Schiff-Base-Modified Chitosan-Metal Complexes as Catalysts in CO₂ Cycloaddition Reactions

Chitosan is a renewable and biodegradable polymer with tunable physicochemical properties, making it an attractive platform for catalytic CO₂ cycloaddition reactions. This study aimed to develop sustainable chitosan-derived Schiff base metal complexes and evaluate their catalytic performance in the cycloaddition of CO₂ to epoxides under mild conditions. Low-molecular-weight chitosan (LMWC) was obtained by chitosan depolymerization and subsequently functionalized with salicylaldehyde to form a Schiff base ligand through a straightforward synthetic route. The modified material was then coordinated with Ni-(II), Pt-(II), and Cu-(II) ions to generate metal-based catalytic systems. The ligand and its metal complexes were characterized by complementary spectroscopic, structural, and microscopic techniques, which confirmed successful functionalization and coordination, as well as improved thermal stability compared to the parent LMWC. Catalytic activity was evaluated in the cycloaddition of CO₂ to styrene oxide under mild conditions (80 °C, 1 bar CO₂, 24 h). The LMWC-derived catalysts exhibited significantly higher conversions (65-82%) than the noncatalyzed reaction (33%), with the Pt-(II) and Ni-(II) complexes showing the best performance. These findings demonstrate the potential of easily prepared chitosan-derived Schiff base metal complexes as sustainable and efficient catalytic platforms for CO₂ valorization.