Th@C2(14)-C86 and Th@C3(18)-C86: two missing C86 isomers stabilized by the encapsulation of thorium†

Two missing cage isomers of C₈₆ have been successfully stabilized by the encapsulation of the actinide ion Th. The two novel isomers of Th@C₈₆ were synthesized using the arc-discharge method and characterized using mass spectrometry, single-crystal X-ray crystallography, UV-vis-NIR spectroscopy and DFT calculations. The molecular structures of the two isomers were unambiguously assigned to Th@C₂(14)-C₈₆ and Th@C₃(18)-C₈₆ by crystallographic analysis. Both of these cage isomers of C₈₆ have been predicted by computational studies but have never been experimentally confirmed until now. DFT computation reveals that both Th@C₂(14)-C₈₆ and Th@C₃(18)-C₈₆ present closed-shell electronic configurations with formally Th(IV), consistent with other previously reported Th based mono-metallofullerenes. Th@C₂(14)-C₈₆ has emerged as the third lowest-energy isomer with significant predicted abundance at temperatures of fullerene formation. Th@C₃(18)-C₈₆, on the other hand, represents an unexpected isomer with low thermodynamic stability, but significant abundance. Its formation was further rationalized as a kinetically trapped intermediate obtained from the lowest-energy and abundant Th@C₂(8)-C₈₄, by a C₂ insertion and a single Stone–Wales transformation. This study highlights the unique impact of actinides on the stabilization of fullerene cage isomers and also provides a more in-depth understanding into the stabilization mechanism of fullerene cage isomers.