A Dysprosium Metallocene Single-Molecule Magnet Functioning at the Axial Limit

Abstraction of a chloride ligand from the dysprosium metallocene [(Cp^ttt )₂ DyCl] (1_Dy Cp^ttt =1,2,4-tri(tert-butyl)cyclopentadienide) by the triethylsilylium cation produces the first base-free rare-earth metallocenium cation [(Cp^ttt )₂ Dy]⁺ (2_Dy ) as a salt of the non-coordinating [B(C₆ F₅ )₄ ]^- anion. Magnetic measurements reveal that [2_Dy ][B(C₆ F₅ )₄ ] is an SMM with a record anisotropy barrier up to 1277 cm^-1 (1837 K) in zero field and a record magnetic blocking temperature of 60 K, including hysteresis with coercivity. The exceptional magnetic axiality of 2_Dy is further highlighted by computational studies, which reveal this system to be the first lanthanide SMM in which all low-lying Kramers doublets correspond to a well-defined M_J value, with no significant mixing even in the higher doublets.