ThCTi@C(6)-C₈₂: Th═C Double Bond in a Mixed Actinide-Transition Metal Cluster

A thorium-carbon double bond that corresponds to the sum of theoretical covalent double bond radii has long been sought after in the study of actinide-ligand multiple bonding as a synthetic target. However, the stabilization of this chemical bond remains a great challenge to date, in part because of a relatively poor energetic matching between 5f-/6d- orbitals of thorium and the 2s-/2p- frontier orbitals of carbon. Herein, we report the successful synthesis of a thorium-carbon double bond in a carbon-bridged actinide-transition metal cluster, i.e., [Th═C═Ti], encapsulated inside a fullerene cage of C₈₂. ThCTi@C(6)-C₈₂ was successfully synthesized by a modified arc discharging method and characterized by mass spectrometry, single-crystal X-ray crystallography, various spectroscopy, and theoretical calculations. X-ray crystallographic analysis reveals a bent μ₂-bridged carbide cluster with a Th-C distance of 2.123(18) Å, which is the shortest reported to date in an isolable compound and is comparable to the sum of the covalent Th═C double bond radii (2.10 Å). In addition, Th═C═Ti takes an unexpected nonlinear configuration with a bond angle of 133.0(10)°. The combined experimental and theoretical investigation further revealed the bonding nature of Th═C, which is polarized toward the bridged carbon but has a notably higher covalency than the Th-C bonds reported previously for organometallic compounds. Moreover, pronounced cage-to-metal donation appears to be stabilizing the encapsulated Th═C═Ti cluster. This work offers a deeper understanding of the bonding behavior of thorium and features the unique ability of fullerene cages to stabilize bonding motifs containing different types of metal-ligand multiple bonds.