Endohedral fullerenes

Endoheral fullerenes

Structures of endohedral fullerenes

One of the most fascinating and unique feature of fullerenes is that there is spherical empty space inside the carbon cage. This hollow space, ranging from 0.4 to 1.0 nm in diameter on going from C₆₀ to C₂₄₀ considering the van der Waals radius of carbon (0.17 nm), is nanometer-scale void and the volume may be varied with the size of fullerene. Such a characteristic of fullerene implies intuitively an idea of stuffing atoms into its empty space so as to alter the
molecular and solid state properties of the fullerenes, resulting in the formation of a brand-new family — endohedral fullerenes.

As the novel form of fullerene-based materials, endohedral fullerenes represent a novel type of nanostructures, which are characterized by a robust fullerene cage with atoms, ions, or clusters trapped in its hollow (see figures below). Because of the electron transfer from the encaged species to the fullerene cage, this new type of molecules has opened many possibilities for research and has been attracting the wide interest not only in physics and chemistry but also in such interdisciplinary areas as materials and biological sciences. By using the contact arc technique, macroscopic quantities of the first endohedral metallofullerene La@C₈₂ was produced in 1990. Because the simple formula of MC_n can not distinguish the fullerene with metal atom (M) inside the carbon cage composed of n carbon atoms from the other fullerenes with M outside the cages, a more explicit symbolism for the endohedral fullerene — M@C_n was constructed, where the symbol @ is used to state that atoms listed to the left of the @ symbol are situated inside the cage which is listed to the right of the @ symbol. From then on this symbol was widely adopted for describing such unique fullerenes based on its virtue of being concise and suggestive.

Conventional endohedral fullerenes are limited to metallofullerenes (left figure), with the structures predominately focused on a single species such as metal ions encaged in the carbon cage. The representative metallofullerenes isolated by our group include Eu@C₇₄, Ce@C₈₂, and Dy₂@C₁₀₀, the largest fullerene cage isolated up to now.

La@C₈₂	La₂@C₈₀	N@C₆₀

During the past few years the success in preparing macroscopic quantities of endohedral fullerenes, thanks to the DC-arc discharge method for fullerene production developed by Krätschmer and co-workers in 1990, has made it possible to isolate a large number of them and characterize their structures and physical properties.
The early study of endohedral fullerenes till 1999 was comprehensively reviewed by Shinohara. In addition to the efforts on preparing conventional endohedral fullerenes at high yields for potential industrial application, exploring the unusual structures of novel endohedral fullerenes has sparkled great interests in this evolving field in recent years. Among them, because of several distinguishable properties of trimetallic nitride endohedral fullerenes, this new family of endohedral fullerenes has been the focus of the fullerene research of our group.

Trimetallic nitride endohedral fullerenes

With the discovery of Sc₃N@C₈₀ in 1999, which is the first member of trimetallic nitride endohedral fullerenes (nitride clusterfullerenes), the world of endohedral fullerenes has changed in all respects. While the ''trimetallic nitride template'' (TNT) process applied for the first description of the Sc₃N@C₈₀ results in the yield of the cluster fullerene in the soot extract ranging from 3 to 5 %, an improved route for clusterfullerene synthesis has to be searched based on other selective nitrogen sources. The breakthrough was achieved by the invention of the ''reactive gas atmosphere'' method in our group. By introducing NH₃ as the reactive gas, for the first time the clusterfullerenes are produced as the dominant products in the soot, while the relative yield of the empty fullerene and conventional metallofullerenes are less than 5%. This makes the isolation of the endohedral fullerenes much more facile as only one separation step even by a simple chromatographic (HPLC) technique is needed. In applying this synthesis method the prerequisite for a successful application is fulfilled.

On the basis of the preferential synthesis of the clusterfullerenes, several large families of clusterfullerenes M₃N@C_2n (M = Ho, Tb, Gd, Dy, Tm; 38≤n≤44) specifically those clusterfullerenes with cages larger than C₈₀ are isolated by our group as well as the mixed clusterfullerenes such as Gd_xSc_3-xN@C₈₀ and Er_xSc_3-xN@C₈₀ and three isomers of Dy₃N@C₈₀ (shown in the next figure).

(klick to enlarge)

Schematic structure model of three isomers of Dy₃N@C₈₀.
(a) Isomer I (I_h);
(b) Isomer II (D_5h).

The most probable symmetry of the third isomer is proposed to be D_5d and shown in two views:
(c) top view along the main C₅ axis assuming that one Dy atom locates on the C₅ axis;
(d) front view.

The Dy and C atoms are drawn in red and grey, respectively. The N atoms are hidden by the central C or Dy atoms.

In specific, the first C₇₀-based clusterfullerene - Sc₃N@C₇₀ - was isolated and proved to have the first non-IPR C₇₀ cage (see next figure).

Non-IPR Sc₃N@C₇₀

*DFT-optimized structure of the non-IPR Sc₃N@C₇₀ (C_2v:7854). Three pairs of the adjacent pentagons are highlighted in black.*

Contact

Prof. Lothar Dunsch

Address:	IFW Dresden
	Helmholtzstraße 20 01069 Dresden
	Germany
Phone:	+49 351 4659 660
Fax:	+49 351 4659 811
Email:	L.Dunsch@ifw-dresden.de

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