Ln3@C80+ (Ln = lanthanide): a new class of stable metallofullerene cations with multicenter metal–metal bonding in the sub-nanometer confined space

Abstract

Among the large number of members in the metallofullerene family, the nitride clusterfullerene M₃N@C₈₀ (M = trivalent metal) is a special one with extraordinarily high stability. It is generally thought that the molecule would be unstable without the nitrogen ion N³⁻ in the center of the M₃ moiety, because N³⁻ can compensate for the Coulomb repulsion between the metal ions M³⁺. Indeed, the tri-metallofullerenes Ln₃@C₈₀ (Ln = trivalent lanthanide) are missing in the family of metallofullerenes to date. In this paper, we provide new insights into the stability of Ln₃@C₈₀ by a combined experimental and theoretical study. Density functional calculations demonstrate that Ln₃@C₈₀ are thermodynamically stable molecules. However, their small HOMO–LUMO gaps can induce severe kinetic instability. Meanwhile, Ln₃@C₈₀ has the smallest ionization energy among the metallofullerene molecules reported so far. Experimental and theoretical studies prove that the Ln₃@C₈₀ molecules can be greatly stabilized by chemical oxidation, because the Ln₃@C₈₀⁺ cation has a large HOMO–LUMO gap comparable to that of Ln₃N@C₈₀. Furthermore, Ln₃@C₈₀⁺ and Ln₃N@C₈₀ have similar molecular geometries and electronic structures. There is a three-center two-electron σ bond in the center of the Ln₃ cluster in Ln₃@C₈₀⁺. This special metal–metal bond significantly compensates for the electrostatic repulsion between the Ln ions and thus stabilizes the cation Ln₃@C₈₀⁺. In previous studies, there are very few examples of metallofullerene cations, because most of the metallofullerene cations are highly unstable. This study provides a strategy for obtaining a new class of stable metallofullerene cations, which can be used to construct a variety of novel ionic compounds Ln₃@C₈₀⁺X⁻.