Yi-Quan Zhang

Zero-field slow magnetic relaxation from single Co(II) ion: a transition metal single-molecule magnet with high anisotropy barrier

An air-stable star-shaped CoIICoIII3 complex with only one paramagnetic Co(II) ion in the D3 coordination environment has been synthesized from a chiral Schiff base ligand. Magnetic studies revealed that this complex exhibits slow magnetic relaxation in the absence of an applied dc field, which is one of the main characteristics of single-molecule magnets (SMMs). The relaxation barrier is as high as 109 K, which is quite large among transition-metal ion-based SMMs. The complex represents the first example of zero-field SMM behavior in a mononuclear six oxygen-coordinate Co(II) complex.

Slow Magnetic Relaxation in a Mononuclear Eight-Coordinate Cobalt(II) Complex

The quest for the single-molecular magnets (SMMs) based on mononuclear transition-metal complexes is focused on the low-coordinate species. No transition-metal complex with a coordination number of eight has been shown to exhibit SMM properties. Here the magnetic studies have been carried out for a mononuclear, eight-coordinate cobalt(II)-12-crown-4 (12C4) complex [Co(II)(12C4)2](I3)2(12C4) (1) with a large axial zero-field splitting. Magnetic measurements show field-induced, slow magnetic relaxation under an applied field of 500 Oe at low temperature. The magnetic relaxation time τ was fitted by the Arrhenius model to afford an energy barrier of Ueff = 17.0 cm(-1) and a preexponential factor of τ0 = 1.5 × 10(-6) s. The work here presents the first example of the eight-coordinate, mononuclear, 3d metal complex exhibiting the slow magnetic relaxation.

Two-Coordinate Co(II) Imido Complexes as Outstanding Single-Molecule Magnets

The pursuit of single-molecule magnets (SMMs) with better performance urges new molecular design that can endow SMMs larger magnetic anisotropy. Here we report that two-coordinate cobalt imido complexes featuring highly covalent Co═N cores exhibit slow relaxation of magnetization under zero direct-current field with a high effective relaxation barrier up to 413 cm-1, a new record for transition metal based SMMs. Two theoretical models were carried out to investigate the anisotropy of these complexes: single-ion model and Co-N coupling model. The former indicates that the pseudo linear ligand field helps to preserve the first-order orbital momentum, while the latter suggests that the strong ferromagnetic interaction between Co and N makes the [CoN]+ fragment a pseudo single paramagnetic ion, and that the excellent performance of these cobalt imido SMMs is attributed to the inherent large magnetic anisotropy of the [CoN]+ core with |MJ = ± 7/2⟩ ground Kramers doublet.

A Family of CoIICoIII3 Single-Ion Magnets with Zero-Field Slow Magnetic Relaxation: Fine Tuning of Energy Barrier by Remote Substituent and Counter Cation

The synthesis, structures, and magnetic properties of a family of air-stable star-shaped Co(II)Co(III)3 complexes were investigated. These complexes contain only one paramagnetic Co(II) ion with the approximate D3 coordination environment in the center and three diamagnetic Co(III) ions in the peripheral. Magnetic studies show their slow magnetic relaxation in the absence of an applied dc field, which is characteristic behavior of single-molecule magnets (SMMs), caused by the individual Co(II) ion with approximate D3 symmetry in the center. Most importantly, it was demonstrated that the anisotropy energy barrier can be finely tuned by the periphery substituent of the ligand and the countercation. The anisotropy energy barrier can be increased significantly from 38 K to 147 K.

Six‐Coordinate Lanthanide Complexes: Slow Relaxation of Magnetization in the Dysprosium(III) Complex

A series of six-coordinate lanthanide complexes {(H3O)[Ln(NA)2]⋅H2O}n (H2NA=5-hydroxynicotinic acid; Ln=Gd(III) (1⋅Gd); Tb(III) (2⋅Tb); Dy(III) (3⋅Dy); Ho(III) (4⋅Ho)) have been synthesized from aqueous solution and fully characterized. Slow relaxation of the magnetization was observed in 3⋅Dy. To suppress the quantum tunneling of the magnetization, 3⋅Dy diluted by diamagnetic Y(III) ions was also synthesized and magnetically studied. Interesting butterfly-like hysteresis loops and an enhanced energy barrier for the slow relaxation of magnetization were observed in diluted 3⋅Dy. The energy barrier (Δ(τ)) and pre-exponential factor (τ0) of the diluted 3⋅Dy are 75 K and 4.21×10(-5) s, respectively. This work illustrates a successful way to obtain low-coordination-number lanthanide complexes by a framework approach to show single-ion-magnet-like behavior.

Cobalt(II) coordination polymer exhibiting single-ion-magnet-type field-induced slow relaxation behavior.

A one-dimensional cobalt(II) coordination polymer, [Co(btm)2(SCN)2·H2O]n [btm = bis(1H-1,2,4-triazol-1-yl)methane], was synthesized and magnetically characterized. The isolated slightly distorted octahedral Co(II) ion displays field-induced slow relaxation with a big positive axial and a negative rhombic magnetic anisotropy (D = 93.9 cm(-1) and E = -10.5 cm(-1)), and the anisotropy energy barrier is 45.4 K.

The slow magnetic relaxation regulated by ligand conformation of a lanthanide single-ion magnet [Hex4N][Dy(DBM)4]

A mononuclear Dysprosium(III) complex [Hex4N][Dy(DBM)4] (1) was synthesized using dibenzoylmethane (DBM) anion ligand with a tetrahexylammonium (Hex4N+) cation balancing the charge. Complex 1 was structurally and magnetically characterized. The local geometry of Dy(III) ions is close to the ideal D4d symmetry. The temperature and frequency-dependent out-of-phase ac susceptibility peaks were observed in the absence of a static dc field. The relaxation energy barrier Ueff = 27.7 K and τ0 = 1.3 × 10−7 s were obtained by Arrhenius fitting. It is interesting that the quantum tunneling of the magnetization was suppressed when two optimum dc fields (300 and 1500 Oe) were applied. Two distinct thermal relaxation processes were observed with Ueff = 56.6 K, τ0 = 6.6 × 10−10 s for 300 Oe and Ueff = 68.1 K, τ0 = 3.4 × 10−11 s and Ueff = 88.0 K, τ0 = 5.0 × 10−10 s for 1500 Oe. The two thermal relaxation processes were also recognized clearly under zero dc field for the analogue with 20 times magnetic site dilution by Y(III). Nevertheless there is only one crystallographically independent Dy(III) ion in this system. Further inspection of the crystallographic structure reveals that the benzene disorder within the conjugated system of the β-diketonate ligand could change the delocalized electron distribution on the carbonyl coordination oxygen atoms and result in small different ligand fields, which account for the multiple relaxation processes. Ab initio calculations confirm the two energy barriers derived from two disordered structures.

Field-Induced Slow Magnetic Relaxation and Gas Adsorption Properties of a Bifunctional Cobalt(II) Compound

A new compound, {[Co(bmzbc)2] · 2DMF}n (JXNU-1, JXNU denotes Jiangxi Normal University), based on the 4-(benzimidazole-1-yl)benzoate (bmzbc(-)) ligand has been synthesized and structurally characterized. The Co(II) ions are bridged by the rod-like bmzbc(-) ligands to give a two-dimensional (2D) sheet wherein the Co(II) ions are spatially separated from each other by the long bmzbc(-) rods. The 2D sheets are further stacked into a 3D framework with 1D channels occluding the guest DMF molecules. Detailed magnetic studies show that the individual octahedral Co(II) ions in JXNU-1 exhibit field-induced slow magnetic relaxation, which is characteristic behavior of single-ion magnets (SIMs). The rarely observed positive value of zero-field splitting (ZFS) parameter D for the Co(II) ion in JXNU-1 demonstrates that JXNU-1 is a unique example of Co(II)-based SIMs with easy-plane anisotropy, which is also confirmed by the calculations. The microporous nature of JXNU-1 was established by measuring CO2 sorption isotherms. The abrupt changes observed in the C3H8 and C2H6 adsorption isotherms indicate that a structural transformation occurred in the gas-loading process. The long connection between the magnetic metal centers in JXNU-1 meets the requirements for construction of porosity and SIM in a well-defined network, harmoniously providing a good candidate of functional molecular materials exhibiting SIM and porosity.

Can Non‐Kramers TmIII Mononuclear Molecules be Single‐Molecule Magnets (SMMs)?

In recent years, plentiful lanthanide-based (Tb(III) , Dy(III) , and Er(III) ) single-molecule magnets (SMMs) were studied, while examples of other lanthanides, for example, Tm(III) are still unknown. Herein, for the first time, we show that by rationally manipulating the coordination sphere, two thulium compounds, 1[(Tp)Tm(COT)] and 2[(Tp*)Tm(COT)] (Tp=hydrotris(1-pyrazolyl)borate; COT=cyclooctatetraenide; Tp*=hydrotris(3,5-dimethyl-1-pyrazolyl)borate), can adopt the structure of non-Kramers SMMs and exhibit their behaviors. Dynamic magnetic studies indicated that both compounds showed slow magnetic relaxation under dc field and a relatively high effective energy barrier (111 K for 1, 46 K for 2). Magnetic diluted 1 a[(Tp)Tm0.05 Y0.95 (COT)] and 2 a[(Tp*)Tm0.05 Y0.95 (COT)] even exhibited magnetic relaxation under zero dc field. Relativistic ab initio calculations combined with single-crystal angular-resolved magnetometry measurements revealed the strong easy axis anisotropy and nearly degenerated ground doublet states. The comparison of 1 and 2 highlights the importance of local symmetry for obtaining Tm SMMs.

π–π Stacking, spin density and magnetic coupling strength

The π-π stacking interaction, one of the main intermolecular forces, sometimes leads to amazing magnetic properties. Although the concept has been raised that spin density is one of the main factors that contribute to the magnetic coupling strength in intermolecular magnetic coupling systems, it has not been confirmed either experimentally or theoretically to date. Herein we present a study on the magnetostructural data of seven unpublished Cu(II) complexes and ten reported radicals. It is confirmed for the first time that the spin density on short contact atoms is a major factor that contributes to the π-π stacking magnetic coupling strength. Based on the reported data to date, when the short contact distance is larger than the default contact radius, medium or relatively strong magnetic coupling strength could be obtained only if the spin density on the short contact atoms is greater than 0.1350; when the C···C short contact is less than the default contact radius of 3.4 Å, but not less than 3.351 Å, and the spin density is less than 0.1, neither medium nor strong magnetic coupling strength could be observed. Further, when the short contact distance decreases with a temperature drop, the spin densities on the relevant short contact atoms increase. In the complexes reported the small spin densities on the relevant short contact atoms are the major factors that result in the weak π-π magnetic coupling strength.