Stabilization of Open-Shell Single Bonds within Endohedral Metallofullerene

The open-shell single covalent bond composed of two electrons is unstable under normal conditions, because the closed-shell electronic configuration is generally beneficial to minimize the energy of the system. This classical rule always governs the chemical bonding of s- and p-block homonuclear diatomic molecules, such as the stable σ2 electron-pair bonds in hydrogen. In this work, surprisingly, we found that the diversified open-shell single bonds between two f-block atoms (e.g., thorium) can be stabilized within a tight “carbon-confined-space” using relativistic quantum chemical calculations. We first identified a stable dithorium endohedral metallofullerene (EMF), Th2 6+@I h -C80 6–, with a Th–Th distance of 3.803 Å inside the I h -C80 cage, which displays a unique spin-polarized σ1π1 2-fold single-electron Th3+–Th3+ bond that is collaboratively dominated by 5f6d7s7p orbitals. The Th3+–Th3+ bond can further evolve into a 5f6d dominated spin-polarized π2 configuration by compressing the Th–Th distance further down to 2.843 Å, within a smaller I h -C60 cage. On the other hand, elongating the Th–Th distance to 4.063 Å by encapsulating Th2 into a long diametric D 3h -C78 fullerene returns the Th3+–Th3+ bond to the normal closed-shell (6d7s7p)­σ2 form. Hence, a new rule is unambiguously revealed through the carbon-confinement induced spin-polarization of a single bond. The key point of this rule is the size of the carbon cage, because the squeezed effect is conducive to the effective overlap of the Th­(5f) orbitals, reducing and further reversing the original large singlet–triplet energy gap of the Th2 6+ unit. This discovery provides pioneering guidance for exploring new chemical bonds and thorium-based endofullerenes.