Influence of lanthanides on spin-relaxation and spin-structure in a family of Fe7Ln4 single molecule magnets

Abstract

A family of isostructural undecanuclear 3d–4f coordination clusters of formula [Fe^III₇Ln^III₄O₄(OH)₃(tea)₂(Htea)₃(Piv)₇(H₂O)₂(NO₃)₃], where Ln = Y (1), Gd (2), Tb (3), Dy (4); PivH ≡ pivalic acid and H₃tea ≡ triethanolamine, was synthesised. The central Fe₇ core of the coordination cluster can be described in terms of two {Fe₄O₂} butterfly motifs sharing a common body Fe atom. The two Fe₄ mean-planes subtend a dihedral angle of ca. 72°. The Tb (3) and Dy (4) compounds show Single Molecule Magnet (SMM) behaviour as confirmed by ac-susceptibility and μ-SQUID measurements. Furthermore, ⁵⁷Fe Mössbauer spectra of 1–4 confirm the presence of high-spin Fe^III sites. The spectra of all complexes in the high temperature range (30–300 K) show broad overlapping doublets which were assigned to the body and wing-tip pairs of metal ions within the Fe₇ core. The low temperature Mössbauer spectra show dependence on the nature of the rare-earth metal as a result of its interaction with the iron sites. Thus, we observed a transition from fast (2), to intermediate (1) and very slow (frozen) (3, 4) spin fluctuation phenomena in these compounds.