Formation of digallium sites in the reaction of trimethylgallium with silica

Article abstract of DOI:10.1021/om051034o

The room-temperature, gas-solid reaction of volatile GaMe₃ with a nonporous silica was studied by elemental and gas-phase analysis, in situ IR and ¹H, ¹³C, and ²⁹Si solid-state NMR, and extended X-ray absorption fine structure (EXAFS) spectroscopy. Most of the grafting (～85%) occurred on Q³ sites, O₃SiOH, but a small amount (～15%) of siloxane (O₃SiO-SiO₃) bond cleavage was also observed. The major, if not the only, gallium product has the empirical formula ≡SiOGaMe₂, but it is not an isolated site. The Ga K-edge EXAFS of GaMe₃-modified silica, recorded at 10 K, reveals that each Ga has a Ga neighbor at 2.97-2.99 A. The sites are best described as [GaMe ₂(μ-OSi≡)]₂. To strengthen this assignment, a molecular analogue, [GaMe₂(μ-OSiPh₃)]₂, was characterized by both single-crystal X-ray diffraction and EXAFS. The Ga ₂O₂ rings in the molecular complex and the silica-supported gallium dimer have very similar dimensions. The gallium dimer is formed on the silica surface regardless of the extent of partial dehydroxylation (varied by pretreatment in vacuo at 100 and 500°C). This result is interpreted in terms of a vicinal disposition for the majority of Q³ grafting sites.