Mechanism of the Cycloaddition of Carbon Dioxide and Epoxides Catalyzed by Cobalt-Substituted 12-Tungstenphosphate

CoII-substituted a-Keggin-type 12-tungstenphosphate [(n- C4H9)4N]4H[PW11Co(H2O)O39]- (PW11Co) is synthesized and used as a single-component, solvent-free catalyst in the cycloaddition reaction of CO2 and epoxides to form cyclic carbonates. The mechanism of the cycloaddition reaction is investigated using DFT calculations, which provides the first computational study of the catalytic cycle of polyoxometalate-catalyzed CO2 coupling reactions. The reaction occurs through a single-electron transfer from the doublet CoII catalyst to the epoxide and forms a doublet CoIIIcarbon radical intermediate. Subsequent CO2 addition forms the cyclic carbonate product. The existence of radical intermediates is supported by free-radical termination experiments. Finally, it is exhilarating to observe that the calculated overall reaction barrier (30.5 kcal?mol-1) is in good agreement with the real reaction rate (83 h-1) determined in the present experiments (at 150?degrees C).