Pd(II) complexes with ONN pincer ligand: Tailored synthesis, characterization, DFT, and catalytic activity toward the Suzuki-Miyaura reaction

Article abstract of DOI:10.1016/j.molstruc.2020.129071

A pincer type ONN tridentate Schiff base ligand, 2-(((pyridin-2-yl)methylimino)methyl)-6-methoxyphenol, (L₁) synthesized by the condensation of 4-hydroxy-3-methoxy-benzaldehyde and (pyridin-2-yl)methanamine. The ligand L₁ and the new Pd(II) heteroleptic complexes of the composition [Pd(L₁)(L₂)]Cl, where L₂ = benzimidazole, imidazole, benzooxazol or pyridine were synthesized and characterized by a set of chemical, spectrometric and spectroscopic analyses. These complexes were named 1 to 4, respectively. The FT-IR and DFT have suggested that ligand is coordinated with metal through azomethine-N and phenolic-O and arranged in square planar fashion around the metal. Correlation coefficients value between 0.995 - 0.993 shows satisfactory agreement in theoretical and experimental ¹H-NMR and ¹³C-NMR. Benzimidazole anchored complex 1 exhibits an excellent catalytic activity. DFT calculated the energy profile diagram of the Suzuki-Miyaura reaction.

Full text of DOI:10.1016/j.molstruc.2020.129071

Journal of Molecular Structure 1225 (2021) 129071
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstr
Pd(II) complexes with ONN pincer ligand: Tailored synthesis,
characterization, DFT, and catalytic activity toward the Suzuki-Miyaura
reaction
a,∗
a
a
b
Satyendra Nath Shukla , Pratiksha Gaur , Sanjay Singh Bagri , Ripul Mehrotra ,
a
a
Bhaskar Chaurasia , Mohan Lal Raidas
a
Coordination Chemistry Research Lab, Department of Chemistry, Government Science College, Jabalpur (M.P.) 482001, India
b
Instituto de Quimica Rosario Area Inorganica Facultad de Cs. Bioquimicas y Farmaceuticas Universidad Nacional de Rosario Suipacha 531 S2002LRK
Rosario, Argentina
a r t i c l e i n f o
a b s t r a c t
Article history:
A pincer type ONN tridentate Schiff base ligand, 2-(((pyridin-2-yl)methylimino)methyl)-6-methoxyphenol,
Received 9 December 2019
Revised 2 August 2020
Accepted 11 August 2020
Available online 17 August 2020
(
L1) synthesized by the condensation of 4-hydroxy-3-methoxy-benzaldehyde and (pyridin-2-
yl)methanamine. The ligand L1 and the new Pd(II) heteroleptic complexes of the composition
[
Pd(L1)(L2)]Cl, where L2 = benzimidazole, imidazole, benzooxazol or pyridine were synthesized and
characterized by a set of chemical, spectrometric and spectroscopic analyses. These complexes were
named 1 to 4, respectively. The FT-IR and DFT have suggested that ligand is coordinated with metal
through azomethine-N and phenolic-O and arranged in square planar fashion around the metal. Cor-
Keywords:
Pincer type ligand
Novel Pd(II) complexes
FT-IR
relation coefficients value between 0.995
experimental 1H-NMR and 13 C-NMR. Benzimidazole anchored complex 1 exhibits an excellent catalytic
- 0.993 shows satisfactory agreement in theoretical and
Catalytic activity
Suzuki-Miyaura reaction
activity. DFT calculated the energy profile diagram of the Suzuki-Miyaura reaction.
©
2020 Elsevier B.V. All rights reserved.
1
. Introduction
be the most popular choice for the palladium-catalyzed cross-
coupling reactions [7]. Sometimes these ligands go through the
P–C bond degradation at elevated temperatures, and cause pal-
ladium aggregation; eventually affects the overall catalytic per-
formance [8]. Recently the application of phosphine-free ligands,
viz. N-heterocyclic carbenes, imines, oxime palladacycles, and diaz-
abutadienes to metal-catalysed synthetic transformations have un-
locked the new opening in catalysis [9–14]. Ligand’s peculiar struc-
ture and donor groups have considerable importance in catalytic
performance and determinative in the stability of the active species
and catalyst lifetime. Recently, pincer complexes have received a
lot of attention due to their proper balance between stability and
reactivity. A modern definition of a pincer-type ligand is chelat-
ing agents that bind tightly to three adjacent co-planar sites on
the metal centre and thus serve as to bind transition metals in a
meridional fashion, giving rise to square planar complexes [15,16].
One of the most attractive features of pincer compounds and their
palladium derivatives is the easy and facile tuning of both their
sterics and electronics to improve their catalytic activity in a cer-
tain way [17]. The inflexibility of the pincer-metal interaction con-
fers high thermal stability to the resulting complexes. Transition
metal pincer complexes with high stability and catalytic activity
have found as the most creative area for the homogeneous cataly-
sis [18].
In organic synthesis, cross-coupling reactions are one of the
2
2
most popular approaches to form sp -sp carbon-carbon bond.
Production of several agrochemicals, pharmaceuticals and ad-
vanced organic material viz. conducting polymers, molecular wires
and liquid crystals has performed using this reaction [1–3]. In all
coupling reactions, the Suzuki-Miyaura cross-coupling reaction is
among the most efficient method, nicknamed as “Grignard reaction
of 21st century” due to a massive impact on both academic and in-
dustrial research [4,5]. The high demand for these new-generation
products derived from C-C bonds under mild conditions gave an
impetus to scientists to develop a more efficient catalyst which can
be synthesized easily and work under mild reaction conditions.
Palladium covers all aspects of catalysis, including homoge-
neous, heterogeneous and nanocatalysis [6]. These compounds not
only serve efficiently under mild conditions and produce high
yields but also contribute to stereo-, regio- and chemoselectiv-
ity. In literature, the phosphine-based ligands have claimed to
∗
Corresponding author.
E-mail addresses: sns1963_1@rediffmail.com, ccrl_2004@rediffmail.com (S.N.
Shukla).
https://doi.org/10.1016/j.molstruc.2020.129071
0
022-2860/© 2020 Elsevier B.V. All rights reserved.

2
S.N. Shukla, P. Gaur and S.S. Bagri et al. / Journal of Molecular Structure 1225 (2021) 129071
Fig. 1. Chemdraw and DFT optimized structure of L1
Since the ligand selection will be a decisive step for the ac-
methoxybenzaldehyde (0.760 g, 5 mmol) was dissolved in
complishment of a catalytic reaction. Initially, a ONN tridentate
pincer type Schiff base ligand (Fig. 1) was synthesized by con-
densation of 4-hydroxy-3-methoxybenzaldehyde with (pyridin-2-
yl)methanamine. Anchoring of complexes is an established strat-
egy to tune the catalytic activity of complexes [19]. Imida-
zole/benzimidazole/benzoxazole is the electron-rich molecule is
selected to anchor the complexes. The synthesized ligand in com-
bination with anchoring moiety was utilized in the synthesis of
four different complexes with the objective to estimate their cat-
alytic potency in Suzuki-Miyaura cross-coupling reaction in diverse
reaction condition. The geometry of ligands and complexes were
optimized by the DFT. The probable mechanism of the reaction and
energy profile diagram has also been studied.
15 mL hot ethanol and added to the solution of (pyridin-2-
yl)methanamine (0.540 g, 5 mmol) dissolved in 20 mL ethanol.
The resulting reaction mixture was initially stirred for 3 h at room
temperature and thereafter refluxed for 9 h under an inert atmo-
sphere. On cooling, the yellow precipitate was obtained which was
filtered off, washed with H O, diethyl ether and recrystallized with
2
hot methanol and dried under vacuum. The ligand was obtained
as a yellow solid (89.7 %, 1085 mg). M.p. 147°C. FTIR (KBr): ν = ν
(O-H) 3371, ν (C-H) 2904, ν (-CH=N) 1629, ν (C=C) 1485, ν (C-O)
−
1
1280, ν (-OCH )asym 1178, ν (-OCH )sym 1082, ν (-CH ) 970 cm
.
3
3
2
−1
−1
UV-Vis (ε in mol
L cm ) in DMSO: 360 (568), 240 (1235) nm.
1
H-NMR (400 MHz, DMSO-d ); δ = 13.10 (s, 1H, OH), 8.77 (s,
6
1H, CH=N), 7.12 (dd, J = 8.8 Hz, J = 3.6 Hz, 1H)
phenolic
, 7.07 (dd,
phenolic
J = 5.2 Hz, J = 2.8 Hz, 1H)
, 6.87 (t, J = 8.8 Hz, 6.8 Hz, 1H)
2-pyridyl
2. Materials and methods
phenolic, 7.47 (t, J = 3.6 Hz, 3.2 Hz, 1H)
-pyridyl
, 7.43 (t, J = 3.6 Hz,
3
.2 Hz, 1H)2
, 7.29 (dd, J = 6.8 Hz, 8.4 Hz, 2H)
, 4.54 (d,
2-pyridyl
Palladium chloride (99 %), benzimidazole (98 %), imidazole (99
13
1
J = 1.6 Hz, 2H), 3.88 (s, 3H, OCH ) ppm. C{ H} NMR (400 MHz,
3
%
), pyridine (99.9 %), chlorobenzene (99 %), bromobenzene (99 %),
DMSO-d6): δ = 162.1 (CH=N), 158.1, 157.3, 150.1, 149.4, 141.2,
2 3
iodobenzene (98 %) were purchased from E. Merck, India. However,
o/m/p-iodoaniline (All 98 %), o/m/p-iodonitrobenzene (All 97 %),
o/m/p-iodoanisole (All 98 %), o/m/p-bromobenzaldehyde (All 98 %),
o/m/p-iodophenol (All 98 %), 4-hydroxy-3-methoxybenzaldehyde
1
30.1, 128.1, 127.5, 122.9, 121.8, 117.8, 61.0 (CH ), 55.2 (OCH ) ppm.
Anal. Calcd. For C14 H14 N O : C, 69.41; H, 5.82; N, 11.56. Found:
2
2
+
+
C, 69.37; H, 5.78; N, 11.51. ESI-Mass spectra m/z [C H N +H ] =
7
+
7
+
2
+
+
1
20.021, [C H O +H ] = 124.060, [C
H
14 14
N O +H ] = 243.268.
7
7
2
2
2
(
99 %) (pyridin-2-yl)methanamine (95 %), phenylboronic acid (98
), benzoxazole (97 %) purchased from Sigma Aldrich were
M τ = 242.273.
%
used as received. Analytical reagent grade(>95 %) methyl alco-
hol, ethyl alcohol, acetonitrile, chloroform, acetone, dimethylfor-
mamide, dimethylsulphoxide and other solvents were used. Elec-
tronic absorption spectra were recorded with EI- 2371 UV-Visible
double beam spectrophotometer equipped with PC. Conductivity
measurements were carried out at 25°C on the EI-181 digital
conductivity bridge with a dipping type cell in DMSO. Fourier-
transformed Infrared spectra were recorded in KBr pellets on a
Shimadzu-8400 PC FT-IR spectrophotometer in the region of 4000-
2
.2. Synthesis of complexes
2
.2.1. [Pd(L )bzimdz]Cl; 1
1
PdCl2 (0.177 g, 1 mmol) dissolved in 10 mL methanol was added
dropwise with stirring for 40 min into a solution of benzimidazole
0.260 g, 1 mmol) dissolved in 15 mL of hot methanol. This mix-
(
ture was added to a solution of L1 (0.242 g, 1 mmol), dissolved
in 20 mL hot methanol. The reaction mixture was stirred for 2 h
at room temperature and thereafter refluxed for 8 h under an inert
atmosphere. Progress of the reaction was observed by TLC. On slow
evaporation a light-brown colour solid was obtained which was fil-
tered, washed with water, diethyl ether and recrystallized from 1:
−
00 cm 1 and the spectra were drawn in IR Solution software (ver-
4
sion 1.20, published 2004) with the number of scan 25 and resolu-
tion of 4 cm−1. Elemental analyses for carbon, hydrogen and nitro-
gen were performed using an Elementar Vario EL III analyzer. 1H-
2
: 3, acetonitrile: acetone: chloroform (v/v/v) solvent mixture to
Nuclear Magnetic Resonance spectra were recorded on a Jeol spec-
trometer at 400 MHz. The 13C-Nuclear Magnetic Resonance spectra
yield light brown crystalline solid, which was dried in a desiccator
over anhydrous calcium chloride under vacuum. Unfortunately, af-
ter several attempts, we did not find any crystal suitable for single-
crystal XRD. Complex 1 was obtained as a light brown solid (77.2%,
were recorded in Bruker Avance-400 (FT-NMR) using Topspin 2.1
software. The Electrospray Ionisation-Mass Spectra were recorded
on Agilent 6520 (QTOF) mass spectrophotometer. The metal con-
tents were analyzed gravimetrically by the literature procedure
3
87 mg). M.p. > 300°C. FTIR (KBr): ν = ν (N-H) 3061, ν (C-H) 2941,
ν (-CH=N) 1608, ν (C=C) 1438, ν (C-O) 1265, ν (-OCH )asym 1184,
[
20]. Thermogravimetric analysis was performed on Perkin Elmer
3
−1
ν (-OCH )sym 1035, ν (–CH ) 972, ν (M-O) 532, ν (M-N) 432 cm
.
TGA 4000 Instrument Type: Pyris 6 TGA by measuring mass losses
3
2
Molar conductance Ʌm at 25°C (ꢀ 1 cm2 mol ): 18 in DMSO),
−
−1
from 30 °C – 900 °C at a rate of 10 °C/min.
−1
−1
1
24 in acetonitrile. UV-Vis (ε in mol
L cm ) in DMSO: 720
2
2
.1. Synthesis of Schiff base ligand
(156), 605 (223), 460 (1257), 400 (1784) nm. 1H-NMR (400 MHz,
-(((pyridin-2-yl)methylimino)methyl)-6-methoxy-phenol; L1
DMSO-d ); δ = 8.89 (s, 1H, CH=N), 11.50 (s, 1H, NH)bzimdz, 8.50 (d,
6
J = 3.2 Hz, 1H)
bzimdz
, 8.40 (d, J = 11.2 Hz, 1H)
, 8.23 (m, 2H)
bzimdz
bzimdz
The ligand was synthesized by the method reported
, 8.11 (s, 1H)
, 7.84 (dd, J = 10.6 Hz, 5.2 Hz, 2H)2-pyridyl,
bzimdz
in the literature [21]. The starting material 4-hydroxy-3-
7.53 (t, J = 2.4 Hz, 2.8 Hz, 1H)2- pyridyl, 7.40 (t, J = 4.8 Hz, 6.4 Hz,

S.N. Shukla, P. Gaur and S.S. Bagri et al. / Journal of Molecular Structure 1225 (2021) 129071
3
1
H)2-pyridyl, 7.27 (dd, J = 10.0 Hz, 5.6 Hz, 2H)
, 7.05 (t, J = 14
(KBr): ν = ν (N-H) 3062, ν (C-H) 2926, ν (-CH=N) 1613, ν (C=C)
1434, ν (C-O) 1262, ν (-OCH )asym 1177, ν (-OCH )sym 1076, ν (–
phenolic
Hz,14.4, 1H)
, 4.54 (d, J = 3.6 Hz, 2H, -CH ), 3.88 (s, 3H, -
phenolic
1
2
3
3
OCH ) ppm. 13C{ H} NMR (400 MHz, DMSO-d6): δ = 169.0 (C=N),
−1
CH2) 976, ν (M-O) 543, ν (M-N) 439 cm . Molar conductance Ʌm
3
at 25°C (ꢀ cm2 mol ): 17 in DMSO, 130 in acetonitrile. UV-Vis
− −1
1
164.7 (C-OH)
, 158.5 (C-O-CH )
, 129.0 (C-C=N)phenolic,
phenolic
3
phenolic
(ε in mol 1 L cm ) in DMSO: 710 (143), 615 (204), 450 (1228),
−
−1
1
23.6 (C-C=N)
, 121.5 (C=C)
2-pyridyl
, 119.7 (C=C)
, 154.5
phenolic
phenolic
phenolic
(
C=N)
, 150.8 (C-N)
, 144.9 (C-C)
2-pyridyl
, 128.1 (C=C-
415 (1724) nm.
2-pyridyl
1
N)
, 120.6 (C=C)
, 146.9 (N=C-NH)
, 146.3 (C-
bzimdz
H-NMR (400 MHz, DMSO-d6): δ = 8.91 (s, 1H, CH=N), 8.48 (d,
2-pyridyl
2-pyridyl
N)bzimdz, 145.4 (C-N)
, 127.4 (C-C)
, 126.9 (C-C)bzimdz, 117.3
bzimdz
J = 2.4 Hz, 1H)bzoxzl, 8.39 (d, J = 15.2 Hz, 2H)bzoxzl, 8.28 (t, J = 2.4
Hz, 2.2 Hz, 1H)bzoxzl, 8.15 (d, J = 2.0 Hz, 1H)bzoxzl, 8.06 (s, 1H),
7.75 (d, J = 2.4 Hz, 1H)2- pyridyl, 7.50 (t, J = 2.0 Hz, 1H)2- pyridyl,
7.38 (t, J = 1.6 Hz,1H)2- pyridyl, 7.29 (d, J = 1.6, 1H) 2- pyridyl, 7.18
(d, J = 3.6 Hz, 1H) phenolic, 7.03 (t, J = 1.6 Hz, 1H) phenolic, 4.52 (d,
bzimdz
(
C=C-N)
, 116.8 (C=C-N)
, 62.9 (-CH ), 55.9 (-OCH ) ppm.
bzimdz
2 3
bzimdz
Anal. Calcd for C
H ClN O Pd: C, 50.22; H, 3.61; N, 8.37; Pd, 21.19
19 4 2
21
Found C, 50.19; H, 3.56; N, 8.34; Pd, 21.15. ESI-Mass spectra m/z
+
+
+ +
[
[
C14 H13N O +H ] = 242.260 [C20H16 ClN O Pd+H ] = 486.257,
2
2
4
2
+
+
13
1
C21H19ClN O Pd+H ] = 502.265. M τ = 501.274.
J = 3.2 Hz, 2H, -CH ) 3.87 (s, 3H, -OCH ) ppm. C{ H} NMR (400
4
2
2
3
MHz, DMSO-d6): δ = 166.3 (C=N), 164.8 (C-OH)phenolic, 155.8 (C-
O-CH3) phenolic, 129.0 (C-C=N) , 123.8 (C-C=N)phenolic, 121.1
2
.2.2. [Pd(L )imdz]Cl; 2
1
phenolic
Palladium chloride (0.177 g, 1 mmol) and imidazole (0.068 g, 1
(C=C)phenolic, 117.5 (C=C)phenolic, 146.9 (C=N)2-pyridyl, 146.3 (C-
mmol) were dissolved in 25 mL methanol by stirring for 40 min.
This mixture was added to a solution of L1 (0.242 g, 1 mmol), dis-
solved in 25 mL hot methanol in a flat bottom flask and stirred
for 2 h at room temperature and thereafter refluxed for 9 h un-
der an inert atmosphere. Progress of the reaction was observed by
TLC. A solid was obtained in each case after refluxing the reac-
tion mixture was filtered and washed several times with acetone.
The solid obtained was recrystallized from 1: 2: 3, acetonitrile:
acetone: chloroform (v/v/v) solvent mixture to yield a dark brown
crystalline solid, which was dried in a desiccator over anhydrous
calcium chloride under vacuum. Complex 2 was obtained as a dark
brown solid (69.8%, 315 mg) M.p. > 300°C. FTIR (KBr): ν = ν (N-
H) 3069, ν (C-H) 2934, ν (-CH=N) 1602, ν (CONH) 1447, ν (C=C)
N) -pyridyl, 144.7 (C=C)2-pyridyl, 127.5 (C=C) 120.7 (C=C)
2
2-pyridyl,
, 157.4 (N=C-O)bzoxzl, 150.8 (C-O)bzoxzl, 145.4 (C-N)bzoxzl
,
2-pyridyl
128.3 (C=C)bzoxzl, 126.6 (C=C)bzoxzl, 119.8 (C=C-N)bzoxzl, 116.2
(C=C-O)bzoxzl, 62.8 (CH ), 55.4 (-OCH3) ppm. Anal. Calcd for
2
C
H ClN O Pd: C, 50.18; H, 3.58; N, 8.33; Pd, 21.14. Found
18 3 3
21
C, 50.22; H, 3.61; N, 8.37; Pd, 21.19. ESI-Mass spectra m/z
+
+
+ +
[C14 H13N O +H ] = 242.269, [C21H18 N O Pd+H ] = 467.849,
2
2
3
3
+
+
[C21H18 ClN O Pd+H ] = 503.265. M τ = 502.258.
3
3
2.2.4. [Pd(L )py]Cl; 4
1
PdCl2 (0.177 g, 1 mmol) and pyridine (0.0791 mL, 1 mmol)
were dissolved in 25 mL methanol by stirring for 50 min. This
mixture was added to a solution of L1 (0.242 g, 1 mmol), dis-
solved in 25 mL hot methanol in a flat bottom flask and stirred
for 2 h at room temperature and then refluxed for 7 h under an
inert atmosphere. Progress of the reaction was observed by TLC.
The brown coloured solid was obtained after a few days, which
was filtered and washed with diethyl ether. The brown solid ob-
tained was recrystallized from 1: 2: 3, acetonitrile: acetone: chlo-
roform (v/v/v) solvent mixture to yield brown crystalline solid,
which was dried in a desiccator over anhydrous calcium chloride
under vacuum. Complex 4 was obtained as a brown solid (69.9%,
319 mg). M.p. >300°C. FTIR (KBr): ν = ν (N-H) 3048, ν (C-H)
1
418.25 (s), ν (C-O) 1271, ν (-OCH )asym 1174, ν (-OCH )sym 1087,
3
3
ν (-CH ) 980, ν (M-O) 538, ν (M-N) 442 cm 1. Molar conductance
−
2
Ʌm at 25°C (ꢀ 1 cm2 mol ): 16 in DMSO, 128 in acetonitrile.
−
−1
−1
−1
UV-Vis (ε in mol
L cm ) in DMSO: 705 (134), 620 (212), 445
(
(
(
1248), 405 (1761) nm. 1H-NMR (400 MHz, DMSO-d6): δ = 8.88
imidz
s, 1H, CH=N), 11.48 (s, 1H, NH), 8.39 (d, J = 2.8 Hz, 1H)
, 8.38
d, J = 2.0 Hz, 1H)
-pyridyl
, 8.24 (s, 1H)
, 7.81 (dd, J = 8.4 Hz, 4.9
, 7.37 (dd, J = 7.2 Hz,
imidz
imidz
Hz, 1H)2
, 7.53 (t, J = 4.4 Hz, 2H)
2-pyridyl
4
.4 Hz, 1H)2
, 7.25 (dd, J = 9.6 Hz, 2.4 Hz, 1H)
phenolic
, 7.17
phenolic
- pyridyl
(dd, J = 7.5 Hz, 2.2 Hz, 1H)
phenolic
, 7.05 (t, J = 6.8 Hz, 6.4 Hz, 1H)
,
4.52 (d, J = 3.8 Hz, 2H, -CH ), 3.84 (s, 3H, -OCH ) ppm.
2938, ν (-CH=N) 1605, ν (C=C) 1448, ν (C-O) 1254, ν (-OCH )asym
2
3
3
13
1
C{ H} NMR (400 MHz, DMSO-d6): δ = 165.6 (C=N), 161.5
phenolic
1171, ν (-OCH3)sym 1089, ν (-CH2) 983, ν (M-O) 528, ν (M-N) 436
cm 1. Molar conductance Ʌm at 25°C (ꢀ
− −1
DMSO, 124 in acetonitrile. UV-Vis (ε in mol
cm2 mol ): 20 in
−1
(
(
C-OH)phenolic, 150.1 (-C-OCH3)phenolic, 129.0 (C-C=N)
, 127.4
−1
−1
C=C)phenolic, 120.5 (C=C)phenolic, 116.3 (C=C)
,
155.5 (C-
L cm ) in DMSO:
phenolic
715 (151), 610 (214), 440 (1237), 410 (1772) nm. 1H NMR (400
MHz, DMSO-d6): δ = 8.89 (s, 1H, CH=N), 8.39 (dd, J = 2.0 Hz,
8.8 Hz, 2H) pyridine, 8.30 (t, J = 2.0 Hz, 2.4 Hz, 2H)pyridine, 8.24
(t, J = 3.6 Hz, 2.0 Hz, 1H)pyridine, 7.82 (dd, J = 7.2 Hz, 2.0 Hz,
1H)2-pyridyl, 7.56 (t, J = 1.2 Hz, 1.6 Hz, 2H)2-pyridyl, 7.35 (dd, J = 1.6
Hz, 1.2 Hz, 1H)2-pyridyl, 7.24 (dd, J = 12.4 Hz, 3.6 Hz, 2H)phenolic,
7.04 (t, J = 1.2 Hz, 1.6 Hz, 1H)phenolic, 4.53 (d, J = 3.2 Hz, 2H, -
N)
, 148.0 (C=N)
, 145.8 (C=C)
, 128.1 (C=C-
2-pyridyl
2-pyridyl
2-pyridyl
N)2-pyridyl, 117.5 (C=C)
, 147.8 (N-C=N)
, 145.1 (C-N)
,
imidz
2-pyridyl
imidz
1
19.8 (C=C-N)
, 62.4 (-CH ), 55.1 (-OCH ) ppm. Anal. Calcd
imidz
2 3
for
C H ClN O Pd: C, 45.25; H, 3.80; N, 12.42; Pd, 23.59.
17 17 4 2
Found C, 45.21; H, 3.77; N, 12.38; Pd, 23.54. ESI-Mass spec-
+
+
+
+ +
tra m/z [C14 H13N O +H ] = 242.265, [C17 H17 N O Pd+H ] =
2
2
4
2
+
416.759, [C17 H17 ClN O Pd+H ] = 452.202. M τ = 451.215.
4
2
13
1
CH ), 3.82 (s, 3H, -OCH ) ppm. C{ H} NMR (400 MHz, DMSO-
2
3
2
.2.3. [Pd(L )bzoxzl]Cl; 3
d6): = 164.4 (CH=N), 162.5 (C-OH)phenolic, 154.5 (C-O-CH3)phenolic
phenolic,
,
1
PdCl2 (0.177 g, 1 mmol) dissolved in 10 mL ethanol was added
128.1 (C-C=N)phenolic, 126.9 (C=C)
116.3 (C=C)phenolic, 147.7 (C-N)
123.6 (C=C)phenolic,
145.6 (C=N) , 144.9
dropwise in a hot ethanolic solution of benzoxazole (0.119 g, 1
mmol). The resulting solution mixture was stirred for 4 h and re-
fluxed for 4 h under an inert atmosphere. This mixture was added
to a solution of L1 (0.242 g, 1 mmol) dissolved in 25 mL hot
ethanol in a flat bottom flask and stirred for 3 h at room tem-
perature and refluxed for 9 h under an inert atmosphere. Progress
of the reaction was observed by TLC. A shiny brown solid was
obtained after slow evaporation of solution, which was filtered,
washed with water, diethyl ether and recrystallized from 1: 2: 3,
acetonitrile: acetone: chloroform (v/v/v) solvent mixture to yield
shiny brown crystalline solid, which was dried in a desiccator over
anhydrous calcium chloride under vacuum. Complex 3 was ob-
tained as a Shiny brown solid (71.5 %, 359 mg). M.p. >300°C. FTIR
2-pyridyl
2-pyridyl
(C=C)
pyridine
, 127.4 (C=C)
pyridine
, 117.4 (C=C)
pyridine
, 150.9 (C-N)
2-pyridyl
2
-pyridyl
2-pyridyl
, 147.8 (C=N)
pyridine
, 129.0 (C=C)
, 120.5 (C=C)pyridine,
120.0 (C=C)
, 62.3 (-CH ), 55.6 (-OCH ) ppm. Anal. Calcd.
2 3
for C
H ClN O Pd: C, 49.33; H, 3.89; N, 9.04; Pd, 22.98. Found:
18 3 2
19
C, 49.37; H, 3.93; N, 9.09; Pd, 23.02. ESI-Mass spectra m/z
+
+
+ +
[C14 H13N O +H ] = 242.262, [C19 H18 N O Pd+H ] = 427.854,
2
2
3
2
+
+
[C19 H18 ClN O Pd+H ] = 463.215. M τ = 462.238.
3
2
2.3. DFT study
To understand the structural characteristics and vibrational
properties of the L1 and their complexes density functional theory

4
S.N. Shukla, P. Gaur and S.S. Bagri et al. / Journal of Molecular Structure 1225 (2021) 129071
Table 1
The calculated quantum chemical parameters of L1 and complex 1-4
Compounds
L1
Complex 1
Complex 2
Complex 3
Complex 4
EHOMO (eV)
ELUMO (eV)
-6.4125
-3.1553
3.2572
4.7839
1.6286
0.6140
-4.7839
0.3070
7.0261
2.9374
5.3514
-1248.2
-5.9247
-3.9631
1.9616
4.9439
0.9808
1.0195
-4.9439
0.5097
12.4603
5.0460
12.7951
-2315.4
-4.6859
-3.3185
1.3674
4.0022
0.6837
1.4626
-4.0022
0.7313
11.7139
5.8537
10.4095
-1902.8
-6.4198
-4.6229
1.7969
5.5213
0.8984
1.1130
-5.5213
0.5565
16.9634
6.1454
11.2438
-2135.7
-6.1464
-4.8458
1.3006
5.4961
0.6503
1.5377
-5.4961
0.7688
18.2255
8.4516
7.6236
-1761.0
ꢀ
E
Mulliken electronegativity χ
Global Hardness η
Absolute softness σ
Chemical Potential μ
Global Softness S
Global Electrophilicity ω
Electronic Charge ꢀN
Dipole Moment μ (Debye)
E (TD-HF/TD-KS)
(
DFT) method of B3LYP with 6-311++G(d,p) and/or 3-21+G∗ basic
structure of L1 and complex 1-4. The elucidated quantum chem-
ical parameters are shown in Table 1. The negative values of both
ELUMO and EHOMO were indicative of the stability of synthesized lig-
and and complexes. The energy gap (ꢁE) value of ligand is high
which indicates its high stability, hard nature, less reactivity and
flexibility in donating electron from donor atom to metal. The
HOMO is outermost filled orbital which behaves as an electron
donor and is directly related to ionization potential, while LUMO
act as an electron acceptor, greater in energy and related to elec-
tron affinity [30]. Since the energy gap, ꢁE for complexes lies in
between 1.3006 – 1.9616 eV, which indicate that the samples can
behave as semiconductors. However, with the energy gap, we are
mainly concern with the reactivity of the molecules. The theoret-
ical chemical shift values were calculated by GAIO method using
TMS HF/6-31G(d)GIAO and TMS B3LYP/6-311+G(2d,p)GIAO level of
theory. The predicted chemical shift values were in good agree-
ment with experimental values.
set for all nonmetallic atoms and Los Alamos National Laboratory
2
double zeta (LANL2DZ) basic set for the central metal atoms in
gas phase was used [22]. No “PCM” model was utilized. Gaussian-
9 software package was employed to carry out all the quantum
0
chemical calculations [23]. The DFT calculations were performed
using the B3LYP parameter, which includes Becke’s gradient ex-
change correction with the Lee, Yang, Parr correlation functional
[
24,25]. Optimized structural parameter of the compounds such
as bond lengths, bond angles and dihedral angles were calculated
with the atom numbering scheme of the molecule. The energy of
the HOMO, LUMO levels, energy gap, absolute hardness (η), ener-
gies (ꢁE), absolute softness (σ), global softness (S), chemical po-
tential (Pi), electronic charge (ꢁNmax), dipole moment (μ), global
electrophilicity (ω), total energy (E-TD-HF/ETD-KS) and Mulliken
electronegativity (χ) have been determined [26]. The 1H-NMR and
13C-NMR chemical shifts of the molecule were calculated by the
gauge-independent atomic orbital (GIAO) method and compared
with experimental results. The 1H-NMR and 13C-NMR chemical
shifts of the molecule were calculated by the gauge-independent
atomic orbital (GIAO) method and compared with the experimen-
tal result [27].
3
.1.2. Mulliken atomic charge analysis
The Mulliken population analyses of L1 and complexes were
done in order to calculate net atomic charges in ligand and com-
plexes [31]. Atomic charge analysis has an important role in the
molecular system because it affects dipole moment, molecular po-
larizability and other properties of the molecule. The result shows
that in L1 the highest positive charge exists on H27 (+0.3407)
because it is attached with the electronegative oxygen atom. The
highly shielded electronegative atom O26 possesses a charge of –
2
.4. Catalytic activity over Suzuki-Miyaura reaction
The solvent/solvent mixture used in the reaction was de-
gassed through the Freeze-pump-claw method [28,29]. A three-
neck round bottom flask equipped with water condenser, magnetic
0.4812. However, in complex 1-4 the highest positive charge was
stir bar and a N -inlet. The flask was charged with degassed sol-
2
observed at Pd metal centre. The other electronegative atoms such
as phenolic oxygen and azomethine nitrogen contain a negative
Mulliken charge because of high electron density on these atoms.
The N atoms of the anchored heterocyclic ring are electron-rich
and possess negative Mulliken charge. Electropositive carbon and
electronegative oxygen/nitrogen exhibit higher Mulliken charge
probably due to the occurrence of back bonding. These results are
consistent with the molecular electrostatic potential (MEP) map.
Mulliken atomic charge plot and pattern of ligand and complex 1
are displayed in Fig. 2 (a) and (b).
vent (100 mL), base i.e. potassium carbonate (0.75 mmol) and 1-4
(
(
0.1-0.6 mmol). The phenylbronic acid (0.6 mmol) and aryl halide
0.5 mmol) was added by glass syringe through a rubber septum.
The mixture was allowed to stir at 25-100 °C for 2-12 h and then
cooled to room temperature. It was further filtered through the
bed of silica. The reaction progress was monitored periodically by
TLC. The product was extracted with ethyl acetate (3 × 10 mL). The
combined organic phase was washed with brine (2 × 10 mL) and
dried over Na SO . The solvent was removed under reduced pres-
2
4
sure to afford crude. The residue obtained was purified through
column chromatography on silica gel eluting with ethyl acetate.
Column chromatography was performed to obtain the pure prod-
3
.1.3. Molecular electrostatic potential analysis (MEP) and contour
map
The molecular electrostatic potential (MEP) is an important pa-
uct with eluting the column ethyl acetate: hexane (5: 95; v/v) sol-
vent mixture. Product was characterized by ESI-MS, 1H-NMR and
13C-NMR.
rameter to explore the different site for electrophilic/nucleophilic
reactions and hydrogen-bonding interactions. The molecular elec-
trostatic potential, (r), at a given point r (x, y, z) in the vicinity of
a molecule is defined in terms of the interaction energy between
the electrical charge generated from the molecule electrons and
nuclei. It also provides a visual method to understand the reactiv-
ity of the molecules and widely used for predicting the sites that
are susceptible to nucleophilic and electrophilic attacks on molec-
3. Results and discussion
3
.1. Quantum chemical study
To attain more insight into the molecular structure, density
functional theory calculations were carried out to optimize the

S.N. Shukla, P. Gaur and S.S. Bagri et al. / Journal of Molecular Structure 1225 (2021) 129071
5
Fig. 2. Mulliken atomic charge plot of (a) L1 and (b) complex 1
Fig. 3. Molecular electrostatic potential (MEP) of (a) L1 and (b) complex 1
ular species [32]. In orde to predict the electrophilic and nucle-
ophilic sites for the title molecule, MEP was calculated using the
B3LYP/LanL2DZ method with the 6-31++G(d,p) basis set and op-
timized geometries. The negative (red) regions of MEP were re-
lated to electrophilic reactivity and the positive (blue) regions to
nucleophilic reactivity. The MEP maps of L1 and complex 1 were
shown in Fig. 3 (a) and (b). The negative regions (deep red) are
mainly over the electronegative atoms such as hydroxyl oxygen
and azomethine nitrogen. The carbon and hydrogen atom bears the
positive charge and shown in blue/green colour. The electrostatic
found to be -3.1553 eV and -3.9631 eV. Since complex 1 exhibit
the highest ꢁE among the three complexes, it is likely to have the
highest stability. From the calculated energies of the highest occu-
pied molecular orbital (HOMO) and lowest unoccupied molecular
orbital (LUMO), it may be concluded that:
1
The difference of FMOs energies in complexes are for EHOMO is
in the order 2 > 1 > 4 > L1 >3, and for ELUMO the order is L1
>
2 > 1 > 3 >4.
2
Complex 4 has low values of the energy gap (ꢁE) suggesting
−
2
−2
lower stability and high reactivity.
potential in ligand ranges between –4.359 × 10 to 4.359 × 10
−2
−2
3 Complex 1 exhibit a high value of the dipole moment, which
au. In complex 1 it ranges from –8.916 × 10 to 8.916 × 10 au.
may favour dipole-dipole interactions between molecules.
Similarly, in Complex 2, 3 and 4 potential values were observed as
−
2
−2
−2
−2
–
8.762 × 10 to 8.762 × 10 au, –8.872 × 10 to 8.872 × 10
−2
−2
au, and –8.845 × 10
to 8.845 × 10
au respectively. The dif-
3.1.5. Bond parameters
ferent values of the electrostatic potential at the surface are rep-
resented by different colours. The potential increases in the order:
red < orange < yellow < green < blue [33].
In the absence of a single crystal, it was essential to optimize
the molecular structure of L1 and complex 1-4 in the ground state.
Table S1 and S2 consist of optimized bond lengths and bond an-
gles of investigated compounds according to the atom numbering
scheme. The comparison of the optimized geometrical parameter
of ligand and complexes has done. The optimized C-O bond length
3
.1.4. Frontier molecular orbitals analysis
HOMO and LUMO are two important molecular orbitals which
˚
are deduced for ligand L1 and complex 1. Chemical stability of the
complexes was established by the difference of energy (ꢁE) be-
tween HOMO and LUMO. Complex with larger energy difference
has a lower chemical reactivity [34]. A suitable cavity was formed
for encapsulating Pd(II) by the Schiff base ligand and the hetero-
cyclic ring was used as anchoring co-ligand. The computed HOMO
energy values of the L1 and complex 1 were -6.4125 eV and -
was 1.4529 A in L but in complexes, it slightly increases, indi-
1,
cating weaker C-O bond. The optimized O-H bond length in L1
˚
was 0.9785 A which diminished in complexes, and a new Pd-O
˚
bond formed with a bond length of ~1.9400 A. The bond length of
˚
azomethine -HC=N in ligand found to be 1.9728 A, which shows
˚
an increase of 0.028-0.064 A in the complexes. This increase in
bond distance was because of the transfer of double bond elec-
tron density towards metal and decrease in the double bond char-
5
.9247 eV (Fig. 4). The LUMO energy of the L1 and complex 1 were

6
S.N. Shukla, P. Gaur and S.S. Bagri et al. / Journal of Molecular Structure 1225 (2021) 129071
Fig. 4. HOMO-LUMO structure with energy level diagram of (a) L1 (b) Complex 1
acter of C=N, which in turn confirms the coordination of metal
with azomethine-N. Unequal bond lengths of Pd-N and Pd-O were
also responsible for the slight deviation. From the bond angle data
trogen with metal was further ascertained by the appearance of a
peak at a lower frequency at ~ 440 cm 1, assign to Pd-N vibration
−
stretching [39]. Complexes also exhibit a band at frequency ~3050
cm 1 assigned for NH vibration. Another band observed at ~2930
−
(
Table S2), it is clear that in complexes N-Pd-N, N-Pd-O bond an-
˚
−1
gles were slightly deviated (1.25-2.11 A). From the bond length and
bond angle data, it is obvious that complexes were almost square
planar with slight distortion.
cm was attributed to C-H vibration of the aromatic ring. The vi-
brational band for C-O stretching was observed at ~1260 cm 1. The
peaks at vibrational frequency ~1180 cm 1 and ~1070 cm
−
−1
−
were
attributed to asymmetric and symmetric C-O stretching of methoxy
group.
3
.2. Characterization of Pd(II) complexes
The experimental (solid phase) and theoretical (gas phase) FT-
IR spectra data of L1 and complex 1 is correlated (Table S3). FT-
IR experimental and theoretical spectrum of L1 is given in Fig. S6.
The calculated IR vibrations were slightly shifted from experimen-
tal values. DFT calculations generally overestimate the frequencies
and neglect the crystal packing effects. However, anharmonicity, as
well as incompleteness of the basis set and dynamic electronic cor-
relation, were also responsible for the above-stated deviations [40].
Therefore, we have derived scaling factors and applied them to get
a satisfactory value. The gas-phase FT-IR frequencies in compounds
were recorded with B3LYP and 6-311G basic set. The scaling factor
for L1 was determined as 1.002 (R2 = 0.999) over a complete range
of spectrum and that in complex 1, it is 0.998 (R2 = 0.999). Cor-
relation of experimental FT-IR spectrum with theoretical IR spec-
trum for L1 and complex 1 is given in Figs. S7-S8. The theoretical
and experimental FT-IR spectrum of complex 1 is shown in Fig. 5.
On comparison it was observed that in the theoretical spectra af-
ter scaling, the peak for ν(N-H), ν(C-H), ν(HC=N), ν(C-O)sym, ν(C-
The stoichiometries of ligand and complexes were in agree-
ment with elemental analyses. ESI-MS of ligand exhibit the
pseudo molecular ion peak at m/z 243.268 attributed
indicates the molecular mass of lig-
=
+
+
for [C14 H14 N O +H ]
2
2
and. Several other peaks of fragments were obtained at m/z:
1
20.021, 124.060, 151.355, 165.071, 212.266 and 226.064 were
+
+
+
+
+ +
assigned for [C H N +H ] , [C H O +H ] , [C H O N+H ] ,
7
+
7
2
7
+
7
+
2
8
8
2
+
+ +
[
C9H10O N+H ] , [C
H
13 11
O N+H ] and [C
H
14 13
N O+H ] re-
2
2
2
spectively. The ESI-MS of L1 is shown in Fig. S1. ESI-Mass spectra of
complexes exhibit several peaks. However, a pseudomolecular ion
+
peak for [M+H ] was inevitably present in each case, which indi-
cates the molecular mass. ESI-MS of complex 1-4 is given in Figs.
S2-S5. The lower value of molar conductance (ꢂm) of metal com-
plexes (16-20 ꢀ cm2 mol ) in DMSO indicates their 1: 1 elec-
−
1
−1
trolytic nature. In acetonitrile molar conductance was high (124 –
2
1
30 ꢀ 1cm mol ) but within the limit of 1:1 electrolyte [35].
− −1
3
.2.1. FT-IR spectra
O)asym, ν(C-H )
, ν(C-H )
ν(Pd-O) and ν(Pd-N) were
out plane,
3
in plane
3
In the FT-IR spectrum of complexes, the band observed at ~3371
observed with a very small deviation of 0.1 %.
cm 1, assigned for -OH vibration in ligands completely vanished,
−
and a new peak at lower frequency appeared at ~530 cm 1, which
was assigned to Pd-O [36,37]. The appearance of this peak indi-
cates the bonding of metal with phenolic oxygen. The band due
to azomethine HC=N groups was shifted downwards by 27 - 16
−
3.2.2. Electronic spectra
In the UV–Vis spectra of ligand an absorption band at ~ 240 nm
was assigned to the intra-ligand π→ π∗ transition and a strong
∗
absorption band at ~ 360 nm was ascribed to n→ π transition as-
−1
cm
in complexes, which confirms the coordination of azome-
sociated with imine moiety. UV-Vis spectrum of the L was given
1
thine nitrogen with metal [38]. The coordination of azomethine ni-
in Fig. S9. The shift of this band (~30 nm) in the spectra of the

S.N. Shukla, P. Gaur and S.S. Bagri et al. / Journal of Molecular Structure 1225 (2021) 129071
7
Fig. 5. FT-IR Experimental (above) and Theoretical (below) spectrum of complex 1
metal complexes suggests the coordination of azomethine nitro-
gen to the metal. The magnetic susceptibility measurements of the
complexes showed that all the complexes are diamagnetic in na-
ture as expected for d8 configuration. The spectra of the metal
complexes show four bands due to three d–d spin allowed transi-
tions. The electronic spectrum of Pd(II) displayed d–d transitions
the 2H of CH2 protons. The J value for these germinal protons was
lower than expected probably due to decrease in s-character by
the presence of 2-pyridyl ring one side and more electronegative
azomethine-N on the other. The singlet at 3.88 ppm was due to the
3H of –OCH3 proton of the vanillin ring. The 1H-NMR spectrum of
the ligand is given in Fig. S11. The theoretical chemical shift values
were calculated by the GIAO method using TMS HF/6-31G(d) GIAO
and TMS B3LYP/6-311+G(2d,p) GIAO level theory. For 1H-NMR of
around 700, 600, 450 and 400 assigned to 1A1g
→
3A2g (ν1),
1A1g
→
1A2g (ν ), 1A1g
→
1
Eg (ν ) and 1A1g
→
1B (ν ) transi-
2
3
1g
4
tions. Electronic spectral data was suggestive of square planar ge-
ometry around Pd(II) ion [41,42]. UV-Vis spectrum of the complex
ligand calculated correlation coefficients is δ = 1.037δexp - 0.089
cal
(R2 = 0.995). 1H-NMR correlation diagram for L1 is given in Fig.
1
, was given in Fig. S10.
S12.
In complexes, the downfield shifting of the azomethine (-
CH=N-) proton signal by 0.11 - 0.13 ppm confirmed the coordina-
tion of the azomethine HC=N group attributed to the discharging
of the electronic cloud towards the Pd(II) ion [44]. The hydroxyl
OH proton signal observed at 13.10 ppm in the ligand disappeared
in the spectra of complexes, indicating deprotonation and coordi-
nation of the oxygen with the metal ion [45]. Two double doublets
centred at ~ 7.80 ppm and ~ 7.40 ppm for one proton each num-
bered as 23 and 26 and a triplet centred at ~ 7.50 ppm for two
protons numbered as 24 and 25, were attributed to 2-pyridyl ring
protons. The three aromatic protons of the phenolic ring appeared
as two doublets centred at ~ 7.25 ppm, ~ 7.17 ppm (J value ~10 Hz
and ~ 2.5 Hz) for one proton each numbered as 12 and 14, and
a triplet at ~ 7.04 ppm for one proton numbered as 13 with a J
value of ~ 9.0 Hz. A doublet centred at ~ 4.50 ppm for two pro-
tons was attributed to CH2 protons. Another singlet at ~ 3.80 ppm
.2.3. 1H-NMR spectra
In 1H-NMR of ligand, the deshielded signal appeared at 13.10
3
ppm was attributed to phenolic–OH. The signal observed at 8.77
ppm for 1H was due to azomethine (-CH=N-) proton. The presence
of this signal was positive evidence for the formation of Schiff base
ligand [43]. The two triplets centred at 7.47 ppm and 7.43 ppm for
one proton each numbered as 8 and 9 with a J value 3.6 Hz and
3
.2 Hz, a double doublet centred at ~ 7.29 ppm for two protons
numbered as 6 and 7 with a J value 6.8 Hz and 8.4 Hz, were as-
signed for pyridyl ring protons. The three aromatic protons of the
phenolic ring appeared as two double doublets centred at 7.12 ppm
(
J value 8.8 Hz, 3.6 Hz) and 7.07 ppm (J value 5.2 Hz, 2.8 Hz) for
one proton each numbered as 23 and 25 and a triplet at 6.87 ppm
for one proton numbered as 24 with J value 8.8 Hz and 6.8 Hz.
A doublet centred at ~ 4.54 ppm with J value 1.6 Hz was due to

8
S.N. Shukla, P. Gaur and S.S. Bagri et al. / Journal of Molecular Structure 1225 (2021) 129071
Table 2
Experimental and calculated 1H chemical shifts (ppm) of L1 and complex 1
L1 1H-NMR
Complex 1 1H-NMR
Experimental
Experimental
Calculated
TMS HF/
RMSD
Calculated
RMSD
TMS B3LYP/
TMS HF/
TMS B3LYP/
6-31G(d) GIAO
6-311+G(2d,p) GIAO
LANL2DZ GIAO
LANL2DZ GIAO
H32
H31
H30
H27
H25
H24
H23
H16
H13
H12
H9
3.883
3.883
3.883
13.107
7.056
6.874
7.074
8.773
4.543
4.534
7.478
7.305
7.226
7.11
3.945
3.941
3.938
13.618
7.242
6.937
7.324
8.945
4.619
4.615
8.184
7.907
7.836
7.384
3.897
3.895
3.886
13.452
7.164
6.919
7.218
8.869
4.617
4.604
7.996
7.875
7.688
7.341
0.026
0.024
0.025
0.212
0.076
0.026
0.102
0.070
0.035
0.035
0.298
0.276
0.259
0.120
H47
H45
H43
H42
H41
H40
H31
H32
H33
H28
H27
H26
H25
H24
H23
H16
H14
H13
H12
11.504
8.053
8.113
8.501
8.407
8.236
3.824
3.824
3.824
4.534
4.526
7.395
7.786
7.797
7.531
8.993
7.176
7.049
7.303
11.862
8.591
8.648
8.99
11.775
8.554
8.616
8.978
8.851
8.714
3.897
3.893
3.89
0.152
0.245
0.245
0.227
0.217
0.231
0.037
0.035
0.034
0.065
0.066
0.249
0.089
0.090
0.151
0.014
0.236
0.210
0.234
8.884
8.738
3.909
3.905
3.902
4.683
4.675
7.954
7.985
7.991
7.867
9.026
7.683
7.531
7.817
4.662
4.656
7.89
H8
H7
7.963
7.986
7.835
8.998
7.674
7.446
7.782
H6
for three protons was assigned to methoxy group attached to the
phenolic ring. The NH proton of benzimidazole/imidazole ring in
complex 1 and 2 resonate at ~ 11.50 ppm [46]. The two doublets
for one proton each, one multiplet for two proton and a singlet for
one proton observed at ~ 8.50 ppm, 8.40 ppm, 8.32 ppm and 8.11
ppm were due to C-H protons numbered as 41, 42, 40 43, and 45
of benzimidazole moiety present as anchoring ligand. In complex
The signal in for -HC=N carbon was downfield shifted to 6.8-
3.4 ppm in the complexes confirming the transfer of one lone pair
electron from nitrogen to the metal and coordination of -HC=N to
metal. The signal for the phenolic carbon linked to the hydroxyl
group was downfield shifted in complexes and appeared in be-
tween 161.4-164.8 ppm. In complex 1, the six signals observed at
164.7 ppm, 158.5 ppm, 128.6 ppm, 123.5 ppm, 121.0 ppm, and
119.6 ppm were ascribed for the six aromatic carbon of pheno-
lic ring. The signal at 164.7 ppm was assigned to carbon liked
with hydroxyl moiety. The five signals at 154.5 ppm, 150.8 ppm,
144.4 ppm, 128.1 ppm and 120.5 ppm were attributed to 2-pyridyl
aromatic carbon. However, the seven signals at 146.9 ppm, 146.2
ppm, 145.4 ppm, 127.3 ppm, 126.8 ppm, 117.3 ppm and 116.7 ppm
were assigned to the seven heteroaromatic carbon of benzimida-
zole moiety present as an adjuvant ligand. In complex 2, six sig-
nals were observed at 161.4 ppm, 150.0 ppm, 128.9 ppm, 127.3
ppm, 120.4 ppm, and 116.2 ppm were attributed to aromatic car-
bon of phenolic moiety. The signal at 161.4 ppm was assigned to
carbon liked with hydroxyl moiety. Five signals observed at 155.5
ppm, 147.9 ppm, 145.7 ppm, 128.1 ppm, and 117.4 ppm were as-
signed for 2-pyridyl aromatic carbon. However, the three signals
observed at 147.8 ppm, 145.1 ppm, and 119.7 ppm were attributed
to the three heteroaromatic carbon of imidazole moiety present as
an adjuvant ligand. In complex 3, the six signals observed at 164.8
ppm, 155.7 ppm, 128.9 ppm, 123.7 ppm, 121.0 ppm and 117.5 ppm
were assigned for aromatic carbon of phenolic moiety. The signal
at 164.8 ppm was assigned to carbon liked with hydroxyl moiety.
The five signals observed at 146.9 ppm, 146.2 ppm, 144.6 ppm,
2
two doublets centred at ~ 8.39 ppm, ~ 8.30 ppm and a singlet
centred at 8.24 for one proton each numbered as 40, 41 and 37
were attributed to imidazole moiety present as anchoring ligand.
In complex 3, two double doublets, two triplets and a singlet cen-
tred at ~ 8.48 ppm, ~ 8.37 ppm, ~ 8.28 ppm, ~ 8.14 ppm and 8.06
ppm for one proton each were assigned for the proton numbered
as 41, 42, 40 and 43 of benzoxazole moiety present as anchoring
unit. In complex 4 the anchoring unit pyridine displayed signal as
a double doublet and a triplet centred at ~ 8.38 ppm, ~ 8.24 ppm
for two protons each numbered as 37, 39, 38, and 42 and another
triplet centred at ~ 8.14 ppm for one proton numbered as 43. The
1H-NMR spectra of complex 1-4 are given in Figs. S13-16. The the-
oretical chemical shift values were calculated by the GIAO method
using TMS HF/6-31G(d) GIAO and TMS B3LYP/6-311+G(2d,p) GIAO
level theory. 1H-NMR correlation diagram for Complex 1 is given
in Fig. S17. The 1H-NMR correlation coefficient R2 for com-
plex 1 was determined as 0.994 (δcal = 1.035δexp - 0.018). Ex-
perimental and theoretical chemical shifts were compared and
given in Table 2. In proton NMR RMSD values range between
0.014 – 0.298.
1
27.4 ppm and 120.7 ppm were attributed to heteroaromatic car-
bon of 2-pyridyl moiety. However, the seven signals observed at
57.3 ppm, 150.8 ppm, 145.4 ppm, 128.2 ppm, 126.5 ppm, 119.8
1
3
.2.4. 13 C{ H}-NMR spectra
In 13C{ H}-NMR, the presence of azomethine moiety in the L
1
1
1
is confirmed by a signal at ~ 162.1 ppm attributed to azomethine
carbon. A signal at 158.0 ppm in was assigned to the carbon con-
nected to hydroxyl moiety. The signals observed between 157.2-
ppm and 116.1 ppm were assigned for the heteroaromatic carbon
of benzoxazole moiety present as an adjuvant ligand. In complex 4,
the six signals observed at 162.4 ppm, 154.4 ppm, 128.1 ppm, 126.8
ppm, 123.5 ppm and 116.2 ppm were assigned for the aromatic
carbon of phenolic moiety. The signal at 162.4 ppm was assigned
to carbon liked with hydroxyl moiety. The five signals observed at
1
30.1 ppm were ascribed to the carbon of pyridyl moiety. Similarly,
phenolic ring aromatic carbon shows signals between 128.1-117.7
ppm. The signal at 61.0 ppm was attributed to methylene carbon.
However, the signal at 55.2 ppm was due to methoxy carbon. The
147.6 ppm, 145.5 ppm, 144.8 ppm, 127.3 ppm and 117.4 ppm were
13C-NMR spectrum of L1
is given in Fig. S18. The correlation coeffi-
attributed for the heteroaromatic carbon 2-pyridyl moiety. How-
ever, another five signal observed at 150.8 ppm, 147.7 ppm, 128.9
ppm, 120.4 ppm and 119.9 ppm were ascribed for the five het-
cient of L1 for 13C-NMR was determined as 0.995 (δcal = 1.010δexp
–
0.299 (R2 = 0.995) and its correlation diagram is given in Fig.
S19.

S.N. Shukla, P. Gaur and S.S. Bagri et al. / Journal of Molecular Structure 1225 (2021) 129071
9
Table 3
Experimental and calculated 13 C chemical shifts (ppm) of L1 and complex 1
L1 13 C-NMR
Complex 1 13 C-NMR
Experimental
Calculated
TMS HF/
RMSD
Experimental
Calculated
RMSD
TMS B3LYP/
TMS HF/
TMS B3LYP/
6-31G(d) GIAO
6-311+G(2d,p) GIAO
LANL2DZ GIAO
LANL2DZ GIAO
C1
157.225
150.08
163.308
158.475
156.512
149.264
143.547
167.034
164.854
137.475
131.573
127.720
124.915
136.048
64.341
158.957
152.459
151.425
143.368
135.069
163.184
160.175
131.198
124.626
123.795
119.789
130.819
61.628
2.558
3.532
3.002
3.405
5.535
2.112
2.832
4.126
3.740
2.468
3.001
3.297
1.443
1.544
C6
116.798
164.740
144.993
120.599
119.689
162.115
146.940
146.292
145.441
154.596
150.807
126.862
123.588
158.504
128.964
128.124
127.357
121.454
117.327
62.884
126.486
172.042
151.853
129.607
127.349
167.731
159.682
156.930
154.729
160.415
157.372
138.081
136.420
164.308
141.725
141.287
134.544
132.108
126.946
67.524
118.057
167.416
146.389
121.742
120.259
163.485
149.612
149.195
147.041
154.986
152.217
129.018
128.156
159.714
130.905
130.691
127.718
125.506
119.794
63.415
4.301
3.016
2.960
4.004
3.484
2.390
5.486
4.489
4.054
2.655
2.821
4.860
5.310
2.500
5.614
5.697
3.306
4.390
4.079
2.073
1.203
C2
C7
C3
149.37
C8
C4
141.205
130.144
162.115
158.077
127.475
122.925
121.775
117.789
128.103
61.021
C9
C5
C10
C15
C20
C21
C22
C28
C29
C34
C35
C36
C37
C38
C39
C44
C11
C17
C30
C16
C17
C18
C19
C20
C21
C22
C11
C29
55.256
58.739
55.719
55.892
58.839
57.318
Fig. 6. TGA curve of complex 1
eroaromatic carbon of pyridine moiety present as an adjuvant lig-
and.
benzimidazole ring. In the second step, in temperature range ~265-
760 °C mass loss of 53.5 % was attributed to the gradual decompo-
¯
The signal observed at ~ 55.2 ppm in complexes was attributed
to methoxy carbon of vanillin moiety. However, the signal at ~ 62.8
ppm in complexes was attributed to methylene carbon. The 13C-
NMR spectra of complex 1-4 are given in Figs. S20-23. The cor-
relation coefficients of 13C-NMR for complex 1 was determined as
sition of ligand and with one Cl moiety. In the end, a metallic ox-
ide residue was formed as PdO (calc. wt 23.50 %). The percentage
of metal in metal oxide residue was determined by an analytical
technique [47,48].
Thus, based on the elemental analysis, ESI-MS, molar conduc-
tance, electronic spectra, FT-IR, 1H-NMR, 13C-NMR, TGA and DFT
studies probable ChemDraw and DFT optimized structures of the
complexes were suggested as below in Fig. 7(a-h). The correspond-
ing cartesian axes (x, y, z) were given in supplementary Fig. S25.
0
.993(δcal = 1.019δexp - 0.853). 13C-NMR correlation diagram for
complex 1 is given in Fig. S24. Experimental and theoretical chem-
ical shifts were compared and given in Table 3. In 13C-NMR calcu-
lated RMSD values ranges between 1.203–5.697.
3
.2.5. Thermogravimetric analysis
3.3. Catalytic activity for the Suzuki-Miyaura reaction
TGA of complex 1 was studied between temperature range from
0-900 °C at a rate of 10 °C min 1 in an inert atmosphere (Fig. 6).
−
The catalytic potential of the synthesized complexes was exam-
ined over the Suzuki-Miyaura C-C cross-coupling reaction. To op-
timize the reaction conditions, i.e. temperature, catalyst loading,
nature of the base, time and solvent, the reaction has been per-
formed using phenylbronic acid, iodobenzene in different solvents
3
The decomposition occurs in two major detectable steps. The TG
curve shows that the thermal degradation pattern of 1 consists of
the initial decomposition steps between 205 and 265 °C. In this
range, a weight loss of 23 % is caused by the elimination of the

10
S.N. Shukla, P. Gaur and S.S. Bagri et al. / Journal of Molecular Structure 1225 (2021) 129071
Fig. 7. Chemdraw and DFT optimized structure of complexes, (a, b) complex 1, (c, d) complex 2, (e, f) complex 3 and (g, h) complex 4
Fig. 8. Optimization of catalysis over Suzuki-Miyaura reaction (a) Effect of temperature, (b) Effect of catalyst loadings, (c) Effect of base, (d) Effect of time (e) Effect of solvent
under the stirring condition with 0.1-0.6 mmol of the Pd(II) com-
plex as a catalyst at 25-100 °C for 1-12 h. The results obtained are
summarized in Tables S4-S8.
possibly due to the oxidative addition step of the catalytic cycle
getting affected in the presence of an increased concentration of
catalyst. It was obvious from the table that the yield of the com-
plexes in order 1>3>2>4.
3
.3.1. Effect of temperature
To study the effect of temperature on the conversion of
3.3.3. Effect of base
iodobenzene, the reaction temperature was varied from 25-100°C
Further, the effect of a base on the reaction performance was
using K CO3 as a base [49]. From the results, it was observed
studied using different bases such as K CO , Na CO , CH COONa,
2 3 2 3 3
2
that the most suitable reaction temperature was found to be 80°C
at which all the complexes gave the highest conversion. Increas-
ing the temperature beyond 100°C (Entries 01-08, Table S4) does
not have a significant beneficial effect on catalyst performance, al-
though complete conversion is reached within 8 h at 80°C. Fig. 8
NaOH, and KOH. As shown in Fig. 8 (c) and (Table S6, entries
5), K CO3 is the most desirable base for the reaction with high
2
product yield. In the presence of another base, such as Na CO ,
2
3
CH COONa and NaOH moderate to low yields are obtained (Table
3
S6, entries 1, 2, 3). It seems that the cation plays an important role
in determining the efficiency of the base to activate the boronic
acid as previously reported [50,51]. Surprisingly, in the presence of
strong KOH base, low yields were obtained.
(
a) shows the effect of temperature on the yield of the reaction.
Based on the results obtained from the temperature study, all sub-
sequent reactions were carried out at 80°C.
3
.3.2. Effect of catalyst concentration on the reaction
3.3.4. Effect of time
Since catalyst loading in a reaction significantly decides the cat-
Moreover, product yield strictly depends on the reaction time. It
was investigated by analyzing the reaction mixture at regular in-
tervals of time. The product conversion was observed to increase
with reaction time till the completion at 2-12 h (Table S7), beyond
which extent of conversion remained almost insignificant. It led us
to choose optimal reaction time for coupling as 8 h. The percent-
age yield at different reaction time is shown in Fig. 8 (d).
alytic efficiency, the amount of catalyst required for effective con-
version was another parameter selected and studied. Fig. 8 (b)
shows the effect of catalyst loading on the yield of the reaction.
The yield was found to increase with the increase in the amount
of catalyst used. However, after the catalyst concentration of 0.4
mmol (Table S5), the yield of the reaction showed a negative trend,

S.N. Shukla, P. Gaur and S.S. Bagri et al. / Journal of Molecular Structure 1225 (2021) 129071
11
Fig. 9. Energy profile of the full catalytic cycle of complex 1 for the Suzuki-Miyaura reaction calculated from DFT
3
.3.5. Effect of solvent
an energy barrier of 3.1068 eV. This step involves the addition of
Ph-I and oxidation of metal center Pd(0) to Pd(II) with the stabi-
lization energy of 3.7416 eV. In the next step base, K CO interacts
Among the different solvents such as DMF, toluene, acetonitrile,
H O, DMF+ H O. The highest catalytic activity was observed with
2
2
2
3
DMF+ H O (Table S8). Moderate to low catalytic activities were
and removes the halogen as KI, introducing carbonate moiety in
the place leading to the formation of transition state TS1. Further,
transmetallation occurs and the phenyl group of boronic acid gets
attached to the metal center and leads to the formation of TS2.
The energy profile of the reaction obtained from the DFT calcula-
tion clearly shows that TS2 is situated at the top in comparison to
TS . Hence, TS is less stable energetically and most favorable for
2
found in other solvents such as DMF, toluene, acetonitrile and wa-
ter. In the case of water, we observed a poor product yield. DMF+
H O was chosen as the optimum solvent for carrying out the reac-
2
tions. The percentage yield is shown in Fig. 8 (e).
3
.3.6. Effect of substituent’s in Suzuki-Miyaura reaction under
1
2
easy conversion into the product by reductive elimination. At the
last original catalyst was obtained by oxidative addition of benz-
imidazole moiety. The probable mechanism of the Suzuki-Miyaura
reaction is shown in Fig. S26.
optimized condition
The effect of substituents in the Suzuki-Miyaura reaction under
optimized condition was studied. The aryl halides (XC H Y; where
6
4
X = Br/Cl/I and Y = H/ o/m/p-NH , o/m/p-NO , o/m/p-OCH , o/m/p-
2
2
3
CHO, o/m/p-OH) eighteen combinations were used. It was obvious
from the (Table S9) iodobenzene combination was most successful
in obtaining the highest yield. Owing to less reactivity of the C-Cl
bond due to higher bond energy, the reaction of chloro-substrates
did not succeed well under optimized conditions, in comparison
to bromo and iodo-substrates. In was interestingly observed that
the substituent in the meta and ortho position of aryl iodide re-
sulted in lower yields than the para-position which may be at-
tributed to the steric hindrance. In aryl iodide substrate substituted
with electron-donating methoxy group reacted well under opti-
mized conditions to give the desired products in good quantitative
yields. It is noticeable here that the deactivated aryl halides like
p-hydroxyiodobenzene, p-aminoiodobenzene also showed moder-
ate to low conversion. However, in the case of substituted aryl io-
dide with electron-withdrawing aldehydes (p, o, m) group lower
yield was observed in comparison to a substrate with an electron-
donating methoxy (p, o, m) group. The highest yield was ob-
served with electron-withdrawing nitro substituted aryl halide and
the lowest yield was obtained with electron-donating hydroxy-
substituted aryl halide substrate.
3.3.8. Characterization of Suzuki-Miyaura reaction products
Spectroscopic techniques characterized Suzuki-Miyaura reaction
products. The details of the 1H-NMR, 13C-NMR and ESI MS data
were given in the Supplementary Table S10-S12. The represen-
tative corresponding, 1H-NMR, 13C-NMR and ESI-MS spectra were
given in Figs. S27 – S62.
4. Conclusion
A pincer type ONN donor ligand L1 was synthesized. The
L1 was employed strategically in the synthesis of four het-
eroleptic palladium complexes anchored with N-donor benzim-
idazole/imidazole/benzoxazole/pyridine. Spectroscopic techniques
were used to characterize ligand and complexes. For a better in-
sight into molecular structure, several quantum chemical calcula-
tions, i.e. Mulliken charge study, HOMO, LUMO, bond parameters
have been deduced, and geometry of complexes was optimized.
The difference between EHOMO and ELUMO (ꢁE) suggest that com-
plex 1 possess good chemical stability and low reactivity. In FTIR
and NMR spectra quite good correlation has been observed in ex-
perimental and theoretical data. Square planar geometry was sug-
gested from spectroscopy. However, bond parameters from DFT
suggest slight distortion in bond length and bond angle. Synthe-
sized complexes were screened for catalytic activity on Suzuki-
Miyaura reaction in different conditions, i.e. temperature, catalyst
loading, base, time and solvent. The complex 1 displayed excel-
lent catalytic activity with a substrate containing iodo substituent.
Computational investigations into of mechanism of Suzuki-Miyaura
catalytic cycles suggest six steps mechanism. DFT shows that the
proposed intermediate TS2 exhibits higher energy than TS1 and
3
.3.7. Energy profile diagram of Suzuki-Miyaura reaction
DFT calculations were performed at the B3LYP/ 6-31G level of
theory to evaluate the C–C coupling reaction using chelated pal-
ladium Schiff base complex 1 as a catalyst. The overall computed
energy profile diagram of this reaction is displayed in Fig. 9. The
energy profile diagram shows that complete Suzuki-Miyaura reac-
tion proceeds via five major steps viz reductive elimination, oxidative
addition, transmetallation, reductive elimination and oxidative addi-
tion. The first step involves the elimination followed by the reduc-
tion of Pd(II) to Pd(0). Reductive elimination is a facile step with

12
S.N. Shukla, P. Gaur and S.S. Bagri et al. / Journal of Molecular Structure 1225 (2021) 129071
situated at the top of the energy profile. This catalytic system tol-
erates various functional group on the phenyl ring and could be
further exploited for the design of promising catalytic material.
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Satyendra Nath Shukla: Conceptualization, Supervision, Project
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Acknowledgement
The authors are grateful to the Principal, Government Science
College, Jabalpur and Head, Chemistry Department, for providing
laboratory facilities. We sincerely thank SAIF-CDRI, Lucknow, for
recording ESI-MS spectra, 1H-NMR, 13C-NMR and IISER Bhopal for
recording TGA. Author “Sanjay Singh Bagri” is grateful to the UGC-
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