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Single-Molecule Magnets of Ferrous Cubes:  Structurally Controlled Magnetic Anisotropy

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posted on 21.07.2004, 00:00 by Hiroki Oshio, Norihisa Hoshino, Tasuku Ito, Motohiro Nakano
Tetranuclear FeII cubic complexes were synthesized with Schiff base ligands bridging the FeII centers. X-ray structural analyses of six ferrous cubes, [Fe4(sap)4(MeOH)4]·2H2O (1), [Fe4(5-Br-sap)4(MeOH)4] (2), [Fe4(3-MeO-sap)4(MeOH)4]·2MeOH (3), [Fe4(sae)4(MeOH)4] (4), [Fe4(5-Br-sae)4(MeOH)4]·MeOH (5), and [Fe4(3,5-Cl2-sae)4(MeOH)4] (6) (R-sap and R-sae were prepared by condensation of salicylaldehyde derivatives with aminopropyl alcohol and aminoethyl alcohol, respectively) were performed, and their magnetic properties were studied. In 16, the alkoxo groups of the Schiff base ligands bridge four FeII ions in a μ3-mode forming [Fe4O4] cubic cores. The FeII ions in the cubes have tetragonally elongated octahedral coordination geometries, and the equatorial coordination bond lengths in 46 are shorter than those in 13. Dc magnetic susceptibility measurements for 16 revealed that intramolecular ferromagnetic interactions are operative to lead an S = 8 spin ground state. Analyses of the magnetization data at 1.8 K gave the axial zero-field splitting parameters (D) of +0.81, +0.80, +1.15, −0.64, −0.66, and −0.67 cm-1 for 16, respectively. Ac magnetic susceptibility measurements for 46 showed both frequency dependent in- and out-of-phase signals, while 13 did not show out-of-phase signals down to 1.8 K, meaning 46 are single-molecule magnets (SMMs). The energy barriers to flip the spin between up- and down-spin were estimated to 28.4, 30.5, and 26.2 K, respectively, for 46. The bridging ligands R-sap2- in 13 and R-sae2- in 46 form six- and five-membered chelate rings, respectively, which cause different steric strain and Jahn−Teller distortions at FeII centers. The sign of the D value was discussed by using angular overlap model (AOM) calculations for irons with different coordination geometry.