Assembly of Zn/Cd coordination polymers containing helixes or polycatenane structures tuned by the tri-pyridyl-bis-amide ligands with different spacer: Syntheses, structures, photoluminescent and photocatalytic properties

Article abstract of DOI:10.1039/c2ce26892b

Four new d¹⁰ metal-organic coordination polymers tuned by the V -shaped tri-pyridyl-bis-amide ligands with different spacers, namely, [Zn(L₁)(BDC)]·H₂O (1), [Cd(L ₁)(BDC)]·H₂O (2), [Zn(L₂)(BDC)] (3) and [Cd(L₂)(BDC)] (4) (L₁ = N,N′-bis(pyridine-3-yl) pyridine-2,6-dicarboxamide, L₂ = N,N′-bis(pyridine-3-yl) pyridine-3,5-dicarboxamide, H₂BDC = 1,4-benzenedicarboxylic acid) have been synthesized under hydrothermal conditions. In complexes 1 and 2, the metal ions are linked by L₁ to form left- and right-helical Zn/Cd-L₁ chains, which are further extended into two-dimensional (2D) wave-like layers by BDC anions. In 3, two L₂ ligands link two Zn^II ions forming the Zn₂(L₂)₂ loops, which are connected by BDC anions to form a (2·6⁵) topological 2D network. In addition, the large Zn₂(L ₂)₂ loops are threaded by the BDC rods from above and below 2D layers so as to form 2D → 3D polyrotaxane and polycatenane structures. In 4, the Cd^II ions are linked by L₂ ligands to generate 1D double chain ribbons with Cd₂(L₂) ₂ loops, which are further connected by the BDC linkers to form a 3D framework. Two identical 3D frameworks interpenetrate each other in a twofold mode, giving rise to a polyrotaxane and polycatenane array, which is relative limited. The diverse structures of complexes 1-4 demonstrate that the tri-pyridyl-bis-amide ligands and the central metals have significant effect on the final structures. The thermal stability and fluorescent properties of complexes 1-4 have been investigated. In addition, the title complexes exhibit photocatalytic activity for dye methylene blue degradation under UV light.

Full text of DOI:10.1039/c2ce26892b

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Assembly of Zn/Cd coordination polymers containing
helixes or polycatenane structures tuned by the tri-
pyridyl–bis-amide ligands with different spacer:
syntheses, structures, photoluminescent and
photocatalytic properties3
Cite this: CrystEngComm, 2013, 15,
1960
Xiuli Wang,* Jingjing Huang, Lianli Liu, Guocheng Liu, Hongyan Lin, Juwen Zhang,
Naili Chen and Yun Qu
Four new d10 metal–organic coordination polymers tuned by the ‘‘V’’-shaped tri-pyridyl–bis-amide ligands
with different spacers, namely, [Zn(L1)(BDC)]?H2O (1), [Cd(L1)(BDC)]?H2O (2), [Zn(L2)(BDC)] (3) and
[Cd(L2)(BDC)] (4) (L1 = N,N9-bis(pyridine-3-yl)pyridine-2,6-dicarboxamide, L2 = N,N9-bis(pyridine-3-yl)pyr-
idine-3,5-dicarboxamide, H2BDC = 1,4-benzenedicarboxylic acid) have been synthesized under hydro-
thermal conditions. In complexes 1 and 2, the metal ions are linked by L1 to form left- and right-helical Zn/
Cd–L1 chains, which are further extended into two-dimensional (2D) wave-like layers by BDC anions. In 3,
two L2 ligands link two ZnII ions forming the Zn2(L2)2 loops, which are connected by BDC anions to form a
(2?65) topological 2D network. In addition, the large Zn2(L2)2 loops are threaded by the BDC rods from
above and below 2D layers so as to form 2D A 3D polyrotaxane and polycatenane structures. In 4, the CdII
ions are linked by L2 ligands to generate 1D double chain ribbons with Cd2(L2)2 loops, which are further
connected by the BDC linkers to form a 3D framework. Two identical 3D frameworks interpenetrate each
other in a twofold mode, giving rise to a polyrotaxane and polycatenane array, which is relative limited.
The diverse structures of complexes 1–4 demonstrate that the tri-pyridyl–bis-amide ligands and the central
metals have significant effect on the final structures. The thermal stability and fluorescent properties of
complexes 1–4 have been investigated. In addition, the title complexes exhibit photocatalytic activity for
dye methylene blue degradation under UV light.
Received 20th November 2012,
Accepted 3rd January 2013
DOI: 10.1039/c2ce26892b
www.rsc.org/crystengcomm
coordination geometry and various experimental conditions.7
Currently, the increasing interest has been concentrated on the
Introduction
The design and construction of metal–organic coordination
polymers (MOCPs) based on metal ions and multifunctional
organic ligands have attracted great interest in the recent years,
not only due to these solids’ multifunctional applications in gas
and selective molecule adsorption,1 luminescence,2 ion
exchange,3 magnetism4 and catalysis,5 but also because of their
complicated molecular architectures and topological features.6
In particular, the design and synthesis of high dimensional
coordination polymers represents a quite active area. It is well
known that the self-assembly process of MOCPs mainly
depends on the combination of several factors, such as the
nature of the organic ligands used, the versatility of the metal
N-donor bridging ligands because it is the most important
factor in the construction of high dimensional networks.
Among various N-donor ligands, pyridyl–amide-based ligands
are attracting more and more attention in the recent years.
Chen et al. and our group have used some flexible bis-pyridyl–
bis-amide ligands constructing many fascinating architectures
and investigated these ligands’ coordination behaviours.8
However, the semi-rigid tri-pyridyl–bis-amide ligands have been
rarely reported to our knowledge.9
As an ongoing work in acylamide-based ligands,10 we try to
explore the effect of the spacer and coordination sites of
pyridyl–amide-based ligands on the assembly and structures of
target complexes, thus in this work, two ‘‘V’’-shaped tri-pyridyl–
bis-amide ligands with different spacers, N,N9-bis(pyridine-3-yl)
pyridine-2,6-dicarboxamide (L1) and N,N9-bis(pyridine-3-yl)
pyridine-3,5-dicarboxamide (L2), were designed and employed
to construct MOCPs. Comparing with the bis-pyridyl–bis-amide
ligands, L1 and L2 ligands not only possess the semi-rigid spacer
Department of Chemistry, Bohai University, Liaoning Province Silicon Materials
Engineering Technology Research Centre, Jinzhou, 121000, P. R. China.
E-mail: wangxiuli@bhu.edu.cn
Electronic supplementary information (ESI) available. CCDC 905091–905094.
3
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c2ce26892b
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1544 m, 1494 m, 1350 s, 1288 w, 1207 w, 889 m, 821 w, 744 w,
657 m.
to tune the flexibility of the whole ligands, but also have more
N-donor coordination sites in the middle junction moieties to
construct high-dimensional frameworks. On the other hand,
acylamide groups of the ligands have the unique potential
capability of providing two types of hydrogen bonding sites: the
–NH moiety acting as an electron acceptor and the –CLO group
as an electron donor.8a
The aromatic dicarboxylates, as a kind of typical multi-
dentate O-donor ligands, are proven to be good candidates for
the self-assembly of coordination polymers because they can
provide both structural scaffolding and necessary charge
equalization.11 Especially, the 1,4-benzenedicarboxylate
(BDC) ligand has been proven useful in constructing high
dimensional coordination polymer networks.12 To the best of
our knowledge, the tri-pyridyl–bis-amide ligands combining
with aromatic dicarboxylates have not been reported in
building MOCPs so far.
Considering the central metal’s effect on the structures of
MOCPs,13 here ZnII and CdII ions were selected as central
metal ions. On the basis of the aforementioned points, four
coordination polymers have been synthesized by using BDC
and L1/L2 mixed ligands under hydrothermal conditions,
namely, [Zn(L1)(BDC)]?H2O (1), [Cd(L1)(BDC)]?H2O (2),
[Zn(L2)(BDC)] (3) and [Cd(L2)(BDC)] (4). The effect of different
N-donor ligands and the central metals on the ultimate
frameworks have been represented and discussed. In addition,
the thermal stability, photoluminescence properties along
with photocatalytic properties of 1–4 have also been investi-
gated in detail.
[Cd(L1)(BDC)]?H2O (2). The same synthetic method as that
for 1 was used except that ZnSO4?7H2O was replaced by
3CdSO4?8H2O (0.052 g, 0.067 mmol). Yellow block-shaped
crystals of 2 were obtained. Yield: 45% (based on Cd).
Elemental anal.(%) Anal. Calc. for C25H19N5O7Cd: C 48.91, H
3.12, N 11.41. Found: C 49.34, H 3.07, N 11.12. IR (KBr, cm21):
3382 m, 3253 w, 1678 s, 1639 m, 1544 s, 1483 w, 1384 m, 1278
w, 1205 w, 889 m, 744 w, 657 m.
[Zn(L2)(BDC)] (3). Complex 3 was prepared in the same way
as 1 except that L2 was used instead of L1. Yellow block-shaped
crystals of 3 were obtained. Yield: 55% (based on Zn).
Elemental anal.(%) Anal. Calc. for C25H17N5O6Zn: C 54.71, H
3.12, N 12.76. Found: C 54.25, H 3.01, N 13.27. IR (KBr, cm21):
3419 w, 3184 w, 1681 s, 1587 s, 1544 m, 1377 s, 1299 w, 1205 w,
1091 w, 890 m, 719 w, 746 m, 657 m.
[Cd(L2)(BDC)] (4). Complex 4 was obtained in the same way
as that described for complex 3, except that ZnSO4?7H2O was
replaced by 3CdSO4?8H2O. Yellow block-shaped crystals of 4
were obtained. Yield: 53% (based on Cd). Elemental anal.(%)
Anal. Calc. for C25H17N5O6Cd: C 50.39, H 2.88, N 11.75. Found:
C 49.93, H 2.76, N 12.08. IR (KBr, cm21): 3249 w, 3062 w, 1687
s, 1548 s, 1483 m, 1386 s, 1374 w, 1286 m, 1101 w, 1047 w, 891
m, 837 w, 742 m, 655 w.
X-ray crystallographic study
Crystallographic data for complexes 1–4 were collected on a
Bruker Smart 1000 CCD diffractometer with Mo Ka radiation
(l = 0.71073 Å) by v and h scan mode at 293 K. The crystal
structures were solved by direct methods using the SHELXS
program of the SHELXTL crystallographic software package
and refined by the full-matrix least-squares methods on F2
using the SHELXTL.14 Non-hydrogen atoms were refined with
anisotropic temperature parameters. The hydrogen atoms of
organic ligands were generated geometrically and refined
isotropically. The detailed crystal data and structures refine-
ment for 1–4 are summarized in Table 1. Selected bond
Experimental section
Materials and characterization
All chemicals were commercially available and used as
purchased. Ligands L1 and L2 were prepared according to the
literature.9f FT-IR spectra were taken on a Varian FT-IR 640
spectrometer (KBr pellets). Elemental analyses (C, H, N) were
lengths and angles are given in Table S1, ESI.
3
carried out on
a Perkin-Elmer 240C element analyzer.
Thermogravimetric analyses (TGA) were recorded on a Pyris–
Diamond TG instrument. The luminescence spectra were
measured on a HITACHI F-4500 Fluorescence Spectrophoto-
meter. UV-Vis absorption spectra were obtained using a SP-1900
UV-Vis spectrophotometer.
Results and discussion
Description of crystal structures
[Zn(L1)(BDC)]?H2O (1). Single crystal X-ray crystallography
reveals that complex 1 features a 2D wave-like layer structure.
The asymmetric unit of 1 contains one crystallographically
independent ZnII ion, one L1 ligand, one BDC anion, and one
crystalline water molecule. As depicted in Fig. 1, the
coordination environment around the ZnII ion is composed
of two nitrogen atoms (N1, N5#1) from two L1 ligands and two
oxygen atoms (O1, O2) from two BDC anions, exhibiting a
distorted tetrahedral geometry. The Zn–O bond distances are
1.9462(18) [Zn(1)–O(1)] and 1.9606(17) Å [Zn(1)–O(2)], and the
Zn–N bond distances are 2.084(2) [Zn(1)–N(1)] and 2.063(2) Å
[Zn(1)–N(5)#1], which are comparable to those of the similar
zinc complexes.8e The BDC anion only adopts one type of bis-
monodentate coordination mode (Chart 1). Adjacent ZnII ions
Preparation of the complexes 1–4
[Zn(L1)(BDC)]?H2O (1). A mixture containing of ZnSO4?7H2O
(0.058 g, 0.2 mmol), L1 (0.032 g, 0.1 mmol), 1,4-H2BDC (0.025
g, 0.15 mmol) and water (8 mL) was placed in a 25 mL Teflon
reactor and the pH value was adjusted around 7 by addition of
0.1 M NaOH solution. The mixture was heated at 120 uC for 6
days, and then the autoclave was gradually cooled to room
temperature. Yellow block-shaped crystals of 1 were obtained
in 54% yield (based on Zn). Elemental anal.(%) Anal. Calc. for
C25H19N5O7Zn: C 52.97, H 3.38, N 12.36. Found: C 52.62, H
3.42, N 12.71. IR (KBr, cm21): 3238 m, 3057 w, 1689 s, 1604 s,
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Table 1 Crystal data and structure refinements for 1–4
Complexes
1
2
3
4
Formula
Formula weight
Crystal system
Space group
C25H19N5O7Zn
566.84
Triclinic
¯
P1
C25H19N5O7Cd
613.86
Triclinic
¯
P1
C25H17N5O6Zn
548.83
Monoclinic
P21/n
C25H17N5O6Cd
595.84
Monoclinic
P21/n
a/Å
b/Å
c/Å
8.8798(6)
9.2592(6)
15.0913(10)
104.7570(10)
99.5260(10)
90.5160(10)
1181.63(14)
2
9.064(3)
9.558(3)
13.644(4)
95.546(4)
91.307(5)
90.209(5)
1176.2(6)
2
11.9526(9)
12.6037(9)
16.3810(12)
90
102.0820(10)
90
2413.1(3)
4
1.511
11.1167(6)
14.3657(7)
15.1482(8)
90
93.2610(10)
90
2415.2(2)
4
1.639
a/u
b/u
c/u
V/Å3
Z
D/g cm23
1.593
1.098
1.733
0.987
m/mm21
1.069
0.956
F(000)
580
6299
4164
0.0120
1.028
0.0333
0.0937
616
6242
4151
0.0225
1.002
0.0271
0.0678
1120
12 710
4373
0.0398
1.005
0.0462
0.1424
1192
15 306
5980
0.0188
1.002
0.0260
0.0691
Total reflections
Unique reflections
Rint
GOF
a
R1 [I . 2s(I)]
b
wR2 (all data)
a
b
2
R1 = S||Fo| 2 |Fc||/S|Fo|. wR2 = S[w(Fo 2 Fc2)2]/S[w(Fo2)2]1/2
.
are bridged by the BDC anions to form a 1D zigzag chain [Zn–
two ZnII ions acts as a ‘V’-shape spacer. Thus, the structure can
¨
BDC] (Fig. S1, ESI ).
be classified as a 4-connected network with Schlafli symbol of
3
n
{44?62} via the Topos40 program15 (Fig. 2c). Just as observed in
An appealing structural feature in 1 is the presence of a bi-
flexural helix, which is constructed by the L1 ligands bridging
the ZnII ions (Fig. 2a). The pitch of the left- and right-handed
helical chain is 8.880 Å. These helical chains are further linked
by BDC anions from [Zn–BDC]n chains to furnish a 2D layer
structure (Fig. 2b). In the 2D network, the ZnII ion is linked by
two L1 ligands and two BDC anions, which can be considered
as a 4-connected node. The BDC anion connects two ZnII ions,
which can be viewed as a linear linker. The L1 ligand bridging
Fig. S2, ESI, the final 3D supramolecular framework of 1 is
3
generated through the hydrogen bonding interactions between
the carbon atoms [C(9)] of L1 ligands and the oxygen atoms
[O(3)] of carboxyl groups with the distance of 3.08 Å [C(9)–
…
H(9A) O(3)].
The similar left- and right-handed helical chains were
obtained by Lu and co-workers,16 in which the chains were
connected to each other by piperazine or pyrazine; while in
complex 1, the chains were further linked by BDC anions. In
addition, both of them are extended into 2D layer structures.
[Cd(L1)(BDC)]?H2O (2). When the ZnII ion was replaced by
CdII ion under the similar synthetic conditions, a structurally
Fig. 1 The coordination environment of the ZnII ion in 1 (at 30% probability
Chart 1 Coordination modes of metal ZnII or CdII ions, the L1, L2 ligands and
dicarboxylate in complexes 1–4.
level). The water molecules are omitted for clarity. Symmetry codes: #1 x 2 1, y,
z.
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Fig. 3 The coordination environment of the CdII ion in 2 (at 30% probability
level). The water molecules are omitted for clarity. Symmetry codes: #1 2x + 2,
2y, 2z.
ions are linked by BDC anions to form a 2D layer structure
(Fig. 4b).
From a topological viewpoint, the CdII ions are coordinated
by two L2 ligands and three BDC ligands, which can be viewed
as a 5-connected node, the L1 ligand is considered as a
‘V’-shape spacer and the BDC anion is regarded as a linear
linker. Then, the 2D corrugated structure can be classified as a
5-connected network with Schlafli symbol of {33?44?53}
¨
(Fig. 4c). The extension of the 2D layer into a 3D supramole-
cular network is accomplished by two kinds of hydrogen
bonding interactions (Fig. S3, ESI ). One is between the oxygen
3
atoms [O(1W)] of lattice water molecules and the oxygen atoms
[O(2)] of carboxyl groups with the distance of 2.88 Å [O(1W)–
…
H(1WA) O(2)]. The other is between the oxygen atoms [O(1W)]
of lattice water molecules and the oxygen atoms [O(9)] of L1
…
ligands with the distance of 2.93 Å [O(1W)–H(1WB) O(9)].
In complex [Cd(BDC)(phen)]?H2O (phen = 1,10-phenanthro-
line) obtained by Rao and co-workers,17 the coordination
modes of BDC anions are the same as those of complex 2.
However, the CdII ions are linked by BDC anions to form a
different 2D layer due to the different angle between the BDC
anions, and the phen ligands just chelate to the CdII ions as
terminal group. While in complex 2, the L1 ligands connect the
CdII ions to form left- and right-handed helical chains
subordinating to the 2D network.
[Zn(L2)(BDC)] (3). When the L2 ligand was used instead of L1
ligand, a completely different framework of 3 was obtained.
Single crystal X-ray diffraction analysis reveals that complex 3
is an interesting 2D A 3D polyrotaxane and polycatenane
structure. The asymmetric unit consists of one crystallogra-
phically independent ZnII ion, one L2 ligand and one BDC
anion. As shown in Fig. 5, each ZnII ion adopts a distorted
tetrahedron geometry, completed by two nitrogen atoms (N1,
N5#1) from two L2 ligands and two oxygen atoms (O1, O2)
from two BDC anions. The Zn–O bond distances are 1.927(3) Å
[Zn(1)–O(1)] and 1.969(3) Å [Zn(1)–O(2)], while the Zn–N bond
distances are 2.057(3) Å [Zn(1)–N(1)] and 2.071(3) Å [Zn(1)–
N(5)#1], respectively.
Fig. 2 (a) The 1D Zn–L1 left-hand helix and the right-hand helix. (b) The 2D
coordination layer of complex 1. (c) Stick and simplified representations of the
2D structure for 1.
different complex 2 was obtained. It crystallizes in the triclinic
¯
space group P1, and the asymmetric unit consists of one
crystallographically independent CdII ion, one L1 ligand, one
BDC anion, and one lattice water molecule. Each CdII ion is
seven-coordinated by five carboxyl oxygen atoms from three
different BDC anions (O1, O2, O3, O4, O3#1) and two nitrogen
atoms (N1, N5) from two L1 ligands in a distorted pentagonal
bipyramidal geometry (Fig. 3). The Cd–O bond distances are in
the range of 2.2476(18)–2.614(2) Å and the Cd–N bond
distances are 2.353(2) [Cd(1)–N(1)] and 2.431(2) [Cd(1)–N(5)] Å.
Interestingly, the L1 ligands bridging adjacent CdII ions also
afford left- and right-handed helical chains with the pitch of
9.558 Å (Fig. 4a). There are two types of BDC anions in complex
2, one adopts a m1-g1:g1-bidentate chelating coordination
mode with the two carboxyl groups and the other shows m2-
g1:g2 coordination modes, respectively (Chart 1). Thus the CdII
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Fig. 5 The coordination environment (at 30% probability level) of the ZnII ion in
3. Symmetry codes: #1 2x, 2y, 2z.
ligands connect two ZnII ions to form a 28-membered Zn2(L2)2
loop with ca. 12 6 12.59 Å2 (Fig. S5a, ESI ), which may
3
constitute a potential factor to generate polyrotaxane and
polycatenane motifs. The adjacent loops are integrated by the
1D Zn–BDC chains to form a 2D network (Fig. 6a). The 2D
sheet could be simplified to a 3-connected net with 63-hcb
topology if the loops are regarded as a linker. However, the
description of the network cannot properly describe the
topology of interpenetration18 as it would require linkers to
pass through the Zn2(L)2 loops. If considering the ZnII ion as a
4-connected node and the L2 ligand as a ‘V’-shape spacer, then
a (2?65) topological net is created. Thus we obtain a network
description that can be used to further describe the
interpenetration. The most peculiar feature of 3 is the
entangled fashion between the Zn2(L2)2 loop and the BDC
anion. The Zn2(L2)2 loops within one sheet are large enough to
be passed by the BDC rods from the above and below 2D
layers, so as to form a 2D A 3D polythreading structure,
further giving rise to a 3D polyrotaxane and polycatenane
array. The entangled fashion is shown in Fig. 6b and c: one
Zn2(L2)2 loop of the yellow layer is threaded by one pink BDC
ion rod of the pink layer, and the loop of the pink layer is
threaded by one light blue rod of the blue layer and so on. A
closer inspection reveals that only two opposite rods perform
such rotaxane-like behavior and the other two rods are free of
such operation.
Sun and co-workers have obtained a similar 2D layer in
[Zn(bmimbp)(tbtpa)]n (bmimbp = 4,49-bis(2-methylimidazol-1-
ylmethyl)biphenyl, H2tbtpa = tetrabromoterephthalic acid).19
The final structures were completely different, although they
all contain flexible N-donors as loop linkers and aromatic
dicarboxylate as rods. The 2D network was interlocked with
each other in a 2D A 2D parallel fashion, thus directly leading
to the formation of a 2D polyrotaxane-like structure containing
rotaxane-like motifs in [Zn(bmimbp)(tbtpa)]n, while in com-
plex 3 the 2D layer was interlocked with each other in a 2D A
3D polyrotaxane and polycatenane array. The structure of
complex 3 is similar to the complex Zn(L)(BDC)?(H2O)0.5 (L =
N,N9-di(pyridin-3-yl)isophthalamide) reported by Luo and co-
Fig. 4 (a) The L1 ligands link the adjoining CdII ions into left- and right-helical
chains. (b) The 2D coordination layer of complex 2. (c) Stick and simplified
representations of the 2D wave-like network of 2.
In complex 3, the BDC anions take the m1-g1:g0 bis-
monodentate coordination mode bridging neighboring ZnII
ions to display a 1D zigzag chain (Fig. S4, ESI ). The two L
3
2
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Fig. 7 The coordination environment (at 30% probability level) of the CdII ion in
4. Symmetry codes: #1 2x + 1, 2y + 1, 2z + 1; #3 2x, 1 2 y, 1 2 z; #5 21 + x, y,
z.
the range of 2.3021(16)–2.5283(17) Å, while the Cd–N bond
distances are from 2.3350(19) to 2.6091(19) Å.
In complex 4, two carboxyl groups of the BDC anions take
the same coordination mode of chelating m1-g1:g1-bidentate to
connect adjacent CdII ions forming a zigzag chain (Fig. 8a).
The L2 ligand adopts a m3-bridging coordination mode
(Chart 1), which is different from that in complex 3. The CdII
ions are linked by L2 ligands to generate a 1D double chain
ribbon with a 28-membered Cd2(L2)2 loop. The dimension of
the loop is ca. 12.37 6 12.39 Å2 (Fig. S5b, ESI ). The 1D double
3
chains connect with the 1D Cd-BDC zigzag chains through
sharing CdII ions to form a 3D architecture, as shown in
Fig. 8a. Each CdII ion is connected to three L2 ligands and two
BDC anions and thus the CdII ion can be considered as a
5-connected node, whereas each L2 ligand is bridged by three
neighboring 5-connected CdII ions, and can be regarded as a
3-connected node. Topological analysis reveals that complex 4
Fig. 6 (a) The 2D layer and 4-connected (2?65) topology matrix of 3. (b) The
representative description of 2D A 3D polyrotaxane and polycatenane array in
3. (c) Stick and simplified representations of 2D A 3D framework in 3.
workers.20 In complex Zn(L)(BDC)?(H2O)0.5, a bis-pyridyl–bis-
amide ligand was used, while in complex 3, we adopted a tri-
pyridyl–bis-amide ligand with an extra N-donor coordination
site in the middle junction moiety. Both the structures contain
Zn2(L2)2 loops with different dimensions. They are ca. 12 6
12.59 Å2 in 3 and ca. 11.69 6 12.90 Å2 in complex
Zn(L)(BDC)?(H2O)0.5. The twist degrees of the N-donor ligands
shown by the dihedral angles between the terminal pyridyl
rings and the central rings are 66.16u and 0u in complex
Zn(L)(BDC)?(H2O)0.5, while the dihedral angles are 63.30u and
10.23u in complex 3.
[Cd(L2)(BDC)] (4). Single crystal X-ray diffraction analysis
reveals that complex 4 consists of two identical 3D frameworks
that are interpenetrated in a parallel fashion, resulting in a
twofold interpenetrating architecture showing both polyrotax-
ane and polycatenane characters. The asymmetric unit of 4
contains a CdII ion, one L2 ligand and one BDC anion. The CdII
anion is coordinated by four carboxyl oxygen atoms (O1, O2,
O3, O4) from two BDC anions and three nitrogen atoms (N1,
N5, N3#1) from three L2 ligands showing distorted pentagonal-
bipyramid geometry in Fig. 7. The Cd–O bond distances are in
¨
forms a binodal 3,5-connected framework with the Schlafli
symbol of (42?65?83)(42?6).21 Furthermore, two identical 3D
frameworks interpenetrate each other in a twofold mode
(Fig. 8c). On the other hand, if we regard the BDC anion as a
single rod, the structure of 4 can be described as a 3D metal–
organic polyrotaxane and polycatenane framework. The
entangled fashion is shown in Fig. 8b: one loop of the blue
3D framework is threaded by one BDC anion rod of the pink
3D net and the loop of the pink 3D framework is threaded by
one blue rod of the blue 3D net.
So far, although a number of interpenetrating networks have
been reported,22 examples showing the polyrotaxane and
polycatenane features have been rarely observed within
coordination polymers.8a,19,23 Recently, Ma’s group have
reported a twofold interpenetrating 3D polyrotaxane frame-
work [Cd1.5(Tipa)(BDC)Cl]2?4H2O (Tipa = tri(4-imidazolylphe-
nyl)amine.24 However, the differences can be found by the
insight into the complicated framework. The single 3D
framework
is
a
polyrotaxane
net
in
complex
[Cd1.5(Tipa)(BDC)Cl]2?4H2O, while in complex 4, the single
3D framework is just a 3,5-connected framework, which
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amide ligands were used to investigate the effect of nitrogen
atoms’ positions of the spacer groups on the structures. In
complex 1 and 2, the L1 ligands link metal ions forming left-
and right-helixes, which are finally extended into 2D layers by
BDC anions. However, the metal ions are connected by L2
ligands to generate M2(L2)2 loops, which are ultimately
threaded by BDC anions into 3D polyrotaxane and polycate-
nane arrays in 3 and 4. All of these may be ascribed to the
different positions of middle pyridyl-N atoms in L1 and L2
ligands. Moreover, the twist degrees of the L1 and L2 ligands
can also influence the final structures of 1–4. The twist degrees
of the N-donor ligands are shown by the dihedral angles
between the three pyridyl rings: h1 represents the dihedral
angle between the pyridyl group containing N1 atom and the
central pyridyl ring; h2 is the dihedral angle between the
pyridyl ring containing N5 atom and the central pyridyl group;
h3 represents the dihedral angle between the pyridyl group
containing N1 atom and the pyridyl ring containing N5 atom.
As shown in Table S2, ESI , the difference in the h angles
3
between 1–4 is mainly attributed to the changes in the
positions of the middle pyridyl-N atoms and the various
conformations of ligands when they coordinate with the
central metal atoms.
On the other hand, the central metals have significant
effects on the resultant structures of the coordination
polymers. The ZnII ion displays four-coordinated distorted
tetrahedral geometry in complexes 1 and 3. Whereas CdII ion
adopts a seven-coordinated mode and can be regarded as
distorted pentagonal-bipyramid geometry in complexes 2 and
4. Besides that, metal ions also influence the coordination
modes of organic ligands. The carboxyl groups of BDC anions
adopt a m1-g1:g0-monodentate coordination mode in 1 and 3,
while carboxyl groups of BDC anions take m1-g1:g1-bidentate
or m2-g1:g2 coordination modes in 2 and 4. In 3, the L2 ligands
show m2-bridging coordination modes to link ZnII ions forming
a Zn2(L2)2 loop, however, in complex 4 the L2 ligands
displaying a m3-bridging coordination mode connect adjacent
CdII ions to form a 1D double chain containing a Cd2(L2)2
loop. So, the difference in the coordination numbers of the
central metals is also the key reason for the structural
diversities.
Thermal stability analysis
Thermal gravimetric analyses (TGA) of complexes 1–4 were
performed to investigate their thermal stabilities. The TG
curves of complexes 1 and 2 exhibit two obvious weight loss
Fig. 8 (a) Single 3D framework constructed by ZnII–BDC chains and ZnII–L2
double chains in 4. (b) The representative description of the entangled net in 4.
(c) Schematic representation of the twofold interpenetrated polyrotaxane and
polycatenane framework of 4.
steps (Fig. S6, ESI ). The first weight loss of 3.12% for 1 from
3
185 to 260 uC and 3.01% for 2 in the region of 185–245 uC is
assigned to the elimination of the lattice water molecules
(calcd: 3.18% for 1, 2.93% for 2). The second weight loss
occurred at 356 uC for 1 and 330 uC for 2. The ZnO residue of
14.21% (calcd: 14.36%) is observed at 600 uC and the CdO
residue of 21.06% (calcd: 20.92%) is occurred at 570 uC, which
corresponds to the decomposition of organic components.
Compared with complexes 1 and 2, the TG curves of 3 and 4
only show one step weight loss process, corresponding to the
loss of BDC and L2 ligands. The overall framework of 3 begins
to collapse at 385 uC and ends at 530 uC with the ZnO residue
of 14.67% (calcd: 14.83%). No obvious weight loss is observed
interpenetrates each other in a twofold mode to obtain a
polyrotaxane and polycatenane array.
Effects of the N-donor ligands and the central metals on the
structures of the complexes
From the structure descriptions above, we can see that the
N-donor ligands play important roles in the construction of
the final structures. In this study, two kinds of tri-pyridyl–bis-
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Fig. 9 Emission spectra of L1, L2 ligands and complexes 1–4 in the solid state at
room temperature.
for complex 4 until the decomposition of the framework
occurs at 370 uC, and the CdO residue of 21.79% (calcd:
21.55%) is obtained at 570 uC.
Fluorescence properties of complexes 1–4
MOFs with d10 metal atoms are promising candidates for
photoactive materials with potential applications such as
chemical sensors, photochemistry and electroluminescent
displays.25 The emission spectra of complexes 1–4 and free
ligands have been investigated in the solid state at room
temperature under the same experimental conditions. As
illustrated in Fig. 9, L1 and L2 exhibit intense emission bands
with a maximum at 431 nm upon excitation at 280 nm and a
maximum at 432 nm upon excitation at 315 nm, respectively.
The emission bands are probably attributable to the p*–p
transitions. Intense emission peaks of complex 1 and complex
2 are found at 412 nm (lex = 307 nm) and 408 nm (lex = 307
nm), which are blue-shifted 19 nm and 23 nm as compared
with those of the pure L1 ligand. The fluorescent emission
peaks can be observed at 414 nm (lex = 325 nm) for 3, which is
blue-shifted 18 nm compared with the free L2 ligand. For
complex 4, the emission band appears at 434 nm (lex = 325
nm), which is similar to that of free L2 ligand. It is well known
that the ZnII and CdII ions are difficult to oxidize or reduce
because of the d10 configuration. Therefore, the emissions of
these complexes can be attributed to intraligand transitions.26
The different emission positions and intensities of 1–4 may be
due to their different structures.
Photocatalytic properties
Fig. 10 (Top and middle) Absorption spectra of the MB solution during the
decomposition reaction under UV irradiation with the presence of complex 1 and
2. (Bottom) Photocatalytic decomposition of MB solution under UV irradiation
with the use of the title complexes and no catalyst under the same conditions.
To study the photocatalytic activity of complexes 1–4, we
selected methylene blue (MB) as a model dye contaminant to
evaluate the photocatalytic activities in the purification of
wastewater. The experiments were investigated through a
typical process: 50 mg powder of the title complexes was
dispersed in a 200 mL MB aqueous solution with the
concentration of 10.0 mg L21, magnetically stirred in the dark
for 0.5 h to ensure the equilibrium of the working solution.
Then the solution was exposed to UV irradiation from a Hg
lamp, kept continuously stirring. 5 mL of suspension was
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taken for centrifugal separation every 30 min. Then the
transparent solution of the top layer was taken out for
analysis. Under the same conditions, the simple photolysis
comparative experiment was also completed without any
Acknowledgements
This work was supported by the Program for New Century
Excellent Talents (NCET-09-0853), the National Natural
Science Foundation of China (No. 20871022 and 21171025)
and Program of Innovative Research Team in University of
Liaoning Province (LT2012020).
catalyst. As illustrated in Fig. 10, Fig. S7 and S8, ESI , the
3
absorption peaks of MB decreased obviously under the UV in
the presence of 1 and 2, while the absorption peaks decreased
much slowly in the presence of 3 and 4. Besides that, changes
in the concentration of MB solution were plotted versus
irradiation time. The calculation results show that the MB
degrades approximately from 100% to 23% for complex 1, to
11% for complex 2, 54% for complex 3 and 74% for complex 4
after 180 min, respectively. It is obvious that the complexes 1
and 2 have higher photocatalytic activities for the degradation
of MB than those of complexes 3 and 4. The possible reason
maybe lies in the difference of components and structures.27
After photocatalysis, the color, the morphology and the IR
spectra of the complexes have not changed, which is
illustrated by microscopy and FT-IR spectrometry (Fig. S9–
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