ε1 = 2.8
×
·
(ε2 = 4.6 × 104 M−1·cm−1), λ3 = 283 nm (shoulder), λ4 = 307 nm (shoulder).
2,5,10,13-Tetrakis(triisopropylsilyl)-2,5,10,13-tetrakis(triisopropylsilyl)hexacosamethyl-tetra-decasilane (13).
Same procedure as described for 11 using: 10 (0.152 mmol) and 1,4-dichlorooctamethyltetrasilane
(51 mg, 0.167 mmol). After 5 h same work up procedure as for
5
yielded 13 as a colorless oil (211 mg,
1
81%, mixture of both isomers). H-NMR (
δ
in ppm): 1.12–1.37 (m, 50H), 0.48 (s, 48H), 0.32 (s, 18H), 0.30
9.1 (Me3Si), 9.2
29.2 (Me2Si), 29.5
119.3 (Siq), 119.4
(s, 36H). 29Si-NMR (
δ
in ppm): 16.9 (iPr3Si), 14.2 (iPr3Si), 14.1 (iPr3Si), 11.1 (iPr3Si),
9.3 (Me3Si), 23.1 (Me2Si), 24.1 (Me2Si), 28.8 (Me2Si),
31.3 (Me2Si), 32.0 (Me2Si), 104.6 (Siq), 106.9 (Siq),
104 M−1
−
−
(Me3Si),
(Me2Si),
−
9.3 (Me3Si),
29.6 (Me2Si),
−
−
−
−
−
−
−
−
−
−
−
−
−
(Siq). UV absorption: λ1 = 220 nm (ε1 = 2.9
×
·
cm−1), λ2 = 268 nm (ε2 = 4.5 × 104 M−1·cm−1
),
λ3 = 293 nm (ε2 = 3.8 × 104 M−1·cm−1), λ4 = 311 nm (shoulder).
2,5,8,11-Tetrakis(trimethylsilyl)icosamethyldodecasilan (14). Same procedure as described for
5 using
2,5-bis(trimethylsilyl)undecamethylhexasilanyl-2-potassium (1.63 mmol) (prepared from: 2,2,5-tris
(trimethylsilyl)undecamethylhexasilane (900 mg, 01.63 mmol), KOtBu (160 mg, 1.71 mmol) in THF
(10 mL)) and 1,2-dichlorotetramethyldisilane (0.854 mmol, 16 mg). After 5 h same work up procedure
as for
1H-NMR (
(s, 36H). 29Si-NMR (
5
yielded colorless crystalline 14 (830 mg, 95%) after recrystallization using pentane/acetone.
δ
in ppm, CDCl3): 0.46 (s, 12H), 0.42 (s, 12H), 0.35 (s, 12H), 0.30 (s, 36H), 0.28 (s, 6H), 0.20
δ
in ppm, CDCl3):
−
9.2 (Me3SiSiMe),
−
11.7 (Me3Si),
−
28.2 (SiMe2),
−
31.2 (SiMe2),
−31.6 (SiMe2), −80.6 (SiMe), −118.3 ((Me3Si)2Si). UV absorption: λ = 265 nm (ε = 5.0 × 104 M−1·cm−1).
2,5-Bis(trimethylsilyl)-2,5-bis(dimethyl-2’-exo-norbornylsilyl)decamethylhexasilane (15). A solution of
tris(trimethylsilyl)silyl potassium (prepared from: tetrakis(trimethylsilyl)silane (2000 mg, 6.23 mmol)
and KOtBu (721 mg, 6.42 mmol) in DME (40 mL) at 60 ◦C) was added dropwise to a DME
(10 mL) solution of dimethyl-2-exo-norbornylchlorosilane (1294 mg, 6.85 mmol). After 24 h same
work up procedure as for
5 and crystallization out of acetone at rt. yielded colorless crystalline
2-(dimethyl-2’-exo-norbornylsilyl)-2-(trimethylsilyl)hexamethyltrisilane (15a) (1285 mg, 51%). Mp.:
◦
55–60 C. 1H-NMR (
0.16 (s, 3H, SiMe2). 13C-NMR (
(SiMe2)
0.5 (SiMe2). 29Si-NMR (
C18H44Si5 (400.97): C 53.92, H 11.06. Found: C 53.61, H 10.96.
δ
in ppm): 2.31 (s, 2H) 1.75–1.13 (m, 9H), 0.26 (s, 27H, 3 SiMe3), 0.21 (s, 3H, SiMe2),
in ppm): 38.8, 37.7, 37.1, 34.2, 33.5, 30.5, 28.8, 2.9 (3 SiMe3), 0.3
in ppm): 5.6 (SiMe2), 10.8 (SiMe3), 136.3 (Siq). Anal. calcd. for
δ
−
−
δ
−
−
−
A solution of 2-dimethyl-2’-exo-norbornylsilylhexamethyltrisilyl-2-potassium (prepared from 15a
(1285 mg, 3.20 mmol) and KOtBu (377 mg, 3.36 mmol) in THF (40 mL) at rt.) is slowly added to a
solution of 1,2-dichlorotetramethylsilane (300 mg, 1.6 mmol) in THF (10 mL). After 24 h same work up
◦
procedure as for
5
and crystallization out of acetone at
−
20 C yielded colorless crystalline 15 (1010 mg,
82%). Mp: 125–127 ◦C. 1H-NMR (δ in ppm): 2.50 (s, 4H) 1.62–0.90 (m, 18H), 0.57 (s, 12H, SiMe2), 0.34
and 0.33 (each s, 18H, SiMe3), 0.31 and 0.24 (each s, 6H, SiMe2). 13C-NMR (
in ppm): 39.9, 38.9, 37.1,
34.1, 33.8, 32.3, 28.8, 3.8 (SiMe3), 1.8 (2 SiMe3), 0.4 and 0.6 (SiMe2). 29Si-NMR (
in ppm): 5.2 (SiMe2),
127.4 (Siq). Anal. calcd. for C34H82Si10 (771.88): C 52.91, H 10.71.
Found: C 52.41, H 10.59. UV absorption: λ = 258 nm (ε = 7.1 × 104 M−1·cm−1).
δ
δ
−
−
9.7 (2 SiMe3),
−
29.1 (2 SiMe2),
−
2,5-Bis(trimethoxysilyl)-2,5-bis(trimethylsilyl)decamethylhexasilane (16). Tris(trimethyl-silyl)]-trimethoxy-
silylsilane (100 mg, 0.272 mmol) and KOtBu (32 mg, 0.285 mmol) were dissolved in THF (2 mL).

Molecules 2016, 21, 1079
20 of 25
After NMR spectroscopy confirmed complete formation of the silanide 16a the solution was added
dropwise to an ice cooled solution of 1,2-dichlorotetramethyldisilane (27 mg, 0.142 mmol) in toluene
(5 mL). After 2 h the solvent was removed and the residue crystallized from a mixture of diethylether
and acetonitrile 2:1. Colorless crystals of 16 (67 mg, 69%) were obtained. Mp: 134–138 ◦C. 1H-NMR
(
δ
in ppm): 3.48 (s, 18H, OMe3), 0.71 (s, 12H, SiMe2), 0.46 (s, 36H, SiMe3). 13C-NMR (
(OMe), 3.3 (SiMe3), 0.4 (SiMe2). 29Si-NMR (
in ppm): 9.2 (SiMe3), 30.5 (SiMe2), 32.7 (Si(OMe)3),
135.7 (SiMe3). Anal. calcd. for C22H66O6Si10 (706.25): C 37.34, H 9.40. Found: C 38.48, H 9.00. UV
δ in ppm): 50.3
δ
−
−
−
−
absorption: λ = 253 nm (ε = 8.4 × 104 M−1·cm−1) in n-hexane.
For the characterization of silanide 16a its formation was undertaken in the presence of 18-crown-6:
A mixture of 50 mg (0.135 mmol, 1 eq) of tris(trimethyl-silyl)]-trimethoxy-silylsilane, KOtBu (16 mg,
0.142 mmol) and 18-crown-6 (38 mg, 0.098 mmol) was dissolved in C6D6 (1 mL) and left for 14 h. After
NMR spectroscopy confirmed formation of methoxysilylpotassium in a pale beige solution, C6D6 was
removed in vacuum after which the residue was dissolved in diethylether. Very sensitive pale beige
1
crystals of 16a were obtained on the walls of the vial. Yield: (80 mg, 100%). H-NMR (d in ppm: 3.74
(s, 9H, OCH3), 3.24 (s, 24H, CH2O), 0.68 (s, 18H, Si(CH3)3). 13C-NMR (d in ppm): 70.1 (CH2O), 50.1
(OMe), 7.7 (SiMe3). 29Si-NMR (d in ppm): 0.9 (Si(OMe)3), −4.3 (Si(SiMe3)3); −220.9 (Si(SiMe3)3).
2,5-Bis(tert-butyldiphenylsilyl)-2,5-bis(trimethylsilyl)decamethylhexasilane (17). Compound 23 (300 mg,
0.616 mmol) and KOtBu (71 mg, 0.635 mmol) were dissolved in THF (3 mL). After NMR spectroscopy
confirmed complete formation the solvent was removed, the residue solved in toluene (3 mL) and
added dropwise over 5 minutes to a solution of 1,2-dichlorotetramethyldisilane (60 mg, 0.323 mmol)
in toluene (2 mL). After 3 h same work up procedure as for
pentane/acetone 2:1 colorless crystalline 25 (210 mg, 72%). Mp: 200–204 C. H-NMR (
CDCl3): 7.66 (m, 8H, Ph), 7.35 (m, 12H, Ph), 1.09 (s, 18H, tBu), 0.47 (s, 12H, SiMe2), 0.11 (s, 36H, SiMe3).
5 yielded after recrystallization from
◦
1
δ
in ppm
13C-NMR (
δ
in ppm CDCl3): 137.38, 137.17, 128.81, 127.16, 29.77 (tBu), 20.39 (tBu), 4.54 (SiMe3), 2.84
(SiMe2). 29Si-NMR (
δ
in ppm CDCl3): 5.4 (SiPh2 Bu),
−
8.7 (Me3Si),
−
23.8 (Me2Si),
−
121.9 (Siq). Anal.
t
calcd. for C48H86O6Si10 (944.07): C 61.07, H 9.18. Found: C 60.94, H 8.93. UV absorption:
(ε = 3.2 × 104 M−1·cm−1), λ = 270 nm (ε = 6.8 × 104 M−1·cm−1) in diethylether.
λ = 240 nm
2,2,5,5-Tetrakis(trimethylsilyl)-3,4-diphenyloctamethylhexasilane
(
18).
To
a
solution of
1,2-dimethyltetraphenyldisilane (775 mg, 1.9 mmol) in CH2Cl2 (10 mL) triflic acid (604 mg,
3.9 mmol) is added slowly. After 12 h the reaction is complete (controlled by 29Si-NMR), the
solvent is removed and the white precipitate is suspended in toluene (10 mL). A solution of
tris(trimethylsilyl)silyl potassium (prepared from tetrakis(trimethylsilyl)silane (1192 mg, 3.7 mmol),
KOtBu (3.9 mmol), and 18-crown-6 (3.9 mmol) in toluene (10 mL)) is added slowly. After 24 h same
work up procedure as for
18 (1.43 g, 98%). Mp: 271–273 C. H-NMR (
(s, 54H, (SiMe3)6). 13C-NMR (
5
and ◦crystallization out of acetone at
−
20 ◦C yielded colorless crystalline
in ppm): 6.70 (m, 10H), 0.39 (s, 6H, (SiMe)2), 0.30
1
δ
δ
in ppm, CDCl3): 138.8, 135.7, 128.2, 127.9, 3.1 (SiMe3),
−
0.6 (SiMePh).
29Si-NMR (
δ
in ppm): 9.5 (SiMe3),
−
−
30.3 (SiMePh),
−
124.2 (Siq). Anal. calcd. for C32H70Si10 (735.76):
C 52.24, H 9.59. Found: C 49.73, H 9.36. UV absorption: λ = 262 nm (ε = 2.4 × 104 M−1·cm−1).
1,5-Bis(pentamethyldisilanyl)-1,1,5,5-tetrakis(trimethylsilyl)hexamethylpentasilane (19). To a solution
of pentamethyldisilanylchloride (260 mg, 1.559 mmol) in benzene (4 mL) a solution of 2,6-bis
(trimethylsilyl)dodecamethylheptasilyl 2,6-dipotassium (prepared from: 2,2,6,6-tetrakis(trimethylsilyl)
dodecamethylheptasilane (390 mg, 0.773 mmol), 18-crown 6 (413 mg, 2.02 equiv) and KOtBu (175 mg,
2.02 equiv) in benzene (5 mL)) was added dropwise over a period of 10 min. After 2 h same work
up procedure as for
5
and colorless waxy 19 (528 mg, 87%) was obtained. Mp: 85–95 ◦C. 1H-NMR
(δ in ppm): 0.54 (s, 12H + 6H), 0.40 (s, 12H), 0.36 (s, 36H), 0.22 (s, 18H). 13C-NMR (δ in ppm): 4.0, 1.6, −0.3,
−0.4, −1.6. 29Si-NMR (δ in ppm): −9.6, −14.4, −29.7, −36.7, −39.0, −122.5. Anal. calcd. for C28H84Si13
(786.083): C 42.78, H 10.77. Found: C 41.23, H 9.99. UV absorption: λ1 = 226 nm (ε1 = 2.1 × 104 M−1·cm−1
λ2 = 230 nm (ε2 = 1.9 × 104 M−1·cm−1), λ3 = 273 nm (ε2 = 3.6 × 104 M−1·cm−1).
),

Molecules 2016, 21, 1079
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2,2,8,8-Tetrakis(trimethylsilyl)hexadecamethylnonasilane (20). A solution of 19 (210 mg, 0.272 mmol)
and Al(Fe)Cl3 (20 mg) in cyclohexane (3 mL) was heated to reflux. After 16 h the reaction was
complete (controlled by 29Si-NMR) and acetone (10 mL) was added. The precipitate was removed by
centrifugation, the solvent removed and the residue crystallized by using acetone. Colorless crystalline
of 20 was obtained (136 mg, 65%). Mp: 182–186 ◦C. 1H-NMR (
0.39 (s, 6H), 0.33 (s, 54H). 13C-NMR ( 3.3. 29Si-NMR (
in ppm): 3.4, 0.9, 2.7,
35.3, 36.6, 129.1. Anal. calcd. for C28H84Si13 (786.083): C 42.78, H 10.77. Found: C 41.23, H 9.99.
δ
in ppm): 0.49 (s, 12H), 0.43 (s, 12H),
in ppm): 9.8, 31.9,
δ
−
−
δ
−
−
−
−
−
UV Absorption: λ1 = 288 nm (ε1 = 4.0
λ3 = 246 nm (ε3 = 1.7 × 104 M−1·cm−1).
×
104 (M−1
·
cm−1)), λ2 = 271 nm (ε2 = 2.7
×
104 M−1 cm−1),
·
1,4-Bis(triisopropylsilyl)-1,4-bis(trimethylsilyl)octamethylcyclohexasilane (21). A dark red solution of the
dianion (prepared from:
(215 g, 0.276 mmol), KOtBu (68 mg, 0.606 mmol), 18-crown-6 (160 mg,
9
0.606 mmol) in benzene (10 mL)) diluted with THF (3 mL) was added dropwise to a solution of
1,2-dichlorotetramethyldisilane (48 mg, 0.256 mmol) in THF (5 mL). After 24 h same work up procedure
as for
5 yielded colorless crystalline 21 (80 mg, 39%) after recrystallization with Et2O. Anal. calcd. for
C32H84Si10 (749.86): C 51.25, H 11.29. Found: C 51.01, H 11.03. The crystals were so large that they
could be separated manually and so it was possible to characterize both isomers.
Cis-1,4-bis(triisopropylsilyl)-1,4-bis(trimethylsilyl)cyclohexasilane (21a): Mp: 229–231 ◦C. 1H-NMR (
ppm): 1.35 (sept, 6H, Si(CH)(CH3)2), 1.20 (d, J = 7.2 Hz, 36H, Si(CH)(CH3)2), 0.48 (s, 12H, SiMe3),
0.47 (s, 12H, SiMe3), 0.40 (s, 18H, SiMe3). 13C-NMR ( in ppm): 20.6 (iPr), 14.6 (iPr), 5.4 (SiMe3), 1.2
8.7 (SiMe3), 37.0 (SiMe2), 128.9 (Siq). UV
δ in
δ
(SiMe3), 0.9 (SiMe2). 29Si-NMR (
δ
in ppm): 22.8 (Si-iPr3),
−
−
−
absorption: λ1 = 251 nm (ε1 = 6.1 × 103 M−1·cm−1), shoulder: 276 nm (ε = 8.3 × 102 M−1·cm−1).
◦
1
Trans-1,4-bis(triisopropylsilyl)-1,4-bis(trimethylsilyl)cyclohexasilane (21b): Mp: 240–242 C. H-NMR
in ppm): 1.40 (sept, 6H, Si(CH)(CH3)2), 1.21 (d, J = 7.2 Hz, 36H, Si(CH)(CH3)2), 0.53 (s, 12H,
SiMe2), 0.46 (s, 12H, SiMe2), 0.40 (s, 18H, SiMe3). 13C-NMR ( in ppm): 20.5 (iPr), 14.8 (iPr), 5.5 (SiMe3),
9.3 (SiMe3), 35.3 (SiMe2), 127.8 (Siq).
(δ
δ
1.6 (SiMe2), 0.8 (SiMe2). 29Si-NMR (
δ
in ppm): 17.2 (Si-iPr3),
−
−
−
UV absorption: λ1 = 250 nm (ε1 = 8.8 × 103 M−1·cm−1), shoulder: 267 nm (ε = 3.2 × 103 M−1·cm−1).
1,1,4,4-Tetrakis(trimethylsilyl)-2,3-diphenyltetramethylcyclopentasilane (22). A benzene (5 mL) solution of
18 (283 mg, 0.38 mmol), KOtBu (88 mg), and 18-crown-6 (208 mg) is stirred for 12 h after which the
conversion to 2,5-bis(trimethylsilyl)-3,4-diphenyloctamethylhexasilyl 2,5-dipotassium (18a) is complete
1
as could be proved by NMR. (18a: H-NMR (
δ
in ppm): 6.70 (m, 10H), 1.11 (s, 6H, SiMePh),
−
0.03
(s, 36H, SiMe3). 29Si-NMR (δ in ppm): −5.0 (SiMe3), −22.7 (SiMePh), −180.5 (Siq).)
This solution was added dropwise to dichlorodimethylsilane (45 mg, 0.35 mmol) in toluene
(2 mL). After 24 h same work up procedure as for ◦
(s, 6H, SiMePh), 0.64 (s, 6H, SiMe2), 0.36 (s, 18H, (SiMe3), 0.14 (s, 18H, SiMe3). 13C-NMR (
5
and crystallization out of acetone at rt. yielded
in ppm): 7.00–7.70 (mp, 10H), 0.81
in ppm):
in ppm): 6.6
colorless crystalline 22 (213 mg, 85%). Mp: 194–197 C. 1H-NMR (
δ
δ
138.7, 135.4, 128.3, 127.8, 4.2 (SiMePh), 4.1 (SiMe3), 3.5 (SiMe2),
−
1.1 (SiMe2). 29Si-NMR (
δ
−
(SiMe3), −8.1 (SiMe3), −23.4 (SiMePh), −20.7 (SiMePh), −127.6 (Siq). Anal. calcd. for C28H58Si9 (647.54):
C 51.94, H 9.03. Found: C 50.73, H 8.86. UV absorption: shoulder: λ = 256 nm (ε = 1.0 × 104 M−1·cm−1).
2-(tert-Butyldiphenylsilyl)-2-(trimethylsilyl)hexamethyltrisilane (23). Tetrakis(trimethyl-silyl)-silane (2.00 g,
6.23 mmol), KOtBu (721 mg, 6.41 mmol) and 18-crown-6 (1.69 g, 6.41 mmol) were dissolved in
toluene (20 mL). After NMR showed complete formation of hypersilyl potassium, this orange solution
◦
was added dropwise to tert-butylchlorodiphenylsilane in toluene (10 mL) at
−
60 C. The reaction
mixture was allowed to warm to room temperature. After 6 h same work up procedure as for
crystallization out of acetone yielded colorless crystalline 23 (3.25 g, 69%). Mp: 342–345 ◦C. 1H-NMR
in ppm, CDCl3): 7.64 (m, 4H), 7.35 (m, 6H), 1.10 (s, 9H, (tBu), 0.12 (s, 27H, (Me3Si).13C-NMR (
in
ppm, CDCl3): 137.2, 137.1, 128.7, 127.2, 29.4 (Me3-C), 20.2 (Me3-C), 3.6 (Me3Si). 29Si-NMR (
in ppm,
5 and
(δ
δ
δ

Molecules 2016, 21, 1079
22 of 25
CDCl3): 3.7 (tBuPh2Si),
−
9.4 (Me3Si),
−
134.1 ((Me3Si)3Si). Anal. calcd. for C25H46Si5 (487.07): C 61.65,
H 9.52. Found: C 60.93, H 9.38. UV absorption: λ = 239 nm (ε = 1.22 × 104 M−1·cm−1).
2-(2’-exo-Norbornyldimethylsilyl)-2-(triisopropylsilyl)hexamethyltrisilane (24). A solution of tris(trimethylsilyl)
triisopropylsilylsilane (639 mg, 1.58 mmol) and KOtBu (186 mg 1.66 mmol) in DME (10 mL) was stirred
◦
at 60 C for 1h. This solution was transferred into a dropping funnel and added slowly to a solution of
dimethyl-2-exo-norbornylchlorosilane in DME (5 mL). After 18 h, the same work-up procedure as for
yielded a colorless oil of 24 (529 mg, 69%). Crystallization with acetone gave colorless plates suitable
for X-ray analysis. Mp: 110–112 ◦C. 1H-NMR (
in ppm): 1.50–2.50 (m, 11H, C7H11Si), 1.28 (sept, 3H,
SiCH(CH3)2), 1.18 (d, 2JHH = 6.9 Hz, 18H, SiCH(CH3)2), 0.34 and 0.33 (s, each 9H, Si(CH3)3) 0.31 and
0.25 (s, each 3H, SiMe2). 13C-NMR (
in ppm): 39.1, 38.0, 37.0, 34.0, 33.9, 29.8, 28.9 (Si(CH3)2C7H11),
20.2 (SiCH(CH3)2), 14.8 (SiCH(CH3)2), 4.35 and 4.31 (Si(CH3)3), 1.2 and 0.9 (Si(CH3)2). 29Si-NMR (
in
ppm): 13.5 (SiiPr3),
4.9 (SiMe2), 10.1 (SiMe3), 134.2 (SiSi4). Anal. calcd. for C24H56Si5 (485.14): C
= 1.5
104 M−1 cm−1),
5
δ
δ
δ
−
−
−
59.42, H 11.64. Found: C 58.58, H 11.67. UV absorption: shoulder: 222 nm (
ε
×
·
λ1 = 274 nm (ε1 = 2.6 × 103 M−1·cm−1), shoulder: 283 nm (ε = 2.3 × 103 M−1·cm−1).
4. Conclusions
One of the interesting aspects of the chemistry of poly- and oligosilanes is the associated property
σ-bond electron delocalization. In recent studies we have shown that terminal tris(trimethylsilyl)silyl
of
groups of short oligosilanes are able to force the molecules to engage in an all-transoid conformation,
which allows a high degree of -bond electron delocalization. In case of longer oligosilanes this
σ
directing effect is diminished and a second conformation becomes energetically accessible. When we
introduced bulky bis(trimethylsilyl)silylene segments inside the chain as conformational amplifiers,
these units produced cisoid turns causing rupture of conjugation. The current study is concerned with
alteration of the steric bulk of the end groups and internal substituents. By exchange of trimethylsilyl
groups against methyl or triisopropyl groups the polysilane chains become either more flexible or
more rigid. This change in conformataional behavior is nicely reflected in the shape and position of the
UV-absorption bands. While flexible molecules exhibit broad bands, indicating a larger conformational
space the rigid molecules show sharp bands which can be associated with the absorption of more
precisely defined conformers. The deliberate introduction of phenyl substituents either in terminal
or internal positions causes a bathochromic shift of the absorption bands, which is much more
pronounced for internal substitution. Subsequent work on these compounds will be concerned with
variable temperature studies of these compounds to investigate their thermochromic behavior.
Supplementary Materials: The following are available online at http://www.mdpi.com/1420-3049/21/8/1079/s1.
Crystallographic information for compounds
crystallographic tables.
9, 14, 15, 16a, 17, 18, 18a, 21a, 21b, 22, 23, and 24 in CIF format and
Acknowledgments: Support of the study was provided by the Austrian Fonds zur Förderung der wissenschaftlichen
Forschung (FWF) via projects I-00669, P-22678 and P-25124.
Author Contributions: The manuscript was written through contributions of all authors. All authors have given
approval to the final version of the manuscript. Johann Hlina, Filippo Stella, and Mohammad Aghazadeh Meshgi
did all the experiments and analyses. Christoph Marschner and Judith Baumgartner provided supervision,
financial support via projects and wrote the paper.
Conflicts of Interest: The authors declare no conflict of interest.
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©
2016 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access
article distributed under the terms and conditions of the Creative Commons Attribution
(CC-BY) license (http://creativecommons.org/licenses/by/4.0/).