Thermally Driven Conformational Tuning of Pyridine Bis-Salicylaldimine for Efficient CO₂ Activation and Cyclic Carbonate Formation under Mild Conditions

Abstract

This study explores the catalytic enhancement of cyclic carbonate synthesis from CO₂ and epoxides by leveraging the conformational dynamics of pyridine bis-salicylaldimine Schiff base catalysts. These multifunctional homogeneous catalysts incorporate phenolic hydrogen bond donors, pyridine nitrogen, and imine moieties, which synergistically drive the efficient cycloaddition of epoxides with CO₂ under mild conditions and atmospheric pressure. The catalytic mechanism is governed by the conformational flexibility of salicylaldimine arms attached to the pyridine ring. Thermally induced rotation of the phenolic –OH disrupts molecular planarity and weakens intramolecular hydrogen bonding, facilitating a reactive geometry that enhances catalytic performance. Fine-tuning hydrogen bond distances between the phenolic –OH and imine nitrogen emerges as a key factor influencing activity. The binary catalytic system, combining rotated pyridine bis-salicylaldimine with tetrabutylammonium iodide (TBAI), achieves exceptional efficiency, enabling cyclic carbonate formation with 82–99% conversion and 99% selectivity using only 0.9 mol% catalyst at 80 °C and 1 bar CO₂. The system’s recyclability highlights its potential for sustainable CO₂ fixation. Mechanistic insights from temperature-dependent NMR, photoluminescence studies, isotopic labelling experiments, insitu IR, HRMS and DFT calculations reveal molecular rotation effects, the role of pyridine nitrogen in CO₂ activation, and optimized pathways for cyclic carbonate synthesis, offering a foundation for improved catalyst design.