A High Energy Barrier Dy^III ₂ Single-Molecule Magnet Supported by a Bulky, Anionic N-O Bridging Ligand

A new dinuclear dysprosium(III) single-molecule magnet (SMM), [Dy₂(hynad)₂(dbm)₄]·DMF (1·DMF), supported by the bulky anionic N-O bridging ligand, N-hydroxy-1,8-naphthalimide (hynad-), has been synthesized. The deprotonated hynad- ligands enforce a robust and nearly planar {Dy₂(µ-OR)₂}⁴⁺ core, which is surrounded by the sterically demanding dibenzoylmethanoate (dbm-) coligands. Each Dy^III center adopts a distorted triangular dodecahedral geometry, generating a highly axial crystal field. AC magnetic measurements reveal zero-field SMM behavior, with slow relaxation of the magnetization persisting up to 16 K, as well as open magnetic hysteresis loops observed by µSQUID magnetometry up to 3.5 K. Analysis of the AC susceptibility data yields an effective energy barrier for magnetization reversal of U_eff = 171 K. µSQUID studies disclose a characteristic double-S-shaped hysteresis, consistent with an antiferromagnetically coupled ground state separated by a small energy gap from a low-lying ferromagnetic excited state. Ab initio calculations confirm the strongly axial m_J = ±15/2 ground Kramers doublets for both Dy^III ions, with the magnetic anisotropy axes oriented nearly perpendicular to the {Dy₂(µ-OR)₂}⁴⁺ plane. The combined experimental and theoretical analysis demonstrates that the Dy···Dy interaction is weak and predominantly dipolar in nature, with thermal population of the excited exchange-coupled state enabling the observed slow magnetic relaxation.