Mixed-valence Ln2@C79N endohedral metallofullerenes: magnetic exchange, magnetic anisotropy, and electric-field control of SMM characteristics

Abstract

The prospect of stabilising mixed-valence 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) provide a robust platform in which the fullerene cage stabilises the otherwise reactive Ln₂ units while simultaneously dictating their electronic structure 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 Ln₂@C₇₉N series (Ln = Ce–Yb, where two lanthanide ions, ranging from Ce to Yb, are confined within a C₇₉N heterofullerene cage), 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 Gd₂@C₇₉N exhibits exceptionally strong exchange, stabilising a high-spin S = 15/2 state, 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 single-ion 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 Ln₂@C₇₉N, which induce bond contraction, enhance covalency, and markedly boost the magnetisation barrier when applied perpendicular to the Ln–Ln direction. These results establish Ln₂@C₇₉N as a model system and highlight complementary chemical and physical strategies for designing ambient-stable, high-performance SMMs for quantum technologies.