Exploring the magnetic anisotropy and slow relaxation in heterometallic Co ^II –Ln ^III –Co ^II complexes: from synthesis to <i>ab initio</i> insights

A series of isostructural trinuclear Co^II-Ln^III-Co^II complexes [Ln = Y (1), Gd (2), Tb (3), Dy (4), and Er (5)] have been synthesized using a pyridine-based compartmental ligand, 2,2'-[{(1<i>E</i>,1'<i>E</i>)-pyridine-2,6-diyl-bis(methaneylylidene)bis(azaneylylidene)diphenol}] (LH₂). Single-crystal X-ray diffraction analysis revealed that the complexes adopt a bent Co-Ln-Co configuration, with an angle of approximately 154°, distinctly differing from the linear arrangement typically reported in the literature. Magnetic studies were carried out to examine the interplay between the anticipated positive axial zero-field splitting (<i>D</i>) of the Co^II ions in a pentagonal bipyramidal geometry and the intrinsic magnetic anisotropy of the Ln^III ions. <i>Ab initio</i> calculations were employed to gain insights into the magnetic properties and anisotropy of the complexes. Compounds 3 (Tb) and 4 (Dy) exhibited slow magnetic relaxation in the absence of an external magnetic field, with the relaxation mechanism governed predominantly by thermally activated processes. The presence of quantum tunneling of magnetization (QTM) was observed in both compounds, with the Orbach process becoming increasingly dominant as the temperature increased. This study underscores the potential of heterometallic 3d-4f complexes for the development of single-molecule magnets and emphasizes the significance of elucidating magneto-structural correlations in such systems.