Electrochemistry of Mono‐ through Hexakis‐adducts of C60

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Abstract
The first systematic electrochemical study by cyclic voltammetry (CV) and rotating‐disk electrode (RDE) of the changes in redox properties of covalent fullerene derivatives (2–11) as a function of increasing number of addends is reported. Dialkynylmethanofullerenes 2–4 undergo multiple, fullerene‐centered reduction steps at slightly more negative potentials than C60 (1; see Table and Fig. 1). The two C‐spheres in the dumbbell‐shaped dimeric fullerene derivative 4 show independent, identical redox characteristics. This highlights the insulating character of the sp3‐C‐atoms in methanofullerenes which prevent through‐bond communication of substituent effects from the methano bridge to the fullerene sphere. In the series of mono‐ through hexakis‐adducts 5–11, formed by tether‐directed remote functionalization, reductions become increasingly difficult and more irreversible with increasing number of addends (see Table and Fig. 2). Whereas, in 0.1M Bu4NPF6/CH2Cl2, the first reduction of mono‐adduct 5 occurs reversibly at −1.06 V vs. the ferrocene/ferricinium couple (Fc/Fc+), hexakis‐adduct 11 is reduced irreversibly only at − 1.87 V. Hence, with incremental functionalization of the fullerene, the LUMO of the remaining conjugated framework is raised in energy. Reduction potentials are also dependent on the relative spatial disposition of the addends on the surface of the fullerene sphere. Observed UV/VIS spectral changes and changes in the chemical reactivity along the series 5–11 are in accord with the results of electrochemical measurements. Further, with increasing number of addends, the oxidation of derivatives 5–11 becomes more reversible. Whereas oxidations are increasingly facilitated upon going from mono‐adduct 5 (+1.22 V) to tris‐adduct 7 (+0.90 V), they occur at nearly the same potential (+0.95 to +0.99 V) in the higher adducts 8–11. This indicates that the oxidations occur in these compounds at a common sub‐structural element, for which a ‘cubic’ cyclophane is proposed (see Fig. 3). This sub‐structure is fully developed in hexakis‐adduct 11.

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