Magnetic relaxation behavior of lanthanide substituted Dawson-type tungstoarsenates

Article abstract of DOI:10.1016/j.jssc.2009.11.013

Two new polyoxometalate compounds [(CH₃)₄N]₈[Ln(H₂O)₈]₂[(α₂-As₂W₁₇O₆₁)Ln(H₂O)₂]₂·nH₂O (Ln=Er (1), Dy (2)) have been prepared by the trivacant Dawson-type anion [α-As₂W₁₅O₅₆]^12- and trivalent rare earth ion and characterized by single-crystal X-ray diffraction, IR spectra, thermogravimetric and electrochemical analyses. The centrosymmetric polyoxoanion, {[(α₂-As₂W₁₇O₆₁)Ln(H₂O)₂]₂}^14-, bounded to each other via Ln³⁺ connecting to terminal W-O oxygen atoms. Furthermore, the polyoxoanions are linked by [Ln(H₂O)₈]³⁺ to form an extensive 3D supramolecular network structure depending on hydrogen bond. The magnetic properties of the two compounds have been studied by measuring their magnetic susceptibilities in the temperature range 2.0-300.0 K, indicating the depopulation of the stark components at low temperature and/or very weak antiferromagnetic interactions between magnetic centers. Low-temperature ac magnetic susceptibility measurements reveal a slow magnetic relaxation behavior for 2.

Full text of DOI:10.1016/j.jssc.2009.11.013

ARTICLE IN PRESS
Journal of Solid State Chemistry 183 (2010) 350–355
Contents lists available at ScienceDirect
Journal of Solid State Chemistry
journal homepage: www.elsevier.com/locate/jssc
Magnetic relaxation behavior of lanthanide substituted
Dawson-type tungstoarsenates
Ã
Lizhen Liu, Fengyan Li, Lin Xu , Xizheng Liu, Guanggang Gao
Key Laboratory of Polyoxometalate Science of Ministry of Education, Department of Chemistry, Northeast Normal University,
Changchun, Jilin 130024, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Two new polyoxometalate compounds [(CH3)4N]8[Ln(H2O)8]2[(
a
2-As2W17O61)Ln(H2O)2]2 ꢀ nH2O (Ln=Er
12ꢁ
and trivalent
Received 15 July 2009
Received in revised form
2 November 2009
(1), Dy (2)) have been prepared by the trivacant Dawson-type anion [
rare earth ion and characterized by single-crystal X-ray diffraction, IR spectra, thermogravimetric and
a
-As2W15O56
]
electrochemical analyses. The centrosymmetric polyoxoanion, {[(
a
2-As2W17O61)Ln(H2O)2]2}14ꢁ
,
Accepted 9 November 2009
Available online 13 November 2009
bounded to each other via Ln3+ connecting to terminal W–O oxygen atoms. Furthermore, the
polyoxoanions are linked by [Ln(H2O)8]3+ to form an extensive 3D supramolecular network structure
depending on hydrogen bond. The magnetic properties of the two compounds have been studied by
measuring their magnetic susceptibilities in the temperature range 2.0–300.0 K, indicating the
depopulation of the stark components at low temperature and/or very weak antiferromagnetic
interactions between magnetic centers. Low-temperature ac magnetic susceptibility measurements
reveal a slow magnetic relaxation behavior for 2.
Keywords:
Crystal structure
Lanthanide
Tungstoarsenate
Magnetic properties
Cyclic voltammetry
& 2009 Elsevier Inc. All rights reserved.
1. Introduction
crystal structures have been determined [17–22]. For example,
Pope et al. obtained the extended polymeric structures of the
8ꢁ
Ln/[
a
-SiW11O39
]
(Ln=LaIII, CeIII) in 2000 [17]. Furthermore,
Polyoxometalates (POMs), a class of metal oxygen clusters
with fascinating structural variety and interesting properties have
been attracting a great deal of attention as a result of their
potential applications in catalysis, medicine and materials science
[1–9]. It is well known that the lacunary POMs can be obtained by
the removal of one or several metal centers and act as polydentate
ligands. Lanthanide (Ln) cations, in virtue of their multiple
coordination numbers and oxophilicity, are suitable to be
incorporated into the lacunary POMs and link polyoxometalates
to form new classes of materials with useful magnetic, lumines-
cent properties, etc. [10–13].
Mialane et al. reported the solid state structures of {Lnn(Si-
W11O39)} Polyoxoanions (Ln=YbIII, NdIII, EuIII, GdIII) in 2003 [18]
and the dimeric K12[(SiW11
complexes in 2004 [19].
In contrast, there are few reports about lanthanide complexes
based on monovacant Dawson-type building block. The only
O39Ln)2(m
-CH3COO)2] (Ln=GdIII, YbIII)
examples are 1:1-type dimers of [{Ce(
[23], [{Ln(
well as 1:2-type dimer of [Lu(
knowledge, the open angle of [
vacant site is larger than that of [
a
1-P2W17O61)(H2O)4}2]14ꢁ
a
2-P2W17O61)(H2O)n}2]14ꢁ (Ln=Ce, Eu, Nd) [24–26] as
a
a
2-P2W17O61)2]17ꢁ [27]. To our
10ꢁ
2-As2W17O61
]
anion in the
10ꢁ, this is more
a
2-P2W17O61
]
Up to now, the two most studied polyoxometalate types have
been the Keggin [14] and Well-Dawson [15] series, which are
favorable to accommodate various lanthanide cations for assem-
bling the type of Ln/[
structures. Until recently, several Ln-containing tungstoarsenates,
10ꢁ
nꢁ
nꢁ
represented by [XM12O40
]
and [X2M18O62
]
(X=PV, SiIV
;
a2-As2W17O61
]
complexes with novel
M=WV,VI, MoV,VI, VIV,V), respectively. Since the first lanthanide
complexes based on [SiW11O39
8ꢁ
10ꢁ
(H3O)[Ln3(H2O)17
(
a
2-As2W17O61)] ꢀ nH2O, with one-dimensional
]
and [P2W17O61
]
were
chain structures, were reported by Zhao et al. [28]. Although the
crystal structures of the Ln-POMs have been well studied in all
previous reports, their magnetic properties are still rarely
explored. In fact, the magnetic properties of lanthanide complexes
closely depend on the coordination situation of Ln3+ ions, such as
the coordination number, the kind of ligand atoms, bond angle,
and bond length. As for lanthanide complexes, even a minor
change of coordination situation could cause obvious alteration of
magnetic properties. Therefore, the exploration of magnetic
prepared by Peacock and Weakley in 1971 [16], much attention
has been focused on developing the polyoxometalate-based
lanthanide complexes by incorporating the Ln ions into lacunary
POMs. In particular, the monovancant Keggin polyoxometalates
linked by Ln3+ have been elaborately investigated and some
Ã
Corresponding author. Fax: +86 431 85099668.
E-mail address: linxu@nenu.edu.cn (L. Xu).
0022-4596/$ - see front matter & 2009 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2009.11.013

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L. Liu et al. / Journal of Solid State Chemistry 183 (2010) 350–355
351
properties for polyoxometalate-based lanthanide complexes
should be of considerable significance. In addition, our research
demonstrated that the rational selection of solvents and counter-
cations was a key factor to realize the successful synthesis.
Herein, we reported two new polyoxometalate-based
3385(m), 951(m), 831(s), 778(s), 522(m). Elemental analyses
calcd (%): W, 59.11; As, 2.83; Er, 6.33; C, 3.63; H, 1.77; N, 1.06.
Found: W, 58.60; As, 2.79; Er, 6.21; C, 3.72; H, 1.85; N, 1.15.
lanthanide complexes assembled by lacunary Dawson-type anion
10ꢁ
[a2-As2W17O61
]
and Ln3+ ions, [(CH3)4N]8[Er(H2O)8]2[(a2
-
2.3. Synthesis of (CH3)4N]8[Dy(H2O)8]2
As2W17O61)Er(H2O)2]2 ꢀ 26H2O (1) and [(CH3)4N]8[Dy(H2O)8]2
[(
netic properties were investigated in detail. These complexes
[(
a
2-As2W17O61)Dy(H2O)2]2 ꢀ 32H2O 2
a
2-As2W17O61)Dy(H2O)2]2 ꢀ 32H2O (2). In particular, their mag-
The preparation procedure was the same as the preparation of
were synthesized by reaction of the metastable, trivacant
complex 1 described above except that DyCl3 ꢀ 6H2O was used
(0.075 g). Yield: 0.017 g (30%, based on W). IR (KBr pellets, cmꢁ1):
3380(m), 952(m), 835(s), 777(s), 521(m). Elemental analyses
calcd (%): W, 58.61; As, 2.81; Dy, 6.10; C, 3.60; H, 1.87; N, 1.05.
Found: W, 58.01; As, 2.75; Dy, 5.97; C, 3.68; H, 1.94; N, 1.12.
12ꢁ
[a-As2W15O56
]
with the Ln3+ to form a [(
a2-As2W17O61)Ln(-
H2O)2]2 dimer. Furthermore, the dimers are linked by
[Ln(H2O)8]3+ to extend into
a
3D supramolecular network
structure in virtue of hydrogen bonds interaction. The static
magnetic measurements on 1 and 2 reveal weak antiferromag-
netic interactions between the Ln ions. Interestingly, the dynamic
magnetic measurements for compound
2 display a slow
relaxation of magnetization, showing a frequency-dependent
susceptibility.
2.4. Single crystal X-ray diffraction
Single-crystal data of compound 1 were collected on a Siemens
SMARTCCD diffractometer with graphite-monochromated MoK
a
2. Experimental
˚
(l
=0.71073 A) radiation at 296 K. Single-crystal data of compound
2 were performed on a R-axis RAPID IP diffractometer equipped
2.1. General methods and materials
with a normal focus, 18 kW sealed-tube X-ray source (Mo KR
˚
radiation,
l=0.71073 A) at 296 K. The structures were solved by
The Na12[a-As2W15O56] ꢀ 21H2O was prepared according to the
direct methods and refined using full-matrix least-squares
methods on F2. All calculations were performed using the
SHELX97 program package [30]. Empirical absorption corrections
were applied. Parts of the non-hydrogen atoms were refined
isotropically. The hydrogen atoms attached to the water mole-
cules were not located in these compounds. The number of lattice
water molecules for 1,2 are determined by thermal gravimetric
(TG) and elemental analyses. Crystal data and structure refine-
ment parameters for 1 and 2 are given in Table 1. Selected bond
lengths and angles are list in supporting information.
literature [29] and confirmed by IR spectroscopy. The ErCl3 ꢀ 6H2O
was prepared by the reaction of Er2O3 and HCl. DyCl3 ꢀ 6H2O was
obtained by similar management. Other reagents were purchased
commercially and used without further purification. The ele-
mental analyses were determined by a Leaman inductively
coupled plasma (ICP) spectrometer. The infrared spectra were
obtained on an Alpha Centaurt FT/IR spectrometer with a pressed
KBr pellet in the range of 400–4000 cmꢁ1. TG analyses were
performed on a Perkin-Elmer TGA7 instrument in N2 atmosphere
with a heating rate of 10 1C minꢁ1. The magnetic susceptibility
measurements for 1 and 2 were carried out using a Quantum
Design MPMS-XL SQUID magnetometer at 1000 Oe. The ac
susceptibility measurements were performed using an oscillating
ac field of 3 Oe and ac frequencies ranging from 100 to 1500 Hz.
Cyclic voltammetry measurements were carried out on a CHI 660
electrochemical workstation with a conventional three-electrode
single compartment cell at room temperature. The working
electrode was a glassy carbon disk electrode. The surface of the
Table 1
Crystal data and structure refinement for 1 and 2.
1
2
Empirical formula
C32H186N8Er4O167As4W34 C32H198N8Dy4O173As4W34
Fw
10 575.06
10 664.11
Triclinic
P-1
CityplaceCrystal system Triclinic
Space group
P-1
ꢀ
˚
13.2330(10)
13.239(3)
glass carbon electrode was polished with 0.3 and 0.05 mm Al2O3
a ðAÞ
ꢀ
˚
18.2319(14)
19.9859(15)
18.267(4)
20.032(6)
powders successively, and washed with distilled water before
each experiment. Platinum wire was used as counter electrode
and SCE was used as reference electrode. The concentrations of 1
and 2 in the measurement of cyclic voltammetric behaviors were
all 1.0 ꢂ 10ꢁ4 M.
b ðAÞ
ꢀ
˚
c ðAÞ
a
b
g
(deg)
(deg)
93.5150(10)
97.6350(10)
96.8940(10)
4729.9(6)
93.421(9)
97.625(9)
96.903(7)
4753(2)
(deg)
3
˚
V (A )
Z
1
1
2.2. Synthesis of (CH3)4N]8[Er(H2O)8]2
Dcalcd (g cmꢁ3
T (K)
)
3.667
296(2)
0.71073
3.643
296(2)
0.71073
[(
a2-As2W17O61)Er(H2O)2]2 ꢀ 26H2O 1
ꢀ
˚
l
ðAÞ
Na12
[a-As2W15O56] ꢀ 21H2O (0.054 g, 0.0125 mmol) was slowly
limiting indices
ꢁ21rkr22
ꢁ24rlr17
ꢁ15rhr16
ꢁ22rkr21
ꢁ24rlr24
1.03–25.601
ꢁ16rhr15
dissolved in the hot (70 1C) solution containing 5 ml acetonitrile
(CH3CN) and 15 ml water with vigorous stirring. Then solid
ErCl3 ꢀ 6H2O (0.076 g, 0.2 mmol) was added. The resulting cloudy
solution was heated at approximately 70 1C for 80 min. Then 0.1 g
tetramethylammonium bromide was added and heated at the
same temperature. After 40 min, the solution was cooled to room
temperature, and the precipitate was removed by filtration. The
filtrate was left to evaporate at room temperature for two days.
Pink pillar-like crystals of 1, suitable for X-ray diffraction, were
collected. Yield: 0.02 g (35%, based on W). IR (KBr pellets, cmꢁ1):
3.09–25.601
16 838/2/949
y
range (deg)
Data/restraints/
17 522/14/1044
parameters
Data collected/unique
Refinement method
Goodness of fit on F2
25 367/17 522
F2
36 289/16 838
F2
1.033
1.027
Final R1,
Final R1,
oR2[I42
s
(I)] 0.0465, 0.1114
0.0549, 0.1247
0.0964, 0.1461
o
R2 (all data) 0.0734, 0.1300
P
P
P
P
Note. R1= ||Fo|ꢁ|Fc||/ |Fo|, wR2={ [w(F2oꢁFc2)2]/[ w(Fo2)2]}1/2
.

ARTICLE IN PRESS
352
L. Liu et al. / Journal of Solid State Chemistry 183 (2010) 350–355
3. Results and discussion
(OW1,OW2).
The
structure
is
similar
to
[Ce2((H2O)6
(a
2-As2W17O61)2]14ꢁ [28]. Due to the smaller size of Er3+ than
Ce3+, it is reasonable that only two water molecules are coordinated
to the Er3+ rather than three water molecules that are found
3.1. Synthesis
coordinated to Ce3+
. Moreover, compared with [Nd((H2O)3(a2-
Two lanthanide complexes were obtained using the similar
synthetic procedure by reaction of the Ln3+ cations with the
As2W17O61)]7ꢁ [28], the As–Er distance (4.121 A) is much shorter
˚
˚
12ꢁ
than the As–Nd distance (4.417 A), may resulting from the larger ‘‘bite
trivacant, metastable [a-As2W15O56
]
polyoxoanions. It is not
angle’’ of polyoxoanion [a
2-As2W17O61)]10ꢁ and smaller radius of
entirely expected that we obtained the monolacunary Dawson-
type fragment in our product. Considering that the reaction
heavy rare earth ions Er3+ than light rare earth ions Nd3+. In addition,
the use of tetramethylammonium cations [(CH3)4N]+ as
countercations can influence on the structural of crystal products.
There is a structural difference in that the previously studied
was approximately carried out at pH=4–5 and 70 1C, the
12ꢁ
[a
-As2W15O56
]
may not be stable in acidic medium and
10ꢁ
rearranges slowly to the more stable [
a
2-As2W17O61
]
[31]. It
compounds (H3O)[Ln3(H2O)17(a2-As2W17O61)] ꢀ nH2O [28] is 1D
was well known that the conventional solution synthesis is a
favorable method for succedent crystal growth. In the conven-
tional synthesis process, different factors can affect the product
formation and succedent crystal growth, such as selection of
solvent, pH values, reaction time, and temperature. In our
experiments, the rational selection of solvent was a key factor
for the crystallization of products. The two title compounds could
not be obtained in water. However, they were synthesized in
mixed solvents of acetonitrile and water system, where the single
crystals produced only in two days. The crystallized time is
shorter than that of the previously reported compounds [28].
polymer, whereas the compound 1 is centrosymmetric dimers. The
difference may be caused by the following reasons. The 1:1-type
LnPOM dimers can be linked into a 1D chainlike structure by using K+
counterions as linker [32]. Because of a competition between the K+
and Ln3+, it is important to eliminate the interference of K+ ions
during the preparation of 1D polymer. In our reaction system, the
surface of anionic clusters [Ln2(H2O)6(a
2-As2W17O61)2]14ꢁ can not be
binded by the large organic counterions [(CH3)4N]+ to form ion pairs
[32] but be surrounded by [(CH3)4N]+ to form a separate dimer. In
result, the Ln3+ ions could not link the dimers into 1D chain structure.
The bond lengths from Er(III) to the oxygen atoms of the
polyoxometalate framework are in the range of 2.247(11)–
˚
2.271(10) A. The bond distances between the Er(III) and the two
3.2. Structure description
˚
water molecules are 2.369(11)–2.406(12) A. The Er–O (oxygen
˚
atom of a terminal WdQO) bond length is 2.313(11) A. The
X-ray diffraction analysis reveals that the solid-state structures
of compounds 1,2 are isomorphic. Therefore, the following
structural discussion is solely exemplified on the basis of
compound 1. It is built on monovacant Dawson-type polyoxoa-
disordered Er(2) cations are coordinated with eight water
molecules. The Er–OW bond distances range from 2.331(19) to
2.379(12) A.
It is noteworthy that oxygen atoms of polyanions are
connected with the oxygen atoms from ligand water molecules
coordinated to the discrete Er atoms by strong hydrogen bonding
interactions with bond lengths in the range of 2.63–2.81 A. As
viewed from the polyhedral diagram along a axis (Fig. S1), the
polyanions are linked by OW7?O33 and OW4?O56 to generate
˚
10ꢁ
10ꢁ
nions [
a
2-As2W17O61
]
and Ln3+ ions. The [
a2-As2W17O61]
polyanion is obtained by removal of a [WO]4+ unit from a ‘‘cap’’
6ꢁ
WO6 polyoxoanion of the parent anion [
of 18{WO6} octahedra. The As–O bond distances are in the range
a-As2W18O62
]
formed
˚
˚
of 1.659(10)–1.694(10) A. The W–O bond distances vary between
˚
1.695(11) and 2.366(10) A and the O–W–O bond angles are within
a
one-dimensional linear chain. In the bc-plane, they are
71.7(4)–173.4(5)1.
connected with OW8?O55, OW9?O7 and OW8?O52 (see Figs.
S2 and S3). The extensive hydrogen bonds can stabilize both
polyanions and the cations, and lead to the formation of a 3D
supramolecular network structure between the chains as shown
in Fig. S4.
The structure of
1
is composed of two [(
a
2-As2W17O61
)
Er(H2O)2]7ꢁ polyoxoanions, two discrete [Er(H2O)8]3+
,
eight
[(CH3)4N]+ and several isolated crystal water molecules. The
polyoxoanion [(
a
2-As2W17O61)Er(H2O)2]7ꢁ is based on the well-
6ꢁ
known PlaceTypePlaceTypeDawson structure of the [a-As2W18O62]
with one W atom substituted by a Er atom. As shown in Fig. 1, the
structure of 1 is a centrosymmertric dimer in a head to cap fashion
3.3. FT-IR spectroscopy
built from two [(
two Er–O–W link. The Er1 cation occupies the vacant site of the
2-As2W17O61)]10ꢁ subunit and it is seven-coordinated, four basic
a
2-As2W17O61)Er(H2O)2]7ꢁ fragments by means of
The IR spectra of two compounds share similarly characteristic
vibrational features to other Dawson-type structures [28] as
shown in Figs. S5 and S6. Asymmetric stretching vibrations of the
different W–O bonds are observed as follows: 952 cmꢁ1 (1) and
951 cmꢁ1 (2) for terminal W–Od; 865 cmꢁ1 (1) and 866 cmꢁ1 (2)
for W–Ob (Ob intrabridges between corner-sharing octahedra);
the W–Oc (Oc intrabridges between edge-sharing octahedra) peak
splits into two peaks: 778, 689 cmꢁ1 (1) and 777, 690 cmꢁ1 (2),
[a
oxygen atoms (O20, O31,O35, O51) surround the vacant site of the
lacunary anion, one terminal oxygen atom (O37) of the other
[(a
2-As2W17O61)Er(H2O)2]7ꢁ moiety and two water molecules
these results show that the polyoxoanions of compounds 1,2 have
6ꢁ
lower symmetry than that of [
a-As2W18O62
]
(D3h) by removing
a [WO]4+ unit. Compared with the As–O bands of Well-Dawson
polyoxoanion [ -As2W18O62
6ꢁ, only one absorption peak could
a
]
be observed at 835 cmꢁ1, but the other peak at 865 cmꢁ1 is
overlapped by the strong peak of W–Ob. A broad band at
3385 cmꢁ1 (1) and 3380 cmꢁ1 (2) associated with the aqua
ligands. Additionally, a series of characteristic bands foꢁr 1the
[(CH3)4N]+ group are 1624, 1482, 1446, 1416, 1286 cm
in
1
and 1623, 1483, 1449, 1415, 1285 cmꢁ1 in
Fig. 1. Ball and stick structure of 1. The hydrogen atoms, discrete Er3+
coordination ions, lattice water molecules and [(CH3)4N]+ are omitted for clarity.
compound
compound 2.

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L. Liu et al. / Journal of Solid State Chemistry 183 (2010) 350–355
353
3.4. Thermal analysis
60
56
52
48
44
40
20
The thermal gravimetric (TG) curve of compound 1 is shown in
Fig. S7. It exhibits a total weight loss of 13.2% in the range of 26–
480 1C, which agrees with the calculated value of 13.3%. In the
range of 26–100 1C, consecutive three-step weight losses were
observed with total weight loss of 4.5% corresponding to the loss
of 25 free water molecules (calcd 4.3%). The weigh loss of 3.1% at
100–250 1C corresponds to the release of 20 coordination water
with Er (calcd 3.4%). The last stage, which occurs from 250 to
480 1C, is attributed to the loss of eight [(CH3)4N]+. The observed
weight loss (5.6%) is in agreement with the calculated value
(calcd 5.6%).
The TG curve of compound 2 is shown in Fig. S8. It exhibits a
total weight loss of 14.28% in the range of 26–480 1C, which agrees
with the calculated value of 14.18%. In the range of 26–100 1C,
consecutive three-step weight losses were observed with total
weight loss of 5.2% corresponding to the loss of 31 free water
molecules (calcd 5.24%). The consecutive three-step weigh loss of
3.4% at 100–240 1C corresponds to the loss of 20 coordination
water molecules with Dy (calcd 3.38%). Then, the weight loss of
5.6% at 240–480 1C corresponds to the loss of all [(CH3)4N]+ (8
[(CH3)4N]+, calcd 5.56%).
15
10
5
0
0
50
100
150
T/K
200
250
300
Fig. 3. Temperature dependence of wm and wmT for 2 at 2–300 K.
(4f10, J ¼ 15 ; S ¼ 25 ; L ¼ 5; g ¼ 34, L15/2) [34]. As the temperature
6
2
decreases, the value of
wmT continuously decreases to
41.58 cm3 molꢁ1 K at 2 K. The plot of 1/wm versus T in the
3.5. Magnetic properties
temperature range 50–300 K obeys the Curie–Weiss law with
C=60.79 cm3 molꢁ1 K and
y=–10.63 K (Fig. S10). All of these
The magnetic behaviors of 1 are shown as plots of the product
results indicate that there is a possible weak antiferromagnetic
interaction between the neighboring Dy(III) ions and/or the
crystal field effects on the Dy(III) Stark sub-levels, similar to
other Dy(III) compounds [35].
w
m versus T and
wmT versus T in Fig. 2. The wm value increases from
0.15 cm3 molꢁ1 at 300 K, reaching a maximum of 15.12 cm3 molꢁ1
at about 2 K as the temperature decreases. The value of
wmT at
room temperature is 46.02 cm3 molꢁ1 K, which is close to the
It is well known that by electronic repulsion interaction and
spin–orbit coupling, the 4fn configuration of a LnIII ion is split into
2S+1LJ states. The crystal-field perturbation further leads to the
splitting into Stark components. At room temperature, the Stark
levels are entirely populated. While the temperature decreases, a
gradual depopulation of these levels takes place [36]. To
investigate the magnetic properties in detail, the temperature
dependence of the alternating current (ac) magnetic susceptibility
for compounds 1 and 2 was thus measured in a zero dc field in the
range of 2–15 K at different frequencies (100–1500 Hz). No out-of-
phase component of ac magnetic susceptibility wm was observed
calculated value (45.92 cm3 molꢁ1 K) for four non-interacting
4
Er(III) in the I15/2 ground state. Upon cooling, the value of
w
mT
remains almost constant until at about 135 K when it decreases
continuously and reaches 30.09 cm3 molꢁ1 K at 2 K. The 1/wm
versus T plot could be fit with Curie–Weiss law from 50 to 300 K,
getting C=46.5 cm3 molꢁ1 K and
decrease in mT and the negative value of
y
= ꢁ2.67 K (Fig. S9). The
might be due to the
w
y
progressive depopulation of the stark components of 4I15/2 ground
state of Er(III) ion and/or the weak antiferromagnetic interaction
between the neighboring Er(III) ions [33].
Similar magnetic behavior to 1 was also revealed for 2 (Fig. 3).
above 2 K for
1 (Fig. S11). Fig. 4 shows the temperature
The w
mT value at room temperature is 59.05 cm3 molꢁ1 K, which is
dependence of the ac magnetic susceptibility for 2. A clearly
frequency-dependent out-of-phase component was observed, but
no Arrhenius plot can be extracted from this measurement, since
no peak was observed in the wm versus T curves.
higher than the value of 56.72 cm3 molꢁ1 K expected for four
Dy(III) ion, including the significant contribution of 4f orbital
To study the thermal relaxation, a small dc field could be
applied to remove the degeneracy of the ms states connected by
the tunnelling mechanism [35,37]. Therefore, the ac susceptibility
measurements were carried out in a 3 Oe ac field oscillating at
100–1500 Hz, under a 1000 Oe static field. In the presence of a
static magnetic field, remarkable changes were observed. The out-
of-phase susceptibilities of the two compounds are all frequency-
dependent, though the maxima of wm for 1 cannot be observed
48
16
44
12
40
8
0
00
(Fig. S12). However, the maxima of both wm and wm (Fig. 5)
appeared for compound 2, indicating a slow magnetic relaxation
36
00
behavior. Analysis of the frequency dependence of the wm peaks
4
through an Arrhenius plot (t=t0 exp(D/kBT)) permits estimation of
32
the energy barrier and the characteristic relaxation time in
compound 2. Best fitting (R=0.991) afforded a barrier height (D/
0
kB) of 11.8 K and a pre-exponential factor (
t
0) of 1.4 ꢂ 10ꢁ6 s (Fig.
28
S13). Because the complexity of the ligand field energy levels for
lanthanide complexes leads to the lack of suitably theoretical
models, more detailed calculations of the magnetic interactions
for 1 and 2 have to be limited in the present status.
0
50
100
150
T/K
200
250
300
Fig. 2. Temperature dependence of wm and wmT for 1 at 2–300 K.

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L. Liu et al. / Journal of Solid State Chemistry 183 (2010) 350–355
14
22
20
18
16
14
12
10
8
100 Hz
300 Hz
500 Hz
800 Hz
1100 Hz
1300 Hz
1500 Hz
100 Hz
300 Hz
500 Hz
800 Hz
1000 Hz
1500 Hz
12
10
8
6
6
4
4
2
0
2
0
2
4
6
8
10 12 14 16 18 20 22
T / K
0
2
4
6
8
10
12
14
16
T / K
2.4
2.2
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
-0.2
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
-0.5
100 Hz
300 Hz
500 Hz
800 Hz
1000 Hz
1500 Hz
100 Hz
300 Hz
500 Hz
800 Hz
1100 Hz
1300 Hz
1500 Hz
0
2
4
6
8
10 12 14 16 18 20 22
T / K
0
2
4
6
8
T / K
10
12
14
16
0
00
0
00
Fig. 4. Plot of the wm (a) and vm (b) vs. T for 2, where vm and vm are in-phase ac
and out-of-phase ac magnetic susceptibilities, respectively, measured with a 3 Oe
ac magnetic field oscillating at the indicated frequencies in a zero applied static
field.
0
00
Fig. 5. Temperature dependence of the wm (a) and vm (b) components of the ac
susceptibility in a dc field of 1000 Oe with a 3 Oe ac magnetic field oscillating at
the indicated frequencies for 2.
3.6. Cyclic voltammetry behavior
4. Conclusions
Cyclic voltammetry (CV) of 1 were carried out in 0.5 M NaAc/
HAc buffer solution at pH=4.3. The three redox processes ascribed
to the oxo-tungsten framework are observed in the range of ꢁ1.1
to 0.4 V. The three redox couples correspond to one one-electron
redox process followed by two two-electron redox processes. No
redox wave on lanthanide ions is observed even if the potential
range from +0.8 to ꢁ1.1 V, which is similar to the redox processes
of [Ln(As2W17O61)2]17ꢁ [38]. The CVs of 1 in pH=4.3 acetate
buffers at different scan rates is shown in Fig. 6, consistent with
that of the [Ln(As2W17O61)2]17ꢁ [38]. When the scan rate is
varying from 100 to 400 mV sꢁ1, the peak currents of 1 are almost
proportional to it, taking the second redox peak as representative
(inset of Fig. 6). This indicates that the redox waves are surface-
controlled redox processes.
In summary, two new lanthanide substituted Dawson-type
tungstoarsenates
[(CH3)4N]8[Er(H2O)8]2[(
a
2-As2W17O61)Er(H2O)2]2
ꢀ 26H2O and [(CH3)4N]8[Dy(H2O)8]2 [(
a2-As2W17O61)Dy(H2O)2]2
ꢀ 32H2O have been synthesized by interaction of the metastable,
12ꢁ
trivacant [a-As2W15O56
]
with the Ln3+. The selection of solvent
and counterions could influence on the formation of crystals and their
structures. In addition, magnetic studies revealed that the weak
antiferromagnetic interactions appear in the two complexes. The
dynamic magnetic measurements show that compound 2 presents a
slow magnetic relaxation behavior. Further work will focus on the
rational adjustment of synthetic condition to isolate multinuclear
polyoxometalate-based lanthanide complexes with diverse coordina-
tion geometry.
Compound
2 shows similar electron transfer processes
Supplementary materials
as 1. The three redox waves correspond to one- and two-electron
transfer processes, respectively. The electrode reaction
process of 2 is also surface-controlled, the same as compound 1
(Fig. S14).
Structural figures, IR, TG, magnetic data, electrochemical proper-
ties and crystallographic data for compounds 1 and 2 are presented.

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Ipa
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