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

Single-molecule magnets (SMMs) based on dysprosocenium cations, [Dy(CpR)2]+ (CpR = substituted cyclopentadienyl), have set record effective energy barriers to magnetic reversal (Ueff) and temperatures at which open magnetic hysteresis is observed (TH), 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 Ueff, but these more flexible ligands can also promote under-barrier magnetic relaxation processes that diminish TH. 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] (1-Dy; Cp* = C5Me5). We find that the mixed ligand system in 1-Dy provides large magnetic anisotropy to give Ueff = 2382(26) K, exceeding any dysprosium CpR-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 (ca. 152.5(2)°) and flexible amide substituents of 1-Dy limits TH to 73 K, far below the record TH 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 Ueff values than bis-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.