"water soluble" palladium nanoparticle engineering for C-C coupling, reduction and cyclization catalysis

Article abstract of DOI:10.1039/c9gc02546d

The use of Pd nanoparticles (Pd-NPs) to realize several important organic reactions allows efficient catalysis with low metal loading (<1000 ppm), hence providing a greener catalytic system. However, to be truly green Pd-NPs need to be synthesized in a sustainable manner and be able to react in aqueous media in order to avoid the use of organic solvents. Here we describe an original and eco-friendly synthesis of Pd-NPs (using benign reactants and simple conditions) perfectly stable in water. Remarkably, this synthesis allows for control over their size and morphology by simply tuning the pH of the stabilizer. We then evaluate the catalytic efficiency of these Pd-NPs on six different model reactions (Suzuki Miyaura, Sonogashira, Heck, nitrophenol reduction and pentynoic cycloisomerization) in aqueous media. We show that the stabilizer structure influences the activity owing to its ability to promote the mass transfer of the organic substrates towards the NP surface in the aqueous environment. Finally, catalytic evaluations show that our nano-catalysts prepared in an eco-friendly manner are among the best catalysts described so far in the literature in each case, with high turnover frequencies reached with a low loading of palladium.

Full text of DOI:10.1039/c9gc02546d

Green Chemistry
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“Water soluble” palladium nanoparticle
engineering for C–C coupling, reduction and
cyclization catalysis†
Cite this: Green Chem., 2019, 21,
6646
A. Iben Ayad,a C. Belda Marín,a,b E. Colaco,b,c C. Lefevre, d C. Méthivier,b
a
A. Ould Driss,a J. Landoulsi b and E. Guénin
*
The use of Pd nanoparticles (Pd-NPs) to realize several important organic reactions allows efficient cataly-
sis with low metal loading (<1000 ppm), hence providing a greener catalytic system. However, to be truly
green Pd-NPs need to be synthesized in a sustainable manner and be able to react in aqueous media in
order to avoid the use of organic solvents. Here we describe an original and eco-friendly synthesis of Pd-
NPs (using benign reactants and simple conditions) perfectly stable in water. Remarkably, this synthesis
allows for control over their size and morphology by simply tuning the pH of the stabilizer. We then evalu-
ate the catalytic efficiency of these Pd-NPs on six different model reactions (Suzuki Miyaura, Sonogashira,
Heck, nitrophenol reduction and pentynoic cycloisomerization) in aqueous media. We show that the
stabilizer structure influences the activity owing to its ability to promote the mass transfer of the organic
substrates towards the NP surface in the aqueous environment. Finally, catalytic evaluations show that our
nano-catalysts prepared in an eco-friendly manner are among the best catalysts described so far in the lit-
erature in each case, with high turnover frequencies reached with a low loading of palladium.
Received 23rd July 2019,
Accepted 29th October 2019
DOI: 10.1039/c9gc02546d
rsc.li/greenchem
More recently, the use of water as a benign solvent in biphasic
systems have been attracting increasing interest to enhance
Introduction
In recent years, metallic nanoparticles have been intensively the sustainability of the catalytic process.18 Nevertheless, two
developed as relevant catalytic systems.1–3 Owing to their high main challenges have to be overcome for the development of
surface/volume ratio and potential recyclability, they are con- such a methodology: first, the direct synthesis of well-con-
sidered as the interface between homogeneous and hetero- trolled Pd-NPs stabilized in water avoiding the tedious “ligand
geneous catalyses.4,5 Moreover, they often combine improved exchange approach”; second, the difficulty in allowing the
activities and better selectivity which qualify them as more mass transfer of organic substrates into the aqueous catalytic
sustainable catalysts.6 Palladium nanoparticles (Pd-NPs) are phase and therefore reaching high efficacy with low Pd
among metallic nanoparticle catalysts, widely studied for loading. On the latter issue, the addition of amphiphilic com-
industrially relevant reactions such as carbon–carbon cross pounds to the reaction media5 to promote the phase exchange
coupling, reduction of nitroarenes, hydrogenation of alkenes has been successfully developed in the last decade by several
and alkynes, and oxidation of primary alcohols.7–14 Usually the groups.19–24 While the results are promising, the NPs are the
use of Pd-NPs in catalysis is performed through immobiliz- best catalysts not synthesized in a sustainable manner (high
ation on a solid support14–17 which could impair their efficacy. temperatures, organic solvents, dry argon, and expensive metal
derivatives). Another option is the use of green bio-reduc-
tants,25 which also act as stabilizers. This more economical
aSorbonne University – Université de Technologie de Compiègne, Laboratoire TIMR
and environmental solution appears nevertheless to yield less
(UTC/ESCOM), EA4297, Centre de recherche de Royallieu, rue du docteur Schweitzer,
reactive catalysts26–29 and reproducibility could also be ques-
CS 60319, 60203 Compiègne cedex, France. E-mail: erwann.guenin@utc.fr
bSorbonne University – Université de Technologie de Compiègne, CNRS, UMR 7338
BioMécanique et BioIngénierie (BMBI), Compiégne cedex, France
cSorbonne University, CNRS – UMR 7197, Laboratoire de Réactivité de Surface, 4
place Jussieu, F-75005 Paris, France
dSorbonne University, Université de Technologie de Compiègne, Service d’Analyse
Physico-Chimique (SAPC), rue du Dr Schweitzer, 60200 Compiègne, France
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
c9gc02546d
tioned owing to the high variability in the bio-extract compo-
sition. Alternatively, other authors proposed the use of ionic
liquids, water-soluble NHC stabilizers, phosphine oxide poly-
mers or other small organic stabilizers30–37 to stabilize the Pd-
NPs in the aqueous media while allowing the mass transfer of
the organic substrates. The latter approach is particularly
attracting, as the strength and types of interactions between
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the stabilizer and the surface could be fine-tuned by an appro- oxide NPs46–51 and shown to have the ability to not only stabil-
priate stabilizer design.38,39
ize metallic NPs (gold or silver) but also allow for their syn-
Herein, we report the straightforward and controlled syn- thesis by reduction of metal salts.52–57 It was hypothesized that
thesis of Pd-NPs perfectly stable in water and able to catalyse the molecules will allow for a moderate coordination to the
organic reactions in aqueous media. Our approach is based on surface (better than carboxylate but weaker than common Pd
an eco-friendly synthesis using benign conditions: the use of stabilizers such as phosphine, thiol or amine) not impairing
sodium ascorbate as a bio-reductant conjugated to bio-compa- catalysis. Additionally, they will permit a good stabilization in
tible phosphonic acid stabilizers. To the best of our knowledge aqueous media as already described for other types of NPs.
phosphonic acids have never been used for Pd-NP synthesis. Among this family of compounds, several molecules listed in
These molecules present several important advantages as NP Scheme 1 were chosen for this study. Compound 1 presenting
stabilizers compared to other small molecules and green bio- an alkene functionality was used for comparison with pre-
reductants. First, they present a unique multidentate robust cedent work on gold NPs.53,54 Compound 2 was chosen as an
and stable binding to the metallic surface and therefore have example of a known FDA approved and commercialized mole-
emerged since three decades as important tools for the prepa- cule (FOSAMAX®) already studied by Benyettou et al. for silver
ration of organic–inorganic nanomaterials with important and NP synthesis.55 Compounds 3 and 4 were chosen as they
various applications.40,41 They have been used for corrosion present aromatic functionalities that could in our opinion
inhibition purposes, to produce a rich variety of hybrid metal– allow for better reactivity of organic substrates, mostly aro-
organic frameworks, to control interfacial properties in matic, at the surface of the Pd-NPs. Compound 5 was chosen
organic-electronic devices and photovoltaic cells, to modify as it is a simple structure that is produced on a large scale and
implanted biomaterials, to produce biosensors, etc. Second, used in industry since a long time. Finally, our protocol was
they can benefit from a great variety of chemical structures also evaluated with a more simple structure of commercial
with relatively low toxicity (compared for example to phos- phosphonic acid (benzyl phosphonic acid 6) and with a com-
phine). The preparation of phosphonates is long known and mercially available polymer bearing acid phosphonic function-
well described and has allowed the production for over a ality 7 already used for the coating of iron oxide NPs (PEOMA
century of numerous commercial structures readily available co MAPC1 ACID).58,59
and studied,42 with some molecules being also used as thera-
We perform NP synthesis in water under aerobic conditions
peutic agents.43 Hence, the second asset permits a perfect as all the molecules 1–3 and 5–7 are readily soluble in water
tuning of the structure of these stabilizers which could allow besides pH = 3. Only compound 4 is soluble besides pH = 9.
for creating a lipophilic environment at the surface, enhancing
the mass transfer of organic substrates. The methodology we
describe using phosphonic acids as stabilizers ensures the pro-
Synthesis and characterization of Pd NPs
In the first approach, based on our precedent experience in
gold NP synthesis, we evaluated the synthesis protocol without
addition of any supplementary reductant. Unlikely to what was
observed with gold the bisphosphonic function was not
sufficient a reductant to allow the formation of Pd-NPs even
under prolonged heating conditions. Therefore, a supplemen-
duction of highly stable and dispersible Pd-NPs in aqueous
media. Then, the great versatility of these nanocatalysts was
reported. We evaluated the same catalyst on an unprecedented
number and types of reactions: 6 different important reactions
(C–C coupling, reduction and cyclization) in an aqueous
medium without any additives (amphiphilic compounds, sup-
plementary ligands, and other metals) and with extreme
efficiency compared to previous solutions, reaching ppm
loading of Pd and high turnover frequencies (TOFs).
Results and discussion
Choice of ligand for the synthesis of Pd NPs
Phosphonic acids and more precisely the hydroxyl methylene
bisphosphonic acid family that are molecules long known for
their chelating properties towards metal ions were chosen as
stabilizers to perform our synthesis. They were industrially
used during the XXth century as water softeners. Following the
work of Fleish in 1960 they have been then studied for their
numerous therapeutic properties towards diverse pathologies
(osteoporosis, cancer, and anti-trypanosoma).44,45 Some of
these compounds are actually commercialized as common
treatment of osteoporosis. More recently they have been uti-
Scheme 1 Phosphorus stabilizers evaluated for the synthesis of Pd-
lized for the coating and functionalization of various metal NPs.
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tary reductant was added to trigger the nucleation. Sodium
ascorbate was chosen because it is environmentally friendly
and biocompatible. This reducing agent has already been used
to synthesize Au-NPs52,60 or Pd-NPs.61,62 Typically, reactions
are performed by mixing a stock solution of Na2PdCl4, the
phosphorous stabilizer in water at three different pH values (3,
7, and 10) and sodium ascorbate. The mixture is then heated
in a microwave reactor for 30 min at 100 °C. The reaction is
run under complete aerobic conditions. Apparition of a brown
to dark brown colour visually assesses the formation of Pd-
NPs. By running UV–vis the complete disappearance of the
2−
band centred at 430 nm characteristic of PdCl4 in water
could be verified (see the ESI†). One must note that Pd-NPs
obtained from the acidic solution (pH = 3) of compound 3
(noted Pd-NP-3A) present a non-negligible absorbance at a
wavelength higher than 700 nm compared to Pd-NPs obtained
from the basic solution (pH = 10) of compound 3 (noted Pd-
NP-3B) (see the ESI†).
The formation of the Pd-NPs was observed whatever the
phosphorus compound used. When realizing the synthesis
without any phosphorus stabilizer the discoloration of the Pd
(II) solution was immediate without the need for heating but
the grey solution produced quickly led to the formation of a
black precipitate. The replacement of the sodium ascorbate by
a solution of sodium borohydride reacted in the presence of
phosphorus stabilizers led also instantly to the formation of a
black solution that precipitates rapidly. Only when treated with
the phosphorus stabilizer and sodium ascorbate does the solu-
tion stay highly stable during time and do not evolve over time
(no colour modification and no precipitation). The variation of
the pH of the stabilizer solution was studied (pH was set at 3,
7 and 10). For most of the compounds studied when realizing
the synthesis with solution at acidic pH, the discoloration of
the reaction media was quicker and, in some cases, Pd-NPs
were formed at room temperature. For solution at pH = 7 or 10
discoloration was only observed when heating the solution for
30 min. The transmission electronic microscopy analysis of
the produced Pd-NPs was realized for each reaction condition.
Whatever the phosphorus stabilizer used the morphology of
the Pd-NPs formed was very similar and depends essentially
on the pH of the starting stabilizer solution (Fig. 1 and 2). The
basic solutions of the phosphorus stabilizer yield small Pd-
NPs <10 nm (from 2 nm to 7 nm depending on the stabilizer)
and monodisperse (Fig. 1).
Fig. 1 TEM micrograph of Pd-NPs obtained with basic solutions of
compound 1: Pd-NP-B1 (A and B), compound 4: Pd-NP-B4 (C and D)
and compound 2: Pd-NP-B2 (E and F).
On the other hand, the acidic or neutral solutions of the
stabilizer yield un-homogeneous population of Pd-NPs with
the formation of larger NPs >50 nm and small NPs (<5 nm) in
the background. In the case of a stabilizer bearing aromatic
side chains (compounds 3, 4 and 6) a particular phenomenon
was observed when reacting under the acidic solutions. The
Fig. 2 TEM micrograph of Pd-NPs obtained with acidic solutions of
compound 3: Pd-NP-A3 (A and B) and compound 6: Pd-NP-A6 (C and
D).
larger particles formed were constituted in a homogeneous defined when performing heating without microwave.
manner of spherical agglomeration of small Pd-NPs in the Moreover, they could be separated from the small NPs in the
form of nanodendrites or nanorasberries (Fig. 2). The for- background by simply concentrating and washing the solution
mation of these anisotropic forms of Pd-NPs can explain the over 100 kDa membranes using centrifugation. This difference
absorption differences noted on UV spectra (see the ESI†). in shape observed for the produced NPs may be due to the
These nanodendrites or nanorasberries were usually better difference in the pH of the phosphonic acid stabilizer and
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Fig. 5 Typical (A, B) P 2p and Pd 3d peaks recorded on (A) compound 3
powder and (B, C) Pd-NP-B3.
Fig. 3 HRTEM micrograph of Pd-NP-B3 (inset: SAED pattern).
ent with phosphonic acid present in the 850–1300 cm−1
fingerprint (νPvO at 1162, 1109 and 1062 cm−1; νs, νas of
PO3 at 1035, 997 and 965 cm−1; and νPOH at 899 cm−1) is
observed to yield one broad band centred around 1090 cm−1
(Fig. 5).
therefore could be due to differences in stabilizer complexing
properties allowing for differences in the growth mechanism
of the NPs.63
P 2p peaks recorded on Pd-NP-B3 and on powder of 3 are
shown in Fig. 5. P 2p consists of a doublet due to the spin–
orbit coupling, P 2p3/2 and P 2p1/2, separated by 0.9 eV and a
ratio of 2 : 1 between their areas.64 This yields a full width at
half maximum (FWHM) equal to 1.5 0.1 eV in all situations.
The P 2p peak shows a poor signal-to-noise ratio on Pd-NP-B3
compared to 3 powder, owing to the low amount deposited on
the flat substrate. The binding energy value of the P 2p3/2 com-
ponent was 134.0 eV on 3 powder (Fig. 5A) and shifted to lower
binding energies, about 132.5 eV, on Pd-NP-B3 (Fig. 5B). This
significant shift (ΔBE ∼ 1.4 eV) suggests the coordination of
the phosphate head group to the metal, as observed on other
phosphate-metal/oxide interfaces.65,66 The Pd 3d peak, includ-
ing Pd 3d5/2 and Pd 3d3/2 doublets, is presented in Fig. 5C. It
shows a main contribution at 335.6 eV due to Pd(0). The dis-
tance between Pd 3d5/2 and Pd 3d3/2 peaks of the Pd 3d
doublet was fixed at 5.25 eV,67 and a ratio of 0.6 was imposed
between their areas. The peak also includes lower contri-
butions at higher binding energies. A component at ∼336.9 eV
is attributed to the PdO compound, indicating a possible oxi-
dation of Pd-NPs. The component at 338.0 eV is also attributed
to Pd oxide, PdO2, but may include a contribution due to the
Further morphological and crystallographic features of Pd
NPs obtained with the basic solution of 3 (Pd-NP-B3) were
studied by HR-TEM. Pd-NP-B3 exhibits spherical NPs of
around 6 nm (Fig. 3). The SAED pattern shows clearly the pres-
ence of the crystal phase through the appearance of (111),
(200), (220) and (311) diffraction planes of the cubic closest
packed structure of palladium. The inter-reticular distance was
assessed using the intensity line profiles revealing lattice
planes distancing by d111 = 2.25 Å as indicated in the HRTEM
micrograph.
All the Pd-NPs were purified by dialysis to remove the
unbound phosphonic stabilizer and lyophilized prior to FTIR
and XPS analyses. The FTIR spectrum of Pd-NP-B3 clearly
shows the vibration bands associated with the phosphonic
moiety bound at the surface of a metal (Fig. 4). Indeed, as
already described on several NP surfaces (iron oxide, gold or
silver)48,54,55 a merging of the sharp absorption bands consist-
2−
PdCl4 compound,67,68 probably originating from the traces
of the unreacted precursor.
The average number of phosphonic molecules per Pd-NP
was deduced by electron dispersive X-ray (EDX) analysis on
which the presence of phosphorus and palladium is clearly
visible (Fig. 6). For example, for Pd-NP-B3 (average diameter,
6 nm) an average number of 270 ( 50) molecules per Pd-NP
was deduced. This represents a surface area of approximately
0.4 nm2 occupied by the bisphosphonic function. This value is
in accordance with the previous results obtained for iron oxide
or gold NPs.
ζ potential analyses were also performed to measure the
surface potential of Pd-NPs covered by phosphonic acids. As
expected at pH = 7 the ζ-potential is clearly negative from −40
Fig. 4 FTIR spectra of the 500–2000 cm−1 area for Pd-NP-B3 (in blue)
and free compound 3 (in black).
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The three cross coupling reactions are by far the most
popular C–C coupling processes often used to describe the
catalytic potency of new catalysts.5 These reactions are still
highly studied as they have many desirable features for large-
scale synthesis and are amenable for the industrial synthesis
of pharmaceuticals and fine chemicals. On the other hand,
nitroarenes are significantly toxic by-products produced from
pesticides, herbicides and synthetic dyes and therefore their
removal from the environment is of high importance. The
reduction of 4-nitrophenol is considered as a model reaction
to evaluate the potency of NP catalysts. The cycloisomerization
of pentynoic acid to the corresponding alkylidene lactones was
chosen as a model reaction of cyclo-isomerization involving
alkyne substrates, as it is a general process that is known to
permit an atom-economical preparation of several hetero-aro-
matic rings.
C–C coupling reactions. All the C–C coupling reactions were
evaluated under similar conditions with different Pd-NPs
(differing by the stabilizer used and the shape of the NPs
formed). For these reactions, 4-halogenonitrobenzene was
chosen as the substrate. The reaction was run in a mixture of
water/ethanol (1/1) in the presence of potassium carbonate as
a cheap base corresponding to green chemistry principles and
by heating the mixture in a microwave reactor at 80 °C.
Conversions were estimated by Gas Chromatography (GC).
Products were purified by column chromatography and charac-
Fig. 6 EDX analysis of Pd-NP-B3.
to −30 mV in accordance with the presence of the phospho-
nate function at pH = 7 at the surface of the NPs.
Nano-catalyst evaluation
1
terized by H and 13C NMR (see the ESI†). Globally all the Pd-
The catalytic abilities of the obtained Pd-NPs were evaluated
on three different C–C cross-coupling reactions (Suzuki–
Miyaura, Sonogashira and Mizoroki-Heck reactions), on two
reduction reactions chosen as model reactions (the reduction
of 4-nitrophenol10,69 and the reduction of styrene)30,34,70,71 and
finally on a cyclization reaction (the pentynoic acid cycloisome-
rization)72 (Scheme 2).
NP catalysts were able to perform the catalysis, and the best
results were always obtained with Pd-NPs of small size pro-
duced from the reaction with stabilizers at basic pH (nano-
raspberries were less reactive for C–C coupling reactions). The
results were similar when varying the phosphorous stabilizer
(differences will be discussed further on) and the best results
were obtained with Pd-NPs obtained with stabilizers 3, 4 and 6
and we mainly focused our study on Pd-NP-B3.
The Suzuki–Miyaura reactions were performed by conden-
sing 4-halogenonitrobenzene with 4-tolyl boronic acid in the
presence of sodium carbonate. From the iodide derivative,
complete conversion was obtained in 30 min by heating to
80 °C. The catalyst loading could be decreased down to
0.002 mol% in palladium which represents a quantity of palla-
dium (in mole) with respect to the substrate of 20 ppm. With
10 ppm of palladium the conversion is still 80%. When using
the less expensive bromide substrate the conversion was com-
plete keeping the same conditions with a Pd loading of
0.01 mol%. Under all these conditions, no secondary products
1
(dehalogenation or homocoupling) were detectable on the H
NMR spectra of the crude mixture, and only the desired
product was formed. It should be pointed out that when con-
version was complete no purification was needed as after
organic extraction the only compound isolated was the
product of Suzuki coupling. With the chloride substrate with
0.1 mol% of Pd loading no conversion was observed. Only
when the reaction time was increased to 120 min traces of the
product were obtained (conversion <20%). Turnover frequen-
cies (considering the surface Pd atom TOFsurf) were calculated
Scheme 2 Reactions catalysed by Pd-NPs: from top to bottom:
Suzuki–Miyaura, Sonogashira, Mizoroki-Heck (with styrene or methyl
acrylate) and 4-nitrophenol reduction.
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(at conversion <50%) and the results correspond for iodide The stabilizer probably allows for improving the substrate con-
and bromide to the TOFsurf values of 130 020 and 91 320 h−1 centration close to the surface. On the other hand, when using
respectively. When working at 0.01 mol% of Pd (100 ppm) a polymer coating that permits more effective steric stabiliz-
using the iodide derivatives the catalyst can be recycled at least ation, the catalysis efficiency is impaired. The latter difference
three times without any decrease in conversion. The recycling could be attributed to the decrease in the accessibility of the
was simply performed by extracting the product from the substrate in the case of a macromolecular stabilizer.
aqueous medium and reintroducing the reactants in the solu-
Mechanism considerations. During the last few years there
tion for the following reaction. Recycled Pd-NPs were evaluated have been important debates on the nature of catalytic species
by TEM (see the ESI†). While extremely low quantity of NPs involved when using Pd-NPs for cross coupling reactions.14
could be found on the TEM grid due to the ppm amount of Pd The question being: does the active catalyst is the Pd-NPs or
used, they did not appear to have been modified during the leached soluble palladium species in the reaction medium?
synthesis. Unfortunately, such a small amount of material Such leaching of Pd species is usually described to happen in
could not allow performing XPS analysis to characterize their the case of prolonged heating and is described to be due to
surface and potential modification in the Pd state.
the oxidative addition of aryl iodide or bromide.14 Another
For the Sonogashira reaction, we conducted the reaction of possible source of Pd leaching species could come from the re-
phenylacetylene with 4-halogenonitrobenzene. It is important dissolution of amorphous PdO on the surface (the presence of
to note that the reactions were performed in the absence of such species was evidenced by XPS). In our case, though it
any additives usually utilized such as copper or phosphine cannot be totally ruled out, the mild conditions and the short
ligands. For the iodide derivative, the conversion was 97% reaction time pledge for a reaction at the surface of the NPs
after 30 min at 0.015 mol% in Pd (150 ppm), and for the rather than through the dissolution of Pd species. Due to the
bromide derivative the conversion was complete after 3 h at high stability of the Pd-NP colloids in the media, the “hot fil-
0.2 mol% in Pd. The TOFsurf (calculated at conversion <50%) tration test” was impossible to further identify the active
corresponds for the iodide and bromide derivatives to 46 440 species. However, when studying Pd-NPs after several reactions
and 180 h−1 respectively. For the chloride derivative under the they still appear spherical in shape with the same size (see the
same conditions as the bromide conversion higher than 20% ESI†). Unfortunately, owing to the low quantity of Pd-NPs
was never reached, even by increasing the reaction time up to used, XPS analysis could not be performed to further analyse
24 h.
them. Moreover, it is generally described that when leaching
For the Mizoroki-Heck reaction two different substrates arises through the oxidative addition of aryl halide a dehalo-
were evaluated on 4-iodonitrobenzene: ethyl acrylate and genation product is formed.14 In our case, no such by-product
styrene. For the reaction with acrylate complete conversion was was observed even when the reaction time and temperature
observed at 0.1 mol% in 2 h, and a TOFsurf of 1200 h−1 was cal- were increased.
culated. For the less reactive styrene the conversion was only
70% at 0.2 mol% in 3 h.
Comparison with similar catalysts. When compared to the
literature (Table 1) it appears that the results of Suzuki–
Influence of the stabilizer on catalytic activity. When evaluat- Miyaura, Sonogashira and Heck C–C coupling obtained for our
ing Pd-NPs bound to any solid support, the activity is depen- Pd-NPs are equivalent to what was described for several other
dent upon the structure and the interactions of the stabilizers “water soluble” (unsupported or supported) Pd-NPs whatever
at the surface because they control the accessibility of the sub- the preparation mode and performed under similar con-
strates31 and it is especially important during the reaction of ditions. Compared to the closest solution consisting of water
organic substrates in an aqueous environment. In order to soluble phosphine ligands the phosphonic acid coated Pd-NPs
evaluate the effect of the stabilizers on the catalysis efficiency, present a clear advantage in terms of catalytic efficiency.32,34
we compared the activity of Pd-NPs obtained from compounds Moreover, our results are even in the same range as the best
3, 5 and 7 at basic pH (Pd-NP-B3, Pd-NP-B5, and Pd-NP-B7) on “homeopathic palladium nanoparticle” water soluble catalysts
the Suzuki coupling of 4-bromonitrobenzene with 4-tolylboro- described in the literature so far21,73 (Table 1). Finally com-
nic acid. These stabilizers led to similar particle size and there- pared to some of the best solutions described in the litera-
fore the nanocatalysts only differ from the stabilizer consti- ture14 our phosphonic acid coated Pd-NPs still appear competi-
tution. Using strictly identical conditions (reaction for 30 min tive and in the range of the best catalysts reported so far for
at 80 °C in the water/ethanol mixture), the best results were Suzuki–Miyaura74 and Sonogashira.75 Concerning the
observed using Pd-NP-B3 that bears a benzyl functionality Sonogashira reaction, one must note that comparable Pd
(80% conversion using 0.001 mol% of Pd). The conversion was based nanocatalysts are often processed with organic solvents
slightly decreased with Pd-NP-B5 bearing a methyl group in (except for the supported solution presented in ref. 75) in con-
place of a benzyl group (60% conversion using 0.001 mol% of trast to our catalyst reaching high efficiency in the water/
Pd), whereas Pd-NP-B7 was far less active (only 40% conversion ethanol mixture.
using 0.01 mol% of Pd). This result clearly indicates that the
Reduction of nitrophenol. The reduction of 4-nitrophenol
constitution of the stabilizer is determinant on the catalyst with Pd-NP-B3 as a catalyst was evaluated in water at room
activity in aqueous media. As we hypothesized, the Pd-NPs temperature with an excess of sodium borohydride and the
bearing aromatic functionalities led to more active catalysts. evolution of the reaction was followed by assessing the dis-
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Table 1 Comparison of Pd-NP reported catalysts on the C–C coupling reaction
mol% Pd forcomplete
conversion
Catalyst
Reaction type Conditions
TOF (h−1
)
Ref.
Pd-NP-B3
SM
S
H2O/EtOH (1 : 1), K2CO3, 80 °C, 30 min
0.002 (iodide)
0.1 (bromide)
0.015 (iodide)
0.2 (bromide)
0.1 (iodide)
130 020
91 320
46 440
180
This work
Pd-NP-B3
H2O/EtOH (1 : 1), K2CO3, 80 °C, 30 min
(iodide) 3 h (bromide)
H2O/EtOH (1 : 1), K2CO3, 80 °C, 2 h
This work
Pd-NP-B3
H
1200
This work
CB[6]-Pd NPs
Pd/MFC
Pd/MFC
SM
SM
H
H2O/EtOH (1 : 1), Na2CO3, 40 °C, 20 min 0.5 (iodide)
N.D.
16.67
16.5
76
77
77
21
EtOH, K2CO3, reflux, 1 h
DMF, K2CO3, 120 °C, 1 h
H2O, 2 wt% TPGS-750-M, S-Phos
(5 mol%) K3PO4, 45 °C, 11 h
H2O/EtOH, K3PO4, 80 °C, 1.5 h
H2O (TBAF), K2CO3, 100 °C, 30 h
H2O/EtOH (1 : 3), K2CO3, 50 °C
H2O, K3PO4, 90 °C, 1 h
0.3 (iodide)
0.3 (iodide)
0.032 (bromide)
Fe–ppm Pd catalysis
SM
N.D.
Dendrimer PdNPs
MEPI-Pd
Branched Pd NCs
Pd NPs with a PEG-tagged stabilizer SM
TPPTS capped Pd NPs
PVP-stabilized PdNPs
SM
SM
SM
0.001 (bromide)
0.000028 (iodide)
0.5
0.25
0.01 (bromide)
0.0008 (iodide)
39 600
119 000
1364
N.D.
N.D.
73
74
78
35
32
79
SM
SM
H2O, K3PO4, 90 °C, 2 h
H2O : EtOH (3 : 1), K3PO4, 90 °C,
6 h under nitrogen
20 625
Tris-imidazolium-stabilized Pd NPs
CPS-MNPs–NNN–Pd
PdCu@Phos.Agarose
Pdnp-nSTDP
S
S
S
S
DMF, piperidine, 130 °C
H2O/DMF (2/1), K2CO3, 90 °C
DMA, DABCO, 50 °C, 24 h
H2O, DIPEA, r.t., 2 h
0.2
0.5
380
31
77
80
81
82
75
0.05 (0.8 mol%Cu)
0.01 (iodide)
4800
S M = Suzuki Miyaura; S = Sonogashira; H = Heck.
Table 2 Comparison of NP reported catalysts on 4-nitrophenol
reduction
knorm a (s−1
,
Catalyst
NaBH4 eq. kapp (s−1
)
mol−1 g−1
)
Ref.
Pd-NP-B3
Pd nanodendrites
Au NPs
Au–Ag NPs
AuNPs
AuNPs@vesicle
Cu NPs
150
27.9
120
7
448
7
0.0006
0.0021
0.0019
0.000475
0.000833
0.0026
907 020.3
681.8
21 149.9
712.5
This work
83
84
85
86
87
88
89
8280.3
Fig. 7 Top: UV-visible absorption spectra (6
s intervals). Bottom:
768 745.4
12 519.7
11 400.2
Absorption dependence on time for the catalytic reduction of 4-nitro-
phenol using Pd-NP-B3 as the catalyst.
167
0.000159
0.000092
Fe3O4@dextran/Pd 1000
a knorm values were calculated from kapp normalized to the molar con-
centration of NaBH4 and the Pd quantity.
appearance of 4-nitrophenol by UV-vis (Fig. 7). The reaction
was complete after 36 min using 0.25 mol% of Pd-NP-B3.
Under these conditions the reaction proceeds with kapp
=
results (Table 3) it could be stated that activities similar to the
best reported systems are observed.
Cycloisomerization of pentynoic acid. Pentynoic acid cycloi-
somerization was evaluated in water with 0.05 mol% of Pd-
NP-B3 as a catalyst followed by GC. Several conditions were
0.6 × 10−3 s−1. In order to compare to literature data we calcu-
lated the constant normalized knorm taking into account the
concentration of sodium borohydride and the quantity of Pd
used. The values of knorm for different catalysts recently
reported in the literature are given in Table 2 and it appears
that our catalyst is clearly efficient in this context.
The reduction of styrene catalysed by Pd-NP-B3 was con-
ducted in water using two reductants: H2 or NaBH4. With
hydrogen under a pressure of 1 bar the reaction was complete
at room temperature after 3 h using 0.8 mol% of catalyst. For
NaBH4 using 6 equivalents the reaction was complete at room
temperature after 3 h using 0.8 mol% of catalyst. TOFsurf
values are identical in both reactions and correspond to
41 h−1. In both cases no formation of Pd precipitates was
observed. Few reports could be found in the literature concern-
ing catalytic hydrogenation of styrene in pure water with Pd-
NPs but when comparing our catalyst to some representative
Table 3 Comparison of Pd-NP reported catalyst efficiency and quantity
on the styrene reduction reaction
mol% Pd for
complete
Catalyst
Conditions
conversion TOF (h−1) Ref.
Pd-NP-B3
H2O, 3 h, 1 bar H2 0.8
or 6 eq. NaBH4
41
This work
NHC PdNPs H2O, 3 h, 1 bar H2 0.42
Pd0@g-Fe2O3 H2O, 3 h, 1 bar H2
IL stabilized H2O, 1.5 h, 50 °C, 4 0.4
Pd NPs bar H2
N.D.
100
N.D.
30
71
34
1
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evaluated and optimal conditions were obtained by running
the reaction at 40 °C for 60 min. Under these conditions, the
conversion was complete without any traces of the ketoacid
Experimental
General methods
byproduct formation. If the reaction was prolonged or the All reagents were purchased from Fisher Scientific and used
temperature was increased the ketoacid byproduct starts to without further purification. Compounds 1, 3, 4 and 5 were
form. To produce the ketoacid with a complete conversion the synthesized using a protocol adapted from the study by
reaction should be run at 90 °C for 3 h. Compared to previous Aufaure et al.54 (see the ESI†). Compound 2 was obtained by
work our catalyst appears competitive, as it was able to following the protocol described by Kieczykowski et al.92
produce the cycloisomerization product selectively under mild Compound 7 (PEOMA-co-MAPC1 ACID) was purchased from
conditions without addition of any base or co-catalyst and with Specific Polymers, France. Nanopure water (conductivity of
a low loading of Pd (2 times less Pd mol% than the best cata- 0.06 mS cm−1) obtained from a Millipore Gradiant Elix-3A10
lyst described so far).72 Moreover, it was also able to realize the system was used to prepare the sample solutions.
subsequent hydrolysis of the cycloisomerization product into Transmission electron microscopy (TEM) was used to charac-
the ketoacid by simply increasing the temperature and the terize the nanoparticles. TEM images were recorded using a
length of reaction time.
JEOL JEM-2100F instrument (Japan) at an acceleration voltage
of 200 kV and visualised with a CCD camera. The sample was
dispersed in ultra-pure water, added drop-wise onto a copper
mesh grid coated with an amorphous carbon film and allowed
to air-dry. Transmission electron microscopy (TEM) images
Conclusions
In conclusion, the preparation of stable Pd-NPs in aqueous were recorded using a JEOL JEM-2100F instrument (Japan) at
media was developed. The proposed synthesis is environmen- an acceleration voltage of 200 kV and visualised with a CCD
tally friendly: using microwaves, no toxic solvents, biocompati- camera. The crystallinity of the NPs was analysed using
ble reductants and commercial and/or nontoxic stabilizers. selected area electron diffraction (SAED) and high resolution
Phosphonic acids were for the first time used as stabilizers for transmission electron microscopy (HRTEM). The sample was
catalytic Pd NPs. This family of (generally) water soluble mole- prepared by depositing a drop of the solution onto a copper
cules is highly valuable due to the fact that they allow a great mesh grid coated with an amorphous carbon film and allowed
variety of chemical structures, with molecules produced at an to air-dry. Infrared spectra were recorded with an Agilent
industrial level and for some compounds relatively low toxicity Technologies Cary 600 Series FTIR. Spectra were recorded
(some are FDA approved drugs) compared to other stabilizers. through the form of thin KBr pellets. X-ray photoelectron spec-
The synthesized Pd-NPs were able to catalyse up to 6 different troscopy (XPS) analyses were performed using an ESCA+
organic reactions in an aqueous medium under benign con- spectrometer (Omicron NanoTechnology), equipped with a
ditions. Moreover, the possibility to finely tune the stabilizer monochromatized aluminum X-ray source (powered at 20 mA
constitution permit us to improve the catalysis efficacy by and 14 kV) and a MCD 128 channeltron detector. Charge stabi-
allowing the surface mass transfer enhancement of organic lization was insured using the CN10 device with an emission
substrates to the surface. The best catalysts enable highly current of 5.0 µA and a beam energy of 1 eV. Analyses were per-
efficient organic reactions in aqueous media with quantities of formed in the sweeping mode; the resulting analyzed area was
Pd down to the ppm level without the need for extra surfac- about 1 mm in diameter. A pass energy of 20 eV was used for
tants or co-catalysts. Evaluations are still ongoing on the exem- narrow scans. Under these conditions, the full width at half
plification of each reaction and on other reaction types but our maximum (FWHM) of the Ag 3d5/2 peak of a clean silver refer-
preliminary results are highly encouraging. In addition, these ence sample was about 0.6 eV. The pressure in the analysis
preliminary results have to be also emphasized by the simpli- chamber was around 10−10 torr. The photoelectron collection
city and eco-compatibility of the synthesis of the NP catalysts angle, θ, between the normal to the sample surface and the
requiring mild conditions (synthesis at mild temperature, analyzer axis was 45°. The following sequence of spectra was
under aerobic conditions and in water) with benign and recorded: survey spectrum, C 1s, N 1s, O 1s, P 2p, Na 1s, Pd
simple reactants. Moreover, another important aspect is the 3d, Cl 2p, and Si 2p. The binding energy scale was set by fixing
stability of the catalyst in time as the solutions of Pd NPs are the C 1s component due to carbon only bound to carbon and
stable by simply storing them in water and under aerobic con- hydrogen at 284.8 eV. The data treatment was performed with
ditions at 4 °C and could be used more than 6 months after CasaXPS software (Casa Software Ltd, UK). DLS and ζ potential
their preparation without any loss in catalytic ability. Finally, analyses were performed on a nano ZS (red badge) ZEN3600
the shape control and formation of anisotropic Pd-NPs can Zetasizer at neutral pH. UV-visible spectra were recorded on a
have interesting catalytic activities in other areas which would Varian Cary 50 Scan UV-Visible spectrophotometer. EDX
be further studied.78,90 Now transposing the synthesis to other microanalyses were performed using a TM 3000 tabletop
metallic NPs or even alloys by using different metallic salts is microscope equipped with a Swift EDX-ray 3000 microanalysis
also envisioned. In addition, such stable aqueous metallic-NPs system (Oxford Instruments). Samples were deposited as
could also find applications in other fields such as in nano- powder on a copper surface, and data were collected using a 15
medicine.91
kV accelerating voltage and studying the ratio of P vs. Pd. Gas
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Chromatography (GC) analyses were performed using
a
General procedure for the Sonogashira reaction of
PerkinElmer Autosystem XL instrument equipped with an phenylacetylene with 4-iodonitrobenzene with Pd-NPs
autosampler and a flame ionization detector and a 30 m AT™-
In a 10 mL MW vial, 43.5 µL of an aqueous solution of Pd-NPs
([Pd] = 0.695 mM) was added to 600 µL of water. 49.8 mg of
4-iodonitrobenzene (0.20 mmol), 24 µL of phenylacetylene
(0.22 mmol) and 400 µL of a 1 M K2CO3 solution were then
added. The glass walls of the vial were rinsed with 1 mL of
ethanol and a cap was fixed on the top of the vial. The vial was
1ht capillary column (0.25 mm diameter, 0.1 μm film thick-
ness), using nitrogen (2 mL min−1) as a carrier gas. The follow-
ing parameters are used: Tinjector = 350 °C and Tcolumn = 40 °C
(2 min) and 40–250 °C (20 °C min−1). 1H NMR spectra
(400 MHz) and proton-decoupled 13C NMR spectra
(100.63 MHz) were recorded on a Bruker Avance III 400
heated with an oil bath or on a microwave apparatus for
spectrometer. Chemical shifts are reported in parts per million
30 min at 80 °C using a ruby thermometer (step 1: heat for
(ppm) on the δ scale. The residual solvent peaks were used as
2 min to 80 °C and step 2: hold at 80 °C for 30 min with the
stirring speed being 1200 rpm for both steps). After cooling
internal references (1H NMR: CHCl3 7.26 ppm; 13C NMR:
CDCl3 77.2 ppm). Data are reported as follows: s = singlet, d =
the solution 1 mL of diethyl ether or ethyl acetate was added
doublet, t = triplet, q = quartet, qt = quintuplet, and m = multi-
and the organic layer was extracted 5 times. The product was
plet and coupling constant(s) are given in Hz. Turnover fre-
then isolated by evaporation and purified by simple filtration
quencies were calculated considering the number of Pd atoms
over silica when conversion was complete or purified by silica
on the surface for reactions at a low conversion rate (Graham).
gel column chromatography using ethyl acetate hexane mix-
tures when starting halogenated reactants remained un-
reacted. 1H NMR (CDCl3, 25 °C): δ = 8.23 (d, J = 9 Hz, 2H); 7.67
General preparation of Pd-NPs
(d, J = 8.8 Hz, 2H); 7.56 (m, 2H); 7.39 (m, 3H) ppm. 13C{1H}
In a 10 mL vial, 2 mL of distilled water, 80 µL of a water solu-
NMR (CDCl3, 25 °C): δ = 146.9; 132.3; 131.9; 130.3; 129.3;
128.6; 123.7; 122.1; 94.7; 87.5 ppm.
tion of Na2PdCl4 (20 mM) and 180 µL of bisphosphonic acid
solution in water (40 mM compounds 1–5) are mixed (for com-
pounds 6 and 7, 360 µL of a 40 mM solution and 180 µL of a
170 mg mL−1 solution are used). Then 40 µL of a solution of
sodium ascorbate (17.6 mg mL−1) were added and the mixture
was heated on a microwave apparatus (Monowave 300, Anton
Paar GmbH) for 30 min at 80 °C using a ruby thermometer
(step 1: heat as fast as possible to 100 °C and step 2: hold at
100 °C for 30 min with the stirring speed being 1200 rpm for
both steps).
General procedure for the Heck reaction of methyl
methacrylate with 4-iodonitrobenzene with Pd@-NPs
In a 10 mL MW vial, 287.7 µL of an aqueous solution of Pd-
NPs ([Pd] = 0.695 mM) was added to 400 µL of water. 49.8 mg
of 4-iodonitrobenzene (0.20 mmol), 27 µL of methyl methacry-
late (0.2 mmol) and 400 µL of a 1 M K2CO3 solution were then
added. The glass walls of the vial were rinsed with 1 mL of
ethanol and a cap was fixed on the top of the vial. The vial was
heated on a microwave apparatus for 2 h at 80 °C using a ruby
thermometer (step 1: heat for 2 min to 80 °C and step 2: hold
at 80 °C for 2 h with the stirring speed being 1200 rpm for
both steps). After cooling the solution 1 mL of diethyl ether or
ethyl acetate was added and the organic layer was extracted 5
times. The product was then isolated by evaporation and puri-
fied by simple filtration over silica when conversion was com-
plete or purified by silica gel column chromatography using
ethyl acetate hexane mixtures when starting halogenated reac-
tants remained un-reacted. NMR 1H (CDCl3, 25 °C): δ = 8.24 (d,
J = 8.8 Hz, 2H); 7.71 (d, J = 16.2 Hz, 1H); 7.66 (d, J = 8.8 Hz,
2H); 6.56 (d, J = 16.2 Hz, 2H); 3.83 (s, 3H) ppm. NMR 13C{1H}
(CDCl3, 25 °C): δ = 166.7; 148.5; 142.0; 140.5; 128.6; 124.5;
122.1; 52.1 ppm.
General procedure for the Suzuki–Miyaura cross coupling
reaction of 4-tolylboronic acid with 4-iodonitrobenzene with
Pd-NPs
In a 10 mL MW vial, 23 µL of an aqueous solution of Pd-NPs
([Pd] = 0.02 µM) was added to 1 mL of water. 49.8 mg of 4-iodo-
nitrobenzene (0.20 mmol), 30 mg of 4-tolylboronic acid
(0.22 mmol) and 400 µL of a 0.1 M K2CO3 solution (0.4 mmol)
were then added. The glass walls of the vial were rinsed with
1 mL of ethanol and a cap was fixed on the top of the vial. The
vial was heated on a microwave apparatus (Monowave 300,
Anton Paar GmbH) for 30 min at 80 °C using a ruby thermo-
meter (step 1: heat for 2 min to 80 °C and step 2: hold at 80 °C
for 28 min with the stirring speed being 1200 rpm for both
steps). After cooling the solution, 1 mL of diethyl ether or ethyl
acetate was added and the organic layer was extracted 5 times.
The product was then isolated by evaporation and purified by
simple filtration over silica when conversion was complete or
General procedure for the reduction of 4-nitrophenol
with Pd-NPs
purified by silica gel column chromatography using ethyl The reaction was performed in a 3 mL quartz cell (Hellma) to
acetate hexane mixtures when starting halogenated reactants perform the kinetic measurements. The solutions of 4-nitro-
1
remained un-reacted. H NMR (CDCl3, 25 °C): δ = 8.28 (d, J = phenol and NaBH4 were freshly prepared before each measure-
8.8 Hz, 2H); 7.72 (d, J = 8.8 Hz, 2H); 7.53 (d, J = 8.8 Hz, 2H); ment, without purging with N2, and 30 μL of 4-nitrophenol (7.4
7.30 (d, J = 8.8 Hz, 2H); 2.42 (s, 3H) ppm. 13C{1H}NMR (CDCl3, × 10−2 mM) solution were added to the reaction medium (2.8 ml
25 °C): δ = 147.8; 147.0; 139.3; 136.0; 130.0; 127.7; 127.4; 124.3; of water), with 200 μL of sodium borohydride (0.167 M, 150 eq.).
21.4 ppm.
The solution was then left undisturbed for some seconds, after
6654 | Green Chem., 2019, 21, 6646–6657
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Paper
which 20 μL of NP-Pd solution (0.028 mM) was added.
Immediately after the addition of Pd nanoparticles, time-depen-
dent absorption spectra were recorded at 6 s intervals.
Acknowledgements
The research described here was sponsored by the European
Union (FEDER), the Région Haut de France and the Université
de Technologie de Compiègne through funding of the chair:
“Green Chemistry and Processes”.
General procedure for the reduction of styrene with Pd-NPs
5.75 mL of the solution of Pd-NP-B3 ([Pd] = 0.68 mM) were
added in a 50 mL boiling flask and purged with H2 for 3 min,
and 57 μL of styrene (0.5 mmol) was added to the reaction
medium and purged a second time with H2 for 4 min. Then,
immediately the flask is closed to perform the reduction reac-
tion at room temperature and 1200 rpm for 3 h. The reaction
evolution was followed by gas chromatography using ethyl
acetate as the solvent.
In the case of use of NaBH4 as a reductant, 114 mg of
sodium borohydride (6 eq.) were added in a 50 mL boiling
flask with 5.75 mL of Pd-NP-B3 ([Pd] = 0.68 mM). The solution
was stirred at 1200 rpm for one minute, after which 57 μL of
styrene (0.5 mmol) was added to the reaction medium. The
reaction was performed at room temperature for 3 h. The reac-
tion evolution was followed by gas chromatography using ethyl
acetate as the solvent.
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General procedure for the cycloisomerization reaction with Pd-
NPs: cycloisomerization product (5-methylenedihydrofuran-2
(3H)-one)
In a 10 mL MW vial, 115 µL of an aqueous solution of Pd-NPs
([Pd] = 0.695 mM) was added to 500 µL of water. 19.6 mg of
pentynoic acid (0.20 mmol) was then added and a cap was
fixed on the top of the vial. The vial was heated on a microwave
apparatus for 60 min at 40 °C using a ruby thermometer (step
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1
of the isolated yield. H NMR (400 MHz, CDCl3) δ 4.72 (dd, J =
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heated on a microwave apparatus for 180 min at 90 °C using a
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