Monosubstituted octasilasesquioxanes

Octasilasesquioxanes are cube-shaped molecules consisting of a Si8O12 core and eight reactive sites, which can, in principle, all be functionalized di fferently. This review article provides an overview of the work, which is c oncerned with the monosubstituted species, R'R7Si8O12, where R' and R are t wo different substituents, such as organic or organometallic groups or sing le atoms (H, CI). Three synthetic routes have been used so far to prepare m onosubstituted 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 discusse d in this review, Various spectroscopic techniques that have been applied t o characterize these molecules - X-ray diffraction, NMR, IR and Raman spect roscopy as well as mass spectrometry - are treated, A main focus is the not ion of ring-opening vibration, which is supposed to be generally applicable to microporous materials, especially zeolites. A simple model for explaini ng the nature of ring-opening vibrations is presented, Based on the new pos sibilities offered by functionalized silasesquioxanes, challenging perspect ives for the synthesis of novel materials are discussed. Copyright (C) 1999 John Wiley & Sons, Ltd.