Encapsulating atoms or molecules inside fullerene cages could give rise to a myriad of novel molecules and materials. The existence of such species is now strongly supported by a growing body of experimental evidence. Fullerene& ndash;metal complexes generally thought to be endohedral are being produced and purified in milligram quantities, and their structure and properties are beginning to be explored

Atoms in carbon cages: the structure and properties of endohedral fullerenes

THE synthesis of fullerenes encapsulating various metal atoms within the carbon cage (endohedral metallofullerenes) has stimulated wide interest1,2 because of their unusual structural and electronic properties. Most of the metallofullerenes prepared so far have been based on C82, and have incorporated lanthanum1,3a¤-5, yttrium6,7, scandium8a¤-10 and most of the lanthanide elements11,12′. Although there has been some debate about the endohedral nature of these compounds2,13,14, observations using scanning tunnelling microscopy15,16, extended X-ray absorption fine structure17,18, transmission electron microscopy19 and electron spin resonance3,6a¤-8,10 have strongly suggested that the metal atoms are indeed inside the fullerene cages; theoretical calculations20,21 also indicate that this is the case. But until now, no structural model has been derived experimentally to confirm the endohedral nature of the metallofullerenes. Here we report the results of a synchrotron X-ray powder diffraction study of Y@C82 that confirms that the yttrium atom is located within the carbon cage. The yttrium atom is displaced from the centre of the C82 molecule and is strongly bound to the carbon cage.

Confirmation by X-ray diffraction of the endohedral nature of the metallofullerene Y@C82

Laser vaporization experiments with graphite in a supersonic cluster beam apparatus indicate that the smallest fullerene to form in substantial abundance is C28. Although ab initio quantum chemical calculations predict that this cluster will favor a tetrahedral cage structure, it is electronically open shell. Further calculations reveal that C28 in this structure should behave as a sort of hollow superatom with an effective valence of 4. This tetravalence should be exhibited toward chemical bonding both on the outside and on the inside of the cage. Thus, stable closed-shell derivatives of C28 with large highest occupied molecular orbital—lowest unoccupied molecular orbital gaps should be attainable either by reacting at the four tetrahedral vertices on the outside of the C28 cage to make, for example, C28H4, or by trapping a tetravalent atom inside the cage to make endothedral fullerenes such as Ti@C28. An example of this second, inside route to C28 stabilization is reported here: the laser and carbon-arc production of U@C28.

https://science.sciencemag.org/content/sci/257/5077/1661.full.pdf

Uranium Stabilization of C28: A Tetravalent Fullerene

Radical reactions of fullerenes: from synthetic organic chemistry to materials science and biology.

FULLERENES with metals encapsulated within the carbon cage have been prepared recently1–6. Laser vaporization of a lanthanum-impregnated graphite rod yielded an air-stable, solvent-extractable species comprising a single La atom inside a C82 cage1, denoted La@ C82; a cluster containing two La atoms (La2@ C82) was later reported3. Similar techniques applied to graphite/yttrium rods have produced Y@ C82 and Y2@ C82 (refs 5, 6). Here we describe the preparation of scandium-containing C82 species, including an encapsulated scandium trimer, Sc3@C82. Mass spectrometry and ESR spectroscopy provide evidence that the Sc3, trimer is indeed trapped within the cage.

A. Jain

Papers

XPS probes of carbon-caged metals