Theoretical Design of Mixed-Metal Endohedral Fullerenes Gd2M@C80 (M = Group IA or IIIA Metal) with Strong Intramolecular Magnetic Coupling

Endohedral metallofullerenes Ln₂@C₈₀ (Ln = lanthanide) represent a fascinating class of molecular magnets due to their unique single-electron metal-metal bond, which strongly interacts with the 4f electrons of the encapsulated lanthanide atoms. However, a critical limitation arises from their unstable triplet ground state, which complicates experimental isolation and practical utilization. To address this challenge, we propose an endohedral doping strategy by introducing a third metal atom (M = group IA or IIIA metal) to form mixed-metal fullerenes Gd₂M@C₈₀. Unlike previous approaches that rely on exohedral functionalization or cage modification, our method preserves the pristine I_h-C₈₀ structure. Our computational studies reveal strong intramolecular magnetic couplings in these metallofullerenes, with the coupling strength showing tunability based on the ionic radius of M. Notably, the Gd₂K@C₈₀ metallofullerene exhibits the largest reported coupling constant among metallofullerenes (+238 cm^-1). A detailed bonding analysis uncovers distinct electronic structures, depending on the dopant: IA metals (Li, Na, and K) form a delocalized three-center-one-electron (3c-1e) bond across the Gd-Gd-M cluster, while IIIA metals (Al, Ga, and In) exhibit a 2c-1e Gd-Gd bond coexisting with a 3c-2e bond involving the IIIA atom. This work establishes an innovative approach to stabilize redox-active Ln₂@C₈₀ systems while remarkably enhancing their intrinsic magnetic characteristics.