Bifunctional picolinate ionic liquids as metal-/halide-free sustainable catalysts for CO2 cycloaddition to epoxides

Abstract

We report the design and synthesis of task-specific room-temperature ionic liquids (RTILs) from picolinic acid, a naturally occurring, non-toxic bulk chemical, via a conventional cation-exchange acid–base reaction. These picolinate-based ionic liquids serve as metal- and halide-free bifunctional organocatalysts for the cycloaddition of CO₂ to epoxides, enabling the efficient synthesis of industrially relevant cyclic carbonates under solvent-free conditions. The presence of a carboxylate group adjacent to the pyridine nitrogen significantly enhances the nucleophilicity of pyridine N, facilitating epoxide activation and CO₂ insertion under mild conditions. Low catalyst loading (∼0.6%) with excellent yields (>99%) and selectivity (>99%) further enhances the utility of the system. Control experiments and DFT calculations revealed that the enhanced activity resulted from pyridine-mediated substrate activation, while the carboxylate group modulated the nucleophilicity of the catalyst. Catalyst recyclability along with a broad substrate compatibility was explored, including the challenging spirocyclic systems. Green chemistry metrics from Chem21 toolkit and EcoScale analysis emphasize the sustainability of this approach, highlighting the facile synthesis, operational simplicity, and minimal waste generation. This work advances the development of halide-free, reusable IL-based organocatalysts for CO₂ valorization.