An in Situ Embedded Square-Planar CuII/NiIIN4 Metalloligand in Coordination Polymers for Visible-Light Photocatalysis

Article abstract of DOI:10.1021/acs.inorgchem.7b03068

Three coordination polymers (CPs) with square-planar Cu^II/Ni^IIN₄ subunits were formed in one step by subcomponent self-assembly, giving rise to an unprecedented linking variety of in situ embedded metalloligands and Cu^I clusters. All CPs exhibit unusual visible-light adsorption. Enhanced photocatalytic activity and high selectivity were observed in the oxidation of benzene under visible-light irradiation.

Full text of DOI:10.1021/acs.inorgchem.7b03068

Communication
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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
An in Situ Embedded Square-Planar CuII/NiIIN4 Metalloligand in
Coordination Polymers for Visible-Light Photocatalysis
Yun-Long Hou,†,‡ Yunhong Pi,‡ Xiao-Ping Zhou,‡ and Dan Li*,§
†Department of Chemistry, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon, Hong Kong, China
‡Department of Chemistry, Shantou University, Shantou, Guangdong 515063, P. R. China
§College of Chemistry and Materials, Jinan University, Guangzhou, Guangdong 510632, China
S
* Supporting Information
harvesting materials.16,17 Given the remarkable function of salen/
ABSTRACT: Three coordination polymers (CPs) with
square-planar CuII/NiIIN4 subunits were formed in one
step by subcomponent self-assembly, giving rise to an
unprecedented linking variety of in situ embedded
metalloligands and CuI clusters. All CPs exhibit unusual
visible-light adsorption. Enhanced photocatalytic activity
and high selectivity were observed in the oxidation of
benzene under visible-light irradiation.
porphyrin metalloligands, it is highly worthwhile to construct
MOFs/CPs with versatile metalloligands featuring square-planar
4-coordinated geometry and accessible metal sites as exhibited by
salen/porphyrin metal complexes.
So far, synthetic strategies compatible with metallosalen/
metalloporphyrin struts include noncovalent encapsulation,18
metalloligand self-assembly,7,12−15 and postsynthetic modifica-
tion/exchange,14 which was developed in the research groups of
Hupp,6,19 Goldberg,20 Zhou,21 Cui,11,13 Ma,16 and others.22−24
Our group has been exploring the CuI-based coordination
chemistry,25−29 introducing metal terpyridine complexes, metal
pyrazolate cluster helicates,30−32 and copper(I) metallosalen33,34
through an in situ assembly and metalloligand strategy. In
addition, the efficient photocatalytic activity of CuI-based
compounds35−37 and MOF/CP materials33,34,38−40 was docu-
mented by us and other groups, including the degradation of
organic pollutants,33,34 hydrogen production,38,39 and organic
transformation40 under visible-light irradiation. In an effort to
explore the synthetic methodologies and properties of the
extended crystalline structures combined with the outstanding
merits of salen/porphyrin analogues and photoactive CuI
clusters, we employ subcomponent self-assembly technology (in
situ formation of coordination and covalent bonds simulta-
neously,41,42 also reported in our work43,44) for the in situ
assembly and embedding of metalloligands (Chart 1b) into the
CuI network, yielding both accessible metal sites and photoactive
CuI clusters. Unlike the predesigned metalloligand approach,
versatile synthetic precursors can be applied in subcomponent self-
assembly, giving diverse supramolecular assemblies. To the best
of our knowledge, this work represents the first example of the in
situ setup of square-planar CuII/NiIIN4 subunits in the CPs from
single CuI ions or mixed metal ions in a one-step reaction.
Here we succeed in building CuII/NiIIN4 components (Chart
1b) into three CPs with 1D/2D coordination structures,
[(NiIIN4)2(CuII)5·DMF]n (1; DMF = dimethylformamide),
[(CuIIN41)(CuII)2]n (2), and [(CuIIN42)2(CuIII2)]n (3). They
have been formulated and characterized on the basis of elemental
analysis, IR spectroscopy, and thermogravimetric (TGA) and
single-crystal X-ray diffraction analyses (see the Supporting
Information, SI, for experimental details). Their optical response
alen metal complexes, metalloporphyrins, and their
S
derivatives with square-planar 4-coordinated structures
have long been of interest to chemists because of their facile
synthesis and unique accessible metal−substrate binding pockets
with controllable steric and electronic properties.1−4 Metal-
losalens/metalloporphyrins (see Chart 1a) can be built into
Chart 1. Structures of Metallosalens and Metalloporphyrins
(a) and Square-Planar CuII/NiIIN4 Metalloligands in This
Work (b)
crystalline coordination architectures5−7 such as metalla-
cycles,3,8,9 rotaxanes,10 cages,11 metal−organic frameworks
(MOFs), and one/two-dimensional (1D/2D) coordination
polymers (CPs).12−16 These supramolecular assemblies inte-
grate the attractive properties of metallosalen/metalloporphyrin
moieties and the well-defined compact or porous crystalline
structures, giving rise to wide applications in fields such as
catalysis,14,15 separation,11,13 photoluminescence,9 and light-
Received: December 7, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.7b03068
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Communication
and visible-light-enhanced photocatalytic properties have been
examined.
A single crystal of complex 1 (Table S1) was prepared by the
four-component self-assembly of 4-formylimidazole, 1,3-diami-
nopropane, CuI, and Ni(NO3)2·6H2O in a molar ratio of 4:2:5:2
in a DMF/ethanol mixture (3:1, v/v) at 120 °C for 3 days (see
Figure 1a and the SI for details). X-ray diffraction analysis reveals
Figure 2. Illustration of the preparation and crystal structures of 1D/2D
CPs 2 and 3: (a) synthesis of 2 and 3 via subcomponent self-assembly;
(b) 1D zigzag chain of 2; (c) 2D layer structure of 3. Color code: I,
purple; CuI, red; CuII, gold; N, blue; C, green; O, red. H atoms are
omitted.
revealed (Table S1) that 2 is a mixed-valence 1D polymer and 3
presents a 2D CuII-based structure. Square-planar 4-coordinated
CuIIN4 metalloligands (Chart 1b) in both polymers involve the
in situ formation of imine bonds and the oxidation of CuI ions in
the presence of O2, with different coordination geometries. In
complex 2, CuIIN41 behaves as a ditopic metalloligand (Figure
S2) and was linked by Cu2I2 clusters along the a axis with a bite
angle of 109.18(8)° (CuI···CuII···CuI), forming 1D zigzag chains
(Figures 2b and S2). In complex 3, CuIIN42 acts as a tritopic
metalloligand (Figure S3). Two adjacent CuIIN42 units were
chelated with the octahedral CuIII2 cluster and bridged through I
anions, giving an extended 2D layer network (Figures 2c and S3).
Compared with the compact structure of 3, CuIIN4 units in 2
feature highly accessible open sites in CuII centers. The successful
one-step in situ construction of two functional moieties in CPs
(NiIIN4 and Cu5I5 in 1; CuIIN4 and Cu2I2 in 2) promotes us to
investigate their light absorption properties and photocatalytic
activities.
All of these complexes were found to be highly stable in air and
various solvents (water, DMF, ethanol, and acetonitrile; <0.5
mg/mL) for weeks, based on powder X-ray diffraction (PXRD)
experiments (Figures S4−S6). Their thermal stabilities rise from
230 °C (1) to 342 °C (3) to 380 °C (2) according to TGA
(Figure S7). The phase purity of the bulk samples was established
by a comparison of their observed and simulated PXRD patterns.
The optical response of 1−3 in the visible-light region was
measured through UV−vis diffuse-reflectance spectra, and the
maximum absorption wavelength ranges from 660 nm for 1 and
762 nm for 3 up to about 815 nm for 2 (Figure 3a), respectively.
The values of the photon energy are listed in Table S2. The
results indicate that the compounds can absorb photons with
energy equal or higher than their highest occupied molecular
orbital−lowest unoccupied molecular orbital gap.
Figure 1. Illustration of the preparation and crystal structure of CP 1:
(a) synthesis of 1 via subcomponent self-assembly; (b) 1D double zigzag
chain; (c) Cu5I5 cluster; (d) Three-dimensional packing of a 1D chain
with trapped DMF molecules. Color code: I, purple; Cu, red; Ni, blue;
C, N, and O, green. H atoms are omitted.
that the NiII atom of the NiIIN4 subunit adopts a square-planar 4-
coordinated configuration, which is coordinated to the discrete
neutral Cu5I5 cluster by two deprotonated N atoms of imidazoles
[CuI−Nimidazole, 1.96853(7)−2.00259(8) Å] in the asymmetric
unit of 1 (Figure S1). In the unique structure of the Cu5I5 cluster
(Figure 1c), four CuI atoms (Cu1, Cu2, Cu3, and Cu4) are
arranged in the pseudocubane coordination geometry, with the
supernumerary Cu5I atom sitting on one of the cubic faces. All
CuI atoms in the Cu5I5 cluster of 1 adopt the tetrahedral 4-
coordinated configuration and are connected by one μ2-I, two μ3-
I, and two μ4-I ions, wherein the Cu3I atoms are coordinated to
four I− anions and the other four CuI atoms act as nodes to bridge
the ditopic NiIIN4 metalloligand with a bite angle of 121.71(4)°
(CuI···NiII···CuI) for the construction of a double zigzag chain
(Figure 1b). Along the a axis, 1D chains stack with each other to
form tubular channels, where DMF molecules are trapped
(Figure 1d).
Efforts were also made to prepare the CuIIN4 motif. The in situ
assembly of CuI, oxalyl dihydrazide, and 4-formylimidazole, in a
molar ratio of 2:1:2, in an ethanol/water mixture (4:1, v/v) at
140 °C for 2 days (see the SI for details) yields dark-green
complex 2. The replacement of 4-formylimidazole with 2-
formylimidazole under the same reaction conditions gave dark-
green 3 (Figure 2a). Single-crystal X-ray diffraction analyses
B
DOI: 10.1021/acs.inorgchem.7b03068
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Accession Codes
CCDC 1589186−1589188 contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2
1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Author
*E-mail: danli@jnu.edu.cn.
■
Figure 3. (a) UV−vis diffuse-reflectance spectra of CPs 1−3. (b)
Catalytic oxidation of benzene by 2 under visible-light irradiation.
ORCID
Xiao-Ping Zhou: 0000-0002-0351-7326
Dan Li: 0000-0002-4936-4599
Because direct and selective transformation of benzene is
economically important and few photocatalysts have been
reported for this reaction,45,46 we here employed the oxidation
of benzene to examine the photocatalytic activity of complexes 1
and 2 (Figure 3b). The reaction was conducted under visible-
light irradiation using a 3 mmol % sample (per the CuIIN4 or
NiIIN4 moiety) and 30% hydrogen peroxide as a catalyst and an
oxidant, respectively (see the SI for details).47,48 It was observed
that only 1−15% conversion of benzene was obtained by 1, 3,
and CuI in the presence or absence of light (Table S3 and Figures
S8 and S9). In contrast to the poor activity of 1, complex 2
exhibits enhanced photocatalytic activity with a moderate
conversion of 37% and an excellent selectivity of 95% of 1,4-
benzoquinone, which is higher than a conversion of 26% in the
absence of light (Table S3). 2 was also recyclable and confirmed
by the phase purity of the recovered sample’s PXRD after three
cycles (Figure S10). The activity comparison of 1 and 2 indicates
that the combination of CuIIN4 moieties and Cu2I2 in the CP 2 is
more favorable for this photoinduced reaction. Compared with
the reported polymeric photocatalysts, the conversion (37%, 2)
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We acknowledge the National Basic Research Program of China
(973 Program 2013CB834803), the National Natural Science
Foundation of China (Grants 21731002, 91222202, 21171114,
and 21371113), Guangdong Natural Science Funds for
Distinguished Young Scholars (Grant 2014A030306042), and
the Training Program for Excellent Young College Teacher of
Guangdong Province.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.7b03068.
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