Modulating Relaxation Dynamics of Dy2 Compounds through Carboxylate Coordination Modes

Article abstract of DOI:10.1002/ejic.201500956

The reaction of the rigid polydentate Schiff-base H₃L with different dysprosium carboxylates afforded two novel centrosymmetrical dinuclear Dy^III compounds, [Dy₂(HL)₂(CH₃COO)₂(DMF)₂

Full text of DOI:10.1002/ejic.201500956

FULL PAPER
DOI:10.1002/ejic.201500956
Modulating Relaxation Dynamics of Dy Compounds
2
through Carboxylate Coordination Modes
Shuang-Yan Lin,[ Jianfeng Wu, Chao Wang, Lang Zhao,
a,b]
[a]
[a]
[a]
and Jinkui Tang*[
a]
Keywords: Lanthanides / Dysprosium / Carboxylate ligands / Magnetic properties / Slow magnetic relaxation
The reaction of the rigid polydentate Schiff-base H
3
L with
two compounds showing dissimilar magnetic behaviour. The
acetato bridging groups with a syn-syn η :η :μ mode in com-
2
pound 1 lead to the disappearance of slow magnetic relax-
1
1
different dysprosium carboxylates afforded two novel centro-
III
symmetrical dinuclear Dy
CH COO) (DMF) (1) and [Dy
DMF (2) with differing carboxylate groups. The two dif-
compounds, [Dy
2
(HL)
2
-
(
4
3
2
2
]
2
(HL) (PhCOO) (DMF)
2
2
2
]·
ation under zero dc field. Nevertheless, typical SMM behav-
2
iour is observed for compound 2 with η -chelating benzoate
fering carboxylate groups affect the magnetic interaction
and/or local anisotropy of the Dy ion, and therefore we have
ligands.
properties of these materials.[5] Thus, in order to improve
our knowledge of magnetostructural relationships of lan-
thanide-based SMMs, it is of primary importance to design
and construct a comprehensive (quasi)isostructural series of
complexes.
Introduction
Recent years have witnessed a burgeoning interest in the
elaboration of high performance single molecular magnets
(SMMs), since the discovery of the first SMM in the begin-
[
1]
Herein, to investigate the effects of carboxylate groups
on the slow magnetic relaxation, two related Dy2 com-
ning of the 1990s, to pursue the goal of technological ap-
[
2]
plications such as high density storage materials. Among
the numerous reported SMMs, lanthanide SMMs (Ln-
SMMs) have an outstanding performance in advancing re-
pounds, [Dy (HL) (CH COO) (DMF) ] (1) and [Dy (HL) -
2
2
3
2
2
2
2
(
PhCOO) (DMF) ]·4DMF (2), based on a rigid ligand
2 2
[
3]
(H L, Scheme 1) with different carboxylate groups, have
laxation energy barriers and blocking temperature. They
benefit from the large intrinsic magnetic anisotropy as a
3
been synthesized and structurally and magnetically charac-
terized. The introduction of two different carboxylate
groups results in dissimilar static and dynamic magnetism.
result of the large unquenched orbital moment.[4] One re-
cent example is an oxido-centred octahedral Dy K com-
4
2
plex reported to have an effective barrier of 692 K – the
[
3i]
largest barrier for multinuclear compounds to date. Re-
3
–
markable results also include a N2 -radical-bridged Tb2
compound, which has the highest blocking temperature of
1
4 K, thanks to the exceptionally strong magnetic ex-
[
3h]
change.
However, the promise of SMMs enhancing the
performance of existing technologies that are indispensable
in modern human life also stimulates the search for new
magnetic systems and the improvement in the comprehen-
sion of the fundamental factors affecting the magnetic
[
a] State Key Laboratory of Rare Earth Resource Utilization,
Changchun Institute of Applied Chemistry, Chinese Academy
of Sciences,
Changchun 130022, P. R. China
E-mail: tang@ciac.ac.cn
http://www.ciac.ac.cn/
[
b] Key Laboratory of Photonic and Electric Band Gap Materials,
Ministry of Education, School of Physics and Electronic
Engineering, Harbin Normal University,
Harbin 150025, P. R. China
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201500956.
Scheme 1. Structure of the H
of the deprotonated form.
3
L ligand and the coordination mode
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Results and Discussion
occupied coordination pockets can coordinate with other
metal ions, thus the compound can be used as a building
block to build new SMMs. Another feature of this complex
is that two rigid ligands, each one with the dihedral angle
between two pyridine rings of 20.013°, are roughly copla-
nar. Within the plane, carbonyl oxygen atoms (O3 and
O3A) of the ligands bridge the two metal centres in their
The rigid Schiff-base H L was prepared by the condensa-
3
tion of 2,6-diformyl-4-methylphenol and picolinohydrazide
(1:2) in methanol. The reaction of H L with dysprosi-
3
um(III) carboxylate in the presence of Et N in the mixture
3
of DMF and CH Cl (v:v = 2:5) produces compounds 1
2
2
and 2 depending on the different carboxylate used. Single-
crystal X-ray studies revealed that both 1 and 2 crystallize
in the triclinic space group P1. Details for the structure
solution and refinement are summarized in Table 1, and se-
lected bond lengths and angles are listed in Table 2.
–
conjugate deprotonated enol form (O ); in the axial direc-
tion, acetato groups connect the two Dy ions in a syn-syn
¯
1
1
η :η :μ mode, giving rise to a four-membered fastened
2
Dy O rhomb with a Dy···Dy distance of 3.7877(12) Å and
2
2
Dy–O–Dy angles of 106.9(3)°. The coordination sphere of
the Dy1 ion fulfilled by one DMF molecule is far from an
idealized geometry and may be described as an intermediate
Table 1. Crystal data and structural refinement for complexes 1 and
2.
1
2
Empirical formula
Dy
2
C
52
H
52
N
14
O
12
2 74 84 18 16
Dy C H N O
–1
M
r
[gmol ]
1390.08
triclinic
1806.59
triclinic
Crystal system
Space group
¯
P1
¯
P1
a [Å]
b [Å]
c [Å]
α [°]
9.773(3)
10.2144(11)
14.3097(16)
15.4432(17)
62.763(2)
85.557(2)
74.062(2)
1926.8(4), 1.557
914, 0.0211
0.0449, 0.1090
0.0553, 0.1276
1.158
12.180(3)
13.358(3)
64.130(3)
73.792(4)
78.527(4)
1368.4(6), 1.687
690, 0.0377
0.0678, 0.2071
0.0815, 0.2294
0.985
β [°]
γ [°] 3
V [Å ], ρcalcd. [Mgm–3]
F(000), Rint
R
R
1
, wR
, wR
2
[IϾ2σ(I)]
(all data)
1
2
GOF
Table 2. Some crucial structural parameters for compounds 1 and
2.
Compound 1
Compound 2
Distance [Å]
Dy1–O1B[a]
Dy1–O1A[a]
Dy1–O4
Dy1–O3A
Dy1–O3
Dy1–O5A
Dy1–O6
Dy1–N4A
Dy1–N6
2.215(8)
2.348(8)
2.348(7)
2.366(7)
2.374(8)
2.398(9)
2.501(10)
2.568(11)
2.194(4)
2.327(4)
2.327(4)
2.350(4)
2.405(4)
2.435(4)
2.483(5)
2.551(5)
Dy1–O2
Dy1–O2B
Dy1–O6
Dy1–O5
Dy1–O4
Dy1–N1B
Dy1–N3
Figure 1. Partially labelled X-ray structures of 1 (top) and 2 (bot-
tom) with H atoms omitted for clarity.
(
Dy–O/N)average 2.390
(Dy–O/N)average 2.384
Dy1–Dy1B 3.8996(7)
Dy1–Dy1A 3.7877(12)
Angle [°]
Dy1–O2–Dy1B 113.84(15)
a] A (symmetry code: –x, –y + 1, –z + 1); B (symmetry code: –x,
y, –z + 2).
Dy1A–O3–Dy1 106.9(3)
[
–
Structures
A structural study of 1 showed it to be a centrosymmet-
rical dinuclear complex (Figure 1) in which two ligands co-
ordinate two dysprosium centres in an antiparallel fashion
[
6]
as in the case of the reported dinuclear complexes. Inter-
estingly, only one pocket of the ligand with the tridentate
unit (O1, N4 and O3) and the bidentate picolinoyl group
Figure 2. Intermediate between square antiprism and dicapped tri-
gonal prism geometry for compound 1 (top) and the hula-hoop-
(O3 and N6) is coordinated, while the other pocket remains
uncoordinated as shown in Figure 1 and Figure S1. The un- like geometry for compound 2.
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Figure 3. Crystal packing of compounds 1 and 2 showing the formation of a supramolecular chain along the crystallographic c axis
through π–π interactions. The distance Cg1···Cg2 is 3.4433(6) Å for 1 and the distance Cg3···Cg4 is 3.7083(3) Å for 2.
between square antiprism with the basal planes made up of (1D) chains are stacked in a parallel manner, producing a
atoms O3, O4, O6, N6 and O3A, N4A, O1A, O5A (sym- framework where the shortest intermolecular Dy···Dy dis-
metry code: –x, –y + 1, –z + 1) and dicapped trigonal prism tance is 8.98 and 9.94 Å for 1 and 2, respectively.
geometry with the basal planes of atoms N4A, O4, O6 and
O3, O5A, N6 (Figure 2).
Similar to compound 1, the centrosymmetrical dinuclear
compound 2 (Figure 1) is surrounded by two partially coor-
Magnetic Properties
dinated ligands. Instead of the axial bridging acetato groups
The static magnetic susceptibility data of compounds 1
1
1
2
(
syn-syn η :η :μ mode) seen in compound 1, two η chelat- and 2 were measured over the temperature range 300–2 K
2
ing benzoato ligands complete the respective coordination in an applied field of 1 kOe, and the plots of χ T vs. T are
M
sphere of Dy ions. Notably, the rigid ligands in 2 are flatter illustrated in Figure 4. At room temperature, the χ T val-
M
3
–1
3
–1
than those in 1 with a dihedral angle between two pyridine ues of 28.8 cm Kmol (compound 1) and 28.1 cm Kmol
rings of 6.200°. Two flat ligands form a flatter plane with (compound 2) are consistent with the expected value for the
III
the coordinated atoms [O2, O2B, N1B, O1B, N3 (symmetry presence of two uncoupled Dy ions (S = 5/2, L = 5,
6
code: –x, –y, –z + 2)] and hence the Dy ion adopts a hula-
H15/2 and g = 4/3). Upon cooling, the χ T products of 1
M
hoop-like geometry (Figure 2). In addition, the Dy···Dy dis- and 2 are nearly constant down to approximately 100 K,
tance is 3.8996(7) Å and the Dy–O–Dy angle is 113.84(15)°. and then show different change trends. For 1, the χ T value
M
3
–1
Examination of the crystal packing reveals that both decreases to 24.9 cm Kmol at 4.5 K, and then slowly in-
3
–1
molecules of 1 and 2 are in contact through π–π interac- creases to a maximum of 25.1 cm Kmol at 2 K. The ther-
tions, generating an infinite supramolecular chain along the mal behaviour is associated with the thermal depopulation
crystallographic c axis (Figure 3). These one-dimensional of the dysprosium(III) excited states and weak ferromag-
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netic interaction. However, the χ T product for 2 gradually the crystal-field effect, is usually populated at low tempera-
M
decreases with the lowering of temperature to reach a mini- tures.
3
–1
mum of 27.6 cm Kmol at about 30 K and then increases
Generally, minor changes in the structure could produce
3
–1
sharply to a maximum value of 33.7 cm Kmol at 2 K, dramatic changes in the magnetic dynamics. To explore fur-
suggesting the presence of stronger ferromagnetic interac- ther, the alternating current (ac) magnetic susceptibilities of
tions in compound 2. The obvious weak ferromagnetic in- 1 and 2 were measured under an ac field of 3 Oe. Under
teraction in 1 is probably due to the presence of additional zero dc field, compound 1 does not exhibit significant slow
bridging acetato groups. Indeed, it is well known that the magnetic relaxation, as it is hard to observe an appreciable
triatomic syn-syn bridging conformation of the carboxylate out-of-phase signal (Figure S2). This behaviour may be at-
group usually mediates antiferromagnetic interactions in tributed to a fast quantum tunnelling of magnetizations
transition metal complexes.[
7]
(QTM), which is commonly reported for lanthanide-con-
taining complexes. Remarkably, ac susceptibility is strongly
affected by a small dc field, highlighting the presence of the
fast QTM, which can be significantly slowed down by the
applied field (Figure 6 and Figure S3). Indeed, the peak in
the χЈЈ as a function of the frequency is observed under a
500 Oe dc field for low temperatures (Figures S4 and S5),
as is characteristic of a field-induced SMM.
M
Figure 4. Temperature dependence of the χ T product at 1000 Oe
for compounds 1 and 2, with χ being the molar susceptibility per
dinuclear unit defined as M/H.
The field dependence of the magnetization (M) has been
measured up to 7 T below 5 K. As shown in Figure 5, the
magnetization rises rapidly at low fields before a gradual
linear increase, which is consistent with the presence of in-
tramolecular ferromagnetic interactions. The lack of a per-
fect superposition of the M vs. H/T data on a single master
Figure 6. Frequency dependence of the out-of-phase ac suscep-
tibility of compound 1 under various dc fields at 2.0 K. Solid lines
curve is most likely attributed to anisotropy and the crystal- are guides for the eyes.
field effect at the Dy ion that eliminates the 16-fold degener-
6
acy of the H
ground state. Indeed, M values reach 11.87
On the contrary, SMM behaviour is observed for 2 as
1
5/2
and 11.13 μ at 1.9 K and 70 kOe for 1 and 2, respectively, evidenced by well-resolved out-of-phase ac susceptibility
B
which is close to the expected value (2ϫ5.23 μ ) for two maxima that vary with temperature (Figure 7 and Fig-
B
uncorrelated Dy ions[ where a ground state, well separated ure S6). The magnetization relaxation time (τ) (Figure 8) is
8]
from the excited states resulting from the anisotropy and derived from the frequency dependence of the out-of-phase
Figure 5. M vs. H/T plots at 1.9, 3.0 and 5 K for compounds 1 (left) and 2 (right). The solid lines are guides for the eyes.
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ac susceptibility between 1.9 and 14 K, which shows a tem- While below 5 K the appearance of the second semicircle
[
11]
perature-independent quantum tunnelling regime at low indicates the onset of another relaxation process.
temperatures and a temperature-dependent thermally acti-
vated regime at temperatures above 9 K. The thermally-ac-
tivated relaxation regime was fitted by an Arrhenius equa- Magneto-Structural Relationships
tion, giving the energy barrier U = 61 K and pre-ex-
eff
It is well known that both the single-ion anisotropy of
Dy ions and magnetic interaction mediate the magnetic re-
–
7
ponential factor τ = 2ϫ10 s. Below 8 K, ln(τ) becomes
0
weakly dependent on T and appears to deviate from the
Arrhenius behaviour, indicating a crossover from a ther-
mally activated Orbach mechanism to a direct or phonon-
induced tunnelling process.[ The Cole–Cole plots of com-
pound 2 are given in Figure 8 and Figure S6, and show an
asymmetric semicircle shape above 6 K, and the emergence
of a second semicircle below 5 K. The semicircles above 6 K
were fitted by the generalized Debye model[10] and α param-
eters were obtained with α = 0.005–0.130, indicating a nar-
row width of relaxation time for this temperature range.
[
6b,9b,12]
laxation of Dy SMMs.
To probe the structure–prop-
erty relationship between compounds 1 and 2, the crucial
structural parameters for both compounds are listed in
Table 2. It is worth noting that both compounds have sim-
ilar average Dy–O/N distances (2.390 Å for 1 and 2.384 Å
for 2), while the Dy···Dy distance in 1 [3.7877(12) Å] is
slightly shorter than that in 2 [3.8996(7) Å]. A large vari-
ance is the Dy–O–Dy angle, which is smaller in 1 (below
9]
110°) than that in 2 (above 110°). This is because a more
acute angle is accessible with additional syn-syn carboxyl-
ates. Indeed, the Dy–O–Dy angles have a great influence on
the magnetic interactions through modifying the overlap of
the magnetic orbitals. When the Dy–O–Dy angle is larger
than 110°, ferromagnetic interactions are usually favour-
[
6b,13]
ed,
which is consistent with the observation that a
stronger ferromagnetic interaction exists in 2.
Further, the significant variance in the coordination ge-
ometry affected by the different carboxylato groups is prob-
ably also responsible for the large divergences of ac mag-
netic properties between them. In 1, acetato groups bridge
1
1
two Dy ions in a syn-syn η :η :μ mode, which shrinks the
2
Dy–O–Dy angle and affects the coordinated environment
of the Dy ion yielding an intermediate between square anti-
prism and dicapped trigonal prism geometry. The distorted
coordination geometry leads to the absence of easy-axis an-
isotropy. However, in 2, benzoato ligands chelate Dy ions
2
2–
in the η form, which puts two rigid HL ligands almost
Figure 7. Frequency dependence of the out-of-phase ac suscep-
tibility of compound 2 under zero dc field. Solid lines are guides
for the eyes.
III
in a plane, yielding eight-coordinate Dy in a hula-hoop-
like geometry. The different coordination geometry of the
Dy ion will alter the nature or directions of the easy axes
through the ligand fields.
[
14]
Therefore, both magnetic interactions and single-ion an-
isotropy mediate the different magnetic relaxations of com-
pounds 1 and 2. For compound 1, the seriously distorted
coordination geometry around the Dy ions and a weaker
ferromagnetic interaction caused by the bridging acetato
groups result in the disappearance of SMM behaviour un-
der a zero field. While typical SMM behaviour in com-
pound 2 is thanks to the hula-hoop-like geometry around
Dy ions that usually generates a suitable crystal field on the
[
6,13b,15]
Dy site leading to SMM behaviour.
Additionally,
the strong intramolecular interactions may suppress QTM
to a certain extent to enhance the SMM behaviour of com-
pound 2.
Conclusions
Figure 8. Fitting of the relaxation time (τ) using Arrhenius law for
compound 2 (inset: Cole–Cole plots under zero-dc field above 5
K, the solid lines indicate the best fits using a generalized Debye
model).
In summary, two novel centrosymmetrical dinuclear
DyIII compounds have been assembled using the rigid H L
3
ligand and different carboxylate groups. The syn-syn
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1
1
η :η :μ -acetato groups in compound 1 lead to a weaker placed by Dy(PhCOO)
3
(0.1 mmol, 58.9 mg), yield 35 mg (39%,
2
based on the metal salt). Dy
2 74 84 18
C H N O16: C 49.19, H 4.69, N
ferromagnetic interaction and seriously distorted coordina-
tion geometry, which results in the disappearance of slow
magnetic relaxation under zero dc field. However, com-
pound 2 displays typical SMM behavior thanks to a hula-
13.95; found C 48.94, H 4.39, N 13.56. IR (KBr): 2926 (w), 2857
(
(
(
w), 1687 (s), 1667 (s), 1653 (s), 1616 (m), 1599 (s), 1556 (s), 1541
s), 1472 (s), 1420 (m), 1385 (m), 1347 (s), 1231 (m), 1163 (m), 1152
m), 1090 (w), 996 (w), 828 (w), 755 (w), 721 (m), 673 (m), 512 (w).
2
hoop-like geometry completed by η chelating benzoate li-
Supporting Information (see footnote on the first page of this arti-
cle): Crystal packing as well as the temperature-, frequency- and
field-dependence of the ac susceptibility of compound 1, and the
frequency-dependence and Cole–Cole plots for compound 2.
gands and stronger ferromagnetic interactions. The investi-
gation into the structure–magnetic properties presents an
opportunity to tune the magnetic properties of SMMs by
modifying the carboxylate ligands and further adjusting the
corresponding symmetry of the ligand field. Meanwhile,
owing to the presence of unoccupied coordination pockets, Acknowledgments
both compounds can be viewed as a building block upon
The authors thank the National Natural Science Foundation of
China (NSFC) (grant numbers 21371166, 21331003 and 21221061)
for financial support.
which further self-assembled SMMs may be built. Current
endeavors are focused on such self-assembly in order to
gain a better understanding of the parameters and rules
that govern SMM construction.
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[
H
Dy
O (0.2 mmol, 71.5 mg) and H
of DMF (4 mL) and CH Cl (10 mL) was stirred for 5 min. Then
Et N (0.2 mmol) was added and the mixture was stirred for 3 h.
2
(HL)
2
(CH
3
COO)
2
(DMF)
2
] (1): A solution of Dy(CH
3
COO)
3
·
[5] a) F. Habib, M. Murugesu, Chem. Soc. Rev. 2013, 42, 3278–
288; b) S.-Y. Lin, Y.-N. Guo, Y. Guo, L. Zhao, P. Zhang, H.
3
2
3
L (0.1 mmol) in a mixed solvent
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2
2
3
The resultant yellow solution was left unperturbed to allow for
slow evaporation of the solvent. Yellow single crystals suitable for
X-ray diffraction analysis, were formed after two weeks, yield
2
8 mg (20%, based on the metal salt). Dy
2 52 52 14
C H N O12: C 44.93,
H 3.77, N, 14.10; found C 44.56, H 3.63, N 13.56. IR (KBr): 3293
(
(
(
(
w), 2983 (w), 2927 (w), 1687 (s), 1656 (s), 1616 (m), 1578 (s), 1537
s), 1474 (s), 1450 (s), 1356 (s), 1348 (s), 1315 (s), 1229 (m), 1154
m), 1109 (m), 1049 (w), 1017 (m), 920 (w), 749 (m), 692 (m), 622
m), 509 (m).
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[
2 2 2 2
[Dy (HL) (PhCOO) (DMF) ]·4DMF (2): A procedure similar to
that for 1 was followed except that Dy(CH
3
COO)
3
·H
2
O was re-
Eur. J. Inorg. Chem. 2015, 5488–5494
5493
© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.eurjic.org
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Published Online: November 5, 2015
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Eur. J. Inorg. Chem. 2015, 5488–5494
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