(375f) Elucidating the Role of Water in Olefin Epoxidation Catalyzed By Methyltrioxorhenium

The activation of MTO by H₂O₂ forms two discrete peroxorhenium complexes, CH₃ReO₂(η²-O₂) and CH₃ReO(η²-O₂)₂(H₂O), via a sequential and reversible reaction. Past reports on the thermodynamics and kinetics of these reactions are inconsistent with each other. New experiments and calculations using density functional theory were conducted to better understand these reactions and to provide a robust experimental foundation for benchmarking computational studies involving MTO and its derivatives. Including solvation and tunneling contributions to the free energies, we compute negative reaction enthalpies for each reaction and correctly predict the hydration state of all complexes in aqueous acetonitrile. New rate constants for each of the forward and reverse reactions were both measured and computed as a function of temperature, providing a complete set of consistent activation parameters. Computed rate constants for a direct ligand-exchange mechanism, and for a mechanism in which water(s) assists ligand-exchange via proton transfer in the transition state, differ by at least seven orders of magnitude. The water-assisted mechanism is predicted to be faster, and is consequently in closer agreement with the experimentally-measured kinetics. Experiments confirm the catalytic role of water: the kinetics of both steps are strongly dependent on the water concentration.