Olefin-Surface Interactions: A Key Activity Parameter in Silica-Supported Olefin Metathesis Catalysts

Abstract

Molecularly defined and classical heterogeneous Mo-based metathesis catalysts are shown to display distinct and unexpected reactivity patterns for the metathesis of long-chain α-olefins at low temperatures (<100 °C). Catalysts based on supported Mo oxo species, whether prepared via wet impregnation or surface organometallic chemistry (SOMC), exhibit strong activity dependencies on the α-olefin chain length, with slower reaction rates for longer substrate chain lengths. In contrast, molecular and supported Mo alkylidenes are highly active and do not display such dramatic dependence on the chain length. State-of-the-art two-dimensional (2D) solid-state nuclear magnetic resonance (NMR) spectroscopy analyses of postmetathesis catalysts, complemented by Fourier transform infrared (FT-IR) spectroscopy and molecular dynamics calculations, evidence that the activity decrease observed for supported Mo oxo catalysts relates to the strong adsorption of internal olefin metathesis products because of interactions with surface Si–OH groups. Overall, this study shows that in addition to the nature and the number of active sites, the metathesis rates and the overall catalytic performance depend on product desorption, even in the liquid phase with nonpolar substrates. This study further highlights the role of the support and active site composition and dynamics on activity as well as the need for considering adsorption in catalyst design.

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

“…14,15 Mo K-edge XAS analysis supports the presence of highly dispersed Mo(VI) dioxo surface sites that can be readily activated for catalysis at relatively low temperatures (30−70 °C) under reducing conditions (Figure 1c) in particular using a molecular reducing agent. 16,17 In fact, this activation process and associated low-temperature activity can also be extended to supported Mo oxide catalysts generated via conventional approaches albeit with lower activity, 18,19 raising questions about the similarities and differences between catalysts synthesized via different routes and the precise nature of the (pre)catalytic active sites.…”

Cationic Molybdenum Imido Alkylidene N‐Heterocyclic Carbene Complexes Confined in Mesoporous Silica: Tuning Transition States Towards Z‐Selective Ring‐Opening Cross‐Metathesis

“…[4] Within the last twenty years, it has been demonstrated that well-defined metathesis catalysts can also be immobilized onto surfaces, and display good reactivity. [5] These immobilized olefin metathesis catalysts not only have the advantages of both homogeneous and surface-supported catalysts, but sometimes benefit from additional changes in reactivity due to increased stability [6] and/or selectivity [7] due to changes in the transition state of the reaction.…”

Advances in the characterization of inorganic solids using NMR correlation experiments

“…For instance, J-HMQC sequence has allowed the selective of observation AlH species on the surface of γ-alumina 175 and OH groups on 17 O-labeled silica and alumina 288,361 . Throughbond and through-space heteronuclear correlation experiments have also been employed to probe 1 H-13 C connectivities and proximities in supported metal complexes and organic functional groups as well as surfactants (see Figure 10) 164,215,[362][363][364] . These experiments were initially recorded for 13 C-labeled samples 362,363 .…”

“…These experiments were initially recorded for 13 C-labeled samples 362,363 . Nevertheless, it was later demonstrated that the sensitivity gain provided by indirect detection via protons allows their acquisition in natural abundance using conventional NMR 215,364 . Similar approaches have been used to probe proximities between protons and other spin-1/2 isotopes, such as 15 N 195,216 , 19 F 365 , 29 Si 90 and 195 Pt 227 , near surfaces (see Figure 18).…”