Mixed-Valent Ln 2 @C 79 N Endohedral Metallofullerenes: Magnetic Exchange, Magnetic Anisotropy, and Electric-Field Control of SMM Characteristics

Abstract

The prospect of stabilising mixed-valent lanthanide species has been especially appealing since the discovery of [(CpiPr5)₂Dy₂I₃], which exhibits a blocking temperature as high as 60 K, however, their immense potential is constrained by the lack of ambient stability, limiting practical applications. Dilanthanide endohedral metallofullerenes (EMFs) offer a unique solution, where the robust fullerene cage stabilises reactive Ln₂ units while delicately dictating their electronic structure and magnetic response. Dilanthanide endohedral metallofullerenes (EMFs) provide a robust platform where the fullerene cage stabilises reactive Ln₂ units while dictating their electronic and magnetic behaviour. In this work, using a combination of computational tools, we present a systematic DFT and ab initio CASSCF study of the complete Ln 2 @C 79 N series (Ln = Ce-Yb), revealing two regimes: early lanthanides (Ce-Sm) show ionic character with spin density localised on lanthanides, while mid-to-late lanthanides (Gd-Yb) stabilise covalent 2c-1e bonds with radical delocalisation, consistent with Robin-Day Class III systems. While Gd 2 @C 79 N exhibits exceptionally strong exchange stabilising a high-spin S = 15/2 state, the Tb and Dy benefit from this strong exchange-mediated quenching of QTM, yielding large anisotropy barriers (>600 cm⁻¹). Despite exhibiting strong Ln-radical exchange, Er and Yb ions fail to show attractive SMM behaviour owing to their inherently limited singleion magnetic anisotropy. Motivated by the fact that azafullerene cages are reported to be excellent molecular rectifiers, we investigated the influence of oriented external electric fields (OEEFs) on Ln 2 @C 79 N, which induce bond contraction, enhance covalency, and markedly boost the magnetisation barrier when it is applied perpendicular Ln-Ln direction. These results establish Ln 2 @C 79 N as a model system and highlight complementary chemical and physical strategies for designing ambient-stable, high-performance SMMs for quantum technologies.