Tuning the Reactivity of Fe^V(O) toward C–H Bonds at Room Temperature: Effect of Water

The presence of an Fe^V(O) species has been postulated as the active intermediate for the oxidation of both C–H and CC bonds in the Rieske dioxygenase family of enzymes. Understanding the reactivity of these high valent iron–oxo intermediates, especially in an aqueous medium, would provide a better understanding of these enzymatic reaction mechanisms. The formation of an Fe^V(O) complex at room temperature in an aqueous CH₃CN mixture that contains up to 90% water using NaOCl as the oxidant is reported here. The stability of Fe^V(O) decreases with increasing water concentration. We show that the reactivity of Fe^V(O) toward the oxidation of C–H bonds, such as those in toluene, can be tuned by varying the amount of water in the H₂O/CH₃CN mixture. Rate acceleration of up to 60 times is observed for the oxidation of toluene upon increasing the water concentration. The role of water in accelerating the rate of the reaction has been studied using kinetic measurements, isotope labeling experiments, and density functional theory (DFT) calculations. A kinetic isotope effect of ∼13 was observed for the oxidation of toluene and <i>d</i>₈-toluene showing that C–H abstraction was involved in the rate-determining step. Activation parameters determined for toluene oxidation in H₂O/CH₃CN mixtures on the basis of Eyring plots for the rate constants show a gain in enthalpy with a concomitant loss in entropy. This points to the formation of a more-ordered transition state involving water molecules. To further understand the role of water, we performed a careful DFT study, concentrating mostly on the rate-determining hydrogen abstraction step. The DFT-optimized structure of the starting Fe^V(O) and the transition state indicates that the rate enhancement is due to the transition state’s favored stabilization over the reactant due to enhanced hydrogen bonding with water