Synthesis, structure and luminescence of lanthanide coordination polymers based on the 1,3-Bis(carboxymethyl) imidazolium salt

Article abstract of DOI:10.1016/j.jssc.2019.120900

The 1,3-bis(carboxymethyl)imidazolium salt (zwitterion) (HBCI) was reacted, under solvothermal conditions, with rare earth chlorides to provide a series of new two-dimensional (2D) coordination polymers, namely [LnBCI) (HCOO) (H₂O)₂Cl]_n [Ln = Sm (1); Eu (2); Gd (3); Tb (4); Dy (5)]. The complexes were characterized by X-ray single crystal diffraction, powder X-ray diffraction, infrared spectroscopy, elemental analysis and thermogravimetric analysis. Compounds of 1, 2, 4 and 5 exhibit the characteristic luminescence of the corresponding lanthanide ions, due to the 4fⁿ-4fⁿ transition. Furthermore, the fluorescence lifetime of 2 and the phosphorescence spectrum of 5 at 77k are discussed.

Full text of DOI:10.1016/j.jssc.2019.120900

Journal Pre-proof
Synthesis, structure and luminescence of lanthanide coordination polymers based on
the 1,3-Bis(carboxymethyl) imidazolium salt
Li-Xin You, Yang Guo, Shi-Yu Xie, Shu-Ju Wang, Gang Xiong, Ileana Dragutan,
Valerian Dragutan, Fu Ding, Ya-Guang Sun
PII:
S0022-4596(19)30395-0
DOI:
https://doi.org/10.1016/j.jssc.2019.120900
YJSSC 120900
Reference:
To appear in: Journal of Solid State Chemistry
Received Date: 26 June 2019
Revised Date: 31 July 2019
Accepted Date: 3 August 2019
Please cite this article as: L.-X. You, Y. Guo, S.-Y. Xie, S.-J. Wang, G. Xiong, I. Dragutan, V. Dragutan,
F. Ding, Y.-G. Sun, Synthesis, structure and luminescence of lanthanide coordination polymers based
on the 1,3-Bis(carboxymethyl) imidazolium salt, Journal of Solid State Chemistry (2019), doi: https://
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Synthesis, Structure and Luminescence of Lanthanide Coordination
Polymers Based on the 1,3-Bis(carboxymethyl) Imidazolium Salt
a
a
a
a
a
*b
Li-Xin You, Yang Guo, Shi-Yu Xie, Shu-Ju Wang, Gang Xiong, Ileana Dragutan, Valerian
b
a
*a
Dragutan, Fu Ding and Ya-Guang Sun
a
Key Laboratory of Inorganic Molecule-Based Chemistry of Liaoning Province, Shenyang University
of Chemical Technology, Shenyang, 110142, China.
b
Institute of Organic Chemistry, Romanian Academy, Bucharest, 060023, Romania.
*
Corresponding authors.
E-mail addresses: sunyaguang@syuct.edu.cn (Y.-G. Sun); idragutan@yahoo.com (I. Dragutan)
ABSTRACT
The 1,3-bis(carboxymethyl)imidazolium salt (zwitterion) (HBCI) was reacted, under solvothermal
conditions, with rare earth chlorides to provide a series of new two-dimensional (2D) coordination
2 2 n
polymers, namely [LnBCI)(HCOO)(H O) Cl] [Ln = Sm (1); Eu (2); Gd (3); Tb (4); Dy (5)]. The
complexes were characterized by X-ray single crystal diffraction, powder X-ray diffraction, infrared
spectroscopy, elemental analysis and thermogravimetric analysis. Compounds of 1, 2, 4 and 5 exhibit
n
n
the characteristic luminescence of the corresponding lanthanide ions, due to the 4f -4f transition.
Furthermore, the fluorescence lifetime of 2 and the phosphorescence spectrum of 5 at 77k are
discussed.
Keywords: lanthanide coordination polymers; 1,3-bis(carboxymethyl) imidazolium salt; crystal
structure; luminescence.
1
. Introduction
Chemical researchers have a strong interest in the study of coordination polymers (CPs) mainly
1
-7
due to the diversity of their structures and wide applications in various fields, such as luminescence,
8-10
11-15
16-18
19,20
chemical sensing,
catalysis,
magnetism,
gas storage and separation,
and drug
21,22
delivery.
topic in recent years.
the structural characteristics of the ligand, the coordination mode of the lanthanide, the ratio ligand to
Therefore, the synthesis and application of functionalized CPs has become a hot research
23-25
However, the synthesis process of CPs is affected by many factors, such as
2
6-31
the metal, the reaction temperature, the nature of the solvent, and the pH of the solution.
Most
n
trivalent lanthanide ions have excellent luminescent properties produced by the 4f electronic
configuration and electronic transitions from the visible to near infrared region of the spectrum. So far,
32-36
many CPs of lanthanides have been extensively studied and applied.
However, due to the high and
variable coordination number of the lanthanide ions and their flexible coordination environment,
synthesizing CPs of lanthanides with predictable and controlled geometries still remains
3
7-41
challenging.
Therefore, the rational selection of organic ligands is particularly important because it
31-33
has strong effect on the arrangement of the molecular packing in the formed CPs.

As a polycarboxylate, the 1,3-bis(carboxymethyl) imidazolium salt (zwitterion) (HBCI) endowed
with two carboxylic groups, may provide, under suitable synthetic conditions, various coordination
modes to construct the CPs. In view of the above, we selected the HBCI ligand to react with different
lanthanide ions and yield under hydrothermal conditions a series of new 2D CPs, i. e.
2 2 n
{[Ln(BCI)(HCOO)(H O) Cl] ; [Ln = Sm (1); Eu (2); Gd (3); Tb (4); Dy (5)]}. The structures were
characterized by single crystal X-ray diffraction analysis, elemental analysis, infrared (IR)
spectroscopy and thermogravimetric analysis (TGA). The luminescent properties of 1, 2, 3, 4 and 5
were studied in detail.
2
. Experimental section
2
.1 Materials
All the chemicals used were of reagent grade and were used without further purification. All
syntheses were carried out in 25 mL teflon-lined autoclaves under autogenous pressure. The reaction
vessels were filled to approximately 50% volume capacity. The water used in the synthesis was
distilled before use.
2
.2 Physical measurements
Elemental analyses (C, H, and N) were carried out with a Perkin-Elmer 240C elemental analyzer.
FT-IR spectra were recorded on a Nicolet IR-470 spectrometer using KBr pellets. Powder X-ray
diffraction (PXRD) patterns of the samples were recorded using an X-ray diffractometer (BRUKER
D8 ADVANCE) with Cu Kα radiation. Thermogravimetric analysis (TGA) experiments were
−
1
performed on a NETZSCH TG 209 instrument applying a heating rate of 10 °C min . Luminescence
spectra were recorded on a FS920 steady state fluorescence spectrometer (Edinburgh Instruments)
equipped with a Hamamatsu R928P red-sensitive photomultiplier for the visible range and a
germanium detector for the near infrared range. Luminescence decay measurements were taken with a
pulsed nitrogen laser (excitation wavelength of 337 nm, pulse length of 800 ps, repetition rate of 1 Hz)
and a 1024-channel Intensified CCD (Andor Technology) attached to a 0.5 m monochromator. At 298
K, the luminescence decay curve was obtained using a FLSP920 fluorescence spectrophotometer, and
an NF900 lamp was used as an excitation source. For low temperature (77K) measurements, samples
were cooled in a liquid nitrogen cryostat (Oxford Instruments), and quantum efficiency (QE) was
measured by an integrating sphere at low temperatures by a FLSP920 photoluminescence quantum
yield measurement system. Then, the QE was calculated according to the method of the software
provided by the manufacturer. The phosphorescence spectrum was measured by FLSP 920 at a low
temperature of 77K.
2
.3 Synthesis of the 1,3-bis(carboxymethyl)imidazolium ligand (HBCI)
The 1,3-bis(carboxymethyl)imidazolium ligand (HBCI) was prepared according to the reported
4
2,43
method.
Glycine (3.75 g, 50 mmol), 40% aqueous glyoxal (2.8 mL, 25 mmol), formaldehyde
solution (40%; 2.5 mL, 25 mmol) and 20 mL water were placed in a 100 mL round bottom flask and
heated at 95 °C for 2h. The resulting solution was left to dry at room temperature to yield colourless
crystals. The impurities were removed by washing with water and methanol and dried in the air to
obtain colourless, transparent, bulk crystals; yield 77.5%. The crystal data are shown in Table S1.
2
.4 Synthesis of complexes 1-5

Compounds 1-5 are synthesized in a similar way, and thus the synthesis method of 1 is taken as a
representative example. SmCl ·6H O (0.1 mmol, 36.48 mg)，HBCI (0.1 mmol, 18.62 mg), DMF (3.0
3
2
mL), ethyl alcohol (7.0 mL) and H
2
O (2.0 mL) were placed in a 25 mL teflon-lined stainless steel
o
vessel, stirred and heated to 110 C for 72 h. The reactor was then cooled to room temperature. The
impurities were removed by washing with DMF and dried in the air to obtain colorless, transparent,
bulk crystals. Anal. Calcd. (%) for 1: C 21.31; H 2.66; N 6.21. Found: C 21.30; H 2.64; N 6.18. Calcd.
for 2: C 21.26; H 2.66; N 6.20. Found: C 21.22; H 2.64; N 6.18. Calcd. for 3: C 21.0; H 2.63; N 6.13.
Found: C 20.98; H 2.62; N 6.09. Calcd. for 4: C 21.06; H 2.63; N 6.14. Found: C 21.05; H 2.61; N
6
.11. Calcd. for 5: C 20.88; H 2.6; N 6.09. Found: C 20.86; H 2.58; N 6.06.
.5 Single crystal X-ray crystallographic study
2
The collection of single crystal crystallographic data was carried out on a Bruker SMART Apex
CCD diffractometer using graphite-monochromated Mo Kα radiation (0.71073 Å) at 293 K in the
ω-2θ scan mode. An empirical absorption correction was applied to the data using the SADABS
44
program. The structures were solved by direct methods and refined by full matrix least-squares
2
45,46
methods on F using the SHELXTL crystallographic software package.
All non-H atoms were
refined anisotropically. The H atoms were placed in calculated positions and refined using a riding
mode. Crystallographic data and structural refinement parameters are shown in Table 1.
Table 1. Crystal data and structure refinement of compounds1-5
Complex
1
2
3
4
5
Formula
Fw
C H SmN O Cl C H EuN O Cl C H GdN O Cl C H TbN O Cl C H DyN O Cl
8 12 2 8 8 12 2 8 8 12 2 8 8 12 2 8 8 12 2 8
450.56
298.15
451.61
298.15
456.90
298.15
455.89
298.15
459.72
298.15
Temp. (K)
Crystal system
Orthorhombic
Orthorhombic
Orthorhombic
Orthorhombic
Orthorhombic
Space group
a (Å)
Cmca
Cmca
Cmca
Cmca
Cmca
17.198(9)
9.123(5)
16.628(8)
2609.0(2)
8
17.204(18)
9.090(9)
16.599(17)
2596.0(5)
8
9.062(9)
17.183(17)
16.490(16)
2568.0(4)
8
17.153(13)
9.055(7)
16.486(13)
2561.0(3)
8
17.100(6)
9.0295(15)
16.403(4)
2532.7(11)
8
b (Å)
c (Å)
3
V (Å)
Z
-3
ρ (g cm )
2.203
2.287
2.364
2.379
2.128
c
-1
µ/mm
4.549
4.906
5.414
5.118
5.917
F(000)
1680.0
11896/1538
1737.0
1752.0
1956.2
1552.0
Rec/unique
12290/1551
12154/1511
12240/1523
11539/1500
2
GOF on F
1.105
1.112
1.087
1.006
1.089
a
R1 [I>2σ(I)]
0.0253
0.0255
0.0261
0.0604
0.0311
b
wR2 [I>2σ(I)]
0.0644
0.0658
0.0635
0.1720
0.0773
a
b
2
o
2 2
2 2 1/2
R =Σ||F | − |F ||/Σ|F |. wR = {Σ[w(F −F ) ]/Σ[w(F ) ]}
1
o
c
o
2
c
o
3
. Results and discussion
3
.1 Structure description

X-ray crystal structure analysis showed that 1-5 are isomorphous, and 1 was taken as an example
to describe the structural details. Compound 1 belongs to the orthorhombic system, Cmca space group.
3
+
-
As shown in Fig. 1, around the Sm ion in 1 there are four BCI ligands, two formate ions and two
3+
coordinating water molecules. Each Sm ion is coordinated by eight oxygen atoms, four from four
-
different BCI ligands (O3, O3D, O4A and O4B), two from coordinating water molecules (O5 and
O5D) and two from the formate ions (O1 and O2C). The formate ions derive from decomposition of
47
the DMF solvent at the high temperature of the hydrothermal synthesis. Therefore, centering on the
3+
Sm ion, the coordination environment can be considered as the geometry of a twisted double-capped
triangular prism. The length of the Sm-O bonds is 2.388(5)-2.561(7) Å, the average bond length is
2
.452(1) Å, and the O-Sm-O bond angle range is 69.43(2)°-144.5 (2)°.
Fig. 1. The coordination environment of the Sm(III) ion in 1 (hydrogen atoms and free ions are
omitted for clarity). Symmetry codes: (A) 1-x, 1-y, 1-z; (B) 1-x, y, z; (C) 1-x, 1.5-y, 0.5+z, (D) x, 1-y,
1
-z.
(
a)
b)
(c)
(d)
(
Fig. 2. (a) Schematic illustration of the 1D chain structure in 1; (b) 2D network of 1; (c) Sm dimers in
are represented by purple balls; (d) The topology of 1.
1
-
3+
Each BCI ligand links to four Sm ions in the coordination mode of [(κ -κ -µ )-(κ -κ -µ )-µ ].
1
1
2
1
1
2
4
3+
Two adjacent Sm ions are connected by the carboxyl group to form a binuclear structure. The
3+
-
distance between two Sm ions is 4.6124(2) Å. The Sm dimers were linked by the BCI ligands to
form one-dimensional (1D) chain, as shown in Fig. 2(a). The 1D chains are connected by formate ions

to form a two-dimensional (2D) network, as shown in Fig. 2(b). In addition, the Sm dimers are
-
considered as nodes (purple balls), while the BCI ligands (blue lines) and formate ions (yellow lines)
are considered as connectors, as shown in Fig. 2(c). The TOPOS 4.0 software was used to describe the
6
6
3
topology of 1 as a 2D network with the Schläfli symbol {3 .4 .5 }, as shown in Fig. 2(d).
3
.2 Infrared Spectroscopy
Infrared spectra were collected in order to detect and characterize the functional groups of HBCI,
NaBCl and compounds 1-5 (Fig. S1 in ESI). Since the results for 1-5 are similar, the infrared
absorption spectra characteristics of these compounds are illustrated taking compound 1 as an example.
-1
The broadband centered at ν = 3432 cm can be assigned to the -OH stretching vibration, indicating
the presence of water molecules in the structure of 1. As expected, the (C=O) and (C-OH) absorption
-1
bands of HBCI, at 1690 and 1253 cm , are not found in the spectrum of 1; the absence of the (C-OH)
absorption indicates that the ligand is completely deprotonated when it is coordinated to the lanthanide
-
1
-1
-
ions. The absorption bands at 1630 cm and 1312 cm are due to the tensile modes of ν (COO )
asym
-
-1
and νsym(COO ) with the difference of ∆ν = 318 cm . The ∆ν value of 1 was greater than that of the
-1
ionic NaBCI (∆ν = 240 cm ). This indicates that the compound 1 adopts the monodentate
coordination mode. All results are consistent with the coordination mode of the carboxyl group given
by the single crystal test.
3
.3 Powder X-ray diffraction (PXRD) and thermogravimetric analysis (TGA)
Powder X-ray diffraction (PXRD) analyses of 1-5 have been performed at room temperature (Fig.
S2, ESI). The patterns for 1-5 are in good agreement with the calculated ones, obtained from the
single-crystal structures, thus confirming that purities equal those of the single crystal samples.
2
Compounds 1-5 were subjected to thermogravimetric analysis (TGA) in a N atmosphere, at a
o
-1
heating rate of 10 C min , aiming to study their structural integrity and stability (Fig. S3, ESI).
Compounds 1-5 exhibited a similar thermal behaviour, and thus 1 was used as example. The TGA
curve indicates that the process of mass loss of 1 proceeds in two stages. The first weight loss, from
o
1
50 to 220 C, was about 6.5% which was assigned to the loss of two coordinating water molecules
(
calculated value: 7.9%). The second weight loss began to collapse at a heating temperature greater
o
than 310 C, quickly and significantly losing weight due to the destruction of the organic ligands.
TGA showed that 1-5 have a certain structural stability.
3
.4 Luminescence
The trivalent lanthanides have excellent fluorescent properties, especially the Sm , Eu , Tb and
3+
3+
3+
3+
Dy ions. Photoluminescence of solid samples of the compounds 1, 2, 4 and 5 was tested with a
FL-1000 fluorometer at room temperature. As shown in Fig. 3, compound 1 was excited at 375 nm
and peaked at 565 nm, 600 nm, 647 nm and 704 nm. This is due to the transition of the excited state
4
6
4
6
4
6
4
6
G
5/2 to the low energy levels H5/2, G5/2→ H7/2, G5/2→ H9/2 and G5/2→ H11/2. The emission intensity
4
6
at 600 nm and 647 nm is slightly stronger. The emission spectrum is dominated by a G → H
9/2
5/2
transition with galvanic polarity. When excited at 405 nm compound 2 exhibited the characteristic
3
+
5
7
emission peaks of the three Eu ions at 591 nm, 612 nm and 703 nm, assigned to D → F (J=1, 2, 4).
0
J
5
7
5
7
D
0
→ F
1 0 2
is a magnetic dipole transition, and D → F is a dipole transition, emitting red light.
3+
Compound 4, excited by 365 nm, exhibits typical Tb sharp emission peaks at 488 nm, 544 nm, 587
5
7
5
7
4 J 4 5
nm, and 623 nm, assigned to D → F (J = 6, 5, 4, and 3). The strongest is the D → F transition,

which emits green light. At 331 nm excitation, compound 5 displayed the typical emission bands of
3
+
4
Dy at 481 nm, 574 nm, 664 nm, and 750 nm, due to the transition of the excited state F to the low
9/2
6
6
6
6
energy levels H15/2, H13/2, H11/2 and H9/2
.
Fig. 3. Emission spectra of compounds 1, 2, 4 and 5.
In addition to the steady-state emission, we also investigated the fluorescence lifetime and
5
3+
quantum yield of compound 2. The decay curve of the D Eu complex was studied by monitoring
0
3+
the emission at 368 nm and the result is shown in Fig 4. It was found that the decay curve of Eu can
be fitted to a double index. The emission decay lifetime was τ =210 µs (11%), τ =540 µs (89%).
1
2
Barium sulfate was chosen as reference, and the quantum efficiency of 2 was measured to be 6.47%.
Fig. 4. Fit attenuation curve for solid composite 2. The sample was excited at 365 nm. Fitted by Fit =
A + B1 × exp( - t /τ ) + B2 × exp( - t /τ ).
1
2

3
+
Usually, the emission phosphorescence spectrum for the Gd complexes allows the identification
of the lowest triplet state of the ligand. Obviously, the singlet energy level (S ) of the HBCI ligand is
1
-
1
-1
3
2258 cm (310 nm) (Fig. S4, ESI). The lowest triplet excited state (T
1
3+
) of HBCI was 22727 cm
4
-1
5
(
440 nm)(Fig. 5), which was significantly higher than G level of Sm (17900 cm ), D level of
5/2
0
3
+
-1
5
3+
-1
4
3+
-1
Eu (17300 cm ), D
phenomenon indicates that HBCI can act as an antenna for the photosensitization of trivalent
lanthanides, transferring energy through T state to the excited states of lanthanide ions. The energy
gap ∆E(S -T ) for HBCI is 9531 cm . It shows that the intersystem crossing is effective in HBCI.
4
level of Tb (20500 cm ) and F9/2 level of Dy (21000 cm ). This
1
-1
1
1
Fig 5. Phosphorescence spectra of compound 5 in the solid-state, at low-temperature (77 K). The
excitation spectra monitored the emission at 440 nm while the emission spectra were recorded upon
excitation at 310 nm.
4
. Conclusions
Five new two-dimensional lanthanide coordination polymers based on the 1,3-bis(carboxymethyl)
imidazolium salt (zwitterion) were prepared under solvothermal conditions, fully characterized, and
their properties studied with emphasis on their luminescence. Compounds 2 and 4 show respectively
n
n
3+
3+
the typical bright red and green luminescence assigned to the 4f -4f transition in Eu and Tb . The
fluorescence lifetime of 2 is shortened by a non-radiative decay due to the presence of coordinated
-1
water molecules. The singlet excited state (32258 cm ) and the lowest triplet state energy level (22727
-1
3+
cm ) of HBCI were calculated based on this ligand and the phosphorescence spectrum of its Gd
complex (3) at 77 K, showing an substantial energy transfer from the HBCI ligand to the lanthanide
ions. Our coordination polymers integrating lanthanides may find applications in advanced therapies
implying antibacterial or anticancer agents.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (21671139), the
Distinguished Professor Project of Liaoning province (2013204), the Natural Science Foundation of
Liaoning Province (20170540713), Program for Liaoning Innovative Research Team in University
(
LT2018012) and the Science and Technology Innovation Program for Middle-aged and Youth
Talents of Shenyang (RC180086). ID and VD acknowledge support from the Institute of Organic
Chemistry, Romanian Academy.
Appendix A. Supplementary material

CCDC Nos. 1904893-1904896 and 1915613, contains the supplementary crystallographic data for
this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif. Supplementary data associated with this article can
be found, in the online version, at http://dx.doi.org/10..............
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3

Research highlights
►
►
Five 2D CPs [LnBCI)(HCOO)(H O) Cl] were prepared and characterized
2 2 n
1,3-Bis(carboxymethyl)imidazolium salt (zwitterion)(HBCI) was used as organic linker
►
►
Compounds 1, 2, 4 and 5 show luminescence of the trivalent lanthanides
The energy transfer processes of the Ln CPs were investigated

Synthesis, Structure and Luminescence of Lanthanide Coordination
Polymers Based on the 1,3-Bis(carboxymethyl) Imidazolium Salt
a
a
a
a
a
*b
Li-Xin You, YangGuo, Shi-Yu Xie, Shu-Ju Wang, Gang Xiong, Ileana Dragutan, Valerian
b
a
*a
Dragutan, Fu Ding and Ya-Guang Sun
Five 2D coordination polymers, namely [LnBCI)(HCOO)(H O) Cl] [Ln = Sm (1); Eu (2); Gd
2
2
n
(3); Tb (4); Dy (5)] [HBCI = 1,3-bis(carboxymethyl)imidazolium salt] were prepared and
structurally characterized. Luminescence properties of 1, 2, 4 and 5, fluorescence lifetime of 2 and
the phosphorescence spectrum of 5 are discussed.