Integration of Diamagnetic and Paramagnetic Ni<sup>II</sup> Ions and Atmospheric CO<sub>2</sub> Fixation in Ni<sub>4</sub>Ln<sub>2</sub> Complexes: Exploring Slow Magnetic Relaxation and Sustainable Catalysis for Epoxide Cycloaddition
Narayan Ch. Jana, Zvonko Jagličić, Kamil Kotrle, Radovan Herchel, Anangamohan Panja
Abstract
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 3 ) 2 ·6H 2 O and Ln(NO 3 ) 3 · x H 2 O (Ln = Gd III, Tb III, Dy III ) in the presence of triethylamine affords a carbonato-bridged family of heterobimetallic Ni 4 Ln 2 complexes: [Ni 4 Ln 2 (L) 2 (L′) 2 (μ-CO 3 ) 2 (NO 3 ) 2 ]· x MeOH· y H 2 O ( 1–3 ). During the complexation reaction, ligand HL undergoes an in situ oxidation, followed by C–C coupling to generate a secondary ligand (H 3 L′). A similar transformation of the ligand was also observed in the isolated square planar Ni II complex [Ni(HL′)]·1.5MeOH·0.5H 2 O. X-ray crystallography confirms that 1–3 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 ab initio studies. Beyond magnetism, these complexes act as efficient catalysts for the fixation of CO 2 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 CO 2 utilization.