Palladium nanoparticles stabilized by metal-carbon covalent bond: An efficient and reusable nanocatalyst in cross-coupling reactions

Article abstract of DOI:10.1016/j.catcom.2013.04.028

Palladium nanoparticles stabilized by PdC_(binaphthyl) covalent bonds have been designed and synthesized. This new class of Pd nanoparticles was efficiently used as reusable catalysts for CC bond forming Heck, Suzuki-Miyaura and Sonogashira cross coupling reactions with high turnover. Even after the several catalytic cycles the Pd NPs had the same reactivity and particle size without any apparent agglomerization.

Full text of DOI:10.1016/j.catcom.2013.04.028

Palladium nanoparticles stabilized by metal-carbon covalent bond: An
efficient and reusable nanocatalyst in cross-coupling reactions
Dhandapani Ganapathy, Govindasamy Sekar
PII:
DOI:
Reference:
S1566-7367(13)00165-9
doi: 10.1016/j.catcom.2013.04.028
CATCOM 3493
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Catalysis Communications
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Accepted date:
28 February 2013
9 April 2013
27 April 2013
Please cite this article as: Dhandapani Ganapathy, Govindasamy Sekar, Palla-
dium nanoparticles stabilized by metal-carbon covalent bond: An efficient and
reusable nanocatalyst in cross-coupling reactions, Catalysis Communications (2013), doi:
10.1016/j.catcom.2013.04.028
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ACCEPTED MANUSCRIPT
Palladium nanoparticles stabilized by metal-carbon covalent bond: an
efficient and reusable nanocatalyst in cross-coupling reactions
Dhandapani Ganapathy, and Govindasamy Sekar*
Department of Chemistry, Indian Institute of Technology Madras, Chennai, Tamil Nadu-600 036, India
Fax: +91(44)22574202; E-mail: gsekar@iitm.ac.in
Abstract
Palladium nanoparticles stabilized by Pd-C(binapthyl) covalent bonds have been designed and
synthesized. This new class of Pd nanoparticles efficiently used as reusable catalysts for C-C
bond forming Heck, Suzuki-Miyaura and Sonogashira cross coupling reactionswith high
turnover. Even after the several catalytic cycles the Pd NPs had the same reactivity and particle
size without any apparent agglomerization.
Keywords
Palladium nanoparticles, Pd-C covalent bond, Nanocatalysis, C-C bond formation, Cross-
coupling reaction.
1. Introduction
Palladium catalysts have been proven to be very efficient in C-C bond forming reactions.[1]
They play an important role in the synthesis of natural products,[2] agro and pharmaceutical
chemicals.[3] In particular, homogeneous palladium catalysts have been extensively studied in
synthetic organic chemistry because of their higher activity and selectivity.[4] However, the uses
of homogeneous Pd catalysts are limited in large-scale reactions owing to the contamination of
products obtained in these reactionswith ligands and palladium metal. Also, the enormously
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tedious separation and recycling of the palladium catalyst involved confer additional
disadvantages to the homogeneous catalysts.
Alternatively, palladium nanoparticles (NPs) have attracted a remarkable interest in academic
and industrial research due to their advantages as semi heterogeneous catalystsand catalysts for
green chemistry.[5,6] In addition, the NPs show enhanced reactivity due to their larger surface-
to-volume ratio.[7] The catalytic property, stability, recoverability and reusability of the NPs
depend on the nature of the stabilizers. Although, several stabilizers such as phosphines,[8]
amine,[9] thiols,[10] and polymers are widely used,[11] these easily oxidizable phosphines and
environmentally malignant thiols are better avoided from the industrial and environmental
aspects.Furthermore, these ligands provide comparatively weak stabilization to the NPs due to
their coordination stabilization. This can lead to agglomerization of the NPs during the catalytic
cycle and result inan inactive catalyst that cannot be reused [12] for further catalytic
cycles.Considering these drawbacks, designing a nanocatalyst which is stabilized by
strongcovalent bonds and utilizing them as catalysts for crucial organic transformations is
essential. Bond strengths offer a clever rationale for designing such a nanocatalyst. For instance,
the metal-carbon bond strength is very high when compared with that of the metal-heteroatom.
(The Pd-C single bond energy is 436 kJ mol-1which is higher than Pd-S bond energy of 380 kJ
mol-1).[13] The Pd NPs stabilized by Pd-C covalent bond are therefore anticipated to be much
more stable than the traditional stabilizers.
In this work, our anticipation leads us to the successfuldesign of attractive and stable Pd NPs
stabilized by Pd-C(binapthyl) covalent bonds. We demonstrate their robust application as easily
recoverable and reusable catalysts for C-C bond forming Heck, Suzuki-Miyaura and Sonogashira
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cross coupling reactions. In addition, the efficient recoverability and reusability of the NPs for all
the reactions without any loss in the catalytic activity are as well presented.
2. Experimental
2.1. General
Infrared spectra were recorded on a FTIR 4000 Series Spectrometer using dry KBr pellet.
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The wave numbers of recoeded IR signals are quoted in cm-1. H and 13C NMR spectra were
recorded in CDCl3 or DMSO-D6 solvent using Bruker AV-400 MHz spectrometer. High
resolution mass spectra (HRMS) were recorded on Q-Tof Micro mass spectrometer and GC-MS
was recorded with JEOL GCMATE II.X-ray photoelectron spectroscopic analysis was performed
by using an ESCA probe/TPD of Omicron Nanotechnology. HRTEM analysis was performed
using a JEOL 3010 microscope. Inductively coupled plasma-optical emission spectrometer (ICP-
OES) from Perkin Elmer Optima 5300 DV was used to identify the Pd content of the
nanoparticles.
2.2. Preparation of Pd-binapthyl nanoparticles
In a 250 mL round bottomed flask potassium tetrachloropalladate (326.5 mg, 1.0 mmol) in
nano pure water (50 mL) was taken and 1,1'-binaphthyl,-2,2'-bis(diazoniumtetrafluoroborate)
1(482.1 mg, 1.0 mmol) and toluene (50 mL) were added to it. The resulting mixture was
vigorously stirred in magnetic stirrer for 1 hour. NaBH4 (226.9 mg, 6.0 mmol) in water (2 mL)
was added dropwise at room temperature to the reaction mixture and the immediate colour
change to dark greenish black signifies the Pd NPs formation. The reaction mixture was stirred at
room temperature for 2 hours. The toluene layer was separated and washed with 0.5M aq. H2SO4
(3 × 50 mL) and 0.5M aq. NaHCO3 (3 × 50 mL) to remove any unreacted metal complexes
(diazonium tetrachloropalladate). Then the solvent was completely evaporated under rotary
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evaporator. The solid particles were subsequently suspended in ethanol (2 × 10 mL) and
sonicated for half an hour at room temperature. The suspension was centrifuged (4500 rpm,
approx. 30 mins). Finally, ethanol was removed and dried in vacuo to furnish the the Pd-
binapthyl nanoparticles (400 mg, 10 Wt% Pd, ICP-OES analysis).
2.3. Typical procedure for synthesis of 1,1'-binaphthyl,-2,2'-is(diazoniumtetrafluoroborate)
Sodium nitrite (1.2 g, 18.0 mmol) in 3 mL of water was added slowly to a stirred solution of
BINAM (1.2 g, 4.0 mmol) and HBF4 (45% in H2O, 10 mL) at 0 oC. Then the reaction was stirred
at 0 oC for 30 min and further stirred at room temperature for 30 min. The resulting yellow solid
was filtered using a Büchner funnel and washed with cold HBF4 (2 × 5 mL), H2O (2 × 10 mL)
and EtOH (10 mL). The diazonium salt 1 solid (1.8 g) was dried under a vaccum desiccator and
stored at 0 oC (yellow solid)
2.4. General procedure for Suzuki-Miyaura coupling reaction and recycling
Aryl halide (0.5 mmol), phenyl boronic acid (182.25 mg, 0.75mmol), KF (116.2 mg, 2.0 mmol)
and Pd NPs 2 (5.3 mg, 0.005mmol, 1 mol %) were taken in an oven dried reaction tube equipped
with magnetic pellet. DMF (1 mL) and H2O (1 mL) were added to the reaction tube and the
reaction mixture was stirred at room temperature under closed atmosphere. The reaction was
monitored by TLC. After the starting material was completely consumed, the reaction was
stopped and then the solution was extracted with diethyl ether (3 × 20 mL). The organic phase
concentrated in vacuum. The crude product was purified by column chromatography using silica
gel.
For recyclability, the reaction was repeated with 4-iodotoluene as substrate in 2.0 mmol scale
maintaining the same conditions as described above, except using the recovered catalyst. After
the completion of the reaction, the reaction mixture was allowed to settle down. The catalyst was
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recovered by simple decantation and washed with water (5 mL) and ethanol (5 mL). Finally the
catalyst was dried under vaccum. The dried catalyst was reused for the next catalytic cycle.
2.5. General procedure for Heck coupling reaction and recycling
Aryl halide (0.5 mmol), ethyl acrylate (65.4 µL, 0.6 mmol), Cs2CO3 (192.4 mg, 1.0 mmol)
and Pd NPs 2 (5.3 mg, 0.005 mmol, 1 mol %) were taken in a oven dried reaction tube equipped
with magnetic pellet. DMF (1 mL) was added to the reaction tube and the reaction mixture was
o
stirred at 110 C temperature under closed atmosphere. The reaction was monitored by TLC.
After complete consumption of the starting material, the reaction was stopped, and cooled to
room temperature. Crude product was extracted with diethyl ether (3 × 20 mL). Then the organic
phase was dried over Na2SO4 and concentrated in vacuum. The product was purified by column
chromatography using silica gel.
For recyclability, the reaction was repeated with 4-iodotoluene as the substrate on a 2.0 mmol
scale maintaining the same conditions as described above, except using the recovered catalyst.
After the reaction completed, the reaction mixture was allowed to cool to room temperature. The
solution was diluted with water (4 mL) and ethanol (4 mL) resulting in solid particles which were
removed by centrifugation, and were dried under vacuum. The dried catalyst was reused for
further catalytic run.
2.6. General procedure for Sonogashira coupling reaction and recycling
Arylhalide (0.6 mmol), phenyl acetylene (52 µl, 0.72 mmol), Cs2CO3 (231.36 mg, 1.2 mmol)
and Pd NPs 2 (5.3 mg, 0.005 mmol, 1 mol %) were taken in an oven dried pressure tube equipped
with magnetic pellet. MeOH (1 mL) was added to the pressure tube and the reaction mixture was
o
stirred at 90 C. After the starting material was completely consumed, the reaction was stopped
and cooled to room temperature. Then the reaction mixture was concentrated in vacuum, the
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product was purified by column chromatography using silica gel. The catalyst was recovered and
reused.
3. Results and discussion
3.1.Synthesis and characterization of Pd-C(binapthyl) NPs
The Pd NPs 2 were prepared by the simple reduction of potassium tetrachloropalladate (1.0
mmol) and 1,1'-binaphthyl,-2,2'-bis(diazoniumtetrafluoroborate) 1 (1.0 mmol) by sodium
borohydride in toluene and water (Sheme 1). 1 was readily prepared by the diazotization of
commercially available 1,1'-binaphthyl-2,2'-diamine. In the first step, the aqueous NaBH4
solution was added to the mixture of K2[PdCl4] and diazonium salt 1 in toluene and water
mixture. Immediately, the colour of the solution was changed from orange-brown to dark
greenish-black signifying the formation of palladium nanoparticles. The PdNPs was purified by
repeatedly washing with 0.5 M H2SO4 followed by 0.5 M NaHCO3. The NPs are seperated by
centrifugation and dried under vaccum. The nanoparticles showed good solubility in
dichloromethane, chloroform and toluene but were completely insoluble in water, ethanol and
methanol. The NPs are stable toward air and moisture.
High resolution transmission electron microscopy (HRTEM) analysis of the Pd NPs (Figure
1) showed a well dispersed homogeneous nanoparticle without any aggregation in the grid which
clearly indicates that the binaphthyl moitey provided good stabilization through Pd-C linkage.
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The particles are found to be spherical with an average diameter of 3.33±0.9 nm. The H NMR
spectrum of the diazonium salt showed sharp peaks at 7.4-9.3 ppm and these peaks got
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broadened in the H NMR spectrum of Pd NPs. Similarly, in 13C NMR of Pd NPs the peaks
completely disappeared in the base line.The broadening of the peaks into the base line isdue to
the facts(a) fast spin–spin relaxation of atoms close to a metal core or (b) size-dependent spin–
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spin relaxation which confimed that the Pd NPs are stabilized M-C(binaphthyl) bond.[14-16] The Pd
NPs were further characterized by FTIR, XPS, ICP-OES, SEM/EDX analyses (see SI for
details).
3.2.Catalytic Studies
In our effort to use these novel Pd NPs as catalysts in crucial organic transformations, the Pd
NPs were initially tested for their catalytic activity in the Suzuki cross coupling reaction.In the
presence of 1 mol% of Pd NPs 2 at room temperature the coupling reaction between the
arylhalide and phenylboronic acid took place smoothly and the results are summarized in Table
1. It is very important to mention the fact that there is no coupling reation in the absence of Pd
NPs 2.Having sucessfully demonstrated the catalytic utility of our Pd NP catalyst, scope of the
new Pd NPs 2 was further extended to the Heck reaction.
The reaction of 4-iodotoluene with ethyl acrylate and Cs2CO3, in DMF solvent in the
presence of 1.0 mol% of Pd NPs 2 at 110 oC afforded 97% of isolated yield (Table 2). Electron-
donating and electron-withdrawing group containing substrates are well tolerated and provided
excellent yields. In particular, sterically hindered ortho-disubstituted aryl iodide provided good
yields of the products (entry 5, Table 2).
The Pd NPs 2 were also successfully used as catalysts for the Sonogashira cross-coupling
reaction (Table 3). The reaction of p-iodotoluene with phenylacetylene and Cs2CO3 in methanol
o
in the presence of 1.0 mol % of Pd NPs 2 at 90 C afforded 91% of isolated yield for the
corresponding product. Minimum catalytic loading have achived for suzuki coupling and the turn
over number is found have 58, 000 (see supporting information).
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3.3.Catalystrecoverability and reusability
The recoveribility and reusability of the catalyst for the Suzuki cross-coupling reaction was then
examined under the optimized reaction condition with 4- iodotoluene and phenylboronic acid as
coupling partners. After the completion of the reaction, the catalyst was successfully recovered
and reused for the next two cycles(Figure 2a). In addition, the catalyst was recovered
quantitatively and reused in the Heck reaction which gave the same yield for three cycles (Figure
2b).
After the successful use of the catalyst in three consecutive cycles, the recovered Pd NPs was
subjected to HRTEM and HRSEM analysis. Gratifyingly, the catalyst showed the same particle
size 3.25±0.85 nm without any apparent aggregation.Also, the SEM-EDX analysis showed the
same wt% of the metal before and after the catalytic cycles. This result strongly confirms that the
Pd NPs are very stable and the interaction between the metal and the stabilizers is very strong.
The wt% of the recovered Pd NPs 2 was confirmed with ICP-OES and it was found to be 10%
by weight(see supporting information). Thus, our catalytic system works very well as a efficient
recoverable and reusable catalyst in cross coupling reactions.
4. Conclusion
In conclusion, we have synthesized a new class of Pd NPs stabilized by stable Pd-C(binaphthyl)
1
covalent bond. The Pd NPs were well characterized by FTIR, H NMR, 13C NMR, HRTEM,
XPS, ICP-OES, SEM/EDX analyses. The Pd NPs were successfully utilized as catalysts in most
important oraganic transformations such as Suzuki, Heck and Sonogashira coupling reactions.
The catalyst was quantitatively recovered and reused for all the three reactions without any loss
in catalytic activity. Very importantly, the strong stabilization of NPs with Pd-C covalent bond
helped the catalyst to retain its activity and particle size and restricts the agglomerization. Thus,
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our rational design of these Pd NPs is expected to have widespread utility in a variety of C-C
forming reactions.
Acknowledgements
We thank DST and CSIR (01(2378)/10/EMR-II), New Delhi for the financial support. D.G.P
thanks CSIR for fellowship.
References
[1] J. Tsuji, Palladium Reagents and Catalysts: New Perspective for the 21st Century, Wiely: 2004.
[2] K. C. Nicolaou, P. G. Bulger, D. SarlahAngew. Chem. Int. Ed. 44 (2005) 4442-4489
[3] C. Torborg, M. Beller, Avd. Synth. Catal. 351(2009) 3027-3043.
[4] A. Zapf, M. Beller, Topics in Catalysis. 19 (2002) 101-109.
[14] D. Astruc, F. Lu, J. R. Aranzaes, Angew. Chem. Int. Ed. 44 (2005) 7852-7872.
[5] D. Astruc, Nanoparticle and Catalysis; VCH: weinheim, 2007.
[6] V. Polshettiwar, R. S. Varma, Green Chem. 12 (2010) 743-754.
[7] J. H. Fendler, Nanoparticles and Nanostructured Films: Preparation, Characterization and Applications,-Wiley-VCH,
Weinhein, Germany, 1998.
[8] R. Tatumi, T. Akitab, H. Fujihara, Chem. Commun. (2006) 3349.
[9] B. J. Gallon, R. W. Koima, R. B. Karner, P. L. Diaconescu, Angew. Chem. Int. Ed. 46 (2007) 7251-7254.
[10] M. Cargnello, N. L. Wieder, P. Canton, T. Montini, G. Giambastiani, A. Benedetti, R. J. Gorte, P. Fornasiero, Chem.
Mater. 23 (2011) 3961-3969.
[11] M. De bruyn, R. Neumann, Adv. Synth. Catal. 349 (2007) 1624-1628.
[12] E. Ramirez, S. Jansat, K. Philippot, P. Lecante, M. Gomez, A. M. Masdeu-Bulto, B. Chaudret, Journal of Organometallic
Chemistry. 689 (2004) 4601-4610.
[13] D. R. Lide, CRC handbook of chemistry and physics: a ready-referencebook of chemical and physical data, CRC Press, Boca
Raton, FL, 2004.
[14] (a) D. Ghosh, S. Chen, J. Mater. Chem. 18 (2008) 755-762.
[15] F.Mirkhalaf, J.Paprotny, D. J. Schiffrin, J. Am. Chem. Soc.128 (2006)7400-7401.
[16] V. K. Ratheesh Kumar, and K. R. Gopidas, Chem. Asian J. 5 (2010) 887 – 896.
Captions to Figures
Figure 1 (a) HRTEM micrographs of Pd-binaphthyl NPs. (b) lattice fringe image. (c) particle
size distribution.
Figure 2 Recoverability and reusability of stable Pd NPs 2 in Suzuki cross coupling reaction.
Captions to Tables
Table 1Suzuki cross-coupling reactions of aryl halides using Pd NPs 2.a
Table 2 Heck cross-coupling reactions of aryl iodides using Pd NPs 2.a
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Table 3Sonagashira cross-coupling reactions of aryl iodides using Pd NPs 2.a
Captions to Scheme
Scheme 1Synthesis of Pd NPs 2 stabilized by Pd-C (binaphthyl) covalent bonds.
Scheme 1
Figure 1
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Figure 2
Table 1
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Table 2
Table 3
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Graphical Abstract
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Highlights
 Pd-C (binapthyl) covalent bonds stabilized Pd NPs are synthesized and characterized.


This Pd NPs were utilized as catalyst for C-C bond forming cross coupling reactions.
Easily recovered and reused without change in catalytic activity of the nanocatalyst.
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