A Bis-Monophospholyl Dysprosium Cation Showing Magnetic Hysteresis at 48 Kelvin

Single-molecule magnets (SMMs) have potential applications in high-density data storage, but magnetic relaxation times at elevated temperatures must be increased to make them practically useful. Bis-cyclopentadienyl lanthanide sandwich complexes have emerged as the leading candidates for SMMs that show magnetic memory at liquid nitrogen temperatures, but the relaxation mechanisms mediated by aromatic C₅ rings have not been fully established. Here we synthesise a bis-monophospholyl dysprosium SMM [Dy(Dtp)₂][Al{OC(CF₃)₃}₄] (1, Dtp = {P(C^tBuCMe)₂}) by the treatment of in situ-prepared “[Dy(Dtp)₂(C₃H₅)]” with [HNEt₃][Al{OC(CF₃)₃}₄]. SQUID magnetometry reveals that 1 has an effective barrier to magnetisation reversal of 1,760 K (1,223 cm^–1) and magnetic hysteresis up to 48 K. Ab initio calculation of the spin dynamics reveal that transitions out of the ground state are slower in 1 than in the first reported dysprosocenium SMM, [Dy(Cp^ttt)₂][B(C₆F₅)₄] (Cp^ttt = C₅H₂^tBu₃-1,2,4), however relaxation is faster in 1 overall due to the compression of electronic energies and to vibrational modes being brought on-resonance by the chemical and structural changes introduced by the bis-Dtp framework. With the preparation and analysis of 1 we are thus able to further refine our understanding of relaxation processes operating in bis-C₅/C₄P sandwich lanthanide SMMs, which is the necessary first step towards rationally achieving higher magnetic blocking temperatures in these systems in future.