Fullerene chemistry


Fullerene chemistry is a field of organic chemistry devoted to the chemical properties of fullerenes. 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 fullerenes with substituents outside the cage and endohedral fullerenes with trapped molecules inside the cage.
This article covers the chemistry of these so-called "buckyballs," while the chemistry of carbon nanotubes is covered in carbon nanotube chemistry.

Chemical properties of fullerenes

Fullerene or C60 is soccer-ball-shaped or Ih 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. 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. or as fullerenes with exohedral stabilization such as C50Cl10 and reportedly C60H8. Fullerenes with fewer than 60 carbons do not obey isolated pentagon rule.
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 π orbitals to a sp2.27 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 π* orbitals also have a high s character.
The double bonds in fullerene are not all the same. Two groups can be identified: 30 so-called double bonds connect two hexagons and 60 bonds connect a hexagon and a pentagon. Of the two the 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 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 139.1 pm for the bond and 145.5 pm for the bond.
C60 fullerene has 60 π 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 salt and then the In this compound the bond length alternation observed in the parent molecule has vanished.

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. In conformity with IUPAC rules, the terms methanofullerene are used to indicate the ring-closed fullerene derivatives, and fulleroid to ring-open structures.

Nucleophilic additions

Fullerenes react as electrophiles with a host of nucleophiles in nucleophilic additions. The intermediary formed carbanion is captured by another 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. Another nucleophilic reaction is the Bingel reaction.
Fullerene reacts with chlorobenzene and aluminium chloride in a Friedel-Crafts alkylation type reaction. In this hydroarylation the reaction product is the 1,2-addition adduct.

Pericyclic reactions

The bonds of fullerenes react as dienes or dienophiles in cycloadditions for instance Diels-Alder reactions. 4-membered rings can be obtained by cycloadditions for instance with benzyne. An example of a 1,3-dipolar cycloaddition to a 5-membered ring is the Prato reaction.

Hydrogenations

Fullerenes are easily hydrogenated by several methods. The smallest perhydrogenated fullerene known is dodecahedrane C20H20, formally derived from the smallest possible but unknown fullerene, C20, which comprises just 12 pentagonal faces.
Examples of hydrofullerenes are C60H18 and C60H36. However, completely hydrogenated C60H60 is only hypothetical because of large strain. Highly hydrogenated fullerenes are not stable, as prolonged hydrogenation of fullerenes by direct reaction with hydrogen gas at high temperature conditions results in cage fragmentation. At the final reaction stage this causes collapse of cage structure with formation of polycyclic aromatic hydrocarbons.
C60 reacts with Li to the weak base −, which is isolated as Li6-9.

Halogenation

Fullerenes can react with halogens. The preferred pattern for addition C60 is calculated to be 1,9- for small groups and 1,7- for bulky groups. C60F60 is a possible structure. C60 reacts with Cl2 gas at 250 °C to a material with average composition C60Cl24, although only C60 can be detected by mass spectrometry. With liquid Br2 C60 yields C60Br24, in which all 24 bromine atoms are equivalent. The only characterized iodine-containing compounds
are intermediates: and 2.

Hydroxylations

Fullerenes can be hydroxylated to fullerenols or fullerols. Water solubility depends on the total number of hydroxyl groups that can be attached. One method is fullerene reaction in diluted sulfuric acid and potassium nitrate to C6015. Another method is reaction in diluted sodium hydroxide catalysed by TBAH adding 24 to 26 hydroxyl groups. Hydroxylation has also been reported using solvent-free NaOH / hydrogen peroxide. C608 was prepared using a multistep procedure starting from a mixed peroxide fullerene. The maximum number of hydroxyl groups that can be attached stands at 36–40.

Electrophilic additions

Fullerenes react in electrophilic additions as well. The reaction with bromine can add up to 24 bromine atoms to the sphere. The record holder for fluorine addition is C60F48. According to in silico predictions the as yet elusive C60F60 may have some of the fluorine atoms in endo positions and may resemble a tube more than it does a sphere.

Eliminations

Protocols have been investigated for removing substituents via eliminations after they have served their purpose. Examples are the retro-Bingel reaction and the retro-Prato reaction.

Carbene additions

Fullerenes react with carbenes to methanofullerenes. The reaction of fullerene with dichlorocarbene was first reported in 1993. A single addition takes place along a bond.

Radical additions

Fullerenes can be considered radical scavengers. With a simple hydrocarbon radical such as the t-butyl radical obtained by thermolysis or photolysis from a suitable precursor the tBuC60 radical is formed that can be studied. The unpaired electron does not delocalize over the entire sphere but takes up positions in the vicinity of the tBu substituent.

Fullerenes as ligands

Fullerene is a ligand in organometallic chemistry. The organometallic chemistry of C60 is dictated by its spherical geometry and localized polyalkene π-electronic structure. All reported derivatives are η2 complexe in which the metal coordinates at a six–six ring fusion with formal double bond. No analogous η4-diene or η6-triene complexes are prepared.
C60 and C70 form complexes with a variety of molecules. In the solid state lattice structures are stabilized by the intermolecular interactions. Charge transfer complexes are formed with weak electron donors. The double bond is electron-deficient and usually forms metallic bonds with η = 2 hapticity. Bonding modes such as η = 5 or η = 6 can be induced by modification of the coordination sphere.
  • 2, in which the C60 molecules are arranged in close-packed layers
  • 3 has C60 molecules trapped in a hydrogen-bonded of 1,4-dihydroquinone molecules
  • The solvated C60 compounds: 4 and , and the intercalate 2, are structurally characterized.
  • 6
  • 2
  • C60 fullerene reacts with tungsten hexacarbonyl W6 to the W5 complex in a hexane solution in direct sunlight.

    Variants

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. 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. In endohedral hydrogen fullerenes the opening, hydrogen insertion and closing back up has already been demonstrated.