Tetranuclear Lanthanide(III) Complexes Containing a Square Grid Core: Synthesis-Structure, and Magnetism

The reactions of Ln(NO3)(3)center dot 5H(2)O (Ln = Dy3+, Tb3+, Ho3+, Er3+) and a multidentate flexible ligand, (E)-N'[2-hydroxy3-(hydroxymethyl)-5-methylbenzylidene]-6-(hydroxymethyl)pic- olinohydrazide (LH4), in the presence of Et3N in a 1:1:3 molar ratio afforded a series of complexes [Ln(4)(LH2)(4)(mu(2)-OH)(4)]center dot xCH(3)OH center dot gamma H2O (Dy3+, x = 2, y = 2; Tb-3+,Tb- x = 4, y = 5; Ho3+, x = 0, y = 13; Er3+, x = 4, y = 6). X-ray diffraction analysis revealed that all the complexes are neutral and possess a distorted [2 x 2] square-grid core [Ln(4)(mu(2)-O)(4)(mu(2)-OH)(4)] anchored by the concerted coordination of four doubly deprotonated ligands, (LH2)(2-), and four mu(2)-OH groups. All the Ln centers adopt a distorted triangular dodecahedral coordination geometry. An ac magnetic susceptibility study revealed undulations of the out of-phase (chi m '') component above 2 K for 1 with a quantum tunnelling of magnetization tail at zero dc field. Surprisingly, a field -induced temperature dependence of the ac frequencies at which the chi m '' maxima occur implies that the slow relaxation is a result of a mixed contribution from more than one Dy3+ center.