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

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 1−6, 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 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 FeII centers. The sign of the D value was discussed by using angular overlap model (AOM) calculations for irons with different coordination geometry

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

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 1−6, 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 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 FeII centers. The sign of the D value was discussed by using angular overlap model (AOM) calculations for irons with different coordination geometry

Antiferromagnetic Fe^III₆ Ring and Single-Molecule Magnet Mn^II₃Mn^III₄ Wheel

Reactions of a quadridentate ligand [N-(2-hydroxy-5-nitrobenzyl)iminodiethanol] with iron and manganese chloride in methanol yielded an antiferromagnetic FeIII6 ring and a single-molecule magnet MnII3MnIII4 wheel, respectively

Rotational Motion and Nuclear Spin Interconversion of H₂O Encapsulated in C₆₀ Appearing in the Low-Temperature Heat Capacity

The heat capacity of H2O encapsulated in fullerene C60 is determined for the first time at temperatures between 0.6 and 200 K. The water molecule in H2O@C60 undergoes quantum rotation at low temperature, and the ortho-H2O and para-H2O isomers are identified by labeling the rotational energy levels with the nuclear spin states. A rounded heat capacity maximum is observed at ∼2 K after rapid cooling due to splitting of the rotational JKaKc = 101 ground state of ortho-H2O. This anomalous feature decreases in magnitude over time, reflecting the conversion of ortho-H2O to para-H2O. Time-dependent heat capacity measurements at constant temperature reveal three nuclear spin conversion processes: a thermally activated transition with Ea ≈ 3.2 meV and two temperature-independent tunneling processes with time constants of τ1 ≈ 1.5 h and τ2 ≈ 11 h

Antiferromagnetic Fe^III₆ Ring and Single-Molecule Magnet Mn^II₃Mn^III₄ Wheel

Reactions of a quadridentate ligand [N-(2-hydroxy-5-nitrobenzyl)iminodiethanol] with iron and manganese chloride in methanol yielded an antiferromagnetic FeIII6 ring and a single-molecule magnet MnII3MnIII4 wheel, respectively

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

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 1−6, 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 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 FeII centers. The sign of the D value was discussed by using angular overlap model (AOM) calculations for irons with different coordination geometry

Coordination-Tuned Single-Molecule-Magnet Behavior of Tb^III−Cu^II Dinuclear Systems

TbIII−CuII-based single-molecule magnet (SMM) and non-SMM were synthesized to investigate the relationship between magnetic anisotropy and the symmetry of the ligand field by the reaction of [TbCu(o-vanilate)2(NO3)3] with methoxypropylamine (MeOC3H6NH2, 1) or ethoxyethylamine (EtOC2H4NH2, 2). In both complexes, TbIII ions have a bicapped square-antiprism coordination geometry. When the TbIII ion is in a less symmetrical ligand field, it has an easy-axis anisotropy and shows SMM behavior, whereas when it is in a more symmetrical environment, it has an easy-plane anisotropy and exhibits non-SMM behavior

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

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 1−6, 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 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 FeII centers. The sign of the D value was discussed by using angular overlap model (AOM) calculations for irons with different coordination geometry

Preparation and Properties of Cyclopentadienyl- and Pentamethylcyclopentadienyl−Titanium(IV) Complexes with the C₈H₄S₈ Ligand, Electrical Conductivities of Their Oxidized Species, and X-ray Crystal Structure of Ti(C₅Me₅)₂(C₈H₄S₈)

Ti(C5H5)2(C8H4S8) (1), Ti(C5Me5)2(C8H4S8) (2), [NMe4][Ti(C5H5)(C8H4S8)2] (3), and [NMe4][Ti(C5Me5)(C8H4S8)2] (4) [C8H4S82- = 2-{(4,5-ethylenedithio)-1,3-dithiole-2-ylidene}-1,3-dithiole-4,5-dithiolate(2−)] were prepared by reaction of Ti(C5H5)2Cl2, Ti(C5Me5)2Cl2, Ti(C5H5)Cl3, or Ti(C5Me5)Cl3 with Li2C8H4S8 or [NMe4]2[C8H4S8] in THF. They were oxidized by iodine, the ferrocenium cation, or TCNQ (7,7,8,8-tetracyano-p-quinodimethane) in CH2Cl2 or in acetone to afford one-electron-oxidized and over-one-electron-oxidized species, [Ti(C5H5)2(C8H4S8)]·I3, [Ti(C5H5)2(C8H4S8)][PF6], [Ti(C5Me5)2(C8H4S8)]·I3, [Ti(C5Me5)2(C8H4S8)][PF6], [Ti(C5H5)(C8H4S8)2]·I0.9, [Ti(C5H5)(C8H4S8)2][TCNQ]0.3, [Ti(C5Me5)(C8H4S8)2]·I2.4, and [Ti(C5Me5)(C8H4S8)2][TCNQ]0.3, with the C8H4S8 ligand-centered oxidation. They exhibited electrical conductivities of 1.6 × 10-1 to 7.6 × 10-4 S cm-1 measured for compacted pellets at room temperature. The crystal structure of 2 was clarified to consist of isolated dimerized units of the molecules through some sulfur−sulfur nonbonded contacts:  monoclinic, P21/c, a = 9.534(2) Å, b = 18.227(2) Å, c = 17.775(2) Å, β = 94.39(1)°, Z = 4

Unexpected Rise of Glass Transition Temperature of Ice Crystallized from Antifreeze Protein Solution

Antifreeze protein (AFP) is known to bind to a single ice crystal composed of hexagonally arranged waters, hexagonal ice. To investigate the effect of the AFP binding to a general ice block that is an assembly of numerous hexagonal ice crystals, thermodynamic properties, dynamics, and the crystal structure of the ice block were examined in the presence of type I AFP (AFP-I). Previously, it was found that hexagonal ice has a glass transition based on the proton ordering in the ice lattice at low temperature. Measurements of heat capacity under adiabatic conditions, dielectric permittivity, and powder X-ray diffraction revealed that the glass transition occurs around 140 K in the ice containing 0.01–1% (w/w) of the AFP-I, which is greater than the value for the pure hexagonal ice (ca. 110 K). These data imply that AFP affects the glass transition kinetics, i.e., the slowness of the proton migration in the ice block. Hence, adsorption of AFP molecules to each hexagonal ice is thought to change the physicochemical properties of the bulk ice