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 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 Å. The sites are best described as [GaMe2(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 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 °C). This result is interpreted in
terms of a vicinal disposition for the majority of Q3 grafting sites.