Magnetic Interaction Affecting the Zero-Field Single-Molecule Magnet Behaviors in Isomorphic {Ni<sup>II</sup><sub>2</sub>Dy<sup>III</sup><sub>2</sub>} and {Co<sup>II</sup><sub>2</sub>Dy<sup>III</sup><sub>2</sub>} Tetranuclear Complexes

Great interest is being shown in investigating magnetic interactions that efficiently influence lanthanide single-molecule magnet behavior. A series of heterometallic complexes [M<sub>2</sub>Ln<sub>2</sub>(Hhms)<sub>2</sub>­(CH<sub>3</sub>COO)<sub>6</sub>­(CH<sub>3</sub>OH)<sub>2</sub>­(H<sub>2</sub>O)<sub>2</sub>]·(NO<sub>3</sub>)<sub>2</sub> (M = Ni<sup>II</sup>, Ln = Dy<sup>III</sup> (<b>1</b>), Gd<sup>III</sup> (<b>2</b>), and Y<sup>III</sup> (<b>3</b>); M = Co<sup>II</sup>, Ln = Dy<sup>III</sup> (<b>4</b>), Gd<sup>III</sup> (<b>5</b>), and Y<sup>III</sup> (<b>6</b>)) have been prepared with a compartmental Schiff-base ligand, 1-(2-hydroxy-3-methoxybenzylidene)-semicarbazide (H<sub>2</sub>hms), featuring a zigzag-shaped M<sup>II</sup>-Ln<sup>III</sup>-Ln<sup>III</sup>-M<sup>II</sup> metallic core arrangement. In complexes <b>1</b>–<b>6</b>, a unique monophenoxo/diacetate asymmetric bridging connects M<sup>II</sup> ion with Ln<sup>III</sup> ion, and four acetates bridge two Ln<sup>III</sup> ions where acetates play essential roles as coligand in generating the tetranuclear units. Magnetic studies reveal the presence of predominant ferromagnetic coupling in Dy<sup>III</sup> and Gd<sup>III</sup> derivatives, and slow relaxation of magnetization is observed for {Ni<sub>2</sub><sup>II</sup>Dy<sub>2</sub><sup>III</sup>} and {Co<sup>II</sup><sub>2</sub>Dy<sup>III</sup><sub>2</sub>} with an energy barrier of 16.0 K for {Ni<sub>2</sub><sup>II</sup>Dy<sub>2</sub><sup>III</sup>} and 6.7 K for {Co<sup>II</sup><sub>2</sub>Dy<sup>III</sup><sub>2</sub>} under zero static field. Compared with the analogue {Co<sup>II</sup><sub>2</sub>Dy<sup>III</sup><sub>2</sub>}, the {Ni<sub>2</sub><sup>II</sup>Dy<sub>2</sub><sup>III</sup>} shows longer relaxation time and an absence of the quantum tunnelling of the magnetization (QTM) at low temperatures. <i>Ab initio</i> calculations suggest that the zero-field QTM of {Ni<sub>2</sub><sup>II</sup>Dy<sub>2</sub><sup>III</sup>} is effectively interrupted thanks to the ferromagnetic exchange coupling generated between Ni<sup>II</sup> and Dy<sup>III</sup> ions. The presence of ferromagnetic exchange between Ni<sup>II</sup> and Dy<sup>III</sup> ions is more conducive to zero-field single-molecule magnet behaviors than in isomorphic {Co<sup>II</sup><sub>2</sub>Dy<sup>III</sup><sub>2</sub>} where the exchange is antiferromagnetic.