Synthesis, spectral characterization, and luminescence properties of a cup-like ligand and its magnesium(II) complex

Article abstract of DOI:10.1007/s11164-013-1103-6

A new tripodal ligand, N,N,N-tri(salicylaldehyde)triaminotriethylamine (1) has been synthesized and characterized by elemental analysis, IR and UV spectroscopy, MS, and X-ray crystallography. X-ray diffraction analysis reveals that the three chains of the

Full text of DOI:10.1007/s11164-013-1103-6

Res Chem Intermed
DOI 10.1007/s11164-013-1103-6
Synthesis, spectral characterization, and luminescence
properties of a cup-like ligand and its magnesium(II)
complex
•
Xi-Shi Tai Wen-Hua Zhao
Received: 4 January 2013 / Accepted: 4 February 2013
Ó Springer Science+Business Media Dordrecht 2013
Abstract A new tripodal ligand, N,N0,N00-tri(salicylaldehyde)triaminotriethyl-
amine (1) has been synthesized and characterized by elemental analysis, IR and UV
spectroscopy, MS, and X-ray crystallography. X-ray diffraction analysis reveals that
the three chains of the ligand form a cup-like structure. The ligand’s magnesium(II)
complex has been synthesized and characterized by elemental analysis, conduc-
tivity, and IR and UV spectroscopy. The luminescence properties of the ligand and
its magnesium(II) complex were investigated in DMF, CH3OH, and CH3CH2OH
solution and in the solid state at room temperature.
Keywords Tripodal ligand ꢀ Magnesium(II) complex ꢀ Synthesis ꢀ
Crystal structure ꢀ Luminescence properties
Introduction
Design and construction of inorganic–organic hybrid materials have attracted
extensive interest in coordination chemistry because of their excellent functional
properties, for example luminescence, magnetism, biological activity, and
catalysis, and their variety of structures and topology [1–7]. Many studies of
inorganic–organic hybrid materials have traditionally focused on transition metal
and rare earth complexes [8–10]. Synthesis of inorganic–organic hybrid materials
containing the magnesium(II) cation is rare [11–13]. Recently, we designed a
series of Mg(II) complexes to investigate their properties. In this work, we
X.-S. Tai (&)
College of Chemistry and Chemical Engineering, Weifang University, Weifang 261061, China
e-mail: taixishi@lzu.edu.cn
W.-H. Zhao
Department of Chemistry, Qinghai Normal University, Xining 810008, China
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X.-S. Tai, W.-H. Zhao
designed and synthesized a new functionalized ligand with tripodal structure,
N,N0,N00-tri(salicylaldehyde)triaminotriethylamine (1), and its Mg(II) complex. The
spectral and luminescence properties of ligand and its Mg(II) complex were also
studied.
Experimental
Apparatus and reagents
Triaminotriethylamine, salicylaldehyde, MgCl2ꢀ6H2O, and other chemicals were
reagent grade and used without further purification.
Elemental analysis (C, H and N) was performed on a Elementar Vario EL III
elemental analyzer. IR spectra were recorded in KBr pellets with a Nicolet 170
SXFT-IR spectrophotometer in the 4,000–400 cm-1 region. Molar conductance
was determined with a DDS-11A conductivity meter with methanol as solvent
(10-3 mol L-1 solution) at 25 °C. Mass spectra were acquired by use of a ZAB-
HS analyzer. UV–visible spectra were acquired by use of a Thermo UV-340
spectrometer in the range of 700–190 nm in CH3CH2OH solution. Luminescence
spectra were obtained with a PE LS-55 fluorescence spectrophotometer.
Synthesis of the ligand
Salicylaldehyde (1.830 g, 15.0 mmol) was slowly added to a solution of triami-
notriethylamine (0.730 g, 5.0 mmol) in ethanol (10 mL). The mixture was heated
under reflux for 3 h with continuous stirring, then the resulting yellow precipitate
was isolated by filtration, washed several times with ethanol, and dried in vacuo.
Yellow crystals suitable for single crystal X-ray structure determination were
obtained by slowly evaporating an ethanol solution of 1 at room temperature. Yield:
76 %. Anal. Calc. For C27H30N4O3 (%): C, 70.66; H, 6.54; N, 12.21. Found (%): C,
70.36; H, 6.37; N, 12.52. IR (KBr pellet; m
1,518 (m), 1,224 (s, C–O). MS (FAB): 459 (M ? 1).
cm-1): 1,665 (s, C=N), 1,594 (m),
max
Synthesis of the Mg(II) complex
N,N0,N00-tri(salicylaldehyde)triaminotriethylamine (0.458 g, 1.0 mmol) and sodium
hydroxide (0.120 g, 3.0 mmol) were added to 15 mL CH3CH2OH. After dissolu-
tion, magnesium chloride (0.690 g, 3.0 mmol) was added to the solution. The
mixture was continuously stirred for 2 h at reflux temperature. The mixture was then
cooled to room temperature and the solid formed was isolated by filtration. Yield
58 %. Anal. Calcd. for C54H66Mg3N8O12: C, 59.45; H, 6.06; N, 10.28. Found: C,
59.05; H, 6.32; N, 9.99. Km (s cm2 mol-1): 8.9. IR m
(cm-1): 1,653 (s, C=N),
max
3,423 (m, H2O), 1,592 (m), 1,517 (m), 1,203 (m, C–O).
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Cup-like ligand and its magnesium(II) complex
X-ray crystallography
A yellow single crystal of dimensions 0.24 mm 9 0.20 mm 9 0.14 mm was placed
on a glass fiber and mounted on a CCD area detector. Diffraction data were
collected in u–x scan mode using graphite-monochromated Mo Ka radiation
˚
(k = 0.71073 A) at 298 K. A total of 22,522 reflections were collected in the range
3.10–27.50°, of which 5,698 were unique (Rint = 0.040) and 3,100 were observed
with I [ 2r(I). The data were corrected for Lp factors. The structure was solved by
direct methods using SHELXL-97 [14] and expanded using Fourier techniques. All
non-hydrogen atoms and hydrogen atoms were refined anisotropically and
isotropically, respectively. Final refinement by the full-matrix least-squares method
was converged at R = 0.0650, and wR = 0.1658 (w = 1/[d2(Fo2) ? (0.0841P)2 ?
0.3728P], P = (Fo2 ? 2Fc2)/3, S = 1.127, (D/r)max = 0.000). The largest peak in the
3
3
˚
˚
final difference Fourier map is 0.394 e/A and the minimum peak is -0.275 e/A .
Molecular graphics were drawn with the crystallographic software package
SHELXTL-97 [15]. The CCDC reference number is 894122. The most relevant
crystal data for the ligand are listed in Table 1. The relevant bond lengths and bond
angles are listed in Table 2.
Table 1 Crystallographic data
Crystal data
and structure refinement for the
ligand
Chemical formula
Formula weight
C27H30N4O3
458.55
Cell setting, space group
Monoclinic, P21/c
9.868 (2)
˚
a (A)
˚
b (A)
11.215 (2)
22.931 (5)
90.00
˚
c (A)
a (°)
b (°)
c (°)
98.82 (3)
90.00
3
˚
Volume (A )
2,507.8 (9)
4
Z
Dc (Mg m-3
)
1.215
Crystal size (mm)
0.24 9 0.20 9 0.14
Mo Ka, 0.71073
3.10–27.50
22,522, 5,698, 0.040
3,100
˚
Radiation (A)
Theta min–max (°)
Total, unique data, R(int)
Observed data [I [ 2.0 sigma(I)]
F (000)
976
R, wR2, S
3
Min. and max. resd. dens. (e/A )
0.0650, 0.1658, 1.127
0.394, -0.275
˚
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X.-S. Tai, W.-H. Zhao
Table 2 Selected bond lengths (nm) and angles (°) for the ligand
˚
Bond length (A)
C7–N2
1.270 (3)
1.451 (3)
1.469 (3)
1.448 (4)
1.270 (3)
1.465 (4)
C19–N1
C20–N3
C21–N3
C18–O2
C23–O3
1.458 (3)
1.462 (3)
1.266 (3)
1.338 (3)
1.345 (3)
C8–N2
C10–N1
C11–N4
C12–N4
C9–N1
Bond angle (°)
C19–N1–C9
C19–N1–C10
C9–N1–C10
C7–N2–C8
C21–N3–C20
112.5 (2)
112.1 (2)
112.3 (2)
120.0 (2)
119.5 (2)
C12–N4–C11
O1–C5–C4
121.1 (3)
119.2 (3)
119.1 (2)
120.7 (2)
O2–C18–C17
O3–C23–C22
Results and discussion
Structure description
The molecular structure of ligand is shown in Fig. 1. It can be seen that the ligand
has a cup-like structure with the tertiary amine nitrogen atoms lying at the bottom.
The three chains make C9–N1–C19, C10–N1–C19, and C10–N1–C9 angles of
112.5(2), 112.1(2), and 112.3(2)°, respectively. The bond distances in the three
˚
chains, except those in the phenyl rings, are in the range of 1.499(4)–1.513(4) A for
˚
˚
C–C, 1.266(3)–1.270(3) A for C=N, and 1.448(4)–1.469(3) A for C–N. The bond
lengths and bond angles of the benzene rings in the molecule are within normal
Fig. 1 Molecular structure of
the ligand. The atoms are
represented by 30 % probability
thermal ellipsoids
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Cup-like ligand and its magnesium(II) complex
ranges. The crystal structure of the ligand is stabilized by intramolecular hydrogen
bonds (Fig. 1).
Properties of the Mg(II) complex
Results from elemental analysis indicated that the composition of the Mg(II)
complex was Mg3L2ꢀ6H2O. The Mg(II) complex is soluble in DMF, DMSO,
methanol, slightly soluble in ethanol and acetone, and insoluble in benzene, diethyl
ether, and THF. The molar conductance of the Mg(II) complex measured in CH3OH
solution (1 9 10-3 mol L-1) at 25 °C is 8.9 scm2 mol-1, showing the Mg(II)
complex is a non-electrolyte [16].
IR spectra
The m (C=N) vibration of the free ligand is at 1,665 cm-1 and for the Mg(II)
complex shifts to 1,653 cm-1, indicating that the nitrogen atoms of the C=N
coordinate to the Mg(II) ions [17]. The band at 1,224 cm-1 in the free ligand is
assigned to m (C–O) vibration of phenolic group; it shifts to 1,203 cm-1 in the
complex, showing that oxygen atoms of phenolic hydroxyl coordinate to Mg(II)
ions. In addition, the band at 3,423 cm-1 shows that the complex contains water
molecules, in accordance with results from elemental analysis.
UV spectra
The spectrum of the free ligand contains two absorption bands at 314 nm and
403 nm. The spectrum of the Mg(II) complex contains two absorption bands at
319 nm and 390 nm. These can be assigned to the p–p* and n–p* transitions.
Luminescence properties
The luminescence characteristics of the ligand and its Mg(II) complex in the solid
state and in organic solvent are listed in Table 3. The emission spectra of the ligand
Table 3 Luminescence data for the ligand and its Mg(II) complex
Compound
Ligand
Solvent
kex (nm)
kem (nm)
RFIa
17.3
Solid
329
368
358
360
326
286
390
398
505
460
478
480
517
441
484
480
DMF
278.9
10.6
CH3OH
CH3CH2OH
Solid
41.1
Mg(II) complex
27.4
DMF
184.1
264.0
2,510.2
CH3OH
CH3CH2OH
a
RFI relative fluorescence intensity
123

X.-S. Tai, W.-H. Zhao
and Mg(II) complex in the solid state and in organic solvent are shown in Figs. 2
and 3, respectively.
Table 3 and Fig. 2 indicate that maximum emission of the free ligand is at
505 nm when excited at 329 nm in the solid state. The effects of organic solvents on
the fluorescence properties of the free ligand were tested. It is apparent that in
organic solvents the maximum emission peak is blue shifted and the maximum
excitation peak is red shifted. The solvents DMF and CH3CH2OH enhance the
fluorescence intensity whereas CH3OH reduces the fluorescence intensity.
Table 3 and Fig. 3 indicate that maximum emission of the Mg(II) complex is at
517 nm when excited at 326 nm in the solid state. Compared with that of the free
ligand, the maximum emission peak is red shifted. The fluorescence quantum yield
of the Mg(II) complex is 0.152 with quinine sulfate as reference. The solvents DMF,
CH3OH, and CH3CH2OH enhance the fluorescence intensity. This may not only be
because of removal of the inner sphere of coordinating water molecules in the
Mg(II) complex but also because of energy transfer from the organic solvents to
N,N0,N00-tri(salicylaldehyde)triaminotriethylamine. So, intramolecular energy trans-
fer is a major factor in the process of enhancing the luminescence of this system
[18]. The enhancing effect of organic solvents on the fluorescence of the Mg(II)
complex is in the order CH3CH2OH [ CH3OH [ DMF. The dielectric constants (e)
of these solvents are 37.6 (DMF), 32.6 (CH3OH), and 24.3 (CH3CH2OH). The order
of fluorescence intensities of the Mg(II) complex is contrary to the e values of these
organic solvents. CH3CH2OH has the strongest enhancing effect of these organic
solvents. The dielectric constant and refractive index of the solvent are correlated
with the Stokes shift. However, It can be assumed polarity is an important factor
here.
Fig. 2 The emission spectra of the ligand in the solid state and in organic solvents (1.0 9
10-5 mol L-1). The excitation and emission slit widths were 5 nm
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Cup-like ligand and its magnesium(II) complex
Fig. 3 The emission spectra of the Mg(II) complex in the solid state and in organic solvents (1.0 9
10-5 mol L-1). The excitation and emission slit widths were 5 nm
Conclusions
In summary, we have synthesized a new tripodal ligand and its Mg(II) complex. The
ligand has been characterized by elemental analysis, IR and UV spectroscopy, MS,
and X-ray crystallography. Its magnesium(II) complex also has been synthesized
and characterized by elemental analysis, conductivity measurement, and IR and UV
spectroscopy. The results show that the Mg(II) ions are coordinated to the nitrogen
and oxygen atoms of the ligand and to water molecules. The fluorescence properties
of ligand and its Mg(II) complex were tested in the solid state and in organic
solvents. On the basis of the results obtained a series of new Mg(II) complex can be
synthesized to optimize the fluorescence properties.
Supplementary material
Crystallographic data for the structure reported in this paper have been deposited
with the Cambridge Crystallographic Data Centre as supplementary publication no.
CCDC 894122. A copy of the data can be obtained free of charge on application to
CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (Fax: ?44-1223-336-033;
E-Mail: deposit@ccdc.cam.ac.uk).
Acknowledgments The authors would like to thank the National Natural Science Foundation of
China (nos 21171132 and 20671073), the Promotive Research Fund for Excellent Young and Middle-
aged Scientists of Shandong Province (2010BSA07004), and the Science Foundation of Weifang
University.
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X.-S. Tai, W.-H. Zhao
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