Effect of a bromo-substituent on the magnetization dynamics of dysprosium coordination polymers constructed using a hybrid-ligand concept

Two novel two-dimensional dysprosium coordination polymers, {[DyL(bpeedo)(ClO₄)]ClO₄·CH₃OH}_<i>n</i> (1) and {[DyL'(bpeedo)_1.5(CH₃OH)](ClO₄)₂·3CH₃OH}_<i>n</i> (2), featuring slow magnetic relaxation under zero dc field, have been successfully assembled using Schiff base ligands <i>N</i>'-(2-hydroxybenzylidene)pyridine-<i>N</i>-oxide-carbohydrazide (HL) and <i>N</i>'-(2-hydroxy-5-bromobenzylidene)pyridine-<i>N</i>-oxide-carbohydrazide (HL') with an O-N-O coordination pocket and the bridging ligand 1,2-bis(4-pyridyl-<i>N</i>-oxide)ethene (bpeedo). Single-crystal X-ray diffraction analysis indicates that both compounds possess similar dimeric structures bridged by Schiff base ligands, with further extension <i>via</i> μ₃- and μ₂-bpeedo linkers, respectively. Notably, the introduction of a bromo-substituent onto the Schiff-base ligand can induce subtle modifications of the coordination environments around Dy³⁺ ions but a great change in the linkage between them, ultimately affecting their magnetic properties, as evidenced by the different energy barriers of 421(8) K and 268(10) K for complexes 1 and 2, respectively. Theoretical calculations and magneto-structural analysis reveal that the higher energy barrier of 1 stems from the Ising-type magnetic anisotropy of Dy³⁺ ions and the strong antiferromagnetic interaction mediated by μ₃-bpeedo, which can effectively suppress the quantum tunneling of magnetization and facilitate the thermally-assisted spin flip through the second-excited state. In contrast, although the structural change induced by the bromo-substituent can slightly enhance the magnetic anisotropy of Dy³⁺ ions in 2, the weak antiferromagnetic coupling between them prevents the relaxation pathway through the second-excited state, resulting in its lower energy barrier and smaller coercive field.