Dehydropolymerization of arylsilanes catalyzed by a novel silylmolybdenum complex

Article abstract of DOI:10.1039/b310663m

A complex [MoH₃[Si(Ph)[Ph₂PCH₂CH ₂P(Ph)C₆H₄-o]₂]] (1) can act as single-component catalyst for dehydrogenative polymerization of ArSiH ₂R (Ar = Ph, p-tolyl, o-tolyl; R = H, Me) to (ArSiR)_n: p- and o- tolylsilane produced the polymers of respectable molecular weights (M_W of 17300 and 6700 respectively) and polymerization of secondary silane, methylphenylsilane, gave a substantial molecular weight (M_W of 1750).

Full text of DOI:10.1039/b310663m

Dehydropolymerization of arylsilanes catalyzed by a novel
silylmolybdenum complex
Makoto Minato,* Takaomi Matsumoto, Miyuki Ichikawa and Takashi Ito*
Department of Materials Chemistry, Graduate School of Engineering, Yokohama National University, 79-5,
Tokiwadai, Hodogaya-ku, Yokohama, 240-8501, Japan. E-mail: minato@ynu.ac.jp; Fax: +81-45-339-3933;
Tel: +81-45-339-3933
Received (in Cambridge, UK) 2nd September 2003, Accepted 21st October 2003
First published as an Advance Article on the web 3rd November 2003
A complex [MoH
can act as single-component catalyst for dehydrogenative
polymerization of ArSiH R (Ar = Ph, p-tolyl, o-tolyl; R =
H, Me) to (ArSiR) : p- and o- tolylsilane produced the
polymers of respectable molecular weights (M of 17300 and
700 respectively) and polymerization of secondary silane,
methylphenylsilane, gave a substantial molecular weight
of 1750).
3
{Si(Ph)[Ph
2
PCH
2
CH
2
P(Ph)C
6
H
4
-o]
2
}] (1)
by the Scheme 2 and the results are presented in this
communication.
2
In a typical experiment, phenylsilane (433 mg, 4.0 mmol)
was added to complex 1 (20 mg, 0.020 mmol) in a Schlenk tube
under argon. The reaction mixture was stirred vigorously under
a stream of argon at 120 °C for 24 h. To the reaction mixture, dry
hexane was added to separate the polymer. The solvent was
decanted; then, remaining phenylsilane was analyzed by GC
using an internal standard. Conversion of phenylsilane was
n
w
6
(
M
w
Polysilanes represent a promising class of electroactive poly-
mers with a high degree of s-delocalization. They are of great
interest as a consequence of their useful application in many
areas, such as organic electroluminescence materials, photo-
conducting materials and photoresists.1 The efficient and
effective synthetic methods of high polymers have been
extensively studied. The catalytic dehydrocoupling of hydro-
2 2
found to be 93%. We found that Ph SiH was produced in 30%
yield, which was confirmed by means of GC-mass spectrome-
try. This finding suggests that disproportionation reactions at
the Si—Ph bond of phenylsilane occur. The amount of Si—H
activation, which yields dehydrocoupling product, was, there-
3
fore, limited to the remaining 40% of PhSiH consumed. In
addition, it was evident from GC-mass spectrometry that dimer
2
silanes, which was introduced by Harrod, provides a clean and
PhH
PhH
2
SiSiH
Si(SiPhH)
2
Ph, trimer PhH
2 2
SiSiPhHSiH Ph and tetramer
simple synthesis for Si—Si bonds. Group IV metallocene
complex catalysts appear to be more efficient in catalytic
performance than the others, although recent reports have
shown that a few other systems can be effective for hydrosilane
2
2
SiH Ph are present. The resultant polymer was
2
washed three times with hexane by decantation and was dried
under vacuum to give dry ivory powders. This feature
differentiated the products from the previously known poly-
phenylsilanes that were prepared by a group IV metallocene-
catalyzed polymerization of phenylsilane. These polyphenylsi-
lanes are usually very viscous oils.7
3
polymerization. We now report an efficient catalytic process
for preparing polysilanes using a novel trihydridosilylmolybde-
num complex.
In previous papers, we reported the unexpected formation of
1
The sample of the polymerizate was analyzed by IR, H NMR
the complex [MoH
which is obtained by the thermal reaction of [MoH
3
{Si(Ph)[Ph
2
PCH
2
CH
2
P(Ph)C
6
H
4
4
-o]
(dppe)
. Complex 1 reacts
2
}] (1),
and 29Si NMR spectroscopies. The IR and NMR data obtained
for the product were very similar to those previously reported
2
] (2,
4
8
dppe = Ph
2
PCH
2
CH PPh ) with PhSiH
2
2
3
for the polyphenylsilanes. The IR spectrum of the product
with a further mole of phenylsilane to give a novel disilyl
complex 3. We found that complex 3 is stable in the solid state
but is transformed into complex 1 quantitatively in solution at
ambient temperature (Scheme 1).
Although the fate of the extruded PhSiH unit is uncertain, 3
is envisioned to be able to release silylene. Actually we
observed that methanolysis of complex 3 results in complex 1
exhibited an intense characteristic band assignable to Si—H
4
21
stretching at 2106 cm , which is a lower frequency than that of
phenylsilane monomer.9 We have been unable to detect
2
–SiPhH end groups in IR spectrum, but this is to be expected
1
given the high molecular masses. The product showed broad H
NMR signals characteristic of linear (4.3–4.9 ppm) and cyclic
(5.2–5.6 ppm) polymers.10 Si NMR spectrum displayed a
narrow set of resonances between 260 and 264 ppm.
29
and PhSi(OMe)
explained by the consecutive methanolysis of PhSiH
3
. The formation of PhSi(OMe)
3
may be
(OMe)
2
The molecular weight distribution of the resultant product
was measured by gel permeation chromatography (GPC)
utilizing polystyrene standards and tetrahydrofuran solvent. The
dehydrocoupling reaction with complex 1 formed polyphenylsi-
that is the expected trapping product of the free silylene PhHSi:
5
with methanol. We thought that the liberated silylene from
complex 3 might also be trapped in the presence of hydrosilanes
because silylenes readily insert into the Si—H bond to form a
w
lane with the average molecular weight M value greater than
6
Si—Si bond. Based on these facts, we attempted the catalytic
9100. This value was found to be comparable to that of the
polyphenylsilanes obtained from a group IV metallocene-
catalyzed polymerization. Analysis of the resultant polymer by
means of GPC revealed the unimodal molecular weight
distribution. It is well-known that GPC profiles for polymers
derived from group IV metallocene-catalysts are bimodal as a
dehydrocoupling of hydrosilanes with complex 1 as expressed
Scheme 1
Scheme 2
2
968
CHEM. COMMUN., 2003, 2968–2969
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result of the presence of both linear and cyclic poly-
phenylsilanes.11 Thus present result poses a somewhat puzzling
situation since H NMR spectrum of the polymer shows both
above, in the dehydrocoupling of phenylsilane using complex 1
the formation of Ph SiH occurred and this product can be
2
2
1
explained through 1,3-migration of the phenyl group in a
linear and cyclic regions as previously described.
(silyl)(silylene)-molybdenum intermediate.15
The effects of other reaction conditions were examined. The
In this communication, we have reported that the use of
complex 1 as a new class of catalyst in the dehydrocoupling of
arylsilanes results in high molecular weight polysilanes. Our
findings show complex 1 has the potential to produce high
molecular weight polysilanes other than polyphenylsilane.
The authors gratefully acknowledge Professor M. Tomoi,
Professor T. Iijima, Dr T. Oyama and their research group for
the use of their GPC instrument. We also thank Professor K.
Osakada and Dr Y. Nishihara of Tokyo Institute of Technology
for recording the 29Si NMR spectrum of the polymer.
w
average molecular weight M , number average molecular
weight M and polydispersity Pd of the polysilane for each of
n
these samples are presented in Table 1. Lowering the tem-
perature from 120 °C to 70 °C markedly slowed the reaction;
moreover, this affected the chain length (entry 2). The M
increased from 9100 to 10400 with increasing monomer
concentration (entry 3). After 48 h of reaction, the M value
w
value
w
remained essentially unchanged (entry 4). In entries 5 and 6, the
reactions were performed in toluene at 110 °C.
Subsequently, based on these results we carried out the
polymerization of other arylsilanes as substrates (entries 7, 8
and 9). p-Tolylsilane was found to produce the polymer with
Notes and references
3
M
w
of 17.3 3 10 (entry 7). Catalytic polymerization reactions
1
For recent reviews, see: (a) R. West, J. Organomet. Chem., 1986, 300,
27; (b) R. D. Miller and J. Michl, Chem. Rev., 1989, 89, 1359; (c) J. M.
of p-tolylsilane using group IV metallocene complexes give a
3
little lower molecular weight polymers compared with phenyl-
Ziegler, Mol. Cryst. Liq. Cryst., 1990, 190, 265; (d) R. West, in
Comprehensive Organometallic Chemistry II, ed. A. G. Davies,
Pergamon, Oxford, 1991, vol. 2, p., 77; (e) H. Yamashita and M.
Tanaka, Bull. Chem. Soc., Jpn., 1995, 68, 403; (f) R. West, in The
Chemistry of Organic Silicon Compounds, ed. Z. Rappoport and Y.
Apeloig, Wiley, New York, 2001, vol. 3, Chapter 9; (g) G. M. Gray and
J. Y. Corey, in Silicon-Containing Polymers: The Science and
Technology of Their Synthesis and Applications, ed. R. G. Jones, W.
Ando and J. Chojnowski, Kluwer Academic Publishers, Dordrecht,
silane; moreover, p-tolylsilane reacts slower than phenyl-
silane.1
2,13
Particularly we were surprised to find that o-
3
tolylsilane produced polymer with M of 6.73 3 10 (entry 8).
w
The dehydrogenative polymerization of hydrosilanes using
group IV metallocene catalysts has been known to be highly
selective: the steric constraints are significant. Ortho substituted
phenylsilanes were found not to undergo dehydropolymer-
ization.1g
2
000, Chapter 14.
It is noteworthy that methylphenylsilane produced the
2
(a) C. Aitken, J. F. Harrod and E. Samuel, J. Organomet. Chem., 1985,
279, C11; (b) J. F. Harrod, Y. Mu and E. Samuel, Polyhedron, 1991, 10,
1239.
3
polymer with M
w
of 1.75 3 10 (ca. 14 Si units, entry 9). In the
2
1
IR spectrum, we observed a band at 2130 cm , which was
assigned to the Si—H stretching of the terminal groups. In the
case of group IV metallocene systems, catalytic dehydropoly-
merization is restricted to primary silanes. With secondary
silanes, this reaction is much slower and provides oligomers
with up to ca. 8 monomer units.14
3 For example see: F.-G. Fontaine and D. Zargarian, Organometallics,
002, 21, 401.
(a) D.-Y. Zhou, M. Minato, T. Ito and M. Yamasaki, Chem. Lett., 1997,
017; (b) T. Ito, Bull. Chem. Soc., Jpn., 1999, 72, 2365.
R. J. P. Corriu, G. F. Lanneau and B. P. S. Chauhan, Organometallics,
993, 12, 2001.
P. Trefonas III, R. West and R. D. Miller, J. Am. Chem. Soc., 1985, 107,
737.
2
4
5
6
1
1
The mechanism of the catalytic cycle seems to be complex.
We think currently that a free silylene or more likely a silylene
complex is involved in the polymerization reaction, although
there is little experimental evidence in this regard. As noted
2
7 See for example:(a) J. P. Banovetz, K. M. Stein and R. M. Waymouth,
Organometallics, 1991, 10, 3430; (b) N. Choi, S. Onozawa, T. Sakakura
and M. Tanaka, Organometallics, 1997, 16, 2765.
8
J. L. Huhmann, J. Y. Corey and N. P. Rath, J. Organomet. Chem., 1997,
Table 1 Dehydropolymerization of arylsilanes using complex 1
533, 61.
9
C. Aitken, J. F. Harrod and U. S. Gill, Can. J. Chem., 1987, 65,
Conditions:
1804.
soln/temp
Yield
10 H.-G. Woo, J. F. Walzer and T. D. Tilley, J. Am. Chem. Soc., 1992, 114,
Entry Monomer (°C)/time (h) (%) M/Sia
M
w
M
n
Pd
7047.
1
1 T. D. Tilley and T. Imori, Polyhedron, 1994, 13, 2231.
1
2
3
4
5
6
7
8
9
a
PhSiH
PhSiH
PhSiH
PhSiH
PhSiH
PhSiH
p-TolSiH
o-TolSiH
PhMeSiH
3
3
3
3
3
3
neat/120/24 18
neat/70/24
neat/120/24 13
1/200
1/500
9150 3030 3.02
4290 2030 2.06
12 (a) H. Hashimoto, S. Obara and M. Kira, Chem. Lett., 2000, 188; (b) B.
J. Grimmond and J. Y. Corey, Organometallics, 2000, 19, 3776.
13 J. P. Banovetz, H. Suzuki and R. M. Waymouth, Organometallics, 1993,
12, 4700.
14 J. Y. Corey, X.-H. Zhu, T. C. Bedard and L. D. Lange, Organometallics,
1991, 10, 924.
15 The 1,3-migration of the aryl group in (silyl)(silylene)metal systems has
been well documented see: (a) T. Sakakura, O. Kumberger, R. P. Tan,
M.-P. Arthur and M. Tanaka, J. Chem. Soc., Chem. Commun., 1995,
193; (b) H. Tobita, K. Ueno, M. Shimoi and H. Ogino, J. Am. Chem.
Soc., 1990, 112, 3415; (c) K. H. Pannell, M.-C. Brun, H. Sharma, K.
Jones and S. Sharma, Organometallics, 1994, 13, 1075.
6
1/500 10400 2890 3.59
neat/120/48
Tol/110/24
Tol/110/48
8
15
21
1/500
1/200
1/200
9800 2710 3.61
6820 2310 2.95
6900 2140 3.22
b
b
c
3
3
neat/110/24 43
neat/120/24 20
neat/110/24 31
b
1/200 17290 5830 2.97
1/200
1/200
d
6730 3680 1.83
1750 1380 1.27
2
Catalyst to monomer ratio. Reactions conducted in toluene (1 mL). c p-
d
Tol = p-CH
3
C
6
H
4
.
o-Tol = o-CH
3
C
6 4
H .
CHEM. COMMUN., 2003, 2968–2969
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