Synthesis, magnetism and Mössbauer studies of tetranuclear heterometallic {FeIII2Ln2}(Ln = Gd, Dy, Tb) complexes: evidence of slow relaxation of magnetization in the terbium analogue

Abstract

A new family of tetranuclear heterometallic assemblies, [Fe^III₂Gd₂(H₂L)₄(η²-NO₃)₂]·2ClO₄·2CH₃OH·2H₂O (1), [Fe^III₂Dy₂(H₂L)₄(η²-NO₃)₂]·2ClO₄·2CH₃OH·2H₂O (2), [Fe^III₂Tb₂(H₂L)₄(η²-NO₃)₂]·2ClO₄·2CH₃OH·2H₂O (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)₂·6H₂O, 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 (Fe^III₂Tb₂). Furthermore, the magnetization dynamics of all the three complexes have been corroborated by Mössbauer spectroscopy.