Syntheses, crystal structures, and photophysical properties of two 2-D coordination polymers

Article abstract of DOI:10.1080/00958972.2012.740022

Two coordination polymers, [Mn(dipt)(m-BDC)₃]_n (1) and [Pb(mip)(1,4-NDC)]_n (2) [dipt=2-(2,4-dichlorophenyl)-1H- imidazo[4,5-f][1,10]phenanthroline, mip=2-(3-methoxyphenyl)- 1H-imidazo[4,5-f][1,10]phenanthroline, m-BDC=isop

Full text of DOI:10.1080/00958972.2012.740022

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Syntheses, crystal structures, and
photophysical properties of two 2-D
coordination polymers
Li Yan a & Chuan-Bi Li a
a College of Chemistry , Jilin Normal University , Siping 136000 ,
People's Republic of China
Accepted author version posted online: 15 Oct 2012.Published
online: 30 Oct 2012.
To cite this article: Li Yan & Chuan-Bi Li (2012) Syntheses, crystal structures, and photophysical
properties of two 2-D coordination polymers, Journal of Coordination Chemistry, 65:24, 4288-4298,
DOI: 10.1080/00958972.2012.740022
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Journal of Coordination Chemistry
Vol. 65, No. 24, 20 December 2012, 4288–4298
Syntheses, crystal structures, and photophysical properties
of two 2-D coordination polymers
LI YAN* and CHUAN-BI LI
College of Chemistry, Jilin Normal University, Siping 136000,
People’s Republic of China
(Received 1 June 2012; in final form 27 August 2012)
Two coordination polymers, [Mn(dipt)(m-BDC)3]n (1) and [Pb(mip)(1,4-NDC)]n (2)
[dipt ¼ 2-(2,4-dichlorophenyl)-1H-imidazo[4,5-f][1,10]phenanthroline, mip ¼ 2-(3-methoxyphe-
nyl)-1H-imidazo[4,5-f][1,10]phenanthroline, m-BDC ¼ isophthalic acid, 1,4-NDC ¼ naphtha-
lene-1,4-dicarboxylic acid], have been synthesized by hydrothermal reactions and characterized
by elemental analysis, thermogravimetric analysis, infrared spectrum, and single-crystal X-ray
diffraction. Single-crystal X-ray diffraction reveals that 1 and 2 have 1-D chain architecture.
Complex 1 has a 2-D-layered structure constructed from C–Hꢀ ꢀ ꢀO hydrogen bonds. Complex 2
has a 2-D-layered structure constructed from N–Hꢀ ꢀ ꢀO hydrogen bonds and ꢀ–ꢀ interactions.
TG analyses suggest 1 and 2 have excellent thermal stabilities from hydrogen bonds and ꢀ–ꢀ
interactions. Mn(II) in 1 has trigonal bipyramidal geometry surrounded by three carboxylate
oxygen atoms from three monodentate bridging m-BDC and two nitrogen atoms from one dipt.
Pb(II) has [:PbN2O4] pentagonal bipyramidal geometry in 2. The luminescent properties for
dipt, mip, 1, and 2 are also presented.
Keywords: Manganese; Lead; Crystal structures; Thermal stability analysis; Fluorescence
1. Introduction
Construction of metal–organic frameworks (MOFs) is an active area of materials
research, driven by interesting network topologies and potential applications in
catalysis, porosity, sensors, magnetism, luminescence, molecular recognition, nonlinear
optics, and electrical conductivity [1–7]. Ligands and metals can be organized by non-
covalent interactions in MOFs. Hydrothermal synthesis is widely used to obtain crystal
structures, addressing problems of ligand solubility and possibilities for enhanced
reactivity of molecules in crystallization. Progress has been made in theoretical
predictions [8–10]. However, it is still a challenge to predict the exact structures and
composition of target products in crystal engineering because formation of MOFs is
determined by coordination nature of metal ions, ligand structures, pH, temperature,
etc. [11]. Multidentate ligands such as poly-carboxylates and N-heterocycles give
structures with desired properties. Among poly-carboxylates, the best studied are
*Corresponding author. Email: yanli820618@163.com
Journal of Coordination Chemistry
ISSN 0095-8972 print/ISSN 1029-0389 online ß 2012 Taylor & Francis
http://dx.doi.org/10.1080/00958972.2012.740022

2-D Coordination polymers
4289
Scheme 1. Syntheses of dipt and mip ligands.
dicarboxylates, tricarboxylates, and biphenyldicarboxylates. 2-(Pyridin-2-yl)pyridine,
4-(pyridin-4-yl)pyridine, and 1,10-phenanthroline can act as terminal ligands and
provide supramolecular interaction sites for molecular recognition. Our strategy is to
employ long-conjugated unsymmetrical ligands dipt (2-(2,4-dichlorophenyl)-1H-imi-
dazo[4,5-f][1,10]phenanthroline) and mip (2-(3-methoxyphenyl)-1H-imidazo[4,5-f][1,10]
phenanthroline) (as shown in scheme 1) because they (1) possess extended conjugated
unsymmetrical aromatic system to provide supramolecular interactions; (2) possess two
nitrogen atoms for bidentate chelating molecules; (3) possess strong and rigid
coordination to metals [12]. Combining N-heterocyclic ligands and anionic O-donor
is an attractive design strategy [13, 14]; investigation of similar N-heterocyclic ligands is
lacking [15–18], with few reports using dipt, mip, isophthalic acid (m-BDC), and
naphthalene-1,4-dicarboxylic acid (1,4-NDC) as mixed ligands to construct coordina-
tion polymers.
In this study, we use m-BDC and 1,4-NDC to obtain [Mn(dipt)(m-BDC)3]n (1) and
[Pb(mip)(1,4-NDC)]n (2). Complexes 1 and 2 exhibit 1-D chain structures, with 2-D
layer structures via hydrogen bonds and inter-chain ꢀ–ꢀ stacking interactions.
2. Experimental
2.1. Materials and physical measurements
The dipt and mip were prepared by literature procedures [19]. All other chemicals
from commercial sources were of AR grade and used without purification. The FT-IR
spectra were recorded on an Alpha Centauri FT-IR spectrophotometer using KBr
discs from 400 to 4000 cmꢁ1. Thermogravimetric (TG) analysis was performed with a
Diamond DSC thermal analyzer at 10ꢂC minꢁ1 in nitrogen. Crystal structures were
determined on a Bruker SMART APEX II CCD X-ray diffractometer. Elemental
analyses (C, H, and N) were performed on a PE-2400 elemental analyzer.
Fluorescence spectra were recorded on an FLSP 920 Edinburgh fluorescence
spectrometer.

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L. Yan and C.-B. Li
2.2. Syntheses of 1 and 2
2.2.1. Synthesis of [Mn(dipt)(m-BDC)3]n (1). A mixture of MnSO4 ꢀ H2O (0.051 g,
0.3 mmol), dipt (0.120 g, 0.3 mmol), m-BDC (0.100 g, 0.3 mmol), and H2O (18 mL) was
stirred at room temperature and the pH adjusted to 7.0 with NaOH. The cloudy
solution was placed in a 30-mL Teflon-lined stainless vessel under autogenous pressure
at 443 K for 72 h and afterwards cooled to room temperature at 5ꢂC hꢁ1. Yellow
crystals of 1 were collected in 78% yield based on Mn. Calcd for C27H14Cl2MnN4O4
(%): C, 55.50; H, 2.41; N, 9.58. Found (%): C, 55.45; H, 2.39; N, 9.51. Infrared (IR)
(KBr, cmꢁ1): 3418(s), 2362(s), 1613(vs), 1383(vs), 1072(s), 731(s), 706(m), 627(m).
2.2.2. Synthesis of [Pb(mip)(1,4-NDC)]n (2). A mixture of Pb(Ac)2 ꢀ 3H2O (0.114 g,
0.3 mmol), 1,4-NDC (0.130 g, 0.6 mmol), mip (0.099 g, 0.3 mmol), and H2O (18 mL) was
stirred at room temperature and the pH was adjusted to 7.0 with NaOH. The cloudy
solution was placed into a 30-mL Teflon-lined stainless vessel under autogenous
pressure at 443 K for 72 h and afterwards cooled to room temperature at 5ꢂC hꢁ1. Small
yellow crystals of 2 were collected in 85% yield based on Pb. Calcd for
C32H21N4O5.50Pb (%): C, 50.78; H, 2.79; N, 7.40. Found (%): C, 50.75; H, 2.66; N,
7.52. IR (KBr, cmꢁ1): 3476(s), 3406(s), 2369(s), 1530(vs), 1403(vs), 1251(s), 837(s),
706(m).
2.3. X-ray crystallography
Single-crystal X-ray diffraction data were collected at room temperature with a Bruker
SMART APEX II CCD diffractometer equipped with graphite-monochromated
ꢂ
˚
Mo-Ka radiation (ꢁ ¼ 0.71073 A) at 293(2) K from 1.65 ꢃ ꢂ ꢃ 26.06 for 1 and
1.68 ꢃ ꢂ ꢃ 26.07ꢂ for 2. Absorption corrections were applied using the multi-scan
technique and all structures were solved by direct methods with SHELXS-97 [20] and
refined with SHELXL-97 [21] by full-matrix least-squares on F2. Non-hydrogen atoms
were refined with anisotropic temperature parameters. Experimental details for
crystallographic data and structure refinement parameters for 1 and 2 are listed in
table 1.
3. Results and discussion
3.1. Description of crystal structures
3.1.1. [Mn(dipt)(m-BDC)3]n (1). The molecular structure of 1 is shown in figure 1.
The 1-D binuclear double-chain structure is shown in figure 2 and the 2-D layer
structure linked by C–Hꢀ ꢀ ꢀO hydrogen bonds is suggested in figure 3. Selected bond
lengths and angles are given in table 2.
As shown in figure 1, the asymmetric unit contains one Mn(II), one dipt, and three
m-BDC. In general, Mn(II) exhibits five-, six-, or seven-coordination with six- or seven-
coordinate more common; five-coordinate Mn(II) complexes are rare [22–25]. In 1,
Mn(II) is five-coordinate with three carboxylate oxygen atoms from three distinct

2-D Coordination polymers
4291
Table 1. Crystal data and details of structure refinement parameters for 1 and 2.
Complex
1
2
Empirical formula
Formula weight
Crystal system
Space group
C
584.26
Monoclinic
C2/c
27H14Cl2MnN4O4
C32H21N4O5.50Pb
756.72
Monoclinic
C2/c
ꢂ
˚
Unit cell dimensions (A, )
a
b
c
18.9697(1)
17.5113(1)
15.6937(1)
113.228(2)
4790.6(8), 8
1.620
0.819
15.531(2)
19.833(3)
17.188(2)
99.098(2)
5227.8(1), 8
1.923
6.510
2936
0.747 ꢄ 0.420 ꢄ 0.338
1.68–26.07
13,827
ꢃ
Volume (A ), Z
3
˚
Calculated density (Mg mꢁ3
)
Absorption coefficient (mmꢁ1
F(000)
)
2360
0.433 ꢄ 0.371 ꢄ 0.308
Crystal size (mm3)
ꢂ range for data collection (ꢂ)
Reflections collected
Unique reflections
1.65–26.06
15,175
4725 [R(int) ¼ 0.0513]
5178 [R(int) ¼ 0.0306]
Goodness-of-fit on F2
Final R indices [I 4 2ꢄ(I)]
R indices (all data)
1.031
1.028
R1 ¼ 0.0612, wR2 ¼ 0.1384
R1 ¼ 0.0967, wR2 ¼ 0.1578
1114 and ꢁ285
R1 ¼ 0.0247, wR2 ¼ 0.0524
R1 ¼ 0.0405, wR2 ¼ 0.0572
683 and ꢁ1242
ꢁ3
˚
Largest difference peak and hole (e A
)
Figure 1. The molecular structure of 1 (hydrogen atoms were omitted).
bridging m-BDC and two nitrogen atoms from one chelating dipt, forming a distorted
trigonal bipyramidal geometry. The O(1)–Mn(1)–O(4)#1 (#1: x, ꢁy, z þ 1/2), O(1)–
Mn(1)–N(1), and O(4)#1–Mn(1)–N(1) angles are 105.91ꢂ, 120.56ꢂ, and 132.60ꢂ, and the
sum is 359.07ꢂ. Mn(1), O(4)#1, O(1), and N(1) define the basal plane, and N(2) and
O(3)#2 (#2: ꢁx, ꢁy, ꢁz þ 1) occupy axial positions. The equation of a plane is: ꢁ14.3474
(0.0219) x ꢁ 10.8300 (0.0147) y þ 7.7566 (0.0190) z ¼ 2.0934 (0.0110). The deviation of
Mn1, O(4)#1, O(1), N(1), N(2), and O(3)#2 to the plane is 0.3868, ꢁ0.0795, ꢁ0.3964,
˚
ꢁ0.0699, ꢁ1.3453, and 1.4768 A, respectively.
˚
The Mn–O bond distances in 1 are 2.023(3) to 2.117(3) A and Mn–N distances are
˚
2.241(3)–2.283(3) A, similar to values reported [26–31]. The average Mn–O bond is
˚
˚
2.059 A, smaller than the average Mn–N bond (2.262 A), which suggests that

4292
L. Yan and C.-B. Li
Figure 2. 1-D binuclear double chain structure of 1 (hydrogen atoms were omitted).
Figure 3. 2-D layer structure of 1 linked by C–Hꢀ ꢀ ꢀO hydrogen bonds (dotted lines represent hydrogen
bonds).
coordination of carboxylate is stronger than dipt. The N(O)–Mn–O(N) angles range
from 88.40(13)ꢂ to 161.82(12)ꢂ.
As shown in figure 2, one m-BDC coordinates to three Mn(II) ions through three
carboxylate oxygen atoms in a mono-bridging fashion, giving a 1-D binuclear wave
double-chain structure. In 1, two types of rings form, an eight-membered ring, in which

2-D Coordination polymers
4293
ꢂ
˚
Table 2. Selected bond lengths (A) and angles ( ) for 1 and 2.
1
Mn(1)–O(1)
Mn(1)–O(3)#2
Mn(1)–N(2)
2.023(3)
2.117(3)
2.283(3)
Mn(1)–O(4)#1
Mn(1)–N(1)
2.037(3)
2.241(3)
O(1)–Mn(1)–O(4)#1
O(4)#1–Mn(1)–O(3)#2
O(4)#1–Mn(1)–N(1)
O(1)–Mn(1)–N(2)
105.91(12)
98.48(13)
132.61(13)
103.67(12)
161.82(12)
O(1)–Mn(1)–O(3)#2
O(1)–Mn(1)–N(1)
O(3)#2–Mn(1)–N(1)
O(4)#1–Mn(1)–N(2)
N(1)–Mn(1)–N(2)
90.68(12)
120.56(12)
90.24(12)
88.40(13)
72.94(12)
O(3)#2–Mn(1)–N(2)
2
Pb(1)–O(3)#1
Pb(1)–O(4)#1
Pb(1)–N(1)
2.465(3)
2.583(3)
2.464(3)
Pb(1)–O(1)
Pb(1)–O(2)
Pb(1)–N(2)
2.505(3)
2.693(3)
2.554(3)
O(3)#1–Pb(1)–O(2)
O(3)#1–Pb(1)–O(4)#1
O(4)#1–Pb(1)–O(2)
N(2)–Pb(1)–O(4)#1
N(1)–Pb(1)–O(1)
O(1)–Pb(1)–N(2)
N(1)–Pb(1)–O(2)
N(1)–Pb(1)–N(2)
131.71(9)
51.14(9)
O(1)–Pb(1)–O(2)
O(1)–Pb(1)–O(4)#1
O(3)#1–Pb(1)–O(1)
N(1)–Pb(1)–O(3)#1
O(3)#1–Pb(1)–N(2)
N(1)–Pb(1)–O(4)#1
N(2)–Pb(1)–O(2)
50.09(8)
135.93(9)
85.80(9)
78.70(10)
121.23(10)
84.90(11)
80.73(9)
155.44(11)
79.83(10)
76.71(10)
124.86(9)
73.35(10)
65.11(10)
#1
#2
Symmetry transformations used to generate equivalent atoms: (1)
ꢁy þ 1/2, z ꢁ 1/2; (2) x, ꢁy, z þ 1/2.
x þ 1/2, ꢁy þ 1/2, z þ 1/2,
x ꢁ 1/2,
#1
ꢂ
˚
the O(3)–Mn(1)–O(4) angle is 98.485 and the distance of Mnꢀ ꢀ ꢀMn is 3.8592 A, and a
ꢂ
˚
16-membered ring with O(3)–Mn(1)–O(1) angle of 90.671 and Mnꢀ ꢀ ꢀMn of 7.9453 A
(figure 3). N-heterocyclic ligands dipt attach to both sides of this chain and the ligands
on the same side are nearly parallel.
Hydrogen-bonding interactions are prevalent in the synthesis of supramolecular
architectures with the most interesting aspect of 1 the intermolecular C–Hꢀ ꢀ ꢀO
interactions. Every pair of molecules is linked by intermolecular C15–H15Aꢀ ꢀ ꢀO2
hydrogen bonds. The existence of C15–H15Aꢀ ꢀ ꢀO2 hydrogen-bond interactions
ꢂ
˚
˚
[H(15A)ꢀ ꢀ ꢀO(2) ¼ 2.52 A, C(15)ꢀ ꢀ ꢀO(2) ¼ 3.265 A and C(15)–H(15A)ꢀ ꢀ ꢀO(2) ¼ 137 ]
lead the 1-D zigzag chain to a 2-D layer structure (figure 3). The two parallel dipt in
˚
the same side have ꢀ–ꢀ stacking interactions with face to face distance of 3.693 A.
Hydrogen bonds and ꢀ–ꢀ stacking interactions reinforce the structure, giving thermal
stability to 1.
3.1.2. [Pb(mip)(1,4-NDC)]n (2). The molecular structure of 2 is shown in figure 4 and
the 1-D arm-shaped chain is shown in figure 5. The 1-D ladder double-chain structure is
illustrated in figure 6 and the 2-D structure is suggested in figure 7. Selected bond
lengths and angles are listed in table 2.
As shown in figure 4, the asymmetric unit of 2 consists of one Pb(II), one mip, and
one 1,4-NDC. Pb(II) is six-coordinate with two nitrogen atoms (N(1), N(2)) from one
chelating mip ligand and four oxygen atoms from two chelating bidentate 1,4-NDC,
furnishing a distorted [:PbN2O4] pentagonal bipyramidal geometry [32]. Pb(1), the
Pb(1), O1, O2, O3, O4, N2 atoms define the basal plane and N1 and the lone pair of
electrons occupy apical positions. The N(O)–Pb–O(N) angles range from 73.35ꢂ to

4294
L. Yan and C.-B. Li
Figure 4. The molecular structure of 2 (hydrogen atoms were omitted).
Figure 5. 1-D arm-shaped chain structure of 2 (hydrogen atoms were omitted).
Figure 6. 1-D ladder chain structure of 2 (hydrogen atoms were omitted).

2-D Coordination polymers
4295
Figure 7. 2-D layer structure of 2 linked by N–Hꢀ ꢀ ꢀO hydrogen bonds and inter-chain ꢀ–ꢀ stacking
interactions (dotted lines represent hydrogen bonds).
ꢂ
˚
124.86 . Pb–O bond distances in 2 are from 2.465(3) to 2.693(3) A and Pb–N distances
˚
are 2.464(3) to 2.554(3) A, similar to reported values [33–38]. The average Pb–O bond is
˚
similar to Pb–N (2.613 A), suggesting that coordination ability of carboxylate is similar
to mip. Lead geometries are holo-directed, in which the bonds to ligands are distributed
throughout the surface of an encompassing globe, or hemi-directed, in which the bonds
to ligands are directed throughout only part of an encompassing globe [39]. The
coordination environment of Pb(II) in 2 is hemi-directed with identifiable gap (or void)
in the distribution of bonds to the ligands.
Each pair of adjacent Pb(II) ions is bridged by 1,4-NDC (bis-chelating mode) to form
˚
an arm-shaped chain structure with Pbꢀ ꢀ ꢀPb distance of 11.5400 A. The mip are
attached to the same side of this chain in a slanted fashion, in which the Pb–Pb–Pb
angle, defined by the orientation of 1,4-NDC in the chain, is 134.257ꢂ (figure 5).
˚
˚
Intermolecular N–Hꢀ ꢀ ꢀO [H(3B)ꢀ ꢀ ꢀO(3) ¼ 2.08 A, N(3)ꢀ ꢀ ꢀO(2) ¼ 2.887 A, and N(3)–
H(3B)ꢀ ꢀ ꢀO(2) ¼ 157ꢂ] interactions help in construction of the 1-D ladder chain (figure 6).
˚
The effective length of each mip of 10.6277 A provides the possibility for stacking.
The double chains recognize each other through aromatic ꢀ–ꢀ stacking interaction of
mip ligands, resulting in a 2-D structure (figure 7). The existence of hydrogen bonds
(N(3)–H(3B)ꢀ ꢀ ꢀO(2)) and ꢀ–ꢀ stacking interactions reinforces the structure.
Different coordination modes of dicarboxylates result in different structures in 1
and 2. Coordination modes (chelating bis-bidentate or bis-monodentate) usually
support 1-D chain or 1-D double-chain structures and diverse coordination modes
(monodentate-bidentate or chelating/bridging bis-bidentate) usually support higher
dimensional structures [40].
3.2. IR spectra
In 1, peaks at 1613 and 1383 cmꢁ1 correspond to antisymmetric and symmetric stretching
of carboxyl. The Dv (vas(COOꢁ)–vs(COOꢁ)) is 230 cmꢁ1 (larger than 200 cmꢁ1),

4296
L. Yan and C.-B. Li
indicating that the carboxylates are monodentate with Mn(II). In 2, the strong absorption
peaks at 1530 cmꢁ1 is the antisymmetric stretching of carboxylate and the absorption at
1403 cmꢁ1 is the symmetric stretch. The separation is 127 cmꢁ1, indicating bidentate
coordination. The IR results are in agreement with the results of their crystal structures.
3.3. Thermal analyses
TG analyses were used to investigate the stabilities of 1 and 2 (Supplementary material).
As expected, 1 exhibits good stability, stable at 360ꢂC. First weight loss of 27.00% from
360ꢂC to 480ꢂC reveals loss of m-BDC (Calcd 28.42%). The second weight loss of
61.20% from 480ꢂC to 670ꢂC corresponds to loss of dipt (Calcd 62.50%); In 2, the first
weight loss of 26.50% from 350ꢂC to 380ꢂC corresponds to loss of 1,4-NDC (Calcd
28.55%). The second weight loss at 380–620ꢂC can be ascribed to release of mip (Obsd
39.20%, Calcd 43.10%). The final formation may be PbO. The analysis results indicate
that 2 exhibits higher thermal stability than similar complexes [41–43]; the existence of
hydrogen bonds and ꢀ–ꢀ stacking interactions reinforce the structure, demonstrating
thermal stability of 2.
3.4. Photoluminescent properties
Luminescent complexes are currently of interest because of their applications in
photochemistry and photophysics [44, 45]. Here, we studied the luminescence of the free
Figure 8. Luminescence spectrum of 1 and dipt in the solid state at room temperature.

2-D Coordination polymers
4297
Figure 9. Luminescence spectrum of 2 and mip in the solid state at room temperature.
ligands, 1 and 2 (figures 8 and 9). Free dipt exhibits one emission band at 388 nm upon
excitation at 325 nm and mip exhibits one emission band at 529 nm upon excitation at
320 nm. Complex 1 shows one strong emission band at 591 nm (excitation at 300 nm),
which is red-shifted by 203 nm relative to that of free dipt. Complex 2 shows one strong
emission at 595 nm (excitation at 320 nm), which is red-shifted by 66 nm relative to that
of free mip. The emission of 1 and 2 can be attributed to a metal-centered transition
involving s and p metal orbitals, as proposed by Vogler et al [46]. The luminescence
results suggest emission bands of the complexes result from poly-carboxylate and
N-heterocyclic ligands interacting on each other. Our complexes exhibit strong
emissions, which may be attributed to rigidity, reducing the loss of energy through a
radiationless pathway. However, the effect of the microenvironment between ligands
and complexes on the luminescence properties still needs further investigations.
Complexes 1 and 2 may be candidates for photoluminescence materials, highly
thermally stable and insoluble in water and common organic solvents.
4. Conclusion
Binuclear 1 and mononuclear 2 have been synthesized by using planar multifunctional
ligands dipt, mip, m-BDC, and 1,4-NDC. Hydrogen bonds and ꢀ–ꢀ interactions reinforce
the stabilities of the complexes. TG analysis reveals that the frameworks of 1 and 2 are
very stable and worthy of further study as potential photoluminescent materials.
Supplementary material
CCDC 745117 and 818553 contain the supplementary crystallographic data for 1 and 2,
respectively. These data can be obtained free of charge from the Cambridge
Crystallographic Data Centre via http://www.ccdc.cam.ac.uk/data_request/cif

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L. Yan and C.-B. Li
Acknowledgments
We thank the National Natural Science Foundation of China (No. 20971019) and the
Postdoctoral Science foundation of China (No. 2005038561) for financial support.
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