Fullerene electron affinities

Thus far the importance of carbon cluster chemistry has been in the discovery of new knowl edge Many scientists feel that the earliest industrial applications of the fullerenes will be based on their novel electrical properties Buckminsterfullerene is an insulator but has a high electron affinity and is a superconductor in its reduced form Nanotubes have aroused a great deal of interest for their electrical properties and as potential sources of carbon fibers of great strength... 

Electron-acceptor end group, 20 504 Electron affinities, of fullerenes, 12 234-235 Electronarcosis, aquaculture applications, 3 224... 

As seen, fullerenes behave as unsaturated ligands. Hence it is expected that coordination to metal fragments might saturate (at least partially) their electron-affinity. Hence it is expected that the electron-withdrawing ability of fullerenes is lowered after complexation with respect to that in their free state. 

A new class of conjugated hydrocarbons is that of the fullerenes [11], which represent an allotropic modification of graphite. Their electrochemistry has been studied in great detail during the last decade [126]. The basic entity within this series is the Ceo molecule (23). Because of its high electron affinity, it can be reduced up to its hexaanion (Fig. 4) [14,127]. Solid-state measurements indicate that the radical anion of Ceo reversibly dimerizes. NMR measurements confirm a u-bond formation between two radical anion moieties [128,129]. 

Several effects can influence the electronic structure of Cjq upon metal complex formation. One is the removal of one double bond from the remaining 29 fullerene double bonds. As in any polyene system, this decreased conjugation is expected to raise the energy of the LUMO and therefore decreases the electron affinity of the system. Conversely, the d-orbital backbonding transfers electron density from the metal into n orbitals of the remaining double bonds, which also decreases the electron affinity. 

Similar calculations were made for the only possible isomer of C70 [11-13] that obeys the isolated pentagon rule [4, 6] and for some of the most stable isomers of the higher fullerenes [11, 14-16]. On the basis of their easily accessible LUMOs and high electron affinities, all stable members of the fullerene family were expected to display very rich cathodic electrochemistry. 

Tab. 1 Calculated and experimental electron affinities (EAc and EAi) and ionization energies (lEc and lEi) of selected fullerenes...

In compounds (15), (16), and (17), attachment of cyano groups to the Cgo substituent has been carried out in an attempt to increase the electron affinity of the fullerene sphere. Indeed, the first... 

The ability of the fullerenes to function as electron acceptors has been recognized since the first investigation of their chemistry (Haufler et al. 1990). Even before the isolation of bulk quantities of C60 (Kratschmer et al. 1990), a large electron affinity was demonstrated for this molecule in gas phase experiments (Curl Smalley 1988). 

This trend has continued with the development of the physics, chemistry and materials science of the fullerenes. In this paper some of the experiments that have thrown light on the ability of the fullerenes to accept electrons are summarized and qualitative explanations for their extraordinary electron affinity are discussed. 

C60 shows an extremely facile reduction profile and there is evidence for the addition of up to 12 electrons to the molecule. The prediction that C60 will exhibit an exceptionally high electron affinity and that the molecule will add up to 12 electrons under suitable conditions (Haddon et al. 1986a) seems to be borne out by the experimental results. Rehybridization plays an important role in determining the electronic structure of the fullerenes and it is the combination of topology and rehybridization which together account for the extraordinary ability of C60 to accept electrons. 

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