Self-assembly of two 2D cobalt(II) coordination polymers constructed from 5-tert-butyl isophthalic acid and flexible bis(benzimidazole)-based ligands

Article abstract of DOI:10.1007/s11243-014-9846-5

Two cobalt(II) coordination polymers, [Co(L1)(tbi)(H₂O)] _n (1) and [Co(L2)(tbi)] _n (2) (L1 = 1,4-bis(benzimidazole) butane, L2 = 1,4-bis(2-methylbenzimidazole)butane, H₂tbi = 5-tert-butyl isophthalic acid) have been synthesized under hydrothermal conditions and characterized by physicochemical and spectroscopic methods as well as by single-crystal X-ray diffraction analysis. Both complexes exhibit similar 2D (4,4) layer structures, constructed from tbi^2- and bis(benzimidazole)-based bridging ligands. The cobalt centers display different coordination environments, with an octahedral geometry in 1 and a distorted square-pyramidal configuration in 2. The thermal stabilities, fluorescence and catalytic properties of both complexes have been investigated.

Full text of DOI:10.1007/s11243-014-9846-5

Transition Met Chem (2014) 39:653–660
DOI 10.1007/s11243-014-9846-5
Self-assembly of two 2D cobalt(II) coordination polymers
constructed from 5-tert-butyl isophthalic acid and flexible
bis(benzimidazole)-based ligands
•
•
•
Xiao Xiao Wang Ying Na Zhao Guang Yue Li
Guang Hua Cui
Received: 30 March 2014 / Accepted: 6 June 2014 / Published online: 25 June 2014
Ó Springer International Publishing Switzerland 2014
Abstract Two cobalt(II) coordination polymers, [Co(L1)-
(tbi)(H2O)]n (1) and [Co(L2)(tbi)]n (2) (L1 = 1,4-bis
(benzimidazole)butane, L2 = 1,4-bis(2-methylbenzimida-
zole)butane, H2tbi = 5-tert-butyl isophthalic acid) have
been synthesized under hydrothermal conditions and char-
acterized by physicochemical and spectroscopic methods as
well as by single-crystal X-ray diffraction analysis. Both
complexes exhibit similar 2D (4,4) layer structures, con-
structed from tbi2- and bis(benzimidazole)-based bridging
ligands. The cobalt centers display different coordination
environments, with an octahedral geometry in 1 and a dis-
torted square-pyramidal configuration in 2. The thermal
stabilities, fluorescence and catalytic properties of both
complexes have been investigated.
significant interest has arisen into the structural tuning of
MOCPs by the rational selection of bridging ligands.
Organic aromatic carboxylates have been widely used as
O-donor bridging ligands to construct novel metal–organic
coordination polymers, owing to their versatile coordina-
tion modes and good stability. In particular, derivatives of
1,3-benzenedicarboxylic acid (isophthalic acid, H2ip), such
as H2tbi (5-tert-butyl isophthalic acid), are of interest. The
latter has a sterically hindered substituent, and the resulting
subtle difference with H2tip may significantly influence the
resulting MOCPs [13, 14]. Flexible bis(benzimidazole)
derivatives have attracted much attention within crystal
engineering and supramolecular chemistry by our and other
groups, due to their good coordination ability and versatile
conformations [15–20]. However, the incorporation of
flexible bis(benzimidazole) ligands into Co(II)-5-tert-bu-
tylisophthalic MOCPs has been rarely reported.
Introduction
Synthetic azo dyes such as Congo red and methyl orange
have versatile applicability in the textile, pharmaceutical,
food and cosmetics industries. Estimates indicate that
approximately 1–15 % of synthetic textile dyes are lost in
wastewater streams during manufacturing or processing
operations [21]. Since most azo dyes are non-biodegrad-
able, as well as toxic and/or carcinogenic, the effective
removal of dyes from waste effluents is of significant
environmental importance [22]. Fenton oxidation technol-
ogies utilize H2O2 which generates hydroxyl radicals that
are very effective in degrading organic pollutants, but the
uncatalyzed reaction rates are generally slow at ambient
temperature, and activation of H2O2 is necessary to
accelerate the process. MOCPs are potential candidates to
catalyze the degradation of such organic pollutants in
Fenton-like processes.
The design and synthesis of metal–organic coordination
polymers (MOCPs) have received much attention in recent
decades due to their fascinating structures and potential
applications in the fields of catalysis, luminescence, con-
ductivity, magnetism and so on [1–5]. Although rapid
progress in the construction of MOCPs has been made, it is
still a challenge to control their final architectures and
properties because the self-assembly of these compounds is
influenced by many factors, such as organic ligands, pH,
choice of metal, temperature, solvent systems [6–9]. In
particular, the appropriate choice of organic ligands is
crucial to obtain target MOCPs [10–12]. Therefore,
X. X. Wang ꢀ Y. N. Zhao ꢀ G. Y. Li ꢀ G. H. Cui (&)
College of Chemical Engineering, Hebei United University,
Tangshan 063009, Hebei, People’s Republic of China
e-mail: tscghua@126.com
In this work, we have selected two related flexible
bis(benzimidazole) ligands (L1 and L2 are bis(benzimidazole)
123

654
Transition Met Chem (2014) 39:653–660
Scheme 1 Two major reactions
of this paper
ligands with aliphatic cores that only differ by a methyl group)
and H2tbi (see Scheme 1) as co-ligand to construct two new
Co(II) MOCPs, namely [Co(L1)(tbi)(H2O)]n (1) and
[Co(L2)(tbi)]n (2) (L1 = 1,4-bis(benzimidazole)butane,
L2 = 1,4-bis(2-methylbenzimidazole)butane, H2tbi = 5-tert-
butyl isophthalic acid). In addition, the fluorescence properties
and catalytic performances of both complexes for the degra-
dation of Congo red dye have been investigated.
Table 1 Crystallographic data and experimental details for 1 and 2
1
2
Empirical formula
Formula weight
Crystal system
C30H32CoN4O5
587.54
C32H34CoN4O4
597.58
Triclinic
P¯ı
Orthorhombic
Pbca
Space group
Unit cell dimensions
˚
a (A)
10.1557(8)
12.2174(11)
12.5774(10)
71.4930(10)
75.1070(11)
77.3160(10)
1,413.8(2)
2
11.3732(10)
19.9007(18)
26.2160(2)
˚
b (A)
Experimental
˚
c (A)
a (°)
b (°)
c (°)
Materials and measurements
All reagents and solvents were obtained from commercial
sources and used without further purification. L1 and L2
were prepared according to the literature procedure [23].
Elemental analyses were obtained on a Perkin-Elmer 240C
elemental analyzer. FTIR spectra were recorded from KBr
pellets in the range of 4,000–400 cm-1 on an Avatar 360
(Nicolet) spectrophotometer. Thermogravimetric analysis
(TGA) was carried out on a NETZSCH TG 209 thermal
analyzer from room temperature to 800 °C with a heating
rate of 10 °C/min under N2 atmosphere. Luminescence
spectra for the powdered solid samples were measured at
room temperature on a Hitachi F-7000 fluorescence spec-
trophotometer. X-ray powder diffraction (XRPD) mea-
surements were made on a Rigaku D/Max-2500PC X-ray
diffractometer using Cu-Ka radiation (k = 0.1,542 nm)
and x - 2h scan mode at 293 K.
3
˚
V (A )
5,933.6(9)
8
Z
qcalcd (g cm-3
Absorption coefficient
(mm-1
F(000)
)
1.380
1.338
0.621
0.653
)
614
2,504
Crystal size (mm)
h range (°)
Index range h, k, l
0.24 9 0.22 9 0.19 0.22 9 0.21 9 0.18
2.10–25.02 1.55–27.46
-12/12, -14/14, - -14/14, -25/17,
14/14
-34/31
Reflections collected
10,780
34,383
Independent reflections 4,969 (0.0199)
(Rint)
6,787 (0.0462)
6,787/120/406
Data/restraint/
parameters
Goodness-of-fit on F2
4,969/0/369
1.034
1.009
Final R1, wR2
(I [ 2r(I))
0.0302, 0.0768
0.0390, 0.0922
Synthesis of complex 1
Largest diff. peak and
hole
0.271, -0.270
0.330, -0.484
A mixture of CoCl2ꢀ6H2O (0.1 mmol, 23.8 mg), L1
(0.1 mmol, 29.0 mg) and H2tbi (0.1 mmol, 22.2 mg) were
dissolved in distilled water (10 mL). The mixture was
sealed in a 25-mL Teflon-lined stainless steel vessel and
a
R1 ¼ RjjFojꢁjFcjj=RjFoj;
n
h
i
h
io
1=2
ꢀ
ꢁ
ꢀ
ꢁ
2
2
b
wR2 ¼ R w Fo2ꢁFc2 =R w Fo2
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Transition Met Chem (2014) 39:653–660
655
˚
using the SADABS program [24]. The structures were
solved by direct methods and refined on F2 by full-matrix
least-squares using the SHELXTL program package [25].
All non-hydrogen atoms were refined anisotropically.
Complex 2 showed disorder in C30, C31 and C32 of the
5-tert-butyl substituent groups; these atoms were refined
with a split model with site occupation factor 0.56, SADI
for restraining distances with related disordered atoms. The
crystallographic data for complexes 1 and 2 are listed in
Table 1, and selected lengths and angles are presented in
Table 2, respectively.
Table 2 Selected bond lengths (A) and angles (°) for 1 and 2
Parameter
Value
Parameter
Value
[Co(L1)(tbi)(H2O)] (1)
n
Co1–N1
2.1482(16) Co1–N3
2.0433(12) Co1–O3
2.0977(13) Co1–O4
2.2045(16)
Co1–O2A
Co1–O1 W
2.2360(13)
2.1335(13)
98.77(5)
O2A–Co1–O1 W
O1 W–Co1–O4
O1 W–Co1–N1
O2A–Co1–N3
O4–Co1–N3
O2A–Co1–O3
N1–Co1–N3
N3–Co1–O3
[Co(L2)(tbi)]n (2)
Co1–O1E
96.12(6)
163.28(5)
91.32(6)
92.93(6)
83.96(6)
158.74(5)
176.82(6)
85.84(5)
O2A–Co1–O4
O2A–Co1–N1
O4–Co1–N1
90.19(6)
96.17(6)
O1 W–Co1–N3
87.75(6)
O1 W–Co1–O3 105.03(5)
O4–Co1–O3
N1–Co1–O3
59.98(5)
91.47(5)
Results and discussion
Synthesis and spectral characterization
of the complexes
1.9680(15) Co1–O4
1.9546(14)
2.0273(17)
Co1–O2E
2.5380(16) Co1–N3
2.0463(17)
Polycarboxylate-bis(benzimidazole) systems provide an
important and flourishing branch of mixed-ligand MOCPs.
Acid and base ligands are ideal partners that can com-
pensate charge balance as well as coordination deficiency.
Such MOCPs are generally synthesized by using solvent to
induce the self-assembly of a regular framework. Such
reactions can be carried out by mixing a solution of the
required metal ion with a solution of the ligands at room
temperature, or under hydrothermal/solvothermal condi-
tions. Hydrothermal reactions have recently been demon-
strated to be a versatile technique for the construction of
MOCPs, and a series of flexible bis(benzimidazole) com-
plexes have been synthesized by this method [15–18]. In
the present work, the two title complexes were synthesized
by the reaction of cobalt chloride with 5-tert-butyl iso-
phthalic acid and flexible bis(benzimidazole in a 1:1:1
molar ratio under hydrothermal conditions. Both com-
plexes 1 and 2 were obtained independent of the reactant
molar ratio and the hydrothermal reaction temperature, as
confirmed by IR spectra and elemental analysis.
Co1–N1
O4–Co1–O1E
O1E–Co1–N3
O1E–Co1–N1
107.19(6)
O4–Co1–N3
O4–Co1–N1
N3–Co1–N1
103.67(7)
96.04(6)
114.43(7)
121.16(7)
110.60(7)
Symmetry transformation used to generate equivalent atoms:
A ¼ ꢁ1 þ x; y; z; E ¼ 0:5 ꢁ x; 0:5 þ y; z
heated at 140 °C for 3 days under autogenous pressure.
After the mixture was cooled to room temperature at a rate
of 5 °C/h, pink block-shaped crystals suitable for single-
crystal X-ray diffraction were obtained with a yield of
35 % (based on CoCl2ꢀ6H2O). Calcd. for C30H32CoN4O5
(Fw = 587.54): C 61.3, H 5.5, N 9.5 %; found: C 61.0, H
5.1, N 9.7 %. FTIR (KBr pellet, cm-1): 3,327 vs, 2,945 m,
1,616 s, 1,548 m, 1,457 m, 1,372 s, 1,255 m, 915 w, 734 s,
659 w.
Synthesis of complex 2
The synthetic method for complex 2 was similar to that for
1, except that L2 (0.1 mmol, 31.8 mg) was used instead of
L1. Purple block crystals of complex 2 suitable for single-
crystal X-ray diffraction were obtained in 52 % yield
(based on CoCl2ꢀ6H2O). Calcd. for C32H34CoN4O4
(Fw = 597.58): C 64.3, H 5.7, N 9.4 %; found: C 64.6, H
5.3, N 9.1 %. FTIR (KBr pellet, cm-1): 2,953 m, 1,627 s,
1,575 m, 1,474 w, 1,342 s, 1,020 w, 749 s, 625 w.
In the IR spectra of the complexes, the main features are
assigned to the carboxylates, water molecules and N-con-
taining ligands. A broad band centered at 3,327 cm-1 for
complex 1 may provide some evidence for the participation
of water molecules in strong hydrogen bonding. Charac-
teristic bands at 1,548 cm-1 for 1, and 1,575 cm-1 for 2
are assigned to the mC=N stretching of the benzimidazole
ring. There are no strong absorption peaks around
1,700 cm-1 for –COOH, demonstrating that all carboxyl
groups of the organic moieties are deprotonated in both
complexes. Strong characteristic bands at 1,616 and
1,457 cm-1 for complex 1 and at 1,627 and 1,474 cm-1 for
complex 2 are attributed to the asymmetric and symmetric
vibrations of the carboxyl groups, respectively. The values
X-ray crystallography
Crystallographic data for complexes 1 and 2 were collected
on a Bruker Smart 1000 CCD diffractometer with graphite-
˚
monochromated Mo-Ka radiation (k = 0.71073 A) at
293 K. Semi-empirical absorption corrections were applied
of Dt[tas(COO)–ts(COO)] are 149 and 153 cm-1
,
123

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Transition Met Chem (2014) 39:653–660
˚
2.043(12)–2.236(13) A, and the Co–N bond distances are
˚
2.148(16) and 2.205(16) A, respectively. The coordination
angles range from 59.98(5) to 176.82(6)°, which are
comparable to values observed for similar cobalt(II) com-
plexes [19].
In complex 1, the L1 ligand adopts trans-conformation
and bridges adjacent Co(II) centers to assemble into infinite
1D [Co(L1)]n cation chains. The adjacent 1D chains are
cross-connected by neighboring tbi2- ligands in mono-
dentate and chelating coordination fashions to generate a
2D (4,4) grid structure, in which each 38-member paral-
lelogram consists of four Co atoms at the corners con-
nected by two L1 ligands and two tbi2- ligands, generating
a [Co4(L1)2(tbi)2] unit for L1 and tbi2- bridging with the
˚
Co_Co distances of 13.988(9) and 10.156(9) A, respec-
tively (Fig. 1b). In addition, the O–HꢀꢀꢀO hydrogen bonds
˚
(O1 W–H1WAꢀꢀꢀO3B: 2.744(2) A, 170°, symmetry codes:
C = 1 - x, 2 - y, 2 - z) between the water ligands and
carboxylate oxygen atoms and p–p stacking interactions
between the imidazole rings from distinct L1 ligands (the
˚
center-to-center separations of 3.820 A and slipping angles
of b (c) of 28.05°; Cg1: N3–C10–N4–C17–C11, symmetry
codes: D = 1 - x, 2 - y, 1 - z) further enhance the sta-
bility of the final framework.
Complex 2 crystallizes in the orthorhombic space group
Pbca and exhibits a 2D coordination network structure.
The asymmetric unit of 2 consists of one Co(II) center, one
L1 ligand and one tbi2- ligand. As shown in Fig. 2a, the
Co atom is five-coordinated by three oxygen atoms (O1E,
O2E, O4) (symmetry code: E = 0.5 - x, 0.5 ? y, z) from
different carboxylic groups of tbi2- ligands and two
nitrogen atoms (N1, N3) from two distinct L2 ligands,
furnishing a distorted square-pyramidal geometry, with a s
index of 0.42 [27]. The Co–O bond lengths vary from
Fig. 1 a Coordination environment of the Co atom in 1: Hydrogen
atoms were omitted for clarity, and symmetry transformation was
used to generating equivalent atoms: A ¼ ꢁ1 þ x; y; z; B ¼
1 þ x; y; ꢁ1 þ z. b 2D (4,4) supramolecular structure in 1
respectively, indicating bridging coordination of the car-
boxylate group to the metal centers [26]. The slightly dif-
ferent separations may suggest the coexistence of mixed
binding modes of the carboxylate groups in the anionic
tbi2- ligands, which is also in agreement with the single-
crystal X-ray diffraction results.
˚
1.955(14) to 1.968(15) A, and the Co–N bond lengths are
˚
2.027(17) and 2.046(17) A, respectively. The bond angles
at the Co center are in the range of 96.04(6)–121.16(7)°.
In complex 2, each L2 ligand adopts a twisted cis-con-
formation with a Ndonor_N–Csp3_Csp3 torsion angle of
163.62°, and the two benzimidazole arms bend in opposite
directions to link two neighboring Co atoms, giving rise to
a 1D [Co(L2)]n zigzag chain with a Co_Co distance of
Structural analysis of the complexes
The structural analysis reveals that complex 1 crystallizes
in the triclinic space group P¯ı. The asymmetric unit con-
tains one Co(II) center, one L1 ligand, one anionic tbi2-
ligand and one coordinated water ligand. As shown in
Fig. 1a, the cobalt atom is six-coordinated by two N atoms
from different L1 ligands, three O atoms from two tbi2-
ligands and one water ligand to give an octahedral geom-
etry. The best equatorial plane is formed by one chelating
tbi2- carboxylate oxygen atom (O3 and O4), one bis-
monodentate tbi2- carboxylate oxygen atom (O2A) (sym-
metry code: A = -1 ? x, y, z) and the oxygen atom from
the aqua ligand (O1 W). The apical positions are occupied
by two nitrogen atoms (N1 and N3) from different L1
ligands. The Co–O bond distances vary in the range of
˚
9.185(6) A (Fig. 2b). Furthermore, the carboxylic groups
act in both monodentate and chelating coordination modes
to connect two neighboring 1D zigzag chains, resulting in a
2D (4,4) network with a through-ligand Co–Co distance of
˚
10.301(9) A (Fig. 2c, d). The corrugated 2D network
consists of one type of parallelograms, where the dimen-
˚
sions are 9.19 9 10.30 A defined by Co_Co separations.
Similar to the present work, some related complexes, namely
{[Co2(L3)2(tbi)2]ꢀ2H2O}n (3) [Co2(L4)(tbi)2(H2tbi)(bpp)(H2-
O)]n (4) and [Co(L5)(tbi)(H2O)]n (5) (L3 = 1,4-bis(imidaz-
ole)butane,
L4 = 1,3-bis(4-pyridyl)propane,
L5 = 4,
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Transition Met Chem (2014) 39:653–660
657
Fig. 2 a Coordination environment of the Co atom in 2: Hydrogen
atoms were omitted for clarity, and symmetry transformation was
used to generating equivalent atoms: E ¼ 0:5 ꢁ x; 0:5 þ y; z; F ¼
ꢁ0:5 þ x; 0:5 ꢁ y; ꢁz. b 1D zigzag chain connected by Co atoms and
L2 ligands in 2. c 2D (4,4) supramolecular structure in 2. d The
uninodal 4-connected 2D net with a point symbol of {44.62} topology
network in 2
40-dipyridylamine) have been described previously [28–30]. In
the present complexes 1 and 2, the tbi2- anions adopt the same
coordination modes, involving one chelating and one bridging
bidentate carboxylate groups. However, the two complexes
show different coordination geometries (octahedral for 1, dis-
torted square pyramidal for 2). The Co(II) centers are bridged
by both tbi2- and flexible N-donor ligands to generate a 2D grid
layer network for complex 1. When the H substituent is
replaced by –CH3, as in complex 2, the result is a 2D corrugated
layer structure. It is possible that the additional Me-group in 2
sterically prevents water from coordinating to the metal atom
and the existence of hydrogen bonds or p–p stacking interac-
tions in 1 may also have an effect on the final crystal structure.
In all of the complexes 1–5, the flexible N-containing
ligands uniformly behave as bis-monodentate linkers to
connect the Co centers, but exhibit different bending and
rotating ability, which leads to different non-bonding
Co_Co distances and distinct coordination geometries. As
shown in Table 3, complex 1 features a 2D grid layer,
while 2 displays a 2D corrugated layer, and 3, 4, 5 have a
2D helical layer, 2D bilayer, 2D twofold parallel layer,
respectively. These results suggest that the different
structures of 1–5 mainly stem from the different N-donor
ligands used in these complexes.
Thermal behaviors and XRPD analysis
To characterize the present complexes more fully in terms
of thermal stability, their thermal behaviors were examined
by TGA (Fig. 3). For complex 1, two weight loss steps
were observed. The first weight loss of 2.7 % can be
attributed to the release of coordination water ligands in the
temperature range of 112–139 °C (calcd. 3.1 %). A sharp
weight loss is observed in the range 366–560 °C, corre-
sponding to the decomposition of the organic ligands to
leave a residue of CoO (calcd. 12.8 %; found. 13.7 %). In
the range 420–590 °C, a weight loss of 88.3 % (calcd.
87.5 %) corresponds to decomposition of the organic
components, leaving a CoO residue of 11.6 % (calcd.
12.5 %) for complex 2.
To confirm the phase purity of both complexes, the XRPD
patterns were obtained at room temperature and they are
comparable to the corresponding simulated patterns calcu-
lated from the single-crystal X-ray diffraction data (Fig. 4).
123

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Transition Met Chem (2014) 39:653–660
Table 3 Comparison of the related complexes of complex 1 and 2
The related complexes
Coordination
number
Conformation Crystal structure
Co_Co separation Geometry
˚
(A)
Reference
[Co(L1)(tbi)(H2O)]n (1)
[Co(L2)(tbi)]n (2)
6
5
4
6
6
Trans
Cis
2D grid layer
13.988(9) and
10.156(9)
octahedral
This work
This work
2D corrugated
layer
9.185(6) and
10.301(9)
distorted square
pyramidal
{[Co2(L3)2(tbi)2]ꢀ2H2O}n (3)
Cis
2D helical layer
9.332(7) and
13.696(3)
distorted tetrahedral 28
octahedral 29
distorted octahedral 30
[Co2(L4)(tbi)2(H2tbi)(H2O)]n
(4)
Cis
2D bilayer
3.534(6) and
3.477(9)
[Co(L5)(tbi)(H2O)]n (5)
Cis
2D twofold
parallel layer
11.833(11) and
11.868(13)
Where L1 = 1,4-bis(benzimidazole)butane, L2 = 1,4-bis(2-methylbenzimidazole)butane, L3 = 1,4-bis(imidazole)butane, L4 = 1,3-bis(4-pyr-
idyl)propane, L5 = 4,40-dipyridylamine, H2tbi = 5-tert-butylisophthalic acid
Fig. 3 TG curves of compounds 1 and 2
Photoluminescence properties
The solid-state photoluminescence properties of complexes
1 and 2 and free ligands were investigated at room tem-
perature. As depicted in Fig. 5, the free ligands L1 and L2
exhibit emission peaks at 399 and 375 nm under excitation
at 335 nm, which can probably be assigned to the p ? p*
transitions [31]. Both complexes exhibit fluorescent emis-
sion under excitation at 250 nm with maxima at 356 nm for
1 and 361 nm for 2, respectively. For both complexes, a
blue-shifted (43 nm for 1, 14 nm for 2) emission band is
observed compared with the free L1 and L2 ligands.
Therefore, the fluorescence behaviors of both complexes can
be tentatively ascribed to ligand-to-metal charge transfer
[32]. However, the emission bands of the carboxylate
Fig. 4 a X-ray powder diffraction patterns of 1. b X-ray powder
diffraction patterns of 2
ligands originating from n - p* transitions are weak, and it
123

Transition Met Chem (2014) 39:653–660
659
Fig. 6 Experimental results of the catalytic degradation of Congo red
CoIIꢁMOF þ H2O2 ! CoIIIꢁMOF þꢀ OH þ OHꢁ
CoIIIꢁMOF þ H2O2 ! HO2ꢀ þ CoIIꢁMOF þ Hþ
azo dye þꢀ OH ! oxidation products
ð1Þ
ð2Þ
ð3Þ
On comparing to the degradation efficiencies of the four
different treatment processes, it is obvious that free Co2?
ions have some catalytic effect, but with a lower efficiency
than both complexes 1 and 2. The higher activity of
complex 1 compared to complex 2 may arise from the
presence of the water ligand in the former, which provides
a labile substitution site for the production of hydroxyl
radicals.
Fig. 5 a Solid-state photoluminescence spectra of free L1 ligand and
1. b Solid-state photoluminescence spectra of free L2 ligand and 2
Conclusion
is considered that the carboxylate ligands have no significant
contribution to the fluorescence emission of complexes 1
and 2 in the presence of the N-donor ligand [33].
In summary, two new Co(II) metal–organic frameworks
with tbi2- ligands and two flexible bis(benzimidazole)
ligands have been synthesized and characterized. The
subtle differences between complexes 1 and 2 in terms of
structure and catalytic properties may result from the
flexible N-bridging ligands bearing different substituent
groups. Both complexes, especially 1, show promising
catalytic activities in the decomposition of Congo red dye.
Catalytic properties
The catalytic degradation experiments were carried out
according to the literature [34], and the results for decol-
orization of Congo red dye by using four different treat-
ment processes are shown in Fig. 6. In the control
experiment, when hydrogen peroxide was added to a
solution of Congo red, there was no evident change for
reaction times up to 130 min, showing that H2O2 alone was
unable to decompose the azo dye effectively. In the pre-
sence of H2O2 and CoCl2, a degradation efficiency of 56 %
was observed within 130 min. When complex 1 or 2 was
introduced into the system, about 85 % or 72 % of the dye
was decolored, respectively, after 130 min. A possible
degradation mechanism can be suggested as follows [35];
Supplementary materials
CCDC 993150 and 993151 contain the supplementary
crystallographic data for the complexes 1 and 2. These data
can be obtained free of charge via http://www.ccdc.cam.ac.
uk/conts/retrieving.html, or from the Cambridge Crystal-
lographic Data Centre, 12 Union Road, Cambridge CB2
1EZ, UK; fax: (?44) 1223-336-033; or e-mail: deposit@
ccdc.cam.ac.uk.
123

660
Transition Met Chem (2014) 39:653–660
18. Zhang JW, Gong CH, Hou LL, Tian AX, Wang XL (2013) J
Solid State Chem 205:104–109
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