Synthesis, structure and photoluminescence of two cadmium coordination polymers with 1,3-benzenedicarboxylate and two flexible bis-imidazole ligands

Article abstract of DOI:10.1016/j.molstruc.2011.02.056

Two new coordination polymers, [Cd₂(2-mBIM)₂(1,3-BDC) ₂]_n (1) and [Cd(BIM)(1,3-BDC)]_n (2) [BIM = bis(imidazol-1-yl)-methane, 2-mBIM = bis(2-methylimidazol-1-yl)methane], were synthesized by reactions

Full text of DOI:10.1016/j.molstruc.2011.02.056

Journal of Molecular Structure 992 (2011) 111–116
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Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Synthesis, structure and photoluminescence of two cadmium coordination
polymers with 1,3-benzenedicarboxylate and two flexible bis-imidazole ligands
⇑
Zhe Zhang, Min Pi, Tao Wang, Chuan-Ming Jin
Hubei Key Laboratory of Pollutant Analysis & Reuse Technology, College of Chemistry and Environmental Engineering, Hubei Normal University, Huangshi, Hubei 435002, 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 coordination polymers, [Cd2(2-mBIM)2(1,3-BDC)2]n (1) and [Cd(BIM)(1,3-BDC)]n (2) [BIM =
bis(imidazol-1-yl)-methane, 2-mBIM = bis(2-methylimidazol-1-yl)methane], were synthesized by reac-
tions of CdCl2 with 2-mBIM or BIM ligand in the presence of 1,3-benzenedicarboxylic acid. The polymers
were then characterized by elemental analyses, IR, TGA, and X-ray diffraction. Compound 1 is a two-
dimensional (2D) twofold parallel interpenetration network with a (4, 4) connected topology. Compound
2 is a 2D double-layer framework containing [Cd2(COO)4] subunits. The results of this study suggest that
subtle changes in the ligand structure may have a great impact on the resulting architectures of the
metal–organic frameworks. Solid-state luminescent spectra of compounds 1 and 2 indicate weak fluores-
cent emissions at ca. 389 and 302 nm, respectively.
Received 4 February 2011
Accepted 27 February 2011
Available online 4 March 2011
Keywords:
Bridging ligand
Crystal engineering
Cadmium
Metal–organic framework
Photoluminescence
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
1,4-di(1-imidazolyl)benzene and 1,3,5-tris(1-imidazolyl) benzene
have been widely used in previously conducted studies [17–20].
In the past decade, a great deal of progress has been made in the
synthesis and structural characterization of metal–organic frame-
works (MOFs), many of which exhibit novel topological structures,
interesting properties, and have the potential for applications in
gas adsorption, ion exchange, magnetism, heterogeneous catalysis,
luminescence, and nonlinear optics [1–10]. The rational design,
synthesis, and characterization of metal–organic compounds with
new architectural characteristics has been elusive and still remains
a major challenge in this field because the self-assembly process is
frequently influenced by various factors such as the number, type,
and spatial disposition of binding sites on the ligand; the stereo-
electronic preferences of the metal ion; and the metal–ligand ratio,
medium, temperature, template, and counterion [1,11–13]. The
most effective and facile approach to the controlled preparation
of functional MOFs is an appropriate choice of well-designed or-
ganic ligands containing multidentate nitrogen and oxygen donor
ligands, together with metal centers that have various coordination
preferences [14–16].
On the other hand, polycarboxylates such as benzenedicarboxylate,
benzenetricarboxylate, biphenyldicarboxylate, and oxalate (OX2ꢀ
)
are the most extensively studied organic ligands containing O do-
nors in the construction of MOFs because of their versatile coordi-
nation modes, structural diversity, and thermodynamic stability
[21–25]. Recent studies have further demonstrated that mixed or-
ganic ligands with more tunable factors, especially polycarboxy-
lates and N-containing compounds, are good candidates for the
construction of novel MOFs [26,27]. The ligand bis(2-methylimi-
dazo-1-yl)methane (2-mBIM) and bis(imidazo-1-yl)methane
(BIM) is a novel flexible N-donor ligand that has attracted a great
deal of attention in the field of crystal engineering [28–30]. Follow-
ing such a mixed ligand strategy, in this study, we evaluated reac-
tions of ligand 2-mBIM and BIM with different carboxylate ligands
and Cd(II) salts. Specifically, a systematic investigation of the im-
pact of subtle changes in the ligand on the structure of the com-
plexes was carried out. Herein, we report the syntheses, crystal
structures, and photoluminescence properties of two novel Cd(II)
coordination polymers [Cd2(2-mBIM)2(1,3-BDC)2]n (1) and
[Cd(BIM)(1,3-BDC)]n (2).
Generally, multidentate organic ligands containing coordina-
tion sites of N and/or O donors play key roles in tailor-made molec-
ular materials as well as in supramolecular self-assembly crystal
engineering owing to their molecular geometry and flexibility.
For example, the ligands 4,40-bipyridine and 1,2-bis(4-pyridyl)eth-
ane as well as the ditopic or tripodal imidazole-containing ligands
2. Experimental section
2.1. Materials and measurements
⇑
All chemicals were obtained commercially available and used as
received without further purification. Ligand bis(imidazol-1-
Corresponding author. Tel.: +86 714 6513829; fax: +86 714 6571397.
E-mail addresses: jincm1999@yahoo.com, cmjin@email.hbnu.edu.cn (C.-M. Jin).
0022-2860/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2011.02.056

112
Z. Zhang et al. / Journal of Molecular Structure 992 (2011) 111–116
yl)methane (BIM) and bis(2-methyl-imidazol-1-yl)methane (2-
mBIM) were prepared by literature’s methods [31]. The FT-IR spec-
tra were recorded from KBr pellets in the range of 4500–500 cmꢀ1
on a Nicolet 5700 infrared spectrometer. Thermogravimetric anal-
ysis (TGA) measurements were made using a Pyris diamond TG/
DTA Thermogravimetric differential Thermal Analyzer. Samples
were heated at 10 °C/min from 40 to 800 °C in a dynamic nitrogen
atmosphere. Elemental analyses were performed on a Perkin–El-
mer 2400 CHN elemental analyzer. The photoluminescence
measurements were carried out on crystalline samples at room
sealed in a 25 mL Teflon-lined stainless steel autoclave, under
autogenous pressure at 160 °C for 2 days and then slowly cooled
to room temperature at a rate of 10 °C/h. Colorless needle crystals
of [Cd(BIM)(1,3-BDC)]n suitable for X-ray analysis were obtained in
76% yield. Anal. Calcd for C15H12CdN4O4: C 42.42, H 2.85, N 13.19;
Found: C 42.79, H 2.96, N 13.27; IR (KBr, cmꢀ1):
t = 3432s, 3133s,
3113 m, 3006w, 1601s, 1551s, 1474w, 1440m, 1388s, 1284m,
1242m, 1208w, 1103m, 1079s, 1026w, 929m, 849s, 747s, 721s,
658s, 613w.
temperature and the spectra were collected with
F-4500 spectrophotometer.
a Hitachi
2.4. X-ray crystallography
Crystals of complexes 1 and 2 were removed from test tube and
covered with a layer of hydrocarbon oil. A suitable crystal was se-
lected, attached to a glass fiber and data for 1 and 2 were collected
at 293(2) K using a Bruker/Siemens SMART APEX instrument (Mo
2.2. Synthesis of [Cd2(2-mBIM)2(1,3-BDC)2]n (1)
The mixture of CdCl2ꢁ2.5H2O(114 mg, 0.5 mmol), 1,3-benzen-
edicarboxylic acid (83 mg, 0.5 mmol), NaOH (40 mg, 1.0 mmol)
and ligand 2-mBIM (89 mg, 0.5 mmol) were dissolved in the solu-
tion of H2O (10 mL), and the resulting mixture was transferred and
sealed in a 25 mL Teflon-lined stainless steel autoclave, under
autogenous pressure at 160 °C for 2 days and then slowly cooled
to room temperature at a rate of 10 °C/h. Colorless needle crystals
of [Cd2(2-mBIM)2(1,3-BDC)2]n suitable for X-ray analysis were ob-
tained in 42% yield. Anal. Calcd for C34H32Cd2N8O8: C 45.10, H 3.56,
Ka radiation, k = 0.71073 Å). An empirical absorption was applied
using SADABS program [32]. The structures were solved by direct
methods using the program SHELXL 97 and refined by full-matrix
least-squares methods on F2 using the SHELXL 97 crystallographic
software package [33]. All of the non-hydrogen atoms were refined
anisotropically. Hydrogen atoms were located from ride mode. The
crystal data and structure refinement of compounds 1 and 2 were
summarized in Table 1. Selected bond lengths and angles of com-
pounds 1 and 2 were listed in Table 2.
N 12.37; Found: C 45.26, H 3.66, N 12.17; IR (KBr, cmꢀ1):
t = 3445s,
3126m, 2992w, 1608s, 1556s, 1499m, 1442w, 1379s, 1262s,
1194w, 1149m, 1138w, 1077m, 999s, 834w, 747s, 673w, 657s,
517w.
3. Results and discussion
3.1. Description of the crystal structures of 1 and 2
2.3. Synthesis of [Cd(BIM)(1,3-BDC)]n (2)
Single crystal X-ray diffraction studies revealed that the asym-
metric unit in 1 contains the basic building block of four indepen-
dent 2-mBIM ligands, four different cadmium centers (Cd1, Cd2,
Cd3 and Cd4), and four 1,3-BDC ligands. The most relevant bond
lengths and angles for compound 1 are given in Table 2. As shown
The mixture of CdCl2ꢁ2.5H2O(114 mg, 0.5 mmol), 1,3-benzen-
edicarboxylic acid (83 mg, 0.5 mmol), NaOH (40 mg, 1.0 mmol)
and ligand BIM (75 mg, 0.5 mmol) were dissolved in the solution
of H2O (10 mL), and the resulting mixture was transferred and
Table 1
Crystallographic data for complexes 1 and 2.
Complex code
1
2
Empirical formula
Formula weight
Temperature (K)
Wavelength (Å)
Crystal system
Space group
A (Å)
C
34H32Cd2N8O8
C15H12CdN4O4
424.69
294(2)
0.71073
Monoclinic
P2(1)/n
9.3613(4)
15.8661(7)
10.1425(4)
90.000(0)
101.646(1)
90.000(0)
1475.43(11)
4
905.48
292(2)
0.71073
Orthorhombic
Pna2(1)
29.1698(11)
10.0178(4)
24.1246(9)
90.000(0)
90.000(0)
90.000(0)
7049.6(5)
8
B (Å)
C (Å)
a
(°)
b (°)
c
(°)
V (Å3)
Z
De (g/cm3)
1.706
1.27
1.912
1.51
l
(mmꢀ1
)
F(000)
3616
840
Crystal size (mm)
Crystal color and habit
h range (°)
0.20 ꢂ 0.12 ꢂ 0.10
Colorless block
2.3–25.3
0.20 ꢂ 0.10 ꢂ 0.10
Colorless needle
2.4–28.0
Index ranges
ꢀ37 6 h 6 37, ꢀ12 6 k 6 12, –30 6 l 6 28
ꢀ12 6 h 6 9, ꢀ20 6 k 6 20, ꢀ12 6 l 6 13
Reflections collected
Independent reflections
Max. and min. transmission
Refinement method
Data/restraints/parameters
Goodness-of-fit on F2
58772
13765
7882 [R(int) = 0.096]
0.8835 and 0.7853
3329 [R(int) = 0.028]
0.8637 and 0.7522
Full-matrix least-squares on F2
7882/1/945
Full-matrix least-squares on F2
3329/0/217
1.03
1.08
Final R indices [I > 2
R indices (all data)
Largest diff. peak and hole
r
(I)]
R1 = 0.0536, wR2 = 0.1171
R1 = 0.0649, wR2 = 0.1230
1.68 and ꢀ0.67 eÅꢀ3
R1 = 0.0312, wR2 = 0.0782
R1 = 0.0358, wR2 = 0.0805
0.64 and ꢀ0.54 eÅꢀ3

Z. Zhang et al. / Journal of Molecular Structure 992 (2011) 111–116
113
Table 2
Selected bond lengths (Å) and angles (°) for complex 1 and 2.
1
Cd(1)AN(5)
Cd(1)AN(3)
2.246(7)
2.257(8)
2.428(9)
2.280(6)
2.363(8)
2.534(8)
2.292(10)
2.339(7)
2.502(9)
2.245(8)
2.257(8)
2.254(7)
2.534(8)
2.256(9)
141.8(3)
100.1(3)
103.3(3)
89.7(3)
135.7(3)
133.3(3)
87.3(3)
141.6(3)
133.2(3)
100.9(3)
110.4(3)
141.8(3)
81.6(3)
Cd(1)AO(1)
Cd(1)AN(1)
Cd(2)AO(8)iii
Cd(2)AN(8)
Cd(2)AO(6)
Cd(3)AO(12)ii
Cd(3)AO(9)
Cd(3)AO(14)
Cd(4)AO(15)
Cd(4)AN(16)v
Cd(4)AO(16)
O(3)ACd(2)i
O(8)ACd(2)ii
2.254(7)
2.283(8)
2.233(8)
2.313(9)
2.415(7)
2.256(9)
2.321(8)
2.430(7)
2.224(7)
2.254(7)
2.478(9)
2.280(6)
2.234(8)
Cd(1)AO(2)
Cd(2)AO(3)iv
Cd(2)AO(5)
Cd(2)AO(4)iv
Cd(3)AN(9)
Cd(3)AO(13)
Cd(3)AO(10)
Cd(4)AN(12)
Cd(4)AN(14)
N(16)ACd(4)vi
O(4)ACd(2)i
O(12)ACd(3)iii
N(5)ACd(1)AO(1)
N(3)ACd(1)AO(1)
N(1)ACd(1)AO(1)
N(5)ACd(1)AO(2)
N(3)ACd(1)AO(2)
O(8)iiiACd(2)AO(3)iv
N(8)ACd(2)AO(3)iv
O(5)ACd(2)AO(3)iv
O(8)iii-Cd(2)AO(6)
N(8)ACd(2)AO(6)
O(8)iiiACd(2)AO(4)iv
N(8)ACd(2)AO(4)iv
O(6)ACd(2)AO(4)iv
O(12)iiACd(3)AO(9)
O(12)iiACd(3)AO(13)
O(13)ACd(3)AO(9)
N(9)ACd(3)AO(14)
O(13)ACd(3)AO(14)
N(9)ACd(3)AO(10)
O(13)ACd(3)AO(10)
O(15)ACd(4)AN(12)
N(12)ACd(4)AN(16)v
N(12)ACd(4)AN(14)
O(15)ACd(4)AO(16)
O(16)ACd(4)AN(16)v
N(5)ACd(1)AN(3)
N(5)ACd(1)AN(1)
N(3)ACd(1)AN(1)
O(2)ACd(1)AO(1)
N(1)ACd(1)AO(2)
O(8)iiiACd(2)AN(8)
O(8)iiiACd(2)AO(5)
N(8)ACd(2)AO(5)
O(6)ACd(2)AO(3)iv
O(5)ACd(2)AO(6)
O(4)ivACd(2)AO(3)iv
O(5)ACd(2)AO(4)iv
O(12)iiACd(3)AN(9)
N(9)ACd(3)AO(9)
N(9)ACd(3)AO(13)
O(12)iiACd(3)AO(14)
O(9)ACd(3)AO(14)
O(12)iiACd(3)AO(10)
O(9)ACd(3)AO(10)
O(14)ACd(3)AO(10)
O(15)ACd(4)AN(16)v
O(15)ACd(4)AN(14)
N(14)ACd(4)AN(16)v
N(12)ACd(4)AO(16)
N(14)ACd(4)AO(16)
94.5(3)
103.6(3)
111.8(3)
55.7(2)
109.9(3)
95.6(3)
79.5(3)
111.9(3)
91.3(3)
53.6(3)
54.5(2)
100.3(3)
96.3(4)
105.5(4)
83.9(3)
108.6(3)
113.0(3)
131.1(3)
53.6(3)
81.3(3)
139.6(3)
98.5(3)
96.2(3)
108.4(3)
136.1(3)
79.6(3)
141.0(3)
138.1(3)
137.0(3)
54.7(3)
108.2(4)
84.5(3)
103.8(3)
105.3(3)
111.6(3)
54.4(3)
Fig. 1. Molecular structure of 1 showing the local coordination environment of the
Cd center with thermal ellipsoids at 30% probability. Hydrogen atoms and other
similar structures (Cd3 and Cd4) were omitted for clarity.
89.8(3)
in Fig. 1, one cadmium atom (Cd1) is coordinated to three nitrogen
atoms of three independent 2-mBIM ligands and two oxygen
atoms of one 1,3-BDC ligand to give a five-coordinated environ-
ment. Another cadmium atom (Cd2) is coordinated to one nitrogen
atom of one independent 2-mBIM ligand and five oxygen atoms of
three independent 1, 3-BDC ligands (one of them in monodentate
and the other two in bidentate mode), forming a six-coordinated
environment. The two cadmium centers have different coordinated
modes, and two neighboring cadmium atoms with the same coor-
dinated mode are bridged by 2-mBIM and 1,3-BDC ligands, respec-
tively, to form two independent 1D zigzag chains along the
direction of the b-axis (Fig. 2). In addition, those 1D chains were
cross-linked by the other bridging bidenate 2-mBIM and 1,3-BDC
ligands alternatively to generate a 2D wavy layer with a 4,4-con-
nected net (Fig. 2). The tetra-coordinated cadmium metal–organic
network can be simplified as a (4,4)-connected net topological
2
Cd(1)AN(1)
2.352(2)
2.237(2)
2.276(2)
2.465(2)
2.309(2)
126.90(9)
84.24(8)
148.35(9)
83.14(8)
90.52(9)
99.88(9)
136.61(8)
95.58(8)
Cd(1)AO(1)
Cd(1)AO(2)
Cd(1)AO(4)iii
Cd(1)ivAO(5)
2.416(2)
2.345(2)
2.465(2)
2.276(2)
Cd(1)AN(4)ii
Cd(1)AO(5)i
Cd(1)iiiAO(4)
Cd(1)vAN(4)
O(5)iACd(1)AN(4)ii
O(5)iACd(1)AO(2)
O(2)ACd(1)AN(4)ii
O(5)iACd(1)AN(1)
N(1)ACd(1)AN(4)ii
O(2)ACd(1)AN(1)
O(5)iACd(1)AO(1)
O(1)ACd(1)AN(4)ii
O(2)ACd(1)AO(1)
N(1)ACd(1)AO(1)
O(5)iACd(1)AO(4)iii
O(4)iiiACd(1)AN(4)ii
O(2)ACd(1)AO(4)iii
N(1)-Cd(1)AO(4)iii
O(1)ACd(1)AO(4)iii
55.28(7)
88.72(8)
100.67(8)
83.30(8)
85.43(8)
173.81(8)
91.75(8)
Symmetry codes: (i) x + 1/2, ꢀy + 3/2, z; (ii) x, y + 1, z; (iii) x, y ꢀ 1, z; (iv) x ꢀ 1/2,
ꢀy + 3/2, z; (v) x + 1/2, ꢀy + 1/2, z; (vi) x ꢀ 1/2, ꢀy + 1/2, z for 1; (i) x, y, z ꢀ 1; (ii)
x + 1, y, z; (iii) ꢀx + 1, ꢀy, ꢀz + 2; (iv) x, y, z + 1; (v) x ꢀ 1, y, z for 2.
Fig. 2. The structure of the 2D wavy layer with a 4,4-connected net in compound 1 (left) and view of the network topology (right).

114
Z. Zhang et al. / Journal of Molecular Structure 992 (2011) 111–116
cis-conformation
trans-conformation
Scheme 1. The two conformations of ligand 2-mBIM in compound 1.
Fig. 3. The 2D twofold parallel interpenetration framework with the (4, 4) connected net topology (left) and schematic representation of the self-penetrating topology (right)
in compound 1.
array with the long and short Schläfli symbols of 4(4).6(2) and 44,
respectively.
through the BIM ligand-coordinated Cd center in the cis-configura-
tion, resulting in formation of a 2D double-layer network (Fig. 6).
The coordinated cadmium metal–organic network can be simpli-
There are two structural characteristics that should be noted.
First, the coordination mode of ligand 2-mBIM in compound 1
adopts two types of conformations (trans- or cis-) as shown in
scheme 1. The dihedral angle between the two imidazole rings in
the same 2-mBIM ligands with the trans- or cis- conformation were
found to be 88.2° and 57.1°, respectively. These findings are com-
parable to previously reported values for corresponding structures
[34]. Second, each 2D (4, 4) layer contains enough empty space to
allow another 2D (4, 4) layer to interpenetrate and form a twofold
parallel interwoven net with the (4, 4) connected topology in the
manner shown schematically in Fig. 3. Here, in complex 1, the flex-
ible structure of 2-mBIM as a V-shaped bidentate nitrogen ligand is
likely necessary for formation of the twofold interpenetration.
When ligand 2-mBIM was replaced by BIM in the reaction, com-
plex 2, which had a different structure, was obtained. Fig. 4 shows
the coordination environment of two independent Cd(II) atoms
with the atom numbering scheme. It was evident that the Cd1
atom was in a distorted octahedral coordination environment with
four oxygen atoms (O1, O2, O4A, and O5B) from three different 1,3-
BDC2ꢀ ligands with Cd1AO1, Cd1AO2, Cd1AO4A, and Cd1AO5B
bond distances of 2.416(2), 2.345(2), 2.465(2), and 2.276(2) Å,
respectively, and two nitrogen atoms (N1 and N4B) from two dif-
ferent BIM ligands with Cd1AN1 and Cd1AN4B bond distances of
2.352(2) and 2.237(2) Å, respectively (Table 2). One 1,3-BDC2ꢀ li-
gand, one BIM ligand, and one Cd center were observed in the
asymmetric unit of 2. One carboxylate group of 1,3-BDC2ꢀ ligand
was coordinated with one Cd center in the bidentate mode,
whereas the other carboxylate group was coordinated with two
Cd centers in the monodentate mode and bridged two adjacent
Cd(II) centers with
a Cd–Cd distance of 4.110 Å to form a
[Cd2(COO)4] subunit that was connected through two bridged
1,3-BDC2ꢀ ligands to make up an infinite [Cd2(1,3-BDC)2]n 1D dou-
ble chain (Fig. 5). Those 1D double chains were further linked
Fig. 4. ORTEP drawing of 2 showing the local coordination environment of Cd(II)
with thermal ellipsoids at 30% probability. Hydrogen atoms were omitted for
clarity.

Z. Zhang et al. / Journal of Molecular Structure 992 (2011) 111–116
115
Fig. 5. A 1D double chain containing the [Cd2(COO)4] subunit in 2.
Fig. 6. A 2D double-layer network in 2 (left) and schematic representation of the network topology (right).
400
389nm
278nm
2000
1500
1000
500
302nm
300
200
100
332nm
250
275
300
325
300
350
400
450
500
Wavelength/nm
Wavelength/nm
Fig. 7. Solid-state emission spectra at room temperature of 1 (left) and 2 (right) upon photoexcitation at 332 nm and 278 nm, respectively.
fied as a (4, 4)-connected net topological array with the long and
short Schläfli symbols of 4(2).6(7).8 and 4.4.6(3).6(3).6(4).
6(4).6(3).6(3), respectively.
and 278 nm, respectively. The main emission peaks are located at
389 nm for 1 and 302 nm for 2. In contrast, the free 2-mBIM and
BIM ligand emits at 396 nm and 338 nm upon the 332 nm and
290 nm, respectively. Generally, the intra-ligand fluorescence
emission wavelength is determined by the energy gap between
3.2. The thermal and photoluminescent properties of 1 and 2
the
related to the extent of
tal complexes, the emission peaks of compound 1 are likely as-
signed to intra-ligand
p⁄ transitions. Which have also been
p
and
p
⁄ molecular orbitals of the free ligand, which is simply
p
conjugation in the system. As for the me-
The thermal analysis (TGA) under nitrogen atmosphere showed
that the complex 1 have the first weight loss of 39.8% from 270 °C
to 410 °C corresponds to the loss of 2-mBIM (calcd: 38.8%). Upon
further heating, an obvious weight loss occurs in the temperature
range of 540 °C to 630 °C, corresponding to the release of organic
anionic ligand and indicating the complete decomposition of 1
with 29.8% residual weight corresponds to CdO (calcd: 28.6%).
Compound 2 have also weight loss of 35.4% occurred in a temper-
ature range from 192 °C to 296 °C, corresponding to the loss of BIM
ligands (calcd: 34.7%). The second-step, which was observed in a
temperature range from 370 °C to 410 °C with a residual weight
30.5%, probably corresponds to the completely decomposition of
complex 2 at 410 °C.
p–
observed in other polymers [35]. And the blue-shifted emissions
of complex 2 compared to free ligand BIM is not only assigned to
the intramolecular
p–
p⁄ and n–p⁄ transitions of the ligands, but
are also attributable to ligand-to-metal charge transfers (LMCT)
[36].
4. Conclusions
In summary, we synthesized and characterized two new cad-
mium coordination polymers using 1,3-benzenedicarboxylic acid
and two different ligands containing N atoms, BIM, and 2-mBIM.
Two interesting different structural motifs were formed in re-
sponse to only a subtle change in the ligand in which 2-mBIM
has a methyl group at the 2-position in the imidazole rings of
The photoluminescent properties of 1 and 2 have been investi-
gated. Emission spectra at room temperature in the solid state are
shown in Fig. 7. It can be observed that compounds 1 and 2 both
exhibit weak photoluminescence upon photoexcitation at 332 nm

116
Z. Zhang et al. / Journal of Molecular Structure 992 (2011) 111–116
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Crystallographic data for the structural analysis have been
deposited with the Cambridge Crystallographic Data Center, CCDC
reference numbers 689345 and 799782. These data can be ob-
tained free of charge at www.ccdc.cam.ac.uk/conts/retrieving.html
(or from the Cambridge Crystallographic Data Center, 12, Union
Road, Cambridge CB21EZ, UK; fax:+44 1223/336 033; e-mail:
deposit@ccdc.cam.ac.uk).
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This work was supported by the Natural Science Foundation of
Hubei Province (2009CDB349), the key research foundations of
HBDE (Z201022001 and CXY2009B028) and the Science and Tech-
nology Foundation for Creative Research Group of HBNU (2009).
[b] Y.Z. Zheng, W. Xue, M.L. Tong, X.M. Chen, S.L. Zheng, Inorg. Chem. 47 (2008)
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