Visible-light-induced tandem reaction of o-aminothiophenols and alcohols to benzothiazoles over Fe-based MOFs: Influence of the structure elucidated by transient absorption spectroscopy

Article abstract of DOI:10.1016/j.jcat.2017.01.014

MIL-100(Fe) and MIL-68(Fe), two Fe-based MOFs, were found to be active for oxidative condensation between alcohols and o-aminothiophenols to form 2-substituted benzothiazoles under visible light irradiation using oxygen (O₂) as oxidant. This reaction can be applied to a wide range of substrates with medium to high yield. Controlled experiments and ESR results revealed a superoxide radical (O₂^-)-mediated pathway, which is derived from the reduction of O₂ by photogenerated Fe²⁺ on Fe–O clusters. The whole multistep reaction is limited by the step of the photo-oxidation of alcohols to aldehydes. MIL-100(Fe) showed catalytic performance superior to that of MIL-68(Fe) because its higher concentration of long-lived (μs time scale) positive holes can be photogenerated over MIL-100(Fe), in contrast to MIL-68(Fe). This study not only provides an economical, sustainable, and thus green process for the production of 2-substituted benzothiazoles, but also illustrates the potential of using transient absorption spectroscopy as an important tool for understanding the photophysics of MOFs, which are believed to show great potential as multifunctional catalysts for light-induced organic transformations.

Full text of DOI:10.1016/j.jcat.2017.01.014

Journal of Catalysis 349 (2017) 156–162
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Visible-light-induced tandem reaction of o-aminothiophenols and
alcohols to benzothiazoles over Fe-based MOFs: Influence of the
structure elucidated by transient absorption spectroscopy
a,
Dengke Wang a, Josep Albero b, Hermenegildo García b, , Zhaohui Li
⇑
⇑
a Research Institute of Photocatalysis, State Key Laboratory of Photocatalysis on Energy and Environment, College of Chemistry, Fuzhou University, Fuzhou 350002, People’s Republic
of China
b Instituto de Tecnología Química, Universitat Politècnica de València, Consejo Superior de Investigaciones Científicas, Avenida de los Naranjos s/n, 46022 Valencia, Spain
a r t i c l e i n f o
a b s t r a c t
Article history:
MIL-100(Fe) and MIL-68(Fe), two Fe-based MOFs, were found to be active for oxidative condensation
between alcohols and o-aminothiophenols to form 2-substituted benzothiazoles under visible light irra-
diation using oxygen (O2) as oxidant. This reaction can be applied to a wide range of substrates with med-
ium to high yield. Controlled experiments and ESR results revealed a superoxide radical (OÅ2ꢀ)-mediated
pathway, which is derived from the reduction of O2 by photogenerated Fe2+ on Fe–O clusters. The whole
multistep reaction is limited by the step of the photo-oxidation of alcohols to aldehydes. MIL-100(Fe)
showed catalytic performance superior to that of MIL-68(Fe) because its higher concentration of long-
Received 10 December 2016
Revised 17 January 2017
Accepted 21 January 2017
Keywords:
Tandem reactions
Benzothiazoles
lived (ls time scale) positive holes can be photogenerated over MIL-100(Fe), in contrast to MIL-68(Fe).
This study not only provides an economical, sustainable, and thus green process for the production of
2-substituted benzothiazoles, but also illustrates the potential of using transient absorption spectroscopy
as an important tool for understanding the photophysics of MOFs, which are believed to show great
potential as multifunctional catalysts for light-induced organic transformations.
Ó 2017 Elsevier Inc. All rights reserved.
Fe-based MOFs
Transient absorption spectroscopy
Light-induced organic transformations
1. Introduction
benzothiazole from oxidative condensation between o-
aminothiophenol and alcohol [12,13]. Unfortunately, the perfor-
As an important class of heterocycles, 2-substituted benzothia-
zoles are building blocks for the generation of many biologically
active products, pharmaceutical compounds, and functional mate-
rials [1–3]. The traditional transition metal-catalyzed intramolecu-
lar cyclization of o-halothiobenzanilides and the condensation
between carbonyl-containing compounds and o-aminothiophenol
to produce 2-substituted benzothiazoles involve either costly
dehydrating reagents or toxic oxidants such as benzoquinones
[4–8]. Direct oxidative condensation of alcohols with o-
aminothiophenols using an environmentally friendly oxidant such
as molecular oxygen (O2) to produce 2-substituted benzothiazoles
is an attractive alternative, since alcohols are readily available sub-
strates, and this process has high atom efficiency and is clean, pro-
ducing water as the only byproduct [9–11]. Both Ru- and Ir-based
homogeneous and noble metal-doped heterogeneous systems have
already been explored for the direct formation of 2-substituted
mance of these catalytic systems for direct oxidative condensation
is still unsatisfactory. In addition to the difficulty of recycling them,
the homogeneous systems also require more than stoichiometric
amounts of basic additives or excess amounts of alcohols, which
results in low atom efficiency. Large catalyst amounts and long
reaction times are necessary for most of the heterogeneous sys-
tems. Moreover, elevated temperatures are required for almost
all these systems, in which by-products due to the overalkylation
of 2-substituted benzothiazoles are produced.
With the aim of developing renewable-energy-based processes,
the utilization of natural light to trigger chemical reactions is
attracting increasing research interest [14–18]. Light-induced
chemical reactions can usually be carried out under mild condi-
tions, which frequently makes them more selective than those car-
ried out following the traditional thermally activated processes,
since some thermally induced undesirable side reactions can be
inhibited. Light-induced formation of 2-substituted benzothiazoles
by reaction of o-aminothiophenols and aldehydes has already been
realized over several photocatalytic systems, such as CdS [19,20],
boron dipyrromethene [21], and tetrazine-based catalytic systems
⇑
Corresponding authors. Fax: +86 591 83779260 (Z. Li).
E-mail addresses: hgarcia@qim.upv.es (H. García), zhaohuili1969@yahoo.com
(Z. Li).
http://dx.doi.org/10.1016/j.jcat.2017.01.014
0021-9517/Ó 2017 Elsevier Inc. All rights reserved.

D. Wang et al. / Journal of Catalysis 349 (2017) 156–162
157
[22]. Considering that alcohols are readily available, the direct use
of alcohols as starting material to react with o-aminothiophenols
to produce 2-substituted benzothiazoles is therefore attractive.
Since this reaction involves the oxidation of alcohols to aldehydes,
condensation of aldehydes with o-aminothiophenols to imine/ben-
zothiazolines, and oxidation of imine/benzothiazolines to 2-
substituted benzothiazoles in consecutive steps (as shown in
Scheme 1), a multifunctional catalyst is required for such a one-
pot multistep reaction [9–11,23–27].
between the structure of MOF-based catalysts and their catalytic
activity.
2. Experimental
2.1. Synthesis
MIL-100(Fe) was prepared following previously reported proce-
dures [81,82]. Fe(NO3)ꢁ9H2O (484 mg, 1.2 mmol) and 1,3,5-
benzenetricarboxylic acid (H3BTC, 210 mg, 1.0 mmol) were dis-
solved in deionized water (5 mL) and were treated thermally in a
stainless steel autoclave at 180 °C for 12 h. The resultant product
was recovered by filtration and washed with water and methanol.
The synthesized MIL-100(Fe) was dried overnight at 60 °C in an
oven.
Metal–organic frameworks (MOFs), a class of 3D crystalline
micro–mesoporous hybrid materials constructed from metal or
metal cluster nodes interconnected with multidentate organic
linkers, have already shown a variety of photocatalytic applications
[28–36]. Ever since previous studies on MOF-5 revealed that metal
clusters in MOFs can be regarded as inorganic semiconductor
quantum entities, while the organic linkers act as antennae to acti-
vate these semiconductor quantum dots via linker-to-metal cluster
charge transfer (LCCT) upon light excitation, the use of MOFs for
photocatalysis has attracted extensive research interest [37,38].
Actually, MOFs are emerging as a new type of promising photocat-
alysts and have already been applied for photocatalytic CO2 reduc-
tion [39–45], organic transformations [46–52], and hydrogen
evolution [53–59], as well as pollutant degradation [60,61]. The
use of MOFs for light-induced organic transformations where pho-
tocatalytic and catalytic steps are consecutively taking place is par-
ticularly appealing [62–72]. The high porosity of MOFs can ensure
fast mass transport, while the presence of different catalytic sites
in different places in the MOF structure enables them to behave
as multifunctional catalysts [52,69–72]. Actually, the use of MOFs,
especially the Matériaux de l’Institut Lavoisier (MILs) for other
types of catalysis, such as Brønsted and Lewis acid catalysis, as well
as tandem reactions, has already been reported previously [73–76].
Among all the reported MOF-based photocatalysts, Fe-containing
MOFs are extremely attractive because Fe is an earth-abundant
element and Fe-based MOFs show relatively intense absorption
in the visible light region localized at the iron-oxo (Fe–O) clusters
[45,51,52,60,77–80].
MIL-68(Fe) was synthesized according to the literature [83,84].
A
mixture
of
FeCl3ꢁ6H2O
(324 mg,
1.2 mmol),
1,4-
benzenedicarboxylic acid (H2BDC, 798 mg, 4.8 mmol), hydrofluoric
acid (5 mol/L, 0.12 mL), and hydrous hydrochloride (1 mol/L,
0.12 mL) was dissolved in N,N-dimethylformamide (18 mL) in a
Teflon-lined autoclave. The resultant mixture was heated at
100 °C for 120 h. The obtained solid product was recovered by fil-
tration, washed with water and methanol, and then dried over-
night at 60 °C in an oven.
2.2. Characterization techniques
X-ray diffraction (XRD) patterns were collected on
Advance X-ray diffractometer (Bruker, Germany) with Cu K
a
D8
a
radi-
ation. XRD patterns were scanned over the angular range of 5°–30°
(2h) with a step size of 0.02°. The UV–vis diffuse reflectance spectra
(UV–vis DRS) were obtained with a UV–vis spectrophotometer
(Varian Cary 500). Barium sulfate (BaSO4) was used as a reference.
BET surface area measurements were carried out on an ASAP
2020M apparatus (Micromeritics Instrument Corp., USA). The sam-
ples were degassed in vacuum at 150 °C for 10 h and then mea-
sured at ꢀ196 °C. The ESR spectra were determined on a Bruker
A300 ESR spectrometer. Fe content in the filtrate was determined
using inductively coupled plasma optical emission spectroscopy
(ICP-OES, Perkin-Elmer, OPTIMA 8000). Before ICP-OES analyses,
the solid was digested in a mixture of HNO3 and milli-Q water.
The reactions were carried out with a 300 W Xe arc lamp (Beijing
Perfectlight, PLS-SXE300c).
In this article, we report direct oxidative condensation between
o-aminothiophenols and alcohols to produce 2-substituted ben-
zothiazoles under visible light irradiations using O2 as an oxidant
over MIL-100(Fe) and MIL-68(Fe), two Fe-based MOFs. It was found
that the structure of MOF influences its catalytic performance,
which was explained well using transient absorption spectroscopy
(TAS). This study not only provides an economical, sustainable, and
thus green process for the production of 2-substituted benzothia-
zoles, but also gives us a better understanding of the relationship
2.3. Light-induced reactions
The synthesis of 2-substituted benzothiazoles from o-
aminothiophenols and alcohols was performed in a sealed Schlenk
tube under visible light irradiation. Typically, a mixture of o-
aminothiophenol (0.1 mmol) and alcohol (0.3 mmol) in acetonitrile
(CH3CN, 2 mL) was saturated with O2 before the mixture was
transferred into a 10 mL tube containing 10 mg of MOFs. The sus-
pension was irradiated with a 300 W Xe lamp equipped with a UV-
cut filter to remove all irradiations with wavelengths less than
420 nm and an IR-cut filter to remove all irradiations with wave-
lengths greater than 800 nm. After the reaction, the suspension
was filtered through a porous membrane (diameter 20 lm) and
the products were analyzed by GC-MS and GC-FID (Shimadzu
GC-2014) with an HP-5 capillary column. The reaction, scaled up
by 10 times, was conducted under similar conditions in a home-
made reactor. A mixture of o-aminothiophenol (1 mmol) and ben-
zyl alcohol (3 mmol) in CH3CN (20 ml) saturated with O2 was
transferred to the homemade reactor containing 100 mg of MIL-
100(Fe). The reactor was irradiated with a 300 W Xe lamp
equipped with both a UV-cut filter and an IR-cut filter.
Scheme 1. Oxidative condensation of alcohols and o-aminothiophenols to produce
2-substituted benzothiazoles.

158
D. Wang et al. / Journal of Catalysis 349 (2017) 156–162
2.4. TAS studies
MIL-100(Fe) and MIL-68(Fe) were dispersed in CH3CN with an
approximate concentration of 0.1 mg/ml by sonication for
30 min. Subsequently, the resultant dispersions were purged with
Ar for 10 min. Transient signals of MIL-100(Fe) and MIL-68(Fe) dis-
persions were acquired at 3 ms upon 532 nm excitation using the
same laser power (25 mJ, 7 ns pulse). Quenching experiments were
carried out by monitoring the transient signals of the MOF disper-
sions at 560 nm upon addition of quenchers, including methanol
(MeOH), ethanol (EtOH), 2-propanol (IPA), dichloromethane (CH2-
Cl2), cerium (IV) ammonium nitrate (Ce(IV)), and oxygen (O2).
Quenching of the transient signals allowed us to build Stern–Vol-
mer plots following the equation
Fig. 2. The DR UV–vis spectra of MIL-100(Fe) and MIL-68(Fe).
I0
I
ꢀ 1 ¼ KSV ꢁ ½Qꢂ
where I0 is the initial and I the after-quencher addition transient
signal intensity, KSV is the Stern–Volmer constant, and [Q] is the
quencher concentration.
Among all the solvents investigated, CH3CN showed the best per-
formance by giving the highest o-aminothiophenol conversion of
78% and a selectivity of 99% to 2-phenylbenzothiazole after 6 h
irradiation (Table 1, entry 3). Benzaldehyde was detected, but no
other reaction intermediates or by-products such as imine/ben-
zothiazoline or other overalkylated tertiary amine were observed.
Since the oxidative condensation between benzyl alcohol and o-
aminothiophenol to produce 2-phenylbenzothiazole involves the
oxidation of alcohol to aldehyde, condensation of aldehyde with
o-aminothiophenol to imine/benzothiazoline, and its transforma-
tion to 2-substituted benzothiazole in consecutive steps (Scheme 1)
[9–13,19–22], the nonobservation of imine/benzothiazoline during
the reaction process indicates that its transformation to 2-
phenylbenzothiazole is fast and that the rate-controlling step in
the overall process is benzyl alcohol photo-oxidation. A prolonged
3. Results and discussion
MIL-100(Fe), a three-dimensional Fe-based MOF material, was
initially chosen to promote photoinduced oxidative condensation
between o-aminothiophenol and alcohol to produce 2-
substituted benzothiazole, due to its very good resistance to water
and organic solvents. MIL-100(Fe) is built up from supertetrahedra
consisting of trimers of FeO6 octahedra sharing a common vertex
l3-O, which delimits two types of mesoporous cages (Fig. S1a)
[81,82]. MIL-100(Fe) was prepared by a solvothermal reaction fol-
lowing a previously reported method [81,82]. Formation of the
pure phase of MIL-100(Fe) was confirmed by the good agreement
between the XRD patterns of the as-prepared MIL-100(Fe) and
those calculated for it (Fig. 1a). The N2 adsorption/desorption mea-
surements on the as-obtained MIL-100(Fe) reveal a specific BET
surface area of 2021 m2/g, comparable to that reported previously
(2050 m2/g) (Fig. S2a) [82]. The UV–vis diffuse reflectance spectra
(DRS) of the as-prepared MIL-100(Fe) show a broad intense
absorption at 200–550 nm, with the absorption edge extending
to around 600 nm (Fig. 2).
The direct oxidative condensation between o-aminothiophenol
and benzyl alcohol was carried out over MIL-100(Fe) in the pres-
ence of O2 in CH3CN under visible light irradiation. It was found
that 2-phenylbenzothiazole was obtained as the main product over
irradiated MIL-100(Fe), while negligible 2-phenylbenzothiazole
was obtained either in the absence of MIL-100(Fe) or in its pres-
ence but in the dark (Table 1, entries 1, 2). The reaction medium
played an important role in this reaction (Table 1, entries 3–7).
reaction time of 10 h led to
a greater conversion of o-
aminothiophenol (91%) without sacrificing the selectivity to 2-
phenylbenzothiazole (98%) (Table 1, entry 8). A filtration test
revealed that no further reaction occurred after MIL-100(Fe) was
removed from the reaction system at 4 h (Table 1, entry 9). The
ICP analysis of the filtrate showed that the amount of Fe3+ was
below the detection limit (ꢃ1 ppm). All the available data indicate
that the formation of 2-phenylbenzothiazole from the reaction
between o-aminothiophenol and benzyl alcohol is a truly hetero-
geneous process induced by the presence of MIL-100(Fe) and
requiring visible light irradiation. Recyclability tests for three runs
showed no obvious decrease in the catalytic activity of MIL-100(Fe)
(Fig. 3). The XRD of MIL-100(Fe) after three runs did not change
and its surface area is comparable to that of the fresh sample
(Figs. 1a and S2b), indicating that MIL-100(Fe) is stable during
the catalytic reaction and is reusable. Furthermore, it was found
that the experiment can be scaled up without obvious decrease
Fig. 1. XRD patterns of (a) as-synthesized MIL-100(Fe), used MIL-100(Fe), and calculated MIL-100(Fe); (b) as-synthesized MIL-68(Fe) and calculated MIL-68(Fe).

D. Wang et al. / Journal of Catalysis 349 (2017) 156–162
159
Table 1
Formation of 2-phenylbenzothiazole from o-aminothiophenol and benzyl alcohol over MIL-100(Fe) under various conditions.
Entry
Solvent
Time/h
Conv. of 2/%
Sel./%
b
1a
2c
3
4
5
6
7
8
CH3CN
CH3CN
CH3CN
ArCF3
Toluene
THF
10
10
6
6
6
1.2
0.5
78
67
47
53
62
91
58
90
3
–
–
b
99
98
98
99
97
98
98
97
98
97
6
6
DMF
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
10
10
10
10
10
9d
10e
11f
12g
66
Notes: Reaction conditions: o-aminothiophenol (0.1 mmol), benzyl alcohol (0.3 mmol), solvent (2 mL), MIL-100(Fe) (10 mg), O2 (1 atm), light irradiation
(800 nm ꢄ k ꢄ 420 nm).
a
Without visible light irradiation.
No products or negligible products were detected.
No catalyst.
The catalyst was filtrated after being irradiated for 4 h.
The reaction was scaled up by 10 times.
The reaction was quenched by benzoquinone.
MIL-68(Fe) was used as catalyst.
b
c
d
e
f
g
Table 2
Conversion and selectivity data for the formation of 2-substituted benzohiazoles from
different o-aminothiophenols and alcohols over MIL-100(Fe).
R1 = aryl, alkyl, heterocycle . . . R2 = –H, –Cl, –CF3 . . .
3a 91% (98%)a
3d 96% (97%)
3b 56% (97%)
3e 92% (99%)
3c 46% (>99%)
3f 78% (97%)
Fig. 3. Recycling of MIL-100(Fe) as catalyst for the oxidative condensation of o-
aminothiophenol with benzyl alcohol to produce 2-phenylbenzothiazole.
3g 80% (98%)
3j 38% (97%)
3m 86% (96%)
3h 85% (98%)
3k 90% (99%)
3n 90% (99%)
3i 42% (98%)
3l 84% (97%)
3o 87% (>99%)
in performance. Comparable o-aminothiophenol conversion (90%)
was still observed with high selectivity of 97% to 2-
phenylbenzothiazole when the reaction was scaled up by a factor
of 10 (Table 1, entry 10).
Under the optimized reaction condition, the substrate scope of
this reaction was also investigated and the results are shown in
Table 2. All the aromatic alcohols with different substituents, ali-
phatic alcohols as well as heterocyclic alcohols, were found to react
with o-aminothiophenol to give the corresponding 2-substituted
benzothiazoles (3a–3m) over irradiated MIL-100(Fe), although
with different activity. As compared with bare benzyl alcohol, aro-
matic alcohols with electron-withdrawing groups such as –Cl and
–NO2 showed lower conversions (46–56%, 3b–3c) in 10 h. In con-
trast, aromatic alcohols bearing electron-donating groups such as
p-OCH3 and p-CH3 exhibited enhanced conversion values (92–
96%, 3d–3e), with the exception observed over benzyl alcohol with
o-OCH3, o-CH3, and p-CH(CH3)2 groups, which exhibited a slightly
lower conversion ratio (78–85%, 3f–3 h) than bare benzyl alcohol.
This suggests the existence of both electronic and steric effects in
this reaction. Aliphatic alcohols can also react with o-
aminothiophenol over irradiated MIL-100(Fe), but with a much
lower conversion of o-aminothiophenol (38–42%) to their corre-
Notes: Reaction conditions: substituted o-aminothiophenol (0.1 mmol), alcohol
(0.3 mmol), CH3CN (2 mL), MIL-100(Fe) (10 mg), O2 (1 atm), light irradiation
(800 nm ꢄ k ꢄ 420 nm), 10 h.
a
The conversion of 2 and the selectivity of 3 in parentheses.
sponding 2-substituted benzothiazoles (3i–3j). The reaction
between heterocyclic alcohols and o-aminothiophenol also
afforded a high conversion rate (84–90%) to 2-substituted benzoth-
iazoles (3k–3m) over MIL-100(Fe). On the other hand, the reactions
between substituted o-aminothiophenols and benzyl alcohol also
occurred. 2-Amino-4-chlorobenzenethiol and 2-amino-4-(trifluoro
methyl)benzenethiol reacted with benzyl alcohol with high con-
version values (87–90%) and a high selectivity (ꢄ99%) to substi-
tuted benzothiazoles (3n–3o). These results indicate that the
light-induced reaction between o-aminothiophenols and alcohols
to synthesize 2-substituted benzothiazoles over MIL-100(Fe) is a
general process applicable to a wide scope of substrates.

160
D. Wang et al. / Journal of Catalysis 349 (2017) 156–162
Previous studies revealed that the visible-light-induced synthe-
sis of 2-substituted benzothiazoles over a homogeneous Ru-
containing catalytic system involves OÅ2ꢀ as an active species [85].
The ESR spectrum of our reaction system, which contained MIL-
100(Fe), benzyl alcohol, and o-aminothiophenol in the presence
of 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as a trapping agent
also shows typical signals for the DMPO–OÅ2ꢀ adduct when irradi-
ated, thus, confirming the formation of OÅ2ꢀ radicals during the cat-
alytic reaction (Fig. 4) [86]. Previous studies revealed that the
oxidative coupling of amines may also go through the singlet oxy-
gen (1O2)-mediated pathway [87]. To investigated whether 1O2
was involved in the reaction, an ESR study using 2,2,6,6-tetrame
thylpiperidinyloxyl (TEMPO) as a 1O2-trapping agent was carried
out. No typical signals for the TEMPO–1O2 adduct were observed
in the ESR spectrum under visible light irradiation (Fig. S3). This
suggested that no 1O2 was involved during the tandem reaction.
Moreover, addition of benzoquinone, an OÅ2ꢀ radical scavenger, into
the catalytic system completely quenched the reaction, further evi-
dence that the synthesis of 2-phenzylbenzothiazole over MIL-100
(Fe) can follow an OÅ2ꢀ radical mediated oxygenation pathway
(Table 1, entry 11). It was proposed that the OÅ2ꢀ radical in this sys-
tem can be generated via the reduction of O2 by Fe2+ formed over
the irradiated MIL-100(Fe). A similar formation of OÅ2ꢀ radicals was
previously observed over NH2-MIL-101(Fe), which was found to be
responsible for the oxidation of benzyl alcohol to realize one-pot
tandem photo-oxidation/Knoevenagel condensation between aro-
matic alcohols and active methylene compounds [77]. In addition,
the reduction of Fe3+ to form Fe2+ upon light irradiation in Fe-based
MOFs has also been reported over MIL-100(Fe) for benzene
hydroxylation as well as MIL-101(Fe) for photocatalytic CO2 reduc-
tion [45,51].
Based on the ESR results, a reasonable mechanism for the
visible-light-induced synthesis of 2-phenylbenzothiazole from o-
aminothiophenols and alcohols over MIL-100(Fe) can be proposed
in Scheme 2. First, an excited charge separation state occurs in
MIL-100(Fe) upon irradiation, with the electron transferred from
O2ꢀ to Fe3+ in the Fe3O clusters to form Fe2+ (step i). The as-
formed Fe2+ can reduce O2 to form O2Åꢀ radicals, while Fe2+ itself
is oxidized back to Fe3+ (step ii). In the meantime, by the quenching
of the hole in MIL-100(Fe), alcohol becomes oxidized through the
benzylic intermediate (step iii), which can further react with the
OÅ2ꢀ radical to form the aldehyde via an oxidative dehydrogenation
process (step iv). Such an OÅ2ꢀ radical-mediated oxidative dehydro-
genation process of alcohols has been previously reported over a
series of MOFs as well as some semiconductor photocatalysts in
oxygenation reactions [14,47,88,89]. The condensation between
the in situ generated aldehyde and o-aminothiophenol can be
promoted by Lewis acidic Fe3+ sites in MIL-100(Fe) to produce
Scheme 2. Proposed mechanism for the visible-light-induced synthesis of 2-
substituted benzothiazoles from o-aminothiophenols and alcohols over MIL-100
(Fe).
2-substituted benzothiazoline, which can be further oxidized by
the active OÅ2ꢀ radicals to form 2-substituted benzothiazole (step
v and vi). According to this mechanistic proposal, MIL-100(Fe)
has to act as a photocatalyst and as a solid Lewis acid to success-
fully promote the tandem reaction.
Based on the above proposed mechanism, it is believed that
other Fe-containing MOFs may also be active in the light-induced
formation of 2-substituted benzothiazole. To explore how the
structure of the MOF can influence the reaction, MIL-68(Fe),
another Fe-containing MOF, was also applied as a catalyst in the
light-assisted reaction between o-aminothiophenol and benzyl
alcohol under otherwise similar conditions. Unlike MIL-100(Fe),
MIL-68(Fe) is assembled from an infinite straight chain of corner-
sharing FeO4(OH)2 octahedra connected through terephthalate
linkers, defining two types of one-dimensional channels
(Fig. S1b) [83,84]. MIL-68(Fe) with high quality, as evidenced from
the XRD pattern and N2 adsorption/desorption, was also obtained
following the previous reported method (Figs. 1b and S4).
Although, as anticipated, the reaction also occurred over MIL-68
(Fe), compared with MIL-100(Fe) (conversion, 91%; selectivity,
98%) (Table 1, entry 8), MIL-68(Fe) showed
a lower o-
aminothiophenol conversion of 66% and a selectivity of 97% to 2-
phenzylbenzothiazole (Table 1, entry 12). Similarly to MIL-100
(Fe), MIL-68(Fe) also showed high stability, as evidenced from
the unchanged XRD patterns of the used MIL-68(Fe) (Fig. S5).
To provide some evidence supporting the photocatalytic activ-
ity of Fe-based MOFs and explain why MIL-100(Fe) shows superior
performance to MIL-68(Fe), TAS studies were carried out over MIL-
100(Fe) and MIL-68(Fe) dispersions in CH3CN under an inert atmo-
sphere. To exclude the influence of their different light absorption
coefficients, the transient spectra of MIL-100(Fe) and MIL-68(Fe)
were acquired using the same laser power (25 mJ, 1 Hz) on both
MOFs dispersions with optically matched dispersions at 532 nm,
the wavelength of the laser excitation. As shown in Fig. 5a,
although both the transient spectra of MIL-100(Fe) and MIL-68
(Fe) show negative signals ranging from 450 to 700 nm centered
at approximately 550 nm, the intensity of the transient signal
observed over the dispersion of MIL-100(Fe) is higher than that
observed over MIL-68(Fe). These signals are related to the bleach-
ing of the ground state absorption and they are in good agreement
with their UV–vis spectra (Fig. 2). Since the laser power over the
dispersions of both MOFs and their light absorption is the same
within experimental error, the higher intensity of the transient sig-
nal observed over the dispersion of MIL-100(Fe) indicates a higher
concentration of excited species reaching the microsecond time
Fig. 4. ESR spectra for DMPO-OÅ2ꢀ adduct in the presence of MIL-100(Fe), benzyl
alcohol, and o-aminothiophenol in the dark and upon irradiation (k ꢄ 420 nm).

D. Wang et al. / Journal of Catalysis 349 (2017) 156–162
161
Fig. 5. (a) Transient spectra of optically matched MIL-100(Fe) (black squares) and MIL-68(Fe) (red dots) dispersions acquired 3 ms upon 532 nm excitation using same laser
power (25 mJ, 7 ns fwhp); (b) Transient kinetics of MIL-100(Fe) (black triangles) and MIL-68(Fe) (red dots) at 560 nm upon 532 nm excitation at 25 mJ. The green and blue
lines correspond to raw data fitting to a single exponential function of MIL-100(Fe) and MIL-68(Fe), respectively.
scale in MIL-100(Fe) than in MIL-68(Fe) upon 532 nm laser excita-
tion. The transient kinetics of MIL-100(Fe) and MIL-68(Fe) disper-
sions acquired at 560 nm upon 532 nm laser excitation can be fit to
a single exponential function (Fig. 5b):
4. Conclusions
MIL-100(Fe) and MIL-68(Fe), two Fe-based MOFs, have been
found to be active for tandem oxidative coupling of o-
aminothiophenols and alcohols to produce 2-substituted benzoth-
iazoles under visible light irradiation using O2 as an oxidant. Due to
the photogeneration of higher concentrations of long-lived tran-
sient species with positive charges over MIL-100(Fe) than over
MIL-68(Fe), MIL-100(Fe) showed superior catalytic performance.
The process has been scaled up to gram amounts. This study not
only provides an economical, sustainable, and thus green process
for the production of 2-substituted benzothiazoles, but also illus-
trates the potential for using TAS to explain the activity data and
understand the photophysics of MOFs, which are believed to show
great potential as multifunctional catalysts for light-induced
organic transformations.
y ¼ yo þ C expðt=
s
Þ;
ð1Þ
where y and yo are the transient signals at time t and 0, respec-
tively, C is a constant, and is the half-life constant. The lifetime
s
(s
) of the excited state generated over MIL-100(Fe) was estimated
to be 1.48 ms, while that for MIL-68(Fe) was only 0.63 ms. These
results indicate that a higher concentration of transient species
reaching microseconds with a longer lifetime is generated over
MIL-100(Fe) as compared with MIL-68(Fe) upon laser excitation
at 532 nm.
To elucidate the nature of the photogenerated transient signals,
quenching experiments were carried out by monitoring the recov-
ery of the ground state for these two MOF dispersions at 560 nm by
adding either electron or hole quenchers (Fig. S6). The Stern–Vol-
mer plots obtained from the quenching results revealed that for
both MOFs, electron donors are better quenchers than electron
acceptors. This suggests that the recovery of the ground state cor-
responding to the negative signal from 450 to 700 nm observed for
both MIL-100(Fe) and MIL-68(Fe) is controlled mainly by the kinet-
ics of the holes. The decay of the holes is believed to be the rate-
determining step in the recovery of the ground state.
The photophysical data provide some rationalization for why
MIL-100(Fe) is more active than MIL-68(Fe) in the light-induced
formation of 2-substituted benzothiazoles. Scheme 2 shows that
alcohols act as electron donors, and by donating electrons to
MIL-100(Fe), alcohols become oxidized through the corresponding
benzylic intermediates, which is an important step in the whole
catalytic cycle. TAS data show that higher concentrations of tran-
sient species with positive charge reaching the relevant microsec-
ond are generated over MIL-100(Fe) than over MIL-68(Fe) upon
laser excitation at 532 nm, indicating that MIL-100(Fe) should be
more active in this photo-oxidation reaction. Actually, our previous
experimental results indicated that MIL-100(Fe) is more active
than MIL-68(Fe) in the photocatalytic oxidation of benzyl alcohol
to form benzaldehyde [46–50]. The photophysical data also imply
that the overall formation of 2-substituted benzothiazoles by the
reaction of o-aminothiophenols and alcohols should be limited
by the ability of alcohols to act as electron donors to quench the
positive holes. This is consistent with our experimental results,
which show the existence of aldehyde as the by-product and the
almost complete transformation of o-aminothiophenol with alde-
hyde to 2-substituted benzothiazole.
Acknowledgments
This work was supported by the 973 Program (2014CB239303),
the NSFC (21273035), the National Key Technologies R&D Program
of China (2014BAC13B03), and an Independent Research Project of
the State Key Laboratory of Photocatalysis on Energy and Environ-
ment (2014A03). Financial support by the Spanish Ministry of
Economy and Competitiveness (Severo Ochoa and CTQ2015-
69153-CO2-1-R) is also gratefully acknowledged. Z. Li thanks the
Award Program for Minjiang Scholar Professorship for financial
support.
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.jcat.2017.01.014.
References
[1] C. Shen, P.F. Zhang, Q. Sun, S.Q. Bai, T.S. Andy Hor, X.G. Liu, Chem. Soc. Rev. 44
(2015) 291.
[2] S. Noël, S. Cadet, E. Gras, C. Hureau, Chem. Soc. Rev. 42 (2013) 7747.
[3] P. Majumdar, A. Pati, M. Patra, R.K. Behera, A.K. Behera, Chem. Rev. 114 (2014)
2942.
[4] I.P. Beletskaya, V.P. Ananikov, Chem. Rev. 111 (2011) 1596.
[5] C. Liu, H. Zhang, W. Shi, A.W. Lei, Chem. Rev. 111 (2011) 1780.
[6] S.H. Cho, J.Y. Kim, J. Kwak, S. Chang, Chem. Soc. Rev. 40 (2011) 5068.
[7] S.G. Modha, V.P. Mehta, E.V. Van der Eycken, Chem. Soc. Rev. 42 (2013) 5042.
[8] T.D. Le, K.D. Nguyen, V.T. Nguyen, T. Truong, N.T.S. Phan, J. Catal. 333 (2016) 94.
[9] A.-C. Gaumont, M. Gulea, J. Levillain, Chem. Rev. 109 (2009) 1371.
[10] Q. Yang, Q.F. Wang, Z.K. Yu, Chem. Soc. Rev. 44 (2015) 2305.
[11] Z.Z. Shi, C. Zhang, C.H. Tang, N. Jiao, Chem. Soc. Rev. 41 (2012) 3381.

162
D. Wang et al. / Journal of Catalysis 349 (2017) 156–162
[12] A.J. Blacker, M.M. Farah, M.I. Hall, S.P. Marsden, O. Saidi, J.M.J. Williams, Org.
Lett. 11 (2009) 2039.
[13] C. Chaudhari, S.M.A.H. Siddiki, K.-I. Shimizu, Tetrahedron Lett. 56 (2015) 4885.
[14] X.J. Lang, X.D. Chen, J.C. Zhao, Chem. Soc. Rev. 43 (2014) 473.
[15] H. Tong, S.X. Ouyang, Y.P. Bi, N. Umezawa, M. Oshikiri, J.H. Ye, Adv. Mater. 24
(2012) 229.
[55] D.R. Sun, W.J. Liu, M. Qiu, Y.F. Zhang, Z.H. Li, Chem. Commun. 51 (2015) 2056.
[56] T.H. Zhou, Y.H. Du, A. Borgna, J.D. Hong, Y.B. Wang, J.Y. Han, W. Zhang, R. Xu,
Energy Environ. Sci. 6 (2013) 3229.
[57] M.A. Nasalevich, R. Becker, E.V. Ramos-Fernandez, S. Castellanos, S.L. Veber, M.
V. Fedin, F. Kapteijn, J.N.H. Reek, J.I. van der Vlugt, J. Gascon, Energy Environ.
Sci. 8 (2015) 364.
[16] J. Schneider, M. Matsuoka, M. Takeuchi, J.L. Zhang, Y. Horiuchi, M. Anpo, D.W.
Bahnemann, Chem. Rev. 114 (2014) 9919.
[17] D. Ravelli, M. Fagnoni, A. Albini, Chem. Soc. Rev. 42 (2013) 97.
[18] N. Corrigan, S. Shanmugam, J.T. Xu, C. Boyer, Chem. Soc. Rev. 45 (2016) 6165.
[19] S. Das, S. Samanta, S.K. Maji, P.K. Samanta, A.K. Dutta, D.N. Srivastava, B.
Adhikary, P. Biswas, Tetrahedron Lett. 54 (2013) 1090.
[20] A.R. Wade, H.R. Pawar, M.V. Biware, R.C. Chikate, Green Chem. 17 (2015) 3879.
[21] Y.R. Zhou, Z.Y. Zhou, Y.J. Li, W.J. Yang, Catal. Commun. 64 (2015) 96.
[22] S. Samanta, S. Das, P. Biswas, J. Org. Chem. 78 (2013) 11184.
[23] C. Robert, C.M. Thomas, Chem. Soc. Rev. 42 (2013) 9392.
[24] J.H. Kim, Y.O. Ko, J. Bouffard, S. Lee, Chem. Soc. Rev. 44 (2015) 2489.
[25] J.C. Wasilke, S.J. Obrey, R.T. Baker, G.C. Bazan, Chem. Rev. 105 (2005) 1001.
[26] M.J. Climent, A. Corma, S. Iborra, Chem. Rev. 111 (2011) 1072.
[27] H. Miyamura, S. Kobayashi, Acc. Chem. Res. 47 (2014) 1054.
[28] Y.J. Cui, Y.F. Yue, G.D. Qian, B.L. Chen, Chem. Rev. 112 (2012) 1126.
[29] T. Zhang, W.B. Lin, Chem. Soc. Rev. 43 (2014) 5982.
[30] W.-Y. Gao, M. Chrzanowski, S.Q. Ma, Chem. Soc. Rev. 43 (2014) 5841.
[31] W.M. Xuan, C.F. Zhu, Y. Liu, Y. Cui, Chem. Soc. Rev. 41 (2012) 1677.
[32] A. Dhakshinamoorthy, A.M. Asiri, H. Garcia, Angew. Chem. Int. Ed. 55 (2016)
5414.
[58] D. Kim, D.R. Whang, S.Y. Park, J. Am. Chem. Soc. 138 (2016) 8698.
[59] J.-D. Xiao, Q.C. Shang, Y.J. Xiong, Q. Zhang, Y. Luo, S.-H. Yu, H.-L. Jiang, Angew.
Chem. Int. Ed. 55 (2016) 9389.
[60] K.G.M. Laurier, F. Vermoortele, R. Ameloot, D.E. De Vos, J. Hofkens, M.B.J.
Roeffaers, J. Am. Chem. Soc. 135 (2013) 14488.
[61] C.C. Wang, J.-R. Li, X.-L. Lv, Y.Q. Zhang, G.S. Guo, Energy Environ. Sci. 7 (2014)
2831.
[62] J.W. Liu, L.F. Chen, H. Cui, J.Y. Zhang, L. Zhang, C.-Y. Su, Chem. Soc. Rev. 43
(2014) 6011.
[63] H.-C. Zhou, S. Kitagawa, Chem. Soc. Rev. 43 (2014) 5415.
[64] A.H. Chughtai, N. Ahmad, H.A. Younus, A. Laypkov, F. Verpoort, Chem. Soc. Rev.
44 (2015) 6804.
[65] Q.-L. Zhu, Q. Xu, Chem. Soc. Rev. 43 (2014) 5468.
[66] Y.B. He, B. Li, M. O’Keeffe, B.L. Chen, Chem. Soc. Rev. 43 (2014) 5618.
[67] P. Silva, S.M.F. Vilela, J.P.C. Tomé, F.A. Almeida Paz, Chem. Soc. Rev. 44 (2015)
6774.
[68] Y. Kuwahara, J. Aoyama, K. Miyakubo, T. Eguchi, T. Kamegawa, K. Mori, H.
Yamashita, J. Catal. 285 (2012) 223.
[69] Z.J. Lin, J. Lu, M.C. Hong, R. Cao, Chem. Soc. Rev. 43 (2014) 5867.
[70] A. Schneemann, V. Bon, I. Schwedler, I. Senkovska, S. Kaskel, R.A. Fischer, Chem.
Soc. Rev. 43 (2014) 6062.
[33] S.B. Wang, X.C. Wang, Small 11 (2015) 3097.
[34] V. Stavila, A.A. Talin, M.D. Allendorf, Chem. Soc. Rev. 43 (2014) 5994.
[35] M.A. Nasalevich, M. van der Veen, F. Kapteijn, J. Gascon, CrystEngComm 16
(2014) 4919.
[36] H.B. Zhang, G.G. Liu, L. Shi, H.M. Liu, T. Wang, J.H. Ye, Nano Energy 22 (2016)
149.
[37] F.X. Llabrés, I Xamena, A. Corma, H. Garcia, J. Phys. Chem. C 111 (2007) 80.
[38] T. Tachikawa, J.R. Choi, M. Fujitsuka, T. Majima, J. Phys. Chem. C 112 (2008)
14090.
[71] P. Deria, J.E. Mondloch, O. Karagiaridi, W. Bury, J.T. Hupp, O.K. Farha, Chem.
Soc. Rev. 43 (2014) 5896.
[72] D.R. Sun, Z.H. Li, J. Phys. Chem. C 120 (2016) 19744.
[73] B.Y. Li, K.Y. Leng, Y.M. Zhang, J.J. Dynes, J. Wang, Y.F. Hu, D.X. Ma, Z. Shi, L.K.
Zhu, D.L. Zhang, Y.Y. Sun, M. Chrzanowski, S.Q. Ma, J. Am. Chem. Soc. 137
(2015) 4243.
[74] G. Akiyama, R. Matsuda, H. Sato, M. Takata, S. Kitagawa, Adv. Mater. 23 (2011)
3294.
[39] Y.H. Fu, D.R. Sun, Y.J. Chen, R.J. Huang, Z.X. Ding, X.Z. Fu, Z.H. Li, Angew. Chem.
Int. Ed. 51 (2012) 3364.
[75] B.Y. Li, M. Chrzanowski, Y.M. Zhang, S.Q. Ma, Coord. Chem. Rev. 307 (2016)
106.
[40] S.Q. Zhang, L.N. Li, S.G. Zhao, Z.H. Sun, J.H. Luo, Inorg. Chem. 54 (2015) 8375.
[41] D.R. Sun, Y.H. Fu, W.J. Liu, L. Ye, D.K. Wang, L. Yang, X.Z. Fu, Z.H. Li, Chem. Eur. J.
19 (2013) 14279.
[42] M.T. Wang, D.K. Wang, Z.H. Li, Appl. Catal. B: Environ. 183 (2016) 47.
[43] H.-Q. Xu, J.H. Hu, D.K. Wang, Z.H. Li, Q. Zhang, Y. Luo, S.-H. Yu, H.-L. Jiang, J. Am.
Chem. Soc. 137 (2015) 13440.
[76] B.Y. Li, D.X. Ma, Y. Li, Y.M. Zhang, G.H. Li, Z. Shi, S.H. Feng, M.J. Zaworotko, S.Q.
Ma, Chem. Mater. 28 (2016) 4781.
[77] D.K. Wang, Z.H. Li, Catal. Sci. Technol. 5 (2015) 1623.
[78] Y. Horiuchi, T. Toyao, K. Miyahara, L. Zakary, D.D. Van, Y. Kamata, T.-H. Kim, S.
W. Leed, M. Matsuoka, Chem. Commun. 52 (2016) 5190.
[79] L. Chi, Q. Xu, X.Y. Liang, J.D. Wang, X.T. Su, Small 12 (2016) 1351.
[80] D.K. Wang, Y.J. Song, J.Y. Cai, L. Wu, Z.H. Li, New J. Chem. 40 (2016) 9170.
[81] P. Horcajada, S. Surblé, C. Serre, D.-Y. Hong, Y.-K. Seo, J.-S. Chang, J.-M.
Grenèche, I. Margiolaki, G. Férey, Chem. Commun. (2007) 2820.
[82] Y.-K. Seo, J.W. Yoon, J.S. Lee, U.-H. Lee, Y.K. Hwang, C.-H. Jun, P. Horcajada, C.
Serre, J.-S. Chang, Micropor. Mesopor. Mater. 157 (2012) 137.
[83] A. Fateeva, P. Horcajada, T. Devic, C. Serre, J. Marrot, J.-M. Grenèche, M.
Morcrette, J.-M. Tarascon, G. Maurin, G. Férey, Eur. J. Inorg. Chem. (2010) 3789.
[84] J.-W. Zhang, H.-T. Zhang, Z.-Y. Du, X.Q. Wang, S.-H. Yu, H.-L. Jiang, Chem.
Commun. 50 (2014) 1092.
[44] Y.Y. Liu, Y.M. Yang, Q.L. Sun, Z.Y. Wang, B.B. Huang, Y. Dai, X.Y. Qin, X.Y. Zhang,
ACS Appl. Mater. Interfaces 5 (2013) 7654.
[45] D.K. Wang, R.K. Huang, W.J. Liu, D.R. Sun, Z.H. Li, ACS Catal. 4 (2014) 4254.
[46] K. Manna, T. Zhang, F.X. Greene, W.B. Lin, J. Am. Chem. Soc. 137 (2015) 2665.
[47] D.R. Sun, L. Ye, Z.H. Li, Appl. Catal. B: Environ. 164 (2015) 428.
[48] J.L. Long, S.B. Wang, Z.X. Ding, S.C. Wang, Y.G. Zhou, L. Huang, X.X. Wang,
Chem. Commun. 48 (2012) 11656.
[49] D.Y. Shi, C. He, B. Qi, C. Chen, J.Y. Niu, C.Y. Duan, Chem. Sci. 6 (2015) 1035.
[50] W.Z. Zhong, T. Qiao, J. Dai, L.Q. Mao, Q. Xu, G.Q. Zou, X.X. Liu, D.L. Yin, F.P. Zhao,
J. Catal. 330 (2015) 208.
[85] C. Yu, K. Lee, Y. You, E.J. Cho, Adv. Synth. Catal. 355 (2013) 1471.
[86] W.W. He, H.M. Jia, W.G. Wamer, Z. Zheng, P.J. Li, J.H. Callahan, J.-J. Yin, J. Catal.
320 (2014) 97.
[51] D.K. Wang, M.T. Wang, Z.H. Li, ACS Catal. 5 (2015) 6852.
[52] D.K. Wang, Z.H. Li, J. Catal. 342 (2016) 151.
[53] C.G. Silva, I. Luz, F.X. Llabrés i Xamena, A. Corma, H. García, Chem. Eur. J. 16
(2010) 11133.
[54] Y. Horiuchi, T. Toyao, M. Saito, K. Mochizuki, M. Iwata, H. Higashimura, M.
Anpo, M. Matsuoka, J. Phys. Chem. C 116 (2012) 20848.
[87] Z.J. Wang, S. Ghasimi, K. Landfester, K.A.I. Zhang, Adv. Mater. 27 (2015) 6265.
[88] M. Zhang, Q. Wang, C.C. Chen, L. Zang, W.H. Ma, J.C. Zhao, Angew. Chem. Int.
Ed. 48 (2009) 6081.
[89] A.N. Pham, G.W. Xing, C.J. Miller, T.D. Waite, J. Catal. 301 (2013) 54.