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

Abstract

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 mu3-mode forming [Fe4O4] cubic cores. The Fe(II) 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-sap2- in 1-3 and R-sae2- 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.

A Family of Enneanuclear Iron(II) Single‐Molecule Magnets

“…Detailed magnetic studies on 1-3 have shown that pseudohalide substitution in 1 has a marked effect on the magnetic properties of complexes 2 and 3, which behave as single-molecule magnets. This family of SMMs represents the second example of an iron(II) SMM family, besides the cubane complexes [Fe 4 (L) 4 A C H T U N G T R E N N U N G (MeOH) 4 ] (L= sae 2À , the dianion of 2-salicylideneamino-1-ethanol, [16] and L = 5-Br-sae or 3,5-Cl 2 -sae, [17] the respective bromo and chloro derivatives thereof), while we have recently reported [Fe 2 A C H T U N G T R E N N U N G (NCO) 3 (acpypentO)] as another Fe II SMM. [18] Results and Discussion…”

“…[16,17] However, no Mçssbauer spectra are available for these. Their preexponential factors were found to be~10 À9 s, with energy barriers of 26-30 K. With these values, we would expect relaxation effects in the Mçssbauer spectra to emerge in the range 10-15 K. Two other similar alkoxo-bridged ferrous cubes have been reported.…”

Radical ligand-containing single-molecule magnets

“…After the discovery of the first single-molecule magnet, cluster compound Mn 12 O 12 (O 2 CMe) 16 (H 2 O) 4 [31], efforts to increase the spin relaxation barrier primarily involved research aimed at synthesizing larger metal clusters with higher values of S, typically featuring oxo bridging ligands between metal centers [31][32][33][34][35][36][37][38][39][40][41][42][43][44][45][46][47][48][49]. Here, the magnetic interactions between the metal centers occur via superexchange coupling through the diamagnetic oxido unit, and these interactions can indeed reach appreciable magnitudes for such one-atom bridging ligands.…”

Synthesis, Structure, and Magnetic Properties of Two Novel Dinuclear Complexes involving Lanthanide‐phenoxo Anion Radical

“…[1][2][3][4][5][6][7] Since the discovering of the first Mn 12 SMM in 1993, [8] many transition-metal-containing (such as Mn, Fe, Ni, V, and d-f) SMMs were obtained and studied deeply. [9][10][11][12][13][14][15] The study proves energy barrier of the SMMs are related to high spin ground state (S) and significant magnetic anisotropy (D). To get SMMs with high energy barrier, S and D need be increased, but Ruiz discloses the S and D can't increase simultaneously.…”