ThC₂@C₈₂versus Th@C₈₄: unexpected formation of triangular thorium carbide cluster inside fullerenes

Synthesis of the first thorium-containing clusterfullerenes, ThC₂@C_s(6)-C₈₂ and ThC₂@C₂(5)-C₈₂, is reported. These two novel actinide fullerene compounds were characterized by mass spectrometry, single-crystal X-ray diffraction crystallography, UV-vis-NIR spectroscopy, and theoretical calculations. Crystallographic studies reveal that the encapsulated ThC₂ clusters in both C_s(6)-C₈₂ and C₂(5)-C₈₂ feature a novel bonding structure with one thorium metal center connected by a C ≡ C unit, forming an isosceles triangular configuration, which has not been hitherto observed for endohedral fullerenes or for solid phase thorium carbides. Electronic structure calculations assign a formal electronic structure of [Th⁴⁺(C₂)²⁻]²⁺@[C₈₂]²⁻, with pronounced donation bonding from (C₂)²⁻ to Th⁴⁺, secondary backbonding from the fullerene to thorium and Th-C double bond character in both compounds. This work presents a new family of endohedral fullerenes, MC₂@C_2n−2, being unexpected isomers of MC_2n, and provides broader understanding of thorium bonding.