Synthesis and infrared and fluorescence spectra of rare earth complexes with a novel amide-based ligand

Article abstract of DOI:10.1016/j.saa.2006.04.040

A novel amide-based open-chain crown ether, N,N′-1,3-propanediyl-bis[2-(benzyl -carbamoyl-methoxy)-benzamide] (L) and its solid complexes with rare earth nitrates and picrates have been prepared. The complexes were characterized by elemental analyses, molar conductivity and IR spectra. The fluorescence properties of the Eu(III) and Tb(III) complexes in solid and in organic solvents were studied. Under the excitation of ultraviolet light, these complexes exhibit characteristic emission of europium and terbium ions. The results show that the ligand favor energy transfers to the emitting energy level of Tb(III). Some factors that influence the fluorescent intensity were also discussed.

Full text of DOI:10.1016/j.saa.2006.04.040

Spectrochimica Acta Part A 66 (2007) 1058–1062
Synthesis and infrared and fluorescence spectra of rare earth
complexes with a novel amide-based ligand
Wei Wang, Yong Huang, Ning Tang∗
College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, Gansu, PR China
Received 3 April 2006; accepted 26 April 2006
Abstract
A novel amide-based open-chain crown ether, N,Nꢀ-1,3-propanediyl-bis[2-(benzyl -carbamoyl-methoxy)-benzamide] (L) and its solid complexes
with rare earth nitrates and picrates have been prepared. The complexes were characterized by elemental analyses, molar conductivity and IR spectra.
The fluorescence properties of the Eu(III) and Tb(III) complexes in solid and in organic solvents were studied. Under the excitation of ultraviolet
light, these complexes exhibit characteristic emission of europium and terbium ions. The results show that the ligand favor energy transfers to the
emitting energy level of Tb(III). Some factors that influence the fluorescent intensity were also discussed.
© 2006 Elsevier B.V. All rights reserved.
Keywords: Amide-based open-chain crown ether; Rare earth complexes; Fluorescence properties
1. Introduction
ies, a new, doubly functionalized and amide-based open-chain
crown ether ligand (L, Scheme 1), N,Nꢀ-1,3-propanediyl-bis[2-
(benzyl-carbamoyl-methoxy)-benzamide], has been designed
and prepared. The coordination and fluorescence properties of
rare earth ions with the ligand have been studied. The results
show that counter anion and the organic solvent notably affected
the fluorescence characteristics of europium and terbium ions.
Rare earth complexes are attracting more and more atten-
tion for their important practical applications in material sci-
ence [1,2]. Their fluorescence properties are of special interest
because they could show long, millisecond-order lifetime, a
large Stokes’ shift, narrow-width emission bond and hypersensi-
tivity to coordinative environment [3]. The fluorescent intensity
of rare earth complexes is strongly dependent on the efficiency
of ligand absorption in the UV region, the efficiency of ligand-
to-metal energy transfer, and the efficiency of rare earth fluo-
rescence [4,5]. Therefore many ligands containing the common
donor groups have been synthesized. Among them open-chain
crown ether containing a high denticity to encapsulate the metal
ion and protect it from solvent interactions offers many advan-
tages in extraction and analysis of rare earth ions [6,7]. Nev-
ertheless, to achieve good fluorescence properties and optimal
binding properties, it is also important to get the right balance
between flexibility and rigidity [8]. Recently, we have designed
a series of poly-functional open-chain crown ethers having both
selective ability to coordinate rare earth ions and enhanced flu-
orescence of rare earth complexes, by selecting amide groups
as proper conjugate groups. As a part of our systematic stud-
2. Experimental
2.1. Materials
N,Nꢀ-1,3-propanediyl-bis[2-hydroxy-benzamide] was pre-
pared according to the literature methods [9]. Other chemicals
were analytical grade and used without further purification.
2.2. Methods
The carbon, hydrogen and nitrogen analyses were obtained
using an Elementar Vario EL elemental analyzer. The rare earth
ions were determined by EDTA titration using xylenol orange
as an indicator. Conductivity measurements were carried out in
10−3 mol cm−3 solutions at 25 ◦C using a DDS-307 conductiv-
ity meter. The IR spectra in the 4000–400 cm−1 region were
measured using KBr pellets on a Nicolet Nexus 670 FTIR spec-
trometer. 1H NMR spectra were recorded on a Varian Mercury
∗
Corresponding author. Tel.: +86 9318912552; fax: +86 9318912582.
E-mail address: tangn@lzu.edu.cn (N. Tang).
1386-1425/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2006.04.040

W. Wang et al. / Spectrochimica Acta Part A 66 (2007) 1058–1062
1059
Scheme 1. The synthetic route for the ligand L.
300 BB, using CDCl3 as a solvent and TMS as an internal stan-
3. Results and discussion
dard. Fluorescence spectra were obtained on a Hitachi F-4500
spectrophotometer at room temperature.
3.1. Properties of the complexes
Analytical data for the complexes, presented in Table 1,
indicate that the nitrate complexes conform to a 1:1 metal-
to-ligand stoichiometry [RE(NO3)3L]·2H2O and the picrate
complexes conform to a 2:3 metal-to-ligand stoichiometry
[RE2(Pic)6L3]·4H2O.
2.3. Synthesis of the ligand L
To
N,Nꢀ-1,3-propanediyl-bis[2-hydroxy-benzamide]
(0.942 g, 3 mmol) in DMF (40 cm3) was added sodium
hydroxide (0.240 g, 6 mmol). The mixture was heated to 80 ◦C
and stirred for about 0.5 h. Then a solution of N-benzyl-2-
chloro-acetamide (1.212 g, 6.6 mmol) and potassium iodide
(0.548 g, 3.3 mmol) in DMF (30 cm3) was dropped at a
constant rate over 1 h. The reaction mixture was stirred at about
80 ◦C for an additional 10 h. The solvent was removed under
vacuum, and then the residue was treated with 50 cm3 water.
The precipitate was collected by filtration and washed with
water. Recrystallization from methanol twice gave white prism
All the nitrate complexes are white powers and soluble in
DMSO, DMF, methanol, ethanol, acetone, acetonitrile and THF,
slightly soluble in ethyl acetate and chloroform, insoluble in
benzene, water and diethyl ether. The molar conductance data
of these complexes in methanol solution (Table 1) indicate that
all complexes are 1:1 ionic compounds [10].
All the picrate complexes are yellow powers and soluble in
DMSO, DMF, methanol, ethanol, acetone, acetonitrile and ethyl
acetate, slightly soluble in diethyl ether and chloroform, insol-
uble in benzene and water, and may be kept in air for a long
time. The molar conductance data of these complexes in ace-
tone solution (Table 1) indicate that all complexes are 2:1 ionic
compounds [10].
crystals. Yield, 67%. mp 144.0–146.0 ◦C. H NMR (CDCl3,
1
ppm): δ 1.70 (m, 2H, C–CH2–C), 3.38 (m, 4H, C–CH2–N),
4.43 (d, 4H, ArCH2N), 4.61 (s, 4H, CH2O), 6.81–7.79 (m,
18H, Ar–H), 7.88 (s, 2H, –CONH–), 8.12 (s, 2H, –CONH–).
Anal. data, calc. for C35H36N4O6: C, 69.06; H, 5.962; N, 9.207.
Found: C, 69.21; H, 6.142; N, 9.368.
3.2. IR Spectra
2.4. Preparation of the complexes
The most significant frequencies in the IR spectral date of the
ligand and its corresponding rare earth complexes are presented
in Tables 2 and 3.
A solution of 0.1 mmol rare earth nitrates (RE = La, Sm, Eu,
Tb, Dy) in 5 cm3 of ethyl acetate was added to a solution of
0.1 mmol L in 5 cm3 of chloroform. The mixture was stirred for
8 h at room temperature. The precipitated white solid complex
was separated from the solution by suction filtration, purified by
washing several times with ethyl acetate and chloroform, and
then dried for 24 h in a vacuum at room temperature.
A solution of 0.1 mmol rare earth picrates (RE = La, Nd, Eu,
Tb, Yb) in 3 cm3 of ethyl acetate was added to a solution of
0.15 mmol L in 3 cm3 of chloroform. The mixture was stirred
for 6 h at room temperature and 3 cm3 of diethyl ether was added
drop wise. After addition the mixture was stirred for another 4 h.
The precipitated yellow solid complex was separated from the
solution by suction filtration, purified by washing several times
with ethyl acetate, chloroform and diethyl ether, and then dried
for 24 h in a vacuum at room temperature.
The nitrate complexes have similar IR spectra which indicate
that they have similar structures. The characteristic absorption
bands of the free ligand due to ν(C O) and ν(Ar–O–C) appear
at 1688, 1629 and 1249 cm−1, respectively, which shift ca. 65,
7 and 27 cm−1, respectively, to lower wave numbers after the
formation of the nitrate complexes. The results clearly show
that the six oxygen atoms in the ligand L participate in the
coordination to the rare earth ions. The characteristic frequen-
cies of the coordinating nitrate groups (C2v) appear at about
1490 cm−1 (ν1), 1030 cm−1 (ν2), 817 cm−1 (ν3), and 1306 cm−1
(ν4) and the difference between the two highest frequency bands
(|ν1 − ν4|) is about 170–190 cm−1, indicating that the coordi-
nated nitrate groups in the complexes are bidentate [11,12]. The
free nitrate groups (D3h) appear at 1384 cm−1 in the spectra of

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W. Wang et al. / Spectrochimica Acta Part A 66 (2007) 1058–1062
Table 1
Elemental analytical and molar conductance date of the complexes
Complexes
Analysis (%), found (Calc.)
ΛM (cm2 ꢀ−1 mol−1
)
C
H
N
RE
[La(NO3)3L]·2H2O
[Sm(NO3)3L]·2H2O
[Eu(NO3)3L]·2H2O
[Tb(NO3)3L]·2H2O
[Dy(NO3)3L]·2H2O
[La2(Pic)6L3]·4H2O
[Nd2(Pic)6L3]·4H2O
[Eu2(Pic)6L3]·4H2O
[Tb2(Pic)6L3]·4H2O
[Yb2(Pic)6L3]·4H2O
43.33 (43.35)
42.73 (42.85)
42.86 (42.78)
42.25 (42.48)
42.43 (42.32)
47.83 (47.78)
47.58 (47.63)
47.61 (47.43)
47.07 (47.25)
46.79 (46.88)
3.96 (4.16)
4.33 (4.11)
3.84 (4.10)
4.26 (4.07)
3.83 (4.06)
3.86 (3.63)
3.44 (3.63)
3.60 (3.61)
3.54 (3.60)
3.78 (3.57)
10.30 (10.11)
10.11 (9.99)
9.87 (9.98)
9.98 (9.91)
9.96 (9.87)
11.90 (11.82)
11.75 (11.82)
11.87 (11.77)
11.64 (11.72)
11.70 (11.63)
13.99 (14.33)
15.30 (15.33)
15.02 (15.46)
16.37 (16.06)
16.20 (16.36)
8.01 (7.84)
8.28 (8.11)
8.54 (8.51)
9.02 (8.87)
9.58 (9.58)
101a
96.5a
93.6a
96.7a
90.8a
205b
173b
159b
186b
199b
a
Measured in methanol.
b
Measured in acetone.
Table 2
The most important IR bands of the free ligand L and the rare earth nitrate complexes (cm−1
)
−
ν(NO3 )
Compound
ν(C O)
ν(Ar–O–C)
ν(O–H)
ν1
ν2
ν3
ν4
|ν1 − ν4|
ν
L
1688, 1629
1621
1622
1622
1623
1249
1222
1222
1220
1226
1221
[La(NO3)3L]·2H2O
[Sm(NO3)3L]·2H2O
[Eu(NO3)3L]·2H2O
[Tb(NO3)3L]·2H2O
[Dy(NO3)3L]·2H2O
3364
3393
3376
3381
3404
1483
1489
1489
1490
1490
1031
1031
1030
1030
1030
818
817
816
816
817
1313
1306
1304
1301
1302
170
183
185
189
188
1384
1384
1384
1384
1384
1623
atoms in the C–O band and the nitro groups of Pic− partici-
pate in the coordination [14]. In addition, broad bands at about
3384 cm−1 indicate the presence of water molecules, confirming
the elemental analysis.
the complexes [13], in agreement with the result of the con-
ductivity experiment. Additionally, expected absorptions due to
the stretching mode of water in the range of 3360–3400 cm−1
indicate the presence of water molecules.
,
In the picrate complexes, after coordination, the ν(Ar–O–C)
absorption bands at 1249 cm−1 shift 28–30 cm−1 and the bands
forν(C O)shiftabout73and14 cm−1 towardslowerwavenum-
bers respectively, indicating that all the six oxygen atoms of the
ligand L take part in coordination. The band at 1265 cm−1 for
ν(C–O) of Pic− is shifted to higher frequency by ca. 10 cm−1
and the bands of the free HPic due to νas(–NO2) and νs(–NO2)
appear at 1555 and 1342 cm−1, respectively, which split into
3.3. Fluorescence studies
The fluorescence characteristics of the ligand L and its Eu3+
and Tb3+ complexes in solid state are listed in Table 4. The solid
complexesexcepttheTbpicratecomplexhavecharacteristicline
emission of f–f transitions of metal ions when they are excited
with UV light. Excited by the absorption band at 328 nm, the
free ligand L exhibits broad emission bands (λmax = 443.6 nm).
After coordination all the complexes show strong fluorescence
two bands at ca. 1575, 1543 cm−1 and ca. 1361, 1330 cm−1
,
respectively, in the complexes. These indicate that some oxygen
Table 3
The most important IR bands of the free ligand L and the rare earth picrate complexes (cm−1
)
Compound
ν(C O)
ν(Ar–O–C)
ν(C–O)
νas(–NO2)
νs(–NO2)
ν(O–H)
L
1688, 1629
1249
HPic
1265
1275
1555
1576
1545
1574
1541
1574
1543
1576
1543
1574
1544
1342
1361
1330
1361
1328
1363
1329
1361
1330
1361
1333
[La2(Pic)6L3]·4H2O
[Nd2(Pic)6L3]·4H2O
[Eu2(Pic)6L3]·4H2O
[Tb2(Pic)6L3]·4H2O
[Yb2(Pic)6L3]·4H2O
1616
1614
1615
1615
1616
1219
1219
1219
1220
1221
3384
3382
3382
3388
3384
1272
1273
1274
1274

W. Wang et al. / Spectrochimica Acta Part A 66 (2007) 1058–1062
1061
Table 4
The fluorescence spectra data of the complexes in the solid state
Compound
Slit (nm)
λex (nm)
328
λem (nm)
RFIa
39.24
5.164
34.13
Assignment
L
1
1
443.6
591.2
614.8
[Eu2(Pic)6L3]·4H2O
5D0–7F1
5D0–7F2
429
[Eu(NO3)3L]·2H2O
[Tb(NO3)3L]·2H2O
592.0
616.2
5.488
7.175
5D0–7F1
5D0–7F2
1
1
397
315
490.4
544.0
582.0
372.1
924.4
48.51
5D4–7F6
5D4–7F5
5D4–7F4
a
RFI is relative fluorescence intensity.
Table 5
The fluorescence spectra data of [Tb(NO3)3L]·2H2O in solution
Compound
Solve
Slit (nm)
λex (nm)
λem (nm)
RFI
Assignment
491.4
545.0
584.4
2754
7401
323.4
5D4–7F6
5D4–7F5
5D4–7F4
CH3CN
5
310
491.0
545.2
584.0
185.7
433.4
26.62
5D4–7F6
5D4–7F4
5D4–7F5
CH3COCH3
DMF
5
5
5
329
333
303
[Tb(NO3)3L]·2H2O
490.2
545.8
583.6
131.3
299.5
25.60
5D4–7F6
5D4–7F5
5D4–7F4
490.6
545.0
584.8
102.6
189.8
19.43
5D4–7F6
5D4–7F5
5D4–7F4
CH3OH
emission and no ligand-based emission bands at about 444 nm,
indicating that the ligand is a good organic chelator to absorb and
transfer energy to rare earth ions. Based on the theory of antenna
effect [15], the intensity of the fluorescence of rare earth com-
plexes is related to the efficiency of the intra-molecular energy
transfer between the triple levels of the ligand and the emit-
ting level of rare earth ions, which depends on the energy gap
between the two levels. From Table 4, we can see that the fluores-
cence intensity of Tb(NO3)3 complex is much stronger than that
of Eu(NO3)3 complex, indicating that the energy gap between
the triple levels of the ligand L and the emitting level of terbium
favor to the energy transfer process for terbium. Additionally the
fluorescence intensity of Eu(Pic)3 complex is stronger than that
of Eu(NO3)3 complex and the Tb(Pic)3 complex do not exhibit
fluorescence, implying that the picrate groups as the counter
anion may influence the triplet state energy level of the ligand
and make the T1 energy be lowered in the picrate complexes,
thus intensify the emission of the Eu(Pic)3 complex and quench
the emission of the Tb(Pic)3 complex.
The influence of solvent on the fluorescence intensities of
the Tb(NO3)3 complex is investigated. As given in Table 5,
we can see that the excitation wavelengths of the Tb(NO3)3
complex are different in the four solvents and the fluorescence
intensity order in different solvents (at the same concentration)
is CH3CN > CH3COCH3 > DMF > CH3OH (Figs. 1 and 2). We
may deduce that with the decrease of the size of the solvent
molecules they become more easily to participate in the coor-
dination instead of water molecules and then the fluorescence
intensities of the complexes become stronger. The fluorescence
intensity in CH3OH is lower than those in other solutions
Table 6
The fluorescence spectra data of [Eu2(Pic)6L3]·4H2O in solution
Compound
Solve
Slit (nm)
λex (nm)
λem (nm)
RFI
Assignment
591.4
615.8
591.2
615.2
590.6
615.8
588.8
615.4
10.69
51.64
6.207
32.92
5.951
23.03
4.885
9.934
5D0–7F1
5D0–7F2
5D0–7F1
5D0–7F2
5D0–7F1
5D0–7F2
5D0–7F1
5D0–7F2
CH3COOCH2CH3
10
10
10
10
446
461
461
446
CH3CN
[Eu2(Pic)6L3]·4H2O
CH3COCH3
CH3OH

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W. Wang et al. / Spectrochimica Acta Part A 66 (2007) 1058–1062
motions consume most of the energy transferred from the lig-
and to the europium ion. The fluorescence intensities of the
Eu(Pic)3 complex in the three solvents have the same order
(CH3CN > CH3COCH3 > CH3OH) that those of the Tb(NO3)3
complex in different solvents have for the same reasons we just
mentioned above. All of these indicate that the solvent molecules
have strong coordination effect and the environment plays an
important role in determining the fluorescence intensity of the
complexes [16].
4. Conclusions
According to the data and discussion above, the novel amide-
based open-chain crown ether could form stable complexes with
rare earth nitrates and picrates. Obvious IR spectrum changes
wereobservedaftercoordinationandrareearthionswerecoordi-
natedtothesixoxygenatomsoftheligandLinallthecomplexes.
Comparing the fluorescence intensities of the nitrate complexes,
we found that the energy gap between the triple levels of the
ligand L and the emitting level of terbium favor to the energy
transfer process for terbium. And comparing the fluorescence
intensity of the Eu(Pic)3 complex with that of the Eu(NO3)3
complex, we also found that the counter anion notably affected
the fluorescence characteristics of rare earth ions. The influence
of solvent on the fluorescence intensities of the complexes indi-
cated that the solvent and the environment played an important
role in determining the fluorescence intensity of the complexes.
Fig. 1. The emission spectrum of the complex [Tb(NO3)3L]·2H2O in CH3CN
solution at room temperature. Concentration: 1.0 × 10−3 mol cm−3
.
Acknowledgments
Fig. 2. The emission spectrum of the complex [Tb(NO3)3L]·2H2O in
CH3COCH3 (line 1), in DMF (line 2) and in CH3OH (line 3) solution at room
The project was supported by National Natural Science Foun-
dation of China (20431010) and the National Science Founda-
tion of Gansu Province (3ZS051-A25-003).
temperature. Concentration: 1.0 × 10−3 mol cm−3
.
because the excited state of rare earth ions is efficiently quenched
by interactions with high-energy vibrations like O–H groups [7].
The influence of solvent on the fluorescence intensities of
the Eu(Pic)3 complex is also studied (Table 6 and Fig. 3).
The fluorescence intensities in other solutions are lower than
that in ethyl acetate. This fact is due to the high coordination
ability of methanol, acetone and acetonitrile, whose oscillatory
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Fig. 3. The emission spectrum of the complex [Eu2(Pic)6L3]·4H2O in
CH3COOCH2CH3 (line 1), in CH3CN (line 2), in CH3COCH3 (line 3)
and in CH3OH (line 4) solution at room temperature. Concentration:
2.5 × 10−4 mol cm−3
.