34 research outputs found
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<sup>II</sup>(12-crown-4)<sub>2</sub>]<sup>2+</sup>. 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<sup>II</sup>(12-crown-4)<sub>2</sub>]<sup>2+</sup> 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 cobaltÂ(II)
complexes, namely, [Co<sup>II</sup>(tdmmb)Â(H<sub>2</sub>O)<sub>2</sub>]Â[BF<sub>4</sub>]<sub>2</sub> (<b>1</b>), [Co<sup>II</sup>(tdmmb)Â(CN)<sub>2</sub>]·2H<sub>2</sub>O (<b>2</b>), [Co<sup>II</sup>(tdmmb)Â(NCS)<sub>2</sub>] (<b>3</b>), and [Co<sup>II</sup>(tdmmb)Â(SPh)<sub>2</sub>] (<b>4</b>) (tdmmb = 1,3,10,12-tetramethyl-1,2,11,12-tetraaza[[3]Â(2,6)Âpyridino[3]Â(2,9)-1,10-phenanthrolinophane-2,10-diene;
SPh<sup>–</sup> = thiophenol anion). Constrained by the rigid
pentadentate macrocyclic ligand tdmmb, the Co<sup>II</sup> 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 Dy<sub>2</sub> complexes (<b>1</b>, <b>2</b>, <b>3</b>) with a similar structural
motif, namely, [Dy<sub>2</sub>Â(HL)<sub>2</sub>Â(NO<sub>3</sub>)<sub>2</sub>Â(CH<sub>3</sub>CN)<sub>2</sub>]·2CH<sub>3</sub>ÂCN (<b>1</b>), [Dy<sub>2</sub>Â(HL)<sub>2</sub>Â(NO<sub>3</sub>)<sub>2</sub>Â(DMF)<sub>2</sub>]·2H<sub>2</sub>O (<b>2</b>), and Dy<sub>2</sub>Â(HL)<sub>2</sub>Â(NO<sub>3</sub>)<sub>2</sub>Â(DMF)<sub>4</sub> (<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 CoÂ(II) complexes was synthesized,
which exhibited field-induced Raman process dominated single-molecule
magnet behavior. Despite the coordination environment of CoÂ(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><sub><i>z</i></sub> and <i>g</i><sub>iso</sub> 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)<sub>3</sub>(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><sub>4d</sub>). 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><sub>B</sub>) 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><sub>z</sub> tensors of two Dy(III) fragments matching the Ising-limit factor (20) anticipated for the pure <i>M</i><sub>J</sub> = ±15/2 state.</p
Magnetic Relaxation Dynamics of a Centrosymmetric <b>Dy<sub>2</sub></b> Single-Molecule Magnet Triggered by Magnetic-Site Dilution and External Magnetic Field
A centrosymmetric <b>Dy</b><sub><b>2</b></sub> single-molecule magnet (SMM) and its doped
diamagnetic yttrium analogues, <b>Dy</b><sub><b>0.19</b></sub><b>Y</b><sub><b>1.81</b></sub> and <b>Dy</b><sub><b>0.10</b></sub><b>Y</b><sub><b>1.90</b></sub>, 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<sup>III</sup> ions and a pair of phenoxido groups, the antiferromagnetically
coupled <b>Dy</b><sub><b>2</b></sub> exhibits a thermal-activated
zero-field effective energy barrier (<i>U</i><sub>eff</sub>) of 277.7 K and negligible hysteresis loop at 2.0 K. The doping
of a diamagnetic Y<sup>III</sup> 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><sub><b>2</b></sub> 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 [DyÂ(<b>L</b>)Â(NO<sub>3</sub>)<sub>2</sub>CH<sub>3</sub>OH]<sub><i>n</i></sub> (<b>1</b>) and a dinuclear compound [Dy<sub>2</sub>(<b>L</b>)<sub>2</sub>(NO<sub>3</sub>)<sub>4</sub>(CH<sub>3</sub>OH)<sub>2</sub>]·2CH<sub>3</sub>OH (<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 [DyLClÂCH<sub>3</sub>ÂOH)]<sub>2</sub> (<b>1</b>) and a one-dimensional compound [DyLÂ(HCOO)Â(CH<sub>3</sub>ÂOH)]<sub><i>n</i></sub> (<b>2</b>) have been
synthesized using an organic ligand of <i>N</i>′-(2-hydroxyÂbenzylÂidene)ÂpicolinoÂhydraÂzide
(H<sub>2</sub>L). Complex <b>1</b> exhibits a symmetric dinuclear
structure, in which the Dy<sup>3+</sup> 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 Dy<sup>3+</sup> ions are situated in an octa-coordinated, hula-hoop-like
coordination geometry. Magnetic studies reveal that ferromagnetic
coupling is found between Dy<sup>3+</sup> 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
Dy<sup>3+</sup>–Dy<sup>3+</sup> magnetic interaction within
and/or between the dimer units