The Smaller Fullerene C50, Isolated as C50Cl10

Smaller isolable fullerenes have long been sought since the discovery of C60. [1] However, fullerenes smaller than C60 should be highly reactive due to their condensed pentagons and increased strain. Although smaller fullerenes can be generated in trace amounts by the traditional Kr!tschmer–Huffman method, in which graphite rods are vaporized by arc-discharge under a low-pressure inert-gas atmosphere, they have not been extracted from the raw soot. The high reactivity also precludes their more conventional chemical syntheses by procedures that gave C60 [3] and its key segments, for example, C20H10, [4] C36H12, [5] and sumanene. Production of the smaller fullerene C36 in macroscopic quantities by DC arc discharge has been claimed, and it was suggested that a fullerene-like C36 cage exists as a covalently bonded cluster assembly in the solid state. However, the definite characterization of a C36based solid is in doubt since no signals for sp carbon atoms were found in the C NMR spectrum. Various attempts to reproduce the production of C36 by arc discharge have not been successful. The synthesis, purification, and identification of C36H6 and C36H6O have also been reported, but the detailed structures are still not clear. Due to its high reactivity, C36H6 forms oligomers or other products immediately after removal of the solvent. The viability of the smallest fullerene C20 was established experimentally by its production in the gas phase and its characterization by anion photoelectron spectroscopy. However, the tremendously reactive C20 has only a fleeting existence in the condensed phase. Recently, it has been reported that a crystallized solid form of C20 was prepared in ultrahigh-molecular-weight polyethylene samples during Ar ion beam irradiation; computations have been performed to assist the characterization. Two important hints to the capture of smaller fullerenes are given by the experience with C36 and C20: 1) When generated, smaller fullerenes have to be stabilized, for example, by partial saturation as in C36H6. 2) Less strained, larger cages are better targets for production and isolation. Following these two guidelines, Xie and co-workers have now reported the landmark achievement of C50Cl10 (Figure 1), [12] the first isolated and fully characterized stable smaller fullerene derivative. C50 itself is distinguished from other smaller fullerenes. First, it has a completely filled electron shell and is thus highly aromatic. Second, it is the smallest cluster for which a cage can be constructed without triplets of directly or sequentially fused pentagons; it contains only isolated singlet and doublet pentagon configurations. The expected enhanced abundance of C50 [14] has been confirmed experimentally in the gas phase by mass spectrometry. Xie and his co-workers prepared C50Cl10 by a graphite arc-discharge process modified by introducing a small amount of carbon tetrachloride (CCl4) in the helium atmosphere. Chlorine not only stabilizes the reactive smaller fullerenes but also is known to catalyze fullerene formation. The availability of samples in milligram quantities facilitated a definite structural assignment. Extensive characterization by mass spectrometry, C NMR, infrared absorption, Raman, UV-Vis absorption, and fluorescence spectroscopies, and theoretical computations established the D5h structure. The elegant architecture of C50Cl10 is the second fullerene “Saturn” after C60F20, [17] which also has a halogen “ring”. The C50 core in C50Cl10 consists of two C20 caps and five C2 units around the equator. The local strain of C50, as assessed by the pyramidalization angle (qp), [18] for the carbon atoms of the C20 caps (10.7–12.88) is comparable to that of C60 (11.68). However, a larger pyramidalization angle (15.58) is responsible for the higher strain of the equatorial C atoms and their readiness to form exohedral adducts. Such structural features result in the instability of a bare C50 cage and the stability of C50Cl10. Figure 1. The B3LYP/6-31G* optimized structure of C50Cl10 (D5h). [*] Dr. Z. Chen Department of Chemistry and Center for Computational Chemistry University of Georgia Athens, GA 30602 (USA) Fax: (1)706-542-0406 E-mail: chen@chem.uga.edu