Direct arylation of heteroarenes by PEPPSI-type palladium–NHC complexes and representative quantum chemical calculations for the compound which the structure was determined by X-ray crystallography

Article abstract of DOI:10.1080/00958972.2019.1692202

In this study, we synthesized five new PEPPSI-type Pd-NHC complexes with high yields around 78–83%. The structures of all complexes were characterized by FT-IR, ¹H NMR, and ¹³C{¹H} NMR spectroscopies. Further, the structur

Full text of DOI:10.1080/00958972.2019.1692202

Journal of Coordination Chemistry
ISSN: 0095-8972 (Print) 1029-0389 (Online) Journal homepage: https://www.tandfonline.com/loi/gcoo20
Direct arylation of heteroarenes by PEPPSI-type
palladium–NHC complexes and representative
quantum chemical calculations for the compound
which the structure was determined by X-ray
crystallography
Neslihan Şahin, Goncagül Serdaroğlu, Serpil Demir Düşünceli, Muhammad
Navaz Tahir, Cengiz Arıcı & İsmail Özdemir
To cite this article: Neslihan Şahin, Goncagül Serdaroğlu, Serpil Demir Düşünceli, Muhammad
Navaz Tahir, Cengiz Arıcı & İsmail Özdemir (2019): Direct arylation of heteroarenes by PEPPSI-
type palladium–NHC complexes and representative quantum chemical calculations for the
compound which the structure was determined by X-ray crystallography, Journal of Coordination
Chemistry, DOI: 10.1080/00958972.2019.1692202
To link to this article: https://doi.org/10.1080/00958972.2019.1692202
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JOURNAL OF COORDINATION CHEMISTRY
https://doi.org/10.1080/00958972.2019.1692202
Direct arylation of heteroarenes by PEPPSI-type
palladium–NHC complexes and representative quantum
chemical calculations for the compound which the
structure was determined by X-ray crystallography
d
b,c
Neslihan S¸ahina,b,c, Goncagul Serdaroglu , Serpil Demir Dus¸unceli
,
€
ꢀ
€ €
b,c
e
f
_
€
Muhammad Navaz Tahir , Cengiz Arıcı and Ismail Ozdemir
b
aFaculty of Education, Department of Basic Education, Cumhuriyet University, Sivas, Turkey; Faculty
c
_
€ €
of Science and Art, Department of Chemistry, Inonu University, Malatya, Turkey; Catalysis Research
d
€ €
and Application Center, Inonu University, Malatya, Turkey; Faculty of Education, Department of
Science Education, Cumhuriyet University, Sivas, Turkey; Department of Physics, University of
Sargodha, Sargodha, Pakistan; The Scientific and Technological Research Council of Turkey,
e
f
Ankara, Turkey
ABSTRACT
ARTICLE HISTORY
Received 17 July 2019
Accepted 23 October 2019
In this study, we synthesized five new PEPPSI-type Pd-NHC com-
plexes with high yields around 78–83%. The structures of all com-
1
plexes were characterized by FT-IR, 1H NMR, and 13Cf Hg NMR
KEYWORDS
spectroscopies. Further, the structure of 2c was determined by
X-ray crystallography. The single-crystal structure of 2c shows that
coordination geometry around Pd is distorted square planar. The
Pd-NHC complexes were efficient catalysts for the direct C5-aryla-
tion of 2-n-butylthiazole, 2-n-butylfuran, and 2-n-butylthiophene
with various aryl-bromides and showed high catalytic activity for
arylation reaction using only 1 mol% catalyst loading at 130ꢀC for
1 h. The conversions for substrates containing electron-withdraw-
ing groups were higher than for substituents containing electron-
donating groups. In order to evaluate the catalytic activity of the
complexes, representative electronic and spectroscopic simula-
tions by DFT method have been performed for 2c. NBO analysis
has revealed that the strongest interaction contributing to the
PEPPSI; N-heterocyclic
carbine; X-ray structure;
direct arylation; quantum
chemical calculations
lowering of the molecular stabilization energy for 2c is predicted
(2)
as p(C11-N27) ! p (N26-C28) with E ¼ 73.93 kcal molꢂ1. In
ꢁ
addition, FMO analysis has implied that 2c is more chemically
reactive because of its lower energy gap value. According to the
other quantum chemical parameters, 2c is softer, better electro-
phile and has more charge-transfer capability than 1c.
_
€
CONTACT Ismail Ozdemir
ismail.ozdemir@inonu.edu.tr
Supplemental data for this article is available online at here.
ß 2019 Informa UK Limited, trading as Taylor & Francis Group

2
N. ŞAHIN ET AL.
1. Introduction
Direct arylation reactions through cleavage of C–H bonds is considered as an environ-
mentally and economically more attractive strategy [1–5]. Consequently, the procedure
provides a valuable and straightforward technique for the synthesis of biaryls. In 1990,
Ohta and co-workers reported the direct 2- or 5-arylation of thiophenes, furans or thia-
zoles, with aryl-halides, in moderate to good yields by using [Pd(PPh3)4] as the catalyst
[6]. Since then, the palladium-catalyzed direct arylation of heteroaryl derivatives with
aryl-halides has proved to be a powerful method for the synthesis of a wide variety of
arylated heterocycles [7–11]. The use of such functional halothiophenes for direct ary-
lation would be useful, as they would give simple access to a wide variety of polyfunc-
tionalized thiophenes useful for material chemistry or pharmaceutical applications. For
example, the 2-arylthiophene derivative Canagliflozin [12] is a drug for the treatment
of type-2 diabetes, and Atliprofen is an anti-inflammatory drug. Several thiophene
derivatives containing a hydroxyalkyl group at C2 have also been found to be bio-
active. Duloxetine is employed against major depressive disorder and penthienate is
antimuscarinic; also several 2-arylated furans display important biological properties.
For example, dantrolene is a muscle relaxant and lapatinib is employed against breast
cancer (Figure 1).
The palladium-catalyzed direct arylation of several heteroaromatics [13–18] via a
C–H bond activation using aryl-halides has led to successes in recent years. Very excit-
ing results have been reported by several groups using thiophenes, furans, pyrroles,
thiazoles, oxazoles, imidazoles or triazoles [19, 20]. Azole-derived metal complexes
have found application in many fields [21–23]. In particular, nucleophilic N-heterocyclic
carbenes (NHCs) have proven as useful ligands for transition metal catalysis [24–28].
Among the most popular catalysts for such reactions are PEPPSI-type palladium-NHC
complexes of the type [PdX2(NHC)(pyridine)] (X ¼ halide, PEPPSI ¼ pyridine enhanced
precatalyst preparation, stabilization, and initiation), which have gained real practical
importance in numerous catalytic processes [29–31]. In 2006, Organ et al. reported
[32] easily handled, air- and moisture-stable Pd–NHC complexes created by using the
PEPPSI method, which featured Pd(II) species bearing an NHC ligand, two halides, and

JOURNAL OF COORDINATION CHEMISTRY
3
Figure 1. Examples of bioactive thiophene or furan derivatives.
a labile ligand such as 3-chloropyridine. A lot of work has been done by many groups
since 2006 on these types of complexes, which are extremely stable in air and moisture.
ꢁ
Nolan and co-workers also showed the preparation of reactivity of [Pd(IPr )(3-Cl-
ꢁ
pyridinyl)Cl2] complex (IPr ¼ 1,3-bis(2,6-bis(diphenylmethyl)-4-methylphenyl)imidazo-2-
ylidene) and catalytic activity in the Buchwald-Hartwig amination reaction [33].
In addition to the synthesis and spectroscopic characterization of the NHC ligands
and their metal complexes, they have been started to be investigated with computa-
tional tools due to the importance of predicting the chemical activities and providing
preliminary information for the compounds to be newly synthesized. In the past, Pd-pyri-
dine/pyrimidine [34], dichloropalladium(II) of 7-azaindole [35] and ferrocenyl-NHC Pd(II)
[36], Pd(II)-NHC complexes [37] were synthesized and characterized by structural and
spectroscopic techniques, and supported by computational tools an insight into experi-
mental results. In another study, Mansour and coworkers [38] investigated the cytotox-
icity and antimicrobial activity of Pd(II) and Pt(II) complexes, and they used the electronic
structure methods in explaining of the experimental activity results. Moreover, the anti-
bacterial and cytotoxic activities of the Ni(II), Pd(II), and Pt(II) complexes were investi-
gated by Bouchoucha and coworkers [39] suggested that the Pd(II) and Pt(II) complexes
had a great inhibition against bacteria tested. In a recent study, the PdCl2 complex [40]
was investigated by spectroscopic and quantum chemical tools to predict the electronic,
optics, and spectroscopic properties of the investigated complex and ligand.
Having the above in mind, first, we report the synthesis and characterization of
new PEPPSI-type palladium(II) complexes. The compounds were characterized by FT-IR
1
1
and H and 13Cf Hg NMR spectroscopies. The structure of 2c was determined by sin-
gle-crystal X-ray diffraction. We then examined the activity of palladium(II) complexes
in the direct C5-arylation of 2-n-butylthiazole, 2-n-butylfuran, and 2-n-butylthiophene
with various aryl bromides. In the computational part of this work, we aim to evaluate
the catalytic activity of 2c with quantum chemical tools. For this purpose, the geometric

4
N. ŞAHIN ET AL.
structures verified by the frequency calculations have been used in further calculations,
including both the spectroscopic and electronic analyses. The results of calculated NMR
and FT-IR spectral analyses have been compared with the corresponding observed values.
Natural bond orbital (NBO) and frontier molecular orbital (FMO) analyses of both com-
pounds have been used in evaluating/explaining the chemical reactivity tendency.
2. Experimental
2.1. Materials and measurements
All manipulations were carried out under argon in flame-dried glassware using standard
Schlenk techniques. Chemicals were obtained from Sigma Aldrich and Fluka. Melting
points were determined in glass capillaries under air with an Electrothermal-9200 melting
point apparatus. Fourier-transform infrared (FTIR) spectra were obtained from 400 to
1
1
4000 cmꢂ1 on a Perkin Elmer Spectrum 100 FT-IR. H NMR and 13Cf Hg NMR spectra were
taken using a Bruker As 400 Mercury spectrometer operating at 400 MHz (1H) and
100 MHz (13Cf Hg) in CDCl3 with tetramethylsilane as the internal reference. Coupling con-
1
stants (J values) are given in hertz. NMR multiplicities are abbreviated as follows:
s¼ singlet, d ¼ doublet, t ¼ triplet, hept ¼ heptet, and m ¼ multiplet signal.
The intensity data were collected using a Bruker Kappa ApexII CCD diffractometer [41]
using Mo-Ka radiation (k ¼ 0.71073 Å). The structure was solved by direct methods using
the solution program SHELXS97 [42] and then refined with full-matrix least-square meth-
ods based on F2SHELXL2014/6 [43]. All non-hydrogen atoms were refined isotropically first
and then with anisotropic atomic displacement parameters by full matrix least square
methods. All hydrogen atoms bonded to carbon atoms were placed geometrically. All
hydrogen atoms were refined as riding with Ueq(H) ¼ 1.2 Uiso(C). All catalytic reactions
were monitored on an Agilent 6890N GC system by GC-FID with an HP-5 column of 30 m
length, 0.32 mm diameter and 0.25 lm film thickness. Column chromatography was per-
formed using silica gel 60 (70-230 mesh). Solvent ratios are given as v/v.
2.2. Synthesis
All compounds were prepared under argon gas atmosphere. Benzimidazole-based N-
heterocyclic carbene ligands (1a–e) and their corresponding Pd(II)-NHC complexes
(2a–e) were synthesized according to literature [44]. Ligands 1a–c and 1e were
reported in our previous studies [45–47]. Ligand 1d was published previously [48].
2.3. Synthesis and characterization of benzimidazolium salts (1a–e)
For the preparation of the benzimidazolium salts, N-substituted benzimidazole
(1.0 mmol) was dissolved in dried dimethylformamide (3 mL) and the alkyl-bromide
(1.0 mmol) was slowly added. The reaction mixture was stirred at 80 ꢀC for 12 h under
argon. After completion of the reaction, all DMF was removed by vacuum, and diethyl
ether (15 mL) was added to obtain a white crystalline solid, which was filtered off. The
solid was washed with diethyl ether (3 ꢃ 10 mL) and dried under vacuum. The crude
product was recrystallized from an ethanol/diethyl ether mixture (1:2, v/v).

JOURNAL OF COORDINATION CHEMISTRY
5
2.3.1. 1-(Cyclobutylmethyl)-3-(methoxyethyl)benzimidazolium bromide, 1a
Yield: 78%, m.p. 136–137 ꢀC. FT-IR ꢀ(CN): 1555 cmꢂ1 1H NMR (400 MHz, CDCl3), d
.
(ppm): 1.90–2.02 (m, 4H, CH2CHC3H6), 2.12–2.22 (m, 2H, CH2CHC3H6), 3.34 (s, 3H,
CH2CH2OCH3) 3.02 (hept., 1H, CH2CHC3H6, J ¼ 8.1 Hz), 4.89 (t, 2H, CH2CH2OCH3,
J ¼ 4.5 Hz), 4.58 (d, CH2CHC3H6, J ¼ 7.8 Hz), 3.93 (t, 2H, CH2CH2OCH3, J ¼ 4.5 Hz),
1
7.60–7.75 (m, 4H, Ar-H), 11.13 (NCHN). 13Cf Hg NMR (100 MHz, CDCl3), d (ppm): 17.9
(CH2CHC3H6), 25.8 (CH2CHC3H6), 34.4 (CH2CHC3H6), 47.7 (CH2CH2OCH3), 52.1
(CH2CHC3H6), 59.1 (CH2CH2OCH3), 70.4 (CH2CH2OCH3), 112.7, 114.1, 127.0, 127.4, 131.1,
132.0 (Ar-C), 142.5 (NCHN).
2.3.2. 1-(Cyclobutylmethyl)-3-(4-methylbenzyl)benzimidazolium bromide, 1b
Yield: 79%, m.p. 210–211 ꢀC. FT-IR ꢀ(CN): 1552 cmꢂ1 1H NMR (400 MHz, CDCl3), d
.
(ppm): 1.87–2.09 (m, 4H, CH2CHC3H6), 2.12–2.20 (m, 2H, CH2CHC3H6), 2.31 (s,
3H,CH2C6H4-4-CH3)), 3.07 (hept., 1H, CH2CHC3H6, J ¼ 7.8 Hz), 4.62 (d, 2H, CH2CHC3H6,
J ¼ 7.8 Hz), 5.88 (s, 2H, CH2C6H4-4-CH3),7.41 (d, 2H, CH2C6H4-4-CH3, J ¼ 8.1 Hz), 7.15(d,
1
2H, CH2C6H4-4-CH3, J ¼ 7.5 Hz), 7.52–7.72(m, 4H, NC6H4N), 11.68 (s, 1H, NCHN). 13Cf Hg
NMR (100 MHz, CDCl3), d (ppm): 17.9 (CH2CHC3H6), 21.2 (CH2C6H4-4-CH3), 25.8
(CH2CHC3H6), 34.5 (CH2CHC3H6), 51.2(CH2C6H4-4-CH3), 52.3 (CH2CHC3H6), 113.0,113.9,
127.1, 127.2,128.3, 129.8, 129.9, 131.1, 131.5, 139.2 (Ar-C), 142.5 (NCHN).
2.3.3. 1-(Cyclobutylmethyl)-3-(4-methoxybenzyl)benzimidazolium bromide, 1c
1
Yield: 73%, m.p.183–184 ꢀC. FT-IR ꢀ(CN): 1514 cmꢂ1. H NMR (400 MHz, CDCl3), d (ppm):
1.88–2.02 (m, 4H, CH2CHC3H6), 2.12–2.22 (m, 2H, CH2CHC3H6), 3.05 (hept., 1H,
CH2CHC3H6, J ¼ 7.8 Hz), 3.75 (s, 3H, CH2C6H4-4-OCH3), 4.58 (d, 2H, CH2CHC3H6,
J ¼ 7.5 Hz), 5.86 (s, 2H, CH2C6H4-4-OCH3), 6.86 (d, 2H, CH2C6H4-4-OCH3, J ¼ 8.7 Hz), 7.52
1
(d, 2H, CH2C6H4-4-OCH3, J ¼ 8.7 Hz), 7.69 (m, 4H, NC6H4N), 11.59 (s, 1H, NCHN). 13Cf Hg
NMR (100 MHz, CDCl3),
d (ppm): 17.9 (CH2CHC3H6), 25.8 (CH2CHC3H6), 25.9
(CH2CHC3H6), 34.4 (CH2CHC3H6), 50.9 (CH2C6H4-4-OCH3), 52.3 (CH2CHC3H6), 55.3
(CH2C6H4-4-OCH3), 113.0, 113.9, 114.6, 124.8, 127.0, 127.1, 130.1, 131.1, 131.5, 160.1
(Ar-C), 142.4 (NCHN).
2.3.4. 1-Benzyl-3-(2,3,5,6-tetramethylbenzyl)benzimidazolium bromide, 1d
Yield: 84%, m.p. 203–204 ꢀC. FT-IR ꢀ(CN): 1554 cmꢂ1 1H NMR (400 MHz, CDCl3), d
.
(ppm): 2.15 and 2.17 (s, 12H, CH2C6H(CH3)4-2,3,5,6), 6.15 (s, 2H, CH2C6H5), 6.23 (s, 2H,
CH2C6H(CH3)4-2,3,5,6), 7.24 (s, 1H, CH2C6H(CH3)4-2,3,5,6), 7.20–7.32 (m, 5H, CH2C6H5),
1
7.53–7.70 (m, 4H, Ar-H),), 11.18 (NCHN). 13Cf Hg NMR (100 MHz, CDCl3), d (ppm): 16.3
and 20.2 (CH2C6H(CH3)4-2,3,5,6), 50.8 (CH2C6H5), 53.3 (CH2C6H(CH3)4-2,3,5,6), 111.2,
111.5, 123.2, 124.5, 128.0, 128.1, 129.1, 129.3, 129.5, 132.5, 134.3, 135.0, 137.9, 138.1
(Ar-C), 142.7 (NCHN).
2.3.5. 1,3-Di-(4-tertbutylbenzyl)benzimidazolium bromide, 1e
Yield: 89%, m.p. 218–219 ꢀC. FT-IR ꢀ(CN): 1550 cmꢂ1 1H NMR (400 MHz, CDCl3) d
.
(ppm): 1.18 (s, 18H, CH2C6H4(C(CH3)3)-4), 5.75 (s, 4H, CH2C6H4(C(CH3)3)-4)), 7.28–7.65
1
(m, 12H, Ar-H), 11.21(NCHN). 13Cf Hg NMR (100 MHz, CDCl3),
d (ppm): 29.8

6
N. ŞAHIN ET AL.
(CH2C6H4(C(CH3)3)-4), 34.1 (CH2C6H4(C(CH3)3)-4), 53.1 (CH2C6H4(C(CH3)3)-4), 113.0, 113.7,
126.2, 126.3, 127.0, 127.1, 128.1, 129.8, 131.7, 152.4 (Ar-C), 143.2 (NCHN).
2.4. Synthesis and characterization of PEPPSI-Type palladium-NHC
complexes (2a–e)
The PEPPSI-type palladium NHC-complexes 2a–d were obtained in moderate to good
yields using the standard procedure initially developed by Organ [32]. A mixture of
K2CO3 (5.0 mmol), pyridine (3 mL), PdCl2 (1.1 mmol), benzimidazolium salt (1.0 mmol),
and KBr (10.0 mmol) was heated at 80 ꢀC for 16 h. The reaction mixture was then fil-
tered through a pad of celite and silica gel to remove the unreacted PdCl2 and benzi-
midazolium salt. The solvent in the reaction medium was then removed. The resulting
complexes were washed with n-pentane (3 ꢃ 5 mL) and dried under vacuum. The
crude product was crystallized in CH2Cl2/pentane and bright yellow Pd-NHC com-
plexes (2a–e) were obtained.
2.4.1. Dibromo[1-(cyclobutylmethyl)-3-(methoxyethyl)benzimidazole-2-ylidene]pyridine
palladium(II), 2a
Yield: 83%, m.p. 218–219 ꢀC. FT-IR ꢀ(CN): 1414 cmꢂ1 1H NMR (400 MHz, CDCl3), d
.
(ppm): 1.89–1.98 (m, 2H, CH2CHC3H6), 2.05–2.12 (m, 4H, CH2CHC3H6), 3.27 (s, 3H,
CH2CH2OCH3) 3.52 (hept., 1H, CH2CHC3H6,J ¼ 8 Hz), 4.18 (t, 2H, CH2CH2OCH3, J ¼ 4 Hz),
4.84 (d, CH2CHC3H6, J ¼ 4 Hz), 4.99 (t, 2H, CH2CH2OCH3, J ¼ 4 Hz), 7.34–7.36, 7.70–7.72
1
and 7.96–8.00 (m, 7H, Ar-H), 8.91 (d, 2H, C5H5, J ¼ 4 Hz). 13Cf Hg NMR (100 MHz,
CDCl3), d (ppm): 18.4 (CH2CHC3H6), 26.6 (CH2CHC3H6), 35.4 (CH2CHC3H6), 48.2
(CH2CH2OCH3), 53.4 (CH2CHC3H6), 58.8 (CH2CH2OCH3), 71.1 (CH2CH2OCH3), 111.3, 112.0,
123.5, 125.7, 126.0, 134.6, 135.2, 139.7, 152.5, 154.4 (Ar-C), 162.7 (Pd-Ccarbene).
2.4.2. Dibromo[1-(cyclobutylmethyl)-3-(4-methylbenzyl)benzimidazole-2-ylidene]pyridine
palladium(II), 2b
Yield: 78%, m.p. 203–204 ꢀC. FT-IR ꢀ(CN): 1407 cmꢂ1 1H NMR (400 MHz, CDCl3), d
.
(ppm): 1.86–1.94 (m, 2H, CH2CHC3H6), 2.01–2.08 (m, 2H, CH2CHC3H6), 2.11–2.19 (m, 2H,
CH2CHC3H6), 2.24 (s, 3H, CH2C6H4(CH3)-4), 3.43 (hept., 1H, CH2CHC3H6, J ¼ 6 Hz), 4.81
(d, 2H, CH2CHC3H6, J ¼ 8 Hz), 6.06 (s, 2H, CH2C6H4(CH3)-4), 6.95–7.02 (m, 2 H, Ar-H),
7.07–7.15 (m, 3H, Ar-H), 7.25–7.30 (m, 2 H, Ar-H), 7.37 (t, 3H, Ar-H, J ¼ 8 Hz), 7.68 (tt, 1H,
1
C5H5, J ¼ 8 Hz), 8.96–8.98 (m, 2H, C5H5). 13Cf Hg NMR (100 MHz, CDCl3), d (ppm): 18.4
(CH2CHC3H6), 21.3 (CH2C6H4(CH3)-4), 27.4 (CH2CHC3H6), 35.5 (CH2CHC3H6), 53.6
(CH2C6H4(CH3)-4), 54.2 (CH2CHC3H6), 110.5, 110.6, 122.9, 124.6, 128.0, 129.5, 132.0,
134.3, 135.4, 137.8, 138.0, 152.7 (Ar-C), 162.7 (Pd-Ccarbene).
2.4.3. Dibromo[1-(cyclobutylmethyl)-3-(4-methoxybenzyl)benzimidazole-2-ylidene]-
pyridine palladium(II), 2c
Yield: 81%, m.p. 206–207 ꢀC. FT-IR ꢀ(CN): 1405 cmꢂ1 1H NMR (400 MHz, CDCl3), d
.
(ppm): 1.89–1.95 (m, 2H, CH2CHC3H6), 2.01–2.08 (m, 2H, CH2CHC3H6), 2.11–2.20 (m, 2H,
CH2CHC3H6), 3.45 (hept., 1H, CH2CHC3H6, J ¼ 8 Hz), 3.70 (s, 3H, CH2C6H4-4-OCH3), 4.81
(d, 2H, CH2CHC3H6, J ¼ 8 Hz), 6.04 (s, 2H, CH2C6H4-4-OCH3), 6.81 (d, 2H, Ar-H, J ¼ 8 Hz),

JOURNAL OF COORDINATION CHEMISTRY
7
6.97–7.03 (m, 2H, Ar-H), 7.13 (td, 1H, Ar-H, J ¼ 6 Hz), 7.28 (t, 2H, Ar-H, J ¼ 8 Hz), 7.36 (d,
1H, Ar-H, J ¼ 8 Hz, 7.45 (d, 2H, Ar-H, J ¼ 8 Hz), 7.70 (tt, 1H, C5H5, J ¼ 8 Hz, 2J ¼ 4 Hz),
1
8.98–9.00 (m, 2H, C5H5). 13Cf Hg NMR (100 MHz, CDCl3), d (ppm): 18.4 (CH2CHC3H6),
27.2 and 27.3 (CH2CHC3H6), 35.5 (CH2CHC3H6), 53.4 (CH2C6H4-4-OCH3), 54.2
(CH2CHC3H6), 55.3 (CH2C6H4-4-OCH3), 110.5, 111.6, 114.2, 114.3, 122.9, 123.0, 124.6,
127.1, 129.5, 134.3, 135.5, 138.0, 152.7, 154.4, 159.4 (Ar-C), 162.7 (Pd-Ccarbene).
2.4.4. Dibromo[1-benzyl-3-(2,3,5,6-tetramethylbenzyl)benzimidazole-2-ylidene]pyri-
dine palladium(II), 2d
Yield: 81%, m.p. 229–230 ꢀC. FT-IR ꢀ(CN): 1418 cmꢂ1 1H NMR (400 MHz, CDCl3), d
.
(ppm): 2.18 and 2.21 (s, 12H, CH2C6H(CH3)4-2,3,5,6), 6.19 (s, 2H, CH2C6H5), 6.24 (s, 2H,
CH2C6H(CH3)4-2,3,5,6), 6.82 (t, 1H, Ar-H, J ¼ 8 Hz), 6.92 (t, 1H, Ar-H, J ¼ 8 Hz), 6.97–7.04
(m, 3H, Ar-H), 7.20–7.32 (m, 5H, Ar-H), 7.51–7.55 (m, 2H, Ar-H), 7.69 (tt, 1H, C5H5,
1
J ¼ 8 Hz), 8.87–8.90 (m, 2H, C5H5N). 13Cf Hg NMR (100 MHz, CDCl3), d (ppm): 16.5
(CH2C6H(CH3)4-2,3,5,6), 20.6 (CH2C6H(CH3)4-2,3,5,6), 50.8 (CH2C6H5), 53.3 (CH2C6H(CH3)4-
2,3,5,6), 111.2, 111.5, 122.8, 123.2, 124.5, 128.0, 128.1, 128.9, 132.5, 134.3, 135.0, 135.2,
138.1, 151.2, 153.4 (Ar-C), 164.5 (Pd-Ccarbene).
2.4.5. Dibromo[1,3-di-(4-tertbutylbenzyl)benzimidazole-2-ylidene]pyridine palladium(II), 2e
Yield: 79%, m.p. 224-225 ꢀC. FT-IR ꢀ(CN): 1407 cmꢂ1 1H NMR (400 MHz, CDCl3), d
.
(ppm): 1.22 (s, 18H, CH2C6H4(C(CH3)3)-4), 6.08–6.19 (m, 4H, CH2C6H4(C(CH3)3)-4)),
6.97–7.08 (m, 4H, Ar-H), 7.24–7.29 (m, 2H, Ar-H), 7.31 (d, 4H, Ar-H, J ¼ 12 Hz), 7.50 (d,
1
4H, Ar-H, J ¼ 8 Hz), 7.66–7.70 (m, 1H, C5H5N), 8.94 (tt, 2H, C5H5, J ¼ 4 Hz). 13Cf Hg NMR
(100 MHz, CDCl3), d (ppm): 31.3 (CH2C6H4(C(CH3)3)-4), 34.6 (CH2C6H4(C(CH3)3)-4), 52.9
and 53.2 (CH2C6H4(C(CH3)3)-4), 111.5, 123.1, 124.5, 125.8, 127.7, 127.8, 132.0, 132.1,
134.6, 134.7, 138.1, 151.1, 151.3, 152.1, 152.7, 153.4, 164.2 (Ar-C), 164.4 (Pd-Ccarbene).
2.5. General procedure for the arylation reaction
KOAc (1.0 mmol), aryl-bromide derivatives (1.0 mmol), heteroaryl derivatives (2-n-
butylthiazole, 2-n-butylthiophene and 2-n-butylfuran) (2.0 mmol) and Pd-NHC–PEPPSI
complexes 2a–e (0.01 mmol) were dissolved in N,N-dimethylacetamide (DMAc) (2 mL)
in a small Schlenk tube under argon as described in literature [49]. The reaction mix-
ture was stirred in an oil bath at 130 ꢀC for 1 h, then cooled to room temperature, and
the solvent was removed under vacuum. The obtained residue was purified by column
chromatography (silica gel 60-120 mesh) by using diethyl ether/n-hexane (1:5) as elu-
ent to afford the pure product. The purity of the compounds was checked by gas
chromatography (GC) and gas chromatography-mass spectrometry (GC-MS).
Conversions were calculated by taking into account the conversion of aryl bromides
to products.
2.6. Computational method
All DFT and HF method calculations have been conducted by using Gaussian 09W [50]
package. The geometry optimization and frequency calculations to verify the

8
N. ŞAHIN ET AL.
Scheme 1. Synthesis of benzimidazolium salts 1a–e.
structures of 1c and 2c have been performed by B3LYP [51, 52], M06-2X [53], G96LYP
[54] level of DFT method and HF method in both the gas and chloroform phases.
Polarized Continuum Model [55, 56] has been used in all simulations performing in
the chloroform phase. In this work, LANL2DZ basis set for Pd atom of the complex
molecule and the standard 6-31 þ G(d,p) basis set for the remaining atoms of both the
complex and ligand molecules has been used in all simulations. NMR spectral analysis
1
to determine 1H and 13Cf Hg chemical shifts have been employed by Gauge-
Independent Atomic Orbital [57, 58] approach by subtracting the shielding constants
of TMS. The potential energy distribution analysis to assign the vibrational modes of
both the complex and ligand have been performed with VEDA [59] program. NBO [60,
61] and FMO [62–64] analyses have been employed in determining the important
inter-/intra-molecular interactions and quantum chemical descriptors to elucidate the
catalytic activity of 2c. Highest occupied molecular orbital (HOMO) and lowest unoccu-
pied molecular orbital (LUMO) amplitudes and molecular electrostatic potential (MEP)
diagrams have been used to show the reactive region of 2c. All quantum chemical cal-
culations to clarify the catalytic activity of 2c have also been employed for 1c and
compared with the results of 2c. GaussView 6.0.16 software [65] has been used to
visualize the FMO amplitudes and MEP diagrams, as well as to illustrate the simulated
IR spectra and to verify the vibrational modes.
3. Results and discussion
3.1. Synthesis and characterization of benzimidazolium salts and PEPPSI type
palladium(II) complexes
The benzimidazolium salts were prepared by the reaction of N-substituted benzimida-
zole with alkyl halides in DMF at 80 ꢀC for 12 h. These salts were prepared according
to the general reaction pathway shown in Scheme 1. The salts are air- and moisture-

JOURNAL OF COORDINATION CHEMISTRY
9
Scheme 2. Synthesis of PEPPSI-type palladium-NHC complexes 2a–e.
1
stable both in the solid state and in solution. The H NMR spectra of the benzimidazo-
lium salts exhibit the signal for the NCHN proton in the d range of 11.13–11.68 ppm.
1
In the 13Cf Hg NMR spectra of 1a–e, the characteristic signals of the imino carbon
(NCHN) were detected as typical singlets at 142.5, 142.5, 142.4, 142.7, and 143.2 ppm,
respectively. These NMR values are in line with those found for other benzimidazolium
salts in literature [66, 67]. The formation of the benzimidazolium salts was also evi-
denced by their IR spectra, which showed CN bond vibration at 1555, 1552, 1514,
1554, and 1550 cmꢂ1 for the respective CN bond vibrations of 1a–e. All compounds
showed good solubility in water and common organic solvents, such as dichlorome-
thane, chloroform, methanol, acetonitrile, and N,N-dimethylformamide.
Imidazole-, benzimidazole-, imidazoline-, and triazole-derived NHCs have been
reported during the last decade [68, 69]. In general, NHC-chemistry is dominated by
imidazole- and imidazoline-based carbene ligands. On the other hand, benzimidazol-
2-ylidenes have received considerably less interest, though in recent years this area
has been developed by Hahn and others [70–72]. However, the catalytic activity of
benzimidazol-2-ylidene metal complexes in C–C coupling reactions and transfer hydro-
genations have been reported [73–75]. We examined the activity of the PEPPSI-type
palladium-NHC complexes in the direct C5-arylation of several heteroaromatic groups,
with various aryl-bromides as coupling partners. The new complexes 2a–2e were pre-
pared using the methodology developed by Organ [32]. The benzimidazolium salts
were mixed with PdCl2 in the presence of a large excess of K2CO3 and KBr in pyridine
at 80 ꢀC for 16 h. After recrystallization, complexes were obtained as air- and moisture-
stable off-bright yellow crystals in a good yield (Scheme 2). Also, complexes 2a–e are
air and moisture stable in solution.

10
N. ŞAHIN ET AL.
These palladium complexes were soluble in different solvents such as dimethylsulf-
oxide, chloroform and dichloromethane. The formation of the complexes was con-
1
firmed by FT-IR, 1H NMR, and 13Cf Hg NMR spectroscopic methods. These
spectroscopic methods, which are seen to be spectroscopically pure, are consistent
1
with the recommended formula. In the H NMR spectra, the benzimidazolium salts as
compared to the PEPPSI Pd-NHC complexes were identified by a characteristic proton
peak at the 2-position (NCHN) of the benzimidazole ring which appeared highly down-
field shifted as sharp singlets at 11.13, 11.68, 11.59, 11.18, and 11.21 ppm for 1a–e,
respectively. It was known that the PEPPSI Pd-NHC complexes had successfully been
1
synthesized because the signal of the acidic benzimidazolium C2-H proton in the H
NMR spectra was not observed. The Pd–Ccarbene resonances of these novel PEPPSI Pd-
1
NHC complexes in the 13Cf Hg NMR spectra appeared highly downfield shifted at
162.7, 162.7, 62.7, 164.5, and 164.4 ppm for 2a–e, respectively. The FT-IR data clearly
indicated the presence of ꢀ(CN) at 1411, 1406, 1405, 1418 and 1401 cmꢂ1 for the
PEPPSI Pd-NHC complexes 2a–e, respectively. (See Supplementary Data for experimen-
tal recorded NMR and FT-IR spectra for the complexes, Figures S1–S10). Also, an
appropriate single-crystal for 2c was obtained and its molecular structure was eluci-
dated through single-crystal X-ray diffraction analyses.
3.2. X-ray crystallographic study
Single-crystals of 2c suitable for diffraction study were obtained by slow diffusion of
n-pentane into a dichloromethane solution of 2c. The structure of 2c was determined
by X-ray crystallography (Figures 2 and 3).
The crystal structure of 2c shows that palladium is coordinated to one C atom [Pd-
C6 1.957(3) Å] of the NHC moiety, one N atom [Pd-N1 2.096(3) Å] of the pyridine
group and two Br ions [Pd-Br1 ¼ 2.4265(5) and Pd-Br2 ¼ 2.4315(5) Å]. Coordination
geometry around Pd is distorted square planar. Pd is located 0.0325(3) Å above the
plane containing N1, C6, Br1, and Br2. The dihedral angle between the Br1/Pd/Br2 and
N1/Pd/C6 moieties is 64.86(2)ꢀ and the dihedral angle between the C6/Pd/Br2 and N1/
Pd/Br1 moieties is 2.61(8)ꢀ.
The details of crystal and experimental data of 2c are given Tables 1 and 2.
Formula ¼ C25H27Br2N3OPd; Molecular weight ¼ 651.71; Crystal system ¼ Monoclinic;
Space group ¼ P21/c system; Unit cell parameters, a ¼ 14.9835(9), b ¼ 13.8697(8),
c ¼ 12.1436(8) Å; a ¼ 90ꢀ, b ¼ 90.843(3)ꢀ, c ¼ 90ꢀ; Cell volume ¼ 2523.4(3) Å3, Z ¼ 4,
d ¼ 1.715 g.cmꢂ3, l ¼ 3.924 mmꢂ1. F(000) ¼ 1288; T ¼ 296(2) K. Crystal size ¼
0.40 ꢃ 0.36 ꢃ 0.24 mm. The measured reflections are 15,347 with 5533 independent
reflections. The observed [I > 2r(I)] reflections are 3964. A final refinement on F2, with
5533 unique intensities and 285 parameters. The value of weight parameter is
w ¼ 0.063 and goodness of fit S ¼ 1.09.
3.3. Quantum chemical calculations on 2c
Here, we have employed representative simulations by both DFT and ab initio meth-
ods for complex 2c and its NHC ligand 1c, because we only have crystallographical

JOURNAL OF COORDINATION CHEMISTRY
11
Figure 2. PLATON drawing of 2c with the atomic numbering scheme. Displacement ellipsoids are
drawn at the 50% probability level.
data of 2c. First, we determined the geometric structures of both molecules and com-
pared them with the X-ray crystallography of the complex. Then, the IR and NMR
spectral analyses have been employed to assign the isotropic chemical shifts and
vibrational frequencies of both compounds. In addition, the reactivity descriptors
obtained from FMO analysis have been used to evaluate/compare with each other the
catalytic activity behavior of both compounds. Also, the non-linear optical tendency of
both compounds has been investigated by NLO analysis.
3.3.1. Geometric optimization
The optimized geometries and ring definitions with original atom labeling of both 1c and
2c are shown in Figure 4. Also, the selected geometric parameters of them have been given
in Table 3, in the chloroform phase. The atomic numbering used in Table 3 has been given
based on 2c, and the corresponding original numbering for 1c can be seen from Figure 4.
According to Table 3, the bond angles and lengths were simulated with similar val-
ues, regardless of the method or approaches used for the simulation. The aromatic
ring (RA; ring A and RB; ring B) C1-C2, C1-N26 and C2-N27 bond lengths for 1c and
2c have been recorded and simulated at 1.4 Å. Moreover, the other N-C bond lengths
for the aromatic ring of the ligand and complex have been observed at N26-

12
N. ŞAHIN ET AL.
Figure 3. H-bonding geometry of 2c. The solid lines show coordination while dotted lines show
H-bonding.
Table 1. Crystallographic data and structure refinement for 2c.
Formula
C25H27Br2N3OPd
651.71
Monoclinic
Formula weight
Crystal system
Space group
P21/c
Unit cell dimensions
a ¼ 14.9835(9) Å
b ¼ 13.8697(8) Å
c ¼ 12.1436(8) Å
Z, V
D (calc)
Mu (Mo Ka) [1/mm]
a ¼ 90ꢀ
b ¼ 90.843(3)ꢀ
c ¼ 90ꢀ
4, 2523.4(3) Å3
1.715 g/cm3
3.924
F (000)
1288
Radiation (Å)
Mo Ka 0.71073
0.40 ꢃ 0.36 ꢃ 0.24
298
0.340, 0.482
2.229, 27.177
0.045
Crystal size [mm]
Temperature (K)
T min-max [1/mm]
Theta min-max
Rint
Tot. uniq. data dataset
Nref, Npar
15347, 5533, 3964
5533, 285
Observed data [I > 2.0 sigma (I)]
R, wR (F2), S
Max. min. resd. dens.
0.035, 0.080, 1.017
0.595, ꢂ0.616 e Åꢂ3
Table 2. Hydrogen bonding geometry (Å,ꢀ) of the Pd complex 2c.
D—H … A
D—H
H … A
D … A
D—H … A
C1-H1 … O1i
0.93
2.52
3.422(5)
163
Symmetry codes: (i) ꢂx,1 ꢂ y,1 ꢂ z.

JOURNAL OF COORDINATION CHEMISTRY
13
Figure 4. The optimized structures of 1c and 2c at B3LYP/6-31 þ G(d,p) level in chloroform.
C23 ¼ 1.5 Å; N27-C11 ¼ 1.5 Å, and N27-C28 ¼ 1.3 Å, where they have been predicted by
the B3LYP level at 1.3, 1.5, and 1.5 Å, respectively. Furthermore, C–O and C–C bond
lengths belonging to the methoxy group of the complex have been observed at 1.4 Å
and 1.5 Å and simulated by the M06-2X level at 1.4 Å and 1.6 Å, respectively. From
Table 3, it can be seen that Pd-C, Pd-Br, and Pd-N bond lengths have been observed
at 2.0, 2.4, and 2.1 Å and predicted by the B3LYP level at 2.0, 2.5, and 2.1 Å, respect-
ively. As it is expected, the aromatic ring bond angle has been recorded at 121.3ꢀ for
the complex and estimated by BYLYP at 121.7ꢀ for the complex and 121.6ꢀ for the lig-
and. Furthermore, the bond angles C1-C2-N27, N26-C28-N27, C28-N26-C23, N26-C23-
C49, C28-N27-C11, and N27-C11-C14 have been observed at 105.8ꢀ, 107.1ꢀ, 125.2ꢀ,
113.8ꢀ, 126.3ꢀ, and 111.9ꢀ and simulated by B3LYP level at 106.3ꢀ, 107.7ꢀ, 124.7ꢀ,
113.8ꢀ, 125.3ꢀ, and 113.8ꢀ, respectively. Also, it should be noticed that bond angles
C17-C20-O44 ¼ 116.7ꢀ and C20-O44-C45 ¼ 118.0ꢀ deviated from the trigonal planar
with 3.3ꢀ and 2.0ꢀ. Similar deviation from the trigonal planar has been observed for
the bond angles N26-C28-Pd ¼ 127.3ꢀ and Pd-N42-C32 ¼ 122.1ꢀ whereas the bond
angle C28-Pd-N42 ¼ 177.6ꢀ has deviated from the linearity with 2.4ꢀ. In fact, it should
be noticed that the recorded dihedral angles for both the ligand and complex have
agreed with the corresponding simulated ones, except for the Pd-RE (Ring E) section
of the complex. Moreover, it can be seen from Table 3 that the dihedral angles C6-C1-
C2-C3 ¼ ꢂ0.2ꢀ, C6-C1-N26-C28 ¼ ꢂ179.6ꢀ, C1-N26-C28-N27 ¼ 1.3ꢀ, N26-C28-N27-C2 ¼
ꢂ1.1ꢀ and N27-C28-N26-C23 ¼ ꢂ178.7ꢀ for 2c have also been simulated with a small
deviation from linearity, while these angles for 1c have been calculated with a little higher
deviation from the linearity compared with the compound. That is, the same dihedral
angles for 1c have been simulated by the B3LYP level at ꢂ0.8ꢀ, ꢂ180.0ꢀ, ꢂ3.0ꢀ, 2.4ꢀ, and
ꢂ175.4ꢀ, respectively. On the other hand, the dihedral angles N26-C28-Pd-N42 and N27-
C28-Pd-N42 for 2c have been observed at 156.0ꢀ and ꢂ20.0ꢀ, while these dihedral angles
have been simulated at 85.3ꢀ and ꢂ93.7ꢀ by B3LYP level; at 141.1ꢀ and ꢂ37.7ꢀ by MO6-
2X level, 98.1ꢀ and ꢂ81.0ꢀ by G96LYP level; 65.8ꢀ and ꢂ113.5ꢀ by HF method. Here, it is
necessary to be noticed that the observed dihedral angle values are different from the
calculated values, especially for RE of 2c, and also the simulated dihedral angles have var-
ied greatly according to the calculation method or approaches used. But still, it can be

14
N. ŞAHIN ET AL.
Table 3. Selected experimental and simulated structural (in the chloroform phase) parameters for
1c and 2c at 6-31 þ G(d,p) basis set.
2c
1c
ꢁ
1.4
Bond length (Å)
Exp .
B3LYP
M06-2X G96LYP
HF
1.4
B3LYP
M06-2X G96LYP
HF
1.4
C1-C2
C1-N26
C2-N27
N27-C28
N26-C23
N27-C11
C20-O44
C49-C51
1.4
1.4
1.4
1.4
1.5
1.5
1.4
1.6
2.0
2.5
2.1
1.3
1.4
1.4
1.4
1.3
1.5
1.5
1.4
1.6
2.0
2.5
2.2
1.3
1.4
1.4
1.4
1.4
1.5
1.5
1.4
1.6
2.0
2.5
2.2
1.4
1.4
1.4
1.4
1.3
1.5
1.5
1.4
1.6
1.4
1.4
1.4
1.3
1.5
1.5
1.4
1.5
1.4
1.4
1.4
1.4
1.5
1.5
1.4
1.6
1.4
1.4
1.3
1.5
1.5
1.4
1.5
2.0
2.4
2.1
1.3
1.4
1.4
1.3
1.5
1.5
1.3
1.5
2.0
2.5
2.2
1.3
1.4
1.4
1.3
1.5
1.5
1.4
1.6
C28-Pd
Pd-Br(30)
Pd-N42
N42-C32
Bond angle (ꢀ)
C1-C2-C3
121.3
105.8
107.1
125.2
113.8
126.3
111.9
116.7
118.0
86.8
127.3
177.6
122.1
91.3
86.9
90.1
121.7
106.3
107.7
124.7
113.8
125.3
113.8
116.0
118.5
88.2
126.0
179.6
120.6
89.1
88.5
91.3
122.0
106.3
108.1
124.9
111.2
126.2
112.0
116.0
117.8
87.8
121.5
106.3
107.4
124.5
114.3
124.7
114.3
115.8
118.4
88.3
121.8
106.2
108.2
125.2
113.7
126.1
113.8
116.2
119.9
88.1
125.5
179.5
120.6
89.2
88.9
91.2
121.6
106.3
110.9
125.4
112.8
124.8
113.6
115.9
118.5
88.3
121.8
106.2
110.9
125.8
111.7
125.2
112.0
116.0
117.7
87.9
121.6
106.4
110.7
124.7
113.2
124.6
113.9
115.7
118.4
88.3
121.7
106.3
111.9
126.8
113.0
124.7
113.7
116.2
119.9
88.3
C1-C2-N27
N26-C28-N27
C28-N26-C23
N26-C23-C49
C28-N27-C11
N27-C11-C14
C17-C20-O44
C20-O44-C45
C49-C51-C53
N26-C28-Pd
C28-Pd-N42
Pd-N42-C32
C28-Pd-Br(31)
C28-Pd-Br(30)
N42-Pd-Br(31)
N42-Pd-Br(30)
Br(30)-Pd-Br(31)
Dihedral angle (ꢀ)
C6-C1-C2-C3
C6-C1-N26-C28
C1-N26-C28-N27
N26-C28-N27-C2
N27-C28-N26-C23
C28-N26-C23-C49
N26-C23-C49-C51
C49-C51-C53-C50
N26-C28-Pd-Br(30)
N27-C28-Pd-Br(30)
N26-C28-Pd-N42
N27-C28-Pd-N42
C28-Pd-N42-C32
C28-Pd-N42-C33
125.8
179.7
120.7
87.0
126.4
179.7
120.6
89.3
89.1
88.8
93.3
93.5
91.1
90.8
91.9
177.5
91.1
177.5
90.7
178.1
173.2
178.0
ꢂ0.2
ꢂ0.5
0.3
179.4
0.5
ꢂ0.9
ꢂ179.4
0.2
ꢂ0.7
ꢂ0.8
ꢂ0.8
ꢂ178.9
ꢂ3.6
ꢂ1.7
ꢂ179.2
ꢂ3.3
ꢂ0.8
179.9
ꢂ2.0
1.3
177.4
ꢂ30.0
ꢂ66.0
18.6
ꢂ179.6 ꢂ179.7
ꢂ179.3 ꢂ180.0
1.3
ꢂ1.1
0.2
ꢂ0.37
179.7
ꢂ87.1
ꢂ75.6
19.2
0.2
ꢂ3.0
ꢂ0.2
ꢂ0.5
ꢂ0.5
2.4
3.0
2.6
ꢂ178.7
ꢂ88.2
ꢂ70.9
20.5
177.4
ꢂ74.8
ꢂ58.2
20.7
ꢂ179.8
ꢂ88.3
ꢂ77.5
19.0
ꢂ180.0 ꢂ175.4
ꢂ174.7
ꢂ23.9
ꢂ62.4
20.4
175.0
ꢂ50.4
ꢂ68.5
18.4
ꢂ88.8
ꢂ76.8
20.0
ꢂ39.6
ꢂ65.7
18.4
98.3
99.5
102.9
ꢂ75.9
141.1
ꢂ37.7
ꢂ75.3
104.4
ꢂ37.2
142.4
98.1
99.9
ꢂ78.5
156.0
ꢂ20.0
177.0
ꢂ1.0
ꢂ79.5
ꢂ81.0
ꢂ79.4
85.3
98.1
65.8
ꢂ93.7
ꢂ99.8
79.8
ꢂ81.0
ꢂ110.5
69.0
ꢂ113.5
ꢂ36.6
142.6
ꢂ70.7
109.1
Br(30)-Pd-N42-C32 ꢂ125.2 ꢂ113.9
ꢂ110.5
Br(31)-Pd-N42-C32
53.3
65.4
69.0
ꢁExperimental data have been given for 2c.
seen from Table 3 that the observed geometric parameters of both 1c and 2c have
mostly agreed with the corresponding simulated values.
3.3.2. NBO analysis
NBO analysis has provided the important information of the molecular systems to elu-
cidate the molecular properties by means of the intra-molecular interactions such as

JOURNAL OF COORDINATION CHEMISTRY
15
resonance, hyperconjugation, cieplak, for example, resulting from the intramolecular
charge transfer, and it has been increasingly used for this purpose. The lowering of
the second order perturbative energy (E(2)) has been defined by Weinhold and cow-
orkers, and it depends on the donor orbital occupancy (qi), the donor and acceptor
orbital energies (ei and ej, diagonal) and NBO Fock matrix element (Fij, off-diagonal)
[60, 61] as follows:
2
ðFijÞ
Eð2Þ ¼ DEij ¼ qi
ðej ꢂ eiÞ
In this study, the results of NBO analysis for 1c and 2c are given in Supplementary
information Table S1 at the B3LYP level in the chloroform phase; the expanded data
including the donor and acceptor orbital hybridization can be seen in Supplementary
information Table S2. As can be seen from Supplementary information Table S1, the
stabilization energy for the resonance interactions predicting in the aromatic RA of
both 1c and 2c are close to each other; the lowering of the stabilization energy for
ꢁ
the electron delocalization from the filled orbital p(C1-C2) to unfilled orbitals p (Cꢂ11-
ꢁ
ꢁ
C2), p (C5-C6), p (N26-C28) have been calculated at 17.34, 17.20, and 13.36 kcal mol
for 1c and at 18.19, 18.19, and 11.58 kcal molꢂ1 for 2c. Moreover, the stabilization
energy lowering for the electron delocalization on the common rings (RA, RB, RC, and
RD) of both the ligand and complex are also similar to each other. Here, it should be
noticed that two of the strong resonance interactions for the ligand are predicted for
the electron movement from the lone pair of the N27 atom to antibonding orbitals of
ꢁ
1c; the resonance energy for the interactions LP(1)N27 ! p (C1-C2) and LP(1)N27!
p (N26-C28) for 1c have been calculated at 34.78 and 82.33 kcal molꢂ1. But, this kind
ꢁ
of interaction has not been simulated for 2c, instead of this, the interactions p(C11-
ꢁ
ꢁ
N27) ! p (C1-C2) and p(C11-N27) ! p (N26-C28) for 2c have been simulated at
32.49 and 73.93 kcal molꢂ1. Additionally, the interactions LP(2)N42! p (C32-C34) and
ꢁ
ꢁ
LP(2)N42! p (C33-C36) for 2c have greatly contributed to the lowering of the
molecular stabilization energy (with the smallest donor orbital occupancy, EDi ¼
1.27829 e); the resonance energies of these interactions have been calculated at 49.40
and 49.30 kcal molꢂ1. The other common interaction for 1c and 2c is predicted as the
ꢁ
resonance interaction LP(O) ! p (C18-C20) with the stabilization energy of 30.27 (EDi
¼ 1.84193 e) and 30.03 (EDi ¼ 1.84400 e) kcal molꢂ1, respectively. It is known that the
strong intramolecular interactions occurred between the electron donating orbital and
accepting orbital cause the increasing of the hyperpolarizability that is an important
tool used in the measuring of the nonlinear optic property of a specific molecular sys-
tem [76]. Here, we can suggest that the newly synthesized 2c can be an available
material for using in the optoelectronic field because of having strong intramolecular
interactions.
3.3.3. Global reactivity descriptors, FMO (frontier molecular orbital) analysis, MEP
Nowadays, from medicine to engineering sciences in many kinds of research fields,
global reactivity identifiers have been used to estimate/evaluate the properties of a
molecular system of interest. As it is known well, Koopmans Theorem [62] defines the
ionization energy (I) and electron affinity (A) according to the frontier molecular orbital

16
N. ŞAHIN ET AL.
Table 4. Calculated quantum chemical parameters for 1c and 2c.
Gas phase
1c
2c
B3LYP
M06-2X
G96LYP
HF
B3LYP
M06-2X
G96LYP
HF
HOMO (-I)
LUMO (-A)
DE (Energy gap)
ꢂ0.198
ꢂ0.054
3.920
ꢂ0.246
ꢂ0.020
6.135
ꢂ0.165
ꢂ0.067
2.675
ꢂ0.292
ꢂ0.223
ꢂ0.075
4.036
ꢂ0.276
ꢂ0.035
6.550
ꢂ0.176
ꢂ0.093
2.247
ꢂ0.311
0.045
0.055
9.158
9.958
v
g
ꢂ3.440
ꢂ3.617
ꢂ3.159
ꢂ3.358
ꢂ4.057
ꢂ4.228
ꢂ3.667
ꢂ3.475
1.960
3.068
1.338
4.579
2.018
3.275
1.124
4.979
x
3.019
2.132
3.730
1.232
4.077
2.729
5.983
1.213
DNmax
1.755
1.179
2.362
0.733
2.010
1.291
3.264
0.698
Chloroform phase
1c
2c
B3LYP
M06-2X
G96LYP
HF
B3LYP
M06-2X
G96LYP
HF
HOMO (-I)
LUMO (-A)
DE (Energy gap)
v
ꢂ0.218
ꢂ0.061
4.263
ꢂ0.269
ꢂ0.026
6.591
ꢂ0.177
ꢂ0.079
2.660
ꢂ0.318
ꢂ0.229
ꢂ0.084
3.946
ꢂ0.280
ꢂ0.045
6.395
ꢂ0.187
ꢂ0.103
2.286
ꢂ0.314
0.055
0.051
10.163
ꢂ3.573
5.081
9.939
ꢂ3.799
ꢂ4.011
ꢂ3.485
ꢂ4.267
ꢂ4.421
ꢂ3.944
ꢂ3.587
g
2.132
3.296
1.330
1.973
3.198
1.143
4.970
x
3.385
2.441
4.566
1.256
4.615
3.056
6.804
1.294
DNmax
1.782
1.217
2.621
0.703
2.163
1.382
3.451
0.722
ꢁDE (Energy Gap), v, g, x and DNmax are in eV; HOMO and LUMO energies are in au.
energies. Also, global reactivity identifiers [63, 64] based on the frontier molecular
orbital energies have provided very useful information of the molecular systems to
elucidate the chemical reactivity behavior of the molecular systems. In this study, we
aim to elucidate the catalytic activity behavior of 2c and compared with that of 1c by
using the global reactivity identifiers. All equations to obtain the global reactivity
parameters via the frontier molecular orbital energies have been defined as follows:
I ¼ ꢂEHOMO
A ¼ ꢂELUMO
I þ A
I ꢂ A
v ¼ ꢂ
g ¼
2
2
l2
x ¼
2g
I þ A
DNmax
¼
2ðI ꢂ AÞ
In many kinds of research fields, from medicine to engineering sciences, computa-
tional tools are used to estimate and evaluate the properties of a molecular system
of interest.
All simulated reactivity descriptors for 1c and 2c are given in Table 4. It is clear
from Table 4 that both the HOMO and LUMO energy of 2c is lower than those of 1c,
at all levels of DFT method and HF method, except for the HF method in the chloro-
form phase. On the other hand, DE (energy gap) value of 2c is lower than that of 1c
in the chloroform phase while it is higher in the gas phase, except for the G96LYP
level. Recently, it was reported that DE value for the ligand was higher than that of
the Pd(II)-ligand complexes [39, 40, 77]. Seyfi et al. [77] predicted DE for the binuclear
palladium complexes and found that DE for the ligand is higher than those of the
Pd(II) complexes. In another study, Bouchoucha and co-workers [39] investigated the
antibacterial and cytotoxicity activities of the Ni(II), Pd(II), and Pt(II) complexes and
suggested that the Pd complex (DE ¼ 3.068 eV) was more toxic than the Pt(II) complex

JOURNAL OF COORDINATION CHEMISTRY
17
Figure 5. HOMO and LUMO amplitudes (isoval:0.02) and MEP diagrams (isoval:0.0004) of 1c and
2c at B3LYP/6-31 þ G(d,p)/LANL2DZ basis set in chloroform phase.
(DE ¼ 3.556 eV) against DNA of the tumor cell. According to Table 4, the chemical
reactivity prediction by quantum chemical parameters depends on the solvent envir-
onment as well as the simulation approaches or method. In the chloroform phase for
all methods, 2c is softer than 1c, in addition to the best electrophilic character and
maximum charge transfer capability. For the chloroform phase simulations, the global
hardness values (g) of 2c/1c have been predicted at 1.973/2.132 eV (B3LYP), 3.198/
3.296 eV (M06-2X), 1.143/1.330 eV (G96LYP), 4.970/5.081 eV (HF). Additionally, the elec-
trophilicity (x) indexes simulated in the chloroform phase 2c/1c have been deter-
mined at 4.615/3.385 eV (B3LYP), 3.056/2.441 eV (M06-2X), 6.804/4.566 eV (G96LYP),
1.294/1.256 eV (HF). Also, it can be seen from Table 4 that the maximum charge trans-
fer capability (DNmax) indexes of 2c/1c have been simulated at 2.163/1.782 eV (B3LYP),
1.382/1.217 eV (M06-2X), 3.451/2.621 eV (G96LYP), 0.722/0.703 eV (HF). These results
explained the catalytic activity of 2c for the direct arylation of various heteroaromatic
compounds with arylbromides, that is, there has not been observed the catalytic activ-
ity for these reactions with 1c even though it has been investigated.
In addition to the simulated global reactivity descriptors, we have dealt with the
frontier orbital amplitudes and MEP diagrams for both 1c and 2c to show the reactive
attack sites of these compounds and Figure 5 presents the visualization of them.
According to Figure 5, HOMO for 2c has mainly spread on the RC and Br-Pd-Br group
around while the HOMO for 1c has mostly Br atom around as well as on the RA and
RB. On the other hand, LUMO for 2c has mostly spread Br-Pd-Br group around as well
as slightly on RB and RE, while LUMO for 1c has quite spread on RA, RB, and H-C-Br
group around. Also, MEP diagrams have given very useful information to estimate the
electrophilic attack and nucleophilic attack sites depending on the charge distribution

18
N. ŞAHIN ET AL.
Figure 6. (a) Experimental and (b) simulated FT-IR spectra of 2c (blue) and 1c (red) at B3LYP/6-
31 þ G(d,p)/LANL2DZ level in the chloroform phase.
on the molecular surface. From Figure 5, the electron-rich regions (red) of 2c and 1c
have been simulated around Br-Pd-Br and H-C-Br, respectively. On the other hand, the
electron-poor regions (blue) of these compounds have been simulated on the RE
(complex) and RB (ligand).
3.3.4. FT-IR spectral analysis
The observed and simulated FT-IR spectra in the spectral region of 3200–500 cmꢂ1 of 2c
are illustrated in Figure 6 that also shows the simulated IR spectrum of 1c. The selected
vibrational mode assignments by using VEDA package for both the complex and ligand

JOURNAL OF COORDINATION CHEMISTRY
19
are given in Supplementary information Table S3 (full data provided in Supplementary
information Table S4). In addition, all computed frequencies of both compounds have
been scaled down by using the factor 0.9648 for B3LYP/6-31þ G(d,p) level to improve the
simulated modes and to compare with the experimental ones [78].
Here, NC bond vibration (ꢀN26C þ ꢀN27C) of 1c has been assigned in 1544 and
1318 cmꢂ1 as a pure mode, also in 1405 and 1366 cmꢂ1 mixed mode with the scissor-
ing rCH2 mode. Otherwise, the same stretching mode of 2c has occurred in
1405 cmꢂ1 and, predicted in 1395 cmꢂ1 mixed with the twisting mode and 1391 cmꢂ1
combined with the scissoring mode. According to these results, it is clear that this
negative shift is an indicator of complex formation. Due to the N–C bond in the other
N-heteroaromatic ring (RE) of the complex molecule, another ꢀN42C stretching mode
for 2c has been predicted in 1594–1002 cmꢂ1 as contaminated with the other stretch-
ing or bending modes. Additionally, the ꢀCO stretching mode(s) for 2c and 1c have
been simulated in 1236/1235 cmꢂ1 and 1237 cmꢂ1 as a mixed mode with the other
vibrational modes, but it has been assigned as not mixed with the other modes in
1017 cmꢂ1 (78%) and 1014 cmꢂ1 (73%), respectively. Recently, it was reported that
ꢀCO stretching mode was observed in 1080 cmꢂ1 and assigned by PED analysis in
1028 cmꢂ1 (71%) [79].
On the other hand, the aromatic C-H stretching modes for the complex and ligand
have been predicted in the range 3119–3067 cmꢂ1 and 3123-3045 cmꢂ1 as expected.
Recently, it was reported that ꢀCH3 mode was observed and simulated by B3LYP level
in 2926 cmꢂ1 [80]. In this work, the symmetric methyl group stretching (vCH3) mode
for the complex has been recorded in 2864 cmꢂ1 and simulated for complex and lig-
and molecules in 2923 and 2925 cmꢂ1, respectively. Moreover, ipb (in-plane bending)
HCC modes for the aromatic rings of 2c have been observed in 1605, 1514, 1476,
1416, 1302, 1250, 1180, and 1141 cmꢂ1 and the corresponding modes have been com-
puted in 1597, 1496, 1467, 1410, 1296, 1235, 1163, and 1100 cmꢂ1, whereas the same
mode for 1c has been simulated in the range 1600–1102 cmꢂ1. But, we should express
that the assignments of some vibrational modes for the common molecular parts of
1c and 2c have given the differences at somewhere in the spectral region of their IR
spectrum; opb (out-of-plane bending) HCC modes for the aromatic rings of 1c and 2c
have been calculated in the range 1001–497 cmꢂ1 and 984–572 cmꢂ1. In previous
work, we have simulated the ipb and opb HCC vibrational modes for the aromatic
rings in 1640–1037 cmꢂ1 and 1006–583 cmꢂ1, for the most stable conformer of
1-(allyl)-3-(2-methylbenzyl) benzimidazolium chloride (NHC ligand) [81]. Here, we can
say that the simulated IR spectral analysis results have a good agreement with the
experimental results recorded in this work, also previous reports (Supplementary
information Figure S6).
1
1
3.3.5. H NMR and 13Cf Hg NMR spectral analysis
In this work, NMR spectra of 2c have been analyzed in CDCl3 and simulated at both
the gas and chloroform phases. Supplementary information Tables S5 and S6 show
the observed and simulated H NMR and 13Cf Hg isotropic shifts for 1c and 2c; the
gas phase simulated NMR chemical shifts for complex and ligand are given in
Supplementary information Table S7. Moreover, the linear regression analysis results
1
1

20
N. ŞAHIN ET AL.
Table 5. Influence of the reaction conditions for 2a catalyzed direct C5-arylation of 2-n-butylthia-
zole and p-bromoacetophenonea.
Entry
Solvent
Base
Time (h)
Temp. (ꢀC)
Conv. (%)
Yield (%)
1
2
3
4
5
6
7
8
DMAc
DMF
Toluene
DMAc
DMF
Toluene
DMAc
DMF
Toluene
DMAc
DMAc
NaOAc
NaOAc
NaOAc
K2CO3
K2CO3
K2CO3
KOAc
KOAc
KOAc
KOAc
KOAc
1
1
1
1
1
1
1
1
1
100
100
100
100
100
100
100
100
100
130
130
65
53
58
35
28
25
78
67
71
89
81
45
50
55
30
20
25
75
55
60
85
78
9
10
11
1
0.5
aConditions: 2-n-butyl thiazole (2.0 mmol), p-bromoacetophenone (1.0 mmol), 2a cat. (0.01 mmol), base (2 mmol),
solvent (2 mL).
including the regression graphics and the correlation equations to show the correl-
ation between the experimental and computed NMR isotropic shifts have been given
1
1
in Supplementary information Figure S11 and Table S8; the observed H and 13Cf Hg
NMR spectra for 2c can be seen from Supplementary information Figure S5.
1
In the H NMR spectrum of 1c, the signal for the NCHN proton was observed at
11.59 ppm. It was known that if the signal of the acidic proton on benzimidazolium is
not observed, 2c is successfully synthesized. This signal has disappeared in the 1H
1
NMR spectra of 2c which it proves the formation of the complex. 13Cf Hg NMR spec-
tra of 1c, the characteristic signal of the NCHN was detected as a typical singlet at
142.4 ppm. NCN-carbene resonance on 2c shifted much downfield region compared to
1
1c and carbene peak of NCN was observed at 162.7 ppm in the 13Cf Hg NMR spec-
1
1
trum of 2c. This is another evidence of complex formation. Moreover, H and 13Cf Hg
NMR shifts for the aromatic rings (RA, RB, RC, and RE) of 2c have been recorded in
the range 6.81–8.99 ppm and 110.5–162.7 ppm and simulated by B3LYP level in the
1
range 6.9–8.9 and 107.5–190.1 ppm. Also, it can be seen that the H NMR chemical
shifts for the H12 and H13 atoms bonded to the carbon atom at the point of connec-
tion between the two aromatic rings (RB and RC) have been recorded in 6.04 ppm,
1
quite high value. As expected, the H NMR chemical shifts for the RD (non-aromatic
ring) of 2c have been observed in 1.9–3.5 ppm and simulated in 1.9–3.8 ppm by
G96LYP level. Depending on the used method or approaches, there are some differen-
1
1
ces in predicting the H and 13Cf Hg NMR shifts. For this reason, we have employed
the regression analysis to look for the best correlation between observed and simu-
lated NMR shifts, and the results can be seen in Supplementary information Table S8.
1
The best correlation for 13Cf Hg NMR shifts of 2c has been determined by M06-2X
1
level with R2 ¼ 0.9850. On the other hand, the best correlation for H NMR shifts of
the complex molecule has been obtained by B3LYP level with R2 ¼ 0.9717. It is clear
that simulated NMR shifts agree with the corresponding observed values. Furthermore,
it can be seen from Supplementary information Table S8 that the best correlation for
1
1
13Cf Hg NMR and H NMR shifts for 1c has been determined in the chloroform phase
by B3LYP level with R2 ¼ 0.9985 and by HF with R2 ¼ 0.9522, respectively. Accordance

JOURNAL OF COORDINATION CHEMISTRY
21
Table 6. Scope of the direct arylation of n-butylthiazole with aryl-bromides.a
Entry
Aryl bromide
Cat.
Product
Conv. (%)
Yield (%)
1
2
3
4
5
6
7
8
2a
2b
2c
2d
2e
2a
2b
2c
2d
2e
2a
2b
2c
2d
2e
2a
2b
2c
2d
2e
2a
2b
2c
2d
2e
2a
2b
2c
2d
2e
100
92
95
99
99
99
97
94
98
97
89
85
63
86
87
93
98
85
92
95
100
98
74
98
97
98
95
60
92
93
99
88
95
95
93
97
92
94
92
96
87
81
63
83
85
90
97
80
90
89
99
95
74
97
92
96
95
60
90
88
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
aConditions: 2-n-butyl thiazole (2.0 mmol), aryl bromide (1.0 mmol), Pd cat. (0.01 mmol), KOAc (2 mmol), N,N-dimethy-
lacetamide (2 mL), 130 ꢀC, 1 h.
with these results, we can say that experimental and simulated NMR results are in
good agreement with each other.
3.4. Using PEPPSI Pd-NHC catalysts, direct C5-Arylation of various
heteroaromatic groups with arylbromides
In 1990, Ohta et al. reported the arylation of thiophenes, furans or thiazoles with aryl
halides, via a C-H bond activation, in moderate to good yields using 5 mol% Pd(PPh3)4
as the catalyst [6]. Since these exciting results, the palladium-catalyzed direct arylation
of heteroaryl derivatives with aryl halides or triflates has proved to be a powerful
method for the synthesis of arylated heterocycles [82–84]. A regioselective C-5 aryla-
tion of 2-furaldehyde using aryl halide has been developed by McClure et al. [85].
Meanwhile, another regioselective monoarylation at either C-2 or C-5 of 3-carboalkoxy
thiophene and furan analogs using different Pd conditions was described [86].
Numerous C-2 substituted thiophenes and furans were successfully arylated at the C-5
position selectively [87, 88]. Doucet reported low Pd-catalyzed loading with phos-
phine-free direct 2-arylation of thiophene with aryl-bromide and aryl-triflate [89–91].

22
N. ŞAHIN ET AL.
Table 7. Scope of the direct arylation of n-butyl thiophene with aryl-bromidesa.
Entry
Aryl bromide
Cat.
Product
Conv. (%)
Yield (%)
1
2
3
4
5
6
7
8
2a
2b
2c
2d
2e
2a
2b
2c
2d
2e
2a
2b
2c
2d
2e
2a
2b
2c
2d
2e
2a
2b
2c
2d
2e
2a
2b
2c
2d
2e
97
92
98
95
83
98
97
98
96
74
56
66
51
66
55
98
95
94
97
95
99
98
92
97
93
98
96
90
97
95
96
88
92
93
82
98
95
84
95
70
50
55
70
58
53
97
92
94
90
94
97
94
98
94
92
95
91
90
96
94
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
aConditions: 2-n-butyl thiophene (2.0 mmol), aryl bromide (1.0 mmol), Pd cat. (0.01 mmol), KOAc (2 mmol), N,N-dime-
thylacetamide (2 mL), 130 ꢀC, 1 h.
Efforts towards selective b-arylation of thiophene and furan were also successful.
Daugulis developed copper catalyzed arylation of heteroarene to prepare various het-
erobiaromatics [92, 93]. Later, Itami reported high regioselectivity in Rh-catalyzed dir-
ect arylation of heteroarenes using aryl-iodide and Ir-catalyzed protocol [94, 95]. Only
a few examples of (NHC)Pd-catalyzed direct arylations of heteroaromatics have been
reported to date [49, 96, 97]. In our previous study, we saw that Pd-NHC complexes
were very effective in the direct arylation of different heteroaromatic groups with vari-
ous aryl halides [98, 99]. Thus, we began by applying these reaction conditions in the
direct arylation of novel PEPPSI Pd-NHC complexes with different aryl bromides
(Tables 5–8). Initially, we commenced our works by attempting to make the direct ary-
lation at the 5-position of 2-n-butylthiazole using PEPPSI Pd-NHC complexes as cata-
lyst. In this screening process, the reaction of 2-n-butylthiazole and p-
bromoacetophenone was used as a model reaction (Table 5).
We initially examined the influence of the nature of the base on the conversion for
this reaction. We tested the coupling of 2-n-butyl thiazole with p-bromoacetophenone
in the presence of NaOAc, K2CO3, and KOAc as base, DMAc as the solvent and com-
plex 2a as catalyst (Table 5). K2CO3 or NaOAc gave low conversions of p-bromoaceto-
phenone (Table 5, entries 1 and 4). The nature of the solvent often modifies the

JOURNAL OF COORDINATION CHEMISTRY
23
Table 8. Scope of the direct arylation of n-butyl furan with aryl-bromidesa.
Entry
Aryl bromide
Cat.
Product
Conv. (%)
Yield (%)
1
2
3
4
5
6
7
8
2a
2b
2c
2d
2e
2a
2b
2c
2d
2e
2a
2b
2c
2d
2e
2a
2b
2c
2d
2e
2a
2b
2c
2d
2e
2a
2b
2c
2d
2e
99
98
91
98
93
65
46
27
48
51
60
52
6
43
41
95
100
100
100
100
100
93
74
96
99
99
98
96
98
97
99
95
91
97
90
58
42
27
45
50
57
50
6
40
40
95
98
100
98
99
98
90
74
92
96
99
95
96
97
97
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
aConditions: 2-n-butyl furan (2.0 mmol), aryl bromide (1.0 mmol), Pd cat. (0.01 mmol), KOAc (2 mmol), N,N-dimethyla-
cetamide (2 mL), 130 ꢀC, 1 h.
catalyst activity in cross-coupling reactions, thus we observed that DMF or toluene in
the presence of 1 mol% 2a with KOAc as the catalyst gave low-to-moderate yields
(Table 5, entries 8 and 9). DMAc in the presence of 0.01 mmol 2a/KOAc led to a high
conversion of p-bromoacetophenone (Table 5, entry 7). In these conditions, increasing
the reaction temperature from 100 to 130 ꢀC had a detrimental effect on the conver-
sion (Table 5, entry 10). When the reaction time was reduced to 0.5 h, the reaction
gave low conversion (Table 5, entry 11). Palladium black formation was observed at
the end of the reaction in all reactions. Finally, the best conditions, leading to the full con-
version of p-bromoacetophenone, with high selectivity in favor of the C5-arylated product,
were obtained when the reaction was carried out in DMAc in the presence of KOAc (2
equiv.) at 130 ꢀC for 1h (Table 5, entry 10). Then, 2-n-butyl thiazole was coupled with sev-
eral aryl-bromides in the presence of PEPPSI type Pd-NHC complexes 2a–e as catalyst,
KOAc as the base in DMAc (Table 6). Since the activity of the catalysts 2a–e is almost
close to each other, in the comments on the catalytic activity in the following sections of
the manuscript were made according to 2a catalyst. Very good conversions are obtained
from the direct arylation at the 5-position with 2-n-butylthiazole of aryl bromides. While
arylation products of bromobenzene, 4-bromotoluene, 4-bromoacetophenone and 1-bro-
monaphthalene with 2-n-butylthiazole were obtained with high yields (Table 6, entries 1,

24
N. ŞAHIN ET AL.
6, 21, and 26), those with 4-bromoanisole and 4-bromobenzaldehyde were obtained lower
yield products (Table 6, entries 11 and 16).
After that, 2-n-butylthiophene was coupled with different aryl-bromides to give the ary-
lated products in excellent conversions (Table 7, entries 1, 6, 16, 21, and 26) except the
arylation reaction product of 4-bromoanisole (Table 7, entry 11). Generally, the conversions
for substrates containing electron-withdrawing groups were higher than those for sub-
stituents containing electron-donating groups as shown in these experiments.
Finally, the scope of the direct C5-arylation of n-butyl furan was investigated with
various aryl-bromides (Table 8). The not good yield was observed using p-bromotoluen
and p-bromoanisole (Table 8, entries 6 and 11).
Among the heteroaromatic groups, 2-n-butylthiazole appears to yield the highest
coupling product. Besides, although small differences in reactivities were observed for
2a–e catalysts due to similar nature of the NHC moieties, it can be said that the most
effective catalyst in the direct arylation of heteroaromatics is 2a containing both cyclo-
butylmethyl and methoxyethyl groups. As a result, generally, good results are obtained
when aryl-bromides together with heteroaromatic groups are used in the reaction.
4. Conclusion
We have prepared a series of benzimidazolium salts. These benzimidazolium salts
were metallated with PdCl2 in pyridine to give easily handled, air and moisture stable
new PEPPSI-type palladium-NHC complex series. All benzimidazolium salts and palla-
1
1
dium complexes were characterized using FT-IR, H NMR, and 13Cf Hg NMR spectros-
copies. The crystal structure determination of 2c was performed by single-crystal X-ray
diffraction method. Synthesized complexes 2a–e were used in the direct C5-arylation
of 2-n-butylthiazole, 2-n-butylfuranand 2-n-butylthiophene with various aryl-bromides.
Results showed that the PEPPSI-type palladium-NHC complexes 2a–e were efficient
and selective catalysts for direct C5-arylation reactions. Also, theoretical studies were
made for 1c and 2c molecules. NBO analysis has revealed that the strongest inter-
action contributing to the lowering of the molecular stabilization energy for 2c is pre-
dicted as p(C11-N27) ! p (N26-C28) with E(2) ¼ 73.93 kcal molꢂ1. In addition, FMO
ꢁ
analysis has implied that 2c is more reactive because of its lower energy gap value.
According to the other quantum chemical parameters, 2c is softer, better electrophile
and has more charge-transfer capability than 1c. Finally, we can say that these results
give a good explanation of the catalytic activity of 2c for the direct arylation of het-
eroaromatic compounds with aryl-bromides.
Disclosure statement
No potential conflict of interest was reported by the author(s).
Funding
This work was supported by Sivas Cumhuriyet University, Scientific Research Projects
ꢀ
Department (Project No: CUBAP: EGT-066). All calculations have been carried out at TUBITAK
ULAKBIM, High Performance and Grid Computing Center (TR-Grid e-Infrastructure).

JOURNAL OF COORDINATION CHEMISTRY
25
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