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Mixed d/f Complexes Zn(H2L)Ln(NO3)3 (Ln=Sm, Tb and Dy): Crystal Structure, Fluorescence and Thermal Stability

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A series of isostructural d/f molecular compounds Zn(H2L)Ln(NO3)3·CH3OH (Ln = Dy (1), Tb (2) and Sm (3)) were synthesized by the introduction of a designed multifunctional ligand N,N′,N″,N″′-tetra(2-hydroxy-3-methoxy-5-methylbenzyl)-1,4,7,10-tetraazacyclododecan (H4L = C44H60N4O8). In the isostructural molecules, each crystallographically independent Zn²⁺ and Ln³⁺ centers are connected by two phenolic oxygen atoms. For the six-coordinate Zn²⁺ ion, the coordination geometry can be viewed as a regular bicapped square pyramid. While for the ten-coordinate Ln³⁺ ion, if each O,O′-chelated nitrate ligand is seen as a single coordination site, the coordination geometry can be viewed as a distorted pentagonal bipyramid. The fluorescent spectra show that compounds 2 and 3 exhibited characteristic sharp emissions of Tb³⁺ and Sm³⁺, respectively, while compound 1 was found to be a single-component white-light-emitting complex in the solid state. Thermal stabilities of the three compounds were investigated by using thermal gravimetric analysis. In addition, the thermal decomposition of compound 1 was confirmed by temperature-dependent powder X-ray diffraction technique.
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35 9 JIEGOU HUAXUE Vol. 35, No. 9
2016. 9 Chinese J. Struct. Chem. 13831390
Mixed d/f Complexes Zn(H2L)Ln(NO3)3 (Ln = Sm, Tb and
Dy): Crystal Structure, Fluorescence and Thermal Stability
WANG Gao-J i a, b WEI Yong-Qinb WU Ke-Chenb
a (College of Chemistry, Fuzhou University, Fuzhou 350116, China)
b (State Key Laboratory of Structural Chemistry, Fujian Institute of Research
on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, China)
ABSTRACT A series of isostructural d/f molecular compounds Zn(H2L)Ln(NO3)3·CH3OH (Ln =
Dy (1), Tb (2) and Sm (3)) were synthesized by the introduction of a designed multifunctional
ligand N,N΄,N΄΄,N΄΄΄-tetra(2-hydroxy-3-methoxy-5-methylbenzyl)-1,4,7,10-tetraazacyclododecan
(H4L = C44H60N4O8). In the isostructural molecules, each crystallographically independent Zn2+ and
Ln3+ centers are connected by two phenolic oxygen atoms. For the six-coordinate Zn2+ ion, the
coordination geometry can be viewed as a regular bicapped square pyramid. While for the
ten-coordinate Ln3+ ion, if each O,O΄-chelated nitrate ligand is seen as a single coordination site,
the coordination geometry can be viewed as a distorted pentagonal bipyramid. The fluorescent
spectra show that compounds 2 and 3 exhibited characteristic sharp emissions of Tb3+ and Sm3+,
respectively, while compound 1 was found to be a single-component white-light-emitting complex
in the solid state. Thermal stabilities of the three compounds were investigated by using thermal
gravimetric analysis. In addition, the thermal decomposition of compound 1 was confirmed by
temperature-dependent powder X-ray diffraction technique.
Keywords: mixed d/f complex, fluorescence, white light, thermal stability;
DOI: 10.14102/j.cnki.0254-5861.2011-1075
1 INTRODUCTION
Using multidentate and multifunctional organic
ligands to construct metal-organic compounds recei-
ved great interest over the last two decades not only
because of their intriguing topological structures, but
also of their potential applications in various areas,
such as chemical sensing, heterogeneous catalysis,
substituting fluorescence or incandescent lamps for
solid state lighting source[1-3]. Currently, white light-
emitting diodes (WLEDs) for solid-state lighting
(SSL) applications have drawn worldwide attention
due to their compactness, high efficiency, good sta-
bility, long operational lifetime, as well as energy
saving and environmental protection[4-9]. Single-
component white-light-emitting materials combined
with near-UV LEDs for WLEDs have been proposed
because of their advantages over the multiple com-
ponent systems such as greater reproducibility, lower
cost preparation, easier modification, and simpler
fabrication processes[10-18].
Herein, we select a designed multidentate ligand
N,N΄,N΄΄,N΄΄΄-tetra(2-hydroxy-3-methoxy-5-methyl-
benzyl)-1,4,7,10-tetraazacyclododecae (H4L =
C44H60N4O8)[19-21], considering that its cyclen group
and four ortho-methoxyphenolic groups can provide
Received 2 December 2015; accepted 6 April 2016 (CCDC 1421859, 1421860 and 1421861 for compounds 1, 3 and 2, respectively)
This project was supported by the Natural Science Foundation of China (No. 21171165, 21201165 and 91122015)
Corresponding authors. Wei Yong-Qin, E-mail: wyq507@fjirsm.ac.cn; Wu Ke-Chen, E-mail: wkc@fjirsm.ac.cn
WANG G. J. et al.: Mixed d/f Complexes Zn(H2L)Ln(NO3)3 (Ln = Sm,
1384 Tb and Dy): Crystal Structure, Fluorescence and Thermal Stability No. 9
competitive coordination sites to the d- or f-block
metal ions[22-28]. In this paper, we report a series of
heterometallic compounds Zn(H2L)Ln(NO3)3·CH3OH
(Ln = Dy (1), Tb (2) and Sm (3)) constructed by
ligand H4L in which Ln3+ selectively binds to the
O-donor site while Zn2+ to the N-donor functionality.
Moreover, the X-ray crystal structures, thermal
stabilities and photoluminescence properties[29-32] of
the three compounds have been discussed. It is noted
that compound Zn(H2L)Dy(NO3)3·CH3OH (1) can
emit white light to the naked eyes.
2 EXPERIMENTAL
2. 1 Physical measurements
All chemicals were obtained from commercial
sources without further purification except for the
ligand H4L which we have previously published[21].
Elemental analyses for C, H and N were carried out
using a Vario MICRO CHNOS elemental analyzer.
All of the fluorescent spectra in the solid state were
measured on a FLS920 fluorescence spectropho-
tometer at room temperature. All the thermal gravi-
metric curves were measured on a TGA/NETZSCH
STA449C instrument heated from room temperature
to 900 under a nitrogen atmosphere at a heating
rate of 10 /min. Powder X-ray diffraction were
measured on a PAN analytical X’pert PRO X-ray
Diffraction using Cu-Kα radiation in the 2θ range of
565°.
2. 2 Synthesis of the molecular Zn/Dy complex (1)
Zn(NO3)2·6H2O (0.2975 g, 1.0 mmol) and
Dy(NO3)3·6H2O (0.4566 g, 1.0 mmol) were added to
a methanol (5 mL) solution of H4L (0.7725 g, 1.0
mmol). The mixture was then stirred at room tem-
perature for 3 minutes, affording a colorless clear
solution. The resulting solution was heated to 70
for 60 minutes and held for 12 hours and then
cooled to room temperature with 10 hours. The
white crystals were obtained by evaporating the
resulting solution for one day. Yield: 83% (based on
H4L). Elemental analysis (%): Anal. Calcd. (%) for
C46H66N7O19DyZn: C, 44.20; H, 5.28; N, 7.85.
Found (%): C, 44.14; H, 5.33; N, 7.77.
2. 3 Synthesis of the molecular Zn/Tb
(2) and Zn/Sm complex (3)
The method is the same as the synthesis of com-
pound 1 but only the Dy(NO3)3·6H2O is replaced by
Tb(NO3)3·6H2O and Sm(NO3)3·6H2O, respectively.
The white crystals of 2 and 3 were obtained by
evaporating the resulting solution for two days. For
compound 2, yield: 78% (based on H4L). Elemental
analysis (%): Anal. Calcd. (%) for C46H66-
N7O19TbZn: C, 44.32; H, 5.30; N, 7.87. Found (%):
C, 44.26; H, 5.38; N, 7.68. For compound 3, yield:
74% (based on H4L). Elemental analysis (%): Anal.
Calcd. (%) for C46H66N7O19SmZn: C, 44.63; H, 5.34;
N, 7.94. Found (%): C, 44.54; H, 5.20; N, 7.88.
2. 4 Crystal structure determination
The single crystals of the three compounds were
mounted on a Rigaku Mercury CCD diffractometer,
respectively. Reflection data were measured at 293(2)
K using a graphite-monochromated Mo-Kα (λ =
0.71073 Å) radiation and an ω-2θ scan mode. Empi-
rical absorption corrections were performed using
the CrystalClear program[33]. Structures were solved
and refined with respect to F2 by the full-matrix
least-squares technique using the SHELX-97 pro-
gram package[34, 35]. Anisotropic thermal parameters
were applied to all non-hydrogen atoms. Hydrogen
atoms were generated geometrically. Both of them
are crystallized in the monoclinic space group C2/c
with a = 17.467(5), b = 13.029(3), c = 24.113(7) Å,
β = 110.616(4)º, Z = 4, V = 5136(2) Å3 for 1, a =
17.472(5), b = 13.031(3), c = 24.118(7) Å, β =
110.612(4)º, Z = 4, V = 5140(2) Å3 for 2, and a =
17.6457(2), b = 13.1769(9), c = 24.185(3) Å, β =
110.087(6)º, Z = 4, V = 5281(3) Å3 for 3. The final R
and wR values for all data are 0.0496 and 0.1475 for
1, 0.0526 and 0.1539 for 2 and 0.0332 and 0.0747
for 3, respectively. Selected bond lengths and bond
angles of 1 are summarized in Table 1.
2016 Vol. 35 学(JIEGOU HUAXUEChinese J. Struct. Chem. 1385
Table 1. Selected Bond Lengths (Å) and Bond Angles (°) of Compound 1
Bond Dist. Bond Dist. Bond Dist.
Dy(1)–O(4) 2.303(3) Dy(1)–O(8) 2.492(4) Zn(1)–N(1) 2.386(4)
Dy(1)–O(4)#1 2.303(3) Dy(1)–O(8)#1 2.492(4) Zn(1)–N(1)#1 2.386(4)
Dy(1)–O(5) 2.628(4) Dy(1)–O(9) 2.500(4) Zn(1)–N(2) 2.196(4)
Dy(1)–O(5)#1 2.628(4) Dy(1)–O(9)#1 2.500(4) Zn(1)–N(2)#1 2.196(4)
Dy(1)–O(6) 2.491(4) Zn(1)–O(4) 2.116(3)
Dy(1)–O(6)#1 2.491(4) Zn(1)–O(4)#1 2.116(3)
Angle (°) Angle (°) Angle (°)
O(4)–Dy(1)–O(4)#1 66.41(17) O(6)#1–Dy(1)–O(9) 111.82(12) O(6)–Dy(1)–O(5)#1 118.01(11)
O(4)–Dy(1)–O(8) 137.24(12) O(4)–Dy(1)–O(9)#1 75.92(12) O(6)#1–Dy(1)–O(5)#1 70.77(11)
O(4)#1–Dy(1)–O(8) 76.17(12) O(4)#1–Dy(1)–O(9)#1 98.67(12) O(9)–Dy(1)–O(5)#1 114.06(12)
O(4)–Dy(1)–O(8)#1 76.17(12) O(8)–Dy(1)–O(9)#1 131.50(12) O(9)#1–Dy(1)–O(5)#1 65.40(12)
O(4)#1–Dy(1)–O(8)#1 137.24(12) O(8)#1–Dy(1)–O(9)#1 50.96(12) O(5)–Dy(1)–O(5)#1 171.06(15)
O(8)–Dy(1)–O(8)#1 145.57(17) O(6)–Dy(1)–O(9)#1 111.82(12) O(4)#1–Zn(1)–O(4) 73.18(18)
O(4)–Dy(1)–O(6) 131.80(12) O(6)#1–Dy(1)–O(9)#1 74.22(11) O(4)#1–Zn(1)–N(2) 123.17(15)
O(4)#1–Dy(1)–O(6) 147.08(12) O(9)–Dy(1)–O(9)#1 173.66(15) O(4)–Zn(1)–N(2) 99.61(14)
O(8)–Dy(1)–O(6) 74.42(12) O(4)–Dy(1)–O(5) 63.42(11) O(4)#1–Zn(1)–N(2)#1 99.61(14)
O(8)#1–Dy(1)–O(6) 74.69(12) O(4)#1–Dy(1)–O(5) 108.48(12) O(4)–Zn(1)–N(2)#1 123.17(15)
O(4)–Dy(1)–O(6)#1 147.08(12) O(8)–Dy(1)–O(5) 113.17(11) N(2)–Zn(1)–N(2)#1 127.2(2)
O(4)#1–Dy(1)–O(6)#1 131.80(12) O(8)#1–Dy(1)–O(5) 69.67(12) O(4)#1–Zn(1)–N(1) 147.69(15)
O(8)–Dy(1)–O(6)#1 74.69(12) O(6)–Dy(1)–O(5) 70.77(11) O(4)–Zn(1)–N(1) 81.58(14)
O(8)#1–Dy(1)–O(6)#1 74.42(12) O(6)#1–Dy(1)–O(5) 118.01(11) N(2)–Zn(1)–N(1) 80.20(15)
O(6)–Dy(1)–O(6)#1 51.70(17) O(9)–Dy(1)–O(5) 65.40(12) N(2)#1–Zn(1)–N(1) 77.49(15)
O(4)–Dy(1)–O(9) 98.67(12) O(9)#1–Dy(1)–O(5) 114.06(12) O(4)#1–Zn(1)–N(1)#1 81.58(14)
O(4)#1–Dy(1)–O(9) 75.92(12) O(4)–Dy(1)–O(5)#1 108.48(12) O(4)–Zn(1)–N(1)#1 147.69(15)
O(8)–Dy(1)–O(9) 50.96(12) O(4)#1–Dy(1)–O(5)#1 63.42(11) N(2)–Zn(1)–N(1)#1 77.49(15)
O(8)#1–Dy(1)–O(9) 131.50(12) O(8)–Dy(1)–O(5)#1 69.67(12) N(2)#1–Zn(1)–N(1)#1 80.20(15)
O(6)–Dy(1)–O(9) 74.22(11) O(8)#1–Dy(1)–O(5)#1 113.17(11) N(1)–Zn(1)–N(1)#1 128.4(2)
Symmetry transformations used to generate the equivalent atoms: #1: x, y, –z+1/2
3 RESULTS AND DISCUSSION
3. 1 Crystal structure description
of the Zn/Ln complexes
The white block crystals of the title compounds
were obtained by solvothermal reaction of
Zn(NO3)2·6H2O (1.0 mmol), Ln(NO3)3·6H2O (1.0
mmol, Ln = Dy, Tb and Sm) and H4L (1.0 mmol) in
a sealed methanol (5 mL) solution at 70 for 1
day. Each of these three compounds contains one
Zn2+ and one Ln3+, respectively. The single-crystal
X-ray analyses revealed that the title compounds
crystallize in the monoclinic space group C2/c. The
molecular structure of compound 1 is depicted as an
example. The symmetric unit of 1 contains crystallo-
graphically independent Zn2+ and Dy3+ centers con-
nected by two phenolic oxygen ions, one H2L2-
ligand and three nitrate ligands (Fig. 1a). If each
O,O΄-chelated nitrate ligand is seen as a single
coordination site, the coordination geometry of
ten-coordinate Dy3+ ion can be viewed as a distorted
pentagonal bipyramid (Fig. 1b) completed by six
oxygen atoms (Ln–O(9) 1.971(5) Å, Ln–O(10)
1.869(5) Å, Ln–O(11) 1.971(5) Å, Ln–O(12)
1.869(5) Å, Ln–O(13) 1.971(5) Å, Ln–O(14)
1.869(5) Å) from three NO3- anions and four oxygen
atoms (Ln–O(1) 1.971(5) Å, Ln–O(2) 1.869(5) Å,
Ln–O(5) 1.971(5) Å, Ln–O(6) 1.869(5) Å) from two
ortho-methoxyphenolic groups of H2L2- ligand. As
shown in Fig. 1c, the coordination geometry of the
six-coordinate Zn2+ can be viewed as a regular
bicapped square pyramid completed by two oxygen
atoms (Zn–O(2) 1.971(5) Å, Zn–O(6) 1.869(5) Å)
from phenolic groups and four nitrogen atoms
(Zn–N(1) 1.971(5) Å, Zn–N(2) 1.869(5) Å, Zn–N(3)
1.971(5) Å, Zn–N(4) 1.869(5) Å) from cyclen ring
of dianion H2L2-. With an intermetallic distance of
3.627 Å, the Zn2+ and Dy3+ ions are connected by
WANG G. J. et al.: Mixed d/f Complexes Zn(H2L)Ln(NO3)3 (Ln = Sm,
1386 Tb and Dy): Crystal Structure, Fluorescence and Thermal Stability No. 9
two phenolate oxygen atoms.
Fig. 1. (a) Structure of compound 1; (b) Coordination environment of Dy3+; (c) Coordination environment of Zn2+.
Hydrogen atoms were omitted for clarity. Legend scheme: Dy yellow, Zn azure, O red, N blue, C gray
3. 2 Fluorescence spectra
The photoluminescence properties of ligand H4L
and compound 1 in the solid state were measured on
a fluorescence spectrometer at room temperature
(Fig. 2a). Monitored at 577 nm, the excitation spec-
trum at 250 to 450 nm of compound 1 consists of
transitions from the ground state 6H15/2 to the excited
states of 4f 9 configuration of Dy3+ ion. It is intere-
sting to note that there is no excitation signal of the
ligand. Upon excitation at 390 nm, compound 1
displays white light to the naked eyes. The emission
spectrum contains a broad band with the maximum
peak at 440 nm and four characteristic sharp emis-
sions of Dy3+ centered at 480, 577, 658 and 749 nm
attributed to the transitions from 4F9/2 level to 6H15/2,
6H13/2, 6H11/2 and 6H9/2, respectively. The emission
color locates at the white-light region with Com-
mission Internationale de L’Eclairage (CIE) coor-
dinates of (0.332, 0.329). The photoluminescence
properties of compounds 2 and 3 were also inves-
tigated. They emit characteristic orange light of
Sm3+ and green light of Tb3+ with CIE coordinates of
(0.462, 0.322) (Fig. 2b) and (0.334, 0.607) (Fig. 2c)
respectively[36, 37].
3. 3 Thermal gravimetric analysis
The thermal stabilities of all these three com-
pounds were investigated by using thermal gravi-
metric analysis (TGA) (Fig. 3). For compound 1,
under N2 atmosphere from 25 to 900 , the TGA
measurement shows that the first weight loss occurs
2016 Vol. 35 学(JIEGOU HUAXUEChinese J. Struct. Chem. 1387
at around 120 due to the departure of lattice
methanol molecules (the observed weight loss of
3.48%, calcd. 3.04%). Compound 1 is stable up to
240 . With the increase of temperature, the TG
curve exhibits the second step of weight loss,
indicating decomposition of organic ligand H2L2-
and the removal of anions NO3-. The final product
around 430 was ZnO and Dy2O3 (the observed
weight loss of 78.84%, calcd. 78.50%). Compounds
2 and 3 exhibit similar properties of thermal stability
and are stable up to 250 . The 77.18% remaining
mass of compound 2 is due to the ZnO and Tb4O7
(calculated 78.13%). While the 80.38% remaining
mass of compound 3 responds to ZnO and Sm2O3
(calculated 79.32%).
Fig. 2. (a) Emission spectra of ligand H4L (black dot) and compound 1 (red line) in the solid state; (b) Excitation
spectrum (monitored by emission of 644 nm) and emission spectrum (upon excitation at 310 nm) of compound 2;
(c) Excitation spectrum (monitored by emission of 546 nm) and emission spectrum (upon excitation at 310 nm) of
compound 3. Insert: the corresponding 1931 commission internationale de L’Eclairage (CIE) diagrams
3. 4 Temperature-dependent powder X-ray
diffraction of complex 1
In addition, the temperature-dependent PXRD
data of compound 1 were collected with a scan
speed of 2 °·min1. The PXRD pattern at 200 is
very similar to that at room temperature (Fig. 3b).
The slight difference of PXRD patterns between 100
and 200 can be attributed to the removal of
lattice methanol molecules. Some diffraction peaks
of the dehydrated sample slightly shift to the larger
2θ value because of the lattice contraction. The TGA
and PXRD measurements indicated that the
structural skeleton is stable up to 240 .
WANG G. J. et al.: Mixed d/f Complexes Zn(H2L)Ln(NO3)3 (Ln = Sm,
1388 Tb and Dy): Crystal Structure, Fluorescence and Thermal Stability No. 9
Fig. 3. TG curves of the title compounds
Fig. 4. Temperature-dependent PXRD patterns of complex 1
4 CONCLUSION
In conclusion, by the introduction of a multifunc-
tional macrocyclic ligand we have synthesized a
series of mixed d/f complexes and investigated their
isomorphic structures and luminescent properties.
The luminescence of complex Zn(H2L)Dy(NO3)3·
CH3OH in the solid state displays pure white-light
emission with CIE coordinates of (0.332, 0.329).
Thermal gravimetric analysis (TGA) indicates that
all of these complexes are stable up to 200 .
2016 Vol. 35 学(JIEGOU HUAXUEChinese J. Struct. Chem. 1389
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