Synthesis, magnetism and Mossbauer studies of tetranuclear heterometallic {Fe2IIILn2}(Ln = Gd, Dy, Tb) complexes: evidence of slow relaxation of magnetization in the terbium analogue

A new family of tetranuclear heterometallic assemblies, [(Fe2Gd2)-Gd-III(H2L)(4)(eta(2)-NO3)(2)]center dot 2ClO(4)center dot 2CH(3)OH center dot 2H(2)O (1), [(Fe2Dy2)-Dy-III(H2L)(4)(eta(2)-NO3)(2)]2ClO(4)center dot 2CH(3)OH center dot 2H(2)O (2), [(Fe2Tb2)-Tb-III(H2L)(4)(eta(2)-NO3)(2)]center dot 2ClO(4)center dot 2CH(3)OH center dot 2H(2)O (3), have been synthesized employing a multi-dentate Schiff-base ligand, (E)-2,2'-(2-hydroxy-3-((2-hydroxyphenylimino)methyl)-5-methylbenzylazanediyl)-diethanol (H4L), Fe(ClO4)(2)center dot 6H(2)O, and Ln(III) nitrate salts. These compounds have been structurally characterized by various analytical and spectroscopic techniques. The molecular structures of 1-3 have been confirmed by single crystal X-ray crystallography. All the three complexes contain two Fe-III ions at the periphery and two Ln(III) ions in the centre. The entire assembly is held together by four doubly deprotonated [LH2](2-) ligands. All the three complexes (1-3) are dicationic in nature and possess an overall Z-type topology. Magnetic measurements reveal the presence of predominant ferromagnetic coupling for all the three compounds at low temperature. The presence of a frequency-dependent out-of-phase signal in the imaginary part of the ac susceptibility plot suggests a slow relaxation of magnetization for 3 ((Fe2Tb2)-Tb-III). Furthermore, the magnetization dynamics of all the three complexes have been corroborated by Mossbauer spectroscopy.