ThC 2 @C 82 versus Th@C 84 : unexpected formation of triangular thorium carbide cluster inside fullerenes.

Synthesis of the first thorium-containing clusterfullerenes, ThC 2 @C s (6)-C 82 and ThC 2 @C 2 (5)-C 82 , 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 ThC 2 clusters in both C s (6)-C 82 and C 2 (5)-C 82 feature a novel bonding structure with one thorium metal center connected by a C[triple bond, length as m-dash]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 [Th 4+ (C 2 ) 2- ] 2+ @[C 82 ] 2- , with pronounced donation bonding from (C 2 ) 2- to Th 4+ , 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, MC 2 @C 2 n -2 , being unexpected isomers of MC 2 n , and provides broader understanding of thorium bonding.