Methyltrioxorhenium interactions with lewis acid sites of an amorphous silica-alumina

Deposition of CH3ReO3 onto the dehydrated surface of an amorphous silica−alumina (Si/Al = 4.8) generates a catalyst for olefin metathesis, although CH3ReO3 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 ReO 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 supp...