Ga/In complexes with Schiff-based ligand as catalysts for CO2 cycloaddition reaction in epoxides: synthesis, characterization and mechanistic insights

Carbon dioxide (CO₂) valorization has attracted considerable attention due to its abundance and environmental impact. Among the available strategies, the cycloaddition of CO₂ to epoxides produces valuable cyclic carbonates used as chemical intermediates, polymer precursors, and battery electrolytes. In this work, nine gallium(III) and indium(III) thiosemicarbazone complexes were synthesized and characterized by spectroscopic, spectrometric, and thermal techniques, while four structures were confirmed by single-crystal X-ray diffraction. Their catalytic performance was evaluated in the cycloaddition of CO₂ to epoxides using a bifunctional catalytic system. Complex 3 showed the highest activity (76%) and 100% selectivity. The catalytic performance depended strongly on temperature, CO₂ pressure, and the coordinated anion. UV–Vis titration suggested the formation of a reaction intermediate, while conductometric studies indicated that anion dissociation is a likely initial step of the catalytic cycle. Ongoing Raman, NMR, ESI-MS, and DFT studies aim to validate the proposed mechanism.