Synthesis, characterization, and crystal structures of 2D cobalt(II) and nickel(II) coordination polymers containing flexible bis(benzimidazole) ligands

Article abstract of DOI:10.1007/s11243-016-0045-4

Three coordination polymers, namely {[Ni(L1)(nip)(H₂O)]·2H₂O}_n (1), [Co(L2)(tbip)]_n (2), and {[Co₂(L3)₂(bptc)]·3H₂O}_n (3) (L1?=?1,4-bis(5,6-dimethylbenzimidazole)butane, L2?=?1,4-bis(5,6-dimethylbenzimidazole)-2-butylene, L3?=?1,3-bis(5,6-dimethylbenzimidazole)propane, H₂nip?=?5-nitro-isophthalic acid, H₂tbip?=?5-tert-butyl-isophthalic acid, H₄bptc?=?biphenyl-3,3′,4,4′-tetracarboxylic acid), have been synthesized under hydrothermal conditions and characterized by physicochemical and spectroscopic methods as well as by single-crystal X-ray diffraction analysis. Complexes 1 and 2 both feature a two-dimensional (4,4) layer with (4⁴?×?6²) topology. Complex 3 possesses a uninodal 4-connected 2D htb network. The fluorescence spectra and catalytic properties of the complexes for the degradation of methyl orange by sodium persulfate in a Fenton-like process are reported.

Full text of DOI:10.1007/s11243-016-0045-4

Transition Met Chem
DOI 10.1007/s11243-016-0045-4
Synthesis, characterization, and crystal structures of 2D cobalt(II)
and nickel(II) coordination polymers containing flexible
bis(benzimidazole) ligands
1
1
1
1
Jing Wang Cui
•
Yong Guang Liu
•
Zeng Chuan Hao
•
Gui Ying Dong
Received: 25 January 2016 / Accepted: 14 March 2016
Ó Springer International Publishing Switzerland 2016
Abstract Three coordination polymers, namely {[Ni(L1)-
absorption, catalysis, luminescence, and magnetism [1–
5]. However, it is still difficult to rationally construct
CPs with specific structures and properties. Many factors,
such as the choice of organic linkers, metal, solvent,
reaction temperatures, and reaction pH value, may sig-
nificantly influence the structures of the resulting CPs [6–
8]. Among these factors, an important key to assemble
CPs is the proper selection of organic bridging ligands
[9, 10]. As excellent multifunctional ligands, aromatic
carboxylic acids offer both versatile coordination modes
and high structural stability [11–13]. For example, 5-ni-
(
nip)(H O)]Á2H O} (1), [Co(L2)(tbip)] (2), and {[Co (L3) -
2
2
n
n
2
2
(
bptc)]Á3H O} (3) (L1 = 1,4-bis(5,6-dimethylbenzimida-
2
n
zole)butane, L2 = 1,4-bis(5,6-dimethylbenzimidazole)-2-
butylene, L3 = 1,3-bis(5,6-dimethylbenzimidazole)propane,
H nip = 5-nitro-isophthalic acid, H tbip = 5-tert-butyl-
2
2
0
0
isophthalic acid, H bptc = biphenyl-3,3 ,4,4 -tetracar-
4
boxylic acid), have been synthesized under hydrothermal
conditions and characterized by physicochemical and spec-
troscopic methods as well as by single-crystal X-ray
diffraction analysis. Complexes 1 and 2 both feature a two-
4
2
dimensional (4,4) layer with (4 9 6 ) topology. Complex 3
possesses a uninodal 4-connected 2D htb network. The flu-
orescence spectra and catalytic properties of the complexes
for the degradation of methyl orange by sodium persulfate in
a Fenton-like process are reported.
tro-isophthalic acid (H nip), 5-tert-butyl-isophthalic acid
2
0
0
(H tbip), and biphenyl-3,3 ,4,4 -tetracarboxylic acid (H
2
4-
bptc) have been used extensively in the synthetic
strategies to develop multidimensional (zero, one, two,
and three dimensional) complexes [14, 15]. Meanwhile,
flexible bis(benzimidazole)s are versatile ligands that can
meet the coordination requirements of the metal centers
by means of their variable conformations [16, 17]. Fur-
thermore, methyl substituent groups can be used to
enhance the electron-donating ability of these bis(benz-
imidazole) derivatives [18–20]. Herein, a mixed-ligand
synthetic strategy has been applied to prepare transition
metal CPs by the judicious selection of neutral N-donor
ligands and aromatic carboxylates. In this way, three new
CPs, with formulas {[Ni(L1)(nip)(H O)]Á2H O} (1),
Introduction
The design and synthesis of coordination polymers (CPs)
are currently attracting much attention, due to their
intriguing topologies and potential applications in
2
2
n
[
Co(L2)(tbip)] (2), and {[Co (L3) (bptc)]Á3H O} (3)
n
2
2
2
n
Electronic supplementary material The online version of this
article (doi:10.1007/s11243-016-0045-4) contains supplementary
material, which is available to authorized users.
(L1 = 1,4-bis(5,6-dimethylbenzimidazole)butane, L2 =
,4-bis(5,6-dimethylbenzimidazole)-2-butylene, L3 = 1,3-
1
bis(5,6-dimethylbenzimidazole)propane), (Scheme 1)
were hydrothermally synthesized and characterized by
single-crystal X-ray diffraction. The solid-state fluores-
cence and catalytic properties of these complexes for the
degradation of methyl orange in a Fenton-like process
are described.
&
Gui Ying Dong
tsdgying@126.com
1
College of Chemical Engineering, North China University of
Science and Technology, 46 West Xinhua Road,
Tangshan 063009, Hebei, People’s Republic of China
123

Transition Met Chem
Experimental
single crystals of complex 1, suitable for X-ray diffraction,
were collected by filtration and washed with deionized
Materials and methods
water. Yield: 35 % (based on Ni(OAc)
2
Á4H
2
O). Anal.
Calcd. for C H N NiO (%): C, 53.9; H, 5.3; N, 10.5.
3
0
35
5
9
-
1
Reagents and solvents of analytical grade were obtained
from commercial sources and used without further purifi-
cation. The ligands L1, L2, and L3 were synthesized
according to the literature procedures [21–23]. FTIR
spectra (KBr pellets) were taken in the range of
Found (%): C, 54.1; H, 5.4; N, 10.4. IR (KBr, cm ): 3430
(m), 3020 (m), 1680 (s), 1600 (m), 1570 (s), 1520 (s), 1470
(s), 1410 (m), 1350 (m), 1190 (w), 1140 (w), 910 (w), 729
(m), 554 (m), 478 (m), 421 (m).
-
1
4
000–400 cm on an Avatar 360 (Nicolet) spectropho-
Synthesis of [Co(L2)(tbip)] (2)
n
tometer. Elemental analyses were obtained on a Perk-
inElmer 240C Elemental Analyzer (C, H and N).
Thermogravimetric analysis (TGA) was performed on a
NETZSCH TG 209 thermal analyzer from room tempera-
ture to 800 °C with a heating rate of 10 °C/min under N2
atmosphere. Luminescence spectra were measured at room
temperature on a Hitachi F-7000 fluorescence spectropho-
tometer. X-ray powder diffraction (XRPD) investigations
were carried out with a Rigaku D/Max-2500PC X-ray
diffractometer using Cu-Ka radiation (k = 0.1542 nm) and
x-2h scan mode at 293 K (Scheme 1).
The synthesis method for complex 2 was similar to that of
1, except that Co(OAc) O (25.0 mg, 0.1 mmol), L2
Á4H
(69.3 mg, 0.2 mmol), and H tbip (54.2 mg, 0.2 mmol)
were used instead of Ni(OAc) O, L1, and H nip. Red
Á4H
block single crystals of 2 were obtained with a yield of
45 % based on Co(OAc) O. Anal. Calcd. for C34
Á4H
CoN (%): C, 65.5, H, 5.8, N, 9.0; Found (%): C, 65.7;
2
2
2
2
2
2
H
36
2
2
O
4 4
-
1
H, 5.9; N, 9.1. IR (KBr, cm ): 2960 (m), 2360 (w), 1620
(s), 1520 (s), 1450 (m), 1350 (s), 1210 (w), 987 (w), 843
(w), 782 (m), 725 (m), 625 (w).
Synthesis of {[Ni(L1)(nip)(H O)]Á2H O} (1)
Synthesis of {[Co
2
(L3)
2
(bptc)]Á3H O} (3)
2 n
2
2
n
A mixture of Ni(OAc) Á4H O (24.9 mg, 0.1 mmol), L1
The synthesis method for complex 3 was similar to that of
2
2
(
35.2 mg, 0.1 mmol), H nip (42.3 mg, 0.2 mmol), and
2
1, except that Co(OAc) Á4H O (25.0 mg, 0.1 mmol), L3
(66.5 mg, 0.2 mmol), and H bptc (66.7 mg, 0.2 mmol)
were used instead of Ni(OAc) O, L1, and H nip,
Á4H
respectively. Red block single crystals of 3 were obtained
with a yield of 33 % based on Co(OAc) O. Anal.
Á4H
Calcd. for C58 11 (%): C, 59.9; H, 5.2; N, 9.6;
2
2
H O (10 mL) was placed in a Teflon-lined stainless steel
2
4
vessel (25 mL). The slurry was kept under magnetic stir-
ring for 30 min at room temperature and then heated at
2
2
2
1
40 °C for 72 h. At the end of this period, the mixture was
2
2
cooled to room temperature at a rate of 5 °C/h. Green block
H60Co N O
2 8
Scheme 1 Structural formulas
of the L1, L2, L3, and organic
acid ligands appeared in
complexes 1–3
1
23

Transition Met Chem
-
Found (%): C, 60.1; H, 5.3; N, 9.8. IR (KBr, cm ): 3350
1
parameters and details of the final refinement parameters
are shown in Table 1. Selected bond lengths and angles for
all three complexes are listed in Table 2.
(
(
m), 2940 (m), 2010 (w), 1660 (s), 1530 (s), 1420 (s), 1280
m), 1160 (w), 1090 (w), 866 (m), 775 (m), 623 (w).
X-ray crystallography
Catalysis experiments
Crystallographic data for complexes 1 and 2 were collected
on a Bruker Smart CCD diffractometer with Mo-Ka radi-
ation (k = 0.71073 A) using an x scan mode, while
crystallographic data for complex 3 were collected on an
Catalytic degradation of the dye was carried out in a 250-
mL round-bottom flask, and the reaction temperature was
controlled at 30 °C by circulating constant temperature
water. A solution of methyl orange (150 mL with 10 mg/L)
plus Na S O (30 mg) was placed in the flask. When the
˚
Agilent Technology SuperNova Atlas Dual System with
˚
2
2 8
Cu-Ka radiation (k = 1.54184 A) using an x scan mode.
temperature was stabilized, the solid catalyst 1, 2, or 3
50 mg) was added. At given time intervals, 5.0 mL ali-
For complexes 1 and 2, semiempirical absorption correc-
tions were applied using the SADABS program [24], while
the CrysAlis program [25] was used for complex 3. All
three structures were solved by direct methods and refined
(
quots of the suspension were collected and separated by
centrifugation at 10,000 rpm for 15 min in order to remove
catalyst particles. The supernatant was then analyzed using
a UV–Vis 1910 spectrophotometer at k = 506 nm. The
degradation efficiency was calculated as the percentage of
discoloration according to Eq. (1) considering the initial
and final absorbance values of the dye solution at a
wavelength of 506 nm (A and A , respectively);
2
on F by full-matrix least-squares using the SHELXL-97
program package [26]. All nonhydrogen atoms were
refined with anisotropic displacement parameters. Two
water molecules in complex 1 and three water molecules in
complex 3 were disordered, and their contribution to the
scattering was taken into account by the application of the
SQUEEZE routine of the PLATON program [27]. Crystal
0
t
Degradation efficiency ¼ ðA À A Þ=A Â 100 %
0
t
0
ð1Þ
Table 1 Crystal and refinement
data for complexes 1–3
Complex
1
2
3
Chemical formula
Formula weight
Crystal system
Space group
C
30
H
35
5
N NiO
9
C
34
H
36CoN
4
O
4
C
58 2 8 11
H60Co N O
668.34
623.60
1163.03
Triclinic
P¯ı
Triclinic
P¯ı
Triclinic
P¯ı
˚
a (A)
9.513(4)
10.1756(8)
10.1990(8)
15.5806(8)
87.801(5)
76.999(6)
76.572(7)
12.9362(6)
14.4598(6)
14.6661(5)
91.108(3)
103.376(3)
94.711(4)
˚
b (A)
10.206(4)
17.749(8)
˚
c (A)
a (°)
b (°)
c (°)
105.987(5)
94.645(6)
107.192(5)
1557.6(12)
2
3
˚
V (A )
1532.29(19)
2
2657.85(19)
2
Z
3
D
calcd (g/cm )
1.425
1.352
0.605
654
1.446
-
1
)
l (mm
F(000)
0.684
5.447
700
1206
Total reflections
11,461
5419
25,427
6264
27,557
9505
Unique reflections
R
int
0.0414
1.078
0.0555
1.048
0.0499
0.1180
1.031
-0.447
0.0774
1.101
2
GOF on F
(I [ 2r(I))
wR (I [ 2r(I))
Dqmax (eA
R
1
0.1108
0.3224
1.925
0.0638
0.1575
0.699
2
-
3
˚
)
-
3
˚
Dqmin (eA
0
-1.117
-0.657
123

Transition Met Chem
Value
Table 2 Selected bond lengths
Parameter
Value
Parameter
[
A˚ ] and angles [°] for
complexes 1–3
1
Ni(1)–O(3)
2.017(7)
2.080(8)
2.115(6)
96.2(3)
90.8(3)
92.6(3)
166.0(3)
92.0(3)
103.8(3)
91.4(3)
62.2(3)
Ni(1)–O(1 W)
2.075(8)
2.096(8)
2.147(7)
87.5(3)
90.4(3)
176.2(3)
97.8(3)
89.2(3)
159.9(3)
86.1(3)
Ni(1)–N(4)
Ni(1)–N(2)A
Ni(1)–O(5)B
Ni(1)–O(6)B
O(3)–Ni(1)–O(1 W)
O(1 W)–Ni(1)–N(4)
O(1 W)–Ni(1)–N(2)A
O(3)–Ni(1)–O(5)B
N(4)–Ni(1)–O(5)B
O(3)–Ni(1)–O(6)B
N(4)–Ni(1)–O(6)B
O(5)B–Ni(1)–O(6)B
2
O(3)–Ni(1)–N(4)
O(3)–Ni(1)–N(2)A
N(4)–Ni(1)–N(2)A
O(1 W)–Ni(1)–O(5)B
N(2)A–Ni(1)–O(5)B
O(1 W)–Ni(1)–O(6)B
N(2)A–Ni(1)–O(6)B
Co(1)–O(1)A
2.0008(19)
2.024(2)
2.397(2)
100.16(8)
100.05(9)
100.18(9)
59.49(7)
88.13(8)
Co(1)–O(3)
Co(1)–N(1)
2.009(2)
2.049(3)
Co(1)–N(3)
Co(1)–O(2)A
O(1)A–Co(1)–O(3)
O(3)–Co(1)–N(3)
O(3)–Co(1)–N(1)
O(1)A–Co(1)–O(2)A
N(3)–Co(1)–O(2)A
3
O(1)A–Co(1)–N(3)
O(1)A–Co(1)–N(1)
O(3)–Co(1)–N(1)
O(3)–Co(1)–O(2)A
N(1)–Co(1)–O(2)A
117.73(9)
113.10(9)
120.07(10)
159.39(8)
91.78(8)
Co(1)–O(2)
1.958(3)
2.042(4)
Co(1)–O(3)A
1.965(3)
2.042(4)
Co(1)–N(1)B
Co(1)–N(8)
Co(2)–O(5)C
1.944(3)
Co(2)–O(8)D
1.950(3)
Co(2)–N(5)
2.020(4)
Co(2)–N(4)
2.034(4)
O(2)–Co(1)–O(3)A
O(3)A–Co(1)–N(1)B
O(3)A–Co(1)–N(8)
O(5)C–Co(2)–O(8)D
O(8)D–Co(2)–N(5)
O(8)D–Co(2)–N(4)
130.60(14)
104.18(14)
98.91(14)
110.08(13)
123.55(15)
93.79(14)
O(2)–Co(1)–N(1)B
O(2)–Co(1)–N(8)
N(1)B–Co(1)–N(8)
O(5)C–Co(2)–N(5)
O(5)C–Co(2)–N(4)
N(5)–Co(2)–N(4)
92.61(14)
119.91(14)
107.59(15)
105.11(14)
120.69(14)
104.51(15)
Symmetry codes for 1: A: x ? 1, y ? 1, z, B: x, y ? 1, z. for 2: A: x, y ? 1, z. for 3: A: -x ? 1, -y ? 1,
-
z ? 2, B: -x ? 2, -y ? 1, -z ? 2, C: x ? 1, y, z, D: -x ? 1, -y, -z ? 1
˚
.147(7) A) from the carboxylate groups of two distinct
2
Results and discussion
2-
nip anions, and one O atom from a coordinated water
˚
ligand (Ni1–O1 W = 2.075(8) A), completing the dis-
Crystal structure of complex 1
torted octahedral geometry. The coordination angles range
from 62.2(3)° to 176.2(3)°, which are similar to those in
other nickel coordination complexes [28].
Structure analysis shows that complex 1 crystallizes in the
triclinic space group P ¯ı . The asymmetric unit contains one
2-
In complex 1, the completely deprotonated nip anions
1
2
-
Ni(II) center, one L1 ligand, one nip anion, and one
water ligand, plus two lattice water molecules (squeezed).
The coordination environment of the Ni center is shown in
Fig. 1a. Each Ni(II) center is six-coordinated by two
2
adopt the l –g :g coordination mode, connecting neigh-
2
boring Ni atoms to construct a 1D linear chain along the b-
˚
axis with a nonbonding NiÁÁÁNi distance of 10.206(4) A.
The adjacent 1D chains are further connected by the L1
˚
nitrogen atoms (Ni1–N2A = 2.096(8) A, Ni1–N4 =
ligands in a l -bridging mode to construct a 2D sheet
2
˚
.080(8) A, symmetry code: A = x ? 1, y ? 1, z)
2
(Fig. 1b), in which the 38-membered parallelogram units
from two different L1 ligands, three oxygen atoms (Ni1–
˚
O3 = 2.017(7) A, Ni1–O5B = 2.115(6) A, Ni1–O6B =
consist of four Ni atoms at the corners connected by two L1
˚
1
23

Transition Met Chem
Fig. 2 a Coordination environment of Co atom in 2. Hydrogen atoms
are omitted for clarity (30 % ellipsoid probability). Symmetry codes:
A: x, y ? 1, z, C: -x ? 1, -y ? 1, -z ? 1. b Ball-and-stick view of
the 2D network structure in 2. c sql topology network of 2
Fig. 1 a Coordination environment of Ni atom in 1. Hydrogen atoms
are omitted for clarity (30 % ellipsoid probability). Symmetry codes:
A: x ? 1, y ? 1, z, B: x, y ? 1, z. b Ball-and-stick view of the 2D
network structure in 1. c sql topology network of 1
coordinated mode, provided by three O atoms from two
˚
and two nip ligands. Meanwhile, the L1 ligand acting in an
H tbip carboxyl groups (Co1–O1A = 2.001(2) A, Co1–
2
anti-conformation connects two Ni atoms with a non-
˚
˚
O2A = 2.397(2) A, Co1–O3 = 2.009(2) A, symmetry
˚
bonding NiÁÁÁNi separation of 11.716(4) A, the dihedral
code: A = x, y ? 1, z) and two N atoms from two L2
angle between its benzimidazole rings of 4.381(3)°. From a
topological viewpoint, the structure of 1 can be simplified
to uninodal 2D (4,4)-network of sql topology with a point
˚
˚
ligands (Co1–N1 = 2.049(3) A, Co1–N3 = 2.024(2) A),
resulting in a distorted trigonal bipyramidal geometry. The
N–Co–O bond angles are in the range from 88.13(8)° to
117.73(9)°, while the O–Co–O bond angles vary from
4
2
symbol of (4 9 6 ) (Fig. 1c), in which the dimensions are
˚
˚
1
0.206(4) A 9 11.716(4) A defined by NiÁÁÁNi separations.
5
9.49(7)° to 159.39(8)°.
The ligand L2 has a bis-monodentate bridging coordi-
Crystal structure of complex 2
nation mode. The nitrogen atoms from two benzimidazole
rings of L2 connect adjacent Co atoms to form an infinite X
Structure analysis reveals that complex 2 has a 2D wave-
like network, crystallizing in the triclinic system P ¯ı space
group. The asymmetric unit consists of one independent
[Co(L2)] chain in which the distances between Co centers
n
˚
are 11.504(1) and 11.852(1) A, and the two benzimidazole
rings of the L2 ligands are parallel to each other. The
2
-
Co(II) center, two halves of L2 ligands, and one tbip
anion (Fig. 2a). The Co(II) center displays a five-
2
-
tbip
anion exhibits two kinds of coordination mode,
123

Transition Met Chem
˚
distances for Co1 are 2.042(4) A, and the Co–O lengths are
˚
1.958(3) and 1.965(3) A; these values are in good agree-
specifically monodentate and bidentate. The oxygen atoms
of tbip ligands bridge neighboring Co(II) atoms to build an
infinite straight chain, in which the Co–Co distance is
˚
ment with those of similar Co(II) complexes [29]. The
coordination environment of Co2 is similar to that of Co1,
the Co2 atom coordinated by two N atoms (N4, N5) from
different L3 ligands with bond distances of 2.020(4) and
1
0.199(2) A. The two 1D chains are cross-linked to com-
pose a 2D wavelike network (Fig. 2b), whose dimensions
˚
˚
are 10.199(1) A 9 11.852(11) A defined by CoÁÁÁCo sep-
arations. In this complex, the Co atom can be simplified as
a 4-connected node, while the two organic ligands are
considered to be linear linkers. As a result, the 2D network
can be best described as a (4,4)-connected network with a
˚
2.034(4) A, plus two O atoms (O5C, O8D, symmetry code:
C = x ? 1, y, z, D = -x ? 1, -y, -z ? 1) from different
4
-
bptc
1.950(3) A.
In complex 3, the bptc anion, which is fully depro-
anions with bond distances of 1.944(3) and
˚
4
2
4-
point symbol of (4 9 6 ) (Fig. 3b).
1
1
1
1
tonated, adopts a l –g : g : g : g coordination mode. The
four-connected bptc anions link adjacent cobalt atoms
Co1 and Co2) to construct a 1D wavelike chain, which is
4
4
-
Crystal structure of complex 3
(
Complex 3 crystallizes in the triclinic crystal system with
P ¯ı space group. The asymmetric unit consists of two Co(II)
further extended into a 2D network (Fig. 3b) via the
bridging L3 ligands. The L3 ligands exhibit a cis-confor-
mation mode with dihedral angles between the benzimi-
dazole rings of 57.990(2)° and 55.900(2)°. Four L3 ligands
connect adjacent cobalt atoms to generate a 40-membered
macrocycle (Fig. 3c), in which the four Co atoms act as
4
-
centers, two L3 ligands, one bptc anion, and three lattice
water molecules (squeezed). The two independent Co(II)
centers both display a distorted tetrahedral coordination
geometry. As shown in Fig. 3a, the Co1 center is four-
coordinated by two O atoms (O2, O3A, symmetry code:
˚
nodes with dimensions of 12.791 9 17.852 A.
4
-
A = -x ? 1, -y ? 1, -z ? 2) from two bptc anions
and two N atoms (N1B, N8, symmetry code: B = -x ? 2,
As discussed above, each Co atom is linked by two
4
bptc anions and two L3 ligands, which can be defined as
-
4
a 4-connected node. Each deprotonated bptc ligand links
-
-
y ? 1, -z ? 2) from two L3 ligands. The Co–N bond
Fig. 3 a Coordination environment of Co atom in 3. Hydrogen atoms
are omitted for clarity (30 % ellipsoid probability). Symmetry codes:
A: -x ? 1, -y ? 1, -z ? 2, B: -x ? 2, -y ? 1, -z ? 2, C: x ? 1,
y, z, D: -x ? 1, -y, -z ? 1. b Ball-and-stick view of the 2D
network structure in 3. c Macrocycle generated by L3 ligands. d htb
topology network of 3
1
23

Transition Met Chem
four Co atoms, which can be simplified as a 4-connected
node, and the L3 ligands can be viewed as linkers. Thus,
the uninodal htb (hexagonal tungsten bronzes) [30]
stretching modes, indicating the presence of both chelating
-
1
-1
and 270 cm ) car-
and monodentate (Dm = 100 cm
boxylic groups. In complex 3, absorptions at ca.
-
1
topology was obtained with
a
point symbol of
3350 cm are assigned to the stretching vibrations of free
water molecules. The antisymmetric and symmetric
stretching vibrations of the carboxylic groups are observed
2
3.4 9 5 .6) (Fig. 3d).
2
(
-
1
Influence of organic ligands on the structures
at 1660 and 1420 cm , respectively, confirming that the
carboxylic groups of complex 3 adopt a monodentate
1
-
In similar hydrothermal reactions, three flexible bis(5,6-
dimethylbenzimidazole) ligands and three aromatic car-
boxylic acids were assembled with Ni(II) and Co(II) atoms,
yielding complexes with diverse structures. In each case,
the flexible bis(5,6-dimethylbenzimidazole) derivatives
adopt a bis-monodentate bridging coordination mode to
link the Ni or Co atoms, but these ligands exhibit different
bending and rotating abilities, leading to different distances
bridging mode with Dm = 240 cm .
The experimental and computer-simulated X-ray pow-
der diffraction (XRPD) patterns of complexes 1–3 are
shown in Fig. S2. There is good agreement between the
patterns, indicating that the bulk samples have the same
structures as the crystals. The different intensities may be
derived from the variation in preferred orientations of the
powder samples during the collection of data [34].
˚
between the metal centers (11.716(4) A for 1, 11.504(1)
˚
˚
and 11.852(1) A for 2, 10.321(1) and 10.630(1) A for 3)
and distinct dihedral angles between the mean planes of the
benzimidazole rings (4.381(3)° for 1, 0° for 2, 57.990(2)°
2-
Thermal and fluorescent properties
Thermogravimetric experiments (TGA) were carried out
for complexes 1–3 under nitrogen atmosphere to investi-
gate their thermal stabilities, with the results shown in
Fig. 4. Complexes 1 and 3 both gave a two-step weight loss
process. The first process of complex 1 occurs from 100 to
180 °C with a weight loss of 8.1 %, which can be attrib-
uted to the loss of water molecules (calcd: 8.2 %). The
second weight loss is assigned to decomposition of the
organic ligands in the region of 290–630 °C. The residual
weight 11.9 % (calcd: 12.0 %) corresponds to NiO. In
complex 2, the single weight loss of 87.9 % (calcd:
87.8 %) from 320 to 630 °C is attributed to decomposition
2
-
and 55.900(2)° for 3). The nip anions in 1 and the tbip
anions in 2 both display the l –g :g coordination mode,
connecting adjacent metal atoms to give infinite 1D chains.
1
In complex 3, the bptc anions adopt a l –g :g :g :g
2
1
2
4
-
1
1
1
4
4
-
coordination mode, in which the four-connected bptc
anions link neighboring cobalt atoms (Co1 and Co2) to
construct a 1D wavelike chain. Finally, the combination of
carboxylate and N-containing co-ligands leads to the con-
struction of two different topological networks: uninodal
2
D (4,4)-network of sql topology for complexes 1 and 2,
and uninodal 2D 4-connected htb topology framework for
complex 3.
of the L2 and H tbip ligands, resulting in the formation of
2
CoO. The TGA curve of complex 3 shows the loss of
lattice water molecules with 4.6 % (calcd: 4.6 %), which
begins at 30 °C and is complete by 203 °C. The residue is
IR and X-ray powder diffraction studies
Figure S1 presents the IR spectra of complexes 1–3 in the
1
-
range of 4000–400 cm . In these spectra, the m(C=N)
breathing vibrations of the benzimidazole rings are
-
observed at 1570, 1520, and 1530 cm , respectively [31].
1
-
1
There is no absorption around 1700 cm for any of these
complexes, indicating that all carboxylic acid groups are
fully deprotonated [32]. For complex 1, a strong broad
-
1
band at around 3400 cm can be assigned to the O–H
stretching vibrations of water molecules or hydrogen
bonds. The IR spectrum of complex 1 also shows the
-
1
carboxylic group at 1680 cm
for the antisymmetric
-
stretching and 1520 plus 1470 cm for the symmetric
1
stretching modes. The separations of mas and ms
-
1
-1
(
Dm = 160 cm and 210 cm ) imply the presence of
both chelating and monodentate carboxylic groups [33].
For complex 2, the strong characteristic band of the car-
-
boxylic groups appears at 1620 cm for the antisymmetric
1
Fig. 4 TGA curves of polymers 1–3 were measured in
atmosphere
N
2
-
stretching and 1520 plus 1350 cm for the symmetric
1
123

Transition Met Chem
stable up to 240 °C and then loses the weight of L3 and
1–3 were tested as heterogeneous catalysts for the activa-
tion of Na S O .
H bptc ligands until up 600 °C. Finally, a CoO remnant of
4
2 2 8
6
.4 % (calcd. 6.5 %) is obtained.
The results of these MO degradation experiments are
shown in Fig. 6. In a control experiment (without added
catalyst), the azo dye and Na S O mixture showed only
The solid-state fluorescence properties of the free
ligands L1–L3 as well as complexes 1–3 have been
investigated at room temperature. As shown in Fig. 5, the
free ligands display strong emission maxima at 369, 359,
and 349 nm upon excitation at 320, 280, and 310 nm for
L1, L2, and L3, respectively, which may be attributed to
p* ? n or p* ? p transitions [35]. For the complexes, the
emission maxima are at 394 nm (kex = 250 nm), 325 nm
2
2 8
19.4 % degradation of MO after 120 min. Upon addition of
complexes 1–3 as heterogeneous catalysts, degradation
efficiencies increased to 73 % for complex 1, 70 % for
complex 2, and 61 % for complex 3 after 120 min. Nev-
ertheless, the catalytic efficiencies of these complexes are
lower than those of other Ni(II) and Co(II) complexes
reported in the literature [42, 43].
(
kex = 320 nm), and 396 nm (kex = 275 nm) for 1, 2, and
, respectively. Compared with the free ligands L1 and L2,
3
the maxima of complexes 1 and 3 are red-shifted, which
may be attributed to intraligand charge transfer transitions
Conclusion
[
36]. However, a blue-shifted emission is observed for
complex 2 with respect to free L2, which can be tentatively
assigned to ligand-to-metal charge transfer transitions [37].
In conclusion, three new coordination polymers based on
flexible bis(5,6-dimethylbenzimidazole) ligands and aro-
matic carboxylate co-ligands have been synthesized and
characterized. The variable coordination modes of the
carboxylate ligands play an essential role in the construc-
tion of these CPs, while the flexible bis(5,6-dimethylben-
zimidazole) ligands display the ability to adjust their
conformations to meet the coordination requirements of the
metal centers. Complexes 1–3 show moderate catalytic
activities in the Fenton-like decomposition of methyl
orange.
Catalytic properties
Methyl orange (MO) is a well-known azo dye that has been
used for textiles, printing, paper, rubber, plastic, and
research [38, 39]. However, it is also potentially harmful to
the environment because of its high chemical stability and
low biodegradability [40]. Therefore, MO was selected as a
model organic pollutant to study the catalytic behavior of
complexes 1–3. An oxidation technology based on strongly
Á-
Supplementary materials
oxidizing sulfate radicals (SO ) has recently been devel-
4
oped for the degradation of organic pollutants resistant to
CCDC depository numbers 1421229 for 1, 1447780 for 2,
and 1446477 for 3 contain the supplementary crystallo-
graphic data for the complexes 1–3. These data can be
obtained free of charge via https://www.ccdc.cam.ac.uk/,
conventional technologies [41]. Thus, persulfate anions can
Á-
be converted into sulfate radicals (SO ), which are
4
effective oxidants for organic pollutants. Here, complexes
Fig. 6 Experiment results of the catalytic degradation of methyl
orange azo dye
Fig. 5 Emission spectra of complexes 1–3 and the free ligands
1
23

Transition Met Chem
or from the Cambridge Crystallographic Data Centre, 12
18. Wang XL, Sui FF, Lin HY, Zhang JW, Liu GC (2014) Cryst
Growth Des 14:3438–3452
Union Road, Cambridge CB2 1EZ, UK; fax: (?44)
1
9. Xiao SL, Li YH, Ma PJ, Cui GH (2013) Inorg Chem Commun
7:54–58
0. Qin L, Li YH, Ma PJ, Cui GH (2013) J Mol Struct 1051:215–220
1
223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
3
2
Acknowledgments The project was supported by the National
Natural Science Foundation of China (51474086), Natural Science
Foundation-Steel and Iron Foundation of Hebei Province
21. Li LL, Li HX, Ren ZG, Liu D, Chen Y, Zhang Y, Lang JP (2009)
Dalton Trans 8567–8573
22. Geng JC, Qin L, Du X, Xiao SL, Cui GH (2012) Z Anorg Allg
Chem 638:1233–1238
(
B2015209299).
2
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3. Liu GC, Yang S, Wang XL, Lin HY, Tian AX, Zhang JW (2012)
Koord Khim 38:55–60
4. Sheldrick GM (1996) SADABS. University of G o¨ ttingen,
Germany
5. Agilent Technologies (2012) CrysAlis PRO Agilent technologies.
Yarnton, England
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