Silsesquioxane isolators

Alkene cross-metathesis has also been recently used for the modification of silsesquioxanes and spherosilicates, by Feher and co-workers [46]. Reaction of vinylsilsesquioxane 28 with a variety of simple functionalised alkenes, in the presence of Schrock s molybdenum catalyst 3, gave complete conversion of the starting material and very good isolated yields of the desired products (75— 100%) (for example Eq. 28). 

A similar reaction sequence starting from the completely condensed silsesquioxane octamer cage (c-C,jI 1i iSi) Oi 2 513 affords the isolated intermediates 514 and 515, eventually providing the octasiloxanediol (r-C4I h iSiHOjoiOH 516 (Scheme 69).471... 

The analogous reaction of 3 with W2(OrBu)6 gave a brown solution, from which a yellow, microcrystalline material could be isolated. Based on spectroscopic data the latter was formulated as [Cy7Si70i2]2W2(p-H)(0tBu) (158). Cleavage of the Mo = Mo triple bond in 157 occurred upon treatment of this compound with NO (two equivalents) to afford the dimeric nitrosyl molybdenum silsesquioxane complex [Cy7Si70i2]2Mo2(NO)2 (159). °°... 

The first iron-containing silsesquioxanes which appeared in the literature were compounds containing ferrocenyl units as side-groups.However, these are not within the scope of this review as iron is not part of the metallasilsesquioxane skeleton. Meanwhile, several ferrasilsesquioxane complexes have been synthesized. The first iron(III) compound of this type was prepared in our laboratory according to Scheme 56. ° In 161, the coordination sphere of iron is completed by TMEDA (N,N,N, N -tetramethylethylenediamine) as a chelating amine ligand. Pale yellow, crystalline 161 was isolated in 80% yield and structurally characterized by X-ray diffraction. This compound was later used by Maxim et to prepare iron... 

Figure 14.1 Silsesquioxanes used as models for Isolated, vicinal and geminal surface silanols.

A crystalline silsesquioxane, octamethyloctasilsesquioxane, was first isolated by Scott (65) among the cracking products of mixed methyl-chlorosilane hydrolyzates. Its molecular weight was not established until some years later (75). This highly symmetrical octamer does not melt below 450° but is readily sublimable in a good vacuum at much lower temperatures. 

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