Single-molecule magnets of ferrous cubes: Structurally controlled magnetic anisotropy

Tetranuclear Fe^II cubic complexes were synthesized with Schiff base ligands bridging the Fe^II 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 1-6, the alkoxo groups of the Schiff base ligands bridge four Fe^II ions in a μ₃-mode forming [Fe₄O ₄] cubic cores. The Fell ions in the cubes have tetragonally elongated octahedral coordination geometries, and the equatorial coordination bond lengths in 4-6 are shorter than those in 1-3. Dc magnetic susceptibility measurements for 1-6 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 1-6, respectively. Ac magnetic susceptibility measurements for 4-6 showed both frequency dependent in- and out-of-phase signals, while 1-3 did not show out-of-phase signals down to 1.8 K, meaning 4-6 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 4-6. The bridging ligands R-sap^2- in 1-3 and R-sae^2- in 4-6 form six- and five-membered chelate rings, respectively, which cause different steric strain and Jahn-Teller distortions at Fe^II centers. The sign of the D value was discussed by using angular overlap model (AOM) calculations for irons with different coordination geometry.