Actinide-ligand multiple bonds are less well-studied than double bonds involving only transition metals and/or main group elements. For example, while there are known compounds featuring Th=E (E = N, As, O, P, Sb) double bonds, stabilizing Th=C bonds with bond lengths close to the sum of theoretical covalent double bond radii has remained challenging.

Jochen Autschbach, University at Buffalo, NY, USA, Ning Chen, Soochow University, Suzhou, China, and colleagues have synthesized a [Th=C=Ti] unit encapsulated in a C₈₂ fullerene cage, which features just such a thorium–carbon double bond. The desired product was prepared via an arc discharge method using graphite rods packed with ThO₂/TiO₂ and graphite powder. The endohedral fullerene ThCTi@C₈₂ was identified among the resulting products using mass spectrometry and isolated using high-performance liquid chromatography (HPLC).

Using X-ray crystallography, the team found that the [Th=C=Ti] unit is non-linear with a bond angle of 133°. It features short Th–C and Ti–C bond lengths. The Ti–C bond length of 1.867 Å is close to that of known Ti=C double bonds, and the Th–C bond length of 2.123 Å is the shortest reported so far in an isolable compound, according to the researchers. Thus, the work suggests that both Ti═C and Th═C double bonds are present. The unique cluster is stabilized by cage-to-metal donation in the fullerene.

• ThCTi@C_s(6)-C₈₂: Th═C Double Bond in a Mixed Actinide-Transition Metal Cluster,
  Zhengkai Cao, Xiaojuan Yu, Yang-Rong Yao, Jochen Autschbach, Ning Chen,
  J. Am. Chem. Soc. 2025.
  https://doi.org/10.1021/jacs.4c15253