The pattern of additions to fullerenes

Summary

Conjugation in C₆₀ is not as extensive as was originally anticipated because, for various reasons, the pentagon rings avoid containing double bonds. As a consequence, there is extensive bond localization and the molecule, which is quite reactive, and displays superalkene rather than superaromatic properties. C₇₀ behaves in a similar fashion; other fullerenes may follow suit. Additions predominate and C₆₀ is particularly susceptible to nucleophilic attack. Added groups may also be readily replaced by nucleophiles, although the reaction mechanism is uncertain at present. The functionalized molecule tends to revert to the parent fullerene at moderate temperatures, and characterization of reaction products by mass spectrometry is thus particularly difficult. This fact, coupled with the complexity of the addition products, makes work with fullerenes exacting. A selection of reactions studied to date and the progress made towards identifying various patterns of addition are described.

Introduction

When spectroscopic evidence for the existence of C₆₀ was first obtained, a view prevailed that it would be a very unreactive molecule. This conclusion was based on the assumption that with a possible 12500 resonance structures (Klein et al. 1986), C₆₀ would be superaromatic. However, the earlier molecular orbital calculations (Bochvar & G'alpern 1973) correctly predicted that there would be substantial bond fixation in the molecule; more recently, the bond lengths have been determined by neutron diffraction studies to be 1.391 Å and 1.455 Å (David et al. 1991); other methods give similar values.

Thus C₆₀ is a superalkene rather than a superarofnatic and readily undergoes additions. Bond fixation arises because structures with double bonds in pentagonal rings are unfavourable in chemistry, due probably to the increase in strain that would result from bdnd-shortening; this phenomenon…