Crystal structure and temperature-dependent fluorescent property of a 2D cadmium (II) complex based on 3,6-dibromobenzene-1,2,4,5-tetracarboxylic acid

Article abstract of DOI:10.1016/j.molstruc.2013.01.027

A new cadmium (II) organic coordination polymers [Cd(dbtec) _0.5(H₂O)₃]·H₂O (1), has been constructed based on 3,6-dibromobenzene-1,2,4,5-tetracarboxylic acid (H ₄dbtec), and characterized by elemental

Full text of DOI:10.1016/j.molstruc.2013.01.027

Journal of Molecular Structure 1038 (2013) 73–77
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Crystal structure and temperature-dependent fluorescent property of a 2D
cadmium (II) complex based on 3,6-dibromobenzene-1,2,4,5-tetracarboxylic acid
⇑
⇑
Liang-Liang Zhang, Yu Guo, Yan-Hui Wei, Jie Guo, Xing-Po Wang , Dao-Feng Sun
Key Lab of Colloid and Interface Chemistry, Ministry of Education, School of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, PR China
h i g h l i g h t s
"
"
"
A new cadmium (II) coordination polymer was synthesized and characterized.
Hydrogen bond and BrÁ Á ÁO halogen bond interactions were observed in 1.
The temperature-dependent fluorescent property of 1 was investigated.
a r t i c l e i n f o
a b s t r a c t
Article history:
A new cadmium (II) organic coordination polymers [Cd(dbtec)0.5(H O) ]ÁH O (1), has been constructed
2
3
2
Received 16 September 2012
Received in revised form 11 January 2013
Accepted 11 January 2013
4
based on 3,6-dibromobenzene-1,2,4,5-tetracarboxylic acid (H dbtec), and characterized by elemental
analysis (EA), infrared spectroscopy (IR), powder X-ray diffraction (PXRD), and single crystal X-ray dif-
1
1
2
2
II
fraction. In 1,
4
l
2
-g
:
g
and
4
l -g :g dbtec ligands link four hepta-coordinated Cd ions to form a 2D
Available online 31 January 2013
4
topological layer structure, which is further connected into an interesting 3D network by hydrogen
bond and BrÁ Á ÁO halogen bond. Moreover, the thermal stabilities, solid ultraviolet spectroscopy and tem-
perature-dependent fluorescent properties of 1 were investigated.
Ó 2013 Elsevier B.V. All rights reserved.
Keywords:
Cadmium (II) complex
Hydrogen and halogen bond
Temperature-dependent fluorescent
1
. Introduction
sites and modes, which may facilitate the formation of novel topol-
4
ogies; (ii) the multidentate ligand H dbtec has two bromine atoms,
With the development of supermolecular chemistry and crystal
which could form halogen bond, directing the resulting supramo-
lecular structure. At the same time, the luminescent properties of
metal-organic frameworks (MOFs) have been intensively studied
[12]. Considering these in mind, we reported a new coordination
engineering, the rational design and synthesis of metal-organic
coordination polymers has attracted much attention, not only be-
cause of their intriguing variety of architectures and topologies
but also owing to their potential applications in catalysis, gas
absorption, ion exchange, luminescent and so on [1–7]. The delib-
erate predict and control of the topology of structure is impossible,
but logical choice of the organic ligands and the central metal
atoms can produce different types of coordination frameworks
with novel topologies. Recently, poly-carboxylate ligands are good
candidates for the assembly of versatile structures.
polymer based on H
4
dbtec and Cd(NO
3
)
2
Á4H
2
O, which featured a
4
two-dimensional 4 layer that was assembled into a three-dimen-
sional framework directed by hydrogen bond and BrÁ Á ÁO halogen
bond. Moreover, the temperature-dependent fluorescent property
of 1 was also investigated.
2
. Experimental procedure
According to our survey of the Cambridge Structural Database
(
CSD), a great number of crystal structures based on 1,2,4,5-
benzenetetracarboxylic acid (H btec) have been reported by other’s
group [8–11], but there is no coordination polymers based on 3,6-
dibromobenzene-1,2,4,5-tetracarboxylic acid (H dbtec) (Scheme
). The ligand was chosen for the remarkable advantages: (i) the li-
gand has four carboxyl groups, providing abundant coordination
2
.1. Materials and methods
4
All the reagents and solvents employed were commercially avail-
4
able and used as received without further purification except H4-
dbtec. Infrared spectra were recorded on a Bruker VERTEX-70
spectrometer as KBr pellets in the frequency range 4000–400 cm
1
À1
.
The elemental analyses (C, H contents) were determined on a CE
instruments EA 1110 analyzer. Photoluminescence measurements
were performed on a Hitachi F-7000 fluorescence spectrophotometer
⇑
Corresponding authors. Fax: +86 531 88364218.
E-mail address: dfsun@sdu.edu.cn (D.-F. Sun).
0
022-2860/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molstruc.2013.01.027

7
4
L.-L. Zhang et al. / Journal of Molecular Structure 1038 (2013) 73–77
Cell parameters were retrieved using SMART software and refined
with SAINT on all observed reflections. Data reduction was per-
formed with the SAINT software and corrected for Lorentz and
polarization effects. Absorption corrections were made using the
SADABS program [14]. The structures were solved using direct
methods and successive Fourier difference synthesis (SHELXS-97)
[
15] and they were refined using the full-matrix least-squares
4
Scheme 1. The reaction pathway of H dbtec.
2
method on F with anisotropic thermal parameters for all non-
hydrogen atoms (SHELXL-97) [16]. Atoms were located from itera-
tive examination of difference F-maps following least squares
refinements of the earlier models. Hydrogen atoms were placed
in calculated positions and included as riding atoms with isotropic
displacement parameters 1.2–1.5 times Ueq of the attached C
atoms. Structure was examined using the Addsym subroutine of
PLATON [17] to assure that no additional symmetry could be ap-
plied to the models. For 1, crystal data and collection and parame-
ters are summarized in Table 1, and selected bond lengths and
angles are given in Table 2.
with solid powder on a 1 cm quartz round plate with a dewar flask
apparatus. TG curves were measured from 30 to 600 °C on a SDT
Q600 instrument at a heating rate 10 °C/min under the N atmo-
2
sphere (100 mL/min). Solid-state UV–vis diffuse reflectance spectra
was obtained at room temperature using agilent Cary 100 spectro-
4
photometer, and BaSO was used as a 100% reflectance standard for
all materials.
2.2. Synthesis of H
4
dbtec ligand
The reaction pathway is shown in Scheme 1. H
4
dbtec ligand was
3
. Results and discussion
prepared according literature [13] and with a little modification. To
a four-necked flask equipped with a reflux condenser and a
mechanical stirrer, 3,6-bromo-tetramethylbenzene (0.171 mol),
3.1. Syntheses and IR
2
.1 L pyridine and 280 mL water were added. The reaction mixture
was heated to 100 °C while stirring. KMnO (0.859 mol) was added
in small portions and the mixture was refluxed for 5 h. The warm
solution was separated from MnO by filtration and the solvent
H
4
dbtec ligand was synthesized by oxidation reaction of
,6-bromo-tetramethylbenzene with KMnO as an oxidant. The
dbtec ligand has been detected by H and C NMR
Fig. S1). Crystallization of the complex 1 was carried out in liquid
4
3
4
1
13
purity of H
4
2
(
evaporated under reduced pressure. 280 mL water and 112 g NaOH
were added to the residual solid. After the combined mixture was
heated up to 100 °C, KMnO
portions and the mixture was refluxed for 5 h. The excess KMnO
was destroyed by the cautious addition of 140 mL ethanol. The
MnO was removed from the hot mixture by filtration. The filtrate
was acidified with aqueous HCl (5 M, 600 mL). After the solvent
was revaporated, the residue was washed with acetone. The resul-
tant solid was dried. The yield was 46 g (66%). H NMR (300 MHz;
diffusion method at room temperature. As shown in Fig. S2, the ob-
served broad peak at 3500 cm in the FT-IR spectrum is attributed
to the OAH stretching vibration of the coordinated water mole-
s
cules and the characteristic asymmetric (mas) and symmetric (m )
absorptions of the carboxylate groups appear as strong bands at
about 1600 and 1400 cm , respectively. The absence of the char-
À1
4
(0.859 mol) was added again in small
4
2
À1
À1
acteristic bands at ꢀ1700 cm attributed to the carboxyl groups
1
indicated the complete deprotonation of all carboxyl groups in 1
II
1
3
upon reaction with Cd ions [18–20]. In order to descript the cor-
rection between structure and temperature, IR spectrum was per-
formed by using the same sample but at different temperature
DMSO-d
1
6 6
) no signals. C NMR (100 MHz; DMSO-d ) dc 115.51,
37.57, 166.27.
(
room temperature 298 K and low temperature 77 K). As shown
2.3. Synthesis of [Cd(dbtec)0.5(H
2
O)
3
]ÁH
2
O (1)
in Fig. S2, The IR absorption peak of 1 at room temperature is
The newcomplex(1)waspreparedby liquiddiffusion method.Ina
1
8 Â 180 mm test tube, Cd(NO Á4H O (0.05 mmol, 15 mg) in 5 mL
3
)
2
2
Table 1
Crystallographic data for 1.
of ethanol was layered onto a solution of 3,6-dibromobenzene-
,2,4,5-tetracarboxylic acid (H dbtec) (0.05 mmol, 20 mg) in 10 mL
O. The resulting mixture was kept at room temperature, and color-
lessrod-likecrystalswereobtainedafter3 weeks. Yield:56%. Elemen-
1
H
4
Empirical formula
5 8 8
C H BrCdO
2
Formula weight
Temperature (K)
Crystal system
Space group
a (Å)
387.42
273.15
Monoclinic
tal analyses for C
5 8
H
BrCdO
8
(387.42): Calcd. C 15.46; H 2.08%; found:
À1
C 15.64; H 2.37%. Selected IR peaks (cm ): 3563 (s), 2922 (w), 1609
P2 /n
1
(
(
s), 1431 (m), 1384 (m), 1329 (s), 1129 (m), 890 (m), 836 (w), 776
w), 709 (w), 654 (w), 576 (w), 513 (w), 422 (w).
10.115(6)
b (Å)
c (Å)
6.421(4)
15.545(9)
90.00
97.435(9)
90.00
1001.1(10)
4
2.570
a
(°)
b (°)
(°)
2
.4. X-ray crystallography
c
3
Volume (Å )
Z
q
m/mm
F(000)
Single crystals of the complex 1 with appropriate dimensions
was chosen under an optical microscope and quickly coated with
high vacuum grease (Dow Corning Corporation) before being
mounted on a glass fiber for data collection. Data for 1 was col-
lected on a Bruker SMART APEX II CCD diffractometer with graph-
3
calc (mg/mm )
À1
6.194
736.0
4.54–49.98°
2H
range for data collection
Index ranges
À11 6 h 6 12, À6 6 k 6 7, À18 6 l 6 15
ite-monochromated MoK
a
radiation source (k = 0.71073 Å)
Reflections collected
Independent reflections
Data/restraints/parameters
Goodness-of-fit on F2
Final R indexes [I P 2
Final R indexes (all data)
Largest diff. peak/hole (e Å
4679
operating at 50 kV and 40 mA by using a multi-scan technique at
room temperature. A preliminary orientation matrix and unit cell
parameters were determined from 3 runs of 12 frames each, each
frame corresponds to a scan in 5 s, followed by spot integration
and least-squares refinement. Data were measured using
for 10 s per frame until a complete hemisphere had been collected.
1760[R(int) = 0.0253]
1760/0/137
1.062
r
(I)]
R
R
1
= 0.0254, wR
= 0.0289, wR
2
= 0.0651
= 0.0669
1
2
x scans
À3
)
0.66/À0.64

L.-L. Zhang et al. / Journal of Molecular Structure 1038 (2013) 73–77
75
Table 2
layered structure (Fig. 2a). To better understand the complicated
structure of 1, the topological analysis of complex 1 has been per-
formed using OLEX [24]. The individual two-dimensional motif
can be simplified as a 4 topological layer if the dbtec ligand is sim-
plified as a four-connected node (Fig. 2b).
Hydrogen and halogen bonds were also investigated, because
the weak forces were suggested to act as a structure-direction tool
2
[25–26]. In complex 1, the coordinated and uncoordinated H O pro-
vided potential hydrogen-bond donors and acceptors. In complex 1,
hydrogen-bond can also be observed between carboxylate oxygen
atoms and coordinated water molecules. The detailed hydrogen-
bonding parameters are listed in Table 3. At the same time, BrÁ Á ÁO
halogen bonds connected adjacent two-dimensional layers with
Selected bond distances (Å) and angles (°) for 1.
CdAO1i
CdAO2W
CdAO3
CdAO3W
CdAO4
2.233 (3)
2.238 (3)
2.315 (3)
2.324 (3)
2.370 (3)
2.400 (3)
2.233 (3)
88.39 (13)
126.67 (11)
142.20 (11)
91.13 (13)
88.75 (11)
102.57 (11)
O2WACdAO4
O3ACdAO4
89.15 (12)
55.35 (10)
90.32 (12)
93.90 (12)
82.73 (12)
81.30 (11)
169.96 (12)
84.28 (12)
154.28 (13)
97.18 (12)
27.70 (12)
81.92 (11)
177.09 (12)
4
O3WACdAO4
i
O1 ACdAO4W
O2WACdAO4W
O3ACdAO4W
O3WACdAO4W
O4ACdAO4W
CdAO4W
ii
O1ACd
i
O1 ACdAO2W
i
i
O1 ACdAO3
O1 ACdAC5
O2WACdAO3
O3WACdAC5
O4ACdAC5
i
O1 ACdAO3W
O2WACdAO3W
O3ACdAO3W
O4WACdAC5
i
O1 ACdAO4
BrÁ Á ÁO2
W
distance of 3.198(4) Å and BrÁ Á ÁO1
W
range from
was
Symmetry code [i: 1 À x, Ày, 1 À z; ii: 1/2 + x, À1/2 À y, À1/2 + z; iii: 1/2 À
x, À1/2 + y, 3/2 À z].
3.563(8) to 3.567(4) Å. The angle
H
1
(\CABrÁ Á ÁO2
W
)
6
0.891(11)°, the geometrical characteristics of BrÁ Á ÁO interactions
may be understood that by assigning a positive polarization in
the polar region of the Br atom and a negative polarization in its
equatorial region. Meanwhile, intramolecular BrÁ Á ÁO halogen bonds
existed with the distance between 3.334(3) and 3.339(6) Å. More-
broadly similar to that of low temperature, but the transmittance
measured in low temperature is much stronger than that of room
temperature, which maybe attribute to the rigidity of ligands and
shortening of bond length at low temperature.
over, weak hydrogen-bonds (O1
W
AH1WBÁ Á ÁBr) were also observed.
Because of the hydrogen bonds and halogen bonds mentioned
above, the structure changed from 2D layer to an interesting 3D
framework (Fig. 2c). As shown in Fig. 3, the layer grids consist of
a kind of large 30-membered-ring hole which contains four H dbtec
4
ligands and four Cd ions. As far as we know, Yang et. al reported a
complex based on btec units and binuclear Cd ions, producing a
step-like 2D layered structure, which contained 14-membered-ring
and 16-membered-ring holes [27]. Based on the structure compar-
3
.2. Crystal structure of [Cd(dbtec)0.5(H
2
O)
3 2
]ÁH O (1)
II
II
Single-crystal X-ray diffraction analysis reveals that each Cd ion
II
is hepta-coordinated by four carboxylate oxygen atoms from two
different dbtec molecules and three oxygen atoms from three coor-
dinated water molecules. As shown in Fig. 1, complex 1 crystallizes
4 4
ison of H dbtec and H btec ligands, we concluded that not only the
in monoclinic P2
ing one crystallographic independent Cd ion, half H
three coordinated and one lattice water molecules. The CdAOdbtec
distances range from 2.233 (3) to 2.370 (3) Å and CdAO distances
1
/n space group with an asymmetric unit contain-
II
steric hindrance but also the electronic effect of Br impacted the
coordination preferences of the dbtec units, consequently, different
structural motifs were produced. Meanwhile, coexistent hydrogen
bond and BrÁ Á ÁO halogen bonds strengthen interactions between
adjacent layers and play important roles in the formation of the
resulting 3D supramolecular frameworks.
4
dbtec ligand,
w
range from 2.238 (3) to 2.400 (3) Å. The bond angles OACdAO range
from 55.37 (10) to 177.14 (12)°, which are comparable to those of
II
reported Cd coordination polymers [21–23]. Because of the syner-
gistic effect of electronic and steric hindrance effect between Br and
carboxylate group, dihedral angles between carboxylate group and
central benzene rings are in the range 67.610 (17)–71.024 (14)°.
3
.3. X-ray Power Diffraction Analyses and Thermogravimetric Analyses
II
In order to check the phase purities of 1, the X-ray powder dif-
The Cd ion in 1 adopts distorted pentagonal bipyramids geometry.
fraction patterns of them was recorded at room temperature. As
shown in Fig. S3, the peak positions of simulated and experimental
patterns are in good agreement with each other, demonstrating the
phase purity of the product. The dissimilarities in intensity may be
due to the preferred orientation of the crystalline powder samples.
The thermal behavior of 1 was studied by TGA (Fig. 4). The
experiments were performed on the samples consisting of numer-
Four carboxylate groups of the dbtec ligand exhibit two kinds of
coordination modes: each of two para-carboxylate groups adopts
monodentate mode, coordinating to one cadmium atom whereas
each of the other two para-carboxylate groups exhibits a chelating
bridging mode linking one cadmium atom, which produces a 2D
ous single crystals under N
2
atmosphere with a heating rate of
À1
1
0 °C min . The TGA curve of 1 displays two step decomposition,
corresponding to the loss of lattice and coordinated water mole-
cules (Calcd. 18.4%, Found 17.8%). The decompositions start at
3
35 °C for 1, accompanying the release of the dbtec ligands. The rel-
ative high thermal stability of 1 may be attributed to its 2D dimen-
sionality connected by hydrogen bond and BrÁ Á ÁO halogen bond.
3.4. UV–vis absorption and photoluminescence properties
The UV–vis absorption spectra of compounds 1 was obtained in
the solid-state at room temperature. As shown in Fig. S4, com-
pounds 1 shows intense absorption peaks at 230–320 nm, which
⁄
can be ascribed to (
p
?
p
) transitions of the ligands. Luminescent
1
0
properties of d transition metal complexes have been well
reported. The organic ligands, their coordination modes and tem-
perature have great effect on the emission wavelengths and lumi-
nescent mechanisms. In order to well understand the luminescent
property of complex 1, the temperature-dependent spectrum of
Fig. 1. The coordination environment of the CdII ions and the linkage modes of
H
4
dbtec ligands in 1 with 50% thermal ellipsoid probability, hydrogen atoms are
omitted for clarity. (i: 1.5 À x, 0.5 + y, 1.5 À z; ii: 1.5 À x, À0.5 + y, 1.5 À z; iii: 2 À x,
À y, 1 À z; iv: 0.5 + x, 1.5 À y, À0.5 + z).
1

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6
L.-L. Zhang et al. / Journal of Molecular Structure 1038 (2013) 73–77
Fig. 2. (a) The 2D layer structure between CdII and ligands. (b) Schematic representation of the 4-connected 44 layer. (c) Hydrogen bonds and halogen bond induced 2D–3D
framework (yellow and blue dotted lines represent hydrogen bonds and halogen bond, respectively). (For interpretation of the references to colour in this figure legend, the
reader is referred to the web version of this article.)
Table 3
Hydrogen-bonding geometrical parameters/Å, ° for compound 1.
DAHÁ Á ÁA
DAH
HÁ Á ÁA
DÁ Á ÁA
\DAHÁ Á ÁA
O2WAH2WAÁ Á ÁO1W
0.85
0.85
0.85
0.85
0.85
0.85
0.85
0.85
1.90
1.86
1.91
2.22
2.03
2.13
1.95
2.18
2.702 (5)
2.700 (4)
2.749 (4)
2.978 (6)
2.852 (5)
2.884 (5)
2.746 (5)
2.983 (5)
157.5
171.6
168.7
148.6
163.2
147.4
156.2
157.1
i
O2WAH2WBÁ Á ÁO4
ii
O3WAH3WAÁ Á ÁO3
iii
O3WAH3WBÁ Á ÁO1W
O4WAH4WAÁ Á ÁO3W
iv
iv
O4WAH4WBÁ Á ÁO2
v
O1WAH1WAÁ Á ÁO1
vi
O1WAH1WBÁ Á ÁO1
Symmetry codes: (i) Àx, Ày, Àz + 1; (ii) Àx + 1/2, y À 1/2, Àz + 3/2; (iii) Àx À 1/2,
y À 1/2, Àz + 3/2; (iv) x, y + 1, z; (v) x À 1, y, z; (vi) Àx + 1/2, y + 1/2, Àz + 3/2.
Fig. 4. TGA curve of 1.
maximum at 310 nm. As the temperature is decreased to 77 K, a
blue shift of 14 nm and surprising fluorescence enhancement is ob-
served with respect to the emission band of 1 at room temperature,
suggesting its high sensitivity to temperature. The emission of H4-
⁄
⁄
dbtec was ascribed to the
p
? n or
p ? p electronic transitions
II
[
28–30]. Because the Cd ion is difficult to oxidize or to reduce
1
0
due to its d electronic configuration, the emissions of these com-
plexes are neither metal-to-ligand charge transfer nor ligand-to-
metal charge transfer. The emission of complex 1 can be assigned
to the intraligand or ligand-to-ligand charge transition as a result
of the resemblance of the emission spectra in comparison with
those of the free ligand [31–32]. The increase in fluorescence can
be attributed to the rigidity of ligands which is favorable for cold
Fig. 3. The 30-member-ring in complex 1 labeled A.
complex 1 and free ligand (H
perature (298 K) and low temperature (77 K) Fig. 5. The solid-state
emission spectrum of 1 and H dbtec at room temperature displays
an emission band centered at 464 nm and 471 nm with excitation
4
dbtec) were measured at room tem-
4
conditions, with the decrease of radiationless decay process of the
⁄
intraligand (
p
?
p
) excited state [33].

L.-L. Zhang et al. / Journal of Molecular Structure 1038 (2013) 73–77
77
in
the
online
version,
at
http://dx.doi.org/10.1016/
j.molstruc.2013.01.027.
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