Catalysis of triethoxysilane disproportionation with oligoethylene glycol ethers

Article abstract of DOI:10.1016/S0022-328X(02)02128-9

Oligoethylene glycol ethers catalyze the disproportionation of triethoxysilane to tetraethoxysilane and silane both at room temperature and upon heating. For comparison, the CsF-catalyzed disproportionation of triethoxysilane has been examined and found m

Full text of DOI:10.1016/S0022-328X(02)02128-9

Journal of Organometallic Chemistry 665 (2003) 246Á249
/
www.elsevier.com/locate/jorganchem
Catalysis of triethoxysilane disproportionation with oligoethylene
glycol ethers
Lidiya N. Parshina *, Lyudmila A. Oparina, Marina Ya. Khil’ko, Boris A. Trofimov
A. E. Favorsky Institute of Chemistry, Russian Academy of Sciences, Siberian Branch, 1, Favorsky Street, 664033 Irkutsk, Russia
Received 11 August 2002; received in revised form 7 November 2002; accepted 7 November 2002
Abstract
Oligoethylene glycol ethers catalyze the disproportionation of triethoxysilane to tetraethoxysilane and silane both at room
temperature and upon heating. For comparison, the CsF-catalyzed disproportionation of triethoxysilane has been examined and
found more facile. Mechanisms of the reactions involving formation of hypervalent silicon complexes have been discussed.
# 2002 Elsevier Science B.V. All rights reserved.
Keywords: Silane; Triethoxysilane; Tetraethoxysilane; Disproportionation; Oligoethylene glycol ethers; Hypervalent silicon complexes
1. Introduction
2. Results and discussion
Although the disproportionation of trialkoxysilanes
in the presence of various homogeneous and hetero-
geneous catalysts was a subject of many investigations
(see Ref. [1]), this reaction keeps being actively studied
The disproportionation of triethoxysilane to tetra-
ethoxysilane and silane under the action of oligoethylene
glycol ethers, such as dimethyl and divinyl ethers of
triethylene glycol or divinyl ether of heptaethylene
glycol, proceeds at ambient temperature. Thus, in the
presence of an equimolar amount of one of the above
mentioned divinyl ethers, 50% conversion of triethox-
[2Á14] due to its practical value since silane, a dispro-
/
portionation product, is used for the production of high
purity silicon. From the theoretical point of view, this
process contributes to a better understanding of hyper-
valence of silicon [13,14], which has attracted much
ysilane is reached in 85Á90 days. In the case of dimethyl
/
ether of triethylene glycol, the same conversion is
attainable in 110 days. A two-fold decrease in the ether
attention in the last two decades [15Á21].
/
concentration results in a 30Á40% conversion of
triethoxysilane for the same period, whereas the use of
a two-fold excess of dimethyl or divinyl ether of
/
Recently, we have found [22,23] that oligoethylene
glycol ethers, including divinyl ethers, catalyze the
homodehydrocondensation of poly(methylhydrosilox-
ane) with the evolution of hydrogen and the formation
triethylene glycol allows a 70Á/75% conversion to be
attained.
of Siꢀ/Si bonds. The reaction was rationalized via
As observed in [22], at higher temperature, the
homodehydrocondensation of poly(methylhydrosilox-
ane) with oligoethylene glycol ethers was slowing
down, which was rationalized in terms of dissociation
intermediate hexacoordinate complexes formed from
siloxane and oligoethylene oxide moieties.
In this work, new data on the catalysis of hydrosilane
reactions by oligoethylene glycol ethers, namely, on
their ability to catalyze triethoxysilane disproportiona-
tion, are presented.
of weak silaneÁ
/
catalyst complexes. Therefore, the same
100 8C the
might be expected in this case. Instead, at 97Á
/
reaction significantly accelerates. Thus, when equimolar
mixtures of triethoxysilane with divinyl ether of triethy-
lene glycol, divinyl ether of heptaethylene glycol or
dimethyl ether of triethylene glycol are heated for 11 h,
the conversion of triethoxysilane to tetraethoxysilane
* Corresponding author. Tel.:
396046
E-mail address: parshina@irioch.irk.ru (L.N. Parshina).
ꢀ
/
7-3952-461931; fax:
ꢀ/7-3952-
0022-328X/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 0 2 2 - 3 2 8 X ( 0 2 ) 0 2 1 2 8 - 9

L.N. Parshina et al. / Journal of Organometallic Chemistry 665 (2003) 246Á
/249
247
Table 1
Disproportionation of triethoxysilane to tetraethoxysilane with oligoethylene glycol ethers
a
Ether
(EtO)3SiH:ether (mol)
Temperature (8C)
Time
(EtO)3SiH conversion (%)
Triethylene glycol dimethyl ether
1:1
2:1
1:2
1:1
1:1.8
1:1
2:1
1:2
1:1
1:1
2:1
1:1
Room
Room
Room
110 days
110 days
110 days
11 h
48
30
69
50
75
49
34
75
43
51
41
60
97Á
/
100
100
97Á
/
11 h
Triethylene glycol divinyl ether
Room
Room
Room
90 days
90 days
90 days
11 h
97Á100
/
Heptaethylene glycol divinyl ether
Room
Room
85 days
85 days
11 h
97Á100
/
a
Conversion is the average of those determined by 29Si-NMR and GLC analysis of the reaction mixture.
and silane is 43, 60 and 50%, respectively. When the
molar portion of triethylene glycol dimethyl ether is 1.8
(for 1 mol of triethoxysilane), the conversion increases
to 75%. Some other representative results are given in
Table 1.
It is likely that the first reaction stage, as in the case of
poly(methylhydrosiloxane) homodehydrocondensation
under the action of oligoethylene glycol ethers [22,23],
probable. Further, these two ions can take part in
disproportionation: hydride ion replaces the ethoxy
group in the next triethoxysilane molecule, whereas
siliconium cation B accepts the ethoxide ion to afford
tetraethoxysilane. It is more likely that these are
concerted processes occurring as a bimolecular reaction
via transition state of type C. The diethoxysilane thus
formed participates further in a similar exchange with
is the formation of intermolecular complex A with the
participation of silicon and oxygen atoms of the
ethylene glycol unit, in which the silicon atom is
triethoxysilane to convert to ethoxysilane, which, like-
wise, finally is transformed to silane.
HSi(OEt)3 ꢀH2Si(OEt)20 Si(OEt)4 ꢀH3SiOEt
HSi(OEt)3 ꢀH3SiOEt 0 Si(OEt)4 ꢀSiH4
hexacoordinated [15Á
/
21]. This makes the Siꢀ/H bond
weaker and thus more prone to dissociation. Due to the
possibility of positive charge redistribution over the five
oxygen atoms, a dissociation leading to siliconium
cation B and, correspondingly, hydride ion, seems
This mechanism is also supported by the fact that
alkoxide ions coordinate readily to organyloxysilanes to
give stable derivatives [19Á
21], such as Kꢀ[HSi(OR)4]ꢁ
/

248
L.N. Parshina et al. / Journal of Organometallic Chemistry 665 (2003) 246Á249
/
(Rꢂ
/
Et, i-Pr, Ph) [24,25], Kꢀ[H2Si(OR)3]ꢁ (Rꢂ
/
Et, i-
Pr) [25], [K, 18-crown-6]ꢀ[PhSi(OCH2CF3)4]ꢁ [26], for
example.
ligands in the formation of hypercoordinate silicon
complexes.
For comparison, the triethoxysilane disproportiona-
tion was carried out in the presence of CsF. As known,
fluoride anion exhibits a strong silicon atom affinity
[19,21], which seems to be responsible for the catalytic
effect of potassium fluoride in the triethoxysilane
disproportionation [14]. At the same time to our knowl-
edge, CsF has been never used in this reaction, though
3. Experimental
3.1. General
IR spectra were recorded on a Specord IR-75 spectro-
meter in thin film. 1H- and 29Si-NMR spectra were
measured on a Bruker DPX 400 instrument (400.1 and
79.5 MHz, respectively) in CDCl3 solutions, HMDS as
an internal standard. Analyses of the reaction mixtures
were performed using GLC on a LCM-80 instrument
(thermal conductivity detector, carrier gas: helium (2 l
the triethoxysilaneꢀ/CsF system was often employed for
the reduction of carbonyl compounds as a hydride ion
donor [19,21]. Despite the fact that pentacoordinate
hydridosilicates were also assumed to be intermediates
in such a reduction [19,21], the authors reported no
disproportionation of triethoxysilane.
hꢁ1 consumption), 3000ꢃ
3 mm column, liquid phase:
/
Since cesium fluoride is insoluble in triethoxysilane,
the disproportionation rate of the latter strongly de-
pends on stirring. Thus, at room temperature in the
presence of 2 mass% of CsF, the full conversion of
triethoxysilane to tetraethoxysilane is reached within a
week without and for 24 h with stirring. Heating of the
Siloxane DS-550, 5% on Chromaton N-AW-HMDS,
analysis temperature: 90 8C).
All experiments were carried out under argon atmo-
sphere.
Oligoethylene glycol divinyl ethers were prepared by
vinylation of the corresponding glycols with acetylene in
reaction mixture to 63Á65 8C upon stirring reduces the
/
the superbase system KOHÁDMSO [27]. Triethoxysi-
/
reaction time to 9 h, the evolved silane being often
exploded even with small amounts of triethoxysilane (1
mmol) and under argon blanket.
lane and dimethyl ether of triethylene glycol are
commercial products (Aldrich and Merck, respectively).
As far as pentacoordinate monofluorosilicates do not
tend to give hexacoordinate species with the second
fluoride anion [21], the pentacoordinated complex D
might be considered as a first probable intermediate for
the CsF-catalyzed disproportionation of triethoxysilane.
The catalytic exchange with the ethoxy group and
hydride ion between two triethoxysilane molecules
may occur in dimeric hexacoordinate intermediate E.
To conclude, oligoethylene glycol ethers are weaker
catalysts of triethoxysilane disproportionation com-
pared with CsF, that indicates their lower activity as
3.2. Disproportionation of triethoxysilane in the presence
of triethylene glycol divinyl ether
A mixture of 1.64 g (10 mmol) of triethoxysilane and
2.02 g (10 mmol) of triethylene glycol divinyl ether
(reactants are soluble in each other) was placed in a glass
flask, filled with argon and sealed with a rubber septum.
The evolving silane volume was periodically measured
by pushing-out of a 2-ml syringe piston. The reaction
was also controlled by GLC, IR and 29Si-NMR spectro-
scopy. A total of 28 ml of the gas was collected after 90

L.N. Parshina et al. / Journal of Organometallic Chemistry 665 (2003) 246Á
/249
249
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at room temperature, as well as experiments in the
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/
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