A photocatalytic ensemble HP-T?Au-Fe3O4: Synergistic and balanced operation in Kumada and Heck coupling reactions

Article abstract of DOI:10.1039/d0gc02082f

A supramolecular catalytic ensemble HP-T?Au-Fe3O4 supported by highly branched assemblies of hexaphenylbenzene (HPB) derivatives has been developed. The as-prepared HP-T?Au-Fe3O4 nanohybrid material serves as an efficient catalytic system to prepare biaryl derivatives through the Kumada cross-coupling reaction using aryl chlorides as one of the coupling partners under mild reaction conditions (visible light irradiation, aqueous media, aerial conditions, short reaction time). Through the cooperative effect of Au NPs and Fe3O4 NPs, dual activation of aryl chlorides for the generation of aryl radical intermediates is achieved. On the other hand, oligomeric assemblies contributed significantly to the enhancement of the reaction rate and yield of the product by facilitating the reductive elimination step. Different mechanistic studies confirm the involvement of Au NPs, Fe3O4 NPs and oligomeric assemblies in the synergistic and balanced operation of HP-T?Au-Fe3O4 nanohybrid materials in the efficient completion of the catalytic cycle of the Kumada coupling reaction. Being magnetic, the catalytic ensemble could be recycled for up to five catalytic cycles. The as-prepared supramolecular photocatalytic ensemble also works efficiently in Heck coupling reactions involving aryl chlorides and aryl iodides as the coupling partner.

Full text of DOI:10.1039/d0gc02082f

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DOI: 10.1039/D0GC02082F
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Photocatalytic ensemble HP-T@ Au-Fe3O4: Synergistic and
balanced operation in Kumada and Heck coupling reactions
Received 00th January 20xx,
Accepted 00th January 20xx
Harpreet Kaur, Manoj Kumar* and Vandana Bhalla*
Supramolecular catalytic ensemble HP-T@Au-Fe3O4 supported by highly branched assemblies of hexaphenylbenzene (HPB)
derivative has been developed. The as prepared HP-T@Au-Fe3O4 nanohybrid material serve as an efficient catalytic system
to prepare biaryl derivatives through Kumada cross coupling reaction using aryl chloride as one of the coupling partners
under mild reaction conditions (visible light irradiation, aqueous media, aerial conditions, short reaction time). Through
the cooperative effect of Au NPs and Fe3O4 NPs dual activation of aryl chlorides for the generation of aryl radical
intermediates is achieved. On the other hand, oligomeric assemblies contributed significantly toward the enhancement of
reaction rate and yield of the product by facilitating the reductive elimination step. Different mechanistic studies confirm
the involvement of Au NPs, Fe3O4 NPs and oligomeric assemblies in the synergistic and balanced operation of HP-T@Au-
Fe3O4 nanohybrid materials in the efficient completion of catalytic cycle of Kumada coupling reaction. Being magnetic,
catalytic ensemble could be recycled upto five catalytic cycles. The as prepared supramolecular photocatalytic ensemble
also work efficiently in Heck coupling reactions involving aryl chlorides as well as for aryl iodides as the coupling partner.
DOI: 10.1039/x0xx00000x
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activate aryl chlorides but the requirement of strong source of
irradiation, inert reaction conditions and dry organic solvent as
the reaction media decrease the economic/environmental
advantages of the strategy. The main problem in the operation
of already reported systems is their inability to deliver a
balanced performance. The endowed steric/electronic
features of the catalytic system which accelerate some part of
Introduction
Kumada cross coupling between aryl halide and Grignard
reagent is one of the most powerful methods to prepare
complex materials having wide range of applications in the
field of supramolecular, medicinal and material chemistry.1a-b
Conventionally, palladium based catalytic systems have been
widely explored for performing these reactions efficiently
using aryl iodides/ bromides as one of the coupling partner in
organic media under thermal conditions.2-3 Besides the high
cost/toxicity of catalytic centre and requirement of heating the
reaction mixture at high temperature, most of these reported
catalytic systems exhibit limited substrate scope with regard to
aryl halides. Despite their easy availability and affordable
prices, aryl chlorides are not preferred coupling partners due
to the low nucleofugality of Cl- ions in Kumada coupling under
the palladium catalysed reaction conditions. To overcome this
limitation, several nickel and iron based catalytic systems have
been developed4,5 which could efficiently activate aryl
chlorides as one of the coupling partners but the reaction
conditions required prolonged heating at high temperature,
extra dried reaction solvents and inert atmosphere. Very
the
catalytic
cycle
(oxidative
addition/reductive
elimination/transmetalation) other way round inhibit the
other part. Consequently, overall harsh reaction conditions are
needed for the completion of the whole catalytic cycle. Thus,
development of a catalytic system showing balanced activities
in oxidative addition (activation of aryl chlorides as coupling
partner), transmetalation and reductive elimination steps
under mild reaction conditions (low reaction temperature,
aerial atmosphere, and wet reaction conditions) is a challenge
to be achieved.
A) Previous Report6:
Fe(acac)3
X
NHC-ligand
THF
R'
.
.
.
Inert atmosphere
Dry organic solvent
Strong source of irradiation
R'MgX
Blue LED
Continous flow
method
(R'- Alkyl)
X- Cl
recently, Fe(acac)2 in combination with
N-heterocyclic
carbene has been utilized for the activation of aryl chlorides in
the presence of continuous flow light generated through blue
LED (Scheme 1A).6 Although the reported system could
Easy to prepare reusable catalyst
Visible light promoted
Mixed aqueous media as reaction
solvent
Wide substrate scope with regard
to aryl chlorides
B) Present Work:
hv
X
R'
HP-T@AuFe3O4
R'MgX
H2O/THF(4:6)
R
R
(R'-Aryl,Alkyl)
X- Cl
a. Department of Chemistry, UGC Sponsored Centre for Advanced Study-II, Guru
Nanak Dev University, Amritsar-143005, Punjab, India.
b. E-mail: mksharmaa@yahoo.co.in , vanmanan@yahoo.co.in
c. Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
Aerial conditions
Scheme 1. Comparison of catalytic activity of HP-T@AuFe3O4 nanohybrid
material in Kumada cross coupling reaction with previous literature report.
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ARTICLE
Journal Name
Our
research
work
involves
development
of acceleration of reductive elimination step11a-c, wVeiewplAarnticnleeOdnlitnoe
DOI: 10.1039/D0GC02082F
catalytic/photocatalytic systems consisting of metallic modify the 1@Au-Fe3O4 NPs by switching the HPB based
centre(s) supported by self-assembled assemblies of designed assemblies with more branched and electron rich assemblies
target specific building blocks for carrying out organic based on designed HPB building block 2 in the supramolecular
transformations under mild and ecofriendly conditions.7 To catalytic ensemble. Based on our experience in the field of
make Kumada coupling ‘more green’, we were interested in development of supramolecular catalytic systems, we
the development of a photocatalytic system which could expected that the assemblies of derivative 2 will serve as
exhibit balanced activity in catalysing the both halves of the nanoreactors for the generation of metal NPs and
catalytic cycle under ecofriendly conditions using aryl chlorides subsequently these assemblies will be oxidized to yield highly
as one of the coupling partners. Already reported iron flexible polymeric/oligomeric species (vide infra). Interestingly,
catalysed Kumada coupling reactions highlight the significant derivative 2 formed J-aggregates in mixed aqueous media
contribution of in situ generated highly reactive iron-Grignard which showed strong interactions toward gold and iron ions to
reagent [Fe-(MgX)2] intermediate8 in activation of aryl generate Au-Fe3O4 nanohybrid materials and during this
chlorides in the coupling reactions. Furthermore, catalytic reduction process oxidation of thiophene groups of derivative
activation of aryl halides including aryl chlorides has also been yielded oligomeric assemblies HP-T (Scheme 3). The
reported by using gold based systems supported by electron supramolecular ensemble HP-T@Au-Fe3O4 could catalyse the
rich ligands.9 Learning from all these literature findings, we reaction between chlorobenzene and p-tolylmagnesium
envisioned that by developing
a nanocatalytic system bromide under visible light irradiation with very high efficiency
comprising of gold and iron species, dual activation of aryl (93% yield) in much lower time (20 min) in mixed aqueous
chlorides may be conveniently achieved in the first half of the media as the reaction solvent (Scheme 1B). The noticeable
cycle (Scheme 2).8 We further expected that by using reduction in reaction time and enhancement in reaction yield
plasmonic Au NPs, the requirement of high energy radiations (93%) in the presence of catalytic ensemble HP-T@Au-Fe3O4
for generation of aryl radicals may be overcome.
points to the significant contribution of oligomeric assemblies
in completion of the catalytic cycle. Unprecedented, the as
prepared catalytic system exhibits better efficiency in the
presence of aryl chlorides as coupling partner in comparison to
aryl iodides/fluoride. The cooperative influence of Au NPs and
Fe3O4 NPs (via generation of Fe-Grignard activated complex) in
the nanocatalytic system resulted in higher activity of aryl
chlorides over aryl iodides. Due to its magnetic nature,
catalytic ensemble HP-T@Au-Fe3O4 could be recycled upto five
Ar-R
Au
Fe
RMgBr
Oxidative
Addition
Ar-Cl
i) Tranmetalation
Au
Fe
(MgBr)2
ii) Reductive
Elimination
Ar
A
Activated Fe-MgBr
complex
(facilitated by
branched,
electron rich
assemblies)
Ar-Cl
a)
H
Scheme 2. Proposed catalytic cycle for Kumada coupling reaction showing dual
activation of Ar-Cl and acceleration of reductive elimination step by using
hypothesized catalytic system.
Au NPs
Au3+ + Fe3+
Fe O NPs
3 4
S
Polythio-
phene
S
H O/THF
2
1@ AuFe3O4
HPB 1
S
S
S
S
S
n
n
n
b)
S
n
n
n
S
S
S
S
n
S
n
S
n
S
Further,
we
envisaged
that
besides
acting
as
S
S
S
n
S
S
S
n
S
≡
n
S
S
S
n
S
reactors/stabilizers for metallic species, supramolecular
assemblies based on electron rich scaffold may also accelerate
the reductive elimination step of the catalytic cycle. Recently,
from our lab, we reported catalytic ensemble consisting of
assemblies of hexaphenylbenzene (HPB) derivative 1 and Au-
Fe3O4 NPs which worked as efficient catalytic system for
carrying out C-H activation of activated substrate.10a-b To check
the validity of our hypothesis, we carried out the reaction
between chlorobenzene and p-tolylmagnesium bromide using
already reported catalytic system 1@Au-Fe3O4 NPs under ‘wet’
reaction conditions (vide infra). The reaction was complete in 2
h to furnish the desired product in 72% yield. Though the
n
n
n
S
S
S
S
S
S
n
n
n
S
S
S
S
S
n
S
S
S
n
S
S
S
n
S
n
THF/H O
n
S
2
S
n
S
S
S
n
S
n
S
S
S
S
n
S
S
S
n
S
n
n
n
Derivative 2
HP-T@AuFe3O4
Aggregates of 2
Au NPs
Fe3O4 NPs
HP-T
Scheme 3. a) Structure of derivative
1
and schematic presentation of
1@AuFe3O4. b) Schematic diagram showing the formation of supramolecular
ensemble HP-T@ AuFe3O4.
activation of chlorobenzene in the presence of Au-Fe cycles. The mechanistic studies confirm the facilitation of
nanohybrid materials was an encouragement but the overall generation of aryl radical by Au-Fe3O4 nanohybrid material and
yield and kinetics of the test reaction was not very impressive reductive elimination step by oligomeric assemblies. Thus, as
in comparison to other literature reports. Since electron rich prepared HP-T@Au-Fe3O4 ensemble presents an excellent
and flexible ligands are known to contribute more significantly example of supramolecular catalysis in which whole system
toward enhancement of reaction kinetics/yield through work synergistically to complete the catalytic cycle and the
2 | J. Name., 2012, 00, 1-3
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catalytic efficiency of system is found to be better than other and Fe3+ (5 equiv.) ions to the solution (Fig. 1). These
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reported systems in literature (Table S1, ESI†). Additionally, absorption studies suggest the simultaDnOeIo: u10s.10in3t9e/Dra0cGtCio0n20s8o2Ff
the as prepared supramolecular ensemble HP-T@Au-Fe3O4 assemblies of derivative 2 with gold and iron ions.
also exhibited a balanced performance as a recyclable catalyst
in Heck coupling reaction using aryl chlorides as one of the
a)
b)
coupling partners. The efficiency shown by supramolecular
catalytic ensemble HP-T@Au-Fe3O4 for activating aryl chlorides
in the Heck coupling reaction is found to be better than the
other reported systems in the literature (Scheme 8 and Table
S2, ESI†). To the best of our knowledge, this is the first report
regarding development of recyclable oligomeric HP-T@Au-
Fe3O4 nanohybrid catalytic material for activation of aryl
chlorides as coupling partners under ‘wet’ and aerial reaction
conditions using visible light radiation as source of energy in
Kumada and Heck coupling reactions.
AuCl3
FeCl3
45
Au-Fe3O4 NPs
540 nm
equiv.
Au3++ Fe3+
ions
Preparation of Au-Fe3O4
nanohybrid material
Fig. 1 (a) UV-vis spectra of assemblies of derivative 2 (5 µM; 10-3 M) upon
simultaneous addition of aqueous solution of Au3+ ions (25 equiv, 10-3 M), and
Fe3+ ions (20 equiv, 10-3 M); inset absorption spectra showing the red shifting of
band at 530 nm to 540 nm. (b) Change in color of solution H2O/THF (6:4) of
derivative 2 upon addition of Au3+ and Fe3+ ions.
Results and Discussion
The derivative 2 was synthesized in 70% yield via Diels-Alder
reaction of 2,5-diphenyl-3,4-di(thiophen-2-yl)cyclopenta-2,4 dien-
1-one12 with 1,2-di(thiophen-2-yl)ethyne under the standard
reaction conditions (Scheme S1, ESI†). The structure of the
The transmission electron microscope (TEM) images of
derivative 2 in the presence of Au3+ and Fe3+ ions revealed the
formation of nanospheres encapsulated into the fibrous
network of organic component (Fig. 2a-b). The TEM images
indicated the average size of nanospheres to be in the range of
1.5-2.0 nm which was also confirmed by DLS studies (Fig. S6,
ESI†).
derivative
2 was confirmed from its spectroscopic and analytical
studies (Fig. S1A-S1D, ESI†).
The UV-vis spectrum of derivative 2 in THF exhibits an
absorption band at 282 nm due to π-π* transitions.13 Upon
increasing the water fraction upto 40% in the THF solution, the
absorption band is red shifted from 282 nm to 289 nm with an
increase in the absorption intensity. In the emission spectrum,
derivative 2 shows an emission band at 408 nm in THF solution
which is red shifted to 426 nm with increase in water fraction.
The UV-vis spectrum of derivative 2 showed a blue shift in the
absorption band from 289 nm to 278 nm in H2O/THF (4:6) on
increasing temperature from 25 oC to 75 oC. The emission
studies and temperature dependent absorption studies of
derivative 2 in different H2O/THF suggest the formation of J-
a)
b)
Fig. 2 (a) TEM image (with scale bar of 20 nm) (b) HR-TEM (with scale bar of 5
nm) of HP-T@Au-Fe3O4 nanohybrid material.
aggregates14 (Fig. S2-S4
, ESI†). To explore the applicability of
The powder X-ray diffraction (XRD) measurements showed the
presence of diffraction peaks at 31.55°, 36.11°, 44.31°, 56.48°,
62.81° corresponding to fcc lattice of Fe3O4 NPs and the
diffraction peaks at 37.99°, 45.39°, 64.50° and 77.45°
corresponding to the Au NPs (Fig. 3a).15,17 The X-ray
photoelectron spectroscopic (XPS) analysis showed the
presence of peak at 84.0 and 87.2 eV corresponding to Au 4f7/2
and Au 4f5/2 of the Au(0) state and another peaks at 712.8 and
724.8 eV confirmed the presence of Fe3+ and Fe2+ species in
the Fe3O4 lattice.18Additionally, peaks were observed
corresponding to S2p, C1s and O1s of organic residue (Fig. 3b).
The Inductively coupled plasma mass spectrometery (ICP-MS)
studies show the presence of 38.4 wt % of gold in the HP-
T@AuFe3O4 nanohybrid material (Table S6, ESI†). The
thermogravimetric analysis (DT-TGA) of the sample showed a
nonlinear continuous weight loss above 200 °C; however, no
weight loss was observed above 600 °C. From the weight loss,
the amount of oligomeric species was estimated to be
derivative 2 for the preparation of Au-Fe3O4 nanohybrid
material, we examined the molecular recognition behaviour of
derivative
2
toward Au3+ and Fe3+ ions by absorption
spectroscopic studies. Upon addition of Au3+ ions (0-5 equiv.)
to the solution of derivative 2, a new band appears at 315 nm.
Upon further addition of Au3+ ions (upto 20 equiv.), a surface
plasmon resonance (SPR) band was observed at 530 nm which
indicates the formation of Au NPs.15 While addition of Fe3+ ions
to the solution of derivative 2, the absorption intensity in the
region from 250 to 600 nm is increased which indicates the
interactions of Fe3+ ions with assemblies of derivative 2 for the
generation of Fe3O4 NPs (Fig. S5A- S5B,
ESI†).16 Encouraged by
these results, we planned to use supramolecular assemblies as
platform for reducing gold and iron species simultaneously to
generate Au-Fe nanohybrid material. Interestingly, upon
addition of Au3+ ions and Fe3+ ions together into the solution of
derivative 2, the absorption spectrum showed the appearance
of new a band at 315 nm. Further, the SPR band at 530 nm is
red shifted to 540 nm upon further additions of Au3+ (5 equiv.)
approximately 34.0% (Fig. S7
,
ESI†). Further, the magnetic
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hysteresis measurements supported the superparamagnetic species and the metallic centres (Fig. S11-S12, ESI†).
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DOI: 10.1039/D0GC02082F
behaviour of Au-Fe3O4 nanohybrid material (Fig. S8 and Table After complete characterization of the catalytic ensemble, we
S3, ESI†). All the above results clearly suggest the existence of examined the catalytic potential of supramolecular ensemble
encapsulated Au-Fe3O4 nanohybrid material.
HP-T@AuFe3O4 (200 µL) for Kumada coupling reaction
between chlorobenzene 3a (1 mmol) and p-tolylmagnesium
bromide 4 (4 mmol, solution in THF) under aerial conditions
using visible light irradiation. We used ‘wet’ THF (2 mL) for
dissolving the reactants and introduced 200 µL of the as
prepared supramolecular ensemble HP-T@Au-Fe3O4 (by
mixing Au3+ ions (0.150 mmol, 10-3 M), Fe3+ ions (0.449 mmol,
10-3 M) and oxidized polymer HP-T (0.062 mmol) in 3000 µL of
H2O/THF (4:6) solvent mixture). The reaction went smoothly
and after 20 min, the desired product, 5a was obtained in 93%
yield (Scheme 4). The catalytic efficiency of HP-T@AuFe3O4 is
found to be better than the 1@AuFe3O4. This may be
attributed to two reasons, first is the smaller size of the
nanohybrid materials in case of HP- T@AuFe3O4 (1.5-2 nm) in
comparison to that of 1@AuFe3O4 (8-12 and 5-10 nm). Second,
we believe that due to their branched morphology and
a)
b)
Au
Fe3O4
31.55
37.99
4500
4000
3500
3000
2500
2000
C1s
Fe3O4 NPs
(Fe2p)
O1s
Au NPs
(Au4f)
44.31
36.11
77.45
64.50
62.8
45.39
S2
p
56.48
30
40
50
60
70
80
2 theta (degree)
Fig. 3 (a) XRD pattern of Au-Fe3O4 nanohybrid material and the blue stars and
green spheres correspond to the fcc planes of Au and Fe3O4 nanohybrid
material, respectively. (b) XPS analysis of HP-T@Au-Fe3O4 nanohybrid
material indicated the presence of Au and Fe3O4 species along with
polythiophene species.
After characterization of Au-Fe3O4 NPs, we analysed the electron rich nature, HP-T assemblies are significantly
oxidation of aggregates of derivative 2. The reaction between facilitating the reductive elimination step and hence, reaction
derivative 2 in H2O/THF solvent mixture in the presence of time and product yield is positively influenced in the presence
Au3+ ions (25 equiv.) and Fe3+ ions (20 equiv.) was repeated of HP-T@ AuFe3O4 ensemble as the catalytic system. To
under lab conditions. After the generation of Au-Fe3O4 understand the role of each component of the supramolecular
nanohybrid material (as indicated by colour change of the ensemble, we performed several experiments using
solution), the solution of aggregates of derivative 2 containing chlorobenzene and p-tolylmagnesium bromide as model
nanoparticles was slowly evaporated. After two days, brown substrates. Using a mixture of AuCl3 and FeCl3 as catalyst in the
precipitates appeared which were separated by filtration and model reaction, yield of the desired product was found to be
then washed with organic solvents (THF and CHCl3) to obtain 22% while by using bare Fe3O4 NPs and bare Au NPs,
Au-Fe3O4 nanohybrid material. The filtrate was evaporated to separately, the desired product was obtained in 39% and 35%
acquire the solid residue and its 1H NMR was recorded. The 1H yield, respectively (Table 1, entries 4-6). This supports our
NMR spectrum of the sample showed upfield shifting and assumption regarding potential of AuNPs and Fe3O4NPs to
broadening of aromatic protons. The FT-IR spectrum of the activate aryl chloride independently. In the presence of bare
oxidized species showed the disappearance of the band at Au-Fe3O4NPs19, yield of the target compound was increased to
3124 cm-1 (Fig. 4). From GPC studies, average molecular weight 48% (Table 1, entry 7). This study highlights the important role
of the oxidized species was determined to be 3512.7 which of supramolecular assemblies in enhancing the activity of
indicates oligomerization of derivative 2 (Fig. S9-S10 ESI†). catalytic ensemble. We also examined the model reaction
Thus, we conclude that the aggregates of derivative 2 reduced using supramolecular assemblies HP-T as the catalyst (Table 1,
the Au3+ ions to Au NPs and Fe3+ ions to Fe3O4 NPs and during entry 3), however, formation of final product was not
the reduction process, the aggregates of derivative 2 were observed. Thus, HP-T assemblies are not activating aryl
oxidized to oligomeric species, HP-T. The oligomer HP-T shows chlorides for the generation of aryl radicals but they are
two absorption bands at 280 nm and 320 nm while in emission assisting the metallic centre by facilitating the reductive
spectrum, a broad emission band in the region of 360 to 680 elimination step.
nm (λex=350 nm) was observed. Upon addition of bare Au- Further, we repeated the model reaction by switching the
Fe3O4 NPs (5 mol %) to the same solution, a decrease in the solvent from THF to dioxane and toluene. Reaction proceeded
emission intensity was observed (~65%). This outcome clearly well in dioxane (around 15-20 min) and the product was
supported the possibility of energy transfer between oxidized
obtained in 93% yield. On the other hand, reaction moved
slowly in toluene (45 min) and yield was found to be 80%
(Table S4, ESI†). These studies highlight the importance of
polar solvent in enhancing the kinetics of the reaction.
Derivative 2
350
300
C-Ha bond stretching
250
200
150
CH3
hv
MgBr
Cl
HP-T@AuFe3O4
100
H2O/THF
HP-T
Aerial conditions
50
CH3
5a
3a
3500
3000
2500
2000
1500
1000
500
4
Wavelength (cm-1
Wavenumber (cm)-1
)
Scheme 4. Kumada coupling between chlorobenzene 3a (1 mmol) and p-
tolylmagnesium bromide 4 (4 mmol) catalysed by HP-T@AuFe3O4 nanohybrid
material.
Fig. 4 FT-IR Spectrum of HP-T indicates the formation of oligomeric
species through C-H bonds of thiophene.
4 | J. Name., 2012, 00, 1-3
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Table 1. Kumada Coupling between chlorobenzene 3a (1 mmol), p-
tolylmagnesium bromide 4 (4 mmol) under different catalytic conditions.
relatively lower yield and longer reaction time in case of aryl
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fluoride (Table 2). Thus, HP-T@AuFe3DOO4I: 1i0s.10a3c9t/ivDa0tGinCg020a8r2yFl
chlorides more in comparison to aryl iodides and aryl fluorides.
We further examined the substrate scope of HP-T@AuFe3O4
with regard to aryl chlorides and reactions worked well with
the substrates having substituent at p-position (Table 3,
entries 2-3). For substrates having substitution at o-/m-
position, reaction kinetics was found to be slow (Table 3,
entries 4-6). Though the catalytic system could catalyze the
reaction of p-nitro chlorobenzene, however, no formation of
desired product was observed when 4- chlorobenzonitrile, 4-
chlorophenol and 4-chlorobenzaldehyde were used as one of
the coupling partners (Table 3, entry 7). We believe that
susceptibility of Grignard reagent mediated reaction of
CN/OH/CHO group is the reason for their no reactivity under
the optimized conditions. We also examined the catalytic
activity of system with regard to cyclohexyl magnesium
bromide as the reaction partner, the desired product was
obtained in good yield but longer time was required for the
completion of the reaction (Table 3, entry 8). All the products
were purified by flash chromatography with hexane as an
Entry
Catalyst
Time
Yield (%)
1.
2.
3.
4.
5.
6.
7.
HP-T@AuFe3O4
1@AuFe3O4
HP-T
20 min
2 h
93%
72%
--
4 h
AuCl3 and FeCl3
Fe3O4NPs
4 h
22%
39%
35%
48%
4 h
Au NPs
4 h
Bare Au-Fe3O4NPs
4 h
Under visible light irradiation.
To rule out the possibility of reaction to occur by means of any eluent.
impurities of palladium source, we carried out the ICP-MS
Table 3. Kumada Coupling between aryl chloride (1 mmol), p-
tolylmagnesium bromide (4 mmol) catalysed by HP-T@AuFe3O4 nanohybrid
material (5 mmol).
analysis of the catalytic ensemble. No palladium was detected
down to ppb level (Table S6 in ESI†). Further, to rule out the
possibility of the reaction due to the presence of traces of
palladium in the reagents, we also carried out ICP-MS analysis
of the reaction mixture (chlorobenzene, p-tolylmagnesium
bromide and catalytic ensemble). As expected, no palladium
was detected even in traces in the reaction mixture. (Table S7,
ESI†).
After optimizing the reaction conditions, we examined the
substrate scope of supramolecular ensemble HP-T@AuFe3O4
(200 µL) as photocatalytic system with regard to different aryl
halides and Grignard reagents. Since catalytic ensemble is
prepared in H2O/THF solvent mixture, hence, for keeping the
solvent uniformity, we preferred THF over dioxane for
dissolving the reaction substrates in various reactions. The
reaction of p-tolylmagnesium bromide also proceeded well
with aryl iodide but the desired product was obtained in
Table 2. Kumada Coupling between chlorobenzene 3a (1 mmol), p-
tolylmagnesium bromide 4 (4 mmol) under different catalytic conditions.
CH3
MgBr
CH3
a) w.r.t Au NPs (mmol) in HP-T@AuFe3O4 nanohybrid material.
hv
X
HP-T@AuFe3O4
H2O/THF
We also investigated the catalytic efficiency of the HP-
T@AuFe3O4 nanohybrid material by varying the loading of
catalyst in the model reaction. Upon increasing the amount of
Au3+ ions (from 0.150 mmol to 0.180 mmol), a slight increase
in yield of the target compound 5a (from 93% to 94%) is
observed (Table S5, entry 1, ESI†). On the other hand, when
the amount of catalytic system was lowered to 0.015 mmol,
the target product is acquired in 72% yield and TON is
improved. (Table S5 entry 6, ESI†). These studies highlight the
R
R
Aryl halide
Yield (%)/Time Yield (%)/Time
for X- I
Yield (%)/Time
for X- F
for X- Cl
X
87/ 20 min
93/ 20 min
89/ 20 min
93/20 min
90/20 min
94/20 min
60/3h
---
X
H3C
X
---
H3CO
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excellent catalytic efficiency of supramolecular ensemble HP- min, the reaction in aliquot X was complete to furnish the
View Article Online
DOI: 10.1039/D0GC02082F
desired product (TLC) while no further progress of reaction
T@AuFe3O4.
To understand the mechanism of the reaction, we performed was observed in case of aliquot Y. These studies show that
several experiments and closely monitored the reaction reaction did not proceed through formation of any activated
between 3a and 4 under various conditions. In the absence of intermediate and continuous light irradiation is a necessity to
visible light source, no desired product was observed however, get the final product.
homocoupled product 6 was formed in high yield (Scheme 5 To understand the dependence of reaction kinetics on
(i)). This study indicates the excellent potential of as prepared Grignard reagent, we examined the model reaction by varying
catalytic ensemble HP-T@AuFe3O4 to activate aryl chloride to the number of equivalents of Grignard reagent. Upon
generate aryl radical even in the absence of light source. Next, decreasing the amount of Grignard reagent from 4 mmol to 2
we repeated the reactions in the absence of the catalyst under mmol, the reaction yield was reduced to 82%, however, no
optimized condition and in the absence of the catalyst under change in the yield was observed if more than 4 mmol of
dark conditions (Scheme 5 (ii) and (iii)). Both the reactions did Grignard reagent is used. These studies support our
not proceed and reactants were recovered, quantitatively assumption that besides acting as coupling partner, Grignard
These studies suggests that presence of the catalyst is reagent is also interacting with catalytic system to generate
essential for the generation of aryl radical. Further, we carried
reactive intermediate. To understand the interactions between
Au-Fe3O4 nanohybrid material and Grignard reagent, we
monitored the model reaction using absorption spectroscopy.
Upon addition of Grignard reagent (4) to the solution of
supramolecular ensemble, the absorption spectrum showed
the presence of slightly broadened band in the region of 245-
290 nm. Further, upon irradiating the reaction mixture for 10
min, an absorption band appeared at 252 nm which suggested
the change in the oxidation states of iron in Fe3O4 NPs.20
Finally upon introducing aryl chloride, reaction was complete
and the band at 252 nm disappeared (Scheme 7a).
On the basis of all the experimental results and literature
reports,8,20 we propose that iron species activate the Grignard
reagent to yield the reactive intermediate A which further
activates the aryl chlorides to generate intermediate B.
Further, aryl chloride is also activated to generate the
Ph
Ph
No light (dark)
O4
HP-T@AuFe3
H3C
(i)
6
77%
0%
2 h
(Major)
MgBr
hv
(ii)
Cl
No Reaction
No Reaction
Ph
Without Catalyst
No light
CH3
(iii)
(iv)
Without Catalyst
2 h
3
Ph
4
O4
HP-T@AuFe3
TEMPO-adduct
hv
H3C
TEMPO (1.2 equiv.)
0%
Traces
Scheme 5. Investigation of reaction between chlorobenzene 3a and p-
tolylmagnesium bromide 4 under different set of conditions.
out the same reaction under visible light irradiation and 200µL intermediate B through the formation of aryl radical by Au
of photocatalytic ensemble HP-T@AuFe3O4 nanohybrid NPs. The dual activation of aryl chlorides by Au NPs as well as
material in the presence of radical scavenger (TEMPO, 1.2 iron-Grignard reactive intermediate is the reason behind their
equiv.). After 30-35 min, formation of TEMPO- adduct of aryl high activity in the coupling reaction. Finally, the target
chloride and homocoupled product of aryl chloride (traces) product was generated through transmetalation and reductive
was observed (Scheme5 (iii)). This study confirms the elimination steps as shown in scheme 7(b). The reductive
formation of aryl radical under the optimized conditions. We elimination step is accelerated due to the presence of highly
repeated this reaction thrice but homocoupled product of branched and electron rich assemblies. Thus, on the basis of all
Grignard reagent or its TEMPO-adduct was never detected.
the above studies, the excellent activity of as prepared HP-
To understand the formation of activated intermediate and T@AuFe3O4 ensemble in Kumada coupling reaction is
need for continuous irradiation of the sample, we repeated attributed to its synergistic and balanced performance in the
Au NPs
the model reaction between 3a and 4 under visible light for 10 catalytic cycHleP-.T@AuFe O
4 + 4 + 3a
Fe3O4 NPs
Fe(II)/(III)
3
HP-T@AuFe3O4 + 4 (w/light for 10 min)
min. Afterwards, we divided the reaction mixture in two equal
portions: aliquots X and Y (Scheme 6). Reaction mixture of
aliquot X was allowed to proceed under light whereas reaction
mixture of aliquot Y was kept under dark. After another 10
HP-T
HP-T@AuFe3O4 + 4 (w/o light)
HP-T@AuFe3O4
Ar-R
RMgBr
(252 nm)
Fe-(MgBr)2
complex
Reductive
Elimination
Ar-X
Oxidative
Addition
(MgBr)2
After 10 mins.
MgBr
R
Cl
Ph
Fe
Ar
(MgBr)2
Ar
Activated
Fe-(MgBr)2
complex
A
CH3
hv
C
H3C
(Major)
Ar-X
Aliquot X
MgBrX
Ar
X
hv
HP-T@AuFe3O4
Dark
Transmetalation
RMgBr
Ph
Fe
Ar
(MgBr)2
Reactants
X
H3C
B
Aliquot Y
Scheme 7. (a) UV-vis studies showing interaction of HP-T@AuFe3O4 ensemble
with Grignard reagent. (b) Proposed catalytic cycle for Kumada Coupling
catalyzed by HP-T@AuFe3O4 nanohybrid material.
Scheme 6. Control experiment examining the role of visible light in the progress
of reaction.
6 | J. Name., 2012, 00, 1-3
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Next, we examined the recyclability of the catalytic system for in 68% yield, while using ensemble HP-T in combination with
View Article Online
DOI: 10.1039/D0GC02082F
Kumada coupling reaction by choosing reaction between Fe3O4 NPs as a photocatalyst, formation of product was not
chlorobenzene and p-tolylmagnesium bromide. After the observed. Further we examined the model reaction in the
completion of the reaction, the catalyst was recycled upto five presence of bare Au NPs and bare Fe3O4 NPs. The desired
O
cycles and the final product was formed in 65% yield after the
5th cycle.
X
R
HP-T@AuFe3O4
R
Since presence of bulky electron rich ligands is also known to
facilitate reductive elimination step in Heck coupling reaction,
we planned to examine the potential of HP-T@ by
supramolecular assemblies of perylene bisimide (AuFe3O4
nanohybrid material in activating aryl chlorides and methyl
acrylates as coupling partners. Recently, we reported the
development of catalytic system based on gold NPs stabilized
PBI) for catalysing Heck coupling.21 The as prepared catalytic
system showed decent catalytic activity in presence of aryl
chlorides as the coupling partner (~68% yield).
O
R
K2CO3, Neat, hv, 2h
R
8
7
3
Scheme 9. Heck Coupling between aryl halide
3 (1mmol), acrylates 7
(1.2mmol).
Table 4. Control Experiments for Heck Coupling between chlorobenzene 3a
(1mmol) and methyl acrylate 7a (1.2mmol).
Entry
Catalyst
Time(h)
Yield (%)
Previous Reports22,23
:
.
.
Toxic Pd catalyst
Microwave radiation
and thermal heating
(120 oC)
EtOH/H2O as solvent.
No recyclability.
A) Cl
a
1.
2.
2.5
2
77%
83%
Pd-Ni NPs
MW(120 oC), K2CO3
HP-T@AuFe3O4
H2O/EtOH
b
R
HP-T@AuFe3O4
R
.
.
B)
c
Cl
3.
4.
HP-T@AuFe3O4
HP-T@Au NPs
3
5
---
O
Co cat., Mn
.
Co(III) catalyst
Mn powder as
reducing agent.
Heating at refluxing.
EtOH as a solvent.
No recyclability
n-Bu
Reflux
n-Bu
.
EtOH
68%
O
R
R
.
.
.
Present Work:
5.
5
---
HP-T@Fe3O4NPs
Cl
C)
O
hv
HP-T@Au-Fe3O4
OR'
O
OR'
6.
7.
8.
Bare Au NPs
24
24
8
40%
---
K2CO3, 40 o
Neat
C
R
.
.
.
Reusable catalyst
Visible light promoted.
No additional solvent
R
R'- Me.Et, n-Bu
Bare Fe3O4NPs
required (almost Neat)
Bare AuFe3O4 +
55%
HP-T
Scheme 8. Comparison of activity of as prepared HP-T@AuFe3O4 ensemble in
Heck coupling reaction with reported systems.
a
b
c
Under light irradiation Under light irradiation, 40 oC No light
(dark)
To examine the efficiency of as prepared supramolecular
ensemble HP-T@AuFe3O4 we carried out reaction between product was obtained in 40% yield using bare Au NPs while no
chlorobenzene 3a (1 mmol) and methyl acrylate 7a (1.2 mmol) reaction was observed using bare Fe3O4 NPs. (Table 4, entries
as a model reaction under photocatalytic conditions in the 4-7). These studies clearly show that the reaction is mainly
presence of supramolecular ensemble HP-T@Au-Fe3O4 and catalysed by Au NPs.
K2CO3 as a base (Scheme 9). The reaction went smoothly in the After the optimization of the reaction conditions, we examined
absence of any additional solvent (only solvent of catalytic the substrate scope of HP-T@Au-Fe3O4 nanohybrid material
ensemble) and after 2.5 h the desired product was obtained in for Heck reaction with respect to different aryl halides and
77% yield. The yield was improved to 83% by raising the acrylates (methyl/ethyl/n-Bu acrylates). The reaction went well
o
temperature of reaction mixture to 40 C. We believe that the with different substrates having electron releasing groups at p-
high efficiency of the catalytic system in activating the aryl position (Table 5, entries 2-3) and slight decrease in reaction
chlorides in Heck coupling over PBI supported Au NPs is due to yield when substituents present at m-position or sterically
the smaller size of HP-T@Au-Fe3O4 (1.5-2 nm) nanohybrid hindered (Table 5, entries 4-5) was observed. Reaction also
material as compared to PBI supported Au NPs (2-10 nm). worked well in the presence of electron withdrawing
Another possible reason for high activity of HP-T@Au-Fe3O4 substituent like –NO2 group on the aryl chloride (Table 5, entry
ensemble is facilitation of the reductive elimination step due 6). Presence of long alkyl groups in alkene also furnished the
to the presence of highly branched and electron rich HP-T desired product in high yield (Table 5, entry 7). All the products
assemblies.11 Under dark condition, the formation of desired were purified by flash chromatography using hexane as eluent.
product was not detected (Table 4, entries 1-3). Using HP- Upon switching aryl chlorides with aryl iodides as the reaction
T@Au NPs as a photocatalyst, target compound was furnished partner, the products were obtained in slightly higher yields
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(Table 5). These results clearly show the potential of Au NPs to
activate the aryl iodides more as compared to aryl chlorides.
Next, we examined the recyclability of the catalytic system for
Heck coupling by choosing the model reaction between
chlorobenzene and methyl acrylate. After the completion of
the reaction, catalyst was recycled upto three cycles with
decrease in yield upto 50%.
Conclusions
View Article Online
DOI: 10.1039/D0GC02082F
In conclusion, the work presented in the manuscript describes
the development of supramolecular ensemble HP-T@Au-Fe3O4
which exhibits high catalytic efficiency in activating the aryl
iodides as coupling partners in Kumada coupling due to
cooperative effect of Au-Fe3O4 nanohybrid material. The
present study also reveals the significant contribution of
oligomeric HP-T assemblies in increasing the rate and yield of
the reactions through facilitation of reductive elimination step.
All the reactions proceed very well under mild reaction
conditions using visible light irradiations and aerial conditions
in a short reaction time. Further, this catalytic system shows
recyclability upto five catalytic cycles due to its magnetic
nature. Moreover, the supramolecular ensemble HP-T@Au-
Fe3O4 also works efficiently as recyclable catalyst in Heck
coupling reaction for activation of aryl iodides as well as aryl
chlorides as coupling partners. To sum up, the present study
describes the excellent activity of HP-T@AuFe3O4 ensemble in
Kumada and Heck coupling reaction due to its synergistic and
balanced performance in the respective catalytic cycle.
Table 5. Heck Coupling between different aryl halides (1mmol) and
acrylates (1.2mmol) using HP-T@Au-Fe3O4 nanohybrid material (5 mmol).
Conflicts of interest
“There are no conflicts to declare”.
Acknowledgements
V.B. is thankful to SERB, New Delhi (ref. no. CRG/2018/001274)
and CSIR, New Delhi (No. 02(0358/19/EMR-II) for financial
support.
Notes and references
On the basis of all the experimental results, we propose that
the reaction begins with activation of aryl chloride by Au-Fe3O4
NPs to generate aryl radical which undergoes further alkene
insertion and subsequent β-hydride elimination to furnish final
product as shown in scheme 10.
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