Mechanistic Investigation into Olefin Epoxidation with H 2 O 2 Catalyzed by Aqua-Coordinated Sandwich-Type Polyoxometalates: Role of the Noble Metal and Active Oxygen Position.

Aqua-coordinated sandwich-type polyoxometalates (POMs), {[WZnTM2 (H2 O)2 ](ZnW9 O34 )2 } n - (TM=RhIII , PdII , and PtII ), catalyze olefin epoxidation with hydrogen peroxide and have been well established, and they present an advance toward the utilization of olefins. To elucidate the epoxidation mechanism, we systematically performed density functional calculations. The reaction proceeds through a two-step mechanism: activation of H2 O2 and oxygen transfer. The aqua-coordinated complexes show two distinct H2 O2 activation pathways: "two-step" and "concerted". The concerted processes are more facile and proceed with similar and rate-determining energy barriers at the Rh-, Pd-, and Pt-containing transition states, which agrees well with the experimental results. Next, the resulting TM-OH-(μ-OOH) intermediate transfers an O atom to olefin to form an epoxide. The higher reactivity of the Rh-containing POM is attributed to more interactions between the Rh and hydroperoxo unit. We also calculated all active oxygen positions to locate the most favorable pathway. The higher reactivity of the two-metal-bonded oxygen position is predominantly ascribed to its lower stereoscopic hindrance. Furthermore, the presence of one and two explicit water solvent molecules significantly reduces the energy barriers, making these sandwich POMs very efficient for the olefin epoxidation with H2 O2 .