Cooperative 3-Pyridonate-Functionalized Zinc(II) Imidazolium Catalysts for CO2 Cycloaddition to Challenging Epoxides
Rhitwika Chowdhury1, Sundharrajan Jegadeesan1, Sharad Kumar Sachan1
1Department of Chemistry, Indian Institute of Technology Kanpur (IITK), Kanpur 208016 Uttar Pradesh, India.
Abstract:
Two zwitterionic 3-pyridonate-functionalized tetrahedral zinc(II) imidazolium complexes (3 and 4) 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 4 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 (Ea: 12.9 kcal mol-1) and Gibbs free energy (ΔG⧧: 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.
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