Surface Reactions in the Aluminum-Catalyzed Direct Synthesis of Alkylsilanes

The formation of alkylsilanes (SiR_xH_4-x, x ≤ 3) from the reaction of triisobutylaluminum (TIBA) or 1-iodopropane with an aluminum-promoted silicon surface at 450-600 K under ultra-high-vacuum conditions is reported. With both reactants, alkylsilane production is accompanied by alkene evolution, and in the case of TIBA aluminum deposition. Isotope labeling studies show that the hydrogen atoms in the alkylsilane product come from the β-carbon of the alkyl intermediate generated on the surface by the dissociative chemisorption of the adsorbate. The reaction kinetics implicate β-hydride elimination from alkyls bound to aluminum as the rate-determining step. When this rate-determining step is circumvented by adding atomic hydrogen to the system, alkylsilanes form at temperatures as low as 340 K. Our results show that the presence of metallic aluminum on the silicon surface is critical for (1) dissociatively adsorbing the alkyl iodide or aluminum alkyl, (2) inducing β-hydride elimination of the adsorbed alkyls to form surface hydrogen atoms, and (3) enhancing the diffusion of hydrogen, alkyls, and silicon on the surface.