Metal-Surface Interactions and Surface Heterogeneity in 'Well-Defined' Silica-Supported Alkene Metathesis Catalysts: Evidences and Consequences

Here we present our observations on a peculiar kinetic behavior observed at low catalyst loadings for the metathesis of internal olefins on a highly active silica-supported tungsten oxo alkylidene prepared through surface organometallic chemistry (SOMC). The observed kinetic profile is comprised of two consecutive regimes, which support the presence of two different active sites on the surface of such catalysts. We propose that the more active site is operational in the beginning of the reaction but deactivates very fast, and the second one initiates over time due to the presence of ethylene formed subsequently under the reaction conditions. A combination of spectroscopic techniques and computations was used to identify the possible nature of these sites. Solid-state NMR, EXAFS and DFT calculations are consistent with the esence of two distinct surface species that likely originate from the occurrence of slightly different local environments on the surface, that is, metal sites with a different number of nearby siloxane bridges. This data highlights the effects that may arise from the amorphous nature of the supports like silica and the difficulties to obtain truly single-site catalysts - one of the ultimate goals of SOMC.