Azo-azomethine based palladium(II) complexes as catalysts for the Suzuki-Miyaura cross-coupling reaction

Article abstract of DOI:10.1016/j.molstruc.2020.128279

Seven azo-azomethine ligands (2-8) have been synthesized from the reaction of (E)-5-((4-chlorophenyl)diazenyl)-2-hydroxybenzaldehyde with different aniline derivatives. Ligand structures were characterized by the combination of IR, UV–Visible spectroscopy

Full text of DOI:10.1016/j.molstruc.2020.128279

Journal Pre-proof
Azo-azomethine based palladium(II) complexes as catalysts for the Suzuki-Miyaura
cross-coupling reaction
Ali Burak Sünbül, Ayşe Inan, Muhammet Köse, Enes Evren, Nevin Gürbüz, İsmail
Özdemir, Mesut Ikiz, Ayşe Kazancı Dağ, Esin İspir
PII:
S0022-2860(20)30604-9
DOI:
https://doi.org/10.1016/j.molstruc.2020.128279
MOLSTR 128279
Reference:
To appear in: Journal of Molecular Structure
Received Date: 18 February 2020
Revised Date: 8 April 2020
Accepted Date: 15 April 2020
Please cite this article as: A.B. Sünbül, Ayş. Inan, M. Köse, E. Evren, N. Gürbüz, İ. Özdemir, M. Ikiz,
Ayş.Kazancı. Dağ, E. İspir, Azo-azomethine based palladium(II) complexes as catalysts for the Suzuki-
Miyaura cross-coupling reaction, Journal of Molecular Structure (2020), doi: https://doi.org/10.1016/
j.molstruc.2020.128279.
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Ali Burak Sünbül, Ayşe Inan, Mesut Ikiz and Ayşe Kazancı Dağ helped the sythesis of
molecules.
Enes, Evren, Nevin Gürbüz and İsmail Özdemir did the catalytic studies.
Muhammet Köse interpreted the X-Ray analysis results
Esin İspir edited the study, interpreted other analysis results and wrote the article.

Azo-azomethine based palladium(II) complexes as catalysts for the Suzuki-
Miyaura Cross-Coupling reaction
Ali Burak Sünbül1, Ayşe Inan1, Muhammet Köse1, Enes, Evren2, Nevin Gürbüz2, İsmail
Özdemir3, Mesut Ikiz4, Ayşe Kazancı Dağ5, Esin İspir 1*
1Department of Chemistry, Faculty of Science and Arts, Kahramanmaraş Sütçü İmam
University, Kahramanmaraş, 46050–9, Turkey
2Department of Chemistry, Inönü University, Malatya, 44069, Turkey
3Catalysis Research and Application Center, İnönü University, Malatya, 44280, Turkey
4Department of Medical Services and Techniques, Vocational School of Health Service,
University of Health Sciences, 34668, İstanbul, Turkey
520th Reg Directorate State Hydraul Works, TR-46100 Kahramanmaraş, Turkey
Abstract
Seven azo-azomethine ligands (2-8) have been synthesized from the reaction of (E)-5-((4-
chlorophenyl)diazenyl)-2-hydroxybenzaldehyde with different aniline derivatives. Ligand
structures were characterized by the combination of IR, UV-Visible spectroscopy, 13C-NMR,
1H-NMR and elemental analyses. The mononuclear Pd(II) complexes of these azo-
azomethine compounds were prepared with sodium tetrachloropalladate(II) and characterized.
According to the UV-visible, FTIR, elemental analyze and HRMS data, the Pd (II) complexes
9-15 are formed by the coordination of N, O atoms of the ligands. Solid state structures of 2, 6
and 7 were determined by single crystal X-ray diffraction studies. Also, Pd(II) complexes
were used catalysts for Suzuki-Miyaura coupling reactions of phenylboronic acid with aryl
bromide in aqueous media. When the catalytic studies were evaluated, it was found that all of
the complexes were suitable for the Suzuki-Miyaura cross-coupling reaction.
KEYWORDS: Azo-azomethine; Pd(II) complexes; Suzuki-Miyaura coupling, C-C bond
formation.
Corresponding author*:
E-mail: esinispir@ksu.edu.tr. (E. İspir).
1

1. INTRODUCTION
Over the past years, the transition-metal complex catalyzed C–C cross-coupling
reactions are excessively effective for the fabricating of new carbon-carbon bonds and draw
huge attention in current organic chemistry [1][2][3]. Amongst them, the Suzuki–Miyaura
reaction, synthesis of unsymmetrical biphenyls from aryl halides and arylboronic acids is one
of the most important reactions catalyzed by palladium [4].
In most of the catalytic reactions that provide C-C bond formation, homogeneous
palladium complexes are used due to their success in achieving high yield and rapid reaction
rates [5][6]. Homogeneous palladium catalysis for the C-C bond formation in the presence of
phosphine ligands is one of the most effective major routes and has attained excellent success.
However, the main disadvantage in the use of phosphine ligands is the difficulties in the
synthesis and requirement of high cost chemicals [7]. Hence the ligand selection has the key
role for the efficient complex design and increases the catalytic characteristics of transition
metal atoms in especially under phosphine-free reaction conditions to overcome mentioned
drawbacks.
Therefore, it is seen that different ligand groups such as carbenes [8][9][10], imidazoline
[11][12], benzimidazole [13][14], bispyridine/pyrazole derivatives, bisimidazole and Schiff
bases are usually used as an alternative to phosphine ligands in the design of the Pd-
complexes used in C-C coupling reactions. Among these ligands, Schiff base ligands are easy
to synthesize, partially cheap and thermally-chemically stable [1][2][15][16]. The Schiff base
derivatives contain an azomethine (-HC=N-) functional group in their structures and have
various properties [17], and hence they can be used in different fields such as organic and
bioinorganic chemistry [18], catalysis [18], synthesis of various compounds [19] and liquid
crystals [20]. The electron pair in nitrogen or carbon atoms can coordinate with metal ions or
various organic groups, and this makes Schiff bases highly flexible in the syntheses and
modifications [21][22].
Numerous effective and selective Pd-catalysts based on Schiff base ligands have been
developed in the last years [23][24] but there are very few examples of reports on the usage of
Pd complexes with azo Schiff base ligands which contain both the azo (-N=N-)
and azomethine (-HC=N-) functional groups in their own structures for Heck reactions [1].
2

On the continuation of our research interest in developing new homogeneous catalysts, herein,
we have synthesized a new series of palladium-Schiff base complexes and used them as a
homogeneous palladium catalyst in Suzuki-Miyaura reactions.
2. EXPERIMENTAL
2.1. Materials and measurements
The reagents and solvents were purchased from chemical suppliers and purified to
match the reported physical and spectroscopic data. The solvents were carefully dried using
standard methods. HRMS analyses were carried out by the Instrumental Analysis Laboratory
of Middle East Technical University and recorded on a Waters Synapt G1 instrument. FTIR
spectra were obtained using KBr discs (4000–400 cm–1) on a Shimadzu 8300 FTIR
spectrophotometer. The electronic spectra in the 200–900 nm range were obtained on a
Shimadzu UV–160 A spectrophotometer. 1H NMR spectra were recorded on a Varian XL-300
NMR instrument. TMS was used as an internal standard and deuterated CDCl3 as solvent.
All catalytic reactions were monitored on a Shimadzu 2010 Plus GC-MS system by
GC-FID with an HP-5 column of 30 m length, 0.32 mm diameter, and 0.25 μm film
thickness.
2.2. Synthesis of (E)-5-((4-chlorophenyl)diazenyl)-2-hydroxybenzaldehyde 1
(E)-5-((4-chlorophenyl)diazenyl)-2-hydroxybenzaldehyde 1 was synthesized according to the
literature method [25].
2.3. General procedure for the synthesis of ligands 2-8
A methanolic solution (25 mL) of (E)-5-((4-chlorophenyl)diazenyl)-2-hydroxybenzaldehyde 1
(0.520 g, 2.00 mmol) was added to the aniline derivatives(p-toluidine for 2, 4-methoxyaniline
for 3, 3,4-dimethoxyaniline for 4, 4-chloroaniline for 5, 4-bromoaniline for 6, 2-
methylpropan-2-amine for 7, cyclohexanaminefor 8 ) in methanol (5 mL) and refluxed for 3
hours. The solution was concentrated on air for two days to ligands 2-8. The product was
filtered and dried on silica.
3

2: Color: Orange. Yield: 0.396 g (54%). m.p. 207 °C. Elemental analysis: Calcd. for
C20H16ClN3O C, 68.67; H, 4.61; N, 12.01. Found: C, 68.63; H, 4.60; N, 12.03. FT-IR (KBr,
1
cm-1): 3424, 2915, 1620 and 1587. UV-vis (λmax, nm): 345, 465 (in 10-4 M DMF). H NMR
(300 MHz, CDCl3) δ 14.07 (s, 1H), 8.75 (s, 1H), 8.03 (s, 2H), 7.86 – 7.80 (m, 2H), 7.50 –
13
7.44 (m, 2H), 7.25 (d, J = 2.3 Hz, 4H), 7.13 (d, J = 8.6 Hz, 1H), 2.40 (s, 3H). C NMR (75
MHz, CDCl3) δ 164.44, 160.99, 150.99, 145.36, 145.04, 137.55, 136.35, 130.16, 129.33,
127.98, 127.32, 123.86, 121.04, 119.00, 118.27, 21.12.
3: Color: Yellow. Yield: 0.403 g (52%). m.p.185-187 °C. Elemental analysis: Calcd. for
C20H16ClN3O2 C, 65.67; H, 4.41; N, 11.49. Found: C, 65.21; H, 4.40; N, 11.35. FT-IR (KBr,
1
cm-1): 3444, 2959, 1617 and 1573. UV-vis (λmax, nm): 352, 464 (in 10-4 M DMF). H NMR
(300 MHz, CDCl3) δ 14.25 (d, J = 74.3 Hz, 1H), 8.74 (d, J = 4.1 Hz, 1H), 8.09 – 7.96 (m,
2H), 7.91 – 7.79 (m, 2H), 7.55 – 7.42 (m, 2H), 7.39 – 7.28 (m, 2H), 7.13 (d, J = 8.5 Hz, 1H),
13
7.02 – 6.95 (m, 2H), 3.86 (s, 3H). C NMR (151 MHz, CDCl3) δ 164.28, 159.63, 159.24,
151.00, 145.38, 140.52, 136.33, 129.34, 127.74, 127.15, 123.86, 122.42, 119.12, 118.18,
114.75, 55.58.
4: Color: Orange. Yield: 0.438 g (53%). m.p.181-182 °C. Elemental analysis: Calcd. for
C21H18ClN3O3 C, 63.72; H, 4.58; N, 10.62. Found: C, 63.65; H, 4.55; N, 10.59. FT-IR (KBr,
1
cm-1): 3442, 2922, 1618 and 1576. UV-vis (λmax, nm): 358, 461 (in 10-4 M DMF). H NMR
(300 MHz, CDCl3) δ 14.05 (s, 1H), 8.75 (s, 1H), 8.03 (dt, J = 8.7, 2.3 Hz, 2H), 7.90 – 7.78
(m, 2H), 7.52 – 7.42 (m, 2H), 7.14 (d, J = 8.7 Hz, 1H), 6.98 – 6.89 (m,3H), 3.95 (d, J = 7.1
Hz, 6H). 13C NMR (75 MHz, CDCl3) δ 164.23, 159.87, 150.98, 149.66, 148.78, 145.41,
140.90, 136.37, 129.34, 127.62, 127.37, 123.85, 119.07, 118.19, 112.97, 111.48, 105.17,
56.17, 56.06.
5: Color: Yellow. Yield: 0.410 g (53%). m.p. 203 °C. Elemental analysis: Calcd. for
C19H13Cl2N3O C, 61.64; H, 3.54; N, 11.35. Found: C, 61.60; H, 3.49; N, 11.33. FT-IR (KBr,
1
cm-1): 3440, 2921, 1616 and 1573. UV-vis (λmax, nm): 345, 465 (in 10-4 M DMF). H NMR
(300 MHz, CDCl3) δ 13.65 (s, 1H), 8.76 (s, 1H), 8.06 (dd, J = 7.0, 2.1 Hz, 2H), 7.89 – 7.83
(m, 2H), 7.53 – 7.48 (m, 2H), 7.48 – 7.42 (m, 2H), 7.32 – 7.30 (m, 1H), 7.28 (s, 1H), 7.20 –
4

13
7.14 (m, 1H). C NMR (75 MHz, CDCl3) δ 164.09, 162.40, 150.91, 146.33, 145.49, 136.50,
133.07, 129.71, 129.36, 128.22, 127.72, 123.90, 122.52, 118.80, 118.31.
6: Color: Yellow. Yield: 0.483 g (56%). m.p.213 °C. Elemental analysis: Calcd. for
C19H13BrClN3O C, 55.03; H, 3.16; N, 10.13. Found: C, 55.01; H, 3.15; N, 10.02. FT-IR (KBr,
1
cm-1): 3442, 2929, 1617 and 1578. UV-vis (λmax, nm): 340, 465 (in 10-4 M DMF). H NMR
(300 MHz, CDCl3) δ 13.62 (s, 1H), 8.76 (s, 1H), 8.10 – 8.04 (m, 2H), 7.89 – 7.84 (m, 2H),
7.63 – 7.57 (m, 2H), 7.55 – 7.47 (m, 2H), 7.25 – 7.21 (m, 2H), 7.20 – 7.14 (m, 1H). 13C NMR
(75 MHz, CDCl3) δ 164.09, 164.06, 162.47, 156.77, 145.50, 136.51, 132.67, 131.87, 129.36,
128.65, 127.76, 123.90, 122.87, 122.36, 118.80
7: Color: Yellow. Yield: 0.366 g (55%). m.p.188 °C. Elemental analysis: Calcd. for
C17H18ClN3O C, 64.66; H, 5.75; N, 13.31. Found: C, 64.31; H, 5.67; N, 13.26. FT-IR (KBr,
1
cm-1): 3447, 2926, 1614 and 1571. UV-vis (λmax, nm): 380, 432 (in 10-4 M DMF). H NMR
(300 MHz, CDCl3) δ 15.16 (s, 1H), 8.36 (d, J = 1.7 Hz, 1H), 7.98 (dd, J = 9.1, 2.5 Hz, 1H),
7.88 (d, J = 2.5 Hz, 1H), 7.83 – 7.77 (m, 2H), 7.48 – 7.42 (m, 2H), 7.01 – 6.94 (m, 1H), 1.42
(s, 9H). 13C NMR (75 MHz, CDCl3) δ 169.82, 159.76, 151.13, 143.82, 135.82, 129.26,
127.18, 123.66, 121.14, 120.23, 116.94, 56.85, 29.51.
8: Color: Orange. Yield: 0.373 g (52%). m.p.162 °C. Elemental analysis: Calcd. for
C19H20ClN3O C, 66.76; H, 5.90; N, 12.29. Found: C, 66.71; H, 5.87; N, 12.02. FT-IR (KBr,
1
cm-1): 3439, 2930, 1616 and 1569. UV-vis (λmax, nm): 340, 435 (in 10-4 M DMF). H NMR
(300 MHz, CDCl3) δ 14.66 (s, 1H), 8.41 (s, 1H), 7.98 (dd, J = 9.0, 2.5 Hz, 1H), 7.88 (d, J =
2.4 Hz, 1H), 7.85 – 7.81 (m, 2H), 7.57 – 7.41 (m, 2H), 7.03 (d, J = 9.0 Hz, 1H), 3.36 (m, J =
9.9, 5.0 Hz, 2H), 1.97 – 1.81 (m, 2H), 1.75 – 1.63 (m, 2H), 1.64 – 1.52 (m, 2H), 1.48 – 1.32
(m, 2H). 13C NMR (75 MHz, CDCl3) δ 167.94, 162.09, 151.07, 144.24, 135.95, 129.27,
128.70, 126.68, 123.73, 119.42, 117.56, 65.84, 34.01, 25.36, 24.28.
2.4. General procedure for the synthesis of complexes 9-15
Na2PdCl4 (147 mg, 0.5 mmol) was added to the ligand solution (2-8) (1mmol) in
methanol (10 mL) in one portion and refluxed for 4 h at nitrogen atmosphere in a schlenk
tube. The precipitate was filtered and washed with diethyl ether (20 mL).
5

9: Color: Dark red. Yield: 0.278 g (56%). m.p. ˃250°C, decompose. Elemental analysis:
Calcd. for C40H30Cl2N6O2Pd C, 59.75; H, 3.76; N, 10.45. Found: C, 59.69; H, 3.71; N, 10.40.
HRMS [PdL2 + H]+: m/z calcd. for [C40H31Cl2N6O2Pd]+ 805.0924; found 805.1006. FT-IR
(KBr, cm-1): 1611, 1566, 597, and 519. UV-vis (λmax, nm): 345, 375, 435, 545 (in 10-4 M
DMF).
10: Color: Dark brown. Yield: 0.302 g (59%). m.p. 312°C. Elemental analysis: Calcd. for
C40H30Cl2N6O4Pd C, 57.47; H, 3.62; N, 10.05. Found: C, 57.29; H, 3.41; N, 10.13. HRMS
[PdL2 + H]+: m/z calcd. for [C40H31Cl2N6O4Pd]+ 837.0823; found 837.0910. FT-IR (KBr, cm-
1): 1601, 1504, 569, and 523. UV-vis (λmax, nm): 335, 360, 462, 543 (in 10-4 M DMF).
11: Color: Light brown. Yield: 0.287 g (53%). m.p. 307°C. Elemental analysis: Calcd. for
C40H34Cl2N6O6Pd C, 56.30; H, 3.82; N, 9.38. Found: C, 56.19; H, 3.72; N, 9.15. HRMS [PdL2
+ H]+: m/z calcd. for [C42H35Cl2N6O6Pd]+ 897.1034; found 897.1049. FT-IR (KBr, cm-1):
1615, 1554, 556, and 513. UV-vis (λmax, nm): 405, 460, 553 (in 10-4 M DMF).
12; Color: Dark red. Yield: 0.287 g (53%). m.p. 307°C. Elemental analysis: Calcd. for
C40H34Cl2N6O6Pd C, 56.30; H, 3.82; N, 9.38. Found: C, 56.19; H, 3.72; N, 9.15. HRMS [PdL2
+ H]+: m/z calcd. for [C38H25Cl4N6O2Pd]+ 844.9816; found 844.9819. FT-IR (KBr, cm-1):
1617, 1543, 587, and 533. UV-vis (λmax, nm): 355, 465, 545 (in 10-4 M DMF).
13: Color: Brown. Yield: 0.336 g (60%). m.p. 270°C. Elemental analysis: Calcd. for
C40H30Cl2N6O2Pd C, 48.88; H, 2.59; N, 9.00. Found: C, 48.25; H, 2.13; N, 9.40. HRMS [L +
H]+: m/z calcd. for [C19H13BrClN3O + H]+ found 415.9978, [PdL2 - Cl]+: m/z calcd. for
[C38H25Br2ClN6O2Pd - Cl]+ found 900.2875, [PdL - Cl]+: m/z calcd. for [C19H13BrN32Pd -
Cl]+ found 485.2871. FT-IR (KBr, cm-1): 1617, 1552, 575, and 520. UV-vis (λmax, nm): 365,
455, 540 (in 10-4 M DMF).
14: Color: Brown. Yield: 0.235 g (51%). m.p. 223°C. Elemental analysis: Calcd. for
C40H30Cl2N6O2Pd C, 55.49; H, 4.66; N, 11.42. Found: C, 55.31; H, 4.63; N, 11.27. HRMS
[PdL2 + H]+: m/z calcd. for [C34H35Cl2N6O6Pd]+ 737.1238; found 737.1235. FT-IR (KBr, cm-
1): 1614, 1519, 587, and 520. UV-vis (λmax, nm): 390, 445, 555 (in 10-4 M DMF).
6

15: Color: Red. Yield: 0.258 g (53%). m.p. 2967°C. Elemental analysis: Calcd. for
C40H30Cl2N6O2Pd C, 57.92; H, 4.86; N, 10.66. Found: C, 57.69; H, 4.71; N, 10.40. HRMS [L
+ H]+: m/z calcd. for [C19H20ClN3O + H]+ found 342.1332, [PdL2 – (C6H3Cl)]+: m/z calcd.
for [C32H35ClN6O2Pd - (C6H3Cl)]+ found 677.4691, [PdL2 – (C6H4NCl)]+: m/z calcd. for
[C32H35ClN6O2Pd - (C6H4NCl)]+ found 663.4535. FT-IR (KBr, cm-1): 1609, 1526, 559, and
516. UV-vis (λmax, nm): 315, 345, 542 (in 10-4 M DMF).
2.5. X-ray structure solution and refinement for 2, 6 and 7
X-ray crystallography data for 2, 6 and 7 were collected on a Bruker D8 QUEST
diffractometer (Mo-Kα radiation (λ= 0.71073 Å) and data reduction weere performed using
Bruker SAINT[26]. SHELXS97 was used to solve and SHELXL2014/6 to refine the
structures [27][28]. The structures of 2, 6 and 7 were solved by direct methods and refined on
F2 using all the reflections. All the non-hydrogen atoms were refined using anisotropic atomic
displacement parameters and hydrogen atoms bonded to carbon and oxygen atoms were
located at calculated positions using a riding model and refined with the temperature factors
riding on the carrying atoms. The crystal data and details of the structure solutions and
refinements are given in Table 1.
2.6. General procedure for Suzuki-Miyaura cross-coupling reaction
Phenylboronic acid (1.5 mmol), aryl chloride (1.0 mmol), K2CO3 (2.0 mmol), palladium
catalyst (0.01 mmol, 1 mol%) (9-15) and mixture of isopropanol/water (1:3; v/v) (4 mL) were
added to a small round bottom flask in air. The mixture was stirred at 80 ° C for 8 h. At the
end of the reaction, the cooled solution to room temperature was extracted with a mixture of
EtOAc/n-hexane (1:5, v/v). The organic phase was separated and dried over anhydrous
MgSO4; the solution passed through the micro silica gel column was concentrated. Product
characterizations were performed by GC. The reaction yields were determined by GC based
on aryl bromide.
3. RESULT AND DISCUSSION
7

In this study, seven azo-azomethine ligands (
2
-
8
) have been synthesized from
with
the reaction of (E)-5-((4- chlorophenyl)diazenyl)-2-hydroxybenzaldehyde
1
different aniline derivatives as shown in Scheme 1. Also, their Pd(II) complexes were
synthesized with sodium tetrachloropalladate(II).
The level of the purity of the ligands and the complexes was checked by T.L.C. on silica
gel-coated plates.
The ligands and complexes are stable at room temperature. The ligand structures were
1
characterized by the combination of IR, UV-Visible spectroscopy, H-13C NMR and
elemental analyses. The mononuclear Pd(II) complexes of these azo-azomethine compounds
were prepared and characterized. According to the UV-visible, IR, elemental analysis and
HRMS data, the Pd (II) complexes 9-15 are formed by the coordination of N, O atoms of the
ligands. The data showed that the complexation ratio is 1:2 [Metal: Ligand]. Single crystals of
2, 6 and 7 were grown from DMF solutions by slow evaporation. Although the single crystals
of the other compounds were not able to isolated, the analytical and spectroscopic data
enables us to propose possible structures as shown in Scheme 1.
The azo-azomethine ligands 2-8 and complexes 9-15 were characterized by the
analytical and spectroscopic methods and the data are listed in the experimental section. The
1H NMR spectra of all ligands are given in the supplementary documents (Fig. S1-7).
In the 1H NMR spectra of the ligands, the signals in the 14.05–15.16 ppm range can be
attributed to the phenolic proton (OH) of the salicylidene rings. In the 1H NMR spectra of the
ligands (2-8), the proton of azomethine group (CH=N) is seen as a singlet in the 8.36-8.76
ppm range [29]. The singlet signals at 2.24 and 1.42 ppm can be assigned to –CH3 group
protons in 2 and 7 structures [38]. In the spectra of 3 and 4, the methoxy groups protons are
observed at 3.86 ppm as a singlet and in 3.95 ppm as a doublet respectively. Besides,
multiplet signals at 3.36 ppm and 1.32-1.97 ppm range are due to the cyclohexane –CH2
group protons in the spectrum of 8 [30].
13
In addition, the C NMR spectra of the ligands also showed a shift in the azomethine
carbon peak from the region 169.82–164.09 ppm for the free ligands. The signals at 21.12-
65.84 range can be assigned to the aliphatic carbon signals for 2, 3, 4, 7 and 8 ligands [31].
Because of the very low solubility of the complexes in organic solvents such as DMSO,
chloroform and methanol, the NMR analyses were not able to perform for the complexes. To
characterize the complex structures, the HRMS analyses were carried out beside the UV and
FTIR analyses.
8

In order to explain the binding of azo-azomethine ligands to the metal ion in the
complexes, the FTIR spectra of the free ligands were compared with those of the Pd(II)
complexes. The characteristic FTIR data of the azo-azomethine ligands and their Pd(II)
complexes are listed in the experimental section. In the spectra of the ligands, the
ligands exhibit bands at 2915–2959 cm–1 and 3424–3447 cm–1 that are assignable to
ν
(Ar-CH) and ν(OH), respectively [20]. In the spectra of Pd(II) complexes, the ν(OH)
vibration disappeared confirming the coordination of the phenolic oxygen to the metal
center. The vibrational bands for ligands at 1620-1614 cm−1 range are assigned for
ν
(C=N) stretching vibration. The
ν
(C=N) stretching frequency shifted to the lower
(C=N) may be
frequency of 1611–1609 cm−1 for the complexes. The shift in the
ν
attributed to the electron withdrawing effects from the coordinated metal, which
weakens the (C=N) bond and thus shifts the stretching frequency to lower values [1].
The characteristic bands of N=N group were observed at the range of 1587-1569 cm-1
range for the ligands and 1556-1504 cm-1 range for Pd(II) complexes. Upon
coordination of the free ligands with the central metal, the new weak bands at 500–600
cm−1 ranges are observed from the spectra of Pd(II) metal complexes can be attributed
to M–O and M–N stretching bands [32].
The electronic absorption spectral data for the ligands and their Pd(II) complexes were
measured in 10–4 M DMF solutions between 260 and 600 nm at room temperature and data
are given in the experimental section. The UV–Vis absorption spectra of the Schiff base
ligands (see Fig. S15). are similar to each other. The absorption peaks at 340-380 nm and
432-465 nm range can be assigned to the characteristic of the overlapping n–π* transitions of
azo (-N=N-) and aldimine (-N=CH-) groups. In the spectra of the complexes (see Fig. S16).,
the bands of the azomethine n→π* transitions are shifted to higher frequencies indicating that
the imine nitrogen atom is involved in coordination with the metal ion [33]. The UV–visible
spectra of the azo-azomethine ligands and their Pd(II) complexes in DMF showed absorption
bands between 486 and 353 nm. The electronic spectra of Pd(II) complexes show an
additional band in the visible region (542-555 nm range), which is attributed to charge
transfer [32].
In the mass spectra of complexes 9, 10, 11, 12 and 14, the peaks that appear at m/z
805.1006, 837.0910, 897.1049, 844.9819 and 737.1235, respectively, have been ascribed to
the [PdL2 + H]+ ion (see Fig. S8–S14). The [PdL2 - Cl]+ molecular ion peak for the complex
9

13 is observed at m/z: 900.2875. The most intense peaks at m/z 415.9978 and 485.2871
corresponds to [L + H]+ and [PdL - Cl]+fragments. Also the [L + H]+ molecular ion peak for
the complex 15 is observed at m/z 342.1332. The peak at m/z 677.4691 may be attributed to
[PdL2 – (C6H3Cl)]+. In the mass spectrum of the complex 15, the peak observed at m/z
663.4535 can be assigned to the fragmentation peak corresponding to [PdL2 – (C6H4NCl)]+.
Single crystals of the ligands 2, 6 and 7 for X-ray diffraction experiments were grown
from slow evaporation of DMF solutions. The structures of all three compounds were solved
in a monoclinic unit cell. The structures of the compounds are similar differing in the group
that attached to the central phenol ring via imine bond. Compound 7 contains a tert-butyl
group while 6 and 2 has the p-bromophenyl and p-methylphenyl ring, respectively. In the
structure of the compounds, the azomethine (N3-C13) and diazo (N1-N2) linkage distances
show characteristic C=N and N=N double bond distances. The X-ray structural data showed
that the compounds adopt phenol-imine tautomeric form in the solid state. Refinement of 7
revealed that the tertbutyl group is highly disordered and this was modeled over two positions
with 70:30 site-occupancy. As expected, the phenyl (C1/C6) and phenol (C7/C12) rings are in
trans conformation. In all three compounds, the phenyl and phenol rings linked by diazo
group (-N=N-) are almost co-planar. However, the p-bromophenyl ring in 6 and p-
methylphenyl ring in 2 are twisted considerably from planarity. The dihedral angles between
the phenyl (C14/C19 and phenol (C7/C12) rings is 48.53(18) and 44.134(12)° for 6 and 2,
respectively.
All three ligands have phenol-imine intramolecular hydrogen (O1H····N3) bonds. In
the structure of 7, two molecules are linked by phenyl-phenol (face to face) stacking
interactions with a centroid-centroid distance of 3.739 Å (Fig. 2). Molecules are further linked
by CH····O and CH····Cl interactions. Molecules of 6 also involves in phenyl-phenyl (offset)
stacking interactions forming π-stacked chains. These chains are linked by halogen····halogen
contacts [34] [Cl····Cl and Br····Br] forming a 2D supramolecular tectons in which molecules
are aligned in a zig-zag arrangement (Fig. 3).
The Cl····Cl and Br····Br contacts are 3.550 and 3.703 Å, respectively. In the structure
of 2, there is no phenyl-phenyl (face to face) stacking interactions. Molecules aligns along the
ab axis via CH····π (phenyl) interactions and these molecules are further connected by weaker
CH····Cl hydrogen bond type interactions forming a 2D supramolecular structure (Fig. 4).
3.1. Catalytic Studies
10

In order to demonstrate the catalytic activity of the Schiff bases-palladium
complexes, we decided to try the usability of complexes in Suzuki-Miyaura cross-
coupling reactions of aryl bromides with phenylboronic acid. The Suzuki-Miyaura
reaction is one of the most efficient and awarded methods for the synthesis of arylated
products which are highly valuable chemicals in many areas, under reaction conditions
from non-toxic comprehensive starting materials.
Optimization of reaction condition by investigating base- and solvent-dependent
conversions of the reaction between 4-bromoacetophenone (1 mmol) with
9 (1 mol%)
o
at 80 C have been tested. To find the best base, different bases (Na2CO3, K2CO3,
i-
Cs2CO3, NaOH, KOH) were tested in the presence of PrOH as solvent at 80 oC for 8
h in the air (Table 3, entries 1-5). When a base of K2CO3 was used, a good yield of a
coupling product was obtained (Table 2, entry 2). It is well-known that the carbonate
bases have critical importance due to good solubility in the water while determining
the efficiency of the Suzuki reaction when aqueous solutions are used as a solvent [9-
12].
Then, to find the best solvent medium, different solvents such as DMF, water, EtOH or
solvent mixtures such as i-PrOH/H2O (1:1; v/v), i-PrOH/H2O (1:3; v/v), i-PrOH/H2O (3:1; v/v)
o
were tested in the presence of K2CO3 as a base at 80 C for 8 h (Table 2, entries 6-11). The
i-
polar solvents such as PrOH, EtOH, and DMF afforded good yields of coupling products.
For this aim, we tried a different combination and ratio of solvents. Although H2O as solvent
afforded lower yields, the highest yields were obtained when a combination of i-PrOH used as
a cosolvent with H2O. When a mixed solvent of i-PrOH/H2O (1:3; v/v) was used, a good yield
was observed (Table 2, entry 10). The addition of water to C-C cross-coupling reactions as
co-solvent may have significant profits. According to the literature [35], the high polarity of
water can have an accelerating effect on the oxidative addition of aryl halides as well as
increase solubility of the base. Also, the addition of water may be causing the displace of
labile ligands such as especially iodide and bromide from the metal center. As a result of the
i-
optimization experiments, a very good yield (99%) was obtained with K2CO3 base and
PrOH/H2O (1:3; v/v) solvent system, at 80 °C for 8 h in the air (Table 2, entry 10). When the
reaction time is regularly reduced from 8 h to 2 h or 4 h at 80 °C, a noticeable effect on the
yield was observed, the yield dropped to 24 an 57% (Table 2, entries 12,13]. Also, When the
reaction temperature was reduced from 80°C to 50 °C in the same condition, no noticeable
11

effect on the yield dropped to 61% (Table 2, entry 14). Palladium black formation was
observed at the end of the reaction in all reactions. When the less reactive aryl chloride was
used as a substrate in the presence of catalyst no conversion was observed.
With the optimized reaction conditions in hand, we screened different aryl bromides for
this catalytic system. As presented in Table 3, both electron-donating and electron-
withdrawing aryl bromides and chlorides were turned to coupled products in good yields (93–
i-
99%) with phenylboronic acid in PrOH/H2O mixture in a reasonable time at room
temperature in presence of 9-15 catalysts (Table 3). It is worth mentioning that this catalytic
system was effective for activated and inactivated aryl bromides. In the presence of the same
amount catalyst loading, good yields were observed Although small differences in reactivities
were observed for 9-15 catalyst due to the similar nature of the Schiff base moieties, it can say
that the most effective catalyst in this study is 13.
A proved mechanism of the reaction is shown in Fig. 5 [36]. A large variety of
palladium(0) complexes are used as catalysts. Palladium(II) complexes are also used as
precursors of palladium(0) complexes in the presence of a reducer [37]. The oxidation of
solvent alcohol molecules causes the reductive Pd(0) active species, indicating that the solvent
alcohol also reacts as a reducing reagent [38]. There are three steps in the catalytic cycle:
oxidative addition, transmetallation and reductive elimination. Based on theoretical researches
[39,40], the steric and electronic properties of ligands could be all essential for catalysis. In
the main ones, the oxidative addition is influenced by electronic parameters. However,
electronic and steric factors could control the transmetallation and reductive elimination
processes.
4. CONCLUSIONS
In this study, seven azo-azomethine ligands and their Pd(II) complexes have been
synthesized and characterized by elemental HRMS analyses, IR, UV-Visible spectroscopy
1
and H-13C NMR spectra. Solid state structures of 2, 6 and 7 were determined by single
crystal X-ray diffraction studies.
The palladium complexes were used as catalysts in the synthesis of biaryls using
Suzuki-Miyaura cross-coupling reaction between the phenylboronic acid and aryl bromides.
When the catalytic studies were evaluated, it was found that all of the complexes were active
12

catalyst for the Suzuki-Miyaura cross-coupling reaction. As a result, these catalytic reactions
provides many advantages such as low catalyst loading, very good yields, green solvent
system, simple and efficient procedure, short reaction times under an air atmosphere.
Acknowledgments
This work was financially supported by the Unit of Coordination of Scientific Research
Projects, Kahramanmaraş Sütçü Imam University, Kahramanmaraş, Turkey (Project
no: 2019/3-29 M). The authors acknowledge Scientific and Technological Research
Application and Research Center, Sinop University, Turkey, for the use of the Bruker D8
QUEST diffractometer and Central Laboratory, METU, Turkey, for the use of the Waters
Synapt G1 instrument.
Appendix A. Supplementary data
Electronic supplementary information (EIS) available: Bond lengths, angles and
hydrogen bond parameters, 1H and 13C-NMR spectra of ligands, HRMS spectra of complexes
and UV-Vis spectra of ligand and complexes.
Crystallographic data (cif files) have been deposited to the Cambridge Crystallographic Data
Centre (CCDC 1960433, 1919987 and 1919988 for 2, 6 and 7, respectively).
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18

Table 2 Optimization of the Suzuki-Miyaura coupling reactions of phenylboronic acid with 4-
bromooacetophenone by complex 9.a
Entry
1
Solvent
Base
Yield
47
TON
47
i-PrOH
Na2CO3
2
3
4
i-PrOH
i-PrOH
i-PrOH
K2CO3
Cs2CO3
NaOH
69
66
58
69
66
58
5
i-PrOH
KOH
49
49
6
7
8
DMF
H2O
K2CO3
K2CO3
K2CO3
24
10
12
24
10
12
EtOH
9
i-PrOH / H2O (1:1; v/v)
i-PrOH / H2O (1:3; v/v)
K2CO3
64
64
10
K2CO3
99
99
11
12
13
14
i-PrOH / H2O (3:1; v/v)
i-PrOH / H2O (1:3; v/v)
i-PrOH / H2O (1:3; v/v)
i-PrOH / H2O (1:3; v/v)
K2CO3
K2CO3
K2CO3
K2CO3
72
72
24
57
61
24b
57c
61d
a
Reaction conditions: complex 9 (0.01 mmol, 1 mol %), 4-bromoacetophenone (1.0 mmol),
phenylboronic acid (1.5 mmol), base (2.0 mmol), solvent (4 mL), 80 oC, 8 h. The yields were
determined by GC for which the calibrations were based on decane. b80 oC, 2 h. 80 oC, 4 h.
c
d50 oC, 8 h.

Table 3 Catalytic activities of complexes (9-15) in the Suzuki-Miyaura coupling reactions of
phenylboronic acid with aryl bromides.a
Entry
1
Aryl bromide
[Pd]
9
Biaryl product
Yield (%) TON
91
96
94
88
95
99
94
93
95
95
95
99
98
94
99
97
98
96
99
94
99
95
94
94
95
99
95
93
91
96
94
88
95
99
94
93
95
95
95
99
98
94
99
97
98
96
99
94
99
95
94
94
95
99
95
93
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
10
11
12
13
14
15
9
10
11
12
13
14
15
9
10
11
12
13
14
15
9
10
11
12
13
14
15
28
a
Reaction conditions: palladium 9-15, (0.01 mmol, 1 mol%), aryl bromide (1.0 mmol),
phenylboronic acid (1.5 mmol), K2CO3 (2.0 mmol), i-PrOH/H2O (1:3; v/v) (4 mL), 80 oC, 8 h.
The yields were determined by GC.

Table 1
Crystallographic data for the compounds.
Identification code
Empirical formula
Crsytal size
2
6
7
C20H16ClN3O
0.14 x 0.10 x 0.05
Block/Red
349.81
C19H13BrClN3O
0.12 x 0.09 x 0.04
Red/plate
414.68
C17H18ClN3O
0.11 x 0.09 x 0.07
Red/plate
315.79
Crystal description
Formula weight
Crystal system
Monoclinic
Pc
Monoclinic
P21/c
Monoclinic
P21/n
Space group
Unit cell
a(Å)
b(Å)
c(Å)
α (°)
β (°)
γ (°)
19.473(2)
7.0627(7)
6.1935(5)
90
32.316(10)
4.3721(12)
12.133(4)
90
6.3774(4)
19.9232(16)
12.8425(10)
90
95.839(3)
90
92.930(15)
90
95.406(3)
90
3
847.37(14)
2
1712.1(9)
4
1624.5(2)
4
Volume (Å )
Z
Abs. coeff. (mm-1)
Refl. collected
0.238
0.256
0.240
18020
19261
28832
Completeness to theta
Ind. Refl. [Rint]
R1, wR2 [I>2σ (I)]
R1, wR2 (all data)
CCDC
99.0%
98.0%
98.9%
3187 [0.0563]
0.0552, 0.1194
0.0747, 0.1271
1960433
3292 [0.0966]
0.0785, 0.1182
0.1313, 0.1309
1919987
3147 [0.0575]
0.0916, 0.1902
0.1200, 0.2040
1919986

Fig. 2. Phenyl-phenol (face to face) stacking interactions in 7.

Fig. 3. Packing diagram of 6 showing stacking and halogen····halogen interactions.

Fig. 4. CH····π (phenyl) and CH····Cl interactions in 2.

This paper reports the preparation, characterization and catalytic activities for some
Palladium(II) complexes of Schiff base ligands that include azo group. The products have
been characterized using elemental analysis, HRMS, FTIR, UV-Vis, NMR (1H, 13C)
spectroscopy and X ray crystallography. These palladium complexes were used as the
catalysts in the synthesis of biaryls using Suzuki-Miyaura cross-coupling reaction. As a result
of these catalytic reactions provides many advantages such as low catalyst loading, very good
yields, green solvent system, simple and efficient procedure, short reaction times under an air
atmosphere

Authors declare that there is no conflict of interest for this article.