Endohedral incorporation

The incorporation of guest molecules can be achieved during their growth or is executed at defect sites and holes via wet chemistry, by surface diffusion and gas-phase transport. Encapsulated fullerenes tend to form chains that are coupled by van der Waals forces. Upon annealing, the encapsulated fullerenes coalesce in the interior of the SWCNTs, resulting in pill-shaped, concentric, endohedral capsules a few nanometers in length [265], The progress of such reactions inside the tubes could be monitored in real time by use of HR-TEM [266],... 

In this chapter on chemically modified fullerenes, I will discuss only a few representative molecules which incorporate fullerenes, and focus attention on the best-characterized compounds. A comprehensive review of chemical reactions with fullerenes has just been published by Taylor and Walton.[Ta93] It is not appropriate here to attempt a synthesis of the emerging field of the chemical properties of the fullerenes. Covered elsewhere in this book are ioni-cally bonded fullerides, such as the superconductor KsCeo (Chapter IV) and endohedral complexes, in which various atoms are captured inside the hollow carbon shell (Chapter VI). 

In certain compounds, the so-caUed endohedral fuUerenes, atoms or small molecules are incorporated into the cage without forming a covalent bond to the fullerene scaffold. The notation M C (pronounced M at C ) is used to indicate the position within the fuUerene, with M representing the atom or molecule inside the cage. In this way, hetero- and endofuUerenes can clearly be distinguished, for... 

Besides the substances mentioned so far, functionalized fuUerenes like the simple Bingel adduct can be intercalated into nanotubes as well (Section 2.5.5.2). The formation of peapods has further been described for metallocenes (e.g., ferrocene), porphyrines (e.g., erbium phthalocyanine complex) and small fragments of nanotubes. The most important prerequisite for the feasibility of inclusion is always a suitable proportion of sizes of both the tube and the structure to be embedded. For example, this effect can be observed for the intercalation of different cobaltocene derivatives into SWNT. The endohedral functionalization only takes place at an internal diameter of 0.92nm or above (Figure 3.100). But there is also an upper limit to successful incorporation. When the diameter of the nanotube is too large, the embedded species can easily diffuse away again from the host. Only few molecules are consequently found inside such a wide tube. 

The significant cavity size present in Cgg ( 3.5 A diameter) and other fuiierenes allows practically any element of the periodic table to be incorporated inside them (Fig. 1). Endohedral complexes of fuiierenes are represented as Mj C2n, where the symbol denotes that the metal M is encapsulated within the carbon cage, in contrast to being attached outside (exohedral) or incorporated within the shell (heterohedral, see pp. 93 this volume. 

Several pure endohedral metallofullerenes have been characterized, including Ln Cg2 (Ln=La, Y, Ce, Nd, Gd, Pr) [12], as well as some incorporating two or even three metal atoms such as La2 C72, La2 Cgo, Sc2 C74, Ce2 Cgo, Sc2 Cg2, Sc2 Cg4, and Sc3 Cg2 [13]. The main obstacles to their study are their low production yields and, for some of them, their high air sensitivity. There are two important questions regarding these species (1) Are the metal atoms within the cages, and (2) what are the charge states of the metals ... 

Recently, remarkable enrichment of the endohedral component up to 400 times has been realized by HPLC of the Cgo/Ar Cgo mixture, giving pg quantities of Ar Cgo samples with greater than 40% purity [33]. The current degree of incorporation achieved during synthesis (0.1%) has been calculated to be far from equilibrium [34]. Ab initio MP2 calculations with an extended basis set give a 2 kcal moL thermodynamic advantage to helium incorporation inside Cgo- The computed equilibrium (RHF/EXT or BLYP/EXT) at 650°C and 3000 atmospheres exceeds 10%. It is clear that suitable catalysts should considerably increase the experimental incorporation yields. 

It was a great thrust for reduced rare earth metal chemistry when it was discovered that sealed metal ampoules (mostly produced from niobium and tantalum tubing by He arc welding) were reaction containers inert enough to the corrosive molten salts to allow for pure products. Still, all constituents of air need to be excluded, which is possible, thanks to modem dry box techniques. The unintended incorporation of ubiquitous hydrogen or carbon led to a whole new world of cluster complexes with endohedral atoms, mostly with extended stmctmes, as in H Lu Cl or in CLu2 Cl2 (see Rare Earth Metal Cluster Complexes). 

For ZRg Xi2R (7-12)-type compounds, the variety of endohedral atoms is larger. Main group atoms from groups 13 to 15 may be incorporated, accounting to 12 to 14 electrons for intracluster bonding with transition metal atoms from groups 7 to 12, there are 16-21 electrons. 

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