Exploring the effect of chain length of bridging ligands in cobalt(II) coordination polymers based on flexible bis(5,6-dimethylbenzimidazole) ligands: Synthesis, crystal structures, fluorescence and catalytic properties

Article abstract of DOI:10.1016/j.molstruc.2013.08.013

Two Co(II) coordination polymers derived from a dicarboxylate and two flexible bis(5,6-dimethylbenzimidazole) ligands with varying chain lengths equipped, namely [Co(bdmbmm)(nip)]_n (1) and [Co₂(bdmbmb) ₂(nip)₂×H₂O]_n (2) (bdmbmm = 1,1′-bis(5,6-dimethylbenzimidazole)methane, H₂nip = 5-nitroisophthalic acid, bdmbmb = 1,4-bis(5,6-dimethylbenzimidazole)butane), have been synthesized by hydrothermal methods and characterized by elemental analyses, IR spectra, thermogravimetric analysis (TGA), X-ray powder diffraction (XRPD) and single-crystal X-ray diffraction. Complex 1 forms a 1D looped-like chain consisting of two kinds of macrocycles, which is further arranged into a 2D supramolecular layer through face-to-face π-π stacking interactions; whereas complex 2 exhibits a 3D framework with a twofold interpenetrating diamondoid topology. The fluorescence and catalytic properties of the complexes for the degradation of methyl orange by sodium persulfate have been investigated.

Full text of DOI:10.1016/j.molstruc.2013.08.013

Journal of Molecular Structure 1051 (2013) 215–220
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Exploring the effect of chain length of bridging ligands in cobalt(II)
coordination polymers based on flexible bis(5,6-dimethylbenzimidazole)
ligands: Synthesis, crystal structures, fluorescence and catalytic
properties
⇑
Li Qin, Yue-Hua Li, Pei-Juan Ma, Guang-Hua Cui
College of Chemical Engineering, Hebei United University, 46 West Xinhua Road, Tangshan 063009, Hebei, People’s Republic of China
g r a p h i c a l a b s t r a c t
h i g h l i g h t s
Two new Co(II) metal–organic coordination polymers have been hydrothermally synthesized using flex-
ible bis(5,6-dimethylbenzimidazole) ligands with varying chain lengths with 5-nitroisophthalic auxiliary
ligand. 1 features a 1D looped-like chain consisting of two kinds of macrocycles alternately, and 2 pos-
sesses a 4-connected twofold interpenetrating diamond network structure. Moreover, the two com-
pounds as a heterogeneous catalysts present high efficiency for the degradation of methyl orange dye
in Fenton-like process.
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
Two CoII coordination polymers have
been characterized.
The influences of chain lengths on
framework were discussed.
TG analysis confirms the stability of
the coordination polymers.
Luminescence properties of
complexes have been investigated.
Complexes show higher activity for
degradation of methyl orange.
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 28 May 2013
Received in revised form 25 July 2013
Accepted 9 August 2013
Available online 14 August 2013
Two Co(II) coordination polymers derived from a dicarboxylate and two flexible bis(5,6-dimethylbenzim-
idazole) ligands with varying chain lengths equipped, namely [Co(bdmbmm)(nip)]n (1) and
[Co2(bdmbmb)2(nip)2ꢁH2O]n (2) (bdmbmm = 1,10-bis(5,6-dimethylbenzimidazole)methane, H2nip = 5-
nitroisophthalic acid, bdmbmb = 1,4-bis(5,6-dimethylbenzimidazole)butane), have been synthesized by
hydrothermal methods and characterized by elemental analyses, IR spectra, thermogravimetric analysis
(TGA), X-ray powder diffraction (XRPD) and single-crystal X-ray diffraction. Complex 1 forms a 1D
looped-like chain consisting of two kinds of macrocycles, which is further arranged into a 2D supramo-
Keywords:
Catalytic property
Cobalt(II) coordination polymers
lecular layer through face-to-face p–p stacking interactions; whereas complex 2 exhibits a 3D framework
Abbreviations: bdmbmm, 1,10-bis(5,6-dimethylbenzimidazole)methane; bdmbmb, 1,4-bis(5,6-dimethylbenzimidazole)butane; L, 1,3-bis(5,6-dimethylbenzimida-
zole)propane; H2nip, 5-nitroisophthalic acid; TGA, thermogravimetric analysis; XRPD, X-ray powder diffraction; MOFs, metal–organic frameworks; AOPs, advance oxidation
processes.
⇑
Corresponding author. Tel.: +86 0315 2592169; fax: +86 0315 2592170.
E-mail address: tscghua@126.com (G.-H. Cui).
0022-2860/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molstruc.2013.08.013

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L. Qin et al. / Journal of Molecular Structure 1051 (2013) 215–220
Crystal structure
Flexible bis(5,6-dimethylbenzimidazole)
Fluorescence property
with a twofold interpenetrating diamondoid topology. The fluorescence and catalytic properties of the
complexes for the degradation of methyl orange by sodium persulfate have been investigated.
Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction
As our ongoing studies, herein we reported the synthesis and crys-
tal structures of two new coordination polymers [Co(bdmbmm)(nip)]n
Rational design and construction of cobalt(II) metal–organic
coordination polymers has been greatly developed, not only be-
cause of their structural diversities, but also owing to their poten-
tial applications in gas storage, catalysis, fluorescence, magnetism
materials [1–4]. Although coordination bonds still remain at the
forefront of crystal engineering strategies, other interactions have
received increasing attention over recent years, particularly halo-
(1) and [Co2(bdmbmb)2(nip)2ꢁH2O]n (2). Both compounds used as het-
erogeneous catalysts for the oxidative degradation of methyl orange
by persulfate as oxidant have also been investigated in detail.
2. Experimental
gen bonds and hydrogen bonding, p–p interactions [5–7]. Coordi-
2.1. Materials and measurements
nation crystalline architectures are intensely influenced by many
factors, such as the coordination geometry of metal center, auxil-
iary ligand, solvent, template, pH value and reaction temperature.
Furthermore, the flexibility of the organic ligand also plays a cru-
cial role in determining the final structure of MOFs [8–10]. There-
fore, significant interest has arisen in the structural tuning of MOFs
through rational selection of bridging ligands. Aromatic polycar-
boxylate ligands as a kind of O-donor bridging ligands have been
extensively used to construct novel metal–organic coordination
polymers owing to their remarkable coordination ability and sta-
bility [11,12]. Especially, 5-nitroisophthalate with the bending an-
gle of ca. 120° between two carboxylate groups may result in
extended multi dimensional frameworks [13–15]. In addition, the
bis(benzimidazole) ligands bearing alkyl spacers are good choices
of N-donor ligand, which can flexibly affect its conformation by
means of the –(CH2)n– group to meet the requirements of the me-
tal atoms in the assembly process. Among the series of benzimid-
azole derivatives, the most prominent compound is 5,6-
dimethylbenzimidazole, which serves as an axial ligand for cobalt
in vitamin B12 [16–18]. Bis(5,6-dimethylbenzimidazole) as a bridg-
ing ligand participating in construction of coordination polymers
can provide more information for us to investigate the influences
of methyl substituted derivative of the benzimidazole on the struc-
tures and properties of resulting complexes. Moreover, to the best
of our knowledge, such metal coordination polymers constructed
from bis(5,6-dimethylbenzimidazole) with carboxylate ligands
have only been scarcely reported [19,20]. Therefore, the introduc-
tion of flexible bis(5,6-dimethylbenzimidazole) ligands into Co(II)-
carboxylates system may be an effective synthetic strategy to con-
struct new functional materials.
All the solvents and reagents for synthesis were obtained from
commercial sources and used without further purification. The li-
gands bdmbmm and bdmbmb were prepared according to litera-
ture procedures (see Chart 1) [26]. C, H, and
N elemental
analyses were performed on a Perkin–Elmer 240C analyzer. FT-IR
spectra were recorded on an Avatar 360 (Nicolet) spectrophotom-
eter between 400 and 4000 cmꢂ1, using the KBr pellet method.
Thermogravimetric analysis (TGA) was conducted on a Netzsch
TG 209 thermal analyzer from room temperature to 800 °C under
N2 with a heating rate of 10 °C/min. The fluorescence spectra were
collected with a Hitachi F-7000 spectrophotometer at room tem-
perature. The X-ray powder diffraction (XRPD) patterns were re-
corded on a Rigaku D/Max-2500 diffractometer at 40 kV, 100 mA
for a Cu-target tube and a graphite monochromator. The concen-
tration of methyl orange solution was measured with TU-1901
UV–vis spectrophotometer.
2.2. Preparation of the complexes 1 and 2
2.2.1. [Co(bdmbmm)(nip)]n (1)
A
mixture of Co(CH3COO)2ꢁ4H2O (0.1 mmol, 24.9 mg),
bdmbmm (0.1 mmol, 30.4 mg), H2nip (0.1 mmol, 21.1 mg) and
H2O (16 mL) was placed in a Teflon-lined stainless steel vessel
(25 mL), then the pH value was adjusted to 6.5 by NaOH
(0.1 mol/L). The mixture was sealed and heated at 160 °C for
3 days; then, the reaction system was cooled to room temperature
at 5 °C/h. Purple block crystals were collected in 20% yield (based
on Co). Anal. Calcd. for C27H23CoN5O6 (Mr = 572.43): C 56.65, H
4.05, N 12.23%. Found: C 56.62, H 4.02, N 12.25%. IR (KBr, cmꢂ1):
Azo dyes represent more than 50% of all dyes in common use
because of their chemical stability and versatility [21]. Most of
them are non-biodegradable, toxic and potentially carcinogenic
in nature and at present are abated by some non-destructive pro-
cesses, such as coagulation, activated carbon adsorption and mem-
brane filtration. Advance oxidation processes (AOPs) are
increasingly used as for the reduction of organic contaminants in
a variety of wastewaters from different industrial plants [22,23].
m
= 3098(w), 3010(w), 1625(s), 1532(m), 1390(m), 1344(m),
1219(w), 1193(w), 733(w), 677(w).
N
N
Persulfate (S2O2ꢂ) is one of the strongest oxidants known in aque-
8
w
ous solutions and has the higher potential (E = 2.01 V) than H2O2
(E = 1.76 V). It offers some advantages over other oxidants as a so-
N
N
w
bdmbmm
lid chemical at ambient temperature with the ease of storage and
transport, high stability, high aqueous solubility and relatively
low cost [24,25]. Among several AOPs, heterogeneous oxidation
is presented as an effective procedure for removing stable organic
compounds including dye molecules. Transition metal complexes
supported on metal–organic framework are used as potentially ac-
N
N
N
N
tive catalysts for the decomposition of S2O2ꢂ and the oxidative
bdmbmb
8
degradation of organic contaminants and dye.
Chart 1. The ligands bdmbmm and bdmbmb.

L. Qin et al. / Journal of Molecular Structure 1051 (2013) 215–220
217
Table 1
2.2.2. [Co2(bdmbmb)2(nip)2ꢁH2O]n (2)
Crystal and refinement data for complexes 1 and 2.
The synthetic procedure of 2 was similar to the synthesis of 1,
except that bdmbmb (0.1 mmol, 34.6 mg) was used instead of
bdmbmm. The purple block crystals of 2 (yield: 35%) were ob-
tained. Anal. Calcd. for C60H60Co2N10O13 (Mr = 1247.06): C 57.79,
H 4.85, N 11.23%. Found: C 57.76, H 4.88, N 11.25%. IR (KBr,
1
2
Empirical formula
Formula weight
Crystal system
Space group
a (Å)
C
27H23CoN5O6
C60H60Co2N10O13
1247.06
Monoclinic
C2/c
16.1902(18)
23.799(3)
16.0948(18)
572.43
Triclinic
P1
ꢀ
cmꢂ1):
m = 3407(vs), 2938(w), 1620(s), 1561(m), 1383(w),
7.1538(8)
12.2333(13)
15.2989(17)
69.8510(10)
83.0780(10)
88.825(2)
1247.5(2)
2
1348(w), 1154(w), 1084(w), 733(w), 623(w).
b (Å)
c (Å)
a
(°)
2.3. Catalytic experimental procedures
b (°)
112.444(2)
c
(°)
The catalytic experiments were performed in 250-mL round-
bottom flask. Reaction solutions were obtained by adding the solid
catalysts 1 (or 2) into aqueous methyl orange solution and the ini-
tial pH of the reaction solution was adjusted with H2SO4 (0.1 mol/
L) solution reaching to 3. Then the reaction was initiated by adding
sodium persulfate (0.2 g/L) into the prepared reaction solutions
with a total volume of 150 mL. At the given time intervals, the
samples were filtered through a 0.45 lm membrane filter before
measurement. All experiments were carried out at a temperature
of 30 °C.
The efficiency of the proposed process was evaluated by moni-
toring methyl orange degradation by measuring absorbance at
506 nm using a double beam UV/vis spectrophotometer. Therefore,
the concentration of the methyl orange in the reaction mixture at
different reaction times was determined by measuring the absorp-
tion intensity at kmax = 506 nm and from a calibration curve. Prior
to the measurement, a calibration curve was obtained by using
the standard methyl orange solution with known concentrations.
Because the reaction continued after sampling, the measurement
of absorbance of reaction solution was done within 1 min.
The degradation efficiency of methyl orange was defined as fol-
lows: [27]
V (Å3)
5731.7(11)
4
1.424
0.651
2552
Z
Dcalc (g/m3)
1.524
0.742
590
l
(mmꢂ1
)
F(000)
Crystal size (mm)
Total reflections
Unique reflections
Rint
GOF
R1 (I > 2
0.15 ꢃ 0.12 ꢃ 0.11
0.22 ꢃ 0.16 ꢃ 0.11
6400
14,602
4346
0.0246
1.074
0.0438
0.0985
0.295
ꢂ0.292
[wF2o ꢂ F2c2]/
5051
0.0690
0.971
0.0638
0.1492
0.906
ꢂ0.469
r
(I))
(I))
wR2 (I > 2
r
D
D
q
q
max (eÅꢂ3
min (eÅꢂ3
)
)
[wF2o2
]
.
1/2
R1
=
R
||Fo|–|Fc||/
R
|Fo|; wR2
=
R
R
tetrahedral geometry {CoN2O2}. The Co–N bond distances are
2.027(3) and 2.062(3) Å, and the Co–O bond lengths are 1.965(2)
and 1.978(2) Å, both of which are in the normal range [31,32].
1 Displays a looped-like chain containing two kinds of macrocy-
cles alternately (Fig. 1b). Two 5-nitroisophthalates adopting bis
(monodentate) coordination mode bridge adjacent Co atoms to
form a 16-membered macrocycle [Co2(nip)2] with the Co1ꢁ ꢁ ꢁCo1#1
distance of 6.5179(9) Å. Another kind of macrocycle [Co2(-
bdmbmm)2]4+ is constructed by two bdmbmm ligands, showing
bis-monodentate mode coordinating to the center atoms with the
Co1ꢁ ꢁ ꢁCo1#2 separation of 8.3793(9) Å (symmetry code #1:
ꢂx + 1, ꢂy + 1, ꢂz; #2: ꢂx + 2, ꢂy, ꢂz). Each bdmbmm ligand in 1
adpots cis-configuration and the dihedral angle between the
mean-planes of the two benzimidazole rings is 67.577°. Further-
more, the adjacent 1D layers are extended into a 2D supramolecular
ðC0 ꢂ CtÞ
Degradation efficiency ¼
ꢃ 100%
ð1Þ
C0
where C0 (mg/L) is the initial concentration of methyl orange, and Ct
(mg/L) is the concentration of methyl orange at reaction time, t
(min).
framework by the
p–p stacking interactions with the center-to-
2.4. X-ray crystallography
center separations of 3.504 Å and 3.6045 Å between neighboring
imidazole rings and benzene rings, and neighboring benzene rings
and benzene rings from benzimidazoles, respectively (Fig. 1c). Be-
sides that, weak intermolecular hydrogen bonding interactions also
exist with a H(11)ꢁ ꢁ ꢁO(2) distance of 2.25 Å and the angle of 150°
[C(11)–H(11)ꢁ ꢁ ꢁO(2)] that further stabilize the crystal packing.
Crystallographic data for 1 and 2 were collected on a Bruker
Smart 1000 CCD diffractometer using graphite-monochromated
Mo Ka radiation (k = 0.71073 Å) at room temperature with x-scan
mode. A semi-empirical absorption correction was applied using
SADABS program [28]. The structures were solved by direct meth-
ods and refined on F2 by full-matrix least-squares using SHELXTL-
97 [29]. All non-hydrogen atoms were refined anisotropically. In
the structure of 2, the one water molecules is disordered. This
structure was refined by the SQUEEZE routine of the PLATON pro-
gram [30]. The crystallographic data for 1 and 2 are listed in Table 1,
and selected lengths and angles parameters for 1 and 2 are pre-
sented in Table S1, respectively.
3.2. Crystal structures of 2
As compared to complex 1, the bdmbmm coligand is replaced by
bdmbmb, and the resulting structure is a twofold interpenetrating
3D diamond framework. Single-crystal X-ray diffraction analysis re-
veals that 2 crystallizes in the monoclinic space group C2/c, and the
asymmetric unit contains one Co(II) atom, one bdmbmb ligand, one
nip dianion and one guest water molecule. The coordination envi-
ronment around Co(II) is shown in Fig. 2a. Each Co(II) atom sits in
a distorted tetrahedron geometry defined by two nitrogen atoms
[Co1–N3 = 2.011(4) Å and Co1–N1 = 2.033(4) Å] of two bdmbmb li-
gands and two oxygen atoms [Co1–O4#1 = 1.954(4) Å and Co1–
O2 = 1.964(3) Å] (symmetry code #1: x + 1/2, ꢂy + 1/2, z + 1/2) from
two separated nip2ꢂ anions. The Co–O/N bond lengths are all consis-
tent with corresponding bond lengths found in the literature [33].
The two carboxylic groups of nip2ꢂ take a uniform coordination
3. Results and discussion
3.1. Crystal structures of 1
ꢀ
Compound 1 crystallizes in the monoclinic space group P1. The
results of crystallographic analysis reveal that the asymmetric unit
of 1 consists of one Co(II) atom, one bdmbmm ligand and one nip2ꢂ
ligand. As shown in Fig. 1a, each Co(II) atom is four-coordinated by
two carboxylate oxygen atoms from two different nip2ꢂ anions and
two nitrogen atoms from two bdmbmm ligands, showing a
mode, all in a l2
ꢂ
g1 1 fashion. Co(II) atoms are connected by nip2ꢂ
:g

218
L. Qin et al. / Journal of Molecular Structure 1051 (2013) 215–220
Fig. 1. (a) The coordination environment of Co(II) atom in complex 1 with 30%
thermal ellipsoids. Hydrogen atoms and the free water molecule are omitted for
clarity. (b) The 1D looped-like chain for 1. (c) 2D supramolecular networks by
p–p
stacking interactions (symmetry code #1: ꢂx + 1, ꢂy + 1, ꢂz; #2: ꢂx + 2, ꢂy, ꢂz).
anions to form a 1D chain. Furthermore, the bdmbmb ligands cross-
linking these 1D chains along thetwo directions, finallyresulting in a
3D framework (Fig. 2b). Analysis of network topology provides a
convenient tool in designing and understanding the complicated
crystal structures of coordination polymers. Such structures can
usually be reduced to simple topological networks with different
connectivity of the components. Thus, the 3D structure of 2 can be
reduced as a classical diamond net (dia) by TOPOS 4.0 program
[34]. Each Co center bridges four other Co atoms, viewed as 4-con-
nected nodes in tetrahedral sphere and nip2ꢂ and bdmbmb ligands
as linkers (Fig. 2c). Notably, the large voids formed by a single 3D
network allow incorporation of the other identical network. There-
fore, the overall structure of 2 has two identical 3D nets, which inter-
penetrate each other to generate a twofold interpenetrating dia
architecture belonging to class IIa (Fig. 2d).
3.3. Effect of the spacer length of flexible bis(5,6-
dimethylbenzimidazole) ligands
Fig. 2. (a) The coordination environment of Co(II) atom in complex 2 with 30%
thermal ellipsoids. Hydrogen atoms and the free water molecule are omitted for
clarity (symmetry code #1: x + 1/2, ꢂy + 1/2, z + 1/2). (b) The 3D framework of 2. (c)
Schematic illustrating the 3D diamond topology in 2. (d) The twofold interpene-
trating diamond architecture of 2.
It is well known that the spacer length of the flexible organic li-
gands play an important role in determining their final structures

L. Qin et al. / Journal of Molecular Structure 1051 (2013) 215–220
219
of the complexes. Compound 3, named [Co(L)(nip)]n (L = 1,3-
bis(5,6-dimethylbenzimidazole)propane) has been previously
synthesized and its structure published [31]. By varying the –
(CH2)n– length (n = 1, 4, 3) of flexible bis(5,6-dimethylbenzimida-
zole) ligands, the three related Co(II) compounds with different
structures (1D, 3D and 2D) were successfully obtained under the
similar synthetic conditions. In complexes 1–3, all of these three
percentage (13.09%) of Co and O components in CoO, indicating
that is the final product. For 2, the first weight loss corresponding
to the release of lattice water molecules is observed in the temper-
ature range of 65 to 102 °C (obsd 1.69%, calcd 1.44%), and the
departure of organic components occurs from 314 °C.
To check the final products of thermal analysis, XRPD was car-
ried out at room temperature (Fig. S3). The experimental XRPD pat-
terns perfectly match the simulated patterns based on the single
crystal X-ray data with CoO.
flexible N-containing ligands present a
l2-bridging coordination
mode, but –(CH2)n– (n = 1, 4, 3) spacers exhibit different bending
and rotating ability, which lead to generating different non-bond-
ing Coꢁ ꢁ ꢁCo distances (8.38 Å for 1, 10.62 Å for 2, 9.83 Å for 3) and
the different dihedral angles (67.58° in 1, 65.63° in 2, 73.89° in 3)
between two benzimidazole rings with the entire bis(5,6-dimeth-
ylbenzimidazole) ligand. Although the three Co(II) complexes con-
3.6. Fluorescence properties
Coordination frameworks with metal centers and N-donor li-
gands linkers are promising candidates for photoactive materials
with potential applications in chemical sensors, photochemistry
and electroluminescent display [37,38]. Thus, the solid state fluo-
rescence properties of Co(II) compounds 1 and 2, together with
the free bdmbmm and bdmbmb ligands were investigated at room
temperature (Fig. 3). The free ligands bdmbmm and bdmbmb exhi-
bit the emission peaks at 318 nm and 370 nm with the excitations
at 313 nm and 270 nm, respectively, which can probably be as-
tain the same dicarboxylate (nip2ꢂ
)
secondary ligand, the
g1 mode
in compounds 1 and 2, a l3 :
g1 g2 mode in compound 3). Thus,
coordination mode of nip2ꢂ anions is different (a l2
ꢂ
g1
:
ꢂ
Co(II) ions are bridged by nip2ꢂ ligands and different flexible
bis(5,6-dimethylbenzimidazole) ligands to generate 1D looped-like
chain for 1, 3D twofold interpenetrating diamond architecture for
2, 2D (4,4) layer for 3. The 2D supermolecular framework for com-
signed to the
p ?
p⁄ transitions [39]. The fluorescence peaks at
pound 1 is formed by
tence of aromatic rings from organic ligands. And hydrogen
bonds or stacking interactions in 2 and 3 are to further stabi-
p–p stacking interactions due to the exis-
about 418 nm for 1 and 423 nm for 2 are found in the emission
spectra when they are excited at 274 nm and 325 nm, respectively.
As previously reported, the solid aromatic carboxylate ligand H2nip
are nearly nonfluorescent in the range 400–600 nm at ambient
p–p
lize the crystal structures.
3.4. IR spectra
The main features of IR spectra for complexes 1 and 2 con-
cerned the carboxylate groups of nip2ꢂ ligand and the bdmbmm
or bdmbmb ligand. No strong absorption peaks around
1700 cmꢂ1 for –COOH are observed, which implies that carboxyl
groups of organic moieties in the title complexes are completely
deprotonated [35,36]. The strong peaks at 1625 and 1390 cmꢂ1
for 1, and 1620 and 1383 cmꢂ1 for 2 may be attributed to the
asymmetric and symmetric vibrations of carboxylate groups. The
presence of the expected characteristic bands at 1532, 1193 and
733 cmꢂ1 for 1, and 1561, 1084 and 733 cmꢂ1 for 2, suggest the
mC–N stretching of the benzimidazole ring of the bdmbmm or
bdmbmb ligand. For 2, it shows strong and broad peak around
3407 cmꢂ1 relating to the O–H stretching vibration modes of water
molecules.
3.5. X-ray Powder diffraction (XRPD) and thermogravimetric analysis
(TGA)
To confirm the phase purity of compounds 1 and 2, the as-syn-
thesized samples were characterized by XRPD at room tempera-
ture, as shown in Fig. S1. Although the experimental patterns
have a few un-indexed diffraction lines and some are slightly
broadened in comparison with those simulated from the single-
crystal models (perhaps they come from the impurity attached to
the surface of crystal samples), it still can be well considered that
the bulk synthesized materials and the crystals used for diffraction
are homogeneous.
To characterize the compounds more fully in terms of thermal
stability, the thermal behaviors of 1 and 2 were examined by
TGA (Fig. S2). The experiments were performed on samples con-
sisting of numerous single crystals under an N2 atmosphere with
a heating rate of 10 °C/h. For complex 1, two weight loss steps were
observed. The continuous one from 270 to 330 °C is attributed to
the loss of the nip2ꢂ ligand. The weight loss is about 33.08% in cor-
respondence with the calculated value of 33.74%. The sharp weight
loss is observed in the range 408–663 °C, corresponding to the
decomposition of bdmbmm ligands (53.04%, calculated value of
53.11%). The remaining weight (13.88%) corresponds to the
Fig. 3. (a) Solid-state photoluminescence spectra of complex 1 and free ligand
bdmbmm. (b) Solid-state photoluminescence spectra of complex 2 and free ligand
bdmbmb.

220
L. Qin et al. / Journal of Molecular Structure 1051 (2013) 215–220
Appendix A. Supplementary material
CCDC 917955 and 917953 contain the supplementary crystallo-
graphic data for the complexes 1 and 2. These data can be obtained
free of charge via http://www.ccdc.cam.ac.uk/conts/retriev-
ing.html, or from the Cambridge Crystallographic Data Centre, 12
Union Road, Cambridge CB21EZ, UK; fax: +44 1223 336 033; or
e-mail: deposit@ccdc.cam.ac.uk. Supplementary data associated
with this article can be found, in the online version, at http://
dx.doi.org/10.1016/j.molstruc.2013.08.013.
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S2O2ꢂ þ CoII ꢂ MOF ! CoIII ꢂ MOF þ SO24ꢂ þ SO4Åꢂ
ð2Þ
8
SOÅ4ꢂ þ CoII ꢂ MOF ! CoIII ꢂ MOF þ SO42ꢂ
Dye þ SOÅ4ꢂ ! oxidation products
ð3Þ
ð4Þ
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Two Co(II) coordination polymers based on bis(5,6-dim-
ethylbenzimidazol) and H2nip ligands have been synthesized
hydrothermally. The structural analysis demonstrates that chain
lengths of the bridging ligands can greatly affect the resulting coor-
dination and supramolecular architectures. Both complexes, espe-
cially 2, show high catalytic activities in the Fenton-like
decomposition of methyl orange.