Integration of Diamagnetic and Paramagnetic Ni

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·xH2O (Ln = GdIII, TbIII, DyIII) in the presence of triethylamine affords a carbonato-bridged family of heterobimetallic Ni4Ln2 complexes: [Ni4Ln2(L)2(L')2(μ-CO3)2(NO3)2]·xMeOH·yH2O (1-3). During the complexation reaction, ligand HL undergoes an in situ 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 NiII complex [Ni(HL')]·1.5MeOH·0.5H2O. X-ray crystallography confirms that 1-3 are isomorphous, featuring a rare combination of both paramagnetic and diamagnetic NiII ions. The magnetic measurements reveal an intramolecular NiII-LnIII ferromagnetic interaction and slow relaxation of magnetization in all three complexes, further supported by DFT and ab initio 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 NiII and LnIII centers, providing valuable guidelines for designing multifunctional catalysts for sustainable CO2 utilization.