Initiation of heterogeneous Schrock-type Mo and W oxide metathesis catalysts: A quantum thermochemical study

Abstract

Three mechanistic pathways were explored theoretically to study the electronic features and thermodynamics behind the initiation of silica-grafted molybdenum and tungsten oxide metathesis pre-catalysts in light olefins (ethylene and propylene) to form Schrock-type carbene structures. At 298 K and 1 atm, the investigated activation process was found to be exothermic for all but one of the carbene species, where the pathway involving a secondary carbene center and liberating acetone as a byproduct was more favorable (by about 10 kcal/mol) than the process involving a primary carbene center and evolving propionaldehyde. The most exothermic carbene to form (−20.9 kcal/mol) had its both propylidene and propoxy groups bonded through their secondary carbon atoms. A competing pathway suggested for an ethylene feedstock and tungsten-based catalyst, which involved the production of acetaldehyde was also viable. The generation of carbene species from the W-based pre-catalyst was less exothermic than the Mo-based pre-catalyst, however. The computational data with the activation pathways at hand stressed the benefits of initiation at low temperatures, the presence of adjacent trapping heterogeneities for the acetone or propanal by-products, and the beneficial role of the presence of an isopropoxy ligand. The localized-orbital locator maps indicated the polar covalent characteristics of the metal–carbene bonds, which were polarized towards the C atom. The analysis of the density of states indicated the destabilization of the highest occupied molecular orbitals (HOMOs) of the pre-catalysts upon carbene generation, where the alkylidene fragment and transition metal both contributed to the HOMO of the carbene complex.