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.