Deposition of CH 3ReO 3 onto the dehydrated surface of an amorphous silica-alumina (Si/Al = 4.8) generates a catalyst for olefin metathesis, although CH 3ReO 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 1H, 13C, and 27Al 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 13C and 1H nuclei. Chemisorption of the intact molecular complex occurs via interaction of one oxo ligand with an Al site, resulting in significant elongation of this Re=O 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 3ReO 3 and related heterogeneous catalysts.
|Number of pages||9|
|State||Published - Apr 24 2006|