Structure and magnetism in Fe-Gd based dinuclear and chain systems: the interplay of weak exchange coupling and zero field splitting effects

The synthesis and characterization of two Fe-Gd systems based on bpca- (Hbpca = bis(2-pyridilcarbonyl)amine) as bridging ligand is presented, taking the systems as a case study for structure-property correlations. Compound 1, [FeLSII(μ-bpca)2Gd(NO3)2(H2O)]NO3·2CH3NO2, is a zigzag polymer, incorporating the diamagnetic low spin FeLS(II) ion. The magnetism of 1 is entirely determined by the weak zero field splitting (ZFS) effect on the Gd(III) ion. Compound 2 is a Fe(III)-Gd(III) dinuclear compound, [FeLSIII(bpca)(μ-bpca)Gd(NO3)4]·4CH3NO2·CH3OH, its magnetism being interpreted as due to the antiferromagnetic coupling between the SFe = 1/2 and SGd = 7/2 spins, interplayed with the local ZFS on the lanthanide center. In both systems, the d-f assembly is determined by the bridging capabilities of the ambidentate bpca- ligand, which binds the d ion by a tridentate moiety with nitrogen donors and the f center by the diketonate side. We propose a spin delocalization and polarization mechanism that rationalizes the factors leading to the antiferromagnetic d-f coupling. Although conceived for compound 2, the scheme can be proposed as a general mechanism. The rationalization of the weak ZFS effects on Gd(III) by multiconfiguration and spin-orbit ab initio calculations allowed us to determine the details of the small but still significant anisotropy of Gd(III) ion in the coordination sites of compounds 1 and 2. The outlined methodologies and generalized conclusions shed new light on the field of gadolinium coordination magnetochemistry.