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Deposition of CH3ReO3 onto the surface of dehydrated, amorphous silica-alumina generates a highly active, supported catalyst for the metathesis of olefins. However, silica-alumina with a high (10 wt %) Re loading is no more active than silica-alumina with low (1 wt %) loading, while CH3ReO3 on silica is completely inactive. Catalysts prepared by grafting CH3ReO3 on silica-alumina contain two types of spectroscopically distinct sites. The more strongly bound sites are responsible for olefin metathesis activity and are formed preferentially at low Re loadings (< or =1 wt %). They are created by two Lewis acid/base interactions: (1) the coordination of an oxo ligand to an Al center of the support and (2) interaction of one of the adjacent bridging oxygens (AlOSi) with the Re center. At higher Re loadings (1-10 wt %), CH3ReO3 also interacts with surface silanols by H-bonding. This gives rise to highly mobile sites, most of which can be observed by 13C solid-state NMR even without magic-angle spinning. Their formation can be prevented by capping the surface hydroxyl groups with hexamethyldisilazane prior to grafting CH3ReO3, resulting in a metathesis catalyst that is more selective, more robust, and more efficient in terms of Re use.

Deposition of CH 3 ReO 3 onto the dehydrated surface of an amorphous silica-alumina (Si/Al ) 4.8) generates a catalyst for olefin metathesis, although CH 3 ReO 3 itself is not active. The nature of the interactions between the silica-alumina surface and the grafted organometallic complex was probed by 1D and 2D 1 H, 13 C, and 27 Al solid-state NMR, IR, EXAFS, and DFT calculations. The methyl ligand remains bound to Re, but grafting alters its symmetry, as well as the shielding of the 13 C and 1 H nuclei. Chemisorption of the intact molecular complex occurs via interaction of one oxo ligand with an Al site, resulting in significant elongation of this RedO bond. Comparison of EXAFS-and DFT-derived bond distances suggests that the participating Lewis acid sites of silica-alumina involve five-coordinate Al. A second surface-organometallic interaction arises by coordination of an adjacent bridging oxygen atom (AlOSi) to the Re center. These insights represent a first step toward understanding the role of solid oxide supports in conferring metathesis activity to CH 3 ReO 3 and related heterogeneous catalysts.