UCTh@ D 3 h (5)-C78: Fullerene-Stabilized Heterobimetallic Cluster with Distinctive Actinide–Carbon Multiple Bonds

Abstract Actinide–carbon multiple bonds are synthetically challenging to access, primarily due to high bond polarization and poor energetic matching between metal and ligand orbitals. This challenge is especially pronounced for thorium, such that thorium–carbon multiple bonds are exceptionally rare. Herein, we report the synthesis of a novel actinide endohedral fullerene, UCTh@D3h(5)-C78, which encapsulates a unique heterobimetallic actinide carbide cluster. Single-crystal X-ray diffraction analysis reveals a carbon-bridged U–C–Th cluster stabilized within a D3h(5)-C78 cage, featuring remarkably short U–C and Th–C distances of 1.962(8) Å and 2.121(8) Å, respectively. Quantum chemical studies further indicate that the U–C bond possesses a rare triple-bond character, while the Th–C bond can be described as a partial triple bond─a bonding mode previously unreported for thorium. This work uncovers a unique bonding motif involving multiple bonds for both uranium and thorium, most notably a rare Th–C interaction with triple-bond character. These findings provide important perspectives toward the design and synthesis of compounds containing thorium-based triple bonds.