Mechanism of the cycloaddition of carbon dioxide and epoxides catalyzed by cobalt-substituted 12-tungstenphosphate

Co ^II-substituted α-Keggin-type 12-tungstenphosphate [(n- C ₄H ₉) ₄N] ₄H[PW ₁₁Co(H ₂O)O ₃₉]- (PW ₁₁Co) is synthesized and used as a single-component, solvent-free catalyst in the cycloaddition reaction of CO ₂ 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 CO ₂ coupling reactions. The reaction occurs through a single-electron transfer from the doublet Co ^II catalyst to the epoxide and forms a doublet Co ^III-carbon radical intermediate. Subsequent CO ₂ 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 °C). Organometallic radical intermediate: Cobalt-substituted α-Keggin-type 12-tungstenphosphate is used as an efficient single-component catalyst for the synthesis of cyclic carbonates from CO ₂ and epoxides. DFT calculations indicate that the cycloaddition reaction occurs through single-electron transfer from the doublet Co ^II cluster to the epoxide, thus leading to homolytic cleavage of the C-O bond and formation of a carbon radical intermediate, followed by cyclization with CO ₂ (see scheme).