Ce0.98Pd0.02O2-δ: Recyclable, ligand free palladium(II) catalyst for Heck reaction

Article abstract of DOI:10.1007/s12039-011-0103-6

Palladium substituted in cerium dioxide in the form of a solid solution, Ce_0.98Pd_0.02O_1.98 is a new heterogeneous catalyst which exhibits high activity and 100% trans-selectivity for the Heck reactions of aryl bromides including heteroaryls with olefins. The catalytic reactions work without any ligand. Nanocrystalline Ce_0.98Pd _0.02O_1.98 is prepared by solution combustion method and Pd is in +2 state. The catalyst can be separated, recovered and reused without significant loss in activity. Indian Academy of Sciences.

Full text of DOI:10.1007/s12039-011-0103-6

J. Chem. Sci., Vol. 123, No. 1, January 2011, pp. 47–54. * Indian Academy of Sciences.
Ce0·98Pd0·02O2-δ: Recyclable, ligand free palladium(II) catalyst
for Heck reaction
S R SANJAYKUMARa, BHASKAR DEVU MUKRIa, SATISH PATILa,
GIRIDHAR MADRASb, and M S HEGDEa*
aSolid State Structural Chemistry Unit, Indian Institute of Science, Bangalore 560012, India
bDepartment of Chemical Engineering, Indian Institute of Science, Bangalore 560012, India
e-mail: mshegde@sscu.iisc.ernet.in
MS received 28 July 2010; revised 10 December 2010; accepted 16 December 2010
Abstract. Palladium substituted in cerium dioxide in the form of a solid solution, Ce0·98 Pd0·02 O1·98 is a
new heterogeneous catalyst which exhibits high activity and 100% trans-selectivity for the Heck reactions of
aryl bromides including heteroaryls with olefins. The catalytic reactions work without any ligand. Nano-
crystalline Ce0·98 Pd0·02 O1·98 is prepared by solution combustion method and Pd is in +2 state. The catalyst
can be separated, recovered and reused without significant loss in activity.
Keywords. Ionic Pd; carbon–carbon coupling; Heck reaction; Pd–ceria; heterogeneous catalysis.
1. Introduction
In recent years, Heck reaction has been catalysed by
palladium metal supported on charcoal8, mesoporous
Palladium catalysed Heck reaction is well-known in carbon9, magnesium oxide10, silica, alumina or tita-
organic synthesis.1 Recent review article2 has summa- nia11, palladium/Nb-MCM-4112, polymers13, zeo-
rized the developments in homogeneous catalysed Heck lites14, polyionic resins.15 Basic supports such as
reaction. Heck reaction is known for its high tolerance layered double hydroxide16, basic zeolites17, alkaline
of functional groups and general applicability.3 The exchanged sepiolites18, mixed oxide19, flourapatite20
palladium catalysed coupling reaction of organic halides have been used because Pd on these supports shows
with olefins allows a one-stepsynthesis of aromatic considerably higher activity towards Heck reaction
olefins4, which are used extensively as biologically compared to neutral supports. Noble metal ion substi-
active compounds, natural products, pharmaceuticals tution in reducible oxides such as CeO2, TiO2 showed
and precursors of conjugated polymers.5 The Heck extraordinarily high activity for a variety of catalytic
reaction proceeds in the presence of homogeneous as reactions.21 We considered it worthwhile to see if Pd
well as heterogeneous palladium catalysts, generated ion substituted CeO2 can be used for carbon–carbon
from either palladium(0) compounds or palladium(II) coupling reactions of aryl halides and substituted
acetate or chloride salts.6 Several ligands such as alkenes in N,N-Dimethylformamide(DMF) using
phosphines, phoshites, carbenes, thioethers have been K2CO3 as a base and without any ligands. We show
successfully employed for this reaction.7 However, here that Pd2+ ion in CeO2 is an excellent Heck
homogeneous catalysis results in problems of recovery coupling heterogeneous catalyst without requiring any
and reuse of the catalyst and also might result in ligand. A wide range of substrates has been screened
palladium contamination of the product. Use of phos- including the less active bromoaryl compounds. The
phines which are generally used as electron donating product is 100% trans selective.
ligands in the Heck reactions is undesirable because
they are toxic as well as moisture sensitive. Therefore,
2. Experimental
there is a need to develop catalysts which do not need
any ligand for Heck coupling.
Ce0·98Pd0·02O1·98 is prepared by solution combustion
method as described earlier.22 In a typical preparation
*For correspondence
47

48
S R Sanjaykumar et al.
9·8 mmol of (NH4)2Ce(NO3)6 (Loba Chemie Pvt. Ltd., Cu Kα radiation at a scan rate of 2θ=2°/min) and XPS
Mumbai, India), 0·20 mmol of PdCl2 (Sigma Aldrich), (Thermoscientific Fisher Multilab 2000 using Al Kα
23·56 mmol C2H6N4O2 (oxalyldihydrazide) were dis- radiation (1486·6 eV)).
solved in 10 ml of water in a borosilicate dish with
All the aryl halides and olefins were obtained from
200 cm3 capacity. The dish containing the redox S.D. Fine Chemicals Ltd. and Spectrochem Pvt. Ltd.
mixture was introduced into a muffle furnace main- Mumbai, India. H NMR and 13C NMR spectra were
1
tained at 350°C. The solution boiled with frothing and recorded using Bruker AV-400 MHz spectrometer.
foaming and ignited to burn with a flame (~ 1000°C) Chemical shifts were given in parts per million and
1
yielding a voluminous solid product within 5 min. coupling constants (J) in Hertz. Chemical shifts of H
Catalysts before and after reactions were characterized NMR are given with respect to the TMS chemical shift
by powder XRD (Phillips X’Pert diffractometer using and of 13C NMR are given with respect to CDCl3
Figure 1. The Rietveld refined XRD patterns of Ce0·98Pd0·02O1·98; (a)
before reaction, (b) after reaction.

Ce0·98Pd0·02
O
2-δ: Recyclable, ligand free palladium(II) catalyst for Heck reaction
49
chemical shift. Flash column chromatography was on silica gel. All the products have been fully
performed on silica gel (100–200 mesh) using ethyl characterized by NMR studies.
acetate and hexane as eluents. The products were
analysed employing gas chromatograph (Varian CP 3. Results and discussions
3800 instrument). The capillary column used was
Varian CP-Sil 8CB Fused silica column 5% Phenyl, Substitution of Pd+2 ion in CeO2 in the form of
95% dimethylpolysiloxane Size: 30 m×0·25 mm (ID), Ce0·98Pd0·02O2-δ solid solution has been shown earlier
0·25μm (Df).
by XRD, XPS, EXAPS studies.22,23 Rietveld refined
XRD profile of freshly prepared compound is shown
in figure 1(a). X-ray pattern corresponds to single phase
Ce0·98Pd0·02O1·98 as reported earlier.23 From the FWHM
2.1 Typical procedure for the Heck reaction
Under an atmosphere of nitrogen, an oven-dried two- of diffraction lines, crystallite size is estimated employ-
necked 25 ml round-bottom flask containing a stir bar ing Schirrer formula (d=0·95λ/βcosθ). Average size is
was charged with an aryl halide (0·50 mmol), olefin 10 0·5 nm and colour of the catalyst is pale yellow.
(2·25 mmol), catalyst Ce0·98Pd0·02O1·98 (0·38 mmol, XPS of the Pd(3d) is shown in figure 2(a). Binding
containing 7·6 μmol of Pd+2 ions) and oven dried energy of Pd(3d5/2) at 337 eV confirms Pd in +2
K2CO3 (1·0 mmol). The mixture was heated and stirred state.22,23 Cerium is in +4 oxidation state in this
at 120–130°C. The progress of the reactions was compound and is confirmed from the XPS of Ce(3d)
monitored by TLC. After completion of the reaction, (not shown). Thus the formula of the catalyst is
the reaction mixture was cooled to room temperature Ce0·98Pd0·02O1·98. Due to lower valent Pd+2 ion
and filtered. The filtrate is extracted with hexane and substitution for Ce+4 ion in CeO2, oxide ion vacancies2
ethyl acetate (3:1) and water. The organic layer was are created. EXAFS study has confirmed that the oxide
washed with water and brine, dried with sodium ion vacancy is found next to Pd+2 ion in the lattice.23
sulphate and evaporated under reduced pressure. The
The activity of nano-crystallite Ce0·98Pd0·02O1·98
residue was finally purified by flash chromatography was examined for the Heck coupling reaction. Effect
of reaction parameters such as time and temperature
on the yield was investigated to determine the optimal
condition with N,N-dimethylformamide (DMF),
dimethylsulphoxide (DMSO), N-methylpyrrolidine
Pd2+(3d5/2
)
(a)
5000
2500
(NMP) as solvents. The highest yield was obtained
Pd2+(3d3/2
)
At 80 C
100
At 125 C
At 150 C
80
0
10000
(b)
60
40
20
0
Pd2+(3d5/2
)
7500
5000
2500
0
Pd2+(3d3/2
)
0
100
200
300
400
500
Time (Min)
334
336
338
340
342
344
Binding energy (eV)
Figure 3. Conversion efficiency of reaction of iodobenzene
and methyl acrylate at different temperatures. Yields are
calculated based on GC results with respect to iodobenzene.
Figure 2. XPS of Pd(3d) core level region of Ce0·98Pd0·02O1·98
(a) before reaction, (b) after reaction.
;

50
S R Sanjaykumar et al.
Table 1. Reactions of different aryl halides with different vinyl compounds.
Temp(oC)
Time(h)
5
Entry Aryl halides
Olefins
Product
Yieldsa
O
I
O
O
120
•
1
82 49
I
b
•
2
130
140
6
7
63 85
Br
b
•
3
51 59
O
I
O
O
•
4
120
120
5
5
83 01
O
O
I
O
O
•
5
86 98
O
O
O
•
6
140
130
10
93 45
O
Br
O
O
Br
O
O
O
•
7
6
95 22
O
O
O
Br
O
O
•
8
9
140
140
10
10
52 02
O
HO
HO
O
Br
O
O
•
46 21
O
O
Br
O
O
•
10
11
O
140
140
10
10
65 23
H2N
H2N
O
O
O
Br
•
48 56
O
O
O
O
•
12
13
86 54
Br
Br
140
130
10
6
O
O
O
O
92%
O
N
N

Ce0·98Pd0·02
O
2-δ: Recyclable, ligand free palladium(II) catalyst for Heck reaction
51
Table 1. (continued)
Temp(oC)
130
Time(h)
6
Entry Aryl halides
Olefins
Product
Yieldsa
87%
O
O
O
14
Br
S
S
N
O
O
O
76%
68%
O
15
140
140
10
12
N
Br
Br
O
O
O
O
16
O
S
S
aThe reaction was carried out with 0·50 mmol of aryl halide, 0·38 mmol of catalyst (7·6 μmol of Pd),
2·50 mmol olefins and 2 equiv. of K2CO3 in DMF at 130oC.
bIsolated yields of pure product based on aryl halides.
in DMF and it was used as a solvent for all the over pure CeO2 as catalyst was carried out and there
reactions. All the reactions were carried out using was no conversion. Thus, Pd ion in the catalyst is the
potassium carbonate (K2CO3) as a base except the active site for the Heck reaction in this catalyst. The
reactions with styrene, for which sodium acetate was C–X bond strength affects the activity towards the
used. Iodobenzene and methylacrylate were chosen as Heck reaction as the general cross coupling reactions
model compounds for the initial study. A number of follow C–I > C–Br > C–Cl. Yields of coupling
reactions have been carried out with Ce0·98Pd0·02O1·98 reactions with alkyl acrylates were found to be more
catalyst and the reactions were performed with 5% compared to styrene. Bromobenzenes with electron
molar excess of methyl acrylate at different temper- withdrawing groups have shown comparable activity
atures. The aryl halides, olefins, products, time, with that of iodobenzene towards Heck reaction. Bro-
temperature and yields are summarized in table 1. mobenzenes substituted with electron donating group
Heck reaction of iodobenzene with methyl acrylate have shown lesser activity. Exclusively trans-coupled
Table 2. Reaction of acrylates with substituted bromobenzenes.
O
Ce0.98 Pd0.02 O1.98
K2CO3
O
Br
O
DMF
O
X
X
Product
(Yield %)b
Time
(h)
Entrya
X
1
2
6
72
95
H
6
6
CHO
3
4
5
COCH3
NH2
92
65
62
6
6
OH
R–methyl, ethyl, t-butyl
a The reaction was carried out with 0·50 mmol of aryl halide, 0·38 mmol of catalyst
(7·6 μmol of Pd), 2·50 mmol olefins and 2 equiv. of K2CO3 in DMF at 130°C.
b Isolated yields of pure product based on aryl halides.

52
S R Sanjaykumar et al.
Table 4. Reuse of catalyst.
product is obtained in each of the reaction as confirmed
from NMR studies (see Supporting Information). The
reaction of para-substituted bromobenzenes was studied
with acrylates (table 2). 4-Formylbromobenzene and 4-
acetylbromobenzene underwent the reaction in 6 h with
95% and 93% yields respectively. But 4-bromoaniline
and 4-bromophenols were relatively less reactive and
the Heck product yields were 65% and 62%, respec-
tively. However, unsubstituted bromobenzene gave
more than 72% yield. The effect of temperature on
conversion was studied with the iodobenzene and
methyl acrylate as shown in figure 3. While ~ 100%
conversion was obtained in 30 min at 150°C, it required
nearly one hour when temperature was 125°C. However,
at 80°C, the reaction was very slow that the conversion
was ~ 60% even after 7 h.
O
Ce0.98 Pd0.02 O1.98
O
I
K2CO3
O
O
DMF, 130oC
Productb
Runa
Recoverability
1
2
3
85 %
82 %
78 %
87 %
86 %
84 %
a The reaction was carried out with 0·50 mmol of iodobenzene, 0·38 mmol
of catalyst (7·6μmol of Pd), 2·25 mmol methyl acrylate and 2 equiv. of
K2CO3 in DMF at 130°C.
b Isolated yields of pure product based on iodobenzene.
It is important to compare the activity of this catalyst
with those Pd substituted ligandless heterogeneous
catalysts for Heck coupling in the literature. Turnover
frequency is one simple way to compare the activity of
the catalysts. Turnover frequency (TOF) is defined
here as the ratio of moles of conversion per unit time
per mole of palladium ion in the catalyst. TOF values
were obtained for the reports in the literature for the
reaction of iodobenzene and methyl acrylate and the
values are compared with our catalyst in table 3.
Clearly TOF of the present catalyst is higher compared
to other Pd substituted ligandless heterogeneous
catalysts for carbon–carbon cross coupling reactions
reported in the literature.24 TOF is comparable with
recently reported immobilized Pd(OAc)2 catalyst in
ionic liquid on silica.25 TOF of our catalyst reported
here is for the first cycle. The same catalyst is reused
and accordingly TOF will be much higher if we take
into account the number of times the same catalyst is
reused for the reaction.
The catalyst can be recycled without significant loss
of activity. After the completion of the reaction, the
catalyst was recovered by filtration or by centrifuga-
tion. The powder catalyst was washed with hexane to
remove organic impurities and then with water to
remove DMF and K2CO3. The dried catalyst was
reused in a fresh reaction. There was no significant loss
in the catalytic activity for the model reaction of
iodobenzene and methyl acrylate (see table 4). In the
same way we could recover and reuse the catalyst in all
the reactions which are listed in table 1. The catalyst
was examined after several cycles of reaction. The
Rietveld refined XRD profile of this reused catalyst is
shown in figure 1(b). Clearly, fluorite structure of the
catalyst remains intact. Thus the catalyst is stable in the
reaction condition. Further, XPS of the spent catalyst
Table 3. Performance of the catalyst in the reaction of
iodobenzene and methyl acrylate.
Entry Catalyst
(equivalents)
Time (h) Yield (%) TOF (h−1)
1a
2b
3c
4d
5e
0·0103
5·5
6·0
8·0
8·5
0·5
91
96
91
94
99
16
30
48
85
132
0·0053
0·0024
0·0013
0·015
a, b, c, d Reported reactions Pd substituted ligandless heterogeneous catalysts
found in literature (reference24).
e Our results: the reaction was carried out with 0·50 mmol of iodobenzene,
0·38 mmol of catalyst (7·6μmol of Pd), 2·25 mmol methyl acrylate and 2
equiv. of K2CO3 in DMF at 150°C.
Figure 4. Mechanism for carbon–carbon cross coupling
reactions with Ce0·98Pd0·02O1·98 catalyst.

Ce0·98Pd0·02
O
2-δ: Recyclable, ligand free palladium(II) catalyst for Heck reaction
53
was recorded and Pd(3d) spectrum given in figure 2(b) thank the NMR Research Centre (NRC), Indian Institute
shows that Pd remains in +2 oxidation state. Pd ion is of Science, for providing NMR spectra.
not reduced to Pd0 state and Ce also remains in +4
state. Further, surface concentration of Pd ion has not
changed indicating that Pd is not leached out. Pd ion is
References
not leached out from the catalyst even in acid (0·1 N
HCl) solution.
1. de Meijere A and Meyer F E 1994 Angew. Chem. Int.
Catalyst surface contains two distinct sites: (i)
Ed. 33 2379
electron deficient Pd2+ ion for adsorption of donor
2. Beletskaya A P and Cheprakov V 2000 Chem. Rev. 100
molecules and (ii) oxide ion vacant sites next to Pd2+
3009; Whitcombe N J and Hii K K M, Gibson S E 2001
Tetrahedron 57 7449
3. Crisp GT 1998 Chem. Soc. Rev. 27 427; Dieck H A and
for the adsorption of acceptor molecules. There was no
Heck reaction on pure CeO2 clearly suggests that Ce4+
and O2− sites are not active for this catalytic reaction.
Heck R F 1974 J. Am. Chem. Soc. 96 1133
There are no other sites for the adsorption of donor and
acceptor molecules in the catalyst. We have extensive-
ly shown that in Ce1−xPdxO2−x the Pd ion and the
oxide ion vacancies next to each other act as red-ox
sites and dual site mechanism has been established.23
Further, redox catalytic reaction such as CO+O2 over
this catalyst is shown to follow modified Langmuir–
Hinshelwood mechanism where, CO is exclusively
adsorbed on Pd2+ site and oxygen on oxide ion
vacancy site.21
A mechanism proposed for this ionic catalyst is
given in figure 4. It is well known that olefins bind
with Pd+2 ion.26 Thus, normal site for adsorption of
olefin is Pd+2 ion in Ce0·98Pd0·02O1·98. Oxide ion
vacancy is a nucleophilic site ideal for accommodation
of –Br, −I of the aryl halide. Elimination of HBr
facilitated by the base leads to C–C coupling. Further
studies are essential to see why only trans coupling is
observed over this catalyst.
4. de Vries Johannes G 2001 Can. J. Chem. 79 1086
5. Burke T R, Liu D G and Gao Y J 2000 J. Org. Chem. 65
6288; Haberli A and Leumann C J 2001 Org. Lett. 3
489; de Meijere A and Meyer F E 1994 Angew. Chem.
106 2473
6. Biffis A, Zecca M, Basato M 2001 J. Mol. Catal. A 173
249
7. Hillier A C, Grasa G A, Viciu M S, Lee H M, Yang C
and Nolan S P 2002 J. Organomet. Chem. 653 69;
Selvakumar K, Zapf A and Beller M 2002 Org. Lett. 4
3031; Yao C, Li H, Wu H, Liu Y and Wu P 2009 Catal.
Commun. 10 1099
8. Hallberg A and Westfelt L 1984 J. Chem. Soc. Perkin
Trans. 1 933
9. Wan Y, Wang H, Zhao Q, Klingstedt M, Terasaki O and
Zhao D 2009 J. Am. Chem. Soc. 131 4541.
10. Kaneda K, Higuchi M and Imanaka T 1990 J. Mol.
Catal. 63 L33
11. Wali A, Pillai S M, Kaushik V K and Satish S 1996
Appl. Catal. A 135 83
12. Mehnert C P, Weaver D W and Ying J Y 1998 J. Am.
Chem. Soc. 120 12289
13. Ding J H and Gin D L 2000 Chem. Mater. 12 22
14. Djakovitch L and Koehler K 2001 J. Am. Chem. Soc.
123 5990
15. Dams M, Drijkoningen L, Pauwels B, van Tendeloo G,
de Vos D E and Jacobs P A 2002 J. Catal. 209 225;
Basu B, Das S, Das P, Mandal B, Banerjee D and
Almqvist F 2009 Synthesis 7 1137
4. Conclusion
A recyclable Pd2+ ion substituted CeO2 (Ce0·98
Pd0·02O1·98) catalyses the carbon–carbon cross cou-
pling of substituted iodobenzene, substituted bromo-
benzenes and bromoheteroaryls with alkylacrylates and
styrenes with good yields.
16. Choudary B M, Madhi S, Chowdari N S, Kantam M L
and Sreedhar B 2002 J. Am. Chem. Soc. 124 14127
17. Corma A, Garcia H, Leyva A and Primo A 2003 Appl.
Catal. A 247 41
18. Ranting T, Shiding M, Zhimin L, Yun X, Buxing H,
Guimin A and Kunlun D 2009 Green Chem. 11 96
19. Cwik A, Hell Z and Figureas F 2006 Adv. Synth. Catal. 348
523
Supporting information
NMR data of line products are given in the supporting
information (see www.ias.ac.in/chemsci).
Acknowledgments
20. Kantam M L, Shiva Kumar K B, Srinivas P and
Sreedhar B 2007 Adv. Synth. Catal. 349 1141
21. Hegde M S, Madras G and Patil K C 2009 Acc. Chem.
Res. 42 704 and references therein
This work was supported by the Department of Science
and Technology (DST) and Council of Scientific and
Industrial Research (CSIR), New Delhi. The authors also

54
S R Sanjaykumar et al.
22. Bera P, Patil K C, Jayaram V, Subbanna G N and Hegde
M S 2000 J. Catal. 196 293
23. Priolkar K R, Bera P, Sarode P R, Hegde M S, Emura
P, Vile J and Williams J M J 2003 Tet. Let. 44 5095;
Phan N T S, Brown D H, Adams H, Spey S E and
Styring P 2004 Dalton Trans. 9 1348
S, Kumashiro R and Lalla N P 2002 Chem. Mater. 14 25. Hagiwara H, Sugawara Y, Isobe K, Hoshi T and Suzuki
2120
T 2004 Org. Lett. 6 2325; Hagiwara H, Sugawara Y,
24. Mori K, Yamaguchi K, Hara T, Muzugaki T, Ebitani K
Hoshi T and Suzuki T 2005 Chem. Commun. 2942
and Kaneda K 2002 J. Am. Chem. Soc. 124 11572; 26. Link JT and Overman L E 1998 Metal-catalyzed cross-
Molnar A, Papp A, Miklos K and Forgo P 2003 Chem.
Commun. 20 2626; Daku K M L, Newton R F, Pearce S
coupling reactions, (eds.: F. Diederich, P. J. Stang)
Weinheim; Wiley-VCHVerlag p. 231