Fullerene chemistry

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Fullerene chemistry is a field of organic chemistry devoted to the chemical properties of fullerenes ^{[1] [2] [3]}. Research in this field is driven by the need to functionalize fullerenes and tune their properties. For example fullerene is notoriously insoluble and adding a suitable group can enhance solubility. By adding a polymerizable group, a fullerene polymer can be obtained. Functionalized fullerenes are divided into two classes: exohedral with substituents outside the cage and endohedral fullerenes with trapped molecules inside the cage.

Fullerene or C₆₀ is isocahedral or I_h with 12 pentagons and 20 hexagons. According to Euler's theorem these 12 pentagons are required for closure of the carbon network consisting of n hexagons and C₆₀ is the first stable fullerene because it is the smallest possible to obey this rule. In this structure none of the pentagons make contact with each other. Both C₆₀ and its relative C₇₀ obey this so-called isolated pentagon rule (IPR). The next homologue C₈₄ has 24 IPR isomers of which several are isolated and another 51,568 non-IPR isomers. Non-IPR fullerenes have thus far only been isolated as endohedral fullerenes such as Tb3N@C₈₄ with two fused pentagons at the apex of an egg-shaped cage ^[4]

Due to its spherical shape the carbon atoms are highly pyramidalized with far reaching consequences for reactivity. It is estimated that strain energy constitutes 80% of the heat of formation. The conjugated carbon atoms respond to deviation from planarity by orbital rehybridization of the sp² orbitals and pi orbitals to a sp^2.27 orbital with a gain in p-character. The p lobes extend further outside the surface than they do into the interior of the sphere and this is one of the reasons a fullerene is electronegative. The other reason is that the empty low-lying pi^* orbitals also have high s character.

The double bonds in fullerene are not all the same. Two groups can be identified: 30 so-called [6,6] double bonds connect two hexagons and 60 [5,6] bonds connect a hexagon and a pentagon. Of the two the [6,6] bonds are shorter with more double-bond character and therefore a hexagon is often represented as a cyclohexatriene and a pentagon as a pentalene or [5]radialene. In other words, although the carbon atoms in fullerene are all conjugated the superstructure is not a super aromatic compound. The X-ray diffraction bond length values are 135.5 pm for the [6,6] bond and 146.7 pm for the [5,6] bond.

C₆₀ fullerene has 60 pi electrons but a closed shell configuration requires 72 electrons. The fullerene is able to acquire the missing electrons by reaction with potassium to form first the K₆C₆₀^6- salt and then the K₁₂C₆₀^12- In this compound the bond length alternation observed in the parent molecule has vanished.

Contents 1 Fullerene reactions 2 Fullerene ligands 3 Nanotube chemistry 4 References

[edit] Fullerene reactions

Fullerenes tend to react as electrophiles. An additional driving force is relief of strain when double bonds become saturated. Key in this type of reaction is the level of functionalization i.e. monoaddition or multiple additions and in case of multiple additions their topological relationships (new substituents huddled together or evenly spaced). Some fullerene chemistry involves the opening of fullerenes by breaking several of the double bonds. The aim is to be able to insert small molecules through the hole, for instance hydrogen in endohedral hydrogen fullerene.

• Fullerenes react as electrophiles with a host of nucleophiles in nucleophilic additions. The intermediary formed carbanion is captured an electrophile. Examples of nucleophiles are Grignard reagents and organolithium reagents. For example the reaction of C₆₀ with methylmagnesium chloride stops quantitatively at the penta-adduct with the methyl groups centered around a cyclopentadienyl anion which is subsequently protonated ^[5]. Another nucleophilic reaction is the Bingel reaction.
• The [6,6] bonds of fullerenes react as dienes or dienophiles in cycloadditions for instance Diels-Alder reactions. 4-membered rings can be obtained by [2+2]cycloadditions for instance with benzyne. An example of a [1,3-dipolar cycloaddition] to a 5-membered ring is the Prato reaction.
• Fullerenes are resistant to hydrogenation, the completely hydrogenated C60H60 is hypothetical because of large strain but hexahydrides are known
• Although more difficult than reduction, oxidation of fullerene is possible for instance with oxygen and osmium tetraoxide
• Fullerenes react in electrophilic additions as well. The reaction with bromine can add up to 24 bromine atoms to the sphere.
• Fullerenes react with carbenes to methanofullerenes

[edit] Fullerene ligands

Fullerene is a ligand in organometallic chemistry. The [6,6] double bond is electron-deficient and forms metallic bonds with η = 2 hapticity. C₆₀ fullerene reacts with tungsten hexacarbonyl W(CO)₆ to the (η²-C₆₀)W(CO)₅ complex in a hexane solution in direct sunlight ^[6].

[edit] Nanotube chemistry

Carbon nanotubes, also part of the fullerene family, consist of graphene sheets rolled up into a cylinder. Unlike the spherical fullerenes made up of hexagons and pentagons, nanotubes only have hexagons present but in terms of reactivity both systems have much in common. Due to electrostatic forces nanotubes have a nasty tendency to cluster together in large bundles and many potential applications require an exfoliation process. One way to do this is by chemical surface modification.

A useful tool for the analysis of derivatised nanotubes is Raman spectroscopy which shows a G-band (G for graphite) for the native nanotubes at 1580 cm^-1 and a D-band (D for defect) at 1280 cm^-1 when the graphite lattice is disrupted with conversion of sp² to sp³ hybridized carbon. The ratio of both peaks I_D/I_G is taken as a measure of functionalization. Other tools are UV spectroscopy where pristine nanaotubes show distinct Van Hove singularities where functionalized tubes do not and simple TGA analysis.

In one type of chemical modification an aniline is oxidized to a diazonium intermediate which after expulsion of nitrogen forms a covalent bond as an aryl radical ^{[7] [8]}:

Also known are protocols for Diels-Alder reactions, one assisted by chromium hexacarbonyl and high pressure ^[9]. The I_D/I_G ratio for reaction with Danishefsky’s diene is 2.6.

[edit] References