Palladium(II) complexes with diaminomaleonitrile-based Schiff-base ligands: Synthesis, characterization and application as Suzuki–Miyaura coupling catalysts

Article abstract of DOI:10.1016/j.poly.2017.06.009

The characterization of synthesized Schiff base ligands (L¹–L⁵); (where L¹?=?N-salicyliden- 2,3-diamino-cis-2-butenedinitrile, L²?=?3- methoxy-N-salicyliden-2,3-diamino-cis-2-butenedinitrile, L³?=?5-bromo-N-salicyliden-2,3-diamino-cis-2-butenedinitrile, L⁴?=?5-nitro-N-salicyliden-2,3-diamino-cis-2-butenedinitrile, and L⁵?=?5-methoxy-N-salicyliden-2,3-diamino-cis-2-butenedinitrile) and their palladium complexes [PdL(PPh₃)] (L¹–L⁵) were carried out by FT-IR, UV–vis, ¹H NMR, ¹³C NMR and elemental analysis. The coordination geometry of [PdL³(PPh₃)] was determined by single crystal X-ray crystallography. In this structure the palladium center was in a partially distorted NNOP square planar coordination environment. The catalytic potential of the synthesized complexes was evaluated in Suzuki–Miyaura cross-coupling reaction by choosing different arylhalides and phenylboronic acid. The results showed that arylhalides with electron withdrawing substituents were more appropriate in this reaction. In all cases, the desired product of cross coupling Suzuki reaction was the major product. It is also worth to mention that the product of homo-coupling reaction was also observed as a minor product. In this regards, a blank reaction with just phenylboronic acid was performed and the results showed the occurrence of the homo-coupling product in a good yield.

Full text of DOI:10.1016/j.poly.2017.06.009

Polyhedron 134 (2017) 65–72
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Palladium(II) complexes with diaminomaleonitrile-based Schiff-base
ligands: Synthesis, characterization and application as Suzuki–Miyaura
coupling catalysts
a
b
b
Zeinab Beigi a, Ali Hossein Kianfar a, , Gholamhossein Mohammadnezhad , Helmar Görls , Winfried Plass
⇑
a Department of Chemistry, Isfahan University of Technology, Isfahan 84156-8311, Iran
b Institute of Inorganic and Analytical Chemistry, Chair of Inorganic Chemistry II, Friedrich Schiller University Jena, Humboldtstr. 8, Jena 07743, Germany
a r t i c l e i n f o
a b s t r a c t
The characterization of synthesized Schiff base ligands (L1–L5); (where L1 = N-salicyliden- 2,3-diamino-
cis-2-butenedinitrile, L2 = 3- methoxy-N-salicyliden-2,3-diamino-cis-2-butenedinitrile, L3 = 5-bromo-N-
salicyliden-2,3-diamino-cis-2-butenedinitrile,
Article history:
Received 5 April 2017
Accepted 8 June 2017
Available online 15 June 2017
L
4 = 5-nitro-N-salicyliden-2,3-diamino-cis-2-butenedini-
trile, and L5 = 5-methoxy-N-salicyliden-2,3-diamino-cis-2-butenedinitrile) and their palladium com-
plexes [PdL(PPh3)] (L1–L5) were carried out by FT-IR, UV–vis, 1H NMR, 13C NMR and elemental
analysis. The coordination geometry of [PdL3(PPh3)] was determined by single crystal X-ray crystallogra-
phy. In this structure the palladium center was in a partially distorted NNOP square planar coordination
environment. The catalytic potential of the synthesized complexes was evaluated in Suzuki–Miyaura
cross-coupling reaction by choosing different arylhalides and phenylboronic acid. The results showed
that arylhalides with electron withdrawing substituents were more appropriate in this reaction. In all
cases, the desired product of cross coupling Suzuki reaction was the major product. It is also worth to
mention that the product of homo-coupling reaction was also observed as a minor product. In this
regards, a blank reaction with just phenylboronic acid was performed and the results showed the occur-
rence of the homo-coupling product in a good yield.
Keywords:
Schiff base
Palladium complexes
X-ray crystallography
Suzuki–Miyaura
Homo-coupling
Ó 2017 Published by Elsevier Ltd.
1. Introduction
with a diversity of ligands, such as phosphines [20], oxime pallada-
cycles [21], imidazole-phenolates [22], Schiff bases [23] and N-
Palladium as an important noble metal with common oxidation
states of +II and +IV, has exclusive properties. It was applied in
many electronics, ceramic capacitors and LCD displays [1–2]. Also,
it has a key role in the technologies used in the fuel cells [3–4]. Its
catalytic application in the organic synthesis was interested during
the twentieth century [5–8]. In this regards, the coupling reactions
in the presence of a palladium-containing catalyst, have arguably
become one of the most utilized tools for the construction of C–C
bonds [9–11]. Especially, the cross-coupling of the organic halides
with aryl boronic acids, which is catalyzed by palladium, is one of
the considerable procedure for the formation of C–C bonds. [12–
14]. Suzuki reaction is taken into consideration due to its high
importance in various fields including pharmaceutical, polymer
and etc. [15–18]. Its superiority over the other coupling reactions
is due to the mild reaction conditions, non-toxic nature of the pro-
cess and stereoselectivity [19]. Soluble palladium(II) complexes
heterocyclic carbines [24] have been utilized as efficient catalysts
in Suzuki reaction. Schiff base compounds have the versatile cat-
alytic application for a wide range of organic reactions [25–26].
The synthesis and catalytic activity of complexes bearing Schiff
base ligands have been widely reported [27]. Also, there is a large
interest to employ nitrogen containing Schiff base of palladium(II)
for Suzuki–Miyaura coupling reactions [28]. They can be used as
the active and selective catalyst for hydrogenation, hydrosilylation,
and Suzuki/Heck C–C bond coupling reactions [29].
Herein, [L1–L5] ligands and [PdL(PPh3)] complexes (where
L1 = N-salicyliden-2,3-diamino-cis-2-butenedinitrile, L2 = 3-meth-
oxy-N-salicyliden-2,3-diamino-cis-2-butenedinitrile, L3 = 5-bromo-
N-salicyliden-2,3-diamino-cis-2-butenedinitrile, L4 = 5-nitro-N-
salicyliden-2,3-diamino-cis-2-butenedinitrile, L5 = 5-methoxy-N-
salicyliden-2,3-diamino-cis-2-butenedinitrile) were synthesized
with the aim of highlighting the influence of substitutions
on the catalytic properties. The complexes were systemati-
cally characterized by various analytical and spectroscopic tech-
niques. The geometry of [PdL3(PPh3)] was determined by X-ray
⇑
Corresponding author. Fax: +98 31 33912350.
E-mail addresses: akianfar@cc.iut.ac.ir, asarvestani@yahoo.com (A.H. Kianfar).
http://dx.doi.org/10.1016/j.poly.2017.06.009
0277-5387/Ó 2017 Published by Elsevier Ltd.

66
Z. Beigi et al. / Polyhedron 134 (2017) 65–72
crystallography. The catalytic activities of the tridentate Schiff base
complexes for Suzuki reaction were also examined. This class of
ligands exhibited a considerable stabilizing effect not only from
the chelate formation, if compared to only phosphines as ligands,
but also from stabilization of different oxidation states of metal
ions. Also, facile and easy synthetic procedures were applied for
their preparation compared to organometallic compounds. The
effect of substitution patterns on the substrates, including elec-
tronic and steric effects, was investigated on the catalytic activities
of the prepared complexes.
ppm): 3.76 (s, 3H, OCH3), 6.84–8.44 (m, 3H, aromatic), 8.58 (s,
2H, NH2), 9.97 (s, 1H, HC@N), 10.69 (s, 1H, OH).
2.3. Synthesis of complexes
Palladium(II) acetate (0.1 mmol, 0.0245 g) and Schiff base
ligand (0.1 mmol) were dissolved in methanol (20 mL) and then,
0.1 mmol of triphenylphosphine (0.0263 g) was added to the solu-
tion. The obtained solution was stirred and heated for 3 h. The resi-
due was isolated by gravity filtration, washed with methanol and
air dried at room temperature. The resulting crystals were formed
from chloroform/methanol by slow evaporation after 2–3 days (see
Scheme 1).
2. Experimental
2.1. General remarks
[PdL1(PPh3)] (1a) (Yield: 88%). Elemental Anal. Calc. For C29H21
-
N4OPPd: C, 59.74%; H, 3.61%; N, 9.61%. Found: C, 59.50%; H, 3.61%;
N, 9.42%. Characteristic IR absorptions: 3391 (NH), 3046 (C–H),
All materials were purchased from Merck and Aldrich used
without further purification. The C, H and N contents were deter-
mined on a CHN-O-Heraeus elemental analyzer. Electronic spectra
were measured on a Perkin Elmer, JASCO V-570 double beam spec-
trophotometer in the range of 200–700 nm. FT-IR spectra were
obtained by using FT-IR JASCO-680 spectrophotometer 400–
4000 cmꢀ1 range using KBr discs at room temperature. The 1H
and 13C NMR spectra were recorded on a Bruker AVANCE 400
and 500 MHz spectrometers. Products of catalytic reactions were
analyzed by an Agilent 6890N gas chromatograph equipped with
a capillary HP-5+column. The column properties were: 30 m long,
2228 and 2182 ðC ꢃ N), 1605 (C@N) cmꢀ1. UV–Vis (kmax),
e
(Lꢁmolꢀ1ꢁcmꢀ1): 248
(
e
= 2.5 ꢂ 102), 318
(
e
= 1.23 ꢂ 103), 426
(e
= 1.77 ꢂ 103), 452
(e
= 2.08 ꢂ 103) and 1H NMR (500 MHz,
DMSO-d6, d, ppm) 6.57–6.58 (d, 1H, Aromatic, J = 5 Hz), 6.69–6.72
(t, 1H, Aromatic, J = 15 Hz), 7.21–7.24 (m, 1H, Aromatic), 7.45–
7.74 (d, 1H, Aromatic), 7.51–7.73(m, 15H, Aromatic), 8.42–8.45
(d, 1H, HC@N, J = 15 Hz).
[PdL2(PPh3)] (2a) (Yield: 88%). Elemental Anal. Calc. For C30H23
-
N4O2PPd: C, 58.77%; H, 3.77%; N, 9.143%. Found: C, 58.72%; H,
3.85%; N, 9.09%. Characteristic IR absorptions: 3391(NH), 3046
0.32 mm in inner diameter, and 0.25
l
m film thickness.
(C–H), 2225 and 2184(C„N), 1606 (C@N) cmꢀ1, UV–Vis (kmax),
e
(Lꢁmolꢀ1ꢁcmꢀ1): 252
(
e
= 2.9 ꢂ 102), 310
(
e
= 1.45 ꢂ 103), 418
2.2. General procedure for the synthesis of ligands
(e
= 1.54 ꢂ 103), 446 (
e
= 1.47 ꢂ 103) and 1H NMR (400 MHz, CDCl3,
Me4Si) 3.72 (s, 3H, OCH3), 6.73–6.77 (t, 1H, Aromatic), 6.90–6.92 (d,
1H, Aromatic, J = 8 Hz), 7.19–7.21 (d, 1H, Aromatic, J = 8 Hz), 7.50–
7.79 (m, 15H, Aromatic), 8.42–8.47 (d, 1H, HC@N, J = 12 Hz). 13C
NMR (CDCl3, d, ppm): 56(C-OCH3), 114–127 (C-Ar), 129–134 (C-
PPh3), 128 and 133 (C@C), 148 (CH@N), 152 and 154 (C„N).
The Schiff base ligands were synthesized by the reaction
between 1,2-diaminomaleonitrile (2,3-diamino-cis-2-butenedini-
trile) and salicylaldehyde or its derivatives (1:1 M ratio) in metha-
nol. The obtained solution was refluxed and stirred overnight.
Finally, the products appeared as colored precipitates which were
filtered and washed with methanol.
[PdL3(PPh3)] (3a) (Yield: 79%). Elemental Anal. Calc. For C29H20
-
N4OBrPPd: C, 52.617%; H, 3.036%; N, 8.467%. Found: C, 52.510%; H,
3.135%; N, 8.364%. Characteristic IR absorptions: 3396 (NH), 3052
L1: (Yield: 80%) C11H8N4O: FT-IR (KBr cmꢀ1
)
v
max, 3416 and
ꢀ
3308 (NH2), 3193 (OH), 2234 and 2205 (C„N), 1625 (C@N). UV–
(C–H), 2225 and 2182(C„N), 1598 (C@N) cmꢀ1, UV–Vis (kmax),
e
Vis, kmax (nm),
e
(Lꢁmolꢀ1ꢁcmꢀ1) (Methanol): 213 (
e
e
= 9.05 ꢂ 103),
(Lꢁmolꢀ1ꢁcmꢀ1): 248
(e
= 6.7 ꢂ 102), 320
(e
= 4.7 ꢂ 102), 432
264 (
e
= 6.6 ꢂ 103), 328 (
e
= 1.03 ꢂ 104) and 379 (
= 1.08 ꢂ 104).
(e
= 3.6 ꢂ 102), 454 (
e
= 3.89 ꢂ 102), and 1H NMR (400 MHz, CDCl3,
1H NMR (DMSO-d6, d, ppm): 6.88–8.47 (m, 4H, aromatic), 8.60 (s,
Me4Si) 6.86–6.87 (d, 1H, Aromatic, J = 4 Hz), 8.10–8.13 (dd, 1H,
Aromatic, J = 12 Hz), 8.41–8.45 (d, 1H, Aromatic, J = 16 Hz), 7.54–
7.72 (m, 15H, Aromatic), 8.61–8.62 (d, 1H, HC@N, J = 4 Hz). 13C
NMR (CDCl3, d, ppm): 126–128 (C-Ar), 129–134 (C-PPh3), 119
and 128 (C@C), 146 (CH@N), 131 and 131 (C„N).
2H, NH2), 10.43 (s, 1H, HC@N), 11.18 (s, 1H, OH).
L2: (Yield: 82%). C12H10N4O2: FT-IR (KBr cmꢀ1
)
max: 3408 (OH),
ꢀ
v
3323 and 3207 (NH2), 3040, 2998 and 2888 (CH3), 2244 and 2204
(C„N), 1643(C@N). UV–Vis, kmax (nm),
e
(Lꢁmolꢀ1ꢁcmꢀ1) (Metha-
nol): 226
(
e
= 5.1 ꢂ 103), 271
(
e
= 2.9 ꢂ 103) and 376
[PdL4(PPh3)] (4a) (Yield: 82%). Elemental Anal. Calc. For C29H20
-
(e
= 6.8 ꢂ 103). 1H NMR (DMSO- d6, d, ppm): 3.86 (s, 3H, OCH3),
N5O3PPd: C, 55.45%; H, 3.20%; N, 11.15%. Found: C, 55.40%; H,
6.84–8.44 (m, 3H, aromatic), 8.62 (s, 2H, NH2), 9.89 (s, 1H, HC@N),
3.18%; N, 10.94%. UV–Vis (kmax),
e
(Lꢁmolꢀ1ꢁcmꢀ1): 252
10.93 (s, 1H, OH).
(
(
e
e
= 2.7 ꢂ 102), 348
(
e
= 2.09 ꢂ 103), 426
(
e
= 1.61 ꢂ 103), 452
L3: (Yield: 80%). C11H7N4OBr: FT-IR (KBr cmꢀ1
)
max: 3403 (OH),
= 1.7 ꢂ 103). Characteristic IR absorptions: 3384 (NH), 3054 (C–
ꢀ
v
3301 and 3196 (NH2), 2246 and 2210 (C„N), 1631(C@ N). UV–Vis,
H), 2228 and 2189 (C„N), 1606 (C@N) cmꢀ1
.
kmax (nm),
e
(Lꢁmolꢀ1ꢁcmꢀ1) (Methanol): 221(
e
= 4.3 ꢂ 103), 245
[PdL5(PPh3)] (5a) (Yield: 89%). Elemental Anal. Calc. For C30H23
-
(e
= 3.3 ꢂ 103), 261 (
e
= 2.9 ꢂ 103), 386 (
e
= 5.9 ꢂ 103). 1H NMR
N4O2PPd: C, 58.77%; H, 3.77%; N, 9.143%. Found: C, 58.44%; H,
3.985%; N, 8.919%. Characteristic IR absorptions: 3395 (NH), 3053
(DMSO- d6, d, ppm): 6.90–8.43 (m, 3H, aromatic), 8.51 (s, 2H,
NH2), 10.67 (s, 1H, HC@N), 11.15 (s, 1H, OH).
(C–H), 2225 and 2183 („N), 1605 (C@N) cmꢀ1, UV–Vis (kmax),
e
L4: (Yield: 80%). C11H7N5O3: FT-IR (KBr cmꢀ1
)
max: 3407 (OH),
(Lꢁmolꢀ1ꢁcmꢀ1): 248 (
e
= 2.9 ꢂ 102), 324 (
e
= 1.196 ꢂ 103), 414
ꢀ
v
3308 and 3203 (NH2), 2237 and 2217 (C„N), 1629 (C@N), 1345
(e
= 1.39 ꢂ 103), 466
(e
= 1.65 ꢂ 103) and 1H NMR (500 MHz,
(NO2). UV–Vis, kmax (nm),
e
(Lꢁmolꢀ1ꢁcmꢀ1
)
(Methanol): 254
DMSO-d6, d, ppm) 3.70 (s, 3H, OCH3), 6.52–6.53 (d, 1H, Aromatic,
J = 5 Hz), 6.92–6.94 (m, 1H, Aromatic), 7.30–7.31 (d, 1H, Aromatic,
J = 5 Hz), 7.44–7.63(m, 15H, Aromatic), 8.45–8.48 (d, 1H, HC@N,
J = 15 Hz).
(e
= 2.34 ꢂ 103), 316
(
e
= 2.36 ꢂ 103), 381
(
e
= 3.95 ꢂ 103). 1H
NMR (DMSO- d6, d, ppm): 6.98–8.74 (m, 3H, aromatic), 8.93 (s,
2H, NH2), 10.28 (s, 1H, HC@N), 11.88 (s, 1H, OH).
L5: (Yield: 77%). C12H10N4O2: FT-IR (KBr cmꢀ1
)
max: 3461 (OH),
ꢀ
v
3338 and 3190 (NH2), 3080, 2991 and 2845 (CH3), 2243 and 2204
2.4. Crystal structure determination
(C„N), 1620 (C@N). UV–Vis, kmax (nm),
e
(Lꢁmolꢀ1ꢁcmꢀ1) (Metha-
nol): 209 (
e
= 3.78 ꢂ 103), 245 (
e
= 3.01 ꢂ 103), 265 (
e
= 2.3 ꢂ 103),
The intensity data were collected on a Nonius Kappa CCD
diffractometer, using graphite-monochromated Mo K radiation.
350 (
e
= 3.7 ꢂ 103), 400 (
e
= 4.3 ꢂ 103). 1H NMR (DMSO- d6, d,

Z. Beigi et al. / Polyhedron 134 (2017) 65–72
67
Table 2
Data were corrected for Lorentz and polarization effects; absorp-
tion was taken into account on a semi-empirical basis using multi-
ple-scans [30–32]. The structure was solved by direct methods
Selected bond distances and angles for 3a.
Bond distances
Bond angles
(
SHELXS [33]) and refined by full-matrix least squares techniques
Pd1–O1
Pd1–N1
Pd1–N2
Pd1–P1
Br1–C4
P1–C12
O1–C1
N1–C7
N1–C8
N2–C9
N3–C10
N4–C11
C1–C6
1.966(3)
2.034(4)
1.975(4)
2.275(12)
1.904(5)
1.812(5)
1.310(6)
1.300(6)
1.399(6)
1.324(6)
1.149(6)
1.140(6)
1.417(7)
1.385(6)
O(1)–Pd(1)–N(1)
O(1)–Pd(1)–N(2)
O(1)–Pd(1)–P(1)
N(1)–Pd(1)–P(1)
N(2)–Pd(1)–P(1)
N(2)–Pd(1)–N(1)
C(7)–N(1)–Pd(1)
C(7)–N(1)–C(8)
C(3)–C(4)–Br(1)
C(9)–C(8)–N(1)
N(1)–C(8)–C(10)
N(2)–C(9)–C(8)
N(3)–C(10)–C(8)
N(4)–C(11)–C(9)
O(1)–C(1)–C(2)
95.56(14)
174.50(16)
95.45(10)
176.95(11)
94.88(13)
82.12(16)
124.7(3)
124.5(4)
118.1(4)
114.2(4)
122.0(4)
120.5(4)
177.9(5)
179.2(5)
116.1(4)
against F2o
(SHELXL-97 [33]). The hydrogen atoms bonded to the
amine-group N2 were located by difference Fourier synthesis and
refined isotropically. All other hydrogen atom positions were
included at calculated positions with fixed thermal parameters.
XP [34] was used for structural representations.
Crystal Data for 3a:
C ,
29H20BrN4OPPd, Mr = 657.77 gmolꢀ1
orange prism, size 0.132 ꢂ 0.128 ꢂ 0.102 mm3, triclinic, space
ꢀ
group P1, a = 8.9663(3), b = 10.9700(3), c = 15.0305(6) Å,
a
= 68.680(1), b = 82.625(2),
c
= 69.914(2)°, V = 1293.45(8) Å3,
T = ꢀ140 °C, Z = 2,
q
calcd. = 1.689 g cmꢀ3, m (Mo K
a ,
) = 23.54 cmꢀ1
C8–C9
multi-scan, transmin: 0.6102, transmax: 0.7456, F(000) = 652,
13,355 reflections in h(ꢀ11/9), k(ꢀ14/14), l(ꢀ18/19), measured in
the range 2.104° ꢄ h ꢄ 27.485°, completeness hmax = 99.3%, 5849
independent reflections, Rint = 0.0345, 4884 reflections with
Fo > 4r(Fo), 338 parameters, 0
restraints, R1obs = 0.0491, wR2-
Table 3
Suzuki cross-coupling of bromobenzene and phenylboronic acid catalyzed by
different catalyst.
obs = 0.0971, R1all = 0.0645, wRa2ll = 0.1057, GOOF = 1.051, largest dif-
ference peak and hole: 1.029/ꢀ1.044 e Åꢀ3. Crystals of 3a were
grown by slow evaporation from chloroform/MeOH solution. The
ORTEP drawing of the inner coordination sphere at the Pd core with
the atom numbering scheme is shown in Fig. 2.
HO
OH
Br
B
Catalyst, K2CO3
70 C, 5h
2.5. General experimental procedure for Suzuki cross-coupling
reaction
+
Toluene
For the homogeneous Suzuki–Miyaura coupling reactions, a
range of substituted aryl halides as substrates was employed. A
mixture of aryl halides (0.5 mmol), phenylboronic acid
(0.75 mmol), base (1.5 mmol), solvent (6 mL) and catalyst
(0.01 mmol) was stirred at 70 °C in the air. Optimization of the
reaction conditions was summarized in Tables 3–7. Also, the reac-
tion mixtures were collected at different time and measured by GC
analysis in order to study the progress of the reaction.
Entry
Catalyst
Yield (%)
1
2
3
4
5
[PdL1(PPh3)] (1a)
[PdL2(PPh3)] (2a)
[PdL3(PPh3)] (3a)
[PdL4(PPh3)] (4a)
[PdL5(PPh3)] (5a)
79
83
86
56
89
Reaction conditions: bromobenzene (0.5 mmol); phenyl boronic acid (0.75 mmol);
K2CO3 (1.5 mmol); catalyst (0.01 mmol); toluene (5 ml); T = 70 °C; t = 5 h.
3. Results and discussions
Table 4
Effect of solvent on Suzuki coupling catalyzed by [PdL5(PPh3)].
3.1. FT-IR spectra
Entry
Solvent
Yield (%)
The FT-IR characteristic bands are seen in Section 2.2 and 2.3 for
the ligands and complexes. The FT-IR spectra of all ligands exhibit
an intense band in the region of 1620–1643 cmꢀ1, which can be
assigned to the stretching vibration of the C@N group. The NH2
1
2
3
4
5
DMF
22
31
50
89
84
MeOH
CH3CN
Toluene
CH2Cl2
Reaction conditions: bromobenzene (0.5 mmol); phenyl boronic acid (0.75 mmol);
K2CO3 (1.5 mmol); catalyst (5a) (0.01 mmol); solvent (5 ml); T = 70 °C; t = 5 h.
Table 1
Selected Crystallographic data for 3a.
Compound
(A)
Table 5
Formula
Formula weight
T (K)
C
29H19BrN4OPPd
Effect of base on Suzuki coupling catalyzed by [PdL5(PPh3)].
656.76
133(2)
0.710
Triclinic
P bar1
2
0.132 ꢂ 0.128 ꢂ 0.102
8.9663 (3)
10.9700 (3)
15.0305 (6)
68.680 (1)
82.625 (2)
69.914 (2)
1.686
Entry
Base
Yield (%)
k (Å)
1
2
3
4
Na2CO3
K2CO3
KOH
78
88
49
52
Crystal system
Space group
Z
Crystal size (mm)
a (Å)
Et3N
Reaction conditions: bromobenzene (0.5 mmol); phenyl boronic acid (0.75 mmol);
Base (1.5 mmol); catalyst (5a) (0.01 mmol); toluene (5 ml); T = 70 °C; t = 5 h.
b (Å)
c (Å)
a
(°)
b (°)
(°)
Dcalc (g cmꢀ1
stretching frequencies are seen in the range of 3190–3416 cmꢀ1
.
c
The hydroxyl vibration band is seen in the 3400 cmꢀ1. As seen in
the FT-IR spectra, the CN stretching shows two bands in the region
of 2204–2217 and 2234–2246 cmꢀ1 which these bands shift to
lower frequencies after coordination of the ligand to the palla-
)
h ranges for data collection
2.10–27.48
650
1293.45(8)
F(000)
V (Å3)

68
Z. Beigi et al. / Polyhedron 134 (2017) 65–72
Table 6
was carried out with the aid of 2D NMR spectra including
Effect of time on Suzuki coupling catalyzed by [PdL5(PPh3)].
{1H–1H} COSY (correlation spectroscopy), and {1H–13C} HSQC
(heteronuclear single quantum correlation) experiments (Figs. 1
and 2). The 1H NMR data of the synthesized complexes has shown
that the aromatic hydrogens (6.73–8.45 ppm) were in the similar
range (6.84–8.47) to the ligands. Also, the assignment of the aro-
matic hydrogens (Scheme 2) was confirmed in (H–H) COSY
(Fig. 1). The NH2 and OH protons were seen at 8.64 and
11.16 ppm, respectively. In the palladium complexes, no signals
were recorded for the phenolic hydrogen, which is an indication
of Schiff bases ortho hydroxyl group deprotonation. The hydrogen
Entry
Time (h)
Yield (%)
1
2
3
4
5
6
7
8
1
2
3
4
5
6
8
12
–
48
70
75
89
89
90
91
Reaction conditions: bromobenzene (0.5 mmol); phenyl boronic acid (0.75 mmol);
Base (1.5 mmol); catalyst (5a) (0.01 mmol); toluene (5 ml); T = 70 °C.
of imine in the free ligands appeared as
a singlet (8.60–
10.67 ppm) while the related signals for palladium(II) complexes
were appeared as a doublet (8.44–8.62 ppm) due to the coupling
with phosphorus atom. In 2a and 5a the OCH3 signal was observed
at 3.72 and 3.70 ppm, respectively.
Table 7
Effect of temperature on Suzuki coupling catalyzed by [PdL5(PPh3)].
The molecular structures of 2a and 3a were also investigated by
13C NMR experiments. The signal of carbon atom of the methoxy
group in the 2a was assigned at 56.34 ppm. The six different sig-
nals of the aromatic carbons related to the Schiff base moiety were
shown in the range of 112–127 ppm, while the chemical shifts of
the triphenylphosphine carbons were seen in the range of 129–
134 ppm. In these complexes, C@C, C„N resonances were shown
as two peaks in the region of 128–131 and 132–154 ppm, respec-
tively. The most relevant features observed in the 13C {1H} NMR
spectra of the synthetic palladium(II) compounds were the upfield
shifts of imine carbon signals, which it was appeared at 147 ppm.
HSQC NMR in Fig. 2 confirms the signals in 2a.
Entry
Temperature (°C)
Yield (%)
1
2
3
4
5
6
25
50
60
70
90
110
15
52
88
89
91
93
Reaction conditions: bromobenzene (0.5 mmol); phenyl boronic acid (0.75 mmol);
Base (1.5 mmol); catalyst (5a) (0.01 mmol); toluene (5 ml); t = 5 h.
dium(II) ion. The stretching frequency of NO2 in L4 ligand is seen at
1345 cmꢀ1. In the [PdL(PPh3)] complexes containing L1–L5, as
shown in the Section 2, the C@N band shifts to the lower frequen-
cies relative to the free ligand in the coordination compounds. A
sharp band observed in the region of 3331–3396 cmꢀ1 assigned
ꢀ
3.3. Structural description of [PdL3(PPh3)]; (3a)
to
v(N–H) which shows that the NH2 functional group has been
The structure of [PdL3(PPh3)] complex was determined by X-ray
changed to the NH after coordination of the Schiff base ligands.
In coordination of ligands to the metal ion the characteristic band
of the hydroxyl group was absence. Also, the bands at about
3053 cmꢀ1 are related to the aromatic C–H vibration of PPh3 in
complexes.
ꢀ
crystallography. The molecule crystallized in the triclinic P1 space
group, with two molecular entities present in the asymmetric unit.
As observed in [PdL3(PPh3)], structure of 3a is given in Fig. 3. Tables
1 and 2 shown X-ray crystallography data, the selected bond
lengths and angles. This complex was contained tridentate ONN
Schiff base and a monodentate triphenylphosphine ligand. Palla-
dium atom was in a slightly distorted square planar coordination
environment consisting of N1, N2, O1 and P1 atoms from ligands.
The Pd–N1, Pd–N2, Pd–O1 and Pd–P1 distances appeared in the
range of 2.035 (4), 1.986 (4), 1.970 (3) and 2.275 (12) Å, respec-
tively. The O1–Pd1–N1, O1–Pd1–P1, N2-Pd1–P1 and N2-Pd1–N1
bond angles (92.60 (1), 90.37 (1), 94.64 (12) and 82.40 (16), respec-
tively) deviate from 90°, the O1–Pd1–N2 and N2–Pd1–N1 angles
(174.82 (16) and 176.98 (12), respectively) deviate from the 180°
angle showed that the coordination geometry of the palladium
ion distorted from the square planar. In [PdL3(PPh3)] complex,
the hydrogen bonding between the C„N nitrogen and the hydro-
gens of triphenyl phosphine (2.56 Å) leads to an extended network
of the complex.
3.2. Electronic spectra
The UV–Vis spectra of L1–L5 ligands and [PdL(PPh3)] (L = L1–L5)
complexes were measured at room temperature in the region of
200–700 nm. The bands which located in the region of 300–
400 nm were involved
group. The electronic intra-ligand
matic rings were observed in the region lower than 300 nm. As it
can be seen, upon complexation these bands were shifted to higher
wavelengths.
p
?
p⁄ transition related to azomethine
p
?
p⁄ transitions related to aro-
3.2.1. 1H NMR and 13C NMR spectra
The NMR spectroscopic data of palladium(II) complexes were
presented in Section 2.3. For 2a, the assignment of the signals
Scheme 1. The structure of Schiff bases and their complexes.

Z. Beigi et al. / Polyhedron 134 (2017) 65–72
69
Fig. 1. The assignment of the aromatic hydrogens in (H–H) COSY.
Fig. 2. The assignment of the aromatic hydrogens in (1H–13C) HSQC.

70
Z. Beigi et al. / Polyhedron 134 (2017) 65–72
In addition, compare to complexes containing phosphorus, the
complexes prepared from tridentate Schiff bases and phosphines
enhanced the catalytic efficiency through the modulating of the
steric crowding around the palladium(II) ion [36]. The
p-acceptor
ability of the phosphine and -donor ability of imine-nitrogen
r
make the metal center more susceptible to catalytic reactions. As
illustrated in Table 3, the best yield was obtained from the [PdL5(-
PPh3)] (5a) complex containing para-methoxy group, while the
worst one was related to the [PdL4(PPh3)] (4a) including para-nitro
group as a catalyst.
Firstly, the solvent effect such as DMF, MeOH, CH3CN, toluene,
and CH2Cl2 in the presence of 0.01 mmol of the catalyst at 70 °C
for 5 h were studied. As could be seen (Table 4), the toluene solvent
for the best solubility of the component with the highest yield and
DMF with the lowest yield were obtained. Similar to reported work
[5], N-donor atoms of DMF compete with the imine of Schiff base
ligand in coordinating to the palladium centre and resultant cata-
lyst could be less active than others. Base is considered to be
important in the Suzuki coupling reaction. The bases, including
Na2CO3, K2CO3, KOH, and Et3N were investigated (Table 5) and
among them, K2CO3 was found to be the best one. The effect of
temperature and the reaction time was also investigated and the
results were listed in Tables 6 and 7. Increasing the temperature
has a significant effect on catalyst performance through increasing
the yield of reaction [5,7]. Also, as shown in Table 8, a wide range of
aryl halides with different functional groups have been assessed. X.
Liu et al. prepared iminopyridylphosphine palladium(II) complexes
which for these complexes containing electron-withdrawing sub-
stituents was investigated in the good yield (75–98%) [37]. in this
work the excellent yields of 80–93% were obtained for the reac-
tants containing electron withdrawing substituents. It can be
inferred that the electronic effects of the substituents on the aryl
bromide show that the electron withdrawing substituents were
more appropriate for the coupling reaction. From the reaction of
aryl halides with different functional groups and boronic acid,
the products of cross coupling (Suzuki reaction) and homo-cou-
pling (HC) of boronic acid were observed at the same time. The
main product with 68–93% yields was obtained from the Suzuki
cross coupling reaction while the biphenyl with 5–24% yield was
homo- coupling (HC) product. Additionally, the catalytic reaction
Scheme 2. The various type of hydrogen atoms in [PdL2(PPh3)] (2a) complex.
3.4. Optimization of the reaction conditions for Suzuki reaction
To check the potency of the catalytic activity of Schiff base pal-
ladium catalysts, they were used in the coupling of bromobenzene
and phenylboronic acid as a model reaction in the Suzuki cross-
coupling reaction. The reaction was achieved at 70 °C by using
K2CO3 as base and the molar ratio of bromobenzene: phenyl-
boronic acid: K2CO3 was 2:3:6.
The catalytic activity of synthesized Palladium(II) complexes
containing Schiff base and triphenylphosphine ligands were inves-
tigated under the same conditions and the results are summarized
in Table 3. Compared with the homogeneous palladium-phosphine
complexes, these complexes are easily synthesized and the use of
Schiff base ligands improved their catalytic properties for Suzuki
coupling reaction [9,35]. The Schiff-base complexes were stable
under the different oxidation states [25], so they have a good effect
in catalytic reactions.
Fig. 3. The crystal structures of the [PdL3(PPh3)].

Z. Beigi et al. / Polyhedron 134 (2017) 65–72
71
Table 8
Suzuki cross-couplings of various aryl halides and phenylboronic acid.
Entry
1
Aryl halide
product
Yield (%) (S/HC)*
95/0
I
2
3
4
89/0
65/0
93/5
Br
Cl
Br
OHC
OHC
5
6
91/8
Br
O2N
O2N
O2N
68/24
Cl
O2N
OHC
7
73/14
Cl
OHC
Reaction conditions: aryl halides (0.5 mmol); phenylboronic acid (0.75 mmol); K2CO3 (1.5 mmol); catalyst (0.01 mmol); toluene (5 ml); T = 70 °C.
*
S = Suzuki product, HC = Homo-coupling product.
was also performed in the absence of aryl halide and 70% yield was
obtained for homo- coupling of boronic acids [38].
Also, this work involved the homo- coupling of phenylboronic acid
without aryl halides as a good yield by common condition of
reaction.
Regarde to Ch. Zhang et al. the Pd2(dba)3 complex had a good
efficiency on coupling of aryl chloride and phenylboronic acid
through the Suzuki reaction. In this complex the imidazol-2-ylide-
nes ligand with high nucleophilic character had a stabilizing effect
over the complex [39]. Similarly to these observation, the [PdL5(-
PPh3)] complex also, is a good catalyst for cross-coupling of aryl
chloride and phenylboronic acid. Although, as shown in (Table 8)
among the coupling reactions, aryl chlorides with high dissociation
energy [25] have lower yields (65–73%), but these results are
noticeable.
Acknowledgements
The authors are grateful to the council of Isfahan University of
Technology (IUT) for providing financial support to undertake this
work. W.P thanks the Deutsche Forschungsgemeinschaft DFG for a
grant (PL 155/14-1).
Appendix A. supplementary data
CCDC 1531112 contains the supplementary crystallographic
data for 3a. These data can be obtained free of charge via http://
dx.doi.org/10.1016/j.poly.2017.06.009, or from the Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ,
UK; fax: (+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
4. Conclusions
In summary, some new palladium(II) complexes containing the
tridentate Schiff base ligands were synthesized and characterized.
The distorted square planer structure was obtained for the [PdL3(-
PPh3)] complex. In this work the Suzuki reaction was carried out in
the presence of an aryl halide and phenyl boronic acid. The yield of
the reactions was obtained via GC analyses. The palladium(II) com-
plexes were used as catalyst for the Suzuki cross-coupling reaction
of several aryl halides with phenylboronic acid. Comparing with
complexes of phosphorus ligands, the advantage of using Schiff
base is that they are air stable and moisture insensitive. Also, tri-
dentate Schiff bases by modulating of the steric around the palla-
dium center are appropriate ligands for catalytic reactions. The
presence of substituents groups on the Schiff base ligands seems
to play a special role in the catalytic activity of the complexes. Also,
excellent yields were obtained in the reaction of aryl halides with
electron withdrawing substituents. The results of the competition
reactions between aryl halides and phenylboronic acid shows that
the desired Suzuki reaction appear as a main product which fol-
lowed by homo-coupling product as a byproduct of the reaction.
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