Formation of digallium sites in the reaction of trimethylgallium with silica

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 H-1, C-13, and Si-29 solid-state NMR, and extended X-ray absorption fine structure (EXAFS) spectroscopy. Most of the grafting (similar to 85%) occurred on Q(3) sites, O3SiOH, but a small amount (similar to 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 [GaM2(mu-OSi )](2). To strengthen this assignment, a molecular analogue, [GaMe2(mu-OSiPh3)](2), was characterized by both single-crystal X-ray diffraction and EXAFS. The Ga2O2 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 degrees C). This result is interpreted in terms of a vicinal disposition for the majority of Q3 grafting sites.