Two Metastable Endohedral\nMetallofullerenes Sc₂C₂@<i>C</i>₁(39656)‑C₈₂ and Sc₂C₂@<i>C</i>₁(51383)‑C₈₄: Direct‑C₂‑Insertion\nProducts from Their Most Stable Precursors

Abstract

Endohedral metallofullerenes (EMFs) are sub-nano carbon\nmaterials\nwith diverse applications, yet their formation mechanism, particularly\nfor metastable isomers, remains ambiguous. The current theoretical\nmethods focus mainly on the most stable isomers, leading to limited\npredictability of metastable ones due to their low stabilities and\nyields. Herein, we report the successful isolation and characterization\nof two metastable EMFs, Sc₂C₂@<i>C</i>₁(39656)-C₈₂ and Sc₂C₂@<i>C</i>₁(51383)-C₈₄, which violate\nthe isolated pentagon rule (IPR). These two non-IPR EMFs exhibit a\nrare case of planar and pennant-like Sc₂C₂ clusters,\nwhich can be considered hybrids of the common butterfly-shaped and\nlinear configurations. More importantly, the theoretical results reveal\nthat despite being metastable, these two non-IPR EMFs survived as\nthe products from their most stable precursors, Sc₂C₂@<i>C</i>_2<i>v</i>(5)-C₈₀ and Sc₂C₂@<i>C</i>_<i>s</i>(6)-C₈₂, via a C₂ insertion\nduring the post-formation annealing stages. We propose a systematic\ntheoretical method for predicting metastable EMFs during the post-formation\nstages. The unambiguous molecular-level structural evidence, combined\nwith the theoretical calculation results, provides valuable insights\ninto the formation mechanisms of EMFs, shedding light on the potential\nof post-formation mechanisms as a promising approach for EMF synthesis.