Aggregates of a hetero-oligophenylene derivative as reactors for the generation of palladium nanoparticles: a potential catalyst in the Sonogashira coupling reaction under aerial conditions

Article abstract of DOI:10.1039/c5cc06158j

The utilization of Pd nanoparticles stabilized by aggregates of hetero-oligophenylene derivative 3 as an excellent catalyst in a copper/amine free Sonogashira coupling reaction under aerial conditions at room temperature has been demonstrated.

Full text of DOI:10.1039/c5cc06158j

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DOI: 10.1039/C5CC06158J
Journal Name
COMMUNICATION
Aggregates of Hetero-oligophenylene Derivative as Reactors for
the Generation of Palladium Nanoparticles: A Potential Catalyst in
Sonogashira Coupling Reaction under Aerial Conditions
Received 00th January 20xx,
Accepted 00th January 20xx
Preet Kamal Walia, Subhamay Pramanik, Vandana Bhalla* and Manoj Kumar*
DOI: 10.1039/x0xx00000x
www.rsc.org/
unactivated substrates under environmentally benign
conditions without the need for any measures for exclusion of
air/moisture still remains a challenge.
The utilization of Pd nanoparticles stabilized by aggregates of
hetero-oligophenylene derivative
3 as excellent catalyst in
copper amine free Sonogashira coupling reactions under aerial
/
Our research interests involve development of fluorescent
supramolecular aggregates for the reductant free preparation
of metal nanoparticles which have further been utilized in
various organic transformations.5a-b Recently, we used
conditions at room temperature has been demonstrated.
Palladium catalyzed Sonogashira coupling reaction is a well-
established method for the synthesis of aryl alkynes which are
important building blocks for the preparation of natural
products, pharmaceutical ingredients, electronic materials and
agrochemicals.1 Under conventional conditions of Sonogashira
coupling reaction, alkynylation of expensive aryl
iodide/bromide is carried out in the presence of copper and
palladium salts. Though these reactions have high synthetic
utility yet the requirement of expensive, unrecoverable, toxic
reagents in the reaction mixture and formation of moisture/air
sensitive copper-acetylide species2 preclude the wide use of
Sonogashira coupling in industry. To overcome these
limitations, new approaches have been developed for carrying
out C-C coupling reactions using metal acetylides and excess of
amine3 instead of copper salts. However, utilization of excess
of amine (even as solvent) in these copper-free reaction
conditions actually decreased the economic and
environmental advantage of the methodology. Furthermore,
most of these reported catalytic systems work efficiently only
in the presence of activated alkynes or aryl iodide/bromide.
During the last few years, enormous efforts have been made
to use aryl chlorides4 in copper free Sonogashira coupling
reactions using highly active palladium catalyst. However,
these reactions suffer from various limitations such as high
temperature and excess amount of aryl chlorides (in some
cases five folds) to get the target compound in considerable
yields. Therefore, development of an efficient catalytic system
for carrying out Sonogashira coupling reaction using
aggregates of pentacenequinone derivative
6 having pyridine
groups as reactors and stabilizers for the preparation of
palladium nanoparticles.5b In continuation of this work, we
were then interested in developing palladium based efficient
catalytic system for carrying out Sonogashira coupling under
mild conditions. Keeping this in mind, we have designed and
synthesized heteroligophenylene derivative
3 having amino
and pyridine groups. Nonplanar heteroligophenylene moiety is
the scaffold of our choice due to its self-assembly behaviour.6
We envisaged that the presence of electron rich hetero-
oligophenylene scaffold could influence the catalytic activity
and stability of in situ generated metal NPs in the Sonogashira
coupling involving unactivated substrates. Pyridine and amino
group were incorporated to enhance the affinity of derivative
3
towards palladium ions.5b,7 As expected derivative
3 formed
fluorescent aggregates in mixed aqueous media which
exhibited sensitive response towards Pd2+ ions and during the
sensing process aggregates of derivative
3 served as reactors,
stabilizers and shape directing agents for the preparation of Pd
NPs. The work being presented in this manuscript shows the
tuning of shape of the in situ generated Pd NPs by changing
the number/topology of binding units in the host molecules.
To our pleasure, in situ generated Pd NPs exhibited high
catalytic efficiency and stability in Sonogashira coupling
reaction involving aryl iodides and furnished the desired
products in high yields under aerial conditions at room
temperature. Isolation of the product from the catalytic
reaction was very easy as all the products were isolated by
simple extraction with organic solvent followed by
recrystallization. Interestingly, the catalytic activity of Pd NPs is
quite impressive in the catalytic reactions involving aryl
chlorides and even when extended to deactivated electron rich
a.Department of Chemistry, UGC Sponsored Centre for Advanced Studies-1, Guru
b.Nanak Dev University, Amritsar-143005, Punjab, India.
c. E-mail: vanmanan@yahoo.co.in; mksharmaa@yahoo.co.in
d.Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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aryl chloride. Various methods have been reported for the increase in the absorbance over the entire spectral range from
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DOI: 10.1039/C5CC06158J
generation of Pd Nps However, most of these methods suffer 250 to 600 nm was observed (Fig. S10(A), ESI†). In the
.
from several limitations of requiring longer reaction times, fluorescence spectrum, the quenching of the emission was
high temperature, presence of reducing agents and additives. observed upon addition of Pd2+ ions (2 equiv.) to the solution
In contrast, the method being reported in the present of aggregates of derivative
3
in mixed aqueous media
also
manuscript for the preparation of Pd NPs at room temperature (H2O/EtOH, v/v, 7:3) (Fig. 1(A)). Aggregates of derivative
3
is fast, facile and free of reducing agent, which makes this displayed chromogenic behaviour towards Pd2+ ions with a
method better than the other methods reported in the colour change from colourless to brown that is clearly visible
literature (Table S1, ESI†). Additionally, the catalytic efficiency to naked eye (Fig. S10(B), ESI†). The time resolved
of in situ generated Pd NPs utilized for Sonogashira reaction is fluorescence studies of derivative
3 in the presence of
better than the other reported systems (Table S2, ESI†). To the palladium ions suggest the static quenching (Fig. S11(B), ESI†).
best of our knowledge, these in situ generated Pd NPs serve as Therefore, we have calculated the Stern Volmer constant
one of the best catalytic systems for carrying out copper and (3.8×105 M-1) from the linear plotting which was observed at
amine free Sonogashira cross coupling reactions at room lower concentrations of Pd2+ ions (upto 2.5 µM) added to the
temperature in mixed aqueous media under aerial conditions.
Heteroligophenylene derivative was synthesized via Diels– detection limit of aggregates
Alder reaction of 4-phenylethynyl-phenylamine8 with 2,5- 6.83 x 10-7 M (Fig. S12, ESI†).
solution of aggregates of derivative 3
. (Fig. S11(A),ESI†).10 The
3
3
for Pd2+ ions was found to be
diphenyl-3,4-di(pyridin-2-yl)cyclopenta-2,4-dienone9
diphenylether at 240ᵒC in 78% yield. The structure of
derivative was confirmed from its spectroscopic and
in
(A)
0 equiv.
Pd2+
2 equiv.
(B)
(C)
(1 1 1)
3
226.68 pm
analytical data (Fig. S1-S3, ESI†).
20 nm
Ph2O
1
Reflux
Fig. 1. (A) Changes in the fluorescence spectrum of derivative
addition of Pd2+ in H2O/EtOH (7:3); λex = 270 nm. (B) TEM image of the in situ
generated Pd NPs of derivative . (C) HR-TEM of the Pd NPs of derivative 3.
3 (5 µM) upon
2
Scheme 1. Synthesis of derivative
3.
3 (78%)
3
The UV-vis spectrum of compound
absorption band at 236 nm with a shoulder band at 270 nm obtained after the evaporation of solution mixture of
due to π-π* and n-π* transitions. Upon addition of water (70% compound
and Pd2+ ions in mixed aqueous media
volume fractions) to the EtOH solution of compound , the (H2O/EtOH, v/v, 7:3) showed the formation of crystal face-
absorption band at 236 nm is red shifted to 240 nm (Fig. S4(A), cantered cubic (fcc) lattice of Pd(0) (Fig. S13, ESI†). The
ESI†). The fluorescence spectrum of compound in EtOH transmission electron microscopy (TEM) image of aggregates
exhibits dual emission peaks at 430 nm and 435 nm when of derivative in the presence of Pd(II) (1:1) ions showed the
excited at 270 nm. Upon addition of water (70% volume formation of spherical nanoparticles of average diameter in
fractions) to the EtOH solution of compound , the emission the range of 2 to 10 nm (Fig. S14, ESI†). On the basis of DLS
3 in EtOH exhibits an The powder X-ray diffraction (XRD) studies of the precipitates
3
3
3
3
3
band at 435 nm is red shifted to 440 nm (Fig. S4(B), ESI†). measurements, an average diameter of 7 nm could be
Thus, the UV-vis and fluorescence studies suggest the assigned to Pd NPs (Fig. S15, ESI†). When ratio of aggregates of
formation of J-type assemblies in case of derivative
temperature-dependent UV-vis studies of derivative
H2O/EtOH (7:3) solvent mixture and the concentration- 1:2 nanorods and nanospheres were observed (Fig. S16, ESI†).
dependent 1H NMR studies of derivative
in CDCl3 support the These studies suggest that by changing the amount of
formation of J-type assemblies (Fig. S5 and S6, ESI†). The aggregates of derivative
3
. The derivative
3
to that Pd2+ ions was increased from 1:1 to 2:1,
3
in nanorods of Pd NPs were observed while switching this ratio to
3
3
, shape of Pd NPs could be
transmission electron microscopy (TEM) image of compound
3
controlled.
in the solvent mixture of H2O/EtOH (v/v, 7:3) shows the
presence of aggregates having flake like morphology (Fig. S7,
ESI†). The DLS studies also indicate the formation of
aggregates having an average diameter in the range of 900 nm
(Fig. S8, ESI†).The solution of aggregates is visibly transparent
and stable at room temperature for several weeks.
4
5
6
Fig. 2. Structures of derivatives 4, 5 and 6.
To get insight into the mechanism of formation of Pd NPs, we
The presence of amino and pyridine moieties prompted us to slowly evaporated the solution of aggregates of derivative
3
evaluate the binding behaviour of aggregates of derivative
3
containing Pd NPs. The precipitates were observed after two
toward different metal ions such as Cu2+, Fe2+, Fe3+, Hg2+, Co2+, days which were filtered and washed with CHCl3 and THF. The
Pb2+, Zn2+, Ni2+, Pd2+ , Cd2+, Ag+, Au3+, Ba2+, Mg2+, K+, Na+ and Li+ 1H NMR spectrum of the residue so obtained after evaporation
as their perchlorates/chloride salts by UV-Vis and fluorescence of the solvent showed the downfield shift of signals
studies (Fig. S9, ESI†). Upon addition of Pd2+ ions (2 equiv.) to corresponding to aromatic protons (Fig. S18, ESI†). On the
the solution of derivative
3
(5 µM) in H2O/EtOH (v/v, 7:3), an basis of above results, we suggest that upon addition of Pd2+
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ions to the solution of aggregates of derivative
COMMUNICATION
3
, the Further, the model reaction between iodobenzene
View Article Oannlinde
DOI: 10.1039/C5CC06158J
palladium ions interact with nitrogen atoms of pyridyl moieties phenylacetylene when carried out in toluene required higher
and that of amino group and get reduced to Pd(0). We believe reaction temperature and the product was obtained in
that the aggregates of derivative
and during this process they themselves get oxidized. To suggest that the polar protic media is more suitable for
confirm the oxidation of aggregates of derivative , we carried carrying out the Sonogashira coupling reactions. Furthermore,
out fluorescence studies of aggregates of derivative in the the model reaction remained unaffected by change in the
3
reduce Pd2+ ions to Pd(0) relatively lower yield (entry 7, table S3, ESI†). These studies
3
3
presence of tert-butyl hydroperoxide a strong oxidizing agent. amount of base (entries 8 and 9, table S3, ESI†). However, the
The fluorescence spectrum exhibited the quenching of the same reaction when carried out in the absence of K2CO3,
emission bands at 430 and 435 nm as was observed in the showed almost no conversion (entry 10, table S3, ESI†). Thus,
presence of Pd2+ ions (Fig. S19(A), ESI†). This result clearly basic medium is essential for carrying out these reactions. We
indicates the reduction of Pd2+ ions to Pd(0) and oxidation of also carried out the reaction between aryl iodide and
derivative
3
. We also carried out the UV-vis studies to confirm phenylacetylene using aggregates of derivative
4 and 5
the reduction of Pd(II) to Pd(0) in the presence of compound
3
stabilized Pd NPs, respectively and in both cases reaction took
(Fig. S19(B), ESI†). The Pd(II) ions in mixed aqueous media longer time for completion and desired product was obtained
(H2O/EtOH, v/v, 7:3) exhibited a band at 420 nm. On addition in lower yields (entries 2 and 3, table S4, ESI†). The faster
of aggregates of derivative
indicating the formation of Pd (0) nanoparticles.11 Thus, stabilized Pd NPs may be attributed to the spherical shape of
aggregates of derivative function as reactors and stabilizing the nanoparticles and spherical shaped NPs provide large
agents for the preparation of Pd NPs at room temperature. surface area for the reaction, consequently rate of reaction is
3, this band gradually disappeared, reaction rate observed in case of aggregates of derivative 3
3
To understand the influence of scaffold on morphology and increased. We also carried out the model reaction between
catalytic activity of generated metal nanoparticles, we also aryl iodide and phenylacetylene using aggregates of
synthesized heteroligophenylene derivatives 48 and 512 (Fig. 2). pentacenequinone derivative
6 stabilized Pd NPs (entry 4,
Interestingly, absorption and emission studies of both the table S4, ESI†). Interestingly, the reaction was complete in 6
derivatives suggest affinity of these molecules towards Pd2+ hours and the product was obtained in 68% yield. This study
ions. The calculated Stern-Volmer constants of aggregates of suggests that spherical Pd NPs stabilized by electron rich
derivatives
and 6×105 M-1, respectively (Fig. S11(C)-(D), ESI†). The TEM carrying out Sonogashira coupling. We also carried out the
image of derivative in the presence of palladium ions showed reaction between aryl iodide and phenylacetylene using 1
the formation of nanospheres and nanocubes whereas the mole % of PdCl2 in H2O/EtOH (v/v, 7:3) solvent system in the
TEM image of derivative showed the presence of nanorods absence of aggregates of derivative (entry 5, table S4, ESI†).
4 and 5 3 are more suitable for
for Pd2+ ions were found to be 5×105 M-1 heteroligophenylene derivative
4
5
3
as well as nanospheres (Fig. S17, ESI†). These studies suggest Under these conditions, the reacting substrates remained
that presence of amino as well as pyridine groups in the unchanged even after 15 hours. On the basis of these studies,
heteroligophenylene scaffold is essential for generation of we conclude that aggregates of derivative
3 stabilized NPs are
spherical NPs. We believe that aggregates of derivative form promising catalysts for carrying out copper and amine free
3
a uniform layer around the metal ions and bind strongly with Sonogashira coupling reaction. For further studies, we selected
palladium ions which increases the rate of reduction for the H2O/EtOH (v/v, 7:3) as the solvent mixture for carrying out
preparation of Pd(0) nanoparticles. Thus, higher number of various reactions (Scheme 2).
nanoparticles is available in this case which leads to uniform
Pd NPs
growth of nanoparticles in all directions resulting in the
formation of spherical nanoparticles.
Having done all this, we were then interested in studying the
K2CO3, H2O/ EtOH (7:3)
7
catalytic efficiency of in situ generated Pd NPs in the
X= I/ Br/ Cl
Sonogashira coupling (Fig. S20, ESI†). For this, we selected aryl
iodide and phenylacetylene as model substrates. We carried
out the coupling reaction between aryl iodide and
phenylacetylene using 1 mole % of in situ generated Pd NPs
and K2CO3 as base (1 mM) in different solvents such as H2O,
EtOH and H2O/EtOH (v/v, 7:3) solvent mixture and also under
solvent free conditions (entries 1, 2, 3 and 4, table S3, ESI†). In
9
X = I (50 mins, r.t., 94%a)
H2N
8
11
X = I (50 mins, r.t., 96%a)
X = I (5h, r.t., 90%a)
10
X= Br (55mins, 60°C, 95%)
X = I (2.5h, r.t., 97%a)
X= Cl (10h, 90°C, 90%)
X= Cl (50mins, 60°C, 92%)
X= Cl (7h, 90°C, 94%)
Scheme 2. Sonogashira coupling of phenyl acetylene with various aryl halides
catalyzed by in situ generated Pd(0) nanoparticles. aunder aerial conditions.
all the cases reaction was complete within 50 mins at room To check the scope of substrates, we utilized the in situ
temperature, whereas on increasing the temperature up to generated Pd NPs as catalyst in coupling reaction between aryl
60°C, the reaction completed within 10 minutes (entry 5, table iodide having electron donating/accepting moiety and
S3, ESI†). Interestingly, the model reaction went smoothly to phenylacetylene under aerial conditions (entries 1 and 2, table
furnish the desired product under aerial conditions at room S5, ESI†). It was found that aryl iodide bearing electron-
temperature. However, the model reaction carried out in DMF donating group provided slightly lower yield of the product.
at 100°C, was complete in 50 minutes (entry 6, table S3, ESI†). The in situ generated Pd NPs also showed high efficiency in the
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reaction for the preparation of symmetrically substituted To evaluate the practical applicability of the in siVtiuewgAertnicelerOantleinde
DOI: 10.1039/C5CC06158J
alkyne (entry 3, table S5, ESI†). Moreover, 2-bromothiophene Pd NPs, we carried out the Sonogashira coupling reaction of
and 2-bromopyridine also furnished the corresponding dibromo perylenediisimide13 with phenylacetylene (Scheme
products in excellent yields (entries 4 and 5, table S5, ESI†).
S2). Earlier the desired product13 was obtained by Sonogashira
coupling of dibromo perylenediisimide with phenylacetylene in
the presence of light sensitive CuI, toxic triethylamine and
carcinogenic THF in 82% yield. It took 14 h for completion of
this reaction. We chose the desired derivative as a candidate
due to its potential applications in the material14 and
supramolecular chemistry.15 Interestingly, as shown in Scheme
S2, in the presence of 1 mole % of in situ generated Pd NPs in
H2O/EtOH (7:3), the reaction was complete in 12 h and the
product was obtained in 86% yield (Fig. S28, ESI†). This study
demonstrates the practical utility of in situ generated Pd NPs
for carrying out Sonogashira coupling reaction.
Table 1: Sonogashira coupling of phenylacetylene with various aryl halides
catalyzed by in situ generated Pd(0) nanoparticles.
In
conclusion,
we
designed
and
synthesized
heteroligophenylene derivative
3
and aggregates of this
derivative served as reactors, stabilizers and shape directing
agents for the preparation of spherical Pd NPs. Furthermore,
the in situ generated Pd NPs worked as excellent catalyst for
carrying out copper and amine free Sonogashira coupling
reactions involving activated/unactivated substrates under
environmentally benign conditions.
aunder aerial conditions. bIsolated yields determined after recrystallization
To broaden the scope of Sonogashira coupling, we also carried We are thankful to DST (ref. no. SR/S1/OC-69/2012) for
out the reaction of aryl chloride and aryl bromide with financial support. We are also thankful to UGC (New Delhi,
phenylacetylene. To our pleasure, both the reactions went India) for the “University with Potential for Excellence” (UPE)
smoothly and desired products were obtained in excellent project. P.K.W. is thankful to UGC for Junior Research
yields (entries 1 and 2, table 1). To check the scope of catalytic fellowship.
efficiency of Pd NPs with regard to chloride, we carried out the
reaction between substituted aryl chlorides and
phenyacetylene using 1 mole % of Pd NPs at 90oC (entries 3
and 4, table 1) and it was found that substituted aryl chlorides
also furnished the desired products in high yields. Further, in
the presence of in situ generated Pd NPs, 1-bromo-4-
iodobenzene reacted with phenylacetylene to furnish 1-
bromo-4-(phenylethynyl)benzene in 94% yield (entry 5, table
1). This result shows the high selectivity of Pd NPs in
Sonogashira reaction. However, Sonogashira coupling
involving aryl bromide/aryl chloride did not complete under
aerial conditions.
Notes and references
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P. J. Stang and F. Diederich, Modern Acetylene Chemistry,
Wiley-VCH: Weinheim, Germany, 1995.
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3
4
K. Sonogashira, J. organomet. Chem., 2002, 653, 46.
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Further, recyclability of Pd NP catalyst was also checked by
monitoring the model reaction between aryl iodide and
phenylacetylene. The product was separated from the reaction
mixture by extracting with organic solvent and the catalyst was
recovered in the aqueous layer which was used as such in the
next cycle of the reaction. The product yield remained
quantitative even after four cycles of the reaction. Thus,
catalyst recycling was very facile since almost all the coupling
products could be readily separated by simple extraction.
The efficiency of the in situ generated Pd NPs was tested for
the reaction between iodobenzene and phenyl acetylene using
various amounts of catalyst, down to 0.0003 mole % of Pd NPs
(table S7, ESI†). This study indicate that the Pd NPs can also be
utilized at such a low concentration (0.0003 mole %) to furnish
the desired product in good yield (83%). Such an extremely
low quantity of palladium has never been successfully used for
Sonogashira coupling before the present study.
R. A. Ammar, N. A. Alarfaj and M. F. El-Tohamy, Int. J.
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