Fullerene addition

For C70 and higher fullerenes addition also occurs only at the [6,6] fusions. However, these sites are no longer all equivalent, and a variety of regioisomers can result. For C70, of the four possible products, as illustrated in Fig. 10, addition occurs exclusively at the polar fusion, A (,14). This is the sterically most accessible site, and energy calculations have shown there is the greatest release of steric strain on com-plexation (91). The equatorial [6,6] fusion, D, has lowest bond order, and correspondingly, no complexations to it have been observed. For C84, complexation with Vaska s compound occurs at the fusion with the highest bond order, as shown by Htickel calculations (28). 

This corroborates observations by Diederich and co-workers who had found analogous results for diastereoisomeric pairs of C70 derivatives combining a uniformly configured chiral side chain with fullerene addition patterns of opposite configuration (cf. Section IV.A.2.e).35,54... 

Similarly to other molecules with isoconstitutional chiral substructures, the combination of distinct fullerene addition patterns with enantiomorphic or homomorphic chiral addends can lead to steric arrangements with a certain complexity. However, these can be easily analyzed with the substitution test delineated in Figure 1.1. In the case of the 1,4-addition pattern of Cgo we may consider the following cases (Figure 1.34) If both chiral addend moieties are homomorphic (structure A), the resulting addition pattern is achiral... 

Fig. 8. CD spectra of a 52a with chiral side chains, hut an achiral fullerene addition pattern, and b the diastereoisomeric 48a and 49a having enantiomeric, inherently chiral fullerene functionalization patterns in addition to the (S)-configured side chains...

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... 

Several studies have demonstrated the successful incoriDoration of [60]fullerene into polymeric stmctures by following two general concepts (i) in-chain addition, so called pearl necklace type polymers or (ii) on-chain addition pendant polymers. Pendant copolymers emerge predominantly from the controlled mono- and multiple functionalization of the fullerene core with different amine-, azide-, ethylene propylene terjDolymer, polystyrene, poly(oxyethylene) and poly(oxypropylene) precursors [63,64,65,66,62 and 66]. On the other hand, (-CggPd-) polymers of the pearl necklace type were fonned via the periodic linkage of [60]fullerene and Pd monomer units after their initial reaction with thep-xy y ene diradical [69,70 and 71]. 

In addition to diamond and amorphous films, nanostructural forms of carbon may also be formed from the vapour phase. Here, stabilisation is achieved by the formation of closed shell structures that obviate the need for surface heteroatoms to stabilise danghng bonds, as is the case for bulk crystals of diamond and graphite. The now-classical example of closed-shell stabilisation of carbon nanostructures is the formation of C o molecules and other Fullerenes by electric arc evaporation of graphite [38] (Section 2.4). 

Chapter 1 contains a review of carbon materials, and emphasizes the stmeture and chemical bonding in the various forms of carbon, including the foui" allotropes diamond, graphite, carbynes, and the fullerenes. In addition, amorphous carbon and diamond fihns, carbon nanoparticles, and engineered carbons are discussed. The most recently discovered allotrope of carbon, i.e., the fullerenes, along with carbon nanotubes, are more fully discussed in Chapter 2, where their structure-property relations are reviewed in the context of advanced technologies for carbon based materials. The synthesis, structure, and properties of the fullerenes and... 

In investigating the metal coating of C70, we will also replace Ba by Ca in the data presented. The coaling of the fullerenes with the latter material is basically identical but exhibits additional interesting features that will be discussed below. Figure 3 shows two mass spectra, the upper one of C oCa, the lower of CvoCa, both obtained under similar conditions as the spec-... 

If it is possible to put one layer of metal around a fullerene molecule, it is tempting to look for the completion of additional layers also. In the spectra in Fig. 3, the sharp edges at CgoCa o and C7oCaJ i4 would be likely candidates for signaling the completion of a second layer. As we will see below, there is, in fact, a very reasonable way of constructing such a second layer with precisely the number of metal atoms observed in the spectrum. 

Of course, after having observed two complete layers of metal around a fullerene, we searched for evidence for the formation of additional layers. Flowever, before looking at experimental data, let us try to con-... 

In addition to chemical or physical properties, a fascinating aspect of fullerene related materials is their central empty space, where atoms, molecules or particles can be enclosed. The enclosed particles are then protected by the robust graphitic layers from chemical or mechanical effects. The very long cavities of CNTs have a special potential due to their high aspect ratio and they can be used as templates to fabricate elongated nanostructures. 

Carbon ean exist in at least 6 erystalline forms in addition to the many newly prepared fullerenes deseribed in Seetion 8.2,4 a- and /3-graphite, diamond, Lonsdaleite (hexagonal... 

Addition reactions — The fullerenes Ceo and C70 react as electron-poor olefins with fairly localized double bonds. Addition occurs preferentially at a double bond common to two annelated 6-membered rings (a 6 6 bond) and a second addition, when it occurs is generally in the opposite hemisphere. The first characteriz-able mono adduct was [C6oOs04(NC5H4Bu )2]. formed by reacting Cgo with an excess of OSO4 in 4-butylpyridine. The structure is shown in... 

Other addition reactions are shown in the scheme. Thus, Ceo reacts as an olefin towards [Pt (PPh3)2] to give the t adduct [Pt(>j -C6o)(PPh3)2]. Indeed six centres can simultaneously be coordinated by a single fullerene cluster to give [C6o M(PEt3)2 6], (M = Ni, Pd, Pt), with the 6M arranged octahedrally about the core. Likewise, reaction... 

An approach for improving the response of conjugated polymcr/fullerene bilayer devices, which is based on an additional excitonic middle layer inserted into the D-A interface, was suggested by Yoshino el al. [94. In the middle layer light absorption produces electron-hole pairs, which migrate towards the interlace and... 

Thilgen C., Cardullo F., Haldimann R., Isaacs L., Seiler P., Diederich F., Boudon C., Gisselbrecht J. P., Gross M. Synthesis of Multiple Adducts of C60 With Specific Addition Patterns by Simple and Reversible (Templated) Tether-Directed Remote Functionalization Proc. - Electrochem. Soc. 1996 96-10 1260-1271 Keywords fullerene C60, regiochemistry... 

In 2000, it was proposed that the regioselectivity of the [3 + 2] cycloaddition of fullerenes could be modified under microwave irradiation. Under conventional heating, N-methylazomethine yhde and fullerene-(C7o) gave three different isomeric cycloadducts because of the low symmetry of C70 vs. Ceo. Using microwave irradiation and o-dichlorobenzene as a solvent, only two isomers were obtained, the major cycloadduct 114 being kinetically favored (Scheme 39) [75]. The same authors had previously reported the 1,3-dipolar cyclo addition of pyrazole nitrile oxides, generated in situ, to Geo under either conventional heating or microwave irradiation. The electrochemical characteristics of the cycloadduct obtained with this method made this product a candidate for photophysical apphcations [76]. 

Scheme 3. J efro-addition of 23 and 24 under MALDI-TOF MS conditions generates the ions of the mono-fullerene adducts of cydo-Ci (25" ) and cydo-C.20 (26" ), respectively [61]...

The state of research on the two classes of acetylenic compounds described in this article, the cyclo[ ]carbons and tetraethynylethene derivatives, differs drastically. The synthesis of bulk quantities of a cyclocarbon remains a fascinating challenge in view of the expected instability of these compounds. These compounds would represent a fourth allotropic form of carbon, in addition to diamond, graphite, and the fullerenes. The full spectral characterization of macroscopic quantities of cyclo-C should provide a unique experimental calibration for the power of theoretical predictions dealing with the electronic and structural properties of conjugated n-chromophores of substantial size and number of heavy atoms. We believe that access to bulk cyclocarbon quantities will eventually be accomplished by controlled thermal or photochemical cycloreversion reactions of structurally defined, stable precursor molecules similar to those described in this review. 

An alternative mode for dehydrobenzoannulene decomposition was recently reported by Vollhardt et al. [58]. Non-planar hybrid 81, prepared in low yield via cyclodimerization of known triyne 82 [Eq.(3)], reacted explosively at ca. 250°C to give a nearly pure carbon residue. Solvent extraction of the black powder failed to yield soluble materials such as fullerenes however, analysis of the residue by TEM showed formation of bucky onions and bucky tubes [59], in addition... 

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