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

Abstraction of a chloride ligand from the dysprosium metallocene [(Cp^ttt)₂DyCl] ( 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)₂Dy]⁺ ( 2_Dy ) as a salt of the non‐coordinating [B(C₆F₅)₄]⁻ anion. Magnetic measurements reveal that [ 2_Dy ][B(C₆F₅)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.