Title: Capturing the Labile Fullerene as C50Cl10. Authors. References and Notes. Figure and Figure Caption. Figure 1 A) Exptl. & Theor. NMR of C 50 Cl 10. B) Structure of C 50 Cl 10.
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Title: Capturing the Labile Fullerene as C50Cl10
References and Notes
Figure and Figure Caption
A) Exptl. & Theor. NMR of C50Cl10.
B) Structure of C50Cl10
All carbon cages (fullerenes) synthesized so far, such as C60 and its larger homologs, faithfully satisfy geometric “isolated pentagon rule” (IPR) (1), that governs the stability of fullerenes comprising hexagons and exactly twelve pentagons (1). The smaller non-IPR fullerenes (2-9), which are predicted to have unusual properties because of their adjacent pentagons and high curvature (2-8), are so labile that their properties and reactivity have only be studied in the gas phase (1-3).
Experimental efforts directed at their bulk synthesis have produced some results(4-6), but complete structural characterization is still under way(7). Here we report the synthesis in milligram quantity of a small non-IPR fullerene C50, a long-sought smaller sister of C60 (1, 3, 7, 8), through the introduction of chlorine in the form of carbon tetrachloride (CCl4) during synthesis from graphite.
C50-containing soot (ca. 90 g) was synthesized in a modified graphite arc-discharge process (9) with 10 Torr carbon tetrachloride being added to 300 Torr helium atmosphere. The toluene-extract from the soot was isolated by a multistage high-performance liquid chromatography (10), and ca. 2 mg C50Cl10 with 99.5% purity was obtained. The C50Cl10 thus-obtained is moderately soluble in some organic solvents, e.g. carbon disulfide, toluene and benzene, as lemon-yellow colored solutions.
Fig. 1A shows the experimental 13C NMR spectrum for the C50Cl10 measured in deuterated benzene. Four distinct signals are located at 161.5, 146.6, 143.0 and 88.7 ppm, respectively, with the intensity ratio of approximately 2:1:1:1; the former three signals are characteristic of sp2-hybridised carbons, whereas the latter one is typical of sp3-hybridised carbons connected to chlorine. This indicates four unique types of carbon atoms in the C50Cl10 with the C(sp2):C(sp3) ratio of 4:1. Among numerous possible structures in the C50 isomers family (3, 8), only the D5h fullerene has four unique types of carbon atoms (I-IV, as illustrated in Fig. 1B).
The ten chlorine atoms should be added to the most reactive CIV sites (i.e., pentagon-pentagon fusions), giving rise to a decachlorofullerene molecule (Fig. 1B). Indeed, the theoretically simulated 13C NMR spectrum (the inset to Fig. 1A) of this Saturn-shaped C50Cl10 structure agrees well with the experimental one. Additionally, the D5h fullerene structure has been further co-characterized by a variety of technologies including mass spectrum, infrared absorption, Raman, UV-Vis absorption and fluorescence spectroscopies (10).
Fullerenes smaller than C60 were predicted to have unusual electronic, magnetic and mechanical properties arising mainly from the significant curvature of their molecular surface (1-7). Hindered by the synthetic difficulty, however, experimental investigation on these properties is scarce. Our successful capture of C50 not only brings into reality a long-sought member of the fullerene family, but also reveals that small non-IPR fullerenes can be obtained in macroscopic quantities by saturating the highly active sites of the otherwise extremely unstable cage. We have chromatographic evidence that other small fullerenes such as C56 and C54 are trapped in the CCl4 arc-discharge process (10).
The chlorinated small fullerenes thus obtained have their curved cage surfaces maintained and, in the meantime, are ready for further chemical manipulations. For example, Cl groups of C50Cl10 can be replaced by reaction with methanol under mild conditions. This result implies that some of the curvature-related unusual properties of small fullerenes are retained in their chlorinated forms, and, more significantly, opens a door for routine experimental investigations of the properties and applications of small fullerenes and their derivatives.