Visible light initiated oxidative coupling of alcohols ando-phenylenediamines to synthesize benzimidazoles over MIL-101(Fe) promoted by plasmonic Au

Article abstract of DOI:10.1039/d1gc01047f

The use of visible light to initiate one-pot synergistic/cascade reactions is a green and energy saving strategy. In this manuscript, we report that MIL-101(Fe) can act as a multifunctional catalyst to realize the oxidative condensation betweeno-phenylenediamines and alcohols to synthesize benzimidazoles under visible light. The deposition of plasmonic Au nanoparticles (Au NPs) on MIL-101(Fe) led to significantly improved activity. Both controlled experiments and electron spin resonance (ESR) results revealed that the production of benzimidazoles fromo-phenylenediamines and alcohols involves three sequential steps,i.e., the oxidative dehydrogenation of alcohols to produce aldehydes, the condensation betweeno-phenylenediamines and the aldehydes to produce Schiff bases and their oxidation to form benzimidazoles,viaa superoxide radical (O₂˙^?)-mediated pathway. The promoting effect of plasmonic Au NPs in this reaction can be ascribed to the effective transfer of the surface plasmon resonance (SPR)-excited hot electrons to the lowest unoccupied molecular orbital (LUMO) of MIL-101(Fe), which led to the generation of more active O₂˙^?radicals. This study not only provides a green and sustainable way for the synthesis of benzimidazoles, but also highlights the great potential of using rationally designed plasmonic metal NP/MOF nanocomposites as multifunctional catalysts for light initiated one-pot tandem/cascade reactions.

Full text of DOI:10.1039/d1gc01047f

Green Chemistry
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Visible light initiated oxidative coupling of
alcohols and o-phenylenediamines to synthesize
benzimidazoles over MIL-101(Fe) promoted by
plasmonic Au†
Cite this: Green Chem., 2021, 23,
4161
Yuhuan Qin, Mingming Hao, Chao Xu * and Zhaohui Li
*
The use of visible light to initiate one-pot synergistic/cascade reactions is a green and energy saving strat-
egy. In this manuscript, we report that MIL-101(Fe) can act as a multifunctional catalyst to realize the oxi-
dative condensation between o-phenylenediamines and alcohols to synthesize benzimidazoles under
visible light. The deposition of plasmonic Au nanoparticles (Au NPs) on MIL-101(Fe) led to significantly
improved activity. Both controlled experiments and electron spin resonance (ESR) results revealed that the
production of benzimidazoles from o-phenylenediamines and alcohols involves three sequential steps,
i.e., the oxidative dehydrogenation of alcohols to produce aldehydes, the condensation between o-phe-
nylenediamines and the aldehydes to produce Schiff bases and their oxidation to form benzimidazoles,
via a superoxide radical (O2•−)-mediated pathway. The promoting effect of plasmonic Au NPs in this reac-
tion can be ascribed to the effective transfer of the surface plasmon resonance (SPR)-excited hot elec-
trons to the lowest unoccupied molecular orbital (LUMO) of MIL-101(Fe), which led to the generation of
Received 26th March 2021,
Accepted 4th May 2021
•−
more active O2 radicals. This study not only provides a green and sustainable way for the synthesis of
DOI: 10.1039/d1gc01047f
benzimidazoles, but also highlights the great potential of using rationally designed plasmonic metal NP/
MOF nanocomposites as multifunctional catalysts for light initiated one-pot tandem/cascade reactions.
rsc.li/greenchem
catalysts in the field of light initiated organic transformations is
extremely appealing since the ordered and tunable pores in
Introduction
Photocatalysis, as a unique type of heterogeneous catalysis, pro- MOFs are beneficial for mass diffusion and substrate–catalyst
vides a promising and sustainable approach to transform solar interactions in MOF-catalyzed chemical reactions.26,27 In
energy into chemical energy.1–10 In particular, photocatalytic addition, different catalytic sites in MOFs, including catalytic
organic transformations, which use light generated photoche- active metal nodes, ligands and pore-encapsulated catalytic
mical potential to produce value-added organics, have attracted species, can also be coupled with MOF-based photocatalysis to
increasing attention recently due to the importance of develop- enable the MOFs to behave as multifunctional catalysts for light
ing renewable energy based processes and reducing environ- initiated one-pot tandem/cascade reactions.28–30
mental pollution associated with the chemical industries.11–16
Surface plasmon resonance (SPR), an optical phenomenon of
Metal–organic frameworks (MOFs), a kind of three-dimensional plasmonic metal nanoparticles (NPs) caused by the collective
crystalline micro-/mesoporous hybrid material constructed from oscillation of conduction band electrons in resonance with the
metal ions or metal clusters interconnected with multi-dentate incoming light field,31–33 endows the plasmonic metal NPs with
organic linkers, are emerging as a new class of promising intensive absorption in the visible light region. Plasmonic metal
photocatalysts due to their tailorable structure that enables the NPs, in particular Au NPs, have been extensively studied in
facile tuning of their light absorption in the visible light photocatalysis due to the light driven generation of hot
region.17–25 In particular, the application of MOF-based photo- electrons,34,35 effective photothermal conversion and highly
focused electromagnetic field.36,37 In particular, the LSPR gener-
ated hot electrons can provide a new pathway to promote chemi-
Research Institute of Photocatalysis, State Key Laboratory of Photocatalysis on
cal reactions by conversion of light and thus have attracted extra-
ordinary research interest recently.38 However, the direct utiliz-
Energy and Environment, College of Chemistry, Fuzhou University, Fuzhou 350116,
P. R. China. E-mail: zhaohuili1969@yahoo.com, cxu@fzu.edu.cn;
ation of the hot electrons generated in the plasmonic metal NPs
Tel: +86-591-22865855; Fax: +86-591-22865855
for chemical reactions shows low efficiency, since the self-ther-
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
malization of these hot electrons leads to their eventual cooling
d1gc01047f
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down by ultrafast electron–phonon scattering.39 An established
strategy to improve the efficiency of the hot electrons in chemi-
cal reactions by alleviating the undesirable energy dissipation is
to fabricate nanocomposites by integration of the plasmonic Au
NPs with semiconductors or by hybridizing them with other
metal NPs.40–43 The obtained plasmonic Au containing nano-
composites have been found to be active for a variety of photo-
catalytic reactions.44,45 Besides acting as photocatalysts, MOFs
are also ideal supports for catalytic active species due to their
high surface areas and the presence of different anchoring
sites.46 Previous studies have revealed that the deposition of
plasmonic Au NPs on Pt@MIL-125(Ti) and NH2-UiO-66(Zr) led
to Pt@MIL-125(Ti)/Au and Au@NH2-UiO-66(Zr),47,48 which show
superior activity for photocatalytic hydrogen evolution and the
oxidation of benzyl alcohol by efficient injection of the hot elec-
trons of plasmonic Au into the LUMO of the MOFs. Considering
that MOFs contain different catalytic active sites in their struc-
ture, it would be anticipated that the integration of MOFs with
plasmonic Au NPs would provide an opportunity for a variety of
valuable catalytic reactions to be realized under visible light.
As an important class of heterocycles, benzimidazoles have
been widely recognized as building blocks in the construction
of biologically active products and pharmaceutical com-
pounds.49 Traditionally, benzimidazoles are obtained by coup-
ling o-phenylenediamines with carboxylic acids or their deriva-
tives, but strongly acidic conditions and temperature higher
than 200 °C are usually required.50 Alternatively, benzimida-
zoles can also be obtained by the condensation of o-phenylene-
diamines with aldehydes, using stoichiometric amounts of
strong oxidants such as DDQ (2,3-dichloro-5,6-dicyano-
1,4-benzoquinone).51 The use of molecular oxygen to realize
this reaction was also reported, but homogeneous catalysts like
metal triflates and a free radical such as 4-methoxy-TEMPO
(4-methoxy-2,2,6,6-tetramethyl-1-piperidinyloxy) were required.52
Benzimidazoles were also reported to be obtained by conden-
sation between o-phenylenediamines and primary amines,
using bioinspired organocatalysts such as pyrogallol, polydopa-
mine and ortho-quinone.67–69 Recently, with the aim of develop-
ing a green process for chemical reactions, the use of light to
initiate the synthesis of benzimidazoles has also been demon-
strated. For example, the reaction between o-phenylenediamines
and aldehydes to synthesize 2-substituted benzimidazoles was
Scheme 1 Illustration of the oxidative condensation of alcohols and
o-phenylenediamines to produce 2-substituted benzimidazoles.
Due to the presence of extensive iron–oxo (Fe–O) clusters,
Fe-based MOFs are highly responsive in the visible light region
and are attractive photocatalytic materials.58–60 In particular,
MIL-101(Fe) (Fe3O(H2O)2Cl(BDC)3, BDC = 1,4-dicarboxyben-
zene), a rigid zeotype structure consisting of quasi-spherical
cages of two modes (2.9 and 3.4 nm) accessible through
windows of ca. 1.2 and 1.6 nm, respectively, has already been
applied in a variety of photocatalytic reactions under visible
light.61,62 In this manuscript, we report that MIL-101(Fe) can
act as a multifunctional catalyst for one-pot oxidative conden-
sation between o-phenylenediamines and alcohols to syn-
thesize benzimidazoles under visible light. In addition, Au/
MIL-101(Fe) nanocomposites, with plasmonic Au NPs of ca.
13 nm deposited on the surface of MIL-101(Fe), exhibited
enhanced activity for visible light induced benzimidazole pro-
duction, due to the effective transfer of the SPR excited hot
electrons in plasmonic Au NPs to the LUMO of MIL-101(Fe),
•−
which led to the generation of more active O2 radicals. This
work not only provides a green and sustainable strategy for the
synthesis of benzimidazoles, but also highlights the great
potential of using rationally designed plasmonic metal NP/
MOF nanocomposites as multifunctional catalysts for light
initiated one-pot tandem/cascade reactions.
Experimental
Preparations
observed over CdSe and fluorescein under visible light.53,54 MIL-101(Fe) was obtained following a previously reported
Considering that alcohols are more stable than aldehydes and method.61 A mixture of FeCl3·6H2O (0.675 g, 2.45 mmol),
are readily available, a direct reaction between alcohols and H2BDC (0.206 g, 1.24 mmol) and 15 mL DMF was heated at
o-phenylenediamines to produce benzimidazoles is more desir- 110 °C for 24 h. After the reaction, the resultant suspension
able, especially when such reactions can be triggered by visible was filtered, washed with DMF and MeOH and vacuum-dried
light using environmentally friendly molecular oxygen (O2) as to obtain a yellow powder. MIL-101(Fe) was activated under
the oxidant.55,56 Generally, the reaction between alcohols and vacuum at 130 °C for 12 h.
o-phenylenediamines to produce benzimidazoles is believed to
Au colloids were synthesized via the Frens method.63
involve three sequential steps, i.e., the oxidative dehydrogena- Trisodium citrate solution (1 wt%, 1.5 mL) was injected into a
tion of alcohols to produce aldehydes, the condensation boiling solution of HAuCl4 (0.01 wt%, 50 mL) and the resultant
between o-phenylenediamines and the aldehydes to produce solution was heated at 120 °C for 20 minutes to obtain a solu-
Schiff bases and their oxidation to form benzimidazoles, as tion of Au colloids. A solution of PVP (0.1 g, Mw = 55 000) in
shown in Scheme 1,57 and therefore, a multifunctional catalyst water (5 mL) was added dropwise to the above Au colloids, and
is required to accomplish such a one-pot tandem reaction.
the resultant colloids were stirred at room temperature for
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12 hours. The resultant PVP-stabilized Au colloids were
obtained by centrifugation at 16 000 rpm for 25 min and were
Results and discussion
redispersed in MeOH, with their concentration determined to MIL-101(Fe) was synthesized following a previously reported
be ca. 1 mg mL−1 procedure.61 Both the XRD patterns and BET results indicated
.
To obtain the 1.0 wt% Au/MIL-101(Fe) nanocomposite, a that MIL-101(Fe) with high quality has been obtained (ESI
solution of Au colloids in MeOH (0.5 mL) was added slowly Fig. S3 and S4†). Its TEM image shows that the as-obtained
into a suspension of activated MIL-101(Fe) (50 mg) in MeOH MIL-101(Fe) is octahedral, with a dimension of about 700 nm
(10 mL). After stirring for 2 h, the product was isolated by fil- (ESI Fig. S5†). Au colloids with a dimension of ca. 13 nm were
tration, washed with MeOH several times, and dried overnight prepared by a citrate-assisted reduction of HAuCl4 using polyvi-
in a vacuum oven. 0.5 wt% Au/MIL-101(Fe) and 1.5 wt% Au/ nylpyrrolidone (PVP) as the capping agent (ESI Fig. S6†). Its
MIL-101(Fe) were obtained similarly, except that the amount of UV-vis absorption spectrum shows an SPR band at ∼520 nm,
Au colloids used was 0.25 mL and 0.75 mL, respectively.
in accordance with the data reported previously (ESI
Fig. S7†).63 Since the zeta potential (ξ) of MIL-101(Fe) in MeOH
is positively charged while that of the as-obtained Au colloids
Characterization
Zeta potential (ζ) measurements were carried out by dynamic is negatively charged due to the presence of carboxylate groups
light scattering analysis on a Zeta sizer 3000HSA at 298 K. (ESI Fig. S8†), the electrostatic self-assembly of Au/MIL-101(Fe)
Powder X-ray diffraction (XRD) patterns were characterized on a occurred when Au colloids were added to a suspension of
Rigaku SmartLab rotating anode X-ray diffractometer with MIL-101(Fe) in MeOH. The successful assembly of Au/MIL-101
graphite-monochromated Cu Kα radiation. XRD patterns were (Fe) via electrostatic attraction between negatively charged Au
scanned over the angular range of 3–50° (2θ) with a step size of colloids and positively charged MIL-101(Fe) was evidenced
0.02°. N2 adsorption/desorption experiments were conducted from the color change of the solid from yellow to reddish
with an ASAP 2020 M instrument (Micromeritics Instrument orange, together with a fading of the color of the supernatant.
Corp.). After the sample was degassed under vacuum at 130 °C The procedure for the fabrication of Au/MIL-101(Fe) is shown
for 10 h, the nitrogen adsorption and desorption isotherms in Scheme 2.
were measured at 77 K. Transmission electron microscopy
The XRD pattern of the as-obtained 1.0 wt% Au/MIL-101
(TEM) images were obtained on an FEI Tecnai G2 F20S-TWIN (Fe) shows all diffraction peaks of MIL-101(Fe) (Fig. 1a). As
instrument. The UV-vis diffuse reflectance spectrum (UV-vis compared with bare MIL-101(Fe), an additional diffraction
DRS) was obtained with a UV-vis spectrophotometer (Varian peak at 38.2° was also observed, which can be assigned to the
Cary 500). Barium sulfate (BaSO4) was used as a reference. The Au (111) crystallographic plane. This suggests that Au colloids
amount of Au was determined on an inductively coupled were successfully incorporated and the framework of MIL-101
plasma emission spectrometer (ICP, PerkinElmer Optima 8000). (Fe) was retained during the self-assembly procedure. The
ESR spectra were obtained on a Bruker A300 ESR spectrometer. TEM image of 1.0 wt% Au/MIL-101(Fe) shows an unchanged
Photoelectrochemical analysis was performed on an electro- octahedral structure of MIL-101(Fe), with Au nanoparticles of
chemical analyzer (Zahner, Germany) in a standard three-elec- ca. 13 nm evenly decorated on the surface (Fig. 1b). Due to the
trode system using the as-obtained sample-coated ITO as the presence of Fe–O clusters in MIL-101(Fe), the UV-vis DRS of
working electrode, a Pt wire as the counter electrode, and Ag/ 1.0 wt% Au/MIL-101(Fe) shows a broad and strong absorption
AgCl (in saturated KCl) as the reference electrode. The transient band in the visible light region, with its edge extending to
photocurrent responses were measured in 0.2 M Na2SO4 600 nm. As compared with bare MIL-101(Fe), an additional
aqueous solution. Electrochemical impedance spectroscopy shoulder peak at 521 nm was observed in 1.0 wt% Au/MIL-101
(EIS) was performed in a mixture of KCl (0.5 M), K3[Fe(CN)6] (Fe), which can be attributed to the SPR excitation of Au col-
(0.01 M) and K4[Fe(CN)6] (0.01 M) aqueous solution.
loids (Fig. 2). The amount of Au in 1.0 wt% Au/MIL-101(Fe)
was determined to be 0.98 wt% by ICP-OES, which is consist-
ent with the amount of Au introduced. The specific surface
Catalytic reactions
The photocatalytic reaction was carried out in a photocatalytic area of 1.0 wt% Au/MIL-101(Fe) (3590 m2 g−1) is only slightly
parallel reaction device (Beijing PerfectLight, PCX50B, 5 W);
the irradiation spectrum of the LED lamp is shown in ESI
Fig. S1.† Typically, o-phenylenediamine (10.8 mg, 0.1 mmol)
in ethanol (2 mL) saturated with O2 was transferred to a
Schlenk tube containing 10 mg of photocatalyst. The reaction
tube was irradiated with an LED lamp. After the reaction, the
suspension was filtered through a porous membrane and the
filtrate was analyzed using a gas chromatograph (Shimadzu
GC-2014) equipped with an HP-5 capillary column. The peaks
of the products in GC were compared to those of the standard
materials. Both the GC graphs of the standard materials and
Scheme 2 Procedures for the fabrication of Au/MIL-101(Fe)
the products are shown in the ESI (Fig. S2†).
nanocomposites.
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Fig. 1 (a) Powder XRD patterns of the as-synthesized MIL-101(Fe) and the 1.0 wt% Au/MIL-101(Fe) nanocomposite; (b) TEM image of the 1.0 wt%
Au/MIL-101(Fe) nanocomposite.
after irradiation for 12 h, with a yield of 45.8% of the desired
product 2-methylbenzimidazole (Table 1, entry 1). In contrast,
the product was detected in a negligible amount when the
reaction was carried out in the absence of either MIL-101(Fe)
or visible light (Table 1, entries 2 and 3). The deposition of
plasmonic Au NPs on MIL-101(Fe) led to improved perform-
ance, even though bare Au colloids show negligible activity for
this reaction, with only 1.3% of o-phenylenediamine converted
(Table 1, entries 4–8). Among the three Au/MIL-101(Fe) nano-
composites investigated, 1.0 wt% Au/MIL-101(Fe) shows the
optimum activity, with 96.5% of o-phenylenediamine con-
verted and a yield of 94.7% of 2-methylbenzimidazole in 12 h
Table 1 Production of 2-methylbenzimidazole from o-phenylene-
diamine and ethanol over Au/MIL-101(Fe) under various conditionsa
Fig. 2 UV-vis DRS spectra of MIL-101(Fe) and the 1.0 wt% Au/MIL-101
(Fe) nanocomposite.
lower than that of bare MIL-101(Fe) (3750 m2 g−1), indicating
Entry Catalysts
Conv. of 1 [%] Yield [%]
that the permanent porosity of MIL-101(Fe) was retained (ESI
Fig. S4†).
1
MIL-101(Fe)
—
MIL-101(Fe)
Au colloids
46.9
—
45.8
—
Due to the intensive absorption of Fe-based MOFs in the
visible light region and the presence of redox active Fe–O clus-
ters, Fe-based MOFs including MIL-101(Fe) have been applied
to a variety of photocatalytic oxidative reactions by activation
of O2 under visible light.64 In addition, the coupling of the
Lewis acid sites in these Fe-based MOFs with photocatalysis
has also enabled a few visible light induced one-pot tandem/
cascade reactions to be realized over these Fe-based MOFs.65
Considering that the reaction between o-phenylenediamines
and alcohols to produce benzimidazoles involves the oxidative
condensation between alcohols and o-phenylenediamines, it is
proposed that Fe-based MOFs may also be active for this reac-
tion under visible light. Therefore, the reaction between
o-phenylenediamine and ethanol under visible light was inves-
tigated. As per our expectation, the use of MIL-101(Fe) as the
catalyst gave a conversion of o-phenylenediamine of 46.9%
2
3b
8.4
1.3
7.8
—
4c
5
6
7
0.5 wt% Au/MIL-101(Fe)
1.0 wt% Au/MIL-101(Fe)
1.5 wt% Au/MIL-101(Fe)
MIL-101(Fe)
1.0 wt% Au/MIL-101(Fe)
1.0 wt% Au/MIL-101(Fe)
MIL-101(Fe)
1.0 wt% Au/MIL-101(Fe)
1.0 wt% Au/MIL-101(Fe)
Mixture – 1.0 wt% Au-MIL-101(Fe) 58.7
61.2
96.5
90.1
17.5
63.8
11.9
1.5
59.3
94.7
87.5
16.6
61.9
9.4
8d
9d
10e
11f
12g
13h
14
—
16.6
Trace
12.3
Trace
57.1
a Reaction conditions: photocatalyst (10 mg), o-phenylenediamine
(0.1 mmol), ethanol (2 ml), O2 (1 atm), 5 W LED visible light
irradiation for 12 h. The amount of the products was calculated based
on GC analyses. b Without irradiation. c 100 μL of Au colloid ethanol
solution (∼1 mg ml−1). d 5 W 520 nm LED visible light irradiation for
12 h. e Without irradiation; 33.5 °C. f The reaction was quenched with
benzoquinone. g Air (1 atm). h N2 (1 atm).
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(Table 1, entries 5–7). The heterogeneous nature of this system in plasmonic Au NPs can generate hot electrons and holes, it
was evidenced from the hot filtrate reaction which showed no is therefore proposed that when these Au NPs are deposited on
further conversion of o-phenylenediamine after Au/MIL-101 MIL-101(Fe), these SPR-excited hot electrons can migrate to
(Fe) was removed from the reaction system after 3 h of the less negative LUMO in MIL-101(Fe) (−0.74 V versus Ag/
irradiation (Fig. 3). All these observations indicate that the AgCl, i.e., −0.54 V vs. NHE),61 leading to the enhanced charge
reaction between o-phenylenediamine and ethanol to produce separation in plasmonic Au NPs. A previous study on Au/
2-methylbenzimidazole is truly induced by photocatalysis over MIL-125(Ti) also revealed a similar migration of the SPR
irradiated MIL-101(Fe), and the presence of plasmonic Au NPs excited hot electrons to MIL-125(Ti), with its LUMO located at
promotes this reaction.
−0.63 V vs. NHE, which shows improved photocatalytic activity
Since the SPR band of plasmonic Au NPs locates at 521 nm, for H2 production.47
to study its effect on the performance, reactions were also
To explore the active species involved in this reaction, the
carried out over both 1.0 wt% Au/MIL-101(Fe) and bare DMPO-trapped ESR spectrum of a reaction system containing
MIL-101(Fe) using a green LED lamp with a wavelength of MIL-101(Fe), ethanol, and o-phenylenediamine was studied.
•−
520 nm (Table 1, entries 8 and 9). It was found that 63.8% of Typical signals corresponding to the DMPO-O2
radical
o-phenylenediamine was converted after irradiation for 12 h adduct were detected when the reaction system was irradiated
•−
over 1.0 wt% Au/MIL-101(Fe), while the conversion over with visible light (Fig. 4).66 The involvement of O2 in the
MIL-101(Fe) was only 17.5%. In addition, reactions were also light initiated reaction between o-phenylenediamine and
carried out at 420 nm, at which plasmonic Au is not supposed ethanol to produce 2-methylbenzimidazole was also confirmed
to be excited to generate hot electrons, using a Xe lamp by the quenching experiment. The introduction of benzo-
•−
equipped with a band pass filter at 420 nm. As per our expec- quinone, an O2 radical scavenger, to the reaction system
tation, only slightly different conversion of o-phenylene- almost completely inhibited the conversion of o-phenylene-
diamine was observed over MIL-101(Fe) (59.3%) and Au/ diamine (Table 1, entry 11). To further confirm the role of O2
MIL-101(Fe) (64.7%), respectively. All these results indicated in this reaction, the light initiated oxidative condensation
that the promoting effect exerted by plasmonic Au NPs is between o-phenylenediamine and ethanol to synthesize benzi-
ascribed to their SPR effect. Although plasmonic Au was pre- midazole over Au/MIL-101(Fe) was also investigated under
viously reported for its photothermal effect, the temperature of either a N2 or an air atmosphere (Table 1, entries 12 and 13).
the reaction system only slightly increased from room tempera- 16.6% of o-phenylenediamine was converted in air under
ture (27.8 °C) to 33.5 °C when irradiated. The slight tempera- otherwise similar reaction conditions, much lower than that
ture increase did not influence the catalytic performance observed in O2 (96.5%). In addition, negligible conversion of
obviously, since the conversion of o-phenylenediamine over o-phenylenediamine was observed in N2. All these results
1 wt% Au/MIL-101(Fe) only slightly increased from the original suggest that O2 plays a pivotal role in this reaction.
8.4% at room temperature to 10.3% at 33.5 °C (Table 1, entry
Based on the observations described above, the mechanism
10), suggesting that the photothermal effect does not play an for the reaction between o-phenylenediamine and ethanol to
important role in this reaction. Since the excitation of the SPR produce 2-methylbenzimidazole over visible light irradiated
Fig. 3 Conversion ratio of o-phenylenediamine as
a function of
irradiation time over (a) 1.0 wt% Au/MIL-101(Fe) and (b) the reaction
system after 1.0 wt% Au/MIL-101(Fe) was removed after irradiation for
3 h.
Fig. 4 DMPO spin-trapping ESR spectra of a reaction system containing
o-phenylenediamine, ethanol, DMPO and the photocatalyst. (a) 1.0 wt%
Au/MIL-101(Fe) and (b) MIL-101(Fe).
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leading to higher photocatalytic activity for the production of
2-methylbenzimidazole. As can be observed, the signals corres-
•−
ponding to DMPO-O2 radicals over Au/MIL-101(Fe) increase
significantly with the light intensity, while the degree of
•−
increase of the signals corresponding to DMPO-O2 radicals
over MIL-101(Fe) is not as obvious as that over Au/MIL-101(Fe)
(ESI Fig. S9†). In the meantime, the holes in plasmonic Au
NPs can also attack ethanol to complete the whole catalytic
cycle. It is obvious that the light initiated reaction between
o-phenylenediamine and ethanol to produce 2-methyl-
benzimidazole involves the oxidation of ethanol to aldehyde,
the condensation between aldehyde and o-phenylenediamine
to produce a Schiff base and its oxidation to produce 2-methyl-
benzimidazole in consecutive steps. A controlled reaction
between o-phenylenediamine and acetaldehyde revealed that
o-phenylenediamine was completely converted within only 1 h,
indicating that the oxidation of ethanol to acetaldehyde is the
rate-limiting step in this one-pot tandem reaction between
o-phenylenediamine and ethanol to produce 2-methyl-
benzimidazole over 1 wt% Au/MIL-101(Fe).
Scheme 3 Proposed mechanism for the synthesis of 2-methyl-
benzimidazole from o-phenylenediamine and ethanol over Au/MIL-101(Fe).
MIL-101(Fe) and Au/MIL-101(Fe) was proposed (Scheme 3).
According to the proposed mechanism, the superior photo-
When MIL-101(Fe) is irradiated, the excited electrons in the catalytic activity observed over Au/MIL-101(Fe) can be ascribed
LUMO are transferred to Fe3+ to generate Fe2+, which reduces to the transfer of the SPR-excited hot electrons in plasmonic
O2 to form O2 radicals, while Fe2+ itself is oxidized back to Au NPs to the LUMO of MIL-101(Fe), leading to the generation
•−
Fe3+. The holes in MIL-101(Fe) can attack the ethanol to gene- of more active O2 radicals. A control experiment carried out
•−
•−
rate the intermediate, which further reacts with the O2
using a simple mechanical mixture of Au colloids and MIL-101
radical to form the aldehyde via an oxidative dehydrogenation (Fe) exhibited a much lower performance, with only 58.7% of
process. The presence of Lewis acidic Fe3+ sites in MIL-101(Fe) o-phenylenediamine converted under otherwise similar con-
can catalyze the condensation between o-phenylenediamine ditions (Table 1, entry 14), which provides strong evidence that
and the as-formed aldehyde to produce the Schiff base, which a good interface between plasmonic Au NPs and MIL-101(Fe)
is further oxidized by the active O2•− radicals to form 2-methyl- is important. The deposition of plasmonic Au NPs on the
benzimidazole. Such an oxidative dehydrogenation process of surface of MIL-101(Fe) also increases the separation of the
•−
alcohols mediated by the O2 radical has been well documen- photogenerated electrons and holes in MIL-101(Fe), as evi-
ted over both semiconductor and MOF based photocatalysts in denced from a much higher photocurrent observed over Au/
some oxygenation reactions.55 By the deposition of plasmonic MIL-101(Fe) as compared with bare MIL-101(Fe) (Fig. 5a). In
Au NPs on MIL-101(Fe), in addition to MIL-101(Fe), plasmonic addition, the EIS of Au/MIL-101(Fe) also exhibits a smaller
Au NPs can also be excited due to the presence of the SPR radius, indicative of a lower charge-transfer resistance (Fig. 5b).
band. As discussed above, the SPR-excited hot electrons in
The visible light initiated oxidative condensation reaction
plasmonic Au NPs can be injected into the LUMO of MIL-101 between o-phenylenediamine and ethanol over Au/MIL-101(Fe)
(Fe), which also contribute to the reduction of Fe3+ to produce was also expanded to a wide range of substrate scopes. Both ali-
Fe2+. As a result, more active O2 radicals can be generated, phatic alcohols and benzyl alcohols with different substituents
•−
Fig. 5 (a) The photocurrent responses of MIL-101(Fe) and the 1.0 wt% Au/MIL-101(Fe) nanocomposite; (b) EIS Nyquist plots of MIL-101(Fe) and the
1.0 wt% Au/MIL-101(Fe) nanocomposite.
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Table 2 Investigation of the production of 2-substituted benzimidazoles from different o-phenylenediamines and alcohols over 1.0 wt% Au/
MIL-101(Fe)
Entry
1
Diamine
Alcohol
Product
Yielda [%]
90.8
CH3OH
2
95.1
94.6
87.1
83.7
81.5
3
4
5b,c
6b,c
7b,c
54.1
94.8
89.5
53.8
8
9
10c
11c
49.5
Reaction conditions: diamine (0.1 mmol), alcohol (2 ml), 1.0 wt% Au/MIL-101(Fe) (10 mg), oxygen (1 atm), visible light irradiation for 12 h.
a Yield was calculated based on diamine. b Alcohol (1 mmol) and CH3CN (2 mL). c Visible light irradiation for 24 h.
in the benzyl ring can react with o-phenylenediamine to
produce the corresponding 2-substituted benzimidazoles,
although with varied yields of the product (Table 2, entries 1–7).
Among the four investigated aliphatic alcohols, the length of
the carbon chain does not significantly influence the perform-
ance. However, lower activity was observed for benzyl alcohols
with electron-withdrawing substituents in their phenyl ring. On
the other hand, the reactions between a series of substituted
o-phenylenediamines and ethanol also occurred successfully
over Au/MIL-101(Fe) under visible light, with yields ranging
from 50.8% to 95.3% (Table 2, entries 8–11). Also, o-phenylene-
diamines with electron-withdrawing substituents like -Cl and
-Br in their phenyl ring show lower reactivity in this reaction.
In addition to its superior photocatalytic activity, Au/
Fig. 6 Recycling of 1.0 wt% Au/MIL-101(Fe) for the oxidative conden-
MIL-101(Fe) also shows high stability in the reaction.
sation between o-phenylenediamine and ethanol to produce
Recyclability tests for three runs showed no obvious decrease 2-methylbenzimidazole.
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Green Chemistry
in the photocatalytic activity of Au/MIL-101(Fe) (Fig. 6). In 11 K. Sun, Q.-Y. Lv, X.-L. Chen, L.-B. Qu and B. Yu, Green
addition, the XRD pattern of Au/MIL-101(Fe) after three runs
Chem., 2021, 23, 232–248.
did not change obviously (ESI Fig. S10†).
12 A. Das and B. König, Green Chem., 2018, 20, 4844–4852.
13 S. H. Kyne, G. Lefèvre, C. Ollivier, M. Petit, V.-A. Ramis
Cladera and L. Fensterbank, Chem. Soc. Rev., 2020, 49,
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18 L. Chen, R. Luque and Y. Li, Chem. Soc. Rev., 2017, 46,
4614–4630.
19 Y.-S. Kang, Y. Lu, K. Chen, Y. Zhao, P. Wang and W.-Y. Sun,
Coord. Chem. Rev., 2019, 378, 262–280.
20 A. Dhakshinamoorthy, Z. Li and H. Garcia, Chem. Soc. Rev.,
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Conclusion
In summary, one-pot oxidative condensation between o-pheny-
lenediamines and alcohols to synthesize benzimidazoles was
realized over MIL-101(Fe) under visible light, in which MIL-101
(Fe) acts not only as a photocatalyst to produce reactive O2
but also as a Lewis acid to promote the condensation between
the oxidized aldehydes and o-phenylenediamines. The depo-
sition of plasmonic Au NPs on the surface of MIL-101(Fe) led
to enhanced activity due to the efficient transfer of the hot
electrons from plasmonic Au NPs to the LUMO of MIL-101(Fe),
•−
,
•−
resulting in the generation of more active O2 radicals. This
work provides a green and sustainable strategy for the syn-
thesis of benzimidazoles. This study also paves the way for the
rational design of plasmonic metal NP/MOF nanocomposites
for their use as multifunctional catalysts for light initiated
one-pot tandem/cascade reactions.
23 J.-D. Xiao and H.-L. Jiang, Acc. Chem. Res., 2019, 52, 356–
366.
24 Q. Wang, Q. Gao, A. M. Al-Enizi, A. Nafady and S. Ma,
Inorg. Chem. Front., 2020, 7, 300–339.
Conflicts of interest
There are no conflicts to declare.
25 Y.-L. Wu, X. Li, Y.-S. Wei, Z. Fu, W. Wei, X.-T. Wu, Q.-L. Zhu
and Q. Xu, Adv. Mater., 2021, 33, 2006965.
26 X. Deng, Z. Li and H. García, Chem. – Eur. J., 2017, 23,
11189–11209.
27 X. Yu, L. Wang and S. M. Cohen, CrystEngComm, 2017, 19,
4126–4136.
Acknowledgements
This work was supported by the NSFC (21872031 and
U1705251). Z. Li thanks the Award Program for Minjiang
Scholar Professorship for financial support.
28 M. Hao and Z. Li, Sol. RRL, 2021, 5, 2000454.
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