Dayu Wu

Field-induced slow relaxation of magnetization in a tetrahedral Co(II) complex with easy plane anisotropy

The mononuclear Co(II) complex CoBr (dmph = 2,9-dimethyl-1,10-phenanthroline) was obtained and X-ray structurally characterized as a distorted tetrahedron environment that is responsible for the moderately strong positive anisotropy of high spin Co(II). In combination with variable-field magnetic susceptibility data at low temperature, high-field electron paramagnetic resonance (HF-EPR) spectroscopy reveals the presence of easy-plane anisotropy (D > 0) in complex CoBr. Slow magnetic relaxation effects were observed for CoBr in the presence of a dc magnetic field. At very low temperatures, ac magnetic susceptibility data show the magnetic relaxation time, τ, to be temperature-independent, while above 2.4 K thermally activated Arrhenius behavior is dominated with Ueff = 22.8(8) cm(-1) and τ0 = 3.7(5) × 10(-10) s. Upon dilution of the complex within a matrix of the isomorphous compound ZnBr, ac susceptibility data reveal the individual molecular nature of the slow magnetic relaxation and indicate that the quantum tunneling pathway observed at low temperatures is likely mediated by intermolecular dipolar interactions.

Tuning Transverse Anisotropy in Co-III-Co-II-Co-III Mixed-Valence Complex toward Slow Magnetic Relaxation

Two cobalt mixed-valence complexes with different substituents have been prepared and structurally characterized by single-crystal X-ray diffraction to alter slow magnetic relaxation by tailoring the transverse anisotropy. The trinuclear complexes [(L(1))4Co3(H2O)2](NO3)4·CH3OH·5H2O (1-NO3) and [(L(2))4Co3(H2O)2](NO3)4·6H2O (2-NO3) feature a distorted octahedral Co(II) strongly hindered in a trinuclear Co(III)-Co(II)-Co(III) mixed-valence array. Detailed magnetic studies of 1-NO3 and 2-NO3 have been conducted using direct- and alternating-current magnetic susceptibility data. In accordance with variable-field magnetic susceptibility data at low temperatures, high-field electron paramagnetic resonance (HF-EPR) spectroscopy reveals the presence of an easy-plane anisotropy (D > 0) with a significant transverse component, E, in complexes 1-NO3 and 2-NO3. These findings indicate that the onset of the variation of distortion within complex 2-NO3 leads to a suppression of quantum tunneling of the magnetization within the easy plane, resulting in magnetic bistability and slow relaxation behavior. Consequently, the anisotropy energy scale associated with the relaxation barrier, 5.46 cm(-1) (τo = 1.03 × 10(-5) s), is determined by the transverse E term. The results demonstrate that slow magnetic relaxation can be switched through optimization of the transverse anisotropy associated with magnetic ions that possess easy-plane anisotropy.

CoII Molecular Square with Single-Molecule Magnet Properties

A new tetranuclear cobalt(II) molecular square in which adjacent Co(II) centers are linked by a mu(2)-bridging oxygen atom and a N-N bridge along the edges of the square has been designed for single-molecule magnets (SMMs) with high anisotropy barriers. The overall intramolecular ferromagnetic coupling at low temperature combined with the slow relaxation at static zero fields suggests a SMM behavior for this molecular square. The zero-field cooled magnetization (ZFCM) and field cooling magnetization (FCM) at 10 Oe illustrate the nonreversibility and bifurcation below 4.5 K. The deviations of magnetization from the saturated value in strong applied fields demonstrate the participation of low-lying excited states. The peaks of the out-of-phase signals are observed corresponding to coincidence of the applied ac field oscillation frequency with the relaxation rate.

Modifying Fe3O4 microspheres with rhodamine hydrazide for selective detection and removal of Hg2+ ion in water

Rhodamine hydrazide modifying Fe(3)O(4) microspheres (Fe(3)O(4)-R6G) was developed for selective detection and removal of mercury ion from water. With a saturation magnetization of 74.2 emu/g, the Fe(3)O(4)-R6G could be simply recollected from water with magnetic separation within a few minutes. The selectivity and adsorption ability of Fe(3)O(4)-R6G for metal cations were studied by fluorophotometry and atomic absorption spectrometry, respectively. The results showed that Fe(3)O(4)-R6G exhibited excellent selectivity for sensing mercury ion over other metal ions in aqueous solution, and also adsorbed 91% of mercury ion. The maximum adsorption capacity of the Fe(3)O(4)-R6G for Hg(2+) ion was 37.4 μmol g(-1). The Fe(3)O(4)-R6G was successfully applied to the determination of Hg(2+) in environmental samples, and could be used repeatedly by treatment with tetrabutylammonium hydroxide.

Colorimetric detection of copper and efficient removal of heavy metal ions from water by diamine-functionalized SBA-15

SBA-15 functionalized with N-[3-(trimethoxysilyl)propyl]ethylene-diamine (TPED) was synthesized and used for the colorimetric detection of Cu(2+) and removal of heavy metal ions in aqueous solutions. Compared to free SBA-15, the adsorption ability of diamine-functionalized SBA-15 (depicted as SBA-TPED) increased remarkably, the maximum adsorption capacity of SBA-TPED for Cu(2+), Pb(2+) and Zn(2+) was 27.22, 96.43 and 12.16 mg g(-1), respectively. Furthermore, SBA-TPED exhibits high selectivity for Cu(2+) with the relative selectivity coefficient of SBA-TPED for Cu(2+)/Pb(2+) being over 10 and for Cu(2+)/Zn(2+) being over 60. The naked-eye detection limit of SBA-TPED for Cu(2+) is 0.95 ppm, and the determination of Cu(2+) in real water samples also displays satisfactory results. Moreover, SBA-TPED possesses fast kinetics for removing Cu(2+) with a saturation time of less than 30 min, and can be regenerated by simple acid treatment.

Metallogrid Single-Molecule Magnet: Solvent-Induced Nuclearity Transformation and Magnetic Hysteresis at 16 K

Structural assembly and reversible transformation between a metallogrid Dy4 SMM (2) and its fragment Dy2 (1) were established in the different solvent media. The zero-field magnetization relaxation was slowed for dysprosium metallogrid (2) with relaxation barrier of Ueff = 61.3 K when compared to Dy2 (1). Both magnetic dilution and application of a moderate magnetic field suppress ground-state quantum tunneling of magnetization and result in an enhanced Ueff of 119.9 and 96.7 K for 2, respectively. Interestingly, the lanthanide metallogrid complex (2) exhibits magnetic hysteresis loop even up to 16 K at a given field sweep rate of 500 Oe/s.

A Highly Stable Nanotubular MOF Rotator for Selective Adsorption of Benzene and Separation of Xylene Isomers.

A remarkably stable tubular 3D Zn-MOF with hexagonal channels and a rare ptr topology was prepared under solvothermal conditions for liquid and vapor phase adsorption and separation of the C6-8 aromatic compounds. The material showed preferential affinity for benzene and can effectively separate benzene from its organic analogues under ambient conditions in both vapor and liquid phases. Furthermore, it exhibited preferable uptake of p-xylene over other C8 xylenes.

An Efficient Blue-Emissive Metal–Organic Framework (MOF) for Lanthanide-Encapsulated Multicolor and Stimuli-Responsive Luminescence

A novel blue-emitting Zn(II) MOF featuring parallel 2D+2D interpenetrated layers and tubelike channels was generated and shown to efficiently accommodate lanthanide(III) cations (Ln3+ = Eu3+, Tb3+, or a mixture of Eu3+/Tb3+), resulting in the Ln3+-encapsulated functional materials with a tunable emission color, including red, green, and nearly pure white light. Furthermore, the thermal-responsive luminescence was investigated for the lanthanide-codoped MOF to exhibit the chromic transition from white at room temperature to blue around liquid nitrogen temperature.

Redox Modulation of Spin Crossover within a Cobalt Metallogrid.

A self-assembled cobalt molecular grid of a pyrazine-bridged bis-tridentate ligand (L(R)), where R = H (1), CH3 (2), and Br (3), was prepared and structurally characterized. Depending on the electronic effects of the substituents on the ligand, the redox of the metal center was systematically modulated, and the magnetic behavior from essentially high-spin Co(II) in 3 versus completely diamagnetic Co(III) in 1 to Co(II) spin-crossover in 2 can be achieved.

Rhodamine-based field-induced single molecule magnets in Yb(III) and Dy(III) series

The reaction between the rhodamine-6G-2-(hydrozinomethyl) quinolin-8-ol (HQR1) ligand and Ln(tta)3·2H2O precursors (tta = 2-thenoyltrifluoroacetone) leads to the formation of a series of mononuclear complexes with formula [Ln(QR1)(tta)2]·(CH3OH)x(H2O)y (3–6) for (Ln = Yb, x = 1, y = 0 for 3; Ln = Dy, x = 1, y = 0.5 for 4; Ln = Tb, x = 2, y = 0 for 5; Ln = Ho, x = 2, y = 0 for 6) together with [Dy(QR1)2][NO3]·(CH3OH)(H2O) (2) and the reported [Yb(QR1)2][NO3]·(CH3OH)(H2O)0.5 (1), for the purpose of magnetic comparison. Their X-ray structures revealed that the coordination environment of each Ln(III) center is filled by two tta carboxylate groups and a tetrachelate N2O2 binding site coming from the deprotonated HQR1 ligand. The Yb and Dy complexes showed the field-induced slow relaxation of magnetization. Both Yb(III)-containing compounds were characterized by X-band EPR and magnetism studies, which revealed the different effective g values and slow paramagnetic relaxation. Comparison between two Yb(III) complexes 1 and 3 shows that the magnetoanisotropy and the barrier height of the magnetic relaxation are sensitive to the subtle change in the coordination environment of the central metal ion. However, in Dy3+ series, QTM is difficult to overcome even under the dc field and the subtle variation of the coordination environment leads to a tiny change in the energy barrier of slow magnetization relaxation. These results show that ligand-donating ability while maintaining molecular symmetry can be controlled to design single molecule magnets with enhanced relaxation barriers.