[K₂(Bi@Pd₁₂@Bi₂₀)]^4-: An Endohedral Inorganic Fullerene with Spherical Aromaticity

Inorganic fullerene clusters have attracted widespread attention due to their highly symmetrical geometric structures and intrinsic electronic properties. However, cage-like clusters composed of heavy metal elements with high symmetry are rarely reported, and their synthesis is also highly challenging. In this study, we present the synthesis of a [K₂(Bi@Pd₁₂@Bi₂₀)]^4- cluster that incorporates a {Bi₂₀} cage with pseudo-I_h symmetry, making it the largest main group metal cluster compound composed of the bismuth element to date. Magnetic characterization and theoretical calculations suggest that the spin state of the overall cluster is a quartet. Quantum chemical calculations reveal that the [Bi₂₀]^3- cluster has a similar electronic configuration to C₆₀^6- and the [Bi@Pd₁₂@Bi₂₀]^6- cluster exhibits a unique open-shell aromatic character.