Other than Sn in zeolite beta, there are few demonstrated examples of tetrahedral oxo-Sn Lewis acid catalysts. A tetrahedral oxo-Sn complex could be synthesized using two silsesquioxane (POSS) as ligands to form a (POSS)Sn(POSS) complex. The tetrahedral coordination is confirmed by single crystal X-ray data, and the complex is a Lewis acid and catalyzes epoxide ring opening and hydride transfer. The complex can be anchored by hydrosilylation onto a silica surface that has been functionalized with silanes, while maintaining its structure. Upon calcining the anchored complex at mild temperatures in a stream of ozone, the alkyl ligands are removed, leaving behind a Sn-oxo cluster in which the Sn maintains the tetrahedral coordination. Because this tetrahedral Sn center is on an open surface, it is accessible to large molecules such as cellobiose. Indeed, this catalyst converts cellobiose in gamma-valerolactone in the presence of dimethylsulfoxide into 5-hydroxymethylfurfural.
The calcined material is also much more active in catalyzing epoxide ring opening than the uncalcined precursor. In the presence of water, acetal is formed. Thus, the catalyst is water tolerant. We speculate that this may be due to the more rigid structure of the (POSS)Sn(POSS) complex and its calcined form that results in weaker water binding.