Electroreductive synthesis of oligosilanes and polysilanes with ordered sequences

Article abstract of DOI:10.1016/S0022-328X(00)00327-2

The stepwise elongation of Si-Si or Si-Ge chain was achieved by the electroreductive cross-coupling reaction of chlorohydrosilanes with dichlorooligosilanes using magnesium electrodes. The electroreductive cross-coupling reaction of chlorodimethylsilane (

Full text of DOI:10.1016/S0022-328X(00)00327-2

www.elsevier.nl/locate/jorganchem
Journal of Organometallic Chemistry 611 (2000) 26–31
Electroreductive synthesis of oligosilanes and polysilanes with
ordered sequences
Manabu Ishifune a, Shigenori Kashimura a,*, Yasumichi Kogai a,
Yasuyoshi Fukuhara a, Tomo Kato a, Hang-Bom Bu a, Natsuki Yamashita a,
Yoshibiro Murai a, Hiroaki Murase b, Ryoichi Nishida b
a Faculty of Science and Technology, Kinki Uni6ersity, 3-4-1 Kowakae, Higashi-Osaka, Osaka 577-8502, Japan
b Applied Research Center, Research and De6elopment Department, Osaka Gas Co. Ltd., 1 Chyudoji-Awata-Machi, Simogyo,
Kyoto 600-8815, Japan
Received 21 February 2000; received in revised form 25 April 2000; accepted 10 May 2000
Abstract
The stepwise elongation of SiꢀSi or SiꢀGe chain was achieved by the electroreductive cross-coupling reaction of chlorohydrosi-
lanes with dichlorooligosilanes using magnesium electrodes. The electroreductive cross-coupling reaction of chlorodimethylsilane
(1) with dichlorodiphenylsilane (2) or dichlorodiphenylgermane (4), for instance, gave 1,3-dihydro-1,1,3,3-tetramethyl-2,2-diphenyl-
trisilane (3) or bis(hydrodimethylsilyl)germane (5) in good yield. Compounds 3 and 5 were readily transformed to the
corresponding 1,3-dichlorotrisilane (6) and bis(chlorosilyl)germane (7), respectively. The electroreductive polymerization of the
resulting dichlorooligosilanes using magnesium electrodes in tetrahydrofuran was successfully applied to the synthesis of
sequence-ordered polysilanes. © 2000 Elsevier Science S.A. All rights reserved.
Keywords: 1,3-Dihydrotrisilanes; 1,3-Dichlorotrisilanes; Polysilanes; Silane–germane copolymers; Electroreductive polymerization; Magnesium
electrode
ture. Several novel methods have opened new ap-
proaches to the synthesis of structure-ordered
1. Introduction
polysilanes, which are essential to the further develop-
Polysilanes and polygermanes have been attracting
ment in the polysilane chemistry [3a]. The anionic
considerable attention due to their potential in the
polymerization of masked disilenes has provided a use-
preparation of new types of material showing conduct-
ful method for preparing some poly(disilanylene)s
ing, photoconducting, and nonlinear optical properties
whose units are ordered in two sequences [3b,c]. The
[1]. They have usually been prepared by the Wurts type
anionic ring-opening polymerization of cyclic oligosi-
coupling reaction of organodichlorosilanes with alkali
lanes is another method for obtaining structure-con-
metals (the Kipping’s method) [2], however, this
method requires rather drastic reaction conditions, and
hence, has a disadvantage in controlling the unit-struc-
trolled polysilanes [4].
Recently we have been studying the synthesis of
polysilanes by using electrochemical methods and it has
been reported in previous papers [5] that the electrore-
duction of organodichlorosilanes with Mg electrodes
led to the formation of polysilanes having monomodal
and sharp molecular weight distribution.
In this paper we wish to report that our electroreduc-
Scheme 1.
tion system was successfully applied to the stepwise
elongation of SiꢀSi chain and the synthesis of structure-
controlled polysilanes.
* Corresponding author. Fax: +81-6-67232721.
E-mail address: r8kashi@cced.kindai.ac.jp (S. Kashimura).
0022-328X/00/$ - see front matter © 2611 Elsevier Science S.A. All rights reserved.
PII: S0022-328X(00)00327-2

M. Ishifune et al. / Journal of Organometallic Chemistry 611 (2000) 26–31
27
Table 1
Electroreductive cross-coupling of chlorosilane 1 with dichlorosilane 2 a
Entry
Chlorosilane 1 (mol l−1
)
Dichlorosilane 2 (mol l−1
)
Electricity (F mol−1
)
Yield of 3 b (%)
1
2
3
4
5
1.69
2.33
2.31
3.30
2.93
0.34
0.33
0.33
0.66
1.00
6
6
4
6
6
41
63
45
43
3
6 c
7 c
2.30
2.30
0.33
0.33
6
4
75
36
a The electroreduction was carried out in a single-compartment cell using Mg electrodes under sonication (47 kHz), and the polarity of
electrodes was alternated with An interval of 15 s.
b Isolated.
c The solution of 1 and 2 was added dropwise during the electroreduction.
2.2. Stepwise elongation of the SiꢀSi chain
2. Results and discussion
The SiꢀH bond was readily transformed to the SiꢀCl
bond by treatment with a catalytic amount of ben-
zoylperoxide in carbon tetrachloride [7]. Using this
2.1. Electroreducti6e cross-coupling reaction of
chlorodimethylsilane 1 with dichlorosilanes 2
We have already reported that the use of Mg elec-
trodes is highly effective for the electroreductive forma-
tion of SiꢀSi bonds under mild reaction conditions [5].
The mildness of the reaction conditions of this elec-
troreductive method is favorable for the synthesis of
polysilanes having various functionalities such as SiꢀH
bonds, which are known to be reactive under radical or
anionic conditions [6]. The electroreductive cross-cou-
pling reaction of chlorodimethylsilane (1) with
dichlorodiphenylsilane (2), in fact, gave the correspond-
ing trisilane 3 (Scheme 1). Some examples of the opti-
mization of reaction conditions are summarized in
Table 1.
method
1,3-dihydro-1,1,3,3-tetramethyl-2,2-dipheny-
trisilane (3) and bis(hydrodimethylsilyl)germane (5)
were transformed into the corresponding chlorides 6
and 7, respectively (Scheme 3). The further electrore-
duction
of
1,3-dichloro-1,1,3,3-tetramethyl-2,2-
diphenyltrisilane (6) with dimethylchlorosilane (1) gave
the corresponding pentasilane 8 (Scheme 4), and these
sequences were utilized for the synthesis of the odd-
numbered oligosilanes. In the same manner, the even-
numbered oligosilanes can be prepared. For instance,
1,4-dichloro-1,1,2,3,3,4,4-heptamethyl-2-phenyltetrasi-
lane (11) can be prepared by the reaction between 1
and 1,2-dichloro-1,1,2-trimethyl-1-phenyldisilane (9)
(Scheme 5). Accordingly, the electroreductive cross-
The electroreduction was carried out under constant
current conditions in
a
single-compartment cell
equipped with a Mg cathode and anode using LiClO4
as a supporting electrolyte and dry tetrahydrofuran
(THF) as a solvent [5]. During the electroreduction the
electrolysis cell was sonicated (47 kHz) and the polarity
of electrodes was alternated with intervals of 15 s. The
optimized supplied electricity was found to be 6 F
mol−1 based on 2 (entries 2–4). In this cross-coupling
reaction the use of an excess amount of chlorodimethyl-
silane 1 relative to dichlorosilane 2 gave a better result
(entries 1, 2), since a high concentration of 2 resulted in
its oligomerization. The best yield of trisilane 3 was
finally obtained when the solution of 2 in THF was
added dropwise into the electrolyte during the reaction
(entry 6). Under optimized reaction conditions the elec-
Scheme 2.
Scheme 3.
troreductive cross-coupling reaction of
1
with
diphenyldichlorogermane (4) was also carried out to
form bis(dimethylsilyl)diphenylgermane (5) in good
yield (Scheme 2).
Scheme 4.

28
M. Ishifune et al. / Journal of Organometallic Chemistry 611 (2000) 26–31
Scheme 5.
coupling reactions followed by the chlorination pro-
vided a powerful method for the stepwise elongation of
the SiꢀSi bonds and synthesis of sequence-controlled
oligosilanes.
provides possibilities for designing oligosilane se-
quences of internal silicon and germanium atoms.
3. Experimental
2.3. Electroreducti6e polymerization of
1,2-dichloro-1,1,2-trimethyl-1-phenyldisilane (9)
IR spectra were obtained on a Hitachi 260-10 spec-
1
trometer. H-NMR and 13C-NMR spectra were mea-
sured on
a
Varian Gemini-200 (200 MHz)
The electroreductive polymerization of the dichloro-
oligosilanes is highly promising for the synthesis of
sequence-ordered polysilanes. The electroreduction of
dichlorodisilane 9 was found to give the corresponding
polysilane 12 consisting of disilane units (Scheme 6).
The electroreductive polymerization was carried out
under a variety of reaction conditions, however, the
yield of the resulting polymer was very low (Table 2)
and low molecular weight oligomers (MnB1000) were
formed. This is probably due to high reactivity of the
disilene intermediate formed by the electroreduction of
9. In fact, the addition of naphthalene, which could
mask disilene [8], into the reaction system remarkably
increased the yield of the polysilane 12 (entry 6).
spectrometer, and the chemical shifts were referenced to
CDCl3 peaks. GC analyses were carried out on a
Shimadzu GC-4C or GC-12A instrument. The gel per-
meation chromatography (GPC) system consisted of a
Shimadzu LC-6A liquid chromatograph, a Shimadzu
SP13-6AV UV–vis spectrophotometric detector, and a
Shodex® GPC A-803 column. Molecular weight values
are relative to the polystyrene standards [Shodex®
STANDARD (SL-105) polystyrene]. The constant elec-
trocurrent was supplied using a Takasago GPO 50-2
regulated DC power supply. The supplied electricity
was counted by a Hokuto Denko Coulomb Am-
perehour Meter HF-201. The sonication of ultrasound
(47 kHz) was performed using a Yarnato Branson
2200.
2.4. Electroreducti6e polymerization of
Chlorodimethylsilane (1), dichlorodiphenylsilane (2)
and dichlorodiphenylgermane (4) are commercially
available from Shin-Etsu Chemical Co. Ltd. and they
were used after distillation. 1,2-Dichloro-1,1,2-
trimethyl-2-phenyldisilane (9) was prepared using a re-
ported method [11]. THF was distilled over
Naꢀbenzophenoneketyl under nitrogen atmosphere.
Benzoyl peroxide (BPO) was recrystallized from ben-
zene–ethanol. Magnesium ingot is commercially avail-
able from Rare Metallic Co. Ltd. and was cut into rods
(b=9 mm, length=4 cm) for electrodes. Mg elec-
trodes were treated with concentrated HCl, and then
washed with water and acetone.
dichlorooligosilanes
Dichlorooligosilanes, such as dichlorotrisilane 6 were
found to be good monomers for the electroreductive
synthesis of the polysilanes having longer sequence
units (Scheme 7). As summarized in Table 3, the tem-
perature control is found to be very important in the
electroreductive polymerization of 6. The reaction at
higher temperature, the backbiting reaction [9] of the
propagating polymer proceeded forming cyclohexa-
silane as a by-product (entry 1). This side reaction was
successfully suppressed when the reaction was carried
out below 0°C, and polysilanes 13 having relatively
high molecular weight were obtained (entries 3, 4).
Under the optimized reaction conditions, the electrore-
duction of dichlorotetrasilane 11 gave the correspond-
ing polysilane 14, units of which were ordered in four
sequences in satisfactory yield (Scheme 8). Moreover,
3.1. Electroreducti6e cross-coupling reaction of
chlorodimethylsilane (1) with dichlorodiphenylsilane (2)
The electroreduction was carried out in a 30-ml,
three-necked flask equipped with an Mg cathode and
the
polymerization
of
bis(chlorodimethylsilyl)-
diphenylgermane (7) gave the corresponding structure-
controlled silane–germane copolymers 15 having the
SiꢀGeꢀSi sequences [10] (Scheme 9). The polymerizabil-
ity of dichlorooligosilanes under electroreduction con-
ditions seems to be mainly affected by the substituents
on the chlorinated terminal silicon atom, and this fact
Scheme 6.

M. Ishifune et al. / Journal of Organometallic Chemistry 611 (2000) 26–31
29
Table 2
Electroreductive polymerization of dichlorodisilane 9 a
Entry
Dichlorodisilane 9 (mol l−1
)
Electricity (F mol−1
)
Polysilane 12
Mw/Mn
Yield (%) b
(
Mn
(
(
1
2
3
4
0.11
0.33
0.67
0.33
0.33
0.33
4
4
4
2
6
4
2900
2.7
2.3
2.9
2.7
3.0
1.81
2.7
2.4
1.0
3.9
1.0
3.0
3600
2100
2800
3000
2500
5
6 c
a The electroreduction was carried out in a single-compartment cell using Mg electrodes under sonication (47 kHz), and the polarity of
electrodes was alternated with an interval of 15 s.
b Isolated.
c The electroreduction was carried out in the presence of naphthalene.
anode, and a three-way stopcock jointed to a balloon of
nitrogen. Into this cell was placed 0.7 g of LiClO4 and
the content of the cell was dried at 50°C in vacuo for 3
h. Chlorotrimethylsilane (0.1 ml) and 15 ml, of dry THF
were then added under a nitrogen atmosphere. After the
solution was magnetically stirred for 3 h, the pre-electrol-
ysis was carried out to remove traces of water and
Scheme 7.
residual chlorotrimethylsilane from the electrolysis sys-
tem. That is, 600 C of electricity was passed through the
cell under the constant current condition (50 mA) and
the polarity of the electrodes was alternated with an
interval of 15 s using a comutator. During the electrol-
ysis, the ultrasound (47 kHz) was sonicated while cooling
with water. Chlorodimethylsilane (1, 5 mmol) and
dichlorodiphenylsilane (2, 35 mmol) were then syringed
into the cell in a stream of nitrogen, and the solution was
further electrolyzed. The progress of the reaction was
monitored by GLC or TLC. After the starting material
was consumed (supplied electricity=ca. 6 F mol−1
based on 2), the reaction mixture was poured into ice cold
1 N HCl (100 ml) and the aqueous solution was extracted
with ether (50 ml×3). The combined organic layer was
washed twice with 50 ml of brine, dried over MgSO4, and
concentrated. The residue was purified by a silica gel
column, eluting with hexane, and then distilled under
reduced pressure (b.p. 136°C/37 mHg) to afford 1,1,3,3-
tetramethyl-2,2-diphenyltrisilane (3). IR (neat) 3067,
Table 3
Electroreductive polymerization of dichlorotrisilane 6 a
(
(
(
Entry
Reaction
temperature
Mn
Mw/Mn Yield of 13
(%) b
1
2
3
4
18
0
−10
−15
3800 1.44
4700 1.87
5500 1.54
4400 1.42
(42) c
50
35
16
a The electroreduction was carried out by using Mg electrodes, and
ultrasound (47 kHz) was applied during the reaction. The anode and
cathode were alternated with an interval of 15 s. [dichlorotrisilane
6]=0.27 mol l−1; [LiClO4]=0.35 mol l−1; supplied electricity, 4 F
mol−1
.
b Purified by reprecipitation from benzene–EtOH.
c Contaminated by 1,1,2,2,4,4,5,5-octamethyl-3,3,6,6-tetraphenylcy-
clohexasilane.
1
2960, 2862, 2095, 1484, 1247, 838, 737 cm−1; H-NMR
(CDCl3) l 0.30 (d, 12H, J=4.5 Hz), 4.13 (sep, 2H,
J=4.5 Hz), 7.30–7.40 (m, 6H), 7.46–7.54 (m, 4H); MS
m/z (relative intensity) 300 [4, M+], 285 [3, M+−Me],
241 (31), 197 (41), 164 (85), 135 (82), 105 [100, PhSi+],
Anal. Calc. for C16H24Si3: C, 63.93; H, 8.05. Found: C,
63.99; H, 7.98%.
Scheme 8.
3.2. Chlorination of
1,1,3,3-tetramethyl-2,2-diphenyltrisilane (3)
A solution of trisilane (3, 30 mmol) in CCl4 (30 ml) and
benzoyl peroxide (0.6 mmol) as an inductor were
Scheme 9.

30
M. Ishifune et al. / Journal of Organometallic Chemistry 611 (2000) 26–31
added into a round-bottomed flask equipped with
reflux condenser jointed to a balloon of nitrogen, and
refluxed for 2 h. Then, the resulting oil was concen-
trated and distilled under reduce pressure (b.p. 159°C/
15 mHg) to give 1,3-dichloro-1,1,3,3-tetramethyl-
2,2-diphenyltrisilane (6). IR (neat) 3070, 2954, 2893,
placed 1 g of LiClO4. and the cell was dried at 50°C in
vacuo for 3 h. Chlorotrimethylsilane (0.1 ml) and 15 ml
of dry THF were then added under a nitrogen atmo-
sphere. After pre-electrolysis was carried out using the
same procedure as for trisilane, 3–5 mmol of the
monomer (9 or 6) was syringed into the cell in a stream
of nitrogen. The electroreduction was performed under
constant current conditions (50 mA), and the polarity
of the electrodes was alternated with an interval of 15 s
using a comutator. During the electrolysis the ultra-
sound (47 kHz) was sonicated while cooling with water.
The supplied electricity was counted by a comutator.
After 4 F mol−1 of electricity was passed through the
cell, the reaction was quenched by EtOH (10 ml). The
mixture was then poured into ice-cold 1 N HCl (100
ml) and the aqueous solution was extracted with ether
(50 ml×3). The combined organic layer was washed
twice with 50 ml of brine, dried over MgSO4, and
concentrated. The resulting crude polymer was dis-
solved in 4 ml of benzene and reprecipitated from
EtOH (100 ml). The molecular weight of the polymer
was determined by GPC with THF as eluent.
1
1483, 1250, 834, 737 cm−1; H-NMR (CDCl3) l 0.60
(s, 1211), 7.30–7.49 (m, 6H), 7.57–7.61 (m, 4H).
HRMS m/e 368.03921 [Calc. for C16H22Cl12Si3,
368.04064].
3.3. Electroreducti6e cross-coupling reaction of
chlorodimethylsilane (1) with dichlorodiphenylgermane
(4) and other dichlorooligosilanes (6, 9)
A solution of chlorosilane (1, 5 mmol) with dichloro-
diphenylgermane (4, 35 mmol) or dichlorooligosilane (6
or 9) in dry THF (15 ml) was electrolyzed by using the
same procedure as for the preparation of trisilane 3 to
afford the corresponding dichlorooligosilanes (supplied
electricity=ca. 6 F mol−1 based on 4). The product
was purified by a silica gel column, eluting with hexane.
Bis(dimethylsilyl)diphenylgermane (5): IR (neat)
Poly[1,2,2-trimethyl-l-phenyldisilane] (12): IR (KBr)
3300, 1652, 1250, 710 cm−1; 1H-NMR (CDCl3) l 0.02–
0.11 (m, 9H), 7.25–7.35 (m, 5H).
3005, 2104, 1422, 1244, 1092, 878 cm−1 1H-NMR
;
(CDCl3) l 0.31 (d, 12H, J=4.3 Hz), 4.26 (sep, 2H,
J=4.3 Hz), 7.28–7.33 (m, 6H), 7.40–7.45 (m, 4H);
HRMS m/e 346.06196 [Calc. for C16H24GeSi2,
346.06283].
Poly[1,1,3,3-tetramethyl-2,2-diphenyltrisilane]
(13):
IR (KBr) 3308, 2981, 1428, 1250, 804, 702 cm−1
;
1H-NMR (CDCl3) l 0.02–0.10 (m, 12H), 7.10–7.45 (m,
5H).
1,1,2,2,4,4,5,5 - Octamethyl - 3,3 - diphenylpentasilane
(8): IR (neat) 3052, 2957, 2896, 2100, 1483, 1250, 838,
Poly[1,1,3,3,4,4 -hexamethyl -2,2-diphenyltetrasilane]
(14): IR (ICBr) 3156, 2958, 1382, 1299, 808 cm−1
;
1
735 cm−1; H-NMR (CDCl3) l 0.01 (d, 12H, J=4.5
1H-NMR (CDCl3) l −0.30 to 0.20 (m, 18H), 7.00–
7.40 (m, 5H).
Hz), 0.31 (s, 12H), 3.78 (sep, 2H, J=4.5 Hz), 7.31–
7.38 (m, 6H), 7.48–7.59 (m, 4H). Anal. Calc. for
C20H36Si5: C, 57.62; H, 830. Found: C, 57.88; H, 839%.
1,1,2,2,3,4,4-Hexamethyl-3-phenyltetrasilane (10): IR
Poly[bis(dimethylsilyl)diphenylgermane] (15): IR
(KBr) 3068, 1431, 1260, 710, 1088 cm−1 1H-NMR
;
(CDCl3) l −0.54 to 0.47 (m, 12H), 6.29–7.52 (m, 5H).
(neat) 3052, 2093, 1598, 1249, 838 cm−1 1H-NMR
;
(CDl3) l 0.02–0.32 (m, 21H), 3.72–3.86 (m, 1H), 3.88–
4.04 (m, 1H), 7.26–7.58 (m, 5H).
Acknowledgements
3.4. Chlorination of bis(dimethylsilyl)diphenylgermane
(5) [10]
The author (S.K.) thanks the Ministry of Education,
Science and Culture Japan for Grant-in Aid for Scien-
tific Research on Priority Areas (No. 11120257).
Bis(dimethylsilyl)diphenylgermane (5) was chlori-
nated using the same procedure as for the chlorination
of trisilane 3 to give bis(chlorodimethylsilyl)diphenyl-
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(m, 4H).
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