Hydrogen-Bond DonorAssisted Cycloaddition of CO2 to Epoxides Catalyzed by Schiff Base Derived Ionic Liquids: Catalytic, Mechanistic, and Kinetic Studies.

Four new salicylaldehyde, o-vanillin, 5-nitrosalicylaldehyde and benzaldehyde derived Schiff base ionic liquids (ILs) with bromide counterion (IL-Sal, IL-Van, IL-NSal, and IL-Bz) have been facilely synthesized. The co-existing phenolic-OH and counteranion, Br- in IL-Sal, IL-Van, and IL-NSal are employed as bifunctional organocatalysts for epoxide-CO2 coupling reaction, where -OH activates the epoxide via hydrogen bonding and Br- promotes ring opening. Experimental evidence from controlled experiments, FT-IR and NMR spectroscopy corroborates a catalytic mechanism involving catalyst-epoxide hydrogen bonding. A control experiment using IL-Bz, lacking phenolic-OH, shows inferior activity compared to IL-Sal [cf. substrate: butyl glycidyl ether; conversion: 9% (cat. IL-Bz); 90% (cat. IL-Sal)], confirming the crucial role of -OH as HBD in epoxide activation. Further, NMR spectroscopic investigation reveals H-bonding via downfield shifts with slightly reduced intensity, supported by FT-IR through shifting of the O-H stretching band. Kinetic studies indicate first-order kinetics with rate constants of 1.2 × 10-5 s-1, 2.2 × 10-5 s-1 and 2.7 × 10-5 s- 1 at 40°C, 60°C, and 80°C, respectively. Activation parameters are ΔH# = 7.50 ± 0.40 kJ·mol-1 and ΔS# = -244.5 ± 1.21 J·mol-1·K-1. The catalyst also shows good recyclability with conversions of 94%, 89%, and 85% in three consecutive cycles.