Rational Design of Cobalt Complexes Based on the trans Effect of Hybrid Ligands and Evaluation of their Catalytic Activity in the Cycloaddition of Carbon Dioxide with Epoxide

A series of cobalt complexes are presented as effective catalysts for the synthesis of cyclic carbonates from epoxides and CO2. The catalytic potentials of the cobalt complexes, in combination with tetrabutylammonium bromide, have been demonstrated to solve some challenges in the synthesis of cyclic carbonates, including the room-temperature conversion of terminal epoxides and activation-challenging substrates such as internal epoxides and fatty acid derived epoxides. A key factor in the success of the strategy is the use of cobalt complexes that are prepared on the basis of the trans effect of hybrid ligands. The trans effect between N-heterocyclic carbenes and acetylacetone has been proved by a number of spectroscopic measurements, including UV–vis, ESI-MS, EPR, and in situ FT-IR and by DFT calculations; these support the notion that acetylacetone prefers to dissociate from the cobalt center, which will result in one coordination site for the activation of a substrate molecule at the cobalt atom and thus give rise to high reactivity.