Silsesquioxane - Silsesquioxane Catalysis

Silsesquioxane Catalysis

Another area involving silsesquioxanes that has experienced increased scientific development recently is the study of metal coordination of silsesquioxanes, resulting in metallasilsesquioxanes, which have found application as catalysts. Incompletely condensed silsesquioxanes like Cy₇Si₇O₉(OH)₃ are similar in structure to β-tridymite and β-cristobalite, making them good models for the silanol sites on silica surfaces. The structures of silsesquioxanes make them ideal for metal coordination as well, due to the fixed orientation of the silanol groups and also the siloxane bridges which can interact with the metal. Additionally the silsesquioxane can be modified for even better metal coordination via simply silylation reactions. Research has shown that silsesquioxanes can bind with numerous main group and transition group metals, including Na, Li, and Be. The most frequently employed starting silsesquioxane for metal complex synthesis is the trisilanol derivative Cy₇Si₇O₉(OH)₃, originally reported by Brown and Vogt, which is synthesized from trichlorocyclohexylsilane, but can take several years to run to completion. This resulted in only a few incompletely condensed silsesquioxanes available in useful quantities for research. Consequently most research has been focused on the trisilanol derivative Cy₇Si₇O₉(OH)₃and its cyclopentyl-substituted analog. However recently Feher et al. have developed an acid-mediated cleavage of fully condensed silsesquioxane frameworks like Cy₈Si₈O₁₂. The process results in silanediols that can further be used to create new metallasilsesquioxanes.

The general preparation for metal-silsesquioxane derivatives involves treating the parent silanol and the desired metal halide in the presence of a base like triethylamine. The product metallasilsesquioxane can frequently be readily isolated by fractional crystallization. Another route of synthesis involves first deprotonating the trisilanol group, however this has proven to be somewhat difficult. Initial attempts by Feher et al. to deprotonate trisilanols with sodium t-butoxide did so, but the products were unstable for an extended period of time. More recently it was found that deprotonated trisilanols could successfully be prepared if the right base was used. Feher et al. determined three equivalents of LiN(SiMe3)2 were effective, with the product potentially even precipitating out depending on the solvent. Aspinall et al. later succeeded in doing the same using three equivalents of n-BuLi in hexanes and further results indicate that alkali metal derivatives of deprotonated silsesquioxanes could also be prepared using alkali metalbis(trimethylsilyl) amides.

Much of modern research is focused on the synthesis of metallasilsesquioxanes that containing metals that have not been done before and the potential application of the metallasilsesquioxanes as catalysts, as it has become accepted that metallasilsesquioxanes are good silica-supported transition metal catalysts. For example Edelmann et al. successfully synthesized and analyzed the first beryllium silsesquioxane, ₂.2THF. The use of silsesquioxanes to make homogeneous models for heterogeneous catalysts is another area research, allowing a better understanding of the system to be reached.