2.1 Carbon nanomaterials  (Page 3/9)

The endohedral He@C ₆₀ and Ne@C ₆₀ form when C ₆₀ is exposed to a pressure of around 3 bar of the appropriate noble gases. Under these conditions it was possible to dope 1 in every 650,000 C ₆₀ cages with a helium atom. Endohedral complexes with He, Ne, Ar, Kr and Xe as well as numerous adducts of the He@C ₆₀ compound have also been proven with operating pressures of 3000 bars and incorporation of up to 0.1 % of the noble gases. The isolation of N@C ₆₀ , N@C ₇₀ and P@C ₆₀ is very unusual and unlike the metal derivatives no charge transfer of the pnictide atom in the center to the carbon atoms of the cage takes place.

Chemically functionalized fullerenes

Although fullerenes have a conjugated aromatic system all the carbons are quaternary (i.e., containing no hydrogen), which results in making many of the characteristic substitution reactions of planar aromatics impossible. Thus, only two types of chemical transformations exist: redox reactions and addition reactions. Of these, addition reactions have the largest synthetic value. Another remarkable feature of fullerene addition chemistry is the thermodymics of the process. Since the sp ² carbon atoms in a fullerene are paramidalized there is significant strain energy. For example, the strain energy in C ₆₀ is ca 8 kcal/mol, which is 80% of its heat of formation. So the relief of this strain energy leading to sp ³ hybridized C atoms is the major driving force for addition reactions ( [link] ). As a consequence, most additions to fullerenes are exothermic reactions.

Cyclic voltammetry (CV) studies show that C ₆₀ can be reduced and oxidized reversibly up to 6 electrons with one-electron transfer processes. Fulleride anions can be generated by electrochemical method and then be used to synthesize covalent organofullerene derivatives. Alkali metals can chemically reduce fullerene in solution and solid state to form M _x C ₆₀ (x = 3 - 6). C ₆₀ can also be reduced by less electropositive metals like mercury to form C ₆₀ ^- and C ₆₀ ^2- . In addition, salts can also be synthesized with organic molecules, for example [TDAE ⁺ ][C ₆₀ ^- ] possesses interesting electronic and magnetic behavior.

Geometric and electronic analysis predicted that fullerene behaves live an electro-poor conjugated polyolefin. Indeed C ₆₀ and C ₇₀ undergo a range of nucleophilic reactions with carbon, nitrogen, phosphorous and oxygen nucleophiles. C60 reacts readily with organolithium and Grignard compounds to form alkyl, phenyl or alkanyl fullerenes. Possibly the most widely used additions to fullerene is the Bingel reaction ( [link] ), where a carbon nucleophile, generated by deprotonation of α-halo malonate esters or ketones, is added to form a cyclopropanation product. The α-halo esters and ketones can also be generated in situ with I ₂ or CBr ₄ and a weak base as 1,8-diazabicyclo[5.4.0]unde-7ene (DBU). The Bingel reaction is considered one of the most versatile and efficient methods to functionalize C ₆₀ .