Insight into the Magnetic Exchange Interactions and Anisotropy in Heterobimetallic Cu^II–Ln^III 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 CuII–LnIII systems, particularly the CuII/GdIII 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 CuII and LnIII 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 DyIII ion on the dynamic magnetic behavior in these systems. Variable-temperature direct-current magnetic susceptibility measurements suggest a ferromagnetic exchange interaction between CuII and LnIII 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 GdIII ion, compound 1 displayed slow relaxation of the magnetization in the presence of an applied field together with its anisotropic counterparts TbIII (2) and DyIII (3). Based on the literature study, this is a rare system in which CuII–GdIII 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 CuII and LnIII ions together with the distinct magnetic dynamics in these systems.