Four cobalt(II) coordination polymers based on a flexible N,N′-bis(3-pyridinecarboxamide)-1,6-hexane ligand: Modulation of crystal architectures through the substituent groups of isophthalates

Article abstract of DOI:10.1016/j.poly.2013.12.044

Four cobalt(II) mixed ligand coordination polymers based on the flexible N,N′-bis(3-pyridinecarboxamide)-1,6-hexane (3-dpyh) and four isophthalates with different substituent groups, namely [Co(3-dpyh)(5-HIP)(H ₂O)₃]·H₂O (1), [Co(3-dpyh)(5-NIP)] ·H₂O (2), [Co(3-dpyh)(5-MIP)]·H₂O (3) and [Co(3-dpyh)_0.5(5-AIP)(H₂O)]·2H₂O (4), have been hydrothermally synthesized and structurally characterized (5-H ₂HIP = 5-hydroxyisophthalic acid, 5-H₂NIP = 5-nitroisophthalic acid, 5-H₂MIP = 5-methylisophthalic acid, 5-H ₂AIP = 5-aminoisophthalic acid). X-ray diffraction analyses reveal that complex 1 is a chain constructed from a [Co-3-dpyh]_n meso-helical chain and 5-HIP anions, in which each 5-HIP anion adopts a monodentate mode coordinating to one Co(II) ion. The isostructural complexes 2 and 3 are assembled from 5-NIP and 5-MIP anions, respectively, featuring a 2D 3,5-connected network with the {4²·6⁷·8} {4²·6} topology, in which the two carboxyl groups of dicarboxylates exhibit μ₁:η¹-η¹ and μ₂:η¹-η¹ modes. When the 5-AIP anion is utilized in 4, a different 2D (3,4)-connected {4²·6 ³·8}{4²·6} network is obtained, in which the two carboxyl groups of AIP exhibit μ₁:η¹- η¹ and μ₁:η¹-η⁰ modes, and the N atom of AIP also coordinates to the Co(II) ion. The influence of the substituent groups of the isophthalates on the structures has been discussed in detail. The fluorescent, electrochemical and photocatalytic properties of complexes 1-4 have also been investigated.

Full text of DOI:10.1016/j.poly.2013.12.044

Polyhedron 71 (2014) 111–118
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Four cobalt(II) coordination polymers based on a flexible
N,N0-bis(3-pyridinecarboxamide)-1,6-hexane ligand: Modulation of
crystal architectures through the substituent groups of isophthalates
⇑
Xiu-Li Wang , Jian Luan, Hong-Yan Lin, Guo-Cheng Liu, Mao Le
Department of Chemistry, Bohai University, Liaoning Province Silicon Materials Engineering Technology Research Centre, Jinzhou 121000, PR China
a r t i c l e i n f o
a b s t r a c t
Four cobalt(II) mixed ligand coordination polymers based on the flexible N,N0-bis(3-pyridinecarboxa-
mide)-1,6-hexane (3-dpyh) and four isophthalates with different substituent groups, namely [Co(3-dpyh)
(5-HIP)(H2O)3]ꢀH2O (1), [Co(3-dpyh)(5-NIP)]ꢀH2O (2), [Co(3-dpyh)(5-MIP)]ꢀH2O (3) and [Co(3-dpyh)0.5
(5-AIP)(H2O)]ꢀ2H2O (4), have been hydrothermally synthesized and structurally characterized (5-H2HIP
= 5-hydroxyisophthalic acid, 5-H2NIP = 5-nitroisophthalic acid, 5-H2MIP = 5-methylisophthalic acid,
Article history:
Received 30 October 2013
Accepted 31 December 2013
Available online 22 January 2014
Keywords:
5-H2AIP = 5-aminoisophthalic acid). X-ray diffraction analyses reveal that complex
1 is a chain
Metal–organic coordination polymer
Bis-pyridyl-bis-amide ligand
Isophthalate
Substituent group
Photocatalytic property
constructed from a [Co-3-dpyh]n meso-helical chain and 5-HIP anions, in which each 5-HIP anion adopts
a monodentate mode coordinating to one Co(II) ion. The isostructural complexes 2 and 3 are assembled
from 5-NIP and 5-MIP anions, respectively, featuring a 2D 3,5-connected network with the {42ꢀ67ꢀ8}{42ꢀ6}
topology, in which the two carboxyl groups of dicarboxylates exhibit l1: – : –
g1 g1 and l2 g1 g1 modes.
When the 5-AIP anion is utilized in 4, a different 2D (3,4)-connected {42ꢀ63ꢀ8}{42ꢀ6} network is obtained,
in which the two carboxyl groups of AIP exhibit l1: – : –
g1 g1 and l1 g1 g0 modes, and the N atom of AIP
also coordinates to the Co(II) ion. The influence of the substituent groups of the isophthalates on the
structures has been discussed in detail. The fluorescent, electrochemical and photocatalytic properties
of complexes 1–4 have also been investigated.
Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction
(BDC = 1,4-benzyldicarboxylate, BPDA = N,N0-bis(4-pyridinyl)-1,4-
benzenedicarboxamide). Recently, our group [9] has obtained a
In the past decades, great efforts have been devoted to the ra-
tional design and synthesis of metal–organic coordination poly-
mers (MOCPs) in the fields of crystal engineering and material
chemistry owing to their intriguing architectures and topologies
as well as potential versatile applications [1–4]. Bis-pyridyl-bis-
amide based coordination polymers, as one kind of multifunctional
complex, have been intensively studied in the fields of lumines-
cence, catalysis, magnetism and gas storage in recent years due
to their unique chemical and physical properties associated with
the easy modification of their molecular structures [5–8]. For
example, Chen’s group [7] has reported four new Ni(II) coordina-
tion polymers based on two flexible bis-pyridyl-bis-amide ligands,
which exhibit various magnetic behaviors. Lu and co-workers [8]
have reported a unique spatial arrangement of amide groups for
CO2 adsorption, which is found in the open-ended channels of
the zinc(II)-organic framework {[Zn4(BDC)4(BPDA)4]ꢀ5DMFꢀ3H2O}n
variety of MOCPs based on two semi-rigid bis-pyridyl-bis-amide li-
gands, which show good selective photocatalytic activities.
As part of our continuous efforts towards new functional com-
plexes based on flexible bis-pyridyl-bis-amide ligands, in this work
we selected N,N0-bis(3-pyridinecarboxamide)-1,6-hexane (3-dpyh)
as the main ligand and 5-hydroxyisophthalic acid (5-H2HIP),
5-nitroisophthalic acid (5-H2NIP), 5-methylisophthalic acid
(5-H2MIP) and 5-aminoisophthalic acid (5-H2AIP) as secondary li-
gands to assemble with the Co(II) ion under hydrothermal condi-
tions, aiming at the construction of new frameworks and the
investigation of the effect of the substituent groups of the isophtha-
lates on the structures and properties of the target complexes. As a
result, a series of crystalline architectures with different properties
have been obtained in similar reaction systems, namely
[Co(3-dpyh)(5-HIP)(H2O)3]ꢀH2O (1), [Co(3-dpyh)(5-NIP)]ꢀH2O (2),
[Co(3-dpyh)(5-MIP)]ꢀH2O (3) and [Co(3-dpyh)0.5(5-AIP)(H2O)]ꢀ2H2O
(4). The influence of the substituent groups of the isophthalate
ligands on the structures has been discussed. The fluorescent,
⇑
Corresponding author. Tel.: +86 416 3400158.
E-mail address: wangxiuli@bhu.edu.cn (X.-L. Wang).
http://dx.doi.org/10.1016/j.poly.2013.12.044
0277-5387/Ó 2014 Elsevier Ltd. All rights reserved.

112
X.-L. Wang et al. / Polyhedron 71 (2014) 111–118
electrochemical and photocatalytic properties of the title complexes
have also been examined.
crystals of 4 suitable for X-ray diffraction were isolated by mechan-
ical separation from the amorphous solid in 37% yield (based on
Co). Anal. Calc. for C17H22CoN3O8: C, 44.80; H, 4.83; N, 9.22. Found:
C, 44.83; H, 4.80; N, 9.25%. mmax (KBr)/cmꢁ1: 3330(m), 1657(s),
1613(s), 1560(s), 1478(m), 1448(m), 1424(m), 1393(s), 1329(m),
1287(m), 1195(w), 1109(w), 1050(m), 961(w), 938(w), 884(w),
778(m), 738(s), 640(w).
2. Experimental
2.1. Materials and measurements
All reagents were obtained from commercial sources and were
used without further purification. The N-donor ligand 3-dpyh was
prepared according to the literature [10].
2.3. Preparation of the 1-CPE, 2-CPE, 3-CPE and 4-CPE
The elemental analyses (C, H and N) were determined on a Per-
kin–Elmer 240C elemental analyzer. FT-IR spectra (KBr pellets)
were taken on a Varian FT-IR 640 spectrometer in the range
500–4000 cm-1. Thermogravimetric data for the complexes 1–4
were carried out on a Pyris Diamond thermal analyzer. Fluorescent
spectra were recorded at room temperature on a Hitachi F-4500
fluorescence/phosphorescence spectrophotometer. A CHI 440 elec-
trochemical workstation connected to a Digital-586 personal com-
puter was used for control of the electrochemical measurements
and for data collection. A conventional three-electrode cell was
used at room temperature. Carbon paste electrodes modified with
complexes 1–4 (1-CPE, 2-CPE, 3-CPE, 4-CPE) were used as the
working electrode. An SCE (saturated calomel electrode) and a
platinum wire were used as the reference and auxiliary electrodes,
respectively. The UV–Vis absorption spectra were obtained using a
SP-1900 UV–Vis spectrophotometer.
1-CPE, 2-CPE, 3-CPE and 4-CPE were fabricated by the following
steps. Graphite powder (0.11 g) and complex 1, 2, 3 or 4 (0.010 g)
were mixed together with an agate mortar for about 30 min to
achieve a mixture; then 0.05 mL paraffin oil was added and stirred
with a glass rod [11]. The homogenized mixture was packed into a
3 mm inner diameter glass tube, and a copper stick was used as the
electrical contact. The surface of the modified electrodes was pol-
ished on a piece of weighing paper to achieve a mirror finish before
use.
2.4. X-ray crystallography
X-ray diffraction data for complexes 1-4 were collected on
a Bruker SMART APEX II diffractometer equipped with a CCD
area detector and graphite-monochromated Mo
K
a
radiation
(k = 0.71073 Å) using the
x
and h scan modes. All of the structures
were solved by direct methods and refined on F2 by full-matrix
least-squares methods using the SHELXS program of the SHELXTL pack-
age [12]. The crystal parameters, data collection, and refinement
results are summarized in Table 1 for complexes 1–4. Selected
bond distances and bond angles are listed in Tables S1–S4.
2.2. Preparation of the complexes
2.2.1. [Co(3-dpyh)(5-HIP)(H2O)3]ꢀH2O (1)
A mixture of CoCl2ꢀ6H2O (0.048 g, 0.20 mmol), 3-dpyh (0.033 g,
0.10 mmol), 5-H2HIP (0.027 g, 0.15 mmol) and NaOH (0.016 g,
0.40 mmol) in 12 mL water was stirred for 30 min at room temper-
ature, and then transferred to a 25 mL Teflon-lined autoclave and
kept at 120 °C for 4 days. After slowly cooling to room temperature,
pink block-shaped crystals of 1 suitable for X-ray diffraction were
3. Results and discussion
3.1. Description of the crystal structures
obtained (yield: 36% based on Co). Anal. Calc. for C26H34CoN4O11
C, 48.94; H, 5.33; N, 8.78. Found: C, 48.89; H, 5.39; N, 8.72%. mmax
(KBr)/cmꢁ1
3270(m), 1637(s), 1602(s), 1546(s), 1483(m),
1417(m), 1371(s), 1321(m), 1288(m), 1224(w), 1172(m), 1108(m),
1051(m), 1004(w), 976(w), 890(m), 786(s), 717(s), 644(w).
:
3.1.1. [Co(3-dpyh)(5-HIP)(H2O)3]ꢀH2O (1)
:
Single crystal X-ray structural analysis reveals that complex 1
has a chain structure (Fig. 1). The asymmetric unit of 1 consists
of one Co(II) ion, one 3-dpyh ligand, one 5-HIP anion, three coordi-
nated water molecules and one lattice water molecule. Fig. 1a
shows the coordination environment of the Co(II) ion, which is
coordinated by two pyridyl nitrogen atoms of the 3-dpyh
ligands [Co1–N1 = 2.132(3), Co1–N2 = 2.137(3) Å], one carboxylic
oxygen atom of one 5-HIP anion [Co1–O1 = 2.011(2) Å] and three
coordinated water molecules [Co1–O1W = 2.114(3), Co1–O2W =
2.160(3), Co1–O3W = 2.093(2) Å], resulting in a distorted octahe-
dral geometry. Each 3-dpyh ligand, acting as a bidentate ligand
2.2.2. [Co(3-dpyh)(5-NIP)]ꢀH2O (2)
Complex 2 was prepared in the same way as 1, except that 5-
H2NIP (0.032 g, 0.15 mmol) was used instead of 5-H2HIP. Pink
block-shaped crystals of 2 suitable for X-ray diffraction were ob-
tained (yield: 40% based on Co). Anal. Calc. for C26H27CoN5O9: C,
50.94; H, 4.41; N, 11.43. Found: C, 50.90; H, 4.48; N, 11.39%. mmax
(KBr)/cmꢁ1
:
3302(m), 1630(s), 1601(m), 1589(s), 1544(s),
1471(m), 1424(m), 1384(s), 1350(m), 1299(m), 1196(m), 1160(m),
1108(m), 1025(w), 925(w), 879(w), 815(m), 787(s), 707(s), 634(w).
with a
l2-bridging mode (via ligation of two pyridyl nitrogen
atoms), connects adjacent Co(II) ions to form a [Co-3-dpyh]n
meso-helical chain, in which the Co1ꢀ ꢀ ꢀCo1A, Co1Aꢀ ꢀ ꢀCo1B and
Co1Bꢀ ꢀ ꢀCo1C separations are 14.95, 16.47 and 14.95 Å, respectively
(Fig. 1b). Each 5-HIP anion coordinates to a Co(II) ion through only
one carboxylic oxygen atom, while the hydroxy oxygen atom and
other carboxylic oxygen atoms are non-coordinated (Fig. S1). The
chains are further connected by hydrogen bonding interactions
between the hydroxy oxygen atom (O5) of a 5-HIP anion and
the carboxyl oxygen atom (O4) from another 5-HIP anion (O5–
H5Bꢀ ꢀ ꢀO4, 2.645 Å, 177°) to form a 2D supramolecular network
(Fig. 1c). Finally, the 2D networks are linked by hydrogen bonding
interactions between the nitrogen atom (N4) of a 3-dpyh ligand
and the oxygen atom (O3) of a carboxyl group from a 5-HIP anion
[N4–H4Aꢀ ꢀ ꢀO3, 2.992 Å, 168°] to complete the 3D supramolecular
structure of 1 (Fig. 1d). The hydrogen bonding data are summa-
rized in Table S5.
2.2.3. [Co(3-dpyh)(5-MIP)]ꢀH2O (3)
The synthetic procedure for 3 was the same as for 1, except that
5-H2MIP (0.027 g, 0.15 mmol) was used instead of 5-H2HIP. Pink
crystals of 3 suitable for X-ray diffraction were isolated by mechan-
ical separation from the amorphous solid in 41% yield (based on
Co). Anal. Calc. for C27H30CoN4O7: C, 55.72; H, 5.16; N, 9.63. Found:
C, 55.77; H, 5.11; N, 9.65%. mmax (KBr)/cmꢁ1: 3301(m), 1667(s),
1617(s), 1586(s), 1546(s), 1474(m), 1423(m), 1370(s), 1305(m),
1246(m), 1197(m), 1115(m), 1030(m), 931(w), 903(w), 835(w),
774(w), 726(m), 701(w), 645(s).
2.2.4. [Co(3-dpyh)0.5(5-AIP)(H2O)]ꢀ2H2O (4)
The synthetic procedure for 4 was the same as for 1, except that
5-H2AIP (0.027 g, 0.15 mmol) was used instead of 5-H2HIP. Pink

X.-L. Wang et al. / Polyhedron 71 (2014) 111–118
113
Table 1
Crystal and refinement data for complexes 1–4.
Empirical formula
C26H34CoN4O11
C26H27CoN5O9
C27H30CoN4O7
C17H22CoN3O8
Fw
637.50
612.46
581.48
455.31
Crystal system
Space group
a (Å)
b (Å)
c (Å)
triclinic
P1
triclinic
P1
triclinic
P1
triclinic
P1
ꢀ
ꢀ
ꢀ
ꢀ
7.9489(14)
11.809(2)
16.288(3)
73.855(3)
81.126(3)
78.589(3)
1431.6(4)
2
9.4910(14)
10.1472(15)
14.520(2)
87.802(3)
89.295(3)
74.722(3)
1348.0(3)
2
9.5483(14)
10.0547(14)
14.570(2)
89.140(2)
88.044(3)
75.873(2)
1355.7(3)
2
8.5954(5)
10.0057(6)
11.1279(7)
81.0020(10)
78.5250(10)
75.6750(10)
902.93(9)
2
a
(°)
b (°)
c
(°)
V (Å3)
Z
T (K)
296(2)
1.479
666
1.019
296(2)
1.509
634
1.017
296(2)
1.424
606
1.001
296(2)
1.675
472
1.049
Dcalc (g cmꢁ3
F(000)
)
Goodness-of-fit (GOF) on F2
Reflections collected
Unique data, Rint
h (°)
7404
6878
6930
4611
5081, 0.0258
1.82–25.14
0.0528
4733, 0.0241
1.40–25.06
0.0615
4759, 0.0312
1.40–25.04
0.0591
3155, 0.0079
1.88–25.00
0.0353
R1 (I > 2r
(I))a
wR2 (all data)a
0.1485
0.1711
0.1562
0.0923
b
P
P
a
R1
wR2
=
||Fo| – |Fc||/ |Fo|.
P
P
b
2
=
[w(Fo ꢁ Fc2)2]/ [w(Fo2)2]1/2
.
Fig. 1. (a) The coordination environment of the Co(II) ion in complex 1 with 30% thermal ellipsoids. (b) The [Co-3-dpyh]n meso-helical chain of 1. (c) The 2D supramolecular
network of 1. (d) View of the 3D supramolecular framework of 1.

114
X.-L. Wang et al. / Polyhedron 71 (2014) 111–118
3.1.2. [Co(3-dpyh)(5-NIP)]ꢀH2O (2) and [Co(3-dpyh)(5-MIP)]ꢀH2O (3)
When the 5-H2NIP/5-H2MIP ligands were used instead of
5-H2HIP as the auxiliary ligand, the isostructural complexes 2
and 3 were obtained in good yields. Their structures are similar, ex-
cept that the bond distances and angles are slightly different
(Fig. 2a and Fig. S2). Thus, the structure of complex 2 is discussed
in detail as an example. X-ray single crystal diffraction indicates
that complex 2 is a 2D network. The asymmetric unit contains
one Co(II) ion, one 3-dpyh ligand, one 5-NIP anion and one lattice
water molecule. The coordination environment of the Co(II) ion is
shown in Fig. 2a. It is coordinated by two nitrogen atoms from two
3-dpyh ligands and four carboxyl oxygen atoms from three 5-NIP
anions, forming a distorted octahedral geometry. The Co–N and
Co–O distances range from 2.140(4) to 2.156(4) Å and 2.030(3) to
Fig. 2. (a) The coordination environment of the Co(II) ion in complex 2 with 30% thermal ellipsoids. (b) The [Co-3-dpyh]n right-helical chain of 2. (c) The [Co-5-NIP]2n ladder-
like chain. (d) The 2D network of 2. (e) The simplified representation of the 2D layer in complex 2.

X.-L. Wang et al. / Polyhedron 71 (2014) 111–118
115
2.293(4) Å, respectively. In complex 2, the adjacent Co(II) ions are
connected by 2-bridging 3-dpyh ligands to construct
[Co-3-dpye]n right-helix chain in one direction (Fig. 2b). The two
carboxyl groups of 5-NIP adopt the l1
1 and l2 1 bridging
coordination modes of the carboxyl groups. In complex 4, the
l
a
5-AIP anion, with -NH2 as the substituent group, was used as the
secondary ligand. The two carboxyl groups adopted the l1
and l1
g0 coordination modes, which are different from those
: –
g1 g1
:
g1
–g
:
g1
–g
: –
g1
modes to link three Co(II) ions to form ladder-like chains (Fig. 2c),
which cross with the [Co-3-dpye]n right-helix chains, resulting in a
layer structure (Fig. 2d). From a topological viewpoint, each Co(II)
is surrounded by two 3-dpyh ligands and three 5-NIP anions,
which can be defined as a 5-connected node. The 3-dpyh and
5-NIP serve as a simple linear linker and a 3-connected node,
respectively. Thus, complex 2 represents a 3,5-connected 2D net-
work with a {42ꢀ67ꢀ8}{42ꢀ6} topology (Fig. 2e). Additionally, the
2D networks are linked by N–Hꢀ ꢀ ꢀO hydrogen bonding interactions
between N4 and O7 from two different 3-dpyh ligands
[N(4)–H(4A)ꢀ ꢀ ꢀO(7), 2.922 Å, 169°], generating a 3D supramolecu-
lar structure (Fig. S3). The hydrogen bonding data are summarized
in Table S5.
of 5-NIP in 2 and 5-MIP in 3. However, the –NH2 group also
coordinated with Co(II) ions, thus 5-AIP connected three Co(II) ions
to form ladder-like chains, which are extended by 3-dpyh to
construct a 2D (3,4)-connected network. The results indicate that
the different substituent groups have
a great effect on the
coordination modes of the dicarboxylates and the final structures
of 1–4.
3.2. Properties
3.2.1. IR spectra
The IR spectra of complexes 1–4 are shown in Fig. S5. The bands
at 3270, 3302, 3301 and 3330 cmꢁ1 are assigned to stretching and
bending vibrations of –OH groups of water molecules for com-
plexes 1–4, respectively [13]. The presence of characteristic bands
at 3608, 1589, 3081 and 3440 cmꢁ1 for complexes 1–4 are attrib-
uted to –OH, –NO2, –CH3 and –NH2, respectively [13,14]. The
strong peaks at 1637, 1630, 1667 and 1657 cmꢁ1 for complexes
3.1.3. [Co(3-dpyh)0.5(5-AIP)(H2O)]ꢀ(H2O)2 (4)
Single crystal X-ray diffraction analysis indicates that complex 4
is a 2D network. The asymmetricunit contains oneCo(II) ion, half of a
3-dpyh ligand, one 5-AIP anion, one coordinated water molecule and
two lattice water molecules. The coordination environment of the
Co(II) ion is shown in Fig. 3a. Each Co(II) ion is coordinated by two
nitrogen atoms from one 3-dpyh ligand [Co1–N1 = 2.164(2) Å] and
one 5-AIP anion [Co1–N3 = 2.253(2) Å], three carboxyl oxygen
atoms from two 5-AIP anions [Co1–O1 = 2.029(2), Co1–O3 =
2.204(2), Co1–O4 = 2.253(1) Å] and one coordinated water molecule
[Co1–O1W = 2.088(3) Å], forming a distorted octahedral geometry.
1–4 are identified mC
@ vibrations of the amide groups [13,14].
O
The presence of characteristic bands at 1602 and 1417 cmꢁ1 for
1, 1601 and 1424 cmꢁ1 for 2, 1617 and 1423 cmꢁ1 for 3, and
1613 and 1424 cmꢁ1 for 4 may be attributed to asymmetric and
symmetric vibrations of carboxyl groups [13–15]. The strong peaks
at 1371 and 1051 cmꢁ1 for 1, 1384 and 1025 cmꢁ1 for 2, 1370 and
1030 cmꢁ1 for 3, and 1393 and 1050 cmꢁ1 for 4 suggest mC–N and
The carboxylic groups of 5-AIP adopt the l1: – : –
g1 g0 and l1 g1 g1
mN–H stretching vibrations of the pyridyl ring of the 3-dpyh ligands
bridging modes to link two Co(II) ions, resulting in a linear chain,
which is different from that of 2 (Fig. 3b). Two adjacent chains are
linked to each other by the N atom of 5-AIP to further form a lad-
der-like chain. The 3-dpyh ligand, acting as bidentate bridging, con-
nects the ladder-like chains in the vertical direction, leading to the
final 2D network (Fig. 3c). From the viewpoint of the network topol-
ogy, such a net can be reduced to a (3,4)-connected {42ꢀ63ꢀ8}{42ꢀ6}
network by considering each Co(II) ion as the 4-connected node,
the 5-AIP anion as the 3-connectednode and the 3-dpyh as the linker
(Fig. 3d). O–HꢀꢀꢀO hydrogen bonding interactions are also found be-
tween the oxygen atom of a coordinated water molecule and the car-
boxyl oxygen atom of 5-AIP [O1W–H1WAꢀ ꢀ ꢀO3, 2.717 Å, 162°],
resulting in a 3D supramolecular framework (Fig. S4).
[13,14].
3.2.2. Thermogravimetric analysis (TGA)
The thermal stabilities of complexes 1–4 were investigated in
the temperature range of 20–790 °C under a N2 atmosphere, as
shown in Fig. S6. Obviously, the first weight loss of 11.68% for 1,
2.12% for 2, 2.80% for 3 and 11.82% for 4, occurring in the temper-
ature range 102–162, 87–158, 90–159 and 99–179 °C, respectively,
can be attributed to the removal of coordinated or lattice water
molecules (calcd. 11.29% for 1, 2.94% for 2, 3.10% for 3 and
11.86% for 4). The second weight loss of about 76.36%, 85.41%,
83.99% and 71.42% (calcd. 76.95%, 84.81%, 84.00% and 71.67%) in
the range 266–575, 285–567, 326–711 and 308–629 °C may sug-
gest decomposition of the organic ligands. The final residues of
11.96%, 12.47%, 13.21% and 16.76% are close to the calculated
values of 11.76%, 12.25%, 12.90% and 16.76%, assuming the CoO
phase as the final residue, respectively.
3.1.4. The effect of the substituent groups of the isophthalates on the
structures of complexes 1–4
In the syntheses of complexes 1–4, 3-dpyh was used as the
main ligand and four type of isophthalates with different substitu-
ent groups were employed as the auxiliary ligands, with the aim to
explore the effects of the different substituent groups of the isoph-
thalates on the assembly and structures of 1–4. According to the
above structural descriptions, 3-dpyh uniformly behaves as a
3.2.3. Fluorescent properties
Metal-organic coordination polymers are promising candidates
for photoactive materials, with potential applications in chemical
sensors, electroluminescent displays, photochemistry and so on
[16–18]. Thus, the solid state fluorescent properties of complexes
1–4, together with the free 3-dpyh ligand, were investigated at
room temperature. The emission spectra of the complexes (slit
width = 10 nm) and the free organic ligand (slit width = 5 nm) are
depicted in Fig. 4. The free ligand 3-dpyh displays an intense emis-
sion band at 398 nm (kex = 320 nm), which can probably be
l2-bridging ligand to connect adjacent Co(II) ions in 1–4, and to
construct different chains in 1–3. In 1, the 5-HIP anion, acting as
a monodentate ligand, coordinates to the Co(II) ion with only one
carboxylic oxygen atom, thus complex 1 shows a chain structure,
which is constructed by Co(II) ions and 3-dpyh ligands. When
the 5-NIP and 5-MIP anions were employed in the synthesis of 2
and 3, respectively, two isostructural 2D networks were obtained.
The 5-NIP or 5-MIP linked three Co(II) ions to construct ladder-like
assigned to
p ?
p⁄ transitions [5]. When the slit width is adjusted
chains, in which two carboxyl groups adopt the l1
:
g1
–
g1 and
to be 10 nm, the emission band of the free organic ligand (3-dpyh)
is very strong and exceeds the experimental range of the intensity
under similar conditions. Thus, it was considered that the carbox-
l2: –
g1
g1 coordination modes, respectively. The ladder-like chains
are crossed with the [Co-3-dpyh]n chains to construct 3,5-con-
nected 2D networks. Though –NO2 in 5-NIP and –CH3 in 5-MIP
are non-coordinated, they still play important roles in the
construction of the 2D networks of 2 and 3 by influencing the
ylic ligands show very weak n ? p⁄ or
p ?
p⁄ transitions and only
contribute a little to the fluorescence of the title complexes at
room temperature [19].

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X.-L. Wang et al. / Polyhedron 71 (2014) 111–118
Fig. 3. (a) The coordination environment of the Co(II) ion in complex 4 with 30% thermal ellipsoids. (b) The [Co-5-AIP]2n ladder-like chain of 4. (c) The 2D network of 4. (d) The
simplified representation of the 2D network in complex 4.
As it is known, Co(II) complexes do not contain d10 metal
centers, but a series of fluorescent Co(II) complexes have been re-
ported [20]. Density functional theory calculations indicate that
the fluorescence of Co(II) coordination complexes may be mainly
attributed to the coupling of ligand-to-metal charge transfers
(LMCT) or ligand-to-ligand charge-transfers (LLCT) [21]. When
the title complexes are excited at 320 nm, their emission peaks
are observed at about 396 nm for 1 and 3, and 410 nm for 2 and
4, as shown in Fig. 4. Compared with the free 3-dpyh ligand,
slightly blue-shifted emission bands (2 nm for 1 and 3), and obvi-
ously red-shifted emission bands (12 nm for 2 and 4) can be
observed. Therefore, the fluorescent behaviors of 1 and 3 can be
best ascribed to the LMCT, while those of complexes 2 and 4 can
be best ascribed to the LLCT [5,19–21].
3.2.4. Electrochemical behaviors of 1-CPE, 2-CPE, 3-CPE and 4-CPE
Cyclic voltammetry (CV) in the solid state is a modern trend in
electrochemical and electrocatalytic research and is mostly used
for the investigation of solid films and composite materials
Fig. 4. The emission spectra of complexes 1–4 and the free ligand 3-dpyh.
[22–26]. The electrochemical behaviors of complexes 1–4

X.-L. Wang et al. / Polyhedron 71 (2014) 111–118
117
bulk-modified carbon paste electrodes (1-CPE, 2-CPE, 3-CPE and 4-
CPE) were carried out in 0.01 M H2SO4 + 0.5 M Na2SO4 aqueous
solution. It can be seen that in the potential range 600 to 100,
450 to 100, 500 to ꢁ300 and 650 to 0 mV (Figs. 5 and S7), a revers-
ible redox peak is observed at the (1–4)-CPE, respectively. The
mean peak potentials E1/2 = (Epa + Epc)/2 are +321 mV for 1-CPE,
+317 mV for 2-CPE, +85 mV for 3-CPE and +260 mV for 4-CPE
(50 mV sꢁ1), which could be attributed to the redox couple of CoIII/
CoII [22–25]. The difference in the mean peak potentials for the ti-
tle complexes may be attributed to their different structures and
substituent group interactions [24,25].
The scan rates effect on the electrochemical behaviors of the
(1–4)-CPEs was also investigated in the same potential ranges and
conditions as outlined above. As shown in Figs. 5 and S7, the peak
potentials of the (1–4)-CPEs change gradually when the scan rates
increase from 10 to 100 mV sꢁ1: the cathodic peak potentials shift
to the negative direction and the corresponding anodic peak poten-
tials shift to the positive direction. Plots of peak currents versus scan
rates are shown in the insets of Figs. 5 and S7. The anodic and catho-
dic peak currents are proportional to the scan rates, indicating that
the redox processes for the (1–4)-CPEs are surface-controlled [27].
It is well known that there is no obvious response on a bare CPE
in 0.01 M H2SO4 + 0.5 M Na2SO4 aqueous solution containing
1.0 mmol/L KNO2 solution under a scan rate of 50 mV sꢁ1 in the po-
tential ranges 600 to 100, 450 to 100, 500 to ꢁ300 and 650 to 0 mV,
as used for (1–4)-CPE, respectively. As shown in Fig. S8, with the
addition of nitrite, the reduction peak currents increase while the
corresponding oxidation peak currents decrease, which suggest
that the (1–4)-CPEs have electrocatalytic activity toward the
reduction of nitrite. It is noted that the (1–4)-CPEs show different
electrocatalytic activities for the reduction of nitrite, which may
be attributed to their different structures.
Fig. 6. Absorption spectra of the MB solution during the decomposition reaction
under UV irradiation in the presence of complex 1.
3.2.5. Photocatalytic properties
Photocatalysis is a ‘‘green’’ technology in purifying wastewater
by thoroughly decomposing organic pollutants [28–30]. Herein, we
investigated the photocatalytic performances of complexes 1–4 for
the photodegradation of methylene blue (MB) under UV irradia-
tion. As shown in Figs. 6 and S9, the absorption peaks of MB de-
creased obviously with reaction time. In addition, the
concentrations of MB (C) versus reaction time (t) using complexes
1–4 are plotted in Fig. 7. The photocatalytic activities of 1–4 were
gradually enhanced with time, which was increased from 0 to
240 min. After 240 min, the degradation ratio of MB reaches 52%
Fig. 7. Photocatalytic decomposition rates of MB solution under UV irradiation with
the title complexes and with no complex under the same conditions.
for 1, 39% for 2, 34% for 3 and 47% for 4. These results suggest that
complexes 1–4 may be candidates for the photocatalytic degrada-
tion of MB. In order to investigate the stability of complexes 1–4 as
photocatalysts, we re-recorded the IR patterns of complexes 1–4
after the photocatalytic experiments, and the IR spectra are almost
identical with those of the as-prepared samples (Fig. S10). Similar
procedures were performed to check the photocatalytic activities
of 1–4 for the degradation of methyl orange (MO). Howerer, there
was no obviously photocatalytic degradation of MO. The results
indicate that complexes 1–4 are selective photocatalysts for MB.
In addition, control experiments under UV irradiation of MB were
carried out. No obvious changes in the degradation of MB were ob-
served under the following reaction conditions: (1) without cata-
lyst; (2) only with CoCl2ꢀ6H2O, 5-H2HIP, 5-H2NIP, 5-H2MIP, 5-
H2AIP and 3-dpyh; (3) in the dark. The results indicate that the title
complexes may be good candidates for the photocatalytic degrada-
tion of MB, and could have potential application in the reduction of
some other organic dyes and for purifying wastewater.
4. Conclusion
Fig. 5. Cyclic voltammograms of the 2-CPE in 0.01 M H2SO4 + 0.5 M Na2SO4
aqueous solution at different scan rates (from inner to outer: 10, 20, 30, 40, 50,
60, 70, 80, 90, 100 mV sꢁ1). The inset shows the plots of the anodic and cathodic
peak currents against the scan rates.
In summary, we have successfully synthesized four cobalt(II)
coordination polymers with the flexible bis-pyridyl-bis-amide

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X.-L. Wang et al. / Polyhedron 71 (2014) 111–118
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5-H2AIP) with different substituent groups as mixed ligands. The
assembly and structures of 1–4 are mainly influenced by the
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In addition, complexes 1–4 exhibit good fluorescent, electrocata-
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The support of the National Natural Science Foundation of
China (No. 20871022, 21171025), New Century Excellent Talents
in University (NCET-09-0853) and Program of the Innovative
Research Team in University of Liaoning Province (LT2012020)
are gratefully acknowledged.
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Appendix A. Supplementary data
CCDC 958974-958977 contains the supplementary crystallo-
graphic data for 1–4. These data can be obtained free of
charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or
from the Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: de-
posit@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.poly.2013.12.044.
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