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(2n−2), being unexpected isomers of MC(2n), and provides broader understanding of thorium bonding.