Syntheses, crystal structures, luminescence and thermal properties of three lanthanide complexes with 2-bromine-5-methoxybenzoate and 2,2:6′,2″-terpyridine

Article abstract of DOI:10.1016/j.poly.2017.12.036

Three new mononuclear lanthanide complexes [Ln(2-Br-5-MOBA)₃(terpy)(H₂O)] (Ln = Gd (1), Tb (2), Er (3); 2-Br-5-MOBA = 2-bromine-5-methoxybenzoate; terpy = 2,2:6′,2″-terpyridine) have been successfully synthesized and characterized by single-crystal X-ray diffraction. The complexes 1–3 were isostructural and eight-coordinated, which is different from the binuclear structure of the previously reported complexes. The molecular structure of complexes 1–3 is very interesting: two adjacent mononuclear units are connected through hydrogen bonding giving rise to a pseudo binuclear structure. These pseudo binuclear units are further linked via O–H?Br hydrogen bonds and slightly offset π–π stacking interactions to from 1D and 2D supramolecular structures. The thermal decomposition mechanism of complexes 1–3 was studied by TG analysis and further authenticated by TG/DSC-FTIR techniques. The solid-state luminescent property of complex 2 was investigated at room temperature. The result indicates that Tb (III) complex appear to be promising candidate for the application as green luminescent material.

Full text of DOI:10.1016/j.poly.2017.12.036

Accepted Manuscript
Syntheses, crystal structures, luminescence and thermal properties of three lan-
thanide complexes with 2-bromine-5-methoxybenzoate and 2,2:6',2''-terpyri-
dine
Zhao Qing-Qing, Ren Ning, Zhang Jian-Jun
PII:
S0277-5387(18)30005-6
https://doi.org/10.1016/j.poly.2017.12.036
POLY 12997
DOI:
Reference:
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29 December 2017
Please cite this article as: Z. Qing-Qing, R. Ning, Z. Jian-Jun, Syntheses, crystal structures, luminescence and thermal
properties of three lanthanide complexes with 2-bromine-5-methoxybenzoate and 2,2:6',2''-terpyridine,
Polyhedron (2018), doi: https://doi.org/10.1016/j.poly.2017.12.036
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Syntheses, crystal structures, luminescence and thermal
properties of three lanthanide complexes with
2-bromine-5-methoxybenzoate and 2,2:6',2''-terpyridine
ZHAO Qing-Qing1,2, REN Ning3,*, ZHANG Jian-Jun1,2,*
(1Testing and Analysis Center, Hebei Normal University, Shijiazhuang 050024, P. R. China;
2College of Chemistry& Material Science, Hebei Normal University, Shijiazhuang 050024, P. R.
China; 3College of Chemical engineering& Material, Handan University, Handan 056005,
Hebei Province, P. R. China;
Abstract
Three
[Ln(2-Br-5-MOBA)3(terpy)(H2O)] (Ln = Gd (1), Tb (2), Er (3);
2-Br-5-MOBA 2-bromine-5-methoxybenzoate; terpy
new
mononuclear
lanthanide
complexes
=
=
2,2:6',2''-terpyridine) have been successfully synthesized and
characterized by single-crystal X-ray diffraction. The complexes 1–3
were isostructural and eight-coordinated, which is different from the
binuclear structure of the previously reported complexes. The molecular
structure of complexes 1–3 is very interesting: two adjacent mononuclear
units are connected through hydrogen bonding giving rise to a pseudo
binuclear structure. These pseudo binuclear units are further linked via
O-H···Br hydrogen bonds and slightly offset π–π stacking interactions to
from 1D and 2D supramolecular structures. The thermal decomposition
mechanism of complexes 1–3 was studied by TG analysis and further
authenticated by TG/DSC-FTIR techniques. The solid-state luminescent
--------------------
*Corresponding author. Email: Email: jjzhang6@126.com; ningren9@163.com;
Tel.: +86-31180786457 Fax: +86-31180786457

property of complex 2 was investigated at room temperature. The result
indicates that Tb (Ш) complex appear to be promising candidate for the
application as green luminescent material.
Keywords
Lanthanide complexes, Crystal structure, Thermal analysis,
Luminescence, 2,2:6',2''-terpyridine
Introduction
In recent years, the search of lanthanide complexes has attracted great
interest for their potential applications in light-emitting diodes [1],
luminescent probes [2, 3], magnetic materials [4, 5], catalysts [6, 7],
thermodynamics [8, 9] and antibacterial [10]. Due to the high
coordination number and flexible coordination geometry, more and more
lanthanide complexes with novel structure have been synthesized [11].
Aromatic carboxylic acid ligands exhibit high affinity for lanthanide
cations because of the presence of the hard donor oxygen atoms [12],
making them become a better choice for the construction of lanthanide
complexes. And the carboxylic acid ligands have many coordination
modes, which makes the lanthanide complex containing
aromatic-carboxylic acids produce a considerable number of structures
[12, 13].
Due to the intense emission of lanthanide ions over narrow wavelength
ranges, fluorescence has been extensively discussed in all applications of

lanthanide complexes [11]. In particular, those containing Eu (III) and Tb
(III) have long-lived emission and have been candidates for red and green
materials. However, the parity inhibition of f-f transitions leads to the low
absorption coefficients of Ln (Ⅲ). Therefore, a suitable chromophore
(antenna) is helpful to trigger their emission through efficiently energy
transfer from the ligand to the rare earth ions [14-17]. Recently, many
researchers have introduced auxiliary organic ligands in the preparation
of complexes, since N-containing heterocyclic auxiliary ligands usually
make lanthanide complexes exhibit higher thermal stability and stronger
luminescence [18].
Herein, we report the synthesis of three coordination complexes
[(Ln(2-Br-5-MOBA)3(terpy)(H2O)] (Ln = Gd (1), Tb (2), Er (3);
2-Br-5-MOBA
=
2-bromine-5-methoxybenzoate;
terpy
=
2,2:6',2''-terpyridine), which are characterized by elemental analysis, IR
spectra, single crystal X-ray diffraction, and thermal stability.
Furthermore, the solid-state luminescent property of complex 2 is also
studied.
Experimental
Materials and general methods
LnCl3·6H2O was prepared via the reaction of Ln2O3 (Ln = Gd, Tb, Er,
Beijing Lanthanide Innovation Technology Co., Ltd, solid, 99.9%) and
hydrochloric acid in aqueous solution, then the liquid was evaporated by

water bath heating at a constant temperature of 353.15 K. Other
analytically pure chemicals were purchased and used without further
purification.
Experimental equipment and conditions
Elemental analysis of the complexes was carried out using the Vario-EL
II element analyzer and the Ln content was assayed by EDTA
chelatometric titration method. The IR spectra were recorded in the range
of 4000–400 cm-1 on a Bruker TENSOR 27 spectrometer using the KBr
medium pellets at room temperature. The single crystal X-ray diffraction
data of complexes 1–3 were obtained on a Smart-1000 diffractometer
with graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å) at 298(2)
K. The structure was resolved by direct methods using the SHELXS-97
program and refined with full-matrix least-squares on F2 using the
SHELXL-97 program.
The thermogravimetric (TG), differential thermogravimetric (DTG) and
Fourier transform infrared (FTIR) analysis of the complexes 1–3 were
conducted at a heating rate of 10 K min−1 (simulated air atmosphere) on a
TG/DSC-FTIR system, which was a NETZSCH STA 449 F3 instrument
with a Bruker TENSOR27 Fourier transform infrared spectrometer. The
NETZSCH STA 449 F3 instrument was linked to the heated gas cell of
the FTIR instrument by means of a heated transfer line kept at 473.15 K.
In addition, the fluorescent spectra of complex 2 was measured on an

F-4600 Hitachi spectrophotometer at room temperature and the solid
fluorescence quantum yields of the complex 2 was measured using
C9920-02G Hamamatsu test system, which was constituted by integrating
sphere of 10-inch diameter and connected to a CCD detector.
Preparation of complexes
A mixture of 2-Br-5-MOHBA (0.6 mmol) and terpy (0.2 mmol) was
dissolved in 6mL 95% ethanol solution and adjust the pH of the mixture
solution to 6-7 with 1 mol L-1 NaOH solution. Then add dropwise the
mixture to the aqueous solution of LnCl3·6H2O (0.2 mmol) under stirring
and continually stir about 7 h. The powders of title complexes were
obtained after depositing overnight, filtering and drying. The single
crystals were collected from the mother liquor after two weeks at room
temperature. Elemental analyses for C39H31Br3N3O10Gd (%), Calcd: C,
42.64; H, 2.84; N, 3.82; Gd, 14.31. Found: C, 42.55; H, 2.81; N, 3.79; Gd,
14.23 (yield: 58.86%). For C39H31Br3N3O10Tb (%), Calcd: C, 42.57; H,
2.84; N, 3.82; Tb, 14.44. Found: C, 42.46; H, 2.82; N, 3.78; Tb, 14.37
(yield: 56.44%). For C39H31Br3N3O10Er (%), Calcd: C, 42.25; H, 2.82; N,
3.79; Er, 15.09. Found: C, 42.20; H, 2.82; N, 3.73; Er, 15.01 (yield:
59.83%).
Results and discussion
Infrared spectroscopy
As we all know, the IR spectra can provide valuable information

regarding the functional group attached to the center metal ions. The
result of infrared spectra for title complexes and ligands indicates that
complexes 1–3 are isostructural, which is verified by single crystal X-ray
diffraction analysis. The main characteristic absorption bands of the
ligands and their complexes 1–3 are presented in Table 1. The IR
spectrum of the free organic carboxylic ligand 2-Br-5-MOHBA has a
strong absorption at 1703 cm-1, which was assigned to the stretch
vibration of the carboxylate group (νC=O). The band disappear in the IR
spectrum of all complexes and appear two new absorbing bands at
1621–1623 cm-1 and 1419–1420 cm-1 , belonging to the asymmetric (νas
(COO–)) and symmetric (νs(COO-)) stretching vibrations, respectively. At the
same time, the band at 417-421 cm-1 in the complexes correspond to the
vibrations of Ln-O. These changes show that oxygen atom of the
carboxylate groups coordinate to the Ln(Ⅲ) ion[19]. What's more, the
peaks of νC=N for terpy are observed at 1559 cm-1 take place red shift to
1522-1523 cm-1, which suggests that three nitrogen atoms of terpy
participate in the coordination to the Ln( III ) ions [20].
Table 1 The main IR absorption bands of the ligands and title complexes.
-
-
ligands/complexes
νC=N
1559
_
δC–H
762
νC=O νas(COO )
νs(COO )
νLn-O
_
Terpy
661
_
_
1703
_
_
_
_
2-Br-5-MOHBA
_
_
_
1522
1522
779
779
653
651
1621
1622
1419
1420
417
420
1
2
_

1523
779
652
_
1622
1419
419
3
Description of Crystal Structures
The single-crystal X-ray structures of three complexes were determined.
The most important crystallographic data and data collection parameters
are shown in Table S1, indicating the three complexes is isostructural and
crystallizes in the triclinic with space group Pī. Therefore, as a
representative example, the crystal structure of the complex 1 will be
discussed in detail.
The asymmetric unit of 1 consists of one Gd (III) ion, three
2-Br-5-MOBA ligands, one terpy ligand and one coordination water
molecule. As shown in Figure 1a, Gd1 ion is eight-coordinated by four
carboxylate oxygenatoms (O1, O4, O7, and O8) from three
2-Br-5-MOBA ligands, one oxygen atoms (O10) from the coordination
water molecule, and three nitrogen atoms from the terpy ligand. Three
2-Br-5-MOBA ligands have two different coordination modes: chelating
monodentate and chelating bidentate. The coordination environment
around the Gd (Ⅲ) ion can be described as a slightly distorted square
antiprismatic molecular geometry structure (Figure 1b). The Gd-O bond
distances range from 2.269(5) to 2.586(5) Å for 2 and the average Gd-N
bond distance is 2.546 Å (Table 2), which are comparable to those
reported complexes. The average lengths of Gd-O and Gd-N are 2.388 Å
and 2.513 Å in {[Gd2(HL)3(H2O)3]·3H2O}n, respectively [11]. It can be

seen that the bond lengths of the Ln-O and Ln-N slightly decrease as the
increase of atomic numbers, which is consistent with lanthanide
contraction rule. In addition, the average bond length for Ln-N is longer
than that of Ln-O. The result is consistent with the first loss of 2,2'-bipy
molecule in the first thermal decomposition process.
Two adjacent units of complex 1 are connected through hydrogen
bonding interactions involving oxygen atoms (O10) from coordinated
water molecules and oxygen atoms (O2 and O5) from carboxylate groups
(d (O10···O2) = 2.874 Å; d (O10···O5) = 2.774 Å) giving rise to a
pseudo binuclear structure, as shown in Figure 2a. The O-H···Br
hydrogen bonds further link these pseudo binuclear units into a 1D
supramolecular chain structure along the x direction (d (O5···Br) = 3.120)
(Figure 2b). At the same time, the slightly offset π–π stacking interactions
between terpy ligands on neighboring units induce the 1D chains into a
2D supramolecular framework viewed along the xy plane (Figure 2c).
These non-covalent interactions are between the centroid of the terpy ring
on one unit and the edge of the terpy ring on the neighboring unit with the
relevant
Cg⊥···Cg⊥···3.5899(2) Å; β = 68.227(399)°.
At 2015, Guang-Cai Zong et al. have reported the structure of
[Er2(2-Br-5-MOBA)6(phen)2] (2-Br-5-MOBA
2-bromine-5-methoxybenzoate, phen = 1,10-phenanthroline) [21]. It is
distances
and
angles:
Cg···Cg···3.3344(88)
Å;
=

binuclear with a coordination number of nine and crystallizes in the
triclinic space group Pī. The carboxylic acid ligands adopt three different
coordination modes: bridging, bidentate chelating, and tridentate
chelating bridging. The difference of auxiliary ligands may affect the
coordination mode of the carboxylic acid ligands so that generate
different molecular structure. The average bond length of the Er-N is
2.533(19) Å for [Er2 (2-Br-5-MOBA)6(phen)2], while the length is
2.497Å for complex 3. The shorter bond length indicates that the terpy
ligand has a stronger affinity to the Ln3+ ions than the phen ligand. At
2016,
Ye
Wang
et
al.
reported
the
complex
[Tb(2,4-DClBA)3(terpy)(H2O)]·H2O[22]. The complex has the same
crystal system and mononuclear structure with complex 2, but the
coordination number is nine. The mononuclear units are stitched together
via O-H···O, C-H···O hydrogen bonding and Cl-π interactions to form
supramolecular 1D chain, 2D sheet and 3D network structures, which is
different from the pseudo binuclear structure of 2. The binuclear
structures
of
[Ln(2,3-DClBA)3(terpy)(H2O)]2
(2,3-DClBA
=
2,3-dichlorobenzoate) were synthesized by Jun-Chen Wu et al.in 2015
[23] are completely different from the mononuclear structures in this
paper. Thus, the acid ligands also play an important role in the formation
of different molecular structures.

Table 2 The selected bond lengths (Å) for complexes 1–3.
Complex 1
Complex 2
Complex 3
Bond length(Å)
2.269(5)
2.381(6)
2.406(5)
2.421(5)
2.532(6)
2.532(7)
2.573(6)
2.586(5)
Bond length(Å)
2.250(5)
2.350(6)
2.392(5)
2.409(5)
2.506(7)
2.523(7)
2.537(6)
2.564(5)
Bond length(Å)
2.214(5)
2.300(5)
2.345(5)
2.375(5)
2.482(6)
2.490(7)
2.504(6)
2.525(5)
Gd(1)-O(4)
Gd(1)-O(1)
Gd(1)-O(10)
Gd(1)-O(7)
Gd(1)-N(1)
Gd(1)-N(3)
Gd(1)-N(2)
Gd(1)-O(8)
Tb(1)-O(1)
Tb(1)-O(7)
Tb(1)-O(10)
Tb(1)-O(5)
Tb(1)-N(1)
Tb(1)-N(3)
Tb(1)-N(2)
Tb(1)-O(4)
Er(1)-O(1)
Er(1)-O(7)
Er(1)-O(10)
Er(1)-O(5)
Er(1)-N(1)
Er(1)-N(3)
Er(1)-N(2)
Er(1)-O(4)
(a)
(b)
Figure 1(a) Crystal structure of complex 1, all H atoms omitted for clarity.
(b) Coordination geometry of Gd3+ ion.

(a)
(b)
(c)
Figure 2 (a) The pseudo binuclear structure of complex 1. (b) 1D chain
structure along x-axis. (c) 2D sheet in about the xy plane via slightly
offset π–π stacking interactions.

Thermal analysis
The thermal stability of complexes 1–3 was examined by
thermogravimetric analysis (TGA) in the temperature range of 300–1050
K in a simulated air atmosphere at a heating rate of 10 K min-1. The
profiles of the TGA, DTG and DSC of the complex 2 are shown in Figure
3 and the date of thermal analysis for the title complexes are listed in
Table S2. Complexes1–3 are isomorphous and exhibit similar TGA
curves, thus only the thermal decomposition process of 2 will be
discussed in detail. At least five main degradation steps can be observed
in the TGA curve of 2 (Figure 3). The first stage of weight loss about
1.65% from 355.15 to 404.15K could be attributed to the loss of one
coordinated water molecules (Calcd: 1.64%), and there is a small
endothermic peak in DSC curve (Tp = 388.85 K; ΔH = 62.4 kJ mol-1).
The second stage of weight loss 37.12% between 404.15 and 689.15 K
corresponds to the release of one terpy ligand and part 2-Br-5-MOBA
ligand (Calcd: 21.20%), which is an exothermic process observed from
the DSC curve (Tp = 600.65 K; ΔH = -101.9kJ mol-1). Then the weight
losses of 2 are divided into three steps at 689.15-1019.15 K,
corresponding to the loss of 2-Br-5-MOBA. There are three exothermic
peaks on the DSC curve (Tp = 775.15 K, ΔH = -260.1 kJ mol-1; Tp =
885.75 K, ΔH = -2057 kJ mol-1; Tp = 922.25 K, ΔH = -259.7 kJ mol-1).
The remaining weight of 17.39% corresponds to the metallic oxide Tb4O7,

which is agreement well with the calculated value of 16.99%. It can be
seen from the data that the thermal stability of the three complexes is not
very good, which may be the result of existing coordination water
molecule.
Figure 3 TG-DTG/DSC curves of the complex 2.
Evolved gas analysis during thermal decomposition
Figure 4 displays the TG-FTIR spectra of the evolved gas products of
thermal decomposition processes for complex 2, which are recorded
under simulated air atmosphere with a heating rate of 10 K/min by
TG/DSC-FTIR system. Complexes 1–3 have similar thermal
decomposition process, thus only the complex 2 will be discussed in
detail. And the IR spectra of the evolved gases for 2 at different
temperature are shown in Figure S3.
The spectrum at T = 391.71 K shown in Figure 5 corresponds to the IR
spectrum of the evolved gas in the first thermal decomposition process.
The absorption band at 1508 cm-1 is attributed to the bending mode in

gaseous H2O and the bands at 3735 and 3649 cm-1 are attributed to the
asymmetric and symmetric stretches of H2O. The mainly evolved gases in
the second stage (T = 601.77 K) can be identified by these characteristic
absorption bands: the CO2 (2300-2385, 667 cm-1), the H2O (3562-3941
cm-1), the terpy (the νC=N (1578 cm-1), the νC-N (1236, 1293 cm-1), in-plane
γ=C-H (771 cm-1) and out-of-plane γ=C-H (1055, 1113 cm-1)) [24], and the
2-Br-5-MOBA (the νC-H, νasOCH3 and νs-OCH3 (2812-3106 cm-1), the νC=C
(1179-1341 cm-1) and the νC=O (1749 cm-1)). Thus, we can draw
conclusions that terpy had been dissociated in the first stage and the
gaseous products also contain dissociated and not dissociated
2-Br-5-MOBA ligands [25]. As shown in Figure 5, there is the similar
absorption at temperature of 773.62, 881.94 and 941.53 K except that the
difference of intensity of absorption band of CO2. The mainly evolved
gases are identified by the characteristic absorption: the CO2 (2252-2385,
667 cm-1), the CO (2109, 2186cm-1) and the H2O (3552-3799 cm-1).
Figure 4 Stacked plot of the FTIR spectra of evolved gases for complex
2.

Luminescent Properties
The fluorescent property of the complex 2 was investigated. The
solid-state luminescent spectra of Tb (Ⅲ) complex at room temperature
were shown in the Figure 5. The excitation spectrum was obtained by
monitoring the emission of the Tb (III) complex at 542 nm, and the
excitation maxima was observed at 349 nm. Four expected peaks are
5
observed in the emission spectrum. They are assigned to D4→7F6 (488
5
5
5
nm), D4→7F5 (542 nm), D4→7F4(584 nm), and D4→7F3 (621 nm) [26].
5
The electric dipole transition D4→7F5 at 542 nm dominates the whole
emission spectrum, which is responsible for the green emission from Tb
(III) complex. There is a broad band between 234 and 398 nm in the
excitation spectrum of Tb (Ш) complex, which corresponds to the π→π*
electronic transition of the organic ligand. What's more, The quantum
yield of the Tb (III) complex is 78.3%. We can draw a conclusion that the
2-Br-5-MOHBA ligand can effective sensitize the luminescence of rare
earth ions (“antenna effect”), according to the emission spectra. In
addition, the Tb (Ш) complex appears to be promising candidates for the
application as green luminescent material.

Figure 5 Solid-state emission (a) and excitation (b) spectra of complex 2.
Conclusions
In summary, three new mononuclear complexes have been successfully
synthesized and characterized. The complexes 1–3 are isostructural and
eight-coordinated, which is different from the binuclear structure of the
previously reported complexes. The pseudo binuclear units constituted by
the mononuclear molecular are stitched together via slightly offset π–π
stacking and O-H···Br hydrogen bonding interactions to form the 1D, 2D
supramolecular structures. The thermal decomposition mechanism for
title complexes was obtained by TG and TG/DSC-FTIR technology. The
thermal stability of the three complexes is not very good due to the
presence of coordination water molecules. The fluorescent spectra
indicate that Tb (Ш) complex appears to be promising candidate for the
application as green luminescent material.
Supporting Information
The numbers of the three complexes (CCDC 1542527 (1), CCDC

1542529 (2), CCDC 1542530 (3)) contain the supplementary
crystallographic data for this paper, which can be obtained free of charge
from
the
Cambridge
Crystallographic
Data
Centre
via
www.ccdc.cam.ac.uk/data_request/cif.
Acknowledgements
The research work was supported by the National Natura Science
Foundation of China (no.21473049) and the Natural Science Foundation
of Hebei Province (no. B2016205207).
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Graphical Abstract
Complexes 1-3 [Ln(2-Br-5-MOBA)3(terpy)(H2O)] (Ln = Gd (1), Tb (2), Er (3)) are
isostructural. The molecular structure of complexes 1-3 is very interesting: two
adjacent mononuclear units are connected through hydrogen bonding giving rise to a
fake binuclear structure. These fake binuclear units are further linked via O-H···Br
hydrogen bonds and slightly offset π–π stacking interactions to from 1D and 2D
supramolecular structures. The solid-state luminescent spectra indicate that Tb (Ш)
complex appear to be promising candidate for the application as green luminescent
material.
Graphical Abstract