Monosubstituted octasilasesquioxanes

Octasilasesquioxanes are cube-shaped molecules consisting of a Si 8 O 12 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′R 7 Si 8 O 12 , 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: co-hydrolysis of trifunctional organo- or hydro-silanes, substitution reactions with retention of the siloxane cage, and corner-capping reactions. These three different strategies are discussed in this review. Various spectroscopic techniques that 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 vibration, 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.