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

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 (<i>U</i>_eff) and temperatures at which open magnetic hysteresis is observed (<i>T</i>_H), due to their highly axial crystal fields (CFs) and rigid ligand frameworks. Dysprosium bis(amide) cations, [Dy(NR₂)]⁺ (R = bulky silyl, aryl), can potentially show superior SMM properties as more charge-dense N-donor atoms can enforce stronger axial CFs to increase <i>U</i>_eff, but these more flexible ligands can also promote under-barrier magnetic relaxation processes that diminish <i>T</i>_H. Here, we combine the favorable SMM properties of each ligand in a single complex, [Dy{N(SiⁱPr₃)₂}(Cp*)][Al{OC(CF₃)₃}₄] (<b>1-Dy</b>; Cp* = C₅Me₅). We find that <b>1-Dy</b> has large magnetic anisotropy, with <i>U</i>_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(SiⁱPr₃)[Si(ⁱPr)₂C(CH₃)=CHCH₃]}{N(SiⁱPr₃)(SiⁱPr₂Et)}][Al{OC(CF₃)₃}₄] (<i>U</i>_eff = 2652(16) K). A combination of the bent N-Dy-Cp*_cent angle (<i>ca</i>. 152.5(2)°) and flexible amide substituents of <b>1-Dy</b> limits <i>T</i>_H to 73 K, which is below the record <i>T</i>_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 <i>U</i>_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.