Encapsuled metal atoms

Endohedral metallofullerenes, which encapsulate metal atoms in a carbon cage, are now becoming one of the attractive key materials for nanotechnology. Since the determination of the endohedral structure of Y C82 in 1995 [1], various endohedral structures of metallofullerenes have been revealed by X-ray structure analysis by our group [2-9]. The obtained results improved understanding of the stability of the metallofullerene endohedral structure and stimulated creation of novel endofullerene-related materials. 

By the MEM charge density studies, the different features of encapsulated metal atoms in C82 were revealed for La C82 [2] and Sc C82 [3]. To compare the three-dimensional shape of the metal-atom charge distribution, a section of the equi-density surface of a La C82 molecule is presented in Fig. 9 together with the result for Sc C82 (Isomer I). The equi-density level is 1.8 e/A3. The number of electrons belonging to the Sc atom of Sc C82 was 18.8(2) e, which is close to the 19.0 e of a divalent state of the scandium atom, Sc2+. This indicates that the Sc C82 (I) is an endohedral metallofullerene whose formal electronic structure is Sc2+C82. This result has brought the long discussion as to whether the Sc atom is in a divalent or trivalent state inside the carbon cage [26-29] to an end experimentally. 

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. 

The encapsulated Co atom is the common vertex of a total of live nearly regular octahedral CoCo4Te units, which are connected via common faces to form the complete CoioCo(//(,-Te)2(//4-Te)5 framework. The mean oxidation state of Co is calculated to be +1.09. This value is in agreement with a mixed-valent state in whieh the encapsulated metal atom is formally divalent and the remaining ones are monovalent. 

For many clusters, the number of valence electrons may be increased as a result of formation of encapsulated compounds containing H, C, N, P, Sb, and S in the interstices between metal atoms (Table 3.12). Recently electron poor clusters containing encapsulated metal atoms such as K and Be and atoms of other elements of the second... 

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. 

Fullerenes are probably the most prominent baU-shaped molecules in organic chemistry. Repeatedly, this class of compounds has astonished the chemical community. One of these surprises was that fullerenes can encapsulate metal atoms as well as noble gases and atomic hydrogen (in formulas symbolized with , e.g. Sc Cg2). Endo-... 

As expected, the reactivity of the encapsuled metal atoms is low. It is well known that yttrium ions react with N2O bearing YO". This reaction also occurs... 

In the last few years several endohedral fullerenes, Mm C , i.e., closed carbon cages C ( = 28, 60, 80, 82) with encapsulated metal atoms M or metal dimers M2, have been investigated by pseudopotential and density functional methods. Scalar-relativistic density functional calculations by Rosch et al. on Ce C2g yield a tetravalent cerium ion in the center of the cage. The C2g cage appears to be sufficiently small that the 4f orbitals contribute significantly... 

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