Preparation and crystal structure of a triple parallel interpenetrated copper coordination polymer based on a flexible bis(imidazole) ligand

Article abstract of DOI:10.1007/s11243-013-9705-9

A Cu(II) coordination polymer, [Cu(L)(bdc)] _n (L = 1,4-bis(imidazole)butane, bdc = 1,3-benzenedicarboxylate) has been hydrothermally synthesized and characterized by elemental analysis, IR spectrum, thermogravimetric analysis, and single-crystal X-ray diffraction. The complex is a rare example of a threefold parallel interpenetrated coordination bonding network based on undulant [Cu₄(μ ₂-L)₂(μ ₂-bdc)₂] parallelograms with 4⁴-sql layer. The heterogeneous catalytic activity of the complex was tested for the degradation of Congo red azo dye in a Fenton-like process in which the degradation efficiency reached 94 % after 100 min. Kinetic analysis indicates that the degradation rate of the dye can be approximated by pseudo-first-order kinetics.

Full text of DOI:10.1007/s11243-013-9705-9

Transition Met Chem (2013) 38:407–412
DOI 10.1007/s11243-013-9705-9
Preparation and crystal structure of a triple parallel
interpenetrated copper coordination polymer based
on a flexible bis(imidazole) ligand
•
•
•
•
Li Qin Yu Gu Guang Yue Li Shu Lin Xiao
Guang Hua Cui
Received: 30 December 2012 / Accepted: 11 February 2013 / Published online: 2 March 2013
Ó Springer Science+Business Media Dordrecht 2013
Abstract A Cu(II) coordination polymer, [Cu(L)(bdc)]n
(L = 1,4-bis(imidazole)butane, bdc = 1,3-benzenedicar-
boxylate) has been hydrothermally synthesized and
characterized by elemental analysis, IR spectrum, ther-
mogravimetric analysis, and single-crystal X-ray diffrac-
tion. The complex is a rare example of a threefold parallel
interpenetrated coordination bonding network based on
undulant [Cu4(l2-L)2(l2-bdc)2] parallelograms with 44-sql
layer. The heterogeneous catalytic activity of the complex
was tested for the degradation of Congo red azo dye in a
Fenton-like process in which the degradation efficiency
reached 94 % after 100 min. Kinetic analysis indicates that
the degradation rate of the dye can be approximated by
pseudo-first-order kinetics.
interest from synthetic chemists. In spite of the best efforts
of crystal engineers, most known examples of such net-
work structures still fill space inefficiently; such structures
seek to minimize the empty space between the networks,
sometimes generating highly interdigitated structures,
especially in 1D and 2D networks [6–10]. A variety of
interpenetrated frameworks have been presented and dis-
cussed in two comprehensive reviews by Batten and
Robson [11, 12]. Nonetheless, rational synthesis of such
networks is often thwarted by interpenetration and archi-
tectural frailty, which arise from the presence of large
linkers and extend into the distances between the nodes.
Here, the control of interpenetration between independent
motifs is still a significant challenge that involves many
factors, including choice of ligand, metal, reaction condi-
tions, use of templates, molar ratio, etc. [13–16].
Introduction
The relatively extended 1,4-bis(imidazole)butane ligand
(L) is a good candidate for the assembly of versatile entan-
gled structures with a propensity to form large voids; the
flexible nature of the spacer allowing this ligand to bend and
rotate when coordinating to metal centers so as to conform to
the coordination preferences of the metal [17–21]. In this
paper, we present the preparation and crystal structure
of a copper(II) complex of L, namely [Cu(L)(bdc)]n (bdc =
1,3-benzenedicarboxylate), which consists of threefold
parallel interpenetrated 44-sql layers. We have also explored
the catalytic activity of this complex for the degradation of
Congo red as a model organic contaminant.
The design and synthesis of coordination polymers exhib-
iting entangled architectures are of great interest. This is
due to their potential as functional materials with specific
properties, such as luminescence, magnetism, absorption,
catalysis, ion-exchange, etc. [1–5]. In the area of entan-
glement, interpenetrating networks have attracted extensive
Electronic supplementary material The online version of this
article (doi:10.1007/s11243-013-9705-9) contains supplementary
material, which is available to authorized users.
L. Qin ꢀ G. Y. Li ꢀ S. L. Xiao ꢀ G. H. Cui (&)
College of Chemical Engineering, Hebei United University,
Tangshan 063009, Hebei, People’s Republic of China
e-mail: tscghua@126.com
Experimental
Materials and measurements
Y. Gu
Qian’an College, Hebei United University, Tangshan 063009,
Hebei, People’s Republic of China
Congo red, purchased from Sinopharm Chemical Reagent
Co., Ltd. (Shanghai, China), was of commercial grade and
123

408
Transition Met Chem (2013) 38:407–412
used without further purification. Other solvents and
starting materials were commercially available and used
without further purification. The free ligand L was prepared
according to the literature method [22]. C, H, and N ele-
mental analyses were performed on a Perkin-Elmer
240C analyzer. IR spectra (KBr pellets) in a range
4,000–400 cm-1 were obtained on a FTIR170 SX (Nicolet)
spectrometer. TG measurements were taken on a NET-
ZSCH TG 209 thermal analyzer from room temperature to
800 °C under N2 with a heating rate of 10 °C min–1. The
X-ray powder diffraction (XRPD) patterns were recorded
on a Rigaku D/Max-2500 diffractometer at 40 kV, 100 mA
for a Cu-target tube and a graphite monochromator. The
concentration of Congo red solution was measured with a
SHANGHAI JINGKE 722N spectrophotometer at its
maximum wavelength 496 nm.
where C0 (mg/L) is the initial concentration of Congo red,
and Ct (mg/L) is the concentration of Congo red at reaction
time, t (min).
X-ray crystallography
X-ray single-crystal diffraction data for the complex were
collected on a Bruker Smart 1000 CCD diffractometer with
˚
graphite-monochromated Mo Ka radiation (k = 0.71073 A)
and x - 2h scan mode at 296 K. A semiempirical
absorption correction was applied using the SADABS
program [24]. The structure was solved by direct methods
and refined anisotropically by full-matrix least-squares
techniques using Bruker SHELXTL program package [25].
Metal atoms were located from the E maps, and other non-
hydrogen atoms were located in successive difference
Fourier syntheses and refined with anisotropic thermal
parameters on F2. The hydrogen atoms of ligand L were
generated theoretically on the specific atoms and refined
isotropically. Crystallographic data and experimental
details for structural analyses are summarized in Table 1,
and the selected bond lengths and angles are listed in
Table 2.
Synthesis of the [Cu(L)(bdc)]n
The complex was prepared by a one-pot hydrothermal
reaction as follows: A mixture of CuSO4ꢀ5H2O (0.1 mmol,
25.1 mg), L (0.1 mmol, 19.0 mg), H2bdc (0.1 mmol,
16.6 mg), water (8 mL), and MeOH (2 mL) was placed in
a Teflon-lined stainless vessel and heated to 140 °C for
3 days, then cooled to room temperature at a rate of 5 °C/h.
After filtration, blue crystals suitable for single-crystal
X-ray diffraction were obtained. Yield: ca. 35 % based on
CuSO4ꢀ5H2O. Calcd. for C18H18CuN4O4 (Fw = 417.90):
C 51.7, H 4.3, N 13.4 %; found C 51.8, H 4.5, N 13.2 %.
FTIR (KBr pellet, cm–1): 3,132 s, 2,939 w, 1,623 vs,
1,570 m, 1,530 m, 1,437 m, 1,329 s, 1,091 s, 952 w,
869 w, 722 s, 660 w.
Table 1 Crystallographic data and experimental details for the
complex
Empirical formula
Formula weight
Crystal system
C18H18CuN4O4
417.90
Monoclinic
C2/c
Space group
Unit-cell dimensions
˚
a (A)
11.6229(10)
22.131(2)
Catalysis experiments
˚
b (A)
˚
c (A)
14.0360(13)
105.1550(10)
3,484.8(5)
8
The catalytic degradation of dye was carried out in a 250-mL
round-bottom flask fitted with a magnetic stirrer, and the
reaction temperature was controlled by circulating constant
temperature water. A solution of Congo red (40 mg, 0.057
mmol) in water (200 mL) was placed in the flask. When the
temperature was constant, a volume of 3.0 mL of thesolution
was collected from the reactor to determine the initial Congo
red concentration, and then a predetermined amount of
the complex (30 mg) was added, followed by addition of
30 % (w/w) H2O2. At given time intervals, 3.0 mL aliquots
of the solution were taken out and centrifuged to remove
the residual catalyst, then analyzed with a UV–Vis spectro-
photometer at an absorption wavelength of 496 nm.
The degradation efficiency of Congo red is defined as
follows: [23]:
b (°)
3
˚
V (A )
Z
qcalcd (g/cm3)
Absorption coefficient (mm-1
1.593
)
1.287
F(000)
1,720
Crystal size (mm)
h range (°)
Index range h, k, l
0.15 9 0.17 9 0.18
1.84–25.81
-14/13, -23/27, -11/17
9,396
Reflections collected
Independent reflections (Rint
Data/restraint/parameters
Goodness-of-fit on F2
)
3,361 (0.0326)
3,361/0/246
1.019
Final R1, wR2 (I [ 2r(I))
Largest diff. peak and hole
0.0328, 0.0813
0.0465, 0.0877
ðC0 ꢁ CtÞ
Degradation efficiency ¼
ꢂ 100 %
ð1Þ
R1 = R||Fo|–|Fc||/R|Fo|; wR2 = {R [w (F2o–Fc2)2]/R [w (Fo2)2]}1/2
C0
123

Transition Met Chem (2013) 38:407–412
409
˚
two nitrogen atoms from two different L ligands and two
oxygen atoms of carboxylate groups from two distinct bdc
anionic ligand. Each bdc ligand serves as a bis-connector
by means of its two monodentate carboxylate groups
bridging Cu1 and Cu2 with Cu1ꢀꢀꢀCu2 separations of
Table 2 The selected bond lengths (A) and angles (°) for the
complex
Parameter
Value
Parameter
Value
1.9730(17)
Cu1–O4A
1.9730(17) Cu1–O4
1.991(2) Cu1–N1A
1.9640(16) Cu2–O1B
Cu1–N1
1.991(2)
1.9640(16)
2.018(2)
91.50(8)
88.67(8)
178.74(12)
89.33(8)
90.67(8)
180.0
˚
9.258(7) A. The Cu–N bond lengths vary from 1.991(2) to
Cu2-O1
˚
2.018(2) A, while the Cu–O bond lengths range from
˚
1.9640(16) to 1.9730(17) A. The coordination angles are in
Cu2–N3B
2.018(2)
164.78(11)
88.67(8)
91.50(8)
180.0
Cu2–N3
O4A–Cu1–O4
O4–Cu1–N1
O4–Cu1–N1A
O1–Cu2–O1B
O1B–Cu2–N3B
O1B–Cu2–N3
O4A–Cu1–N1
O4A–Cu1–N1A
N1–Cu1–N1A
O1–Cu2–N3B
O1–Cu2–N3
N3B–Cu2–N3
the range of 88.67(8) to 91.50(8) for the trans angles, and
164.78(11) to 180° for the cis angles.
The Cu(II) centers are connected by L and bridging bdc
ligands, to form an infinite 1D chain structure. The two
kinds of chains link through each other to form 2D 44-sql
layers, in which each 38-membered circuit consists of four
copper(II) atoms at the corners connected by two L ligands
and two bdc ligands that make up four edges (Fig. 1b). The
90.67(8)
89.33(8)
Symmetry transformations used to generate equivalent atoms:
A = -x ? 2, y, -z ? 1/2; B = -x ? 1/2, -y ? 1/2, -z
˚
length of the L edge is 14.036(13) A, and length of the bdc
˚
edge is 9.258(7) A. In each circuit, Cu(II) centers are
Results and discussion
bridged by ligands in the sequence of –Cu–L–Cu–bdc–.
The cycles have parallelogram configurations and build
into a threefold interpenetrated undulant quadrilateral (4,4)
net (Fig. 2). The interlocking of the three independent 2D
networks takes place in such a way that every ring crosses
two rings of the other two networks. Hence, if one of the
2D networks could be removed, the remaining two cova-
lent networks would be still interlocked (Fig. S1). Thus,
each layer encloses three identical and independent parallel
interpenetrated 2D coordination layers, and this situation
provides a rare example of a threefold interpenetrated (4,4)
network [27]. Parallel interpenetration requires undulation
of individual sheets and has been observed for both (4,4)
and (6,3) coordination networks. To the best of our
knowledge, of the (4,4) coordination networks that exhibit
parallel interpenetration, only this example is triply inter-
penetrated, with all other known examples being doubly
interpenetrated [11, 12]. It is worth noting that Ma et al.
[29] reported a compound using the same mixed ligands,
namely {[Cu(L)(bdc)(H2O)]ꢀ5H2O}n, which forms an
unprecedented 3D four-connected topology of an 86 net,
IR spectrum and crystal structure
The IR spectrum of the complex exhibits the antisymmetric
stretching vibration mas(COO-) at 1,623 cm–1 and the
symmetric stretching vibration ms(COO-) at 1,329 cm–1,
with a separation Dm of 294 cm–1, as expected for the
observed monodentate coordination mode of the carbox-
ylate groups [26, 27]. Bands in the range of
3,132–2,939 cm–1 are assigned to the C–H stretches, while
band at 1,530 cm–1 could be assigned to the mC=N absorp-
tions of the imidazole rings in the ligand L. The equivalent
band is observed at 1,509 cm–1 for the free ligand L.
The complex crystallizes in a centrosymmetric space
group C2/c with one repeating metal complex structural
motif. The asymmetric unit consists of one Cu(II) center,
one L ligand plus one bdc dianionic ligand. As depicted in
Fig. 1a, atoms Cu1 and Cu2 are four-coordinated in a
square-planar coordination environment with the value of
the s4 factor being 0.1 and 0 [28], respectively, defined by
Fig. 1 a Coordination
environment of the Cu(II) atom
in the complex with thermal
ellipsoids drawn at 30 %
probability. Hydrogen atoms
were omitted for clarity, and
symmetry transformations were
used to generating equivalent
atoms: A = -x ? 2, y, -z ?
1/2; B = -x ? 1/2, -y ? 1/2,
-z. b The 2D layer connected
by L in the complex
123

410
Transition Met Chem (2013) 38:407–412
demonstrating that temperature and solvent also have a
significant effect on different topologies of the resulting
complexes.
the large amounts of sludge produced require further
treatment, which is expensive [31]. To overcome the dis-
advantages of Fenton type processes, heterogeneous Fen-
ton-like catalysts have recently received much attention
[32–34]. In particular, MOFs possess inherent advantages
since no additional support is needed for the heterogeni-
zation and the microenvironment provided by the organic
framework often allows the metal active sites to behave in
a manner reminiscent of enzymes [35].
Thermal and XRPD analysis
The thermal properties of the complex were examined by
thermogravimetric analysis under a nitrogen atmosphere
with a heating rate of 10 °C/min from room temperature to
800 °C. As shown in Fig. S2, the complex is thermally
stable until around 260 °C and then decomposition begins.
A continuous mass loss occurs in the temperature range
from 260 to 455 °C, which is assigned to the loss of L and
bdc ligands. The remaining residual mass of 20.6 % cor-
responds to the formation of CuO (calcd. 19.1 %).
In order to investigate the catalytic activity of the
complex, degradation experiments of Congo red with
hydrogen peroxide were investigated. As shown in Fig. 3,
when the complex is added as a catalyst, rapid degradation
ensues and the degradation efficiency can reach 81 %
within 20 min; after which Congo red oxidation proceeds
at a reduced rate, reaching up to 94 % after 100 min.
However, in the absence of catalyst, the degradation effi-
ciency of control experiments is low, reaching 46 % after
100 min under the same conditions. Hence, the complex
has a positive effect on the degradation of Congo red.
The observed degradation efficiency decreased with
increasing concentrations of Congo red, as illustrated in
Fig. 4. This is probably due to the coverage of the catalyst
surface with adsorbed Congo red molecules as the con-
centration of the dye is increased. This would have an
inhibiting effect on the generation of ꢀOH species, due to
retardation of the interaction between H2O2 and the cata-
lyst [36]. Nevertheless, with Congo red concentrations up
to 200 mg/L, the degradation efficiency reached 74 % after
100 min.
The observed and simulated powder XRD patterns of
the complex are depicted in Fig. S3. It is obvious that the
measured powder XRD pattern is in good agreement with
the pattern simulated from the X-ray single-crystal data,
indicating phase purities of the sample. The difference in
reflection intensities between the simulated and experi-
mental patterns is due to the different orientation of the
crystals in the bulk powder sample.
Catalytic degradation experiments
Waste waters generated by the textile industry contain
considerable amounts of non-fixed dyes, especially azo
dyes [30]. Among these, Congo red (sodium salt of ben-
zidinediazo-bis-1-naphthylamine-4 sulfonic acid) is one of
the important, used for coloring of paper products, and
unaffected by conventional degradation system. The clas-
sical Fenton reagent consisting of a homogeneous solution
of Fe2?/Fe3? and H2O2 has been extensively studied, but
two major drawbacks exist in the process: The pH of
operation must be strictly controlled at around pH = 3; and
We next explored the effect of initial pH on the degra-
dation by adjusting the initial pH of the solution in the
range of 3.2–10.3. The complex showed high catalytic
activity over a wide pH range, as presented in Fig. 5. It is
found that during the first 20 min of reaction, the initial pH
100
80
60
40
20
control experiment
1
as catalyst
0
0
20
40
60
80
100
Time (min)
Fig. 3 The control experiment of the catalytic degradation of Congo
red
Fig. 2 Interpenetration of three units in the layer for the complex
123

Transition Met Chem (2013) 38:407–412
411
100
100
80
60
40
20
0
80
60
40
20
0
20 mg/L
100 mg/L
140 mg/L
200 mg/L
0
20
40
60
80
100
120
0
20
40
60
80
100
Time (min)
Time (min)
Fig. 6 Effect of temperature on the catalytic degradation of Congo red
Fig. 4 Effect of initial dye on the catalytic degradation of Congo red
100
-2.6
ln k = -837.6223/T -0.1117
80
60
40
R2 = 0.9731
-2.7
-2.8
-2.9
pH=3.2
pH=4.0
pH=6.5
20
pH=8.0
pH=10.3
0
0
20
40
60
80 100
0.0030
0.0031
0.0032
0.0033
Time (min)
1/ T
Fig. 5 Effect of initial pH on the catalytic degradation of Congo red
Fig. 7 Arrhenius plot of ln k versus 1/T for the degradation of Congo
red
has a pronounced effect on the degradation efficiency.
Nevertheless, the degradation efficiencies tend to slow
down over time. After 100 min, the degradation efficien-
cies are 53, 41, 94, 96, and 96 % at pH values of 3.2, 4.0,
6.5, 8.0, and 10.3, respectively. Hence, the optimal pH
range is 6.5–10.3. Good efficiency at near-neutral pH is
desirable since operation under near-neutral conditions
would be required for most wastewaters. On the other
hand, acidic conditions significantly enhance the leaching
of active components [37]. In addition, higher or lower pH
values are undesirable since they lead to deactivation of the
catalyst. The following experiments were carried out at pH
6.5, the natural pH value of Congo red.
efficiency of Congo red reaches about 95 % after 120 min.
The increase is attributed to higher reaction rate at elevated
temperature producing more ꢀOH species; however, still
higher temperatures may result in decomposition of H2O2
and so reduce the efficiency of H2O2 usage [38].
Kinetics and mechanism of the degradation reaction
A pseudo-first-order kinetic model can be used to describe
the degradation of Congo red in this system, as follows:
ꢀ
ꢁ
C0
C
Next, we studied the effect of reaction temperature on
the degradation of Congo red in a series of experiments at
temperatures from 30 to 60 °C, as displayed in Fig. 6. As
expected, lower temperatures cause incomplete degrada-
tion of Congo red, while higher temperature increases the
catalytic degradation efficiency. At 60 °C, the degradation
ln
¼ kt
ð2Þ
where the slope of the plot of ln(C0/C) as a function of time
yields the value of k. To understand the relationship
between temperature and the rate of degradation, the
k values are assumed to obey the Arrhenius equation.
123

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Transition Met Chem (2013) 38:407–412
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Hence, a plot of ln k versus 1/T should be a straight line
with a slope of -Ea/R, as shown in Fig. 7. A good linear
relationship is indeed observed. From the slope of the plot,
the activation energy Ea = 7 kJ/mol (R2 = 0.973) over the
temperature ranges of 30–60 °C, indicating that degrada-
tion of Congo red solution by this complex requires a
relatively low activation energy.
Based on the experimental findings, a reaction mecha-
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CuII - MOF þ H2O2 ! HO2 ꢀ þCuI - MOF þ Hþ
CuI - MOF þ H2O2 ! CuII - MOF þ ꢀOH þꢁ OH
Dye þ ꢀOH ! oxidation products
ð3Þ
ð4Þ
ð5Þ
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4782
In this work, a coordination polymer [Cu(L)(bdc)]n has
been hydrothermally synthesized, and its crystal structure
is a rare example of a triple interpenetrated, four-connected
coordinative-bonded 2D network. The complex exhibits
high activity in the Fenton-like degradation of Congo red;
additionally, the results show that the degradation of Congo
red follows first-order kinetics.
23. Zhan YZ, Li HL, Chen YL (2010) J Hazard Mater 180:481–485
24. Sheldrick GM (1996) SADABS (version 2.03), Program for
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Supplementary materials
¨
versity of Gottingen, Germany
CCDC 881022 contains the supplementary crystallographic
data for the complex. These data can be obtained free of
charge via http://www.ccdc.cam.ac.uk/conts/retrieving.
html, or from the Cambridge Crystallographic Data Cen-
tre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (?44)
1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
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Acknowledgments The authors thank the financial support from the
National Natural Science Foundation of China (Nos. 11247281), and
Excellent Youth Fund of Hebei Province Department of Education
(Y2012010).
˙
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