Synthesis, Magnetism, and 57 Fe Mö ssbauer Spectroscopic Study of a Family of [Ln 3 Fe 7 ] Coordination Clusters (Ln = Gd, Tb, and Er) Ghulam Abbas, †,‡ Yanhua Lan, † Valeriu Mereacre, † Gernot Buth, § Moulay T. Sougrati, ∥ Fernande Grandjean, ∥ Gary J. Long,* ,⊥ Christopher E. Anson, † and Annie K. Powell* ,†,¶ † Institut fü r Anorganische Chemie, Karlsruhe Institute of Technology, Engesserstrasse 15, D-76131 Karlsruhe, Germany § Institut fü r Synchrotronstrahlung and ¶ Institut fü r Nanotechnologie, Karlsruhe Institute of Technology, Hermann-von-Helmholtz-Platz 1, D-76344 Eggenstein-Leopoldshafen, Germany ∥ Department of Physics, University of Lie ̀ ge, B-4000 Sart-Tilman, Belgium ⊥ Department of Chemistry, Missouri University of Science and Technology, University of Missouri, Rolla, Missouri 65409-0010, United States * S Supporting Information ABSTRACT: The reaction of N-methydiethanolamine (mdeaH 2 ), benzoic acid, FeCl 3 , and Ln(NO 3 ) 3 ·6H 2 O or LnCl 3 ·xH 2 O yields a series of decanuclear coordination clusters, [Ln 3 Fe 7 (μ 4 -O) 2 (μ 3 - OH) 2 (mdea) 7 (μ-benzoate) 4 (N 3 ) 6 ]·4MeCN·H 2 O, where Ln = Gd III ( 1 ) or Tb III ( 2 ), and [Er 3 Fe 7 ( μ 4 -O) 2 ( μ 3 -OH) 2 (mdea) 7 ( μ - benzoate) 4 (N 3 ) 5 (MeOH)]Cl·7.5H 2 O·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 Fe III ions are pseudooctahedrally coordinated, whereas the seventh has a trigonal-bipyramidal coordination geometry. Temperature-depend- ent direct-current magnetic susceptibility studies indicate that intra- cluster 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 Tb III ions. Between 30 and 295 K, the 57 Fe Mö ssbauer spectra reveal paramagnetic behavior with six partially resolved quadrupole doublets, one for the trigonal-bipyramidal Fe III site and five for the six pseudooctahedral Fe III sites. The Mö ssbauer spectra of 2 and 3 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. 1. INTRODUCTION The study of high-nuclearity coordination clusters has attracted much attention in recent years with the discovery that such molecules can display the phenomenon of single-molecule magnets (SMMs), 1 providing a significant boost to efforts on developing synthetic routes to such molecules as well as understanding the relationship between their molecular structures and magnetic properties. The characteristic magnetic behavior of SMMs is that a slow relaxation of the magnetization of purely molecular origin is observed, and this usually derives from the combination of a nonzero ground-state spin (S) and the large and negative magnetoanisotropy of an Ising and easy-axis type, as can be quantified by an overall molecular axial zero-field- splitting parameter D. 1,2 These molecules behave as magnets below a blocking temperature (T B ) and exhibit hysteresis in magnetization versus direct-current (dc) field scans. These hysteresis loops display increasing coercivity with decreasing temperature and increasing field sweep rates, which is taken as the characteristic signature of SMMs. Recently, particular attention has been directed toward synthesizing heterometallic complexes featuring both 3d and 4f block elements, and the distinct coordination behaviors of the different metal ions have been observed in a large number of stunningly beautiful complexes. 3 Although incorporation of lanthanide ions repre- sents an effective way of introducing magnetic anisotropy into a coordination cluster because of the large orbital contributions from such ions, a major drawback is the weak coupling between the molecular building blocks. 4 In general, the coupling between atomic spins arising from unpaired d electrons is typically on the order of 10−100 K, whereas the coupling between atomic spins arising from unpaired f electrons is ca. 1 K and the coupling between atomic spins arising from unpaired d and f electrons is expected to be less than 10 K. The first investigation of the magnetic properties of a heterometallic 3d−4f complex was reported by Gatteschi et al., who thoroughly characterized and Received: April 28, 2013 Published: October 3, 2013 Article pubs.acs.org/IC © 2013 American Chemical Society 11767 dx.doi.org/10.1021/ic401011d | Inorg. Chem. 2013, 52, 11767−11777