Design and synthesis of functionalized coordination polymers as recyclable heterogeneous photocatalysts

Article abstract of DOI:10.1039/c8dt00902c

The functionalized ligand 9,10-anthraquinone-1,4-dicarboxylate acid (H₂AQDC) was designed and synthesized in order to develop metal-organic coordination polymers as heterogeneous catalysts with a photosensitizing feature. Two major considerations of the ligand design are anthraquinone moieties for photosensitizing to harvest light and carboxylate groups for polymeric coordination toward less solubility. A series of transition metal complexes based on this ligand were synthesized subsequently, namely {Co(AQDC)(H₂O)₃·2H₂O}_n (Co-AQDC), {Ni(AQDC)(H₂O)₃·2H₂O}_n (Ni-AQDC), {[Cu(AQDC)(H₂O)₃][Cu(AQDC)(H₂O)₂(DMF)]·(H₂O)₄}_n (Cu-AQDC), {Zn_1.5(AQDC)(OH)(H₂O)₂·H₂O}_n (Zn-AQDC), {Ag₂(AQDC)(CH₃OH)}_n (Ag-AQDC). Both the ligand and its transition metal complexes are able to catalyze the visible-light driven oxidation reactions of alkynes into 1,2-diketones in air under mild conditions, in which compound Ni-AQDC demonstrates the best activity. This catalyst can be easily isolated from the reaction mixture by filtration with a trace amount of loss in solution and is ready for recycled use after simple washing and drying without any need for regeneration. Remarkably, the catalyst shows no loss of activity after five catalytic cycles and X-ray powder diffraction proves no change in the structure after five runs. This designed metal-organic coordination polymer represents an environmentally friendly, economical and recyclable photocatalyst, constituting a good candidate for photocatalytic organic syntheses in terms of green chemistry.

Full text of DOI:10.1039/c8dt00902c

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1
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1
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Design and Synthesis of Functionalized Coordination
Polymers as Recyclable Heterogeneous Photocatalysts
a
a
a
b
a
a
a
a
a
CaiꢀPing Ye, Gan Xu, Zheng Wang, Jie Han, Lian Xue, FuꢀYu Cao, Qi Zhang, LinꢀFeng Yang, LiꢀZhou Lin,
a,
XuꢀDong Chen *
a
Jiangsu Collaborative Innovation Center of Biomedical Functional Materials and Jiangsu Key Laboratory of Biofunctional Materials, School of
Chemistry and Materials Science, Nanjing Normal University, Nanjing 210023, China
b
School of Science & Technology, The Open University of Hong Kong, Kowloon, Hong Kong SAR, PR China
*
Correspondence eꢀmail: xdchen@ninu.edu.cn
Abstract: The functionalized ligand 9,10ꢀanthraquinoneꢀ1,4ꢀdicarboxylate acid (H
2
AQDC), has been designed
and synthesized in order to develop metalꢀorganic coordination polymers as heterogeneous catalyst with
photosensitizing feature. Two major considerations of ligand design are anthraquinone moieties for photosensitizing to
harvest light and carboxylate groups for polymeric coordination toward less solubility. A series of transition metal
complexes based on this ligand have been synthesized subsequently, namely {Co(AQDC)(H
2
O)
3
—2H
2
O}
n
(CoꢀAQDC),
(CuꢀAQDC),
{
Ni(AQDC)(H
2
O)
3
—2H
2
O}
n
(NiꢀAQDC),
{[Cu(AQDC)(H
2
O)
3
][Cu(AQDC)(H
2
O)
2
(DMF)]—(H O) }
2
4 n
{
Zn1.5(AQDC)(OH)(H
2
O)
2
—H
2
n
O} (ZnꢀAQDC), {Ag
2
(AQDC)(CH
3
OH)} (AgꢀAQDC). Both the ligand itself and its
n
transition metal complexes are able to catalyze the visibleꢀlight driven oxidation reactions of alkynes into
,2ꢀdiꢀketones in air under mild conditions, in which compound NiꢀAQDC demonstrates the best activity. This catalyst
1
can be easily isolated from the reaction mixture by filtration with trace amount of loss in solution and is ready for
recycled use after simple washing and drying without any need for regeneration. Remarkably, the catalyst shows no
loss of activity after five catalytic cycles and Xꢀray powder diffraction proves no change of structure after five runs. This
designed metalꢀorganic coordination polymer represents an environmentally friendly economic and recyclable
photocatalyst, constituting a good candidate for photocatalytic organic syntheses in terms of green chemistry.
Introduction
Coordination polymers constructed by organic linkers and inorganic nodes has seen rapid development in the field of
1
ꢀ4
supramolecular catalysis, attributed to their features such as easy isolation and potentially high atom economy. The
intrinsic characteristics of coordination polymers, for example low solubility in common solvents, highly ordered
structures and easy functionalization through design and modification, make them perfect candidates for
5
ꢀ9
heterogeneous catalysis. However, their applications in photocatalytic organic transformations have been scarce.
Visibleꢀlight driven organic transformations has in recent years attracted intense research interests owing to the
1
0ꢀ14
II
III
I
II
natural abundance of light which is renewable and sustainable.
Metal (Ru , Ir , Re and Os ) polypyridine
represent two typical categories of photocatalysts for visibleꢀlight driven
photoredox processes, and have been extensively studied in homogenous synthetic organic systems. Heterogeneous
15ꢀ17
18ꢀ19
complexes
and organic dyes
20,21
22
photocatalysts sensitizing visible light, including MOFs
and semiconductive metal oxide, have also been
investigated for their unique feature of easy isolation as well as the potential of recycled usage. However, most of
these photocatalysts suffer from disadvantages in such aspects as difficulties in efficient recovering and significant
1

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loss of activity during recycling. It is still highly desirable to develop efficient heterogeneous photocatalysts with good
recyclability in terms of green chemistry.
In seeking photocatalysts that is recyclable, our attention has been focused in the development of coordination
polymers with functionalized ligands, which might provide the possibility of heterogeneous photocatalysis. Carboxylate
groups represent a very important category of binding sites in organic ligands for coordination polymers with desired
2
3
structures and thus becomes an inevitable choice of our ligand design. The idea of ligand functionalization was
2
4ꢀ25
inspired by the important role of quinones in biological systems as photosensitizer
involving processes such as single electron transfer and protonꢀcoupled electron transfer.
,10ꢀanthraquinoneꢀ1,4ꢀdicarboxylic acid (H AQDC) has been designed and synthesized. The expectation is that
and their rich redox properties
2
6ꢀ27
Therefore, ligand
9
2
anthraquinone portion behaves as photosensitizer to harvest light and carboxylate groups bind metal ions to form
coordination polymers. Subsequently, a series of transition metal complexes based on this ligand have been
synthesized as potential photocatalysts.
To investigate the photocatalytic property of the metal complexes thus synthesized, the reaction of
photocatalytic oxidation of diarylethyne analogues into 1,2ꢀdiketones has been selected as the substrate reaction.
1,2ꢀDiketones are important organic molecules for synthesizing heterocyclic compounds with biological activity, which
2
8ꢀ29
30ꢀ31
32
have attracted intense synthetic interests using catalysts such as metal complexes involving Pd,
Ru,
Au, and
33
34
Cu, and organic dye eosin Y as well. But same issue exists that the catalysts are not recyclable, despite the large
amount of efforts in this area. Our investigation has demonstrated that both the H AQDC ligand itself and its transition
2
metal complexes are able to catalyze the airꢀoxidation reactions of alkynes to produce 1,2ꢀdiketones under the
irradiation of visible light, in which the Ni(II) complex exhibits the best catalytic activity. This catalyst can be readily
recovered without loss of mass and change in structure, and remarkably, it can be efficiently recycled to sustain at
least five catalytic cycles without loss of activity.
Experimental Section
General materials and measurements
All reagents and solvents were purchased from commercial sources and used without further purification. The ligand
3
5
H
2
AQDC, was synthesized using a modified procedure as reported. Elemental analyses were performed using a
PEꢀ240C elemental analyzer. Thermal analyses were carried out on a MettlerꢀToledo TGA/DSC STARe system in the
−1
temperature range of 28 to 800 °C with a heating rate of 10 °C—min , under nitrogen flow. Infrared spectra were
−
1
recorded on a VECTOR 22 spectrometer with pressed KBr pellets in the range of 4000 to 400 cm . Powder Xꢀray
diffraction (PXRD) was done with a MiniFlex 600 Xꢀray powder diffractometer equipped with a Cu sealed tube (λ =
1
13
1
3
.54178 Å) at 40 KV and 40 mA. H and C NMR spectra were obtained in CDCl solvent on a Bruker 400 MHz
spectrometer.
Synthesis of metal complexes
{
Co(AQDC)(H
.05 mmol) were dissolved into 5.0 mL water. After stirring, the resultant solution was allowed to evaporate at room
temperature for 15 days to afford pink needleꢀlike crystals, which were collected by filtration and washed with water
and acetone (15.1 mg, yield: 68%). Anal. Calcd for CoꢀAQDC (C16 11Co): C, 43.36; H 3.64. Found: C, 43.38; H,
.69. IR (KBr cm ): 1672(s, C=O), 1573(vs, carboxylate C=O), 1265(s, CꢀO).
2
O)
3
—2H
2
O}
n
2 2 2
(CoꢀAQDC). A mixture of Co(OAc) —4H O (12.4 mg, 0.05 mmol) and H AQDC (14.8 mg,
0
16
H O
ꢀ
1
3
2

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{
Ni(AQDC)(H
except that Ni(OAc)
filtration and washed with water and acetone (11.7 mg yield: 53%). Anal. Calcd for NiꢀAQDC (C16
2
O)
3
—2H
2
O}
n
(NiꢀAQDC). This complex was obtained by the same procedure as that for CoꢀAQDC
2
—4H
2
O was used instead of Co(OAc)
2
—4H
2
O as starting material. Green crystals were obtained by
16
H O
11Ni): C 43.38, H
ꢀ1
3
.64; Found: C 43.37, H 3.97. IR (KBr, cm ): 1670(s, C=O), 1591(s, carboxylate C=O), 1265(s, CꢀO).
{
[Cu(AQDC)(H
mg, 0.1 mmol) was added H
green crystals (34.7 mg, yield: 64%). Anal. Calcd for CuꢀAQDC (C35
2
O)
3
][Cu(AQDC)(H
2
O)
2
(DMF)]—(H
2
O)
4
}
n
2 2
(CuꢀAQDC). To 3.0 mL DMF solution of CuCl —2H O (17.1
2
AQDC (29.6 mg, 0.1mmol) in 3.0 mL DMF. Filtration followed by ether diffusion afforded
37 2
H O22NCu
): C 44.22, H 3.92. Found: C 44.18, H
ꢀ1
3
.89. IR (KBr, cm ): 1672(s, C=O), 1585(s, carboxylate C=O), 1265(s, CꢀO).
{
Zn1.5(AQDC)(OH)(H
14.8 mg, 0.05 mmol ) and NaOH (4.0 mg, 0.10 mmol ) were mixed together, followed by acetone diffusion to give
orange crystals (17.4 mg, yield: 75%). Anal. Calcd for ZnꢀAQDC (C16 10Zn1.5): C 41.48, H 2.83. Found: C 41.43, H
.83. IR (KBr, cm ): 1670(s, C=O), 1583(s, carboxylate C=O), 1265(s, CꢀO).
2
O)
2
—H
2
O}
n
3 2 2 2
(ZnꢀAQDC). Aqueous solution of Zn(NO ) —6H O (11.9 mg, 0.05 mmol), H AQDC
(
13
H O
ꢀ1
2
{
Ag
methanol and this solution was carefully layered over 2.0 mL aqueous solution of AgClO
.0 mL buffer layer (1:1 water and methanol). Light yellow needleꢀlike crystals were obtained after two days (26.1 mg,
2
(AQDC)(CH
3
OH)}
n
(AgꢀAQDC). The ligand H
2
AQDC (14.8 mg, 0.05 mmol) was dissolved into 3.0 mL
4
(20.7 mg, 0.1 mmol), with
5
ꢀ1
yield: 55%). Anal. Calcd for AgꢀAQDC (C17
10 7 2
H O Ag ): C 37.67, H 1.86. Found: C 37.70, H 1.86. IR (KBr, cm ): 1664(s,
C=O), 1591(s, carboxylate C=O), 1269(s, CꢀO).
Typical procedure for photocatalytic experiment
A mixture of photocatalyst (0.02 mmol), 1,2ꢀdiarylethyne (0.2 mmol) and 4ꢀchlorobenzenethiol (57.8 mg, 0.4 mmol)
was dissolved into 2.0 mL MeCN. The reaction mixture was stirred at room temperature under the irradiation of blue
LED for 72h in ambient air. Filtration was done after the reaction is complete. The insoluble photocatalyst thus isolated
is ready for recycled use after dichloromethane washing and vacuum drying. The filtrate containing final product was
evaporated to dryness and subjected to column chromatography (silica gel, petroleum ether/ethyl acetate) to afford
pure 1,2ꢀdiketone product. The yields reported are based on isolated pure product and characterization was done by
1
13
H and C NMR spectroscopy (see Supporting Information for spectra and product details).
Single crystal Xꢀray crystallography
Single crystal Xꢀray diffraction data of all five complexes were collected at 293K on Bruker APEX II CCD diffractometer
operating at 50 KV and 30 mA using Mo Ka radiation (λ = 0.71073 Å). Data was integrated by SAINT and scaled with
3
6
either a numerical or multiꢀscan absorption correction using SADABS. All structures were solved by direct methods
37
2
38
39
or Patterson maps and refined by fullꢀmatrix least squares on F using SHELXLꢀ2014 and OLEX2 program. All
nonꢀhydrogen atoms were refined anisotropically. Nonꢀwater hydrogen atoms were added at calculated positions and
refined using a riding model. Hydrogen atoms of water molecules were located by different Fourier map and refined
using riding model at restrained distance of 0.86 Å. Crystallographic data for all five complexes are summarized in
Table 1 and selected bond lengths and angles are listed in Table S1. CCDC 1821544 (CoꢀAQDC), 1821545
(
NiꢀAQDC), 1821548 (CuꢀAQDC), 1821546 (ZnꢀAQDC), and 1821547 (AgꢀAQDC) containing the supplementary
crystallographic data can be obtained free of charge from the Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
2
–
Table 1 Crystallographic data for metal complexes based on AQDC ligand.
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CoꢀAQDC
NiꢀAQDC
CuꢀAQDC
ZnꢀAQDC
AgꢀAQDC
Empirical formula
C16H16CoO11
443.22
C16H16NiO11
443.00
C
35
H
37Cu
2
NO22
C
16
H
13
O
10Zn1.5
C17H10Ag O7
2
Fw
950.73
monoclinic
P2 /c
463.32
monoclinic
P2 /c
542.00
Crystal system
monoclinic
monoclinic
orthorhombic
Pbca
Space group
a(Å)
b(Å)
c(Å)
P2
1
/c
P2
1
/c
1
1
10.693(2)
21.451(4)
7.7724(14)
90
10.6610(18)
21.487(4)
7.76743(13)
90
6.6984(11)
21.754(4)
26.573(4)
90
6.8037(11)
14.962(2)
15.720(3)
90
11.383(6)
9.236(5)
30.142(15)
90
α(°)
β(°)
102.437(2)
90
102.258(2)
90
96.302(2)
90
98.280(2)
90
90
γ(°)
90
V(Å)
Z
1740.9(6)
4
1717.9(5)
4
3846.7(11)
4
1555.0(4)
4
3169(3)
8
ꢀ
3
D
c
(g—cm )
1.691
1.713
1.642
1.979
2.272
ꢀ
1
ꢁ
(mm )
1.048
1.192
1.198
2.394
2.511
F(000)
908
912
1952
936
2083
a
b
b
R
R
1
1
, wR
, wR
2
2
[I > 2σ(I)]
(all data)
0.0328, 0.0865
0.0456, 0.0937
0.89, ꢀ0.54
0.0317, 0.0764
0.0419, 0.0825
0.67, ꢀ0.44
0.0718, 0.1368
0.1391, 0.1602
1.05, ꢀ1.04
0.0271, 0.0643
0.0361, 0.0678
0.47, ꢀ0.52
0.0307, 0.0619
0.0506, 0.0686
0.72, ꢀ0.84
a
−
3
(
∆ρ)max, (∆ρ)min (e Å )
Results and Discussion
Design and Synthesis:
2
4ꢀ25
Quinones are important photosensitive compounds in biological systems.
Natural benzoquinones and
anthraquinones participate in Type I and Type II photosensitization to produce singlet oxygen and superoxide anion
4
0ꢀ41
for photooxidation reactions.
The nature and position of substituents determine the photochemical properties of
anthraquinone derivatives, by affecting the electronic states and electron transitions. For example, 2ꢀClꢀanthraquinone
has shown exceptional reactivity over its analogs as photosensitizer in catalyzing certain organic transformations
4
2ꢀ45
under the irradiation of fluorescent lamp.
Anthraquinone moiety has thus been chosen as the functional group of
our designed ligand for developing metalꢀorganic coordination polymer with photosensitizing activity.
On the other hand, ligand with carboxylate groups has proven to be very successful in constructing coordination
19
polymers such as metalꢀorganic frameworks. So the combination of carboxylate groups with anthraquinone moiety
might constitute perfect candidate for our designed ligand. Regarding the positions of substituting carboxylate groups,
a preferable arrangement is paraꢀpositioned for polymeric coordination. And furthermore, close arrangement of the
carboxylate and carbonyl groups will lead to significant steric hindrance between them, so that carboxylate groups will
hold angles to the phenyl rings. As a result, the conjugation between anthraquinone plane and carboxylate groups will
be broken, and the photochemical properties of the anthraquinone portion might be better maintained.
Therefore, the functionalized ligand 9,10ꢀanthraquinoneꢀ1,4ꢀdicarboxylic acid has been synthesized as a white
3
1
solid in three steps using a modified procedure of the literature method. . And transition metal complexes involving
2
–
Co(II), Ni(II), Cu(II), Zn(II) and Ag(I) based on AQDC ligand have been prepared as crystalline solid by common
solution method for synthesizing coordination compounds. All metal complexes were characterized by means of
singleꢀcrystal and powder Xꢀray diffractions, infrared spectra and elemental analyses and their thermoꢀstabilities were
studied (See Supporting Information for PXRD patterns and TGA curves).
Structure Analysis:
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M
O
O
O
O
Cu
O
O
O
O
Cu
M
O
O
O
O
M = Co or Ni
Ag
Ag
Zn
Zn
O
O
O
O
Zn Ag
Zn Ag
O
O
O
O
Ag
Ag
O
O
O
O
Ag
2
ꢀ
Scheme 1. Versatile coordination modes of the AQDC ligand in its metal complexes.
A common feature of these coordination complexes is that they are all polymeric, indicating less solubility in
common organic solvents and therefore dictating the possibility of heterogeneous catalysis and potential of easy
2–
recycling. Ascribed to rich coordination chemistry of carboxylate groups, the AQDC ligand exhibits various
coordination modes in this series of transition metal complexes (Scheme 1), resulting in versatile polymeric structures.
Complexes CoꢀAQDC and NiꢀAQDC are isostructural, crystallizing in monoclinic space group P2
1
/c. In both
complexes, each metal center adopts a distorted octahedral geometry, coordinating to two carboxylate oxygen atoms
2
–
2–
from two symmetry related AQDC ligands and four coordinated water molecules (Fig. 1a). The AQDC ligands take
ꢁ
2
ꢀbridging mode with both carboxylate groups participating into coordination. 2D layers in (010) plane has been
2ꢀ
formed through the connection of AQDC ligands and ꢁ
2
ꢀbridging water molecules, which further packed together
through the interactions of hydrogen bonding and πꢀπ stacking (Fig. 1b and 1c).
(
a)
(b)
(c)
Fig. 1 (a) Coordination geometry of Ni(II) center and binding mode of ligand in NiꢀAQDC. (b) 2D network in
i
ii
iii
NiꢀAQDC. (c) Packing diagram of NiꢀAQDC. Symmetry codes: xꢀ1, –y+1/2, zꢀ1/2; x, –y+1/2, zꢀ1/2; x, –y+1/2,
iv
z+1/2; x+1, –y+1/2, z+1/2.
(
a)
b)
(
c)
(
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Fig. 2 (a) Coordination geometries of Cu(II) centers and binding mode of ligand in CuꢀAQDC. (b) 1ꢀD chain structure
i
ii
in CuꢀAQDC with pseudoꢀisostructural units. (c) Packing diagram of CuꢀAQDC. Symmetry code: x, y+1, z; x, yꢀ1, z.
As shown in Fig. 2, the Cu(II) centers in CuꢀAQDC adopt pyramidal coordination mode, with water ligands sitting
on the apex of the pyramid at elongated coordination distances (Fig. 2a). Two carboxylate O atoms from oppositely
2
–
positioned AQDC ligands and one water molecule occupy three of the four corners of the pyramid base. The other
corner is occupied by water and dimethylformamide alternatively to form pseudoꢀisostructural motifs, which are
connected alternatively to form 1D chain structure along [010] direction (Fig. 2b). Hydrogen bonding and πꢀπ stacking
further assemble the chains into higher dimension (Fig. 2c).
There are two symmetry independent Zn(II) cations in ZnꢀAQDC, both taking slightly distorted octahedral
2–
geometry. Four symmetry related AQDC ligands and two hydroxyl ions constitute the coordination sphere of one
2–
Zn(II) ion. And the other Zn(II) ion binds with two symmetry related AQDC ligands, two water molecules and two
hydroxyl ions (Fig. 3a). The hydroxyl ion exhibits a ꢁ ꢀbridging mode, connecting three adjacent Zn(II) atoms to form a
Zn motif, which are further linked together by AQDC ligand to generate a 3D nonꢀporous supramolecular
architecture (Fig. 3b). The AQDC ligand links four Zn(II) cations through its carboxylate groups (Scheme 1).
3
2–
3
2–
(
a)
(b)
Fig. 3 (a) Coordination geometries of Zn(II) centers and binding mode of ligand in ZnꢀAQDC. (b) Packing diagram of
i
ii
iii
iv
v
ZnꢀAQDC. Symmetry codes: x, –y+1/2, zꢀ1/2; –x+1, y+1/2, –z+1/2; –x+1, –y+1, –z; –x, –y+1, –z; –x, yꢀ1/2, –
z+1/2.
(
a)
(b)
(c)
Fig. 4 (a) Coordination geometries of Ag(I) centers and binding mode of ligand in AgꢀAQDC. (b) 2ꢀD bilayered
i
ii
iii
structure in AgꢀAQDC. (c) The 3D network structure of AgꢀAQDC. Symmetry code: –x+1/2, y+1/2, z; x+1, y, z;
–
iv
v
x+1, –y+1, –z+1; –x+1/2, yꢀ1/2, z; xꢀ1, y, z.
Complex AgꢀAQDC crystallizes in orthorhombic space group Pbca, in which short argentophilic interaction at
2.869 Å has been observed (Fig. 4a). The dimeric AgꢀAg moiety containing two crystallographically independent Ag(I)
2
–
cations is surrounded by five symmetry related AQDC ligands and one coordinated methanol molecule, which link
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the dimeric moieties into 2D bilayer sheet (Fig. 4b). The layered structures further stack by Van der Waals interactions
into 3D structure (Fig. 4c).
2
ꢀ
In accord with our ligand design, the two carboxylate groups of AQDC ligand are not in the plane of the
conjugated system due to steric hindrance with neighboring carbonyl groups, bearing dihedral angles ranging from
55.4° to 89.2° for this family of coordination polymers (Table 2). The ligand is slightly bent in it coordination polymers,
as indicated by the dihedral angles of the two carbonyl groups and the central sixꢀmember ring and the deviation of
atoms of the anthraquinone plane (Table 2).
Ascribed to the existence of coordinated and lattice water molecules, rich intraꢀ and interꢀ molecular hydrogen
bonds exist in the crystal packing of these compounds except that of the Ag(I) complex, leading to more stabilized
stacking as solid materials (Table 3). Some of the hydrogen bonds are pretty strong with donor to acceptor distances
between 2.5 to 2.6 Å.
Table 2 Metrics of ligand conformation in metal complexes.
Dihedral angels (°)
Atomic deviation of the
anthraquinone plane (Å)
carboxylate groups VS
attached benzyl ring
carbonyl groups VS
attached sixꢀmember ring
Mean
Maximum
CoꢀAQDC
NiꢀAQDC
55.4(2)
55.5(2)
86.1(2)
86.1(2)
86.3(2)
87.4(2)
76.6(1)
74.9(1)
2.3(2)
2.1(2)
2.9(1)
1.8(1)
3.6(1)
3.2(3)
12.1(1)
11.7(1)
2.7(1)
1.8(1)
2.0(1)
9.3(3)
0.134(3)
0.130(3)
0.048(7)
0.028(6)
0.054(2)
0.132(6)
ꢀ0.350(2)
0.340(2)
0.098(5)
0.066(5)
ꢀ0.112(2)
ꢀ0.301(4)
8
8
8.1(2)
9.2(2)
CuꢀAQDC
ZnꢀAQDC
AgꢀAQDC
80.1(1)
64.2(2)
2
–
Table 3 Hydrogen bonding parameters in metal complexes of AQDC ligand (Å and deg).
ꢁ
DꢀH
A
d(H———A)
CoꢀAQDC
d(D———A)
∠DHA
O10WꢀH10WA———O1
O10WꢀH10WB———O11W
O9WꢀH9WB———O10W
2.154
2.142
2.063
1.780
1.751
1.863
2.001
1.869
2.933
2.979
2.905
2.585
2.576
2.705
2.848
2.701
150.300
164.470
166.062
154.794
159.894
165.982
168.359
162.360
i
O7WꢀH7WB———O3
ii
OW7ꢀHW7A———O5
iii
O8WꢀH8WB———O6
iv
O8WꢀH8WA———O11W
v
O9WꢀH9WB———O4
i
ii
iii
iv
Symmetry codes: x+1,
–
–
y+3/2, z+1/2; x,
–
y+3/2, zꢀ1/2; x,
–
y+3/2, z+1/2; –x+2, y+1/2,
v
z+3/2; x+1, y, z.
NiꢀAQDC
O8WꢀH8WB———O11W
O9WꢀH9WB———O10W
O10WꢀH10WA———O1
1.992
2.061
2.072
2.089
1.752
1.750
1.851
2.835
2.896
2.921
2.933
2.573
2.563
2.702
2.697
166.484
163.418
168.737
166.727
158.801
156.915
170.113
i
O11WꢀH11WB———O2
i
O7WꢀH7WB———O3
ii
O7WꢀH7WA———O5
iii
O8WꢀH8WA———O6
iv
O9WꢀH9WA———O4
1.858
164.820
i
ii
iii
iv
Symmetry codes: xꢀ1,
–
y+1/2, zꢀ1/2; x,
–
y+1/2, z+1/2; x,
–
y+1/2, zꢀ1/2; xꢀ1, y, z.
7

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CuꢀAQDC
O13WꢀH13WB———O7
1.955
1.876
1.818
1.878
1.966
1.988
1.949
1.962
2.810
2.597
2.669
2.673
2.809
2.775
2.732
2.804
170.958
140.334
170.098
152.855
166.373
151.954
146.128
165.840
O14WꢀH14WB———O20W
O14WꢀH14WA———O21W
O17WꢀH17WB———O18W
i
O18WꢀH18WA———O5
i
O19WꢀH19WB———O4
i
O20WꢀH18WB———O12
i
O21WꢀH21WA———O9
i
Symmetry codes: –x+2, –y+1, –z+1.
ZnꢀAQDC
1.775
1.906
O9WꢀH9WB———O10W
2.635
2.734
2.931
2.815
2.856
2.819
176.233
161.009
153.444
166.963
158.443
170.130
i
O8WꢀH8WB———O6
i
O9WꢀH9WA———O6
2.137
1.970
2.038
1.968
ii
O8WꢀH8WA———O3
iii
O10WꢀH10WB———O8
iv
O10WꢀH10WA———O2
i
ii
iii
iv
Symmetry codes:
–
x+1, y, z; xꢀ1,
–y+1/2, zꢀ1/2
–
x, y+1/2,
–
z+1/2; –x+1, –y+1, –z+1.
Catalytic Studies:
Diphenylethyne is used as the substrate to optimize the reaction conditions (Table 4). An initial investigation
revealed that the ligand itself is capable of catalyzing the reaction at 63% yield (Entry 1). Screening of different
solvents as reaction medium revealed that acetonitrile (MeCN) performs much better than tetrahydrofuran (THF),
dichloromethane (DCM), and methanol (MeOH) in mediating the reaction (Entries 2ꢀ5). Water contents of the solvent
was found to affect the yield of the reaction significantly. Neither anhydrous acetonitrile (Entry 6) nor those doped with
excess amount of water (Entries 7 and 8) as the reaction solvent performs better than commercially available
acetonitrile with 0.13% water content (determined by CarlꢀFischer titration). Minimum amount of photocatalyst
required for satisfactory yield is 0.1 equivalent (entries 2, 9ꢀ11). Further studies indicate that almost no reactions
occurs in absence of any one of the elements including photocatalyst, 4ꢀchlorobenzenethiol, blue LED and air (entries
12ꢀ15).
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Table 4 Optimization of Reaction Conditions
Entry
1
Photocatalyst (mol%)
AQDC (10)
Solvent
MeCN
yield(%)
63
H
2
2
3
4
5
6
7
8
9
NiꢀAQDC (10)
NiꢀAQDC (10)
NiꢀAQDC (10)
NiꢀAQDC (10)
NiꢀAQDC (10)
NiꢀAQDC (10)
NiꢀAQDC (10)
NiꢀAQDC (3)
NiꢀAQDC (5)
NiꢀAQDC (7)
NiꢀAQDC (10)
NiꢀAQDC (10)
—
MeCN
THF
81
0
DCM
MeOH
trace
trace
37
a
MeCN
b
MeCN
74
c
MeCN
51
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
trace
52
1
0
11
71
d
e
1
1
1
1
2
3
4
5
0
trace
0
f
NiꢀAQDC (10)
0
Reaction conditions (unless otherwise specified): diphenylethyne (0.2 mmol), 4ꢀchlorobenzenethiol (0.4
mmol), photocatalyst, solvent (2 mL), irradiated by 20 W blue LED (450nmꢀ470nm) in air at room
temperature for 72 h. Commercially available solvents containing trace amount of H O were used
2
a
b
c
unless otherwise specified. Anhydrous. Doped with 50 ꢂL water (3% water content). Doped with
00 ꢂL water (24% water content). In the dark. Without 4ꢀchlorobenzenethiol. Under N
d
e
f
5
2
atmosphere
(
1 atm).
2
–
As shown in Table 5, screening of transition metal complexes of AQDC ligand shows that ZnꢀAQDC and
AgꢀAQDC exhibit comparable catalytic activities (57ꢀ68%, Entry 18 and 19) as that for the ligand, while CoꢀAQDC and
CuꢀAQDC complexes catalyze the reaction at only 35ꢀ37% yield (Entry 16 and 17). It is notable that isostructural pair
CoꢀAQDC and NiꢀAQDC behaves very different in promoting the photooxidation reaction, therefore we can conclude
that structures of these complexes and coordination modes of metal centers/ligand are not decisive factors for the
2
–
catalytic activity of the complexes. There might be some synergistic effect of AQDC ligand and Ni(II) center in
NiꢀAQDC that enhances the catalytic activity of the complex.
Table 5 Screening of photocatalysts.
Entry
1
Photocatalyst (mol%)
AQDC (10)
yield(%)
63
H
2
2
NiꢀAQDC (10)
CoꢀAQDC (10)
CuꢀAQDC (10)
ZnꢀAQDC (10)
AgꢀAQDC (10)
81
37
35
68
65
1
1
1
1
6
7
8
9
Reaction conditions same as that for Entry 2 in Table 4.
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Under the optimized reaction conditions, the generality and scope of this photocatalytic system using NiꢀAQDC
as catalyst were then examined with a series of diarylethynes as substrates (Table 6). Moderate to good yields have
been obtained for the reaction series with products listed in Table 6. It is notable that easily oxidized formyl group
remains intact during the oxidation process (2l in Table 6), indicating good oxidizing selectivity in functional groups.
Diarylethyne comprising thienyl group could also be oxidized into corresponding 1,2ꢀdiketones at 72% yield.
Table 6 Scope of diarylethynes 1.
Reaction conditions same as that for Entry 2 in Table 4. Isolation yields reported.
The NiꢀAQDC catalyst therefore represents an efficient heterogeneous catalyst for this reaction system and can
be easily recovered by filtration after the reaction, attributed to very poor solubility in reaction medium. To determine
the amount of catalyst loss during the catalytic process, we determined the concentration of Ni element remaining in
the solution after the reaction, through inductive coupled plasma emission spectrum (ICP). The result shows that only
0.04 ꢂg/mL of Ni element presents in the filtrate, accounting for 0.0068% loss of the catalyst, which means that the
photocatalyst can be efficiently recovered. Powder Xꢀray diffraction patterns of the NiꢀAQDC catalyst before and after
the catalytic reaction reveal no change in the material structure, indicating the fact that the catalyst stays intact during
the catalytic process. The high recovery yield and good stability of the catalyst dictate the possibility of recycled use.
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Fig. 5 (a) Recyclability of NiꢀAQDC catalyst. (b) Powder Xꢀray diffraction patterns of the NiꢀAQDC catalyst before
and after recycling.
Consequently, the recyclability and reusability of NiꢀAQDC catalyst is evaluated by such oxidation reactions
using diphenylethyne as the substrate. The catalyst is recovered from the reaction system by filtration, washed with
dichloromethane, dried under vacuum, and subjected to catalytic use again without any operation of regeneration.
Upon four times of recycled use, the NiꢀAQDC complex is still able to efficiently catalyze the reaction without
significant loss of activity, demonstrating good recyclability as a photocatalyst (Fig. 5a). The yields with recycled
catalyst are comparable to that with asꢀsynthesized catalyst, all at ~80% level. Powder Xꢀray diffraction (PXRD)
pattern of the photocatalyst after five catalytic cycles is in good agreement with that of the asꢀsynthesized NiꢀAQDC
complex (Fig. 5b).
2
H O
R
Ar
Ar
R
(
B )
OH
O
2
ArꢀS ( A )
NiꢀAQDC(H)
ArꢀSH
O
O
O
R
R
Ar
(
C )
O
H
2
O
ArꢀSH
NiꢀAQDC*
NiꢀAQDC
Ar
O
Blue LED
Scheme 2. Proposed reaction mechanism.
In presence of 2.0 equivalent 2,2,6,6ꢀtetramethylꢀ1ꢀpiperidinyloxy (TEMPO), no product was detected from the
reaction system under standard conditions, being a sign of possible radical pathway for the transformation. Based on
this and above mentioned observations, a plausible reaction mechanism has been proposed (Scheme 2). Upon
irradiation by visible light, the NiꢀAQDC catalyst converts to excited state, which undergoes reductive quenching by
thiophenol to generate its semiquinone form accompanied by the formation of thiophenyl radical (A). Radical addition
2
involving thiophenyl radical, water and diphenylethyne affords intermediate B, which reacts with O to generate
•
intermediate C. Redox reaction between semiquinone NiꢀAQDC(H) and intermediate C gives rise to the formation of
1,2ꢀdiketone and ground state NiꢀAQDC.
1
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Conclusions
To develop novel heterogeneous photocatalysts, the quinone functionalized dicarboxylate ligand
,10ꢀanthraquinoneꢀ1,4ꢀdicarboxylate acid has been designed and synthesized, followed by the syntheses of a family
9
of transition metal complexes. The ligand shows versatile coordination modes in these metal complexes to result in
various polymeric structures. These coordination polymers are able to catalyze the reaction of visibleꢀlight driven
aerobic oxidation of alkynes into 1,2ꢀdiꢀketones under ambient conditions. The NiꢀAQDC complex exhibits enhanced
catalytic activity as compared to the ligand itself, representing the best one among the family of metal complexes over
a broad scope of substrates with moderate to good yield. Recovery of the catalyst is as simple as filtration, washing
and drying, with only 0.0068% loss in solution determined by ICP. Remarkably, the recovered catalyst can be used
again to catalyze the reaction without any need of regeneration for at least five times with persistently good activity.
Xꢀray powder diffractions before and after five catalytic runs reveal no structure change of the NiꢀAQDC catalyst,
demonstrating good stability. This catalyst represents an environmentally friendly economic and recyclable
photocatalyst with good performance, which might be used conveniently in lightꢀirradiated flow reactors as fixed bed
catalyst. This ligand design strategy is potentially useful in the development of functional metalꢀorganic hybrid
materials with versatile applications.
Conflicts of interest
There are no conflicts of interest to declare.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (21671104), Jiangsu Key Technology
R&D Program (BE2016010 and BE2016010ꢀ1), Natural Science Foundation of Jiangsu Higher Education Institutions
of China (17KJA430010 and 15KJA350002), the Priority Academic Program Development of Jiangsu Higher
Educational Institutions, and Jiangsu Collaborative Innovation Center of Biomedical Functional Materials.
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