Azo-amide palladium(II) complexes: Synthesis, characterization and application in C–C cross-coupling reactions

Article abstract of DOI:10.1016/j.poly.2017.06.055

The newly designed tridentate ligands bis-2,2′-(N-alkylamino)azobenzene, 1a-1d, have been prepared by the reaction between 2,2′-diaminoazobenzene and alkyl halides in the presence of K₂CO₃. These ligands were reacted with Na₂[PdCl₄] in a 1:1 ratio in methanol to give the new Pd(II) complexes 2a–2d. All the compounds were characterized by ¹H NMR, IR spectroscopy and elemental analysis. Furthermore, the solid-state structures of the ligand 1a and two complexes (2a and 2c) were determined using single crystal X-ray diffraction analysis. The diffraction analysis revealed that the ligands bind with the Pd(II) ion in a monoanionic tridentate (N,N,N) fashion, offering a distorted square planar geometry where the fourth position is occupied by one chloride ligand. The air/moisture stable complex 2a was employed as an efficient catalyst for the Suzuki and Heck reactions under mild conditions. The catalyst exhibits high catalytic activities for the coupling of several aryl halides with phenyl boronic acid and styrene, providing excellent yields. Further, the catalyst can be easily recovered by simple chromatographic separation and reused up to three times without significant loss of its catalytic activity.

Full text of DOI:10.1016/j.poly.2017.06.055

Accepted Manuscript
Azo-amide palladium(II) complexes: synthesis, characterization and application
in C-C cross-coupling reactions
Jahar Lal Pratihar, Paritosh Mandal, Chia-Her Lin, C-K. Lai, Dasarath Mal
PII:
S0277-5387(17)30487-4
DOI:
http://dx.doi.org/10.1016/j.poly.2017.06.055
POLY 12740
Reference:
To appear in:
Polyhedron
Received Date:
Accepted Date:
30 May 2017
16 June 2017
Please cite this article as: J.L. Pratihar, P. Mandal, C-H. Lin, C-K. Lai, D. Mal, Azo-amide palladium(II) complexes:
synthesis, characterization and application in C-C cross-coupling reactions, Polyhedron (2017), doi: http://
dx.doi.org/10.1016/j.poly.2017.06.055
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers
we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and
review of the resulting proof before it is published in its final form. Please note that during the production process
errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Azo-amide palladium(II) complexes: synthesis, characterization
and application in C-C cross-coupling reactions
a*
a
b
c
d
Jahar Lal Pratihar , Paritosh Mandal , Chia-Her Lin , C-K. Lai , Dasarath Mal
a
Department of Chemistry, Kandi Raj College, Murshidabad 742137, India
b
Department of Chemistry, Chung Yuan Christian University, Chung-Li 320, Taiwan
c
Department of Chemistry, Central National University, Jhong-Li 32001, Taiwan
d
Departamento de Química, CICECO, Universidade de Aveiro, 3810-193 Aveiro, Portugal
________________________________________________________________________________
Abstract
The newly designed tridentate ligands bis-2,2'-(N-alkylamino)azobenzene, 1a-1d, have been
prepared by the reaction between 2,2'-diaminoazobenzene and alkyl halides in the presence of
K
2
CO
3
. These ligands were reacted with Na
2
[PdCl
4
] in a 1:1 ratio in methanol to give the new
1
Pd(II) complexes 2a-2d. All the compounds were characterized by H NMR, IR spectroscopy and
elemental analysis. Furthermore, the solid-state structures of the ligand 1a and two complexes (2a
and 2c) were determined using single crystal X-ray diffraction analysis. The diffraction analysis
revealed that the ligands bind with the Pd(II) ion in a monoanionic tridentate (N,N,N) fashion,
offering a distorted square planar geometry where the fourth position is occupied by one chloride
ligand. The air/moisture stable complex 2a was employed as an efficient catalyst for the Suzuki and
Heck reactions under mild conditions. The catalyst exhibits high catalytic activities for the coupling
of several aryl halides with phenyl boronic acid and styrene, providing excellent yields. Further, the
catalyst can be easily recovered by simple chromatographic separation and reused up to three times
without significant loss of its catalytic activity.
_______________________________________________________________________________
Keywords: Bis-2,2'-(N-alkylamino)azobenzene, Crystal structure, Palladium(II), Catalysis, Suzuki
and Heck Reaction
_______________________________________________________________________________
Corresponding author. Fax.: +91 3484 255230; E-mail address: jlpr@rediffmail.com (J. L. Pratihar)
1

1. Introduction
Palladium-catalyzed Suzuki cross-coupling and Heck reactions have been considered as a
very powerful, versatile and popular method for selective construction of carbon-carbon bonds in
organic chemistry [1-10]. Palladium complexes containing various ligands have been used for the
synthesis of biaryl compounds, which are important structural substructures in numerous polymers,
agrochemicals, pharmaceuticals, materials chemistry, synthetic chemistry and natural products [1-
6
,11-15]. These reactions are very popular in the pharmaceutical and chemical industries, due to the
benefits of functional group compatibility, excellent yield of products and mild reaction conditions
8-9,16-23], however, most of the coupling reactions are usually carried out in organic solvents in
[
the presence of bulky electron-rich phosphines as ancillary ligands [1-2,8-9,24-26]. These catalysts
are expensive, toxic and unrecoverable, as well as air and water/moister sensitive, thus requiring air
free handling to minimize ligand oxidation or stable oxidative addition products [26-28]. Therefore,
catalysis under phosphine-free conditions is a challenge of high importance, and a number of
phosphine-free palladium catalysts for C-C coupling reactions have been reported [8-9,29-34]. The
improvement of the catalytic properties of transition-metal complexes depends on the nature of
their ligands and the donor groups [35-39]. Thus, ligand design is necessary to improve the catalytic
property of transition-metal complexes. A small number of palladium(II) complexes of azoimine
ligands have been found to be useful as catalysts for Suzuki and Heck coupling reactions [40-42].
Recently, we have published a paper concerning the coordination chemistry of 2,2'-
diaminoazobenzene and a homogeneous catalysis where phosphine was an additional ligand [42].
In our attempts to evaluate phosphine-free systems in C-C coupling reactions, an N-alkyl
substituted 2,2'-diaminoazobenzene has been chosen for this purpose.
In the present work, we report the synthesis of phosphine-free palladium complexes with
bis-2,2'-(N-alkylamino)azobenzene ligands. All the ligands and complexes were characterized by
1
UV-vis, IR and H NMR spectroscopy. The structures of the ligand 1d and complexes 2a and 2c
2

were determined by X-ray crystallography. The new complex 2a has been successfully applied in
Suzuki cross-coupling and Heck reactions in the presence of air and moisture.
2. Experimental
2.1. Materials
The solvents used in the reactions were of reagent grade (E. Marck, Kolkata, India) and
were purified and dried by general procedures [43]. Palladium chloride, sodium chloride, benzyl
chloride, ethyl bromide and potassium carbonate were purchased from E. Mark, India. Iodobenzene,
bromobenzene, 1-iodo-2-nitrobenzene, 1-iodo-3-nitrobenzene, 1-iodo-3,5-dimethylbenzene, 1-iodo-
3,4-dimethylbenzene, 1-bromo-3,5-dimethylbenzene, 4-iodotoluene, 4-nitroiodobenzene, 4-
nitrobromobenzene, 4-bromotoluene, 1-bromo-2-nitrobenzene, phenyl boronic acid and styrene
were purchased from Aldrich. Allyl chloride and methyl iodide were purchased from SRL,
Mumbai, India. Sodium tetrachloropalladate and 2,2'-diaminoazobenzene were prepared following
reported procedures [9,42].
2.2. Physical measurements
Microanalysis (C, H, N) was performed using a Perkin-Elmer 2400 C, H, N, S / O series II
elemental analyzer. Infrared spectra were recorded on a Perkin Elmer BX-1 FT-IR spectrometer
with the samples prepared as KBr pellets. Electronic spectra were recorded on a Shimadzu UV-
1
1
800 PC spectrophotometer. H NMR spectra were obtained on Bruker 300 and Bruker 400 NMR
spectrometers in CDCl using TMS as the internal standard.
3
2.3. Syntheses
2.3.1. Synthesis of the ligand
A mixture of 2,2'-diaminoazobenzene (0.5 g, 2.36 mmol), benzyl chloride (0.6 g, 4.72
3
mmol), 4 g of K
2
CO
3
in 30 cm dry ethanol was refluxed for 5h. The orange solid mass that was
obtained after evaporation of the solvent, afforded the ligand, 1a, which was isolated by column
3

chromatography on silica gel (60-120 mesh) using the eluent petroleum ether–toluene (75/25 v/v).
Upon evaporation of the solvent after chromatography, an orange-red solid of pure 1a was obtained.
1a. Yield: 62%. Anal. Calc. for C26
H
24
N
4
: C, 79.56; H, 6.16; N, 14.27. Found: C, 79.24; H, 6.10; N,
2
-1
1
4.12%. UV/VIS (λmax / nm (ε / dm mol ), dichloromethane): 500 (18890), 315 (12665), 265
−
1
1
(
23105), 230 (36335). IR (KBr, cm ): 3237 ν(NH), 1495 ν(N=N). H NMR (400 MHz, CDCl
ppm: 4.57 (CH , d, 2H), 6.79-6.85 (ArH, m, 2H), 7.35-7.42 (ArH, m, 4H), 7.54 (ArH, t, 1H), 7.80
ArH, d, 1H), 7.91 (ArH, d, 1H), 9.40 (NH, s, 1H).
The ligands 1b, 1c and 1d were prepared following the same procedure as described in the
3
) δ,
2
(
case of 1a, but using allyl chloride, ethyl bromide and methyl iodide in place of benzyl chloride
respectively. A catalytic amount of KI (0.01 g, 0.06 mmol) was added in the case of 1c. The
refluxing time was 6 and 8 h for 1b and 1c, but 1d was prepared under stirring (10 h) at 50 °C.
1
b. Yield: 55%. Anal. Calc. for C18
H
20
N
4
: C, 73.94; H, 6.89; N, 19.16. Found: C, 73.57; H,
2
-1
6
2
.75; N, 18.96%. UV/VIS (λmax / nm (ε / dm mol ), dichloromethane): 500 (19345), 315 (13855),
−
1
1
65 (21360), 230 (35240). IR (KBr, cm ): 3259 ν(NH), 1504 ν(N=N). H NMR (300 MHz, CDCl
, s, 2H), 5.23 (CH , d, 1H), 5.35 (CH , d, 1H), 5.96-6.08 (CH, m, 1H), 6.76 (ArH,
t, 2H), 7.23 (ArH, t, 1H), 7.66 (ArH, d, 1H), 8.16 (NH, s, 1H).
3
)
δ, ppm: 3.94 (CH
2
2
2
1
c. Yield: 47%. Anal. Calc. for C16
H
20
4
N : C, 71.61; H, 7.51; N, 20.88. Found: C, 71.75; H,
2
-1
7
2
.28; N, 20.67%. UV/VIS (λmax / nm (ε / dm mol ), dichloromethane): 500 (20270), 315 (13775),
−
1
1
65 (19865), 230 (33305). IR (KBr, cm ): 3240 ν(NH), 1508 ν(N=N). H NMR (300 MHz, CDCl
, m, 3H), 3.28-3.35 (CH , m, 2H), 6.72-6.78 (ArH, m, 2H), 7.23 (ArH, t,
H), 7.64 (ArH, d, 1H), 8.07 (NH, s, 1H).
d. Yield: 50%. Anal. Calc. for C14
3
)
δ, ppm: 1.34-1.39 (CH
3
2
1
1
H
16
N
4
: C, 69.97; H, 6.71; N, 23.32. Found: C, 69.73; H,
2
-1
6
2
.55; N, 23.45%. UV/VIS (λmax / nm (ε / dm mol ), dichloromethane): 500 (18960), 315 (12940),
−
1
1
70 (17950), 235 (29850). IR (KBr, cm ): 3263 ν(NH), 1507 ν(N=N). H NMR (300 MHz,
4

CDCl
3
): 2.98 (CH , s, 3H), 6.74 (ArH, t, 2H), 7.26 (ArH, t, 1H), 7.65 (ArH, d, 1H), 8.08 (NH, s,
3
1H).
2.3.2. Synthesis of the complexes
All the complexes 2a-2d were prepared following similar procedures. A representative
procedure for 2a is given below.
3
A solution of 1a (0.135 g, 0.34 mmol) in 10 cm methanol was added to a solution of
3
Na
2
[PdCl
4
] (0.1 g, 0.34 mmol) in 10 cm methanol. The mixture was stirred for 4 h. The dark solid
precipitate that formed was separated by filtration and purified by column chromatography using
silica gel (60-120 mesh). The eluent was toluene-acetonitrile (85/15 v/v) mixed solvent. Upon
evaporation of the solvent, a blue solid of pure 2a was obtained.
2
a. Yield: 70%. Anal. Calc. for C26
H
23
4
N ClPd: C, 58.55; H, 4.35; N, 10.50. Found: C,
2
-1
5
8.36; H, 4.45; N, 10.14%. UV/VIS (λmax/nm (ε/ dm mol ), dichloromethane): 615 (11325), 575
−
1
(
12160), 360 (10475), 270 (28590), 235 (52750). IR (KBr, cm ): 3067 ν(NH), 1736 ν(C=N), 1401
1
ν(N=N). H NMR (400 MHz, CDCl
3
) δ, ppm: 4.12 (CH
2
, d, 2H), 4.39 (CH , d, 2H), 5.57 (NH, d,
2
1
4
H), 6.42-6.38 (ArH, m, 1H), 7.09-7.06 (ArH, t, 3H), 7.20-7.14 (ArH, m, 3H), 7.31-7.22 (ArH, m,
H), 7.38 (ArH, d, 3H), 7.54(ArH, d, 1H), 7.64 (ArH, d, 1H).
b. Yield: 72%. Anal. Calc. for C18 ClPd: C, 49.90; H, 4.42; N, 12.93. Found: C,
0.16; H, 4.60; N, 12.72%. UV/VIS (λmax / nm (ε / dm mol ), dichloromethane): 610 (8630), 570
2
H
19
N
4
2
-1
5
−
1
(
10850), 360 (9855), 260 (26755), 230 (37920). IR (KBr, cm ): 3076 ν(NH), 1742 ν(C=N), 1401
1
ν(N=N). H NMR (400 MHz, CDCl
3
) δ, ppm: 3.475-3.550 (CH
, m, 1H), 4.706 (CH , d, 1H), 4.909-4.960 (CH
m, 2H), 5.665-5.734 (CH, m, 1H), 6.059-6.152 (CH, m, 1H), 6.449 (ArH, t, 1H), 6.656 (ArH, d,
2
, m, 1H), 3.663-3.715 (CH
2
, m,
1
H), 4.535-4.590 (CH
2
2
2
, m, 2H), 4.961-5.076 (CH ,
2
1
2
H), 6.702 (NH, s, 1H), 7.051-7.101 (ArH, m, 1H), 7.120-7.146 (ArH, t, 2H), 7.330-7.390 (ArH, m,
H), 7.692-7.716 (ArH, t, 1H).
5

2
c. Yield: 64%. Anal. Calc. for C16
H
19
N
4
ClPd: C, 46.96; H, 4.68; N, 13.69. Found: C,
2
-1
4
6.58; H, 4.54; N, 13.47%. UV/VIS (λmax / nm (ε / dm mol ), dichloromethane): 610 (9380), 575
−
1
(
11510), 360 (8715), 260 (22955), 230 (36530). IR (KBr, cm ): 3071 ν(NH), 1744 ν(C=N), 1403
1
ν(N=N). H NMR (300 MHz, CDCl
3
) δ, ppm: 1.21 (CH
, m, 2H), 6.0 (NH, s, 1H), 6.53 (ArH, t, 1H), 6.96 (ArH, d, 1H), 7.16-7.21
ArH, m, 2H), 7.25-7.28 (ArH, m, 1H), 7.36 (ArH, d, 1H), 7.52 (ArH, d, 1H), 7.97 (ArH, d, 1H).
d. Yield: 70%. Anal. Calc. for C14 ClPd: C, 44.11; H, 3.97; N, 14.70. Found: C,
4.24; H, 4.07; N, 14.52%. UV/VIS (λmax / nm (ε / dm mol ), dichloromethane): 575 (10420), 360
3
, t, 3H), 1.26 (CH
3
, t, 3H), 3.16-3.21 (CH
2
,
m, 2H), 4.01-4.07 (CH
2
(
2
H
15
N
4
2
-1
4
−
1
1
(
8325), 260 (24450), 230 (36730). IR (KBr, cm ): 3062 ν(NH), 1739 ν(C=N), 1400 ν(N=N). H
NMR (300 MHz, CDCl ) δ, ppm: 3.22 (CH , d, 3H), 3.45-3.53 (CH , m, 3H), 5.30 (NH, s, 1H),
.58 (ArH, t, 1H), 6.99 (ArH, t, 1H), 7.20-7.28 (ArH, m, 2H), 7.32 (ArH, t, 1H), 7.39 (ArH, d, 1H),
.57 (ArH, d, 1H), 8.07 (ArH, d, 1H).
.3.5. General procedures for the Suzuki reaction
To the catalyst (1.0 mol%) dissolved in 10 ml THF in a 25 ml round-bottom flask, aryl
halide (1.0 mmol), phenyl boronic acid (1.2 mmol) and K CO (2.0 mmol) were all added. The
3
3
3
6
7
2
2
3
mixture was stirred at refluxing temperature for 4 h. After completion of the reaction, the mixture
was evaporated to dryness, water was added and the product was extracted with dichloromethane.
The organic layer was washed with brine, dried over Na
2
SO , filtered, passed through a silica
4
column (60-120 mesh), where the complex remained trapped. Upon removal of the solvent, pure
solid products were obtained. The yields of the products obtained (based on the aryl halides) from
1
all the reactions were determined after isolation, and the products were characterized by H NMR
spectra and melting points by a comparison with the literature data.
2.3.6. General procedures for the Heck reaction
To the catalyst (1.0 mol%) dissolved in 10 ml 2-propanol in a 25 ml round-bottom flask,
aryl halide (1.0 mmol), styrene (1.1 mmol) and K
2
CO (2.0 mmol) were all added. The mixture was
3
6

stirred at a temperature of 60 °C for 6 h. After the completion of the reaction, the mixture was
evaporated to dryness, water was added and the product was extracted with dichloromethane. The
organic layer was washed with brine, dried over Na
2
SO
4
, filtered, passed through silica column (60-
120 mesh), where the complex remained trapped. Upon removal of the solvent, pure solid products
were obtained. The yields of the products obtained (based on the aryl halides) from all the reactions
1
were determined after isolation, and the products were characterized by H NMR spectra and
melting points by a comparison with the literature data.
2
.4. Crystallography
Single crystals of 1d and 2a were grown by diffusion of dichloromethane into petroleum
ether at 25 °C and for 2c by slow evaporation of a methanol solution at room temperature. Data
were collected by the ω-scan technique on a Bruker Smart CCD diffractometer with Mo-K
α
graphite crystal monochromated radiation. Structure solution was carried out by direct methods
2
with the SHELXS–97 program [44,45]. Full matrix least square refinements on F were performed
using the SHELXL–97 program [44,45]. All non-hydrogen atoms were refined anisotropically
using reflections I > 2σ (I). The C-bound hydrogen atoms were included in calculated positions and
refined as riding atoms and N-bound hydrogen atoms were assigned and refined. Data collection
parameters and relevant crystal data are collected in Table 1.
-
-
-----------------------------------------------------------------------------------------------------------------------
Please insert Table 1 here
-----------------------------------------------------------------------------------------------------------------------
3. Results and discussion
3.1. Synthesis
The new ligands bis-2,2'-(N-alkylamino)azobenzene, 1a-1d, were prepared by substituting
one of the amino protons of each amino group of 2,2'-diaminoazobenzene with alkyl halides in the
presence of potassium carbonate in refluxing (except 1d) dry ethanol (Scheme 1). The ligands were
isolated as orange-red solids after column chromatographic purification.
7

-
-
-----------------------------------------------------------------------------------------------------------------------
Please insert Scheme 1 here
-----------------------------------------------------------------------------------------------------------------------
The reactions of 1a-1d with Na
coloured complexes 2a-2d (Scheme 1). In all the complexes 2a-2d, the ligand binds in a tridentate
N, N, N) fashion through one amido, one azo and one amino nitrogen atom, forming two fused
2
[PdCl
4
] in methanol at room temperature afforded the blue
(
rings - one six membered and the other five membered. The fourth coordination site of the
palladium atom in the complexes is occupied by a chloride ligand. The presence of alkyl
substituents at the amino nitrogen atom of 2,2'-diaminoazobenzene is expected to stabilize the
chloride complex by changing the electronic and steric properties, rather than the phosphine
complex [42]. The compositions of the ligands and complexes were supported by the
1
microanalytical data and H NMR spectra.
3.2. Characterization
The ligands 1a-1d and complexes 2a-2d are soluble in common organic solvents and
furnished orange-red and blue-violet colours respectively. The UV-vis spectra of the ligands and the
complexes were recorded in dichloromethane solutions. Representative examples for 1a and 2a are
shown in Fig. 1 and the spectral data are collected in experimental section.
-
-
-----------------------------------------------------------------------------------------------------------------------
Please insert Fig. 1 here
-----------------------------------------------------------------------------------------------------------------------
The absorption bands observed in the spectra of the free ligand are shifted to a lower energy region
in the spectra of complexes due to the coordination of the ligand with the metal ion. All the
transitions are mainly due to intra ligand charge transfer transitions [40-42, 46-47]. A characteristic
low energy absorption band for 2b-2d at 570-575 nm is assigned as a mixed metal ligand charge
transfer (see DFT result in the supplementary material S25) [40-42].
8

-
The ligands 1a–1d displayed an N-H absorption as a single band in the range 3263-3237 cm
1
-1
,
whereas for the complexes 2b–2d the N-H band appeared within the range 3076–3062 cm in the
-1
IR spectra [42,48]. The N=N band (1508-1504 cm ) of all the ligands shifted to a lower frequency
-1
(
1406-1400 cm ) in the complexes, consistent with coordination of the azo nitrogen atom [40-
-1
4
2,48-49]. A new band near 1744-1736 cm in the IR spectra of the complexes, unlike the ligands,
has been attributed to imine formation [41-42,49]. The spectral data for all the ligands and
complexes are collected in the experimental section.
The NMR spectra of all the compounds were recorded in the non-coordinating solvent
1
CDCl
3
at room temperature. The ligands 1a-1d and complexes 2a-2d show highly resolved H-
NMR spectra, which are in complete agreement with the X-ray structures of the ligand and the
complexes. The eight aromatic protons of the ligands 1a-1d show three distinct sets (for one, one
and two protons respectively) of aromatic signals in the range δ 6.72-7.91 ppm (excluding the Ar-H
protons of the benzyl group of 1a) due to the centrosymmetric nature of the ligands (see the X-ray
structure). Those protons appeared in the complexes 2a-2d (again except for the Ar-H protons of
the benzyl group of 2a) as seven separate set of signals in the range δ 6.42-8.07 ppm due to the
formation of two unsymmetrical chelate rings with the ligands. A sharp singlet in the range δ 8.074-
8.398 ppm is assigned to the NH proton of the ligands, which is shifted to δ 5.300-6.702 ppm in the
complexes. A similar situation is observed for the non-aromatic protons of all the ligands and
complexes. The spectral data for the ligands and complexes are collected in the experimental
section.
3.3. X-ray crystallography
The crystal structure of the ligand 1d has been determined by the X-ray diffraction method.
The crystal data and structure refinement details are given in Table 1. The molecular structure of 1d
9

is shown in Fig. 2. Selected bond lengths and bond angles are given in Table 2. The ligand 1d is
monoclinic and crystallizes in the space group P121/n1 with Z = 4. The centrosymmetric structure of
-
-
-----------------------------------------------------------------------------------------------------------------------
Please insert Fig.2 and Tables 1 and 2 here
-----------------------------------------------------------------------------------------------------------------------
1d reveals the planar geometry of the ligand, with intra molecular hydrogen bonding between the
amino proton and azo nitrogen atom [N(1)-H….N(2)#]. All the non-hydrogen atoms (except for the
methyl carbon atom) of 1d make a good plane (mean deviation 0.017 Å). The N(1)-C(1), N(2)-C(7)
and N(2)-N(2)# bond lengths of 1.3664(11), 1.4041(13) and 1.2829(14) Å are in agreement with the
reported values [42].
The structures of 2a and 2c have been determined by the single crystal X-ray diffraction
method and crystal data and structure refinement details are given in Table 1. The molecular
structures of 2a and 2c are shown in Figs. 3 and 4. Selected bond lengths and bond angles are given
in Tables 3 and 4. Complexes 2a and 2c are monoclinic and crystallize in the space group C2/c and
P21/n with Z = 8 and 4, respectively. The ligands are coordinated with the palladium atom for 2a and
-
-
-----------------------------------------------------------------------------------------------------------------------
Please insert Fig. 3 & 4 and Table 3 & 4 here
-----------------------------------------------------------------------------------------------------------------------
2c in a monoanionic tridentate (N,N,N) mode, forming a six membered chelate ring with the imino
and azo nitrogen atoms, and a five membered ring with the amino and azo nitrogen atoms. The
fourth coordination site of the palladium atoms in the complexes is occupied by a chloride ligand.
The crystal structures of 2a and 2c have a similar geometry and therefore only the bond parameters
for 2a are considered for structure analysis. The geometry about the Pd(II) ion is near square planar,
with N(1)–Pd–N(4) and N(3)–Pd–Cl(1) bond angles of 174.29(8) and 169.53(6)°. The Pd–N(1),
Pd–N(4), Pd–N(3) and Pd–Cl(1) bond lengths of 2.001(2), 2.063(19), 1.947(2) and 2.3512(6) Å are
within the normal range [40-42,49-50]. The C(1)-N(1) and C(6)-N(2) lengths (~1.34 Å) are shorter
than the C-N single bonds of C(7)-N(3) and C(12)-N(4), (~1.44 Å) in the same molecule, and are
10

close to the imine bond distance [40-42,48-49]. The bond lengths within the ligand frame indicate
the presence of extensive delocalization of the negative charge along the ligand backbone [41-
42,49]. Thus the carbon–carbon bond lengths on the phenyl ring (adjacent to the six membered
chelate rings) of the ligand (C(2)-C(3) (1.353(4) Å) and C(4)–C(5) (1.351(4) Å)) are appreciably
shorter than those of the other four bonds (C(1)–C(2) (1.445(3) Å), C(3)–C(4) (1.407(4), C(5)–C(6)
(
1.421(4) Å) and C(6)–C(1) (1.446(3) Å)) [41-42,50]. The dimeric packing structures of 2a and 2c
are stabilized through three-dimensional NH···Cl hydrogen bonds to form boat and chair-like
arrangements (Fig. 5). Interestingly two benzyl fragments in the structure of 2a are positioned on
the same side of the ring plane due to steric hindrance of the bulky benzyl group, but in the
structure of 2c the two small ethyl groups are on opposite sides of the ring plane.
-
-
-----------------------------------------------------------------------------------------------------------------------
Please insert Fig. 5 here
-----------------------------------------------------------------------------------------------------------------------
3.4. The Suzuki and Heck coupling reactions
Palladium-catalyzed carbon-carbon bond-forming processes are known as versatile tools in
organic synthesis. Suzuki coupling is an efficient method for the production of biaryls, which are
important in agrochemistry, pharmaceuticals, herbicides, materials and synthetic chemistry [1-15].
Thus, the design of new palladium catalysts and the search for new catalytic systems for the
production of biaryls derivatives are continuously on-going. The catalytic performance of 2a in the
Suzuki cross coupling reaction was investigated by the conversion of different iodo and bromo
benzenes with phenyl boronic acid in refluxing THF solvent and with K
2
CO as a base (Eq. 1). The
3
isolated yields of the products varied from 70 to 92%, depending on the aryl halide substrate, and
the conversion results are listed in Table 5. All the reactions were carried out in the presence of air
-
-
-----------------------------------------------------------------------------------------------------------------------
Please insert Eq. 1 and Table 5 here
-----------------------------------------------------------------------------------------------------------------------
11

and moisture. After the completion of the catalytic cycle, the palladium complex was intact and
isolated from the reaction mixture easily by column chromatography. The isolated catalyst 2a was
reused for the next cycle. This reusability of the catalyst was examined up to three times without
sufficient loss of its catalytic activity, keeping the other conditions intact.
The palladium complex 2a was tested as a catalyst in one of the most important carbon-
carbon bond forming processes in synthetic organic chemistry, known as the Heck reaction, the
palladium-catalyzed coupling of aryl halides with styrene [1-10]. The catalytic performance of 2a in
the Heck reaction was investigated by the conversion of different iodo and bromo benzenes with
styrene in stirring in 2-propanol solvent (60 °C) with K
2
CO as the base (Eq. 2). The isolated yields
3
of the products varied from 65 to 90%, depending upon the aryl halide substrate, and the conversion
results are listed in Table 6. All the reactions were carried out in presence of air and moisture.
Similarly, as mentioned for the Suzuki coupling reactions, at the completion of the reaction the
catalyst was isolated and reused in the next cycle. This reusability of the catalyst was examined up
to three times without significant loss of its catalytic activity, keeping the other conditions intact.
-
-
-----------------------------------------------------------------------------------------------------------------------
Please insert Eq. 2 and Table 6 here
-----------------------------------------------------------------------------------------------------------------------
4. Conclusion
A series of NNN tridentate azo ligands and their corresponding palladium(II) complexes have been
synthesized. In the new complexes 2a-2d, the NNN ligands behave as monoanionic tridentate and
the square planar coordination is completed by a chloride ion. Characterization of all the ligands
1
and complexes was accomplished by analytical and spectral (UV-vis, FT-IR and H NMR)
methods. Spectral studies and solid state X-ray crystallography confirmed the coordination mode of
the ligand to the metal center through NNN donors and revealed the square planar geometry around
12

the palladium center. Complex 2a is a highly active and efficient catalyst for promoting the Suzuki
and Heck coupling reactions of various aryl halides to produce the corresponding products with
high yields under relatively mild experimental conditions. The procedure for carrying out the
catalytic reactions is quite simple and efficient towards various aryl bromides and iodides. The
complex 2a is stable under the conditions of the Suzuki cross-coupling and Heck reactions,
indicating the catalytic activity of the molecular complex itself.
Acknowledgments
Special thanks are due to Professor S. Chattopadhyay, Department of Chemistry, University of
Kalyani for his valuable suggestions throughout this work. The author acknowledges the University
Grants Commission under Minor Research Project, Sanction No. F.PSW-164/15-16(ERO).
Appendix A. Supplementary data
1
Figures S1–S16 shows the UV/Vis and IR spectra and Figures S17–S24 the H NMR spectra of all
the ligands and complexes. Figure S-23 shows the DFT plot of complex 2a; CCDC 1550572,
1550568 and 1552565 contain the supplementary crystallographic data for compounds 1d, 2a and
2c. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
References
[
[
[
1]
2]
3]
Á. Molnár, Chem. Rev. 111 (2011) 2251.
A. R. Kapdi, I. J. S. Fairlamb, Chem. Soc. Rev. 43 (2014) 4751.
J. J. Davis, C. B. Bagshaw, K. L. Busuttil, Y. Hanyu, K. S. Coleman, J. Am. Chem. Soc.
1
28 (2006) 14135.
[
4]
J-P. Corbet, G. Mignani, Chem. Rev. 106 (2006) 2651.
13

[
[
5]
6]
N. Selander, K.J. Szabá, Chem. Rev. 111 (2011) 2048.
S. M. S. Hussain, M. B. Ibrahim, A. Fazal, R. Suleiman, M. Fettouhi, B. E. Ali,
Polyhedron 70 (2014) 39.
[
7]
C. Valente, S. Çalimsiz, K-H. Hoi, D. Mallik, M. Sayah, M.G. Organ, Angew. Chem. Int.
Ed. 51 (2012) 3314.
[
[
8]
9]
R. F. Heck, Palladium Reagents in Organic Synthesis, Academic Press, London, 1985.
E. Negishi, A. de Meijere, Handbook of Organopalladium Chemistry for Organic Synthesis;
Wiley-Interscience: New York, 2002.
[
10] A. Ghorbani-Choghamarani, A. Naghipour, H. Babaee, B. Notash, Polyhedron 119 (2016)
17.
5
[
[
[
[
[
[
11] Y. Ding, M. A. Clark, ACS Comb. Sci. 17 (2015) 1.
12] Y. Ding, J. L. DeLorey, M. A. Clark, Bioconjugate Chem. 27 (2016) 2597.
13] D. Zhang, Q. Wang, Coord. Chem. Rev. 286 (2015) 1.
14] P. Ryberg, Org. Process Res. Dev. 12 (2008) 540.
15] S. Rizzo, H. Waldmann, Chem. Rev. 114 (2014) 4621.
16] I. D. Kostas, A-C. Tenchiu, C. Arbez-Gindre, V. Psycharis, C. P. Raptopoulou, Catal.
Commun. 51 (2014) 15.
[
[
[
[
[
17] K. Karami, N. Rahimi, M. B. Shehni, Tetrahedron Lett. 53 (2012) 2428.
18] T. Ishiyama, K. Nishijima, N. Miyaura, A. Suzuki, J. Am. Chem. Soc. 115 (1993) 7219.
19] V. Farina, Adv. Synth. Catal. 346 (2004) 1553.
20] J. Magano, J. R. Dunetz, Chem. Rev. 111 (2011) 2177.
21] M. Gholinejad, H. R. Shahsavari, M Razeghi, M. Niazi, F. Hamed, J. Organomet.
Chem. 796 (2015) 3.
[
22] Y. Yuen, S. M. Wang, K. F. Chan, C. M. So, F.Y. Kwong, Synthesis 46 (2014)
2
826.
14

[
[
[
23] B. Kaboudin, H. Salemi, R. Mostafalu, F. Kazemi, T. Yokomatsu, J. Organomet. Chem. 818
2016) 195.
23] C. Gábler, J. M. Speck, M. Korb, D. Schaarschmidt, H. Lang, J. Organomet. Chem. 813
2016) 26.
(
(
24] S. J. Sabounchei, M. Ahmadi, M. Panahimehr, F. A. Bagherjeri, Z. Nasri, J. Mol. Catal. A:
Chemical 383–384 (2014) 249.
[
[
[
[
[
[
[
25] S. Mohanty, B. Punji, M. S. Balakrishna, Polyhedron 25 (2006) 815.
26] J-Y. Li, A-J. Yu, Y-J. Wu, Y. Zhu, C-X. Du, H-W. Yang, Polyhedron 26 (2007) 2629.
27] T. Yasukawa, H. Miyamura, S. Kobayashi, Chem. Soc. Rev. 43 (2014) 1450.
28] J. Mao, J. Guo, F. Fang, S-J. Ji, Tetrahedron 64 (2008) 3905.
29] S. R. Borhade, S. B. Waghmode, Tetrahedron Lett. 49 (2008) 3423.
30] T. Zhang, W. Wang, X. Gu, M. Shi, Organometallics 27 (2008) 753.
31] R. Garrido, P. S. H-M. Á. Gordillo, P. Gómez-Sal, C. López-Mardomingo, E. de Jesús,
Organometallics 34 (2015) 1855.
[
[
32] H-X. Wang, H-F. Wu, X-L. Yang, N. Ma, L. Wan, Polyhedron 26 (2007) 3857.
33] L. Naya, D. Vázquez-García, A. Fernández, M. López-Torres, I. Marcos, O. A. Lenis, M. T.
Pereira, J. M. Vila, J. J. Fernández, J. Organomet. Chem. 772–773 (2014) 192.
34] M. Micksch, M. Tenne, T. Strassner, Organometallics 33 (2014) 3966.
35] T. E. Barder, S. D. Walker, J. R. Martinelli, S. L. Buchwald, J. Am. Chem. Soc. 127 (2005)
[
[
4
685.
[
[
[
[
36] Y. Yang, S. L. Buchwald, J. Am. Chem. Soc. 135 (2013) 10642.
37] M. A. Düfert, K. L. Billingsley, S. L. Buchwald, J. Am. Chem. Soc. 135 (2013) 12877.
38] D. Zhang, Q. Wang, Coord. Chem. Rev. 286 (2015) 1.
39] S. Budagumpi, R. A. Haque, A. W. Salman, Coord. Chem. Rev. 256 (2012) 1787.
15

[
[
[
[
40] P. Pattanayak, S. P. Parua, D. Patra, C.-K. Lai, P. Brandão, V. Felix, S.
Chattopadhyay, Inorg. Chim. Acta 429 (2015) 122.
41] J. L. Pratihar, P. Pattanayak, D. Patra, C-H. Lin, S. Chattopadhyay, Polyhedron 33
(
2012) 67.
42] P. Mandal, C-H. Lin, P. Brandão, D. Mal, V. Felix, J. L. Pratihar, Polyhedron 106 (2016)
71.
1
43] D. D. Perrin and W.L.F. Armarego, Purification of laboratory chemicals, 3rd edn,
Pergamon, New York, 1988.
[
[
44] G. M. Sheldrick, Acta Crystallogr A. 64 (2008) 112.
45] G. M. Sheldrick, SHELXL-97, Program for the Refinement of Crystals Structures from
Diffraction Data, University of Göttingen, Göttingen,Germany, 1997.
[
[
46] S. Durmus, A. Atahan, M. Zengin, Spectrochim. Acta Part A 84 (2011) 1.
47] S. Anbu, S. Kamalraj, B. Varghese, J. Muthumary, M. Kandaswamy, Inorg. Chem. 51
(
2012) 5580.
[
[
48] J. l. Pratihar, N. Maiti, P. Pattanayak, S. Chattopadhyay, Polyhedron 24 (2005) 1953.
49] J. L. Pratihar, B. Shee, P. Pattanayak, D. Patra, A. Bhattacharyya, V. G. Puranik, C-H.
Hung, S. Chattopadhyay, Eur. J. Inorg. Chem. (2007) 4272.
16

Scheme 1
R
R
N
N
NH2
Cl
H
N
N
N
RX
Na [PdCl ]
Pd
N
2
4
N
N
2
N
Ethanol
K CO
H
N
Methanol
R
H N
2
2
3
R
H
1
A
Complex (2)
2a
Ligand (1)
R
CH Ph
1
1
1
1
a
b
c
2
CH CHCH
2
2
2b
2c
CH CH
2
3
d
CH3
2d
Scheme 1. Synthesis of the ligands and the palladium complexes
Equation (1) and (2)
THF
Reflux
X
+
B(OH)2
(1)
K CO3
2
R
R
X = I or Br
R = H, Me, NO2
MeOH
Reflux
X
+
K CO
(2)
2
3
R
R
X = I or Br
R = H, Me, NO2
17

Table 1
Crystallographic data for1d, 2a and 2c
Chemical formula
Formula weight
C
14
H
16
N
4
C
26
H
23ClN
4
Pd
C
16
H
19ClN Pd
4
240.31
533.33
409.22
Crystal system
space group
a / Å
Monoclinic
P1 21/n 1
10.6531(2)
5.16620(10)
10.7994(2)
90
Monoclinic
C2/c
Monoclinic
P 21/n
17.3420(13)
17.6844(13)
14.8272(12)
90
9.0285(3)
10.2829(4)
17.2562(7)
90
b / Å
c / Å



/ deg
/ deg
/ deg
92.7910(10)
90
95.279(2)
90
95.542(2)
90

/ Å
0.71073
593.651(19)
256
0.71073
4528.0(6)
2160
0.71073
3
V / Å
1594.56(10)
824
F(000)
Crystal size / mm
0.20 x 0.15 x 0.15 0.50 x 0.13 x 0.13 0.20 x 0.15 x 0.15
Z
4
8
4
T / K
100(2)
150(2)
100(2)
-3
D / Mg m
1.344
1.565
1.705
-1

/ mm
0.084
0.959
1.333
Theta Min-Max. / deg
Reflections measured
Unique reflections (Rint)
No. of reflections used [I> 2σ(I)]
R1,wR2 (all data)
2.62, 28.32
5471
1.65, 27.50
14545
2.31, 28.48
14935
1472, 0.0171
1306
5202, 0.0351
4467
4003
3756
0.0418, 0.01010
0.0370, 0.0977
1.084
0.0417, 0.0774
0.0330, 0.0728
1.001
0.0618, 0.1470
0.0591, 0.1460
1.335
R1, wR2[I> 2(I)]
GOF
18

Table 2
Selected bond distances (Å) and angles (deg) for compound 1d.
Distances
N(1)-C(1)
N(2)-C(6)
1.3659(12)
1.4041(13)
C(2)-C(3)
1.3835(13)
C(3)-C(4)
C(4)-C(5)
1.3980(15)
1.3798(14)
1
.4129(14)
C(1)-C(2)
N(2)-N(2)#
1.2823(16)
1.4497(13)
C(5)-C(6)
C(1)-C(6)
1.4069(13)
1.4312(14)
N(1)-C(7)
Angles
N(2)#1-N(2)-C(6) 116.90(10)
C(1)-N(1)-C(7)
122.19(8)
N(1)-C(1)-C(6)
N(1)-C(1)-C(2)
121.75(9)
120.58(9)
N(2)-C(6)-C(1)
N(2)-C(6)-C(5)
127.54(9)
113.32(8)
19

Table 3
Selected bond distances (Å) and angles (deg) for compound 2a.
Distances
Pd-N(1)
Pd-N(3)
N(2)-C(6)
N(3)-C(7)
1.355(3)
1.431(3)
2
.001(2)
1.947(2)
2.0630(19)
2.3512(6)
1.334(3)
1.478(3)
1.280(3)
Pd-N(4)
N(4)-C(12)
N(4)-C(20)
C(1)-C(2)
C(2)-C(3)
C(3)-C(4)
C(4)-C(5)
C(7)-C(12)
1.453(3)
1.502(3)
1.445(3)
1.353(4)
1.407(4)
1.351(4)
1.387(4)
Pd-Cl(1)
N(1)-C(1)
N(1)-C(13)
N(2)-N(3)
C(1)-C(6)
C(5)-C(6)
C(7)-C(8)
1
.446(3)
1.421(4)
1.390(3)
Angles
N(3)-Pd-N(1)
N(3)-Pd-N(4)
91.59(8)
83.77(8)
C(1)-N(1)-Pd
C(13)-N(1)-Pd
123.53(16)
119.03(16)
N(1)-Pd-N(4)
N(3)-Pd-Cl(1)
N(1)-Pd-Cl(1)
N(4)-Pd-Cl(1)
C(1)-N(1)-C(13)
C(12)-N(4)-Pd
174.29(8)
169.53(6)
97.69(6)
87.23(6)
117.4(2)
108.69(15)
N(3)-N(2)-C(6)
N(2)-N(3)-C(7)
N(2)-N(3)-Pd
C(7)-N(3)-Pd
121.7(2)
114.7(2)
131.05(16)
114.05(16)
C(12)-N(4)-C(20) 112.49(19)
C(20)-N(4)-Pd 113.91(15)
20

Table 4
Selected bond distances (Å) and angles (deg) for compound 2c.
Distances
Pd(1)-C(l1)
Pd(1)-N(1)
N(4)-C(12)
N4-C15
1.337(8)
1.484(9)
2
.3476(15)
2.078(5)
1.932(5)
2.001(6)
1.454(7)
1.498(8)
1.265(7)
Pd(1)-N(2)
Pd(1)-N(4)
N(1)-C(1)
N(1)-C(13)
N(2)-N(3)
N(2)-C(6)
N(3)-C(7)
C(7)-C(8)
1.418(9)
1.362(9)
1.413(10)
1.351(10)
1.456(9)
1.441(10)
1.505(10)
C(8)-C(9)
C(9)-C(10)
C(10)-C(11)
C(7)-C(12)
C(11)-C(12)
C(13)-C(14)
1
.446(8)
1.372(8)
Angles
Cl(1)-Pd(1)-N(1)
N(2)-Pd(1)-N(4)
89.09(13)
91.0(2)
C(1)-N(1)-C(13) 112.7(5)
Pd(1)-N(4)-C(12) 124.0(4)
Cl(1)-Pd(1)-N(2)
Cl(1)-Pd(1)-N(4)
N(1)-Pd(1)-N(2)
N(1)-Pd(1)-N(4)
Pd(1)-N(1)-C(1)
172.98(16)
95.98(16)
83.9(2)
N(2)-N(3)-C(7)
N(3)-N(2)-C(6)
Pd(1)-N(2)-C(6)
120.3(5)
112.6(5)
114.3(4)
174.9(2)
108.7(4)
Pd(1)-N(2)-N(3) 133.1(4)
Pd(1)-N(4)-C(15) 118.3(4)
Pd(1)-N(1)-C(13) 116.2(4)
21

Table 5
Suzuki cross coupling reaction of various aryl halides with phenyl boronic acid catalyzed by 2a
Entry
Product
Isolated Yield
92
I
9
8
0
9
I
I
9
2
I
NO2
NO2
9
9
3
0
I
O N
O N
2
2
O N
2
O N
I
2
8
8
8
6
I
Br
Br
7
8
Br
7
0
Br
NO2
8
8
2
0
NO2
O N
O N
Br
2
2
22

Table 5
Heck reaction of various aryl halides with styrene catalyzed by 2a
Entry
Product
Isolated Yield
I
90
I
8
8
I
I
86
9
0
NO2
NO2
I
89
90
O N
2
O N
2
O N
2
O N
I
2
Br
80
Br
6
5
Br
NO2
8
3
NO2
O N
Br O N
75
2
2
23

Fig. 1 UV-Vis spectra of 1a(―) and 2a(―)in dichloromethane
24

Fig. 2 Molecular structure of 1d (80% probability ellipsoids) with the atom numbering scheme
25

Fig. 3 Molecular structure of 2a (80% probability ellipsoids) with the atom numbering scheme.
Hydrogen atoms are omitted for clarity.
26

Fig. 4 Molecular structure of 2c (80% probability ellipsoids) with the atom numbering scheme.
Hydrogen atoms are omitted for clarity.
27

(
i)
(ii)
Fig. 5 Dimeric hydrogen bonded structures of 2a (i) and 2c (ii). (…) Lines indicate hydrogen
bonds.
28

Graphical Synopsis
Azo-amide palladium(II) complexes: synthesis, characterization
and application in C-C cross-coupling reactions
a*
a
b
c
d
Jahar Lal Pratihar , Paritosh Mandal , Chia-Her Lin , C-K. Lai , Dasarath Mal
a
Department of Chemistry, Kandi Raj College, Murshidabad 742137, India
b
Department of Chemistry, Chung Yuan Christian University, Chung-Li 320, Taiwan
c
Department of Chemistry, Central National University, Jhong-Li 32001, Taiwan
d
Departamento de Química, CICECO, Universidade de Aveiro, 3810-193 Aveiro, Portugal
_______________________________________________________________________________
X
X
R
R
+
+
B(OH)2
R
R
/
The newly designed tridentate ligands 2,2 -(N-alkyl bisdiamino)azobenzene and their corresponding
palladium(II) complexes were synthesized and structurally characterized. The new palladium amido
complexes were formed through beta proton elimination from the ligand precursor. One of the
newly synthesized air and moisture stable palladium(II) amido complexes acts as a potential
catalyst towards C-C bond formation for a variety of substrates under mild conditions.
29