Exchange-driven slow relaxation of magnetization in Ni(2)(II)Ln(2)(III) (Ln(III) = Y, Gd, Tb and Dy) butterfly complexes: experimental and theoretical studies

The tetranuclear Ni(2)(II)Ln(2)(III) complexes, [{L'(2){Ni(MeOH)(mu-OAc)}2(mu(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, H3L [H3L = 3-{(2-hydroxy-3-methoxybenzylidene)amino}-2-(2-hydroxy- 3-methoxyphenyl)-2,3-dihydroquinazolin-4(1H)-one, where {L'}(3-) is the deprotonated open structure of H3L]. 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; tau(0) = 4.1(7) x 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.