Ligand controlled structure of cadmium(II) metal-organic frameworks for fluorescence sensing of Fe3+ ion and nitroaromatic compounds

Article abstract of DOI:10.1016/j.cclet.2018.12.009

Three cadmium(II) metal-organic frameworks (MOFs) based on tetracarboxylate ligands, namely [Cd₂(TTTA)(DMF)₃]·2DMF (1), [Cd₂(TB)(H₂O)₄]·3DMF·H₂O (2) and [Cd(TEB)_0.5]·2DMF·4H₂O (3) have been designed and synthesized. Complex 1 is a 2-dimensional (2D) 3,4-connected network with 3,4L13 topology, complex 2 features a 3-dimensional (3D) 3,4-connected tfa topology with a 2-fold interpenetrating structure and complex 3 has a 3D 4-connected dia topology with a 4-fold interpenetrating structure. Interestingly, 2 exhibits permanent pores and selective adsorption of CO₂ over CH₄. In addition, 2 shows fluorescence sensing of Fe³⁺ ion and rapid detection of nitroaromatic compounds (NACs) through fluorescence quenching.

Full text of DOI:10.1016/j.cclet.2018.12.009

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CCLET 4746 No. of Pages 5
Chinese Chemical Letters xxx (2018) xxx–xxx
Contents lists available at ScienceDirect
Chinese Chemical Letters
journal homepage: www.elsevier.com/locate/cclet
Communication
Ligand controlled structure of cadmium(II) metal-organic frameworks
for fluorescence sensing of Fe3+ ion and nitroaromatic compounds
Xia Wanga,1, Weidong Fana,1, Ming Zhanga, Yizhu Shanga, Yutong Wanga, Di Liub,
a
Hailing Guoc, Fangna Daia, , Daofeng Sun
*
a
School of Materials Science and Engineering, College of Science, China University of Petroleum (East China), Qingdao 266580, China
College of Chemical and Environmental Engineering, Shandong University of Science and Technology, Qingdao 266590, China
State Key Laboratory of Heavy Oil Processing, China University of Petroleum (East China), Qingdao 266580, China
b
c
A R T I C L E I N F O
A B S T R A C T
Article history:
Three cadmium(II) metal-organic frameworks (MOFs) based on tetracarboxylate ligands, namely
Received 18 September 2018
Received in revised form 1 November 2018
Accepted 6 December 2018
Available online xxx
[Cd2(TTTA)(DMF)3]
ꢀ
2DMF (1), [Cd2(TB)(H2O)4]
ꢀ
3DMF
ꢀ
H2O (2) and [Cd(TEB)0.5
]
ꢀ
2DMF 4H2O (3) [H4TTTA
ꢀ
= 2,20,20',20”-(2,20,4,40,6,60-hexamethylbiphenyl-3,30,5,50-tetrayl)tetrakis(methylene))tetrakis(sulfane-
diyl))tetrabenzoic acid, H4TB = 3,30,5,50-tetra((4-carboxyphenyl)bimesityl, and H4TEB = 3,30,5,50-tetra(4-
ethynylbenzoic acid)bimesityl) have been designed and synthesized. Complex 1 is a 2-dimensional (2D)
3,4-connected network with 3,4L13 topology, complex 2 features a 3-dimensional (3D) 3,4-connected tfa
topology with a 2-fold interpenetrating structure and complex 3 has a 3D 4-connected dia topology with
a 4-fold interpenetrating structure. Interestingly, 2 exhibits permanent pores and selective adsorption of
CO2 over CH4. In addition, 2 shows fluorescence sensing of Fe3+ ion and rapid detection of nitroaromatic
compounds (NACs) through fluorescence quenching.
Keywords:
Metal-organic frameworks
Selective adsorption
Fluorescence sensing
Fe3+ ion
© 2018 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences.
Published by Elsevier B.V. All rights reserved.
Nitroaromatic compounds
Metal–organic frameworks (MOFs) have emerged as useful
materials in gas separation, fluorescence, catalysis, magnetism,
drug delivery, and so on [1–4]. MOFs can be readily self-assembled
from metal ions/clusters and organic linkers. To achieve predict-
able structures with new properties, the matching of pre-designed
ligand’s geometry and metal ion’s coordination mode are needed
[5–8]. The past researches have found that the construction of
MOFs by rigid or flexible carboxylate ligands have different
advantages [9–11]. For example, rigid carboxylate ligands are easy
to construct robust networks with permanent pores, flexible ones
can change their conformations to meet the coordination
requirement of metal ions and build more unexpected topologies
[12,13].
As known, heavy metal ions and nitroaromatic compounds
(NACs) produced by industrial production have done great damage
to the living environment of human beings. However, effective
detection of those pollution is a challenging work. There are some
reports about detecting the presence of metal ions by different
methods. For example, Zhang and Lin’s group reported supramo-
supramolecular organic frameworks, which can detect ions and
molecules through different mechanisms [14–18]. Zeng’s group
reported electrochemical sensing strategy to realize the highly
sensitive and selective detection of Pb2+ [19,20]. Fe3+ ions are
important for most organisms, Therefore, detection of Fe3+ ions for
life science is of great significance [21–23]. It is reported that a
novel supramolecular organoꢁgel (P5N-OG) showing ultrasensitive
response for Fe3+ and H2PO4 through the competition between
cation–p and exo-wall p–p interactions [24]. Among these
different kinds of sensing materials, the selective sensing of
supramolecular compounds benefitting from the dynamic and
reversible nature of supramolecular interactions, including
C@Hꢀꢀꢀp pꢀꢀꢀp, cationꢀꢀꢀp, etc. [16]. The sensitive electrochemical
,
lead ion sensors have good charge-transport capacity, low
detection limit and might exist complexity of design. MOFs
combine the advantages of the highly regular porous structures,
various luminescence mechanisms and diversified host-guest
interactions, which make they reflect simple and quick detection,
high selectivity, easy design and operation.
lecular
compounds
including
supramolecular
polymer,
Recently, by using chromophores organic ligands or fluorescent
metal ions, fluorescence MOFs have been reported [25,26]. Sun’s
group have reported serials of 3D porous MOFs, which have
showed rapid and selective detection of metal ions and NACs
[27,28]. It is insufficient to be limited to metal ions for constructing
fluorescent MOFs, the role of ligand is more essential. Based on this
* Corresponding author.
E-mail address: fndai@upc.edu.cn (F. Dai).
1
These authors contributed equally to this work.
https://doi.org/10.1016/j.cclet.2018.12.009
1001-8417/© 2018 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences. Published by Elsevier B.V. All rights reserved.
Please cite this article in press as: X. Wang, et al., Ligand controlled structure of cadmium(II) metal-organic frameworks for fluorescence
sensing of Fe3+ ion and nitroaromatic compounds, Chin. Chem. Lett. (2018), https://doi.org/10.1016/j.cclet.2018.12.009

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CCLET 4746 No. of Pages 5
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X. Wang et al. / Chinese Chemical Letters xxx (2018) xxx–xxx
consideration, a new flexible tetracarboxylate ligand (H4TTTA
= 2,20,20',20”-(2,20,4,40,6,60-hexamethylbiphenyl-3,30,5,50-tetrayl)
tetrakis(methylene)tetrakis(sulfanediyl)tetrabenzoic acid) and
two new rigid tetracarboxylate ligands (H4TB = 3,30,5,50-tetra((4-
carboxyphenyl)bimesityl, and H4TEB = 3,30,5,50-tetra(4-ethynyl-
benzoic acid)bimesityl) have been designed and synthesized as
the assembly organic ligands (Fig. 1). The three new ligands have
some specific characteristics: (i) Four carboxyl groups can bridge
metal ions with multiple coordination modes; (ii) The steric
hindrance of methyl will trigger rearrangement of adjacent
benzene rings [29–31]; (iii) The flexibility conformational of
ligands may offer more possibility to construct new topology
networks; (iv) The polycyclic aromatic conjugated ligands will
induce fluorescent properties of MOFs. Herein, three new
complexes [Cd2(TTTA)(DMF)3]
3DMF H2O (2) and [Cd(TEB)0.5
ꢀ
2DMF (1), [Cd2(TB)(H2O)4]
ꢀ
ꢀ
]
ꢀ
2DMF 4H2O (3) are constructed
ꢀ
by the above three ligands and Cd2+ ion. Complex 1 is a 2D 3,4-
connected network with 3,4L13 topology. Complex 2 is a 3D
framework with a 2-fold interpenetrating tfa topology, which
exhibits selective adsorption of CO2 over CH4 and displays
interesting fluorescent sensing properties for Fe3+ ion and rapid
detection of NACs. Complex 3 has a 3D 4-fold interpenetrating 4-
connected dia topology.
Fig. 2. (a) Coordination environment of the dimeric Cd2+ ions in 1; (b) Configuration
and coordination modes of TTTA4ꢁ ligand; (c) and (d) The 2D layer structure and 2D
network with 3,4L13 topology for 1.
By using rigid ligand of H4TB and H4TEB, complexes of 2
H4TTTA, H4TB and H4TEB are synthesized by Suzuki and
Sonogashira coupling reaction followed by hydrolysis with dilute
HCl (Scheme S1 in Supporting information). Detailed synthesizing
procedures of the three complexes are depicted in Supporting
information.
{[Cd2(TB)(H2O)4]
ꢀ
3DMF
ꢀ
H2O} and
3
{[Cd(TEB)0.5
]
ꢀ
2DMF 4H2O}
ꢀ
were obtained. Single crystal X-ray diffraction reveals that 2
crystallizes in the triclinic p-1 space group. The asymmetrical unit
of 2 contains two Cd(II) ions, one fully deprotonated TB4ꢁ ligand
and four H2O molecules. Cd1 is six-coordinated by three oxygen
atoms from three different TB4ꢁ ligands and three coordinated
water molecules; Cd2 is seven-coordinated by six oxygen atoms
from four different TB4ꢁ ligands, and one coordinated water
molecule (Fig. 3a). All the carboxyl groups of H4TB ligand were
deprotonated during the reaction: one adopt bidentate mode to
link a Cd2+ ion, one adopt chelating mode to connect a Cd2+ ion, and
the remaining two adopt mono-dentate mode to connect four Cd2+
ions (Fig. 3b). Then, the dimeric SBUs are connected by TB4ꢁ
ligands to generate an open framework with 3D channel which
similar pore features in a, b, c axis with the size of 16.0 ꢃ18.7 Å2
(Fig. S2 in Supporting information). Due to the existence of large
channel in the single net of 2, two such nets interpenetrate
mutually to provide a 3D structure with the channels of 7.5 ꢃ7.8 Å2
(Fig. 3c). Topologically, the H4TB ligands can be simplified as a 4-
connected linker and the dimeric SBU is taken as a 4-connected
node. Then, the 3D structure can be classified as a classical tfa
topology (Fig. 3d).
Complex 1 is constructed by using flexible H4TTTA ligand.
Single crystal X-ray diffraction analysis reveals that 1 crystallizes in
the triclinic p-1 space group. The asymmetrical unit of 1 contains
two Cd(II) ions, one fully deprotonated TTTA4ꢁ ligand and four DMF
molecules. Cd1 is bonded to five carboxylate oxygen atoms from
three different TTTA4ꢁ ligands and the remaining two sites are
occupied by oxygen atoms from two different coordinated DMF
molecules; Cd2 bridging six oxygen atoms from four different
TTTA4ꢁ ligands and the residue one oxygen atom from
a
coordinated DMF molecule (Fig. 2a). H4TTTA ligand is deproto-
nated during the self-assembly process, the average dihedral angle
between the side benzene ring and the adjacent central benzene
ring is 75.4ꢂ (Fig. 2b). There are two kinds of rectangular
macrocycles in this network: one is connected by two half of
TTTA4- ligands and two SBUs with the dimensions of 9.2 ꢃ 9.4 Å2
(Fig. S1a in Supporting information) and the other is made up of
two benzene rings of the rigid center and two SBUs with the
dimensions of 15.0 ꢃ 7.9 Å2 (Fig. S1b in Supporting information).
The metal-macrocycles are further connected one another through
the expansion of ligands to generate a 2D layer (Fig. 2c).
Topologically, by treating the dimeric Cd SBUs as a 4-connected
nodes and the TTTA4- ligands as double Y-shaped linkers, the 2D
structure can be classified as 3,4L13 topology (Fig. 2d).
Single crystal X-ray diffraction reveals that 3 crystallizes in the
tetragonal I4122 space group. The asymmetrical unit of 3 contains
one independent Cd(II) ion and half fully deprotonated TEB4ꢁ
ligand. Cd2+ is eight-coordinated by eight oxygen atoms from four
different TEB4- ligands (Fig. 4a). One Cd2+ ion is bonded to four
TEB4- ligands affording an extended open framework with window
Fig. 1. The three ligands designed and used in this paper.
Please cite this article in press as: X. Wang, et al., Ligand controlled structure of cadmium(II) metal-organic frameworks for fluorescence
sensing of Fe3+ ion and nitroaromatic compounds, Chin. Chem. Lett. (2018), https://doi.org/10.1016/j.cclet.2018.12.009

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3
Fig. 5. (a) and (b) N2 and H2 sorption isotherms for 2 at 77 K. (c) The CO2/CH4
sorption isotherms for 2 at 273 K. (d) The CO2/CH4 selectivity for 2 at 273 K,
calculated by the IAST method (CO2/CH4: 50/50).
Fig. 3. (a) Coordination environment of Cd2+ ions in 2; (b) Configuration and
coordination modes of organic ligands in 2; (c) The 2-fold interpenetrating 3D
framework; (d) The 3D network with tfa topology for 2.
of 21.7 ꢃ22.1 Å2 along b axis (Fig. 4b), and four frameworks are
interpenetrated with each other to form a rectangular channel
with dimensions of 16.5 ꢃ14.0 Å2 (Fig. 4c). Topologically, the Cd2+
ions and H4TEB ligands can be considered as two 4-connected
nodes, 3 reveals a typically diamondoid framework (Fig. S3 in
Supporting information). The four identical diamondoid networks
interpenetrated to giving rise to a 4-fold interpenetrating dia
topology (Fig. 4d).
As described above, the interpenetration dimensionality
significantly influenced the porosity of the final framework, which
can be further confirmed by gas-uptake measurements [32]. The
N2 sorption isotherms of 2 shows the reversible type I, indicating
its permanent porosity. The BET surface area calculated from N2
isotherm is 165.8 m2/g, with the pore volume of 0.08 cm3/g
(Fig. 5a). The H2 adsorption isotherms at 77 K also exhibits
reversible type I behavior with the largest adsorption amounts of
28.3 cm3/g (Fig. 5b). In addition, the uptake of CO2 is of 18.2 cm3/g
at 273 K, which is much higher than CH4 (5.35 cm3/g at 273 K)
(Fig. 5c). It is obvious that 2 exhibits selective adsorption of CO2
over CH4, although the adsorption amounts are somewhat lower
than those reported results [33–35]. To further investigate the CO2
selectivity, the CO2/ CH4 selectivity at 273 K is calculated by the
ideal adsorbed solution theory (IAST) method. As shown in Fig. 5d,
the selectivity has a downward trend in the range of 0–109 kPa. The
values of selectivity are all above 9.6, which are comparable with
many reported MOFs [36–38]. The selective adsorption may
possibly attribute to the combined effects of the pore feature and
host-guest interactions.
As described above, complex 2 features stable porous 2-fold
interpenetrating network, which can be used as a sensor to identify
metal ions and NACs through fluorescence alternation [39]. In the
measurement, 2.0 mg freshly prepared samples of 2 are ground
and suspended in 3.0 mL DMF solution, to which is dropwise added
different metal ions in DMF (10.0 mmol/L). We experimented three
parallel times and obtained experimental data finally. The
fluorescence properties are recorded and listed in Fig. 6a (excita-
tion at 330 nm). Specifically, the fluorescence intensity of 2 at
418 nm decreases slightly when Li+, Ag+, Cd2+, Zn2+, Mg2+, Cu2+ Co2+
,
Ni2+, Ca2+, Pb2+ ions are added, but decreases significantly when Fe3
+ ions are added (Fig. 6b), the quenching efficiency is higher than
UPC-19(Zn-MOF) and UPC-20(Zn-MOF) [40]. The color change
after adding Fe3+ is shown in Fig. S13 (Supporting information).
Furthermore, the detection limit (LOD) is calculated based on the
equation: LOD = 3s/k, s value is calculated from 11 groups of blank
test and K value is based on the linear regression. The calculated
detection limit, LOD, is about 0.011 mmol/L for Fe3+, which further
confirm the high sensitivity of 2 on the fluorescent detection of Fe3
+. Room-temperature emission spectra for complex 2 and free
H4TEB ligand are presented in Fig. S5 (Supporting information).
The luminescence was generated from H4TEB ligand. In Fe3+
detection, in the view of crystal structures, 2 is microporous and
metal ions can enter into the channels, coordination environment
of Cd2+ ions in complex 2 are uncoordinated and provide binding
Fig. 4. (a) Configuration and coordination modes of organic ligands in 3; (b) The 1D
channel of 3 along b axis; (c) The 4-fold interpenetrating 3D framework; (d) The 3D
network with dia topology for 3.
Fig. 6. (a) Fluorescence intensity of different metal ions in the DMF-emulsion of 2.
(b) Effect on the emission spectra of 2 upon incremental addition of Fe3+. Inert:
Stern-Volmer plot of I0/I versus the Fe3+ concentration in DMF.
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sensing of Fe3+ ion and nitroaromatic compounds, Chin. Chem. Lett. (2018), https://doi.org/10.1016/j.cclet.2018.12.009

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sites for metal ions and the relatively small ionic radius of Fe3+
results in easier combination with complex 2 than other metal ion.
With the Fe3+ added, complex 2 and Fe3+ is expected to perturb the
luminescence of ligand and leads to fluorescence quenching at last.
On the other hand, aromatic compounds are potentially
neurotoxic and carcinogenic in nature. Rapid and selective
detection of benzene compounds, especially for nitroaromatic
compounds (NACs) is important [41]. Hence, different benzene
compounds in DMF (10 mmol/L) are gradually added into the DMF-
suspensions of complex 2 to determine the sensing properties.1,3-
Dinitrobenzene (1,3-DNB), 2,4-dinitrotoluene (2,4-DNT), 1,4-dini-
trobenzene (1,4-DNB), 4-nitrophenol (4-NP), 4-nitroaniline (4-
NA), 1-methyl-4-nitrobenzene (1-M-4-NB), (4-nitrophenyl)-hy-
drazine (4-NPH) were selected as the investigated reagents. The
results show that all NACs can weaken the fluorescence intensity of
2 in suspension, but the quenching percentage show great
differences (Fig. 7a). Meanwhile, the fluorescence intensity has
relative little change when adding the 1,3-DNB, 2,4-DNT, 1,4-DNB
and 1-M-4-NB solution, but the fluorescence intensity produce
significant quenching when introducing the 4-NA, 4-NP and 4-NPH
solution (Figs. 7b–d). At the same time, the color change after
adding NACs is shown in Fig. S14 (Supporting information).
In order to further compare the efficiency of the sensor, the
fluorescence quenching efficiency was calculated using the Stern-
Volmer (SV) equation [42], (I0/I) = Ksv [A] + 1, where I0 is the initial
fluorescence intensity, I is the fluorescence intensity after adding
the analyte, [A] is the molar concentration of analyte [43], and Ksv
is the quenching coefficient. According to the equation, the
quenching efficiency for the NACs in DMF was found to be the
following order: 4-NPH>4-NA>4-NP>>1,4-DNB>2,4-DNT>1-M-4-
NB>1,3-DNB > NB (Fig. S16 in Supporting information). The
quenching efficiency of 4-NPH is lower than Cd-PDA [44], but
appeared. The highly acidic NH2NH-, NH2- and OH- group can
interact strongly with the fluorophore via electrostatic interac-
tions, and the quenching effect can be maintained over a long range
due to the energy transfer mechanism. However, because the
electron-withdrawing capacity in these functional groups are
different: -NHNH2>-NH2>ꢁOH, the quenching efficiency for the
NACs in DMF was found to be the following order: 4-NPH>4-NA>4-
NP. Thus, the comprehensive reasonwith electrostatic interactions,
favorable electrons, and the energy transfer mechanism results in a
higher quenching effect.
In conclusion, through design three new methyl-functionality
flexible and rigid ligands, three new Cd-based MOFs have been
synthesized. Complex 1 is a 2D layer with two rectangular
macrocycles and 3 has a 3D 4-connected dia network with a 4-fold
interpenetrating, whereas 2 exhibits a 3D 3, 4-connected tfa
network with a 2-fold interpenetrating. Complex 2 is porous and
exhibits selective adsorption of CO2 over CH4, indicating its
potential applications in selective gas separation. Besides that,
complex 2 can sense Fe3+ ions in DMF-suspension with a higher
selectivity. Meanwhile, 2 shows strong sensing of 4-NPH, 4-NA and
4-NP through fluorescence quenching.
Acknowledgments
This work was supported by the National Natural Science
Foundation of China (NSFC, No. 21771191), the Shandong Natural
Science Fund (No. ZR2017QB012), the Applied Basic Research
Projects of Qingdao (No.16-5-1-95-jch), the Fundamental Research
Funds for the Central Universities (Nos.16CX05015A, 18CX06003A,
YCX2018071), and the Foundation of State Key Laboratory of
Structural Chemistry (No. 20160006).
higher than [Pr(LOMe)(H2O)4]
ꢀ
2.5DMA
ꢀ
3H2O [45]. The Stern-
Appendix A. Supplementary data
Volmer plots are typically linear at low concentrations for 4-
NPH, 4-NA and 4-NP. The calculated Ksv are 1.68 ꢃ 105, 3.07 ꢃ104
and 2.05 ꢃ104 L/mol (Fig. 7), and the calculated LOD are about
0.004, 0.025, and 0.027 mmol/L, respectively. The results indicate a
higher selectivity for 4-NPH, 4-NA and 4-NP, which is possible due
to the presence of the NHNH2-, NH2- and OH- group. In NACs
detection, due to the strong electron-withdrawing of nitrogen
atoms, the competition of absorption of the light source energy and
the electronic interaction between the NACs and H4TEB ligands
Supplementarymaterialrelatedtothisarticlecanbefound, inthe
online version, at doi:https://doi.org/10.1016/j.cclet.2018.12.009.
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Fig. 7. (a) Percentage of fluorescence quenching, obtained for introducing different
NACs into the DMF-emulsion of 2. (b, c and d) Effect on the emission spectra of 2
upon incremental addition of a 4-NPH, 4-NA and 4-NP DMF solution (10 mmol/L).
Insert: Stern–Volmer plot of I0/I versus the 4-NPH (b), 4-NA (c) and 4-NP (d).
Please cite this article in press as: X. Wang, et al., Ligand controlled structure of cadmium(II) metal-organic frameworks for fluorescence
sensing of Fe3+ ion and nitroaromatic compounds, Chin. Chem. Lett. (2018), https://doi.org/10.1016/j.cclet.2018.12.009

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Please cite this article in press as: X. Wang, et al., Ligand controlled structure of cadmium(II) metal-organic frameworks for fluorescence
sensing of Fe3+ ion and nitroaromatic compounds, Chin. Chem. Lett. (2018), https://doi.org/10.1016/j.cclet.2018.12.009