Synthesis, structures, and properties of two helical structures from rigid carboxylate ligand and flexible N-bridging ligands

Article abstract of DOI:10.1002/zaac.201100519

Two novel coordination polymers based on mixed ligands, [Zn(dpb)(bdc)(H₂O)]_n (1) and [Cd(dpb)(bbdc)(H ₂O)(DMF)]_n (2) [dpb = 1, 4-bis(pyridin-3-ylmethoxy) benzene, H₂bdc = 1, 4-benzenedicarboxylate, H₂bbdc = 4, 4'-dibenzenedicarboxylate], were synthesized under hydrothermal conditions. Compound 1 forms meso-helical chain and shows three fold interpenetrating architecture with 4-connected net {6 6} diamond topology. Compound 2 is a left- and right-handed helical layer, which are interacted by π-π stacking interactions to construct a 3D framework. The luminescent properties of the compounds are discussed. Copyright

Full text of DOI:10.1002/zaac.201100519

ARTICLE
DOI: 10.1002/zaac.201100519
Synthesis, Structures, and Properties of two Helical Structures from Rigid
Carboxylate Ligand and Flexible N-Bridging Ligands
Xia Li,[a] Wenjie Zhao,[b] Yue Zhang,[a] Yu Zhang,[a] Jinting Tan,[a] Yingli Lu,[a]
Xing Feng,[a] and Xuwu Yang*[a]
Keywords: Helical structure; Porous framework; Three-fold interpenetrating architecture; Luminescence; Zinc; Cadmium
Abstract. Two novel coordination polymers based on mixed ligands, and shows three fold interpenetrating architecture with 4-connected net
[Zn(dpb)(bdc)(H2O)]n (1) and [Cd(dpb)(bbdc)(H2O)(DMF)]n (2) [dpb {66} diamond topology. Compound 2 is a left- and right-handed heli-
= 1,4-bis(pyridin-3-ylmethoxy)benzene, H2bdc = 1,4-benzenedicar-
cal layer, which are interacted by π–π stacking interactions to construct
boxylate, H2bbdc = 4,4Ј-dibenzenedicarboxylate], were synthesized a 3D framework. The luminescent properties of the compounds are
under hydrothermal conditions. Compound 1 forms meso-helical chain discussed.
coordination polymers with versatile topologies was obtained.
Introduction
These ligands have the following structural characters: (i) the
Recently, metal-organic helical coordination polymers have
neutral bis(pyridine) ligands exhibit the special ability to coor-
received much attention because of their molecular self-as-
dinate to various central metal atoms in multiform coordination
sembly process and potential applications in asymmetric catal-
fashions to formulate the compounds. (ii) The pyridine rings
ysis, non-linear optical materials, and biomimetic chemistry.[1]
can freely twist around the –O–CH2– group to meet the
Until now, many single-, double-, triple-, and even multiple-
requirements of the coordination arrangements of metal atoms
stranded helices as well as circular and cylindrical helices have
been prepared and studied.[2] However, the meso-helices and
left- and right-handed helices with interpenetrating architecture
or porous framework are infrequent.[3] Therefore, the major
in the assembly process, which often lead to helical structures,
and can easily produce new classes of compounds. (iii) The
large aromatic system could provide potential supramolecular
recognition sites for π–π stacking interactions that could be
used to govern the process of self-assembly.[7]
task for synthesizing such helical structures is to choose appro-
priate flexible N-bridging ligands and metal cations.[4] Further
research is necessary to enrich and develop this field.
On the basis of the above considerations, we select 1,
4-bis(pyridin-3-ylmethoxy)benzene (dpb) as main ligand, and
introduce 1,4-benzenedicarboxylate and 4,4Ј-biphenyldime-
thanoic acid with different spacer angles and lengths as ancil-
lary ligands.[8,11] Two fascinating metal-organic helical coordi-
nation polymers, namely, [Zn(dpb)(bdc)(H2O)]n (1) and
[Cd(dpb)(bbdc)(H2O)(DMF)]n (1) were synthesized under hy-
drothermal conditions. Compound 1 forms threefold interpen-
etrating architecture and 2 forms 3D framework by π–π stack-
ing interactions. Both of them represent the rare examples of
metal-organic helical coordination polymers with similar flexi-
ble N-bridging ligands. In addition, the luminescence proper-
ties of two compounds in the solid state have also been investi-
gated.
In accord with the literatures[5,6] the flexible N-bridging li-
gands, such as 1,2-bis(2-pyridyloxy)benzene,[5a] 1,3-bis(2-pyr-
idyloxy)benzene,[5a]
1,4-bis(2-pyridylmethoxy)benzene,[5a]
1,4-bis(2-pyridyloxymethyl)-benzene,[5a–c] 1,4-bis(pyridin-3-
ylmethoxy)benzene,[6g]
1,4-bis(pyridin-4-ylmethoxy)benz-
ene,[6a,b] 4,4Ј-bis(pyridin-2-ylmethoxy)biphenyl, and bis[(4-
pyridin-2-ylmethoxy)phenyl]methane[5d] were investigated for
the construction of metal-organic coordination polymers.
Using these flexible N-bridging ligands or their metal-organic
* Prof. Dr. X. Yang
Fax: +86-29-88303798
E-Mail: yangxuwu@nwu.edu.cn
[a] College of Chemistry and Materials Science
Agricultural Chemical Technology Institute
Shaanxi Key Laboratory of Physico-Inorganic Chemistry
Key Laboratory of Synthetic and Natural Functional
Molecule Chemistry of Ministry of Education
Northwest University
Results and Discussion
Description of Crystal Structures
XiЈan 710069, P R China
[b] Department of Biochemical Engineering
Xian yang Vocational Technical College
Xian yang, 712000, P. R. China
[Zn(dpb)(bdc)(H2O)]n (1)
Compound 1 crystallizes in the monoclinic space group
C2/c. The structure unit consists of one ZnII, half of each dpb
ligands, half of each deprotonated bdc ligands, and one coordi-
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/zaac.201100519 or from the au-
thor.
Z. Anorg. Allg. Chem. 2012, 638, (5), 785–790
© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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X. Yang et al.
ARTICLE
nated water molecule. The ZnII ion is five-coordinate in a dis-
torted square-pyramidal arrangement, defined by one O1 atom
from one coordinated water molecules, two oxygen atoms (O2,
O2#1) from two half separated H2bdc ligands, respectively,
and two nitrogen atoms (N1, N1#1) from half of each different
dpb ligands in the axial direction. The Zn–O bond lengths
range from 1.979(2) to 2.024(3) Å, and Zn–N bond length is
2.174(3) Å. The Zn–O and Zn–N distances are quite similar to
the normal Zn–O and Zn–N distances.[9,10] Each deprotonated
carboxylic group of bdc2– anion adopts monodentate mode to
coordinate with adjacent ZnII ions. Both dpb ligands adopt
trans-monodentate bridging mode (Scheme 1a) to connect two
adjacent ZnII ions by the nitrogen atoms (Figure 1). Three dpb
ligands are linked by four adjacent zinc ions to form a “Z”
shape with a ZnII···ZnII···ZnII···ZnII torsion angle of 93°. Such
“Z” shapes are bridged by dpb to form a extraordinary meso-
helical chain with Zn···Zn distances of 23.06 Å (Figure 2a).
The helical chains are further linked by deprotonated bdc li-
gands to construct a 3D wavelike structure (Figure 2b). Inter-
estingly, the compound has mutual interpenetration of the inde-
pendent equivalent frameworks in a normal mode, giving rise
to a threefold interpenetrating architecture with 4-connected
net {66} diamond topology (Figure 2c).
Scheme 1. Coordination mode of dpb ligand, (a) trans-monodentate
bridging mode, (b) cis-monodentate bridging mode.
Figure 1. Coordination environment of the ZnII ion in 1 (hydrogen
atoms are omitted for clarity).
Figure 2. (a) “Z” shapes are bridged by dpb to form a extraordinary
meso-helical chain. (b) Zn–bdc linear chains are linked by dpb ligands
to form a 3D structure along c direction. (c)The compound has mutual
interpenetration, giving rise to a threefold interpenetrating architecture.
[Cd(dpb)(bbdc)(H2O)(DMF)]n (2)
Compound 2 crystallizes in the monoclinic space group P21/ each distinct dpb ligands in the apical direction and in axial
c. The asymmetric unit consists of one CdII, half of each dpb positions, four oxygen atoms (O1, O2, O3, and O4) from half
ligands, half of each deprotonated bbdc ligands, one coordi- of each different carboxylic groups, and O8 from one coordi-
nated water molecule and one guest DMF molecule. The CdII nation water molecule (Figure 3). The Cd–N bond lengths are
ion adopts a distorted pentagonal bipyramidal arrangement by in the range of 2.331(7) to 2.347(7) Å, and the Cd–O bond
coordinating with two nitrogen atoms (N1, N2) from half of lengths range from 2.390(6) to 2.458(6) Å, which are in the
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Z. Anorg. Allg. Chem. 2012, 785–790

Helical Structures from Rigid Carboxylate and Flexible N-Bridging Ligands
normal range.[10,11] Based on these coordination modes, each ions of the single helix form parallelogram (Figure 4b). Inter-
dpb ligand adopts cis-monodentate bridging mode
(Scheme 1b) to connect two adjacent CdII ions via the nitrogen
atoms and each bbdc ligands chelated two CdII ions via the
carboxylate oxygen atoms. Two kinds of different ligands and
CdII ions by appropriate coordination environments alternate
one another form an infrequent left-handed right-handed heli-
ces (Figure 4a). The dpb–Cd–bbdc angle is 26.967°, four CdII
estingly, the pitch of the single helix is different, which dpb–
bbdc–dpb is 19.3393 Å and bbdc–dpb–bbdc is 15.5873 Å.
Moreover, left-handed right-handed helical 2D layers form a
3D framework by π–π stacking interactions (Figure 4c). In
contrast to 1, the dpb adopts cis-monodentate bridging mode
owing to the fact that pyridin rings can freely twist around
the –O–CH2– group.
IR Spectroscopy
The IR spectra of compounds 1 and 2 were performed as
KBr pellets in the range 4000–400 cm–1. The peaks of
1589 cm–1 and 1673 cm–1 are assigned to the skeleton vi-
bration of C=N in 1 and 2, respectively, which display certain
shifts in contrast with 1594 cm–1 in the ligand (Figure S1, Sup-
porting Information). It is thus assumed that nitrogen atoms in
the ligand coordinate to metal atoms. The absorption band of
C=O group at 1684 cm–1 disappeared in the compounds. The
compound 1 shows the characteristic bands of the carboxylate
group in the usual region[12] at 1360–1410 cm–1 for the sym-
metric vibration and at 1550–1630 cm–1 for the asymmetric,
and the Δ[νas(COO–) – νs(COO–)] Ͼ 200 cm–1 reveals that the
carboxylate group is coordinated in a monodentate fashion.[13]
And the compound 2 displayed both symmetric and asymmet-
ric stretching vibrations of COO– at 1388–1472 cm–1 and
1520–1578 cm–1, respectively. The Δ[νas(COO–) – νs(COO–)]
Ͻ 200 cm–1 reveals that the carboxylate group is coordinated
in a bidentate chelating coordination mode.[14] These proposals
are in accordance with the crystal structures.
Figure 3. Coordination environment of CdII ion in 2 (hydrogen atoms
and DMF molecules are omitted for clarity).
Luminescent Properties
Taking into account the excellent luminescent properties of
d10 metal compounds, the luminescence of 1 and 2 were inves-
tigated (Figure 5). Additionally, to further comprehend the na-
ture of these emission bands, the fluorescent spectra of dpb,
H2bdc, and H2bbdc ligands were tested. The intense band in
the emission spectra for 1 and 2 are observed at 419.5 nm (λex
= 280 nm) and 409.7 nm (λex = 280 nm), whereas the pure
H2bdc and H2bbdc ligands exhibit fluorescent emission band
at 409.3 nm (λex = 280 nm) and 430.5 nm (λex = 280 nm),
respectively. The emission peak of 1 is red-shifted by about
10.2 nm compared to pure H2bdc ligand (Figure 5a), but 2 is
blue-shifted by about 20.8 nm relative to that of the H2bbdc
ligand (Figure 5b). Since the ZnII and CdII ions are difficult to
be oxidized or reduced.[15] Thus, the significant red or blue
shift of the band in contrast to that of ancillary ligand should
be attributed to the intraligand (π–π*) fluorescent emission.
Thermogravimetric Analysis and PXRD Patterns
In order to examine the thermal stability of the two com-
pounds, thermal gravimetric (TG) analyses were carried out
for 1 and 2 (Figure S2, Supporting Information). The TG curve
of 1 exhibits three weight loss stages in the temperature ranges
of 160–228 (3.2%), 228–333 (29.3%), and 333–590
Figure 4. (a) View of the infrequent left-handed right-handed helical
build by two different ligands and CdII ions alternate one another. (b)
View the single helix form parallelogram. (c) View of the 3D frame-
work by stacking of 2D layers.
Z. Anorg. Allg. Chem. 2012, 785–790
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X. Yang et al.
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Figure 5. Luminescence spectra of compounds 1 and 2 in the solid state.
(0.013 g, 0.05 mmol) was dissolved in distilled water (8 mL), and se-
aled into a 25mL Teflon-lined autoclave. The mixture was heated to
180 °C for 4 h and kept under autogenous pressure at 180 °C for 3
days. After cooling to room temperature at a rate 2 K·h–1, yellow
block-shaped crystals of 1 were filtered out and washed with distilled
water. Yield about 45% based on Zn. Anal. C20H22N2O7Zn: calcd. C
51.35; H 4.74; N 5.99%; found: C 51.26; H 4.62; N 5.91%. IR (KBr):
ν˜ = 3729 (s), 3099 (w), 3054 (w), 1589 (m), 1504 (vs), 1427 (s), 1374
(s), 1234(vs), 1120 (m), 1052 (s), 806 (s), 757 (s), 694 (s) cm–1.
(52.72%), corresponding to the release of water molecules,
dpb, and bdc ligands, respectively. The residue is ZnO
(15.16%).
The TG curve of 2 exhibits two weight loss stages in the
temperature ranges of 100–190 (12.38%), 254–641 (72.36%),
corresponding to the release of water and DMF molecules, dpb
and bbdc ligands. The residue is CdO (17.31%). Additionally,
to confirm the phase purity of the compounds 1 and 2, the
original samples are both characterized by X-ray powder dif-
fraction (XRPD) at room temperature. The peak positions sim-
ulated from the single-crystal X-ray data of 1 and 2 are in
good agreement with those observed (Figure S3, Supporting
Information).
Preparation of [Cd(dpb)(bbdc)(H2O)(DMF)]n (2):A mixture of
CdCl2·2.5H2O (0.014 g, 0.065 mmol), H2bbdc (0.0124 g, 0.05 mmol)
and dpb (0.013 g, 0.05 mmol) was dissolved in distilled water (5 mL)
and DMF (3 mL). The mixture was heated to 150 °C for 4 h and kept
under autogenous pressure at 150 °C for 3 days. After cooling to room
temperature at a rate 2 K·h–1, yellow block-shaped crystals of 2 were
filtered out and washed with distilled water. Yield about 50% based
on Cd. Anal. C35H31N2O8Cd: calcd. C 58.38; H 4.34; N 3.89%; found:
C 58.29; H 4.45; N 3.78%. IR (KBr): ν˜ = 3415 (vs), 3060 (w), 2917
(w), 1941 (m), 1673 (vs), 1581 (vs), 1523 (vs), 1386 (vs), 1238 (vs),
1199 (m), 1047 (s), 838 (s), 767 (s), 684 (s) cm–1.
Conclusions
We have presented a rational synthetic strategy that success-
fully provides two metal-organic helical coordination polymers
by combining long flexible N-bridging ligands and rigid car-
boxylic group ancillary ligands. The structure of two com-
pounds provides the rare examples of metal-organic helical co-
ordination polymers with similar flexible N-bridging ligands.
The solid-state fluorescent analyses show that compounds 1
and 2 exhibit luminescence properties.
X-ray Crystallography: Intensity data were collected with a Bruker
Smart APEX II CCD diffractometer with graphite-monochromated
Mo-Kα radiation (λ = 0.71073 Å) at room temperature. The structure
was solved by direct methods and refined with full-matrix least-
squares on F2 with the SHELXL-97 program package.[16] All non-
hydrogen atoms were located with difference Fourier synthesis, and
the hydrogen atoms were generated geometrically. Crystal data collec-
tion and refinement parameters for compounds are shown in Table 1,
and the selected bond lengths and angles are listed in Table S1 (Sup-
porting Information).
Experimental Section
Materials and Physical Measurements: All reagents were obtained
commercially and used without further purification. The C, H, and N
contents were measured with a Vario EL III elemental analyzer (Ger-
many). The standard deviation for C, H, and N is Ͻ 0.1%. The thermal
analysis TG experiment of the complex was performed with a Perkin–
Elmer thermogravimetric instrument in a nitrogen atmosphere at a flow Crystallographic data (excluding structure factors) for the structures in
rate of 60 cm3·min–1. The heating rate used was 10 K·min–1 from am-
this paper have been deposited with the Cambridge Crystallographic
bient temperature to 1000 K, with a sample weight of about 5 mg. X-
ray power diffraction (XRPD) data were recorded with a Rigaku
RU200 diffractometer at 60 kV, 300 mA for Cu-Kα radiation (λ =
1.5406 Å), with a scan speed of 2 K·min–1 and a step size of 0.02° in
2θ. Fluorescence spectra were measured with a Hitachi F-7000 fluores-
cence spectrophotometer at room temperature.
Data Centre, CCDC, 12 Union Road, Cambridge CB21EZ, UK. Copies
of the data can be obtained free of charge on quoting the depository
numbers CCDC-784391 and CCDC-842458 (Fax: +44-1223-336-033;
E-Mail: deposit@ccdc.cam.ac.uk, http://www.ccdc.cam.ac.uk).
Supporting Information (see footnote on the first page of this article):
Preparation of [Zn(dpb)(bdc)(H2O)]n (1): A mixture of Zn(NO3) IR spectra, TG curves, Powder X-ray diffraction patterns, and bond
2·6H2O (0.0195 g, 0.08 mmol), H2bdc (0.0085 g, 0.05 mmol), and dpb lengths and angles of compounds 1 and 2.
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Z. Anorg. Allg. Chem. 2012, 785–790

Helical Structures from Rigid Carboxylate and Flexible N-Bridging Ligands
Table 1. Crystallographic data and structure refinement parameters for compounds 1 and 2.
compounds
1
2
Empirical formula
Formula weight
Temperature /K
Wavelength /Å
C26H20ZnN2O7
537.81
296(2)
C35H31CdN3O8
734.03
296(2)
0.71073
0.71073
Crystal system
monoclinic
monoclinic
Space group
C2/c
P21/c
Unit cell dimensions /Å, °
a = 18.326(3)
b = 5.9882(9)
c = 21.995(3)
α = 90
a = 6.7730(13)
b = 28.525(5)
c = 17.297(3)
α = 90
β = 101.910(2)
γ = 90
2254.8(6)
β = 101.967(2)
γ = 90
3120.5(10)
Volume /Å3
Z
4
4
Calculated density /mg·m–3
Absorption coefficient /mm–1
F (000)
1.584
1.142
1104
1.562
0.759
1496
Crystal size /mm
Range for data collection /°
Limiting indices
0.30ϫ0.25ϫ0.14
1.98 to 25.09
–21 Յ h Յ 21
–7 Յ k Յ 7
–26 Յ l Յ 18
5379 / 1992[R (int) = 0.0244]
99.5%
0.30ϫ0.28ϫ0.16
1.43 to 25.10
–8 Յ h Յ 8
–34 Յ k Յ 30
–20 Յ l Յ 13
15488/5550[R (int) = 0.0563]
99.5%
Reflections collected/unique
Completeness to θ = 25.10
Refinement method
Full-matrix least-squares on F2
3002 / 2 / 256
1.061
Full-matrix least-squares on F2
5522 / 1 / 416
1.161
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I Ͼ2 sigma (I)]
R indices (all data)
R1 = 0.0358, wR2 = 0.0862
R1 = 0.0473, wR2 = 0.0956
0.618, –0.307
R1 = 0.0867, wR2 = 0.2218
R1 = 0.1026, wR2 = 0.2
3.766, –1.293
Largest diff. peak, hole /e·Å–3
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