Effect of immobilization on catalytic characteristics of saturated Pd-N-heterocyclic carbenes in Mizoroki-Heck reactions

Article abstract of DOI:10.1016/j.jorganchem.2006.03.012

A saturated Pd-N-heterocyclic complex was immobilized on an amorphous silica. The complex itself is of very high thermal stability. However, TEM observations, hot filtration, reusability, and poisoning tests all revealed that the complex acted only as a precatalyst to highly active Pd species in Mizoroki-Heck reactions when immobilized. The complex appears more stable when used under homogeneous reaction conditions. The immobilized complex afforded high turnover numbers, 10⁴-10⁵. The higher turnover frequencies were realized at the lower Pd concentrations, which is a characteristic property of ligand-free Pd catalyzed reactions.

Full text of DOI:10.1016/j.jorganchem.2006.03.012

Journal of Organometallic Chemistry 691 (2006) 3027–3036
www.elsevier.com/locate/jorganchem
Effect of immobilization on catalytic characteristics of saturated
Pd-N-heterocyclic carbenes in Mizoroki–Heck reactions
a
b
a,
b
c
Ozge Aksın , Hayati Turkmen , Levent Artok *, Bekir C¸ etinkaya , Chaoying Ni ,
¨
¨
d
a
¨
Orhan Buyukgungor , Erhan Ozkal
¨ ¨
¨
¨
a
Department of Chemistry, Faculty of Science, Izmir Institute of Technology, Urla 35430 Izmir, Turkey
b
Department of Chemistry, Faculty of Science, Ege University, Bornova 35100, Izmir, Turkey
Department of Materials Science and Engineering, University of Delaware, Newark, DE 19716, USA
Department of Physics, Faculty of Science and Art, Ondokuz Mayıs University, Kurupelit 55139, Samsun, Turkey
c
d
Received 13 December 2005; received in revised form 27 February 2006; accepted 8 March 2006
Available online 14 March 2006
Abstract
A saturated Pd-N-heterocyclic complex was immobilized on an amorphous silica. The complex itself is of very high thermal stability.
However, TEM observations, hot filtration, reusability, and poisoning tests all revealed that the complex acted only as a precatalyst to
highly active Pd species in Mizoroki–Heck reactions when immobilized. The complex appears more stable when used under homoge-
neous reaction conditions. The immobilized complex afforded high turnover numbers, 104–105. The higher turnover frequencies were
realized at the lower Pd concentrations, which is a characteristic property of ligand-free Pd catalyzed reactions.
ꢀ 2006 Elsevier B.V. All rights reserved.
Keywords: Mizoroki–Heck reaction; C–C coupling; Heterocyclic carbene; Immobilized palladium; Heterogeneous catalyst, Palladium leaching
1. Introduction
in the product solution, respectively. To this end, immobi-
lization of Pd-catalysts onto a support allows heteroge-
neous catalyst with decreased leaching of Pd to the
solution. When an NHC is complexed with a metal, it
exhibits extraordinary thermal stability, making it a rea-
sonable candidate for immobilization. Likewise, due to
the stability of the metal complex, it is presumed that the
metal will remain in its immobilized state when anchored
to a support prior to reaction making it be less likely to
leach into solution [3c,5].
According to the proposed reaction mechanism for cou-
pling reactions involving haloarenes, the oxidative Pd addi-
tion to haloarenes is believed to be the rate-determining
step with the activity of catalyst being determined by the
electron density on palladium. Therefore, in the paper pre-
sented here, the use of saturated NHCs was preferred
because of their electron-donating ability as compared with
their unsaturated counterparts.
¨
Since their first introduction by Ofele and Wanzlick [1],
N-heterocyclic carbenes (NHCs) have become universally
accepted ligands in organometallic and inorganic coordina-
tion chemistries. Due to their being strong r-donors and
low p-acceptors, NHCs have even been proposes as
replacements for the more commonly used phosphane
ligands in palladium catalyzed coupling reactions, because
they resemble organophosphanes [2]. Furthermore, NHCs
are easily acquired, non-toxic and exhibit high thermal sta-
bility when compared with phosphanes [3]. These proper-
ties have been exploited in a broad range of catalytic
applications [4].
As a catalyst, Pd is very important in the pharmaceutical
industry; however, because of its expense and toxicity, it is
particularly important to reduce both its loss and presence
*
In contrast to view that metals of NHCs are less likely
to leach into solution, results will be shown here which
Corresponding author. Tel.: +90 232 7507529; fax: +90 232 7507509.
E-mail address: leventartok@iyte.edu.tr (L. Artok).
0022-328X/$ - see front matter ꢀ 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2006.03.012

¨
O. Aksın et al. / Journal of Organometallic Chemistry 691 (2006) 3027–3036
3028
indicate that immobilization of a thermally stable Pd–
NHC complex [5e], led to the liberation of Pd into solu-
tion, thus giving rise to highly reactive Pd species during
Mizoroki–Heck (M–H) reactions.
of solvent and reagents were adjusted depending on the
amount of catalyst that had been recovered from the previ-
ous run. The results are presented in Table 2.
The immobilized complex 2 (catalyst 2) appears to be
reusable for 10 cycles for the reaction of 4-iodotoluene;
however, longer reaction periods are required for reactions
after the sixth cycle. A reaction period of 7–8 h seems to be
sufficient for the first six cycles, nevertheless, it took nearly
14 h to obtain high yield formation for the tenth cycle.
The first cycle showed a higher rate of reaction for 4-
bromoacetophenone as compared to 4-iodotoluene; how-
ever, longer reaction times were required progressively
for each subsequent cycle to attain the trans-coupling prod-
uct at yields greater than 90%. While trans-coupling prod-
uct formation was achieved at yields of 95%, after 2 h for
the reaction of 4-bromoacetophenone with styrene during
its first use, it took, however, 18 h to obtain the same
amount of yield during its seventh use, and only 64% prod-
uct formation could be achieved after 28.5 h when the cat-
alyst had been used for the eight time, indicating that the
catalyst underwent gradual deactivation at each cycle.
Trends were also similar with the immobilized complex 1.
Atomic absorption spectrometry (AAS) analyses
revealed no detectible amounts of Pd within the filtrates
after the reactions. However, a significant amount of Pd
was determined in a solution recovered upon hot filtration:
The reaction solution of 2 mmol 4-bromoacetophenone
with styrene which had been performed under standard
conditions was recovered upon hot filtration under dry
N2 atmosphere after 15 min of reaction, where the conver-
sion and trans-yields were 16% and 9%, respectively. Fur-
ther AAS analysis revealed the presence of 3.6 ppm Pd in
the solution, this being about 3.4% of the Pd charged ini-
tially. Moreover, the reaction proceeded to give a 39% con-
version and to yield 34% of the trans-coupling product
when the base was added to the filtrate and heated for an
additional 3.75 h. Though these results do not rule out
the possibility that the anchored complex might have been
involved in the reaction in a heterogeneous way, it is evi-
dent that the catalyst released Pd species into the solution
that were capable of catalyzing the reaction.
2. Results and discussion
The amorphous silica supported Pd–NHC complexes, 1
and 2, were the catalysts tested in this study. The molar
N/C ratio of the samples indicated that no ethoxy group
remained after anchoring the complex.
OH OH OH
O
O
O
OH
OH
Si
Bz
N
Cl
Pd
Cl
N
N
Bz: benzyl (1) or 2,4,6-trimethylbenzyl (2)
N
Bz
Si
OH
OH OH OH
OH
O
O
O
Initial trials were performed to determine the optimal
base type for the coupling reaction of bromobenzene with
styrene over catalyst 2 (0.066 mmol Pd/g silica) with a Pd
concentration of 0.5% in DMF solvent at 140 ꢁC. Catalyst
2 demonstrated very low activity with NaOAc and Cs2CO3
(Table 1). Na2CO3 was somewhat more effective than
K3PO4.
The catalysts were tested for their recyclabilities for M–
H reactions involving styrene. For each cycle, the amounts
Table 1
The effect of base on M–H reactions of bromobenzene with styrene over
catalyst 2a
Base
(6mmol)
Pd
(0.02 mmol)
Br
+
140˚C
DMF
(10ml)
4mmol
Base
4.8 mmol
TEM analyses revealed the presence of Pd particles on
the silica support of the used catalysts which were recovered
after the first and eight runs with 4-bromoacetophenone
and styrene (Fig. 1). After its first use, the Pd agglomerates
were considerably large, extending to diameters as large as
90 nm, with an average size of 36.7 nm. Nevertheless, the
catalyst after the eighth run contained a higher quantity
Time (h)
Conversion (%)
Yield (%) trans
CH3CO2Na
Na2CO3
Cs2CO3
K3PO4
a
14
7
18
14
40
87
46
25
78
12
70
>99
With 0.066 mmol Pd/g silica.
Table 2
Recyclability test of the immobilized complex 2 for M–H reactiona
Aryl halide/cycle
trans-Yield % (time, h)
1
2
3
4
5
6
7
8
9
10
4-CH3C6H5I
4-CH3COC6H4Br
87 (7.3)
95 (2)
85 (6.8)
95 (3)
88 (6.6)
96 (4)
91 (7.5)
96 (4.7)
89 (8)
93 (7.3)
88 (7.4)
83 (11.2)
88 (10.5)
93 (18)
88 (11.5)
65 (28.5)
90 (11.3)
–
86 (14.3)
–
a
Reaction conditions. 4 mmol aryl halide, 4.8 mmol styrene, 6 mmol Na2CO3, 10 ml DMF, 0.5% Pd, 140 ꢁC, with 0.066 mmol Pd/g silica.

¨
O. Aksın et al. / Journal of Organometallic Chemistry 691 (2006) 3027–3036
3029
Fig. 1. The TEM micrographs with histograms of palladium particle sizes of catalyst 2 recovered from the reaction of 4-bromoacetophenone with styrene
after the first run (upper figure) and the eight run (lower figure). Bright spots in the images are due to the contrast from Pd.
of particles with smaller average size (8.7 nm), and the par-
ticles were located less distant with respect to each other on
the silica when compared to those of the former catalyst.
These results seem to demonstrated that the immobilized
complex decomposed gradually at each cycle, however, it
was not possible to be certain whether the loss in activity
is due to the complex decomposition or the consumption
of a source of the soluble active Pd species (i.e., whether
the M–H reaction is catalyzed by free or immobilized palla-
dium). On the other hand, it was surprising somewhat, for
us, that Pd dissociated from the complex, because it was
previously determined that complex 2 was stable up to
250 ꢁC during heat treatment [5e].
In an afford to determine the relative fraction of the
M–H reaction that was catalyzed by the leached Pd from
an immobilized Pd–SCS pincer complex, You et al. per-
formed an M–H reaction in the presence of an insoluble
cross-linked poly(vinylpyridine) polymer (PVPy) [6]. PVPy
strongly coordinates to soluble Pd(II) species, but it is inca-
pable of interacting with solid Pd. They found that the
reaction was completely inhibited in the presence of PVPy,
supporting the hypothesis that the reaction had been cata-
lyzed by only soluble Pd species leached from the solid
catalyst.
The same methodology was applied herein. Fig. 2 com-
pares the effect of PVPy addition on the conversion kinetics
100
80
60
40
20
Catalyst 2
Complex 2
Catalyst 2 + PVPy
Complex 3
0
0
1
2
3
4
5
6
7
8
Time, h
Fig. 2. Conversion kinetics of the M–H reactions of 4-bromoacetophe-
none and styrene. Pd: 0.05%, with 0.013 mmol Pd/g silica for catalyst 2.

¨
O. Aksın et al. / Journal of Organometallic Chemistry 691 (2006) 3027–3036
3030
for the reaction of 4-bromoacetophenone. Yet, these reac-
tions were performed at a lower Pd concentration, 0.05%,
than those of the recyclability tests, since the higher Pd
concentration could mask various stages of the reaction
(e.g., an induction period).
Fig. 2 also illustrates the kinetic profile for the reaction
catalyzed by another type of Pd–NHC complex (complex
3) in which all nitrogen atoms of pentacylic rings of
NHC ligands are functionalized by 2,4,6-trimethylbenzyl
groups [10]:
In the absence of PVPy, the conversion was 90% after
4 h of reaction and no induction period was observable
from the conversion curve (Fig. 2). However, in the pres-
ence of PVPy, the activity of the catalyst was significantly
reduced to give only 28% of conversion after 5 h of reac-
tion. This difference demonstrated that the reaction was
catalyzed mainly by the dissolved Pd.
R
R
Cl
Pd
Cl
N
N
N
N
R:CH2C6H2(Me)3-2,4,6
R
R
For comparison, a homogeneous reaction was per-
formed over complex 2. Interestingly, the kinetic profile of
the homogeneous reaction was very similar to that of the
reaction performed over immobilized complex–PVPy com-
bination (Fig. 2). This similarity may imply that probably a
similar type of Pd species catalyzed the reactions in both
reaction media. It has been shown previously that PVPy
supported Pd nano-clusters can catalyze the M–H reaction
[7]. Then, the M–H catalysis should have taken place either
on the surface of Pd aggregates or via the ligand-immobi-
lized Pd with the catalyst 2–PVPy combination.
The Hg poisoning test is a popular method to probe the
catalyst for its status in a reaction (homogeneous or heter-
ogeneous) [4c,4e,8]. Metallic mercury deteriorates the
activity of the colloidal Pd by forming an amalgam. The
addition of Hg (300 equiv.) to the reaction medium, either
in the presence of free complex or catalyst 2, completely
prohibited the M–H reaction of 4-bromoacetophenone
and styrene. A negative result for the mercury test can con-
firm a homogeneous catalytic system [4c,4e,8a,8c], and a
positive result can essentially confirm a reaction proceeding
through the Pd(0)–Pd(II) cycle, but does not warrant heter-
ogeneous colloidal Pd catalysis [8c].
Under homogeneous conditions (with complex 2), the
solution turned from a light yellow to a yellow-orange
color towards the end of the reaction, and no palladium
black formation was observed, whereas the solid catalyst
turned to gray, consistent with the formation of Pd aggre-
gates. In the former case, NHC ligands should have been
available to coordinate with Pd, preventing Pd black for-
mation, but on the other hand, lowering Pd activity by con-
fining it. Immobilization of the palladium complexes may
restrict them from adopting the most stable geometries
required for efficient reaction [9]. This would place high
strain to temporary structures and, therefore, lead to
decomposition of the complex. The less strongly coordi-
nated Pd species would effectively catalyze the reaction
until they precipitate to the less active agglomerates on
the silica surface. In conclusion, the above experiments
show that the reactions over the immobilized Pd–NHC
complex were catalyzed predominantly by dissolved Pd
species. Nevertheless by saying this, we do not mean that
there would be no contribution of the immobilized com-
plex, and/or by other Pd species to the overall activity of
the catalyst.
Complex 3
It is apparent that though this catalyst afforded the reac-
tion in higher reaction rates than did complex 2 under
homogeneous protocol, however, it is less active than the
immobilized complex 2.
M–H reactions were also performed at low Pd concen-
trations to realize the limits of this catalytic system. The
overall silica content of each reaction medium was brought
to 300 mg upon addition of silica that contained no Pd
originally. This enabled us to recover the catalyst more
effectively, otherwise it would have been impractical to
recover the very small amounts of catalyst used for the
studies performed at low Pd concentrations.
As can be seen in Table 3, catalyst 2 showed high activities
when used also in lower proportions for the M–H reactions
with styrene. Interestingly, the reactions of 4-iodoanisole
proceeded at comparable rates over the both palladium con-
centration of 0.05% and 0.5% (entries 1 and 2). Very high
turnover numbers (TON) and turnover frequencies (TOF),
which were calculated on the basis of trans-coupling product
formation, were achieved for iodo- and bromoarenes.
Higher TOF values were obtained at lower catalyst concen-
trations. TON values were as high as 166,000 and 19,000,
and TOF values of ꢀ6900 and ꢀ1200 hÀ1 were obtained
with excellent yield of trans-coupling products from the reac-
tions of 4-iodoanisole and 4-bromoacetophenone with sty-
rene, respectively, and a good yield (66%) was obtained at
a very high TON value (66,000) for the reaction using 4-
bromoacetophenone (Table 3, entries 1–11), whereas the
total TON and TOF values were only 1422 and 18 hÀ1
respectively, for 4-bromoacetophenone after eight consecu-
tive runs were conducted.
,
Catalyst 2, which was subjected to the M–H reaction of
4-bromoacetophenone at a Pd concentration of 0.005%,
showed reduced activity during second use of the catalyst,
giving 48% of the trans-coupling product after 38 h of reac-
tion time (Table 2, entry 10).
The sterically congested 2-iodoanisole required compar-
atively longer reaction times (Table 3, entry 6). Another
activated aryl bromide, 4-bromonitrobenzene afforded rel-
atively lower trans-coupling products at a Pd concentration
of 0.005%, giving rise to 78% trans-coupling product with a
TON value of 15,600 (Table 2, entry 12). As a model

¨
O. Aksın et al. / Journal of Organometallic Chemistry 691 (2006) 3027–3036
3031
Table 3
The M–H reactions of aryl halides with styrene over the catalyst 2a
Entry
Aryl halide
Pd (%)
Time (h)
Conversion (%)
Yield (%) trans
TONb
166
1640
16,400
8800
166,000
180
TOFb (hÀ1
)
1
2
3
4
5
6
7
8
9
4-CH3OC6H4Ic
4-CH3OC6H4I
4-CH3OC6H4Id
4-CH3OC6H4Id,e
4-CH3OC6H4If
2-CH3OC6H4Ic
4-CH3COC6H4Brc
4-CH3COC6H4Br
4-CH3COC6H4Brf
4-CH3COC6H4Bre,f
4-CH3COC6H4Brf
4-NO2C6H4Brd
3-BrC6H5N
3-BrC6H5N
C6H5Br
C6H5Br
C6H5Br
4-CH3C6H4Br
4-CH3C6H4Br
4-CH3C6H4Br
4-CH3OC6H4Br
4-CH3OC6H4Brf
4-CH3COC6H4Cl
5 · 10À1
5 · 10À2
5 · 10À3
5 · 10À3
5 · 10À4
5 · 10À1
5 · 10À1
5 · 10À2
5 · 10À3
5 · 10À3
1 · 10À3
5 · 10À3
5 · 10À2
1 · 10À2
5 · 10À1
5 · 10À2
1 · 10À2
5 · 10À1
5 · 10À2
1 · 10À2
5 · 10À1
5 · 10À2
5 · 10À1
6
5.3
14.5
22
24
23
2
98
>99
100
50
94
96
>99
90
99
54
77
93
>99
82
87
84
57
69
73
35
83
82
82
44
83
90
95
87
93
48
66
78
87
60
78
59
28
63
64
29
19
14
15
28
309
1131
400
6917
8
190
85
4
1740
18,600
9600
66,000
15,600
1740
6000
156
1180
2800
126
1280
2900
38
450
1240
253
1784
709
109
171
22
15
38
37
22
16
35
7
15
8
10.7
15
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
a
79
350
12
85
322
7.6
47
5
6
19
23
15
28
280
30
2
[Styrene]/[aryl halide] = 1.2, [Na2CO3]/[aryl halide] = 1.5, with 0.013 mmol Pd/g silica, 4 mmol aryl halide, 10 ml DMF, 140 ꢁC, under inert atmo-
sphere, sufficient amount of silica was added to the reaction medium to contain 300 mg of silica overall.
b
TON = [trans-yield]/[Pd], TOF = TON/h.
The catalyst contained 0.066 mmol Pd/g silica, with no additional silica.
6.7 mmol aryl halide.
Second use.
c
d
e
f
20 mmol aryl halide, 15 mL DMF.
molecule to hetero-aryl halide compounds, 3-bromopyri-
dine was also converted effectively, yielding 87% and 60%
trans-coupling product for Pd concentrations of 0.05%
and 0.01%, respectively (Table 3, entries 13–14).
acrylate reagent than with styrene (total isomeric products
were usually <1%). The higher TOF values with the lower
catalyst amounts were also a characteristic trend for the
coupling reaction of this reagent with aryl halides. Cata-
lyst 2 gave high coupling product formation with extre-
mely high TONs for all the activated aryl bromides
employed, being as high as 500,000; 700,000; and
840,000 for 4-bromobenzonitrile; 4-nitrobromobenzene;
and 4-bromoacetophenone, respectively (Table 4, entries
1–12). However, the catalyst was air-sensitive; the trans-
coupling product formation was 63% after 9 h of reaction
of 4-bromoacetophenone in air versus 95% after 6.8 h in
an inert atmosphere.
For butyl acrylate, the catalyst was ineffective during sec-
ond use under low Pd concentrations (Table 4, entry 5).
Catalyst 2 was also very active for the reactions involving
either 3-bromopyridine or 2-bromonaphthalene, yielding
very high coupling products with low Pd concentrations
(Table 4, entries 13–15). A moderate coupling product for-
mation was obtained for Pd-catalyzed M–H reaction
involving bromobenzene and 4-bromotoluene (Table 4,
entries 16–19).
Non-activated bromoarenes, bromobenzene, and 4-
bromotoluene gave moderate to high coupling yields at
Pd concentrations of 0.5% and 0.05% (entries 15–22). Cat-
alyst 2 was, however, low in its activity towards M–H reac-
tions of 4-bromoanisole and 4-chloroacetophenone (Table
2, entries 21–23). Nevertheless, it should be noted that
trans-coupling formation was approximately comparable
and ceased at the fifth and sixth h of reaction of 4-bromo-
anisole for Pd concentrations of both 0.5% and 0.05%,
respectively.
1,1-Diarylethene was the major isomeric by-product of
the M–H reactions when styrene was used as an olefin
source. Its formation varied between 3% and 6% for reac-
tions involving bromoarenes, while iodoarenes gave higher
yields of this isomeric product, (10–12%). cis-Coupling
isomer formation was generally observed at yields of less
than 1%.
Reactions with low Pd amounts were also performed
with butyl acrylate as an olefin reagent. As expected, the
catalyst was more active with this olefin (Table 4). The
reaction was more selective to trans-coupling product with
de Vries and Reetz pointed out that the irreversible cor-
relation of TOF values with Pd concentrations is an indica-
tion for the presence of an equilibrium between less active

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O. Aksın et al. / Journal of Organometallic Chemistry 691 (2006) 3027–3036
3032
Table 4
The M–H reactions of aryl halides with butyl acrylate over the catalyst 2a
Entry
Aryl halide
Pd (%)
Time (h)
Conversion (%)
Yield (%) trans
TONb
TOFb (hÀ1
)
1
2
3
4
5
6
7
8
9
4-CH3COC6H4Brc
4-CH3COC6H4Brd
4-CH3COC6H4Brd,e
4-CH3COC6H4Brd
4-CH3COC6H4Brd,f
4-CH3COC6H4Brg
4-NO2C6H4Brh
4-NO2C6H4Brd
4-NO2C6H4Brd
4-CNC6H4Brd
4-CNC6H4Brd
4-CNC6H4Brd
3-BrC6H5Ni
5 · 10À3
5 · 10À4
5 · 10À4
2 · 10À4
2 · 10À4
1 · 10À4
5 · 10À4
2 · 10À4
1 · 10À4
5 · 10À4
2 · 10À4
1 · 10À4
1 · 10À2
5 · 10À3
5 · 10À3
5 · 10À2
1 · 10À2
5 · 10À3
5 · 10À2
2.5
6.8
9
100
97
69
73
30
87
77
78
77
98
89
53
99
81
97
74
64
39
52
98
95
63
68
12
84
74
71
70
96
80
50
98
80
94
64
54
31
48
19,600
190,000
126,000
340,000
60,000
840,000
148,000
355,000
700,000
192,000
400,000
500,000
9800
7840
27,941
14,000
28,333
2143
30,000
6167
14,792
28,000
8727
16,667
20,833
613
12
28
28
24
24
25
22
24
24
16
28
20
7.8
9
10
11
12
13
14
15
16
17
18
19
a
3-BrC6H5Nc
16,000
18,000
1280
5400
6200
571
940
164
600
310
82
2-Bromonaphthalenec
C6H5Bri
C6H5Bri
C6H5Brc
20
11
4-CH3C6H4Bri
960
[butyl acrylate]/[aryl halide] = 1.2; [Na2CO3]/[aryl halide] = 1.5, with 0.013 mmol Pd/g silica, under inert atmosphere, sufficient amount of silica was
added to the reaction medium to contain 300 mg of silica overall.
b
TON = [trans-yield]/[Pd], TOF = TON/h.
6.67 mmol aryl halide, 10 mL DMF.
20 mmol aryl halide, 15 mL DMF.
Under air.
Second use.
40 mmol, 20 mL DMF.
10 mmol aryl halide, 10 mL DMF.
4 mmol aryl halide, 10 mL DMF.
c
d
e
f
g
h
i
Pd aggregates and highly active Pd species, lowering the
Pd-to-aryl halide ratio shifts the equilibrium in favor of
the latter species [11].
Various supported Pd catalysts [12], some pincer com-
plexes [6,13], palladacycles [6a,11a], and simple Pd com-
pounds [11a,14] were also shown to act as Pd reservoirs
when used for M–H reactions.
these reports, true active state of metal which catalyzed
the reaction is unknown.
A catalyst, such as the one used in this study, that deliv-
ers metal to the reaction solution, would not be favored.
However, possibly the complete recovery of Pd at the end
reactions may be an advantage of the catalyst used here.
If metal recovery is one of the most important criteria in
choosing a catalyst, it would be more preferable to use a
better accessible, lower cost catalyst such as a metal oxide,
zeolite or carbon supported metals, provided that the pro-
cess works at high TONs and TOFs, and that the catalyst
leaves no remnants of metal within the end product [12,15],
since the transition metal contamination is highly regulated
in the pharmaceutical industry.
Our results are also consistent with previously reported
results of various supported unsaturated Pd–NHC com-
plexes. Schwarz et al. demonstrated that a polymer sup-
ported N-heterocyclic dicarbene chelate complex of
palladium was recyclable up to 15 times for the M–H reac-
tion of 4-bromoacetophenone with styrene at a Pd concen-
tration of 0.15%, while its activity gradually decreased after
the fifth use when the Pd concentration was 0.02% in the
reaction medium [3c]. Moreover, several other types of
polymer bounded Pd–NHC complexes also revealed lower
activities in subsequent uses for M–H [5a] and Suzuki reac-
tions [5b].
In agreement with the reports by You et al. [6], where
immobilized Pd–SCS complexes were shown to act as cat-
alyst precursors rather than as true heterogeneous cata-
lysts, this study, also revealed that even thermally stable
complexes, such as NHC ligated metals, may decompose
during a reaction process. In fact, there have been numer-
ous reports that claimed immobilized metal complexes as
reusable heterogeneous catalysts; however, for most of
3. Conclusion
Silica anchored saturated Pd-N-heterocyclic carbenes
were tested for their catalytic behavior in M–H reactions
of aryl halides. Activity of the Pd–NHC complex was
greatly promoted upon immobilization, functioning at very
high TONs and TOFs. This is related to the decomposition
of the immobilized complex to give rise to the highly active
Pd species, which precipitates onto the silica surface at the
end of the reaction. Pd–NHC complex seems to be highly
stable when used at homogeneous procedures. However,
it lost its stability upon immobilization, probably due to

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O. Aksın et al. / Journal of Organometallic Chemistry 691 (2006) 3027–3036
3033
its restricted mobility in adopting stable geometries of tem-
porary structures in a reaction cycle.
lography technique (Fig. 3). The characterization of the
2,4,6-trimethylbenzyl substituted complex (complex 2) is
given elsewhere [5e].
4. Experimental section
4.3. Anchoring of Pd–NHC complexes
Diethyl ether (J.T. Baker) was distilled under nitrogen
from sodium benzophenone ketyl just before use. DMF
(J.T. Baker) was dried over molecular sieve 4A. Chloroform
(Riedel-de Hae¨n, extra pure) was dried by refluxing with
anhydrous CaCl2. All other reagents which were obtained
from commercial sources were used without further purifi-
cation. All the syntheses of the 1,3-disubstituted imidazolin-
ium salts and the Pd–NHC complexes were performed
under a nitrogen atmosphere with the use of standard
Schlenk techniques. Amorphous silica was synthesized as
reported earlier [16]. Accordingly, to a 3 N, 500 mL solu-
tion of H2SO4 was added sodium silicate solution (5%
NaOH, 13.5% SiO2, Riedel-de Hae¨n) at a flow rate of
10 mL minÀ1 by means of a peristaltic pump until a pH of
3.65 was attained. Gelation occurred in 5 min after addition
of the silicate solution was completed. The synthesized silica
was washed with water until the solution conductivity
showed no change. The BET surface area was 490 m2/g;
the single point total pore volume was 0.75 cm3/g. Average
A dry CHCl3 solution (20 mL/g silica) which contained
Pd–NHC complex in an amount furnishing 0.2 or 1 wt%
Pd with respect to the silica amount was added drop wise
to the amorphous silica which was evacuated overnight
at 120 ꢁC previous to the anchoring process. The resulting
suspension was refluxed for 24 h under inert atmosphere
while being magnetically stirred. After filtration, it was
washed with CH2Cl2 thoroughly. AAS analysis of the sam-
ple showed a palladium loading of 0.013 mmol or
0.066 mmol gÀ1, respectively.
4.4. Characterization of the catalysts
Specific surface areas of the catalysts and the pure sup-
port material were measured using a static process by
means of a Micrometrics ASAP 2010 instrument using
nitrogen at 77 K. The specific surface area was calculated
by the BET method and average pore diameters and pore
size distributions were calculated from the adsorption
branch of the isotherm using the Barrett, Joyner and Hel-
enda (BJH) method. All samples were degassed overnight
at 423 K. Elemental analyses of samples were done by
the Scientific and Technical Research Council of Turkey
using a LECO CHNS-932 instrument. The palladium con-
tent of the catalysts was determined by AAS (Solaar AA
Spectrometer Thermo Elemental). The samples (20 mg)
were digested in a mixture of HNO3 (65%, 3 mL), HF
(40%, 2 mL) and HClO4 (65%, 1 mL) using an Ethos Plus
Microwave Labstation furnace. TEM observations were
performed using a JEM-2010F field emission transmission
microscope operated in the scanning mode at 200 kV and a
high angle annular dark field detector was utilized.
˚
pore diameter by BET was 61 A.
4.1. Synthesis of 1-benzyl or 1-(2,4,6-trimethylbenzyl)-3-
(propyltriethoxysilane)-imidazolinium chloride
The preparation of the imidazolinium salts were readily
accomplished by the quarternization of 1-(3-triethoxysilyl-
propyl)-2-imidazoline (imeo). To a 5 mL of DMF solution
of imeo (3.99 g, 14.78 mmol) was added 14.79 mmol of
benzyl chloride or 2,4,6-trimethylbenzyl chloride and the
resulting mixture was stirred at room temperature (RT)
for 24 h. Diethyl ether (20 mL) was added to obtain a white
crystalline solid. The product was filtered, washed with
Et2O (2 · 15 mL) and dried under vacuum. The salt was
colorless, hygroscopic and freely soluble in CHCl3, but
insoluble in hexane or diethyl ether. The structural charac-
terizations for this sample are given elsewhere [17].
4.2. Synthesis of bis[1-(benzyl or 2,4,6-trimethylbenzyl)-3-
(propyltriethoxysilane)-imidazolidin-2-
ylidene]dichloropalladium-II complexes
To a CH2Cl2 (10 mL) solution of the 1.0 mmol corre-
sponding imidazolinium salt synthesized, 0.5 mmol Ag2O
was added. After stirring the mixture for 48 h at RT,
0.13 g (0.53 mmol) of PdCl2(CH3CN)2 was added and it
was further stirred for 48 h at RT. The precipitated AgCl
was separated from the mixture by filtration. The complex
was recrystallized from ether, giving 62% yield. Analytical
calculation for C38H64Cl2N4PdSi2: C, 50.367.12%; N,
6.18%. Found: C, 48.8%; H, 6.8%; N, 6.1% for the benzyl
substituted complex (complex 1). The structural elucida-
tion of the complex was established through X-ray crystal-
˚
Fig. 3. X-ray structure of complex 1. Selected bond distances (A) and
angles (ꢁ): Pd1–C1 2.031, Pd–Cl1 2.311; C1–Pd1–Cl1 90.71(7), C1–Pd1–
Cl1i 89.29.

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O. Aksın et al. / Journal of Organometallic Chemistry 691 (2006) 3027–3036
3034
4.5. Typical procedure for the Heck reactions
4.5.4. 2-Methoxy-trans-stilbene
Hexane:DCM (9:1); pale yellow plates; m.p.: 59.5–
65.5 ꢁC; >99%; H NMR (400 MHz, CDCl3) d: 7.46 (d,
1H, J = 8.0 Hz), 7.40 (d, 2H, J = 7.2 Hz), 7.35 (d, 1H,
J = 16.8 Hz), 7.21 (t, 2H, J = 7.6 Hz), 7.06–7.13 (m, 2H),
6.97 (d, 1H, J = 16.4 Hz), 6.83 (t, 1H, J = 7.6 Hz), 6.74
(d, 1H, J = 8.0 Hz), 3.76 (s, 3H), 13C NMR (100 MHz,
CDCl3) d: 157.2, 138.3, 129.3, 128.8, 127.6, 126.9, 126.8,
126.7, 123.9, 121.0, 111.1, 55.6; MS: 210 (M+).
1
Aryl halide, olefin, base and hexadecane (as an internal
standard) and DMF were introduced into a 3-necked
round bottomed flask which was equipped with a con-
denser and a septum and contained the catalyst which
was predried at 110 ꢁC for 2 h under vacuum. The reactor
was then placed in a preheated oil bath at 140 ꢁC and vig-
orously stirred during the reaction under inert atmosphere.
Small amounts of samples were periodically withdrawn by
syringe during the reaction, diluted in CH2Cl2 and ana-
lyzed by GC. The course of the reaction was followed until
no further increase in the yield was observed. After cooling
to room temperature, the catalyst was recovered by filter-
ing through a membrane filter with 0.2 lm porosity.
All products are known compounds and products were
analyzed by GC and GC/MS and isolated by column chro-
matography on silica gel.
In order to remove adsorbed organic substrates and
bases, the catalyst was first washed with CH2Cl2 and then
with CH3OH for NaOAc or with water for other bases
after the reaction. For the recycling studies, the recovered
catalyst was dried at 110 ꢁC under vacuum for at least
2 h and reused without further treatment. A small sample
of the reaction solution was collected for GC analysis
and the remaining solution was kept for Pd analysis. The
filtrate was digested in a Teflon vessel in the presence of
H2O2 and HNO3 (conc.) after all the organic solvent was
completely evaporated for AAS analysis.
4.5.5. 4-Methyl-trans-stilbene
Hexane; white plates; m.p.: 117–118 ꢁC; purity >99%;
1H NMR (400 MHz, CDCl3) d: 7.38 (d, 2H, J = 8.4 Hz),
7.29 (d, 2H, J = 8.0 Hz), 7.23 (t, 2H, J = 7.6 Hz), 7.12 (t,
1H, J = 6.4 Hz), 7.043 (d, 2H, J = 8.0 Hz), 6.97 (d, 1H,
J = 16.5 Hz), 6.93 (d, 1H, J = 16.3 Hz), 2.27 (s, 3H); 13C
NMR (100 MHz, CDCl3) d: 137.7, 137.3, 134.8, 129.5,
128.8, 128.7, 127.9, 127.5, 126.6, 126.6, 21.5; MS: 194
(M+), 179, 178.
4.5.6. trans-3-Styrylpyridine
Hexane:ether (2:3); pale yellow plates; m.p.: 78.6–
81.6 ꢁC; purity: >99%; 1H NMR (400 MHz, CDCl3) d:
8.62 (s, 1H), 8.38 (d, 1H, J = 4.8 Hz), 7.70 (d, 1H,
J = 8.4 Hz), 7.40 (d, 2H, J = 7.2 Hz), 7.26 (t, 2H,
J = 7.2 Hz), 7.12–7.20 (m, 2H), 7.05 (d, 1H, J = 16.4 Hz),
6.95 (d, 1H, J = 16.0 Hz); 13C NMR (100 MHz, CDCl3)
d: 148.7, 148.6, 136.8, 133.0, 132.6, 131.0, 128.9, 128.3,
126.8, 125.0, 123.5; MS: 181 (M+).
4.5.1. 4-Nitro-trans-stilbene
4.5.7. trans-4-Nitrocinnamic acid butyl ester
Hexane:DCM (2:1); yellow plates; m.p.: 156–157 ꢁC;
purity: >99%; 1H NMR (400 MHz, CDCl3) d: 8.21 (d,
2H, J = 9.2 Hz), 7.61 (d, 2H, J = 9.2 Hz), 7.52 (d, 2H,
J = 7.2 Hz), 7.28–7.42 (m, 3H), 7.24 (d, 1H, J = 16 Hz),
7.12 (d, 1H, J = 16.4 Hz); 13C NMR (100 MHz, CDCl3)
d: 147.2, 143.9, 136.4, 133.5, 129.1, 129.0, 127.2, 127.0,
126.5, 124.3; MS: 225 (M+), 178.
Hexane; yellow plates; m.p.: 59.6–64.4 ꢁC; purity:
97.3%; 1H NMR (400 MHz, CDCl3) d: 8.24 (d, 2H,
J = 8.8 Hz), 7.65–7.71 (m, 3H), 6.55 (d, 1H, J = 16 Hz),
4.23 (t, 2H, J = 6.8 Hz), 1.68 (quint, 2H, J = 7.6 Hz),
1.43 (sixet, 2H, J = 7.6 Hz), 0.96 (t, 3H, J = 7.4 Hz); 13C
NMR (100 MHz, CDCl3) d: 166.3, 148.7, 141.8, 140.8,
128.8, 124.4, 122.9, 65.1, 30.9, 19.4, 13.9; MS: 249 (M+),
194, 176, 130.
4.5.2. 4-Acetyl-trans-stilbene
Hexane:DCM (4:1); white plates; m.p.: 138.7–144.8 ꢁC;
purity: >99%; 1H NMR (400 MHz, CDCl3) d: 7.92 (d, 2H,
J = 8.0 Hz), 7.55 (d, 2H, J = 8.0 Hz), 7.50 (d, 2H, J =
8.0 Hz), 7.24–7.38 (m, 3H), 7.19 (d, 1H, J = 16.4 Hz), 7.04
(d, 1H, J = 16.4 Hz), 2.28 (s, 3H); 13C NMR (100 MHz,
CDCl3) d: 196.6, 142.1, 136.9, 136.3, 131.7, 129.0, 129.0,
128.5, 127.7, 127.0, 126.7, 26.6; MS: 222 (M+), 207, 178.
4.5.8. trans-4-Acetylcinnamic acid butyl ester
Hexane:ethyl acetate (gradient elution, 8:2 and 6:4); col-
orless, oily; 99.1%; 1H NMR (400 MHz, CDCl3) d: 7.93 (d,
2H, J = 8.8 Hz), 7.64 (d, 1H, J = 16 Hz), 7.58 (d, 2H,
J = 8.4 Hz), 6.47 (d, 1H, J = 16 Hz), 4.19 (t, 2H,
J = 7.2 Hz), 2.57 (s, 3H), 1.70 (quint, 2H, J = 7.2 Hz),
1.42 (sixet, 2H, J = 7.2 Hz), 0.96 (t, 3H, J = 7.4 Hz); 13C
NMR (100 MHz, CDCl3) d: 196.3, 166.2, 143.0, 139.0,
138.2, 129.0, 128.2, 121.1, 64.6, 31.0, 26.6, 19.4, 14.0.
MS: 246 (M+), 231, 190, 175, 131, 115.
4.5.3. 4-Methoxy-trans-stilbene
Hexane:DCM (4:1); pale yellow plates; m.p.: 135.4–
1
137.1 ꢁC; purity: >99%; H NMR (400 MHz, CDCl3) d:
7.37 (d, 2H, J = 7.2 Hz), 7.33 (d, 2H, J = 8.4 Hz), 7.23 (t,
2H, J = 7.2 Hz), 7.12 (t, 1H, J = 6.8 Hz), 6.95 (d, 1H,
J = 16.0 Hz), 6.85 (d, 1H, J = 16.4 Hz), 6.78 (d, 2H,
J = 8.4 Hz), 3.73 (s, 3H); 13C NMR (100 MHz, CDCl3)
d: 159.4, 137.8, 130.3, 128.7, 128.4, 127.8, 127.2, 126.8,
126.4, 114.2, 55.2; MS: 210 (M+), 165.
4.5.9. trans-3-(3-Pyridinyl)acrylic acid butyl ester
Hexane:ethyl acetate (2:1); oily; colorless; purity: 99.1%;
1H NMR (400 MHz, CDCl3) d: 8.72 (s, 1H), 8.60 (m, 1H),
7.82 (d, 1H, J = 8.4 Hz), 7.64 (d, 1H, J = 16 Hz), 7.31 (m,
1H), 6.49 (d, 1H, J = 16.4 Hz), 4.2 (t, 2H, J = 6.6 Hz), 1.67
(quint, 2H, J = 7.2 Hz), 1.41 (sixet, 2H, J = 7.2 Hz), 0.94

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O. Aksın et al. / Journal of Organometallic Chemistry 691 (2006) 3027–3036
3035
(t, 3H, J = 7.4 Hz); 13C NMR (100 MHz, CDCl3) d: 166.6,
Appendix A. Supplementary data
150.9, 149.7, 140.9, 134.6, 130.6, 124.0, 120.9, 64.9, 30.9,
21.2, 19.4, 13.9; MS: 205 (M+), 190, 176, 150, 132, 121,
104, 77.
Supplementary data associated with this article can
be found, in the online version, at doi:10.1016/
j.jorganchem.2006.03.012.
4.5.10. trans-4-Methyl-cinnamic acid butyl ester
Hexane:ethyl acetate (12:1); oily; colorless; purity: 97%;
1H NMR (400 MHz, CDCl3) d: 7.66 (d, 1H, J = 16 Hz),
7.42 (d, 2H, J = 8.4 Hz), 7.19 (d, 2H, J = 8.4 Hz), 6.39
(d, 1H, J = 16 Hz), 4.20 (t, 2H, J = 6.4 Hz), 2.37 (s,
3H), 1.68 (quint, 2H, J = 6 Hz), 1.45 (sixet, 2H, J =
7.2 Hz), 0.97 (t, 3H, J = 7.4 Hz); 13C NMR (100 MHz,
CDCl3) d: 167.5, 144.8, 140.8, 132.0, 129.8, 128.3, 117.5,
64.5, 31.0, 21.7, 19.4, 14.0; MS: 218 (M+), 162, 145, 115,
91.
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