Dysprosium metallocene complexes of the type DyL₂, featuring either substituted monoanionic cyclopentadienyl or dianionic borolide ligands, exhibit slow magnetic relaxation and magnetic blocking at temperatures near and below cryogenic levels (77 K). The remarkable properties of these state-of-the-art single molecule magnets are primarily due to the strong axial ligand field induced by the five-membered ring ligands. Consequently, increasing the ligand field strength by positioning the ligands closer to the Dy³⁺ ion could further enhance the blocking temperatures. Here, we report the successful synthesis of a charge-neutral heteroleptic borolide/cyclopentadienyl dysprosium compound, Dy(BC₄Ph₅)(Cp^iPr5) (1) (Ph = phenyl, Cp = cyclopentadienyl, iPr = isopropyl), with a Dy³⁺ ion coordinated to a dianionic pentaphenylborolide and a monoanionic pentaisopropylcyclopentadienyl ligand. Variable-field magnetization revealed an open magnetic hysteresis loop up to 72 K, and AC magnetic susceptibility measurement and DC relaxation revealed a high U_eff of 1523(11) cm^-1and a 100 s blocking temperature of 65 K, both among the highest reported for single-molecule magnets. A shorter Dy-borolide centroid distance of 2.268(1) Å as well as a higher U_eff than the homoleptic analogs suggest a stronger axial ligand field originating from the ideal electrostatic interaction in 1, with a stronger Dy-ligand attraction from the dianionic borolide ligand and weaker ligand-ligand repulsion from the monoanionic cyclopentadienyl ligand. Comparison to the homoleptic analogs, bis-pentaisopropylcyclopentadienyl and bis-pentaphenylborolide Dy (III) complexes, reveals a trend in Raman relaxation rate with bis-borolide Dy (III) complex possessing the slowest, indicating that phenyl substituents are more structurally rigid than alkyl ones and thus lead to slower Raman relaxation. Apart from magnetism, the newly-discovered synthetic route for 1 may also be applicable to the synthesis of other heteroleptic complexes with different dianionic or monoanionic ligands, such as cyclopentadienyl-cyclobutadienyl, amide-borolide, phenoxide-borolide, and other combinations. A new family of asymmetric heteroleptic compounds with exciting magnetism could be accessible.

Randall McClain

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