Synthesis and Isolation of the Titanium-Scandium Endohedral Fullerenes-Sc₂TiC@I_h-C₈₀, Sc₂TiC@D_5h-C₈₀ and Sc₂TiC₂@I_h-C₈₀: Metal Size Tuning of the Ti^IV/Ti^III Redox Potentials

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/CH4 and Ti/CH4 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/CH₄ system forms mainly empty cage fullerenes. In contrast, the main fullerene products in the Sc/CH₄ system are Sc₄C₂@C₈₀ (the most abundant EMF from this synthesis), Sc₃C₂@C₈₀, isomers of Sc₂C₂@C₈₂, and the family Sc₂C_2n (2n=74, 76, 82, 86, 90, etc.), as well as Sc₃CH@C₈₀. The Sc-Ti/CH₄ system produces the mixed-metal Sc₂TiC@C_2n (2n=68, 78, 80) and Sc₂TiC₂@C_2n (2n=80) clusterfullerene families. The molecular structures of the new, transition-metal-containing endohedral fullerenes, Sc₂TiC@I_h-C₈₀, Sc₂TiC@D_5h-C₈₀, and Sc₂TiC₂@I_h-C₈₀, were characterized by NMR spectroscopy. The structure of Sc₂TiC@I_h-C₈₀ was also determined by single-crystal X-ray diffraction, which demonstrated the presence of a short Ti=C double bond. Both Sc₂TiC- and Sc₂TiC₂-containing cluster-fullerenes 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.