Active Site Descriptors from ⁹⁵Mo NMR Signatures of Silica-Supported Mo-Based Olefin Metathesis Catalysts

The olefin metathesis activity of silica-supported molybdenumoxidesdepends strongly on metal loading and preparation conditions, indicatingthat the nature and/or amounts of the active sites vary across compositionallysimilar catalysts. This is illustrated by comparing Mo-based (pre)-catalystsprepared by impregnation (2.5-15.6 wt % Mo) and a model material(2.3 wt % Mo) synthesized via surface organometallic chemistry (SOMC).Analyses of FTIR, UV-vis, and Mo K-edge X-ray absorption spectrashow that these (pre)-catalysts are composed predominantly of similarisolated Mo dioxo sites. However, they exhibit different reactionproperties in both liquid and gas-phase olefin metathesis with theSOMC-derived catalyst outperforming a classical catalyst of a similarMo loading by x1.5-2.0. Notably, solid-state Mo-95 NMR analyses leveraging state-of-the-art high-field (28.2 T) measurementconditions resolve four distinct surface Mo dioxo sites with distributionsthat depend on the (pre)-catalyst preparation methods. The intensityof a specific deshielded Mo-95 NMR signal, which is mostprominent in the SOMC-derived catalyst, is linked to reducibilityand catalytic activity. First-principles calculations show that Mo-95 NMR parameters directly manifest the local strain andcoordination environment: acute (SiO-Mo-(O)(2)-OSi)angles and low coordination numbers at Mo lead to highly deshielded Mo-95 chemical shifts and small quadrupolar coupling constants,respectively. Natural chemical shift analyses relate the Mo-95 NMR signature of strained species to low LUMO energies, whichis consistent with their high reducibility and corresponding reactivity.The Mo-95 chemical shifts of supported Mo dioxo sites arethus linked to their specific electronic structures, providing a powerfuldescriptor for their propensity toward reduction and formation ofactive sites.