Silver nanoparticles doped TiO2 catalyzed Suzuki-coupling of bromoaryl with phenylboronic acid under visible light

Article abstract of DOI:10.1016/j.jphotobiol.2020.111807

The formation of the carbon?carbon bond in the synthetic chemistry explored in many ways. Suzuki-cross coupling is one of the ways to make bonds between two carbon atoms of similar molecules or different molecules. C–C bond was successfully formed between two aryl rings of aryl halides and phenylboronic acid at room temperature and atmospheric pressure under the visible illuminance. In this work we report, an in-situ synthesis of silver nanoparticles doped TiO₂ nanoparticles (NPs) and studied its catalytic activity as an eco-friendly, simple, recyclable and efficient catalyst for one-pot Suzuki-coupling of bromoaryl with phenylboronic acid under visible light. Only, 45 mg of the catalyst resulted in a 98% conversion of p-ethyl bromobenzene with a 97% yield of p-ethyl biphenyl using toluene as the solvent in the presence of visible light at atmospheric pressure. The electron-donating groups (e.g., ethyl group) substituted bromobenzene resulted in the maximum yields than that of the substitution with the electron-withdrawing groups. The catalyst shown significant catalytic activity up to seven recycling runs without any loss. The doping of silver nanoparticles boosted the catalytic activity at titanium dioxide surface as well as inside the pores. The high surface area of the semiconductor support provides the sites for accommodated silver nanoparticles and shows enhanced reactivity towards the coupling reaction of bromoaryl with phenylboronic acid. The as-synthesized catalyst was thoroughly characterized by XRD, TEM, EDX, XPS, FTIR, TGA, UV–vis, Raman and BET analysis. The high recyclability of the photocatalyst remarked the footprints in the C–C coupling reactions.

Full text of DOI:10.1016/j.jphotobiol.2020.111807

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Silver nanoparticles doped TiO2 catalyzed Suzuki-coupling of
bromoaryl with phenylboronic acid under visible light
Yuning Chen, Li Feng
PII:
S1011-1344(19)31596-9
DOI:
https://doi.org/10.1016/j.jphotobiol.2020.111807
JPB 111807
Reference:
To appear in:
Journal of Photochemistry & Photobiology, B: Biology
Received date:
Revised date:
Accepted date:
21 November 2019
4 January 2020
24 January 2020
Please cite this article as: Y. Chen and L. Feng, Silver nanoparticles doped TiO2 catalyzed
Suzuki-coupling of bromoaryl with phenylboronic acid under visible light, Journal of
Photochemistry
&
Photobiology, B: Biology(2019), https://doi.org/10.1016/
j.jphotobiol.2020.111807
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Silver nanoparticles doped TiO2 catalyzed Suzuki-coupling of bromoaryl with
phenylboronic acid under visible light
Yuning Chen, Li Feng*
School of Civil and Transportation Engineering, Guangdong University of Technology,
No100, Waihuan Xi Road, Guangzhou, Higher Education Mega Center, Panyu District,
Guangzhou-510006, Guangdong, China.
*Corresponding author. Tel: +86-13594617802; fax: +86-02039322515
E-mail: fl19860314@126.com (L. Feng).
Abstract
The formation of the carbon-carbon bond in the synthetic chemistry explored in many ways.
Suzuki-cross coupling is one of the ways to make bonds between two carbon atoms of similar
molecules or different molecules. C-C bond was successfully formed between two aryl rings
of aryl halides and phenylboronic acid at room temperature and atmospheric pressure under
the visible illuminance. In this work we report, an in-situ synthesis of silver nanoparticles
doped TiO2 nanoparticles (NPs) and studied its catalytic activity as an eco-friendly, simple,
recyclable and efficient catalyst for one‐pot Suzuki-coupling of bromoaryl with
phenylboronic acid under visible light. Only, 45 mg of the catalyst resulted in a 98%
conversion of p-ethyl bromobenzene with a 97% yield of p-ethyl biphenyl using toluene as
the solvent in the presence of visible light at atmospheric pressure. The electron-donating
groups (e.g., ethyl group) substituted bromobenzene resulted in the maximum yields than that
of the substitution with the electron-withdrawing groups. The catalyst shown significant
catalytic activity up to seven recycling runs without any loss. The doping of silver
nanoparticles boosted the catalytic activity at titanium dioxide surface as well as inside the
pores. The high surface area of the semiconductor support provides the sites for
accommodated silver nanoparticles and shows enhanced reactivity towards the coupling
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reaction of bromoaryl with phenylboronic acid. The as-synthesized catalyst was thoroughly
characterized by XRD, TEM, EDX, XPS, FTIR, TGA, UV-vis, Raman and BET analysis.
The high recyclability of the photocatalyst remarked the footprints in the C-C coupling
reactions.
Keywords: Metal oxide; Supported nanoparticle; Photocatalysis; Cross-coupling.
1. Introduction
Cross-coupling is one of the promising approaches for making carbon-carbon single bond but
has tedious protocols as developed [1]. It has versatile characteristics in a different field or
organic chemistry, such as intermediates in natural products [2], bioactive compounds, etc.
[3]. The coupling reaction occurred even at a high temperature >100°C due to the high
activation energy barrier associated with the rate-limiting step [4]. The coupling of two
carbon atoms is a challenging task explored in the last few decades [5]. The coupling reaction
usually occurred with Palladium metal at high temperatures [6]. Other noble metals have
been scrutinized to explore the coupled products. Akira Suzuki first reported Cross-coupling
in the late 1970s over Pd-nanoparticles. He shared the noble prize with Richard F. Heck and
Ei-ichi Negishi for their effort for the discovery and development of palladium-catalyzed
cross couplings in organic synthesis. After the discovery, a plethora of research has been
carried out over expensive noble metals and at high temperatures. The tedious recovery of
homogeneous palladium metal and high cost limits the scope of the coupling reaction [7]. In
the last few decades, the carbon-carbon coupling proved it as an essential approach to
synthesize a variety of products in organic synthesis [8-11]. Support materials play crucial
roles in synthetic chemistry. The efforts over cheap metal support materials for the
accommodation of noble metals, such as nickel and copper to couple carbon-carbon atoms,
have attracted the society.
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High-temperature and high pressure darken the glow of the Suzuki coupling reaction.
It requires a massive amount of energy to conduct the experiments. Also, the homogeneous
catalysts have the drawbacks of separation and stability in the reaction for a long. An
alternate, photocatalysis is a promising approach over thermal C―C coupling reactions due
to their low cost of equipment [12]. The photocatalytic reactions proceeded through the
radical-based mechanism driven by photogenerated electron-hole pairs at room temperature.
Also, Suzuki-coupling reported in photocatalysis using homogenous as well as heterogeneous
catalysts in the presence of visible light, but the recovery issue of the homogenous catalysts
darkens their use in the coupling reactions. In this context, Li et al. reported the room-
temperature Suzuki-coupling reaction over mesoporous carbon nitride supported Pd-
nanoparticles a Mott-Schottky photocatalyst with a maximum of 99% yield of the coupled
product under the visible illuminance [13]. Wang et al. reported Pd nanoparticle immobilized
conjugated microporous polymer as a catalyst for the coupling of aryl halides with
phenylboronic acid with 98% yield if the coupled product in the presence of visible light at
room temperature [14]. Xiao et al. performed the coupling reaction of an aryl halide with
phenylboronic acid over Au-Pd alloy with >99% yield of the coupled products in the
presence of visible light [15]. Jiao et al. synthesized the Mott-Schottky-type Pd/SiC catalyst
for the coupling of aryl halide and phenylboronic acid under the visible illuminance [16]. Sun
et al. reported the visible light assisted Suzuki-coupling reaction over Pd nanoclusters
encapsulated inside the cavity of NH2―Uio-66(Zr) MOF with >99% yield of the coupled
product [17]. Xiao et al. carried the lower temperature cross-coupling reaction using
palladium and gold alloy nanoparticles as the photocatalyst under the visible illuminance
[18]. Koohgard et al. reported the plasmonic property and catalytic performance of Pd
anchored to TiO2 for Suzuki-Miyaura coupling in the presence of visible light [19]. Although,
the above-reported protocols used the supports synthesized by tedious methods, such as
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MOFs, graphene oxide form modified Hummer’s method, which are very expensive and
undesirable for the practical applications; cross-coupling is also being studied with silver
nanoparticles and other supports synthesized from cheap transition metals, such as Ti, Ni, and
Cu [20, 21]. However, silver doped nanoparticles have been used with a series of coupling
reactions.
Recently, Yim et al. described the ultrathin WO3 nanosheets supported PdO
nanoclusters catalyzed C-C coupling under visible illuminance [22]. Wang et al. performed
the Suzuki-coupling reactions over graphitic carbon nitride supported palladium
photocatalyst and got a TOF of 47.3 h-1 [23]. Sahoo et al. reported the visible-light assisted
Suzuki-coupling using GO/LDH supported plasmonic AuPd bimetallic nanoalloy hybrid
nanocomposite resulted in high yields of the corresponding biphenyls [24]. Although, silver
doped catalysts have the advantage over the Pd-metal relatively less expensive and easily
synthesized. Silver doped photocatalysts reported in a variety of coupling reactions [25]. In
this regard, Sharma et al. reported Ag@Cu2O core-shell NPs as the catalysts applied to the
Suzuki-cross-coupling reaction in the presence of visible light [26]. Gao et al. manufactured
graphene oxide-palladium modified Ag-AgBr photocatalyst for cross-coupling of aryl halides
and phenylboronic acid in the presence of visible light [27].
However, few reports are available with the first transition series metals supports. In
this regard, based on the previous literature, we are focusing on low-cost metal supports, such
as titanium, manganese, iron, cobalt, nickel, and copper for the Suzuki coupling in the
presence of visible light. Herein, we report the most studied semiconductor TiO2 as
semiconductor support for the doping of Ag nanoparticles catalyzed cross-coupling of aryl
halides and phenylboronic acid in the presence of visible light. We thought that this work
would play an important role in the formation of carbon-carbon bond for the industrial point
of view.
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Scheme 1. Suzuki-coupling of bromobenzene and phenylboronic acid using Ag/TiO2 under
the visible illuminance.
2. Experimental section
2.1 Synthesis of Ag/TiO2 photocatalyst
The desired photocatalyst was synthesized by the little modification in the method reported in
the literature [28]. 5 mL of titanium tetrachloride solution was mixed with 5 mL of
triethanolamine and 10 mL of ethanol at 0℃. Subsequently, the obtained white solid was
dissolved in 50 mL of de-ionized water with continuous stirring for two h at room
temperature. Silver nitrate (0.5-4 wt%) added to this homogeneous mixture and maintain the
pH seven by the dropwise addition of ammonium hydroxide. The obtained precipitates were
filtered, washed with distilled water by several times, followed by ethanol, and finally dried
at 60℃ for 24 h in a vacuum oven. The dried material was calcined at 550℃ for four h in the
presence of air in a muffle furnace. The as-synthesized catalyst was used for the
photocatalytic reaction. For the comparative study, TiO2 also prepared following a similar
procedure without the use of silver salt.
2.2 Characterizations
UV-Vis absorption spectrum of the materials was recorded using a Perkin Elmer Lambda-19
UV-VIS-NIR spectrophotometer with a 10-mm quartz cell and BaSO4 as a reference. X-ray
diffraction (XRD) pattern was recorded to determine the crystallinity of the materials using
Bruker D8 Advance diffractometer at 40 kV and 40 mA with Cu K radiation
(nm). Thermogravimetric analysis was carried out to find out the stability of the
composites using a thermal analyzer TA-SDT Q-600. The thermal degradation pattern for the
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materials was determined by thermogravimetric analysis in the temperature range of 40 to
800°C under nitrogen flow with a 10°C/min heating rate. Fourier transform infrared
spectroscopy (FT-IR) used to determine the stretching and bending vibrations on a Perkin–
Elmer spectrum RX-1 IR spectrophotometer having a potassium bromide window. All the
samples were degassed at 200℃ for three hours under an N2 atmosphere. High-resolution
transmission electron microscopy (HR-TEM) was used to analyze the nanocomposite and
estimated the interplanar distances in the materials, using a JEM 2100 (JEOL, Japan)
microscope. X-ray photoelectron spectroscopy (XPS) analysis was used to estimate the
oxidation state and binding energy of elements in the composites using ESCA+ equipment
(omicron nanotechnology, Oxford Instrument Germany) equipped with monochromator
Aluminum Source (Al ka radiation hv =1486.7 eV). ICP-AES was used to estimate the metal
content in the final catalyst using Inductively Coupled Plasma Atomic Emission
Spectrometer (ICP-AES, DRE, PS-3000UV, Leeman Labs Inc, USA). BET surface area,
pore-volume, and mean pore diameter of composites were estimated using N2 adsorption-
desorption isotherm at 77 K by using Micromeritics ASAP 2010.
2.3 Photocatalytic experiment of Suzuki-coupling
Suzuki-coupling reaction was carried out in a 25 mL round bottom (RB) flask. Initially, RB
was cleaned with water and then rinsed with acetone followed by drying at 60℃ in an oven
for one hour. One mmol of bromobenzene and two mmol of phenylboronic acid were poured
into RB. Subsequently, 2 mL of toluene was inserted into the RB and closed with a rubber
septum. The reaction solution was purged with N2 gas for 30 min for the complete removal of
air from the mixture. After that, RB was charged with 30 mg of Ag/TiO2 photocatalyst, and
the vessel was sealed with the rubber septum. The reaction mixture again purged with the N2
gas for the residual air and then irradiated with the 20 W white LED (λ ~ 420 nm) for 24 h.
The sample was analyzed with TLC after every two-hour interval. The conversion and the
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yield were determined with GC-FID (Thermo equipped with Stabilwax® w/Integra- Guard®
1
13
30-meter-long column), and confirmation was obtained by the H and C NMR of the pure
samples.
3. Results and Discussions
The photocatalyst was synthesized using silver nitrate and titanium tetrachloride as the
precursor of silver and titanium. The synthesis of the photocatalyst was demonstrated in
scheme 2.
Scheme 2. A synthetic protocol of Ag/TiO2 from silver nitrate and titanium tetrachloride.
Crystallinity, crystal size, and planes of the materials were determined by X-ray diffraction
spectroscopy (Figure 1). The diffraction pattern of TiO2 indicated the characteristic
diffraction peaks correspond to the anatase form of TiO2, and the intensity of the diffraction
peaks suggested that the materials were highly crystalline (Figure 1a). TiO2 semiconductor
has the tetragonal structure with a body-centered crystal lattice and follows the JCPDS Card
No. 89-4921 [29]. The loading of the silver nanoparticles onto the TiO2 semiconductor shows
the signal of reduced AgNPs. 2%Ag/TiO2 shows a diffraction peak at 44.22° for the reduced
silver nanoparticles (JCPDS Card No. 89-3722) which corresponds to (200) plane (Figure 1b)
[30]. The rest of the diffractogram remains as it is for TiO2 semiconductor support.
4%Ag/TiO2 also shows a diffraction signal at a similar angle for reduced Ag nanoparticles
but in little bit enhanced intensity confirms the successful loading of the silver nanoparticles
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at TiO2 semiconductor support. This also reveals that the doping of the silver nanoparticles
did not change the character and the crystal lattice of TiO2 support (Figure 1c).
Figure. 1. X-ray diffractogram of (a) TiO2, (b) 4%Ag/TiO2 and (c) 4% Ag/TiO2
photocatalyst synthesized and calcined at 550℃.
The stretching and bending vibrations of TiO2, 2%Ag/TiO2, and 4%Ag/TiO2 photocatalyst
were determined by the FT-IR technique using potassium bromide as reference material
(Figure 2). The vibrational peak at 1078 cm-1 corresponds to the characteristic Ti-O stretching
in the crystal lattice of TiO2 semiconductor (Figure 2a). The next two vibrations at 1450 and
1635 cm-1 were due to the intercalated water molecule in the semiconductor support while the
later broad signal obtained at 3391 cm-1 attributed to the O-H stretching of the hydroxyl
moieties suggested that the semiconductor absorbs moisture from the atmosphere [31]. This
indicated that the oxygen atoms present at the surface of TiO2 semiconductor are in the form
of -OH groups. The insertion of silver nanoparticles in the semiconductor shows the
vibrational signals in the spectra with 2% loading. The doping of 2% AgNPs onto TiO2
semiconductor show slightly shifted stretching vibration with a shoulder at 1674 cm-1
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exhibited the Ag-O stretching reveals the incorporation of the silver nanoparticles in TiO2
semiconductor (Figure 2b) [32]. Another signal at 3245 cm-1 attributed to the Ag-OH
stretching due to the moisture content in the final catalyst. Rest of the spectrum for
2%Ag/TiO2 photocatalyst remains the same as for TiO2 semiconductor. Further, loading of
4%AgNPs shows peaks similar to the 2% loading but the slightly increased intensity of the
vibrational peaks indicating the higher loading of AgNPs in the nanocomposite (Figure 2c).
Figure. 2. Furrier transform infra-red (FTIR) spectra of (a) TiO2, (b) 2%Ag/TiO2 and (c)
4%Ag/TiO2 using KBr as standard for the instrument.
The absorption maximum for TiO2, 4%Ag/TiO2, and 4%Ag/TiO2 photocatalyst were
estimated by UV-Vis spectrophotometer using barium sulfate as the standard (Figure 3). UV-
vis spectrum of TiO2 shows broadband in UV region 246-384 nm, indicating that the TiO2
had strong absorption in the UV region, not in the visible region (Figure 3a) [33]. However,
the doping of silver nanoparticles onto the semiconductor makes the nanocomposite capable
of absorbing visible light. Loading of 2 wt% of AgNPs on TiO2 shows a hump in the range
between 400-500 nm along with the broad characteristic signal of Ag (Figure 3b) [34]. The
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nanocomposite contains both the TiO2 and AgNPs absorption confirms the successful
incorporation. The absorption maximum of the nanocomposite in the visible region shows the
activity for the visible-light-induced reaction. Higher loading, 4 wt% of AgNPs onto TiO2
also exhibited similar hump at the same wavelength, but slightly high intensity suggested the
more absorption with higher loadings of AgNPs (Figure 3c).
Figure. 3. Ultra-Violet-visible (UV-vis) spectra of (a) TiO2, (b) 2%Ag/TiO2 and (c)
4%Ag/TiO2 photocatalyst using BaSO4 as the reference material.
HR-TEM images, SAED pattern, and EDX pattern for 2%Ag/TiO2 photocatalyst are
demonstrated (Figure 4). The sample was prepared at a lacey carbon grid using ethanol as the
solvent. The nanocomposite shows uniformly distributed granules at a 100 nm scale (Figure
4a). The doping of AgNPs onto TiO2 observed in the nanocomposite where the silver
nanoparticles lie in the range from 1.5-5 nm at 5 nm scale (Figure 4b) [35]. SAED pattern
shows the clear, bright spots which confirm that the nanocomposite is crystalline (Figure 4c).
The loading of the silver nanoparticles observed in the EDX pattern of the 2%Ag/TiO2
photocatalyst and found 1.87 wt%, which is close to 2% loading of silver nanoparticles.
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Figure. 4. (a, b). HR-TEM images, (c) SAEd pattern and (d) EDX pattern of 2%Ag/TiO2
photocatalyst.
The electronic properties for TiO2 and 2%Ag/TiO2 photocatalyst were determined by Raman
spectroscopy at wavelength 532 nm (Figure 5). Anatase TiO2 shows five characteristic
vibrational signals at 150, 201, 393, 509, and 633 cm-1 attributed to the Eg, Eg, B1g, A1g/B1g,
and Eg Raman active modes and confirmed the tetragonal space group D4h (Figure 5a) [36-
39]. The doping of silver nanoparticles onto TiO2 did not show any effect in 2% Ag/TiO2
photocatalyst (Figure 5b). The lower doping of the silver nanoparticles onto TiO2
semiconductor did not show any vibration along with the semiconductor support suggested
that 2%Ag/TiO2 photocatalyst did not change the electronic properties and behave as usual.
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Figure. 5. Raman frequencies of (a) TiO2 and (b) 2%Ag/TiO2 photocatalyst.
The surface chemical properties of 2%Ag/TiO2 photocatalyst was determined by X-ray
photoelectron spectroscopy (Figure 6). XPS spectra of Ag-3d shows two peaks for 3d3/2 and
3d5/2 attributed at 373.6 eV and 368.3 eV, respectively correspond to the metallic silver Ag(0)
in the 2%Ag/TiO2 nanocomposite and free to bind with the oxygen atoms (Figure 6a). The
metallic silver was in a good agreement with the literature reported by Paul et al. [40]. Also,
XPS of Ti-2p exhibited two binding energies of 464.3 eV and 458.9 eV corresponds to the
Ti-2p1/2 and Ti-2p3/2, respectively which suggested that the titanium is present in +4 oxidation
state (Ti4+) (Figure 6b) [41].
Figure. 6. XPS analysis of (a) Ag-3d and (b) Ti-2p for 2%Ag/TiO2 photocatalyst.
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The BET surface area, pore-volume, and the average pore size were determined by the N2
adsorption-desorption isotherm at -196℃ (Figure 7). All the samples were degassed at 300℃
for 3 h in the presence of N2 gas. The N2 adsorption-desorption isotherm for TiO2 shows type
IV isotherm with the H3 hysteresis loop (Figure 7a) [42]. The hysteresis of the isotherm and
the pore size distribution reveals the mesoporous nature of TiO2 semiconductor support. The
surface area, pore diameter and pore volume determined for TiO2 were 45 m2-g-1, 13 nm, and
0.013 m3-g-1. The average pore diameter of 13 nm lies in the mesoporous range of the pore
and confirms the mesopores in the semiconductor. After deposition of 2 wt% silver
nanoparticles onto the TiO2 semiconductor, the N2 adsorption-desorption isotherm neither
changed from type IV nor changed the hysteresis loop from H3 (Figure 7b). The surface area
of 2%Ag/TiO2 nanocomposite decreased up to 29 m2-g-1 due to the deposition of the silver
nanoparticles at the surface of TiO2. The average pore size decreased to 11 nm, but the pore
volume slightly increased to 0.019 m3-g-1 due to the deposition of the AgNPs into the pores
of the semiconductor [43]. The decrement in the pore size of 2%Ag/TiO2 photocatalyst does
not influence the range of mesoporosity and remains intact as TiO2 semiconductor. More
loading, i.e., 4 wt% of AgNPs at TiO2, reveals the similar N2 adsorption-desorption isotherm
and hysteresis loop as well. The increased loading of AgNPs slightly decreased the surface
area (26 m2-g-1) and average pore diameter (10 nm), but slightly increase pore volume (0.021
m3-g-1) in a small extent (Figure 7c).
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Figure. 7. N2 adsorption-desorption isotherm of (a) 2%Ag/TiO2, (b) TiO2 and (c) silver
nanoparticles (AgNPs).
The weight loss of TiO2, 2%Ag/TiO2, and 4%Ag/TiO2 were determined by thermal
gravimetric analysis (TGA) and derivative thermal analysis under N2 atmosphere (Figure 8).
The thermogram of TiO2 exhibited 11% initial weight loss at 101℃attributed to the adsorbed
water content from the atmosphere (Figure 8a). No weight loss occurred at a higher
temperature and remained 89% of the semiconductor at 800℃, indicating the stability of TiO2
[44]. The incorporation of AgNPs into the semiconductor slightly changes the thermal
behavior 2%Ag/TiO2 nanocomposite (Figure 8b). The initial weight loss (5%) occurred at
60℃ for the evaporation of the water molecules. Further heating of the nanocomposite shows
a steady loss of weight at 800℃ and 84% of the material remains at the end of the analysis.
On heating to 4%Ag/TiO2, 5% weight loss occurred at 60℃, which corresponds to the
evaporation of water at a lower temperature (Figure 8c). Further heating the material, 4%
weight loss from 88-84 on the weight % scale found at 500℃was attributed to the elimination
of nitrate precursor AgNPs [45]. This weight loss also supported by the DTA, shows a sharp
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signal at 517℃ corresponds to the weight loss at 500℃. The rest of the TGA shows small,
steady loss of weight and 82% of total weight remained at 800℃.
Figure. 8. Thermogravimetric and derivative weight loss per min. for (a) TiO2, (b) 2%
Ag/TiO2 and (c) 4%Ag/TiO2 plotted against the temperature from 30-800℃under N2
atmosphere.
Photocatalytic activity of the catalyst
The coupling reaction was performed in a 25 mL RB charged with bromoaryl, phenylboronic
acid, toluene, and photocatalyst. The closed RB purged by N2 gas for 30 min and then
removed, followed by irradiation by white LED light. The catalyst was separated from the
reaction mixture with the help of a syringe filter at the end of the reaction, and 1 μL of the
reaction mixture was injected into the GC-FID to estimate the conversion and yield of the
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1
substrate and the product, respectively. The products were further confirmed using H and
13C NMR spectroscopy.
Suzuki-coupling reaction was screened with variation in the loading of silver onto
TiO2 using different solvents at room temperature in the presence of visible light. At first,
water was used as the reaction solvent for the coupling reaction using 2%Ag/TiO2
photocatalyst. The conversion of the substrate and yield of the product were moderate. While
using ethanol as a solvent, a slight increase in the yield observed under the identical reaction
condition. Like ethanol, similar results were obtained with acetonitrile shows that polar
solvents poorly proceeded the coupling reaction. A significant increase in yield of the product
was observed with DMF as the reaction medium up to 76 yields of the coupled product.
Toluene, a less polar solvent efficiently converted bromobenzene into the corresponding
coupled product in 92% yield, indicating that toluene is the suitable solvent for the Suzuki-
coupling reaction. Further, to optimize the loading of silver onto TiO2, a variation from 0.5-
3.0% loading of Ag studied for the reaction. Less loading of the noble metal shows the low
efficiency of the catalyst under the identical reaction condition. 0.5%Ag/TiO2 shows the
lowest yield of 52% of the product. An increase in the loading of Ag increased the efficiency
of the reaction and afforded a higher yield. 2%Ag/TiO2 resulted in the highest 92% yield of
the corresponding product in toluene while a further increase of Ag loading shows no effect
or slightly decreases the yield of the product. In the absence of light, there was no product
detected in GC-FID. The reaction proceeded at TiO2 but gave poor results while no reaction
was found in the absence of the photocatalyst. Commercial anatase and rutile TiO2 used to
compare with the synthesized anatase TiO2 under similar reaction conditions, but the results
were not as good as-synthesized TiO2.
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Table 1. Optimization of parameters for the photocatalytic coupling of bromobenzene and
phenylboronic acid.a
Sr. No. Catalyst
Solvent
Water
Conv. (%)
Yield (%)b
1.
2.
2%Ag/TiO2
57
63
64
79
95
23
53
79
94
93
89
89
-
52
61
60
76
92
20
51
78
91
91
88
85
-
2%Ag/TiO2
2% Ag/TiO2
2% Ag/TiO2
2% Ag/TiO2
0.5% Ag/TiO2
1% Ag/TiO2
1.5% Ag/TiO2
2.5% Ag/TiO2
3% Ag/TiO2
3.5% Ag/TiO2
4% Ag/TiO2
2%Ag/TiO2
TiO2
Ethanol
Acetonitrile
DMF
3.
4.
5.
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
6.
7.
8.
9.
10.
11.
12.
13.c
14.
15.
16.
11
-
09
-
No catalyst
TiO2 (Commercial Toluene
anatase)
08
06
17.
TiO2 (Commercial Toluene
rutile)
05
04
aReaction condition: Bromobenzene (1 mmol), phenylboronic acid (2 mmol), catalyst (45 mg), solvent 2 mL for
24 h, at room temperature, white light 20 W LED (λ > 420 nm). bIsolated yield by GC-FID. cNo light.
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Figure. 9. A plot of yield % against the loading of AgNPs for the Suzuki-cross-coupling
reaction.
The scope of the reaction was elaborated for the substituted bromoaryls under
identical reaction conditions. 2%Ag/TiO2 found to be the optimum photocatalyst for the
coupling of bromoaryl and phenylboronic acid in the presence of visible light. Bromoaryls
with electron-donating groups at para position afforded the excellent yield than the non-
substituted bromobenzene up to 97% yield of the corresponding product (entry 2-3). p-methyl
bromobenzene resulted in a 95% yield of 4-methyl-1,1’-biphenyl (entry 2). Substitution of an
ethyl group at para position enhanced the efficiency and gave 97% yield of 4-ethyl-1,1’-
biphenyl (entry 3). The substitution of electron-withdrawing groups at the para position in
bromobenzene retarded the yield of the corresponding coupled biphenyl compounds up to
83% (entry 4-10). Bromobenzene with amino substitution at a para-position slightly reduced
the yield of 94% for (1,1’-biphenyl)-4-amine (entry 4). This decrease also found in para
hydroxy-substituted bromobenzene afforded a 91% yield of (1,1’-biphenyl)-4-ol (entry 5).
While the methoxy-substituted bromobenzene provided higher yield 94% of 4-methoxy-1,1’-
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biphenyl due to the electron-donating methyl group at the oxygen of the methoxy group
(entry 6). Phenoxy substituted bromobenzene further reduced the yield of 4-phenoxy-1,1’-
biphenyl about 93% indicating that the conjugated phenyl ring slightly restricts the product
formation. Highly electronegative and strong electron-withdrawing groups resulted in the
lowest yield amongst the substrate used. Chloro-substituted bromobenzene gave a 90% yield
of 4-chloro-1,1’-biphenyl due to the high electronegativity of chlorine (entry 8). Strong
electronegative NO2 substituted bromobenzene afforded much lower yield 83% of 4-nitro-
1,1’-biphenyl suggested that the electron pulling tendency of nitro group slightly retards the
coupling of phenyl rings of 4-bromo nitrobenzene and phenylboronic acid. Further, 4-bromo
benzaldehyde afforded somehow 87% yield of (1,1’-biphenyl)-4-carbaldehyde showing
better efficiency than the product yield of 4-nitro-1,1’-biphenyl. The overall summary found
that the biphenyls with electron-donating groups at para position show excellent activity than
that with electron-withdrawing groups for the Suzuki-coupling of substituted bromoaryls and
phenylboronic acid in the presence of visible light.
Table 2. Suzuki-coupling of bromoaryls and phenylboronic acid under the optimized
condition.
Sr.
No.
Substrate
Product
Conversion
(%)b
Yield (%)c
1.
95
96
92
2.
95
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3.
4.
98
97
97
94
5
95
96
93
91
87
91
94
90
90
83
6.
7.
8.
9.
10.
89
87
aReaction: 1 mmol of Substrate, 2 mmol of phenylboronic acid, 45 mg of photocatalyst, 2 mL of
b
toluene, reaction time 24 h, 20 W white LED (λ > 420 nm), room temperature. Conversion by GC-
FID. cIsolated yield by GC-FID.
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Further, the photocatalyst was recycled after the completion of the cross-coupling
reaction. The photocatalyst was separated by filtration, thoroughly washed with ethanol for
the complete removal of toluene through the centrifugation, and dried in a vacuum oven at
60℃ for 24 h. The recycled photocatalyst was reused for the coupling reaction up to seven
times with a small or negligible loss of activity (Figure 10). This shows that the recovered
photocatalyst was highly efficient and productive for the coupling reactions.
Figure. 10. A plot of yield against the recovered catalysts in each cycle for the Suzuki-cross-
coupling reaction.
Furthermore, the superiority of the developed protocol established by the comparison
of the already reported literature for the Suzuki-coupling reactions based on the silver doped
photocatalysts in the presence of visible-light in Table 3. The table showed that the highest
coupled biphenyls obtained with Ag/TiO2 photocatalyst reported in the present report. The
main advantage of the catalytic system is that there was no use of base or the reaction
proceeded without the co-catalyst.
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Entry
Photocatalyst Base
Time (h)
Yield (%)
Ref.
1.
Pd/Ag/SBA-
15
K2CO3
K2CO3
-
2
80
Verma
al.[46]
et
et
2.
3.
Ag@Cu2O
12
24
89
97
Sharma
al.[26]
Ag/TiO2
This work
The mechanism of the Suzuki-coupling reaction is well established. However, here
we have proposed the mechanism for coupling of substituted bromoaryls and phenylboronic
acid using Ag/TiO2 as a photocatalyst in the presence of visible light. TiO2 is an excellent
known photocatalyst in the photocatalysis and mostly studied for maximum reactions. TiO2
alone is UV-active as well as shows moderate activity in the presence of visible light but, the
doping of noble metal nanoparticles makes it a suitable catalyst for the photo-responsive
reaction [47-49]. The noble metal nanoparticles show plasmonic resonance through which the
electrons are quickly transferred to the molecule which has to be reduced. Silver
nanoparticles showed the plasmonic resonance through which the yield of the biphenyls
increased. In the presence of visible light, TiO2 semiconductor alone performs small
excitation of the electrons from the valence band (VB) to conduction band (CB) in the visible
light. In combination with the silver nanoparticles, the combined photocatalyst 2%Ag/TiO2
shown activity towards the coupling reactions. The excitation of electrons enhanced by the
doping of AgNPs. The electrons of VB of TiO2 get excited to CB in the presence of visible
light and produced electrons and holes. These holes utilized to oxidize the quaternary
phenylboronic acid at VB and generated the phenyl radical. The excited electrons of CB then
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transferred to the silver nanoparticles, where the silver nanoparticles utilized these electron
through plasmonic resonance for the aryl halides [50]. Then, both the species react with each
other and generated the biphenyls as the coupled product.
Scheme. 3. A plausible mechanism of the Suzuki-coupling reaction catalyzed by 2%Ag/TiO2
photocatalyst in the presence of visible light.
4. Conclusion
Suzuki-coupling of bromoaryls has been successfully carried out using Ag/TiO2 catalyst
under the visible illuminance. Ag/TiO2 photocatalyst exhibited the superior efficiency for
coupling of bromoaryls with phenylboronic acid into the corresponding bi-aryls up to 98%
yield in toluene in 24 h of reaction time. The silver nanoparticles recycled easily by simple
centrifugation of the reaction mixture and used up to six times with a small decrease in the
reactivity. The synergistic effect of Silver and Titania has advantages over both homogeneous
and heterogeneous photocatalyst for the Suzuki-coupling reaction.
Acknowledgments
The authors are grateful for the financial support provided by the National Key Research and
Development Program of China (No. 2017YFC040470002) and Guangdong Provincial
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Natural Science Fund (No. 2017A030313321).
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