Magnetic Interaction Affecting the Zero-Field Single-Molecule Magnet Behaviors in Isomorphic {NiII2DyIII2} and {CoII2DyIII2} 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 = NiII, Ln = DyIII (1), GdIII (2), and YIII (3); M = CoII, Ln = DyIII (4), GdIII (5), and YIII (6)) have been prepared with a compartmental Schiff-base ligand, 1-(2-hydroxy-3-methoxybenzylidene)-semicarbazide (H2hms), featuring a zigzag-shaped MII-LnIII-LnIII-MII metallic core arrangement. In complexes 16, a unique monophenoxo/diacetate asymmetric bridging connects MII ion with LnIII ion, and four acetates bridge two LnIII ions where acetates play essential roles as coligand in generating the tetranuclear units. Magnetic studies reveal the presence of predominant ferromagnetic coupling in DyIII and GdIII derivatives, and slow relaxation of magnetization is observed for {Ni2IIDy2III} and {CoII2DyIII2} with an energy barrier of 16.0 K for {Ni2IIDy2III} and 6.7 K for {CoII2DyIII2} under zero static field. Compared with the analogue {CoII2DyIII2}, the {Ni2IIDy2III} 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 {Ni2IIDy2III} is effectively interrupted thanks to the ferromagnetic exchange coupling generated between NiII and DyIII ions. The presence of ferromagnetic exchange between NiII and DyIII ions is more conducive to zero-field single-molecule magnet behaviors than in isomorphic {CoII2DyIII2} where the exchange is antiferromagnetic.