Photoactive Metal-Organic Frameworks for the Selective Synthesis of Thioethers: Coupled with Phosphine to Modulate Thiyl Radical Generation

Article abstract of DOI:10.1021/acs.inorgchem.1c00642

Metal-organic framework (MOF) materials are intriguing photocatalysts to trigger radical-mediated chemical transformations. We report herein the synthesis and characterization of a series of isomorphic MOFs which show a novel structure, wide visible-light absorption, high chemical stability, and specific redox potential. The prepared MOFs were explored for the photoinduced single-electron oxidation of thiol compounds, generating reactive thiyl radicals to afford thioethers via a convenient thiol-olefin reaction. Importantly, we provide a widely applicable strategy by combing a photoactive MOF with phosphine to modulate the generation of thiyl radical in the reaction, thereby producing a single product of the thioether without the formation of a disulfide byproduct due to the dimerization of thiyl radicals. The photocatalytic reaction takes advantage of this strategy, showing great generality where tens of thiols and olefins have been examined as coupling partners. In addition, the strategy has also been demonstrated to be effective for the reactions catalyzed by other MOFs. Mechanism studies reveal that the selective synthesis of C-S products relies on a synergy between the photoinduced generation of a thiyl radical over the MOF and the in situ cleavage of S-S bond into a S-H bond by phosphine. It is notable that the synthesized MOFs show advanced performance in comparison with classical MOFs. The work not only provides a series of novel MOF photocatalysts that are capable of photoinduced thiol-olefin coupling but also indicates the great potential of MOFs for photochemical transformations mediated by reactive radicals.

Full text of DOI:10.1021/acs.inorgchem.1c00642

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Article
Photoactive Metal−Organic Frameworks for the Selective Synthesis
of Thioethers: Coupled with Phosphine to Modulate Thiyl Radical
Generation
Zhifen Guo, Xin Liu, Rong Bai, Yan Che, Yanhong Chi, Chunyi Guo, and Hongzhu Xing*
Cite This: Inorg. Chem. 2021, 60, 8672−8681
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ABSTRACT: Metal−organic framework (MOF) materials are
intriguing photocatalysts to trigger radical-mediated chemical
transformations. We report herein the synthesis and character-
ization of a series of isomorphic MOFs which show a novel
structure, wide visible-light absorption, high chemical stability, and
specific redox potential. The prepared MOFs were explored for the
photoinduced single-electron oxidation of thiol compounds,
generating reactive thiyl radicals to afford thioethers via a
convenient thiol−olefin reaction. Importantly, we provide a widely
applicable strategy by combing a photoactive MOF with phosphine
to modulate the generation of thiyl radical in the reaction, thereby
producing a single product of the thioether without the formation
of a disulfide byproduct due to the dimerization of thiyl radicals.
The photocatalytic reaction takes advantage of this strategy, showing great generality where tens of thiols and olefins have been
examined as coupling partners. In addition, the strategy has also been demonstrated to be effective for the reactions catalyzed by
other MOFs. Mechanism studies reveal that the selective synthesis of C−S products relies on a synergy between the photoinduced
generation of a thiyl radical over the MOF and the in situ cleavage of S−S bond into a S−H bond by phosphine. It is notable that the
synthesized MOFs show advanced performance in comparison with classical MOFs. The work not only provides a series of novel
MOF photocatalysts that are capable of photoinduced thiol−olefin coupling but also indicates the great potential of MOFs for
photochemical transformations mediated by reactive radicals.
highly reactive radical species are typically generated to initiate
target reactions. Fox example, the photoinduced electron
transfer of an MOF to a ubiquitous oxygen molecule to form a
superoxide radical has been widely observed for dye
degradation.17 In addition, the superoxide radical has also
been recognized as a reactive intermediate to trigger the
photocatalytic oxidation of amines and thioethers to produce
valuable imines and sulfoxides, respectively.13−15 Despite these
successful investigations, important issues such as the
activation of diverse substrates by MOFs to generate different
reactive radicals and the control of the reaction to afford a
single-phase product have still been less addressed.
INTRODUCTION
■
One approach toward photoinduced chemical transformations
to address energy and environment concerns is the photo-
catalytic technique, in which the synthesis of the photocatalyst
and the exploration of its reactivity have always been at the
frontiers of photocatalysis studies.1−5 Recently, the use of a
metal−organic framework (MOF) as a photocatalyst has
attracted much attention in the field of photocatalysis due to
the rapid development of coordination chemistry in the past
few decades.6 The great flexibility of components, structures
and properties of a MOF material makes it a designable
platform to prepare novel heterogeneous photocatalysts with
excellent photophysical properties and desirable catalytic
activities. Successful studies have demonstrated that method-
ologies including linker modification, dye inclusion, and others
are feasible to construct visible-light-responsive MOFs, thereby
achieving various photocatalytic reactions such as dye
degradation, water splitting, carbon dioxide reduction, and
organic transformation.7−16 Studies reveal that these photo-
catalytic reactions depend primarily on the ability of an excited
MOF to either remove an electron from or donate an electron
to a substrate molecule under mild conditions. As a result,
Among the various candidates that could be activated by
photoactive MOFs, the oxidation of a thiol to generate a thiyl
radical is of much interest to us, since thiol compounds are
Received: March 3, 2021
Published: June 8, 2021
https://doi.org/10.1021/acs.inorgchem.1c00642
© 2021 American Chemical Society
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cheap and easily available. More importantly, thiyl radicals
have been demonstrated as versatile intermediates to construct
C−S bonds via convenient thiol−olefin reactions that have
been widely adopted for the synthesis of natural products,
pharmaceuticals, polymers, and bioconjugated materials.18−23
In addition, thiyl radicals have also been recognized as
hydrogen atom transfer (HAT) species to activate carbon−
hydrogen and heteroatom−hydrogen bonds.24−26 Convention-
ally, the generation of radicals from thiols can be achieved by
heating a radical initiator, using strong ultraviolet irradiation,
or adding oxidants to the reaction.20,21,27 Recent studies
showed that thiols could be oxidized expediently by photo-
active IrIII(ppy)3 and Ru(bpz)3 to afford thiyl radicals.19,28
2+
However, a literature survey suggests that the activation of
thiols by a photoactive MOF has been rarely studied so far. We
envisaged that the generation of thiyl radicals would be
achievable using a MOF photocatalyst, and the MOF-mediated
reaction would be useful to prepare functional complexes and
materials via convenient thiol−olefin reactions under mild
conditions.
We report herein a series of novel MOFs with properties of
wide visible-light absorption, high chemical stability, and
specific redox potential to activate thiol substrates via a single-
electron-oxidation process, generating reactive thiyl radicals to
couple with olefins for the synthesis of thioethers. In addition,
we provide a facile and widely applicable strategy by combing a
photoactive MOF with organophosphine catalysis to modulate
the photogeneration of the thiyl radical in the reaction, thus
providing a single thioether product without the formation of a
disulfide byproduct due to the rapid dimerization of generated
thiyl radicals.
Figure 1. (a) Coordination mode of lanthanide ion in the structure.
(b) Structure of the MOFs showing a dense stacking between
different layers. (c) Single layer in the MOFs in which the
coordinated DMF molecules are omitted. Gray, blue, red, and
cyanine represent carbon, nitrogen, oxygen, and lanthanide atoms,
respectively. All hydrogen atoms are omitted for clarity.
appear in pairs with a face to face distance of 3.64 Å (Figure 1c
and Figure S2).
Scanning electron microscopy (SEM) suggests that the
prepared MOF compounds are strip-shaped crystals (Figure
S3). The microcrystalline samples of these compounds were
then studied by powder X-ray diffraction (PXRD). Simulated
PXRD patterns of the isostructural Ln-MOFs are almost the
same as the experimental patterns. As shown in Figure 2a, the
experimental PXRD patterns of the MOFs fit well with those
simulated from the crystal structures, suggesting that the
prepared samples are in a single phase. In addition, the
chemical stability of the prepared MOF was also studied by
PXRD (Figure 2b). When the MOF was soaked in different
solvents such as methanol, acetonitrile, DMF, chloroform, and
dioxane for 1 day, it showed original the PXRD pattern with
high intensity, indicating the good chemical stability of the
MOF in common organic solvents. FTIR spectra of the MOFs
show very similar absorption peaks (Figure S4). The
characteristic absorption of the alkynyl group in ADBEB is
observed at 2193 cm−1. In addition, the absorption of the
phenyl group in ADBEB appears at 861, 787, and 758 cm−1.
These peaks indicate the existence of an aromatic ligand in the
structure. The vibration peaks of carboxylate groups in the
structure are observed at 1568 and 1408 cm−1.39 The presence
of DMF is confirmed by a series of weak peaks from 3056 to
2846 cm−1 (νC−H) along with an intense peak at 1625 cm−1
(νCO).40
RESULTS AND DISCUSSION
■
Synthesis, Structures, and Characterizations of
MOFs. To achieve visible-light absorption of the prepared
MOFs, the conjugated anthracene ligand (4,4′-anthracene-
9,10-diylbis(ethyne-2,1-diyl))dibenzoic acid (ADBEB) was
synthesized according to previous reports (Figure S1).29,30
Then, a series of novel orange lanthanide MOFs were prepared
via solvothermal reactions between ADBEB and lanthanide
nitrate (Ln = Gd3+, Eu3+, Sm3+) at 120 °C for 2 days (see the
Experimental Section). It is expected that the strong
coordination interaction between the carboxylate group and
the lanthanide ion would be helpful to synthesize MOFs with
good stability.31−34 The crystal structures of the prepared
MOFs (1Gd, 1Eu, and 1Sm) were determined by single-
crystal X-ray diffraction. The structure analysis indicates that
these compounds crystallize in the monoclinic P21/c space
group (Table S1), showing an isomorphic framework structure
with a general formula of Ln(ADBEB)(DMF)(HCOO). In the
structure, the trivalent lanthanide ion coordinates with
ADBEB, DMF, and HCOO−, in which HCOO− results from
the decomposition of the DMF solvent molecule (Figure
1a).35,36 Adjacent lanthanide ions in the structure are
interconnected via oxygen atoms to form dimers containing
Ln−O−Ln bonds (Figure 1a). The Ln−O bond lengths are in
the ranges of 2.331(5)−2.622(4) Å for 1Gd, 2.358(7)−
2.628(6) Å for 1Eu, and 2.377(5)−2.635(6) Å for 1Sm. These
values are in good agreement with those reported for other
lanthanide MOFs.37,38 As shown in Figure 1b, the coordination
between ADBEB and the lanthanide ion constructs the two-
dimensional MOFs, where the conjugated ADBEB ligands
Ultraviolet−visible diffuse reflectance spectroscopy (UV−vis
DRS) measurements shows that the synthesized MOF exhibits
intense absorption before 520 nm in the visible-light region,
followed by weak and broadened absorption extending to 700
nm (Figure 2c). As the optical absorption of lanthanide ions
falls usually into the UV region, the visible-light absorption of
the MOF should result primarily from the aromatic ADEBE
(Figure S1).41−43 In comparison with the transition absorption
of ADBEB, the optical absorption of the prepared MOF is
broadened, due possibly to a ligand to ligand charge transfer
interaction.44,45 The band gap energies of the MOF were
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Figure 2. (a) Experimental PXRD of the MOFs and the simulated PXRD of 1Gd, which is isostructural with 1Eu and 1Sm. (b) PXRD patterns for
1Gd soaked in organic solvents. (c) UV−vis DRS spectra of the MOFs. (d) Band-gap energy of the MOFs.
estimated to be 2.23 eV for 1Gd, 2.26 eV for 1Eu, and 2.24 eV
for 1Sm from Tauc plots (Figure 2d); it is believed that the
electronic bands in the MOFs are contributed primarily from
the HOMO−LUMO levels of the aromatic ligand.46−49
Photocatalytic Reactions. Visible-light-induced oxidation
of thiol and its subsequent coupling with olefin was studied
under ambient conditions using 1Gd as a prototype. At first,
the coupling between benzyl thiol (1) and styrene (2) in
acetonitrile was carried out with a commercially available blue
LED (32 W) as the light source (Table S2, entry 1). The
reaction at a scale of 0.1 mmol of the olefin gave a 76% yield of
the thioether. The formation of the target product suggests the
successive oxidation of thiol by excited 1Gd, generating a
reactive thiyl radical to initiate a thiol−olefin reaction.
However, a GC-MS analysis indicated that the reaction suffers
from a certain amount of disulfide byproduct with an S−S
bond. The presence of the disulfide byproduct hindered the
promotion of the reaction yield and complicated product
purification. Solvent optimization was then carried out to
diminish the formation of disulfide. As shown in Table S2, the
reaction in either polar or nonpolar solvents such as methanol,
DMF, acetonitrile, chloroform, dioxane, and their mixture
showed a major product of the thioether, accompanied by a
certain amount (4−9%) of disulfide. Among these solvents, the
reaction in the mixture of DMF/MeCN gave an ideal
performance (89% for thioether) in comparison to that carried
out in other solvents. Control experiments were carried out in
DMF/MeCN as the model condition to study the nature of
the reaction. The results show that the reaction is photo-
catalytic, where 1Gd and visible light are essential in the
system (Table 1, entries 2 and 3).
Table 1. Visible-Light-Induced Thiol−Olefin Coupling
a
under Different Conditions
entry
reaction
typical
no light
thioether (%)a
disulfide (%)a
1
2
89
0
4
0
3
4
5
6
7
8
9
10
11
12
13
14
15
16
no 1Gd
15
92
72
58
50
95
79
72
2.5
38
31
8
12
7
2
3
2
5
3
3
88
23
11
92
87
0
thiol/olefin (4/1)
thiol/olefin (1/1)
thiol/olefin (1/2)
thiol/olefin (1/4)
1Gd (15 mol %)
1Gd (5 mol %)
1Gd (2.5 mol %)
TEA (1 equiv)
KI (1 equiv)
NH2-UiO-66
MIL-101(Fe)
NH2-MIL-101(Fe)
TCEP (1 equiv)
12
99
Reaction conditions: MOF (7.4 mg, 10 mol %), MeCN/DMF (V/V
1/1, 500 μL), benzyl thiol (24.8 mg, 0.2 mmol), styrene (11.5 μL, 0.1
a
mmol), blue LED (32 W), N2 atmosphere, 8 h. The yields were
determined by GC.
ratio from 4 to 0.25 seems to be effective to decrease the
relative content of disulfide from 7% to 2%, whereas the yield
of the thioether decreased significantly from 92% to 50%
(Table 1, entries 4−7). Similarly, the optimization of
photocatalyst dosage exhibits the same trend, where the yield
of thioether and disulfide decreased simultaneously when the
dosage of 1Gd was lowered gradually (Table 1, entries 8−10).
It is well-known that the generation of disulfide in thiol−olefin
To enhance the selectivity of the reaction to produce a
thioether, parameters such as the thiol/olefin ratio and
photocatalyst dosage were optimized to avoid the generation
of disulfide (Table 1, entries 4−10). Reducing thiol/olefin
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a
Table 2. Scope of Thiols for 1Gd-Mediated Reactions
a
Reaction conditions: 1Gd (7.4 mg), MeCN/DMF (V/V 1/1, 500 μL), thiol (0.2 mmol), styrene (0.1 mmol), TCEP (28.6 mg, 0.1 mmol), blue
LED (32 W), N2 atmosphere, 8 h.
coupling is commonly encountered due to the presence of
excessive thiyl radicals in the system,20,21 in which the
dimerization between thiyl radicals occurs rapidly since the
reaction exhibits a large rate constant of ca. 109 M−1 s−1.27,50,51
In this context, we attempted to control the photoinduced
generation of the thiyl radical by the addition of a hole
scavenger such as triethylamine (TEA) or potassium iodide
(KI). However, the addition of these chemicals depressed the
target reaction (Table 1, entries 11 and 12). The results
showed that it is difficult to directly eliminate the formation of
the disulfide byproduct via optimization experiments. There-
after, photoinduced thiol−olefin coupling was studied using
other visible-light-responsive MOFs, such as NH2-UiO-66,
MIL-101(Fe) and NH2-MIL-101(Fe) (Table 1, entries 13−
15). Under the same conditions as with the 1Gd-mediated
reaction, NH2-UiO-66 gave a major product of thiother (31%),
but this was accompanied by relatively more disulfide (11%).
MIL-101(Fe) and NH2-MIL-101(Fe) produced the major
product disulfide (92% and 87%) and minor product thioether
(8% and 12%). Further optimization of the thiol/olefin ratio
and catalyst dosage in these reactions has been demonstrated
to be unable to produce a single product of thioether. These
experiments suggest that the heterogeneity of the reaction may
be responsible for the generation of disulfide. A comparison
between the performance shown by 1Gd and classical MOFs
indicates that the synthesized MOF is more advanced for
thioether synthesis via a photoinduced thiol−olefin reaction.
Then, we assorted to promote the reaction by cleaving the
disulfide into the starting thiol compound to cause it to be
involved in the reaction again. Previous reports have disclosed
that organophosphine tris(2-carboxyethyl)phosphine (TCEP)
is a promising agent for the rapid and irreversible reduction of
disulfides into thiols.52 It has been recently adopted for the
cleavage of S−S bonds on proteins and peptides.53 However,
the use of TCEP to modulate thiol−olefin coupling has rarely
been explored. A preliminary study was inspiring as the
reaction exhibited an excellent yield of 99% of the thioether
upon the addition of TCEP in a molar stoichiometry, without
the formation of disulfide (Table 1, entry 16). The role of
TCEP in the reaction was then studied by contrasting
experiments where dibenzyl disulfide was used as the substrate,
instead of benzyl thiol (Table S3). In the absence of TCEP, no
thioether formed in the 1Gd-mediated reaction. In contrast,
the photocatalytic reaction exhibits a 70% yield of thioether
upon the addition of TCEP, due to the in situ cleavage of an
S−S bond into an S−H bond. This has been affirmed by the
direct observation of the irreversible reduction of dibenzyl
disulfide into benzyl thiol (Figure S5).
Under these optimized conditions, the versatility of the
1Gd-mediated photocatalytic reaction was then explored using
a diverse set of reaction partners. As shown in Table 2, a wide
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a
Table 3. Scope of Olefins for 1Gd-Mediated Reactions
a
Reaction conditions: 1Gd (7.4 mg), MeCN/DMF (V/V 1/1, 500 μL), benzyl thiol (0.2 mmol), olefin (0.1 mmol), TCEP (28.6 mg, 0.1 mmol),
blue LED (32 W), N2 atmosphere, 8 h.
range of thiols (8−13) including benzyl thiols, thiophenol, and
alkanethiols (14−17) have been demonstrated to be efficient
partners to couple with styrene. The reactions of these thiols
exhibit excellent isolated yields of 90−96% to produce the
corresponding thioethers (8−17). Reactions with secondary
and tertiary thiols exhibit moderate yields of 75% and 70% (18,
19), due probably to the steric effect of the thiol. Beyond thiol
substrates, a series of olefins were also examined to further
highlight the generality of 1Gd-mediated photocatalytic
reactions. As shown in Table 3, styrene derivatives with
methoxy and halide substituents gave excellent yields of 90−
95% (20−23). For the derivatives of p-α-methylstyrene (24,
25), the substrate modified with an electron-donating group
showed a much higher yield (91%) in comparison to that with
an electron-withdrawing group (58%), suggesting that the
electronic effect of the olefin substrate is important for the
reaction. Cyclohexene and other substrates with a vinyl group
on the terminal position exhibit moderate yields of 56−85%
(26−31).
It is worth noting that there was no disulfide detected in any
of these reactions. Comparative experiments exhibit that the
same reaction produced both a thioether and a disulfide if
TCEP was absent (Table S4). These studies indicate the
combination of 1Gd and TCEP has wide applicability for the
synthesis of a thioether as a single product via a thiol−olefin
reaction. Cycling experiments demonstrated that 1Gd is stable
enough to survive five cycles (Figure S6), maintaining its
crystal structure and catalytic activity (Figures S7−S9).
Moreover, an enlarged model reaction has been performed
with a lower photocatalyst loading of 1.9 mol %. As a result, 1.3
g of the thioether was isolated, suggesting that the synthesized
MOF is promising for the preparation of the target product in
large amounts.
To explore the feasibility of the strategy by combining the
photoactive MOF with TCEP, visible-light-induced thiol−
olefin reactions over the MOFs of NH2-UiO-66, MIL-101(Fe),
and NH2-MIL-101(Fe) were studied in the presence of TCEP.
For the reaction of NH2-UiO-66, the yield of the thioether
increased from 31% to 48% upon the addition of equimolar
TCEP, accompanied by a decrease in disulfide from 11% to 4%
(Table S5). For the reactions of MIL-101(Fe) and NH2-MIL-
101(Fe), they exhibit the same trend to increase the amount of
thioether (from 8−12% to 18−25%) when TCEP was
introduced (Table S5). These results indicate that the strategy
is widely feasible for other heterogeneous photocatalysts to
prepare thioethers via thiol−olefin reactions. The same
strategy has also been applied for the 1Gd analogues 1Eu
and 1Sm. As shown in Table S6, the MOFs exhibited
comparable photocatalytic performance when various coupling
partners of thiols and olefins were used to prepare thioethers,
indicating that these isostructural MOFs are promising
photocatalysts to prepare a single thioether in the presence
of TCEP. To understand the photocatalytic activity of the
synthesized Ln-MOFs, control experiments using lanthanide
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Figure 3. (a) Transient photocurrent response under visible light (420 ≤ λ ≤ 800 nm). (b) Mott−Schottky plots at different frequencies. (c)
Redox potentials of 1Gd and PhCH2SH.
Figure 4. (a) EPR trapping of thiyl radical from benzyl thiol. (b) EPR trapping of thiyl radical from disulfide in the presence of TCEP. The
simulated aH and aN of values of the thiyl−DMPO adduct are 13.78 and 13.26 G, respectively. (c) Proposed mechanism for TCEP-mediated
synthesis of a thioether over the MOF.
nitrates as photocatalysts were conducted under the same
conditions (Table S7). The results show that these metal salts
are incapable of carrying out the target reaction. In contrast, a
photocatalytic reaction using ADBEB as the catalyst shows a
moderate yield of 62% (Table S7). Then mixtures of the ligand
ADBEB and lanthanide nitrates were tested for the photo-
catalytic thiol−ene coupling. These reactions showed a 6−10%
increase in the yield in comparison with the reaction using
solely ADBEB (Table S7). These control experiments suggest
the photocatalytic activity of the synthesized MOFs results
from the photoactive ligand, and the interaction between
ligand and lanthanide ion plays a role in the reaction.
Mechanistic Studies. In addition to the photocatalytic
experiments, more physical measurements have been carried
out on 1Gd to understand the photoinduced activation of thiol
initiating the thiol−olefin reaction. The transient photocurrent
response of the photocatalyst was studied at first under
ambient conditions. As shown in Figure 3a, 1Gd exhibits a
significant and reproducible photocurrent signal, suggesting an
efficient photoinduced electron−hole separation upon visible-
light excitation, leading to the subsequent oxidation of thiol on
its valence band (VB). To evaluate the redox potential of the
synthesized MOF, Mott−Schottky measurements were con-
ducted. Positive slopes of C2 versus the applied potential were
observed at 1500−2500 Hz, indicating a semiconductor-like
behavior (Figure 3b).13 The conduction band (CB) of 1Gd is
ca. −1.17 V versus the Ag/AgCl electrode. Hence, the valence
band (VB) of 1Gd was estimated to be 1.06 V versus the Ag/
AgCl electrode: i.e., 1.26 V versus the normal hydrogen
electrode (NHE). This value is much more positive than that
of the thiol compound (+0.70 V versus NHE, benzyl thiol),19
indicating that the excited 1Gd is thermodynamically feasible
to oxidize thiols, generating thiyl radicals (Figure 3c).
To gain a deeper insight into the reaction, the photoinduced
generation of the thiyl radical has been studied by electron
paramagnetic resonance (EPR). The EPR response of a
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suspension containing benzyl thiol and 1Gd was tested at
room temperature, in which the spin-trapping agent 5,5-
dimethyl-1-pyrroline N-oxide (DMPO) was added to capture
the resulting thiyl radicals. As shown in Figure 4a, the
suspension shows no EPR response under dark conditions. In
contrast, the same suspension exhibits characteristic EPR peaks
belonging to a DMPO−thiyl adduct under visible-light
irradiation from a xenon lamp.54 The measured EPR spectrum
fits well with that simulated by SpinFit, and the parameters aH
and aN were calculated to be 13.78 and 13.26 G, respectively.
EPR results show clearly the visible-light-induced generation of
thiyl radical by 1Gd. In addition, the influence of TCEP on the
1Gd-mediated reaction was also studied by EPR, where
dibenzyl disulfide was adopted instead of benzyl thiol. As
shown in Figure 4b, the suspension containing 1Gd, disulfide,
and DMPO is EPR-silent under visible-light irradiation,
suggesting that the photoinduced generation of thiyl radical
from disulfide cannot be directly achieved in the absence of
TCEP. When TCEP was added to the suspension, significant
EPR signals belonging to a DMPO−thiyl adduct were
observed, indicating the generation of a thiyl radical from
disulfide due to the synergistic effect of photoactive MOF and
TCEP.
CONCLUSION
■
In summary, a series of visible-light-responsive MOFs with
novel structure, excellent visible-light absorption, high
chemical stability, and specific redox potential have been
successfully synthesized. Photocatalytic studies suggest that
they are intriguing photocatalysts for the synthesis of
thioethers via thiol−olefin reactions. We provide here a facile
strategy to promote the reaction by combining a photoactive
MOF and organophosphine catalysis, resulting in the
generation of a single product of the thioether, without the
formation of disulfide byproduct due to the self-coupling of
thiyl radicals. By this strategy, tens of thioethers have been
synthesized using the synthesized MOF as a photocatalyst.
Photocatalytic experiments indicate the strategy is also
effective for photoinduced thiol-olefin coupling catalyzed by
other MOFs to produce thioether. Mechanism studies reveal
that the generation of a single product in C−S bonding
depends on the synergy between the photoinduced generation
of a thiyl radical by MOFs and the in situ cleavage of an S−S
bond into an S−H bond by phosphine to regenerate a thiyl
radical. The work provides a series of rarely reported MOFs
serving as heterogeneous photocatalysts for the photoinduced
synthesis of thioethers via thiol−olefin reactions, showing the
great potential of photoactive MOFs for radical-mediated
transformations.
According to our experimental findings and previous reports,
a plausible mechanism for the reaction was then proposed, as
shown in Figure 4c. The excited 1Gd oxidizes the thiol
compound upon visible-light irradiation, resulting in the
generation of reactive thiyl radicals (4). The thiyl radical
then attacks the vinyl group on an olefin in anti-Markownikoff
fashion to form a transient carbon-centered radical (5). This
radical species (5) would then be reduced by 1Gd via a
proton-coupled electron transfer (PCET) mechanism to
produce a thioether (6) containing a C−S bond.55 However,
the rapid dimerization of photogenerated thiyl radicals
produces an undesirable disulfide (7) with an S−S bond.
The addition of organophosphine irreversibly reduces 7 into
the starting thiol ,which can be oxidized by 1Gd to regenerate
thiyl radicals (4). In this case, the synergy between the
photoactive MOF and the organophosphine provides a single
product of the thioether in C−S bonding. Alternatively, the
alkyl radical (5) could participate in a classical radical chain
transfer pathway to produce the thiol−ene product (Figure
4c). This has been confirmed by the measurement of the
quantum yield of the reaction (Table S8).56−60 The quantum
yields under blue-light irradiation were estimated to be 1.13
(450 nm) and 1.46 (475 nm), indicating the existence of the
radical chain transfer pathway. In addition, we studied the
influence of the light source on the reaction. As shown in Table
S8, a model reaction using a blue LED (440−485 nm) with
lower powers of 8 and 24 W gave lower yields of 70% and 90%,
respectively. The reaction under a green LED (495−530 nm)
gave a relatively low yield of 53%. The quantum yield of the
reaction under green light at 520 nm was measured to be 0.43.
These studies suggest that the photophysical properties of the
MOF are vital to control the photocatalytic performance. In
addition, it is worth noting that the solvent also plays an
important role in the reaction (Table S2). Upon the detection
of thiyl radical, it is conjectured that influence of the solvent on
the reaction might be attributed to the relative stabilization of
the thiyl radical induced by solvents of different nature.27,61
EXPERIMENTAL SECTION
■
Synthesis of the MOFs [Ln(ADBEB)(DMF)(HCOO)]. Typically,
the MOF was prepared via a solvothermal reaction as follows: a
mixture of the organic ligand ADBEB (0.26 mmol, 120 mg),
gadolinium nitrate hexahydrate (1.04 mmol, 469 mg), 2-fluorobenzoic
acid (10 mmol, 1.4 g), and nitric acid (0.3 M, 3 mL) was stirred in
DMF (15 mL) for 10 min. Then the mixture was transferred to a
Teflon-lined autoclave and heated to 120 °C for 2 days under
autogenous pressure. After the solvothermal reaction, the autoclave
was cooled naturally to ambient temperature. The orange crystals of
1Gd were washed with DMF and ethanol and then dried under
ambient conditions for further use. The yield of 1Gd is calculated to
be 85.8% on the basis of the ligand. 1Eu and 1Sm were prepared by
the same solvothermal method, where the lanthanide (Eu/Sm) nitrate
hexahydrate was used. The yields for 1Eu and 1Sm were 74.6% and
78.6% on the basis of the ligand, respectively. A structure analysis
suggests that they possess isomorphic structures on the basis of the
single-crystal X-ray diffraction (SCXRD) data in Table S1. IR (KBr,
cm−1): 3056−2846 (m), 2193 (s), 1625 (s), 1568 (s), 1408 (s), 861
(s), 787 (s) and 758 (s). Elemental analysis for the synthesized MOFs
is as follows. Anal. Found for [Gd(ADBEB)(DMF)(HCOO)]: C,
58.34%; H, 3.54%; N, 1.93%; Gd, 21.35%. Calcd: C, 58.46%; H,
3.25%; N, 1.89%; Gd, 21.24%. Found for [Eu(ADBEB)(DMF)-
(HCOO)]: C, 58.91%; H, 3.82%; N, 2.04%; Eu, 20.05%. Calcd: C,
58.85%; H, 3.27%; N, 1.91%; Eu, 20.68%. Found for [Sm(ADBEB)-
(DMF)(HCOO)]: C, 59.10%; H, 3.62%; N, 1.96%; Sm, 19.95%.
Calcd: C, 59.02%; H, 3.28%; N, 1.91%; Sm, 20.49%.
Electrochemical Measurements. Photocurrent measurements
were performed in a standard three-electrode system with MOF-
coated ITO as the working electrode, a Pt plate as the counter
electrode, and Ag/AgCl as the reference electrode.13 A solution of 0.2
M Na2SO4 was used as the electrolyte with nitrogen bubbling for 30
min. A suspension of the MOF was made by adding 40 mg of the
MOF and 5 mg of polyvinylidene fluoride (PVDF) to 1.0 mL of
ethanol. Then the working electrode was prepared by dropping the
suspension (20 μL) onto the surface of an ITO plate covering
approximately 1 cm2 and then drying at 85 °C for 2 h. The
photocurrent signal of the working electrode was measured under a
300 W Xe lamp, where optical filters were used to remove light of less
than 420 nm and greater than 800 nm. Mott−Schottky measurements
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Article
were conducted on the basis of the same electrochemical setting.
During the test, the frequencies of the impedance were set to be 1500,
2000, and 2500 Hz, respectively.
ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.1c00642.
General Photocatalytic Reactions. Photocatalytic Thiol−
Olefin Coupling. Typically, the MOF (7.4 mg, 10.0 μmol), thiol
(0.2 mmol), TCEP (28.6 mg, 0.1 mmol), anhydrous MeCN (99.8%,
250 μL), and DMF (99.8%, 250 μL) were mixed under an N2
atmosphere in a vial (4 mL). Then, the olefin (0.1 mmol) was injected
with a syringe. After that, the reaction mixture was irradiated with a 32
W blue LED and fanned to maintain the temperature (ca. 25 °C) for
8 h. Then the filtrate was extracted three times with EtOAc, and the
extracts were washed with NaCl aqueous solution. Then the organic
phase was dried by anhydrous magnesium sulfate for 30 min, followed
by concentration and purification by flash chromatography on silica
gel with a hexane/EtOAc mixture to obtain the isolated yield. The
structures of the synthesized thioethers were determined by 1H NMR
spectra in reference to structures reported in previous studies.19,62−64
To confirm the correctness of the structures, we also measured the
13C NMR and HR-MS data for thioethers such as 3, 12, 16, and 17.
Materials, single-crystal structure analysis, supporting
tables and figures, experimental methods, and com-
pound characterization data (PDF)
Accession Codes
CCDC 1963656−1963658 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
The data are provided in the Supporting Information, and they
confirm the correctness of the molecular structure of the products.
The large-scale photocatalytic reaction was performed as follows:
the MOF (105 mg, 0.14 mmol, 1.9 mol %), benzyl thiol (15 mmol),
TCEP (429 mg, 1.5 mmol, 0.2 equiv), anhydrous MeCN (99.8%,
18.75 mL), and DMF (99.8%, 18.75 mL) were mixed under an N2
atmosphere in a round-bottom flask (50 mL). Then, styrene (7.5
mmol) was injected with a syringe. After that, the reaction was
irradiated with a 32 W blue LED and fanned to maintain the
temperature at about 25 °C for 8 h. The product was purified by flash
chromatography on silica gel to isolate the thioether (1.3 g, yield
76%).
Photocatalytic Reaction of Dibenzyl Disulfide with Styrene. The
MOF (7.4 mg, 10.0 μmol), TCEP (28.6 mg, 0.1 mmol), dibenzyl
disulfide (24.6 mg, 0.1 mmol), anhydrous MeCN (99.8%, 250 μL),
and DMF (99.8%, 250 μL) were mixed in a vial (4 mL) under a
nitrogen atmosphere. Then styrene (11.5 μL, 0.1 mmol) was injected
with a syringe. The reaction was irradiated with a 32 W blue LED and
fanned to maintain the temperature at ca. 25 °C for 8 h. The resulting
suspension was filtered using a membrane (22 μm in diameter) and
analyzed by GC.
Electron Paramagnetic Resonance (EPR) Measurements.
The EPR response of the reaction solutions was recorded on the a
spectrometer (X-band) operated at a frequency of 9.41 GHz at 298 K.
General spectrometer parameters are as follows: scan range, 200 G;
center field set, 3359.0 G; conversion time, 34.18 ms; sweep time,
35.00023 s; modulation amplitude, 1.0 G; modulation frequency, 100
kHz; receiver gain, 1.00 × 20; microwave power, 19.1 mW.54
EPR Detection of Radical Generated from Thiol. An oven-
dried tube (4 mL) containing MeCN (500 μL, nitrogen bubbled for
30 min) was equipped with a stirring bar under an N2 atmosphere.
Then benzyl thiol (24.8 mg, 0.2 mmol) and 1Gd (7.4 mg, 10.0 μmol)
were placed in the tube with stirring in the dark. Before the EPR
measurement, 0.2 mmol of DMPO was injected into the system as a
trapping agent for thiyl radicals. A small portion of the mixture was
taken out quickly using a capillary and placed into a quartz EPR tube
(4 mm diameter) for EPR measurement. There was no EPR response.
Thereafter, the mixture was irradiated by a 300 W Xe lamp where
light of less than 420 nm and greater than 800 nm was removed by an
optical filter. An EPR signal for the DMPO−thiyl adduct was
observed.
EPR Detection of Radical Generated from Dibenzyl
Disulfide. At first, a mixture of 1Gd (7.4 mg, 10.0 μmol), dibenzyl
disulfide (24.6 mg, 0.1 mmol), and 0.2 mmol of DMPO in N2-
saturared MeCN (500 μL) was tested under visible-light irradiation.
There was no EPR response in this case, indicating that the direct
generation of a thiyl radical from disulfide is not achievable in the
presence of only 1Gd. TCEP (28.4 mg, 0.1 mmol) was then added to
the system, whereupon an EPR signal for the DMPO−thiyl adduct
was detected, indicating the generation of a thiyl radical from
disulfide.
■
Corresponding Author
Hongzhu Xing − Laboratory of Advanced Energy Materials,
College of Chemistry, Northeast Normal University,
Changchun 130021, People’s Republic of China;
orcid.org/0000-0001-7179-0394; Email: xinghz223@
nenu.edu.cn
Authors
Zhifen Guo − Laboratory of Advanced Energy Materials,
College of Chemistry, Northeast Normal University,
Changchun 130021, People’s Republic of China
Xin Liu − Laboratory of Advanced Energy Materials, College
of Chemistry, Northeast Normal University, Changchun
130021, People’s Republic of China
Rong Bai − Laboratory of Advanced Energy Materials, College
of Chemistry, Northeast Normal University, Changchun
130021, People’s Republic of China
Yan Che − Laboratory of Advanced Energy Materials, College
of Chemistry, Northeast Normal University, Changchun
130021, People’s Republic of China
Yanhong Chi − Laboratory of Advanced Energy Materials,
College of Chemistry, Northeast Normal University,
Changchun 130021, People’s Republic of China
Chunyi Guo − Laboratory of Advanced Energy Materials,
College of Chemistry, Northeast Normal University,
Changchun 130021, People’s Republic of China
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.1c00642
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by Natural Science Foundation of
Jilin Province (20180101290JC) and the Fundamental
Research Funds for the Central Universities.
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Inorg. Chem. 2021, 60, 8672−8681