Synthesis, structural characterization, and catalytic behaviour in Heck coupling of palladium(ii) complexes containing pyrimidine-functionalized N-heterocyclic carbenes

Article abstract of DOI:10.1039/b801264d

[Pd(L1)₂(CH₃CN)](PF₆)₂ (L1 = 1-n-butyl-3-(2-pyrimidyl)imidazolylidene, 3) and [Pd(L2)₂](PF ₆)₂ (L2 = 1-(2-picolyl)-3-(2-pyrimidyl)imidazolylidene 4), prepared via carbene transfe

Full text of DOI:10.1039/b801264d

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PAPER
www.rsc.org/dalton | Dalton Transactions
Synthesis, structural characterization, and catalytic behaviour in Heck
coupling of palladium(II) complexes containing pyrimidine-functionalized
N-heterocyclic carbenes†
Jiansheng Ye,a Wanzhi Chen*a,b and Daqi Wangb
Received 24th January 2008, Accepted 2nd May 2008
First published as an Advance Article on the web 19th June 2008
DOI: 10.1039/b801264d
[Pd(L1)2(CH3CN)](PF6)2 (L1 = 1-n-butyl-3-(2-pyrimidyl)imidazolylidene, 3) and [Pd(L2)2](PF6)2 (L2 =
1-(2-picolyl)-3-(2-pyrimidyl)imidazolylidene 4), prepared via carbene transfer reactions of [Ag(L1)2]PF6
(1) and [Ag2(L2)2](PF6)2 (2) with palladium salts, respectively, have been fully characterized by 1H and
13C NMR spectroscopy and elemental analysis. The X-ray crystal structures of complexes 1–4 are
reported. Complex 3 is an unusual pentacoordinated palladium complex, in which the palladium is
coordinated by two imidazolylidene, two pyridine, and one acetonitrile molecule in a square-pyramidal
geometry. The apical position is occupied by a pyrimidine nitrogen atom with a relatively long Pd–N
˚
distance (2.762(6) A). Complex 4 is a typical square-planar palladium complex with palladium
surrounded by two pairs of cis-arranged pyridine and imidazolylidene ligands. The complexes exhibit
good catalytic activities in the Heck coupling reaction of aryl bromides and activated aryl chlorides
under mild conditions.
Previous reports demonstrate that variation of the N-
Introduction
substituents of NHCs has a limited effect on the electronic
density of the carbenic carbon atom.10 However, variation of
the substituents offers an avenue for tuning the catalytic activity
of metal–NHC complexes by altering the steric bulk of the
NHCs.1b,6 Provided that one or two heteroaryls is incorporated
at the N-positions, the situation would be changed since such
hemilable ligands may exert electronic influence on the metal
through coordination. The labile character of the heteroaryl
group ensures the ease of creation of an unsaturated coordination
site and stabilization of the catalytically active species after
reductive elimination in the catalytic cycle. The employment of
hemilable bidentate NHC ligands would also be a good choice
for the stabilization of the in situ generated active species.11
As an extension of our studies on the chemistry of heteroaryl
functionalized NHC ligands,12 herein we describe the synthesis and
structural characterization of palladium complexes of pyrimidine
functionalized NHC ligands, [Pd(L1)2(CH3CN)](PF6)2 (L1 =
1-n-butyl-3-(2-pyrimidyl)imidazolylidene, 3) and [Pd(L2)2](PF6)2
(L2 = 1-(2-picolyl)-3-(2-pyrimidyl)imidazolylidene 4), which were
prepared via carbene transfer reactions with [Ag(L1)2]PF6 (1) and
[Ag2(L2)2](PF6)2 (2), respectively. Both palladium complexes are
efficient catalysts for the Heck coupling reaction of aryl bromides
and activated aryl chlorides.
The palladium-catalyzed Heck coupling reaction of aryl halides
and olefins has proven to be a powerful method for the preparation
of aryl substituted olefins.1 Although Heck reactions catalyzed by
palladium–phosphine systems have achieved great success over
the last decade, in some cases the phosphine ligands are not easily
available, air-sensitive and quite expensive.2 These disadvantages
of such catalysts set limits to the wide application in practical
synthetic processes. The development of catalyst systems with non-
phosphine ligands3 or ligandless catalysts4 is of practical interest
and thus has been receiving great attention.
Heck coupli◦ng reactions often require high temperature (nor-
mally 110–180 C) to proceed, even with activated aryl bromides.1,2
Only a few catalyst systems can catalyze Heck–Mizoroki reactions
◦
at a temperature below 100 C.3a,5 The high thermal stability of
transition metal complexes of N-heterocyclic carbenes (NHCs)
makes these complexes particularly suitable for Heck coupling
reactions.6 The strong r-donating character of NHCs has proven
to be responsible for the stability and enhanced catalytic activity
of metal–NHC complexes.6,7 The air and moisture stability of such
complexes provides more convenience in handling especially for
industrial application. The first application of Pd–NHC complexes
for the Heck reaction of aryl bromides and activated aryl chlorides
was reported by Herrmann et al. in 1995.8 Since then, a number
of monodentate and bidentate NHC ligands have been shown to
have good activity in Pd-catalyzed Heck reactions.9
Results and discussion
The two imidazolium salts, [HL1]PF6 and [HL2]PF6, could be
easily prepared from the reactions of 2-chloropyrimidine and
N-n-butylimidazole and N-picolylimidazole in refluxing toluene,
and subsequent treatment of the resultant imidazolium chlorides
with NH4PF6. They are isolated as colorless solids. As shown
in Scheme 1, deprontonation of the imidazolium salts with
Ag2O in acetonitrile at room temperature yielded [Ag(L1)2]PF6
aDepartment of Chemistry, Zhejiang University, Xixi Campus, Hangzhou
310028, P. R. China. E-mail: chenwzz@zju.edu.cn; Fax: +86-571-88273314
bDepartment of Chemistry, Liaocheng University, Liaocheng 252059,
P. R. China
† CCDC reference numbers 675961–675964 for 1–4, respectively. For
crystallographic data in CIF or other electronic format see DOI:
10.1039/b801264d
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that the complex in solution may lose one molecule of acetonitrile
and form a normal square-planar complex similar to 4.
Complexes 1–4 have been further characterized by X-ray diffrac-
tion analysis. As shown in Fig. 1, complex 1 is mononuclear. The
central silver is bicoordinated by two imidazolylidenes in a linear
fashion. The Ag–C bond distances are normal for Ag(NHC)2
species.13 The two imidazolylidene rings coordinated to the same
silver atom are nearly coplanar as evidenced by the small dihedral
angle of 4.12◦. The two pyrimidine rings are also approximately
coplanar with their attached imidazolylidene rings with the
dihedral angles of 9.59 and 4.53◦, respectively. Another structural
feature of the compound is that the two n-butyl groups directed
towards the same side of the coordination plane. Usually the two
imidazolylidene rings of silver complexes having linear Ag(NHC)2
conformation are bisected.13 These structural characteristics of
the complex allow close approach of two molecules (Fig 1b). The
intermolecular Ag · · · Ag and Ag · · · C (imidazolylidene) distances
Scheme 1
(1) and [Ag2(L2)2](PF6)2 (2), respectively. Palladium complexes,
[Pd(L1)2(CH3CN)](PF6)2 (3) and [Pd(L2)2](PF6)2 (4), were ob-
tained from the carbene transfer reactions of the silver–NHC
complexes and Pd(CH3CN)2Cl2. These complexes have been
characterized by NMR spectroscopy and elemental analyses. The
formation of the Ag– and Pd–NHC complexes was confirmed
1
by the absence of the H NMR resonance for the acidic NCHN
protons at 10.2 ppm, where the resonance signals of [HL1]PF6
and [HL2]PF6 were found. In the 13C NMR spectra of the silver
complexes, the resonances ascribed to the carbenic carbon atoms
appeared at 182.5 and 180.2 ppm, respectively, which fall in
the range of 13C chemical shifts for Ag–C complexes.13 For the
two palladium complexes, the resonance signals assignable to the
carbenic carbons were observed at 171.5 and 159.7 ppm for 3
and 4, respectively. The 13C chemical shifts of known Pd–NHC
complexes appear in the range of 182.4–149.5 ppm depending
upon the ancillary ligands,14
Fig. 1 Perspective view of (a) the molecular structure of [Ag(L1)2]2+
Thermal ellipsoids are drawn at 30% probability level. (b) The weak inter-
.
Although the pentacoordinate complex contains two inequiv-
alent 1-n-butyl-3-(2-pyrimidyl)imidazolylidene ligands in its solid
state (see Fig. 3), its 1H and 13C NMR spectra show only one set
of resonance signals assigned to the ligand. This result indicates
˚
molecular Ag–p and Ag–Ag interaction. Selected bond distances [A] and
angles [◦]: Ag(1)–C(1) 2.097(3), Ag(1)–C(12) 2.104(4), C(1)–Ag(1)–C(12)
175.77(13).
4016 | Dalton Trans., 2008, 4015–4022
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˚
˚
are 3.275 A and 3.509 A, indicating somewhat weak Ag–p and Ag–
Ag interactions. Both intermolecular and intramolecular Ag–Ag
contacts are often observed for silver–NHC complexes.15 The Ag–
p interaction is also seen in a pyrazole-functionalized NHC silver
complex.16
The molecular structure of 2 is shown in Fig. 2. Two
silver atoms are bound together by two 1-(2-picolyl)-3-(2-
pyrimidyl)imidazolylidene ligands forming a metallacyle. Similar
to 1, the pyrimidine groups are not coordinated, and thus 1-(2-
picolyl)-3-(2-pyrimidyl)imidazolylidene acts as a bidentate ligand
rather than as the expected tridentate pincer ligand. Each silver
ion is bicoordinated by a pyridine and a imidazolylidene group.
The Ag–C and Ag–N bond distances are consistent with those of
silver complexes having C–Ag–N moieties. The silver atoms are
˚
closely contacted with a Ag(1)–Ag(2) distance of 3.131(1) A, which
shows a very weak Ag–Ag interaction. Similar disilver complexes
containing bis(NHC) ligands have been reported previously.17
Fig.
3
Perspective view of the molecular structure of
[Pd(L1)2(CH3CN)]2+. Thermal ellipsoids are drawn at 30% probability
◦
˚
level. Selected bond distances [A] and angles [ ]: Pd(1)–C(1) 1.987(7),
Pd(1)–C(12) 1.987(7), Pd(1)–N(8) 2.051(6), Pd(1)–N(1) 2.074(5),
Pd(1)–N(4) 2.762(6), C(1)–Pd(1)–C(12) 98.0(3), C(1)–Pd(1)–N(8)
171.0(3), C(12)–Pd(1)–N(8) 87.1(2), C(1)–Pd(1)–N(1) 80.6(3), C(12)–
Pd(1)–N(1) 177.6(2), N(8)–Pd(1)–N(1) 94.6(2), C(1)–Pd(1)–N(4) 100.3(2),
C(12)–Pd(1)–N(4) 69.5(2), N(8)–Pd(1)–N(4) 88.5(2), N(1)–Pd(1)-N(4)
108.7(2).
Pd(II) complexes are uncommon. A few such complexes with
distorted trigonal bipyramidal or square pyramidal environments
supported by phenanthroline19 or polyphosphine20 ligands are
known. The Pd(II) complexes of N-heterocyclic carbenes are
usually tetracoordinated displaying square-planar geometry.13,21
Complex 3 represents the first example of pentacoordinated
palladium(II) complex containing NHC ligands.
Complex 4 is a typical square-planar palladium complex with
palladium surrounded by two pyridine and two imidazolylidene
ligands. A structural drawing of the cation is provided in Fig. 4.
Fig. 2 Perspective view of the molecular structure of [Ag2(L2)2]2+
.
Thermal ellipsoids are drawn at 30% probability level. Selected bond
◦
˚
distances [A] and angles [ ]: Ag(1)–Ag(2) 3.131(1), Ag(1)–C(1) 2.108(6),
Ag(1)–N(10) 2.187(5), Ag(2)–C(14) 2.100(6), Ag(2)–N(5) 2.149(5),
C(1)–Ag(1)–N(10) 175.2(2), C(14)–Ag(2)–N(5) 178.0(2).
Complex 3 is an unusual pentacoordinated palladium complex,
in which the palladium is surrounded by two imidazolylidene, two
pyridine, and one acetonitrile molecule in a square-pyramidal ge-
ometry. The structure of 3 is shown in Fig. 3. Both two 1-n-butyl-3-
(2-pyrimidyl)imidazolylidene ligands are coordinated in bidentate
fashion. The square plane is formed by atoms C(1) and N(1) of
one 1-n-butyl-3-(2-pyrimidyl)imidazolylidene molecule, the C(12)
atom of the second 1-n-butyl-3-(2-pyrimidyl)imidazolylidene lig-
and, and the N(8) atom of the acetonitrile molecule; the
apical position is occupied by atom N(4) of the second 1-n-
butyl-3-(2-pyrimidyl)imidazolylidene ligand. The Pd–C and Pd–
N(equatorial) bond distances do not show distinct differences
from the known typical Pd–NHC complexes.13 The crystal struc-
˚
ture indicates a long range interaction (2.762(6) A) between the
planar palladium and the apical nitrogen atom. The presence of
the five-coordinate species is suggested assuming a weak apical
Fig. 4 Perspective view of the molecular structure of [Pd(L2)2]2+
.
Thermal ellipsoids are drawn at 30% probability level. Selected
◦
Pd–N interaction on the basis of Pd–N bond distances in the
˚
bond distances [A] and angles [ ]: Pd(1)–C(1) 1.963(6), Pd(1)–N(3)
18–20
˚
range 2.576(4)–2.805(5) A.
2.088(5), C(1)#1–Pd(1)–C(1) 92.7(4), C(1)–Pd(1)–N(3)#1 179.2(2),
C(1)–Pd(1)–N(3) 86.5(2), N(3)#1–Pd(1)–N(3) 94.3(3). Symmetry trans-
formations used to generate equivalent atoms: #1 −x + 1, y, −z + 3/2.
The coordination and organometallic compounds of Pd(II) tend
to adopt square-planar coordination geometry. Pentacoordinate
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Table 1 Heck coupling of aryl halides with olefinsa
Entry
Aryl halide
Olefin
Cat. 3 (mol%)
Additive
T/◦C
Time/h
GC yield (isolated yield) (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
4-Bromoacetophenone
4-Bromoacetophenone
4-Bromoacetophenone
4-Bromoacetophenone
4-Bromobenzaldehyde
4-Bromobenzaldehyde
4-Bromobenzaldehyde
4-Bromobenzaldehyde
Bromobenzene
Bromobenzene
Bromobenzene
4-Bromotoluene
4-Bromotoluene
4-Bromotoluene
4-Bromotoluene
4-Bromotoluene
2-Bromotoluene
2-Bromotoluene
4-Bromoanisole
4-Bromoanisole
4-Chloroacetophenone
4-Chloroacetophenone
n-butyl acrylate
n-butyl acrylate
styrene
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.1
0.1
0.1
1
1
1
1
1
1
1
1
1
none
none
none
none
none
none
none
none
none
none
none
none
120
80
120
80
120
80
120
80
120
120
120
120
120
120
140
140
140
140
140
140
140
140
1
6
1
6
1
6
1
6
1
100 (95)
100
100 (98)
100
100 (93)
100
100 (91)
100
styrene
n-butyl acrylate
n-butyl acrylate
styrene
styrene
n-butyl acrylate
n-butyl acrylate
styrene
n-butyl acrylate
n-butyl acrylate
n-butyl acrylate
n-butyl acrylate
styrene
n-butyl acrylate
styrene
n-butyl acrylate
styrene
64
6
6
6
100 (95)
100 (95)
17.5
58
90
none
20
20
20
20
20
20
20
20
20
20
n-Bu4NBr
n-Bu4NBr
n-Bu4NBr
n-Bu4NBr
n-Bu4NBr
n-Bu4NBr
n-Bu4NBr
n-Bu4NBr
n-Bu4NBr
100 (90)
96 (90)/4b
80 (78)
85 (80)
100 (97)
90 (89)/10b
100
n-butyl acrylate
styrene
1
88/12b
a Reaction conditions: aryl halide (1.0 mmol), olefin (1.5 mmol), catalyst 3, NaOAc (2.0 mmol), DMAc (3 mL). b Yield of cis- product.
Table 2 Heck coupling of aryl halides with olefinsa
Entry
Aryl halide
Olefin
Cat. 4 (mol%)
Additive
T/◦C
Time/h
Yieldb (%)
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
4-Bromoacetophenone
4-Bromoacetophenone
4-Bromoacetophenone
4-Bromoacetophenone
4-Bromobenzaldehyde
4-Bromobenzaldehyde
4-Bromobenzaldehyde
4-Bromobenzaldehyde
Bromobenzene
Bromobenzene
Bromobenzene
4-Bromotoluene
4-Bromotoluene
4-Bromotoluene
2-Bromotoluene
2-Bromotoluene
4-Bromoanisole
4-Bromoanisole
4-Chloroacetophenone
4-Chloroacetophenone
n-butyl acrylate
n-butyl acrylate
styrene
0.01
(0.01)
0.01
0.01
0.01
0.01
0.01
0.01
0.01
1
1
1
1
1
1
1
1
1
1
1
none
none
none
none
none
none
none
none
120
80
120
80
120
80
120
80
120
120
120
120
140
140
140
140
140
140
140
140
2
20
2
20
2
20
2
20
20
20
20
20
20
20
20
20
20
20
20
20
100
100
100
100
100
100
100
100
36
100
100
90
100
92/8c
69
styrene
n-butyl acrylate
n-butyl acrylate
styrene
styrene
n-butyl acrylate
n-butyl acrylate
styrene
n-butyl acrylate
n-butyl acrylate
styrene
n-butyl acrylate
styrene
n-butyl acrylate
styrene
none
n-Bu4NBr
n-Bu4NBr
n-Bu4NBr
n-Bu4NBr
n-Bu4NBr
n-Bu4NBr
n-Bu4NBr
n-Bu4NBr
n-Bu4NBr
n-Bu4NBr
n-Bu4NBr
80
85
75/11c
87
n-butyl acrylate
styrene
83/6c
a Reaction conditions: aryl halide (1.0 mmol), olefin (1.5 mmol), catalyst 4, NaOAc (2.0 mmol), DMAc (3 mL). b GC yield. c Yield of cis- product.
Two pyrimidine groups are not coordinated. Both the two pyridine
and two imidazolylidene ligands are cis-positioned. The Pd–
C and Pd–N distances are not unusual, and are similar to
those of known palladium complexes.14 The imidazolylidene ring
and its trans-positioned pyridine ring are almost coplanar as
indicated by the small dihedral angle of 3.35◦ between the two
rings. The C–Pd–C angle (92.7(4)◦) is slightly smaller than that
of N–Pd–N (94.3(3)◦) due to the geometric constraint of the
ligand.
Catalytic activities for Heck coupling reaction
The palladium-catalyzed Heck coupling reaction has been widely
employed for the preparation of aryl olefins. The Heck coupling
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reactions of aryl bromides with n-butyl acrylate and styrene were
examined by using 3 and 4 as the catalyst precursors. The results
were summarized in Tables 1 and 2. Under the conditions often
used for other palladium catalysts, i.e. using NaOAc as the base
and dimethylacetamide (DMAc) as the solvent,22 both 3 and
4 exhibit excellent catalytic activities for aryl bromides bearing
electron-withdrawing substituents.
For the coupling reactions of activated aryl bromides, complex
4 showed higher catalytic activities than similar complexes which
have both chelating hemilabile ligands.28 Without any additives,
at 80 ◦C and 0.01 mol% catalyst loading, both styrene and n-
butyl acrylate could be coupled in 100% conversion within 20 h
(entries 24, 26, 28, and 30). Raising the temperature to 120 ◦C,
the reaction time could be shortened to 2 hours to reach 100%
conversion. Complex 4 also exhibits high activity towards various
aryl bromides with moderate to excellent yields (entries 31–40)
with catalyst loading of 1 mol%. The catalyst also works for
activated chlorides and afforded the coupled products in good
yields (entries 41 and 42). Unfortunately, the catalyst shows
no catalytic activity for aryl chlorides bearing electron-donating
substituents.
In summary, we have described the synthesis and structural
characterization of two palladium complexes containing pyrim-
idine functionalized N-heterocyclic carbenes. Complex 3 has
an unusual square-pyramidal structure, which represents the
first example of penta-coordinated Pd–NHC complex. These
palladium complexes show high catalytic activity for the Heck
olefination of aryl bromides without the need of additional ligands
under mild conditions. The methodology is applicable to a variety
of electron-deficient aryl bromides and chlorides and electron-rich
bromides. Good to excellent yields were obtained.
Generally, the Heck coupling reaction requires reaction tem-
◦
peratures higher than 110 C.1 As can be seen from Table 1,
our catalysts are so active that allows the olefination reactions of
activated bromides bearing acetyl or formyl groups with n-butyl
acrylate or styrene proceed at 80 ◦C by using only 0.01 mol% of 3
or 4. These reactions could be completed within 6 hours with 100%
conversion (entries 2, 4, 6, and 8). Raising the reaction temperature
to 120 ◦C, these reactions could achieve 100% conversion even
within 1 hour at the same catalyst loadings (entries 1, 3, 5, and
7). The reactions of styrene and n-butyl acrylate did not show
distinctly different reactivities (entries 1–8). The corresponding
coupled products could be isolated in 91–98% yields. In the
case of n-butyl acrylate, only the trans isomer was detected,
whereas for the coupling reactions of styrene cis olefins were also
observed as minor products in not more than 15% yields. For the
activated bromide substrates, the palladium–NHC catalysts are
even more effective than most of known palladium catalysts such
as phospha-palladacycle23 and Pd–phosphine–imidazolium salt
systems.24
Experimental section
Complex 3 has also been applied to the cross coupling of
electron-neutral and electron-rich bromides. As expected, the
reactivities of these two bromides are comparati◦vely lower. For
instance, at 0.01 mol% catalyst loading and 120 C the reaction
only gave 64% conversion of phenyl bromide within 1 hour. To
reach complete conversion, the coupling reaction requires higher
catalyst loading (0.1 mol%) and long reaction time (entries 9–11).
When deactivated p-tolyl bromide was used as the substrate, only
58% of the bromide could be coupled to the product even using
1 mol% of 3 at 120 ◦C within 20 hours (entry 13). To ensure
the reactions go to completion, a further increase of the reaction
General procedures
All the chemicals were obtained from commercial suppliers
and used without further purification. N-Picolylimidazole29 was
prepared according to the known procedure. The C, H, and N
elemental analyses were carried out with a Carlo Erba 1106
elemental analyzer. H and 13C NMR spectra were recorded on
Bruker Avance-400 (400 MHz) spectrometer. Chemical shifts (d)
are expressed in ppm downfield to TMS at d = 0 ppm and coupling
constants (J) are expressed in Hz.
1
◦
temperature to 140 C and addition of n-Bu4NBr as an additive
Synthesis
are needed. It has been known that addition of n-Bu4NBr as a
cocatalyst enhances the activity of many catalytic systems,22d,25
and n-Bu4NBr is able to activate and stabilize the in situ generated
palladium(0) species, which is believed to be the real catalytically
active species. Actually, the reaction yields could be improved by
adding 20 mol% of n-Bu4NBr (entries 14–16). As expected, o-tolyl
bromide displays much lower reactivity than other bromides due to
steric hindrance (entries 17 and 18). The catalyst is also efficient for
the coupling of deactivated p-bromoanisole with n-butyl acrylate
[HL1](PF6).
A solution of 2-chloropyrimidine (3.0 g,
26.2 mmol) in toluene (15 mL) was added to N-butylimidazole
(3.9 g, 31.4 mmol). The mixture was refluxed overnight to yield
a brown solid. The solid was filtered and washed with 10 mL
of diethyl ether and dried. The solid was dissolved in 10 mL
of water and slowly added to an aqueous solution of NH4PF6
in a 1 : 1.1 molar ratio. The mixture was stirred for 30 min at
room temperature and the resulting precipitate was separated by
filtration and dried under vacuum. Yield: 9.12 g, 75%. Anal. Calcd
for C11H15F6N4P: C, 37.94; H, 4.34; N, 16.09. Found: C, 38.07;
◦
and styrene at the elevated temperature (140 C, entries 19–20),
and the corresponding coupling products were obtained in good
yields.
1
H, 4.35; N, 16.40%. H NMR (400 MHz, DMSO-d6): 10.2 (s,
Attempts had also been made to test the reactivity of aryl
chlorides which are cheaper and more easily available starting
materials. It is noteworthy that the catalyst works well for activated
aryl chloride using 1 mol% palladium at 140 ◦C (21 and 22) giving
quantitative yields. In the case of styrene, a trans- : cis- ratio
of 88 : 12 was observed. Unfortunately, the catalyst is totally
ineffective for deactivated aryl chlorides. Actually, only a few
catalyst systems show good catalyst activity for the olefination
of aryl chlorides.9a,26,27
NCHN, 1H), 9.04 (d, m-C4H3N2, J = 5.2, 2H), 8.48, 8.03 (both s,
NCHCHN, each 1H), 7.75 (t, p-C4H3N2, J = 4.8, 1H), 4.33 (t,
CH2CH2CH2CH3, J = 7.2, 2H), 1.87 (m, CH2CH2CH2CH3, 2H),
1.31 (m, CH2CH2CH2CH3, 2H), 0.93 (t, CH2CH2CH2CH3, J =
5.6, 3H).
[HL2](PF6). The compound was prepared according to
the same procedure as for [HL1](PF6) starting from N-
pyridylmethylimidazole and 2-chloropyrimidine. Yield: 72%.
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Anal. Calcd for C13H12F6N5P: C, 40.74; H, 3.16; N, 18.27. Found:
C, 40.95; H, 3.35; N, 18.40%. H NMR (400 MHz, DMSO-d6):
through Celite and the filtrate was concentrated to 5 mL. Addition
of 20 mL diethyl ether gave a pale yellow solid. Yield: 125 mg,
74%. Anal. Calcd for C24H31F12N9P2Pd: C, 34.24; H, 3.71; N,
14.97. Found: C, 34.43; H, 3.90; N, 15.13%. 1H NMR (400 MHz,
DMSO-d6): 9.09 (d, m-C4H3N2, J = 4.2, 4H), 8.44, 7.91 (both d,
NCHCHN, J = 2.0, each 2H), 7.81 (t, p-C4H3N2, J = 4.8, 2H),
3.99, 3.59 (both d, CH2CH2CH2CH3, each 2H), 2.07 (s, CH3CN,
3H), 1.59, 1.39 (both m, CH2CH2CH2CH3, each 2H), 1.11 (m,
CH2CH2CH2CH3, 4H), 0.77 (t, CH2CH2CH2CH3, J = 7.2, 6H).
13C NMR (100 MHz, DMSO-d6): 171.5 (Pd–C), 160.0, 155.2,
154.6, 125.3, 121.6, 120.7, 51.4, 32.6, 19.5, 13.8, 1.5.
1
10.3 (s, NCHN, 1H), 9.06 (d, m-C4H3N2, J = 4.8, 2H), 8.56 (d,
m-C5H4N, J = 4.4, 1H), 8.51, 8.03 (both s, NCHCHN, each 1H),
7.9 (t, m-C5H4N, J = 7.6, 1H), 7.78 (t, p-C4H3N2, J = 4.8, 1H),
7.55 (d, o-C5H4N, J = 7.2, 1H), 7.41 (t, p-C5H4N, J = 6.0, 1H),
5.73 (s, CH2, 2H).
[Ag(L1)2](PF6) (1). To a slurry of Ag2O (24 mg, 0.1 mmol) in
10 mL of CH3CN was added [HL1](PF6) (69.6 mg, 0.2 mmol).
The mixture was protected from light and stirred at ambient
temperature until the Ag2O was dissolved. The solution was
filtered through Celite to remove a small amount of precipitate.
The clear filtrate was then evaporated to dryness. The residue
was washed with diethyl ether and dried. Suitable crystals for
X-ray diffraction analysis were obtained by slow diffusion of
diethyl ether to its CH3CN solution. Yield: 56 mg, 85%. Anal.
Calcd for C22H28AgF6N8P: C, 40.20; H, 4.29; N, 17.05. Found:
[Pd(L2)2](PF6)2·CH3CN (4). The compound was prepared
according to the same procedure as for 3 starting from silver
carbene 2 and Pd(CH3CN)2Cl2. Yield: 68%. Anal. Calcd for
C28H25F12N11P2Pd: C, 36.88; H, 2.76; N, 16.90. Found: C, 37.01;
1
H, 2.77; N, 17.34%. H NMR (400 MHz, DMSO-d6): 8.83 (d,
1
m-C4H3N2, J = 4.8, 2H), 8.50 (d, m-C5H4N, J = 5.6, 1H), 8.28 (t,
m-C5H4N, J = 7.6, 1H), 8.05 (d, o-C5H4N, J = 7.6, 1H), 7.69, 7.64
(both d, NCHCHN, J = 1.6, each 1H), 7.67 (t, p-C5H4N, J = 6.0,
1H), 7.54 (t, p-C4H3N2, J = 4.8, 1H), 6.21, 5.97 (both d, CH2, J =
14.8, each 1H). 13C NMR (100 MHz, DMSO-d6): 159.7 (Pd–C),
159.3, 154.2, 153.6, 153.1, 142.1, 126.6, 126.4, 124.1, 121.4, 121.1,
118.5, 55.7, 1.5.
C, 40.43; H, 4.35; N, 17.19%. H NMR (400 MHz, DMSO-d6):
8.82 (d, m-C4H3N2, J = 4.8, 4H), 8.29, 7.82 (both d, NCHCHN,
J = 1.6, each 2H), 7.60 (t, p-C4H3N2, J = 5.2, 2H), 4.30 (t,
CH2CH2CH2CH3, J = 7.2, 4H), 1.87 (m, CH2CH2CH2CH3, 4H),
1.29 (m, CH2CH2CH2CH3, 4H), 0.87 (t, CH2CH2CH2CH3, J =
7.2, 6H). 13C NMR (100 MHz, DMSO-d6): 182.5 (Ag–C), 159.7,
155.5, 123.7, 121.5, 119.9, 118.4, 52.3, 32.9, 19.4, 13.7, 1.4.
[Ag2(L2)2](PF6)2·CH3CN (2). The compound was prepared
according to the same procedure as for 1 starting from [HL2](PF6)
and Ag2O. Yield: 72%. Anal. Calcd for C28H25Ag2F12N11P2: C,
32.93; H, 2.47; N, 15.09. Found: C, 32.95; H, 2.63; N, 15.24%. 1H
NMR (400 MHz, DMSO-d6): 8.68 (d, m-C4H3N2, J = 4.4, 4H),
8.61 (d, m-C5H4N, J = 4.0, 2H), 8.28, 7.80 (both d, NCHCHN, J =
1.6, each 2H), 7.97 (t, m-C5H4N, J = 7.6, 2H), 7.64 (d, o-C5H4N,
J = 7.6, 2H), 7.53 (t, p-C4H3N2, J = 4.8, 2H), 7.47 (t, p-C5H4N,
J = 6.0, 2H), 5.70 (s, CH2, 4H), 2.07 (s, CH3CN, 3H). 13C NMR
(100 MHz, DMSO-d6): 180.2 (Ag–C), 159.5, 154.7, 154.3, 152.1,
140.4, 126.3, 125.4, 124.1, 121.4, 121.2, 118.4, 57.8, 1.4.
X-Ray structural determination
Single-crystal X-ray diffraction data were collected at 298(2)
K on a Siemens Smart/CCD area-detector diffractometer with
˚
a MoKa radiation (k = 0.71073 A) by using an x–2h scan
mode. Unit-cell dimensions were obtained with least-squares
refinement. Data collection and reduction were performed using
the SMART and SAINT software.30 The structures were solved
by direct methods, and the non-hydrogen atoms were subjected
to anisotropic refinement by full-matrix least squares on F2 using
SHELXTXL package.31 Hydrogen atom positions for all of the
structures were calculated and allowed to ride on their respective
[Pd(L1)2(CH3CN)](PF6)2 (3). A sample of Pd(CH3CN)2Cl2
(52 mg, 0.2 mmol) was added to acetonitrile (10 mL) and silver
carbene 1 (137 mg, 0.2 mmol) was added. The solution was stirred
for 10 h at ambient temperature. The reaction mixture was filtered
˚
C atoms with C–H distances of 0.93–0.97 A and Uiso(H) =
−1.2–1.5Ueq(C). Further details of the structural analyses are
summarized in Table 3.
Table 3 Summary of X-ray crystallographic data for complexes 1–4
1
2
3
4
Formula
M
C22H28AgF6N8P
657.36
monoclinic
C2/c
11.3286(2)
19.3969(4)
25.0274(5)
101.8260(10)
5382.78(18)
8
1.622
30316
4752, 0.0180
1.073
0.0375, 0.1070
0.0414, 0.1126
C28H25Ag2F12N11P2
1021.27
monoclinic
P21/n
14.645(2)
18.615(2)
15.032(2)
112.860(2)
3776.0(9)
4
1.796
18562
6660, 0.0326
1.066
0.0433, 0.1024
0.0786, 0.1302
C24H31F12N9P2Pd
841.94
monoclinic
P21/c
C30H28F12N12P2Pd
952.98
monoclinic
C2/c
20.944(2)
12.853(1)
14.223(2)
96.152(2)
3806.7(7)
4
1.663
9235
3352, 0.0453
1.135
0.0615, 0.1809
0.0884, 0.2169
Crystal system
Space group
˚
a/A
9.467(1)
˚
b/A
24.486(2)
14.774(2)
101.008(2)
3361.7(6)
4
1.664
16800
5928, 0.0359
1.085
0.0615, 0.1547
0.0895, 0.1787
˚
c/A
b/deg
3
˚
V/A
Z
D
calc/Mg m−3
Reflections collected
Reflections unique, Rint
Goodness-of-fit on F2
R (I > 2rI)
R (all data)
4020 | Dalton Trans., 2008, 4015–4022
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General procedure for the Heck reactions
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A Schlenk tube was charged with aryl halide (1.0 mmol), olefin
(1.5 mmol), NaOAc (2.0 mmol), an appropriate amount of Pd
catalyst with or without 20 mol% of n-Bu4NBr, and DMAc (3 mL).
The solution was stirred at an appropriate temperature under
nitrogen. After several hours, the mixture was then allowed to
cool to room temperature and added to 20 mL of water. The
product was extracted with CH2Cl2 (3 × 10 mL). The combined
organic layer was dried with anhydrous MgSO4 and concentrated
in vacuo. The residue was purified by chromatography on silica gel
using petroleum ether as eluent to give the desired product. The
detailed reaction conditions were given in Table 1.
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Acknowledgements
We are grateful to the support of the NSF of China (20572096),
NSF of Zhejiang Province (R405066) and Qianjiang Project
(2007R10006) for financial support.
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4022 | Dalton Trans., 2008, 4015–4022
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The Royal Society of Chemistry 2008
©