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

Through successive one-electron reduction processes, a coordinatively unsaturated thiolate-bridged CoIICoII 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.