Synthesis, crystal structures, luminescence properties of two metal coordination polymers derived from 5-substituted isophthalate and flexible bis (triazole) ligands

Article abstract of DOI:10.1016/j.saa.2014.03.025

Two new metal complexes, [Ni(btx)(nip)(H₂O)]_n (1), {[Cd(btx)(mip)(H₂O)]·H₂O}_n (2) (btx = 1,4-bis(1,2,4-triazol-1-ylmethyl)benzene, H₂nip = 5-nitroisophthalic acid, H₂mip = 5-methyisophthalic acid) were synthesized under hydrothermal conditions and characterized by single-crystal X-ray diffraction methods, IR spectroscopy, TGA and elemental analysis. Complex 1 features a 3D metal-organic framework with three-fold interpenetrating CdSO₄-type topology. Complex 2 exhibits a 2D network with square grid units, which is further extended into a rare 3,5T1 three-dimensional supramolecular network via three modes of classical O-H?O hydrogen bonds. In addition, luminescence properties of 1 and 2 have also been investigated in the solid state.

Full text of DOI:10.1016/j.saa.2014.03.025

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 129 (2014) 125–130
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Synthesis, crystal structures, luminescence properties of two metal
coordination polymers derived from 5-substituted isophthalate
and flexible bis (triazole) ligands
Chun-lun Ming a, Li-na Wang a, Kristof Van Hecke b, Guang-hua Cui a,
⇑
a College of Chemical Engineering, Hebei United University, 46 West Xinhua Road, Tangshan 063009, Hebei, PR China
b Department of Inorganic and Physical Chemistry, Ghent University, Krijgslaan 281 S3, B-9000 Ghent, Belgium
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
ꢀ
ꢀ
ꢀ
Two new metal coordination
polymers have been synthesized.
IR, PXRD, TG technique for the
polymers.
X-ray single-crystal structure
analyses and discussion for two
polymers.
ꢀ
Luminescence properties of two
complexes have been investigated.
a r t i c l e i n f o
a b s t r a c t
Article history:
Two new metal complexes, [Ni(btx)(nip)(H2O)]n (1), {[Cd(btx)(mip)(H2O)]ÁH2O}n (2) (btx = 1,4-bis(1,2,4-
triazol-1-ylmethyl)benzene, H2nip = 5-nitroisophthalic acid, H2mip = 5-methyisophthalic acid) were syn-
thesized under hydrothermal conditions and characterized by single-crystal X-ray diffraction methods, IR
spectroscopy, TGA and elemental analysis. Complex 1 features a 3D metal-organic framework with three-
fold interpenetrating CdSO4-type topology. Complex 2 exhibits a 2D network with square grid units,
which is further extended into a rare 3,5T1 three-dimensional supramolecular network via three modes
of classical OAHÁ Á ÁO hydrogen bonds. In addition, luminescence properties of 1 and 2 have also been
investigated in the solid state.
Received 17 December 2013
Received in revised form 4 March 2014
Accepted 18 March 2014
Available online 27 March 2014
Keywords:
Crystal structure
Bis (triazole) ligand
Metal–organic framework
Luminescence
Ó 2014 Elsevier B.V. All rights reserved.
Introduction
such intriguing topologies and functional materials, the crucial
step is to employ appropriate organic building blocks as well as
Metal organic coordination polymers (MOCPs) assembled from
the linkage of metal center atoms through organic ligands have vi-
tal applications in luminescent devices, gas storage, ion exchange,
separations, biomedicine, catalysis and so on [1–6]. In order to get
metal ions. However, construction of MOCPs with expected
structures and properties in hydrothermal reactions is still a chal-
lenge, owing to the fact that the assembly of such complexes can
be easily influenced by the geometrical and electronic properties
of metal ions and ligands, temperature, pH value of the solution,
etc. In recent years, 1,2,4-triazole and its derivatives have often
been chosen as ligands for the construction of interesting and
⇑
Corresponding author. Tel.: +86 0315 2592169; fax: +86 0315 2592170.
E-mail address: tscghua@126.com (G.-h. Cui).
http://dx.doi.org/10.1016/j.saa.2014.03.025
1386-1425/Ó 2014 Elsevier B.V. All rights reserved.

126
C.-l. Ming et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 129 (2014) 125–130
fascinating structures [7–11]. Especially, the flexible 1,4-bis(1,2,4-
triazol-1-ylmethyl)-benzene (btx) not only possesses the merits of
triazole, but can also freely bend and rotate to interact with metal
ions. [12–14]. A series of Cd–btx complexes with the third-order
NLO (nonlinear optics) properties and Co-btx coordination poly-
mers with novel topologies have been studied [15,16]. In addition,
aromatic carboxylates such as 1,2,4,5-benzenetetracarboxylate,
1,3,5-benzenetricarboxylate, terephthalate, 5-substituted iso-
phthalate, ligands are of particular interest due to their multi-car-
boxylate groups and the versatile coordination modes of these
carboxylate groups, such as monodentate, bis-monodentate or
chelating modes, leading to diverse structures of the resulting
MOCPs [17]. The combination of flexible btx and aromatic polycar-
boxylate ligands may induce novel architectures and versatile
properties. Wang and co-workers have presented a rare example
of entangled coordination polymer containing both interpenetrat-
ing and polythreading features coordination polymer [Zn(CH3O-
ip)(btx)]2ÁH2O (CH3O-ip = 5-methoxyisophthalate) [18]. We have
reported three unprecedented binodal high-connected metal–
organic frameworks constructed from btx and 1,2,4,5-benzenetet-
racarboxylate [19,20]. As a part of our ongoing studies on ternary
transition metal complexes based on btx and aromatic carboxylate
ligands, two new coordination polymers with interesting
luminescence properties were prepared and structurally character-
ized. [Ni(btx)(nip)(H2O)]n (1) (H2nip = 5-nitroisophthalic acid,
H2mip = 5-methyisophthalic acid) reveals a 3-fold interpenetrating
3D CdSO4-type network, while {[Cd(btx)(mip)(H2O)]ÁH2O}n (2)
shows a 2D (4, 4) sheet-like network, which is finally extended into
a 3D 3,5T1 supramolecular framework through multiple strong
hydrogen bond interactions. To the best of our knowledge, coordi-
nation polymers exhibiting such 3,5T1 supramolecular network
have only rarely been reported [21].
{[Cd(btx)(mip)(H2O)]ÁH2O}n (2)
The synthetic procedure of 2 was analogous to the synthesis of
1, except that H2nip and Ni(CH3COO)2Á4H2O were used instead of
H2mip (0.1 mmol, 18.0 mg) and Cd(CH3COO)2Á2H2O (0.1 mmol,
26.7 mg), respectively. Colorless block-shaped crystals of 2 (yield:
43%) could be obtained. Anal. Calcd. for C21H22CdN6O6 (%): C,
44.50, H, 3.91, N, 14.83. Found (%): C, 44.36, H, 3.82, N, 14.55. IR
(KBr, cm–1): 3427(vs), 3103(w), 2948(w), 1640(s), 1542(s),
1483(s), 1348(s), 1284(w), 1064(s), 733(w), 683(w).
X-ray crystallography
The crystallographic data for the single crystal of compound 1
was collected at 100 K on an Agilent SuperNova Dual Source dif-
fractometer with Atlas CCD detector, microfocus sources and
focusing multilayer optics, using Mo K
a radiation (k = 0.71073 Å).
CrysAlis PRO [23] software was used to collect, index, scale and ap-
ply analytical absorption correction based on faces of the crystal
with multi-scan mode. X-ray diffraction data for 2 was collected
on a Bruker Smart 1000 CCD diffractometer using graphite-mono-
chromated Mo K
a radiation (k = 0.71073 Å) at room temperature
with -scan mode. A semi-empirical absorption correction was ap-
x
plied using the SADABS program [24]. Both structures were solved
by direct methods with SHELXS-97 and refined by full-matrix least
squares on F2 with SHELXTL [25]. All non-hydrogen atoms were re-
fined with anisotropic thermal parameters. The H-atoms of organic
ligands were generated theoretically onto the specific atoms and
refined isotropically. The aqua hydrogen atoms in 1 and 2 were lo-
cated from difference Fourier maps and refined with isotropic dis-
placement parameters. The crystallographic data for 1 and 2 are
listed in Table 1, and selected length and angle parameters for 1
and 2 are presented in Table 2, respectively.
Results and discussion
Experimental
Crystal structures
Materials and measurements
Crystal structures of compound 1
The structure of 1 is a 3D MOF. X-ray analysis reveals that 1
crystallized in the monoclinic space group Cc. The asymmetric unit
All reagents were obtained from commercial sources and used
without further purification. The ligand btx was prepared accord-
ing to literature procedures [22]. Elemental analysis (C, H, and N)
was performed on a Perkin–Elmer 240C Elemental Analyzer. Power
X-ray diffraction (XRD) data were recorded on a Rigaku D/Max-
2500 diffractometer at 40 kV, 100 mA for a Cu-target tube and a
graphite monochromator. FT-IR spectra were recorded on an
Avatar 360 (Nicolet) spectrophotometer between 400 and
Table 1
Crystal and refinement data for complexes 1 and 2.
1
C
2
Empirical formula
Formula weight
Crystal system
Space group
a (Å)
20H17N7NiO7
C21H22CdN6O6
566.85
Triclinic
4000 cm–1
, using the KBr pellet method. Thermogravimetric
526.12
Monoclinic
Cc
16.1760(3)
16.9691(3)
7.3806(2)
90
analyses (TGA) were conducted on a Netzsch TG 209 thermal ana-
lyzer in N2 environmental at a heating rate of 10 °C/min up to
800 °C. The fluorescence spectra were collected with a Hitachi
F-4500 spectrophotometer at room temperature.
Pı
¯
10.2021(7)
11.4351(8)
11.8614(8)
69.2300(8)
68.8065(8)
67.0920(7)
1150.98(14)
2
b (Å)
c (Å)
a
(deg)
b (deg)
94.459(2)
90
2019.77(7)
4
1.730
1.025
c
(deg)
Preparation of the complexes 1 and 2
[Ni(btx)(nip)(H2O)]n (1)
V (Å3)
Z
Dcalc (g/m3)
1.636
0.999
572
(mm–1
)
A
mixture of Ni(CH3COO)2Á4H2O (0.1 mmol, 24.9 mg), btx
l
(0.1 mmol, 24.0 mg), H2nip (0.1 mmol, 105.5 mg) was added in
20 mL H2O. It was then sealed in a 25 mL Teflon-lined stainless
steel autoclave and heated at 160 °C for 3 days; then, the reaction
system was cooled to room temperature at 5 °C/h. Green block-
shaped crystals were collected by filtration and washed with dis-
tilled water in 45% yield (based on Ni(CH3COO)2Á4H2O). Anal.
Calcd. for C20H17N7NiO7 (%): C, 45.66, H, 3.26, N, 18.64. Found
(%): C, 45.32, H, 3.12, N, 18.39. IR (KBr, cm–1): 3420(vs), 3120(w),
2368(w), 1617(s), 1532(s), 1461(s), 1353(s), 1282(s), 1125(w),
733(w), 677(w).
F(000)
1080
Crystal size (mm)
Total reflections
Unique reflections
Rint
0.28 Â 0.21 Â 0.15
21,862
4784
0.0635
0.851
0.27 Â 0.23 Â 0.22
7039
5062
0.0164
1.002
0.0254
0.0622
0.341
–0.421
GOF
R1 (I > 2
r
(I))
0.0366
0.0805
0.393
wR2 (I > 2
r(I))
D
D
q
q
max (eÅ–3
min (eÅ–3
)
)
–0.330
[w F2o À F2c 2]/
R
[w F2o
]
.
2
1/2
R1
=
R
||Fo|–|Fc||/
R
|Fo|; wR2
=
R

C.-l. Ming et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 129 (2014) 125–130
127
Table 2
Selected bond lengths (Å) and angles (°) for complexes 1 and 2.
Complex 1
N1ANi1
Ni1AO1W
Ni1AO6A
Ni1AN4B
Ni1AO1
Ni1AO2
O1WANi1AO6A
O1WANi1AN1
O6AANi1AN1
O1WANiAN4B
2.071(3)
2.036(2)
2.056(2)
2.085(2)
2.095(2)
2.229(2)
96.34(8)
91.75(9)
87.66(8)
95.04(9)
O6AANi1AN4B
N1ANi1AN4B
O1WANi1AO1
O6AANi1AO1
N1ANi1AO1
N4BANi1AO1
O1WANi1AO2
O6AANi1AO2
N1ANi1AO2
87.55(8)
172.08(10)
91.67(8)
171.97(8)
91.48(8)
92.38(8)
152.91(8)
110.75(7)
89.25(8)
86.55(8)
N4BANi1AO2
Complex 2
Cd1AO3A
Cd1AN3
Cd1AN6
Cd1AO2
2.2953(17)
2.2957(19)
2.301(2)
2.3228(15)
2.3445(18)
2.5275(15)
2.5707(17)
91.31(7)
169.55(5)
92.80(7)
99.88(8)
167.14(7)
83.84(6)
92.18(7)
O3AACd1AO1W
N3ACd1AO1W
N6ACd1AO1W
O2ACd1AO1W
O3AACd1AO1
N3ACd1AO1
129.60(7)
83.97(8)
86.28(8)
146.40(7)
137.38(5)
86.55(6)
85.56(7)
53.66(5)
92.76(6)
52.88(5)
91.23(7)
94.70(7)
136.70(6)
76.85(7)
Cd1AO1W
Cd1AO1
Cd1AO4A
N6ACd1AO2
O1ACd1AO4A
O3AACd1AN3
O3AACd1AN6
N3ACd1AN6
O3AACd1AO2
N3ACd1AO2
N6ACd1AO1
O2ACd1AO1
O1WACd1AO1
O3AACd1AO4A
N3ACd1AO4A
N6ACd1AO4A
O2ACd1AO4A
O1WACd1AO4A
Symmetry transformations used to generate equivalent atoms in 1: A x + 1/2,
Ày + 1/2, z + 1/2; B x + 1/2, y + 1/2, z À 1. In 2: A x À 1, y, z; B x + 1, y, z.
of 1 contains one Ni(II) cation, one btx ligand, one nip anion and
one coordinated water molecule. As depicted in Fig. 1a, the coordi-
nation sphere around the six-coordinated Ni(II) ion is composed of
two nip2À ligands, one water and two btx ligands. The equatorial
plane is formed by two chelating carboxylate oxygen atoms (O1
and O2) from one nip2À, one bis-monodentate carboxylate oxygen
atom (O6) from a symmetry-equivalent nip2À and one oxygen
atom from a coordinated aqua molecule (O1W). The apical posi-
tions are occupied by two nitrogen atoms (N1 and N4B) from
two distinct btx ligands (B = x + 1/2, y + 1/2, zÀ1). The NiAO bond
distances are in the range of 2.038(2)–2.229(2) Å, and the NiAN
bond distances are 2.073(3) and 2.084(3) Å, respectively, which
are all similar to those found in other Ni(II) compounds [26].
In 1, the btx ligands adopt a trans-conformation and bridge the
adjacent two Ni(II) centers to assemble into an infinite 1D
[Ni(btx)]n ribbon-like chain along the b direction, wherein the in-
tra-chain adjacent NiÁ Á ÁNi separation is 14.183(3) Å. For the btx
containing N1 and N4, the dihedral angles between the triazole
ring and the phenyl plane are 71.851(9)° and 63.191(1)°, respec-
tively. The torsion angle (C5AC4AC3AN3) is 48.906(4)° for the for-
mer kind of btx, whereas the corresponding torsion angle
(C6AC7AC12AN5) is enhanced to 68.077(3)° for the latter type
of btx. The flexible skeleton is conducive to such conformational
transformation. Moreover, nip anions exhibit bridging ligands with
one monodentate carboxylic group and one chelating carboxylic
Fig. 1. (a) The coordination environment for Ni(II) ion in complex 1, showing
thermal displacement ellipsoids at the 30% probability level. (b) Perspective view of
the 3D framework for complex 1. (c) 3D framework with 4-connected cds topology.
contribute to the stability of the resultant three dimensional supra-
molecular framework 1.
Crystal structures of compound 2
Compound 2 is a 3,5-connected binodal 3D supramolecular net-
work and crystallized in the space group of Pı. The asymmetric unit
¯
of 2 consists of one Cd(II) ion, one mip anion, one btx ligand, one
coordination and one lattice water molecules. As depicted in
Fig. 2a, each Cd(II) ion coordinates to two bridging btx ligands
through N-coordination (CdAN3 = 2.296(2) Å, CdAN6 = 2.301(2)
Å) in an almost linear configuration (N3ACdAN6 = 167.143(7)°),
two chelating mip2À carboxylate oxygen atoms (the bond lengths
of CdAO range from 2.295(2) to 2.571(2) Å) and one H2O solvate
oxygen atom to form a pentagonal bipyramid geometry. Two con-
formational btx ligands act as bidentate trans-configuration ligands
in which the dihedral angles between two triazole ring planes of
btx ligand are 73.9° and 75.9°, respectively. The btx ligands link
Cd(II) ions to form a 1D zigzag [Cd(btx)]n cation chain. The adjusted
1D chains are parallel to each other and further connected by bis-
chelating mip2À ligands to assemble into a 2D (4, 4) grid structure,
in which each 42-member parallelogram consists of four Cd(II)
group to connect two Ni(II) ions, generating
a 1D zigzag
[Ni(nip)(H2O)]n neutral chain along the a direction. Two kinds of
chains are interconnected along different directions to generate a
3D framework (Fig. 1b). As for the topological geometry, each Ni(II)
ion connecting four neighboring Ni(II) ions can be considered as 4-
connected node, btx and nip2À bridging ligands can be regarded as
linkers, thus the structure of 1 can be simplified into a (65.8)-cds
topology. Because the single 3D cds network has large spacious
voids, it allows two other identical cds networks to interpenetrate
giving rise to a 3-fold interpenetrating 3D cds network (Fig. 1c). In
addition, one classical OAHÁ Á ÁN hydrogen bond interaction be-
tween the coordinated water molecule and 2-position triazole
nitrogen atoms (O1WÁ Á ÁN2 = 2.879(3) Å, O1 WAH1WÁ Á ÁN2 = 161°)

128
C.-l. Ming et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 129 (2014) 125–130
Table 3
Hydrogen-bonding geometry for 2.
DAHÁ Á ÁA
DAH (Å)
HÁ Á ÁA (Å)
DÁ Á ÁA (Å)
DAHÁ Á ÁA (°)
O1WAH1AÁ Á ÁO1
O1WAH1BÁ Á ÁO2 W
O2WAH2AÁ Á ÁO4
O2WAH2BÁ Á ÁO4
0.83(4)
0.82(3)
0.82
1.90(4)
2.04(3)
2.22
2.711(3)
2.855(5)
2.912(4)
2.817(4)
169(4)
170(4)
141
0.82
2.00
171
Symmetry transformations used to generate equivalent atoms in 2: A x À 1, y, z; B
x + 1, y, z.
connect two Cd(II) ions and one mip2À can be considered as a
3-connected node, and each btx ligand act as linker, giving a
(3,5)-connected dinodal 3D 3,5T1 supramolecular network
(Fig. 2c). To the best of our knowledge, only few complexes with
this topology have been reported [21].
Comparison of complexes 1 with 2
The above structural descriptions clearly reveal that it was evi-
dent that the coordination geometries of the central metal ions and
the coordination modes of the organic ligands have an important
on the formation and dimensions of the final structures. Compari-
son of complexes 1 with 2, the coordination geometries of metal
centers may play a crucial key role in determining the dimension
of the resulting structures. The nickel atom in 1, centers a [NiO4N2]
octahedral coordination geometry, while [CdO3N2] in 2 with a pen-
tagonal bipyramid geometry (The coordination sphere of 1 and 2
both contain one coordinated water molecule). Meanwhile, the
5-substituted isophthalates ligands adopt two different coordina-
tion modes as a l2
– :
g2 g1 coordination mode for nip2À ligand in
1, one l2 :
–
g2 g2 coordination pattern connecting two Cd(II) atoms
for mip2À in 2. As a result, the nip2À ligand links the Ni(II) centers
together to form a 1D ribbon-like polymeric chain in 1, while a zig-
zag polymeric chain in 2. These chains are further extended by btx
ligand to produce 3D or 2D network.
IR and XPRD
There is no absorption peak between 1760–1680 cmÀ1 for 1 and
2, indicating that all carboxyl groups of the organic moieties are
deprotonated [27]. The strong peaks at 1617 cm–1 and 1461 cm–1
for 1, and 1640 cm–1 and 1483 cm–1 for 2 may be attributed to
the asymmetric and symmetric vibrations of carboxylate groups.
Dt[tas(COO)–t , indicating
s(COO)] are 156 cm–1 and 157 cm-1
Fig. 2. (a) Local coordination geometry of the central Cd(II) cation in the complex 2,
showing thermal displacement ellipsoids at the 30% probability level. All hydrogen
atoms are omitted for clarity. (b) 2D 4-connected grid structure consisting of 1D btx
line chains. (c) 3D framework with binodal (3, 5)-connected 3,5T1 topology.
bridging coordination of the carboxylate group to the metal center
[28]. For 1 and 2, the strong broad bands center at around 3000–
3500 cm–1, contributing to the OAH stretching vibration modes
of water molecules or hydrogen bonds. The IR spectrum determi-
nation shows that tas(C@N) and tas(N@N) of the triazole ring vibra-
tions in 1 and 2 are at 1532 cm–1 and 1282 cm–1, 1542 cm–1 and
1284 cm–1, respectively [29].
The simulated and experimental XRPD patterns of two poly-
mers, obtained at room temperature, are shown in Fig. 3a and b.
Their peak positions are in good consistency with each other, indi-
cating the phase purity of the as-synthesized samples.
ions at the corners connected by two btx ligands and two mip2À li-
gands. In each parallelogram, the CdÁ Á ÁCd distances are 14.717(7)
or 13.085(7) Å bridged by distinct conformational btx ligands,
10.202(7) Å separated by mip2À ligands, respectively (Fig. 2b).
More interesting, the neighboring (4, 4) layers are extended to a
3D supramolecular framework by three kinds of OAHÁ Á ÁO hydro-
gen bond modes between water molecules and carboxylate oxygen
atoms. One mode of hydrogen bonds is formed between the coor-
dinated water molecules and chelating carboxylate oxygen atoms
with O1WÁ Á ÁO1 distance of 2.711(3) Å, a second mode is assembled
between lattice water molecules and chelating carboxylate oxygen
atoms with O2WÁ Á ÁO4 distance of 2.912(4) Å and 2.817(4) Å, and a
third mode is built between the coordinated water and lattice
water molecules with O1WÁ Á ÁO2W distance of 2.855(5) Å. The re-
lated hydrogen-bonding geometries are given in Table 3. For a
topological view, each Cd(II) ion links five neighboring Cd(II) ions,
which can be regarded as a 5-connected node, each mip2À ligand
Thermal and photoluminescent properties
Thermogravimetric experiments under nitrogen atmosphere
were carried out to investigate the thermal stability of two com-
plexes. 1-2 possessed a two-step weight loss process, which was
performed in the curves of TG and DTG (Fig. 4a and b). The first-
step occurred from 216 °C to 285 °C with a weight loss of 3.54%
for 1 and 110 °C to 140 °C with a weight of 6.43% for 2, which
can be attributed to the dehydration of the crystallized water
molecules (calcd.: 3.43% for 1, 6.36% for 2). While the second step

C.-l. Ming et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 129 (2014) 125–130
129
Fig. 4. (a and b) The TG and DTG curves of polymer 1–2 measured in N2
atmosphere.
Fig. 3. (a and b) The powder X-ray diffraction patterns calculated from the single-
crystal data and that obtained from the experiments for polymer
respectively.
1 and 2,
occurred in a temperature range from 346 °C to 668 °C for complex
1 and 280 °C to 521 °C for complex 2, corresponding to the release
of the btx and aromatic acid ligands and indicating the complete
decomposition of complexes with 85.20% for 1 and 77.15% for 2,
residual weight corresponds to MO (M = Ni or Cd) (calcd.: 85.8%
for 1, 77.35% for 2). A comparison of the two complexes indicates
that complex 1 is more stable than 2. That is because the nickel
atomic radius is smaller than the atomic radius of cadmium, which
leads to the nickel binding capacity being much stronger than
cadmium.
It is well known that inorganic–organic hybrid coordination
compounds, especially with d10 metal centers, have been found
to present photoluminescent properties and may have potential
application as fluorescence-emitting materials [30,31]. The photo-
luminescent properties of complex 1–2 along with btx in the solid
state at room temperature have been investigated (Fig. 5). The btx
ligand exhibits an intense emission with a maximum at 432 nm
upon excitation at 365 nm. The emission band of complex 1 ap-
pears in 430 nm (kex = 300 nm), since it is similar to that of btx,
the emission can probably be assigned to the intraligand transition
of coordinated btx ligands. Obviously, large blue shifted emission
occurs in 2 (kem = 350 nm, kex = 270 nm) with respect to the free
btx ligand and such broad band may be tentatively attributable
to ligand-to-metal charge transfer (LMCT) as reported 2D cad-
mium(II) metal–organic frameworks [32]. However, the emission
bands of the carboxylate ligands originated from the nÀp* transi-
tion are weak, and it is considered that the carboxylate ligands
have no significant contribution to the fluorescence emission of
complexes 1 and 2 in the presence of the N-donor ligand [33].
Fig. 5. Emission spectra of complex 1–2 and the free btx ligand.
Conclusion
Two new coordination polymers have been successfully isolated
under hydrothermal conditions by reactions of two 5-substituted
isophthalate, btx and metal salt. Complex 1 forms a 3-fold inter-
penetrating 4-connected 65, 8-cds 3D network. Complex 2 exhibits
a 3D supramolecular network. The structural differences of com-
plexes 1 and 2 may result from the different coordination geome-
try of metal ions, and the distinct 5-substituted group with
isophthalate ligands may impose remarkable steric and/or elec-
tronic effects on the coordination modes of organic carboxylate

130
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