Th@C2(8)-C84 and Th@Cs(15)-C84: impact of actinide metal ions on the electronic structures of actinide endohedral metallofullerenes

Abstract

Two novel thorium-based endohedral metallofullerenes (EMFs), Th@C₂(8)-C₈₄ and Th@C_s(15)-C₈₄, have been successfully synthesized and fully characterized by MALDI-TOF mass spectroscopy, single-crystal X-ray diffraction, UV-vis-NIR spectroscopy, cyclic voltammetry, and DFT computations. The molecular structures of Th@C₂(8)-C₈₄ and Th@C_s(15)-C₈₄ were unambiguously determined by single-crystal X-ray crystallography, which differ from the previous theoretical prediction that the most stable structure among the Th@C₈₄ isomers is Th@C_s(10)-C₈₄. These results, together with the previous reports of the isomers of U@C₈₄, show that the C₂(8)-C₈₄ cage can only be stabilized by the encapsulation of actinide metals (U and Th). Detailed structural analysis shows that, in these structures, the encapsulated actinide atom has a unique interaction with the sumanene-type hexagon, which likely contributes to the stabilization of the endohedral molecular structures. Moreover, the two thorium-based EMFs show UV-vis-NIR spectra and redox behaviors that are notably different from those of the previously reported U@C₈₄ isomers with the same carbon cages, suggesting that, different from lanthanide-based EMFs, endohedral actinide metal atoms have an important impact on the electronic structures of actinide metallofullerenes.