Capturing unconventional metallofullerene M@C60 through activation of the unreactive [5,6] bond toward Diels–Alder reaction

Abstract

Full control of the regioselectivity in the functionalization of fullerenes is important for production of fullerene derivatives with desirable properties. Cycloaddition reactions of C₆₀ usually take place at the hexagon–hexagon ring junction, i.e. the [6,6] bond of the fullerene cage, whereas the [5,6] bond is generally unreactive. The activation of the [5,6] bond toward Diels–Alder reactions is difficult because of its longer bond length than the [6,6] bond. In this study, we computationally demonstrate that the [5,6] bond of C₆₀ can be efficiently activated by encapsulation of a divalent metal atom such as Ca or Sm. Electron transfer from the metal atom to the fullerene cage and the interaction between the metal cation and the cage play critical roles in enhancing the reactivity of the [5,6] bond. The physical origin of the reactivity enchancement of the [5,6] bond is investigated quantitatively by using the activation strain model and the energy decomposition method. The change in the orbital interaction energy along the intrinsic reaction coordinate has a major effect on the thermodynamics and kinetics of the reactions between Ca@C₆₀ and cyclopentadiene. Both mono- and bis-addition reactions of cyclopentadiene with Ca@C₆₀ prefer to take place at the [5,6] bonds of the fullerene cage thermodynamically, which is distinct from the case of pristine C₆₀. The HOMO–LUMO energy gap of Ca@C₆₀ is remarkably enlarged upon mono- and bis-functionalization with cyclopentadienes. Therefore, the covalent derivatization strategy can be used to capture the unconventional, missing metallofullerene M@C₆₀.