Insight into ferromagnetic interactions in Cu^II-Ln^III dimers with a compartmental ligand

In the last two decades, efforts have been devoted to obtaining insight into the magnetic interactions between Cu-II and Ln(III) utilizing experimental and theoretical means. Experimentally, it has been observed that the exchange coupling (J) in Cu-II-Ln(III) systems is often found to be ferromagnetic for >= 4f(7) metal ions. However, exchange interactions at sub-Kelvin temperatures between Cu-II and the anisotropic/isotropic Ln(III) ions are not often explored. In this report, we have synthesized a series of heterobimetallic [CuLn(HL)(mu-piv)(piv)(2)] complexes (Ln(III) = Gd (1), Tb (2), Dy (3) and Er (4)) from a new compartmental Schiff base ligand, N,N'-bis(3-methoxy-5-methylsalicylidene)-1,3-diamino-2-propanol (H3L). X-ray crystallographic analysis reveals that all four complexes are isostructural and isomorphous. Magnetic susceptibility measurements reveal a ferromagnetic coupling between the Cu-II ion and its respective Ln(III) ion for all the complexes, as often observed. Moreover, mu-SQUID studies, at sub-Kelvin temperatures, show S-shaped hysteresis loops indicating the presence of antiferromagnetic coupling in complexes 1-3. The antiferromagnetic interaction is explained by considering the shortest Cu & ctdot;Cu distance in the crystal structure. The nearly closed loops for 1-3 highlight their fast relaxation characteristics, while the opened loops for 4 might arise from intermolecular ordering. CASSCF calculations allow the quantitative assessment of the interactions, which are further supported by BS-DFT calculations.