The preparation of encapsulated fluoride anion in T8 silsesquiocane cages

Pourny, Manuel (2005). The preparation of encapsulated fluoride anion in T8 silsesquiocane cages. PhD thesis The Open University.



A great number of reactions leading to the formation of polyhedral silsesquioxanes are known. Because this is a complex and multistep process, these reactions also lead to polymers and oligomers, which may include polyhedral silsesquioxanes and some homo derivatives. Our work has focused on the synthesis of octasilsesquioxanes by the hydrolytic condensation of trifunctional monomers using tetra-n-butylammonium fluoride as a catalyst. We have studied the influence of solvent and the nature of the alkoxy leaving group in the starting material, trialkoxysilane, on the yields in the synthesis of octacyclopentyloctasilsesquioxanes and octaphenyloctasilsesquioxanes. Using the TBAF route, we also prepared a phenyl-T8, a vinyl-T8 and para-tolyl-T8 cage with a fluoride anion which is perfectly centred in the middle of the cage. For all of these fluoride encapsulated cages, we have obtained single crystal X-Ray structures as shown below.
We were also able to synthesize octa-pflra-chloromethylphenyloctasilsesquioxane cage with an encapsulated fluoride anion. For this latter structure we only obtained 29Si and 19F NMR data and Mass Spectrometry data which are in agreement with data from the phenyl and vinyl cages. The four tetra-H-butylammonium octasilsesquioxane fluoride cages obtained so far all have an sp2 carbon atom bonded to the silicon atom. We thus attempted to prepare fluoride-encapsulated cages where the R group at silicon is bound by an sp3 carbon, such as cyclohexyl, cyclopentyl, isobutyl, n-octyl or benzyl. Unfortunately using these groups, we have only been able to isolate the conventional octasilsesquioxane fluoride ion free cages. However, experimental attempts to synthesize Q8 cages with fluoride ion inside have been successful and even though we were unable to obtain a single X-ray crystal structure, 29Si and 19F NMR spectroscopy and Mass Spectrometry data demonstrated that tetra-w -butylammonium fluoride Q8 cages were the main product of the reaction and were obtained in reasonable yields. In order to understand the encapsulation mechanism we investigated the possibility of the entrapment of different anions using other sources of catalyst. None of these investigations showed the encapsulation of an anion other than fluoride ion. This highlighted the peculiar role of TBAF and so we looked further at the mechanism of the encapsulation and its requirements in terms of physical and environmental parameters.
Finally we carried out some physical studies and in particular Thermo Gravimetric Analysis (TGA) of some of materials synthesized by our research group, namely pure hexa, octa and dodecasilsesquioxane cages and compared them with the corresponding fluoride-encapsulated cages where possible.

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