Endohedral metal fullerenes

Structural Proposetls for Endohedral Metal-Fullerene Complexes, Chem. Phys. Lett. 187, 1-7 (1991). 

Rubin, Y. Parker, T.C. Kahn S.I. Holliman, C.L. McElvany S.W. Precursors to endohedral metal fullerene complexes synthesis and x-ray structure of a flexible macrocyclic acetylenic cyclophane CgiH h- J. Am. Chem. Soc. 1996, 35(22), 5308-5309. 

Endohedral metal fullerenes can be detected in relatively small amounts in the mass spectra in the laser vaporization cluster beams (vide supra). However, macroscopic quantities of these compounds may be produced rather readily either by vaporization in a laser furnace apparatus or by arc-burning of a composite rod of graphite and the corresponding metal oxide. In Fig. 4.50, a mass spectrum which illustrates the formation of a series of fullerene endohedral yttrium complexes obtained by laser vaporization of a composite graphite/ Y2O3 rod at 1200 ""C is reproduced. Among these species there is also one, Y2 Cs2, which corresponds to the inclusion of a metal cluster in the fullerene ball. 

Manolopoulos, D. E., 8c Fowler, P. W. (1991). Structural proposals for endohedral metal-fullerene complexes. Chemical Physics Letters, 187,1-7. 

Y. Rubin, Ring Opening Reactions of Fullerenes Designed Approaches to Endohedral Metal Complexes , Top. Curr. Chem. 1999,199, 67-91. 

In contrast to carbon, which forms structures derived from both sp2 and sp3 bonds, silicon is unable to form sp2 related structures. Since one out of four sp3 bonds of a given atom is pointing out of the cage, the most stable fullerene-like structure in this case is a network of connected cages. This kind of network is realized in alkali metal doped silicon clathrate (19), which were identified to have a connected fullerene-like structure (20). In these compounds, Si polyhe-dra of 12 five-fold rings and 2 or 4 more six-fold rings share faces, and form a network of hollow cage structures, which can accommodate endohedral metal atoms. Recently, the clathrate compound (Na,Ba), has been synthesized and demonstrated a transition into a superconductor at 4 K (21). The electronic structure of these compounds is drastically different from that of sp3 Si solid (22). 

These examples demonstrate the well-known process of polymerization initiated by anion-radicals. Our next consideration is devoted to an unusual case of initiation. Intercalation of fullerenes by metals results in the formation of fullerene-metal derivatives. Paramagnetic metallofullerenes (anion-radicals) are the fullerenes doped with endohedral metal. According to calculations and structural studies, LaCs2, for example, contains La in the center of one hexagonal ring of the fuller-ene cage (Akasaka et al. 2000, Nishibori et al. 2000, Nomura et al. 1995). Intrafullerene electron transfer in metallofullerenes is possible (Okazaki et al. 2001). 

The electrochemical properties of TNT-EMFs, M3N C2n n > 39) differ from those of the empty cage fullerenes (see Fig. 6) due to the interaction of the metal cluster with the carbon cage and because the structure of these carbon cages are generally different. As a consequence, the reductive processes are electrochemically irreversible but chemically reversible. The oxidative processes occur at lower potentials because the HOMO orbital is mainly localized on the trimetallic nitride clusters and the HOMO-LUMO gaps in solution are smaller [25,58]. The endohedral metallo-fullerenes M C2n show similar behavior but even smaller HOMO-LUMO gaps [59]. 

Yamada M, Akasaka T, Nagase S (2010) Endohedral metal atoms in pristine and functionalized fullerene cages Acc Chem Res 43 92-102... 

Cardona, C.M., Kitaygorodskiy, A., and Echegoyen, L. (2005) Trimetallic nitride endohedral metallo-fullerenes reactivity dictated by the encapsulated metal cluster. Journal of the American Chemical Society, 127, 10448-10453. 

The endohedral (metal trapped inside the carbon cage) nature of the metallofullerenes has been questioned, but gas phase photofragmentation experiments of ions99,100 and solid-state measurements indicate that the metals are endohedral not exohedral.101 Exohedral metallofullerenes have been prepared in the gas phase but they have different properties to the endohedral metallofullerenes prepared by the arc or laser methods.102-104 The symbol has been used to denote a species inside the fullerene cage and this symbol will be now used to denote endohedral metallofullerenes. 

Ring Opening Reactions of Fullerenes Designed Approaches to Endohedral Metal Complexes... 

Since the discovery of Cgo (Kroto et al. 1985) empty fullerenes and fullerenes with encapsulated metal atoms have attracted much attention of theoreticians and experimentalists. We caimot even cite all important articles in this quickly expanding field and we merely concentrate on the few quantum-chemical studies of endohedral lanthanide-fullerene complexes. 

Complexation of metals to curved carbon n surfaces has attracted continuous interest since the discovery of fullerenes. So far, fullerenes have been found to act as an -coordination ligand to various transition metals from its eYO- r-surface [1, 2], Other coordination modes in exo complexes were reported in the modified fullerene it systems [3], such as buckyferrocene [4] Fe(CgoMe5)Cp (Cp = cyclopentadienyl). Fullerenes are also known to form the endohedral metal complexes to provide potential materials in carbon chemistry [5, 6], On the other hand, fullerene fragments, termed as n bowls (we use this term in the present chapter, whereas they have been also called open geodesic polyarenes [7] or buckybowls [8]), also possess curved carbon n surfaces, n Bowls have attracted interest in the science of nonplanar -conjugated carbon systems [9-14], From the viewpoint of coordination chemistry, such molecules are quite unique because they can provide not only a convex surface but also an open concave surface for binding (a schematic illustration is shown in Fig. 35.1), which was first addressed by the computational study of hemifullerene (C30H12) in 1993 [15],... 

Metal-carbide clusters are relevant to the fonnation of both endohedral fullerenes and carbon nanotubes [1351. There also exists a class of apparently stable metal-carbide cluster ions, = Ti, V, Cr, Zr and Hf), called... 

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