Insight into the Magnetic Exchange Interactions and Anisotropy in Heterobimetallic CuII–LnIII Complexes: A Rare Example of Cu–Gd Single-Molecule Magnets

The exploration of heterometallic complexes containing both 3d and 4f metal ions has been on the rise with advancements in the field of 3d–4f single-molecule magnets. The motivation stemmed from early investigations into the Cu^II–Ln^III systems, particularly the Cu^II/Gd^III one, that revealed a ferromagnetic interaction between these ions, irrespective of the complex’s topology or nuclearity. In this context, we have synthesized three new isostructural and isomorphous [CuLn(L)(NO₃)₃(H₂O)] complexes (Ln = Gd (1), Tb (2), and Dy (3)) derived from a compartmental Schiff base ligand H₂L. All three complexes were structurally characterized in which Cu^II and Ln^III ions occupy inner N₂O₂ and outer O₂O′₂ compartments of the doubly deprotonated Schiff base ligand, respectively. Moreover, another closely related complex [Cu(L)]₂[CuDy(L)(OAc)(NO₃)(H₂O)](NO₃)·5H₂O (4) has been synthesized starting from the same Schiff base ligand H₂L to examine the effect of the coordination environment around the Dy^III ion on the dynamic magnetic behavior in these systems. Variable-temperature direct-current magnetic susceptibility measurements suggest a ferromagnetic exchange interaction between Cu^II and Ln^III ions, especially in 1–3, which is further supported by DFT calculations. However, alternating-current magnetic susceptibility measurements resulted in the most interesting part of this report. Remarkably, despite the isotropic nature of the Gd^III ion, compound 1 displayed slow relaxation of the magnetization in the presence of an applied field together with its anisotropic counterparts Tb^III (2) and Dy^III (3). Based on the literature study, this is a rare system in which Cu^II–Gd^III complexes showed single-molecule magnet behavior. Moreover, theoretical calculations were also performed by means of DFT and CASSCF levels of theory to obtain precise information on the magnetic exchange coupling between Cu^II and Ln^III ions together with the distinct magnetic dynamics in these systems.