Monosubstituted Octasilasesquioxanes

Claudia Marcolli and Gion Calzaferri
Department of Chemistry and Biochemistry, University of Bern, Freiestrasse 3, CH-3000 Bern 9, Switzerland


Octasilasesquioxanes are cube shaped molecules consisting of a Si8O12 core and eight reactive sites, which can, in principle, all be functionalized differently. This review article provides an overview of the work, which is concerned with the monosubstituted species, R'R7Si8O12, where R' and R are two different substituents, such as organic or organometallic groups or single atoms (H, Cl). Three synthetic routes have been used so far to prepare monosubstituted silasesquioxanes: cohydrolysis of trifunctional organo- or hydrosilanes, substitution reactions under retention of the siloxane cage and corner capping reactions. These three different strategies are discussed in this review. Different spectroscopic techniques, which have been applied to characterize these molecules - X-ray diffraction, NMR, IR and Raman spectroscopy as well as mass spectrometry - are treated. A main focus is the notion of ring-opening vibrations, which is supposed to be generally applicable to microporous materials, especially zeolites. A simple model for explaining the nature of ring-opening vibrations is presented. Based on the new possibilities offered by functionalized silasesquioxanes, challenging perspectives for the synthesis of novel materials are discussed.

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