Effect of bromo-substituent on magnetization dynamics of dysprosium coordination polymers constructed by hybrid-ligand conception

Two novel two-dimensional dysprosium coordination polymers, {[DyL(bpeedo)(ClO₄)]ClO₄•CH₃OH}n (1) and {[DyL′(bpeedo)_1.5(CH₃OH)](ClO₄)₂•3CH₃OH}n (2), featuring slow magnetic relaxation under zero dc field, have been successfully assembled using Schiff base ligands of N′-(2-hydroxybenzylidene)pyridine-N-oxide-carbohydrazide (HL) and N′-(2-hydroxy-5-bromobenzylidene)pyridine-N-oxide-carbohydrazide (HL′) with O-N-O coordination pocket and the bridging ligand of 1,2-bis(4-pyridyl-N-oxide)ethene (bpeedo). Single-crystal X-ray diffraction analysis indicates that both compounds possess similar dimeric structure bridged by Schiff base ligand, with further extension via μ₃- and μ₂-bpeedo linkers, respectively. Notably, the introduction of bromo-substituent on the Schiff-base ligand can induce subtle modification of the coordination environments around Dy³⁺ ions while great change of 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 thermal-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.