Fine-Tuning of Magnetic Anisotropy in Tetranuclear M2Dy2 Complexes with Zig-Zag Topology: The Impact of 3d Metal Selection

This study presents the design, synthesis, and magnetic characterization of two novel heterometallic tetranuclear complexes, [M₂Dy₂(Hheb)₂(heb)₄]·4MeOH (H₂heb = (E)-N′-(1-(2-hydroxyphenyl)ethylidene)benzohydrazide; M = Ni (1), Cu(2)). These complexes exhibit a rare zig-zag core topology induced by the rigid Hheb/heb^2– ligands. A subtle interplay between the incorporated 3d metal ions (Ni²⁺ and Cu²⁺) and the magnetic properties is evidenced. Notably, the choice of the 3d metal plays a crucial role in modulating the Dy³⁺ ion’s coordination environment and axiality, as supported by theoretical calculations. While both complexes exhibit rapid Quantum Tunneling of Magnetization (QTM), complex 1 (Ni²⁺) demonstrates markedly enhanced slow relaxation dynamics compared to complex 2. This difference is attributed to the stronger axiality indirectly induced by Ni²⁺ in complex 1, whereas the Cu²⁺-induced distortions and ferromagnetic interactions in complex 2 negatively affect the slow relaxation behavior.