Agro waste derived nanosilica supported Pd(ll) complex: A protocol for copper free Sonogashira reaction in water

Article abstract of DOI:10.1016/j.jorganchem.2019.06.015

A palladium (II) complex immobilized onto nanosilica(Pd-imine@nanoSiO₂) has been developed and evaluated as a highly efficient, retrievable catalyst for carbon-carbon triple bond activation reactions between aryl halides and terminal alkynes. Nanosilica has been derived from rice husk by simple and eco-compatible methodology. The catalyst has been extensively characterized by techniques such as FT-IR, UV–vis, powder XRD, XPS, SEM-EDX, thermogravimetric analysis, BET surface area measurement. The catalyst can be reused for five consecutive runs without compromising much with the activity. Easy preparation, its long shelf life, air-stability, wide substrate scope, ‘in water’ reactions, easy separability and good recyclability make it an ideal system for Sonogashira cross-coupling reaction. Moreover, various alkyne substrates were efficiently cross-coupled with a broad range of aryl iodides and aryl bromides to afford diaryl alkynes, providing improved yields with low catalyst loading in water. This protocol is also suitable for aliphatic alkynes.

Full text of DOI:10.1016/j.jorganchem.2019.06.015

Accepted Manuscript
Agro waste derived nanosilica supported Pd(ll) complex: A protocol for copper free
Sonogashira reaction in water
Rajjyoti Gogoi, Rituraj Saikia, Geetika Borah
PII:
S0022-328X(19)30254-2
DOI:
https://doi.org/10.1016/j.jorganchem.2019.06.015
JOM 20827
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 7 December 2018
Revised Date: 11 June 2019
Accepted Date: 14 June 2019
Please cite this article as: R. Gogoi, R. Saikia, G. Borah, Agro waste derived nanosilica supported Pd(ll)
complex: A protocol for copper free Sonogashira reaction in water, Journal of Organometallic Chemistry
(2019), doi: https://doi.org/10.1016/j.jorganchem.2019.06.015.
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Agro Waste DerivedNanosilica SupportedPd(ll) Complex: A Protocol for
Copper Free Sonogashira Reaction in Water
RajjyotiGogoi, RiturajSaikia&Geetika Borah*
Department of Chemistry, Dibrugarh University, Dibrugarh, Assam, India
email: geetikachem@yahoo.co.inOrcid Id:orcid.org/0000-0002-5781-1694
Abstract: A palladium (II) complex immobilized onto nanosilica(Pd-imine@nanoSiO2) has been
developed and evaluated as a highly efficient, retrievable catalyst for carbon-carbon triple bond
activation reactions between aryl halides and terminal alkynes. Nanosilica has been derived from
rice husk by simple and eco-compatible methodology. The catalyst has been extensively
characterized by techniques such as FT-IR, UV-vis, powder XRD, XPS, SEM-EDX,
thermogravimetric analysis, BET surface area measurement.The catalyst can be reused for five
consecutive runs without compromising much with the activity.Easy preparation, its long shelf
life, air-stability, wide substrate scope, ‘in water’ reactions,easy separability and good
recyclability make it an ideal system for Sonogashira cross-coupling reaction. Moreover, various
alkyne substrates were efficiently cross-coupled with a broad range of aryl iodides and aryl
bromides to afford diaryl alkynes, providing improved yields with low catalyst loading in water.
This protocol is also suitable for aliphatic alkynes.
Keywords:Aliphatic alkyne; Aryl halide; Diaryl alkyne; Sonogashira cross-coupling;Terminal
alkyne;Thermogravimetric.
Introduction:
Sonogashira reaction belongs to the most powerful tool for the construction of C(sp)-C(sp2) bond
allowing the synthesis of different acetylene derivatives such as, pharmaceuticals, polymers,
dyes , sensors, etc.[1-4]The acetylene derivatives find tremendous applications in material
chemistry due to their unique electrical and optical properties.Therefore, remarkable efforts have
been devoted to improve the synthetic methodologies of the reaction and also to increase its
efficiency. Traditionally, this reaction was catalyzed by palladium salt in the presence of
different ligands viz. phosphine based, N-based, Schiff-base, etc. along with a copper co-catalyst
in an amine as a solvent. In the recent decades, however, several copper-free Sonogashira
reaction methodologies have been proposed, since from environmental perspectives copper-
mediated protocol is no longer viable as it produces a significant amount of unwanted homo

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coupling product of the terminal alkynes along with the desired product and needs an extra non
environment friendly chemical CuX.[5-10]As a result several alternative methodologies were
proposed time to time including designing of different ligand systems, use of additives, viz.
tetra-n-butylammonium salt, sodium sulphide, Ag, Zn etc. However, most of these protocols
were homogeneous in nature with advantages of higher yields but with distinct drawbacks like
non-reusability, contamination of the products, catalyst loss and tedious workup procedures.
These prompted researchers to develop ecofriendly, cheap and reusable heterogeneous
systems.[11]Till now various heterogeneous catalysts such as silica, clay, zeolites and SBA-15
immobilized Pd complex/ Pd(0)nanoparticles ;bimetallic Pd-nanoparticles; polymer-supported
N-heterocyclic carbene–palladium catalyst; poly(vinyl chloride)-supported Pd(II) complex;
Pd(II) Schiff-base complex supported on multiwalled carbon nanotubes; Pd(II) supported on
MCM-41 were reported for Sonogashira reactions.[12-16]Although significant advancements
have been made, as per our literature review bio-derived silica supported Pd(II) complex has not
been reported as catalyst for Sonogashira reaction till date. However, because of some specific
characteristics like low cost, wide availability, high thermal stability and surface reactivity, high
surface area, silica is one of the best candidates as solid support material.[17-22] Silica is found
in various natural sources with different proportions. Agro waste such as rice husk ash is one of
the cheapest and most eco-friendly sources of silica with 90-97%silica content.[23]So extraction
of silica particle from rice husk ash benefits not only of producing valuable silica powder but
also of reducing disposal and pollution problems.
Recently we have reported the efficiency of rice husk derived nanosilicasupported Pd(II) and
Ru(III)complexes and montmorillonite K-10 supportedPd(0) nanoparticles in Suzuki–Miyaura,
hydration of nitriles and selective oxidation of benzyl alcohols reactions, respectively.[23-24]So
in order to extend the scope of bio-derived nanosilicaanchoredPd(II) complex, here we have
reported a simple and efficient protocol for the Sonogashira reaction of aryl iodides in water
under mild conditions .
Experimental
Materials and Methods

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All the chemicals were commercially obtained and used as received. The substrates for
Sonogashira reactions, bases and solvents were purchased from Merck, Sigma Aldrich,
LobaChemie and Rankem and 3-aminopropyltriethoxysilane (APTES) and palladium acetate
were from Sigma-Aldrich. Silica gel (particle size 60–120 mesh) used for column
chromatography was purchased from Rankem, India.
FT-IR spectra were recorded using KBr pellets on a Shimadzu IR Prestige-21 FT-IR
spectrophotometer(400-4000cm-1). UV-vis (200-800nm) spectra were recorded on a Jasco V-750
spectrometer in solid state. The powder XRD patterns were recorded on RikaguUltima IV
diffractometer with Cu-Kα (λ=1.541 Å) radiation. The SEM images were obtained with a JEOL,
JSM IT-300 operating at an accelerating voltage of 20 kV. The EDX spectra were also recorded
in the same instrument attached to the scanning electron microscope. The surface area
measurement at liquid nitrogen temperature was obtained from the linear plot following
Brunauer-Emmett-Teller (BET) method using Quantachrome instrument (Boynton Beach, FL
33426, USA).The X-ray photoelectron spectrum (XPS) of (Pd-imine@nanoSiO2) was recorded on
a XPS-AES Module, Model: PHI 5000 Versa Prob II. The C (1s) electron binding energy
corresponding to graphitic carbon was used for calibration of the Pd (3d) core-level binding
energy. Thermogravimetric analyses (TGA) were done with a Perkin Elmer STA 8000 thermal
analyzer in the temperature range 35 ºC-850 ºC at a heating rate 20 ºC min-1 in air. The palladium
content of the catalyst was determined by Inductively Coupled Plasma Atomic Emission
Spectrometric (ICP-AES) analysis with a ARCOS, Simultaneous ICP Spectrometer at SAIF, IIT
Bombay. The reaction products were characterized by recording1H NMR spectra and comparing
with authentic sample.
Synthesis of materials
Synthesis of nanosilica: From green chemistry and eco-compatibility point of view, the use of
waste material is one of the major concerns in recent time. Keeping this in mind, we have used
rice husk derived nanosilica[17] for immobilization of the Pd (II) complex. This process mainly
involves the following steps and can be schematically represented as shown in Scheme 1.[25]
Synthesis of Pd-imine@nanoSiO2catalyst
Step I: Immobilization of APTES onto nanosilica( APTES@nanoSiO2):4g of nanosilica
(dried at 250ºC for 20 h) was added to a 100 mL of dry toluene followed by addition of APTES

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(0.04g, 2.0 mmol). The resulting mixture was refluxed under N2 for 6 h, followed by cooling,
whereupon a solid product was formed. It was separated by filtration, washed repeatedly through
Soxhlet extraction with toluene and finally dried at 120oC for 24 h to obtain APTES immobilized
nanosilica.
Step II: Synthesis of imine funtionalizednanosilica [imine@nanoSiO2]: In a suspension of 2.3
g of APTES@nanoSiO2 in 60mL ethanol, 0.033g (0.21 mmol) of quinoline-2-carboxyaldehyde
was added and was refluxed for 6 h. A solid precipitated was formed, which was filtered off, and
washed several times through Soxhlet extraction with ethanol and acetone. The resulting
compound was dried at 120oC for 24 h and designated as imine@nanoSiO2.The amount of the
imine ligand attached to the imine@nanoSiO2 was measured from the TGA analysis and was
found to be 2.14 mmol/g.
StepIII: Immobilizationof Pd metal onto imine funtionalizednanosilica [Pd-
imine@nanoSiO2]:0.2 g of imine@nanoSiO2 and0.060 g (0.026mmol) of Pd(OAc)2were added
in 40 mL acetone and stirred at room temperature for 24 h. The product was filtered off and
washed through soxhlet extraction with acetone. The product was dried in an oven at 70⁰C for
24h, whereupon a light brown color material was obtained and was designated as Pd-
imine@nanoSiO2.The palladium content of Pd-imine@nanoSiO2 based on ICP analysis was
estimated to be 0.9 mol % per 10mg.
Scheme 1: Synthesis of Pd-imine@nanoSiO2
Procedure for Sonogashira Cross-Coupling reaction

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In a typical reaction, a mixture of alkyne (1.1mmol),aryl halide(1.3mmol), amine (2 mmol),
catalyst (10mg, 0.9 mol% of Pd ) were added in water (4mL) and stirred at 70⁰C for required
time.After completion of the reaction (monitored by GC-MS at different time intervals) it was
cooled to room temperature and the catalyst was separated by simple centrifugal precipitation.
The filtrated was diluted with water and extracted with 15mL acetic acid. To obtain the desired
product the organic phase was washed with brine(2×10mL) and dried over anhydrous Na2SO4,
then filtered and evaporated under reduced pressure. The products were isolated by silica gel
1
column chromatography using hexane as eluent. The FT-IR and H NMR data of isolated
products were compared with previously reported literatures.[26-27]To check the reusability of
the catalyst, the recovered catalyst was dried overnight at 100
reaction under identical conditions.
⁰C and reused in a new coupling
Results and Discussion
The X-ray diffraction pattern of the complex (Fig 1) is in good agreement with the reported
literatures.[28-30]The PXRD of silica exhibits peaks at 2 value of 19.26°, 29.82° , 39.62° ,
θ
49.04° and 60.68° corresponds to (100), (101), (012), (112) and (211) planes of hexagonal unit
cell[29,31].The XRD patterns of the silica, nanosilica and the Pd-imine@nanoSiO2 revealed that
crystalline nature of silica changes to amorphous in both nanosilica and the complex. A broad
peak corresponding to the silica phase was observed at 22⁰ for both the materials, nanosilica and
complex [25].The XRD patterns of nanosilica and the complex are similar, however a prominent
decrease in peak intensity for the complex is consistent with the immobilization of Pd and ligand
onto the nanosilica as reported by Komura et al.[29].

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Fig.1a: XRD pattern of silica
Fig.1b :XRD patterns of nanosilica and Pd-imine@nanoSiO2
FT-IR study:The FTIR spectra of nanosilicaand silica are almost similar, however the bands for
υSi-O-Si, γSi-O-Si, υOH and γOH are shifted slightly either higher or lower frequency compared to that
of silica(Fig 2a& 2b)[23,24]. The spectrum of APTES@nanoSiO2 demonstrated a peak at 1621
cm-1 due to γNH indicating immobilization of APTES on nanosilica(Fig 2c) .The FTIR spectrum
2
of imine@nanoSiO2 showed a new band at 1600 cm-1 which could be assigned to νC=N, suggesting
the formation of Schiff base anchored nanosilica (Fig 2d). Interestingly, it was observed that this
band was shifted from 1600 cm-1 to 1528cm-1on complexation with Pd(II)( Fig 2e). In the far-IR
region a band at 455 cm-1(νPd-O) and 585 cm-1 (νPd-N) clearly suggest that Pd is coordinated
through N-and O-atom[23,24].

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Fig. 2 : FT-IR spectra of (a) silica, (b) nanosilica, (c) APTES@nanoSiO2, (d) imine@nanoSiO2,
(e) Pd-imine@nanoSiO2
BET analysis: With the exception of nanosilica, the surface area of silica, APTES@nanoSiO2and
Pd-imine@nanoSiO2 decreases successively, indicating successful immobilization of APTES,
imine and Pd onto the silica. As compared to the catalysts reported by Nikoorazm et al. and
Komura et al., we have observed higher decrease of surface area of nanosilica with the successive
immobilization of the ligand and Pd metal[28,29].

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Table 1: BET surface area measurements of the silica based materials
Entry
Materials
Surface Area(m2/g)
1
2
3
4
Silica
186
280
135
111
Nanosilica
APTES@nanoSiO2
Pd-imine@nanoSiO2
SEM-EDX and ICP analysis: Surface morphology of nanosilica was changed after
immobilization of APTES, imine and Pd onto nanosilica (Fig 3a & Fig 3b).EDX patterns of
nanosilica and Pd-imine@nanoSiO2(Fig 3c & Fig 3d) invariably showed the presence of Pd, N
elements and suggested the successful grafting of Pd complex with the anchored ligand. ICP-
AES analysis revealed 0.9 mol% of Pd content per 10mg of Pd-imine@nanoSiO2.
Fig. 3a: SEM image of nanosilica
Fig. 3b: SEM image of Pd-imine@nanoSiO2

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Fig. 3c: EDX of nanosilica
X-ray photoelectron spectroscopy study:
Fig. 3d: EDX of Pd-imine@nanoSiO2
The XPS study for Pd 3d core of Pd-imine@nanoSiO2 is demonstrated in Fig 4. The catalyst
exhibited two peaks at binding energy 337.4eV and 342.6eV, could be assigned to the Pd2+ 3d
core-level peaks corresponding to the palladium 5/2and 3/2 spin-orbit components, respectively.
This enumerates the presence of Pd(II)in the complex.[11,28,29,32]
Fig 4: Pd 3d core level XPS spectrum Pd-imine@nanoSiO2
TGA: The TGA analysis of nanosilica,APTES@nanoSiO2 and imine@nanoSiO2 revealed a
initial weight loss of approximately 4.5, 5.3 and 4.6%, respectively in the temperature range 100-
250°C, could be attributed to adsorbed water(Fig S1-S3). [28,33]With further increase in
temperature the APTES@nanoSiO2 and imine@nanoSiO2 showed 3.80 and 5.94% wt loss
corresponding to the decomposition of APTES and APTES- imine, respectively. The quantity of
imine ligand attached to the Pd was measured from the TGA analysis and was found to be 2.14
wt% or 0.013 mmol in the catalyst (ESI).

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UV-vis spectra: The electronic spectrum of the complex was recorded in the range 200-800nm
in solid state. The bands observed in the region 220-250 nm and 355 nm could be assigned to the
intraligand transitions (Fig 5).
Fig 5: Solid UV-vis spectrum for Pd-imine@nanoSiO2
Pd-imine@nanoSiO2 Catalyzed Sonogashira Cross-Coupling reaction:
After characterization of the complex extensively we want to check the potentiality of the
complex as a catalyst for Sonogashira cross-coupling reaction with various alkynes and aryl
halides.For this purpose a model reaction was carried out between phenylacetylene and
iodobenzene at different temperatures using triethylamine as an additive in copper free condition.
As we know solvent plays an important role in product formation we have screened various
solvents for the reaction. From the table it is clear that water is the most suitable solvent for the
reaction as other solvents gave comparatively low percentage yield (table 2, entry1). Water may
readily solvate other substrates present in the reaction medium due to high polarity index. We
have also performed the reaction at different temperatures, viz. room temperature (25 ºC), 50 ºC,
70 ºC, 80 ºC, 100 ºC and observed that80 ºC was the optimum temperature for better product
yield (table 2, entry1). Likewise, a number of runs were performed with a various amounts of Pd
loadings such as 0.45, 0.9, 1.08, 1.35, 1.8 and 2.25 mol% and found that 0.9 mol% of Pd was the
sufficient amount to produce the highest yield in water (table 2, entry1). It was clear that increase
of Pd loading did not improve the product yield, while decrease of Pd loading to 0.45 mol% led

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to decrease the yield significantly (table 2, entries 13-17). In the absence of catalyst and solvent
only trace amount of product was formed (table 2, entries 9, 10 & 11).
Table2:Optimization of the reaction conditions for the Sonogashira reaction of phenyl acetylene
and iodobenzene using Pd-imine@nanoSiO2 as a catalyst a
Catalyst
Isolated yield(%)
⁰
Entry
1
Solvent
H2O
(mg; Pd
loading)
10
Temperature( C)
Time (h)*
(0.9mol%
Pd)
80
2
95
2
3
4
5
6
7
8
Isopropanol
DMF
10 (
″
)
80
2
2
65
55
10(
10(
10(
10(
10(
10(
″
)
80
EtOH
″
″
″
″
″
)
)
)
)
)
80
4
50
CH3CN
EtOH/H2O
H2O
80
2
70
100
25 (r.t.)
50
3
85
24
4
trace
60
H2O
9
H2O
H2O
-
-
100
100
100
70
2
trace
trace
trace
80
10
11
12
-
4
10(
″
)
24
4
H2O
10(
″)
5(0.45
mol% Pd)
12(1.08
13
H2O
80
5
65
14
15
H2O
H2O
80
80
2
2
95
95
mol% Pd)
15(1.35

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12
mol% Pd)
20(1.80
80
16
17
H2O
H2O
2
2
95
95
mol% Pd)
25(2.25
80
mol% Pd)
aReaction conditions: Aryl halide (1.3 mmol), phenyl acetylene (1.0 mmol), triethylamine (2.0
mmol), catalyst (Pd-imine@nanoSiO2) and solvent (3ml) .
*all reactions were carried out for 24h however, the time required to get maximum conversion is
reported here.
Previous studies suggested that bases play a significant role in absorption of hydrogen halide in
this reaction as well as inhibits the formation of homo-coupling product[34]. The reaction did
not occur in absence of any base (Table 3, entry 5). We have examined both organic and
inorganic bases like K2CO3, NaHCO3, NaOH, Et3N and water extract of banana (WEB) to find
out the most efficient base for the reaction and found Et3N as the most efficient base for the
reaction (Table 3, entry 4).This might be due to the fact that amines trap hydrogen halide readily
compared to inorganic bases[35-36] . The results are summarized in the Table-3.

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Table 3: Optimization of bases for the Sonogashira cross-coupling reactiona
Entry
Base
Isolatedyield(%)
1
2
3
4
5
6
K2CO3
NaHCO3
NaOH
Et3N
60
55
50
95
-
-
WEBb
30
aReaction conditions: iodobenzene(1.0 mmol),phenyl acetylene(1.1 mmol)], H2O(2 ml), Pd-
imine@nanoSiO2 (10mg ; 0.9 mol% of Pd) at 80
C for 2 h; b Water extract of banana (WEB)
⁰
To widen the scope of the reaction we have carried out Sonogashira cross-coupling reaction
between a wide range of substrates with Pd-imine@nanoSiO2 as the catalyst. Water as a solvent
tolerates very hardly both aryl iodides and aryl bromides as substrate to give good yield, however
our catalytic system showed high tolerance for both aryl iodides and aryl bromides. Aryl
bromides gave comparatively lower isolated yield than the aryl iodides (Table 4, entries 6-10 &
16-20) and aryl iodides with electron donating groups gave higher yield compared to electron
withdrawing groups (Table 4, entries 4,5&15).Aliphatic alkynes coupled less effectively with
aryl halides and afforded low yields, might be attributed to the presence of less acidic terminal
hydrogen which results slow coordination of alkyneto aryl palladium intermediate to form aryl
alkynyl palladium species (Table 4, entries 20-25).
Table 4: Sonogashira cross- coupling reactions of different aryl halides and alkynes in watera
Entry
Aryl halide
Product
Isolated yield

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14
Alkyne
(%)
1
I
C
C
85
80
HC
HC
2
3
O2N
I
I
C
C
75
92
HC
HC
O2N
I
4
I
5
6
7
8
C
C
C
C
96
85
87
85
HC
HC
Br
Br
HC
HC
HC
Br
O
Br
9
C
C
88
77
H2N
O2N
Br
10
HC
11
12
13
90
85
87
I
O2N
I
I
O2N

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15
I
14
15
16
17
18
19
85
90
80
82
77
81
I
Br
Br
Br
O
Br
H2N
O2N
Br
20
21
22
81
78
80
8
8
8
I
O2N
I
I
23
78
8
O2N
Br
24
25
73
75
8
8
Br
aReaction conditions: aryl halide(1.0 mmol), alkyne(1.1 mmol) , Pd-imine@nanoSiO2(10mg ;
0.9 mol% of Pd) ,Et3N(2.0 mmol), H2O (2ml) .

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Catalyst leaching and reusability of the catalyst
Hot filtration test
We have carried out a hot filtration test to know whether these catalytic reactions are truly
heterogeneous or not by using phenyl acetylene and iodobenzene in the presence of Pd-
imine@nanoSiO2 catalyst (10mg; 0.9mol% of Pd) and stirred at 80 ⁰C. After 30 min the reaction
was stopped and the catalyst was filtered off (55% yield found from GC-MS). The reaction was
allowed to progress without the catalyst for another 1and1/2 h, which eventually showed no
increase in product yield. This clearly suggested non-leaching of Pd during reaction which was
further confirmed by ICP-AES analysis with the filtrate.
Reusability test
Retrievability of a catalyst is of utmost concern to evaluate the efficiency of the catalyst as well
as from the economic point of view. We therefore conducted the recyclability experiment by
taking iodobenzene and phenyl acetylene as coupling partners with10 mg (0.9 mol% of Pd) of
the catalyst. After completion of the reaction (monitored by GC-MS) the catalyst was separated
with centrifugal precipitation and dried at temperature 120⁰C in an oven for 24h. Then the
catalyst was reused in a new coupling reaction .To our delight the catalyst showed good
recyclability for five consecutive runs without significant loss in activity (Fig 6).
To check the chemical change of the catalyst during subsequent runs we have performed a SEM-
EDX analysis of the catalyst recovered after the fifth catalytic cycle (Fig 7a & 7b). It revealed
that the catalyst retains its initial chemical composition even after 5th catalytic cycle.

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Fig 6: Reusability of Pd-imine@nanoSiO2 in the Sonogashira cross-coupling reaction
Reaction conditions: Catalyst (10 mg;0.9 mol %);iodobenzene (6.3 mmol); phenyl acetylene (6.0
mmol);(Et)3N (12mmol); H2O (9 ml); temperature: 80 ◦C.
Fig.7a: SEM of catalyst after 5th cycle Fig.7b: EDX spectrum of the catalyst

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A plausible mechanism for the catalytic performance of Pd-imine@nanoSiO2 in copper free
condition for Sonogashira reaction is demonstrated in Scheme 2.In analogy with the earlier
reports , the mechanism initially may be started with the activation of Pd-imine@nanoSiO2 and
generating acetate free Pd(0) real catalytic species I in basic medium[4]. This was followed by
the oxidative addition of aryl halide to the species I through the oxidation of Pd(0) to Pd(II). The
coordination of terminal alkyne to intermediate II producing palladium acetylide complex III .
At the final step Sonogashira reaction product was released through the reductive elimination of
complex III with the simultaneous release of the active catalyst for the next catalytic cycle. To
confirm the nature of the real catalytic species we have recorded TEM image of the catalyst after
completion of the reaction, which clearly showed the formation of Pd(0) nanoparticles ( Fig S4).
Scheme 5: The probable mechanism of copper free Sonogashira reaction
To know the actual catalytic performance of our catalyst with respect to the reported ones we
have performed a literature survey (Table 5) and our catalyst was found to be effective with
wide substrate scope at mild reaction conditions.
Table 5: Comparative study on Sonogashira cross-coupling reactions with different catalytic
systems
Catalyst
Reaction Conditions
Yield/Conversion
Literature reference
(%)
Pd(II)Cl2-
DIPEA, DMF/H2O,RT,
80-95
[4]

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BTP@TMSP-nSiO2
Nano-Pd(0)/SDPP
2-3h,0.15mol%
K2CO3, PEG 200,
50-91
[3]
⁰
100 C, 1.5-4
h,0.5mol%
Hexamine, H2O, reflux,
10-15 h,0.52 mol%
(Et)3N,H2O,reflux,4-
12h, 0.085mol%
Piperidinen,NMP,800C,
3h,1.8mol%
Pd-LHMS-3
70-90
82-96
49-100
92-97
70-96
45-80
64-92
71-96
85-93
50-97
[2]
[5]
nSiO2-Dendrimer-
Pd(0)0
Pd-2QC-MCM
[29]
Pd-ABA-MCM-41
CuNPs@MP-3
K2CO3, PEG 200,
60oC,1-7h,1.6 mol%
Et3N, H2O, 400C, 8h,1
mol%
[28]
[35]
Co-MS@MNPs/CS KOH,DMSO,1400C,10-
[37]
12h, 1.1 mol %
Pd/ICCP
K2CO3,EtOH,1000C,4h,
1 mol %
Et3N,DMF,1000C,1.5-
[38]
Pd-BIP-
γ
-
[39]
Fe2O3@SiO2
6h,0.5 mol %
Pd(II)-PMO-SBA-16 DBU,CuI,H2O,900C,4-
10h, 0.9 mol %
[40]
Pd-imine@nanoSiO2
Our work
(Et)3N, H2O, 80
⁰C,2h,
0.9 mol%
Conclusion:
We have developed an eco-friendly, novel protocol for Sonogashira cross-coupling reaction
using Pd-imine@nanoSiO2. The catalyst exhibited excellent catalytic efficiency for carbon-
carbon triple bond activation reactions between arylhalides(both iodides and aryl bromides) and
terminal alkynes in water. Our catalyst could be easily recovered from the reaction mixture by

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20
simple centrifugal precipitation and reused for five consecutive catalytic cycles without profound
loss in activity. This protocol is also moderately suitable for aliphatic alkynes.
Acknowledgments
The authors thank SAIF, STIC, Kochi University, Kochi for ICP-AES analysis, SAIF, NEHU,
1
Shillong for HNMR facilities and ACMS, IIT Kanpur for X-ray photoelectron spectroscopic
analytical facilities. The authors express their gratitude to UGC, New Delhi for the SAP-DRS-I
grant (2016-2021) awarded to the department of Chemistry, Dibrugarh University.
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Highlights
•
•
Rice husk derived nanosilica supported Pd-imine@nanoSiO2 catalyst can be
synthesized easily.
This is the first example of a nanosilica anchored Pd catalyst for Sonogashira
cross-coupling reaction in aqueous medium in copper free condition and gave
good to excellent isolated yields of the products.
•
•
Activate both aryl iodides and aryl bromides to afford diaryl alkynes. This
protocol was also suitable for aliphatic alkynes.
The catalyst could be easily recovered and can be reused for five consecutive
cycles without profound loss in activity.