The preparation of encapsulated fluoride anion in T8 silsesquiocane cages

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

Fluoride-ion encapsulated within a silsesquioxane cage

Octaspherosilicate structures have found use as scaffolds for dendrimer synthesis, as models for aluminosilicate structures, and as supports for organometallic catalysts. Space-filling models of such cages suggest there is a void at the center of the cube, but that it is questionable whether it could be filled by an atomic species. Päch and Stosser have shown that g-irradiation of octaalkyl octasilsesquioxane or octatrialkyl siloxyoctasilsesquioxane leads to hydrogenencapsulated T8 or Q8 cages. ESR spectroscopy confirmed that the hydrogen originated from the organic substituents of the cage

Fluoride Ion Entrapment in Octasilsesquioxane Cages as Models for ion entrapment in Zeolites. Further examples, X-ray Crystal Structure studies, and investigations into how and why they may be formed

We have successfully prepared two further fluoride-encapsulated octasilsesquioxane cage compounds using our TBAF and scare water catalysed route as important molecular models for studying ion entrapment in siliceous zeolites. Analysis by X-ray crystal structure and solution 19F/29Si NMR spectroscopy reveal very similar environments for the encapsulated fluoride ion perfectly centred within each cage with evidence of weak silicon fluoride ion van der Waals interactions. By comparing our NMR and X-ray data with non-encapsulated cage analogues from the literature we observe the effect the fluoride ion has on the geometry of the cage and use the findings of related studies in the literature to suggest reasons why fluoride ion may be becoming entrapped in the silsesquioxane core

Further studies of fluoride ion entrapment in octasilsesquioxane cages; X-ray crystal structure studies and factors that affect their formation

A range of fluoride-encapsulated octasilsesquioxane cage compounds have been prepared using the TBAF route. Our studies suggest that whilst it is relatively straightforward to prepare fluoride-encapsulated octasilsesquioxane cage compounds with adjacent sp(2) carbons, leading to a range of aryl and vinyl substituted compounds, the corresponding sp(3) carbon derivatives are more capricious, requiring an electron withdrawing group that can stabilize the cage whilst not acting as a leaving group. Analysis by X-ray crystallography and solution F-19/Si-29 NMR spectroscopy of R8T8@F- reveal very similar environments for the encapsulated fluoride octasilsesquioxane cages. Migration of a fluoride ion from inside the cage to outside the cage without breaking the T-8 framework and the possibility of encapsulating other anions within silsesquioxane cages have been also investigated