Synthesis and characterization of bidentate NHC-Pd complexes and their role in amination reactions

Article abstract of DOI:10.1016/j.poly.2010.10.015

The new well-defined and air-stable ortho-xylyl-linked N-heterocyclic carbene (NHC) Pd complexes (2a-d) have been synthesized and characterized by elemental analysis, ¹H NMR, ¹³C NMR, IR spectroscopy, and single crystal X-ray diffraction studies. The palladium atom in the complex 2a lies on a crystallographic mirror plane and can be described as having a square-planar coordination environment with the carbene atoms of the benzimidazole rings of the ligand occupying two coordination sites in cis positions. Two further coordination sites are occupied by chloride ligands. The benzimidazole rings are connected to each other by an ortho-xylyl bridge. The catalytic activity of these palladium complexes has been tested in the coupling reactions of various N-containing substrates with bromobenzene. A preliminary catalytic study shows that the bis(NHC)-Pd complexes are highly active in the Buchwald-Hartwig amination reaction.

Full text of DOI:10.1016/j.poly.2010.10.015

Polyhedron 30 (2011) 195–200
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Synthesis and characterization of bidentate NHC–Pd complexes and their role
in amination reactions
a
a,
⇑
b
c
d
_
Serpil Demir , Ismail Özdemir , Bekir Çetinkaya , Hakan Arslan , Don VanDerveer
a
b
c
_
Department of Chemistry, Faculty of Science and Art, Inönü University, Malatya, TR 44280, Turkey
_
Department of Chemistry, Faculty of Science, Ege University, Bornova-Izmir, TR 35100, Turkey
Department of Chemistry, Faculty of Arts and Science, Mersin University, Mersin, TR 33343, Turkey
Department of Chemistry, Clemson University, Clemson, SC 29634, USA
d
a r t i c l e i n f o
a b s t r a c t
Article history:
The new well-defined and air-stable ortho-xylyl-linked N-heterocyclic carbene (NHC) Pd complexes
Received 10 May 2010
Accepted 12 October 2010
Available online 4 November 2010
1
13
(2a–d) have been synthesized and characterized by elemental analysis, H NMR, C NMR, IR spectros-
copy, and single crystal X-ray diffraction studies. The palladium atom in the complex 2a lies on a crystal-
lographic mirror plane and can be described as having a square-planar coordination environment with
the carbene atoms of the benzimidazole rings of the ligand occupying two coordination sites in cis
positions. Two further coordination sites are occupied by chloride ligands. The benzimidazole rings are
connected to each other by an ortho-xylyl bridge. The catalytic activity of these palladium complexes
has been tested in the coupling reactions of various N-containing substrates with bromobenzene. A pre-
liminary catalytic study shows that the bis(NHC)–Pd complexes are highly active in the Buchwald–
Hartwig amination reaction.
Keywords:
Chelating N-heterocyclic carbene
Palladium complexes
C–N bond formation
Catalysis
Amination
Ó 2010 Elsevier Ltd. All rights reserved.
1
. Introduction
ligands containing NHCs have found widespread applications
[
12]. It is expected that when the NHC orientation is perturbed
Since Arduengo’s report on stable imidazol-2-ylidenes [1], the
in space, the catalytic activity of the linked metal site should be
greatly modified. This influence leads to the design of new chelat-
ing NHC complexes. Examples of bis-NHC carbenes [13], mixed
pyridine-carbene [14] and oxazoline-carbene [15] complexes have
already been reported. Theoretical calculations indicated that cer-
tain bidentate chelating ligands could favor the oxidative-addition
process [16] and improve the stability of the catalyst, which may
exclude the side pathway of b-H elimination.
first use by Herrmann and co-workers of N-heterocyclic carbene
complexes in catalysis [2] and the preparation of the Grubbs’ sec-
ond generation catalyst and related catalysts [3], there has been a
growing interest in the use of these NHC ligands as supporting
ligands in both organometallic chemistry and homogeneous catal-
ysis [4]. The NHCs have gained great popularity as they are, in most
cases, better
r-donors than phosphines and in general possess
higher thermal stability, and the use of NHC as a supporting ligand
usually leads to better complex stability [5]. This is especially true
in the area of palladium-mediated cross-coupling reactions.
NHC-containing Pd-catalysts [6] have proven to be an excellent
alternative to catalytic systems involving tertiary phosphines [7].
Cross-coupling reactions catalyzed by palladium species are
increasingly important for the formation of C–Y bonds (Y = C, O
and N) [8]. During the last decade, efficient catalytic C–N bond
forming methods have been developed, for example palladium
catalyzed amination of aryl halides [9], hydroamination [10] and
hydroaminomethylation of olefins or alkynes [11]. Bidentate
We are interested in bidentate NHC ligands based on ortho-
xylyl-linked azolium systems [17]. In this article, some of the
benzimidazolium salts that are precursors to these NHC ligands
are readily synthesized [17d] and the subsequent formation of
mononuclear complexes of Pd(II) has proved facile. The reaction
2
of the NHC precursors with Pd(OAc) in DMSO afforded the
bis(NHC)–Pd complexes. All the synthesized compounds were
1
13
characterized by H NMR, C NMR, IR and elemental analysis tech-
niques, which support the proposed structures. The molecular and
0
crystal structure of the cis-dichloro{1,1 -di[2,3,4,5,6-pentameth-
0
0
ylbenzyl]-3,3 -o-xylyldibenzimidazol-2,2 -diylidene} palladium(II)
complex was determined by the single crystal X-ray diffraction
technique. In addition, we examined the activity of the synthesized
palladium complexes in the Buchwald–Hartwig amination of vari-
ous N-containing substrates with bromobenzene in this study.
⇑
Corresponding author.
E-mail address: iozdemir@inonu.edu.tr (I. Özdemir).
_
0
277-5387/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2010.10.015

1
96
S. Demir et al. / Polyhedron 30 (2011) 195–200
2
. Results and discussion
2.2. X-ray diffraction studies
2
.1. Synthesis and characterization of bis(NHC)–Pd complexes
A crystal of 2a suitable for X-ray single crystal diffraction anal-
ysis was obtained from a chloroform solution layered with diethyl
ether. The crystal and molecular structure of complex 2a has been
confirmed by X-ray single crystal diffractometry. The molecular
structure of complex 2a, with the atom-numbering scheme, is
shown in Fig. 1 [19].
The complex crystallizes in the orthorhombic, space group Cmca
(Z = 8). The palladium atom in the title compound lies on a crystal-
lographic mirror plane and is coordinated by the two carbene
atoms of the benzimidazole rings and two chlorine atoms. The
structure of 2a shows a minor distortion from an ideal square
planar geometry (Fig. 1). The palladium atom is displaced slightly
at a distance of 0.040(1) Å from the coordination plane defined
by the atoms Cl1, Cl1A, C1, and C1A.
In the compound, the two benzimidazole units are linked by an
ortho-xylyl bridge. So, the formation of a 9-membered chelate ring
imposes distortion around the palladium atom. Both carbene
atoms and chlorine atoms are cis to each other, with C1–Pd1–
C1A and Cl1–Pd1–Cl1A angles of 94.7(4) and 91.46(9)°, respec-
tively. The Cl1–Pd1–Cl1A and Ccarben–Pd–Ccarben angles are larger
than the expected value of 90°. This distorted square planar geom-
etry is typical of this class of compounds with a cis substitution
pattern around the central palladium atom [20].
The NHC precursors were prepared by established procedures
[
17d]. The reaction of ortho-xylyl-linked bis(benzimidazolium)
chloride with Pd(OAc) in DMSO, according to Scheme 1, led to
the formation of the desired mononuclear complexes of Pd(II) in
8–82% yield. The crude product was recrystallized from a dichlo-
2
6
romethane:diethyl ether mixture (1:2) at room temperature,
which afforded the corresponding crystals. The new complexes
were characterized by H NMR, C NMR, IR and elemental analysis
techniques, which support the proposed structures.
1
13
The air and moisture-stable palladium–carbene complexes
(2a–d) were soluble in halogenated solvents and insoluble in
non-polar solvents. The palladium complexes exhibit a characteris-
tic
Pd–NHC complexes was confirmed by the absence of the H NMR
resonance signal of the acidic benzimidazolium C2–H. The
NMR spectra of the bis(NHC)–Pd complexes exhibit a resonance
signal in the 173.5–175.7 ppm range ascribed to the carbenic car-
bon atom, which is consistent with reported values for Pd–NHC
complexes. The C chemical shifts of known Pd–NHC complexes
appear in the range 182.4–149.5 ppm, depending upon the ancil-
lary ligands [18].
ꢀ1
m
(NCN) band, typically at 1394–1413 cm . The formation of the
1
1
3
C
1
3
N
N
N
N
Pd
Cl Cl
2a
N
N
Pd
N
Cl Cl
N
N + N
Cl-
R
R
2b
2
DMSO/Pd(OAc)2
N
N
+
1
N
N
Pd
N
Cl Cl
N
2c
N
N
N
Pd
Cl Cl
N
N
N
2d
Scheme 1. Synthesis of o-xylyl linked palladium(II) N-heterocyclic carbene complexes.

S. Demir et al. / Polyhedron 30 (2011) 195–200
197
2.3. Catalytic applications of Pd(NHC) complexes (2a–2d) in
amination reactions
Among cross-coupling reactions, N-aryl amination has emerged
as a practical and popular method for the formation of C–N bonds
[
[
28]. Since pioneering work, notably by Buchwald and Hartwig
29], palladium catalyzed N-aryl amination has gathered increasing
interest, especially because this reaction has filled the deficiencies
of classic organic methodology in this area [30]. Therefore, having
new bis-NHC containing palladium complexes on hand, we initi-
ated a study focusing on their catalytic activity for aryl amination
using bromobenzene and various anilines or amines. The target
product was obtained in not more than 20% yield when K
2 3
CO
and Cs CO were used as bases. However, the reactions could be
2
3
t
dramatically improved in the presence of KOBu as a base under
the same conditions. It was observed that dimethoxyethane
(
DME) was effective in providing higher conversions, whereas sol-
vents like NMP, toluene, DMF, dioxane showed lower conversions.
The effect of temperature on the coupling of aniline with bromo-
benzene was studied. It was observed that the reaction was very
slow at 50 °C but an increase in the temperature to 80 °C signifi-
cantly improved the conversion. No further increase in the yield
of the amination product was observed even when the reaction
time was increased. With these optimized conditions in hand, we
continued our investigation of the amination reactions of other
substrates. The results are summarized in Table 2. We have dem-
onstrated conditions for higher yields of the Pd–NHC catalyzed
amination of aniline, p-methoxyaniline and p-methylaniline to tri-
arylamines (Table 2, entries 3, 8, and 12). With p-methoxyaniline
and p-methylaniline, containing electron donating groups at the
para position, the amination reaction proceeds with a considerable
increase in the yield, up to 77–95% (Table 2, entries 5–12). When
no catalyst was added, the blank reaction with the solvent DME
Fig. 1. ORTEP drawing of complex 2a. Displacement ellipsoids are drawn at the 50%
probability level.
Table 1
Selected bond lengths (Å) and angles (°) of the Pd complex.
Bond lengths (Å)
Pd(1)–C(1)
Pd(1)–Cl(1)
N(1)–C(1)
N(1)–C(2)
N(1)–C(20)
N(2)–C(1)
1.979(7)
2.3390(19)
1.331(9)
1.380(10)
1.500(10)
1.379(10)
N(2)–C(7)
N(2)–C(8)
C(2)–C(7)
C(8)–C(9)
C(20)–C(21)
1.418(10)
1.488(9)
1.343(11)
1.463(11)
1.496(11)
t
and KO Bu as a base exhibited extremely low reactivity towards
Bond angles (°)
the yield of the triarylamine, with many byproducts from the aryl
halide substrate. We also examined the amination of bromoben-
zene (1.2 mmol) with cycloheptyl amine, cyclohexylamine, cyclo-
pentylamine, morpholine and 2-aminopyridine (1 mmol) under
the same reaction conditions. As shown in Table 2, moderate to
good yields were obtained (entries 13, 15, 18, 22, 25, 26, 27, and
C(1)A–Pd(1)–C(1)
C(1)A–Pd(1)–Cl(1)
C(1)–Pd(1)–Cl(1)
Cl(1)A–Pd(1)–Cl(1)
C(1)–N(1)–C(2)
C(1)–N(1)–C(20)
C(2)–N(1)–C(20)
C(1)–N(2)–C(7)
94.7(4)
86.9(2)
C(1)–N(2)–C(8)
C(7)–N(2)–C(8)
N(1)–C(1)–N(2)
N(1)–C(1)–Pd(1)
N(2)–C(1)–Pd(1)
C(9)–C(8)–N(2)
C(21)–C(20)–N(1)
122.5(6)
129.8(6)
106.5(6)
127.5(6)
125.8(5)
114.3(6)
114.0(6)
177.3(2)
91.46(9)
111.2(7)
122.9(6)
124.7(6)
107.7(6)
3
2). With piperazine (1 mmol) and bromobenzene (2.4 mmol),
the amination reaction proceeds with a considerable increase in
the yield, up to 61–79% (Table 2, entries 33–36). We observed that
the influence of the alkyl on the N atom of the palladium complex
is not important.
In
corresponding Pd–Ccarben bond length values are 1.983(5) and
.971(5) Å in [21], 1.990(3) and 1.978(4) Å in [20], and 1.993(2)
and 1.971(2) Å in [22], following a similar trend to those in complex
a. However, the Pd–Ccarben distances are slightly shorter than other
Pd–Ccarben single bond distances (2.036(5) Å in [23], 2.037(1) Å in
a series of related palladium carbene complexes, the
1
3. Experimental
2
3.1. General considerations
[
[
24], 2.067(10) Å in [25], 2.016(5) Å in [26], and 2.035(2) Å in
27]). The molecular structure and selected geometrical parameters
The syntheses were carried out by using standard Schlenk
techniques under an inert argon atmosphere with previously dried
of the palladium complex are presented in Fig. 1 and Table 1,
respectively.
The benzimidazole ring is almost planar and the maximum
deviations from planarity are 0.039(6) Å for the atom N1 and,
1
13
1
solvents. H and C NMR were performed in CDCl
3
or DMSO-d
6
. H
1
3
NMR and C NMR spectra were recorded using a Varian As 400
1
13
Merkur spectrometer operating at 400 MHz ( H), 100 MHz ( C).
Chemical shifts (d) are given in ppm relative to TMS, coupling con-
stants (J) in Hz. Infrared spectra were recorded as KBr pellets in the
0
.038(8) Å for the atom C1. The dihedral angle of the two
benzimidazole moieties is 71.54(6)°. In addition, the benzimid-
azole moiety forms dihedral angles of 87.5(3)° and 55.8(4)° with
the mean planes through the 2,3,4,5,6-pentamethylbenzyl and
ꢀ1
range 400–4000 cm
on an ATI UNICAM 1000 spectrometer.
Melting points were measured in open capillary tubes with an
Electrothermal-9200 melting point apparatus and are uncorrected.
Elemental analyses were performed by the Turkish Research
Council (Ankara, Turkey) Microlab. Single crystal X-ray data were
collected on a Rigaku AFC8S Mercury CCD diffractometer [31]
1
,2-phenylene rings, respectively.
The crystal structure of 2a is stabilized by intermolecular
C–Hꢁ ꢁ ꢁCl hydrogen bonds which link the molecules into a three-
i
dimensional molecular network (C4–H4ꢁ ꢁ ꢁCl1 : C4–H4 0.96 Å,
H4–Cl1 2.76 Å, C4ꢁ ꢁ ꢁCl1 3.691(10) Å, C4–H4ꢁ ꢁ ꢁCl1 164°; Symmetry
code (i): 1/2 + x, 3/2 ꢀ y, 1 ꢀ z).
using monochromated Mo K
a radiation. The structure was solved
by direct and conventional Fourier methods [32]. Full-matrix

Table 2
Buchwald–Hartwig amination reaction using o-xylyl-linked NHC–Pd complexes.
R
NH
R
NH
R = H, OMe, Me
Pd-NHC
2
or
RNH
2
, morpholine, piperazine
O
N
Br
R
N
Pd-NHC
or
N
N
Entry
Cat.
Amine
Product
Yield (%)
1
2
3
4
2a
2b
2c
2d
67
88
92
71
NH
2
N
5
6
7
8
2a
2b
2c
2d
84
82
93
95
MeO
NH
2
MeO
N
9
2a
2b
2c
2d
85
77
80
90
Me
NH
2
1
1
1
0
1
2
Me
N
1
1
1
1
3
4
5
6
2a
2b
2c
2d
98
92
98
95
NH
2
NH
1
1
1
2
7
8
9
0
2a
2b
2c
2d
93
98
75
83
NH
2
NH
2
2
2
2
1
2
3
4
2a
2b
2c
2d
81
99
94
77
NH2
NH
2
2
2
2
2
3
3
3
5
6
7
8
9
0
1
2
2a
2b
2c
2d
2a
2b
2c
2d
99
99
98
93
62
59
68
75
O
NH
O
N
N
N
NH2
NH
N
H
3
3
3
3
3
4
5
6
2a
2b
2c
2d
73
79
61
67
N
N
NH
t
Reaction conditions: catalyst (2a–d) (0.01 mmol), KOBu (1.5 mmol), aniline (1 mmol), bromobenzene (2.4 mmol) (entries 1–12 and 33–36); 1.2 mmol (entries 13–32), DME (2 mL), 80 °C-15 h. Yields are based on anilines. All
reactions were monitored by TLC and GC.

S. Demir et al. / Polyhedron 30 (2011) 195–200
199
0
0
Table 3
3.2.2. Dichloro{1,1 -[di-2,3,5,6-tetramethylbenzyl]-3,3 -o-
Crystal data and structure refinement for complex 2a.
0
xylyldibenzimidazolidin-2,2 -diylidene}palladium (II), 2b
m
(CN): 1400 cm 1. 1H NMR
ꢀ
Empirical formula
Formula weight
Temperature (K)
Wavelength (Å)
Crystal system, space group
Unit cell dimensions
a (Å)
C
46
H
50Cl
2
N
4
Pd
Yield: 0.59 g (73%). M.p.: 287–288 °C,
(399.9 MHz, CDCl ) d: 7.97–8.01 [m, 2H, –CH
m, 2H, –CH CH –], 7.89 [d, 2H, J = 8.1 Hz, NC
and 7.21–7.28 [m, 4H, NC N], 7.08 [s, 2H, CH
.87 [t, 2H, J = 8.1 Hz, NC N], 6.78, 6.73, 6.10, 6.07, 6.05, 5.61, and
5.56 [s, 8H, CH H(CH -2,3,5,6 and –CH CH –], 2.23 and
.10 [s, 24H, CH H(CH -2,3,5,6]. C NMR (100.5 MHz, CDCl
d: 175.7 (Ccarbene), 134.8, 134.6, 133.6, 132.9, 129.6, 129.4, 123.5,
23.1, 112.0, and 111.2 [CH H(CH -2,3,5,6; –CH CH –,
and NC N], 52.0 and 51.1 [–CH CH – and CH H(CH
2,3,5,6], 20.6 and 16.5 [CH H(CH -2,3,5,6]. Anal. Calc. for
: C, 65.39; H, 5.74; N, 6.93. Found: C, 65.41; H,
.70; N, 6.96%.
836.20
158(2)
0.71073
C
2 6
H
4
CH
–], 7.45–7.48
N], 7.49–7.61
H(CH -2,3,5,6],
3
2
[
C
2 6
H
4
2
6 4
H
6
H
4
C
2 6
3 4
)
orthorhombic, Cmca
6
6 4
H
18.741(4)
18.476(4)
29.049(6)
90
90
90
10058(3)
8, 1.104
0.505
2
C
C
6
3
)
4
2
C
H
6 4
2
1
3
b (Å)
c (Å)
2
2
6
3
)
4
3
)
a
(°)
b (°)
(°)
Volume (Å )
1
2
C
6
3
)
4
2
C
6
H
4
2
c
6
H
4
C
2 6
H
4
2
2
C
6
3 4
) -
3
C
2 6
3 4
)
3
Z, Dcalc (mg/m )
44 46 4 2
C H N PdCl
Absorption coefficient (mmꢀ1)
F(0 0 0)
5
3472
Crystal size (mm)
h Range for data collection (°)
Limiting indices
0.26 ꢂ 0.05 ꢂ 0.05
2.59–25.60
ꢀ20 ꢃ h ꢃ 22
0
0
3
.2.3. Dichloro{1,1 -[dicyclohexylbenzyl]-3,3 -o-
0
xylyldibenzimidazolidin-2,2 -diylidene}palladium (II), 2c
ꢀ
ꢀ
12 ꢃ k ꢃ 22
35 ꢃ l ꢃ 33
(CN): 1407 cm 1. 1H NMR
N], 8.13–
CH –],
N], 7.25,
ꢀ
Yield: 0.58 g (82%). M.p.: 286–287 °C,
(399.9 MHz, DMSO-d ) d: 8.23 [d, 2H, J = 8.1 Hz, NC
8.16 [m, 2H, –CH CH –], 7.45–7.42 [m, 2H, –CH
.73 [d, 2H, J = 7.8 Hz, NC N], 7.39–7.28 [m, 4H, NC
7.21, 5.81, and 5.76 [s, 4H, –CH CH –], 4.64–4.71 and 4.33–
.41 [m, 4H, CH 11], 2.73–2.79 and 1.03–1.73 [m, 22H,
11]. C NMR (100.5 MHz, DMSO-d ) d: 173.5 (Ccarbene),
35.3, 134.3, and 123.7 [–CH CH –], 133.5, 129.6, and 112.5
NC N], 54.5 [–CH CH –], 50.6 [CH 11], 37.6, 30.9, 30.9,
6.2, 25.8, and 25.5 [CH 11]. Anal. Calc. for C36 PdCl : C,
1.07; H, 5.98; N, 7.91. Found: C, 61.11; H, 5.93; N, 7.85%.
m
Reflections collected/unique
Completeness to h = 25.60
Absorption correction
Maximum and minimum transmission
Refinement method
22953/4873 [Rint = 0.1152]
99.7%
REQAB (multi-scan)
6
6 4
H
2
6
C H
4
2
2
C
6
H
4
2
7
6
H
4
6 4
H
0.9752 and 0.8798
Full-matrix least-squares on F2
2
C H
6 4
2
4
CH
1
[
2
6
2 6
C H
Data/restraints/parameters
Goodness-of-fit on F
4873/0/246
0.983
2
13
C
2 6
H
6
Final R indices [I > 2
R indices (all data)
r
(I)]
R
R
1
= 0.0896, wR
= 0.1340, wR
2
2
= 0.2275
= 0.2562
2
C
6
H
4
2
1
Largest difference in peak and hole, (e Åꢀ3) 1.713 and ꢀ0.758
6
H
4
C
2 6
H
4
2
2 6
C H
2
C
6
H
H
42
N
4
2
2
least-squares refinement was based on F [32]. Apart from hydro-
0
0
3
.2.4. Dichloro{1,1 -[di-(2-diizopropylaminoethyl)]-3,3 -o-
gen, all atoms were refined anisotropically; hydrogen atom coordi-
nates were calculated at idealized positions and refined using a
riding model. Further details concerning data collection and refine-
ment are given in Table 3. All catalytic reactions were monitored
on an Agilent 6890 N GC system by GC-FID with a HP-5 column
of 30 m length, 0.32 mm diameter, and 0.25
Column chromatography was performed using silica gel 60 (70–
30 mesh).
0
xylyldibenzimidazolidin-2,2 -diylidene}palladium (II), 2d
(CN): 1413 cmꢀ1. 1H NMR
CH –], 7.83–7.86
–], 7.28–7.48 [m,
CH –],
Yield: 0.60 g (78%). M.p.: 258–259 °C,
399.9 MHz, CDCl ) d: 7.97–8.01 [m, 2H, –CH
m, 2H, NC N], 7.51–5.54 [m, 2H, –CH
N], 5.41, 5.36, 5.27, and 4.32 [s, 4H, –CH
.72–3.42 [m, 8H, CH CH NCH(CH ], 0.76–1.45 [m, 28H,
]. C NMR (100.5 MHz, CDCl ) d: 174.6 (Ccarbene),
34.7, 134.3, and 123.6 [–CH CH –], 134.1, 129.7, and 111.8
–], 50.7 [CH CH NCH(CH ], 49.3
], 44.2 [CH CH NCH(CH ], 21.0 [CH CH N-
]. Anal. Calc. for C38 PdCl : C, 59.26; H, 6.81; N,
0.91. Found: C, 59.30; H, 6.77; N, 10.95%.
m
(
[
6
2
CH
3
C
2 6
H
4
2
6
H
4
2 6 4 2
C H CH
H, NC
6
H
4
2
C
6
H
4
2
lm film thickness.
2
2
3 2
)
13
2
CH
2
NCH(CH
3
)
2
3
2
1
2
C
6
H
4
2
[
[
NC
CH
6
H
4
N], 52.0 [–CH
C
2 6
H CH
4 2
2
2
3 2
)
3
.2. General procedure for the preparation of palladium complexes
2
CH NCH(CH
2
3
)
2
2
2
3
)
2
2
2
(2a–2d)
CH(CH
1
3
)
2
H
52
N
6
2
About 1.0 mmol of bisbenzimidazolium salt (1) and Pd(OAc)
0.22 g, 1.0 mmol) were dissolved in 5 mL DMSO and stirred at
room temperature for 5 h. Afterwards the mixture was heated for
2
(
3
.3. General procedure for the catalytic C–N bond formation
5
h at 50 °C, then for 2 h at 90 °C and then for 1 h 120 °C. The vol-
t
Under argon, 1.5 mmol of KO Bu, 1 mol% of catalyst, 1 mmol
atile compounds were removed in vacuo and the precipitate was
washed twice with 5 mL THF. The complexes were recrystallized
from dichloromethane:diethylether (1:2 ratio) at room tempera-
ture (Scheme 1).
aniline, 2.4 mmol bromobenzene (for diarylation) or 1.2 mmol bro-
mobenzene (for monoarylation), and 2 mL dimethoxyethane
DME) were added into an oven-dried Schlenk tube. The mixture
was stirred at 80 °C for 15 h. The reaction mixture was allowed
to cool to room temperature, quenched by filtering through a short
silica column (eluent: ethyl acetate) and then concentrated under
reduced pressure. After purification by flash chromatography
(
0
0
3.2.1. Dichloro{1,1 -[di-2,3,4,5,6-pentamethylbenzyl]-3,3 -o-
0
xylyldibenzimidazolidin-2,2 -diylidene}palladium (II), 2a
(CN): 1394 cm 1. 1H NMR
ꢀ
Yield: 0.57 g (68%). M.p.: 282–283 °C,
399.9 MHz, DMSO-d ) d: 8.09–8.14, 7.36–7.48, 7.17–7.22, 8.82–
.90, and 6.55–6.62 [m, 12H, NC N and –CH CH –], 5.96–
.15, 5.69–5.82, and 5.45–5.60 [m, 8H, CH (CH -2,3,4,5,6 and
CH CH –], 2.25, 2.24, 2.17, 2.14, and 1.99 [s, 30H,
-2,3,4,5,6]. C NMR (100.5 MHz, DMSO-d
carbene), 136.1, 135.1, 134.3, and 132.9 [CH (CH
34.8, 134.1, and 123.4 [–CH CH –], 134.2, 133.9, 129.4,
27.5, 112.2, and 111.9 [NC N], 54.7 [–CH CH –], 52.4
(CH -2,3,4,5,6], 17.8, 17.4, and 17.0 [CH (CH
,3,4,5,6]. Anal. Calc. for C46 PdCl : C, 66.07; H, 6.03; N, 6.70.
m
(
eluent: ethyl acetate/hexane), the yield was calculated based on
(
6
the aniline.
6
6
–
6
H
4
2
6
C H
4
2
2
C
6
3 5
)
2
C
6
H
4
2
4. Conclusion
13
CH
2
C
6
(CH
3
)
5
6
) d: 175.0
-2,3,4,5,6],
(C
2
C
6
3
)
5
We have prepared o-xylyl-linked palladium–NHC complexes
1
13
1
1
2
C
H
6 4
2
(2a–d), whose structures were confirmed by H NMR, C NMR,
IR and elemental analysis, and complex 2a has been characterized
by single crystal X-ray diffraction studies. These complexes are
effective catalysts for palladium catalyzed C–N coupling reactions.
6
H
4
C
2 6
H
4
2
[
CH
2
C
6
3
)
5
C
2 6
3 5
) -
2
H
50
N
4
2
t
Found: C, 66.11; H, 6.09; N, 6.74%.
The catalyst–KOBu –DME system was found to be very efficient for

2
00
S. Demir et al. / Polyhedron 30 (2011) 195–200
amination of bromobenzene with a variety of primary and second-
ary amines. Research in our lab is currently on-going to extend the
coordination chemistry of functionalized NHCs to other transition
metals, such as Ru, Rh, Ag, and Au, and to explore their potential
applications in catalysis. In addition our further research will focus
on the development of more efficient catalytic systems for aryl
chlorides and various aryl bromides in this reaction.
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1
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Acknowledgements
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c) M. Poyatos, J.A. Mata, E. Peris, Chem. Rev. 109 (2009) 3677;
d) M.C. Jahnke, M. Hussain, F. Hupka, T. Pape, S. Ali, F.E. Hahn, K.J. Cavell,
This work was financially supported by the Technological and
_
Scientific Research Council of Turkey TUBITAK-BOSPHORUS
Tetrahedron 65 (2009) 909.
_
_
(
France) [109T605] and Inönü University Research Fund (I.Ü.
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Appendix A. Supplementary data
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2003) 1110;
c) F.E. Hahn, M.C. Jahnke, V. Gomez-Benitez, D. Morales-Morales, T. Pape,
CCDC 771186 contains the supplementary crystallographic data
for complex 2a. These data can be obtained free of charge via http://
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