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), Th₂⁶⁺@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 Th₂⁶⁺ unit. This discovery provides pioneering guidance for exploring new chemical bonds and thorium-based endofullerenes.