Palladium(II)-N-heterocyclic carbene complexes: Synthesis, characterization and catalytic application

Article abstract of DOI:10.1002/aoc.3143

N-Heterocyclic carbenes (NHCs) are of great importance and are powerful ligands for transition metals. A new series of sterically hindered benzimidazole-based NHC ligands (LHX) (2a, 2b, 2c, 2d, 2e, 2f), silver-NHC complexes (3a, 3b, 3c, 3d, 3e, 3f) and pa

Full text of DOI:10.1002/aoc.3143

Full Paper
Received: 16 January 2014
Revised: 23 February 2014
Accepted: 23 February 2014
Published online in Wiley Online Library: 20 April 2014
(wileyonlinelibrary.com) DOI 10.1002/aoc.3143
Palladium(II)-N-heterocyclic carbene complexes:
synthesis, characterization and
catalytic application
Suzan Çekirdeka, Sedat Yaşara* and İsmail Özdemirb
N-Heterocyclic carbenes (NHCs) are of great importance and are powerful ligands for transition metals. A new series of
sterically hindered benzimidazole-based NHC ligands (LHX) (2a–f), silver–NHC complexes (3a–f) and palladium–NHC
complexes (4a-f) have been synthesized and characterized using appropriate spectroscopic techniques. Studies have focused
on the development of a more efficient catalytic system for the Suzuki coupling reaction of aryl chlorides. Catalytic performance
of Pd–NHC complexes and in situ prepared Pd(OAc)2/LHX catalysts has been investigated for the Suzuki cross-coupling reaction
under mild reaction conditions in aqueous N,N-dimethylformamide (DMF). These complexes smoothly catalyzed the
Suzuki–Miyaura reactions of electron-rich and electron-poor aryl chlorides. Copyright © 2014 John Wiley & Sons, Ltd.
Additional supporting information may be found in the online version of this article at the publisher’s web-site.
Keywords: Suzuki reaction; N-heterocyclic carbene; palladium–NHC; phenyl boronic acid
the most powerful and widely preferred reactions for CÀC bond
formation, which plays an important role in the synthesis of fine
chemicals, medicinal chemistry and natural products.[10,18–22]
The advantages of the Suzuki reaction are the use of non-toxic,
air- and moisture-stable reagents such as phenylboronic acid
and its derivatives. Furthermore, mild reactions conditions, the
use of green solvents and substrate tolerance are other superior
features of the Suzuki reaction. Recently, research has focused
on the development of new catalyst precursors which can
catalyze the coupling reactions of unactivated and cheaper aryl
chlorides with low catalyst concentrations and convenient sol-
vents.[23–27] Using mixtures of water and organic solvents in the
Suzuki reaction promotes solvation of mineral bases to support
the Suzuki reaction by activating arylboronic acid and has the
ability to dissolve organic substrates and corresponding
palladium catalysts. At the same time water has economic and
environmental advantages because it is inexpensive, abundant,
non-toxic, non-flammable and is easily separable from organic
compounds. Furthermore, water was tolerated in the synthesis
of di- or tri-substituted biaryls at ambient temperatures.[28] There
are several studies of the Suzuki reaction being performed in
water as solvent,[23–26,29] but to our knowledge there are compar-
atively few studies in the literature regarding the Suzuki reaction
catalyzed in aqueous media by Pd–NHC. In 2010, Ying et al.
discovered a novel, highly active pre-catalyst for the coupling of
deactivated aryl chlorides and phenyl boronic acid at room
Introduction
N-Heterocyclic carbenes (NHCs) have great importance as ligands
for transition metals chemistry owing to their tunable steric and
electronic properties. The first NHC complexes were reported by
Öfele and Wanzlick.[1] After isolation and characterization of the
first stable free NHC in 1991 by Arduengo et al.,[2] a great many
NHC-ligated metal complexes have been synthesized,[3] leading
to numerous applications in the field of transition metal catalysis
such as hydrosilylation,[4] Ru-catalyzed furan synthesis,[5] transfer
hydrogenation,[6] olefin metathesis[7] and cross-coupling reac-
tions.[8,9] Because of the stability of imidazole-based carbenes,
metal complexes of this class of ligand and their catalytic
applications have been examined frequently.[10] Recently, benz-
imidazole-based NHCs and their different metal complexes have
been synthesized and their catalytic activities investigated.[11,12]
Most of these NHC–metal complexes possess bulky carbene
substitutents. Bulky substituents have beneficial properties such
as strong σ-electron donation to the metal center, which leads
to a strong metal–ligand bond and a highly stable metal com-
plex. These properties enhance the catalytic performance of
these metal complexes.[13,14] Sterically demanding NHCs might
facilitate the formation of a catalytically active species[15] and en-
hance the reductive elimination step.[16] On the other hand, oxi-
dative addition or transmetalation steps might be slowed by
steric demand.
An important purpose in organic and inorganic chemistry is
the improvement of green processes that use minimum energy
and raw materials, maximize fine resources and eliminate the
use of hazardous chemicals. Evidently, these useful features are
not possible without catalysis. In this regard, catalysis is pivotal.
Carbon–carbon and carbon–heteroatom bond-forming reac-
tions have been catalyzed by structurally different palladium
complexes. Numerous reviews on this topic have been published
by different research groups.[10,17] The Suzuki coupling is one of
*
Correspondence to: Sedat Yaşar, Department of Chemistry, Faculty of Science and
Art, Gaziosmanpaşa University, 60150 Tokat, Turkey. E-mail: syasar44@gmail.com
a Department of Chemistry, Faculty of Science and Art, Gaziosmanpaşa University,
60150 Tokat, Turkey
b Department of Chemistry, Faculty of Science and Art, Inönü University, 44280
Malatya, Turkey
Appl. Organometal. Chem. 2014, 28, 423–431
Copyright © 2014 John Wiley & Sons, Ltd.

S. Çekirdek et al.
temperature by using non-anhydrous solvents (dioxane/H2O,
THF/H2O) in air.[29] In 2011, Shao et al. reported that Pd–NHC
complexes derived from proline could catalyze the Suzuki
coupling reaction of aryl bromide and iodide in water at room
temperature.[26] Shao et al. reported in a different study that
Pd–NHC complexes derived from readily available starting mate-
rials such as IPr.HCl or IMes.HCl, showed high catalytic activity in
the Suzuki–Miyaura coupling reactions of aryl chlorides at room
temperature.[27] However the procedures reported in the litera-
ture focused mainly on the Suzuki coupling of aryl bromides
and iodides in organic solvents. For all these reasons we pre-
ferred to use aqueous DMF (3ml H2O/DMF) as a solvent for the
Suzuki reaction of aryl chlorides. We report herein a simple and
efficient catalytic system for rapid Suzuki coupling of aryl chlo-
rides in good yields. This catalytic system, using Pd–NHC or Pd
(OAc)2/LHX, KOtBu and aqueous DMF, was applicable to
unactivated aryl chlorides under mild reaction conditions. Pd
(OAc)2/LHX catalysts, when prepared in situ by deprotonation of
the NHC precursors with appropriate base, leads to significantly
better results than the use of the pre-formed palladium carbene
complexes. The results we found are consistent with the
literature.[30] Herein, we now wish to report the synthesis of air-
stable benzimidazole-based NHCs and their Pd–NHC complexes,
and study their use as co-catalysts in the Suzuki–Miyaura
reaction. This catalytic system could be applied to the Suzuki
coupling of unactivated aryl chlorides.
N
+
N
Cl-
N
Cl-
N
+
2a
N
N
+
N
Cl-
O
2f
CH3
N
N
2b
N
+
N
Cl-
N
+
N
Cl-
N
+
N
Cl-
2e
2c
2d
Figure 1. Synthesis of benzimidazole-based NHC ligand precursors,
2a–f.
Experimental
1-Methyl-3-(2-methylbenzyl)benzimidazolium chloride, 2a
Materials and Methods
Yield 86% (2.4 g); m.p. 134–136°C. ν(CN) = 1490 cmÀ1 1H NMR
.
(399.9 MHz, CDCl3) δ (ppm) = 2.39 [s, 3H, CH2C6H4(CH3)-2],
4.30 [s, 3H, NCH3], 5.84 [s, 2H, CH2C6H4(CH3)-2], 7.07–7.75
[m, 8H, C6H4(CH3)-2 and C6H4], 11.47 [s, 1H, NCHN]. 13C NMR
Unless stated otherwise, all procedures were carried out under an
inert atmosphere using standard Schlenk line techniques.
Chemicals and solvents were purchased from Sigma-Aldrich Co.
(Gillingham, UK). The solvents used were purified by distillation
over the drying agents indicated and were transferred under Ar:
Et2O (Na/K alloy), CH2Cl2 (CaH2), hexane, toluene (Na). Elemental
analyses were performed by the Turkish Research Council
(Ankara, Turkey) Microlab.
(100 MHz, CDCl3)
δ (ppm) = 19.5 [CH2C6H4(CH3)-2]; 33.8
[NCH3]; 49.7 [CH2C6H4(CH3)-2]; 112.9, 113.5[C6H4-o],113.6,
126.7 [C6H4-m], 131.2, 132.2 [C6H4-i], 127.2[CH2C6H4(CH3)-2,
C4], 127.3[CH2C6H4(CH3)-2, C5], 128.1[CH2C6H4(CH3)-2, C6],
129.2[CH2C6H4(CH3)-2, C3], 130.5[CH2C6H4(CH3)-2, C2], 136.4
[CH2C6H4(CH3)-2, C1]; 144.2 [NCHN]. Anal. Calcd for
NMR and IR Spectroscopy
C
16H17N2Cl: C, 70.45; H, 7.36; N, 8.90. Found: C, 70.63; H,
1H NMR and 13C NMR spectra were recorded using a Varian As
400 Merkur spectrometer operating at 400 MHz (1H) and
100 MHz (13C) in CDCl3 with TMS as an internal reference. The
NMR studies were carried out in high-quality 5 mm NMR tubes.
Signals are quoted in parts per million as δ downfield from TMS
(δ 0.00) as an internal standard. Coupling constants (J values)
are given in hertz. NMR multiplicities are abbreviated as follows:
s = singlet, d = doublet, t = triplet, m = multiplet, h = heptet.
FT-IR spectra were recorded on a Mattson 1000 spectropho-
7.56; N, 9.09.
1-Methyl-3-(4-methylbenzyl)benzimidazolium chloride, 2b
Yield 95% (2 g); m.p. 223–225°C. ν(CN) = 1452 cmÀ1 1H NMR
.
(399.9 MHz, CDCl3) δ (ppm) = 2.26 [s, 3H, CH2C6H4(CH3)-4], 4.22 [s, 3H,
NCH3], 5.75 [s, 2H, CH2C6H4(CH3)-4], 7.12 [d, J = 8 Hz, 2H, C6H4
(CH3)-4], 7.32 [d, J = 8 Hz, 2H, C6H4(CH3)-4],7.53-7.65 [m, 4H,
C6H4], 11.67 [s, 1H, NCHN]. 13C NMR (100 MHz, CDCl3) δ (ppm) =
20.8 [CH2C6H4(CH3)-4], 33.3 [NCH3], 50.4 [CH2C6H4(CH3)-4],
112.7, 112.9[C6H4-o], 113.4,126.7[C6H4-m], 130.6, 131.8 [C6H4-i],
126.8[CH2C6H4(CH3)-4, C1], 128.0[CH2C6H4(CH3)-4, C2,6] 129.8
[CH2C6H4(CH3)-4, C3,5], 138.5 [CH2C6H4(CH3)-4, C4]; 142.7 [NCHN].
Anal. Calcd. for C16H17N2Cl: C, 70.45; H, 7.36; N, 8.90. Found: C,
70.60; H, 7.51; N, 9.03
tometer, wavenumbers in cmÀ1
.
General Preparation of 1,3-Dialkybenzlimidazolium Salts
The synthetic procedure for 2a–f benzimidazolium salts is given
below. A solution of 1-methyl benzimidazole (1.32 g, 10.0 mmol)
in DMF (5 ml) was added slowly to corresponding aryl chlorides
(12.0 mmol) and the resulting mixture was stirred at 80°C for
2–3 days (Fig. 1). The white crystalline solid that formed was
filtered and washed with diethyl ether (2 × 10 ml). The white solid
was crystallized with methyl alcohol–ether (1:3).
1-Methyl-3-(2,4,6-trimethylbenzyl)benzimidazolium chloride, 2c
This known compound was synthesized and characterized by
1
m.p, IR, H and 13C NMR techniques. All data have been given as
supporting information. Results were consistent with the literature.[31]
wileyonlinelibrary.com/journal/aoc
Copyright © 2014 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2014, 28, 423–431

Benzimidazole-based Pd–NHC complexes and Suzuki coupling reaction
1-Methyl-3-(2,3,5,6-tetramethylbenzyl)benzimidazolium chloride, 2d
benzimidazolium salts (2a–f) in 20 ml CH2Cl2. Mixtures were
stirred for 2 days at room temperature (Fig. 2) and filtered
through celite. Diethyl ether was added to solutions to obtain
white crystals. Molecular structures of Ag–NHC complexes were
determined by NMR, IR and microanalyses.
Yield 94% (1.6 g); m.p. 256–258°C. ν(CN) = 1471 cmÀ1 1H NMR
.
(399.9 MHz, CDCl3) δ (ppm) = 2.15 [s, 6H, CH2C6H(CH3)4-3,5], 2.20
[s, 6H, CH2C6H(CH3)4-2,6], 4.23 [s, 3H, NCH3], 5.74 [s, 2H, CH2C6H
(CH3)4-2,3,5,6], 6.97 [s, 1H, CH2C6H(CH3)4-2,3,5,6], 7.36 [d, J = 8.4
Hz, 1H, C6H4ÀH1], 7.46 [dd, J = 8.4 Hz and 7.4 Hz, 1H,
C6H4ÀH2],7.55 [dd, J = 8.4 Hz and 7.4 Hz, 1H, C6H4ÀH3], 7.72
[d, J = 8.4 Hz, 1H, C6H4ÀH4], 10.73 [s, 1H, NCHN]. 13C NMR
(100 MHz, CDCl3) δ (ppm) = 15.0, 16.0, 19.4 and 20.5 [CH2C6H
(CH3)4-2,3,5,6], 33.9 [NCH3], 47.3 [CH2C6H(CH3)4-2,3,5,6]; 111.9,
112.3 [C6H4-o], 113.4, 126.2 [C6H4-m], 130.2, 132.2 [C6H4-i] ,
127.5 [CH2C6H(CH3)4-2,3,5,6, C4 ], 133.5 [CH2C6H(CH3)4-2,3,5,6,
Chloro-[1-methyl-3-(2-methylbenzyl)benzimidazole-2-ylidene]silver(I), 3a
Yield 70% (0.35 g); m.p. 190–192 °C. ν(CN) = 1602 cmÀ1. H NMR
1
(399.9 MHz, CDCl3) δ (ppm) = 2.40 [s, 3H, CH2C6H4(CH3)-2], 4.10
[s, 3H, NCH3], 5.60 [s, 2H, CH2C6H4(CH3)-2], 7.10–7.24 [m, 4H,
C6H4(CH3)-2],7.31 [d, J = 8.4 Hz, 1H, C6H4ÀH1], 7.34 [dd, J = 7.2
Hz and 8.4 Hz, 1H, C6H4-H2],7.43 [dd, J = 7.2 Hz and 8.4 Hz, 1H,
C6H4ÀH3], 7.52 [d, J = 8.4 Hz, 1H, C6H4ÀH4]. 13C NMR (100 MHz,
C
2,6], 133.9 [CH2C6H(CH3)4-2,3,5,6, C3,5], 135.0 [CH2C6H(CH3)4-2,
CDCl3)
δ (ppm) = 19.6 [CH2C6H4(CH3)-2]; 36.0 [NCH3]; 51.2
3,5,6, C1], 143.0 [NCHN]. Anal. Calcd for C19H23N2Cl: C, 72.48; H,
7.36; N, 8.90. Found: C, 72.60; H, 7.45; N, 9.02
[CH2C6H4(CH3)-2]; 111.3, 112.0 [C6H4-o], 126.5, 126.6 [C6H4-m],
130.9, 132.7 [C6H4-i], 124.3 [CH2C6H4(CH3)-2, C4], 128.3[CH2C6H4
(CH3)-2, C5], 133.8 [CH2C6H4(CH3)-2, C6], 134.5 [CH2C6H4(CH3)-2,
C3], 132.5[CH2C6H4(CH3)-2, C2],135.4 [CH2C6H4(CH3)-2, C1] ; 189.4
[NCN]. Anal. Calcd for C16H16N2ClAg: C, 50.62; H, 4.25; N, 7.38.
Found: C, 50.69; H, 4.37; N, 7.51.
1-Methyl-3-(2,3,4,5,6-pentamethylbenzyl)benzimidazolium chloride, 2e
1
Yield 98% (2.84 g); m.p. 257°C. ν(CN) = 1467 cmÀ1. H NMR (399.9
MHz, CDCl3) δ (ppm) = 2.21 [s, 6H, CH2C6(CH3)5-3,5], 2.25 [s, 6H,
CH2C6(CH3)5-2,6], 2.26 [s, 3H, CH2C6(CH3)5-4], 4.28 [s, 3H, NCH3],
5.77 [s, 2H, CH2C6(CH3)5-2,3,4,5,6], 7.40 [d, J = 7.4 Hz, 1H,
C6H4-H1], 7.51 [dd, J = 8 Hz and 7.4 Hz, 1H, C6H4ÀH2],7.60 [dd, J = 8
Hz and 7.4 Hz, 1H, C6H4ÀH3], 7.73 [d, J = 7.4 Hz, 1H, C6H4ÀH4],
10.50 [s, 1H, NCHN]. 13C NMR (100 MHz, CDCl3) δ (ppm)= 16.9,
17.0, 17.2 [CH2C6(CH3)5-2,3,4,5,6], 34.0 [NCH3], 47.8 [CH2C6(CH3)5-2,
3,4,5,6]; 112.9, 113.4 [C6H4-o], 124.9, 127.2 [C6H4-m], 131.2,
132.3[C6H4-i], 127.3 [CH2C6(CH3)5-2,3,4,5,6, C4],133.6 [CH2C6(CH3)5-2,
3,4,5,6, C2,6], 133.9 [CH2C6(CH3)5-2,3,4,5,6, C3,5], 137.3 [CH2C6(CH3)
5-2,3,4,5,6, C1], 143.0 [NCHN]. Anal. Calcd for C20H25N2Cl: C, 73.04;
H, 7.66; N, 8.52. Found: C, 73.10; H, 7.80; N, 8.65.
Chloro-[1-methyl-3-(4-methylbenzyl)benzimidazole-2-ylidene]silver(I), 3b
Yield 67% (0.4 g); m.p. 202–206°C. ν(CN) = 1604 cmÀ1 1H NMR
.
(399.9 MHz, CDCl3) δ (ppm) = 2.29 [s, 3H, CH2C6H4(CH3)-4], 4.12
[s, 3H, NCH3], 5.73 [s, 2H, CH2C6H4(CH3)-4], 7.02–7.22 [m, 4H,
C6H4(CH3)-4],7.23–7.35 [m, 4H, C6H4]. 13C NMR (100 MHz, CDCl3)
δ (ppm) = 20.5 [CH2C6H4(CH3)-4], 35.1 [NCH3], 52.0 [CH2C6H4
(CH3)-4], 110.4, 111.1 [C6H4-o], 111.2, 126.5 [C6H4-o], 128.9, 132.2
[C6H4-i], 123.1 [CH2C6H4(CH3)-4, C1], 129.4 [CH2C6H4(CH3)-4,
C
2,6], 133.1 [CH2C6H4(CH3)-4, C3,5], 134.2, [CH2C6H4(CH3)-4, C4],
not observed [NCN]. Anal. Calcd for C16H16N2ClAg: C, 50.62; H,
4.25; N, 7.38. Found: C, 50.67; H, 4.34; N, 7.50.
1-Methyl-3-(2-morpholinethyl)benzimidazolium chloride, 2f
Chloro-[1-methyl-3-(2,4,6-trimethylbenzyl)benzimidazole-2-ylidene]silver(I), 3c
Yield 87% (4.6 g); m.p. 218–220°C. ν(CN) = 1459 cmÀ1 1H NMR
.
1
Yield 50% (0.15 g); m.p. 224–226°C. ν(CN) = 1608 cmÀ1. H NMR
(399.9 MHz, CDCl3) δ (ppm) = 2.54 [t, J = 4 Hz, 4H, CH2CH2N
(CH2CH2)2O], 2.90 [t, J = 6 Hz, 2H, CH2CH2N(CH2CH2)2O], 3.61
[t, J = 4 Hz, 4H, CH2CH2N(CH2CH2)2O], 4.28 [s, 3H, NCH3],
4.72 [t, J = 6 Hz, 2H, CH2CH2N(CH2CH2)2O], 7.62–7.78 [m, 4H,
C6H4], 11.36 [s, 1H, NCHN]. 13C NMR (100 MHz, CDCl3) δ (ppm) = 33.6
[CH2CH2N(CH2CH2)2O], 44.3 [CH2CH2N(CH2CH2)2O], 53.5 [CH2CH2N
(CH2CH2)2O], 56.6 [NCH3], 66.8 [CH2CH2N(CH2CH2)2O], 112.9, 113.0
[C6H4-o], 127.1, 127.2 [C6H4-m], 131.2, 131.8 [C6H4-i], 144.1 [NCHN].
Anal. Calcd for C14H20N3OCl: C, 59.67; H, 7.15; N, 14.91. Found: C,
59.77; H, 7.32; N, 15.04.
(399.9 MHz, CDCl3) δ (ppm) = 2.24 [s, 6H, CH2C6H2(CH3)3-2,6],
2.35 [s, 3H, CH2C6H2(CH3)3-4], 4.02 [s, 3H, NCH3], 5.47 [s, 2H,
CH2C6H2(CH3)3-2,4,6], 6.99 [s, 2H, CH2C6H2(CH3)3-2,4,6], 7.33–7.49
[m, 4H, C6H4]. 13C NMR (100 MHz, CDCl3) δ (ppm) = 20.3
[CH2C6H2(CH3)-2,6], 21.2 [CH2C6H2(CH3)-4], 36.4 [NCH3], 47.6
[CH2C6H2(CH3)3-2,4,6], 111.3, 111.5 [C6H4-o], 124.3, 124.4 [C6H4-m],
130.3, 134.1 [C6H4-i], 126.6 [CH2C6H2(CH3)3-2,4,6, C3,5], 134.5
[CH2C6H2(CH3)3-2,4,6, C4], 137.5 [CH2C6H2(CH3)3-2,4,6,
C2,6],
139.6 [CH2C6H2(CH3)3-2,4,6, C1], Not observed [NCN]. Anal. Calcd
for C18H20N2ClAg: C, 53.03; H, 4.94; N, 6.87. Found: C, 53.10; H,
5.01; N, 7.02.
General Method for the Preparation of the Silver–NHC
Complexes (3a–f)
Chloro-[1-methyl-3-(2,3,5,6-tetramethylbenzyl)benzimidazole-2-ylidene]
silver(I), 3d
The synthetic procedure for Ag–NHC complexes reported by
Wang and Lin[32] was followed. The Ag–NHC complexes were
Yield 70% (0.19 g); m.p: 227–229°C. ν(CN) = 1602 cmÀ1. H NMR
1
prepared by reaction of Ag2O and
2
equiv. of dialkyl
(399.9 MHz, CDCl3) δ (ppm) = 2.16 [s, 6H, CH2C6H(CH3)4-3,5], 2.31
[s, 6H, CH2C6H(CH3)4-2,6], 4.00 [s, 3H, NCH3], 5.47 [s, 2H,
CH2C6H(CH3)4-2,3,5,6], 7.16 [s, 1H, CH2C6H(CH3)4-2,3,5,6],
7.43–7.51 [m, 4H, C6H4]. 13C NMR (100 MHz, CDCl3)
δ (ppm) = 16.1 and 20.7 [CH2C6H(CH3)4-2,3,5,6], 36.6
[NCH3], 47.2 [CH2C6H(CH3)4-2,3,5,6]; 111.2, 111.3 [C6H4-o],
124.1, 124.4 [C6H4-m], 133.2, 133.4 [C6H4-i], 129.6
[CH2C6H(CH3)4-2,3,5,6, C4 ], 134.2 [CH2C6H(CH3)4-2,3,5,6,
C2,6 ], 134.4 [CH2C6H(CH3)4-2,3,5,6, C3,5], 135.3 [CH2C6H
(CH3)4-2,3,5,6, C1 ], not observed [NCN]. Anal. Calcd for
C29H22N2ClAg: C, 55.86; H, 5.71; N, 6.61. Found: C, 55.92;
H, 5.23; N, 6.74.
Figure 2. Synthesis of silver–NHC complexes, 3a–f.
Appl. Organometal. Chem. 2014, 28, 423–431
Copyright © 2014 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/aoc

S. Çekirdek et al.
Chloro-[1-methyl-3-(2,3,4,5,6-pentamethylbenzyl)benzimidazole-2-ylidene]
silver(I), 3e
[NCN]. Anal. Calcd for C32H32N4Cl2Pd: C, 59.13; H, 4.96; N, 8.62.
Found: C, 59.19; H, 5.04; N, 8.72.
Yield 68% (0.42 g); m.p. 301–302°C. ν(CN) = 1604 cmÀ1. H NMR
1
cis/trans-Dichloro-(di-[1-methyl-3-(4-methylbenzyl)benzimidazole-2-ylidene])
(399.9 MHz, CDCl3) δ (ppm) = 2.21 [s, 6H, CH2C6(CH3)5-3,5], 2.26
[s, 3H, CH2C6(CH3)5-4], 2.30 [s, 6H, CH2C6(CH3)5-2,6], 3.99 [s, 3H,
NCH3], 5.48 [s, 2H, CH2C6(CH3)5-2,3,4,5,6], 7.44–7-52 [m, 4H,
C6H4]. 13C NMR (100 MHz, CDCl3) δ (ppm) = 16.8, 17.0, 17.3
[CH2C6(CH3)5-2,3,4,5,6], 36.6 [NCH3], 47.7 [CH2C6(CH3)5-2,3,4,5,6];
111.2, 112.8 [C6H4-o], 124.1, 124.3 [C6H4-m], 133.0, 133.6 [C6H4-i],
126.7 [CH2C6(CH3)5-2,3,4,5,6, C4],134.1 [CH2C6(CH3)5-2,3,4,5,6, C2,6],
134.5 [CH2C6(CH3)5-2,3,4,5,6, C3,5], 137.1 [CH2C6(CH3)5-2,3,4,5,6, C1 ],
not observed [NCN]. Anal. Calcd for C20H24N2ClAg: C, 55.13; H,
5.55; N, 6.43. Found: C, 55.19; H, 5.63; N, 6.52.
palladium(II), 4b
Yield 48% (0.13 g); m.p. 264–265°C. ν(CN) = 1612 cmÀ1. H NMR
1
(399.9 MHz, CDCl3) δ (ppm) = 2.29 [s, 6H, CH2C6H4(CH3)-4], 4.27
[s, 6H, NCH3], 6.06 [s, 4H, CH2C6H4(CH3)-4], 7.01–7.26 [m, 8H,
C6H4(CH3)-4],7.33-7.56 [m, 8H, C6H4]. 13C NMR (100 MHz, CDCl3)
δ (ppm) = 21.1 [CH2C6H4(CH3)-4], 35.1 [NCH3], 51.7 [CH2C6H4
(CH3)-4], 111.4, 112.0 [C6H4-o], 123.6, 124.3 [C6H4-m], 128.4,
133.4 [C6H4-i], 129.7 [CH2C6H4(CH3)-4, C1], 133.4 [CH2C6H4(CH3)-4,
C2,6], 134.9 [CH2C6H4(CH3)-4, C3,5],137.8, [CH2C6H4(CH3)-4, C4],
167.1 [NCN]. Anal. Calcd for C32H32N4Cl2Pd: C, 59.13; H, 4.96; N,
8.62. Found: C, 59.18; H, 5.01; N, 8.69.
Chloro-[1-methyl-3-(2-morpholinethyl)benzimidazole À2-ylidene]silver(I), 3f
Yield 54% (0.13 g); m.p. 184–186°C. ν(CN) = 1605 cmÀ1. 1H NMR (399.9
MHz, CDCl3) δ (ppm) = 2.55 [t, J= 4.4 Hz, 4H, CH2CH2N(CH2CH2)2O],
2.82 [t, J= 6.4 Hz, 2H, CH2CH2N(CH2CH2)2O], 3.72 [t, J= 4.4 Hz, 4H,
CH2CH2N(CH2CH2)2O], 4.06 [s, 3H, NCH3], 4.50 [t, J= 6.4 Hz, 2H,
CH2CH2N(CH2CH2)2O], 7.44–7.53 [m, 4H, C6H4]. 13C NMR (100 MHz,
cis/trans-Dichloro-(di-[1-methyl-3-(2,4,6-trimethylbenzyl)benzimidazole-2-
ylidene])palladium(II), 4c
Yield 76% (0.64 g); m.p. 276–278°C. ν(CN) = 1612 cmÀ1 1H NMR
.
(399.9 MHz, CDCl3) δ (ppm) = 2.34 [s, 12H, CH2C6H2(CH3)3-2,6],
2.43 [s, 6H, CH2C6H2(CH3)3-4], 4.55 [s, 6H, NCH3], 6.32 [s, 4H,
CH2C6H2(CH3)3-2,4,6], 6.96 [s, 4H, CH2C6H2(CH3)3-2,4,6],
7.19–7.39 [m, 8H, C6H4]. 13C NMR (100 MHz, CDCl3) δ (ppm) = 20.9
[CH2C6H2(CH3)-2,6], 21.0 [CH2C6H2(CH3)-4], 34.6 [NCH3], 49.7
[CH2C6H2(CH3)3-2,4,6], 109.9, 111.6 [C6H4-o], 122.9, 123.7 [C6H4-m],
128.2, 133.6 [C6H4-i], 126.4 [CH2C6H2(CH3)3-2,4,6, C3,5], 129.6
CDCl3)
δ (ppm) = 30.0 [CH2CH2N(CH2CH2)2O], 46.8 [CH2CH2N
(CH2CH2)2O], 54.0 [CH2CH2N(CH2CH2)2O], 58.3 [NCH3], 66.8
[CH2CH2N(CH2CH2)2O], 111.3, 111.5 [C6H4-o], 124.1, 124.3 [C6H4-m],
133.6, 134.2 [C6H4-i], 188.7 [NCHN]. Anal. Calcd for C14H19N3OClAg:
C, 43.27; H, 4.93; N, 10.81. Found: C, 43.32; H, 4.99; N, 10.92.
[CH2C6H2(CH3)3-2,4,6, C4], 135.3 [CH2C6H2(CH3)3-2,4,6,
C2,6],
General Method for the Preparation of the Palladium–NHC
Complexes (4a–f)
138.4 [CH2C6H2(CH3)3-2,4,6, C1], 173.8 [NCN]. Anal. Calcd for
C36H40N4Cl2Pd: C, 61.24; H, 5.71; N, 7.94. Found: C, 61.27; H, 5.75;
N, 8.02.
The Pd–NHC complexes (4a–f) were synthesized by transmetalation
via Ag–NHC complexes (3a–f) using the method reported by
Wang and Lin.[32] One equiv. of PdCl2(CH3CN)2 and 2 equiv. of
Ag–NHC complexes were added to a Schlenk tube; 20 mL DCM
was added and the mixture was stirred for 24 h at room
temperature (Fig. 3). The solution was filtered through celite,
and crystallized from dichloromethane–diethyl ether (1:2) at room
temperature. Molecular structures of Pd–NHC complexes were
determined by NMR, IR and micro analyses.
cis/trans-Dichloro-(di-[1-methyl-3-(2,3,5,6-tetramethylbenzyl)benzimidazole-
2-ylidene])palladium(II), 4d
1
Yield 60% (0.18 g); m.p. 284–286°C. ν(CN) = 1631 cmÀ1. H NMR
(399.9 MHz, CDCl3) δ (ppm) = 2.22 [s, 24H, CH2C6H(CH3)4-2,3,5,6],
4.60 [s, 6H, NCH3], 5.41 [s, 4H, CH2C6H(CH3)4-2,3,5,6], 6.38 [s, 2H,
CH2C6H(CH3)4-2,3,5,6], 7.90–7.30 [m, 8H, C6H4]. 13C NMR (100
MHz, CDCl3) δ (ppm) =20.5 [CH2C6H(CH3)4-2,3,5,6], 36.3 [NCH3],
50.9 [CH2C6H(CH3)4-2,3,5,6], 110.1, 111.7 [C6H4-o], 123.4, 123.7
[C6H4-m], 129.3, 132.9 [C6H4-i], 129.6 [CH2C6H(CH3)4-2,3,5,6, C4],
133.8 [CH2C6H(CH3)4-2,3,5,6, C2,6], 134.6 [CH2C6H(CH3)4-2,3,5,6,
C3,5 ], 135.3[CH2C6H(CH3)4-2,3,5,6, C1], 173.8 [NCN]. Anal. Calcd
for C38H44N4Cl2Pd: C, 62.17; H, 6.04; N, 7.63. Found: C, 62.21; H,
6.10; N, 7.74.
cis/trans-Dichloro-(di-[1-methyl-3-(2-methylbenzyl)benzimidazole-2-ylidene])
palladium(II), 4a
1
Yield 58% (0.18 g); m.p. 340–342°C. ν(CN) = 1631 cmÀ1. H NMR
(399.9 MHz, CDCl3) δ (ppm) = 2.51 [s, 6H, CH2C6H4(CH3)-2], 4.30
[s, 6H, NCH3], 5.98 [s, 4H, CH2C6H4(CH3)-2], 6.87–7.26 [m, 8H,
C6H4(CH3)-2],7.34–7.86 [m, 8H, C6H4]. 13C NMR (100 MHz, CDCl3)
δ (ppm) = 19.7 [CH2C6H4(CH3)-2]; 35.2 [NCH3]; 49.0 [CH2C6H4
(CH3)-2]; 111.4, 111.6 [C6H4-o], 112.0, 123.9 [C6H4-m], 124.6,
130.9 [C6H4-i], 126.6 [CH2C6H4(CH3)-2, C4], 127.7 [CH2C6H4(CH3)-
2, C5], 128.3 [CH2C6H4(CH3)-2, C6], 134.3 [CH2C6H4(CH3)-2, C3],
134.5[CH2C6H4(CH3)-2, C2], 136.4 [CH2C6H4(CH3)-2, C1], 167.4
cis/trans-Dichloro-(di-[1-methyl-3-(2,3,4,5,6-pentamethylbenzyl)benzimid-
azole-2-ylidene])palladium(II), 4e
Yield:58%(0.18g);m.p. 340–342°C. ν(CN) =1631cmÀ1.1HNMR(399.9
MHz, CDCl3) δ (ppm) = 2.08 [s, 6H, CH2C6(CH3)5-3,5], 2.19 [s, 6H, CH2C6
(CH3)5-4], 2.30 [s, 12H, CH2C6(CH3)5-2,6], 4.59 [s, 6H, NCH3], 5.39 and
6.34 [d, 4H, CH2C6(CH3)5-2,3,4,5,6], 6.15 [d, J= 8.4 Hz, 2H,
C6H4-H1], 6.92 [dd, J= 7.6 Hz and 8.4 Hz, 2H, C6H4-H2], 7.21
[dd, J= 7.6 Hz and 8.4 Hz, 2H, C6H4-H3], 7.36 [d, J= 8.4 Hz, 2H,
C6H4-H4]. 13C NMR (100 MHz, CDCl3) δ (ppm) = 16.8, 17.1,17.2,
[CH2C6(CH3)5-2,3,4,5,6], 36.3 [NCH3], 51.4 [CH2C6(CH3)5-2,
3,4,5,6]; 110.0, 111.9 [C6H4-o], 123.4, 123.6 [C6H4-m], 126.7,
133.9 [C6H4-i], 126.9 [CH2C6(CH3)5-2,3,4,5,6, C4], 135.4
[CH2C6(CH3)5-2,3,4,5,6, C2,6], 135.5 [CH2C6(CH3)5-2,3,4,5,6,
C3,5], 136.7 [CH2C6(CH3)5-2,3,4,5,6, C1], 173.6 [NCN]. Anal.
Calcd for C40H48N4Cl2Pd: C, 63.03; H, 6.35; N, 7.35. Found: C,
63.09; H, 6.43; N, 7.44
Figure 3. Synthesis of cis/trans-palladium–NHC complexes, 4a–f.
wileyonlinelibrary.com/journal/aoc
Copyright © 2014 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2014, 28, 423–431

Benzimidazole-based Pd–NHC complexes and Suzuki coupling reaction
cis/trans-Dichloro-(di-[1-methyl-3-(2-morpholinoethyl)benzimidazole-2-
ylidene])palladium(II), 4f
salts of 2a–f. The IR data for benzimidazolinium salts of 2a–f
clearly indicate the presence of the CÀN group with a ν(CÀN)
vibration at 1490, 1452, 1455, 1471, 1467 and 1459 cmÀ1. The
NMR values of 2a–f are similar to other synthesized and
characterized benzimidazolium salts.[34–39]
1
Yield 52% (0.18 g); m.p. 214–216°C. ν(CN) = 1646 cmÀ1. H NMR
(399.9 MHz, CDCl3) δ (ppm) = 2.67-2.72 [m, 8H, CH2CH2N(CH2CH2)
2O], 3.28 [t, J= 8 Hz, 4H, CH2CH2N(CH2CH2)2O], 3.70–3.75 [m, 8H,
CH2CH2N(CH2CH2)2O], 4.41 [s, 6H, NCH3], 5.04 [t, J = 8 Hz, 4H,
CH2CH2N(CH2CH2)2O], 7.31–7.49 [m, 8H, C6H4]. 13C NMR (100 MHz,
Preparation of Silver–Carbene Complexes 3a–f and
Palladium–Carbene Complexes 4a–f
CDCl3)
δ (ppm) = 33.8 [CH2CH2N(CH2CH2)2O], 45.1 [CH2CH2N
(CH2CH2)2O], 53.3 [CH2CH2N(CH2CH2)2O], 57.3 [NCH3], 66.3
[CH2CH2N(CH2CH2)2O], 109.8, 111.0 [C6H4-o], 122.2, 123.1 [C6H4-m],
133.8, 134.1 [C6H4-i], 180.6 [NCHN]. Anal. Calcd for C28H38N6O2Cl2Pd:
C, 50.35; H, 5.73; N, 12.58. Found: C, 50.41; H, 5.83; N, 12.69.
Ag–NHC complexes are widely used as transfer agents. Therefore, a se-
ries of Ag–NHC complexes, 3a–f, were prepared by addition of Ag2O
to the solution of benzimidazolium salts (2a–f) in CH2Cl2 (DCM) (Fig. 2).
The resulting suspension was stirred at room temperature over-
night, filtered through celite and crystallized with DCM–ether.
White crystals (in 50–70% yield) were washed and dried in vacuo.
Low field signals of 2a–f attributed to the acidic NCHN protons
General Procedure for Suzuki Cross-Coupling Reactions
Pd–NHC (4a–f) or Pd(OAc)2 (1.0 mmol%), 1,3-dialkylben-
zimidazolinium salts, 2a–f (2 mmol%), aryl chloride (1.0 mmol),
phenylboronic acid (1.5 mmol), KOtBu (2 mmol), water (3 ml)–DMF
(3 ml) were added to a small round-bottom flask under argon or
nitrogen gas and the mixture was heated at 50°C for 3 h. The
reaction mixture was then cooled to room temperature and
extracted with Et2O, filtered through a pad of silica with copious
washings, concentrated and purified by flash chromatography
on silica. The purity of the compounds was checked by NMR
and yields are based on aryl chloride.
1
were not observed in the H NMR spectra of Ag–NHC complexes
(2a–f). This observation confirmed that the benzimidazole silver
complexes were produced. Also, coordination of NHC ligands to
the metal center leads to low field resonance signals for carbene car-
bons at around δ 187–190 ppm. Signals for carbene carbons of some
Ag–NHC complexes (3b,3c, 3d, 3e) were not detected. This case has
been reported in the literature by different researchers.6a,40–44 The
structures of 3a–f were determined from their characteristic NMR
and IR spectroscopic data and elemental analyses.
Pd–NHC complexes were synthesized by addition of PdCl2(CH3CN)2
to the solution of Ag–NHC complexes in the dark as shown in Fig. 3;
complexes 3a–f were obtained in good yield as stable yellow solids.
The low field resonance signals of AgÀCcarb shifted to high field
167.4, 181.4, 181.6, 173.8, 173.6, 180.6 ppm, respectively, for 4a–f,
when the NHC ligands bonded to the palladium metal center. The
structures of 4a–f were determined from their characteristic NMR
and IR spectroscopic data and elemental analyses.
Results and Discussion
Preparation of 1-Methyl-Benzimidazolinium Salts
According to Scheme 1, 1-methyl-3-arylbenzimidazolinium salts,
2, are classic NHC precursors and were synthesized in
approximately quantitative yield by quaternization of 1-methyl-
benzimidazole in DMF with aryl and morpholine chlorides (Fig. 1).[33]
The structures of 2a–f were determined from their characteristic
In order to demonstrate the utility of these NHCs and their
palladium complexes, we used them as co-catalysts in Suzuki
coupling reactions with boronic acids and unactivated aryl
chlorides under mild reaction conditions.
1
spectroscopic data and elemental analyses. 13C NMR and H NMR
chemical shifts were consistent with the proposed structures; the im-
ino carbons were detected as a typical singlet with the 1H-decoupled
mode at 144.2, 142.7, 143.2, 143.0, 143.0 and 144.1 ppm. The reso-
nances of NCHN were observed as sharp singlets at 11.47, 11.67,
10.98, 10.73, 10.50, 11.36 ppm respectively for benzimidazolinium
Suzuki Coupling Reaction
To find optimal conditions, a series of Suzuki coupling reactions
were performed with 4-chloroacetophenone and phenylboronic
Table 1. Effect of solvent ratio on Suzuki coupling reaction of 4-chloroacetophenone with
phenylboronic acid catalyzed by 4aa
Entry
Solvent
DMF(6 ml)
Yieldb (%)
40
1
2
3
4
5
DMF/H2O (4/2 ml)
DMF/H2O (3/3 ml)
DMF/H2O (2/4 ml)
H2O ( 6 ml)
68
90
85
22
aReaction conditions: complex 4a (1 mol%), 4-chloroacetophenone (1 mmol), Ph(OH)2 (1.5
mmol), KOtBu (2 mmol), 50°C, 3 h.
bIsolated yields.
Appl. Organometal. Chem. 2014, 28, 423–431
Copyright © 2014 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/aoc

S. Çekirdek et al.
Table 2. Suzuki coupling reaction of aryl chlorides with phenylboronic acid catalyzed by LHX (2a–f)/Pd(OAc)2
Entry
R
Product
LHX
Yielda (%)
1
COMe
PhÀPhÀp-COMe
—
2a
2b
2c
2d
2e
2f
25e
99
99
95
92
90
94
30e
89
83
84
87
94
99
20e
93
83
70
69
60
58
15e
67
81
57
70
86
94
15e
55
77
93
92
70
60
2
3
4
5
6
7
8
COH
OMe
Me
PhÀPhÀp-COH
Ph―Ph―p-OMe
PhÀPhÀp-Me
PhÀPh
—
2a
2b
2c
2d
2e
2f
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
—
2a
2b
2c
2d
2e
2f
—
2a
2b
2c
2d
2e
2f
ÀH
—
2a
2b
2c
2d
2e
2f
aReaction conditions: 1.0 mmol p-RÀC6H4Cl, 1.5 mmol phenylboronic acid, 2 mmol KOtBu, 1 mol% Pd(OAc)2, 2 mmol% 2a–f,
water (3 ml)–DMF (3 ml).
bPurity of compounds was checked by NMR and yields are based on aryl chloride.
cAll reactions were monitored by TLC.
dTemperature 50°C, 3 h.
eNo LHX (2a–f).
acid as a model reaction. The effect of solvent on yield was thor-
oughly investigated and is summarized in Table 1. A trace
amount of product (4-phenylacetophenone) was obtained when
pure DMF was used as the solvent. However, addition of an incre-
mental amounts of water led to increasing amounts of product;
the highest yield was obtained when the DMF/H2O ratio was
1:1. The results demonstrated that the ratio of water and DMF
was very important for Suzuki coupling reactions of unactivated
aryl chlorides.
We tested the effect of common mineral bases such as Cs2CO3,
K2CO3, NaOH and KOtBu for the Suzuki coupling reactions of
unactivated aryl chlorides; KOtBu showed the best performance
in these catalytic systems. We also investigated the role of NHC li-
gand precursors and Pd–NHC complexes with control experiments
(Table 2, entries 1, 8, 15, 22, 29; and Table 3, entries 1, 8, 15, 22, 29).
As expected, low or trace yields were obtained in control
experiments. The no-LHX control experiment yields are higher than
the no-Pd–NHC catalyst control experiments. It is likely that
wileyonlinelibrary.com/journal/aoc
Copyright © 2014 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2014, 28, 423–431

Benzimidazole-based Pd–NHC complexes and Suzuki coupling reaction
Table 3. Suzuki coupling reaction of aryl chlorides with phenylboronic acid catalyzed by Pd–NHC complexes (4a–f)
Entry
R
Product
Pd–NHC
Yielda (%)
1
COMe
PhÀPhÀp-COMe
—
4a
4b
4c
4d
4e
4f
1e
90
81
80
79
83
81
3e
2
3
4
5
6
7
8
COH
OMe
Me
PhÀPhÀp-COH
PhÀPhÀp-OMe
PhÀPhÀp-Me
PhÀPh
—
4a
4b
4c
4d
4e
4f
9
80
77
76
74
85
90
0e
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
—
4a
4b
4c
4d
4e
4f
90
85
70
62
62
54
1e
—
4a
4b
4c
4d
4e
4f
62
77
59
64
76
88
2e
ÀH
—
4a
4b
4c
4d
4e
4f
54
71
87
88
67
61
aReaction conditions: 1.0 mmol p-R―C6H4Cl, 1.5 mmol phenylboronic acid, 2 mmol KOtBu, 1 mmol% Pd–NHC (4a–f),
water (3 ml)–DMF (3 ml).
bPurity of compounds was checked by NMR and yields are based on aryl chloride.
cAll reactions were monitored by TLC.
dTemperature 50°C, 3 h.
eNo Pd–NHC(4a–f).
some palladium black was produced and Pd(0) nanoparticles may
be responsible for catalysis. Thus results showed that NHCs are
significant ligands for Suzuki cross-coupling reactions.
After determining the optimum cross-coupling reaction condi-
tions, the scope of the reaction and efficiencies of the catalyst
system were evaluated by investigating the coupling of
phenylboronic acid with p-substituted aryl chlorides (Tables 2 and
3). With this catalytic system, unactivated p-substituted aryl chlo-
rides and chlorobenzene reacted very cleanly with phenylboronic
acid in good yields (Tables 2 and 3). In all cases, around 10%
homocoupling of the boronic acid was observed, probably originat-
ing from the reduction of the Pd(II) pre-catalyst. Both the electron-
rich and electron-poor unactivated aryl chlorides were coupled
successfully with boronic acid in good yields to give desirable
products by Pd(OAc)2/NHC precursors and Pd–NHC complexes
under mild reaction conditions. The catalytic activity of Pd–NHC
complexes strongly depends on the steric bulk of the NHC ligand.
Herein, we reported sterically similar benzimidazole-based NHC
Appl. Organometal. Chem. 2014, 28, 423–431
Copyright © 2014 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/aoc

S. Çekirdek et al.
[7] a) T. Weskamp, W. C. Schattenmann, M. Spiegler, W. A. Herrmann,
Angew. Chem. Int. Ed. 1998, 37, 2490; b) J. Huang, E. D. Stevens, S.
P. Nolan, J. L. Petersen, J. Am. Chem. Soc. 1999, 121, 2674.
[8] a) C. W. K. Gstöttmayr, V. P. W. Böhm, E. Herdtweck, M. Grosche,
W. A. Hermann, Angew. Chem. Int. Ed. 2002, 41, 1363; b) M. F. Lappert,
J. Organomet. Chem. 1988, 358, 185; c) C. Zhang, J. Huang, M. L. Trudell,
S. P. Nolan, J. Org. Chem. 1999, 64, 3804; d) A. Fürstner, A. Leitner, Synlett
2001, 290.
[9] a) D. Bourissou, O. Guerret, F. P. Gabbai, G. Bertrand, Chem. Rev.
2000, 100, 39; b) W. A. Herrmann, T. Wescamp, V. P. W. Böhm, Adv.
Organomet. Chem. 2001; 48, 1.
[10] a) J. Hassan, M. Sevignon, C. Gozzi, E. Schulz, M. Lemaire, Chem. Rev.
2002, 102, 1359; b) N. Miyaura, Top. Curr. Chem. 2002, 219, 11; c)
J. Y. Shin, B. S. Lee, Y. Jung, S. J. Kim, S. Lee, Chem. Commun. 2007,
5238; d) H. Doucet, J. Eur. Org. Chem. 2008, 12, 2013; e) V. L. Budarin,
J. H. Clark, R. Luque, D. J. Macquarrie, R. J. White, Green Chem. 2008,
10, 382; f) S. Kotha, K. Lahiri, D. Kashinath, Tetrahedron 2002, 58,
9633.
ligands. The catalytic performance of (2a–e) and (4a–e) is within an
interval due to similar steric effects of ligands. Also we wanted to
see the effect of aromatic and non-aromatic cyclic groups on N-sub-
stituents. Although results were quite close to each other, some-
times 2e showed better (Table 2 and Table 3, entries 14 and 28)
or worse catalytic activity. From these results, we assume that high
catalytic activity is dependent upon substrate/catalyst matching.
It should be noted that ‘in situ’ formation of Pd–NHC complex
gave significantly better results than the use of the pre-formed
Pd–NHC complex (Table 2). Reduction of Pd(II) to Pd(0) is very
important for the Suzuki reaction. It is envisaged that induction
periods for Suzuki reactions are supported by Pd(OAc)2/LHX. Dur-
ing these induction periods Pd(II)/NHC complexes were formed
and then reduced slowly to catalytically active palladium–NHC
complexes.[45]
[11] a) N. Hadei, E. A. B. Kantchev, C. J. O’Brien, M. G. Organ, Org. Lett.
2005, 7, 1991; b) C. J. O’Brien, E. A. B. Kantchev, G. A. Chass, N. Hadei,
A. C. Hopkinson, M. G. Organ, D. H. Setiadi, T. H. Tang, D. C. Fang,
Tetrahedron 2005, 61, 9723.
As a result, these benzimidazolium ligands and their palladium
complexes were found to be useful catalysts for the formation of
biaryls from unactivated aryl chlorides.
[12] a) H. V. Huynh, J. H. H. Ho, T. C. Neo, L. L. J. Koh, Organomet. Chem. 2005,
690, 3854; b) C. Metallinos, F. B. Barrett, J. L. Chaytor, M. E. A. Heska, Org.
Lett. 2004, 6, 3641.
[13] a) L. Cavallo, A. Correa, C. Costabile, H. J. Jacobsen, Organomet.
Chem. 2005, 690, 5407; b) O. Kuhl, J. Coord. Chem. Rev. 2009,
253, 2481.
[14] M. S. Viciu, O. Navarro, R. F. Germaneau, R. A. Kelly, W. Sommer, N. Marion,
E. D. Stevens, L. Cavallo, S. P. Nolan, Organometallics 2004, 23, 1629.
[15] a) U. Christman, R. Vilar, Angew. Chem. Int. Ed. 2005, 44, 366; b)
E. Galardon, S. Ramdeehul, J. M. Brown, A. Cowley, K. K. M. Hii,
A. Jutand, Angew. Chem. Int. Ed. 2002, 41, 1760.
[16] a) J. P. Wolfe, R. A. Singer, B. H. Yang, S. L. Buchwald, J. Am. Chem.
Soc. 1999, 121, 9550; b) G. A. Grasa, M. S. Viciu, J. Huang, C. Zhang,
M. L. Trudell, S. P. Nolan, Organometallics 2002, 21, 2866.
[17] a) A. J. Arduengo, R. L. Harlow, M. J. Kline, Am. Chem. Soc. 1991, 113, 361;
b) A. J. Arduengo, Acc. Chem. Res. 1999, 32, 913; c) D. Bourissou, O.
Guerret, F. P. Gabbai, G. Bertrand, Chem. Rev. 2000, 100, 39; d)
G. C. Fortman, S. P. Nolan, Chem. Soc. Rev. 2011, 40, 5151; e) J. L. Moore,
T. Rovis, Top. Curr. Chem. 2010, 291, 77; f) C. Valente, S. Calimsiz, K. H. Hoi,
D. Mallik, M. Sayah, M. G. Organ, Angew. Chem. Int. Ed. 2012, 51, 3314.
[18] N. Miyaura, A. Suzuki, Chem. Rev. 1995, 95, 2457.
Conclusions
In the last couple of years, NHCs have been a commonly used
ligand for many transition metals and catalytic transformations.
Sterically demanding NHCs have often been found valuable
due to their unique impact on the metal coordination sphere.
The design of NHC ligands and their palladium complexes with
steric properties that might be independently varied have been
highlighted in this study. This study has supported the hypothe-
sis that in situ formation of the active catalyst by deprotonation of
the benzimidazolinium chlorides with appropriate base leads to
significantly better results than the pre-formed benzimidazole–
palladium carbene complexes. We have synthesized highly
active, easily synthesized and environmentally friendly NHC
precursors and their palladium complexes. We tested catalysis
for palladium-mediated the Suzuki cross-coupling in aqueous me-
dia using benzimidazolidin-2-ylidene ligands/Pd–NHC complexes.
The catalytic system is simple and efficiently couples various
unactivated aryl chlorides.
[19] A. J. Suzuki, Organomet. Chem. 1999, 576, 147.
[20] S. P. Stanforth, Tetrahedron 1998, 54, 263.
[21] N. J. Whitcombe, K. K. Hii, S. E. Gibson, Tetrahedron 2001, 57, 7449.
[22] G. T. Crisp, Chem. Soc. Rev. 1998, 27, 427.
[23] a) A. F. Littke, G. C. Fu, Angew. Chem. Int. Ed. Engl. 2002, 41, 4176; b)
A. J. Suzuki, Organomet. Chem. 2002, 653, 83; c) L. Xu, W. Chen, J. J. Xiao,
Mol. Catal. A 2002, 187, 189; d) J. Yin, M. P. Rainka, X. X. Zhang,
S. L. Buchwald, J. Am. Chem. Soc. 2002, 124, 1162.
Acknowledgment
[24] a) S. D. Walker, T. E. Barder, J. R. Martinelli, S. L. Buchwald, Angew.
Chem. Int. Ed. Engl. 2004, 43, 1871; b) A. F. Littke, C. Dai, G. C. J. Fu,
Am. Chem. Soc. 2000, 122, 4020; c) G. R. Rosa, G. Ebeling, J. Dupont,
A. L. Monteiro, Synthesis 2003, 18, 2894; d) D. Bourissou, O. Guerret,
F. P. Gabbai, G. Bertrand, Chem. Rev. 2000, 100, 39; e) W. A. Herrmann,
T. Wescamp, V. P. W. Bohm, Adv. Organomet. Chem. 2001, 48, 1; f) C.
Zhang, J. Huang, M. L. Trudell, S. P. Nolan, J. Org. Chem. 1999, 64, 3804;
g) A. Furstner, A. Leitner, Synlett 2001, 14, 290.
This work was financially supported by Gaziosmanpasa University
Research Fund (GOP. B.A.P:2013/44, 2011/51 and 2011/105).
References
[1] a) K. Öfele, J. Organomet. Chem. 1968, 12, 42; b) H. W. Wanzlick,
H. J. Schonher, Angew. Chem. Int. Ed. 1968, 7, 141.
[25] G. R. Peh, E. A. B. Kantchev, J. C. Er, J. Y. Ying, Chem. Eur. J. 2010, 16, 4010.
[26] Y. Q. Tang, H. Lv, J. M. Lu, L. X. Shao, J. Organomet. Chem. 2011, 696, 2576.
[27] Y. Q. Tang, J. M. Lu, L. X. Shao, J. Organomet. Chem. 2011, 696, 3741.
[28] G. Altenhoff, R. Goddard, C. W. Lehmann, F. Glorius, Angew. Chem.
Int. Ed. 2003, 42, 3690.
[29] a) J. P. Genet, M. Savignac, J. Organomet. Chem. 1999, 576, 305; b) H.
Sakurai, T. Sukuda, T. Hirao, J. Org. Chem. 2002, 67, 2721; c) S. Parisot,
R. Kolodziuk, C.G. Henry, A. Iourtchenko, D. Sinou, Tetrahedron Lett.
2002, 43, 7397.
[30] a) A. C. Hillier, G. A. Grasa, M. S. Viciu, H. M. Lee, C. Yang, S. P. Nolan,
J. Organomet. Chem. 2002, 653, 69; b) C. Zhang, M. L. Trudell,
Tetrahedron Lett. 2000, 41, 595.
[31] İ. Özdemir, S. Demir, S. Günal, C. Arıcı, D. Ülkü, Inorg. Chim. Acta
2010, 363, 3803.
[2] a) A. J. Arduengo, R. L. Harlow, M. Kline, J. Am. Chem. Soc. 1991, 113,
361; b) A. J. Arduengo, Acc. Chem. Res. 1999, 32, 913.
[3] a) S. P. Nolan, N-Heterocyclic Carbenes in Synthesis, Wiley-VCH,
Weinheim, 2006; b) F. Glorius, N-Heterocyclic Carbenes in Transition
Metal Catalysis, Topics in Organometallic Chemistry, Springer, Berlin,
2007; c) W. A. Herrmann, C. Kocher, Angew. Chem. Int. Ed. 1997, 36,
2162; d) W. A. Herrmann, Angew. Chem. Int. Ed. 2002, 41, 1290;
e) S. D. Gonzalez, N. Marion, S. P. Nolan, Chem. Rev. 2009, 109,
3612; f) R. H. Crabtree, Coord. Chem. Rev. 2007, 251, 595.
[4] W. A. Herrmann, L. T. Goosen, C. Köcher, G. R. J. Artus, Angew. Chem.
Int. Ed. Engl. 1996, 35, 2805.
[5] H. Küçükbay, B. Cetinkaya, G. Salaheddine, P. H. Dixneuf, Organometallics
1996, 15, 2434.
[6] a) S. Yaşar, K. J. Cavell, B. D. Ward, B. Kariuki, Appl. Organomet. Chem.
2011, 25, 374; b) N. Gurbuz, I. Ozdemir, S. Demir, B. Cetinkaya, J. Mol.
Catal. A: Chem. 2004, 209, 23.
[32] H. M. J. Wang, I. J. B. Lin, Organometallics 1998, 17, 972.
[33] F. Demirhan, O. Yildirim, B. Cetinkaya, Trans. Metal. Chem. 2003, 28, 558.
wileyonlinelibrary.com/journal/aoc
Copyright © 2014 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2014, 28, 423–431

Benzimidazole-based Pd–NHC complexes and Suzuki coupling reaction
[34] Y. Gök, N. Gürbüz, I. Özdemir, B. Çetinkaya, E. Çetinkaya, Appl.
Organomet. Chem. 2005, 19, 870.
[42] X. Hu, I. C. Rodriquez, K. Olsen, K. Meyer, Organometallics 2004,
23, 755.
[35] N. Gürbüz, S. Yasar, E. Ö. Özcan, I. Özdemir, B. Çetinkaya, Eur. J. Inorg.
Chem. 2010, 3051.
[43] K. S. Coleman, H. T. Chamberlayne, S. Turberville, M. L. H. Gren,
A. R. Cowley, Dalton Trans. 2003, 2917.
[36] S. Köytepe, T. Seçkin, S. Yaşar, I. Özdemir, Des. Monomers Polym.
2008, 11, 409.
[44] K. J. Cavell, K. J. Nielsen, B. W. Skelton, A. H. White, Inorg. Chim. Acta
2003, 352, 143.
[37] S. Demir, I. Özdemir, B. Çetinkaya, Appl. Organomet. Chem. 2009, 23, 520.
[38] I. Özdemir, Y. Gök, N. Gürbüz, E. Çetinkaya, B. Çetinkaya,
J. Heterocyclic Chem. 2005, 42, 303.
[45] D. S. McGuinness, K. J. Cavell, Organometallics 2000, 19, 741.
[39] S. Grupta, B. Basu, S. Das, Tetrahedron 2013, 69, 122.
[40] J. Pytkowicz, S. Roland, P. Mangeney, J. Organomet. Chem. 2001, 631,
157.
Supporting Information
Additional supporting information may be found in the online
version of this article at the publisher's web-site.
[41] H. M. Lee, P. L. Chiu, C. H. Hu, C. L. Lai, Y. C. Chou, J. Organomet.
Chem. 2005, 690, 403.
Appl. Organometal. Chem. 2014, 28, 423–431
Copyright © 2014 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/aoc