Selective Photocatalytic Oxidation of Sulfides in Lanthanide Metal -Organic Frameworks Incorporating Ru(2,2′-bpy)3 photosensitizer

Article abstract of DOI:10.1002/asia.202100482

Three isostructural lanthanide metal-organic frameworks (Ln-MOFs) were synthesized with uncoordinated N^N site, and the Ru(N^N)₃ photosensitizer was introduced via coordination link. These functionalized frameworks showed excellent performance

Full text of DOI:10.1002/asia.202100482

doi.org/10.1002/asia.202100482
Communication
Selective Photocatalytic Oxidation of Sulfides in Lanthanide Metal
-
Organic Frameworks Incorporating Ru(2,2’-bpy) photosensitizer
3
[a]
Xiaobang Zhang, Xiaomei Wei, Sheng-Li Huang,* and Guo-Yu Yang*
Dedicated to Professor T. S. Andy. Hor on the occasion of his 65th birthday.
difficult to obtain good photocatalytic effect. Indirect sensitiza-
tion is usually used to produce better photocatalytic efficiency.
Abstract: Three isostructural lanthanide metal-organic
frameworks (Ln-MOFs) were synthesized with uncoordi-
nated N^N site, and the Ru(N^N)3 photosensitizer was
introduced via coordination link. These functionalized
frameworks showed excellent performance in the photo-
catalytic oxidation of sulfides with good conversion and
high sulfoxide selectivity.
Ru(N^N)
enjoyed a rich history in photocatalysis. The metalloligand or
post-synthesis strategy could produce Ru(N^N) functionalized
units are easily excited by visible light and have
3
3
coordination frameworks, and they have been applied to
various photocatalytic reactions, such as arylboronic acids
[27]
oxidation and oxidative cross-coupling. The combination of
Ru(N^N) and Ln-MOFs imparts special photocatalytic activity.
3
Three isostructural complexes Ln-1 (Ln=Tb, Gd or Eu) were
synthesized from the reaction of 2,2’-bipyridine-5,5’-dicarboxylic
acid (H bpydc) and Ln(NO ) . The harder carboxylate groups
Introduction
2
3 3
Porous coordination frameworks have been widely explored in
preferentially coordinate oxyphilic Ln ions and keep the
[1–8]
[9–13]
the fields of gas adsorption and separation,
catalysis,
due to their
uncoordinated N^N sites. The Ru(N^N) unit could be intro-
3
[14–17]
[18–19]
[20–23]
sensors,
magnetic,
biomedicine,
duced into the frameworks via coordination driven connection.
Sulfoxide is one kind of important reagents in the
pharmaceutical industry, and oxidation of sulfides into sulf-
oxides is by far the most common method. Traditionally, it can
be achieved through a thermal oxidation process using diverse
oxidants such as trifluoroperacetic acid, hydrogen peroxide,
sodium periodate, hypochlorite, and iodobenzene diacetate.
However, the product selectivity is low because of uncontrol-
lable over-oxidation to sulfones, and these thermal catalytic
systems are energy wasteful and environmentally harmful.
Photocatalytic synthesis compared with traditional synthetic
method, has moderate reaction conditions (room temperature
and atmospheric pressure), simple procedures, and environ-
mental friendliness, hence photocatalytic selective oxidation of
designable topology, adjustable porosity and tunable function-
ality. Coordination frameworks as catalyst possess special
advantages over the traditional porous materials, such as
uniformly dispersed catalytic active sites, tunable pore sizes and
environments. However, the low chemical tolerance of coordi-
nation frameworks limits their application as heterogeneous
catalysts. Lanthanide (Ln) ions have high coordination number,
flexible coordination modes and strong lewis acidity. The
coordination frameworks incorporating Ln oxygen cluster as
metal nodes not only expand the topology diversity of frame-
works but also increase their chemical stability. Some Ln-MOFs
[24]
have been reported, however few attention was focused on
their functionalities. Particularly, as photocatalyst, only a few
[
25]
[28]
examples were applied in the reduction of CO2
and
sulfides into sulfoxides has attracted great attention. To the
[26]
production of H2.
The parity forbidden 4f-4f molar absorption coefficient of
Ln is very low due to the shielding effect of sublayer electrons
on 4f orbits. Moreover, the band gap between the ground state
best of our knowledge, Ln-MOFs photocatalyzed oxidation of
sulfides has never been reported.
3
+
and the excited state of rare earth ions is small, and the energy Results and Discussion
is easily lost by non-radiative dissipation. Direct excitation of
rare earth ions is difficult to obtain high energy excited states
and produce highly separated electrons and holes, so it is
3
+
3+
3+
Solvothermal reactions of Ln(NO ) ·xH O (Ln=Tb , Gd , Eu )
3 3 2
and H bpydc afforded corresponding Ln-1. Tb-1 was formu-
2
lated by single-crystal X-ray diffraction (SCXRD) studies as
[
(CH ) NH ] [Tb (μ -OH) (bpydc) ]·x(Solvent). Gd-1 and Eu-1 are
[
a] X. Zhang, X. Wei, Prof. S.-L. Huang, Prof. G.-Y. Yang
MOE Key Laboratory of Cluster Science, Photoelectronic/Electrophotonic
Conversion Materials
School of Chemistry and Chemical Engineering
Beijing Institute of Technology, Beijing, 100081 (P. R. China)
E-mail: huangsl@bit.edu.cn
3 2 2 2 6 3 8 6
isostructural with Tb-1 as confirmed by their same powder X-
ray diffraction data (PXRD) with Tb-1.
Single crystal analysis demonstrates that Tb-1 crystallizes in
3
+
�
cubic crystal system with Fm3m space group. Each Tb was
surrounded with four μ -OH groups and four oxygen atoms
from the carboxylate groups (η :μ -CO ). Six adjacent Tb
cations were aggregated together by eight μ -OH groups and
ygy@bit.edu.cn
3
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/asia.202100482
This manuscript is part of a special collection on Metals in Functional Ma-
terials and Catalysis.
1
À
2
3+
2
3
formed a hexanuclear cluster (Figure 1a). Each cluster was
Chem Asian J. 2021, 16, 2031–2034
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© 2021 Wiley-VCH GmbH

Communication
photosensitizer was observed, and two extra double peaks
2
À
between 8.3–8.45 ppm are characteristic peaks of bpydc
Figure S5). Energy-dispersion X-ray (EDX) mapping demon-
(
strates that the incorporating Ru(N^N)3 units were evenly
embedded in Ln-1 framework (Figure 2b). To further confirm
the loading of Ru-centers is coordination link, not the
adsorption process, the X-ray photoelectron spectroscopy (XPS)
spectrum was carefully investigated (Figure 3). All shifts for
three samples, Ru-Tb-1, Ru-Gd-1, Ru-Eu-1, were corrected by
normalization of C(1 s) binding energy to 284.8 eV. Low loading
amount of Ru-center resulted in weak characteristic peaks of Ru
3
P3/2 and Ru 3P1/2. Through the data fitting of binding energy of
Ru 3d and Ru 3p, only one set of peaks was respectively
observed, indicating non-existence of other phases of Ru
complex. Collectively, it can be confirmed that Ru(N^N) have
3
been successfully introduced into Ln-1. And energy dispersive
spectrometer (EDS) analysis in random areas demonstrated that
Ru(N^N)3 units were evenly loaded in Ln-1, as well as the
formation of Ru-Ln-1 catalyst (Table S2–S4).
Figure 1. a) Hexanuclear cluster [Tb
in Tb-1. c) Tetrahedral cage. d) Octahedral cage.
6
(μ
3
-OH)
8
(COOÀ )12]. b) two types of cages
The photocatalytic sulfide oxidation of Ru-Ln-1 were
performed under O atmosphere and blue LED light (450 nm).
2
To test whether Ru-Ln-1 has the capability of photocatalytic
sulfide oxidation, methyl phenyl sulfide was initially used based
on its moderately oxidizable state. For all three Ru-Ln-1
complexes, 9 hours later, almost all methyl phenyl sulfide
substrate was converted to sulfoxide with 100% selectivity
(entries 1–3, Table 1). As shown in Figure S10, three Ru-Ln-1
frameworks still have high catalytic activity after five cycles.
PXRD patterns of the recycled catalyst showed that the
structural integrity of Ru-Ln-1 was maintained after reactions
(Figure S11), indicating the enough stability of these framework
catalysts. Using Ln-1 catalyst, only very small percentage of
substrate was converted (entries 4–6, Table 1), and the loading
of Ru(N^N)3 photosensitizer paly a decisive role in photo-
catalytic process. There is no substrate conversion excluding
the use of any catalyst (entry 7, Table 1). The sulfide conversion
rate is also zero when the reactions were performed under dark
or Ar atmosphere (entries 8–9, Table 1). In the photocatalysis
2
À
linked with 12 independent bpydc ligands thus forming a 12-
connected node, finally producing the porous structure with
fcu topology (Figure 1b). There are two types of polyhedral
cages in Tb-1 including one octahedron cage and one
tetrahedral cage, where [Tb (μ -OH) (O CÀ ) ] cluster and
6
3
8
2
12
2
À
bpydc respectively acts as vertex and edge of polyhedron.
The diameter of inner cavity of tetrahedral cage and octahedral
cage is estimated to be 1.2 and 1.6 nm, respectively (Figure 1c–
1
d).
For three complexes Ln-1, both their phase purity and
isostructural nature were confirmed as evidenced from PXRD
data (Figure 2a). Thermogravimetric analysis (TGA) of them
indicates that all three complexes have strong thermostability
Figure S3). The photosensitizer Ru(N^N) was easily incorpo-
rated into the Ln-1 framework via the coordination link of N^N
(
3
bis-cheating site and Ru center. After Ru(N^N) -metallization,
3
the UV-vis adsorption red-shift of Ru-Ln-1 framework expand its
absorption band, making Ru-Ln-1 framework has the capability
of visible light driven photocatalysis. The UV-vis adsorption
process, Ru-Ln-1 catalyst, light and O are all indispensable.
2
change initially indicated the incorporation of Ru(N^N) photo-
3
sensitizer (Figure S4). The incorporation of Ru(N^N) unit into
3
1
Ln-1 could be further confirmed by the H NMR analysis of
digested Ru-Tb-1. The characteristic peaks assigned to Ru
Figure 2. a) Powder patterns of different complexes; b) SEM images of Ru-
Tb-1, Ru-Gd-1, Ru-Eu-1 and corresponding EDX mapping images of Ru.
Figure 3. XPS spectrumof Ru 3d (a, b, c) and Ru 4p (d, e, f) corresponding to
Ru-Tb-1, Ru-Gd-1, Ru-Eu-1, respectively.
Chem Asian J. 2021, 16, 2031–2034
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Communication
[a]
[a]
Table 1. Optimal Photo-oxidation Conditions.
Table 2. Photocatalytic oxidation of different sulfides.
Entry
Catalyst
Time/[h]
Yield/[%]
Selectivity/[%]
Entry
Catalyst
Time/[h]
Yield/[%]
Selectivity/[%]
1
2
3
4
5
6
7
8
9
1
1
1
1
1
Ru-Tb-1
Ru-Gd-1
Ru-Eu-1
Tb-1
Gd-1
Eu-1
Ru-Ln-1
Ru-Ln-1
Ru-Ln-1
Ru-Gd-1
Ru-Gd-1
Ru-Gd-1
Ru-Gd-1
Ru-Gd-1
9
9
9
12
12
12
12
12
12
9
9
9
9
9
>99
>99
>99
5
6
3
0
0
0
20
97
96
83
99
100
100
100
100
100
100
100
100
100
100
100
100
100
100
1
2
5.5
6
>99
>99
100
100
3
4
5
9
>99
96
100
100
100
[
[
[
b]
c]
d]
[
12
12
e]
f]
96
0
1
2
3
4
[
[
[
[
g]
h]
i]
6
7
12
30
96
90
100
100
[a] Conditions: sulfide (0.25 mmol) and catalyst (5 mg) in 5 mL MeOH with
O
2
atmosphere under 10 W blue LED at room temperature. [b] No light.
[c] No catalyst. [d] Under Ar atmosphere. [e] 2 equiv. of benzoquinone
(BQ). [f] 2 equiv. of iso-propyl alcohol (IPA). [g] 2 equiv. of diazabicyclo
[2.2.2]octane (DABCO). [h] 2 equiv. of KI. [i] 0.1 g of catalase.
[
a] Conditions: sulfide (0.25 mmol) and Ru-Gd-1 (5 mg) in 5 mL MeOH
with O atmosphere under 10 W blue LED at room temperature.
2
*
À
+
In oxidation of methyl phenyl sulfide, three complexes Ru-
erated O2 is the main reactive oxygen species and h also
Ln-1 exhibited similar catalytic capability, and Ru-Gd-1 was
applied in the oxidation sulfides bearing various groups. In
contrast to methyl group, ethyl group caused ethyl phenyl
sulfide was more difficult to be oxidized (entries 3–4, Table 2).
When methyl phenyl sulfide substrates involving electron-
donating groups were applied, Ru-Gd-1 still exhibited high
catalytic activity, 96% yield for both Br and methoxy derivatives
at 12 h (entries 5–6, Table 2). Although the reaction time was
extended to 30 h, only 90% yield was obtained for NO2
takes effect in auxiliary oxidation. Ru-Ln-1 is excited by visible
light and generates electron and hole. O accepts electron,
2
*
À
producing O2 species, which attack sulfide and form active
intermediate persulfoxide with the assist of hole. Methanol can
stabilize the intermediate persulfides as protic solvent through
[29]
hydrogen bond interaction,
and finally, the electrophilic
intermediate reacts with another sulfide molecule, yielding
sulfoxide products (Figure S12).
derivative due to strong electron-withdrawing of NO group
2
(entry 7, Table 2). The oxidation of alkyl sulfides is easier than Conclusion
phenyl sulfides, and how to controllably oxide alkyl sulfides into
sulfoxides remains a great challenge because overoxidation of
sulfoxides produce sulfones. Luckily, for alkyl sulfides, almost all
sulfide substrates were transformed at less time (about 6 h)
along with 100% sulfoxide/sulfone selectivity (entries 1–2,
Table 2). Previously reported coordination frameworks photo-
catalyzed sulfide oxidations were all performed under strong
light source, herein irradiated by weak light (10 W), Ru-Ln-1 still
In summary, we synthesized three isostructural Ln-MOFs with
fcu topology, the Ru(N^N) photosensitizer was introduced via
3
coordination link, producing the heterogeneous photocatalyst
Ru-Ln-1. At room temperature and O2 atmosphere, Ru-Ln-1
photocatalyzed sulfide oxidation in high yield along with 100%
sulfoxide/sulfone selectivity. These exciting results revealed
some novel functions of photosensitized Ln-MOFs, however the
designed construction of functional Ln-MOFs as well as their
application exploration is still in its infancy.
[28]
exhibited excellent photocatalytic activity.
In photocatalytic aerobic oxidation, superoxide radical
*
À
*
1
anions ( O ), hydroxyl radicals ( OH), singlet oxygen ( O ), holes
h ) and H O are all active species to oxide substrates. 1,4-
2 2
*
2
2
+
(
À
benzoquinone (BQ, a scavenger of O ), isopropanol (IPA, a Acknowledgements
2
*
scavenger of OH), diazabicyclo[2.2.2]octane (DABCO, a scav-
1
+
enger of O ), KI (KI, a scavenger of h ), and catalase (a
This work was supported by the National Natural Science
Foundation of China (21971011 and 21831001), the Fundamental
Research Funds for the Central Universities. Technical support
from staff at Analysis & Testing Center, Beijing Institute of
Technology is also appreciated.
2
scavenger of H O ) were used to respectively detect the
2
2
formation of reactive species and verify the photocatalytic
mechanism (entries 10–14, Table 1). A series of reactive inter-
mediate scavengers were applied to investigate which reactive
species play a key role in photocatalytic sulfide oxidation. When
BQ was added into the reaction system, 79% decrease of
catalytic efficiency was observed. The addition of KI caused a Conflict of Interest
6% shutdown of photocatalytic activity. However, only slight
1
weakness of catalytic performance was observed following the
addition of IPA, DABCO and catalase. Collectively, photogen-
The authors declare no conflict of interest.
Chem Asian J. 2021, 16, 2031–2034
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Communication
Keywords: Rare-earth metal-organic frameworks · Ru(N^N)3
photosensitizer · photocatalysis · sulfide oxidation
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