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.