Nd─Nd Bond in I_h and D_5h Cage Isomers of Nd₂ @C₈₀ Stabilized by Electrophilic CF₃ Addition

Synthesis of molecular compounds with metal-metal bonds between 4f elements is recognized as one of the fascinating milestones in lanthanide metallochemistry. The main focus of such studies is on heavy lanthanides due to the interest in their magnetism, while bonding between light lanthanides remains unexplored. In this work, the Nd─Nd bonding in Nd-dimetallofullerenes as a case study of metal-metal bonding between early lanthanides is demonstrated. Combined experimental and computational study proves that pristine Nd₂ @C₈₀ has an open shell structure with a single electron occupying the Nd─Nd bonding orbital. Nd₂ @C₈₀ is stabilized by a one-electron reduction and further by the electrophilic CF₃ addition to [Nd₂ @C₈₀ ]^- . Single-crystal X-ray diffraction reveals the formation of two Nd₂ @C₈₀ (CF₃ ) isomers with D_5h -C₈₀ and I_h -C₈₀ carbon cages, both featuring a single-electron Nd─Nd bond with the length of 3.78-3.79 Å. The mutual influence of the exohedral CF₃ group and endohedral metal dimer in determining the molecular structure of the adducts is analyzed. Unlike Tb or Dy analogs, which are strong single-molecule magnets with high blocking temperature of magnetization, the slow relaxation of magnetization in Nd₂ @I_h -C₈₀ (CF₃ ) is detectable via out-of-phase magnetic susceptibility only below 3 K and in the presence of magnetic field.