Methyltrioxorhenium Interactions with Lewis Acid Sites of an Amorphous Silica−Alumina

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 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 3 ReO 3 and related heterogeneous catalysts.