Fullerene, higher

In addition to the most abundant fullerene, namely [60]fullerene, a number of higher fullerenes have also been isolated and characterized, including [70] (point group D . ), chiral [76] (point group Dj), the D and Cj isomers... 

The electrochemical features of the next higher fullerene, namely, [70]fullerene, resemble the prediction of a doubly degenerate LUMO and a LUMO + 1 which are separated by a small energy gap. Specifically, six reversible one-electron reduction steps are noticed with, however, a larger splitting between the fourth and fifth reduction waves. It is important to note that the first reduction potential is less negative than that of [60]fullerene [31]. 

Parallel to the shift that the reduction of higher fullerenes shows, oxidation of the latter is also made easier... 

Azamar-Barrios J A, Munoz E P and Penicaud A 1997 Electrochemical generation of the higher fullerene radicals Tmoisture-free conditions and their observation by EPR J. Chem. Soc.,... 

Repeated chromatography on neutral alumina yields minor quantities of solid samples of C76, Cg4, C90 and C94 believed to be higher fullerenes. A stable oxide C70O has been identified. Chromatographic procedures for the separation of these compounds are reported. [Science 2S2 548 7997.]... 

For C70, molecular orbital calculations [60] reveal a large number of closely-spaced orbitals both above and below the HOMO-LUMO gap [60]. The large number of orbitals makes it difficult to assign particular groups of transitions to structure observed in the solution spectra of C70. UV-visible solution spectra for higher fullerenes (C n = 76,78,82,84,90,96) have also been reported [37, 39, 72]. 

Not too long ago, graphite and diamond were the only two known modifications of carbon. That changed dramatically with the discovery of in 1985 and all the higher fullerenes soon thereafter. Nevertheless, this breakthrough did not stand alone in paving the way to the new era of chemical and physical research into carbon rich compounds that we are now enjoying. 

Ceo and higher fullerenes are distinguished from other allotropes of carbon, diamond and graphite, in that they exist as discrete molecules. The spherical or ellipsoidal nature of the monotropes opens up the possibility of intriguing new areas of chemistry. Here we are only interested in the hydrogen (or muonium) adducts, although this study has important implications to the very vigorous and extensive research in fullerene chemistry. 

Following the discovery of a bulk fullerene preparation process in 1990, the covalent chemistry of these carbon allotropes has developed at a phenomenal pace. Frontier orbital (LUMO) and tether-directed functionalization concepts have been successfully applied to the regio- and stereoselective preparation of multiple covalent adducts of C60. These have found increasing applications in the construction of functional supramol-ecules. More recently, the sequence of Bingel reaction - retro-Bingel reaction has provided an elegant access to isomerically pure higher fullerenes and, in particular, to pure carbon enantiomers. 

For origins of fullerene chirality and definitions of configurational description factors fC and fA (f = fullerene, C = clockwise, A = anticlockwise), see C. Thilgen, A. Herrmann, F. Diederich, The Covalent Chemistry of Higher Fullerenes C70 and Beyond , Angew. Chem Int. Ed. Engl. 1997,36,2269-2280. 

Both calculated values are in fair agreement with the experimental value. Therefore, with this approach it is possible to calculate also the molar volumes of higher fullerenes, whose density is not known. Table 13.1 reports the calculated molar volumes for a series of fullerenes. 

Table 13.1 reports the solubihty parameters of a series of fullerenes. Higher fullerene homologues up to C90 are predicted to have a solubility parameter <5t=<5d=20MPa1/2, thus, not far from the value calculated for the most common fullerenes CL and CL. 

Higher fullerenes are also able to coordinate one or more metal fragments, as is the case of the monoadducts [Ir(CO)(7/2-C7o)Cl(PPh3)2], [Ir(CO)((/2-C84)Cl(PPh3)2],33 34 and the diadduct [(ij2-C70) Ir(CO)Cl (PMe2Ph)2 2].35 Also in this case the coordination to the metal fragment occurs through the double bond of the pyracylene unit. 

The photochemical addition of some cyclic oligosilanes to Ceo has also been found interesting. Scheme 8.8 shows some examples of such a transformation. Irradiation (X > 300 nm) of a toluene solution of disilirane 36 with Ceo afforded the fullerene derivative 37 in a 82% yield [37]. The reaction mechanism is still unknown. When toluene is replaced by benzonitrile the bis-silylated product of the solvent was obtained in good yields. In these experiments a photoinduced electron transfer between 36 and Ceo is demonstrated, indicating the role of Ceo as sensitizer [38]. The photoinduced reactions of disilirane 36 with higher fullerenes such as C70, Cv8(C2v)and CuiDi) have also been reported... 

Fullerenes (Ceo) (Fig- 3) have a structure similar to that of truncated icosahedron, made out of five- and six-member rings of sp carbons. Higher fullerenes are also made of five- and six-member carbon rings. 

In another nomenclature recommendation it was suggested that fullerenes be named in the same way as annulenes, for which the number of C-atoms is indicated in square brackets in front of the word [4]. For fullerenes the number of C-atoms is accompanied by the point group symmetry and by the number of the isomer (using capital Roman) in cases were there are more than one. This is especially important for higher fullerenes. Thus, for Buckminsterfullerene the full description is... 

It can be also very important for determining the number of isomers of higher fullerenes of regioisomers of exohedral fullerene adducts and for carrying out mechanistic investigations on the escape of the endohedral guest and reversible addition reactions [125]. The Xe NMR spectrum ofXe C5Q has also been reported [126]. 

The second approach is based on the idea of synthesizing bowl-shaped hydrocarbons in which curved networks of trigonal C-atoms map out the same patterns of five- and six-membered rings as those found on the surfaces of Cjq and/or the higher fullerenes [145-152]. An example for such an open geodesic polyarene is circum-trindene (5), generated by flash-vacuum pyrolysis (FVP) of trichlorodecacyclene 4 (Scheme 1.5) [152], Circumtrindene represents 60% of the framework of Cjq. 

A completely automated system with reinjection/sample collection and solvent recovery allows the separation of up to 10 g of extract a day. The Cjq is obtained in very high purity and the recovery is nearly 100%. A simple benchtop method for the enrichment of preparative amounts of Cjq, C7Q and higher fullerenes (up to Cjqq) from a crude fullerene mixture is based on a single elution through a column of poly(dibromostyrene)-divinylbenzene using chlorobenzene as mobile phase [208]. 

Figure 1.20 A selection of structurally characterized higher fullerenes.

---