Relaxation of magnetization in Dy2 and Tb2 dimers with single-electron lanthanide-lanthanide bonds: the effect of cage perturbation and its ultimate symmetry.

Covalent bonding between lanthanide ions dramatically enhances exchange interactions between their magnetic moments and leads to fascinating magnetic properties. Particularly attractive for this goal are single-electron lanthanide-lanthanide bonds, which were first realized in metal dimers encapsulated in endohedral metallofullerenes. So far, stabilization of the single-electron bond in fullerenes required modification of the carbon cage by replacing one carbon with nitrogen or by an exohedral attachment of a radical group, which inevitably perturbed the cage symmetry and thereby affected the magnetic anisotropy. Here, we report on the isolation of [M2@I h-C80]- anions (M = Tb, Dy) with single-electron M-M bonds, for which we developed a chromatography-free separation protocol. Thorough investigation of their dynamic magnetic properties and relaxation mechanisms in comparison to those of M2@C80(CF3), M2@C80(CH2Ph), and M2@C79N showed that highly symmetric [M2@C80]- anions exhibit the highest blocking temperatures in the series. Magnetization relaxation parameters depend considerably on the mode of the cage functionalization, and Dy2 is more susceptible to the variation of its environment than Tb2. Our work opens the way to capture elusive anions of endohedral metallofullerenes without perturbing their cage symmetry, facilitating deep investigation of these fascinating molecules.