Magnetic Hysteresis up to 73 K in a Dysprosium Cyclopentadienyl-Amide Single-Molecule Magnet

Abstract

Single-molecule magnets (SMMs) based on dysprosocenium cations, [Dy(Cp R ) 2 ] + (Cp R = substituted cyclopentadienyl), have set record effective energy barriers to magnetic reversal ( U eff ) and temperatures at which open magnetic hysteresis is observed ( T H ), due to their highly axial crystal fields (CFs) and rigid ligand frameworks. Dysprosium bis(amide) cations, [Dy(NR 2 )] + (R = bulky silyl, aryl), can potentially show superior SMM properties as more charge-dense N-donor atoms can enforce stronger axial CFs to increase U eff, but these more flexible ligands can also promote under-barrier magnetic relaxation processes that diminish T H . Here, we combine the favorable SMM properties of each ligand in a single complex, [Dy{N(Si i Pr 3 ) 2 }(Cp*)][Al{OC(CF 3 ) 3 } 4 ] ( 1-Dy; Cp* = C 5 Me 5 ). We find that 1-Dy has large magnetic anisotropy, with U eff = 2191(33) K; this is comparable with the best-performing dysprosium Cp R -based SMMs, but lower than the dysprosium bis(amide)-alkene complex [Dy{N(Si i Pr 3 )[Si( i Pr) 2 C(CH 3 )=CHCH 3 ]}{N(Si i Pr 3 )(Si i Pr 2 Et)}][Al{OC(CF 3 ) 3 } 4 ] ( U eff = 2652(16) K). A combination of the bent N–Dy–Cp* cent angle ( ca . 152.5(2)°) and flexible amide substituents of 1-Dy limits T H to 73 K, which is below the record T H value of 100 K for the bis(amide)-alkene. Together, this work shows that dysprosium SMMs containing one π-aromatic and one monodentate ligand can have comparable U eff values to bis-π-aromatic complexes, but in common with dysprosium bis(amide) complexes, they show a greater sensitivity of interligand angle toward under-barrier relaxation processes. This new class of dysprosium complexes are promising candidates for high-temperature SMMs, and it is likely that large improvements on this first example can be made with exquisite control of molecular geometry.

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