Magnetic Interaction Affecting the Zero-Field Single-Molecule Magnet Behaviors in Isomorphic {Ni^II₂Dy^III₂} and {Co^II₂Dy^III₂} Tetranuclear Complexes

Great interest is being shown in investigating magnetic interactions that efficiently influence lanthanide single-molecule magnet behavior. A series of heterometallic complexes [M₂Ln₂(Hhms)₂­(CH₃COO)₆­(CH₃OH)₂­(H₂O)₂]·(NO₃)₂ (M = Ni^II, Ln = Dy^III (1), Gd^III (2), and Y^III (3); M = Co^II, Ln = Dy^III (4), Gd^III (5), and Y^III (6)) have been prepared with a compartmental Schiff-base ligand, 1-(2-hydroxy-3-methoxybenzylidene)-semicarbazide (H₂hms), featuring a zigzag-shaped M^II-Ln^III-Ln^III-M^II metallic core arrangement. In complexes 1–6, a unique monophenoxo/diacetate asymmetric bridging connects M^II ion with Ln^III ion, and four acetates bridge two Ln^III ions where acetates play essential roles as coligand in generating the tetranuclear units. Magnetic studies reveal the presence of predominant ferromagnetic coupling in Dy^III and Gd^III derivatives, and slow relaxation of magnetization is observed for {Ni₂^IIDy₂^III} and {Co^II₂Dy^III₂} with an energy barrier of 16.0 K for {Ni₂^IIDy₂^III} and 6.7 K for {Co^II₂Dy^III₂} under zero static field. Compared with the analogue {Co^II₂Dy^III₂}, the {Ni₂^IIDy₂^III} shows longer relaxation time and an absence of the quantum tunnelling of the magnetization (QTM) at low temperatures. Ab initio calculations suggest that the zero-field QTM of {Ni₂^IIDy₂^III} is effectively interrupted thanks to the ferromagnetic exchange coupling generated between Ni^II and Dy^III ions. The presence of ferromagnetic exchange between Ni^II and Dy^III ions is more conducive to zero-field single-molecule magnet behaviors than in isomorphic {Co^II₂Dy^III₂} where the exchange is antiferromagnetic.