Cage within a cage

A 'Russian doll' endohedral fullerene has been made, comprising a carbon dimer within a scandium tetrahedron, all encased by a C₈₀ cage.

Credit: © 2009 ACS

There is a broad range of compounds that consist of fullerene cages enclosing various metal atoms — known as endohedral metallofullerenes. They range from C₆₀ cages with one or two small atoms inside, to large C₈₀ to C₈₄ cages containing several metal atoms, such as Sc₄O₂@C₈₄. A cage with the formula Sc₄@C₈₂ has recently been predicted¹ to in fact be a C₈₀ cage, with the extra two carbon atoms to be found inside.

Now, Tai-Shan Wang and colleagues from Beijing and Xiamen have confirmed² their predictions by synthesising Sc₄C₂@C₈₀. They further discovered that the structure is 'doubly caged', with a C₂ dimer inside a Sc₄ tetrahedron that is itself within the C₈₀ icosahedron. This 'Russian doll' compound was made by the Krächmer–Huffman method, Soxhlet-extracted in toluene and then purified by HPLC.

Carbon-13 nuclear magnetic resonance spectroscopy confirmed that the fullerene was C₈₀, with only two carbon environments; no C₈₂ cage could provide such a spectrum. The metal carbide endofullerene is very stable, with cyclic voltammetry experiments and density functional theory (DFT) calculations suggesting a bandgap of around 1.6 eV. Three bands in the infrared spectrum confirmed the presence of three types of scandium–carbon bonds, confirming the 'Russian doll' structure predicted by DFT.