Cadmium(II) and zinc(II) coordination polymers with mixed building blocks of benzenedicarboxyl and 2,5-bipyridyl-1,3,4-oxadiazole: Syntheses, crystal structures, and properties

Article abstract of DOI:10.1016/j.ica.2011.08.069

Three Cd(II) and Zn(II) coordination polymers, including {[Cd(3-bpo)(mip)(H₂O)](H₂O)₂}_n (1), {[Cd(4-bpo)(hip)(H₂O)](H₂O)₄}_n (2), and {[Zn(4-bpo)(tp)](CH₃OH)}_n (3) were synthesized from the reactions of Cd^II or Zn^II nitrate with mixed organic ligands [3-bpo = 2,5-bis(3-pyridyl)-1,3,4-oxadiazole, H₂mip = 5-methylisophthalic acid, 4-bpo = 2,5-bis(4-pyridyl)-1,3,4-oxadiazole, H ₂hip = 5-hydroxylisophthalic acid, H₂tp = terephthalic acid] under the similar layered diffusion condition. The resulting crystalline materials 1-3 were characterized by IR, microanalysis, powder X-ray diffraction (PXRD) techniques. Single-crystal X-ray diffraction indicates a 1-D tubular motif for 1, a 1-D dual-track array for 2, and a 2-D grid-like pattern for 3, constructed via different metal-ligand coordination contacts. Higher-dimensional supramolecular architectures are further assembled in 1-3 via H-bonding and aromatic stacking interactions. In addition, thermal stability and fluorescence of these polymeric complexes were also investigated and discussed.

Full text of DOI:10.1016/j.ica.2011.08.069

Inorganica Chimica Acta 378 (2011) 206–212
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Inorganica Chimica Acta
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Cadmium(II) and zinc(II) coordination polymers with mixed building blocks
of benzenedicarboxyl and 2,5-bipyridyl-1,3,4-oxadiazole: Syntheses, crystal
structures, and properties
⇑
Jing Chen, Shang-Yuan Liu, Cheng-Peng Li
College of Chemistry, Tianjin Key Laboratory of Structure and Performance for Functional Molecule, Tianjin Normal University, Tianjin 300387, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 5 January 2011
Received in revised form 10 August 2011
Accepted 31 August 2011
Available online 8 September 2011
Three Cd(II) and Zn(II) coordination polymers, including {[Cd(3-bpo)(mip)(H2O)](H2O)2}n (1),
{[Cd(4-bpo)(hip)(H2O)](H2O)4}n (2), and {[Zn(4-bpo)(tp)](CH3OH)}n (3) were synthesized from the reac-
tions of CdII or ZnII nitrate with mixed organic ligands [3-bpo = 2,5-bis(3-pyridyl)-1,3,4-oxadiazole,
H2mip = 5-methylisophthalic acid, 4-bpo = 2,5-bis(4-pyridyl)-1,3,4-oxadiazole, H2hip = 5-hydroxylisoph-
thalic acid, H2tp = terephthalic acid] under the similar layered diffusion condition. The resulting crystal-
line materials 1–3 were characterized by IR, microanalysis, powder X-ray diffraction (PXRD) techniques.
Single-crystal X-ray diffraction indicates a 1-D tubular motif for 1, a 1-D dual-track array for 2, and a 2-D
grid-like pattern for 3, constructed via different metal–ligand coordination contacts. Higher-dimensional
supramolecular architectures are further assembled in 1–3 via H-bonding and aromatic stacking interac-
tions. In addition, thermal stability and fluorescence of these polymeric complexes were also investigated
and discussed.
Keywords:
Coordination polymer
Crystal structure
Benzenedicarboxylic acid
2,5-Bipyridyl-1,3,4-oxadiazole
Secondary interaction
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
types, have been generally regarded as the most familiar and
reliable candidates to construct the desired coordination architec-
The design and preparation of coordination supramolecular
chemistry has become an attractive research field in recent years,
producing a large number of coordination complexes which are
generally assembled from fine-tuning molecular building tectons
with unambiguous intermolecular interactions [1–14]. In
particular, their ease of structural regulation and potential physico-
chemical properties, such as guest absorption, optics, ion exchange,
chirality, catalysis, magnetism, etc., provide good explanations to
structure–function relationship thereof [15–26]. Construction of
such supramolecular systems can be achieved by the combination
of metal ions and organic ligands in a well-rounded manner
[27–31].
tures in recent years [33–35]. In this direction, as a continuation
of our research, we selected the modified bent dipyridyl ligands,
namely, 2,5-bis(3-pyridyl)-1,3,4-oxadiazole (3-bpo) and its 4-pyri-
dyl analog (4-bpo), as well as different benzenedicarboxylic acids
[5-methylisophthalic acid (H2mip), 5-hydroxylisophthalic acid
(H2hip), and terephthalic acid (H2tp)] as building tectons, to assem-
ble with Cd(II) or Zn(II) nitrates under ambient conditions. First, the
paired analog bpo ligands can be viewed as not only the extension
of traditionally employed 4,40-bipyridine ligand, but also the mod-
ification types with longer and bent spacers, flexible conformations,
as well as various coordination modes [36]. Second, these benzen-
edicarboxylic isomers contain two carboxyl groups at distinct posi-
tions, which may engender significant spatial effect and thus
influence the structural assembly with multiform coordination
fashions. Third, the cooperation of bpo and benzenedicarboxylic
counterparts can produce directional conformation of network
structures via coordination bonds and also noncovalent cooperative
forces, such as hydrogen bonding and/or aromatic stacking. In this
context, three novel coordination polymers, {[Cd(3-bpo)(mi-
p)(H2O)](H2O)2}n (1), {[Cd(4-bpo)(hip)(H2O)](H2O)4}n (2), and
{[Zn(4-bpo)(tp)](CH3OH)}n (3) are prepared, which display differ-
ent 1-D and 2-D coordination motifs and supramolecular networks
via secondary interactions. In addition, their thermal stability and
fluorescence were also investigated.
As for the d10 metal elements, especially for Cd and Zn, they pre-
fer the +2 oxidation state in most of their complexes, and com-
monly possess the coordination numbers from 4 to 7 and the
corresponding geometries, which can be utilized to prepare multi-
form metallosupramolecular complexes [32]. On the other hand, to
meet the requirement of metal–ligand binding preference and the
energetic consideration of overall crystal packing, mixed-ligand
assembling strategy may be the most effective approach. A variety
of organic ligands, especially for polycarboxylate and polypyridyl
⇑
Corresponding author.
E-mail address: tjnulicp@gmail.com (C.-P. Li).
0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2011.08.069

J. Chen et al. / Inorganica Chimica Acta 378 (2011) 206–212
207
2.2.3. {[Zn(4-bpo)(tp)](CH3OH)}n (3)
2. Experimental
A mixture of 4-bpo (11.2 mg, 0.05 mmol) and H2tp (8.3 mg,
0.05 mmol) in CH3OH (5 mL) was layered onto a water solution
(5 mL) of Zn(NO3)2ꢀ6H2O (29.7 mg, 0.1 mmol). Yellow block single
crystals of 3 were observed on the wall of test tube after ca. one
week and collected in a yield of 71% (17.2 mg, based on 4-bpo).
Anal. Calc. for C21H16ZnN4O6 (3): C, 51.92; H, 3.32; N, 11.53. Found:
C, 52.04; H, 3.29; N, 11.65%. IR (cmꢁ1): 3242b, 1692w, 1617m,
1588s, 1541vs, 1493s, 1422s, 1385vs, 1286s, 1058w, 1011s, 840s,
787s, 736s, 536w, 497w.
2.1. Materials and general methods
With the exception of the ligands 3-bpo and 4-bpo, which were
synthesized according to the reported procedures [37], all reagents
and solvents for synthesis and analysis were commercially
available and used as received. Fourier transform (FT) IR spectra
(KBr pellets) were taken on an AVATAR-370 (Nicolet) spectrome-
ter. Elemental analyses of C, H, and N were performed on a
CE-440 (Leemanlabs) analyzer. Thermogravimetric analysis (TGA)
curves were carried out on a NETZSCH TG209 (Siemens) thermal
analyzer in the range of 25–600 °C at a heating rate of 10 °C/min
under N2 atmosphere. Powder X-ray diffraction (PXRD) patterns
were recorded on a Rigaku D/Max-2500 diffractometer at 40 kV
and 100 mA for a Cu-target tube (k = 1.5406 Å). The calculated
PXRD patterns were obtained from the single-crystal diffraction
data using the PLATON software. Fluorescence spectra of the solid
samples were measured on a Cary Eclipse spectrofluorimeter
(Varian) at room temperature.
2.3. X-ray single crystal diffraction
Single-crystal X-ray diffraction data for complexes 1–3 were
collected on a Bruker Apex II CCD diffractometer at room temper-
ature with Mo Ka radiation (k = 0.71073 Å). There was no evidence
of crystal decay during data collection. Generally, a semi-empirical
absorption correction (SADABS) was applied and the program SAINT
was used for integration of the diffraction profiles [38]. The struc-
tures were solved by direct methods using the SHELXS program of
the SHELXTL package and refined with SHELXL [39]. The final refine-
ments were performed by full-matrix least-squares methods on
F2 with anisotropic thermal parameters for all non-H atoms.
Hydrogen atoms attached to carbon were generated geometrically,
and those of hydroxyl of hip, and lattice methanol or water were
first located in difference Fourier syntheses and then treated as rid-
ing. Isotropic displacement parameters of H were derived from
their parent atoms. As for 1, both lattice aqua molecules of O8
and O9 are disordered over two sites, with each part possessing
the quarter site-occupancy. All H atoms of the lattice water mole-
cules cannot be located. For 2, O10 was treated using a disordered
model over two sites, with the partial site-occupancies of 0.20/
0.80, the H atoms of which were also not located. A summary of
the crystallographic data of 1–3 are listed in Table 1. Selected bond
parameters and H-bonding geometries are shown in Tables S1 and
S2 (in Supplementary material).
2.2. Synthesis of complexes 1–3
2.2.1. {[Cd(3-bpo)(mip)(H2O)](H2O)2}n (1)
A mixture containing 3-bpo (11.2 mg, 0.05 mmol) and H2mip
(9.0 mg, 0.05 mmol) in CH3OH (5 mL) was layered onto an aqueous
solution (5 mL) of Cd(NO3)2ꢀ8H2O (30.8 mg, 0.1 mmol). Colorless
block single crystals of 1 were found on the wall of test tube after
ca. a month and collected in a yield of 56% (15.8 mg, based on 3-
bpo). Anal. Calc. for C21H20CdN4O8 (1): C, 44.34; H, 3.54; N, 9.85.
Found: C, 44.55; H, 3.69; N, 10.03%. IR (cmꢁ1): 3447b, 1605s,
1549w, 1428s, 1331m, 1261vs, 1116b, 1031vs, 960w, 823s,
731m, 689m, 634m, 459m.
2.2.2. {[Cd(4-bpo)(hip)(H2O)](H2O)4}n (2)
A mixed solution of 4-bpo (11.2 mg, 0.05 mmol) and H2hip
(9.1 mg, 0.05 mmol) in CH3OH (5 mL) was layered onto an aqueous
solution (5 mL) of Cd(NO3)2ꢀ8H2O (30.8 mg, 0.1 mmol). Colorless
block single crystals of 2 were observed on the wall of test tube
after ca. 1 week and collected in 55% yield (16.8 mg, based on 4-
bpo). Anal. Calc. for C20H22CdN4O11 (2): C, 39.59; H, 3.65; N, 9.23.
Found: C, 39.86; H, 3.89; N, 9.18%. IR (cmꢁ1): 3434b, 1618w,
1562vs, 1386vs, 1275m, 1219s, 1110w, 1002s, 785s, 729s, 585m.
3. Results and discussion
3.1. Synthesis and general characterization
In the syntheses of complexes 1–3, a layer-separation diffusion
approach in water-methanol readily facilitates the slow growth of
Table 1
Crystallographic data and structural refinement summary for complexes 1–3.
Compound reference
1
2
3
Chemical formula
Formula mass
Crystal system
a (Å)
b (Å)
c (Å)
C
21H20CdN4O8
C20H22CdN4O11
606.82
triclinic
C21H16ZnN4O6
485.77
568.81
monoclinic
9.820(2)
18.193(4)
25.679(6)
90.00
90.132(4)
90.00
4587.7(17)
296(2)
C2/c
monoclinic
9.5268(10)
15.3121(16)
15.5250(12)
90.00
117.816(5)
90.00
2003.0(3)
296(2)
7.7332(15)
9.7404(18)
17.238(3)
91.833(3)
97.011(3)
109.782(3)
1208.9(4)
296(2)
a
(°)
b (°)
(°)
c
Unit cell volume (Å3)
Temperature (K)
ꢀ
Space group
P21/c
P1
Number of formula units per unit cell (Z)
8
2
4
Absorption coefficient (
l
/mmꢁ1
)
1.007
11633
4065
0.0173
0.0257
0.0726
0.0299
0.0748
1.082
0.969
6250
4249
0.0111
0.0245
0.0650
0.0271
0.0668
1.055
1.275
10197
3546
0.0180
0.0270
0.0707
0.0338
0.0752
Number of reflections measured
Number of independent reflections
Rint
Final R1 values (I > 2
r
(I))
Final wR (F2) values (I > 2
Final R1 values (all data)
r(I))
Final wR (F2) values (all data)
Goodness of fit (GOF) on F2
1.037

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J. Chen et al. / Inorganica Chimica Acta 378 (2011) 206–212
well-shaped single crystals suitable for X-ray single-crystal diffrac-
tion. Instead of this, only a mass of microcrystalline solids can be
obtained from the direct metal–ligand assemblies in the homoge-
neous solutions. Although the metal/dicarboxyl/bpo ratio of 2:1:1
was used for the starting materials in each assembling case, the
resulting complexes all possess the equivalent metal/dicarboxyl/
bpo ratio, as confirmed by elemental analysis and single crystal
X-ray diffraction. As for the IR spectra of 1–3, the broad bands cen-
tered at ca. 3400 cmꢁ1 reveal the O–H characteristic stretching
vibrations of water, hydroxyl, or methanol. No obvious adsorption
peak at ca. 1700 cmꢁ1 is observed, indicating a complete deproto-
nation of the dicarboxyl ligands in all complexes. Correspondingly,
the peaks at 1562–1605 and 1385–1428 cmꢁ1 can be properly as-
signed to the antisymmetric and symmetric stretching vibrations
of the deprotonated carboxylate groups. The phase purity of the
bulk crystalline sample was also confirmed by PXRD technique
for each complex, showing a good agreement of the experimental
and calculated patterns (see Fig. S1 in Supplementary material).
are observed in this case, with the separation of Cdꢀ ꢀ ꢀCd thereof
being 4.070(1) Å. A side view from the [010] direction shows that
the terminal pyridyl rings of extended 4-bpo pendants engender
the H-bonding interactions with the hydroxyl groups of hip
(O6–H6ꢀ ꢀ ꢀN4), resulting in a 2-D H-bonding layer (see Fig. 2c).
Furthermore, such 2-D patterns are linked by O7–H7Aꢀ ꢀ ꢀO3 bonds
between the aqua ligands and carboxylates of hip to form a 3-D
supramolecular network (see Fig. 2d). In addition, there also exist
O–Hꢀ ꢀ ꢀO and O–Hꢀ ꢀ ꢀN bonds between the lattice water molecules
and the 3-D network (see Table S2 for details), as well as multiple
aromatic stacking contacts between the aromatic planes of 4-bpo
and hip ligands.
3.2.3. {[Zn(4-bpo)(tp)](CH3OH)}n (3)
The 2-D layered network of 3 is composed of the familiar pad-
dle-wheel [Zn2(COO)4] building units, in which the ZnII ion has a
distorted square–pyramidal coordination sphere, being completed
by four carboxylate O atoms from four distinct tp and one pyridyl
N donor (see Fig. 3a). Within this bimetallic unit, the Znꢀ ꢀ ꢀZn dis-
3.2. Structural analysis of 1–3
tance is 2.921(1) Å and each carboxylate group in tp adopts the l-
3.2.1. {[Cd(3-bpo)(mip)(H2O)](H2O)2}n (1)
O,O0 bridging mode. Along the bc plane, the dimeric subunits are
extended by the tp ligands to form an infinite 2-D layered net
with the dimension of 10.903(1) ꢂ 10.903(1) Å2 (see Fig. 3b).
Moreover, the lattice methanol molecules are attached to the 2-
D layer via O6–H6Aꢀ ꢀ ꢀN4 bonds (see Table S2 for details). Notably,
the 2-D motif is decorated with the unidentate 4-bpo ligands as
side arms at both sides. Of further interest, these long prominent
4-bpo arms, with a length of 9.947(3) Å between the two pyridyl
N donors, penetrate into the void grids of the adjacent 2-D layers
to form a novel 3-D polythreading supramolecular architecture
with finite components (see Fig. 3c) [40]. The adjacent layers take
a parallel stacking, in which each mesh of a layer is occupied
oppositely by two 4-bpo lateral pendants coming from different
Single-crystal X-ray diffraction indicates that complex 1 shows
a 1-D polymeric tubular motif. The asymmetric unit of 1 is com-
posed of one Cd(II) ion, one mip dianion, one 3-bpo, as well as
one coordinated and two lattice water molecules. As depicted in
Fig. 1a, the coordination sphere of each Cd(II) center can be por-
trayed as a distorted pentagonal–bipyramid, in which the metal
ion is coordinated by five O atoms from two mip and one water li-
gands in the equatorial plane, and paired trans-pyridyl N donors of
3-bpo located at the axial positions. The carboxylates of mip both
take the chelating coordination fashion. In this 1-D polymeric mo-
tif, two adjacent Cd(II) centers (Cdꢀ ꢀ ꢀCd = 8.223(1) Å) are doubly
bridged by a pair of 3-bpo ligands to feature a bimetallic macrocy-
clic unit. The dimeric units are further interconnected by paired
mip ligands to produce a tubular array (see Fig. 1b), in which the
lattice water molecules of O9 are located and interact with the
carboxylates of mip (O9ꢀ ꢀ ꢀO4 = 2.553(1) Å). In addition, the coordi-
nated water molecules of O6 are involved in the intrachain
O6–H6Aꢀ ꢀ ꢀO3 and interchain O6–H6Aꢀ ꢀ ꢀN3 hydrogen bonding
(see Table S2 for details), which extend the 1-D arrays into a 2-D
supramolecular layered framework (see Fig. 1c). Moreover, the rich
aromatic 3-bpo systems facilitate multiple aromatic stacking
contacts between the oxadiazole and pyridyl rings, with the
centroid-to-centroid distances and the dihedral angles between
the corresponding groups of 3.556–3.752 Å and 2.0–5.6°. Further
investigation of the crystal packing indicates that these parallel
H-bonding 2-D layers are arranged in an offset mode, without
any significant secondary interactions between them.
layers, with the presence of
their pyridyl rings (centroid-to-centroid distance: 3.658 Å and
p p stacking interactions between
ꢀ ꢀ ꢀ
dihedral angle: 7.3°). In addition, the phenyl groups of tp from
the in-between layer are also involved in
p p stacking interac-
ꢀ ꢀ ꢀ
tions with the oxadiazole rings of 4-bpo from the adjacent layers,
with the centroid-to-centroid distance and the dihedral angle of
3.613 Å and 3.6°, which further stabilize the 3-D supramolecular
network.
3.3. Structural diversity of coordination polymers 1–3
Complexes 1–3 were obtained under similar synthetic condi-
tions and their structural differences can be exclusively attributed
to different metal ions and mixed-ligands used in the assembled
processes. A comparison of these structures reveals that the CdII
and ZnII ions tend to adopt the pentagonal–bipyramidal and
square–pyramidal coordination geometries, respectively. In 1–3,
the 3-bpo ligand exhibits the bridging function, whereas 4-bpo be-
haves as terminal coordination, constructing the resulting 1-D and
2-D coordination patterns. As for 1, in favor of the bridging role of
3-bpo and mip ‘‘struts’’, a 1-D tubular motif can be achieved and
rich aromatic packing interactions are found between the 3-bpo
and mip ligands. In 2, the polymeric 1-D dual-track array is consti-
tuted by the hip ‘‘struts’’ and 4-bpo side arms, and O–Hꢀ ꢀ ꢀN bonds
between the 4-bpo and hip from different chains further extend
the resultant supramolecular structure. With regard to 3, the tp
ligands extend the paddle-wheel dinuclear units along two
directions to result in a 2-D grid-like framework with the 4-bpo
as pendent arms. Moreover, the rich pyridyl and phenyl rings in
the mixed-ligand system facilitate the formation of aromatic
stacking interactions to stabilize the 3-D supramolecular network.
3.2.2. {[Cd(4-bpo)(hip)(H2O)](H2O)4}n (2)
Complex 2 displays a polymeric dual-track array that is com-
pletely different from that of 1. In this structure, the asymmetric
unit consists of one CdII, one 4-bpo, one hip, as well as one coordi-
nated and four lattice water molecules. Each CdII ion is surrounded
by four carboxylate O atoms from a pair of hip and one pyridyl N
donor to constitute the equatorial plane, as well as two water li-
gands in the axial positions (see Fig. 2a). Similar to that of 1, the
local coordination geometry of CdII can also be described as a dis-
torted pentagonal–bipyramid, although they have different coordi-
nation donors (CdN2O5 for 1 and CdNO6 for 2). For each hip ligand,
the two carboxylate groups take the chelating and
bridging coordination modes. As a result, the adjacent CdII centers
are connected by the 3-hip linkers to generate a 1-D dual-track ar-
ray, with the unidentate 4-bpo ligands outspread at both sides (see
Fig. 2b). Interestingly, the familiar dimeric [Cd2(
2-COOꢁ)2] units
l-O,O-g
-O,O0
l
l

J. Chen et al. / Inorganica Chimica Acta 378 (2011) 206–212
209
Fig. 1. Crystal structure of 1. (a) A local view showing the coordination geometry of CdII (symmetry codes: A = ꢁx + 1, ꢁy, ꢁz + 1, B = x ꢁ 1/2, y + 1/2, z). (b) Views of the 1-D
tubular coordination motif with the accommodation of lattice water molecules (shown as red balls). (c) 2-D H-bonding layer constructed via interchain O–Hꢀ ꢀ ꢀO interactions
(shown as red dashed lines). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
3.4. Thermal stability of complexes 1–3
22.58%). In the TGA curve of 2, the first weight loss of 11.39% in the
range of 35–80 °C reveals the expulsion of four lattice water mol-
ecules (calculated: 11.86%). Then, the residual framework keeps
stable until 370 °C with two consecutive weight losses that stop
at 520 °C. Further heating to 600 °C indicates no weight loss and
the remaining solid (20.79% of the total sample) appears to be
CdO (calculated: 21.09%). With regard to 3, the first weight loss
of 6.39% in the region of 100–146 °C can be ascribed to the release
of lattice methanol molecule (calculated: 6.59%). After that, the
residue suffers a series of consecutive weight losses, which end
The thermal stability of 1–3 was studied by TGA experiments
(see Fig. S2 for TGA curves). Complex 1 is thermally stable until
heating to ca. 220 °C, upon which a very sharp weight loss is ob-
served and ends at ca. 360 °C. The loss of lattice water moiety is
not observed, which may be concomitant with the collapse of the
coordination framework [27,33]. Further heating to 600 °C indi-
cates no obvious weight loss, and the final solid holds a weight
of 23.31% of the total sample, which seems to be CdO (calculated:

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J. Chen et al. / Inorganica Chimica Acta 378 (2011) 206–212
Fig. 2. Crystal structure of 2. (a) A local view showing the coordination geometry of CdII (symmetry codes: A = x + 1, y + 1, z, B = ꢁx + 1, ꢁy, ꢁz + 1). (b) Views of the 1-D dual-
track coordination array. (c) 2-D hydrogen-bonding layer via interchain O–Hꢀ ꢀ ꢀN interactions (shown as red dashed lines). (d) 3-D packing diagram highlighting the interlayer
H-bonding interactions (shown as red broken lines). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
at 431 °C. No obvious decomposition is observed until heating to
600 °C and the residue holds a weight of 16.28%, corresponding
to that of ZnO (calculated: 16.68%). All the residual of metallic oxi-
des were confirmed by PXRD (see Fig. S3).

J. Chen et al. / Inorganica Chimica Acta 378 (2011) 206–212
211
Fig. 3. Crystal structure of 3. (a) A local view showing the coordination geometry of ZnII. (b) Views of the 2-D coordination layer decorated with the terminal 4-bpo arms
outside. (c) 3-D packing diagram highlighting the aromatic stacking interactions (shown as green broken lines). (For interpretation of the references to color in this figure
legend, the reader is referred to the web version of this article.)
3.5. Fluorescence of complexes 1–3
nent of photoluminescent, electroluminescent and non-linear opti-
cal materials [41]. Thus, solid-state fluorescent properties of
complexes 1–3 were studied at room temperature to explore their
potential for hybrid luminescent materials. Excitation of the CdII
samples 1 and 2 at 350 nm leads to the maximum fluorescent
Notably, 1,3,4-oxadiazole system shows good electron affinity
and electron-transmission ability for improvement of luminescent
efficiency, which has been widely applied as the essential compo-

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J. Chen et al. / Inorganica Chimica Acta 378 (2011) 206–212
Appendix A. Supplementary material
1
CCDC 805649–805651 contain the supplementary crystallo-
graphic data for complexes 1–3, respectively. These data can be ob-
tained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif. Supplementary
data associated with this article can be found, in the online version,
at doi:10.1016/j.ica.2011.08.069.
2
3-bpo
4-bpo
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350
400
450
Wavelehgth (nm)
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Fig. 4. Solid state fluorescent emissions of complexes 1 and 2, as well as 3-bpo and
4-bpo ligands.
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emission bands at 425 and 378 nm, respectively (see Fig. 4).
However, no obvious emission is found for the ZnII complex 3 un-
der this excitation, which is probably due to the quenching effect of
high-energy C–H and/or O–H oscillators from the CH3OH solvent
[42]. In addition, the maximal emissions of 3-bpo and 4-bpo li-
gands are observed at 393 and 417 nm (kex = 345 and 370 nm).
However for H2mip or H2hip ligand, no obvious emission peak is
found under the similar conditions. Therefore, the emission peaks
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of CdII complexes should be ascribed to the intraligand
and/or n ? p⁄ transitions with either 3-bpo or 4-bpo involved. Fur-
thermore, the significant red-shift for 1 ( k = 32 nm) and blue-
shift for 2 ( k = 39 nm) as well as the differences in luminescent
p ?
p⁄
D
D
intensity, compared with that of the corresponding free 3-bpo or
4-bpo ligand may be caused by the incorporation of metal–ligand
coordination interactions. The fluorescent emission-shift phenom-
ena are prevalently observed in the coordination complexes with
3-/4-bpo and benzenedicarboxylic acids [43–45].
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4. Conclusion
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This work presents three new CdII and ZnII coordination polymers
based on bent dipyridyl building blocks 3-/4-bpo and different ben-
zenedicarboxylic acids under similar synthetic conditions. Appar-
ently, their different network structures are largely dependent on
the choice of metal ions and organic tectons, affording two 1-D CdII
(tubular or dual-track motif) and one 2-D ZnII (grid-like layer) coor-
dination patterns, in which the 3-bpo and 4-bpo tectons exhibits the
bidentate and unidentate coordination fashions, respectively, and
all dicarboxylates act as the bridges to connect the metal centers.
Additionally, diverse higher-dimensional supramolecular lattices
are furnished via secondary interactions (H-bonding and aromatic
stacking), which are also critical to stabilize the overall structures.
Further efforts on such network-based metallosupramolecular
assemblies with mixed ligands are underway.
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Acknowledgment
This work was financially supported by Tianjin Normal
University (No. 52X09004).