A Combined Synthetic, Magnetic, and Theoretical Study on Enhancing Ligand-Field Axiality for Dy(III) Single-Molecule Magnets Supported by Ferrocene Diamide Ligands

Molecular design is crucial for improving the performance of single-molecule magnets (SMMs). For dysprosium­(III) SMMs, enhancing ligand-field axiality is a well-suited strategy to achieve high-performance SMMs. We synthesized a series of dysprosium­(III) complexes, (NNTIPS)­DyBr­(THF)2 (1, NNTIPS = fc­(NSi i Pr3)2; fc = 1,1′-ferrocenediyl, THF = tetrahydrofuran), [(NNTIPS)­Dy­(THF)3]­[BPh4] (2), (NNTIPS)­DyI­(THF)2 (3), and [(NNTBS)­Dy­(THF)3]­[BPh4] (4, NNTBS = fc­(NSi t BuMe2)2), supported by ferrocene diamide ligands. X-ray crystallography shows that the rigid ferrocene backbone enforces a nearly axial ligand field with weakly coordinating equatorial ligands. Dysprosium­(III) complexes 1–4 all exhibit slow magnetic relaxation under zero fields and possess high effective barriers (U eff) around 1000 K, comparable to previously reported (NNTBS)­DyI­(THF)2 (5). We probed the influences of structural variations on SMM behaviors by theoretical calculations and found that the distribution of negative charges defined by r q, i.e., the ratio of the charges on the axial ligands to the charges on the equatorial ligands, plays a decisive role. Moreover, theoretical calculations on a series of model complexes 1′–5′ without equatorial ligands unveil that the axial crystal-field parameters B 2 0 are directly proportional to the N–Dy–N angles and support the hypothesis that enhancing the ligand-field axiality could improve SMM performance.