Synthesis, characterization, crystal structure and catalytic activity of amido azo palladium(II) complex

Article abstract of DOI:10.1007/s11243-020-00407-7

Abstract: The newly designed tridentate ligand, 2-((2-aminophenyl)diazenyl)-N-benzylaniline, 1 has been synthesized by the reaction between 2,2′-diaminoazobenzene and benzyl chloride in presence of K₂CO₃. This ligand was reacted with Na₂[PdCl₄] in methanol to give the new Pd(II) complex?2. Both the ligand and complex were characterized by usual spectroscopic techniques. Furthermore, the solid-state structure of complex 2 was determined using single crystal X-ray diffraction analysis. It revealed that the ligand binds with Pd(II) in dianionic tridentate (N,N,N) fashion offering distorted square planar geometry where fourth position is occupied by one phosphine ligand. The performance of the Pd(II) phosphine complex as catalyst was evaluated in the homogenous Suzuki and Heck reactions under mild conditions in presence of air and moisture. The Pd(II) complex showed good catalytic activities for the coupling of several aryl halides (iodides and bromides) with phenyl boronic acid and styrene providing excellent yields. After catalytic reactions, the catalyst has been recovered by simple chromatographic separation and reused for next reaction and its activity checked up to three cycles without sufficient loss. Graphic abstract: The newly designed tridentate ligand, 2-((2-aminophenyl)diazenyl)-N-benzylaniline and its corresponding palladium(II) complex were synthesized and structurally characterized. The neutral palladium(II) amido complex with phosphine as ancillary ligand was formed through two amino proton elimination from ligand precursor. The newly synthesized Palladium(II) complex acts as potential catalyst toward C–C bond formation for a variety of substrate under mild conditions in presence of air and moisture.[Figure not available: see fulltext.].

Full text of DOI:10.1007/s11243-020-00407-7

Transition Metal Chemistry
https://doi.org/10.1007/s11243-020-00407-7
Synthesis, characterization, crystal structure and catalytic activity
of amido azo palladium(II) complex
1
1
2
3
Jahar Lal Pratihar · Paritosh Mandal · Dasarath Mal · Chia‑Her Lin
Received: 15 January 2020 / Accepted: 3 June 2020
©
Springer Nature Switzerland AG 2020
Abstract
The newly designed tridentate ligand, 2-((2-aminophenyl)diazenyl)-N-benzylaniline, 1 has been synthesized by the reaction
between 2,2′-diaminoazobenzene and benzyl chloride in presence of K CO . This ligand was reacted with Na [PdCl ] in
2
3
2
4
methanol to give the new Pd(II) complex 2. Both the ligand and complex were characterized by usual spectroscopic tech-
niques. Furthermore, the solid-state structure of complex 2 was determined using single crystal X-ray diꢀraction analysis. It
revealed that the ligand binds with Pd(II) in dianionic tridentate (N,N,N) fashion oꢀering distorted square planar geometry
where fourth position is occupied by one phosphine ligand. The performance of the Pd(II) phosphine complex as catalyst
was evaluated in the homogenous Suzuki and Heck reactions under mild conditions in presence of air and moisture. The
Pd(II) complex showed good catalytic activities for the coupling of several aryl halides (iodides and bromides) with phenyl
boronic acid and styrene providing excellent yields. After catalytic reactions, the catalyst has been recovered by simple
chromatographic separation and reused for next reaction and its activity checked up to three cycles without suꢁcient loss.
Introduction
as an alternative [14–20]. The unique steric and electronic
properties of some ligands as well as their speciꢂc coordi-
nation mode and their strong binding ability to the forma-
tion of highly active and stable metal complexes which are
more active catalyst for coupling reactions [21–23]. Still
palladium phosphines are an important catalyst for cross-
coupling reactions because phosphines control the catalytic
cycle to maintain the steric and electronic properties to favor
both the oxidative addition of the aryl halide and the reduc-
tive elimination of the coupled product [24–26]. The forma-
tion of amine complexes depends on the steric and electronic
inꢃuences of the substituents that are conjugated with the
chelate unit. N-alkyl substituents can control substantial
steric eꢀects on structure, reactivity and selectivity of the
complexes. Optimistic to these points a balance between the
steric and electronic properties of the ligand is an impor-
tant part. Moreover, few reports are available on azo-imine
ligands with palladium(II) and their applications as catalyst
in cross-coupling reactions [27–31]. Therefore, we are inter-
ested in continuing our work focused on the synthesis of
new catalysts, which are inexpensive and stable in catalytic
reaction conditions. This work originates from our previous
observations of ligands 2,2′-diaminoazobenzene and bis-
In the last three decades, palladium-catalyzed cross-coupling
reactions leading to formation of C–C, C–N, C–O, and
C–S bonds have become widely used in laboratory prepa-
ration, as well as industrial ꢂne chemical synthesis [1–6].
Most of the works reported an eꢁcient catalytic system for
cross-coupling reactions based on the sterically demand-
ing electron-rich phosphines, and its analogs [1, 2, 7–10].
However, palladium phosphines have several disadvantages,
including toxicity, sensitivity to air and moisture, and dif-
ꢂcult separation from the products [11–13]. To avoid this
disadvantage several phosphine free systems have emerged
Electronic supplementary material The online version of this
article (https://doi.org/10.1007/s11243-020-00407-7) contains
supplementary material, which is available to authorized users.
*
Jahar Lal Pratihar
jlpr@rediꢀmail.com
1
2
3
Department of Chemistry, Kandi Raj College,
Murshidabad 742137, India
Department of Chemistry, Vijaygarh Jyotish Ray College,
Kolkata 700032, India
2
,2′-(N-alkylamino)azobenzene with palladium and their
application as catalyst [28, 29]. The stability of the chlo-
ride or phosphine complex depends on the alkyl substituent
Department of Chemistry, National Taiwan Normal
University, Ting-Chow Rd., Taipei, Taiwan
Vol.:(0
1
123456789)
3

Transition Metal Chemistry
at the amino nitrogen of 2,2′-diaminoazobenzene [28, 29].
The present study where the ligand, 2-((2-aminophenyl)
diazenyl)-N-benzylaniline, contains one alkyl substituent
at the amino nitrogen of 2,2′-diaminoazobenzene gives a
neutral phosphine complex with the ligand coordinated as a
dianion, which is diꢀerent from our previous observations
(ArH, m, 1H), 7.19–7.23 (ArH, m, 1H), 7.28–7.31 (ArH, m,
1H), 7.36 (ArH, t, 2H), 7.40 (ArH, t, 2H), 7.54 (ArH, d, 1H),
7.72 (ArH, d, 1H), 8.55 (NH, s, 1H).
Preparation of complex 2
[
28, 29].
A solution of 1 (0.103 g, 0.34 mmol) in 10 mL methanol
was added to a solution of Na PdCl (0.1 g, 0.34 mmol) in
Herein, we report the synthesis of new ligand 2-((2-ami-
2
4
nophenyl)diazenyl)-N-benzylaniline and its palladium com-
1
0 mL methanol. The mixture was stirred for 4 h and then
plex. Both the ligand and complex were characterized by
added PPh (0.52 mmol). The dark solid precipitate was
3
1
usual (UV–Vis, IR and H NMR) spectroscopy. The struc-
separated by ꢂltration and puriꢂed by thin layer chromatog-
ture of the complex 2 was determined by X-ray crystallog-
raphy. The Pd(II) complex was investigated as a catalyst in
Suzuki cross-coupling and Heck reactions in the presence of
air and moisture for a variety of aryl halides.
raphy (R =0.3) on silica gel using the eluent toluene. Yield:
f
6
8
5%. Anal. Calc. for PdC H N P: C, 66.42; H, 4.67; N,
3
7
31
4
.37. Found: C, 66.65; H, 4.42; N, 8.35%. UV/VIS (λ /nm
max
2
−1
(
ε/dm mol ), dichloromethane): 585 (3450), 345 (28,750),
−
1
2
1
30 (65,560). IR (KBr, cm ): 3346 ν(NH), 1653 ν(C=N),
1
484 ν(N=N), 1478, 692 ν(PPh ). H NMR (400 MHz,
3
Experimental
CDCl ): 4.66 (CH , s, 2H), 5.21 (NH, d, 1H), 6.31 (ArH,
3
2
d, 1H), 6.53 (ArH, d, 2H), 6.65 (ArH, t, 1H), 6.71 (ArH, t,
H), 6.86–6.94 (ArH, m, 2H), 7.01–7.07 (ArH, m, 2H), 7.23
ArH, t, 1H), 7.30–7.34 (ArH, m, 6H(PPh )), 7.43 (ArH,
The solvents used in the reactions were of reagent grade
2
(
Merck, India) and were puriꢂed and dried by general proce-
(
3
dure [32]. Palladium chloride, sodium chloride, benzyl chlo-
ride and potassium carbonate were purchased from Merck,
India. Iodobenzene, Bromobenzene, 1-Iodo-2-nitroben-
zene, 1-Iodo-3-nitrobenzene, 1-Iodo-3,5-dimethylbenzene,
t, 3H(PPh )), 7.76–7.70 (ArH, m, 6H(PPh )), 8.07(ArH, d,
3
3
1
H), 8.67 (ArH, d, 1H).
General procedures for the Suzuki reaction
4
4
-Iodotoluene, 4-Nitroiodobenzene, 4-Nitrobromobenzene,
-Bromotoluene, 1-Bromo-2-nitrobenzene, Phenyl boronic
To the catalyst (1.0 mol%) dissolved in 10 mL THF, aryl
halide (1.0 mmol), phenyl boronic acid (1.2 mmol), K CO
acid and Styrene were purchased from Aldrich. Sodium
tetrachloropalladate and 2,2′-diaminoazobenzene were pre-
pared following reported procedure [29, 33]. Microanalysis
2
3
(
2.0 mmol) were all inserted in a 25 mL round-bottom ꢃask.
The mixture was heated at 80 °C under stirring for 3 h. The
reaction was monitored using thin layer chromatography
(
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
(
R =0.4–0.7) on silica gel. After the completion of the reac-
f
tion 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 SO ,
1
recorded on a Shimadzu UV-1800 PC spectrophotometer. H
2
4
NMR spectra were obtained on Bruker 400 NMR spectrom-
ꢂltered, passed through silica column (60–120 mesh), the
complex remain trapped. Upon removal of the solvent, solid
of pure products were obtained. The yields of the products
obtained (based on the aryl halides) from all the reactions
eter in CDCl using TMS as the internal standard.
3
Preparation of ligand 1
1
were determined after isolation, and characterized by H
A mixture of 2,2′-diaminoazobenzene (0.5 g, 2.36 mmol),
NMR spectra and melting points with the literature data.
benzyl chloride (0.3 g, 2.36 mmol), 5 g of K CO in 40 mL
2
3
dry ethanol was reꢃuxed for 2 h. The orange solid mass that
was obtained after evaporation of the solvent, aꢀorded the
ligand, 1, which was isolated by thin layer chromatography
General procedures for the Heck reaction
(
R =0.6) on silica gel using the eluent petroleum ether–tol-
A similar procedure to that described for the Suzuki reaction
was followed to carry out the Heck reaction using aryl halide
(1.0 mmol), styrene (1.1 mmol), K CO (2.0 mmol) with
f
uene (70/30 v/v). Yield: 50%. Anal. Calc. for C H N :
1
9
18
4
C, 75.47; H, 6.00; N, 18.53. Found: C, 75.18; H, 6.13; N,
2
3
2
−1
1
8.35%. UV/VIS (λ /nm (ε/dm mol ), dichloromethane):
(1.0 mol%) catalyst and 10 mL of 2-propanol. The reaction
mixture was heated to 70 °C under stirring during 4 h. The
yields of the products obtained (based on the aryl halides)
from all the reactions were determined after isolation, and
max
4
80 (25,290), 315 (21,810), 265 (33,520), 230 (45,760). IR
−
1
1
(
KBr, cm ): 3382,3271 ν(NH, NH ), 1433 ν(N=N). H
2
NMR (400 MHz, CDCl ): 4.46 (CH , s, 2H), 4.70 (NH , s,
3
2
2
1
H), 5.50 (NH , s, 1H), 6.70–6.78 (ArH, m, 4H), 7.11–7.13
2
1
3

Transition Metal Chemistry
1
characterized by H NMR spectra and melting points with
using reꢃections I>2σ(I). The C & N bound hydrogen atoms
were included in calculated positions and reꢂned as riding
atoms. Data collection parameters and relevant crystal data
are collected in Table 1.
the literature data.
X‑ray structure analysis
CCDC 1958999 contains the supplementary crystallo-
graphic data for compound 2. These data can be obtained
free of charge from the Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
Single crystal of 2 was grown by diꢀusion of dichlorometh-
ane into petroleum ether at room temperature. Data were col-
lected by ω-scan technique on a Bruker Smart CCD diꢀrac-
tometer with Mo-K radiation monochromated by graphite
α
crystal. Structure solution was done by direct method with
SHELX program [34, 35]. Full matrix least square reꢂne-
Results and discussion
2
ments on F were performed using SHELX program [34,
3
5]. All non-hydrogen atoms were reꢂned anisotropically
Synthesis
The new ligand 2-((2-aminophenyl)diazenyl)-N-benzylani-
line, 1 was prepared by substituting one of the amino proton
of one amino group of 2,2′-diaminoazobenzene with benzyl
chloride in the presence of potassium carbonate in reꢃuxing
dry ethanol (Scheme 1). The ligand was isolated as orange-
red liquid after chromatographic puriꢂcation.
Table 1 X-ray data of compound 2
Formula
PdC H N P
37 31 4
Mol. wt
669.05
Monoclinic
P21/c
15.4207(6)
21.7425(10)
9.0226(3)
90
96.728(2)
90
0.71073
3004.3(2)
1368
0.11×0.14×0.26
4
296
1.479
0.705
1.3, 28.5
27,136
5738, 0.054
3830
0.0535, 0.0732
0.0423, 0.0754
1.01
Crystal system
Space group
A (Å)
B (Å)
C (Å)
α (°)
β (°)
γ (°)
Wavelength (Å)
Volume (Å)
F(000)
Crystal dimensions (mm)
Z
The reaction of ligand 1 with Na [PdCl ] in methanol at
2
4
room temperature in the presence of PPh aꢀorded a blue
3
colored complex of composition 2 (Scheme 1). In the com-
plex 2, the ligand binds in a tridentate (N,N,N) fashion
through amido nitrogen, azo nitrogen and amino nitrogen
atoms forming both six membered and ꢂve membered che-
late rings. The fourth coordination site of the palladium in
the complexes is occupied by a PPh ligand. Reactions of
3
2
,2′-diaminoazobenzene with Pd(II) gives ionic phosphine
complex [29], whereas the presence of alkyl substituents
at both the amino nitrogen, i.e., bis-2,2′-(N-alkylamino)
azobenzene gives phosphine free neutral complex [28].
The presence of only one benzyl substituent in the ligand,
Temperature (K)
−
3
Density/mg/m
2
-((2-aminophenyl)diazenyl)-N-benzylaniline, is expected
Abs. coeꢀ (mm−1)
to stabilize the dianionic neutral phosphine complex by
changing electronic and steric properties, which is further
supported by two other unsuccessful reactions of 2-((2-ami-
nophenyl)diazenyl)-N-alkylaniline, where alkyl substituents
are N–Me or Et.
Theta Min–Max (°)
Reꢃections measured
Unique reꢃections (Rint)
No. of reꢃections used [I>2σ(I)]
Final R (wR ) (all)
1
2
Final R , (wR ) [I>2σ(I)]
1
2
Goodness of ꢂt
Scheme 1 Synthesis of ligand
and Palladium(II) complex
1
3

Transition Metal Chemistry
Characterization
resulting from the unsymmetric nature of the protons due to
hydrogen bonding. In the complex only one NH resonance
appeared at 5.21 ppm as a doublet indicating the dissocia-
tion of amino proton on complexation [28]. A sharp singlet
Both the ligand and complex are soluble in common
organic solvents and furnished orange-red and blue color.
UV–Vis spectra of the ligand and complex were recorded
in dichloromethane solutions and these spectra are given
in Fig. 1. The absorption spectrum of the free ligand and
its complex consists of four and three absorption peaks in
the range 230–480 nm and 230–585 nm respectively. First
three (230, 265 and 315 nm) high energy bands of the ligand
can be assigned to π–π* transitions of the phenyl rings and
at 4.46 ppm is assigned for CH protons of the ligand which
2
is shifted to 4.66 in the complex. The aromatic protons (thir-
teen) appeared in the range of 6.70–7.73 ppm, and appeared
as two doublets, three set of triplets, and three set of mul-
tiplets, which are matched well with the composition of
the ligand in solution. In the complex the aromatic protons
(except PPh ) appeared in the range 6.31–8.67 ppm for thir-
3
*
the last one (480 nm) characteristic of n–π transitions of azo
teen protons as separate doublets, triplets and multiplets.
The structure of 2 is determined by single crystal X-ray
diꢀraction method and relevant crystallographic parameters
for data collection and crystal structure solution and reꢂne-
ment details are given in Table 1 and selected bond lengths
and bond angles are given in Tables 2. The completeness
was low (75.4%), but 5738 unique reꢃections were collected,
enough to reꢂne 388 structural parameters. The Goodness
of Fit covered to 1.012 [I>2σ(I)], suggesting that the data
reꢂnement was reasonable. The crystal structure of 2 is
shown in Fig. 2. Complex 2 is monoclinic and crystallized
(–N=N–) function [28–31, 36, 37]. Upon coordination of the
ligand, the π–π* transitions of the phenyl rings in the com-
plex were shifted to diꢀerent wavelengths (230 and 345 nm),
which indicated the coordination of the palladium. A char-
acteristic broad low intense absorption peak was observed
at 585 nm in the electronic spectrum of the complex and can
be assigned to a mixed metal ligand charge transfer transition
[
28–31, 36, 37].
The IR spectrum of the ligand in solid KBr support the
presence of ν
and ν
which appeared as a twin band at
NH2
N–H
−
1
3
382 and 3271 cm , whereas in the complex only one ν
in the space group P2 with Z = 4. The palladium atom
NH
1/c
−
1
appeared as singlet band at 3346 cm indicating the dis-
is located in a distorted square planar environment (mean
deviation 0.08 Å), surrounded by the three nitrogen atoms
of the ligand and the fourth coordination site of the palla-
sociation of two amino protons on complexation [28–31].
−
1
The ν
band of the ligand 1484 cm shifted to lower
N=N
−
1
frequency in the complex 1433 cm , consistent with coor-
dium is occupied by a P atom of the PPh ligand. Ligand 1 is
3
dination of the azo nitrogen [28–31]. Two new bands near
coordinated with palladium in a dianionic tridentate (N,N,N)
fashion to construct two chelate rings (six membered and
ꢂve membered) with amino nitrogen (N4) and azo nitrogen
(N3) and amido nitrogen (N1) atoms.
−
1
−1
1
478 cm and 692 cm of complex 2 are assigned for the
coordinated triphenylphosphine [29, 38]. A new band at
−
1
1
653 cm in the IR spectrum of the complex unlike the
ligand has been attributed to imine formation [28–31].
The geometry about Pd(II) is near square planar with
N(1)–Pd–N(4), N(3)–Pd–Cl(1) bond angles 170.61(11) and
178.90(7). The Pd–N(1), Pd–N(4), Pd–N(3) and Pd–P bond
lengths 1.968(3), 2.016(3), 2.009(3), 2.2862(9) Å are com-
parable with similar Pd(II) complexes [28, 29, 38–41]. The
C(1)–N(1) and C(6)–N(2) lengths (~1.35 Å) is shorter than
the C–N single bond C(7)–N(3) 1.41 Å in the same molecule
is close to the imine distance [28, 29] and C(12)–N(4) dis-
tance 1.343(4) Å is comparably shorter due to dissociation
of amino proton upon complexation. The bond distances
within the ligand frame adjacent to six membered chelate
rings (C(2)–C(3) and C(4)–C(5) (~1·35 Å)) are appreciably
shorter than that of other four bonds (C(1)–C(2), C(3)–C(4),
C(5)–C(6) and C(6)–C(1) (~1·41) Å)), which indicates the
presence of extensive delocalization of the negative charge
along the ligand backbone [28–31].
1
Both the ligand and complex shows highly resolved H-
NMR spectra in CDCl , which are in complete agreement
3
with the structure of the ligand and complex. A sharp singlet
at 8.55 ppm is assigned for an NH proton of the ligand which
is absent in the complex. The NH resonance of the ligand
2
appeared as two singlets at 5.50 ppm and 4.70 ppm, probably
The Suzuki and Heck coupling reactions
The palladium-catalyzed Suzuki and Heck cross-coupling
reactions are a powerful strategy to form carbon–car-
bon bonds, and have been utilized in synthesizing biaryl
Fig. 1 UV–Vis spectra of 1 (dashed line) and 2(solid line) in dichlo-
romethane
1
3

Transition Metal Chemistry
Table 2 Selected bond distances (Å) and angles (°) of compound 2
Distances
Pd–P
2.2862(10)
1.967(3)
2.009(3)
2.016(3)
1.329(4)
1.290(4)
1.372(4)
1.411(4)
1.343(4)
P–C20
C1–C6
C1–C2
C2–C3
C3–C4
C4–C5
C5–C6
C13–C14
N4–C13
1.827(3)
1.417(5)
1.430(4)
1.352(6)
1.391(6)
1.355(5)
1.416(5)
1.515(4)
1.457(4)
Pd–N1
Pd–N3
Pd–N4
N1–C1
N2–N3
N2–C6
N3–C7
N4–C12
Angles
P–Pd–N1
P–Pd–N3
P–Pd–N4
N1–Pd–N3
N1–Pd–N4
N3–Pd–N4
Pd–N4–C13
91.50(9)
178.90(7)
97.88(8)
88.65(11)
170.61(11)
81.99(11)
130.4(2)
Pd–N1–C1
N3–N2–C6
Pd–N3–N2
Pd–N3–C7
N2–N3–C7
Pd–N4–C12
C12–N4–C13
128.0(2)
121.4(3)
131.0(2)
112.8(2)
116.2(3)
112.5(2)
116.9(3)
and air-stable azo-imine based palladium(II)-catalysts for
C–C coupling reactions [29–31].
The complex 2 was screened for the catalytic Suzuki cou-
pling reactions (Eq. 1). In this study we report the coupling
reactions of several aromatic halides under atmospheric
pressure with changing three parameters base, solvent and
reaction temperature. Before detailed study on the cata-
lytic reactions, we ꢂrst optimized the reaction conditions
by choosing a model substrate (iodobenzene with phenyl
boronic acid). The important three factors for the reactions,
base (NEt and K CO ), temperature (25, 40 and 65/80 °C)
3
2
3
and solvents (THF, MeOH and CH CN) were compared
3
for the purpose of attaining the optimum conditions. The
progress of the reaction was monitored by TLC. Among
the preliminary studies, K CO was found to be best base
2
3
and the THF was found to be appropriate solvent at 80 °C
(
Table S1). After optimization of the reaction conditions,
the reactions of several aryl halides (iodide and bromide)
with diꢀerent substituent in the aromatic ring were also
examined and moderate to good yields were obtained using
ꢂxed amount of catalysts (1.0 mol%). The results obtained
are given in Table 3. All the reactions were carried out in
presence of air and moisture. At the completion of the reac-
tion the catalyst was isolated and reused for the next cycle.
The reusability of the catalyst was checked up to three times
for a reaction without suꢁcient loss of its catalytic activity
where all other conditions remained intact.
Fig. 2 Molecular structure of 2 (40% probability ellipsoids) with
atom numbering scheme. Hydrogen atoms are omitted for clarity
except at the N1 atom
compounds which are utilized in various ꢂelds such as phar-
macology, medicine, and agriculture [1, 2, 7–9]. In most
cases, phosphine ligands, used in combination with Pd pre-
catalyst, have demonstrated eꢁcient catalytic activity. In
continuation of our eꢀort toward the sustainable chemistry
employing metal catalysis under mild reaction conditions,
we investigated eꢁcient, less expensive, easily accessible
1
3

Transition Metal Chemistry
Table 3 Suzuki cross-coupling reaction of various aryl halides with
phenyl boronic acid catalyzed by 2
Table 4 Heck reaction of various aryl halides with styrene catalyzed
by 2a
moderate to good yields (Table 4) were obtained using ꢂxed
amount of catalysts (1.0 mol%). All the reactions were car-
ried out in presence of air and moisture. Similarly as men-
tioned in Suzuki coupling reactions, at the end of the reac-
tion the catalyst was isolated and reused for the next cycle.
The reusability of the catalyst was checked up to three times
for a reaction without suꢁcient loss of its catalytic activity
where all other conditions remained intact.
(
2)
(1)
In the similar way, the complex 2 was tested as catalyst
in another important carbon–carbon bond forming reaction
in synthetic organic chemistry, known as the Heck reac-
tion (Eq. 2). Before a detailed study on the catalytic reac-
tions was initiated, we ꢂrst optimized the reaction conditions
by choosing a model substrate, iodobenzene with styrene,
keeping other parameters (base, solvent and temperature)
as mentioned in Suzuki reaction. After optimization, K CO
Conclusion
In summary, a tridentate azo ligand and its palladium(II)
complex have been synthesized and characterized. The
eꢀect of steric and electronic inꢃuences on the chelating
tendency of 2-((2-aminophenyl)diazenyl)-N-benzylani-
line with Palladium(II) ion has been investigated using
N-substituted 2-((2-aminophenyl)diazenyl)-N-alkylaniline.
The easily accessible and air-stable palladium(II) azo-
imine catalyst is highly active and eꢁcient catalyst for
the formation of unsymmetrical biaryls diversities with
2
3
was found to be best base and 2-propanol was found to be
an appropriate solvent at 80 °C (Table S2). The reactions
of several aryl halides (iodide and bromide) with diꢀerent
substituents in the aromatic ring were also examined and
1
3

Transition Metal Chemistry
high yields under comparatively mild experimental con-
ditions. The eꢀects of bases, temperature and solvents on
the model reaction are also screened. The catalytic reac-
tions are simple and eꢁcient at low catalyst loading. The
complex is stable under the condition of Suzuki and Heck
reactions, signifying the catalytic activity of the molecular
complex itself.
18. Desnoyer AN, Love JA (2017) Chem Soc Rev 46:197–238
1
2
9. Liu P, Feng X-J, He R (2010) Tetrahedron 66:631–636
0. Kostas ID, Coutsolelos AG, Charalambidis G, Skondra A (2007)
Tet Let 48:6688–6691
21. Yang Y, Buchwald SL (2013) J Am Chem Soc 135:10642–10644
22. Zhang D, Wang Q (2015) Coord Chem Rev 286:1–16
2
3. Düfert MA, Billingsley KL, Buchwald SL (2013) J Am Chem Soc
1
35:12877–12885
2
4. Keles M, Keles T, Serindag O (2008) Trans Met Chem
3
3:717–720
Acknowledgements Special thanks to Professor K. Ghosh, Depart-
ment of Chemistry, University of Kalyani, for his spectral help. Author
acknowledged to University Grants Commission under Minor Research
Project, Sanction No.F.PSW-164/15-16(ERO).
25. Kayan C, Biricik N, Aydemir M (2011) Trans Met Chem
36:513–520
26. Christmann U, Vilar R (2005) Angew Chem Int Ed 44:366–374
27. Oloyede HO, Woods JAO, Gorls H, Plass W, Eseola AO (2020) J
Mol Struct 1199:127030
2
8. Pratihar JL, Mandal P, Lin C-H, Lai C-K, Mal D (2017) Polyhe-
dron 135:224–230
References
2
9. Mandal P, Lin C-H, Brandão P, Mal D, Felix V, Pratihar JL (2016)
Polyhedron 106:171–177
1
2
3
.
.
.
Barnard C (2008) Palladium-catalyzed C–C coupling: then and
now. Platinum Metals Rev 52:38–45
30. Pratihar JL, Pattanayak P, Patra D, Lin C-H, Chattopadhyay S
(2012) Polyhedron 33:67–73
Molnár Árpád (2013) Palladium-catalyzed coupling reactions:
practical aspects and future developments. Wiley, Hoboken
Mpungose PP, Vundla ZP, Maguire GEM, Friedrich HB (2018)
Molecules 23:1676
31. Pratihar JL, Mandal P, Lai CK, Chattopadhyay S (2019) Polyhe-
dron 161:317–324
32. Armarego WLF (2017) Puriꢂcation of laboratory chemicals, 8th
edn. Butterworth-Heinemann, Oxford
4
5
.
.
Ruiz-Castillo P, Buchwald SL (2016) Chem Rev 116:12564–12649
Gowrisankar S, Sergeev AG, Anbarasan P, Spannenberg A, Neu-
mann H, Beller M (2010) J Am Chem Soc 132:11592–11598
Kuhn M, Falk FC, Paradies J (2011) Org Lett 13:4100–4103
Aranyos A, Old DW, Kiyomori A, Wolfe JP, Sadighi JP, Buchwald
SL (1999) J Am Chem Soc 121:4369–4378
33. Negishi E, de Meijere A (2002) Handbook of organopalladium
chemistry for organic synthesis. Wiley, New York
34. Sheldrick GM (2015) Acta Crystallogr A 71:3–8
35. Sheldrick GM (2015) Acta Crystallogr C 71:3–8
36. Durmus S, Atahan A, Zengin M (2011) Spectrochimica Acta A
84:1–5
6
7
.
.
8
9
.
.
Fleckenstein CA, Plenio H (2010) Chem Soc Rev 39:694–711
Anderson KW, Tundel RE, Ikawa T, Altman RA, Buchwald SL
37. Anbu S, Kamalraj S, Varghese B, Muthumary J, Kandaswamy M
(2012) Inorg Chem 51:5580–5592
(
2006) Angew Chem Int Ed Engl 45:6523–6527
38. Tamizh MM, Cooper BFT, Macdonald CLB, Karvembu R (2013)
Inorg Chim Acta 394:391–394
1
1
1
0. Liao C-Y, Chan K-T, Tu C-Y, Chang Y-W, Hu C-H, Lee HM
(
2009) Chem Eur J 15:405–417
39. Lobana TS, Kumari P, Butcher RJ, Akitsu T, Aritake Y, Perles J,
Fernandez FJ, Vega MC (2012) J Organomet Chem 701:17–26
40. Jain VK, Jain L (2005) Coord Chem Rev 249:3075–3197
41. Jain VK, Jain L (2010) Coord Chem Rev 254:2848–2903
1. Li J-Y, Yu A-J, Wu Y-J, Zhu Y, Du C-X, Yang H-W (2007) Poly-
hedron 26:2629–2637
2. Yasukawa T, Miyamura H, Kobayashi S (2014) Chem Soc Rev
4
3:1450–1461
1
1
3. Mao J, Guo J, Fang F, Ji S-J (2008) Tetrahedron 64:3905–3911
4. González-Sebastián L, Morales-Morales D (2019) J Organomet
Chem 893:39–51
Publisher’s Note Springer Nature remains neutral with regard to
jurisdictional claims in published maps and institutional aꢁliations.
1
1
1
5. Das P, Linert W (2016) Coord Chem Rev 311:1–23
6. Kumbhar A (2017) J Organomet Chem 848:22–28
7. Takise R, Muto K, Yamaguchi J (2017) Chem Soc Rev
4
6:5864–5888
1
3