Exploring magnetic anisotropy and exchange coupling in FeIII2DyIII heterotrimetallic assemblies displaying slow relaxation of magnetization

Abstract

Three new Fe^III₂Dy^III heterotrinuclear complexes were synthesized using structurally related, yet distinct, Schiff base ligands in combination with acetate, phenoxide, or methoxide bridging groups. X-Ray crystallographic analyses reveal that, despite variations in the ligand backbones and donor atoms, all complexes share a common Fe₂Dy core, exhibiting only subtle differences in the Fe–O–Dy angles and in the coordination environment around the Dy^III center. Direct-current (DC) magnetic measurements establish high-spin Fe^III ions (S = 5/2) and an overall ferromagnetic Fe–Dy exchange, while alternating-current (AC) data show clear out-of-phase signals under zero DC field, consistent with single-molecule magnet (SMM) behavior. The effective energy barriers for magnetization reversal (U_eff = 17–26 K) depend on the nature of the bridging ligand and the coordination environment around Dy^III ion. Ab initio CASSCF and broken-symmetry DFT calculations support the experimental observations, demonstrating that axial Dy^III anisotropy is preserved while the strength of the Fe–Dy coupling subtly adjusts the relaxation dynamics. These results highlight that the minor structural variations at the bridging ligands markedly influence the balance between exchange interactions and crystal-field effects, thereby advancing the understanding of Fe–Ln based SMMs.