Controlled Cleavage of Fully-Condensed Silsesquioxane Frameworks: A Revolutionary New Method for Manufacturing Precursor
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FRANK J. FEHER AND DARAVONG SOULIVONG Department of Chemistry, University of California, Irvine, CA 92697-2025 e-mail: [email protected]
ABSTRACT The reaction of readily available Cy8Si8Ol2 (a) with triflic acid (TfOH) can produce two different ditriflates derived from controlled cleavage of Si-O-Si linkages. These ditriflates can be subsequently hydrolyzed to four different incompletely condensed silsesquioxanes, which are attractive precursors to hybrid inorganic/organic materials. INTRODUCTION Discrete polyhedral clusters containing silicon and oxygen have attracted widespread attention as precursors to new families of hybrid inorganic/organic materials. 1 -6 Two broad families of polyhedral Si/O clusters exist: (1) spherosilicates 8
7
(e.g., 1), which are most often prepared by silylation of silicate
solutions; and (2) polyhedral silsesquioxanes (e.g., 2), which are usually obtained from hydrolytic condensation reactions of trifunctional organosilicon monomers (RSiX 3 ).9 Both families have excellent potential as building blocks for hybrid inorganic/organic materials if cost-effective methods can be developed to manufacture appropriately functionalized Si/O frameworks on a large scale. In this paper we outline a strategy for preparing four incompletely-condensed silsesquioxanes from Cy8Si8O1 2 (a), a single fully-condensed silsesquioxane framework that is available in large quantities from readily available starting materials.
RR\ Si-O
R -Si-
R i----/-, /6 o-- Si.--/Y R
0/ R
oN" R
1 R = OSiR R 2R alkyl, aryl, vinyl, H 2 c-C 61H 11 3
111
Mat. Res. Soc. Symp. Proc. Vol. 576 © 1999 Materials Research Society
•£_
The base-catalyzed hydrolytic
PhSiX 3 or [PhSiO 312]n
condensation of CySi(OMe) 3 in benzene affords good yields of Cy8Si80l 2 (1).10
I•'qi 1-')
,"
--
Ph 8Si 8 0
12
refe
1
r1fO0
CySi(OMe)3
3
Excellent yields of 3 can via catalytic also be obtained
hydrogenation of Ph 8 Si8 0 12 ,11 which is readily available in nearly quantitative yield 12 13 from a variety of PhSiX 3 precursors. , The reaction of 3 with HBF 4 "OMe2 (4.6 eq) and BF 3 "OEt 2 (6 eq, C6 H 6 , 60 °C, 30 min) or triflic acid (TfOH, 5 eq, C 6 H 6 , 25 °C, 30 min) in a non-coordinating solvent (e.g., CH 2Cl 2 or benzene) induces cleavage of a single Si-O-Si linkage. 14 In the case of 3 and HBF 4 /BF 3, this reaction produces 4 via the mechanism proposed in Scheme 1. No other Si-containing products are formed, but the reaction typically stops at 700 conversion because H 2 0 produced by the reaction lowers the activity of HBF 4 /BF 3 to a point where protonation of Si-O-Si is no longer favorable. The Si-F 8 %
groups in 4 are remarkably resistant to hydrolysis, but hydrolysis to 5 can be achieved with complete retention of stereochemistry at Si via sequential treatment with excess Me 3 SnOH and aqueous HC1.14-16 Scheme 1
Si
0.i.
PO /y s• -' -o - "s --H '+ "/si-°-s cY I'i C,,
+
C
Y-S
"o--/
S
B
PC
Oi---- --- i"• CY
H
i_
-H20
0 Cy /S
I/
--
2. aq HC,
Cy
6_i _
Cy
2 a
I/--cy -
0
Cy
Cy/ 2.q C 1. tMe3SnO
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