a Sc2C2 carbide cluster is included in a D2d-C84 cage (Fig. 2a). Meanwhile, dimetallofullerenes of scandium often reduce the larger cage size by forming a 6 Sc2C2 cluster. As the existence of such a carbide cluster fullerene was also proved 7,8 for a diyttrium structure with Y2C2, one asks whether only scandium and yttrium metals can form such carbides. Nevertheless the structures isolated so far show that metal carbide endohedral fullerenes represent a type of endohedral fullerenes with a cluster inside. Cages other than C82 demonstrably also encase the metal carbide inside. Among the multimetallofullerenes, the Sc3@C82 structure was important as the three metal ions were shown by electron spin resonance (ESR) spectroscopy to be completely equivalent in the position and redox The Recent State of state of each of the scandium ions.9 Furthermore the position of the Endohedral Fullerene Research scandium ions inside the cage can be changed at lower temperatures as by Lothar Dunsch and Shangfeng Yang detected by ESR spectroscopy.10 As the equivalence of the three scandium ions inside the cage seemed ince the last general reviews in cages have been isolated. This group to point to a balanced ordering of them, the field of endohedral fullerenes of multimetallofullerenes considerably it was surprising that in a recent study by Shinohara in 20001 and by extended the scope of the field by S a carbide endohedral fullerene was Akasaka and Nagase in 2002,2 the giving rise to completely new types of proposed by the group of Akasaka.11 variety of endohedral structures has endohedral fullerenes. By NMR spectroscopy the authors have been extended tremendously. With shown that a Sc C @C fullerene may the turn of the millennium the world The Non-IPR World 3 2 80 exist with icosahedral cage symmetry of endohedral fullerenes has changed With the discovery of the (Fig. 2b). In this way the existence of in all respects. The research on the dimetallofullerene, Sc2@C66, it was a five atomic cluster in an endohedral conventional endohedral fullerenes was shown for the first time by Shinohara’s fullerene has been proved. The question predominately focused on structures group that a dimetallo structure is arises as to how large the cluster inside with a single species such as metal sufficient to break the isolated pentagon a fullerene cage can be. Is it a matter of ions encaged in the carbon cage like rule (IPR), which is the stability rule for size or a matter of chemical structure La@C . Such structures have been 82 conventional fullerenes. As the C66 cage which determines the existence of a treated as a special type of a closed cannot form any IPR structure, there cluster inside the fullerene cage? intercalation compound characterized are fused pentagons in the non-IPR C66 by a doping process in which the 3 (C2v) cage (Fig. 1). Meanwhile, the same Extending the Cage Size electron transfer from the encaged group showed that even fullerenes being While it is expected that enlargement metal ions to the carbon structure able to form IPR cages are stabilized in a of the caged cluster could also stabilize occurs. The charge separation in the non-IPR form by a dimetallo structure: 3+ 3- larger cage sizes it was demonstrated form of La @C82 results in non- the endohedral La @C fullerene.4a 2 72 in the frame of the synthesis of cluster dissociating salts. The dissociation in The most recent non-IPR fullerene fullerenes by Yang and Dunsch that this case was blocked for sterical reasons. cage is based on a type of encaged instead, the di- and trimetallofullerenes Besides these monometallofullerenes species: the M C cluster. (Sc C )@C 2 2 2 2 68 (multimetallofullerenes) can result which have been isolated as the first as reported by Wang for which et al., in the stabilization of a large series endohedral structures, noble gases a Sc C carbide cluster is encaged in a 2 2 of higher fullerene cages. Thus from He to Kr and nonmetal atoms like non-IPR C -C cage.4b Interestingly 2v 68 they isolated for the first time the N and P also have been incorporated. the isomeric structure of the non-IPR largest fullerene cage to date, a These structures form preferably C60 C cage proposed for (Sc C )@C 68 2 2 68 stable Dy @C dimetallofullerene endohedrals, which are prepared under was different from that existing in the 2 100 at high yield as well as the largest high pressure or by ion bombardment family of nitride cluster fullerenes and is of the preformed fullerene cages. Unlike trimetallofullerene ever isolated, discussed below in detail by Wang, et al. 12 conventional metallofullerenes, all these Dy3@C98. Therefore, the extension endohedral fullerenes are uncharged. The Carbide Structure of the sizes of both the conventional multimetallofullerenes and the encaged How the charge at the fullerene cage The story of stabilizing a C2 clusters opens new avenues in fullerene can be changed by the incorporation moiety was first demonstrated for the research. By extending the fullerene of more than one metal ion as dimetallofullerene, Sc2@C86, which was cage size with large structures of metals multicharged empty fullerenes exist Sc2C2@C84 as demonstrated by a nuclear or clusters inside, another situation in strong doping reactions, e.g., by magnetic resonance (NMR) study.5 arises: the number of IPR-isomers is potassium, was important to know. The fullerene cage of (Sc C )@C was 2 2 84 increasing (for C100, there are 450 Thus many di- and trimetallofullerene first reported by Wang et al., in which isomers) and the variety of structures 34 The Electrochemical Society Interface • Summer 2006 of the metallofullerenes increases (a) (b) dramatically. Thus for Dy2@C100 at least six cage isomers must be taken into consideration (Fig. 3). While the variation of the number of cage isomers is not a preferable situation, the influence of cage size on physical properties and the behavior upon charging the fullerene is of high interest. A New World of Cluster Fullerenes A new world of cluster fullerenes was introduced by a synthesis which occurred by chance. While it was accepted in the fullerene community to avoid nitrogen as a cooling gas, the element was the key for a type of cluster fullerenes. Thus a class of endohedral fullerenes with a cluster inside was introduced as the trimetallic nitride endohedral fullerenes, with Sc3N@C80 being the first most abundant member. This structure was discovered in FIG. 1. (a) X-ray structure of the IPR-violating Sc2@C66 (C2v) 1999 at a yield higher than all other fullerene, showing a top view along the C2 axis and a side view. (b) endohedral fullerenes by introducing a Calculated Sc2@C66 structures. (Adapted from Ref. 3.) small portion of nitrogen gas into the Krätschmer-Huffman generator during vaporization of graphite rods containing metal oxides.13 This method is called (a) (b) the trimetallic nitride template (TNT) process as proposed by the authors.13 In this nitride cluster fullerene as a type of endohedral fullerenes, the carbon cage adopts a caged trimetal nitride cluster structure which is not stable as a single molecule. The carbon cage of such an endohedral fullerene has an isomeric form which has not been isolated as an empty cage so far. The icosahedral C80 cage is stabilized by the electron transfer from the nitride cluster. Therefore, the trimetal nitride cluster structure and the icosahedral carbon cage appear to be stabilized by each other. The stabilizing effect of the nitride cluster is a further subject of detailed studies on the nitride cluster fullerenes to understand why such fullerenes are stabilized in these two ways. Based on this method, several cluster fullerenes were produced like Er Sc N@C (x = 0-3)13,14 and x 3-x 80 FIG. 2. (a) Schematic representation of the (Sc2C2)@C84 (D2d) molecule. (Adapted from AxSc3-xN@C68 (x = 0-2; A = Tm, Er, Ref. 5.); (b) Structures calculated for Sc3C2@C80 with two views. (Adapted from Ref. 11.) Gd, Ho, La).15 In the latter case the trimetallic nitride is trapped in a non- 15 was made by Yang et al. on the basis of IPR C68 cage. The Sc3N@C68 with structures described in the past high-yield synthesis by the reactive gas the Sc3N cluster encapsulated in a D3 resulted in very low yields. Generally, atmosphere procedure described below (isomer 6140) non-IPR C68 cage, as endohedral metallofullerenes yield determined by X-ray crystallography and a facile isolation.20 2% or less from the fullerene soot.1,2 and NMR spectroscopy (Fig. 4a),15,16 Furthermore the following nitride The first description of the Sc3N@C80 makes this fullerene particularly cluster structures have been synthesized fullerene formation by the TNT process interesting. Besides the recent studies on by the TNT method despite the claimed to secure a higher yield of the theoretical calculations of the molecular relatively low yield of the nitride cluster cluster fullerene in the soot extract structure of Sc N@C on the basis of 21 22,23 ranging from 3 to 5%.13 While in the 3 68 fullerenes: Sc3N@C78, Lu3N@C80, 13 17-19 23 original work, air traces were used the C NMR spectrum, a detailed Lu3-xAxN@C80 (x = 0-2; A = Gd, Ho), study on its electronic and vibrational 24 as a source of nitrogen, an improved and Y3N@C80. structures (Fig. 4b) with density route for nitride cluster fullerene must The standard arc discharge functional theory (DFT) computations production of endohedral fullerene (continued on next page) The Electrochemical Society Interface • Summer 2006 35 Dunsch and Yang (continued from previous page) be searched based on other selective nitrogen sources.