Synthesis, characterization and catalytic activity of N-heterocyclic carbene ligated Schiff base palladacycles

Article abstract of DOI:10.1016/j.ica.2021.120360

A series of N-heterocyclic carbene (NHC) ligated Schiff base palladacycles were synthesized by the reaction of μ-acetato-bridged Schiff base palladacycles (PdL_1-2) with imidazolium salts (a-c). The new NHC ligated palladacycles were fully characterized using ¹H NMR, ¹³C NMR, infrared spectroscopy, MS analysis, and single-crystal X-ray diffraction for one complex (PdL₁). Further exploration of the catalytic application of the palladacycles for Suzuki-Miyaura cross-coupling reactions of aryl bromides with phenylboronic acid was carried out. It was determined that the new dimeric imine-palladacycle (PdL₂) exhibits the highest catalytic activity among the coupling compounds in this study.

Full text of DOI:10.1016/j.ica.2021.120360

Inorganica Chimica Acta 522 (2021) 120360
Contents lists available at ScienceDirect
Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Research paper
Synthesis, characterization and catalytic activity of N-heterocyclic carbene
ligated Schiff base palladacycles
a
a
b
a,*
¨
˙
Ilknur Babahan , Rukiye Fırıncı , Namık Ozdemir , M. Emin Günay
a Department of Chemistry, Aydın Adnan Menderes University, 09010 Aydın, Turkey
b Department of Mathematics and Science Education, Ondokuz Mayıs University, 55139 Samsun, Turkey
A R T I C L E I N F O
A B S T R A C T
A series of N-heterocyclic carbene (NHC) ligated Schiff base palladacycles were synthesized by the reaction of
Keywords:
Imine-palladacycle
N-heterocyclic carbene
Schiff base
μ
-acetato-bridged Schiff base palladacycles (PdL1-2) with imidazolium salts (a-c). The new NHC ligated palla-
dacycles were fully characterized using 1H NMR, 13C NMR, infrared spectroscopy, MS analysis, and single-crystal
X-ray diffraction for one complex (PdL1). Further exploration of the catalytic application of the palladacycles for
Suzuki-Miyaura cross-coupling reactions of aryl bromides with phenylboronic acid was carried out. It was
determined that the new dimeric imine-palladacycle (PdL2) exhibits the highest catalytic activity among the
coupling compounds in this study.
Imidazol-2-ylidene
Catalysis
Suzuki–Miyaura cross-coupling reaction
1. Introduction
could be combining Schiff base palladacycles with N-heterocyclic car-
benes (NHCs) which are better σ-donor ligands. However, in spite of the
In recent years, Pd-catalyzed processes that assist the formation of
widespread use of Pd(II)-NHCs as catalysts, the number of cyclo-
palladated derivatives with these ligands is rather limited [7–11],
particularly those in which the metallated moiety is a Schiff base and an
NHC ligand [12,13]. Some of these have shown to be active pre-catalysts
in the Suzuki-Miyaura, Heck-Mizoroki, Buchwald-Hartwig reactions
[14–16].
–
C
C bonds have seen great progress [1] and reaction such as Suzuki-
Miyaura cross-coupling is employed extensively in synthetic routes for
a large number of natural products, new materials, and pharmaceuticals
[2]. The use of cyclopalladated complexes, sometimes called pallada-
cycles featuring bidentate ligands with a general structure C-X (X = N, P,
S, etc.), as precatalysts have led to significant advances [3]. In this re-
gard, phospha-containing palladacycles and Pd(II)-N-heterocyclic car-
Schiff bases are well known as multidentate ligands in catalytic re-
actions among varied N-based ligands. According to the literature, there
have been numerous studies regarding Schiff base ligands employment
in palladium-catalyzed Suzuki-Miyaura cross-coupling reactions [17].
The utilization of nitrogen-containing ligands particularly Schiff base
ligands in the Suzuki-Miyaura reaction instead of phosphine ligands is
getting popular. Since nitrogen-based ligands are cheap, air-stable,
having an easy handle and synthesis, they are preferred by researchers
for the Suzuki-Miyaura reaction. It is favorable for the researchers that
Schiff bases are obtained by a simple synthetic route with the com-
mercial compounds. It is also worth mentioning that they also display
high performance under a wide range of redox reactions and having
readily varied electronic and steric properties [18–21]. With regard to
imine-based palladacycles, Weissman and Milstein first reported the use
of a dinuclear phosphine-free complex in the Suzuki coupling of
nonactivated aryl bromides [22]. Bedford and co-workers then reported
that the phosphine adducts of imine-based palladacycles exhibited
–
C
bene (NHC) complexes were successfully used as catalysts for C
coupling reactions. Since the first phosphapalladacyles were empha-
sized by Herrmann et al. [4], many new palladacyclic complexes have
arisen. In addition to these complexes, “pincer” palladacycles containing
functional groups bound by N, P, or S have also shown their efficiency in
the Suzuki-Miyaura cross-coupling reaction [5].
Many groups have focused their research on the modification of
palladacycles in order to tune and improve their catalytic properties.
The electronic and steric properties of palladacycles are influenced by
various factors, such as the size of the metallocyclic ring, the type of the
donor atoms, and the nature of the ancillary ligand [6]. The activities of
the palladacycles can often be modulated by ancillary ligands; the role is
frequently performed by tertiary phosphines. However, the latter at
present a certain disadvantage such as that they are the difficulties in
removing the ligands and their degradation. An interesting approach
* Corresponding author.
E-mail address: megunay@adu.edu.tr (M. Emin Günay).
https://doi.org/10.1016/j.ica.2021.120360
Received 27 November 2020; Received in revised form 16 March 2021; Accepted 22 March 2021
Available online 26 March 2021
0020-1693/© 2021 Elsevier B.V. All rights reserved.

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I. Babahan et al.
Inorganica Chimica Acta 522 (2021) 120360
higher catalytic activities than dinuclear phosphine-free palladacycles in
Suzuki coupling and elegantly highlighted the role played by the
phosphine and the anionic X ligand in terms of both catalyst activity and
lifetime [23]. The imine palladacycles have known as highly stabile
catalysts for many cross-coupling reactions. Additionally, the imine
palladacycles have the potential to be transformed into a diverse range
of functionalized product. Therefore, this ability makes them easily
recoverable and recyclable catalysts. These imine palladacycles are
synthesized by the complexation of Pd precursors with imine which is
synthesized via a condensation reaction between amines and aldehydes
or ketones [24].
Ar
N
N
N
a: -C6H2-(2,4,6-CH3)
b: -C6H-(2,3,5,6-CH3)
c: -C6-(2,3,4,5,6-CH3)
BrCH2Ar
RT
+
H Br-
N
Ar
a-c
Scheme 1. Synthesis of imidazolium salts (a-c).
In this study, we aimed to obtain new coupling reagents that can
work more efficiently with Pd-catalysts at very low metal loads in the
Suzuki-Miyaura reaction under milder reaction conditions. For this
Hz, NCHCHNCH3), 6.90 (s, 2H, C6H2(CH3)3), 7.46 (d, 1H, J = 1.6 Hz,
NCHCHNCH3), 10.16 (s, 1H, NCHN). 13C NMR (100 MHz, CDCl3): δ =
20.0 (C6H2(CH3)3), 21.2 (C6H2(CH3)3), 37.2 (NCH3), 48.1
(NCH2C6H2(CH3)3), 120.9 (NCHCHNCH3), 124.0 (NCHCHNCH3),
125.5, 130.1, 137.0, 138.3 (C6H2(CH3)3), 140.1 (NCHN). Elemental
analyses (%) calc. for C14H19BrN2 (Mw : 295.2181): C, 56.96; H, 6.49; N,
9.49; found: C, 56.89; H, 6.27; N, 9.55.
purpose, two new acetato-bridge imine-palladacyclic dimers (PdL1-2
)
from Schiff base ligands (L1-2) were first synthesized. Subsequently,
these two acetato-bridge imine-palladacyclic complexes synthesized
were reacted with NHC precursors (a-c) to obtain NHC ligated Schiff
base palladacycles (PdL1a-c and PdL2a-c). Finally, catalytic activities of
all synthesized complexes in the Suzuki-Miyaura cross-coupling reaction
were investigated. The Suzuki-Miyaura reaction has proved on the uti-
lization of these complexes (PdL1-2, PdL1a-c, and PdL2a-c) as an effective
precatalyst for the cross-coupling of aryl bromides with phenylboronic
acid at 82 ◦C temperature. It can be said that both imine-palladacycles
(PdL1-2) and N-heterocyclic carbene ligated Schiff base palladacycles
(PdL1a-c and PdL2a-c) displayed a very important role in this situation.
This study has been also included preliminary results on the very effi-
cient performance of Schiff base palladacycle (PdL1-2) as precatalysts in
arylation reactions of aryl bromides.
Compound b
Yield: 5.13 g, 83%, m.p.: 123–125 ◦C. Color: White. 1H NMR (400
MHz, CDCl3): δ = 2.16 (s, 6H, C6H(CH3)4), 2.21 (s, 6H, C6H(CH3)4), 4.09
(s, 3H, NCH3), 5.59 (s, 2H, NCH2C6H(CH3)4), 6.90 (d, 1H, J = 1.6 Hz,
NCHCHNCH3), 7.01 (s, 1H, C6H(CH3)4), 7.51 (d, 1H, J = 1.6 Hz,
NCHCHNCH3), 10.02 (s, 1H, NCHN). 13C NMR (100 MHz, CDCl3): δ =
16.1 (C6H(CH3)4), 20.6 (C6H(CH3)4), 37.3 (NCH3), 48.8 (NCH2C6H
(CH3)4), 121.1 (NCHCHNCH3), 123.8 (NCHCHNCH3), 128.1, 133.8,
134.3, 135.2 (C6H(CH3)4), 137.2 (NCHN). Elemental analyses (%) calc.
for C15H21BrN2 (Mw : 309.2447): C, 58.26; H, 6.84; N, 9.06; found: C,
58.35; H, 6.57; N, 9.23.
2. Experimental
Compound c
Yield: 6.08 g, 94%, m.p.: 192–194 ◦C. Color: White. 1H NMR (400
MHz, CDCl3): δ = 2.17 (s, 6H, C6(CH3)5), 2.18 (s, 6H, C6(CH3)5), 2.21 (s,
3H, C6(CH3)5), 4.08 (s, 3H, NCH3), 5.57 (s, 2H, NCH2C6(CH3)5), 6.91 (d,
1H, J = 1.6 Hz, NCHCHNCH3), 7.54 (d, 1H, J = 1.6 Hz, NCHCHNCH3),
9.94 (s, 1H, NCHN). 13C NMR (100 MHz, CDCl3): δ = 17.0 (C6(CH3)5),
17.1 (C6(CH3)5), 17.4 (C6(CH3)5), 37.2 (NCH3), 49.3 (NCH2C6(CH3)5),
121.1 (NCHCHNCH3), 123.9 (NCHCHNCH3), 125.4, 133.8, 133.9, 136.9
(C6(CH3)5), 137.5 (NCHN).). Elemental analyses (%) calc. for
C16H23BrN2 (Mw : 323.2713): C, 59.45; H, 7.17; N, 8.67; found: C, 59.38;
H, 7.07; N, 8.74.
2.1. Material and general methods
All reagents used were purchased from Merck. All reactions carried
out under Argon in flame-dried glassware using a standard Schlenk type
flask. Solvents were dried and freshly distilled prior to use. All other
chemicals were used as received. Benzyl bromide derivatives were
synthesized according to the literature [25]. 1H and 13C NMR spectrum
of compounds was collected on Varian AS 400 spectrometer operating at
400 and 100 MHz, respectively. Chemical shifts were measured in ppm
relative to tetramethylsilane for 1H and 13C NMR spectra. All catalytic
reactions were monitored with an Agilent 6890 N GC system using GC
flame ionization detection with an HP-5 column of 30 m length, 0.32
2.3. Synthesis of the Schiff base ligands (L1 and L2)
mm diameter, and 0.25
μm film thickness. Melting points were
measured in open capillary tubes with a Stuart SMP 30 melting point
apparatus. Single-crystal X-ray diffraction data of PdL1 was recorded
with an STOE IPDS II diffractometer.
The Schiff base ligands were synthesized according to previous re-
ports as shown in Scheme 2 [26]. 4-(diethylamino)benzaldehyde/4-
(dimethylamino)benzaldehyde ethanolic solution was refluxed with 2,4-
dimethylaniline/2,4-dimethoxylaniline (1:1 mmol ratio) with a few
drops of an acetic acid catalyst, respectively. The resulting yellow pre-
cipitates of the Schiff base ligands were filtered, and then the yellow
products were washed with the cold ethanol–water mixture and
recrystallized from ethanol.
2.2. General method of synthesis of imidazolium salts (a-c)
The imidazolium salts were synthesized prepared according to pre-
vious reports [10]. The benzyl bromide (2,4,6-trimethylbenzyl-, 2,3,5,6-
tetramethylbenzyl- and 2,3,4,5,6-pentamethylbenzyl bromides) de-
rivatives (20 mmol) and 1-methylimidazole (1.6 mL, 20 mmol) were
stirred in toluene (15 mL) for 3 h at room temperature as shown in
Scheme 1. The volume of the solution was reduced to 5 mL, diethyl ether
was added to the remaining solution, which was vigorously shaken and
then decantated. The solid residue was washed with diethyl ether (3x20
mL) to obtain a white solid, which was recrystallized from ethanol/
diethyl ether (3 mL/15 mL).
2.4. Synthesis of the PdL1 and PdL2 complexes
A solution of Schiff base ligands (1 mmol L1 or L2), which had been
synthesized according to the previous studies, [26] were reacted with
palladium(II) acetate (1 mmol) dissolved in 10 mL of dry acetonitrile as
shown in Scheme 3. Subsequently, the reaction was refluxed under Ar
atmosphere. Finally, yellow precipitates were formed after 2 h. The
crystals of the final product were collected by recrystallization from
dichloromethane and diethylether as seen in Fig. 1 and, characterized by
using FT-IR, 1H and 13C NMR, and single-crystal X-ray diffraction.
PdL1: Yield: 75%, m.p.: 230–231 ◦C. Color: Red. FT-IR (KBr cmꢀ 1):
Compound a
Yield: 5.37 g, 91%, m.p.: 117–118 ◦C. Color: White. 1H NMR (400
MHz, CDCl3): δ = 2.26 (s, 6H, C6H2(CH3)3), 2.38 (s, 3H, C6H2(CH3)3),
4.10 (s, 3H, NCH3), 5.53 (s, 2H, NCH2C6H2(CH3)3), 6.87 (d, 1H, J = 1.6
2

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Inorganica Chimica Acta 522 (2021) 120360
R3,4
R1,2
R3,4
R1,2
EtOH
N
N
H2N
CHO
R3,4
+
N
R3,4
R1,2
R1,2
L
L
1: R1 = -CH2CH3, R3 = -CH3
2: R2 = -CH3, R4 = -OCH3
Scheme 2. Synthesis of the Schiff base ligands (L1 and L2).
R3,4
R1,2
R1,2
R3,4
dry CH3CN
reflux
N
+
Pd(OAc)2
N
N
R3,4
N
R3,4
R1,2
R1,2
Pd
OAc
AcO
L
L
1: R1 = -CH2CH3, R3 = -CH3
2: R2 = -CH3, R4 = -OCH3
2
PdL1/PdL2
Scheme 3. Synthesis of the imine-palladacycles (PdL1 and PdL2 complexes).
OCH3), 2.87 (s, 6H, –COO-CH3), 3.75 [s, 6H, (CH3)2NC6H3)], 5.91, 6.30
(s, 2H, Ar-H), 6.28, 7.07 (d, 2H, J = 8 Hz, Ar-H), 5.97, 6.22 (d, 2H, J =
7.4 Hz, Ar-H), 7.48 (s, 1H, –CH = N). 13C NMR (100 MHz, CDCl3): δ =
55.4 (C6H3(OCH3)2-p-OCH3), 55.9 (C6H3(CH3)2-o-CH3), 40.0
(CH3)2NC6H3), 24.2 (–CO-CH3), 98.2, 102.9, 106.5, 115.2, 125.4, 128.4,
–
131.6, 134.4, 149.9, 153.6, 157.1, 158.3 (Ar–C), 173.2 (-C N), 179.3
–
(–CO-). HRMS (ESI): m/z calcd for [C38H44N4O8Pd2]: 897.6180 (M±),
found: 897.8201.
2.5. Crystal structure determination of PdL1
X-ray diffraction data of PdL1 were recorded with an STOE IPDS II
diffractometer at room temperature using graphite-monochromated Mo
Kα radiation by applying the ω-scan method. Data collection and cell
refinement were carried out using X-AREA [27] while data reduction
was applied using X-RED32 [27a]. The structures were solved by direct
methods with SIR2019 [27b] and refined by means of the full-matrix
full-matrix least-squares calculations on F2 using SHELXL-2018 [27c].
All H atoms were located in difference electron-density map and then
treated as riding atoms in geometrically idealized positions, fixing the
bond lengths at 0.93, 0.97, and 0.96 Å for CH, CH2, and CH3 atoms,
respectively. The displacement parameters of the H atoms were fixed at
Fig. 1. Molecular structure of PdL1 showing the atom numbering scheme.
Displacement ellipsoids are drawn at the 30% probability level. Hydrogens
have been omitted for clarity and only the major part of disordered fragment
is drawn.
U
iso(H) = 1.2Ueq (1.5Ueq for methyl) of their parent atoms. In the
complex, one of the two diethylamide moieties was disordered over two
positions, and the refined site-occupancy factors of the disordered part
are 0.635(19) and 0.365(19)% for N4/C37-C40. The disordered atoms
were refined using the SIMU, DELU, and SAME restraints of SHELXL-
2018, which resulted in the best parameters. The crystallographic data
and refinement parameters are summarized in Table 1. The molecular
graphic was created by using OLEX2 [27d].
1
–
–
–
–
1565 (C N), 1515 (C C), 1203 (C O). H NMR (400 MHz, CDCl3): δ
–
= 1.17 (t, 6H, (CH3CH2)2NC6H3), 1.77 (s, 3H, C6H3(CH3)2-p-CH3), 2.21
(s, 3H, C6H3(CH3)2-o-CH3), 2.78 (s 6H, –COO-CH3), 3.23–3.36 (t, 4H,
(CH3CH2)2NC6H3), 5.88, 6.83 (s, 2H, Ar-H), 6.34, 6.98 (d, 2H, J = 8 Hz,
Ar-H), 6.43, 6.58 (d, 2H, J = 7.4 Hz, Ar-H), 7.38 (s, 1H, –CH = N). 13
C
NMR (100 MHz, CDCl3):
δ
= 12.6 (C6H3(CH3)2-p-CH3), 18.3
(C6H3(CH3)2-o-CH3), 21.0 (CH3CH2)2NC6H3), 23.9 (–CO-CH3), 44.3
(CH3CH2)2NC6H3), 106.0, 114.9, 126.1, 128.2, 128.8, 129.0, 130.0,
2.6. General methods of synthesis of the PdL1a-c and PdL2a-c
–
–
133.2, 134.7, 145.7, 148.4, 157.1 (Ar–C), 171.5 (-C N), 179.4 (–CO-).
HRMS (ESI): m/z calcd for [C42H52N4O4Pd2]: 889.7267 (M±), found
A sample of PdL1 or PdL2 (0.20 mmol) was refluxed with one of the
compounds of a-c (0.40 mmol) in toluene (10 mL) at 110 ◦C for 2 h. The
solvent was removed in vacuo, the remaining precipitate was then dis-
solved in dichloromethane (2 mL) and recrystallization from dichloro-
methane/toluene afforded the complexes of PdL1a-c and PdL2a-c. The
crystals of these complexes were characterized by using FT-IR, 1H and
889.3178.
PdL2: Yield: 72%, m.p. 250–251 ◦C. Color: Brown. FT-IR (KBr cmꢀ 1):
1
–
–
–
–
–
1566 (C N), 1528 (C C), 1202 (C O). H NMR (400 MHz, CDCl3): δ
= 1.82 (s, 3H, C6H3(OCH3)2-p-OCH3), 3.75 (s, 3H, C6H3(OCH3)2-o-
3

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Inorganica Chimica Acta 522 (2021) 120360
13C NMR as shown in Scheme 4.
Table 1
Crystal data and structure refinement parameters for PdL1.
Compound PdL1a
CCDC depository
1,947,705
Yield: 68%, m.p.:190–191 ◦C. Color: Brown. FT-IR (KBr cmꢀ 1): 1568
Color/shape
Red/prism
(C N), 1521 (C C). 1H NMR (400 MHz, CDCl3): δ = 1.06 (s, 3H,
–
–
–
–
Chemical formula
Formula weight
Temperature (K)
Wavelength (Å)
Crystal system
Space group
[Pd2(C19H23N2)2(C2H3O2)2]
C6H3(CH3)2-p-CH3), 1.08, 2.31 (t, 6H, (CH3CH2)2NC6H3), 2.27 (s, 3H,
C6H2(CH3)3-p-CH3), 2.51 (s, 3H, C6H3(CH3)2-o-CH3), 3.98 (s, 3H,
C6H2(CH3)3-o-CH3), 3.23 (q, 4H, (CH3CH2)2NC6H3), 4.12 (s, 3H,
–NCH3), 5.34, 5.80 (d, 2H, J = 6.6 Hz, –NCH2-Ar), 5.84, 6.32 (d, 2H, J =
14.0 Hz, –NCHCHNCH3), 5.74, 6.84, 6.90 (s, 3H, Ar-H), 6.34, 6.90 (d,
2H, J = 8.0 Hz, Ar-H), 5.44, 6.39 (d, 2H, J = 7.4 Hz, Ar-H), 7.89 (s, 1H,
–CH = N). 13C NMR (100 MHz, CDCl3): δ = 12.7 (C6H3(CH3)2-p-CH3),
21.0 (C6H2(CH3)3-p-CH3), 19.5 (C6H3(CH3)2-o-CH3), 20.2 (C6H2(CH3)3-
o-CH3), 20.8 (CH3CH2)2NC6H3), 38.7 (–NCH3), 44.4 (CH3CH2)2NC6H3),
49.5 (-N-CH2-Ar), 121.1, 122.5 (–NCHCHNCH3), 106.3, 117.7, 119.2,
126.6, 128.0, 129.2, 130.1, 130.9, 131.9, 134.7, 135.3, 138.4, 138.6,
889.67
296(2)
0.71073 Mo K
Triclinic
α
P1 (No. 2)
Unit cell parameters
a, b, c (Å)
11.4991(11), 12.7569(12), 14.5849(14)
α
, β, γ (◦)
75.288(7), 80.481(7), 86.757(8)
Volume (Å3)
2040.6(3)
2
Z
D
calc. (g/cm3)
1.448
μ
(mmꢀ 1
)
0.926
Absorption correction
Integration
0.7200, 0.8911
912
–
–
147.4, 149.3, 161.2 (Ar–C), 173.9 (-C N),186.5 (Pd–Ccarbene). HRMS
T
min., Tmax.
(ESI): m/z calcd. for [C33H41BrN4Pd]±: 680.0294 (M±), 601.2370 (M±-
F000
Crystal size (mm3)
Diffractometer
0.41 × 0.28 × 0.14
STOE IPDS II
Br), found; 601.1334 (M±-Br).
Measurement method
Index ranges
ω scan
Compound PdL1b
ꢀ 13 ≤ h ≤ 13, ꢀ 15 ≤ k ≤ 15, ꢀ 17 ≤ l ≤ 17
2.131 ≤ θ ≤ 25.049
28,592
Yield: 65%, m.p.: 227–228 ◦C. Color: Brown. FT-IR (KBr cmꢀ 1): 1570
θ range for data collection (◦)
Reflections collected
Independent/observed reflections
Rint.
(C N), 1520 (C C). 1H NMR (400 MHz, CDCl3): δ = 1.06 (s, 3H,
–
–
–
–
7220/4086
C6H3(CH3)2-p-CH3), 1.08, 2.18 (t, 6H, (CH3CH2)2NC6H3), 1.57 (s, 6H,
C6H(CH3)4-m-CH3), 2.50 (s, 3H, C6H3(CH3)2-o-CH3), 3.99 (s, 6H, C6H
(CH3)4-o-CH3), 3.24 (q, 4H, (CH3CH2)2NC6H3), 4.13 (s, 3H, –NCH3),
5.43, 5.90 (d, 2H, J = 6.6 Hz, –NCH2-Ar), 5.82, 6.32 (d, 2H, J = 12.0 Hz,
–NCHCHNCH3), 5.87, 6.83, 7.02 (s, 3H, Ar-H), 6.34, 6.96 (d, 2H, J = 8.0
Hz, Ar-H), 5.45, 6.39 (d, 2H, J = 7.4 Hz, Ar-H), 7.90 (s, 1H, –CH = N).
13C NMR (100 MHz, CDCl3): δ = 12.7 (C6H3(CH3)2-p-CH3), 16.2 (C6H
(CH3)4-m-CH3), 19.5 (NC6H3(CH3)2-o-CH3), 20.4 (C6H(CH3)4-o-CH3),
21.0 (CH3CH2)2NC6H3), 38.6 (–NCH3), 44.4 (CH3CH2)2NC6H3, 50.2 (-N-
CH2-Ar), 120.9, 122.5 (NCHCHNCH3), 106.3, 117.7, 119.5, 126.6,
130.1, 130.8, 130.9, 131.9, 132.2, 134.2, 134.6, 134.7, 135.3, 147.4,
0.1510
Refinement method
Data/restraints/parameters
Goodness-of-fit on F2
Full-matrix least-squares on F2
7220/83/527
0.951
Final R indices [I > 2
R indices (all data)
σ
(I)]
R1 = 0.0652, wR2 = 0.1352
R1 = 0.1197, wR2 = 0.1558
1.13, ꢀ 0.51
Δ
ρmax., Δρmin. (e/Å3)
Table 2
Selected geometric parameters for PdL1.
Bond lengths (Å)
–
149.3, 161.2 (Ar–C), 173.8 (-C N), 186.4 (Pd–Ccarbene). HRMS (ESI): m/
–
z calcd. for [C34H43BrN4Pd]±: 694.0560 (M±), 615.2527 (M±-Br),
Pd1─C15
1.977(7)
2.027(6)
2.035(5)
2.137(5)
Pd2─C36
Pd2─N3
Pd2─O2
Pd2─O3
1.956(7)
2.027(6)
2.070(5)
2.137(5)
found; 615.8601 (M±-Br).
Pd1─N1
Pd1─O4
Pd1─O1
Bond angles (◦)
C15─Pd1─N1
C15─Pd1─O4
N1─Pd1─O4
C15─Pd1─O1
N1─Pd1─O1
O4─Pd1─O1
Compound PdL1c
Yield: 62%, m.p. 266–267 ◦C. Color: Brown. FT-IR (KBr cmꢀ 1): 1567
81.0(3)
C36─Pd2─N3
C36─Pd2─O2
N3─Pd2─O2
C36─Pd2─O3
N3─Pd2─O3
O2─Pd2─O3
80.7(3)
92.3(3)
172.3(2)
174.5(3)
95.0(2)
91.7(2)
(C N), 1518 (C C). 1H NMR (400 MHz, CDCl3): δ = 1.06 (s, 3H,
92.8(3)
–
–
–
–
173.7(2)
174.3(3)
95.0(2)
C6H3(CH3)2-p-CH3), 1.08, 2.32 (t, 6H, (CH3CH2)2NC6H3), 1.60 (s, 6H,
C6(CH3)5-m-CH3), 2.27 (s, 6H, C6(CH3)5-p-CH3), 2.49 (s, 3H,
C6H3(CH3)2-o-CH3), 3.98 (s, 6H, C6(CH3)5-o-CH3), 3.23 (q, 4H,
(CH3CH2)2NC6H3), 4.12 (s, 3H, (–NCH3), 5.45, 5.86 (d, 2H, J = 6.6 Hz,
(–NCH2-Ar), 5.84, 6.33 (d, 2H, J = 12.0 Hz, (–NCHCHNCH3), 5.88, 6.82
(s, 2H, Ar-H), 6.33, 6.96 (d, 2H, J = 8.0 Hz, Ar-H), 5.48, 6.71 (d, 2H, J =
7.4 Hz, Ar-H), 7.90 (s, 1H, –CH = N). 13C NMR (100 MHz, CDCl3): δ =
91.30(19)
R1,2
R3,4
R1,2
R3,4
N
+
N
PhMe
reflux
N
H Br-
N
R3,4
+
N
R3,4
R1,2
N
R1,2
Pd
Pd
Ar
Ar
OAc
AcO
N
Br
2
a-c
N
PdL1/PdL2
Ar
a: -C6H2-(2,4,6-CH3)
b: -C6H-(2,3,5,6-CH3)
c: -C6-(2,3,4,5,6-CH3)
PdL1a-c/PdL2a-c
Scheme 4. Synthesis of the imine-palladacycles (PdL1a-c and PdL2a-c).
4

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I. Babahan et al.
Inorganica Chimica Acta 522 (2021) 120360
12.6 (C6H3(CH3)2-p-CH3), 12.8 (C6(CH3)5-p-CH3), 17.1 (C6(CH3)5-m-
CH3), 19.4 (C6H3(CH3)2-o-CH3), 20.2 (C6(CH3)5-o-CH3), 21.0
(CH3CH2)2NC6H3), 38.5 (–NCH3), 44.3 (CH3CH2)2NC6H4), 50.2 (-N-
CH2-Ar), 122.3, 121.2 (–NCHCHNCH3), 106.0, 126.6, 128.7, 130.0,
130.9, 133.0, 133.2, 134.2, 134.3, 134.7, 135.3, 135.8, 147.4, 148.4,
with palladium(II) complexes (PdL1, PdL2, PdL1a-c, and Pd2a-c, 0.5 mol
%), Cs2CO3 (1.5 mmol), phenylboronic acid (0.75 mmol), aryl bromide
(0.5 mmol), diethylene glycol di-n-butyl ether as an internal standard
(0.3 mmol) in iPrOH (IPA, 3.0 mL). The flask was placed in a pre-heated
oil bath at 82 ◦C. The reaction was followed by GC at a selected period of
times (0.5 h, 1 h, 2 h, 4 h). After the sample was taken from the flask,
iPrOH (2.0 mL) was added to the mixture, centrifuged and the solution
was transferred to vials. The yields of the product obtained (based on the
aryl bromides) were estimated by the area of GC chromatogram peaks.
–
149.3, 161.3 (Ar–C), 173.5 (-C N), 186.1 (Pd–Ccarbene). HRMS (ESI): m/
–
z calcd for [C35H45BrN4Pd]±: 708.0826 (M±), 628.1786 (M±-Br), found;
628.3020 (M±-Br).
Compound PdL2a
Yield: 58%, m.p.: 150–151 ◦C. Color: Brown. FT-IR (KBr cmꢀ 1): 1568
3. Results and discussion
(C N), 1528 (C C). 1H NMR (400 MHz, CDCl3): δ = 2.43 (s, 3H,
–
–
–
–
C6H3(OCH3)2-p-OCH3), 2.36 (s, 6H, (CH3)2NC6H3), 2.18 (s, 6H,
C6H2(CH3)3-p-CH3), 2.83 (s, 3H, C6H3(OCH3)2-o-OCH3), 3.87 (s, 6H,
C6H2(CH3)3-o-CH3), 4.02 (s, 3H, –NCH3), 5.49, 5.81 (d, 2H, J = 6.6 Hz,
–NCH2-Ar), 5.84, 6.30 (d, 2H, J = 12.0 Hz, –NCHCHNCH3), 5.81, 6.93,
7.38 (s, 3H, Ar-H), 6.52, 6.92 (d, 2H, J = 8.0 Hz, Ar-H), 5.58, 6.69 (d,
2H, J = 7.4 Hz, Ar-H), 8.04 (s, 1H, –CH = N). 13C NMR (100 MHz,
CDCl3): δ = 20.4 (C6H2(CH3)3-p-CH3), 21.2 (C6H2(CH3)2-o-CH3), 40.3
((CH3)2NC6H3), 55.6 (C6H3(OCH3)2-p-OCH3), 56.3 (s, 3H,
C6H3(OCH3)2-o-OCH3), 39.0 (–NCH3), 49.8 (-N-CH2-Ar), 121.8, 121.4
(–NCHCHNCH3), 100.0, 103.8, 107.1, 111.2, 118.4, 119.4, 126.4,
In this study, initially, the synthesized Schiff base ligands (L1-2),
derived from 4-(diethylamino)benzaldehyde/4-(dimethylamino)benz-
aldehyde and 2,4-dimethylaniline/2,4-dimethoxylaniline, were reacted
with palladium(II) acetate to give two novel palladacycle dimers (PdL1
and PdL2, Fig. 1), which are stable in the air. The structure of PdL1 has
been also determined by single-crystal X-ray diffraction as shown in
Fig. 1. Palladacycle dimers having acetato-bridged can be readily
divided by using N-heterocyclic carbenes (NHCs) to form novel mono-
meric complexes. Therefore, N-heterocyclic carbenes (NHCs (a-c),
Scheme 1) were easily reacted with the dinuclear palladium(II) acetate
(PdL1 and PdL2) complexes. The popular approach is to combine pal-
ladacycles as convenient Pd sources and special ligands in the catalytic
system, e.g. NHC ligands, and to use a preformed 1:1 complex. These
complexes are air-stable, and thus are more convenient in handling than
toxic phosphines derivatives. The general route to the target pallada-
cyclic complexes bearing NHC ligand is shown in Scheme 4 and the
structures of the new catalysts are given in Scheme 5.
128.5, 129.6, 130.3, 138.9, 138.9, 153.3, 158.7, 161.2, 169.5 (Ar–C),
–
174.5 (-C N), 186.1 (Pd–Ccarbene). HRMS (ESI): m/z calcd. for
–
[C31H37BrN4O2Pd2]+: 683.9751 (M+), 603.1951, found; 603.1932 (M±-
Br).
Compound PdL2b
Yield: 64%, m.p.: > 300 ◦C. Color: Brown. FT-IR (KBr cmꢀ 1): 1564
(C N), 1528 (C C). 1H NMR (400 MHz, CDCl3): δ = 2.18 (s, 3H,
Mononuclear N-heterocyclic carbene-based imine palladacycles
(PdL1a-c and PdL2a-c) were prepared from the imidazolium salts (a-c) by
in situ deprotonation with palladium dimer [10]. Both the bridge-
splitting reaction and replacement of acetato with bromide reaction
occurred together to form new mononuclear NHC ligated C,N-pallada-
cycle complexes (PdL1a-c and PdL2a-c). GC was used to evaluate the
activity of the novel Pd(II) complexes in the Suzuki–Miyaura reaction for
the coupling of phenylboronic acid with aryl bromides as seen in
Table 3.
–
–
–
–
C6H3(OCH3)2-p-OCH3), 2.27 (s, 6H, (CH3)2NC6H3), 1.71 (s, 6H, C6H
(CH3)4-m-CH3), 2.20 (s, 3H, C6H3(OCH3)2-p-OCH3), 2.85 (s, 3H,
C6H3(OCH3)2-o-OCH3), 3.85 (s, 6H, C6H(CH3)4-o-CH3), 3.99 (s, 3H,
–NCH3), 5.49, 5.88 (d, 2H, J = 6.6 Hz, –NCH2-Ar), 5.84, 6.34 (d, 2H, J =
12.0 Hz, –NCHCHNCH3), 5.52, 6.82, 7.00 (s, 3H, Ar-H), 6.52, 6.82 (d,
2H, J = 8.0 Hz, Ar-H), 5.63, 6.72 (d, 2H, J = 7.4 Hz, Ar-H), 8.05 (s, 1H,
(–CH = N). 13C NMR (100 MHz, CDCl3): δ = 19.4 (C6H(CH3)4-o-CH3),
15.1
(C6H(CH3)4-m-CH3), 55.6 (C6H3(OCH3)2-p-OCH3), 56.2
(C6H3(OCH3)2-o-OCH3), 40.3 ((CH3)2NC6H3), 38.9 (–NCH3), 49.1 (-N-
CH2-Ar), 121.0, 121.9 (–NCHCHNCH3), 102.6, 114.5, 118.5, 120.9,
130.2, 130.7, 131.3, 133.6, 134.7, 138.6, 138.2, 143.8, 147.6, 149.5,
3.1. Preparation of the imidazolium salts (a-c)
–
–
161.3, 166.8 (Ar–C), 174.2 (-C N), 186.2 (Pd–Ccarbene). HRMS (ESI): m/
The general pathway to the target NHC ligand precursors and pal-
ladacyclic complexes are shown in Scheme 1. Unsymmetrical imidazo-
lium salts (a–c) were synthesized by the reaction of 1-methylimidazole
with 2,4,6-trimethylbenzyl, 2,3,5,6-tetramethylbenzyl or 2,3,4,5,6-pen-
tamethylbenzyl bromides, respectively (Scheme 1). The imidazolium
protons display at 10.16, 10.02, and 9.94 ppm in the 1H NMR spectra of
(a–c), respectively. The 13C NMR shift of the NCN sp2 carbon atoms in
(a–c) appears between 140.1 and 137.5 ppm, respectively. As expected,
signals at 48.1–49.3 ppm correspond to the benzylic methylene carbon
are observed for (a–c). The NMR spectroscopic data of (a–c) confirms
the proposed structures and also the 1H and 13C NMR chemical shift data
are consistent with those reported in the literature [28].
z calcd. for [C32H39BrN4O2Pd]±: 698.0017 (M±), 617.2108 (M±-Br),
found; 617.2090 (M±-Br).
Compound PdL2c
Yield: 60%, m.p.: 253–254 ◦C. Color: Brown. FT-IR (KBr cmꢀ 1): 1571
(C N), 1529 (C C). 1H NMR (400 MHz, CDCl3):δ = 2.20 (s, 3H,
–
–
–
–
(C6H3(OCH3)2-p-OCH3), 2.36 (s, 6H, (CH3)2NC6H3), 1.82 (s, 6H,
C6(CH3)5-m-CH3), 2.20 (s, 3H, C6(CH3)5-p-CH3), 2.84 (s, 3H,
C6H3(OCH3)2-o-OCH3), 3.88 (s, 3H, C6(CH3)5-o-CH3), 4.00 (s, 3H,
NCH3), 5.51, 5.91 (d, 2H, J = 6.6 Hz, –NCH2-Ar), 5.84, 6.32 (d, 2H, J =
12.0 Hz, –NCHCHNCH3), 5.87, 6.82, 7.38 (s, 3H, Ar-H), 6.54, 6.82 (d,
2H, J = 8.0 Hz, Ar-H), 5.52, 6.41 (d, 2H, J = 7.4 Hz, Ar-H), 8.04 (s, 1H,
–CH = N). 13C NMR (100 MHz, CDCl3): δ = 17.0 (C6(CH3)5-p-CH3), 17.3
(C6(CH3)5-o-CH3), 20.0 (C6(CH3)5-m-CH3), 55.6 (C6H3(OCH3)2-p-
OCH3), 56.3 (C6H3(OCH3)2-o-OCH3), 40.2 ((CH3)2NC6H3), 38.9
(–NCH3), 50.9 (-N-CH2-Ar), 121.1, 121.2 (–NCHCHNCH3), 100.1, 103.9,
107.0, 115.9, 118.8, 119.9, 126.7, 128.5, 129.2, 130.3, 133.2, 134.4,
3.2. Synthesis of and characterization of PdL1-2, PdL1a-c and PdL2a-c
3.2.1. FT-IR spectra
It is well known that the Schiff base based ligands and their metal
complexes are seen that the coordination of the azomethine nitrogen to
the palladium metal produces the shift of imine stretching frequencies to
the lower value in the FT-IR spectra. On complexation, this band was
–
135.9, 138.3, 142.3, 158.7 (Ar–C), 174.4 (-C N), 186.6 (Pd–Ccarbene).
–
HRMS (ESI): m/z calcd. for [C33H41BrN4O2Pd]±: 712.0282 (M±),
631.2264 (M±-Br), found; 631.2244 (M±-Br).
shifted to lower frequency values in all the complexes [29]. Therefore, it
–
was observed that the Schiff base ligand (L1-2) showed ν(C
azome-
N)
–
2.7. General procedure for Suzuki-Miyaura cross-coupling reaction
thine band at 1618 cmꢀ 1 for L1 and 1606 cmꢀ 1 for L2 and, palladium
metal complexes displayed ν(C N) azomethine bands at 1565 cmꢀ 1 for
–
–
Under Argon atmosphere, a two-necked 25 mL flask was charged
PdL1 and 1566 cmꢀ 1 for PdL2. These results comply with the previous
5

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Inorganica Chimica Acta 522 (2021) 120360
N
N
N
N
N
N
Pd
Pd
Pd
N
Br
N
N
Br
Br
N
N
N
PdL1b
PdL1c
PdL1a
H3CO
N
H3CO
N
H3CO
N
N
N
N
OCH3
OCH3
OCH3
Pd
Pd
Pd
N
N
N
Br
Br
Br
N
N
N
PdL2c
PdL2a
PdL2b
Scheme 5. The structure of the new catalysts (PdL1a-c/PdL2a-c).
Table 3
Suzuki-Miyaura Cross-Coupling Reactions with Aryl Bromidesa
Cat. (0.5 mol%)
Cs2CO3, IPA, 82 oC, 0.5 h
Cat.
Br
R
B(OH)2
R
Entry
Product
GC Yield (%)b,c
TOF (h¡1
)
1
PdL1a
PdL1b
PdL1c
PdL1
90
98
98
98
48
73
78
98
42
36
16
45
10
21
21
99
40
67
71
75
76
80
80
99
180
196
196
196
96
2
H
3
4
5
PdL2a
PdL2b
PdL2c
PdL2
6
146
156
196
84
7
8
9
PdL1a
PdL1b
PdL1c
PdL1
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
72
H3C
33
90
PdL2a
PdL2b
PdL2c
PdL2
20
42
42
198
80
PdL1a
PdL1b
PdL1c
PdL1
134
142
150
152
160
160
198
PdL2a
PdL2b
PdL2c
PdL2
a
Reagents: an aryl bromide (0.50 mmol), PhB(OH)2 (0.75 mmol), Cs2CO3 (1.50 mmol), diethyleneglicol di-n-butyl ether (0.3 mmol, internal standard), catalyst (0.5
mol %), and IPA (3.0 mL).
b
Yields based on the aryl halide and average of two runs.
c
All reactions were followed by GC.
6

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I. Babahan et al.
Inorganica Chimica Acta 522 (2021) 120360
studies. In the previous studies, it has been displayed that the charac-
atom of the phenyl ring and one imine nitrogen atom of the Schiff base
ligand and one oxygen atom from each of the two bridging acetate li-
gands and therefore has a slightly distorted square-planar geometry. The
most noticeable deviation from the ideal square-planar arrangement is
mainly observed in the C─Pd─N bite angles of 81.0(3)◦ and 80.7(3)◦.
The distance of the Pd atoms from the plane through the four coordi-
nating atoms is small [0.0395(6) Å for Pd1 and 0.0638(6) Å for Pd2].
The four-coordinate geometry index for the complex, τ4 [33], is 0.01 for
atom Pd1 and 0.04 for atom Pd2 (where 0 would be perfectly square-
planar, and 1 is perfectly tetrahedral). These values also indicate that
the coordination polyhedron of the palladium atoms is a distorted
square planar.
–
teristic ν(C N) band in the Pd complexes was shifted to a lower wave
–
number, compared to that of the free ligand, due to π-back donation of
electron density from metal ion to the
π
* orbital of imine group [29a-g].
–
The ν(C C) and ν(C O) vibrational frequencies could be seen in the range
–
–
of 1515 and 1203 cmꢀ 1 for PdL1 and 1528 and 1202 cmꢀ 1 for PdL2. The
–
ν(C
bands are exhibited in the range of 1521–1518 for PdL1a-c and
C)
–
1529–1528 for PdL2a-c. It can be said that the absence of ν(COO) bands
in the IR spectrum for PdL1 and PdL2 supported substitution of the ex-
pected acetate moiety, MeCO-O, by a bromide ligand and comply with
the previous studies [11].
3.2.2. 1H and 13C NMR spectra
The Pd⋅⋅⋅Pd distance of 2.9970(10) Å is at the upper limit accepted
for a palladium-palladium single bond length, which is between 2.5 and
3.0 Å [33b]. The sum of angles around the palladium atoms is approx-
imately 360◦. The Pd─C bond lengths vary from 1.956(7) to 1.977(7) Å,
as both the Pd─N distances are equal to 2.027(6) Å. The Pd─O bond
distances trans to the aryl carbon donor are slightly longer (ca. 0.09 Å)
than those trans to the nitrogen donor due to the trans lengthening in-
The 1H and 13C NMR data of the new complexes, PdL1-2, PdL1a-c, and
PdL2a-c, are given in the experimental section. The studies of the 1H
NMR data of PdL1 displayed that the methyl groups were existence in
the form of a single peak at 1.77 ppm for (C6H3(CH3)2-p-CH3), 2.21 ppm
for (C6H3(CH3)2-o-CH3) and
a triplet peak at 1.17 ppm for
(CH3CH2)2NC6H3). While the methyl groups of PdL2 have been observed
at 2.94 ppm for ((CH3)2NC6H3-), the methoxy groups of PdL2 have been
shown at 1.82 ppm for (C6H3(OCH3)2-p-OCH3) and at 3.75 ppm for
(C6H3(OCH3)2-o-OCH3).
fluence of carbon
σ-donors [33c]. The bond lengths of C9─N1 and
C30─N3 [1.288(9) Å] are characteristic for C═N double bonds. All bond
lengths and angles are normal and in agreement with values previously
reported for similar complexes [33d-h].
The chemical shifts for the methyl groups atom were observed at
1.06 ppm for (C6H3(CH3)2-p-CH3), in the range 2.49–2.51 ppm for
(C6H3(CH3)2-o-CH3) and in the range 1.06–1.08 and 2.18–2.32 ppm as
two different triplets for ((CH3CH2)2NC6H3) for PdL1a-c and in the range
of 2.27–2.36 ppm as singlets for ((CH3)2NC6H3) for PdL2a-c. The
methoxy groups of PdL2a-c displayed as singlets in the range of
2.18–2.43 ppm for (C6H3(OCH3)2-p-OCH3) and 2.83–2.85 ppm for
(C6H3(OCH3)2-o-OCH3).
3.3. Catalytic studies of Pd complexes (PdL1-2, PdL1a-c and PdL2a-c
)
Awareness of the coupling of organoboron reagents with organic
halides in the Suzuki-Miyaura reaction has been growing since 1979. Pd-
catalysts, the most powerful catalysts in these reactions, are used for the
2
installation of biaryls compounds obtained by the formation of new Csp
-
3
2
2
Csp and Csp -Csp bonds [34].
As it was expected that the aromatic hydrogen atoms could be seen in
the range of 5.88–6.98 and 5.91–7.07 ppm and the imine hydrogen
atoms (HC = N) were observed at 7.38 ppm and 7.48 ppm for PdL1 and
PdL2, respectively. The aromatic hydrogen atoms and the imine
hydrogen atoms (HC = N) for their NHC imidazolium salts (PdL1a-c and
PdL2a-c) exhibited similarly in the range of 5.44–7.02 ppm, 7.89–7.90
ppm for PdL1a-c, 5.58–7.38 ppm, and 8.04–8.05 ppm for PdL2a-c [30].
The chemical shifts for the carbene carbon atoms were exhibited in
the range 186.5–186.1 ppm and 186.6–186.1 ppm for PdL1a-c and
The activities of Pd(II) complexes were investigated in the Suzuki-
Miyaura cross-coupling reactions of various aryl bromides with phe-
nylboronic acid yielding the corresponding biphenyl products. The re-
sults are summarized in Table 3. The Suzuki–Miyaura reaction was
employed by using three different substrates to compare. For this pur-
pose, 4-bromotoluene as an electron-rich substrate, 4-bromoacetophe-
none as an electron-poor substrate, and 4-bromobenzene as an electron-
neutral substrate have been used. It is well known that the optimum
temperature range is 80–100 ◦C with for aryl halide for the catalytic
tests. Therefore, the catalytic activity reactions were carried out in IPA,
Cs2CO3 as base and temperature at 82 ◦C following reaction optimiza-
tion for the synthesized complexes (PdL1, PdL2, PdL1a-c, and PdL2a-c).
The reaction was carried out with 0.5 mol% catalysts to achieve a high
yield. Aryl bromides with different functional groups adequately reacted
with phenylboronic acid using Cs2CO3 and IPA (3.0 mL) at 82 ◦C tem-
perature in the presence of the palladium(II) complexes as a catalyst.
Interestingly, the most facile reaction is between 4-bromobenzene,
which is electron-neutral, and phenylboronic acid with yields of
48–98% (Table 3, entries 1–8). As expected, the aryl bromide substituted
by an electron-withdrawing group (MeCO-, entries 17–24) in the para-
position was more reactive than those substituted by an electron-
donating group (Me-, entries 9–15). The reaction between the
electron-rich p-bromotoluene and phenylboronic acid proved to be more
difficult and the formation of the coupling products in moderate yields
(Table 3, entries 9–15).
–
PdL2a-c and, azomethine carbons (C N) 173.9–173.5 ppm for PdL
–
1a-c
and 174.4–174.2 ppm for PdL2a-c. It is also observed that the carbene C2
resonance was shifted to the lower field, ca. 46 ppm when compared to
the carbene resonance in unreacted imidazolium salts [31].
1
–
–
In studying in H NMR, the proof that the C N resonances of the
metallated ring in of PdL1-2 were not affected by reaction with the
imidazolium salts could be seen with the similarity of the peak which is
–
–
–
–
attributed to C N of PdL1a-c and PdL2a-c. Though C N resonances of
metallated ring usually shift to the higher field, as compared to the
spectrum of the imidazolium salt, it does not display any difference as
for the shift of the azomethine in PdL1 and PdL2 [32].
Absences of acetate bands in PdL1a-c and PdL2a-c complexes in IR, 1H
and 13C NMR spectra displayed of replacing acetate moiety in PdL1-2
complexes with bromide. The substation reaction type of replacing ac-
etate by bromide reaction in Pd(II) dimeric complexes is known to be
labile and inclined to be replaced by bromide under mild conditions and
short reaction times [31].
When the catalytic activities of palladacyclic complexes were
compared, the acetato-bridge palladium complexes (PdL1 and PdL2)
were found to be more than the active catalyst NHC-ligated Schiff base
palladacycles (PdL1a-c and PdL2a-c) (Table 3, entries 4,8,12,16,20,24).
Among them (PdL1 and PdL2), it was concluded that PdL2 is the most
active catalyst in Suzuki-Miyaura cross-coupling reactions (Table 3,
entries 8,16,24).
3.2.3. Description of the crystal structure
The molecular diagram of PdL1 with the adopted atom-labeling
scheme is shown in Fig. 1, while important geometric parameters are
listed in Table 2. The dinuclear complex PdL1 adopts a U-shaped or
closed-book conformation where the two five-membered chelate rings of
the Schiff base ligands stack on top of each other. The tilting of the
complex along the bridging acetate groups is indicated by the inter-
planar angle of 35.6(3)◦ between the coordination planes of the two
palladium ions. Each palladium atom is surrounded by one ortho carbon
There is a lot of data in the literature for Suzuki-coupling reactions
catalyzed by palladacyclic complexes under a variety of different
experimental conditions. With respect to the related Milstein,[22]
7

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I. Babahan et al.
Inorganica Chimica Acta 522 (2021) 120360
Bedford,[35] Yang,[7] Shaughnessy,[36] and Liu [37] in the Suzu-
ki–Miyaura reaction,[22] our results show that Pd(II) complexes exhibit
moderate to good catalytic activity.
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In conclusion, we have synthesized and fully characterized novel air-
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complexes. The structure of the dinuclear complex PdL1 was also
determined by X-ray analysis. The coordination around the PdII ion is
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Acknowledgments
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