First application of homogeneous Pd nanoparticles prepared by pulsed laser ablation in liquid to a Suzuki-type reaction

Article abstract of DOI:10.1016/j.catcom.2017.06.052

Palladium nanoparticles are a valuable alternative to molecular catalysts for not requiring costly and toxic ligands. In this study palladium nanoparticles were produced by either pulsed laser ablation in liquid or direct reactions between ascorbic acid and a palladium salt. The obtained nanoparticles have been characterized by TEM, XRD and XPS techniques and they have been applied to the catalysis of the ligand-free Suzuki-type cross-coupling reaction. Methyl (E)-4-bromocrotonate has been coupled to different aryl and heteroarylboronic acids with yield up to 81% and selectivity between 85 and 90%. Investigations on catalytic reusability of the NPs have shown that they could be used for three or ten catalytic cycles without loss of activity, respectively for LAL or chemically synthesized Pd NPs.

Full text of DOI:10.1016/j.catcom.2017.06.052

Accepted Manuscript
First application of homogeneous Pd nanoparticles prepared by
pulsed laser ablation in liquid to a Suzuki-type reaction
Angela De Bonis, Rosarita D'Orsi, Maria Funicello, Paolo
Lupattelli, Antonio Santagata, Roberto Teghil, Lucia
Chiummiento
PII:
DOI:
Reference:
S1566-7367(17)30289-3
doi: 10.1016/j.catcom.2017.06.052
CATCOM 5118
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Catalysis Communications
Received date:
Revised date:
Accepted date:
11 May 2017
28 June 2017
29 June 2017
Please cite this article as: Angela De Bonis, Rosarita D'Orsi, Maria Funicello, Paolo
Lupattelli, Antonio Santagata, Roberto Teghil, Lucia Chiummiento , First application of
homogeneous Pd nanoparticles prepared by pulsed laser ablation in liquid to a Suzuki-
type reaction. The address for the corresponding author was captured as affiliation for all
authors. Please check if appropriate. Catcom(2017), doi: 10.1016/j.catcom.2017.06.052
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ACCEPTED MANUSCRIPT
First application of homogeneous Pd nanoparticles prepared by pulsed
laser ablation in liquid to a Suzuki-type reaction
Angela De Bonisa, Rosarita D’Orsia, Maria Funicelloa, Paolo Lupattellia, Antonio Santagatab,
Roberto Teghila and Lucia Chiummientoa,
aDepartment of Science, Via dell’ateneo Lucano, 10, University of Basilicata, 85100 Potenza, Italy.
bCNR-ISM, UOS Tito Scalo Unit, C.da Santa Loja, 85010 Tito Scalo (PZ), Italy.
 corresponding author: lucia.chiummiento@unibas.it;phone: +39 0971 205492

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Abstract
Palladium nanoparticles are a valuable alternative to molecular catalysts for not requiring costly and toxic ligands. In this study
palladium nanoparticles were produced by either pulsed laser ablation in liquid or direct reactions between ascorbic acid and a
palladium salt. The obtained nanoparticles have been characterized by TEM, XRD and XPS techniques and they have been
applied to the catalysis of the ligand-free Suzuki-type cross-coupling reaction. Methyl (E)-4-bromocrotonate has been coupled to
different aryl and heteroarylboronic acids with yield up to 81% and selectivity between 85-90%. Investigations on catalytic
reusability of the NPs have shown that they could be used for three or ten catalytic cycles without loss of activity, respectively for
LAL or chemically synthesized Pd NPs.
Keywords : Pd nanoparticles, laser ablation in liquid, ascorbic acid, Suzuki cross-coupling, ligand-free reaction, methyl 4-
bromocrotonate.

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1. Introduction:
The interest in nanoparticle (NP) catalysis has considerably increased for being one of the most promising systems for reactions
under mild, environmentally benign conditions. The most commonly used metallic and oxide NPs are prepared by chemical
synthesis, where solvents, ligands and high temperature conditions are usually needed. Catalytic studies of these NPs involve
either homogeneous or heterogeneous systems where metal oxides such as silica, alumina or carbon allotropes including carbon
nanotubes are usually used as supports.[1, 2] Recently, Pulsed Laser Ablation in Liquid (PLAL or LAL) has been proposed as
flexible and efficient one-step method to produce metallic and alloy NPs and nanostructures by a one-step synthesis.[3, 4] During
the interaction of an intense laser pulse with a solid submerged in liquid the formation of a plasma confined in a cavitation bubble
can be observed. Inside the bubble, the rapid quenching of the high temperature laser induced plasma provides the formation of
polycrystalline NPs.[5] It has been showed that varying target composition and physical-chemical properties of the surrounding
liquid, it is possible to tune both properties and the composition of the obtained particles, whereas laser parameters such as laser
fluence, repetition rate and ablation time play a relevant role in determining both the observed particle size distribution and
colloidal solution concentration.[6] Catalytic performances of LAL synthesized NPs have been investigated in many reactions
such as water splitting [7], hydrocarbon oxidation [8] and photocatalytic dyes degradation [9]. Moreover, it has been proved that
due to their surface chemical features, LAL synthesized NPs are prone to couple with oxide such as TiO2, CeO2 and graphene
oxide and to form heterogeneous catalysts[10]. In particular, the production by LAL of palladium nanoparticles (Pd NPs), with
potential applications as heterogeneous catalyst [11], has been widely investigated varying several experimental parameters
(solvents, pulse length, laser wavelengths) [12-14]. It has been showed that the metal ablation in organic solvents gets to the
formation of a carbon matrix around the particles [12,14] whereas by using water as liquid medium the formation of a skin oxide
layer has been observed [12-15]. Since different ablation mechanism are effective varying the laser source wavelength, it has been
proved that the amount of palladium oxide results to be lower by using IR laser source with respect to UV laser [13].
Commonly Pd NPs catalysts are produced by chemical synthesis, using solvents and potentially toxic chemical reactants.
Moreover, the obtained particles aggregate easily. Since LAL technique offers an universal approach to produce clean surfactant-
free metallic NPs, that form a stable colloidal solution due to electrostatic interaction, we decided to investigate the potential use
of LAL synthesized Pd NPs in a Suzuki-type reaction. Nowadays, palladium appears to be as the most frequently investigated
metal for catalytic coupling in the synthesis of C-C bonds [16-19] and Suzuki cross-coupling reactions using Pd NPs [20,21] or
PdO NPs [22-24] have been carried out to get biaryl systems.
Since there are no data on the use of LAL synthesized Pd NPs in cross-coupling reactions and due to the great interest in
developing practical and simple approaches to the synthesis of allylated arenes [25-28], we decided to investigate in such a
direction.
Thus laser synthesized Pd NPs were applied to the formation of new C-C bonds between methyl (E)-4-bromo-but-2-enoate and
several arylboronic acids according to a useful ligand-free palladium-catalyzed Suzuki-type cross-coupling protocol [29].
Moreover, a parallel preparation of Pd NPs, which reactivity is known in the classical Suzuki reaction, was performed with
ascorbic acid (AA) in which AA reduces Pd2+ to Pd0 to form Pd NPs directly by its oxidation to 2,3-dehydroascorbic acid (DHA)
and to a series of fragment species of DHA.[30] In order to compare the efficiency of the two catalytic systems these latter NPs
were used in the same reaction conditions.
2. Experimental part
2.1. Synthesis of the catalyst
2.1.1. Preparation of Pd NPs by LAL (method A)
LAL experiments were performed using a Ti:sapphire laser source (Spitfire – Spectra Physics) operating at repetition rate of
1kHz, with laser duration, wavelength and energy of 100fs, 800nm and 3 mJ, respectively. A metallic Palladium (Goodfellow)

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target was ablated for 10 min in 10 mL of water (Bi-distilled MilliQ) which means a liquid column of 2 cmabove its surface. A 50
mm focal plane lens mounted on a micrometric translation stage was used to focus carefully the laser beam perpendicularly on the
target surface. In these conditions non linear optical effects were minimized. The colloidal solution obtained was decanted,
centrifuged and the palladium nanoparticles were dried in order to obtain the heterogeneous catalyst.
2.1.2. Preparation of Pd NPs by reduction of Pd(OAc)2 with AA (method B)
Pd NPs were synthesized by adding palladium acetate (1mmol, 0.224 g) and AA (3mmol, 0.528 g) into a mortar and grinding
at roomtemperature; the prepared powder was dispersed in water and transferred into the vial and kept there for 2 hours to finish
the reaction. The obtained black powder was decanted and filtrated by Buchner, then dried in the oven at 100°C for a night to
afford the heterogeneous catalyst.
2.2. Physicochemical characterization
As soon as the colloidal solutions by LAL had been obtained (method A), some drops of it were casted on Holey carbon film
copper grids for Transmission Electron Microscopy (TEM) measurements. TEM observations of the Pd NPs obtained by method
B were performed preparing an alcoholic colloidal solution of these NPs and adding a drop on copper grids.
TEM images were acquired by a FEI TECNAI microscope, operating at 200kV. More than 300 NPs were considered to evaluate
size dimension of the formed NPs obtained by each experimental approach. XRD measurements of dried NPs were performed by
an ADXRD X-Perth-Pro Philips instrument. XPS spectra of dried NPs were acquired by LH Leybold X1 spectometer using Mg
Kα radiation and operating at a costant power of 260 W. The acquired XPS spectra of Pd 3d region were analyzed using Origin 9.6
to obtain curve-fitting of photo-peaks in the form of a Voigt function and the relative associated background.
2.3. Ligand-free Suzuki reaction
In a two-neck flask boronic acid 1 (1.5 mmol) was dissolved into the solvent (10 mL) under argon atmosphere with KF (3 mmol),
NPs (0.1mmol) and methyl 4-bromocrotonate 2 (1 mmol). The mixture was kept to temperature and for the time reported in Table
2. At the end the NPs were filtered and washed with water (2x 3 mL); the mixture was extracted with ethyl acetate (2x 10 mL) and
dried over anhydrous MgSO4. Finally the organic phase was concentrated under reduced pressure and purified by column
chromatography on silica gel to afford product 3. All products are known in literature,[29] except product 3f whose
characterization is reported in the Supplementary material.
2.4. Catalytic recyclability forPd NPs
Recyclability of a catalyst plays an important role for industrial applications. Coupling between methyl 4-bromocrotonate and
phenyl boronic acid 1a was chosen as a model reaction. In a two-neck flask phenylboronic acid 1a (0.7 mmol) was dissolved in a
mixture of dioxane/water 1:1 (5 mL) under argon atmosphere with KF (1.5 mmol), Pd NPs (0.05 mmol) and methyl 4-bromo-
crotonate 2 (0.5 mmol). The mixture was heated at the temperature of 70°C. The reaction was monitored by GC-MS every 2
hours. After maximum conversion, the reactants were added in the same proportions to the original reaction mixture. After the
completion of the catalytic cycles the reaction mixture was cooled to roomtemperature, Pd NPs were filtered by Buchner and t he
product was extracted with ethyl acetate. The organic phases were washed with brine and dried over Na2SO4. The crude was
obtained by evaporation off of the solvent and analyzed by GC-MS.
3. Results and discussion
TEM image of NPs obtained by LAL (Figure 1.a) shows the presence of quite spherical NPs, some of them result to be partially
aggregated. The formation of palladium oxide during laser ablation of a palladium target in water has been widely reported [15]

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and the Pd(0): Pd(II) ratio has expected to vary in different ablation conditions. Mortazavi et al [13] have used Nd:YAG and ArF
laser sources, showing a higher amount of palladiumoxide by using the UV laser with respect to the IR source. In this contest, we
can argue that the thin amorphous layer that can be observed in TEM image, could be related to palladium oxide.
Figure 1. (a) TEM image of Pd NPs obtained by LAL. (b) TEM image of Pd NPs obtained with AA. (c) HRTEM images showing
visible crystalline domains. d) Particle size distribution of Pd NPs prepared by LAL referring to TEM image. e) Particle size
distribution of Pd NPs prepared with AA referring to TEM image.
The particle size distribution, evaluated considering more than 800 particles, is reported in Figure 1.d. The estimated mean particle
size was of 27 nm. TEM micrograph in Figure 1.c shows the presence of crystalline nanodomains where the lattice plane distance,
calculated by Fast Fourier Transform (FFT), is 0.22 nm, which is in good agreement with the lattice distance reported for (111)
planes of crystalline ccp palladium[5].
TEM micrograph of the Pd NPs synthesized by reduction of Pd(OAc)2 with AA, Figure 1.b, shows that the obtained NPs are
essentially aggregated and quite irregular; this effect hinder the detection of small particles which distribution might be
misleading, although this datum can be considered as a rough estimation it was of 33 nm (Figure 1.e).
Features of crystal faces are present in some particles and the planes’ distance, obtained by FFT, is equal to 0.22 nm. These results
match with XRD spectra (Figure 2), where the diffraction patterns of LAL and chemically synthesized Pd NPs are shown. In both,
spectra signals occur at 40.1° and 46.7°. These signals are assigned to (111) and (200) planes in agreement with standard
diffraction data of palladium ( JPCDS # 00-005-0681). Crystalline sizes, evaluated by Scherrer methods considering the Pd (111)
signal, have been estimated of 18 nm and 9 nm for Pd NPs obtained by LAL and chemical reduction synthesis, respectively. The
mean particle size measured by TEM analysis is larger than the crystalline size evaluated by XRD spectra, suggesting that the
obtained particles are polycrystalline and/orthat an amount of amorphous phase is present.
Figura 2. XRD spectra of Pd NPs obtained by LAL and with AA.
Pd NPs were used as catalyst in the Suzuki-type reaction. Phenylboronic acid 1a was employed as the model reagent for the
ligand-free cross-coupling with methyl 4-Br-crotonate 2 to prepare product 3a varying reaction parameters as temperature,
solvents and base (Table 1).

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Table 1: Effects of temperature, solvent and base on the efficiency of Pd NPs in the Suzuki reaction.
Entry
1
Pd (0.1 eq) a
Base (3 eq)
Conditions
Conversion % b
(Yield % of 3ac)
KF
18 (15)
Pd NPs (A)
dioxane, r.t., 2h
2d
3e
4
Pd NPs (B)
Pd NPs (A)
Pd NPs (B)
Pd NPs (B)
Pd NPs (B)
Pd NPs (B)
Pd NPs (B)
Pd NPs (A)
KF
Na2CO3
Na2CO3
Na2CO3
K2CO3
KF
dioxane, r.t., 2h
H2O, TBAI, 80°C, 24h
H2O, TBAI, 80°C, 24h
H2O, 80°C, 24h
37 (31)
10 <10
14 (10)
12 (10)
13 (10)
45 (40)
69 (50)
72 (58)
5
6
H2O, 80°C, 24h
7
H2O, 80°C, 6h
8f
9f
KF
dioxane/H2O, 70° C, 2h
dioxane/H2O, 70° C, 2h
KF
a) Method A: Pd NPs produced by LAL. Method B: Pd NPs produced with AA; b) conversion determined by GC analysis versus a
calibrated internal standard (average of two experiments) c) isolated yield (average of two experiments); d) If reaction runs for
24h the yield increases, but with higher isomers’ production; e) TBAI: tetrabutylammonium iodide (0.3 equiv); f) dioxane/ H2O
1:1
Under our reported conditions (KF, dioxane, rt and Pd(OAc)2 as the catalyst) [29] the Suzuki-type reactions catalyzed by Pd NPs
didn’t afford good yields (Table 1, entries 1 and 2). Since it is known that in presence of ammonium salt the halide anions
stabilize the first layers surrounding the transition-metal nanoparticles allowing Suzuki cross-coupling reactions to occur in water
at high temperature [31], we performed the reaction in presence of tetrabutylammonium iodide (TBAI) in water, but also under
these conditions the yields of reaction remained disappointingly low (Table 1, entries 3 and 4). Comparing the results obtained
with different bases it was found that potassium fluoride (KF) was the best one (Table 1, entries 5-7). In particular, good yields
were obtained under optimized conditions (T = 70 °C, 2 hours in dioxane/H2O 1:1) for both Pd NPs (Table 1, entries 8 and 9) with
traces of the two possible isomers of 3a (the cis isomer and the regioisomer with the rearranged double bond), not higher than
20%. This results showed that selectivity of the catalyst exceeds 80%. Moreover the solvents mixture makes catalyst more stable
in aqueous solution and reagents more soluble in organic solvent.
Then, we applied the optimized reaction conditions described before for the coupling reaction of bromide 2 with different
arylboronic acids 1b-f using both Pd NPs catalysts (Table 2, entries 1-10).
Table 2:Cross-coupling of methyl 4-bromocrotonate 2 with aryl and heteroaryl-boronic acids 1b-f.

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Conversion
%
Entry
ArB(OH)2
Catalyst
Product 3
(Yield % )a
1
2
Pd NPs (A)
Pd NPs (B)
72 (61)
86 (81)
3b
1b
1c
3
4
5
Pd NPs (A)
Pd NPs (B)
Pd NPs (A)
46 (41)
59 (52)
44 (36)
3c
1d
1e
3d
6
7
Pd NPs (B)
Pd NPs (A)
58 (49)
51 (44)
3e
8
9
Pd NPs (B)
Pd NPs (A)
Pd NPs (B)
65 (53)
42 (37)
3f
1f
10
88 (82)
a) isolated yield (average of two experiments)
Versability of catalysts was investigated using arylboronic acids with different substituents, electron donating groups to enhance
the nucleophilic activity of boronic acid, and electron withdrawing groups which could disadvantage this process. Good yields
were obtained (up to 81%) using 3,5-dimethoxyphenyl boronic acid 1b (Table 2, entries 1 and 2) and moderate were obtained
using boronic acids 1c and 1d (up to 52%, Table 2, entries 3-6), moreover no homocoupling products were recovered using
boronic acid 1e (Table 2, entries 7 and 8). Also using an heteroaryl boronic acid 1f (Table 2, entries 9 and 10) results showed the
wide applicability of the synthesized NPs. It can be noted that higher yields were always obtained using Pd NPs prepared with
AA. Since the external surface area, evaluated by the size distribution of NPs obtained by LAL and chemical synthesis , are
comparable (about 2 and 4 m2/g, respectively) the observed better catalytic performance are probably due to two factors: the shape
of Pd NPs and the different amount of PdO. In fact, Pd NPs showed different shape, LAL nanoparticles are quite spherical, instead
nanoparticles obtained by method B have a sharp shape, as can be seen in TEM images (figure 1). Different amount of PdO can be
seen in Pd 3d region of XPS data (Figure S4) equal to 20% and 12%, in Pd NPs obtained by LAL and by AA, respectively.
Palladium oxide is usually reported as a good catalyst in the Suzuki cross-coupling [ 22-24], present in both Pd NPs and in
amorphous form, probably here, it does not take place to the coordination with the allylic moiety of the substrate and affects
negatively the catalyst reactivity.
Experimental data on the recyclability of Pd NPs catalysts showed that Pd NPs synthesized by method B were used for 10 cycles
without loss of catalytic activity, whereas Pd NPs obtained by method A retained the activity and selectivity for three catalytic
cycles and at the end of the fourth cycle the yield decreased by approximately 50% producing a greater amount of the cis isomer

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of compound 3a. The greater amount of oxide lowers the catalytic activity of the cross-coupling reaction enabling the
coordination of Pd(0) to the already formed product increasing the production of the isomer compound. TEM images of Pd NPs
after their catalytic cycles are reported in Figure S3 where metallic particles embedded in an amorphous matrix can be observed.
The modification in particles shape and size distribution as well as the increasing of about 10% in particles diameter can be related
to the leaching mechanism effect [31].
4. Conclusion
Pd NPs synthesized by two methods, characterized by TEM, XRD and XPS, were used as catalyst in the Suzuki-type coupling
reaction between methyl 4-bromocrotonate and different EWG- and EDG-substituted aryl- and heteroarylboronic acids, affording
cross-coupled products in good yields (40-80%). LAL proved to be an efficient method for the preparation of NPs with defined
morphology, avoiding reducing agents necessary in chemical methods. Pd NPs catalysts showed better results using chemically
prepared Pd NPs compared to those produced by LAL. Both Pd NPs were used for the first time as catalyst in such allylic
substitution with arylboronic acid [27,28]. The reaction could occur via π-allyl palladium species trapped by nucleophilic boronic
derivatives, in which there is no evidence of branched or bis-arylated products and where Pd oxide seems to affect negatively the
reactivity of the catalytic system. Nevertheless, forthcoming challenges have to be faced in order to improve catalytic efficiency of
LAL Pd NPs catalyst and to understand the role of Pd oxide during this reaction.

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Fig. 1A
Fig. 1B
Fig. 1C

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Fig. 1D
Fig. 1E

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Fig. 2

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Highlights



Pd NPs prepared underenvironmentally benign conditions and used as catalyst.
Firstuse of Pd NPs synthesized by LALin the Suzuki cross-coupling.
Suzuki couplingstudy, usingan unusual bromide and arylboronicacids.