Synthesis, crystal structure and properties of two ternary rare earth complexes with aromatic acid and 1,10-phenanthroline

Article abstract of DOI:10.1016/j.jallcom.2007.09.008

Two dimeric rare-earth complexes [Eu(o-MOBA)₃phen]₂·2H₂O (1), [Tb(o-MOBA)₃phen]₂·2H₂O (2), (where o-MOBA = o-methoxybenzoate, phen = 1,10-phenanthroline) were synthesized and structurally c

Full text of DOI:10.1016/j.jallcom.2007.09.008

Journal of Alloys and Compounds 463 (2008) 338–342
Synthesis, crystal structure and properties of two ternary rare earth
complexes with aromatic acid and 1,10-phenanthroline
Na Zhaoa, Shu-Ping Wanga, Rui-Xia Maa, Zhi-Hua Gaoa,
Rui-Fen Wanga,∗, Jian-Jun Zhangb
a Department of Chemistry, Hebei Normal University, Shijiazhuang 050016, PR China
b Experimental Center, Hebei Normal University, Shijiazhuang 050016, PR China
Received 13 June 2007; received in revised form 30 August 2007; accepted 2 September 2007
Available online 6 September 2007
Abstract
Two dimeric rare-earth complexes [Eu(o-MOBA)3phen]2·2H2O (1), [Tb(o-MOBA)3phen]2·2H2O (2), (where o-MOBA = o-methoxybenzoate,
phen = 1,10-phenanthroline) were synthesized and structurally characterized. Both of them consist of neutral dimeric molecules, crystallize in
¯
triclinic system, space group P1. Each RE(III) ion is nine-coordinated with one 1,10-phenanthroline molecule, one bidentate chelating carboxylate
group, two bidentate bridging carboxylate groups and two tridentate bridging carboxylate groups. Complex 1 shows bright red luminescence, 2
shows green luminescence under UV light at room temperature, respectively. The thermal analysis indicates that they are all quite stable to heat.
© 2007 Elsevier B.V. All rights reserved.
Keywords: Crystal structure; Rare earth; Phenanthroline; Aromatic acid; Complex
1. Introduction
nation of a conjugated ␲-electron system to the metal ions.
On the other hand, the species of organic carboxylate (even
the different substituents on them) also influence the lumines-
cent intensity and stability. In view of the fascinating properties
and potential applications of lanthanide carboxylate, we syn-
thesized two ternary complexes of rare-earth methoxybenzoic
acids with 1,10-phenanthroline: [Eu(o-MOBA)3phen]2·2H2O
(1), [Tb(o-MOBA)3phen]2·2H2O (2). Their crystal struc-
tures, luminescent properties and thermal analysis were also
reported.
The design and construction of efficient photoluminescent
lanthanide complexes continues to be an active area of research.
Several classes of ligand have been utilized for the prepara-
tion of such complexes, such as cryptands [1,2], podands [3,4],
␤-diketones [5–7], carboxylic acid derivatives [8–13]. Most of
them emit red or green light (Eu(III) and Tb(III) luminescence,
respectively). Efficient luminescent materials may have several
applications: as new rare-earth fluorescent materials, as lumi-
nescent probes in biomedical assays [14,15], as emitters in
electroluminescent (EL) devices [16–18], etc.
2. Experimental
Variation of the coordination geometries of central metal
ion, and also of the coordination modes of carboxylate in the
lanthanide carboxylate complexes, are interesting themes of
structural chemistry. Over the past 20 years, many studies of
lanthanide complexes have reported the diverse coordination
types of carboxylates (such as monodentate, chelating biden-
tate, bridging bidentate and bridging tridentate) [19–26]. The
common feature for strong luminescence is the direct coordi-
2.1. General
All solvents and reagents for the synthesis were commercially available and
were used as received. EuCl3·6H2O and TbCl3·6H2O were prepared by dis-
solving their oxides in hydrochloric acid, and then the solutions were dried by
water-bath heating. The Elemental Analysis was carried out on a Flash EA 1112
elemental analyzer. The infrared spectrum was taken on a FT-IR spectrometer
in the 3500–350 cm−1 regions, using KBr pellets. The fluorescence proper-
ties of powder samples were determined using a Hitachi F-4500 fluorescence
spectrophotometer at room temperature. The TG and DTG experiment were
performed using a Perkin-Elmer TGA7 thermogravimetric analyzer in nitrogen
atmosphere.
∗
Corresponding author. Tel.: +86 311 86263569.
E-mail address: wruifen@mail.hebtu.edu.cn (R.-F. Wang).
0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2007.09.008

N. Zhao et al. / Journal of Alloys and Compounds 463 (2008) 338–342
339
2.2. Preparation of complexes 1 and 2
plexes 1 and 2 are listed in Table 2 and Table S1 in Supplementary Materials,
respectively.
CCDC 294235 and 605411 contain the supplementary crystallographic
data for this paper. These data can be obtained free of charge via
http://www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge crys-
tallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: +44
1223 336033).
Two complexes were synthesized by a similar procedure. A stoichiometric
amount of RECl3·6H2O (RE = Eu or Tb), o-methoxybenzoic acid and 1,10-
phenanthroline were dissolved in 95% C2H5OH, respectively. The pH of the
o-methoxybenzoic acid solution was adjusted in a range of 6–7 with 1 mol L−1
NaOH solution. The C2H5OH solutions of two ligands were mixed and then the
mixturewasaddeddropwisetotheethanolicRECl3 solutionunderstirring, while
a white precipitate formed. The solution was stirred for 4 h at room temperature
and filtered. The colorless transparent crystals suitable for X-ray analysis were
obtained by the slow evaporation of the filtrates.
Anal. Calcd. for C36H31N2O10Eu (1): C 53.81; H 3.89; N 3.49%. Found:
C 53.69; H 3.86; N 3.64%. Important IR absorptions (KBr pellet, cm−1): 1608
(νas(COO)), 1402 (νs(COO)), 1518 (ν(C N)).
Anal. Calcd. for C72H62N4O20Tb2 (2): C 53.34; H 3.85; N 3.46%. Found:
C 52.70; H 3.72; N 3.38%. Important IR absorptions (KBr pellet, cm−1): 1610
(νas(COO)), 1404 (νs(COO)), 1518 (ν(C N)).
3. Result and discussion
3.1. Crystal structures of complexes 1 and 2
The molecular structure and atomic numbering of [Eu(o-
MOBA)3phen]2·2H2O (1) are shown in Fig. 1. Complex 1
is a solvate, and consists of dimeric units. The two Eu(III)
ions are linked together by four o-MOBA groups through their
bi- and tridentate bridging modes, form a dimeric unit with
crystallographic inversion center, and the distance between
2.3. Crystallographic data collection and structure determination
˚
two Eu(III) ions is 4.0047(10) A. Each Eu(III) ion is coordi-
The single crystal of complex 1 was put on a Bruker Apex II CCD area detec-
tor and that of complex 2 was put on a Bruker Smart Apex CCD area detector,
nated to nine atoms, of which five oxygen atoms are from the
bridging carboxylates, two oxygen atoms from the bidentate
chelating carboxylate group, and two nitrogen atoms from a
1,10-phenanthroline molecule. The analysis of structural fea-
tures indicates that the central Eu(III) ion adopts a distorted
mono-capped square antiprism geometry (Fig. 2). The under-
side plane of the square antiprism for Eu1 is formed by N1, N2,
O3, O4, and the deviations of N1, N2, O3 and O4 from the plane
˚
equipped with a graphite-monochromated Mo K␣ radiation (λ = 0.71073 A).
Data were collected at room temperature by ϕ–ω scan mode. The details of
crystallographic data and structure refinement parameters are summarized in
Table 1.
The structures were solved with direct methods using SHELXS-97 pro-
gram [27]. All nonhydrogen atoms were refined anisotropically. The H atoms
were assigned with common isotropic displacement factors and included in the
final refinement by use of geometrical restraints. A full matrix least-squares
refinement on F2 was carried out using SHELXL-97 [28]. Reliability fac-
˚
ꢀ
ꢀ
are 0.1132, −0.1101, −0.1416, 0.1384 A, respectively. The top-
tors were defined as R1 = (||Fo| − |Fc|)/ |Fo|, and the function minimized
ꢁ
ꢂ
side plane is formed by O2, O5, O1A, O6A, and their deviations
1/2
ꢀ
ꢀ
was wR2
=
[w(Fo2 − Fc2)2]/ [w(Fo2)2]
, where in the least squares
˚
from the plane are 0.0971, −0.1135, 0.1235, −0.1071 A, respec-
calculation the unit weight was used. Selected bond distances and angles of com-
tively. The capped position is occupied by O1, which is displaced
Table 1
Crystallographic data and structure refinement parameters for complex 1, 2
Complex
1
2
Empirical formula
Formula weight
Temperature (K)
C
36H31N2O10Eu
C72H62N4O20Tb2
1621.10
293(2)
803.59
293(2)
˚
Wavelength (A)
0.71073
Triclinic
P1
0.71073
Triclinic
P1
Crystal system
Space group
¯
¯
◦
◦
˚
˚
˚
˚
Unit cell dimensions
a = 11.436(4) A, α = 66.818(4)◦
a = 11.4300(19) A, α = 66.717(3)
b = 12.759(4) A, β = 83.533(4)◦
b = 12.752(2), β = 83.435(3)◦
c = 13.147(2) A, γ = 70.488(3)
◦
˚
c = 13.139(4) A, γ = 70.335(3)
3
˚
Volume (A ), Z
1659.1(9), 2
1.609
1.953
808
1658.8(5), 1
1.623
2.194
Calculated density (g/cm3)
Absorption coefficient (mm−1
F(0 0 0)
)
812
Crystal size (mm3)
0.32 × 0.12 × 0.10
0.389 × 0.196 × 0.188
θ range for data collection (◦)
Limiting indices
1.69–25.03
1.69–28.23
−13 ≤ h ≤ 13, −15 ≤ k ≤ 15, −7 ≤ l ≤ 15
9125/5795 [R(int) = 0.0167]
5173
SADABS
5795/3/445
−14 ≤ h ≤ 15, −15 ≤ k ≤ 16, −17 ≤ l ≤ 15
9901/7232 [R(int) = 0.0995]
4089
SADABS
7232/2/454
Reflections collected/unique
Observed reflections
Absorption correction
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I > 2σ(I)]
R indices (all data)
1.048
0.836
R1 = 0.0226, wR2 = 0.0555
R1 = 0.0269, wR2 = 0.0564
0.575 and −0.424
R1 = 0.0635, wR2 = 0.1241
R1 = 0.1115, wR2 = 0.1687
1.911 and −1.695
−3
˚
Largest diffraction peak and hole (e A
)

340
N. Zhao et al. / Journal of Alloys and Compounds 463 (2008) 338–342
Fig. 1. Dimeric structure for [Eu(o-MOBA)3phen]2·2H2O (1) with 50% thermal ellipsoids.
position, which is from the thridentate bridging carboxylate.
˚
by 1.6283 A above the topside plane. The Eu1–O1 bond is
the longest bond in the coordination polyhedron of Eu(III) ion
˚
The mean Eu–O bond length is 2.452(2) A, and the average
˚
Eu–N distance is 2.603(2) A. The fact indicates that the strength
˚
(Eu1–O1 = 2.688(2) A).
In complex 1, most of the Eu–O bonds formed by the bridg-
ing carboxylates (2.357(2) to 2.436(2) A) are slightly shorter
of Eu–O bond is stronger than that of Eu–N bond in the
complex.
˚
The structures of complexes 1 and 2 are isomorphous.
The molecular structure and atomic numbering of [Tb(o-
MOBA)3phen]2·2H2O (2) are shown in Fig. S1. It also consists
of dimeric molecules, in which the distance between two Tb(III)
than the corresponding bonds formed by the terminal chelat-
ing carboxylate groups (2.461(2) and 2.482(2) A), except the
longest Eu1–O1 bond formed by the oxygen atom at the capped
˚
˚
ions is 3.9928(11) A. Each Tb(III) ion is coordinated to nine
atoms (TbO7N2). The Tb(III) ion also adopts a distorted mono-
capped square antiprism geometry (Fig. S2).
In both 1 and 2, the RE–O (chelating carboxylate) bond dis-
tances are longer than those of the bridging carboxylates groups,
which clearly shows that the coordination of RE(III) ion with
the chelating carboxylate is weaker than that of RE(III) ion with
the bridging carboxylates groups, due to the formation of an
unstable four-membered ring [10].
Compared with the analogous binuclear molecules
[Eu(BA)3phen]2 (C.N. = 8) [29] and [Tb(BA)3phen]2 (C.N. = 9,
˚
Tb· · ·Tb = 4.056 A) [30], the coordination environment of
Eu(III) ion has been changed, while that of Tb(III) ion remains.
The distance between two Tb(III) ions is shortened. The results
are related to the electronic effect and steric effect of the
methoxy substituent on the benzene ring.
3.2. Luminescent properties
All excitation and luminescence of powder complexes were
performed under identical conditions and instrumental param-
eters at room temperature (as shown in Figs. 3 and 4). The
excitation of two complexes were effected in a range of
Fig. 2. The coordination polyhedron of Eu1 ion in [Eu(o-MOBA)3phen]2·2H2O
(1).

N. Zhao et al. / Journal of Alloys and Compounds 463 (2008) 338–342
341
Table 2
◦
˚
Selected bond lengths (A) and angles ( ) for complex [Eu(o-MOBA)
3(phen)]2·2H2O (1)
Bond lengths
Eu(1)–O(1)#1
Eu(1)–O(5)
Eu(1)–O(3)
Eu(1)–N(1)
Eu(1)–O(1)
Eu(1)–O(6)#1
Eu(1)–O(2)
Eu(1)–O(4)
Eu(1)–N(2)
2.357(2)
2.376(2)
2.461(2)
2.609(2)
2.688(2)
2.3648(19)
2.436(2)
2.482(2)
2.597(2)
Bond angles
O(1)#1–Eu(1)–O(6)#1
O(1)#1–Eu(1)–O(2)
O(1)#1–Eu(1)–O(4)
O(1)#1–Eu(1)–O(1)
O(6)#1–Eu(1)–O(2)
O(6)#1–Eu(1)–O(4)
O(6)#1–Eu(1)–N(1)
O(6)#1–Eu(1)–O(1)
O(5)–Eu(1)–O(3)
O(5)–Eu(1)–N(2)
O(5)–Eu(1)–O(1)
O(2)–Eu(1)–O(4)
O(2)–Eu(1)–N(1)
O(3)–Eu(1)–O(4)
O(3)–Eu(1)–N(1)
O(4)–Eu(1)–N(2)
O(4)–Eu(1)–O(1)
N(2)–Eu(1)–O(1)
O(1)#1–Eu(1)–O(5)
O(1)#1–Eu(1)–O(3)
O(1)#1–Eu(1)–N(2)
O(1)#1–Eu(1)–N(1)
O(6)#1–Eu(1)–O(5)
O(6)#1–Eu(1)–O(3)
O(6)#1–Eu(1)–N(2)
O(5)–Eu(1)–O(2)
O(5)–Eu(1)–O(4)
O(5)–Eu(1)–N(1)
O(2)–Eu(1)–O(3)
O(2)–Eu(1)–N(2)
O(2)–Eu(1)–O(1)
O(3)–Eu(1)–N(2)
O(3)–Eu(1)–O(1)
O(4)–Eu(1)–N(1)
N(2)–Eu(1)–N(1)
N(1)–Eu(1)–O(1)
75.04(7)
125.29(6)
78.67(7)
75.10(7)
88.89(7)
128.89(7)
78.04(7)
71.61(7)
128.56(7)
77.28(7)
68.59(7)
141.41(7)
70.56(7)
52.52(7)
71.81(8)
71.79(7)
140.15(6)
116.26(7)
75.87(7)
88.09(7)
143.86(7)
148.01(7)
135.32(6)
83.36(7)
140.48(7)
81.09(7)
76.30(7)
136.12(7)
142.38(7)
72.99(7)
50.28(6)
90.08(7)
152.62(7)
106.00(7)
62.96(7)
112.20(7)
Fig. 3. Luminescence spectra of complex [Eu(o-MOBA)3phen]2·2H2O (1).
Fig. 4. Luminescence spectra of complex [Tb(o-MOBA)3phen]2·2H2O (2).
transition. Both complexes show intense luminescent properties
at room temperature, their emission lifetime reached 1.56 ms for
1 and 1.24 ms for 2, respectively.
3.3. Thermal decomposition
The TG and DTG curves of [Eu(o-MOBA)3phen]2·2H2O (1)
are shown in Fig. 5. Complex 1 decomposed via intermediates
to give europium oxide as an end product. The results of ther-
mal analysis indicate that complex 1 begins to decompose at
the temperature of 35 ◦C and its decomposition ends at about
693 ◦C. The TG degradation reveals four decomposition stages,
which consists with the result of DTG curve. There are maxi-
mum weight losses rates at 97, 308, 455 and 566 ◦C in the DTG
curve. The first weight loss of 2.0% occurs from 35 ◦C to 110 ◦C,
which approximately corresponds to the loss of 2 mol uncoordi-
nated H2O (theoretical loss is 2.24%). The second degradation
stage is in the range of 277–380 ◦C, which corresponds to the
loss of 2 mol phen. This degradation can be explained that
the Eu–N bonds are less stable and easy to be broken down.
The third stage of temperature 380–490 ◦C corresponds to the
loss of 2 mol MOBA. This degradation can be explained based
Symmetry transformations used to generate equivalent atoms: #1 −x + 2, −y,
−z + 2.
200–400 nm. The fluorescence was observed in a range of
400–700 nm by selective excitation at 328 nm.
The emission spectrum for complex 1 is composed of the
characteristic emission peaks of Eu(III) arising from the tran-
5
7
5
7
5
7
sition D0 → F0 (579 nm), D0 → F1 (592 nm), D0 → F2
(612, 619 nm), respectively. The intensity of emitting band aris-
7
ing from 5D0 → F2 transition is the most intense. The emission
spectrum for complex 2 is composed of four characteristic
emission peaks of Tb(III): 5D4 → F6 (489 nm), 5D4 → F5
7
7
5
7
5
7
(545 nm), D4 → F4 (585 nm), D4 → F3 (620 nm), respec-
5
7
tively. The most intense emitting band arises from D4 → F5

342
N. Zhao et al. / Journal of Alloys and Compounds 463 (2008) 338–342
Department (No. 2006125) and Hebei Normal University, PR
China.
Appendix A. Supplementary data
Supplementary data associated with this article can be found,
in the online version, at doi:10.1016/j.jallcom.2007.09.008.
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on the Eu–O bond distances. The Eu–O (terminal chelating
carboxylate) bonds are longer than the Eu–O (bridging carboxy-
late) bonds, so two terminal o-methoxybenzoates are removed
firstly. The last stage degradation temperature is in the range of
490–693 ◦C, in which the remained four MOBA are removed
while the complex was completely degraded into Eu2O3 with a
total loss of 76.42 wt% (theoretical loss is 78.10%). The ther-
mal decomposition of [Eu(o-MOBA)3phen]2·2H2O (1) can be
described as follows:
[Eu(o-MOBA)3phen]2·2H2O → [Eu(o-MOBA)3phen]2
→ Eu2(o-MOBA)6 → Eu2(o-MOBA)4 → Eu2O3
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is similar to 1 (Fig. S3.), with four decomposition stages of tem-
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respectively. Its thermal decomposition can be described as fol-
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[Tb(o-MOBA)3phen]2·2H2O → [Tb(o-MOBA)3phen]2
→ Tb2(o-MOBA)6 → Tb2(o-MOBA)4 → (1/2)Tb4O7
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Two dimeric rare-earth complexes [Eu(o-MOBA)3phen]
2·2H2O (1), [Tb(o-MOBA)3 phen]2·2H2O (2) have been synthe-
sized. Complex 1 shows bright red luminescence, 2 shows green
luminescence under UV light at room temperature, respectively.
The thermal analysis indicates that they are all quite stable to
heat.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (No.20773034), Natural Science Founda-
tion of Hebei Province (No. B2007000237), Hebei Education