A mononuclear tantalum catalyst with a peroxocarbonate ligand for olefin epoxidation in compressed CO 2

A new class of tantalum-based peroxocarbonate ionic liquid ([P 4,4,4,4 ] 3 [Ta(η 2 -O 2 ) 3 (CO 4 )]) has been generated through the reaction of pressurized CO 2 with [P 4,4,4,4 ] 3 [Ta(O) 3 (η 2 -O 2 )] in the presence of H 2 O 2 during the reaction process. The newly formed species has been verified by NMR, FT-IR, HRMS and density functional theory (DFT) calculations. The CO 2 -induced monomeric peroxocarbonate anion-based ionic liquid is more advantageous than the monomeric peroxotantalate analogue for the epoxidation of olefins under very mild conditions. Interestingly, the transformation between peroxotantalate and peroxocarbonate species is completely reversible, and CO 2 can actually act as a trigger agent for epoxidation reaction. The further mechanism studies by DFT calculation reveal that peroxo η 2 -O 2 (site a ) affords higher reactivity towards the CC bond than that of peroxocarbonate–CO 4 (site b ). These quantitative illustrations of the relationship between structural properties and kinetic consequences enable rational design for an efficient and environmental IL catalyst for the epoxidation of olefins.