Palladium nanoparticle-cored G1-dendrimer stabilized by carbon-Pd bonds: Synthesis, characterization and use as chemoselective, room temperature hydrogenation catalyst

Article abstract of DOI:10.1016/j.tetlet.2011.04.011

Palladium nanoparticle-cored Fréchet type G₁-dendrimer (Pd-G₁) stabilized by Pd-carbon bonds is synthesized and characterized by IR, NMR, UV-Vis and TEM. Pd-G₁ was found to be a highly efficient, chemoselective and reusable catalyst for the room temperature hydrogenation of carbon-carbon multiple bonds. Reducible functionalities like CHO, CO, COOR, CN, NO₂ and halogens were unaffected. Pd-G₁ is projected as an efficient catalyst for the selective hydrogenation of carbon-carbon multiple bonds in multifunctional organic molecules.

Full text of DOI:10.1016/j.tetlet.2011.04.011

Tetrahedron Letters 52 (2011) 3102–3105
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Palladium nanoparticle-cored G1-dendrimer stabilized by carbon–Pd bonds:
synthesis, characterization and use as chemoselective, room temperature
hydrogenation catalyst
⇑
Venugopal K. Ratheesh Kumar, Karical R. Gopidas
Photosciences and Photonics Section, Chemical Sciences and Technology Division, National Institute for Interdisciplinary Science and Technology (NIIST),
Council of Scientific and Industrial Research (CSIR), Trivandrum 695 019, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Palladium nanoparticle-cored Fréchet type G1-dendrimer (Pd-G1) stabilized by Pd–carbon bonds is syn-
thesized and characterized by IR, NMR, UV–Vis and TEM. Pd-G1 was found to be a highly efficient, chemo-
selective and reusable catalyst for the room temperature hydrogenation of carbon–carbon multiple
bonds. Reducible functionalities like CHO, CO, COOR, CN, NO2 and halogens were unaffected. Pd-G1 is pro-
jected as an efficient catalyst for the selective hydrogenation of carbon-carbon multiple bonds in multi-
functional organic molecules.
Received 15 February 2011
Revised 29 March 2011
Accepted 1 April 2011
Available online 9 April 2011
Keywords:
Ó 2011 Elsevier Ltd. All rights reserved.
Nanoparticle-cored dendrimer
Diazodendron
Palladium
Catalysis
Hydrogenation
Nanoparticle-cored dendrimers (NCD) are core-shell materials
possessing nanometer-sized metal cluster at the core and a shell
made of dendrons of different generations, which are attached
radially to the core.1–4 Because of the conical shape and steric
requirements of the dendrons, their assembly on the metal surface
is decided by a sterically induced stoichiometry, leaving large
number of voids at the metal surface as shown in Figure 1. The
unpassivated metal surface at the voids can function as catalytic
sites. NCDs with different metals and dendrons were synthesized
and characterized in the past and several of them have been stud-
ied for their catalytic activities.3,5,6
Dendron
Voids
Metal
One of us in collaboration with Fox and Whitesell designed and
synthesized the first catalytically active NCD.3 Our idea was to de-
sign a Pd catalyst that is capable of catalyzing several reactions. We
observed that the Heck and Suzuki reactions catalyzed by Pd–NCD
were successful but it failed as a hydrogenation catalyst due to the
hydrogenolysis of the NCD, leading to its decomposition and pre-
cipitation of Pd. We felt that if the Pd–S bonds in the NCD are re-
placed by Pd–C bonds, then hydrogenolysis could be prevented
and the NCD might function as good hydrogenation catalyst also.
In this Letter, we report the synthesis and characterization of a
Pd-cored G1-dendrimer (Pd-G1), where the Fréchet–type G1-den-
drons are linked to the Pd core through Pd–carbon bonds. We also
show that Pd-G1 is a very effective catalyst for the chemoselective
Figure 1. Schematic of a NCD.
hydrogenation of olefinic and acetylenic bonds at room tempera-
ture and atmospheric pressure in the presence of other reducible
groups like CHO, NO2 and halogens.
We have recently reported the synthesis and characterization of
Au–NCDs, where the Fréchet dendrons are linked to the gold clus-
ter through Au–C covalent bonds.7 We employed a similar strategy
⇑
Corresponding author. Tel.: +91 471 2515390, fax: +91 471 2490186.
E-mail address: gopidaskr@rediffmail.com (K.R. Gopidas).
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.04.011

V. K. R. Kumar, K. R. Gopidas / Tetrahedron Letters 52 (2011) 3102–3105
3103
for the synthesis8 of Pd-G1 which involved the simultaneous
reduction of G1-diazadendron and Pd(II) phase-transferred into
dichloromethane using TOAB by NaBH4.7,9–11 During reduction
Pd(II) is reduced to palladium atoms, which through nucleation
and growth process grows into small Pd clusters. The diazoden-
dron upon reduction forms a phenyl radical terminated dendron.
At some stage of reduction, the dendron radicals get attached to
the growing Pd nanoparticles to give Pd-G1 as shown in Scheme 1
(see SI for synthesis). Pd-G1, obtained as a black powder, was freely
soluble in chloroform, dichloromethane, tetrahydrofuran and
insoluble in alcohols, diethyl ether and water. It was very stable
for several months both in the solid state and in solution.
TEM image of the Pd-G1 thus prepared is shown in Figure 2. Fig-
ure 2 shows that the Pd core in Pd-G1 has average diameter of
2.7 nm. Thermogravimetric analysis showed that nearly 60% of
the weight of Pd-G1 is due to Pd metal (ꢀ700 Pd atoms, see SI).
A simple calculation1–3 would show that approximately 131 den-
drons are linked to the Pd core in Pd-G1.
Pd-G1 was characterized by UV–Vis, FTIR, and NMR spectrosco-
pies (see SI for UV, IR and NMR spectra). The UV–Visible spectrum
of Pd-G1 gave a progressively decreasing absorption profile with no
plasmon band, consistent with earlier reports.3 The G1-diazoden-
dron exhibited vibrational stretching of the diazo group at
2237 cmÀ1, which was absent in the IR spectrum of Pd-G1 and ex-
cept for this difference, IR spectra of Pd-G1 and G1-diazodendron
were identical. 1H NMR of G1-diazodendron showed signals at d
8.2 and 7.0 due to the diazophenyl ring protons7,9 and upon reduc-
tion, these peaks disappeared along with considerable signal
broadening. Broadening of the NMR signals due to fast spin-spin
relaxation of atoms close to metal core or size-dependent spin-spin
relaxation clearly support the metal core-organic shell type struc-
ture of Pd-G1.1–4,7,12–14 The signals at d <3 ppm in the 1H NMR of
Pd-G1 can be due to trapped TOAB.7 In the 13C NMR spectrum of
G1-diazodendron signals at d 168, 135 and 118 ppm are identified
as belonging to the diazo bearing phenyl group, which disappeared
upon reduction to Pd-G1. Similar observations were made earlier
for monolayer protected gold cluster where Au is attached directly
to the phenyl group and it was suggested that the signals due to
phenyl ring carbons linked directly to the metal have broadened
so much that they are indistinguishable from the background.7,9–14
XRD studies indicated the presence of diffraction peak at 40° (2h)
corresponding to the {111} plane of metallic palladium fcc lattice.
However, calculation of particle size from diffraction line broaden-
ing was difficult due to their low intensity.
Having characterized Pd-G1, we evaluated its catalytic activity
in hydrogenation reactions. We have attempted hydrogenation15
of different substrates listed in Table 1 using catalytic amounts of
Pd-G1. The yields reported in Table 1 are isolated product yields
and purity has been confirmed by NMR and GC–MS analysis (see
SI). It may be noted that for entries 1–13 in Table 1, product yields
are nearly quantitative and reaction times were low. The reaction
conditions employed15 were very mild compared to hydrogenation
reactions of similar substrates using other palladium catalysts
where complete reduction of substrates including aromatic rings
were reported.16 Inspection of Table 1 also reveals that Pd-G1 is a
very selective catalyst for the reduction of C@C and C„C at room
temperature and mild hydrogen pressure, even in the presence of
sensitive groups like nitro (entries 5–13 in Table 1). Selective
reduction of C–C multiple bonds in multifunctional organic mole-
cules is always challenging especially in the presence of groups like
NO2 and CHO which can also undergo reduction under the hydro-
genation conditions.16–20 In Pd-G1 catalyzed hydrogenations, these
groups remain unaffected. Entries 14–15 show that halogens (Cl, Br
and I) and endocyclic double bonds are also unaffected under the
reaction conditions. We have calculated turnover number (TON =
moles of product formed/moles of palladium atoms) and turnover
frequency (TOF = moles of product formed/moles of palladium
atom/hours) for Pd-G1 and the values are given in Table 1.
Designing an efficient hydrogenation catalyst that can perform
under mild reaction conditions with high chemoselectivity is still
an active challenge in organic synthesis. The commonly employed
hydrogenation catalyst Pd/C do not normally exhibit selectivity in
the reduction of multifunctional organic compounds.17,19,20 Table 1
shows that Pd-G1 can catalyze chemoselective reduction of C@C in
the presence of NO2 group (substrates 7 and 12). Reduction of NO2
did not occur even after reaction time was extended to 8 h (see SI).
Our control experiments with these substrates showed that hydro-
genation with Pd/C (under same reaction conditions with respect
to time and Pd content in the catalyst) led to reduction of both ni-
tro and C@C moieties (entries 7e and 12e in Table 1). With only
5 mg Pd/C (reaction time 6 h), mixture of products arising from
the reduction of C@C (80%) and C@C plus nitro (20%) were ob-
tained. It is reported that chemoselectivity can be observed with
Pd/C if suitable catalyst poisons were employed.17,19,20 But, the
presence of these additives may demand their separation from
the final products after reaction. Thus, if chemoselective hydroge-
nation is desired Pd-G1 is a better choice. Our studies show that
endocyclic double bonds are insensitive to hydrogenation condi-
tions with Pd-G1 and at present we are not certain about the rea-
son for this selectivity which requires further understanding of
the reaction mechanism. In addition to the substrates reported in
Table 1, we have carried out reduction of compounds containing
isolated carbon-carbon multiple bonds such as 1-dodecyne, 1-oc-
tyne and hex-5-ene-2-one. In these cases GC–MS analysis of reac-
tion mixtures indicated complete reduction of carbon-carbon
multiple bonds and formation of corresponding products. How-
ever, these products being volatile liquids could not be isolated
completely using the standard procedure employed here. Hence
these reactions are not reported in Table 1.
-
N2+BF4
O
O
O
TOAB, DCM
NaBH4,
K2PdCl4
Pd
O
O
O
m
Pd-G1
-
G1N2+BF4
Scheme 1. Synthesis of Pd-G1.
20
10
0
D = 2.7 nm
0
2
4
6
8
10
Diameter, nm
We observed that most of the Pd-G1 could be recovered and
reused a few times without loss of catalytic activity. Catalyst
Figure 2. (a) TEM image (b) core-size histogram of Pd-G1.

3104
V. K. R. Kumar, K. R. Gopidas / Tetrahedron Letters 52 (2011) 3102–3105
Table 1
Hydrogenation using Pd-G1
a
Entry
Substrate
Product
Yieldb (%)
TONc
Time (h)
TOFd (hÀ1
)
1
2
90
92
77.6
83.0
4
4
19.4
20.8
3
4
99
98
19.6
19.8
6
6
3.3
3.3
COOH
CHO
COOH
CHO
5
6
95
81
23.3
29.0
6
7
3.9
4.2
7
7e
8
98
75
98
15.7
11.7
16.9
6
6
6
2.6
1.9
2.8
O2N
O2N
H2N
O2N
H3CO
H3CO
O
O
9
10
11
99
98
97
17.2
21.7
14.8
6
5
6
2.9
4.3
2.5
COOCH3
O
COOCH3
O
O
O
12
95
12.2
6
6
2.1
O2N
O2N
O2N
H2N
O
O
12e
95
94
11.9
11.8
1.9
1.7
O
O
13
14
7
No reaction
No reaction
24
O
X
X = Cl, Br, I, -COCH3, -CH2CN,
-OCH3, -OH, -COOH
15
24
a
b
c
Substrate and Pd-G1 (2.8 Â 10À5 mol Pd) were taken in ethyl acetate (15 mL) and stirred under hydrogen atmosphere (hydrogen balloon) for the indicated time.
Isolated product yield.
TON = moles of product/moles of Pd atom.
TOF = moles of product / moles of Pd atom/hour.
Control experiment using 10% Pd/C (2.8Â10À5 mol Pd).
d
e
recovery studies with phenylacetylene (2 mmol) and Pd-G1 (5 mg)
showed nearly 92% conversion efficiency for three cycles after
which product yields decreased, most probably due to incomplete
recovery of catalyst in the previous cycles (see SI for experimental
details).
The importance of transition metals in hydrogenation catalysis
is well documented in the literature.20–29 Although there are sev-
eral reports dealing with hydrogenations catalyzed by dendri-
mer-encapsulated Pd nanoparticles,21,22 and Pd/Pt nanoparticle-
cored dendrimers,5,6 there are few reports dealing with chemose-
lective hydrogenations using these catalysts. Previously reported
Pd–NCDs required elevated temperature and pressure conditions
to carry out hydrogenation reactions.6 Catalyst recovery and recy-
cling were also not possible with these systems. In these cases, the
Pd nanoparticles were stabilized by either noncovalent interac-
tions or weak coordinate bonds. In Pd-G1, the dendrons are linked
to Pd cluster by very strong Pd-C bonds (bond energy
ꢀ436 kJMÀ1).11 These materials are expected to be very stable
and active compared to other NCDs.
In conclusion, present work describes the synthesis and
characterization of a palladium nanoparticle-cored Fréchet type
G1-dendrimer, Pd-G1, where the dendrons are linked to Pd core
through Pd–C bonds. Pd-G1 was characterized using IR, NMR
and UV–visible spectroscopies. TEM analysis indicated the pres-
ence of nearly spherical, polydisperse particles with average
diameter of 2.7 nm. Preliminary studies show that Pd-G1 meets
all the requirements to act as a highly efficient, chemoselective,
room temperature hydrogenation catalyst with good recyclability
and short reaction times giving excellent product yields. We are
interested in developing nanoparticle catalysts that are capable
of catalyzing several reactions. In this context we are exploring
the catalytic efficiency of Pd-G1 in other C–C bond forming

V. K. R. Kumar, K. R. Gopidas / Tetrahedron Letters 52 (2011) 3102–3105
3105
9. Mirkhalaf, F.; Paprotny, J.; Schiffrin, D. J. J. Am. Chem. Soc. 2006, 128, 7400–7401.
10. Pinson, J.; Podvorica, F. Chem. Soc. Rev. 2005, 34, 429–439.
11. Ghosh, D.; Chen, S. J. Mater. Chem. 2008, 18, 755–762.
reactions and the results are very encouraging. These studies are
in progress in our laboratory.
12. Terrill, R. H.; Postlethwaite, T. A.; Chen, C.-H.; Poon, C.-D.; Terzis, A.; Chen, A.;
Hutchison, J. E.; Clark, M. R.; Wignall, G.; Londono, J. D.; Superfine, R.; Flavo, M.;
Johnson, C. S.; Samulski, E. T.; Murray, R. W. J. Am. Chem. Soc. 1995, 117, 12537–
12548.
13. Badia, A.; Singh, S.; Demers, L.; Cuccia, L.; Brown, G. R.; Lennox, R. B. Chem. Eur.
J. 1996, 2, 359–363.
14. Hostetler, M. J.; Wingate, J. E.; Zhong, C.-J.; Harris, J. E.; Vachet, R. W.; Clark, M. R.;
Londono, J. D.; Green, S. J.; Stokes, J. J.; Wignal, G. D.; Glish, G. L.; Porter, M. D.;
Evans, N. D.; Murray, R. W. Langmuir 1998, 14, 17–30.
15. Procedure for hydrogenation reaction: The substrate (actual concentration
varied from 0.5 to 2.5 mmol depending up on the solubility in ethyl acetate,
see SI for exact concentration of each substrate) and Pd-G1 (5 mg) were
dissolved in ethyl acetate (15 mL) and the mixture was stirred under slight
positive pressure of hydrogen (hydrogen balloon) for 4–7 h. Progress of the
reaction was monitored using GC–MS analysis. After reaction, solvent was
evaporated and the products were extracted into ether (2 Â 15 mL). The
ether extract, containing insoluble Pd-G1, was decanted and passed through
Acknowledgments
The authors thank the Council of Scientific and Industrial
Research (CSIR) and the Department of Science and Technology
(DST grant No. SR/S5/OC-15/2003), Government of India, for
financial support. V.K.R.K. thanks University Grants Commission
(UGC), Government of India, for research fellowship. This is contri-
bution number NIIST-PPG-309.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2011.04.011.
a
small pad of silica to remove any suspended impurities. Ether was
removed to get pure products.
16. Kogan, V.; Aizenshtat, Z.; Neumann, R. New J. Chem. 2002, 26, 272–274.
17. Maegawa, T.; Takahashi, T.; Yoshimura, M.; Suzuka, H.; Monguchi, Y.; Sajiki, H.
Adv. Synth. Catal. 2009, 351, 2091–2095.
18. Callis, N. M.; Thiery, E.; Bras, J. L.; Muzart, J. Tetrahedron Lett. 2007, 48, 8128–
8131.
19. Mori, A.; Miyakawa, Y.; Ohashi, E.; Haga, T.; Maegawa, T.; Sajiki, H. Org. Lett.
2006, 8, 3279–3281.
20. Mori, A.; Mizusaki, T.; Kawase, M.; Maegawa, T.; Monguchi, Y.; Takao, S.;
Takagi, Y.; Sajiki, H. Adv. Synth. Catal. 2008, 350, 406–410.
21. Crooks, R. M.; Zhao, M.; Sun, L.; Chechik, V.; Yeung, L. K. Acc. Chem. Res. 2001,
34, 181–190.
22. Scott, R. W. J.; Wilson, O. M.; Crooks, R. M. J. Phys. Chem. B 2005, 109, 692–704.
23. Astruc, D.; Lu, F.; Aranzaes, J. R. Angew. Chem., Int. Ed. 2005, 44, 7852–7872.
24. Vasylyev, M. V.; Maayan, G.; Hovav, Y.; Haimov, A.; Neumann, R. Org. Lett.
2006, 8, 5445–5448.
25. Maegawa, T.; Fujita, Y.; Sakurai, A.; Akashi, A.; Sato, M.; Oono, K.; Sajiki, H.
Chem. Pharm. Bull. 2007, 55, 837–839.
26. Ornelas, C.; Ruiz, J.; Salmon, L.; Astruc, D. Adv. Synth. Catal. 2008, 350, 837–845.
27. Bhattacharjee, S.; Dotzauer, D. M.; Bruening, M. L. J. Am. Chem. Soc. 2009, 131,
3601–3610.
References and notes
1. Gopidas, K. R.; Whitesell, J. K.; Fox, M. A. J. Am. Chem. Soc. 2003, 125, 6491–
6502.
2. Gopidas, K. R.; Whitesell, J. K.; Fox, M. A. J. Am. Chem. Soc. 2003, 125, 14168–
14180.
3. Gopidas, K. R.; Whitesell, J. K.; Fox, M. A. Nano Lett. 2003, 3, 1757–1760.
4. Shon, Y.-S.; Choi, D. Curr. Nanosci. 2007, 3, 245–254.
5. Yang, P.; Zhang, W.; Du, Y.; Wang, X. J. Mol. Catal. A: Chem. 2006, 260, 4–10.
6. Wu, L.; Li, B.-L.; Huang, Y.-Y.; Zhou, H.-F.; He, Y.-M.; Fan, Q. H. Org. Lett. 2006, 8,
3605–3608.
7. Kumar, V. K. R.; Gopidas, K. R. Chem. Asian J. 2010, 5, 887–896.
8. Procedure for the synthesis of Pd-G1: To
À
a
stirred solution of G1N2+BF4
(1.4 mmol) in dichloromethane, aqueous K2PdCl4 (1.4 mmol) was added
followed by tetra-n-octylammonium bromide (TOAB) (2.1 mmol). Organic
phase was separated, washed with water and cooled to À30 °C followed by the
addition of 20 mL methanolic NaBH4 (14 mmol). The mixture was stirred at
À30 °C for 4 h and at room temperature for 20 h. The organic phase was
separated and washed with 0.5 M H2SO4, 0.5 M Na2CO3 and finally with water.
The crude product was purified by column chromatography (silica gel 100–200
mesh). The column was first eluted with dichloromethane to remove organic
impurities and then with methanol to obtain Pd-G1. (see Supplementary data
for characterization).
ˇ
28. Jurcík, V.; Nolan, S. P.; Cazin, C. S. J. Chem. Eur. J. 2009, 15, 2509–2511.
29. Jayamurugan, G.; Umesh, C. P.; Jayaraman, N. J. Mol. Catal. A: Chem. 2009, 307,
142–148.