Elucidation of the two-step relaxation processes of a tetranuclear dysprosium molecular nanomagnet through magnetic dilution

Article abstract of DOI:10.1039/c8dt01870g

A new centrosymmetric tetranuclear aggregate [Dy₄(L)₂(OAc)₈(CH₃OH)₂] (1) was assembled using a unique symmetrical Schiff base ligand 1,5-bis(salicylidene)-carbohydrazide (H₂L). Magnetic stu

Full text of DOI:10.1039/c8dt01870g

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Joseph T. Hupp, Omar K. Farha et al.
Efficient extraction of sulfate from water using
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Elucidation the two-step relaxation processes of a
tetranuclear dysprosium molecular nanomagnet through
magnetic dilution
Received 00th January 20xx,
Accepted 00th January 20xx
d
Yu Fang,a XiaoꢀQin Ji,a Jin Xiong,c Guanzheng Li,a, Fang Ma,a HaoꢀLing Sun,*a YiꢀQuan
DOI: 10.1039/x0xx00000x
Zhang*b and Song Gao*c
www.rsc.org/
A new centrosymmetric tetranuclear aggregate [Dy4(L)2(OAc)8(CH3OH)2] (1) was assembled using a unique
symmetrical Schiff base ligand 1,5ꢀbis(salicylidene)ꢀcarbohydrazide (H2L). Magnetic studies reveal
ferromagnetic interactions between dysprosium ions, two obvious relaxation processes under zero dc field
with effective energy barriers Ueff of 38 K and 223 K, the highest among reported tetranuclear dysprosium
molecular nanomagnets. To obtain further evidence on the origination of the slow magnetic relaxation,
the diamagnetic yttrium analogue [Y4(L)2(OAc)8(CH3OH)2]
(
2)
and diluted sample
[(Dy0.06Y0.94)4(L)2(OAc)8(CH3OH)2] ( ) were synthesized. And further magnetic studies on diluted
3
sample combined with theoretical calculations indicate that the twoꢀstep magnetic relaxation processes in
complex 1 originate from the singleꢀion magnetic behaviors of dysprosium ions with different
coordination environments.
succession.3,4
Since the discovery of trinuclear dysprosium molecular
nanomagnets with fantastic spin chirality in 2006,7a plenty of efforts
Introduction
Molecular nanomagnets characterized by quantum tunnelling effects
and slow magnetic relaxation, have been attached great importance
for their potential applications in highꢀdensity information data
storage, molecular spintronics and quantum computation.1 Currently,
lanthanide ions have been considered one of the best candidates of
spin carriers due to their strong spin–orbital coupling effect and large
magnetic anisotropy, which can generate high effective energy
barrier (Ueff) and blocking temperature (TB) for the slow magnetic
relaxation.2ꢀ4 A number of lanthanideꢀbased molecular nanomagnets
complexes have been reported in recent years.2ꢀ8 Among them,
mononuclear lanthanide molecular nanomagnets have been studied
thoroughly for their symmetry and uniaxial magnetic anisotropy are
apt to be manipulated, and both the Ueff and TB records of
mononuclear lanthanide complexes have been broken in
have been focus on the construction of magnetically interesting
lanthanide building blocks.7,8 However, in contrast to the wellꢀ
behaved mononuclear lanthanide molecular nanomagnets, the
magnetic properties of reported polynuclear lanthanide molecular
nanomagnets haven’t been very prominent. According to reported
researches, rare wellꢀperformed polynuclear lanthanide complexes
have been successfully constructed because it is more difficult to
control the ligand field and the magnetic anisotropy of lanthanide
ions in polynuclear lanthanide complexes.7,8 Meanwhile it’s more
challenging to control the easyꢀaxis arrangement of the spin carriers
and obtain a strong uniaxial magnetic anisotropy for polynuclear
complexes. What’s more, the origination of the magnetic relaxation
in polynuclear molecular nanomagnets still remains poorly
understood. Distinct multiple anisotropic centers usually exist in
building blocks so as to produce complicated magnetic interactions
making it confused to trace the origin of relaxation processes.
Therefore, the creation of intriguing polynuclear building blocks is
of fundamental importance to enlarge the available database and thus
further reveal the origination of the magnetic relaxation in lanthanide
polynuclear molecular nanomagnets.
Investigations proved that the choosing organic ligands is very
important in constructing lanthanide molecular nanomagnets.
Appropriate organic ligands can affect the magnetic behaviors of
building blocks not only by controlling the nuclearity and topology
of the clusters, but also by promoting the magnetic interactions
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between the lanthanide ions and altering the nature or directions of based on the metal salt). Elemental analysis (%) calcd for
the easy axes. Many ligands, such as phthalocyanine,2a,9 Schiff C48H56Dy4N8O24: C, 32.38, N, 6.30, H, 3.15; found C, 32.32, N, 6.29
base,5b,10 βꢀdiketone,2c,11 carboxylic acid2g,12 and their H, 3.05. IR (KBr, cmꢀ1): 3178(br), 1627(s), 1550(s), 1476(m),
derivatives have been successfully adopted to construct 1456(m), 1399(m), 1328(w), 1158(w), 1072(m), 922(w), 755(m),
different dysprosium
versatile organic ligands, we are particularly interested in
m
olecular nanomagnets
.
Among the 668(m), 642(m), 538(w).
Synthesis of [Y4(L)2(OAc)8(CH3OH)2] (2). Compound 2 was
aroylhydrazone Schiff base ligands because they can provide prepared following the same procedure as that for 1, with the
O,N,Oꢀbased multiꢀchelating sites that are especially favourable exception of using Y(OAc)3·6H2O instead of Dy(OAc)3·6H2O.
to possess a variety of coordination characteristics and bridging Yield: 44.7 mg (60.2 % based on the metal salt). Elemental analysis
modes.10b,13 Moreover, the phenyl oxygen atom of this kind of (%) calcd for C48H56Y4N8O24: C, 38.80, N, 7.54, H, 3.77; found C,
ligands can form strong coordination bond with lanthanide ion 38.61, N, 7.59 H, 3.76. IR (KBr, cmꢀ1): 3180(br), 1628(s), 1553(s),
that is helpful to get its single axial magnetic anisotropy. 1479(m), 1455(m), 1397(m), 1330(w), 1205(w), 1157(w), 1079(m),
According to previous studies, major aroylhydrazone Schiff 925(w), 759(m), 670(m), 640(m), 537(w).
base ligands utilized on the formations of polynuclear
Synthesis of [(Dy0.06Y0.94)4(L)2(OAc)8(CH3OH)2] (3). Compound
dysprosium complexes are asymmetrical. While symmetrical
3 was obtained following a method similar to that of 1 but with
aroylhydrazone Schiff base ligands can offer more ideal O,N,Oꢀ
Dy(OAc)3·6H2O and Y(OAc)3·6H2O (molar ratio of 1:19) used in
donor chelating pockets, which could efficiently enhance
place of Dy(OAc)3·6H2O. Yield: 43.2 mg (55.4 % based on the
coordination capacities of the ligands, making it easier to coordinate
metal salt). Elemental analysis (%) calcd for C48H56Dy0.24Y3.76N8O24:
with dysprosium ions and increase the symmetry of compounds.
C, 37.90, N, 7.37, H, 3.68; found C, 37.95, N, 7.45 H, 3.79. IR (KBr,
Accordingly, we choose a symmetrical polydentate Schiff base
cmꢀ1): 3175(br), 1625(s), 1551(s), 1475(m), 1453(m), 1395(m),
ligand 1,5ꢀbis(salicylidene)ꢀcarbohydrazide (H2L) with two identical
1326(w), 1201(w), 1155(w), 1070(m), 922(w), 754(m), 665(m),
O,N,Oꢀtriꢀdentate chelating pockets that can coordinate to two
642(m), 535(w).
different lanthanide ions and thus might result in polynuclear
lanthanide complexes with strong magnetic anisotropy and slow
magnetic relaxation. Herein we report the synthesis and the magnetic
X-ray crystallography
studies of a novel ferromagnetic tetranuclear dysprosium complex
constructed by the Schiff base ligand of H2L and carboxylate with
slow magnetic relaxation, elucidate the origin of its magnetic
relaxation by the dilution method and theoretical calculation.
Intensity data for crystal of 1 and 2 were collected on a rigaku
SuperNova, Dual, AtlasS2 diffractometer with graphiteꢀ
monochromated Mo Kα radiation (0.71073 Å) or Cu Kα radiation
(1.54184 Å) at 100 K. Using Olex2, the structures were solved with
the olex2.solve structure solution program using Charge Flipping
and refined with the olex2.refine refinement package using Gaussꢀ
Newton minimisation. All nonꢀhydrogen atoms were refined
anisotropically. Hydrogen atoms were placed at the computational
positions. The details of crystallographic data and selected bond
parameters for compound 1 and 2 are listed in Table 1 and Table
S1(†).
Experimental section
Materials and measurements
The Schiff base H2L ligand is synthesized by condensation of
carbohydrazide and salicyladehyde in 1:2 ratios in methanol. All the
other starting chemicals were commercially available reagents for
analytical grade and used without any further purification. Powder
Xꢀray diffraction (XRD) analyses were performed on a Rigaku
Dmaxꢀ2000 Xꢀray diffractometer with Cu Kα (λ = 1.54059 Å)
radiation (Fig. S1†). Elemental analyses of carbon, hydrogen, and
nitrogen were carried out with an Elementar Vario EL analyser.
FTIR spectra were recorded in the range of 4000 to 400 cmꢀ1 on an
AVATAR 360 Nicolet 380 FT/IR spectrometer using KBr pellets.
Inductively coupled plasma (ICP) measurements were carried out
with inductively coupled plasma atomic emission spectroscopy.
Variableꢀtemperature magnetic susceptibility measurements of 1 and
3 were performed on the MPMS3 and PPMS magnetometers. The
experimental susceptibilities for all compounds were corrected for
the diamagnetism of the constituent atoms (Pascal’s tables) and the
background of the sample holder.
Results and discussion
Table 1 Crystallographic Data and Structure Refinement for complex
1 ꢀ 2
compound
Formula
Mr
1
2
C48H56Dy4N8O24
1779.00
C48H56Y4N8O24
1484.65
Crystal system
Space group
T [K]
Triclinic
Triclinic
P1
P
1
100
10.580(5)
11.830(5)
13.232(6)
110.986(4)
104.860(4)
105.422(4)
1372.2(1)
1
100
10.734(1)
11.874(2)
13.261(2)
111.052(1)
104.623(2)
105.043(3)
1406.8(4)
1
a (Å)
b (Å)
c (Å)
a (°)
β (°)
γ (°)
V (Å3)
Synthesis of [Dy4(L)2(OAc)8(CH3OH)2] (1).
A methanolic
solution (4.0 mL) of H2L (0.1 mmol, 29.8 mg) was neutralized with
NaOH solution (2.0 mL, 0.05 mol·Lꢀ1), solid Dy(OAc)3·6H2O (0.2
mmol, 82.2 mg) was then added, which was continually stirred for
20 min at room temperature. The mixture was transferred to a 20 mL
Teflon reactor and heated to 70 for 72 h under autogenous
pressure, then cooled to room temperature at a rate of 5 ·hꢀ1. The
yellowish block crystals of 1 were obtained. Yield: 51.4 mg (57.9 %
Z
ꢁ (mmꢀ1)
F(000)
5.475
856
6.130
748
GOF
1.049
0.971
Data collected
Unique
Rint
11904
5914
0.0730
8766
4963
0.0862
R1, wR2 [I >2σ(I)]
R1, wR2 [all data]
0.0536, 0.0865
0.0851, 0.0965
0.0515, 0.1265
0.0922, 0.1367
2 | J. Name., 2012, 00, 1-3
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Structural details of complex 1
The crystal structure of complex 1 is established by singleꢀcrystal
Xꢀray diffraction and presented in Fig. 1. Compound 1 crystallizes
in the triclinic space group P1 and displays a novel tetranuclear
cluster structure sustained by four bridging acetate groups. The
asymmetric unit of
1
is composed of two Dy3+ ions, one
deprotonated L2ꢀ ligand, four acetate groups and one methanol. To
determine the geometric symmetry of the coordination sphere around
Dy3+ ions, CShM's values for two octaꢀcoordinated Dy3+ ions were
calculated by using the SHAPE 2.0 program (Table 2).14 As shown
in Table 2, the smallest values for both Dy3+ ions are settled on
triangular dodecahedron geometry, indicating that the coordination
polyhedra around Dy3+ ions are distorted triangular dodecahedron.
As shown in Fig. 1, an interesting aspect of assembling the dinuclear
unit is that each Dy3+ ion is located in one of the O,N,Oꢀchelating
pocket from symmetrical ligand while Dy1 and Dy2 have different
coordination environments. The coordination sphere of Dy1 is
composed of a half L2ꢀ ligand, one chelating acetate group and three
bridging acetate groups. And Dy2 is coordinated by the other half L2ꢀ
ligand, one chelating acetate group, two bridging acetate group and
one methanol molecule. The Dy1 and Dy2 ions are held together to
form a dimer unit by two oxygen atoms of O2 and O6, among which
O2 belongs to a carbonyl group and O6 belongs to a bridging acetate
Fig. 1 The tetranuclear structure of complex 1.
Table 2. The calculated results for Dy3+ ions configuration of complex 1 by
SHAPE 2.0 software.a
TDDꢀ8
2.825
1.683
BTPRꢀ8
3.477
2.307
JBTPRꢀ8
3.708
2.928
Dy1
Dy2
aTDDꢀ8: triangular dodecahedron. BTPRꢀ8: biaugmented trigonal prism. JBTPRꢀ
8: biaugmented trigonal prism J50.
Magnetic behaviors
group respectively with the intradimer Dy⋯Dy distance of 3.982 Å.
Directꢀcurrent (dc) magnetic susceptibility was measured on
microcrystalline samples of 1 in the temperature range of 2ꢀ300 K at
1 kOe. As shown in Fig. 2, at room temperature, the observed χMT
values of 56.35 cm3molꢀ1K is slightly lower than the expected value
Furthermore, the two dinuclear units in 1 are linked with each other
by four acetate bridging groups in two forms (η1:η1:ꢁ2 and η2:η1:ꢁ3),
resulting in a novel tetranuclear structure with interdimer Dy
distance of 4.930 Å (Dy1 Dy2), 5.061 Å (Dy1 Dy1) and 7.397 Å
(Dy2 Dy2). The Dy–O bond lengths range from 2.172 Å to 2.465
⋯Dy
⋯
⋯
of 56.68 cm3molꢀ1K for four uncoupled Dy3+ ions (6H15/2
= 15/2 and
,
S
= 5/2,
J
⋯
g
= 4/3).15 Upon cooling, the χMT value decreases
Å and the Dy–N bond lengths are from 2.512 Å to 2.566 Å, which
are similar to the reported Dy3+ complexes based on Schiff ligands
(Table 3).10 It is worth noticing that bond lengths of Dy2–O1 (2.169
Å) and Dy1–O3 (2.209 Å) are shorter than those distances between
Dy3+ and any other coordination atoms in this molecule, which is
possibly caused by the stronger coordination affinity of the phenyl
oxygen with negative charge. These shorter Dy–O bonds may play a
significant role in adjusting the dynamic magnetic behaviors of
gradually in a wide temperature range from 300 K to 50 K and then
reaches a minimum of 47.86 cm3molꢀ1K at 18 K, which is mostly
due to the thermal depopulation of the excited Stark sublevels for
single Dy3+ ion. The sharp increase in the χMT value below 18 K
suggests a ferromagnetic interaction between the Dy3+ ions.
60
complex 1. Along the bꢀaxis, neighbouring tetranuclear units are
H
dc
= 1 kOe
parallelly arranged and connected by hydrogen bonds between the
uncoordinated imino nitrogen and the chelating acetate forming a
oneꢀdimensional (1D) chain (Table S2†) with the shortest interꢀ
20
45
30
15
2 K
3 K
5 K
aggregate Dy
axis are further crossꢀlinked to construct a layer structure through πꢀ
stackings between interdigitating phenyl with he shortes
interchain Dy Dy distance of 7.425 Å (Fig. S3 and S4†).
⋯Dy distance of 7.374 Å. Adjacent chains along the cꢀ
10
π
t
t
8 K
M
⋯
10 K
5
T 15
χ
χ
χ
χ
0
0
5
10
15
-1
20
25
-1
HT / kOe K
0
0
50
100
200
250
300
T 1/50K
Fig. 2 Temperature dependence of χMT for complex
M−H/T at 2, 3, 5, 8 and 10 K.
1. Inset: Plots of
The field dependence of the magnetization of complex 1 from 0 to
50 kOe at 2, 3, 5, 8 and 10 K is shown Fig. 2 (inset). A rapid
increase of the magnetization is observed at low magnetic fields,
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-4
followed by a linear increase up to 18.0 Nβ at 2 K, which is clearly
lower than the expected saturation value for four uncorrelated Dy3+
ions (40 Nβ). The magnetization of 1 shows nonꢀsuperimposition of
the temperatureꢀdependence under different fields, suggesting the
presence of a significant magnetic anisotropy in this system.
Moreover, variableꢀfield magnetization measurements have been
Quantum regime
-6
-8
Hdc = 0 Oe
Thermally activated
done on microcrystalline samples using
a
SQUIDꢀVSM
SR
FR
Fit
magnetometer at low temperatures. As shown in Fig. S5, the
butterflyꢀshaped loops are clearly visible below 6 K at a field sweep
rate of 500 Oe/s, which is typical for molecular nanomagnets. When
the temperature falls off to 2 K, a more visible opening hysteresis
behaviour can be observed, also implying the characteristic slow
magnetization behaviors of 1. Besides, the hysteresis loops exhibit a
strong dependence of the temperature and field sweep rates, also
supporting the existence of slow magnetic relaxation in 1. At 2 K
with sweep rates of 500 Oe/s, a small coercive field about 92 Oe and
a small remnant magnetization of 0.19 Nβ was clearly observed.
In order to further study the dynamic magnetic behaviour of
complex 1, the temperature dependence of alternating current (ac)
susceptibility was measured under zero dc field (Fig. 3 and Fig. S6†).
Below 20 K, both the inꢀphase (χM′) and outꢀofꢀphase (χM")
susceptibilities of 1 demonstrate a strong temperature and frequency
dependence, clearly indicating the existence of slow magnetic
relaxation in 1. As the temperature decreases, an increase of χM" was
observed and it appeared two distinct peaks. The two separate peaks
can be observed around 6 K and 15 K in the outꢀofꢀphase ac
susceptibility at 10 kHz, indicating the possible occurrence of two
relaxation processes, which is similar to that found in the several
polynuclear dysprosium molecular nanomagnets with different Dy3+
centers.8 Below 7 K, two clear semiꢀcircle shapes coexist in the
QTM process
Ramam process
Orbach process
-10
0.1
0.2
0.3
0.4
0.5
T
-1 / K-1
Fig. 4 The ln(τ) verus Tꢀ1 plots of complex 1. The red scatter and black scatter are
fitted with two modified Debye model. (Blue lines represent the fitting results
).
To further probe the relaxation behavior of complex 1, relaxation
times for each process were extracted from the frequencyꢀdependent
data using the sum of two modified Debye functions at low
temperature range (2.0 – 7.5 K) or a Debye function at high
temperature range (7.5 – 16.0 K) (Fig. 4 and Table S3†). For fast
relaxation (FR) process, utilizing a thermally activated model, the
high temperature data can be fitted well with Arrhenius law and
gained the effective energy barrier 38 K with the preꢀexponential
factor 1.2×10ꢀ7 s. When attempting to fit it through combining the
thermal activated process with the Raman process or QTM process,
we cannot get reasonable results. However, for slow relaxation (SR)
process, considering the combination of relaxation pathways
including Orbach, Raman and QTM processes, we can fit ln(τ)
versus Tꢀ1 plots well, giving n = 4.93, C = 0.010 sꢀ1Kꢀ4.5, τQTM
=
Cole–Cole plots of χM′ versus χM". And then, upon heating, they
0.008 s, Ueff = 223 K and τ0 = 2.3×10ꢀ11 s. For the SR, at the high
temperature, the relaxation process is dominated by Orbach
mechanism. The n value (n = 4.93) is between 4 to 9, which is
reasonable for a Raman process. The relatively small C value further
indicates that the Raman mechanism dominates at low temperature,
and the curvature in the Arrhenius plots are resulted from the
transition from Orbach to Raman relaxation. As temperature
decreases, the relaxation time reaches a plateau (8 ms), which
suggests the presence of a QTM process. It is worth noticing that the
latter effective barrier of 223 K is higher than most tetranuclear Dy4
molecular nanomagnets.8
gradually change to a semiꢀcircles one (Fig. S7†).
6
100 Hz
316 Hz
631 Hz
1000 Hz
1585 Hz
2512 Hz
3981 Hz
6310 Hz
10000 Hz
Hdc=0 Oe
631 Hz
1000 Hz
1585 Hz
2512 Hz
3981 Hz
6310 Hz
10000 Hz
2.0
4
2
0
1.5
1.0
0.5
0.0
4
8
16
T
/ K12
Magnetic dilution studies
To obtain further evidence on the origination of the twoꢀstep
slow magnetic relaxation, we synthesized the diamagnetic
4
8
16
20
24
T
12/ K
yttrium analogue [Y4(L)2(OAc)8(CH3OH)2] (
samples of [(Dy0.06Y0.94)4(L)2(OAc)8(CH3OH)2] (
the paramagnetic Dy3+ ions into the diamagnetic lattice. The
exact ratio of Dy:Y in compound was confirmed by
2) and diluted
3
) by doping
Fig. 3 Temperature dependence of the outꢀofꢀphase ac susceptibility signals under
the zero field for complex 1.
3
inductively coupled plasma (ICP) measurements. The found
concentration of dysprosium dilution is 6%, which is slightly
higher than the value of 5% employed in the synthesis
procedure. XꢀRay singleꢀcrystal and powder diffraction data
indicate that compounds
1 and Fig. S1†). Moreover, the Xꢀray powder diffraction pattern
of matches well with that of and the simulation pattern from
2 and 3 are isoꢀstructural with 1 (Table
3
1
4 | J. Name., 2012, 00, 1-3
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single crystal data of
samples used for the magnetic measurements.
1
, which confirms the phase purity of the
As shown in Fig. 5, directꢀcurrent (dc) magnetic susceptibility
was also measured on microcrystalline samples of 3. It can be seen
clearly that the χMT value descends very slowly in a wide
60
temperature
range
H
dc
= 1 kOe
20
15
45
30
15
0
2 K
5 K
10 K
15 K
20 K
25 K
10
M
5
0
T
0
5
10
15
20
25
HT-1/ kOeK-1
0
50
100
150 200
250
300
T
/ K
Fig. 5 Temperature dependence of χMT for complex 3. Inset: Plots of M−H/T at 2, 5,
10, 15, 20 and 25 K.
Table 3. Reported Dy3+ tetranuclear complexes with slow magnetic relaxation under zero dc filed.
complex
the shortest Dy–O bond (Å)
2.169
Ueff / K (zero dc field)
ref.
[Ln4(L)2(OAc)8(CH3OH)2] (1)
38/223
143/204
190
this work
2.211
2.252
2.243
2.298
2.206
2.322
2.285
2.285
2.323
2.289
2.327
[Dy4(OH)2(bpt)4(NO3)4(OAc)2] (2)
8a
8b
8b
8c
8d
8e
[Dy4Cl2(ꢁ3ꢀOH)2(ꢁꢀOH)2(2,2ꢀbpt)4(H2O)4]Cl2·2H2O·4EtOH (3)
[Dy4Br2(ꢁ3ꢀOH)2(ꢁꢀOH)2(2,2ꢀbpt)4(H2O)4]Cl2·2H2O·4EtOH (4)
[Dy4(ꢁ3ꢀOH)2(bmh)2(msh)4Cl2] (5)
197
9.7/170
158
[Dy4(vht)4(MeOH)8](NO3)4·8.07MeOH·0.65H2O (6)
[Dy4(ꢁ3ꢀbpt)4(ꢁ3ꢀOH)2(ꢁꢀOMe)2(NO3)4]·3MeOH (7)
116
[Dy4(ꢁ3ꢀOH)2L6(acac)4]·2.5CH3CN (8)
[Dy4(ꢁ3ꢀOH)2L6(tmhd)4]·2CH3CN (9)
99
65
8f
8f
[Dy4(ꢁ3ꢀOH)2L6(beac)4]·CH3CN (10)
61
97
8f
8g
8h
[Dy4(bzhdepꢀ2H)4(H2O)4(NO3)4]·6CH3OH·6H2O (11)
[Dy4(HL)2(L)4(ꢁ3ꢀOH)2]·5(MeOH)2·7H2O (12)
3/84
5
-8
-9
100 Hz
316 Hz
631 Hz
1000 Hz
1585 Hz
2512 Hz
3981 Hz
6310 Hz
10000 Hz
H
dc=0 Oe
4
3
2
1
0
Hdc = 0 Oe
2.5
2.0
1.5
1.0
0.5
0.0
1000 Hz
1585 Hz
2512 Hz
3981 Hz
6310 Hz
10000 Hz
2 K
18 K
-10
-11
4
8
16
T
/ K12
-12
0.1
0.2
0.4
0.5
T -1 /0K.3-1
4
8
16
20
24
T
12/ K
Fig. 7 The ln(τ) verus Tꢀ1 plots of complex 3, the black scatter are fitted with
modified Debye model.
Fig. 6 Temperature dependence of the outꢀofꢀphase ac susceptibility signals
under the zero field for complex 3.
from 300 K to 100 K. The very small declines of the χMT products
suggest that the excited Kramer’s doublets are very well isolated
from the ground state. While below 100 K, it shows a more rapid fall
and reaches a minimum of 48.50 cm3molꢀ1K at 2 K, which is quite
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different from the increase of the χMT value below 18 K found for 1, L2− ligand) and J2 (between Dy1 and Dy2 ions through carboxylate)
19
further conforming the ferromagnetic interaction between the Dy3+ interactions within Lines model
(Fig. S13 † ) are both
ions in 1 and suggesting the weakening of magnetic exchange by the antiferromagnetic, and the J3 (between two Dy1 ions through
dilution method. carboxylate) and J4 (between two Dy2 ions through carboxylate)
The ac susceptibility measurements were also conducted for 3 and interactions are both ferromagnetic. We also gave the exchange
obvious temperatureꢀ and frequencyꢀdependence are found in inꢀ energies and the main values of the gz for the lowest eight exchange
phase (χM′) and outꢀofꢀphase (χM") signals (Fig. 6, Fig. S8 and Fig. doublets of complex 1 in Table S7, where the gz value of the ground
S9†). As shown in Fig. 6, similar to that found in 1, the χM" data of exchange state for complex 1 is equal to 49.010.
compound 3 at 10000 Hz also show two shoulder at ca. 6 K and 16
K, indicating the existence of two relaxation processes in the diluted
sample. By using a generalized Debye relaxation model, relaxation
times can be extracted from the frequencyꢀdependent data in the
temperature range of 2.0 – 7.5 K and 12.5 – 18 K (Fig. 7, Table S4
and Fig. S10†). However, we failed to extract reasonable relaxation
times from the frequencyꢀdependent data between 8 and 12 K, which
may be ascribed to the complicated relaxation progresses existing in
this region. Above 15 K, the relaxation time can be well fitted by the
Arrhenius law with a preꢀexponential factor of 7.5×10ꢀ11 s and an
effective barrier Ueff of 203 K, which is comparable to that found in
compound 1 for the SR process, indicating that the SR process in 1
is originated from the single ion nature of individual Dy3+ ion.
Within 1, there are two types of Dy3+ ions with different
coordination environments. We presume that the SR of 1 comes
from the single ion behaviour of Dy2 because it forms the strongest
Fig. 8 Orientations of the local main magnetic axes of the ground doublets on four
coordination bond with phenyl oxygen atom (O1) among the other Dy3+ ions of complex 1.
coordination atoms with the bond distance of 2.169 Å. And the
shortest Dy2–O1 bond can form a strong interaction between the
negative charge of O1 and the fꢀelectron of Dy3+ ion, resulting in the
strong Ising type magnetic anisotropy of Dy2 ions. As for the other
FR process at low temperature region, it is a very complicated task
to definitely explain the relaxation mechanism. Although we can see
a relaxation process at around 6 K in the ac signals, we cannot get its
relaxation time and energy barrier, which prevent us to finger out its
origination. For FR process, it is probably due to the single ion
magnetic behavior of Dy1 ion with relative weaker magnetic
anisotropy compared with Dy2.
It’s well known that the coordination environment of dysprosium
centers can strongly influence their magnetic anisotropy as well as
the relaxation behavior of the resulting compounds.2ꢀ8 Within 1,
compared with Dy1 ion, Dy2 forms the strongest coordination bond
with phenyl oxygen atom with the shortest bond length of 2.169 Å,
resulting in the stronger magnetic anisotropy of Dy2 with larger gz
value of 19.487. And the main magnetic axes on Dy2 ions are very
close to the direction of Dy2ꢀO1 bond. Around Dy1 ion, as indicated
by the theoretical calculation results, the coordination bond length in
the axial sites are obviously longer than that found for Dy2,
indicating a weaker axial ligand field. While, in the equatorial
positions, Dy1 has a stronger ligand field as the average coordination
bond length around Dy1 (2.371 Å) is shorter than that found for Dy2
ion (2.420 Å). All these factors have proved that Dy2 ion has
stronger magnetic anisotropy than Dy1 ion, since Dy1 ion has not
only a weaker axial ligand field but also a stronger equatorial ligand
field.
We also calculated energies of the Kramers doublets (KDs) of the
individual Dy3+ fragments for complex 1 to further discuss the
mechanism of magnetic relaxation (Table S5†). The calculated
energy gap between the ground Kramers doublets and the first
excited one for Dy1 ion is found to be 39.3 cm−1 (56.5 K), which is
reasonable consistent with the extracted experimental energy barrier
of 38 K for fast relaxation because the experimental Ueff is usually
smaller than the corresponding calculated energy gap due to the
quantum tunnelling magnetization between the ground Kramers
doublets. Meanwhile we note that the energy gap between the
ground Kramers doublets and the second excited state for Dy2 ion is
160.6 cm−1 (230.9 K), which is well coincided with the experimental
energy barrier of slow relaxation (223 K).
Theoretical Calculations
In order to gain more insight into the magnetic anisotropy of Dy3+
ions, completeꢀactiveꢀspace selfꢀconsistent field (CASSCF)
calculations on individual Dy3+ fragments of Dy1(Dy1a) and
Dy2(Dy2a) for complex 1 on the basis of Xꢀray determined
16
geometries have been carried out with MOLCAS 8.2
Single_Aniso
and
17
programs (Fig. S11†, see Supporting Information
for details). The lowest eight Kramers doublets (KDs) energies and
the corresponding g tensors of individual Dy3+ fragments of complex
1 are shown in Table S5. The calculated ground gz values of
individual Dy3+ fragments for Dy1(Dy1a) and Dy2(Dy2a) are both
close to 20, thus the Dy3+ꢀDy3+ exchange interactions for complex 1
were approximately regarded as the Ising type during the fitting.
17
The program Poly_Aniso
was used to fit the magnetic
susceptibilities of complex 1 using the exchange parameters from
Table S6. All parameters were calculated with respect to the
pseudospin
= 1/2 of the Dy3+ ions. For complex 1, the total
%
S
coupling parameters J (dipolar and exchange) were included into
fitting the magnetic susceptibilities. The calculated and experimental
χMT versus T plots of complex 1 are shown in Fig. S12, where the
calculated χMT values match reasonably with the experimental
data.18 From Table S6, the J1 (between Dy1 and Dy2 ions through
To further investigate the influences of coordination geometry and
ligand field effects for 1, we compared it with the reported Dy3+ꢀonly
m
olecular nanomagnets with different tetranuclear clusters,8 some of
6 | J. Name., 2012, 00, 1-3
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2
(
a
) N. Ishikawa, M. Sugita, T. Ishikawa, S. Koshihara and Y.
which also revealed two discernible relaxation processes (Table 3).
Through careful analysis of these clusters, it’s not difficult to find
that molecule symmetry is very important for similar tetranuclear
clusters. Complexes with energy barriers higher than 150 K are all
centrosymmetric Dy4 aggregates constructing by a symmetrical
ligand, which guarantees the architectural symmetry, successfully
controls the easyꢀaxis arrangement of the spin carriers and thus
modulates the magnetic anisotropies of the clusters. What’s more,
the shortest Dy–O bond is one of the key factors determining the
anisotropic axes of dysprosium ions, which have been evidenced by
many previous reports and can be roughly perceived from data of
Table 3. The shortest Dy–O bond (2.169 Å) in complex 1 implies a
strong ligand field generated by the symmetrical Schiff base ligand
H2L, producing a relatively strong magnetic anisotropy and leading
to the highest effective energy barrier within the reported
tetranuclear dysprosium clusters.
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In this work, we have reported the synthesis and structural
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a centrosymmetric tetranuclear dysprosium
complex constructed by a novel symmetrical Schiff base ligand and
acetate groups. Magnetic studies indicate ferromagnetic interaction
between dysprosium ions and two relaxation processes under zero dc
field with the effective energy barriers of 38 K and 223 K, for FR
and SR respectively. Further magnetic studies on magnetic diluted
sample and theoretical calculation indicate that the slow magnetic
relaxation process originates from the single ion magnetic behaviors
of dysprosium ion. And, to the best of our knowledge, the effective
energy barrier for this compound is the largest among reported
tetranuclear dysprosium complexes, which might be due to the
strong coordination DyꢀO bond and/or relatively high symmetry
originated from designed symmetrical Schiff base ligand.
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Acknowledgements
This work was supported by the National Natural Science
Foundation of China (No 21671024) and National Key Basic
Research Program of China (2013CB933402).
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