A Zn(II)/anthracene coordination polymer showing highly efficient photocatalytic Cr(VI) reduction in aqueous solution

Article abstract of DOI:10.1016/j.inoche.2019.01.006

Reaction of Zn(OAc)₂·2H₂O with the visible light responsive organic ligand 9,10-bis(4′ -pyridylethynyl)-anthracene (BPEA) gave rise to a Zn(II)/anthracene coordination polymer [Zn(OAc)₂(BPEA)]_n. Compound 1 has a one-dimensional (1D) chain structure in which the binuclear {Zn₂(OAc)₂} cores are linked by the BPEA ligands using the pyridyl N atoms to give to the 1D ladder-like network. The as-prepared compound 1 shows broad-range visible light absorption and good water stability, which are highly demandable for its application in the photocatalysis. The photocatalytic property of 1 were investigated by reduction of Cr(VI) to Cr(III) in aqueous solution under visible light, which reveals its high efficiency and stability in this photocatalytic process.

Full text of DOI:10.1016/j.inoche.2019.01.006

InorganicChemistryCommunications101(2019)52–56
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Inorganic Chemistry Communications
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Short communication
A Zn(II)/anthracene coordination polymer showing highly efficient
photocatalytic Cr(VI) reduction in aqueous solution
⁎
⁎
Jing Lia, Wei Goub, Yan Zhouc, Zhen-Fang Lind, , Fei Penge,
a School of Chemical Engineering, Xi'an University, Xi'an, Shaanxi, China
b Department of Neurology, The Seventh People's Hospital of Chengdu, Chengdu, Sichuan, China
c 2nd TB Endemic Area, The Public Health Clinical Center of Chengdu, Chengdu, Sichuan, China
d Department of Neurology, Sichuan Provincial Rehabilitation Hospital Affiliated with Chongqing medical University & Sichuan Bayi Rehabilitation Center, Chengdu,
Sichuan, China
e Department of Pharmacy, People's Hospital of Changshou Chongqing, Chongqing, China
G R A P H I C A L A B S T R A C T
Reaction of Zn(OAc)2·2H2O with the visible light responsive organic ligand 9,10-bis(4′ -pyridylethynyl)-anthracene (BPEA) gave rise to a Zn(II)/anthracene co-
ordination polymer with a one-dimensional (1D) chain structure. The as-prepared compound shows broad-range visible light absorption and good water stability, and
could be applied as the photocatalyst with high efficiency and stability.
A R T I C L E I N F O
A B S T R A C T
Keywords:
Reaction of Zn(OAc)2·2H2O with the visible light responsive organic ligand 9,10-bis(4′ -pyridylethynyl)-an-
thracene (BPEA) gave rise to a Zn(II)/anthracene coordination polymer [Zn(OAc)2(BPEA)]n. Compound 1 has a
one-dimensional (1D) chain structure in which the binuclear {Zn2(OAc)2} cores are linked by the BPEA ligands
using the pyridyl N atoms to give to the 1D ladder-like network. The as-prepared compound 1 shows broad-range
visible light absorption and good water stability, which are highly demandable for its application in the pho-
tocatalysis. The photocatalytic property of 1 were investigated by reduction of Cr(VI) to Cr(III) in aqueous
solution under visible light, which reveals its high efficiency and stability in this photocatalytic process.
Coordination polymer
Anthracene-based ligand
Photocatalysis
Cr(VI) reduction
Nowadays, there is growing concern about environmental protec-
tion and human health because more and more areas of the earth are
facing serious water pollution [1]. Among all the pollutants, heavy
metals effluents from industrial discharge has awakened the public
concern over the last decade because they are usually highly toxic
which are not easy to be degraded biologically directly [2]. For in-
stance, inorganic hexavalent chromium Cr(VI) is with high levels of
toxicity and nonbiodegradable, and it could also cause DNA damage
⁎ Corresponding authors.
E-mail addresses: zhenfang_lin666@yeah.net (Z.-F. Lin), fei_peng666@yeah.net (F. Peng).
https://doi.org/10.1016/j.inoche.2019.01.006
Received 8 December 2018; Received in revised form 3 January 2019; Accepted 4 January 2019
Availableonline11January2019
1387-7003/©2019ElsevierB.V.Allrightsreserved.

J. Li et al.
InorganicChemistryCommunications101(2019)52–56
Fig. 1. (a) The coordination environment of the Zn(II) ion and the {Zn2(AcO)4} unit; (b) The 1D chain-like network of 1 showing the π-π interactions.
and cancers. In comparison, Cr(III) is nontoxic and served as an es-
sential trace metal for human health [3]. In this case, the reduction of
Cr(VI) to Cr(III) is an effective method to reduce the toxicity of Cr(VI) to
the environment and human beings. Reported studies categorize the
reduction into three types: chemical reduction, microbial reduction and
photocatalytic reduction [4–6]. Among them, the photocatalytic re-
duction under visible light is green and cost-effective for the waste-
water treatment, due to the abundant visible light (50%) in the solar
energy [7].
(III) in aqueous solution under visible light, which reveals its high ef-
ficiency and stability in this photocatalytic process.
Single-crystal X-ray diffraction measurement manifests that com-
plex 1 belongs to the triclinic space group P-1 and reveals a 1D ladder-
like structure. The molecular unit of 1 contains one Zn(II) ion, one
BPEA ligand and two coordinated AcO− groups. The Zn(II) ion is six-
coordinated with two N atoms from two BPEA ligands and four O atoms
from three different AcO− groups, forming a distorted octahedral
geometry (Fig. 1a). The Zn(II)-N bond distances are in the range of
2.174(9) to 2.181(9) Å and the Zn(II)-O bond distances are in the range
of 2.014(8) to 2.194 (1) Å. The adjacent Zn(II) ions are connected with
each other via the AcO− groups to afford the {Zn2(AcO)4} secondary
building unit, which is further connected by the BPEA ligands to afford
the 1D ladder-like network. The 1D chains are further stabilized via the
π-π interactions between the phenyl rings of the parallel BPEA ligands
(Fig. 1b). Due to the tightly packed of the 1D chains, the unit cell of 1
shows no solvent accessible void as calculated via the software PLATON
[24].
As an emerging class of crystalline hybrid material, coordination
polymers (CPs) have undergone a booming development in the last ten
years because they could be applied in various fields, including fluor-
escent materials and heterogeneous catalysis [8–17]. Some CPs show
their semiconductors behavior under irradiation, implying that they are
potentially useful as photocatalysts [18,19]. However, most CPs-based
photocatalyst such as ZIF-8, MOF-5 and UiO-67 can merely harvest UV
light owing to the large band gap and the narrow-range adsorption
which could not adsorb the light from the visible region [20,21]. To
tackle this problem, some strategies such as linker decoration, dye
sensing and combination with other semiconductor were adopted to
reduce the band gap so as to enhance the solar light utilization [20].
Among them, a feasible way to improve the photocatalytic activity is to
introduce the visible light responsive organic ligand. For instance, Li
et al. successfully achieved in the synthesis of a visible-light responsive
CP through substitution of benzenedicarboxylate (BDC) in UV-re-
sponsive MIL-125(Ti) by visible-light-active 2-aminoterephthalate
(H2ATA) [22]; Xing et al. have prepared a visible-light responsive CP
based on a visible-light harvesting pillar [23]. Bearing these in mind, in
The phase purity of the as-prepared complex 1 has been confirmed
via the PXRD measurement. As shown in the Fig. S3, the well-defined
diffraction peaks revealed the high crystallinity of the products, which
are in good agreement with the simulated pattern from the crystal data.
In view of the following Cr(VI) reduction reactions, it is necessary to
check the framework stability of 1 in water. The stability of 1 in water
has been confirmed via measuring the PXRD patterns of 1 after being
soaked in water for three days, which reveal good agreement with that
of the as-prepared samples. The TGA curve of 1 reveals no obvious
weight loss until the temperature of 287 °C, which is in accordance with
that observed from the crystal data (Fig. S4). The optical absorbance of
complex 1 as well as the BPEA ligand was measured by UV − vis
spectrometer, which shows that the absorption edge of 1 locates at ca.
670 nm in the visible light region (Fig. 2a). Compared with the BPEA
linker, the absorption band of complex 1 is obviously red-shift, prob-
ably due to the close stacking interactions between conjugated BPEA
ligands in the framework of 1 [25]. The absorption spectrum of 1 is re-
plotted in accordance with the frequency-dependent relationship
αhν = (hν − Eg)1/2 to evaluate the band gap of 1, where α and Eg are
the absorption coefficient and the band gap energy. The plot of (αhν)2
this work,
a
Zn(II)/anthracene coordination polymer [Zn
(OAc)2(BPEA)]n has been successfully prepared via reaction of Zn
(OAc)2·2H2O with a visible light responsive organic ligand 9,10-bis(4′-
pyridylethynyl)-anthracene (BPEA). Compound 1 has a one-dimen-
sional (1D) chain structure in which the binuclear {Zn2(OAc)2} cores
are linked by the BPEA ligands using the pyridyl N atoms to give to the
1D ladder-like network. The as-prepared compound 1 shows broad-
range visible light absorption and good water stability, which are
highly demandable for its application in the photocatalysis. The pho-
tocatalytic property of 1 were investigated by reduction of Cr(VI) to Cr
53

J. Li et al.
InorganicChemistryCommunications101(2019)52–56
Fig. 2. (a) The UV–vis spectra for the BPEA ligand and complex 1; (b) The SPV spectra of BPEA ligand and complex 1.
versus photon energy displays a well-fitted linear dependence near the
absorption edge, and the band gap energy of 1 is thus estimated to be
ca. 2.12 eV, implying that 1 possesses the nature of semiconductivity.
The SPV technique is a nondestructive approach to study heterogeneous
photocatalysts by monitoring the surface voltage resulting from light-
induced charge generation. As shown in Fig. 2b, complex 1 displays a
wide-range SPV response from 420 nm to 550 nm with a peak at
480 nm, indicating its visible-light-induced charge generation and ef-
ficient photoinduced surface charge generation. It could be noticed that
the SPV spectrum of complex 1 is similar to that of the BPEA ligand,
suggesting that the photoinduced charge generation in 1 is based on the
visible light responsive ligand. All the above experimental results in-
dicate that complex 1 might serve as photocatalysts to carry out visible-
light-driven photochemical reactions.
Cr(VI) by CP-based photocatalysts. When the pH value was adjusted to
3, nearly 96% of the Cr(VI) in the solution could be reduced after
30 min. Compared with reported CP-based photocatalysts for Cr(VI)
reduction, complex 1 has the following advantages such as it does not
need the high energy UV light and it does not need to be composite with
other semiconductor materials [27–29]. The stability is also of great
importance to evaluate the photocatalyst. Fig. S6 shows the durability
of complex 1 toward the reduction of Cr(VI) to Cr(III) under visible-
light illumination. The photocatalytic activity of 1 does not obviously
decrease after four cycles, showing that complex 1 exhibits high cata-
lytic stability. Furthermore, the framework integrality after photo-
catalytic process has also been confirmed via the PXRD measurements.
These results suggest that complex 1 shows good catalytic reusability
and stability in this photocatalytic reaction.
The photocatalytic activities of complex 1 were studied the reduc-
tion of Cr(VI) to Cr(III) in an aqueous solution of K2Cr2O7 under the
simulated solar light exposure. A blank experiment without a photo-
catalyst or sunlight was also performed to verify the concentration of Cr
(VI), and the results were found to remain constant, which reflects the
insignificant photolysis and relatively stable content of K2Cr2O7 (Fig.
S5). Before being exposed to light, the Cr(VI) solution was stirred in the
dark at about 30 min to ensure the establishment of an ad-
sorption−desorption equilibrium between the photocatalyst and the Cr
(VI) solution. The previous study reported that the reduction rate of
aqueous Cr(VI) over photocatalysts is greatly influenced by pH values of
the solution and the hole scavenger [26]. In the present study, the pH
value of the reaction system was adjusted by H2SO4 (aq, 0.2 M) and
0.2 mL of MeOH was used as the hole scavenger. As shown in Fig. 3, it
can be seen that the photo-reduction efficiencies of Cr(VI) were greatly
improved by the decreasing the pH values. When the pH value in the
solution decrease from 7 to 4, the reduction ratio of Cr(VI) increase
rapidly from 23% to 92% under the same irradiation time. This result of
the experiment is consistent with other reports on aqueous reduction of
On the grounds of all the experimental results and associate studies,
a possible mechanism for the enhanced photoactivity of Cr(VI) reduc-
tion can be proposed [26–29]. As illustrated in Fig. 4, when the visible
light irradiates, the BPEA ligands adsorb light and the electrons (e−) in
the valance band (VB) of are easily photoexcited to the conduction band
(CB), leaving the holes (h+) in the VB. Then the photogenerated
electrons in the CB are transferred to reduce the Cr(VI) ions on the
surface of while the holes oxidize the hole scavenger MeOH to form CO2
and H2O.
In conclusion, by utilizing a visible light responsive organic ligand
9,10-bis(4′-pyridylethynyl)-anthracene (BPEA), we have successfully
achieved a one-dimensional (1D) chain structure which shows broad-
range visible light absorption and good water stability. The photo-
catalytic property of 1 were investigated by reduction of Cr(VI) to Cr
(III) in aqueous solution under visible light, which reveals its high ef-
ficiency and stability in this photocatalytic process. This work also
highlights the importance of the visible light responsive organic ligand
in the construction of CP-based photocatalyst.
Fig. 3. (a) Reduction profiles of photocatalytic reduction of Cr(VI) over 1 under different pH conditions; (b) The UV–vis spectra for the photocatalytic reduction of Cr
(VI) at pH value of 4.
54

J. Li et al.
InorganicChemistryCommunications101(2019)52–56
Fig. 4. Schematic illustration of photocatalytic reduction Cr(VI) over 1 under visible light irradiation.
Appendix A. Supplementary material
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CCDC 1880705 (1) contains the supplementary crystallographic
data for the title compound. This 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: deposit@ccdc.cam.
ac.uk. Supplementary data associated with this article can be found, in
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