Sol-Gel Chemistry of Hydrogenosiliconates: The Role of Hypervalent Silicon Species
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V.BELOT, R. CORRIU. C. GUERIN, B. HENNER, D. LECLERCQ, H. MUTIN, A. VIOUX, Q.WANG Unitd Mixte CNRS, Rhbne Poulenc, USTL n0 44 Universitd de Montpellier II Place E. Bataillon 34095 Montpellier Cedex 05
The hydrolysis of organic silicates ( mainly Si(OEt)4: [TEOS] and Si(OMe)4: [TMOS]) is the key reaction in sol gel process for preparation of silica. The mechanism of this reaction is not well known because of the competition taking place between hydrolysis itself and condensation reactions (homo or heterocondensations) but also because of redistribution reactions which take place competitively. Furthermore the competition between these steps depends also on the catalysis (acid,basic or nucleophilic) [ 1, 3]. 2 (RO)
Si OH
3
z-
2(RO)2Si(OH) 2
•
(RO) 2 Si(OH) 2
+
Si (OR ) 4
(RO) 3 SiOH
+
ROSi(OH) 3
etc
ROH
(RO) 3 S(' R¢3-R "S__2 i 0S' , ROI
I
( 0R "6i
C
OR
~~H20
R0
'"O H
(RO )3 Si 0 Si ( OR )3
1H20
i0S(OR) i ( O R ) 3 + ÷iS ROH
(RO )2
OR
OH
4/NRO"H. (OR)2
Si(OR 1 H20 0 OH HO Si OR
#
., Si ORO)
(
*-O
/ SROH (RO)2Si ONsirO0> Si(OR) 2
(O)2I
(HO)3 Si OR ,1
/t
2
OSi (OR )3
( RO )2 i 0 Si (OR )3 OR I
Nsk( o0*R)2
I I M.
etc
O
Dimensional
The lack of kinetic studies originates from the great difficulty to have an accurate measure of the concentration of the species which are formed at each step. . Concerning the nature of the intermediates formed during the first step in basic or acidic conditions, some indications can be obtained from previous studies performed in the field of organosilicon- chemistry in the case of mono and dialkoxysilanes [4]. Both acid and basic catalysis involve the nucleophilic attack of water on the electrophilic silicon. This is commonly accepted by organosilicon chemists and the experimental facts
Mat. Res. Soc. Symp. Proc. Vol. 180. 01990 Materials Research Society
4
obtained by AELION [ 2 ] in the hydrolysis of silicates as well as those obtained by GRUBB [3] concerning siloxanes are in agreement with this mechanism. Hydrolysis 1.
H +
+
2.
11.
+
-.., H 2 HO
^.
I
fast
-A Si--OR
sla
*+. H
"o Si"--OH + ROH
+ H+
-'0HR
Condensation
2. ";Si-OH
+
---U0R
f•st
- Si-- OR
+
H
1.
.,H
-
.
-
-
ROH
In acid catalysis the first step is a fast and reversible protonation of the alkoxy group which increases the capability of this group to be displaced by nucleophilic reagent. This SN2 type reaction in acid catalysis is well illustrated by EABORN [5] who observed a racemisation rate two times faster than the exchange rate in labelling exchange reactions of optically active alkoxysilanes. Some authors have proposed the possibility of an SN 1 reaction involving tricoordinate silicenium ions (RO)3 Si+ [6]. I& ?Et#4 Et%-OEt
EtOle
on
?Et +OEt 1i O~
.Et
Eto-,
wmm
H
+
2
Et OH HN0
EtO/ \OEt
Et
7Et 9IO -O H +
EtOI
+
This proposition cannot be eliminated stricto sensu because of the lack of experimental studies on tetraalkoxysilanes. However it is unlikely in view of the general literature of organosilicon chemistry [7-10]. Silicenium
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