Syntheses, crystal structures, thermal stability, and fluorescence properties of zinc(II) complexes with tridentate Schiff bases

Article abstract of DOI:10.1080/15533174.2012.740742

Two new mononuclear Schiff base zinc(II) complexes, [Zn (NMP)(MeCOO)] (1) and [ZnBr₂(MP)]·MeOH·H₂O(2) (NMP=4-nitro-2-[(3-morpholin-4-ylpropylimino)methyl]phenolate; MP = 2-[(3-morpholin-4-ylpropylimino)methyl]phenolate), have been sy

Full text of DOI:10.1080/15533174.2012.740742

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Syntheses, Crystal Structures, Thermal Stability and
Fluorescence Properties of Zinc(II) Complexes with
Tridentate Schiff Bases
Shao-Song Qian a , Hai-Hua Li b , Hui Zhu a , Zhong-Ming Yang a , Zhong-Lu You b & Hai-Liang
Zhu a
a School of Life Sciences, Shandong University of Technology, ZiBo, 255049, P. R. China
b Department of Chemistry and Chemical Engineering, Liaoning Normal University, Dalian,
116029, P. R. China
Accepted author version posted online: 04 Dec 2012.
To cite this article: Shao-Song Qian , Hai-Hua Li , Hui Zhu , Zhong-Ming Yang , Zhong-Lu You & Hai-Liang Zhu (2012):
Syntheses, Crystal Structures, Thermal Stability and Fluorescence Properties of Zinc(II) Complexes with Tridentate Schiff
Bases, Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry, DOI:10.1080/15533174.2012.740742
To link to this article: http://dx.doi.org/10.1080/15533174.2012.740742
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Syntheses, Crystal Structures, Thermal Stability and Fluorescence Properties of Zinc(II)
Complexes with Tridentate Schiff Bases
Shao-Song Qiana, Hai-Hua Lib, Hui Zhua, Zhong-Ming Yanga, Zhong-Lu Youb, Hai-Liang
Zhua,*
a School of Life Sciences, Shandong University of Technology, ZiBo 255049, P. R. China
b
Department of Chemistry and Chemical Engineering, Liaoning Normal University, Dalian
116029, P. R. China
Corresponding author. Email: hailiang_zhu@163.com
Abstract
Two new mononuclear Schiff base zinc(II) complexes, [Zn(NMP)(MeCOO)] (1) and
[ZnBr2(MP)]·MeOH·H2O
(2)
(NMP
=
=
4-nitro-2-[(3-morpholin-4-ylpropylimino)methyl]phenolate;
MP
2-[(3-morpholin-4-ylpropylimino)methyl]phenolate), have been synthesized from the similar
tridentate Schiff base ligands. The complexes were characterized by elemental analysis, infrared
spectra, and single-cyrstal X-ray diffraction. The Zn atom in 1 is five-coordinated in a distorted
square pyramidal geometry by the NNO donor set of NMP and two O atoms of an acetate ligand.
The Zn atom in 2 is four-coordinated in a tetrahedral geometry by the imino N and phenolate O
atoms of MP and two Br atoms. The thermal stability and fluorescence properties of the
complexes have also been determined.
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Keywords Schiff base, zinc(II) complex, crystal structure, thermal stability, fluorescence
INTRODUCTION
Metal complexes containing Schiff bases have been of great interest for a long time due to their
importance in the fields of coordination chemistry related to catalysis and enzymatic reactions,
magnetism and molecular architectures.[1-5] Zinc is an important element in biological systems,
functions as the active site of hydrolytic enzymes, such as carboxypeptidase and carbonic
anhydrase where it is in a hard-donor coordination environment of nitrogen and oxygen.[6] Zinc
complexes with Schiff bases have been proved to possess interesting fluorescence properties.[7-10]
As a continuation of our work on the Schiff base complexes,[11-13] we report in this paper the
syntheses, crystal structures, thermal stability, and fluorescene properties of two new
mononuclear
Schiff
base
zinc(II)
complexes,
[Zn(NMP)(MeCOO)]
(1)
and
=
[ZnBr2(MP)]·MeOH·H2O
(2)
(NMP
MP
4-nitro-2-[(3-morpholin-4-ylpropylimino)methyl]phenolate;
=
2-[(3-morpholin-4-ylpropylimino)methyl]phenolate; Scheme 1).
O2N
N
N
N
N
O
O
OH
OH
Scheme 1. The Schiff bases HNMP (left) and HMP (right).
EXPERIMENTAL
Materials and Measurements
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Commercially
available
5-nitrosalicylaldehyde,
salicylaldehyde,
and
3-morpholin-4-ylpropylamine were purchased from Aldrich and used without further purification.
Other solvents and reagents were made in China and used as received. C, H and N elemental
analyses were performed with a Perkin-Elmer elemental analyser. The infrared spectra were
recorded on a Nicolet AVATAR 360 spectrometer as KBr pellets in the 4000–400 cm–1 region. 1H
NMR spectra were obtained from solution in CDCl3 with Me4Si as internal standard using a
Bruker AVANCE 500 MHz analyzer. Thermal stability analysis was performed on a Perkin-Elmer
Pyris Diamond TG-DTA thermal analyses system. Fluorescence spectra were recorded with an
F-2500 FL Spectrophotometer analyzer.
Synthesis of the Schiff Base HNMP
5-Nitrosalicylaldehyde (1.0 mmol, 0.167 g) and 3-morpholin-4-ylpropylamine (1.0 mmol,
0.144 g) were dissolved in EtOH (20 mL) with stirring. The mixture was stirred for about 30 min
at room temperature to give a yellow solution. The solvent was evaporated to give yellow gummy
product of HNMP. Anal. Calcd. for C14H19N3O4 (FW 293.3): C, 57.3; H, 6.5; N, 14.3. Found: C,
57.1; H, 6.6; N, 14.5%. 1H NMR (ppm): δ: 1.73 (m, 2H), 2.37 (m, 4H), 2.45 (t, 2H), 3.65 (m, 4H),
3.69 (t, 2H), 7.26 (d, 1H), 8.07 (d, 1H), 8.37 (s, 1H), 8.53 (s, 1H), 11.32 (s, 1H).
Synthesis of the Schiff Base HMP
HMP was prepared by the similar procedure as described for HNMP, with
5-nitrosalicylaldehyde replaced by salicylaldehyde (1.0 mmol, 0.122 g). Anal. Calcd. for
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1
C14H20N2O2 (FW 248.3): C, 67.7; H, 8.1; N, 11.3. Found: C, 67.5; H, 8.2; N, 11.2%. H NMR
(ppm): δ: 1.73 (m, 2H), 2.37 (m, 4H), 2.45 (t, 2H), 3.65 (m, 4H), 3.70 (t, 2H), 7.01–7.10 (m, 2H),
7.50–7.68 (m, 2H), 8.54 (s, 1H), 10.89 (s, 1H).
Synthesis of [Zn(NMP)(MeCOO)] (1)
A MeOH solution (5 mL) of Zn(MeCOO)2.2H2O (0.1 mmol, 22.0 mg) was added to a MeOH
solution (10 mL) of HNMP (0.1 mmol, 29.3 mg) with stirring. The mixture was stirred for 20 min
to give a clear colourless solution. The resulting solution was allowed to stand in air for a week.
Colourless block-shaped crystals suitable for X-ray single crystal analysis were formed at the
bottom of the vessel. The isolated product was washed three times with cold EtOH, and dried in a
vacuum over anhydrous CaCl2. Anal. Calcd. for C16H21N3O6Zn (FW 416.7): C, 46.1; H, 5.1; N,
10.1. Found: C, 46.3; H, 5.1; N, 9.9%. Yield, 17.2 mg (41.2% on the basis of HNMP).
Synthesis of [ZnBr2(MP)]·MeOH·H2O (2)
Complex 2 was prepared by the similar procedure as described for 1, with HNMP replaced by
HMP (0.1 mmol, 24.8 mg), and with Zn(MeCOO)2.2H2O replaced by ZnBr2 (0.1 mmol, 22.5 mg).
Colourless block-shaped crystals suitable for X-ray single crystal analysis were obtained and
dried in a vacuum over anhydrous CaCl2. Anal. Calcd. for C15H26Br2N2O4Zn (FW 523.6): C, 34.4;
H, 5.0; N, 5.4. Found: C, 34.5; H, 4.9; N, 5.3%. Yield, 25.0 mg (47.7% on the basis of HMP).
Data Collection, Structural Determination and Refinement
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Diffraction intensities for the complexes were collected at 298(2) K using a Bruker SMART 1K
area-detector with MoK radiation ( = 0.71073 Å). The collected data were reduced using the
SAINT program,[14] and multi-scan absorption corrections were performed using the SADABS
program.[15] The structures were solved by direct methods and refined against F2 by full-matrix
least-squares methods using the SHELXTL version 5.1.[16] All of the non-hydrogen atoms were
refined anisotropically. The water and amino H atoms in 2 were located in a difference Fourier
map and refined isotropically, with O–H, N–H, and H···H distances restrained to 0.85(1), 0.90(1),
and 1.37(2) Å, respectively. All other H atoms were placed in idealized positions and constrained
to ride on their parent atoms. The crystallographic data for the complexes are summarized in
Table 1. Selected bond lengths and angles are given in Table 2. Crystallographic data for the four
complexes have been deposited with the Cambridge Crystallographic Data Centre (CCDC
866687 and 866688).
RESULTS AND DISCUSSION
Structure Description of the Complex 1
The molecular structure of 1 is shown in Figure 1. The Zn atom in the complex is coordinated
by one Schiff base ligand and one acetate ligand. The Schiff base acts as a tridentate ligand and
coordinates to the zinc atom through the phenolate O, imine N, and morpholine N atoms. The
acetate ligand coordinates to the zinc atom through the two O atoms. The stereochemistry around
the Zn atom in the complex can be described as a distorted square-pyramidal configuration, with
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the phenolate O, imine N, and the two acetate O atoms defining the basal plane, and with the
morpholine O atom located at the apical position. The angles between the apical and the four basal
donor atoms range from 94.9(1) to 122.8(1)°, and the perpendicular angles of the basal plane
range from 57.1(1) to 99.4(1)°, indicating the difference from the ideal values of 90°. The
distorsion of the square pyramid is mainly caused by the tensile force of the four-membered
chelate ring Zn1-O5-C16-O6. The Zn-O and Zn-N bond lengths in the complex are similar to the
corresponding values observed in other Schiff base zinc(II) complexes.[17,18] As expected, the
bond involving amine atom N2 is longer than that involving imine atoms N1.[19]
The coordination number 5 for zinc(II) complexes is very common and usually presents either a
square-pyramidal or a trigonal-bipyramidal geometry, or any of the distorted intermediate
geometries. For a pentacoordinated metal center, the distortion of the coordination environment
from square-pyramidal to trigonal-bipyramidal can be calculated from the Addison distortion
index, τ, which defined as τ = (β – α)/60, where α and β are the two largest coordinate angles, and
τ = 0 represents perfect square-pyramidal, τ = 1 represents perfect trigonal-bipyramidal.[20] In the
present complex, the calculated τ value is 0.113, suggesting a distorted square-pyramidal
geometry for the Zn atom. The Zn atom is displaced from the mean basal plane towards the apical
N2 atom by 0.609(2) Å.
Structure Description of the Complex 2
The molecular structure of 2 is shown in Figure 2. The compound contains a mononuclear zinc
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complex molecule, a methanol molecule, and one quarter of a water molecule. The Zn atom in the
complex is coordinated by one Schiff base ligand and two Br atoms. The Schiff base acts as a
bidentate ligand and coordinates to the zinc atom through the phenolate O and imine N atoms. The
stereochemistry around the Zn atom in the complex can be described as a tetrahedral
configuration. The angles related to the Zn atom range from 94.6(1) to 113.3(1)°, indicating the
difference from the ideal values of 109°28’ for an ideal tetrahedral geometry. The Zn-O, Zn-N,
and Zn-Br bond lengths in the complex are similar to the corresponding values observed in other
Schiff base zinc(II) complexes.[21,22]
In the crystal structure of 2, the mononuclear zinc complex molecules are linked by the water
and methanol molecules through intermolecular O–H···O, N–H···O, and O–H···Br hydrogen
bonds (Table 3), to form a 3D network, as shown in Figure 3.
IR Spectra
The i.r. spectra of the Schiff base H2L and the complexes provide information about the
metal-ligand bonding. The weak bands centered at 3618 and 3529 cm–1 in the spectrum of 2 prove
the presence of the methanol and water molecules. The strong absorption bands at 1655 cm–1 for
HNMP and 1632 cm–1 for HMP are assigned to the azomethine groups, ν(C=N). The bands are
shifted to lower wave numbers in the complexes, 1646 cm–1 for 1 and 1616 cm–1 for 2, what can
be attributed to the coordination of the nitrogen atoms of the azomethine groups to the Zn atoms.
The phenolic ν(Ar-O) in the free Schiff base ligands exhibit strong bands at 1208 cm–1 for HNMP
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and 1202 cm–1 for HMP. However, in the complexes, the bands appear at 1188 cm–1 for 1, and
1192 cm–1 for 2, which may be assigned to the skeletal vibrations related to the phenolic oxygen
of the Schiff base ligands. The weak bands located at the low numbers may be assigned to the
ν(Zn–O), ν(Zn–N), and ν(Zn–Br).
Thermal Stability
Differential thermal (DT) and thermal gravimetric analyses (TGA) were conducted to examine
the stability of the complexes. For the DT-TGA of 1, the first step started at about 252 °C and was
completed at about 280 °C, corresponding to the loss of the acetate ligand. The observed weight
loss of 14.5% is very close to the calculated value (14.2%). The second step, from 280 °C to 581
°C, corresponds to the loss of the Schiff base ligand, and the formation of the final product (ZnO).
The observed weight loss of 67.5% in this step is very close to the calculated value (66.2%). The
total weight loss of 82.0% is close to the calculated value (80.3%). For the DT-TGA of 2, the first
step started at about 100 °C and was completed at about 128 °C, corresponding to the loss of the
water and methanol molecules. The observed weight loss of 6.7% is very close to the calculated
value (7.0%). The second step, from 219 °C to 450 °C, corresponds to the loss of the two Br
ligands. The observed weight loss of 30.8% is close to the calculated value (31.4%). The last step,
from 450 °C to 635 °C, corresponds to the loss of the Schiff base ligand, and the formation of the
final product (ZnO). The observed weight loss of 46.5% in this step is close to the calculated
value (45.3%). The total weight loss of 85.3% is close to the calculated value (84.1%).
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Fluorescence Property
Owing to the excellent luminescent properties of d10 metal ions, the solid state
photoluminescence of the complexes were investigated at room temperature. For 1, upon
excitation at 367 nm, the emission spectrum of the complex shows three emission bands at 417,
471, and 505 nm, respectively (Figure 6). For 2, upon excitation at 368 nm, the emission spectrum
of the complex shows one emission band at 458 nm (Figure 7).
CONCLUSION
We report in this paper two new d10 metal Schiff base complexes exhibiting interesting
photoluminescence properties at room temperature. Characterization of the two complexes was
carried out by elemental analyses, infrared spectra, thermal studies, and X-ray structural analyses.
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TABLE 1
Crystallographic and experimental data for complexes 1 and 2
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Complexes
1
C16H21N3O6Zn
416.7
2
C15H24.5Br2N2O3.25Zn
510.1
Formula
FW
Crystal shape/color
block/colourless
0.23×0.21×0.20
Monoclinic
P21/c
block/colourless
0.12×0.10×0.10
Orthorhombic
Pbca
Crystal size/ mm
Crystal system
Space group
a/Å
7.841(3)
14.633(6)
15.820(6)
102.722(3)
1770.5(12)
4
14.441(3)
15.824(3)
17.391(3)
90
b/Å
c/Å
/°
V/Å3
3974.1(13)
8
Z
T/K
298(2)
298(2)
/mm-1(Mo-K)
Dc/g cm-3
1.426
5.273
1.563
1.705
Reflections/parameters
Restraints
3849/235
0
4150/222
1
2766
2565
Observed reflections [I 
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2(I)]
F(000)
864
2036
Tmin
0.7351
0.5703
Tmax
0.7636
0.6207
Goodness of fit on F2
R1, wR2 [I  2(I)]a
R1, wR2 (all data)a
1.035
1.003
0.0377, 0.0809
0.0368, 0.0782
0.0848, 0.0978
0.0628, 0.0915
2
2 2 1/2
a R1 = Fo-Fc/Fo, wR2 = [w(Fo -Fc2)2/w(Fo ) ] .
TABLE 2
Selected bond lengths (Å) and angles (°)
1
Zn1-O1
1.932(2)
Zn1-N1
Zn1-O5
2.008(2)
Zn1-N2
2.103(2)
2.032(2)
93.4(1)
143.1(1)
94.9(1)
136.3(1)
57.1(1)
2.374(3)
Zn1-O6-
O1-Zn1-N1
N1-Zn1-O6
N1-Zn1-N2
O1-Zn1-O5
O6-Zn1-O5
O1-Zn1-O6
O1-Zn1-N2
O6-Zn1-N2
N1-Zn1-O5
N2-Zn1-O5
99.4(1)
122.8(1)
106.5(1)
90.2(1)
100.2(1)
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2
Zn1-O1
1.941(3)
2.368(1)
94.6(2)
Zn1-N1
2.026(4)
2.378(1)
111.3(1)
110.0(1)
113.0(1)
Zn1-Br1
Zn1-Br2
O1-Zn1-N1
N1-Zn1-Br1
N1-Zn1-Br2
O1-Zn1-Br1
O1-Zn1-Br2
Br1-Zn1-Br2
TABLE 3
113.3(1)
113.2(1)
Hydrogen-bond geometry for 2 (Å, °)
D–H···A
D–H
H···A
1.86
D···A
D–H···A
167
O3–H3···O1i
0.82
2.662(4)
3.189(13)
3.423(11)
2.733(4)
O4–H4B···O2ii 0.85
O4–H4B···Br2iii 0.85
2.54
134
2.96
116
N2–H2···O3iv
0.90(1)
1.85(2)
164(5)
Symmetry codes: (i) x, 1 + y, z; (ii) x, 3/2 – y, 1/2 + z; (iii) 1/2 – x, 1 – y, 1/2 + z; (iv) 1/2 – x, –1/2
+ y, z.
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Figure 1. Molecular structure of 1 at 30% probability displacement.
Figure 2. Molecular structure of 2 at 30% probability displacement.
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Figure 3. The crystal structure of 2, viewed along the c axis. Hydrogen bonds are shown as dashed
lines.
100
80
60
40
20
40
30
20
10
0
-10
-20
-30
-40
200
400
600
Temperature (oC)
Figure 4. DT-TGA curve of 1.
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Temperature (oC)
Figure 5. DT-TGA curve of 2.
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417
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Wavelength [nm]
Figure 6. Solid state fluorescent emission spectrum of 1.
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458
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Wavelength [nm]
Figure 7. Solid state fluorescent emission spectrum of 2.
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