Exchange-driven slow relaxation of magnetization in NiII2LnIII2 (Ln = Y, Gd, Tb and Dy) butterfly complexes: experimental and theoretical studies.

The tetranuclear NiII2LnIII2 complexes, [{L'{Ni(MeOH)(μ-OAc)}(μ-MeO)Ln}, Ln = Y (1), Gd (2), Tb (3), and Dy (4)], were prepared using a Schiff base ligand, HL [HL = 3-{(2-hydroxy-3-methoxybenzylidene)amino}-2-(2-hydroxy-3-methoxyphenyl)-2,3-dihydroquinazolin-4(1)-one, where {L'} is the deprotonated open structure of HL]. X-ray crystallographic analysis of 1-4 revealed that all the complexes crystallized in the orthorhombic () space group, and possessed an isostructural tetranuclear butterfly or defect dicubane like core. Direct current magnetic susceptibility measurements performed on 2-4 revealed that all these complexes show an intramolecular ferromagnetic exchange coupling. Well resolved zero-field out-of-phase signals in ac magnetic susceptibility measurements were observed only in the case of 3 ( = 13.4 K; = 4.1(7) × 10 s). This was attributed to the comparatively strong Ni-Tb magnetic exchange coupling. DFT and calculations were carried out on 1-4 to ascertain the nature of the ferromagnetic Ni-Ln () and Ln-Ln () interactions. Magnetic anisotropy and magnetic relaxation mechanisms were discussed in detail for 3 and 4. Theoretical studies provide a rationale for the slow relaxation of magnetization in 3.