High-Nuclearity Mixed-Chelate Ferric Complexes from a New Family of Polynuclear Precursors

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.