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 H-1, C-13, and Al-21 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 C-13 and H-1 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 CH3ReO3 and related heterogeneous catalysts.