Synthesis and infrared and luminescence spectra of rare earth complexes with a new hexapodal ligand

Article abstract of DOI:10.1016/j.saa.2005.04.045

Solid complexes of rare earth nitrates and picrates with a new hexapodal ligand, 1,2,3,4,5,6-hexa{[(2′-benzylamino-formyl)phenoxyl]methyl}-benzene (L) have been prepared. These complexes were characterized by elemental analysis, IR and molar conductivity.

Full text of DOI:10.1016/j.saa.2005.04.045

Spectrochimica Acta Part A 63 (2006) 164–168
Synthesis and infrared and luminescence spectra of
rare earth complexes with a new hexapodal ligand
Yu Tang, De-Bo Liu, Wei-Sheng Liu, Min-Yu Tan∗
College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, PR China
Received 18 March 2005; accepted 26 April 2005
Abstract
Solid complexes of rare earth nitrates and picrates with a new hexapodal ligand, 1,2,3,4,5,6-hexa{[(2ꢀ-benzylamino-formyl)phenoxyl]
methyl}-benzene (L) have been prepared. These complexes were characterized by elemental analysis, IR and molar conductivity. At the same
time, the luminescence properties of the Eu(III) and Tb(III) nitrates and picrates complexes in solid state were also investigated. Under the
excitation of UV light, these complexes exhibited characteristic emission of europium and terbium ions. The influence of the counter anion
on the luminescent intensity was also discussed.
© 2005 Elsevier B.V. All rights reserved.
Keywords: Hexapodal ligand; Rare earth complexes; Luminescence properties
1. Introduction
ous podands which have demonstrated their potential use
in functional supramolecular chemistry [8–10], amide type
podands have been attracted more attention in preparing the
rare earth complexes possessing strong luminescence prop-
erties. It is expected that the amide type podands, which are
flexible in structure and have ‘terminal-group effects’ [11],
will shield the encapsulated rare earth ion from interaction
with the surroundings effectively, and thus to achieve strong
luminescent properties. As a part of our systematic studies, a
new hexapodal ligand (L, Scheme 1), 1,2,3,4,5,6-hexa{[(2ꢀ-
benzylaminoformyl)phenoxyl]-methyl}-benzene, has been
designed and prepared successfully with the goal of expand-
ing the pool of ligand type and studying the coordination and
luminescence properties of rare earth ions with the hexapo-
dal ligand, which were not very familiar. The results indicated
that counter anion notably affected the luminescence charac-
teristics of europium and terbium ions.
The specific spectroscopic and magnetic properties of
rare earth ions have made them essential components in the
preparation of new materials and ideal as probes in stud-
ies of biological systems [1]. Macrocyclic, macrobicyclic
(cryptand) and podand-type ligands have been extensively
used for these purposes [2–5]. These complexes have been
studied as supramolecular devices, as contrast-enhancing
agents in magnetic resonance imaging, as effective chela-
tors in the separation and purification of rare earths, and
as fluorescent probes and luminescent labels in fluoroim-
munoassays. Among these studies, the luminescence prop-
erties of rare earth complexes are of special interest because
these complexes could show narrow emission bands, a large
Stokes’ shift and long luminescence decay times [6]. Podand-
type ligands have drawn much attention in recent years,
mainly due to their possessing spheroidal cavities and bind-
ing sites that are hard, therefore, stabilizing their complexes
and shielding the encapsulated ion from interaction with the
surroundings [7]. Ligands containing many of the common
donor groups have been synthesized. Among these numer-
2. Experimental
2.1. Materials
∗
N-Benzylsalicylamide was prepared according to the
Corresponding author. Tel.: +86 9318912552; fax: +86 9318912582.
E-mail address: tangyu@lzu.edu.cn (M.-Y. Tan).
literature methods [12]. Other chemicals were obtained
1386-1425/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2005.04.045

Y. Tang et al. / Spectrochimica Acta Part A 63 (2006) 164–168
165
Scheme 1. The synthetic route for the hexapodal ligand L.
sium carbonate (1.0 g, 3.6 mmol) and DMF (20 cm3) were
warmed to ca. 60 ◦C and 1,2,3,4,5,6-hexakis(bromomethyl)-
benzene (0.48 g, 0.75 mmol) was added. The reaction mixture
was stirred at 60–65 ◦C for 2 h. After cooling down, the mix-
ture was poured into water (100 cm3). The resulted solid was
treated with column chromatography on silica gel [petroleum
ether:ethyl acetate (2:1)] to get hexapodal ligand L, yield
78%, mp 175–178 ◦C; 1H NMR (CD3COCD3, 200 MHz) δ:
4.17 (d, 12H, –CH2–N), 5.43 (s, 12H, –CH2–O), 7.02–7.34
(m, 54H, Ar–H), H(N–H) not detected; IR (KBr) ν: 2922, 1648,
1451, 1218, 1105 cm−1; anal. data, calc. for C96H84N6O12:
C, 76.17; H, 5.59; N, 5.55. Found: C, 76.31; H, 5.84; N, 5.40.
from commercial sources and used without further purifi-
cation.
2.2. Methods
The RE(III) ion was determined by EDTA titration using
xylenol-orange as an indicator. Carbon, nitrogen and hydro-
gen were determined using an Elementar Vario EL (see
Table 1). Conductivity measurements were carried out with
a DDS-307 type conductivity bridge using 10−3 mol cm−3
solutions at 25 ◦C. The IR spectra were recorded in the
4000–400 cm−1 regionusingKBrpelletsandaNicoletNexus
670 FTIR spectrometer (see Tables 2 and 3). 1H NMR spec-
tra were measured on a Varian Mercury 200 spectrometer
in CD3COCD3 solutions, with TMS as internal standard.
Luminescence spectra were obtained on a Hitachi F-4500
spectrophotometer at room temperature.
2.4. Preparation of the complexes
A solution of 0.10 mmol L in 10 cm3 of ethyl acetate was
added drop wise to a solution of 0.15 mmol rare earth nitrates
(RE = La, Nd, Eu, Tb, Ho, Lu) in 10 cm3 of ethyl acetate.
The mixture was stirred at room temperature for 4 h. The
precipitated white solid complex was filtered, washed with
ethyl acetate and dried in vacuo over P4O10 for 48 h and
submitted for elemental analysis, yield 65%.
2.3. Preparation of hexapodal ligand L
The synthetic route for the hexapodal ligand is shown in
Scheme 1. N-Benzylsalicylamide (1.08 g, 4.75 mmol), potas-
Table 1
Analytical and molar conductance data of the complexes (calculated values in parentheses)
Complexes
Analysis (%)
C
ꢀm (cm2 (ꢁ mol)−1
)
H
N
RE
9.60 (9.77)
[La3(NO3)9L2]·3C4H8O2
[Nd3(NO3)9L2]·3C4H8O2
[Eu3(NO3)9L2]·3C4H8O2
[Tb3(NO3)9L2]·3C4H8O2
[Ho3(NO3)9L2]·3C4H8O2
[Lu3(NO3)9L2]·3C4H8O2
[La4(Pic)12L3]·C2H5OH
[Nd4(Pic)12L3]·C2H5OH
[Eu4(Pic)12L3]·C2H5OH
[Tb4(Pic)12L3]·C2H5OH
[Ho4(Pic)12L3]·C2H5OH
[Er4(Pic)12L3]·C2H5OH
[Y4(Pic)12L3]·C2H5OH
57.61 (57.43)
57.33 (57.21)
56.85 (56.91)
56.54 (56.63)
56.35 (56.40)
56.28 (56.01)
55.27 (55.18)
54.86 (55.03)
54.74 (54.81)
54.42 (54.62)
54.35 (54.46)
54.52 (54.39)
56.48 (56.61)
4.24 (4.54)
4.48 (4.52)
4.80 (4.49)
4.40 (4.47)
4.33 (4.45)
4.56 (4.42)
3.47 (3.61)
3.55 (3.60)
3.50 (3.58)
3.53 (3.57)
3.60 (3.56)
3.63 (3.56)
3.77 (3.70)
6.86 (6.89)
6.75 (6.87)
6.52 (6.83)
6.89 (6.80)
6.81 (6.77)
6.39 (6.72)
9.50 (9.60)
9.54 (9.57)
9.37 (9.54)
9.47 (9.50)
9.56 (9.47)
9.29 (9.46)
9.74 (9.85)
60.4a
65.0a
57.8a
75.1a
50.9a
63.7a
63.7b
51.8b
42.0b
55.7b
47.5b
39.7b
37.5b
10.27 (10.10)
10.37 (10.59)
10.86 (11.02)
11.10 (11.39)
11.88 (12.00)
7.33 (7.05)
7.51 (7.30)
7.50 (7.66)
7.75 (7.99)
8.01 (8.26)
8.09 (8.37)
4.79 (4.63)
a
Measured in acetone.
Measured in methanol (10−3 mol L−1).
b

166
Y. Tang et al. / Spectrochimica Acta Part A 63 (2006) 164–168
Table 2
The most important IR bands of the rare earth nitrate complexes (cm−1
)
−
ν(NO3 )
Compound
ν(C O)
ν(C–O–C)
ν1
ν4
|ν1–ν4|
ν2
ν3
L
1649s
1105m
1108m
[La3(NO3)9L2]·3C4H8O2
1609s
1638s
1451s
1482s
1299s
1300s
1299s
1299s
1300s
1299s
152
183
1029w
1029w
1028w
1029w
1028w
1029w
813w
814w
814w
813w
816w
814w
[Nd3(NO3)9L2]·3C4H8O2
[Eu3(NO3)9L2]·3C4H8O2
[Tb3(NO3)9L2]·3C4H8O2
[Ho3(NO3)9L2]·3C4H8O2
[Lu3(NO3)9L2]·3C4H8O2
1610s
1638s
1107m
1108m
1108m
1108m
1108m
1451s
1481s
151
181
1609s
1638s
1450s
1481s
151
182
1609s
1638s
1451s
1482s
152
183
1610s
1638s
1451s
1481s
151
181
1609s
1638s
1450s
1481s
151
182
Table 3
The most important IR bands of the rare earth picrate complexes (cm−1
)
Compound
ν(C O)
ν(C–O–C)
ν(C–O)
νas(–NO2)
νs(–NO2)
ν(OH)
HPic
1265s
1555m
1342s
L
1649s
1613s
1111m
1108m
[La4(Pic)12L3]·C2H5OH
1274m
1275m
1274m
1274m
1275m
1274m
1274m
1574m
1542m
1363m
1326m
3403w
3404w
3402w
3402w
3403w
3404w
3401w
[Nd4(Pic)12L3]·C2H5OH
[Eu4(Pic)12L3]·C2H5OH
[Tb4(Pic)12L3]·C2H5OH
[Ho4(Pic)12L3]·C2H5OH
[Er4(Pic)12L3]·C2H5OH
[Y4(Pic)12L3]·C2H5OH
1613s
1614s
1613s
1613s
1613s
1613s
1108m
1108m
1108m
1108m
1108m
1108m
1575m
1542m
1364m
1328m
1574m
1542m
1366m
1330m
1574m
1542m
1363m
1326m
1575m
1543m
1367m
1330m
1575m
1543m
1363m
1327m
1574m
1542m
1364m
1327m
A solution of 0.10 mmol L in 10 cm3 of ethanol was added
drop wise to a solution of 0.14 mmol rare earth picrates
(RE = La, Nd, Eu, Tb, Ho, Er, Y) in 10 cm3 of ethanol. The
mixture was stirred at room temperature for 6 h. The precip-
itated yellow solid complex was filtered, washed with ethyl
ethanol and dried in vacuo over P4O10 for 48 h and submitted
for elemental analysis, yield 62%. Analytical data and molar
conductance values of these rare earth nitrates and picrates
complexes are given in Table 1.
plexes conform to a 3:2 metal-to-ligand stoichiome-
try [RE3(NO3)9L2]·3C4H8O2 and seven picrates com-
plexes conform to a 4:3 metal-to-ligand stoichiometry
[RE4(Pic)12L3]·C2H5OH.
All the rare earth nitrates complexes are soluble in DMF,
DMSO, methanol, ethanol, acetone and THF, but slightly sol-
uble in AcOEt and CHCl3. The molar conductances of the
complexes in acetone (see Table 1) indicate that all complexes
act as noneletrolytes [13], implying that all nitrate groups are
in coordination sphere.
All the rare earth picrates complexes are soluble in
DMF, DMSO, methanol, acetone and acetonitrile, but
slightly soluble in ethanol, AcOEt and CHCl3. The
molar conductances of the complexes in methanol (see
Table 1) indicate that all complexes act as noneletrolytes
[13], implying that all picrate groups are in coordination
sphere.
3. Results and discussion
3.1. Properties of the complexes
Analytical data for the newly synthesized complexes,
listed in Table 1, indicate that the six nitrates com-

Y. Tang et al. / Spectrochimica Acta Part A 63 (2006) 164–168
167
Pic− at 1265 cm−1 is shifted toward higher frequency by
ca. 10 cm−1 in the complexes. This is due to the following
two effects. First, the hydrogen atom of the OH group is
replaced by RE(III), increasing the ␲-bond character in the
C–O bond. Secondly, coordination of the nitrogen atom of
L to RE(III) decreases the ␲-character. The free HPic has
νas(−NO2) and νs(−NO2) at 1555 and 1342 cm−1, respec-
tively, which split into two bands at ca. 1575, 1542 cm−1
and ca. 1365, 1330 cm−1, respectively, in the complexes.
This indicates that some O-atoms in the nitro group of Pic−
take part in coordination [16]. Additionally, broad bands at
3400 cm−1 could be attributed to the O–H stretching vibra-
tions of ethanol molecule.
3.2. IR spectra
The most important IR peaks of the ligand and the com-
plexes are reported in Tables 2 and 3.
The “free” ligand L exhibit two absorption bands at
1649 and 1105 cm−1 which are assigned to ν(C O) and
ν(C O C), respectively. In the nitrate complexes, the low-
energy band remains unchanged, but the high-energy band
red shifts to about 1609 cm−1 (ꢂν = 40 cm−1) as compared
to its counterpart for the “free” ligand, indicating that only
the oxygen atom of C O takes part in coordination to the
metal ions.
The characteristic frequencies of the coordinating nitrate
groups (C2v) appear at ca. 1450 and 1480 cm−1 (ν1),
1300 cm−1 (ν4), 1030 cm−1 (ν2) and 815 cm−1 (ν3) cm−1
[14], and the difference between two strongest absorptions
3.3. Luminescence properties of the complexes
(ν1 and ν4) of the nitrate group−s is about 150–180 cm−1
,
Ligand-based excitations cause the characteristic emis-
sion of rare earth complexes while the ligand fluorescence is
completely quenched. This indicates the presence of ligand-
to-metal energy transfer [17]. The ability to transfer energy
from ligand to metal is important in the design of RE(III)
supramolecular photonic devices [18,19]. The luminescence
spectra of the ligand and its Eu3+ and Tb3+ complexes in solid
state were recorded at room temperature. Among these four
complexes, luminescence of the complexes of Eu, Tb nitrates
and Eu picrate were observed but not for the Tb picrate com-
plexes. The emission spectra of the ligand and complexes are
shown in Fig. 1.
Excitedbytheabsorptionbandat325 nm, the“free”ligand
exhibits broad emission bands (λmax = 450 nm). It is shown
in Fig. 1 that the complexes show the characteristic emissions
of Eu3+ or Tb3+. This indicates that the ligand L is a good
organic chelator to absorb and transfer energy to metal ions.
clearly establishing that the NO3 groups in the solid com-
plexes coordinate to the rare earth ion as bidentate ligands
[15]. Additionally, no bands at 1380, 820 and 720 cm−1 in
the spectra of complexes indicates that free nitrate groups
(D3h) are absent. Additionally, band at 1638 cm−1 indicates
that ethyl acetate is existent in the complexes, confirming the
elemental analysis.
In the picrate complexes, after the formation of the com-
plex, the characteristic frequency of the free ligand ν(C O) at
1649 cm−1 shifts ca. 35 cm−1 towards lower wave numbers
and the ν(C–O–C) absorption band also remains unchanged,
indicating that only the oxygen atom of C O coordinate to
the rare earth ion.
The OH out-of-plane bending vibration of the free HPic
at 1151 cm−1 disappears, indicating that the H-atom of the
OH group is replaced by RE(III). The vibration νC–O of
Fig. 1. (A)EmissionspectraofLinsolidstate;(B)emissionspectraof[Eu3(NO3)9L2]·3C4H8O2 insolidstate;(C)emissionspectraof[Tb3(NO3)9L2]·3C4H8O2
in solid state; (D) emission spectra of [Eu4(Pic)12L3]·C2H5OH in solid state.

168
Y. Tang et al. / Spectrochimica Acta Part A 63 (2006) 164–168
In the spectrum of Eu(NO3)3 complex, the relative
complexes is very essential in determining the luminescent
properties of the rare earth complexes by influencing the
electrostatic factors in the ligand–metal bonding.
5
7
5
7
intensity of D0 → F2 is stronger than that of D0 → F1,
showing that the Eu(III) ion is not in a centro-symmetric
coordination site [20]. In addition, the Tb(NO3)3 and
Eu(Pic)3 complexes show no ligand-based emission bands
in ca. 450 nm, but this emission bands in the Eu(NO3)3
complex are still visible which means the energy transfer
in the Tb(NO3)3 and Eu(Pic)3 complexes are more efficient
than that in the Eu(NO3)3 complex. Intramolecular energy
transfer from the triplet state of the ligand to the resonance
level of the RE(III) ion is one of the most important processes
influencing the luminescence quantum yields of RE(III)
complexes [21]. The energy difference between the triplet
state energy level of the ligand and the lowest excited state
level of RE(III) cannot be too large or too small. So consid-
ering the emission spectra of these three complexes, it can be
concluded that the triplet energy of L is in an appropriate level
to Tb3+ in the Tb(NO3)3 complex and Eu3+ in the Eu(Pic)3
complex, which makes the energy transition from the ligand
to RE(III) more easily in these two complexes. We deduced
that the electrostatic factors in the ligand–metal bonding,
which may be affected by the different counter anion in these
two series complexes, influenced the triplet state energy level
of the ligand L [6] and made this T1 energy be lowered in
the picrates complexes, thus intensified the emission of the
Eu(III) and quenched the emission of the Tb(Pic)3 complex.
Acknowledgments
This work was supported by the National Natural Sci-
ence Foundation of China (Grant 20401008) and the Doctoral
Research Foundation of Lanzhou University.
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4. Conclusions
In this paper, we report the preparation, IR aspectra and
luminescent properties of the solid complexes of rare earth
nitrates and picrates with a new hexapodal ligand, 1,2,3,4,5,6-
hexa{[(2ꢀ-benzylamino-formyl)phenoxyl]methyl}-benzene
(L). Comparing the luminescence spectra of the nitrate
complexes of europium(III) and terbium(III), we found
that the hexapodal ligand L in Tb(NO3)3 complex is more
effective in energy-transfer than in Eu(NO3)3 complex.
The Tb(NO3)3 complex show no emission bands from
the ligand L in ca. 450 nm, but the emission bands in the
Eu(NO3)3 complex are still visible (Fig. 1). And the compare
between the luminescence spectra of the Eu(NO3)3 complex
and the Eu(Pic)3 complex showed that the ligand L in
Eu(Pic)3 complex is more effective in energy-transfer than
in Eu(NO3)3 complex. This is maybe due to the suitable
lowest triplet energy of L in its Eu(Pic)3 complex. And the
Tb(Pic)3 complex does not show its characteristic emission
spectrum. So, we may deduce that the counter anion of the
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