Palladium complexes derived from benzoylthiourea ligands: Synthesis, crystal structure, and catalytic application in Suzuki C–C coupling reactions

Article abstract of DOI:10.1002/aoc.6348

[PdCl₂(HL¹-κS)₂] and [PdCl₂(HL²-κS)₂] complexes were formed from neutral monodentate modes of HL¹ and HL² ligands, which are coordinated to the palladium(II) center, r

Full text of DOI:10.1002/aoc.6348

Received: 16 April 2021
DOI: 10.1002/aoc.6348
Revised: 31 May 2021
Accepted: 13 June 2021
R E S E A R C H A R T I C L E
Palladium complexes derived from benzoylthiourea ligands:
Synthesis, crystal structure, and catalytic application in
Suzuki C–C coupling reactions
Ummuhan Solmaz1
| Ilkay Gumus2,3
| Mustafa Kemal Yilmaz1
|
Simay Ince2 | Hakan Arslan1
1Department of Chemistry, Faculty of Arts
and Science, Mersin University, Mersin,
Turkey
2Department of Nanotechnology and
Advanced Materials, Institute of Science,
Mersin University, Mersin, Turkey
3Department of Basic Sciences, Faculty of
Maritime, Mersin University, Mersin,
Turkey
Abstract
[PdCl2(HL1-κS)2] and [PdCl2(HL2-κS)2] complexes were formed from neutral
monodentate modes of HL1 and HL2 ligands, which are coordinated to the
palladium(II) center, respectively. [Pd(L1-κ2S,O)2] and [Pd(L2-κ2S,O)2] com-
plexes were obtained by recrystallization of corresponding [PdCl2(HL1-κS)2]
and [PdCl2(HL2-κS)2] complexes as anionic bidentate coordination modes. All
palladium(II) complexes were characterized by elemental analysis, FT-IR, and
NMR (1H and 13C) techniques. The molecular structure of [Pd(L1-κ2S,O)2] and
[Pd(L2-κ2S,O)2] was also confirmed by single-crystal X-ray diffraction method.
The roles and behaviors of the prepared four palladium(II) complexes in cata-
lytic investigations for the Suzuki C–C coupling reaction were examined for
the first time. The complexes acted as the excellent catalyst precursor and
showed highly catalytic activity for the Suzuki C–C coupling reaction of vari-
Correspondence
Hakan Arslan, Department of Chemistry,
Faculty of Arts and Science, Mersin
University, Mersin 33343, Turkey.
Email: hakan.arslan@mersin.edu.tr
Funding information
Mersin University Research Fund, Grant/
Award Number: 2018-3-AP4-3090
ous arylhalides with phenylboronic acid at low catalyst loading (0.01 mol%).
Highlights
• All compounds were structurally characterized by FT-IR, H NMR, and 13C
1
NMR spectroscopies.
• [Pd(L-κ2O,S)2] complex was obtained by recrystallization of [PdCl2(HL-κS)2]
complex.
• The molecular structures of [Pd(L1-κ2S,O)2] and [Pd(L2-κ2S,O)2] complexes
were confirmed by X-ray diffraction analysis in single crystal.
• The structure of the compounds was stabilized by hydrogen bond
interactions.
• All palladium precursors performed excellent catalytic activity in the Suzuki
C–C coupling reaction.
K E Y W O R D S
benzoylthiourea, catalytic activity, crystal structure, palladium complexes, Suzuki C–C
coupling reaction
Appl Organomet Chem. 2021;e6348.
wileyonlinelibrary.com/journal/aoc
© 2021 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.6348

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SOLMAZ ET AL.
1 | INTRODUCTION
2 | EXPERIMENTAL
2.1 | Instrumentation
Palladium metal is considered as a soft metal (Class B)
and known with the rich chemical properties. Compre-
hensive applications of palladium chemistry have been
developed with various organic transformations such
as Suzuki, Heck, Sonogashira, Corriud–Kumada, Stille,
Tsuji–Trost, Ullmann, and Hiyama through the C–C
coupling reactions.[1–5] Palladium-catalyzed Suzuki C–C
coupling reactions of arylhalides with arylboronic acids
have been considered as one of the most valuable syn-
thetic methods for the preparation of symmetric and
nonsymmetric biaryl compounds that are widely used
in numerous natural products, polymers, agrochemi-
cals, pharmaceuticals, materials, and synthetic chemis-
try.[6–16] In addition, palladium complexes that can be
varied with many ligands, such as phosphines,[17–20]
1H and 13C NMR spectra were obtained on a Bruker
Avance III 400 MHz Ultrashield Plus Biospin spectrome-
ter. The deuterated solvent CDCl3 was used as purchased.
FT-IR spectra were recorded on a PerkinElmer Spectrum
100 series FT-IR spectrometer (4000–400 cmꢀ1; number
of scans: 250; resolution: 1 cmꢀ1) and were reported in
cmꢀ1 units. Carbon, hydrogen, and nitrogen analyses
were carried out on a Carlo Erba MOD 1106 elemental
analyzer.
The X-ray single-crystal diffraction data were
recorded on a Bruker APEX II CCD diffractometer. A
suitable crystal was selected and coated with Paratone oil
and mounted onto a Nylon loop on a Bruker APEX II
CCD diffractometer. The crystal was kept at T = 100 K
during the collection of the data. The data were collected
with CuKα (λ = 1.54178 Å) radiation for [Pd(L1-κ2S,O)2]
and MoKα (λ = 0.71073 Å) radiation for [Pd(L2-κ2S,O)2]
imidazole,[21]
pyridine,[22]
dibenzylideneacetone,[23]
porphyrins,[24] phthalocyanines,[25] and thiols,[26] were
successfully carried out as catalyst for this coupling
reaction. As compared with benzoylthiourea ligands,
some of the ligands are relatively more expensive, less
thermally and chemically stable.[27–37] Moreover, more
amounts of catalyst or longer time may be required for
catalytic conversions.[22,38,39] On the other hand,
benzoylthiourea ligands are generally stable in air and
moisture, and their catalytic properties can be easily
controlled by changing the N-substitution.[34,37,40,41]
Moreover, N-substituted benzoylthioureas are the
organic compounds with donor atoms that provide a
large number of bonding possibilities such as nitrogen,
sulfur, and oxygen atoms.[30,42,43] Thus, these ligands
can be coordinated with various transition metals or
ions as neutral monodentate[44] or anionic bidentate
coordination modes.[45,46] Although these ligands
exhibit common anionic O,S-bidentate coordination
modes, the unusual neutral monodentate coordination
through only sulfur atoms is reported in some stud-
ies.[44,47,48] In the catalytic reactions and other organic
transformations, there are a number of applications of
benzoylthiourea-derived compounds due to their
different coordination modes. The benzoylthiourea
ligands have only been applied in the Sonogashira
coupling reactions, and also there has been no
report on the Suzuki C–C coupling reaction so
far.[30] Therefore, in this study, we wanted to apply
benzoylthiourea-palladium(II) complexes as catalysts in
the Suzuki C–C coupling reactions, and we found that
these ligand systems were very active, robust, and suit-
able for the formation of biaryl units with very low
catalyst loading. Besides, the different coordination
modes of these ligands were also investigated by using
X-ray diffraction analysis.
at
a crystal-to-detector distance of 40 mm. Using
Olex2,[49] the structure was solved with the Superflip[50–52]
structure solution program using the charge flipping
solution method and refined by full-matrix least-squares
technique on F2 using ShelXL[53] with refinement of F2
against all reflections. Hydrogen atoms were constrained
by different maps and were refined isotropically; in addi-
tion, all non-hydrogen atoms were refined anisotropically.
The molecular structure plots were prepared using
PLATON or Olex2.[49–54]
Gas chromatography (GC) analyses were carried out
on a Shimadzu GC-2010 Plus series gas chromatograph
equipped with a 30 m ꢁ 0.25 mm ꢁ 0.25 μm film thick-
ness Rxi-5ms (5% diphenyl:95% dimethylpolysiloxane)
capillary column. Thin layer chromatography (TLC) was
monitored on a silica gel plate (Merck Kieselgel 60 F254).
The transmission electron microscopy (TEM) mea-
surement was carried out with a FEI Talos F200S, oper-
ated at 200 kV. The particle size and standard deviation
were determined by counting 200 particles from enlarged
TEM images.
2.2 | Synthesis of ligands
All chemicals used for the preparation of the ligand were
purchased from Sigma-Aldrich, and the chemicals
were of reagent grade quality. 4-Chloro-N-(di-n-
ethylcarbamothioyl)benzamide (HL1) and 4-chloro-N-(di-
n-propylcarbamothioyl)benzamide (HL2) were prepared
according to previously published method.[55–58] A solu-
tion of 4-chlorobenzoyl chloride (5 ꢁ 10ꢀ2 mol) in

SOLMAZ ET AL.
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acetone (50 mL) was added dropwise to a suspension of
potassium thiocyanate (5 ꢁ 10ꢀ2 mol) in acetone
(30 mL). The reaction mixture was heated under reflux
for 30 min and then cooled to room temperature. A solu-
tion of dialkylamine (diethylamine or di-n-propylamine)
(5 ꢁ 10ꢀ2 mol) in acetone (10 mL) was added, and the
resulting mixture was stirred for 2 h. Hydrochloric acid
(0.1 N, 300 mL) was added, and the solution was filtered.
The solid product was washed with water and purified by
recrystallization from ethanol/dichloromethane mixture
(1:1, v:v) (Scheme 1).
748.84 (w). 1H NMR (400 MHz, CDCl3, δ, ppm): 8.29
(s, 1H, NH), 7.78 (d, J = 8.2 Hz, 2H, Ar–H), 7.45
(d, J = 8.5 Hz, 2H, Ar–H), 3.94 (t, J = 12.8, 7.4 Hz, 2H,
N–CH2), 3.49 (t, J = 192.3, 178.5 Hz, 2H, N–CH2), 1.88–1.79
(m, 2H, CH2), 1.76–1.66 (m, 2H, CH2), 1.02 (t, J = 7.5,
0.9 Hz, 3H, CH3,), 0.89 (t, J = 7.5, 0.4 Hz, 3H, CH3). 13C
NMR (100 MHz, CDCl3, δ, ppm): 179.8 (C=S), 162.6
(C=O), 139.4, 131.2, 129.2 (C–Ar), 55.1 (C–N), 21.5
(CH2), 19.8 (CH2), 11.3 (CH3). Anal. calcd. for
C14H19ClN2OS: C, 56.27; H, 6.41; N, 9.37. Found: C,
56.25; H, 6.39; N, 9.31%.
4-Chloro-N-(di-ethylcarbamothioyl)benzamide (HL1)
Color: White. Yield: 80% (1.083 g). M.p.: 153–155ꢂC.
FT-IR (ATR, ν, cmꢀ1): ν(N-H) 3275 (w); ν (Ar–H) 3056
(w); ν(C–H) 2978, 2933, 2875 (w); ν(C=O) 1675 (s);
2.3 | Synthesis of the palladium(II)
complexes
1
ν(C=S) 1281 (s); ν(C–Cl) 749 (w). H NMR (400 MHz,
CDCl3, δ, ppm): 8.27 (s, 1H, NH), 7.78 (d, J = 8.5 Hz, 2H,
Ar–H), 7.45 (d, J = 8.5 Hz, 2H, Ar–H), 4.03
(d, J = 4.8 Hz, 2H, N–CH2), 3.59 (d, J = 7.6 Hz, 2H,
N–CH2), 1.33 (td, J = 29.0, 6.3 Hz, 6H, CH3). 13C NMR
(100 MHz, CDCl3, δ, ppm): 179.1 (C=S), 162.8 (C=O),
139.4, 131.1, 129.2 (C–Ar), 48.1 (C–N), 47.8 (C–N), 13.2
(CH3), 11.5 (CH3). Anal. calcd. for C12H15ClN2OS: C,
53.23; H, 5.58; N, 10.35. Found: C, 53.19; H, 5.50; N, 10.25%.
4-Chloro-N-(di-n-propylcarbamothioyl)benzamide
(HL2)
2.3.1 | Synthesis of dichloro bis[4-chloro-N-
(di-ethylcarbamothioyl)benzamido-κS]
palladium(II) [PdCl2(HL1-κS)2]
4-Chloro-N-(di-ethylcarbamothioyl)benzamide
(HL1)
(1.1 ꢁ 10ꢀ2 mol) dissolved in acetonitrile was added
dropwise to an acetonitrile solution of PdCl2
(5 ꢁ 10ꢀ3 mol) at room temperature. The resulting mix-
ture was stirred overnight. The orange powder that
formed was filtered and washed with diethyl ether and
then dried in vacuum[59] (Scheme 2).
Color: White. Yield: 75% (1.12 g). M.p.: 98–99ꢂC. FT-IR
(ATR, ν, cmꢀ1): ν(N–H) 3276, 3212 (w); ν(C–H) 2966,
2933, 2876 (w); ν(C=O) 1690 (s); ν(C=S) 1257 (s); ν(C–Cl)
Color: Orange. Yield: 78%. FT-IR (ATR, ν, cmꢀ1): ν
(NH) 3122, 3087 (w); ν (CH) 2961, 2935, 2874 (w);
SCHEME 1 Synthesis mechanism of the HL1 and HL2 ligands
SCHEME 2 Synthesis of the palladium complexes

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SOLMAZ ET AL.
1
ν(C=O) 1680 (s); ν(C=S) 1258 (s); ν(C–Cl) 750 (s). H
NMR (400 MHz, CDCl3, δ, ppm): 11.41 (s, 1H, NH), 10.98
(s, 1H, NH), 8.06 (d, J = 8.2 Hz, 2H, Ar–H), 7.96 (d,
J = 8.4 Hz, 2H, Ar–H), 7.43 (d, J = 8.1 Hz, 4H, Ar–H),
3.96 (q, J = 7.1 Hz, 4H, CH2), 3.55 (q, J = 7.2 Hz, 4H,
CH2), 1.43 (t, J = 7.2 Hz, 6H, CH3), 1.31 (t, J = 7.2 Hz,
6H, CH3). 13C NMR (100 MHz, CDCl3, δ, ppm): 177.6
(2C, C=S), 162.4 (2C, C=O), 140.2, 130.3, 129.3 (12C,
C–Ar), 49.9, 48.2 (4C, C–N), 12.7, 11.8 (4C, CH3).
Anal. calcd. for C24H30Cl4N4O2PdS2: C, 40.10; H,
4.21; N, 7.80. Found: C, 40.50; H, 4.20; N, 7.80%.
(dt, J = 25.0, 7.1 Hz, 12H, CH3). 13C NMR (100 MHz,
CDCl3, δ, ppm): 171.4 (2C, C–S), 169.7 (2C, C–O), 137.8,
135.7, 131.0, 128.2 (12C, C–Ar), 47.3, 46.2 (4C, C–N), 13.1,
13.6 (4C, CH3). Anal. calcd. for C24H28Cl2N4O2PdS2: C,
44.63; H, 4.37; N, 8.67. Found: C, 44.51; H, 4.32; N, 8.59%.
2.3.4 | Bis[4-chloro-N-(di-n-
propylcarbamothioyl)benzamido-κ2O,S]
palladium(II), [Pd(L2-κ2S,O)2]
[Pd(L2-κ2O,S)2] was obtained by recrystallization of
[PdCl2(HL2-κS)2] from dichloromethane: ethanol mixture
(1:2, v:v) or hot acetonitrile (Scheme 2). Color: Yellow.
FT-IR (ATR, ν, cmꢀ1): ν(C–H) 2960, 2925, 2869 (w); ν(C–N)
1582 (w); ν(C-O) 1482 (s), ν(C–S) 1082 (s), ν(C–Cl) 749 (s).
1H NMR (400 MHz, CDCl3, δ, ppm): 8.13 (dt, J = 5.0, 2.5,
2.4 Hz, 4H, Ar–H), 7.38 (dt, J = 4.7, 2.3, 2.2 Hz, 4H, Ar–H),
3.73 (ddd, J = 9.7, 7.8, 5.5 Hz, 8H, N–CH2), 1.85–1.76
(m, 4H, CH2), 1.75–1.67 (m, 4H, CH2), 0.99 (dt, J = 17.6,
7.4 Hz, 12H, CH3). 13C NMR (100 MHz, CDCl3, δ, ppm):
171.9 (2C, C–S), 169.6 (2C, C–O), 137.7, 135.7, 131.0, 128.2
(12, C–Ar), 54.8, 53.6 (4C, C–N), 21.3, 20.8 (4C, CH2), 11.4,
11.4 (4C, CH3). Anal. calcd. for C28H36Cl2N4O2PdS2: C,
47.90; H, 5.17; N, 7.98. Found: C, 47.75; H, 5.10; N, 7.91%.
2.3.2 | Synthesis of dichloro bis[4-chloro-N-
(di-n-propylcarbamothioyl)benzamido-κS]
palladium(II) [PdCl2(HL2-κS)2]
4-Chloro-N-(di-n-propylcarbamothioyl)benzamide (HL2)
(1.1 ꢁ 10ꢀ2 mol) dissolved in acetonitrile were added
dropwise to an acetonitrile solution of PdCl2
(5 ꢁ 10ꢀ3 mol) at room temperature. The resulting mix-
ture was stirred overnight. The orange powder that
formed was filtered and washed with diethyl ether and
then dried in vacuum[59] (Scheme 2).
Color: Orange. Yield: 82%. FT-IR (ATR, ν, cmꢀ1): ν
(NH) 3077 (w); ν (ArH) 3020 (w); ν (CH) 2966, 2931, 2877
(w); ν(C=O) 1694 (s); ν(C–N) 1593 (w); ν(C=S) 1212 (s);
ν(C–Cl) 747 (s). 1H NMR (400 MHz, CDCl3, δ, ppm):
11.40 (s, 1H, NH), 11.04 (s, 1H, NH), 8.02 (d, J = 8.4 Hz,
4H, Ar–H), 7.43 (dt, J = 13.7, 2.0 Hz, 4H, Ar–H), 3.86 (td,
J = 15.2 Hz, 4H, N–CH2), 3.43 (td, J = 8.1, 7.3 Hz, 4H,
N–CH2), 1.89 (h, J = 7.4 Hz, 4H, CH2), 1.69 (h,
J = 7.5 Hz, 4H, CH2), 1.06 (t, J = 7.4 Hz, 6H, CH3), 0.86
(t, J = 7.4 Hz, 6H, CH3). 13C NMR (100 MHz, CDCl3, δ,
ppm): 178.2 (2C, C=S), 162.0 (2C, C=O), 140.0,
130.2, 129.3 (12C, C–Ar), 57.0, 55.2 (4C, C–N), 21.1,
20.1 (4C, CH2), 11.2 (4C, CH3). Anal. calcd. for
C28H38Cl4N4O2PdS2: C, 43.40; H, 4.94; N, 7.20. Found: C,
43.60; H, 4.90; N, 7.30%.
2.4 | Typical procedure for the Suzuki
C–C coupling reaction
Arylbromide (1.0 mmol), phenylboronic acid (1.2 mmol),
base (1.2 mmol), organic solvent H2O (2:2, v:v), Bu4NBr
(0.02 mmol), and palladium catalyst (0.2 mol%) were
mixed in a sealed tube, and the reaction mixture was
stirred at 110ꢂC for 18 h under aerobic conditions. At the
end of this period, the reaction mixture was cooled, and
then, dodecane was added to the mixture as an internal
standard. The mixture was extracted with chloroform
(20 mL) and washed with saturated ammonium chloride
and brine. The organic phase was separated and dried
over anhydrous sodium sulfate, and the solvent was evap-
orated. The crude product was chromatographed on silica
gel, and the isolated biphenyl product was characterized
by 1H NMR and GC analyses.
2.3.3 | Bis[4-chloro-N-
(diethylcarbamothioyl)benzamido-κ2O,S]
palladium(II), [Pd(L1-κ2S,O)2]
[Pd(L1-κ2O,S)2] was obtained by recrystallization of
[PdCl2(HL1-κS)2] from dichloromethane/ethanol mixture
(1:2, v:v) or hot acetonitrile (Scheme 2). Color: Yellow.
FT-IR (ATR, ν, cmꢀ1): ν(C–H) 2973, 2929, 2864 (w); ν(C–N)
1583 (w); ν(C–O) 1485 (s); ν(C–S) 1076 (s); ν(C–Cl)
3 | RESULTS AND DISCUSSION
3.1 | Synthesis of palladium(II)
complexes
1
750 (s). H NMR (400 MHz, CDCl3, δ, ppm): 8.15 (dt,
J = 5.0, 2.0 Hz, 4H, Ar–H), 7.38 (dt, J = 5.0, 2.3, 2.3 Hz,
4H, Ar–H), 3.84 (q, J = 7.1 Hz, 8H, N–CH2), 1.27
HL1 and HL2 were synthesized according to the proce-
dures given in the literature.[55–58] [PdCl2(HL1-κS)2] and

SOLMAZ ET AL.
5 of 16
[PdCl2(HL2-κS)2] were prepared by adding dropwise ace-
tonitrile solutions of HL1 and HL2, respectively, to aceto-
nitrile solution of PdCl2. [PdCl2(HL1-κS)2] and
[PdCl2(HL2-κS)2] were formed as orange powder. [Pd(L1-
κ2O,S)2] and [Pd(L2-κ2O,S)2] were obtained yellow crys-
tals by recrystallization of [PdCl2(HL1-κS)2] and
[PdCl2(HL2-κS)2] from dichloromethane/ethanol mixture
(1:2, v:v) or hot acetonitrile, respectively. When
[PdCl2(HL1-κS)2] and [PdCl2(HL2-κS)2] were obtained as
powder form, unfortunately, we could not crystallize in
the same structure because these palladium complexes
were completely converted to [Pd(L1-κ2O,S)2] and [Pd(L2-
κ2O,S)2] structures after recrystallization.
3.2 | FT-IR and NMR spectroscopic
studies
In Section 2, FT-IR spectra of the desired compounds are
given to determine the most important infrared
(IR) spectral bands of ligands and palladium(II) com-
plexes. The vibration frequencies between ligands and
complexes are described by comparative analyses,
and the analyses of the complexes obtained in different
binding modes are compared with each other (Figure 1).
In Figure 1, the FT-IR spectrum exhibited a strong
stretching absorption band at 3275 cmꢀ1, which indicates
the presence of N–H group in structure of ligand HL1.
Because the ligand exhibits neutral monodentate
(hypodentate) coordination via the sulfur atom to the
palladium(II) ion, the deprotonation did not occur in this
complex. Therefore, the related N–H stretching mode
was observed at 3122 and 3087 cmꢀ1 in [PdCl2(HL1-κS)2]
and [PdCl2(HL1-κS)2] complexes, respectively. Although
[PdCl2(HL1-κS)2] complex converted to [Pd(L1-κ2O,S)2]
complex during the crystallization, it has been seen that
deprotonation of amidic protons (C(O)–NH–C(S) moiety)
occured; hence, the ligand coordinated to the
palladium(II) center via the sulfur and oxygen atoms in
monoanionic bidentate form. The presence of C=O
groups in HL1 and [PdCl2(HL1-κS)2] was determined with
the observation of a strong absorption band at 1675 and
1680 cmꢀ1, respectively. Absorption bands belonging to
C=S moiety in HL1 and [PdCl2(HL1-κS)2] compounds
were observed as medium intensity bands at 1281 and
1258 cmꢀ1 in the FT-IR spectrum. This situation
The palladium(II) ions in [PdCl2(HL1-κS)2] and
[PdCl2(HL2-κS)2] complexes were coordinated to the
ligands as neutral monodentate coordination through
the only sulfur atoms. However, in [Pd(L1-κ2O,S)2] and
[Pd(L2-κ2O,S)2] complexes, palladium(II) ions were coor-
dinated to the corresponding ligands as monoanionic bid-
entate coordination through the sulfur and oxygen
atoms.
All new palladium(II) complexes are air stable and
soluble in common organic solvent such as acetone,
dichloromethane, chloroform, 1,4-dioxane, dimethyl
sulfoxide, and dimethylformamide (DMF), but insoluble
in n-hexane, ethanol, and water.
The prepared palladium(II) complexes were charac-
terized by elemental analyses, FT-IR, 1H NMR, 13C NMR,
and X-ray diffraction analysis. The analytical and spectro-
scopic data are consistent with the proposed structures
given in Scheme 2.
FIGURE 1 The comparison FT-IR spectra of HL1, [PdCl2(HL1-κS)2], and [Pd(L1-κ2O,S)2] compounds

6 of 16
SOLMAZ ET AL.
indicates that the carbonyl moiety is in a similar environ-
ment with the ligand and its complex, and thus, it was
free from coordination. But the band belonging to the
C=S group appeared in the lower frequency region in the
IR spectrum of the [PdCl2(HL1-κS)2] complex; thus, it
was observed that the sulfur atom of the thiocarbonyl
group is involved in coordination. C=S and C=O
stretching modes observed in HL1 and [PdCl2(HL1-κS)2]
compounds disappeared in the [Pd(L1-κ2O,S)2] complex,
and new ν(C–O) and ν(C–S) streching modes were
observed in 1485 and 1076 cmꢀ1, respectively.
The eight proton signals corresponding to the CH2
protons were observed as two quartets at δ 3.96 and
3.55 ppm, and the 12 proton signals assigned to the CH3
protons were observed as again two doublets at δ 1.43
1
and 1.31 ppm in the H NMR spectrum of [PdCl2(HL1-
κS)2]. In the [Pd(L1-κ2O,S)2] complex, N–CH2 proton sig-
nals appeared as single quartets at δ 3.84 ppm, and the
signals corresponding to the CH3 protons were observed
1
as two triplets at δ 1.34 and 1.27 ppm. In the H NMR
spectrum of [PdCl2(HL2-κS)2], two triplets at δ 3.86 and
3.43 ppm, two multiplets in the regions δ 1.93–1.84 ppm
and δ 1.74–1.65 ppm, and two triplets at δ 1.06 and
0.86 ppm were observed, which were assigned to the
methylene protons of N–CH2 groups, CH2 protons, and
terminal methyl protons in the aliphatic chain,
respectively.
NH vibration bands exhibited at 3276 and 3077 cmꢀ1
in HL2 and [PdCl2(HL2-κS)2], respectively. In addition,
the stretching frequencies assigned to C=O and C=S
groups were appeared at 1690, 1694 cmꢀ1 and 1257,
1212 cmꢀ1, respectively. In the [Pd(L2-κ2O,S)2] complex,
the frequencies related to C=O and C=S groups in HL2
and [PdCl2(HL2-κS)2] disappeared, and new C–O and C–S
stretching vibration modes at 1482 and 1082 cmꢀ1 were
observed. These results showed us that similar situations
in Figure 1 were realized.
The observation of signals at δ 179.08 and 177.55 ppm
and at δ 162.82 and 162.37 ppm in the 13C NMR spectra
of HL1 and [PdCl2(HL1-κS)2] confirmed the presence of
C=S and C=O groups, respectively. The signals assigned
to the C–S and C–O groups in [Pd(L1-κ2O,S)2] were
appeared in δ 171.40 and 166.72 ppm. In the 13C NMR
spectrum, C=S and C=O carbons of HL2 and
[PdCl2(HL2-κS)2] were observed also at δ 179.76 and
178.23 ppm and δ 162.61 and 162.04 ppm, respectively.
In the [Pd(L2-κ2O,S)2] complex, carbon atoms of the same
groups were shown at δ 171.94 and 169.56 ppm. The
cause of chemical shifts in [Pd(L-κ2O,S)2] complexes
relates to the neutral bidentate coordination to the
palladium(II) center through the sulfur and oxygen
atoms.
In both [Pd(L-κS)2] complexes, signals of the
thiocarbonyl carbons in corresponding ligands were
shifted after each ligand coordinated to the palladium
center, which indicates that coordination occured only
through the sulfur atoms for the formation of neutral
monodentate palladium(II) complexes. On the other
hand, the signals of the other hydrogen and carbon atoms
of all ligands and were appeared in the expected regions.
Thus, as expected, all the results that we found support
our suggested molecular and bonding structures
(Sections 2.2 and 2.3).
Molecular structures of the complexes were also exam-
ined by NMR (1H and 13C) spectroscopy, and comparative
analysis on the basis of spectroscopic data of both ligands
and palladium(II) complexes was examined in a similar
1
manner to the FT-IR analysis described above. In the H
NMR spectra of the HL1 and HL2 ligands, proton signals
assigned to NH group were appeared as broad signals at δ
8.27 and δ 8.29 ppm, respectively. NH proton signals of
[PdCl2(HL1-κS)2] and [PdCl2(HL2-κS)2] complexes were
observed at δ 10.98 and δ 11.40 ppm, respectively. The NH
signals of the palladium(II) complexes shifted downfield
from their positions compared with free ligands. This situ-
ation is due to the intramolecular hydrogen bonding
between the N–H groups and the chloro atoms coordi-
nated to palladium metal.[44] However, the NH proton sig-
nals that existed in the ligands and [PdCl2(HL-κS)2]
complexes disappeared in those of [Pd(L1-κ2O,S)2] and [Pd
(L2-κ2O,S)2] complexes due to the coordination of the both
sulfur and oxygen donors to the palladium center. The
deprotonation of the N–H group was also observed from
FT-IR analysis.
In the [Pd (L1-κ2O,S)2] and [Pd (L2-κ2O,S)2] com-
plexes, the disappearance of the N–H proton after the
ligands are coordinated to the palladium metal is related
to the increase in the electron density of the C–N bond in
the complexes.[60-62] Therefore, a slight downfield shift
was also observed in the aromatic protons of [Pd(L1-
κ2O,S)2] and [Pd(L2-κ2O,S)2] complexes, compared with
the chemical shift of free ligands. The aromatic protons
appeared in the region δ 7.78–7.45 ppm for the ligands in
the region δ 8.15–7.38 ppm for the corresponding
palladium(II) complexes.
3.3 | Crystal structure analysis
X-ray quality crystals of the prepared complexes were
obtained from dichloromethane:/ethanol mixture (1:2, v:v)
or hot acetonitrile (Figure 2). Block-shaped orange-
colored crystals of complexes [Pd(L1-κ2O,S)2] and [Pd
(L2-κ2O,S)2] crystallize in a triclinic crystal system with
P-1 space group (Table 1). The asymmetric unit of the
complex consists of one Pd(II) center with coordination

SOLMAZ ET AL.
7 of 16
FIGURE 2 The crystal structures of the compounds [Pd(L1-κ2O,S)2] and [Pd(L2-κ2O,S)2]
by two sulfur and two oxygen atoms from two mono-
anionic ligand moieties (Lꢀ). Total of angles around
Pd(II) centers of the complexes [Pd(L1-κ2O,S)2] and [Pd
(L2-κ2O,S)2] are 360ꢂ and 359ꢂ, indicating almost
undistorted square planar geometry around the
Pd(II) center, respectively. The bond lengths and bite
angles are very similar to those observed for other
Pd(II) complexes.[63] The chelate bite angles (ꢂ) for the
six-membered ring in complexes are S1–Pd1–
O1 = 93.87(4) and S2–Pd1–O2 = 92.67(4)ꢂ for complex
[Pd(L1-κ2O,S)2] and S1–Pd1–O1 = 93.99(5) and S2–Pd1–
O2 = 93.88(5)ꢂ for complex [Pd(L2-κ2O,S)2] (Table 2).
Bond lengths in both complexes are similar. The C(O)
and C(S) bond lengths in the complexes [Pd(L1-κ2O,S)2]
and [Pd(L2-κ2O,S)2] are slightly longer (1.299(3) and 1.299
(3) Å for complex [Pd(L1-κ2O,S)2] and 1.299(3) and 1.299
(3) Å for complex [Pd(L2-κ2O,S)2], respectively) than the
corresponding distance found in the free ligand due to
coordination to Pd(II) (Table 2).[48,51] These bond lengths
reflect a relatively extent of electron delocalization over
the six-membered Pd-SCNCO chelate ring, because the
corresponding thiocarbonyl and carbonyl formal double
bond is substantially longer in the free ligand.[31,63,64]
On the other hand, single-crystal X-ray diffraction
analysis of complexes indicates significant intra- and
intermolecular interactions that appear to stabilize the
solid state. Intra- and intermolecular hydrogen bonds
and geometrical parameters are given in Tables 3 and 4
for complexes. In the molecular packing structure of the
complex [Pd(L1-κ2O,S)2], the molecules are linked
through intermolecular C–HꢃꢃꢃCl, C–HꢃꢃꢃS interactions
and πꢃꢃꢃπ stacking interactions to form supramolecular
structure (Figure 3). The intermolecular C–HꢃꢃꢃCl interac-
tions occur between the chlorine atoms and the aromatic
ring hydrogen atoms, forming dimeric R2 (28) and syn-
2
thons, whereas the intermolecular C–HꢃꢃꢃS interactions
occur between the thiocarbonyl sulfur atoms and the ali-
phatic hydrogen atoms, forming dimeric R2 (14). The
2
molecular structure of complex [Pd(L1-κ2O,S)2] also
shows significant intermolecular πꢃꢃꢃπ stacking interac-
tions between the planar six-membered Pd-SCNCO che-
late ring and the phenyl ring of the neighboring
molecule. These πꢃꢃꢃπ stacking interactions are face-to-
face, and the average interplanar distance between these
π-stacked aromatic moieties is 3.405 Å with symmetry
code: i = ꢀx, 1-y, ꢀz (Figure 3).
Examination of the packing for the complex [PdL2-
κ2O,S2] showed intermolecular C–HꢃꢃꢃCl interactions
between the chlorine atom and the ethyl or aromatic ring
hydrogen atom of an adjacent molecule. The C–HꢃꢃꢃCl
interactions provide contribution to the polymeric chain
along the c axis, leading to the formation of fused dimeric
R2 (28) and tetrameric R4 (30) synthons (Figure 4). On
2
4
the other hand, unlike complex [Pd(L1-κ2O,S)2], the mol-
ecules of complex [Pd(L2-κ2O,S)2] are assembled through
π-stacking interactions slightly offset relative to one
another between the aromatic rings of thiourea ligands.
Corresponding distances for these interactions are
3.939 Å with symmetry code i = ꢀ1 + x, y, z (Figure 5).
Furthermore, additional C–Hꢃꢃꢃπ stacking interactions
occur between Pd–S–C–N–C–O chelate ring and aliphatic
hydrogen atoms, and these interactions lead to the stabi-
lization of three-dimensional supramolecular structure of
the complex [Pd(L2-κ2O,S)2] (Figure 5).

8 of 16
SOLMAZ ET AL.
TABLE 1 Crystal data and details of the structure refinement for complexes [Pd(L1-κ2O,S)2] and [Pd(L2-κ2O,S)2]
Compound
Empirical formula
Formula weight
Temperature (K)
Crystal system
Space group
a (Å)
[Pd(L1-κ2O,S)2]
C24H28Cl2N4O2PdS2
645.92
[Pd(L2-κ2O,S)2]
C28H36Cl2N4O2PdS2
702.03
100.0
100.01
Triclinic
Triclinic
P-1
P-1
7.7253(5)
9.7269(3)
12.8369(4)
13.3640(5)
66.157(2)
87.520(2)
84.892(2)
1520.15(9)
2
b (Å)
13.7144(9)
13.8434(10)
112.837(2)
92.382(2)
c (Å)
α (ꢂ)
β (ꢂ)
γ (ꢂ)
Volume (Å3)
95.896(2)
1339.24(16)
2
Z
ρ
calc (g/cm3)
1.602
1.534
μ (mmꢀ1
)
1.078
8.086
F(000)
656.0
720.0
Crystal size (mm3)
0.45 ꢁ 0.14 ꢁ 0.09
MoKα (λ = 0.71073)
5.868 to 50.224
0.27 ꢁ 0.26 ꢁ 0.21
CuKα (λ = 1.54178)
7.232 to 133.184
Radiation
2Θ range for data collection (ꢂ)
Index ranges
ꢀ9 ≤ h ≤ 9
ꢀ11 ≤ h ≤ 11
ꢀ15 ≤ k ≤ 15
ꢀ15 ≤ l ≤ 15
ꢀ16 ≤ k ≤ 16
ꢀ16 ≤ l ≤ 16
Reflections collected
50,601
49,145
Independent reflections
Data/restraints/parameters
Goodness of fit on F2
4747 [Rint = 0.0531, Rsigma = 0.0176]
4747/0/321
5371 [Rint = 0.0994, Rsigma = 0.0370]
5371/0/356
1.066
1.085
Final R indexes [I ≥ 2σ (I)]
Final R indexes [all data]
Largest diff. peak/hole (e Åꢀ3
R1 = 0.0193, wR2 = 0.0471
R1 = 0.0217, wR2 = 0.0483
0.34/ꢀ0.56
R1 = 0.0320, wR2 = 0.0775
R1 = 0.0339, wR2 = 0.0789
0.69/ꢀ1.34
)
3.4 | The Suzuki C–C coupling reaction
DMF, good yield (67%) of coupling product was obtained
after 18 h (Entry 1). Under the same conditions, the best
result was obtained with 1,4-dioxane as the solvent with
a good isolated yield (93%) of biphenyl (Entry 4). It
seemed that polar aprotic solvents such as 1,4-dioxane
and DMF played an important role in the Suzuki C–C
coupling reactions as they allow for the best solubility of
both the catalysts and substrates used. In addition to this,
when comparing the results obtained from 1,4-dioxane
and DMF, it can also be said that because the nitrogen
donor in DMF is a softer base than oxygen donors in
1,4-dioxane and this solvent is more likely to coordinate
to the Pd(II) center, it is more likely to interfere with cat-
alyst activity.[65] After determining the best solvent, we
also investigated the effect of the bases such as NaOAc,
In order to evaluate the optimal reaction conditions, the
effect of solvent, base, substrate to pre-catalyst ratio
(S/C), and using tetrabutylammonium bromide (Bu4NBr)
as an additive were investigated on the Suzuki C–C cou-
pling reaction between bromobenzene and phe-
nylboronic acid in the presence of pre-catalyst
[PdCl2(HL2-κS)2]. The reactions were performed with
0.2 mol% of pre-catalyst for initial tests and carried out
under aerobic conditions in all cases. After screening
1,4-dioxane, DMF, toluene, and 2-propanol as solvent, in
the presence of KOH, we found 11% and 44% conversion
for 18 h of the reaction when using 2-propanol and tolu-
ene as solvent, respectively (Table 5, Entries 2 and 3). In

SOLMAZ ET AL.
9 of 16
TABLE 2 Selected bond lengths (Å), bond angles, and torsion angles (ꢂ) for [Pd(L1-κ2O,S)2] and [Pd(L2-κ2O,S)2]
Atom Atom Length (Å)
Atom Atom Atom Angle (ꢂ)
Atom Atom Atom Atom Angle (ꢂ)
[Pd(L1-κ2O,S)2]
Pd1
Pd1
Pd1
Pd1
S1
S1
2.2386(5)
2.2435(5)
2.0141(12)
2.0183(12)
1.7421(18)
1.7420(18)
1.267(2)
S1
Pd1
Pd1
Pd1
Pd1
Pd1
Pd1
S1
S2
89.075(17)
93.87(4)
S2
Pd1
Pd1
Pd1
Pd1
O2
O2
C1
S1
C8
167.24(1)
S2
O1
O1
O1
O2
O2
C8
C20
C1
C8
C8
C9
C13
S1
O2
O1
S1
S2
C20
C8
ꢀ21.81(1)
ꢀ11.80(1)
158.36(1)
21.18(1)
O1
O2
C8
C20
C1
C13
C1
C8
C8
C9
C11
S2
176.91(4)
84.38(5)
S1
O2
S1
S2
C20
S2
178.24(4)
92.67(4)
C13
C13
N1
C2
O1
N1
S1
Pd1
Pd1
O1
N1
N1
C2
N1
S1
S2
S2
O1
Pd1
C8
ꢀ159.78(1)
ꢀ14.36(1)
178.01(1)
ꢀ2.92(1)
2.40(1)
O1
O2
N1
N1
N2
N2
N2
Pd1
Pd1
C8
107.46(6)
106.71(6)
126.61(16)
124.35(15)
120.46(15)
115.20(15)
125.85(16)
1.264(2)
S2
C1
1.325(2)
N1
C1
C8
1.343(2)
N2
C9
C1
C7
1.338(2)
N2
C11
C11
C20
C8
C1
7.32(1)
1.461(2)
N2
N2
S1
C8
Pd1
C9
ꢀ176.61(1)
3.86(1)
1.475(2)
N3
C8
N2
[Pd(L2-κ2O,S)2]
Pd1
Pd1
Pd1
Pd1
Cl1
Cl2
S1
S1
2.2431(7)
2.2407(7)
2.0219(18)
2.0205(18)
1.749(3)
1.749(3)
1.743(3)
1.748(3)
1.267(3)
1.260(3)
1.350(4)
1.322(3)
1.341(3)
1.481(3)
1.465(3)
S2
Pd1
Pd1
Pd1
Pd1
Pd1
Pd1
S1
S1
87.82(2)
93.88(5)
177.11(6)
178.09(5)
93.99(5)
84.28(7)
107.38(9)
107.96(9)
127.55(17)
129.16(17)
125.0(2)
121.5(2)
123.4(2)
115.0(2)
116.0(2)
S2
Pd1
Pd1
Pd1
Pd1
S1
S1
C1
ꢀ175.16(1)
7.17(12)
177.78(1)
ꢀ164.2(2)
ꢀ23.2(3)
15.3(2)
S2
O1
O1
O2
O2
O2
C1
C15
C2
C16
C2
C1
C1
C12
N2
S1
O1
S1
S1
C1
O1
O2
C6
S2
S2
C15
C2
S1
O2
Pd1
S1
O1
C1
O1
S2
S2
N1
C2
C20
C1
O1
Pd1
Pd1
Pd1
Pd1
C1
C9
C12
C9
S1
Pd1
Pd1
Pd1
Pd1
S1
O2
S2
C15
C16
C16
N2
N3
N4
N3
C17
S1
ꢀ2.82(12)
ꢀ11.6(2)
166.1(2)
162.49(1)
13.4(3)
S2
C15
C2
S2
O2
O2
C1
C15
C15
C16
C16
C1
O1
O2
N1
N1
N2
N2
N2
O1
O2
N1
N2
N2
N2
C1
O1
Pd1
Pd1
Pd1
Pd1
Pd1
C2
C16
C1
S2
C2
S2
ꢀ168.69(1)
20.7(4)
C1
O2
C9
O2
ꢀ159.35(1)
16.6(4)
C12
N1
TABLE 3 Intra- and intermolecular
hydrogen bonds for complexes [Pd(L1-
κ2O,S)2] and [Pd(L2-κ2O,S)2] (Å,ꢂ)a
Compound
D-HꢃꢃꢃA
C(7)-H(7)ꢃꢃꢃN(1)
d(D-H)
d(HꢃꢃꢃA)
d(DꢃꢃꢃA)
2.7772(2)
3.6685(3)
2.9467(2)
2.9286(2)
2.936(3)
2.931(3)
∠ (D-HꢃꢃꢃA)
[Pd(L1-κ2O,S)2]
0.95
0.99
0.99
0.99
0.99
0.99
2.46
2.85
2.49
2.51
2.51
2.46
100
141
108
105
106
109
C(9)-H(9A)ꢃꢃꢃS(2) i
C(9)-H(9B)ꢃꢃꢃS(1)
C(23)-H(23B)ꢃꢃꢃS(2)
C(12)-H(12)ꢃꢃꢃS(1)
C(26)-H(26A)ꢃꢃꢃS(2)
[Pd(L2-κ2O,S)2]
aSymmetry codes for co-crystal: i = 1-x, 1-y, 1-z.
K2CO3, Cs2CO3, and Et3N, but we could not obtain a sig-
nificant beneficial effect on the pre-catalyst performance
(Entries 5–8). These results support that strong bases are
more effective to neutralize the hydrogen halide and to
generate the catalytically active species for facilities the
transmetallation step in Suzuki C–C coupling

10 of 16
SOLMAZ ET AL.
TABLE 4 Geometrical parameters
of C–Hꢃ ꢃ ꢃπ interactions for complexes
[Pd(L1-κ2O,S)2] and [Pd(L2-κ2O,S)2]
(Å,ꢂ)a
C-HꢃꢃꢃCg(J)b
HꢃꢃꢃCg
H-perpc
∠C-HꢃꢃꢃCg
γd
CꢃꢃꢃCge
[Pd(L1-κ2O,S)2]
C23-H23AꢃꢃꢃCg(1)
[Pd(L2-κ2O,S)2]
2.76
2.68
115
13.86
3.3030(2)
C10-H10BꢃꢃꢃCg(2)
C24-H24AꢃꢃꢃCg(2)
2.81
2.83
ꢀ2.79
2.82
118
127
6.10
5.95
3.383(3)
3.516(4)
Note: Cg(1) is the centroids of the rings Pd1-S1-C8-N1-C1-O1, and Cg(2) is the centroids of the rings
Pd1-S2-C15-N3-C16-O2.
aSymmetry codes: i = 1 + x, y, z; for complex [Pd(L1-κ2O,S)2]; ii = ꢀ1 + x, y, z and iii = 2-x, 1-y, 1-z.
bCenter of gravity of ring J (plane number above).
cPerpendicular distance of H to the ring plane J.
dAngle between Cg-H vector and the ring J normal.
eDistance between C atom and the nearest carbon atom in the benzene ring.
FIGURE 3 C–HꢃꢃꢃCl intramolecular interactions and πꢃꢃꢃπ stacking interactions in [Pd(L1-κ2O,S)2] complex
reaction.[66] Thus, the best solvent and the best base were
found to be 1,4-dioxane and KOH, respectively. Then, we
used these conditions to evaluate the effect of
tetrabutylammonium bromide (Bu4NBr) as an additive. It
is well known that quaternary ammonium salts show a
beneficial effect on the coupling of arylhalides with phe-
nylboronic acids.[67] Entry 9 showed that the coupling of
bromobenzene with phenylboronic acid gave a full biphe-
nyl product (with TON 500) when 10 mol% Bu4NBr is
added to the reaction mixture.
To extend the catalytic tests of the pre-catalyst
[PdCl2(HL2-κS)2], the effect of the catalyst amount for the
coupling of bromobenzene with phenylboronic acid was
also studied by varying the pre-catalyst loading from 0.2
to 0.001 mol% under optimized conditions. We found that
the reduction of the catalyst loading from 0.20 to
0.01 mol% led in the reaction of bromobenzene with phe-
nylboronic acid to a product yield of >99% with TON
>9900 but further decreasing the catalyst loading from
0.01 to 0.001 mol% led to a decrease in the catalytic activ-
ity (54%) (compare Entries 10 and 11, Table 5). These
excellent catalytic activities showed that even using very
low pre-catalyst loading conditions was capable of effec-
tively catalyzing the coupling of bromobenzene and phe-
nylboronic acid at 110ꢂC. Thus, the optimized reaction
conditions are summarized as follows: 0.01 mol% of cata-
lyst, 110ꢂC, KOH in 1,4-dioxane.
The catalytic activities of [PdCl2(HL1-κS)2], [Pd(L1-
κ2S,O)2], and [Pd(L2-κ2S,O)2] were also investigated fol-
lowing identical optimized reaction conditions for the
model coupling reaction, and excellent yield and selectiv-
ity were obtained over the same period (Table 6, Entries
1, 3, and 7).
At this stage, Hg(0) poisoning tests were performed
for all pre-catalysts to understand whether the catalyti-
cally active species is the molecular benzoylthiourea-

SOLMAZ ET AL.
11 of 16
FIGURE 4 Consecutive the formation dimeric R2 (28) and tetrameric R4 (30) synthons generated through C–HꢃꢃꢃCl intermolecular
2
4
interactions
palladium(II) complex or nanoparticles (colloidal species
or other Pd(0) particles) due to the formation of the black
colloidal precipitates in all runs. It is known that the
Hg(0) poisoning test is the most widely used and simple
test of homogeneous versus heterogeneous catalysis. If
the catalytic reaction were supported with palladium
(0) species such as nanoclusters or colloidal palladium(0),
the catalyst activity would be dramatically decreased by
the Hg(0) addition. It was observed that addition of
20 equivalents Hg(0) at the beginning of the reaction
completely suppressed the catalytic activity for all of the
pre-catalysts (Table 6, Entries 2, 4, 6 and 8). Despite that
the only Hg(0) poisoning test has been used to assess the
homogeneity or heterogeneity of our catalyst system,
obtained results strongly suggest that the heteroge-
neously active Pd(0) species play an important role and
the possibility of a homogeneous pathway cannot be pre-
sumed. These results also indicate that our thiourea
ligands are highly capable of stabilizing the active cata-
lyst; in other words, synthesized benzoylthiourea-
palladium complexes are necessary intermediates for the
formation of active Pd(0) species in the reaction
medium.[65,68,69] After determining the catalytic pathway,
the reusability of black precipitates that produced by pre-
catalyst [PdCl2(HL2-κS)2] during the reaction under the
optimized conditions was studied. At the end of each
reaction, the resulting black precipitates were recovered
by centrifugation and washed with chloroform,
methanol, and water, respectively. Then, recovered pre-
cipitates were dried under high vacuum and used with
fresh bromobenzene and phenylboronic acid. The results
showed that the active catalyst could be reused for four
consecutive cycles without significant loss of catalytic
activity. It is worth nothing that the morphological char-
acterization of the recovered precipitates after four suc-
cessive runs was performed using TEM image analysis
and the average diameter of the nanoparticles is in the
range 3.6 0.1 nm without obvious aggregation (see
Supporting Information).
With our optimized conditions in hand, we exam-
ined a range of substituted arylhalides in the Suzuki C–C
coupling reaction with phenylboronic acid. The
results are summarized in Table 7. We have discovered
that catalyst systems can serve as an effective catalyst
considering this process for activated, sterically hin-
dered, and deactivated arylbromides. For example, the
reaction of 4-bromobenzaldehyde containing electron-
withdrawing group at para position, which could accel-
erate the rate-determining oxidative addition step in the
catalytic cycle due to the reduced electron density of C–Br
bond, gave biphenyl-4-carboxaldehyde as the principal
product in excellent yields with all catalysts (Entry 3).[65,70]
Our pre-catalysts are also effective toward the
coupling of 2-bromobenzaldehyde, 2-bromoanisole, and
1-bromo-2-nitrobenzene, which have sterically bulky
-formyl, -methoxy, and -nitro groups at the ortho

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SOLMAZ ET AL.
FIGURE 5 C–Hꢃꢃꢃπ and πꢃꢃꢃπ stacking interactions in [Pd(L2-κ2O,S)2] complex
position (Entries 2, 7, and 8). In spite of this steric hin-
drance, [Pd(L1-κ2S,O)2] gave biphenyl-2-carboxaldehyde as
the major product with >99% conversion and 87%
selectivity, whereas other complexes gave the conver-
sion in the range of 74%–84%, and thus, steric effect
was well tolerated. Generally, for the Suzuki C–C cou-
pling reactions, the use of deactivated arylbromides,
substituted with electron-donating groups such as
-methyl, -naphthyl, or -methoxy, reduce the rate of the
reaction (Entries 9–11).[19,71] However, the coupling of
4-bromoanisole and phenylboronic acid gave desired
coupled product with good to excellent isolated yields
by all pre-catalysts (up to 96%), achieving a TON of
9900 (Entry 1). The catalytic activities of all
palladium(II) complexes were also tested in the Suzuki
C–C coupling of 1-chloro-4-nitrobenzene and hetero-
aromatic substrate 2-bromopyridine with phenylboronic
acid, and then, moderate to good coupling products
were obtained (12%–77%; Entries 4 and 12).
These results clearly indicate that Suzuki C–C cou-
pling reactions can catalyze with excellent conversion
and selectivity using the benzoylthiourea-based palla-
dium precursors even at very low catalyst loadings.
Thus, this type of molecular structures bearing
benzoylthiourea-palladium complexes as pre-catalyst
can be further developed in Suzuki C–C coupling reac-
tion by changing their electronic structure and
geometry.

SOLMAZ ET AL.
13 of 16
TABLE 5 Optimization of Suzuki C–C coupling reaction conditiona
Entry
Base
Solvent
Conversion (%)b
Yield (%)b
Selectivity (%)
TONc
1
KOH
KOH
KOH
KOH
NaOAc
Cs2CO3
Et3N
DMF
74
67
90
370
220
55
2
Toluene
44
44
>99
45
3
2-Propanol
1,4-Dioxane
1,4-Dioxane
1,4-Dioxane
1,4-Dioxane
1,4-Dioxane
1,4-Dioxane
1,4-Dioxane
1,4-Dioxane
11
5
4
93
93 (92d)
>99
>99
>99
>99
>99
>99
>99
>99
465
245
65
5
49
49
6
13
13
7
49
49
245
30
8
K2CO3
KOH
KOH
KOH
6
6
9
>99e
>99e,f
54e,g
>99 (97d)
>99 (97d)
54
500
>9900
54000
10
11
aReaction conditions: bromobenzene (1.0 mmol), phenylboronic acid (1.2 mmol), base (1.2 mmol), catalyst ([PdCl2(HL2-κS)2]) (0.002 mmol), solvent (2 mL),
H2O (2 mL), temperature: 110ꢂC, time: 18 h.
bYields and conversions were determined with GC analysis based on bromobenzene using dodecane as an internal standard.
cTON: mole product/mole catalyst.
dIsolated yield after silica gel chromatography.
eBu4NBr (relative to the complex used, 10%).
fPre-catalyst (0.01 mol%), from the stock solution of the pre-catalyst (0.01 M).
gPre-catalyst (0.001 mol%), from the stock solution of the pre-catalyst (0.01 M).
TABLE 6 Hg(0) poisoning effect on pre-catalysts for Suzuki C–C coupling reaction between bromobenzene and phenylboronic acida
Entry
Catalystb
[PdCl2(HL1-κS)2]
Hg(0)c
ꢀ
Conversion (%)d
Yield (%)d
>99 (96f)
<1
Selectivity (%)
TONe
9900
ꢀ
1
2
3
4
5
6
7
8
>99
<1
>99
<1
+
[Pd(L1-κ2S,O)2]
[PdCl2(HL2-κS)2]
[Pd(L2-κ2S,O)2]
ꢀ
+
>99
<1
>99 (97f)
>99
<1
9900
<1
ꢀ
>9900
ꢀ
ꢀ
+
>99
<1
>99
>99
<1
<1
>99 (97f)
ꢀ
+
>99
1
>99
<1
9900
1
ꢀ
aReaction conditions: bromobenzene (1.0 mmol), phenylboronic acid (1.2 mmol), KOH (1.2 mmol), catalyst (0.0001 mmol), Bu4NBr (10%), 1,4-dioxane (2 mL),
H2O (2 mL), temperature: 110ꢂC, time: 18 h.
bFrom stock solution of the catalyst (0.01 M).
c20 equivalent of Hg(0) (relative to the complex used) was added at the beginning of the reaction.
dYields and conversions were determined by GC analysis based on bromobenzene using dodecane as an internal standard.
eTON: moles of product/moles of catalyst.
fIsolated yield after silica gel chromatography.

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SOLMAZ ET AL.

SOLMAZ ET AL.
15 of 16
4 | CONCLUSIONS
Ilkay Gumus https://orcid.org/0000-0002-9398-0057
Mustafa Kemal Yilmaz https://orcid.org/0000-0002-
9969-3956
Simay Ince https://orcid.org/0000-0002-9941-7281
Hakan Arslan https://orcid.org/0000-0003-0046-9442
Both coordination modes of the palladium(II) complexes
have been successfully synthesized and characterized. The
molecular structure of the complexes has been confirmed
by several analytical techniques and as well as X-ray crys-
tallography of [Pd(L1-κ2S,O)2] and [Pd(L2-κ2S,O)2]. In addi-
tion, the catalytic activities of the synthesized palladium(II)
complexes have been tested for Suzuki C–C coupling reac-
tion. Our catalyst system is highly efficient and selective
for the Suzuki C–C coupling reaction of activated,
deactivated, and sterically hindered arylbromides with phe-
nylboronic acid in reasonable reaction times (18 h), even
with substrate to catalyst ratio of 10000/1. Furthermore,
considering our results and the discussions above for the
catalytic reactions, benzoylthiourea-palladium complexes
were excellent precursors that decompose to the actual
active Pd(0) species during the catalytic process.
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DATA AVAILABILITY STATEMENT
CCDC-1986148 ([Pd(L1-κ2S,O)2]) and CCDC-1986149 ([Pd
(L2-κ2S,O)2]) contain the supplementary crystallographic
data for this paper. These data can be obtained free of
charge via https://www.ccdc.cam.ac.uk/structures/, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting
The Cambridge Crystallographic Data Centre, 12 Union
Road, Cambridge CB2 1EZ, UK; Fax: +44(0)1223-336033.
Samples of the compounds are available from the author.
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How to cite this article: U. Solmaz, I. Gumus, M.
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