Synthesis, crystal structures, and characterizations of two Zn(II) coordination polymers based on a reduced Schiff base ligand

Article abstract of DOI:10.1080/15533174.2013.862707

Two coordination polymers base on a reduced Schiff base ligand, namely [Zn₂((OOC)₂C₆H₃NHCH₂C₆H₄COOH)₂(C₁₀H₈N₂)(H₂O)₂

Full text of DOI:10.1080/15533174.2013.862707

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Synthesis, Crystal Structures and Characterizations of
Two Zn(II) Coordination Polymers Base on a Reduced
Schiff-base Ligand
Shao-Ming Yinga & Xiao-Hui Huanga
a Department of chemistry, Ningde Normal University, Ningde, 352100, P. R. China
Accepted author version posted online: 01 May 2015.
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To cite this article: Shao-Ming Ying & Xiao-Hui Huang (2015): Synthesis, Crystal Structures and Characterizations of Two Zn(II)
Coordination Polymers Base on a Reduced Schiff-base Ligand, Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-
Metal Chemistry, DOI: 10.1080/15533174.2013.862707
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Synthesis, Crystal Structures and Characterizations of Two Zn(II) Coordination Polymers
Base on a Reduced Schiff-base Ligand
Shao-Ming Ying*, Xiao-Hui Huang
Department of chemistry, Ningde Normal University, Ningde, 352100, P. R. China
*Corresponding author. E-mail: yingshaoming@126.com
ABSTRACT
Two coordination polymers base on a reduced Schiff-base ligand, namely,
[Zn2((OOC)2C6H3NHCH2C6H4COOH)2(C10H8N2)(H2O)2]·3H2O (1) and
[Zn2((OOC)2C6H3NHCH2C6H4COO)(C10H8N2)2(HCOO)(H2O)]·H2O (2) have been obtained by
hydrothermal method and structurally characterized. These two compounds crystallize in the
P21/c space group. Compound 1 exhibits a double-chain structure while compound 2 exhibit a
layer structure. Photoluminescent investigation shows that compounds 1 and 2 displays strong
emission in the yellow region.
Keywords: Reduced Schiff-bases; Photoluminescence; Crystal structure.
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INTRODUCTION
In recent years, metal coordination polymers(CPs) which are formed by metal ions and organic
ligands have been put much attention because of their potential applications in catalysis, ion
exchange, proton conductivity, intercalation chemistry, photochemistry, gas sorption, selective
separation, magnetic, electronic, nonlinear optical and materials chemistry [1-9]. Lots of CPs
which exhibit a variety of infinite or discrete frameworks of 1D, 2D or 3D structures have been
reported [10-12]. We can divide the organic ligands into two categories: rigid (such as Terephthalic
acid [13]) and flexible (semi-rigid or non-rigid, such as succinic acid [14]) ligands according to the
properties of the organic ligands. Compared to the rigid ligands, using flexible ligands to
construct CPs is more difficult because of the less predictable assembly process. However, due to
the sensitivity and diversification of the flexible ligands, they can afford good opportunities to
investigate the details of the self-assembly process and provide more information on the
directional synthesis of the target CPs [15-16]. The reduced Schiff-bases is a sort of good flexible
ligands. Their flexible skeleton can freely twist around the –NH-CH2–groups. Up to now, to the
best of our knowledge, the coordination polymers base on the reduced Schiff-bases which
formed by the aminobenzoic acid and the carboxybenzaldehyde is still few. Ligands of this type
have two or three carboxylate groups at the terminal positions and a secondary amine group
between the phenyl groups, so they can show various coordination modes. Professor Jian-Fang
Ma's group has reported a series compounds base on the reduced Schiff-base of this type and the
flexible 1,4-bis(1H-imidazol-1-yl)butane ligand [17]. In order to study the coordination chemistry
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of reduced Schiff-base ligands of this type, in this paper, we have synthesized a reduced Schiff-
base ligand which formed by 5-aminoisophthalic acid and 4-carboxybenzaldehyde. Two Zinc(II)
compounds have been obtained base on this ligand with the present of 4,4'- bipyridine or 2,2'-
bipyridine. Herein we report their syntheses, characterizations, and crystal structures.
EXPERIMENTAL
Materials and instruments
All the chemicals were obtained from commercial sources and used without further purification.
Elemental analyses of C, H, N were performed on a German Elementary Vario EL III
instrument. The FT-IR spectra were recorded on a Perkin-Elmer Spectrum 2000 FT-IR
spectrometer using KBr pellets in the range of 4000-400 cm-1. Thermogravimetric analyses were
carried out on a NETZSCH STA 449C unit at a heating rate of 10oC /min under a nitrogen
atmosphere in the range of room temperature to 900°C. Photoluminescence spectra were
collected on an Edinburgh FLS920 spectrometer equipped with a continuous Xe900 Xenon lamp
at room temperature.
Synthesis of (HOOC)2C6H3NHCH2C6H4(COOH), H3L
H3L was synthesized according to the procedures previously described [18-20]. A mixture of KOH
(50 mmol, 2.80 g) and 5-Aminoisophthalic acid (25 mmol, 4.48 g) in 30 mL CH3OH/H2O (V:V
= 5:1) was stirred for 30 minutes at room temperature. A mixture of KOH (25 mmol, 1.40 g) and
4-carboxybenzaldehyde (25 mmol, 3.75 g) in 40 mL CH3OH was stirred for 30 minutes at room
temperature, too. Then the latter was added slowly into the former by stirring. The resulting
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solution was refluxed for 6 h. Then excess NaBH4 was added after the solution was cooled in an
ice bath. After 30 minutes, the solution was acidified with concentrated HCl to a pH of 3.5-5.0.
The resulting solid was filtered off, washed with water and ethanol, and recrystallized from
water/ethanol (1:1) (yield 75%). ESI-MS (methanol) m/z: 313.9, [M-H]-.
Synthesis of [Zn2(HL)2(4,4'-bipy)(H2O)2]·3H2O (1)
A mixture of Zn(NO3)2•6H2O (0.089 g, 0.3 mmol), H3L (0.063 g, 0.2 mmol), 4,4'-bipyridine
(0.031 g, 0.2 mmol), 1 mL N, N-dimethylformamide (DMF) and 9 mL deionized water was
sealed into a steel bomb equipped with a Teflon liner (15 mL), and then heated at 120 ºC for 4
days. Light brown needle crystals of compound 1 were recovered in ca. 45% yield based on Zn.
Elemental analyses for 1, C42H40N4O17Zn2 (Mr = 1003.52): C, 50.08; H, 4.17; N, 5.53%; Calcd.:
C, 50.22; H, 3.99; N, 5.58%. IR data (KBr, cm-1): 3441(s), 1610(s), 1550(s), 1460(s), 1418(s),
1364(s), 1221(m), 1148(m), 1072(m), 1016(m), 818(m), 787(m), 762(m), 729(m), 692(m),
642(m), 629(m).
Synthesis of [Zn2(L)(2,2'-bipy)2(HCOO)(H2O)]·H2O (2)
A mixture of Zn(NO3)2•6H2O (0.060 g, 0.2 mmol), H3L (0.032 g, 0.1 mmol), 2,2'-bipyridine
(0.031 g, 0.2 mmol), 1 mL DMF and 9 mL deionized water was sealed into a steel bomb
equipped with a Teflon liner (15 mL), and then heated at 120 ºC for 3 days. A few light brown
block crystals of compound 2 were recovered in ca. 30% yield based on Zn. Elemental analyses
for 2, C37H31N5O10Zn2 (Mr = 836.41): C, 53.22; H, 3.87; N, 8.43%; Calcd.: C, 53.08; H, 3.71; N,
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8.37%. IR data (KBr, cm-1): 3443(m), 3314(m), 1703(s), 1589(s), 1418(s), 1364(m), 1315(m),
1252(m), 1175(m), 1105(m), 1030 (w), 868(m), 766(s), 733(m), 704(m).
Structure determination
Single crystal of compounds 1 (size: 0.40*0.06*0.04 mm) and 2 (size: 0.25*0.22*0.18 mm) were
mounted on a Bruker Smart Apex CCD diffractometer equipped with a graphite-monochromated
Mo-Ka radiation (λ = 0.71073 Å) respectively. Intensity data for the two compounds were
collected using ω-scans at 293 K. The data sets were corrected for Lorentz and polarization
factors as well as for absorption by SADABS program. The structure was solved by direct
methods using SHELXS-97[21] and refined on F2 by full-matrix least-squares procedure with
SHELXL-97 program[22]. The difference Fourier maps based on these atomic positions yield the
non-hydrogen atoms. The hydrogen atoms were located at the geometrically calculated positions.
Crystallographic data and structural refinements are summarized in Table 1. Selected bond
lengths are listed in Table 2 for compounds 1 and 2. More details about the crystallographic data
have been deposited as supplementary materials.
RESULTS AND DISCUSSION
Crystal structure
As shown in Fig. 1, compound 1 contains one Zn(II) ion, a HL anion and one-half 4,4'-bipy, a
coordinated water molecule and one and one and a half lattice water molecules in its an
asymmetric unit. The Zn(II) ions are four-coordinated by two oxygen atoms from two HL
anions, one oxygen atoms from the coordinated water molecule and one nitrogen atoms from a
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4,4'-bipy ligand. The Zn-O distances ranges from 1.904(7) to 2.006(7) Å, the Zn-N distances is
2.006(8)Å, which are comparable to those reported for other Zn(II) compounds [23, 24]. The
reduced Schiff-base ligand is bidentate. Two COO- group of the reduced Schiff-base ligand
bridged with a Zn(II) ion respectively. By the bridge of the reduced Schiff-base ligands and the
4,4'-bipyridine, a double-chain structure has formed (Fig. 2). These double-chains are further
link into layer structure by the hydrogen bond [O(1)w…O(6), symmetry code: 1-x, y-0.5, 0.5-z,
bond distance is 2.675(9) Å] and the ••• interactions between the pyridyl rings of adjacent
4,4'-bipy [the cg•••cg (center of gravity of the pyridyl ring) distance is 3.66 Å]. Then these layers
are stacking into a "V-shape" supramolecular structure (Fig. 3) by the hydrogen bond
[O(1)w…O(2), symmetry code: x, y-1, z, bond distance is 2.713(10) Å].
Compound 2 contains two Zn(II) ions, two L anions, a 2,2'-bipy, a formyl (come from the
decompose of DMF), a coordinated water molecule and a lattice water molecules in its an
asymmetric unit (Fig. 4). Zn(1) is six-coordinated by two nitrogen atoms from a 2,2'-bipy ligand,
an oxygen atom from the HCOO- anions(come from the decomposition of the DMF), three
oxygen atoms from two L anions. Zn(2) is six-coordinated((including a weak Zn-O coordinated
bond of length 2.469(2) Å) by two nitrogen atoms from a 2,2'-bipy ligand, two oxygen atoms
from a HCOO- anion, an oxygen atom from a L anion and an oxygen atom from the coordinated
water molecule. The reduced Schiff-base ligand is tetradentate. Two COO- group of the reduced
Schiff-base ligand bridged with a Zn(II) ion respectively, and one COO- group chelate with a
Zn(II) ion. By the bridge of the reduced Schiff-base ligands, a layer structure has formed (Fig. 5).
These layers are further linked into supramolecular structure (Fig. 6) by the hydrogen bonds
[O(2)w…O(3), symmetry code: -x, y-0.5, 0.5-z, bond distance is 2.746(3) Å; O(2)w…O(7),
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symmetry code: x, 1.5-y, 0.5+z, bond distance is 2.833(3) Å] and the ••• interactions between
the pyridyl rings of adjacent 2,2'-bipy [the cg•••cg (center of gravity of the pyridyl ring) distance
is 3.72 or 3.74 Å respectively].
TGA studies
The thermal behavior of compounds 1 and 2 were studied to reveal their thermal stability. The
thermal curves are exhibited in Fig.7. The TGA curves of compound 1 exhibits three stages
weight loss dues to the release of lattice water molecules (from room temperature to 130 ºC,
expt.: 5.2%, calcd.: 5.4%), the coordinated water molecules (from 170 ºC to 240 ºC, expt.: 3.6%,
calcd.: 3.5%) and the decomposes of the organic ligands (from 290 ºC up to 900 ºC). For 2, it can
stable up to 320 ºC. Then it exhibits a stage of weight loss dues to release or decomposes of the
water molecules and the organic ligands. The weight loss of 69.9% is similar to that
calculated(70.1%), if the residua is ZnCO3.
Photoluminescent properties
The solid state luminescent emission spectrum of compounds 1, 2 and H3L at room temperature
are depicted in Fig. 8. The H3L displays emission peaks at 415 nm upon excitation at 315 nm.
The compound 1 exhibit emission peaks at 556 nm upon excitation at 372 nm. The compound 2
exhibit emission peaks at 517 nm upon excitation at 370 nm. Compared with the emission
spectrum of the H3L, red shift emissions of compound 1 and 2 have been observed. The emission
bands of compounds 1 and 2 are probably originated from the intraligand luminescent emission
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[25]. These results indicate that compounds 1 and 2 would be candidates for potential photoactive
materials.
CONCLUSION
In summary, by hydrothermal method, we have successfully synthesized two new Zn(II)
compounds base on the reduced Schiff-bases ligand, The structural diversities of the these two
compounds maybe due to the difference of the present of the second ligand---4,4'-bipy or 2,2'-
bipy. The former is a bridged ligand while the latter is a chelated ligand. We hope that other new
coordination polymers with intriguing structures as well as physical properties may also be
synthesized base on the reduced Schiff-base multidentate carboxylates.
ACKNOWLEDGEMENTS
This work was supported by the Research Program of Ningde Normal University (2011H103,
and, 2012H1201) and the Research Program of Department of education of Fujian Province(No.
JK2013058)
Supplementary material
CCDC 870367 to 870368 contain the supplementary crystallographic data for this paper. These
data can be obtained free of charge at www.ccdc.cam.ac.uk/conts/retrieving.html [or from the
Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax:
(internat.)C44 1223 336 033; E-mail: deposit@ccdc.cam.ac.uk].
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Table 1 Summary of crystal data and structural refinements for compounds 1 and 2.
Compound
2
1
Formula
C42H40N4O17Zn2
1003.52
C37H31N5O10Zn2
Fw
836.41
Space group
P21/c
P21/c
a/Å
9.5164(9)
14.344(1)
b/Å
c/Å
β/
5.6425(4)
17.542(1)
13.836(1)
102.309(1)
3401.6(5)
4
39.571(3)
93.576(3)
2120.7(3)
2
V/Å3
Z
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Dc/g•cm-3
1.572
1.633
(Mo-K)/mm-1
Goodness-of-fit on F2
F(000)
1.213
1032
1.480
1.017
1.033
1712
1.86-
2 range (°)
26.00
3.07-26.00
Reflections collected
11242
17301
Independent, observed reflections (Rint)
Largest and Mean /
3977, 1674(0.1196)
0.000/0.000
6658, 5617(0.0218)
0.001/0.000
 (max/min) (e/ Å3)
0.822/-0.839
1.114/-0.729
R1, wR2 [I>2(I)]
0.0395/0.1107
0.0846/0.2135
R1, wR2 (all data)
0.0477/0.1177
0.1939/0.3148
R1 = Fo -Fc/Fo, wR2 = {w[(Fo)2 –(Fc)2]2/w[(Fo)2]2}1/2
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Table 2. Selected bond lengths [Å] for compounds 1 and 2.
Compound 1
Zn(1)-O(4)#1
2.006(7)
1.904(7)
2.006(8)
2.675(9)
Zn(1)-O(5) 1.938(8)
Zn(1)-O(1W)
Zn(1)-N(2)
Hydrogen bond
O1W...O6#2
O1W...O2#3 2.713(10)
Compound 2
Zn(1)-O(2)
2.125(2)
2.0257(19)
2.130(2)
1.965(2)
2.141(2)
Zn(1)-O(8) 2.059(2)
Zn(1)-O(6)#4
Zn(1)-N(5)
2.360(2)
Zn(1)-N(4) 2.168(2)
Zn(2)-O(1W) 2.056(2)
Zn(2)-N(2) 2.147(2)
Zn(1)-O(5)#4
Zn(2)-N(3)
Zn(2)-O(8)
Zn(2)-O(4)#5
2.117(2)
Zn(2)-O(7)
2.469(2)
Hydrogen bond
O2W...O3#6
2.746(3)
O2W...O7#7 2.833(3)
Symmetry transformations used to generate equivalent atoms: #1: x-1, y, z; #2: x, y-1, z; #3: -
x+1, -y-1, -z; #4: 1-x, y-0.5, 0.5-z; #4: -x, y-0.5, 0.5-z; #6: -x, -y-1, -z; #7: -x, 1.5 -y, 0.5+z.
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Fig. 1 Asymmetric unit of compound 1 drawn at 30% probability thermal ellipsoids. Hydrogen
atoms were omitted for clarity. Symmetry codes for the generated atoms: A, 1-x, -y, -z; B, 1+x,
y, z; C, x-1, y, z.
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Fig. 2 The double chain structure of compound 1. Hydrogen atoms and lattice water molecules
were omitted for clarity
Fig. 3 View of the structure of compound 1 down the a-axis. Hydrogen atoms and lattice water
molecules were omitted for clarity. The hydrogen bonds are represented by the black dashed
lines.
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Fig. 4 Asymmetric unit of compound 2 drawn at 30% probability thermal ellipsoids. Hydrogen
atoms were omitted for clarity. Symmetry codes for the generated atoms: A, -x, y-0.5, 0.5-z; B,
1-x, y-0.5, 0.5-z; C, 1-x, 0.5+y, 0.5-z; D, -x, 0.5+y, 0.5-z.
Fig. 5 The layer structure of compound 2. Hydrogen atoms and lattice water molecules were
omitted for clarity
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Fig. 6 View of the structure of compound 2 down the a-axis. Hydrogen atoms were omitted for
clarity. The lattice water molecules are represented by the red ball. The hydrogen bonds are
represented by the black dashed lines
100
compound 2
80
compound 1
60
40
20
0
0
100
200
300
400
500
600
700
800
900
T/centigrade
Fig. 7 The TGA curve of compounds 1 and 2
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compound 1
H3L
compound 2
360
420
480
540
600
660
720
wavelength(nm)
Fig. 8 The solid-state luminescent emission spectrum of compounds 1, 2 and H3L.
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