Two-Layer Wrapping for Selective Fullerene Functionalization

Fullerenes, such as C₆₀, can be functionalized and used, e.g., in organic solar cells. However, the synthesis of precisely functionalized fullerenes is challenging due to the formation of a large number of different isomers and/or by-products. This leads to a need for time-consuming purification steps and limits the widespread application of fullerenes.

Max von Delius, Ulm University, Germany, Xavi Ribas, University of Girona, Spain, and colleagues have developed a highly selective method for selectively functionalizing C₆₀ to give trans-3 fullerene bis-adducts. The team used a multi-shell, nested, matryoshka-doll-like nanoreactor (pictured schematically) to direct the reagents to the desired positions. The fullerene sits at the center of this reactor and is surrounded by a [10]cycloparaphenylene ([10]CPP) nanohoop, which shields parts of the fullerene surface from the reagents.

The resulting complex is embedded in a nanocage that forms the outer layer of the reactor. The reaction space has “windows” on its sides through which the functionalization reagents can enter the nanoreactor. The two caps of the nanocages are based on Zn(II)-containing 5,10,15,20-(tetra-4-carboxyphenyl)porphyrins, which are connected with Pd(II) or Cu(II)-containing macrocyclic linkers via a self-assembly process. The team assembled the overall matryoshka-like structure by mixing the nanocage and C₆₀ in solution and then adding the [10]CPP nanohoop. The structure restricts the usually complex functionalization chemistry of C₆₀ to two positions with a 120° angle between them.

To functionalize the “wrapped” fullerene, the researchers added an excess of diethyl bromomalonate and NaH in CH₃CN at room temperature and obtained a complex containing the desired trans-3 fullerene bis-adduct. The product was removed from its shells by washing with CHCl₃ and the addition of unfunctionalized C₆₀ to displace the product from its [10]CPP complex. A single bis-adduct regioisomer was obtained in a total yield of 90 % without any complex purification steps.