CO2 fixation and transformation on a thiolate-bridged dicobalt scaffold under oxidising conditions

Abstract

Through successive one-electron reduction processes, a coordinatively unsaturated thiolate-bridged Co^IICo^II complex [Cp*Co(μ-SEt)₂CoCp*] (1) was facilely generated in high yield. In the presence of O₂, complex 1 can smoothly take up CO₂, and then install it at the Co₂S₂ scaffold in the form of a carbonate bridge to give complex [Cp*Co(μ-SEt)₂(μ–η¹:η¹-CO₃)CoCp*] (2). The carbonate group plays the role of a robust bridging ligand in a rare μ–η¹:η¹ fashion, which remains unchanged even upon treatment by protonation to bicarbonate or methylation to a carbonate monoester. Furthermore, in the presence of a one-electron oxidant Fc·PF₆, this Co₂S₂ platform can facilitate CO₂ functionalization with primary alcohols or amines by the C–O or C–N bond formation process. The resultant deprotonated carbonate monoester or carbamate is bridged between the two cobalt centers to generate complexes [Cp*Co(μ-SEt)₂(μ–η¹:η¹-O₂COR)CoCp*][PF₆] (5a[PF₆], R = Me; 5b[PF₆], R = Et) or [Cp*Co(μ-SEt)₂(μ–η¹:η¹-O₂CNR¹R²)CoCp*][PF₆] (6a[PF₆], R¹ = H, R² = ⁿBu; 6b[PF₆], R¹ = H, R² = ⁱPr; 6c[PF₆], R¹ = Et, R² = Et), and serve as a chelating ligand through the two oxygen atoms. This system thus illustrates a new dinuclear cobalt complex for CO₂ activation and transformation under oxidised conditions by the synergistic effect.