Diamines as interparticle linkers for silica-titania supported PdCu bimetallic nanoparticles in Chan-Lam and Suzuki cross-coupling reactions

Article abstract of DOI:10.1039/c8nj05050c

A series of highly efficient amine functionalized SiO₂-TiO₂ supported bimetallic PdCu catalysts with varied metal composition have been synthesized. Ethane-1,2-diamine, butane-1,4-diamine and hexane-1,6-diamine were employed as interparticle linkers for amine functionalization of a SiO₂-TiO₂ support material so as to study the effect of pendant chain length on stabilization and immobilization of bimetallic nanoparticles. The shortest carbon chain length on the support provided the best results, which may be due to the trapping of metal nanoparticles more efficiently by the basic nitrogen sites. The catalytic activities of these materials were evaluated for C-N and C-C coupling reactions. The most active catalyst, Pd₁Cu₁@12DA-STS, was characterized by various techniques including SEM, HR-TEM, ICP-AES, XRD, FTIR, EDX, CHN analysis and TGA studies. Moreover, the synthesized catalyst was found to be recyclable for up to five runs without significant loss of activity.

Full text of DOI:10.1039/c8nj05050c

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Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
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Diamines as interparticle linkers for silica-titania supportDeOdI: 10P.10d39C/C8uNJ05050C
bimetallic nanoparticles in Chan-Lam and Suzuki cross-coupling
reactions
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Babita Jamwal,a Manpreet Kaur,a Harsha Sharma,a Chhavi Khajuria,a Satya
Paul,a,* J. H. Clark,b
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aDepartment of Chemistry, University of Jammu, Jammu Tawi-180006, India
*Corresponding author: E-mail address: paul7@rediffmail.com, Fax: +91-191-2431365
bGreen Chemistry Centre of Excellence, Department of Chemistry, University of York, Heslington
York YO10 5DD UK, Email: james.clark@york.ac.uk, Tel: + 44 (0) 1904 322559
Keywords: Pd-Cu nanoparticles; silica-titania; heterogeneous catalysis; amines;
Suzuki coupling; Chan-Lam cross-coupling.
Abstract
A series of highly efficient amine functionalized SiO2-TiO2 supported bimetallic PdCu
catalysts with varied metal composition have been synthesized. Ethane-1,2-diamine, butane-
1,4-diamine and hexane-1,6-diamine were employed as interparticle linkers for amine
functionalization of a SiO2-TiO2 support material so as to study the effect of pendant chain
length on stabilization and immobilization of bimetallic nanoparticles. The shortest carbon
chain length on the support provided the best results which may be due to the trapping of
metal nanoparticles more efficiently by the basic nitrogen sites. The catalytic activities of
these materials were evaluated for C–N and C–C coupling reactions. The most active
catalyst, Pd1Cu1@12DA-STS was characterized by various techniques including SEM, HR-
TEM, ICP-AES, XRD, FTIR, EDX, CHN analysis and TGA studies. Moreover, the
synthesized catalyst was found to be recyclable for up to five runs without loss of significant
activity.
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Introduction
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Bimetallic nanocatalysis has emerged as a spectacular and exciting field of catalysis since it
offers a way to modify the catalytic properties of metals. Several studies have demonstrated
that bimetallic nanoparticles can go beyond their monometallic counterparts by exhibiting a
significantly higher catalytic performance in terms of activity, selectivity and stability1.
Heterogeneous bimetallic catalysts2,3 are highly distinctive and offer incredible opportunities
for tuning catalytic activity by controlling the size, shape, chemical composition and
dispersity of metal nanoparticles4,5. Their enhanced catalytic behaviour is attributed to the
synergistic effect that exists between the two metals6. The heterogeneous bimetallic
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catalysts, for instance Ir-Au/TiO2 , Pd-Cu/TiO2–CeO2 , Cu-Ni/AC9, Pd-Ni/ZrO210 and many
others have recently been explored for their multifunctional applications. Though there has
been a tremendous progress in the field of bimetallic catalysis, still there is much scope for
the improvement of the current strategies and techniques to arrive at the best catalytic
system in terms of activity and selectivity11. Various organic or inorganic supports such as
carbon12, graphene13, metal oxides14,15, zeolites16,17, EDTA18 are being used to immobilize
bimetallic nanoparticles for their higher stability, dispersion and stabilization19. The
combination of two nanoscale inorganic moieties have gained much attention as they yield a
composite with high surface area and desirable porosities to improve their catalytic
performance20,21. Abundantly available silica being highly stable can trap the metal
nanoparticles easily and hence is widely used as a catalyst support22. But the surface of the
silica cannot hold metal nanoparticles for long when used in a reaction, thereby decreasing
the activity of the catalyst after even a small number of runs23. Recently, titania, has
emerged as a feasible support for the immobilization of metal nanoparticles due to its good
biological compatibility, non-toxicity, cost-effectiveness and easy preparation7, 24. Studies
have been devoted to the synthesis of silica-titania composites with well-controlled texture
as they have considerable potential for applications in chemical industry and biomedical
technology25, 26
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Although, SiO2-TiO2 composite shows improved properties, it suffers from some
disadvantages such as leaching of metal nanoparticles and thus, decreases the likelihood of
making the catalyst reusable. Recently, different binding agents such as phosphine, imine,
acetylacetonate, oleic acid, L-dopa27 and amine moieties28 have been reported which
stabilizes the metal nanoparticles against aggregation and also enhance their catalytic
properties29. A strong interaction between metal nanoparticles and the amine functionalized
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support is believed to account for the enhanced stability of these catalyst materials30. InVtiehweArticle Online
DOI: 10.1039/C8NJ05050C
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present work, bimetallic heterogeneous catalytic systems based on Pd-Cu nanoparticles
supported onto amine functionalized silica-titania have been developed. The developed
catalysts were tested for Chan-Lam and Suzuki cross-coupling reactions. Earlier, there are
reports in literature where Chan-Lam process31-33 of C-N bond formation has been carried
out using different copper salts in the presence of various ligands, which include square
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pyramidal [Cu(DMAP)4I]I34,
Cu-doped CoFe2O4
nanoparticles35, Copper
iminoarylsulfonate complexes36, Ni- and Cu-based Catalysts37 etc. We intend to report a
bimetallic heterogeneous catalytic system where different diamines were used as
interparticle linker and incorporated onto silica-titania support in order to study the effect of
pendant chain length on stabilization and immobilization of bimetallic nanoparticles. Pd and
Cu nanoparticles with varied metal composition were immobilized onto the support in order
to investigate the effect of Pd/Cu ratio on the stability of the catalytic system. Out of the
various catalysts, Pd1Cu1@12DA-STS showed superior activity for Chan-Lam and Suzuki
cross-coupling reactions and was fully characterized by FT-IR, TGA, SEM, HR-TEM,
CHN, EDX, ICP-AES, XRD and XPS.
Experimental
Materials and Characterization
All the chemicals used were purchased from Aldrich Chemical Company or Merck. Silca gel
(Merck grade 7734, pore size 60 ᵒA, 70-230 mesh) was used in the present study. Perkin-
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Elmer FTIR spectrophotometer was used to record the FTIR spectra of the samples. The H
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and C NMR data were recorded in CDCl3 on Bruker Avance III (400 and 100 MHz) and
mass spectral data on Bruker Esquires 3000 (ESI). X-ray diffratograms (XRD) were recorded
in 2 theta range of 10–80ο on a Bruker AXSDB X-ray diffractometer using Cu Kα radiations.
BET (Brunauer-Emmett-Teller) surface area was determined from N2 adsorption-desorption
isotherms using Autosorb-IQ MP analyzer. XPS spectra of the catalyst were recorded on
KRATOS ESCA model AXIS 165 (Resolution). TGA was recorded on Perkin Elmer,
Diamond TG/DTA. EDX analysis was carried out using JEOL Model JED-2300 and the
amount of metal in catalyst was determined by ICP-AES analysis using ARCOS from M/s.
Spectro, Germany. CHN analysis was recorded on Thermo Finnigan FLASH EA 1112 series.
SEM images were recorded using JEOL Model JSM-6390LV Scanning Electron Microscope
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and High Resolution Transmission Electron Micrographs (HR-TEM) were recorded on
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JEOL, JEM-2100F.
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Catalyst preparation
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Synthesis of SiO2-TiO2 support [STS]
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Activated SiO2 (1.0 g) was dispersed into a mixture of ethanol (40 mL) and ammonium
hydroxide (5 mL) and ultrasonicated for 30 min. The solution was then transferred into a
round bottom flask (100 mL) and heated at 45 °C under continuous stirring. The solution of
tetrabutyl orthotitanate (TBOT, 4 mL) in ethanol (30 mL) was added into the suspension
dropwise at a rate of 3 mL min−1, and stirring was continued to proceed at 45 °C for 24 h.
The resultant SiO2-TiO2 composite was filtered, washed with distilled water (3×10 mL) and
ethanol (3×10 mL), and dried in an oven at 60 °C overnight.
Synthesis of amine-functionalized SiO2-TiO2 support [DA-STS]
To a mixture of SiO2-TiO2 composite (1g) and ethane-1,2-diamine (0.3 g, 5 mmol) [or
butane-1,4-diamine (0.44 g, 5 mmol) or hexane-1,6-diamine (0.68 g, 5 mmol)] in a round
bottom flask (50 mL), deionized water (10 mL) was added and the reaction mixture was
stirred at 120 °C for 8 h. Diamine functionalized SiO2-TiO2 support [12DA-STS prepared
from ethane-1,2-diamine; 14DA-STS prepared from butane-1,4-diamine; 16DA-STS
prepared from hexane-1,6-diamine] was filtered, washed with water (3×10 mL) and ethanol
(3×10 mL), and dried under vacuum at room temperature to get light grey powder.
Synthesis of palladium-copper (1:1) onto amine functionalized SiO2-TiO2 support
[Pd1Cu1@12DA-STS, Pd1Cu1@14DA-STS and Pd1Cu1@16DA-STS]
To a solution of Pd(OAc)2 (0.023g, 0.1 mmol) in ethanol (10 mL), Cu(OAc)2 (0.020g, 0.1
mmol) in ethanol (10 mL) was added and stirred for 2 h. To this solution, ethane-1,2-
diamine functionalized SiO2-TiO2 support (1.0 g) was added and the reaction mixture was
stirred at room temperature for another 3 h. Finally, freshly prepared aqueous solution of
NaBH4 (1.5 mmol, 5 mL) was added dropwise over a period of 3 h followed by further
stirring for 8 h at room temperature. The reaction mixture was then filtered and the catalyst
was successfully washed with distilled water (3×10 mL) and ethanol (3×10 mL)
respectively. The prepared grey coloured Pd1Cu1@12DA-STS was dried in vacuum at room
temperature overnight.
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Similarly, Pd1Cu1@14DA-STS and Pd1Cu1@16DA-STS were prepared by similar
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procedure using 14DA-STS and 16DA-STS respectively.
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Synthesis of palladium-copper [1:3 and 3:1] onto ethane-1,2-diamine functionalized SiO2-
TiO support [Pd1Cu3@12DA-STS] and Pd3Cu1@12DA-STS]
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To a solution of Pd(OAc)2 (0.0112g, 0.05 mmol) in ethanol (10 mL), Cu(OAc)2 (0.030g,
0.15 mmol) in ethanol (10 mL) was added in case of Pd1Cu3@12DA-STS and [or Pd(OAc)2
(0.0337g, 0.15 mmol) and Cu(OAc)2 (0.010g, 0.05 mmol) in case of Pd3Cu1@12DA-STS]
and the resulting mixture was stirred for 2 h. To this solution, ethane-1,2-diamine
functionalized SiO2-TiO2 support (1 g) was added and the reaction mixture was stirred at
room temperature for another 3 h. Finally, freshly prepared aqueous solution of NaBH4 (1.5
mmol, 5 mL) was added dropwise over a period of 3 h followed by further stirring for 8 h at
room temperature. The reaction mixture was then filtered and the catalyst was successfully
washed with distilled water (3×10 mL) and ethanol (3×10 mL) respectively. The prepared
catalysts Pd1Cu3@12DA-STS and Pd3Cu1@12DA-STS were dried in vacuum at room
temperature overnight.
Similarly, monometallic Pd(0)@)12DA-STS and Cu(0)@)12DA-STS were also prepared by
using 0.1 mmole of Pd(OAc)2 and Cu(OAc)2 seperately following the same procedure.
Catalytic test
General procedure for Pd1Cu1@12DA-STS catalyzed Chan-Lam cross-coupling of
aromatic amines with phenylboronic acid
Aryl amine (1 mmol), phenylboronic acid (1.2 mmol), K2CO3 (1.2 mmol) and
Pd1Cu1@12DA-STS (0.1g) were dispersed in water:ethanol (3:1, 8 mL) and the reaction
mixture was stirred at 70 °C for the appropriate time. After completion of the reaction
(monitored by TLC), the catalyst was filtered, washed with ethyl acetate (35 mL) followed
by deionized water (210 mL) and dried under vacuum. The reaction mixture was cooled
and extracted with ethyl acetate (20 mL). The organic layer was washed with deionized
water (320 mL) and dried over anhydrous Na2SO4. The product was obtained after passing
through column of silica gel and elution with EtOAc-pet. ether.
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General procedure for Pd1Cu1@12DA-STS catalyzed Suzuki coupling of aryl halides with
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phenylboronic acid
To a mixture of aryl halide (1 mmol), phenylboronic acid (1.2 mmol), K2CO3 (0.25 mmol)
and Pd1Cu1@12DA-STS (0.1 g) in a round bottom flask (25 mL), water:ethanol (3:1, 8 mL)
was added and the reaction mixture was stirred for the appropriate time at 70 °C. After
completion of the reaction (monitored by TLC), the reaction mixture was diluted with ethyl
acetate (10 mL) and filtered. The organic layer was washed with water (50 mL) and dried
over anhydrous Na2SO4. Finally, the product was obtained by column chromatography on
silica gel (EtOAc:pet.ether). The catalyst was washed with EtOAc (35 mL) and double
distilled water (310 mL) and dried under vaccum overnight at room temperature for further
use. The structures of the products were confirmed by 1H and 13C NMR spectral data.
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SiO2
Activated SiO2
Ethanol/NH4OH
SiO2-TiO2
Ultrasound, TBOT
Ethane-1,2-
diamine/H2O
Pd(OAc)
2, Cu (OAc)2
NaBH4
Pd1Cu1@12DA-STS
12DA-STS
Ethane-1,2-diamine moiety
Cu
Pd
Scheme 1. General procedure for the synthesis of Pd1Cu1@12DA-STS.
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3. Results and discussion
Characterization of Pd1Cu1@12DA-STS
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Bimetallic Pd1Cu1@12DA-STS, Pd1Cu1@14DA-STS and Pd1Cu1@16DA-STS were
synthesized by changing the amine moiety. Further, monometallic Pd(0)@)12DA-STS,
Cu(0)@)12DA-STS and bimetallic Pd1Cu3@12DA-STS and Pd3Cu1@12DA-STS were
prepared by changing the composition of Pd and Cu (Scheme 1). All the catalysts were
characterized by Fourier Transform Infrared Spectroscopy (FT-IR) and Thermogravimetric
Analysis (TGA). While the most active heterogeneous catalyst was further characterized by
Scanning Electron Microscopy (SEM), High Resolution Transmission Electron Microscopy
(HR-TEM), Carbon, Hydrogen and Nitrogen (CHN) analysis, BET (Brunauer-Emmett-
Teller) analysis, Energy Dispersive X-ray analysis (EDX), Inductively Coupled Plasma
Atomic Emission Spectroscopy (ICP-AES), Powder X-ray Diffraction (XRD) and X-ray
Photoelectron Spectroscopy (XPS).
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The FT-IR spectral analysis of the synthesized catalysts was done to study their surface
chemical structure. All the catalysts i.e. Pd1Cu1@14DA-STS, Pd1Cu1@16DA-STS
Pd1Cu1@12DA-STS, Pd1Cu3@12DA-STS and Pd3Cu1@12DA-STS showed almost similar
peaks due to the presence of same functionalities in them. The FT-IR spectra of STS, 12DA-
STS and Pd1Cu1@12DA-STS are shown in S1 and Fig. 1 respectively. The peaks at 1085,
795 and 474 cm−1 are probably due to asymmetric stretching (νas), symmetric stretching (νs)
and deformation modes of Si-O-Si respectively. A typical band at 943 cm−1 has been
assigned to the asymmetric stretching of Si-O-Ti, clearly indicating the formation of SiO2-
TiO2 composite20, 24, and 39. A broad band in the range of 3400–3100 cm−1 can be attributed to
the stretching vibrations of surface O-H and N-H groups on the surface of the material as
well as adventitious water. The characteristic peaks at 2919 cm−1 and 1633 cm−1 were
assigned to the stretching vibrations of the C-H stretching of the CH2 group and the bending
vibrations of the N-H of the NH2 group respectively, signifying the presence of an ethylene
diamine moiety on the catalyst. The peaks at 1085 cm−1 and 1299 cm−1 were attributed to C–
H deformation and C–N stretching vibrations respectively. To investigate the thermal
stabilities of the catalysts, a thermo-gravimetric analysis (TGA) was carried out by heating
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the material at a rate of 10◦C min− . The difference in thermal stabilities of all the three
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Fig. 1 FTIR of Pd1Cu1@12DA-STS.
bimetallic catalysts may be due to the different ratios of metals thus affecting the covalent
bonding between the silica-titania support and the amine moiety. TGA of Pd1Cu1@12DA-
STS, the most active catalyst, showed an initial weight loss of 0.75% up to 160 °C which is
likely to be due to the loss of adsorbed solvent or water from the catalyst. It was followed by
continuous weight loss of ca. 8% upto 321°C, which could be due to the thermal
decomposition of chemisorbed molecules of diamine moiety present in the catalyst. Further
weight loss from 530 to 900°C was attributed to the bulk pyrolysis of silica-titania
composite. The analysis indicated that the catalyst is stable upto 160°C, thus confirming that
it could be safely used for carrying out the reactions under optimum conditions (upto 80°C).
The major weight loss of 12DA-STS, Pd1Cu3@12DA-STS and Pd3Cu1@12DA-STS are
presented in S2. The shape and surface morphology of Pd1Cu1@12DA-STS was studied by
scanning electron microscopy at different magnifications (Fig. 3). SEM images revealed that
the synthesized catalyst is homogeneous in nature and Pd and Cu nanoparticles were
uniformly spread over the surface of amine functionalized silica-titania support with partial
surface exposure (Fig. 3b). Their spherical shape results in low tendency of nanoparticles to
undergo agglomeration, thereby increasing the surface area of the catalyst and hence the
observed increased catalytic efficiency.
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Fig. 2 TGA of Pd1Cu1@12DA-STS.
Fig. 3 SEM images (a, b) of Pd1Cu1@12DA-STS.
The morphology and distribution of Pd and Cu onto 1,2-diamine functionalized silica-titania
support was further verified by high resolution transmission electron microscopy (Fig. 4).
The high magnification HR-TEM images of Pd1Cu1@12DA-STS shows black coloured
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spots, which could be ascribed to the presence of Pd(0) and Cu(0) nanoparticlesDfOoIr: m10.e10d3V9ia/eCws8ANartJic0le5O05nl0inCe
result of the coordination of Pd and Cu with the surface of the support (Fig. 4b). The average
size of PdCu nanoparticles was 5~7 nm as depicted by the histogram. The nanoparticles are
dispersed homogeneously without any noticable aggregation onto the surface of 1,2-diamine
functionalized silica-titania support.
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Fig. 4 HR-TEM images (a, b) of Pd1Cu1@12DA-STS.
The elemental composition of Pd1Cu1@12DA-STS was analyzed by energy dispersive
spectroscopy (EDX). The EDX analysis reveals that the catalyst is composed of C, O, Si, Ti,
Cu and Pd which indicate that PdCu nanoparticles were successfully incorporated over the
amine functionalized silica-titania support (Fig. 5). CHN analysis further confirms the
presence of amino group in Pd1Cu1@12DA-STS [S3]. The porous properties and the surface
area estimation of the prepared catalyst was determined by nitrogen adsorption-desorption
analysis3a (Fig. 5). The resulting plot can be considered as a composite of type II and type IV
isotherm with an H3-type hysteresis loop, which are associated to the microporous and
mesoporous materials with slit like pores. The BET surface area of the Pd1Cu1@12DA-STS
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was found to be 86.335 m2 g− and a total pore volume as 0.34 cm3 g-.1
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Fig. 5 Nitrogen adsorption-desorption isotherm of Pd1Cu1@12DA-STS.
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8
9
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Fig. 6 EDX of Pd1Cu1@12DA-STS.
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DOI: 10.1039/C8NJ05050C
In order to determine the content of palladium and copper loaded onto the support material,
5
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7
8
the synthesised catalyst was analysed by inductively coupled plasma atomic emission
spectroscopy (ICP-AES). The results indicated that palladium and copper content loaded
onto 0.1g of Pd1Cu1@12DA-STS was found to be 0.478 wt% and 0.972 wt% respectively.
To get further insight into the structure of the catalyst, the presence of palladium and copper
nanoparticles onto Pd1Cu1@12DA-STS was also confirmed by powder X-ray diffraction
(XRD) analysis (Fig. 6). The XRD pattern of Pd1Cu1@12DA-STS exhibits the diffraction
patterns at 2θ = 43.4, 50.5, and 74.2° which can be indexed as [111], [200] and [220] planes
of copper. A strong diffraction peak at 42.4° along with a weak peak at 49.5° was observed
between the diffraction peaks from (111) and (200) facets of monometallic Pd and Cu,
which indicates the formation of PdCu nanoalloy40, 41. Fairly broad peaks were observed
around 41°, 46.7° and 68.1° which are related to Pd-Cu alloy in the fcc phase. Diffraction
peaks at 25.2, 48.1 and 53.8° correspond to the [101], [200] and [105] planes of the cubic
phase of TiO2, while the peak at 22° is assigned to crystalline silica. The average particle
size of the catalyst, determined using Scherrer equation (5.931 nm) was found to be in close
agreement with the particle size obtained from HR-TEM studies.
9
10
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12
13
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Fig.7 XRD of Pd1Cu1@12DA-STS.
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X-ray photoelectron spectroscopy (XPS) was employed to further understand the surface of
5
6
7
8
the catalyst and elemental states in bimetallic Pd1Cu1@12DA-STS, such as the oxidation
state and the binding energy of the core electrons of the palladium and copper. The results
shown in Fig.7a represent the overall survey spectrum of Pd1Cu1@12DA-STS.
9
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13
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Fig. 8 XPS of Pd1Cu1@12DA-STS.
The peaks corresponding to carbon 1s (284.8 eV), oxygen 1s (532.3 eV), nitrogen 1s (401.4
eV), silica 2p (105.3 eV), titanium 2p (456.02-459.8 eV), palladium 3d (336.7 eV) and
copper 3d (932.9 eV) are clearly observed. Fig.7b shows the typical Pd(0) absorptions at
336.7 and 339.2 eV for 3d5/2 and 3d3/2 respectively which is slightly shifted to higher
binding energies as compared to the monometallic Pd 3d5/241-43. While comparing the XPS
13

New Journal of Chemistry
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1
2
3
4
5
6
7
8
9
of Cu 2p3/2 for monometallic Cu44 to those obtained for the bimetallDicOI: 1c0a.1t0a3Vl9iy/eCws8tANrtJic0le5O05nl0inCe
(Pd1Cu1@12DA-STS), a progressive shift to lower binding energy was observed. Fig. 7c
shows the typical Cu(0) absorptions at 932.1 and 952.5 eV, attributed to the binding energy
of Cu 2p3/2 and Cu 2p5/2 respectively45, which may be related to Cu-M, where M can be
either Pd or Cu.
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Catalyst testing for C–N bond formation
The catalytic performance of the synthesized bimetallic catalysts and their monometallic
analogues was studied in C-N bond forming reactions. We investigated the effect of pendent
chain length by changing the amine moiety in order to find the most active catalyst among
the supported PdCu bimetallic catalysts studied (Table 1). We selected the reaction of 4-
methoxyaniline and phenylboronic acid as the model reaction. It was found that shortest
carbon chain length on the support provided the best results compared to other amine
moieties (Table 1, entries 6, 9, 12). This may be due to the trapping of metal nanoparticles
more efficiently by the basic nitrogen sites of ethane-1,2-diamine present on the support
material. We also examined the influence of the metal composition on the reaction
efficiency. The model reaction was carried out using supported PdCu catalysts with different
composition of Pd and Cu. It was found that catalysts with pure copper or mainly copper
gave lower product yields (Table 1, entry 9 and 16). In contrast, catalysts consisting
predominently of palladium (Table 1, entry 6 and 15) showed better results. The catalytic
system containing equal amounts of Pd and Cu showed higher activity (Table 1, entry 12)
as compared to the other metal compositions (Table 1, entries 13-16) and their
monometallic counterparts. During our studies on exploring the catalytic performance of
Pd1Cu1@12DA-STS for C-N cross-coupling reaction, various reaction parameters such as,
reaction temperature, catalyst screening, the presence of base and the effect of solvents were
investigated. Initially, the model reaction was performed using 0.1g of different supported
monometallic and bimetallic catalysts for optimization of the appropriate catalyst amount,
while other conditions remained the same. The results are summarized in Table 1. When the
model reaction was attempted with only 0.05g of the same catalyst, it led to a slight decrease
in product yield (Table 1, entry 17). The effect of solvent was studied by carrying out the
model reaction in different solvents, including water, acetonitrile, toluene, ethanol,
methanol, H2O/EtOH (3:1), H2O/EtOH (1:1) and H2O/EtOH (1:3). The results are
summarized in Table 2. With water, acetonitrile, toluene and methanol, the reaction did not
14

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progress to completion even with longer reaction times (Table 2, entries 1–5 and 7). VBiewuAtrticle Online
DOI: 10.1039/C8NJ05050C
5
6
7
8
we got good results when ethanol was used as a solvent. The model reaction was also carried
out in ethanol:water mixtures in different ratios and we found that H2O/EtOH (3:1) was
superior in terms of both the yield and the reaction time (Table 2, entry 10). In order to
study the importance of the base, the model reaction was tested in the absence of base giving
only 25-40% of product (Table 2, entries 1, 9, 13, 15). When the reaction was performed in
the presence of K2CO3 as base, the reaction gave more satisfactory results at 70 ᵒC (Table 2,
entry 10). After optimization of the reaction conditions, the generality of the developed
protocol was studied with respect to different aromatic amines and overall good results were
obtained (Table 3). Further, the reaction was also carried out using aliphatic amines but only
trace amounts of products were obtained that too after long reaction times, except for butyl
amine which showed good results (Table 3, entry 2m). The reaction proceeds faster with
better yield in the case of 4-methoxyaniline (Table 3, entry 2b) as compared to 2-
methoxyaniline (Table 3, entry 2c). With 4-fluoroaniline (Table 3, entry 2f) and 2-
fluoroaniline (Table 3, entry 2j), the coupling products were obtained in good yields,
whereas in the reaction of 4-bromoaniline with phenyl boronic acid (Table 3, entry 2g), we
witnessed the formation of a side product (C-C coupling product, Table 3, entry 2h) though
only in small amount along with the C-N coupling product. Similar results were obtained for
4-chlorophenylboronic acid (Table 3, entry 2r), wherein C-C coupling product 2s was also
formed as the side product. Reaction with 2-iodoaniline (Table 3, entry 2k) led to
significant decrease in C-N product formation because of the good leaving nature of the iodo
group. Instead it formed C-C coupling product (Table 3, entry 2l) as the major product.
Low yield was obtained in case of diphenylamine (Table 3, entry 2o) and diethylamine
(Table 3, entry 2t) with longer reaction time, whereas, imidazole and indoles did not form
the expected coupling products. Further, 2-thiomethylphenylboronic acid fails to couple
with 4-methoxyaniline, which may be due to the presence of sulphur in the substrate. Only
trace amount of expected product was detected with 2-thiopheneboronic acid, which could
not be isolated.
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Table 1. Optimization of the catalyst amount for C-N cross-coupling reaction, comparison
5
6
7
of the catalytic activity of various supported monometallic and bimetallic catalysts, and
effect of metal composition on reaction efficiencya
8
9
Entry
Catalyst
Amount (g)
Time (h)
Yieldb (%)
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
1
2
3
4
No catalyst
STS
Pd@STS
Cu@STS
0.1g
0.1g
0.1
8
8
6
6
N.R.
Traces
45
0.1
50
5
6
PdCu@STS
Pd@12DA-STS
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.05
6
6
58
72
60
64
70
65
68
90
80
74
74
72
86
7
8
9
Pd@14DA-STS
Pd@16DA-STS
Cu@12 DA-STS
Cu@14DA-STS
Cu@16DA-STS
Pd1Cu1@12DA-STS
Pd1Cu1@14 DA-STS
Pd1Cu1@16 DA-STS
Pd3Cu1@12 DA-STS
Pd1Cu3@12 DA-STS
Pd1Cu1@12 DA-STS
6
5.5
5.5
6
10
11
12
13
14
15
16
17
6
2.5
5.5
5.5
5.5
4.0
4.5
aReaction conditions: 4-methoxybenzenamine (1 mmol), phenylboronic acid (1.2 mmol), K2CO3 (1.2 mmol,
0.165 g) in H2O/EtOH (3:1, 8 mL) at 70 °C; bChromatographic yield.
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Table 2. Effect of various reaction parameters for C–N and C–C couplings in the presence
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of Pd1Cu1@12DA-STS
7
8
9
S.No.
Solvent
Base
Temperature
(ͦC)
C-N
Time
(h)
Couplinga
Yieldc
(%)
25
C-C
Time
Couplingb
Yieldc
(%)
42
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(h)
3.5
3.5
3.0
2.5
3.0
3.0
3.5
2.0
2.5
0.25
2.0
0.5
2.5
1.75
3.0
0.75
1
2
Water
-
RT
RT
100
RT
RT
70
8.0
8.0
6.0
6.5
6.0
5.5
6.0
6.0
6.0
2.5
6.0
4.0
5.5
6.0
6.0
4.5
Water
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
-
25
52
3
Water
65
82
4
Acetonitrile
Toluene
35
70
5
35
60
6
Ethanol
65
78
7
Methanol
RT
RT
70
35
74
8
H2O/EtOH (3:1)
H2O/EtOH (3:1)
H2O/EtOH (3:1)
H2O/EtOH (1:1)
H2O/EtOH (1:1)
H2O/EtOH (1:1)
H2O/EtOH (1:3)
H2O/EtOH (1:3)
H2O/EtOH (1:3)
55
80
9
35
54
10
11
12
13
14
15
16
K2CO3
K2CO3
K2CO3
-
70
90
88
RT
70
55
78
82
88
70
40
52
K2CO3
-
RT
70
40
84
35
48
K2CO3
70
86
86
aReaction conditions: 4-methoxybenzenamine (1 mmol), phenylboronic acid (1.2 mmol), K2CO3 (1.2 mmol,
0.165 g), Pd1Cu1@12DA-STS (0.1 g , 0.75 mol% Cu, 0.91 mol% Pd). Reaction conditions: bromobenzene (1
b
mmol), phenyl boronic acid (1.2 mmol), K2CO3 (0.25 mmol, 0.165 g), Pd1Cu1@12DA-STS (0.1 g, 0.75 mol%
Cu, 0.91 mol% Pd). cChromatographic yield.
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Table 3. Pd1Cu1@12DA-STS catalyzed Chan-Lam cross-coupling reaction of aDmOiI:n1e0.s10w39/iCth8NJ05050C
arylboronic acids in water/ethanol mixturea,b
6
7
8
9
H
N
B(OH)2
R1
Pd1Cu1@12DA-STS
H2O:EtOH(3:1)
K2CO3 , 70 oC
R1 NH2
R2
R2
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
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2
1
OMe
H
H
H
N
H
N
N
N
MeO
OMe
2d: 5h, 30%
2c: 3.5h, 85%
2b: 2.5h, 90%
2a: 2h, 82%
H
N
NH2
H
N
H
N
F
Me
Br
2e: 3.5h, 80%
2f: 3h, 82%
2h: 2h, 25%
2g: 2h, 72%
F
I
NH2
H
N
H
H
N
N
O2N
2i: 3h, 84%
2l: 1.0h, 75%
2j: 4h, 84%
2k: 3.5h, 15%
C6H5
N
H
N
H
N
H
N
MeO
F
2o: 8h, 40%
2p: 4.0h, 88%
Et Et
2m: 4.5h, 72%
2n: 5h, 78%
N
NH2
H
N
H
N
F
MeO
Cl
Br
Cl
2t: 5h, 60%
2s: 4.5h, 20%
2q: 3.5h, 90%
2r: 4.5h, 70%
aReaction conditions: aryl amine (1 mmol), arylboronic acid (1.2 mmol) Pd1Cu1@12DA-STS (0.1 g, 0.75
mol% Cu, 0.91 mol% Pd.), K2CO3 (1.2 mmol, 0.165 g) in H2O/EtOH (3:1, 8 mL) at 70 °C; bChromatographic
yield.; 2h, 2l, 2s: C-C coupling takes place along with C-N cross-coupling.
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DOI: 10.1039/C8NJ05050C
Catalyst testing for C–C bond formation
5
6
7
8
Encouraged by the catalytic proficiency of our new designed bimetallic catalyst,
Pd1Cu1@12DA-STS for C-N bond formation, its catalytic activity was further explored in
Suzuki couplings. The reaction parameters such as catalyst amount, temperature and solvent
were optimized based on the reaction of bromobenzene and phenylboronic acid as a test
reaction. The optimum conditions were found to be: bromobenzene (1 mmol),
phenylboronic acid (1.2 mmol), Pd1Cu1@12DA-STS (0.1 g, 0.75 mol% Cu, 0.91 mol% Pd),
H2O/EtOH (3:1) as solvent, 70 °C and with K2CO3 as the base. Suzuki coupling also occurs
in the presence of a base but since the synthesised catalyst, Pd1Cu1@12DA-STS is base
functionalized, the test reaction was attempted in the absence of base. We did not get very
good results and the corresponding products were obtained in only 42-54% yield (Table 2,
entries 1, 9, 13, 15). The test reaction was tried with only 0.25 mmole of K2CO3 which did
give good results. These results supported the observation that the base functionalised
catalyst requires a limited amount of base - about eight times lower than the reported Suzuki
coupling reaction46. In order to evaluate the generality of the developed method, the C-C
coupling reaction was extended to different aryl halides and phenylboronic acids, substituted
with both electron-donating and electron-withdrawing groups. The reaction proceeded
smoothly to give the products in quantitative yields (Table 4). The reaction with indole also
worked well but lower yields were obtained as compared to aryl halides (Table 4, entry 3j-
l). It is pertinent to mention that exclusively C-C coupling take place and there is no
evidence for the C-N coupling reaction under the selected conditions.
9
10
11
12
13
14
15
16
17
18
19
20
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22
23
24
25
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31
32
33
34
35
36
37
38
39
40
41
42
43
44
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46
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Table 4. Pd1Cu1@12DA-STS catalysed Suzuki coupling between aryl hDaOliI:d1e0s.103a9/nCd8NJ05050C
arylboronic acidsa,b
6
7
8
9
10
11
12
13
14
15
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51
52
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aReaction conditions: aryl halide (1 mmol), arylboronic acid (1.2 mmol), Pd1Cu1@12DA-STS (0.1 g, 0.75
mol% Cu, 0.91 mol% Pd), K2CO3 (0.25 mmol, 0.0345 g) in H2O/EtOH (3:1, 8 mL) at 70 °C;
bChromatographic yield.
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DOI: 10.1039/C8NJ05050C
Recyclability
5
6
7
8
The recovery of the catalyst is most crucial in order to save energy, time and resources for
sustainable organic synthesis. The reusability of Pd1Cu1@12DA-STS catalytic system was
checked in case of entry 2b, Table 3 and entry 3g, Table 4 under the selected conditions.
After completion of the reaction, the catalyst was easily separated by simple filtration,
washed with ethyl acetate and distilled water, dried and used in the next cycle. The activity
of the catalyst was investigated for five consecutive runs and the results are presented in
Table 5. These clearly indicate that the catalyst is recyclable upto five runs without any
significant loss of catalytic activity.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
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39
40
41
42
43
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45
46
47
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49
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Table 5. Recyclability of Pd1Cu1@12DA-STS for C–N and C–C coupling reaction
C-N couplinga
C-C Couplingb
Catalytic
runs
Catalyst amount (g)
Yield (%)c
Catalyst amount (g)
Yield (%)c
1
0.1
90
88
86
84
78
0.1
88
88
87
84
81
2
3
4
5
0.092
0.088
0.082
0.076
0.094
0.090
0.086
0.081
aReaction conditions: 4-methoxybenzenamine (1 mmol), phenylboronic acid (1.2 mmol), Pd1Cu1@12DA-STS
(0.1 g , 0.75 mol% Cu, 0.91 mol% Pd ), K2CO3 (1.2 mmol, 0.165 g) in H2O/EtOH (3:1, 8 mL) at 70 °C.
bReaction conditions: bromobenzene (1 mmol), phenylboronic acid (1.2 mmol), Pd1Cu1@12DA-STS (0.1 g,
0.75 mol% Cu, 0.91 mol% Pd ), K2CO3 (0.25 mmol) in H2O/EtOH (3:1, 8 mL) at 70 °C;
cChromatographic yield.
Conclusion
Amine functionalized SiO2-TiO2 supported bimetallic PdCu catalysts have been synthesised
and characterized for carrying out C-N and C-C coupling reactions. Amine functionalization
of SiO2-TiO2 support was done by ethane-1,2-diamine, butane-1,4-diamine and hexane-1,6-
diamine so as to study the effect of pendant chain length on stabilization of bimetallic
nanoparticles. PdCu metal alloy nanoparticles immobilized onto ethane-1,2-diamine
functionalized SiO2-TiO2 support with metal ratio of 1:1 was found to be the most active
catalytic system. The remarkable synergetic effect between the two metals and the presence
of basic nitrogen sites onto the support effectively stabilize and enhance the activity by
preventing agglomeration. Our catalyst exhibits excellent activity under less basic
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conditions, than normal and with excellent yields for the C–C coupling reaction.DMOI:o10r.e10o3v9/eCr8,NJ05050C
5
6
the developed catalyst ensures easy recovery and reusability upto five consecutive runs.
7
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Acknowledgements
The authors gratefully acknowledge the Head, SAIF, IIT Bombay for HR-TEM, SEM, ICP-
AES, EDX, and CHN studies; Head, ACMS, IIT Kanpur for XPS study; CIF, IIT Guwahati
for BET studies and Head, SAIF, Punjab University Chandigarh for XRD. Financial
assistance from UGC, New Delhi (FIP to one of the authors, BJ) is gratefully acknowledged.
Appendix A. Supplementary data
References
[1].
(a) J. R. Renzas, W. Huang, Y. Zhang, M. E. Grass, G. A. Somorjai, Catal. Lett.
2011, 141, 235-241.
(b) G. J. Hutchings, Catal. Today. 2014, 238, 69-73.
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Diamines as interparticle linkers for silica-titania supported PdCu bimetallic
nanoparticles in Chan-Lam and Suzuki cross-coupling reactions
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Babita Jamwal,a Manpreet Kaur,a Harsha Sharma,a Chhavi Khajuria,a Satya Paul,a,* J. H.
Clarkb
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aDepartment of Chemistry, University of Jammu, Jammu Tawi-180006, India
∗Corresponding author: E-mail address: paul7@rediffmail.com, Fax: +91-191-2431365
bGreen Chemistry Centre of Excellence, Department of Chemistry, University of York, Heslington
York YO10 5DD UK, Email: james.clark@york.ac.uk, Tel: + 44 (0) 1904 322559
The remarkable synergetic effect between Pd and Cu, and basic nitrogen sites onto the
support effectively stabilize the nanoparticles and enhance the catalytic activity.