Integration of Diamagnetic and Paramagnetic Ni^II Ions and Atmospheric CO₂ Fixation in Ni₄Ln₂ 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(NO₃)₂·6H₂O and Ln(NO₃)₃·<i>x</i>H₂O (Ln = Gd^III, Tb^III, Dy^III) in the presence of triethylamine affords a carbonato-bridged family of heterobimetallic Ni₄Ln₂ complexes: [Ni₄Ln₂(L)₂(L')₂(μ-CO₃)₂(NO₃)₂]·<i>x</i>MeOH·<i>y</i>H₂O (<b>1-3</b>). During the complexation reaction, ligand HL undergoes an <i>in situ</i> oxidation, followed by C-C coupling to generate a secondary ligand (H₃L'). A similar transformation of the ligand was also observed in the isolated square planar Ni^II complex [Ni(HL')]·1.5MeOH·0.5H₂O. X-ray crystallography confirms that <b>1-3</b> are isomorphous, featuring a rare combination of both paramagnetic and diamagnetic Ni^II ions. The magnetic measurements reveal an intramolecular Ni^II-Ln^III ferromagnetic interaction and slow relaxation of magnetization in all three complexes, further supported by DFT and <i>ab initio</i> studies. Beyond magnetism, these complexes act as efficient catalysts for the fixation of CO₂ 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^II and Ln^III centers, providing valuable guidelines for designing multifunctional catalysts for sustainable CO₂ utilization.