Colette Boskovic, Hans U. Güdel, Gaël Labat, Antonia Neels, Wolfgang Wernsdorfer,
Boujemaa Moubaraki and Keith S. Murray
High-Nuclearity Mixed-Chelate Ferric Complexes from a New Family of Polynuclear Precursors
Inorg. Chem. 44, 3181-3189 (2005)
Abstract:
The syntheses, structures, and magnetochemical characterization of two novel
mixed-chelate undeca- and dodecanuclear ferric complexes are reported. Preformed
tri- and pentanuclear ferric complexes that possess tridentate Schiff base
(L2- and (L')2-) and acetate ligands
were reacted with 1,1,1-tris(hydroxymethyl)ethane (H3thme) to afford
[Fe11O3(OH)(O2CMe)8(thme)2(L)6]
(1) and
[Fe12O4(O2CMe)8(thme)2(NH2(CH2)2O)2(L')6]
(2), respectively, following structural agglomeration and rearrangement
associated with ligand substitution. The incorporation of more than one type of
ligand that can both chelate and bridge the Fe centers gives rise to the
complicated molecular structures displayed by 1 and 2. As a result
of the tripodal conformation of thme3-, the cores of both
molecules incorporate several face-shared defect
{Fe3O4}+ cuboidal subunits.
Variable-temperature dc and ac magnetic susceptibility studies, together with
low-temperature magnetization measurements, are consistent with S = 5/2
and S = 0 ground-state spins for 1 and 2, respectively, and
suggest that excited states with higher spin values lie relatively close in
energy to the ground state for both species. Low-temperature micro-SQUID
measurements on oriented single crystals of 1 confirm the easy-axis type
magnetic anisotropy suggested by conventional SQUID magnetometry. However
magnetization hysteresis is not observed down to 0.04 K, which is ascribed to
rapid quantum tunneling of the magnetization associated with transverse
interactions.