Water-Catalyzed Activation of H2O2 by Methyltrioxorhenium: A Combined Computational–Experimental Study

Abstract

The formation of peroxorhenium complexes by activation of H2O2 is key in selective oxidation reactions catalyzed by CH3ReO3 (methyltrioxorhenium, MTO). Previous reports on the thermodynamics and kinetics of these reactions are inconsistent with each other and sometimes internally inconsistent. New experiments and calculations using density functional theory with the ωB97X-D and augmented def2-TZVP basis sets were conducted to better understand these reactions and to provide a strong experimental foundation for benchmarking computational studies involving MTO and its derivatives. Including solvation contributions to the free energies as well as tunneling corrections, we compute negative reaction enthalpies for each reaction and correctly predict the hydration state of all complexes in aqueous CH3CN. 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. New, independent measurements of equilibrium constants do not indicate strong cooperativity in peroxide ligand binding, as was previously reported. The free energy barriers for formation of both CH3ReO2(η(2)-O2) (A) and CH3ReO(η(2)-O2)2(H2O) (B) are predominantly entropic, and the former is much smaller than a previously reported value. Computed rate constants for a direct ligand-exchange mechanism, and for a mechanism in which a water molecule facilitates ligand-exchange via proton transfer in the transition state, differ by at least 7 orders of magnitude. The latter, water-assisted mechanism is predicted to be much faster and is consequently in much closer agreement with the experimentally measured kinetics. Experiments confirm the predicted catalytic role of water: the kinetics of both steps are strongly dependent on the water concentration, and water appears directly in the rate law.