Magnetic hysteresis up to 73 K in a dysprosium cyclopentadienyl-amide single-molecule magnet

<p dir="ltr">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 aim to combine the favourable SMM properties of each ligand in a single complex, namely [Dy{N(SiⁱPr₃)₂}(Cp*)][Al{OC(CF₃)₃}₄] (<b>1-Dy</b>; Cp* = C₅Me₅). We find that the mixed ligand system in <b>1-Dy</b> provides large magnetic anisotropy to give <i>U</i>_eff = 2382(26) K, exceeding any dysprosium Cp^R-based SMM reported to date, and only bettered by the dysprosium bis(amide)-alkene complex [Dy{N(SiⁱPr₃)[Si(ⁱPr)₂C(CH₃)=CHCH₃]}{N(SiⁱPr₃)(SiⁱPr₂Et)}][Al{OC(CF₃)₃}₄]. However, a combination of the bent N–Dy–Cp*_cent (<i>ca</i>. 152.5(2)°) and flexible amide substituents of <b>1-Dy</b> limits <i>T</i>_H to 73 K, far below the record <i>T</i>_H value of 100 K for the bis(amide)-alkene. This work shows that dysprosium SMMs containing one π-aromatic and one monodentate ligand can have larger <i>U</i>_eff values than <i>bis</i>-π-aromatic complexes, but in common with dysprosium bis(amide) complexes they show a greater sensitivity of inter-ligand angle towards under-barrier relaxation processes. Hence, this new class of dysprosium complexes are promising candidates for high-temperature SMMs, but the full potential of this strategy will only be realized with exquisite control of molecular geometry.</p>