Synthesis, crystal structures, and properties of two novel cadmium(II)-organic frameworks based on asymmetric dicarboxylate and N-donor ligands

Article abstract of DOI:10.1016/j.molstruc.2014.05.028

Two novel cadmium(II)-organic frameworks with asymmetric dicarboxylate and N-donor ligands, namely [Cd(cpa)(phen)]_n (1) and {[Cd ₂(cpa)₂(bpy)_1.5]·0.5H₂O} _n (2) (H₂cpa = 3-(4-benzoic)propionic acid, phen = 1,10-phenanthroline, bpy = 4,4′-bipyridine) have been hydrothermally synthesized and characterized by elemental analyses, FT-IR spectra, single-crystal X-ray diffraction analyses, TGA, powder XRD and fluorescent measurements. 1 displays a double zigzag chain structure containing 8-number and 22-number circles. 2 Shows a 6-connected 3D polymer network based on tetranuclear cadmium cluster units. The most striking feature of 2 is that a pair of identical 3D networks are interlocked with each other to the form a 2-fold interpenetrated 3D α-Po structural topology. The diverse structures of two complexes indicate that the skeleton of N-donor ligands plays a great role in the assembly of such different frameworks. In addition, the thermal stabilities, XRD and photoluminescence properties of 1-2 were also studied.

Full text of DOI:10.1016/j.molstruc.2014.05.028

Journal of Molecular Structure xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Synthesis, crystal structures, and properties of two novel
cadmium(II)–organic frameworks based on asymmetric
dicarboxylate and N-donor ligands
⇑
Xiaoli Chen , Loujun Gao, Xiaoge Zhang, Xuhua Han, Yao Wang, Rong Sun
School of Chemistry and Chemical Engineering, Shaanxi Key Laboratory of Chemical Reaction Engineering, Yanan University, Yan’an 716000, China
g r a p h i c a l a b s t r a c t
h i g h l i g h t s
Two novel cadmium coordination polymers have been synthesized and characterized. 1 is a double zigzag
ꢀ
Two cadmium(II) complexes with
asymmetric dicarboxylate and N-
donor ligands.
chain structure. 2 Displays a 2-fold interpenetration 3D
a
-Po structural topology. The thermal stabilities
and luminescence properties were also studied.
ꢀ
ꢀ
ꢀ
The synthesis shows that N-donor
ligand plays the key role.
1 Displays a double zigzag chain
structure.
O
OH
2 Shows a 2-fold interpenetrated 3D
a-Po structural topology.
Cd
Cd
O
phen
bpy
OH
a r t i c l e i n f o
a b s t r a c t
Article history:
Two novel cadmium(II)–organic frameworks with asymmetric dicarboxylate and N-donor ligands,
namely [Cd(cpa)(phen)]n (1) and {[Cd2(cpa)2(bpy)1.5]ꢁ0.5H2O}n (2) (H2cpa = 3-(4-benzoic)propionic acid,
phen = 1,10-phenanthroline, bpy = 4,40-bipyridine) have been hydrothermally synthesized and character-
ized by elemental analyses, FT-IR spectra, single-crystal X-ray diffraction analyses, TGA, powder XRD and
fluorescent measurements. 1 displays a double zigzag chain structure containing 8-number and 22-num-
ber circles. 2 Shows a 6-connected 3D polymer network based on tetranuclear cadmium cluster units. The
most striking feature of 2 is that a pair of identical 3D networks are interlocked with each other to the
Received 9 April 2014
Received in revised form 12 May 2014
Accepted 12 May 2014
Available online xxxx
Keywords:
Cadmium(II)–organic framework
Crystal structure
form a 2-fold interpenetrated 3D a-Po structural topology. The diverse structures of two complexes indi-
cate that the skeleton of N-donor ligands plays a great role in the assembly of such different frameworks.
In addition, the thermal stabilities, XRD and photoluminescence properties of 1–2 were also studied.
Ó 2014 Elsevier B.V. All rights reserved.
2-Fold interpenetrated network
Introduction
because of their intriguing aesthetic structures and topological
features as well as their promising applications as functional
The crystal engineering of metal–organic frameworks (MOFs)
has grown into a highly active area that attracts significant interest
materials [1–8]. In order to build these molecular architectures
with interesting compositions and topologies, judicious selection
of appropriate polydentate organic ligands and metal ions could
be the key factors of a successful approach [9–13]. According to
⇑
Corresponding author. Tel./fax: +86 911 2332037.
E-mail address: chenxiaoli003@163.com (X. Chen).
http://dx.doi.org/10.1016/j.molstruc.2014.05.028
0022-2860/Ó 2014 Elsevier B.V. All rights reserved.
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2
X. Chen et al. / Journal of Molecular Structure xxx (2014) xxx–xxx
previous literature, the construction of MOFs mainly depends on
the nature of the organic ligands (spacers) and metal ions (nodes)
[14–15]. Especially organic ligands play crucial roles for the
designed synthesis of some interesting coordination networks,
such as the donating type, the flexibility, and the geometry of the
organic ligands [16–18]. Among various organic ligands, many
polycarboxylate ligands are often employed as bridging ligands
to construct metal–organic frameworks, due to their extension
ability both in covalent bonding and in supramolecular interac-
tions (H-bonding and aromatic stacking) [19–26]. However, in con-
trast to symmetrical carboxylates, the use of unsymmetrical
carboxylate ligands has been reported infrequently [27–31].
3-(4-Benzoic) propionic acid (H2cpa) is such an asymmetric
dicarboxylate, having two different substituent groups attached
to the benzene ring. One is carboxyl, the other is propionyloxy.
So H2cpa ligand has four potential donor atoms, which can induce
rich coordination modes and many interesting structure with
higher dimensions. Furthermore, they have two carboxyl groups
that may be completely or partially deprotonated, and can provide
hydrogen bond donors and acceptors. Meanwhile, it possesses both
rigidity and flexibility, since propionyloxy can freely rotate around
the C–C bonds according to the small change in the coordination
environment in order to minimize steric hindrance. However, to
the best of our knowledge, compound based on H2cpa ligand have
been documented very little to date [32–35]. In this regard, Wang
et al. have reported a serial of divalent metal (cobalt, nickel, znic,
cadmium and copper) frameworks with H2cpa ligand. Due to
semirigid carboxyphenylpropionate of this ligand, it enables the
formation of microporous and interpenetrating metal–organic
frameworks [33–34]. More recently, Ag(I) and Co(II) carbo-
xyphosphonates with a 1D chain structure have also been obtained
by our group [35]. On the other hand, d10 metal (particularly of ZnII
and CdII) complexes have attracted extensive interest in recent
years in that they not only exhibit appealing structures but also
possess photoluminescent properties [36]. But there has been only
three reported polymers constructed by CdII metal and cpa ligand
hitherto. Furthermore, the combination of N-donor ligands with
polycarboxylate is a good choice for the construction of novel
topology and networks. The introduction of N-containing auxiliary
ligands, such as phen, bpy into the [M-cpa] system may lead to
new structural evolution and fine-tuning the structural motif of
these metal–organic hybrid compounds. In the meanwhile, the
bulky aromatic rings on phen help to direct the spatial arrange-
analyses (TGA) were performed in a nitrogen atmosphere with a
heating rate of 10 °C minꢂ1 with a NETZSCHSTA 449C thermogravi-
metric analyzer. The X-ray powder diffraction pattern was
recorded with a Rigaku D/Max 3III diffractometer.
Synthesis
Synthesis of [Cd(cpa)(phen)]n (1)
A
mixture of Cd(NO3)2ꢁ4H2O (0.0308 g, 0.1 mmol), H2cpa
(0.0194 g, 0.1 mmol) and phen (0.0198 g, 0.1 mmol) in molar ratio
1:1:1 and water (10 mL) was stirred for 30 min in air, then sealed
in a 25 mL Telfon-lined stainless steel container, which was heated
to 160 °C for 96 h. After cooling to room temperature at a rate of
5 °C hꢂ1, the colorless block crystals were obtained in ca. 65% yield
(based on Cd). Anal. (%) calcd for C22H13N2O4Cd: C, 54.85; H, 2.72;
N, 5.81; O, 13.28%. Found: C, 54.88; H, 2.74; N, 5.77; O, 13.24. IR
(KBr cmꢂ1): 3419 m, 3057 w, 2930 w, 1589 s, 1541 s, 1396 s,
1230 w, 1101 w, 848 m, 777 w, 729 m, 638 w.
Synthesis of {[Cd2(cpa)2(bpy)1.5]ꢁ0.5H2O}n (2)
An identical procedure with 1 was followed to prepare 3 except
that phen was changed to bpy (0.0182 g, 0.1 mmol). The colorless
block crystals were obtained in ca. 58% yield (based on Cd). Anal.
(%) calcd for C35H29N3O8.5Cd2: C, 49.31; H, 3.43; N, 4.93; O, 15.95.
Found: C, 49.34; H, 3.40; N, 4.89; O, 15.94%. IR (KBr cmꢂ1): 3440
m, 3051 w, 1605 s, 1547 s, 1407 s, 1211 m, 1069 m, 1007 w, 874
w, 813 m, 627 m.
Single crystal X-ray crystallography
Intensity data were collected on a Bruker Smart APEX II CCD
diffractometer with graphite-monochromated MoK
a radiation
(k = 0.71073 Å) at room temperature. Empirical absorption correc-
tions were applied using the SADABS program. The structures were
solved by direct methods and refined by the full-matrix least-
squares based on F2 using SHELXTL-97 program [37]. All non-
hydrogen atoms were refined anisotropically and the hydrogen
atoms of organic ligands were generated geometrically. Crystal
data and structural refinement parameters for
1 and 2 are
Table 1
Crystal data and structure refinement for complexes 1 and 2.
Complex
1
2
ment of the bridging groups and form extensive
pꢁ ꢁ ꢁp interactions
Empirical formula
Formula weight
Crystal system
Space group
a (Å)
C
22H13CdN2O4
C35H29Cd2N3O8.5
852.41
Monoclinic
P21/c
11.7616(8)
20.4431(14)
17.2742(8)
90.00
between coordination layers. However, it is accompanied by even
more uncertain factors. Thus, the prediction of mixed-ligand archi-
tectures is a challenging scientific endeavor. So we have recently
engaged in the research of synthesizing the novel compounds with
H2cpa and N-donor mixed ligands. Fortunately, we have now
isolated two novel compounds, [Cd(cpa)(phen)]n (1) and
{[Cd2(cpa)2(bpy)1.5]ꢁ0.5H2O}n (2). In this paper, we report their syn-
thesis, characterization, crystal structure, and photoluminescence.
481.74
Monoclinic
C2/c
25.811(2)
10.3107(9)
18.5708(15)
90.00
b (Å)
c (Å)
a
(°)
b (°)
130.6910(10)
90.00
3747.4(5)
8
1.708
1.198
127.810(3)
90.00
3281.4(4)
4
2.076
1.355
c
(°)
V (Å3)
Z
D calc Mg m3
Experimental section
l
(mmꢂ1
)
F (000)
1912
1696
Materials and methods
Crystal size (mm)
h Range (°)
k (MoKa) Å
Reflections collected
Unique reflections
Parameters
0.24 ꢃ 0.15 ꢃ 0.18
2.08–26.41
0.71073
3824
3038
245
0.21 ꢃ 0.13 ꢃ 0.15
2.41–28.40
0.71073
7971
7272
418
1.062
0.0482, 0.1170
0.0524, 0.1196
1.925 and ꢂ1.054
All chemicals and reagents were used as received from com-
mercial sources without further purification. All reactions were
carried out under hydrothermal conditions. Elemental analyses
(C, H, N) were determined with a Elementar Vario EL III elemental
analyzer; IR spectra were recorded as KBr pellets on a Bruker EQUI-
NOX55 spectrophotometer in the 4000–400 cmꢂ1 region. Fluores-
cence spectra were performed on a Hitachi F-4500 fluorescence
spectrophotometer at room temperature. Thermogravimetric
S on F2
R1, wR2 [I > 2
1.047
a
r(I)]
0.0393, 0.0973
0.0506, 0.1041
0.604 and ꢂ0.479
a
R1, wR2 (all data)
D
q
max and min (e Åꢂ3
)
R1 = R||Fo|–|Fc||/R|Fo|, wR2 = [R R .
w(Fo2–F2c)2/ w(F2o)2]1/2
a
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X. Chen et al. / Journal of Molecular Structure xxx (2014) xxx–xxx
3
Fig. 1. (a) The coordination environment of the CdII ion in 1, all H atoms are omitted for clarity; (b) View of the 1D double zigzag chain. (c) View of the 2D bilayer
supramolecular network. (d) View of the 3D supramolecular structure by hydrogen bonds and C–Hꢁ ꢁ ꢁ interaction.
p
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4
X. Chen et al. / Journal of Molecular Structure xxx (2014) xxx–xxx
summarized in Table 1. Selected bond distances and bond angles
are listed in Table S1.
Results and discussion
Crystal structure of [Cd(cpa)(phen)]n (1)
X-ray single-crystal diffraction analysis reveals that compound
1 crystallizes in the monoclinic, C2/c space group with an asym-
metric unit that contains one crystallogra-phically unique CdII
ion, one (cpa)2ꢂ ion and one phen ligand. As shown in Fig. 1a,
the environment of the coordination sphere of the CdII ion features
a distorted octahedron geometry consisting of two oxygen atoms
of chelating bidentate carboxylic groups, two oxygen atoms of
monodentate bridging carboxylic groups, and two nitrogen atoms
deriving from one chelating phen ligand. The Cd(II) atom exhibits
a distorted octahedron geometry. Four atoms (N2, O2A, O3B and
O4B) comprise the equatorial plane, while another two atoms
(O1, N1) occupy in the axial position. The bond lengths of CdꢂO
are comparable to the published ones [38], varying between
2.208(3) and 2.366(4) Å.
Scheme 1. The coordination modes of cpa2ꢂ ligand in 1 and 2.
geometry. O3C, O6A and O7 form the top plane of the trigonal
prism while the bottom plane is completed by O4C, O8 and N2A.
The Cd2–O bond distances range from 2.260(3) to 2.410(4) Å,
which are also comparable to those in the literatures [40].
It worth to noting that the (cpa)2ꢂ ligand adopts a pentadentate
chelating and bridging coordination mode; one carboxylate group
chelate with one CdII ion bidentately, whereas the other carboxyl-
ate group adopts a chelate/bridge tridentate coordination mode
connecting two CdII ions (Scheme 1b), which is different from
those of compound 1. To our knowledge, up to now this coordina-
tion mode has never been observed in other metal–cpa complexes
[32–35]. Based on these connection modes, a pair of Cd1 centers
and two Cd2 centers are bridged by carboxylate groups of
eight (cpa)2ꢂ ligands to form a tetranuclear cadmium cluster
unit{Cd4O18N4}. Four cadmium atoms (Cd1A, Cd2A, Cd1 and Cd2)
do not locate in a line with the distances of Cd1ꢁ ꢁ ꢁCd1A (3.799 Å)
and Cd1ꢁ ꢁ ꢁCd2 (4.346 Å), respectively, in which the polyhedrons
of Cd1 and Cd1A centers are in an edge sharing, while the polyhe-
drons of Cd1 and Cd2 centers are in vertex-sharing mode (Fig. 2b).
If the tetranuclear cadmium unit is considered as a secondary
building unit (SBU), the adjacent tetranuclear cadmium units are
interlinked by eight (cpa)2ꢂ ligands into 2D network running along
four directions. Interestingly, the adjacent Cd1, Cd2 centers are
connected by four carboxylate groups of two (cpa)2ꢂ ligands along
Interestingly, the (cpa)2ꢂ ligand assume bridging bidentate/
chelating bidentate fashion (Scheme 1a). One carboxylate group
chelates with one CdII ion bidentately, while the other adopts a
bidentate coordination mode and connect with two CdII ions. Cd1
and the symmetry formed Cd2+ ion are connected by two bridging
bidentate carboxylic groups from two independent (cpa)2ꢂ ligands
to generate a binuclear unit and form a {Cd2O4C2N2} 8-numbered
circle. (based on the distances of Cdꢁ ꢁ ꢁCd 4.008 Å). The adjacent
dimeric Cd2 units are linked by four carboxylate groups of four
(cpa)2ꢂ ligands generating a double zigzag chain (Fig. 1b), thus
forming a novel {Cd2O4C16} 22-membered ring containing a type
of pore with size of ca. 9.536 Å ꢃ 10.459 Å based on the distances
of Cd1ꢁ ꢁ ꢁCd1 and C8ꢁ ꢁ ꢁC8. The double zigzag chain is decorated
with two phen ligands at the same side. The neighboring double
zigzag chains recognize each other to generate a 2D bilayer supra-
molecular network via the
to-edge distances of 3.307 Å between phenyl rings of adjacent
p p stacking interactions with edge-
ꢁ ꢁ ꢁ
phen ligands, hydrogen bonding interaction [C19–H19ꢁ ꢁ ꢁO4] =
2.623 Å] and C–Hꢁ ꢁ ꢁ
p
interaction (C15–H15ꢁ ꢁ ꢁ
p = 2.764 Å)
(Fig. 1c). Then 2D bilayer supramolecular network are found to
two different alternating directions to form a novel {Cd4O6C16}
26-membered ring containing a type of pore (I) with size of
ca.10.911 Å ꢃ 12.103 Å based on the distances of Cd2ꢁ ꢁ ꢁCd2 and
O6ꢁ ꢁ ꢁO6. While the adjacent Cd2 centers are connected by four car-
boxylate groups of two (cpa)2ꢂ ligands along two different alter-
nating directions to form a novel {Cd2O4C16} 22-membered ring.
Then the 2D layer is further interconnected by bpy ligands to form
a 3D framework (Fig. 2c). Further topological analysis suggests the
pack on top of each other, displaying a slipped geometry stabilized
through C–Hꢁ ꢁ ꢁO hydrogen bonds interactions and C–Hꢁ ꢁ ꢁ
p inter-
action [C4–H4ꢁ ꢁ ꢁ
p
= 2.832 Å], which is developed into 3D supra-
molecular structure (Fig. 1d).
Crystal structure of {[Cd2(cpa)2(bpy)1.5]ꢁ0.5H2O}n (2)
a
-Po framework for it, if we define the tetranuclear Cd4 unit as a
As compared to polymer 1, the phen ligand is replaced by bpy,
and the resulting structure is a 2-fold interpenetrated 3D -Po
node. More remarkably, due to the flexible conformation of
cpa2ꢂ ligands, the {Cd2O4C16} 22-membered ring pass through
{Cd4O6C16} 26-membered ring from different networks to inter-
locked with each other, thus directly leading to the formation of
a 2-fold interpenetrated 3D architecture (Fig. 2d).
a
structural topology. The asymmetric unit of 2 has two crystallo-
graphically independent CdII ions, two (cpa)2ꢂ anions, one and half
bpy ligands and half lattice water molecules (Fig. 2a). Cd1 center is
seven-coordinated and exhibits a distorted pentagonal bipyramid
environment supplied by two nitrogen atoms [Cd1–N1 =
2.336(4), Cd1–N3 = 2.326(4) Å] from two bridging bpy ligands, five
oxygen atoms of three separated (cpa)2ꢂ ligands, in which five oxy-
gen atoms O1, O2, O6, O5 and O1A in the equatorial plane and two
nitrogen atoms N1 and N3 in the axial positions. The Cd1–O bond
lengths are in the range of 2.275(4)–2.456(4) Å, in good agreement
with previous studies [39]. Whereas the Cd2 center ligated by two
pairs of chelating oxygen atoms from two different (cpa)2ꢂ ligands,
one bridging oxygen atom from one carboxylate group of one
(cpa)2ꢂ ligand and one nitrogen atom [Cd2–N2 = 2.322(4) Å] from
one bridging bpy ligand to complete a distorted trigonal prism
Thermal properties and XRPD measurement of complexes 1–2
To identify the thermal stabilities of these complexes, the TGA
measurements were carried out (Fig. S1). The TGA result indicates
that 1 does not decompose up to 302 °C. Then followed by a con-
tinuously two-step weight loss of 76.35% from 302 °C to 610 °C
(calcd. 76.26%), corresponding to the loss of (cpa)2ꢂ and phen
ligands. The remaining weight of 27.11% is cadmium oxide that
is in agreement with the calculated value of 26.65%. Compound 2
first lost its lattice water molecule below 135 °C, the weight loss
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X. Chen et al. / Journal of Molecular Structure xxx (2014) xxx–xxx
5
Fig. 2. (a) The coordination environment of the CdII ion in 2, all H atoms are omitted for clarity. (b) Perspective view of the tranuclear cadmium unit along a-axis. (c) View of
the 3D network along a-axis. (d) View of the 2-fold interpenetrating 3D -Po net of 2 along a-axis.
a
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X. Chen et al. / Journal of Molecular Structure xxx (2014) xxx–xxx
architectures through the control of auxiliary ligands. Complexes
1–2 show photoluminescence at room temperature.
Acknowledgement
This work was supported by the National Natural Science
Foundation of China (Grant No. 21101133).
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.molstruc.2014.05.
028.
Crystallographic data for the structural analyses have been
deposited with the Cambridge Crystallographic Data Centre, CCDC
no. 958163 (1) and 958164 (2). Copies of the data can be obtained
free of charge on application to CCDC, 12 Union Road, Cambridge
CB2 1EZ, UK (fax: (internat.) +44 1223 336 033; e-mail: depos-
it@ccdc.cam.ac.uk). Supplementary data associated with this arti-
cle can be found, in the online version, at the website.
Fig. 3. The emission spectra of H2cpa ligand, 1 and 2 in the solid state at room
temperature.
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Taking into account the excellent luminescent properties of d10
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r-donation from the carboxylate
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