Synthesis, Magnetism, and ⁵⁷Fe Mössbauer Spectroscopic Study of a Family of [Ln₃Fe₇] Coordination Clusters (Ln = Gd, Tb, and Er)

The reaction of <i>N</i>-methydiethanolamine (mdeaH₂), benzoic acid, FeCl₃, and Ln­(NO₃)₃·6H₂O or LnCl₃·<i>x</i>H₂O yields a series of decanuclear coordination clusters, [Ln₃Fe₇(μ₄-O)₂(μ₃-OH)₂(mdea)₇(μ-benzoate)₄(N₃)₆]·4MeCN·H₂O, where Ln = Gd^III (<b>1</b>) or Tb^III (<b>2</b>), and [Er₃Fe₇(μ₄-O)₂(μ₃-OH)₂(mdea)₇(μ-benzoate)₄(N₃)₅(MeOH)]­Cl·7.5H₂O·11.5MeOH (<b>3</b>). The isostructural compounds <b>1</b>–<b>3</b> all crystallize isotypically in the triclinic space group <i>P</i>1̅ with <i>Z</i> = 2, as does the previously reported dysprosium analogue <b>4</b>. Six of the Fe^III ions are <i>pseudo</i>octahedrally coordinated, whereas the seventh has a trigonal-bipyramidal coordination geometry. Temperature-dependent direct-current magnetic susceptibility studies indicate that intracluster antiferromagnetic interactions are dominant in <b>1</b>–<b>3</b>. The frequency-dependent out-of-phase (χ″) alternating-current susceptibility reveals that <b>2</b> undergoes a slow relaxation of its magnetization, presumably resulting from anisotropy of the Tb^III ions. Between 30 and 295 K, the ⁵⁷Fe Mössbauer spectra reveal paramagnetic behavior with six partially resolved quadrupole doublets, one for the trigonal-bipyramidal Fe^III site and five for the six <i>pseudo</i>octahedral Fe^III sites. The Mössbauer spectra of <b>2</b> and <b>3</b> obtained between 3 and 30 K are consistent with the presence of Fe^III intracluster antiferromagnetic coupling with slow magnetic relaxation relative to the Larmor precession time. Further, the observed changes in the effective magnetic field values in the spectra measured at 3 K with increasing applied field are consistent with the effect of the local spin polarization along the applied magnetic field direction, a behavior reminiscent of antiparallel spin-coupled iron molecular paramagnetic systems