The various architectures and properties of a series of coordination polymers tuned by the central metals

Article abstract of DOI:10.1039/c4dt00064a

Five new metal-organic coordination polymers based on the mixed ligands of a semi-rigid bis-pyridyl-bis-amide N,N′-bis(3-pyridinecarboxamide)-1,2- cyclohexane (3-bpah) and 1,4-benzenedicarboxylic acid (1,4-H₂BDC), namely [Co(3-bpah)(1,4-BDC)(H₂O)₃] (1), [Ni(3-bpah)(1,4-BDC)(H₂O)₃] (2), [Cu(3-bpah)(1,4-BDC)] (3), [Zn(3-bpah)(1,4-BDC)]·H₂O (4) and [Cd(3-bpah)(1,4-BDC) (H₂O)] (5), have been hydrothermally synthesized and structurally characterized. Complexes 1 and 2 are isostructural and display the similar 1D infinite chains, which are further linked via hydrogen-bonding interactions to generate 3D supramolecular frameworks. Complex 3 features a 3D polymeric framework with CdSO₄-like topology. Complexes 4 and 5 show two similar 2D (2,4)-connected networks with (4·8⁵)(4) topology, in which the 3-bpah ligands adopt different μ₂-bridging coordination modes via the ligation of two pyridyl nitrogen atoms in 4 and via the ligation of one pyridyl nitrogen and one amide oxygen atom in 5. In addition, the central metals show different coordination geometries in 4 and 5. The adjacent layers of complexes 4 and 5 are finally extended into 3D supramolecular architectures through hydrogen-bonding interactions. The effects of the central metals on the structures and properties of complexes 1-5 have been discussed. The electrochemical properties of complexes 1-3 and fluorescent sensing behaviors of 4-5 toward ethanol and nitrobenzene have been investigated in detail. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4dt00064a

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The various architectures and properties of a
series of coordination polymers tuned by the
central metals†
Cite this: Dalton Trans., 2014, 43,
8072
Xiu-Li Wang,* Jian Luan, Hong-Yan Lin, Mao Le and Guo-Cheng Liu
Five new metal–organic coordination polymers based on the mixed ligands of a semi-rigid bis-pyridyl-
bis-amide N,N’-bis(3-pyridinecarboxamide)-1,2-cyclohexane (3-bpah) and 1,4-benzenedicarboxylic acid
(1,4-H2BDC), namely [Co(3-bpah)(1,4-BDC)(H2O)3] (1), [Ni(3-bpah)(1,4-BDC)(H2O)3] (2), [Cu(3-bpah)(1,4-
BDC)] (3), [Zn(3-bpah)(1,4-BDC)]·H2O (4) and [Cd(3-bpah)(1,4-BDC)(H2O)] (5), have been hydrothermally
synthesized and structurally characterized. Complexes 1 and 2 are isostructural and display the similar 1D
infinite chains, which are further linked via hydrogen-bonding interactions to generate 3D supramolecular
frameworks. Complex 3 features a 3D polymeric framework with CdSO4-like topology. Complexes 4 and
5 show two similar 2D (2,4)-connected networks with (4·85)(4) topology, in which the 3-bpah ligands
adopt different μ2-bridging coordination modes via the ligation of two pyridyl nitrogen atoms in 4 and via
the ligation of one pyridyl nitrogen and one amide oxygen atom in 5. In addition, the central metals show
different coordination geometries in 4 and 5. The adjacent layers of complexes 4 and 5 are finally
extended into 3D supramolecular architectures through hydrogen-bonding interactions. The effects of
the central metals on the structures and properties of complexes 1–5 have been discussed. The electro-
chemical properties of complexes 1–3 and fluorescent sensing behaviors of 4–5 toward ethanol and
nitrobenzene have been investigated in detail.
Received 8th January 2014,
Accepted 26th February 2014
DOI: 10.1039/c4dt00064a
www.rsc.org/dalton
luminescent properties when coordinated by multidentate
Introduction
ligands.18,19 Li’s group has reported two Zn-based MOCPs with
The design and synthesis of novel metal–organic coordination a 1D channel through multi-component self-assembly at room
polymers (MOCPs) using crystal engineering concepts have temperature, namely {[Zn(OCA–OH)2(4,4′-bipy)0.5]·1.67H2O}n
become increasingly popular during the past few decades.1–4 (OCA–OH = (E)-3-(2-hydroxyl-phenyl)-acrylic acid); 4,4′-bipy =
These novel and various networks possess a wide range of pro- 4,4′-bipyridine) and {[Zn(MCA–OH)2(4,4′-bipy)0.5]·0.5CH3OH·
perties, which make them potential materials for applications H2O}n (MCA–OH = (E)-3-(3-hydroxyl-phenyl)-acrylic acid). Their
in luminescence, electrochemistry, catalysis, magnetism, gas guest-free forms demonstrate their potential as a potential
storage and separation.5–12 Typically, luminescent and electro- luminescent probe or sensor for small molecules, and the
chemical materials have been extensively explored for their MOCPs also show exceptional fluorescence quenching and
diverse functionalities and applications in lighting, display, enhancement behaviors for different types of aromatic mole-
sensing, electrocatalysis and optical devices.13–17 The suitable cules.20 In addition, various efforts have been made to study
selection of metal ions is one of the most important prerequi- the electrochemical and electrocatalytic activities of MOCPs
sites for constructing MOCPs with luminescent and electro- with Co(II)/Ni(II)/Cu(II) ions.16,17 From the synthesis point of
chemical properties, and a large number of such MOCPs have view, Co(II)/Ni(II)/Cu(II)-containing MOCPs have attracted sig-
been synthesized.13–17 For example, the Zn/Cd ions are the nificant interest in both the scientific and industrial fields
most commonly reported and are widely used to construct because of their low background current, easy fabrication and
luminescent MOCPs because the d10 metal ions not only good performance.16,17 Benvidi and co-workers prepared a
possess various coordination numbers, but also show excellent Co(II) bis(benzoylacetone)ethylenediimino multi-wall carbon
nanotube
modified
carbon
paste
electrode
(Co(II)
L-MWCNT-CPE), which was applied to the determination of
hydroxylamine in water and pharmaceutical samples.21
Recently, our group has been working on the syntheses of
transition metal complexes containing rigid/semi-rigid/flexible
bis-pyridyl-bis-amide ligands, and a series of coordination
Department of Chemistry, Bohai University, Jinzhou, 121000, P. R. China.
E-mail: wangxiuli@bhu.edu.cn; Fax: +86-416-3400158; Tel: +86-416-3400158
†Electronic supplementary information (ESI) available: IR spectra, TG and
additional figures. CCDC numbers 962356–962360. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c4dt00064a
8072 | Dalton Trans., 2014, 43, 8072–8082
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a platinum wire were used as the reference and auxiliary elec-
trodes, respectively.
X-ray crystallographic study
The X-ray diffraction data for complexes 1–5 were collected on
a Bruker SMART APEX II diffractometer equipped with a CCD
area detector and graphite-monochromated Mo Kα (λ =
0.71073 Å) by the ω and θ scan mode at 296(2) K. 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 package.26 For complexes 1–5, the crystal para-
meters, data collection and refinement results are summarized
in Table 1. Selected bond distances and bond angles are listed
in Table S1–S5.† The hydrogen-bonding geometries of com-
plexes 1, 4 and 5 are summarized in Table S6.†
Scheme 1 The semi-rigid bis-pyridyl-bis-amide N,N’-bis(3-pyridinecar-
boxamide)-1,2-cyclohexane (3-bpah) and 1,4-benzenedicarboxylic acid
(1,4-H2BDC) used in this work.
polymers with various properties have been constructed.22–24
As a continuation of our previous study, in this contribution,
five transition metal ions (Co(II), Ni(II), Cu(II), Zn(II), Cd(II))
were selected as the central metals, and a semi-rigid bis-
pyridyl-bis-amide ligand N,N′-bis(3-pyridinecarboxamide)-1,2-
cyclohexane (3-bpah) and 1,4-benzenedicarboxylic acid (1,4-
H2BDC) have been used as the mixed ligands (Scheme 1), to
constructing novel MOCPs with good luminescent sensing or
electrochemical properties. We investigated the effect of the
metal ions on the structures and properties of the title com-
plexes. As a result, five new MOCPs with different dimensional-
ities, namely [Co(3-bpah)(1,4-BDC)(H2O)3] (1), [Ni(3-bpah)(1,4-
BDC)(H2O)3] (2), [Cu(3-bpah)(1,4-BDC)] (3), [Zn(3-bpah)(1,4-
BDC)]·H2O (4) and [Cd(3-bpah)(1,4-BDC)(H2O)] (5) have been
obtained. The crystal structures of the five MOCPs have been
represented and discussed. The fluorescence properties of the
coordination polymers 1–5, the fluorescent selectivities and
fluorescent sensing behaivours of 4 and 5, and the electrocata-
lytic activities of 1–3 have been investigated in detail.
The preparation of the title complexes
The synthesis of [Co(3-bpah)(1,4-BDC)(H2O)3] (1). A mixture
of CoCl2·6H2O (0.048 g, 0.2 mmol), 3-bpah (0.032 g,
0.10 mmol), 1,4-H2BDC (0.025 g, 0.15 mmol) and NaOH
(0.016 g, 0.40 mmol) in 12 mL water was stirred for 30 min at
room temperature, transferred to a 25 mL Teflon-lined auto-
clave 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 obtained (yield: 27% based on Co). If the
molar ratio of the reactance was changed to 1 : 1 : 1, the yield is
up to 55% (based on Co). Anal. calcd for C26H30CoN4O9: C
51.87, H 4.99, N 9.31. Found: C 51.83, H 4.95, N 9.37%. νmax
(KBr)/cm−1: 3300 (m), 3079 (m), 2935 (w), 1649 (s), 1598 (s),
1549 (s), 1482 (m), 1422 (m), 1364 (s), 1326 (m), 1292 (m), 1218
(w), 1199 (w), 1145 (w), 1119 (w), 1053 (w), 931 (w), 862 (w), 755
(s), 698 (s), 644 (m), 598 (w), 514 (w).
The synthesis of [Ni(3-bpah)(1,4-BDC)(H2O)3] (2). Complex
2 was prepared in the same way as 1, except that NiCl2·6H2O
(0.048 g, 0.2 mmol) was used instead of CoCl2·6H2O. Green
block-shape crystals of 2 suitable for X-ray diffraction were
obtained (yield: 29% based on Ni). If the molar ratio of the
Experimental section
Materials and measurements
All reagents were obtained from commercial sources without reactance was changed to 1 : 1 : 1, the yield is up to 63% (based
further purification. The N-donor ligand 3-bpah was prepared on Ni). Anal. calcd for C26H30N4NiO9: C 51.89, H 4.99, N 9.31.
according to the literatures.25 The elemental analyses (C, H Found: C 51.93, H 5.01, N 9.30%. νmax (KBr)/cm−1: 3299 (m),
and N) were determined on a Perkin-Elmer 240C elemental 3083 (w), 2936 (w), 1649 (s), 1599 (s), 1550 (s), 1483 (m), 1422
analyzer. The FT-IR spectra (KBr pellets) were taken on a (m), 1365 (s), 1325 (m), 1237 (w), 1199 (w), 1145 (w), 1119 (w),
Varian 640-IR spectrometer in the range of 500–4000 cm−1
Powder XRD investigations were carried out with an Ultima IV 598 (w), 513 (w).
.
1092 (w), 1050 (w), 930 (w), 868 (w), 814 (m), 755 (s), 698 (m),
with a D/teX Ultra diffractometer at 40 kV, 40 mA with Cu Kα
The synthesis of [Cu(3-bpah)(1,4-BDC)] (3). The synthetic
(λ = 1.5406 Å) radiation. The thermogravimetric data for the procedure for 3 was the same as for 1, except that CuCl2·2H2O
complexes 1–5 were obtained using a Pyris Diamond thermal (0.034 g, 0.2 mmol) was used instead of CoCl2·6H2O. Blue crys-
analyzer. The fluorescence spectra were recorded at room tals of 3 suitable for X-ray diffraction were isolated by mechan-
temperature on
a Hitachi F-4500 fluorescence/phosphor- ical separation from the amorphous solid in 30% yield (based
escence spectrophotometer. A CHI 440 electrochemical work- on Cu). If the molar ratio of the reactance was changed to
station connected to a Digital-586 personal computer was used 1 : 1 : 1, the yield is up to 47% (based on Cu). Anal. calcd for
for the control of the electrochemical measurements and for C26H24CuN4O6: C 56.52, H 4.35, N 10.14. Found: C 56.48, H
data collection. A conventional three-electrode cell was used at 4.39, N 10.18%. νmax (KBr)/cm−1: 3525 (w), 2923 (w), 1651 (s),
room temperature. The carbon paste electrodes bulk-modified 1607 (s), 1562 (s), 1485 (s), 1431 (m), 1369 (s), 1309 (s), 1242
with complexes 1–3 (1-CPE, 2-CPE, 3-CPE) were used as the (w), 1198 (m), 1144 (m), 1090 (w), 1016 (w), 973 (w), 891 (w),
working electrodes. An SCE (saturated calomel electrode) and 835 (m), 752 (s), 699 (s), 657 (m), 594 (m), 507 (w).
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Table 1 The crystal data and structure refinement for complexes 1–5
Complex
1
2
3
4
5
Empirical formula
Formula weight
Crystal system
Space group
a (Å)
b (Å)
c (Å)
C
26H30CoN4O9
C26H30N4NiO9
601.25
Triclinic
ˉ
P1
8.7723(3)
12.7797(5)
13.4540(5)
76.1920(10)
85.5170(10)
72.7230(10)
1398.57(9)
2
C26H24CuN4O6
552.03
Monoclinic
C2/c
19.719(3)
9.0200(12)
16.077(2)
90
105.865(2)
90
2750.6(6)
2
C26H26ZnN4O7
571.88
Triclinic
ˉ
P1
9.3074(5)
11.8484(6)
13.2041(6)
108.8050(10)
97.7350(10)
112.4420(10)
1217.51(11)
2
C26H26CdN4O7
618.91
Triclinic
ˉ
P1
7.8559(4)
11.5298(6)
16.1232(8)
82.5120(10)
81.0530(10)
73.3020(10)
1376.12(12)
2
601.47
Triclinic
ˉ
P1
8.7946(5)
12.8230(8)
13.5008(8)
76.2220(10)
85.6630(10)
72.7250(10)
1411.99(15)
2
α (°)
β (°)
γ (°)
V (Å3)
Z
D/g cm−3
1.415
0.666
1.428
0.752
1.33
0.839
1.560
1.065
1.494
0.843
μ/mm−1
F(000)
626
7133
4921
368
0.0077
1.047
0.0451
628
7121
4882
368
0.0083
1.063
0.0436
1140
6765
2418
169
0.0265
1.020
0.0559
592
6230
4247
343
0.0094
1.038
0.0258
0.0756
628
6984
4824
343
0.0086
1.020
0.0211
Reflection collected
Unique reflections
Parameters
Rint
GOF
R1 a [I > 2σ(I)]
wR2b (all data)
0.1315
0.1273
0.1265
0.0569
a R1 = ∑||Fo| − |Fc||/∑|Fo|. b wR2 = ∑[w(Fo2 – Fc2)2]/∑[w(Fo2)2]1/2
.
The synthesis of [Zn(3-bpah)(1,4-BDC)]·H2O (4). Complex 4
was synthesized with same procedure as for 1, using Zn-
(NO3)2·6H2O (0.060 g, 0.2 mmol) in place of CoCl2·6H2O,
leading to the formation of colorless block crystals of 4 (yield:
Results and discussion
Description of the structures
[Co(3-bpah)(1,4-BDC)(H2O)3] (1) and [Ni(3-bpah)(1,4-BDC)-
36% based on Zn). If the molar ratio of the reactance was (H2O)3] (2). The single crystal X-ray structural analysis reveals
changed to 1 : 1 : 1, the yield is up to 63% (based on Zn). Anal. that complexes 1 and 2 crystallize in the triclinic space group
ˉ
calcd for C26H26ZnN4O7: C 54.56, H 4.55, N 9.79. Found: C P1, and they have an isostructural 3D supramolecular architec-
54.60, H 4.51, N 9.74%. νmax (KBr)/cm−1: 3280 (m), 2943 (w), ture based on a 1D polymeric chain [Co(3-bpah)(1,4-BDC)-
1656 (s), 1601 (s), 1542 (s), 1475 (m), 1428 (m), 1402 (m), 1382 (H2O)3] and [Ni(3-bpah)(1,4-BDC)(H2O)3] (Fig. 1 and Fig. S1†).
(s), 1354 (s), 1245 (w), 1208 (w), 1132 (m), 1092 (w), 1054 (m), Therefore, complex 1 is employed as a representative structure
882 (w), 829 (s), 752 (s), 697 (s), 654 (w), 592 (w), 502 (w).
to be described in detail. The structure of 1 contains two crys-
The synthesis of [Cd(3-bpah)(1,4-BDC)(H2O)] (5). Complex 5 tallographically unique Co(II) ions, as depicted in Fig. 1a. The
was synthesized with same procedure as for 1, using Co1 ion takes a distorted octahedral geometry via coordinating
CdCl2·2.5H2O (0.045 g, 0.2 mmol) in place of CoCl2·6H2O, to two pyridyl nitrogen atoms (N1 and N1#1) of 3-bpah and
leading to the formation of colorless block crystals of 5 (yield: four oxygen atoms (O1W, O1W#1, O2W and O2W#1) of four
28% based on Cd). If the molar ratio of the reactance was coordination water molecules, whereas the octahedral Co2
changed to 1 : 1 : 1, the yield is up to 57% (based on Cd). Anal. center is coordinated by two pyridyl nitrogen atoms (N2 and
calcd for C26H26CdN4O7: C 50.41, H 4.20, N 9.05. Found: C N2#1) of 3-bpah and two oxygen atoms (O1, O1#1) of two 1,4-
50.47, H 4.24, N 9.07%. νmax (KBr)/cm−1: 3412 (m), 2939 (w), BDC anions plus two oxygen atoms (O3W, O3W#1) of coordi-
1652 (s), 1624 (s), 1582 (s), 1502 (m), 1433 (s), 1367 (s), 1199 nation water molecules [Co–N = 2.174(2)–2.237(3) Å, Co–O =
(w), 1141 (m), 1091 (w), 1018 (w), 886 (w), 834 (w), 778 (m), 744 2.0599(19)–2.0889(18) Å]. The adjacent Co1 and Co2 centers
(s), 646 (w), 541 (w).
are alternately linked by the μ2-bridging 3-bpah (via ligation of
The preparation of 1–3-CPE. The complexes 1–3 bulk-modi- two pyridyl nitrogen atoms, Table S7†) ligands to form a 1D
fied carbon paste electrodes (1-CPE, 2-CPE, 3-CPE) were fabri- [Co(3-bpah)]n meso-helical chain (Fig. 1b), the non-bonding
cated by the following steps. Graphite powder 0.11 g and Co1⋯Co2 distance is 10.67 Å. Then, each 1,4-BDC anion
complex 1, 2 or 3 0.010 g were mixed together with an agate acting as a monodentate ligand coordinates to the Co2 ion
mortar for about 30 min to achieve a mixture; then 0.05 mL through only one carboxylic oxygen atom, leaving the other
paraffin oil was added and stirred with a glass rod.27 The homo- oxygen atoms non-coordinated and resulting in a fish-bone
genized mixture was packed into a 3 mm inner diameter glass chain (Fig. S2a†). The hydrogen-bonding interactions between
tube, and a copper stick was used as the electrical contact. The the coordinated water molecules and the carboxyl oxygen
surface of the modified electrodes was polished on a piece of atoms O1W–H1WA⋯O4 (2.7023 Å) are found to assemble
weighing paper to achieve a mirror finish before use.
these 1D chains into a 2D layer (Fig. 1c). Additionally, the
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Fig. 1 (a) The coordination environment of the Co(II) ions in complex 1
with 50% thermal ellipsoids. Symmetry codes: #1 −x + 2, −y + 1, −z −
1. (b) The 1D [Co(3-bpah)]n meso-helical chain. (c) A view of the 2D
supramolecular layer in complex 1.
O1W–H1WB⋯O2 (2.6892 Å) hydrogen-bonding interactions
interlink the adjacent layers to form a 3D supramolecular
framework (Fig. S2b†). The solvent-accessible voids by
PLATON28 calculations are 3.6% for 1 and 3.5% for 2.
[Cu(3-bpah)(1,4-BDC)] (3). Complex 3 crystallizes in the
monoclinic space group C2/c with one crystallographically
unique Cu(II) ion. The Cu(II) center maintains a quadrilateral
Fig. 2 (a) The coordination environment of the Cu(II) ion in complex 3
with 50% thermal ellipsoids. Symmetry codes: #1 −x + 1/2, −y + 3/2, −z
+ 1. (b) A view of the 1D [Cu(3-bpah)]n meso-helical chain in complex 3.
(c) A view of the 3D framework in complex 3. (d) A view of the CdSO4-
like topology of complex 3.
coordination environment, as shown in Fig. 2a, and is co- meso-helical chains are further linked by each other to form a
ordinated by two nitrogen atoms of distinct 3-bpah ligands, 3D framework (Fig. 2c). Topological analysis was carried out to
and two oxygen atoms from two different 1,4-BDC anions. The simplify the 3D architecture. The 3-bpah/1,4-BDC is viewed as
lengths of the Cu–N bonds are both 2.000(3) Å, while the Cu–O a linear rod, and the Cu(II) ion could be regarded as a 4-con-
bonds are 1.947(2) Å. In 3, 1,4-BDC acts as a μ2-bridging nected node. In such a way, the whole structure illustrates a
ligand, with both carboxyl groups of 1,4-BDC adopting the uninodal 4-connected framework with the Schläfli symbol of
same coordination mode: μ1–η1:η0-mode, which interlink the (65·8), which is topologically identical to the augmented
Cu(II) ions to produce a 1D [Cu(1,4-BDC)]n chain with a non- square CdSO4 array (Fig. 2d). The PLATON28 calculations show
bonding Cu⋯Cu distance of 10.84 Å (Fig. S3†). The 3-bpah that the largest solvent-accessible void is 10.4%.
ligand adopts a μ2-bridging mode and connects the Cu(II) ions
[Zn(3-bpah)(1,4-BDC)]·H2O (4). The single crystal X-ray ana-
[Cu⋯Cu distance: 10.89 Å, Table S7†] to build a 1D [Cu(3- lysis reveals that 4 is a 2D coordination network formed by
bpah)]n meso-helical chain (Fig. 2b), which is similar to that in Zn(II) ions, 3-bpah ligands and 1,4-BDC anions. Fig. 3a shows
complex 1. The 1D [Cu(1,4-BDC)]n chains and [Cu(3-bpah)]n the coordination environment of the Zn(II) ion, which is four-
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coordinated by two pyridyl nitrogen atoms of two different (Fig. 3d). In complex 4, each Zn(II) center acts as a 4-connected
3-bpah ligands [Zn1–N1 = 2.0461(16) Å, Zn1–N2 = 2.0755(16) node to connect two 3-bpah ligands and two 1,4-BDC anions.
Å], and two oxygen atoms of two 1,4-BDC anions [Zn1–O1 = The 3-bpah ligands serve as the 2-connected nodes and the
1.9240(14) Å, Zn1–O3 = 1.9465(13) Å] to give a distorted tetra- 1,4-BDC anions act as linkers. Thus, the topological analysis
hedral coordination geometry. In 4, two μ2-bridging 3-bpah reveals that complex 4 represents a (2,4)-connected network
ligands through their pyridyl nitrogen atoms (Table S7†) (Fig. 3e) with the Schläfli symbol (4·85)(4). Finally, the 2D
connect two Zn(II) ions to form a [Zn2(3-bpah)2] binuclear layers are further connected by two kinds of O–H⋯O hydro-
subunit with the dimensional size of 14.35 × 10.40 Å2 (Fig. 3b). gen-bonding interactions to generate a 3D supramolecular
The 1,4-BDC acts as a bis(monodentate) bridging ligand, with framework (Fig. S4†). One hydrogen-bonding interaction is
two carboxyl groups adopting the same μ1–η1:η0-mode to link between the carbonyl oxygen atom (O5) of 3-bpah and the
the Zn(II) ions, constructing a 1D [Zn(1,4-BDC)]n zigzag chain oxygen atom (O1W) from the coordinated water molecule with
with non-bonding Zn1⋯Zn1A and Zn1A⋯Zn1B distances of a distance of 3.0119 Å, and the other is between the carboxyl
10.96 and 10.95 Å, respectively (Fig. 3c). The adjacent 1D oxygen atom (O4) of 1,4-BDC and the oxygen atom (O1W) from
[Zn(1,4-BDC)]n zigzag chains are linked by [Zn2(3-bpah)2] the coordinated water molecule with a distance of 3.0340 Å.
binuclear subunits to form
a
2D coordination network Moreover, by padding the water molecules into the large
porous channels, there is no residual solvent accessible void in
the whole structure. After removing the H2O molecules, the
voids are found to be 3.9% in the final network by using
PLATON28 calculations.
[Cd(3-bpah)(1,4-BDC)(H2O)] (5). Complex 5 is a 2D Cd(II)
coordination framework bridged by organic ligands 3-bpah
and 1,4-BDC. The fundamental building unit of 5 contains one
Cd(II) ion, one 3-bpah ligand, one 1,4-BDC anion and one co-
ordinated water molecule (Fig. 4a). The Cd(II) center takes a
distorted octahedron geometry, in which one nitrogen atom
(N1) from the 3-bpah ligand [Cd(1)–N(1) = 2.3200(15) Å] and
three oxygen atoms (O1, O3, and O4) from two 1,4-BDC anions
[Cd(1)–O(1) = 2.2784(16) Å, Cd(1)–O(3) = 2.5293(13) Å, Cd(1)–
O(4) = 2.3037(13) Å] form the equatorial plane. The apical posi-
tions are occupied by one amide oxygen atom (O5) from a
3-bpah ligand [Cd(1)–O(5) = 2.3538(13) Å] and one oxygen
atom (O1W) from one coordinated water molecule [Cd(1)–
O(1W) = 2.2925(14) Å]. Two 3-bpah ligands with a μ2-bridging
mode (via the ligation of one pyridyl nitrogen and one amide
oxygen atom) link two Cd(II) ions to generate a [Cd2(3-bpah)2]
binuclear subunit with the dimensional size of 5.30 × 3.57 Å2
(Fig. 4b). The dimensional size of the binuclear subunit in 5 is
obviously smaller than that in 4, which is attributed to the
different coordination modes of the 3-bpah ligands
(Table S7†). In 5, the 1,4-BDC anions possess two different
coordination modes. One acts as a bis(chelating) bridging
ligand linking two Cd(II) ions, in which each carboxyl group
adopts a μ1–η1:η1-mode, while the other acts as a bis(mono-
dentate) bridging ligand, in which each carboxyl group with a
μ1–η1:η0-mode connects two Cd(II) ions. The adjacent Cd(II)
ions are bridged by 1,4-BDC anions with different coordination
modes to form a 1D [Cd(1,4-BDC)]n zigzag chain (Fig. 4c). Such
1D chains are further interlinked by a [Cd2(3-bpah)2] loop to
form a 2D layer (Fig. 4d). Topologically, complex 5 displays a
2D (2,4)-connected (4·85)(4) structure, which is similar to that
of complex 4 (Fig. 4e). The 2D layers are further linked by
hydrogen-bonding interactions between the coordinated water
Fig. 3 (a) The coordination environment of the Zn(II) ion in complex 4
with 50% thermal ellipsoids. (b) A view of the [Zn2(3-bpah)2] loop in
complex 4. (c) A view of the 1D zigzag chain in complex 4. (d) A view of
the 2D layer of complex 4. (e) A view of the 2D (2,4)-connected topology
of complex 4.
molecules O1W and the carboxyl O atoms of 1,4-BDC [O(1W)–
H(1WA)⋯O(2) = 2.8327 Å] to generate a 3D supramolecular
architecture (Fig. S5†). In addition, the voids are found to be
9.6% in the final networks by using PLATON28 calculations.
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μ2-bridging ligands to link the Cu(II) ions producing two kinds
of 1D chains. In complex 4, the Zn(II) ions also show the
coordination number of 4 and the 1,4-BDC ligands also act as
μ2-bridging ligands, however, a 2D network was constructed.
The 3-bpah ligands adopt a μ2-bridging mode (via ligation of
two pyridyl nitrogen atoms) to connect two Zn(II) ions con-
structing a [Zn2(3-bpah)2] binuclear subunit but not a 1D
chain, which is different from that in 1–3. When the Cd(II)
ions, which belong to the same IIB group as the Zn(II) ions but
have a bigger atomic radius, were employed as the central
metal in complex 5, the coordination number of the Cd(II) ions
was 6. In addition, though complex 5 shows a similar 2D
network to 4, the 3-bpah ligands adopt a different μ2-bridging
mode (via the ligation of one pyridyl nitrogen and one amide
oxygen atom) to link two Cd(II) ions constructing a smaller
[Cd2(3-bpah)2] binuclear subunit. It is the central metal that
determines the final structure of complexes 1–5, as well as the
influence the coordination modes of the 3-bpah and 1,4-BDC
ligands. The differences in the coordination numbers and the
atomic radii of the central metal ions are the key reasons for
producing diverse structures. In addition, the 3-bpah ligands
act as μ2-bridging ligands in complexes 1–5 but exhibit
different conformations and coordination modes, resulting in
the formation of different metal-3-bpah subunits, which indi-
cates that the semi-rigid 3-bpah is an excellent structural con-
structor to design and build novel complexes.
IR spectra
The IR spectra of complexes 1–5 are shown in Fig. S6.† The
bands at 3300, 3299, 3280 and 3412 cm−1 are assigned to the
stretching and bending vibrations of the –OH groups of water
molecules for complexes 1, 2, 4 and 5, respectively.29,30 The
strong peaks at 1649, 1649, 1651, 1656 and 1652 cm−1 for com-
plexes 1–5 are identified as the νCvO vibrations of the amide
groups.29–31 The presence of the characteristic bands at 1598
and 1422 cm−1 for 1, 1599 and 1422 cm−1 for 2, 1607 and
1431 cm−1 for 3, 1601 and 1428 cm−1 for 4 and 1624 and
1433 cm−1 for 5, may be attributed to the asymmetric and sym-
metric vibrations of the carboxyl groups.29–31 The strong peaks
Fig. 4 (a) The coordination environment of the Cd(II) ion in complex 5
with 50% thermal ellipsoids. (b) A view of the [Cd2(3-bpah)2] loop in
complex 5. (c) A view of the 1D zigzag chain in complex 5. (d) A view of
the 2D layer of complex 5. (e) A view of the 2D (2,4)-connected topology
in complex 5.
It is well known that the central metal ions play an impor- at 1364 and 1053 cm−1 for 1, 1365 and 1050 cm−1 for 2, 1369
tant role in the formation of the final frameworks and various and 1016 cm−1 for 3, 1382 and 1054 cm−1 for 4 and 1367 and
structures. In this work, five coordination polymers based on 1018 cm−1 for 5, suggest the νC–N stretching vibrations of the
3-bpah and 1,4-BDC mixed ligands and different metal centers pyridyl ring of the 3-bpah ligands.29–31
(Co(II), Ni(II), Cu(II), Zn(II), Cd(II)) have been synthesized under
Powder X-ray diffraction and thermogravimetric analyses
similar hydrothermal conditions, which exhibit distinct
coordination networks. In complexes 1 and 2 the Co(II)/Ni(II) The powder X-ray diffraction (PXRD) patterns for complexes
ions, which belong to the VIIIB group and have similar atomic 1–5 are presented in Fig. S7.† The as-synthesized patterns are
radii, were used as the central metals and showed a coordi- in good agreement with the corresponding simulated ones,
nation number of 6. As a result, isostructural 1D chains 1 and indicating the phase purity of the samples.
2 have been obtained, in which the 3-bpah ligands act as
Thermogravimetric analysis was carried out for complexes
μ2-bridging ligands to connect the Co(II)/Ni(II) ions yielding a 1–5 in order to investigate their thermal stability. As shown in
1D meso-helical chain with monodentate 1,4-BDC ligands at Fig. S8,† the first weight loss of 8.72% for 1, 8.49% for 2,
both sides in an outward fashion. When the Cu(II) ions were 2.98% for 4 and 2.88% for 5, occurred in the range of
selected as the central metal in 3, a 3D CdSO4 framework was 151–195 °C, 136–206 °C, 52–118 °C and 117–173 °C, respect-
formed, in which the Cu(II) ions exhibited the coordination ively, and can be attributed to the removal of water molecules
number of 4, both the 3-bpah and 1,4-BDC ligands act as (calcd 8.98% for 1, 8.98% for 2, 3.15% for 4 and 2.91% for 5).
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Then, the second weight loss of about 78.75%, 79.23%, band has been observed compared with the free 3-bpah
82.36% and 76.18% (calcd 78.55%, 78.55%, 82.69% and ligand. The fluorescence behaviors of 4 and 5 can be best
76.41%) in the range of 251–684 °C, 262–511 °C, 311–639 °C ascribed to the ligand-to-metal charge-transfer (LMCT).18,37
and 384–567 °C, may suggest the decomposition of the
In order to investigate the effect of the guest molecules on
organic ligands. Complex 3 does not contain water molecules, the fluorescence properties, the fluorescence spectra of the
and it is stable up to 205 °C, after which the framework begins powder samples of 4a and 5a (the guest-free form of 4 and 5)
to decompose with a continuous weight loss of 85.27% up to were recorded. Desolvated 4a and 5a were prepared as follows:
620 °C, which could be attributed to the loss of the organic after immersion in CH3OH for 3 days with the soaking solu-
ligands (calcd 85.51%). For the five complexes, the final resi- tion being replaced every 6 h, the solvent-exchanged samples
dues of 12.53%, 12.28%, 14.73%, 14.66% and 20.94% are were evacuated at 60 °C for 1 h and then 120 °C for over 3 h.
close to the calculated values of 12.47%, 12.47%, 14.49%, Interestingly, the fluorescence spectrum of 4a reveals an
14.16% and 20.68% by assuming the CoO, NiO, CuO, ZnO and obvious fluorescence enhancement as shown in Fig. S9,†
CdO phases as the final residues, respectively.
which indicates that the guest water molecules in 4 have a
great effect on the fluorescence intensity. The fluorescence
spectrum of 5a is same as that of 5, indicating that there are
no guest molecules in the voids of network 5, which is consist-
ent with the formula of 5. In order to further confirm the
effect of the guest molecules on the fluorescence properties,
the solid-state fluorescence properties of 4a and 5a containing
various solvent molecules (designated as 4a-solvent and
5a-solvent, respectively) were investigated at room temperature.
The complexes 4a-solvent and 5a-solvent were prepared by
introducing 5.0 mg of 4a and 5a fine powder into 5.0 mL of
MeOH, EtOH, CH3CN, CH2Cl2, DMF, DMSO, THF, distilled
water, diethyl ether or cyclohexane for 72 h, respectively. After
sonication treatment, aging for over 24 h, shaking, filtering
and drying in air, the fluorescence samples were obtained.
Then the fluorescence spectra were measured.
As shown in Fig. 6 and Fig. S10,† the fluorescence intensity
of 4a-EtOH was enhanced. In contrast, for the other 4a-solvent,
such as MeOH, CH3CN, CH2Cl2, DMF, DMSO, distilled water,
diethyl ether and cyclohexane, different degrees of a decrease
in the fluorescence properties can be observed. Furthermore,
there was no obvious change for 4a-THF. However, for
5a-MeOH, EtOH, CH3CN and CH2Cl2, the fluorescence intensi-
ties were enhanced; for 5a-DMSO, no fluorescence intensity
change could be observed; while the other 5a-solvents show
different degrees of a fluorescence intensity decrease. The
results indicate that the guest solvent molecules in the voids of
network 4a or 5a exhibit a great effect on their fluorescence
intensity. The fluorescence response of 4a-EtOH and 5a-EtOH
was the largest, while the fluorescence intensity of 4a-cyclo-
hexane and 5a-cyclohexane was the lowest.
Fluorescence property
The fluorescence properties of the transition metal complexes
1–5, together with the free 3-bpah ligand were studied in the
solid-state at room temperature. The emission spectra of the
complexes and the free ligand have been measured under the
same conditions (slit width = 2.5 nm) and are depicted in
Fig. 5. Obviously, the free ligand 3-bpah exhibits the emission
peaks at 386 nm with the excitation at 315 nm, which can
probably be assigned to the π→π* or n→π* transitions.32,33
The fluorescence peaks at about 386 nm for 1–3, 420 nm for 4
and 424 nm for 5 are found in the emission spectra when they
are excited at 320 nm, respectively. It was considered that the
1,4-BDC anion has no significant contribution to the fluo-
rescence emission of the coordination polymers with the pres-
ence of the N-donor ligands.34,35 Although the Co(II)/Ni(II)/
Cu(II) complexes do not contain d10 metal centers, a series of
weak fluorescence Co(II)/Ni(II)/Cu(II) complexes have been
reported.34–36 Here, complexes 1–3 also show weak fluo-
rescence bands. The density functional theory calculations
indicate that the weak fluorescence may be a ligand-based
emission, which may be ascribed to the π→π* or n→π* tran-
sitions.18 For complexes 4 and 5, it is clear that an obvious
red-shifted (34 nm for 4 and 38 nm for 5) strong emission
The above results show that 4a and 5a may be applied as
fluorescence sensors for some organic molecules with a high
sensitivity and selectivity. Specifically, an amount of ethanol
can be easily sensed because of its significant fluorescence
enhancement, which is dependent on its content. Fig. S11†
shows the fluorescence spectra of 4a-EtOH–H2O and 5a-EtOH–
H2O with various amounts of ethanol. It can be clearly seen
that the fluorescence intensity gradually increase with the
increase of the ethanol content, which indicates that 4a and 5a
are promising fluorescent probes for detecting both EtOH
molecules and water molecules simultaneously.
In addition, we also investigated the solid-state fluorescence
properties of the 4a- and 5a-aromatic molecules, which
Fig. 5 The emission spectra of complexes 1–5 and the free ligand 3-
bpah.
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Fig. 6 The solid-state fluorescence spectra of complexes 4a-solvent
and 5a-solvent.
revealed a unique fluorescence enhancement and quenching
behavior upon immersing in methylbenzene, benzene and
nitrobenzene, respectively (Fig. 7). Among them, methylben-
zene enhanced the emission intensity most significantly
(60.6% for 4a and 78.9% for 5a), followed by benzene (54.4%
for 4a and 49.5% for 5a). Oppositely, nitrobenzene is a fluo-
rescence quencher, which quenched the emission intensity as
much as 81.8% for 4a and 95.9% for 5a. This is in accordance
with the trend of the electron-withdrawing groups. The results
demonstrate the ability of 4a and 5a as fluorescence sensors to
detect some aromatic molecules. As shown in Fig. 8, with the
increase of the nitrobenzene content, the emission intensity of
4a- and 5a-nitrobenzene–ethanol decreases gradually, which
indicates that 4a and 5a are promising fluorescent probes for
detecting both nitrobenzene and ethanol simultaneously.
Fig. 7 (a, b) The solid-state fluorescence spectra of 4a and 5a, as well
as 4a- and 5a-methylbenzene, benzene and nitrobenzene. (c) A view of
the percentage of fluorescence enhancement and quenching of 4a and
5a by three different analytes.
1–3-CPE at different scan rates, respectively. It can be seen
clearly that in the potential range of 650 to 150 mV, 800 to
−100 mV and 400 to −600 mV, a quasi-reversible redox peak is
observed at the 1–3-CPE, respectively, which could be attribu-
ted to the redox couple of CoIII/CoII, NiIII/NiII and CuII/CuI,
respectively.18,19 The mean peak potentials E1/2 = (Epa + Epc)/2
Electrochemical properties
The electrochemical studies of 1-CPE, 2-CPE and 3-CPE were
carried out in a 0.01 M H2SO4 + 0.5 M Na2SO4 aqueous solu-
tion. Fig. 9 and Fig. S12† show the cyclic voltammograms of
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Fig. 9 The cyclic voltammograms of 3-CPE in a 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 and 100 mV s−1). The inset shows the plots
of the anodic and cathodic peak currents against the scan rates.
peak potentials shift to the positive direction. The plots of the
peak current versus scan rate are shown in the inset of Fig. 9
and Fig. S12.† The anodic and cathodic peak currents are pro-
portional to the scan rates, indicating that the redox processes
for 1–3-CPE are surface-controlled.16,26
In addition, we investigated the electrocatalytic activity of
1–3-CPE toward the reduction of nitrite. No obvious response
on a bare CPE in a 0.01 M H2SO4 + 0.5 M Na2SO4 aqueous
solution containing a 1.0 mmol L−1 KNO2 solution can be
observed in the potential ranges of 650 to 150 mV for 1, 800 to
−100 mV for 2 and 400 to −600 mV for 3. As shown in Fig. 10
and Fig. S13,† with the addition of nitrite, the reduction peak
currents of the 1–3-CPEs increase while the corresponding
oxidation peak currents decrease, which indicates that the
Fig. 8 The solid-state fluorescence spectra of 4a (a) and 5a (b)
immersed in ethanol in the presence of various amounts of nitroben-
zene (the insert is graph of the fluorescence intensity of 4a and 5a-
ethanol as a function of the nitrobenzene content). The photographs
show 4a (c) and 5a (d) immersed in different ratios of nitrobenzene/
ethanol.
are +378 mV for 1-CPE, +326 mV for 2-CPE, and −30 mV for
3-CPE (50 mV s−1), respectively.
The effect of the scan rates on the electrochemical behavior
of 1–3-CPE can be seen from Fig. 9 and Fig. S12,† with the
scan rates increasing from 10 to 100 mV s−1, the peak poten-
tials of 1–3-CPE change gradually: the cathodic peak potentials
shift to the negative direction and the corresponding anodic
Fig. 10 The cyclic voltammograms of the bare CPE in a 0.01 M H2SO4
+ 0.5 M Na2SO4 aqueous solution containing 1.0 mmol L−1 KNO2, 3-CPE
in a 0.01 M H2SO4 + 0.5 M Na2SO4 aqueous solution containing: 0.0 (a),
2.0 (b), 4.0 (c), 6.0 (d) and 8.0 (e) mmol L−1 KNO2, respectively. Scan
rate: 50 mV s−1
.
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1–3-CPEs have an electrocatalytic activity toward the reduction
of nitrite.
6 E. Ribeiro and Y. Gushikem, Electroanalysis, 1999, 11,
1280.
Although the electrochemical behaviors of some MOCPs
based on Co(II)/Ni(II)/Cu(II) ions have been reported, their elec-
trochemical properties are different.16,38 Mukherjee et al.16
reported the electrochemical behaviors of a series of Co(II),
Ni(II) and Cu(II) complexes with a hexadentate pyridine amide
ligand, in which the Co(II)/Ni(II)/Cu(II) complexes revealed a
7 A. Dhakshinamoorthy and H. Garcia, Chem. Soc. Rev., 2012,
41, 5262.
8 Y. B. Huang, T. F. Liu, J. X. Lin, J. Lü, Z. J. Lin and R. Cao,
Inorg. Chem., 2011, 50, 2191.
9 Y. B. Huang, T. Ma, P. Huang, D. S. Wu, Z. J. Lin and
R. Cao, ChemCatChem, 2013, 5, 1877.
one-electron
response with E1/2 = 0.48/1.17/−0.88 V in CH2Cl2 (0.1 M in
TBAP) at a platinum electrode, respectively. The differences of 11 N. Chilton, S. Langley, B. Moubaraki, A. Soncini, S. Battena
the mean peak potential among the corresponding transition and K. Murray, Chem. Sci., 2013, 4, 1719.
quasi-reversible
CoIII–CoII/NiIII–NiII/CuII–CuI 10 Y. B. Huang, S. J. Liu, Z. J. Lin, W. J. Li, X. F. Li and R. Cao,
J. Catal., 2012, 292, 111.
metal complexes may be ascribed to the different electrolyte 12 C. H. Lee, H. Y. Huang, Y. H. Liu, T. T. Luo, G. H. Lee,
solutions. In addition, our group38 have also obtained three
isostructural Co(II)/Ni(II)/Cu(II) coordination polymers, which
S. M. Peng, J. C. Jiang, I. Chao and K. L. Lu, Inorg. Chem.,
2013, 52, 3962.
exhibited a reversible redox peak, respectively. Unfortunately, 13 S. J. Liu, Y. B. Huang, Z. J. Lin, X. F. Li and R. Cao, RSC
they have no electrocatalytic activity toward the reduction of
nitrite.
Adv., 2013, 3, 9279.
14 C. Yan, K. Li, S. C. Wei, H. P. Wang, L. Fu, M. Pan and
C. Y. Su, J. Mater. Chem., 2012, 22, 9846.
15 S. L. Huang, X. X. Li, X. J. Shi, H. W. Hou and Y. T. Fan,
J. Mater. Chem., 2010, 20, 5695.
16 S. Pandey, P. P. Das, A. K. Singh and R. Mukherjee, Dalton
Trans., 2011, 40, 10758.
17 T. Mitkina, N. Zakharchuk, D. Naumov, O. Gerasko,
D. Fenske and V. Fedin, Inorg. Chem., 2008, 47, 6748.
18 Y. J. Cui, Y. F. Yue, G. D. Qian and B. L. Chen, Chem. Rev.,
2012, 112, 1126.
19 Y. C. Chang and S. L. Wang, J. Am. Chem. Soc., 2012, 134,
9848.
20 G. L. Liu, Y. J. Qin, L. Jing, G. Y. Wei and H. Li, Chem.
Commun., 2013, 49, 1699.
21 A. Benvidi, P. Kakoolaki, A. Gorji, M. Mazloum-Arda-
kani, H. Zare and R. Vafazadeh, Anal. Methods, 2013,
5, 6649.
Conclusion
In summary, we have successfully synthesized five new metal–
organic coordination polymers based on five metal(II) ions and
the mixed ligands of semi-rigid bis-pyridyl-bis-amide ligand
3-bpah and 1,4-BDC. The diverse structures of the title com-
plexes indicate that the central metal ions play an important
role in the construction of the final framework. Complexes 4
and 5 exhibit excellent fluorescence properties and fluorescent
sensing behaviors toward ethanol and nitrobenzene. The
central metals also show great effects on the various properties
of the target complexes. Moreover, the fluorescence selectivity
of 4 and 5 make them promising candidates for use as fluo-
rescent sensor materials.
22 X. L. Wang, H. Y. Lin, B. Mu, A. X. Tian and G. C. Liu,
Dalton Trans., 2010, 39, 6187.
23 X. L. Wang, J. Luan, H. Y. Lin, C. Xu, G. C. Liu, J. W. Zhang
and A. X. Tian, CrystEngComm, 2013, 15, 9995.
24 X. L. Wang, J. Luan, H. Y. Lin, Q. L. Lu, C. Xu and
G. C. Liu, Dalton Trans., 2013, 42, 8375.
25 H. Chen, H. Y. An, X. Liu, H. L. Wang, Z. F. Chen, H. Zhang
and Y. Hu, Inorg. Chem. Commun., 2012, 21, 65.
26 G. M. Sheldrick, Acta Crystallogr., Sect. A: Found. Crystal-
logr., 2008, 64, 112.
Acknowledgements
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 Innova-
tive Research Team in University of Liaoning Province
(LT2012020) is gratefully acknowledged.
27 X. L. Wang, H. Y. Zhao, H. Y. Lin, G. C. Liu, J. N. Fang and
B. K. Chen, Electroanalysis, 2008, 20, 1055.
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