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

The reaction of N-methydiethanolamine (mdeaH(2)), benzoic acid, FeCl3, and Ln(NO3)(3)center dot 6H(2)O or LnCl(3)center dot xH(2)O yields a series of decanuclear coordination clusters, [Ln(3)Fe(7)(mu(4)-O)(2)(mu(3)-OH)(2)(mdea)(7)(mu-benzoate)(4)(N-3)(6)]center dot 4MeCN center dot H2O, where Ln = Gd-III (1) or Tb-III (2), and [Er3Fe7(mu(4)-O)(2)(mu(3)-OH)(2)(mdea)(7)(mu-benzoate)(4)(N-3)(5)(MeOH)]Cl center dot 7.5H(2)O center dot 11.5MeOH (3). The isostructural compounds 1-3 all crystallize isotypically in the triclinic space group P (1) over bar 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-dependent direct-current magnetic susceptibility studies indicate that intracluster antiferromagnetic interactions are dominant in 1-3. The frequency-dependent out-of-phase (chi '') 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 Fe-57 Mossbauer 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 Mossbauer spectra of 2 and 3 obtained between 3 and 30 K are consistent with the presence of Fe ill 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.