A mononuclear tantalum catalyst with a peroxocarbonate ligand for olefin epoxidation in compressed CO2

Abstract

A new class of tantalum-based peroxocarbonate ionic liquid ([P_4,4,4,4]₃[Ta(η²-O₂)₃(CO₄)]) has been generated through the reaction of pressurized CO₂ with [P_4,4,4,4]₃[Ta(O)₃(η²-O₂)] in the presence of H₂O₂ during the reaction process. The newly formed species has been verified by NMR, FT-IR, HRMS and density functional theory (DFT) calculations. The CO₂-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 CO₂ can actually act as a trigger agent for epoxidation reaction. The further mechanism studies by DFT calculation reveal that peroxo η²-O₂ (site a) affords higher reactivity towards the CC bond than that of peroxocarbonate–CO₄ (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.