The synthesis, structure, magnetic and luminescent properties of a new tetranuclear dysprosium (III) cluster

Article abstract of DOI:10.1016/j.jssc.2011.10.033

The synthesis and characterization of [Dy₄(dhampH ₃)₄(NO₃)₂](NO₃) ₂ (1), a new tetranuclear dysprosium (III) complex, is described. The compound was characterized by its X-ray structure, magnetic properties as well as the luminescent spectra. The compound crystallizes in a P1 space group with a zig-zag linear form of geometry. The ac magnetic susceptibilities of the molecule indicate that it is a magnetic molecule with a slow magnetization relaxation. The molecule also exhibits an emission spectrum that was confirmed to be ligand based. These results indicate that this molecule has both a slow magnetization relaxation (that could be potentially a SMM) and luminescent properties.

Full text of DOI:10.1016/j.jssc.2011.10.033

Journal of Solid State Chemistry 185 (2012) 166–171
Contents lists available at SciVerse ScienceDirect
Journal of Solid State Chemistry
journal homepage: www.elsevier.com/locate/jssc
The synthesis, structure, magnetic and luminescent properties
of a new tetranuclear dysprosium (III) cluster
n
Yen-Han Chen a, Yun-Fan Tsai a, Gene-Hsian Lee b, En-Che Yang a,
a Department of Chemistry, Fu Jen Catholic University, Hsinchuang, Taipei, 24205 Taiwan, ROC
b Instrument Centre, College of Science, National Taiwan University, Taipei, 106 Taiwan, ROC
a r t i c l e i n f o
a b s t r a c t
Article history:
The synthesis and characterization of [Dy4(dhampH3)4(NO3)2](NO3)2 (1), a new tetranuclear dyspro-
sium (III) complex, is described. The compound was characterized by its X-ray structure, magnetic
Received 10 July 2011
Received in revised form
11 October 2011
¯
properties as well as the luminescent spectra. The compound crystallizes in a P1 space group with a zig-
zag linear form of geometry. The ac magnetic susceptibilities of the molecule indicate that it is a
magnetic molecule with a slow magnetization relaxation. The molecule also exhibits an emission
spectrum that was confirmed to be ligand based. These results indicate that this molecule has both a
slow magnetization relaxation (that could be potentially a SMM) and luminescent properties.
& 2011 Elsevier Inc. All rights reserved.
Accepted 18 October 2011
Available online 12 November 2011
Keywords:
Slow magnetization relaxation
Magnetic properties
Luminescence
Dysprosium
1. Introduction
complexes have been developed through association with a novel
phenomenon, such as two-step magnetization relaxation [8].
The synthesis of new metal clusters is of great interest,
because of their wide variety of architectural features [1].
Through different topological arrangements, metal ions can inter-
act with each other differently and thus trigger fascinating
physical properties, including luminescence and/or magnetism.
Magnetic metal clusters, which have a sluggish magnetization
relaxation, are attractive because they are potential candidates for
use in memory devices [2]. One extremely successful example are
single molecule magnets (SMMs) [3], which have a large mole-
cular spin and zero-field splitting (zfs) and can be utilized
quantum computing, spintronics and magnetic refrigeration [4].
Lanthanides, because they have a large number of unpaired
electrons and un-quenched 4f orbitals, are considered to be
excellent building blocks for creating clusters with slow magne-
tization relaxations. In many cases, chemists have attempted to
prepare 3d–4f mixed metal clusters by incorporating 3d metals
such as Cr, Mn, Fe, Co and Ni with lanthanides [5]. However,
chemists have also began to focus on pure lanthanide-based
clusters as well [6]. A wide variety of 4f metals with different
molecular geometries have been reported in the literature.
Among these, Dy is known to produce more clusters with
frequency-dependent ac signals than other lanthanides [6,7]. In
particular, in the past year, two linear tetra-nuclear Dy4
Luminescence has been applied in a wide variety of molecular
materials, such as metal–organic frameworks (MOF) [9]. Although
not many cases have been reported, magnetic molecules which
also have fluorescence properties have attracted interest in recent
years [10]. One of the advantages of fluorescent magnetic mole-
cules is that the luminescence provides a means for their detec-
tion. This property would facilitate the construction of actual
molecular devices. Molecular luminescence is known to be
applicable to light-emitting diodes (LED), organic light-emitting
diodes (OLED) [11], tunable lasers and for optical storage.
A molecule showing both magnetism and luminescence would
be quite interesting. However, to the best of our knowledge, a
single molecule that has both luminescent and slow magnetiza-
tion relaxation (SMMs-like) behavior is quite rare. In the present
paper, we report on the preparation and characterization of a new
linear Dy4 molecule that simultaneously behaves as an SMM-like
and luminescent molecular material [10].
Because the magnetic properties of a metal cluster are strongly
dependent on the geometry of the ligand, the choice of an appro-
priate ligand is important. From literature reports, it is apparent
that many phenol-containing ligands have been used in many
SMMs studies [12]. Also, amino-alcohol groups have been utilized
in the synthesis of a wide variety of metal clusters [13]. Our goal
here was to combine these two fragments into a single ligand.
To accomplish this, we synthesized a new molecule, 2,6-bis[N,N-
di(2-hydroxylethyl)aminomethyl]phenol (dhampH5). The synthetic
strategy for the synthesis is shown in Scheme 1. We then attempted
n Corresponding author.
E-mail address: 071549@mail.fju.edu.tw (E.-C. Yang).
0022-4596/$ - see front matter & 2011 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2011.10.033

Y.-H. Chen et al. / Journal of Solid State Chemistry 185 (2012) 166–171
167
diffusion. Pale yellow crystals suitable for X-ray structure determina-
tion were obtained after one week. ATR-IR,
/cmꢁ1: 3218m, 2851m,
n
1594w, 1462s, 1383m, 1289s, 1238s, 1089s, 1030m, 979s, 903s,
863w, 757w. Anal. Calcd. For (C64H104Dy4N12O32): C, 34.88; H, 4.77;
N, 7.63%. Found: C, 34.57; H, 4.97; N, 7.62%.
2.2. Physical property measurements
NMR spectra of the compound were recorded with a Bruker
AV-300 spectrometer. All chemicals were dissolved in CDCl3.
Infared spectra were recorded on KBr pellets using Perkin Elmer
1600 spectrometer in the 650–4000 cmꢁ1 range. Elemental ana-
lysis (C, H, N) and dc magnetic susceptibility data were measured
by the National Taiwan University of Instrument Centre College of
Science. Part of the ac magnetic susceptibility measurements
were performed at the National Chiao Tung University.
Scheme 1. Synthetic strategy used to prepare dhampH5: (i) AcCl, NEt3, CH2Cl2;
(ii) NBS, CHCl3, IR-irradiation 14 h reflux and (iii) diethanolamine, (NMe4)2CO3,
NMe4HCO3, MeOH.
to use this ligand to develop a new metal cluster with dysprosium
and to examine its magnetic and luminescent properties.
2.3. X-ray crystallography
2. Experimental section
A light yellow crystal of complex 1, dimension 0.50 ꢂ 0.25
ꢂ 0.20 mm was selected for X-ray analysis on a Bruker SMART
2.1. Synthesis
CCD diffractometer using MoKa radiation. The temperature of the
Tetramethylammonium hydrogencarbonate [14] and 2,6-
dimethylphenoxy acetate [15] were prepared as described in
the literature.
crystal was controlled using an Oxford Cryosystems Cryostream
Cooler at 150 K. The detector was located at a distance of 5.00 cm
from the crystal. The crystallographic data were collected over a
hemisphere of reciprocal space, by a combination of four sets of
2.1.1. [2,6-Bis(bromomethyl)phenyl ester]
exposures. Each set had a different
each exposure of 5 s covered 0.301 in
j
angle for the crystal and
. Crystal decay was
2,6-Dimethylphenoxy acetate (6.60 g, 36.9 mmol) and N-bro-
mosuccinimide (13.13 g, 73.8 mmol) in 130 ml chloroform were
refluxed under an atmosphere of nitrogen with continuous
irradiation with a 500 W IR lamp. The resulting mixture was
washed with 250 ml of an aqueous 1 M NaOH solution three
o
monitored by repeating the initial 50 frames at the end of the
data collection period and analyzing the duplicate reflections. No
decay was observed. An empirical absorption was based on the
symmetry-equivalent reflections and the data examined using the
SADABS [16] program. The structure analysis was completed
using the SHELXTL [17] program on PC computer. The compound
times and once with 30 ml of
a saturated brine solution.
The organic layer was dried over MgSO4. After removing the
chloroform by evaporation, the resulting solid was dissolved in a
10 ml CH2Cl2/13 ml hexanes solution and the solution stored in a
refrigerator overnight. Yellow crystals of the product were
obtained. Yield: 35%. (1H NMR in CDCl3, 300 MHz)
phenyl-H), 7.23 (t, 2H, phenyl-H), 4.38 (s, 4H, CH2-Br), 2.46 (s,
3H,CH3CO); 13C (75 MHz)
20.8, 27.5, 127.0, 131.2, 131.6, 147.7
˚
¯
crystallized in the space group P1, triclinic, a¼16.0192(9) A,
˚
˚
b¼18.3060(10) A, c¼18.3287(10) A,
a¼64.826(2)1, b¼65.240(2)1,
3
˚
˚
g
¼74.174(2)1, V¼4387.8(4) A , z¼2,
l
¼0.71073 A,
m(MoK
a
)¼
d 7.43(d, 2H,
3.457 mmꢁ1, F(000)¼2312 and T¼150(2) K. A total of 47,081
reflections were collected ranging from 1.241r
15,450 unique reflections and 13,534 with IZ2
y
r25.001, of which
s(I) were collected
d
and 168.6. HREI-MS: 321.9026. Anal. Calcd. For (C10H10O2Br2):
C, 37.30; H, 3.14; Found: C, 37.43; H, 3.12%.
for the analysis. The structure was solved using the Shelxs-97
program and refined using Shelxl-97 program by full-matrix least
squares on F2 values. Hydrogen atoms were fixed at the calculated
positions and refined using a riding mode. The final indices were
R1¼0.0399, wR2¼0.1193 with goodness-of-fit¼1.034. Detailed
crystallography data can be found in Table 1.
2.1.2. [2,6-Bis[N,N-di(2-hydroxyethyl)aminomethyl]phenol]
NMe4HCO3 (2.9 g, 21.4 mmol) and (NMe4)2OH (1.3 g, 14.3 mmol)
were dissolved in 35 ml of methanol and the solution was stirred for
3 h. A solution of diethanolamine (1.5 g, 14.3 mmol) and 3,5-
Bis(bromomethyl)phenyl ester (2.3 g, 7.14 mmol) in 35 ml methanol
was added to this solution, followed by stirring overnight under a
nitrogen atmosphere. After removing the methanol, 60 ml CH2Cl2
was added. The organic layer was extracted twice with 20 ml
portions of distilled water. The aqueous layer was then washed
with diethyl-ether several times and then extracted with CH2Cl2.
After drying the CH2Cl2, the ligand dhampH5 was obtained in 20%
yield. (1H NMR in CDCl3, 300 MHz)
(t, 1H, phenyl-H), 3.82 (s, 4H, PhCH2N), 3.68 (t, 8H, CH2OH), 2.73
(t, 8H, NCH2CH2); 13C (75 MHz)
55.8, 57.2, 59.4, 119.0, 124.0, 129.7
and 156.3. HRESI-MS: 329.2056. Anal. Calcd. For (C16H28N2O5): C,
58.51; H, 8.59; N, 8.53. Found: C, 58.40; H, 8.56; N, 8.69%.
3. Results and discussion
3.1. Structure description
¯
Complex 1 crystallized in space group P1. There are two
symmetry independent molecules in the unit cell. The structure
of complex 1 is depicted in Fig. 1, where the four Dy(III) ions adopt
a zig-zag form with Dy–Dy–Dy angles of 116.531 for one molecule
and 117.761 for the other. The molecules have centro-symmetry
with an inversion center between Dy1 and Dy1A. All of the Dy ions
are eight-coordinates with a highly distorted square antiprism
geometry. There are two sets of Dy ions, one set located in the
d 7.02(d, 2H, phenyl-H), 6.75
d
2.1.3. [Dy4(dhampH3)4(NO3)2] ꢀ 2(NO3) ꢀ 3CH3OH ꢀ H2O (1)
middle, numbered Dy1 and Dy1A, that are m2-bridged to two
The ligand (0.33 g, 1 mmol) was dissolved in 20 ml methanol,
and 0.46 ml (4 mmol) triethylamine and Dy(H2O)6(NO3)3 (0.44 g,
1 mmol) was then added to this solution. The solution was
refluxed for 4.5 h under a nitrogen atmosphere. After cooling,
the solution was filtered and the filtrate was treated by diethyl-ether
alkoxide groups from two dhampH23ꢁ ligands, and have nitrate
ligands bi-dentate coordinated with each of them. The other set of
Dy ions are located on both ends of the molecule, numbered Dy2
and Dy2A, which are ligated by two dhampH23ꢁ ligands, and are
m2-bridged to Dy1 and Dy1A by a deprotonated phenol group and

168
Y.-H. Chen et al. / Journal of Solid State Chemistry 185 (2012) 166–171
Table 1
3.2. DC magnetic property
Crystallographic data for complex 1.
Variable temperature dc magnetic susceptibility measurements
of complex 1 were performed in a 1000 G magnetic field in
Complex
Formula
1
C67 H118 Dy4 N12 O36
2317.73
Fw (g molꢁ1
T (K)
)
temperatures that ranged from 2 to 300 K. Fig. 2 illustrates the
150(2)
w
MT vs. T plot. The w
MT value was 57.9 cm3 K molꢁ1 at 300 K, which
¯
is quite close to the theoretical value of 56.7 cm3 K molꢁ1 for the
Space group
P1
16.0192(9)
18.3060(10)
18.3287(10)
˚
four uncoupled Dy(III) ions (6H15/2, g¼4/3). The
wMT values are then
a (A)
˚
slightly decreased to 49.0 cm3 K molꢁ1 at 30 K and significantly
decreased to 40.1 cm3 K molꢁ1 at 2 K, which can be attributed to
the depopulation of excited Stark sublevels [18]. Fig. 3 shows the
reduced magnetization data recording under a 10–60,000 G mag-
netic field. The magnetizations measured at different magnetic field
strengths do not superimpose. According to the literature reports,
this characteristic is usually attributed to the presence of significant
zfs and/or low-lying excited states [19]. The magnetizations reading
from 5 T and 6 T are quite close. This can be because the system is
approaching to its saturation value. From this fact, as well as the
small splitting of magnetization under different magnetic fields, the
feature of the non-superimposed reduced magnetization curves
seem to more likely be the result of zfs. We also found the
b (A)
˚
c (A)
a
b
g
(deg.)
64.826(2)
65.240(2)
74.174(2)
4387.8(4)
(deg.)
(deg.)
3
˚
Volume (A )
Z
2
F(000)
2312
Density (calcd) (mg mꢁ3
Absorption coefficient
Absorption correction
Transmission max/min
Reflns, measured
)
1.754
3.457
SADABS
0.5447/0.2768
47081
Reflns, independent
15450 [R(int)¼0.0333]
15450/9/1028
1.034
Data/ restraints/ parameters
Goodness-of-fit on F2
R indices [I42
R indices (all data)a,b
d
(I)]a,b
maximum value of reduced magnetization is 20.76
b
, which is very
R1¼0.0399, wR2¼0.1193
R1¼0.0466, wR2¼0.1292
close to Powell’s calculation (4 ꢂ 5.23
b
¼20.92
b) for four uncorre-
lated Dy(III) ions [20].
P
P
a
R¼ :F09ꢁ9Fc:/ 9F09.
P
P
b
R(
o
F2)¼{ [w(F20ꢁFc2)2]/
[
o
(F20)2]}1/2
,
o
¼1/[
s
2(F20)þ(aP)2þbP], where
P¼[2F2c þmax(F0,0)]/3.
Fig. 2.
wMT vs. T plot of complex 1, collected in a 1000 G magnetic field.
Fig. 1. (a) Molecular structure of the [Dy4(dhampH3)4(NO3)2]2þ (1) cluster.
Hydrogen atoms, counter ions and packing solvent molecules are omitted for
the sake of clarity. (b) Binding modes of the ligand (dhampH3)2ꢁ in the middle of
the complex and (c) binding modes of the ligand (dhampH3)2ꢁ on both ends of the
complex.
an alkoxide arm. From the charge of this compound, it seemed
reasonable to assume that all ligands take the form dhampH32ꢁ
with all the phenols and m2-bridging alcohol groups deprotonated,
while all the mono-dentate alcohol groups are not deprotonated.
The ligands in the middle coordinate to two Dy(III) ions, and the
binding mode is shown in Fig. 1(b). The ligands on both ends of the
complex have only one Dy ion in one bis-ethanol-amine arm while
the other arm is left empty (see Fig. 1(c)). For detailed structure
data, please refer to the supplemental information.
Fig. 3. Reduced magnetization for complex 1 under magnetic fields 10–60,000 G
and temperature 2–4 K.

Y.-H. Chen et al. / Journal of Solid State Chemistry 185 (2012) 166–171
169
3.3. AC magnetic susceptibility
Alternating current susceptibility data in the temperature range
1.9–10 K were determined by applying a zero dc magnetic field
and a 3.5 G ac field with frequencies of 50–1000 Hz. Both in-phase
and out-of phase signals are shown in Fig. 4. The frequency
dependent behavior is observed at temperatures below 5 K, which
suggests that slow magnetization relaxation processes are involved.
To actually locate the position of the peak for the out-of-phase
signals, we extended the oscillating frequency to 10,000 Hz. For the
further analysis of the Cole–Cole plot, a frequency-scan method was
employed in this experiment, i.e. the ac frequency was varied with
the temperature held constant. Fig. 5 shows the magnetic suscept-
ibilities vs. the logarithm of the frequency plot for both in-phase and
out-of-phase signals. As the temperature is increased, the out-of-
phase signals clearly shift to higher frequencies. Again, this fre-
quency-dependent feature indicates that this molecule has slow
magnetization relaxation characterteristics, suggesting that it could
be potentially a SMM. The ac data were thus extracted to estimate
the kinetic energy barrier for the magnetization reversal. The inset
of Fig. 5 shows an Arrhenius plot, where the solid line represents the
least-square fit of the experimental data to the formula (ln
t
¼Ueff
/
Fig. 5. ac Magnetic susceptibilities of complex 1 in a 3.5 G oscillating field and
zero dc field with temperature hold constant and scanning with variable
frequencies. The in-phase signals are uppermost and the out-of-phase signals
are on the bottom. (inset: the Arrhenius plot of ac measurements. The solid line
represents the fit that gives Ueff¼1.5 cmꢁ1).
kTþlnt0). The calculation from this fit gives the effective energy
barrier as Ueff¼1.5 cmꢁ1. Although it is small, this value for an
energy barrier is quite normal for many poly-dysprosium clusters
[21]. For further explore the blocking temperature for complex 1,
magnetization hysteresis loops and field cooling/zero-field cooling
(FC/ZFC) experiments were carried out. (see supporting information
Figs. 1S and 2S.) Neither hysteresis (above 1.9 K) nor the splitting of
FC/ZFC (above 1.8 K) were detected. This indicates that the blocking
temperature is likely far below the temperature that the traditional
SQUID can access.
3.4. Cole–Cole plot
To examine the process of magnetization relaxation, ac magnetic
susceptibilities were employed to construct a Cole–Cole (or Argand)
plot [22]. The results are shown in Fig. 6, in which the solid lines
represent the least-square fits of the theoretical curve based on Eqs.
Fig. 6. Cole–Cole plot for complex 1 in the temperature range 1.8–3.0 K. The solid
lines are the least-square fit of the data to the distribution of single relaxation
processes.
(1) and (2)
a
ð
wT
ꢁ
wSÞ½1þðot 1ꢁ
Þ
sinð1=2Þap
ꢃ
w0
ð
o
Þ ¼ wS
þ
ð1Þ
ð2Þ
1þ2ðot 1ꢁ
Þ
sinð1=2Þapþðot 2ð1ꢁ
Þ
a
aÞ
a
ð
wT
ꢁ
wSÞ½ðot 1ꢁ
Þ
cosð1=2Þap
ꢃ
w00
ðoÞ ¼
1þ2ðot 1ꢁ
Þ
sinð1=2Þapþðot 2ð1ꢁ
Þ
a
aÞ
where wT is the isothermal susceptibility, ws is the adiabatic
susceptibility, and is the magnetization relaxation time. The
parameter is an indicator of the distribution of relaxation time,
i.e. the larger values correspond to a wider distribution of the
relaxation times. An analysis of the Cole–Cole plots at different
temperatures show parameters ranging from 0.27 to 0.31. Because
t
a
a
a
these values are deviated from zero, the magnetization relaxation
seem more likely to involve a distribution of relaxation processes.
The ac data were further analyzed by the frequency-dependent shift
Fig. 4. ac Magnetic susceptibilities of complex 1 in a 3.5 G oscillating field and
zero dc field with frequencies held constant and scanning under different
temperatures. The in-phase signals are uppermost and the out-of-phase signals
are at the bottom.
of the freezing temperatures according to F¼
DTf/(TfDlogo), giving

170
Y.-H. Chen et al. / Journal of Solid State Chemistry 185 (2012) 166–171
Acknowledgments
The authors thank Prof. Hui-Lien Tsai of National Cheng Kung
University for assistance with the ac measurements. This work
was supported by the National Science Council of Taiwan (NSC98-
2113-M-030-003-MY2).
Appendix A. Supplementary materials
Supplementary materials associated with this article can be
found in the online version at doi:10.1016/j.jssc.2011.10.033.
References
Fig. 7. Emission spectra for complex 1 (black line) in solid state and the ligand
(red line) in the liquid state at room temperature. (For interpretation of the
references to colour in this figure legend, the reader is referred to the web version
of this article.)
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