Theoretical Modeling of the Magnetic Behavior of Thiacalix[4]arene Tetranuclear MnII 2GdIII 2 and CoII 2EuIII 2 Complexes

• Published in: Inorganic chemistry Vol. 55; no. 7; pp. 3566 - 3575
• Main Authors: Aldoshin, Sergey M., Sanina, Nataliya A., Palii, Andrew V., Tsukerblat, Boris S.
• Format: Journal Article
• Language: English
• Published: American Chemical Society 04.04.2016
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/acs.inorgchem.6b00065

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 MnII 2GdIII 2 and CoII 2EuIII 2. The energy pattern of the MnII 2GdIII 2 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 χT product observed for the MnII 2GdIII 2 complex is attributed to the competitive influence of the ferromagnetic Mn–Gd and antiferromagnetic Mn–Mn exchange interactions, the latter being stronger (J(Mn, Mn) = −1.6 cm–1, J s(Mn, Gd) = 0.8 cm–1, g = 1.97). The model for the CoII 2EuIII 2 complex includes uniaxial anisotropy of the seven-coordinate CoII ions and an isotropic exchange interaction in the CoII 2 pair, while the EuIII ions are diamagnetic in their ground states. Best-fit analysis of χT versus T showed that the anisotropic contribution (arising from a large zero-field splitting in CoII ions) dominates (weak-exchange limit) in the CoII 2EuIII 2 complex (D = 20.5 cm–1, J = −0.4 cm–1, g Co = 2.22). This complex is concluded to exhibit an easy plane of magnetization (arising from the CoII 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 CoII 2 pair in the weak-exchange limit.