Syntheses, crystal structures and photocatalytic properties of transition metal complexes based on 9,10-anthraquinone-1,3-dicarboxylate

Article abstract of DOI:10.1007/s11243-019-00334-2

Five transition metal complexes based on the?ligand 9,10-anthraquinone-1,3-dicarboxylate acid (1,3-H₂AQDC) have been synthesized and characterized by physico-chemical and spectroscopic methods. Single-crystal analyses show that the?complexes {M

Full text of DOI:10.1007/s11243-019-00334-2

Transition Metal Chemistry
https://doi.org/10.1007/s11243-019-00334-2
Syntheses, crystal structures and photocatalytic
properties of transition metal complexes based
on 9,10‑anthraquinone‑1,3‑dicarboxylate
Cai‑Ping Ye1 · Rong Ling1 · Lin‑Feng Yang1 · Qi Zhang1 · Jie Han2 · Xu‑Dong Chen1
Received: 12 April 2019 / Accepted: 11 May 2019
© Springer Nature Switzerland AG 2019
Abstract
Five transition metal complexes based on the ligand 9,10-anthraquinone-1,3-dicarboxylate acid (1,3-H2AQDC) have been
synthesized and characterized by physico-chemical and spectroscopic methods. Single-crystal analyses show that the com-
plexes {Mn(1,3-AQDC)(H2O)4·(H2O)}n, {Zn(1,3-AQDC)(H2O)4·(H2O)}n, {Ni(1,3-AQDC)(4,4′-bpy)(H2O)2(CH3OH)}n and
{Co(1,3-AQDC)(4,4′-bpy)(H2O)2(CH3OH)}n (4,4′-bpy=4,4′-bipyridine) comprise one-dimensional (1D) chains in their
crystal structures, while [Ni(1,3-AQDC)(H2O)4·(H2O)3]2 bears dimeric units, and these secondary building units are further
linked by hydrogen bonding to form three-dimensional structures. These compounds proved to be able to catalyze visible-
light-driven air-oxidation reactions of diarylethyne into diketones under mild conditions.
Introduction
and have been widely used as coordination sites to fabricate
novel metal–organic frameworks. Therefore, ligands with
carboxylate groups are considered as excellent candidates
for constructing versatile fnite and infnite structures [9].
Quinones are good photosensitizers and play impor-
tant roles in biological systems [10, 11], exhibiting rich
redox properties in such processes as single electron trans-
fer and proton-coupled electron transfer [12, 13]. In this
study, the proligand 9,10-anthraquinone-1,3-dicarboxylic
acid (1,3-H2AQDC) has been synthesized, with the anth-
raquinone portion as photosensitizer to harvest light and
carboxylate groups as coordination sites to bind metal
ions. Five transition metal complexes based on this ligand
have then been synthesized, which were characterized by
single-crystal/powder X-ray difraction, infrared spectra
and thermal gravimetric analyses. Among them, the com-
plexes {Mn(1,3-AQDC)(H2O)4·(H2O)}n (1), {Zn(1,3-
AQDC)(H2O)4·(H2O)}n (2), {Ni(1,3-AQDC)(4,4′-bpy)
(H2O)2(CH3OH)}n (4) and {Co(1,3-AQDC)(4,4′-bpy)
(H2O)2(CH3OH)}n (5) (4,4′-bpy=4,4′-bipyridine) bear one-
dimensional (1D) chains in their crystal structures, while
[Ni(1,3-AQDC)(H2O)4·(H2O)3]2 (3) comprises dimeric
units. Hydrogen bonding interactions exist widely in these
complexes, which link the dimers/chains into three-dimen-
sional frameworks.
The design and construction of functionalized coordination
compounds has been one of the central themes in coordi-
nation chemistry, crystal engineering and supramolecular
chemistry, ascribed to their established and/or potential
applications in such felds as gas adsorption/separation,
catalysis, sensors, molecular probes and magnetism [1–7].
Some general methods for functionalization of coordination
complexes toward desired physical properties have been
developed, such as wise selection of metal ions as nodes
and/or organic ligands as linkers, rational design and syn-
thesis of ligands/metal complexes, and post-synthetic modi-
fcations [8]. Carboxylate groups have very rich coordina-
tion modes that may result in diverse framework structures
* Jie Han
chan@ouhk.edu.hk
* Xu-Dong Chen
xdchen@njnu.edu.cn
1
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, People’s Republic of China
Inspired by our recent studies on the photocatalytic activi-
ties of metal complexes derived from 9,10-anthraquinone-
1,4-dicarboxylic acid [14], we investigated the photocatalytic
2
School of Science and Technology, The Open
University of Hong Kong, Kowloon, Hong Kong SAR,
People’s Republic of China
Vol.:(0123456789)
1 3

Transition Metal Chemistry
properties of these complexes in promoting organic reac-
tions. As an important category of organic molecules used
for synthesizing biologically active heterocyclic compounds,
1,2-diketones have attracted lots of synthetic eforts utiliz-
ing diferent catalysts based on metal complexes comprising
Pd [15, 16], Au [17], Ru [18, 19] and Cu [20], as well as
the organic dye eosin Y [21]. Along this line, the air-oxi-
dation reactions of alkynes to produce 1,2-diketones under
the irradiation of visible light were selected as the template
reaction to verify the photocatalytic activity of complexes
1–5. They proved to be good heterogeneous catalysts, being
able to catalyze visible-light-driven oxidation reactions of
diphenylethyne into benzil in air under mild conditions, in
diferent yields.
(ppm) 13.72 (s, 2H), 8.69 (d, J=2.0 Hz, 1H), 8.19 (m, 3H),
7.95 (m, 2H). Anal. Calcd for C16H8O6: C, 64.87; H, 2.72.
Found: C, 64.91; H, 2.70.
Synthesis of complex 1
{Mn(1,3-AQDC)(H2O)4·(H2O)}n (1): A mixture of
Mn(OAc)2·4H2O (12.2 mg, 0.05 mmol) and 1,3-H2AQDC
(14.8 mg, 0.05 mmol) was dissolved into 5.0 mL water with
stirring. The reaction solution was fltrated and allowed to
evaporate at room temperature for 1 week to give yellow
crystals, which were collected by filtration and washed
with acetone (20.3 mg, yield: 46%). Anal. Calcd for
C16H16MnO11: C, 43.75; H, 3.67. Found: C, 43.58; H, 3.75.
IR (KBr, cm−1): 1670(s, C=O), 1580(s, carboxylate C=O),
1280(s, C–O).
Experimental
Synthesis of complex 2
Materials and methods
{Zn(1,3-AQDC)(H2O)4·(H2O)}n (2): This complex
was obtained by the same procedure as that for 1 using
Zn(OAc)2·2H2O instead of Mn(OAc)2·4H2O. Yellow needle-
like crystals were obtained by fltration and washed with
water and acetone (39.1 mg yield: 87%). Anal. Calcd for
C16H16O11Zn: C, 42.74; H, 3.59. Found: C, 43.01; H, 3.65.
IR (KBr, cm−1): 1670(s, C=O), 1590(s, carboxylate C=O),
1280(s, C–O).
All reagents and solvents were obtained from commercial
suppliers and used without further purifcation. The proli-
gand 1,3-H2AQDC was synthesized using a modifed pro-
cedure as reported [22]. Thermal analyses were carried out
under nitrogen fow on a Mettler-Toledo TGA/DSC STARe
system in the temperature range of 25 to 800 °C with a heat-
ing rate of 10 °C min−1. Elemental analyses were performed
using a PE-240C elemental analyzer. Infrared spectra were
recorded on a VECTOR 22 spectrometer with pressed KBr
pellets in the range of 4000 to 400 cm−1. 1H and 13C NMR
spectra were obtained in CDCl3 on a Bruker 400 MHz
spectrometer.
Synthesis of complex 3
[Ni(1,3-AQDC)(H2O)4·(H2O)3]2 (3): A mixture of
Ni(OAc)2·4H2O (24.8 mg, 0.10 mmol) and 1,3-H2AQDC
(14.8 mg, 0.05 mmol) was dissolved into 5.0 mL water and
stirred together. Filtration followed by acetone difusion
aforded green plate-like crystals in several days (36.4 mg,
yield: 76%). Anal. Calcd for C16H20NiO13: C, 40.12; H, 4.21.
Found: C, 40.31; H, 4.17. IR (KBr, cm−1) 1670(s, C=O)
1590(s, carboxylate C=O), 1280(s, C–O).
Synthesis of 9,10‑anthraquinone‑1,3‑dicarboxylate
acid
1,3-Dimethylanthraquinone (7.55 g, 32.0 mmol) was dis-
solved into a mixed solvent of pyridine (300 ml) and water
(200 ml) and heated to refux. Then, KMnO4 (161.70 g,
125 mmol) was added gram by gram in 48 h. After being
cooled to room temperature, the precipitated MnO2 was fl-
tered of. The fltrate was then subjected to rotary evapora-
tion to yield a yellow oil, and the fltration bed was extracted
with a 20% aqueous solution of KOH (5 × 100 ml). The
basic extractions were combined together and concentrated
to approximately one-third of their original volume with
a rotary evaporator. This residue was then combined with
the above-mentioned yellow oil, which was then acidifed
to pH = 1 with concentrated hydrochloric acid to precipi-
tate a yellow powder. The yellow powder was collected by
fltration and treated with diethyl ether. After being dried
in air, pure 9,10-anthraquinone-1,3-dicarboxylate acid was
Synthesis of complex 4
{Ni(1,3-AQDC)(4,4′-bpy)(H2O)2(CH3OH)}n (4): A mixture
of 1,3-H2AQDC (14.8 mg, 0.05 mmol), Ni(OAc)2·4H2O
(24.8 mg, 0.10 mmol) and a 1.0 mL methanol solution of
4,4′-bpy (15.6 mg, 0.10 mmol) was placed in a Parr Tefon-
lined stainless steel vessel (25.0 mL) and heated to 120 °C
for 3 days. Green crystals were obtained in 48 h on cooling
to room temperature slowly, which were collected by fltra-
tion and washed with methanol (43.4 mg, yield: 87%). Anal.
Calcd for C22H18NNiO9: C, 52.95; H, 3.64; N, 2.81. Found:
C, 52.80; H, 3.55; N, 2.99. IR (KBr, cm−1): 1670(s, C=O),
1580(s, carboxylate C=O), 1420(s, C–N), 1280(s, C–O).
1
obtained in 33% yield. H NMR (400 MHz, d6-DMSO) δ
1 3

Transition Metal Chemistry
4H). 13C NMR (100 MHz, CDCl3) δ (ppm) 194.6, 134.9,
Synthesis of complex 5
133.0, 129.9, 129.0.
{Co(1,3-AQDC)(4,4′-bpy)(H2O)2(CH3OH)}n (5): Com-
pound 5 was obtained by the same procedure as that
for 4 except that Co(OAc)2·4H2O was used instead of
Ni(OAc)2·4H2O as starting material. Pink crystals were
obtained by fltration and washed with methanol (26.4 mg,
yield: 53%). Anal. Calcd for C22H18CoNO9: C, 52.92; H,
3.63; N, 2.81. Found: C, 52.66; H, 3.84; N, 2.90. IR (KBr,
cm−1): 1668(s, C=O), 1579(s, carboxylate C=O), 1417(m,
C–N), 1282(m, C–O).
X‑ray crystallography
Single-crystal X-ray difraction data of all fve complexes
were collected at 293 K on a Bruker APEX II CCD dif-
fractometer operating at 50 kV and 30 mA using Mo Ka
radiation (λ=0.71073 Å). Data were integrated by SAINT
and scaled with either a numerical or multi-scan absorption
correction using SADABS [23]. All structures were solved
by direct methods or Patterson maps [24] and refned by
full-matrix least squares on F2 using the SHELXL-2014
[25] and OLEX2 [26] programs. All non-hydrogen atoms
were refned anisotropically. Non-water hydrogen atoms
were added at calculated positions and refned using a rid-
ing model. Hydrogen atoms of water molecules were located
by diference Fourier maps and refned using a riding model
at restrained distance of 0.86 Å. Crystallographic data for all
fve complexes are summarized in Table 1.
Typical procedure for photocatalytic experiments
The photocatalyst (0.02 mmol), 1,2-diphenylethyne
(35.6 mg, 0.2 mmol) and 4-chlorobenzenethiol (57.8 mg,
0.4 mmol) were mixed in 2.0 mL MeCN. The reaction mix-
ture was stirred at room temperature under the irradiation
of a blue LED for 84 h in ambient air. Filtration was done
on completion of the reaction. The thus collected fltrate
containing the fnal product was evaporated to dryness and
subjected to column chromatography (silica gel, petroleum
ether/ethyl acetate) to aford pure benzil as a yellow solid.
The catalyst isolated by fltration was washed by dichlo-
romethane and dried in air before being subjected to recy-
cled use. 1H NMR of benzil (400 MHz, CDCl3) δ (ppm) 7.90
(d, J=8.8 Hz, 4H), 7.60 (t, J=7.4 Hz, 2H), 7.51 (t, J=8.0,
Results and discussion
Crystal structures of complexes 1 and 2
Complexes {Mn(1,3-AQDC)(H2O)4·(H2O)}n (1) and
{Zn(1,3-AQDC)(H2O)4·(H2O)}n (2) are isostructural,
Table 1 Crystallographic data for complexes 1–5
1
2
3
4
5
Empirical formula
Fw
C16H16MnO11
439.23
C16H16O11Zn
449.66
C16H20NiO13
479.03
C22H18NNiO9
499.08
C22H18NCoO9
499.30
Crystal system
Space group
a (Å)
b (Å)
c (Å)
Triclinic
̄
P1
Triclinic
̄
P1
Monoclinic
P21/n
7.2689(4)
24.5673(12)
10.7500(5)
90
90.350(3)
90
1919.67(17)
4
Monoclinic
P21/c
10.7127(18)
15.200(3)
12.976(2)
90
106.545(2)
90
2025.4(6)
4
Monoclinic
P21/c
10.770(8)
15.222(12)
12.995(10)
90
106.953(11)
90
2038(3)
4
9.583(5)
9.851(5)
10.187(5)
116.985(11)
97.827(8)
92.689(7)
842.5(7)
2
9.6470(19)
9.7839(19)
10.110(2)
117.171(2)
98.057(2)
92.512(2)
834.3(3)
2
α (°)
β (°)
γ (°)
V (Å)
Z
Dc (g·cm−3
)
1.731
0.848
1.790
1.537
1.657
1.081
1.637
1.106
1.627
0.900
μ (mm−1
)
F(000)
450
460
992
1028
1024
Ra1, wRb2 [I>2σ (I)]
0.0295, 0.0841
0.0325, 0.0867
0.385/− 0.367
0.0283, 0.0866
0.0337, 0.0908
0.35/− 0.26
0.0327, 0.0630
0.0549, 0.0714
0.33/− 0.52
0.0369, 0.0975
0.0431, 0.1016
0.48/− 0.87
0.0525, 0.1074
0.0963, 0.1285
0.89/− 0.90
Ra1, wRb2 (all data)
(Δρ) max, (Δρ) min (e Å−3
)
/
∑
b wR2 =
∑
a
��
Fo − Fc
��
�
�
��
��
�
�
�
R1 =
Fo
�
�
�
�
[
]/
[
w(F2)2
o
]}
∑
∑
1∕ 2
w(F2 − F2)2
o
c
1 3

Transition Metal Chemistry
̄
contributing to the assembly of the dimers into a 3D supra-
molecular structure (Table 2).
both crystallize in triclinic space group P1. One metal
atom, which is Mn(II) in 1 and Zn(II) in 2, one 1,3-
AQDC2− ligand, four water ligands and one lattice water
molecule constitute the asymmetric unit in the crystal
structure. The metal centers adopt octahedral geometry
composed of two carboxylate oxygen atoms from two
symmetry-related ligands and four water ligands (Fig. 1
with compound 2 as example). The Mn–O bond distances
in complex 1 are between 2.136(2) and 2.209(2) Å, while
the Zn–O bond lengths in compound 2 fall in the range
of 2.052(2)–2.167(7) Å. The dihedral angles between the
carboxylate groups (1- and 3-positions) and the attached
phenyl rings are 69.3(1)° and 18.2(1)° for 1, and 69.5(1)°
and 17.4(1)° for 2, respectively, indicating the most con-
strained ligand conformation in this family of metal com-
plexes. The μ2-briding 1,3-AQDC2− ligands connect the
six-coordinated metal centers into 1D chains, which fur-
ther assemble into a 3D network via intermolecular hydro-
gen bonding (Table 2).
Crystal structures of complexes 4 and 5
Complexes {Ni(1,3-AQDC)(4,4′-bpy)(H2O)2(CH3OH)}n
(4) and {Co(1,3-AQDC)(4,4′-bpy)(H2O)2(CH3OH)}n
(5) are isostructural, both crystallizing in monoclinic
space group P21/c. In their crystal structures, each asym-
metric unit contains one divalent metal atom, one 1,3-
AQDC2− ligand, half a 4,4′-bpy ligand, one methanol ligand
and two water ligands. The metal center is six-coordinated
by two carboxylate O atoms from two symmetry-related 1,3-
AQDC2− ligands, one N atom from 4,4′-bpy, and three O
atoms from coordinated methanol and water ligands (Fig. 3a
with 4 as the example). The 1- and 3-positioned carboxylate
groups of the ligand also hold diferent dihedral angles to the
attached phenyl ring, being 84.4(1)° and 11.4(1)° for 4, and
83.1(1)°and 11.0(1)° for 5, respectively. In 4, the Ni–O/N
bond lengths fall in the range of 2.033(2)–2.103(2) Å and
the nonlinear O–Ni–O/N bond angles are between 83.15(7)
and 93.96(7)°. These parameters in 5 are comparable with
those in 4, with the Co–O/N bond distances ranging from
2.042(3) to 2.155(4) Å and the nonlinear O–Co–O/N bond
angles between 82.99(13)° and 94.75(13)°. It is interesting
that these complexes also bear dimeric units similar as that
in compound 3, which are linked by the auxiliary ligands
4,4′-bpy to form 1D stair-like chains (Fig. 3b with 4 as the
example). The 1D chains are interconnected by hydrogen
bonding interactions to form a 3D structure (Table 2).
Crystal structure of complex 3
Single-crystal X-ray diffraction reveals that [Ni(1,3-
AQDC)(H2O)4·(H2O)3]2 (3) crystalizes in monoclinic
space group P21/n. The asymmetric unit of 3 contains one
Ni(II) atom, one 1,3-AQDC2− anion, four water ligands
and two lattice water molecules. The distorted octahedron
confguration of the Ni(II) center is constituted by oxygen
atoms from four water ligands and two carboxylate groups
of two symmetry-related 1,3-AQDC2− ligands. The Ni–O
bond contacts range from 2.025(2) to 2.097(2) Å. The
dihedral angles between the carboxylate groups and their
attached phenyl rings are 98.5(1)° and 28.5(1)° for the
1- and 3-positions, respectively. The 1,3-AQDC2− ligands
take μ2-briding mode to form dimeric moieties that are
further stabilized by intramolecular hydrogen bonding
(Fig. 2). Very rich intermolecular hydrogen bonding
motifs also exist in the crystal packing of compound 3,
Structural comparison and solubility
Compared to previously reported metal complexes based on
the isomeric ligand 9,10-anthraquinone-1,4-dicarboxylate
acid (1,4-AQDC), complexes of 1,3-AQDC exhibit lower
dimensionalities (0D and 1D for complexes of 1,3-AQDC
v.s. 1D, 2D and 3D for complexes of 1,4-AQDC). This might
be related to the structural diferences of the ligand asso-
ciated with their intrinsic restraint generated from steric
hindrance between the carboxylate groups and adjacent
Fig. 1 Metal coordination
sphere and 1D chain structure in
complex 2. Symmetry codes: ix,
−1+y, z
1 3

Transition Metal Chemistry
Table 2 Hydrogen bonding parameters in complexes 1-5 (Å and deg)
Table 2 (continued)
Complex D-H…A
D(H…A) D(D…A) ∠DHA
Complex D-H…A
D(H…A) D(D…A) ∠DHA
Symmetry codes: i1−x, 1−y, 1−z; ii1−x,−1/2+y, 3/2−z; iii1+x,
1
O7 W-H7 WA…O5i
2.04
2.06
1.87
2.11
1.89
2.00
1.90
2.06
2.01
2.864(2) 162.8
2.814(2) 145.7
2.713(2) 170.7
2.830(2) 142.3
2.721(2) 164.0
2.718(2) 141.5
2.750(2) 170.4
2.855(2) 156.6
2.848(2) 169.5
+y, +z
O7W-H7WB…O3ii
O8 W-H8 WA…O6iii
O8W-H8WB…O4iv
O9 W-H9WA…O11 W
O9 W-H9WB…O6
O10 W-H10A…O1i
O10 W-H10B…O4iii
O11 W-H11B…O4iii
5
O8 W-H8 WA…O3
O8W-H8WB…O6i
O9 W-H9 WA…O2ii
O9 W-H9WB…O1i
O9W-H9WB…O6i
O7-H7…O6iii
1.83
1.90
2.00
2.20
2.15
1.93
2.668(4) 164.6
2.748(5) 167.9
2.854(5) 175.0
2.829(5) 129.4
2.864(4) 140.4
2.670(5) 154.0
Symmetry codes: i1−x, 1/2+y, 3/2−z; ii1+x, +x, +z; iii1−x,
1−y, 1−z
Symmetry codes: i1−x, 2−y, 1−z; ii1−x, 1−y, 1−z; iii1−x,
1−y, −z; iv +x, 1+y, +z
2
O7 W-H7WA…O11 W
O7 W-H7WB…O4
O8 W-H8 WA…O4i
O8W-H8WB…O6ii
O9 W-H9 WA…O3iii
O9W-H9WB…O5iv
O10 W-H10A…O6i
O10 W-H10B…O2iii
O11 W-H11B…O6i
1.88
1.93
1.90
2.04
2.04
2.13
2.11
1.93
2.03
2.722(3) 164.7
2.677(3) 144.6
2.729(3) 162.5
2.775(3) 143.6
2.875(3) 163.4
2.829(3) 138.3
2.902(3) 152.8
2.765(3) 163.1
2.859(3) 162.1
Symmetry codes: i1−x, 1−y, 2−z; ii +x, −1+y, +z; iii1−x, −y,
1−z; iv1−x, 1−y, 1−z.
3
O7 W-H7 WA…O2i
O7W-H7WB…O5ii
O8 W-H8WA…O12 W
1.94
1.82
1.94
2.788(2) 168.1
2.670(2) 170.3
2.771(2) 163.0
2.751(2) 166.2
2.693(2) 163.2
2.630(2) 159.5
2.791(2) 177.1
2.828(3) 175.4
2.712(2) 155.2
2.763(3) 161.4
2.875(2) 151.2
2.825(3) 171.9
2.754(2) 167.7
O8 W-H8WB…O11Wiii 1.91
O9 W-H9 WA…O5
O9W-H9WB…O3iv
1.86
1.81
O10 W-H10A…O12Wv 1.93
Fig. 2 The dimeric unit in complex 3 showing coordination geometry
of Ni(II) center and binding mode of 1,3-AQDC2− ligand. Symmetry
codes: i−x+1, −y+1, −z+1
O10 W-H10B…O13Wvi 1.97
O11 W-H11A…O3
O11 W-H11B…O13vii
O12 W-H12A…O1viii
1.91
1.93
2.09
Conjugation of the 3-positioned carboxylate group with an
attached phenyl ring makes it less favorable for higher con-
nectivity at this site and thus resulted in diferent binding
modes of the ligand 1,3-AQDC as compared to 1,4-AQDC,
leading to metal complexes of diferent dimensionalities.
Complexes 1–5 are all slightly soluble in DMSO but not
soluble in water and other common organic solvents includ-
ing methanol, DMF, acetonitrile, acetone, tetrahydrofuran
and dichloromethane. It is interesting that complex 3 with
discrete dimeric structural units exhibits similar poor solu-
bility as that of the coordination polymers in this family,
which might be ascribed to very rich hydrogen bonding
interactions in its crystal packing.
O12 W-H12B…O11Wiv 1.97
O13 W-H13A…O9 W
1.91
Symmetry codes: i1−x, 1−y, 1−z; ii1+x, +y, +z; iii3/2−x,
1/2+y, 1/2−z; iv1−x, 1−y, −z; v −1/2+x, 3/2−y, 1/2+z;
vi1/2+x, 3/2−y, 1/2+z; vii1/2−x,−1/2+y,1/2−z; viii1/2+x,
3/2−y,−1/2+z
4
O7-H7A…O5i
1.86
2.22
2.20
2.03
1.91
1.83
2.666(3) 151.4
2.833(3) 128.3
2.861(3) 133.7
2.865(3) 162.6
2.746(3) 162.3
2.646(3) 156.9
O8 W-H8 WA…O1ii
O8 W-H8WA…O5ii
O8W-H8WB…O2iii
O9 W-H9 WA…O5ii
O9W-H9WB…O3
carbonyl groups. Regarding the dihedral angles of carboxy-
late groups with attached phenyl rings, those for the 3-posi-
tioned ones in 1,3-AQDC are much smaller than the other
ones in complexes of both 1,3-AQDC and 1,4-AQDC.
Thermogravimetric analyses
Thermal analyses for complexes 1–5 were performed from
25 to 800 °C under a nitrogen atmosphere at a heating rate
1 3

Transition Metal Chemistry
Fig. 3 a Coordination geometry of Ni(II) center and binding mode of ligand in complex 4; b 1D chain structure in complex 4. Symmetry codes:
i−x+1, −y+1, −z+1
Table 3 Studies of the catalytic performance of the complexes
Entry Photocatalyst (mol %)
Yield (%)
1
2
3
4
5
6
7
8
9
Complex 1 (10)
47
58
71
49
33
0
Complex 2 (10)
Complex 3 (10)
Complex 4 (10)
Complex 5 (10)
Complex 3 (10) (in the dark)
Complex 3 (10) (without 4-chlorobenzenethiol) Trace
–
0
0
Fig. 4 TGA curves for complexes 1–5
Complex 3 (10) (under 1 atm N2 atmosphere)
of 10 °C min−1 (Fig. 4). All water molecules (including
the coordinated and lattice ones) of compound 1 were lost
between 68 and 149 °C (obsd 21.54%, calcd 20.50%). How-
ever, the solvent molecules in crystalline samples of com-
plexes 2 and 3 are not stable at room temperature, resulting
in weight loss before heating, which makes it hard to corre-
late the weight loss percentage with the loss of coordinated
or lattice solvents. It is interesting that the coordinated sol-
vent molecules in 4 are gone between 121 and 180 °C (obsd
12.52%, calcd 13.62%), while those in 5 are gone between
83 and 117 °C (obsd 12.11%, calcd 13.62%), although the
two complexes are isostructural. Further decomposition
temperatures are 300 °C for compound 4 and 312 °C for 5.
Therefore, we investigated the photocatalytic activity of
these complexes based on 9,10-anthraquinone-1,3-dicarbox-
ylate. We found that these complexes are able to promote the
air-oxidation of diarylethyne into diketones at ambient con-
ditions when irradiated by a blue LED. These heterogeneous
catalysts hold the advantages of easy isolation and recycling
as compared to conventional homogeneous photocatalysts
such as complexes with precious metals and organic dyes
[15–21].
All of the complexes were subjected to the evaluation of
catalytic activities for promoting the above-mentioned photo-
oxidation reaction of 1,2-diphenylethyne into benzil (Entries
1–5 in Table 3). The results reveal that they all exhibit moder-
ate activities except complex 5. However, the catalytic activi-
ties of these complexes are not as good as our previously
reported Ni(II) complex [14], which gives 81% isolated yield
on this substrate. For further comparison, Ni(II) complex 3
showing the best efciency was used as the catalyst for a few
other substrates under the same reaction conditions, as shown
in Table 4. These reactions further demonstrate that complex
3 is less efcient in catalyzing these photo-oxidation reactions
Photocatalytic activity
In our recent studies of the photocatalytic activities of metal
complexes based on 9,10-anthraquinone-1,4-dicarboxylate
[14], we have demonstrated that these kinds of anthraqui-
none-functionalized coordination complexes are able to
catalyze the visible-light-driven air-oxidation reactions of
diarylalkynes into 1,2-diketones under mild conditions.
1 3

Transition Metal Chemistry
Table 4 A few typical substrates with complex 3 as catalyst
subsequently, and their crystal structures have been inves-
tigated. The metal centers of all of these complexes have
aqua ligands in their coordination spheres and form one-
dimensional chains or dimeric units through the connections
of the anionic ligand, which are further linked by versatile
hydrogen bonding motifs to generate three-dimensional net-
works. These metal complexes could be used as heterogene-
ous photocatalyst for the photo-oxidation of diarylalkynes
into 1,2-diketones. Current research is focused on explora-
tion of new reaction systems in which these complexes could
be used as more efcient heterogeneous photocatalysts.
Supplementary materials
CCDC 1870373 and 1832026-1832029 contain the supple-
mentary crystallographic data for complexes 1–5. These data
can be obtained free of charge from the Cambridge Crystal-
lographic Data Centre via www.ccdc.cam.ac.uk/data_reque
st/cif.
as compared to our previous reported Ni(II) complex. Experi-
mental observations suggest that the reaction mechanism
should be the same as the one reported in our previous study,
because further studies revealed that no reaction was observed
in the absence of the factors of photocatalyst, 4-chloroben-
zenethiol, blue LED and air (entry 6–9 in Table 3) [14].
These complexes are easily isolated from the reaction
mixture by fltration, and could be recycled in catalyzing
the photo-oxidation reaction with no obvious loss of activ-
ity. In the case that 3 was recycled for further catalysis with
1,2-diphenylethyne as substrate, benzil could be obtained in
63–67% yields for three cycles. Only trace amounts of metal
ions were detected in the fltrate obtained after the reaction
using ICP analysis, indicating less than 0.01% catalyst loss
for all complexes. As has been proposed in the description of
solubility, very rich hydrogen bonding interactions in com-
plex 3 decrease its solubility signifcantly to the same level as
that for other coordination polymers in this series, which also
accounts for the same level of metal leaching after catalysis.
The recyclability of Ni(II) complexes in this work is about the
same as that of the Ni(II) complex previously reported, but the
catalytic efciency is not as good as the previously reported
one. A plausible explanation is that steric hindrance between
carboxylate groups and adjacent carbonyl groups is diferent in
1,3- and 1,4-anthraquinone dicarboxylate, resulting in difer-
ent conjugation systems in the ligands and distinct electronic
properties regarding coordination, which subsequently afects
catalytic properties of their metal complexes.
Acknowledgements This work was supported by the National Natu-
ral Science Foundation of China (21671104), Jiangsu Key Technol-
ogy R&D Program (BE2016010 and BE2016010-1), Natural Sci-
ence Foundation of Jiangsu Higher Education Institutions of China
(17KJA430010 and 15KJA350002) and the Priority Academic Program
Development of Jiangsu Higher Educational Institutions.
Compliance with ethical standards
Conflict of interest The authors declare that they have no confict of
interest.
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