Metal-Organic Framework Based on Heptanuclear Cu-O Clusters and Its Application as a Recyclable Photocatalyst for Stepwise Selective Catalysis

Article abstract of DOI:10.1021/acs.inorgchem.9b02084

Visible-light driven photoreactions using metal-organic frameworks (MOFs) as catalysts are promising with regard to their environmental friendly features such as the use of renewable and sustainable energy of visible light and potential catalyst recyclability. To develop potential heterogeneous photocatalysts, a family of three copper(II) coordination polymers bearing different Cu-O assemblies have been synthesized with the ligand 4,4-disulfo-[1,1-biphenyl]-2,2-dicarboxylate acid (H₄DSDC), namely, {[Cu₇(DSDC)₂(OH)₆(H₂O)₁₀]·xH₂O}_n (1), {[Cu₄(DSDC)(4,4-bpy)₂(OH)₄]·2H₂O}_n (2), and {Cu₂(DSDC)(phen)₂(H₂O)₂}_n (3) (4,4-bpy = 4,4-bipyridine and phen = 1,10-phenanthroline). Complex 1 represents a metal-organic framework featuring a NbO type topology constructed from the infinite linkage of heptanuclear [Cu₇(μ₃-OH)₆(H₂O)₁₀]⁸⁺ clusters by deprotonated DSDC^4- ligands, comprising one-dimensional hexagonal channels of a diameter around 11 ? that are filled with water molecules. The infinite waving {[Cu₂(OH)₂]²⁺}_n ladderlike chains in complex 2 are bridged by DSDC^4- and 4,4-bpy ligands into a three-dimensional framework. A two-dimensional layered structure is formed in complex 3 due to the existence of terminal phenanthroline ligands. All of the coordination polymers 1-3 are able to catalyze the visible-light driven oxidation of alcohols at mild conditions using hydrogen peroxide as an oxidant, in which complex 1 demonstrates satisfactory efficiency. Significantly for this photoreaction catalyzed by 1, the extent of oxidation over aryl primary alcohols is fully controllable with time-resolved product selectivity, giving either corresponding aldehydes or carboxylate acids in good yields. It is also remarkable that the photocatalyst could be recovered almost quantitatively on completion of the catalytic cycle without any structure change, and could be recycled for catalytic use for at least five cycles with constant efficiency. This photocatalyst with time-resolved selectivity for different products may provide new insight into the design and development of novel catalytic systems.

Full text of DOI:10.1021/acs.inorgchem.9b02084

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pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Metal−Organic Framework Based on Heptanuclear Cu−O Clusters
and Its Application as a Recyclable Photocatalyst for Stepwise
Selective Catalysis
Jie Zhou,† Xu Huang-Fu,† Yang-Ying Huang,† Chu-Ning Cao,† Jie Han,‡ Xiao-Li Zhao,*,§
and Xu-Dong Chen*,†
†Jiangsu Key Laboratory of Biofunctional Materials and Jiangsu Collaborative Innovation Center of Biomedical Functional Materials,
School of Chemistry and Materials Science, Nanjing Normal University, Nanjing 210023, China
‡School of Science & Technology, The Open University of Hong Kong, Kowloon, Hong Kong SAR, China
§Shanghai Key Laboratory of Green Chemistry and Chemical Processes, Department of Chemistry, East China Normal University,
Shanghai 200062, China
S
* Supporting Information
ABSTRACT: Visible-light driven photoreactions using metal−
organic frameworks (MOFs) as catalysts are promising with
regard to their environmental friendly features such as the use of
renewable and sustainable energy of visible light and potential
catalyst recyclability. To develop potential heterogeneous photo-
catalysts, a family of three copper(II) coordination polymers
bearing different Cu−O assemblies have been synthesized with
the ligand 4,4′-disulfo-[1,1′-biphenyl]-2,2′-dicarboxylate acid
(H4DSDC), namely, {[Cu7(DSDC)2(OH)6(H2O)10]·xH2O}n
(1), {[Cu4(DSDC)(4,4′-bpy)2(OH)4]·2H2O}n (2), and
{Cu2(DSDC)(phen)2(H2O)2}n (3) (4,4′-bpy = 4,4′-bipyridine
and phen = 1,10-phenanthroline). Complex 1 represents a
metal−organic framework featuring a NbO type topology
constructed from the infinite linkage of heptanuclear [Cu7(μ3-OH)6(H2O)10]8+ clusters by deprotonated DSDC4− ligands,
comprising one-dimensional hexagonal channels of a diameter around 11 Å that are filled with water molecules. The infinite
waving {[Cu2(OH)2]2+}n ladderlike chains in complex 2 are bridged by DSDC4− and 4,4′-bpy ligands into a three-dimensional
framework. A two-dimensional layered structure is formed in complex 3 due to the existence of terminal phenanthroline ligands.
All of the coordination polymers 1−3 are able to catalyze the visible-light driven oxidation of alcohols at mild conditions using
hydrogen peroxide as an oxidant, in which complex 1 demonstrates satisfactory efficiency. Significantly for this photoreaction
catalyzed by 1, the extent of oxidation over aryl primary alcohols is fully controllable with time-resolved product selectivity,
giving either corresponding aldehydes or carboxylate acids in good yields. It is also remarkable that the photocatalyst could be
recovered almost quantitatively on completion of the catalytic cycle without any structure change, and could be recycled for
catalytic use for at least five cycles with constant efficiency. This photocatalyst with time-resolved selectivity for different
products may provide new insight into the design and development of novel catalytic systems.
mild reaction conditions and product selectivity and hence is
growing into a powerful tool in organic synthesis.29−32 Metal
(RuII, IrIII, ReI, and OsII) polypyridine complexes and organic
dyes are two typical categories of photocatalysts that have
attracted intense research interests.33−37 Metal−organic frame-
works (MOFs) have demonstrated their success as heteroge-
neous catalysts in organic transformations, whereas their
applications in heterogeneous photocatalysis have been
scarce.24,38−40 It is highly desirable to develop efficient
heterogeneous photocatalysts with high catalytic activity, good
selectivity, and perfect recyclability in terms of green
INTRODUCTION
■
The polymeric coordination of metal cations and organic ligands
into highly ordered structures provides us with a promising
category of functional materials, and the so-called coordination
polymers (e.g., metal−organic frameworks) have wide applica-
tions in many fields such as proton conduction,1−5 gas
storage,6−10 separation,11−18 photoluminescence,19−23 organic
catalysis,24−28 and so on. Their typical features, such as flexible
coordination geometry, facile functionalization through design
and modification, easy isolation due to poor solubility in
common solvents, and potentially high atom economy, make
them perfect candidates as heterogeneous catalysts.
Photocatalysis utilizing the renewable and sustainable energy
of visible light holds intrinsic advantages in the aspects of both
Received: July 13, 2019
© XXXX American Chemical Society
A
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chemistry.41 Therefore, our attention has been focused on the
development of novel coordination polymers as efficient
heterogeneous photocatalysts.
°C. The reaction mixture was held at 130 °C for 30 min and cooled to
room temperature slowly to generate light blue crystals, which were
collected by filtration, washed with CH3OH, and dried at ambient
temperature. Yield: 60% based on H4DSDC. Elemental analysis for
C14H19Cu3.5O18S2, calcd (%): C, 22.07; H, 2.51. Found: C, 22.10; H,
2.46. IR spectrum (cm−1, KBr pellet): 3411 (s), 1596 (s), 1543 (s),
1411 (s), 1367 (m), 1217 (m), 1164 (s), 1147 (m), 1093 (m), 1041 (s),
996 (m), 828 (w), 802 (w), 785 (w), 757 (w), 670 (m), 617 (m), 572
(s), 554 (s), 476 (s).
Coordination complexes comprising Cu−O clusters have
been widely used as catalysts in molecular polymerization and
oxidation reactions.42−44 According to the feature that
carboxylate groups would ligate with Cu(II) cations under
basic conditions to generate Cu−O cluster structures,42,45,46 the
ligand 4,4′-disulfo-[1,1′-biphenyl]-2,2′-dicarboxylate acid has
therefore been synthesized, which bears both carboxylate and
sulfonate groups. The expectation is that rich coordinating
groups would allow not only the formation of cluster structures
but also infinite linkage of such clusters into a polymeric
framework, which would qualify as a good candidate for
heterogeneous catalysis. On the basis of this ligand, and in
certain cases together with auxiliary ligands, a series of
coordination polymers bearing different Cu−O cluster
structures have been synthesized, and their applications in
catalyzing oxidation of primary aryl alcohols using hydrogen
peroxide as an oxidant upon irradiation of visible light at room
temperature have thus been examined.
Remarkably, this photocatalyst could be recovered almost
quantitatively on completion of the reaction without any
structure change, which proved capable of recycled use for at
least five catalytic cycles with constant activity. With aryl primary
alcohols as the substrate, the oxidation product proved to be
either aldehyde or carboxylic acid, indicating a unique time-
resolved oxidation process with a good product selectivity. This
type of restorable and recyclable photocatalyst capable of
selective catalysis over successive reactions may shed light on the
design and development of novel heterogeneous catalysts
toward challenging organic reactions.
Synthesis of {[Cu4(DSDC)(4,4′-bpy)2(OH)4]·2H2O}n (2). A mixture of
Cu(NO3)2·3H2O (24.2 mg, 0.1 mmol), 4,4′-bipyridine (15.6 mg, 0.1
mmol), and H4DSDC (20.1 mg, 0.05 mmol) was dissolved into a mixed
solvent of dimethylacetamide/water (3.0/1.0 mL) in a 10 mL glass vial.
After the addition of 1.0 mL of 0.2 M NaOH solution, the vessel was
sealed with a cap and then heated to 130 °C. The reaction mixture was
held at 130 °C for 12 h and cooled to room temperature slowly to give
green crystals, which were collected by filtration, washed with CH3OH,
and dried at ambient temperature. Yield: 72% based on H4DSDC.
Elemental analysis for C17H15Cu2N2O8S, calcd (%): C, 38.20; H, 2.83;
N, 5.24. Found: C, 38.25; H, 2.78; N, 5.20. IR spectrum (cm−1, KBr
pellet): 3454 (m), 3102 (w), 3045 (w), 1614 (m), 1558 (s), 1422 (s),
1393 (s), 1365 (s), 1223 (s), 1195 (s), 1171 (s), 1165 (m), 1143 (m),
1120 (w), 1103 (w), 1074 (w), 1035 (s), 990 (m), 961 (w), 842 (w),
819 (s), 768 (w), 734 (w), 666 (m), 643 (m), 608 (s), 450 (s).
Synthesis of {Cu2(DSDC)(phen)2(H2O)2}n (3). Cu(NO3)2·3H2O
(24.2 mg, 0.1 mmol), 1,10-phenanthroline (18.0 mg, 0.1 mmol), and
H4DSDC (20.1 mg, 0.05 mmol) were dissolved into a mixed solvent of
dimethylformamide/water (2.0/1.0 mL) in a 10 mL glass vial. After the
addition of 1.0 mL of 0.2 M NaOH solution, the vessel was sealed with a
cap and then heated to 130 °C. The reaction mixture was held at 130 °C
for 2 days and cooled to room temperature slowly to give navy blue
crystals, which were collected by filtration, washed with CH3OH, and
dried at ambient temperature. Yield: 81% based on H4DSDC.
Elemental analysis for C38H26Cu2N4O12S2, calcd (%): C, 49.51; H,
2.84; N, 6.08. Found: C, 49.51; H, 2.77; N, 6.09. IR spectrum (cm−1,
KBr pellet): 3414 (s), 3108 (m), 3073 (m), 3057 (m), 1603 (s), 1574
(s), 1500 (m), 1467 (m), 1432 (s), 1410 (m), 1353 (s), 1120 (m), 1098
(m), 1035 (s), 990 (m), 910 (vw), 865 (vw), 836 (vw), 802 (w), 768
(s), 734 (m), 671 (m), 620 (s), 541 (w), 404 (w).
EXPERIMENTAL SECTION
■
General Materials and Measurements. All reagents and solvents
were purchased from commercial sources and used without further
purification. The ligand H4DSDC was synthesized using a modified
procedure as reported.47,48 Possible Cu(II) cations leaching in reaction
solution were measured on a PLASMASPEC (I) ICP atomic emission
spectrometer. Thermal analyses were carried out on a Mettler-Toledo
TGA/DSC STARe system in the temperature range from room
temperature to 800 °C with a heating rate of 10 °C·min−1, under a
nitrogen flow. Infrared spectra were recorded on a VECTOR 22
spectrometer and an Alpha Centaur Fourier transform infrared (FTIR)
spectrometer with pressed KBr pellets in the range 4000−400 cm−1.
Powder X-ray diffraction (PXRD) was performed using a MiniFlex 600
X-ray powder diffractometer equipped with a Cu sealed tube (λ =
Typical Procedure for a Photocatalytic Experiment. Into a 25
mL Schlenk tube equipped with a magnetic stir bar were added the
catalyst (0.04 mmol), MeCN (2.0 mL), and benzyl alcohol (0.4 mmol).
After the addition of 30% hydrogen peroxide (1.2 mmol), the reaction
mixture was stirred at room temperature under the irradiation of a blue
LED. In one of the parallel runs, the oxidation product was isolated after
24 h. The reaction mixture was filtrated, and the filtrate was quenched
by water (2.0 mL) and extracted with EtOAc (20 mL × 3). The
combined organic phase was washed with brine (20 mL × 2), dried over
Na2SO4 and MgSO4, evaporated to dryness, and subjected to column
chromatography (silica gel, petroleum ether/ethyl acetate) to furnish
1
the product, which was characterized by H and 13C NMR to be
1
1.54178 Å) at 40 kV and 40 mA. H and 13C NMR spectra were
benzaldehyde (74% yield). When benzaldehyde was not isolated and
the reaction carried on for 60 successive hours, the reaction mixture was
filtrated to afford a microcrystalline solid and the filtrate. The solid was
washed by dichloromethane, dried in vacuum, and subjected to powder
XRD analysis to prove a material identical to the fresh prepared catalyst,
which was good for recycled use of the oxidation reaction. The filtrate
was quenched by NaOH solution and washed by EtOAc (20 mL × 3).
The aqueous phase was then acidified by concentrated hydrochloric
acid to precipitate the benzoic acid, which was collected by filtration,
washed with small amount of water, and dried to afford benzoic acid in
88% yield. All yields reported are based on isolated pure products and
obtained on a Bruker 400 MHz spectrometer.
Synthesis of 4,4′-Disulfo-[1,1′-biphenyl]-2,2′-dicarboxylate acid
(H4DSDC). 9,10-Phenanthraquinone-2,7-disulfonic acid (7.36 g) and
acetic acid (15.0 mL) were placed in a two-necked flask, and the mixture
was heated to 85 °C while stirring. Then 40.0 mL of 30% H2O2 solution
was added dropwise into the mixture over a period of 2 h. After stirring
at 80 °C for another 12 h, the mixture was cooled to room temperature
and concentrated on a rotavapor. The product was obtained at 81%
yield as a white powder upon the addition of acetonitrile into the
concentrated solution, which was collected by filtration and dried at
ambient temperature. 1H NMR (400 MHz, D2O, δ, ppm): 8.34 (s, 2H),
7.99 (d, 2H), 7.44 (dd, 2H). 13C NMR (400 MHz, D2O, δ, ppm):
169.44, 144.46, 142.12, 130.99, 129.79, 128.91, 126.90.
Synthesis of {[Cu7(DSDC)2(OH)6(H2O)10]·xH2O}n (1). A mixture of
Cu(NO3)2·3H2O (24.2 mg, 0.1 mmol) and H4DSDC (20.1 mg, 0.05
mmol) was dissolved into a mixed solvent of dimethylacetamide/water
(2.0/1.0 mL) in a 10 mL glass vial. After the addition of 1.0 mL of 0.2 M
NaOH solution, the vessel was sealed with a cap and then heated to 130
1
characterization was performed using H and 13C NMR spectroscopy
(see Supporting Information for spectra).
Single-Crystal X-ray Crystallography. Single-crystal X-ray
diffraction data of all three complexes were collected at 293 K using a
Bruker APEX II CCD diffractometer operating at 50 kV and 30 mA
using Mo Kα radiation (λ = 0.71073 Å). Data were integrated using
SAINT and scaled with either a numerical or multiscan absorption
correction using SADABS49 All structures were solved by direct
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Table 1. Crystallographic Data for Complexes 1−3
1
2
3
empirical formula
Fw
C14H19Cu3.5O18S2
761.80
C17H15Cu2N2O8S
534.45
C38H26Cu2N4O12S2
921.83
crystal system
space group
a (Å)
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
V (Å3)
trigonal
R3
31.674(4)
31.674(4)
16.052(4)
90
90
120
13947(5)
18
1.633
2.575
6849
1.072
0.0418, 0.1023
0.0667, 0.1133
0.875, −0.428
monoclinic
C2/c
14.295(2)
18.460(3)
15.322(2)
90
115.887(2)
90
3637.5(11)
8
1.952
2.507
2152
1.036
0.0286, 0.0702
0.0362, 0.0745
0.493, −0.376
orthorhombic
Pccn
28.604(4)
14.5604(18)
17.081(2)
90
90
90
7114.0(16)
8
1.721
1.388
3744
Z
D calcd (g·cm−3
)
μ (mm−1
F(000)
)
goodness-of-fit on F2
1.017
a
b
R1, wR2 [I > 2σ(I)]
0.0319, 0.0767
0.0450, 0.0826
0.32, −0.36
a
b
R1, wR2 (all data)
(Δρ)max, (Δρ)min (e·Å−3
)
R1 = ∑ ||F | − |F||/∑ |F |. wR2 = {∑ [w(F 2 − F 2)2]/∑ [w(F 2)2]}
.
a
b
1/2
o
c
o
o
c
o
Figure 1. (a) Heptanuclear [Cu7(μ3-OH)6(H2O)10]8+ cluster in complex 1 surrounded by DSDC4− ligands, showing coordination geometries of the
Cu(II) centers. (b) Packing diagram of complex 1 with one-dimensional channels along the [001] direction. Disordered guest molecules are omitted
for clarity. (c) Topological representation of the framework in complex 1. The blue nodes and lines represent the [Cu7(μ3-OH)6(H2O)10]8+ SBUs and
DSDC4− ligands, respectively. Symmetry code: i 1 − x, 1 − y, 2 − z; ii 1/3 − y + x, −1/3 + x, 5/3 − z.
methods or Patterson maps50 and refined by full-matrix least squares on
F2 using the SHELXL-201451 and OLEX252 programs. All non-
hydrogen atoms were refined anisotropically. Non-water hydrogen
atoms were added at calculated positions and refined using a riding
model. Hydrogen atoms of water molecules and hydroxyl groups were
located by difference Fourier maps and refined using a riding model at a
restrained distance of 0.86 Å. The disordered water molecules in the
channels of the framework in complex 1 could not be modeled very well
and were treated by the SQUEEZE routine, resulting in the absence of
hydrogen bonding acceptor for H5wB. For complex 2, one of the lattice
water molecules (O8) suffered from disorder, which led to uncertainty
in the position. Crystallographic data of all three complexes are
summarized in Table 1, and selected bond lengths and angles are listed
in Table S1 in the Supporting Information.
RESULTS AND DISCUSSION
■
Syntheses and Structures of Candidate Photocata-
lysts. Versatile coordination groups in the designed DSDC4−
ligand dictate rich potential coordination modes that may
facilitate the formation of versatile Cu−O moieties and lead to
various polymeric complexes. As for a systematic comparison,
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Figure 2. (a) One-dimensional ladderlike belt in complex 2, showing the coordination geometry of metal centers and bridging mode of the DSDC4−
ligand. (b) Three-dimensional framework in complex 2 assembled from one-dimensional belts. Symmetry codes: i −x, +y, 3/2 − z; ii −x, 1 − y, 1 − z; iii
−x, +y, 1/2 − z; iv −1 + x, 1 − y, −1/2 + z; v −1/2 + x, 3/2 − y, −1/2 + z.
complexes 2 and 3 with additional different auxiliary ligands
have also been synthesized and their photocatalytic activities
have been studied and compared with that of complex 1
regarding the same reaction. The selection of the auxiliary
ligands is based on the consideration of either good connectivity
for better stability and limited solubility or the potential of
increasing photosensitivity of the metal complex for better
photocatalytic activity. All three Cu(II) complexes synthesized
are revealed to be polymeric according to the X-ray single-crystal
structure analyses, demonstrating poor solubility in common
organic solvents that may facilitate heterogeneous catalysis and
easy recycling. Their phase purities were confirmed by
experimental powder X-ray diffractions in comparison with
simulated ones, as shown in Figure S1.
Single-crystal X-ray diffraction indicated that complex
{[Cu7(DSDC)2(OH)6(H2O)10]·xH2O}n (1) crystallizes in the
trigonal space group R3 (Table 1). Each asymmetric unit is
constituted by three and a half Cu(II) cations, one tetraanionic
DSDC4− ligand, three bridging hydroxyl groups, and five aqua
ligands. The DSDC4− ligand bridges four Cu(II) cations by four
distinct carboxylate oxygen atoms while both sulfonate groups of
the ligand remain free from metal coordination (Figure 1a). All
Cu(II) cations are six-coordinated and bind with carboxylate
oxygens, μ3-bridging hydroxyl groups, and μ2-bridging or
terminal aqua ligands to form distorted octahedral coordination
geometries. As a result, the nonlinear O−Cu−O angles fall in the
range 79.70(13)−106.18(14)° for all symmetry-independent
Cu(II) cations. The equatorial Cu−O bond lengths are between
1.914(3) and 2.007(3) Å [1.914(3)−1.974(4) Å for Cu1;
1.944(3)−1.981(3) Å for Cu2; 1.917(3)−1.988(4) Å for Cu3;
1.964(3)−2.007(3) Å for Cu4] while the axial Cu−O distances
range from 2.437(4) to 2.617(4) Å [2.437(4)/2.617(4) Å for
Cu1; 2.433(4)/2.590(4) Å for Cu2; 2.448(4)/2.521(4) Å for
Cu3; 2.495(3)/2.495(3) Å for Cu4]. Significant elongation of
the axial Cu−O bond lengths has been observed for all Cu(II)
centers due to the Jahn−Teller effect, which exists widely in
copper-based coordination complexes.
0.0336 Å of the plane constituted by seven Cu(II) cations in the
7Cu cluster. Each 7Cu cluster is supported by four DSDC4−
ligands, which further bridge the 7Cu clusters into a three-
dimensional framework (Figure 1b). From a topological point of
view, each 7Cu cluster as the secondary building unit (SBU)
constitutes a four-connected node surrounded by four DSDC4−
ligands and each DSDC4− ligands bridges two neighboring 7Cu
SBU. Therefore, a NbO type topology can be induced for the
4
2
̈
framework, whose Schafli notation is expressed as (6 ·8 )
(Figure 1c). The framework contains a one-dimensional
hexagonal channel extending along the [001] direction, with a
diameter of around 11 Å. Uncoordinated sulfonate groups and
terminal aqua ligands protrude into the channels, interacting
with guest water molecules via versatile hydrogen bonding.
Complex {[Cu4(DSDC)(4,4′-bpy)2(OH)4]·2H2O}n (2)
crystallizes in monoclinic space group C2/c, bearing an
asymmetrical unit constituted by two symmetry-independent
Cu(II) cations, half of a DSDC4− ligand, two hydroxyl groups,
one 4,4′-bpy, and a lattice water molecule. Two carboxylate O
atoms, three hydroxyl groups, and one pyridyl N atom account
for the distorted octahedral coordination geometry of Cu1,
whereas the Cu2 center adopts a pyramidal coordination
environment consisting of one sulfonate O atom, three hydroxyl
groups, and one pyridyl N atom. The nonlinear O−Cu−O
angles are between 79.08(8) and 106.15(8) Å for Cu1 and
84.02(9) and 101.53(9) Å for Cu2, indicating that coordination
geometries of both metal centers are significantly distorted. The
Cu−O bond lengths vary in the range 1.934(2)−2.620(2) Å for
Cu1 and 1.946(2)−2.288(2) Å for Cu2, and Cu−N distances
are 2.001(2) Å for Cu1 and 2.028(2) Å for Cu2. Notably, the
Jahn−Teller effect is also observed in the metal coordination of
complex 2, leading to obvious longer Cu−O bond lengths in the
axial direction. Each DSDC4− ligand takes the μ6-coordination
mode utilizing all four carboxylate O atoms and two sulfonate O
atoms, interconnecting six Cu(II) atoms. Adjacent symme-
try−independent Cu(II) atoms are bridged by hydroxyl groups
to form a waving {[Cu2(OH)2]2+}n ladderlike belt (Figure 2a).
The belt is also supported by alternate coordination of the
DSDC4− ligand through its carboxylate groups. The one-
dimensional belts are further assembled together into a three-
dimensional framework through the coordination linkage of
DSDC4− ligand using its sulfonate groups and auxiliary ligands
(Figure 2b).
A disclike centrosymmetric heptanuclear [Cu7(μ3-
OH)6(H2O)10]8+ (7Cu) cluster could be identified in the
structure of complex 1, in which the Cu4 atom locates on the
inversion center, as shown in Figure 1a. The central Cu4 atom is
homocoordinated to six hydroxyl groups. The Cu···Cu
separations in the 7Cu cluster range from 3.0222(7) to
3.2143(7) Å and Cu−Ohydroxyl/water−Cu angles are between
92.44(12) and 116.17(16)°. The mean atomic deviation is
Complex {Cu2(DSDC)(phen)2(H2O)2}n (3) crystallizes in
orthorhombic space group Pccn according to single-crystal X-ray
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analysis (Table 1). With two symmetry-independent Cu(II)
cations, one DSDC4− ligand, two 1,10-phenanthroline, and two
aqua ligands in the asymmetric unit, complex 3 features a two-
dimensional infinite network. There are two types of Cu(II)
centers with different coordination geometries, bound by two
symmetry independent phenanthroline ligands. Cu1 binds to a
pair of O atoms from two carboxylate groups of the same
DSDC4− ligands, one sulfonate O atom from another symmetry
related DSDC4− ligand, one aqua ligand, and two N atoms of a
single phenanthroline ligand, leading to a distorted octahedral
geometry as reflected by nonlinear O/N−Cu1−O/N bond
angles between 81.25(8) and 102.89(8) ° (Figure 3a). The
coordinates to another Cu(II) cation using an alternate O atom.
As a typical terminal ligand, the 1,10-phenanthroline precludes
the formation of the three-dimensional framework; thus, the
metal−organic coordination in this complex leads to only a two-
dimensional network spreading over the (010) plane (Figure
3b).
Versatile coordination groups in the designed DSDC4− ligand
dictate rich potential coordination modes, which may lead to
various polymeric complexes (Table 2). All three Cu(II)
complexes thus synthesized are revealed to be polymeric
according to X-ray single-crystal structure analyses, demonstrat-
ing poor solubility in common solvents such as water,
acetonitrile, acetone, methanol, dichloromethane, tetrahydro-
furan, and toluene, which may facilitate heterogeneous catalysis
and easy recycling.
Photocatalytic Studies. Guided by the objective of
developing new photocatalysts, all complexes were subjected
to the evaluation of their catalytic activities toward the oxidation
of alcohols irradiated by visible light. The oxidation of alcohols
to the corresponding carbonyl compounds is a critical
transformation in organic synthesis.53−58 Many catalytic
systems, homogeneous or heterogeneous, have been developed
for the oxidation of alcohols, forming corresponding aldehydes,
carboxylic acids, or esters depending on the level of oxidation
and esterification, with poor or satisfactory yields depending on
the catalytic system.29,53,59−61 Despite the progresses made
toward this end, available catalysts are still limited with versatile
disadvantages, such as the need of harsh reaction conditions, the
involvement of noble or toxic metals, the use of costly
cocatalysts, and poor recyclability.62−64 To the best of our
knowledge, there are no attempts utilizing visible light to
mediate the oxidation reaction. The development of a
photocatalytic system utilizing a highly efficient heterogeneous
catalyst with good recyclability is of great importance in terms of
green chemistry.65−67
The study began with the survey and optimization of reaction
conditions. With benzyl alcohol as the substrate, reaction
conditions with varied factors including photocatalysts, solvents,
oxidant, and visible light have been systematically investigated.
An initial study revealed that the ligand itself did not catalyze the
reaction (Table 3, entry 1). Acetonitrile was found to be the best
solvent to mediate this reaction (Table 3, entries 2−3). Other
solvents giving poor yields and product selectivity were not
listed. No occurrence of any reactions was observed with the
absence of either blue LED or oxidant (Table 3, entries 4−5).
Figure 3. (a) Coordination geometries of Cu(II) centers and binding
mode of the ligand in complex 3. (b) Two-dimensional layered
structure in complex 3. Symmetric codes: i 1/2 − x, +y, 1/2 + z; ii 1 − x,
1 − y, 1 − z; iii 1/2 − x, +y, −1/2 + z.
equatorial Cu1−O/N bond lengths fall in the range 1.962(2)−
2.016(2) Å, and the axial Cu1−O distances are 2.459(2) and
2.491(2) Å due to the Jahn−Teller effect. Cu2 adopts pyramidal
coordination geometry, with four basic corners occupied by two
N atoms of a second phenanthroline ligand, one carboxylate O
atom, and one aqua ligand. A sulfonate O atom locates at the
apex of the pyramid at elongated bond distances of 2.219(2) Å
due to the Jahn−Teller effect (Figure 3a). The Cu2−O/N bond
lengths between 1.937(2) and 2.219(2) Å and the nonlinear O/
N−Cu2−O/N bond angles between 82.18(10) and 105.65(9)°
suggest a distorted pyramid. The DSDC4− ligand bridges four
Cu(II) cations with all carboxylate and sulfonate groups
involved in coordination. Both sulfonate groups bind to distinct
Cu(II) cations in monodentate mode, and two carboxylate
groups chelate with one Cu(II) cation, one of which also
Table 2. Coordination Modes of DSDC4− Ligand in Complexes 1−3 and Key Structural Parameters Defining the Ligand
Conformations
dihedral angles (deg)
carboxylate group vs
distance between the two carboxylate C atoms (Å)
between phenyl rings
attached benzyl ring
carboxylate group vs carboxylate group
1
2
3
4.00
4.67
2.93
85.5
73.1
48.5
31.9
21.3
31.9
41.9
21.3
42.3
78.6
89.4
18.5
E
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Inorganic Chemistry
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a
Table 3. Optimization of Reaction Conditions
product and yield (%, isolation/in situ)
entry
photocatalyst (mol %)
solvent
visible light
oxidant
PhCHO (at 24 h)
PhCOOH (at 60 h)
1
2
3
4
5
6
7
8
9
H4DSDC (10)
1 (10)
1 (10)
1 (10)
1 (10)
1 (10)
1 (5)
2 (10)
3 (10)
MeCN
MeCN
MeOH
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
blue LED
blue LED
blue LED
−
blue LED
blue LED
blue LED
blue LED
blue LED
H2O2
H2O2
H2O2
H2O2
−
−
−
74/85
45/55
−
88/90
53/56
−
−
−
O2
trace
66/76
−
trace
78/80
41/45
23/25
H2O2
H2O2
H2O2
−
a
Some other reaction conditions: benzyl alcohol (0.4 mmol), H2O2 (1.2 mmol, wherever applied), solvent (2 mL), 20 W blue LED used (450−470
nm).
Figure 4. (a) Powder X-ray diffraction patterns of photocatalyst 1 before and after five catalytic recycles. (b) Recyclability of the photocatalyst 1. The
blue bar is the isolation yield of benzaldehyde at 24 h, and the green bar is the isolation yield of benzoic acid at 60 h for a separate run. All yields are
based on parallel reactions.
When molecular oxygen was used as the oxidant, only a trace
amount of product was observed (Table 3, entry 6). The
minimum amount of photocatalyst required for a satisfactory
yield was found to be 0.1 equiv (Table 3, entries 2 and 7).
Therefore, the best reaction conditions for the photocatalytic
oxidation of benzyl alcohol is the application of 10 mol %
complex 1 as the photocatalyst, with H2O2 as the oxidant, and
MeCN as the solvent, utilizing the irradiation of blue LED at
room temperature (Table 3, entry 2).
Significantly for this photocatalytic system, the reaction
exhibits stepwise oxidation to generate first benzaldehyde in the
early stage and then benzoic acid with an elongated reaction
time, fabricating a unique catalytic system with perfect time-
resolved product selectivity. As shown by entry 2 in Table 1,
after 24 h of reaction, benzaldehyde dominates the product with
an isolation yield at 74% (85% in situ), while in a parallel run
without the isolation of benzaldehyde, benzoic acid represents
the major product at 88% isolation yield (90% in situ) after 60 h
of reaction. A systematic investigation on the in situ yields of
benzaldehyde and benzoic acid during reaction process have
been carried out. The yield of benzaldehyde increases in the first
24 h of the reaction while very little benzoic acid is generated
during this period. Then benzaldehyde starts to be oxidized
further into benzoic acid, resulting in a decreasing amount of
benzaldehyde and an increasing amount of benzoic acid (see
Supporting Information for graphic representation and corre-
sponding data regarding the time-resolved yields of the two
products). To the best of our knowledge, this case represents the
sole example that a catalyst shows time-resolved multistage
selective catalysis for different products with satisfactory
isolation yields.
Complexes 2 and 3 exhibited poor catalytic activity and no
stepwise oxidation was observed (Table 3, entries 8 and 9),
which is out of our expectation. Further studies revealed that the
UV−vis absorption of complex 1 differs significantly from those
of the ligand and complexes 2 and 3, and this might be an
explanation of the distinct catalytic properties for these
complexes. The utilization of a bridging auxiliary ligand (4,4′-
bipyridine) led to a less stable complex 2, which compared to 1 is
also less efficient in catalysis (Table 3, entries 2 and 8). The
introduction of an auxiliary ligand (1,10-phenanthroline)
capable of facilitating light absorption made no contribution
to the catalytic activity of 3, whose catalytic efficiency in
promoting the reaction decreased dramatically compared to that
of 1 (Table 3, entries 2 and 9). The use of auxiliary ligands might
be able to enhance the physical property of the metal complexes
F
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Article
a
Table 4. Substrates Scope
a
Note: Oxidation of aryl primary alcohols into aldehydes took 24 h and oxidation of secondary alcohols into ketones took 48 h. Reaction
conditions (unless otherwise specified): primary alcohols (0.4 mmol), H2O2 (1.2 mmol), complex 1 (0.04 mmol), CH3CN (2.0 mL), irradiated by
20 W blue LEDs (450 nm−470 nm) in air at room temperature.
in certain aspects, but this may not always contribute positively
to the catalytic activity of the complex. Factors that might
potentially affect the catalytic activities of these complexes in this
photooxidation reaction include distinct porosities that may
contribute substantially to selectivity and yield, different
bulkiness of the auxiliary ligands that may block catalytic center,
various intrinsic features of different Cu−O cluster moieties, and
a potential synergetic effect between the DSDC4− ligand and
Cu−O clusters. Further studies in this area may be able to
provide more information in the future.
One of the remarkable features of this catalytic system is that
photocatalyst 1 could be recovered almost quantitatively on
completion of the reaction with the structure being intact. To
determine possible metal leaching into the remaining solution
on completion of the reaction, the reaction mixture was filtrated
to remove the solid catalyst and the filtrate was subjected to an
analysis using inductively coupled plasma atomic emission
spectroscopy. The result indicated that the concentration of Cu
element in the filtrate was 0.01 μg·mL−1, which accounts for a
0.00286‰ loss of the catalyst, suggesting negligible metal
leaching, which provides a perfect recovery rate and the
possibility of catalyst recycling.
substrate. After each reaction cycle (60 h without the isolation of
benzaldehyde), photocatalyst 1 was isolated by filtration,
washed with acetone, dried in vacuum, and subjected to
recycled use in another catalytic cycle without any need of
activation or regeneration. A series of paralleled runs revealed
that after five catalytic cycles, photocatalyst 1 remains
unchanged according to almost constant powder XRD patterns
and IR spectra before and after catalysis (Figure 4a and Figure
S3), and most importantly there was no obvious dropping of the
catalytic efficiency (Figure 4b).
The substrate scope of the oxidation reaction was then
investigated with both primary and secondary alcohols, as shown
in Table 4. For primary alcohols with different aromatic
functional groups, no matter if it was a phenyl ring with either
an electron withdrawing or electron donating group or even if it
was a heterocyclic ring, all these substrates exhibited good
adaptability over this oxidation reaction using complex 1 as the
photocatalyst, giving corresponding aldehydes or carboxylic
acids in good yields (Table 4, entries 1−7). The photocatalyst
showed perfect compatibility with selective time-resolved
control and constant recyclability upon reactions of these
substrates. When the aliphatic primary alcohol was subjected to
this photocatalytic reaction, no stepwise oxidation was observed
and only the corresponding carboxylate acid was isolated in
The recyclability and reusability of complex 1 as a catalyst in
this reaction were then evaluated with benzyl alcohol as the
G
DOI: 10.1021/acs.inorgchem.9b02084
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Article
medium yield (Table 4, entry 8). Regarding the secondary
alcohols, a direct oxidation into ketones occurred with
satisfactory yields (Table 4, entries 9−13).
Xiao-Li Zhao: 0000-0002-4458-6432
Xu-Dong Chen: 0000-0002-0152-5410
Notes
The authors declare no competing financial interest.
CONCLUSIONS
■
A family of three coordination polymers comprising various
Cu−O assemblies has been synthesized, which can be used as
heterogeneous photocatalysts for the visible-light driven
oxidation reactions of alcohols. Complex 1 exhibits the best
catalytic efficiency, which is constituted by the infinite linkage of
heptanuclear [Cu7(μ3-OH)6(H2O)10]8+ clusters via deproto-
nated DSDC4− ligands into a NbO type topology. The reaction
conditions required for this catalytic system are environment
friendly, such as the use of visible light (blue LED) at room
temperature, the application of hydrogen peroxide as an oxidant,
easy isolation and recyclability of the catalyst, etc.
With primary aryl alcohols as the reaction substrate, different
oxidation products of aldehydes or carboxylic acids could be
generated at different stages of the catalytic reaction and could
be isolated in good yield, constituting a unique example of
stepwise catalysis with perfect time-resolved control. Signifi-
cantly, this MOF photocatalyst 1 could be quantitatively
recovered as a crystalline material with negligible metal leaching
on completion of the reaction, being structurally unchanged and
recyclable for at least five catalytic cycles with constant efficiency
and consistent activity of stepwise catalysis. This Cu-based MOF
1 represents the first example of a photocatalyst capable of
catalyzing successive reactions with good time-resolved control
of products. According to the reaction process, it is possible to
design a flow reactor toward low-cost large-scale production on
the basis of the efficiency and recyclability of the photocatalyst.
This novel catalytic system of stepwise catalysis with good
multistage selectivity gives new insight into the design and
development of new types of catalysts for both fundamental
studies and industrial uses toward challenging organic trans-
formations.
ACKNOWLEDGMENTS
■
This work was supported by the National Natural Science
Foundation of China (21671104), the Jiangsu Key Technology
R&D Program (BE2016010 and BE2016010-1), the Natural
Science Foundation of Jiangsu Higher Education Institutions of
China (17KJA430010 and 15KJA350002), the Priority
Academic Program Development of Jiangsu Higher Educational
Institutions, the Jiangsu Collaborative Innovation Center of
Biomedical Functional Materials, and the State Key Laboratory
of Coordination Chemistry in Nanjing University. We thank the
staff of the BL17B beamline of the National Facility for Protein
Science Shanghai (NFPS) at the Shanghai Synchrotron
Radiation Facility (SSRF).
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.9b02084.
■
S
PXRD data (Figure S1), TGA curves (Figure S2), IR
spectra of complex 1 before and after catalysis (Figure
S3), time-resolved yields comparison (Table S1 and
Figure S4), UV−vis spectra (Figure S5), selected bond
lengths and angles (Table S2), product yields and NMR
spectra (PDF)
Accession Codes
CCDC 1903006−1903008 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 email-
ing data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road, Cam-
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AUTHOR INFORMATION
Corresponding Authors
*Email: xlzhao@chem.ecnu.edu.cn (X.L.Z.).
*Email: xdchen@njnu.edu.cn (X.D.C.).
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