Unexpectedly Strong Magnetic Anisotropy in a Mononuclear Eight-Coordinate Cobalt(II) Complex: a Theoretical Exploration

Ab initio methods have been used to explore the unexpectedly strong magnetic anisotropy and the magnetostructural correlations in mononuclear eight-coordinate complex [Co^II(12-crown-4)₂]²⁺. Our calculations showed that both decreasing α and increasing φ may enhance its magnetic anisotropy, which was rationalized by the qualitative theory proposed by Long and co-workers. Moreover, we deduced that the |<i>D</i>| value of [Co^II(12-crown-4)₂]²⁺ with α = 52° and φ = 43° is the largest one

Series of Benzoquinone-Bridged Dicobalt(II) Single-Molecule Magnets

Mononuclear complexes within a particular coordination geometry have been well recognized for high-performance single-molecule magnets (SMMs), while the incorporation of such well-defined geometric ions into multinuclear complexes remains less explored. Using the rigid 2-(di(1H-pyrazol-1-yl)methyl)-6-(1H-pyrazol-1-yl)pyridine (PyPz3) ligand, here, we prepared a series of benzoquinone-bridged dicobalt(II) SMMs [{(PyPz3)Co}2(L)][PF6]2, (1, L = 2,5-dioxo-1,4-benzoquinone (dhbq2–); 2, L = chloranilate (CA2–); and 3, L = bromanilate (BA2–)), in which each Co(II) center adopts a distorted trigonal prismatic (TPR) geometry and the distortion increases with the sizes of 3,6-substituent groups (H (1) 2) 3)). Accordingly, the magnetic study revealed that the axial anisotropy parameter (D) of the Co ions decreased from −78.5 to −56.5 cm–1 in 1–3, while the rhombic one (E) increased significantly. As a result, 1 exhibited slow relaxation of magnetization under a zero dc field, while both 2 and 3 showed only the field-induced SMM behaviors, likely due to the increased rhombic anisotropy that leads to the serious quantum tunneling of the magnetization. Our study demonstrated that the relaxation dynamics and performances of a multinuclear complex are strongly dependent on the coordination geometry of the local metal ions, which may be engineered by modifying the substituent groups

Probing the Effect of Axial Ligands on Easy-Plane Anisotropy of Pentagonal-Bipyramidal Cobalt(II) Single-Ion Magnets

We herein reported the synthetic, structural, computational, and magnetic studies of four air-stable heptacoordinated mononuclear cobalt­(II) complexes, namely, [Co^II(tdmmb)­(H₂O)₂]­[BF₄]₂ (<b>1</b>), [Co^II(tdmmb)­(CN)₂]·2H₂O (<b>2</b>), [Co^II(tdmmb)­(NCS)₂] (<b>3</b>), and [Co^II(tdmmb)­(SPh)₂] (<b>4</b>) (tdmmb = 1,3,10,12-tetramethyl-1,2,11,12-tetraaza[[3]­(2,6)­pyridino[3]­(2,9)-1,10-phenanthrolinophane-2,10-diene; SPh^– = thiophenol anion). Constrained by the rigid pentadentate macrocyclic ligand tdmmb, the Co^II centers in all of these complexes are in the heptacoordinated pentagonal-bipyramidal geometry. While the equatorial environments of these complexes remain very similar to each other, the axial ligands are systematically modified from C to N to O to S atoms. Analyses of the magnetic data and the ab initio calculations both reveal large easy-plane magnetic anisotropy (<i>D</i> > 0) for all four complexes. While the experimentally obtained <i>D</i> values do not show any clear tendency when the axial coordinated atoms change from C to N to O atoms (complexes <b>1</b>–<b>3</b>), the largest value is for the heavier and softer S-atom-coordinated complex <b>4</b>. Because of significant magnetic anisotropy, all four complexes are field-induced single-ion magnets. This work represents a delicate modification of the magnetic anisotropy by tuning the chemical environment of the metal centers

Five-Coordinated Dysprosium Single-Molecule Magnet Functionalized by the SMe Group

A five-coordinate mononuclear Dy(III) complex with a C4v geometry (square-pyramid), [Dy(X)(DBP)2(TMG(H))2] [X = 3-(methylthio)-1-propoxide, DBP = 2,6-di-tert-butylphenoxide, and TMG(H) = 1,1,3,3-tetramethylguanidine] (1), was designed and synthesized. The complex displays a large anisotropy barrier of 432 cm–1 in the absence of a dc magnetic field benefiting from the strong interaction between the phenolate and Dy(III) ion. Ab initio calculations reveal that the most possible relaxation pathway is going through the second excited state. The terminal SMe group in the apical position furnishes the possibility of depositing it on the Au surface by the strong Au–S bond

Single-Molecule Magnet Behavior Enhanced by Synergic Effect of Single-Ion Anisotropy and Magnetic Interactions

As the simplest entity carrying intramolecular magnetic interactions, a dinuclear lanthanide complex serves as a model to investigate the effects of magnetic interactions on relaxation of magnetization, and importantly, it proves to be an efficient method to obtain robust single-molecule magnets via improving the communication between lanthanide centers. Here, three Dy₂ complexes (<b>1</b>, <b>2</b>, <b>3</b>) with a similar structural motif, namely, [Dy₂­(HL)₂­(NO₃)₂­(CH₃CN)₂]·2CH₃­CN (<b>1</b>), [Dy₂­(HL)₂­(NO₃)₂­(DMF)₂]·2H₂O (<b>2</b>), and Dy₂­(HL)₂­(NO₃)₂­(DMF)₄ (<b>3</b>), were successfully assembled. One critical difference found in this series of complexes is that the Dy center in complex <b>3</b> is coordinated by one more solvent molecule. Surprisingly, complex <b>3</b> exhibits the best magnet-like behavior, as evidenced by the high effective barrier and butterfly-type hysteresis, although the crystal field effect around Dy ions is weakened heavily. <i>Ab initio</i> calculations revealed the crucial reason is the significant synergic effect between single-ion anisotropy and magnetic interactions, i.e., not only the axiality of the Dy ion is improved efficiently but also the exchange magnetic interactions increased to the same order of magnitude to the dipolar interaction in <b>3</b>. This effect mainly benefits from the elaborate modification of the local coordinate environment around the Dy ion, which results in a special arrangement of anisotropy axes different from the other two complexes. It demonstrates that the magnetic interactions could be effectively enhanced by means of deliberate local structural modulation

Magnetic Anisotropy from Trigonal Prismatic to Trigonal Antiprismatic Co(II) Complexes: Experimental Observation and Theoretical Prediction

A family of trigonal antiprismatic Co­(II) complexes was synthesized, which exhibited field-induced Raman process dominated single-molecule magnet behavior. Despite the coordination environment of Co­(II) being of similar symmetry, the four complexes exhibit distinct dynamic magnetic properties owing to their packing arrangements and dipole–dipole interactions. On the basis of computational results we have demonstrated that the <i>g</i>_<i>z</i> and <i>g</i>_iso values follow a cosine relation with respect to the rotated angle φ (twist angle φ defined as the rotation angle of one coordination square away from the eclipse conformation to the other)

A new <i>β</i>-diketonate Dy(III) single‒ion magnet featuring multiple magnetic relaxation processes

<p>A <i>β</i>-diketonate mononuclear dysprosium compound, [Dy(TFNB)₃(bpy)] (<b>1</b>) (TFNB = 4,4,4-trifluoro-1-(2-naphthyl)-1,3-butanedione, bpy = 2,2′-bipyridine), has been prepared and structurally and magnetically characterized. X-ray crystallographic analysis reveals that <b>1</b> contains two crystallographically equivalent Dy(III) ions of which the eight-coordinate geometries uniformly behave as distorted square antiprismatic configurations (<i>D</i>_4d). Magnetic investigations demonstrate that <b>1</b> displays dual relaxation processes of SMMs behavior with the effective barrier (Δ<i>E</i>/<i>k</i>_B) of 23.44 K under 1200 Oe DC field, corresponding to the coexistence of two metal centers in the structure of the compound. The comparative studies of some Dy(III)-based SIMs with TFNB ligand have been conducted as well. <i>Ab initio</i> studies demonstrate that the Kramers doublet ground state is predominantly axial with the <i>g</i>_z tensors of two Dy(III) fragments matching the Ising-limit factor (20) anticipated for the pure <i>M</i>_J = ±15/2 state.</p

Magnetic Relaxation Dynamics of a Centrosymmetric <b>Dy₂</b> Single-Molecule Magnet Triggered by Magnetic-Site Dilution and External Magnetic Field

A centrosymmetric <b>Dy</b>_<b>2</b> single-molecule magnet (SMM) and its doped diamagnetic yttrium analogues, <b>Dy</b>_<b>0.19</b><b>Y</b>_<b>1.81</b> and <b>Dy</b>_<b>0.10</b><b>Y</b>_<b>1.90</b>, were solvothermally synthesized to investigate the effects of intramolecular exchange coupling and quantum tunneling of magnetization (QTM) on the magnetic relaxation dynamics. Constructed from two hula-hoop-like Dy^III ions and a pair of phenoxido groups, the antiferromagnetically coupled <b>Dy</b>_<b>2</b> exhibits a thermal-activated zero-field effective energy barrier (<i>U</i>_eff) of 277.7 K and negligible hysteresis loop at 2.0 K. The doping of a diamagnetic Y^III matrix with 90.5% and 95.0% molar ratios reveals the single-ion origin of the Orbach channel, increases the relaxation time by partially quenching the QTM process, and induces an open hysteresis loop until 5.0 K. In contrast, an optimal dc field of 1.0 kOe improves the barrier height up to 290.1 K through complete elimination of the QTM and delays the relaxation time of the direct relaxation pathway. More interestingly, the collaborative dual effects of magnetic-site dilution and external magnetic field make the effective energy barrier and relaxation time increase 8.1% and 49 times, respectively. Thus, the overall magnetization dynamics of the <b>Dy</b>_<b>2</b> system systematically elaborate the inherent interplay of the QTM and Orbach processes on the effective energy barrier, highlighting the vital role of the relaxation time on the coercive hysteresis loop

Modulating Slow Magnetic Relaxation of Dysprosium Compounds through the Position of Coordinating Nitrate Group

A chain complex [Dy­(<b>L</b>)­(NO₃)₂CH₃OH]_<i>n</i> (<b>1</b>) and a dinuclear compound [Dy₂(<b>L</b>)₂(NO₃)₄(CH₃OH)₂]·2CH₃OH (<b>2</b>) were synthesized by the assembly of a novel pyridine-<i>N</i>-oxide-containing ligand with dysprosium nitrate under different reaction temperatures, where two coordinating nitrates are located in para or ortho position with respect to each other around dysprosium ions. Magnetic studies indicate that the chain complex with two para-coordinating nitrates shows fast quantum tunnelling of the magnetization under zero direct-current field, while the dinuclear complex with two ortho-coordinating nitrates exhibits a thermal-activated process with an effective energy barrier of 51 K. Theoretical and magneto-structural correlation studies indicate that position change of coordinating nitrates can significantly modulate the crystal field around dysprosium ion and further lead to their different relaxation behaviors

Assembling Dysprosium Dimer Units into a Novel Chain Featuring Slow Magnetic Relaxation via Formate Linker

A dinuclear complex [DyLCl­CH₃­OH)]₂ (<b>1</b>) and a one-dimensional compound [DyL­(HCOO)­(CH₃­OH)]_<i>n</i> (<b>2</b>) have been synthesized using an organic ligand of <i>N</i>′-(2-hydroxy­benzyl­idene)­picolino­hydra­zide (H₂L). Complex <b>1</b> exhibits a symmetric dinuclear structure, in which the Dy³⁺ centers reside in a pentagonal-bipyramidal coordination environment. In <b>2</b>, the dinuclear units of <b>1</b> are strung into chains by formate anions, in which Dy³⁺ ions are situated in an octa-coordinated, hula-hoop-like coordination geometry. Magnetic studies reveal that ferromagnetic coupling is found between Dy³⁺ ions in both compounds. Complexes <b>1</b> and <b>2</b> exhibit slow magnetic relaxation under zero dc field with effective energy barriers of 88.4 and 175.8 K, respectively. Magnetic study combined with ab initio calculations indicates that the better performance of <b>2</b> is related to the unique molecular geometry and relatively stronger Dy³⁺–Dy³⁺ magnetic interaction within and/or between the dimer units