Two Metastable Endohedral Metallofullerenes Sc₂C₂@C ₁(39656)-C₈₂ and Sc₂C₂@C ₁(51383)-C₈₄: Direct-C₂-Insertion Products from Their Most Stable Precursors

Endohedral metallofullerenes (EMFs) are sub-nano carbonmaterialswith diverse applications, yet their formation mechanism, particularlyfor metastable isomers, remains ambiguous. The current theoreticalmethods focus mainly on the most stable isomers, leading to limitedpredictability of metastable ones due to their low stabilities andyields. Herein, we report the successful isolation and characterizationof two metastable EMFs, Sc2C2@C (1)(39656)-C-82 and Sc2C2@C (1)(51383)-C-84, which violatethe isolated pentagon rule (IPR). These two non-IPR EMFs exhibit arare case of planar and pennant-like Sc2C2 clusters,which can be considered hybrids of the common butterfly-shaped andlinear configurations. More importantly, the theoretical results revealthat despite being metastable, these two non-IPR EMFs survived asthe products from their most stable precursors, Sc2C2@C (2v )(5)-C-80 and Sc2C2@C ( s )(6)-C-82, via a C-2 insertionduring the post-formation annealing stages. We propose a systematictheoretical method for predicting metastable EMFs during the post-formationstages. The unambiguous molecular-level structural evidence, combinedwith the theoretical calculation results, provides valuable insightsinto the formation mechanisms of EMFs, shedding light on the potentialof post-formation mechanisms as a promising approach for EMF synthesis.