Crystal structures and DFT analysis of Palladium(II) complexes with Schiff bases derived from N,N-dialkyl-p-phenylenediamines

Article abstract of DOI:10.1016/j.molstruc.2019.127549

Herein, new palladium(II) complexes with Schiff bases derived from salicylaldehyde and N,N-dialkyl-p-phenylenediamines were obtained. The ligands 2-((4-(N,N-dimethylamino)aniline)formimidoyl)phenol (N,N-metph-sOH - L₁) and 2-((4-(N,N-diethylamino)aniline)formimidoyl)phenol (N,N-etph-sOH - L₂) and complexes [Pd(N,N-metph-sO)₂] (C₁) and [Pd(N,N-etph-sO)₂] (C₂) were characterized by elemental analysis, infrared (IR) and UV–Vis spectroscopy, ¹H and ¹³C nuclear magnetic resonance (NMR), thermogravimetry (TG) and differential thermal analysis (DTA), and the results agree with the proposed structures. The crystal structures data showed the formation of N,O-chelate with the pairs of phenolate O and imine N occupying the trans positions. Hirshfeld surface analysis was employed to elucidate the intermolecular contacts which drive the different supramolecular assemblies of C₁ and C₂. In addition, Density Functional Theory (DFT) calculations were also performed to better understand the electronic properties. Additional DFT calculations were also devoted to elucidate the stereochemistry of these palladium complexes toward the cis-trans isomerism among these square-planar Pd(II) N_,O-Schiff base complexes. Time-dependent DFT (TD-DFT) analysis was applied to shed light on the nature of the electronic transitions determined in the UV–Vis spectra.

Full text of DOI:10.1016/j.molstruc.2019.127549

Journal Pre-proof
Crystal structures and DFT analysis of Palladium(II) complexes with Schiff bases
derived from N,N-dialkyl-p-phenylenediamines
Luís Eduardo Sarto, Wladimir Pereira Duarte Badaró, Elba Pereira de Gois, Marília
Imaculada Frazão Barbosa, Claudia Torres, Rommel Bezerra Viana, João Honorato,
Eduardo Ernesto Castellano, Eduardo Tonon de Almeida
PII:
S0022-2860(19)31658-8
DOI:
https://doi.org/10.1016/j.molstruc.2019.127549
MOLSTR 127549
Reference:
To appear in: Journal of Molecular Structure
Received Date: 24 June 2019
Revised Date: 25 November 2019
Accepted Date: 6 December 2019
Please cite this article as: Luí.Eduardo. Sarto, W.P.D. Badaró, E.P. de Gois, Marí.Imaculada.Frazã.
Barbosa, C. Torres, R.B. Viana, Joã. Honorato, E.E. Castellano, E.T. de Almeida, Crystal
structures and DFT analysis of Palladium(II) complexes with Schiff bases derived from N,N-
dialkyl-p-phenylenediamines, Journal of Molecular Structure (2020), doi: https://doi.org/10.1016/
j.molstruc.2019.127549.
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-1-
Crystal structures and DFT analysis of Palladium(II)
complexes with Schiff bases derived from
N,N-dialkyl-p-phenylenediamines
Luís Eduardo Sarto¹, Wladimir Pereira Duarte Badaró², Elba Pereira de Gois²,
Marília Imaculada Frazão Barbosa², Claudia Torres², Rommel Bezerra Viana², João
3
4
Honorato , Eduardo Ernesto Castellano and Eduardo Tonon de Almeida² *
¹
Instituto Federal de Educação Ciência e Tecnologia do Sul de Minas Gerais –
IFSULDEMINAS, Machado-MG, 37750-000, Brazil.
Universidade Federal de Alfenas - UNIFAL-MG, Instituto de Química – IQ, Alfenas –
²
MG, 37130-001, Brazil
3
Universidade Federal de São Carlos - UFSCar, Departamento de Química - DQ, São
Carlos - SP, CP 676, 13565-905, Brazil,
4
Universidade de São Paulo - USP Departamento de Física e Informática, Instituto de
Física de São Carlos - IFSC, São Carlos - SP, CP 369, 13560-970, Brazil.
*
To whom correspondence should be addressed. e-mail: tonon@unifal-mg.edu.br
(Eduardo Tonon de Almeida). Tel.: +55 35 3701-9712

-2-
Graphical Abstract
Highlights
•
•
Two new Schiff base Pd(II) N,O-chelate complexes were synthetized.
Used ligands were derived from N,N-dialkyl-p-phenylenediamines and
salicylaldehyde.
•
•
•
Spectroscopic, thermal and elemental analyzes agree with the proposed
structures.
The crystal structures and Hirshfeld surface analysis of the new complexes were
determined by X-ray diffraction.
Pd(II) complexes stereochemistry and UV-Vis spectra were elucidated by DFT.

-3-
ABSTRACT
Herein, new palladium(II) complexes with Schiff bases derived from salicylaldehyde
and N,N-dialkyl-p-phenylenediamines were obtained. The ligands
-((4-(N,N-dimethylamino)aniline)formimidoyl)phenol (N,N-metph-sOH - L ) and
2
2
1
-((4-(N,N-diethylamino)aniline)formimidoyl)phenol (N,N-etph-sOH
-
L2) and
complexes [Pd(N,N-metph-sO) ] (C ) and [Pd(N,N-etph-sO) ] (C ) were characterized
2
1
2
2
1
13
by elemental analysis, infrared (IR) and UV-Vis spectroscopy, H and C nuclear
magnetic resonance (NMR), thermogravimetry (TG) and differential thermal analysis
(DTA), and the results agree with the proposed structures. The crystal structures data
showed the formation of N,O-chelate with the pairs of phenolate O and imine N
occupying the trans positions. Hirshfield surface analysis was employed to elucidate the
intermolecular contacts which drive the different supramolecular assemblies of C and
1
C . In addition, Density Functional Theory (DFT) calculations were also performed to
2
better understand the electronic properties. Additional DFT calculations were also
devoted to elucidate the stereochemistry of these palladium complexes toward the cis-
trans isomerism among these square-planar Pd(II) N O-Schiff base complexes. Time-
,
dependent DFT (TD-DFT) analysis was applied to shed light on the nature of the
electronic transitions determined in the UV-Vis spectra.
Keywords: Pd(II) complexes, Schiff bases, Single crystal XRD, Thermal studies, DFT.

-4-
1
. INTRODUCTION
The coordination chemistry of palladium complexes with Schiff base ligands has
excited great interest among chemists in recent years, due to the applications of these
compounds in catalysis and their relevance to bioinorganic systems in a wide variety of
biological applications, such as antimicrobial [1,2], antifungal [3] and antitumoral
activity [4,5,6] as well as other biochemical and pharmacological proprieties.
In bioinorganic systems, palladium complexes were introduced after the success
of platinum(II) compounds [7], since the coordinating geometry and ionic radius of
palladium(II) are similar to platinum(II) ions [8]. However, palladium(II) compounds
5
demonstrate ligand-exchange kinetics that are 10 times faster than the analogous
platinum(II), which can facilitate the hydrolysis of the leaving groups that dissociate
readily in solution before the complex reaches the pharmacological target [8].
Chelating ligands have been used to overcome this high lability, forming
thermodynamically stable and kinetically inert palladium(II) complexes [9,10]. In this
context, Schiff bases are an important class of ligands in Coordination Chemistry,
because these compounds are versatile ligands in the formation of transition metal
complexes [11,12].
Schiff bases are obtained by the condensation reaction of aldehydes and amines
under specific conditions [13,14]. The presence of the iminic group (-C = N-) allows
these molecules to easily bind to metal ions by the nitrogen atom, once this group has
electron donor properties. The existence of other regions that are suitable for
coordination (SH or OH) enables the formation of the chelate compound with increased
kinetic and thermodynamic stability [15,16].
As a part of our on-going research work with interest in the structural chemistry
of palladium compounds, we herein report the synthesis, spectral characterization,
single cristal X-ray diffraction and theoretical studies of new Pd(II) complexes with
Schiff bases (L1 and L2) (Scheme 1) obtained by reaction of salicylaldehyde with N,N-
dialkyl-p-phenylenediamines. These ligands were chosen based on their similarity to
other ligands that showed biological activity when free [17] or coordinated to palladium
[
4,18] or other metals [19]. Our research group is dedicated to synthesis,
characterization and investigation of biological activity of new transition metal
complexes candidates to metallodrugs, like antitumoral [5,20,21], antileishmanial

-5-
[
22,23,24], antifungal [24] and tuberculostatic [20,24,25]. Ligands and complexes were
1
13
1
characterized by IR, UV-Vis, H and C{ H} NMR spectroscopies as well as elemental
and thermal analysis. In addition, the crystal structures of the complexes (C and C )
1
2
were determined, allowing us to unambiguously confirm the geometry of the
complexes. Hirshfeld surface analysis (HS) was carried out to a better understanding of
crystallographic interactions and packing nature within their crystal structures.
Moreover, density functional theory (DFT) calculations were also performed to better
understand the electronic properties, and time-dependent DFT (TD-DFT) analysis was
applied to shed light on the nature of the electronic transitions determined in the UV-
Vis spectra. A detailed characterization of obtained compounds in this work was
necessary in order to allow the reproducibility and reliability in the bioassays that will
be performed a posteriori.
Scheme 1. Synthetic route for Schiff bases (L and L ) and corresponding palladium(II)
1
2
complexes (C and C ).
1
2

-6-
2
. MATERIAL AND METHODS
2
.1. Preparations
All reagents and solvents were obtained from commercial suppliers and used
without further purification. The lithium tetrachloropalladate(II), palladium chloride,
salicylaldehyde and N,N-dialkyl-p-phenylenediamines were used as received from
Aldrich.
2
.2. Instrumentation
Infrared spectra (IR) were recorded using KBr disk on a Shimadzu-IR-Prestige-
-1
2
1 spectrophotometer in the spectral range 400-4,000 cm . The percentages of C, H and
N were determined by elemental analysis using a Leco Instruments Ltda, model
TruSpec CHNS-O instrument. The %Pd was obtained by thermogravimetric analysis
(TG). NMR spectra were recorded on a Bruker DPX-300 spectrometer operating at 300
1
13
MHz ( H) and 75 MHz ( C), using CDCl as solvent. Thermogravimetry (TG) and
3
differential thermal analysis (DTA) data were obtained from a TA Instruments
-1
equipment - Q 600, under dry synthetic air flow (100 mL min ), mass sample of
-1
approximately 10 mg, alumina crucible and heating rate of 20 ºC min from room
temperature to 900 ºC. The reference substance used in DTA measurements was pure α-
alumina. The electronic absorption was recorded by a THERMO Scientific-Evolution
6
0, using MeCN as solvent. X-ray powder diffraction (XRD) patterns were obtained by
Rigaku Ultima IX equipment using KαCu wavelength (λ=1.5418 Å) setting of 34 kV
and 20 mA.
2
.3. X-ray crystallography
Yellow (C ) and orange (C ) single-crystals were grown from evaporation of a
1
2
solution of the complex in CDCl . Well-shaped single crystals were chosen for the X-
3
ray diffraction experiment that was performed at 293 K on an automatic diffractometer
Rigaku XtaLAB mini II diffractometer (graphite monochromator) with Mo Kα radiation
(λ = 0.71073 Å). CrysAlisPro [26] were used for data collection, cell refinement, data

-7-
reduction, and multi-scan method absorption correction. The structures of C and C
1
2
were solved and refined using the software SHELXT2014 [27] from the OLEX2 suite
[
28]. All atoms, except hydrogen, were clearly identified and refined by least squares
2
full matrix F with anisotropic thermal parameters. The crystallographic tables were
generated by OLEX2 [28] and the structure representations by Mercury software [29].
The main crystal data collections and structure refinement parameters for C and C are
1
2
summarized in Table 1, as well as the CCDC deposit numbers for supplementary
crystallographic data. These data can be obtained free of charge via
http://www.ccdc.cam.ac.uk/conts/retrieving.html,
Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: (+44)
1223-336-033). Relevant interatomic bond lengths and angles are listed in
or
from
the
Cambridge
0
Table 2. The Hirshfeld surfaces (HS) and the 2D-fingerprint plots were carried out with
CrystalExplorer package [50]. The 2D-fingerprint plots were performed from a fraction
of points in the Hirshfeld surface based on the normalized contact distances defined as
d (distance from the surface to the nearest atom in another molecule) and d (distance
i
e
from the surface to the nearest atom in the molecule itself) across the metal complexes.
2
.4. Synthesis of N,N-metph-sOH - (L1)
The used Schiff bases have been reported in the literature [31,32], however, a
new methodology was applied to obtain them, as described in Scheme 1 (the indicative
numbers on the atoms were used for the NMR assignments). To a solution of 1.0710 g
(19.09 mmol) of KOH in heated methanol (70 mL) were added 1.9865 g (9.50 mmol) of
N,N-dimethyl-p-phenylenediamine dihydrochloride and 1.00 mL (9.51 mmol) of
salicylaldehyde. The mixture was stirred at room temperature (approximately 25 ºC) for
3
h. Then, the obtained precipitate was filtered off, washed with water and dried in
vacuum. Finally, it was recrystallized with methanol. Yield: 78%. Anal. Calc. for
-1
C H N O (%): C 75.0, H 6.7, N 11.7; Found: C 74.9, H 6.7, N 11.8. IR (KBr, cm ):
15
16
2
2
889 w (νC-H), s 1614 s (νC=N), 1591 s (ν ), 1518(ν ), 1456 m (ν ), 1366 s
ring ring ring
1
(
γOH), 1280 s (νC-N), 1196 s (νC-O), 810 s (γp-ring), 756 s (γo-ring). H NMR δppm
(multiplicity; integration; J; assignment): 13.78 (s; 1H; 1), 8.61 (s; 1H; 8), 7.38-7.32 (m;
2
H; 4, 6), 7.29 (d; 2H; 9.1 Hz; 10), 7.01 (d; 1H; 8.1 Hz; 3), 6.92 (t; 1H; 8.0 Hz; 5), 6.75
1
3
(
d; 2H; 9.0 Hz; 11), 3.00 (s; 6H; 13). C NMR δ
(assignment): 160.95 (2), 157.63
ppm
(8), 149.94 (12), 137.10 (9), 132.04 (4), 131.56 (6), 122.23 (10), 119.81 (7), 118.85 (5),

-8-
1
3
1
1
17.05 (3), 112.78 (11), 40.63 (13). C NMR DEPT 135 δ (assignment): 157.56 (8),
ppm
32.06 (4), 131.63 (6), 122.27 (10), 118.90 (5), 117.06 (3), 112.76 (11), 40.64 (13).
2
.5. Synthesis of N,N-etph-sOH - (L2)
The Schiff base L was obtained as described in section 2.4, however the N,N-
2
dimethyl-p-phenylenediamine dihydrochloride was replaced by 2.4920 g (9.50 mmol)
of N,N-diethyl-p-phenylenediamine sulfate (Scheme 1). Yield: 85%. Anal. Calc. for
-1
C H N O (%): C 76.1, H 7.5, N 10.4; Found: C 75.9, H 7.5, N 10.2. IR (KBr, cm ):
17
20
2
2
964 w (νC-H), 1614 s (νC=N), 1591 s (ν ), 1517 s (ν ), 1458 m (ν )1404 s
ring ring ring
1
(
δCH ), 1360 s (γOH) 1269 s(νC-N), 1196 s (νC-O), 818 s (γp-ring), 756 s (γo-ring). H
2
NMR δppm (multiplicity; integration; J; assignment): 13.86 (s; 1H; 1), 8.62 (s; 1H; 8),
7
.40-7.31 (m; 2H; 4, 6), 7.28 (d; 2H; 9.0 Hz; 10), 7.02 (d; 1H; 8.1 Hz; 3), 6.92 (t; 1H;
1
3
7
.4 Hz; 5), 6.71 (d; 2H; 9.0 Hz; 11), 3.40 (q; 8H; 7.0; 13), 1.20 (t; 6H; 7.0 Hz; 5). C
NMR δppm (assignment): 160.92 (2), 156.84(8), 147.30 (12), 136.05 (9), 131.84 (4),
1
1
1
31.44 (6), 122.50 (10), 119.91 (7), 118.82 (5), 117.02 (3), 112.05 (11), 44.61 (13),
1
3
2.68 (14). C NMR DEPT 135 δ
(assignment): 156.77 (8), 131.90 (4), 131.55 (6),
ppm
22.60 (10), 118.90 (5), 117.05 (3), 112.01 (11), 44.63 (13), 12.73 (14).
2
.6. Synthesis of [Pd(N,N-metph-sO) ] - (C )
2
1
To a solution of lithium tetrachloropalladate(II), Li [PdCl ], prepared as described
2
4
in the literature [33] with 200.2 mg (1.13 mmol) of PdCl and 106.9 mg (2.52 mmol) of
2
LiCl in methanol (40 mL), was added a L solution (543.1 mg, 2.26 mmol) in methanol
1
(40 mL). Then, a solution of triethylamine (315 µL, 2.26 mmol) in methanol (5 mL)
was added slowly and the reaction mixture was stirred for 24 h at room temperature.
The precipitate was filtered off, washed (with methanol, water and ethyl ether), dried in
vacuum and recrystallized from chloroform (Scheme 1). Yield: 62%. Anal. Calc. for
C H N O Pd (%): C 61.6, H 5.2, N 9.6, Pd 18.19; Found: C 61.4, H 5.2, N 9.3,
30
30
4
2
-1
Pd 18.53. IR (KBr, cm ): 2794 w (νC-H), 1610 s(νC=N), 1514 s (ν ), 1440 s (ν ),
ring
ring
1
1
319 s(νC-N), 1190 s (νC-O), 824 m (γp-ring), 756 s (γo-ring). H NMR δ (multiplicity;
ppm
integration; J; assignment): 7.71 (s; 2H; 8), 7.22 (d; 4H; 8.9 Hz; 10), 7.16-7.07 (m; 4H;
4
, 6), 6.75 (d; 4H; 8.8 Hz; 11), 6.48 (t; 2H; 7.3 Hz; 5), 6.27 (d; 2H; 8.4 Hz; 3), 3.00 (s;
13
1
2H; 13). C NMR δ (assignment): 165.16 (2), 162.16 (8), 149.36 (12), 139.52 (9),
ppm

-9-
1
4
1
34.65 (4), 134.24 (6), 125.22 (10), 120.70 (7), 120.52 (5), 114.86 (3), 111.92 (11),
1
3
1.02 (13). C NMR DEPT 135 δ (assignment): 162.16 (8), 134.67 (4), 134.29 (6),
ppm
25.22 (10), 120.53 (5), 114.90 (3), 111.92 (11), 41.05 (13).
2
.7. Synthesis of complex [Pd(N,N-etph-sO) ] - (C )
2
2
The C complex was prepared as described in section 2.6, using 606.5 mg (2.26
2
mmol) of L instead of L (Scheme 1). Yield: 65%. Anal. Calc. for C H N O Pd (%):
2
1
34 38
4
2
-1
C 63.7, H 6.0, N 8.7, Pd 16.60; Found: C 63.4, H 6.3, N 8.5, Pd 16.91. IR (KBr, cm ):
2
8
7
1
1
1
1
1
1
976 w (νC-H), 1607(νC=N), 1514 s (ν ), 1440 s (ν ), 1321 s(νC-N), 1186 s (νC-O),
ring ring
1
18 m (γp-ring), 756 s (γo-ring). H NMR δppm (multiplicity; integration; J; assignment):
.72 (s; 2H; 8), 7.19 (d; 4H; 8.9 Hz; 10), 7.16-7.06 (m; 4H; 4, 6), 6.69 (d; 4H; 8.8 Hz;
1), 6.48 (t; 2H; 7.3 Hz; 5), 6.29 (d; 2H; 8.4 Hz; 3), 3.41 (q; 8H; 7.0 Hz; 13), 1.21 (t;
13
2H; 7.0 Hz; 14). C NMR δ
(assignment): 165.15 (2), 161.84 (8), 146.39 (12),
ppm
38.37 (9), 134.49 (4), 134.20 (6), 125.43 (10), 120.87 (7), 120.52 (5), 114.82 (3),
1
3
11.32 (11), 44.84 (13), 12.59 (14). C NMR DEPT 135 δ (assignment): 161.83 (8),
ppm
34.45 (4), 134.19 (6), 125.40 (10), 120.49 (5), 114.80 (3), 111.18 (11), 44.75 (13),
2.58 (14).
2
.8. Computational details
The calculations were performed leveraging density functional theory (DFT) and
the PBE0 (also known as PBE1PBE) functional [34,35] used in the Gaussian 09
program package [36]. The stationary point on the potential energy surface was fully
optimized, while the harmonic vibration frequencies were used to characterize the
optimized structures as true energy minima based on the absence of imaginary
frequencies. The LANL2TZ basis set [37,38] was carried out for the palladium atom,
and 6-31G(d,p) [39,40] was used for the other atoms. Nevertheless, def2-QVZPPD
basis sets [41] were applied for all elements in single-point calculations with the
purpose to refine the electronic energy evaluating the dimeric assembly and the energy
difference between cis/trans configurations [in these cases, the zero-point vibrational
energy was obtained with LANL2TZ/6-31G(d,p)]. In addition, the acetonitrile solvation
effect was accounted for with the integral equation formalism variant of the polarizable
continuum model (IEF-PCM) [42]. Natural population analysis (NPA) [43] was

-10-
employed to evaluate the atomic charge distribution, and the Mayer bond index [44]
was used to predict the bond order on the coordination sphere. The NBO6 program
package [45] was used to carry out the natural bond orbital (NBO) analysis. The results
were analyzed using Avogadro [46] and VMD [47] softwares.
3
. RESULTS AND DISCUSSION
Synthesis of Schiff bases L1 and L2 was carried out by heating
a solution of N,N-dialkyl-p-phenylenediamine salt (N,N-dimethyl-p-phenylenediamine
dihydrochloride or N,N-diethyl-p-phenylenediamine sulfate) and salicylaldehyde in
methanol with KOH for 3 h (Scheme 1). The choice of N,N-dialkyl-p-
phenylenediamines allows for fine tuning of electronic and steric properties of ligands,
therefore influencing the resulting metal complexes properties.
As depicted in the experimental section, the lithium tetrachloropalladate(II),
Li [PdCl ] precursor was employed for the synthesis of the complexes C : [Pd(N,N-
2
4
1
metph-sO) ] and C : [Pd(N,N-etph-sO) ] (Scheme 1). These complexes were isolated in
2
2
2
good yield and purity, and the structures were characterized by elemental analysis
1
3
1
1
(
experimental section), C{ H} and H NMR spectroscopy, UV-Vis and IR spectra and
TG-DTA. In addition, the structures of C and C were determined by X-ray
1
2
crystallography.
3
.1. Single Crystal X-ray Analysis
The crystallographic structures (Figure 1) confirm the proposed C and C
1
2
structures and coordination mode for L and L , where the coordination sphere is
1
2
square-planar with the pairs of phenolate O and imine N occupying the trans positions.
The X-ray experimental data and structure refinement for the reported structures are
summarized in Table 1, and the selected bond lengths and angles are presented in
Table 2. For both complexes the Pd–O and Pd–N bond distances are 1.92 and 2.02 Å
respectively, similar to those seen in related complexes [48-50]. The O-Pd-N and
O-Pd-O bond angles are close to 90° and 180°, respectively, as expected due to the low
tensioned chelate formed. The crystal structure of compound C presents two
2
symmetry-independent molecules (C A and C B), although the two molecules are
2
2

-11-
structurally very similar, as can be seen from the overlap shown in Figure 2, and do not
display significative differences for bond angles and lengths.
Table 1. Crystal data and structure refinement for (C and C )
1
2
Complex
C1
C2
CCDC code
1916130
1916131
Empirical formula
C H N O Pd
C H N O Pd
3
0
30
4
2
34 38
4
2
Formula weight
Temperature/K
Crystal system
Space group
585.00
293(2)
orthorhombic
Pbca
641.08
293(2)
triclinic
P-1
a/Å
b/Å
c/Å
α/°
β/°
9.1540(6)
12.0400(8)
23.9393(12)
90
10.2574(4)
10.6494(4)
14.9306(6)
70.211(3)
83.816(3)
76.144(3)
1489.34(11)
2
90
90
γ/°
3
Volume/Å
2638.4(3)
4
Z
3
ρ g/cm
calc
1.473
1.43
-1
ꢀ
/mm
0.738
0.66
F(000)
1200
664
3
Crystal size/mm
Radiation
0.463 × 0.311 × 0.054
MoKα (λ = 0.71073)
0.623 × 0.512 × 0.340
MoKα (λ = 0.71073)
2
Θ range for data
5
.602 to 54.206
5.156 to 54.204
collection/°
-
11 ≤ h ≤ 11, -14 ≤ k ≤ 15, -30 -13 ≤ h ≤ 13, -13 ≤ k ≤ 13, -19
Index ranges
≤
l ≤ 30
≤ l ≤ 19
Reflections collected
17376
32749
2
916 [R = 0.0358, R
=
6569 [R = 0.0270, R
=
sigma
int
sigma
int
Independent reflections
0
.0247]
0.0162]
Data/restraints/paramet
ers
2
916/0/159
6569/0/377
2
Goodness-of-fit on F
1.119
1.18
Final R indexes [I>=2σ
R = 0.0381, wR = 0.0891
R = 0.0281, wR = 0.0669
1 2
1
2
(
I)]
Final R indexes [all
data]
R = 0.0743, wR = 0.1124
R = 0.0392, wR = 0.0782
1
2
1
2
Largest diff. peak/hole
-3
0.40/-0.36
0.46/-0.61
/
e Å
Table 2. A comparative analysis between experimental and the calculated values with

-12-
PBE0 functional involving selected bond lengths (in Å) and angles (in °) for C and C
1
2
complexes.
C1
C2
Bond
Exp
1.972(2)
1.972(2)
2.013(3)
PBE0
1.992
1.992
2.045
2.045
Exp
PBE0
1.996
1.996
2.047
2.047
1.992
1.992
2.047
2.047
Pd -O1
1.9768(16)
1.9768(16)
2.0275(18)
2.0275(18)
1.9666(17)
1.9666(17)
2.0253(19)
2.0253(19)
1
1
Pd -O1
1
1
Pd -N1
1
Pd -N1
2.013(3)
1
Pd -O2
-
-
-
-
2
2
Pd -O2
2
Pd -N3
2
2
Pd -N3
2
Angle
1
O Pd O
180.0
87.65(10)
92.35(10)
92.35(10)
87.65(10)
180.0
180.00
88.06
91.94
91.94
88.06
180.00
180.00(3)
90.98(7)
89.02(7)
90.98(7)
89.02(7)
180.00(11)
180.00(6)
92.48(8)
87.52(8)
92.48(8)
87.52(8)
180.0
180.00
91.78
92.20
91.78
92.20
180.00
180.00
92.07
91.96
92.07
91.96
180.00
1
1
1
O Pd N
1
1
1
1
O Pd N
1
1
1
1
1
O Pd N
1
1
1
1
O Pd N
1
1
1
1
N Pd N
1
1
1
2
2
O Pd O
2
-
-
-
-
-
-
2
O Pd N
3
2
2
2
O Pd N
2
2
3
2
2
O Pd N
3
2
2
2
O Pd N
2
2
3
1
N Pd N
1
1
1

-13-
Figure 1. ORTEP view of C and C (ellipsoids at 30% of probability).
1
2
Figure 2. Structural overlap of two symmetry-independent molecules in compound C2

-14-
The fingerprint plots of C and the independent conformers in C (A and B) are
1
2
shown in Figure 3-1 (see also Figures S1-S3 in the supplementary material). It can be
noted that the fingerprints do not show prominent spikes which are mainly due the
absence of strong donor/acceptor electrons among intermolecular interactions. The
H⋯H intermolecular contacts (see Figure 4) are the major contributions to the Hirshfeld
surface areas exhibiting values around 60% indicating that the crystal packing of these
metal complexes is governed by van der Waals contacts. The C⋯H interactions are
represented by internal spikes in the bottom area corresponding to 28% of HS, including
a C-H⋯Cg one (π-Ring) founded in C (see Figure S4 in the supplementary material).
2
In contrast, the N⋯H and O⋯H interactions are not extensive with small contributions
around 3% and 5%, respectively (Figure 4-1). A Pd⋯H contact was also identified
displaying 2.9%, 1.6% and 1.0% of C , C A and C B HS, respectively. These contacts
1
2
2
(i.e. Pd⋯H, O⋯H and N⋯H) appeared as sharp spikes in the fingerprint plots indicating
that these interactions are significantly short.
Figure 3. 1- 2D-plot (fingerprints) derived from the Hirshfeld surface summarizing the
contribution of intermolecular interactions in the nearest environment of C , C A and
1
2
C B. 2- Comparative quantitative analysis (%) of the intermolecular contacts derived
2
from the Hirshfeld Surfaces (HS) of the complexes

-15-
Figure 4. 2D-fingerprint plots derived from the Hirshfeld surface of C for the H…H,
1
C…H, O…H and Pd…H contacts.
Examining the molecular electrostatic potential maps, Pd⋯H contacts were
considerably visible showing red spots over the Hirshfeld surfaces. These Pd⋯H red
spots are visible only in C , while the main spots in C A and C B are assigned by the
1
2
2
C…H contacts (see Figures S5 and S6). This occurs due to the small torsion angles
between the ligand rings in C of 88.55° against 125.49° and 131.08° in C A and C B,
1
2
2
respectively. Thereby, these metal⋯H interactions modulate the molecular packing and
conferring supramolecular differences between C and C .
1
2

-16-
Figure 5. Hirshfeld Surface displaying the electrostatic potential map of C . The Pd…H
1
and C…H interactions are evidenced.
3
.2. Infrared spectra
The IR spectra of free Schiff bases L and L show bands related to aromatic and
1
2
−1
aliphatic C‒H stretching vibrations in the range 2830‒3100 cm , while C=N, C=C and
−1
C‒O stretching vibrations were detected between 1150 and 1620 cm [51,52]. As
expected, the latter bands shift to lower wavenumbers after coordination of these
−1
ligands to palladium. For example, the band at ~1614 cm associated with the
−1
stretching frequency of the imine C=N bond in L shifted to ~1604 cm in C , which is
1
1
indicative of imine nitrogen coordination to the metal center, as reported for other
palladium Schiff bases in the literature [48,51-53]. The υ(C-O) bands in the ligands L1
−1
and L were observed at approximately 1196 cm [54], and after coordination, they
2
−1
shifted to 1190 and 1186 cm , respectively, due to the coordination of an oxygen atom
to palladium. The changes observed for υ(C=N) and υ(C-O) suggests the formation of
N,O-chelate complexes, as previously described for other Schiff base palladium
complexes [55]. Figure 6 shows the vibrational spectrum in the fingerprint infrared
region of the L and L ligands and C and C complexes. The complete infrared
1
2
1
2
spectra are available in the supplementary material (figure S8).

-17-
Figure 6. Fingerprint region IR spectra of ligands L and L , and complexes C and C ,
1
2
1
2
in KBr.
1
13
3
.3. H and C NMR spectra
1
13
1
1
Figures 7 and 8 show, respectively, H and C{ H} NMR spectra (complete H
1
NMR spectra are available in the supplementary material). In the H NMR spectrum of

-18-
free ligands (L and L ), signals δ(ppm) at approximately 13.8 and 8.6 were observed,
1
2
which were assigned to hydroxyl group [56] and –CH=N– (imine proton), respectively
53-56]. Aromatic hydrogen atom resonances were observed in the range of 6.00–7.40
[
and were attributed to the protons present in the aromatic region. N-alkyl hydrogens
signals were observed in the range of 2.80–3.60. After coordination to Pd(II), an upfield
chemical shift was observed (to approximately 7.71) for the imine signal in the C and
1
C complexes. Palladium-N coordination reduces the double-bond current effects in the
2
–
HC=N functional group (anisotropic effect), decreasing the deshielding [4]. This shift
is also observed in all phenyl ring hydrogens, with the biggest displacements of H3 and
H5. This is because these regions present more electronic density by the presence of
electron donating group OH [57]. Hence, these hydrogens are more likely to changes
caused by coordination. Others palladium complexes also presented the same behavior
[
4,48,58]. Furthermore, the OH signal was not observed in the spectra of the complexes,
as expected, suggesting the chelating O-N formation [56].
1
Figure 7. Detailed H NMR spectra of all synthesized compounds

-19-
1
3
Figure 8. C NMR spectra of all synthesized compounds
1
3
In the L and L2 C NMR spectrum, signals δ(ppm) at approximately 161 and
1
1
57 were observed, attributed to oxygen-bonded aromatic carbon and iminic carbon,
respectively [55]. After coordination to Pd(II), C2 and C8 for C and C presented
1
2
1
3
deshielded signals as expected at approximately 165 and 162, respectively. C DEPT
35 experiments agree with the proposed assignments (see Figure S10 in the
supplementary material).
1
3
.4. Thermal Analysis
In order to investigate the thermal behavior of the ligands and complexes,
TG-DTA curves were obtained. Figure 9 presents the TG-DTA curves of all synthesized
compounds. The thermal decomposition steps, mass loss (calculated and experimental)
and final residue percentages are summarized in the supplementary material (Table S1).
The final residues were analyzed by X-ray powder diffraction (XRD) (see Figure S11 in
0
the supplementary material), and peaks were identified on the basis of ICSD to Pd
(648680) [59].

-20-
Figure 9. TG-DTA curves of all synthesized compounds

-21-
The TG curve of compound L shows two decomposition steps, resulting in a
1
residual mass of 0.14%, which indicates complete compound decomposition.
Furthermore, the DTA curve also shows an endothermic peak at 142 ºC before
decomposition, corresponding to the melting of L . The thermal behavior of ligand L is
1
2
very similar to that of L , and the residual mass at the end of the decomposition was
1
0
.27%. An endothermic peak (105 ºC) before the decomposition start, which
corresponds to the L melting, is also observed in DTA curve.
2
The TG-DTA curves of complexes C1 and C2 show a very similar
decomposition pattern. Unlike ligands, which have well-defined melting points, the
complexes begin to decompose without melting point. Both TG curves present three
main decomposition stages. The resulting mass after the first two decomposition steps
(
20.88% for C and 19.81% for C ) is compatible with the formation of PdO (calc.
1 2
2
0.93% for C and 19.10% for C ). The mass loss between 760 and 850 °C, associated
1 2
with the observation of an endothermic peak, in the DTA curve, at 816 ºC, indicates the
0
reduction of PdO to Pd . This behavior, presented in the last step, is characteristic of
most palladium complexes and has already been widely discussed in the literature
0
[
52,53,60,61]. The residual mass of Pd obtained was 18.53% (calc. 18.19%) for C and
1
1
6.91% (calc. 16.60%) for C2.
Thermal study was also essential to determine the thermal stability of
synthesized compounds, because this is an important property for possible
pharmacological application. It is observed that coordination did not cause a significant
difference in thermal stability with respect to free ligands. All compounds are stable up
to 150 ° C. Thermal stability of the compounds can be ordered in the following
sequence: L < C < L < C .
1
1
2
2
3
.5. Computational analysis
A good accordance in the prediction of the Pd-N O coordination spheres and
2
2
among the chelate rings was observed when comparing the experimental and calculated
geometries of the palladium complexes. In C , slight changes can be seen in the
1
orientation of aniline rings and N-methyl groups, while in C , only the N-ethyl groups
2
are out of the plane (see Figure S12 in the supplementary material). Other DFT methods
including B3LYP [62,63], M06 [64], M06-2X [64], wB97XD [65] and B97D [66] were
also employed in order to compare their accuracy to predict the experimental geometry

-22-
of the structures investigated here (see Figure S13, S14 and Table S2). The differences
among the DFT methods were lower than 10% in the mean absolute error (MAE)
among bonds, angles and the dihedral angles, which points out the small influence from
the functional employed. An additional analysis on PBE0 was devoted to evaluate the
influence of more polarized basis sets functions toward the ligand [LANL2TZ/6-
3
1G(2df,2pd)], the metal [def2-TZVP/6-31G(d,p)] and also for the whole system [def2-
TZVP/def2-TZVP]. However, the small MAE values only mirrors that the PBE0
functional is less sensitive to the basis set changes (see Figure S15).
The frontier molecular orbitals shapes can be seen in Figure 10. The highest
occupied molecular orbitals (HOMO) of the Pd-complexes have an electron density
distribution that is exclusively toward the π-orbitals localized on the phenyl rings,
*
whereas the lowest unoccupied molecular orbitals (LUMO) are predominantly π in
nature and are localized in the chelate ring with a minor participation from the Pd 4d-
orbitals. In the ligands, their HOMOs are mainly composed by π-orbitals distributed in
the aniline rings and iminic moieties and with a contribution from the lone pair
*
electrons of the nitrogen atom in the amine group, while their LUMOs are π in nature
and more delocalized. Hence, no significant difference can be noted on the HOMO-
LUMO (HL) energy gap values between ligands and the metal-complexes, which occurs
due to the similar nature among the frontier molecular orbitals. The HL energy
difference of the palladium complexes was around 3.7 eV, which is in good agreement
with other Pd complexes [67-69]. Moreover, molecular electrostatic potential (MEP)
maps were also applied to gain a deeper insight into the charge distribution in the ligand
and metal-complex structures (see Figure 11). The negative charges in the ligands are
localized along the phenyl rings and the iminic groups with the positive regions mostly
on alkyl groups. In contrast, these Pd-complexes showed positive regions mainly
distributed in the coordination sphere, while negative regions are observed toward the
N,N-dialkyl-p-phenylenediamine groups and the phenyl rings of salicylaldehyde.

-23-
Figure 10. Electronic density (with an isovalue of 0.05 a.u.) of the frontier orbitals
HOMO and LUMO) with the energy difference (in eV) for the structures L and C
(
1
1
calculated in the gas phase with the PBE0 functional.

-24-
Figure 11. The molecular electrostatic potential (with an isovalue of 0.001 a.u.) of the
structures C and L mapped where the regions in blue and red indicated positive and
1
1
negative potentials, respectively. The top side view shows the front of the solid surface,
and the bottom side view shows the back of the surface, where the molecular disposition
is also displayed
Two asymmetric independent molecules were observed in the unit cell of
complex C . In this regard, a DFT simulation was performed using the crystallographic
2
coordinates as the starting point to the optimization procedure to evaluate the electronic
properties of this dimeric assemble (see Figure 12). The interaction energy was
computed considering the energy difference between the dimer motif energy and two
−1
times the energy obtained for the C structure showing a value of −4.5 kcal mol . This
2
small interaction energy only enhances that van der Waals forces govern the formation
of this self-assembled dimer similarly as can been seen elsewhere [70-73]. By viewing
the MEP surface, any change can be seen in the charge distribution with the interaction
of another C complex that mirrors the small interaction between both molecules. This
2
is in good accordance with NPA charges, which assigned the same charge distribution
in each molecule of the self-assembled dimer (as shown in Table S3 in the

-25-
supplementary material). Indeed, a very slight change is also observed when examining
the HL gap energy. In this assembly, HOMO is still characterized by π-orbitals on
aniline aromatic rings, although it is distributed along one of the phenylamine rings of
each molecule, while LUMO shows the same composition and is localized in one of the
palladium complexes. Moreover, it was possible to characterize the main interactions
involving the N-alkyl groups of the two molecules applying the second-order
perturbation theory analysis of the Fock matrix (E ) implemented in the NBO program.
2
The main interactions involve the charge transfer process from the Pd d-orbital in to the
*
σ(C−H) anti-bonding of the alkyl group [Pd-d→σ (C−H)alkyl], which is followed by the
donor-acceptor interaction between the π(C=C) bonds of the phenolate group and
*
*
σ (C−H)
[π(C=C)→σ (C−H)alkyl] as well as between the π(C=C) bonds of aniline
alkyl
*
rings and σ (C−H)alkyl. These three donor-acceptor interactions represent the total
−1
interaction energy of 1.5 kcal mol , while the other interactions represent very small
−1
contributions of 0.1 kcal mol .
Figure 12. The optimized structure of the two asymmetric independent molecules
-1
observed in the unit cell of complex C with the interaction energy (in kcal mol )
2
calculated in the gas phase with the PBE0 functional (in the left side). In the middle is
showed the molecular electrostatic potential (with an isovalue of 0.0001 a.u.). In the
right side can be seen the electronic density (with an isovalue of 0.05 a.u.) of the
frontier orbitals (HOMO and LUMO) with the energy difference (in eV).
Additional DFT calculations were also devoted to elucidate the stereochemistry
of these palladium complexes toward the cis-trans isomerism among these square-

-26-
planar Pd(II) N O -Schiff base complexes (see Figure 13). Despite that, the trans-
2
2
configuration is the usual structural arrangement in the crystal molecular packings of
palladium complexes [74-79], there are some cases where the structures can assume two
crystallographic independent cis- and trans-configurations in the crystal packing [67-
6
9] or even both forms in solution [80]. In this way, DFT simulations were also
performed to understand the main differences between these palladium complexes with
trans and cis-forms. Considering the electric dipole moments, cis-arrangements
presented values close to 11 Debye, while the trans ones have zero dipole moments due
to their symmetric structures. The energy difference between the cis-trans stereoisomers
−1
are around 5 kcal mol , which only reinforces the trans-configurations as the most
stable isomer. These energy gaps between both forms are associated with a small
increase in the electrostatic and steric repulsions. Further calculations on the solvent
effect with the polarized continuum solvent model showed any influence in the energy
gap among cis-trans orientations. In addition, a comparative analysis of the MEP
surfaces assigned any change among the stereoisomers along the localization of positive
and negative regions as well as in the NPA charge distribution. Similar molecular
orbital shapes were also predicted between both configurations, which explained the
same HL gap energies. On the other hand, slight increases in the Pd-O bond order
indices were predicted and followed by a slight decrease in the Pd-N ones.

-27-
-1
Figure 13. The relative stability (in kcal mol ) between cis-trans configurations of
complexes C and C calculated in the gas phase and on the solvent effect with the IEF-
1
2
PCM solvent model for acetonitrile (in brackets) with the PBE0 functional.
3
.6. UV-Vis and TD-DFT analysis
Time-dependent DFT (TD-DFT) calculations were carried out in order to
elucidate the UV-Vis spectra (as shown in Table S5 in the supplementary material). In
the ligands, it can be noted that the UV-Vis spectra present a large intense band varying
from 295 to 488 nm. This former band involves the transition from HOMO to LUMO,
*
*
thereby indicating mixed transitions involving π→π and η→π charge transfer
processes. These bands showed a shoulder around 330 nm, which is also centered on
mixed transitions. This mixed composition is also predicted for the bands observed
around 248 nm. In contrast, the Pd-complexes showed absorption bands at 414 nm that
are mainly assigned by a H→L transition corresponding to a ligand-to-(metal-ligand)
charge transfer (LMLCT) process with a minor contribution of a ligand-to-ligand charge
transfer (LLCT) based in the H−1→L+1 transition with a weak oscillator strength.

-28-
Another broad absorption band is detected at 315 nm in the complex C with
1
assignments based on a LMLCT and a LLCT in character, followed by transitions with
a weak oscillator strength attributed to (metal-ligand)-to-(metal-ligand) (MLMLCT) and
ligand-to-metal (LMCT) charge transfer processes. In C , this band is observed at
2
3
27 nm with a shoulder at 366 nm. The different shape of this band in C can be
2
explained by a major contribution involving a LMLCT character with different picture
among the minor contributions that also stem from a (metal-ligand)-to-ligand charge
transfer (MLLCT). In addition, the third and most intense band is around 248 nm and
can be described with a mixed LLCT/LMCT contribution. Furthermore, the results
presented here are different when compared with other Pd-complexes [67-69,81,82].
4
. CONCLUSIONS
In this work, we have successfully synthesized and characterized two new
palladium complexes, C and C with Schiff bases derived from salicylaldehyde and
1
2,
N,N-dialkyl-p-phenylenediamines. FT-IR and NMR experiments showed N,O-chelate
coordination compounds, as expected. Elemental analysis data (C,H,N) and %Pd
obtained by TG agree with the proposed structures, thus indicating the formation of two
new Schiff base palladium chelate complexes. Crystal structures of C and C were
1
2
obtained and determined by X ray diffraction confirming that coordination sphere is
square-planar with the pairs of phenolate O and imine N occupying the trans positions.
Furthermore, HS analysis showed that H...H, H...C and Pd…H are present in the
interactions in the packages. These results showed that the different torsion angles
present in the ligands may contribute to different percentages of each contacts, such as
the observed for Pd...H interactions. DFT analysis was performed to understand the
electron density distribution of the frontier molecular orbitals shapes and the molecular
electrostatic potential maps were applied the charge distribution in the ligand and metal-
complex structures. Moreover, DFT calculations toward the two asymmetric
independent molecules observed in the unit cell of complex C showed an interaction
2
−1
energy of −4.5 kcal mol , reinforcing that van der Waals forces govern the formation of
this self-assembled dimer. DFT simulations involving the cis-trans isomerism showed
−1
an energy difference around 5 kcal mol which are associated with a small increase in
the electrostatic and steric repulsions, enhancing the trans-configurations as the most
stable isomer.

-29-
5
. ACKNOWLEDGEMENTS
This study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de
Nível Superior - Brasil (CAPES) - Finance Code 001, CNPq (Conselho Nacional de
Desenvolvimento Científico e Tecnológico), FINEP (Financiadora de Estudos e
Projetos), and FAPEMIG (Fundação de Amparo à Pesquisa do Estado de Minas
Gerais) for financial support (Project: CEX - APQ-01984-14). This work is also a
collaboration research project of a member of RQ-MG (Rede Mineira de Química)
supported by FAPEMIG (Project: REDE-113/10). This research was supported by
resources supplied by the Center for Scientific Computing (NCC/GridUNESP) of the
Universidade Estadual Paulista (UNESP) and by CENAPAD-SP (Centro Nacional de
Processamento de Alto Desempenho em São Paulo).
6
. REFERENCES
[
1]
A. Palanimurugan, A. Kulandaisamy, DNA, in vitro antimicrobial/anticancer
activities and biocidal based statistical analysis of Schiff base metal complexes derived
from salicylalidene-4-imino-2, 3-dimethyl-1-phenyl-3-pyrazolin-5-one and 2-
aminothiazole, J. Organomet. Chem. 861 (2018) 263-274
https://doi.org/10.1016/j.jorganchem.2018.02.051.
[
2]
L.H. Abdel‐ Rahman, A.M Abu‐ Dief, H. Moustafa, S.K. Hamdan, Ni (II) and
Cu (II) complexes with ONNO asymmetric tetradentate Schiff base ligand: synthesis,
spectroscopic characterization, theoretical calculations, DNA interaction and
antimicrobial studies, Appl. Organomet. Chem. 31 (2017) e3555.
https://doi.org/10.1002/aoc.3555.
[
3]
S. Torabi, M. Mohammadi, M. Shirvani, Antidiabetic, Antioxidant,
Antibacterial, and Antifungal Activities of Vanadyl Schiff Base Complexes, Trends in
Pharm. Sci. 4 (2018) 87-94.
[
4]
Z. Faghih, A. Neshat, A. Wojtczak, Z. Faghih, Z. Mohammadi, S. Varestan,
Palladium (II) complexes based on Schiff base ligands derived from ortho -vanillin;
synthesis, characterization and cytotoxic studies, Inorg. Chim. Acta. 471 (2018) 404-
4
12. https://doi.org/10.1016/j.ica.2017.11.025
[
5] N.C. Campanella, M.D.S. Demartini, C. Torres, E.T. de Almeida, C.M.C.P.
Gouvêa, The cytotoxic and growth inhibitory effects of palladium (II) complexes on
MDA-MB-435 cells, Genet. Mol. Biol. 35 (2012) 159-163.
http://dx.doi.org/10.1590/S1415-47572012005000016.