Fullerene chemistry

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.

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Chemical properties of fullerenes

See also: Endohedral fullerenes

Fullerene or C₆₀ is soccer-ball-shaped 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]

Because of the molecule's spherical shape the carbon atoms are highly pyramidalized, which has 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.

Fullerene reactions

See also: Bingel reaction and Prato reaction

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).

• 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.
• Fullerene reacts with chlorobenzene and aluminum chloride in a Friedel-Crafts alkylation type reaction. In this hydroarylation the reaction product is the 1,2-addition adduct (Ar-CC-H) ^[6].
• 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 easily hydrogenated by several methods with C₆₀H₁₈ and C₆₀H₃₆ being most studied hydrofullerenes^{[citation needed]}. However, completely hydrogenated C₆₀H₆₀ is only hypothetical because of large strain. Highly hydrogenated fullerenes are not stable, prolonged hydrogenation of fullerenes by direct reaction with hydrogen gas at high temperature conditions results in collapse of cage structure with formation of polycyclic aromatic hydrocarbons.
• 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

Fullerenes as 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 ^[7].

Multistep fullerene synthesis

Although the procedure for the synthesis of the C60 fullerene is well established (generation of a large current between two nearby graphite electrodes in an inert atmosphere) a 2002 study described an organic synthesis of the compound starting from simple organic compounds ^{[8] [9]}.

In the final step a large polycyclic aromatic hydrocarbon consisting of 13 hexagons and three pentagons is submitted to flash vacuum pyrolysis at 1100°C and 0.01 Torr. The three carbon chlorine bonds serve as free radical incubators and the ball is stiched up in a no-doubt complex series of radical reactions. The chemical yield is low: 0.1 to 1%. This method is considered by many specialists as a variation of cumbustion method of fullerenes production rather than as organic synthesis^{[citation needed]}. A small percentage of fullerenes is formed in any process which involves burning of hydrocarbons, e.g. in candle burning. The yield through a combustion method is often above 1%. The method proposed above does not provide any advantage for synthesis of fulerenes compared to usual combustion method, and therefore, the organic synthesis of fullerenes remains a challenge for chemistry.

Open-cage fullerenes

A part of fullerene research is devoted to so-called open-cage fullerenes whereby one or more bonds are removed chemically exposing an orifice ^[10]. In this way it is possible to insert into it small molecules such as hydrogen, helium or lithium. The first such open-cage fullerene was reported in 1995 ^[11]. In endohedral hydrogen fullerenes the opening, hydrogen insertion and closing back up has already been demonstrated.

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 ^{[12] [13]}:

Also known are protocols for Diels-Alder reactions, one assisted by chromium hexacarbonyl and high pressure ^[14]. The I_D/I_G ratio for reaction with Danishefsky’s diene is 2.6.

Fullerene purification

Fullerene purification is the process of obtaining a fullerene compound free of contamination. In fullerene production mixtures of C₆₀, C₇₀ and higher homologues are always formed. Fullerene purification is key to fullerene science and determines fullerene prices and the success of practical applications of fullerenes. The first available purification method for C₆₀ fullerene was by HPLC from which small amounts could be generated at large expense.

A practical laboratory-scale method for purification of soot enriched in C₆₀ and C₇₀ starts with extraction in toluene followed by filtration with a paper filter. The solvent is evaporated and the residue (the toluene-soluble soot fraction) redissolved in toluene and subjected to column chromatography. C60 elutes first with a purple color and C70 is next displaying a reddish-brown color ^[15].

In nanotube processing the established purification method for removing amorphous carbon and metals is by competitive oxidation (often a sulfuric acid / nitric acid mixture). It is assumed that this oxidation creates oxygen containing groups (hydroxyl, carbonyl, carboxyl) on the nanotube surface which electrostatically stabilize them in water and which can later be utilized in chemical functionalization. One report ^[16] reveals that the oxygen containing groups in actuality combine with carbon contaminations absorbed to the nanotube wall that can be removed by a simple base wash. Cleaned nanotubes are reported to have reduced D/G ratio indicative of less functionalization, and the absence of oxygen is also apparent from IR spectroscopy and X-ray photoelectron spectroscopy.

Experimental purification strategies

• A recent kilogram scale fullerene purification strategy was demonstrated by Nagata et al ^[17]. In this method C₆₀ was separated from a mixture of C₆₀, C₇₀ and higher fullerene compounds by first adding the amidine compound DBU to a solution of the mixture in 1,2,3-trimethylbenzene. DBU as it turns out only reacts to C₇₀ fullerenes and higher which reaction products separate out and can be removed by filtration. C₆₀ fullerenes do not have any affinity for DBU and are subsequently isolated. Other diamine compounds like DABCO do not share this selectivity.
• C₆₀ but not C₇₀ forms a 1:2 inclusion compound with cyclodextrin (CD). A separation method for both fullerenes based on this principle is made possible by anchoring cyclodextrin to colloidal gold particles through a sulfur - sulfur bridge ^[18]. The Au/CD compound is very stable and soluble in water and selectively extracts C₆₀ from the insoluble mixture after refluxing for several days. The C₇₀ fullerene component is then removed by simple filtration. C₆₀ is driven out from the Au/CD compound by adding adamantol which has a higher affinity for the cyclodextrin cavity. Au/CD is completely recycled when adamantol in turn is driven out by adding ethanol and ethanol removed by evaporation. 50 mg of Au/CD captures 5 mg of C₆₀ fullerene.

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