Synthesis, structure, infrared and fluorescence spectra of new rare earth complexes with 6-hydroxy chromone-3-carbaldehyde benzoyl hydrazone

Article abstract of DOI:10.1016/j.saa.2005.05.007

A novel 6-hydroxy chromone-3-carbaldehyde benzoyl hydrazone ligand and its four complexes, [LnL₂(NO₃)₂]NO₃ [Ln = Eu(1), Sm(2), Tb(3), Dy(4)], were synthesized. The complexes were characterized by the elemental a

Full text of DOI:10.1016/j.saa.2005.05.007

Spectrochimica Acta Part A 63 (2006) 213–219
Synthesis, structure, infrared and fluorescence spectra of new rare earth
complexes with 6-hydroxy chromone-3-carbaldehyde benzoyl hydrazone
Bao-Dui Wang, Zheng-Yin Yang∗, Ding-Wa Zhang, Yan Wang
College of Chemistry and Chemical Engineering and State Key Laboratory of Applied Organic Chemistry, Lanzhou University,
Lanzhou 730000, PR China
Received 25 February 2005; received in revised form 30 April 2005; accepted 10 May 2005
Abstract
A novel 6-hydroxy chromone-3-carbaldehyde benzoyl hydrazone ligand and its four complexes, [LnL2(NO3)2]NO3 [Ln = Eu(1), Sm(2),
Tb(3), Dy(4)], were synthesized. The complexes were characterized by the elemental analyses, molar conductivity and IR spectra. The crystal
and molecular structure of Sm(III) complex was determined by single-crystal X-ray diffraction: crystallized in the triclinic system, space
˚
˚
˚
group P-1, Z = 1, a = 11.037(4) A, b = 14.770(5) A, c = 15.032(7) A, α = 60.583(4), β = 75.528(7), γ = 88.999(4), R1 = 0.0349. The fluorescence
properties of complexes in the solid state and in the organic solvent were studied in detail, respectively. Under the excitation of ultraviolet
light, strong red fluorescence of solid europium complex was observed. But the green fluorescence of solid terbium complex was not observed.
These observations show that the ligand favor energy transfers to the emitting energy level of Eu3+. Some factors that influence the fluorescent
intensity were also discussed.
© 2005 Elsevier B.V. All rights reserved.
Keywords: Synthesis; Complexes; X-ray structure; Fluorescence properties; 6-Hydroxy chromone-3-carbaldehyde benzoyl hydrazone
1. Introduction
a series of Schiff base, which can enhance their luminescence
by providing proper conjugate absorption groups for suitable
energy transfer, and be used as a luminescent device. In the
present work, a new and doubly functionalized Schiff-base
ligand, 6-hydroxy chromone-3-carbaldehyde benzoyl hydra-
zone has been synthesized, and the fluorescence properties of
europium and terbium complexes with the new ligand have
been studied. The results show that the Eu(III) with the lig-
and can emit intrinsic spectrum of Eu(III) under excitation
of ultraviolet light, and that the organic solvent can affect the
fluorescence characteristics of the Eu(III) complex.
Because they have long fluorescence lifetimes and their
excited states have strong fluorescence emission, europium
(III) and terbium(III) have extensive applications in biologi-
cal medicine, especially in the fluorescence imagery cancer
radiation treatment [1], fluorescence mark, and fluorescence
analysis [2].
The bioactivity and magnetic performance of Schiff base
and their complexes with rare earth ions have been stud-
ied extensively. However, their fluorescence properties have
rarely been studied due to their low luminescence [3,4]. So
far, chemists have realized that it is essential to design appro-
priate ligands to optimize the luminescence properties of lan-
thanide ions by facilitating the well-known light conversion
process, which show to be efficient ligand-to-metal energy-
transfer process (antenna effect). Recently, we have designed
2. Experimental
2.1. Materials
Acetic anhydride, benzoyl hydrazine, and hydroquinone
were produced in China. All material and solvents employed
in this study were analytical reagents. Absolute ethanol was
∗
Corresponding author. Tel.: +86 9318913515; fax: +86 9318912582.
E-mail address: yangzy@lzu.edu.cn (Z.-Y. Yang).
1386-1425/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2005.05.007

214
B.-D. Wang et al. / Spectrochimica Acta Part A 63 (2006) 213–219
Table 1
derived and distilled by standard method. The rare earth(III)
nitrates were derived from their oxide acquired from Nong
Hua (PR China).
Crystal data and structure refinement for Sm(III) complex
Empirical formula
C72 H66 N14 O39 Sm2
Formula weight
T (K)
Crystal system
Space group
2052.09
298(2)
Triclinic
P-1
2.2. Physical measurements
The metal ions were determined by EDTA titration using
xylenol oranges as an indicator. Carbon, nitrogen and hydro-
gen analyses were performed using a Vario EL elemental ana-
lyzer. Infrared spectra (4000–400 cm−1) were obtained with
KBr discs on a Therrno Mattson FTIR spectrometer. 1H NMR
spectra were recorded on a Varian Mercury Plus 300 BB,
using TMS as an internal standard in DMSO-d6. Mass spec-
tra (fast atom bombardment) were recorded on a VG ZAB-HS
mass spectrometer. Fluorescence measurements were made
on a Hitachi RF-540 spectrofluorophotometer equipped with
quartz cuvettes of 1 cm path length at room temperature. The
molar conductance values measurements were carried out in
DMF on a model DSS-11A conductivity meter.
˚
a (A)
11.037(4)
14.770(5)
15.032(7)
60.583(4)
75.528(7)
88.999(4)
2050.6(14)
1
10694
7082 [R(int) = 0.0226]
1.009
R1 = 0.0349, wR2 = 0.0786
1.141, −0.539
˚
b (A)
˚
c (A)
α (◦)
β (◦)
γ (◦)
3
˚
V (A )
Z
Reflections collected
Independent reflections
Goodness-of-fit on F2
Final R indices [I > 2σ(I)]
−3
˚
Largest difference peak, hole/e A
2.4. Synthesis of the complexes
2.3. Synthesis of ligand
The ligand (1 mmol, 0.308 g) and the Ln(NO3)3·6H2O
(0.5 mmol) were added to the ethanol (10 mL). The mixtures
were stirred at 60 ◦C. After 5 min, the mixtures solution was
filtrated to move reside and continued stirring for 24 h at room
temperature. A white precipitated, the Ln(III) complex, was
separated from the solution by suction filtration, purified by
washing several times with ethanol, and dried for 24 h in a
vacuum. All the complexes are white powers and stable in air,
but the crystals of the solvated Sm(III) complex is a light yel-
low and recrystallized from CH3OH with slow evaporation
at room temperature.
The compounds of 1–4 (Fig. 1) were prepared according
to the literature [5].
2.3.1. Preparation of 5
An ethanol solution containing benzoyl hydrazine (1.36 g,
10 mmol) was added dropwise to another ethanol solution
containing 4 (1.90 g, 10 mmol). The mixture was stirred for
2 h at room temperature and a white precipitate formed.
The precipitate was collected by filtration and washed with
ethanol. Recrystallization from 1:1 (v/v) DMF/H2O gave
the ligand, which was dried in a vacuum. Yield, 85%. M.P.
2.5. X-ray crystallography
1
209–211 ◦C. H NMR (DMSO-d6, ppm): δ 11.95 (1H, br,
A light yellow crystal of Sm(III) complex (0.42 mm ×
0.19 mm × 0.13 mm) was determined with a Bruker Smart-
1000 CCD diffractometer by a graphite monochromatic
NH), 10.18 (1H, s, OH), 8.79 (1H, s, 2-H), 8.65 (1H,
s, CH N), 7.95–7.25 (8H, m, Ph–H, 5,7,8-H). FAB MS:
m/z = 309 (M + H).
Fig. 1. Scheme of the synthesis of the ligand.

B.-D. Wang et al. / Spectrochimica Acta Part A 63 (2006) 213–219
215
the oxygen of carbonyl has formed a coordinative bond with
the rare earth ions [7]. The band at 1598 cm−1 for the free
ligand is assigned to the ν (C N) stretch, which shifts to
1571–1573 cm−1 for its complexes. Weak bands at 427 cm−1
are assigned to ν (M N). These further confirm that the nitro-
gen of the imino-group bonds to the rare earth ions [7]. The
absorption bands of the coordinated nitrates were observed at
about 1480 (νas) and 840 (νs) cm−1. The ν3 (Eꢀ) free nitrates
appear at 1384 cm−1 in the spectra of the complexes [8], in
agreement with the result of the conductivity experiments. In
addition, the separation of the two highest frequency bands
|ν4 − ν1| is approximately 154 cm−1, indicating that the coor-
dinated nitrate groups in the complexes are bidentate.
˚
Mo K␣ radiation (λ = 0.71073 A) at 298(2) K. The crys-
tallographic data are given in Table 1. The intensity data
were collected by the ω scan mode within 2.14 < θ < 25.03◦
for hkl (−13 ≤ h ≤ 12, −17 ≤ k ≤ 17, −17 ≤ l ≤ 17). The
structure was solved by direct method, the positions of
rest non-hydrogen atoms were determined from successive
Fourier syntheses, and the hydrogen atoms except water
hydrogens were placed in the geometrically calculated posi-
tions. The positions and anisotropic thermal parameters of
all non-hydrogen atoms were refined on F2 by full-matrix
least-squares techniques with SHELX-97 program package.
Absorption correction was employed using semi-empirical
from equivalents.
3.3. Crystal structure of Sm(III) complex
3. Result and discussion
The coordination of the hydrazone with lanthanide(III)
results in the formation of five-membered (two SmN-
NCO) and six-membered (two SmNOCCO) chelating
rings in which the lanthanide ion is ten-coordinated.
The complex 2 crystallized in a triclinic lattice with a
space group P-1. Each unite cell contains two molecules.
ORTEP drawing with the atoms numbering scheme for
[SmL2(NO3)2]NO3·0.5H2O·2CH3OH are shown in Fig. 2. In
fact, the formula of Sm complexshouldbe double molecule in
the unit cell (Fig. 3). Main bond lengths and angles are sum-
marized in Table 4. The coordination polyhedron for complex
2 is described by two nitrate group coordinated as bidentate,
six donors atoms (four iminolic oxygen and two nitrogen) of
the twoligands. There are five typesofthe coordination bonds
in the complex 2; its bond lengths are: Sm O (hydrazonic)
3.1. Properties of the complexes
Analytical data for the complexes, presented in Table 2,
conform to Ln(NO3)3L2. All of the complexes are soluble
in DMSO, DMF, methanol and methanol/chloroform (1:1),
slightly soluble in ethanol, insoluble in benzene, water and
diethyl ether, and may be kept in air for along time. Conduc-
tivity measurements for these complexes in DMF solution
(Table 2) indicate that all complexes are 1:1 ionic compounds
[6].
3.2. IR spectra
The IR spectra of the complexes are similar. Table 3 shows
the characteristic bands of ligand and its complexes. The
ν(carbony) (C O) and ν(hydrazonic) (C O) vibrations of the free
ligand are at 1632 and 1649 cm−1, respectively; for the com-
plexes these peaks shift to 1609 and 1634 cm−1 or so, ꢀν
(ligand-complexes) is equal to 23–33 and 12–15 cm−1. The band
at 591 cm−1 is assigned to ν (M O). These demonstrate that
˚
2.476(3) and 2.510(3) A, Sm O (carbonyl) 2.438(3) and
˚
˚
˚
2.367(3) A, Sm N (hydrazonic) 2.700(3) and 2.672(4) A,
Sm O (nitrate) 2.499(3), 2.479(3), 2.574(3) and 2.570(3) A.
The Sm O (carbony) distance (mean 2.402 A) is significantly
shorterthantheSm O(hydrazonic)distance(mean2.493 A),
˚
˚
which suggests that the Sm O (carbony) bond is stronger
Table 2
Analytical and molar conductance data for the complexes
Complexes
C% found (calc.)
H% found (calc.)
N% found (calc.)
Ln% found (calc.)
Λm (s cm2 mol−1
)
1
2
3
4
42.94(42.77)
42.46(42.86)
42.20(42.46)
42.67(42.30)
2.27(2.51)
2.36(2.52)
2.32(2.50)
2.31(2.49)
10.86(10.27)
10.78(10.29)
10.09(10.20)
9.89(10.16)
15.89(15.93)
15.52(15.70)
16.85(16.54)
16.14(16.85)
92
121.8
102
104.8
Table 3
IR spectral data of the free ligands and its complexes (cm−1
)
Compounds
␯
(C O)
␯
(C O) ␯ (C N) ␯ (NO3) ␯ (NO3) ␯ (NO3) ␯ (NO3) ␯ (NO3) ␯ (M–O) ␯ (M–N)
(carbony)
(hydrazonic)
1
2
3
4
0
L
1
2
3
4
1632
1609
1609
1609
1609
1649
1634
1636
1634
1637
1598
1573
1571
1572
1572
1480
1480
1480
1480
1187
1187
1187
1186
840
838
840
838
1325
1325
1326
1325
1383
1380
1384
1382
596
595
591
596
426
427
427
429

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B.-D. Wang et al. / Spectrochimica Acta Part A 63 (2006) 213–219
Fig. 2. ORTEP drawing of Sm(III) complex.
than the Sm O (hydrazonic) bond, in agreement with the IR
spectral data.
3.4. Fluorescence studies
In complex the C11–N2 and C28–N4 distances are con-
sistent with a double bond while the C11–O4 and C28–O11
distances are consistent with a single bond.
The fluorescence characteristics of the complexes in solid
are listed in Table 5. The solid complexes have characteristic
line emission of f–f transitions of metal ions when they are
Fig. 3. Packing diagram in the unit cell showing hydrogen bonding.

B.-D. Wang et al. / Spectrochimica Acta Part A 63 (2006) 213–219
217
Table 4
Select bond lengths (A) and angles ( ) for complex 2
◦
˚
Sm(1) O(2)
Sm(1) O(6)
Sm(1) O(4)
Sm(1) O(10)
2.367(3)
2.438(3)
2.476(3)
2.479(3)
2.499(3)
148.36(9)
122.91(11)
67.70(9)
76.94(11)
78.37(11)
78.37(11)
74.36(12)
75.99(10)
73.18(10)
117.59(10)
51.13(11)
70.06(11)
104.40(10)
165.96(10)
Sm(1) O(8)
Sm(1) O(13)
Sm(1) O(12)
Sm(1) N(3)
116.37(11)
68.63(10)
73.21(10)
2.510(3)
2.570(3)
2.574(3)
O(4) Sm(1) O(12)
O(10) Sm(1) O(12)
O(9) Sm(1) O(12)
O(8) Sm(1) O(12)
O(2) Sm(1) N(3)
O(6) Sm(1) N(3)
O(4) Sm(1) N(3)
O(10) Sm(1) N(3)
O(9) Sm(1) N(3)
O(8) Sm(1) N(3)
O(13) Sm(1) N(3)
O(12) Sm(1) N(3)
O(2) Sm(1) N(1)
O(6) Sm(1) N(1)
O(4) Sm(1) N(1)
O(10) Sm(1) N(1)
O(6) Sm(1) O(13)
O(10) Sm(1) O(13)
66.68(11)
139.79(11)
163.22(10)
103.72(10)
127.26(10)
64.73(9)
106.59(11)
138.25(11)
97.73(10)
59.43(10)
74.60(10)
65.93(10)
66.97(10)
127.32(10)
60.30(10)
80.34(11)
135.07(9)
145.99(11)
Sm(1) O(9)
Sm(1) N(1)
O(2) Sm(1) O(6)
O(2) Sm(1) O(4)
O(6) Sm(1) O(4)
O(2) Sm(1) O(10)
O(6) Sm(1) O(10)
O(6) Sm(1) O(10)
O(4) Sm(1) O(10)
O(2) Sm(1) O(9)
O(6) Sm(1) O(9)
O(4) Sm(1) O(9)
O(10) Sm(1) O(9)
O(2) Sm(1) O(8)
O(6) Sm(1) O(8)
O(4) Sm(1) O(8)
O(10) Sm(1) O(8)
O(9) Sm(1) O(8)
O(2) Sm(1) O(13)
O(9) Sm(1) N(1)
O(8) Sm(1) N(1)
O(13) Sm(1) N(1)
O(12) Sm(1) N(1)
N(3) Sm(1) N(1)
O(13) Sm(1) O(12)
O(4) Sm(1) O(13)
O(9) Sm(1) O(13)
O(8) Sm(1) O(13)
O(2) Sm(1) O(12)
O(6) Sm(1) O(12)
2.672(4)
2.700(3)
124.39(10)
128.22(10)
73.31(11)
72.25(10)
137.65(10)
49.07(10)
109.06(10)
132.80(10)
67.68(10)
116.35(10)
95.28(9)
Table 5
Fluorescence data of the complexes at room temperature
Complexes
State
Solid
Slit (nm)
λex (nm)
λem (nm)
RFIa
Assignment
7
1
2.5
344
579.2
591.6
615.8
129.9
829.8
1818
5D0 → F0
7
5D0 → F1
5D0 → F2
7
6
2
Solid
2.5
333
562.4
594.6
641.0
37.96
45.71
12.34
4G5/2 → H5/2
4G5/2 → H7/2
6
4G5/2 → H7/2
6
a
RFI is relative fluorescence intensity.
excited with UV light. The solid complexes of Eu and Sm
(Figs. 4 and 5) show strong fluorescence emission, and the
fluorescence intensity of Eu(III) complexes is the strongest.
Based on the theory of antenna effect [9,10], the intensity
of the luminescence of Ln3+ complexes is related to the
efficiency of the intramolecular energy transfer between the
triple level of the ligand and the emitting level of the ions,
which depends on the energy gap between the two levels.
In the solid state, probably the energy gap between the lig-
ands triplet levels and the emitting levels of the europium
favor the energy transfer process for europium. This makes
the europium complex show the most intense red fluores-
cence. However, the Dy and Tb(III) complexes do not exhibit
fluorescence, which indicates that energy transfer process
between the ligands triplet levels and the emitting levels of
the Dy and Tb(III) does not favor.
The solid fluorescence emission peaks of Eu(III) com-
plex are all from 5D0 → FJ transition, and 5D0 → F2
7
7
Fig. 4. The fluorescence emission spectra of the Eu(III) complex.
Fig. 5. The fluorescence emission spectra of the Sm(III) complex.

218
B.-D. Wang et al. / Spectrochimica Acta Part A 63 (2006) 213–219
Table 6
Fluorescence data of the complex 1 at room temperature
Complex
C1
Solve
Slit (nm)
5.0
λex (nm)
λem (nm)
RFIa
Assignment
7
DMSO
394
591
613
650
691
11.3
6.97
0.66
1.12
5D0 → F1
5D0 → F2
7
7
5D0 → F3
7
5D0 → F4
5D0 → F0
7
C1
C1
CH3OH/CHCl3
DMF
5.0
5.0
391
394
572
591
615
3.32
4.61
8.36
5D0 → F1
7
7
5D0 → F2
5D0 → F1
7
591
616
0.61
0.84
5D0 → F2
7
C represents concentration: 5.0 × 10−4 mol L−1
.
a
RFI is relative fluorescence intensity.
transition is an electric dipole transition. It can be detected
as a relatively strong peak when Eu3+ does not lie in cen-
and Fig. 6, the fluorescence intensities of the Eu(III) com-
plex in organic solvent are weaker than that of powder. This
may be due to the quench process of solvent molecules in
the solution. The fluorescence intensity order of the Eu(III)
complex (at the same concentration) in different solvents
is DMSO > CH3OH/CHCl3 (1:1) > DMF. The fluorescence
intensity measured in CH3OH/CHCl3 is lower than the one
measured in DMSO because radiationless energy transfer
competes with the radiative processes through coupling of the
emissive states of Eu3+ ion to the O–H vibrational overstones.
Besides, in DMF, which has a higher dielectric constant than
CH3OH/CHCl3 (1:1), the europium complex exhibits the
lowest intensity. This fact is due to the high coordination
ability of DMF, whose oscillatory motions consume most of
the energy transferred from the hydrazone to the metal ion.
We also see that the emission spectra of the Eu(III)
complex exhibit different bands in different solvents. Inter-
estingly, the Eu complex exhibits four bands in DMSO,
trosymmetric ligand field. 5D0 → F1 transition is a magnetic
7
dipole transition, and its fluorescence intensity becomes the
most intensive only when Eu3+ ion is center of inversion. In
Tables 5 and 6, it can be seen that the intensity of 5D0 → F2
7
5
7
transition is far stronger than that of D0 → F1 in solid,
DMF and CH3OH/CHCl3 (1:1, v/v), and a higher value for
7
7
ηEu = 5D0 → F2/5D0 → F1 (ηEu = 2.19 (solid), 1.38 (DMF)
and 1.82 (CH3OH/CHCl3 (1:1, v/v)) indicates that the line-
like emission spectra results from intra-4f transitions of
predominantly electric dipole character, and thus Eu3+ lies
in a non-centrosymmetric ligand field [11]. However, in
the DMSO, a lower value for ηEu = 5D0 → F2/5D0 → F1
(ηEu = 0.62) indicates that the line-like emission spectra
results from intra-4f transitions of predominantly magnetic
dipole character, and thus Eu3+ lies in a centrosymmetric
ligand field. From the above discussion, we can draw a con-
clusion that the geometry of the complex has been changed
by DMSO coordination.
7
7
7
7
7
corresponding to 5D0 → F1, 5D0 → F2, 5D0 → F3 and
5D0 → F4 transitions. Comparing to the fluorescence of
7
solid complex, the 5D0 → F and 5D0 → F4 emission peaks
were be observed at about 650 and 691 nm. These indicate
that solvent molecules have strong coordination effect, and
that the environment plays an important role in determining
the fluorescence intensity of the complexes [12].
7
7
The influence of solvent on the fluorescence intensities of
the Eu(III) complex was investigated. As given in Table 6
Acknowledgements
This work is supported by the National Natural Science
Foundation of China (20475023) and Gansu NSF.
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