Integration of Diamagnetic and Paramagnetic Ni Ions and Atmospheric CO Fixation in NiLn Complexes: Exploring Slow Magnetic Relaxation and Sustainable Catalysis for Epoxide Cycloaddition.

We report the synthesis and characterization of a new Schiff base ligand (HL), derived from 2-picolylamine and 2-hydroxy-3-methoxy-5-methylbenzaldehyde. Its reaction with Ni(NO)·6HO and Ln(NO)·HO (Ln = Gd, Tb, Dy) in the presence of triethylamine affords a carbonato-bridged family of heterobimetallic NiLn complexes: [NiLn(L)(L')(μ-CO)(NO)]·MeOH·HO (). During the complexation reaction, ligand HL undergoes an oxidation, followed by C-C coupling to generate a secondary ligand (HL'). A similar transformation of the ligand was also observed in the isolated square planar Ni complex [Ni(HL')]·1.5MeOH·0.5HO. X-ray crystallography confirms that are isomorphous, featuring a rare combination of both paramagnetic and diamagnetic Ni ions. The magnetic measurements reveal an intramolecular Ni-Ln ferromagnetic interaction and slow relaxation of magnetization in all three complexes, further supported by DFT and studies. Beyond magnetism, these complexes act as efficient catalysts for the fixation of CO into cyclic carbonates. Epoxide conversion proceeds with low catalyst loading, affording high yields (82-98%) at 70-100 °C within 4-6 h. Gram-scale reactions validate practical utility, while green metrics highlight sustainability (E-factor 3.73, excellent Eco-scale). Substrate scope includes aliphatic, phenoxyalkyl, and disubstituted oxiranes. Mechanistic insights underscore the cooperative roles of Ni and Ln centers, providing valuable guidelines for designing multifunctional catalysts for sustainable CO utilization.