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

The reaction of N-methydiethanolamine (mdeaH2), benzoic acid, FeCl3, and Ln­(NO3)3·6H2O or LnCl3·xH2O 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 = GdIII (1) or TbIII (2), and [Er3Fe7(μ4-O)2(μ3-OH)2(mdea)7(μ-benzoate)4(N3)5(MeOH)]­Cl·7.5H2O·11.5MeOH (3). The isostructural compounds 1–3 all crystallize isotypically in the triclinic space group P1̅ with Z = 2, as does the previously reported dysprosium analogue 4. Six of the FeIII ions are pseudooctahedrally 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 1–3. The frequency-dependent out-of-phase (χ″) alternating-current susceptibility reveals that 2 undergoes a slow relaxation of its magnetization, presumably resulting from anisotropy of the TbIII ions. Between 30 and 295 K, the 57Fe Mössbauer spectra reveal paramagnetic behavior with six partially resolved quadrupole doublets, one for the trigonal-bipyramidal FeIII site and five for the six pseudooctahedral FeIII sites. The Mössbauer spectra of 2 and 3 obtained between 3 and 30 K are consistent with the presence of FeIII 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.