Sifting Through Metallofullerene Mixtures

Metallofullerenes are buckyball-like molecules that enclose metals or metallic compounds. Some chemists think the carbon cages show promise for a wide range of applications, such as imaging contrast agents and components of organic solar cells. However, current synthesis techniques result in mixtures of metallofullerenes that are hard to separate, thus limiting their use.

Now, chemists at Indiana University-Purdue University, Fort Wayne, have introduced a method that separates metallofullerenes from mixtures based on their reactivity, preparing them for finer separation with high-performance liquid chromatography (Inorg. Chem. 2013, DOI: 10.1021/ic4013476). Steven Stevenson and coworker Khristina A. Rottinger turned to a weak Lewis acid, CuCl₂, a compound that binds to a metallofullerene’s carbon cage by accepting a pair of electrons. Once a sufficient number of the copper compounds grab onto a metallofullerene—also called an endofullerene—it becomes insoluble, Stevenson says. “The most reactive endofullerenes will precipitate first.” The Lewis acid can even differentiate between structural isomers because they have different reactivities, he adds.

The researchers tested CuCl₂ on a soot containing more than 50 different types of fullerenes—some had trapped metals, others were metal-free. Based on analysis by HPLC, the compounds that precipitated out of the CuCl₂-treated solution were the most reactive ones in the soot.

Stevenson thinks that the remaining solution could be treated with a series of slightly stronger Lewis acids, such as AlCl₃ and FeCl₃, to again precipitate out molecules with the highest reactivity. “You get three fractions that can all then be sent through chromatography columns for further separation,” Stevenson says. The researchers are now testing the multistep separation process.