Field enhanced thermally activated mechanism in a square Dy4 aggregate

Article abstract of DOI:10.1039/c2cc31864d

A rare μ₄-OH centred square Dy₄ aggregate displays a field enhanced thermally activated mechanism with 3-fold increase in the single-molecule magnetic relaxation barrier upon application of an optimal field. The Royal Society of Chem

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Field enhanced thermally activated mechanism in a square Dy4
aggregate
Shufang Xue,a,b Lang Zhao,a Yun-Nan Guo,a,b Xiao-Hua Chenc and Jinkui Tang*a
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
process.7a, 9
A rare µ4-OH centred square Dy4 aggregate displays field
enhanced thermally activated mechanism with 3-fold increase
in the single-molecule magnetic relaxation barrier upon
application of an optimal field.
10 Since the discovery that slow relaxation of the magnetization
above cryogenic temperature can occur in the celebrated
molecule (Bu4N)[Tb(Pc)2] (H2Pc = phthalocyanine),1 the
elaboration of high performance single molecular magnets
(SMMs) featuring heavier lanthanides has rejuvenated the
15 interest of physicists and chemists because of their envisioned
technological applications.2 Notably, the success of such
application hinges upon the inherent molecular spin-inversion
barrier and blocking temperature. There have been remarkable
outcomes, such as (Bu4N)[Tb(Pc)2] with effective barrier
20 accessible to 330 K.1 Equally remarkable has been attributable
to slow relaxation observed in the notable Dy3 molecule in
spite of its non-magnetic ground state.3 Interest in the
polynuclear lanthanides complexes thus has pushed the slow
relaxation to 40 K in a pyramidal Dy5 molecule.4 Moreover,
50
Fig. 1 Tetranuclear unit of 1. Hydrogen atoms, counter ions and solvents
are omitted for clarity. In the left corner the coordination polyhedra of
mono capped square-antiprismatic geometry for Dy1.
In the present work, we describe a rare ꢀ4-OH centred Dy4
55 square
molecule,
[Dy4(ꢀ4-OH)(Hhpch-
)8)]·(ClO4)3·2CH3CN·MeOH·4H2O (1) (where H2hpch = 2-
hydroxylbenzaldehyde (pyridine-4-carbonyl) hydrazone), with
3-
25 the exceptionally strong magnetic exchange occurs in a N2
quasi
mono
capped
square-antiprism
coordination
radical bridged Tb2 compound with a record magnetic
environment around the metallic centre. This compound
60 displays field enhanced thermally activated mechanism with
3-fold increase in the single-molecule magnetic relaxation
barrier upon application of 1 kOe field.
The reaction between H2hpch and Dy(ClO4)3·6H2O in 1:3
methanol/acetonitrile, in the presence of NaHCO3 (see
65 Experimental Section), yielded yellow crystalline plates of
blocking temperature of 14 K.5
To date, however, a crucial roadblock to the isolation of
zero-field lanthanide-based SMMs with large barriers is fast
30 quantum tunneling of the magnetization (QTM) resulted from
the mixing of MS levels via hyperfine coupling, dipolar spin-
spin interactions, or transverse zero-field splitting (E).2a The
latter two factors can be minimized by design a perfectly
high-symmetrical coordination environment of a Kramers ion
35 as well as ensuring a large separation between molecules.
Rationally optimizing the ligand field around metallic centre
is therefore beholden to the study of magnetic relaxation. The
[Dy4(ꢀ4-OH)(Hhpch-)8)]·(ClO4)3·2CH3CN·MeOH·4H2O
(1)
over three days. Single-crystal X-ray studies revealed that 1
crystallized in the monoclinic space group P21/c.
A
perspective view of the square core-shaped molecule is
70 represented in Fig. 1. The central core of this aggregate
consists of four dysprosium cations arranged in a corner
fashion around the central ꢀ4-OH group (O17) as well as eight
Hhpch- ligands like “petals” enclosing the core alternately.
Each pair of Hhpch- ligands coordinates to two dysprosium
75 (III) centres in an antiparallel or “head-to-tail” fashion with
deprotonated phenol oxygen atoms (O2 and O6), imine
nitrogen atoms (N3 and N9) and the keto-form carbonyl
oxygen atoms (O1 and O5) (Fig.S1). Furthermore, all the
deprotonated phenol groups derived from ligands function as
80 ꢀ2-bridge along one of the edges of the square plane and
effect of coordination sphere with a high-order single axis,
6
such as S8 in the square antiprism (SAP),1,
C4 in the
4
40 coordinated symmetry of C4v and C3 in the trigonal prism
environment,7 is that of breaking the spherical symmetry,
splitting the (2J+1) degenerate ground states into new
sublevels, thus achieving an easy axis of magnetization and
Ising-type anisotropy. Additionally, recent advances have
45 manifested that for many SMMs, the slow relaxation
behaviour varies significantly with applied field, as it can
predominantly reduce the efficiency of the underbarrier
process8 or bring about the occurrence of another relaxation
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create a twelve-membered Dy4O8 central core with the Dy–O–
Dy angles falling in the range 97.13(18)–99.73(19)° and the
average Dy–O distance of 2.356 Å. When attention is diverted
to this ꢀ4-OH centred tetranuclear skeleton, it is reminiscent
with a second set around 3 K (Fig. 2, left). Evidence of
possible multiple relaxation processes has been observed in
several reported SMMs due to the different anisotropic
centres13a, 14 or the existence of the isomers or conformers in
5
of those reported for 4f compounds with a
rhombus 55 the crystal.15 For 1, the slight differences in the local
arrangement Ln4 unit linked by ꢀn-O group (n = 2, 4)10 as well
coordination spheres at the DyIII centres are probably
responsible for two processes. Therefore, Arrhenius analysis
of the relaxation time was extracted from χ"(υ) plot (Fig. 2,
right) by fitting the data to two relaxation processes using the
as for transition metal-based MOFs.11
All Dy sites are nine-coordinated with contributions from
four phenol atoms, two carbonyl atoms and two imine atoms
10 but the distance between the metallic centre and ꢀ4-OH atom 60 sum of two modified Debye functions.16 The two limiting
is a little longer than that of the ꢀ2-O groups, making them
with the geometry of quasi mono capped square-antiprism
(Fig. 1). The shortest Dy···Dy distance between adjacent
molecules is 11.883 Å.
values for the energy barrier and relaxation times obtained by
a linear fit of the high and low temperature data, respectively,
are reported in Table 1. At high temperature (HT, T > 7 K),
the similar values of preexponential factor indicate that the
15
Direct current (dc) magnetic susceptibility studies of a 65 relaxation times of two relaxation processes will get close to
polycrystalline sample (Fig. S2) reveal a room-temperature
χMT value equals to 56.05 cm3 K mol-1, approaching the
expected value for four uncoupled DyIII ions (C = 14.17 cm3
each other with increasing temperature; as we can see, the
Cole-Cole plots go through a fusion of FR and SR at the
frequency range of the test (Fig. S4).
6
K mol-1), which indicates the Stark sublevels of H15/2 ground
20 state split by the ligand field are statistically populated at
room temperature, and the free-ion approximation applies.12
With lowering the temperature, however, the components of
higher energy are successively depopulated, thus the χMT
value of 1 gradually decreases deviated from the Curie Law.
25 After reaching the minimum (50.58 cm3 K mol-1) at 10 K, the
χMT value slightly increases and reaches 51.1 cm3 K mol-1 at
2.0 K. Reduced magnetization data do not lie on a single
master-curve below
7 K, suggesting the presence of
significant magneto-anisotropy in the dysprosium (III) system
30 (Fig. S3, inset).
70 Fig. 3 Magnetization relaxation time, τ, versus T-1 plot for 1 under 0 and 1
kOe. The solid line is fitted with the Arrhenius law. ● The relaxation time
was extracted from the χ"(υ) plot; ○ The relaxation time was extracted
from the plot of χ"/χ against temperature
T (Fig. S3).
Table
1 Characteristic dynamic parameters extracted from the
75 Arrhenius law (τ = τ0exp(ꢁ/T) for 1
Hdc/ Oe
(SR) 6.2
(FR) 3.3
6.7
ꢁLT / K
τ0 LT / 10-6 s
ꢁHT / K
τ0 HT / 10-6 s
1960
182
30.3
16
30
37
0.2
0
1000
5850
92
Fig. 2 Left: Temperature dependence of the out-of-phase ac susceptibility
(χ") of 1 in zero static field. Right: Frequency dependence of the out-of-
phase ac susceptibility (χ") of 1 collected at indicated temperature in zero
35 static field. Solid lines are guides for the eye.
To better understand the nature of the dynamics,
measurements at various applied dc fields at 3.5 K were
carried out and the application of dc fields indeed
80 dramatically alter the profile of the χ" frequency scan, as
shown in Fig. 4, left. Here, the peak centred at high
frequencies under zero field rapidly diminishes with a
concomitant increase of a much lower-frequency peak. This
change continues until 1 kOe, at which point the high-
85 frequency peak has completely disappeared. Strong frequency
dependence of ac susceptibility with varying temperature was
observed under this field (Fig. 4, right). The data plotted as
Cole-Cole plots (Fig. S5) at fixed temperature below 17 K
show a relatively symmetrical shape and can be fitted to the
90 generalized Debye model,17 with α parameters below 0.30 (α
= 0 for a Debye model).
Temperature dependences of the alternating current (ac)
susceptibilities in zero static field reveal a slowly relaxing
magnetic moment below ~20 K, that is hallmarks of typical
SMM behaviour of the “freezing” of the spins by the
40 anisotropy barriers (Fig. 2, left). In comparison with another
similar analogue Dy4 reported by Bi,10d for which the slow
magnetic relaxation was observed only at low temperatures
(below 8 K), relatively high-temperature relaxation occurs in
1. The difference in magnetic behaviour could be due to the
45 disparate local coordination geometry around the metallic
centre in light of the detail bond lengths and slightly acute
bridging angles, which presumably influence the axial
anisotropy.13
At low temperature (LT, T < 4 K), the activation energies
(ꢁLT) are comparable for the two fields (ꢁLT = 6.2 (SR), 3.3
(FR) and 6.7 K for 0 and 1 kOe, respectively). However, with
The out-of-phase component χ" for higher frequencies
50 shows a series of frequency-dependent peaks around 13 K,
2
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c College of Chemistry and Materials Science, Fujian Normal University,
50 Fuzhou 350007, China
temperature increasing, the Arrhenius fitting between 12 K
and 16 K (solid line, Fig. 3) gives a strongly enhanced barrier
of Ueff = 92 K, which is more than triple that of zero dc field.
This suggests a thermally activated mechanism seems to be
strongly affected by dc-field. Usually in SMMs the
application of a static field results in the suppression of the
underbarrier process deduced from transverse anisotropy
terms8a, 18 rather than the enhancement of the effective barriers
as occurred in 1.
† Electronic Supplementary Information (ESI) available:. Experimental
details and additional figures. CCDC reference number 870479. For ESI
or other electronic format see DOI: 10.1039/b000000x/.
5
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10
All the above magnetic analysis in combination with Field-
cooled and Zero-field cooled measurements (Fig. S6)
indicates that the magnetic relaxation phenomenon of 1
basically originates from an intrinsic molecular properties of
the compound, rather than intermolecular interactions and
15 long-range magnetic order. One possible scenario of magnetic
dynamics in 1 is that dc-field alters the thermally activated
relaxation pathway, and plays a key role in the switch of low-
lying excited states in 1. After all, the energy ꢁ of about 30 K
under zero dc field is not comparable to the one of above 90 K
20 under 1 kOe dc-field. It’s noteworthy that simple magneto-
structural relationships based on the coordination environment
seem hardly to prognose not only the correct magnetic
anisotropy, but also the dynamic behaviour of lanthanide
complex with nonideal symmetry environment.19 The ab initio
25 calculations are thus planned in order to provide more precise
information on crystal field splitting of this complex as well
as on the role played by anisotropy.
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Fig. 4 Top: Out-of-phase ac susceptibility (χ") collected on 1 at 3.5 K
30 under indicated dc field. Bottom: Frequency dependence of χ" of 1 under
an applied dc field of 1 kOe.
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In conclusion, we report a peculiar ꢀ4-OH centred square
Dy4 aggregate featuring SMM behaviour. The most striking
property evidenced in this study is the optimal field has
35 unparalleled impact on the thermally activated regime with
the remarkable enhancement of effective barrier up to triple of
magnitude. It has proven to be a novel protype for further
investigations of magneto-structural correlations to shed light
on the complicated relaxation mechanism for pure 4f systems.
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40 Acknowledgment
We thank the National Natural Science Foundation of China
(Grants 91022009 and 20921002) for financial support.
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Notes and references
a State Key Laboratory of Rare Earth Resource Utilization, Changchun
45 Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun
130022, China; E-mail: tang@ciac.jl.cn.
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b Graduate School of the Chinese Academy of Sciences, Beijing, 100039,
China
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Graphical Abstract
A rare ꢀ4-OH centred square Dy4 aggregate displays field
enhanced thermally activated mechanism with 3-fold increase in
the single-molecule magnetic relaxation barrier upon application
of an optimal field.
5
4
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This journal is © The Royal Society of Chemistry [year]