Conjugated mesoporous polyazobenzene–Pd(II) composite: A potential catalyst for visible-light-induced Sonogashira coupling

Article abstract of DOI:10.1016/j.jcat.2019.07.031

We herein report the direct harnessing of visible light energy by a novel semiconducting azobenzene-based colloidal porous organic polymer, B₃-Azo₄ amenable to post-synthetic Pd(II)-complexation for photo-induced Sonogashira coupling using polymer → metal energy transfer process. Completely mesoporous nano-sized particles of the obtained organic network manifested excellent Pd(II) coordinating ability and afforded desired catalytic products at room temperature under aerobic condition in aqueous medium. The protocol remained valid for different aromatic and aliphatic substrates as well. A possible reaction mechanism for photo-mediated Sonogashira coupling catalyzed by Pd-B₃-Azo₄ is also proposed.

Full text of DOI:10.1016/j.jcat.2019.07.031

Journal of Catalysis 377 (2019) 183–189
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Conjugated mesoporous polyazobenzene–Pd(II) composite: A potential
catalyst for visible-light-induced Sonogashira coupling
Ipsita Nath a,b, Jeet Chakraborty a,b, , Anish Khan a,b, Muhammad N. Arshad , Naved Azum c,d,
⇑
c,d
c,d
c,d
c,d
a,e,f,
⇑
Malik A. Rab , Abdullah M. Asiri , Khalid A. Alamry , Francis Verpoort
a
Laboratory of Organometallics, Catalysis and Ordered Materials, State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University
of Technology, Wuhan 430070, China
b
School of Resources and Environmental Engineering, Wuhan University of Technology, Wuhan 430070, China
Chemistry Department, Faculty of Science, King Abdulaziz University, P.O. Box 80203, Jeddah 21589, Saudi Arabia
c
d
Center of Excellence for Advanced Materials Research, King Abdulaziz University, P.O. Box 80203, Jeddah 21589, Saudi Arabia
e
National Research Tomsk Polytechnic University, Lenin Avenue 30, Tomsk 634050, Russia
f
Centre for Environmental and Energy Research, Ghent University Global Campus, 119 Songdomunhwa-Ro, Yeonsu-Gu, Songdo, Incheon 406-840, South Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
We herein report the direct harnessing of visible light energy by a novel semiconducting azobenzene-
Received 29 April 2019
Revised 12 July 2019
Accepted 15 July 2019
3 4
based colloidal porous organic polymer, B -Azo amenable to post-synthetic Pd(II)-complexation for
photo-induced Sonogashira coupling using polymer ? metal energy transfer process. Completely meso-
porous nano-sized particles of the obtained organic network manifested excellent Pd(II) coordinating
ability and afforded desired catalytic products at room temperature under aerobic condition in aqueous
medium. The protocol remained valid for different aromatic and aliphatic substrates as well. A possible
Keywords:
Azobenzene
Colloidal porous organic polymer
Energy transfer
3 4
reaction mechanism for photo-mediated Sonogashira coupling catalyzed by Pd-B -Azo is also proposed.
Ó 2019 Elsevier Inc. All rights reserved.
Visible and sunlight induced Sonogashira
Mechanism
1
. Introduction
Nonetheless, heterogenizing the catalysts solved some of these
perennial problems e.g. separation, reusability etc., other aspects
have hardly been addressed [12,13].
Sonogashira coupling is one of the major carbon-carbon bond
forming methodologies practiced to date for synthesizing aryl
alkynes and conjugated enynes, the key precursors for preparing
various natural products, pharmaceuticals, fine chemicals, and
organic molecular materials [1–6]. The traditional approach to fur-
nish Sonogashira products under homogeneous conditions
demand the use of Pd catalysts and Cu co-catalysts under harsh
reaction conditions involving high reaction temperature, corrosive
solvents, and additional expensive and often harmful organic
ligands [7–10]. Moreover, other drawbacks such as high energy
consumption due to prolonged reaction span, and post-
production difficulty in separating and reusing the catalysts etc.
all together adds to the environmentally unfavorable aspect of the-
ses reaction protocols limiting their wide applications [11].
As photo-irradiation can shift the apparent thermodynamic
equilibrium of reactions to favor the chemical transformations at
lower temperature avoiding thermo-induced side reactions, low
energy consuming photocatalytic approaches to trigger versatile
reactions are acquiring global attentions lately [14,15]. Moreover,
owing to the mild reaction conditions, these methods often help
to avoid potential reactant and product decompositions as well
as maintain the integrity of the catalysts [16]. Though rarely,
photo-induced Sonogashira couplings have also been targeted in
homogeneous media using transition metal catalysts such as CuCl,
[Ru(bpy)
3
6
](PF )
2
/Pd(CH
3
CN)
2
Cl
2
etc. [17,18]. The methods
remained exceptionally active generating desired products in high
yields within short reaction period. Efforts have been given very
recently to synthesize heterogeneous catalysts susceptible to per-
form similar catalysis as these can effectively solve all the existing
shortcomings. In pursuit of this idea, semiconductor/CuO pair-
based composites [19,20], photo-responsive Pd nanoparticles
⇑
Corresponding authors at: Laboratory of Organometallics, Catalysis and Ordered
Materials, State Key Laboratory of Advanced Technology for Materials Synthesis and
Processing, Wuhan University of Technology, Wuhan 430070, China.
E-mail addresses: jeet.chakraborty@hotmail.com (J. Chakraborty), francis.ver-
poort@ghent.ac.kr (F. Verpoort).
[
16], Au-Pd plasmonic composite [21], and SiC/Pd-Cu alloy have
recently been surfaced as viable photocatalysts for Sonogashira
https://doi.org/10.1016/j.jcat.2019.07.031
0
021-9517/Ó 2019 Elsevier Inc. All rights reserved.

1
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I. Nath et al. / Journal of Catalysis 377 (2019) 183–189
couplings [22]. However, potential drawback of the Cu-catalyzed
reactions originating from their inherent activity towards Glaser
homocoupling of terminal alkynes reduces the effective yields of
desired products [23]. Additionally, Cu-salts have also shown inhi-
bitory activity towards Sonogashira coupling in certain cases
3 4
Pd(II) centers in semiconducting B -Azo skeleton can manifest a
similar effect through polymer ? metal intra-molecular energy
transfer. Moreover, apart from the obvious photo-responsive
contribution, azo-benzenes were also expected to play a key role
in coordinating Pd(II)-centers through azo-N owing to their
well-known transition metal complexation properties [37,38].
[
24,25]. Hence, Cu-free couplings prompted as an applicable alter-
native [26–28]. Given the wide spread activity of Pd-complexes to
afford Sonogashira products [29], and based on our recent under-
standing on the photocatalytic efficiency of Pd/semiconductor sys-
tems [30], we have targeted to synthesize novel porous organic
polymer (POP)-Pd hetero-composite for catalyzing photo-induced
Cu-free Sonogashira coupling.
Conjugated porous organic polymers are an emerging class of
permanently nano-porous, commonly insoluble, 3D materials pos-
sessing excellent thermal and chemical stability, high surface area,
3 4
Additionally, colloidal synthesis of B -Azo was exercised to
achieve the targeted catalytic activity in aqueous medium and to
impart enhanced mesoporosity in the catalytic skeleton beneficial
for easy mass transport in heterogeneous catalysis [30]. Interest-
ingly, following this synthetic technique we have been able to
achieve a completely mesoporous polymer architecture. To the
best of our knowledge, this is first ever report on photoactive con-
jugated POP-Pd system for successfully catalyzing Sonogashira
coupling. Furthermore, apart from a single report of homogeneous
catalysis dated back in 2007 [17], this is, to date, the only work
highlighting substantial efficiency of semiconductor-Pd(II) systems
in Sonogashira coupling, and in general overall photocatalysis.
and infinite
p–delocalized skeleton [31,32]. Due to highly conju-
gated structures, these networks featuring suitable band gaps often
demonstrate semiconducting ability on visible light irradiation
[
33,34]. Moreover, the choice over synthetic methodologies and
synthons used for preparing these polymers allow manual control
on their properties such as HOMO-LUMO potentials, physical
appearance, porosity distribution pattern, solvent dispersiblity
etc. making them extremely suitable candidate for different cat-
alytic applications.
Our research involved the use of azo-benzenes as precursors for
photo-active polymer synthesis using different synthetic tech-
niques such as high dilution method, colloidal approach etc. for
new-age applications [30,35,36]. As a continuation of this work,
we became interested in developing novel azo-benzene based
2
. Result and discussion
2.1. Synthesis and characterizations of B
3
-Azo
4
and Pd-B
3 4
-Azo
The conjugated polymeric network B
prepared by Sonogashira-Hagihara cross-coupling of (E)-1,2-bis(3,
-dibromophenyl)diazene and triethynyl benzene following a pre-
3
-Azo (Fig. 1c) was
4
5
viously reported 1:9 oil-in-water miniemulsion method [30].
Addition of 1% aqueous CTAB solution to a mixture of 1:1 toluene
and triethyl amine dissolving the reactants and catalysts, followed
by strong ultrasonication generated thermodynamically stable
emulsified reaction mixture (Scheme S1). As the solvolytic attrac-
tions between nonpolar organic solvents and oligomeric organic
intermediates generated during polymerization are known to pro-
duce extremely intertwined cross-coupled skeleton of the final
3 4
polymer-Pd catalytic system Pd-B -Azo susceptible to harvest
solar energy for carrying out Sonogashira coupling in aqueous
media under aerobic condition at room temperature. Given the
superior activity of homogeneous Pd(II) complex in presence of
suitable photosensitizer [17], we envisioned that immobilizing
Fig. 1. (a) N
2 3 4 3 4 3 4
-sorption plot, (b) NLDFT porosity distribution pattern, and (c) skeletal backbone of B -Azo ; (d) comparative FT-IR pattern of B -Azo and Pd-B -Azo
highlighting the differences observed after metal loading.

I. Nath et al. / Journal of Catalysis 377 (2019) 183–189
185
networks, a trend in decreased microporosity is usually observed
39,40]. Owing to an efficient cross-coupling between randomly
claim of ionic Pd species comprehended from FT-IR analysis. A
1.0 eV negative shift of the Pd(II)3d5/2 peak compared to free Pd
[
oriented tetra-connected azo-monomer and tris-connected alkynyl
co-monomer in the highly concentrated nonpolar organic phase,
B -Azo was also anticipated to show supressed microporosity.
3 4
Though, substantial mesoporosity was expected due to the col-
loidal synthetic approach.
2
(OAc) (338.4 eV) suggested an increased electron density over
Pd possibly arising from the azo-N atoms of the polymeric back-
bone, inferring a more stable coordination environment provided
by the organic network [44]. A comparison of the N1s spectrum
3 4
of Pd-B -Azo and that of pristine polymer demonstrated almost
Following our assumption, the non-local density functional
theory (NLDFT) revealed completely mesoporous distribution of
500 unit decrease in the N-C peak area and 0.3 eV positive shift
after Pd loading (Fig. 2c). These observations are further indicative
of the strong coordination between Pd(II) and azo-N-atoms present
in the network. Such coordination of Pd ions with azo-N are pre-
vailing in literature [37,38]. Moreover, apart from the usual C@C
and CAN peaks appearing at 284.7 and 286.3 eV respectively,
another shakeup peak was observed at 288.5 eV in the C1s region
as well (Fig. 2b). This is attributed to the OACAO units arising from
the acetate counter anions of Pd(II).
2
ꢀ1
B
3
-Azo
mmett–Teller (BET) surface area and 0.14 cm g pore volume of
the polymer. Corresponding Type-V N -sorption plot also supports
this porosity distribution pattern (Fig. 1a) [41]. Adsorption of N in
4
(Fig. 1b) resulting in relatively low 31 m g Brunauer–E
3
ꢀ1
2
2
the micropores is accounted as a major contributing factor to
attain high BET value. However, the micropores present in the
polymers obtained by colloidal technique often remain too small
for the gas molecules to access due to highly interwoven network
3 4 3 4
The FESEM images of B -Azo and Pd-B -Azo showed identical
[
39,40]. Therefore, no identifiable gaseous monolayer can form
during adsorption, lowering the overall surface area. A similar
explanation can be drawn for B -Azo as well. Despite apparently
morphology of both materials (Figs. S2 and S3), while TEM images
confirmed relatively homogeneous distribution of Pd in amor-
phous polymeric nano-sheets (Figs. S4 and S5). Gaining a control
over polymeric particle size in the nano-scale range, typically
within 200–500 nm, has been ascribed to the colloidal synthesis
approach of organic networks. Such methods have previously been
proven as a promising route for attaining desirable particle sizes
[30,43]. Average size of Pd particles were found to be 3.8 nm. The
3
4
low BET surface area, good mesoporosity is generally desirable
for catalytic applications as it allows efficient reactant and product
mass-transport without imposing any steric constrain.
3 4
Successful synthesis of B -Azo network was confirmed by solid
1
3
state C CP/MAS NMR spectroscopy. Appearance of peaks at
d = 151.8 and 90.1 ppm proved the respective presence of CAN
and C„C bonds in the polymer (Fig. S1), while the absence of
any residual signals for unreacted alkynes, which typically appear
at ꢁ80–82 ppm, inferred a high degree of polymerization. Forma-
tion of C„C can also be seen from the weak stretching peaks at
3 4 3 4 2
TGA profile of both B -Azo and Pd-B -Azo obtained under N
atmosphere confirmed their excellent thermal stability till 350 °C
with high residual mass even at 1000 °C (Fig. S6).
2.2. Photophysical and electrochemical properties
ꢀ
ꢀ
1
1
ꢁ
2200 cm
3 4
in the FT-IR spectra of B -Azo , whereas peaks at
ꢁ
3000 cm can be ascribed to typical aromatic C-H stretching
EPR analysis displayed an increase in the Lorentzian signal
intensity upon visible light illumination on both materials, reveal-
ing their substantial photo-responsive character (Figs. S7 and S8).
(
Fig. 1d). Pd was loaded onto this organic network (Pd-B
3
-Azo
4
)
using acetone-water dual solvent technique. ICP analysis showed
ꢀ1
0
.293 mmol g
Pd-B -Azo demonstrated the immergence of new peaks at
1670 and 1080 cm (Fig. 1d). These peaks were postulated to
loading of Pd in Pd-B
3
-Azo
4
. FTIR spectrum of
Accordingly, the steady-state UV–vis absorption spectra of B
3
-
3
4
Azo and Pd-B -Azo also showed high visible light absorption
4
3
4
ꢀ1
ꢁ
cross-sections (Fig. 3a). A significant absorbance tail is attributed
to efficient coupling of monomer and co-monomer leading to
originate from C@O and CAO stretches inferring the possible pres-
ence of acetate moieties in the network. This, in turn, suggests that
Pd might be present in ionic states in the polymer where acetates
remained as counter anions. Disappearance of C„C peaks com-
increased
p–conjugation. Certain decrease in absorption maxima
after Pd-incorporation occurred, inferring possible existence of a
polymer to metal energy transfer process [19]. Interestingly,
despite their well-accounted photo-responsive nature, no fluores-
cence were observed for pristine as well as Pd-loaded networks
after repeated trials with variable excitation wavelengths. The
3 4
pared to parent B -Azo also supports this idea of ionic Pd species,
as these metals ions are well known for their coordination ability
with alkynes [42,43]. The otherwise identical FTIR patterns of both
materials demonstrated the retention of structural integrity of the
polymeric skeleton after metal incorporation.
Oxidation state of Pd was confirmed by XPS analysis where
peaks for Pd(II)3d3/2 and Pd(II)3d5/2 were appeared at 342.7 and
3 4
absence of any fluorescence for B -Azo suggest a feasible inter
system crossing of the excited electron to available low-energy tri-
plet states possessing relatively extended life-span, which would
further ease the chances of polymer to metal energy transfer in
3
37.4 eV respectively (Fig. 2a). This observation supports our initial
3 4
Pd-B -Azo upon photo-illumination [45,48].
Fig. 2. (a) Pd3d and (b) C1s XPS peaks of Pd-B
3 4
-Azo showing the position of each peaks; (c) comparative XPS pattern of N1s peak before (top) and after (bottom) Pd-loading
demonstrating slight shift and decrease in peak area after metal incorporation inferring N-Pd coordination.

1
86
I. Nath et al. / Journal of Catalysis 377 (2019) 183–189
3 4 3 4
Fig. 3. Comparative (a) UV–Vis absorption pattern, and (b) HOMO-LUMO energy state potentials of B -Azo and Pd-B -Azo .
The electrochemical potentials (vs SCE) of the polymers were
obtained by cyclic voltammogram experiments. Conduction band
CB) potentials for the parent and the Pd-incorporated networks
Choosing iodobenzene and phenyl acetylene as coupling part-
ners, successive screening tests were performed to optimize the
standard reaction condition for Pd-B -Azo (Table S1). Only
3 4
(
were calculated as ꢀ0.62 V and ꢀ0.56 V respectively from the
reduction edges, while maximum reduction potentials of ꢀ1.09 V
and ꢀ0.89 V were acquired (Figs. S10 and S12). Similarly, 1.92 V
and 1.93 V respective valance band (VB) potentials were achieved
from oxidation edges (Figs. S9 and S11). Based on these data, a
0.08 mol% catalyst was found enough to furnish the desired pro-
duct in 94% yield at room temperature within 3 h using water as
solvent. We presume that the solvolytic repulsions asserted by
water to the hydrophobic reactants might compel them to prefer-
entially transport to the organic skeleton of the catalyst. While col-
loidal nature of the catalyst can be accounted for its dispersibility
in reaction medium, CTAB played the role of phase transfer
catalyst.
bandgap of 2.54 eV was obtained for B
3 4
-Azo which slightly
decreased to 2.49 eV after Pd loading (Fig. 3b).
To test the substrate scope of our catalyst, a combination of dif-
ferent organo halides and terminal alkynes were subjected to the
standard condition. While bromobenzene resulted the desired pro-
duct in high yield, the catalyst remained tolerant towards different
electron donating and withdrawing substituents as well (Table 1,
Entries 2–6). Moreover, Pd-B -Azo performed successfully even
3 4
when Cl was used as leaving group, though demonstrated compet-
itive preference towards C-I bond cleavage (Table 1, Entries 7–13,
2
.3. Photocatalytic activity
Owing to the suitably oriented VB-CB energy state potentials
and photoactivity of Pd-B -Azo , we examined its catalytic activity
3
4
towards visible-light-induced Sonogashira-Hagihara cross-
coupling reactions. Despite many reports on traditional homoge-
neous and heterogeneous Sonogashira coupling reactions involving
high temperature and environmentally hazardous reaction condi-
tions, examples of photocatalytic green Sonogashira are particu-
larly rare.
1
8). Despite well-known inactivity of aryl chlorides towards tradi-
tional coupling reactions, the ability of our catalyst to successfully
Table 1
3 4
Photocatalytic performances of Pd-B -Azo
for Sonogashira cross-coupling of various organo halides with alkynes a.
0
Yieldb
Entry
R
R
X
Time (h)
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
Ph
4-Me-Ph
4-OMe-Ph
4-CN-Ph
4-F-Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph-CH
Ph-CH
n-Bu
Ph
I
I
I
I
I
Br
Cl
Cl
Cl
Cl
1,4-Cl
I
4-Cl,1-I
1,4-I
I
I
3
5
7
5
5
4
7
12
12
8
24
4
16
7
4
94
94
99
99
98
89
68
63
58
65
53
87
51
75
69
55
52
58
>99
Ph
4-Me-Ph
4-CN-Ph
4-Cl-Ph
4-Cl-Ph
4-Cl-Ph
4-Cl-Ph
4-I-Ph
Ph
Ph
Ph
Ph-CH
Ph
0
1
2
3
4
5
6
7
8
9
c
c
c
2
2
-CH
2
5
12
5
I
Cl
I
2
d
Ph
4
a
Reaction conditions: organo halides, 0.25 mmol; alkynes, 0.25 mmol; K
2
3 3 4
CO , 5 equiv.; Pd-B -Azo , 0.08 mol%; water, 3 mL; CTAB, 0.25 mmol; reaction temperature, 25 °C;
water bath; PL-XQ 350 W Xe lamp with 420 nm UV cut-off.
b
Isolated yields after chromatography.
Phenyl acetylene, 0.5 mmol.
Reaction performed in natural sunlight.
c
d

I. Nath et al. / Journal of Catalysis 377 (2019) 183–189
187
perform Sonogashira cross-coupling of theses substrates with alky-
nes is highly recommendable. However, bis-coupled product was
isolated on subjecting dihalo benzene with 2 equivalent terminal
alkynes (Table 1, Entries 11, 13 and 14). Inspired by excellent cat-
alytic activity, Sonogashira coupling involving more challenging
aliphatic substrates were tried. To our delight, the desired products
were obtained in moderate to good yields in each case (Table 1,
Entries 15–18). No trace of 1,3-diynes, the major side-product of
Sonogashira obtained by Cu-catalysed oxidative homo-coupling
of terminal alkynes, were found under any investigated conditions.
As a visual proof of the clean nature of our reaction protocol, a rep-
resentative reaction mixture of iodobenzene and phenyl acetylene
was analysed at different intervals by gas chromatogram. The com-
parative chromatogram results given in supporting information
neous azobenzenes under photo-illumination [37], the retention
of original electronic properties of our catalyst can be attributed
to: (i) the use of light having higher wavelength (low energy) than
the excitation wavelength of the material, and (ii) the extreme
structural rigidity of the POP, that confines azobenzenes in their
initial trans conformation. TEM image of the reused catalyst also
showed similar Pd distribution in the organic network (Fig. S13).
These studies collectively proved that the integrity of the catalyst
remained under our catalytic condition.
2.5. Mechanism
Control experiments were performed to assess the mechanism
of the reaction. The coupling failed to proceed in the absence of
base, and produced trace amounts of product in absence of light,
signifying important roles of either components (Table S2, Entries
(Fig. S20) shows the appearance of only desired product peak with-
out any additional peaks for by-products.
2
3 4
,3). On the other hand, B -Azo alone could not afford the desired
2
.4. Adaptability of Pd-B
3
-Azo
4
product, while commercial Pd-acetate resulted in only 5% of pro-
duct in due time (Table S2, Entries 4,5). Using Pd/C as supported
commercial catalyst under our standardised condition also
resulted in poor yield (Table S2, Entry 6). These observations con-
firm that neither polymer nor Pd-acetate alone is susceptible to
manifest the preferred reaction; a Pd(II)-polymer composite is
necessary.
To check the practical applicability of Pd-B
3 4
-Azo , a prototypal
Sonogashira coupling between iodobenzene and phenyl acetylene
was probed under natural solar irradiance (Fig. S14). To our plea-
sure, 1,2-biphenyl acetylene was obtained quantitatively within
4
h on a transiently cloudy day with average outside temperature
of 28 °C (Table 1, Entry 19). Furthermore, the effect of light inten-
Judging from these observations, and the photophysical and
sity on overall catalytic activity was also tested probing the same
electrochemical properties of Pd-B
3 4
-Azo , a mechanism involving
2
model reaction. Compared to the initial 0.15 W/cm , when the
conjugated B -Azo ? Pd energy transfer is proposed (Fig. 4). Exis-
3
4
intensity was decreased to 0.10 W/cm2 Pd-B
-Azo
resulted in
tence of similar energy transfer processes have been claimed
recently by other authors as well [17,19]. Upon photo-
illumination, an energy transfer occurs from polymer to Pd. These
high energy Pd centres can attract the electronic clouds of the
3
4
7
5% product yield in 3 h. Continuing the reaction for 3 more hours
ultimately furnished 89% yield of the desired compound. Further
2
decrease of irradiance intensity to 0.05 W/cm led to 43% product
sp
yield at desired time while 78% yield was observed after 12 h reac-
tion period.
alkyne units making the terminal C-H more acidic. The base then
3 4
deprotonates the alkyne to form an alkyne-Pd-B -Azo intermedi-
Intrigued by excellent catalytic ability of Pd-B
interested to check if it can be used for preparation of B
3
-Azo
4
, we were
-Azo net-
ate acting as a secondary photocatalyst. Aromatic alkynes them-
selves being photo-responsive, cases of in situ generated
alkynated-metal complex enacting the role of photosensitizers
have been extensively documented in literature [16,18–20].
Darkening the colour of our reaction mixtures during photo-
irradiation suggests the participation of aromatic alkynes for plau-
sible formation of such photo-active complexes as well [46]. Upon
excitation of this secondary photocatalyst, the high energy Pd cen-
tre forms a transient interaction with the CAX bonds of organoha-
lides making it more labile which reacting with the alkynyl
counterpart ultimately produces the Sonogashira coupling product
[16,18,20].
3
4
work itself from respective monomers under photo-induced condi-
tion. Gladly, the product was obtained as brown insoluble solid
after 3 days in aqueous medium (Scheme 1). To the best of our
knowledge, this is the first ever report of photocatalytic organic
polymer synthesis in aqueous medium using POP as catalyst. The
resultant network was analysed by FTIR and UV–Vis spectroscopy
Figs. S17 and S18) confirming the synthesis of desired structure.
The catalyst was successfully reused five times with negligible
change in reactivity (Fig. S15). XPS analysis of recycled Pd-B
(
3
-
Azo demonstrated retention of Pd(II) oxidation state (Fig. S16),
4
while ICP analysis confirmed almost no change in Pd concentra-
tion. It is noteworthy to mention that despite some literature
reports on the reduction of transition metal cations by homoge-
It is evident that the CB potential of the catalyst is not sufficient
enough to cleave the C-X bonds completely, generating aromatic
free radicals and halide anions. Among the organohalides used as
Scheme 1. Synthesis of
B
3
-Azo
4
by visible-light-induced Sonogashira protocol using Pd-B
3 4
-Azo in aqueous medium. Reaction condition: 1 eqv. (E)-1,2-bis(3,5-
dibromophenyl)diazene, 1.3 eqv. triethynylbenzene, 3 mg Pd-B
3
-Azo , 0.1 mmol CTAB, aqueous medium, visible light, room temperature, 3 day.
4

1
88
I. Nath et al. / Journal of Catalysis 377 (2019) 183–189
3 4
Fig. 4. Proposed mechanism of the photo-induced Sonogashira coupling catalyzed by Pd-B -Azo .
substrates, p-cyanoiodobenzene possessed the lowest reduction
potential ꢀ1.89 V vs SCE [47], which is still higher than the LUMO
3 4
The colloidal organic network, B -Azo , appeared as small nano-
sized particles demonstrated complete mesoporosity. Homoge-
neously distributed Pd(II) strongly coordinated to polymer back-
of Pd-B
3
-Azo
4
(ꢀ0.56 V vs SCE). Ergo, we postulate that upon acety-
lation, the in situ generated catalyst becomes suitable to tran-
siently ‘‘activate” the C-X bonds for further reactions [16,18,20].
bone was prepared resulting Pd-B
demonstrated substantial visible-light response, while existence
of B -Azo ? Pd energy transfer upon photo-illumination was pro-
posed. Using such photo-triggered energy transfer process, visible
light mediated Sonogashira coupling catalyzed by Pd-B -Azo was
3
-Azo
4
.
Both materials
ꢀ
An additional test was made to confirm the generation of I in
3
4
reaction medium as an indirect proof of our said mechanism. As
shown in Fig. S25, when the reaction mixture containing iodoben-
3
4
zene and phenyl acetylene as substrates is added to a H
acid solution, brown coloration was observed confirming the gen-
eration of I . This mechanistic explanation accords well with rela-
2
O
2
/acetic
targeted. The catalytic protocol remained suitable for successful
coupling of organo halides (I, Br, Cl) having different electron
donating and withdrawing substitutions, and alkynes possessing
both aromatic and aliphatic chains. Consulting relevant literatures,
a plausible mechanism for the catalysis was also suggested. Identi-
cal catalytic activity was retained in natural solar irradiance as
well.
2
tively low reaction yields observed for aliphatic alkynes, as the
alkynated intermediates originated in those situations did not pos-
sess any additional secondary photoactivity. Subjecting p-benzo-
quinone as an electron scavenger in the reaction of iodobenzene
and phenyl acetylene with an expectation that it might block the
formation of highly energetic Pd species, a mere 23% yield of biphe-
nyl acetylene was obtained in due time endorsing our proposed
mechanism (Table S2, Entry 8).
Acknowledgement
The authors would like to acknowledge State Key Laboratory of
Advanced Technology for Materials Synthesis and Processing,
Wuhan University of Technology for the financial support. F.V.
acknowledges the support from the Tomsk Polytechnic University
Competitiveness Enhancement Program grant (VIU-2019).
To elucidate the role of azo-group in POP skeleton, another pre-
viously reported network CMP-1 containing only benzene units
was prepared and loaded with Pd(II) [35,43]. Owing to the poor
3
coordination environment offered by CMP-1 compared to B -
ꢀ1
4
Azo ,
only 0.103 mmol g
loading of Pd(II) was observed
(
Fig. S21). Subjecting this material to our standard catalytic condi-
Appendix A. Supplementary material
tion probing the reaction between iodobenzene and phenyl acety-
lene as a model, only 13% product was obtained within 3 h
(
alytic run showed no trace of Pd as confirmed from the XPS and
TEM results (Figs. S22–S24). ICP analysis of reused Pd-CMP-1 also
revealed that the metals have mostly leached out of system as only
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.jcat.2019.07.031.
Table S2, Entry 7). Moreover, the polymer collected after first cat-
References
8
2 ppm Pd-concentration was calculated. These experiments all
[
1] K. Shekarrao, P.P. Kaishap, S. Gogoi, R.C. Boruaha, Adv. Synth. Catal. 357 (2015)
1187.
together proved that the azo-group played important role in Pd-
coordination as well as in endowing enhanced photo-responsive
activity in the polymer network.
[2] B.-N. Lin, S.-H. Huang, W.-Y. Wu, C.-Y. Mou, F.-Y. Tsai, Molecules 15 (2010)
157.
[
9
3] R.R. Tykwinski, Angew. Chem. Int. Ed. 42 (2003) 1566.
[
4] J.-H. Li, J.L. Li, D.-P. Wang, S.-F. Pi, Y.-X. Xie, M.-B. Zhang, X.-C. Hu, J. Org. Chem.
72 (2007) 2053.
3
. Conclusion
[5] S. Cacchi, G. Fabrizi, Chem. Rev. 105 (2005) 2873.
[
[
[
6] R. Chinchilla, C. Najera, Chem. Soc. Rev. 40 (2011) 5084.
7] R. Chinchilla, C. Najera, Chem. Rev. 107 (2007) 874.
8] F. Monnier, F.O. Turtaut, L. Duroure, M. Taillefer, Org. Lett. 10 (2008) 3203.
In conclusion, a new photo-active conjugated porous organic
polymer having azo-benzene in its backbone was synthesized.
[9] P. Saejueng, C.G. Bates, D. Venkataraman, Synthesis 10 (2005) 1706.

I. Nath et al. / Journal of Catalysis 377 (2019) 183–189
189
[
[
[
10] E. Zuidema, C. Bolm, Chem. - Eur. J. 16 (2010) 4181.
11] D.J. Cole-Hamilton, Science 299 (2003) 1702.
12] R. Bernini, S. Cacchi, G. Fabrizi, G. Forte, F. Petrucci, A. Prastaro, S. Niembro, A.
Shafird, A. Vallribera, Green Chem. 12 (2010) 150.
[34] N. Kang, J.H. Park, K.C. Ko, J. Chun, E. Kim, H.-W. Shin, S.M. Lee, H.J. Kim, T.K.
Ahn, J.Y. Lee, S.U. Son, Angew. Chem. Int. Ed. 52 (2013) 6228.
[35] I. Nath, J. Chakraborty, P.M. Heynderickx, F. Verpoort, Appl. Catal. B: Environ.
227 (2018) 102.
[
[
[
[
[
[
[
[
[
13] M.J. Jin, D.H. Lee, Angew. Chem. Int. Ed. 49 (2010) 1119.
14] X.J. Lang, X.D. Chen, J.C. Zhao, Chem. Soc. Rev. 43 (2014) 473.
15] S. Sarina, E.R. Waclawik, H.Y. Zhu, Green Chem. 15 (2013) 1814.
16] A. Elhage, A.E. Lanterna, J.C. Scaiano, ACS Sustain. Chem. Eng. 6 (2018) 1717.
17] M. Osawa, H. Nagai, M. Akita, Dalton Trans. (2007) 827.
18] A. Sagadevan, K.C. Hwang, Adv. Synth. Catal. 354 (2012) 3421.
19] H. Deol, S. Pramanik, M. Kumar, I.A. Khan, V. Bhalla, ACS Catal. 6 (2016) 3771.
20] Y. Li, X. Feng, Z. Li, Catal. Sci. Technol. 9 (2019) 377.
[36] J. Chakraborty, I. Nath, F. Verpoort, Data Brief 21 (2018) 1157.
[37] H. Nishihara, Coord. Chem. Rev. 249 (2005) 1468.
[38] B. Tylkowski, A. Trojanowska, V. Marturano, M. Nowak, L. Marciniak, M.
Giamberini, V. Ambrogi, P. Cerruti, Coord. Chem. Rev. 351 (2017) 205.
[39] K. Zhang, Z. Vobecka, K. Tauer, M. Antonietti, F. Vilela, Chem. Commun. 49
(2013) 11158.
[40] Z.J. Wang, S. Ghasimi, K. Landfester, K.A.I. Zhang, Chem. Commun. 50 (2014)
8177.
[41] M. Thommes, K. Kaneko, A.V. Neimark, J.P. Olivier, F. Rodriguez-Reinoso, J.
Rouquerol, K.S.W. Sing, Pure Appl. Chem. 87 (2015) 1051.
[42] T. Hasell, C.D. Wood, R. Clowes, J.T.A. Jones, Y.Z. Khimyak, D.J. Adams, A.I.
Cooper, Chem. Mater. 22 (2010) 557.
21] Q. Xiao, S. Sarina, A. Bo, J. Jia, H. Liu, D.P. Arnold, Y. Huang, H. Wu, H. Zhu, ACS
Catal. 4 (2014) 1725.
[
22] B. Wang, Y. Wang, J. Li, X. Guo, G. Bai, X. Tong, G. Jin, X. Guo, Catal. Sci. Technol.
8
(2018) 3357.
[
[
[
[
[
[
[
[
[
[
[
23] K. Balaraman, V. Kesavan, Synthesis (2010) 3461.
[43] P. Zhang, Z. Weng, J. Guo, C. Wang, Chem. Mater. 23 (2011) 5243.
[44] S.-Y. Ding, J. Gao, Q. Wang, Y. Zhang, W.-G. Song, C.-Y. Su, W. Wang, J. Am.
Chem. Soc. 133 (2011) 19816.
[45] K. Zhang, D. Kopetzki, P.H. Seeberger, M. Antonietti, F. Vilela, Angew. Chem.
Int. Ed. 52 (2013) 1432.
[46] S. Protti, M. Fagnoni, A. Albini, Angew. Chem. Int. Ed. 44 (2005) 5675.
[47] S. Ghasimi, S.A. Bretschneider, W. Huang, K. Landfester, K.A.I. Zhang, Adv. Sci. 4
(2017) 1700101.
[48] W. Wu, S. Xu, G. Qi, H. Zhu, F. Hu, Z. Liu, D. Zhang, B. Liu, Angew. Chem. Int. Ed.
58 (2019) 3062.
24] D. Gelman, S. Buchwald, Angew. Chem. Int. Ed. 42 (2003) 5993.
25] M. Aufiero, F. Proutiere, F. Schoenebeck, Angew. Chem. Int. Ed. 51 (2012) 7226.
26] K. Heuze, D. Mery, D. Gauss, D. Astruc, Chem. Commun. 18 (2003) 2274.
27] X. Pu, H. Li, T.J. Colacot, J. Org. Chem. 78 (2013) 568.
28] N. Li, R.K. Lim, S. Edwardraja, Q. Lin, J. Am. Chem. Soc. 133 (2011) 15316.
29] E. Negishi, L. Anastasia, Chem. Rev. 103 (2003) 1979.
30] J. Chakraborty, I. Nath, F. Verpoort, Chem. Eng. J. 358 (2019) 580.
31] G. Zhang, Z.-A. Lan, X. Wang, Angew. Chem. Int. Ed. 55 (2016) 15712.
32] A.I. Cooper, Adv. Mater. 21 (2009) 1291.
33] L. Chen, Y. Honsho, S. Seki, D. Jiang, J. Am. Chem. Soc. 132 (2010) 6742.