Lanthanide Fluoride Monomer and Dimer in Carbon Cages: Steering Single-Molecule Magnet Property via Terminal and Bridging Ligands

Diatomic lanthanide halide LnX (Ln: lanthanide, X: halogen) and its dimer (LnX)2 offer ideal models to unveil the role of metal–halogen ionic bonds on their magnetic properties. However, the synthesis of such highly ionic species in the isolated form with terminal and bridging ligand coordination remains challenging. Here, we report solid-state syntheses of discrete diatomic lanthanide fluoride and its dimer clusters encapsulated within isomeric fullerene cages, namely, DyF@C2v(9)-C82 and Dy2F2@Cs(6)-C82. Crystallographic characterization revealed a terminal fluoride ligand and a short Dy–F ionic bond length within DyF@C2v(9)-C82, which give rise to a strong uniaxial ligand field and thus single-molecule-magnet behavior. Dimerization of the DyF cluster results in an unchanged original Dy position but a shift of the central F atom, leading to a quasi-rhombic (DyF)2 dimer with elongated Dy–F ionic bond lengths in Dy2F2@Cs(6)-C82 and, consequently, significantly reduced magnetic anisotropy of Dy3+. These results reveal the critical role of terminal ligand coordination in achieving an axial local symmetry toward the design of high-performance single-molecule magnets.