ONO pincer type binuclear Pd(II) complex: Synthesis, crystal structure and catalytic utilization of the resulting organopalladium complex in catalytic copper-free Sonogashira coupling reaction

Article abstract of DOI:10.1016/j.ica.2016.07.032

New binuclear palladium (II) complex featuring ONO pincer type hydrazone ligand was synthesized and characterized by spectroscopic and single-crystal XRD analysis. This complex showed excellent catalytic activity towards the copper-free Sonogashira cross coupling reaction. The main advantages over previous methodologies include low catalyst loading, less problematic reaction media (H₂O–DMSO) and reusability in homogenous systems for optimal performance.

Full text of DOI:10.1016/j.ica.2016.07.032

Accepted Manuscript
ONO pincer type binuclear Pd(II) complex: synthesis, crystal structure and cat-
alytic utilization of the resulting organopalladium complex in catalytic copper-
free Sonogashira coupling reaction
Mojtaba Bagherzadeh, Narges-alsadat Mousavi, Maryam Zare, Sirous Jamali,
Arkady Ellern, L. Keith Woo
PII:
S0020-1693(16)30410-8
http://dx.doi.org/10.1016/j.ica.2016.07.032
ICA 17167
DOI:
Reference:
Inorganica Chimica Acta
To appear in:
Received Date:
Revised Date:
Accepted Date:
2 May 2016
10 July 2016
18 July 2016
Please cite this article as: M. Bagherzadeh, N-a. Mousavi, M. Zare, S. Jamali, A. Ellern, L. Keith Woo, ONO pincer
type binuclear Pd(II) complex: synthesis, crystal structure and catalytic utilization of the resulting organopalladium
complex in catalytic copper-free Sonogashira coupling reaction, Inorganica Chimica Acta (2016), doi: http://
dx.doi.org/10.1016/j.ica.2016.07.032
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ONO pincer type binuclear Pd(II) complex: synthesis, crystal structure and
catalytic utilization of the resulting organopalladium complex in catalytic
copper-free Sonogashira coupling reaction
Mojtaba Bagherzadeha*, Narges-alsadat Mousavia, Maryam Zareb, Sirous Jamalia, Arkady Ellernc
and L. Keith Wooc
a Department of Chemistry, Sharif University of Technology, P.O. Box 11155-3516, Tehran, Iran
bDepartment of Basic Sciences, Golpayegan University of Technology, P.O. Box 8771765651, Golpayegan, Iran.
c1711 Gilman Hall, Iowa State University, Molecular Structure Lab, Ames, IA, 50011, USA
Abstract
New binuclear palladium (II) complex featuring ONO pincer type hydrazone ligand was
synthesized and characterized by spectroscopic and single-crystal XRD analysis. This complex
showed excellent catalytic activity towards the copper-free Sonogashira cross coupling reaction.
The main advantages over previous methodologies include low catalyst loading, less problematic
reaction media (H2O–DMSO) and reusability in homogenous systems for optimal performance.
Introduction
The chemistry of palladium complexes much attention receive because of the catalytic properties
of palladium-based systems in many of industrially crucial reactions [1,2], involving palladium-
catalyzed carbon–carbon and carbon–heteroatom bond formations.
In the case of palladium-catalyzed C–C and C–heteroatom bond forming reactions, palladium
2−
starting materials [like PdCl4 , PdCl2,Pd2(DBA)3] are regularly used as starting reagents for the
synthesis of reactive organo-palladium species that contain a Pd–C bond during catalysis[3].
Synthesis of transition metal pincer type complexes with high stability and catalytic activity has
been a most productive area of homogeneous catalysis and coordination chemistry. There is
* Corresponding author. Tel.: +98 21 66165354; fax: +98 21 66012983.
E-mail address:bagherzadeh@sharif.edu (M. Bagherzadeh)

significant equilibrium between stability and reactivity of the pincer complexes, which can be
controled by exact ligand design and following structure of metal complexes. As a result,
research on the development of pincer ligands bearing CCC[4], CNC[4], PNP[5], SNS[6],
NNN[7], NCN[8] and SCS[9] donor atoms got much attention. However, studies on ONO pincer
ligands and corresponding palladium complexes are limited in the literature[10]. Acid hydrazides
R–CO–NH–NH2, and their corresponding aroylhydrazones, R–CO–NH–N=CH–R′ have a
specific place between organic ligands due to their variety of donor atoms. Of course, they can
have different dentacity depending on reaction condition. Many of them are biologically active
and their activity can be enhanced by coordinating to biometals[11]. According to the nature of
heterocyclic ring substituents attached to the hydrazone unit, these ligands can act as bidentate,
tridentate or tetradentate ligands. And due to their facile keto–enol tautomerization, they can
coordinate with metals. Hydrazides and hydrazones have interesting ligation properties due to
presence of several coordination sites[12].
Some complexes of 5-bromosalicylaldehyde N-benzoylhydrazone with copper[11],
vanadium[12], manganese [13], ruthenium[14], and iron[15] were reported, up to now. But, no
palladium complex of this hydrazine base ligand is reported. Our interest in the area of palladium
catalysis coupling reactions[16–18], centers on the synthesis of new palladium-based systems
that contain a pincer ligand based on a Schiff base platform. These compounds have the ability to
use as precursors for preparation of reactive palladium precatalysts in popular catalytic
transformations. In this manuscript, for the first time, we describe the binuclear palladium
complex of 5-bromosalicylaldehyde N-benzoylhydrazone, bearing one ONO pincer like
palladium center and another Pd-C center, and its catalytic activity towards Sonogashira
coupling reaction.
The Sonogashira cross-coupling reaction between an aryl halide and a terminal alkyne has
become known as one of the most widely used carbon–carbon bond forming reactions in organic
chemistry. The most usual catalytic systems used for this coupling reaction include
[PdCl2(PPh3)2], PdCl2/PPh3, and [Pd(PPh3)4] together with copper(I) salt as the co-catalyst.
However, if the reactions are not done under inert conditions, copper salts can enforce Glaser–
Hay type homocoupling of terminal alkynes to give dienes, which are difficult to separate from
the favorable products[19]. So, development of copper-free systems has received notable

attention[20]. Herein we report our data on the synthesis of these new [Pd2L(SMe2)2(Cl)]
complex, along with its characterization by a combination of spectroscopic methods, and X-ray
diffraction analyses, and the ability of this cyclopalladated specie to serve as precursor in
catalytic Sonogashira cross-coupling is discussed.
Results and discussion
Syntheses and spectroscopic characterizations
The Schiff base condensation reaction of 5-bromosalicylaldehyde (1 g, 5 mmol) in acetic acid
(25 mL) with benzhydrazide (681 mg, 5 mmol) lead to the formation ligand precursor. The
binuclear palladium (II) complexe was synthesized by reacting the ligand precursor and PdCl2
in DMSO as solvent. This ligand precursor and binuclear palladium(II) complex were
characterized by elemental analysis, IR, 1H NMR and UV–Vis spectroscopies
.
Figure 1. 5-Bromosalicylaldehyde N-benzoylhydrazone
The IR spectrum of the free ligand exhibit v(N-H) absorption bands at ca. 3275 and ca. 3210 cm-
1 and v(C=O) at ca. 1670 cm-1, suggesting that the ligand exist in the keto form in the solid state
(figure 1) [21]. The absorption band in ca. 3052 cm-1 is due to the vibration of OH group which
is omitted in IR spectra of the complex indicating the deprotonation of OH followed by
coordination to metal ion. A broad band around 1460 to 1530cm−1 in spectrum of complex may
be attributed to the stretching of C=N functional group of Schiff-base ligand, which can be seen
in 1659 cm-1 in ligand spectrum[21]. The band observed in the complex in the region 530 cm-1 is
attributed to v(Pd-N)[22]. A medium band observed within 1500 cm-1 is assigned to the
conjugated –C=N-N=C- fragment of the tridentate ligand[14].

In 1HNMR spectra of ligand, the Schiff-base proton (–CH=N) signal was observed at 8.58 ppm.
This signal was shifted down field in the spectra of the binuclear complexes (8.62 ppm). The
hydrogens of SMe2 can be seen at 3.82 ppm, but solvent peak obscures the signal arising from
protons of methyl groups attached to S.
The UV-Vis absorption spectra of the ligand precursor and the binuclear Pd(II) complex were
recorded in DMSO solution. The absorption band of the complex is red-shifted relative to the
free ligand. In the UV region (400-200 nm), the free ligand, exhibit absorption bands at ca. 339
(ɛ = 2142 L mol-1 cm-1), 291 (ɛ = 2714 L mol-1 cm-1) and 250 (ɛ = 1440 L mol-1 cm-1) nm which
are assigned to n→σ*, π→π* and n→π* transitions. For complex, a broad band in 415 (ɛ = 7143
L mol-1 cm-1) nm is assigned for MLCT. In the visible region of the square planar palladium
complexes, three spin-allowed d-d transitions were considered. These transitions were expected
to correspond to the transitions 1A1g→1A2g, 1A1g→1B1g and 1A1g→1Eg, but the presence of sulfur
covers these bands, because of its very intense charge transfer band[23]. The spectra of the
complex show intense absorption bands in the range of 278-248 (ɛ > 103 L mol-1 cm-1) which are
attributed to intraligand transitions (Figure 2).

Figure 2. UV-Vis spectra of the precursor ligand and the dimer Pd complex
Crystal Structure of [Pd2L(SMe2)2(Cl)]
A perspective view of binuclear PdII complex [Pd2L(SMe2)2(Cl)] with the atom numbering
scheme is shown in figure 3. Relevant interatomic bond distances and angles are listed in Table
2. The intramolecular Pd(1)···Pd(2) separation is 4.90(11) Å in this molecule. Both palladium
(II) ions are four-coordinated with an NO2S donor set of atoms for Pd(1) and an NCSCl donor
set of atoms for Pd(2). The Pd─O, Pd─N, Pd─C, Pd─S and Pd─Cl bonds are within the normal
ranges and are comparable to those observed in similar compounds[24],[25],[26].

Figure 3 Molecular structure of [Pd2L(SMe2)2(Cl)], showing the atom-labelling system and 50%
thermal ellipsoids.
In the crystal packing of complex [Pd2L(SMe2)2(Cl)], the intermolecular π–π interactions with
centroid to centroid distances 3.480(5)–3.748(6) Ǻ, linked the neighboring molecules into
individual dimers (Fig. 4), which are further connected through the intermolecular C–H…Br and
C–H…S interactions into extended layer parallel to bc-plane (Fig. 5)

Figure 4 Intermolecular π–π stacking interactions in complex [Pd2L(SMe2)2(Cl)] in the solid
state. Hydrogen atoms are omitted for clarity.

Figure 5. The crystal packing of [Pd2L(SMe2)2(Cl)] viewed down the a-axis showing individual
dimer formation by π–π interactions which are further extended into a 2-D layer through the
intermolecular C–H…Br and C–H…S interactions parallel to bc-plane. Dashed lines show the
intermolecular interactions.
Table 1. Crystal data and structure refinement for the complex [Pd2L(SMe2)2(Cl)]
Empirical formula
Formula weight
C18H21BrClN2O2Pd2S2
695.42
Temperature ( K)
173 (2) K

Wavelength (Å)
Crystal system
Space group
Crystal colour, shape
Crystal size (mm)
a (Å)
0.71073 Å
Monoclinic
C1 2/c1
Red, needle
0.14×0.21×0.28
25.184(3)
b (Å)
12.3739(17)
16.817(2)
c (Å)
α (°)
90
β (°)
122.032(2)
γ (°)
90
4442.7(10)
V (
)
-
1
3.795
µ, mm
Dcalc, g/mL
2.079
Z
F(0 0 0)
8
2688
Theta range for data collection
Index ranges
1.90 to 21.97°
-26≤ h ≤26
-13≤ k ≤13
-17≤ l ≤17
Maximum and minimum transmission
Refinement method
0.6186 and 0.4163
Full-matrix least-squares on F2
1.041
Goodness-of-fit on
R1 and wR2 indices [I > 2σ(I)]
R1 and wR2 indices (all data)
Largest difference in peak and hole (e
R1 = 0.0495, wR2 = 0.1250
R1 = 0.0709, wR2 = 0.1352
1.281 and -1.332 eÅ-3
)
Table 2. Selected interatomic bond distances (Å) and angles (°) for the complex
[Pd2L(SMe2)2(Cl)]
BOND LENGTHS (Å)
N1
BOND ANGLES (°)
N1-Pd1-O3
Pd1
Pd1
Pd1
─
─
─
1.954(8)
1.969(6)
1.993(6)
2.305(3)
2.010(11)
95.3(3)
175.0(3)
90.4(2)
80.3(4)
175.1(3)
O3
O1
O3-Pd1-O1
O3-Pd1-Cl3
C3-Pd2-N2
N2-Pd2-Cl2B
Pd1
─
S1
Pd2
─
C3

Pd2
─
N2
2.085(8)
2.210(8)
2.403(4)
2.414(8)
1.919(11)
1.400(10)
N2-Pd2-Cl1
C3-Pd2-Cl2A
Cl2B-Pd2-Cl2A
N1-Pd1-O1
101.7(2)
80.3(4)
Pd2─
S2b
Pd2─
Cl1
18.90(19)
80.6(3)
Pd2─
S2a
Br1
─
C11
N2
N1-Pd1-Cl3
O1-Pd1-Cl3
174.3(2)
93.7(2)
N1─
2.3. Catalytic Activity
We evaluated catalytic property of [Pd2L(SMe2)2(Cl)] in a Sonogashira cross coupling reaction.
The reaction of iodo benzen with phenyl acetylene achieving 1,2-diphenyl acetylene has been
chosen as a model system for the Sonogashira coupling. Initially, the catalytic reaction was
studied with various solvents (Table 3), where DMSO provides the coupled product in 61% yield
(entry1).
Table 3. Solvent optimization for Sonogashira cross coupling
GC Yield
Entry
Solvent
CuI (mmol)
24h
10
6
48h
71
42
37
0
1
2
3
4
5
6
7
8
DMSO
DMF
0
0
NEt3
0
0
31
0
H2O
DMSO/H2O
THF
0
73
0
90
0
0
DMSO
THF
0.002
0.002
-
62
89
-
aIodo benzene (1 mmol), phenyl acetylene (1.1 mmol), Base: NEt3 (3mmol), Cat. (0.001mmol), T= 100 °C
When THF was used as solvent, no coupling product was seen (entry 6), but by adding CuI as
co-catalyst, 89% yield achieved (entry 8). No reaction was observed in neat water, because of
catalyst insolubility (entry 4). This problem was circumvented by adding DMSO as organic
solvent to improve dissolving the catalyst in the reaction media. A 50:50% mixture of H2O–

DMSO was found to be most effective for the titled coupling reaction (entry 5). To further
optimization of the coupling reaction, various bases were used (Table 4).
Table 4. Base and Catalysis amount optimization for Sonogashira cross coupling
Iodo benzene (1 mmol), phenyl acetylene (1.1 mmol), Base (3mmol), T=110 °C, Solvent: DMSO/H2O (1/1 mL)
GC Yield (%)
Cat. amount
(mmol)
0.01
Entry
Base
24 h
-
48h
51
90
72
0
1
2
3
4
5
6
K2CO3
NaOAc
KOH
80
-
0.01
0.01
NaHCO3
NEt3
-
0.01
73
>99
90
-
0.01
NaOAc
0.02
Adding NaOAc as base to the coupling reaction, increase the yield up to 90% in 48h (entry 2).
Excessing the catalysis amount from 0.01mmol to 0.02mmol (2 mol %), provides the coupled
product to 99% (entry 6).
To further explore the scope and application of the reaction, we studied the reaction of various
aryl halides with phenyl acetylene under the optimized condition mention above (Table 5).
Table 5. Sonogashira coupling reaction of phenyl acetylene with substituted aryl halides
Entry
1
GC Yeild
59
2
31

3
4
30
71
5
68
6
7
100
100
8
9
98
0
It was observed that aryl bromides containing electron-withdrawing groups (nitro, acetyl and 4-
formyl) could effectively couple (entries 4, 7-8) and provide the corresponding products in
excellent yields. Bromobenzene (entry 3) and aryl bromides containing electron-donating groups
(4-methyl, 4-methoxy) (entries 1-2) gave moderate amount of the corresponding internal
alkynes. While significant advances in palladium-based systems to catalyze the reaction of aryl
halides and terminal alkynes have been made, very few reports for the coupling of alkyl or
benzyl halides have been reported[27]. Using this catalytic system, benzyl chloride and phenyl
acetylene were efficiently combined providing the benzyl alkyne in 100% yield (entry 6). It
seems that, the use of the electron-rich bulky ligand, suppress formation of the proposed Pd
bound benzyl alkynyl species and prevent formation of the product[27]. Moreover, 1-bromo
naphthalene also led to good yield with phenyl acetylene (entry 5). For checking the possibility

of Glaser homocoupling reactions as seen in entry 9, no homocoupling product was seen[28].
Although the mechanism of the Sonogashira reaction is not well established, it is generally
accepted that the formation of a Pd(0) species (e.g., by reductive elimination of the product from
an Ar–Pd–C≡C–Ar′ intermediate) and the following oxidative addition of Ar–X to regenerate a
Pd(II) species are the key steps in the catalytic cycle[29].
Figure 6. Reusability of the precatalyst.
Reusability of the precatalyst, 2 mol% screened and the trend of the GC yield is summarized in
Figure 5. A 20 mL round bottom flask was charged with 1 mmol iodo benzene, 1.1 mmol phenyl
acetylene and 3 mmol NaOAc in H2O-DMSO. To this reaction mixture was added 2 mol% of the
precatalyst and heated to 110 °C. After completion of the reaction, the mixture was extracted
with ethyl acetate and the organic layer was injected to the GC chromatography. In order to
recover, the catalyst solution was washed with ethyl acetate for several times, utilized for the
next cycle under same reaction conditions. The first cycle afforded 96% of the corresponding
coupled product, but yield decreased steadily in subsequent cycles, 85%, 77% and finally 63%
(Figure 6).
Experimental
Material

5-bromosalicylaldehyde, benzhydrazide and PdCl2 were obtained from Merck and Sigma-
Aldrich, and used without further purification. Solvents were distilled before use.
Physical measurements
Elemental analyses were performed on a Fison equipment, model EA 1108. UV–Vis spectra
were recorded with a CARY 100 Bio VARIAN UV–Vis spectrophotometer using quartz cells
having a 1.0 cm path length. Infrared spectra (4000-400 cm-1) of solid samples were taken as a
1
1% dispersion in KBr pellets using a Unicam Matson 1000 FT-IR spectrophotometer. H NMR
spectra were recorded on a Bruker 500-DRX Avance spectrometer. Chemical shifts for NMR
spectra are referenced to TMS. Gas chromatography (GC) analyses were performed on an
Agilent Technologies 6890N, equipped with a 19019 J-413 HP-5, 5% phenyl methyl siloxane,
capillary column (60.0m×250µ m×1.00µm).
Crystal Structure Determination. The crystal structures of complex [Pd2L(SMe2)2(Cl)] was
established using X-ray diffraction. Crystallographic data and structural refinements are given in
Table 1. Selected bond lengths and bond angles are listed in Table 2. Diffraction data were
collected on a BRUKER APEX2 CCD diffractometer. The frames were integrated with the
Bruker SAINT software package using a narrow-frame algorithm. The integration of the data
using a monoclinic unit cell yielded a total of 13164 reflections to a maximum θ angle of 21.97°
(0.95 Å resolution), of which 2705 were independent (average redundancy 4.867, completeness
= 100.0%, Rint = 9.62%, Rsig = 7.04%) and 1973 (72.94%) were greater than 2σ(F2). Data were
corrected for absorption effects using the multi-scan method (SADABS). The ratio of minimum
to maximum apparent transmission was 0.744. The calculated minimum and maximum
transmission coefficients (based on crystal size) are 0.4163 and 0.6186.
SMe2 groups are disordered by two positions. All non-hydrogen atoms were refined in full-
matrix anisotropic approximation based on F2. All expected hydrogen atoms were placed on a
calculated positions and were refined in isotropic approximation using "riding" model.
The Uiso(H) values have been set at 1.2 -1.5 times the Ueq value of the carrier atom.
General procedure for the synthesis of the ligand

Aroylhydrazones of o-hydroxyaldehyde possess a third potent coordination site, which can make
them tridentate ligands. 5-bromosalicylaldehyde N-benzoylhydrazone was synthesized by
reaction between the 5-bromosalicylaldehyde (1 g, 5 mmol) and benzhydrazide (681 mg, 5
mmol) in refluxing methanol (25 mL). The reaction mixture was stirred for 15 min and then
poured into cold water (15 mL). The resulting solid was filtered, washed with water, triturated
with pentane, filtered and dried under vacuum. The compound was obtained as a white solid
(1.36 g, 4.25 mmol, 85 %) (Figure 1) [30].
1HNMR: (500 MHz, DMSO- d6, ppm): 12.1 (s, 1H, NH), 11.2 (s, 1H, OH), 8.58 (s, 1H, CH=N),
7.9-6.8 (8H, ph).
Synthesis of the binuclear palladium complex
For synthesis of the dimer complex of Pd, PdCl2 (2 mmol) was dissolved in DMSO (25 mL), by
refluxing the solution overnight. Then a hot solution of the ligand (1 mmol) in methanol was
added to the solution of the PdCl2 with constant stirring. The solution was refluxed for 5 h. The
black depositions which formed during the reaction were filtered off. Then, by reducing the
solvent and adding the excess methanol, orange-red solids are formed. Precipitates were
collected by filtration, washed with methanol and dried at 80 °C under vacuum. Yield: (60%)
1HNMR: (d6, DMSO): 3.9 (S, SMe2), 6.8-7.9 (m, 7H aromatic), 8.36 (s, H Schiff-base)
General Procedure for the Sonogashira Cross-coupling Reaction
A mixture of aryl halide (1.0 mmol), phenyl acetylene (1.5 mmol), precatalyst (2 mol %), 2mL
solvent and 3 mmol base was stirred under air atmosphere at 110 °C for the desired time until
complete consumption of starting material as monitored by TLC. After the mixture was washed
with water, extracted with ethyl acetate (2*5 mL), dried over MgSO4, and injected to the GC
chromatography. All of compounds have been characterized by 1H-NMR spectra.
Conclusion

Herein, we reported the synthesis of a new dinuclear organo-palladium complex bearing ONO
1
pincer type ligand which characterized by HNMR and single crystal XRD techniques. This
complex showed strong performance in copper-free Sonogashira coupling reaction between aryl
halides and phenyl acetylene. This homogenous precatalyst showed good reusability in this
condition.
Acknowledgements
M.B. acknowledges the research council of Sharif University of Technology for the research
funding of this project.
Appendix A. Supplementary material
CCDC 1475257 contain the supplementary crystallographic data for the complex. This data can
be obtained free of charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif. The NMR of products of the Sonogashira coupling
reactions are available in Supplementary data associated with this article.
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Highlights
1. A new binuclear palladium (II) -ONO pincer type complex.
2. The catalytic activity towards the copper-free Sonogashira cross coupling reaction.
3. Good reusability and low catalyst loading for the catalytic system.

Graphical Abstract- Synopsis
ONO pincer type binuclear Pd(II) complex: synthesis, crystal structure and
catalytic utilization of the resulting organopalladium complex in catalytic
copper-free Sonogashira coupling reaction
Mojtaba Bagherzadeh, Narges-alsadat Mousavi, Maryam Zare, Sirous Jamali, Arkady Ellern and
L. Keith Woo
New binuclear palladium (II) complex featuring ONO pincer type hydrazone ligand was
synthesized and characterized by spectroscopic and single-crystal XRD analysis. This complex
showed strong performance in copper-free Sonogashira coupling reaction between aryl halides
and phenyl acetylene.