Regioselective Benzyl Radical Addition to an Open-Shell Cluster Metallofullerene. Crystallographic Studies of Cocrystallized Sc₃C₂@<i>I</i>_<i>h</i><i>‑</i>C₈₀ and Its Singly Bonded Derivative

Abstract

The endohedral fullerene once erroneously identified as Sc₃@C₈₂ was recently shown to be Sc₃C₂@<i>I</i>_<i>h</i>-C₈₀, the first example of an open-shell cluster metallofullerene. We herein report that benzyl bromide (<b>1</b>) reacts with Sc₃C₂@ <i>I</i>_<i>h</i>-C₈₀ via a regioselective radical addition that affords only one isomer of the adduct Sc₃C₂@<i>I</i>_<i>h</i>-C₈₀(CH₂C₆H₅) (<b>2</b>) in high yield. An X-ray crystallographic study of <b>2</b> demonstrated that the benzyl moiety is singly bonded to the fullerene cage, which eliminates the paramagnetism of the endohedral in agreement with the ESR results. Interestingly, X-ray results further reveal that the 3-fold disordered Sc₃C₂ cluster adopts two different configurations inside the cage. These configurations represent the so-called “planar” form and the computationally predicted, but not crystallographically characterized, “trifoliate” form. It is noteworthy that this is the first crystallographic observation of the “trifoliate” form for the Sc₃C₂ cluster. In contrast, crystallographic investigation of a Sc₃C₂@<i>I</i>_<i>h</i>-C₈₀/Ni­(OEP) cocrystal, in which the endohedral persists in an open-shell structure with paramagnetism, indicates that only the former form occurs in pristine Sc₃C₂@ <i>I</i>_<i>h</i>-C₈₀. These results demonstrate that the cluster configuration in EMFs is highly sensitive to the electronic structure, which is tunable by exohedral modification. In addition, the electrochemical behavior of Sc₃C₂@<i>I</i>_<i>h</i>-C₈₀ has been markedly changed by the radical addition, but the absorption spectra of the pristine and the derivative are both featureless. These results suggest that the unpaired electron of Sc₃C₂@<i>I</i>_<i>h</i>-C₈₀ is buried in the Sc₃C₂ cluster and does not affect the electronic configuration of the cage