DFT studies of the full mechanistic Suzuki–Miyaura reaction: synthesis, structural analysis and cytotoxicity of P,C-chelated palladium(II) and platinum(II) complexes

Article abstract of DOI:10.1080/00958972.2020.1836624

The new Pd/Pt(II) complexes, [MBr₂(Ph₂PCH₂PPh₂C(H)C(O)C₆H₄-m-Br)] (M = Pd (1); M = Pt (2)), with unsymmetrical phosphorus ylide [Ph₂PCH₂PPh₂=C(H)C(O)C₆H₄Br] (Y) have been synthesized through a simple procedure and were screened in a search for novel antitumor agents. These compounds were characterized by FT-IR, NMR (¹H, ¹³C and ³¹P) spectroscopic methods and density functional theory methods. The crystal structures of 1 and 2 were elucidated by single-crystal X-ray diffraction. The results indicated that the complexes were P,C-chelated. Also, a theoretical study on the structure of the complexes has been investigated at the BP86/def2-SVP level of theory. The nature of metal-ligand bonds in the complexes was analyzed using EDA and ETS-NOCV analyses. In particular, a mechanistic pathway of the Suzuki–Miyaura reaction catalyzed by Pd(II) complex 1 with a P,C-donor ligand has been investigated by DFT studies, which the computational results were in agreement with those of experiments. Moreover, the cytotoxic effects of the compounds were studied in four human carcinoma cell lines: PC3, A2780, MCF-7 and PC-12 by MTT assay. Compound 1 proved to be an outstanding potent cytotoxic agent against PC3 cell line and can be considered as a promising lead in cancer drug discovery and development.

Full text of DOI:10.1080/00958972.2020.1836624

Journal of Coordination Chemistry
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DFT studies of the full mechanistic Suzuki–Miyaura
reaction: synthesis, structural analysis and
cytotoxicity of P,C-chelated palladium(II) and
platinum(II) complexes
Khadijeh Badpa , Seyyed Javad Sabounchei , Leila Hosseinzadeh & Robert W.
Gable
To cite this article: Khadijeh Badpa , Seyyed Javad Sabounchei , Leila Hosseinzadeh & Robert
W. Gable (2020): DFT studies of the full mechanistic Suzuki–Miyaura reaction: synthesis, structural
analysis and cytotoxicity of P,C-chelated palladium(II) and platinum(II) complexes, Journal of
Coordination Chemistry, DOI: 10.1080/00958972.2020.1836624
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JOURNAL OF COORDINATION CHEMISTRY
https://doi.org/10.1080/00958972.2020.1836624
DFT studies of the full mechanistic Suzuki–Miyaura
reaction: synthesis, structural analysis
and cytotoxicity of P,C-chelated palladium(II)
and platinum(II) complexes
a
a
b
Khadijeh Badpa , Seyyed Javad Sabounchei , Leila Hosseinzadeh and
c
Robert W. Gable
a
b
Faculty of Chemistry, Bu-Ali Sina University, Hamedan, Iran; Pharmaceutical Sciences Research
c
Center, Faculty of Pharmacy, Kermanshah University of Medical Sciences, Kermanshah, Iran; School
of Chemistry, University of Melbourne, Victoria, Australia
ABSTRACT
ARTICLE HISTORY
The new Pd/Pt(II) complexes, [MBr (Ph PCH PPh C(H)C(O)C H -m-
Received 16 April 2020
Accepted 18 September 2020
2
2
2
2
6 4
Br)] (M ¼Pd (1); M ¼Pt (2)), with unsymmetrical phosphorus ylide
[
Ph PCH PPh Br] (Y) have been synthesized
2
2
2
¼C(H)C(O)C
6 4
H
KEYWORDS
Pd/Pt-cyclic complexes; X-
ray; DFT; catalyst; cytotoxic
through a simple procedure and were screened in a search for
novel antitumor agents. These compounds were characterized by
1
13
31
FT-IR, NMR ( H, C and P) spectroscopic methods and density
functional theory methods. The crystal structures of 1 and 2 were
elucidated by single-crystal X-ray diffraction. The results indicated
that the complexes were P,C-chelated. Also, a theoretical study on
the structure of the complexes has been investigated at the
BP86/def2-SVP level of theory. The nature of metal-ligand bonds
in the complexes was analyzed using EDA and ETS-NOCV analy-
ses. In particular, a mechanistic pathway of the Suzuki–Miyaura
reaction catalyzed by Pd(II) complex 1 with a P,C-donor ligand
has been investigated by DFT studies, which the computational
results were in agreement with those of experiments. Moreover,
the cytotoxic effects of the compounds were studied in four
human carcinoma cell lines: PC3, A2780, MCF-7 and PC-12 by
MTT assay. Compound 1 proved to be an outstanding potent
cytotoxic agent against PC3 cell line and can be considered as a
promising lead in cancer drug discovery and development.
CONTACT Seyyed Javad Sabounchei
jsabounchei@Yahoo.co.uk
Faculty of Chemistry, Bu-Ali Sina University,
Hamedan, 65174, Iran
Supplemental data for this article is available online at https://doi.org/10.1080/00958972.2020.1836624.
ß 2020 Informa UK Limited, trading as Taylor & Francis Group

2
K. BADPA ET AL.
1. Introduction
Unsymmetrical a-keto stabilized ylides derived from diphosphines have shown more
useful applications in organometallic chemistry due to their ambidentate character as
ligands [1–5]. Also juxtaposition of the keto group and carbanion in the phosphorus
ylides allows for the resonance delocalization of the ylidic electron density providing
additional stabilization to the ylide species [6–9]. The coordination behavior of these
compounds towards several metal ions has been studied and it was found that phos-
phorus ylides can coordinate to metal ions in different modes [10, 11]. Although many
bonding modes are possible for a-keto ylides [12], coordination through carbon is
more predominant. The bidentate (or chelating) coordination mode occurs when a
metal ion (for example, Pd(II) or Cu(I)) interacts with two P and C atoms of the ylidic
group to give the P,C-chelated complexes [13, 14]. Hg(II) or Ag(I) interacts only with
the free P atom of the diphosphine group [15, 16], whereas O-coordination dominates
when the metals involved are hard, e.g. Ti(IV), Zr(IV) and Hf(IV) [17].
Phosphorus compounds such as phosphorus ylides and their transition metal com-
plexes, notably Pd(II) and Pt(II) cyclometallated complexes, have been known for 3
decades, but recently have attracted much more attention, especially in the synthesis
of these products with biological, pharmacological and catalytic activities [18–20].
Nevertheless, cisplatin as a metal-based anticancer drug has raised interest to develop
new series generation of platinum-based anticancer agents [21–23]. The significant
similarity between the coordination chemistry of palladium(II) and platinum(II) com-
pounds has advocated studies of Pd(II) complexes as antitumor drugs [24]. Researches
2
þ
2þ
have shown that the complex compounds of both Pd and Pt ions are effective in
cell apoptosis, however, the platinum(II) compounds have shown more activity [25,
26]. The ligand exchange kinetics is a key factor that might explain why platinum
compounds are the most useful.

JOURNAL OF COORDINATION CHEMISTRY
3
A wide variety of palladium-catalyzed carbon ꢀcarbon bond forming methodologies
are available through reactions catalyzed by Pd(0) compounds believed to have particular
significance in the synthesis of biaryl derivatives [27]. The Suzuki-Miyaura cross-coupling
(
SMC) reaction is arguably the most important and widely used method for the construc-
2
2
tion of sp -sp carbon-carbon bonds [28]. The use of cyclometallated Pd(II) complexes as
precatalysts has led in part to some important advances. Mononuclear palladium(II) com-
plexes of P,C-donor ligands of the phosphorus ylides were successfully used as catalysts
and show their efficiency in Suzuki among other cross-coupling reactions [29].
Herein, we describe Pt(II) and Pd(II) complexes incorporating bidentate ligands hav-
ing both phosphorus and carbon donor atoms and exhibiting intriguing functionalities
including in vitro activity. Experimental and theoretical studies of the reactivity and
coordination chemistry of carbonyl stabilized phosphorus ylides are an important
research field of our group [30, 31]. In this report to prove the molecular structures of
1
and 2, DFT theoretical studies by using EDA and ETS-NOCV analysis have been per-
formed to elucidate the physical and chemical nature of interactions. Also a computa-
tional study with the Becke3LYP DFT functional is carried out on the cross-coupling
reaction of aryl bromides and phenylboronic acid catalyzed by [PdBr (Ph2
2
PCH PPh C(H)C(O)C H -m-Br)] (1) in the presence of an excess of base.
2
2
6 4
2. Experimental
2.1. Materials and apparatus
All synthetic reactions were carried out under dry nitrogen using standard Schlenk tech-
0
niques. 2-Bromo-3 -bromoacetophenone and dppm were purchased from commercial
sources and used without further purification. [MBr (cod)] (M ¼Pd or Pt) complexes were
2
prepared according to the published procedures [32]. Phosphorus ylide Y was synthe-
sized and characterized previously [33, 34]. Toluene, n-hexane and chloroform were used
1
13
as reagent grade and dried over Na/Benzophenone and CaCl subsequently. The H,
C
2
3
1
and P NMR spectra were recorded on 250 MHz Bruker and 90 MHz Jeol spectrometers
with CDCl and DMSO as solvents at 25 C. IR spectra were recorded with KBr pellets
ꢁ
3
ꢀ
1
using a Shimadzu 435-U04 spectrophotometer from 4000–400 cm
.
2.2. Synthesis of Pd/Pt complexes
2
.2.1. Synthesis of [PdBr (Ph PCH PPh C(H)C(O)C H -m-Br)] (1)
2
2
2
2
6 4
To a dichloromethane solution of [PdBr (cod)] (0.5 mmol, 5 mL) while stirring, a solution
2
of ylide (0.5 mmol) (5 mL, CH Cl ) was added. The resulting solution was stirred for 2 h at
2
2
room temperature and then concentrated to a ca. 2 mL under reduced pressure and
treated with n-hexane (5 mL) to obtain the title compound as a yellow solid of the Pd
complex of desired diphosphine ylide. X-ray quality crystals were obtained by vapor-dif-
fusion of diethyl ether into a concentrated solution of 1 in dichloromethane. Yield: 0.4 g
ꢁ
ꢀ1
1
(
94%), M.p. 205 C. IR (KBr disk), m(cm ): 1620 (C ¼O). H NMR (CDCl ), d (ppm): 4.64 (br,
3
31
CH , 2H); 6.15 (br, 1H, PCH); 7.19–8.47 (m, 24H, Ph). P NMR (CDCl ), d (ppm): 28.16 (d,
2
3
1
2
13
PPh , J ¼37.37 Hz); 38.53 (d, PCH, J ¼38.38 Hz). C NMR (CDCl ), d (ppm): 30.34
2
P-P
P-P
3
(
br, CH ); 34.44 (br, PCH); 121–140.14 (m, Ph); 195.49 (s, CO).
2

4
K. BADPA ET AL.
2
.2.2. Synthesis of [PtBr (Ph PCH PPh C(H)C(O)C H -m-Br)] (2)
2
2
2
2
6 4
A solution of [Ph PCH PPh ¼C(H)C(O)C H -m-Br] (Y) (290 mg, 0.5 mmol) in CH Cl
2
2
2
6
4
2
2
(
5 mL) was added dropwise to a [PtBr (cod)] (231 mg, 0.5 mmol) dichloromethane solu-
2
tion while stirring. The reaction was stirred for an additional 2 h and then concen-
trated in vacuo, resulting in the precipitation of the product as an off-white crystalline
solid. X-ray quality crystals were obtained by vapor-diffusion of diethyl ether into a
ꢁ
concentrated solution of 2 in dichloromethane. Yield: 0.4 g (86%), M.p. 210 C. IR (KBr
ꢀ
1
1
disk), m(cm ): 1625 (C ¼O). H NMR (CDCl ), d (ppm): 4.59 (br, 2H, CH ); 6.11 (br, H,
3
2
31
1
PCH); 7.17–8.29 (m, 24H, Ph). P NMR (CDCl ), d (ppm): 6.64 (td, PPh ,
J
Pt-P
¼
3
2
2
13
3
860.22 Hz); 42.97 (d, PCH, J ¼40.41 Hz). C NMR (CDCl , ppm), d (ppm): 26.96 (s,
P-P
3
CH ); 29.40 (d, PCH); 121.72–139.75 (m, Ph); 195.60 (s, CO).
2
2.3. Crystallography
Single-crystals of 1 (C H Br OP Pd) and 2 (C H Br OP Pt) were crystallized by
33
27
3
2
33 27
3
2
vapor-diffusion of diethyl ether into a concentrated solution of dichloromethane. Data
collection for 1 and 2 were carried out on a Rigaku Oxford Diffraction SuperNova,
Dual, Cu at zero, Atlas diffractometer using mirror monochromated Cu Ka radiation
(k ¼1.54184 Å); the crystals were kept at 130 K during data collection. Data collection
and reduction was carried out using CrysalisPro, with the data for both structures
being corrected for absorption using Gaussian integration. Using Olex2 [35], the struc-
tures were solved with the ShelXT [36] structure solution program using Intrinsic
Phasing and refined with the ShelXL [37] refinement package using least-squares mini-
mization. All non-hydrogen atoms were refined with anisotropic displacement parame-
ters. Hydrogens were included, constrained to geometrical estimates and refined using
the riding model. For 2, the 2-(3-bromophenyl)-2-oxoethyl group of one of the two
crystallographically independent molecules was disordered over two positions; the
final occupancy factors were 0.831(5) and 0.169(5).
CCDC 1818847 (for 1) and 1818849 (for 2) contain the supplementary crystallo-
graphic data. Crystal data and structure refinement details for the complexes are sum-
marized in Table 1.
2.4. Computational studies
Recently, we investigated the structures and nature of metal-ligand bond in complexes
of five-membered rings P,C-chelated of Pd(II) and Pt(II) by a density functional theory
(DFT) [38]. For the purpose of geometry optimization of pallada- and platinacycle com-
plexes, the calculations were performed using the Gaussian 09 program at the BP86/
SVP level of theory [39]. For bonding analyses, the terms of energy decomposition
analysis (EDA) were carried out at BP86 [40, 41] / TZ2P(ZORA)//BP86/def2-SVP [42]
with C1 symmetry. It has been shown that in former studies the BP86 is a suitable
level for calculation of bonding situation between the M L in such as these com-
plexes [43–45]. The basis sets for all elements have triple-f quality augmented by one
set of polarization functions (ADF basis set TZ2P (ZORA)) with the program pack-
age ADF2009.01.

JOURNAL OF COORDINATION CHEMISTRY
5
Table 1. Crystal data and structure refinement for 1 and 2.
1
2
Empirical formula
Formula weight
T [K]
Crystal system
Space group
a [Å]
C
33
H
27Br
3
OP
2
Pd
C
33
H
3 2
27Br OP Pt
847.61
130.00(10)
Monoclinic
936.30
130.01(10)
Triclinic
P2
1
/n
P-1
13.6516(2)
15.6747(2)
15.1812(2)
90
11.1794(2)
16.5381(5)
17.3168(5)
83.876(2)
83.427(2)
84.961(2)
b [Å]
c [Å]
ꢁ
a [ ]
ꢁ
b [ ]
107.9424(16)
ꢁ
c [ ]
90
3
V [Å ]
3090.55(8)
4
3153.14(14)
4
Z
q
3
calcg/cm
1.822
1.972
-
1
l [mm ]
F(000)
10.568
1656.0
13.935
1784.0
Radiation
CuKa (k ¼1.54184)
7.624 to 154.188
ꢀ17 ꢃh ꢃ15,
CuKa (k ¼1.54184)
7.092 to 154.162
ꢀ14 ꢃh ꢃ8,
ꢁ
2
h range [ ]
Index ranges
ꢀ
19 ꢃk ꢃ19,
ꢀ20 ꢃk ꢃ20,
ꢀ21 ꢃl ꢃ20
ꢀ
14 ꢃl ꢃ19
6464 [Rint¼0.0358,
Independent reflections
12970 [Rint¼0.0268,
R
sigma¼0.0359]
R
sigma¼0.0360]
Final R indexes [I> ¼2r (I)]
R
R
1
¼0.0300, wR
¼0.0371, wR
1.019
2
¼0.0761
¼0.0803
R
R
1
¼0.0294, wR
¼0.0332, wR
1.075
2
¼0.0750
¼0.0780
Final R indexes [all data]
1
2
1
2
2
Goodness-of-fit on F
Largest diff. peak/hole /e Å
-3
0.79/ꢀ1.02
1.68/ꢀ1.24
2.5. 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay
In this study, PC3 (human prostate cancer cell line), A2780 (human ovarian cancer cell
line), MCF-7 (human breast cancer cell line), and PC-12 (rat pheochromocytoma-derived
cells) were supplied from Pasteur Institute of Iran (Tehran, Iran). These cancer cell lines
were incubated in Dulbecco’s modified Eagle’s medium (DMEM-F12) supplemented with
inactivated fetal bovine serum (10% v/v) and 1% penicillin/streptomycin (100 U/mL:
ꢁ
1
00 U/mL) for cell growth at 37 C in a humidified incubator containing 5% CO . The cell
2
culture medium was replenished at predetermined time intervals (2–3 days) until the cell
proliferation had reached 70–80% confluence. The cells were seeded in a 96-well culture
3
plate while the number of cells in each plate was 15–20 ꢂ10 cell/well. Then, different
concentrations of each compound were prepared by dissolving in DMSO. At appropriate
time intervals, the medium was replenished with 0.5 mg/mL MTT solution and plates
ꢁ
were further incubated for 3 h at 37 C. After the incubation period, and to solubilize the
formazan crystals, 100 mL DMSO was added to each well. The optical density at 570 nm
(
(
OD570) was detected (reference wavelength 630 nm) using an Eliza micro plate reader
BioTek Instruments, USA) and cell viability percentage was determined. The IC50 value
was obtained, corresponding to the concentration required for killing 50% of cells. All
the MTT assays were conducted in triplicate.
2.6. Typical procedure for the Suzuki reaction
To a round-bottom flask equipped with a magnetic stirring bar, a mixture of Pd cata-
lyst (0.005 mmol), phenylboronic acid (0.75 mmol), aryl halide (0.5 mmol) and K CO3
2

6
K. BADPA ET AL.
a
Table 2. Selected experimental results.
ꢁ
b
Cat. loading [mol%]
Aryl bromides
Arylboronic acid
Temp. ( C)
Yield [%]
0
0
0
0
0
0
0
0
a
.005
.005
.005
.005
.0005
.0005
.0005
.0005
3a
3a
3b
3b
3a
3a
3b
3b
4a
4b
4a
4b
4a
4b
4a
4b
90
130
110
85
85
94
73
70
54
69
53
45
85
130
110
90
Reaction conditions for the Suzuki-Miyaura coupling reaction: aryl bromides (0.5 mmol), aryl boronic acid
ꢁ
b(0.75 mmol), K
2
CO
3
(1 mmol), DMF (2 mL), catalyst (0.005 mol%), 130 C, 2 h in the air.
Yield of isolated product (5).
(1 mmol) in DMF (2 mL) were added. The mixture was heated at different temperatures
for completing the reaction for 2 h in the presence of air, and the progress was moni-
tored by thin-layer chromatography (TLC). After completing the cross-coupling reac-
tion, the mixture was diluted with n-hexane (15 mL) and water (15 mL). The organic
layer was then dried over CaCl and evaporated under reduced pressure to give a
2
1
13
crude product which was analyzed by H and C NMR spectroscopy. A number of
experimental results are available for these palladium complexes derived from bulky
electron-rich diphosphine ligands (Figures S9–S16). The best results were found to be
catalyst loading, 0.005 (mol%) of the palladacycle complex 1 as an efficient catalyst for
the reactions described above (Table 2).
Aryl bromides (3a) and phenyl boronic acid (4b) were selected as substrates for the
computational study to allow comparison with the experimental data from our investi-
gations (see Supporting Information, data for A to D).
3. Results and discussion
3.1. Synthesis
Reaction of Y with [MBr (cod)] (M ¼Pd/Pt) in equimolar ratio yielded the new P,C-che-
2
lated pallada- and platinacycle [MBr (Ph P(CH ) PPh C(H)C(O)C H Br)] (M ¼Pd (1);
2
2
2 n
2
6 4
M ¼Pt (2)) without any evidence of by-product formation (Scheme 1). All complexes
are moderately soluble in dichloromethane and insoluble in non-polar solvents, such
as n-hexane.

JOURNAL OF COORDINATION CHEMISTRY
7
Scheme 1. Total illustration of the synthesis procedure of metallacycle complexes 1 and 2.
Table 3. Spectroscopic data for Y, 1 and 2.
ꢀ
1
1
13
31
Compound
IR (cm );
v(CO)
H NMR (ppm)
d(PCH)
C NMR (ppm);
P NMR (ppm);
d(CO)
2
d(PCH) and (PPh )
Y
1
2
1572
1620
1625
4.31
6.15
6.11
183.07
195.49
195.60
14.53, ꢀ26.82
38.53, 28.16
42.97, 6.64
3.2. Spectroscopy
1
13
31
The structure of products was characterized by H, C and P NMR spectroscopies
and other conventional techniques such as X-ray diffraction and Fourier transform
infrared (FT-IR). Table 3 shows the brief summary of these collected data sets.
3.3. Infra-red spectra
As noted in the literature [11], coordination of the phosphorus ylide through the car-
bon (chelating mode) causes a significant increase in the v(CO) frequency. The IR spec-
ꢀ
1
tra of 1 and 2 both show a sharp v(CO) band at 1620 and 1625 cm , respectively.
The absorption bands are shifted to higher-energy from the v(CO) stretching bands of
ꢀ
1
ꢀ1
their precursors Y at 1572 cm . Presence of v(CO) bands around 1620–1625 cm in
the IR spectra of these complexes indicates the products (P,C-chelated complexes)

8
K. BADPA ET AL.
3
1
Figure 1. P NMR spectra of 1 and 2.
were formed, and there are not any significant amounts of P,P-coordinated complexes
as side products.
3.4. NMR spectral data
3
1
The P chemical shift values for 1 and 2 appear to be shifted downfield with respect
31
1
to the parent ylide, indicating that coordination of the ylide occurred. The PfHg
NMR spectrum of 1 shows two doublet peaks at d ¼28.16 and 38.53 ppm that are
assigned to PPh (P ) and PCH (P ), respectively, which was similarly reported for
2
a
b
other palladium complexes [38] (Figure 1). Formation of the platinacycle 2 is apparent
3
1
by distinctive P signal splittings for two phosphorus atoms; a virtual triplet for PPh
2
Pa) resonance at 6.64 ppm with a large one-bond 1 Pt– P coupling constant ( J
95
31
1
(
Pt–P
¼
3860.22 Hz) and a doublet for PCH (P ) resonance at 42.97 ppm (Figure 1) [38]. The
b
larger downfield shifts for PPh and PCH phosphorus atoms demonstrate the P,C-coor-
2
dinated mode of phosphorus ylide in 1 and 2.
1
The H NMR spectra of 1 and 2 exhibit characteristic shifts in the methinic proton
signals. This is interesting because the complexation of the ylide (Y) to Pd/Pt through
1
PPh group could not change the chemical shift values of H NMR significantly, while
2
coordination through carbanion causes the shift of PCH peaks to higher frequency
1
around 6.11–6.15 ppm. The H NMR chemical shift values for these complexes appear
to be shifted downfield with respect to the parent ylide, indicating also that the
coordination of the ylide through P and Ca atoms has occurred.
1
3
1
The C fHgNMR spectra of 1 and 2 show downfield shift of CO, PCH and PCH
2
groups in respect to the parent ylide Y. These observations also confirm that coordin-
ation of ylide Y has occurred through P and Ca atoms.
3.5. Crystallography
The molecular structures of [C H Br OP Pd] (1) and [C H Br OP Pt] (2) were con-
33
27
3
2
33 27
3
2
firmed by X-ray crystal structure determination as shown in Figures 2 and 3, respect-
ively. Selected bond lengths and angles are given in Table 4. The Pd atom in 1 and
the Pt atom in 2 are both in a square planar environment, coordinated by one

JOURNAL OF COORDINATION CHEMISTRY
9
Figure 2. ORTEP view of X-ray crystal structure 1.
Figure 3. ORTEP view of X-ray crystal structure 2.
phosphorous and one carbon atom, from the chelating ylide, and by two cis-coordi-
nating bromides.
The structure of platinum complex 2 contains two crystallographically independent
molecules, while the asymmetric unit of palladium complex 1 comprises a single mol-
ecule. All three molecules have similar structural features: the metal atom is in a tetra-
hedrally-distorted square planar environment with two cis-coordinating Br atoms, one
phosphorous and an ylide carbon. For one molecule of 2, the rms deviation of the
four coordinating atoms is 0.1503 Å, with Pt1 lying 0.0464(11) Å out of the plane,
while, for the second molecule of 2, the rms deviation of the four coordinating atoms
is 0.0609 Å, with Pt2 lying 0.0088(11) Å out of the plane; for 2-2 the rms deviation of

1
0
K. BADPA ET AL.
ꢁ
Table 4. Selected bond lengths [Å] and angles [ ] for 1 and 2.
Compound 2
Compound 1
Mol 2-1
Mol 2-2
Bond distances
Pd
Pd
Pd
Pd
1
1
1
1
-Br
-Br
1
2.4703(4)
2.4876(4)
2.2447(7)
2.108(3)
1.220(4)
1.498(4)
1.774(3)
1.843(3)
Pt
Pt
Pt
Pt
O
C
1
-Br
-Br
1
2
2.4651(4)
2.4937(4)
Pt
Pt
2
-Br
-Br
-P
2 3
-C34
-C35
4
5
2.4694(4)
2.4829(4)
2.2004(10)
2.104(4)
1.228(5)
1.509(10)
1.804(4)
2
1
2
-P
-C
1
1
-P
-C
1
2.2031(10)
2.111(4)
1.226(5)
1.512(5)
1.800(4)
1.808(4)
Pt
Pt
1
1
1
2
O
1
-C
-C
-C
2
1
-C
-C
2
O
2
C
1
2
1
2
C
34-C35
P
P
2
1
P
2
-C21
-C28
P
4
-C54
-C61
1
-C21
P
1
P
3
1.808(4)
Bond angles
Br -Pd -Br
-P
-Br
2
1
1
93.414(12)
90.89(7)
86.64(7)
89.15(2)
174.68(2)
Br
C
2
-Pt
-Pt
1
-Br
-P
1
90.149(15)
90.77(11)
174.71(10)
89.18(3)
Br
34-Pt
34-Pt
4
-Pt
2
5
91.760(15)
91.43(11)
176.82(11)
86.89(3)
C
C
1
1
-Pd
-Pd
1
1
1
1
1
1
C
C
2
3
2
C
P
P
1
-Pt
1
-Pt
1
-Pt
1
-Br
-Br
-Br
1
2
4
P
P
1
-Pd
-Pd
1
-Br
-Br
1
2
1
1
1
2
3
P -Pt
-Pt
2
2
1
1
170.28(3)
P
3
176.29(3)
the four coordinating atoms is 0.0580 Å, with Pd1 lying 0.0384(8) Å out of the plane.
For all three molecules, the M-Br distance trans to the ylide carbon is shorter than the
M-Br distance trans to the coordinating phosphorous atom. The main difference
between the molecules lies in the conformation of the 5-membered metallacyclic rings
and in the orientation of the bromo-phenyl ring. For both molecules of 2, the five-
membered platinacycle ring is in an envelope conformation with the ylide carbon
atoms, C1 and C34, lying significantly out of the plane of the other four atoms, the dif-
ferences between the molecules being that, for molecule 2-1, the carbonyl oxygen
ꢁ
and the bromine atom are in a trans arrangement (O1-C2 … C5-Br3 178.5(4) ), while
for molecule 2-2 the carbonyl oxygen and the bromine atom are in a cis arrangement
ꢁ
(
O2-C35 … C38-Br6-21(1) ). The five-membered palladacycle ring in 1 is also in an
envelope conformation; however, the atom that lies significantly out of the plane of
the other four atoms is the phosphorous atom that is not coordinated to the palla-
dium atom, P2. This leads to differences in the orientation of all phenyl rings attached
to P2, as well as in the orientation of the bromo-phenyl ring, the torsion angle O1-
ꢁ
C2 … C5-Br3 being 136.3(3) . This latter conformation of the 5-membered metallacyclic
ring is displayed by the second molecule, as well as in the structures of (2-(biphenyl-4-
yl)-1-(((diphenylphosphino)methyl)-(diphenyl)phosphonio)-2-oxoethyl)-(dichloro)-palla-
dium [34] and dichloro(3-methoxybenzoylmethylenediphenyldiphenylphosphino-meth-
ylphosphorane)palladium [38]. However, in this latter case, the 5-membered
metallocyclic rings adopt half-chair conformations. In both structures, the molecules
are linked by weak C-H … Br hydrogen bonds which lie along the c-axis for 2-2 and
the b-axis for 2-1.
3.6. Theoretical studies
3.6.1. Energy decomposition analysis (EDA) of 1 and 2
A computational study on structures and nature of metal-ligand (Y!M) bonds in
[
LMBr ] (M ¼Pd (1), Pt (2), L¼Y) complexes has been reported at the BP86 level of
2

JOURNAL OF COORDINATION CHEMISTRY
11
Figure 4. Optimized structures of [YMBr ] (M ¼Pd, Pt) complexes at the BP86/SVP level of theory.
2
theory using def2-SVP basis set. The crystal structures of these complexes were deter-
mined by single-crystal X-ray crystallography (Figures 2 and 3). The X-ray crystal struc-
tures obtained in this work and previous experimental data [38] show that these
complexes formed monomeric P,C-chelated complexes with five-membered rings with
a square planar geometry around the metal center. The observed geometry of [Pd(II)
and Pt(II)] was used as a basis for the DFT calculations of [YMBr ] (M ¼Pd, Pt) com-
2
plexes at the mentioned level of theory.
The optimized structures of the [YMBr ] (M ¼Pd, Pt) complexes at BP86/Def2-SVP
2
are shown in Figure 4 and the trends for the variation of the corresponding bond
lengths and angles and their comparison with experimental data are given in Figure 5.
The agreement between the theoretical and experimental bond lengths and angles of
the P,C-chelated palladium(II) and platinum(II) complexes is quite good (Figure 5).
For studying the nature of metal-ligand bond and/or possible interactions in
[
YPdBr ] and [YPtBr ] complexes, the NBO analysis was performed [46]. The values of
2 2
the Wiberg (WBI) and partial charges on M, P and C (ylide) atoms involved in the
bonding interactions between MBr and Y fragments in these complexes as well as
2
the total charge amounts of the MBr2 fragment (M ¼Pd, Pt) were also evaluated
through natural population analysis (Table 5). The result showed that the calculated
partial charge (q) on the P atom of the P!M bond is positive, but on both the M (Pd,
Pt) and C atoms in the M ꢀC bonds are negative. Also, the negative value of q(C) is
greater than that for q(M) (Table 5). The Wiberg bond indices (WBIs) for C!M and
P!M bonds of 1 and 2 are also compared in Table 5. As can be seen, the value of
WBI of P!M bond in the complexes is larger than C!M bond. Also with the same L
ligand and changing the M atoms from Pd to Pt, the values of WBIs of C!M and
P!M bonds in the complexes are increased.
Morokuma [47] and Ziegler [48] developed EDA in the 1970s. With the help of this
technique, a quantitative computational pattern for the explanation of the strength of

1
2
K. BADPA ET AL.
ꢁ
Figure 5. Bond length [Å] and angles [ ] of metal–ligand in Pd and Pt complexes at the BP86/SVP
level of theory and experimental data.
Table 5. Wiberg bond indices (WBI) of C!M and P!M bonds and natural charges of C, P, M
atoms and MBr fragment in the complexes investigated here.
2
Complexes
YPdBr
YPtBr
WBI C-M
WIB P-M
M
C(ylide)
P
MBr
2
2
0.376
0.47
0.55
0.69
ꢀ0.075
ꢀ0.242
ꢀ0.83
ꢀ0.82
1.04
1.11
ꢀ0.736
ꢀ0.83
2
Table 6. EDA [Kcal/mol] analysis (BP86-D3/TZ2P(ZORA)//BP86/def2-SVP) of [YMBr ] (M ¼Pd, Pt)
2
complexes with the C1 symmetry.
Complex
DE(int)
DE(pauli)
DE(elec)
DE(orb)
DE(disp)
YPdBr
2
ꢀ115.05
ꢀ148.70
265.57
343.15
ꢀ219.64
ꢀ286.07
ꢀ140.70
ꢀ180.69
ꢀ20.28
ꢀ25.09
YPtBr
2
M
L r-donation, and M!L back-bonding in the main group and transition metal
complexes with different types of ligands is in hand [49–51]. For better analysis of the
nature of the M–C bonding between the Y and [MBr ] fragments in [YPdBr ] and
2
2
[
YPtBr ] complexes, the quantum chemical calculations in the terms of energy-decom-
2
position analysis (EDA) at the BP86D3/TZ2P(ZORA)//BP86/def2-SVP with C1 symmetry
by using the program package ADF ADF2009.01 were carried out. The results of the
EDA analyses for aforementioned complexes are given in Table 6. The EDA decom-
poses the interaction energy between two considered fragments in one system into
four terms which can be interpreted in a chemically meaningful way; the quasi-clas-
sical electrostatic interaction between the frozen charges of the fragments (DEelec), the

JOURNAL OF COORDINATION CHEMISTRY
13
exchange (Pauli) repulsion between electrons possessing the same spin (DEPauli), the
orbital interaction term (DEorb) and the dispersion contributions (DEdisp). In the EDA,
the bonding formation of interacting fragments would result from two main compo-
nents as follows:
DEin ¼DEelec þDEpauli þDEorb þDEdisp
The results of EDA confirm that the main portions of DE , about 57–58%, in the
int
complexes are allocated to DEelec (Table 6).
Also, the visualization of natural orbitals for chemical valence NOCV pairs (Dq)
between the donor orbitals of phosphorus ylide (Y) and the acceptor orbitals of the
MBr fragment are shown in Figure 6. As can be seen, the dominant term of DE for
2
orb
[
YPdBr ] and [YPtBr ] complexes arise from Dq1 and Dq2, showing r donation from
2 2
C(ylide) and P atoms of Y fragments to [Pd/PtBr ] in [YPdBr ] and [YPtBr ] complexes.
2
2
2
Figure 6 also displays the deformation density Dq3, Dq4 and Dq5 which show the p
back-donation from the [Pd/PtBr ] fragment to Y ligand in latter complexes. In the
2
above theoretical studies, the effect of R group (R ¼Br) substituted on aromatic part
of ylide (Y) in [YPdBr ] and [YPtBr ] complexes is negligible.
2
2
3.6.2. Study of the full catalytic cycle of the Suzuki-Miyaura cross-coupling
This model is defined by 4-bromobenzaldehyde, 3a, phenylboronic acid, 4b, as reac-
tants and [PdBr (Ph PCH PPh C(H)C(O)C H -m-Br)], 1, as catalyst. All calculations were
2
2
2
2
6 4
performed with the Gaussian09 program at the B3LYP/SVP level of theory. All geome-
tries were fully optimized without any constraints and reported in the ESI. Solvent
effects were introduced in selected cases through PCM single-point calculations [52]
on gas phase optimized geometries. In order to consider the solvent effects, based on
the Experimental section, the solvation energies (Gsolv) were calculated with the SMD
model [53] using the resulting data for N,N-dimethylformamide (DMF) as solvent, for
each segment of the catalytic cycles, that there are; oxidative addition (i), transmetalla-
tion (ii), and reductive elimination (iii) (Scheme 2).
3.7. Prereaction complex
One of the reasons behind the rapid ascendance of Pd is its tendency to undergo a
clean “two electron”-based reduction of Pd(II) complexes to Pd(0). This reduction is
the key step in the activation of many versatile precatalysts, but also a fundamental
part of the mechanism of cross-coupling or C-H activation processes. The primary
reduction of Pd(II) to Pd(0) is most likely accomplished by phosphine in the phos-
phine-assisted catalytic cycles [54]. The reduction is assisted by hard nucleophiles, of
which the most common are hydroxide [55] and alkoxide ions [56], water [57], and
water and acetate ion [58], though in special cases even fluoride in the presence of
water can play a role [59].
Aryl halides, which can act as electron-accepting ligands, could form stable com-
plexes with the electron-rich, unsaturated [Pd(P-C)] fragment and were thus investi-
gated as potential prereaction complexes in Ar-X activation. By employing the
phosphine-based ligand, aryl halides and base as reducing agents, the active Pd(0)
species were generated [54, 60] in an exothermic reaction (Scheme 2).

1
4
K. BADPA ET AL.
Figure 6. Deformation densities (Dq) associated with the most important orbital interactions in
[
YPdBr ] and [YPtBr ] complexes.
2 2
3.8. Oxidative addition
The classic catalytic cycle for a cross-coupling reaction starts from the coordinatively
10
unsaturated palladium(0)-diphosphine, d -complex 1, which has a square planar
geometry, with the [Ph PCH PPh ¼C(H)C(O)C H Br] ligand in an eclipsed arrangement
2
2
2
6 4
(Figure 7). In the oxidative addition step, the bond between the organic group and
the halide atom breaks, and two new bonds are formed with palladium, which in this
way increases its oxidation state from 0 to þ2. The transition state for this process,
shown in Figure 7, is labeled TSOA. The criteria for the labeling, which will be fol-
lowed for all intermediate and transition states throughout the article, are TS for

JOURNAL OF COORDINATION CHEMISTRY
15
Scheme 2. The proposed mechanism for Pd-catalyzed cross-coupling reactions.
Figure 7. Energy profile for the oxidative addition step in the dissociative mechanism.

1
6
K. BADPA ET AL.
Scheme 3. Energy profile for the transmetalation step in the dissociative mechanism.
transition state, O for oxidative, A for addition and IT for intermediate. Before reaching
TSAO, the initial approach between the reactants produces intermediate 6, has a
2
square-planar coordination where the alkene is coordinated g to the metal center,
with Pd-C distances of 2.00 and 2.04 Å more distant from Pd than the other one (P(1)
2.244 Å, Pd(1)-C(1) 2.108 Å).
3.9. Transmetallation
The next step in the Suzuki-Miyaura cross-coupling is the transmetallation (Scheme 3),
where the base reacts initially with the organoboronic acid to afford the correspond-
ing organoborate anion, which then reacts with the palladium complex and organic
group originally attached to the boron atom replaces the bromide in the palladium
coordination sphere. The reaction starts with the formation of complex IT1. After that,
the B center binds to the OH group attached to Pd. This step going through the
TSTA1 transition state (Scheme 3) produces the intermediate oxo-palladium IT2. The
process can be clearly followed by the evolution of the bond distances. The Pd-Br dis-
tance changes from 2.487 Å in IT1 to 2.530 Å in TSTA1; the distance of the breaking
bond Pd–Br and the forming Pd–O are computed as 4.35 Å and 3.92 Å, respectively.
The energies of the next transition state TSTA2 and intermediate IT2 were calculated
as ꢀ32.30 kcal/mol and ꢀ38.15 kcal/mol below the separated reactants, respectively. In
the subsequent intermediate IT2 the bromide leaves the palladium coordination
2
sphere. The second step corresponds to the formation of a g alkene complex TSTA2.
The energy barrier to reach the transition state of this last step (TSTA3) (Scheme 3),
which has the highest energy in the profile, was calculated to be only 28.88 kcal/mol

JOURNAL OF COORDINATION CHEMISTRY
17
Figure 8. Optimized structures of transition state (TSE) for the reductive elimination step in the
dissociative mechanism.
below the separated reactants. The distances of the forming Pd–C and breaking Pd–O
are 2.031 and 4.012 Å, respectively.
3.10. Reductive elimination
In this associative mechanism, the reductive elimination step (E) is initiated by the for-
mation of a bond between the two organic groups attached to the metal, leading to
2
a g p-complex of palladium with R-R΄ (TSE) and decreases the oxidation state of
metal, from þ2 to 0. This step proceeds through release of the R-R΄ molecule from
the YPd(R)(R΄) complex. In our model system, this reductive elimination takes place dir-
ectly from the four-coordinate complex 8. In the corresponding transition state (TSE)
(
1
Figure 8), the distance C-C between the organic groups evolves from 2.750 Å in 8 to
.468 Å in TSE and the Pd-C bonds are 2.039 Å to 2.064 Å in the corresponding transi-
tion state (TSE).
3
.11. In vitro cytotoxicity
The cell viability assay was used to determine the viability of three lines of cancer cells
PC-3, MCF-7, and A2780) and PC-12 cell line as a model of normal cells [61] after
(
exposure of different concentrations of the compounds and investigate whether treat-
ment of cells with these concentrations of the compounds can decrease the viability
of these cancer cells. Figure 9 reveals that 1 can potentially inhibit the proliferation of
PC-3, A2780, and MCF-7 cancer cell lines. Exposure of 1 with concentrations lower
than 5 mg/mL and 25 mg/mL caused a significant cytotoxic effect on PC-3, A2780, and
MCF-7 cancer cells, respectively. On the other hand, the cytotoxic potential of 1 was
not dose-dependent at concentrations higher than 5 mg/mL for PC-3, 25 mg/mL for
A2780 and MCF-7 cells, because the viability percentage of these cancer cell lines
reached plateau at above concentrations. The IC50 values of 1 toward these cells were

1
8
K. BADPA ET AL.
Figure 9. Comparative cytotoxicity profiles of 1 and 2 for (a) MCF-7, (b) PC-12, (c) PC-3, and (d)
A2780 cell lines. The cells were incubated with different concentrations of selected compounds for
24 h. The cell proliferation inhibition was determined by the MTT assay as described in “Materials
and Methods”. Data are presented as mean S.E.M. (n ¼3).
Figure 10. The cytotoxic effect of ligand against cancer cell lines. The cells were incubated with
different concentrations of ligand for 24 h. The cell proliferation inhibition was determined by the
MTT assay as described in “Materials and Methods”. Data are presented as mean S.E.M. (n ¼3).
calculated separately and were 1.75 mg/mL and 12.25 mg/mL, respectively, toward PC-3
and A2780 cell lines. After 24 h exposure the MCF-7 cell line shows a significant drop in
cell viability to 65% at a concentration of 25 mg/mL of 1. Moreover, the cytotoxic effect of
1
was also evaluated on PC-12 cells (Figure 9) as a model of normal cells. The obtained
results showed that this compound was not cytotoxic to PC-12 cells. These results indi-
cated that these compounds are not able to induce cytotoxic effect in the normal cells.
The IC50 values were much higher than those observed in A2780, MCF-7, and PC-3 at the
same incubation period (Figure 9). It must be noted that 2 did not have any cytotoxic
effect on the cancer cell lines. The cytotoxic effect of ligand alone was also evaluated in
the cancer cells. As seen in Figure 10, the ligand had the ability to reduce cell viability.
The IC50 of ligand in PC12, PC3, A2780, and MCF7 cells was, respectively, 9.72, 10.46,

JOURNAL OF COORDINATION CHEMISTRY
19
Table 7. Comparison of cytotoxic effects against PC-3 cell line between 1 and other com-
plexes reported.
Entry
Compound
IC50 (lg/mL)
Ref.
a
1
2
3
4
5
6
7
a
[Pd(Mpy)
[(bipy)Pd(Pcurc)][CF
PdCl (XPy)
(where Py ¼pyridine; X ¼H, CH
Chiral diimine based cyclopalladated complexes
trans-[Pt(2-hmpy) (sac)
]ꢄ3H
[PdBr (Ph PCH PPh C(H)C(O)C
Cisplatin
2
(PPh
3
)
2
]
80.10
22.2
not cytotoxic
51.5
[62]
[63]
[64]
[65]
[66]
3
3
SO ]
2
2
3
or Cl)
2
2
2
O
72.2
1.75
5.18
2
2
2
2
6
H
4
-m-Br)]
This work
This work
Mpy ¼2-mercaptopyridine.
Figure 11. Representative photomicrograph shows morphological changes of the PC-3 cells. Cells
were treated with (a) control, (b) 5 mg/mL and (c) 25 mg/mL then imaged by an inverted phase
contrast microscope.
1
2.65 and 25.2 mg/mL. Moreover, we assayed cytotoxic effect of cisplatin on PC3, the
most sensitive cancer cell line. The result revealed that 1 is more potent against PC3 cells
in comparison with cisplatin (IC50 ¼5.18 mg/mL) (Table 7).
Figure 11 shows the effects of the compounds on the morphology of the most sen-
sitive cell line PC-3, after treatment with 5 lg/mL and 25 mg/mL of 1 while cells treated
with the compounds showed signs of growth inhibition, shrinkage, vacuolization and
moderate cytoplasmic granulations. In our previous study, we evaluated cytotoxicity of
cisplatin, a platinum-based antineoplastic drug, in A2780 cell line [62]. Our results
revealed that cisplatin has a potent anti-proliferative effect against the growth of
ovary human carcinoma cell lines (IC50 ¼2.86 mM).
It seems that 1, [PdBr (Ph PCH PPh C(H)C(O)C H -m-Br)], is favorable to the cyto-
2
2
2
2
6 4
toxic activity against PC-3 cells. Table 7 presents a comparison between the cytotoxic
effects against the PC-3 cell line for 1 and some other Pd/Pt-based complexes
reported [63–67]. The results revealed that 1 exhibits higher cytotoxic activity than
those of palladium and platinum(II) complexes. On the whole, based on the results of
cytotoxic experiments and comparative study, it can be concluded that the metal ions
have important effects on anticancer activity and also primary ligands can exert add-
itional effects on the cytotoxicity.
4. Conclusion
The present study describes the simple and efficient synthesis of two mononuclear
Pd(II) and Pt(II) complexes involving bidentate P,C-donor ligands. The structure of the

2
0
K. BADPA ET AL.
1
13
31
products was characterized by spectroscopic techniques such as H, C, P NMR and
IR spectroscopies, and X-ray analysis. On the basis of the spectroscopic data, we have
proposed that ligands herein exhibit a chelate P-C coordination behavior to the metal
center affording five-membered chelate rings. Also, we used 1 as highly active and
efficient catalysts for promoting the Suzuki reaction of aryl bromides and phenylbor-
onic acid to produce the corresponding products in high yields. Accordingly, density
functional theory calculations have been utilized to clarify a detailed mechanism of
the palladium-catalyzed Suzuki reaction and the nature of metal-ligand bonds in the
complexes. The mechanism for the full catalytic cycle of the Suzuki-Miyaura cross-cou-
pling between benzaldehyde bromides and phenylboronic acid has been analyzed by
Becke3LYP DFT calculations. The nature of metal-ligand bonds in the complexes was
analyzed using EDA, and the extended transition state combined with the natural orbi-
tals for chemical valence (ETS-NOCV). In fact, these results strongly suggest that differ-
ent detailed mechanisms will occur in different experimental systems. In this study we
also demonstrated that 1 is cytotoxic against PC-3, A2780, and MCF-7 cell lines, with-
out any cytotoxic effect on PC12 cells as a normal cell model. According to the per-
formance of these compounds against cancer cells, these compounds had good
stability in solution. Overall, based on the results of cytotoxic experiments, it can be
concluded that the metal ions have an important effect on the anticancer activity and
also, more stability of palladium complexes can exert additional effects on the cytotox-
icity. Also, these compounds remain unchanged for at least 30 days at room tempera-
ture, suggesting their stability.
Supplementary material
31
13
1
Physical measurements and selected P, C and H NMR spectra of some compounds
can be found in the online version. These data can be obtained free of charge via
www.ccdc.cam.ac.uk/data_request/cif or from the Cambridge Crystallographic Data
Center, 12 Union Road, Cambridge CB2 1EZ, UK. Tel.: þ44 0 1223 762911; or
deposit@ccdc.cam.ac.uk.
Acknowledgements
Funding of our research from the Bu-Ali Sina University is gratefully acknowledged.
Disclosure statement
No potential conflict of interest was reported by the author(s).
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