Magnetically recoverable Fe3O4@Au-coated nanoscale catalysts for the A3-coupling reaction

Article abstract of DOI:10.1039/c7dt00058h

The utility of novel Fe₃O₄@Au nanoparticles as magnetically separable and recyclable heterogeneous catalysts for the A³-coupling reaction of aldehydes, amines and terminal alkynes to yield the corresponding propargylamines

Full text of DOI:10.1039/c7dt00058h

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Efficient extraction of sulfate from water using
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Magnetically recoverable Fe3O4@Au-coated nanoscale catalysts
for A3-coupling reaction
Alaa M. Munshi,a Mingwen Shi,a Sajesh P. Thomas,a Martin Saunders,b Mark A. Spackman,a K.
Swaminathan Iyera,* and Nicole M. Smitha,*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
The utility of novel Fe3O4@Au nanoparticles as magnetically integration of metal nanocatalysts with catalyst supports such as
separable and recyclable heterogeneous catalysts for the A3- metal oxides or porous and carbonaceous materials, offer the
15
coupling reaction of aldehydes, amines, and terminal alkynes to possibility of green and sustainable catalysis.14,
Among the
yield the corresponding propargylamines is demonstrated. Herein various metal nanocatalysts that have been investigated for A3-
we present a comprehensive analysis of the experimentally coupling reactions, Au nanoparticles are considered the most
observed trends in the conversions with computational analysis effective due to their ability to activate the alkyne C–H bonds in the
using LUMO density on molecular isosurfaces and the electrostatic A3-coupling reaction.16 Following the report by Kidwai et al. on the
potential (ESP) effects estimated using DFT calculations.
use of Au nanoparticles as a catalyst for A3-coupling reaction,
significant research has been pursued with the aim of preventing
aggregation of the catalyst nanoparticles.17 Datta et al. developed a
novel catalyst by embedding Au nanoparticles in mesoporous
carbon nitride, to prevent the agglomeration of Au nanoparticles.18
More recently, Abahmane et al. reported a promising catalyst
comprising two Au nanocatalysts supported on alumina supports.19
Magnetic nanoparticles such as magnetite (Fe3O4) are an interesting
class of catalyst supports over conventional nonmagnetic colloidal
systems, which enable facile separation of the nanocatalyst from
Propargylamines are essential building blocks for biologically active
compounds and are crucial intermediates in the production of
pharmaceuticals and natural products.1-3 The three-component
coupling (A3-coupling) reaction of an aldehyde, an amine, and a
terminal alkyne with a transition metal catalyst for C–H bond
activation is considered superior to traditional methods such as
nucleophilic addition of Grignard reagents or lithium acetylides to
imines, which involve multistep synthesis/purification and the use
of moisture sensitive agents in a regulated reaction condition.4,
5
21
the reaction mixture using an external magnetic field.20,
Various homogeneous and heterogeneous metal catalysts have
been reported for A3-coupling reactions, which include gold, silver,
copper, iron salts and gold and iridium complexes that exhibit high
catalytic activity and ease of optimisation.6-11 However, these suffer
from the difficulty in separating the catalysts from the reaction
mixture which in turn results in multistep purification.12 Colloidal
nanocatalysts have emerged as a particularly desirable alternative
as they operate at the intersection of homogeneous and
heterogeneous catalysis. They are similar to homogeneous catalysts
in their accessibility and large surface area and mimic
heterogeneous catalysts regarding durability and recyclability.13 In
addition, the size, shape, and composition of metal nanocatalysts
can be controlled by tuning their unique properties. Furthermore,
Furthermore, core- shell nanostructures have a substantial effect in
the catalysis of various reactions.22 Thus different approaches have
been developed to generate core-shell nanocatalysts, with
diameters ranging from 100 to 240 nm, such as
25
Au@CeO2 nanocomposites,23
Au@SiO2
nanoparticles,24,
Ag@Fe3O4 nanocomposites,26 Fe3O4 @MgAl-layered double
hydroxides (LDH)@Au and
nanoparticles27
Au/Fe3O4@polydopamine (PDA) nanoparticles.28 Herein, we report
catalyst system comprising superparamagnetic Fe3O4
a
a
nanoparticle core coated with Au (Fe3O4@Au) for use as a catalyst
in propargylamine synthesis via the A3-coupling reaction. We
demonstrate that this catalyst has excellent recyclability and
permits ease of separation from the reaction mixture using an
external magnetic field. Finally, we discuss the correlation between
the selected aldehyde structures, electron density distribution,
Lowest Unoccupied Molecular Orbital (LUMO) density and the
electrostatic potential (ESP) at the reaction sites on molecular
isosurfaces, and the corresponding conversions using DFT
calculations performed on the aldehyde reactants.

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Fig. 2 Recycling of Fe3O4@Au nanoparticles catalyst in the A3-coupling reaction of
benzaldehyde, piperidine, and phenylacetylene in toluene. Percent conversions were
determined by 1H NMR analysis of the crude reaction mixture.
optimised conditions were determined to be in toluene at 100 °C
for 48 h, which gave a conversion of 94% for this reaction. We note
that in the absence of Fe3O4@Au nanoparticles, no benzaldehyde
conversion was observed and poor conversion results were
obtained when Au or Fe3O4 nanoparticles (7% and 32% respectively)
were used independently as catalysts compared to Fe3O4@Au
nanoparticles as reported in our previous study.30 We next
evaluated the recyclability of the Fe3O4@Au nanoparticles under
the optimised conditions for up to five successive runs. After each
run, the catalyst was separated using an external magnet and
washed with toluene and acetone, then air dried for reuse without
further purification. For the first four cycles, the loss of activity was
negligible (Figure 2), while after the fifth cycle, there was a slight
reduction of 13% from the initial catalytic efficiency. This decrease
could possibly be attributed to leaching of the catalyst, as indicated
by ICP (Inductively Coupled Plasma) analysis (Supporting
Information).
Fig. 1 TEM image of a) PEI coated Fe3O4 (Fe3O4-PEI) nanoparticles; b) Bright-field TEM
image and c) corresponding dark-field TEM images of Au seed decorated Fe3O4-PEI
nanoparticles; d) Bright-field TEM image and e) corresponding dark-field TEM images of
Au-coated Fe3O4 (Fe3O4@Au) nanoparticles. Scale bars a), d) and e) 100 nm; b) and c)
200 nm.
Fe3O4@Au nanoparticles with sizes of 100 ± 2 nm (average ±
standard error mean) were fabricated in a multistep assembly
process (described in Supporting Information).29 Briefly, stable core
Fe3O4–PEI (polyethyleneimine) nanoparticles were formed with
sizes of 88 ± 2 nm (Figure 1 A). To facilitate the Au shell formation,
the Fe3O4 nanoparticles were first decorated with 2 nm Au seeds
(Figures 1 B and C), followed by Au shell formation. The formation
of uniform Au-coated Fe3O4 nanoparticles was validated using
transmission electron microscopy (TEM) and energy-dispersive X-
ray spectroscopy (EDS) (Figures 1D, 1E and Supporting Information
Figure S1).
With the optimised conditions, we next examined the scope of the
A3-coupling reaction for various aldehydes with piperidine and
phenylacetylene (Table 1). Formaldehyde and benzaldehyde
showed high conversion (95% and 94% respectively) (Table 1,
entries 1 and 2). In the case of 1-naphthaldehyde (Table 1, entry 3)
the lower conversion (28%) relative to benzaldehyde can be
attributed to the bulkier substituent. Furthermore, 4-
methoxybenzaldehyde (Table 1, entry 5) resulted in
a lower
The reaction medium plays a crucial role in the A3-coupling
reaction; hence, a variety of solvents (methanol, chloroform, water,
and acetonitrile) were screened to optimise the catalytic activity of
the Fe3O4@Au nanoparticles (Supporting Information, Table S1).17
The A3-coupling reaction of benzaldehyde (1 mmol), piperidine (1
mmol), and phenylacetylene (1 mmol) was initially performed with
10 mol% Fe3O4@Au nanoparticles as the catalyst under a nitrogen
atmosphere using above-mentioned solvents. Among the solvents
tested, an excellent conversion of 70% (determined by 1H NMR
analysis) was observed in toluene at 100 °C after 24 h. The overall
conversion (25%) compared to 4-methylbenzaldehyde (63%) (Table
1, entry 4). This can be due to the stronger electron-donating
resonance effect and increased bulkiness of the methoxy (-OCH3)
substituent at the para position relative to the latter with a methyl
(-CH3) substituent. Similarly, 4-chlorobenzaldehyde (Table 1, entry
7) had a lower conversion (29%) than 4-fluorobenzaldehyde (67%)
(Table 1, entry 6), which could be partly due to the lower
electronegativity and larger radius of the Cl atom. 3-
nitrobenzaldehde (Table 1, entry 8) exhibited the lowest conversion
(7%), which could be attributed to the presence of the strong
2 | J. Name., 2012, 00, 1-3
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Reaction conditions: aldehyde (1 mmol), piperidine (1 mmol), phenylacetylene (1
electron withdrawing nitro group. For the heterocyclic aldehydes, 2-
furaldehyde and 2-thiophenecarboxaldehyde (Table 1, entries 9 and
10 respectively), the latter displayed a lower conversion
a
mmol) and Fe3O4@Au nanoparticales (10 mol %) in Toluene (3 mL). LUMO
density of carbonyl carbon for aldehydes modelled by B3LYP, 6-311++G (d, p)
basis set. Superscript indicates the rank of LUMO associated with carbonyl carbon
compared with other atoms in the molecule. b Electrostatic potential value of the
Table
piperidine and phenylacetylene, LUMO density and ESP of aldehydes.
1
Fe3O4@Au nanoparticles catalysed A3-coupling reaction of aldehydes,
c
carbonyl carbon. Conversions determined by 1H NMR of crude reaction
mixtures. au, atomic units.
(26%) which may be due to the lower electronegativity and larger
radius of the S atom compared to the O atom. Finally, in the case of
the pyridinecarboxaldehydes with nitrogen in the ortho, meta and
para positions (Table 1, entries 11, 12, and 13 respectively)
LUMO densitya
ESP/aub
Conversion (%)c
displayed
a range of conversion rates 63%, 20%, and 53%,
Entry
R
respectively. This owing to the role of nitrogen as an ortho/para-
director. Herein the electron density is increased in the meta
position but decreased in the ortho and para positions via
resonance effects in the pyridinecarboxaldehyde.
1
H
0.126
95
0.199 au1
0.139 au1
In order to determine the influence of the ESP and LUMO density of
the aldehydes in determining the overall conversion, DFT
calculations were performed at the B3LYP/6-311++G (d, p) level
2
3
4
Ph
0.103
0.099
0.099
94
28
63
(Table 1). Since the carbonyl carbons in the aldehydes are more
32
susceptible to nucleophilic addition by piperidine,31,
the
1-Naphthyl
4-MeC6H4
optimised geometries and LUMO density localised near the
carbonyl carbons (electrophilic centres) were evaluated. The
propensity of nucleophilic attack may be understood in terms of the
localisation of the LUMO at the electrophilic site, which we
estimate here as the ESP and LUMO density on the promolecular
isoelectron density surface using CrystalExplorer software.33 It is
apparent that the ESP is altered with different substituents. For
example, benzaldehyde (Table 1, entry 2) has a low ESP (0.103 au)
that could be attributed to the weak electron-donating properties
of benzene (-Ph). In addition, 4-methylbenzaldehyde and 4-
methoxybenzaldehyde (Table 1, entries 4 and 5) have comparable
ESPs (0.099 and 0.094 au respectively), the lower ESPs compared to
benzene may be due to the stronger electron-donating effects of
the methoxy (-OCH3) and methyl (-CH3) substituents at the para
position. However, 3-nitrobenzaldehde (Table 1, entry 8) exhibited
the highest ESP (0.122 au) which could be attributed to the
presence of the strong electron withdrawing nitro group.
Accordingly, based on the computational analysis the more electron
donating the substituent group is, the lower the ESP at the carbonyl
carbon. In addition, the common feature among these different
aldehydes is that the majority have the highest LUMO density at
carbonyl carbon, indicating that that the carbonyl carbon is the
favoured position for nucleophilic (piperidine) attack. The two
aldehydes that deviate from this trend are 1-naphthaldehyde and 3-
nitrobenzaldehyde, which have the 2nd and 7th highest LUMO
density respectively at the carbonyl carbon. However, no obvious
correlation between conversion and LUMO density as well as ESP
was deducible. Therefore, we conclude that in order to fully
understand the nature of the reactants and the experimentally
observed conversions, it is important to take into account the
important role and nature of the nanocatalyst in the calculations.
However, due to the limitations of DFT we were unable to
incorporate the nanocatalyst in this study.
0.108 au2
0.136 au1
5
6
7
8
4-MeOC6H4
4-FC6H4
0.094
0.106
0.111
0.122
25
67
29
7
0.138 au1
0.137 au1
0.131 au1
4-ClC6H4
3-NO2C6H4
0.050 au7
9
2-Furfuryl
0.102
78
0.139 au1
0.126 au1
0.130 au1
0.130 au1
0.132 au1
10
11
12
13
2-Thiophenyl
2-Pyridyl
0.102
0.108
0.114
0.119
26
63
20
53
3-Pyridyl
4-Pyridyl
Finally, we propose a mechanism for the A3-coupling reaction
catalysed by Fe3O4@Au nanoparticles (Scheme 1). The reaction is
initiated by the coordination of the terminal phenylacetylene to
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Fe3O4@Au nanoparticles to activate the C-H bond. The Au- funded by the University, State and Commonwealth Governments.
phenylacetylide intermediate is formed on the surface of the A. M. Munshi would like to thank Umm Al-Qurah University,
Fe3O4@Au nanoparticles, due to the high alkynophilicity of Au Makkah, Saudi Arabia for a postgraduate scholarship. Support of
35
metal.34,
Meanwhile, the aldehyde and piperidine form the the work from the Australian Research Council and The Perth Mint
iminium ion in situ, which then reacts with the Au-acetylide by is also acknowledged.
nucleophilic addition to give the desired propargylamine and
regenerate the active nanoparticle catalyst for further reactions.
Notes and references
The authors declare no competing financial interests.
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The authors acknowledge the Australian Microscopy
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Magnetically recoverable and recyclable Fe3O4@Au nanocatalysts for A3-
coupling reactions