The chemistry of polyhedral oligomeric silsesquioxanes (POSS) has taken a tremendous leap over the past decade. New synthetic approaches have been developed, and new complexes based on elements throughout the periodic table have been synthesized and applied in catalytic processes such as alkene epoxidation, Oppenauer oxidation, alkene polymerization, Diels−Alders reactions and alkene metathesis. The use of silsesquioxane ligands as models for silica surface has provided an intimate knowledge of the reactions on the surface of this much‐used support. New methods have been developed to incorporate silsesquioxanes into new materials. Encapsulation in siloxane polymers has provided precision supports for catalysts and controlled calcination has produced materials with well‐defined metal oxide particles. This paper reviews the history and the latest developments of silsesquioxane chemistry. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)

Calcination of mixtures of (c-C 5 H 9 ) 7 Si 7 O 9 (OH) 3 , 1, and (c-C 5 H 9 ) 7 Si 7 O 12 Fe(tmeda), 2 (tmeda ) N,N,N′,N′tetramethylethylenediamine), led to microporous amorphous Fe-Si-O materials with adjustable iron content in the range 1-11 wt %. A set of different complementary techniques including N 2 physisorption, XRD, XPS, DRUV-vis, RS, IR, HRTEM, and Mo ¨ssbauer spectroscopy was used to follow the variation of the textural properties, metal dispersion, and speciation with the iron content along the whole mixing series. The calcination of these mixtures produced Fe-Si-O materials having basically the same properties as those observed for the individually calcined iron silsesquioxane. The N 2 physisorption indicates high surface areas, rather large pore volumes, and a very narrow pore size distribution with an average pore size diameter around 6-7 Å. The TEM and the spectroscopic analysis of the Fe-Si-O materials indicate that the iron is present mainly as small iron oxide particles highly dispersed throughout silica and to a minor extent as clustered and isolated species. The particle size distribution was estimated to be about 2-8 nm for 11% Fe-Si-O and 2-4 nm for samples with lower iron content. These materials showed catalytic activity in NH 3 oxidation and N 2 O decomposition.

The heterogenization of a titanium(IV) silsesquioxane complex in an MCM-41 molecular sieve, by tailoring the polarity of MCM-41, results in self-assembled materials (see the space-filling model on the right) that are active, truly heterogeneous, and recyclable catalysts for liquid-phase alkene epoxidation.

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