New Pd-NHC-complexes for the Mizoroki-Heck reaction

Article abstract of DOI:10.1016/j.jorganchem.2006.11.050

The synthesis and structural characterization of novel chelating N-aryl substituted palladium(II)-biscarbene-complexes is reported: 1,1′-bis(4-bromophenyl)-3,3′-methylene-diimidazoline-2,2′-diylidene-palladium(II)-dibromide, 1,1′-bis(4-methoxyphenyl)-3,3′-methylene-diimidazoline-2,2′-diylidene-palladium(II)-dibromide and 1,1′-bis(4-n-butoxyphenyl)-3,3′-methylenediimidazoline-2,2′-diylidene-palladium(II)-dibromide have been synthesized in good yields. The catalytic activity of these 1,1′-aryl-3,3′-methylenediimidazoline-2,2′-diylidene-palladium(II)-dihalogenide complexes was tested for the Mizoroki-Heck reaction in comparison to 1,1′-(bis)methyl-3,3′-methylenediimidazoline-2,2′-diylidene-palladium(II)-dihalogenide complexes and to 1,1′-bis(phenyl)-3,3′-methylene-diimidazoline-2,2′-diylidene-palladium(II)-dibromide. The activity of the aryl substituted catalysts is significantly higher compared to the methyl substituted NHC complexes. They also allow the coupling of arylchlorides with olefins.

Full text of DOI:10.1016/j.jorganchem.2006.11.050

Journal of Organometallic Chemistry 692 (2007) 1519–1529
www.elsevier.com/locate/jorganchem
New Pd–NHC-complexes for the Mizoroki–Heck reaction
a
a
a
a,1
Maria A. Taige , Alexander Zeller , Sebastian Ahrens , Sigrid Goutal
,
Eberhardt Herdtweck b,1, Thomas Strassner
a,
*
a
Physikalische Organische Chemie, Technische Universita¨t Dresden, Bergstrasse 66, 01062 Dresden, Germany
Department Chemie - Lehrstuhl fu¨r Anorganische Chemie, Technische Universita¨t Mu¨nchen, Lichtenbergstrasse 4, 85747 Garching, Germany
b
Received 24 August 2006; received in revised form 28 November 2006; accepted 30 November 2006
Available online 9 December 2006
Abstract
The synthesis and structural characterization of novel chelating N-aryl substituted palladium(II)–biscarbene-complexes is reported:
1,10-bis(4-bromophenyl)-3,30-methylene-diimidazoline-2,20-diylidene-palladium(II)-dibromide, 1,10-bis(4-methoxyphenyl)-3,30-methylene-
diimidazoline-2,20-diylidene-palladium(II)-dibromide and 1,10-bis(4-n-butoxyphenyl)-3,30-methylenediimidazoline-2,20-diylidene-palla-
dium(II)-dibromide have been synthesized in good yields. The catalytic activity of these 1,10-aryl-3,30-methylenediimidazoline-2,
20-diylidene-palladium(II)-dihalogenide complexes was tested for the Mizoroki–Heck reaction in comparison to 1,10-(bis)methyl-3,
30-methylenediimidazoline-2,20-diylidene-palladium(II)-dihalogenide complexes and to 1,10-bis(phenyl)-3,30-methylene-diimidazoline-
2,20-diylidene-palladium(II)-dibromide. The activity of the aryl substituted catalysts is significantly higher compared to the methyl substi-
tuted NHC complexes. They also allow the coupling of arylchlorides with olefins.
ꢀ 2006 Elsevier B.V. All rights reserved.
Keywords: Mizoroki–Heck reaction; Palladium; N-heterocyclic carbene; Styrene; Bromoacetophenone; Chloroacetophenone; Bromoanisole; Methylac-
rylate
1. Introduction
[19,20]. Promising approaches for the activation of chloro-
and bromoarenes have been palladium complexes with N-
heterocyclic carbene ligands [21–31], carbocyclic carbene
ligands [32], PCP pincer ligands [33,34], CNC pincer
ligands [35,36], cyclometallated phosphines [37–40], phos-
phites [41], phosphonium salts [42–44], bulky Lewis basic
phosphines [45,46], phosphinous acid ligands [47,48], ani-
line ligands [49] as well as cyclometalated sulfur, amine
and imine ligands [50–55] as catalysts. The use of immobi-
lized [56] and non immobilized [57] ligand free palladium
salts for the Mizoroki–Heck reaction with chloro- and bro-
moarenes is also discussed in the literature. Recently, Gib-
son et al. [58] as well as Jones et al. [59] showed that
palladium complexes with chiral ligands are well suited as
catalysts for the asymmetric Heck reaction. Despite the
progress in the design of homogeneous catalysts for the
Mizoroki–Heck reaction, the nature of the catalytically
active species of this reaction is still not determined [60–66].
Due to the high stability of metal complexes of imida-
zoline-2-ylidenes towards heat, oxygen and moisture [67],
The Mizoroki–Heck reaction has an enormous synthetic
potential for generating substituted aryl-olefins [1–6], toler-
ating a wide range of functional groups [7,8]. During the
last decade, important progress has been made in homoge-
neous Heck catalysis, which is summarized in recent
reviews [3,9–15]. Very high turnover numbers (TON) could
already be observed for the Heck coupling of iodobenzene
with styrene [16–18], while the search for effective catalysts
for the coupling of bromo- and especially chloroarenes
with olefins is ongoing. The development of catalysts with
high TONs for the carbon–carbon coupling of olefins with
bromo- and chloroarenes is highly important, since these
are interesting substrates for industrial applications
*
Corresponding author. Tel.: +49 351 46338571; fax: +49 351
46339679.
E-mail address: thomas.strassner@chemie.tu-dresden.de (T. Strassner).
X-ray analysis.
1
0022-328X/$ - see front matter ꢀ 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2006.11.050

1520
M.A. Taige et al. / Journal of Organometallic Chemistry 692 (2007) 1519–1529
they have long been subject of catalytic studies. In addition
to the palladium catalyzed Mizoroki–Heck reaction, they
have been used for the catalysis of other C–C coupling
reactions [68], hydroformylations [69,70], polymerization
reactions [71,72], olefin metathesis [73–76], and CH-activa-
tion [77–79]. Crucial for this development was the improve-
ment of the synthetic methodology for the preparation of
N-heterocyclic carbene (NHC) complexes, which has been
reported in recent reviews [80,81]. The high synthetic rele-
vance of catalysts for the Heck coupling of bromo- and
chloroarenes with olefins makes NHC–palladium com-
plexes promising candidates for these reactions.
cedures [71,78,85,87], while complexes 2–4 were obtained
by conversion of the imidazolium salt with Pd(OAc)2 in
DMSO at temperatures up to 130 ꢁC (Scheme 2). Decom-
position of the N-aryl substituted complexes 1–4 is not
observed below 300 ꢁC, demonstrating the high thermal
stability of these complexes.
2.2. Spectroscopic properties
The 1H NMR spectra of the N-aryl substituted Pd-com-
plexes 1 and 2 show an AB spin system for the protons of
1
the methylene bridge, while in the H NMR spectra of N-
alkylsubstituted Pd-complexes (5 and 6) and of the N-aryl
substituted Pd-complexes (3 and 4) these protons show a
singlet [78]. Therefore, the flip of the boat conformation
of the central six-membered palladacycle (Scheme 3) is slow
on the NMR time-scale at room temperature. This is also
observed in the case of sterically demanding alkyl-substitu-
ents like tert-butyl [71,88].
2. Results and discussion
2.1. Preparation and structural properties of the
Palladium(II)–bis(carbene) complexes
The catalytic activity of Pd–NHC-complexes (Scheme 1)
was investigated for the Mizoroki–Heck coupling of aryl-
halides with olefins and compared to previously published
examples. It could be shown that analogous N-aryl substi-
tuted Pt-complexes not only show a significantly higher sta-
bility than the corresponding N-methyl substituted Pt-
complexes, but that the electronic situation of the metal
can be changed depending on the substituent in para-posi-
tion of the aromatic ring [82]. Therefore, we were especially
interested in the catalytic activity of N-aryl substituted Pd–
NHC-complexes compared to N-methyl substituted Pd–
NHC-complexes, which have been studied previously as
it is well known that strongly electron donating ligands
improve the activity of Heck catalysts [26,27,83–86]. Com-
pounds 1–4 allow the evaluation of the electronic effect of
substituents in the para-position of the phenyl ring. Com-
plexes 1, 5 and 6 were prepared following the literature pro-
2.3. Solid state structures
We were able to obtain crystals suitable for X-ray diffrac-
tion of complexes 2 and 3 by cooling a hot DMSO solution
or condensing THF into DMSO, respectively. Complex 2
crystallizes as a DMSO solvate with two DMSO molecules
per Pd-compound, while the DMSO solvate of complex 3
shows only one DMSO molecule per Pd-compound. The
ORTEP style plots of 2 and 3 are illustrated in Figs. 1
and 2.
The disparity of the structures at the central bis-NHC-
units of compounds 2 and 3 is minimal. The most striking
difference is the torsional angle between the plane of the
aromatic ring and the plane of the imidazole ring. These
torsional angles (ꢀ29.2(5)ꢁ and 40.5(4)ꢁ) are slightly smal-
ler for the bromide substituted phenyl ring in complex 2
than in complex 3 (ꢀ54.7(4)ꢁ and 45.6(4)ꢁ), which contains
the methoxy substituted phenyl ring. It is not possible to
distinguish between electronic and packing effects, how-
ever, DFT calculations described below and illustrated in
Fig. 3, showed only small electronic effects. Comparing
the bond lengths and angles of complexes 2 and 3 with
the structure of the corresponding methyl substituted com-
pound 5 [89] only negligible differences in the geometry of
Br
OMe
N
N
N
Br
Br
N
N
N
N
Br
Br
N
N
Br
Br
Pd
N
Pd
Pd
N
N
Br
OMe
2
1
3
R
R
OBu
N
N
N
N
N
N
Br
Br
Br
Br
N
N
N
N
N
N
Br
N
N
Br
Br
N
Cl
Cl
Pd(OAc)2
- 2 HOAc
Yield:
Pd
N
Pd
Pd
Pd
2: 72%
3: 61%
4: 71%
Br
N
N
N
N
OBu
R
R
4
5
6
Scheme 2. Synthesis of palladium–biscarbene-complexes with aromatic
Scheme 1. Pd–NHC-complexes for the Mizoroki–Heck reaction.
side chains (2: R = Br, 3: R = OMe, 4: R = OBu).

M.A. Taige et al. / Journal of Organometallic Chemistry 692 (2007) 1519–1529
1521
H
H
C
Pd
C
N
N
C
Scheme 3. Illustration of the boat conformation and the flip.
Fig. 3. Superposition of the DFT calculated structures 2 and 3.
the palladium–bis-NHC structure could be observed. This
is another evidence that the electronic influence of the side
chains does not really affect the structure of the central bis-
NHC unit.
Fig. 1. ORTEP style plot of complex 2 in the solid state. Thermal
ellipsoids are drawn at the 50% probability level. Hydrogen atoms are
plotted as balls of arbitrary radius. The DMSO molecules are omitted for
clarity.
To get additional insight concerning the electronic influ-
ence of the substituent in para-position of the phenyl ring
on the geometry of the palladium–bis-NHC structure, we
performed DFT geometry optimizations. The calculations
were carried out using GAUSSIAN 03 [90]. Details of the
DFT calculations are given in Section 4. The results of
the geometry optimizations of structures 2 and 3 are illus-
trated in Fig. 3 as superpositions of the geometries.
The results obtained from the X-ray diffraction study
and the DFT calculations are very similar. The computed
structures of the palladium–bis-NHC complexes 2 and 3
differ only slightly in the torsion angle between the plane
of the aromatic ring and the plane of the imidazole ring
(55.79 and ꢀ55.79 for 2, 52.01 and ꢀ52.01 for 3). These
small discrepancies are due to the different electronic prop-
erties of the substituents at the phenyl ring. Electron donat-
ing substituents like the methoxy group seem to reduce the
torsional angle between the aromatic side chain and the
NHC ring slightly in comparison to electron withdrawing
substituents like bromine.
2.4. Catalysis
Fig. 2. ORTEP style plot of complex 3 in the solid state. Thermal
ellipsoids are drawn at the 50% probability level. Hydrogen atoms are
plotted as balls of arbitrary radius. The DMSO molecule is omitted for
clarity.
For a better comparison of the catalytic activity we used
standard conditions with styrene as olefin and sodium ace-
tate as base (Table 1).

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M.A. Taige et al. / Journal of Organometallic Chemistry 692 (2007) 1519–1529
Table 1
Influence of the arylhalide on the catalytic reactivity in the Mizoroki–Heck reaction: coupling of different arylhalides with styrene
R
R
cat. [Pd]
+
+ HX
140 ˚C, NaOAc,
DMAc
X
Entry
R
X
Catalyst
mol% Catalyst
t (h)
Conversion (%)
TON
49
TOF (hꢀ1
)
1
2
3
4
5
6
7
8
9
C(O)CH3
C(O)CH3
C(O)CH3
C(O)CH3
C(O)CH3
C(O)CH3
C(O)CH3
C(O)CH3
C(O)CH3
C(O)CH3
C(O)CH3
C(O)CH3
C(O)CH3
C(O)CH3
OCH3
OCH3
OCH3
OCH3
OCH3
OCH3
OCH3
OCH3
OCH3
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Cl
Cl
Cl
Cl
Cl
Cl
1
2
3
4
5
6
3
4
6
3
4
3
4
4
1
2
3
4
5
6
3
4
3
3
4
4
3
4
3
4
3
4
0.5
0.5
0.5
0.5
0.5
0.5
0.01
0.01
0.01
0.001
0.001
0.0001
0.0001
0.0001
0.5
0.5
0.5
0.5
0.5
8
8
3
8
8
8
6
8
8
26
26
28
8
28
8
8
8
8
8
8
8
8
8
30
8
30
30
30
30
30
30
30
34
52
99
6
9
47
16
10
74
141
126
80
114
88 (4% cis)
56
80 (5% cis)
100
94 (2% cis)
64 (2% cis)
41
93 (3% cis)
33
14
7143
6714
4571
29286
66429
235714
342857
714286
49
1191
839
571
1126
2555
8418
42857
25510
6
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
48
100 (4% cis)
34
66 (6% cis)
52 (6% cis)
88 (4% cis)
32
30 (7% cis)
21
26 (2% cis)
4
26 (4% cis)
15 (4% cis)
51 (4% cis)
36 (4% cis)
5
94
74
126
46
12
9
16
6
0.5
43
5
0.01
0.01
0.001
0.001
0.001
0.001
1.0
1.0
0.5
0.5
0.01
0.01
1500
1857
2857
18571
10714
36428
26
188
232
357
619
1339
1214
0.9
0.1
1.5
1.6
38
OCH3
OCH3
OCH3
C(O)CH3
C(O)CH3
C(O)CH3
C(O)CH3
C(O)CH3
C(O)CH3
4
44
49
31
34 (4% cis)
16
–
1143
–
–
These results confirm that our complexes are highly
active catalysts for the Heck-coupling of bromoacetophe-
none or bromoanisole with styrene. Turnover numbers
(TONs) up to 700000 (catalyst 4) could be observed for
the coupling of bromoacetophenone without addition of
co-catalysts or other additives (Table 1, entry 14). Due to
the low solubility of the Pd-complex 4 the conversion of
bromoacetophenone with styrene in high concentration is
marginally lower compared to the Pd-complex 3 (Table 1,
entry 3/4 and entry 7/8). But at lower catalyst concentra-
tions, complex 4 shows a surprising behaviour and a signif-
icantly higher TON compared to complex 3 (Table 1, entry
10/11 and entry 12/14). Complexes 1 and 2 are less active
and show significantly lower TOFs for the Heck reaction
of styrene with bromoacetophenone than the other com-
plexes. Complex 2 was included for comparison although
the bromophenyl group of the catalyst itself might react
in the Heck reaction leading to catalyst deactivation. In
the NMR spectra of the catalyst after the reaction small
alkene signals could be found.
Another reason for the lower activity are obviously the
different electronic properties of the substituents, although
the activity of complexes 1 and 2 is also hampered by sol-
ubility problems. Comparison of the Pd-complexes with
aryl substituted ligands to the alkyl complexes 5 and 6 indi-
cate that the aryl substituent stabilizes the catalytically
active species. As expected, the +M-effect of the methoxy-
and butoxy-substituent stabilizes the catalytically active
Pd0 species. The influence of the counterion on the TON
is only small as can be seen from Table 1, entries 19/20.
Bromoanisole is an example for a haloarene with elec-
tron-donating substituents and is well known for smaller
reaction rates than what is usually observed for bromoace-
tophenone. Nevertheless, we could obtain TONs up to
35000 for the Heck coupling of bromoanisole with styrene
with our catalysts (Table 1, entry 26). Again, complex 4 is

M.A. Taige et al. / Journal of Organometallic Chemistry 692 (2007) 1519–1529
1523
the most active catalyst. The observed differences in the
catalytic activity are comparable to the ones obtained for
the reactions with bromoacetophenone.
0.01 mol% catalyst 3, cesium acetate
100
80
60
40
20
0
0.01 mol% catalyst 3, sodium acetate
0.01 mol% catalyst 3, calcium hydroxide
0.01 mol% catalyst 3, sodium carbonate
As expected, the TONs for the coupling of chloroarenes
with styrene are clearly lower than those for the coupling of
bromoarenes (Table 1). However, compared to literature
results [54,91,92], catalyst 3 is also a very active catalyst
for the Heck coupling of chloroacetophenone (TON >
1000) (Table 1, entry 31). Increasing the amount of catalyst
3 from 0.5 mol% (entry 29) to 1 mol% (entry 27) does not
really affect the conversion per reaction time, instead the
formation of the cis-product is observed.
0
200
400
600
800
1000 1200 1400 1600
Reaction time [min]
Fig. 4. Conversion of bromoacetophenone during the Heck-coupling with
styrene at 140 ꢁC using different bases in the presence of catalyst 3.
A great advantage of our catalysts is their high selectiv-
ity. Table 1 shows that the amount of cis-product formed
during the conversion of bromoacetophenone with styrene
is very small for catalysts 4 and 6 and below the detection
limit for all other catalysts. For the reaction of bromoani-
sole with styrene the amount of the cis-product is slightly
higher. In this case it is also formed during the reaction cat-
alyzed with complexes 2 and 3. But when the amount of
catalyst 3 was reduced from 0.5 to 0.01 mol% only the
trans-product was formed. Together with their high cata-
lytic activity this makes our Pd–NHC-complexes interest-
ing Heck catalysts.
We also looked at the effects of different reaction condi-
tions on the catalytic activity of the Pd–NHC-complexes.
The screening of several bases showed good results for
inexpensive NaOAc (Table 2 and Fig. 4). Stronger bases
like KOH lead to quick decomposition of the catalyst
and therefore lower conversions. Sodium carbonate is less
soluble in DMAc, leading to longer reaction times. To
reach 100% conversion it took approximately four times
longer for the same reaction conditions with sodium car-
bonate than for sodium acetate. In addition, the formation
of a byproduct (5% of cis-stilbene) was observed in the case
of sodium carbonate (Table 2, entry 3). In the presence of
Ca(OH)2 the reaction time to reach complete conversion
was slightly shorter than with NaOAc, but here even more
byproduct was formed (6% cis-isomer) (Table 2, entry 4).
Using Cs2CO3 partial decomposition of the catalyst was
observed and the reaction time was clearly longer than with
NaOAc (Table 2, entry 2). Thus, sodium acetate turned out
to be the optimal base for our catalysts.
Different olefins do not much affect the reaction rate.
Only small differences of the TONs using methylacrylate
or styrene were observed (Table 3). Therefore we decided
to use styrene as olefin in the standard reaction system
for comparison of the catalytic activity of our palladium
complexes.
In our case, the use of co-catalysts like NBu4Br did not
improve the catalytic results as it was reported in the lit-
erature for the reaction of bromoacetophenone and sty-
rene together with 20 mol% of NBu4Br [38]. The
advantage of our catalysts is their ability to reach full
conversion even with concentrations of 0.01 mol%, while
very few examples in the literature do reach full conver-
sion. Mostly only with relatively high catalyst concentra-
tions of up to 1% full conversions are observed [93]. The
Heck reaction of styrene with bromoanisole often uses
higher reaction temperatures, but the results observed
with our catalysts are still good in comparison to those
reported in the literature. Full conversion for the reaction
of bromoanisole with styrene is difficult to achieve, only
few examples get close [94].
Summarizing the comparison of our catalysts with liter-
ature results, it becomes apparent that our Pd–NHC-com-
plexes are very good catalysts for the Mizoroki–Heck
reaction, their biggest advantage being the full conversion
at very low concentrations, which leads to TONs and TOFs
Table 2
Mizoroki–Heck reaction: coupling of bromoacetophenone with styrene in the presence of different bases
R
R
3
+
+ HX
140 ˚C, base,
X
DMAc
Entry
R
X
mol% 3
Base
t (h)
Conversion (%)
TON
1
2
3
4
C(O)CH3
C(O)CH3
C(O)CH3
C(O)CH3
Br
Br
Br
Br
0.01
0.01
0.01
0.01
NaOAc
Cs2CO3
Na2CO3
Ca(OH)2
6
23
22
5
100
51
96 (5% cis)
100 (6% cis)
7143
3643
6857
7143

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M.A. Taige et al. / Journal of Organometallic Chemistry 692 (2007) 1519–1529
Table 3
Influence of the olefin on the catalytic reactivity in the Mizoroki–Heck reaction
R
R
3
R'
+
+ HX
140 ˚C, NaOAc,
DMAc
X
R'
Entry
R
X
R0
mol% 3
t (h)
Conversion (%)
TON
1
2
3
4
5
6
C(O)CH3
C(O)CH3
OCH3
OCH3
C(O)CH3
C(O)CH3
Br
Br
Br
Br
Cl
Cl
C(O)OCH3
Phenyl
C(O)OCH3
Phenyl
C(O)OCH3
Phenyl
0.01
0.01
0.01
0.01
0.5
7
6
22
19
28
28
100
100
39
22
28
7143
7143
2785
1571
40
0.5
30
43
which have not been reported before in the reaction with
bromoacetophenone.
5. Experimental
5.1. General procedure
3. Conclusion
All reactions were carried out under an atmosphere of
We present the synthesis and structural characteriza-
tion of N-aryl substituted palladium biscarbene com-
plexes. The electronic influences of different substituents
at the aryl rings on the structure of the Pd–NHC-complex
is minimal, but important for their catalytic activity. This
allows us to further fine tune the catalysts. We have inves-
tigated the performance of N-aryl substituted Pd–NHC
complexes (1–4) as catalysts for the Mizoroki–Heck reac-
tion in comparison to N-methyl substituted Pd–NHC
complexes (5, 6). We demonstrated that 1,10-bis(4-meth-
oxyphenyl)-3,30-methylenediimidazoline-2,20-diylidene-pal-
ladium(II)-dibromide (3) and 1,10-bis(4-n-butoxyphenyl)-
3,30-methylenediimidazoline-2,20-diylidene-palladium(II)-
dibromide (4) are very active and highly selective catalysts
in the Heck coupling reaction of haloarenes with styrene
and methylacrylate. These results show that bis-NHC–
palladium(II)-complexes are useful catalysts in C–C cou-
pling reactions and therefore open new routes for the
design of new chelating ligands based on N-heterocyclic
carbenes.
dry nitrogen. N,N-dimethylacetamide was distilled prior
use and stored over 4 A molecular sieve under nitrogen.
˚
GC–MS spectra were measured on a Hewlett-Packard
GC G1800A system. The conversion was measured using
diethylene glycol di-n-butyl-ether as internal standard.
The turnover number (TON) is defined as (mol product)/
(mol Pd), while the turnover frequency (TOF) is defined
as (mol product)/[(mol Pd) · (reaction time in h)]. The
amount of catalyst is defined regarding the amount of the
olefin. All catalytic experiments were repeated at least
twice, the TON and TOF given is the average value of sev-
eral GC measurements.
1H and 13C NMR spectra were recorded with a Bruker
AC 300 P, Jeol JNM GX-400 or Jeol JNM GX-270 spec-
trometer. The spectra were referenced internally to the res-
onances of solvent (1H, 13C). Elemental analyses were
performed by the microanalytical laboratory of our insti-
tute using an EuroVektor Euro EA-300 Elemental Ana-
lyzer. Melting and decomposition points of the complexes
1–4 were determined with a melting point apparatus (Elec-
trothermal 9100) and are uncorrected.
4. Computational details
1-R-substituted imidazoles (R: 4-bromophenyl, 4-meth-
oxyphenyl) [82], 1,10-R-disubstituted-3,30-methylenediimi-
dazolium-dibromide (R: phenyl, 4-bromophenyl, 4-meth-
oxyphenyl) [82,87], 1,10-bisphenyl-3,30-methylene-diimi-
dazoline-2,20-diylidene-palladium(II)-dibromide (1) [87],
1,10-bismethyl-3,30-methylene-diimidazoline-2,20-diylidene-
palladium(II)-dibromide (5) [71,85] and 1,10-bismethyl-3,
30-methylene-diimidazoline-2,20-diylidene-palladium(II)-
dichloride (6) [103] were prepared according to the litera-
ture procedures.
All calculations were carried out using GAUSSIAN 03
[90]. The geometry optimizations of the structures were
performed using B3LYP [95] combined with the split
valence double-f (DZ) basis set 6-31G(d) [96,97] for the
non-metal atoms except bromine, the split valence triple-
f (TZ) basis set 6-311G(d) [98–100] for bromine and the
Hay–Wadt-ECP [101] for palladium. No symmetry or
internal coordinate constraints were applied during opti-
mizations. Vibrational frequency calculations were carried
out for all structures to verify them as true minima by the
absence of imaginary eigenvalues. The superposition of
the geometries was carried out using the program YASARA
[102].
5.2. Synthesis of 1-(4-n-butoxyphenyl)-imidazole
The synthesis follows previously published protocols
[82,104]. 4-Butoxyaniline (5.96 g, 36 mmol) in methanol

M.A. Taige et al. / Journal of Organometallic Chemistry 692 (2007) 1519–1529
1525
(20 mL) was treated with 30% aq. glyoxal (5.8 mL,
36 mmol) for 16 h at r.t. until a yellowish precipitate
was formed. NH4Cl (3.85 g, 72 mmol) was added, fol-
lowed by 37% aq. formaldehyde (5.8 mL, 72 mmol).
The mixture was diluted with methanol (150 mL) and
the resulting mixture refluxed for 1 h. H3PO4 (5 mL,
85%) was added over a period of 10 min. The resulting
mixture was then stirred at reflux temperature for addi-
tional 4 h. After removal of the solvent, the dark residue
was poured onto ice (100 g) and treated with aq. 40%
KOH solution until pH 9. The resulting mixture was
extracted with dichloromethane (three times 250 mL).
The organic phases were combined, washed with H2O,
brine and dried over MgSO4. The solvent was removed
and the resulting dark residue was subjected to sublima-
Half of the solvent was removed and a white solid was
formed after addition of 15 mL ethanol. The solid was fil-
trated and washed with 10 mL ethanol. (Yield: 718 mg,
72%), m.p.: 335 ꢁC (decomp.). 1H NMR (300 MHz,
DMSO-d6): d 7.86–7.83 (m, 6H, NCH and Ar), 7.77–7.73
(m, 6H, NCH and Ar), 6.52 (d, J = 14.0 Hz, 1H, NCH2N),
6.45 (d, J = 12.4 Hz, 1H, NCH2N) ppm. 13C NMR
(75.5 MHz, DMSO-d6): d 159.5 (NCN), 138.6 (i-C of Ar),
131.6 (m-C of Ar), 127.1 (o-C of Ar), 122.6 (NCH), 122.4
(C-Br), 120.9 (NCH), 63.1 (NCH2N) ppm. Anal. Calc.
for C19H14Br4N4Pd: C, 31.50; H, 1.95; N, 7.73. Found:
C, 31.22; H, 1.99; N, 7.58%.
5.5. Synthesis of 1,10-bis(4-methoxyphenyl)-3,30-methylene-
diimidazoline-2,20-diylidene-palladium(II)-dibromide (3)
1
tion in vacuum at 160 ꢁC (Yield: 4.88 g, 63%). H NMR
(300 MHz, CDCl3): d 7.77 (s, 1H, NCHN), 7.26 (d,
J = 9.0 Hz, 2H, Ar), 7.18 (s, 1H, NCH), 7.16 (s, 1H,
NCH), 6.94 (d, J = 8.9 Hz, 2H, Ar), 3.96 (t,
J = 6.5 Hz, 2H, OCH2), 1.77 (m, 2H, CH2), 1.48 (m,
2H, CH2), 0.96 (t, J = 7.4 Hz, 3H, CH3) ppm. 13C
NMR (75.5 MHz, CDCl3): d 158.5 (C-OnBu), 135.7
(NCHN), 130.3 (i-C of Ar), 129.7 (NCH), 123.0 (m-C
of Ar), 118.7 (NCH), 115.3 (o-C of Ar), 68.0 (OCH2),
31.1 (CH2), 19.1 (CH2), 13.7 (CH3) ppm. GC–MS: m/z
216 (M), 160 (MꢀnBu + H). Anal. Calc. for
C13H16N2O: C, 72.19; H, 7.46; N, 12.95. Found: C,
72.07; H, 7.60; N, 12.96%.
A solution of 430 mg 1,10-bis(4-methoxyphenyl)-3,30-
methylene-diimidazolium-dibromide (0.82 mmol) and
184 mg palladium(II) acetate (0.82 mmol) in 6 mL DMSO
was heated stepwise during 6 h to 110 ꢁC and stirred for
2 h at this temperature. After the removal of the solvent,
the residue was washed two times with dichloromethane
(5 mL) and with acetonitrile (5 mL). The product was
obtained as a brown solid (Yield: 316 mg, 61%), m.p.:
328 ꢁC (decomp.). 1H NMR (300 MHz, DMSO-d6): d
7.82 (s, 2H, NCH), 7.68–7.65 (m, 6H, NCH and Ar),
7.14 (d, J = 8.7 Hz, 4H, Ar), 6.46 (s, 2H, NCH2N), 3.85
(s, 6H, OCH3) ppm. 13C NMR (75.5 MHz, DMSO-d6): d
158.7 (C-OCH3), 144.0 (NCN), 132.3 (i-C of Ar), 126.7
(o-C of Ar), 122.7 (NCH), 121.8 (NCH), 113.7 (m-C of
Ar), 63.0 (NCH2N), 55.3 (OCH3) ppm. Anal. Calc. for
C21H20Br2N4O2Pd: C, 40.25; H, 3.22; N, 8.94. Found: C,
40.34; H, 3.17; N, 9.06%.
5.3. Synthesis of 1,10-bis(4-n-butoxyphenyl)-3,30-methylene-
diimidazolium-dibromide
In
a
pressure tube (4-n-butoxyphenyl)-imidazole
(4 mmol, 865 mg) and dibromomethane (3 mmol, 521 mg)
were heated neat to 130 ꢁC for 48 h. A brown solid forms,
which was washed twice with 5 mL of cold THF (Yield:
5.6. Synthesis of 1,10-bis(4-n-butoxyphenyl)-3,30-
methylenediimidazoline-2,20-diylidene-palladium(II)-
dibromide (4)
1
527 mg, 43%). H NMR (300 MHz, DMSO-d6): d 10.28
(s, 2H, NCHN), 8.39 (s, 2H, NCH), 8.32 (s, 2H, NCH),
7.74 (d, J = 9.0 Hz, 4H, Ar), 7.22 (d, J = 9.0 Hz, 4H,
Ar), 6.90 (s, 2H, NCH2N), 4.07 (t, J = 6.4 Hz, 4H,
OCH2), 1.75 (m, 4H, CH2), 1.45 (m, 4H, CH2), 0.94 (t,
J = 7.4 Hz, 6H, CH3) ppm. 13C NMR (75.5 MHz,
DMSO-d6): d 159.6 (C–OnBu), 136.7 (NCN), 127.2 (i-C
of Ar), 123.4 (m-C of Ar), 123.4 (NCH), 121.7 (NCH),
115.6 (o-C of Ar), 67.7 (NCH2N), 58.1 (OCH2), 30.4
(CH2), 18.5 (CH2), 13.5 (CH3) ppm. Anal. Calc. for
C27H34Br2N4O2: C, 53.48; H, 5.65; N, 9.24. Found: C,
53.50; H, 5.69; N, 9.21%.
303 mg 1,10-bis(4-n-butoxyphenyl)-3,30-methylene-diimi-
dazolium-dibromide (0.5 mmol) and 112 mg palladium(II)
acetate (0.5 mmol) dissolved in 8 mL DMSO were stirred
at 60 ꢁC for 2 h, at 80 ꢁC for 2 h and finally at 110 ꢁC for
2 h. After removal of the solvent, the residue was washed
twice with THF (5 mL) and dichloromethane (5 mL). The
product was obtained as a white solid (251 mg; Yield:
71%), m.p.: 326 ꢁC (decomp.). 1H NMR (500 MHz,
DMSO-d6): d 7.81 (s, 2H, NCH), 7.68 (s, 2H, NCH),
7.64 (d, J = 8.5 Hz, 4H, o-H of Ar), 7.13 (d, J = 8.7 Hz,
4H, m-H of Ar), 6.45 (s, 2H, NCH2N), 4.06 (t,
J = 6.0 Hz, 4H, OCH2), 1.75 (m, 4H, CH2), 1.48 (m, 4H,
CH2), 0.97 (t, J = 7.4 Hz, 6H, CH3) ppm. 13C NMR
(162 MHz, DMSO-d6): d 158.4 (C–OnBu), 144.9 (NCN),
132.4 (i-C of Ar), 126.1 (o-C of Ar), 123.0 (NCH), 122.1
(NCH), 114.4 (m-C of Ar), 67.5 (OCH2), 63.2 (NCH2N),
30.8 (CH2), 18.8 (CH2), 13.8 (CH3) ppm. Anal. Calc. for
C27H32Br2N4O2Pd: C, 45.62; H, 4.54; N, 7.88. Found: C,
45.73; H, 4.56; N, 7.74%.
5.4. Synthesis of 1,10-bis(4-bromophenyl)-3,30-methylene-
diimidazoline-2,20-diylidene-palladium(II)-dibromide (2)
857 mg 1,10-bis(4-bromophenyl)-3,30-methylene-diimi-
dazolium-dibromide (1.38 mmol) and 310 mg palla-
dium(II) acetate (1.38 mmol) were suspended in 10 mL
DMSO. The reaction mixture was heated stepwise to
120 ꢁC during 6 h and stirred for 2 h at this temperature.

1526
M.A. Taige et al. / Journal of Organometallic Chemistry 692 (2007) 1519–1529
Table 4
5.7. General procedure for the Mizoroki–Heck reaction
Crystal data and crystallographic details for compounds 2 and 3
2
3
A 10 mL Schlenk flask equipped with a reflux condenser
was degassed and put under an atmosphere of nitrogen. It
was charged with aryl halides (1 mmol), styrene or methyl-
acrylate (1.4 mmol), anhydrous sodium acetate (1.1 mmol),
diethylene glycol di-n-butyl-ether, the denoted amount of
catalyst and 5 mL of N,N-dimethylacetamide. After that
the flask was placed in a preheated oil bath at 140 ꢁC. Ali-
quots (0.2 mL) were removed from the reaction mixture
after a fixed time period and added to dichloromethane
(10 mL) to observe the reaction progress. The organic por-
tion was washed four times with 5 mL of water and dried
with anhydrous MgSO4. The solution was filtered, diluted
with 5 mL of dichloromethane and analyzed by gas
chromatography.
Measurement
Formula
E. Herdtweck
C19H14Br4N4Pd Æ
2C2H6OS
S. Goutal
C21H20Br2N4O2Pd Æ
C2H6OS
704.77
Colorless/block
Triclinic
Formula weight
Color/shape
Crystal system
Space group
880.64
Colorless/needle
Monoclinic
P21/n (No. 14)
9.6311(1)
21.1983(1)
18.7303(1)
90
95.2463(2)
90
3808.01(5)
4
ꢀ
P1 (No. 2)
9.823(1)
˚
a (A)
˚
b (A)
10.054(1)
14.073(1)
101.29(1)
106.14(1)
96.16(1)
1289.8(2)
2
1.815
˚
c (A)
a (ꢁ)
b (ꢁ)
c (ꢁ)
3
˚
V (A )
Z
qcalc (g/cm3)
1.536
4.816
l (mmꢀ1
)
3.931
5.8. Structure determination of compound 2
Diffractometer
Nonius kappa-CCD
(area diffraction system);
Mo Ka; rotating anode
Nonius kappa-CCD
(area diffraction
system); Mo Ka;
sealed tube
198
0.71073
3.63/25.00
36285
Crystal data and details on the structure determination
are presented in Table 4. Table 5 summarizes selected bond
distances and bond angles. Suitable single crystals for the
X-ray diffraction study were grown by cooling a saturated,
hot solution of 2 in DMSO (see Fig. 1). A clear colorless
needle (0.10 · 0.13 · 0.58 mm) was stored under perfluori-
nated ether, transferred in a Lindemann capillary, fixed
and sealed. Preliminary examination and data collection
were carried out on an area detection system (NONIUS,
kappa-CCD) at the window of a rotating anode (NON-
IUS, FR591) and graphite-monochromated Mo Ka radia-
Temperature (K)
123
˚
k (A)
0.71073
1.45/25.34
88284
hmin/max (ꢁ)
Reflections
integrated
Independent
reflections (all
data)
6963
4534
3766
Observed reflections 6232
[I > 2r(I)]
Parameter refined
R1 (observed/all
data)
wR2 (observed/all
data)
Goodness-of-fit
(observed/all
data)
329
0.0284/0.0321
311
0.0245/0.0378
˚
tion (k = 0.71073 A). The unit cell parameters were
obtained by full-matrix least-squares refinement of 7179
reflections. Data collection were performed at 123 K
0.0713/0.0725
1.086/1.086
0.0458/0.0491
1.003/1.003
(OXFORD CRYOSYSTEMS) within
a h-range of
1.45ꢁ < h < 25.34ꢁ. Nine data sets were measured in rota-
tion scan modus with Du/Dx = 1.0ꢁ. A total number of
88284 intensities were integrated. Raw data were corrected
for Lorentz, polarization, and, arising from the scaling pro-
cedure, for latent decay and absorption effects. After merg-
ing, Rint = 0.046, a sum of 6963 (all data) and 6232
[I > 2r(I)] reflexes remained. The structure was solved by
a combination of direct methods and difference Fourier
syntheses. All non-hydrogen atoms were refined with
anisotropic displacement parameters. Methyl hydrogen
atoms were refined as part of rigid rotating groups, with
Residual electrꢀo3n
ꢀ0.93/1.29
ꢀ0.43/0.70
˚
density (e A
)
formed on an Intel Pentium II PC, with the STRUX-V sys-
tem, including the programs SHELXL-97, SIR92, and PLATON
[105–110]. In addition, two molecules DMSO become
apparent in the final difference Fourier maps but the severe
disorder could not modelled properly. This problem was
solved by using the PLATON calc squeeze procedure.
˚
C–H = 0.98 A and Uiso(H) = 1.5Ueq(C). Other H atoms were
placed in calculated positions and refined using a riding
model, with methylene and aromatic C–H distances of
5.9. Structure determination of compound 3
˚
˚
0.99 A and 0.95 A, respectively, and Uiso(H) = 1.2Ueq(C)
.
Suitable single crystals for the X-ray diffraction study
were grown by cooling a saturated, hot solution of 3 in
DMSO/THF (see Fig. 2). A clear colorless block (0.30 ·
0.12 · 0.10 mm) was stored under perflourinated ether,
transferred on a glass capillary and fixed. Preliminary
examination and data collection were carried out on an
area detecting system (kappa-CCD; Nonius) at the window
of a sealed X-ray tube (Nonius, FR590) and graphite
Full-matrix least-squares refinements with 329 parameters
2
were carried out by minimizing RwðF 2o ꢀ F c2Þ with the
SHELXL-97 [105] weighting scheme and stopped at shift/
err < 0.001. The final residual electron density maps
showed no remarkable features. Neutral atom scattering
factors for all atoms and anomalous dispersion corrections
for the non-hydrogen atoms were taken from International
Tables for Crystallography. All calculations were per-
˚
monochromated Mo Ka radiation (k = 0.71073 A). The

M.A. Taige et al. / Journal of Organometallic Chemistry 692 (2007) 1519–1529
1527
Table 5
Selected bond length (A) and angles (ꢁ) of compound 2 and 3
conts/retrieving.html, or from the Cambridge Crystallo-
graphic Data Centre, 12 Union Road, Cambridge CB2
1EZ, UK; fax: (+44) 1223-336-033; or e-mail: depos-
it@ccdc.cam.ac.uk. Supplementary data associated with
this article can be found, in the online version, at
doi:10.1016/j.jorganchem.2006.11.050.
˚
2
3
Pd–C1
Pd–C5
Pd–Br1
Pd–Br2
C1–N1
C1–N2
C5–N3
C5–N4
C1–Pd–C5
C1–Pd–Br1
C1–Pd–Br2
C5–Pd–Br1
C5–Pd–Br2
Br1–Pd–Br2
N1–C1–N2
N3–C5–N4
C1–N1–C11–C16
C5–N4–C21–C26
1.979(3)
1.971(3)
2.4828(5)
2.4690(4)
1.358(4)
1.343(4)
1.345(4)
1.358(4)
83.42(12)
171.59(9)
92.19(9)
91.04(9)
173.89(9)
92.84(1)
105.0(3)
105.1(3)
ꢀ29.2(5)
40.5(4)
1.983(3)
1.971(3)
2.4811(4)
2.4935(4)
1.350(4)
1.351(4)
1.353(4)
1.355(4)
84.14(13)
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