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

The tetranuclear NiII2LnIII2 complexes, [{L' 2 {Ni(MeOH)(μ-OAc)} 2 (μ 3 -MeO) 2 Ln 2 }, Ln III = Y III (1), Gd III (2), Tb III (3), and Dy III (4)], were prepared using a Schiff base ligand, H 3 L [H 3 L = 3-{(2-hydroxy-3-methoxybenzylidene)amino}-2-(2-hydroxy-3-methoxyphenyl)-2,3-dihydroquinazolin-4(1 H )-one, where {L'} 3- is the deprotonated open structure of H 3 L]. X-ray crystallographic analysis of 1-4 revealed that all the complexes crystallized in the orthorhombic ( Pbcn ) 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 ( U eff = 13.4 K; τ 0 = 4.1(7) × 10 -7 s). This was attributed to the comparatively strong Ni II -Tb III magnetic exchange coupling. DFT and ab initio calculations were carried out on 1-4 to ascertain the nature of the ferromagnetic Ni II -Ln III ( J Ni-Ln ) and Ln III -Ln III ( J 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.