Rare earth endohedral metallofullerenes are an interesting class of full- erenes because electron transfer from the encaged metal atom to the carbon cage has been known to occur and this dramatically alters elec- tronic and magnetic properties of the fullerenes.
A week after the first observation of the ‘‘magic number’’ soccerball- shaped C60 in a laser-vaporized cluster beam mass spectrum by Kroto et al. (1985), the same research group also found a magic number feature due to LaC60in a mass spectrum prepared by laser vaporization of a LaCl3 impregnated graphite rod (Heath et al., 1985). They observed a series of Cnþand LaCnþion species with LaC60þas a magic number ion in the mass spectrum (Figure 1) and concluded that a La atom was encaged within the (then hypothetical) soccerball-shaped C60. This was obviously the first proposal of the so-called ‘‘endohedral metallofullerene’’ concept based on experiments. They first tried Fe with no success and found that La is a correct atom for encapsulation within fullerenes. It is interesting to note that even today Fe has not been encapsulated by fullerenes.
Further circumstantial (not direct) evidence that metal atoms may be encaged in C60was also reported by the Rice group, showing that LaC60þ
ions did not react with H2, O2, NO, and NH3(Weiss et al., 1988), suggest- ing that reactive metal atoms are protected from the surrounding gases and are indeed trapped inside the C60cage.
The first direct evidence of the soccerball (truncated icosahedron) C60
was amply demonstrated in 1990 by a historical experiment done by Kraetschmer, Huffman and co-workers. They succeeded in producing macroscopic quantities of soccerball-shaped C60by using resistive heating of graphite rods under a He atmosphere (Kraetschmer et al., 1990a,b). The resistive heating method was then superseded by the so-called contact arc discharge method (Haufler et al., 1991) since the arc discharge method can produce fullerenes order of magnitudes larger than by resistive heating.
Since then, the arc discharge method has become a standard method for fullerene synthesis.
The first production of macroscopic quantities of endohedral metallo- fullerenes were also reported by the Rice group (Chai et al., 1991). They used high-temperature laser vaporization of La2O3/graphite composite rods and the corresponding contact arc technique to produce various sizes of La-metallofullerenes. Contrary to expectation, only the La@C82fuller- ene survived in a solvent and was extractable by toluene even though La@C60and La@C70were also seen in the mass spectra of the sublimed film from soot. In other words, the major La-metallofullerene with air stability is La@C82, and La@C60and La@C70are somehow unstable in air and in solvents.
The symbol @ is conventionally used to indicate that atoms listed to the left of the @ symbol are encaged in the fullerenes. For example, a C60-encaged metal species (M) is then written as M@C60(Chai et al., 1991).
The corresponding IUPAC nomenclature is different from this conven- tional M@C60representation. It is recommended by IUPAC that La@C82
should be called [82] fullerene-incar-lanthanum and be written iLaC82
(Godly and Taylor, 1997). However, throughout this review the conven- tional M@C2n description is used for endohedral metallofullerenes for brevity, unless otherwise noted.
C60
C60La
C60La2
40 50
Carbon atoms (n) per CnLa cluster
60 70
C70
FIGURE 1 Laser-vaporization supersonic cluster-beam time-of-flight mass spectrum of various lanthanum–carbon clusters. LaC60is seen as an enhanced (magic number) peak.
1.1 The first detection and synthesis of endohedral metallofullerenes
Figure 2 shows a FT-ICR mass spectrum of hot toluene extract of fullerene materials produced by laser vaporization of a 10% La2O3/graphite composite rod (Chai et al., 1991). In addition to empty fullerenes, only the La@C82 metallofullerene is seen and La@C60 and La@C70 are completely absent in the mass spectrum of the solvent extracts. The speciality of the La@C82fullerene was soon confirmed by Whetten and co-workers (Alvarez et al., 1991). However, they also observed that at relatively high loading ratios of La2O3in composite rods a di-lanthano- fullerene, La2@C80, was also produced by the resistive-heating method and found to be another solvent-extractable major lanthanofullerene (Alvarez et al., 1991; Yeretzian et al., 1992).
The first important information on the electronic structure of La@C82
was provided by the IBM Almaden research group. The charge state of the encaged La atom was studied by Johnson et al. (1992) using electron spin resonance (ESR). The ESR hyperfine splitting (hfs) analysis of La@C82
revealed that the La atom is in the 3þcharge state and that the formal charge state of La@C82is written as La3þ@C823: three outer electrons of La transferring to the C82cage (Bethune et al., 1993).
Several other research groups extended their work to endohedral yttrium compounds. The Rice–Minnesota University (Weaver et al., 1992) and Nagoya University (Shinohara et al., 1992a) research groups also reported solvent-extractable Y@C82 and Y2@C82 fullerenes and observed the ESR hfs of Y@C82. From the hfs analyses both groups concluded that the charge state of the Y atom is 3þand that a similar
660 720 780 840 900 (amu) C60
C70
C84 C96
(La@C82)
960 1020 1080 1140 1200
FIGURE 2 An FT-ICR mass spectrum of hot toluene extract of fullerene soot produced by high-temperature laser vaporization of a 10% La2O3/graphite composite rod.
intra-fullerene electron transfer was taking place in Y@C82as in La@C82. These results were also confirmed by the NRL group (Ross et al., 1992).
In addition, they also reported the production of mixed di-metallofuller- enes like (LaY)@C80. McElvany (1992) reported the production of a series of yttrium fullerenes, Ym@Cnincluding Y@C82, by direct laser vaporiza- tion of samples containing graphite, yttrium oxide and fullerenes in the gas phase.
Scandium metallofullerenes were also produced in macroscopic quan- tity and solvent-extracted by Shinohara et al. (1992b) and Yannoni et al.
(1992). The Sc fullerenes exist in extracts as a variety of species (mono-, di-, tri-and even tetra-scandium fullerenes), typically as Sc@C82, Sc2@C74, Sc2@C82, Sc2@C84, Sc3@C82, and Sc4@C82. It was found that Sc3@C82was also an ESR-active species whereas di-and tetra-scandium fullerenes like Sc2@C84 and Sc4@C82 were ESR-silent. (See Section 5 for the present correct assignment for some of the di- and tri-scandium metallofuller- enes.) A detailed discussion on the electronic structures of the scandium fullerenes accrued from these ESR experiments is given in Section 6.1.
The formation of lanthanide metallofullerenes R@C82(R¼Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb, and Lu) was also reported by the UCLA (Gillan et al., 1992) and SRI international (Moro et al., 1993) groups. These metallofullerenes were also based on the C82fullerene.
In addition to group 3 (Sc, Y, La) and the lanthanide metallofullerenes, group 2 metal atoms (Ca, Sr, Ba) were also found to form endohedral metallofullerenes, and have been produced and isolated in milligram quantity (Dennis and Shinohara, 1997a,b, 1998; Dennis et al., 1998; Wan et al., 1997, 1998; Xu et al., 1996). These metal atoms have been encaged not only by C82and C84but also by such smaller fullerenes as C72, C74, and C80. Furthermore, group 4 metallofullerenes (Ti, Zr, Hf) were synthesized and isolated (Cao et al., 2001, 2002).
Other important C60-based endohedral fullerenes which have been produced are Ca@C60 (Guo et al., 1992; Kubozono et al., 1995; L Wang et al., 1993a,b; Y Wang et al., 1993f) and U@C60 (Guo et al., 1992). The Ca@C60and U@C60fullerenes are unique metallofullerenes in which Ca and U atoms are encaged by C60, and are quite different from group 3 and lanthanide, R@C82type, metallofullerenes. Anab initioSCF Hartree–Fock calculation indicates that the Ca ion in Ca@C60is displaced by 0.7 A˚ from the center and that the electronic charge of Ca is 2þ (Scuseria, 1992, L Wang et al., 1993a). A similar theoretical prediction has been made on Sc@C60 by Scuseria and co-workers (Guo et al., 1994). Metallofullerenes based on C60 are known to be unstable in air and in normal fullerene solvents such as toluene and carbon disulfide. We will discuss the stabil- ity and properties of C60-based metallofullerenes together with an inabil- ity to extract and purify these in Section 10. The metal atoms which have been reported to form endohedral metallofullerenes are shown in Table 1.
In the following sections, the major advances in production, separa- tion, structures, electronic/magnetic and solid state properties of endo- hedral metallofullerenes will be discussed in an effort to shed light on this fascinating new class of fullerene-related materials.