Synthesis, crystal structure and luminescence properties of two novel lanthanide coordination polymers containing double chain

Article abstract of DOI:10.1016/j.ica.2007.09.013

Two novel lanthanide(III) two-dimensional (2D) coordination polymers [Ln₂(PDC)₂(OH)₂(H₂O)₂] · H₂O (Ln = Eu (1) and Tb (2), H₂PDC = pyridine-3,4-dicarboxylic acid) have been prepa

Full text of DOI:10.1016/j.ica.2007.09.013

Available online at www.sciencedirect.com
Inorganica Chimica Acta 361 (2008) 1421–1425
www.elsevier.com/locate/ica
Synthesis, crystal structure and luminescence properties of two
novel lanthanide coordination polymers containing double chain
*
Hui-Hua Song , Ya-Juan Li
College of Chemistry and Material Sciences, Hebei Normal University, Shijiazhuang 050016, PR China
Received 31 July 2007; received in revised form 9 September 2007; accepted 11 September 2007
Available online 19 September 2007
Abstract
Two novel lanthanide(III) two-dimensional (2D) coordination polymers [Ln2(PDC)2(OH)2(H2O)2] Æ H2O (Ln = Eu (1) and Tb (2),
H2PDC = pyridine-3,4-dicarboxylic acid) have been prepared under hydrothermal conditions and characterized by elemental analysis,
ꢀ
IR, TGA and single-crystal X-ray diffraction. Compounds 1 and 2 crystallize in the triclinic system, space group P1, they are isostruc-
tural and exhibit the same two-dimensional topological network constructed by PDC-connected Ln–O–Ln double chains. Photolumines-
cence properties of the compounds 1 and 2 have been investigated in the solid state at room temperature.
Ó 2007 Elsevier B.V. All rights reserved.
Keywords: Coordination polymer; Hydrothermal synthesis; Double chain; Lanthanide; Fluorescent properties; Carboxylate
1. Introduction
hand, investigations of the pyridine-3,4-dicarboxylic acid
complexes have mainly focused on the d-block transition–
metal [10–16], f-block lanthanide ions have received com-
paratively less attention than transition–metal ions. How-
ever, due to their ability of high coordination number,
special magnetic and fluorescence properties, lanthanide
complexes is likely to bring unprecedented crystal structures
and unique properties [17–20]. Therefore, a further research
in this area is still a especially attractive target. Up to now,
Frameworks constructed by PDC and lanthanide ion are
still limited [21–27]. Herein, we report two new lanthanide
metal coordination polymers [Ln2(PDC)2(OH)2(H2O)2] Æ
H2O (Ln = Eu (1) and Tb (2)), in which Ln–O–Ln double
chains are cross-linked by PDC ligands to form interesting
two-dimensional framework structures. They present, to
the best of our knowledge, the first examples of PDC com-
plexes containing double chain.
The rapidly expanding field of crystal engineering of two-
and three-dimensional coordination polymers have recently
attracted great interest for both the structural and topolog-
ical novelty as well as for their potential applications to
material science as catalytic, conductive, luminescent, mag-
netic, non-linear optical or porous materials [1–7]. Thus so
far, much attention has been drawn to the assembly of
metal–organic framework (MOF) and significant develop-
ment has been witnessed in high-dimensional coordination
polymers. The symmetrical multicarboxylate ligands have
been generally used in this field. Miao Du et al succeeded
in preparing a series of high-dimensional metal–organic
coordination with pyridine-2,6-dicarboxylic acid [8]. Whit-
field et al. reported 3D complex Co(pydc)(H2O)2 and 2D
layer structure Ni(pydc)(H2O) (pydc = 3,5-pyridinedicarb-
oxylate) [9]. While pyridine-3,4-dicarboxylic acid (H2PDC),
as a member of asymmetric ligands, has also drawn more
and more attention in crystal engineering. On the other
2. Experimental
2.1. Materials and methods
*
Compound LnCl3 Æ 6H2O (Ln = Eu, Tb) were pre-
Corresponding author. Tel.: +86 86268311.
pared by dissolving their respective oxides in concen-
E-mail address: songhuihua@mail.hebtu.edu.cn (H.-H. Song).
0020-1693/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2007.09.013

1422
H.-H. Song, Y.-J. Li / Inorganica Chimica Acta 361 (2008) 1421–1425
trated hydrochloric acid followed by drying. All the other
reagents are commercially available and were used as
received without further purification. Elemental analyses
(C, H, N) were determined with an Elemental Vario
EL elemental analyzer. IR spectra were (KBr pellets)
recorded in the 4000–400 cmꢀ1 range with a FTIR-8900
spectrometer. Thermogravimetric analysis (TGA) was
2.2.2. [Tb2(PDC)2(OH)2(H2O)2] Æ H2O (2)
Compound 2 was prepared using a similar method to
that employed for the synthesis of compound 1, with
TbCl3 Æ 6H2O (0.2 mmol) in place of EuCl3 Æ 6H2O. Color-
less needle-like crystals of compound 2 suitable for X-ray
determination were obtained in 45% yield. The initial pH
value of the reactive solution was 4.7 and the final pH value
was still 4.7. Anal. Calc. for C14H14Tb2N2O13: C, 22.84; H,
1.92; N, 3.81. Found: C, 22.93; H, 1.60; N, 3.95%. IR spec-
tra (cmꢀ1): 3525 (m), 2360 (s), 1573 (s), 1490 (m), 1424 (s),
1393 (s), 1171 (w), 824 (w), 701 (m), 675 (m), 651 (w).
performed under nitrogen with
a heating rate of
10 °C minꢀ1 using a TGA-7 system. Fluorescent spectra
were measured with a Hitachi F-4500 luminescence
spectrometer.
2.3. Crystal structure determination
2.2. Preparations
Suitable single crystals of 1 and 2 were carefully selected
under an optical microscope and glued to thin glass fibers.
Crystallographic data for all compounds were collected
with a Bruker SMART-CCD area detector diffractometer
with graphite-monochromated Mo Ka radiation (k =
2.2.1. [Eu2(PDC)2(OH)2(H2O)2] Æ H2O (1)
A mixture of EuCl3 Æ 6H2O (0.2 mmol), pyridine-3,4-
dicarboxylic acid (0.2 mmol), 4,40-bipyridine (0.2 mmol)
and NaOH (0.4 mmol) in H2O (8.0 ml) was mixed in a
15 ml Teflon reactor, which was heated to 180 °C for
3d. The resulting solutions were cooled slowly to room
temperature. Colorless needle-like crystals of compound
1 suitable for X-ray determination were obtained in 37%
yield. The initial pH value of the reactive solution was
4.7 and the final pH value was still 4.7. Anal. Calc. for
C14H14Eu2N2O13:C, 23.28; H, 1.95; N, 3.88. Found: C,
23.54; H, 1.60; N, 4.00%. IR spectrum (cmꢀ1): 3526
(m), 2360 (s), 2342 (m), 1573 (s), 1490 (m), 1429 (s),
1395 (s), 1171 (w), 824 (w), 670 (m), 642 (w).
˚
0.71073 A) at T = 294(2) K. Absorption corrections were
made using SADABS program [28]. The structures were
solved using direct method and refined by full-matrix
least-squares methods on F2 by using SHELX-97 program
package [29]. All non-hydrogen atoms and the hydrogen
atoms of water molecules are found from the different Fou-
rier maps and refined anisotropically. A summary of the
crystallographic data and refinement parameters are given
in Table 1. Selected bond lengths for 1 and 2 are given in
Table 2.
Table 1
Crystal data and structure refinement details
Compound
1
2
Empirical formula
Formula weight
Temperature (K)
C
722.19
294(2)
14H14Eu2N2O13
C14H14Eu2N2O13
736.11
294(2)
˚
Wavelength (A)
0.71073
0.71073
Crystal system
Space group
a (A)
triclinic
triclinic
ꢀ
ꢀ
P1
P1
˚
8.0134(9)
9.4793(10)
12.6667(14)
76.637(2)
85.157(2)
83.073(2)
927.70(18)
2
7.9774(17)
9.472(2)
12.658(3)
76.408(3)
84.883(3)
82.958(3)
920.9(3)
2
˚
b (A)
˚
c (A)
a (°)
b (°)
c (°)
3
˚
V (A )
Z
Calculated density (Mg/m3)
F(000)
2.585
684
6.776
2.655
692
7.695
Absorption coefficient (mmꢀ1
)
h Range for data collection (°)
Limiting indices
Reflections collected/unique [Rint
2.22–25.02
1.66–25.02
ꢀ9 6 h 6 9, ꢀ11 6 k 6 10, ꢀ11 6 k 6 10
4824/3262 [0.0233]
99.4%
ꢀ8 6 h 6 9, ꢀ15 6 l 6 12, ꢀ15 6 l 6 15
4728/3211 [0.0234]
98.8%
]
Completeness to h = 25.02°
Maximum and minimum transmission
Data/restraints/parameters
Goodness-of-fit on F2
1.000000 and 0.430601
3262/9/280
1.021
1.00000 and 0.664934
3211/9/280
1.028
Final R indices [I > 2r(I)]
R indices (all data)
Largest differences in peak and hole (e A
R1 = 0.0335, wR2 = 0.0953
R1 = 0.0372, wR2 = 0.0980
2.809 and ꢀ2.061
R1 = 0.0251, wR2 = 0.0644
R1 = 0.0303, wR2 = 0.0677
1.009 and ꢀ1.335
ꢀ3
˚
)

H.-H. Song, Y.-J. Li / Inorganica Chimica Acta 361 (2008) 1421–1425
1423
Table 2
˚
Selected bond lengths (A) for compounds 1 and 2
Compound 1
Eu(1)–O(3)#1
Eu(1)–O(10)
Eu(1)–O(11)
Eu(2)–O(8)#4
Eu(2)–O(10)#3
Eu(2)–O(12)
O(4)–Eu(2)#1
O(8)–Eu(2)#4
2.343(4)
2.396(4)
2.488(4)
2.385(4)
2.435(4)
2.584(5)
2.404(4)
2.385(4)
Eu(1)–O(7)#2
Eu(1)–O(1)
Eu(1)–O(9)
Eu(2)–O(4)#1
Eu(2)–O(2)#3
O(2)–Eu(2)#3
O(6)–Eu(1)#1
O(10)–Eu(2)#3
2.356(4)
2.401(4)
2.508(5)
2.404(4)
2.443(4)
2.443(4)
2.446(4)
2.435(4)
Eu(1)–O(11)#1
Eu(1)–O(6)#1
Eu(2)–O(11)
Eu(2)–O(5)
Eu(2)–O(10)
O(3)–Eu(1)#1
O(7)–Eu(1)#5
O(11)–Eu(1)#1
2.419(4)
2.446(4)
2.342(4)
2.416(4)
2.471(4)
2.343(4)
2.356(4)
2.419(4)
Compound 2
Tb(1)–O(3)#1
Tb(1)–O(1)
Tb(1)–O(11)
Tb(2)–O(8)#3
Tb(2)–O(10)#4
Tb(2)–O(12)
O(4)–Tb(2)#1
O(8)–Tb(2)#3
2.306(4)
2.379(4)
2.470(3)
2.362(4)
2.408(3)
2.538(4)
2.378(4)
2.362(4)
Tb(1)–O(7)#2
Tb(1)–O(11)#1
Tb(1)–O(9)
Tb(2)–O(4)#1
Tb(2)–O(2)#4
O(2)–Tb(2)#4
O(6)–Tb(1)#1
O(10)–Tb(2)#4
2.332(3)
2.396(3)
2.491(4)
2.378(4)
2.422(4)
2.422(4)
2.418(3)
2.408(3)
Tb(1)–O(10)
Tb(1)–O(6)#1
Tb(2)–O(11)
Tb(2)–O(5)
Tb(2)–O(10)
O(3)–Tb(1)#1
O(7)–Tb(1)#5
O(11)–Tb(1)#1
2.366(3)
2.418(3)
2.331(3)
2.391(4)
2.455(3)
2.306(4)
2.331(3)
2.396(3)
Symmetry transformations used to generate equivalent atoms:
1: #1: ꢀx, ꢀy + 1, ꢀz + 2; #2: x, y ꢀ 1, z; #3: ꢀx + 1, ꢀy + 1, ꢀz + 2; #4: ꢀx + 1, ꢀy + 2, ꢀz + 2; #5: x, y + 1, z.
2: #1: ꢀx, ꢀy + 1, ꢀz + 2; #2: x, y ꢀ 1, z; #3: ꢀx + 1, ꢀy + 2, ꢀz + 2; #4: ꢀx + 1, ꢀy + 1, ꢀz + 2; #5: x, y + 1, z.
˚
3. Results and discussion
[Eu(2)–O(PDB) = 2.385(4)–2.443(4) A], three binding oxygen
atoms from three OHꢀ groups [Eu(2)–O(OHꢀ) = 2.342(4)–
˚
3.1. Description of the structures
2.471(4) A] and one oxygen atom from the terminal water
molecular [Eu(2)–O(H2O) = 2.584(5) A]. The distance of
˚
˚
Single-crystal X-ray analysis reveals that compound 1 is a
unique two-dimensional layered framework containing one-
dimensional Eu–O–Eu double chain, in which the asymmet-
ric unit consists of two europium atoms, two PDC ligands,
two OHꢀ groups, two aqua ligands, and one isolated water
molecular. The local coordination environment around
Eu(III) centers (Eu(1), Eu(2)) is depicted in Fig. 1. They
all exhibit distorted bicapped trigonal prism geometries.
Eu(1) is coordinated by two pairs of carboxylate oxygen
atoms from two different PDB ligands [Eu(1)–O(PDB) =
Eu(1)ꢁ ꢁ ꢁEu(2) is 3.9163(5) A, which is comparable to the dis-
tance found in the compound [Na2[Ln2(sal)4(CF3SO3)2-
˚
(H2O)4](CF3SO3)2]n (3.8545(8) A) [30], which is possibly
related to that OHꢀ groups participate in coordination.
Eu(1) and Eu(2) centers are connected together via l3-oxy-
gen atoms of OHꢀ groups in an edge-sharing mode to form
a one-dimensional Eu–O–Eu double chain along an axis
with adjacent Euꢁ ꢁ ꢁEu distance of 4.2278(6) and
˚
4.2987(6) A. These chains are further linked via PDC ligand
to a two-dimensional network (Fig. 2). In other words, the
2D layer can be best described as being constructed by
PDC-connected Eu–O–Eu double chain. Furthermore, the
adjacent layers are further linked via hydrogen bonds
˚
2.343(4)–2.446(4) A], threꢀe binding oxygen atoms from three
OHꢀ groups [Eu–O(OH ) = 2.396(4)–2.488(4) A] and one
˚
oxygen atom from the terminal water molecular [Eu(1)–
˚
O(H2O) = 2.508(5) A]; Eu(2) is also defined by two pairs of
carboxylate oxygen atoms from two different PDB ligands
Fig. 1. ORTEP view of the coordination environment around the
Fig. 2. Two-dimensional layered structures of 1 constructed by PDC-
Europium centers for 1.
connected Eu–O–Eu double chains.

1424
H.-H. Song, Y.-J. Li / Inorganica Chimica Acta 361 (2008) 1421–1425
Fig. 3. Hydrogen bonds interactions linking layers into a 3D supermo-
lecular network in 1.
Fig. 4. The emission spectra (emission at 340 nm) of compound 1 and 2 in
the solid state.
between pyridyl N atoms, coordinated water molecule and
isolated water to form a three-dimensional supramolecular
network (Fig. 3). It is interesting that the PDC ligand exhib-
its tetradenate coordination modes, all the carboxyl groups
adopt a monodenate bridging mode to connect four Eu
atoms, pyridine nitrogen atoms are uncoordinated.
and O components, indicating that the final product is
Eu2O3.
3.3. Fluorescence properties of 1 and 2
It is also noteworthy that OHꢀ bridging lanthanide clus-
ters as the building unit to form high-dimensional network
in respect of lanthanide pyridinedicarboxylate coordina-
tion polymers have only one structure [Tb2(Dinic)2(phe-
n)2(OH)2(H2O)]n Æ 3nH2O reported [25]. What is different
from the above coordination polymer, which possesses a
one-dimensional chain structure, is the compound 1 has
a two-dimensional layer structure. This could be related
to the fact that phen in [Tb2(Dinic)2(phen)2(OH)2
(H2O)]n Æ 3nH2O serves a chelating ligand serving a ‘passiv-
ating’ role by occupying coordination sites on the metal
centers and providing steric constraints, thus preventing
spatial extension of skeleton to higher dimensions [31].
X-ray single-crystal diffraction studies reveal that com-
pound 2 adopts a structure very similar to that of 1 (Figs.
S1–S3), as shown by the detailed structural data listed in
the supplementary materials.
Fig. 4 shows the emission spectra of 1 and 2 in the solid
state at room temperature. Excitation at 340 nm into the
lowest energy ligand-centered absorption band results in
the luminescence characteristic of Ln3+ ion (Ln = Eu, Tb).
The emission peaks of 1 at 579, 591, 614 and 618 nm can
7
be assigned to 5D0 ! FJ (J = 0, 1, 2) transitions, while the
emission peaks of 2 at 490, 543, 584 and 621 nm can be
5
7
assigned to D4 ! FJ (J = 6, 5, 4, 3) transitions, respec-
tively [32–34]. However, these characteristic emission
bands indicate that the ligand-to-metal energy transfer is
moderately efficient under the experimental conditions
[35]. For compound 1, the symmetric forbidden emission
7
5D0 ! F0 found at 579 nm reveals that Eu3+ occupy sites
with low symmetry and without an inversion center. The
7
strongest emission is in the 5D0 ! F2 transition region
5
7
and the D0 ! F2 peak was split into two levels at 614
7
and 618 nm. It is well known that the 5D0 ! F2 transition
induced by the electric dipole moment is hypersensitive to
7
3.2. Thermogravimetric analysis
the environment of the Eu ion, while the 5D0 ! F1 transi-
tion is a magnetic dipole transition, which is fairly insensi-
tive to the coordination environment of the Eu ion. The
To investigate their thermal stabilities, thermogravimet-
ric analyses (TGA) of 1 and 2 were carried out at a heating
rate of 10 °C minꢀ1. TG analysis shows that the thermal
decomposition behavior of compounds 1 and 2 was similar.
The first weight loss in 1 started at ca. 70 up to 150 °C giv-
ing a weight loss of 2.16%, corresponding to the loss of lat-
tice water molecules (2.49% calculated). On further
heating, it lost two coordinated water molecules and two
OHꢀ groups between 150 and 250 °C (10.34% observed,
9.69% calculated). In the range of 250–900 °C, the weight
loss of 38.78% should correspond to the decomposition
of organic ligands (calc. 39.09%). The remaining weight
of 48.72% corresponds to the percentage (48.73%) of Eu
7
7
intensity ratio I(5D0 ! F2)/I(5D0 ! F1) is equal to ca.
1.8, which indicates that Eu3+ ions are not at an inversion
center [36]. This is in agreement with the result of the sin-
gle-crystal X-ray.
4. Conclusion
In summary, we have synthesized two novel lanthanide
metal coordination polymers by hydrothermal reactions.
The title complexes represent the first examples of PDC
complexes containing double chain constructed from
OHꢀ bridging building unit. The fluorescent spectra show

H.-H. Song, Y.-J. Li / Inorganica Chimica Acta 361 (2008) 1421–1425
1425
[12] Y.F. Zhou, C.Y. Yue, D.Q. Yuan, L. Chen, J.T. Chen, A.J. Lan, F.L.
Jiang, M.C. Hong, Eur. J. Inorg. Chem. (2006) 4852.
[13] X.L. Wang, C. Qin, E.b. Wang, Y.G. li, N. Hao, C.W. Hu, L. Xu,
Chem. Commun. (2004) 378.
[14] X.L. Wang, C. Qin, E.b. Wang, Y.G. li, N. Hao, C.W. Hu, L. Xu,
Inorg. Chem. 43 (2004) 1850.
that compounds 1 and 2 exhibit the characteristic emission
transitions of Eu3+ and Tb3+ ion, respectively.
Acknowledgements
This work was supported by the Natural Science Foun-
dation of China (Grant No. 20601007), the Natural Science
Foundation of Hebei Education Department (Grant No.
ZH2006002).
[15] C. Qin, X.L. Wang, Y.G. li, E.B. Wang, Z.M. Su, L. Xu, R. Cle´rac,
Dalton Trans. (2005) 2609.
[16] W. Chen, Q. Yue, C. Chen, H.M. Yuan, W. Xu, J.S. Chen, S.N.
Wang, Dalton Trans. (2003) 28.
[17] L. Pan, X.P. Huang, J. Li, Y.G. Wu, N.G. Zheng, Angew. Chem.,
Int. Ed. 39 (2000) 527.
Appendix A. Supplementary material
[18] Y.Q. Sun, J. Zhang, Y.M. Chen, Y. Yang, Angew. Chem., Int. Ed. 44
(2005) 5814.
[19] X.J. Gu, D.F. Xue, Cryst. Growth Des. 6 (2006) 2551.
[20] X.J. Gu, D.F. Xue, Inorg. Chem. 45 (2006) 9257.
[21] Y.Z. Zheng, M.L. Tong, X.M. Chen, Eur. J. Inorg. Chem. (2005) 4109.
[22] X.L.Wang,C.Qin,E.B.Wang,L.Xu,Inorg.Chim.Acta359(2006)417.
[23] Z.B. Han, X.N. Cheng, X.F. Li, X.M. Chen, Z. Anorg. Allg. Chem.
631 (2005) 937.
[24] D. Ang, G.B. Deacon, P.C. Junk, D.R. Turner, Polyhedron 26 (2007)
385.
[25] Z.L. Liu, D.Q. Zhang, C.M. Liu, D.B. Zhu, Chem. Lett. (2004) 33.
[26] Z.B. Han, Y.K. He, Acta Crystallogr. E62 (2006) m2374.
[27] F. Wang, Y.K. He, Y. Ma, Z.B. Han, Acta Crystallogr. E62 (2006)
m2505.
CCDC 648468 and 648469 contain the supplementary
crystallographic data for 1 and 2. These data can be
obtained free of charge from The Cambridge Crystallo-
graphic Data Centre http://www.ccdc.cam.ac.uk/data_re-
quest/cif. Supplementary data associated with this article
can be found, in the online version, at doi:10.1016/
j.ica.2007.09.013.
References
[28] G.M. Sheldrick, SADABS, Program for Area Detector Adsorption
Correction, Institute for Inorganic Chemistry, University of Gottin-
gen, Germany, 1996.
[1] M.J. Zaworotko, Chem. Rev. 101 (2001) 1629.
[2] S.-L. Zheng, M.-L. Tong, X.-M. Chen, Coord. Chem. Rev. 246 (2003)
185.
[29] G.M. Sheldrick, SHELXL-97, Program for Solution of Crystal Struc-
tures, University of Gottingen, Germany, 1997.
[30] M.A. Singh-Wilmot, R.U. Richard-Johnson, T.N. Dawkins, A.J.
Lough, Inorg. Chim. Acta 360 (2007) 3727.
[31] C. Qin, X.L. Wang, E.b. Wang, L. Xu, J. Mol. Struct. 738 (2005)
91.
[32] V. Patroniak, P.N.W. Baxter, J.-M. Lehn, Z. Hnatejko, M. Kubicki,
Eur. J. Inorg. Chem. (2004) 2379.
[3] S.R. Batten, R. Robson, Angew. Chem., Int. Ed. 37 (1998) 1460.
[4] A.J. Blake, N.R. Champness, P. Hubberstey, W.-S. Li, M.A.
Withersby, M. Schro¨der, Coord. Chem. Rev. 183 (1999) 117.
[5] C.N.R. Rao, S. Natarajan, R. Vaidhyanathan, Angew. Chem., Int.
Ed. 43 (2004) 1466.
[6] S. Kitagawa, R. Kitaura, S.-i. Noro, Angew. Chem., Int. Ed. 43
(2004) 2334.
[7] M.L. Tong, S. Kitagawa, H.C. Chang, M. Ohba, Chem. Commun.
(2004) 418.
[33] C. Tedeschi, J. Aze’ma, H. Gornitzka, P. Tisnes, C. Picard, Dalton
Trans. (2003) 1738.
[8] M. Du, H. Cai, X.J. Zhao, Inorg. Chim. Acta 359 (2006) 673.
[9] T. Whitfield, L.M. Zheng, X.Q. Wang, A.J. Jacobson, Solid State Sci.
3 (2001) 829.
[34] K.A. Thiakou, V. Bekiari, C.P. Raptopoulou, V. Psycharis, P.
Lianos, S.P. Perlepes, Polyhedron 25 (2006) 2869.
[35] A.B. Dias, S. Viswanathan, Chem. Commun. 4 (2004) 1024.
[36] Y.B. Wang, X.J. Zheng, W.J. Zhang, L.P. Jin, Eur. J. Inorg. Chem.
(2003) 1355.
[10] Q.Z. Zhang, C.Z. Lu, C.K. Xia, Inorg. Chem. Commun.
(2005) 304.
8
[11] M.L. Tong, J. Wang, S. Hu, J. Solid State Chem. 178 (2005) 1518.