Silica nanosphere-supported palladium(II) furfural complex as a highly efficient and recyclable catalyst for oxidative amination of aldehydes

Article abstract of DOI:10.1039/c3dt51928g

The present work reports the fabrication of a novel and highly efficient silica nanospheres-based palladium catalyst (SiO₂@APTES@Pd-FFR) via immobilization of a palladium complex onto silica nanospheres functionalized with 3-aminopropyltriethoxysilane (APTES), and its catalytic application for the oxidative amination of aldehydes to yield commercially important amides. The structure of the nano-catalyst was confirmed by Solid-state ¹³C CPMAS and ²⁹Si CPMAS NMR spectroscopy, Brunauer-Emmett-Teller (BET) surface area analysis, Fourier transform infrared spectroscopy (FT-IR), Energy dispersive X-ray fluorescence spectroscopy (ED-XRF), Atomic absorption spectroscopy (AAS), Transmission electron microscopy (TEM) and elemental analysis. The nano-catalyst was found to be highly effective for the oxidative amination of aldehydes using hydrogen peroxide as an environmentally benign oxidant to give amides. The effect of various reaction parameters such as temperature, amount of catalyst, reaction time, type of solvent, oxidant used, substrate to oxidant ratio etc. have been demonstrated to achieve high catalytic efficacy. Moreover, this nanostructured catalyst could be recovered with simplicity and reused for several cycles without any significant loss in its catalytic activity. In addition, the stability of the reused nano-catalyst was proved by FT-IR and HRTEM techniques. It is worth noting that the features of mild reaction conditions, simple work-up procedure, high product yield, no use of toxic organic solvents, high turn-over frequency (TOF), and easy recovery and reusability of the present quasi-homogeneous nano-catalyst make this protocol an attractive alternative to the existing catalytic methods for the oxidative amination of aldehydes to furnish industrially important amides. The Royal Society of Chemistry 2014.

Full text of DOI:10.1039/c3dt51928g

Dalton
Transactions
View Article Online
View Journal | View Issue
PAPER
Silica nanosphere-supported palladium(II) furfural
complex as a highly efficient and recyclable
catalyst for oxidative amination of aldehydes†
Cite this: Dalton Trans., 2014, 43,
1292
R. K. Sharma* and Shivani Sharma
The present work reports the fabrication of a novel and highly efficient silica nanospheres-based palla-
dium catalyst (SiO2@APTES@Pd-FFR) via immobilization of a palladium complex onto silica nanospheres
functionalized with 3-aminopropyltriethoxysilane (APTES), and its catalytic application for the oxidative
amination of aldehydes to yield commercially important amides. The structure of the nano-catalyst was
confirmed by Solid-state 13C CPMAS and 29Si CPMAS NMR spectroscopy, Brunauer–Emmett–Teller (BET)
surface area analysis, Fourier transform infrared spectroscopy (FT-IR), Energy dispersive X-ray fluor-
escence spectroscopy (ED-XRF), Atomic absorption spectroscopy (AAS), Transmission electron
microscopy (TEM) and elemental analysis. The nano-catalyst was found to be highly effective for the oxi-
dative amination of aldehydes using hydrogen peroxide as an environmentally benign oxidant to give
amides. The effect of various reaction parameters such as temperature, amount of catalyst, reaction time,
type of solvent, oxidant used, substrate to oxidant ratio etc. have been demonstrated to achieve high cata-
lytic efficacy. Moreover, this nanostructured catalyst could be recovered with simplicity and reused for
several cycles without any significant loss in its catalytic activity. In addition, the stability of the reused
nano-catalyst was proved by FT-IR and HRTEM techniques. It is worth noting that the features of mild
reaction conditions, simple work-up procedure, high product yield, no use of toxic organic solvents, high
turn-over frequency (TOF), and easy recovery and reusability of the present quasi-homogeneous nano-
catalyst make this protocol an attractive alternative to the existing catalytic methods for the oxidative
amination of aldehydes to furnish industrially important amides.
Received 16th July 2013,
Accepted 6th October 2013
DOI: 10.1039/c3dt51928g
www.rsc.org/dalton
for novel recyclable catalysts that can bridge the gap between
homogeneous and heterogeneous catalysis continues to be a
Introduction
The development of green and economical routes for the large challenging mission in field of catalysis. In this context, the
scale production of fine chemicals is one of the major chal- covalent immobilization of existing homogeneous catalysts
lenges at the industrial level. The field of catalysis significantly onto solid support materials has emerged as a potential
contributes to solving this problem by using highly active and approach to overcome these issues, giving rise to recyclable
selective catalysts.1,2 Although homogeneous catalysis is con- and reusable catalysts, which have conquered new horizons in
sidered to be one of the most proficient and environmentally green catalysis.4–6 These third generation catalysts have
sustainable routes to synthesize high added-value chemicals, uniform and precisely engineered active sites similar to those
the practical applications of many homogeneous catalysts in of their homogeneous counterparts, and therefore combine
industrial processes are hindered by their high costs, multi- the best attributes of both homogeneous (high activity and
step preparation and difficult separation from the reaction selectivity) and heterogeneous (easy separation from the reac-
mixture.3 On the contrary, heterogeneous catalysis allows tion mixture) catalysts. In fact, the last decade has witnessed
facile separation and recycling of the catalyst, yet it suffers an emerging attention to the development of heterogenized
from several drawbacks such as low activity and selectivity as catalysts employing several types of solid support materials
compared to its homogeneous counterpart. Hence, the quest such as polymers, inorganic supports and nanostructured
materials.7–11 Among various solid support materials, nano-
structures are considered to be the most promising support as
they offer several advantages over bulk supports, such as
Green Chemistry Network Centre, Department of Chemistry, University of Delhi,
New Delhi-110007, India. E-mail: rksharmagreenchem@hotmail.com;
increased surface area, high complex loading, and excellent
Fax: +91-011-27666250; Tel: +91-011-27666250
†Electronic supplementary information (ESI) available. See DOI:
stability.12–16 From this perspective, silica nanospheres have
attracted a great deal of attention in the field of catalysis due
10.1039/c3dt51928g
1292 | Dalton Trans., 2014, 43, 1292–1304
This journal is © The Royal Society of Chemistry 2014

View Article Online
Dalton Transactions
Paper
to their inherent outstanding properties such as nanometre (Merck) and furfural (Spectrochem Pvt. Ltd, India) were
size, large surface area to volume ratio, excellent thermal and commercially obtained, and used as received without further
mechanical stability and the presence of silanol groups on the purification. All other starting materials and reagents used in
surface, which allow the incorporation of a wide variety of the reactions were purchased from Alfa Aesar and Sigma
functionalities.17–20 Moreover, these novel “quasi-homogeneous” Aldrich.
catalysts can easily be dispersed in several solvents which facili-
tate the accessibility of the reactants to the catalytic active sites,
Characterization techniques
thereby avoiding the diffusion problems associated with hetero- Solid-state 13C and 29Si cross-polarization magic-angle spin-
geneous bulk catalytic systems. Hence, the fabrication of silica ning (CPMAS) NMR spectra were obtained using a Bruker
nanospheres-supported catalysts is currently undergoing exciting DSX-300 NMR spectrometer. Energy dispersive X-ray fluore-
developments with increasing achievements.21–26
scence (ED-XRF) spectroscopy was performed using a Fischer-
In the present study, we have prepared a silica nano- scope X-Ray XAN-FAD BC. The palladium content in the
spheres-supported palladium nano-catalyst (SiO2@APTES@Pd- catalyst and in the supernatant was estimated by flame atomic
FFR), and investigated its catalytic activity for the oxidative absorption spectroscopy (FAAS) on a LABINDIA AA 7000
amination of aldehydes to yield amides. The importance of atomic absorption spectrometer using an acetylene flame. The
amides in organic chemistry is well recognized as they are inte- optimum parameters for the palladium measurements are:
gral parts of polymers, natural products, and pharmaceuti- wavelength = 244.8 nm; lamp current = 2 mA; slit width =
cals.27,28 Hence, a variety of catalytic methods have been 0.2 nm; fuel flow rate = 0.2 L min−1. Fourier transform infrared
developed for the synthesis of amide functionality from (FT-IR) spectra were collected through a Perkin Elmer Spec-
aldehydes.29–40 However, these methodologies suffer from trum 2000 FT-IR spectrometer. The spectra of the samples
various drawbacks such as harsh reaction conditions, low were obtained using a KBr pellets method in the range of
product yield, tedious work-up, and the use of excess amounts 4000–400 cm−1 under atmospheric conditions with a resolu-
of catalyst, expensive reagents and solvents which drastically tion of 1 cm−1. Elemental analysis (CHN) was performed using
restrict their application in large scale synthesis. Conse- an Elementar Analysensysteme GmbH VarioEL V3.00. Trans-
quently, one of the major challenges in chemical synthesis is mission electron microscopy (TEM) images of the nanomater-
to develop alternative methods of amide bond formation that ials were observed on a TECNAI G2 T30 microscope with an
circumvent these problems. Apart from this, the green syn- accelerating voltage of 300 kV. The samples for TEM analysis
thesis of products without using volatile and toxic organic sol- were prepared by ultrasonically dispersing the nanoparticles
vents is the subject of significant interest in organic synthesis into absolute ethanol and drying a drop of this suspension on
as it often leads to short reaction times, increased yields, easy the carbon-coated copper TEM grids. Energy-dispersive X-ray
workup procedures, and may enhance the chemo-, regio- and (EDX) analysis (equipped with the TEM instrument) was done
stereoselectivity of reactions.41
for detailed composition characterization of the nanomater-
Thus, in continuation of our research work on the design ials. The BET surface areas were determined using a Gemini-
and synthesis of silica-based materials, and their applications V2.00 instrument (Micromeritics Instrument Corp.). Prior to
as metal scavengers, sensors and catalysts for several organic the measurement, solid samples were outgassed under
transformations,42–52 herein we describe the fabrication, vacuum in order to eliminate physisorbed moisture. The amin-
characterization and application of a silica nanospheres-based ation products obtained were analyzed and confirmed on an
heterogeneous palladium nano-catalyst (SiO2@APTES@ Agilent gas chromatograph (6850 GC) with a quadrupole mass
Pd-FFR) for the oxidative amination of aldehydes. High TOFs, filter equipped 5975 mass selective detector (MSD) using
high product yield, short reaction time, mild reaction con- helium as the carrier gas (rate 0.9 ml min−1).
ditions, solvent free conditions, facile preparation and re-
Preparation of the silica nanospheres-based palladium
nano-catalyst (SiO2@APTES@Pd-FFR)
usability of the catalyst are the noteworthy advantages which
make the present catalytic protocol superior to previously
reported ones. Furthermore, use of hydrogen peroxide (H2O2) Firstly, the monodisperse silica nanospheres (SiO2) were syn-
as an oxidant is quite attractive because water is formed as the thesized by hydrolysis of TEOS in ethanol medium in the pres-
sole by-product. To the best of our knowledge this is the first ence of ammonium hydroxide as a catalyst.53 1 mL of TEOS
report wherein the silica nanospheres-based nanostructured was added into 10 mL of ethanol under sonication. While
palladium catalyst has been applied in oxidative amination of sonicating, 10 mL of 25% ammonium hydroxide and 10 mL of
aldehydes to furnish industrially important amides.
ethanol were added to the reaction mixture synchronously
after 5 min. Sonication was further continued for 60 minutes
to get a white turbid suspension, and then the reaction
mixture was centrifuged. The separated silica nanospheres
were washed with water and ethanol respectively, and dried
under vacuum in an oven.
Experimental
Materials and reagents
Tetra-ethyl orthosilicate (TEOS) (99.9%) (Sigma Aldrich), 3-amino-
Then, silica nanospheres were functionalized with APTES to
propyltriethoxysilane (APTES) (98%) (Fluka), palladium acetate give aminopropylated silica nanospheres (SiO2@APTES). For
This journal is © The Royal Society of Chemistry 2014
Dalton Trans., 2014, 43, 1292–1304 | 1293

View Article Online
Paper
Dalton Transactions
that, 1 mL of APTES was added to 50 mL of a vigorously stirred
dispersion of SiO2 in ethanol, and the mixture was stirred over-
night at room temperature.54 The SiO2@APTES was purified by
centrifugation and re-dispersion in ethanol which was
Results and discussion
Catalyst characterization
FT-IR spectroscopy. The FT-IR spectral assignments of
repeated three times. After that, the mixture of SiO2@APTES nanomaterials viz. SiO2, SiO2@APTES and SiO2@APTES@
and furfuraldehyde (FFR) was stirred for 24 h in ethanol solu- Pd-FFR are shown in Fig. 1. The spectra show typical vibration
tion at 60 °C.55 The product was centrifuged and washed with bands of silica materials such as the Si–O–Si asymmetric
ethanol thoroughly to remove unreacted furfuraldehyde and stretching band at 1095 cm−1 and Si–O symmetric stretching
was dried in a vacuum oven. It was then reacted with the solu- band at 805 cm−1. Further, the aliphatic –CH2 stretching
tion of palladium acetate in acetone. The mixture was stirred vibration bands of the propyl chain of the silylating agent
for 24 h at room temperature (Scheme 1) and the catalyst was (APTES) appear at 2927 cm−1 and 2855 cm−1 in the IR spec-
centrifuged and washed thoroughly with acetone. The catalyst trum of SiO2@APTES, that are absent in the case of the silica
was dried in a vacuum at 120 °C.
nanospheres, which clearly confirms that 3-APTES has been
anchored on the silica nanospheres.56 It should be noted that
the band corresponding to the Si–OH group which appears at
960 cm−1 is found to be a little weaker in the spectrum of
SiO2@APTES compared to that in the spectrum of the silica
nanospheres. The reason behind this is the reaction between
the surface Si–OH groups of SiO2 and ethoxy groups of APTES
during the modification reaction. It is also worth noting that
there are no significant changes observed in the SiO2 structure
sensitive vibrations after its modification with APTES. Hence,
it may be inferred that the basic silica framework remains
intact. The spectrum of the SiO2@APTES@Pd-FFR nano-cata-
lyst shows a large number of bands which merely appear as
shoulders because of their superimposition by a broad
General procedure of the catalytic experiments
Aromatic aldehyde (1 mmol) and aromatic amine (1.5 mmol)
were refluxed at 70 °C in the presence of the SiO2@
APTES@Pd-FFR nano-catalyst (20 mg) using hydrogen peroxide
(2 mmol). After completion of the reaction (monitored by TLC,
ethyl acetate–hexane, 1.2 : 1), the reaction mixture was allowed
to cool to room temperature, quenched with water, and
extracted with ethyl acetate. The nano-catalyst was recovered by
centrifugation, and washed with ethyl acetate. The combined
organic layers were dried over Na2SO4, and solvent was
removed under vacuum. The products obtained were analyzed
and confirmed by GC-MS.
Scheme 1 Preparation of the silica nanospheres-based palladium nano-catalyst (SiO2@APTES@Pd-FFR).
1294 | Dalton Trans., 2014, 43, 1292–1304
This journal is © The Royal Society of Chemistry 2014

View Article Online
Dalton Transactions
Paper
Table 1 Physico-chemical parameters of SiO2, SiO2@APTES and
SiO2@APTES@Pd-FFR
Elemental analysis
BET surface
Nanomaterial
C (%)
H (%)
N (%)
area (m2 g−1
)
SiO2
—
5.73
11.35
—
1.21
2.51
—
1.95
3.59
241.23
153.80
79.52
SiO2@APTES
SiO2@APTES@Pd-FFR
SiO2@APTES@Pd-FFR, this was subjected to AAS determi-
nation. For that, the sample was digested with 5 mL aqua regia
in a microwave oven at 400 watt for 15 min. The volume of the
filtrate was then adjusted to 50 mL using double deionized
water. Reference solutions were made with a high degree of
analytical purity in order to get the calibration curves for the
palladium measurement. 0.117 mmol g−1 palladium content
in the catalyst was quantified using a calibration curve in
duplicate for each sample. Further, to support the above obser-
vation, the SiO2@APTES@Pd-FFR nano-catalyst was also sub-
jected to energy dispersive X-ray fluorescence spectroscopy.
The sample was pressed as a homogeneous tablet of the com-
pressed powder. The well resolved peak of palladium was
observed in the ED-XRF spectrum (Fig. 2).
TEM analysis. The nanoparticles were analyzed by using
transmission electron microscopy with a field emission gun at
a working voltage of 300 kV. Fig. 3 shows the TEM micrographs
which provide the information about the shape and approxi-
mate size of the synthetic nanoparticles. TEM micrographs
reveal that these particles are monodisperse and spherical in
shape and have distinct uniformity which is maintained
throughout. Moreover, they do not agglomerate into clusters. It
should also be noted that the morphological homogeneity that
characterized these nanoparticles was maintained even after
the functionalization of organic moieties onto the silica nano-
spheres. Furthermore, the size of functionalized as well as
non-functionalized silica nanospheres was found to be
∼250 nm. EDX peaks due to nitrogen have been detected on
SiO2@APTES which is consistent with the presence of APTES
at the surface of the SiO2 nanospheres (ESI†); this result was
Fig. 1 FT-IR spectra of (a) SiO2 (b) SiO2@APTES (c) SiO2@APTES@
Pd-FFR and (d) recovered SiO2@APTES@Pd-FFR.
polymer band at 1088 cm−1. In addition, the expected
vibration band of the azomethine group appears at 1629 cm−1
.
Thus, the covalent grafting of APTES on SiO2, and successive
immobilization of FFR on SiO2@APTES is confirmed by FT-IR
spectroscopy. Moreover, the spectrum of the recycled nano-
catalyst is also recorded in the range of 4000–400 cm−1 in
order to compare its vibrations with the fresh heterogeneous
nano-catalyst. The results indicate that both recycled as well as
fresh heterogeneous nano-catalysts possess similar properties.
Elemental and BET surface area analysis. The surface area
of the synthesized silica nanospheres obtained by the BET
method is found to be 241.23 m2 g−1. However, the immobili-
zation of organic groups onto the silica nanospheres blocks the
access of nitrogen gas molecules, which ultimately results in a
decrease in the surface area with increases in surface modifi-
cation following this sequence SiO2 > SiO2@APTES > SiO2@
APTES@Pd-FFR.57 This confirms the functionalization of SiO2
with APTES to give SiO2@APTES, and its further modification
with palladium complex to yield SiO2@APTES@Pd-FFR. A nin-
hydrin test was also carried out for the qualitative testing of
accessible –NH2 groups on the surface of the SiO2@APTES. As
expected, the colour of the nanomaterial (SiO2@APTES)
changes from white to blue on addition of the ninhydrin solu-
tion which provides clear evidence for the presence of the
accessible –NH2 group (ESI†). Notably, the test was found to be
negative for the silica nanospheres. Further, the quantitative
estimation of the organic functional groups covalently tethered
onto the surface of silica nanospheres was performed using
elemental analysis, and results are presented in Table 1. The
amount of the APTES attached to the silica nanospheres was
also calculated based on nitrogen percentage, and it was
found to be 1.39 mmol g−1 of support. The observed carbon
and nitrogen contents of SiO2@APTES shows the C/N ratio to
be ∼3. From this, it can be inferred that the APTES has been
successfully incorporated onto the surface of the silica nano-
spheres. In order to quantify the amount of palladium in the
Fig. 2 ED-XRF spectrum of the nano-catalyst (SiO2@APTES@Pd-FFR).
Dalton Trans., 2014, 43, 1292–1304 | 1295
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
Dalton Transactions
Fig. 3 TEM micrographs of (a) SiO2 and (b) SiO2@APTES@Pd-FFR. (c) Recovered SiO2@APTES@Pd-FFR. (d) EDX spectrum of SiO2@APTES@Pd-FFR.
further confirmed by elemental analysis performed on the denoted as the C5 and C4 carbon atoms. Fig. 4(b) presents the
same samples (Table 1). Along with it, the peaks due to palla- solid-state 29Si NMR spectrum of the SiO2@APTES@Pd-FFR
dium in the EDX spectrum of the palladium nano-catalyst nano-catalyst. This spectrum exhibits several peaks which
(Fig. 3(d)) clearly give evidence for the surface modification of correspond to Qn-type groups as well as Tn-type groups. Two
the silica nanospheres with the palladium complex. In
addition, the TEM micrographs were also taken for the recov-
ered nano-catalyst. Interestingly, it was observed that the
shape and size of the particles remain unchanged which ulti-
mately provides evidence that the morphology of the catalyst
remains the same even after the surface modification reaction.
Solid state 13C CPMAS and 29Si CPMAS NMR spectro-
scopy. 29Si and 13C CPMAS solid-state NMR spectroscopy is
considered to be the most powerful technique to determine
the presence of organic functionality on the solid support
material. It provides evidence of the covalent bonding between
the organic functionalities and the silane linking group, i.e.
APTES. So, in order to confirm the immobilization of organic
moieties onto the surface of the silica nanospheres, 29Si and
13C CPMAS solid-state NMR spectra were recorded. The 13C
CPMAS NMR spectrum of SiO2@APTES which is shown in
Fig. 4(a) presents three resonance peaks at δ = 10.7, 23.8 and
44.2 ppm attributed to three carbons (C1, C2 and C3 respecti-
vely) of the functionalized silane linking agent which authenti-
cate that the structural integrity of the APTES group is
preserved even after its immobilization onto the silica nano-
spheres. Apart from this, the spectrum also presents two low
Fig. 4 (a) 13C CPMAS NMR spectrum of SiO2@APTES and (b) 29Si
intensity peaks at δ = 16.8 and 58.1 ppm due to the presence
of small amounts of un-reacted ethoxy groups from APTES
CPMAS NMR spectrum of SiO2@APTES@Pd-FFR.
1296 | Dalton Trans., 2014, 43, 1292–1304
This journal is © The Royal Society of Chemistry 2014

View Article Online
Dalton Transactions
Paper
resonance peaks at −58 ppm and −69 ppm can be ascribed to
the Si atoms of different environments in the organosilane
APTES viz. Si–OH of the C–Si(OSi)2(OH) group (T2) and
C–Si(OSi)3 group (T3) respectively, which showed that the
SiO2@APTES@Pd-FFR nano-catalyst comprises of highly
condensed siloxane network with aminopropyl linking group
covalently tethered to the silica nanospheres. In addition, the
spectrum also possesses two other signals corresponding to
the inorganic polymeric structure of the silica nanospheres at
−110.8 ppm and −101.8 ppm due to the Si(OSi)4 group (Q4)
and Si(OSi)3OH (Q3) respectively.58
Table 2 Oxidative amination of benzaldehyde with piperidine using
various catalystsa
Entry
Catalyst
Conversionb (%)
TON (TOF)c
1
2
3
SiO2
Pd(OAc)2
SiO2@APTES@Pd-FFR
—
98
97
—
d
418 (278)
414 (276)
a Reaction conditions: benzaldehyde (1 mmol), piperidine (1.5 mmol)
and oxidant (2 mmol), catalyst (20 mg), reflux at 70 °C. b Conversion
was determined using GC-MS. c TON is the number of moles of
product per mol of catalyst and TOF = TON per hour. d 0.0023 mmol.
Catalytic activity tests
decomposed immediately after the reaction and could not be
reused. In particular, the SiO2@APTES@Pd-FFR system has
been developed via immobilization onto silica nanospheres to
acquire the advantages of a heterogeneous catalytic system so
that it could be easily recovered and reused for several catalytic
cycles. In addition, the heterogenized catalyst is devoid of the
diffusion problems typically associated with heterogeneous
bulk catalytic systems. This is because the silica nanospheres
can easily be dispersed in several solvents, thereby facilitating
the accessibility of the reactants to the catalytic active sites.
Effect of various solvents. It is well-known that the nature of
the solvent plays a very important role in organic reactions.
Hence, the effect of various solvents on the oxidative ami-
nation of benzaldehyde with piperidine was examined. The reac-
tion was also carried out under solvent-free conditions as it is
considered to be quantitative and waste free. The results are
summarized in Fig. 6. Only 25% and 36% conversions were
obtained when the reactions were performed in chloroform
and toluene respectively. However, in comparison to the non-
polar solvents, polar solvents such as tetrahydrofuran, water,
dichloromethane, ethanol and acetonitrile resulted in better
conversions. Further investigation indicated that the catalyst
showed the best performance with maximum percentage
Effect of the amount of nano-catalyst. In the first set of
experiments, the effect of the amount of nano-catalyst on the
direct transformation of aldehydes to amides was established.
For that, benzaldehyde and piperidine were employed as test
substrates using hydrogen peroxide as the oxidant. The results
are shown in Fig. 5. The conversion of benzaldehyde increased
on increasing the amount of catalyst up to 20 mg. Further
increases in the amount of catalyst did not produce any con-
siderable increase in the conversion. This may be attributed to
the hindrance of the mass transfer between the active sites
and reagent caused by the low dispersity of the excess catalyst
in the reaction media. The best results were obtained when
the reaction was performed using 20 mg of the nano-catalyst.
Comparison of various catalysts. In order to examine the
effects of the catalyst on the direct transformation of aldehydes
to corresponding amides, the model reaction was performed
using various catalysts and the results are presented in
Table 2. As can be seen, the use of simple silica nanospheres
as a catalyst showed quite poor activity. However, the use of
homogeneous Pd(OAc)2 as catalyst gave higher conversion
in comparison to the heterogenized SiO2@APTES@Pd-FFR
catalyst. But, this expensive homogeneous catalyst
Fig. 5 Effect of the amount of catalyst on the model reaction (reaction
conditions: benzaldehyde (1 mmol), piperidine (1.5 mmol) and H2O2
(2 mmol) reflux at 70 °C).
Fig. 6 Effect of various solvents on the oxidative amination of benz-
aldehyde (reaction conditions: benzaldehyde (1 mmol), piperidine
(1.5 mmol), H2O2 (2 mmol), catalyst (20 mg) reflux).
This journal is © The Royal Society of Chemistry 2014
Dalton Trans., 2014, 43, 1292–1304 | 1297

View Article Online
Paper
Dalton Transactions
Table 3 Effect of various oxidants on the oxidative amination of
aldehydesa
Entry
Oxidant
Conversionb (%)
TON (TOF)c
1
2
3
4
5
No oxidant
Pyridine-N-oxide
m-CPBA
TBHP
Hydrogen peroxide
5
49
56
89
97
21 (14)
209 (139)
239 (155)
380 (253)
414 (276)
a Reaction conditions: benzaldehyde (1 mmol), piperidine (1.5 mmol)
and oxidant (2 mmol), catalyst (20 mg), reflux at 70 °C. b Conversion
was determined using GC-MS. c TON is the number of moles of
product per mol of catalyst and TOF = TON per hour.
amination of aldehydes due to the obtained higher conversion
while all other oxidants failed to produce satisfactory results.
Hence hydrogen peroxide was preferred in comparison to
other oxidants. Moreover, it is an environmentally friendly
oxidant, shows good selectivity and produces only water as a
Fig. 7 Effect of time and temperature on the catalytic efficacy of
SiO2@APTES@Pd-FFR for the oxidative amination of aldehyde (reaction
conditions: benzaldehyde (1 mmol), piperidine (1.5 mmol), H2O2 by-product.
(2 mmol), catalyst (20 mg)).
Effect of oxidant to substrate molar ratio. In addition, the
influence of H2O2 to benzaldehyde molar ratio on the oxidative
amination of benzaldehyde was also studied (Fig. 8). While
keeping all the other parameters fixed, three different H2O2–
benzaldehyde molar ratios (1 : 1, 2 : 1 and 3 : 1) were used and
it was found that the 2 : 1 molar ratio gave the maximum per-
centage conversion. If the molar ratio is reduced to 1 : 1, the
overall percentage conversion was lowered. Hence, 2 : 1 molar
ratio is the minimum requirement for the effective oxidative
amination of benzaldehyde. Further, increases in H2O2 con-
centration may enhance the side reactions which clearly
suggest that a large concentration of oxidant is not an essen-
tial condition to maximize benzaldehyde transformation.
SiO2@APTES@Pd-FFR catalyzed oxidative amination reac-
tion. In order to assess the scope and generality of this trans-
formation,awiderangeofaldehydesweresubjectedtoamination
reactions under the optimized reaction conditions (Table 4).
conversion when the reaction was carried out without any
solvent. This may be attributed to the competitive adsorption
of solvent molecules with the substrate molecules on the cata-
lyst surface. Hence, solvent-free conditions are best suited for
this reaction and therefore catalytic studies were carried out in
absence of any solvent.
Effect of time and temperature. It was noticed that the oxi-
dative amination of benzaldehyde is strongly influenced by
variations in the reaction time and temperature. Hence, to
investigate the effect of temperature and time on the catalyst’s
efficacy, the model reaction was performed at diverse range of
temperatures (30–100 °C). Fig. 7 represents the conversion of
benzaldehyde as a function of time and temperature. The
results showed that there was no change in the conversion of
benzaldehyde at room temperature even if the reaction was
continued for 1.5 h. Therefore, the reaction temperature was
raised, and found that the conversion of benzaldehyde
increased with the increasing reaction temperature. The
maximum conversion of benzaldehyde was obtained when the
reaction was carried out at 70 °C. Further increases in reaction
temperature led to a decrease in the conversion of benz-
aldehyde because of the self-decomposition of hydrogen per-
oxide at higher temperatures. Hence, in all cases, neat
reactions were carried out at 70 °C using H2O2 as the oxidant
in the presence of 20 mg of SiO2@APTES@Pd-FFR catalyst
under solvent-free conditions.
Effect of various oxidants. The oxidizing ability of various
oxidants, like pyridine N-oxide, TBHP, hydrogen peroxide and
m-CPBA, were investigated for the oxidative amination of benz-
aldehyde with piperidine using SiO2@APTES@Pd-FFR as a
nano-catalyst. It is evident from Table 3 that the conversion
percentage of benzaldehyde was negligible when the reaction
was carried out without any oxidant. The results showed that
hydrogen peroxide is the best oxidizing agent for the oxidative
Fig. 8 Effect of H2O2–benzaldehyde ratio on the oxidative amination of
benzaldehyde as a function of time (reaction conditions: benzaldehyde
(1 mmol), piperidine (1.5 mmol), catalyst (20 mg), reflux at 70 °C).
1298 | Dalton Trans., 2014, 43, 1292–1304
This journal is © The Royal Society of Chemistry 2014

View Article Online
Dalton Transactions
Paper
Table 4 SiO2@APTES@Pd-FFR nano-catalyst catalyzed oxidative amination of aldehydes with aminesa
Entry
1
Aldehyde
Amine
Product
Time (h)
1.5
Conversionb (%)
97
Selectivityc (%)
99
TON (TOF)d
414 (276)
2
3
4
1.5
2.5
1.5
88
89
92
92
91
95
376 (250)
380 (152)
393 (262)
5
6
2.5
1
88
92
89
97
376 (150)
393 (393)
7
8
1
86
87
93
91
367 (367)
371 (247)
1.5
9
1.5
2
93
89
95
90
95
89
97
89
397 (264)
380 (190)
405 (135)
384 (192)
10
11
12
3
2
This journal is © The Royal Society of Chemistry 2014
Dalton Trans., 2014, 43, 1292–1304 | 1299

View Article Online
Paper
Dalton Transactions
Table 4 (Contd.)
Entry
13
Aldehyde
Amine
Product
Time (h)
1.5
Conversionb (%)
85
Selectivityc (%)
TON (TOF)d
363 (242)
84
88
81
14
15
1.5
3
89
80
380 (253)
341 (114)
a Reaction conditions: aldehyde (1 mmol), amine (1.5 mmol) and H2O2 (2 mmol), catalyst (20 mg), reflux at 70 °C. b Conversion and selectivity
were determined using GC-MS c Selectivity of the amide was calculated based on the aldehyde conversion. d TON is the number of moles of
product per mol of catalyst and TOF = TON per hour.
It was found that in all the cases, aldehydes could be smoothly amide with the help of the palladium nano-catalyst. The entire
converted to the desired products. In general, the reaction pro- mechanism is illustrated as Scheme 2.
ceeded well for substituted aromatic benzaldehydes and
Heterogeneity test. To test the heterogeneity of the catalyst,
amines. For example, benzaldehyde was almost quantitatively the oxidative amination of benzaldehyde with piperidine was
converted to N-benzoylpiperidine (Table 3, entry 1). Aldehydes performed using the SiO2@APTES@Pd-FFR catalyst. Then, the
having electron withdrawing groups (Table 3, entries 2, 3, 4, 7, catalyst was removed after 1 h from the reaction media and the
8, 9, 11, 13 & 14) underwent amination at faster rate in com- resulting supernatant was subjected to further heating. GC-MS
parison to those having electron donating groups (Table 3, analysis of the supernatant obtained after the removal of the
entries 5, 10 & 15). The reason behind this may be the increase solid catalyst was done and the results revealed that the reac-
in the electron deficiency of the carbonyl group of aldehydes tion did not proceed further, once the catalyst was removed
due to the electronic and mesomeric effects, thereby making (Table 5). UV-vis spectroscopy and atomic absorption spectro-
nucleophilic attack by amines easy. The important features of scopic (AAS) analysis was also done to ensure the absence of
the protocol are that the reaction is devoid of unwanted pro- palladium in the supernatant. As, anticipated, the UV-vis spec-
ducts, produces the desired amides in good to excellent yields, trum of the supernatant did not show any absorption peaks
and the nano-catalyst can be reused for seven consecutive characteristic of palladium metal. No traces of palladium were
cycles without any significant loss in its activity. Hence, the detected by AAS analysis of the supernatant after removal of
obtained results highlight the development of a simple proto- the catalyst. These studies demonstrated that the palladium
col using a silica nanospheres-based palladium nano-catalyst was intact to a considerable extent with the solid support
(SiO2@APTES@Pd-FFR) which endows excellent oxidative which clearly indicates the true heterogeneous nature of the
amination of various substituted aromatic aldehydes with catalyst.
different amines affording good to excellent yield of the
Reusability of the catalyst. From an industrial point of view,
desired amide products.
reusability is one of the most important parameters for
Plausible reaction mechanism. Mechanistically, it seems measuring the utility of a heterogeneous catalyst, which is
plausible that aldehyde is first oxidized to the corresponding largely dependent on keeping the active sites of the catalyst
carboxylic acid, which then reacts with amine to form an intact. Thus, the recyclability of the SiO2@APTES@Pd-FFR
amide. However, when benzoic acid is used instead of benz- heterogeneous nano-catalyst for the model reaction was
aldehyde under the same conditions, no sign of amide for- demonstrated successfully. Upon completion of the reaction,
mation was observed. Therefore, it can be assumed that the nano-catalyst was recovered from the reaction mixture by
oxidative amination does not proceed via carboxylic acid as an simple centrifugation and washed with ethyl acetate to remove
intermediate. It is most probable that the oxidative amination traces of the previous reaction mixture. The recovered catalyst
of aldehydes proceeds through carbinolamine intermediate was used for further runs for the same reaction. As evident
formation which subsequently oxidizes to the corresponding from Fig. 9, the catalyst could be reused efficiently several
1300 | Dalton Trans., 2014, 43, 1292–1304
This journal is © The Royal Society of Chemistry 2014

View Article Online
Dalton Transactions
Paper
Scheme 2 Proposed mechanism for the oxidative amination of aldehydes.
times without much considerable reduction in the rate of
reaction under similar experimental conditions. The
recovered nano-catalyst showed only a very slight decrease in
its catalytic activity. The slight loss of catalytic efficiency
was most likely due to the loss of catalyst during the handling
and centrifugation. Further, on comparing the IR spectra
and TEM micrographs of the fresh and recovered catalysts,
it was found that the structural properties of the silica
Table 5 Conversions for the oxidative amination of benzaldehyde
under optimized conditions (with and without the nano-catalyst)
Entry
Time (h)
Conversion (%)
Selectivity (%)
1a
2b
1
3
82
82
85
85
a Reaction conditions: benzaldehyde (1 mmol), piperidine (1.5 mmol),
H2O2 (2 mmol), and catalyst (20 mg), reflux at 70 °C. b GC-MS analysis
of the supernatant after the removal of the nano-catalyst.
nanospheres-based
nanostructured
palladium
catalyst
remained unaltered after being used in the oxidative ami-
nation reaction. Apart from this, the adsorption capacity of the
reused catalyst after seven catalytic cycles was found to be
0.112 mmol g−1
.
Comparison of the catalytic activity of the SiO2@APTES@Pd-
FFR nano-catalyst with the literature precedents. The advan-
tages of the present protocol over existing methods could be
seen by comparing our results with those of some reported lit-
erature precedents. It is clearly evident from Table 6 that the
silica nanospheres-based palladium nano-catalyst showed
good results in terms of the reaction conditions and product
yield in comparison to homogeneous catalysts reported in lit-
erature so far. This may be attributed to the high surface to
volume ratio of the nano-catalyst in comparison to the bulk
catalytic systems. Further, it should also be noted that all the
catalysts which have been previously reported in the literature
for the concerned reaction were homogeneous. These catalysts
decomposed immediately after the reaction and could not be
recovered. On the contrary, the silica nanospheres-based palla-
dium nano-catalyst can be recovered simply by centrifugation
and reused for several cycles without any significant loss of its
catalytic activity.
Fig. 9 Catalytic reusability test for the oxidative amination of benz-
aldehyde with piperidine (reaction conditions: benzaldehyde (1 mmol),
piperidine (1.5 mmol), H2O2 (2 mmol), catalyst (20 mg) reflux at 70 °C).
This journal is © The Royal Society of Chemistry 2014
Dalton Trans., 2014, 43, 1292–1304 | 1301

View Article Online
Paper
Dalton Transactions
Table 6 Comparison of the catalytic activity of the SiO2@APTES@Pd-FFR nano-catalyst with other catalysts reported in literature for the oxidative
amination of benzaldehyde with various amines
Entry
1
Aldehydes
Amines
Catalytic conditions
Time (h)
24
Reusability
Yield (%)
92
Reference
30
Palladium acetate/aryl bromide triphenylphosphine/
K2CO3/1,2-dimethoxy ethane/reflux under argon
—
2
3
[Rh(COD)2]BF4/100 °C/toluene
La[N(TMS)2]3/C6D6/25 °C
8
—
—
78
38
31
33
24
4
PdCl2/Xantphos/H2O2-urea/MeOH/AcOH/50 °C
20
—
Trace
34
5
6
7
KI/TBHP/H2O/80 °C
15
15
24
12
—
—
—
61
63
66
35
35
36
KI/TBHP/H2O/80 °C
RuH2(PPh3)4/NaH/CH3CN
NaOCl/Bu4NHSO4/PEG-400/120 °C
8
9
—
—
81
65
37
38
(i) NCS, CH3CN, rt
(i) 3
(ii) Cu(Ac)2, TBHP, reflux
(ii) 50 min
10
11
12
FeSO4·7H2O, TBHP, CaCO3, MeCN, 60 °C, under
inert atmosphere
6
—
—
74
78
96
39
40
CuSO4·5H2O, TBHP, CaCO3, MeCN, 60 °C, under
argon atmosphere
6
SiO2@APTES@Pd-FFR/solvent free/70 °C/H2O2
1.5
Up to seven
cycles
Present
study
which suggested that the nano-catalyst catalyzes the oxidative
amination in a truly heterogeneous manner. The additional
Conclusion
We have developed a novel, highly selective and recyclable pal- advantages of the present protocol include simplicity, an easy
ladium nanostructured catalyst through the immobilization of work-up procedure, mild reaction conditions, use of a green
a palladium complex onto silica nanospheres functionalized oxidant (H2O2), economic viability, high turn-over frequencies
with 3-aminopropyltriethoxysilane for the oxidative amination (TOFs) and excellent product yields, which make it highly
of aldehydes. The oxidative amination proceeded successfully attractive for addressing the industrial requirements and
in the absence of a solvent without any additive other than the environmental concerns.
nano-catalyst. Furthermore, the activity of the nano-catalyst
was found to be convincingly superior in terms of reaction
time, yield, cost, selectivity and turn over frequency as com-
pared to reported catalysts. This may be attributed to the nano-
metre size of the inorganic support material which allowed the
immobilization of a higher amount of the complex and
Acknowledgements
enhanced the dispersion of the catalytic active sites in the reac- One of the authors, Shivani Sharma, thanks the Council of
tion medium, thus improving their accessibility to the sub- Scientific & Industrial Research (CSIR), New Delhi, India, for
strate and oxidant species. Moreover, this quasi-homogeneous the award of a Junior Research fellowship. The authors also
catalyst could be recovered simply by centrifugation and thank IISc, Bangalore, India for carrying out the solid state
recycled several times without any significant loss in its NMR analysis and USIC-CLF, DU, Delhi, India for the NMR
activity. In addition, a hot filtration test was also performed data.
1302 | Dalton Trans., 2014, 43, 1292–1304
This journal is © The Royal Society of Chemistry 2014

View Article Online
Dalton Transactions
Paper
27 J. M. Humphery and A. R. Chamberlin, Chem. Rev., 1997,
97, 2243–2266.
Notes and references
1 A. Corma and H. Garcia, Catal. Today, 1997, 38, 257– 28 R. K. Mylavarapu, G. C. M. Kondaiah, N. Kolla,
308.
R. Veeramalla, P. Koilkonda, A. Bhattacharya and
2 P. T. Anastas, M. M. Kirchhoff and T. C. Williamson, Appl.
Catal., A, 2001, 221, 3–13.
R. Bandnewichhor, Org. Process Res. Dev., 2007, 11, 1065–
1068.
3 D. J. Cole-Hamilton, Science, 2003, 299, 1702–1706.
4 N. T. S. Phan and C. W. Jones, J. Mol. Catal. A: Chem., 2006,
253, 123–131.
29 W. J. Yoo and C. J. Li, J. Am. Chem. Soc., 2006, 128, 13064–
13065.
30 T. Yoshinao, Y. Yoshimi and Z. Yoshida, Synthesis, 1983,
5 J.-Y. Park, Y.-J. Lee, P. K. Khanna, K.-W. Jun, J. W. Bae and
Y. H. Kim, J. Mol. Catal. A: Chem., 2010, 323, 84–90.
6 M. Moghadama, S. Tangestaninejada, V. Mirkhania,
474–476.
31 A. Tillack, I. Rudolff and M. Beller, Eur. J. Org. Chem., 2001,
523–528.
I. M. Baltorka, A. Mirjafari and N. S. Mirbagheria, J. Mol. 32 T. Naota and S. Murahashi, Synlett, 1991, 693–694.
Catal. A: Chem., 2010, 329, 44–49.
7 M. Kaya, M. Zahmakiran, S. Ozkar and M. Volkan, ACS
Appl. Mater. Interfaces, 2012, 4(8), 3866–3873.
8 S. Miao, C. Zhang, Z. Liu, B. Han, Y. Xie, S. Ding and
Z. Yang, J. Phys. Chem. C, 2008, 112(3), 774–780.
9 S. Yu, H. J. Yun, D. M. Lee and J. Yi, J. Mater. Chem., 2012,
22, 12629–12635.
33 S. Seo and T. J. Marks, Org. Lett., 2008, 10, 317–
319.
34 Y. Suto, N. Yamagiwa and Y. Torisawa, Tetrahedron Lett.,
2008, 49, 5732–5735.
35 K. R. Reddy, C. U. Maheswari, M. Venkateshwar and
M. L. Kantam, Eur. J. Org. Chem., 2008, 3619–3622.
36 S. Muthaiah, S. C. Ghosh, J.-E. Jee, C. Chen, J. Zhang and
S. H. Hong, J. Org. Chem., 2010, 75, 3002–3006.
10 N. Suzuki, J. Yu, N. Shioda, H. Asami, T. Nakamura,
T. Huhn, A. Fukuoka, M. Ichikawa, M. Saburi and 37 J. Liang, J. Lv and Z. Shang, Tetrahedron, 2011, 67, 8532–
Y. Wakatsuki, Appl. Catal., A, 2002, 224, 63–75. 8535.
11 B. Yuan, X. He, Y. Chen and K. Wang, Macromol. Chem. 38 R. Cadoni, A. Porcheddu, G. Giacomelli and L. D. Luca,
Phys., 2011, 212, 2378–2388. Org. Lett., 2012, 14, 5014–5017.
12 I. I. Slowing, B. G. Trewyn, S. Giri and V. S.-Y. Lin, Adv. 39 S. C. Ghosh, J. S. Y. Ngiam, C. L. L. Chai, A. M. Seayad,
Funct. Mater., 2007, 17, 1225–1236.
T. T. Dang and A. Chen, Adv. Synth. Catal., 2012, 354, 1407–
13 I. I. Slowing, J. L. Vivero-Escoto, B. G. Trewyn and
V. S.-Y. Lin, J. Mater. Chem., 2010, 20, 7924–7937.
14 M. Pagliaro, R. Ciriminna and G. Palmisano, J. Mater.
Chem., 2009, 19, 3116–3126.
1412.
40 S. C. Ghosh, J. S. Y. Ngiam, A. M. Seayad, D. T. Tuan,
C. L. L. Chai and A. Chen, J. Org. Chem., 2012, 77(18),
8007–8015.
15 H. E. Bergna and W. O. Roberts, Colloidal Silica: Fundamen- 41 A. H. Jadhav and H. Kim, RSC Adv., 2013, 3, 5131–
tals and Applications, CRC Press, Taylor & Francis, Boca
Raton, USA, 2006.
16 J. M. Rosenholm, C. Sahlgren and M. Linden, Nanoscale,
2010, 2, 1870–1883.
17 Y. L. Choi, J. H. Lee, J. Jaworski and J. H. Jung, J. Mater.
Chem., 2012, 22, 9455–9457.
5140.
42 R. Sharma and S. Dhingra, Designing and Synthesis of Func-
tionalized Silica Gels and their Applications as Metal Scaven-
gers, Sensors, and Catalysts: A Green Chemistry Approach,
LAP Lambert Academic Publishing LAP Lambert Academic
Publishing, Germany, 2011.
18 N. Z. Knezevic, B. G. Trewyn and V. S. Y. Lin, Chem. 43 R. K. Sharma, Y. Monga, A. Puri and G. Gaba, Green Chem.,
Commun., 2011, 47, 2817–2819. 2013, 15, 2800–2809.
19 A. J. Moro, J. Schmidt, T. Doussineau, A. Lapresta-Fernan- 44 R. K. Sharma, S. Sharma, S. Gulati and A. Pandey, Anal.
dez, J. Wegener and G. J. Mohr, Chem. Commun., 2011, 47,
6066–6606.
20 B. Ou, G. Yang, Y. Xiao, X. Zeng, Z. Zhou, Q. Liu, X. Zhang
Methods, 2013, 5, 1414–1426.
45 R. K. Sharma, Y. Monga and A. Puri, Catal. Commun., 2013,
35, 110–114.
and D. Li, J. Macromol. Sci., Part A: Pure Appl. Chem., 2013, 46 R. K. Sharma, S. Gulati, A. Pandey and A. Adholeya, Appl.
50, 25–28.
Catal., B, 2012, 125, 247–258.
21 W. He, F. Zhang and H. Li, Chem. Sci., 2011, 2, 961–966.
22 R. H.-Y. Chang, J. Jang and K. C.-W. Wu, Green Chem.,
2011, 13, 2844–2850.
47 R. K. Sharma and Y. Monga, Appl. Catal., A., 2013, 454,
1–10.
48 R. K. Sharma, A. Pandey, S. Gulati and A. Adholeya,
J. Hazard. Mater., 2012, 209–210, 285–292.
23 X. Du and J. He, Nanoscale, 2012, 4, 852–859.
24 S. Banerjee, J. Das, R. P. Alvareza and S. Santra, New 49 R. K. Sharma, D. Rawat and G. Gaba, Catal. Commun.,
J. Chem., 2010, 34, 302–306. 2012, 19, 31–36.
25 H.-T. Chen, S. Huh, J. W. Wiench, M. Pruski and 50 R. K. Sharma and S. Gulati, J. Mol. Catal. A: Chem., 2012,
V. S.-Y. Lin, J. Am. Chem. Soc., 2005, 127, 13305–13311. 363–364, 291–303.
26 P. Veerakumar, M. Velayudham, K.-L. Lub and 51 R. K. Sharma and P. Pant, J. Hazard. Mater., 2009, 163, 137–
S. Rajagopal, Catal. Sci. Technol., 2011, 1, 1512–1525.
142.
This journal is © The Royal Society of Chemistry 2014
Dalton Trans., 2014, 43, 1292–1304 | 1303

View Article Online
Paper
Dalton Transactions
52 R. K. Sharma and C. Sharma, Catal. Commun., 2011, 12, 56 D. Tang, L. Zhang, Y. Zhang, Z.-A. Qiao, Y. Liu and Q. Huo,
327–331.
J. Colloid Interface Sci., 2012, 369, 338–343.
57 K. B. Sidhpuria, A. L. Daniel-da-Silva, T. Trindade and
J. A. P. Coutinho, Green Chem., 2011, 13, 340–349.
58 C. Pereira, J. F. Silva, A. M. Pereira, J. P. Araujo, G. Blanco,
J. M. Pintadoc and C. Freire, Catal. Sci. Technol., 2011, 1,
784–793.
53 Y. He and X. Yu, Mater. Lett., 2007, 61, 2071–2074.
54 V. Mahalingam, S. Onclin, M. Peter, B. J. Ravoo, J. Huskens
and D. N. Reinhoudt, Langmuir, 2004, 20, 11756–11762.
55 S. Paul and J. H. Clark, J. Mol. Catal. A: Chem., 2004, 215,
107–111.
1304 | Dalton Trans., 2014, 43, 1292–1304
This journal is © The Royal Society of Chemistry 2014