Cooperative 3-Pyridonate-Functionalized Zinc(II) Imidazolium Catalysts for CO ₂ Cycloaddition to Challenging Epoxides at Ambient Conditions: A Pathway to Renewable Cyclic Carbonates

Two zwitterionic 3-pyridonate-functionalized tetrahedral zinc(II) imidazolium complexes (<b>3</b> and <b>4</b>) were synthesized and characterized thoroughly by various spectroscopic techniques. They consist of the pendant imidazolium group with potential hydrogen bonding sites, invoking a metal-ligand cooperativity during catalysis and activating and stabilizing the CO₂-bound intermediate, which may increase the catalytic activity. Among them, complex <b>4</b> efficiently catalyzes the CO₂ cycloaddition reaction toward challenging terminal and internal epoxides, such as styrene oxide, cyclohexene oxide, and limonene oxide, with very high selectivity under solvent-free conditions in the presence of TBAB as a cocatalyst at ambient conditions. Kinetic and thermodynamic studies were performed to elucidate the reaction's activation energy (<i>E</i>_a: 12.9 kcal mol^-1) and Gibbs free energy (Δ<i>G</i>^⧧: 23.85 kcal/mol at 313 K). Besides, various spectroscopic tools were used to understand the reaction's catalytic pathway. Further, this catalyst can be reused for at least six cycles without losing significant yields for styrene oxide, adding an advantage over the other known zinc(II)-based catalysts.