Reaction of Organosilicon Hydrides with Solid Surfaces:  An Example of Surface-Catalyzed Self-Assembly

The solution-phase reactions of octadecylsilane (C18H37SiH3) with 10 high surface area metal oxides (groups II−VIII) were investigated. C18H37SiH3 reacted with most metal oxides at room temperature and produced supported monolayers (self-assembled monolayers, SAMs) with a high grafting density of C18, ∼4.5−5 groups/nm2. According to the FTIR and 29Si NMR spectra, molecules in the SAMs demonstrated “horizontal” cross-linking (Si−O−Si and Si−OH···HO−Si bonds) and little or no “vertical” bonds with the metal oxide forming an amorphous, yet ordered film. Also, ∼3 mol of H2 was formed per each mole of grafted C18, indicating complete hydrolysis of C18H37SiH3 during the reaction. On the basis of the activity of different metal oxides, we concluded that the hydrolysis of C18H37SiH3, the key step in the reaction mechanism, is catalyzed by water adsorbed on acidic and basic centers (Lewis and Brönsted) of the surface of metal oxide. Metal oxides and solids with weak acidic and basic properties, like silica, carbon, and organic polymers, do not react with C18H37SiH3. Increasing the temperature of the reaction or doping neutral surfaces with acids or bases greatly increases their activity in the reaction with RSiH3.