Synthesis, crystal structure and properties of a chiral 2D Zn(II) coordination polymer with helical chains

Article abstract of DOI:10.5560/ZNB.2013-3027

A chiral two-dimensional (2D) coordination polymer {[Zn(cmmb)(1,4-bbi)] H₂O}_n (1) has been synthesized by a hydrothermal reaction of Zn(NO₃)₂ 6H₂O with 4-(carboxymethoxy)-2- methylbenzoic acid (H2cmmb) and 1,1'-(1,4-butanediyl)bis(imidazole) (1,4-bbi). It has been characterized by elemental analysis, X-ray crystallography, IR spectroscopy and thermogravimetry, and also by its fluorescence properties. The metal-organic layer of the complex is held together with its neighbors via C-H O hydrogen bonds to give rise to a chiral three-dimensional supramolecular network.

Full text of DOI:10.5560/ZNB.2013-3027

Note
403
Synthesis, Crystal Structure and
Properties of a Chiral 2D Zn(II)
Coordination Polymer with Helical
Chains
Ting-Ting Kang, Hao Tang, Yong-Li Wei, and
Shuang-Quan Zang
view, we use 1,10-(1,4-butanediyl)bis(imidazole) (1,4-
bbi) as the auxiliary ligand. Another noteworthy point
is that coordination polymers containing metal ions
with a d10 configuration, such as Zn(II), Cd(II) and
Hg(II), are potential materials for optical applications,
such as fluorescence probes and nonlinear optical ma-
terials [16 – 19].
Helical and homochiral structures received much at-
tention owing to their relation to helical arrays in DNA
chains. This kind of complexes have practical impli-
cations in multidisciplinary areas, such as optical de-
vices, biomimetic chemistry, asymmetric catalysis, and
structural biology [20 – 23].
The College of Chemistry and Molecular Engineering,
Zhengzhou University, Zhengzhou 450001, P. R. China
Reprint requests to Dr. Yong-Li Wei. Fax: (+86)-371-6778
0136. E-mail: weiyongli@zzu.edu.cn
Z. Naturforsch. 2013, 68b, 403 – 407
DOI: 10.5560/ZNB.2013-3027
Received January 28, 2013
In this paper we report the synthesis, structure,
thermogravimetric analysis (TGA), and photolumines-
cence properties of a two-dimensional metal-organic
polymer {[Zn(cmmb)(1,4-bbi)]·H2O}n (1) with 4-
(carboxymethoxy)-2-methylbenzoate (cmmb2−) and
A chiral two-dimensional (2D) coordination polymer
{[Zn(cmmb)(1,4-bbi)]·H2O}n (1) has been synthesized
by a hydrothermal reaction of Zn(NO3)2·6H2O with 4-
(carboxymethoxy)-2-methylbenzoic acid (H2cmmb) and 1,10-(1,4-butanediyl)bis(imidazole) (1,4-bbi) ligands.
1,10-(1,4-butanediyl)bis(imidazole) (1,4-bbi). It has been
characterized by elemental analysis, X-ray crystallography,
IR spectroscopy and thermogravimetry, and also by its fluo-
Experimental Section
rescence properties. The metal-organic layer of the complex
Materials and measurements
is held together with its neighbors via C–H···O hydrogen
bonds to give rise to a chiral three-dimensional supramolec-
All the reagents and solvents were commercially avail-
ular network.
able at analytical grade and were used without further pu-
Key words: Helical, Chiral, Supramolecule, Hydrogen
Bonding, Luminescence, Zinc, Benzoate
Ligands
rification or with purification by standard methods prior to
use. Elemental analyses for C, H and N were carried out on
a Perkin-Elmer 240 elemental analyzer. The Fourier trans-
form infrared (FT-IR) spectra were obtained in the range
of 4000 – 400 cm−1 from KBr pellets on a Bruker VEC-
TOR 22 spectrometer. Luminescence spectra for the solid
samples were recorded on a Hitachi 850 fluorescence spec-
trophotometer. Thermogravimetric measurements were car-
ried out from r. t. to 700 ◦C on crystalline samples in a nitro-
gen stream using a SDT 2960 thermal analyzer at a heating
Introduction
In recent years, more and more attention has been
focused on the construction of polymeric frameworks
by reactions of metal salts with organic ligands, due
to their intriguing structures and interesting proper-
ties [1 – 5]. Among the reported studies, those with
carboxylate ligands are especially interesting because
they can adopt a variety of coordination modes result-
ing in diverse multidimensional architectures [6 – 9].
The use of rigid carboxylate ligands has been re-
ported frequently, while we focus our attention on flex-
ible carboxylate ligands [10 – 12]. The use of auxil-
iary ligands is also an effective method for the for-
mation of coordination polymers [13]. The auxiliary
ligands can satisfy and even mediate the coordination
geometry of the metal center and consequently gener-
rate of 20 ◦C min−1
.
Synthesis of 4-(carboxymethoxy)-2-methylbenzoic acid
The mixture of chloroacetic acid (1.290 g, 13.65 mmol)
and 4-hydroxy-3-methylbenzoic acid (1.976 g, 13 mmol) in
ethanol (10 mL) and NaOH (2.527 g, 63.18 mmol) in H2O
(40 mL) was refluxed at 100 ◦C for 12 h and then fil-
tered after cooling to ambient temperature. A large amount
of a colorless precipitate was obtained upon adding 1 M
hydrochloric acid to the solution to reach pH = 1 and
collected by filtration, washed with water and acetone,
and dried in an oven at 105 ◦C for 10 h (89% yield).
ate designed architectures [14, 15]. From this point of M. p. > 300 ◦C. – C10H10O5 (210.20): calcd. C 57.14, H
© 2013 Verlag der Zeitschrift fu¨r Naturforschung, Tu¨bingen · http://znaturforsch.com

404
Note
˚
4.76; found C 57.09, H 4.71. − IR (KBr): ν = 3434(m), Table 2. Selected bond lengths (A) and bond angles (deg) for
the coordination polymer 1a.
2924(s), 1714(vs), 1607(s), 1576(m), 1231(m), 1155(w),
1
950(w), 918(s), 873(m), 773(m), 749(s), 615(s) cm−1. − H
Zn(1)–O(2)
Zn(1)–O(5)#1
O(2)–Zn(1)–O(5)#1
O(2)–Zn(1)–N(4)#2
1.946(5) Zn(1)–N(4)#2
1.967(5) Zn(1)–N(1)
115.5(2) O(2)–Zn(1)–N(1)
1.998(5)
2.015(6)
118.3(2)
NMR ([D6]DMSO): δ = 7.81 (d, H, benzene), 6.76 (m, 2H,
benzene), 4.59 (s, 2H, methylene), 2.49 (s, 3H, methyl) ppm.
113.9(2) O(5)#1–Zn(1)–N(1) 100.5(2)
Preparation of {[Zn(cmmb)(1,4-bbi)]·H2O}n
O(5)#1–Zn(1)–N(4)#2 100.2(2) N(4)#2–Zn(1)–N(1) 106.1(2)
A mixture of Zn(NO3)2·6H2O (0.05 mmol, 14.9 mg),
4-(carboxymethoxy)-2-methylbenzoic acid (H2cmmb;
0.05 mmol, 10.5 mg) and 1,4-bbi (0.05 mmol, 9.5 mg) in
distilled water (4 mL) and ethanol (2 mL) was placed in
a Teflon-lined stainless-steel vessel. The vessel was sealed
and heated to 120 ◦C for 3 d, and then cooled to room tem-
perature. Colorless block-shaped crystals of complex 1 were
obtained. Yield: 70% (based on zinc). − C20H24ZnN4O6
(481.80): calcd. C 49.81, H 4.98, N 11.62; found C 49.77, H
4.96, N 11.58. − IR (KBr): ν = 3566(m), 3430(m), 3148(w),
3120(m), 2949(m), 1608(vs), 1370(vs), 1281(m), 1229(s),
1171 (s), 1103(s), 939(w), 913(w), 818(w), 747(m), 710(s),
a Symmetry codes: #1 −x−1/2, −y+1, z−1/2; #2 −x+3/2, −y+
1, z+1/2.
ful1-matrix least-squares techniques using SHELXS-97 and
SHELXL-97, respectively [24, 25]. Anisotropic displacement
parameters were assigned to all non-hydrogen atoms. Analyt-
ical expressions of neutral-atom scattering factors were em-
ployed, and anomalous dispersion corrections were incorpo-
rated. The hydrogen atoms were assigned common isotropic
displacement factors and included in the final refinement by
using geometrical constraints. Crystal data and further infor-
mation on the structure determination are summarized in Ta-
ble 1. Relevant bond lengths and bond angles are given in
Table 2.
630(m) cm−1
.
X-Ray crystallography
CCDC 865627 contains the supplementary crystallo-
graphic data for this paper. These data can be obtained free
of charge from The Cambridge Crystallographic Data Centre
via www.ccdc.cam.ac.uk/data request/cif.
Single-crystal X-ray diffraction analysis of complex 1
was carried out at room temperature on a Bruker SMART
APEX CCD diffractometer and investigated with graphite-
monochromatized MoKα radiation at room temperature us-
ing ω-scans. The structure was solved by Direct Meth-
ods, completed by difference Fourier maps and refined by
Results and Discussion
Description of the crystal structure
Table 1. Crystallographic data for the coordination poly-
mer 1.
The single-crystal X-ray structure analysis revealed
that 1 crystallizes in the asymmetric orthorhombic
space group P212121, and consists of right-handed he-
lical chains. The metal coordination and atom label-
ing is depicted in Fig. 1. In each unit, there are one
Zn2+ dication, one cmmb2− dianion, one 1,4-bbi lig-
and and one free water molecule. The Zn atom is
four-coordinated with two O atoms and two N atoms
to form a distorted tetrahedral coordination geometry,
with two oxygen atoms from two cmmb2− ligands and
two nitrogen atoms from two 1,4-bbi ligands. The Zn–
Compound
1
Empirical formula
Formula weight
Color, habit
C20H24N4O6Zn
481.80
colorless, block
0.21 × 0.20 × 0.19
orthorhombic
P212121
Crystal size, mm3
Crystal system
Space group
˚
a, A
8.2832(4)
15.7072(10)
16.3589(9)
2128.4(2)
4
1.50
1.2
293(2)
1000
˚
b, A
˚
c, A
3
˚
V, A
Z
Dcalcd., g cm−3
˚
O bond lengths are 1.946(5) and 1.967(5) A, while the
Absorption coeff., mm−1
T, K
˚
Zn–N bond lengths are 1.998(5) and 2.015(6) A. The
cmmb2− ligands are monodentate in coordination with
the Zn cations, and the uncoordinated carboxylic oxy-
gen atoms have only weak interactions with the Zn
F(000), e
Reflections collected / independent / Rint
Independent reflections
R1 / wR2 [I > 2σ(I)]
R1 / wR2 (all refls.)
Goodness-of-fit (GOF)
x (Flack)
5319 / 3665 / 0.0303
0.0604 / 0.1416
0.0737 / 0.1509
1.035
˚
atoms (Zn1–O1 = 2.8416(62) A).
As shown in Fig. 2, there are two kinds of helical
chains in the layer. The anionic cmmb2− ligands con-
nect the zinc cation ions along the c axis to generate
−0.0313(17)
−3
˚
Largest difference peak / hole, e A
0.49 / −0.24

Note
405
Fig. 1 (color online). Metal coordi-
nation and atom labeling in complex
1. For symmetry operations see Ta-
ble 2.
Fig. 2 (color online). A layer formed by right-
handed helical chains.
Fig. 3 (color online). 3D supramolecular frame-
work formed by hydrogen bonding interactions.
˚
a single right-handed helical chain. The 1,4-bbi ligands is 16.358 A. The most salient structural feature of com-
bridge the zinc ions of parallel helical chains in the plex 1, however, is the chiral coordination polymer as-
c direction. The two kinds of helical chains have the sembled from achiral components in right-handed he-
same nearest Zn–Zn distance. The pitch of the helices lices.

406
Note
Fig. 4 (color online). Solid-state emission spectrum of
complex 1 in the solid at r. t.
Fig. 5. Thermogravimetric analysis (TGA) of complex 1.
As shown in Fig. 3, there are O–H···O hydrogen by 133 nm with respect to the band shown by the
bonds between the O atoms from the uncoordinated free ligand. The spectrum can tentatively be as-
water molecule (O6W) and the carboxylate group (O2, signed to the intraligand n-π∗ or π-π∗ fluorescence
O4) with O6W···O2 and O6W···O4 distances of 2.975 emission [28].
˚
and 2.792 A, respectively. Hydrogen bonding interac-
tions further consolidate the crystal structure, as in ad- Thermogravimetric analysis of complex 1
dition, C–H···O hydrogen bonds are also observed in
the crystal. There is a C(19)–H(19)···O(3) hydrogen
Thermogravimetric analysis (TGA) was conducted
bond between the imidazole carbon atom C(19) of the to study the thermal stability of the title complex,
1,4-bbi ligand and the O(3) atom of the cmmb2− lig- which is an important aspect for metal-organic frame-
and of the adjacent layer. The C···O distance and the works [29]. The TGA curve has three degradation
◦
C–H···O angle are 3.310(8) A and 161.002(4) , re- steps in the range 29 – 694 ◦C. As depicted in Fig. 5,
spectively. It is well-known that hydrogen bonding is the first gradual weight loss of 3.5% occurs between
generally very important for generating supramolec- 30 and 202 ◦C (calcd. 3.7%), corresponding to the
ular architectures [26]. With the help of these inter- loss of the uncoordinated water molecule per formula
chain hydrogen bonds, the adjacent layers give a 3D unit. Then, a plateau region is observed from 202 to
˚
structure.
302 ◦C. On raising the temperature further, two con-
secutive decompositions take place in the tempera-
ture range of 302 – 587 ◦C. The calculated and ob-
served overall losses correlate quite well (found 83%,
Photoluminescence properties of complex 1
Luminescent compounds composed of d10 metal calcd. 83.2%). Above 587 ◦C, no weight loss is ob-
centers and organic ligands are of great interest be- served, and the final residue obtained corresponds
cause of their potential applications in the areas of to the formation of zinc oxide (obsd: 17%, calcd.
chemical sensors and photochemistry [27]. Thus, we 16.8%).
studied the photoluminescence properties of com-
plex 1 and of the neutral acid H2cmmb in the solid Conclusion
state at room temperature. The complex shows lu-
minescence with an emission maximum at 481 nm
In this paper, we have presented a chiral
upon excitation at 318 nm (Fig. 4). The emission two-dimensional Zn(II) coordination polymer
and excitation peaks of H2cmmb have their max- {[Zn(cmmb)(1,4-bbi)]·H2O}n
assembled
from
ima at 348 and 318 nm, respectively. The emis- cmmb2− and 1,4-bbi ligands under hydrothermal
sion maximum for complex 1 is thereforered-shifted conditions. Our results indicate that the cmmb2−

Note
407
ligand has the potential to form chiral metal-organic Acknowledgement
frameworks when it reacts with metal ions, but also
illustrate that hydrogen bonds have an influence on the
formation of the supramolecular architecture.
We gratefully acknowledge financial support by the Na-
tional Natural Science Foundation of China (no. 20901070)
and Zhengzhou University (P. R. China).
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