Ligand Exchange-Mediated Activation and Stabilization of a Re-Based Olefin Metathesis Catalyst by Chlorinated Alumina

Extensive chlorination of gamma-Al2O3 results in the formation of highly Lewis acidic surface domains depleted in surface hydroxyl groups. Adsorption of methyltrioxorhenium (MTO) onto these chlorinated domains serves to activate it as a low temperature, heterogeneous olefin metathesis catalyst and confers both high activity and high stability. Characterization of the catalyst reveals that the immobilized MTO undergoes partial ligand exchange with the surface, whereby some Re sites acquire a chloride ligand from the modified alumina while donating an oxo ligand to the support. More specifically, Re L-III-edge EXAFS and DFT calculations support facile ligand exchange between MTO and Cl-Al2O3 to generate [CH3ReO2Cl+] fragments that interact with a bridging oxygen of the support via a Lewis acid-base interaction. According to IR and solid-state NMR, the methyl group remains intact, and does not evolve spontaneously to a stable methylene tautomer. Nevertheless, the chloride-promoted metathesis catalyst is far more active and productive than MTO/gamma-Al2O3, easily achieving a TON of 100 000 for propene metathesis in a flow reactor at 10 degrees C (compared to TON < 5000 for the nonchlorinated catalyst). Increased activity is a consequence of both a larger fraction of active sites and a higher intrinsic activity for the new sites. Increased stability is tentatively attributed to a stronger interaction between MTO and chlorinated surface regions, as well as extensive depletion of the Bronsted acidic surface hydroxyl population. The reformulated catalyst represents a major advance for Re-based metathesis catalysts, whose widespread use has thus far been severely hampered by their instability.