Synthesis and Isolation of the Titanium–Scandium Endohedral Fullerenes—Sc ₂TiC@ I _h ‐C ₈₀, Sc ₂TiC@ D _{5 h} ‐C ₈₀ and Sc ₂TiC ₂@ I _h ‐C ₈₀: Metal Size Tuning of the Ti ^IV/Ti ^III Redox Potentials

Abstract

The formation of endohedral metallofullerenes (EMFs) in an electric arc is reported for the mixed‐metal Sc–Ti system utilizing methane as a reactive gas. Comparison of these results with those from the Sc/CH ₄ and Ti/CH ₄ systems as well as syntheses without methane revealed a strong mutual influence of all key components on the product distribution. Whereas a methane atmosphere alone suppresses the formation of empty cage fullerenes, the Ti/CH ₄ system forms mainly empty cage fullerenes. In contrast, the main fullerene products in the Sc/CH ₄ system are Sc ₄C ₂@C ₈₀ (the most abundant EMF from this synthesis), Sc ₃C ₂@C ₈₀, isomers of Sc ₂C ₂@C ₈₂, and the family Sc ₂C _{2  n} (2  n=74, 76, 82, 86, 90, etc.), as well as Sc ₃CH@C ₈₀. The Sc–Ti/CH ₄ system produces the mixed‐metal Sc ₂TiC@C _{2  n} (2  n=68, 78, 80) and Sc ₂TiC ₂@C _{2  n} (2  n=80) clusterfullerene families. The molecular structures of the new, transition‐metal‐containing endohedral fullerenes, Sc ₂TiC@ I _h ‐C ₈₀, Sc ₂TiC@ D _{5 h} ‐C ₈₀, and Sc ₂TiC ₂@ I _h ‐C ₈₀, were characterized by NMR spectroscopy. The structure of Sc ₂TiC@ I _h ‐C ₈₀ was also determined by single‐crystal X‐ray diffraction, which demonstrated the presence of a short Ti=C double bond. Both Sc ₂TiC‐ and Sc ₂TiC ₂‐containing clusterfullerenes have Ti‐localized LUMOs. Encapsulation of the redox‐active Ti ion inside the fullerene cage enables analysis of the cluster–cage strain in the endohedral fullerenes through electrochemical measurements.