Tetranuclear lanthanide complexes showing magnetic refrigeration and single molecule magnet behavior

Article abstract of DOI:10.1039/c8nj05109g

Three tetranuclear lanthanide complexes: {[Ln₄(L)₆(pbd)₄(μ₃-OH)₂]·2CH₃CN} (Ln = Gd (1), Tb (2), Dy (3); HL = 5-(benzylideneamino)quinolin-8-ol, pbd = 1-phenylbutane-1,3-dione) were fabricated and structurally characterized. Compounds 1-3 are isostructural belonging to the triclinic system with space group P1. The core of the complexes contains a tetranuclear arrangement of Ln^III ions which is held together by two pyramidal μ₃-OH^- ions and six μ₂ phenol hydroxyl oxygen atoms derived from six L^- ligands. Magnetic studies indicated that 1 exhibits cryogenic magnetic refrigeration properties (maximum -ΔS_m = 21.41 J K^-1 kg^-1, ΔH = 7 T at 2.0 K), whereas compound 3 exhibits slow relaxation of the magnetization with an energy barrier (ΔE/k_B) of 81.48 K and τ₀ = 6.48 × 10^-8 s.

Full text of DOI:10.1039/c8nj05109g

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ISSN 1144-0546
PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
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Tetranuclear lanthanide complexes showing magnetic
refrigeration and single molecule magnet behavior
a
b
b
a
Hong Yu, Jia-Xing Yang, Ju-Qing Han, Peng-Fei Li, Yin-Ling
1
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d,
b,
a, c,
Hou, * Wen-Min Wang, * Ming Fang
Abstract：
Three tetranuclear lanthanide complexes: {[Ln (L) (pbd) (µ -OH) ]·2CH CN} (Ln =
4
6
4
3
2
3
Gd (1), Tb (2), Dy (3); HL = 5-(benzylideneamino)quinolin-8-ol, pbd =
1
-phenylbutane-1,3-dione) were fabricated and structurally characterized. Compounds
1
-3 are isostructural belonging to the triclinic system with space group P-1. The core
of the complexes contain a tetranuclear arrangement of LnIII ions which is held
-
together by two pyramidal μ -OH ions and six µ phenol hydroxyl oxygen atoms
3
2
−
derived from six L ligands. Magnetic studies indicated that 1 exhibits cryogenic
−1
−1
magnetic refrigeration property (maximum −ΔS = 21.41 J K kg , ΔH = 7 T at 2.0
m
K), whereas compound 3 exhibits slow relaxation of the magnetization with energy
-8
barrier (ΔE/k ) of 81.48 K and τ = 6.48×10 s.
B
0
Keyword: Lanthanide, Tetranuclear compounds, Magnetocaloric effect,
Single-molecule magnet behaviour
Introduction
During the past two decades, lanthanide (III) polynuclear clusters have attracted
a Department of Chemistry, Hebei Normal University of Science & Technology, Qinhuangdao
0
66004, Hebei province, P. R.China. E-mail:fangmingchem@163.com
b Department of Chemistry, Taiyuan Normal University, Jinzhong 030619, PR China. E-mail:
wangwenmin0506@126.com
c Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), College of
Chemistry, Nankai University, Tianjin 300071, China
d School of Life and Health Science, Kaili University, Kaili, 556011, China, hyl0506@126.com
†
Electronic supplementary information (ESI) available: Selected bond lengths and angles. CCDC
1871768-1871770. For ESI and crystallographic data in CIF or other electronic format see DOI:
0.1039/c000000x/
1

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[1]
great interest because of their potential applications in molecular magnetic materials,
such as molecular magnetic refrigeration and single-molecule magnets (SMMs).[2, 3]
Molecular magnetic refrigeration exhibiting an enhanced magnetocaloric effect (MCE)
are proposed to replace the expensive and increasingly rare He-3 in ultralow
temperature refrigeration.[4] To our knowledge, the magnetocaloric effect (MCE)
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represents the change of isothermal magnetic entropy (ΔS ) and adiabatic temperature
m
[
5]
(
ΔT ) with external magnetic field variation. In general, an outstanding molecular
ad
magnetic refrigeration requires a large spin multiplicity associated with a negligible
anisotropy.[6] Thus, Gd is a common constituent element for molecular refrigerant
materials, because it is favored by it having a 8S7/2 ground-state term (S=7/2, L=0),
with high spin and zero orbital momentum, and therefore no spin-orbit coupling is
possible. These theoretical perspectives were further supported by a large amount of
Gd-containing clusters have been reported,[7] such as, the Gd104 with large magnetic
3+
-1
-1
[6a]
entropy change of ΔS = 46.9 J kg K (T = 2.0 K and ΔH = 7 T) and Gd with
m
60
-
1
-1
[8]
large ΔS = 66.5 J kg K (T = 3.0 K and ΔH = 7 T).
m
Furthermore, since the discovery of Ishikawa et al.’s double-decker compound
(Bu N)[Tb(Pc) ] (H Pc = phthalocyanine) shows slow relaxation of the magnetization
4
2
2
at low temperature,[9] considerable attentions have been paid toward introducing 4f
ions into SMMs in either mixed 3d/4f or pure 4f systems because of their inherent
magnetic anisotropy arising from a large, unquenched orbital angular momentum.[10-13]
In particular, dysprosium plays a crucial role because of its unparalleled single-ion
anisotropy and the spinparity effect for a Kramers ion, which is responsible for
numerous ground-breaking results.[12,13] Such as, Layfield group reported a
dysprosium metallocene with a record energy barrier U = 1837 K in zero field and a
eff
blocking temperature of 60 K,[12] exceeding the previous one (U = 1815 K) reported
eff
by Zheng’s group.[13] These excellent results attracted more and more researcher focus
on the study of Ln-SMMs.
The Dy (III) ions are ideal for constructing SMMs with high energy barriers,
benefiting from both the high moment and the high anisotropy of the spin–orbit
coupled Dy (III) Kramers doublet ground state.[14] However, the magnetic exchange

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effect between Dy (III) ions is weakened by the shielding effect of 5s and 5p orbitals,
meanwhile, quantum tunneling phenomenon often occurs in such complexes,
resulting in the reduction of effective potential energy barrier. Therefore, how to
increase the magnetic exchange between ions and suppress the quantum tunneling
effect is the key to improving the energy barrier of Dy-SMMs. The ligand field
surrounding lanthanide ions play an important role in tuning the magnetic properties
1
1
1
1
1
1
1
1
1
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of SMMs, and a slight difference in the structure may have an effect on the directions
of easy axes, thus resulting in different slow relaxation behaviors.[15] In a quest for
excellent magnetic behaviors for Ln(III)-based polynuclear species, an attractive
8
-hydroxyquinoline Schiff base derivative ligand was employed to construct
lanthanide polynuclear compounds, which have excellent structural features: (i) it can
easily coordinate with Ln(III) ions through N and O donor atoms; (ii) the phenoxy
atom of the ligand can act as a bridge between Ln(III) ions centers, which can
transmit magnetic exchange efficiently. Taking advantage of this ligand, three
tetranuclear
lanthanide
complexes
with
a
general
formula
{[Ln (L) (pbd) (µ -OH) ]·2CH CN} (Ln = Gd (1), Tb (2), Dy (3); HL =
4
6
4
3
2
3
5-(benzylideneamino)quinolin-8-ol, pbd = 1-phenylbutane-1,3-dione) were fabricated
and structurally characterized. Magnetic study reveals that 1 exhibits cryogenic
−
1
−1
magnetic refrigeration property with maximum −ΔS = 21.41 J K kg for ΔH = 7 T
m
at 2.0 K, whereas 3 displays slow relaxation of the magnetization (ΔE/k = 61.6 K and
B
-6
τ = 2.77×10 s).
0
Experimental section
Materials and Physical Measurements.
All chemicals purchased were of reagent grade and used without further
purification. Water used in the reactions is distilled water. Analyses for C, H and N
were carried out on a Perkin-Elmer analyzer. IR spectra were collected in the range of
00−4000 cm− with a Bruker TENOR 27 spectrophotometer using KBr pellets. 1H
1
4
NMR spectrum was performed on a Bruker Avance III HD 500 MHz instrument.
PXRD data were examined on a Rigaku Ultima IV instrument with Cu Kα radiation

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o
-1
o
(
λ = 1.54056 Å), with a scan speed of 10 min in the range of 2θ = 5–50 .
Luminescence properties were recorded on an F-4500 FL spectrophotometer with a
xenon arc lamp as the light source. Magnetic susceptibilities were performed on a
Quantum Design MPMS-XL7 and a PPMS-9 ACMS magnetometer. Diamagnetic
corrections were made with Pascal’s constants for all the constituent atoms.
Crystallographic studies.
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Single-crystal X-ray diffraction measurements of 1 - 3 were carried out at room
temperature on a CCD X-ray diffractometer equipped with graphite monochromated
MoK radiation ( = 0.71073 Å). Lorentz polarization and absorption corrections
were applied. The structure was solved by direct methods and refined with a
full-matrix least-squares technique based on F2 using the SHELXS-97 and
SHELXL-97 programs.[16] All the nonhydrogen atoms were refined with anisotropic
parameters while H atoms were placed in calculated positions and refined using a
riding model. The disorder of solvent molecules is serious, so squeeze was used to
process this data, and molecular formula of 1-3 is determined by elemental analysis.
Crystallographic data for 1 – 3 were summarized in Table 1. Selected bond lengths
and angles were summarized in Table S1-S3.
Table 1 Crystallographic Data and Structure Refinements for 1-3
1
2
3
Formula
C
140
H
110Gd
4
N
14
O
16
C
140
H
110
N
14
O
16Tb
4
C
140
H
4 14 16
110Dy N O
−
1
Mr (g mol )
Cryst syst
Space group
a (Å)
2873.41
triclinic
P-1
2880.09
triclinic
P-1
2894.5
triclinic
P-1
14.9021(8)
14.9151(7)
15.4503(8)
15.5107(8)
80.272
13.7157(6)
16.2542(8)
17.1198(7)
87.098
b (Å)
15.4352(10)
15.5643(10)
80.276
67.618
68.757
3083.4(3)
1
c (Å)

()
 ()
()
67.467
89.370

68.699
86.006
V (Å3)
3074.1(3)
1
3802.4(3)
1
Z
−
3
D
c
(g cm )
1.547
1.556
1.264
−
1
μ (mm )
2.195
2.345
1.998

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θ/°
2.448 to 25.010
2.430 to 26.418
1436
2.186 to 25.372
1396
F(000)
1432
35405
Reflns collected
Unique reflns
50754
56611
10790
12629
13934
R
int
0.0711
0.0988
0.1160
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GOF on F2
1.073
1.010
1.033
R
1
, wR
2
[I >2σ(I)]
0.0909, 0.2220
0.1415, 0.2668
0.0478, 0.0870
0.0938, 0.0982
0.0455, 0.0935
0.0948, 0.1067
R
1
, wR
2
(all date)
Synthesis of 5-aminoquinolin-8-ol.
A mixture of 5-nitroquinolin-8-ol (50 mmol) and 5 % Pd/C (0.75 g), which was used
as catalyst, in a 1.3 % ratio in absolute isopropanol was heated to 65 °C, and then 12
mL of 85 % hydrazine hydrate was dropped into the mixture over 1 h. It was heated to
9
0 °C and refluxed for 6 h. Finally, the solvent was removed, and dichloromethane
was used to wash the grass green solid product (yield: 50.8%). Elemental analysis (%):
Calcd for C H ON (F = 160.42): C 67.32, H 5.00, N 17.50. Found: C 67.28, H 4.97,
9
8
2
w
N 17.82.
Scheme 1. The synthesis of HL.
Synthesis of 5-(benzylideneamino)quinolin-8-ol (HL)
The Schiff base ligand 5-(benzylideneamino)quinolin-8-ol (HL) was synthesized in a
simple aldimine condensation reaction of 5-aminoquinolin-8-ol (10 mmol) with
benzaldehyde (10 mmol) in 50 mL ethanol with the catalyze of 5 drops formic acid
(
Scheme 1). The reaction mixture was stirred for 5 h at 85 ℃. The product was
isolated from the mixture and it was purified by recrystallization from a mixed solvent
of ethanol and acetone (3/1, v/v). The purified product was obtained as a green solid
(
yield 2.0 g, 80.6%). Elemental analysis (%): Calcd for C H ON (F = 248.28): C
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7.40, H 4.87, N 11.28. Found: C 77.28, H 4.97, N 11.22. The IR spectra and 1H
NMR spectrum (500 MHz, CDCl ) of the ligand are shown in Figure S1 and Figure
3
S2.
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Synthesis of Complexes 1−3
The compounds 1-3 were obtained by the reaction of Ln(pbd) ·2H O (0.025 mmol;
3
2
Ln(III) = Gd (1), Tb (2), Dy (3)) and HL (0.025 mmol) in mixture solution containing
8 mL acetonitrile and 4 mL dichloromethane. The solution was sealed in a 20 mL vial
and heated at 70 °C for 24 h, followed by cooling to room temperature slowly. Yellow
block shaped crystals suitable for single-crystal X-ray data collection was isolated.
Yield: 64%, 62%, and 58% based on HL for 1-3, respectively. Elemental analysis
found (%) for C H Gd N O (1): C 58.56, H 3.77, N 6.88; for C H N O Tb
4
1
40 110
4
14 16
140 110 14 16
(2): C 58.44, H 3.89, N 6.85; for C H Dy N O (3): C 58.05, H 3.76, N 6.79.
140 110 4 14 16
Results and Discussion
Crystal structures of compounds 1–3
Single crystal X-ray diffraction analyses reveal that the three compounds are
isostructural, and crystallize in the triclinic space group P−1 (Table 1) , with Z = 1.
The structure of 3 (Ln = Dy) will be described here as a representative example of the
series. The molecular structure (Figure 1) consists of a precisely coplanar [Dy ] core
4
−
with crystallographic inversion symmetry (Figure 2), in which two pyramidal µ −OH
3
oxygen atoms (O8) are located at the opposite sides of the [Dy ] plane and are
4
displaced out of that plane by 0.89 Å. The bond distances of Dy−O8 are 2.347(4),
2
9
.340(4) and 2.352(4) Å, and the Dy−O8−Dy angles 110.45(16), 110.82(16) and
8.08(15), and a Dy—Dy distance of 3.8593(6), 3.5434(4) and 3.8582(5) Å. The
tetranuclear [Dy ] ions are further bridged by six µ phenol hydroxyl oxygen atoms
4
2
−
−
(
O1, O2, O3) derived from six L ligands, generating a [Dy O ] core. The L ligand
4 8
serves as a didentate ligand and chelates the central Dy(III) ions through one phenol
hydroxyl oxygen atom (O1/O2/O3) and one pyridyl nitrogen (N2/N4/N6), and the
-
oxygen atom bridge to another Dy(III) ions; and each pbd chelates the Dy atom
through two oxygen atoms. Thus, Dy1 is eight−coordinated, chelates by one L−

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-
−
ligands and one pbd , and also coordinate with two µ −OH oxygen atoms and two µ
3
2
phenol hydroxyl oxygen atoms from the ligands. While Dy2 is chelated by two L−
-
−
ligands and one pbd , and coordinate with one µ −OH oxygen atom and one µ
3
2
phenol hydroxyl oxygen atom. The Dy−O bond lengths are in the range of
.290(4)−2.421(4) Å; the Dy−N bond lengths are 2.533(5), 2.558(5) and 2.525(5) Å.
1
1
1
1
1
1
1
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These values are comparable to those of the already reported lanthanide complexes.[17]
The coordination geometries for Dy(III) ions in an asymmetry unit were performed by
continuous-shape measurements (CShM) using SHAPE 2.0 software. The calculated
3+
results listed in Table S4 reveal that the centre Dy ions possess square antiprism
geometries.
Figure 1. Molecular structure of 3 (left) (all hydrogen atoms are omitted for clarity), the [Dy4]
−
core bridged by µ −OH oxygen atom andµ phenol hydroxyl oxygen atom (right).
3
2
Figure 2. The side view of the [Dy O ] core of 3 (all hydrogen atoms are omitted for clarity).
4
8
Powder X-ray diffraction (PXRD)
The purity of crystalline powders of 1-3 were confirmed by powder X-ray

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diffraction (PXRD) (Figure S3). The observed PXRD patterns are in good agreement
with the results simulated from the single crystal data, suggesting the purity of the
solid samples. The differences in intensity could be due to the preferred orientation of
the crystalline powder samples.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Luminescence properties of 2
The solid-state luminescence property of 2 was measured at the exciting
wavelength of 300 nm at room temperature. As shown in Figure S4, the emission
III
spectrum of 2 exhibits the characteristic emissions of Tb ion. The emission spectrum
presents four peaks at 492, 546, 588 and 621 nm which are assigned to the transition
5
7
5
7
of D to F (J = 6, 5, 4, 3). Among them, the emission peak at 546 nm ( D → F ) is
4
J
4
5
the strongest.
Figure 3. The plots of χ T versus T for 1(■), 2(●) and 3(▲) under 1000 Oe field.
M
Magnetic Properties of 1-3
The magnetic susceptibility of 1-3 were measured on the microcrystalline samples
in the temperature range from 2 to 300 K and under 1000 Oe field, as shown in Figure
3
-1
3. At room temperature, the χ T value for 1-3 is 31.59, 47.33 and 56.73 cm K mol
M
3
which is close to the corresponding theoretical value of 31.52, 47.28 and 56.68 cm K

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-
1
3+
3+
8
mol expected for four magnetically Ln ions: Gd in the S ground state (g = 2)
7/2
3
+
7
3+
6
for 1, Tb in the F ground state (g = 3/2) for 2 and Dy in the H ground state (g
6
15/2
=
4/3) for 3.[18] For 1, the χ T value stays almost constant in the temperature range
M
3
−1
3
00−30 K and then decreases rapidly to a minimum value of 16.27 cm K mol at 2
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
K, which indicates the presence of a weak antiferromagnetic interaction between the
3
+
[19]
adjacent Gd ions. The magnetic interaction was analyzed by means of a dimer
law based on the Hamiltonian Ĥ = -JŜ Ŝ with Ŝ = Ŝ = 7/2, a corresponding
A
B
A
B
equation as following was deduced, and the magnetic properties of 1 can be
appropriately described by it.[20]
2
2
J / kT
3J / kT
6J / kT
10J / kT
15J / kT
21J / kT
28J / kT
28J / kT
2
Ng 
e
 5e
14e
 30e
 55e
 91e
13e
140e
15e
M  2*

J / kT
3J / kT
6J / kT
10J / kT
15J / kT
21J / kT
kT
1 3e
 5e
 7e
 9e
11e
-1
Least-squares fitting of the experimental data leads to J = -0.15 cm , g = 2.01, and
the agreement factor R, defined as R = ∑(χobsd–χcalcd)2/∑(χobsd) , is 1.4×10 . The
2
−2
negative and small J value indicates very weak antiferromagnetic coupling interaction
3
+
between adjacent Gd . For lanthanide ions, the 4f electrons are well shielded by
outer-shell electrons, compared with 3d electrons, the interaction of 4f electrons from
adjacent lanthanide ions is rather weak.
3
For 2, as the temperature is lowered, χ T values decreases slightly to 43.54 cm
M
−
1
K mol at 60 K which may attribute to thermal depopulation of Stark sublevels, with
3
-1
further cooling the χ T values decrease quickly to 13.65 cm K mol which may
M
3
+
[21]
ascribe to antiferromagnetic interactions between adjacent Tb ions.
For 3, the χMT
3
-1
value decreases slowly to 53.57 cm K mol on cooling from 300 to 20 K, which may
attribute to thermal depopulation of Stark sublevels dominates the magnetic properties
3
above 20 K. With further cooling the χ T values increases quickly to 71.63 cm K
M
-1
mol at 2 K. This behavior is indicative of the presence of ferromagnetic interactions
between adjacent Dy(III) ions.[21]
The magnetization data of 1 is carried out at a field of 0−8 T between 2 and 10 K.
As shown in Figure 4 (left), the M versus H curves display a gradual increase with the
increasing field and saturation values of 28.11 Nβ for 1 at 80 kOe and 2 K, which is
extremely close to the theoretical value of 28 Nβ for four individual Gd(III) (S = 7/2,

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1
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2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
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g = 2) ions. Magnetic entropy changes ΔS of 1 can be calculated from the M versus
m
H data to evaluate the MCE. ΔS are calculated by using the Maxwell equation:
m
ΔS (T) = ∫ [∂M(T, H)/∂T]H dH
(1)
m
According to eq 1,[22] the −ΔS values of 1 can be obtained; the plots of −ΔS
m
m
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
versus T are shown in Figure 4 (right). For 1, the maximum value of −ΔS is 21.41 J
m
−
1
−1
−1
−1
K kg (calculated as 4Rln(2S+1), expected maximum −ΔS is 24.05 J K kg for a
m
field change ΔH = 7 T at 2.0 K. The difference of −ΔS between the experimental and
m
theoretical values for 1 might be due to the antiferromagnetic interaction in 1.[23] The
maximum −ΔS of 1 is smaller than the antiferromagnetic {Gd } complexes which
m
4
have been reported:[24] [Gd (µ -OH) (L) L L (HOCH ) ]·11H O {H2L
=
=
=
4
3
2
2
1
2
3 2
2
2
,3-bis((E)-(2-hydroxy-3-methoxy benzylidene) amino) maleonitrile, HL1
(
(
2-amino-3-((E)-(2-hydroxy-3-methoxy benzylidene)amino)maleonitrile), H3L2
(1E,3Z,8Z,10E)-1,6,11-tris(2-hydroxy-3-methoxyphenyl)-2,5,7,10-tetraazaundeca-1,
-1
-1
3,8,10-tetraene-3,4,8,9-tetracarbonitrile), −ΔS = 27.2 J kg K for ΔH = 7 T at 3 K},
m
[Ln L (OH) ](OAc) ·4H O
{H2L
=
butanedihydrazidebridgedbis
4
4
2
2
2
(3-ethoxysalicylaldehyde), −ΔSm = 24.4 J kg-1 K-1 for ΔH = 7 T at 3 K},
[Gd (CO )(L) (acac) (H O) ]·2CH CN
(A)
and
4
3
4
2
2
4
3
[Ln (CO )(L) (acac) (CH OH) (H O) ]·CH OH·H O
(B)
{H2L
4
3
4
2
3
2
2
2
3
2
=
2-(hydroxyimino)-2-[(3-methoxyl-2-hydroxyphenyl)methylene]hydrazide, Hacac =
-1
-1
acetylacetone, −ΔS (A) = 31.23 J kg K for ΔH = 7 T at 2 K, −ΔS (B) = 27.06 J
m
m
-
1
-1
[25]
kg K for ΔH = 7 T at 2.5 K); however, it is larger than some Gd clusters:
4
[
Gd (μ -OH) L (acac) ]·2CH CN
{HL
=
5-(4-o-hydroxybenzylidene)-8-
4
3
2
6
4
3
-1
-1
hydroxylquinoline, acac = acetylacetone, −ΔS =18.85 J kg K for ΔH = 7 T at 2.5
m
−
1
−1
K} and [Gd (dbm) (L) (μ -OH) ]·5CH CN(−ΔS = 16.35 J K kg for ΔH = 7.0 T at
4
4
6
3
2
3
m
3
1
.0 K; HL = 5-(4-pyridinecarboxaldehyde)amino-8-hydroxylquinoline, dbm =
,3-diphenyl-1,3-propanedione).

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1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Figure 4. (Left) M versus H plots for 1 at T = 2.0−10.0 K and H = 0−80 kOe. (Right) Temperature
dependence of magnetic entropy change (−ΔS ) as calculated from the magnetization data of 1 at
m
T = 2−10.0 K and 0−7 T.
The variation of the magnetization with the applied magnetic field of 2 and 3 has
been measured at 2 K in the field between 0 and 80 kOe, and shows a gradual increase
of the magnetization at low fields. With the increasing of the field, 2 shows a
saturation of 25 Nβ at 80 kOe, while 3 exhibits a lack of saturation even at 80 kOe
(
Fig. S5). This behavior of 3 may be explained as the presence of magnetic anisotropy
and/or the lack of a well defined ground state suggesting the presence of low-lying
excited states that might be populated when a field is applied.[26]
In order to investigate the dynamics of magnetization, the temperature
dependences alternating current (AC) magnetic susceptibility measurements on 2 and
3
were performed in the temperature range 2−20 K and in the 111−2311 Hz frequency
range with a 3 Oe ac magnetic field and under 0 dc field, however compound 2 (Tb4)
does not exhibit a frequency dependence under these conditions (Figure S6). As show
in Figure 5, compound 3 displays clear frequency-dependence at low temperature,
indicative of the possibility of SMM behaviour. Both the in-phase (χ′) and
out-of-phase (χ″) signals of 3 show an increasing trend in the low temperature zone,
which may suggest the quantum tunneling of magnetization (QTM) were occurred in
3. The difference of dynamics of magnetization between 2 and 3 may originate from
the difference of intrinsic properties of lanthanide ions.

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1
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1
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2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
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1
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4
5
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7
8
9
0
1
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5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Figure 5. Temperature dependence of the in-phase (χ′) and out-of-phase (χ″) ac susceptibilities for
3 in H = 0 Oe with an oscillation of 3.0 Oe.
dc
’
’
Figure 6. Frequency dependence of the in-phase (χ′) and out-of-phase (χ ) ac susceptibilities for 3
at 2.0−16.0 K in H = 0 Oe.
dc
Figure 7. The cole-cole plot at different temperature in H = 0 Oe.
dc

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In addition, to further understand the dynamics of magnetization of 3, frequency
dependence ac susceptibilities (Figure 6) were measured at 2-15 K under the zero dc
fields, and the shape and frequency dependence further confirm the presence of
single-molecule magnet behavior in 3.[27] The Cole–Cole plots of 3 are shown in
Figure 7. A generalized Debye model was used in an attempt to fit the Cole-Cole
diagram of 3 (Figure 7), and in the temperature range 2-15 K, α parameters are in the
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
range of 0.25−0.40. The relatively small distribution coefficients α of the Dy4
complexes indicate that there is a narrow distribution of relaxation time in 3.[28]
The relaxation time τ of 3 can be accessed from the frequency-dependent data, and
the Arrhenius plot exhibit an obvious curvature (Figure 8), indicating that a
combination of multiple relaxation pathways has to be taken into account. Data in the
entire 3–14 K temperature range were analyzed by using the following equation:[29]
n
-1
lnτ = -ln[AT + CT + τ exp(-U / k T)]
0
eff
B
n
-1
where AT, CT , and τ exp(U / k T) represent direct, Raman, and Orbach
0
eff
B
-1
relaxation processes, respectively. Least-squares fitting of lnτ versus T leads to A
-8
=
19.42, n = 3.9, C = 0.03, U = 81.48 K, τ = 6.48×10 s and the agreement factor
eff 0
R, defined as R = ∑(χobsd–χcalcd)2/∑(χobsd) , is 3.29×10 . The obtained pre-exponential
2
−3
−
6
-12
factor (τ ) is consistent with the reported values of 10 −10 s for the Dy clusters
0
4
SMMs.[30] The anisotropic energy barrier (ΔE/k ) of 3 is smaller than some reported
B
Dy4
compounds:
[Dy (dbm) (L) (μ -OH) ]·4CH CN·2H O
(L=
4
4
6
3
2
3
2
-(4-pyridinecarboxaldehyde)amino-8-hydroxylquinoline, U /k = 89.38 K),[25b]
5
eff
B
[
Dy (OH) (bpt) (NO ) (OAc) ] (Hbpt = 3,5-bis(pyridin-2-yl)-1,2,4-trizole, U /k =
4 2 4 3 4 2 eff B
2
06 K);[31] however, it is larger than some related tetranuclear dysprosium compounds
literature: [Ln (L) (C H COO) (MeOH) ] (HL
2,6-bis((furan-2-ylmethylimino)methyl)-4-methylphenol, U /k = 17.2 K),[32a]
in
=
4
2
6
5
12
4
eff
B
[Ln (μ -OH) L (acac) ]·2(CH CN) (L = N,N’-bis(salicylidene)-1,2-ethanediamine,
4
3
2
2
6
3
acac
=
acetylacetonate,
U /k
=
13.95
(LH3
K),[32b]
=
eff
B
[{(LH) Dy }(μ -O) ](H O) ·2CH OH·8H O
2
4
2
4
2
8
3
2
6
-hydroxymethyl)-N’-((8-hydroxyquinolin-2-yl)methylene)-picolinohydrazide, U /k
eff B

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1
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2
2
2
2
2
2
2
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2
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3
3
3
3
3
3
3
3
3
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=
54.2 K).[32c]
0
1
2
3
4
5
6
7
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0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Figure 8. lnτ versus T−1 plots for 3. The red solid lines represent the least-squares fits of the
experimental data to the Arrhenius law.
Conclusions
In summary, three tetranuclear lanthanide coordination complexes based on
5
-(benzylideneamino)quinolin-8-ol have been constructed and structurally
characterized. Systematic magnetic studies for them have been performed, 1 exhibits
−
1
−1
cryogenic magnetic refrigeration property with maximum −ΔS = 21.41 J K kg for
m
a field change ΔH = 7 T at 2.0 K); compound 3 exhibits slow relaxation of the
-
8
magnetization with U = 81.48 K and the preexponential factor τ = 6.48×10 s.
eff
0
Acknowledgments
We gratefully acknowledge the National Natural Science Foundation of China
21501043, 21501079), the Natural Science Foundation of Education Department of
(
Guizhou Province(KY[2015]454).
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