Insight into the\nMagnetic Exchange Interactions and\nAnisotropy in Heterobimetallic Cu<sup>II</sup>–Ln<sup>III</sup> Complexes: A Rare Example of Cu–Gd Single-Molecule Magnets

The exploration of heterometallic\ncomplexes containing\nboth 3d\nand 4f metal ions has been on the rise with advancements in the field\nof 3d–4f single-molecule magnets. The motivation stemmed from\nearly investigations into the Cu<sup>II</sup>–Ln<sup>III</sup> systems, particularly the Cu<sup>II</sup>/Gd<sup>III</sup> one,\nthat revealed a ferromagnetic interaction between these ions, irrespective\nof the complex’s topology or nuclearity. In this context, we\nhave synthesized three new isostructural and isomorphous [CuLn(L)(NO<sub>3</sub>)<sub>3</sub>(H<sub>2</sub>O)] complexes (Ln = Gd (<b>1</b>), Tb (<b>2</b>), and Dy (<b>3</b>)) derived from a compartmental\nSchiff base ligand H<sub>2</sub>L. All three complexes were structurally\ncharacterized in which Cu<sup>II</sup> and Ln<sup>III</sup> ions occupy\ninner N<sub>2</sub>O<sub>2</sub> and outer O<sub>2</sub>O′<sub>2</sub> compartments of the doubly deprotonated Schiff base ligand,\nrespectively. Moreover, another closely related complex [Cu(L)]<sub>2</sub>[CuDy(L)(OAc)(NO<sub>3</sub>)(H<sub>2</sub>O)](NO<sub>3</sub>)·5H<sub>2</sub>O (<b>4</b>) has been synthesized starting\nfrom the same Schiff base ligand H<sub>2</sub>L to examine the effect\nof the coordination environment around the Dy<sup>III</sup> ion on\nthe dynamic magnetic behavior in these systems. Variable-temperature\ndirect-current magnetic susceptibility measurements suggest a ferromagnetic\nexchange interaction between Cu<sup>II</sup> and Ln<sup>III</sup> ions,\nespecially in <b>1</b>–<b>3</b>, which is further\nsupported by DFT calculations. However, alternating-current magnetic\nsusceptibility measurements resulted in the most interesting part\nof this report. Remarkably, despite the isotropic nature of the Gd<sup>III</sup> ion, compound <b>1</b> displayed slow relaxation\nof the magnetization in the presence of an applied field together\nwith its anisotropic counterparts Tb<sup>III</sup> (<b>2</b>) and Dy<sup>III</sup> (<b>3</b>). Based on the literature\nstudy, this is a rare system in which Cu<sup>II</sup>–Gd<sup>III</sup> complexes showed single-molecule magnet behavior. Moreover,\ntheoretical calculations were also performed by means of DFT and CASSCF\nlevels of theory to obtain precise information on the magnetic exchange\ncoupling between Cu<sup>II</sup> and Ln<sup>III</sup> ions together\nwith the distinct magnetic dynamics in these systems.