Fullerenes dimerization

On the other hand, the mechanochemical solid-state reaction was found to be the most suitable for this purpose. Thus, when the solid-state reaction was conducted for Cgo in the presence of one equivalent or less of KCN under the HSVM conditions for 30 min, a clean reaction took place to give the [2-1-2] fullerene dimer C120 (3) in 30% yield while 70% of Cgo was recovered unchanged (Scheme 2) [20]. It is to be noted that no cyanated fullerene such as 4 was obtained in comparison to the result of a liquid-phase reaction in o-dichloroben-zene (ODCB)-DMF [21]. This is apparently ascribed to the difference in reactivity of the initially formed cyanated Cgo anion with or without solvation. 

In addition to the dumbbell-type structure 3, fullerene dimer structures having the two fullerene cages connected by a single bond and a methylene or an oxa bridge such as 7a and 7b are possible, and have been actually reported... 

It was also found that the HSVM reaction of Cgo with JV-methylglycine can proceed even without the presence of any carbonyl compound to give JV-methylfulleropyrrolidine 36a (R=Me) in 20-30% yield [47]. In this reaction, the fullerene dimer C120 (3) was also formed in a yield comparable to that of 36a... 

Scheme 12.2 Synthesis of the aza[60]fullerene dimer 2 starting from the cluster opened ketolactam 6.

Fullerenes have also been chemically modified. The reactivity of Ceo is closer to that of an olefin rather than a benzene ring. Solid-state reaction of the fullerene with potassium cyanite results in a fullerene dimer (a dumbbellshaped fullerene see Fig. 3.3). Because the fullerene intermediate anion (with a CN substituent) is reactive in the solid state, dimerization of the fullerene occurs. In contrast, this anion is stabilized by solvation, and so dimerization does not proceed. 

The dimers may then be linked through further [2+2] cycloadditions into polyfullerenes. Although all-carbon polyfiillerenes have been obtained (see below), fullerene dimers as the species that are most important to the proposed [24-2] cycloaddition mechanism for fullerene photopolymerization are yet to he isolated and characterized. 

This summarizes all the theoretical and gas-phase preparation work that has been done up to now with respect to aza[ 60] fullerene, except for the developments that led to the realization of actual bulk preparation of [60]- and aza[70]fullerene dimers and derivatives and the investigations of these products. This work will be described in detail in the remainder of this section. 

Synthesis of Aza[70]fullerene Dimers (Cg9N)2 and Aza[70]fullerene Derivatives... 

The groups of Hirsch and Wudl both applied their own synthetic methods for the preparation of aza[60]fullerene dimer to C70. Since both methods start with the preparation of aza[70]fulleroids and bisaza[70]fulleroids as precursors, we will summarize these results first. 

Nuber and Hirsch, in the meantime, prepared bisaza[70]fulleroid 60 in a one-step synthesis from C70 with MEM azide in ODCB at 120°C. They observed that while 60 was formed preferentially, another bisadduct isomer [presumably the (l,6) (l,9)-isomer, according to modern lUPAC numbering] was formed as a byproduct. When this material was subjected to the reaction conditions that afforded aza[60]fullerenes from 8 (see Sect. 2.2.5), an analogous aza[70]fullerene dimer, most likely with the structure 61, and the methoxyethoxy derivative 62 could be isolated from the mixture in 10 and 15% yield, respectively (Fig. 26)... 

Heterodimers 70 and 71 are the first examples of what is theoretically an enormous family of possible (hetero)fullerene heterodimers. It is expected that fullerene heterodimers will exhibit interesting electronic (optical, electron accepting) properties. Nuber and Hirsch recognized the fact that they had obtained a mixture of three aza[70] fullerene dimers upon treatment of a mixture of two isomeric bisaza[70]fulleroids. One of their products is most likely a mixed dimer [8]. This is a hetero -dimer in a sense that differs from 70 and 71 ... 

Komatsu K, Wang G-W, Murata Y, Tanaka T, Fujiwara K. Mechanochemical synthesis and characterization of the fullerene dimer C]2o. J Org Chem 1998 63 9358-66. 

Komatsu K, Murata Y, Fujiwara K, Suzuki M, Murata M. Synthesis of new fullerene dimers and open-cage fullerenes by solid-state and liquid-phase reaction of Cgo with N-containing aromatics and with organosilicon compounds. Electrochem Soc Proc 2002 12 291-7. 

Komatsu K, Fujiwara K, Tanaka T, Murata Y. The Fullerene dimer C120 and related carbon aUotropes. Carbon 2000 38 1529-34. 

Equally effective was the complexation of C70 by HSVM (lOmin, Cjq j-cyclodextrin 1 8), while the other functionalized C50 derivatives afforded water-soluble 1 1 complexes, since functional groups prevent the formation of the bicapped 1 2 complexes. By mechanochemical treatment, fullerene 50 also complexes with sulfocalix[8]arene (lOmin, equimolar amounts), and the obvious advantage of the solid-state complexation is illustrated by the complexation of sulfocalix [8]arene with fullerene dimer (also prepared by mechanosynthesis, see chapter Applications of Ball Milling in Nanocarbon Material Synthesis). Dimer is hardly soluble in most common organic solvents, but the mechanochemical treatment of an equimolar mixture of sulfocalix[8]arene and fullerene dimer for lOmin afforded bicapped complex, which is about twice more soluble in water than in commonly used ODCB. 

Shvartsburg, A.A., Hudgins, R.R., Dugourd, P., Jarrold, M.F., Stractural elucidation of fullerene dimers by high-resolution ion mobility measurements and trajectory calculation simulations. J. Phys. Chem. A 1997,101, 1684. 

Koshino M, Niimi Y, Nakamura E, Kataura H, Okazaki T, Suenaga K, lijima S. Analysis of the reactivity and selectivity of fullerene dimerization reactions at the atomic level. Nat Chem 2010 2 117-24. 

Single-bonded fullerene dimers RCso-CsqR. featuring a direct covalent bond between the two C50 cages, are unusual structures with potentially interesting properties due to the interaction of two adjacent fullerene cages. These dimers are formed as a mixture of racemic and meso isomers because of the lack of symmetry of their 1,4 addition pattern, which are in equilibrium with the monomer radical (RCsqC ) in solution [33]. 

Scheme 34.4 Copper(II)-mediated synthesis of [60]fullerene dimers.

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