What Can Be Encapsulated Inside Fullerenes

The exohedral derivatives of fullerenes with organometallic compounds have received extensive attention and appropriate materials have been generated as catalysts, molecular devices, liquid crystals, and so on [10-13]. The discussion in this chapter will not address such materials but will only specifically examine endohedral metallofullerenes because they are viewed as more unique, fascinating, and promising due to the strong interactions between the encapsulated metallic species and the carbon cages [2]. 

Among all the endofullerenes produced to date, such species containing rare earth elements (Sc, Y, and the lanthanides) are particularly outstanding and intriguing, not only because of their 

The interior cavities of fullerenes are so spacious that they can encapsulate more than one atom. In fact, from one to six atoms have reportedly been trapped inside [5, 7, 31]. According to the number of encapsulated metal atoms, the isolated EMFs are roughly categorized into three types. 

Fullerenes with only one metal atom inside - mono-metallofullerenes. All rare earth metals can form mono-metallofullerenes. Although the carbon cages that can incorporate one metal atom range from Ceo to bigger than Cioo, as detected by mass spectrometry, the most extractable and stable species are exclusively M Cs2 normally, more than one cage isomer can be isolated. To date, only those mono-metallofullerenes with cages of C72 [32], C74 [33,34], and Cs2 [35] have been determined structurally. 

Fullerenes with two metal atoms inside - di-metallofuUerenes. M2 C2 type EMFs are also found for most rare earth metals, but the isolated examples are less than the corresponding mono-metallofullerenes. The most stable and extractable species are always M2 Cso [7] recently, M2 C2n (2n = 66, 72, 76, 78, 82, 84) [36-39] were also isolated and structurally characterized. Di-metallofullerenes with two different metals were also produced inside one cage, such as HoTm Cs2 [40], but its structure has not been determined yet. 

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