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(C 6 F 5 ) 4 ] − anion. Magnetic measurements reveal that [ 2 Dy ][B(C 6 F 5 ) 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.