Self-assembly and structures of new lanthanide coordination polymers with 1,3-phenylenebis(oxy)diacetic acid

Article abstract of DOI:10.1016/j.poly.2014.01.030

Seven Ln(III) coordination polymers, namely [Ln(PODA)(FA)(H ₂O)]_n [Ln = La(1), Ce(2), Pr(3), Nd(4), Eu(5)] and [Ln(PODA)_1.5(phen)]_n [Ln = La(6), Pr(7)] (H ₂PODA = 1,3-phenylenebis(oxy)diacetic acid, HFA = formic acid, phen = 1,10-phenanthroline) have been synthesized under hydrothermal conditions and characterized by IR spectroscopy, elemental analysis, single crystal X-ray crystallography, UV-Vis spectroscopy and powder X-ray diffraction analysis (PXRD). Structural analysis reveals that 1-5 are isostructural and exhibit 3-D frameworks with a (3².4².5²)₂(3 ⁸.4²⁰.5²⁴.6¹⁴) topology, which comprise single- and quadruple-strand helical chains; while 6 and 7 show similar layer structures with a (4⁴.6²) topology, however, they only consist of one kind of single-strand helical chain. In addition, the thermal stabilities of complexes 3 and 7 and the luminescent properties of complex 5 have been briefly investigated.

Full text of DOI:10.1016/j.poly.2014.01.030

Polyhedron 72 (2014) 83–89
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Self-assembly and structures of new lanthanide coordination polymers
with 1,3-phenylenebis(oxy)diacetic acid
a
b
c,
Gao-Shan Yang a, Lin Li a, Chong-Bo Liu a, , Hong Liu , Yun-Han Wen , Hui-Liang Wen
⇑
⇑
a School of Environment and Chemical Engineering, Nanchang Hangkong University, Nanchang 330063, PR China
b Department of Chemistry and Biochemistry, The Ohio State University-Columbus, Columbus, OH 43210, United States
c State Key Laboratory of Food Science and Technology, Nanchang University, Nanchang 330047, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Seven Ln(III) coordination polymers, namely [Ln(PODA)(FA)(H2O)]n [Ln = La(1), Ce(2), Pr(3), Nd(4), Eu(5)]
and [Ln(PODA)1.5(phen)]n [Ln = La(6), Pr(7)] (H2PODA = 1,3-phenylenebis(oxy)diacetic acid, HFA = formic
acid, phen = 1,10-phenanthroline) have been synthesized under hydrothermal conditions and character-
ized by IR spectroscopy, elemental analysis, single crystal X-ray crystallography, UV–Vis spectroscopy
and powder X-ray diffraction analysis (PXRD). Structural analysis reveals that 1–5 are isostructural
and exhibit 3-D frameworks with a (32.42.52)2(38.420.524.614) topology, which comprise single- and qua-
druple-strand helical chains; while 6 and 7 show similar layer structures with a (44.62) topology, how-
ever, they only consist of one kind of single-strand helical chain. In addition, the thermal stabilities of
complexes 3 and 7 and the luminescent properties of complex 5 have been briefly investigated.
Ó 2014 Published by Elsevier Ltd.
Received 2 December 2013
Accepted 23 January 2014
Available online 6 February 2014
Keywords:
Lanthanide coordination polymers
1,3-Phenylenebis(oxy)diacetic acid
Co-ligand
Crystal structure
Helix chain
1. Introduction
acid) was applied as a ligand in this work because it contains
two carboxylic groups with an angle of 60° that can provide versa-
The construction of novel coordination polymers is the current
interest in the field of supramolecular chemistry and crystal engi-
neering stems from their potential applications as functional mate-
rials as well as their intriguing variety of architectures and
molecular topologies [1–10]. Among them, the construction of
helical coordination polymers has aroused a great deal of attention
owing to the wide appearance of helical structures in, for example,
proteins, collagens, quartz, single-walled carbon nanotubes and
many more natural or artificial fiber-type derivatives [11–13].
Until now, many single-, double-, triple- and higher-order stranded
helical complexes have been generated by a self-assembly process
[14–18]. Previously, our group synthesized a series of pyrazole acid
compounds as ligands to coordinate with transition metal ions, and
obtained a lot of complexes with various fascinating helical struc-
tures [19–22]. The crucial step for the design of helical coordina-
tion polymers is the reasonable choice of ligands. The ligands
should be capable of bridging metal atoms in multi-directions with
appropriate angles and contain steric information that can be
interpreted by the arrangement of the bound metal centers, which
help to result in the formation of helical structures. With this
understanding, H2PODA (H2PODA = 1,3-phenylenebis(oxy)diacetic
tile coordination modes and substituted carboxyl groups may act
as both hydrogen-bond donors and acceptors, which can lead to
remarkable classes of complexes bearing helical architectures
and potential functions. Besides the natural flexibility of the
original ligands, other subtle factors, such as coordination geome-
tries of the metal ions, reaction conditions and ancillary ligands,
are also important for the construction of helical coordination
polymers [23–27]. Lanthanide ions have a larger radii and higher
coordination numbers than transition metals, and they have un-
ique luminescent properties and specific magnetic properties,
which have aroused more and more scientists’ interest [28–31].
In addition, considering the high coordination number of lantha-
nide ions, ancillary ligands can be employed to occupy some coor-
dination sites and prevent the interpenetration of frameworks. HFA
(HFA = formic acid) has a strong coordination ability and can adopt
various bridging modes to link metal ions and/or construct stable
metal-carboxylate secondary building units [32–34], which can
contribute to form helical and high-dimensional structures;
phen (phen = 1,10-phenanthroline) has a rigid planar structure
and its two nitrogen atoms are apt to coordinate to metal ions in
chelating mode, it is also a good
construct stable supramolecular structures via C–HÁ Á ÁO (N) and
C–HÁ Á Á hydrogen bonds and stacks. A series of copper(II),
zinc(II) and cadmium(II) complexes based on H2PODA form
p electron acceptor and helps to
p
p–p
⇑
Corresponding authors.
E-mail address: cbliu@nchu.edu.cn (C.-B. Liu).
http://dx.doi.org/10.1016/j.poly.2014.01.030
0277-5387/Ó 2014 Published by Elsevier Ltd.

84
G.-S. Yang et al. / Polyhedron 72 (2014) 83–89
low-dimensional structures, such as chain structures [35–37].
However, there are few reports about lanthanide coordination
polymers based on H2PODA. Herein, we report the syntheses and
structures of seven lanthanide coordination polymers with
H2PODA and HFA or phen: [Ln(PODA)(FA)(H2O)]n [Ln = La (1), Ce
(2), Pr (3), Nd (4), Eu (5)] and [Ln(PODA)1.5(phen)]n [Ln = La (6),
Pr (7)]. In addition, the photoluminescence properties of complex
5 in the solid state have also been investigated.
2.2.2. Syntheses of complexes 6 and 7
A mixture of 3 mL ethanol and 7 mL distilled water containing
H2PODA (0.0452 g, 0.2 mmol), NaOH (0.4 mmol, 0.65 mol/L),
1,10-phenanthroline (0.0198 g, 0.1 mmol), LaCl3Á6H2O for
6
(0.0353 g, 0.1 mmol) and PrCl3Á6H2O for 7 (0.0355 g, 0.1 mmol),
was sealed in a Teflon-lined stainless reactor (23 mL), heated at
150 °C for 72 h under autogenous pressure and then cooled to
room temperature. Block crystals were obtained.
[La(PODA)1.5(phen)]n (6) Yield: 31.3% based on La. Anal. Calc. for
C
27H19LaN2O9: C 49.57; H 2.93; N 4.28. Found: C 49.53; H 2.97; N
4.19%. IR data (KBr pellet,
m
/cmÀ1): 3131 m, 1616 s, 1493 m, 1402
2. Experimental
m, 1326 m, 1287 m, 1184 m, 1156 m, 1087 w, 1061 w, 953 m, 854
m, 819 w, 777 w, 728 w, 625 w, 599 w.
2.1. Materials and physical measurements
[Pr(PODA)1.5(phen)]n (7) Yield: 25.6% based on Pr. Anal. Calc. for
C
27H19PrN2O9: C 49.42; H 2.92; N 4.27. Found: C 49.38; H 2.94; N
All the materials and reagents were obtained commercially and
used without further purification. The elemental analysis of C, H
and N were carried out with a Flash EA 1112 elemental analyzer.
The IR spectra were recorded with a Nicolet Avatar 360 FT-IR
spectrometer using the KBr pellet technique in the range
4000–400 cmÀ1. UV–Vis spectra were recorded on a Hitachi Model
U-3900 H spectrophotometer. Powder X-ray diffraction (PXRD)
measurements were performed on a Bruker D8 Advance X-ray dif-
4.29%. IR data (KBr pellet,
m, 1616 s, 1494 s, 1423 w, 1328 s, 1287 s, 1183 m, 1157 m, 1089 m,
1061 m, 953 m, 854 m, 775 m, 726 w, 626 w.
m
/cmÀ1): 3445 m, 3070 m, 2916 m, 2856
2.3. X-ray crystallographic study
The single-crystal X-ray data of 1–7 were collected on a Brucker
APEX II area detector diffractometer with graphite-monochromat-
ed Mo Ka radiation (k = 0.71073 Å). Semi-empirical absorption cor-
rections were applied to the title complexes using the SADABS
program [38]. The structures were solved by direct methods [39]
and refined by full-matrix least squares on F2 using SHELXL-97
[40]. All non-hydrogen atoms were refined anisotropically. The
water H atoms were located from difference Fourier maps, the
other hydrogen atoms were placed in geometrically calculated
positions. Experimental details for the X-ray data collection of 1–
7 are presented in Table 1 and selected bond lengths and angles
are listed in Table S1. Hydrogen bonding lengths and angles are
listed in Table S2. In complexes 6 and 7, C24 and O9 are disordered.
fractometer using Cu K
a radiation (k = 1.54056 Å), in which the
X-ray tube was operated at 40 kV and 40 mA. Thermogravimetric
curves were recorded with a Perkin-Elmer Diamond TG/DTA Ther-
mal Analyzer. The fluorescent spectra were measured on an Hitachi
Model F-7000 FL spectrophotometer.
2.2. Syntheses of the complexes
2.2.1. Syntheses of complexes 1–5
A mixture of 1 mL ethanol and 9 mL distilled water containing
H2PODA (0.0226 g, 0.1 mmol), NaOH (0.3 mmol, 0.65 mol/L), for-
mic acid (0.1 mmol, 0.65 mol/L), LaCl3Á6H2O for
1 (0.0353 g,
0.1 mmol), Ce(NO3)3Á6H2O for 2 (0.0434 g, 0.1 mmol), PrCl3Á6H2O
for 3 (0.0355 g, 0.1 mmol), NdCl3Á6H2O for 4 (0.0358 g, 0.1 mmol)
and Eu(NO3)3Á6H2O for 5 (0.0446 g, 0.1 mmol) was placed in a Tef-
lon-lined stainless reactor (23 mL), heated at 120 °C for 72 h under
autogenous pressure and then cooled to room temperature. Block
crystals were obtained.
3. Results and discussion
3.1. Description of the structures
The single-crystal X-ray diffraction revealed that complexes 1,
2, 3, 4 and 5, and 6 and 7 are isostructural, respectively. So only
the structures of complexes 3 and 7 will be discussed in detail.
La(PODA)(FA)(H2O)]n (1) Yield: 50.5% based on La. Anal. Calc. for
C11H11LaO9: C 31.00; H 2.60. Found: C 30.88; H 2.51%. IR data (KBr
pellet, m
/cmÀ1): 3388 s, 2932 m, 1595 vs, 1426 s, 1343 s, 1280 s,
1183 s, 1089 m, 1054 m, 976 w, 810 w, 732 w, 687 w, 629 w,
3.1.1. Crystal structure of [Pr(PODA)(FA)(H2O)]n (3)
575 w.
The single-crystal X-ray diffraction revealed that complex 3
crystallizes in the monoclinic system with the space group P21/c,
exhibiting a three-dimensional framework. Each asymmetric unit
of 3 contains one Pr(III) ion, one fully deprotonated PODA2À ligand,
one formate ion and one coordinated water molecule. As shown in
Fig. 1, the metal center Pr(III) is coordinated by nine oxygen atoms:
five oxygen atoms from four PODA2À ligands, three oxygen atoms
from three formate ions and one oxygen atom from one water mol-
ecule, displaying a tricapped trigonal-prismatic geometry. In com-
plex 3, the formate ions adopt a kind of tridentate coordination
mode (see Scheme 1a), and each formate ligand joins three Pr(III)
ions; the H2PODA ligands show one kind of coordination mode,
one carboxylate group acts in a bidentate bridging fashion and
the other one functions in a chelating bridging fashion (see
Scheme 1b), and every PODA2À ligand links four Pr(III) ions. Every
Pr(III) ion is bridged to eight adjacent Pr(III) ions by four PODA2À
ligands, which results in a 2-D infinite network with left- and
right-handed helical chains (Fig. 2a), the pitch of the helix running
along the b axis is the same as its length (7.80 Å), and formic mol-
ecules further link these layers into a 3-D metal–organic frame-
work (Fig. 2b), in which it is noteworthy that four kinds of
Ce(PODA)(FA)(H2O)]n (2) Yield: 43.3% based on Ce. Anal. Calc.
for C11H11CeO9: C 30.92; H 2.59. Found: C 30.79; H 2.46%. IR data
(KBr pellet,
1427 s, 1344 s, 1280 s, 1183 s, 1090 m, 1055 m, 976 m, 812 w,
785 w, 732 m, 688 w, 630 w, 577 w.
m
/cmÀ1): 3398 s, 2874 w, 1594 vs, 1498 s, 1464 s,
Pr(PODA)(FA)(H2O)]n (3) Yield: 35.4% based on Pr. Anal. Calc. for
C11H11PrO9: C 30.86; H 2.59. Found: C 30.80; H 2.53%. IR data (KBr
pellet,
m
/cmÀ1): 3407 s, 2877 w, 1595 vs, 1499 s, 1427 s, 1343 s,
1281 s, 1183 s, 1091 m, 1056 m, 975 m, 813 w, 784 w, 732 m,
688 w, 631 w, 578 w.
Nd(PODA)(FA)(H2O)]n (4) Yield: 46.8% based on Nd. Anal. Calc.
for C11H11NdO9: C 30.62; H 2.57. Found: C 30.56; H 2.48%. IR data
(KBr pellet, m
/cmÀ1): 3417 s, 3137 s, 1598 vs, 1497 m, 1465 m, 1450
m, 1401 s, 1341 m, 1285 m, 1184 m, 1154 w, 1094 m, 1058 m, 978
w, 961 w, 812 w,780 w, 733 w, 688 w, 630 w, 582 w.
Eu(PODA)(FA)(H2O)]n (5) Yield: 35.6% based on Eu. Anal. Calc.
for C11H11EuO9: C 30.08; H 2.52. Found: C 29.98; H 2.37%. IR data
(KBr pellet,
m
/cmÀ1): 3414 s, 2881 w, 1596 vs, 1499 s, 1426 s,
1342 s, 1281 m, 1183 m, 1091 m, 1056 m, 974 w, 812 w, 732 w,
688 w, 630 w.

G.-S. Yang et al. / Polyhedron 72 (2014) 83–89
85
Table 1
Crystal data for complexes 1–7.
Complex
1
2
3
4
5
6
7
Empirical formula
Formula weight
T (K)
C
11H11LaO9
C11H11CeO9
427.32
296(2)
C11H11PrO9
428.11
296(2)
C11H11NdO9
431.44
296(2)
C11H11EuO9
439.16
296(2)
C27H19LaN2O9
654.35
296(2)
C27H19PrN2O9
656.35
296(2)
426.11
296(2)
Crystal system
Space group
Unit cell dimensions
a (Å)
b (Å)
c (Å)
monoclinic
P21/c
monoclinic
P21/c
monoclinic
P21/c
monoclinic
P21/c
monoclinic
P21/c
triclinic
P1
triclinic
P1
ꢀ
ꢀ
12.3254(8)
7.9281(5)
13.0742(8)
90
90
90
12.308(3)
7.8806(18)
13.049(3)
90
90
90
12.312(5)
7.845(3)
13.049(5)
90
90
90
12.2814(7)
7.8064(5)
13.0193(8)
90
90.9050(10)
90
12.2745(6)
7.8023(4)
13.0090(7)
90
90.8530(10)
90
9.187(3)
11.843(4)
12.804(4)
116.160(3)
92.132(4)
93.283(4)
1245.3(6)
2
9.1377(11)
12.0833(15)
12.6995(16)
117.1160(10)
92.017(2)
93.299(2)
1243.0(3)
2
a
(°)
b (°)
c
(°)
V (Å3)
Z
1277.57(14)
4
1265.7(5)
4
1260.4(9)
4
1248.05(13)
4
1245.73(11)
4
F(000)
DCalc (g cmÀ3
h (°)
824
2.215
3.00–25.49
3.390
828
2.242
3.02–27.49
3.642
1.072
832
2.256
3.03–25.50
3.912
1.121
836
2.296
3.04–25.50
4.207
1.174
848
2.342
3.04–25.50
5.082
1.078
648
1.745
2.75–25.50
1.775
1.078
652
1.754
2.24–25.50
2.020
1.046
)
l
(mmÀ1
)
Goodness-of-fit on F2 1.054
Reflections/collected
unique [Rint
R1, wR2 [I > 2
9408/2375
0.0182
0.0134, 0.0323
0.0139, 0.0325
10668/2903
0.0231
0.0158, 0.0410
0.0169, 0.0418
9203/2337
0.0334
0.0215, 0.0576
0.0221, 0.0580
5176/2276
0.0140
0.0173, 0.0431
0.0178, 0.0433
9177/2318
0.0171
0.0180, 0.0475
0.0183, 0.0477
8930/4541
0.0400
0.0486, 0.1174
0.0789, 0.1397
9550/4582
0.0324
0.0366, 0.0796
0.0559, 0.0899
]
r
(I)]
R1, wR2 (all data)
Largest diff. peak
0.465 and À0.243 0.822 and À0.651 1.381 and À1.016 0.643 and À0.514 0.794 and À0.500 1.897 and À1.398 0.938 and À0.560
and hole (e ÅÀ3
)
groups, while III and IV are made up of metal ions and oxygen
atoms, which are different from the quadruple-strand helical
chains reported by us previously [20,22]. In the quadruple-strand
helical chains, I and IV both are right-handed, while II and III both
are left-handed, and the whole crystal is racemic and does not ex-
hibit chirality. The pitches of the four helical chains are the same,
all being the length of the b axis (7.80 Å). Better insight into this
complicated 3-D architecture can be achieved by topology analysis.
In this framework, two Pr(III) ions are associated by two formate
ions to form a tetramer [Pr2FA2], and from a topological point of
view, the [Pr2FA2] unit and the PODA ligand can be considered
as 12-connected (Fig. 3 rose color) and 4-connected (Fig. 3 blue col-
or) nodes respectively. The overall structure can be simplified as
having an sqc376 {32.42.52}2{38.420.524.614} topology, as shown in
Fig. 3. It is exciting to find that such a topological structure has
not been reported previously. To date, frameworks with a connec-
tion of over eight are rarely observed [41,42]. Employing polynu-
clear metal clusters as nodes is an effective route to generate
higher connected topology nets.
Fig. 1. The coordination environment of the Pr(III) ion in complex 3 with 30%
thermal ellipsoids.
helical chains can be observed. These four helices are formed by
metal ions and carboxyl groups of PODA2À ligands [I: (–Pr1–O2–
C8–O3–Pr1–O2A–C8A–O3A–)n, II: (–Pr1–O5B–C10B–O6B–Pr1–
O5C–C10C–O6C–)n], metal ions and carboxyl oxygen atoms of
PODA2À ligands [III: (–Pr1–O5B–Pr1–O5C–)n] and metal ions and
carboxyl oxygen atoms of formate ligands [IV: (–Pr1–O9D–Pr1–
O9E–)n], and they intertwined with each other to generate a un-
ique four-stranded helix which is united at the Pr(III) ions
(Fig. 2c). The helices I and II are made up of metal ions and carboxyl
3.1.2. The crystal structure of [Pr(PODA)1.5(phen)]n (7)
Crystallographic analysis revealed that complex 7 belongs to
ꢀ
the triclinic system with the space group P1. Each asymmetric unit
Scheme 1. The coordination modes of the H2PODA and HFA ligands in the title complexes.

86
G.-S. Yang et al. / Polyhedron 72 (2014) 83–89
Fig. 2. (a) The 2-D structure bridging by PODA2À ligands of 3 with left-handed (turquoise) helices and right-handed (blue) helices along the c axis; (b) the 3-D metal–organic
framework of 3 with single- and quadruple-stranded helical chains along the b axis and (c) view of the quadruple-stranded helical chains of complex 3 along the c axis. (Color
online.)
Fig. 3. Schematic representation of the (4,12)-connected framework with
Fig. 4. Coordination environment of the Pr(III) ion in complex 7 with 30% thermal
{32.42.52}2 {38.420.524.614
(Color online.)
} topology (blue: PODA; rose: Pr2FA2) for complex 3.
ellipsoids. The O9 and C24 were modeled as disordered around two positions in a
0.5:0.5 ratio.
of complex 7 contains one unique Pr(III) ion, one and half PODA2À
ligands, and one phen molecule. Pr(III) is nine coordinated by seven
oxygen atoms (O1, O7, O8, O2A, O4B, O5B, O4C) from five PODA2À
ligands and two nitrogen atoms (N1, N2) from one phen molecule,
left- and right-handed helix chains can be observed (Fig. 5). The
pitch of the helix running along the a axis is the same as its length
(9.19 Å). To best understand the structure of complex 7, a topology
analysis is employed to describe the architecture. It is interesting
that two adjacent Pr(III) atoms are linked by two carboxylate oxy-
gen atoms to generate a dimeric SBU with a PrÁ Á ÁPr distance of
4.06 Å. Each dimeric SBU is surrounded by four dimeric SBUs
through H2PODA ligands. In accordance with the simplification
principle, the dinuclear SBUs can be considered as four-connected
nodes, and the architecture of 7 can be described as a 2-D uninodal
displaying
a distorted tricapped trigonal prismatic structure
(Fig. 4). In complex 7, the H2PODA ligands are all completely
deprotonated and exhibit two kinds of coordination modes: one
is the same as that in complex 3 and the other is a bis-chelating
coordination mode (see Scheme 1b and c), and each PODA2À ligand
links four or two Pr(III) ions, forming a layer structure, in which

G.-S. Yang et al. / Polyhedron 72 (2014) 83–89
87
(a)
(b)
(c)
Fig. 5. (a) The 2-D network of 7 along the b axis; (b) View of the helical chains with left-handed (pink) helices and (c) right-handed (bright green) helices in complex 7 along
the c axis. (Color online.)
4-connected network with (44.62) topology (Fig. 6). The layer struc-
ture of complex 7 is further linked into a 3-D supramolecular struc-
ture through C–HÁ Á ÁO hydrogen bonds between the C–H groups of
phen molecules and carboxylate oxygen atoms (as shown in
Table S2).
structures with the PODA2À ligand [43,49], Cu(II) and Zn(II) poly-
mers exhibit 1-D chain structures based on the PODA2À ligand
[36,37], while the Ln(III) coordination polymers display layer struc-
tures based on the PODA2À ligand, showing the larger radii, coordi-
nation number and higher valance of the lanthanide ions need
more PODA2À ligands, which lead to the high-dimensional struc-
tures of the lanthanide coordination polymers.
The crystal structures of complexes 3 and 7 are different even
though the PODA2À ligands link the same Pr(III) ion to generate
layer structures. In complex 3, formic acid acts as a bridging ligand
and in complex 7 phen acts as a chelating terminal ligand, which
result in complex 3 exhibiting a 3-D metal–organic framework,
while complex 7 displays a layer structure. The results reveal that
the auxiliary ligands play an important role in determining the
structural diversity in the coordination polymers, and selecting
an appropriate co-ligand helps to construct high-dimensional
structures.
In addition, complex 3 not only owns one kind of single-strand
helical chain with the participation of PODA2À ligands, but also has
one kind of quadruple-strand helical chain together with the help
of the formate ion and the PODA2À ligand, and complex 7 only dis-
plays one kind of single-strand helical chain without the associa-
tion of phen. This shows that formic acid, with a multi-dentate
bridging coordination mode, helps to form multi-stranded helical
structures.
3.2. Comparison of the structures
The PODA2À ligands in the titled seven complexes exhibit two
kinds of coordination modes, presenting only one l4
coordination mode in complexes 1–5, as shown in Scheme 1b, while
displaying l4
g2 and l2 g1 coordination
– : : :
g1 g1 g1 g2
–
g1
:
g1
:
g1
:
– : : :
g1 g1 g1
modes in complexes 6 and 7 (see Scheme 1b and c), which are differ-
ent from that reported by Shan Gao et al. [43–48]. Complexes 1–5
feature 2-D wave-like layer structures, with every PODA2À ligand
joining four metal ions, whilst complexes 6 and 7 display different
2-D rectangle-like structures with the PODA2À ligands joining four
or two metal ions, showing that the flexible carboxylate groups of
the PODA2À ligands can display different types of bridging confor-
mations resulting in the different structures of complexes 1–5 and
6–7.
Compared with the structures of transition metal complexes re-
ported previously, Mg(II) and Co(II) polymers display 0-D
The average Ln–O bond lengths (2.583 Å for 1, 2.558 Å for 2,
2.542 Å for 3, 2.524 Å for 4, 2.522 Å for 5, 2.544 Å for 6 and
2.510 Å for 7) and Ln–N bond lengths (2.719 Å for 6 and 2.677 Å
for 7) decrease with the increase of the lanthanide atomic number,
which can be ascribed to the lanthanide contraction effect.
3.3. UV–vis absorption spectra
The solid-state UV–Vis spectra of H2PODA and complexes 1–7
were recorded at room temperature in the region 250–700 nm.
As shown in Fig. 7, complexes 1–7 all exhibit a large broad band
Fig. 6. Schematic representation of the 4-connected network with
topology for complex 7.
a
(44.62)

88
G.-S. Yang et al. / Polyhedron 72 (2014) 83–89
Fig. 7. The solid-state UV–Vis spectra of complexes 1–5 (a), the H2PODA ligand and 6–7 (b) at room temperature.
in the range 250–300 nm for the absorption of the PODA2À ligand
and the bands in the region 300–700 nm originate from the char-
acteristic 4f–4f transitions from the ground state to the excited
ones of the Ln(III) ions (3, 4, 5 and 7). Bands at 276 nm for 1,
275 nm for 2, 277 nm for 3, 273 nm for 4, 274 nm for 5, 273 nm
for 6 and 270 nm for 7 belong to the p–
p⁄ transition of the ligand,
and the bands are slightly blue-shifted compared with that of
H2PODA (278 nm), indicating that the coordination with the Ln(III)
ions enlarges the energy gaps in the conjugated systems of
PODA2À. As the f–f transitions are inner shell transitions, the effect
of the ligands will be negligible and the electronic spectra of the
lanthanide complexes resemble those of the free ions. The hyper-
sensitive transition bands of complex 3 at ca. 444, 471, 486 and
591 nm originate from the 3H4 ground state of the Pr(III) ion to var-
ious excited states: 3P2, 3P1, 3P0 and 1D2. The absorption spectrum
of complex 4, with bands at ca. 427, 464, 482 and 580 nm, is con-
2
4
sistent with the absorption bands P1/2, ,
4G11/2 4G9/2 and G5/2 of
thte Nd(III) ion. Complex 5 revealed several absorption peaks
around 352, 406 and 464 nm, which correspond to the f–f elec-
tronic transitions from the 7F0 ground state of Eu(III) to the higher
excited states: 5D4, 5D3 and 5D2. The absorption spectrum of com-
plex 7 displays several bands, at 443, 469, 482 and 590 nm, as-
signed to the corresponding electronic transitions of Pr(III) from
the ground state 3H4 to the excited states: 3P2, 3P1, 3P0 and 1D2.
However, the absorption spectra of complexes 1, 2 and 6 do not
show bands from the f–f transitions of the Ln(III) ion in the visible
region, and only one band from the ligand is observed in the range
250–300 nm.
Fig. 8. The luminescence spectra of the H2PODA ligand and complex 5 at room
temperature.
weight loss of 26.46% between 236 and 285 °C corresponds to
the loss of phen, which is consistent with the calculated value of
27.42%. Upon increasing the temperature, the frameworks of 3
and 7 commence to collapse and the final residuals were not
characterized.
3.4. Powder X-ray diffraction and thermal analysis
3.5. Luminescent properties
Complexes 3 and 7 were selected as representatives to check
the phase purity of these complexes and the powder X-ray diffrac-
tion (PXRD) patterns were recorded at room temperature, as show
in Fig. S1 (Supporting information). The bulk samples of 3 and 7
corroborate well with those simulated from the single crystal data
respectively.
Complexes 3 and 7 were also chosen as examples to character-
ize the thermal stability of the complexes and thermogravimetric
analyses (TGA) were performed under an N2 atmosphere with a
heating rate of 10 °C/min from room temperature to 800 °C, as
shown in Fig. S2 (see Supporting information). For 3, the first step,
in the range 185–274 °C, can be ascribed to the loss of the coordi-
nated water molecule (obsd 4.42%, calcd 4.20%). For 7, the first
Considering the excellent luminescent property of Eu(III) ions,
the solid-state photoluminescent spectra of complex 5 and the
H2PODA ligand were measured at room temperature. The solid
H2PODA ligand shows an emission band at 415 nm upon excitation
at 342 nm, which may be assigned to the p⁄ ? n electron transi-
tion. As shown in Fig. 8, the emission spectrum of 5 upon excitation
at 396 nm displays intense red luminescence and exhibits the
characteristic transitions of 5D0 ? 7Fn (n = 0–4) of Eu(III) ions at
580, 591, 619, 644 and 683 nm, respectively. It is worth noting that
these characteristic emission bands indicate that the ligand to me-
tal energy transfer is moderately efficient under the experimental
conditions used. Moreover, the intensity of the hypersensitive
transition 5D0 ? 7F2 is larger than that of the 5D0 ? 7F1 transition,

G.-S. Yang et al. / Polyhedron 72 (2014) 83–89
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89
indicating a highly polarizable chemical environment around the
Eu(III) ion. The intensity ratio of the 5D0 ? 7F2 transition (electric
dipole transition) to the 5D0 ? 7F1 transition (magnetic dipole
transition) of 5 is ca. 5.41, which indicates that the Eu(III) ion in
5 is not located at a center of inversion and the symmetry of the
Eu(III) ion site is low.
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In summary, seven new lanthanide supramolecular complexes
with diverse helical characters have been obtained under hydro-
thermal conditions. Complexes 1–5 exhibit 3-D metal–organic
frameworks containing single- and quadruple-stranded helical
chains with the help of formate and PODA2À ligands, while
complexes
6 and 7 display layer networks containing only
single-stranded helical chains. The results demonstrate that the
PODA2À ligand, with two flexible carboxylate groups at a suitable
angle, is apt to construct helical structures with lanthanide ions,
and formic acid, with multi-dentate bridging coordination modes,
as a co-ligand helps to construct multi-stranded helical and high-
dimensional structures. The results of photoluminescent measure-
ments indicate that complex 5 may be a potential fluorescent
material.
Acknowledgements
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This work was supported by the National Science Foundation of
China (Grant Nos. 21264011, 20961007), the Aviation Fund (Grant
No. 2010ZF56023) and the Young Scientists Program of Jiangxi
Province (Grant No. 2008DQ00600).
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Appendix A. Supplementary data
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CCDC 971286–971292 contain the supplementary crystallo-
graphic data for complexes 1–7. These data can be obtained free
of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or
from the Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: de-
posit@ccdc.cam.ac.uk. Supplementary data associated with this
article can be found, in the online version, at http://dx.doi.org/
10.1016/j.poly.2014.01.030.
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