Theoretical Modeling of the Magnetic Behavior of Thiacalix[4]arene Tetranuclear Mn^II₂Gd^III₂ and Co^II₂Eu^III₂ Complexes

In view of a wide perspective of 3d–4f complexes in single-molecule magnetism, here we propose an explanation of the magnetic behavior of the two thiacalix[4]­arene tetranuclear heterometallic complexes Mn^II₂Gd^III₂ and Co^II₂Eu^III₂. The energy pattern of the Mn^II₂Gd^III₂ complex evaluated in the framework of the isotropic exchange model exhibits a rotational band of the low-lying spin excitations within which the Landé intervals are affected by the biquadratic spin–spin interactions. The nonmonotonic temperature dependence of the χ<i>T</i> product observed for the Mn^II₂Gd^III₂ complex is attributed to the competitive influence of the ferromagnetic Mn–Gd and antiferromagnetic Mn–Mn exchange interactions, the latter being stronger (<i>J</i>(Mn, Mn) = −1.6 cm^–1, <i>J</i>_s(Mn, Gd) = 0.8 cm^–1, <i>g</i> = 1.97). The model for the Co^II₂Eu^III₂ complex includes uniaxial anisotropy of the seven-coordinate Co^II ions and an isotropic exchange interaction in the Co^II₂ pair, while the Eu^III ions are diamagnetic in their ground states. Best-fit analysis of χ<i>T</i> versus <i>T</i> showed that the anisotropic contribution (arising from a large zero-field splitting in Co^II ions) dominates (weak-exchange limit) in the Co^II₂Eu^III₂ complex (<i>D</i> = 20.5 cm^–1, <i>J</i> = −0.4 cm^–1, <i>g</i>_Co = 2.22). This complex is concluded to exhibit an easy plane of magnetization (arising from the Co^II pair). It is shown that the low-lying part of the spectrum can be described by a highly anisotropic effective spin-¹/₂ Hamiltonian that is deduced for the Co^II₂ pair in the weak-exchange limit

Single-Ion Magnet Et₄N[Co^II(hfac)₃] with Nonuniaxial Anisotropy: Synthesis, Experimental Characterization, and Theoretical Modeling

In this article we report the synthesis and structure of the new Co­(II) complex Et₄N­[Co^II­(hfac)₃] (<b>I</b>) (hfac = hexafluoroacetylacetonate) exhibiting single-ion magnet (SIM) behavior. The performed analysis of the magnetic characteristics based on the complementary experimental techniques such as static and dynamic magnetic measurements, electron paramagnetic resonance spectroscopy in conjunction with the theoretical modeling (parametric Hamiltonian and ab initio calculations) demonstrates that the SIM properties of <b>I</b> arise from the nonuniaxial magnetic anisotropy with strong positive axial and significant rhombic contributions