Formation of Defect-Dicubane-Type Ni^II ₂Ln^III ₂ (Ln = Tb, Er) Clusters: Crystal Structures and Modeling of the...

In the field of molecular nanoclusters, cubane and defect-dicubane, or butterfly structures, are typical examples of tetranuclear metal core architectures. In this work, a halogenated and anionic Schiff-base ligand (L^2-) is utilized as it is predisposed to chelate within a cluster core to both 3d and 4f metal ions, in different binding configurations (H₂L = 4-chloro-2-(2-hydroxy-3-methoxybenzyliden amino)phenol). The phenolate oxygen atoms of the deprotonated ligand can act in μ-O and μ₃-O bridging binding modes for the intramolecular assembly of metal ions. Based on that, two tetranuclear and isostructural compounds [Ni₂Tb₂(L)₄(NO₃)₂(DMF)₂]·2CH₃CN (1) and [Ni₂Er₂(L)₄(NO₃)₂(DMF)₂]·0.5CH₃CN (2) were synthesized and structurally characterized. Magnetic susceptibility and magnetization data indicate the occurrence of dominant intramolecular ferromagnetic interactions between the spin centers. Particular emphasis is given to the theoretical description of the magnetic behavior, taking into account the Ln-Ni and Ni-Ni coupling paths and the magnetic anisotropy of the Ln^III and Ni^II ions. The study is distinguished for its discussion of two distinct models, whereby model A relies on the uniaxial B ₂₀ Stevens term describing the lanthanide anisotropy and model B is based on point-charge model calculations. Importantly, the physical meaning of the obtained parameters for both models was critically scrutinized.