Structural Design of Easy‐Axis Magnetic Anisotropy and Determination of Anisotropic Parameters of LnIIICuIISingle‐Molecule Magnets

Four dinuclear LnIIICuIIcomplexes with Ln=Tb (1), Dy (2), Ho (3), and Er (4) were synthesized to investigate the relationship between their respective magnetic anisotropies and ligand‐field geometries. These complexes were crystallographically isostructural, and a uni‐axial ligand field was achieved by using three phenoxo oxygen groups. Complexes 1and 2displayed typical single‐molecule magnet (SMM) behaviors, of which the out‐of‐phase susceptibilities were observed in the temperature range of 1.8–5.0 K (1) and 1.8–20.0 K (2). The Cole–Cole plots exhibited a semicircular shape with αparameters in the range of 0.08–0.18 (2.6–4.0 K) and 0.07–0.24 (3.5–7.0 K). The energy barriers Δ/kBwere estimated from the Arrhenius plots to be 32.9(4) K for 1and 26.0(5) K for 2. Complex 3displayed a slow magnetic relaxation below 3.0 K, whereas complex 4did not show any frequency‐dependent behavior for both in‐phase and out‐of‐phase susceptibilities, which indicates that easy‐axis anisotropy was absent. The temperature dependence of the dc susceptibilities for the field‐aligned samples of 1–3revealed that the χMTvalue continuously increased as the temperature was lowered, which indicates the presence of low‐lying Stark sublevels with the highest |Jz| values. In contrast, complex 4displayed a smaller and temperature‐independent χMTvalue, which also indicates that easy‐axis anisotropy was absent. Simultaneous analyses were carried out for 1–3to determine the magnetic anisotropy parameters on the basis of the Hamiltonian that considers B20, B40, and B60.