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, (NN TIPS )DyBr(THF) 2 ( 1 , NN TIPS = fc(NSi i Pr 3 ) 2 ; fc = 1,1'-ferrocenediyl, THF = tetrahydrofuran), [(NN TIPS )Dy(THF) 3 ][BPh 4 ] ( 2 ), (NN TIPS )DyI(THF) 2 ( 3 ), and [(NN TBS )Dy(THF) 3 ][BPh 4 ] ( 4 , NN TBS = fc(NSi t BuMe 2 ) 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 (NN TBS )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.